1

International Geology Review Vol 46 2004 p 1ndash27Copyright copy 2004 by V H Winston amp Son Inc All rights reserved

0020-6814047081-27 $2500

Global UHP Metamorphism and ContinentalSubductionCollision The Himalayan Model

J G LIOU1

Department of Geological and Environmental Sciences Stanford University Stanford California 94305-2115

T TSUJIMORI Department of Geological and Environmental Sciences Stanford University Stanford California 94305-2115 and Research Insti-

tute of Natural Sciences Okayama University of Science Okayama 700-0005 Japan

R Y ZHANG Department of Geological and Environmental Sciences Stanford University Stanford California 94305-2115

I KATAYAMA Department of Geology and Geophysics Yale University New Haven CT 06520 and Department of Earth and Planetary Sciences

Tokyo Institute of Technology Meguro-ku Tokyo 152-8551 Japan

AND S MARUYAMA

Department of Earth and Planetary Sciences Tokyo Institute of Technology Meguro-ku Tokyo 152-8551 Japan

Abstract

Continental crust (density ~28 g cmndash3) resists subduction into the earthrsquos mantle (~33 g cmndash3)because of buoyancy However more than 20 recognized ultrahigh-pressure (UHP) terranes havebeen documented these occurrences demonstrate that not only is continental crust subducted todepths as great as 150 km but also that some supracrustal rocks were then exhumed to the earthrsquossurface UHP terranes are composed of mainly supracrustal rocks that contain minor amounts ofminerals such as coesite or diamond indicative of P gt 25 GPa In general quartzofeldspathic unitsare thoroughly back reacted and only mafic eclogite lenses and boudins retain scattered UHPphases These index minerals are restricted to micron-scale inclusions in chemically and mechani-cally resistant zircon garnet and a few other strong container minerals and are difficult to identifyby conventional petrologic studies The continental rocks were subjected to UHP metamorphism atT ranging from ~700 to 950degC and P gt 28 to 50 GPa corresponding to depths of ~100 to 150 kmThese UHP units were subsequently exhumed to crustal depths and subjected to intense hydrationand amphibolite-facies overprint Widespread Barrovian-type metamorphism in many collisionalorogens may mask an earlier higher-pressure metamorphic history We suspect that coesite-bearingUHP rocks were once generated in the majority of exhumed collisional orogens

The recent finding of coesite inclusions in rare Himalayan eclogites and country rock gneisses isa typical example We use the Himalayan model to illustrate UHP metamorphism and subduction ofcontinental crustal rocks to mantle depths and later Barrovian-type overprint during exhumationHimalayan UHP eclogites and adjacent gneisses were formed at mantle depths gt 100 km at 46 to 52Ma These rocks were exhumed to crustal depths and subjected to Barrovian amphibolite- to granu-lite-facies metamorphism associated magmatism occurred at 30 to 15 Ma The Himalayan metamor-phic belt was domally uplifted and the mountain-building process initiated since 11 Ma whenunderthrusting of the Indian tectosphere beneath the Lesser Himalayas occurred

Introduction

ULTRAHIGH-PRESSURE (UHP) metamorphism refersto the metamorphism of crustal rocks (both conti-nental and oceanic) brought to pressures high

enough to crystallize index minerals such as coesiteat a minimum P gt 27 GPa at T gt 600degC andor dia-mond Prior to the initial discoveries of coesite insupracrustal rocks (Chopin 1984 Smith 1984)coesite and diamond were thought to occur onlyin meteorite impact craters and mantle xenolithsFigure 1 shows the relevant P-T conditions defining1Corresponding author email lioupangeastanfordedu

2 LIOU ET AL

UHP high-pressure (HP) and low-pressure (LP)metamorphism in addition geotherms of about5degC kmndash1 (extreme high PT) and 20degC kmndash1 (oldplates) are illustrated

The objective of the present article is topresent a summary to the literature of continentalcrust subduction and UHP metamorphism andillustrate recent petrochemical research of variousUHP rocks To honor our teacher and mentor WG Ernst on the occasion of his retirement wehave prepared this review article to honor hiscontributions to the studies of both HP and UHPsubduction-zone metamorphism and tectonics We

emphasize the interactions between metamor-phism and tectonics in continental collision beltsusing the Himalayan orogen as an example Distri-bution and mineralogical and petrological charac-teristics of other UHP terranes in the world arealso described Similar reviews for recent mineral-ogical-petrochemical-tectonic studies of globalUHP rocks are included in Chopin (2003) Zhenget al (2003) Rumble et al (2003) and variouschapters of the book Ultrahigh-Pressure Metamor-phism published by the European MineralogicalUnion and edited by Carswell and Compagnoni(2003)

FIG 1 P-T regimes assigned to various metamorphic types (1) ultrahigh-P (2) high-P and (3) low-P Geotherms of5degCkm and 20degCkm are indicated Stabilities of diamond coesite glaucophane jadeite + quartz aragonite kyanitesillimanite andalusite paragonite and the minimum meltings of granite and tonalite are shown P-T boundaries ofvarious metamorphic facies [granulite amphibolite epidote amphibolite greenschist and subgreenschist facies] andsubdivision of the eclogite field into amphibole (Amp) eclogite epidote (Ep) eclogite lawsonite (Lw) eclogite and dryeclogite are indicated (for abbreviations see Liou et al 2000)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 3

The stabilities of coesite and other UHP mineralsin metamorphic regime require abnormally low geo-thermal gradients less than approximately 7degC kmndash1Such environments can be attained only by the sub-duction of old cold oceanic crustndashcapped litho-sphere plusmn pelagic sediments or ancient continentalcrust Eclogites with compositions of mid-oceanicridge basalt (MORB) + H2O have been experimen-tally studied (eg Schmidt and Poli 1998 Okamotoand Maruyama 1999) the results shown in Figure 1subdivide P-T regimes for the eclogite facies intothose for amphibole eclogite epidote eclogitelawsonite eclogite and dry eclogite UHP and HPmetamorphism can be separated conveniently bythe P-T boundary for the quartz-coesite equilibriumEclogitic rocks formed at pressures greater or lessthan the transition boundary have been called ldquohotrdquoor ldquocoldrdquo eclogite respectively The equilibriumboundary for the graphite-diamond transitionfurther subdivides UHP regimes into diamond gradeand coesite grade for appropriate compositions andfO2 and XCO2 conditions microdiamond plusmn coesiteoccur in diamond-grade UHP rocks

We have classified global HP and UHP belts intotwo types according to protoliths (Maruyama et al1996) In active margins of the Pacific typeprotoliths for HP metamorphism consist of rockassemblages forming an accretionary complexincluding bedded cherts MORB-origin green-stones seamount fragments and enclosing trenchturbidites On the other hand protoliths of the colli-sion-type HP-UHP rocks include continental base-ment complexes and the overlying sediment +volcanoclastic rocks of a variety of tectonic settingsFigure 2 is a schematic diagram showing the con-trasting tectonic settings for accumulation of A-typeprotoliths in a rifted continental margin and B-typeprotoliths in an active continental margin

Most UHP rocks have A-type protoliths and arecharacterized by occurrences of continental shelfcarbonates bimodal volcanics peraluminous sedi-ments and granite-gneiss basement rocks Eclogitesand garnet peridotites although volumetricallysmall are widespread in all recognized UHP ter-ranes of the world They are the most significantcomponents inasmuch as they preserve most com-pletely UHP index minerals such as coesite andmicrodiamond These mafic-ultramafic rocks mayhave originated from different tectonic settings andwere subjected to similar UHP metamorphismdeformation and retrogression along with theenclosing granitic gneisses and supracrustal rocks

Because of better preservation of the effect of UHPmetamorphism compared with adjacent gneissicrocks eclogites and garnet peridotites have receivedthe most intensive study accordingly they provideimportant petrochemical and isotopic constraints ontectonic models of continental subduction collisionand exhumation for UHP terranes

Global Distribution of UHP Metamorphic Terranes

Occurrences of UHP rocks have been increas-ingly recognized and reviewed extensively (egLiou 1999 2002 Chopin 2003 Carswell andCompagnoni 2003) Thus far more than 20 UHPterranes shown in Figure 3 have been documentedThese UHP terranes lie within major continentalplate collision belts and extend for several hundredkm or more many are in Eurasia but a few are inAfrica South America or Antarctica They sharecommon structural and lithological characteristics(1) UHP records are preserved mainly in eclogitesand garnet peridotites enclosed as pods and slabswithin gneissic units A few of these rocks containminute inclusions of coesite in zircon garnet andomphacite and microdiamonds in garnet and zir-con (2) Most lithologies are continental in chemicalcomposition (3) Exhumed UHP units are nowpresent in the upper continental crust as thin sub-horizontal slabs bounded by normal faults on topand reverse faults on bottom and sandwiched inamongst HP or lower-grade metamorphic units(4) Coeval island-arc volcanic and plutonic rocks donot occur whereas post-collisional or late-stageanorogenic granitic plutons are common in someoccurrences

In this section classical (Ernst and Liou 2000)less intensively studied (Liou 1999) and recentlyrecognized UHP terranes are described belowLithological and tectonic characteristics includingindex minerals P-T conditions size and peakmetamorphic ages of these UHP terranes are sum-marized in Table 1

Classical UHP Terranes

Several classical UHP terranes including (1) theDora Maira massif of the Western Alps (Chopin1984) (2) the Western Gneiss Region of Norway(Smith 1984) (3) the Dabie-Sulu terrane of east-central China (Wang et al 1989) (4) the Kokchetavmassif of northern Kazakhstan (Sobolev andShatsky 1990) and (5) the Bohemian massif

4 LIOU ET AL

(Bakun-Czubarow 1991) have been well studiedand extensively reviewed (eg Ernst and Liou2000) Except for the Dora Maira massif occur-rences of microdiamond in these UHP terranes havebeen reported petrochemical characteristics ofthese terranes are summarized below

Dora Maira massif Western Alps The Dora Mairamassif consists of Late Paleozoic and older conti-nental basement rocks metamorphosed under UHPand HP conditions as a result of MesozoicndashCenozoicconvergence of the European and African plates (fora recent review see Compagnoni and Rolfo 2003)The HP + UHP belt is a stack of four major unitsseparated by low-angle faults These units havebeen affected by Cenozoic HP metamorphism andlater re-equilibration at lower pressures The UHP

unit consists of dark- and light-colored eclogitespyrope quartzite with phengite schist and jadeite-rich rock inclusions as well as orthogneiss countryrocks and undeformed metagranites Boudins ofpyrope quartzite contain UHP relics such as coesiteand other UHP minerals This entity was subdividedinto two metamorphic subunits (a) a coesite-bear-ing UHP subunit contains kyanite-eclogites withrelict coesite and coesite pseudomorphs and hasestimated peak P-T conditions of 37 GPa and790degC within the diamond stability field (Schertl etal 1991) and (b) a lower-P eclogite subunit wasmetamorphosed at 15 GPa and 500degC The time ofUHPHP metamorphism has been considered tohave culminated between 90 to 125 Ma and retro-graded between 35 to 41 Ma However recent

FIG 2 Schematic cross-section contrasting the tectonic setting of generation and exhumation of (A) Collision (A)-type and (B) Pacific (B)-type HP-UHP metamorphic belts Characteristics of the Pacific type include subduction ofoceanic lithosphere and development of an accretionary complex forearc basin huge tonalite-trondhjeimite-granitoid(TTG) belt and volcanic arc Collision-type belts which are associated with continental collision lack those character-istics and the high-P belt is generally of a much large extent (after Maruyama et al 1996)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 5

FIG

3 D

istr

ibut

ion

and

peak

met

amor

phic

age

of r

ecog

nize

d U

HP

terr

anes

in th

e w

orld

(mod

ified

aft

er L

iou

et a

l 2

000)

6 LIOU ET ALTA

BL

E 1

Sum

mar

y of

Pet

roch

emic

al F

eatu

res

of R

ecog

nize

d U

HP

Ter

rane

s in

the

Wor

ld

UH

P te

rran

eU

HP

rock

type

Age

sP

-T e

stim

ates

Spec

ial f

eatu

res

Cla

ssic

al te

rran

es

Dor

a M

aira

Mas

sif

Wes

tern

Alp

s I

taly

Ecl

ogit

e P

yrop

e qu

artz

ite

Pel

ites

Whi

tesc

hist

354

plusmn 1

0 M

a (p

eak)

30 M

a (r

etro

grad

e)37

kba

r 79

0degC

1 P

yrop

e w

hite

schi

st c

onta

ins

Mg-

Al s

ilica

tes

2 N

o ga

rnet

per

idot

ite

Wes

t Gne

iss

Reg

ion

Nor

way

Ecl

ogit

eG

arne

t per

idot

ite

Dia

mon

d-be

arin

ggn

eiss

408ndash

425

Ma

(pea

k)37

4ndash37

8 M

a (r

etro

grad

e)P

gt 2

8 kb

arT

gt 7

90degC

1 R

are

diam

ond

in c

ount

ry r

ock

gnei

ss a

nd g

arne

t per

idot

ite

2 M

ajor

itic

garn

et in

per

idot

ite

form

ed a

t man

tle

~17

00 M

a

Dab

ie-S

ulu

terr

ane

Eas

t-C

entr

al C

hina

Ecl

ogit

eG

arne

t per

idot

ite

Jade

ite q

uart

zite

Min

or w

hite

schi

st

220ndash

240

Ma

(pea

k)E

clog

ite

26ndash

40 k

bar

700ndash

850deg

CG

arne

t per

idot

ite

50-6

0 kb

ar 7

00ndash8

50degC

1 A

bund

ant c

oesi

te in

gar

net

omph

acit

e k

yani

te z

oisi

te z

irco

n 2

Rar

e di

amon

d in

ecl

ogit

e3

Ecl

ogite

and

gar

net p

erid

otit

e co

ntai

n U

HP

hydr

ous

phas

es4

Dua

l ori

gin

for

garn

et p

erid

otit

e 5

Maj

oriti

c ga

rnet

in e

clog

ite

Kok

chet

av M

assi

fno

rthe

rn K

azak

hsta

nE

clog

ite

Dia

mon

d-be

arin

ggn

eiss

and

mar

ble

Whi

tesc

hist

537plusmn

9 M

a (p

eak)

507plusmn

9 M

a (r

etro

grad

e)D

iam

ond

grad

e 4

0ndash60

kbar

920

ndash100

0degC

Coe

site

gra

de 3

6 kb

ar

750ndash

800deg

C

1 M

icro

diam

ond

is a

bund

ant i

n so

me

garn

et g

neis

s an

d do

lom

ite

mar

ble

2 C

oesi

te in

clus

ions

in w

hite

schi

st e

clog

ite

and

gar

net g

neis

s3

Rar

e ga

rnet

per

idot

ite

Boh

emia

n M

assi

fE

clog

iteG

arne

t per

idot

ite

379ndash

395

Ma

gt 30

kba

r 70

0ndash80

0degC

Gar

net p

erid

otit

e yi

elds

muc

h hi

gher

P th

an c

oesi

te-e

clog

ite

Les

s in

tens

ivel

y st

udie

d te

rran

es

Zer

mat

t-Sa

as a

rea

Sw

itze

rlan

dE

clog

ite

Mn-

quar

tzit

e52

plusmn 1

8 M

a (p

eak)

26ndash3

0 kb

ar 5

90ndash6

30degC

1 P

roto

lith

s ar

e oc

eani

c af

fini

ty2

Coe

site

incl

usio

n in

the

man

tle

of g

arne

t whe

re g

arne

t cor

e ha

s qu

artz

Saxo

nian

Erz

gebi

rge

Ger

man

yE

clog

ite

Dia

mon

d-be

arin

g gn

eiss

360

plusmn 7

Ma

(pea

k)34

8ndash35

5 M

a (r

etro

grad

e)gt

42 k

bar

900ndash

1000

degC1

Inc

lusi

ons

of m

icro

diam

ond

in z

irco

n k

yani

te a

nd g

arne

t fro

m g

neis

s2

Rut

ile

with

-P

bO2

stru

ctur

e

Mal

i A

fric

aE

clog

ite

620

Ma

(pea

k)gt2

7 kb

ar 7

00ndash7

50degC

1 O

ldes

t age

of U

HP

met

amor

phis

m

Mak

bal

wes

tern

K

yrgy

zsta

nE

clog

ite

Gar

net-

bear

ing

peli

tic s

chis

t

482

Ma

gt26

kba

r 61

0ndash68

0degC

1 I

nclu

sion

s of

coe

site

pse

udom

orph

in g

arne

t fro

m e

clog

ite a

nd s

chis

t

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 7A

tbas

hy K

azak

hsta

nE

clog

ite27

0 M

agt

25 k

bar

660deg

C1

Inc

lusi

ons

of c

oesi

te in

gar

net

Cen

tral

Ind

ones

ia S

W

Sula

wes

iE

clog

ite

Gar

net p

erid

otite

115ndash

120

Ma

gt27

kbar

720

ndash760

degC33

ndash35

kbar

115

0degC

1 I

nclu

sion

s of

coe

site

in z

irco

n an

d co

esit

e ps

eudo

mor

ph in

jade

ite

Rec

entl

y re

cogn

ized

terr

anes

Fre

nch

Alp

sE

clog

ite

Met

apel

ites

400ndash

420

Ma

(pea

k)36

0ndash38

0 M

a (r

etro

grad

e)gt

28 k

bar

750deg

C1

Inc

lusi

ons

of c

oesi

te in

gar

net a

nd o

mph

acit

e

Nor

thea

st G

reen

land

Ecl

ogite

400ndash

440

Ma

24ndash2

8 kb

ar 8

20degC

1 I

nclu

sion

s of

coe

site

pse

udom

orph

in g

arne

t

Sout

heas

t Bra

zil

Ecl

ogit

e63

0 M

agt

27 k

bar

800deg

C1

Inc

lusi

ons

of c

oesi

te in

zir

con

Chu

acus

Com

plex

no

rthe

rn G

uate

mal

aE

clog

ite

Ban

ded

gnei

ss48

ndash72

Ma

(ret

rogr

ade)

20ndash3

0 kb

ar 7

00ndash8

00degC

1 C

oesi

te p

seud

omor

ph in

gar

net a

nd k

yani

te

Rho

dope

Mas

sif

Gre

ece

Ecl

ogit

eO

rtho

gnei

ssPe

litic

gne

iss

Ultr

amaf

ic r

ocks

30ndash4

2 M

a (p

eak)

8ndash12

Ma

(ret

rogr

ade)

gt 30

kba

r ~

1200

degC1

Mic

rodi

amon

d in

clus

ion

in g

arne

t por

phyr

obla

st2

Sili

ca r

util

e an

d ap

atit

e ro

ds in

gar

net

Lan

term

an R

ange

A

ntar

ctic

aE

clog

ite

Fels

ic g

neis

s50

0 M

a (p

eak)

486ndash

490

Ma

(ret

rogr

ade)

gt 29

kba

r gt

850

degC1

Inc

lusi

ons

of c

oesi

te a

nd it

s ps

eudo

mor

ph in

gar

net

Wes

tern

Tia

n-Sh

an

nort

hwes

t Chi

naE

clog

ite

Mar

ble

lt 3

10 plusmn

5 M

a (p

eak)

26ndash2

7 kb

ar 5

00ndash6

00degC

1 I

nclu

sion

s of

coe

site

and

its

pseu

dom

orph

in g

arne

t2

Pos

sibl

e co

exis

tenc

e of

mag

nesi

te +

ara

goni

te

Qai

dam

wes

tern

Chi

naE

clog

ite

Gne

iss

Gar

net p

erid

otite

495

plusmn 7

Ma

(pea

k)46

7 plusmn

1 M

a (r

etro

grad

e)gt

27 k

bar

700deg

C1

Inc

lusi

ons

of c

oesi

te in

zir

con

from

par

agne

iss

Alt

un M

ount

ains

w

este

rn C

hina

Ecl

ogit

eG

rt-l

herz

olit

e50

4 plusmn

5 M

a (p

eak)

28ndash3

2 kb

ar 8

20ndash8

50dego C

1 C

oesi

te p

seud

omor

ph in

gar

net

Nor

th Q

inli

ng M

tns

C

entr

al C

hina

Ecl

ogit

eG

neis

s50

7 plusmn

38

Ma

400

plusmn 1

6 M

agt

26 k

bar

590ndash

760deg

C1

Coe

site

in g

arne

t2

Mic

rodi

amon

d in

zir

cons

from

ecl

ogit

e an

d gn

eiss

Kag

han

vall

ey P

akis

tan

Him

alay

asE

clog

ite

Gne

iss

46 plusmn

1 M

a (p

eak)

44 plusmn

1 M

a (r

etro

grad

e)27

ndash29

kbar

690

ndash750

degC1

Inc

lusi

ons

of c

oesi

te a

nd it

s ps

eudo

mor

ph in

om

phac

ite

and

zirc

on

from

ecl

ogit

e an

d in

zir

con

from

gne

iss

Tso

Mor

ari

Indi

an H

imal

ayas

Ecl

ogit

eG

neis

s55

Ma

(pea

k)47

Ma

and

30 M

a(r

etro

grad

e)

gt 28

kba

r 70

0ndash80

0degC

1 I

nclu

sion

s of

coe

site

and

its

pseu

dom

orph

in g

arne

t fro

m e

clog

ite

8 LIOU ET AL

SHRIMP dating of zircons extracted from gneissschist pyrope inclusion and pyrope quartzite indi-cates the occurrence of 240ndash275 Ma old zirconcores and newly formed 35 Ma rims for some zircons(Gebauer et al 1997) Thus UHP metamorphismmust have taken place during Oligocene time withan estimated exhumation rate of 2 to 24 cm yrndash1

Western Gneiss Region Norway The WesternGneiss Region lies within an Early Paleozoic colli-sion zone the gneissic unit about 300 km long and150 km wide consists of interlayered pelite andmigmatite marble quartzite and amphibolite withtectonic inclusions of gabbro and peridotite (for arecent review see Carswell and Cuthbert 2003) Thegneissic unit exhibits mainly amphibolite-faciesassemblages but relics of HP assemblages alsooccur Eclogite boudins are widespread Coesite wasfirst reported from Grytting (Smith 1984) severalnew localities of coesite and coesite pseudomorphshave been recognized recently in both eclogites andthe adjacent gneissic rocks (Wain 1997 Cuthbert etal 2000 Wain et al 2000 Carswell et al 2003)Microdiamond grains 20ndash50 microns across havebeen described from residues separated from twogneisses (Dobrzhinetskaya et al 1995) the associ-ated kyanite-eclogites contain inclusions of coesitepseudomorphs in garnet Relict majoritic garnetsshowing exsolution of pyroxene lamellae in peridot-ite bodies have been recently discovered on theislands of Otrogy and Flemsoy (van Roermund andDrury 1998 van Roermund et al 2000) Theseperidotite bodies originally must have had an evenhigher-pressure deeper-mantle origin probablywithin a rising mantle diapir at ~1700 Ma In thesegarnet peridotites diamond inclusions in symple-tetic spinel after garnet have been discovered (vanRoermund et al 2002) Radiometric ages for eclog-ites of the Western Gneiss Region are 408ndash425 Mafor the peak metamorphism and 374ndash378 Ma forthe retrogressive mid-crustal metamorphic over-printing (for review see Carswell et al 1999) Car-swell et al (2003) provide new petrographicevidence and a review of the latest radiometric agedata and conclude that the UHP metamorphism ofeclogites occurred at 400ndash410 Ma significantlyyounger than the previous widely accepted age of425 Ma A two-stage exhumation process is sug-gested an initial exhumation to about 35 km depthby about 395 Ma at a mean rate of about 10 m Mandash1and subsequent exhumation to 8ndash10 km by about375 Ma at a much slower rate of about 13 m Mandash1

The suture zone occupied by UHPHP rocks ofthe Western Gneiss Region reflects collision of theeclogite-bearing Greenland sialic crust-cappedlithospheric plate with Fennoscandia during theCaledonian orogeny (Gilotti 1993 Krogh andCarswell 1995 Brueckner and Medaris 1998)Inclusions of coesite pseudomorph in garnet andomphacite from kyanite eclogites have beenrecently documented in the East Greenland eclogiteprovince (Gilotti and Ravna 2002) this new UHPHP terrane is remote but has excellent exposuresand requires close examination

Dabie-Sulu terrane of east-central China TheDabie-Sulu terrane occupies a Triassic collisionzone between the Sino-Korean and Yangtze plates(for a recent review see Hirajima and Nakamura2003) The central UHP coesite-bearing eclogitebelt (P = 27ndash5 GPa) is flanked to the north by amigmatite zone (P lt 2 GPa) and to the south by abelt of blueschist epidote amphibolite and eclog-ite-facies rocks (P = 05ndash12 GPa) Prevalent rocktypes include felsic gneiss orthogneiss marble andquartzite Protolith ages range from Proterozoic toOrdovician but the majority of reported ages areNeoproterozoic (Li et al 1993 Hacker et al 2000)Blocks boudins and layers of eclogites and garnetperidotites occur as enclaves in gneisses in the UHPunit The Dabie-Sulu UHP rocks show several char-acteristics (1) Widespread coesite with hydrousminerals such as talc zoisiteepidote and phengiteoccur in eclogites (2) Two distinct types of mantle-and crustal-derived garnet peridotites are present(3) Abundant exsolution textures were identified inUHP minerals from garnet peridotite and eclogite(4) Diamond separates from both eclogite and garnetperidotite have been reported from few restrictedregions (eg Xu et al 1992 2003) Some of thesefindings are not confirmed by thin-section observa-tion and extensive studies of mineral inclusions inzircon (for review see Liou et al 2002) (5) Somegarnet peridotites record much higher pressure thanassociated coesite-bearing eclogites reaching 5ndash6GPa (Yang et al 1993 Zhang R Y et al 20002003) (6) Various isotopic age dating methods forUHP minerals and rocks from Dabie-Sulu terranegives 220ndash240 Ma (Li et al 1993 Hacker et al2000)

Eclogites in the Huwan shear zone from thenorthwestern Dabie Mountains have SHRIMP UPbdates of 309 plusmn 3 Ma (Sun et al 2002) suggesting adiscrete Carboniferous subductioncollisionbetween North and South China Farther to the west

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 9

in North Qinling inclusions of microdiamond werediscovered in zircons from both eclogite and itsgneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyielded 507 plusmn 38 Ma for metamorphic rims and1200ndash1800 Ma for relic magmatic or old cores sug-gesting an Early Paleozoic UHP metamorphic eventThese new data suggest that episodic subductionand collision of continents occurred during stages ofclosures of paleo Tethys prior to the well-docu-mented major Triassic continent-continent collisionfor the Dabie-Sulu terrane

Kokchetav massif The Kokchetav massif north-ern Kazakhstan is the type site for the UHPdiamond-eclogite regional metamorphic facies(Sobolev and Shatsky 1990 Parkinson et al 2002Shatsky and Sobolev 2003) Diamonds occur asminute inclusions in garnet diopside and zirconporphyroblasts in marble pyroxene-carbonate-gar-net rock and garnet-biotite gneiss and schist Inclu-sions of coesite and coesite pseudomorphs occur ingarnet and zircon from eclogite and diamond-bear-ing gneiss The UHPHP unit extends NW-SE for atleast 80 km and is about 17 km wide The UHP slabis structurally overlain by a weak- to low-grademetamorphic unit and is underlain by a low-P meta-morphic andalusite-staurolite facies unit (Kaneko etal 2000) The UHP slab is composed of felsicgneiss with locally abundant eclogite lenses andminor orthogneiss metacarbonate and rare garnetperidotite with a range of Proterozoic protolith agesA well-constrained P-T-time path deduced frommineral inclusions and SHRIMP geochronology ofzircons is illustrated in Figure 4 Zoned zircons fromdiamond-bearing gneiss yield the following ages aquartz-bearing inherited core with 1100ndash1400 Maas the protolith age coesite-bearing mantle with 537plusmn 9 Ma for UHP metamorphism and the plagio-clase-bearing rim with 507 plusmn 8 Ma for amphibolite-facies overprint (Katayama et al 2001) Manydiamond-grade marbles contain clinopyroxene withexsolution lamellae of quartz K-feldspar phengiteand phlogopite (Katayama et al 2002 Zhu andOgasawara 2002) and titanite with coesite lamellae(Ogasawara et al 2002) these occurrences suggestthat UHP metamorphism occurred at P gt 6 GPa and1000degC

Cndash13 isotopic data of diamond suggest biogenicsources inclusions of water carbonate and nano-metric oxide in diamonds indicate that metamorphicmicrodiamonds in this and other UHP terranes wereprecipitated from C-O-H supercritical fluids (eg

Stockhert et al 2001) Ogasawara et al (2002)further suggested a two-stage growth mechanism formicrodiamonds Masago et al (2003) reported nega-tive 18O values (-39 per mil) for minerals in eclogiteand whiteschist of the Kokchetav Massif similar tothe Chinese UHP rocks first described by Yui et al(1995) The Kokchetav Massif is the second recog-nized UHP region that preserves a significant effectof the interaction of cold meteoric water with theprotolith prior to subduction

Saxonian Erzgebirge Germany The ErzgebirgeCrystalline Complex (ECC) at the northern margin ofthe Bohemian massif is in fault contact with a low-grade Paleozoic sequence at its margins (for a recentreview see Massonne and OrsquoBrien 2003) TheVariscan ECC consists of abundant gneisses thatinclude numerous lenses of eclogites The ldquoGneiss-Eclogite Unitrdquo at the core of this massif containsUHP rocks typified by inclusions of coesite pseudo-morph in garnets from eclogite (Schmadicke 1991)and inclusions of microdiamonds in garnet kyaniteand zircon from gneisses (Massonne 1999) Thusfar the diamondiferous gneisses seem to occur onlyin a 1 km long strip near the eastern shore of theSaidenbach Reservoir Diamond micro-inclusionsoccur exclusively in garnet kyanite and zircon ofseveral gneissic rocks Besides completely pre-served diamonds with grain sizes between 1 to 25microm partially graphitized diamonds and graphitepseudomorphs after diamond occur Such microdia-mond inclusions in garnet and zircon are similar tothose in Kokchetav diamondiferous biotite gneissSome of these inclusions contain additional fluid-bearing phases including apatite phengitic micaand possible fluid inclusions the morphologies ofinclusions and mineral associations suggest thatmicrodiamonds from both the Erzgebirge and theKokchetav were crystallized from supercritical flu-ids under UHP conditions (Stockhert et al 2001Hwang et al 2001 Dobrzhnetskaya et al 20012003)

Schmadicke et al (1995) reported Sm-Nd isoch-rons for garnet-Cpx-WR at 360 plusmn 7 Ma for eclogiteand 353 plusmn 6 Ma for garnet pyroxenite from Erzge-birge UHP rocks 40Ar39Ar spectra of phengite fromtwo eclogite samples give plateau ages of 348 plusmn 2and 355 plusmn 2 Ma Diamond-bearing zircons yieldSHRIMP UPb dates at 336 plusmn 2 Ma (Massonne2001) Such similarity in ages for garnet peridotitesand eclogite enclosed in gneiss suggests a coevalVariscan UHP in-situ metamorphism of the ECCaround 340ndash360 Ma

10 LIOU ET AL

Less Intensively Studied UHP terranes

Several UHP terranes described below exceptfor the Maksyvtov Complex have received lessintensive study mainly due to remoteness and diffi-cult access despite their long recognition

Maksyutov Complex Southern Urals TheMaksyutov Complex is an elongated (10 times 120 km)N-S belt between the Russian and Siberian cratonsin the southern Ural Mountains (Coleman and Wang1995 Dobretsov et al 1996) and contains blocky

graphitized diamond() in pelitic schist (Leech andErnst 1998) The complex is the locality from whichquartz aggregates within garnet exhibiting radialcracks were first recognized as coesite-pseudo-morphs by Chesnokov and Popov (1965) nearly 20years prior to the independent finding of coesiteinclusions by Chopin (1984) and Smith (1984) Theeclogite-bearing unit contains boudins of eclogitelayers of eclogitic gneiss and rare ultramafic bodieswithin host metasedimentary mica schist andquartzite

FIG 4 A Schematic diagram showing the growth of detrital zircon (stage 1) through subduction (stage 2 and peakstage 3) and exhumation (stage 4) of supracrustal rocks Mineral inclusions in various stages of zircon growth are shownB SHRIMP U-Pb dating of zoned zircons from a Kokchetav diamond-bearing gneiss yields a well constrained P-T-timepath (for details see Katayama et al 2001)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 11

Some Maksyutov eclogites were recrystallized at594ndash637degC with a minimum pressure from 15 to17 GPa (Beane et al 1995 Leech and Ernst 1998)or up to 27 GPa if coesite pseudomorphs (Ches-nokov and Popov 1965 Dobretsov and Doretsova1988) are indeed present However subsequentstudies have not revealed the occurrence of inclu-sions of coesite or coesite pseudomorph in garnetand zircon (Beane et al 1995 Shatsky et al 1995)Leech and Ernst (1998) described unusual graphiteaggregates (up to 13 mm edge length) that may bediamond pseudomorphs This suggestion was basedon the blocky morphology of the cuboid graphiteaggregates cross-cutting rock foliation pressureshadows biogenic isotopic signatures and otherspectroscopic characters similar to those graphi-tized diamonds from the Beni Bousera massifRecent study of several microdiamond inclusions ingarnet has yielded further suggestive possibility thatthe eclogitic unit of the Maksyutov Complex experi-enced diamond-grade UHP metamorphism (Bosticket al 2003) The UHPHP metamorphism with peakmetamorphic ages of 370ndash374 Ma (Shatsky et al1995 Beane and Connelly 1998) may be related tothe collision of the Russian platform with a fragmentof the Siberian craton during Devonian time butwas affected by the Triassic collision of the Kazakh-stan block with the Russian platform (eg Matte1998)

Zermatt-Saas area Western Alps In contrast tomost other UHP terranes in which the protolith iscontinental crust eclogites and metasediments atLago di Cignana of the Western Alps were derivedfrom oceanic crust and constitute the ophiolitesequence of the Zermatt-Saas zone Inclusions ofcoesite and coesite pseudomorphs occur in tourma-line and garnet of manganiferous quartzite and inomphacite and garnet of the underlying mafic eclog-ite (Reinecke 1991) The metasediments are mainlygarnet-phengite-quartz schist with variableamounts of garnet-clinopyroxene quartzite andpiedmontite-phengite-quartz schist Eclogites andretrograded eclogites with preserved UHP mineralparageneses consist of garnet omphacite glau-cophane zoisite phengite and dolomite Zoned gar-nets preserve prograde quartz crystals in their coresand coesite inclusions in mantles and rims Coesiteinclusions are much more common in omphacitethan in garnet UHP conditions were estimated at26ndash30 GPa and 590ndash630degC Sm-Nd isotopic anal-yses of eclogites yield UHP metamorphism at 52 plusmn18 Ma (Bowtell et al 1994) consistent with a

Tertiary age of eclogitization recently established forAlpine metamorphism in the Western Alps(Gebauer et al 1997)

Thus far the recognized areal extent of thecoesite-bearing rocks is ~ 2 km2 The apparentthickness of the UHP slice does not exceed a fewhundred meters This area is the best example forsubduction of oceanic lithosphere to mantle depthand later exhumation as a fragmented tectonic slabSimilarly the Allalin metagabbro part of the oce-anic Piemonte zone contains magnesite + talc +kyanite + garnet recrystallized at ~600degC and 35GPa with an estimated geothermal gradient of 5ndash6degC kmndash1 however inclusions of coesite or coesitepseudomorphs have not been reported from theAllalin metagabbro

Mali Africa Eclogitic rocks in the Pan-Africancollision zone cropping out in northern Mali containinclusions of coesite in omphacite from mafic nod-ules within a calc-silicate layer enclosed in impuremarble and coesite pseudomorphs in garnets fromthe surrounding eclogitic metasediments (Caby1994) These UHP eclogitic mica schists have P-Testimates of 700ndash750degC and gt 27 GPa phengiteyields a well-defined plateau 39Ar40Ar age of 1045plusmn 9 Ma Recent SmNd ages of 620 Ma wereobtained by Jahn et al (2001) for peak UHP meta-morphism This may be the oldest UHP terrane inthe world

The coesite-bearing UHP eclogitic unit about 3km thick lies within an internal nappe as a flat thinslice bounded on the top by a thick passive-marginshaly formation and on the bottom by low-gradegreenschist-facies phyllites Caby (1994) suggestedthat a large portion of the terrigenous metasedimentsof passive-continental-margin affinity of the WestAfrican plate was subducted eastward to mantledepths (gt90 km) beneath an oceanic domain Colli-sion of the West African plate with an island arcresulted in a low-angle westward subhorizontalextrusion of this subducted slice of sialic materialsto its present position The coesite-bearing eclogiteunit may also crop out 1500 km to the south in Togowhere kyanite-bearing eclogites occur as part of thepassive-margin assemblage

Makbal (480 Ma) and Atbashy (270 Ma) Kazak-hstan The Tian-Shan is divided into three mountainrangesmdashnorthern middle and southern Within theKyrgyzstan Tian-Shan UHP eclogite localities withcoesite pseudomorph inclusions in garnet andomphacite from the northern and southern Tian-Shan represent Caledonian and Hercynian orogenic

12 LIOU ET AL

belts respectively (Tagiri and Bakirov 1990 Tagiriet al 1995) The Makbal Formation in the westernKyrgyz Ridge of the northern Tian-Shan consists ofquartzose schist alternating with pelitic schistsintercalated with thin layers or lenses of marbleeclogite and amphibolitized eclogite Tagiri andBakirov (1990) found inclusions of coesite pseudo-morphs in garnet from a garnet-chloritoid-talc schistwith a peak metamorphic assemblage of coesite +almandine + chloritoid + talc + phengite (Si = 342pfu) + phlogopite + rutile Makbal coesite-gradeeclogite has a paragonite KAr age of 480 Ma

The eclogite-bearing complex of Atbashy Ridgefrom the southern Tian-Shan is composed of peliticand siliceous schists alternating with thin UHPeclogitic layers and is unconformably overlain byUpper Paleozoic molasse and silicic volcanic rocksInclusions of coesite pseudomorphs occur inomphacite cores and in garnet Eclogites from theAtbashy area preserve prograde and retrogradepaths with a peak metamorphic condition at 660degC25 GPa The Atbashy coesite-eclogites yielded aRbSr mineral isochron age of 270 Ma (Tagiri et al1995)

Central Indonesia UHP terrane The pre-Tertiarybasement of the Indonesian region comprises a vari-ety of imbricate terranes meacutelange ophiolite andvariably metamorphosed accretionary complexessome contain HP to UHP metamorphic rocks result-ing from the collision of an Australia-derived conti-nent with Eurasia (Parkinson 2003) HP rocksincluding eclogites and garnet peridotites are widelydistributed in Cretaceous accretionary complexesMany of these rocks occur as imbricate slices of car-bonate quartzite and pelitic schist of shallow-marine or continental-margin parentage interthrustwith subordinate mafic schist and serpentinitesome yield mica KAr ages of 110ndash120 Ma (Parkin-son 2003)

HP and UHP rocks are sporadically exposed astectonic blocks throughout the Cretaceous accre-tionary complexes They include eclogite garnet-glaucophane rock (P = 18ndash24 GPa T = 580ndash620degC) and jadeite-garnet quartzite (P gt 27 GPa T =720ndash760degC) in Bantimala southwest Sulawesieclogite and garnet granulite in west-centralSulawesi eclogite and jadeite-glaucophane-quartzrock (P ~22 GPa T ~530degC) in central Java Mgchloritoidndashbearing whiteschists (P ~18 GPa) insoutheast Kalimantan garnet lherzolites in east-central Sulawesi (P = 22ndash28 GPa T = 1000ndash1100degC) west-central Sulawesi (P = 16ndash20 GPa T =

1050ndash1100degC) and garnet pyroxenite (P ~2 GPa T~ 850degC) in Sabah northeast Borneo Evidence forUHP rocks include (1) inclusions of coesite inzircon and coesite pseudomorphs in jadeite fromjadeite quartzite and eclogite of the BantimalaComplex of south Sulawesi (Parkinson 2003) and(2) P-T estimates of peak-stage recrystallization at27ndash35 GPa and 1000ndash1100degC for most garnetperidotites (Kardarusman and Parkinson 2000)Many of these rocks were probably recrystallized ina N-dipping subduction zone at the margin of theSundaland craton in the Early Cretaceous Exhuma-tion may have been facilitated by the collision of aGondwana continental fragment with the Sundalandmargin at ~120ndash115 Ma

Recently Recognized UHP Terranes

Several new UHP terranes were recently identi-fied inasmuch as they contain partially preservedtrace index minerals in strong containers such aszircon or garnet In fact zircon has been consideredto be the best container and many new terranes werediscovered through positive identification of inclu-sions of coesite or coesite pseudomorphs ordiamond in zircons Detailed examination of mineralinclusions in core mantle and rims of zircon sepa-rates from eclogites and their enclosing country-rock gneisses have yielded both prograde andretrograde P-T-time paths for various UHP terranesmentioned above (see Katayama et al 2001 fordetails also see the section on the Himalayan eclog-ite for an example)

The latest recognized UHP terranes include thefollowing (1) coesite in the French Massif Central(Lardeaux et al 2001) (2) coesite and diamondfrom the Greek Rhodope metamorphic province(Mposkos and Kostopoulos 2001) (3) coesitepseudomorphs in the Northeast Greenland EclogiteProvince (Gilotti and Ravna 2002) (4) coesite ingranulite-facies overprinting eclogites of SoutheastBrazil (Parkinson et al 2001) (5) coesite in Hima-layan eclogite from the Upper Kaghan Valley Paki-stan (OrsquoBrien et al 2001 Kaneko et al 2003) andcoesite from the Tso-Morari crystalline complex ofIndia (Mukherjee et al 2003) (6) coesite inclusionsin gneissic rocks from the North Qaidam belt west-ern China (Yang et al 2001 Song et al 2003) (7)diamond inclusions in garnets of eclogite and gneissfrom North Qinling (Yang et al 2002a) (8) inclu-sions of quartz pseudomorphs and minor coesite ingarnets of mafic eclogites from the LantermanRange in Antarctica (Ghiribelli et al 2002) (9)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 13

inclusions of calcite pseudomorphs after aragonite +magnesite in dolomite in metapelites from the west-ern Tian-Shan inferred as metamorphism in a UHPregion of dolomite decomposition (Zhang L et al2002) and (10) inclusion of possible coesitepseudomorphs in garnet lamellar inclusions in gar-net kyanite and high sodium content of garnet fromeclogites of the Chuacus Complex of north-centralGuatemala (Solari et al 2003) The results of petro-chemical and geochronological data for these newterranes are summarized below The HimalayanUHP terrane in the Upper Kaghan Valley and theTso-Morari complex are used to illustrate the pro-cesses of continental subduction and collision in alater section

French Massif Central The French Massif Cen-tral in the western part of the Variscan Belt hasexperienced Late SilurianndashEarly Devonian to LateCarboniferous orogenic events (eg Matte 1986)High-pressure metamorphic rocks occur mainly inthe upper gneissic unit within the leptyno-amphibo-lite group The latter consists of an association ofmetagreywacke mica schist metabasalt leptynitemetagranite and peridotite Eclogites are associatedwith silicic and mafic HP granulites (Lardeaux etal 2001) and also with spinel andor garnet lherzo-lites (Gardien et al 1990) Inclusions of coesite andcoesite pseudomorphs occur in garnets of kyanite-bearing eclogite (Lardeaux et al 2001) Only twocoesite grains are preserved as relics most coesitegrains are completely transformed into polycrystal-line radial quartz (palisade texture) or into polygonalquartz surrounded by radial cracks Metamorphictemperatures were estimated to be 740-780degC andsubsequent amphibolite-facies overprinting at750degC at 15ndash17 GPa during decompression UHPmetamorphism occurred between 420 and 400 Ma(Paquette et al 1995) 40Ar39Ar data from amphib-ole separates from retrogressed eclogites yielded339 plusmn 4 Ma (Costa et al 1993)

Northeast Greenland Caledonides The first evi-dence for UHP metamorphism in the GreenlandCaledonites was reported from kyanite eclogites andassociated host gneisses on an island in Jokelbugt(Gilotti and Ravna 2002) Inclusions of quartzpseudomorphs exhibiting palisade structure andradiating fractures occur in garnet and omphacite ofeclogites and in garnet of the host gneisses P-T esti-mates of peak-stage metamorphism of eclogite at~972degC and 36 GPa lie well within the coesite sta-bility field SHRIMP UPb dates of 403 plusmn 5 Ma forzircon rims from HP metapsammite and 404 plusmn 4 Ma

for zircons from anatectic melt derived frommetapelite are coeval with the estimated ages ofexhumation of UHP terranes in the ScandinavianCaledonides described in a previous section(McClelland and Gilotti 2003)

Neoproterozoic nappes in Southeast BrazilNeoproterozoic nappes of Southeast Brazil consistmainly of coarse-grained kyanite-garnet granulitewith intercalations of impure quartzite calc-silicaterock and minor lenses and sills of mafic-ultramaficrocks including eclogite Early eclogitic phaseswere strongly obliterated during granulite-faciesoverprinting during a major Pan-African continentcollision at 640ndash630 Ma (Campos Neto and Caby1999) Rare ultramafic lenses consist of phlogopite-orthopyroxene rocks and garnet clinopyroxeniteCores of granulite boudins retain eclogitic assem-blages some quartz inclusions within the mantleregions of zoned garnet porphyroblasts are sur-rounded by intense radial fractures suggestive ofthe former presence of coesite In fact coarse-grained zircons in some granulites contain numer-ous micro-inclusions of K-feldspar + kyanite + coes-ite as confirmed by Raman spectroscopy (Parkinsonet al 2001) Many exsolution microstructuresobserved in other UHP terranes also occur Thisarea represents the first coesite-grade UHP rocks inthe Americas and it can be correlated with the MaliUHP terrane of equatorial Africa (Caby 1994) Ithas been suggested that eclogitic assemblages andprobable UHPM components may be present in sev-eral other correlative nappe systems at the marginsof the Sao Francisco craton in Brazil

Chuacus Complex north-central Guatemala HPeclogites within banded gneisses schists andmigmatites of the Chuacus Complex contain relicttextures and mineralogical features that suggestUHP metamorphism (Solari et al 2003) Theseinclude inclusions of possible coesite pseudomor-phs in garnet and kyanite lamellar inclusions ofrutileilmenite in garnet high sodium content ofsome garnets (up to 1200 ppm) and preliminary P-T estimates of 700ndash800degC and 2ndash3 GPa AlthoughK-Ar dating of mica and hornblende yield ages of48ndash72 Ma possible for amphibolite facies retrogres-sion regional and structural data support a pre-Mesozoic age for the multi-stage HP-UHP recrystal-lization Such a discovery opens up a new interpre-tation regarding the tectonic evolution of Maya-Chortis-Oaxaquia microcontinental blocks formingthe structural backbone of Mesoamerica

14 LIOU ET AL

Greek Rhodope The Greek Rhodope Metamor-phic Province (RMP) at the border between Greeceand Bulgaria represents a syn-metamorphic nappesystem of Alpine age that was formed during theCretaceous to Mid-Tertiary collision of Apulia andpaleo-Europe The Kimi nappe complex comprisescrustal eclogites leucocratic orthogneisses peliticgneisses and volumetrically minor mantle-derivedultramafic rocks these units contain many mineral-ogical indicators of UHP metamorphism (Mposkosand Kostopoulos 2001) The ultramafic lithologiesinclude garnet spinelndashbearing lherzolite and layersof spinel-garnet clinopyroxenite garnet pyroxeniteand clinopyroxene garnetite These rocks were sub-jected to UHP recrystallization at P-T conditions of~1200degC and gt30 GPa The pelitic gneiss containsmicrodiamond inclusions in garnet porphyroblastsas well as rods or needles of silica rutile biotiteand apatite in sodic garnet suggesting the prioroccurrence of majoritic garnet The growth of suchsupersilicic garnet was suggested to be at a pressureof ~7 GPa Inclusions of polycrystalline quartzaggregates surrounded by radial cracks occur ineclogitic garnet Exsolution lamellae of quartz ineclogitic clinopyroxene also indicate an UHP pre-cursor of supersilicic clinopyroxene

Lanterman Range Antarctica The LantermanRange in northern Victoria Land of Antarctica con-sists of several metamorphic complexes The Gate-way Hills Metamorphic Complex is a thindiscontinuous belt more than 50 km long consistingof mafic and ultramafic rocks including lenses andpods of eclogite within felsic gneiss Some less foli-ated eclogites contain inclusions of coesite andcoesite pseudomorphs in garnet porphyroblasts asconfirmed by in situ Raman spectroscopy as well asradial fractures around the inclusions (Ghiribelli etal 2002) The eclogites were subjected to UHPmetamorphism at T gt 850degC and P gt 29 GPa thenamphibolite-facies overprinting during isothermaldecompression The UHP event is dated by SmNdand 238U206Pb data at 500 Ma 40Ar39Ar ages of490ndash486 Ma for Ca-amphibole (amphibolite-faciesoverprinting) suggest rapid cooling with an averageexhumation rate of 3ndash4 km_Ma-1 for the UHP ter-rane (Di Vincenzo and Palmeri 2001)

Western Tian-Shan China Caledonian eclogitesof the western Tian-Shan China have been classi-fied into three types (Zhang L et al 2002) Type Ieclogite pods are layered with mafic blueschists andType II eclogites preserved pillow structureswhereas Type III eclogites are banded with marbles

This 200 km long eclogite-bearing belt extendswestward to connect with the Atbashy UHP belt inKazakhstan (Tagiri et al 1995) Reported UHP evi-dence includes inclusions of coesite pseudomorphsin garnets from Type I and III eclogites quartzexsolution lamellae in omphacite from Type IIeclogites and relict magnesite within matrix dolo-mite in Type III eclogites (Zhang J X et al 2002Zhang L et al 2002) P-T estimates of the peakUHP stage of metamorphism are T = 500ndash600degCand P = 26ndash27 GPa (Wei et al 2003)

North Qaidam Mountains western China Manylenses and layers of garnet peridotite and eclogiteoccur in an amphibolite-facies terrane in the NorthQaidam Mountains along the northeastern rim of theQaidam Basin This belt comprises mainly felsicgneiss quartz schist garnet-bearing gneissamphibolite and minor eclogite + garnet peridotiteit extends westward for more than 1000 km to theAltun Mountains and was displaced by the left-lat-eral Altyn-Tagh fault At the eastern end of thisUHP belt at Dulan occurrences of coesite inclu-sions in zircon separates from paragneiss inclusionsof coesite pseudomorph in garnet and omphacite ofeclogites quartz rods in eclogitic omphacite andP-T estimates of eclogites (P = 287ndash317 GPa andT = 631ndash687degC) demonstrate that this is a typicalUHP metamorphic terrane (Yang et al 2001 Songet al 2003a 2003b) The Dulan eclogites have anSm-Nd mineral isochron age of 497 plusmn 87 Ma and aSHRIMP UPb zircon age of 495 Ma (Yang et al2002b)

Altun Mountains western China This UHP ter-rane about 200 km long in the southern margin ofthe Altun Mountains is the western extension of theNorth Qaidam belt Eclogitic lenses and blocks ofvarious sizes together with garnet-lherzolite and cli-nopyroxenite are enclosed within an Early Protero-zoic sequence of amphibolite amphibole schistgarnet-bearing granitic gneiss and pelitic schistGarnet lherzolite contains magnesite that reacted toform dolomite P-T estimates of the peak magnesite-bearing assemblage yield P gt 36 GPa and 850degC(Liu et al 2001) Garnet porphyroblasts includeexsolution lamellae of clinopyroxene and rutile sim-ilar to that of eclogite reported by Ye et al (2000) asmajoritic garnet The associated eclogites containinclusions of coesite pseudomorphs in garnet withcharacteristic radial fractures this together with theexsolved quartz rods in omphacite and P-T esti-mates of 820ndash850degC and 28 to 32 GPa provideadditional evidence for UHP (Zhang J X et al

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 15

2001) An SmNd mineral isochron for eclogiticgarnet omphacite and the whole rock yields 500 plusmn10 Ma whereas an UPb isochron for zircon sepa-rates from eclogite gives an age of 504 plusmn 5 Ma(Zhang J X et al 2000)

North Qinling Mountains central China Lensesand blocks of eclogitic rocks in northern Qinling areenclosed in amphibolite-facies gneiss and garnet-bearing quartz + phengite mica schist Inclusions ofcoesite in eclogitic garnet (Hu et al 1995) andmicrodiamonds in zircon separates from both eclog-ite and gneiss (Yang et al 2002a) have beenrecently reported Although individual outcrops areno more than a few meters wide the belt of eclogitesextends more than 10 km P-T estimates of amphib-olitized coesite-bearing eclogite yield T = 590ndash758degC and P gt 26 GPa An Sm-Nd isochron for gar-net omphacite rutile amphibole and whole-rockyields 400 plusmn 16 Ma Microdiamonds were recentlydiscovered as inclusions in zircon from both eclogiteand its gneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyields 507 plusmn 38 Ma for peak UHP metamorphism(Yang et al 2002a) These data indicate that two

discrete collision events occurred between theNorth China and Yangtze cratons (1) an Early Pale-ozoic North Qinling UHP suturing and (2) a Triassicsuturing along the Dabie-Sulu UHP-HP belt TheEarly Paleozoic belt extends westward to the NorthQaidamndashAltun Mountains for more than 4000 km(Yang et al 2002b)

Himalayan UHP Eclogites

The Himalayan orogen has long been considereda classic locality of continental collision betweenthe Indian and Eurasian continents (Fig 5) Severaleclogite-facies HP rocks in the Himalayas havebeen reported since the late 1980s (eg Chaudaryand Ghazanfar 1987 Pognante and Spencer 1991de Sigoyer et al 1997 Lombardo and Rolfo 2000)Inclusions of coesite were recently discovered inomphacite and garnet in several Himalayan eclog-ites from the upper Kaghan Valley Pakistan(OrsquoBrien et al 2001 Kaneko et al 2003) Subse-quent findings of additional coesite localities in theTso Morari area (Sachan et al 2001 Mukherjee etal 2003) have established the presence of a UHPbelt in the Himalayas an active ongoing collision

FIG 5 A simplified tectonic map of the western Himalaya Hindu Kush Pamir Karakoran and western Tibet mod-ified after Figure 3 of Searle et al (2001) This map shows the locations of eclogite and UHP eclogites from both Hima-layan and North Qaidam Indus Suture Zone (ISZ) ophiolites and major structures

16 LIOU ET AL

since the Eocene (Massonne and OrsquoBrien 2003)However UHP relics are volumetrically very minorconstituents of the metamorphic belts dominatedmostly by later Barrovian overprintings In theserocks hydration related to Barrovian-zone metamor-phism has resulted in remarkably pervasive recrys-tallization during the later exhumation this eventhas almost entirely masked the preceding UHPrecord

The finding of HP-UHP evidence has led to newinsights into the Himalayan tectonic modelEngland and Houseman (1986) showed that in amodel of homogeneous thickening of the Hima-layan-Tibet boundary the maximum crustal thick-ness should be less than 70 km However theoccurrences of coesite-bearing eclogites andgneisses in the Himalayan orogen cannot beexplained by a homogeneous thickening of the crustbut rather by continental subduction as describedbelow According to available geochronologicaldata (eg Treloar et al 2003) the exhumation rateof the Himalayan UHP rocks has been estimated torange from ~1 cm yrndash1 (OrsquoBrien et al 2001) to gt3cm yrndash1 (Treloar et al 2003 Massone and OrsquoBrien2003) These estimates are higher than those calcu-lated from other UHP-HP terranes This requirestectonic emplacement of the UHP-HP Himalayanslab into shallow crust following isostatic rebounddue to slab decoupling between the Indian continentand the oceanic crust initiated during the MiddleEocene UHP rocks from both the Kaghan Valleyand Tso-Mori are described below

Kaghan Valley The Kaghan nappe in the north-western Himalayan syntaxis lies between the Indussuture (also called main mantle thrust MMT) to thenorth and the main central thrust (MCT) to thesouthwest (Pognante and Spear 1991 Spencer1993) (Fig 5) The Cretaceous Kohistan island-arcsequence lies north of the Indus suture It developedas a result of intra-oceanic subduction The maincentral thrust separates an underlying Salkhala unitfrom an overlying higher Himalayan crystalline(HHC) unit The Kaghan nappe consists mainly ofgranitic gneiss and paragneiss with minor inter-calated layers of amphibolite marble and quartziteEclogites are preserved in the cores of maficboudins (less than 1 m thick) in gneiss and aresignificantly overprinted by amphibolite assem-blages They contain fine-grained (lt 1 mm) garnetand omphacite together with aggregates of quartzchains of rutile rimmed by titanite and mm-sizedphengitic micas overgrown by randomly oriented

amphiboles up to 1 cm in length Coesite and coesitepseudomorphs occur as inclusions in omphaciteand show the characteristic palisade-quartz aggre-gate texture fractures radiating from these inclu-sions are characteristically developed in the hostpyroxene

Garnet pyroxenendashphengite barometry wasapplied to the eclogites and yielded peak P-T con-ditions of 27ndash29 GPa and 690ndash750degC (OrsquoBrien etal 2001) (Fig 6) Retrograde textures are commonin the eclogites including symplectic intergrowthsof augite amphibole and plagioclase after omphac-ite thermobarometric analyses of these assemblagesyielded 10ndash13 GPa and 600ndash710degC (Kaneko et al2003) On the other hand the dominant graniticgneisses and metapelites of the higher Himalayancrystalline unit contain typical Barrovian-zoneamphibolite-facies assemblages represented bykyanite- and staurolite-bearing garnetiferousmetapelites (Treloar 1995) Extensive overprintinghas mostly obliterated the peak assemblages matrixminerals of the gneisses yielded P-T conditions of07ndash11 GPa and 600ndash700degC (Treloar 1995) con-sistent with retrogression of the eclogites Howeverrelict coesite is preserved as inclusions in zirconthe inclusions are ovoid and approximately 10 microm indiameter (Fig 7A) This discovery of coesite-bear-ing zircons from Himalayan quartzofeldspathicgneisses demonstrates subduction of Indian conti-nental crust to the depths of UHP conditions(Kaneko et al 2003)

Geochronological studies using SmNd (garnet-omphacite) RbSr (phengite) and UPb (rutile zir-con) methods suggest that the eclogite-facies eventtook place at 40ndash50 Ma (Tonarini et al 1993 Spen-cer and Gebauer 1996) 40Ar39Ar cooling ages forhornblende and mica from gneisses were reported tobe ~43 Ma and ~25 Ma respectively (Chamberlainet al 1991) Recent SHRIMP UPb dates of zonedzircons with mineral inclusions formed at differentstages indicate the ages of non-UHP mineral-bear-ing mantle domains of zircon and UHP mineral-bearing rims are at about 50 and 46 Ma respectively(Kaneko et al 2003) A new U-Pb age of 44 plusmn 11Ma was obtained for rutile from Kaghan coesite-bearing eclogite (Treloar et al 2003) this togetherwith available data implies that exhumation of theUHP rocks from mantle depth to mid-crustal leveloccurs within a few million years

Tso Morari The Tso Morari crystalline dome islocated between the Indus suture zone on the northbordered by the Zildat detachment fault and the

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 17

Zanskar sedimentary unit on the south (de Sigoyer etal 1997) The dome represents an internal massif ofthe northern part of the higher Himalaya The TsoMorari area has been considered a typical IndianTethyan margin because of a stratigraphic succes-sion similar to the Nimaling antiform in the core ofthe dome a metamorphic basement consists of Cam-bro-Ordovician augen gneisses (Stutz 1988) Thiscomplex is covered by a metasedimentary series ofCambrian to Devonian quartzite schist and con-glomerate and is overlain by Lower Carboniferousto Triassic marble and metapelite Eclogites occuras lenses or irregular bodies within the basementand cover gneisses and are strongly overprinted byamphibolite-facies assemblages de Sigoyer et al(1997) reported peak P-T conditions for the eclog-ites at 17ndash23 GPa and 520ndash640degC (Fig 7) How-ever relics of coesite 30ndash80 microm in size were

recently found as inclusions in garnet (Sachan et al2001 Mukherjee et al 2003) typified by radialfractures (Figs 7B and 7C) for the SiO2 inclusionsa characteristic Raman spectrum is centered at 523cm-1 (Fig 7D) The country rock metasedimentswith jadeite + chloritoid + paragonite + garnetassemblages have peak P-T estimates of 18ndash22GPa and 500ndash600degC and record retrogression at theeclogite-blueschist facies transition of 13ndash18 GPaand 490ndash590degC (Guillot et al 1997)

The geochronology of the Tso Morari eclogiteshas been extensively studied and subdivided intothree different stages (de Signoyer et al 2000)Eclogitization at ~55 Ma was obtained by Lu-Hf andSm-Nd methods on garnet omphacite glaucophaneand the whole rock An amphibolite-facies overprintoccurred at 47 Ma judging by RbSr dating onphengite apatite and whole rock from metapelites

FIG 6 P-T estimates of peak- and retrograde-stage metamorphism for coesite-bearing eclogites and the country-rockgneisses from the Kaghan Valley and Tso Morari

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

2 LIOU ET AL

UHP high-pressure (HP) and low-pressure (LP)metamorphism in addition geotherms of about5degC kmndash1 (extreme high PT) and 20degC kmndash1 (oldplates) are illustrated

The objective of the present article is topresent a summary to the literature of continentalcrust subduction and UHP metamorphism andillustrate recent petrochemical research of variousUHP rocks To honor our teacher and mentor WG Ernst on the occasion of his retirement wehave prepared this review article to honor hiscontributions to the studies of both HP and UHPsubduction-zone metamorphism and tectonics We

emphasize the interactions between metamor-phism and tectonics in continental collision beltsusing the Himalayan orogen as an example Distri-bution and mineralogical and petrological charac-teristics of other UHP terranes in the world arealso described Similar reviews for recent mineral-ogical-petrochemical-tectonic studies of globalUHP rocks are included in Chopin (2003) Zhenget al (2003) Rumble et al (2003) and variouschapters of the book Ultrahigh-Pressure Metamor-phism published by the European MineralogicalUnion and edited by Carswell and Compagnoni(2003)

FIG 1 P-T regimes assigned to various metamorphic types (1) ultrahigh-P (2) high-P and (3) low-P Geotherms of5degCkm and 20degCkm are indicated Stabilities of diamond coesite glaucophane jadeite + quartz aragonite kyanitesillimanite andalusite paragonite and the minimum meltings of granite and tonalite are shown P-T boundaries ofvarious metamorphic facies [granulite amphibolite epidote amphibolite greenschist and subgreenschist facies] andsubdivision of the eclogite field into amphibole (Amp) eclogite epidote (Ep) eclogite lawsonite (Lw) eclogite and dryeclogite are indicated (for abbreviations see Liou et al 2000)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 3

The stabilities of coesite and other UHP mineralsin metamorphic regime require abnormally low geo-thermal gradients less than approximately 7degC kmndash1Such environments can be attained only by the sub-duction of old cold oceanic crustndashcapped litho-sphere plusmn pelagic sediments or ancient continentalcrust Eclogites with compositions of mid-oceanicridge basalt (MORB) + H2O have been experimen-tally studied (eg Schmidt and Poli 1998 Okamotoand Maruyama 1999) the results shown in Figure 1subdivide P-T regimes for the eclogite facies intothose for amphibole eclogite epidote eclogitelawsonite eclogite and dry eclogite UHP and HPmetamorphism can be separated conveniently bythe P-T boundary for the quartz-coesite equilibriumEclogitic rocks formed at pressures greater or lessthan the transition boundary have been called ldquohotrdquoor ldquocoldrdquo eclogite respectively The equilibriumboundary for the graphite-diamond transitionfurther subdivides UHP regimes into diamond gradeand coesite grade for appropriate compositions andfO2 and XCO2 conditions microdiamond plusmn coesiteoccur in diamond-grade UHP rocks

We have classified global HP and UHP belts intotwo types according to protoliths (Maruyama et al1996) In active margins of the Pacific typeprotoliths for HP metamorphism consist of rockassemblages forming an accretionary complexincluding bedded cherts MORB-origin green-stones seamount fragments and enclosing trenchturbidites On the other hand protoliths of the colli-sion-type HP-UHP rocks include continental base-ment complexes and the overlying sediment +volcanoclastic rocks of a variety of tectonic settingsFigure 2 is a schematic diagram showing the con-trasting tectonic settings for accumulation of A-typeprotoliths in a rifted continental margin and B-typeprotoliths in an active continental margin

Most UHP rocks have A-type protoliths and arecharacterized by occurrences of continental shelfcarbonates bimodal volcanics peraluminous sedi-ments and granite-gneiss basement rocks Eclogitesand garnet peridotites although volumetricallysmall are widespread in all recognized UHP ter-ranes of the world They are the most significantcomponents inasmuch as they preserve most com-pletely UHP index minerals such as coesite andmicrodiamond These mafic-ultramafic rocks mayhave originated from different tectonic settings andwere subjected to similar UHP metamorphismdeformation and retrogression along with theenclosing granitic gneisses and supracrustal rocks

Because of better preservation of the effect of UHPmetamorphism compared with adjacent gneissicrocks eclogites and garnet peridotites have receivedthe most intensive study accordingly they provideimportant petrochemical and isotopic constraints ontectonic models of continental subduction collisionand exhumation for UHP terranes

Global Distribution of UHP Metamorphic Terranes

Occurrences of UHP rocks have been increas-ingly recognized and reviewed extensively (egLiou 1999 2002 Chopin 2003 Carswell andCompagnoni 2003) Thus far more than 20 UHPterranes shown in Figure 3 have been documentedThese UHP terranes lie within major continentalplate collision belts and extend for several hundredkm or more many are in Eurasia but a few are inAfrica South America or Antarctica They sharecommon structural and lithological characteristics(1) UHP records are preserved mainly in eclogitesand garnet peridotites enclosed as pods and slabswithin gneissic units A few of these rocks containminute inclusions of coesite in zircon garnet andomphacite and microdiamonds in garnet and zir-con (2) Most lithologies are continental in chemicalcomposition (3) Exhumed UHP units are nowpresent in the upper continental crust as thin sub-horizontal slabs bounded by normal faults on topand reverse faults on bottom and sandwiched inamongst HP or lower-grade metamorphic units(4) Coeval island-arc volcanic and plutonic rocks donot occur whereas post-collisional or late-stageanorogenic granitic plutons are common in someoccurrences

In this section classical (Ernst and Liou 2000)less intensively studied (Liou 1999) and recentlyrecognized UHP terranes are described belowLithological and tectonic characteristics includingindex minerals P-T conditions size and peakmetamorphic ages of these UHP terranes are sum-marized in Table 1

Classical UHP Terranes

Several classical UHP terranes including (1) theDora Maira massif of the Western Alps (Chopin1984) (2) the Western Gneiss Region of Norway(Smith 1984) (3) the Dabie-Sulu terrane of east-central China (Wang et al 1989) (4) the Kokchetavmassif of northern Kazakhstan (Sobolev andShatsky 1990) and (5) the Bohemian massif

4 LIOU ET AL

(Bakun-Czubarow 1991) have been well studiedand extensively reviewed (eg Ernst and Liou2000) Except for the Dora Maira massif occur-rences of microdiamond in these UHP terranes havebeen reported petrochemical characteristics ofthese terranes are summarized below

Dora Maira massif Western Alps The Dora Mairamassif consists of Late Paleozoic and older conti-nental basement rocks metamorphosed under UHPand HP conditions as a result of MesozoicndashCenozoicconvergence of the European and African plates (fora recent review see Compagnoni and Rolfo 2003)The HP + UHP belt is a stack of four major unitsseparated by low-angle faults These units havebeen affected by Cenozoic HP metamorphism andlater re-equilibration at lower pressures The UHP

unit consists of dark- and light-colored eclogitespyrope quartzite with phengite schist and jadeite-rich rock inclusions as well as orthogneiss countryrocks and undeformed metagranites Boudins ofpyrope quartzite contain UHP relics such as coesiteand other UHP minerals This entity was subdividedinto two metamorphic subunits (a) a coesite-bear-ing UHP subunit contains kyanite-eclogites withrelict coesite and coesite pseudomorphs and hasestimated peak P-T conditions of 37 GPa and790degC within the diamond stability field (Schertl etal 1991) and (b) a lower-P eclogite subunit wasmetamorphosed at 15 GPa and 500degC The time ofUHPHP metamorphism has been considered tohave culminated between 90 to 125 Ma and retro-graded between 35 to 41 Ma However recent

FIG 2 Schematic cross-section contrasting the tectonic setting of generation and exhumation of (A) Collision (A)-type and (B) Pacific (B)-type HP-UHP metamorphic belts Characteristics of the Pacific type include subduction ofoceanic lithosphere and development of an accretionary complex forearc basin huge tonalite-trondhjeimite-granitoid(TTG) belt and volcanic arc Collision-type belts which are associated with continental collision lack those character-istics and the high-P belt is generally of a much large extent (after Maruyama et al 1996)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 5

FIG

3 D

istr

ibut

ion

and

peak

met

amor

phic

age

of r

ecog

nize

d U

HP

terr

anes

in th

e w

orld

(mod

ified

aft

er L

iou

et a

l 2

000)

6 LIOU ET ALTA

BL

E 1

Sum

mar

y of

Pet

roch

emic

al F

eatu

res

of R

ecog

nize

d U

HP

Ter

rane

s in

the

Wor

ld

UH

P te

rran

eU

HP

rock

type

Age

sP

-T e

stim

ates

Spec

ial f

eatu

res

Cla

ssic

al te

rran

es

Dor

a M

aira

Mas

sif

Wes

tern

Alp

s I

taly

Ecl

ogit

e P

yrop

e qu

artz

ite

Pel

ites

Whi

tesc

hist

354

plusmn 1

0 M

a (p

eak)

30 M

a (r

etro

grad

e)37

kba

r 79

0degC

1 P

yrop

e w

hite

schi

st c

onta

ins

Mg-

Al s

ilica

tes

2 N

o ga

rnet

per

idot

ite

Wes

t Gne

iss

Reg

ion

Nor

way

Ecl

ogit

eG

arne

t per

idot

ite

Dia

mon

d-be

arin

ggn

eiss

408ndash

425

Ma

(pea

k)37

4ndash37

8 M

a (r

etro

grad

e)P

gt 2

8 kb

arT

gt 7

90degC

1 R

are

diam

ond

in c

ount

ry r

ock

gnei

ss a

nd g

arne

t per

idot

ite

2 M

ajor

itic

garn

et in

per

idot

ite

form

ed a

t man

tle

~17

00 M

a

Dab

ie-S

ulu

terr

ane

Eas

t-C

entr

al C

hina

Ecl

ogit

eG

arne

t per

idot

ite

Jade

ite q

uart

zite

Min

or w

hite

schi

st

220ndash

240

Ma

(pea

k)E

clog

ite

26ndash

40 k

bar

700ndash

850deg

CG

arne

t per

idot

ite

50-6

0 kb

ar 7

00ndash8

50degC

1 A

bund

ant c

oesi

te in

gar

net

omph

acit

e k

yani

te z

oisi

te z

irco

n 2

Rar

e di

amon

d in

ecl

ogit

e3

Ecl

ogite

and

gar

net p

erid

otit

e co

ntai

n U

HP

hydr

ous

phas

es4

Dua

l ori

gin

for

garn

et p

erid

otit

e 5

Maj

oriti

c ga

rnet

in e

clog

ite

Kok

chet

av M

assi

fno

rthe

rn K

azak

hsta

nE

clog

ite

Dia

mon

d-be

arin

ggn

eiss

and

mar

ble

Whi

tesc

hist

537plusmn

9 M

a (p

eak)

507plusmn

9 M

a (r

etro

grad

e)D

iam

ond

grad

e 4

0ndash60

kbar

920

ndash100

0degC

Coe

site

gra

de 3

6 kb

ar

750ndash

800deg

C

1 M

icro

diam

ond

is a

bund

ant i

n so

me

garn

et g

neis

s an

d do

lom

ite

mar

ble

2 C

oesi

te in

clus

ions

in w

hite

schi

st e

clog

ite

and

gar

net g

neis

s3

Rar

e ga

rnet

per

idot

ite

Boh

emia

n M

assi

fE

clog

iteG

arne

t per

idot

ite

379ndash

395

Ma

gt 30

kba

r 70

0ndash80

0degC

Gar

net p

erid

otit

e yi

elds

muc

h hi

gher

P th

an c

oesi

te-e

clog

ite

Les

s in

tens

ivel

y st

udie

d te

rran

es

Zer

mat

t-Sa

as a

rea

Sw

itze

rlan

dE

clog

ite

Mn-

quar

tzit

e52

plusmn 1

8 M

a (p

eak)

26ndash3

0 kb

ar 5

90ndash6

30degC

1 P

roto

lith

s ar

e oc

eani

c af

fini

ty2

Coe

site

incl

usio

n in

the

man

tle

of g

arne

t whe

re g

arne

t cor

e ha

s qu

artz

Saxo

nian

Erz

gebi

rge

Ger

man

yE

clog

ite

Dia

mon

d-be

arin

g gn

eiss

360

plusmn 7

Ma

(pea

k)34

8ndash35

5 M

a (r

etro

grad

e)gt

42 k

bar

900ndash

1000

degC1

Inc

lusi

ons

of m

icro

diam

ond

in z

irco

n k

yani

te a

nd g

arne

t fro

m g

neis

s2

Rut

ile

with

-P

bO2

stru

ctur

e

Mal

i A

fric

aE

clog

ite

620

Ma

(pea

k)gt2

7 kb

ar 7

00ndash7

50degC

1 O

ldes

t age

of U

HP

met

amor

phis

m

Mak

bal

wes

tern

K

yrgy

zsta

nE

clog

ite

Gar

net-

bear

ing

peli

tic s

chis

t

482

Ma

gt26

kba

r 61

0ndash68

0degC

1 I

nclu

sion

s of

coe

site

pse

udom

orph

in g

arne

t fro

m e

clog

ite a

nd s

chis

t

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 7A

tbas

hy K

azak

hsta

nE

clog

ite27

0 M

agt

25 k

bar

660deg

C1

Inc

lusi

ons

of c

oesi

te in

gar

net

Cen

tral

Ind

ones

ia S

W

Sula

wes

iE

clog

ite

Gar

net p

erid

otite

115ndash

120

Ma

gt27

kbar

720

ndash760

degC33

ndash35

kbar

115

0degC

1 I

nclu

sion

s of

coe

site

in z

irco

n an

d co

esit

e ps

eudo

mor

ph in

jade

ite

Rec

entl

y re

cogn

ized

terr

anes

Fre

nch

Alp

sE

clog

ite

Met

apel

ites

400ndash

420

Ma

(pea

k)36

0ndash38

0 M

a (r

etro

grad

e)gt

28 k

bar

750deg

C1

Inc

lusi

ons

of c

oesi

te in

gar

net a

nd o

mph

acit

e

Nor

thea

st G

reen

land

Ecl

ogite

400ndash

440

Ma

24ndash2

8 kb

ar 8

20degC

1 I

nclu

sion

s of

coe

site

pse

udom

orph

in g

arne

t

Sout

heas

t Bra

zil

Ecl

ogit

e63

0 M

agt

27 k

bar

800deg

C1

Inc

lusi

ons

of c

oesi

te in

zir

con

Chu

acus

Com

plex

no

rthe

rn G

uate

mal

aE

clog

ite

Ban

ded

gnei

ss48

ndash72

Ma

(ret

rogr

ade)

20ndash3

0 kb

ar 7

00ndash8

00degC

1 C

oesi

te p

seud

omor

ph in

gar

net a

nd k

yani

te

Rho

dope

Mas

sif

Gre

ece

Ecl

ogit

eO

rtho

gnei

ssPe

litic

gne

iss

Ultr

amaf

ic r

ocks

30ndash4

2 M

a (p

eak)

8ndash12

Ma

(ret

rogr

ade)

gt 30

kba

r ~

1200

degC1

Mic

rodi

amon

d in

clus

ion

in g

arne

t por

phyr

obla

st2

Sili

ca r

util

e an

d ap

atit

e ro

ds in

gar

net

Lan

term

an R

ange

A

ntar

ctic

aE

clog

ite

Fels

ic g

neis

s50

0 M

a (p

eak)

486ndash

490

Ma

(ret

rogr

ade)

gt 29

kba

r gt

850

degC1

Inc

lusi

ons

of c

oesi

te a

nd it

s ps

eudo

mor

ph in

gar

net

Wes

tern

Tia

n-Sh

an

nort

hwes

t Chi

naE

clog

ite

Mar

ble

lt 3

10 plusmn

5 M

a (p

eak)

26ndash2

7 kb

ar 5

00ndash6

00degC

1 I

nclu

sion

s of

coe

site

and

its

pseu

dom

orph

in g

arne

t2

Pos

sibl

e co

exis

tenc

e of

mag

nesi

te +

ara

goni

te

Qai

dam

wes

tern

Chi

naE

clog

ite

Gne

iss

Gar

net p

erid

otite

495

plusmn 7

Ma

(pea

k)46

7 plusmn

1 M

a (r

etro

grad

e)gt

27 k

bar

700deg

C1

Inc

lusi

ons

of c

oesi

te in

zir

con

from

par

agne

iss

Alt

un M

ount

ains

w

este

rn C

hina

Ecl

ogit

eG

rt-l

herz

olit

e50

4 plusmn

5 M

a (p

eak)

28ndash3

2 kb

ar 8

20ndash8

50dego C

1 C

oesi

te p

seud

omor

ph in

gar

net

Nor

th Q

inli

ng M

tns

C

entr

al C

hina

Ecl

ogit

eG

neis

s50

7 plusmn

38

Ma

400

plusmn 1

6 M

agt

26 k

bar

590ndash

760deg

C1

Coe

site

in g

arne

t2

Mic

rodi

amon

d in

zir

cons

from

ecl

ogit

e an

d gn

eiss

Kag

han

vall

ey P

akis

tan

Him

alay

asE

clog

ite

Gne

iss

46 plusmn

1 M

a (p

eak)

44 plusmn

1 M

a (r

etro

grad

e)27

ndash29

kbar

690

ndash750

degC1

Inc

lusi

ons

of c

oesi

te a

nd it

s ps

eudo

mor

ph in

om

phac

ite

and

zirc

on

from

ecl

ogit

e an

d in

zir

con

from

gne

iss

Tso

Mor

ari

Indi

an H

imal

ayas

Ecl

ogit

eG

neis

s55

Ma

(pea

k)47

Ma

and

30 M

a(r

etro

grad

e)

gt 28

kba

r 70

0ndash80

0degC

1 I

nclu

sion

s of

coe

site

and

its

pseu

dom

orph

in g

arne

t fro

m e

clog

ite

8 LIOU ET AL

SHRIMP dating of zircons extracted from gneissschist pyrope inclusion and pyrope quartzite indi-cates the occurrence of 240ndash275 Ma old zirconcores and newly formed 35 Ma rims for some zircons(Gebauer et al 1997) Thus UHP metamorphismmust have taken place during Oligocene time withan estimated exhumation rate of 2 to 24 cm yrndash1

Western Gneiss Region Norway The WesternGneiss Region lies within an Early Paleozoic colli-sion zone the gneissic unit about 300 km long and150 km wide consists of interlayered pelite andmigmatite marble quartzite and amphibolite withtectonic inclusions of gabbro and peridotite (for arecent review see Carswell and Cuthbert 2003) Thegneissic unit exhibits mainly amphibolite-faciesassemblages but relics of HP assemblages alsooccur Eclogite boudins are widespread Coesite wasfirst reported from Grytting (Smith 1984) severalnew localities of coesite and coesite pseudomorphshave been recognized recently in both eclogites andthe adjacent gneissic rocks (Wain 1997 Cuthbert etal 2000 Wain et al 2000 Carswell et al 2003)Microdiamond grains 20ndash50 microns across havebeen described from residues separated from twogneisses (Dobrzhinetskaya et al 1995) the associ-ated kyanite-eclogites contain inclusions of coesitepseudomorphs in garnet Relict majoritic garnetsshowing exsolution of pyroxene lamellae in peridot-ite bodies have been recently discovered on theislands of Otrogy and Flemsoy (van Roermund andDrury 1998 van Roermund et al 2000) Theseperidotite bodies originally must have had an evenhigher-pressure deeper-mantle origin probablywithin a rising mantle diapir at ~1700 Ma In thesegarnet peridotites diamond inclusions in symple-tetic spinel after garnet have been discovered (vanRoermund et al 2002) Radiometric ages for eclog-ites of the Western Gneiss Region are 408ndash425 Mafor the peak metamorphism and 374ndash378 Ma forthe retrogressive mid-crustal metamorphic over-printing (for review see Carswell et al 1999) Car-swell et al (2003) provide new petrographicevidence and a review of the latest radiometric agedata and conclude that the UHP metamorphism ofeclogites occurred at 400ndash410 Ma significantlyyounger than the previous widely accepted age of425 Ma A two-stage exhumation process is sug-gested an initial exhumation to about 35 km depthby about 395 Ma at a mean rate of about 10 m Mandash1and subsequent exhumation to 8ndash10 km by about375 Ma at a much slower rate of about 13 m Mandash1

The suture zone occupied by UHPHP rocks ofthe Western Gneiss Region reflects collision of theeclogite-bearing Greenland sialic crust-cappedlithospheric plate with Fennoscandia during theCaledonian orogeny (Gilotti 1993 Krogh andCarswell 1995 Brueckner and Medaris 1998)Inclusions of coesite pseudomorph in garnet andomphacite from kyanite eclogites have beenrecently documented in the East Greenland eclogiteprovince (Gilotti and Ravna 2002) this new UHPHP terrane is remote but has excellent exposuresand requires close examination

Dabie-Sulu terrane of east-central China TheDabie-Sulu terrane occupies a Triassic collisionzone between the Sino-Korean and Yangtze plates(for a recent review see Hirajima and Nakamura2003) The central UHP coesite-bearing eclogitebelt (P = 27ndash5 GPa) is flanked to the north by amigmatite zone (P lt 2 GPa) and to the south by abelt of blueschist epidote amphibolite and eclog-ite-facies rocks (P = 05ndash12 GPa) Prevalent rocktypes include felsic gneiss orthogneiss marble andquartzite Protolith ages range from Proterozoic toOrdovician but the majority of reported ages areNeoproterozoic (Li et al 1993 Hacker et al 2000)Blocks boudins and layers of eclogites and garnetperidotites occur as enclaves in gneisses in the UHPunit The Dabie-Sulu UHP rocks show several char-acteristics (1) Widespread coesite with hydrousminerals such as talc zoisiteepidote and phengiteoccur in eclogites (2) Two distinct types of mantle-and crustal-derived garnet peridotites are present(3) Abundant exsolution textures were identified inUHP minerals from garnet peridotite and eclogite(4) Diamond separates from both eclogite and garnetperidotite have been reported from few restrictedregions (eg Xu et al 1992 2003) Some of thesefindings are not confirmed by thin-section observa-tion and extensive studies of mineral inclusions inzircon (for review see Liou et al 2002) (5) Somegarnet peridotites record much higher pressure thanassociated coesite-bearing eclogites reaching 5ndash6GPa (Yang et al 1993 Zhang R Y et al 20002003) (6) Various isotopic age dating methods forUHP minerals and rocks from Dabie-Sulu terranegives 220ndash240 Ma (Li et al 1993 Hacker et al2000)

Eclogites in the Huwan shear zone from thenorthwestern Dabie Mountains have SHRIMP UPbdates of 309 plusmn 3 Ma (Sun et al 2002) suggesting adiscrete Carboniferous subductioncollisionbetween North and South China Farther to the west

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 9

in North Qinling inclusions of microdiamond werediscovered in zircons from both eclogite and itsgneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyielded 507 plusmn 38 Ma for metamorphic rims and1200ndash1800 Ma for relic magmatic or old cores sug-gesting an Early Paleozoic UHP metamorphic eventThese new data suggest that episodic subductionand collision of continents occurred during stages ofclosures of paleo Tethys prior to the well-docu-mented major Triassic continent-continent collisionfor the Dabie-Sulu terrane

Kokchetav massif The Kokchetav massif north-ern Kazakhstan is the type site for the UHPdiamond-eclogite regional metamorphic facies(Sobolev and Shatsky 1990 Parkinson et al 2002Shatsky and Sobolev 2003) Diamonds occur asminute inclusions in garnet diopside and zirconporphyroblasts in marble pyroxene-carbonate-gar-net rock and garnet-biotite gneiss and schist Inclu-sions of coesite and coesite pseudomorphs occur ingarnet and zircon from eclogite and diamond-bear-ing gneiss The UHPHP unit extends NW-SE for atleast 80 km and is about 17 km wide The UHP slabis structurally overlain by a weak- to low-grademetamorphic unit and is underlain by a low-P meta-morphic andalusite-staurolite facies unit (Kaneko etal 2000) The UHP slab is composed of felsicgneiss with locally abundant eclogite lenses andminor orthogneiss metacarbonate and rare garnetperidotite with a range of Proterozoic protolith agesA well-constrained P-T-time path deduced frommineral inclusions and SHRIMP geochronology ofzircons is illustrated in Figure 4 Zoned zircons fromdiamond-bearing gneiss yield the following ages aquartz-bearing inherited core with 1100ndash1400 Maas the protolith age coesite-bearing mantle with 537plusmn 9 Ma for UHP metamorphism and the plagio-clase-bearing rim with 507 plusmn 8 Ma for amphibolite-facies overprint (Katayama et al 2001) Manydiamond-grade marbles contain clinopyroxene withexsolution lamellae of quartz K-feldspar phengiteand phlogopite (Katayama et al 2002 Zhu andOgasawara 2002) and titanite with coesite lamellae(Ogasawara et al 2002) these occurrences suggestthat UHP metamorphism occurred at P gt 6 GPa and1000degC

Cndash13 isotopic data of diamond suggest biogenicsources inclusions of water carbonate and nano-metric oxide in diamonds indicate that metamorphicmicrodiamonds in this and other UHP terranes wereprecipitated from C-O-H supercritical fluids (eg

Stockhert et al 2001) Ogasawara et al (2002)further suggested a two-stage growth mechanism formicrodiamonds Masago et al (2003) reported nega-tive 18O values (-39 per mil) for minerals in eclogiteand whiteschist of the Kokchetav Massif similar tothe Chinese UHP rocks first described by Yui et al(1995) The Kokchetav Massif is the second recog-nized UHP region that preserves a significant effectof the interaction of cold meteoric water with theprotolith prior to subduction

Saxonian Erzgebirge Germany The ErzgebirgeCrystalline Complex (ECC) at the northern margin ofthe Bohemian massif is in fault contact with a low-grade Paleozoic sequence at its margins (for a recentreview see Massonne and OrsquoBrien 2003) TheVariscan ECC consists of abundant gneisses thatinclude numerous lenses of eclogites The ldquoGneiss-Eclogite Unitrdquo at the core of this massif containsUHP rocks typified by inclusions of coesite pseudo-morph in garnets from eclogite (Schmadicke 1991)and inclusions of microdiamonds in garnet kyaniteand zircon from gneisses (Massonne 1999) Thusfar the diamondiferous gneisses seem to occur onlyin a 1 km long strip near the eastern shore of theSaidenbach Reservoir Diamond micro-inclusionsoccur exclusively in garnet kyanite and zircon ofseveral gneissic rocks Besides completely pre-served diamonds with grain sizes between 1 to 25microm partially graphitized diamonds and graphitepseudomorphs after diamond occur Such microdia-mond inclusions in garnet and zircon are similar tothose in Kokchetav diamondiferous biotite gneissSome of these inclusions contain additional fluid-bearing phases including apatite phengitic micaand possible fluid inclusions the morphologies ofinclusions and mineral associations suggest thatmicrodiamonds from both the Erzgebirge and theKokchetav were crystallized from supercritical flu-ids under UHP conditions (Stockhert et al 2001Hwang et al 2001 Dobrzhnetskaya et al 20012003)

Schmadicke et al (1995) reported Sm-Nd isoch-rons for garnet-Cpx-WR at 360 plusmn 7 Ma for eclogiteand 353 plusmn 6 Ma for garnet pyroxenite from Erzge-birge UHP rocks 40Ar39Ar spectra of phengite fromtwo eclogite samples give plateau ages of 348 plusmn 2and 355 plusmn 2 Ma Diamond-bearing zircons yieldSHRIMP UPb dates at 336 plusmn 2 Ma (Massonne2001) Such similarity in ages for garnet peridotitesand eclogite enclosed in gneiss suggests a coevalVariscan UHP in-situ metamorphism of the ECCaround 340ndash360 Ma

10 LIOU ET AL

Less Intensively Studied UHP terranes

Several UHP terranes described below exceptfor the Maksyvtov Complex have received lessintensive study mainly due to remoteness and diffi-cult access despite their long recognition

Maksyutov Complex Southern Urals TheMaksyutov Complex is an elongated (10 times 120 km)N-S belt between the Russian and Siberian cratonsin the southern Ural Mountains (Coleman and Wang1995 Dobretsov et al 1996) and contains blocky

graphitized diamond() in pelitic schist (Leech andErnst 1998) The complex is the locality from whichquartz aggregates within garnet exhibiting radialcracks were first recognized as coesite-pseudo-morphs by Chesnokov and Popov (1965) nearly 20years prior to the independent finding of coesiteinclusions by Chopin (1984) and Smith (1984) Theeclogite-bearing unit contains boudins of eclogitelayers of eclogitic gneiss and rare ultramafic bodieswithin host metasedimentary mica schist andquartzite

FIG 4 A Schematic diagram showing the growth of detrital zircon (stage 1) through subduction (stage 2 and peakstage 3) and exhumation (stage 4) of supracrustal rocks Mineral inclusions in various stages of zircon growth are shownB SHRIMP U-Pb dating of zoned zircons from a Kokchetav diamond-bearing gneiss yields a well constrained P-T-timepath (for details see Katayama et al 2001)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 11

Some Maksyutov eclogites were recrystallized at594ndash637degC with a minimum pressure from 15 to17 GPa (Beane et al 1995 Leech and Ernst 1998)or up to 27 GPa if coesite pseudomorphs (Ches-nokov and Popov 1965 Dobretsov and Doretsova1988) are indeed present However subsequentstudies have not revealed the occurrence of inclu-sions of coesite or coesite pseudomorph in garnetand zircon (Beane et al 1995 Shatsky et al 1995)Leech and Ernst (1998) described unusual graphiteaggregates (up to 13 mm edge length) that may bediamond pseudomorphs This suggestion was basedon the blocky morphology of the cuboid graphiteaggregates cross-cutting rock foliation pressureshadows biogenic isotopic signatures and otherspectroscopic characters similar to those graphi-tized diamonds from the Beni Bousera massifRecent study of several microdiamond inclusions ingarnet has yielded further suggestive possibility thatthe eclogitic unit of the Maksyutov Complex experi-enced diamond-grade UHP metamorphism (Bosticket al 2003) The UHPHP metamorphism with peakmetamorphic ages of 370ndash374 Ma (Shatsky et al1995 Beane and Connelly 1998) may be related tothe collision of the Russian platform with a fragmentof the Siberian craton during Devonian time butwas affected by the Triassic collision of the Kazakh-stan block with the Russian platform (eg Matte1998)

Zermatt-Saas area Western Alps In contrast tomost other UHP terranes in which the protolith iscontinental crust eclogites and metasediments atLago di Cignana of the Western Alps were derivedfrom oceanic crust and constitute the ophiolitesequence of the Zermatt-Saas zone Inclusions ofcoesite and coesite pseudomorphs occur in tourma-line and garnet of manganiferous quartzite and inomphacite and garnet of the underlying mafic eclog-ite (Reinecke 1991) The metasediments are mainlygarnet-phengite-quartz schist with variableamounts of garnet-clinopyroxene quartzite andpiedmontite-phengite-quartz schist Eclogites andretrograded eclogites with preserved UHP mineralparageneses consist of garnet omphacite glau-cophane zoisite phengite and dolomite Zoned gar-nets preserve prograde quartz crystals in their coresand coesite inclusions in mantles and rims Coesiteinclusions are much more common in omphacitethan in garnet UHP conditions were estimated at26ndash30 GPa and 590ndash630degC Sm-Nd isotopic anal-yses of eclogites yield UHP metamorphism at 52 plusmn18 Ma (Bowtell et al 1994) consistent with a

Tertiary age of eclogitization recently established forAlpine metamorphism in the Western Alps(Gebauer et al 1997)

Thus far the recognized areal extent of thecoesite-bearing rocks is ~ 2 km2 The apparentthickness of the UHP slice does not exceed a fewhundred meters This area is the best example forsubduction of oceanic lithosphere to mantle depthand later exhumation as a fragmented tectonic slabSimilarly the Allalin metagabbro part of the oce-anic Piemonte zone contains magnesite + talc +kyanite + garnet recrystallized at ~600degC and 35GPa with an estimated geothermal gradient of 5ndash6degC kmndash1 however inclusions of coesite or coesitepseudomorphs have not been reported from theAllalin metagabbro

Mali Africa Eclogitic rocks in the Pan-Africancollision zone cropping out in northern Mali containinclusions of coesite in omphacite from mafic nod-ules within a calc-silicate layer enclosed in impuremarble and coesite pseudomorphs in garnets fromthe surrounding eclogitic metasediments (Caby1994) These UHP eclogitic mica schists have P-Testimates of 700ndash750degC and gt 27 GPa phengiteyields a well-defined plateau 39Ar40Ar age of 1045plusmn 9 Ma Recent SmNd ages of 620 Ma wereobtained by Jahn et al (2001) for peak UHP meta-morphism This may be the oldest UHP terrane inthe world

The coesite-bearing UHP eclogitic unit about 3km thick lies within an internal nappe as a flat thinslice bounded on the top by a thick passive-marginshaly formation and on the bottom by low-gradegreenschist-facies phyllites Caby (1994) suggestedthat a large portion of the terrigenous metasedimentsof passive-continental-margin affinity of the WestAfrican plate was subducted eastward to mantledepths (gt90 km) beneath an oceanic domain Colli-sion of the West African plate with an island arcresulted in a low-angle westward subhorizontalextrusion of this subducted slice of sialic materialsto its present position The coesite-bearing eclogiteunit may also crop out 1500 km to the south in Togowhere kyanite-bearing eclogites occur as part of thepassive-margin assemblage

Makbal (480 Ma) and Atbashy (270 Ma) Kazak-hstan The Tian-Shan is divided into three mountainrangesmdashnorthern middle and southern Within theKyrgyzstan Tian-Shan UHP eclogite localities withcoesite pseudomorph inclusions in garnet andomphacite from the northern and southern Tian-Shan represent Caledonian and Hercynian orogenic

12 LIOU ET AL

belts respectively (Tagiri and Bakirov 1990 Tagiriet al 1995) The Makbal Formation in the westernKyrgyz Ridge of the northern Tian-Shan consists ofquartzose schist alternating with pelitic schistsintercalated with thin layers or lenses of marbleeclogite and amphibolitized eclogite Tagiri andBakirov (1990) found inclusions of coesite pseudo-morphs in garnet from a garnet-chloritoid-talc schistwith a peak metamorphic assemblage of coesite +almandine + chloritoid + talc + phengite (Si = 342pfu) + phlogopite + rutile Makbal coesite-gradeeclogite has a paragonite KAr age of 480 Ma

The eclogite-bearing complex of Atbashy Ridgefrom the southern Tian-Shan is composed of peliticand siliceous schists alternating with thin UHPeclogitic layers and is unconformably overlain byUpper Paleozoic molasse and silicic volcanic rocksInclusions of coesite pseudomorphs occur inomphacite cores and in garnet Eclogites from theAtbashy area preserve prograde and retrogradepaths with a peak metamorphic condition at 660degC25 GPa The Atbashy coesite-eclogites yielded aRbSr mineral isochron age of 270 Ma (Tagiri et al1995)

Central Indonesia UHP terrane The pre-Tertiarybasement of the Indonesian region comprises a vari-ety of imbricate terranes meacutelange ophiolite andvariably metamorphosed accretionary complexessome contain HP to UHP metamorphic rocks result-ing from the collision of an Australia-derived conti-nent with Eurasia (Parkinson 2003) HP rocksincluding eclogites and garnet peridotites are widelydistributed in Cretaceous accretionary complexesMany of these rocks occur as imbricate slices of car-bonate quartzite and pelitic schist of shallow-marine or continental-margin parentage interthrustwith subordinate mafic schist and serpentinitesome yield mica KAr ages of 110ndash120 Ma (Parkin-son 2003)

HP and UHP rocks are sporadically exposed astectonic blocks throughout the Cretaceous accre-tionary complexes They include eclogite garnet-glaucophane rock (P = 18ndash24 GPa T = 580ndash620degC) and jadeite-garnet quartzite (P gt 27 GPa T =720ndash760degC) in Bantimala southwest Sulawesieclogite and garnet granulite in west-centralSulawesi eclogite and jadeite-glaucophane-quartzrock (P ~22 GPa T ~530degC) in central Java Mgchloritoidndashbearing whiteschists (P ~18 GPa) insoutheast Kalimantan garnet lherzolites in east-central Sulawesi (P = 22ndash28 GPa T = 1000ndash1100degC) west-central Sulawesi (P = 16ndash20 GPa T =

1050ndash1100degC) and garnet pyroxenite (P ~2 GPa T~ 850degC) in Sabah northeast Borneo Evidence forUHP rocks include (1) inclusions of coesite inzircon and coesite pseudomorphs in jadeite fromjadeite quartzite and eclogite of the BantimalaComplex of south Sulawesi (Parkinson 2003) and(2) P-T estimates of peak-stage recrystallization at27ndash35 GPa and 1000ndash1100degC for most garnetperidotites (Kardarusman and Parkinson 2000)Many of these rocks were probably recrystallized ina N-dipping subduction zone at the margin of theSundaland craton in the Early Cretaceous Exhuma-tion may have been facilitated by the collision of aGondwana continental fragment with the Sundalandmargin at ~120ndash115 Ma

Recently Recognized UHP Terranes

Several new UHP terranes were recently identi-fied inasmuch as they contain partially preservedtrace index minerals in strong containers such aszircon or garnet In fact zircon has been consideredto be the best container and many new terranes werediscovered through positive identification of inclu-sions of coesite or coesite pseudomorphs ordiamond in zircons Detailed examination of mineralinclusions in core mantle and rims of zircon sepa-rates from eclogites and their enclosing country-rock gneisses have yielded both prograde andretrograde P-T-time paths for various UHP terranesmentioned above (see Katayama et al 2001 fordetails also see the section on the Himalayan eclog-ite for an example)

The latest recognized UHP terranes include thefollowing (1) coesite in the French Massif Central(Lardeaux et al 2001) (2) coesite and diamondfrom the Greek Rhodope metamorphic province(Mposkos and Kostopoulos 2001) (3) coesitepseudomorphs in the Northeast Greenland EclogiteProvince (Gilotti and Ravna 2002) (4) coesite ingranulite-facies overprinting eclogites of SoutheastBrazil (Parkinson et al 2001) (5) coesite in Hima-layan eclogite from the Upper Kaghan Valley Paki-stan (OrsquoBrien et al 2001 Kaneko et al 2003) andcoesite from the Tso-Morari crystalline complex ofIndia (Mukherjee et al 2003) (6) coesite inclusionsin gneissic rocks from the North Qaidam belt west-ern China (Yang et al 2001 Song et al 2003) (7)diamond inclusions in garnets of eclogite and gneissfrom North Qinling (Yang et al 2002a) (8) inclu-sions of quartz pseudomorphs and minor coesite ingarnets of mafic eclogites from the LantermanRange in Antarctica (Ghiribelli et al 2002) (9)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 13

inclusions of calcite pseudomorphs after aragonite +magnesite in dolomite in metapelites from the west-ern Tian-Shan inferred as metamorphism in a UHPregion of dolomite decomposition (Zhang L et al2002) and (10) inclusion of possible coesitepseudomorphs in garnet lamellar inclusions in gar-net kyanite and high sodium content of garnet fromeclogites of the Chuacus Complex of north-centralGuatemala (Solari et al 2003) The results of petro-chemical and geochronological data for these newterranes are summarized below The HimalayanUHP terrane in the Upper Kaghan Valley and theTso-Morari complex are used to illustrate the pro-cesses of continental subduction and collision in alater section

French Massif Central The French Massif Cen-tral in the western part of the Variscan Belt hasexperienced Late SilurianndashEarly Devonian to LateCarboniferous orogenic events (eg Matte 1986)High-pressure metamorphic rocks occur mainly inthe upper gneissic unit within the leptyno-amphibo-lite group The latter consists of an association ofmetagreywacke mica schist metabasalt leptynitemetagranite and peridotite Eclogites are associatedwith silicic and mafic HP granulites (Lardeaux etal 2001) and also with spinel andor garnet lherzo-lites (Gardien et al 1990) Inclusions of coesite andcoesite pseudomorphs occur in garnets of kyanite-bearing eclogite (Lardeaux et al 2001) Only twocoesite grains are preserved as relics most coesitegrains are completely transformed into polycrystal-line radial quartz (palisade texture) or into polygonalquartz surrounded by radial cracks Metamorphictemperatures were estimated to be 740-780degC andsubsequent amphibolite-facies overprinting at750degC at 15ndash17 GPa during decompression UHPmetamorphism occurred between 420 and 400 Ma(Paquette et al 1995) 40Ar39Ar data from amphib-ole separates from retrogressed eclogites yielded339 plusmn 4 Ma (Costa et al 1993)

Northeast Greenland Caledonides The first evi-dence for UHP metamorphism in the GreenlandCaledonites was reported from kyanite eclogites andassociated host gneisses on an island in Jokelbugt(Gilotti and Ravna 2002) Inclusions of quartzpseudomorphs exhibiting palisade structure andradiating fractures occur in garnet and omphacite ofeclogites and in garnet of the host gneisses P-T esti-mates of peak-stage metamorphism of eclogite at~972degC and 36 GPa lie well within the coesite sta-bility field SHRIMP UPb dates of 403 plusmn 5 Ma forzircon rims from HP metapsammite and 404 plusmn 4 Ma

for zircons from anatectic melt derived frommetapelite are coeval with the estimated ages ofexhumation of UHP terranes in the ScandinavianCaledonides described in a previous section(McClelland and Gilotti 2003)

Neoproterozoic nappes in Southeast BrazilNeoproterozoic nappes of Southeast Brazil consistmainly of coarse-grained kyanite-garnet granulitewith intercalations of impure quartzite calc-silicaterock and minor lenses and sills of mafic-ultramaficrocks including eclogite Early eclogitic phaseswere strongly obliterated during granulite-faciesoverprinting during a major Pan-African continentcollision at 640ndash630 Ma (Campos Neto and Caby1999) Rare ultramafic lenses consist of phlogopite-orthopyroxene rocks and garnet clinopyroxeniteCores of granulite boudins retain eclogitic assem-blages some quartz inclusions within the mantleregions of zoned garnet porphyroblasts are sur-rounded by intense radial fractures suggestive ofthe former presence of coesite In fact coarse-grained zircons in some granulites contain numer-ous micro-inclusions of K-feldspar + kyanite + coes-ite as confirmed by Raman spectroscopy (Parkinsonet al 2001) Many exsolution microstructuresobserved in other UHP terranes also occur Thisarea represents the first coesite-grade UHP rocks inthe Americas and it can be correlated with the MaliUHP terrane of equatorial Africa (Caby 1994) Ithas been suggested that eclogitic assemblages andprobable UHPM components may be present in sev-eral other correlative nappe systems at the marginsof the Sao Francisco craton in Brazil

Chuacus Complex north-central Guatemala HPeclogites within banded gneisses schists andmigmatites of the Chuacus Complex contain relicttextures and mineralogical features that suggestUHP metamorphism (Solari et al 2003) Theseinclude inclusions of possible coesite pseudomor-phs in garnet and kyanite lamellar inclusions ofrutileilmenite in garnet high sodium content ofsome garnets (up to 1200 ppm) and preliminary P-T estimates of 700ndash800degC and 2ndash3 GPa AlthoughK-Ar dating of mica and hornblende yield ages of48ndash72 Ma possible for amphibolite facies retrogres-sion regional and structural data support a pre-Mesozoic age for the multi-stage HP-UHP recrystal-lization Such a discovery opens up a new interpre-tation regarding the tectonic evolution of Maya-Chortis-Oaxaquia microcontinental blocks formingthe structural backbone of Mesoamerica

14 LIOU ET AL

Greek Rhodope The Greek Rhodope Metamor-phic Province (RMP) at the border between Greeceand Bulgaria represents a syn-metamorphic nappesystem of Alpine age that was formed during theCretaceous to Mid-Tertiary collision of Apulia andpaleo-Europe The Kimi nappe complex comprisescrustal eclogites leucocratic orthogneisses peliticgneisses and volumetrically minor mantle-derivedultramafic rocks these units contain many mineral-ogical indicators of UHP metamorphism (Mposkosand Kostopoulos 2001) The ultramafic lithologiesinclude garnet spinelndashbearing lherzolite and layersof spinel-garnet clinopyroxenite garnet pyroxeniteand clinopyroxene garnetite These rocks were sub-jected to UHP recrystallization at P-T conditions of~1200degC and gt30 GPa The pelitic gneiss containsmicrodiamond inclusions in garnet porphyroblastsas well as rods or needles of silica rutile biotiteand apatite in sodic garnet suggesting the prioroccurrence of majoritic garnet The growth of suchsupersilicic garnet was suggested to be at a pressureof ~7 GPa Inclusions of polycrystalline quartzaggregates surrounded by radial cracks occur ineclogitic garnet Exsolution lamellae of quartz ineclogitic clinopyroxene also indicate an UHP pre-cursor of supersilicic clinopyroxene

Lanterman Range Antarctica The LantermanRange in northern Victoria Land of Antarctica con-sists of several metamorphic complexes The Gate-way Hills Metamorphic Complex is a thindiscontinuous belt more than 50 km long consistingof mafic and ultramafic rocks including lenses andpods of eclogite within felsic gneiss Some less foli-ated eclogites contain inclusions of coesite andcoesite pseudomorphs in garnet porphyroblasts asconfirmed by in situ Raman spectroscopy as well asradial fractures around the inclusions (Ghiribelli etal 2002) The eclogites were subjected to UHPmetamorphism at T gt 850degC and P gt 29 GPa thenamphibolite-facies overprinting during isothermaldecompression The UHP event is dated by SmNdand 238U206Pb data at 500 Ma 40Ar39Ar ages of490ndash486 Ma for Ca-amphibole (amphibolite-faciesoverprinting) suggest rapid cooling with an averageexhumation rate of 3ndash4 km_Ma-1 for the UHP ter-rane (Di Vincenzo and Palmeri 2001)

Western Tian-Shan China Caledonian eclogitesof the western Tian-Shan China have been classi-fied into three types (Zhang L et al 2002) Type Ieclogite pods are layered with mafic blueschists andType II eclogites preserved pillow structureswhereas Type III eclogites are banded with marbles

This 200 km long eclogite-bearing belt extendswestward to connect with the Atbashy UHP belt inKazakhstan (Tagiri et al 1995) Reported UHP evi-dence includes inclusions of coesite pseudomorphsin garnets from Type I and III eclogites quartzexsolution lamellae in omphacite from Type IIeclogites and relict magnesite within matrix dolo-mite in Type III eclogites (Zhang J X et al 2002Zhang L et al 2002) P-T estimates of the peakUHP stage of metamorphism are T = 500ndash600degCand P = 26ndash27 GPa (Wei et al 2003)

North Qaidam Mountains western China Manylenses and layers of garnet peridotite and eclogiteoccur in an amphibolite-facies terrane in the NorthQaidam Mountains along the northeastern rim of theQaidam Basin This belt comprises mainly felsicgneiss quartz schist garnet-bearing gneissamphibolite and minor eclogite + garnet peridotiteit extends westward for more than 1000 km to theAltun Mountains and was displaced by the left-lat-eral Altyn-Tagh fault At the eastern end of thisUHP belt at Dulan occurrences of coesite inclu-sions in zircon separates from paragneiss inclusionsof coesite pseudomorph in garnet and omphacite ofeclogites quartz rods in eclogitic omphacite andP-T estimates of eclogites (P = 287ndash317 GPa andT = 631ndash687degC) demonstrate that this is a typicalUHP metamorphic terrane (Yang et al 2001 Songet al 2003a 2003b) The Dulan eclogites have anSm-Nd mineral isochron age of 497 plusmn 87 Ma and aSHRIMP UPb zircon age of 495 Ma (Yang et al2002b)

Altun Mountains western China This UHP ter-rane about 200 km long in the southern margin ofthe Altun Mountains is the western extension of theNorth Qaidam belt Eclogitic lenses and blocks ofvarious sizes together with garnet-lherzolite and cli-nopyroxenite are enclosed within an Early Protero-zoic sequence of amphibolite amphibole schistgarnet-bearing granitic gneiss and pelitic schistGarnet lherzolite contains magnesite that reacted toform dolomite P-T estimates of the peak magnesite-bearing assemblage yield P gt 36 GPa and 850degC(Liu et al 2001) Garnet porphyroblasts includeexsolution lamellae of clinopyroxene and rutile sim-ilar to that of eclogite reported by Ye et al (2000) asmajoritic garnet The associated eclogites containinclusions of coesite pseudomorphs in garnet withcharacteristic radial fractures this together with theexsolved quartz rods in omphacite and P-T esti-mates of 820ndash850degC and 28 to 32 GPa provideadditional evidence for UHP (Zhang J X et al

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 15

2001) An SmNd mineral isochron for eclogiticgarnet omphacite and the whole rock yields 500 plusmn10 Ma whereas an UPb isochron for zircon sepa-rates from eclogite gives an age of 504 plusmn 5 Ma(Zhang J X et al 2000)

North Qinling Mountains central China Lensesand blocks of eclogitic rocks in northern Qinling areenclosed in amphibolite-facies gneiss and garnet-bearing quartz + phengite mica schist Inclusions ofcoesite in eclogitic garnet (Hu et al 1995) andmicrodiamonds in zircon separates from both eclog-ite and gneiss (Yang et al 2002a) have beenrecently reported Although individual outcrops areno more than a few meters wide the belt of eclogitesextends more than 10 km P-T estimates of amphib-olitized coesite-bearing eclogite yield T = 590ndash758degC and P gt 26 GPa An Sm-Nd isochron for gar-net omphacite rutile amphibole and whole-rockyields 400 plusmn 16 Ma Microdiamonds were recentlydiscovered as inclusions in zircon from both eclogiteand its gneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyields 507 plusmn 38 Ma for peak UHP metamorphism(Yang et al 2002a) These data indicate that two

discrete collision events occurred between theNorth China and Yangtze cratons (1) an Early Pale-ozoic North Qinling UHP suturing and (2) a Triassicsuturing along the Dabie-Sulu UHP-HP belt TheEarly Paleozoic belt extends westward to the NorthQaidamndashAltun Mountains for more than 4000 km(Yang et al 2002b)

Himalayan UHP Eclogites

The Himalayan orogen has long been considereda classic locality of continental collision betweenthe Indian and Eurasian continents (Fig 5) Severaleclogite-facies HP rocks in the Himalayas havebeen reported since the late 1980s (eg Chaudaryand Ghazanfar 1987 Pognante and Spencer 1991de Sigoyer et al 1997 Lombardo and Rolfo 2000)Inclusions of coesite were recently discovered inomphacite and garnet in several Himalayan eclog-ites from the upper Kaghan Valley Pakistan(OrsquoBrien et al 2001 Kaneko et al 2003) Subse-quent findings of additional coesite localities in theTso Morari area (Sachan et al 2001 Mukherjee etal 2003) have established the presence of a UHPbelt in the Himalayas an active ongoing collision

FIG 5 A simplified tectonic map of the western Himalaya Hindu Kush Pamir Karakoran and western Tibet mod-ified after Figure 3 of Searle et al (2001) This map shows the locations of eclogite and UHP eclogites from both Hima-layan and North Qaidam Indus Suture Zone (ISZ) ophiolites and major structures

16 LIOU ET AL

since the Eocene (Massonne and OrsquoBrien 2003)However UHP relics are volumetrically very minorconstituents of the metamorphic belts dominatedmostly by later Barrovian overprintings In theserocks hydration related to Barrovian-zone metamor-phism has resulted in remarkably pervasive recrys-tallization during the later exhumation this eventhas almost entirely masked the preceding UHPrecord

The finding of HP-UHP evidence has led to newinsights into the Himalayan tectonic modelEngland and Houseman (1986) showed that in amodel of homogeneous thickening of the Hima-layan-Tibet boundary the maximum crustal thick-ness should be less than 70 km However theoccurrences of coesite-bearing eclogites andgneisses in the Himalayan orogen cannot beexplained by a homogeneous thickening of the crustbut rather by continental subduction as describedbelow According to available geochronologicaldata (eg Treloar et al 2003) the exhumation rateof the Himalayan UHP rocks has been estimated torange from ~1 cm yrndash1 (OrsquoBrien et al 2001) to gt3cm yrndash1 (Treloar et al 2003 Massone and OrsquoBrien2003) These estimates are higher than those calcu-lated from other UHP-HP terranes This requirestectonic emplacement of the UHP-HP Himalayanslab into shallow crust following isostatic rebounddue to slab decoupling between the Indian continentand the oceanic crust initiated during the MiddleEocene UHP rocks from both the Kaghan Valleyand Tso-Mori are described below

Kaghan Valley The Kaghan nappe in the north-western Himalayan syntaxis lies between the Indussuture (also called main mantle thrust MMT) to thenorth and the main central thrust (MCT) to thesouthwest (Pognante and Spear 1991 Spencer1993) (Fig 5) The Cretaceous Kohistan island-arcsequence lies north of the Indus suture It developedas a result of intra-oceanic subduction The maincentral thrust separates an underlying Salkhala unitfrom an overlying higher Himalayan crystalline(HHC) unit The Kaghan nappe consists mainly ofgranitic gneiss and paragneiss with minor inter-calated layers of amphibolite marble and quartziteEclogites are preserved in the cores of maficboudins (less than 1 m thick) in gneiss and aresignificantly overprinted by amphibolite assem-blages They contain fine-grained (lt 1 mm) garnetand omphacite together with aggregates of quartzchains of rutile rimmed by titanite and mm-sizedphengitic micas overgrown by randomly oriented

amphiboles up to 1 cm in length Coesite and coesitepseudomorphs occur as inclusions in omphaciteand show the characteristic palisade-quartz aggre-gate texture fractures radiating from these inclu-sions are characteristically developed in the hostpyroxene

Garnet pyroxenendashphengite barometry wasapplied to the eclogites and yielded peak P-T con-ditions of 27ndash29 GPa and 690ndash750degC (OrsquoBrien etal 2001) (Fig 6) Retrograde textures are commonin the eclogites including symplectic intergrowthsof augite amphibole and plagioclase after omphac-ite thermobarometric analyses of these assemblagesyielded 10ndash13 GPa and 600ndash710degC (Kaneko et al2003) On the other hand the dominant graniticgneisses and metapelites of the higher Himalayancrystalline unit contain typical Barrovian-zoneamphibolite-facies assemblages represented bykyanite- and staurolite-bearing garnetiferousmetapelites (Treloar 1995) Extensive overprintinghas mostly obliterated the peak assemblages matrixminerals of the gneisses yielded P-T conditions of07ndash11 GPa and 600ndash700degC (Treloar 1995) con-sistent with retrogression of the eclogites Howeverrelict coesite is preserved as inclusions in zirconthe inclusions are ovoid and approximately 10 microm indiameter (Fig 7A) This discovery of coesite-bear-ing zircons from Himalayan quartzofeldspathicgneisses demonstrates subduction of Indian conti-nental crust to the depths of UHP conditions(Kaneko et al 2003)

Geochronological studies using SmNd (garnet-omphacite) RbSr (phengite) and UPb (rutile zir-con) methods suggest that the eclogite-facies eventtook place at 40ndash50 Ma (Tonarini et al 1993 Spen-cer and Gebauer 1996) 40Ar39Ar cooling ages forhornblende and mica from gneisses were reported tobe ~43 Ma and ~25 Ma respectively (Chamberlainet al 1991) Recent SHRIMP UPb dates of zonedzircons with mineral inclusions formed at differentstages indicate the ages of non-UHP mineral-bear-ing mantle domains of zircon and UHP mineral-bearing rims are at about 50 and 46 Ma respectively(Kaneko et al 2003) A new U-Pb age of 44 plusmn 11Ma was obtained for rutile from Kaghan coesite-bearing eclogite (Treloar et al 2003) this togetherwith available data implies that exhumation of theUHP rocks from mantle depth to mid-crustal leveloccurs within a few million years

Tso Morari The Tso Morari crystalline dome islocated between the Indus suture zone on the northbordered by the Zildat detachment fault and the

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 17

Zanskar sedimentary unit on the south (de Sigoyer etal 1997) The dome represents an internal massif ofthe northern part of the higher Himalaya The TsoMorari area has been considered a typical IndianTethyan margin because of a stratigraphic succes-sion similar to the Nimaling antiform in the core ofthe dome a metamorphic basement consists of Cam-bro-Ordovician augen gneisses (Stutz 1988) Thiscomplex is covered by a metasedimentary series ofCambrian to Devonian quartzite schist and con-glomerate and is overlain by Lower Carboniferousto Triassic marble and metapelite Eclogites occuras lenses or irregular bodies within the basementand cover gneisses and are strongly overprinted byamphibolite-facies assemblages de Sigoyer et al(1997) reported peak P-T conditions for the eclog-ites at 17ndash23 GPa and 520ndash640degC (Fig 7) How-ever relics of coesite 30ndash80 microm in size were

recently found as inclusions in garnet (Sachan et al2001 Mukherjee et al 2003) typified by radialfractures (Figs 7B and 7C) for the SiO2 inclusionsa characteristic Raman spectrum is centered at 523cm-1 (Fig 7D) The country rock metasedimentswith jadeite + chloritoid + paragonite + garnetassemblages have peak P-T estimates of 18ndash22GPa and 500ndash600degC and record retrogression at theeclogite-blueschist facies transition of 13ndash18 GPaand 490ndash590degC (Guillot et al 1997)

The geochronology of the Tso Morari eclogiteshas been extensively studied and subdivided intothree different stages (de Signoyer et al 2000)Eclogitization at ~55 Ma was obtained by Lu-Hf andSm-Nd methods on garnet omphacite glaucophaneand the whole rock An amphibolite-facies overprintoccurred at 47 Ma judging by RbSr dating onphengite apatite and whole rock from metapelites

FIG 6 P-T estimates of peak- and retrograde-stage metamorphism for coesite-bearing eclogites and the country-rockgneisses from the Kaghan Valley and Tso Morari

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 3

The stabilities of coesite and other UHP mineralsin metamorphic regime require abnormally low geo-thermal gradients less than approximately 7degC kmndash1Such environments can be attained only by the sub-duction of old cold oceanic crustndashcapped litho-sphere plusmn pelagic sediments or ancient continentalcrust Eclogites with compositions of mid-oceanicridge basalt (MORB) + H2O have been experimen-tally studied (eg Schmidt and Poli 1998 Okamotoand Maruyama 1999) the results shown in Figure 1subdivide P-T regimes for the eclogite facies intothose for amphibole eclogite epidote eclogitelawsonite eclogite and dry eclogite UHP and HPmetamorphism can be separated conveniently bythe P-T boundary for the quartz-coesite equilibriumEclogitic rocks formed at pressures greater or lessthan the transition boundary have been called ldquohotrdquoor ldquocoldrdquo eclogite respectively The equilibriumboundary for the graphite-diamond transitionfurther subdivides UHP regimes into diamond gradeand coesite grade for appropriate compositions andfO2 and XCO2 conditions microdiamond plusmn coesiteoccur in diamond-grade UHP rocks

We have classified global HP and UHP belts intotwo types according to protoliths (Maruyama et al1996) In active margins of the Pacific typeprotoliths for HP metamorphism consist of rockassemblages forming an accretionary complexincluding bedded cherts MORB-origin green-stones seamount fragments and enclosing trenchturbidites On the other hand protoliths of the colli-sion-type HP-UHP rocks include continental base-ment complexes and the overlying sediment +volcanoclastic rocks of a variety of tectonic settingsFigure 2 is a schematic diagram showing the con-trasting tectonic settings for accumulation of A-typeprotoliths in a rifted continental margin and B-typeprotoliths in an active continental margin

Most UHP rocks have A-type protoliths and arecharacterized by occurrences of continental shelfcarbonates bimodal volcanics peraluminous sedi-ments and granite-gneiss basement rocks Eclogitesand garnet peridotites although volumetricallysmall are widespread in all recognized UHP ter-ranes of the world They are the most significantcomponents inasmuch as they preserve most com-pletely UHP index minerals such as coesite andmicrodiamond These mafic-ultramafic rocks mayhave originated from different tectonic settings andwere subjected to similar UHP metamorphismdeformation and retrogression along with theenclosing granitic gneisses and supracrustal rocks

Because of better preservation of the effect of UHPmetamorphism compared with adjacent gneissicrocks eclogites and garnet peridotites have receivedthe most intensive study accordingly they provideimportant petrochemical and isotopic constraints ontectonic models of continental subduction collisionand exhumation for UHP terranes

Global Distribution of UHP Metamorphic Terranes

Occurrences of UHP rocks have been increas-ingly recognized and reviewed extensively (egLiou 1999 2002 Chopin 2003 Carswell andCompagnoni 2003) Thus far more than 20 UHPterranes shown in Figure 3 have been documentedThese UHP terranes lie within major continentalplate collision belts and extend for several hundredkm or more many are in Eurasia but a few are inAfrica South America or Antarctica They sharecommon structural and lithological characteristics(1) UHP records are preserved mainly in eclogitesand garnet peridotites enclosed as pods and slabswithin gneissic units A few of these rocks containminute inclusions of coesite in zircon garnet andomphacite and microdiamonds in garnet and zir-con (2) Most lithologies are continental in chemicalcomposition (3) Exhumed UHP units are nowpresent in the upper continental crust as thin sub-horizontal slabs bounded by normal faults on topand reverse faults on bottom and sandwiched inamongst HP or lower-grade metamorphic units(4) Coeval island-arc volcanic and plutonic rocks donot occur whereas post-collisional or late-stageanorogenic granitic plutons are common in someoccurrences

In this section classical (Ernst and Liou 2000)less intensively studied (Liou 1999) and recentlyrecognized UHP terranes are described belowLithological and tectonic characteristics includingindex minerals P-T conditions size and peakmetamorphic ages of these UHP terranes are sum-marized in Table 1

Classical UHP Terranes

Several classical UHP terranes including (1) theDora Maira massif of the Western Alps (Chopin1984) (2) the Western Gneiss Region of Norway(Smith 1984) (3) the Dabie-Sulu terrane of east-central China (Wang et al 1989) (4) the Kokchetavmassif of northern Kazakhstan (Sobolev andShatsky 1990) and (5) the Bohemian massif

4 LIOU ET AL

(Bakun-Czubarow 1991) have been well studiedand extensively reviewed (eg Ernst and Liou2000) Except for the Dora Maira massif occur-rences of microdiamond in these UHP terranes havebeen reported petrochemical characteristics ofthese terranes are summarized below

Dora Maira massif Western Alps The Dora Mairamassif consists of Late Paleozoic and older conti-nental basement rocks metamorphosed under UHPand HP conditions as a result of MesozoicndashCenozoicconvergence of the European and African plates (fora recent review see Compagnoni and Rolfo 2003)The HP + UHP belt is a stack of four major unitsseparated by low-angle faults These units havebeen affected by Cenozoic HP metamorphism andlater re-equilibration at lower pressures The UHP

unit consists of dark- and light-colored eclogitespyrope quartzite with phengite schist and jadeite-rich rock inclusions as well as orthogneiss countryrocks and undeformed metagranites Boudins ofpyrope quartzite contain UHP relics such as coesiteand other UHP minerals This entity was subdividedinto two metamorphic subunits (a) a coesite-bear-ing UHP subunit contains kyanite-eclogites withrelict coesite and coesite pseudomorphs and hasestimated peak P-T conditions of 37 GPa and790degC within the diamond stability field (Schertl etal 1991) and (b) a lower-P eclogite subunit wasmetamorphosed at 15 GPa and 500degC The time ofUHPHP metamorphism has been considered tohave culminated between 90 to 125 Ma and retro-graded between 35 to 41 Ma However recent

FIG 2 Schematic cross-section contrasting the tectonic setting of generation and exhumation of (A) Collision (A)-type and (B) Pacific (B)-type HP-UHP metamorphic belts Characteristics of the Pacific type include subduction ofoceanic lithosphere and development of an accretionary complex forearc basin huge tonalite-trondhjeimite-granitoid(TTG) belt and volcanic arc Collision-type belts which are associated with continental collision lack those character-istics and the high-P belt is generally of a much large extent (after Maruyama et al 1996)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 5

FIG

3 D

istr

ibut

ion

and

peak

met

amor

phic

age

of r

ecog

nize

d U

HP

terr

anes

in th

e w

orld

(mod

ified

aft

er L

iou

et a

l 2

000)

6 LIOU ET ALTA

BL

E 1

Sum

mar

y of

Pet

roch

emic

al F

eatu

res

of R

ecog

nize

d U

HP

Ter

rane

s in

the

Wor

ld

UH

P te

rran

eU

HP

rock

type

Age

sP

-T e

stim

ates

Spec

ial f

eatu

res

Cla

ssic

al te

rran

es

Dor

a M

aira

Mas

sif

Wes

tern

Alp

s I

taly

Ecl

ogit

e P

yrop

e qu

artz

ite

Pel

ites

Whi

tesc

hist

354

plusmn 1

0 M

a (p

eak)

30 M

a (r

etro

grad

e)37

kba

r 79

0degC

1 P

yrop

e w

hite

schi

st c

onta

ins

Mg-

Al s

ilica

tes

2 N

o ga

rnet

per

idot

ite

Wes

t Gne

iss

Reg

ion

Nor

way

Ecl

ogit

eG

arne

t per

idot

ite

Dia

mon

d-be

arin

ggn

eiss

408ndash

425

Ma

(pea

k)37

4ndash37

8 M

a (r

etro

grad

e)P

gt 2

8 kb

arT

gt 7

90degC

1 R

are

diam

ond

in c

ount

ry r

ock

gnei

ss a

nd g

arne

t per

idot

ite

2 M

ajor

itic

garn

et in

per

idot

ite

form

ed a

t man

tle

~17

00 M

a

Dab

ie-S

ulu

terr

ane

Eas

t-C

entr

al C

hina

Ecl

ogit

eG

arne

t per

idot

ite

Jade

ite q

uart

zite

Min

or w

hite

schi

st

220ndash

240

Ma

(pea

k)E

clog

ite

26ndash

40 k

bar

700ndash

850deg

CG

arne

t per

idot

ite

50-6

0 kb

ar 7

00ndash8

50degC

1 A

bund

ant c

oesi

te in

gar

net

omph

acit

e k

yani

te z

oisi

te z

irco

n 2

Rar

e di

amon

d in

ecl

ogit

e3

Ecl

ogite

and

gar

net p

erid

otit

e co

ntai

n U

HP

hydr

ous

phas

es4

Dua

l ori

gin

for

garn

et p

erid

otit

e 5

Maj

oriti

c ga

rnet

in e

clog

ite

Kok

chet

av M

assi

fno

rthe

rn K

azak

hsta

nE

clog

ite

Dia

mon

d-be

arin

ggn

eiss

and

mar

ble

Whi

tesc

hist

537plusmn

9 M

a (p

eak)

507plusmn

9 M

a (r

etro

grad

e)D

iam

ond

grad

e 4

0ndash60

kbar

920

ndash100

0degC

Coe

site

gra

de 3

6 kb

ar

750ndash

800deg

C

1 M

icro

diam

ond

is a

bund

ant i

n so

me

garn

et g

neis

s an

d do

lom

ite

mar

ble

2 C

oesi

te in

clus

ions

in w

hite

schi

st e

clog

ite

and

gar

net g

neis

s3

Rar

e ga

rnet

per

idot

ite

Boh

emia

n M

assi

fE

clog

iteG

arne

t per

idot

ite

379ndash

395

Ma

gt 30

kba

r 70

0ndash80

0degC

Gar

net p

erid

otit

e yi

elds

muc

h hi

gher

P th

an c

oesi

te-e

clog

ite

Les

s in

tens

ivel

y st

udie

d te

rran

es

Zer

mat

t-Sa

as a

rea

Sw

itze

rlan

dE

clog

ite

Mn-

quar

tzit

e52

plusmn 1

8 M

a (p

eak)

26ndash3

0 kb

ar 5

90ndash6

30degC

1 P

roto

lith

s ar

e oc

eani

c af

fini

ty2

Coe

site

incl

usio

n in

the

man

tle

of g

arne

t whe

re g

arne

t cor

e ha

s qu

artz

Saxo

nian

Erz

gebi

rge

Ger

man

yE

clog

ite

Dia

mon

d-be

arin

g gn

eiss

360

plusmn 7

Ma

(pea

k)34

8ndash35

5 M

a (r

etro

grad

e)gt

42 k

bar

900ndash

1000

degC1

Inc

lusi

ons

of m

icro

diam

ond

in z

irco

n k

yani

te a

nd g

arne

t fro

m g

neis

s2

Rut

ile

with

-P

bO2

stru

ctur

e

Mal

i A

fric

aE

clog

ite

620

Ma

(pea

k)gt2

7 kb

ar 7

00ndash7

50degC

1 O

ldes

t age

of U

HP

met

amor

phis

m

Mak

bal

wes

tern

K

yrgy

zsta

nE

clog

ite

Gar

net-

bear

ing

peli

tic s

chis

t

482

Ma

gt26

kba

r 61

0ndash68

0degC

1 I

nclu

sion

s of

coe

site

pse

udom

orph

in g

arne

t fro

m e

clog

ite a

nd s

chis

t

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 7A

tbas

hy K

azak

hsta

nE

clog

ite27

0 M

agt

25 k

bar

660deg

C1

Inc

lusi

ons

of c

oesi

te in

gar

net

Cen

tral

Ind

ones

ia S

W

Sula

wes

iE

clog

ite

Gar

net p

erid

otite

115ndash

120

Ma

gt27

kbar

720

ndash760

degC33

ndash35

kbar

115

0degC

1 I

nclu

sion

s of

coe

site

in z

irco

n an

d co

esit

e ps

eudo

mor

ph in

jade

ite

Rec

entl

y re

cogn

ized

terr

anes

Fre

nch

Alp

sE

clog

ite

Met

apel

ites

400ndash

420

Ma

(pea

k)36

0ndash38

0 M

a (r

etro

grad

e)gt

28 k

bar

750deg

C1

Inc

lusi

ons

of c

oesi

te in

gar

net a

nd o

mph

acit

e

Nor

thea

st G

reen

land

Ecl

ogite

400ndash

440

Ma

24ndash2

8 kb

ar 8

20degC

1 I

nclu

sion

s of

coe

site

pse

udom

orph

in g

arne

t

Sout

heas

t Bra

zil

Ecl

ogit

e63

0 M

agt

27 k

bar

800deg

C1

Inc

lusi

ons

of c

oesi

te in

zir

con

Chu

acus

Com

plex

no

rthe

rn G

uate

mal

aE

clog

ite

Ban

ded

gnei

ss48

ndash72

Ma

(ret

rogr

ade)

20ndash3

0 kb

ar 7

00ndash8

00degC

1 C

oesi

te p

seud

omor

ph in

gar

net a

nd k

yani

te

Rho

dope

Mas

sif

Gre

ece

Ecl

ogit

eO

rtho

gnei

ssPe

litic

gne

iss

Ultr

amaf

ic r

ocks

30ndash4

2 M

a (p

eak)

8ndash12

Ma

(ret

rogr

ade)

gt 30

kba

r ~

1200

degC1

Mic

rodi

amon

d in

clus

ion

in g

arne

t por

phyr

obla

st2

Sili

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term

an R

ange

A

ntar

ctic

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ite

Fels

ic g

neis

s50

0 M

a (p

eak)

486ndash

490

Ma

(ret

rogr

ade)

gt 29

kba

r gt

850

degC1

Inc

lusi

ons

of c

oesi

te a

nd it

s ps

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gar

net

Wes

tern

Tia

n-Sh

an

nort

hwes

t Chi

naE

clog

ite

Mar

ble

lt 3

10 plusmn

5 M

a (p

eak)

26ndash2

7 kb

ar 5

00ndash6

00degC

1 I

nclu

sion

s of

coe

site

and

its

pseu

dom

orph

in g

arne

t2

Pos

sibl

e co

exis

tenc

e of

mag

nesi

te +

ara

goni

te

Qai

dam

wes

tern

Chi

naE

clog

ite

Gne

iss

Gar

net p

erid

otite

495

plusmn 7

Ma

(pea

k)46

7 plusmn

1 M

a (r

etro

grad

e)gt

27 k

bar

700deg

C1

Inc

lusi

ons

of c

oesi

te in

zir

con

from

par

agne

iss

Alt

un M

ount

ains

w

este

rn C

hina

Ecl

ogit

eG

rt-l

herz

olit

e50

4 plusmn

5 M

a (p

eak)

28ndash3

2 kb

ar 8

20ndash8

50dego C

1 C

oesi

te p

seud

omor

ph in

gar

net

Nor

th Q

inli

ng M

tns

C

entr

al C

hina

Ecl

ogit

eG

neis

s50

7 plusmn

38

Ma

400

plusmn 1

6 M

agt

26 k

bar

590ndash

760deg

C1

Coe

site

in g

arne

t2

Mic

rodi

amon

d in

zir

cons

from

ecl

ogit

e an

d gn

eiss

Kag

han

vall

ey P

akis

tan

Him

alay

asE

clog

ite

Gne

iss

46 plusmn

1 M

a (p

eak)

44 plusmn

1 M

a (r

etro

grad

e)27

ndash29

kbar

690

ndash750

degC1

Inc

lusi

ons

of c

oesi

te a

nd it

s ps

eudo

mor

ph in

om

phac

ite

and

zirc

on

from

ecl

ogit

e an

d in

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con

from

gne

iss

Tso

Mor

ari

Indi

an H

imal

ayas

Ecl

ogit

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neis

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Ma

(pea

k)47

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and

30 M

a(r

etro

grad

e)

gt 28

kba

r 70

0ndash80

0degC

1 I

nclu

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m e

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8 LIOU ET AL

SHRIMP dating of zircons extracted from gneissschist pyrope inclusion and pyrope quartzite indi-cates the occurrence of 240ndash275 Ma old zirconcores and newly formed 35 Ma rims for some zircons(Gebauer et al 1997) Thus UHP metamorphismmust have taken place during Oligocene time withan estimated exhumation rate of 2 to 24 cm yrndash1

Western Gneiss Region Norway The WesternGneiss Region lies within an Early Paleozoic colli-sion zone the gneissic unit about 300 km long and150 km wide consists of interlayered pelite andmigmatite marble quartzite and amphibolite withtectonic inclusions of gabbro and peridotite (for arecent review see Carswell and Cuthbert 2003) Thegneissic unit exhibits mainly amphibolite-faciesassemblages but relics of HP assemblages alsooccur Eclogite boudins are widespread Coesite wasfirst reported from Grytting (Smith 1984) severalnew localities of coesite and coesite pseudomorphshave been recognized recently in both eclogites andthe adjacent gneissic rocks (Wain 1997 Cuthbert etal 2000 Wain et al 2000 Carswell et al 2003)Microdiamond grains 20ndash50 microns across havebeen described from residues separated from twogneisses (Dobrzhinetskaya et al 1995) the associ-ated kyanite-eclogites contain inclusions of coesitepseudomorphs in garnet Relict majoritic garnetsshowing exsolution of pyroxene lamellae in peridot-ite bodies have been recently discovered on theislands of Otrogy and Flemsoy (van Roermund andDrury 1998 van Roermund et al 2000) Theseperidotite bodies originally must have had an evenhigher-pressure deeper-mantle origin probablywithin a rising mantle diapir at ~1700 Ma In thesegarnet peridotites diamond inclusions in symple-tetic spinel after garnet have been discovered (vanRoermund et al 2002) Radiometric ages for eclog-ites of the Western Gneiss Region are 408ndash425 Mafor the peak metamorphism and 374ndash378 Ma forthe retrogressive mid-crustal metamorphic over-printing (for review see Carswell et al 1999) Car-swell et al (2003) provide new petrographicevidence and a review of the latest radiometric agedata and conclude that the UHP metamorphism ofeclogites occurred at 400ndash410 Ma significantlyyounger than the previous widely accepted age of425 Ma A two-stage exhumation process is sug-gested an initial exhumation to about 35 km depthby about 395 Ma at a mean rate of about 10 m Mandash1and subsequent exhumation to 8ndash10 km by about375 Ma at a much slower rate of about 13 m Mandash1

The suture zone occupied by UHPHP rocks ofthe Western Gneiss Region reflects collision of theeclogite-bearing Greenland sialic crust-cappedlithospheric plate with Fennoscandia during theCaledonian orogeny (Gilotti 1993 Krogh andCarswell 1995 Brueckner and Medaris 1998)Inclusions of coesite pseudomorph in garnet andomphacite from kyanite eclogites have beenrecently documented in the East Greenland eclogiteprovince (Gilotti and Ravna 2002) this new UHPHP terrane is remote but has excellent exposuresand requires close examination

Dabie-Sulu terrane of east-central China TheDabie-Sulu terrane occupies a Triassic collisionzone between the Sino-Korean and Yangtze plates(for a recent review see Hirajima and Nakamura2003) The central UHP coesite-bearing eclogitebelt (P = 27ndash5 GPa) is flanked to the north by amigmatite zone (P lt 2 GPa) and to the south by abelt of blueschist epidote amphibolite and eclog-ite-facies rocks (P = 05ndash12 GPa) Prevalent rocktypes include felsic gneiss orthogneiss marble andquartzite Protolith ages range from Proterozoic toOrdovician but the majority of reported ages areNeoproterozoic (Li et al 1993 Hacker et al 2000)Blocks boudins and layers of eclogites and garnetperidotites occur as enclaves in gneisses in the UHPunit The Dabie-Sulu UHP rocks show several char-acteristics (1) Widespread coesite with hydrousminerals such as talc zoisiteepidote and phengiteoccur in eclogites (2) Two distinct types of mantle-and crustal-derived garnet peridotites are present(3) Abundant exsolution textures were identified inUHP minerals from garnet peridotite and eclogite(4) Diamond separates from both eclogite and garnetperidotite have been reported from few restrictedregions (eg Xu et al 1992 2003) Some of thesefindings are not confirmed by thin-section observa-tion and extensive studies of mineral inclusions inzircon (for review see Liou et al 2002) (5) Somegarnet peridotites record much higher pressure thanassociated coesite-bearing eclogites reaching 5ndash6GPa (Yang et al 1993 Zhang R Y et al 20002003) (6) Various isotopic age dating methods forUHP minerals and rocks from Dabie-Sulu terranegives 220ndash240 Ma (Li et al 1993 Hacker et al2000)

Eclogites in the Huwan shear zone from thenorthwestern Dabie Mountains have SHRIMP UPbdates of 309 plusmn 3 Ma (Sun et al 2002) suggesting adiscrete Carboniferous subductioncollisionbetween North and South China Farther to the west

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 9

in North Qinling inclusions of microdiamond werediscovered in zircons from both eclogite and itsgneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyielded 507 plusmn 38 Ma for metamorphic rims and1200ndash1800 Ma for relic magmatic or old cores sug-gesting an Early Paleozoic UHP metamorphic eventThese new data suggest that episodic subductionand collision of continents occurred during stages ofclosures of paleo Tethys prior to the well-docu-mented major Triassic continent-continent collisionfor the Dabie-Sulu terrane

Kokchetav massif The Kokchetav massif north-ern Kazakhstan is the type site for the UHPdiamond-eclogite regional metamorphic facies(Sobolev and Shatsky 1990 Parkinson et al 2002Shatsky and Sobolev 2003) Diamonds occur asminute inclusions in garnet diopside and zirconporphyroblasts in marble pyroxene-carbonate-gar-net rock and garnet-biotite gneiss and schist Inclu-sions of coesite and coesite pseudomorphs occur ingarnet and zircon from eclogite and diamond-bear-ing gneiss The UHPHP unit extends NW-SE for atleast 80 km and is about 17 km wide The UHP slabis structurally overlain by a weak- to low-grademetamorphic unit and is underlain by a low-P meta-morphic andalusite-staurolite facies unit (Kaneko etal 2000) The UHP slab is composed of felsicgneiss with locally abundant eclogite lenses andminor orthogneiss metacarbonate and rare garnetperidotite with a range of Proterozoic protolith agesA well-constrained P-T-time path deduced frommineral inclusions and SHRIMP geochronology ofzircons is illustrated in Figure 4 Zoned zircons fromdiamond-bearing gneiss yield the following ages aquartz-bearing inherited core with 1100ndash1400 Maas the protolith age coesite-bearing mantle with 537plusmn 9 Ma for UHP metamorphism and the plagio-clase-bearing rim with 507 plusmn 8 Ma for amphibolite-facies overprint (Katayama et al 2001) Manydiamond-grade marbles contain clinopyroxene withexsolution lamellae of quartz K-feldspar phengiteand phlogopite (Katayama et al 2002 Zhu andOgasawara 2002) and titanite with coesite lamellae(Ogasawara et al 2002) these occurrences suggestthat UHP metamorphism occurred at P gt 6 GPa and1000degC

Cndash13 isotopic data of diamond suggest biogenicsources inclusions of water carbonate and nano-metric oxide in diamonds indicate that metamorphicmicrodiamonds in this and other UHP terranes wereprecipitated from C-O-H supercritical fluids (eg

Stockhert et al 2001) Ogasawara et al (2002)further suggested a two-stage growth mechanism formicrodiamonds Masago et al (2003) reported nega-tive 18O values (-39 per mil) for minerals in eclogiteand whiteschist of the Kokchetav Massif similar tothe Chinese UHP rocks first described by Yui et al(1995) The Kokchetav Massif is the second recog-nized UHP region that preserves a significant effectof the interaction of cold meteoric water with theprotolith prior to subduction

Saxonian Erzgebirge Germany The ErzgebirgeCrystalline Complex (ECC) at the northern margin ofthe Bohemian massif is in fault contact with a low-grade Paleozoic sequence at its margins (for a recentreview see Massonne and OrsquoBrien 2003) TheVariscan ECC consists of abundant gneisses thatinclude numerous lenses of eclogites The ldquoGneiss-Eclogite Unitrdquo at the core of this massif containsUHP rocks typified by inclusions of coesite pseudo-morph in garnets from eclogite (Schmadicke 1991)and inclusions of microdiamonds in garnet kyaniteand zircon from gneisses (Massonne 1999) Thusfar the diamondiferous gneisses seem to occur onlyin a 1 km long strip near the eastern shore of theSaidenbach Reservoir Diamond micro-inclusionsoccur exclusively in garnet kyanite and zircon ofseveral gneissic rocks Besides completely pre-served diamonds with grain sizes between 1 to 25microm partially graphitized diamonds and graphitepseudomorphs after diamond occur Such microdia-mond inclusions in garnet and zircon are similar tothose in Kokchetav diamondiferous biotite gneissSome of these inclusions contain additional fluid-bearing phases including apatite phengitic micaand possible fluid inclusions the morphologies ofinclusions and mineral associations suggest thatmicrodiamonds from both the Erzgebirge and theKokchetav were crystallized from supercritical flu-ids under UHP conditions (Stockhert et al 2001Hwang et al 2001 Dobrzhnetskaya et al 20012003)

Schmadicke et al (1995) reported Sm-Nd isoch-rons for garnet-Cpx-WR at 360 plusmn 7 Ma for eclogiteand 353 plusmn 6 Ma for garnet pyroxenite from Erzge-birge UHP rocks 40Ar39Ar spectra of phengite fromtwo eclogite samples give plateau ages of 348 plusmn 2and 355 plusmn 2 Ma Diamond-bearing zircons yieldSHRIMP UPb dates at 336 plusmn 2 Ma (Massonne2001) Such similarity in ages for garnet peridotitesand eclogite enclosed in gneiss suggests a coevalVariscan UHP in-situ metamorphism of the ECCaround 340ndash360 Ma

10 LIOU ET AL

Less Intensively Studied UHP terranes

Several UHP terranes described below exceptfor the Maksyvtov Complex have received lessintensive study mainly due to remoteness and diffi-cult access despite their long recognition

Maksyutov Complex Southern Urals TheMaksyutov Complex is an elongated (10 times 120 km)N-S belt between the Russian and Siberian cratonsin the southern Ural Mountains (Coleman and Wang1995 Dobretsov et al 1996) and contains blocky

graphitized diamond() in pelitic schist (Leech andErnst 1998) The complex is the locality from whichquartz aggregates within garnet exhibiting radialcracks were first recognized as coesite-pseudo-morphs by Chesnokov and Popov (1965) nearly 20years prior to the independent finding of coesiteinclusions by Chopin (1984) and Smith (1984) Theeclogite-bearing unit contains boudins of eclogitelayers of eclogitic gneiss and rare ultramafic bodieswithin host metasedimentary mica schist andquartzite

FIG 4 A Schematic diagram showing the growth of detrital zircon (stage 1) through subduction (stage 2 and peakstage 3) and exhumation (stage 4) of supracrustal rocks Mineral inclusions in various stages of zircon growth are shownB SHRIMP U-Pb dating of zoned zircons from a Kokchetav diamond-bearing gneiss yields a well constrained P-T-timepath (for details see Katayama et al 2001)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 11

Some Maksyutov eclogites were recrystallized at594ndash637degC with a minimum pressure from 15 to17 GPa (Beane et al 1995 Leech and Ernst 1998)or up to 27 GPa if coesite pseudomorphs (Ches-nokov and Popov 1965 Dobretsov and Doretsova1988) are indeed present However subsequentstudies have not revealed the occurrence of inclu-sions of coesite or coesite pseudomorph in garnetand zircon (Beane et al 1995 Shatsky et al 1995)Leech and Ernst (1998) described unusual graphiteaggregates (up to 13 mm edge length) that may bediamond pseudomorphs This suggestion was basedon the blocky morphology of the cuboid graphiteaggregates cross-cutting rock foliation pressureshadows biogenic isotopic signatures and otherspectroscopic characters similar to those graphi-tized diamonds from the Beni Bousera massifRecent study of several microdiamond inclusions ingarnet has yielded further suggestive possibility thatthe eclogitic unit of the Maksyutov Complex experi-enced diamond-grade UHP metamorphism (Bosticket al 2003) The UHPHP metamorphism with peakmetamorphic ages of 370ndash374 Ma (Shatsky et al1995 Beane and Connelly 1998) may be related tothe collision of the Russian platform with a fragmentof the Siberian craton during Devonian time butwas affected by the Triassic collision of the Kazakh-stan block with the Russian platform (eg Matte1998)

Zermatt-Saas area Western Alps In contrast tomost other UHP terranes in which the protolith iscontinental crust eclogites and metasediments atLago di Cignana of the Western Alps were derivedfrom oceanic crust and constitute the ophiolitesequence of the Zermatt-Saas zone Inclusions ofcoesite and coesite pseudomorphs occur in tourma-line and garnet of manganiferous quartzite and inomphacite and garnet of the underlying mafic eclog-ite (Reinecke 1991) The metasediments are mainlygarnet-phengite-quartz schist with variableamounts of garnet-clinopyroxene quartzite andpiedmontite-phengite-quartz schist Eclogites andretrograded eclogites with preserved UHP mineralparageneses consist of garnet omphacite glau-cophane zoisite phengite and dolomite Zoned gar-nets preserve prograde quartz crystals in their coresand coesite inclusions in mantles and rims Coesiteinclusions are much more common in omphacitethan in garnet UHP conditions were estimated at26ndash30 GPa and 590ndash630degC Sm-Nd isotopic anal-yses of eclogites yield UHP metamorphism at 52 plusmn18 Ma (Bowtell et al 1994) consistent with a

Tertiary age of eclogitization recently established forAlpine metamorphism in the Western Alps(Gebauer et al 1997)

Thus far the recognized areal extent of thecoesite-bearing rocks is ~ 2 km2 The apparentthickness of the UHP slice does not exceed a fewhundred meters This area is the best example forsubduction of oceanic lithosphere to mantle depthand later exhumation as a fragmented tectonic slabSimilarly the Allalin metagabbro part of the oce-anic Piemonte zone contains magnesite + talc +kyanite + garnet recrystallized at ~600degC and 35GPa with an estimated geothermal gradient of 5ndash6degC kmndash1 however inclusions of coesite or coesitepseudomorphs have not been reported from theAllalin metagabbro

Mali Africa Eclogitic rocks in the Pan-Africancollision zone cropping out in northern Mali containinclusions of coesite in omphacite from mafic nod-ules within a calc-silicate layer enclosed in impuremarble and coesite pseudomorphs in garnets fromthe surrounding eclogitic metasediments (Caby1994) These UHP eclogitic mica schists have P-Testimates of 700ndash750degC and gt 27 GPa phengiteyields a well-defined plateau 39Ar40Ar age of 1045plusmn 9 Ma Recent SmNd ages of 620 Ma wereobtained by Jahn et al (2001) for peak UHP meta-morphism This may be the oldest UHP terrane inthe world

The coesite-bearing UHP eclogitic unit about 3km thick lies within an internal nappe as a flat thinslice bounded on the top by a thick passive-marginshaly formation and on the bottom by low-gradegreenschist-facies phyllites Caby (1994) suggestedthat a large portion of the terrigenous metasedimentsof passive-continental-margin affinity of the WestAfrican plate was subducted eastward to mantledepths (gt90 km) beneath an oceanic domain Colli-sion of the West African plate with an island arcresulted in a low-angle westward subhorizontalextrusion of this subducted slice of sialic materialsto its present position The coesite-bearing eclogiteunit may also crop out 1500 km to the south in Togowhere kyanite-bearing eclogites occur as part of thepassive-margin assemblage

Makbal (480 Ma) and Atbashy (270 Ma) Kazak-hstan The Tian-Shan is divided into three mountainrangesmdashnorthern middle and southern Within theKyrgyzstan Tian-Shan UHP eclogite localities withcoesite pseudomorph inclusions in garnet andomphacite from the northern and southern Tian-Shan represent Caledonian and Hercynian orogenic

12 LIOU ET AL

belts respectively (Tagiri and Bakirov 1990 Tagiriet al 1995) The Makbal Formation in the westernKyrgyz Ridge of the northern Tian-Shan consists ofquartzose schist alternating with pelitic schistsintercalated with thin layers or lenses of marbleeclogite and amphibolitized eclogite Tagiri andBakirov (1990) found inclusions of coesite pseudo-morphs in garnet from a garnet-chloritoid-talc schistwith a peak metamorphic assemblage of coesite +almandine + chloritoid + talc + phengite (Si = 342pfu) + phlogopite + rutile Makbal coesite-gradeeclogite has a paragonite KAr age of 480 Ma

The eclogite-bearing complex of Atbashy Ridgefrom the southern Tian-Shan is composed of peliticand siliceous schists alternating with thin UHPeclogitic layers and is unconformably overlain byUpper Paleozoic molasse and silicic volcanic rocksInclusions of coesite pseudomorphs occur inomphacite cores and in garnet Eclogites from theAtbashy area preserve prograde and retrogradepaths with a peak metamorphic condition at 660degC25 GPa The Atbashy coesite-eclogites yielded aRbSr mineral isochron age of 270 Ma (Tagiri et al1995)

Central Indonesia UHP terrane The pre-Tertiarybasement of the Indonesian region comprises a vari-ety of imbricate terranes meacutelange ophiolite andvariably metamorphosed accretionary complexessome contain HP to UHP metamorphic rocks result-ing from the collision of an Australia-derived conti-nent with Eurasia (Parkinson 2003) HP rocksincluding eclogites and garnet peridotites are widelydistributed in Cretaceous accretionary complexesMany of these rocks occur as imbricate slices of car-bonate quartzite and pelitic schist of shallow-marine or continental-margin parentage interthrustwith subordinate mafic schist and serpentinitesome yield mica KAr ages of 110ndash120 Ma (Parkin-son 2003)

HP and UHP rocks are sporadically exposed astectonic blocks throughout the Cretaceous accre-tionary complexes They include eclogite garnet-glaucophane rock (P = 18ndash24 GPa T = 580ndash620degC) and jadeite-garnet quartzite (P gt 27 GPa T =720ndash760degC) in Bantimala southwest Sulawesieclogite and garnet granulite in west-centralSulawesi eclogite and jadeite-glaucophane-quartzrock (P ~22 GPa T ~530degC) in central Java Mgchloritoidndashbearing whiteschists (P ~18 GPa) insoutheast Kalimantan garnet lherzolites in east-central Sulawesi (P = 22ndash28 GPa T = 1000ndash1100degC) west-central Sulawesi (P = 16ndash20 GPa T =

1050ndash1100degC) and garnet pyroxenite (P ~2 GPa T~ 850degC) in Sabah northeast Borneo Evidence forUHP rocks include (1) inclusions of coesite inzircon and coesite pseudomorphs in jadeite fromjadeite quartzite and eclogite of the BantimalaComplex of south Sulawesi (Parkinson 2003) and(2) P-T estimates of peak-stage recrystallization at27ndash35 GPa and 1000ndash1100degC for most garnetperidotites (Kardarusman and Parkinson 2000)Many of these rocks were probably recrystallized ina N-dipping subduction zone at the margin of theSundaland craton in the Early Cretaceous Exhuma-tion may have been facilitated by the collision of aGondwana continental fragment with the Sundalandmargin at ~120ndash115 Ma

Recently Recognized UHP Terranes

Several new UHP terranes were recently identi-fied inasmuch as they contain partially preservedtrace index minerals in strong containers such aszircon or garnet In fact zircon has been consideredto be the best container and many new terranes werediscovered through positive identification of inclu-sions of coesite or coesite pseudomorphs ordiamond in zircons Detailed examination of mineralinclusions in core mantle and rims of zircon sepa-rates from eclogites and their enclosing country-rock gneisses have yielded both prograde andretrograde P-T-time paths for various UHP terranesmentioned above (see Katayama et al 2001 fordetails also see the section on the Himalayan eclog-ite for an example)

The latest recognized UHP terranes include thefollowing (1) coesite in the French Massif Central(Lardeaux et al 2001) (2) coesite and diamondfrom the Greek Rhodope metamorphic province(Mposkos and Kostopoulos 2001) (3) coesitepseudomorphs in the Northeast Greenland EclogiteProvince (Gilotti and Ravna 2002) (4) coesite ingranulite-facies overprinting eclogites of SoutheastBrazil (Parkinson et al 2001) (5) coesite in Hima-layan eclogite from the Upper Kaghan Valley Paki-stan (OrsquoBrien et al 2001 Kaneko et al 2003) andcoesite from the Tso-Morari crystalline complex ofIndia (Mukherjee et al 2003) (6) coesite inclusionsin gneissic rocks from the North Qaidam belt west-ern China (Yang et al 2001 Song et al 2003) (7)diamond inclusions in garnets of eclogite and gneissfrom North Qinling (Yang et al 2002a) (8) inclu-sions of quartz pseudomorphs and minor coesite ingarnets of mafic eclogites from the LantermanRange in Antarctica (Ghiribelli et al 2002) (9)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 13

inclusions of calcite pseudomorphs after aragonite +magnesite in dolomite in metapelites from the west-ern Tian-Shan inferred as metamorphism in a UHPregion of dolomite decomposition (Zhang L et al2002) and (10) inclusion of possible coesitepseudomorphs in garnet lamellar inclusions in gar-net kyanite and high sodium content of garnet fromeclogites of the Chuacus Complex of north-centralGuatemala (Solari et al 2003) The results of petro-chemical and geochronological data for these newterranes are summarized below The HimalayanUHP terrane in the Upper Kaghan Valley and theTso-Morari complex are used to illustrate the pro-cesses of continental subduction and collision in alater section

French Massif Central The French Massif Cen-tral in the western part of the Variscan Belt hasexperienced Late SilurianndashEarly Devonian to LateCarboniferous orogenic events (eg Matte 1986)High-pressure metamorphic rocks occur mainly inthe upper gneissic unit within the leptyno-amphibo-lite group The latter consists of an association ofmetagreywacke mica schist metabasalt leptynitemetagranite and peridotite Eclogites are associatedwith silicic and mafic HP granulites (Lardeaux etal 2001) and also with spinel andor garnet lherzo-lites (Gardien et al 1990) Inclusions of coesite andcoesite pseudomorphs occur in garnets of kyanite-bearing eclogite (Lardeaux et al 2001) Only twocoesite grains are preserved as relics most coesitegrains are completely transformed into polycrystal-line radial quartz (palisade texture) or into polygonalquartz surrounded by radial cracks Metamorphictemperatures were estimated to be 740-780degC andsubsequent amphibolite-facies overprinting at750degC at 15ndash17 GPa during decompression UHPmetamorphism occurred between 420 and 400 Ma(Paquette et al 1995) 40Ar39Ar data from amphib-ole separates from retrogressed eclogites yielded339 plusmn 4 Ma (Costa et al 1993)

Northeast Greenland Caledonides The first evi-dence for UHP metamorphism in the GreenlandCaledonites was reported from kyanite eclogites andassociated host gneisses on an island in Jokelbugt(Gilotti and Ravna 2002) Inclusions of quartzpseudomorphs exhibiting palisade structure andradiating fractures occur in garnet and omphacite ofeclogites and in garnet of the host gneisses P-T esti-mates of peak-stage metamorphism of eclogite at~972degC and 36 GPa lie well within the coesite sta-bility field SHRIMP UPb dates of 403 plusmn 5 Ma forzircon rims from HP metapsammite and 404 plusmn 4 Ma

for zircons from anatectic melt derived frommetapelite are coeval with the estimated ages ofexhumation of UHP terranes in the ScandinavianCaledonides described in a previous section(McClelland and Gilotti 2003)

Neoproterozoic nappes in Southeast BrazilNeoproterozoic nappes of Southeast Brazil consistmainly of coarse-grained kyanite-garnet granulitewith intercalations of impure quartzite calc-silicaterock and minor lenses and sills of mafic-ultramaficrocks including eclogite Early eclogitic phaseswere strongly obliterated during granulite-faciesoverprinting during a major Pan-African continentcollision at 640ndash630 Ma (Campos Neto and Caby1999) Rare ultramafic lenses consist of phlogopite-orthopyroxene rocks and garnet clinopyroxeniteCores of granulite boudins retain eclogitic assem-blages some quartz inclusions within the mantleregions of zoned garnet porphyroblasts are sur-rounded by intense radial fractures suggestive ofthe former presence of coesite In fact coarse-grained zircons in some granulites contain numer-ous micro-inclusions of K-feldspar + kyanite + coes-ite as confirmed by Raman spectroscopy (Parkinsonet al 2001) Many exsolution microstructuresobserved in other UHP terranes also occur Thisarea represents the first coesite-grade UHP rocks inthe Americas and it can be correlated with the MaliUHP terrane of equatorial Africa (Caby 1994) Ithas been suggested that eclogitic assemblages andprobable UHPM components may be present in sev-eral other correlative nappe systems at the marginsof the Sao Francisco craton in Brazil

Chuacus Complex north-central Guatemala HPeclogites within banded gneisses schists andmigmatites of the Chuacus Complex contain relicttextures and mineralogical features that suggestUHP metamorphism (Solari et al 2003) Theseinclude inclusions of possible coesite pseudomor-phs in garnet and kyanite lamellar inclusions ofrutileilmenite in garnet high sodium content ofsome garnets (up to 1200 ppm) and preliminary P-T estimates of 700ndash800degC and 2ndash3 GPa AlthoughK-Ar dating of mica and hornblende yield ages of48ndash72 Ma possible for amphibolite facies retrogres-sion regional and structural data support a pre-Mesozoic age for the multi-stage HP-UHP recrystal-lization Such a discovery opens up a new interpre-tation regarding the tectonic evolution of Maya-Chortis-Oaxaquia microcontinental blocks formingthe structural backbone of Mesoamerica

14 LIOU ET AL

Greek Rhodope The Greek Rhodope Metamor-phic Province (RMP) at the border between Greeceand Bulgaria represents a syn-metamorphic nappesystem of Alpine age that was formed during theCretaceous to Mid-Tertiary collision of Apulia andpaleo-Europe The Kimi nappe complex comprisescrustal eclogites leucocratic orthogneisses peliticgneisses and volumetrically minor mantle-derivedultramafic rocks these units contain many mineral-ogical indicators of UHP metamorphism (Mposkosand Kostopoulos 2001) The ultramafic lithologiesinclude garnet spinelndashbearing lherzolite and layersof spinel-garnet clinopyroxenite garnet pyroxeniteand clinopyroxene garnetite These rocks were sub-jected to UHP recrystallization at P-T conditions of~1200degC and gt30 GPa The pelitic gneiss containsmicrodiamond inclusions in garnet porphyroblastsas well as rods or needles of silica rutile biotiteand apatite in sodic garnet suggesting the prioroccurrence of majoritic garnet The growth of suchsupersilicic garnet was suggested to be at a pressureof ~7 GPa Inclusions of polycrystalline quartzaggregates surrounded by radial cracks occur ineclogitic garnet Exsolution lamellae of quartz ineclogitic clinopyroxene also indicate an UHP pre-cursor of supersilicic clinopyroxene

Lanterman Range Antarctica The LantermanRange in northern Victoria Land of Antarctica con-sists of several metamorphic complexes The Gate-way Hills Metamorphic Complex is a thindiscontinuous belt more than 50 km long consistingof mafic and ultramafic rocks including lenses andpods of eclogite within felsic gneiss Some less foli-ated eclogites contain inclusions of coesite andcoesite pseudomorphs in garnet porphyroblasts asconfirmed by in situ Raman spectroscopy as well asradial fractures around the inclusions (Ghiribelli etal 2002) The eclogites were subjected to UHPmetamorphism at T gt 850degC and P gt 29 GPa thenamphibolite-facies overprinting during isothermaldecompression The UHP event is dated by SmNdand 238U206Pb data at 500 Ma 40Ar39Ar ages of490ndash486 Ma for Ca-amphibole (amphibolite-faciesoverprinting) suggest rapid cooling with an averageexhumation rate of 3ndash4 km_Ma-1 for the UHP ter-rane (Di Vincenzo and Palmeri 2001)

Western Tian-Shan China Caledonian eclogitesof the western Tian-Shan China have been classi-fied into three types (Zhang L et al 2002) Type Ieclogite pods are layered with mafic blueschists andType II eclogites preserved pillow structureswhereas Type III eclogites are banded with marbles

This 200 km long eclogite-bearing belt extendswestward to connect with the Atbashy UHP belt inKazakhstan (Tagiri et al 1995) Reported UHP evi-dence includes inclusions of coesite pseudomorphsin garnets from Type I and III eclogites quartzexsolution lamellae in omphacite from Type IIeclogites and relict magnesite within matrix dolo-mite in Type III eclogites (Zhang J X et al 2002Zhang L et al 2002) P-T estimates of the peakUHP stage of metamorphism are T = 500ndash600degCand P = 26ndash27 GPa (Wei et al 2003)

North Qaidam Mountains western China Manylenses and layers of garnet peridotite and eclogiteoccur in an amphibolite-facies terrane in the NorthQaidam Mountains along the northeastern rim of theQaidam Basin This belt comprises mainly felsicgneiss quartz schist garnet-bearing gneissamphibolite and minor eclogite + garnet peridotiteit extends westward for more than 1000 km to theAltun Mountains and was displaced by the left-lat-eral Altyn-Tagh fault At the eastern end of thisUHP belt at Dulan occurrences of coesite inclu-sions in zircon separates from paragneiss inclusionsof coesite pseudomorph in garnet and omphacite ofeclogites quartz rods in eclogitic omphacite andP-T estimates of eclogites (P = 287ndash317 GPa andT = 631ndash687degC) demonstrate that this is a typicalUHP metamorphic terrane (Yang et al 2001 Songet al 2003a 2003b) The Dulan eclogites have anSm-Nd mineral isochron age of 497 plusmn 87 Ma and aSHRIMP UPb zircon age of 495 Ma (Yang et al2002b)

Altun Mountains western China This UHP ter-rane about 200 km long in the southern margin ofthe Altun Mountains is the western extension of theNorth Qaidam belt Eclogitic lenses and blocks ofvarious sizes together with garnet-lherzolite and cli-nopyroxenite are enclosed within an Early Protero-zoic sequence of amphibolite amphibole schistgarnet-bearing granitic gneiss and pelitic schistGarnet lherzolite contains magnesite that reacted toform dolomite P-T estimates of the peak magnesite-bearing assemblage yield P gt 36 GPa and 850degC(Liu et al 2001) Garnet porphyroblasts includeexsolution lamellae of clinopyroxene and rutile sim-ilar to that of eclogite reported by Ye et al (2000) asmajoritic garnet The associated eclogites containinclusions of coesite pseudomorphs in garnet withcharacteristic radial fractures this together with theexsolved quartz rods in omphacite and P-T esti-mates of 820ndash850degC and 28 to 32 GPa provideadditional evidence for UHP (Zhang J X et al

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 15

2001) An SmNd mineral isochron for eclogiticgarnet omphacite and the whole rock yields 500 plusmn10 Ma whereas an UPb isochron for zircon sepa-rates from eclogite gives an age of 504 plusmn 5 Ma(Zhang J X et al 2000)

North Qinling Mountains central China Lensesand blocks of eclogitic rocks in northern Qinling areenclosed in amphibolite-facies gneiss and garnet-bearing quartz + phengite mica schist Inclusions ofcoesite in eclogitic garnet (Hu et al 1995) andmicrodiamonds in zircon separates from both eclog-ite and gneiss (Yang et al 2002a) have beenrecently reported Although individual outcrops areno more than a few meters wide the belt of eclogitesextends more than 10 km P-T estimates of amphib-olitized coesite-bearing eclogite yield T = 590ndash758degC and P gt 26 GPa An Sm-Nd isochron for gar-net omphacite rutile amphibole and whole-rockyields 400 plusmn 16 Ma Microdiamonds were recentlydiscovered as inclusions in zircon from both eclogiteand its gneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyields 507 plusmn 38 Ma for peak UHP metamorphism(Yang et al 2002a) These data indicate that two

discrete collision events occurred between theNorth China and Yangtze cratons (1) an Early Pale-ozoic North Qinling UHP suturing and (2) a Triassicsuturing along the Dabie-Sulu UHP-HP belt TheEarly Paleozoic belt extends westward to the NorthQaidamndashAltun Mountains for more than 4000 km(Yang et al 2002b)

Himalayan UHP Eclogites

The Himalayan orogen has long been considereda classic locality of continental collision betweenthe Indian and Eurasian continents (Fig 5) Severaleclogite-facies HP rocks in the Himalayas havebeen reported since the late 1980s (eg Chaudaryand Ghazanfar 1987 Pognante and Spencer 1991de Sigoyer et al 1997 Lombardo and Rolfo 2000)Inclusions of coesite were recently discovered inomphacite and garnet in several Himalayan eclog-ites from the upper Kaghan Valley Pakistan(OrsquoBrien et al 2001 Kaneko et al 2003) Subse-quent findings of additional coesite localities in theTso Morari area (Sachan et al 2001 Mukherjee etal 2003) have established the presence of a UHPbelt in the Himalayas an active ongoing collision

FIG 5 A simplified tectonic map of the western Himalaya Hindu Kush Pamir Karakoran and western Tibet mod-ified after Figure 3 of Searle et al (2001) This map shows the locations of eclogite and UHP eclogites from both Hima-layan and North Qaidam Indus Suture Zone (ISZ) ophiolites and major structures

16 LIOU ET AL

since the Eocene (Massonne and OrsquoBrien 2003)However UHP relics are volumetrically very minorconstituents of the metamorphic belts dominatedmostly by later Barrovian overprintings In theserocks hydration related to Barrovian-zone metamor-phism has resulted in remarkably pervasive recrys-tallization during the later exhumation this eventhas almost entirely masked the preceding UHPrecord

The finding of HP-UHP evidence has led to newinsights into the Himalayan tectonic modelEngland and Houseman (1986) showed that in amodel of homogeneous thickening of the Hima-layan-Tibet boundary the maximum crustal thick-ness should be less than 70 km However theoccurrences of coesite-bearing eclogites andgneisses in the Himalayan orogen cannot beexplained by a homogeneous thickening of the crustbut rather by continental subduction as describedbelow According to available geochronologicaldata (eg Treloar et al 2003) the exhumation rateof the Himalayan UHP rocks has been estimated torange from ~1 cm yrndash1 (OrsquoBrien et al 2001) to gt3cm yrndash1 (Treloar et al 2003 Massone and OrsquoBrien2003) These estimates are higher than those calcu-lated from other UHP-HP terranes This requirestectonic emplacement of the UHP-HP Himalayanslab into shallow crust following isostatic rebounddue to slab decoupling between the Indian continentand the oceanic crust initiated during the MiddleEocene UHP rocks from both the Kaghan Valleyand Tso-Mori are described below

Kaghan Valley The Kaghan nappe in the north-western Himalayan syntaxis lies between the Indussuture (also called main mantle thrust MMT) to thenorth and the main central thrust (MCT) to thesouthwest (Pognante and Spear 1991 Spencer1993) (Fig 5) The Cretaceous Kohistan island-arcsequence lies north of the Indus suture It developedas a result of intra-oceanic subduction The maincentral thrust separates an underlying Salkhala unitfrom an overlying higher Himalayan crystalline(HHC) unit The Kaghan nappe consists mainly ofgranitic gneiss and paragneiss with minor inter-calated layers of amphibolite marble and quartziteEclogites are preserved in the cores of maficboudins (less than 1 m thick) in gneiss and aresignificantly overprinted by amphibolite assem-blages They contain fine-grained (lt 1 mm) garnetand omphacite together with aggregates of quartzchains of rutile rimmed by titanite and mm-sizedphengitic micas overgrown by randomly oriented

amphiboles up to 1 cm in length Coesite and coesitepseudomorphs occur as inclusions in omphaciteand show the characteristic palisade-quartz aggre-gate texture fractures radiating from these inclu-sions are characteristically developed in the hostpyroxene

Garnet pyroxenendashphengite barometry wasapplied to the eclogites and yielded peak P-T con-ditions of 27ndash29 GPa and 690ndash750degC (OrsquoBrien etal 2001) (Fig 6) Retrograde textures are commonin the eclogites including symplectic intergrowthsof augite amphibole and plagioclase after omphac-ite thermobarometric analyses of these assemblagesyielded 10ndash13 GPa and 600ndash710degC (Kaneko et al2003) On the other hand the dominant graniticgneisses and metapelites of the higher Himalayancrystalline unit contain typical Barrovian-zoneamphibolite-facies assemblages represented bykyanite- and staurolite-bearing garnetiferousmetapelites (Treloar 1995) Extensive overprintinghas mostly obliterated the peak assemblages matrixminerals of the gneisses yielded P-T conditions of07ndash11 GPa and 600ndash700degC (Treloar 1995) con-sistent with retrogression of the eclogites Howeverrelict coesite is preserved as inclusions in zirconthe inclusions are ovoid and approximately 10 microm indiameter (Fig 7A) This discovery of coesite-bear-ing zircons from Himalayan quartzofeldspathicgneisses demonstrates subduction of Indian conti-nental crust to the depths of UHP conditions(Kaneko et al 2003)

Geochronological studies using SmNd (garnet-omphacite) RbSr (phengite) and UPb (rutile zir-con) methods suggest that the eclogite-facies eventtook place at 40ndash50 Ma (Tonarini et al 1993 Spen-cer and Gebauer 1996) 40Ar39Ar cooling ages forhornblende and mica from gneisses were reported tobe ~43 Ma and ~25 Ma respectively (Chamberlainet al 1991) Recent SHRIMP UPb dates of zonedzircons with mineral inclusions formed at differentstages indicate the ages of non-UHP mineral-bear-ing mantle domains of zircon and UHP mineral-bearing rims are at about 50 and 46 Ma respectively(Kaneko et al 2003) A new U-Pb age of 44 plusmn 11Ma was obtained for rutile from Kaghan coesite-bearing eclogite (Treloar et al 2003) this togetherwith available data implies that exhumation of theUHP rocks from mantle depth to mid-crustal leveloccurs within a few million years

Tso Morari The Tso Morari crystalline dome islocated between the Indus suture zone on the northbordered by the Zildat detachment fault and the

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 17

Zanskar sedimentary unit on the south (de Sigoyer etal 1997) The dome represents an internal massif ofthe northern part of the higher Himalaya The TsoMorari area has been considered a typical IndianTethyan margin because of a stratigraphic succes-sion similar to the Nimaling antiform in the core ofthe dome a metamorphic basement consists of Cam-bro-Ordovician augen gneisses (Stutz 1988) Thiscomplex is covered by a metasedimentary series ofCambrian to Devonian quartzite schist and con-glomerate and is overlain by Lower Carboniferousto Triassic marble and metapelite Eclogites occuras lenses or irregular bodies within the basementand cover gneisses and are strongly overprinted byamphibolite-facies assemblages de Sigoyer et al(1997) reported peak P-T conditions for the eclog-ites at 17ndash23 GPa and 520ndash640degC (Fig 7) How-ever relics of coesite 30ndash80 microm in size were

recently found as inclusions in garnet (Sachan et al2001 Mukherjee et al 2003) typified by radialfractures (Figs 7B and 7C) for the SiO2 inclusionsa characteristic Raman spectrum is centered at 523cm-1 (Fig 7D) The country rock metasedimentswith jadeite + chloritoid + paragonite + garnetassemblages have peak P-T estimates of 18ndash22GPa and 500ndash600degC and record retrogression at theeclogite-blueschist facies transition of 13ndash18 GPaand 490ndash590degC (Guillot et al 1997)

The geochronology of the Tso Morari eclogiteshas been extensively studied and subdivided intothree different stages (de Signoyer et al 2000)Eclogitization at ~55 Ma was obtained by Lu-Hf andSm-Nd methods on garnet omphacite glaucophaneand the whole rock An amphibolite-facies overprintoccurred at 47 Ma judging by RbSr dating onphengite apatite and whole rock from metapelites

FIG 6 P-T estimates of peak- and retrograde-stage metamorphism for coesite-bearing eclogites and the country-rockgneisses from the Kaghan Valley and Tso Morari

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

4 LIOU ET AL

(Bakun-Czubarow 1991) have been well studiedand extensively reviewed (eg Ernst and Liou2000) Except for the Dora Maira massif occur-rences of microdiamond in these UHP terranes havebeen reported petrochemical characteristics ofthese terranes are summarized below

Dora Maira massif Western Alps The Dora Mairamassif consists of Late Paleozoic and older conti-nental basement rocks metamorphosed under UHPand HP conditions as a result of MesozoicndashCenozoicconvergence of the European and African plates (fora recent review see Compagnoni and Rolfo 2003)The HP + UHP belt is a stack of four major unitsseparated by low-angle faults These units havebeen affected by Cenozoic HP metamorphism andlater re-equilibration at lower pressures The UHP

unit consists of dark- and light-colored eclogitespyrope quartzite with phengite schist and jadeite-rich rock inclusions as well as orthogneiss countryrocks and undeformed metagranites Boudins ofpyrope quartzite contain UHP relics such as coesiteand other UHP minerals This entity was subdividedinto two metamorphic subunits (a) a coesite-bear-ing UHP subunit contains kyanite-eclogites withrelict coesite and coesite pseudomorphs and hasestimated peak P-T conditions of 37 GPa and790degC within the diamond stability field (Schertl etal 1991) and (b) a lower-P eclogite subunit wasmetamorphosed at 15 GPa and 500degC The time ofUHPHP metamorphism has been considered tohave culminated between 90 to 125 Ma and retro-graded between 35 to 41 Ma However recent

FIG 2 Schematic cross-section contrasting the tectonic setting of generation and exhumation of (A) Collision (A)-type and (B) Pacific (B)-type HP-UHP metamorphic belts Characteristics of the Pacific type include subduction ofoceanic lithosphere and development of an accretionary complex forearc basin huge tonalite-trondhjeimite-granitoid(TTG) belt and volcanic arc Collision-type belts which are associated with continental collision lack those character-istics and the high-P belt is generally of a much large extent (after Maruyama et al 1996)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 5

FIG

3 D

istr

ibut

ion

and

peak

met

amor

phic

age

of r

ecog

nize

d U

HP

terr

anes

in th

e w

orld

(mod

ified

aft

er L

iou

et a

l 2

000)

6 LIOU ET ALTA

BL

E 1

Sum

mar

y of

Pet

roch

emic

al F

eatu

res

of R

ecog

nize

d U

HP

Ter

rane

s in

the

Wor

ld

UH

P te

rran

eU

HP

rock

type

Age

sP

-T e

stim

ates

Spec

ial f

eatu

res

Cla

ssic

al te

rran

es

Dor

a M

aira

Mas

sif

Wes

tern

Alp

s I

taly

Ecl

ogit

e P

yrop

e qu

artz

ite

Pel

ites

Whi

tesc

hist

354

plusmn 1

0 M

a (p

eak)

30 M

a (r

etro

grad

e)37

kba

r 79

0degC

1 P

yrop

e w

hite

schi

st c

onta

ins

Mg-

Al s

ilica

tes

2 N

o ga

rnet

per

idot

ite

Wes

t Gne

iss

Reg

ion

Nor

way

Ecl

ogit

eG

arne

t per

idot

ite

Dia

mon

d-be

arin

ggn

eiss

408ndash

425

Ma

(pea

k)37

4ndash37

8 M

a (r

etro

grad

e)P

gt 2

8 kb

arT

gt 7

90degC

1 R

are

diam

ond

in c

ount

ry r

ock

gnei

ss a

nd g

arne

t per

idot

ite

2 M

ajor

itic

garn

et in

per

idot

ite

form

ed a

t man

tle

~17

00 M

a

Dab

ie-S

ulu

terr

ane

Eas

t-C

entr

al C

hina

Ecl

ogit

eG

arne

t per

idot

ite

Jade

ite q

uart

zite

Min

or w

hite

schi

st

220ndash

240

Ma

(pea

k)E

clog

ite

26ndash

40 k

bar

700ndash

850deg

CG

arne

t per

idot

ite

50-6

0 kb

ar 7

00ndash8

50degC

1 A

bund

ant c

oesi

te in

gar

net

omph

acit

e k

yani

te z

oisi

te z

irco

n 2

Rar

e di

amon

d in

ecl

ogit

e3

Ecl

ogite

and

gar

net p

erid

otit

e co

ntai

n U

HP

hydr

ous

phas

es4

Dua

l ori

gin

for

garn

et p

erid

otit

e 5

Maj

oriti

c ga

rnet

in e

clog

ite

Kok

chet

av M

assi

fno

rthe

rn K

azak

hsta

nE

clog

ite

Dia

mon

d-be

arin

ggn

eiss

and

mar

ble

Whi

tesc

hist

537plusmn

9 M

a (p

eak)

507plusmn

9 M

a (r

etro

grad

e)D

iam

ond

grad

e 4

0ndash60

kbar

920

ndash100

0degC

Coe

site

gra

de 3

6 kb

ar

750ndash

800deg

C

1 M

icro

diam

ond

is a

bund

ant i

n so

me

garn

et g

neis

s an

d do

lom

ite

mar

ble

2 C

oesi

te in

clus

ions

in w

hite

schi

st e

clog

ite

and

gar

net g

neis

s3

Rar

e ga

rnet

per

idot

ite

Boh

emia

n M

assi

fE

clog

iteG

arne

t per

idot

ite

379ndash

395

Ma

gt 30

kba

r 70

0ndash80

0degC

Gar

net p

erid

otit

e yi

elds

muc

h hi

gher

P th

an c

oesi

te-e

clog

ite

Les

s in

tens

ivel

y st

udie

d te

rran

es

Zer

mat

t-Sa

as a

rea

Sw

itze

rlan

dE

clog

ite

Mn-

quar

tzit

e52

plusmn 1

8 M

a (p

eak)

26ndash3

0 kb

ar 5

90ndash6

30degC

1 P

roto

lith

s ar

e oc

eani

c af

fini

ty2

Coe

site

incl

usio

n in

the

man

tle

of g

arne

t whe

re g

arne

t cor

e ha

s qu

artz

Saxo

nian

Erz

gebi

rge

Ger

man

yE

clog

ite

Dia

mon

d-be

arin

g gn

eiss

360

plusmn 7

Ma

(pea

k)34

8ndash35

5 M

a (r

etro

grad

e)gt

42 k

bar

900ndash

1000

degC1

Inc

lusi

ons

of m

icro

diam

ond

in z

irco

n k

yani

te a

nd g

arne

t fro

m g

neis

s2

Rut

ile

with

-P

bO2

stru

ctur

e

Mal

i A

fric

aE

clog

ite

620

Ma

(pea

k)gt2

7 kb

ar 7

00ndash7

50degC

1 O

ldes

t age

of U

HP

met

amor

phis

m

Mak

bal

wes

tern

K

yrgy

zsta

nE

clog

ite

Gar

net-

bear

ing

peli

tic s

chis

t

482

Ma

gt26

kba

r 61

0ndash68

0degC

1 I

nclu

sion

s of

coe

site

pse

udom

orph

in g

arne

t fro

m e

clog

ite a

nd s

chis

t

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 7A

tbas

hy K

azak

hsta

nE

clog

ite27

0 M

agt

25 k

bar

660deg

C1

Inc

lusi

ons

of c

oesi

te in

gar

net

Cen

tral

Ind

ones

ia S

W

Sula

wes

iE

clog

ite

Gar

net p

erid

otite

115ndash

120

Ma

gt27

kbar

720

ndash760

degC33

ndash35

kbar

115

0degC

1 I

nclu

sion

s of

coe

site

in z

irco

n an

d co

esit

e ps

eudo

mor

ph in

jade

ite

Rec

entl

y re

cogn

ized

terr

anes

Fre

nch

Alp

sE

clog

ite

Met

apel

ites

400ndash

420

Ma

(pea

k)36

0ndash38

0 M

a (r

etro

grad

e)gt

28 k

bar

750deg

C1

Inc

lusi

ons

of c

oesi

te in

gar

net a

nd o

mph

acit

e

Nor

thea

st G

reen

land

Ecl

ogite

400ndash

440

Ma

24ndash2

8 kb

ar 8

20degC

1 I

nclu

sion

s of

coe

site

pse

udom

orph

in g

arne

t

Sout

heas

t Bra

zil

Ecl

ogit

e63

0 M

agt

27 k

bar

800deg

C1

Inc

lusi

ons

of c

oesi

te in

zir

con

Chu

acus

Com

plex

no

rthe

rn G

uate

mal

aE

clog

ite

Ban

ded

gnei

ss48

ndash72

Ma

(ret

rogr

ade)

20ndash3

0 kb

ar 7

00ndash8

00degC

1 C

oesi

te p

seud

omor

ph in

gar

net a

nd k

yani

te

Rho

dope

Mas

sif

Gre

ece

Ecl

ogit

eO

rtho

gnei

ssPe

litic

gne

iss

Ultr

amaf

ic r

ocks

30ndash4

2 M

a (p

eak)

8ndash12

Ma

(ret

rogr

ade)

gt 30

kba

r ~

1200

degC1

Mic

rodi

amon

d in

clus

ion

in g

arne

t por

phyr

obla

st2

Sili

ca r

util

e an

d ap

atit

e ro

ds in

gar

net

Lan

term

an R

ange

A

ntar

ctic

aE

clog

ite

Fels

ic g

neis

s50

0 M

a (p

eak)

486ndash

490

Ma

(ret

rogr

ade)

gt 29

kba

r gt

850

degC1

Inc

lusi

ons

of c

oesi

te a

nd it

s ps

eudo

mor

ph in

gar

net

Wes

tern

Tia

n-Sh

an

nort

hwes

t Chi

naE

clog

ite

Mar

ble

lt 3

10 plusmn

5 M

a (p

eak)

26ndash2

7 kb

ar 5

00ndash6

00degC

1 I

nclu

sion

s of

coe

site

and

its

pseu

dom

orph

in g

arne

t2

Pos

sibl

e co

exis

tenc

e of

mag

nesi

te +

ara

goni

te

Qai

dam

wes

tern

Chi

naE

clog

ite

Gne

iss

Gar

net p

erid

otite

495

plusmn 7

Ma

(pea

k)46

7 plusmn

1 M

a (r

etro

grad

e)gt

27 k

bar

700deg

C1

Inc

lusi

ons

of c

oesi

te in

zir

con

from

par

agne

iss

Alt

un M

ount

ains

w

este

rn C

hina

Ecl

ogit

eG

rt-l

herz

olit

e50

4 plusmn

5 M

a (p

eak)

28ndash3

2 kb

ar 8

20ndash8

50dego C

1 C

oesi

te p

seud

omor

ph in

gar

net

Nor

th Q

inli

ng M

tns

C

entr

al C

hina

Ecl

ogit

eG

neis

s50

7 plusmn

38

Ma

400

plusmn 1

6 M

agt

26 k

bar

590ndash

760deg

C1

Coe

site

in g

arne

t2

Mic

rodi

amon

d in

zir

cons

from

ecl

ogit

e an

d gn

eiss

Kag

han

vall

ey P

akis

tan

Him

alay

asE

clog

ite

Gne

iss

46 plusmn

1 M

a (p

eak)

44 plusmn

1 M

a (r

etro

grad

e)27

ndash29

kbar

690

ndash750

degC1

Inc

lusi

ons

of c

oesi

te a

nd it

s ps

eudo

mor

ph in

om

phac

ite

and

zirc

on

from

ecl

ogit

e an

d in

zir

con

from

gne

iss

Tso

Mor

ari

Indi

an H

imal

ayas

Ecl

ogit

eG

neis

s55

Ma

(pea

k)47

Ma

and

30 M

a(r

etro

grad

e)

gt 28

kba

r 70

0ndash80

0degC

1 I

nclu

sion

s of

coe

site

and

its

pseu

dom

orph

in g

arne

t fro

m e

clog

ite

8 LIOU ET AL

SHRIMP dating of zircons extracted from gneissschist pyrope inclusion and pyrope quartzite indi-cates the occurrence of 240ndash275 Ma old zirconcores and newly formed 35 Ma rims for some zircons(Gebauer et al 1997) Thus UHP metamorphismmust have taken place during Oligocene time withan estimated exhumation rate of 2 to 24 cm yrndash1

Western Gneiss Region Norway The WesternGneiss Region lies within an Early Paleozoic colli-sion zone the gneissic unit about 300 km long and150 km wide consists of interlayered pelite andmigmatite marble quartzite and amphibolite withtectonic inclusions of gabbro and peridotite (for arecent review see Carswell and Cuthbert 2003) Thegneissic unit exhibits mainly amphibolite-faciesassemblages but relics of HP assemblages alsooccur Eclogite boudins are widespread Coesite wasfirst reported from Grytting (Smith 1984) severalnew localities of coesite and coesite pseudomorphshave been recognized recently in both eclogites andthe adjacent gneissic rocks (Wain 1997 Cuthbert etal 2000 Wain et al 2000 Carswell et al 2003)Microdiamond grains 20ndash50 microns across havebeen described from residues separated from twogneisses (Dobrzhinetskaya et al 1995) the associ-ated kyanite-eclogites contain inclusions of coesitepseudomorphs in garnet Relict majoritic garnetsshowing exsolution of pyroxene lamellae in peridot-ite bodies have been recently discovered on theislands of Otrogy and Flemsoy (van Roermund andDrury 1998 van Roermund et al 2000) Theseperidotite bodies originally must have had an evenhigher-pressure deeper-mantle origin probablywithin a rising mantle diapir at ~1700 Ma In thesegarnet peridotites diamond inclusions in symple-tetic spinel after garnet have been discovered (vanRoermund et al 2002) Radiometric ages for eclog-ites of the Western Gneiss Region are 408ndash425 Mafor the peak metamorphism and 374ndash378 Ma forthe retrogressive mid-crustal metamorphic over-printing (for review see Carswell et al 1999) Car-swell et al (2003) provide new petrographicevidence and a review of the latest radiometric agedata and conclude that the UHP metamorphism ofeclogites occurred at 400ndash410 Ma significantlyyounger than the previous widely accepted age of425 Ma A two-stage exhumation process is sug-gested an initial exhumation to about 35 km depthby about 395 Ma at a mean rate of about 10 m Mandash1and subsequent exhumation to 8ndash10 km by about375 Ma at a much slower rate of about 13 m Mandash1

The suture zone occupied by UHPHP rocks ofthe Western Gneiss Region reflects collision of theeclogite-bearing Greenland sialic crust-cappedlithospheric plate with Fennoscandia during theCaledonian orogeny (Gilotti 1993 Krogh andCarswell 1995 Brueckner and Medaris 1998)Inclusions of coesite pseudomorph in garnet andomphacite from kyanite eclogites have beenrecently documented in the East Greenland eclogiteprovince (Gilotti and Ravna 2002) this new UHPHP terrane is remote but has excellent exposuresand requires close examination

Dabie-Sulu terrane of east-central China TheDabie-Sulu terrane occupies a Triassic collisionzone between the Sino-Korean and Yangtze plates(for a recent review see Hirajima and Nakamura2003) The central UHP coesite-bearing eclogitebelt (P = 27ndash5 GPa) is flanked to the north by amigmatite zone (P lt 2 GPa) and to the south by abelt of blueschist epidote amphibolite and eclog-ite-facies rocks (P = 05ndash12 GPa) Prevalent rocktypes include felsic gneiss orthogneiss marble andquartzite Protolith ages range from Proterozoic toOrdovician but the majority of reported ages areNeoproterozoic (Li et al 1993 Hacker et al 2000)Blocks boudins and layers of eclogites and garnetperidotites occur as enclaves in gneisses in the UHPunit The Dabie-Sulu UHP rocks show several char-acteristics (1) Widespread coesite with hydrousminerals such as talc zoisiteepidote and phengiteoccur in eclogites (2) Two distinct types of mantle-and crustal-derived garnet peridotites are present(3) Abundant exsolution textures were identified inUHP minerals from garnet peridotite and eclogite(4) Diamond separates from both eclogite and garnetperidotite have been reported from few restrictedregions (eg Xu et al 1992 2003) Some of thesefindings are not confirmed by thin-section observa-tion and extensive studies of mineral inclusions inzircon (for review see Liou et al 2002) (5) Somegarnet peridotites record much higher pressure thanassociated coesite-bearing eclogites reaching 5ndash6GPa (Yang et al 1993 Zhang R Y et al 20002003) (6) Various isotopic age dating methods forUHP minerals and rocks from Dabie-Sulu terranegives 220ndash240 Ma (Li et al 1993 Hacker et al2000)

Eclogites in the Huwan shear zone from thenorthwestern Dabie Mountains have SHRIMP UPbdates of 309 plusmn 3 Ma (Sun et al 2002) suggesting adiscrete Carboniferous subductioncollisionbetween North and South China Farther to the west

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 9

in North Qinling inclusions of microdiamond werediscovered in zircons from both eclogite and itsgneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyielded 507 plusmn 38 Ma for metamorphic rims and1200ndash1800 Ma for relic magmatic or old cores sug-gesting an Early Paleozoic UHP metamorphic eventThese new data suggest that episodic subductionand collision of continents occurred during stages ofclosures of paleo Tethys prior to the well-docu-mented major Triassic continent-continent collisionfor the Dabie-Sulu terrane

Kokchetav massif The Kokchetav massif north-ern Kazakhstan is the type site for the UHPdiamond-eclogite regional metamorphic facies(Sobolev and Shatsky 1990 Parkinson et al 2002Shatsky and Sobolev 2003) Diamonds occur asminute inclusions in garnet diopside and zirconporphyroblasts in marble pyroxene-carbonate-gar-net rock and garnet-biotite gneiss and schist Inclu-sions of coesite and coesite pseudomorphs occur ingarnet and zircon from eclogite and diamond-bear-ing gneiss The UHPHP unit extends NW-SE for atleast 80 km and is about 17 km wide The UHP slabis structurally overlain by a weak- to low-grademetamorphic unit and is underlain by a low-P meta-morphic andalusite-staurolite facies unit (Kaneko etal 2000) The UHP slab is composed of felsicgneiss with locally abundant eclogite lenses andminor orthogneiss metacarbonate and rare garnetperidotite with a range of Proterozoic protolith agesA well-constrained P-T-time path deduced frommineral inclusions and SHRIMP geochronology ofzircons is illustrated in Figure 4 Zoned zircons fromdiamond-bearing gneiss yield the following ages aquartz-bearing inherited core with 1100ndash1400 Maas the protolith age coesite-bearing mantle with 537plusmn 9 Ma for UHP metamorphism and the plagio-clase-bearing rim with 507 plusmn 8 Ma for amphibolite-facies overprint (Katayama et al 2001) Manydiamond-grade marbles contain clinopyroxene withexsolution lamellae of quartz K-feldspar phengiteand phlogopite (Katayama et al 2002 Zhu andOgasawara 2002) and titanite with coesite lamellae(Ogasawara et al 2002) these occurrences suggestthat UHP metamorphism occurred at P gt 6 GPa and1000degC

Cndash13 isotopic data of diamond suggest biogenicsources inclusions of water carbonate and nano-metric oxide in diamonds indicate that metamorphicmicrodiamonds in this and other UHP terranes wereprecipitated from C-O-H supercritical fluids (eg

Stockhert et al 2001) Ogasawara et al (2002)further suggested a two-stage growth mechanism formicrodiamonds Masago et al (2003) reported nega-tive 18O values (-39 per mil) for minerals in eclogiteand whiteschist of the Kokchetav Massif similar tothe Chinese UHP rocks first described by Yui et al(1995) The Kokchetav Massif is the second recog-nized UHP region that preserves a significant effectof the interaction of cold meteoric water with theprotolith prior to subduction

Saxonian Erzgebirge Germany The ErzgebirgeCrystalline Complex (ECC) at the northern margin ofthe Bohemian massif is in fault contact with a low-grade Paleozoic sequence at its margins (for a recentreview see Massonne and OrsquoBrien 2003) TheVariscan ECC consists of abundant gneisses thatinclude numerous lenses of eclogites The ldquoGneiss-Eclogite Unitrdquo at the core of this massif containsUHP rocks typified by inclusions of coesite pseudo-morph in garnets from eclogite (Schmadicke 1991)and inclusions of microdiamonds in garnet kyaniteand zircon from gneisses (Massonne 1999) Thusfar the diamondiferous gneisses seem to occur onlyin a 1 km long strip near the eastern shore of theSaidenbach Reservoir Diamond micro-inclusionsoccur exclusively in garnet kyanite and zircon ofseveral gneissic rocks Besides completely pre-served diamonds with grain sizes between 1 to 25microm partially graphitized diamonds and graphitepseudomorphs after diamond occur Such microdia-mond inclusions in garnet and zircon are similar tothose in Kokchetav diamondiferous biotite gneissSome of these inclusions contain additional fluid-bearing phases including apatite phengitic micaand possible fluid inclusions the morphologies ofinclusions and mineral associations suggest thatmicrodiamonds from both the Erzgebirge and theKokchetav were crystallized from supercritical flu-ids under UHP conditions (Stockhert et al 2001Hwang et al 2001 Dobrzhnetskaya et al 20012003)

Schmadicke et al (1995) reported Sm-Nd isoch-rons for garnet-Cpx-WR at 360 plusmn 7 Ma for eclogiteand 353 plusmn 6 Ma for garnet pyroxenite from Erzge-birge UHP rocks 40Ar39Ar spectra of phengite fromtwo eclogite samples give plateau ages of 348 plusmn 2and 355 plusmn 2 Ma Diamond-bearing zircons yieldSHRIMP UPb dates at 336 plusmn 2 Ma (Massonne2001) Such similarity in ages for garnet peridotitesand eclogite enclosed in gneiss suggests a coevalVariscan UHP in-situ metamorphism of the ECCaround 340ndash360 Ma

10 LIOU ET AL

Less Intensively Studied UHP terranes

Several UHP terranes described below exceptfor the Maksyvtov Complex have received lessintensive study mainly due to remoteness and diffi-cult access despite their long recognition

Maksyutov Complex Southern Urals TheMaksyutov Complex is an elongated (10 times 120 km)N-S belt between the Russian and Siberian cratonsin the southern Ural Mountains (Coleman and Wang1995 Dobretsov et al 1996) and contains blocky

graphitized diamond() in pelitic schist (Leech andErnst 1998) The complex is the locality from whichquartz aggregates within garnet exhibiting radialcracks were first recognized as coesite-pseudo-morphs by Chesnokov and Popov (1965) nearly 20years prior to the independent finding of coesiteinclusions by Chopin (1984) and Smith (1984) Theeclogite-bearing unit contains boudins of eclogitelayers of eclogitic gneiss and rare ultramafic bodieswithin host metasedimentary mica schist andquartzite

FIG 4 A Schematic diagram showing the growth of detrital zircon (stage 1) through subduction (stage 2 and peakstage 3) and exhumation (stage 4) of supracrustal rocks Mineral inclusions in various stages of zircon growth are shownB SHRIMP U-Pb dating of zoned zircons from a Kokchetav diamond-bearing gneiss yields a well constrained P-T-timepath (for details see Katayama et al 2001)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 11

Some Maksyutov eclogites were recrystallized at594ndash637degC with a minimum pressure from 15 to17 GPa (Beane et al 1995 Leech and Ernst 1998)or up to 27 GPa if coesite pseudomorphs (Ches-nokov and Popov 1965 Dobretsov and Doretsova1988) are indeed present However subsequentstudies have not revealed the occurrence of inclu-sions of coesite or coesite pseudomorph in garnetand zircon (Beane et al 1995 Shatsky et al 1995)Leech and Ernst (1998) described unusual graphiteaggregates (up to 13 mm edge length) that may bediamond pseudomorphs This suggestion was basedon the blocky morphology of the cuboid graphiteaggregates cross-cutting rock foliation pressureshadows biogenic isotopic signatures and otherspectroscopic characters similar to those graphi-tized diamonds from the Beni Bousera massifRecent study of several microdiamond inclusions ingarnet has yielded further suggestive possibility thatthe eclogitic unit of the Maksyutov Complex experi-enced diamond-grade UHP metamorphism (Bosticket al 2003) The UHPHP metamorphism with peakmetamorphic ages of 370ndash374 Ma (Shatsky et al1995 Beane and Connelly 1998) may be related tothe collision of the Russian platform with a fragmentof the Siberian craton during Devonian time butwas affected by the Triassic collision of the Kazakh-stan block with the Russian platform (eg Matte1998)

Zermatt-Saas area Western Alps In contrast tomost other UHP terranes in which the protolith iscontinental crust eclogites and metasediments atLago di Cignana of the Western Alps were derivedfrom oceanic crust and constitute the ophiolitesequence of the Zermatt-Saas zone Inclusions ofcoesite and coesite pseudomorphs occur in tourma-line and garnet of manganiferous quartzite and inomphacite and garnet of the underlying mafic eclog-ite (Reinecke 1991) The metasediments are mainlygarnet-phengite-quartz schist with variableamounts of garnet-clinopyroxene quartzite andpiedmontite-phengite-quartz schist Eclogites andretrograded eclogites with preserved UHP mineralparageneses consist of garnet omphacite glau-cophane zoisite phengite and dolomite Zoned gar-nets preserve prograde quartz crystals in their coresand coesite inclusions in mantles and rims Coesiteinclusions are much more common in omphacitethan in garnet UHP conditions were estimated at26ndash30 GPa and 590ndash630degC Sm-Nd isotopic anal-yses of eclogites yield UHP metamorphism at 52 plusmn18 Ma (Bowtell et al 1994) consistent with a

Tertiary age of eclogitization recently established forAlpine metamorphism in the Western Alps(Gebauer et al 1997)

Thus far the recognized areal extent of thecoesite-bearing rocks is ~ 2 km2 The apparentthickness of the UHP slice does not exceed a fewhundred meters This area is the best example forsubduction of oceanic lithosphere to mantle depthand later exhumation as a fragmented tectonic slabSimilarly the Allalin metagabbro part of the oce-anic Piemonte zone contains magnesite + talc +kyanite + garnet recrystallized at ~600degC and 35GPa with an estimated geothermal gradient of 5ndash6degC kmndash1 however inclusions of coesite or coesitepseudomorphs have not been reported from theAllalin metagabbro

Mali Africa Eclogitic rocks in the Pan-Africancollision zone cropping out in northern Mali containinclusions of coesite in omphacite from mafic nod-ules within a calc-silicate layer enclosed in impuremarble and coesite pseudomorphs in garnets fromthe surrounding eclogitic metasediments (Caby1994) These UHP eclogitic mica schists have P-Testimates of 700ndash750degC and gt 27 GPa phengiteyields a well-defined plateau 39Ar40Ar age of 1045plusmn 9 Ma Recent SmNd ages of 620 Ma wereobtained by Jahn et al (2001) for peak UHP meta-morphism This may be the oldest UHP terrane inthe world

The coesite-bearing UHP eclogitic unit about 3km thick lies within an internal nappe as a flat thinslice bounded on the top by a thick passive-marginshaly formation and on the bottom by low-gradegreenschist-facies phyllites Caby (1994) suggestedthat a large portion of the terrigenous metasedimentsof passive-continental-margin affinity of the WestAfrican plate was subducted eastward to mantledepths (gt90 km) beneath an oceanic domain Colli-sion of the West African plate with an island arcresulted in a low-angle westward subhorizontalextrusion of this subducted slice of sialic materialsto its present position The coesite-bearing eclogiteunit may also crop out 1500 km to the south in Togowhere kyanite-bearing eclogites occur as part of thepassive-margin assemblage

Makbal (480 Ma) and Atbashy (270 Ma) Kazak-hstan The Tian-Shan is divided into three mountainrangesmdashnorthern middle and southern Within theKyrgyzstan Tian-Shan UHP eclogite localities withcoesite pseudomorph inclusions in garnet andomphacite from the northern and southern Tian-Shan represent Caledonian and Hercynian orogenic

12 LIOU ET AL

belts respectively (Tagiri and Bakirov 1990 Tagiriet al 1995) The Makbal Formation in the westernKyrgyz Ridge of the northern Tian-Shan consists ofquartzose schist alternating with pelitic schistsintercalated with thin layers or lenses of marbleeclogite and amphibolitized eclogite Tagiri andBakirov (1990) found inclusions of coesite pseudo-morphs in garnet from a garnet-chloritoid-talc schistwith a peak metamorphic assemblage of coesite +almandine + chloritoid + talc + phengite (Si = 342pfu) + phlogopite + rutile Makbal coesite-gradeeclogite has a paragonite KAr age of 480 Ma

The eclogite-bearing complex of Atbashy Ridgefrom the southern Tian-Shan is composed of peliticand siliceous schists alternating with thin UHPeclogitic layers and is unconformably overlain byUpper Paleozoic molasse and silicic volcanic rocksInclusions of coesite pseudomorphs occur inomphacite cores and in garnet Eclogites from theAtbashy area preserve prograde and retrogradepaths with a peak metamorphic condition at 660degC25 GPa The Atbashy coesite-eclogites yielded aRbSr mineral isochron age of 270 Ma (Tagiri et al1995)

Central Indonesia UHP terrane The pre-Tertiarybasement of the Indonesian region comprises a vari-ety of imbricate terranes meacutelange ophiolite andvariably metamorphosed accretionary complexessome contain HP to UHP metamorphic rocks result-ing from the collision of an Australia-derived conti-nent with Eurasia (Parkinson 2003) HP rocksincluding eclogites and garnet peridotites are widelydistributed in Cretaceous accretionary complexesMany of these rocks occur as imbricate slices of car-bonate quartzite and pelitic schist of shallow-marine or continental-margin parentage interthrustwith subordinate mafic schist and serpentinitesome yield mica KAr ages of 110ndash120 Ma (Parkin-son 2003)

HP and UHP rocks are sporadically exposed astectonic blocks throughout the Cretaceous accre-tionary complexes They include eclogite garnet-glaucophane rock (P = 18ndash24 GPa T = 580ndash620degC) and jadeite-garnet quartzite (P gt 27 GPa T =720ndash760degC) in Bantimala southwest Sulawesieclogite and garnet granulite in west-centralSulawesi eclogite and jadeite-glaucophane-quartzrock (P ~22 GPa T ~530degC) in central Java Mgchloritoidndashbearing whiteschists (P ~18 GPa) insoutheast Kalimantan garnet lherzolites in east-central Sulawesi (P = 22ndash28 GPa T = 1000ndash1100degC) west-central Sulawesi (P = 16ndash20 GPa T =

1050ndash1100degC) and garnet pyroxenite (P ~2 GPa T~ 850degC) in Sabah northeast Borneo Evidence forUHP rocks include (1) inclusions of coesite inzircon and coesite pseudomorphs in jadeite fromjadeite quartzite and eclogite of the BantimalaComplex of south Sulawesi (Parkinson 2003) and(2) P-T estimates of peak-stage recrystallization at27ndash35 GPa and 1000ndash1100degC for most garnetperidotites (Kardarusman and Parkinson 2000)Many of these rocks were probably recrystallized ina N-dipping subduction zone at the margin of theSundaland craton in the Early Cretaceous Exhuma-tion may have been facilitated by the collision of aGondwana continental fragment with the Sundalandmargin at ~120ndash115 Ma

Recently Recognized UHP Terranes

Several new UHP terranes were recently identi-fied inasmuch as they contain partially preservedtrace index minerals in strong containers such aszircon or garnet In fact zircon has been consideredto be the best container and many new terranes werediscovered through positive identification of inclu-sions of coesite or coesite pseudomorphs ordiamond in zircons Detailed examination of mineralinclusions in core mantle and rims of zircon sepa-rates from eclogites and their enclosing country-rock gneisses have yielded both prograde andretrograde P-T-time paths for various UHP terranesmentioned above (see Katayama et al 2001 fordetails also see the section on the Himalayan eclog-ite for an example)

The latest recognized UHP terranes include thefollowing (1) coesite in the French Massif Central(Lardeaux et al 2001) (2) coesite and diamondfrom the Greek Rhodope metamorphic province(Mposkos and Kostopoulos 2001) (3) coesitepseudomorphs in the Northeast Greenland EclogiteProvince (Gilotti and Ravna 2002) (4) coesite ingranulite-facies overprinting eclogites of SoutheastBrazil (Parkinson et al 2001) (5) coesite in Hima-layan eclogite from the Upper Kaghan Valley Paki-stan (OrsquoBrien et al 2001 Kaneko et al 2003) andcoesite from the Tso-Morari crystalline complex ofIndia (Mukherjee et al 2003) (6) coesite inclusionsin gneissic rocks from the North Qaidam belt west-ern China (Yang et al 2001 Song et al 2003) (7)diamond inclusions in garnets of eclogite and gneissfrom North Qinling (Yang et al 2002a) (8) inclu-sions of quartz pseudomorphs and minor coesite ingarnets of mafic eclogites from the LantermanRange in Antarctica (Ghiribelli et al 2002) (9)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 13

inclusions of calcite pseudomorphs after aragonite +magnesite in dolomite in metapelites from the west-ern Tian-Shan inferred as metamorphism in a UHPregion of dolomite decomposition (Zhang L et al2002) and (10) inclusion of possible coesitepseudomorphs in garnet lamellar inclusions in gar-net kyanite and high sodium content of garnet fromeclogites of the Chuacus Complex of north-centralGuatemala (Solari et al 2003) The results of petro-chemical and geochronological data for these newterranes are summarized below The HimalayanUHP terrane in the Upper Kaghan Valley and theTso-Morari complex are used to illustrate the pro-cesses of continental subduction and collision in alater section

French Massif Central The French Massif Cen-tral in the western part of the Variscan Belt hasexperienced Late SilurianndashEarly Devonian to LateCarboniferous orogenic events (eg Matte 1986)High-pressure metamorphic rocks occur mainly inthe upper gneissic unit within the leptyno-amphibo-lite group The latter consists of an association ofmetagreywacke mica schist metabasalt leptynitemetagranite and peridotite Eclogites are associatedwith silicic and mafic HP granulites (Lardeaux etal 2001) and also with spinel andor garnet lherzo-lites (Gardien et al 1990) Inclusions of coesite andcoesite pseudomorphs occur in garnets of kyanite-bearing eclogite (Lardeaux et al 2001) Only twocoesite grains are preserved as relics most coesitegrains are completely transformed into polycrystal-line radial quartz (palisade texture) or into polygonalquartz surrounded by radial cracks Metamorphictemperatures were estimated to be 740-780degC andsubsequent amphibolite-facies overprinting at750degC at 15ndash17 GPa during decompression UHPmetamorphism occurred between 420 and 400 Ma(Paquette et al 1995) 40Ar39Ar data from amphib-ole separates from retrogressed eclogites yielded339 plusmn 4 Ma (Costa et al 1993)

Northeast Greenland Caledonides The first evi-dence for UHP metamorphism in the GreenlandCaledonites was reported from kyanite eclogites andassociated host gneisses on an island in Jokelbugt(Gilotti and Ravna 2002) Inclusions of quartzpseudomorphs exhibiting palisade structure andradiating fractures occur in garnet and omphacite ofeclogites and in garnet of the host gneisses P-T esti-mates of peak-stage metamorphism of eclogite at~972degC and 36 GPa lie well within the coesite sta-bility field SHRIMP UPb dates of 403 plusmn 5 Ma forzircon rims from HP metapsammite and 404 plusmn 4 Ma

for zircons from anatectic melt derived frommetapelite are coeval with the estimated ages ofexhumation of UHP terranes in the ScandinavianCaledonides described in a previous section(McClelland and Gilotti 2003)

Neoproterozoic nappes in Southeast BrazilNeoproterozoic nappes of Southeast Brazil consistmainly of coarse-grained kyanite-garnet granulitewith intercalations of impure quartzite calc-silicaterock and minor lenses and sills of mafic-ultramaficrocks including eclogite Early eclogitic phaseswere strongly obliterated during granulite-faciesoverprinting during a major Pan-African continentcollision at 640ndash630 Ma (Campos Neto and Caby1999) Rare ultramafic lenses consist of phlogopite-orthopyroxene rocks and garnet clinopyroxeniteCores of granulite boudins retain eclogitic assem-blages some quartz inclusions within the mantleregions of zoned garnet porphyroblasts are sur-rounded by intense radial fractures suggestive ofthe former presence of coesite In fact coarse-grained zircons in some granulites contain numer-ous micro-inclusions of K-feldspar + kyanite + coes-ite as confirmed by Raman spectroscopy (Parkinsonet al 2001) Many exsolution microstructuresobserved in other UHP terranes also occur Thisarea represents the first coesite-grade UHP rocks inthe Americas and it can be correlated with the MaliUHP terrane of equatorial Africa (Caby 1994) Ithas been suggested that eclogitic assemblages andprobable UHPM components may be present in sev-eral other correlative nappe systems at the marginsof the Sao Francisco craton in Brazil

Chuacus Complex north-central Guatemala HPeclogites within banded gneisses schists andmigmatites of the Chuacus Complex contain relicttextures and mineralogical features that suggestUHP metamorphism (Solari et al 2003) Theseinclude inclusions of possible coesite pseudomor-phs in garnet and kyanite lamellar inclusions ofrutileilmenite in garnet high sodium content ofsome garnets (up to 1200 ppm) and preliminary P-T estimates of 700ndash800degC and 2ndash3 GPa AlthoughK-Ar dating of mica and hornblende yield ages of48ndash72 Ma possible for amphibolite facies retrogres-sion regional and structural data support a pre-Mesozoic age for the multi-stage HP-UHP recrystal-lization Such a discovery opens up a new interpre-tation regarding the tectonic evolution of Maya-Chortis-Oaxaquia microcontinental blocks formingthe structural backbone of Mesoamerica

14 LIOU ET AL

Greek Rhodope The Greek Rhodope Metamor-phic Province (RMP) at the border between Greeceand Bulgaria represents a syn-metamorphic nappesystem of Alpine age that was formed during theCretaceous to Mid-Tertiary collision of Apulia andpaleo-Europe The Kimi nappe complex comprisescrustal eclogites leucocratic orthogneisses peliticgneisses and volumetrically minor mantle-derivedultramafic rocks these units contain many mineral-ogical indicators of UHP metamorphism (Mposkosand Kostopoulos 2001) The ultramafic lithologiesinclude garnet spinelndashbearing lherzolite and layersof spinel-garnet clinopyroxenite garnet pyroxeniteand clinopyroxene garnetite These rocks were sub-jected to UHP recrystallization at P-T conditions of~1200degC and gt30 GPa The pelitic gneiss containsmicrodiamond inclusions in garnet porphyroblastsas well as rods or needles of silica rutile biotiteand apatite in sodic garnet suggesting the prioroccurrence of majoritic garnet The growth of suchsupersilicic garnet was suggested to be at a pressureof ~7 GPa Inclusions of polycrystalline quartzaggregates surrounded by radial cracks occur ineclogitic garnet Exsolution lamellae of quartz ineclogitic clinopyroxene also indicate an UHP pre-cursor of supersilicic clinopyroxene

Lanterman Range Antarctica The LantermanRange in northern Victoria Land of Antarctica con-sists of several metamorphic complexes The Gate-way Hills Metamorphic Complex is a thindiscontinuous belt more than 50 km long consistingof mafic and ultramafic rocks including lenses andpods of eclogite within felsic gneiss Some less foli-ated eclogites contain inclusions of coesite andcoesite pseudomorphs in garnet porphyroblasts asconfirmed by in situ Raman spectroscopy as well asradial fractures around the inclusions (Ghiribelli etal 2002) The eclogites were subjected to UHPmetamorphism at T gt 850degC and P gt 29 GPa thenamphibolite-facies overprinting during isothermaldecompression The UHP event is dated by SmNdand 238U206Pb data at 500 Ma 40Ar39Ar ages of490ndash486 Ma for Ca-amphibole (amphibolite-faciesoverprinting) suggest rapid cooling with an averageexhumation rate of 3ndash4 km_Ma-1 for the UHP ter-rane (Di Vincenzo and Palmeri 2001)

Western Tian-Shan China Caledonian eclogitesof the western Tian-Shan China have been classi-fied into three types (Zhang L et al 2002) Type Ieclogite pods are layered with mafic blueschists andType II eclogites preserved pillow structureswhereas Type III eclogites are banded with marbles

This 200 km long eclogite-bearing belt extendswestward to connect with the Atbashy UHP belt inKazakhstan (Tagiri et al 1995) Reported UHP evi-dence includes inclusions of coesite pseudomorphsin garnets from Type I and III eclogites quartzexsolution lamellae in omphacite from Type IIeclogites and relict magnesite within matrix dolo-mite in Type III eclogites (Zhang J X et al 2002Zhang L et al 2002) P-T estimates of the peakUHP stage of metamorphism are T = 500ndash600degCand P = 26ndash27 GPa (Wei et al 2003)

North Qaidam Mountains western China Manylenses and layers of garnet peridotite and eclogiteoccur in an amphibolite-facies terrane in the NorthQaidam Mountains along the northeastern rim of theQaidam Basin This belt comprises mainly felsicgneiss quartz schist garnet-bearing gneissamphibolite and minor eclogite + garnet peridotiteit extends westward for more than 1000 km to theAltun Mountains and was displaced by the left-lat-eral Altyn-Tagh fault At the eastern end of thisUHP belt at Dulan occurrences of coesite inclu-sions in zircon separates from paragneiss inclusionsof coesite pseudomorph in garnet and omphacite ofeclogites quartz rods in eclogitic omphacite andP-T estimates of eclogites (P = 287ndash317 GPa andT = 631ndash687degC) demonstrate that this is a typicalUHP metamorphic terrane (Yang et al 2001 Songet al 2003a 2003b) The Dulan eclogites have anSm-Nd mineral isochron age of 497 plusmn 87 Ma and aSHRIMP UPb zircon age of 495 Ma (Yang et al2002b)

Altun Mountains western China This UHP ter-rane about 200 km long in the southern margin ofthe Altun Mountains is the western extension of theNorth Qaidam belt Eclogitic lenses and blocks ofvarious sizes together with garnet-lherzolite and cli-nopyroxenite are enclosed within an Early Protero-zoic sequence of amphibolite amphibole schistgarnet-bearing granitic gneiss and pelitic schistGarnet lherzolite contains magnesite that reacted toform dolomite P-T estimates of the peak magnesite-bearing assemblage yield P gt 36 GPa and 850degC(Liu et al 2001) Garnet porphyroblasts includeexsolution lamellae of clinopyroxene and rutile sim-ilar to that of eclogite reported by Ye et al (2000) asmajoritic garnet The associated eclogites containinclusions of coesite pseudomorphs in garnet withcharacteristic radial fractures this together with theexsolved quartz rods in omphacite and P-T esti-mates of 820ndash850degC and 28 to 32 GPa provideadditional evidence for UHP (Zhang J X et al

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 15

2001) An SmNd mineral isochron for eclogiticgarnet omphacite and the whole rock yields 500 plusmn10 Ma whereas an UPb isochron for zircon sepa-rates from eclogite gives an age of 504 plusmn 5 Ma(Zhang J X et al 2000)

North Qinling Mountains central China Lensesand blocks of eclogitic rocks in northern Qinling areenclosed in amphibolite-facies gneiss and garnet-bearing quartz + phengite mica schist Inclusions ofcoesite in eclogitic garnet (Hu et al 1995) andmicrodiamonds in zircon separates from both eclog-ite and gneiss (Yang et al 2002a) have beenrecently reported Although individual outcrops areno more than a few meters wide the belt of eclogitesextends more than 10 km P-T estimates of amphib-olitized coesite-bearing eclogite yield T = 590ndash758degC and P gt 26 GPa An Sm-Nd isochron for gar-net omphacite rutile amphibole and whole-rockyields 400 plusmn 16 Ma Microdiamonds were recentlydiscovered as inclusions in zircon from both eclogiteand its gneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyields 507 plusmn 38 Ma for peak UHP metamorphism(Yang et al 2002a) These data indicate that two

discrete collision events occurred between theNorth China and Yangtze cratons (1) an Early Pale-ozoic North Qinling UHP suturing and (2) a Triassicsuturing along the Dabie-Sulu UHP-HP belt TheEarly Paleozoic belt extends westward to the NorthQaidamndashAltun Mountains for more than 4000 km(Yang et al 2002b)

Himalayan UHP Eclogites

The Himalayan orogen has long been considereda classic locality of continental collision betweenthe Indian and Eurasian continents (Fig 5) Severaleclogite-facies HP rocks in the Himalayas havebeen reported since the late 1980s (eg Chaudaryand Ghazanfar 1987 Pognante and Spencer 1991de Sigoyer et al 1997 Lombardo and Rolfo 2000)Inclusions of coesite were recently discovered inomphacite and garnet in several Himalayan eclog-ites from the upper Kaghan Valley Pakistan(OrsquoBrien et al 2001 Kaneko et al 2003) Subse-quent findings of additional coesite localities in theTso Morari area (Sachan et al 2001 Mukherjee etal 2003) have established the presence of a UHPbelt in the Himalayas an active ongoing collision

FIG 5 A simplified tectonic map of the western Himalaya Hindu Kush Pamir Karakoran and western Tibet mod-ified after Figure 3 of Searle et al (2001) This map shows the locations of eclogite and UHP eclogites from both Hima-layan and North Qaidam Indus Suture Zone (ISZ) ophiolites and major structures

16 LIOU ET AL

since the Eocene (Massonne and OrsquoBrien 2003)However UHP relics are volumetrically very minorconstituents of the metamorphic belts dominatedmostly by later Barrovian overprintings In theserocks hydration related to Barrovian-zone metamor-phism has resulted in remarkably pervasive recrys-tallization during the later exhumation this eventhas almost entirely masked the preceding UHPrecord

The finding of HP-UHP evidence has led to newinsights into the Himalayan tectonic modelEngland and Houseman (1986) showed that in amodel of homogeneous thickening of the Hima-layan-Tibet boundary the maximum crustal thick-ness should be less than 70 km However theoccurrences of coesite-bearing eclogites andgneisses in the Himalayan orogen cannot beexplained by a homogeneous thickening of the crustbut rather by continental subduction as describedbelow According to available geochronologicaldata (eg Treloar et al 2003) the exhumation rateof the Himalayan UHP rocks has been estimated torange from ~1 cm yrndash1 (OrsquoBrien et al 2001) to gt3cm yrndash1 (Treloar et al 2003 Massone and OrsquoBrien2003) These estimates are higher than those calcu-lated from other UHP-HP terranes This requirestectonic emplacement of the UHP-HP Himalayanslab into shallow crust following isostatic rebounddue to slab decoupling between the Indian continentand the oceanic crust initiated during the MiddleEocene UHP rocks from both the Kaghan Valleyand Tso-Mori are described below

Kaghan Valley The Kaghan nappe in the north-western Himalayan syntaxis lies between the Indussuture (also called main mantle thrust MMT) to thenorth and the main central thrust (MCT) to thesouthwest (Pognante and Spear 1991 Spencer1993) (Fig 5) The Cretaceous Kohistan island-arcsequence lies north of the Indus suture It developedas a result of intra-oceanic subduction The maincentral thrust separates an underlying Salkhala unitfrom an overlying higher Himalayan crystalline(HHC) unit The Kaghan nappe consists mainly ofgranitic gneiss and paragneiss with minor inter-calated layers of amphibolite marble and quartziteEclogites are preserved in the cores of maficboudins (less than 1 m thick) in gneiss and aresignificantly overprinted by amphibolite assem-blages They contain fine-grained (lt 1 mm) garnetand omphacite together with aggregates of quartzchains of rutile rimmed by titanite and mm-sizedphengitic micas overgrown by randomly oriented

amphiboles up to 1 cm in length Coesite and coesitepseudomorphs occur as inclusions in omphaciteand show the characteristic palisade-quartz aggre-gate texture fractures radiating from these inclu-sions are characteristically developed in the hostpyroxene

Garnet pyroxenendashphengite barometry wasapplied to the eclogites and yielded peak P-T con-ditions of 27ndash29 GPa and 690ndash750degC (OrsquoBrien etal 2001) (Fig 6) Retrograde textures are commonin the eclogites including symplectic intergrowthsof augite amphibole and plagioclase after omphac-ite thermobarometric analyses of these assemblagesyielded 10ndash13 GPa and 600ndash710degC (Kaneko et al2003) On the other hand the dominant graniticgneisses and metapelites of the higher Himalayancrystalline unit contain typical Barrovian-zoneamphibolite-facies assemblages represented bykyanite- and staurolite-bearing garnetiferousmetapelites (Treloar 1995) Extensive overprintinghas mostly obliterated the peak assemblages matrixminerals of the gneisses yielded P-T conditions of07ndash11 GPa and 600ndash700degC (Treloar 1995) con-sistent with retrogression of the eclogites Howeverrelict coesite is preserved as inclusions in zirconthe inclusions are ovoid and approximately 10 microm indiameter (Fig 7A) This discovery of coesite-bear-ing zircons from Himalayan quartzofeldspathicgneisses demonstrates subduction of Indian conti-nental crust to the depths of UHP conditions(Kaneko et al 2003)

Geochronological studies using SmNd (garnet-omphacite) RbSr (phengite) and UPb (rutile zir-con) methods suggest that the eclogite-facies eventtook place at 40ndash50 Ma (Tonarini et al 1993 Spen-cer and Gebauer 1996) 40Ar39Ar cooling ages forhornblende and mica from gneisses were reported tobe ~43 Ma and ~25 Ma respectively (Chamberlainet al 1991) Recent SHRIMP UPb dates of zonedzircons with mineral inclusions formed at differentstages indicate the ages of non-UHP mineral-bear-ing mantle domains of zircon and UHP mineral-bearing rims are at about 50 and 46 Ma respectively(Kaneko et al 2003) A new U-Pb age of 44 plusmn 11Ma was obtained for rutile from Kaghan coesite-bearing eclogite (Treloar et al 2003) this togetherwith available data implies that exhumation of theUHP rocks from mantle depth to mid-crustal leveloccurs within a few million years

Tso Morari The Tso Morari crystalline dome islocated between the Indus suture zone on the northbordered by the Zildat detachment fault and the

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 17

Zanskar sedimentary unit on the south (de Sigoyer etal 1997) The dome represents an internal massif ofthe northern part of the higher Himalaya The TsoMorari area has been considered a typical IndianTethyan margin because of a stratigraphic succes-sion similar to the Nimaling antiform in the core ofthe dome a metamorphic basement consists of Cam-bro-Ordovician augen gneisses (Stutz 1988) Thiscomplex is covered by a metasedimentary series ofCambrian to Devonian quartzite schist and con-glomerate and is overlain by Lower Carboniferousto Triassic marble and metapelite Eclogites occuras lenses or irregular bodies within the basementand cover gneisses and are strongly overprinted byamphibolite-facies assemblages de Sigoyer et al(1997) reported peak P-T conditions for the eclog-ites at 17ndash23 GPa and 520ndash640degC (Fig 7) How-ever relics of coesite 30ndash80 microm in size were

recently found as inclusions in garnet (Sachan et al2001 Mukherjee et al 2003) typified by radialfractures (Figs 7B and 7C) for the SiO2 inclusionsa characteristic Raman spectrum is centered at 523cm-1 (Fig 7D) The country rock metasedimentswith jadeite + chloritoid + paragonite + garnetassemblages have peak P-T estimates of 18ndash22GPa and 500ndash600degC and record retrogression at theeclogite-blueschist facies transition of 13ndash18 GPaand 490ndash590degC (Guillot et al 1997)

The geochronology of the Tso Morari eclogiteshas been extensively studied and subdivided intothree different stages (de Signoyer et al 2000)Eclogitization at ~55 Ma was obtained by Lu-Hf andSm-Nd methods on garnet omphacite glaucophaneand the whole rock An amphibolite-facies overprintoccurred at 47 Ma judging by RbSr dating onphengite apatite and whole rock from metapelites

FIG 6 P-T estimates of peak- and retrograde-stage metamorphism for coesite-bearing eclogites and the country-rockgneisses from the Kaghan Valley and Tso Morari

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 5

FIG

3 D

istr

ibut

ion

and

peak

met

amor

phic

age

of r

ecog

nize

d U

HP

terr

anes

in th

e w

orld

(mod

ified

aft

er L

iou

et a

l 2

000)

6 LIOU ET ALTA

BL

E 1

Sum

mar

y of

Pet

roch

emic

al F

eatu

res

of R

ecog

nize

d U

HP

Ter

rane

s in

the

Wor

ld

UH

P te

rran

eU

HP

rock

type

Age

sP

-T e

stim

ates

Spec

ial f

eatu

res

Cla

ssic

al te

rran

es

Dor

a M

aira

Mas

sif

Wes

tern

Alp

s I

taly

Ecl

ogit

e P

yrop

e qu

artz

ite

Pel

ites

Whi

tesc

hist

354

plusmn 1

0 M

a (p

eak)

30 M

a (r

etro

grad

e)37

kba

r 79

0degC

1 P

yrop

e w

hite

schi

st c

onta

ins

Mg-

Al s

ilica

tes

2 N

o ga

rnet

per

idot

ite

Wes

t Gne

iss

Reg

ion

Nor

way

Ecl

ogit

eG

arne

t per

idot

ite

Dia

mon

d-be

arin

ggn

eiss

408ndash

425

Ma

(pea

k)37

4ndash37

8 M

a (r

etro

grad

e)P

gt 2

8 kb

arT

gt 7

90degC

1 R

are

diam

ond

in c

ount

ry r

ock

gnei

ss a

nd g

arne

t per

idot

ite

2 M

ajor

itic

garn

et in

per

idot

ite

form

ed a

t man

tle

~17

00 M

a

Dab

ie-S

ulu

terr

ane

Eas

t-C

entr

al C

hina

Ecl

ogit

eG

arne

t per

idot

ite

Jade

ite q

uart

zite

Min

or w

hite

schi

st

220ndash

240

Ma

(pea

k)E

clog

ite

26ndash

40 k

bar

700ndash

850deg

CG

arne

t per

idot

ite

50-6

0 kb

ar 7

00ndash8

50degC

1 A

bund

ant c

oesi

te in

gar

net

omph

acit

e k

yani

te z

oisi

te z

irco

n 2

Rar

e di

amon

d in

ecl

ogit

e3

Ecl

ogite

and

gar

net p

erid

otit

e co

ntai

n U

HP

hydr

ous

phas

es4

Dua

l ori

gin

for

garn

et p

erid

otit

e 5

Maj

oriti

c ga

rnet

in e

clog

ite

Kok

chet

av M

assi

fno

rthe

rn K

azak

hsta

nE

clog

ite

Dia

mon

d-be

arin

ggn

eiss

and

mar

ble

Whi

tesc

hist

537plusmn

9 M

a (p

eak)

507plusmn

9 M

a (r

etro

grad

e)D

iam

ond

grad

e 4

0ndash60

kbar

920

ndash100

0degC

Coe

site

gra

de 3

6 kb

ar

750ndash

800deg

C

1 M

icro

diam

ond

is a

bund

ant i

n so

me

garn

et g

neis

s an

d do

lom

ite

mar

ble

2 C

oesi

te in

clus

ions

in w

hite

schi

st e

clog

ite

and

gar

net g

neis

s3

Rar

e ga

rnet

per

idot

ite

Boh

emia

n M

assi

fE

clog

iteG

arne

t per

idot

ite

379ndash

395

Ma

gt 30

kba

r 70

0ndash80

0degC

Gar

net p

erid

otit

e yi

elds

muc

h hi

gher

P th

an c

oesi

te-e

clog

ite

Les

s in

tens

ivel

y st

udie

d te

rran

es

Zer

mat

t-Sa

as a

rea

Sw

itze

rlan

dE

clog

ite

Mn-

quar

tzit

e52

plusmn 1

8 M

a (p

eak)

26ndash3

0 kb

ar 5

90ndash6

30degC

1 P

roto

lith

s ar

e oc

eani

c af

fini

ty2

Coe

site

incl

usio

n in

the

man

tle

of g

arne

t whe

re g

arne

t cor

e ha

s qu

artz

Saxo

nian

Erz

gebi

rge

Ger

man

yE

clog

ite

Dia

mon

d-be

arin

g gn

eiss

360

plusmn 7

Ma

(pea

k)34

8ndash35

5 M

a (r

etro

grad

e)gt

42 k

bar

900ndash

1000

degC1

Inc

lusi

ons

of m

icro

diam

ond

in z

irco

n k

yani

te a

nd g

arne

t fro

m g

neis

s2

Rut

ile

with

-P

bO2

stru

ctur

e

Mal

i A

fric

aE

clog

ite

620

Ma

(pea

k)gt2

7 kb

ar 7

00ndash7

50degC

1 O

ldes

t age

of U

HP

met

amor

phis

m

Mak

bal

wes

tern

K

yrgy

zsta

nE

clog

ite

Gar

net-

bear

ing

peli

tic s

chis

t

482

Ma

gt26

kba

r 61

0ndash68

0degC

1 I

nclu

sion

s of

coe

site

pse

udom

orph

in g

arne

t fro

m e

clog

ite a

nd s

chis

t

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 7A

tbas

hy K

azak

hsta

nE

clog

ite27

0 M

agt

25 k

bar

660deg

C1

Inc

lusi

ons

of c

oesi

te in

gar

net

Cen

tral

Ind

ones

ia S

W

Sula

wes

iE

clog

ite

Gar

net p

erid

otite

115ndash

120

Ma

gt27

kbar

720

ndash760

degC33

ndash35

kbar

115

0degC

1 I

nclu

sion

s of

coe

site

in z

irco

n an

d co

esit

e ps

eudo

mor

ph in

jade

ite

Rec

entl

y re

cogn

ized

terr

anes

Fre

nch

Alp

sE

clog

ite

Met

apel

ites

400ndash

420

Ma

(pea

k)36

0ndash38

0 M

a (r

etro

grad

e)gt

28 k

bar

750deg

C1

Inc

lusi

ons

of c

oesi

te in

gar

net a

nd o

mph

acit

e

Nor

thea

st G

reen

land

Ecl

ogite

400ndash

440

Ma

24ndash2

8 kb

ar 8

20degC

1 I

nclu

sion

s of

coe

site

pse

udom

orph

in g

arne

t

Sout

heas

t Bra

zil

Ecl

ogit

e63

0 M

agt

27 k

bar

800deg

C1

Inc

lusi

ons

of c

oesi

te in

zir

con

Chu

acus

Com

plex

no

rthe

rn G

uate

mal

aE

clog

ite

Ban

ded

gnei

ss48

ndash72

Ma

(ret

rogr

ade)

20ndash3

0 kb

ar 7

00ndash8

00degC

1 C

oesi

te p

seud

omor

ph in

gar

net a

nd k

yani

te

Rho

dope

Mas

sif

Gre

ece

Ecl

ogit

eO

rtho

gnei

ssPe

litic

gne

iss

Ultr

amaf

ic r

ocks

30ndash4

2 M

a (p

eak)

8ndash12

Ma

(ret

rogr

ade)

gt 30

kba

r ~

1200

degC1

Mic

rodi

amon

d in

clus

ion

in g

arne

t por

phyr

obla

st2

Sili

ca r

util

e an

d ap

atit

e ro

ds in

gar

net

Lan

term

an R

ange

A

ntar

ctic

aE

clog

ite

Fels

ic g

neis

s50

0 M

a (p

eak)

486ndash

490

Ma

(ret

rogr

ade)

gt 29

kba

r gt

850

degC1

Inc

lusi

ons

of c

oesi

te a

nd it

s ps

eudo

mor

ph in

gar

net

Wes

tern

Tia

n-Sh

an

nort

hwes

t Chi

naE

clog

ite

Mar

ble

lt 3

10 plusmn

5 M

a (p

eak)

26ndash2

7 kb

ar 5

00ndash6

00degC

1 I

nclu

sion

s of

coe

site

and

its

pseu

dom

orph

in g

arne

t2

Pos

sibl

e co

exis

tenc

e of

mag

nesi

te +

ara

goni

te

Qai

dam

wes

tern

Chi

naE

clog

ite

Gne

iss

Gar

net p

erid

otite

495

plusmn 7

Ma

(pea

k)46

7 plusmn

1 M

a (r

etro

grad

e)gt

27 k

bar

700deg

C1

Inc

lusi

ons

of c

oesi

te in

zir

con

from

par

agne

iss

Alt

un M

ount

ains

w

este

rn C

hina

Ecl

ogit

eG

rt-l

herz

olit

e50

4 plusmn

5 M

a (p

eak)

28ndash3

2 kb

ar 8

20ndash8

50dego C

1 C

oesi

te p

seud

omor

ph in

gar

net

Nor

th Q

inli

ng M

tns

C

entr

al C

hina

Ecl

ogit

eG

neis

s50

7 plusmn

38

Ma

400

plusmn 1

6 M

agt

26 k

bar

590ndash

760deg

C1

Coe

site

in g

arne

t2

Mic

rodi

amon

d in

zir

cons

from

ecl

ogit

e an

d gn

eiss

Kag

han

vall

ey P

akis

tan

Him

alay

asE

clog

ite

Gne

iss

46 plusmn

1 M

a (p

eak)

44 plusmn

1 M

a (r

etro

grad

e)27

ndash29

kbar

690

ndash750

degC1

Inc

lusi

ons

of c

oesi

te a

nd it

s ps

eudo

mor

ph in

om

phac

ite

and

zirc

on

from

ecl

ogit

e an

d in

zir

con

from

gne

iss

Tso

Mor

ari

Indi

an H

imal

ayas

Ecl

ogit

eG

neis

s55

Ma

(pea

k)47

Ma

and

30 M

a(r

etro

grad

e)

gt 28

kba

r 70

0ndash80

0degC

1 I

nclu

sion

s of

coe

site

and

its

pseu

dom

orph

in g

arne

t fro

m e

clog

ite

8 LIOU ET AL

SHRIMP dating of zircons extracted from gneissschist pyrope inclusion and pyrope quartzite indi-cates the occurrence of 240ndash275 Ma old zirconcores and newly formed 35 Ma rims for some zircons(Gebauer et al 1997) Thus UHP metamorphismmust have taken place during Oligocene time withan estimated exhumation rate of 2 to 24 cm yrndash1

Western Gneiss Region Norway The WesternGneiss Region lies within an Early Paleozoic colli-sion zone the gneissic unit about 300 km long and150 km wide consists of interlayered pelite andmigmatite marble quartzite and amphibolite withtectonic inclusions of gabbro and peridotite (for arecent review see Carswell and Cuthbert 2003) Thegneissic unit exhibits mainly amphibolite-faciesassemblages but relics of HP assemblages alsooccur Eclogite boudins are widespread Coesite wasfirst reported from Grytting (Smith 1984) severalnew localities of coesite and coesite pseudomorphshave been recognized recently in both eclogites andthe adjacent gneissic rocks (Wain 1997 Cuthbert etal 2000 Wain et al 2000 Carswell et al 2003)Microdiamond grains 20ndash50 microns across havebeen described from residues separated from twogneisses (Dobrzhinetskaya et al 1995) the associ-ated kyanite-eclogites contain inclusions of coesitepseudomorphs in garnet Relict majoritic garnetsshowing exsolution of pyroxene lamellae in peridot-ite bodies have been recently discovered on theislands of Otrogy and Flemsoy (van Roermund andDrury 1998 van Roermund et al 2000) Theseperidotite bodies originally must have had an evenhigher-pressure deeper-mantle origin probablywithin a rising mantle diapir at ~1700 Ma In thesegarnet peridotites diamond inclusions in symple-tetic spinel after garnet have been discovered (vanRoermund et al 2002) Radiometric ages for eclog-ites of the Western Gneiss Region are 408ndash425 Mafor the peak metamorphism and 374ndash378 Ma forthe retrogressive mid-crustal metamorphic over-printing (for review see Carswell et al 1999) Car-swell et al (2003) provide new petrographicevidence and a review of the latest radiometric agedata and conclude that the UHP metamorphism ofeclogites occurred at 400ndash410 Ma significantlyyounger than the previous widely accepted age of425 Ma A two-stage exhumation process is sug-gested an initial exhumation to about 35 km depthby about 395 Ma at a mean rate of about 10 m Mandash1and subsequent exhumation to 8ndash10 km by about375 Ma at a much slower rate of about 13 m Mandash1

The suture zone occupied by UHPHP rocks ofthe Western Gneiss Region reflects collision of theeclogite-bearing Greenland sialic crust-cappedlithospheric plate with Fennoscandia during theCaledonian orogeny (Gilotti 1993 Krogh andCarswell 1995 Brueckner and Medaris 1998)Inclusions of coesite pseudomorph in garnet andomphacite from kyanite eclogites have beenrecently documented in the East Greenland eclogiteprovince (Gilotti and Ravna 2002) this new UHPHP terrane is remote but has excellent exposuresand requires close examination

Dabie-Sulu terrane of east-central China TheDabie-Sulu terrane occupies a Triassic collisionzone between the Sino-Korean and Yangtze plates(for a recent review see Hirajima and Nakamura2003) The central UHP coesite-bearing eclogitebelt (P = 27ndash5 GPa) is flanked to the north by amigmatite zone (P lt 2 GPa) and to the south by abelt of blueschist epidote amphibolite and eclog-ite-facies rocks (P = 05ndash12 GPa) Prevalent rocktypes include felsic gneiss orthogneiss marble andquartzite Protolith ages range from Proterozoic toOrdovician but the majority of reported ages areNeoproterozoic (Li et al 1993 Hacker et al 2000)Blocks boudins and layers of eclogites and garnetperidotites occur as enclaves in gneisses in the UHPunit The Dabie-Sulu UHP rocks show several char-acteristics (1) Widespread coesite with hydrousminerals such as talc zoisiteepidote and phengiteoccur in eclogites (2) Two distinct types of mantle-and crustal-derived garnet peridotites are present(3) Abundant exsolution textures were identified inUHP minerals from garnet peridotite and eclogite(4) Diamond separates from both eclogite and garnetperidotite have been reported from few restrictedregions (eg Xu et al 1992 2003) Some of thesefindings are not confirmed by thin-section observa-tion and extensive studies of mineral inclusions inzircon (for review see Liou et al 2002) (5) Somegarnet peridotites record much higher pressure thanassociated coesite-bearing eclogites reaching 5ndash6GPa (Yang et al 1993 Zhang R Y et al 20002003) (6) Various isotopic age dating methods forUHP minerals and rocks from Dabie-Sulu terranegives 220ndash240 Ma (Li et al 1993 Hacker et al2000)

Eclogites in the Huwan shear zone from thenorthwestern Dabie Mountains have SHRIMP UPbdates of 309 plusmn 3 Ma (Sun et al 2002) suggesting adiscrete Carboniferous subductioncollisionbetween North and South China Farther to the west

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 9

in North Qinling inclusions of microdiamond werediscovered in zircons from both eclogite and itsgneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyielded 507 plusmn 38 Ma for metamorphic rims and1200ndash1800 Ma for relic magmatic or old cores sug-gesting an Early Paleozoic UHP metamorphic eventThese new data suggest that episodic subductionand collision of continents occurred during stages ofclosures of paleo Tethys prior to the well-docu-mented major Triassic continent-continent collisionfor the Dabie-Sulu terrane

Kokchetav massif The Kokchetav massif north-ern Kazakhstan is the type site for the UHPdiamond-eclogite regional metamorphic facies(Sobolev and Shatsky 1990 Parkinson et al 2002Shatsky and Sobolev 2003) Diamonds occur asminute inclusions in garnet diopside and zirconporphyroblasts in marble pyroxene-carbonate-gar-net rock and garnet-biotite gneiss and schist Inclu-sions of coesite and coesite pseudomorphs occur ingarnet and zircon from eclogite and diamond-bear-ing gneiss The UHPHP unit extends NW-SE for atleast 80 km and is about 17 km wide The UHP slabis structurally overlain by a weak- to low-grademetamorphic unit and is underlain by a low-P meta-morphic andalusite-staurolite facies unit (Kaneko etal 2000) The UHP slab is composed of felsicgneiss with locally abundant eclogite lenses andminor orthogneiss metacarbonate and rare garnetperidotite with a range of Proterozoic protolith agesA well-constrained P-T-time path deduced frommineral inclusions and SHRIMP geochronology ofzircons is illustrated in Figure 4 Zoned zircons fromdiamond-bearing gneiss yield the following ages aquartz-bearing inherited core with 1100ndash1400 Maas the protolith age coesite-bearing mantle with 537plusmn 9 Ma for UHP metamorphism and the plagio-clase-bearing rim with 507 plusmn 8 Ma for amphibolite-facies overprint (Katayama et al 2001) Manydiamond-grade marbles contain clinopyroxene withexsolution lamellae of quartz K-feldspar phengiteand phlogopite (Katayama et al 2002 Zhu andOgasawara 2002) and titanite with coesite lamellae(Ogasawara et al 2002) these occurrences suggestthat UHP metamorphism occurred at P gt 6 GPa and1000degC

Cndash13 isotopic data of diamond suggest biogenicsources inclusions of water carbonate and nano-metric oxide in diamonds indicate that metamorphicmicrodiamonds in this and other UHP terranes wereprecipitated from C-O-H supercritical fluids (eg

Stockhert et al 2001) Ogasawara et al (2002)further suggested a two-stage growth mechanism formicrodiamonds Masago et al (2003) reported nega-tive 18O values (-39 per mil) for minerals in eclogiteand whiteschist of the Kokchetav Massif similar tothe Chinese UHP rocks first described by Yui et al(1995) The Kokchetav Massif is the second recog-nized UHP region that preserves a significant effectof the interaction of cold meteoric water with theprotolith prior to subduction

Saxonian Erzgebirge Germany The ErzgebirgeCrystalline Complex (ECC) at the northern margin ofthe Bohemian massif is in fault contact with a low-grade Paleozoic sequence at its margins (for a recentreview see Massonne and OrsquoBrien 2003) TheVariscan ECC consists of abundant gneisses thatinclude numerous lenses of eclogites The ldquoGneiss-Eclogite Unitrdquo at the core of this massif containsUHP rocks typified by inclusions of coesite pseudo-morph in garnets from eclogite (Schmadicke 1991)and inclusions of microdiamonds in garnet kyaniteand zircon from gneisses (Massonne 1999) Thusfar the diamondiferous gneisses seem to occur onlyin a 1 km long strip near the eastern shore of theSaidenbach Reservoir Diamond micro-inclusionsoccur exclusively in garnet kyanite and zircon ofseveral gneissic rocks Besides completely pre-served diamonds with grain sizes between 1 to 25microm partially graphitized diamonds and graphitepseudomorphs after diamond occur Such microdia-mond inclusions in garnet and zircon are similar tothose in Kokchetav diamondiferous biotite gneissSome of these inclusions contain additional fluid-bearing phases including apatite phengitic micaand possible fluid inclusions the morphologies ofinclusions and mineral associations suggest thatmicrodiamonds from both the Erzgebirge and theKokchetav were crystallized from supercritical flu-ids under UHP conditions (Stockhert et al 2001Hwang et al 2001 Dobrzhnetskaya et al 20012003)

Schmadicke et al (1995) reported Sm-Nd isoch-rons for garnet-Cpx-WR at 360 plusmn 7 Ma for eclogiteand 353 plusmn 6 Ma for garnet pyroxenite from Erzge-birge UHP rocks 40Ar39Ar spectra of phengite fromtwo eclogite samples give plateau ages of 348 plusmn 2and 355 plusmn 2 Ma Diamond-bearing zircons yieldSHRIMP UPb dates at 336 plusmn 2 Ma (Massonne2001) Such similarity in ages for garnet peridotitesand eclogite enclosed in gneiss suggests a coevalVariscan UHP in-situ metamorphism of the ECCaround 340ndash360 Ma

10 LIOU ET AL

Less Intensively Studied UHP terranes

Several UHP terranes described below exceptfor the Maksyvtov Complex have received lessintensive study mainly due to remoteness and diffi-cult access despite their long recognition

Maksyutov Complex Southern Urals TheMaksyutov Complex is an elongated (10 times 120 km)N-S belt between the Russian and Siberian cratonsin the southern Ural Mountains (Coleman and Wang1995 Dobretsov et al 1996) and contains blocky

graphitized diamond() in pelitic schist (Leech andErnst 1998) The complex is the locality from whichquartz aggregates within garnet exhibiting radialcracks were first recognized as coesite-pseudo-morphs by Chesnokov and Popov (1965) nearly 20years prior to the independent finding of coesiteinclusions by Chopin (1984) and Smith (1984) Theeclogite-bearing unit contains boudins of eclogitelayers of eclogitic gneiss and rare ultramafic bodieswithin host metasedimentary mica schist andquartzite

FIG 4 A Schematic diagram showing the growth of detrital zircon (stage 1) through subduction (stage 2 and peakstage 3) and exhumation (stage 4) of supracrustal rocks Mineral inclusions in various stages of zircon growth are shownB SHRIMP U-Pb dating of zoned zircons from a Kokchetav diamond-bearing gneiss yields a well constrained P-T-timepath (for details see Katayama et al 2001)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 11

Some Maksyutov eclogites were recrystallized at594ndash637degC with a minimum pressure from 15 to17 GPa (Beane et al 1995 Leech and Ernst 1998)or up to 27 GPa if coesite pseudomorphs (Ches-nokov and Popov 1965 Dobretsov and Doretsova1988) are indeed present However subsequentstudies have not revealed the occurrence of inclu-sions of coesite or coesite pseudomorph in garnetand zircon (Beane et al 1995 Shatsky et al 1995)Leech and Ernst (1998) described unusual graphiteaggregates (up to 13 mm edge length) that may bediamond pseudomorphs This suggestion was basedon the blocky morphology of the cuboid graphiteaggregates cross-cutting rock foliation pressureshadows biogenic isotopic signatures and otherspectroscopic characters similar to those graphi-tized diamonds from the Beni Bousera massifRecent study of several microdiamond inclusions ingarnet has yielded further suggestive possibility thatthe eclogitic unit of the Maksyutov Complex experi-enced diamond-grade UHP metamorphism (Bosticket al 2003) The UHPHP metamorphism with peakmetamorphic ages of 370ndash374 Ma (Shatsky et al1995 Beane and Connelly 1998) may be related tothe collision of the Russian platform with a fragmentof the Siberian craton during Devonian time butwas affected by the Triassic collision of the Kazakh-stan block with the Russian platform (eg Matte1998)

Zermatt-Saas area Western Alps In contrast tomost other UHP terranes in which the protolith iscontinental crust eclogites and metasediments atLago di Cignana of the Western Alps were derivedfrom oceanic crust and constitute the ophiolitesequence of the Zermatt-Saas zone Inclusions ofcoesite and coesite pseudomorphs occur in tourma-line and garnet of manganiferous quartzite and inomphacite and garnet of the underlying mafic eclog-ite (Reinecke 1991) The metasediments are mainlygarnet-phengite-quartz schist with variableamounts of garnet-clinopyroxene quartzite andpiedmontite-phengite-quartz schist Eclogites andretrograded eclogites with preserved UHP mineralparageneses consist of garnet omphacite glau-cophane zoisite phengite and dolomite Zoned gar-nets preserve prograde quartz crystals in their coresand coesite inclusions in mantles and rims Coesiteinclusions are much more common in omphacitethan in garnet UHP conditions were estimated at26ndash30 GPa and 590ndash630degC Sm-Nd isotopic anal-yses of eclogites yield UHP metamorphism at 52 plusmn18 Ma (Bowtell et al 1994) consistent with a

Tertiary age of eclogitization recently established forAlpine metamorphism in the Western Alps(Gebauer et al 1997)

Thus far the recognized areal extent of thecoesite-bearing rocks is ~ 2 km2 The apparentthickness of the UHP slice does not exceed a fewhundred meters This area is the best example forsubduction of oceanic lithosphere to mantle depthand later exhumation as a fragmented tectonic slabSimilarly the Allalin metagabbro part of the oce-anic Piemonte zone contains magnesite + talc +kyanite + garnet recrystallized at ~600degC and 35GPa with an estimated geothermal gradient of 5ndash6degC kmndash1 however inclusions of coesite or coesitepseudomorphs have not been reported from theAllalin metagabbro

Mali Africa Eclogitic rocks in the Pan-Africancollision zone cropping out in northern Mali containinclusions of coesite in omphacite from mafic nod-ules within a calc-silicate layer enclosed in impuremarble and coesite pseudomorphs in garnets fromthe surrounding eclogitic metasediments (Caby1994) These UHP eclogitic mica schists have P-Testimates of 700ndash750degC and gt 27 GPa phengiteyields a well-defined plateau 39Ar40Ar age of 1045plusmn 9 Ma Recent SmNd ages of 620 Ma wereobtained by Jahn et al (2001) for peak UHP meta-morphism This may be the oldest UHP terrane inthe world

The coesite-bearing UHP eclogitic unit about 3km thick lies within an internal nappe as a flat thinslice bounded on the top by a thick passive-marginshaly formation and on the bottom by low-gradegreenschist-facies phyllites Caby (1994) suggestedthat a large portion of the terrigenous metasedimentsof passive-continental-margin affinity of the WestAfrican plate was subducted eastward to mantledepths (gt90 km) beneath an oceanic domain Colli-sion of the West African plate with an island arcresulted in a low-angle westward subhorizontalextrusion of this subducted slice of sialic materialsto its present position The coesite-bearing eclogiteunit may also crop out 1500 km to the south in Togowhere kyanite-bearing eclogites occur as part of thepassive-margin assemblage

Makbal (480 Ma) and Atbashy (270 Ma) Kazak-hstan The Tian-Shan is divided into three mountainrangesmdashnorthern middle and southern Within theKyrgyzstan Tian-Shan UHP eclogite localities withcoesite pseudomorph inclusions in garnet andomphacite from the northern and southern Tian-Shan represent Caledonian and Hercynian orogenic

12 LIOU ET AL

belts respectively (Tagiri and Bakirov 1990 Tagiriet al 1995) The Makbal Formation in the westernKyrgyz Ridge of the northern Tian-Shan consists ofquartzose schist alternating with pelitic schistsintercalated with thin layers or lenses of marbleeclogite and amphibolitized eclogite Tagiri andBakirov (1990) found inclusions of coesite pseudo-morphs in garnet from a garnet-chloritoid-talc schistwith a peak metamorphic assemblage of coesite +almandine + chloritoid + talc + phengite (Si = 342pfu) + phlogopite + rutile Makbal coesite-gradeeclogite has a paragonite KAr age of 480 Ma

The eclogite-bearing complex of Atbashy Ridgefrom the southern Tian-Shan is composed of peliticand siliceous schists alternating with thin UHPeclogitic layers and is unconformably overlain byUpper Paleozoic molasse and silicic volcanic rocksInclusions of coesite pseudomorphs occur inomphacite cores and in garnet Eclogites from theAtbashy area preserve prograde and retrogradepaths with a peak metamorphic condition at 660degC25 GPa The Atbashy coesite-eclogites yielded aRbSr mineral isochron age of 270 Ma (Tagiri et al1995)

Central Indonesia UHP terrane The pre-Tertiarybasement of the Indonesian region comprises a vari-ety of imbricate terranes meacutelange ophiolite andvariably metamorphosed accretionary complexessome contain HP to UHP metamorphic rocks result-ing from the collision of an Australia-derived conti-nent with Eurasia (Parkinson 2003) HP rocksincluding eclogites and garnet peridotites are widelydistributed in Cretaceous accretionary complexesMany of these rocks occur as imbricate slices of car-bonate quartzite and pelitic schist of shallow-marine or continental-margin parentage interthrustwith subordinate mafic schist and serpentinitesome yield mica KAr ages of 110ndash120 Ma (Parkin-son 2003)

HP and UHP rocks are sporadically exposed astectonic blocks throughout the Cretaceous accre-tionary complexes They include eclogite garnet-glaucophane rock (P = 18ndash24 GPa T = 580ndash620degC) and jadeite-garnet quartzite (P gt 27 GPa T =720ndash760degC) in Bantimala southwest Sulawesieclogite and garnet granulite in west-centralSulawesi eclogite and jadeite-glaucophane-quartzrock (P ~22 GPa T ~530degC) in central Java Mgchloritoidndashbearing whiteschists (P ~18 GPa) insoutheast Kalimantan garnet lherzolites in east-central Sulawesi (P = 22ndash28 GPa T = 1000ndash1100degC) west-central Sulawesi (P = 16ndash20 GPa T =

1050ndash1100degC) and garnet pyroxenite (P ~2 GPa T~ 850degC) in Sabah northeast Borneo Evidence forUHP rocks include (1) inclusions of coesite inzircon and coesite pseudomorphs in jadeite fromjadeite quartzite and eclogite of the BantimalaComplex of south Sulawesi (Parkinson 2003) and(2) P-T estimates of peak-stage recrystallization at27ndash35 GPa and 1000ndash1100degC for most garnetperidotites (Kardarusman and Parkinson 2000)Many of these rocks were probably recrystallized ina N-dipping subduction zone at the margin of theSundaland craton in the Early Cretaceous Exhuma-tion may have been facilitated by the collision of aGondwana continental fragment with the Sundalandmargin at ~120ndash115 Ma

Recently Recognized UHP Terranes

Several new UHP terranes were recently identi-fied inasmuch as they contain partially preservedtrace index minerals in strong containers such aszircon or garnet In fact zircon has been consideredto be the best container and many new terranes werediscovered through positive identification of inclu-sions of coesite or coesite pseudomorphs ordiamond in zircons Detailed examination of mineralinclusions in core mantle and rims of zircon sepa-rates from eclogites and their enclosing country-rock gneisses have yielded both prograde andretrograde P-T-time paths for various UHP terranesmentioned above (see Katayama et al 2001 fordetails also see the section on the Himalayan eclog-ite for an example)

The latest recognized UHP terranes include thefollowing (1) coesite in the French Massif Central(Lardeaux et al 2001) (2) coesite and diamondfrom the Greek Rhodope metamorphic province(Mposkos and Kostopoulos 2001) (3) coesitepseudomorphs in the Northeast Greenland EclogiteProvince (Gilotti and Ravna 2002) (4) coesite ingranulite-facies overprinting eclogites of SoutheastBrazil (Parkinson et al 2001) (5) coesite in Hima-layan eclogite from the Upper Kaghan Valley Paki-stan (OrsquoBrien et al 2001 Kaneko et al 2003) andcoesite from the Tso-Morari crystalline complex ofIndia (Mukherjee et al 2003) (6) coesite inclusionsin gneissic rocks from the North Qaidam belt west-ern China (Yang et al 2001 Song et al 2003) (7)diamond inclusions in garnets of eclogite and gneissfrom North Qinling (Yang et al 2002a) (8) inclu-sions of quartz pseudomorphs and minor coesite ingarnets of mafic eclogites from the LantermanRange in Antarctica (Ghiribelli et al 2002) (9)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 13

inclusions of calcite pseudomorphs after aragonite +magnesite in dolomite in metapelites from the west-ern Tian-Shan inferred as metamorphism in a UHPregion of dolomite decomposition (Zhang L et al2002) and (10) inclusion of possible coesitepseudomorphs in garnet lamellar inclusions in gar-net kyanite and high sodium content of garnet fromeclogites of the Chuacus Complex of north-centralGuatemala (Solari et al 2003) The results of petro-chemical and geochronological data for these newterranes are summarized below The HimalayanUHP terrane in the Upper Kaghan Valley and theTso-Morari complex are used to illustrate the pro-cesses of continental subduction and collision in alater section

French Massif Central The French Massif Cen-tral in the western part of the Variscan Belt hasexperienced Late SilurianndashEarly Devonian to LateCarboniferous orogenic events (eg Matte 1986)High-pressure metamorphic rocks occur mainly inthe upper gneissic unit within the leptyno-amphibo-lite group The latter consists of an association ofmetagreywacke mica schist metabasalt leptynitemetagranite and peridotite Eclogites are associatedwith silicic and mafic HP granulites (Lardeaux etal 2001) and also with spinel andor garnet lherzo-lites (Gardien et al 1990) Inclusions of coesite andcoesite pseudomorphs occur in garnets of kyanite-bearing eclogite (Lardeaux et al 2001) Only twocoesite grains are preserved as relics most coesitegrains are completely transformed into polycrystal-line radial quartz (palisade texture) or into polygonalquartz surrounded by radial cracks Metamorphictemperatures were estimated to be 740-780degC andsubsequent amphibolite-facies overprinting at750degC at 15ndash17 GPa during decompression UHPmetamorphism occurred between 420 and 400 Ma(Paquette et al 1995) 40Ar39Ar data from amphib-ole separates from retrogressed eclogites yielded339 plusmn 4 Ma (Costa et al 1993)

Northeast Greenland Caledonides The first evi-dence for UHP metamorphism in the GreenlandCaledonites was reported from kyanite eclogites andassociated host gneisses on an island in Jokelbugt(Gilotti and Ravna 2002) Inclusions of quartzpseudomorphs exhibiting palisade structure andradiating fractures occur in garnet and omphacite ofeclogites and in garnet of the host gneisses P-T esti-mates of peak-stage metamorphism of eclogite at~972degC and 36 GPa lie well within the coesite sta-bility field SHRIMP UPb dates of 403 plusmn 5 Ma forzircon rims from HP metapsammite and 404 plusmn 4 Ma

for zircons from anatectic melt derived frommetapelite are coeval with the estimated ages ofexhumation of UHP terranes in the ScandinavianCaledonides described in a previous section(McClelland and Gilotti 2003)

Neoproterozoic nappes in Southeast BrazilNeoproterozoic nappes of Southeast Brazil consistmainly of coarse-grained kyanite-garnet granulitewith intercalations of impure quartzite calc-silicaterock and minor lenses and sills of mafic-ultramaficrocks including eclogite Early eclogitic phaseswere strongly obliterated during granulite-faciesoverprinting during a major Pan-African continentcollision at 640ndash630 Ma (Campos Neto and Caby1999) Rare ultramafic lenses consist of phlogopite-orthopyroxene rocks and garnet clinopyroxeniteCores of granulite boudins retain eclogitic assem-blages some quartz inclusions within the mantleregions of zoned garnet porphyroblasts are sur-rounded by intense radial fractures suggestive ofthe former presence of coesite In fact coarse-grained zircons in some granulites contain numer-ous micro-inclusions of K-feldspar + kyanite + coes-ite as confirmed by Raman spectroscopy (Parkinsonet al 2001) Many exsolution microstructuresobserved in other UHP terranes also occur Thisarea represents the first coesite-grade UHP rocks inthe Americas and it can be correlated with the MaliUHP terrane of equatorial Africa (Caby 1994) Ithas been suggested that eclogitic assemblages andprobable UHPM components may be present in sev-eral other correlative nappe systems at the marginsof the Sao Francisco craton in Brazil

Chuacus Complex north-central Guatemala HPeclogites within banded gneisses schists andmigmatites of the Chuacus Complex contain relicttextures and mineralogical features that suggestUHP metamorphism (Solari et al 2003) Theseinclude inclusions of possible coesite pseudomor-phs in garnet and kyanite lamellar inclusions ofrutileilmenite in garnet high sodium content ofsome garnets (up to 1200 ppm) and preliminary P-T estimates of 700ndash800degC and 2ndash3 GPa AlthoughK-Ar dating of mica and hornblende yield ages of48ndash72 Ma possible for amphibolite facies retrogres-sion regional and structural data support a pre-Mesozoic age for the multi-stage HP-UHP recrystal-lization Such a discovery opens up a new interpre-tation regarding the tectonic evolution of Maya-Chortis-Oaxaquia microcontinental blocks formingthe structural backbone of Mesoamerica

14 LIOU ET AL

Greek Rhodope The Greek Rhodope Metamor-phic Province (RMP) at the border between Greeceand Bulgaria represents a syn-metamorphic nappesystem of Alpine age that was formed during theCretaceous to Mid-Tertiary collision of Apulia andpaleo-Europe The Kimi nappe complex comprisescrustal eclogites leucocratic orthogneisses peliticgneisses and volumetrically minor mantle-derivedultramafic rocks these units contain many mineral-ogical indicators of UHP metamorphism (Mposkosand Kostopoulos 2001) The ultramafic lithologiesinclude garnet spinelndashbearing lherzolite and layersof spinel-garnet clinopyroxenite garnet pyroxeniteand clinopyroxene garnetite These rocks were sub-jected to UHP recrystallization at P-T conditions of~1200degC and gt30 GPa The pelitic gneiss containsmicrodiamond inclusions in garnet porphyroblastsas well as rods or needles of silica rutile biotiteand apatite in sodic garnet suggesting the prioroccurrence of majoritic garnet The growth of suchsupersilicic garnet was suggested to be at a pressureof ~7 GPa Inclusions of polycrystalline quartzaggregates surrounded by radial cracks occur ineclogitic garnet Exsolution lamellae of quartz ineclogitic clinopyroxene also indicate an UHP pre-cursor of supersilicic clinopyroxene

Lanterman Range Antarctica The LantermanRange in northern Victoria Land of Antarctica con-sists of several metamorphic complexes The Gate-way Hills Metamorphic Complex is a thindiscontinuous belt more than 50 km long consistingof mafic and ultramafic rocks including lenses andpods of eclogite within felsic gneiss Some less foli-ated eclogites contain inclusions of coesite andcoesite pseudomorphs in garnet porphyroblasts asconfirmed by in situ Raman spectroscopy as well asradial fractures around the inclusions (Ghiribelli etal 2002) The eclogites were subjected to UHPmetamorphism at T gt 850degC and P gt 29 GPa thenamphibolite-facies overprinting during isothermaldecompression The UHP event is dated by SmNdand 238U206Pb data at 500 Ma 40Ar39Ar ages of490ndash486 Ma for Ca-amphibole (amphibolite-faciesoverprinting) suggest rapid cooling with an averageexhumation rate of 3ndash4 km_Ma-1 for the UHP ter-rane (Di Vincenzo and Palmeri 2001)

Western Tian-Shan China Caledonian eclogitesof the western Tian-Shan China have been classi-fied into three types (Zhang L et al 2002) Type Ieclogite pods are layered with mafic blueschists andType II eclogites preserved pillow structureswhereas Type III eclogites are banded with marbles

This 200 km long eclogite-bearing belt extendswestward to connect with the Atbashy UHP belt inKazakhstan (Tagiri et al 1995) Reported UHP evi-dence includes inclusions of coesite pseudomorphsin garnets from Type I and III eclogites quartzexsolution lamellae in omphacite from Type IIeclogites and relict magnesite within matrix dolo-mite in Type III eclogites (Zhang J X et al 2002Zhang L et al 2002) P-T estimates of the peakUHP stage of metamorphism are T = 500ndash600degCand P = 26ndash27 GPa (Wei et al 2003)

North Qaidam Mountains western China Manylenses and layers of garnet peridotite and eclogiteoccur in an amphibolite-facies terrane in the NorthQaidam Mountains along the northeastern rim of theQaidam Basin This belt comprises mainly felsicgneiss quartz schist garnet-bearing gneissamphibolite and minor eclogite + garnet peridotiteit extends westward for more than 1000 km to theAltun Mountains and was displaced by the left-lat-eral Altyn-Tagh fault At the eastern end of thisUHP belt at Dulan occurrences of coesite inclu-sions in zircon separates from paragneiss inclusionsof coesite pseudomorph in garnet and omphacite ofeclogites quartz rods in eclogitic omphacite andP-T estimates of eclogites (P = 287ndash317 GPa andT = 631ndash687degC) demonstrate that this is a typicalUHP metamorphic terrane (Yang et al 2001 Songet al 2003a 2003b) The Dulan eclogites have anSm-Nd mineral isochron age of 497 plusmn 87 Ma and aSHRIMP UPb zircon age of 495 Ma (Yang et al2002b)

Altun Mountains western China This UHP ter-rane about 200 km long in the southern margin ofthe Altun Mountains is the western extension of theNorth Qaidam belt Eclogitic lenses and blocks ofvarious sizes together with garnet-lherzolite and cli-nopyroxenite are enclosed within an Early Protero-zoic sequence of amphibolite amphibole schistgarnet-bearing granitic gneiss and pelitic schistGarnet lherzolite contains magnesite that reacted toform dolomite P-T estimates of the peak magnesite-bearing assemblage yield P gt 36 GPa and 850degC(Liu et al 2001) Garnet porphyroblasts includeexsolution lamellae of clinopyroxene and rutile sim-ilar to that of eclogite reported by Ye et al (2000) asmajoritic garnet The associated eclogites containinclusions of coesite pseudomorphs in garnet withcharacteristic radial fractures this together with theexsolved quartz rods in omphacite and P-T esti-mates of 820ndash850degC and 28 to 32 GPa provideadditional evidence for UHP (Zhang J X et al

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 15

2001) An SmNd mineral isochron for eclogiticgarnet omphacite and the whole rock yields 500 plusmn10 Ma whereas an UPb isochron for zircon sepa-rates from eclogite gives an age of 504 plusmn 5 Ma(Zhang J X et al 2000)

North Qinling Mountains central China Lensesand blocks of eclogitic rocks in northern Qinling areenclosed in amphibolite-facies gneiss and garnet-bearing quartz + phengite mica schist Inclusions ofcoesite in eclogitic garnet (Hu et al 1995) andmicrodiamonds in zircon separates from both eclog-ite and gneiss (Yang et al 2002a) have beenrecently reported Although individual outcrops areno more than a few meters wide the belt of eclogitesextends more than 10 km P-T estimates of amphib-olitized coesite-bearing eclogite yield T = 590ndash758degC and P gt 26 GPa An Sm-Nd isochron for gar-net omphacite rutile amphibole and whole-rockyields 400 plusmn 16 Ma Microdiamonds were recentlydiscovered as inclusions in zircon from both eclogiteand its gneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyields 507 plusmn 38 Ma for peak UHP metamorphism(Yang et al 2002a) These data indicate that two

discrete collision events occurred between theNorth China and Yangtze cratons (1) an Early Pale-ozoic North Qinling UHP suturing and (2) a Triassicsuturing along the Dabie-Sulu UHP-HP belt TheEarly Paleozoic belt extends westward to the NorthQaidamndashAltun Mountains for more than 4000 km(Yang et al 2002b)

Himalayan UHP Eclogites

The Himalayan orogen has long been considereda classic locality of continental collision betweenthe Indian and Eurasian continents (Fig 5) Severaleclogite-facies HP rocks in the Himalayas havebeen reported since the late 1980s (eg Chaudaryand Ghazanfar 1987 Pognante and Spencer 1991de Sigoyer et al 1997 Lombardo and Rolfo 2000)Inclusions of coesite were recently discovered inomphacite and garnet in several Himalayan eclog-ites from the upper Kaghan Valley Pakistan(OrsquoBrien et al 2001 Kaneko et al 2003) Subse-quent findings of additional coesite localities in theTso Morari area (Sachan et al 2001 Mukherjee etal 2003) have established the presence of a UHPbelt in the Himalayas an active ongoing collision

FIG 5 A simplified tectonic map of the western Himalaya Hindu Kush Pamir Karakoran and western Tibet mod-ified after Figure 3 of Searle et al (2001) This map shows the locations of eclogite and UHP eclogites from both Hima-layan and North Qaidam Indus Suture Zone (ISZ) ophiolites and major structures

16 LIOU ET AL

since the Eocene (Massonne and OrsquoBrien 2003)However UHP relics are volumetrically very minorconstituents of the metamorphic belts dominatedmostly by later Barrovian overprintings In theserocks hydration related to Barrovian-zone metamor-phism has resulted in remarkably pervasive recrys-tallization during the later exhumation this eventhas almost entirely masked the preceding UHPrecord

The finding of HP-UHP evidence has led to newinsights into the Himalayan tectonic modelEngland and Houseman (1986) showed that in amodel of homogeneous thickening of the Hima-layan-Tibet boundary the maximum crustal thick-ness should be less than 70 km However theoccurrences of coesite-bearing eclogites andgneisses in the Himalayan orogen cannot beexplained by a homogeneous thickening of the crustbut rather by continental subduction as describedbelow According to available geochronologicaldata (eg Treloar et al 2003) the exhumation rateof the Himalayan UHP rocks has been estimated torange from ~1 cm yrndash1 (OrsquoBrien et al 2001) to gt3cm yrndash1 (Treloar et al 2003 Massone and OrsquoBrien2003) These estimates are higher than those calcu-lated from other UHP-HP terranes This requirestectonic emplacement of the UHP-HP Himalayanslab into shallow crust following isostatic rebounddue to slab decoupling between the Indian continentand the oceanic crust initiated during the MiddleEocene UHP rocks from both the Kaghan Valleyand Tso-Mori are described below

Kaghan Valley The Kaghan nappe in the north-western Himalayan syntaxis lies between the Indussuture (also called main mantle thrust MMT) to thenorth and the main central thrust (MCT) to thesouthwest (Pognante and Spear 1991 Spencer1993) (Fig 5) The Cretaceous Kohistan island-arcsequence lies north of the Indus suture It developedas a result of intra-oceanic subduction The maincentral thrust separates an underlying Salkhala unitfrom an overlying higher Himalayan crystalline(HHC) unit The Kaghan nappe consists mainly ofgranitic gneiss and paragneiss with minor inter-calated layers of amphibolite marble and quartziteEclogites are preserved in the cores of maficboudins (less than 1 m thick) in gneiss and aresignificantly overprinted by amphibolite assem-blages They contain fine-grained (lt 1 mm) garnetand omphacite together with aggregates of quartzchains of rutile rimmed by titanite and mm-sizedphengitic micas overgrown by randomly oriented

amphiboles up to 1 cm in length Coesite and coesitepseudomorphs occur as inclusions in omphaciteand show the characteristic palisade-quartz aggre-gate texture fractures radiating from these inclu-sions are characteristically developed in the hostpyroxene

Garnet pyroxenendashphengite barometry wasapplied to the eclogites and yielded peak P-T con-ditions of 27ndash29 GPa and 690ndash750degC (OrsquoBrien etal 2001) (Fig 6) Retrograde textures are commonin the eclogites including symplectic intergrowthsof augite amphibole and plagioclase after omphac-ite thermobarometric analyses of these assemblagesyielded 10ndash13 GPa and 600ndash710degC (Kaneko et al2003) On the other hand the dominant graniticgneisses and metapelites of the higher Himalayancrystalline unit contain typical Barrovian-zoneamphibolite-facies assemblages represented bykyanite- and staurolite-bearing garnetiferousmetapelites (Treloar 1995) Extensive overprintinghas mostly obliterated the peak assemblages matrixminerals of the gneisses yielded P-T conditions of07ndash11 GPa and 600ndash700degC (Treloar 1995) con-sistent with retrogression of the eclogites Howeverrelict coesite is preserved as inclusions in zirconthe inclusions are ovoid and approximately 10 microm indiameter (Fig 7A) This discovery of coesite-bear-ing zircons from Himalayan quartzofeldspathicgneisses demonstrates subduction of Indian conti-nental crust to the depths of UHP conditions(Kaneko et al 2003)

Geochronological studies using SmNd (garnet-omphacite) RbSr (phengite) and UPb (rutile zir-con) methods suggest that the eclogite-facies eventtook place at 40ndash50 Ma (Tonarini et al 1993 Spen-cer and Gebauer 1996) 40Ar39Ar cooling ages forhornblende and mica from gneisses were reported tobe ~43 Ma and ~25 Ma respectively (Chamberlainet al 1991) Recent SHRIMP UPb dates of zonedzircons with mineral inclusions formed at differentstages indicate the ages of non-UHP mineral-bear-ing mantle domains of zircon and UHP mineral-bearing rims are at about 50 and 46 Ma respectively(Kaneko et al 2003) A new U-Pb age of 44 plusmn 11Ma was obtained for rutile from Kaghan coesite-bearing eclogite (Treloar et al 2003) this togetherwith available data implies that exhumation of theUHP rocks from mantle depth to mid-crustal leveloccurs within a few million years

Tso Morari The Tso Morari crystalline dome islocated between the Indus suture zone on the northbordered by the Zildat detachment fault and the

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 17

Zanskar sedimentary unit on the south (de Sigoyer etal 1997) The dome represents an internal massif ofthe northern part of the higher Himalaya The TsoMorari area has been considered a typical IndianTethyan margin because of a stratigraphic succes-sion similar to the Nimaling antiform in the core ofthe dome a metamorphic basement consists of Cam-bro-Ordovician augen gneisses (Stutz 1988) Thiscomplex is covered by a metasedimentary series ofCambrian to Devonian quartzite schist and con-glomerate and is overlain by Lower Carboniferousto Triassic marble and metapelite Eclogites occuras lenses or irregular bodies within the basementand cover gneisses and are strongly overprinted byamphibolite-facies assemblages de Sigoyer et al(1997) reported peak P-T conditions for the eclog-ites at 17ndash23 GPa and 520ndash640degC (Fig 7) How-ever relics of coesite 30ndash80 microm in size were

recently found as inclusions in garnet (Sachan et al2001 Mukherjee et al 2003) typified by radialfractures (Figs 7B and 7C) for the SiO2 inclusionsa characteristic Raman spectrum is centered at 523cm-1 (Fig 7D) The country rock metasedimentswith jadeite + chloritoid + paragonite + garnetassemblages have peak P-T estimates of 18ndash22GPa and 500ndash600degC and record retrogression at theeclogite-blueschist facies transition of 13ndash18 GPaand 490ndash590degC (Guillot et al 1997)

The geochronology of the Tso Morari eclogiteshas been extensively studied and subdivided intothree different stages (de Signoyer et al 2000)Eclogitization at ~55 Ma was obtained by Lu-Hf andSm-Nd methods on garnet omphacite glaucophaneand the whole rock An amphibolite-facies overprintoccurred at 47 Ma judging by RbSr dating onphengite apatite and whole rock from metapelites

FIG 6 P-T estimates of peak- and retrograde-stage metamorphism for coesite-bearing eclogites and the country-rockgneisses from the Kaghan Valley and Tso Morari

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

6 LIOU ET ALTA

BL

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Zer

mat

t-Sa

as a

rea

Sw

itze

rlan

dE

clog

ite

Mn-

quar

tzit

e52

plusmn 1

8 M

a (p

eak)

26ndash3

0 kb

ar 5

90ndash6

30degC

1 P

roto

lith

s ar

e oc

eani

c af

fini

ty2

Coe

site

incl

usio

n in

the

man

tle

of g

arne

t whe

re g

arne

t cor

e ha

s qu

artz

Saxo

nian

Erz

gebi

rge

Ger

man

yE

clog

ite

Dia

mon

d-be

arin

g gn

eiss

360

plusmn 7

Ma

(pea

k)34

8ndash35

5 M

a (r

etro

grad

e)gt

42 k

bar

900ndash

1000

degC1

Inc

lusi

ons

of m

icro

diam

ond

in z

irco

n k

yani

te a

nd g

arne

t fro

m g

neis

s2

Rut

ile

with

-P

bO2

stru

ctur

e

Mal

i A

fric

aE

clog

ite

620

Ma

(pea

k)gt2

7 kb

ar 7

00ndash7

50degC

1 O

ldes

t age

of U

HP

met

amor

phis

m

Mak

bal

wes

tern

K

yrgy

zsta

nE

clog

ite

Gar

net-

bear

ing

peli

tic s

chis

t

482

Ma

gt26

kba

r 61

0ndash68

0degC

1 I

nclu

sion

s of

coe

site

pse

udom

orph

in g

arne

t fro

m e

clog

ite a

nd s

chis

t

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 7A

tbas

hy K

azak

hsta

nE

clog

ite27

0 M

agt

25 k

bar

660deg

C1

Inc

lusi

ons

of c

oesi

te in

gar

net

Cen

tral

Ind

ones

ia S

W

Sula

wes

iE

clog

ite

Gar

net p

erid

otite

115ndash

120

Ma

gt27

kbar

720

ndash760

degC33

ndash35

kbar

115

0degC

1 I

nclu

sion

s of

coe

site

in z

irco

n an

d co

esit

e ps

eudo

mor

ph in

jade

ite

Rec

entl

y re

cogn

ized

terr

anes

Fre

nch

Alp

sE

clog

ite

Met

apel

ites

400ndash

420

Ma

(pea

k)36

0ndash38

0 M

a (r

etro

grad

e)gt

28 k

bar

750deg

C1

Inc

lusi

ons

of c

oesi

te in

gar

net a

nd o

mph

acit

e

Nor

thea

st G

reen

land

Ecl

ogite

400ndash

440

Ma

24ndash2

8 kb

ar 8

20degC

1 I

nclu

sion

s of

coe

site

pse

udom

orph

in g

arne

t

Sout

heas

t Bra

zil

Ecl

ogit

e63

0 M

agt

27 k

bar

800deg

C1

Inc

lusi

ons

of c

oesi

te in

zir

con

Chu

acus

Com

plex

no

rthe

rn G

uate

mal

aE

clog

ite

Ban

ded

gnei

ss48

ndash72

Ma

(ret

rogr

ade)

20ndash3

0 kb

ar 7

00ndash8

00degC

1 C

oesi

te p

seud

omor

ph in

gar

net a

nd k

yani

te

Rho

dope

Mas

sif

Gre

ece

Ecl

ogit

eO

rtho

gnei

ssPe

litic

gne

iss

Ultr

amaf

ic r

ocks

30ndash4

2 M

a (p

eak)

8ndash12

Ma

(ret

rogr

ade)

gt 30

kba

r ~

1200

degC1

Mic

rodi

amon

d in

clus

ion

in g

arne

t por

phyr

obla

st2

Sili

ca r

util

e an

d ap

atit

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ds in

gar

net

Lan

term

an R

ange

A

ntar

ctic

aE

clog

ite

Fels

ic g

neis

s50

0 M

a (p

eak)

486ndash

490

Ma

(ret

rogr

ade)

gt 29

kba

r gt

850

degC1

Inc

lusi

ons

of c

oesi

te a

nd it

s ps

eudo

mor

ph in

gar

net

Wes

tern

Tia

n-Sh

an

nort

hwes

t Chi

naE

clog

ite

Mar

ble

lt 3

10 plusmn

5 M

a (p

eak)

26ndash2

7 kb

ar 5

00ndash6

00degC

1 I

nclu

sion

s of

coe

site

and

its

pseu

dom

orph

in g

arne

t2

Pos

sibl

e co

exis

tenc

e of

mag

nesi

te +

ara

goni

te

Qai

dam

wes

tern

Chi

naE

clog

ite

Gne

iss

Gar

net p

erid

otite

495

plusmn 7

Ma

(pea

k)46

7 plusmn

1 M

a (r

etro

grad

e)gt

27 k

bar

700deg

C1

Inc

lusi

ons

of c

oesi

te in

zir

con

from

par

agne

iss

Alt

un M

ount

ains

w

este

rn C

hina

Ecl

ogit

eG

rt-l

herz

olit

e50

4 plusmn

5 M

a (p

eak)

28ndash3

2 kb

ar 8

20ndash8

50dego C

1 C

oesi

te p

seud

omor

ph in

gar

net

Nor

th Q

inli

ng M

tns

C

entr

al C

hina

Ecl

ogit

eG

neis

s50

7 plusmn

38

Ma

400

plusmn 1

6 M

agt

26 k

bar

590ndash

760deg

C1

Coe

site

in g

arne

t2

Mic

rodi

amon

d in

zir

cons

from

ecl

ogit

e an

d gn

eiss

Kag

han

vall

ey P

akis

tan

Him

alay

asE

clog

ite

Gne

iss

46 plusmn

1 M

a (p

eak)

44 plusmn

1 M

a (r

etro

grad

e)27

ndash29

kbar

690

ndash750

degC1

Inc

lusi

ons

of c

oesi

te a

nd it

s ps

eudo

mor

ph in

om

phac

ite

and

zirc

on

from

ecl

ogit

e an

d in

zir

con

from

gne

iss

Tso

Mor

ari

Indi

an H

imal

ayas

Ecl

ogit

eG

neis

s55

Ma

(pea

k)47

Ma

and

30 M

a(r

etro

grad

e)

gt 28

kba

r 70

0ndash80

0degC

1 I

nclu

sion

s of

coe

site

and

its

pseu

dom

orph

in g

arne

t fro

m e

clog

ite

8 LIOU ET AL

SHRIMP dating of zircons extracted from gneissschist pyrope inclusion and pyrope quartzite indi-cates the occurrence of 240ndash275 Ma old zirconcores and newly formed 35 Ma rims for some zircons(Gebauer et al 1997) Thus UHP metamorphismmust have taken place during Oligocene time withan estimated exhumation rate of 2 to 24 cm yrndash1

Western Gneiss Region Norway The WesternGneiss Region lies within an Early Paleozoic colli-sion zone the gneissic unit about 300 km long and150 km wide consists of interlayered pelite andmigmatite marble quartzite and amphibolite withtectonic inclusions of gabbro and peridotite (for arecent review see Carswell and Cuthbert 2003) Thegneissic unit exhibits mainly amphibolite-faciesassemblages but relics of HP assemblages alsooccur Eclogite boudins are widespread Coesite wasfirst reported from Grytting (Smith 1984) severalnew localities of coesite and coesite pseudomorphshave been recognized recently in both eclogites andthe adjacent gneissic rocks (Wain 1997 Cuthbert etal 2000 Wain et al 2000 Carswell et al 2003)Microdiamond grains 20ndash50 microns across havebeen described from residues separated from twogneisses (Dobrzhinetskaya et al 1995) the associ-ated kyanite-eclogites contain inclusions of coesitepseudomorphs in garnet Relict majoritic garnetsshowing exsolution of pyroxene lamellae in peridot-ite bodies have been recently discovered on theislands of Otrogy and Flemsoy (van Roermund andDrury 1998 van Roermund et al 2000) Theseperidotite bodies originally must have had an evenhigher-pressure deeper-mantle origin probablywithin a rising mantle diapir at ~1700 Ma In thesegarnet peridotites diamond inclusions in symple-tetic spinel after garnet have been discovered (vanRoermund et al 2002) Radiometric ages for eclog-ites of the Western Gneiss Region are 408ndash425 Mafor the peak metamorphism and 374ndash378 Ma forthe retrogressive mid-crustal metamorphic over-printing (for review see Carswell et al 1999) Car-swell et al (2003) provide new petrographicevidence and a review of the latest radiometric agedata and conclude that the UHP metamorphism ofeclogites occurred at 400ndash410 Ma significantlyyounger than the previous widely accepted age of425 Ma A two-stage exhumation process is sug-gested an initial exhumation to about 35 km depthby about 395 Ma at a mean rate of about 10 m Mandash1and subsequent exhumation to 8ndash10 km by about375 Ma at a much slower rate of about 13 m Mandash1

The suture zone occupied by UHPHP rocks ofthe Western Gneiss Region reflects collision of theeclogite-bearing Greenland sialic crust-cappedlithospheric plate with Fennoscandia during theCaledonian orogeny (Gilotti 1993 Krogh andCarswell 1995 Brueckner and Medaris 1998)Inclusions of coesite pseudomorph in garnet andomphacite from kyanite eclogites have beenrecently documented in the East Greenland eclogiteprovince (Gilotti and Ravna 2002) this new UHPHP terrane is remote but has excellent exposuresand requires close examination

Dabie-Sulu terrane of east-central China TheDabie-Sulu terrane occupies a Triassic collisionzone between the Sino-Korean and Yangtze plates(for a recent review see Hirajima and Nakamura2003) The central UHP coesite-bearing eclogitebelt (P = 27ndash5 GPa) is flanked to the north by amigmatite zone (P lt 2 GPa) and to the south by abelt of blueschist epidote amphibolite and eclog-ite-facies rocks (P = 05ndash12 GPa) Prevalent rocktypes include felsic gneiss orthogneiss marble andquartzite Protolith ages range from Proterozoic toOrdovician but the majority of reported ages areNeoproterozoic (Li et al 1993 Hacker et al 2000)Blocks boudins and layers of eclogites and garnetperidotites occur as enclaves in gneisses in the UHPunit The Dabie-Sulu UHP rocks show several char-acteristics (1) Widespread coesite with hydrousminerals such as talc zoisiteepidote and phengiteoccur in eclogites (2) Two distinct types of mantle-and crustal-derived garnet peridotites are present(3) Abundant exsolution textures were identified inUHP minerals from garnet peridotite and eclogite(4) Diamond separates from both eclogite and garnetperidotite have been reported from few restrictedregions (eg Xu et al 1992 2003) Some of thesefindings are not confirmed by thin-section observa-tion and extensive studies of mineral inclusions inzircon (for review see Liou et al 2002) (5) Somegarnet peridotites record much higher pressure thanassociated coesite-bearing eclogites reaching 5ndash6GPa (Yang et al 1993 Zhang R Y et al 20002003) (6) Various isotopic age dating methods forUHP minerals and rocks from Dabie-Sulu terranegives 220ndash240 Ma (Li et al 1993 Hacker et al2000)

Eclogites in the Huwan shear zone from thenorthwestern Dabie Mountains have SHRIMP UPbdates of 309 plusmn 3 Ma (Sun et al 2002) suggesting adiscrete Carboniferous subductioncollisionbetween North and South China Farther to the west

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 9

in North Qinling inclusions of microdiamond werediscovered in zircons from both eclogite and itsgneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyielded 507 plusmn 38 Ma for metamorphic rims and1200ndash1800 Ma for relic magmatic or old cores sug-gesting an Early Paleozoic UHP metamorphic eventThese new data suggest that episodic subductionand collision of continents occurred during stages ofclosures of paleo Tethys prior to the well-docu-mented major Triassic continent-continent collisionfor the Dabie-Sulu terrane

Kokchetav massif The Kokchetav massif north-ern Kazakhstan is the type site for the UHPdiamond-eclogite regional metamorphic facies(Sobolev and Shatsky 1990 Parkinson et al 2002Shatsky and Sobolev 2003) Diamonds occur asminute inclusions in garnet diopside and zirconporphyroblasts in marble pyroxene-carbonate-gar-net rock and garnet-biotite gneiss and schist Inclu-sions of coesite and coesite pseudomorphs occur ingarnet and zircon from eclogite and diamond-bear-ing gneiss The UHPHP unit extends NW-SE for atleast 80 km and is about 17 km wide The UHP slabis structurally overlain by a weak- to low-grademetamorphic unit and is underlain by a low-P meta-morphic andalusite-staurolite facies unit (Kaneko etal 2000) The UHP slab is composed of felsicgneiss with locally abundant eclogite lenses andminor orthogneiss metacarbonate and rare garnetperidotite with a range of Proterozoic protolith agesA well-constrained P-T-time path deduced frommineral inclusions and SHRIMP geochronology ofzircons is illustrated in Figure 4 Zoned zircons fromdiamond-bearing gneiss yield the following ages aquartz-bearing inherited core with 1100ndash1400 Maas the protolith age coesite-bearing mantle with 537plusmn 9 Ma for UHP metamorphism and the plagio-clase-bearing rim with 507 plusmn 8 Ma for amphibolite-facies overprint (Katayama et al 2001) Manydiamond-grade marbles contain clinopyroxene withexsolution lamellae of quartz K-feldspar phengiteand phlogopite (Katayama et al 2002 Zhu andOgasawara 2002) and titanite with coesite lamellae(Ogasawara et al 2002) these occurrences suggestthat UHP metamorphism occurred at P gt 6 GPa and1000degC

Cndash13 isotopic data of diamond suggest biogenicsources inclusions of water carbonate and nano-metric oxide in diamonds indicate that metamorphicmicrodiamonds in this and other UHP terranes wereprecipitated from C-O-H supercritical fluids (eg

Stockhert et al 2001) Ogasawara et al (2002)further suggested a two-stage growth mechanism formicrodiamonds Masago et al (2003) reported nega-tive 18O values (-39 per mil) for minerals in eclogiteand whiteschist of the Kokchetav Massif similar tothe Chinese UHP rocks first described by Yui et al(1995) The Kokchetav Massif is the second recog-nized UHP region that preserves a significant effectof the interaction of cold meteoric water with theprotolith prior to subduction

Saxonian Erzgebirge Germany The ErzgebirgeCrystalline Complex (ECC) at the northern margin ofthe Bohemian massif is in fault contact with a low-grade Paleozoic sequence at its margins (for a recentreview see Massonne and OrsquoBrien 2003) TheVariscan ECC consists of abundant gneisses thatinclude numerous lenses of eclogites The ldquoGneiss-Eclogite Unitrdquo at the core of this massif containsUHP rocks typified by inclusions of coesite pseudo-morph in garnets from eclogite (Schmadicke 1991)and inclusions of microdiamonds in garnet kyaniteand zircon from gneisses (Massonne 1999) Thusfar the diamondiferous gneisses seem to occur onlyin a 1 km long strip near the eastern shore of theSaidenbach Reservoir Diamond micro-inclusionsoccur exclusively in garnet kyanite and zircon ofseveral gneissic rocks Besides completely pre-served diamonds with grain sizes between 1 to 25microm partially graphitized diamonds and graphitepseudomorphs after diamond occur Such microdia-mond inclusions in garnet and zircon are similar tothose in Kokchetav diamondiferous biotite gneissSome of these inclusions contain additional fluid-bearing phases including apatite phengitic micaand possible fluid inclusions the morphologies ofinclusions and mineral associations suggest thatmicrodiamonds from both the Erzgebirge and theKokchetav were crystallized from supercritical flu-ids under UHP conditions (Stockhert et al 2001Hwang et al 2001 Dobrzhnetskaya et al 20012003)

Schmadicke et al (1995) reported Sm-Nd isoch-rons for garnet-Cpx-WR at 360 plusmn 7 Ma for eclogiteand 353 plusmn 6 Ma for garnet pyroxenite from Erzge-birge UHP rocks 40Ar39Ar spectra of phengite fromtwo eclogite samples give plateau ages of 348 plusmn 2and 355 plusmn 2 Ma Diamond-bearing zircons yieldSHRIMP UPb dates at 336 plusmn 2 Ma (Massonne2001) Such similarity in ages for garnet peridotitesand eclogite enclosed in gneiss suggests a coevalVariscan UHP in-situ metamorphism of the ECCaround 340ndash360 Ma

10 LIOU ET AL

Less Intensively Studied UHP terranes

Several UHP terranes described below exceptfor the Maksyvtov Complex have received lessintensive study mainly due to remoteness and diffi-cult access despite their long recognition

Maksyutov Complex Southern Urals TheMaksyutov Complex is an elongated (10 times 120 km)N-S belt between the Russian and Siberian cratonsin the southern Ural Mountains (Coleman and Wang1995 Dobretsov et al 1996) and contains blocky

graphitized diamond() in pelitic schist (Leech andErnst 1998) The complex is the locality from whichquartz aggregates within garnet exhibiting radialcracks were first recognized as coesite-pseudo-morphs by Chesnokov and Popov (1965) nearly 20years prior to the independent finding of coesiteinclusions by Chopin (1984) and Smith (1984) Theeclogite-bearing unit contains boudins of eclogitelayers of eclogitic gneiss and rare ultramafic bodieswithin host metasedimentary mica schist andquartzite

FIG 4 A Schematic diagram showing the growth of detrital zircon (stage 1) through subduction (stage 2 and peakstage 3) and exhumation (stage 4) of supracrustal rocks Mineral inclusions in various stages of zircon growth are shownB SHRIMP U-Pb dating of zoned zircons from a Kokchetav diamond-bearing gneiss yields a well constrained P-T-timepath (for details see Katayama et al 2001)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 11

Some Maksyutov eclogites were recrystallized at594ndash637degC with a minimum pressure from 15 to17 GPa (Beane et al 1995 Leech and Ernst 1998)or up to 27 GPa if coesite pseudomorphs (Ches-nokov and Popov 1965 Dobretsov and Doretsova1988) are indeed present However subsequentstudies have not revealed the occurrence of inclu-sions of coesite or coesite pseudomorph in garnetand zircon (Beane et al 1995 Shatsky et al 1995)Leech and Ernst (1998) described unusual graphiteaggregates (up to 13 mm edge length) that may bediamond pseudomorphs This suggestion was basedon the blocky morphology of the cuboid graphiteaggregates cross-cutting rock foliation pressureshadows biogenic isotopic signatures and otherspectroscopic characters similar to those graphi-tized diamonds from the Beni Bousera massifRecent study of several microdiamond inclusions ingarnet has yielded further suggestive possibility thatthe eclogitic unit of the Maksyutov Complex experi-enced diamond-grade UHP metamorphism (Bosticket al 2003) The UHPHP metamorphism with peakmetamorphic ages of 370ndash374 Ma (Shatsky et al1995 Beane and Connelly 1998) may be related tothe collision of the Russian platform with a fragmentof the Siberian craton during Devonian time butwas affected by the Triassic collision of the Kazakh-stan block with the Russian platform (eg Matte1998)

Zermatt-Saas area Western Alps In contrast tomost other UHP terranes in which the protolith iscontinental crust eclogites and metasediments atLago di Cignana of the Western Alps were derivedfrom oceanic crust and constitute the ophiolitesequence of the Zermatt-Saas zone Inclusions ofcoesite and coesite pseudomorphs occur in tourma-line and garnet of manganiferous quartzite and inomphacite and garnet of the underlying mafic eclog-ite (Reinecke 1991) The metasediments are mainlygarnet-phengite-quartz schist with variableamounts of garnet-clinopyroxene quartzite andpiedmontite-phengite-quartz schist Eclogites andretrograded eclogites with preserved UHP mineralparageneses consist of garnet omphacite glau-cophane zoisite phengite and dolomite Zoned gar-nets preserve prograde quartz crystals in their coresand coesite inclusions in mantles and rims Coesiteinclusions are much more common in omphacitethan in garnet UHP conditions were estimated at26ndash30 GPa and 590ndash630degC Sm-Nd isotopic anal-yses of eclogites yield UHP metamorphism at 52 plusmn18 Ma (Bowtell et al 1994) consistent with a

Tertiary age of eclogitization recently established forAlpine metamorphism in the Western Alps(Gebauer et al 1997)

Thus far the recognized areal extent of thecoesite-bearing rocks is ~ 2 km2 The apparentthickness of the UHP slice does not exceed a fewhundred meters This area is the best example forsubduction of oceanic lithosphere to mantle depthand later exhumation as a fragmented tectonic slabSimilarly the Allalin metagabbro part of the oce-anic Piemonte zone contains magnesite + talc +kyanite + garnet recrystallized at ~600degC and 35GPa with an estimated geothermal gradient of 5ndash6degC kmndash1 however inclusions of coesite or coesitepseudomorphs have not been reported from theAllalin metagabbro

Mali Africa Eclogitic rocks in the Pan-Africancollision zone cropping out in northern Mali containinclusions of coesite in omphacite from mafic nod-ules within a calc-silicate layer enclosed in impuremarble and coesite pseudomorphs in garnets fromthe surrounding eclogitic metasediments (Caby1994) These UHP eclogitic mica schists have P-Testimates of 700ndash750degC and gt 27 GPa phengiteyields a well-defined plateau 39Ar40Ar age of 1045plusmn 9 Ma Recent SmNd ages of 620 Ma wereobtained by Jahn et al (2001) for peak UHP meta-morphism This may be the oldest UHP terrane inthe world

The coesite-bearing UHP eclogitic unit about 3km thick lies within an internal nappe as a flat thinslice bounded on the top by a thick passive-marginshaly formation and on the bottom by low-gradegreenschist-facies phyllites Caby (1994) suggestedthat a large portion of the terrigenous metasedimentsof passive-continental-margin affinity of the WestAfrican plate was subducted eastward to mantledepths (gt90 km) beneath an oceanic domain Colli-sion of the West African plate with an island arcresulted in a low-angle westward subhorizontalextrusion of this subducted slice of sialic materialsto its present position The coesite-bearing eclogiteunit may also crop out 1500 km to the south in Togowhere kyanite-bearing eclogites occur as part of thepassive-margin assemblage

Makbal (480 Ma) and Atbashy (270 Ma) Kazak-hstan The Tian-Shan is divided into three mountainrangesmdashnorthern middle and southern Within theKyrgyzstan Tian-Shan UHP eclogite localities withcoesite pseudomorph inclusions in garnet andomphacite from the northern and southern Tian-Shan represent Caledonian and Hercynian orogenic

12 LIOU ET AL

belts respectively (Tagiri and Bakirov 1990 Tagiriet al 1995) The Makbal Formation in the westernKyrgyz Ridge of the northern Tian-Shan consists ofquartzose schist alternating with pelitic schistsintercalated with thin layers or lenses of marbleeclogite and amphibolitized eclogite Tagiri andBakirov (1990) found inclusions of coesite pseudo-morphs in garnet from a garnet-chloritoid-talc schistwith a peak metamorphic assemblage of coesite +almandine + chloritoid + talc + phengite (Si = 342pfu) + phlogopite + rutile Makbal coesite-gradeeclogite has a paragonite KAr age of 480 Ma

The eclogite-bearing complex of Atbashy Ridgefrom the southern Tian-Shan is composed of peliticand siliceous schists alternating with thin UHPeclogitic layers and is unconformably overlain byUpper Paleozoic molasse and silicic volcanic rocksInclusions of coesite pseudomorphs occur inomphacite cores and in garnet Eclogites from theAtbashy area preserve prograde and retrogradepaths with a peak metamorphic condition at 660degC25 GPa The Atbashy coesite-eclogites yielded aRbSr mineral isochron age of 270 Ma (Tagiri et al1995)

Central Indonesia UHP terrane The pre-Tertiarybasement of the Indonesian region comprises a vari-ety of imbricate terranes meacutelange ophiolite andvariably metamorphosed accretionary complexessome contain HP to UHP metamorphic rocks result-ing from the collision of an Australia-derived conti-nent with Eurasia (Parkinson 2003) HP rocksincluding eclogites and garnet peridotites are widelydistributed in Cretaceous accretionary complexesMany of these rocks occur as imbricate slices of car-bonate quartzite and pelitic schist of shallow-marine or continental-margin parentage interthrustwith subordinate mafic schist and serpentinitesome yield mica KAr ages of 110ndash120 Ma (Parkin-son 2003)

HP and UHP rocks are sporadically exposed astectonic blocks throughout the Cretaceous accre-tionary complexes They include eclogite garnet-glaucophane rock (P = 18ndash24 GPa T = 580ndash620degC) and jadeite-garnet quartzite (P gt 27 GPa T =720ndash760degC) in Bantimala southwest Sulawesieclogite and garnet granulite in west-centralSulawesi eclogite and jadeite-glaucophane-quartzrock (P ~22 GPa T ~530degC) in central Java Mgchloritoidndashbearing whiteschists (P ~18 GPa) insoutheast Kalimantan garnet lherzolites in east-central Sulawesi (P = 22ndash28 GPa T = 1000ndash1100degC) west-central Sulawesi (P = 16ndash20 GPa T =

1050ndash1100degC) and garnet pyroxenite (P ~2 GPa T~ 850degC) in Sabah northeast Borneo Evidence forUHP rocks include (1) inclusions of coesite inzircon and coesite pseudomorphs in jadeite fromjadeite quartzite and eclogite of the BantimalaComplex of south Sulawesi (Parkinson 2003) and(2) P-T estimates of peak-stage recrystallization at27ndash35 GPa and 1000ndash1100degC for most garnetperidotites (Kardarusman and Parkinson 2000)Many of these rocks were probably recrystallized ina N-dipping subduction zone at the margin of theSundaland craton in the Early Cretaceous Exhuma-tion may have been facilitated by the collision of aGondwana continental fragment with the Sundalandmargin at ~120ndash115 Ma

Recently Recognized UHP Terranes

Several new UHP terranes were recently identi-fied inasmuch as they contain partially preservedtrace index minerals in strong containers such aszircon or garnet In fact zircon has been consideredto be the best container and many new terranes werediscovered through positive identification of inclu-sions of coesite or coesite pseudomorphs ordiamond in zircons Detailed examination of mineralinclusions in core mantle and rims of zircon sepa-rates from eclogites and their enclosing country-rock gneisses have yielded both prograde andretrograde P-T-time paths for various UHP terranesmentioned above (see Katayama et al 2001 fordetails also see the section on the Himalayan eclog-ite for an example)

The latest recognized UHP terranes include thefollowing (1) coesite in the French Massif Central(Lardeaux et al 2001) (2) coesite and diamondfrom the Greek Rhodope metamorphic province(Mposkos and Kostopoulos 2001) (3) coesitepseudomorphs in the Northeast Greenland EclogiteProvince (Gilotti and Ravna 2002) (4) coesite ingranulite-facies overprinting eclogites of SoutheastBrazil (Parkinson et al 2001) (5) coesite in Hima-layan eclogite from the Upper Kaghan Valley Paki-stan (OrsquoBrien et al 2001 Kaneko et al 2003) andcoesite from the Tso-Morari crystalline complex ofIndia (Mukherjee et al 2003) (6) coesite inclusionsin gneissic rocks from the North Qaidam belt west-ern China (Yang et al 2001 Song et al 2003) (7)diamond inclusions in garnets of eclogite and gneissfrom North Qinling (Yang et al 2002a) (8) inclu-sions of quartz pseudomorphs and minor coesite ingarnets of mafic eclogites from the LantermanRange in Antarctica (Ghiribelli et al 2002) (9)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 13

inclusions of calcite pseudomorphs after aragonite +magnesite in dolomite in metapelites from the west-ern Tian-Shan inferred as metamorphism in a UHPregion of dolomite decomposition (Zhang L et al2002) and (10) inclusion of possible coesitepseudomorphs in garnet lamellar inclusions in gar-net kyanite and high sodium content of garnet fromeclogites of the Chuacus Complex of north-centralGuatemala (Solari et al 2003) The results of petro-chemical and geochronological data for these newterranes are summarized below The HimalayanUHP terrane in the Upper Kaghan Valley and theTso-Morari complex are used to illustrate the pro-cesses of continental subduction and collision in alater section

French Massif Central The French Massif Cen-tral in the western part of the Variscan Belt hasexperienced Late SilurianndashEarly Devonian to LateCarboniferous orogenic events (eg Matte 1986)High-pressure metamorphic rocks occur mainly inthe upper gneissic unit within the leptyno-amphibo-lite group The latter consists of an association ofmetagreywacke mica schist metabasalt leptynitemetagranite and peridotite Eclogites are associatedwith silicic and mafic HP granulites (Lardeaux etal 2001) and also with spinel andor garnet lherzo-lites (Gardien et al 1990) Inclusions of coesite andcoesite pseudomorphs occur in garnets of kyanite-bearing eclogite (Lardeaux et al 2001) Only twocoesite grains are preserved as relics most coesitegrains are completely transformed into polycrystal-line radial quartz (palisade texture) or into polygonalquartz surrounded by radial cracks Metamorphictemperatures were estimated to be 740-780degC andsubsequent amphibolite-facies overprinting at750degC at 15ndash17 GPa during decompression UHPmetamorphism occurred between 420 and 400 Ma(Paquette et al 1995) 40Ar39Ar data from amphib-ole separates from retrogressed eclogites yielded339 plusmn 4 Ma (Costa et al 1993)

Northeast Greenland Caledonides The first evi-dence for UHP metamorphism in the GreenlandCaledonites was reported from kyanite eclogites andassociated host gneisses on an island in Jokelbugt(Gilotti and Ravna 2002) Inclusions of quartzpseudomorphs exhibiting palisade structure andradiating fractures occur in garnet and omphacite ofeclogites and in garnet of the host gneisses P-T esti-mates of peak-stage metamorphism of eclogite at~972degC and 36 GPa lie well within the coesite sta-bility field SHRIMP UPb dates of 403 plusmn 5 Ma forzircon rims from HP metapsammite and 404 plusmn 4 Ma

for zircons from anatectic melt derived frommetapelite are coeval with the estimated ages ofexhumation of UHP terranes in the ScandinavianCaledonides described in a previous section(McClelland and Gilotti 2003)

Neoproterozoic nappes in Southeast BrazilNeoproterozoic nappes of Southeast Brazil consistmainly of coarse-grained kyanite-garnet granulitewith intercalations of impure quartzite calc-silicaterock and minor lenses and sills of mafic-ultramaficrocks including eclogite Early eclogitic phaseswere strongly obliterated during granulite-faciesoverprinting during a major Pan-African continentcollision at 640ndash630 Ma (Campos Neto and Caby1999) Rare ultramafic lenses consist of phlogopite-orthopyroxene rocks and garnet clinopyroxeniteCores of granulite boudins retain eclogitic assem-blages some quartz inclusions within the mantleregions of zoned garnet porphyroblasts are sur-rounded by intense radial fractures suggestive ofthe former presence of coesite In fact coarse-grained zircons in some granulites contain numer-ous micro-inclusions of K-feldspar + kyanite + coes-ite as confirmed by Raman spectroscopy (Parkinsonet al 2001) Many exsolution microstructuresobserved in other UHP terranes also occur Thisarea represents the first coesite-grade UHP rocks inthe Americas and it can be correlated with the MaliUHP terrane of equatorial Africa (Caby 1994) Ithas been suggested that eclogitic assemblages andprobable UHPM components may be present in sev-eral other correlative nappe systems at the marginsof the Sao Francisco craton in Brazil

Chuacus Complex north-central Guatemala HPeclogites within banded gneisses schists andmigmatites of the Chuacus Complex contain relicttextures and mineralogical features that suggestUHP metamorphism (Solari et al 2003) Theseinclude inclusions of possible coesite pseudomor-phs in garnet and kyanite lamellar inclusions ofrutileilmenite in garnet high sodium content ofsome garnets (up to 1200 ppm) and preliminary P-T estimates of 700ndash800degC and 2ndash3 GPa AlthoughK-Ar dating of mica and hornblende yield ages of48ndash72 Ma possible for amphibolite facies retrogres-sion regional and structural data support a pre-Mesozoic age for the multi-stage HP-UHP recrystal-lization Such a discovery opens up a new interpre-tation regarding the tectonic evolution of Maya-Chortis-Oaxaquia microcontinental blocks formingthe structural backbone of Mesoamerica

14 LIOU ET AL

Greek Rhodope The Greek Rhodope Metamor-phic Province (RMP) at the border between Greeceand Bulgaria represents a syn-metamorphic nappesystem of Alpine age that was formed during theCretaceous to Mid-Tertiary collision of Apulia andpaleo-Europe The Kimi nappe complex comprisescrustal eclogites leucocratic orthogneisses peliticgneisses and volumetrically minor mantle-derivedultramafic rocks these units contain many mineral-ogical indicators of UHP metamorphism (Mposkosand Kostopoulos 2001) The ultramafic lithologiesinclude garnet spinelndashbearing lherzolite and layersof spinel-garnet clinopyroxenite garnet pyroxeniteand clinopyroxene garnetite These rocks were sub-jected to UHP recrystallization at P-T conditions of~1200degC and gt30 GPa The pelitic gneiss containsmicrodiamond inclusions in garnet porphyroblastsas well as rods or needles of silica rutile biotiteand apatite in sodic garnet suggesting the prioroccurrence of majoritic garnet The growth of suchsupersilicic garnet was suggested to be at a pressureof ~7 GPa Inclusions of polycrystalline quartzaggregates surrounded by radial cracks occur ineclogitic garnet Exsolution lamellae of quartz ineclogitic clinopyroxene also indicate an UHP pre-cursor of supersilicic clinopyroxene

Lanterman Range Antarctica The LantermanRange in northern Victoria Land of Antarctica con-sists of several metamorphic complexes The Gate-way Hills Metamorphic Complex is a thindiscontinuous belt more than 50 km long consistingof mafic and ultramafic rocks including lenses andpods of eclogite within felsic gneiss Some less foli-ated eclogites contain inclusions of coesite andcoesite pseudomorphs in garnet porphyroblasts asconfirmed by in situ Raman spectroscopy as well asradial fractures around the inclusions (Ghiribelli etal 2002) The eclogites were subjected to UHPmetamorphism at T gt 850degC and P gt 29 GPa thenamphibolite-facies overprinting during isothermaldecompression The UHP event is dated by SmNdand 238U206Pb data at 500 Ma 40Ar39Ar ages of490ndash486 Ma for Ca-amphibole (amphibolite-faciesoverprinting) suggest rapid cooling with an averageexhumation rate of 3ndash4 km_Ma-1 for the UHP ter-rane (Di Vincenzo and Palmeri 2001)

Western Tian-Shan China Caledonian eclogitesof the western Tian-Shan China have been classi-fied into three types (Zhang L et al 2002) Type Ieclogite pods are layered with mafic blueschists andType II eclogites preserved pillow structureswhereas Type III eclogites are banded with marbles

This 200 km long eclogite-bearing belt extendswestward to connect with the Atbashy UHP belt inKazakhstan (Tagiri et al 1995) Reported UHP evi-dence includes inclusions of coesite pseudomorphsin garnets from Type I and III eclogites quartzexsolution lamellae in omphacite from Type IIeclogites and relict magnesite within matrix dolo-mite in Type III eclogites (Zhang J X et al 2002Zhang L et al 2002) P-T estimates of the peakUHP stage of metamorphism are T = 500ndash600degCand P = 26ndash27 GPa (Wei et al 2003)

North Qaidam Mountains western China Manylenses and layers of garnet peridotite and eclogiteoccur in an amphibolite-facies terrane in the NorthQaidam Mountains along the northeastern rim of theQaidam Basin This belt comprises mainly felsicgneiss quartz schist garnet-bearing gneissamphibolite and minor eclogite + garnet peridotiteit extends westward for more than 1000 km to theAltun Mountains and was displaced by the left-lat-eral Altyn-Tagh fault At the eastern end of thisUHP belt at Dulan occurrences of coesite inclu-sions in zircon separates from paragneiss inclusionsof coesite pseudomorph in garnet and omphacite ofeclogites quartz rods in eclogitic omphacite andP-T estimates of eclogites (P = 287ndash317 GPa andT = 631ndash687degC) demonstrate that this is a typicalUHP metamorphic terrane (Yang et al 2001 Songet al 2003a 2003b) The Dulan eclogites have anSm-Nd mineral isochron age of 497 plusmn 87 Ma and aSHRIMP UPb zircon age of 495 Ma (Yang et al2002b)

Altun Mountains western China This UHP ter-rane about 200 km long in the southern margin ofthe Altun Mountains is the western extension of theNorth Qaidam belt Eclogitic lenses and blocks ofvarious sizes together with garnet-lherzolite and cli-nopyroxenite are enclosed within an Early Protero-zoic sequence of amphibolite amphibole schistgarnet-bearing granitic gneiss and pelitic schistGarnet lherzolite contains magnesite that reacted toform dolomite P-T estimates of the peak magnesite-bearing assemblage yield P gt 36 GPa and 850degC(Liu et al 2001) Garnet porphyroblasts includeexsolution lamellae of clinopyroxene and rutile sim-ilar to that of eclogite reported by Ye et al (2000) asmajoritic garnet The associated eclogites containinclusions of coesite pseudomorphs in garnet withcharacteristic radial fractures this together with theexsolved quartz rods in omphacite and P-T esti-mates of 820ndash850degC and 28 to 32 GPa provideadditional evidence for UHP (Zhang J X et al

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 15

2001) An SmNd mineral isochron for eclogiticgarnet omphacite and the whole rock yields 500 plusmn10 Ma whereas an UPb isochron for zircon sepa-rates from eclogite gives an age of 504 plusmn 5 Ma(Zhang J X et al 2000)

North Qinling Mountains central China Lensesand blocks of eclogitic rocks in northern Qinling areenclosed in amphibolite-facies gneiss and garnet-bearing quartz + phengite mica schist Inclusions ofcoesite in eclogitic garnet (Hu et al 1995) andmicrodiamonds in zircon separates from both eclog-ite and gneiss (Yang et al 2002a) have beenrecently reported Although individual outcrops areno more than a few meters wide the belt of eclogitesextends more than 10 km P-T estimates of amphib-olitized coesite-bearing eclogite yield T = 590ndash758degC and P gt 26 GPa An Sm-Nd isochron for gar-net omphacite rutile amphibole and whole-rockyields 400 plusmn 16 Ma Microdiamonds were recentlydiscovered as inclusions in zircon from both eclogiteand its gneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyields 507 plusmn 38 Ma for peak UHP metamorphism(Yang et al 2002a) These data indicate that two

discrete collision events occurred between theNorth China and Yangtze cratons (1) an Early Pale-ozoic North Qinling UHP suturing and (2) a Triassicsuturing along the Dabie-Sulu UHP-HP belt TheEarly Paleozoic belt extends westward to the NorthQaidamndashAltun Mountains for more than 4000 km(Yang et al 2002b)

Himalayan UHP Eclogites

The Himalayan orogen has long been considereda classic locality of continental collision betweenthe Indian and Eurasian continents (Fig 5) Severaleclogite-facies HP rocks in the Himalayas havebeen reported since the late 1980s (eg Chaudaryand Ghazanfar 1987 Pognante and Spencer 1991de Sigoyer et al 1997 Lombardo and Rolfo 2000)Inclusions of coesite were recently discovered inomphacite and garnet in several Himalayan eclog-ites from the upper Kaghan Valley Pakistan(OrsquoBrien et al 2001 Kaneko et al 2003) Subse-quent findings of additional coesite localities in theTso Morari area (Sachan et al 2001 Mukherjee etal 2003) have established the presence of a UHPbelt in the Himalayas an active ongoing collision

FIG 5 A simplified tectonic map of the western Himalaya Hindu Kush Pamir Karakoran and western Tibet mod-ified after Figure 3 of Searle et al (2001) This map shows the locations of eclogite and UHP eclogites from both Hima-layan and North Qaidam Indus Suture Zone (ISZ) ophiolites and major structures

16 LIOU ET AL

since the Eocene (Massonne and OrsquoBrien 2003)However UHP relics are volumetrically very minorconstituents of the metamorphic belts dominatedmostly by later Barrovian overprintings In theserocks hydration related to Barrovian-zone metamor-phism has resulted in remarkably pervasive recrys-tallization during the later exhumation this eventhas almost entirely masked the preceding UHPrecord

The finding of HP-UHP evidence has led to newinsights into the Himalayan tectonic modelEngland and Houseman (1986) showed that in amodel of homogeneous thickening of the Hima-layan-Tibet boundary the maximum crustal thick-ness should be less than 70 km However theoccurrences of coesite-bearing eclogites andgneisses in the Himalayan orogen cannot beexplained by a homogeneous thickening of the crustbut rather by continental subduction as describedbelow According to available geochronologicaldata (eg Treloar et al 2003) the exhumation rateof the Himalayan UHP rocks has been estimated torange from ~1 cm yrndash1 (OrsquoBrien et al 2001) to gt3cm yrndash1 (Treloar et al 2003 Massone and OrsquoBrien2003) These estimates are higher than those calcu-lated from other UHP-HP terranes This requirestectonic emplacement of the UHP-HP Himalayanslab into shallow crust following isostatic rebounddue to slab decoupling between the Indian continentand the oceanic crust initiated during the MiddleEocene UHP rocks from both the Kaghan Valleyand Tso-Mori are described below

Kaghan Valley The Kaghan nappe in the north-western Himalayan syntaxis lies between the Indussuture (also called main mantle thrust MMT) to thenorth and the main central thrust (MCT) to thesouthwest (Pognante and Spear 1991 Spencer1993) (Fig 5) The Cretaceous Kohistan island-arcsequence lies north of the Indus suture It developedas a result of intra-oceanic subduction The maincentral thrust separates an underlying Salkhala unitfrom an overlying higher Himalayan crystalline(HHC) unit The Kaghan nappe consists mainly ofgranitic gneiss and paragneiss with minor inter-calated layers of amphibolite marble and quartziteEclogites are preserved in the cores of maficboudins (less than 1 m thick) in gneiss and aresignificantly overprinted by amphibolite assem-blages They contain fine-grained (lt 1 mm) garnetand omphacite together with aggregates of quartzchains of rutile rimmed by titanite and mm-sizedphengitic micas overgrown by randomly oriented

amphiboles up to 1 cm in length Coesite and coesitepseudomorphs occur as inclusions in omphaciteand show the characteristic palisade-quartz aggre-gate texture fractures radiating from these inclu-sions are characteristically developed in the hostpyroxene

Garnet pyroxenendashphengite barometry wasapplied to the eclogites and yielded peak P-T con-ditions of 27ndash29 GPa and 690ndash750degC (OrsquoBrien etal 2001) (Fig 6) Retrograde textures are commonin the eclogites including symplectic intergrowthsof augite amphibole and plagioclase after omphac-ite thermobarometric analyses of these assemblagesyielded 10ndash13 GPa and 600ndash710degC (Kaneko et al2003) On the other hand the dominant graniticgneisses and metapelites of the higher Himalayancrystalline unit contain typical Barrovian-zoneamphibolite-facies assemblages represented bykyanite- and staurolite-bearing garnetiferousmetapelites (Treloar 1995) Extensive overprintinghas mostly obliterated the peak assemblages matrixminerals of the gneisses yielded P-T conditions of07ndash11 GPa and 600ndash700degC (Treloar 1995) con-sistent with retrogression of the eclogites Howeverrelict coesite is preserved as inclusions in zirconthe inclusions are ovoid and approximately 10 microm indiameter (Fig 7A) This discovery of coesite-bear-ing zircons from Himalayan quartzofeldspathicgneisses demonstrates subduction of Indian conti-nental crust to the depths of UHP conditions(Kaneko et al 2003)

Geochronological studies using SmNd (garnet-omphacite) RbSr (phengite) and UPb (rutile zir-con) methods suggest that the eclogite-facies eventtook place at 40ndash50 Ma (Tonarini et al 1993 Spen-cer and Gebauer 1996) 40Ar39Ar cooling ages forhornblende and mica from gneisses were reported tobe ~43 Ma and ~25 Ma respectively (Chamberlainet al 1991) Recent SHRIMP UPb dates of zonedzircons with mineral inclusions formed at differentstages indicate the ages of non-UHP mineral-bear-ing mantle domains of zircon and UHP mineral-bearing rims are at about 50 and 46 Ma respectively(Kaneko et al 2003) A new U-Pb age of 44 plusmn 11Ma was obtained for rutile from Kaghan coesite-bearing eclogite (Treloar et al 2003) this togetherwith available data implies that exhumation of theUHP rocks from mantle depth to mid-crustal leveloccurs within a few million years

Tso Morari The Tso Morari crystalline dome islocated between the Indus suture zone on the northbordered by the Zildat detachment fault and the

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 17

Zanskar sedimentary unit on the south (de Sigoyer etal 1997) The dome represents an internal massif ofthe northern part of the higher Himalaya The TsoMorari area has been considered a typical IndianTethyan margin because of a stratigraphic succes-sion similar to the Nimaling antiform in the core ofthe dome a metamorphic basement consists of Cam-bro-Ordovician augen gneisses (Stutz 1988) Thiscomplex is covered by a metasedimentary series ofCambrian to Devonian quartzite schist and con-glomerate and is overlain by Lower Carboniferousto Triassic marble and metapelite Eclogites occuras lenses or irregular bodies within the basementand cover gneisses and are strongly overprinted byamphibolite-facies assemblages de Sigoyer et al(1997) reported peak P-T conditions for the eclog-ites at 17ndash23 GPa and 520ndash640degC (Fig 7) How-ever relics of coesite 30ndash80 microm in size were

recently found as inclusions in garnet (Sachan et al2001 Mukherjee et al 2003) typified by radialfractures (Figs 7B and 7C) for the SiO2 inclusionsa characteristic Raman spectrum is centered at 523cm-1 (Fig 7D) The country rock metasedimentswith jadeite + chloritoid + paragonite + garnetassemblages have peak P-T estimates of 18ndash22GPa and 500ndash600degC and record retrogression at theeclogite-blueschist facies transition of 13ndash18 GPaand 490ndash590degC (Guillot et al 1997)

The geochronology of the Tso Morari eclogiteshas been extensively studied and subdivided intothree different stages (de Signoyer et al 2000)Eclogitization at ~55 Ma was obtained by Lu-Hf andSm-Nd methods on garnet omphacite glaucophaneand the whole rock An amphibolite-facies overprintoccurred at 47 Ma judging by RbSr dating onphengite apatite and whole rock from metapelites

FIG 6 P-T estimates of peak- and retrograde-stage metamorphism for coesite-bearing eclogites and the country-rockgneisses from the Kaghan Valley and Tso Morari

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 7A

tbas

hy K

azak

hsta

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clog

ite27

0 M

agt

25 k

bar

660deg

C1

Inc

lusi

ons

of c

oesi

te in

gar

net

Cen

tral

Ind

ones

ia S

W

Sula

wes

iE

clog

ite

Gar

net p

erid

otite

115ndash

120

Ma

gt27

kbar

720

ndash760

degC33

ndash35

kbar

115

0degC

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nclu

sion

s of

coe

site

in z

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d co

esit

e ps

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Rec

entl

y re

cogn

ized

terr

anes

Fre

nch

Alp

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clog

ite

Met

apel

ites

400ndash

420

Ma

(pea

k)36

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0 M

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etro

grad

e)gt

28 k

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750deg

C1

Inc

lusi

ons

of c

oesi

te in

gar

net a

nd o

mph

acit

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Nor

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st G

reen

land

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ogite

400ndash

440

Ma

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8 kb

ar 8

20degC

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sion

s of

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site

pse

udom

orph

in g

arne

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Sout

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ogit

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27 k

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of c

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Chu

acus

Com

plex

no

rthe

rn G

uate

mal

aE

clog

ite

Ban

ded

gnei

ss48

ndash72

Ma

(ret

rogr

ade)

20ndash3

0 kb

ar 7

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00degC

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te p

seud

omor

ph in

gar

net a

nd k

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te

Rho

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Mas

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Gre

ece

Ecl

ogit

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litic

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iss

Ultr

amaf

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ocks

30ndash4

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(ret

rogr

ade)

gt 30

kba

r ~

1200

degC1

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rodi

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ion

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Fels

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490

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(ret

rogr

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gt 29

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850

degC1

Inc

lusi

ons

of c

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tern

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Mar

ble

lt 3

10 plusmn

5 M

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7 kb

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sion

s of

coe

site

and

its

pseu

dom

orph

in g

arne

t2

Pos

sibl

e co

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te

Qai

dam

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tern

Chi

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Gne

iss

Gar

net p

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495

plusmn 7

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etro

grad

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27 k

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700deg

C1

Inc

lusi

ons

of c

oesi

te in

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agne

iss

Alt

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ains

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este

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Ecl

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4 plusmn

5 M

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28ndash3

2 kb

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oesi

te p

seud

omor

ph in

gar

net

Nor

th Q

inli

ng M

tns

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entr

al C

hina

Ecl

ogit

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s50

7 plusmn

38

Ma

400

plusmn 1

6 M

agt

26 k

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590ndash

760deg

C1

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in g

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Mic

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d in

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cons

from

ecl

ogit

e an

d gn

eiss

Kag

han

vall

ey P

akis

tan

Him

alay

asE

clog

ite

Gne

iss

46 plusmn

1 M

a (p

eak)

44 plusmn

1 M

a (r

etro

grad

e)27

ndash29

kbar

690

ndash750

degC1

Inc

lusi

ons

of c

oesi

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nd it

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and

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on

from

ecl

ogit

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Indi

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Ecl

ogit

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(pea

k)47

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and

30 M

a(r

etro

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gt 28

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orph

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clog

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8 LIOU ET AL

SHRIMP dating of zircons extracted from gneissschist pyrope inclusion and pyrope quartzite indi-cates the occurrence of 240ndash275 Ma old zirconcores and newly formed 35 Ma rims for some zircons(Gebauer et al 1997) Thus UHP metamorphismmust have taken place during Oligocene time withan estimated exhumation rate of 2 to 24 cm yrndash1

Western Gneiss Region Norway The WesternGneiss Region lies within an Early Paleozoic colli-sion zone the gneissic unit about 300 km long and150 km wide consists of interlayered pelite andmigmatite marble quartzite and amphibolite withtectonic inclusions of gabbro and peridotite (for arecent review see Carswell and Cuthbert 2003) Thegneissic unit exhibits mainly amphibolite-faciesassemblages but relics of HP assemblages alsooccur Eclogite boudins are widespread Coesite wasfirst reported from Grytting (Smith 1984) severalnew localities of coesite and coesite pseudomorphshave been recognized recently in both eclogites andthe adjacent gneissic rocks (Wain 1997 Cuthbert etal 2000 Wain et al 2000 Carswell et al 2003)Microdiamond grains 20ndash50 microns across havebeen described from residues separated from twogneisses (Dobrzhinetskaya et al 1995) the associ-ated kyanite-eclogites contain inclusions of coesitepseudomorphs in garnet Relict majoritic garnetsshowing exsolution of pyroxene lamellae in peridot-ite bodies have been recently discovered on theislands of Otrogy and Flemsoy (van Roermund andDrury 1998 van Roermund et al 2000) Theseperidotite bodies originally must have had an evenhigher-pressure deeper-mantle origin probablywithin a rising mantle diapir at ~1700 Ma In thesegarnet peridotites diamond inclusions in symple-tetic spinel after garnet have been discovered (vanRoermund et al 2002) Radiometric ages for eclog-ites of the Western Gneiss Region are 408ndash425 Mafor the peak metamorphism and 374ndash378 Ma forthe retrogressive mid-crustal metamorphic over-printing (for review see Carswell et al 1999) Car-swell et al (2003) provide new petrographicevidence and a review of the latest radiometric agedata and conclude that the UHP metamorphism ofeclogites occurred at 400ndash410 Ma significantlyyounger than the previous widely accepted age of425 Ma A two-stage exhumation process is sug-gested an initial exhumation to about 35 km depthby about 395 Ma at a mean rate of about 10 m Mandash1and subsequent exhumation to 8ndash10 km by about375 Ma at a much slower rate of about 13 m Mandash1

The suture zone occupied by UHPHP rocks ofthe Western Gneiss Region reflects collision of theeclogite-bearing Greenland sialic crust-cappedlithospheric plate with Fennoscandia during theCaledonian orogeny (Gilotti 1993 Krogh andCarswell 1995 Brueckner and Medaris 1998)Inclusions of coesite pseudomorph in garnet andomphacite from kyanite eclogites have beenrecently documented in the East Greenland eclogiteprovince (Gilotti and Ravna 2002) this new UHPHP terrane is remote but has excellent exposuresand requires close examination

Dabie-Sulu terrane of east-central China TheDabie-Sulu terrane occupies a Triassic collisionzone between the Sino-Korean and Yangtze plates(for a recent review see Hirajima and Nakamura2003) The central UHP coesite-bearing eclogitebelt (P = 27ndash5 GPa) is flanked to the north by amigmatite zone (P lt 2 GPa) and to the south by abelt of blueschist epidote amphibolite and eclog-ite-facies rocks (P = 05ndash12 GPa) Prevalent rocktypes include felsic gneiss orthogneiss marble andquartzite Protolith ages range from Proterozoic toOrdovician but the majority of reported ages areNeoproterozoic (Li et al 1993 Hacker et al 2000)Blocks boudins and layers of eclogites and garnetperidotites occur as enclaves in gneisses in the UHPunit The Dabie-Sulu UHP rocks show several char-acteristics (1) Widespread coesite with hydrousminerals such as talc zoisiteepidote and phengiteoccur in eclogites (2) Two distinct types of mantle-and crustal-derived garnet peridotites are present(3) Abundant exsolution textures were identified inUHP minerals from garnet peridotite and eclogite(4) Diamond separates from both eclogite and garnetperidotite have been reported from few restrictedregions (eg Xu et al 1992 2003) Some of thesefindings are not confirmed by thin-section observa-tion and extensive studies of mineral inclusions inzircon (for review see Liou et al 2002) (5) Somegarnet peridotites record much higher pressure thanassociated coesite-bearing eclogites reaching 5ndash6GPa (Yang et al 1993 Zhang R Y et al 20002003) (6) Various isotopic age dating methods forUHP minerals and rocks from Dabie-Sulu terranegives 220ndash240 Ma (Li et al 1993 Hacker et al2000)

Eclogites in the Huwan shear zone from thenorthwestern Dabie Mountains have SHRIMP UPbdates of 309 plusmn 3 Ma (Sun et al 2002) suggesting adiscrete Carboniferous subductioncollisionbetween North and South China Farther to the west

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 9

in North Qinling inclusions of microdiamond werediscovered in zircons from both eclogite and itsgneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyielded 507 plusmn 38 Ma for metamorphic rims and1200ndash1800 Ma for relic magmatic or old cores sug-gesting an Early Paleozoic UHP metamorphic eventThese new data suggest that episodic subductionand collision of continents occurred during stages ofclosures of paleo Tethys prior to the well-docu-mented major Triassic continent-continent collisionfor the Dabie-Sulu terrane

Kokchetav massif The Kokchetav massif north-ern Kazakhstan is the type site for the UHPdiamond-eclogite regional metamorphic facies(Sobolev and Shatsky 1990 Parkinson et al 2002Shatsky and Sobolev 2003) Diamonds occur asminute inclusions in garnet diopside and zirconporphyroblasts in marble pyroxene-carbonate-gar-net rock and garnet-biotite gneiss and schist Inclu-sions of coesite and coesite pseudomorphs occur ingarnet and zircon from eclogite and diamond-bear-ing gneiss The UHPHP unit extends NW-SE for atleast 80 km and is about 17 km wide The UHP slabis structurally overlain by a weak- to low-grademetamorphic unit and is underlain by a low-P meta-morphic andalusite-staurolite facies unit (Kaneko etal 2000) The UHP slab is composed of felsicgneiss with locally abundant eclogite lenses andminor orthogneiss metacarbonate and rare garnetperidotite with a range of Proterozoic protolith agesA well-constrained P-T-time path deduced frommineral inclusions and SHRIMP geochronology ofzircons is illustrated in Figure 4 Zoned zircons fromdiamond-bearing gneiss yield the following ages aquartz-bearing inherited core with 1100ndash1400 Maas the protolith age coesite-bearing mantle with 537plusmn 9 Ma for UHP metamorphism and the plagio-clase-bearing rim with 507 plusmn 8 Ma for amphibolite-facies overprint (Katayama et al 2001) Manydiamond-grade marbles contain clinopyroxene withexsolution lamellae of quartz K-feldspar phengiteand phlogopite (Katayama et al 2002 Zhu andOgasawara 2002) and titanite with coesite lamellae(Ogasawara et al 2002) these occurrences suggestthat UHP metamorphism occurred at P gt 6 GPa and1000degC

Cndash13 isotopic data of diamond suggest biogenicsources inclusions of water carbonate and nano-metric oxide in diamonds indicate that metamorphicmicrodiamonds in this and other UHP terranes wereprecipitated from C-O-H supercritical fluids (eg

Stockhert et al 2001) Ogasawara et al (2002)further suggested a two-stage growth mechanism formicrodiamonds Masago et al (2003) reported nega-tive 18O values (-39 per mil) for minerals in eclogiteand whiteschist of the Kokchetav Massif similar tothe Chinese UHP rocks first described by Yui et al(1995) The Kokchetav Massif is the second recog-nized UHP region that preserves a significant effectof the interaction of cold meteoric water with theprotolith prior to subduction

Saxonian Erzgebirge Germany The ErzgebirgeCrystalline Complex (ECC) at the northern margin ofthe Bohemian massif is in fault contact with a low-grade Paleozoic sequence at its margins (for a recentreview see Massonne and OrsquoBrien 2003) TheVariscan ECC consists of abundant gneisses thatinclude numerous lenses of eclogites The ldquoGneiss-Eclogite Unitrdquo at the core of this massif containsUHP rocks typified by inclusions of coesite pseudo-morph in garnets from eclogite (Schmadicke 1991)and inclusions of microdiamonds in garnet kyaniteand zircon from gneisses (Massonne 1999) Thusfar the diamondiferous gneisses seem to occur onlyin a 1 km long strip near the eastern shore of theSaidenbach Reservoir Diamond micro-inclusionsoccur exclusively in garnet kyanite and zircon ofseveral gneissic rocks Besides completely pre-served diamonds with grain sizes between 1 to 25microm partially graphitized diamonds and graphitepseudomorphs after diamond occur Such microdia-mond inclusions in garnet and zircon are similar tothose in Kokchetav diamondiferous biotite gneissSome of these inclusions contain additional fluid-bearing phases including apatite phengitic micaand possible fluid inclusions the morphologies ofinclusions and mineral associations suggest thatmicrodiamonds from both the Erzgebirge and theKokchetav were crystallized from supercritical flu-ids under UHP conditions (Stockhert et al 2001Hwang et al 2001 Dobrzhnetskaya et al 20012003)

Schmadicke et al (1995) reported Sm-Nd isoch-rons for garnet-Cpx-WR at 360 plusmn 7 Ma for eclogiteand 353 plusmn 6 Ma for garnet pyroxenite from Erzge-birge UHP rocks 40Ar39Ar spectra of phengite fromtwo eclogite samples give plateau ages of 348 plusmn 2and 355 plusmn 2 Ma Diamond-bearing zircons yieldSHRIMP UPb dates at 336 plusmn 2 Ma (Massonne2001) Such similarity in ages for garnet peridotitesand eclogite enclosed in gneiss suggests a coevalVariscan UHP in-situ metamorphism of the ECCaround 340ndash360 Ma

10 LIOU ET AL

Less Intensively Studied UHP terranes

Several UHP terranes described below exceptfor the Maksyvtov Complex have received lessintensive study mainly due to remoteness and diffi-cult access despite their long recognition

Maksyutov Complex Southern Urals TheMaksyutov Complex is an elongated (10 times 120 km)N-S belt between the Russian and Siberian cratonsin the southern Ural Mountains (Coleman and Wang1995 Dobretsov et al 1996) and contains blocky

graphitized diamond() in pelitic schist (Leech andErnst 1998) The complex is the locality from whichquartz aggregates within garnet exhibiting radialcracks were first recognized as coesite-pseudo-morphs by Chesnokov and Popov (1965) nearly 20years prior to the independent finding of coesiteinclusions by Chopin (1984) and Smith (1984) Theeclogite-bearing unit contains boudins of eclogitelayers of eclogitic gneiss and rare ultramafic bodieswithin host metasedimentary mica schist andquartzite

FIG 4 A Schematic diagram showing the growth of detrital zircon (stage 1) through subduction (stage 2 and peakstage 3) and exhumation (stage 4) of supracrustal rocks Mineral inclusions in various stages of zircon growth are shownB SHRIMP U-Pb dating of zoned zircons from a Kokchetav diamond-bearing gneiss yields a well constrained P-T-timepath (for details see Katayama et al 2001)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 11

Some Maksyutov eclogites were recrystallized at594ndash637degC with a minimum pressure from 15 to17 GPa (Beane et al 1995 Leech and Ernst 1998)or up to 27 GPa if coesite pseudomorphs (Ches-nokov and Popov 1965 Dobretsov and Doretsova1988) are indeed present However subsequentstudies have not revealed the occurrence of inclu-sions of coesite or coesite pseudomorph in garnetand zircon (Beane et al 1995 Shatsky et al 1995)Leech and Ernst (1998) described unusual graphiteaggregates (up to 13 mm edge length) that may bediamond pseudomorphs This suggestion was basedon the blocky morphology of the cuboid graphiteaggregates cross-cutting rock foliation pressureshadows biogenic isotopic signatures and otherspectroscopic characters similar to those graphi-tized diamonds from the Beni Bousera massifRecent study of several microdiamond inclusions ingarnet has yielded further suggestive possibility thatthe eclogitic unit of the Maksyutov Complex experi-enced diamond-grade UHP metamorphism (Bosticket al 2003) The UHPHP metamorphism with peakmetamorphic ages of 370ndash374 Ma (Shatsky et al1995 Beane and Connelly 1998) may be related tothe collision of the Russian platform with a fragmentof the Siberian craton during Devonian time butwas affected by the Triassic collision of the Kazakh-stan block with the Russian platform (eg Matte1998)

Zermatt-Saas area Western Alps In contrast tomost other UHP terranes in which the protolith iscontinental crust eclogites and metasediments atLago di Cignana of the Western Alps were derivedfrom oceanic crust and constitute the ophiolitesequence of the Zermatt-Saas zone Inclusions ofcoesite and coesite pseudomorphs occur in tourma-line and garnet of manganiferous quartzite and inomphacite and garnet of the underlying mafic eclog-ite (Reinecke 1991) The metasediments are mainlygarnet-phengite-quartz schist with variableamounts of garnet-clinopyroxene quartzite andpiedmontite-phengite-quartz schist Eclogites andretrograded eclogites with preserved UHP mineralparageneses consist of garnet omphacite glau-cophane zoisite phengite and dolomite Zoned gar-nets preserve prograde quartz crystals in their coresand coesite inclusions in mantles and rims Coesiteinclusions are much more common in omphacitethan in garnet UHP conditions were estimated at26ndash30 GPa and 590ndash630degC Sm-Nd isotopic anal-yses of eclogites yield UHP metamorphism at 52 plusmn18 Ma (Bowtell et al 1994) consistent with a

Tertiary age of eclogitization recently established forAlpine metamorphism in the Western Alps(Gebauer et al 1997)

Thus far the recognized areal extent of thecoesite-bearing rocks is ~ 2 km2 The apparentthickness of the UHP slice does not exceed a fewhundred meters This area is the best example forsubduction of oceanic lithosphere to mantle depthand later exhumation as a fragmented tectonic slabSimilarly the Allalin metagabbro part of the oce-anic Piemonte zone contains magnesite + talc +kyanite + garnet recrystallized at ~600degC and 35GPa with an estimated geothermal gradient of 5ndash6degC kmndash1 however inclusions of coesite or coesitepseudomorphs have not been reported from theAllalin metagabbro

Mali Africa Eclogitic rocks in the Pan-Africancollision zone cropping out in northern Mali containinclusions of coesite in omphacite from mafic nod-ules within a calc-silicate layer enclosed in impuremarble and coesite pseudomorphs in garnets fromthe surrounding eclogitic metasediments (Caby1994) These UHP eclogitic mica schists have P-Testimates of 700ndash750degC and gt 27 GPa phengiteyields a well-defined plateau 39Ar40Ar age of 1045plusmn 9 Ma Recent SmNd ages of 620 Ma wereobtained by Jahn et al (2001) for peak UHP meta-morphism This may be the oldest UHP terrane inthe world

The coesite-bearing UHP eclogitic unit about 3km thick lies within an internal nappe as a flat thinslice bounded on the top by a thick passive-marginshaly formation and on the bottom by low-gradegreenschist-facies phyllites Caby (1994) suggestedthat a large portion of the terrigenous metasedimentsof passive-continental-margin affinity of the WestAfrican plate was subducted eastward to mantledepths (gt90 km) beneath an oceanic domain Colli-sion of the West African plate with an island arcresulted in a low-angle westward subhorizontalextrusion of this subducted slice of sialic materialsto its present position The coesite-bearing eclogiteunit may also crop out 1500 km to the south in Togowhere kyanite-bearing eclogites occur as part of thepassive-margin assemblage

Makbal (480 Ma) and Atbashy (270 Ma) Kazak-hstan The Tian-Shan is divided into three mountainrangesmdashnorthern middle and southern Within theKyrgyzstan Tian-Shan UHP eclogite localities withcoesite pseudomorph inclusions in garnet andomphacite from the northern and southern Tian-Shan represent Caledonian and Hercynian orogenic

12 LIOU ET AL

belts respectively (Tagiri and Bakirov 1990 Tagiriet al 1995) The Makbal Formation in the westernKyrgyz Ridge of the northern Tian-Shan consists ofquartzose schist alternating with pelitic schistsintercalated with thin layers or lenses of marbleeclogite and amphibolitized eclogite Tagiri andBakirov (1990) found inclusions of coesite pseudo-morphs in garnet from a garnet-chloritoid-talc schistwith a peak metamorphic assemblage of coesite +almandine + chloritoid + talc + phengite (Si = 342pfu) + phlogopite + rutile Makbal coesite-gradeeclogite has a paragonite KAr age of 480 Ma

The eclogite-bearing complex of Atbashy Ridgefrom the southern Tian-Shan is composed of peliticand siliceous schists alternating with thin UHPeclogitic layers and is unconformably overlain byUpper Paleozoic molasse and silicic volcanic rocksInclusions of coesite pseudomorphs occur inomphacite cores and in garnet Eclogites from theAtbashy area preserve prograde and retrogradepaths with a peak metamorphic condition at 660degC25 GPa The Atbashy coesite-eclogites yielded aRbSr mineral isochron age of 270 Ma (Tagiri et al1995)

Central Indonesia UHP terrane The pre-Tertiarybasement of the Indonesian region comprises a vari-ety of imbricate terranes meacutelange ophiolite andvariably metamorphosed accretionary complexessome contain HP to UHP metamorphic rocks result-ing from the collision of an Australia-derived conti-nent with Eurasia (Parkinson 2003) HP rocksincluding eclogites and garnet peridotites are widelydistributed in Cretaceous accretionary complexesMany of these rocks occur as imbricate slices of car-bonate quartzite and pelitic schist of shallow-marine or continental-margin parentage interthrustwith subordinate mafic schist and serpentinitesome yield mica KAr ages of 110ndash120 Ma (Parkin-son 2003)

HP and UHP rocks are sporadically exposed astectonic blocks throughout the Cretaceous accre-tionary complexes They include eclogite garnet-glaucophane rock (P = 18ndash24 GPa T = 580ndash620degC) and jadeite-garnet quartzite (P gt 27 GPa T =720ndash760degC) in Bantimala southwest Sulawesieclogite and garnet granulite in west-centralSulawesi eclogite and jadeite-glaucophane-quartzrock (P ~22 GPa T ~530degC) in central Java Mgchloritoidndashbearing whiteschists (P ~18 GPa) insoutheast Kalimantan garnet lherzolites in east-central Sulawesi (P = 22ndash28 GPa T = 1000ndash1100degC) west-central Sulawesi (P = 16ndash20 GPa T =

1050ndash1100degC) and garnet pyroxenite (P ~2 GPa T~ 850degC) in Sabah northeast Borneo Evidence forUHP rocks include (1) inclusions of coesite inzircon and coesite pseudomorphs in jadeite fromjadeite quartzite and eclogite of the BantimalaComplex of south Sulawesi (Parkinson 2003) and(2) P-T estimates of peak-stage recrystallization at27ndash35 GPa and 1000ndash1100degC for most garnetperidotites (Kardarusman and Parkinson 2000)Many of these rocks were probably recrystallized ina N-dipping subduction zone at the margin of theSundaland craton in the Early Cretaceous Exhuma-tion may have been facilitated by the collision of aGondwana continental fragment with the Sundalandmargin at ~120ndash115 Ma

Recently Recognized UHP Terranes

Several new UHP terranes were recently identi-fied inasmuch as they contain partially preservedtrace index minerals in strong containers such aszircon or garnet In fact zircon has been consideredto be the best container and many new terranes werediscovered through positive identification of inclu-sions of coesite or coesite pseudomorphs ordiamond in zircons Detailed examination of mineralinclusions in core mantle and rims of zircon sepa-rates from eclogites and their enclosing country-rock gneisses have yielded both prograde andretrograde P-T-time paths for various UHP terranesmentioned above (see Katayama et al 2001 fordetails also see the section on the Himalayan eclog-ite for an example)

The latest recognized UHP terranes include thefollowing (1) coesite in the French Massif Central(Lardeaux et al 2001) (2) coesite and diamondfrom the Greek Rhodope metamorphic province(Mposkos and Kostopoulos 2001) (3) coesitepseudomorphs in the Northeast Greenland EclogiteProvince (Gilotti and Ravna 2002) (4) coesite ingranulite-facies overprinting eclogites of SoutheastBrazil (Parkinson et al 2001) (5) coesite in Hima-layan eclogite from the Upper Kaghan Valley Paki-stan (OrsquoBrien et al 2001 Kaneko et al 2003) andcoesite from the Tso-Morari crystalline complex ofIndia (Mukherjee et al 2003) (6) coesite inclusionsin gneissic rocks from the North Qaidam belt west-ern China (Yang et al 2001 Song et al 2003) (7)diamond inclusions in garnets of eclogite and gneissfrom North Qinling (Yang et al 2002a) (8) inclu-sions of quartz pseudomorphs and minor coesite ingarnets of mafic eclogites from the LantermanRange in Antarctica (Ghiribelli et al 2002) (9)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 13

inclusions of calcite pseudomorphs after aragonite +magnesite in dolomite in metapelites from the west-ern Tian-Shan inferred as metamorphism in a UHPregion of dolomite decomposition (Zhang L et al2002) and (10) inclusion of possible coesitepseudomorphs in garnet lamellar inclusions in gar-net kyanite and high sodium content of garnet fromeclogites of the Chuacus Complex of north-centralGuatemala (Solari et al 2003) The results of petro-chemical and geochronological data for these newterranes are summarized below The HimalayanUHP terrane in the Upper Kaghan Valley and theTso-Morari complex are used to illustrate the pro-cesses of continental subduction and collision in alater section

French Massif Central The French Massif Cen-tral in the western part of the Variscan Belt hasexperienced Late SilurianndashEarly Devonian to LateCarboniferous orogenic events (eg Matte 1986)High-pressure metamorphic rocks occur mainly inthe upper gneissic unit within the leptyno-amphibo-lite group The latter consists of an association ofmetagreywacke mica schist metabasalt leptynitemetagranite and peridotite Eclogites are associatedwith silicic and mafic HP granulites (Lardeaux etal 2001) and also with spinel andor garnet lherzo-lites (Gardien et al 1990) Inclusions of coesite andcoesite pseudomorphs occur in garnets of kyanite-bearing eclogite (Lardeaux et al 2001) Only twocoesite grains are preserved as relics most coesitegrains are completely transformed into polycrystal-line radial quartz (palisade texture) or into polygonalquartz surrounded by radial cracks Metamorphictemperatures were estimated to be 740-780degC andsubsequent amphibolite-facies overprinting at750degC at 15ndash17 GPa during decompression UHPmetamorphism occurred between 420 and 400 Ma(Paquette et al 1995) 40Ar39Ar data from amphib-ole separates from retrogressed eclogites yielded339 plusmn 4 Ma (Costa et al 1993)

Northeast Greenland Caledonides The first evi-dence for UHP metamorphism in the GreenlandCaledonites was reported from kyanite eclogites andassociated host gneisses on an island in Jokelbugt(Gilotti and Ravna 2002) Inclusions of quartzpseudomorphs exhibiting palisade structure andradiating fractures occur in garnet and omphacite ofeclogites and in garnet of the host gneisses P-T esti-mates of peak-stage metamorphism of eclogite at~972degC and 36 GPa lie well within the coesite sta-bility field SHRIMP UPb dates of 403 plusmn 5 Ma forzircon rims from HP metapsammite and 404 plusmn 4 Ma

for zircons from anatectic melt derived frommetapelite are coeval with the estimated ages ofexhumation of UHP terranes in the ScandinavianCaledonides described in a previous section(McClelland and Gilotti 2003)

Neoproterozoic nappes in Southeast BrazilNeoproterozoic nappes of Southeast Brazil consistmainly of coarse-grained kyanite-garnet granulitewith intercalations of impure quartzite calc-silicaterock and minor lenses and sills of mafic-ultramaficrocks including eclogite Early eclogitic phaseswere strongly obliterated during granulite-faciesoverprinting during a major Pan-African continentcollision at 640ndash630 Ma (Campos Neto and Caby1999) Rare ultramafic lenses consist of phlogopite-orthopyroxene rocks and garnet clinopyroxeniteCores of granulite boudins retain eclogitic assem-blages some quartz inclusions within the mantleregions of zoned garnet porphyroblasts are sur-rounded by intense radial fractures suggestive ofthe former presence of coesite In fact coarse-grained zircons in some granulites contain numer-ous micro-inclusions of K-feldspar + kyanite + coes-ite as confirmed by Raman spectroscopy (Parkinsonet al 2001) Many exsolution microstructuresobserved in other UHP terranes also occur Thisarea represents the first coesite-grade UHP rocks inthe Americas and it can be correlated with the MaliUHP terrane of equatorial Africa (Caby 1994) Ithas been suggested that eclogitic assemblages andprobable UHPM components may be present in sev-eral other correlative nappe systems at the marginsof the Sao Francisco craton in Brazil

Chuacus Complex north-central Guatemala HPeclogites within banded gneisses schists andmigmatites of the Chuacus Complex contain relicttextures and mineralogical features that suggestUHP metamorphism (Solari et al 2003) Theseinclude inclusions of possible coesite pseudomor-phs in garnet and kyanite lamellar inclusions ofrutileilmenite in garnet high sodium content ofsome garnets (up to 1200 ppm) and preliminary P-T estimates of 700ndash800degC and 2ndash3 GPa AlthoughK-Ar dating of mica and hornblende yield ages of48ndash72 Ma possible for amphibolite facies retrogres-sion regional and structural data support a pre-Mesozoic age for the multi-stage HP-UHP recrystal-lization Such a discovery opens up a new interpre-tation regarding the tectonic evolution of Maya-Chortis-Oaxaquia microcontinental blocks formingthe structural backbone of Mesoamerica

14 LIOU ET AL

Greek Rhodope The Greek Rhodope Metamor-phic Province (RMP) at the border between Greeceand Bulgaria represents a syn-metamorphic nappesystem of Alpine age that was formed during theCretaceous to Mid-Tertiary collision of Apulia andpaleo-Europe The Kimi nappe complex comprisescrustal eclogites leucocratic orthogneisses peliticgneisses and volumetrically minor mantle-derivedultramafic rocks these units contain many mineral-ogical indicators of UHP metamorphism (Mposkosand Kostopoulos 2001) The ultramafic lithologiesinclude garnet spinelndashbearing lherzolite and layersof spinel-garnet clinopyroxenite garnet pyroxeniteand clinopyroxene garnetite These rocks were sub-jected to UHP recrystallization at P-T conditions of~1200degC and gt30 GPa The pelitic gneiss containsmicrodiamond inclusions in garnet porphyroblastsas well as rods or needles of silica rutile biotiteand apatite in sodic garnet suggesting the prioroccurrence of majoritic garnet The growth of suchsupersilicic garnet was suggested to be at a pressureof ~7 GPa Inclusions of polycrystalline quartzaggregates surrounded by radial cracks occur ineclogitic garnet Exsolution lamellae of quartz ineclogitic clinopyroxene also indicate an UHP pre-cursor of supersilicic clinopyroxene

Lanterman Range Antarctica The LantermanRange in northern Victoria Land of Antarctica con-sists of several metamorphic complexes The Gate-way Hills Metamorphic Complex is a thindiscontinuous belt more than 50 km long consistingof mafic and ultramafic rocks including lenses andpods of eclogite within felsic gneiss Some less foli-ated eclogites contain inclusions of coesite andcoesite pseudomorphs in garnet porphyroblasts asconfirmed by in situ Raman spectroscopy as well asradial fractures around the inclusions (Ghiribelli etal 2002) The eclogites were subjected to UHPmetamorphism at T gt 850degC and P gt 29 GPa thenamphibolite-facies overprinting during isothermaldecompression The UHP event is dated by SmNdand 238U206Pb data at 500 Ma 40Ar39Ar ages of490ndash486 Ma for Ca-amphibole (amphibolite-faciesoverprinting) suggest rapid cooling with an averageexhumation rate of 3ndash4 km_Ma-1 for the UHP ter-rane (Di Vincenzo and Palmeri 2001)

Western Tian-Shan China Caledonian eclogitesof the western Tian-Shan China have been classi-fied into three types (Zhang L et al 2002) Type Ieclogite pods are layered with mafic blueschists andType II eclogites preserved pillow structureswhereas Type III eclogites are banded with marbles

This 200 km long eclogite-bearing belt extendswestward to connect with the Atbashy UHP belt inKazakhstan (Tagiri et al 1995) Reported UHP evi-dence includes inclusions of coesite pseudomorphsin garnets from Type I and III eclogites quartzexsolution lamellae in omphacite from Type IIeclogites and relict magnesite within matrix dolo-mite in Type III eclogites (Zhang J X et al 2002Zhang L et al 2002) P-T estimates of the peakUHP stage of metamorphism are T = 500ndash600degCand P = 26ndash27 GPa (Wei et al 2003)

North Qaidam Mountains western China Manylenses and layers of garnet peridotite and eclogiteoccur in an amphibolite-facies terrane in the NorthQaidam Mountains along the northeastern rim of theQaidam Basin This belt comprises mainly felsicgneiss quartz schist garnet-bearing gneissamphibolite and minor eclogite + garnet peridotiteit extends westward for more than 1000 km to theAltun Mountains and was displaced by the left-lat-eral Altyn-Tagh fault At the eastern end of thisUHP belt at Dulan occurrences of coesite inclu-sions in zircon separates from paragneiss inclusionsof coesite pseudomorph in garnet and omphacite ofeclogites quartz rods in eclogitic omphacite andP-T estimates of eclogites (P = 287ndash317 GPa andT = 631ndash687degC) demonstrate that this is a typicalUHP metamorphic terrane (Yang et al 2001 Songet al 2003a 2003b) The Dulan eclogites have anSm-Nd mineral isochron age of 497 plusmn 87 Ma and aSHRIMP UPb zircon age of 495 Ma (Yang et al2002b)

Altun Mountains western China This UHP ter-rane about 200 km long in the southern margin ofthe Altun Mountains is the western extension of theNorth Qaidam belt Eclogitic lenses and blocks ofvarious sizes together with garnet-lherzolite and cli-nopyroxenite are enclosed within an Early Protero-zoic sequence of amphibolite amphibole schistgarnet-bearing granitic gneiss and pelitic schistGarnet lherzolite contains magnesite that reacted toform dolomite P-T estimates of the peak magnesite-bearing assemblage yield P gt 36 GPa and 850degC(Liu et al 2001) Garnet porphyroblasts includeexsolution lamellae of clinopyroxene and rutile sim-ilar to that of eclogite reported by Ye et al (2000) asmajoritic garnet The associated eclogites containinclusions of coesite pseudomorphs in garnet withcharacteristic radial fractures this together with theexsolved quartz rods in omphacite and P-T esti-mates of 820ndash850degC and 28 to 32 GPa provideadditional evidence for UHP (Zhang J X et al

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 15

2001) An SmNd mineral isochron for eclogiticgarnet omphacite and the whole rock yields 500 plusmn10 Ma whereas an UPb isochron for zircon sepa-rates from eclogite gives an age of 504 plusmn 5 Ma(Zhang J X et al 2000)

North Qinling Mountains central China Lensesand blocks of eclogitic rocks in northern Qinling areenclosed in amphibolite-facies gneiss and garnet-bearing quartz + phengite mica schist Inclusions ofcoesite in eclogitic garnet (Hu et al 1995) andmicrodiamonds in zircon separates from both eclog-ite and gneiss (Yang et al 2002a) have beenrecently reported Although individual outcrops areno more than a few meters wide the belt of eclogitesextends more than 10 km P-T estimates of amphib-olitized coesite-bearing eclogite yield T = 590ndash758degC and P gt 26 GPa An Sm-Nd isochron for gar-net omphacite rutile amphibole and whole-rockyields 400 plusmn 16 Ma Microdiamonds were recentlydiscovered as inclusions in zircon from both eclogiteand its gneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyields 507 plusmn 38 Ma for peak UHP metamorphism(Yang et al 2002a) These data indicate that two

discrete collision events occurred between theNorth China and Yangtze cratons (1) an Early Pale-ozoic North Qinling UHP suturing and (2) a Triassicsuturing along the Dabie-Sulu UHP-HP belt TheEarly Paleozoic belt extends westward to the NorthQaidamndashAltun Mountains for more than 4000 km(Yang et al 2002b)

Himalayan UHP Eclogites

The Himalayan orogen has long been considereda classic locality of continental collision betweenthe Indian and Eurasian continents (Fig 5) Severaleclogite-facies HP rocks in the Himalayas havebeen reported since the late 1980s (eg Chaudaryand Ghazanfar 1987 Pognante and Spencer 1991de Sigoyer et al 1997 Lombardo and Rolfo 2000)Inclusions of coesite were recently discovered inomphacite and garnet in several Himalayan eclog-ites from the upper Kaghan Valley Pakistan(OrsquoBrien et al 2001 Kaneko et al 2003) Subse-quent findings of additional coesite localities in theTso Morari area (Sachan et al 2001 Mukherjee etal 2003) have established the presence of a UHPbelt in the Himalayas an active ongoing collision

FIG 5 A simplified tectonic map of the western Himalaya Hindu Kush Pamir Karakoran and western Tibet mod-ified after Figure 3 of Searle et al (2001) This map shows the locations of eclogite and UHP eclogites from both Hima-layan and North Qaidam Indus Suture Zone (ISZ) ophiolites and major structures

16 LIOU ET AL

since the Eocene (Massonne and OrsquoBrien 2003)However UHP relics are volumetrically very minorconstituents of the metamorphic belts dominatedmostly by later Barrovian overprintings In theserocks hydration related to Barrovian-zone metamor-phism has resulted in remarkably pervasive recrys-tallization during the later exhumation this eventhas almost entirely masked the preceding UHPrecord

The finding of HP-UHP evidence has led to newinsights into the Himalayan tectonic modelEngland and Houseman (1986) showed that in amodel of homogeneous thickening of the Hima-layan-Tibet boundary the maximum crustal thick-ness should be less than 70 km However theoccurrences of coesite-bearing eclogites andgneisses in the Himalayan orogen cannot beexplained by a homogeneous thickening of the crustbut rather by continental subduction as describedbelow According to available geochronologicaldata (eg Treloar et al 2003) the exhumation rateof the Himalayan UHP rocks has been estimated torange from ~1 cm yrndash1 (OrsquoBrien et al 2001) to gt3cm yrndash1 (Treloar et al 2003 Massone and OrsquoBrien2003) These estimates are higher than those calcu-lated from other UHP-HP terranes This requirestectonic emplacement of the UHP-HP Himalayanslab into shallow crust following isostatic rebounddue to slab decoupling between the Indian continentand the oceanic crust initiated during the MiddleEocene UHP rocks from both the Kaghan Valleyand Tso-Mori are described below

Kaghan Valley The Kaghan nappe in the north-western Himalayan syntaxis lies between the Indussuture (also called main mantle thrust MMT) to thenorth and the main central thrust (MCT) to thesouthwest (Pognante and Spear 1991 Spencer1993) (Fig 5) The Cretaceous Kohistan island-arcsequence lies north of the Indus suture It developedas a result of intra-oceanic subduction The maincentral thrust separates an underlying Salkhala unitfrom an overlying higher Himalayan crystalline(HHC) unit The Kaghan nappe consists mainly ofgranitic gneiss and paragneiss with minor inter-calated layers of amphibolite marble and quartziteEclogites are preserved in the cores of maficboudins (less than 1 m thick) in gneiss and aresignificantly overprinted by amphibolite assem-blages They contain fine-grained (lt 1 mm) garnetand omphacite together with aggregates of quartzchains of rutile rimmed by titanite and mm-sizedphengitic micas overgrown by randomly oriented

amphiboles up to 1 cm in length Coesite and coesitepseudomorphs occur as inclusions in omphaciteand show the characteristic palisade-quartz aggre-gate texture fractures radiating from these inclu-sions are characteristically developed in the hostpyroxene

Garnet pyroxenendashphengite barometry wasapplied to the eclogites and yielded peak P-T con-ditions of 27ndash29 GPa and 690ndash750degC (OrsquoBrien etal 2001) (Fig 6) Retrograde textures are commonin the eclogites including symplectic intergrowthsof augite amphibole and plagioclase after omphac-ite thermobarometric analyses of these assemblagesyielded 10ndash13 GPa and 600ndash710degC (Kaneko et al2003) On the other hand the dominant graniticgneisses and metapelites of the higher Himalayancrystalline unit contain typical Barrovian-zoneamphibolite-facies assemblages represented bykyanite- and staurolite-bearing garnetiferousmetapelites (Treloar 1995) Extensive overprintinghas mostly obliterated the peak assemblages matrixminerals of the gneisses yielded P-T conditions of07ndash11 GPa and 600ndash700degC (Treloar 1995) con-sistent with retrogression of the eclogites Howeverrelict coesite is preserved as inclusions in zirconthe inclusions are ovoid and approximately 10 microm indiameter (Fig 7A) This discovery of coesite-bear-ing zircons from Himalayan quartzofeldspathicgneisses demonstrates subduction of Indian conti-nental crust to the depths of UHP conditions(Kaneko et al 2003)

Geochronological studies using SmNd (garnet-omphacite) RbSr (phengite) and UPb (rutile zir-con) methods suggest that the eclogite-facies eventtook place at 40ndash50 Ma (Tonarini et al 1993 Spen-cer and Gebauer 1996) 40Ar39Ar cooling ages forhornblende and mica from gneisses were reported tobe ~43 Ma and ~25 Ma respectively (Chamberlainet al 1991) Recent SHRIMP UPb dates of zonedzircons with mineral inclusions formed at differentstages indicate the ages of non-UHP mineral-bear-ing mantle domains of zircon and UHP mineral-bearing rims are at about 50 and 46 Ma respectively(Kaneko et al 2003) A new U-Pb age of 44 plusmn 11Ma was obtained for rutile from Kaghan coesite-bearing eclogite (Treloar et al 2003) this togetherwith available data implies that exhumation of theUHP rocks from mantle depth to mid-crustal leveloccurs within a few million years

Tso Morari The Tso Morari crystalline dome islocated between the Indus suture zone on the northbordered by the Zildat detachment fault and the

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 17

Zanskar sedimentary unit on the south (de Sigoyer etal 1997) The dome represents an internal massif ofthe northern part of the higher Himalaya The TsoMorari area has been considered a typical IndianTethyan margin because of a stratigraphic succes-sion similar to the Nimaling antiform in the core ofthe dome a metamorphic basement consists of Cam-bro-Ordovician augen gneisses (Stutz 1988) Thiscomplex is covered by a metasedimentary series ofCambrian to Devonian quartzite schist and con-glomerate and is overlain by Lower Carboniferousto Triassic marble and metapelite Eclogites occuras lenses or irregular bodies within the basementand cover gneisses and are strongly overprinted byamphibolite-facies assemblages de Sigoyer et al(1997) reported peak P-T conditions for the eclog-ites at 17ndash23 GPa and 520ndash640degC (Fig 7) How-ever relics of coesite 30ndash80 microm in size were

recently found as inclusions in garnet (Sachan et al2001 Mukherjee et al 2003) typified by radialfractures (Figs 7B and 7C) for the SiO2 inclusionsa characteristic Raman spectrum is centered at 523cm-1 (Fig 7D) The country rock metasedimentswith jadeite + chloritoid + paragonite + garnetassemblages have peak P-T estimates of 18ndash22GPa and 500ndash600degC and record retrogression at theeclogite-blueschist facies transition of 13ndash18 GPaand 490ndash590degC (Guillot et al 1997)

The geochronology of the Tso Morari eclogiteshas been extensively studied and subdivided intothree different stages (de Signoyer et al 2000)Eclogitization at ~55 Ma was obtained by Lu-Hf andSm-Nd methods on garnet omphacite glaucophaneand the whole rock An amphibolite-facies overprintoccurred at 47 Ma judging by RbSr dating onphengite apatite and whole rock from metapelites

FIG 6 P-T estimates of peak- and retrograde-stage metamorphism for coesite-bearing eclogites and the country-rockgneisses from the Kaghan Valley and Tso Morari

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

8 LIOU ET AL

SHRIMP dating of zircons extracted from gneissschist pyrope inclusion and pyrope quartzite indi-cates the occurrence of 240ndash275 Ma old zirconcores and newly formed 35 Ma rims for some zircons(Gebauer et al 1997) Thus UHP metamorphismmust have taken place during Oligocene time withan estimated exhumation rate of 2 to 24 cm yrndash1

Western Gneiss Region Norway The WesternGneiss Region lies within an Early Paleozoic colli-sion zone the gneissic unit about 300 km long and150 km wide consists of interlayered pelite andmigmatite marble quartzite and amphibolite withtectonic inclusions of gabbro and peridotite (for arecent review see Carswell and Cuthbert 2003) Thegneissic unit exhibits mainly amphibolite-faciesassemblages but relics of HP assemblages alsooccur Eclogite boudins are widespread Coesite wasfirst reported from Grytting (Smith 1984) severalnew localities of coesite and coesite pseudomorphshave been recognized recently in both eclogites andthe adjacent gneissic rocks (Wain 1997 Cuthbert etal 2000 Wain et al 2000 Carswell et al 2003)Microdiamond grains 20ndash50 microns across havebeen described from residues separated from twogneisses (Dobrzhinetskaya et al 1995) the associ-ated kyanite-eclogites contain inclusions of coesitepseudomorphs in garnet Relict majoritic garnetsshowing exsolution of pyroxene lamellae in peridot-ite bodies have been recently discovered on theislands of Otrogy and Flemsoy (van Roermund andDrury 1998 van Roermund et al 2000) Theseperidotite bodies originally must have had an evenhigher-pressure deeper-mantle origin probablywithin a rising mantle diapir at ~1700 Ma In thesegarnet peridotites diamond inclusions in symple-tetic spinel after garnet have been discovered (vanRoermund et al 2002) Radiometric ages for eclog-ites of the Western Gneiss Region are 408ndash425 Mafor the peak metamorphism and 374ndash378 Ma forthe retrogressive mid-crustal metamorphic over-printing (for review see Carswell et al 1999) Car-swell et al (2003) provide new petrographicevidence and a review of the latest radiometric agedata and conclude that the UHP metamorphism ofeclogites occurred at 400ndash410 Ma significantlyyounger than the previous widely accepted age of425 Ma A two-stage exhumation process is sug-gested an initial exhumation to about 35 km depthby about 395 Ma at a mean rate of about 10 m Mandash1and subsequent exhumation to 8ndash10 km by about375 Ma at a much slower rate of about 13 m Mandash1

The suture zone occupied by UHPHP rocks ofthe Western Gneiss Region reflects collision of theeclogite-bearing Greenland sialic crust-cappedlithospheric plate with Fennoscandia during theCaledonian orogeny (Gilotti 1993 Krogh andCarswell 1995 Brueckner and Medaris 1998)Inclusions of coesite pseudomorph in garnet andomphacite from kyanite eclogites have beenrecently documented in the East Greenland eclogiteprovince (Gilotti and Ravna 2002) this new UHPHP terrane is remote but has excellent exposuresand requires close examination

Dabie-Sulu terrane of east-central China TheDabie-Sulu terrane occupies a Triassic collisionzone between the Sino-Korean and Yangtze plates(for a recent review see Hirajima and Nakamura2003) The central UHP coesite-bearing eclogitebelt (P = 27ndash5 GPa) is flanked to the north by amigmatite zone (P lt 2 GPa) and to the south by abelt of blueschist epidote amphibolite and eclog-ite-facies rocks (P = 05ndash12 GPa) Prevalent rocktypes include felsic gneiss orthogneiss marble andquartzite Protolith ages range from Proterozoic toOrdovician but the majority of reported ages areNeoproterozoic (Li et al 1993 Hacker et al 2000)Blocks boudins and layers of eclogites and garnetperidotites occur as enclaves in gneisses in the UHPunit The Dabie-Sulu UHP rocks show several char-acteristics (1) Widespread coesite with hydrousminerals such as talc zoisiteepidote and phengiteoccur in eclogites (2) Two distinct types of mantle-and crustal-derived garnet peridotites are present(3) Abundant exsolution textures were identified inUHP minerals from garnet peridotite and eclogite(4) Diamond separates from both eclogite and garnetperidotite have been reported from few restrictedregions (eg Xu et al 1992 2003) Some of thesefindings are not confirmed by thin-section observa-tion and extensive studies of mineral inclusions inzircon (for review see Liou et al 2002) (5) Somegarnet peridotites record much higher pressure thanassociated coesite-bearing eclogites reaching 5ndash6GPa (Yang et al 1993 Zhang R Y et al 20002003) (6) Various isotopic age dating methods forUHP minerals and rocks from Dabie-Sulu terranegives 220ndash240 Ma (Li et al 1993 Hacker et al2000)

Eclogites in the Huwan shear zone from thenorthwestern Dabie Mountains have SHRIMP UPbdates of 309 plusmn 3 Ma (Sun et al 2002) suggesting adiscrete Carboniferous subductioncollisionbetween North and South China Farther to the west

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 9

in North Qinling inclusions of microdiamond werediscovered in zircons from both eclogite and itsgneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyielded 507 plusmn 38 Ma for metamorphic rims and1200ndash1800 Ma for relic magmatic or old cores sug-gesting an Early Paleozoic UHP metamorphic eventThese new data suggest that episodic subductionand collision of continents occurred during stages ofclosures of paleo Tethys prior to the well-docu-mented major Triassic continent-continent collisionfor the Dabie-Sulu terrane

Kokchetav massif The Kokchetav massif north-ern Kazakhstan is the type site for the UHPdiamond-eclogite regional metamorphic facies(Sobolev and Shatsky 1990 Parkinson et al 2002Shatsky and Sobolev 2003) Diamonds occur asminute inclusions in garnet diopside and zirconporphyroblasts in marble pyroxene-carbonate-gar-net rock and garnet-biotite gneiss and schist Inclu-sions of coesite and coesite pseudomorphs occur ingarnet and zircon from eclogite and diamond-bear-ing gneiss The UHPHP unit extends NW-SE for atleast 80 km and is about 17 km wide The UHP slabis structurally overlain by a weak- to low-grademetamorphic unit and is underlain by a low-P meta-morphic andalusite-staurolite facies unit (Kaneko etal 2000) The UHP slab is composed of felsicgneiss with locally abundant eclogite lenses andminor orthogneiss metacarbonate and rare garnetperidotite with a range of Proterozoic protolith agesA well-constrained P-T-time path deduced frommineral inclusions and SHRIMP geochronology ofzircons is illustrated in Figure 4 Zoned zircons fromdiamond-bearing gneiss yield the following ages aquartz-bearing inherited core with 1100ndash1400 Maas the protolith age coesite-bearing mantle with 537plusmn 9 Ma for UHP metamorphism and the plagio-clase-bearing rim with 507 plusmn 8 Ma for amphibolite-facies overprint (Katayama et al 2001) Manydiamond-grade marbles contain clinopyroxene withexsolution lamellae of quartz K-feldspar phengiteand phlogopite (Katayama et al 2002 Zhu andOgasawara 2002) and titanite with coesite lamellae(Ogasawara et al 2002) these occurrences suggestthat UHP metamorphism occurred at P gt 6 GPa and1000degC

Cndash13 isotopic data of diamond suggest biogenicsources inclusions of water carbonate and nano-metric oxide in diamonds indicate that metamorphicmicrodiamonds in this and other UHP terranes wereprecipitated from C-O-H supercritical fluids (eg

Stockhert et al 2001) Ogasawara et al (2002)further suggested a two-stage growth mechanism formicrodiamonds Masago et al (2003) reported nega-tive 18O values (-39 per mil) for minerals in eclogiteand whiteschist of the Kokchetav Massif similar tothe Chinese UHP rocks first described by Yui et al(1995) The Kokchetav Massif is the second recog-nized UHP region that preserves a significant effectof the interaction of cold meteoric water with theprotolith prior to subduction

Saxonian Erzgebirge Germany The ErzgebirgeCrystalline Complex (ECC) at the northern margin ofthe Bohemian massif is in fault contact with a low-grade Paleozoic sequence at its margins (for a recentreview see Massonne and OrsquoBrien 2003) TheVariscan ECC consists of abundant gneisses thatinclude numerous lenses of eclogites The ldquoGneiss-Eclogite Unitrdquo at the core of this massif containsUHP rocks typified by inclusions of coesite pseudo-morph in garnets from eclogite (Schmadicke 1991)and inclusions of microdiamonds in garnet kyaniteand zircon from gneisses (Massonne 1999) Thusfar the diamondiferous gneisses seem to occur onlyin a 1 km long strip near the eastern shore of theSaidenbach Reservoir Diamond micro-inclusionsoccur exclusively in garnet kyanite and zircon ofseveral gneissic rocks Besides completely pre-served diamonds with grain sizes between 1 to 25microm partially graphitized diamonds and graphitepseudomorphs after diamond occur Such microdia-mond inclusions in garnet and zircon are similar tothose in Kokchetav diamondiferous biotite gneissSome of these inclusions contain additional fluid-bearing phases including apatite phengitic micaand possible fluid inclusions the morphologies ofinclusions and mineral associations suggest thatmicrodiamonds from both the Erzgebirge and theKokchetav were crystallized from supercritical flu-ids under UHP conditions (Stockhert et al 2001Hwang et al 2001 Dobrzhnetskaya et al 20012003)

Schmadicke et al (1995) reported Sm-Nd isoch-rons for garnet-Cpx-WR at 360 plusmn 7 Ma for eclogiteand 353 plusmn 6 Ma for garnet pyroxenite from Erzge-birge UHP rocks 40Ar39Ar spectra of phengite fromtwo eclogite samples give plateau ages of 348 plusmn 2and 355 plusmn 2 Ma Diamond-bearing zircons yieldSHRIMP UPb dates at 336 plusmn 2 Ma (Massonne2001) Such similarity in ages for garnet peridotitesand eclogite enclosed in gneiss suggests a coevalVariscan UHP in-situ metamorphism of the ECCaround 340ndash360 Ma

10 LIOU ET AL

Less Intensively Studied UHP terranes

Several UHP terranes described below exceptfor the Maksyvtov Complex have received lessintensive study mainly due to remoteness and diffi-cult access despite their long recognition

Maksyutov Complex Southern Urals TheMaksyutov Complex is an elongated (10 times 120 km)N-S belt between the Russian and Siberian cratonsin the southern Ural Mountains (Coleman and Wang1995 Dobretsov et al 1996) and contains blocky

graphitized diamond() in pelitic schist (Leech andErnst 1998) The complex is the locality from whichquartz aggregates within garnet exhibiting radialcracks were first recognized as coesite-pseudo-morphs by Chesnokov and Popov (1965) nearly 20years prior to the independent finding of coesiteinclusions by Chopin (1984) and Smith (1984) Theeclogite-bearing unit contains boudins of eclogitelayers of eclogitic gneiss and rare ultramafic bodieswithin host metasedimentary mica schist andquartzite

FIG 4 A Schematic diagram showing the growth of detrital zircon (stage 1) through subduction (stage 2 and peakstage 3) and exhumation (stage 4) of supracrustal rocks Mineral inclusions in various stages of zircon growth are shownB SHRIMP U-Pb dating of zoned zircons from a Kokchetav diamond-bearing gneiss yields a well constrained P-T-timepath (for details see Katayama et al 2001)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 11

Some Maksyutov eclogites were recrystallized at594ndash637degC with a minimum pressure from 15 to17 GPa (Beane et al 1995 Leech and Ernst 1998)or up to 27 GPa if coesite pseudomorphs (Ches-nokov and Popov 1965 Dobretsov and Doretsova1988) are indeed present However subsequentstudies have not revealed the occurrence of inclu-sions of coesite or coesite pseudomorph in garnetand zircon (Beane et al 1995 Shatsky et al 1995)Leech and Ernst (1998) described unusual graphiteaggregates (up to 13 mm edge length) that may bediamond pseudomorphs This suggestion was basedon the blocky morphology of the cuboid graphiteaggregates cross-cutting rock foliation pressureshadows biogenic isotopic signatures and otherspectroscopic characters similar to those graphi-tized diamonds from the Beni Bousera massifRecent study of several microdiamond inclusions ingarnet has yielded further suggestive possibility thatthe eclogitic unit of the Maksyutov Complex experi-enced diamond-grade UHP metamorphism (Bosticket al 2003) The UHPHP metamorphism with peakmetamorphic ages of 370ndash374 Ma (Shatsky et al1995 Beane and Connelly 1998) may be related tothe collision of the Russian platform with a fragmentof the Siberian craton during Devonian time butwas affected by the Triassic collision of the Kazakh-stan block with the Russian platform (eg Matte1998)

Zermatt-Saas area Western Alps In contrast tomost other UHP terranes in which the protolith iscontinental crust eclogites and metasediments atLago di Cignana of the Western Alps were derivedfrom oceanic crust and constitute the ophiolitesequence of the Zermatt-Saas zone Inclusions ofcoesite and coesite pseudomorphs occur in tourma-line and garnet of manganiferous quartzite and inomphacite and garnet of the underlying mafic eclog-ite (Reinecke 1991) The metasediments are mainlygarnet-phengite-quartz schist with variableamounts of garnet-clinopyroxene quartzite andpiedmontite-phengite-quartz schist Eclogites andretrograded eclogites with preserved UHP mineralparageneses consist of garnet omphacite glau-cophane zoisite phengite and dolomite Zoned gar-nets preserve prograde quartz crystals in their coresand coesite inclusions in mantles and rims Coesiteinclusions are much more common in omphacitethan in garnet UHP conditions were estimated at26ndash30 GPa and 590ndash630degC Sm-Nd isotopic anal-yses of eclogites yield UHP metamorphism at 52 plusmn18 Ma (Bowtell et al 1994) consistent with a

Tertiary age of eclogitization recently established forAlpine metamorphism in the Western Alps(Gebauer et al 1997)

Thus far the recognized areal extent of thecoesite-bearing rocks is ~ 2 km2 The apparentthickness of the UHP slice does not exceed a fewhundred meters This area is the best example forsubduction of oceanic lithosphere to mantle depthand later exhumation as a fragmented tectonic slabSimilarly the Allalin metagabbro part of the oce-anic Piemonte zone contains magnesite + talc +kyanite + garnet recrystallized at ~600degC and 35GPa with an estimated geothermal gradient of 5ndash6degC kmndash1 however inclusions of coesite or coesitepseudomorphs have not been reported from theAllalin metagabbro

Mali Africa Eclogitic rocks in the Pan-Africancollision zone cropping out in northern Mali containinclusions of coesite in omphacite from mafic nod-ules within a calc-silicate layer enclosed in impuremarble and coesite pseudomorphs in garnets fromthe surrounding eclogitic metasediments (Caby1994) These UHP eclogitic mica schists have P-Testimates of 700ndash750degC and gt 27 GPa phengiteyields a well-defined plateau 39Ar40Ar age of 1045plusmn 9 Ma Recent SmNd ages of 620 Ma wereobtained by Jahn et al (2001) for peak UHP meta-morphism This may be the oldest UHP terrane inthe world

The coesite-bearing UHP eclogitic unit about 3km thick lies within an internal nappe as a flat thinslice bounded on the top by a thick passive-marginshaly formation and on the bottom by low-gradegreenschist-facies phyllites Caby (1994) suggestedthat a large portion of the terrigenous metasedimentsof passive-continental-margin affinity of the WestAfrican plate was subducted eastward to mantledepths (gt90 km) beneath an oceanic domain Colli-sion of the West African plate with an island arcresulted in a low-angle westward subhorizontalextrusion of this subducted slice of sialic materialsto its present position The coesite-bearing eclogiteunit may also crop out 1500 km to the south in Togowhere kyanite-bearing eclogites occur as part of thepassive-margin assemblage

Makbal (480 Ma) and Atbashy (270 Ma) Kazak-hstan The Tian-Shan is divided into three mountainrangesmdashnorthern middle and southern Within theKyrgyzstan Tian-Shan UHP eclogite localities withcoesite pseudomorph inclusions in garnet andomphacite from the northern and southern Tian-Shan represent Caledonian and Hercynian orogenic

12 LIOU ET AL

belts respectively (Tagiri and Bakirov 1990 Tagiriet al 1995) The Makbal Formation in the westernKyrgyz Ridge of the northern Tian-Shan consists ofquartzose schist alternating with pelitic schistsintercalated with thin layers or lenses of marbleeclogite and amphibolitized eclogite Tagiri andBakirov (1990) found inclusions of coesite pseudo-morphs in garnet from a garnet-chloritoid-talc schistwith a peak metamorphic assemblage of coesite +almandine + chloritoid + talc + phengite (Si = 342pfu) + phlogopite + rutile Makbal coesite-gradeeclogite has a paragonite KAr age of 480 Ma

The eclogite-bearing complex of Atbashy Ridgefrom the southern Tian-Shan is composed of peliticand siliceous schists alternating with thin UHPeclogitic layers and is unconformably overlain byUpper Paleozoic molasse and silicic volcanic rocksInclusions of coesite pseudomorphs occur inomphacite cores and in garnet Eclogites from theAtbashy area preserve prograde and retrogradepaths with a peak metamorphic condition at 660degC25 GPa The Atbashy coesite-eclogites yielded aRbSr mineral isochron age of 270 Ma (Tagiri et al1995)

Central Indonesia UHP terrane The pre-Tertiarybasement of the Indonesian region comprises a vari-ety of imbricate terranes meacutelange ophiolite andvariably metamorphosed accretionary complexessome contain HP to UHP metamorphic rocks result-ing from the collision of an Australia-derived conti-nent with Eurasia (Parkinson 2003) HP rocksincluding eclogites and garnet peridotites are widelydistributed in Cretaceous accretionary complexesMany of these rocks occur as imbricate slices of car-bonate quartzite and pelitic schist of shallow-marine or continental-margin parentage interthrustwith subordinate mafic schist and serpentinitesome yield mica KAr ages of 110ndash120 Ma (Parkin-son 2003)

HP and UHP rocks are sporadically exposed astectonic blocks throughout the Cretaceous accre-tionary complexes They include eclogite garnet-glaucophane rock (P = 18ndash24 GPa T = 580ndash620degC) and jadeite-garnet quartzite (P gt 27 GPa T =720ndash760degC) in Bantimala southwest Sulawesieclogite and garnet granulite in west-centralSulawesi eclogite and jadeite-glaucophane-quartzrock (P ~22 GPa T ~530degC) in central Java Mgchloritoidndashbearing whiteschists (P ~18 GPa) insoutheast Kalimantan garnet lherzolites in east-central Sulawesi (P = 22ndash28 GPa T = 1000ndash1100degC) west-central Sulawesi (P = 16ndash20 GPa T =

1050ndash1100degC) and garnet pyroxenite (P ~2 GPa T~ 850degC) in Sabah northeast Borneo Evidence forUHP rocks include (1) inclusions of coesite inzircon and coesite pseudomorphs in jadeite fromjadeite quartzite and eclogite of the BantimalaComplex of south Sulawesi (Parkinson 2003) and(2) P-T estimates of peak-stage recrystallization at27ndash35 GPa and 1000ndash1100degC for most garnetperidotites (Kardarusman and Parkinson 2000)Many of these rocks were probably recrystallized ina N-dipping subduction zone at the margin of theSundaland craton in the Early Cretaceous Exhuma-tion may have been facilitated by the collision of aGondwana continental fragment with the Sundalandmargin at ~120ndash115 Ma

Recently Recognized UHP Terranes

Several new UHP terranes were recently identi-fied inasmuch as they contain partially preservedtrace index minerals in strong containers such aszircon or garnet In fact zircon has been consideredto be the best container and many new terranes werediscovered through positive identification of inclu-sions of coesite or coesite pseudomorphs ordiamond in zircons Detailed examination of mineralinclusions in core mantle and rims of zircon sepa-rates from eclogites and their enclosing country-rock gneisses have yielded both prograde andretrograde P-T-time paths for various UHP terranesmentioned above (see Katayama et al 2001 fordetails also see the section on the Himalayan eclog-ite for an example)

The latest recognized UHP terranes include thefollowing (1) coesite in the French Massif Central(Lardeaux et al 2001) (2) coesite and diamondfrom the Greek Rhodope metamorphic province(Mposkos and Kostopoulos 2001) (3) coesitepseudomorphs in the Northeast Greenland EclogiteProvince (Gilotti and Ravna 2002) (4) coesite ingranulite-facies overprinting eclogites of SoutheastBrazil (Parkinson et al 2001) (5) coesite in Hima-layan eclogite from the Upper Kaghan Valley Paki-stan (OrsquoBrien et al 2001 Kaneko et al 2003) andcoesite from the Tso-Morari crystalline complex ofIndia (Mukherjee et al 2003) (6) coesite inclusionsin gneissic rocks from the North Qaidam belt west-ern China (Yang et al 2001 Song et al 2003) (7)diamond inclusions in garnets of eclogite and gneissfrom North Qinling (Yang et al 2002a) (8) inclu-sions of quartz pseudomorphs and minor coesite ingarnets of mafic eclogites from the LantermanRange in Antarctica (Ghiribelli et al 2002) (9)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 13

inclusions of calcite pseudomorphs after aragonite +magnesite in dolomite in metapelites from the west-ern Tian-Shan inferred as metamorphism in a UHPregion of dolomite decomposition (Zhang L et al2002) and (10) inclusion of possible coesitepseudomorphs in garnet lamellar inclusions in gar-net kyanite and high sodium content of garnet fromeclogites of the Chuacus Complex of north-centralGuatemala (Solari et al 2003) The results of petro-chemical and geochronological data for these newterranes are summarized below The HimalayanUHP terrane in the Upper Kaghan Valley and theTso-Morari complex are used to illustrate the pro-cesses of continental subduction and collision in alater section

French Massif Central The French Massif Cen-tral in the western part of the Variscan Belt hasexperienced Late SilurianndashEarly Devonian to LateCarboniferous orogenic events (eg Matte 1986)High-pressure metamorphic rocks occur mainly inthe upper gneissic unit within the leptyno-amphibo-lite group The latter consists of an association ofmetagreywacke mica schist metabasalt leptynitemetagranite and peridotite Eclogites are associatedwith silicic and mafic HP granulites (Lardeaux etal 2001) and also with spinel andor garnet lherzo-lites (Gardien et al 1990) Inclusions of coesite andcoesite pseudomorphs occur in garnets of kyanite-bearing eclogite (Lardeaux et al 2001) Only twocoesite grains are preserved as relics most coesitegrains are completely transformed into polycrystal-line radial quartz (palisade texture) or into polygonalquartz surrounded by radial cracks Metamorphictemperatures were estimated to be 740-780degC andsubsequent amphibolite-facies overprinting at750degC at 15ndash17 GPa during decompression UHPmetamorphism occurred between 420 and 400 Ma(Paquette et al 1995) 40Ar39Ar data from amphib-ole separates from retrogressed eclogites yielded339 plusmn 4 Ma (Costa et al 1993)

Northeast Greenland Caledonides The first evi-dence for UHP metamorphism in the GreenlandCaledonites was reported from kyanite eclogites andassociated host gneisses on an island in Jokelbugt(Gilotti and Ravna 2002) Inclusions of quartzpseudomorphs exhibiting palisade structure andradiating fractures occur in garnet and omphacite ofeclogites and in garnet of the host gneisses P-T esti-mates of peak-stage metamorphism of eclogite at~972degC and 36 GPa lie well within the coesite sta-bility field SHRIMP UPb dates of 403 plusmn 5 Ma forzircon rims from HP metapsammite and 404 plusmn 4 Ma

for zircons from anatectic melt derived frommetapelite are coeval with the estimated ages ofexhumation of UHP terranes in the ScandinavianCaledonides described in a previous section(McClelland and Gilotti 2003)

Neoproterozoic nappes in Southeast BrazilNeoproterozoic nappes of Southeast Brazil consistmainly of coarse-grained kyanite-garnet granulitewith intercalations of impure quartzite calc-silicaterock and minor lenses and sills of mafic-ultramaficrocks including eclogite Early eclogitic phaseswere strongly obliterated during granulite-faciesoverprinting during a major Pan-African continentcollision at 640ndash630 Ma (Campos Neto and Caby1999) Rare ultramafic lenses consist of phlogopite-orthopyroxene rocks and garnet clinopyroxeniteCores of granulite boudins retain eclogitic assem-blages some quartz inclusions within the mantleregions of zoned garnet porphyroblasts are sur-rounded by intense radial fractures suggestive ofthe former presence of coesite In fact coarse-grained zircons in some granulites contain numer-ous micro-inclusions of K-feldspar + kyanite + coes-ite as confirmed by Raman spectroscopy (Parkinsonet al 2001) Many exsolution microstructuresobserved in other UHP terranes also occur Thisarea represents the first coesite-grade UHP rocks inthe Americas and it can be correlated with the MaliUHP terrane of equatorial Africa (Caby 1994) Ithas been suggested that eclogitic assemblages andprobable UHPM components may be present in sev-eral other correlative nappe systems at the marginsof the Sao Francisco craton in Brazil

Chuacus Complex north-central Guatemala HPeclogites within banded gneisses schists andmigmatites of the Chuacus Complex contain relicttextures and mineralogical features that suggestUHP metamorphism (Solari et al 2003) Theseinclude inclusions of possible coesite pseudomor-phs in garnet and kyanite lamellar inclusions ofrutileilmenite in garnet high sodium content ofsome garnets (up to 1200 ppm) and preliminary P-T estimates of 700ndash800degC and 2ndash3 GPa AlthoughK-Ar dating of mica and hornblende yield ages of48ndash72 Ma possible for amphibolite facies retrogres-sion regional and structural data support a pre-Mesozoic age for the multi-stage HP-UHP recrystal-lization Such a discovery opens up a new interpre-tation regarding the tectonic evolution of Maya-Chortis-Oaxaquia microcontinental blocks formingthe structural backbone of Mesoamerica

14 LIOU ET AL

Greek Rhodope The Greek Rhodope Metamor-phic Province (RMP) at the border between Greeceand Bulgaria represents a syn-metamorphic nappesystem of Alpine age that was formed during theCretaceous to Mid-Tertiary collision of Apulia andpaleo-Europe The Kimi nappe complex comprisescrustal eclogites leucocratic orthogneisses peliticgneisses and volumetrically minor mantle-derivedultramafic rocks these units contain many mineral-ogical indicators of UHP metamorphism (Mposkosand Kostopoulos 2001) The ultramafic lithologiesinclude garnet spinelndashbearing lherzolite and layersof spinel-garnet clinopyroxenite garnet pyroxeniteand clinopyroxene garnetite These rocks were sub-jected to UHP recrystallization at P-T conditions of~1200degC and gt30 GPa The pelitic gneiss containsmicrodiamond inclusions in garnet porphyroblastsas well as rods or needles of silica rutile biotiteand apatite in sodic garnet suggesting the prioroccurrence of majoritic garnet The growth of suchsupersilicic garnet was suggested to be at a pressureof ~7 GPa Inclusions of polycrystalline quartzaggregates surrounded by radial cracks occur ineclogitic garnet Exsolution lamellae of quartz ineclogitic clinopyroxene also indicate an UHP pre-cursor of supersilicic clinopyroxene

Lanterman Range Antarctica The LantermanRange in northern Victoria Land of Antarctica con-sists of several metamorphic complexes The Gate-way Hills Metamorphic Complex is a thindiscontinuous belt more than 50 km long consistingof mafic and ultramafic rocks including lenses andpods of eclogite within felsic gneiss Some less foli-ated eclogites contain inclusions of coesite andcoesite pseudomorphs in garnet porphyroblasts asconfirmed by in situ Raman spectroscopy as well asradial fractures around the inclusions (Ghiribelli etal 2002) The eclogites were subjected to UHPmetamorphism at T gt 850degC and P gt 29 GPa thenamphibolite-facies overprinting during isothermaldecompression The UHP event is dated by SmNdand 238U206Pb data at 500 Ma 40Ar39Ar ages of490ndash486 Ma for Ca-amphibole (amphibolite-faciesoverprinting) suggest rapid cooling with an averageexhumation rate of 3ndash4 km_Ma-1 for the UHP ter-rane (Di Vincenzo and Palmeri 2001)

Western Tian-Shan China Caledonian eclogitesof the western Tian-Shan China have been classi-fied into three types (Zhang L et al 2002) Type Ieclogite pods are layered with mafic blueschists andType II eclogites preserved pillow structureswhereas Type III eclogites are banded with marbles

This 200 km long eclogite-bearing belt extendswestward to connect with the Atbashy UHP belt inKazakhstan (Tagiri et al 1995) Reported UHP evi-dence includes inclusions of coesite pseudomorphsin garnets from Type I and III eclogites quartzexsolution lamellae in omphacite from Type IIeclogites and relict magnesite within matrix dolo-mite in Type III eclogites (Zhang J X et al 2002Zhang L et al 2002) P-T estimates of the peakUHP stage of metamorphism are T = 500ndash600degCand P = 26ndash27 GPa (Wei et al 2003)

North Qaidam Mountains western China Manylenses and layers of garnet peridotite and eclogiteoccur in an amphibolite-facies terrane in the NorthQaidam Mountains along the northeastern rim of theQaidam Basin This belt comprises mainly felsicgneiss quartz schist garnet-bearing gneissamphibolite and minor eclogite + garnet peridotiteit extends westward for more than 1000 km to theAltun Mountains and was displaced by the left-lat-eral Altyn-Tagh fault At the eastern end of thisUHP belt at Dulan occurrences of coesite inclu-sions in zircon separates from paragneiss inclusionsof coesite pseudomorph in garnet and omphacite ofeclogites quartz rods in eclogitic omphacite andP-T estimates of eclogites (P = 287ndash317 GPa andT = 631ndash687degC) demonstrate that this is a typicalUHP metamorphic terrane (Yang et al 2001 Songet al 2003a 2003b) The Dulan eclogites have anSm-Nd mineral isochron age of 497 plusmn 87 Ma and aSHRIMP UPb zircon age of 495 Ma (Yang et al2002b)

Altun Mountains western China This UHP ter-rane about 200 km long in the southern margin ofthe Altun Mountains is the western extension of theNorth Qaidam belt Eclogitic lenses and blocks ofvarious sizes together with garnet-lherzolite and cli-nopyroxenite are enclosed within an Early Protero-zoic sequence of amphibolite amphibole schistgarnet-bearing granitic gneiss and pelitic schistGarnet lherzolite contains magnesite that reacted toform dolomite P-T estimates of the peak magnesite-bearing assemblage yield P gt 36 GPa and 850degC(Liu et al 2001) Garnet porphyroblasts includeexsolution lamellae of clinopyroxene and rutile sim-ilar to that of eclogite reported by Ye et al (2000) asmajoritic garnet The associated eclogites containinclusions of coesite pseudomorphs in garnet withcharacteristic radial fractures this together with theexsolved quartz rods in omphacite and P-T esti-mates of 820ndash850degC and 28 to 32 GPa provideadditional evidence for UHP (Zhang J X et al

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 15

2001) An SmNd mineral isochron for eclogiticgarnet omphacite and the whole rock yields 500 plusmn10 Ma whereas an UPb isochron for zircon sepa-rates from eclogite gives an age of 504 plusmn 5 Ma(Zhang J X et al 2000)

North Qinling Mountains central China Lensesand blocks of eclogitic rocks in northern Qinling areenclosed in amphibolite-facies gneiss and garnet-bearing quartz + phengite mica schist Inclusions ofcoesite in eclogitic garnet (Hu et al 1995) andmicrodiamonds in zircon separates from both eclog-ite and gneiss (Yang et al 2002a) have beenrecently reported Although individual outcrops areno more than a few meters wide the belt of eclogitesextends more than 10 km P-T estimates of amphib-olitized coesite-bearing eclogite yield T = 590ndash758degC and P gt 26 GPa An Sm-Nd isochron for gar-net omphacite rutile amphibole and whole-rockyields 400 plusmn 16 Ma Microdiamonds were recentlydiscovered as inclusions in zircon from both eclogiteand its gneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyields 507 plusmn 38 Ma for peak UHP metamorphism(Yang et al 2002a) These data indicate that two

discrete collision events occurred between theNorth China and Yangtze cratons (1) an Early Pale-ozoic North Qinling UHP suturing and (2) a Triassicsuturing along the Dabie-Sulu UHP-HP belt TheEarly Paleozoic belt extends westward to the NorthQaidamndashAltun Mountains for more than 4000 km(Yang et al 2002b)

Himalayan UHP Eclogites

The Himalayan orogen has long been considereda classic locality of continental collision betweenthe Indian and Eurasian continents (Fig 5) Severaleclogite-facies HP rocks in the Himalayas havebeen reported since the late 1980s (eg Chaudaryand Ghazanfar 1987 Pognante and Spencer 1991de Sigoyer et al 1997 Lombardo and Rolfo 2000)Inclusions of coesite were recently discovered inomphacite and garnet in several Himalayan eclog-ites from the upper Kaghan Valley Pakistan(OrsquoBrien et al 2001 Kaneko et al 2003) Subse-quent findings of additional coesite localities in theTso Morari area (Sachan et al 2001 Mukherjee etal 2003) have established the presence of a UHPbelt in the Himalayas an active ongoing collision

FIG 5 A simplified tectonic map of the western Himalaya Hindu Kush Pamir Karakoran and western Tibet mod-ified after Figure 3 of Searle et al (2001) This map shows the locations of eclogite and UHP eclogites from both Hima-layan and North Qaidam Indus Suture Zone (ISZ) ophiolites and major structures

16 LIOU ET AL

since the Eocene (Massonne and OrsquoBrien 2003)However UHP relics are volumetrically very minorconstituents of the metamorphic belts dominatedmostly by later Barrovian overprintings In theserocks hydration related to Barrovian-zone metamor-phism has resulted in remarkably pervasive recrys-tallization during the later exhumation this eventhas almost entirely masked the preceding UHPrecord

The finding of HP-UHP evidence has led to newinsights into the Himalayan tectonic modelEngland and Houseman (1986) showed that in amodel of homogeneous thickening of the Hima-layan-Tibet boundary the maximum crustal thick-ness should be less than 70 km However theoccurrences of coesite-bearing eclogites andgneisses in the Himalayan orogen cannot beexplained by a homogeneous thickening of the crustbut rather by continental subduction as describedbelow According to available geochronologicaldata (eg Treloar et al 2003) the exhumation rateof the Himalayan UHP rocks has been estimated torange from ~1 cm yrndash1 (OrsquoBrien et al 2001) to gt3cm yrndash1 (Treloar et al 2003 Massone and OrsquoBrien2003) These estimates are higher than those calcu-lated from other UHP-HP terranes This requirestectonic emplacement of the UHP-HP Himalayanslab into shallow crust following isostatic rebounddue to slab decoupling between the Indian continentand the oceanic crust initiated during the MiddleEocene UHP rocks from both the Kaghan Valleyand Tso-Mori are described below

Kaghan Valley The Kaghan nappe in the north-western Himalayan syntaxis lies between the Indussuture (also called main mantle thrust MMT) to thenorth and the main central thrust (MCT) to thesouthwest (Pognante and Spear 1991 Spencer1993) (Fig 5) The Cretaceous Kohistan island-arcsequence lies north of the Indus suture It developedas a result of intra-oceanic subduction The maincentral thrust separates an underlying Salkhala unitfrom an overlying higher Himalayan crystalline(HHC) unit The Kaghan nappe consists mainly ofgranitic gneiss and paragneiss with minor inter-calated layers of amphibolite marble and quartziteEclogites are preserved in the cores of maficboudins (less than 1 m thick) in gneiss and aresignificantly overprinted by amphibolite assem-blages They contain fine-grained (lt 1 mm) garnetand omphacite together with aggregates of quartzchains of rutile rimmed by titanite and mm-sizedphengitic micas overgrown by randomly oriented

amphiboles up to 1 cm in length Coesite and coesitepseudomorphs occur as inclusions in omphaciteand show the characteristic palisade-quartz aggre-gate texture fractures radiating from these inclu-sions are characteristically developed in the hostpyroxene

Garnet pyroxenendashphengite barometry wasapplied to the eclogites and yielded peak P-T con-ditions of 27ndash29 GPa and 690ndash750degC (OrsquoBrien etal 2001) (Fig 6) Retrograde textures are commonin the eclogites including symplectic intergrowthsof augite amphibole and plagioclase after omphac-ite thermobarometric analyses of these assemblagesyielded 10ndash13 GPa and 600ndash710degC (Kaneko et al2003) On the other hand the dominant graniticgneisses and metapelites of the higher Himalayancrystalline unit contain typical Barrovian-zoneamphibolite-facies assemblages represented bykyanite- and staurolite-bearing garnetiferousmetapelites (Treloar 1995) Extensive overprintinghas mostly obliterated the peak assemblages matrixminerals of the gneisses yielded P-T conditions of07ndash11 GPa and 600ndash700degC (Treloar 1995) con-sistent with retrogression of the eclogites Howeverrelict coesite is preserved as inclusions in zirconthe inclusions are ovoid and approximately 10 microm indiameter (Fig 7A) This discovery of coesite-bear-ing zircons from Himalayan quartzofeldspathicgneisses demonstrates subduction of Indian conti-nental crust to the depths of UHP conditions(Kaneko et al 2003)

Geochronological studies using SmNd (garnet-omphacite) RbSr (phengite) and UPb (rutile zir-con) methods suggest that the eclogite-facies eventtook place at 40ndash50 Ma (Tonarini et al 1993 Spen-cer and Gebauer 1996) 40Ar39Ar cooling ages forhornblende and mica from gneisses were reported tobe ~43 Ma and ~25 Ma respectively (Chamberlainet al 1991) Recent SHRIMP UPb dates of zonedzircons with mineral inclusions formed at differentstages indicate the ages of non-UHP mineral-bear-ing mantle domains of zircon and UHP mineral-bearing rims are at about 50 and 46 Ma respectively(Kaneko et al 2003) A new U-Pb age of 44 plusmn 11Ma was obtained for rutile from Kaghan coesite-bearing eclogite (Treloar et al 2003) this togetherwith available data implies that exhumation of theUHP rocks from mantle depth to mid-crustal leveloccurs within a few million years

Tso Morari The Tso Morari crystalline dome islocated between the Indus suture zone on the northbordered by the Zildat detachment fault and the

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 17

Zanskar sedimentary unit on the south (de Sigoyer etal 1997) The dome represents an internal massif ofthe northern part of the higher Himalaya The TsoMorari area has been considered a typical IndianTethyan margin because of a stratigraphic succes-sion similar to the Nimaling antiform in the core ofthe dome a metamorphic basement consists of Cam-bro-Ordovician augen gneisses (Stutz 1988) Thiscomplex is covered by a metasedimentary series ofCambrian to Devonian quartzite schist and con-glomerate and is overlain by Lower Carboniferousto Triassic marble and metapelite Eclogites occuras lenses or irregular bodies within the basementand cover gneisses and are strongly overprinted byamphibolite-facies assemblages de Sigoyer et al(1997) reported peak P-T conditions for the eclog-ites at 17ndash23 GPa and 520ndash640degC (Fig 7) How-ever relics of coesite 30ndash80 microm in size were

recently found as inclusions in garnet (Sachan et al2001 Mukherjee et al 2003) typified by radialfractures (Figs 7B and 7C) for the SiO2 inclusionsa characteristic Raman spectrum is centered at 523cm-1 (Fig 7D) The country rock metasedimentswith jadeite + chloritoid + paragonite + garnetassemblages have peak P-T estimates of 18ndash22GPa and 500ndash600degC and record retrogression at theeclogite-blueschist facies transition of 13ndash18 GPaand 490ndash590degC (Guillot et al 1997)

The geochronology of the Tso Morari eclogiteshas been extensively studied and subdivided intothree different stages (de Signoyer et al 2000)Eclogitization at ~55 Ma was obtained by Lu-Hf andSm-Nd methods on garnet omphacite glaucophaneand the whole rock An amphibolite-facies overprintoccurred at 47 Ma judging by RbSr dating onphengite apatite and whole rock from metapelites

FIG 6 P-T estimates of peak- and retrograde-stage metamorphism for coesite-bearing eclogites and the country-rockgneisses from the Kaghan Valley and Tso Morari

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 9

in North Qinling inclusions of microdiamond werediscovered in zircons from both eclogite and itsgneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyielded 507 plusmn 38 Ma for metamorphic rims and1200ndash1800 Ma for relic magmatic or old cores sug-gesting an Early Paleozoic UHP metamorphic eventThese new data suggest that episodic subductionand collision of continents occurred during stages ofclosures of paleo Tethys prior to the well-docu-mented major Triassic continent-continent collisionfor the Dabie-Sulu terrane

Kokchetav massif The Kokchetav massif north-ern Kazakhstan is the type site for the UHPdiamond-eclogite regional metamorphic facies(Sobolev and Shatsky 1990 Parkinson et al 2002Shatsky and Sobolev 2003) Diamonds occur asminute inclusions in garnet diopside and zirconporphyroblasts in marble pyroxene-carbonate-gar-net rock and garnet-biotite gneiss and schist Inclu-sions of coesite and coesite pseudomorphs occur ingarnet and zircon from eclogite and diamond-bear-ing gneiss The UHPHP unit extends NW-SE for atleast 80 km and is about 17 km wide The UHP slabis structurally overlain by a weak- to low-grademetamorphic unit and is underlain by a low-P meta-morphic andalusite-staurolite facies unit (Kaneko etal 2000) The UHP slab is composed of felsicgneiss with locally abundant eclogite lenses andminor orthogneiss metacarbonate and rare garnetperidotite with a range of Proterozoic protolith agesA well-constrained P-T-time path deduced frommineral inclusions and SHRIMP geochronology ofzircons is illustrated in Figure 4 Zoned zircons fromdiamond-bearing gneiss yield the following ages aquartz-bearing inherited core with 1100ndash1400 Maas the protolith age coesite-bearing mantle with 537plusmn 9 Ma for UHP metamorphism and the plagio-clase-bearing rim with 507 plusmn 8 Ma for amphibolite-facies overprint (Katayama et al 2001) Manydiamond-grade marbles contain clinopyroxene withexsolution lamellae of quartz K-feldspar phengiteand phlogopite (Katayama et al 2002 Zhu andOgasawara 2002) and titanite with coesite lamellae(Ogasawara et al 2002) these occurrences suggestthat UHP metamorphism occurred at P gt 6 GPa and1000degC

Cndash13 isotopic data of diamond suggest biogenicsources inclusions of water carbonate and nano-metric oxide in diamonds indicate that metamorphicmicrodiamonds in this and other UHP terranes wereprecipitated from C-O-H supercritical fluids (eg

Stockhert et al 2001) Ogasawara et al (2002)further suggested a two-stage growth mechanism formicrodiamonds Masago et al (2003) reported nega-tive 18O values (-39 per mil) for minerals in eclogiteand whiteschist of the Kokchetav Massif similar tothe Chinese UHP rocks first described by Yui et al(1995) The Kokchetav Massif is the second recog-nized UHP region that preserves a significant effectof the interaction of cold meteoric water with theprotolith prior to subduction

Saxonian Erzgebirge Germany The ErzgebirgeCrystalline Complex (ECC) at the northern margin ofthe Bohemian massif is in fault contact with a low-grade Paleozoic sequence at its margins (for a recentreview see Massonne and OrsquoBrien 2003) TheVariscan ECC consists of abundant gneisses thatinclude numerous lenses of eclogites The ldquoGneiss-Eclogite Unitrdquo at the core of this massif containsUHP rocks typified by inclusions of coesite pseudo-morph in garnets from eclogite (Schmadicke 1991)and inclusions of microdiamonds in garnet kyaniteand zircon from gneisses (Massonne 1999) Thusfar the diamondiferous gneisses seem to occur onlyin a 1 km long strip near the eastern shore of theSaidenbach Reservoir Diamond micro-inclusionsoccur exclusively in garnet kyanite and zircon ofseveral gneissic rocks Besides completely pre-served diamonds with grain sizes between 1 to 25microm partially graphitized diamonds and graphitepseudomorphs after diamond occur Such microdia-mond inclusions in garnet and zircon are similar tothose in Kokchetav diamondiferous biotite gneissSome of these inclusions contain additional fluid-bearing phases including apatite phengitic micaand possible fluid inclusions the morphologies ofinclusions and mineral associations suggest thatmicrodiamonds from both the Erzgebirge and theKokchetav were crystallized from supercritical flu-ids under UHP conditions (Stockhert et al 2001Hwang et al 2001 Dobrzhnetskaya et al 20012003)

Schmadicke et al (1995) reported Sm-Nd isoch-rons for garnet-Cpx-WR at 360 plusmn 7 Ma for eclogiteand 353 plusmn 6 Ma for garnet pyroxenite from Erzge-birge UHP rocks 40Ar39Ar spectra of phengite fromtwo eclogite samples give plateau ages of 348 plusmn 2and 355 plusmn 2 Ma Diamond-bearing zircons yieldSHRIMP UPb dates at 336 plusmn 2 Ma (Massonne2001) Such similarity in ages for garnet peridotitesand eclogite enclosed in gneiss suggests a coevalVariscan UHP in-situ metamorphism of the ECCaround 340ndash360 Ma

10 LIOU ET AL

Less Intensively Studied UHP terranes

Several UHP terranes described below exceptfor the Maksyvtov Complex have received lessintensive study mainly due to remoteness and diffi-cult access despite their long recognition

Maksyutov Complex Southern Urals TheMaksyutov Complex is an elongated (10 times 120 km)N-S belt between the Russian and Siberian cratonsin the southern Ural Mountains (Coleman and Wang1995 Dobretsov et al 1996) and contains blocky

graphitized diamond() in pelitic schist (Leech andErnst 1998) The complex is the locality from whichquartz aggregates within garnet exhibiting radialcracks were first recognized as coesite-pseudo-morphs by Chesnokov and Popov (1965) nearly 20years prior to the independent finding of coesiteinclusions by Chopin (1984) and Smith (1984) Theeclogite-bearing unit contains boudins of eclogitelayers of eclogitic gneiss and rare ultramafic bodieswithin host metasedimentary mica schist andquartzite

FIG 4 A Schematic diagram showing the growth of detrital zircon (stage 1) through subduction (stage 2 and peakstage 3) and exhumation (stage 4) of supracrustal rocks Mineral inclusions in various stages of zircon growth are shownB SHRIMP U-Pb dating of zoned zircons from a Kokchetav diamond-bearing gneiss yields a well constrained P-T-timepath (for details see Katayama et al 2001)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 11

Some Maksyutov eclogites were recrystallized at594ndash637degC with a minimum pressure from 15 to17 GPa (Beane et al 1995 Leech and Ernst 1998)or up to 27 GPa if coesite pseudomorphs (Ches-nokov and Popov 1965 Dobretsov and Doretsova1988) are indeed present However subsequentstudies have not revealed the occurrence of inclu-sions of coesite or coesite pseudomorph in garnetand zircon (Beane et al 1995 Shatsky et al 1995)Leech and Ernst (1998) described unusual graphiteaggregates (up to 13 mm edge length) that may bediamond pseudomorphs This suggestion was basedon the blocky morphology of the cuboid graphiteaggregates cross-cutting rock foliation pressureshadows biogenic isotopic signatures and otherspectroscopic characters similar to those graphi-tized diamonds from the Beni Bousera massifRecent study of several microdiamond inclusions ingarnet has yielded further suggestive possibility thatthe eclogitic unit of the Maksyutov Complex experi-enced diamond-grade UHP metamorphism (Bosticket al 2003) The UHPHP metamorphism with peakmetamorphic ages of 370ndash374 Ma (Shatsky et al1995 Beane and Connelly 1998) may be related tothe collision of the Russian platform with a fragmentof the Siberian craton during Devonian time butwas affected by the Triassic collision of the Kazakh-stan block with the Russian platform (eg Matte1998)

Zermatt-Saas area Western Alps In contrast tomost other UHP terranes in which the protolith iscontinental crust eclogites and metasediments atLago di Cignana of the Western Alps were derivedfrom oceanic crust and constitute the ophiolitesequence of the Zermatt-Saas zone Inclusions ofcoesite and coesite pseudomorphs occur in tourma-line and garnet of manganiferous quartzite and inomphacite and garnet of the underlying mafic eclog-ite (Reinecke 1991) The metasediments are mainlygarnet-phengite-quartz schist with variableamounts of garnet-clinopyroxene quartzite andpiedmontite-phengite-quartz schist Eclogites andretrograded eclogites with preserved UHP mineralparageneses consist of garnet omphacite glau-cophane zoisite phengite and dolomite Zoned gar-nets preserve prograde quartz crystals in their coresand coesite inclusions in mantles and rims Coesiteinclusions are much more common in omphacitethan in garnet UHP conditions were estimated at26ndash30 GPa and 590ndash630degC Sm-Nd isotopic anal-yses of eclogites yield UHP metamorphism at 52 plusmn18 Ma (Bowtell et al 1994) consistent with a

Tertiary age of eclogitization recently established forAlpine metamorphism in the Western Alps(Gebauer et al 1997)

Thus far the recognized areal extent of thecoesite-bearing rocks is ~ 2 km2 The apparentthickness of the UHP slice does not exceed a fewhundred meters This area is the best example forsubduction of oceanic lithosphere to mantle depthand later exhumation as a fragmented tectonic slabSimilarly the Allalin metagabbro part of the oce-anic Piemonte zone contains magnesite + talc +kyanite + garnet recrystallized at ~600degC and 35GPa with an estimated geothermal gradient of 5ndash6degC kmndash1 however inclusions of coesite or coesitepseudomorphs have not been reported from theAllalin metagabbro

Mali Africa Eclogitic rocks in the Pan-Africancollision zone cropping out in northern Mali containinclusions of coesite in omphacite from mafic nod-ules within a calc-silicate layer enclosed in impuremarble and coesite pseudomorphs in garnets fromthe surrounding eclogitic metasediments (Caby1994) These UHP eclogitic mica schists have P-Testimates of 700ndash750degC and gt 27 GPa phengiteyields a well-defined plateau 39Ar40Ar age of 1045plusmn 9 Ma Recent SmNd ages of 620 Ma wereobtained by Jahn et al (2001) for peak UHP meta-morphism This may be the oldest UHP terrane inthe world

The coesite-bearing UHP eclogitic unit about 3km thick lies within an internal nappe as a flat thinslice bounded on the top by a thick passive-marginshaly formation and on the bottom by low-gradegreenschist-facies phyllites Caby (1994) suggestedthat a large portion of the terrigenous metasedimentsof passive-continental-margin affinity of the WestAfrican plate was subducted eastward to mantledepths (gt90 km) beneath an oceanic domain Colli-sion of the West African plate with an island arcresulted in a low-angle westward subhorizontalextrusion of this subducted slice of sialic materialsto its present position The coesite-bearing eclogiteunit may also crop out 1500 km to the south in Togowhere kyanite-bearing eclogites occur as part of thepassive-margin assemblage

Makbal (480 Ma) and Atbashy (270 Ma) Kazak-hstan The Tian-Shan is divided into three mountainrangesmdashnorthern middle and southern Within theKyrgyzstan Tian-Shan UHP eclogite localities withcoesite pseudomorph inclusions in garnet andomphacite from the northern and southern Tian-Shan represent Caledonian and Hercynian orogenic

12 LIOU ET AL

belts respectively (Tagiri and Bakirov 1990 Tagiriet al 1995) The Makbal Formation in the westernKyrgyz Ridge of the northern Tian-Shan consists ofquartzose schist alternating with pelitic schistsintercalated with thin layers or lenses of marbleeclogite and amphibolitized eclogite Tagiri andBakirov (1990) found inclusions of coesite pseudo-morphs in garnet from a garnet-chloritoid-talc schistwith a peak metamorphic assemblage of coesite +almandine + chloritoid + talc + phengite (Si = 342pfu) + phlogopite + rutile Makbal coesite-gradeeclogite has a paragonite KAr age of 480 Ma

The eclogite-bearing complex of Atbashy Ridgefrom the southern Tian-Shan is composed of peliticand siliceous schists alternating with thin UHPeclogitic layers and is unconformably overlain byUpper Paleozoic molasse and silicic volcanic rocksInclusions of coesite pseudomorphs occur inomphacite cores and in garnet Eclogites from theAtbashy area preserve prograde and retrogradepaths with a peak metamorphic condition at 660degC25 GPa The Atbashy coesite-eclogites yielded aRbSr mineral isochron age of 270 Ma (Tagiri et al1995)

Central Indonesia UHP terrane The pre-Tertiarybasement of the Indonesian region comprises a vari-ety of imbricate terranes meacutelange ophiolite andvariably metamorphosed accretionary complexessome contain HP to UHP metamorphic rocks result-ing from the collision of an Australia-derived conti-nent with Eurasia (Parkinson 2003) HP rocksincluding eclogites and garnet peridotites are widelydistributed in Cretaceous accretionary complexesMany of these rocks occur as imbricate slices of car-bonate quartzite and pelitic schist of shallow-marine or continental-margin parentage interthrustwith subordinate mafic schist and serpentinitesome yield mica KAr ages of 110ndash120 Ma (Parkin-son 2003)

HP and UHP rocks are sporadically exposed astectonic blocks throughout the Cretaceous accre-tionary complexes They include eclogite garnet-glaucophane rock (P = 18ndash24 GPa T = 580ndash620degC) and jadeite-garnet quartzite (P gt 27 GPa T =720ndash760degC) in Bantimala southwest Sulawesieclogite and garnet granulite in west-centralSulawesi eclogite and jadeite-glaucophane-quartzrock (P ~22 GPa T ~530degC) in central Java Mgchloritoidndashbearing whiteschists (P ~18 GPa) insoutheast Kalimantan garnet lherzolites in east-central Sulawesi (P = 22ndash28 GPa T = 1000ndash1100degC) west-central Sulawesi (P = 16ndash20 GPa T =

1050ndash1100degC) and garnet pyroxenite (P ~2 GPa T~ 850degC) in Sabah northeast Borneo Evidence forUHP rocks include (1) inclusions of coesite inzircon and coesite pseudomorphs in jadeite fromjadeite quartzite and eclogite of the BantimalaComplex of south Sulawesi (Parkinson 2003) and(2) P-T estimates of peak-stage recrystallization at27ndash35 GPa and 1000ndash1100degC for most garnetperidotites (Kardarusman and Parkinson 2000)Many of these rocks were probably recrystallized ina N-dipping subduction zone at the margin of theSundaland craton in the Early Cretaceous Exhuma-tion may have been facilitated by the collision of aGondwana continental fragment with the Sundalandmargin at ~120ndash115 Ma

Recently Recognized UHP Terranes

Several new UHP terranes were recently identi-fied inasmuch as they contain partially preservedtrace index minerals in strong containers such aszircon or garnet In fact zircon has been consideredto be the best container and many new terranes werediscovered through positive identification of inclu-sions of coesite or coesite pseudomorphs ordiamond in zircons Detailed examination of mineralinclusions in core mantle and rims of zircon sepa-rates from eclogites and their enclosing country-rock gneisses have yielded both prograde andretrograde P-T-time paths for various UHP terranesmentioned above (see Katayama et al 2001 fordetails also see the section on the Himalayan eclog-ite for an example)

The latest recognized UHP terranes include thefollowing (1) coesite in the French Massif Central(Lardeaux et al 2001) (2) coesite and diamondfrom the Greek Rhodope metamorphic province(Mposkos and Kostopoulos 2001) (3) coesitepseudomorphs in the Northeast Greenland EclogiteProvince (Gilotti and Ravna 2002) (4) coesite ingranulite-facies overprinting eclogites of SoutheastBrazil (Parkinson et al 2001) (5) coesite in Hima-layan eclogite from the Upper Kaghan Valley Paki-stan (OrsquoBrien et al 2001 Kaneko et al 2003) andcoesite from the Tso-Morari crystalline complex ofIndia (Mukherjee et al 2003) (6) coesite inclusionsin gneissic rocks from the North Qaidam belt west-ern China (Yang et al 2001 Song et al 2003) (7)diamond inclusions in garnets of eclogite and gneissfrom North Qinling (Yang et al 2002a) (8) inclu-sions of quartz pseudomorphs and minor coesite ingarnets of mafic eclogites from the LantermanRange in Antarctica (Ghiribelli et al 2002) (9)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 13

inclusions of calcite pseudomorphs after aragonite +magnesite in dolomite in metapelites from the west-ern Tian-Shan inferred as metamorphism in a UHPregion of dolomite decomposition (Zhang L et al2002) and (10) inclusion of possible coesitepseudomorphs in garnet lamellar inclusions in gar-net kyanite and high sodium content of garnet fromeclogites of the Chuacus Complex of north-centralGuatemala (Solari et al 2003) The results of petro-chemical and geochronological data for these newterranes are summarized below The HimalayanUHP terrane in the Upper Kaghan Valley and theTso-Morari complex are used to illustrate the pro-cesses of continental subduction and collision in alater section

French Massif Central The French Massif Cen-tral in the western part of the Variscan Belt hasexperienced Late SilurianndashEarly Devonian to LateCarboniferous orogenic events (eg Matte 1986)High-pressure metamorphic rocks occur mainly inthe upper gneissic unit within the leptyno-amphibo-lite group The latter consists of an association ofmetagreywacke mica schist metabasalt leptynitemetagranite and peridotite Eclogites are associatedwith silicic and mafic HP granulites (Lardeaux etal 2001) and also with spinel andor garnet lherzo-lites (Gardien et al 1990) Inclusions of coesite andcoesite pseudomorphs occur in garnets of kyanite-bearing eclogite (Lardeaux et al 2001) Only twocoesite grains are preserved as relics most coesitegrains are completely transformed into polycrystal-line radial quartz (palisade texture) or into polygonalquartz surrounded by radial cracks Metamorphictemperatures were estimated to be 740-780degC andsubsequent amphibolite-facies overprinting at750degC at 15ndash17 GPa during decompression UHPmetamorphism occurred between 420 and 400 Ma(Paquette et al 1995) 40Ar39Ar data from amphib-ole separates from retrogressed eclogites yielded339 plusmn 4 Ma (Costa et al 1993)

Northeast Greenland Caledonides The first evi-dence for UHP metamorphism in the GreenlandCaledonites was reported from kyanite eclogites andassociated host gneisses on an island in Jokelbugt(Gilotti and Ravna 2002) Inclusions of quartzpseudomorphs exhibiting palisade structure andradiating fractures occur in garnet and omphacite ofeclogites and in garnet of the host gneisses P-T esti-mates of peak-stage metamorphism of eclogite at~972degC and 36 GPa lie well within the coesite sta-bility field SHRIMP UPb dates of 403 plusmn 5 Ma forzircon rims from HP metapsammite and 404 plusmn 4 Ma

for zircons from anatectic melt derived frommetapelite are coeval with the estimated ages ofexhumation of UHP terranes in the ScandinavianCaledonides described in a previous section(McClelland and Gilotti 2003)

Neoproterozoic nappes in Southeast BrazilNeoproterozoic nappes of Southeast Brazil consistmainly of coarse-grained kyanite-garnet granulitewith intercalations of impure quartzite calc-silicaterock and minor lenses and sills of mafic-ultramaficrocks including eclogite Early eclogitic phaseswere strongly obliterated during granulite-faciesoverprinting during a major Pan-African continentcollision at 640ndash630 Ma (Campos Neto and Caby1999) Rare ultramafic lenses consist of phlogopite-orthopyroxene rocks and garnet clinopyroxeniteCores of granulite boudins retain eclogitic assem-blages some quartz inclusions within the mantleregions of zoned garnet porphyroblasts are sur-rounded by intense radial fractures suggestive ofthe former presence of coesite In fact coarse-grained zircons in some granulites contain numer-ous micro-inclusions of K-feldspar + kyanite + coes-ite as confirmed by Raman spectroscopy (Parkinsonet al 2001) Many exsolution microstructuresobserved in other UHP terranes also occur Thisarea represents the first coesite-grade UHP rocks inthe Americas and it can be correlated with the MaliUHP terrane of equatorial Africa (Caby 1994) Ithas been suggested that eclogitic assemblages andprobable UHPM components may be present in sev-eral other correlative nappe systems at the marginsof the Sao Francisco craton in Brazil

Chuacus Complex north-central Guatemala HPeclogites within banded gneisses schists andmigmatites of the Chuacus Complex contain relicttextures and mineralogical features that suggestUHP metamorphism (Solari et al 2003) Theseinclude inclusions of possible coesite pseudomor-phs in garnet and kyanite lamellar inclusions ofrutileilmenite in garnet high sodium content ofsome garnets (up to 1200 ppm) and preliminary P-T estimates of 700ndash800degC and 2ndash3 GPa AlthoughK-Ar dating of mica and hornblende yield ages of48ndash72 Ma possible for amphibolite facies retrogres-sion regional and structural data support a pre-Mesozoic age for the multi-stage HP-UHP recrystal-lization Such a discovery opens up a new interpre-tation regarding the tectonic evolution of Maya-Chortis-Oaxaquia microcontinental blocks formingthe structural backbone of Mesoamerica

14 LIOU ET AL

Greek Rhodope The Greek Rhodope Metamor-phic Province (RMP) at the border between Greeceand Bulgaria represents a syn-metamorphic nappesystem of Alpine age that was formed during theCretaceous to Mid-Tertiary collision of Apulia andpaleo-Europe The Kimi nappe complex comprisescrustal eclogites leucocratic orthogneisses peliticgneisses and volumetrically minor mantle-derivedultramafic rocks these units contain many mineral-ogical indicators of UHP metamorphism (Mposkosand Kostopoulos 2001) The ultramafic lithologiesinclude garnet spinelndashbearing lherzolite and layersof spinel-garnet clinopyroxenite garnet pyroxeniteand clinopyroxene garnetite These rocks were sub-jected to UHP recrystallization at P-T conditions of~1200degC and gt30 GPa The pelitic gneiss containsmicrodiamond inclusions in garnet porphyroblastsas well as rods or needles of silica rutile biotiteand apatite in sodic garnet suggesting the prioroccurrence of majoritic garnet The growth of suchsupersilicic garnet was suggested to be at a pressureof ~7 GPa Inclusions of polycrystalline quartzaggregates surrounded by radial cracks occur ineclogitic garnet Exsolution lamellae of quartz ineclogitic clinopyroxene also indicate an UHP pre-cursor of supersilicic clinopyroxene

Lanterman Range Antarctica The LantermanRange in northern Victoria Land of Antarctica con-sists of several metamorphic complexes The Gate-way Hills Metamorphic Complex is a thindiscontinuous belt more than 50 km long consistingof mafic and ultramafic rocks including lenses andpods of eclogite within felsic gneiss Some less foli-ated eclogites contain inclusions of coesite andcoesite pseudomorphs in garnet porphyroblasts asconfirmed by in situ Raman spectroscopy as well asradial fractures around the inclusions (Ghiribelli etal 2002) The eclogites were subjected to UHPmetamorphism at T gt 850degC and P gt 29 GPa thenamphibolite-facies overprinting during isothermaldecompression The UHP event is dated by SmNdand 238U206Pb data at 500 Ma 40Ar39Ar ages of490ndash486 Ma for Ca-amphibole (amphibolite-faciesoverprinting) suggest rapid cooling with an averageexhumation rate of 3ndash4 km_Ma-1 for the UHP ter-rane (Di Vincenzo and Palmeri 2001)

Western Tian-Shan China Caledonian eclogitesof the western Tian-Shan China have been classi-fied into three types (Zhang L et al 2002) Type Ieclogite pods are layered with mafic blueschists andType II eclogites preserved pillow structureswhereas Type III eclogites are banded with marbles

This 200 km long eclogite-bearing belt extendswestward to connect with the Atbashy UHP belt inKazakhstan (Tagiri et al 1995) Reported UHP evi-dence includes inclusions of coesite pseudomorphsin garnets from Type I and III eclogites quartzexsolution lamellae in omphacite from Type IIeclogites and relict magnesite within matrix dolo-mite in Type III eclogites (Zhang J X et al 2002Zhang L et al 2002) P-T estimates of the peakUHP stage of metamorphism are T = 500ndash600degCand P = 26ndash27 GPa (Wei et al 2003)

North Qaidam Mountains western China Manylenses and layers of garnet peridotite and eclogiteoccur in an amphibolite-facies terrane in the NorthQaidam Mountains along the northeastern rim of theQaidam Basin This belt comprises mainly felsicgneiss quartz schist garnet-bearing gneissamphibolite and minor eclogite + garnet peridotiteit extends westward for more than 1000 km to theAltun Mountains and was displaced by the left-lat-eral Altyn-Tagh fault At the eastern end of thisUHP belt at Dulan occurrences of coesite inclu-sions in zircon separates from paragneiss inclusionsof coesite pseudomorph in garnet and omphacite ofeclogites quartz rods in eclogitic omphacite andP-T estimates of eclogites (P = 287ndash317 GPa andT = 631ndash687degC) demonstrate that this is a typicalUHP metamorphic terrane (Yang et al 2001 Songet al 2003a 2003b) The Dulan eclogites have anSm-Nd mineral isochron age of 497 plusmn 87 Ma and aSHRIMP UPb zircon age of 495 Ma (Yang et al2002b)

Altun Mountains western China This UHP ter-rane about 200 km long in the southern margin ofthe Altun Mountains is the western extension of theNorth Qaidam belt Eclogitic lenses and blocks ofvarious sizes together with garnet-lherzolite and cli-nopyroxenite are enclosed within an Early Protero-zoic sequence of amphibolite amphibole schistgarnet-bearing granitic gneiss and pelitic schistGarnet lherzolite contains magnesite that reacted toform dolomite P-T estimates of the peak magnesite-bearing assemblage yield P gt 36 GPa and 850degC(Liu et al 2001) Garnet porphyroblasts includeexsolution lamellae of clinopyroxene and rutile sim-ilar to that of eclogite reported by Ye et al (2000) asmajoritic garnet The associated eclogites containinclusions of coesite pseudomorphs in garnet withcharacteristic radial fractures this together with theexsolved quartz rods in omphacite and P-T esti-mates of 820ndash850degC and 28 to 32 GPa provideadditional evidence for UHP (Zhang J X et al

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 15

2001) An SmNd mineral isochron for eclogiticgarnet omphacite and the whole rock yields 500 plusmn10 Ma whereas an UPb isochron for zircon sepa-rates from eclogite gives an age of 504 plusmn 5 Ma(Zhang J X et al 2000)

North Qinling Mountains central China Lensesand blocks of eclogitic rocks in northern Qinling areenclosed in amphibolite-facies gneiss and garnet-bearing quartz + phengite mica schist Inclusions ofcoesite in eclogitic garnet (Hu et al 1995) andmicrodiamonds in zircon separates from both eclog-ite and gneiss (Yang et al 2002a) have beenrecently reported Although individual outcrops areno more than a few meters wide the belt of eclogitesextends more than 10 km P-T estimates of amphib-olitized coesite-bearing eclogite yield T = 590ndash758degC and P gt 26 GPa An Sm-Nd isochron for gar-net omphacite rutile amphibole and whole-rockyields 400 plusmn 16 Ma Microdiamonds were recentlydiscovered as inclusions in zircon from both eclogiteand its gneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyields 507 plusmn 38 Ma for peak UHP metamorphism(Yang et al 2002a) These data indicate that two

discrete collision events occurred between theNorth China and Yangtze cratons (1) an Early Pale-ozoic North Qinling UHP suturing and (2) a Triassicsuturing along the Dabie-Sulu UHP-HP belt TheEarly Paleozoic belt extends westward to the NorthQaidamndashAltun Mountains for more than 4000 km(Yang et al 2002b)

Himalayan UHP Eclogites

The Himalayan orogen has long been considereda classic locality of continental collision betweenthe Indian and Eurasian continents (Fig 5) Severaleclogite-facies HP rocks in the Himalayas havebeen reported since the late 1980s (eg Chaudaryand Ghazanfar 1987 Pognante and Spencer 1991de Sigoyer et al 1997 Lombardo and Rolfo 2000)Inclusions of coesite were recently discovered inomphacite and garnet in several Himalayan eclog-ites from the upper Kaghan Valley Pakistan(OrsquoBrien et al 2001 Kaneko et al 2003) Subse-quent findings of additional coesite localities in theTso Morari area (Sachan et al 2001 Mukherjee etal 2003) have established the presence of a UHPbelt in the Himalayas an active ongoing collision

FIG 5 A simplified tectonic map of the western Himalaya Hindu Kush Pamir Karakoran and western Tibet mod-ified after Figure 3 of Searle et al (2001) This map shows the locations of eclogite and UHP eclogites from both Hima-layan and North Qaidam Indus Suture Zone (ISZ) ophiolites and major structures

16 LIOU ET AL

since the Eocene (Massonne and OrsquoBrien 2003)However UHP relics are volumetrically very minorconstituents of the metamorphic belts dominatedmostly by later Barrovian overprintings In theserocks hydration related to Barrovian-zone metamor-phism has resulted in remarkably pervasive recrys-tallization during the later exhumation this eventhas almost entirely masked the preceding UHPrecord

The finding of HP-UHP evidence has led to newinsights into the Himalayan tectonic modelEngland and Houseman (1986) showed that in amodel of homogeneous thickening of the Hima-layan-Tibet boundary the maximum crustal thick-ness should be less than 70 km However theoccurrences of coesite-bearing eclogites andgneisses in the Himalayan orogen cannot beexplained by a homogeneous thickening of the crustbut rather by continental subduction as describedbelow According to available geochronologicaldata (eg Treloar et al 2003) the exhumation rateof the Himalayan UHP rocks has been estimated torange from ~1 cm yrndash1 (OrsquoBrien et al 2001) to gt3cm yrndash1 (Treloar et al 2003 Massone and OrsquoBrien2003) These estimates are higher than those calcu-lated from other UHP-HP terranes This requirestectonic emplacement of the UHP-HP Himalayanslab into shallow crust following isostatic rebounddue to slab decoupling between the Indian continentand the oceanic crust initiated during the MiddleEocene UHP rocks from both the Kaghan Valleyand Tso-Mori are described below

Kaghan Valley The Kaghan nappe in the north-western Himalayan syntaxis lies between the Indussuture (also called main mantle thrust MMT) to thenorth and the main central thrust (MCT) to thesouthwest (Pognante and Spear 1991 Spencer1993) (Fig 5) The Cretaceous Kohistan island-arcsequence lies north of the Indus suture It developedas a result of intra-oceanic subduction The maincentral thrust separates an underlying Salkhala unitfrom an overlying higher Himalayan crystalline(HHC) unit The Kaghan nappe consists mainly ofgranitic gneiss and paragneiss with minor inter-calated layers of amphibolite marble and quartziteEclogites are preserved in the cores of maficboudins (less than 1 m thick) in gneiss and aresignificantly overprinted by amphibolite assem-blages They contain fine-grained (lt 1 mm) garnetand omphacite together with aggregates of quartzchains of rutile rimmed by titanite and mm-sizedphengitic micas overgrown by randomly oriented

amphiboles up to 1 cm in length Coesite and coesitepseudomorphs occur as inclusions in omphaciteand show the characteristic palisade-quartz aggre-gate texture fractures radiating from these inclu-sions are characteristically developed in the hostpyroxene

Garnet pyroxenendashphengite barometry wasapplied to the eclogites and yielded peak P-T con-ditions of 27ndash29 GPa and 690ndash750degC (OrsquoBrien etal 2001) (Fig 6) Retrograde textures are commonin the eclogites including symplectic intergrowthsof augite amphibole and plagioclase after omphac-ite thermobarometric analyses of these assemblagesyielded 10ndash13 GPa and 600ndash710degC (Kaneko et al2003) On the other hand the dominant graniticgneisses and metapelites of the higher Himalayancrystalline unit contain typical Barrovian-zoneamphibolite-facies assemblages represented bykyanite- and staurolite-bearing garnetiferousmetapelites (Treloar 1995) Extensive overprintinghas mostly obliterated the peak assemblages matrixminerals of the gneisses yielded P-T conditions of07ndash11 GPa and 600ndash700degC (Treloar 1995) con-sistent with retrogression of the eclogites Howeverrelict coesite is preserved as inclusions in zirconthe inclusions are ovoid and approximately 10 microm indiameter (Fig 7A) This discovery of coesite-bear-ing zircons from Himalayan quartzofeldspathicgneisses demonstrates subduction of Indian conti-nental crust to the depths of UHP conditions(Kaneko et al 2003)

Geochronological studies using SmNd (garnet-omphacite) RbSr (phengite) and UPb (rutile zir-con) methods suggest that the eclogite-facies eventtook place at 40ndash50 Ma (Tonarini et al 1993 Spen-cer and Gebauer 1996) 40Ar39Ar cooling ages forhornblende and mica from gneisses were reported tobe ~43 Ma and ~25 Ma respectively (Chamberlainet al 1991) Recent SHRIMP UPb dates of zonedzircons with mineral inclusions formed at differentstages indicate the ages of non-UHP mineral-bear-ing mantle domains of zircon and UHP mineral-bearing rims are at about 50 and 46 Ma respectively(Kaneko et al 2003) A new U-Pb age of 44 plusmn 11Ma was obtained for rutile from Kaghan coesite-bearing eclogite (Treloar et al 2003) this togetherwith available data implies that exhumation of theUHP rocks from mantle depth to mid-crustal leveloccurs within a few million years

Tso Morari The Tso Morari crystalline dome islocated between the Indus suture zone on the northbordered by the Zildat detachment fault and the

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 17

Zanskar sedimentary unit on the south (de Sigoyer etal 1997) The dome represents an internal massif ofthe northern part of the higher Himalaya The TsoMorari area has been considered a typical IndianTethyan margin because of a stratigraphic succes-sion similar to the Nimaling antiform in the core ofthe dome a metamorphic basement consists of Cam-bro-Ordovician augen gneisses (Stutz 1988) Thiscomplex is covered by a metasedimentary series ofCambrian to Devonian quartzite schist and con-glomerate and is overlain by Lower Carboniferousto Triassic marble and metapelite Eclogites occuras lenses or irregular bodies within the basementand cover gneisses and are strongly overprinted byamphibolite-facies assemblages de Sigoyer et al(1997) reported peak P-T conditions for the eclog-ites at 17ndash23 GPa and 520ndash640degC (Fig 7) How-ever relics of coesite 30ndash80 microm in size were

recently found as inclusions in garnet (Sachan et al2001 Mukherjee et al 2003) typified by radialfractures (Figs 7B and 7C) for the SiO2 inclusionsa characteristic Raman spectrum is centered at 523cm-1 (Fig 7D) The country rock metasedimentswith jadeite + chloritoid + paragonite + garnetassemblages have peak P-T estimates of 18ndash22GPa and 500ndash600degC and record retrogression at theeclogite-blueschist facies transition of 13ndash18 GPaand 490ndash590degC (Guillot et al 1997)

The geochronology of the Tso Morari eclogiteshas been extensively studied and subdivided intothree different stages (de Signoyer et al 2000)Eclogitization at ~55 Ma was obtained by Lu-Hf andSm-Nd methods on garnet omphacite glaucophaneand the whole rock An amphibolite-facies overprintoccurred at 47 Ma judging by RbSr dating onphengite apatite and whole rock from metapelites

FIG 6 P-T estimates of peak- and retrograde-stage metamorphism for coesite-bearing eclogites and the country-rockgneisses from the Kaghan Valley and Tso Morari

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

10 LIOU ET AL

Less Intensively Studied UHP terranes

Several UHP terranes described below exceptfor the Maksyvtov Complex have received lessintensive study mainly due to remoteness and diffi-cult access despite their long recognition

Maksyutov Complex Southern Urals TheMaksyutov Complex is an elongated (10 times 120 km)N-S belt between the Russian and Siberian cratonsin the southern Ural Mountains (Coleman and Wang1995 Dobretsov et al 1996) and contains blocky

graphitized diamond() in pelitic schist (Leech andErnst 1998) The complex is the locality from whichquartz aggregates within garnet exhibiting radialcracks were first recognized as coesite-pseudo-morphs by Chesnokov and Popov (1965) nearly 20years prior to the independent finding of coesiteinclusions by Chopin (1984) and Smith (1984) Theeclogite-bearing unit contains boudins of eclogitelayers of eclogitic gneiss and rare ultramafic bodieswithin host metasedimentary mica schist andquartzite

FIG 4 A Schematic diagram showing the growth of detrital zircon (stage 1) through subduction (stage 2 and peakstage 3) and exhumation (stage 4) of supracrustal rocks Mineral inclusions in various stages of zircon growth are shownB SHRIMP U-Pb dating of zoned zircons from a Kokchetav diamond-bearing gneiss yields a well constrained P-T-timepath (for details see Katayama et al 2001)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 11

Some Maksyutov eclogites were recrystallized at594ndash637degC with a minimum pressure from 15 to17 GPa (Beane et al 1995 Leech and Ernst 1998)or up to 27 GPa if coesite pseudomorphs (Ches-nokov and Popov 1965 Dobretsov and Doretsova1988) are indeed present However subsequentstudies have not revealed the occurrence of inclu-sions of coesite or coesite pseudomorph in garnetand zircon (Beane et al 1995 Shatsky et al 1995)Leech and Ernst (1998) described unusual graphiteaggregates (up to 13 mm edge length) that may bediamond pseudomorphs This suggestion was basedon the blocky morphology of the cuboid graphiteaggregates cross-cutting rock foliation pressureshadows biogenic isotopic signatures and otherspectroscopic characters similar to those graphi-tized diamonds from the Beni Bousera massifRecent study of several microdiamond inclusions ingarnet has yielded further suggestive possibility thatthe eclogitic unit of the Maksyutov Complex experi-enced diamond-grade UHP metamorphism (Bosticket al 2003) The UHPHP metamorphism with peakmetamorphic ages of 370ndash374 Ma (Shatsky et al1995 Beane and Connelly 1998) may be related tothe collision of the Russian platform with a fragmentof the Siberian craton during Devonian time butwas affected by the Triassic collision of the Kazakh-stan block with the Russian platform (eg Matte1998)

Zermatt-Saas area Western Alps In contrast tomost other UHP terranes in which the protolith iscontinental crust eclogites and metasediments atLago di Cignana of the Western Alps were derivedfrom oceanic crust and constitute the ophiolitesequence of the Zermatt-Saas zone Inclusions ofcoesite and coesite pseudomorphs occur in tourma-line and garnet of manganiferous quartzite and inomphacite and garnet of the underlying mafic eclog-ite (Reinecke 1991) The metasediments are mainlygarnet-phengite-quartz schist with variableamounts of garnet-clinopyroxene quartzite andpiedmontite-phengite-quartz schist Eclogites andretrograded eclogites with preserved UHP mineralparageneses consist of garnet omphacite glau-cophane zoisite phengite and dolomite Zoned gar-nets preserve prograde quartz crystals in their coresand coesite inclusions in mantles and rims Coesiteinclusions are much more common in omphacitethan in garnet UHP conditions were estimated at26ndash30 GPa and 590ndash630degC Sm-Nd isotopic anal-yses of eclogites yield UHP metamorphism at 52 plusmn18 Ma (Bowtell et al 1994) consistent with a

Tertiary age of eclogitization recently established forAlpine metamorphism in the Western Alps(Gebauer et al 1997)

Thus far the recognized areal extent of thecoesite-bearing rocks is ~ 2 km2 The apparentthickness of the UHP slice does not exceed a fewhundred meters This area is the best example forsubduction of oceanic lithosphere to mantle depthand later exhumation as a fragmented tectonic slabSimilarly the Allalin metagabbro part of the oce-anic Piemonte zone contains magnesite + talc +kyanite + garnet recrystallized at ~600degC and 35GPa with an estimated geothermal gradient of 5ndash6degC kmndash1 however inclusions of coesite or coesitepseudomorphs have not been reported from theAllalin metagabbro

Mali Africa Eclogitic rocks in the Pan-Africancollision zone cropping out in northern Mali containinclusions of coesite in omphacite from mafic nod-ules within a calc-silicate layer enclosed in impuremarble and coesite pseudomorphs in garnets fromthe surrounding eclogitic metasediments (Caby1994) These UHP eclogitic mica schists have P-Testimates of 700ndash750degC and gt 27 GPa phengiteyields a well-defined plateau 39Ar40Ar age of 1045plusmn 9 Ma Recent SmNd ages of 620 Ma wereobtained by Jahn et al (2001) for peak UHP meta-morphism This may be the oldest UHP terrane inthe world

The coesite-bearing UHP eclogitic unit about 3km thick lies within an internal nappe as a flat thinslice bounded on the top by a thick passive-marginshaly formation and on the bottom by low-gradegreenschist-facies phyllites Caby (1994) suggestedthat a large portion of the terrigenous metasedimentsof passive-continental-margin affinity of the WestAfrican plate was subducted eastward to mantledepths (gt90 km) beneath an oceanic domain Colli-sion of the West African plate with an island arcresulted in a low-angle westward subhorizontalextrusion of this subducted slice of sialic materialsto its present position The coesite-bearing eclogiteunit may also crop out 1500 km to the south in Togowhere kyanite-bearing eclogites occur as part of thepassive-margin assemblage

Makbal (480 Ma) and Atbashy (270 Ma) Kazak-hstan The Tian-Shan is divided into three mountainrangesmdashnorthern middle and southern Within theKyrgyzstan Tian-Shan UHP eclogite localities withcoesite pseudomorph inclusions in garnet andomphacite from the northern and southern Tian-Shan represent Caledonian and Hercynian orogenic

12 LIOU ET AL

belts respectively (Tagiri and Bakirov 1990 Tagiriet al 1995) The Makbal Formation in the westernKyrgyz Ridge of the northern Tian-Shan consists ofquartzose schist alternating with pelitic schistsintercalated with thin layers or lenses of marbleeclogite and amphibolitized eclogite Tagiri andBakirov (1990) found inclusions of coesite pseudo-morphs in garnet from a garnet-chloritoid-talc schistwith a peak metamorphic assemblage of coesite +almandine + chloritoid + talc + phengite (Si = 342pfu) + phlogopite + rutile Makbal coesite-gradeeclogite has a paragonite KAr age of 480 Ma

The eclogite-bearing complex of Atbashy Ridgefrom the southern Tian-Shan is composed of peliticand siliceous schists alternating with thin UHPeclogitic layers and is unconformably overlain byUpper Paleozoic molasse and silicic volcanic rocksInclusions of coesite pseudomorphs occur inomphacite cores and in garnet Eclogites from theAtbashy area preserve prograde and retrogradepaths with a peak metamorphic condition at 660degC25 GPa The Atbashy coesite-eclogites yielded aRbSr mineral isochron age of 270 Ma (Tagiri et al1995)

Central Indonesia UHP terrane The pre-Tertiarybasement of the Indonesian region comprises a vari-ety of imbricate terranes meacutelange ophiolite andvariably metamorphosed accretionary complexessome contain HP to UHP metamorphic rocks result-ing from the collision of an Australia-derived conti-nent with Eurasia (Parkinson 2003) HP rocksincluding eclogites and garnet peridotites are widelydistributed in Cretaceous accretionary complexesMany of these rocks occur as imbricate slices of car-bonate quartzite and pelitic schist of shallow-marine or continental-margin parentage interthrustwith subordinate mafic schist and serpentinitesome yield mica KAr ages of 110ndash120 Ma (Parkin-son 2003)

HP and UHP rocks are sporadically exposed astectonic blocks throughout the Cretaceous accre-tionary complexes They include eclogite garnet-glaucophane rock (P = 18ndash24 GPa T = 580ndash620degC) and jadeite-garnet quartzite (P gt 27 GPa T =720ndash760degC) in Bantimala southwest Sulawesieclogite and garnet granulite in west-centralSulawesi eclogite and jadeite-glaucophane-quartzrock (P ~22 GPa T ~530degC) in central Java Mgchloritoidndashbearing whiteschists (P ~18 GPa) insoutheast Kalimantan garnet lherzolites in east-central Sulawesi (P = 22ndash28 GPa T = 1000ndash1100degC) west-central Sulawesi (P = 16ndash20 GPa T =

1050ndash1100degC) and garnet pyroxenite (P ~2 GPa T~ 850degC) in Sabah northeast Borneo Evidence forUHP rocks include (1) inclusions of coesite inzircon and coesite pseudomorphs in jadeite fromjadeite quartzite and eclogite of the BantimalaComplex of south Sulawesi (Parkinson 2003) and(2) P-T estimates of peak-stage recrystallization at27ndash35 GPa and 1000ndash1100degC for most garnetperidotites (Kardarusman and Parkinson 2000)Many of these rocks were probably recrystallized ina N-dipping subduction zone at the margin of theSundaland craton in the Early Cretaceous Exhuma-tion may have been facilitated by the collision of aGondwana continental fragment with the Sundalandmargin at ~120ndash115 Ma

Recently Recognized UHP Terranes

Several new UHP terranes were recently identi-fied inasmuch as they contain partially preservedtrace index minerals in strong containers such aszircon or garnet In fact zircon has been consideredto be the best container and many new terranes werediscovered through positive identification of inclu-sions of coesite or coesite pseudomorphs ordiamond in zircons Detailed examination of mineralinclusions in core mantle and rims of zircon sepa-rates from eclogites and their enclosing country-rock gneisses have yielded both prograde andretrograde P-T-time paths for various UHP terranesmentioned above (see Katayama et al 2001 fordetails also see the section on the Himalayan eclog-ite for an example)

The latest recognized UHP terranes include thefollowing (1) coesite in the French Massif Central(Lardeaux et al 2001) (2) coesite and diamondfrom the Greek Rhodope metamorphic province(Mposkos and Kostopoulos 2001) (3) coesitepseudomorphs in the Northeast Greenland EclogiteProvince (Gilotti and Ravna 2002) (4) coesite ingranulite-facies overprinting eclogites of SoutheastBrazil (Parkinson et al 2001) (5) coesite in Hima-layan eclogite from the Upper Kaghan Valley Paki-stan (OrsquoBrien et al 2001 Kaneko et al 2003) andcoesite from the Tso-Morari crystalline complex ofIndia (Mukherjee et al 2003) (6) coesite inclusionsin gneissic rocks from the North Qaidam belt west-ern China (Yang et al 2001 Song et al 2003) (7)diamond inclusions in garnets of eclogite and gneissfrom North Qinling (Yang et al 2002a) (8) inclu-sions of quartz pseudomorphs and minor coesite ingarnets of mafic eclogites from the LantermanRange in Antarctica (Ghiribelli et al 2002) (9)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 13

inclusions of calcite pseudomorphs after aragonite +magnesite in dolomite in metapelites from the west-ern Tian-Shan inferred as metamorphism in a UHPregion of dolomite decomposition (Zhang L et al2002) and (10) inclusion of possible coesitepseudomorphs in garnet lamellar inclusions in gar-net kyanite and high sodium content of garnet fromeclogites of the Chuacus Complex of north-centralGuatemala (Solari et al 2003) The results of petro-chemical and geochronological data for these newterranes are summarized below The HimalayanUHP terrane in the Upper Kaghan Valley and theTso-Morari complex are used to illustrate the pro-cesses of continental subduction and collision in alater section

French Massif Central The French Massif Cen-tral in the western part of the Variscan Belt hasexperienced Late SilurianndashEarly Devonian to LateCarboniferous orogenic events (eg Matte 1986)High-pressure metamorphic rocks occur mainly inthe upper gneissic unit within the leptyno-amphibo-lite group The latter consists of an association ofmetagreywacke mica schist metabasalt leptynitemetagranite and peridotite Eclogites are associatedwith silicic and mafic HP granulites (Lardeaux etal 2001) and also with spinel andor garnet lherzo-lites (Gardien et al 1990) Inclusions of coesite andcoesite pseudomorphs occur in garnets of kyanite-bearing eclogite (Lardeaux et al 2001) Only twocoesite grains are preserved as relics most coesitegrains are completely transformed into polycrystal-line radial quartz (palisade texture) or into polygonalquartz surrounded by radial cracks Metamorphictemperatures were estimated to be 740-780degC andsubsequent amphibolite-facies overprinting at750degC at 15ndash17 GPa during decompression UHPmetamorphism occurred between 420 and 400 Ma(Paquette et al 1995) 40Ar39Ar data from amphib-ole separates from retrogressed eclogites yielded339 plusmn 4 Ma (Costa et al 1993)

Northeast Greenland Caledonides The first evi-dence for UHP metamorphism in the GreenlandCaledonites was reported from kyanite eclogites andassociated host gneisses on an island in Jokelbugt(Gilotti and Ravna 2002) Inclusions of quartzpseudomorphs exhibiting palisade structure andradiating fractures occur in garnet and omphacite ofeclogites and in garnet of the host gneisses P-T esti-mates of peak-stage metamorphism of eclogite at~972degC and 36 GPa lie well within the coesite sta-bility field SHRIMP UPb dates of 403 plusmn 5 Ma forzircon rims from HP metapsammite and 404 plusmn 4 Ma

for zircons from anatectic melt derived frommetapelite are coeval with the estimated ages ofexhumation of UHP terranes in the ScandinavianCaledonides described in a previous section(McClelland and Gilotti 2003)

Neoproterozoic nappes in Southeast BrazilNeoproterozoic nappes of Southeast Brazil consistmainly of coarse-grained kyanite-garnet granulitewith intercalations of impure quartzite calc-silicaterock and minor lenses and sills of mafic-ultramaficrocks including eclogite Early eclogitic phaseswere strongly obliterated during granulite-faciesoverprinting during a major Pan-African continentcollision at 640ndash630 Ma (Campos Neto and Caby1999) Rare ultramafic lenses consist of phlogopite-orthopyroxene rocks and garnet clinopyroxeniteCores of granulite boudins retain eclogitic assem-blages some quartz inclusions within the mantleregions of zoned garnet porphyroblasts are sur-rounded by intense radial fractures suggestive ofthe former presence of coesite In fact coarse-grained zircons in some granulites contain numer-ous micro-inclusions of K-feldspar + kyanite + coes-ite as confirmed by Raman spectroscopy (Parkinsonet al 2001) Many exsolution microstructuresobserved in other UHP terranes also occur Thisarea represents the first coesite-grade UHP rocks inthe Americas and it can be correlated with the MaliUHP terrane of equatorial Africa (Caby 1994) Ithas been suggested that eclogitic assemblages andprobable UHPM components may be present in sev-eral other correlative nappe systems at the marginsof the Sao Francisco craton in Brazil

Chuacus Complex north-central Guatemala HPeclogites within banded gneisses schists andmigmatites of the Chuacus Complex contain relicttextures and mineralogical features that suggestUHP metamorphism (Solari et al 2003) Theseinclude inclusions of possible coesite pseudomor-phs in garnet and kyanite lamellar inclusions ofrutileilmenite in garnet high sodium content ofsome garnets (up to 1200 ppm) and preliminary P-T estimates of 700ndash800degC and 2ndash3 GPa AlthoughK-Ar dating of mica and hornblende yield ages of48ndash72 Ma possible for amphibolite facies retrogres-sion regional and structural data support a pre-Mesozoic age for the multi-stage HP-UHP recrystal-lization Such a discovery opens up a new interpre-tation regarding the tectonic evolution of Maya-Chortis-Oaxaquia microcontinental blocks formingthe structural backbone of Mesoamerica

14 LIOU ET AL

Greek Rhodope The Greek Rhodope Metamor-phic Province (RMP) at the border between Greeceand Bulgaria represents a syn-metamorphic nappesystem of Alpine age that was formed during theCretaceous to Mid-Tertiary collision of Apulia andpaleo-Europe The Kimi nappe complex comprisescrustal eclogites leucocratic orthogneisses peliticgneisses and volumetrically minor mantle-derivedultramafic rocks these units contain many mineral-ogical indicators of UHP metamorphism (Mposkosand Kostopoulos 2001) The ultramafic lithologiesinclude garnet spinelndashbearing lherzolite and layersof spinel-garnet clinopyroxenite garnet pyroxeniteand clinopyroxene garnetite These rocks were sub-jected to UHP recrystallization at P-T conditions of~1200degC and gt30 GPa The pelitic gneiss containsmicrodiamond inclusions in garnet porphyroblastsas well as rods or needles of silica rutile biotiteand apatite in sodic garnet suggesting the prioroccurrence of majoritic garnet The growth of suchsupersilicic garnet was suggested to be at a pressureof ~7 GPa Inclusions of polycrystalline quartzaggregates surrounded by radial cracks occur ineclogitic garnet Exsolution lamellae of quartz ineclogitic clinopyroxene also indicate an UHP pre-cursor of supersilicic clinopyroxene

Lanterman Range Antarctica The LantermanRange in northern Victoria Land of Antarctica con-sists of several metamorphic complexes The Gate-way Hills Metamorphic Complex is a thindiscontinuous belt more than 50 km long consistingof mafic and ultramafic rocks including lenses andpods of eclogite within felsic gneiss Some less foli-ated eclogites contain inclusions of coesite andcoesite pseudomorphs in garnet porphyroblasts asconfirmed by in situ Raman spectroscopy as well asradial fractures around the inclusions (Ghiribelli etal 2002) The eclogites were subjected to UHPmetamorphism at T gt 850degC and P gt 29 GPa thenamphibolite-facies overprinting during isothermaldecompression The UHP event is dated by SmNdand 238U206Pb data at 500 Ma 40Ar39Ar ages of490ndash486 Ma for Ca-amphibole (amphibolite-faciesoverprinting) suggest rapid cooling with an averageexhumation rate of 3ndash4 km_Ma-1 for the UHP ter-rane (Di Vincenzo and Palmeri 2001)

Western Tian-Shan China Caledonian eclogitesof the western Tian-Shan China have been classi-fied into three types (Zhang L et al 2002) Type Ieclogite pods are layered with mafic blueschists andType II eclogites preserved pillow structureswhereas Type III eclogites are banded with marbles

This 200 km long eclogite-bearing belt extendswestward to connect with the Atbashy UHP belt inKazakhstan (Tagiri et al 1995) Reported UHP evi-dence includes inclusions of coesite pseudomorphsin garnets from Type I and III eclogites quartzexsolution lamellae in omphacite from Type IIeclogites and relict magnesite within matrix dolo-mite in Type III eclogites (Zhang J X et al 2002Zhang L et al 2002) P-T estimates of the peakUHP stage of metamorphism are T = 500ndash600degCand P = 26ndash27 GPa (Wei et al 2003)

North Qaidam Mountains western China Manylenses and layers of garnet peridotite and eclogiteoccur in an amphibolite-facies terrane in the NorthQaidam Mountains along the northeastern rim of theQaidam Basin This belt comprises mainly felsicgneiss quartz schist garnet-bearing gneissamphibolite and minor eclogite + garnet peridotiteit extends westward for more than 1000 km to theAltun Mountains and was displaced by the left-lat-eral Altyn-Tagh fault At the eastern end of thisUHP belt at Dulan occurrences of coesite inclu-sions in zircon separates from paragneiss inclusionsof coesite pseudomorph in garnet and omphacite ofeclogites quartz rods in eclogitic omphacite andP-T estimates of eclogites (P = 287ndash317 GPa andT = 631ndash687degC) demonstrate that this is a typicalUHP metamorphic terrane (Yang et al 2001 Songet al 2003a 2003b) The Dulan eclogites have anSm-Nd mineral isochron age of 497 plusmn 87 Ma and aSHRIMP UPb zircon age of 495 Ma (Yang et al2002b)

Altun Mountains western China This UHP ter-rane about 200 km long in the southern margin ofthe Altun Mountains is the western extension of theNorth Qaidam belt Eclogitic lenses and blocks ofvarious sizes together with garnet-lherzolite and cli-nopyroxenite are enclosed within an Early Protero-zoic sequence of amphibolite amphibole schistgarnet-bearing granitic gneiss and pelitic schistGarnet lherzolite contains magnesite that reacted toform dolomite P-T estimates of the peak magnesite-bearing assemblage yield P gt 36 GPa and 850degC(Liu et al 2001) Garnet porphyroblasts includeexsolution lamellae of clinopyroxene and rutile sim-ilar to that of eclogite reported by Ye et al (2000) asmajoritic garnet The associated eclogites containinclusions of coesite pseudomorphs in garnet withcharacteristic radial fractures this together with theexsolved quartz rods in omphacite and P-T esti-mates of 820ndash850degC and 28 to 32 GPa provideadditional evidence for UHP (Zhang J X et al

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 15

2001) An SmNd mineral isochron for eclogiticgarnet omphacite and the whole rock yields 500 plusmn10 Ma whereas an UPb isochron for zircon sepa-rates from eclogite gives an age of 504 plusmn 5 Ma(Zhang J X et al 2000)

North Qinling Mountains central China Lensesand blocks of eclogitic rocks in northern Qinling areenclosed in amphibolite-facies gneiss and garnet-bearing quartz + phengite mica schist Inclusions ofcoesite in eclogitic garnet (Hu et al 1995) andmicrodiamonds in zircon separates from both eclog-ite and gneiss (Yang et al 2002a) have beenrecently reported Although individual outcrops areno more than a few meters wide the belt of eclogitesextends more than 10 km P-T estimates of amphib-olitized coesite-bearing eclogite yield T = 590ndash758degC and P gt 26 GPa An Sm-Nd isochron for gar-net omphacite rutile amphibole and whole-rockyields 400 plusmn 16 Ma Microdiamonds were recentlydiscovered as inclusions in zircon from both eclogiteand its gneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyields 507 plusmn 38 Ma for peak UHP metamorphism(Yang et al 2002a) These data indicate that two

discrete collision events occurred between theNorth China and Yangtze cratons (1) an Early Pale-ozoic North Qinling UHP suturing and (2) a Triassicsuturing along the Dabie-Sulu UHP-HP belt TheEarly Paleozoic belt extends westward to the NorthQaidamndashAltun Mountains for more than 4000 km(Yang et al 2002b)

Himalayan UHP Eclogites

The Himalayan orogen has long been considereda classic locality of continental collision betweenthe Indian and Eurasian continents (Fig 5) Severaleclogite-facies HP rocks in the Himalayas havebeen reported since the late 1980s (eg Chaudaryand Ghazanfar 1987 Pognante and Spencer 1991de Sigoyer et al 1997 Lombardo and Rolfo 2000)Inclusions of coesite were recently discovered inomphacite and garnet in several Himalayan eclog-ites from the upper Kaghan Valley Pakistan(OrsquoBrien et al 2001 Kaneko et al 2003) Subse-quent findings of additional coesite localities in theTso Morari area (Sachan et al 2001 Mukherjee etal 2003) have established the presence of a UHPbelt in the Himalayas an active ongoing collision

FIG 5 A simplified tectonic map of the western Himalaya Hindu Kush Pamir Karakoran and western Tibet mod-ified after Figure 3 of Searle et al (2001) This map shows the locations of eclogite and UHP eclogites from both Hima-layan and North Qaidam Indus Suture Zone (ISZ) ophiolites and major structures

16 LIOU ET AL

since the Eocene (Massonne and OrsquoBrien 2003)However UHP relics are volumetrically very minorconstituents of the metamorphic belts dominatedmostly by later Barrovian overprintings In theserocks hydration related to Barrovian-zone metamor-phism has resulted in remarkably pervasive recrys-tallization during the later exhumation this eventhas almost entirely masked the preceding UHPrecord

The finding of HP-UHP evidence has led to newinsights into the Himalayan tectonic modelEngland and Houseman (1986) showed that in amodel of homogeneous thickening of the Hima-layan-Tibet boundary the maximum crustal thick-ness should be less than 70 km However theoccurrences of coesite-bearing eclogites andgneisses in the Himalayan orogen cannot beexplained by a homogeneous thickening of the crustbut rather by continental subduction as describedbelow According to available geochronologicaldata (eg Treloar et al 2003) the exhumation rateof the Himalayan UHP rocks has been estimated torange from ~1 cm yrndash1 (OrsquoBrien et al 2001) to gt3cm yrndash1 (Treloar et al 2003 Massone and OrsquoBrien2003) These estimates are higher than those calcu-lated from other UHP-HP terranes This requirestectonic emplacement of the UHP-HP Himalayanslab into shallow crust following isostatic rebounddue to slab decoupling between the Indian continentand the oceanic crust initiated during the MiddleEocene UHP rocks from both the Kaghan Valleyand Tso-Mori are described below

Kaghan Valley The Kaghan nappe in the north-western Himalayan syntaxis lies between the Indussuture (also called main mantle thrust MMT) to thenorth and the main central thrust (MCT) to thesouthwest (Pognante and Spear 1991 Spencer1993) (Fig 5) The Cretaceous Kohistan island-arcsequence lies north of the Indus suture It developedas a result of intra-oceanic subduction The maincentral thrust separates an underlying Salkhala unitfrom an overlying higher Himalayan crystalline(HHC) unit The Kaghan nappe consists mainly ofgranitic gneiss and paragneiss with minor inter-calated layers of amphibolite marble and quartziteEclogites are preserved in the cores of maficboudins (less than 1 m thick) in gneiss and aresignificantly overprinted by amphibolite assem-blages They contain fine-grained (lt 1 mm) garnetand omphacite together with aggregates of quartzchains of rutile rimmed by titanite and mm-sizedphengitic micas overgrown by randomly oriented

amphiboles up to 1 cm in length Coesite and coesitepseudomorphs occur as inclusions in omphaciteand show the characteristic palisade-quartz aggre-gate texture fractures radiating from these inclu-sions are characteristically developed in the hostpyroxene

Garnet pyroxenendashphengite barometry wasapplied to the eclogites and yielded peak P-T con-ditions of 27ndash29 GPa and 690ndash750degC (OrsquoBrien etal 2001) (Fig 6) Retrograde textures are commonin the eclogites including symplectic intergrowthsof augite amphibole and plagioclase after omphac-ite thermobarometric analyses of these assemblagesyielded 10ndash13 GPa and 600ndash710degC (Kaneko et al2003) On the other hand the dominant graniticgneisses and metapelites of the higher Himalayancrystalline unit contain typical Barrovian-zoneamphibolite-facies assemblages represented bykyanite- and staurolite-bearing garnetiferousmetapelites (Treloar 1995) Extensive overprintinghas mostly obliterated the peak assemblages matrixminerals of the gneisses yielded P-T conditions of07ndash11 GPa and 600ndash700degC (Treloar 1995) con-sistent with retrogression of the eclogites Howeverrelict coesite is preserved as inclusions in zirconthe inclusions are ovoid and approximately 10 microm indiameter (Fig 7A) This discovery of coesite-bear-ing zircons from Himalayan quartzofeldspathicgneisses demonstrates subduction of Indian conti-nental crust to the depths of UHP conditions(Kaneko et al 2003)

Geochronological studies using SmNd (garnet-omphacite) RbSr (phengite) and UPb (rutile zir-con) methods suggest that the eclogite-facies eventtook place at 40ndash50 Ma (Tonarini et al 1993 Spen-cer and Gebauer 1996) 40Ar39Ar cooling ages forhornblende and mica from gneisses were reported tobe ~43 Ma and ~25 Ma respectively (Chamberlainet al 1991) Recent SHRIMP UPb dates of zonedzircons with mineral inclusions formed at differentstages indicate the ages of non-UHP mineral-bear-ing mantle domains of zircon and UHP mineral-bearing rims are at about 50 and 46 Ma respectively(Kaneko et al 2003) A new U-Pb age of 44 plusmn 11Ma was obtained for rutile from Kaghan coesite-bearing eclogite (Treloar et al 2003) this togetherwith available data implies that exhumation of theUHP rocks from mantle depth to mid-crustal leveloccurs within a few million years

Tso Morari The Tso Morari crystalline dome islocated between the Indus suture zone on the northbordered by the Zildat detachment fault and the

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 17

Zanskar sedimentary unit on the south (de Sigoyer etal 1997) The dome represents an internal massif ofthe northern part of the higher Himalaya The TsoMorari area has been considered a typical IndianTethyan margin because of a stratigraphic succes-sion similar to the Nimaling antiform in the core ofthe dome a metamorphic basement consists of Cam-bro-Ordovician augen gneisses (Stutz 1988) Thiscomplex is covered by a metasedimentary series ofCambrian to Devonian quartzite schist and con-glomerate and is overlain by Lower Carboniferousto Triassic marble and metapelite Eclogites occuras lenses or irregular bodies within the basementand cover gneisses and are strongly overprinted byamphibolite-facies assemblages de Sigoyer et al(1997) reported peak P-T conditions for the eclog-ites at 17ndash23 GPa and 520ndash640degC (Fig 7) How-ever relics of coesite 30ndash80 microm in size were

recently found as inclusions in garnet (Sachan et al2001 Mukherjee et al 2003) typified by radialfractures (Figs 7B and 7C) for the SiO2 inclusionsa characteristic Raman spectrum is centered at 523cm-1 (Fig 7D) The country rock metasedimentswith jadeite + chloritoid + paragonite + garnetassemblages have peak P-T estimates of 18ndash22GPa and 500ndash600degC and record retrogression at theeclogite-blueschist facies transition of 13ndash18 GPaand 490ndash590degC (Guillot et al 1997)

The geochronology of the Tso Morari eclogiteshas been extensively studied and subdivided intothree different stages (de Signoyer et al 2000)Eclogitization at ~55 Ma was obtained by Lu-Hf andSm-Nd methods on garnet omphacite glaucophaneand the whole rock An amphibolite-facies overprintoccurred at 47 Ma judging by RbSr dating onphengite apatite and whole rock from metapelites

FIG 6 P-T estimates of peak- and retrograde-stage metamorphism for coesite-bearing eclogites and the country-rockgneisses from the Kaghan Valley and Tso Morari

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 11

Some Maksyutov eclogites were recrystallized at594ndash637degC with a minimum pressure from 15 to17 GPa (Beane et al 1995 Leech and Ernst 1998)or up to 27 GPa if coesite pseudomorphs (Ches-nokov and Popov 1965 Dobretsov and Doretsova1988) are indeed present However subsequentstudies have not revealed the occurrence of inclu-sions of coesite or coesite pseudomorph in garnetand zircon (Beane et al 1995 Shatsky et al 1995)Leech and Ernst (1998) described unusual graphiteaggregates (up to 13 mm edge length) that may bediamond pseudomorphs This suggestion was basedon the blocky morphology of the cuboid graphiteaggregates cross-cutting rock foliation pressureshadows biogenic isotopic signatures and otherspectroscopic characters similar to those graphi-tized diamonds from the Beni Bousera massifRecent study of several microdiamond inclusions ingarnet has yielded further suggestive possibility thatthe eclogitic unit of the Maksyutov Complex experi-enced diamond-grade UHP metamorphism (Bosticket al 2003) The UHPHP metamorphism with peakmetamorphic ages of 370ndash374 Ma (Shatsky et al1995 Beane and Connelly 1998) may be related tothe collision of the Russian platform with a fragmentof the Siberian craton during Devonian time butwas affected by the Triassic collision of the Kazakh-stan block with the Russian platform (eg Matte1998)

Zermatt-Saas area Western Alps In contrast tomost other UHP terranes in which the protolith iscontinental crust eclogites and metasediments atLago di Cignana of the Western Alps were derivedfrom oceanic crust and constitute the ophiolitesequence of the Zermatt-Saas zone Inclusions ofcoesite and coesite pseudomorphs occur in tourma-line and garnet of manganiferous quartzite and inomphacite and garnet of the underlying mafic eclog-ite (Reinecke 1991) The metasediments are mainlygarnet-phengite-quartz schist with variableamounts of garnet-clinopyroxene quartzite andpiedmontite-phengite-quartz schist Eclogites andretrograded eclogites with preserved UHP mineralparageneses consist of garnet omphacite glau-cophane zoisite phengite and dolomite Zoned gar-nets preserve prograde quartz crystals in their coresand coesite inclusions in mantles and rims Coesiteinclusions are much more common in omphacitethan in garnet UHP conditions were estimated at26ndash30 GPa and 590ndash630degC Sm-Nd isotopic anal-yses of eclogites yield UHP metamorphism at 52 plusmn18 Ma (Bowtell et al 1994) consistent with a

Tertiary age of eclogitization recently established forAlpine metamorphism in the Western Alps(Gebauer et al 1997)

Thus far the recognized areal extent of thecoesite-bearing rocks is ~ 2 km2 The apparentthickness of the UHP slice does not exceed a fewhundred meters This area is the best example forsubduction of oceanic lithosphere to mantle depthand later exhumation as a fragmented tectonic slabSimilarly the Allalin metagabbro part of the oce-anic Piemonte zone contains magnesite + talc +kyanite + garnet recrystallized at ~600degC and 35GPa with an estimated geothermal gradient of 5ndash6degC kmndash1 however inclusions of coesite or coesitepseudomorphs have not been reported from theAllalin metagabbro

Mali Africa Eclogitic rocks in the Pan-Africancollision zone cropping out in northern Mali containinclusions of coesite in omphacite from mafic nod-ules within a calc-silicate layer enclosed in impuremarble and coesite pseudomorphs in garnets fromthe surrounding eclogitic metasediments (Caby1994) These UHP eclogitic mica schists have P-Testimates of 700ndash750degC and gt 27 GPa phengiteyields a well-defined plateau 39Ar40Ar age of 1045plusmn 9 Ma Recent SmNd ages of 620 Ma wereobtained by Jahn et al (2001) for peak UHP meta-morphism This may be the oldest UHP terrane inthe world

The coesite-bearing UHP eclogitic unit about 3km thick lies within an internal nappe as a flat thinslice bounded on the top by a thick passive-marginshaly formation and on the bottom by low-gradegreenschist-facies phyllites Caby (1994) suggestedthat a large portion of the terrigenous metasedimentsof passive-continental-margin affinity of the WestAfrican plate was subducted eastward to mantledepths (gt90 km) beneath an oceanic domain Colli-sion of the West African plate with an island arcresulted in a low-angle westward subhorizontalextrusion of this subducted slice of sialic materialsto its present position The coesite-bearing eclogiteunit may also crop out 1500 km to the south in Togowhere kyanite-bearing eclogites occur as part of thepassive-margin assemblage

Makbal (480 Ma) and Atbashy (270 Ma) Kazak-hstan The Tian-Shan is divided into three mountainrangesmdashnorthern middle and southern Within theKyrgyzstan Tian-Shan UHP eclogite localities withcoesite pseudomorph inclusions in garnet andomphacite from the northern and southern Tian-Shan represent Caledonian and Hercynian orogenic

12 LIOU ET AL

belts respectively (Tagiri and Bakirov 1990 Tagiriet al 1995) The Makbal Formation in the westernKyrgyz Ridge of the northern Tian-Shan consists ofquartzose schist alternating with pelitic schistsintercalated with thin layers or lenses of marbleeclogite and amphibolitized eclogite Tagiri andBakirov (1990) found inclusions of coesite pseudo-morphs in garnet from a garnet-chloritoid-talc schistwith a peak metamorphic assemblage of coesite +almandine + chloritoid + talc + phengite (Si = 342pfu) + phlogopite + rutile Makbal coesite-gradeeclogite has a paragonite KAr age of 480 Ma

The eclogite-bearing complex of Atbashy Ridgefrom the southern Tian-Shan is composed of peliticand siliceous schists alternating with thin UHPeclogitic layers and is unconformably overlain byUpper Paleozoic molasse and silicic volcanic rocksInclusions of coesite pseudomorphs occur inomphacite cores and in garnet Eclogites from theAtbashy area preserve prograde and retrogradepaths with a peak metamorphic condition at 660degC25 GPa The Atbashy coesite-eclogites yielded aRbSr mineral isochron age of 270 Ma (Tagiri et al1995)

Central Indonesia UHP terrane The pre-Tertiarybasement of the Indonesian region comprises a vari-ety of imbricate terranes meacutelange ophiolite andvariably metamorphosed accretionary complexessome contain HP to UHP metamorphic rocks result-ing from the collision of an Australia-derived conti-nent with Eurasia (Parkinson 2003) HP rocksincluding eclogites and garnet peridotites are widelydistributed in Cretaceous accretionary complexesMany of these rocks occur as imbricate slices of car-bonate quartzite and pelitic schist of shallow-marine or continental-margin parentage interthrustwith subordinate mafic schist and serpentinitesome yield mica KAr ages of 110ndash120 Ma (Parkin-son 2003)

HP and UHP rocks are sporadically exposed astectonic blocks throughout the Cretaceous accre-tionary complexes They include eclogite garnet-glaucophane rock (P = 18ndash24 GPa T = 580ndash620degC) and jadeite-garnet quartzite (P gt 27 GPa T =720ndash760degC) in Bantimala southwest Sulawesieclogite and garnet granulite in west-centralSulawesi eclogite and jadeite-glaucophane-quartzrock (P ~22 GPa T ~530degC) in central Java Mgchloritoidndashbearing whiteschists (P ~18 GPa) insoutheast Kalimantan garnet lherzolites in east-central Sulawesi (P = 22ndash28 GPa T = 1000ndash1100degC) west-central Sulawesi (P = 16ndash20 GPa T =

1050ndash1100degC) and garnet pyroxenite (P ~2 GPa T~ 850degC) in Sabah northeast Borneo Evidence forUHP rocks include (1) inclusions of coesite inzircon and coesite pseudomorphs in jadeite fromjadeite quartzite and eclogite of the BantimalaComplex of south Sulawesi (Parkinson 2003) and(2) P-T estimates of peak-stage recrystallization at27ndash35 GPa and 1000ndash1100degC for most garnetperidotites (Kardarusman and Parkinson 2000)Many of these rocks were probably recrystallized ina N-dipping subduction zone at the margin of theSundaland craton in the Early Cretaceous Exhuma-tion may have been facilitated by the collision of aGondwana continental fragment with the Sundalandmargin at ~120ndash115 Ma

Recently Recognized UHP Terranes

Several new UHP terranes were recently identi-fied inasmuch as they contain partially preservedtrace index minerals in strong containers such aszircon or garnet In fact zircon has been consideredto be the best container and many new terranes werediscovered through positive identification of inclu-sions of coesite or coesite pseudomorphs ordiamond in zircons Detailed examination of mineralinclusions in core mantle and rims of zircon sepa-rates from eclogites and their enclosing country-rock gneisses have yielded both prograde andretrograde P-T-time paths for various UHP terranesmentioned above (see Katayama et al 2001 fordetails also see the section on the Himalayan eclog-ite for an example)

The latest recognized UHP terranes include thefollowing (1) coesite in the French Massif Central(Lardeaux et al 2001) (2) coesite and diamondfrom the Greek Rhodope metamorphic province(Mposkos and Kostopoulos 2001) (3) coesitepseudomorphs in the Northeast Greenland EclogiteProvince (Gilotti and Ravna 2002) (4) coesite ingranulite-facies overprinting eclogites of SoutheastBrazil (Parkinson et al 2001) (5) coesite in Hima-layan eclogite from the Upper Kaghan Valley Paki-stan (OrsquoBrien et al 2001 Kaneko et al 2003) andcoesite from the Tso-Morari crystalline complex ofIndia (Mukherjee et al 2003) (6) coesite inclusionsin gneissic rocks from the North Qaidam belt west-ern China (Yang et al 2001 Song et al 2003) (7)diamond inclusions in garnets of eclogite and gneissfrom North Qinling (Yang et al 2002a) (8) inclu-sions of quartz pseudomorphs and minor coesite ingarnets of mafic eclogites from the LantermanRange in Antarctica (Ghiribelli et al 2002) (9)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 13

inclusions of calcite pseudomorphs after aragonite +magnesite in dolomite in metapelites from the west-ern Tian-Shan inferred as metamorphism in a UHPregion of dolomite decomposition (Zhang L et al2002) and (10) inclusion of possible coesitepseudomorphs in garnet lamellar inclusions in gar-net kyanite and high sodium content of garnet fromeclogites of the Chuacus Complex of north-centralGuatemala (Solari et al 2003) The results of petro-chemical and geochronological data for these newterranes are summarized below The HimalayanUHP terrane in the Upper Kaghan Valley and theTso-Morari complex are used to illustrate the pro-cesses of continental subduction and collision in alater section

French Massif Central The French Massif Cen-tral in the western part of the Variscan Belt hasexperienced Late SilurianndashEarly Devonian to LateCarboniferous orogenic events (eg Matte 1986)High-pressure metamorphic rocks occur mainly inthe upper gneissic unit within the leptyno-amphibo-lite group The latter consists of an association ofmetagreywacke mica schist metabasalt leptynitemetagranite and peridotite Eclogites are associatedwith silicic and mafic HP granulites (Lardeaux etal 2001) and also with spinel andor garnet lherzo-lites (Gardien et al 1990) Inclusions of coesite andcoesite pseudomorphs occur in garnets of kyanite-bearing eclogite (Lardeaux et al 2001) Only twocoesite grains are preserved as relics most coesitegrains are completely transformed into polycrystal-line radial quartz (palisade texture) or into polygonalquartz surrounded by radial cracks Metamorphictemperatures were estimated to be 740-780degC andsubsequent amphibolite-facies overprinting at750degC at 15ndash17 GPa during decompression UHPmetamorphism occurred between 420 and 400 Ma(Paquette et al 1995) 40Ar39Ar data from amphib-ole separates from retrogressed eclogites yielded339 plusmn 4 Ma (Costa et al 1993)

Northeast Greenland Caledonides The first evi-dence for UHP metamorphism in the GreenlandCaledonites was reported from kyanite eclogites andassociated host gneisses on an island in Jokelbugt(Gilotti and Ravna 2002) Inclusions of quartzpseudomorphs exhibiting palisade structure andradiating fractures occur in garnet and omphacite ofeclogites and in garnet of the host gneisses P-T esti-mates of peak-stage metamorphism of eclogite at~972degC and 36 GPa lie well within the coesite sta-bility field SHRIMP UPb dates of 403 plusmn 5 Ma forzircon rims from HP metapsammite and 404 plusmn 4 Ma

for zircons from anatectic melt derived frommetapelite are coeval with the estimated ages ofexhumation of UHP terranes in the ScandinavianCaledonides described in a previous section(McClelland and Gilotti 2003)

Neoproterozoic nappes in Southeast BrazilNeoproterozoic nappes of Southeast Brazil consistmainly of coarse-grained kyanite-garnet granulitewith intercalations of impure quartzite calc-silicaterock and minor lenses and sills of mafic-ultramaficrocks including eclogite Early eclogitic phaseswere strongly obliterated during granulite-faciesoverprinting during a major Pan-African continentcollision at 640ndash630 Ma (Campos Neto and Caby1999) Rare ultramafic lenses consist of phlogopite-orthopyroxene rocks and garnet clinopyroxeniteCores of granulite boudins retain eclogitic assem-blages some quartz inclusions within the mantleregions of zoned garnet porphyroblasts are sur-rounded by intense radial fractures suggestive ofthe former presence of coesite In fact coarse-grained zircons in some granulites contain numer-ous micro-inclusions of K-feldspar + kyanite + coes-ite as confirmed by Raman spectroscopy (Parkinsonet al 2001) Many exsolution microstructuresobserved in other UHP terranes also occur Thisarea represents the first coesite-grade UHP rocks inthe Americas and it can be correlated with the MaliUHP terrane of equatorial Africa (Caby 1994) Ithas been suggested that eclogitic assemblages andprobable UHPM components may be present in sev-eral other correlative nappe systems at the marginsof the Sao Francisco craton in Brazil

Chuacus Complex north-central Guatemala HPeclogites within banded gneisses schists andmigmatites of the Chuacus Complex contain relicttextures and mineralogical features that suggestUHP metamorphism (Solari et al 2003) Theseinclude inclusions of possible coesite pseudomor-phs in garnet and kyanite lamellar inclusions ofrutileilmenite in garnet high sodium content ofsome garnets (up to 1200 ppm) and preliminary P-T estimates of 700ndash800degC and 2ndash3 GPa AlthoughK-Ar dating of mica and hornblende yield ages of48ndash72 Ma possible for amphibolite facies retrogres-sion regional and structural data support a pre-Mesozoic age for the multi-stage HP-UHP recrystal-lization Such a discovery opens up a new interpre-tation regarding the tectonic evolution of Maya-Chortis-Oaxaquia microcontinental blocks formingthe structural backbone of Mesoamerica

14 LIOU ET AL

Greek Rhodope The Greek Rhodope Metamor-phic Province (RMP) at the border between Greeceand Bulgaria represents a syn-metamorphic nappesystem of Alpine age that was formed during theCretaceous to Mid-Tertiary collision of Apulia andpaleo-Europe The Kimi nappe complex comprisescrustal eclogites leucocratic orthogneisses peliticgneisses and volumetrically minor mantle-derivedultramafic rocks these units contain many mineral-ogical indicators of UHP metamorphism (Mposkosand Kostopoulos 2001) The ultramafic lithologiesinclude garnet spinelndashbearing lherzolite and layersof spinel-garnet clinopyroxenite garnet pyroxeniteand clinopyroxene garnetite These rocks were sub-jected to UHP recrystallization at P-T conditions of~1200degC and gt30 GPa The pelitic gneiss containsmicrodiamond inclusions in garnet porphyroblastsas well as rods or needles of silica rutile biotiteand apatite in sodic garnet suggesting the prioroccurrence of majoritic garnet The growth of suchsupersilicic garnet was suggested to be at a pressureof ~7 GPa Inclusions of polycrystalline quartzaggregates surrounded by radial cracks occur ineclogitic garnet Exsolution lamellae of quartz ineclogitic clinopyroxene also indicate an UHP pre-cursor of supersilicic clinopyroxene

Lanterman Range Antarctica The LantermanRange in northern Victoria Land of Antarctica con-sists of several metamorphic complexes The Gate-way Hills Metamorphic Complex is a thindiscontinuous belt more than 50 km long consistingof mafic and ultramafic rocks including lenses andpods of eclogite within felsic gneiss Some less foli-ated eclogites contain inclusions of coesite andcoesite pseudomorphs in garnet porphyroblasts asconfirmed by in situ Raman spectroscopy as well asradial fractures around the inclusions (Ghiribelli etal 2002) The eclogites were subjected to UHPmetamorphism at T gt 850degC and P gt 29 GPa thenamphibolite-facies overprinting during isothermaldecompression The UHP event is dated by SmNdand 238U206Pb data at 500 Ma 40Ar39Ar ages of490ndash486 Ma for Ca-amphibole (amphibolite-faciesoverprinting) suggest rapid cooling with an averageexhumation rate of 3ndash4 km_Ma-1 for the UHP ter-rane (Di Vincenzo and Palmeri 2001)

Western Tian-Shan China Caledonian eclogitesof the western Tian-Shan China have been classi-fied into three types (Zhang L et al 2002) Type Ieclogite pods are layered with mafic blueschists andType II eclogites preserved pillow structureswhereas Type III eclogites are banded with marbles

This 200 km long eclogite-bearing belt extendswestward to connect with the Atbashy UHP belt inKazakhstan (Tagiri et al 1995) Reported UHP evi-dence includes inclusions of coesite pseudomorphsin garnets from Type I and III eclogites quartzexsolution lamellae in omphacite from Type IIeclogites and relict magnesite within matrix dolo-mite in Type III eclogites (Zhang J X et al 2002Zhang L et al 2002) P-T estimates of the peakUHP stage of metamorphism are T = 500ndash600degCand P = 26ndash27 GPa (Wei et al 2003)

North Qaidam Mountains western China Manylenses and layers of garnet peridotite and eclogiteoccur in an amphibolite-facies terrane in the NorthQaidam Mountains along the northeastern rim of theQaidam Basin This belt comprises mainly felsicgneiss quartz schist garnet-bearing gneissamphibolite and minor eclogite + garnet peridotiteit extends westward for more than 1000 km to theAltun Mountains and was displaced by the left-lat-eral Altyn-Tagh fault At the eastern end of thisUHP belt at Dulan occurrences of coesite inclu-sions in zircon separates from paragneiss inclusionsof coesite pseudomorph in garnet and omphacite ofeclogites quartz rods in eclogitic omphacite andP-T estimates of eclogites (P = 287ndash317 GPa andT = 631ndash687degC) demonstrate that this is a typicalUHP metamorphic terrane (Yang et al 2001 Songet al 2003a 2003b) The Dulan eclogites have anSm-Nd mineral isochron age of 497 plusmn 87 Ma and aSHRIMP UPb zircon age of 495 Ma (Yang et al2002b)

Altun Mountains western China This UHP ter-rane about 200 km long in the southern margin ofthe Altun Mountains is the western extension of theNorth Qaidam belt Eclogitic lenses and blocks ofvarious sizes together with garnet-lherzolite and cli-nopyroxenite are enclosed within an Early Protero-zoic sequence of amphibolite amphibole schistgarnet-bearing granitic gneiss and pelitic schistGarnet lherzolite contains magnesite that reacted toform dolomite P-T estimates of the peak magnesite-bearing assemblage yield P gt 36 GPa and 850degC(Liu et al 2001) Garnet porphyroblasts includeexsolution lamellae of clinopyroxene and rutile sim-ilar to that of eclogite reported by Ye et al (2000) asmajoritic garnet The associated eclogites containinclusions of coesite pseudomorphs in garnet withcharacteristic radial fractures this together with theexsolved quartz rods in omphacite and P-T esti-mates of 820ndash850degC and 28 to 32 GPa provideadditional evidence for UHP (Zhang J X et al

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 15

2001) An SmNd mineral isochron for eclogiticgarnet omphacite and the whole rock yields 500 plusmn10 Ma whereas an UPb isochron for zircon sepa-rates from eclogite gives an age of 504 plusmn 5 Ma(Zhang J X et al 2000)

North Qinling Mountains central China Lensesand blocks of eclogitic rocks in northern Qinling areenclosed in amphibolite-facies gneiss and garnet-bearing quartz + phengite mica schist Inclusions ofcoesite in eclogitic garnet (Hu et al 1995) andmicrodiamonds in zircon separates from both eclog-ite and gneiss (Yang et al 2002a) have beenrecently reported Although individual outcrops areno more than a few meters wide the belt of eclogitesextends more than 10 km P-T estimates of amphib-olitized coesite-bearing eclogite yield T = 590ndash758degC and P gt 26 GPa An Sm-Nd isochron for gar-net omphacite rutile amphibole and whole-rockyields 400 plusmn 16 Ma Microdiamonds were recentlydiscovered as inclusions in zircon from both eclogiteand its gneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyields 507 plusmn 38 Ma for peak UHP metamorphism(Yang et al 2002a) These data indicate that two

discrete collision events occurred between theNorth China and Yangtze cratons (1) an Early Pale-ozoic North Qinling UHP suturing and (2) a Triassicsuturing along the Dabie-Sulu UHP-HP belt TheEarly Paleozoic belt extends westward to the NorthQaidamndashAltun Mountains for more than 4000 km(Yang et al 2002b)

Himalayan UHP Eclogites

The Himalayan orogen has long been considereda classic locality of continental collision betweenthe Indian and Eurasian continents (Fig 5) Severaleclogite-facies HP rocks in the Himalayas havebeen reported since the late 1980s (eg Chaudaryand Ghazanfar 1987 Pognante and Spencer 1991de Sigoyer et al 1997 Lombardo and Rolfo 2000)Inclusions of coesite were recently discovered inomphacite and garnet in several Himalayan eclog-ites from the upper Kaghan Valley Pakistan(OrsquoBrien et al 2001 Kaneko et al 2003) Subse-quent findings of additional coesite localities in theTso Morari area (Sachan et al 2001 Mukherjee etal 2003) have established the presence of a UHPbelt in the Himalayas an active ongoing collision

FIG 5 A simplified tectonic map of the western Himalaya Hindu Kush Pamir Karakoran and western Tibet mod-ified after Figure 3 of Searle et al (2001) This map shows the locations of eclogite and UHP eclogites from both Hima-layan and North Qaidam Indus Suture Zone (ISZ) ophiolites and major structures

16 LIOU ET AL

since the Eocene (Massonne and OrsquoBrien 2003)However UHP relics are volumetrically very minorconstituents of the metamorphic belts dominatedmostly by later Barrovian overprintings In theserocks hydration related to Barrovian-zone metamor-phism has resulted in remarkably pervasive recrys-tallization during the later exhumation this eventhas almost entirely masked the preceding UHPrecord

The finding of HP-UHP evidence has led to newinsights into the Himalayan tectonic modelEngland and Houseman (1986) showed that in amodel of homogeneous thickening of the Hima-layan-Tibet boundary the maximum crustal thick-ness should be less than 70 km However theoccurrences of coesite-bearing eclogites andgneisses in the Himalayan orogen cannot beexplained by a homogeneous thickening of the crustbut rather by continental subduction as describedbelow According to available geochronologicaldata (eg Treloar et al 2003) the exhumation rateof the Himalayan UHP rocks has been estimated torange from ~1 cm yrndash1 (OrsquoBrien et al 2001) to gt3cm yrndash1 (Treloar et al 2003 Massone and OrsquoBrien2003) These estimates are higher than those calcu-lated from other UHP-HP terranes This requirestectonic emplacement of the UHP-HP Himalayanslab into shallow crust following isostatic rebounddue to slab decoupling between the Indian continentand the oceanic crust initiated during the MiddleEocene UHP rocks from both the Kaghan Valleyand Tso-Mori are described below

Kaghan Valley The Kaghan nappe in the north-western Himalayan syntaxis lies between the Indussuture (also called main mantle thrust MMT) to thenorth and the main central thrust (MCT) to thesouthwest (Pognante and Spear 1991 Spencer1993) (Fig 5) The Cretaceous Kohistan island-arcsequence lies north of the Indus suture It developedas a result of intra-oceanic subduction The maincentral thrust separates an underlying Salkhala unitfrom an overlying higher Himalayan crystalline(HHC) unit The Kaghan nappe consists mainly ofgranitic gneiss and paragneiss with minor inter-calated layers of amphibolite marble and quartziteEclogites are preserved in the cores of maficboudins (less than 1 m thick) in gneiss and aresignificantly overprinted by amphibolite assem-blages They contain fine-grained (lt 1 mm) garnetand omphacite together with aggregates of quartzchains of rutile rimmed by titanite and mm-sizedphengitic micas overgrown by randomly oriented

amphiboles up to 1 cm in length Coesite and coesitepseudomorphs occur as inclusions in omphaciteand show the characteristic palisade-quartz aggre-gate texture fractures radiating from these inclu-sions are characteristically developed in the hostpyroxene

Garnet pyroxenendashphengite barometry wasapplied to the eclogites and yielded peak P-T con-ditions of 27ndash29 GPa and 690ndash750degC (OrsquoBrien etal 2001) (Fig 6) Retrograde textures are commonin the eclogites including symplectic intergrowthsof augite amphibole and plagioclase after omphac-ite thermobarometric analyses of these assemblagesyielded 10ndash13 GPa and 600ndash710degC (Kaneko et al2003) On the other hand the dominant graniticgneisses and metapelites of the higher Himalayancrystalline unit contain typical Barrovian-zoneamphibolite-facies assemblages represented bykyanite- and staurolite-bearing garnetiferousmetapelites (Treloar 1995) Extensive overprintinghas mostly obliterated the peak assemblages matrixminerals of the gneisses yielded P-T conditions of07ndash11 GPa and 600ndash700degC (Treloar 1995) con-sistent with retrogression of the eclogites Howeverrelict coesite is preserved as inclusions in zirconthe inclusions are ovoid and approximately 10 microm indiameter (Fig 7A) This discovery of coesite-bear-ing zircons from Himalayan quartzofeldspathicgneisses demonstrates subduction of Indian conti-nental crust to the depths of UHP conditions(Kaneko et al 2003)

Geochronological studies using SmNd (garnet-omphacite) RbSr (phengite) and UPb (rutile zir-con) methods suggest that the eclogite-facies eventtook place at 40ndash50 Ma (Tonarini et al 1993 Spen-cer and Gebauer 1996) 40Ar39Ar cooling ages forhornblende and mica from gneisses were reported tobe ~43 Ma and ~25 Ma respectively (Chamberlainet al 1991) Recent SHRIMP UPb dates of zonedzircons with mineral inclusions formed at differentstages indicate the ages of non-UHP mineral-bear-ing mantle domains of zircon and UHP mineral-bearing rims are at about 50 and 46 Ma respectively(Kaneko et al 2003) A new U-Pb age of 44 plusmn 11Ma was obtained for rutile from Kaghan coesite-bearing eclogite (Treloar et al 2003) this togetherwith available data implies that exhumation of theUHP rocks from mantle depth to mid-crustal leveloccurs within a few million years

Tso Morari The Tso Morari crystalline dome islocated between the Indus suture zone on the northbordered by the Zildat detachment fault and the

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 17

Zanskar sedimentary unit on the south (de Sigoyer etal 1997) The dome represents an internal massif ofthe northern part of the higher Himalaya The TsoMorari area has been considered a typical IndianTethyan margin because of a stratigraphic succes-sion similar to the Nimaling antiform in the core ofthe dome a metamorphic basement consists of Cam-bro-Ordovician augen gneisses (Stutz 1988) Thiscomplex is covered by a metasedimentary series ofCambrian to Devonian quartzite schist and con-glomerate and is overlain by Lower Carboniferousto Triassic marble and metapelite Eclogites occuras lenses or irregular bodies within the basementand cover gneisses and are strongly overprinted byamphibolite-facies assemblages de Sigoyer et al(1997) reported peak P-T conditions for the eclog-ites at 17ndash23 GPa and 520ndash640degC (Fig 7) How-ever relics of coesite 30ndash80 microm in size were

recently found as inclusions in garnet (Sachan et al2001 Mukherjee et al 2003) typified by radialfractures (Figs 7B and 7C) for the SiO2 inclusionsa characteristic Raman spectrum is centered at 523cm-1 (Fig 7D) The country rock metasedimentswith jadeite + chloritoid + paragonite + garnetassemblages have peak P-T estimates of 18ndash22GPa and 500ndash600degC and record retrogression at theeclogite-blueschist facies transition of 13ndash18 GPaand 490ndash590degC (Guillot et al 1997)

The geochronology of the Tso Morari eclogiteshas been extensively studied and subdivided intothree different stages (de Signoyer et al 2000)Eclogitization at ~55 Ma was obtained by Lu-Hf andSm-Nd methods on garnet omphacite glaucophaneand the whole rock An amphibolite-facies overprintoccurred at 47 Ma judging by RbSr dating onphengite apatite and whole rock from metapelites

FIG 6 P-T estimates of peak- and retrograde-stage metamorphism for coesite-bearing eclogites and the country-rockgneisses from the Kaghan Valley and Tso Morari

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

12 LIOU ET AL

belts respectively (Tagiri and Bakirov 1990 Tagiriet al 1995) The Makbal Formation in the westernKyrgyz Ridge of the northern Tian-Shan consists ofquartzose schist alternating with pelitic schistsintercalated with thin layers or lenses of marbleeclogite and amphibolitized eclogite Tagiri andBakirov (1990) found inclusions of coesite pseudo-morphs in garnet from a garnet-chloritoid-talc schistwith a peak metamorphic assemblage of coesite +almandine + chloritoid + talc + phengite (Si = 342pfu) + phlogopite + rutile Makbal coesite-gradeeclogite has a paragonite KAr age of 480 Ma

The eclogite-bearing complex of Atbashy Ridgefrom the southern Tian-Shan is composed of peliticand siliceous schists alternating with thin UHPeclogitic layers and is unconformably overlain byUpper Paleozoic molasse and silicic volcanic rocksInclusions of coesite pseudomorphs occur inomphacite cores and in garnet Eclogites from theAtbashy area preserve prograde and retrogradepaths with a peak metamorphic condition at 660degC25 GPa The Atbashy coesite-eclogites yielded aRbSr mineral isochron age of 270 Ma (Tagiri et al1995)

Central Indonesia UHP terrane The pre-Tertiarybasement of the Indonesian region comprises a vari-ety of imbricate terranes meacutelange ophiolite andvariably metamorphosed accretionary complexessome contain HP to UHP metamorphic rocks result-ing from the collision of an Australia-derived conti-nent with Eurasia (Parkinson 2003) HP rocksincluding eclogites and garnet peridotites are widelydistributed in Cretaceous accretionary complexesMany of these rocks occur as imbricate slices of car-bonate quartzite and pelitic schist of shallow-marine or continental-margin parentage interthrustwith subordinate mafic schist and serpentinitesome yield mica KAr ages of 110ndash120 Ma (Parkin-son 2003)

HP and UHP rocks are sporadically exposed astectonic blocks throughout the Cretaceous accre-tionary complexes They include eclogite garnet-glaucophane rock (P = 18ndash24 GPa T = 580ndash620degC) and jadeite-garnet quartzite (P gt 27 GPa T =720ndash760degC) in Bantimala southwest Sulawesieclogite and garnet granulite in west-centralSulawesi eclogite and jadeite-glaucophane-quartzrock (P ~22 GPa T ~530degC) in central Java Mgchloritoidndashbearing whiteschists (P ~18 GPa) insoutheast Kalimantan garnet lherzolites in east-central Sulawesi (P = 22ndash28 GPa T = 1000ndash1100degC) west-central Sulawesi (P = 16ndash20 GPa T =

1050ndash1100degC) and garnet pyroxenite (P ~2 GPa T~ 850degC) in Sabah northeast Borneo Evidence forUHP rocks include (1) inclusions of coesite inzircon and coesite pseudomorphs in jadeite fromjadeite quartzite and eclogite of the BantimalaComplex of south Sulawesi (Parkinson 2003) and(2) P-T estimates of peak-stage recrystallization at27ndash35 GPa and 1000ndash1100degC for most garnetperidotites (Kardarusman and Parkinson 2000)Many of these rocks were probably recrystallized ina N-dipping subduction zone at the margin of theSundaland craton in the Early Cretaceous Exhuma-tion may have been facilitated by the collision of aGondwana continental fragment with the Sundalandmargin at ~120ndash115 Ma

Recently Recognized UHP Terranes

Several new UHP terranes were recently identi-fied inasmuch as they contain partially preservedtrace index minerals in strong containers such aszircon or garnet In fact zircon has been consideredto be the best container and many new terranes werediscovered through positive identification of inclu-sions of coesite or coesite pseudomorphs ordiamond in zircons Detailed examination of mineralinclusions in core mantle and rims of zircon sepa-rates from eclogites and their enclosing country-rock gneisses have yielded both prograde andretrograde P-T-time paths for various UHP terranesmentioned above (see Katayama et al 2001 fordetails also see the section on the Himalayan eclog-ite for an example)

The latest recognized UHP terranes include thefollowing (1) coesite in the French Massif Central(Lardeaux et al 2001) (2) coesite and diamondfrom the Greek Rhodope metamorphic province(Mposkos and Kostopoulos 2001) (3) coesitepseudomorphs in the Northeast Greenland EclogiteProvince (Gilotti and Ravna 2002) (4) coesite ingranulite-facies overprinting eclogites of SoutheastBrazil (Parkinson et al 2001) (5) coesite in Hima-layan eclogite from the Upper Kaghan Valley Paki-stan (OrsquoBrien et al 2001 Kaneko et al 2003) andcoesite from the Tso-Morari crystalline complex ofIndia (Mukherjee et al 2003) (6) coesite inclusionsin gneissic rocks from the North Qaidam belt west-ern China (Yang et al 2001 Song et al 2003) (7)diamond inclusions in garnets of eclogite and gneissfrom North Qinling (Yang et al 2002a) (8) inclu-sions of quartz pseudomorphs and minor coesite ingarnets of mafic eclogites from the LantermanRange in Antarctica (Ghiribelli et al 2002) (9)

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 13

inclusions of calcite pseudomorphs after aragonite +magnesite in dolomite in metapelites from the west-ern Tian-Shan inferred as metamorphism in a UHPregion of dolomite decomposition (Zhang L et al2002) and (10) inclusion of possible coesitepseudomorphs in garnet lamellar inclusions in gar-net kyanite and high sodium content of garnet fromeclogites of the Chuacus Complex of north-centralGuatemala (Solari et al 2003) The results of petro-chemical and geochronological data for these newterranes are summarized below The HimalayanUHP terrane in the Upper Kaghan Valley and theTso-Morari complex are used to illustrate the pro-cesses of continental subduction and collision in alater section

French Massif Central The French Massif Cen-tral in the western part of the Variscan Belt hasexperienced Late SilurianndashEarly Devonian to LateCarboniferous orogenic events (eg Matte 1986)High-pressure metamorphic rocks occur mainly inthe upper gneissic unit within the leptyno-amphibo-lite group The latter consists of an association ofmetagreywacke mica schist metabasalt leptynitemetagranite and peridotite Eclogites are associatedwith silicic and mafic HP granulites (Lardeaux etal 2001) and also with spinel andor garnet lherzo-lites (Gardien et al 1990) Inclusions of coesite andcoesite pseudomorphs occur in garnets of kyanite-bearing eclogite (Lardeaux et al 2001) Only twocoesite grains are preserved as relics most coesitegrains are completely transformed into polycrystal-line radial quartz (palisade texture) or into polygonalquartz surrounded by radial cracks Metamorphictemperatures were estimated to be 740-780degC andsubsequent amphibolite-facies overprinting at750degC at 15ndash17 GPa during decompression UHPmetamorphism occurred between 420 and 400 Ma(Paquette et al 1995) 40Ar39Ar data from amphib-ole separates from retrogressed eclogites yielded339 plusmn 4 Ma (Costa et al 1993)

Northeast Greenland Caledonides The first evi-dence for UHP metamorphism in the GreenlandCaledonites was reported from kyanite eclogites andassociated host gneisses on an island in Jokelbugt(Gilotti and Ravna 2002) Inclusions of quartzpseudomorphs exhibiting palisade structure andradiating fractures occur in garnet and omphacite ofeclogites and in garnet of the host gneisses P-T esti-mates of peak-stage metamorphism of eclogite at~972degC and 36 GPa lie well within the coesite sta-bility field SHRIMP UPb dates of 403 plusmn 5 Ma forzircon rims from HP metapsammite and 404 plusmn 4 Ma

for zircons from anatectic melt derived frommetapelite are coeval with the estimated ages ofexhumation of UHP terranes in the ScandinavianCaledonides described in a previous section(McClelland and Gilotti 2003)

Neoproterozoic nappes in Southeast BrazilNeoproterozoic nappes of Southeast Brazil consistmainly of coarse-grained kyanite-garnet granulitewith intercalations of impure quartzite calc-silicaterock and minor lenses and sills of mafic-ultramaficrocks including eclogite Early eclogitic phaseswere strongly obliterated during granulite-faciesoverprinting during a major Pan-African continentcollision at 640ndash630 Ma (Campos Neto and Caby1999) Rare ultramafic lenses consist of phlogopite-orthopyroxene rocks and garnet clinopyroxeniteCores of granulite boudins retain eclogitic assem-blages some quartz inclusions within the mantleregions of zoned garnet porphyroblasts are sur-rounded by intense radial fractures suggestive ofthe former presence of coesite In fact coarse-grained zircons in some granulites contain numer-ous micro-inclusions of K-feldspar + kyanite + coes-ite as confirmed by Raman spectroscopy (Parkinsonet al 2001) Many exsolution microstructuresobserved in other UHP terranes also occur Thisarea represents the first coesite-grade UHP rocks inthe Americas and it can be correlated with the MaliUHP terrane of equatorial Africa (Caby 1994) Ithas been suggested that eclogitic assemblages andprobable UHPM components may be present in sev-eral other correlative nappe systems at the marginsof the Sao Francisco craton in Brazil

Chuacus Complex north-central Guatemala HPeclogites within banded gneisses schists andmigmatites of the Chuacus Complex contain relicttextures and mineralogical features that suggestUHP metamorphism (Solari et al 2003) Theseinclude inclusions of possible coesite pseudomor-phs in garnet and kyanite lamellar inclusions ofrutileilmenite in garnet high sodium content ofsome garnets (up to 1200 ppm) and preliminary P-T estimates of 700ndash800degC and 2ndash3 GPa AlthoughK-Ar dating of mica and hornblende yield ages of48ndash72 Ma possible for amphibolite facies retrogres-sion regional and structural data support a pre-Mesozoic age for the multi-stage HP-UHP recrystal-lization Such a discovery opens up a new interpre-tation regarding the tectonic evolution of Maya-Chortis-Oaxaquia microcontinental blocks formingthe structural backbone of Mesoamerica

14 LIOU ET AL

Greek Rhodope The Greek Rhodope Metamor-phic Province (RMP) at the border between Greeceand Bulgaria represents a syn-metamorphic nappesystem of Alpine age that was formed during theCretaceous to Mid-Tertiary collision of Apulia andpaleo-Europe The Kimi nappe complex comprisescrustal eclogites leucocratic orthogneisses peliticgneisses and volumetrically minor mantle-derivedultramafic rocks these units contain many mineral-ogical indicators of UHP metamorphism (Mposkosand Kostopoulos 2001) The ultramafic lithologiesinclude garnet spinelndashbearing lherzolite and layersof spinel-garnet clinopyroxenite garnet pyroxeniteand clinopyroxene garnetite These rocks were sub-jected to UHP recrystallization at P-T conditions of~1200degC and gt30 GPa The pelitic gneiss containsmicrodiamond inclusions in garnet porphyroblastsas well as rods or needles of silica rutile biotiteand apatite in sodic garnet suggesting the prioroccurrence of majoritic garnet The growth of suchsupersilicic garnet was suggested to be at a pressureof ~7 GPa Inclusions of polycrystalline quartzaggregates surrounded by radial cracks occur ineclogitic garnet Exsolution lamellae of quartz ineclogitic clinopyroxene also indicate an UHP pre-cursor of supersilicic clinopyroxene

Lanterman Range Antarctica The LantermanRange in northern Victoria Land of Antarctica con-sists of several metamorphic complexes The Gate-way Hills Metamorphic Complex is a thindiscontinuous belt more than 50 km long consistingof mafic and ultramafic rocks including lenses andpods of eclogite within felsic gneiss Some less foli-ated eclogites contain inclusions of coesite andcoesite pseudomorphs in garnet porphyroblasts asconfirmed by in situ Raman spectroscopy as well asradial fractures around the inclusions (Ghiribelli etal 2002) The eclogites were subjected to UHPmetamorphism at T gt 850degC and P gt 29 GPa thenamphibolite-facies overprinting during isothermaldecompression The UHP event is dated by SmNdand 238U206Pb data at 500 Ma 40Ar39Ar ages of490ndash486 Ma for Ca-amphibole (amphibolite-faciesoverprinting) suggest rapid cooling with an averageexhumation rate of 3ndash4 km_Ma-1 for the UHP ter-rane (Di Vincenzo and Palmeri 2001)

Western Tian-Shan China Caledonian eclogitesof the western Tian-Shan China have been classi-fied into three types (Zhang L et al 2002) Type Ieclogite pods are layered with mafic blueschists andType II eclogites preserved pillow structureswhereas Type III eclogites are banded with marbles

This 200 km long eclogite-bearing belt extendswestward to connect with the Atbashy UHP belt inKazakhstan (Tagiri et al 1995) Reported UHP evi-dence includes inclusions of coesite pseudomorphsin garnets from Type I and III eclogites quartzexsolution lamellae in omphacite from Type IIeclogites and relict magnesite within matrix dolo-mite in Type III eclogites (Zhang J X et al 2002Zhang L et al 2002) P-T estimates of the peakUHP stage of metamorphism are T = 500ndash600degCand P = 26ndash27 GPa (Wei et al 2003)

North Qaidam Mountains western China Manylenses and layers of garnet peridotite and eclogiteoccur in an amphibolite-facies terrane in the NorthQaidam Mountains along the northeastern rim of theQaidam Basin This belt comprises mainly felsicgneiss quartz schist garnet-bearing gneissamphibolite and minor eclogite + garnet peridotiteit extends westward for more than 1000 km to theAltun Mountains and was displaced by the left-lat-eral Altyn-Tagh fault At the eastern end of thisUHP belt at Dulan occurrences of coesite inclu-sions in zircon separates from paragneiss inclusionsof coesite pseudomorph in garnet and omphacite ofeclogites quartz rods in eclogitic omphacite andP-T estimates of eclogites (P = 287ndash317 GPa andT = 631ndash687degC) demonstrate that this is a typicalUHP metamorphic terrane (Yang et al 2001 Songet al 2003a 2003b) The Dulan eclogites have anSm-Nd mineral isochron age of 497 plusmn 87 Ma and aSHRIMP UPb zircon age of 495 Ma (Yang et al2002b)

Altun Mountains western China This UHP ter-rane about 200 km long in the southern margin ofthe Altun Mountains is the western extension of theNorth Qaidam belt Eclogitic lenses and blocks ofvarious sizes together with garnet-lherzolite and cli-nopyroxenite are enclosed within an Early Protero-zoic sequence of amphibolite amphibole schistgarnet-bearing granitic gneiss and pelitic schistGarnet lherzolite contains magnesite that reacted toform dolomite P-T estimates of the peak magnesite-bearing assemblage yield P gt 36 GPa and 850degC(Liu et al 2001) Garnet porphyroblasts includeexsolution lamellae of clinopyroxene and rutile sim-ilar to that of eclogite reported by Ye et al (2000) asmajoritic garnet The associated eclogites containinclusions of coesite pseudomorphs in garnet withcharacteristic radial fractures this together with theexsolved quartz rods in omphacite and P-T esti-mates of 820ndash850degC and 28 to 32 GPa provideadditional evidence for UHP (Zhang J X et al

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 15

2001) An SmNd mineral isochron for eclogiticgarnet omphacite and the whole rock yields 500 plusmn10 Ma whereas an UPb isochron for zircon sepa-rates from eclogite gives an age of 504 plusmn 5 Ma(Zhang J X et al 2000)

North Qinling Mountains central China Lensesand blocks of eclogitic rocks in northern Qinling areenclosed in amphibolite-facies gneiss and garnet-bearing quartz + phengite mica schist Inclusions ofcoesite in eclogitic garnet (Hu et al 1995) andmicrodiamonds in zircon separates from both eclog-ite and gneiss (Yang et al 2002a) have beenrecently reported Although individual outcrops areno more than a few meters wide the belt of eclogitesextends more than 10 km P-T estimates of amphib-olitized coesite-bearing eclogite yield T = 590ndash758degC and P gt 26 GPa An Sm-Nd isochron for gar-net omphacite rutile amphibole and whole-rockyields 400 plusmn 16 Ma Microdiamonds were recentlydiscovered as inclusions in zircon from both eclogiteand its gneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyields 507 plusmn 38 Ma for peak UHP metamorphism(Yang et al 2002a) These data indicate that two

discrete collision events occurred between theNorth China and Yangtze cratons (1) an Early Pale-ozoic North Qinling UHP suturing and (2) a Triassicsuturing along the Dabie-Sulu UHP-HP belt TheEarly Paleozoic belt extends westward to the NorthQaidamndashAltun Mountains for more than 4000 km(Yang et al 2002b)

Himalayan UHP Eclogites

The Himalayan orogen has long been considereda classic locality of continental collision betweenthe Indian and Eurasian continents (Fig 5) Severaleclogite-facies HP rocks in the Himalayas havebeen reported since the late 1980s (eg Chaudaryand Ghazanfar 1987 Pognante and Spencer 1991de Sigoyer et al 1997 Lombardo and Rolfo 2000)Inclusions of coesite were recently discovered inomphacite and garnet in several Himalayan eclog-ites from the upper Kaghan Valley Pakistan(OrsquoBrien et al 2001 Kaneko et al 2003) Subse-quent findings of additional coesite localities in theTso Morari area (Sachan et al 2001 Mukherjee etal 2003) have established the presence of a UHPbelt in the Himalayas an active ongoing collision

FIG 5 A simplified tectonic map of the western Himalaya Hindu Kush Pamir Karakoran and western Tibet mod-ified after Figure 3 of Searle et al (2001) This map shows the locations of eclogite and UHP eclogites from both Hima-layan and North Qaidam Indus Suture Zone (ISZ) ophiolites and major structures

16 LIOU ET AL

since the Eocene (Massonne and OrsquoBrien 2003)However UHP relics are volumetrically very minorconstituents of the metamorphic belts dominatedmostly by later Barrovian overprintings In theserocks hydration related to Barrovian-zone metamor-phism has resulted in remarkably pervasive recrys-tallization during the later exhumation this eventhas almost entirely masked the preceding UHPrecord

The finding of HP-UHP evidence has led to newinsights into the Himalayan tectonic modelEngland and Houseman (1986) showed that in amodel of homogeneous thickening of the Hima-layan-Tibet boundary the maximum crustal thick-ness should be less than 70 km However theoccurrences of coesite-bearing eclogites andgneisses in the Himalayan orogen cannot beexplained by a homogeneous thickening of the crustbut rather by continental subduction as describedbelow According to available geochronologicaldata (eg Treloar et al 2003) the exhumation rateof the Himalayan UHP rocks has been estimated torange from ~1 cm yrndash1 (OrsquoBrien et al 2001) to gt3cm yrndash1 (Treloar et al 2003 Massone and OrsquoBrien2003) These estimates are higher than those calcu-lated from other UHP-HP terranes This requirestectonic emplacement of the UHP-HP Himalayanslab into shallow crust following isostatic rebounddue to slab decoupling between the Indian continentand the oceanic crust initiated during the MiddleEocene UHP rocks from both the Kaghan Valleyand Tso-Mori are described below

Kaghan Valley The Kaghan nappe in the north-western Himalayan syntaxis lies between the Indussuture (also called main mantle thrust MMT) to thenorth and the main central thrust (MCT) to thesouthwest (Pognante and Spear 1991 Spencer1993) (Fig 5) The Cretaceous Kohistan island-arcsequence lies north of the Indus suture It developedas a result of intra-oceanic subduction The maincentral thrust separates an underlying Salkhala unitfrom an overlying higher Himalayan crystalline(HHC) unit The Kaghan nappe consists mainly ofgranitic gneiss and paragneiss with minor inter-calated layers of amphibolite marble and quartziteEclogites are preserved in the cores of maficboudins (less than 1 m thick) in gneiss and aresignificantly overprinted by amphibolite assem-blages They contain fine-grained (lt 1 mm) garnetand omphacite together with aggregates of quartzchains of rutile rimmed by titanite and mm-sizedphengitic micas overgrown by randomly oriented

amphiboles up to 1 cm in length Coesite and coesitepseudomorphs occur as inclusions in omphaciteand show the characteristic palisade-quartz aggre-gate texture fractures radiating from these inclu-sions are characteristically developed in the hostpyroxene

Garnet pyroxenendashphengite barometry wasapplied to the eclogites and yielded peak P-T con-ditions of 27ndash29 GPa and 690ndash750degC (OrsquoBrien etal 2001) (Fig 6) Retrograde textures are commonin the eclogites including symplectic intergrowthsof augite amphibole and plagioclase after omphac-ite thermobarometric analyses of these assemblagesyielded 10ndash13 GPa and 600ndash710degC (Kaneko et al2003) On the other hand the dominant graniticgneisses and metapelites of the higher Himalayancrystalline unit contain typical Barrovian-zoneamphibolite-facies assemblages represented bykyanite- and staurolite-bearing garnetiferousmetapelites (Treloar 1995) Extensive overprintinghas mostly obliterated the peak assemblages matrixminerals of the gneisses yielded P-T conditions of07ndash11 GPa and 600ndash700degC (Treloar 1995) con-sistent with retrogression of the eclogites Howeverrelict coesite is preserved as inclusions in zirconthe inclusions are ovoid and approximately 10 microm indiameter (Fig 7A) This discovery of coesite-bear-ing zircons from Himalayan quartzofeldspathicgneisses demonstrates subduction of Indian conti-nental crust to the depths of UHP conditions(Kaneko et al 2003)

Geochronological studies using SmNd (garnet-omphacite) RbSr (phengite) and UPb (rutile zir-con) methods suggest that the eclogite-facies eventtook place at 40ndash50 Ma (Tonarini et al 1993 Spen-cer and Gebauer 1996) 40Ar39Ar cooling ages forhornblende and mica from gneisses were reported tobe ~43 Ma and ~25 Ma respectively (Chamberlainet al 1991) Recent SHRIMP UPb dates of zonedzircons with mineral inclusions formed at differentstages indicate the ages of non-UHP mineral-bear-ing mantle domains of zircon and UHP mineral-bearing rims are at about 50 and 46 Ma respectively(Kaneko et al 2003) A new U-Pb age of 44 plusmn 11Ma was obtained for rutile from Kaghan coesite-bearing eclogite (Treloar et al 2003) this togetherwith available data implies that exhumation of theUHP rocks from mantle depth to mid-crustal leveloccurs within a few million years

Tso Morari The Tso Morari crystalline dome islocated between the Indus suture zone on the northbordered by the Zildat detachment fault and the

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 17

Zanskar sedimentary unit on the south (de Sigoyer etal 1997) The dome represents an internal massif ofthe northern part of the higher Himalaya The TsoMorari area has been considered a typical IndianTethyan margin because of a stratigraphic succes-sion similar to the Nimaling antiform in the core ofthe dome a metamorphic basement consists of Cam-bro-Ordovician augen gneisses (Stutz 1988) Thiscomplex is covered by a metasedimentary series ofCambrian to Devonian quartzite schist and con-glomerate and is overlain by Lower Carboniferousto Triassic marble and metapelite Eclogites occuras lenses or irregular bodies within the basementand cover gneisses and are strongly overprinted byamphibolite-facies assemblages de Sigoyer et al(1997) reported peak P-T conditions for the eclog-ites at 17ndash23 GPa and 520ndash640degC (Fig 7) How-ever relics of coesite 30ndash80 microm in size were

recently found as inclusions in garnet (Sachan et al2001 Mukherjee et al 2003) typified by radialfractures (Figs 7B and 7C) for the SiO2 inclusionsa characteristic Raman spectrum is centered at 523cm-1 (Fig 7D) The country rock metasedimentswith jadeite + chloritoid + paragonite + garnetassemblages have peak P-T estimates of 18ndash22GPa and 500ndash600degC and record retrogression at theeclogite-blueschist facies transition of 13ndash18 GPaand 490ndash590degC (Guillot et al 1997)

The geochronology of the Tso Morari eclogiteshas been extensively studied and subdivided intothree different stages (de Signoyer et al 2000)Eclogitization at ~55 Ma was obtained by Lu-Hf andSm-Nd methods on garnet omphacite glaucophaneand the whole rock An amphibolite-facies overprintoccurred at 47 Ma judging by RbSr dating onphengite apatite and whole rock from metapelites

FIG 6 P-T estimates of peak- and retrograde-stage metamorphism for coesite-bearing eclogites and the country-rockgneisses from the Kaghan Valley and Tso Morari

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 13

inclusions of calcite pseudomorphs after aragonite +magnesite in dolomite in metapelites from the west-ern Tian-Shan inferred as metamorphism in a UHPregion of dolomite decomposition (Zhang L et al2002) and (10) inclusion of possible coesitepseudomorphs in garnet lamellar inclusions in gar-net kyanite and high sodium content of garnet fromeclogites of the Chuacus Complex of north-centralGuatemala (Solari et al 2003) The results of petro-chemical and geochronological data for these newterranes are summarized below The HimalayanUHP terrane in the Upper Kaghan Valley and theTso-Morari complex are used to illustrate the pro-cesses of continental subduction and collision in alater section

French Massif Central The French Massif Cen-tral in the western part of the Variscan Belt hasexperienced Late SilurianndashEarly Devonian to LateCarboniferous orogenic events (eg Matte 1986)High-pressure metamorphic rocks occur mainly inthe upper gneissic unit within the leptyno-amphibo-lite group The latter consists of an association ofmetagreywacke mica schist metabasalt leptynitemetagranite and peridotite Eclogites are associatedwith silicic and mafic HP granulites (Lardeaux etal 2001) and also with spinel andor garnet lherzo-lites (Gardien et al 1990) Inclusions of coesite andcoesite pseudomorphs occur in garnets of kyanite-bearing eclogite (Lardeaux et al 2001) Only twocoesite grains are preserved as relics most coesitegrains are completely transformed into polycrystal-line radial quartz (palisade texture) or into polygonalquartz surrounded by radial cracks Metamorphictemperatures were estimated to be 740-780degC andsubsequent amphibolite-facies overprinting at750degC at 15ndash17 GPa during decompression UHPmetamorphism occurred between 420 and 400 Ma(Paquette et al 1995) 40Ar39Ar data from amphib-ole separates from retrogressed eclogites yielded339 plusmn 4 Ma (Costa et al 1993)

Northeast Greenland Caledonides The first evi-dence for UHP metamorphism in the GreenlandCaledonites was reported from kyanite eclogites andassociated host gneisses on an island in Jokelbugt(Gilotti and Ravna 2002) Inclusions of quartzpseudomorphs exhibiting palisade structure andradiating fractures occur in garnet and omphacite ofeclogites and in garnet of the host gneisses P-T esti-mates of peak-stage metamorphism of eclogite at~972degC and 36 GPa lie well within the coesite sta-bility field SHRIMP UPb dates of 403 plusmn 5 Ma forzircon rims from HP metapsammite and 404 plusmn 4 Ma

for zircons from anatectic melt derived frommetapelite are coeval with the estimated ages ofexhumation of UHP terranes in the ScandinavianCaledonides described in a previous section(McClelland and Gilotti 2003)

Neoproterozoic nappes in Southeast BrazilNeoproterozoic nappes of Southeast Brazil consistmainly of coarse-grained kyanite-garnet granulitewith intercalations of impure quartzite calc-silicaterock and minor lenses and sills of mafic-ultramaficrocks including eclogite Early eclogitic phaseswere strongly obliterated during granulite-faciesoverprinting during a major Pan-African continentcollision at 640ndash630 Ma (Campos Neto and Caby1999) Rare ultramafic lenses consist of phlogopite-orthopyroxene rocks and garnet clinopyroxeniteCores of granulite boudins retain eclogitic assem-blages some quartz inclusions within the mantleregions of zoned garnet porphyroblasts are sur-rounded by intense radial fractures suggestive ofthe former presence of coesite In fact coarse-grained zircons in some granulites contain numer-ous micro-inclusions of K-feldspar + kyanite + coes-ite as confirmed by Raman spectroscopy (Parkinsonet al 2001) Many exsolution microstructuresobserved in other UHP terranes also occur Thisarea represents the first coesite-grade UHP rocks inthe Americas and it can be correlated with the MaliUHP terrane of equatorial Africa (Caby 1994) Ithas been suggested that eclogitic assemblages andprobable UHPM components may be present in sev-eral other correlative nappe systems at the marginsof the Sao Francisco craton in Brazil

Chuacus Complex north-central Guatemala HPeclogites within banded gneisses schists andmigmatites of the Chuacus Complex contain relicttextures and mineralogical features that suggestUHP metamorphism (Solari et al 2003) Theseinclude inclusions of possible coesite pseudomor-phs in garnet and kyanite lamellar inclusions ofrutileilmenite in garnet high sodium content ofsome garnets (up to 1200 ppm) and preliminary P-T estimates of 700ndash800degC and 2ndash3 GPa AlthoughK-Ar dating of mica and hornblende yield ages of48ndash72 Ma possible for amphibolite facies retrogres-sion regional and structural data support a pre-Mesozoic age for the multi-stage HP-UHP recrystal-lization Such a discovery opens up a new interpre-tation regarding the tectonic evolution of Maya-Chortis-Oaxaquia microcontinental blocks formingthe structural backbone of Mesoamerica

14 LIOU ET AL

Greek Rhodope The Greek Rhodope Metamor-phic Province (RMP) at the border between Greeceand Bulgaria represents a syn-metamorphic nappesystem of Alpine age that was formed during theCretaceous to Mid-Tertiary collision of Apulia andpaleo-Europe The Kimi nappe complex comprisescrustal eclogites leucocratic orthogneisses peliticgneisses and volumetrically minor mantle-derivedultramafic rocks these units contain many mineral-ogical indicators of UHP metamorphism (Mposkosand Kostopoulos 2001) The ultramafic lithologiesinclude garnet spinelndashbearing lherzolite and layersof spinel-garnet clinopyroxenite garnet pyroxeniteand clinopyroxene garnetite These rocks were sub-jected to UHP recrystallization at P-T conditions of~1200degC and gt30 GPa The pelitic gneiss containsmicrodiamond inclusions in garnet porphyroblastsas well as rods or needles of silica rutile biotiteand apatite in sodic garnet suggesting the prioroccurrence of majoritic garnet The growth of suchsupersilicic garnet was suggested to be at a pressureof ~7 GPa Inclusions of polycrystalline quartzaggregates surrounded by radial cracks occur ineclogitic garnet Exsolution lamellae of quartz ineclogitic clinopyroxene also indicate an UHP pre-cursor of supersilicic clinopyroxene

Lanterman Range Antarctica The LantermanRange in northern Victoria Land of Antarctica con-sists of several metamorphic complexes The Gate-way Hills Metamorphic Complex is a thindiscontinuous belt more than 50 km long consistingof mafic and ultramafic rocks including lenses andpods of eclogite within felsic gneiss Some less foli-ated eclogites contain inclusions of coesite andcoesite pseudomorphs in garnet porphyroblasts asconfirmed by in situ Raman spectroscopy as well asradial fractures around the inclusions (Ghiribelli etal 2002) The eclogites were subjected to UHPmetamorphism at T gt 850degC and P gt 29 GPa thenamphibolite-facies overprinting during isothermaldecompression The UHP event is dated by SmNdand 238U206Pb data at 500 Ma 40Ar39Ar ages of490ndash486 Ma for Ca-amphibole (amphibolite-faciesoverprinting) suggest rapid cooling with an averageexhumation rate of 3ndash4 km_Ma-1 for the UHP ter-rane (Di Vincenzo and Palmeri 2001)

Western Tian-Shan China Caledonian eclogitesof the western Tian-Shan China have been classi-fied into three types (Zhang L et al 2002) Type Ieclogite pods are layered with mafic blueschists andType II eclogites preserved pillow structureswhereas Type III eclogites are banded with marbles

This 200 km long eclogite-bearing belt extendswestward to connect with the Atbashy UHP belt inKazakhstan (Tagiri et al 1995) Reported UHP evi-dence includes inclusions of coesite pseudomorphsin garnets from Type I and III eclogites quartzexsolution lamellae in omphacite from Type IIeclogites and relict magnesite within matrix dolo-mite in Type III eclogites (Zhang J X et al 2002Zhang L et al 2002) P-T estimates of the peakUHP stage of metamorphism are T = 500ndash600degCand P = 26ndash27 GPa (Wei et al 2003)

North Qaidam Mountains western China Manylenses and layers of garnet peridotite and eclogiteoccur in an amphibolite-facies terrane in the NorthQaidam Mountains along the northeastern rim of theQaidam Basin This belt comprises mainly felsicgneiss quartz schist garnet-bearing gneissamphibolite and minor eclogite + garnet peridotiteit extends westward for more than 1000 km to theAltun Mountains and was displaced by the left-lat-eral Altyn-Tagh fault At the eastern end of thisUHP belt at Dulan occurrences of coesite inclu-sions in zircon separates from paragneiss inclusionsof coesite pseudomorph in garnet and omphacite ofeclogites quartz rods in eclogitic omphacite andP-T estimates of eclogites (P = 287ndash317 GPa andT = 631ndash687degC) demonstrate that this is a typicalUHP metamorphic terrane (Yang et al 2001 Songet al 2003a 2003b) The Dulan eclogites have anSm-Nd mineral isochron age of 497 plusmn 87 Ma and aSHRIMP UPb zircon age of 495 Ma (Yang et al2002b)

Altun Mountains western China This UHP ter-rane about 200 km long in the southern margin ofthe Altun Mountains is the western extension of theNorth Qaidam belt Eclogitic lenses and blocks ofvarious sizes together with garnet-lherzolite and cli-nopyroxenite are enclosed within an Early Protero-zoic sequence of amphibolite amphibole schistgarnet-bearing granitic gneiss and pelitic schistGarnet lherzolite contains magnesite that reacted toform dolomite P-T estimates of the peak magnesite-bearing assemblage yield P gt 36 GPa and 850degC(Liu et al 2001) Garnet porphyroblasts includeexsolution lamellae of clinopyroxene and rutile sim-ilar to that of eclogite reported by Ye et al (2000) asmajoritic garnet The associated eclogites containinclusions of coesite pseudomorphs in garnet withcharacteristic radial fractures this together with theexsolved quartz rods in omphacite and P-T esti-mates of 820ndash850degC and 28 to 32 GPa provideadditional evidence for UHP (Zhang J X et al

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 15

2001) An SmNd mineral isochron for eclogiticgarnet omphacite and the whole rock yields 500 plusmn10 Ma whereas an UPb isochron for zircon sepa-rates from eclogite gives an age of 504 plusmn 5 Ma(Zhang J X et al 2000)

North Qinling Mountains central China Lensesand blocks of eclogitic rocks in northern Qinling areenclosed in amphibolite-facies gneiss and garnet-bearing quartz + phengite mica schist Inclusions ofcoesite in eclogitic garnet (Hu et al 1995) andmicrodiamonds in zircon separates from both eclog-ite and gneiss (Yang et al 2002a) have beenrecently reported Although individual outcrops areno more than a few meters wide the belt of eclogitesextends more than 10 km P-T estimates of amphib-olitized coesite-bearing eclogite yield T = 590ndash758degC and P gt 26 GPa An Sm-Nd isochron for gar-net omphacite rutile amphibole and whole-rockyields 400 plusmn 16 Ma Microdiamonds were recentlydiscovered as inclusions in zircon from both eclogiteand its gneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyields 507 plusmn 38 Ma for peak UHP metamorphism(Yang et al 2002a) These data indicate that two

discrete collision events occurred between theNorth China and Yangtze cratons (1) an Early Pale-ozoic North Qinling UHP suturing and (2) a Triassicsuturing along the Dabie-Sulu UHP-HP belt TheEarly Paleozoic belt extends westward to the NorthQaidamndashAltun Mountains for more than 4000 km(Yang et al 2002b)

Himalayan UHP Eclogites

The Himalayan orogen has long been considereda classic locality of continental collision betweenthe Indian and Eurasian continents (Fig 5) Severaleclogite-facies HP rocks in the Himalayas havebeen reported since the late 1980s (eg Chaudaryand Ghazanfar 1987 Pognante and Spencer 1991de Sigoyer et al 1997 Lombardo and Rolfo 2000)Inclusions of coesite were recently discovered inomphacite and garnet in several Himalayan eclog-ites from the upper Kaghan Valley Pakistan(OrsquoBrien et al 2001 Kaneko et al 2003) Subse-quent findings of additional coesite localities in theTso Morari area (Sachan et al 2001 Mukherjee etal 2003) have established the presence of a UHPbelt in the Himalayas an active ongoing collision

FIG 5 A simplified tectonic map of the western Himalaya Hindu Kush Pamir Karakoran and western Tibet mod-ified after Figure 3 of Searle et al (2001) This map shows the locations of eclogite and UHP eclogites from both Hima-layan and North Qaidam Indus Suture Zone (ISZ) ophiolites and major structures

16 LIOU ET AL

since the Eocene (Massonne and OrsquoBrien 2003)However UHP relics are volumetrically very minorconstituents of the metamorphic belts dominatedmostly by later Barrovian overprintings In theserocks hydration related to Barrovian-zone metamor-phism has resulted in remarkably pervasive recrys-tallization during the later exhumation this eventhas almost entirely masked the preceding UHPrecord

The finding of HP-UHP evidence has led to newinsights into the Himalayan tectonic modelEngland and Houseman (1986) showed that in amodel of homogeneous thickening of the Hima-layan-Tibet boundary the maximum crustal thick-ness should be less than 70 km However theoccurrences of coesite-bearing eclogites andgneisses in the Himalayan orogen cannot beexplained by a homogeneous thickening of the crustbut rather by continental subduction as describedbelow According to available geochronologicaldata (eg Treloar et al 2003) the exhumation rateof the Himalayan UHP rocks has been estimated torange from ~1 cm yrndash1 (OrsquoBrien et al 2001) to gt3cm yrndash1 (Treloar et al 2003 Massone and OrsquoBrien2003) These estimates are higher than those calcu-lated from other UHP-HP terranes This requirestectonic emplacement of the UHP-HP Himalayanslab into shallow crust following isostatic rebounddue to slab decoupling between the Indian continentand the oceanic crust initiated during the MiddleEocene UHP rocks from both the Kaghan Valleyand Tso-Mori are described below

Kaghan Valley The Kaghan nappe in the north-western Himalayan syntaxis lies between the Indussuture (also called main mantle thrust MMT) to thenorth and the main central thrust (MCT) to thesouthwest (Pognante and Spear 1991 Spencer1993) (Fig 5) The Cretaceous Kohistan island-arcsequence lies north of the Indus suture It developedas a result of intra-oceanic subduction The maincentral thrust separates an underlying Salkhala unitfrom an overlying higher Himalayan crystalline(HHC) unit The Kaghan nappe consists mainly ofgranitic gneiss and paragneiss with minor inter-calated layers of amphibolite marble and quartziteEclogites are preserved in the cores of maficboudins (less than 1 m thick) in gneiss and aresignificantly overprinted by amphibolite assem-blages They contain fine-grained (lt 1 mm) garnetand omphacite together with aggregates of quartzchains of rutile rimmed by titanite and mm-sizedphengitic micas overgrown by randomly oriented

amphiboles up to 1 cm in length Coesite and coesitepseudomorphs occur as inclusions in omphaciteand show the characteristic palisade-quartz aggre-gate texture fractures radiating from these inclu-sions are characteristically developed in the hostpyroxene

Garnet pyroxenendashphengite barometry wasapplied to the eclogites and yielded peak P-T con-ditions of 27ndash29 GPa and 690ndash750degC (OrsquoBrien etal 2001) (Fig 6) Retrograde textures are commonin the eclogites including symplectic intergrowthsof augite amphibole and plagioclase after omphac-ite thermobarometric analyses of these assemblagesyielded 10ndash13 GPa and 600ndash710degC (Kaneko et al2003) On the other hand the dominant graniticgneisses and metapelites of the higher Himalayancrystalline unit contain typical Barrovian-zoneamphibolite-facies assemblages represented bykyanite- and staurolite-bearing garnetiferousmetapelites (Treloar 1995) Extensive overprintinghas mostly obliterated the peak assemblages matrixminerals of the gneisses yielded P-T conditions of07ndash11 GPa and 600ndash700degC (Treloar 1995) con-sistent with retrogression of the eclogites Howeverrelict coesite is preserved as inclusions in zirconthe inclusions are ovoid and approximately 10 microm indiameter (Fig 7A) This discovery of coesite-bear-ing zircons from Himalayan quartzofeldspathicgneisses demonstrates subduction of Indian conti-nental crust to the depths of UHP conditions(Kaneko et al 2003)

Geochronological studies using SmNd (garnet-omphacite) RbSr (phengite) and UPb (rutile zir-con) methods suggest that the eclogite-facies eventtook place at 40ndash50 Ma (Tonarini et al 1993 Spen-cer and Gebauer 1996) 40Ar39Ar cooling ages forhornblende and mica from gneisses were reported tobe ~43 Ma and ~25 Ma respectively (Chamberlainet al 1991) Recent SHRIMP UPb dates of zonedzircons with mineral inclusions formed at differentstages indicate the ages of non-UHP mineral-bear-ing mantle domains of zircon and UHP mineral-bearing rims are at about 50 and 46 Ma respectively(Kaneko et al 2003) A new U-Pb age of 44 plusmn 11Ma was obtained for rutile from Kaghan coesite-bearing eclogite (Treloar et al 2003) this togetherwith available data implies that exhumation of theUHP rocks from mantle depth to mid-crustal leveloccurs within a few million years

Tso Morari The Tso Morari crystalline dome islocated between the Indus suture zone on the northbordered by the Zildat detachment fault and the

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 17

Zanskar sedimentary unit on the south (de Sigoyer etal 1997) The dome represents an internal massif ofthe northern part of the higher Himalaya The TsoMorari area has been considered a typical IndianTethyan margin because of a stratigraphic succes-sion similar to the Nimaling antiform in the core ofthe dome a metamorphic basement consists of Cam-bro-Ordovician augen gneisses (Stutz 1988) Thiscomplex is covered by a metasedimentary series ofCambrian to Devonian quartzite schist and con-glomerate and is overlain by Lower Carboniferousto Triassic marble and metapelite Eclogites occuras lenses or irregular bodies within the basementand cover gneisses and are strongly overprinted byamphibolite-facies assemblages de Sigoyer et al(1997) reported peak P-T conditions for the eclog-ites at 17ndash23 GPa and 520ndash640degC (Fig 7) How-ever relics of coesite 30ndash80 microm in size were

recently found as inclusions in garnet (Sachan et al2001 Mukherjee et al 2003) typified by radialfractures (Figs 7B and 7C) for the SiO2 inclusionsa characteristic Raman spectrum is centered at 523cm-1 (Fig 7D) The country rock metasedimentswith jadeite + chloritoid + paragonite + garnetassemblages have peak P-T estimates of 18ndash22GPa and 500ndash600degC and record retrogression at theeclogite-blueschist facies transition of 13ndash18 GPaand 490ndash590degC (Guillot et al 1997)

The geochronology of the Tso Morari eclogiteshas been extensively studied and subdivided intothree different stages (de Signoyer et al 2000)Eclogitization at ~55 Ma was obtained by Lu-Hf andSm-Nd methods on garnet omphacite glaucophaneand the whole rock An amphibolite-facies overprintoccurred at 47 Ma judging by RbSr dating onphengite apatite and whole rock from metapelites

FIG 6 P-T estimates of peak- and retrograde-stage metamorphism for coesite-bearing eclogites and the country-rockgneisses from the Kaghan Valley and Tso Morari

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

14 LIOU ET AL

Greek Rhodope The Greek Rhodope Metamor-phic Province (RMP) at the border between Greeceand Bulgaria represents a syn-metamorphic nappesystem of Alpine age that was formed during theCretaceous to Mid-Tertiary collision of Apulia andpaleo-Europe The Kimi nappe complex comprisescrustal eclogites leucocratic orthogneisses peliticgneisses and volumetrically minor mantle-derivedultramafic rocks these units contain many mineral-ogical indicators of UHP metamorphism (Mposkosand Kostopoulos 2001) The ultramafic lithologiesinclude garnet spinelndashbearing lherzolite and layersof spinel-garnet clinopyroxenite garnet pyroxeniteand clinopyroxene garnetite These rocks were sub-jected to UHP recrystallization at P-T conditions of~1200degC and gt30 GPa The pelitic gneiss containsmicrodiamond inclusions in garnet porphyroblastsas well as rods or needles of silica rutile biotiteand apatite in sodic garnet suggesting the prioroccurrence of majoritic garnet The growth of suchsupersilicic garnet was suggested to be at a pressureof ~7 GPa Inclusions of polycrystalline quartzaggregates surrounded by radial cracks occur ineclogitic garnet Exsolution lamellae of quartz ineclogitic clinopyroxene also indicate an UHP pre-cursor of supersilicic clinopyroxene

Lanterman Range Antarctica The LantermanRange in northern Victoria Land of Antarctica con-sists of several metamorphic complexes The Gate-way Hills Metamorphic Complex is a thindiscontinuous belt more than 50 km long consistingof mafic and ultramafic rocks including lenses andpods of eclogite within felsic gneiss Some less foli-ated eclogites contain inclusions of coesite andcoesite pseudomorphs in garnet porphyroblasts asconfirmed by in situ Raman spectroscopy as well asradial fractures around the inclusions (Ghiribelli etal 2002) The eclogites were subjected to UHPmetamorphism at T gt 850degC and P gt 29 GPa thenamphibolite-facies overprinting during isothermaldecompression The UHP event is dated by SmNdand 238U206Pb data at 500 Ma 40Ar39Ar ages of490ndash486 Ma for Ca-amphibole (amphibolite-faciesoverprinting) suggest rapid cooling with an averageexhumation rate of 3ndash4 km_Ma-1 for the UHP ter-rane (Di Vincenzo and Palmeri 2001)

Western Tian-Shan China Caledonian eclogitesof the western Tian-Shan China have been classi-fied into three types (Zhang L et al 2002) Type Ieclogite pods are layered with mafic blueschists andType II eclogites preserved pillow structureswhereas Type III eclogites are banded with marbles

This 200 km long eclogite-bearing belt extendswestward to connect with the Atbashy UHP belt inKazakhstan (Tagiri et al 1995) Reported UHP evi-dence includes inclusions of coesite pseudomorphsin garnets from Type I and III eclogites quartzexsolution lamellae in omphacite from Type IIeclogites and relict magnesite within matrix dolo-mite in Type III eclogites (Zhang J X et al 2002Zhang L et al 2002) P-T estimates of the peakUHP stage of metamorphism are T = 500ndash600degCand P = 26ndash27 GPa (Wei et al 2003)

North Qaidam Mountains western China Manylenses and layers of garnet peridotite and eclogiteoccur in an amphibolite-facies terrane in the NorthQaidam Mountains along the northeastern rim of theQaidam Basin This belt comprises mainly felsicgneiss quartz schist garnet-bearing gneissamphibolite and minor eclogite + garnet peridotiteit extends westward for more than 1000 km to theAltun Mountains and was displaced by the left-lat-eral Altyn-Tagh fault At the eastern end of thisUHP belt at Dulan occurrences of coesite inclu-sions in zircon separates from paragneiss inclusionsof coesite pseudomorph in garnet and omphacite ofeclogites quartz rods in eclogitic omphacite andP-T estimates of eclogites (P = 287ndash317 GPa andT = 631ndash687degC) demonstrate that this is a typicalUHP metamorphic terrane (Yang et al 2001 Songet al 2003a 2003b) The Dulan eclogites have anSm-Nd mineral isochron age of 497 plusmn 87 Ma and aSHRIMP UPb zircon age of 495 Ma (Yang et al2002b)

Altun Mountains western China This UHP ter-rane about 200 km long in the southern margin ofthe Altun Mountains is the western extension of theNorth Qaidam belt Eclogitic lenses and blocks ofvarious sizes together with garnet-lherzolite and cli-nopyroxenite are enclosed within an Early Protero-zoic sequence of amphibolite amphibole schistgarnet-bearing granitic gneiss and pelitic schistGarnet lherzolite contains magnesite that reacted toform dolomite P-T estimates of the peak magnesite-bearing assemblage yield P gt 36 GPa and 850degC(Liu et al 2001) Garnet porphyroblasts includeexsolution lamellae of clinopyroxene and rutile sim-ilar to that of eclogite reported by Ye et al (2000) asmajoritic garnet The associated eclogites containinclusions of coesite pseudomorphs in garnet withcharacteristic radial fractures this together with theexsolved quartz rods in omphacite and P-T esti-mates of 820ndash850degC and 28 to 32 GPa provideadditional evidence for UHP (Zhang J X et al

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 15

2001) An SmNd mineral isochron for eclogiticgarnet omphacite and the whole rock yields 500 plusmn10 Ma whereas an UPb isochron for zircon sepa-rates from eclogite gives an age of 504 plusmn 5 Ma(Zhang J X et al 2000)

North Qinling Mountains central China Lensesand blocks of eclogitic rocks in northern Qinling areenclosed in amphibolite-facies gneiss and garnet-bearing quartz + phengite mica schist Inclusions ofcoesite in eclogitic garnet (Hu et al 1995) andmicrodiamonds in zircon separates from both eclog-ite and gneiss (Yang et al 2002a) have beenrecently reported Although individual outcrops areno more than a few meters wide the belt of eclogitesextends more than 10 km P-T estimates of amphib-olitized coesite-bearing eclogite yield T = 590ndash758degC and P gt 26 GPa An Sm-Nd isochron for gar-net omphacite rutile amphibole and whole-rockyields 400 plusmn 16 Ma Microdiamonds were recentlydiscovered as inclusions in zircon from both eclogiteand its gneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyields 507 plusmn 38 Ma for peak UHP metamorphism(Yang et al 2002a) These data indicate that two

discrete collision events occurred between theNorth China and Yangtze cratons (1) an Early Pale-ozoic North Qinling UHP suturing and (2) a Triassicsuturing along the Dabie-Sulu UHP-HP belt TheEarly Paleozoic belt extends westward to the NorthQaidamndashAltun Mountains for more than 4000 km(Yang et al 2002b)

Himalayan UHP Eclogites

The Himalayan orogen has long been considereda classic locality of continental collision betweenthe Indian and Eurasian continents (Fig 5) Severaleclogite-facies HP rocks in the Himalayas havebeen reported since the late 1980s (eg Chaudaryand Ghazanfar 1987 Pognante and Spencer 1991de Sigoyer et al 1997 Lombardo and Rolfo 2000)Inclusions of coesite were recently discovered inomphacite and garnet in several Himalayan eclog-ites from the upper Kaghan Valley Pakistan(OrsquoBrien et al 2001 Kaneko et al 2003) Subse-quent findings of additional coesite localities in theTso Morari area (Sachan et al 2001 Mukherjee etal 2003) have established the presence of a UHPbelt in the Himalayas an active ongoing collision

FIG 5 A simplified tectonic map of the western Himalaya Hindu Kush Pamir Karakoran and western Tibet mod-ified after Figure 3 of Searle et al (2001) This map shows the locations of eclogite and UHP eclogites from both Hima-layan and North Qaidam Indus Suture Zone (ISZ) ophiolites and major structures

16 LIOU ET AL

since the Eocene (Massonne and OrsquoBrien 2003)However UHP relics are volumetrically very minorconstituents of the metamorphic belts dominatedmostly by later Barrovian overprintings In theserocks hydration related to Barrovian-zone metamor-phism has resulted in remarkably pervasive recrys-tallization during the later exhumation this eventhas almost entirely masked the preceding UHPrecord

The finding of HP-UHP evidence has led to newinsights into the Himalayan tectonic modelEngland and Houseman (1986) showed that in amodel of homogeneous thickening of the Hima-layan-Tibet boundary the maximum crustal thick-ness should be less than 70 km However theoccurrences of coesite-bearing eclogites andgneisses in the Himalayan orogen cannot beexplained by a homogeneous thickening of the crustbut rather by continental subduction as describedbelow According to available geochronologicaldata (eg Treloar et al 2003) the exhumation rateof the Himalayan UHP rocks has been estimated torange from ~1 cm yrndash1 (OrsquoBrien et al 2001) to gt3cm yrndash1 (Treloar et al 2003 Massone and OrsquoBrien2003) These estimates are higher than those calcu-lated from other UHP-HP terranes This requirestectonic emplacement of the UHP-HP Himalayanslab into shallow crust following isostatic rebounddue to slab decoupling between the Indian continentand the oceanic crust initiated during the MiddleEocene UHP rocks from both the Kaghan Valleyand Tso-Mori are described below

Kaghan Valley The Kaghan nappe in the north-western Himalayan syntaxis lies between the Indussuture (also called main mantle thrust MMT) to thenorth and the main central thrust (MCT) to thesouthwest (Pognante and Spear 1991 Spencer1993) (Fig 5) The Cretaceous Kohistan island-arcsequence lies north of the Indus suture It developedas a result of intra-oceanic subduction The maincentral thrust separates an underlying Salkhala unitfrom an overlying higher Himalayan crystalline(HHC) unit The Kaghan nappe consists mainly ofgranitic gneiss and paragneiss with minor inter-calated layers of amphibolite marble and quartziteEclogites are preserved in the cores of maficboudins (less than 1 m thick) in gneiss and aresignificantly overprinted by amphibolite assem-blages They contain fine-grained (lt 1 mm) garnetand omphacite together with aggregates of quartzchains of rutile rimmed by titanite and mm-sizedphengitic micas overgrown by randomly oriented

amphiboles up to 1 cm in length Coesite and coesitepseudomorphs occur as inclusions in omphaciteand show the characteristic palisade-quartz aggre-gate texture fractures radiating from these inclu-sions are characteristically developed in the hostpyroxene

Garnet pyroxenendashphengite barometry wasapplied to the eclogites and yielded peak P-T con-ditions of 27ndash29 GPa and 690ndash750degC (OrsquoBrien etal 2001) (Fig 6) Retrograde textures are commonin the eclogites including symplectic intergrowthsof augite amphibole and plagioclase after omphac-ite thermobarometric analyses of these assemblagesyielded 10ndash13 GPa and 600ndash710degC (Kaneko et al2003) On the other hand the dominant graniticgneisses and metapelites of the higher Himalayancrystalline unit contain typical Barrovian-zoneamphibolite-facies assemblages represented bykyanite- and staurolite-bearing garnetiferousmetapelites (Treloar 1995) Extensive overprintinghas mostly obliterated the peak assemblages matrixminerals of the gneisses yielded P-T conditions of07ndash11 GPa and 600ndash700degC (Treloar 1995) con-sistent with retrogression of the eclogites Howeverrelict coesite is preserved as inclusions in zirconthe inclusions are ovoid and approximately 10 microm indiameter (Fig 7A) This discovery of coesite-bear-ing zircons from Himalayan quartzofeldspathicgneisses demonstrates subduction of Indian conti-nental crust to the depths of UHP conditions(Kaneko et al 2003)

Geochronological studies using SmNd (garnet-omphacite) RbSr (phengite) and UPb (rutile zir-con) methods suggest that the eclogite-facies eventtook place at 40ndash50 Ma (Tonarini et al 1993 Spen-cer and Gebauer 1996) 40Ar39Ar cooling ages forhornblende and mica from gneisses were reported tobe ~43 Ma and ~25 Ma respectively (Chamberlainet al 1991) Recent SHRIMP UPb dates of zonedzircons with mineral inclusions formed at differentstages indicate the ages of non-UHP mineral-bear-ing mantle domains of zircon and UHP mineral-bearing rims are at about 50 and 46 Ma respectively(Kaneko et al 2003) A new U-Pb age of 44 plusmn 11Ma was obtained for rutile from Kaghan coesite-bearing eclogite (Treloar et al 2003) this togetherwith available data implies that exhumation of theUHP rocks from mantle depth to mid-crustal leveloccurs within a few million years

Tso Morari The Tso Morari crystalline dome islocated between the Indus suture zone on the northbordered by the Zildat detachment fault and the

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 17

Zanskar sedimentary unit on the south (de Sigoyer etal 1997) The dome represents an internal massif ofthe northern part of the higher Himalaya The TsoMorari area has been considered a typical IndianTethyan margin because of a stratigraphic succes-sion similar to the Nimaling antiform in the core ofthe dome a metamorphic basement consists of Cam-bro-Ordovician augen gneisses (Stutz 1988) Thiscomplex is covered by a metasedimentary series ofCambrian to Devonian quartzite schist and con-glomerate and is overlain by Lower Carboniferousto Triassic marble and metapelite Eclogites occuras lenses or irregular bodies within the basementand cover gneisses and are strongly overprinted byamphibolite-facies assemblages de Sigoyer et al(1997) reported peak P-T conditions for the eclog-ites at 17ndash23 GPa and 520ndash640degC (Fig 7) How-ever relics of coesite 30ndash80 microm in size were

recently found as inclusions in garnet (Sachan et al2001 Mukherjee et al 2003) typified by radialfractures (Figs 7B and 7C) for the SiO2 inclusionsa characteristic Raman spectrum is centered at 523cm-1 (Fig 7D) The country rock metasedimentswith jadeite + chloritoid + paragonite + garnetassemblages have peak P-T estimates of 18ndash22GPa and 500ndash600degC and record retrogression at theeclogite-blueschist facies transition of 13ndash18 GPaand 490ndash590degC (Guillot et al 1997)

The geochronology of the Tso Morari eclogiteshas been extensively studied and subdivided intothree different stages (de Signoyer et al 2000)Eclogitization at ~55 Ma was obtained by Lu-Hf andSm-Nd methods on garnet omphacite glaucophaneand the whole rock An amphibolite-facies overprintoccurred at 47 Ma judging by RbSr dating onphengite apatite and whole rock from metapelites

FIG 6 P-T estimates of peak- and retrograde-stage metamorphism for coesite-bearing eclogites and the country-rockgneisses from the Kaghan Valley and Tso Morari

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 15

2001) An SmNd mineral isochron for eclogiticgarnet omphacite and the whole rock yields 500 plusmn10 Ma whereas an UPb isochron for zircon sepa-rates from eclogite gives an age of 504 plusmn 5 Ma(Zhang J X et al 2000)

North Qinling Mountains central China Lensesand blocks of eclogitic rocks in northern Qinling areenclosed in amphibolite-facies gneiss and garnet-bearing quartz + phengite mica schist Inclusions ofcoesite in eclogitic garnet (Hu et al 1995) andmicrodiamonds in zircon separates from both eclog-ite and gneiss (Yang et al 2002a) have beenrecently reported Although individual outcrops areno more than a few meters wide the belt of eclogitesextends more than 10 km P-T estimates of amphib-olitized coesite-bearing eclogite yield T = 590ndash758degC and P gt 26 GPa An Sm-Nd isochron for gar-net omphacite rutile amphibole and whole-rockyields 400 plusmn 16 Ma Microdiamonds were recentlydiscovered as inclusions in zircon from both eclogiteand its gneissic country rocks (Yang et al 2002a)SHRIMP UPb dating of zircons from granitic gneissyields 507 plusmn 38 Ma for peak UHP metamorphism(Yang et al 2002a) These data indicate that two

discrete collision events occurred between theNorth China and Yangtze cratons (1) an Early Pale-ozoic North Qinling UHP suturing and (2) a Triassicsuturing along the Dabie-Sulu UHP-HP belt TheEarly Paleozoic belt extends westward to the NorthQaidamndashAltun Mountains for more than 4000 km(Yang et al 2002b)

Himalayan UHP Eclogites

The Himalayan orogen has long been considereda classic locality of continental collision betweenthe Indian and Eurasian continents (Fig 5) Severaleclogite-facies HP rocks in the Himalayas havebeen reported since the late 1980s (eg Chaudaryand Ghazanfar 1987 Pognante and Spencer 1991de Sigoyer et al 1997 Lombardo and Rolfo 2000)Inclusions of coesite were recently discovered inomphacite and garnet in several Himalayan eclog-ites from the upper Kaghan Valley Pakistan(OrsquoBrien et al 2001 Kaneko et al 2003) Subse-quent findings of additional coesite localities in theTso Morari area (Sachan et al 2001 Mukherjee etal 2003) have established the presence of a UHPbelt in the Himalayas an active ongoing collision

FIG 5 A simplified tectonic map of the western Himalaya Hindu Kush Pamir Karakoran and western Tibet mod-ified after Figure 3 of Searle et al (2001) This map shows the locations of eclogite and UHP eclogites from both Hima-layan and North Qaidam Indus Suture Zone (ISZ) ophiolites and major structures

16 LIOU ET AL

since the Eocene (Massonne and OrsquoBrien 2003)However UHP relics are volumetrically very minorconstituents of the metamorphic belts dominatedmostly by later Barrovian overprintings In theserocks hydration related to Barrovian-zone metamor-phism has resulted in remarkably pervasive recrys-tallization during the later exhumation this eventhas almost entirely masked the preceding UHPrecord

The finding of HP-UHP evidence has led to newinsights into the Himalayan tectonic modelEngland and Houseman (1986) showed that in amodel of homogeneous thickening of the Hima-layan-Tibet boundary the maximum crustal thick-ness should be less than 70 km However theoccurrences of coesite-bearing eclogites andgneisses in the Himalayan orogen cannot beexplained by a homogeneous thickening of the crustbut rather by continental subduction as describedbelow According to available geochronologicaldata (eg Treloar et al 2003) the exhumation rateof the Himalayan UHP rocks has been estimated torange from ~1 cm yrndash1 (OrsquoBrien et al 2001) to gt3cm yrndash1 (Treloar et al 2003 Massone and OrsquoBrien2003) These estimates are higher than those calcu-lated from other UHP-HP terranes This requirestectonic emplacement of the UHP-HP Himalayanslab into shallow crust following isostatic rebounddue to slab decoupling between the Indian continentand the oceanic crust initiated during the MiddleEocene UHP rocks from both the Kaghan Valleyand Tso-Mori are described below

Kaghan Valley The Kaghan nappe in the north-western Himalayan syntaxis lies between the Indussuture (also called main mantle thrust MMT) to thenorth and the main central thrust (MCT) to thesouthwest (Pognante and Spear 1991 Spencer1993) (Fig 5) The Cretaceous Kohistan island-arcsequence lies north of the Indus suture It developedas a result of intra-oceanic subduction The maincentral thrust separates an underlying Salkhala unitfrom an overlying higher Himalayan crystalline(HHC) unit The Kaghan nappe consists mainly ofgranitic gneiss and paragneiss with minor inter-calated layers of amphibolite marble and quartziteEclogites are preserved in the cores of maficboudins (less than 1 m thick) in gneiss and aresignificantly overprinted by amphibolite assem-blages They contain fine-grained (lt 1 mm) garnetand omphacite together with aggregates of quartzchains of rutile rimmed by titanite and mm-sizedphengitic micas overgrown by randomly oriented

amphiboles up to 1 cm in length Coesite and coesitepseudomorphs occur as inclusions in omphaciteand show the characteristic palisade-quartz aggre-gate texture fractures radiating from these inclu-sions are characteristically developed in the hostpyroxene

Garnet pyroxenendashphengite barometry wasapplied to the eclogites and yielded peak P-T con-ditions of 27ndash29 GPa and 690ndash750degC (OrsquoBrien etal 2001) (Fig 6) Retrograde textures are commonin the eclogites including symplectic intergrowthsof augite amphibole and plagioclase after omphac-ite thermobarometric analyses of these assemblagesyielded 10ndash13 GPa and 600ndash710degC (Kaneko et al2003) On the other hand the dominant graniticgneisses and metapelites of the higher Himalayancrystalline unit contain typical Barrovian-zoneamphibolite-facies assemblages represented bykyanite- and staurolite-bearing garnetiferousmetapelites (Treloar 1995) Extensive overprintinghas mostly obliterated the peak assemblages matrixminerals of the gneisses yielded P-T conditions of07ndash11 GPa and 600ndash700degC (Treloar 1995) con-sistent with retrogression of the eclogites Howeverrelict coesite is preserved as inclusions in zirconthe inclusions are ovoid and approximately 10 microm indiameter (Fig 7A) This discovery of coesite-bear-ing zircons from Himalayan quartzofeldspathicgneisses demonstrates subduction of Indian conti-nental crust to the depths of UHP conditions(Kaneko et al 2003)

Geochronological studies using SmNd (garnet-omphacite) RbSr (phengite) and UPb (rutile zir-con) methods suggest that the eclogite-facies eventtook place at 40ndash50 Ma (Tonarini et al 1993 Spen-cer and Gebauer 1996) 40Ar39Ar cooling ages forhornblende and mica from gneisses were reported tobe ~43 Ma and ~25 Ma respectively (Chamberlainet al 1991) Recent SHRIMP UPb dates of zonedzircons with mineral inclusions formed at differentstages indicate the ages of non-UHP mineral-bear-ing mantle domains of zircon and UHP mineral-bearing rims are at about 50 and 46 Ma respectively(Kaneko et al 2003) A new U-Pb age of 44 plusmn 11Ma was obtained for rutile from Kaghan coesite-bearing eclogite (Treloar et al 2003) this togetherwith available data implies that exhumation of theUHP rocks from mantle depth to mid-crustal leveloccurs within a few million years

Tso Morari The Tso Morari crystalline dome islocated between the Indus suture zone on the northbordered by the Zildat detachment fault and the

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 17

Zanskar sedimentary unit on the south (de Sigoyer etal 1997) The dome represents an internal massif ofthe northern part of the higher Himalaya The TsoMorari area has been considered a typical IndianTethyan margin because of a stratigraphic succes-sion similar to the Nimaling antiform in the core ofthe dome a metamorphic basement consists of Cam-bro-Ordovician augen gneisses (Stutz 1988) Thiscomplex is covered by a metasedimentary series ofCambrian to Devonian quartzite schist and con-glomerate and is overlain by Lower Carboniferousto Triassic marble and metapelite Eclogites occuras lenses or irregular bodies within the basementand cover gneisses and are strongly overprinted byamphibolite-facies assemblages de Sigoyer et al(1997) reported peak P-T conditions for the eclog-ites at 17ndash23 GPa and 520ndash640degC (Fig 7) How-ever relics of coesite 30ndash80 microm in size were

recently found as inclusions in garnet (Sachan et al2001 Mukherjee et al 2003) typified by radialfractures (Figs 7B and 7C) for the SiO2 inclusionsa characteristic Raman spectrum is centered at 523cm-1 (Fig 7D) The country rock metasedimentswith jadeite + chloritoid + paragonite + garnetassemblages have peak P-T estimates of 18ndash22GPa and 500ndash600degC and record retrogression at theeclogite-blueschist facies transition of 13ndash18 GPaand 490ndash590degC (Guillot et al 1997)

The geochronology of the Tso Morari eclogiteshas been extensively studied and subdivided intothree different stages (de Signoyer et al 2000)Eclogitization at ~55 Ma was obtained by Lu-Hf andSm-Nd methods on garnet omphacite glaucophaneand the whole rock An amphibolite-facies overprintoccurred at 47 Ma judging by RbSr dating onphengite apatite and whole rock from metapelites

FIG 6 P-T estimates of peak- and retrograde-stage metamorphism for coesite-bearing eclogites and the country-rockgneisses from the Kaghan Valley and Tso Morari

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

16 LIOU ET AL

since the Eocene (Massonne and OrsquoBrien 2003)However UHP relics are volumetrically very minorconstituents of the metamorphic belts dominatedmostly by later Barrovian overprintings In theserocks hydration related to Barrovian-zone metamor-phism has resulted in remarkably pervasive recrys-tallization during the later exhumation this eventhas almost entirely masked the preceding UHPrecord

The finding of HP-UHP evidence has led to newinsights into the Himalayan tectonic modelEngland and Houseman (1986) showed that in amodel of homogeneous thickening of the Hima-layan-Tibet boundary the maximum crustal thick-ness should be less than 70 km However theoccurrences of coesite-bearing eclogites andgneisses in the Himalayan orogen cannot beexplained by a homogeneous thickening of the crustbut rather by continental subduction as describedbelow According to available geochronologicaldata (eg Treloar et al 2003) the exhumation rateof the Himalayan UHP rocks has been estimated torange from ~1 cm yrndash1 (OrsquoBrien et al 2001) to gt3cm yrndash1 (Treloar et al 2003 Massone and OrsquoBrien2003) These estimates are higher than those calcu-lated from other UHP-HP terranes This requirestectonic emplacement of the UHP-HP Himalayanslab into shallow crust following isostatic rebounddue to slab decoupling between the Indian continentand the oceanic crust initiated during the MiddleEocene UHP rocks from both the Kaghan Valleyand Tso-Mori are described below

Kaghan Valley The Kaghan nappe in the north-western Himalayan syntaxis lies between the Indussuture (also called main mantle thrust MMT) to thenorth and the main central thrust (MCT) to thesouthwest (Pognante and Spear 1991 Spencer1993) (Fig 5) The Cretaceous Kohistan island-arcsequence lies north of the Indus suture It developedas a result of intra-oceanic subduction The maincentral thrust separates an underlying Salkhala unitfrom an overlying higher Himalayan crystalline(HHC) unit The Kaghan nappe consists mainly ofgranitic gneiss and paragneiss with minor inter-calated layers of amphibolite marble and quartziteEclogites are preserved in the cores of maficboudins (less than 1 m thick) in gneiss and aresignificantly overprinted by amphibolite assem-blages They contain fine-grained (lt 1 mm) garnetand omphacite together with aggregates of quartzchains of rutile rimmed by titanite and mm-sizedphengitic micas overgrown by randomly oriented

amphiboles up to 1 cm in length Coesite and coesitepseudomorphs occur as inclusions in omphaciteand show the characteristic palisade-quartz aggre-gate texture fractures radiating from these inclu-sions are characteristically developed in the hostpyroxene

Garnet pyroxenendashphengite barometry wasapplied to the eclogites and yielded peak P-T con-ditions of 27ndash29 GPa and 690ndash750degC (OrsquoBrien etal 2001) (Fig 6) Retrograde textures are commonin the eclogites including symplectic intergrowthsof augite amphibole and plagioclase after omphac-ite thermobarometric analyses of these assemblagesyielded 10ndash13 GPa and 600ndash710degC (Kaneko et al2003) On the other hand the dominant graniticgneisses and metapelites of the higher Himalayancrystalline unit contain typical Barrovian-zoneamphibolite-facies assemblages represented bykyanite- and staurolite-bearing garnetiferousmetapelites (Treloar 1995) Extensive overprintinghas mostly obliterated the peak assemblages matrixminerals of the gneisses yielded P-T conditions of07ndash11 GPa and 600ndash700degC (Treloar 1995) con-sistent with retrogression of the eclogites Howeverrelict coesite is preserved as inclusions in zirconthe inclusions are ovoid and approximately 10 microm indiameter (Fig 7A) This discovery of coesite-bear-ing zircons from Himalayan quartzofeldspathicgneisses demonstrates subduction of Indian conti-nental crust to the depths of UHP conditions(Kaneko et al 2003)

Geochronological studies using SmNd (garnet-omphacite) RbSr (phengite) and UPb (rutile zir-con) methods suggest that the eclogite-facies eventtook place at 40ndash50 Ma (Tonarini et al 1993 Spen-cer and Gebauer 1996) 40Ar39Ar cooling ages forhornblende and mica from gneisses were reported tobe ~43 Ma and ~25 Ma respectively (Chamberlainet al 1991) Recent SHRIMP UPb dates of zonedzircons with mineral inclusions formed at differentstages indicate the ages of non-UHP mineral-bear-ing mantle domains of zircon and UHP mineral-bearing rims are at about 50 and 46 Ma respectively(Kaneko et al 2003) A new U-Pb age of 44 plusmn 11Ma was obtained for rutile from Kaghan coesite-bearing eclogite (Treloar et al 2003) this togetherwith available data implies that exhumation of theUHP rocks from mantle depth to mid-crustal leveloccurs within a few million years

Tso Morari The Tso Morari crystalline dome islocated between the Indus suture zone on the northbordered by the Zildat detachment fault and the

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 17

Zanskar sedimentary unit on the south (de Sigoyer etal 1997) The dome represents an internal massif ofthe northern part of the higher Himalaya The TsoMorari area has been considered a typical IndianTethyan margin because of a stratigraphic succes-sion similar to the Nimaling antiform in the core ofthe dome a metamorphic basement consists of Cam-bro-Ordovician augen gneisses (Stutz 1988) Thiscomplex is covered by a metasedimentary series ofCambrian to Devonian quartzite schist and con-glomerate and is overlain by Lower Carboniferousto Triassic marble and metapelite Eclogites occuras lenses or irregular bodies within the basementand cover gneisses and are strongly overprinted byamphibolite-facies assemblages de Sigoyer et al(1997) reported peak P-T conditions for the eclog-ites at 17ndash23 GPa and 520ndash640degC (Fig 7) How-ever relics of coesite 30ndash80 microm in size were

recently found as inclusions in garnet (Sachan et al2001 Mukherjee et al 2003) typified by radialfractures (Figs 7B and 7C) for the SiO2 inclusionsa characteristic Raman spectrum is centered at 523cm-1 (Fig 7D) The country rock metasedimentswith jadeite + chloritoid + paragonite + garnetassemblages have peak P-T estimates of 18ndash22GPa and 500ndash600degC and record retrogression at theeclogite-blueschist facies transition of 13ndash18 GPaand 490ndash590degC (Guillot et al 1997)

The geochronology of the Tso Morari eclogiteshas been extensively studied and subdivided intothree different stages (de Signoyer et al 2000)Eclogitization at ~55 Ma was obtained by Lu-Hf andSm-Nd methods on garnet omphacite glaucophaneand the whole rock An amphibolite-facies overprintoccurred at 47 Ma judging by RbSr dating onphengite apatite and whole rock from metapelites

FIG 6 P-T estimates of peak- and retrograde-stage metamorphism for coesite-bearing eclogites and the country-rockgneisses from the Kaghan Valley and Tso Morari

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 17

Zanskar sedimentary unit on the south (de Sigoyer etal 1997) The dome represents an internal massif ofthe northern part of the higher Himalaya The TsoMorari area has been considered a typical IndianTethyan margin because of a stratigraphic succes-sion similar to the Nimaling antiform in the core ofthe dome a metamorphic basement consists of Cam-bro-Ordovician augen gneisses (Stutz 1988) Thiscomplex is covered by a metasedimentary series ofCambrian to Devonian quartzite schist and con-glomerate and is overlain by Lower Carboniferousto Triassic marble and metapelite Eclogites occuras lenses or irregular bodies within the basementand cover gneisses and are strongly overprinted byamphibolite-facies assemblages de Sigoyer et al(1997) reported peak P-T conditions for the eclog-ites at 17ndash23 GPa and 520ndash640degC (Fig 7) How-ever relics of coesite 30ndash80 microm in size were

recently found as inclusions in garnet (Sachan et al2001 Mukherjee et al 2003) typified by radialfractures (Figs 7B and 7C) for the SiO2 inclusionsa characteristic Raman spectrum is centered at 523cm-1 (Fig 7D) The country rock metasedimentswith jadeite + chloritoid + paragonite + garnetassemblages have peak P-T estimates of 18ndash22GPa and 500ndash600degC and record retrogression at theeclogite-blueschist facies transition of 13ndash18 GPaand 490ndash590degC (Guillot et al 1997)

The geochronology of the Tso Morari eclogiteshas been extensively studied and subdivided intothree different stages (de Signoyer et al 2000)Eclogitization at ~55 Ma was obtained by Lu-Hf andSm-Nd methods on garnet omphacite glaucophaneand the whole rock An amphibolite-facies overprintoccurred at 47 Ma judging by RbSr dating onphengite apatite and whole rock from metapelites

FIG 6 P-T estimates of peak- and retrograde-stage metamorphism for coesite-bearing eclogites and the country-rockgneisses from the Kaghan Valley and Tso Morari

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

18 LIOU ET AL

40Ar39Ar ages of biotite and muscovite at ~30 Masuggest that the Tso Morari unit rose to upper crustallevels and recrystallized at the end of the exhuma-tion (de Signoyer et al 2000) Recent SHRIMPUPb dating of zoned zircons from country gneissicrocks to eclogite from the Tso Morari yields 48 plusmn 1Ma for the peak eclogite-facies metamorphism andprotolith ages of 700 plusmn 6 Ma to 1668 plusmn 14 Ma (Leechet al 2003) consistent with the 46 plusmn 1 Ma UHPages for Kaghan eclogite (Kaneko et al 2003) Dif-fusion modeling of garnet overgrowth compositionsteps by OrsquoBrien and Sachan (2000) yields an exhu-mation rate of 23ndash45 mm yrndash1 from the UHP stage to

low greenschist-facies stage occuring within about 3Ma (45ndash48 Ma) (Massone and OrsquoBrien 2003) Thesimilarity in the tectonic setting lithologies andUHP ages suggest they belong to a single UHP belt

Continental Subduction And Collision(the Himalayan Model)

Continental subduction and UHP metamorphism at 45 to 52 Ma

The Himalayan orogeny was controlled princi-pally by subduction of the Indian continental crustbeneath the Asian continental lithosphere this may

FIG 7 A Inclusions of low-P phases (quartz and muscovite) at core and coesite at rim from zircon separates ofgneissic rocks from Kaghan Valley (Kaneko et al 2003) B Photomicrograph of coesite inclusion in garnet from TsoMorari eclogite showing thin palisade quartz around coesite and radial fractures in host garnet (from Fig 2 of Mukherjeeet al 2003) C Inclusion of coesite in eclogitic garnet from Tso Morari (from Fig 2 of Mukherjee et al 2003) D LaserRaman spectra of coesite inclusion in zircon separates of Figure 7A

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 19

provide a present-day analog for UHP metamor-phism So far numerous tectonic models for theHimalayan orogeny have been proposed (for areview see Hodges 2000) most contributions havefocused on the dynamics of unusual crustal thicken-ing (up to 75 km) that resulted in a regional Bar-rovian-zone metamorphic sequence In general theclosure of Neo-Tethys to form the Indus suture wasfollowed by the India-Asia continental collision at52ndash50 Ma (Fig 8) Both Asian and Indian marginswere thickened and metamorphosed and the intra-

oceanic convergence generated a continental suture(eg Rowley 1996) During the infant stage of theHimalayan orogeny (~80ndash50 Ma) a Pacific-typearc-trench system was produced and huge multiplevolcanicplutonic calc-alkaline arcs formed coredby the Kohistan-Ladakh and Trans-Himalayanbatholiths Pacific-type subduction formed a Creta-ceous accretionary complex along the southernmargin of the Kohistan-Ladakh arc the accretionarycomplex contains 80 Ma blueschists (e g Anczkiewicz et al 2000) Granitic arc-magmatism

FIG 8 Subduction and collision of the Indian and Eurasian continents showing the subduction of microcontinent forformation of UHP rocks at 52 Ma (1) and exhumation of UHP rocks from the Himalayan metamorphic belt Exhumationof the UHP slab includes (2) wedge extrusion from mantle depth to crustal level at gt27 Ma and (3) a doming event at 11ndash9 Ma Paleogeographic positions of the Indian continent at 84 52 45 27 and 11ndash9 Ma are shown together with the rateestimates of northeastward motion of the Indian plate The doming event was responsible for the mountain-buildingprocesses for the High Himalayan Mountain chain

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

20 LIOU ET AL

continued until 50 Ma at Ladakh (Weinberg andDunlap 2000) when collision was initiated at thewestern syntax and propagated to the SE Sincethen subduction of the Indian continent hasoccurred and the India-Asia convergence rate dras-tically decreased from 19 to 5 cm yrndash1 at about Mid-dle Eocene time (eg Klootwijk et al 1992)Continental collision closed the intervening oceanand terminated arc magmatism (Fig 8) hence 45Ma lacustrine deposits on the Indian subcontinentcontain detritus derived from the Asian mainland

Recent seismologic studies of the ongoing Hima-layan orogeny have provided important constraintson its tectonic evolution In the western Himalayathe Hindu Kush region is seismically most activewith abundant intermediate-depth seismicity(Pegler and Das 1998) The seismic zone extends toa depth of 100-250 km and steepens with increas-ing depth A seismic reflection profile through theregion shows the presence of subducted continentalcrust and a trapped oceanic basin at depth On thebasis of the available seismic data Searle et al(2001) proposed a continental subduction model forthe formation of UHP metamorphic rocks beneaththe Hindu Kush Their model suggests that a coldlower crust of the Indian block comprising Precam-brian granulite-facies basement and a coversequence of PaleozoicndashLower Mesozoic supraconti-nental sediments was subducting beneath the HinduKush subsequently the crustal rocks underwentUHP metamorphism at about 150 km depth and~800ndash900degC at 4ndash6 GPa Kaneko et al (2003)obtained a sinking rate of 12ndash16 cm yrndash1 for thedowngoing Indian crust to ~110 km depth based onSHRIMP UPb ages of zoned zircon in UHP rocksfrom the Kagan Valley These data suggest a time-integrated subduction angle of 18ndash22deg similar tothe estimated angle of the present Moho between theHimalaya foreland and southern edge of the TethyanHimalaya (Zhao et al 1993) The timing of UHPmetamorphism at 46 Ma (Kaneko et al 2003) issynchronous with that of the rapid cooling (Wein-berg and Dunlap 2000) of the youngest plutons ofthe Ladakh batholith

Barrovian metamorphic overprint at gt30ndash15 Ma

The Himalayan orogeny involves crustal thick-ening that resulted in a regional Barrovian-typekyanite-sillimanite series metamorphism Thismetamorphism is well documented in metapelites ofthe Higher Himalaya (including the Zanskar Hima-layan series) and the north Himalayan gneiss

domes Most Oligocene kyanite-grade rocks weremetamorphosed at P = 06ndash10 GPa and T = 450ndash600degC Early Miocene sillimanite-grade rocks wereformed at higher T (650ndash770degC) but lower P (04ndash07 GPa) The UPb and SmNd geochronology inthe Zansker and Lahoul regions yields ~37ndash29 Mafor regional Barrovian-type kyanite-grade metamor-phism (eg Vance and Harris 1999 Walker et al1999) The 40Ar39Ar cooling ages of muscovite inthe metapelites yield 22ndash21 Ma (Walker et al1999) and overlap those in the Himalayan leucog-ranite (24ndash17 Ma Searle et al 1988) Monazitefrom sillimanite-bearing metapelites of the greaterHimalayan series in the Everest region yielded UPbages of 32 Ma (Simpson et al 2000) monazite UPbages of 23 Ma from sillimanite-cordieritendashbearinggranitic gneiss represent the timing of cordierite-grade low-P metamorphism related to crustalmelting

The kyanite- and staurolite-bearing mineralassemblage occurs as a retrograde phase of the TsoMorari eclogitic metapelites (Guillot et al 1997) inthe largest gneiss dome of the Himalayan chain40Ar39Ar plateau ages of 31ndash29 Ma from low-Simuscovite and biotite of retrograded metapelitesrepresent thermal relaxation at upper crustal levels(de Sigoyer et al 2000) A series of gneiss domes(north Himalayan gneiss domes) that are mantled byBarrovian-type kyanite-grade metamorphic rockscan be traced for at least 2000 km their cooling ageis similar to the Barrovian overprint of the TsoMorari UHP eclogites

Domal uplift and mountain-building since 11 Ma

The uplift of the higher Himalayas apparentlyoccurred in the Late Miocene as documented bysedimentary flooding demonstrated on ODP Leg116 by on-land geology and by climate change incentral Asia Drilling of the distal Bengal fan in thecentral Indian Ocean revealed that the sedimenta-tion rate drastically increased at 109 Ma (Amanoand Taira 1992) Paleontological evidence suggeststhat glaciation began in the higher Himalayas atabout 7 Ma (Lakhanpal et al 1983) The loess-paleosol sequence of the Chinese Loess Plateau isknown to have formed by eolian transport of parti-cles from the inland deserts of northwestern Chinaand possibly Central Asia In recent years uplift ofthe Himalayan-Tibetan Plateau (Manabe and Broc-coli 1990 An et al 2001) and changes in land-seadistribution (Ramstein et al 1997) have beenconsidered principal driving forces for long-term

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

Amano K and Taira A 1992 Two-phase uplift ofHigher Himalayas since 17 Ma Geology v 20 p 391ndash394

An Z S Kutzbach J E Prell W L and Porter S C2001 Evolution of Asian monsoons and phased upliftof the Himalaya-Tibetan Plateau since Late Miocenetimes Nature v 411 p 62ndash66

Ankiewicz R Burg J P Villa I M and Meier M2000 Late Cretaceous blueschist metamorphism inthe Indus suture zone Shangla region Pakistan Hima-laya Tectonophysics v 324 p 111ndash134

Bakun-Czubarow N 1991 On the possibility of occur-rence of quartz pseudomorphs after coesite in theeclogite-granulite rock series of the Zlote Mountains inthe Sudetes SW Poland Archiwum Mineralogy v 48p 3ndash25

Beane R J and Connelly J N 1998 40Ar39Ar U-Pband Sm-Nd constraints on the timing of metamorphicevents in the Maksyutov Complex southern UralMountains Journal of the Geological Society of Lon-don v 157 p 811ndash822

Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

Bowtell S A Cliff R A and Barnicoat A C 1994Sm-Nd isotopic evidence on the age of eclogitization inthe Zermatt-Saas ophiolite Journal of MetamorphicGeology v 12 p 187ndash196

Brueckner H K and Medaris L G Jr 1998 A tale oftwo orogens The contrasting T-P-t history andgeochemical evolution of mantle in high- and ultra-high-pressure metamorphic terranes of the NorwegianCaledonides and the Czech Variscides SchweizerischeMineralogische und Petrographische Mitteilungen v78 p 293ndash307

Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 21

Cenozoic climatic change A very surprising resulthas just been published in Nature (Guo et al 2003)which recounts the discovery of a 22 Ma old loessdeposit in Gansu Province and the onset of Asiandesertification has been linked to the uplift of theTibetan Plateau

The doming event at 15ndash11 Ma is also suggestedby mica 40Ar39Ar thermochronology of the Kangmardome (Maluski et al 1988 Lee et al 2000) Thisdoming event raised the Himalayan Mountain chainmore than 5 km in elevation active erosion has con-tinued and thick flysch sequences have accumu-lated in the Bengal fan The major driving force forthe domal uplift is not well understood TheMiocene underthrusting of the Indian tectospherebeneath the Lesser Himalayas may have elevatedthe Higher Himalaya

These data clearly show a two-step exhumationprocess for the Himalayan UHP and high-grademetamorphic terrane (1) Emplacement of the Hima-layan high-grade metamorphic rocks at shallowcrustal levels occurred at gt 30 to 20 Ma when notopographic mountain building was apparent and(2) Himalayan mountain building began at 109 Mawhen extensive surface erosion commenced Figure8 schematically shows such events together with thepaleogeography of the Indian and Eurasian conti-nents since 80 Ma

Acknowledgments

We submit this review article for the retirementcelebration of W G Ernst who has documentedsubduction-zone metamorphism for both Pacific-type and Alpine-type blueschist terranes Thismanuscript was initiated in 2000ndash2001 while SMaruyama held a sabbatical position as a Cox Pro-fessor at Stanford both T Tsujimori and I Katayamawere supported by a JSPS Research Fellowship forYoung Scientists and a Grant-in-Aid for JSPS Fel-lows The paper represents a product of the USndashJapan project supported by the NSF Global Partner-ship Fellowship INT-9820171 and the NSF US-China project EAR 0003355 We thank Y Kanekoand Himanshu Sachan for preprints and photomi-crograph of coesite inclusions in UHP rocks respec-tively from the Kaghan Valley of Pakistan and Tso-Mori We appreciate a constructive review of themanuscript by our mentor W G Ernst

REFERENCES

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Beane R J Liou J G Coleman R G and LeechM L 1995 Petrology and retrograde P-T path foreclogites of the Maksyutov complex southern UralMountains Russia The Island Arc v 4 p 254ndash266

BostickB Jones R E Ernst W G Chen C LeechM L and Beane R J 2003 Low-temperaturemicrodiamond aggregates in the Maksyutov metamor-phic complex south Ural Mountains Russia Ameri-can Mineralogist v 88 p 1709ndash1717

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Caby R 1994 Precambrian coesite from N Mali Firstrecord and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt EuropeanJournal of Mineralogy v 6 p 235ndash244

Carswell D A Brueckner H K Cuthbert S J MehtaK and OrsquoBrien P J 2003 The timing of stabilisationand the exhumation rate for ultra-high pressure rocksin the Western Gneiss Region Journal of MetamorphicGeology v 21 p 601ndash612

Carswell D A and Compagnoni R eds 2003 Ultra-high pressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 51ndash74

Carswell D A and Cuthbert S J 2003 Reviews of rep-resentative UHPM terranes The Western Gneiss

22 LIOU ET AL

Region of Norway in Carswell D A and Compag-noni R eds Ultra-high pressure metamorphismEuropean Mineralogical Union Notes in Mineralogyv 5 p 51ndash74

Carswell D A Cuthbert S J and Krogh Ravna E J1999 Ultrahigh-pressure metamorphism in the west-ern Gneiss Region of the Norwegian CaledonidesInternational Geology Review v 41 p 955ndash966

Campos Neto M da C and Caby R 1999 Tectonic con-straint on Neoproterozoic high-pressure metamor-phism and nappe system south of the San Franciscocraton southeast Brazil Precambrian Research v 97p 3ndash26

Chamberlain C P Zeitler P K and Erickson E 1991Constraints on the tectonic evolution of the northwest-ern Himalaya from geochronologic and petrologicstudies of Babusar Pass Pakistan Journal of Geologyv 99 p 829ndash849

Chaudary M N and Ghazanfar M 1987 Geology struc-ture and geomorphology of upper Kaghan valleyNorthwestern Himalaya Pakistan Geological Bulle-tin University of Punjab v 22 p 13ndash57

Chesnokov B V and Popov V A 1965 Increasing vol-ume of quartz grains in eclogites of the south UralsDoklady Akademii Nauk v 162 p 176ndash178 (inRussian)

Chopin C 1984 Coesite and pure pyrope in high-gradeblueschists of the Western Alps A first record andsome consequences Contributions to Mineralogy andPetrology v 86 p 107ndash118

______ 2003 Ultrahigh-pressure metamorphism Trac-ing continental crust into the mantle Earth and Plane-tary Science Letters v 212 p 1ndash14

Coleman R G and Wang X 1995 Ultrahigh pressuremetamorphism Cambridge UK Cambridge Univer-sity Press

Compagnoni R and Rolfo F 2003 Reviews of repre-sentative UHPM terranes The Western Alps in Car-swell D A and Compagnoni R eds Ultra-highpressure metamorphism European MineralogicalUnion Notes in Mineralogy v 5 p 13ndash50

Costa S Maluski A and Lardeaux J M 1993 40-39Archronology of Variscan tectono-metamorphic events inan exhumed crustal nappe The Monts du Lyonnaiscomplex (Massif Central France) Chemical Geology v105 p 339ndash359

Cuthbert S J Carswell D A Krogh-Ravna E J andWain A 2000 Eclogites and eclogites in the WesternGneiss Region Norwegian Caledonides Lithos v 52p 165ndash195

de Sigoyer J Chavagnac V Blichert-Toft J Villa I MLuais B Guillot S Cosca M and Mascle G 2000Dating the Indian continental subduction and colli-sional thickening in the northwest Himalaya Multi-chronology of the Tso Morari eclogites Geology v 28p 487ndash490

de Sigoyer J Guillot S Laudeaux J M and Mascle G1997 Glaucophane-bearing eclogites in the TsoMorari dome eastern Ladakh NW Himalaya Euro-pean Journal of Mineralogy v 9 p 1073ndash1083

Di Vincenzo G and Palmeri R 2001 An 40Ar-39Arinvestigation of high-pressure metamorphism and theretrogressive history of mafic eclogites from the Lan-terman Range Antarctica Evidence against a simpletemperature control on argon transport in amphiboleContributions to Mineralogy and Petrology v 141 p15ndash35

Dobretsov N L and Dobretsova L V 1988 New miner-alogic data on the Maksyutov eclogite-glaucophaneschist complex southern Urals Doklady AkademiiNauk v 300 p 111ndash116

Dobretsov N L Shatsky V S Coleman R G LennykhV I Valizer P M Liou J G Zhang R Y andBeane R J 1996 Tectonic setting and petrology ofultrahigh-pressure metamorphic rocks in theMaksyutov Complex Ural Mountains Russia Interna-tional Geology Review v 38 p 136ndash160

Dobrzhinetskaya L F Eide E A Larsen R B SturtB A Tronnes R G Smith D C Taylor W R andPosukhova T V 1995 Microdiamond in high-grademetamorphic rocks of the Western Gneiss region Nor-way Geology v 23 p 597ndash600

Dobrzhinetskaya L F Green H W Mitchell T E andDickerson R M 2001 Metamorphic diamondsMechanism of growth and inclusion of oxides Geologyv 29 p 263ndash266

Dobrzhinetskaya L F Green H W Weschler MDarus M Wang Y C Massonne H and StoumlckertB 2003 Focused ion beam technique and transmis-sion electron microscope studies of microdiamondsfrom the Saxonian Erzgebirge Germany Earth andPlanetary Science Letter in press

England P C and Houseman G A 1986 Finite straincalculations of continental deformation II Applica-tion to the India-Asia plate collision Journal of Geo-physical Research v 91 p 3664ndash3676

Ernst W G and Liou J G eds 2000 Ultrahigh-pres-sure metamorphism and geodynamics in collision-typeorogenic belts Geological Society of America Interna-tional Book Series v 4 293 p

Gardien V Tegyey M Lardeaux J M Misseri M andDufour E 1990 Crustal-mantle relationship inFrench Variscan chain The example of the southernMonts du Lyonnais unit (eastern French Massif Cen-tral) Journal of Metamorphic Geology v 8 p 477ndash492

Gebauer D Schertl H P Brix M and Schreyer W1997 35 Ma old ultrahigh-pressure metamorphismand evidence for very rapid exhumation in the DoraMaira Massif western Alps Lithos v 41 p 5ndash24

Ghiribelli B Frezzotti M-L and Palmeri R 2002Coesite in eclogites of the Lanterman range Antarc-

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 23

tica Evidence from textural and Raman studies Euro-pean Journal of Mineralogy v 14 p 355ndash360

Gilotti J A 1993 Discovery of a medium-temperatureeclogite province in the Caledonides of North-EastGreenland Geology v 21 p 523ndash526

Gilotti J A and Ravna E J K 2002 First evidence forultrahigh-pressure metamorphism in the NortheastGreenland Caledonides Geology v 30 p 551ndash554

Guillot S de Sigoyer J Lardeaux J M and Mascle G1997 Eclogitic metasediments from the Tso Morariarea Ladakh Himalaya Evidence for continentalsubduction during India-Asia convergence Contribu-tions to Mineralogy and Petrology v 128 p 197ndash212

Guo Z T Ruddiman W F Hao Q Z Wu H B QiaoY S Zhu R X Peng S Z Wei J J Yuan B Y andLiu T S 2003 Onset of Asian desertification by 22Myr ago inferred from loess deposits in China Naturev 416 p 159ndash162

Hacker B R Ratschbacher L Webb L McWilliamsM O Ireland T Calvert A Dong S Wenk H-Rand Chateigner D 2000 Exhumation of ultrahigh-pressure continental crust in East-central China LateTriassicndashEarly Jurassic tectonic unroofing Journal ofGeophysical Research v 105 p 13339ndash13364

Hirajima T and Nakamura D 2003 Reviews of repre-sentative UHPM terranes The Dabie Shan and Suluregion of China in Carswell D A and CompagnoniR eds Ultra-high pressure metamorphism Euro-pean Mineralogical Union Notes in Mineralogy v 5p 105ndash144

Hodges K V 2000 Tectonics of the Himalaya and south-ern Tibet from two perspectives Geological Society ofAmerica Bulletin v 112 p 324ndash350

Hu N Zhao D Xu B and Wang T 1995 Petrographyand metamorphic study on high-ultrahigh pressureeclogite from Guanpo area northern Qinling Moun-tain Journal of Mineralogy and Petrology v 15 p 1ndash9(in Chinese)

Hwang S L Shen P Chu H T Yui T F and LinC C 2001 Genesis of microdiamonds from melt andassociated multiphase inclusions in garnet of ultra-high-pressure gneiss from Erzegebirge GermanyEarth and Planetary Science Letters v 188 p 9ndash15

Jahn B M Caby R and Monie P 2001 The oldestUHP eclogites of the World Age of UHP metamor-phism nature of protoliths and tectonic implicationsChemical Geology v 178 p 143ndash158

Kaneko Y Katayama I Yamamoto H Misawa KIshikawa M Rehman H U Kausar A B andShiraishi K 2003 Timing of Himalayan ultrahigh-pressure metamorphism Sinking rate and subductionangle of the Indian continental crust beneath AsiaJournal of Metamorphic Geology v 21 p 589ndash599

Kaneko Y Maruyama S Terabayashi M YamamotoH Ishikawa M Anma R Parkinson C D Ota TNakajima Y Katayama I Yamamoto J and Yamau-chi K 2000 Geology of the Kokchetav UHP-HP

metamorphic belt Northern Kazakhstan The IslandArc v 9 p 264ndash283

Kardarusman A and Parkinson C D 2000 Petrologyand P-T evolution of garnet peridotites from centralSilawesi Indonesia Journal of Metamorphic Geologyv 18 p 193ndash209

Katayama I Maruyama S Parkinson C D Terada Kand Sano Y 2001 Ion micro-probe U-Pb zircon geo-chronology of peak and retrograde stages of ultrahigh-pressure metamorphic rocks from the Kokchetav mas-sif northern Kazakhstan Earth and Planetary ScienceLetters v 188 p 185ndash198

Katayama I Ohta M and Ogasawara Y 2002 Mineralin zircon from diamond-bearing marble in theKokchetav massif northern Kazakhstan EuropeanJournal of Mineralogy v 14 p 1103ndash1108

Krogh E J and Carswell D A 1995 HP and UHPeclogites and garnet peridotites in the ScandinavianCaledonides in Coleman R G and Wang X edsUltrahigh pressure metamorphism Cambridge UKCambridge University Press p 244ndash298

Klootwijk C Gee J Peirce J Smith G and McFad-den P 1992 An early India-Asia contact Paleomag-netic constraints from the Ninetyeast Ridge ODP Leg121 Geology v 20 p 395ndash398

Lakhanpal R N Sah S C D Kewal K Sharma K Kand Guleria J S 1983 Occurrence of Livistona inthe Hemis conglomerate horizon of Ladakh in ThakurV C and Sharma K K eds Geology of Indus suturezone of Ladakh New Delhi India Hindustan BookPublishing Corporation p 179ndash185

Lardeaux J M Ledru P Daniel I and Duchene S2001 The Variscan French Massif Centralmdasha newaddition to the ultrahigh pressure metamorphicldquoclubrdquo Exhumation processes and geodynamic conse-quences Tectonophysics v 332 p 143ndash167

Lee J Hacker B R Dinklage W S Gans P BCalvert A Wang Y Wan J and Chen W 2000Evolution of the Kangmar Dome southern TibetStructural petrologic and thermochronologic con-straints Tectonics v 19 p 872ndash895

Leech M L and Ernst W G 1998 Graphite pseudo-morphs after diamond A carbon isotope and spectro-scopic study of graphite cuboids from the Maksyutovcomplex south Ural Mountains Russia Geochimicaet Cosmochimica Acta v 62 p 2143ndash2154

Leech M L Singh S Jain A K and ManickavasagamR M 2003 New U-Pb SHRIMP ages for the UHPTso-Morari crystallines eastern Ladakh India [abs]Geological Society of America Abstracts with Pro-grams v 34 p 637

Li S Chen Y Cong B Zhang Z Zhang R Liou DHart S R and Ge N 1993 Collision of the NorthChina and Yangtze blocks and formation of coesite-bearing eclogites Timing and processes ChemicalGeology v 109 p 70ndash89

24 LIOU ET AL

Liou J G 1999 Petrotectonic summary of less-inten-sively studied UHP regions International GeologyReview v 41 p 571ndash586

Liou J G and Zhang R Y 2002 Ultrahigh-pressuremetamorphic rocks Encyclopedia of Physical sciencesand technology third ed v 17 p 227ndash244 TarzanaCA Academia Press

Liou J G Zhang R Y Katayama I Maruyama S andErnst W G 2002 Petrotectonic characterization ofthe Kokchetav Massif and the Dabie-Sulu terranesmdashUltrahigh-P metamorphism in the so-called P-T For-bidden-Zone Western Pacific Earth Sciences v 2 p119ndash148

Liu F Xu Z Katayama I Yang J S Maruyama Sand Liou J G 2001 Mineral inclusions in zircons ofpara- and orthogneiss from pre-pilot drillhole CCSD-PP1 Chinese Continental Scientific Drilling ProjectLithos v 59 p 199ndash215

Lombardo B and Rolfo F 2000 Two contrasting eclog-ite types in the Himalaya implications for the Hima-layan orogeny Journal of Geodynamics v 30 p 37ndash60

Maluski H Matte P and Brunel M 1988 Argon 39-argon 40 dating of metamorphic and plutonic events inthe North and High Himalaya belts (southern Tibet-China) Tectonics v 7 p 299ndash326

Manabe S and Broccoli AJ 1990 Mountains and aridclimates of middle latitudes Science v 247 p 192ndash195

Maruyama S Liou J G and Terabayashi M 1996Blueschists and eclogites of the world and their exhu-mation International Geology Review v 38 p 485ndash594

Masago H Rumble D Ernst W G Parkinson C andMaruyama S 2003 O18 depletion in eclogites fromthe Kokchetav massif northern Kazakhstan Journal ofMetamorphic Geology v 21 p 579ndash587

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______ 2001 First find of coesite in the ultrahigh-pres-sure metamorphic region of the Central ErzgebirgeGermany European Journal of Mineralogy v 13 p565ndash570

Massonne H J and OrsquoBrien P 2003 Reviews of repre-sentative UHPM terranes The Bohemian Massif andthe NW Himalaya in Carswell D A and Compag-noni R (eds) Ultra-high pressure metamorphismEuropean Mineralogical Union Notes in Mineralogyv 5 p 145ndash188

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Matte P 1998 Continental subduction and exhumationof HP rocks in Paleozoic orogenic belts Uralides and

Variscides Geologiska Foreningen i StockholmForhandlingar v 20 p 209ndash222

McClelland W C and Gilotti J A 2003 Late-stageextensional exhumation of high-pressure granulites inthe Greenland Caledonides Geology v 31 p 259ndash262

Mposkos E D and Kostopoulos D K 2001 Diamondformer coesite and supersilicic garnet in metasedi-mentary rocks from the Greek Rhodope A new ultra-high-pressure metamorphic province establishedEarth and Planetary Science Letters v 192 p 497ndash506

Mukherjee B K Sachan H K Ogasawara Y MukoA and Yoshioka N 2003 Carbonate-bearing UHPMrocks from the Tso-Morari region Ladakh India Pet-rological implications International Geology Reviewv 45 p 49ndash69

OrsquoBrien P J and Sachan H K 2000 Diffusion model-ling in garnet from Tso Morari eclogite and implica-tions for exhumation models Earth Science Frontier(China University of Geoscience Beijing) v 7 p 25ndash27

OrsquoBrien P J Zotov N Law R Khan M A and JanM Q 2001 Coesite in Himalayan eclogite and impli-cations for models of India-Asia collision Geology v29 p 435ndash438

Ogasawara Y Fukasawa K and Maruyama S 2002Coesite exsolution from supersilicic titanite in UHPmarble from the Kokchetav Massif northern Kazakh-stan American Mineralogist v 87 p 454ndash461

Ogasawara Y Ohta M Fukasawa K Katayama I andMaruyama S 2000 Diamond-bearing and diamond-free metacarbonate rocks from Kumdy-Kol in theKokchetav massif northern Kazakhstan The IslandArc v 9 p 400ndash416

Okamoto K and Maruyama S 1999 The high-pressuresynthesis of lawsonite in the MORB + H2O systemAmerican Mineralogist v 84 p 362ndash373

Paquette J L Monchoux P and Couturier M 1995Geochemical and isotopic study of a norite-eclogitetransition in the European Variscan belt Implicationsfor U-Pb zircon systematics in metabasic rocksGeochimica et Cosmochimica Acta v 59 p 1611ndash1622

Parkinson C D 2003 Coesite-bearing quartzites fromSulawesi Indonesia A first record of UHP metamor-phism in SE Asia Tectonophysics in press

Parkinson C D Katayama I Liou J G and MaruyamaS eds 2002 The diamond-bearing Kokchetav MassifKazakhstan Petrochemistry and tectonic evolution ofan unique ultrahigh-pressure metamorphic terraneTokyo Japan Universal Academy Press Inc 527 p

Parkinson C D Motoki A Onishi C T andMaruyama S 2001 Ultrahigh-pressure pyrope-kyan-ite granulites and associated eclogites in Neoprotero-zoic Nappes of Southeast Brazil UHPM Workshop2001 Waseda University p 87ndash90

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 25

Pegler G and Das S 1998 An enhanced image of thePamirndashHindu Kush seismic zone from relocated earth-quake hypocentres Geophysical Journal Internationalv 134 p 573ndash595

Pognante U and Spear D A 1991 First record ofeclogites from the High Himalayan belt Kaghan Val-ley (northern Pakistan) European Journal of Mineral-ogy v 3 p 613ndash618

Ramstein G Fluteau F Besse J and Joussaume S1997 Effect of orogeny plate motion and land-seadistribution on Eurasian climate change over the past30 million years Nature v 386 p 788ndash795

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Rowley D Y 1996 Age of initiation of collision betweenIndia and Asia A review of stratigraphic data Earthand Planetary Science Letters v 145 p 1ndash13

Rumble D Liou J G and Jahn B M 2003 Continen-tal crust subduction and UHP metamorphism in Rud-nick R L ed The crust Oxford UK ElsevierTreatise on Geochemistry v 3 (Holland H D andTurekian K K eds) p 293ndash319

Sachan H Mucherjee B K Ishida H Muko AYoshioka N Ogasawara Y and Maruyama S 2001New discovery of coesite from the Indian Himalaya inFluidslabmantle interactions and ultrahigh-P miner-als UHPM Workshop 2001 Waseda Japan p 124ndash128

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Schmadicke E 1991 Quartz pseudomorphs after coesitein eclogites from the Saxonian Erzgebirge EuropeanJournal of Mineralogy v 3 p 231ndash238

Schmadicke E Mezger K Cosca M A and OkruschM 1995 Variscan Sm-Nd and Ar-Ar ages of eclogitefacies rocks from the Erzgebirge Bohemian MassifJournal of Metamorphic Geology v 13 p 537ndash552

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Shatsky V S and Sobolev N V 2003 Reviews of repre-sentative UHPM terranes The Kokchetav massif

Kazakhstan in Carswell D A and Compagnoni Reds Ultra-high pressure metamorphism EuropeanMineralogical Union Notes in Mineralogy v 5 p 75ndash104

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Solari L Ortega F Sole-vinas J Gomez-Tuena AOrtega-Obregon C Reher-Salas M Martens U andMoran S 2003 Petrologic evidence for possibleultrahigh pressure metamorphism in the ChuacusComplex of north-central Guatemala [abs] GeologicalSociety of America Abstracts with Programs v 34 p639

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Song S G Yang J S Xu Z Q Liou J G and ShiR D 2003b Metamorphic evolution of the coesite-bearing ultrahigh-pressure terrane in the NorthQaidam Northern Tibet NW China Journal of Meta-morphic Geology v 21 p 631ndash644

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Stoumlckhert B Duyster J Trepmann C and MassonneH J 2001 Microdiamond daughter crystals precipi-tated from supercritical COH + silicate fluids includedin garnet Erzegebirge Germany Geology v 29 p391ndash394

26 LIOU ET AL

Stutz E 1988 Geologie de la chaine du Nyimaling auxconfines du Ladakh et du Rupshu NW-HimalayaIndesmdashevolution paleogeographique et tectoniquedrsquoun segment de la marge nord-indienne Memorie deGeologie Lausanne no 3 149 p

Sun W D Williams I S and Li S G 2002 Carbonif-erous and Triassic eclogites in the western DabieMountains east-central China Evidence for pro-tracted convergence of the North and South ChinaBlocks Journal of Metamorphic Geology v 20 p873ndash886

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Tagiri M Yano T Bakirov A Nakajima T and Uchi-umi S 1995 Mineral parageneses and metamorphicP-T paths of ultrahigh-pressure eclogites fromKyrghyzstan Tien-Shan The Island Arc v 4 p 280ndash292

Tonarini S Villa I M Oberli F Meier M SpencerD A Pognante U and Ramsay J R 1993 Eoceneage of eclogite metamorphism in Pakistan HimalayaImplications for India-Eurasia collision Terra Nova v5 p 13ndash20

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Treloar P OrsquoBrien P J Parrish R R and Khan M A2003 Exhumation of early Tertiary coesite-bearingeclogites from the Pakistan Himalaya Journal of theGeological Society of London v 160 p 367ndash376

van Roermund H L M Carswell D A Drury M Rand Heijboer T C 2002 Microdiamonds in a mega-crystic garnet websterite pod from Bardane on theisland of Fjortoft western Norway Evidence fordiamond formation in mantle rocks during deep conti-nental subduction Geology v 30 p 959ndash962

van Roermund H L M and Drury M R 1998 Ultra-high pressure (P gt 6 Gpa) garnet peridotites in westernNorway Exhumation of mantle rocks from gt 185 kmdepth Terra Nova v 10 p 295ndash301

van Roermund H L M Drury M R Barnhoom A andRonde A 2000 Super-silicic garnet microstructuresfrom an orogenic garnet peridotite evidence for anultra-deep gt 6 GPa Origin Journal of MetamorphicGeology v 18 p 135ndash148

Vance D and Harris N B W 1999 The timing of pro-grade metamorphism in the Zanskar Himalaya Geol-ogy v 27 p 395ndash398

Wain A 1997 New evidence for coesite in eclogite andgneisses Defining an ultrahigh-pressure province inthe Western Gneiss Region of Norway Geology v 25p 927ndash930

Wain A Waters D Jephcoat A and Olijynk H 2000The high-pressure to ultrahigh-pressure eclogite tran-

sition in the Western Gneiss Region Norway Euro-pean Journal of Mineralogy v 12 p 667ndash687

Walker J Martin M W Bowring S A Searle M PWaters D J and Hodges K V 1999 Metamor-phism melting and extension Age constraints fromthe High Himalayan slab of southeast Zanskar andnorthwest Lahaul Journal of Geology v 107 p 473ndash495

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Wei C J Powell R and Zhang L F 2003 Eclogitesfrom the south Tianshan NW China Petrologicalcharacteristic and calculated mineral equilibria in theNa2O-CaO-FeO-MgO-Al2O3-SiO2-H2O system Jour-nal of Metamorphic Geology v 21 p 163ndash180

Weinberg R F and Dunlap W 2000 Growth and defor-mation of the Ladakh Batholith NW HimalayasImplications for timing of continental collision and ori-gin of calc-akaline batholiths Journal of Geology v108 p 303ndash320

Xu S Liu Y Chen G Compagnoni R Rolfo F HeM and Liu H 2003 New findings of microdiamondsin eclogites from Dabie-Sulu region in central-easternChina Chinese Science Bulletin v 48 p 988ndash994

Xu W Okay A I Ji S Sengor A M C Su W andJiang L 1992 Diamond from the Dabie Shan meta-morphic rocks and its implication for tectonic settingScience v 256 p 80ndash82

Yang J J Godard G Kienast J R Lu Y and Sun J1993 Ultrahigh-pressure 60 kbar magnesite-bearinggarnet peridotite from Northeastern Jiangsu ChinaJournal of Geology v 101 p 541ndash554

Yang J Xu Z Pei X Shi R Wu C Zhang J Li HMeng F and Rong H 2002a Discovery of diamondin North Qiling Evidence for a giant UHPM beltacross central China and Recognition of Paleozoic andMesozoic dual deep subduction between North Chinaand Yangtze plates Acta Geologica Sinica v 76 p484ndash495 (in Chinese)

Yang J S Xu Z Song S Zhang J Shi R Li H andBrunel M 2001 Discovery of coesite in the NorthQaidam Early Paleozoic ultrahigh pressure UHPmetamorphic belt NW China Comptes Rendus delrsquoAcademie des Sciences Paris Sciences de la Terreet des Planets v 333 p 719ndash724

Yang J S Xu Z Zhang J Chu C Zhang R and LiouJ G 2001 Tectonic significance of early Paleozoichigh-pressure rocks in Altun-Qaidam-Qilian Moun-tains northwest China in Hendrix M S and DavisG A eds Paleozoic and Mesozoic tectonic evolutionof central Asia From continental assembly to intrac-ontinental deformation Geological Society of AmericaMemoir v 194 p 151ndash170

Yang J S Xu Z Zhang J Song S Wu C Shi R LiH and Brunel M 2002b Early Palaeozoic NorthQaidam UHP metamorphic belt on the north-eastern

UHP METAMORPHISM AND CONTINENTAL SUBDUCTION 27

Tibetan Plateau and a paired subduction model TerraNova v 14 p 397ndash404

Ye K Cong B and Ye D 2000 The possible subduc-tion of continental material to depths greater than 200km Nature v 407 p 734ndash736

Yui T F Rumble D and Lo C H 1995 Unusually lowd18O ultrahigh-pressure metamorphic rocks from Su-Lu terrane China Geochimica et Cosmochimica Actav 59 p 2859ndash2864

Zhang J X Yang J S Xu Z Q Meng F C Li H Band Shi R 2002 Evidence for UHP metamorphism ofeclogite from the Altun Mountains Chinese ScienceBulletin v 47 p 751ndash755

Zhang J X Zhang Z M Xu Z Q Yang J S and CuiJ W 2001 Petrology and geochronology of eclogitesfrom the western segment of the Altyn Tagh northwest-ern China Lithos v 56 p 187ndash206

Zhang L Ellis D J Arculus R J and Jiang W 2003ldquoForbidden zonerdquo subduction of sediments to 150 kmdepthmdashthe reaction of dolomite to magnesite + arago-nite in the UHPM metapelites from western TianshanChina Journal of Metamorphic Geology v 21 p 523ndash529

Zhang L Ellis D J and Jiang W 2002 Ultrahigh-pressure metamorphism in western Tianshan China

Part I Evidence from inclusions of coesite pseudomor-phs in garnet and from quartz exsolution lamellae inomphacite in eclogites Part II Evidence from magne-site in eclogite American Mineralogist v 87 p 853-860 860ndash866

Zhang R Y Liou J G and Yang J S 2000 Petro-chemical constraints for dual origin of garnet peridot-ites of the Dabie-Sulu UHP terrane China Journal ofMetamorphic Geology v 18 p 149ndash166

Zhang R Y Liou J G Yang J and Ye K 2003 Ultra-high-pressure metamorphism in the forbidden zoneThe Xugou garnet peridotite Sulu terrane easternChina Journal of Metamorphic Geology v 21 p 539ndash550

Zheng Y F Fu B Gong B and Li L 2003 Stableisotope geochemistry of ultrahigh-pressure metamor-phic rocks from the Dabie-Sulu orogen in ChinaImplications for geodynamics and fluid regime EarthScience Review v 62 p 105ndash161

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