7 petrography, mineral chemistry and p t conditions … 7.pdfchapter 7 ‐ 91 ‐ 7 petrography,...

CHAPTER 7

‐ 91 ‐

7 Petrography, mineral chemistry and P‐T conditions of HP/UHP

metamorphic rocks in the Xitieshan terrane, North Qaidam

The Xitieshan eclogite‐gneiss terrane, named for the Xitie Mountain range, is located in

the central segment of the North Qaidam Orogen (Figure 7.1a and b). The rocks of the

Xitieshan terrane are underlain mainly by rocks belonging to the Dakendaban metamorphic

complex and the Tanjianshan Group, which is composed of early Paleozoic

volcano‐sedimentary rocks (Figure 7.1c). The Dakendaban Group in the Xitieshan area

consists predominantly of paragneiss and orthogneiss and is overthrust by rocks of the

Tanjianshan Group and locally intruded by granitic plutons. The relative age relationship

between the para‐ and orthogneiss is difficult to distinguish in the field. Eclogites and

ultramafic rocks occur as lenses or blocks in the gneiss and schist.

Figure 7.1 (a) Map of China showing the location of North Qaidam Orogen; (b) distribution of UHP

metamorphic terranes in the North Qaidam Orogenic belt; (c) regional map of the Xitieshan terrane

and studied samples localities. Modified from Zhang et al. (2005a).

In this study, ten samples representing various rock types from the Huangyanggou area

of the Xitieshan terrane (Figure 7.1c) were examined petrographically. The modal

abundances of minerals in the samples that have been studied in detail are presented in

Table 7.1.

This chapter was not part of the Chinese version of the thesis.

Xitieshan Town

Huangyanggou

4 km

Quaternary-TertiaryOrdovician volcanic rocksGraniteDakendaban GneissEclogite and retrograde eclogiteFoliationStretching lineation

AnticlineSynclineThrust fault

95°42′E

38°18′N

95°38′E95°34′E

38°22′N

38°26′N

(a)

(b)

E105

N35

1000 km

Qilian BlockDa Qaidam

Yuka UHPterrane

Luliangshangarnet-peridotite

terrane Xitieshanterrane

Delingha Ulan

Dulan

Dulan UHPterrane

Qaidam Block100 km

Figure. 7-1c

Sulu

Dabie

QinlingNorth Qaidam

West Tianshan

Altun

China

(c)

09NQ4209NQ4309NQ44

09NQ50

09NQ5109NQ52

09NQ5509NQ45

09NQ39

09NQ60

PETROGRAPHY OF HP/UHP ROCKS IN THE XITIESHAN TERRANE

‐ 92 ‐

Table 7.1 Modal abundance of minerals in Xitieshan samples (vol. %)

Sample Lithology Grt Cpx Amp Ms Bt Qz Fsp Zo Sym Zrn Chl Rt Ttn

09NQ39 Qz‐Grt‐Amphibolite 55 – 30 – – 15 1 – – – –

09NQ42 Schist – – 10 15 30 10 35 – – – – – –

09NQ43 Amphibolite – – 55 – – 5 25 – – – 10 – –

09NQ44 Grt‐Amphibolite 25 5 50 – – 5 15 1 – – 1 1

09NQ45 Grt‐Amphibolite 30 5 50 – – 5 15 – 20 – – 1 1

09NQ50 Amphibolite – – 70 – – 5 10 – – – – – –

09NQ51 Gneiss – – – 25 10 40 25 – – 1 – – –

09NQ52 Gneiss – – – 15 15 40 30 – – 1 – – –

09NQ55 Granitic gneiss – – 10 – 10 40 40 – – – – – 1

09NQ60 Eclogite 30 30 20 – – 3 – 20 1 – 3 3

Note: Mineral abbreviations according to Whitney and Evans (2010) except Sym=symplectite.

7.1 Field occurrences

In the Xitieshan terrane, eclogitic rocks occur as lenses, boudins and blocks enclosed in

gneisses and micaschists (Figure 7.2a and c). Based on mineral assemblages, two types of

eclogite are recognized: phengite‐bearing eclogite and bimineralic eclogite (Zhang et al.,

2011a). Compared with the eclogites in the Yuka terrane located further northwest, the

Xitieshan eclogites are much more extensively retrograded. We1l‐preserved eclogite mineral

assemblages are only preserved locally in the cores of large boudins characterized by a

successive transition from the core outward of eclogite (Grt+Cpx), garnet granulite

(Grt+Cpx+Pl±Bt), garnet amphibolite (Grt+Amp+Pl), and finally amphibolite (Amp+Pl). Coesite

relics and their quartz pseudomorphs occur as inclusions in zircon and omphacite grains in

garnet amphibolite (Liu et al., 2012) and bimineralic eclogite (Zhang et al., 2011a),

respectively. In addition, the eclogite lenses are usually strongly sheared along their margins

and the deformation is clearly linked to retrograde amphibolization of the eclogite. The

foliated amphibolite rims of eclogite lenses are characterized by parallel to the foliation of

the enclosing gneisses, indicating that the eclogites and their country rocks experienced a

common late‐stage metamorphic and deformation event during exhumation (Zhang et al.,

2006).

The country rock para‐ and orthogneisses contain upper‐amphibolite to high‐pressure

granulite‐facies mineral assemblages and show extensive migmatization (Figure 7.2c). Two

types of orthogneiss have been recognized on the basis of their field color and mineral

assemblages: dark‐colored biotite‐rich gneiss and light‐colored muscovite gneiss, which are

usually separated by clear boundaries (Figure 7.2d). A fabric of oriented muscovite, biotite

and elongated feldspar, quartz usually defines the foliation.

CHAPTER 7

‐ 93 ‐

Figure 7.2 Photographs showing field relationships between gneiss and enclosed eclogitic rocks in

Xitieshan terrane, North Qaidam, NW China. (a) Retrogressed eclogite lens within gneiss. (b)

Amphibolite lens within gneiss and surrounded by felsic veins. (c) Garnet amphibolite lens within

migmatitic gneiss, leucosome within gneiss contains coarse‐grained feldspar and amphibole. (d)

Macroscopically distinct dark‐colored and light‐colored gneisses with a sharp contact. The length of

the hammer used in (a) and (d) is 41 cm; the geologist in (b) is 1.80 m; the pen in (c) is 14 cm.

7.2 Sample description and Petrography

The main lithological units and sample localities of the Xitieshan terrane are shown in

Figure 7.1c. The rock units include eclogite, garnet amphibolite, amphibolite, orthogneiss

and micaschist. Their petrographic characteristics are described below.

7.2.1 Eclogite

Sample 09NQ60 from the Xitieshan terrane is a medium‐ to coarse‐grained eclogite with

a granoblastic texture and a mineral assemblage comprising mainly garnet and clinopyroxene

with retrograde diopside + plagioclase symplectite and calcic‐amphibole, as well as traces of

rutile quartz and zircon (Figure 7.3a). Garnet is subhedral to euhedral, ~0.5 to 2 mm in

diameter and is typically rimmed by plagioclase and diopside coronas. It commonly contains

inclusions of clinopyroxene (Cpx‐I), quartz, amphibole, rutile and titanite. Clinopyroxene in

the matrix (Cpx‐II) occurs as subhedral intergrowths with amphibole. Some clinopyroxene

(Cpx‐III) occurs as symplectites completely enclosed within single grains of optically

continuous clinopyroxene and as diopside‐plagioclase symplectites at clinopyroxene grain

boundaries. Amphibole occurs as a retrograde product in the matrix and as small inclusions

in garnet cores. Plagioclase occurs as rims surrounding garnet and as rod‐shaped

components of symplectites. Rutile usually occurs in the matrix or as inclusions within garnet

and omphacite, and is rimmed by titanite.


‐ 94 ‐

Figure 7.3 Photomicrographs showing the textural relationships in samples from the Xitieshan terrane.

(a) Eclogite (09NQ60, XPL); garnet and clinopyroxene are rimmed by thin Di+Pl coronas and some

clinopyroxenes have been completely altered to vermicular Di+Pl symplectites. (b) Quartz garnet

amphibolite (09NQ39, XPL); subhedral and euhedral garnet is intergrown with quartz and amphibole.

(c) Garnet amphibolite (09NQ45, PPL); garnet is surrounded by amphibole, plagioclase and biotite

coronas, biotite also fills fractures in garnet mantles. (d) Garnet amphibolite (09NQ44, PPL); garnet

with medium‐grained subhedral amphibole inclusions is surrounded by amphibole and plagioclase

coronas, vermicular Di+Pl symplectite is overgrown by green amphibole. (e) Amphibolite (09NQ43,

XPL) exhibits granoblastic texture, consisting of amphibole, feldspar, chlorite and minor titanite. (f)

Micaschist (09NQ42, XPL) shows a well‐developed foliation defined by biotite and elongated

plagioclase.

7.2.2 Garnet amphibolite

Quartz bearing garnet amphibolite 09NQ39 is fine to medium grained with mosaic

granoblastic texture (Figure 7.3b). The mineral assemblage consists of garnet, amphibole and

quartz, with titanite, zircon and rare rutile as accessory minerals. Garnet is generally

CHAPTER 7

‐ 95 ‐

subhedral, and locally occurs as coalesced aggregates of two or more grains. Amphibole

occurs as subhedral crystals 1‐3 mm in size and as an irregular network around many garnet

and quartz inclusions forming a sieve texture. Two modes of quartz occurrence were

observed: inclusions in garnet and amphibole, and irregular crystal intergrowths with garnet.

Garnet amphibolite samples 09NQ44 and 09NQ45 contain predominantly garnet,

amphibole, plagioclase and symplectites of clinopyroxene + plagioclase and amphibole +

plagioclase. They are characterized by an intensive granulite/upper amphibolite facies

overprint during decompression before returning to the surface (Figure 7.3c and d). Garnet

usually occurs as idioblastic porphyroblasts with sizes of 0.2 – 2 mm in diameter, containing

inclusions of quartz, plagioclase, biotite and amphibole. The margins of garnet commonly

display conspicuous coronas of symplectite intergrowths of amphibole + plagioclase. No

omphacite relics have been recognized in these samples. Clinopyroxene only occurs as

diopside, in fine‐grained symplectites with plagioclase. Amphibole is the major phase in

these samples. The amphiboles generally can be divided into three types based on their

petrographic positions: (1) inclusions in garnet porphyroblasts (Amp‐I); (2) fine crystals

in vermicular symplectite surrounding garnet (Amp‐II); and (3) coronas around garnet or

intergrown with plagioclase in the matrix (Amp‐III). Three modes of plagioclase

occurrence were identified: 1) inclusions in garnet (Pl‐I); 2) fine grains in

amphibole‐plagioclase and diopside‐plagioclase symplectites, and coronas around garnet

(Pl‐II), and 3) irregular shaped in matrix (Pl‐III). Biotite occurs as inclusions in garnet mantles

or as coronas intergrown with amphibole in garnet rims (Figure 7.3c).

7.2.3 Amphibolite

Sample 09NQ43 is a fine‐grained amphibolite, comprising equigranular amphibole,

feldspar, chlorite and minor titanite (Figure 7.3e). The overall texture of the rock is

granoblastic. Amphibole usually occurs as anhedral laths (0.1 – 1 mm in grain size). Feldspar

occurs as subhedral tabular crystals (0.1 – 0.5 mm in grain size), is often slightly rounded and

exhibits Carlsbad and lamellar twins. Chlorite partly replaces amphibole.

Sample 09NQ50 is a medium‐grained amphibolite that predominantly consists of

amphibole, feldspar and minor titanite. Amphibole grains usually exhibit sieve texture, in

which irregular fine‐grained plagioclase crystals and subhedral titanites are enclosed in

coarse‐grained amphibole. Feldspar occurs as irregular patches at amphibole grain

boundaries.

7.2.4 Schist

Sample 09NQ42 is a medium grained, strongly foliated biotite schist that is mainly

composed of biotite and feldspar with muscovite, amphibole and quartz (Figure 7.3f). The

foliation is defined by alignment of biotite crystals up to 3 mm in size. Biotite is usually

rimmed by chlorite and has straight grain boundaries with feldspar. Feldspar and quartz are

statically intergrown in lenticular polygonal aggregates. Potassic feldspar shows Carlsbad

twinning and local seritization of the K‐feldspar can be observed.


‐ 96 ‐

Figure 7.4 Photomicrographs showing the texture and paragenesis of gneisses from the Xitieshan

terrane, North Qaidam orogen. (a) In light‐colored gneiss (09NQ51, XPL) muscovites form‐ coarse

mica fish. (b) Granitic gneiss (09NQ55, XPL); muscovite and biotite occur as small subhedral platy

crystals in intergranular areas in the quartzofeldspathic matrix.

7.2.5 Gneiss

The medium‐grained, poorly foliated gneisses (09NQ51 and 09NQ52) contain similar

assemblages of muscovite, feldspar, quartz and biotite (Figure 7.4a). Accessory minerals are

titanite, zircon and apatite. Sample 09NQ51 is light‐colored in the field due to its lower

biotite and higher quartz and feldspar contents (Figure 7.2d). Feldspar occurs as subhedral to

anhedral crystals up to 2 mm in size, with small quartz and biotite as inclusions. Quartz

occurs as anhedral crystals and is often intergrown with feldspar. Muscovite and biotite

crystals are subhedral plates and their orientations define a moderate foliation. Some

muscovites are characterized by coarse fish‐shaped crystals (Figure 7.4a).

Granitic gneiss sample 09NQ55 is medium‐grained and comprises feldspar, quartz,

biotite, amphibole and muscovite, with accessory titanite and zircon (Figure 7.4b). The

texture is broadly equigranular. Alkali feldspars are euhedral to subhedral crystals up to 2 mm

in size, and exhibit indistinct tartan twining. Plagioclase crystals are subhedral and up to 1

mm in size and exhibit albite twinning. Quartz occurs as anhedral and intergranular crystals

up to 2 mm in size and in places exhibits undulose extinction. Muscovite and biotite occur as

small subhedral platy crystals in intergranular areas (Figure 7.4b).

7.3 Mineral composition

The minerals compositions were analyzed using a JEOL JXA‐8800M electron microprobe

at the Petrology department of VU University Amsterdam. The analyses were performed at

an accelerating voltage of 15.0 kV and a beam current of approximately 20 nA. Counting

times were 20 − 30 s on peaks and half this on backgrounds. The data were regressed using

an oxide‐ZAF correction program supplied by JEOL. Molecular formula for amphibole was

CHAPTER 7

‐ 97 ‐

calculated according to Holland and Blundy (1994). All iron is assumed to be Fe2+ in white

mica and biotite. Representative mineral compositions for all minerals analyzed are listed in

Table 7.2.

7.3.1 Garnet

Garnet formulae were normalized to 12 oxygen atoms. Garnet occurs in both eclogite

and garnet amphibolite. In eclogite 09NQ60, individual garnet grains usually have a uniform

composition from the core to the rim. This indicates that the rocks probably stay long under

high temperature conditions, which caused homogenization of garnet. The general

composition of garnet porphyroblasts is Prp35‐40Alm38‐41Grs18‐23Adr0.1‐3.3Sps0.8‐1. In the

pyrope–(almandine+spessartine)–(grossular+andradite) ternary diagram (Figure 7.5a) garnet

compositions plot within the group B‐type eclogite field of Coleman et al. (1965).

Figure 7.5 Ternary diagrams showing garnet and clinopyroxene compositions of the Xitieshan terrane.

(a) Garnet plotted after Coleman et al. (1965). (b) Clinopyroxene plotted after Morimoto et al. (1988).

For quartz garnet amphibolite (09NQ39) garnet compositions are in the range

Prp16‐17Alm56‐60Grs22‐27And0.1‐0.2Sps1‐1.5 and garnet amphibolites (09NQ44 and 09NQ45) have a

garnet composition range of Prp26‐30Alm42‐44Grs23‐28And2‐3Sps0.9‐1.1. In these samples,

individual garnet grains usually have uniform compositions, except for the outermost 50 to

100μm of grains. An EMPA transect across a coarse garnet crystal of sample 09NQ45

indicates a slight increase in the FeO component associated with a decrease in the CaO and

MgO components at the contacts with its plagioclase corona (Figure 7.6). These features

indicate that the garnet grains were homogenized at high temperature, followed by

retrograde Fe/Mg re‐equilibration related to volume diffusion in garnet and adjacent

minerals (O'Brien, 1997; Zhang et al., 2005a). In the pyrope–(almandine+spessartine)–

(grossular+andradite) ternary diagram (Figure 7.5a), garnet compositions again plot mainly

within the group C‐type eclogite field of Coleman et al. (1965).

(a) (b)Qz-Grt-Amphibolite(09NQ39)

Grt-Amphibolite(09NQ44&09NQ45)

Eclogite(09NQ60)

Alm+Sps

PrpGrs+Adr

C-type B-type A-type%Fs%En

Aug

%Wo

Hd

En

Pgt

Fs

Di


‐ 98 ‐

Figure 7.6 BSE image and composition profile of garnet of the Xitieshan garnet amphibolite 09NQ45.

7.3.2 Clinopyroxene

Clinopyroxene was found in eclogite and garnet amphibolite. The clinopyroxene

analyses were calculated for 6 oxygens. The jadeite contents were calculated as the sodium

content minus the amount of acmite (ferric iron) (Brouwer et al., 2002). Ferric iron in

clinopyroxene is estimated using the scheme of Droop (1987) and the nomenclature for

sodic‐calcic pyroxene is based on Morimoto et al. (1988). The composition of clinopyroxene

in the eclogite and garnet amphibolites is shown in the ternary diagram of Figure 7.5b.

Clinopyroxene has a high calcium‐tschermak and a non‐omphacitic jadeite composition (<20%

jadeite), and almost all clinopyroxene analyses plot well within the diopside field.

Clinopyroxene in the matrix (Cpx‐II) of eclogite (09NQ60) has a content of 15–21 mol%

jadeite, whereas the inclusions (Cpx‐I) in garnet and fine grains (Cpx‐III) in

diopside‐plagioclase symplectites have much more lower jadeite contents of ~14 and 7–12

mol%, respectively. Clinopyroxene in the garnet amphibolites (09NQ44 and 09NQ45) only

occurs as fine grains in diopside‐plagioclase symplectites and has a very low jadeite content

of 3–8 mol%.

7.3.3 Amphibole

Amphibole is a major constituent in almost all studied samples and shows a large

variation in grain size, modal abundance and composition. Amphibole in eclogite 09NQ60 is

calcic‐amphibole according to Leake’s classification (Leake et al., 1997). It generally can be

divided into two types based on its position in the petrographic fabric, but both types have

similar compositions. The first type (Amp‐I) occurs as inclusions and is associated with quartz

in garnet and the second type (Amp‐III) shows sharp grain boundaries with intergrowth

garnet and clinopyroxene in the matrix. Both are classified as magnesio‐hornblende (Figure

7.7a).

Amphibole (Amp‐I) inclusions in garnet in garnet amphibolites (09NQ44 and 09NQ45)

CHAPTER 7

‐ 99 ‐

are tschermakite; Amp‐II in the amphibole‐plagioclase symplectites are actinolite; and the

large poikilitic retrograde amphibole porphyroblasts (Amp‐III) formed during granulite or

upper amphibolite facies metamorphism are magnesio‐hornblende (Figure 7.7a). Amphibole

in amphibolites 09NQ43 and 09NQ50 is classified as edenite, pargasite and

magnesio‐hornblende (Figure 7.7a and b). For the granitic gneiss (09NQ55), the amphibole

porphyroblast phase is edenite (Figure 7.7b). Amphibole from the biotite schist (09NQ42) is

actinolite (Figure 7.7a).

Figure 7.7 Amphibole compositions plotted in standard end‐member diagrams (nomenclature after

Leake et al. (1997))

7.3.4 Mica

White mica formulae were calculated by normalizing to 11 oxygen atoms and all Fe was

assumed to be ferrous. In the Xitieshan terrane, phengite inclusions in eclogite garnet have

been reported by Zhang et al. (2011a), but was not observed in this study. White mica has

only been identified in gneissic and schistose country rocks. As shown in Figure 7.8, white

mica in light‐colored and dark‐colored gneisses (09NQ51 and 09NQ52) and granitic gneiss

(09NQ55) has similar compositions, with Si of 3.04‐3.22 per formula unit (p.f.u.), within the

“muscovite” series based on the classification of Rieder et al. (1999).

pargasite

ferro-pargasite

edenite

ferro-edenite

0.0

0.5

1.0

5.56.06.57.07.5

Mg/

(Mg+

Fe2+

)

Si (p.f.u)

Calcic amphiboles

ferroactinolite ferrohornblende

magnesiohornblende

ferrotschermakite

tschermakite

actinolite

tremolite

0.0

0.5

1.0

5.56.06.57.07.58.0

Mg/

(Mg+

Fe2+

)

Si (p.f.u)

Calcic amphiboles

(a)

(b)

in Grt (Amp-I) Eclogite Amphibolite

GneissSchist

in Matrix (Amp-III)Amp-IIIAmp-IIIAmp-III

}in Grt (Amp-I) Sym (Amp-II) Grt-Amphibolitein Matrix (Amp-III)

}


‐ 100 ‐

Figure 7.8 Muscovite compositions in Xitieshan gneisses. Data obtained by microprobe analysis.

Biotite is ubiquitous in Xitieshan schist 09NQ42 and dark‐colored gneiss 09NQ52, but is

a trace phase in granitic gneiss 09NQ55. Biotite formulae were normalized to 11 oxygen

atoms. All Fe is assumed to be ferrous. Microprobe analyses show that these biotites are

characterized by high TiO2 (2.6–3.5%) and low Al2O3 (~16%) contents. The Mg/(Mg+Fe) ratios

range from 0.48 to 0.57. Moreover, biotite in granitic gneiss shows higher FeO content (18–

20%) than that in schist (16–18%).

7.3.5 Feldspar

In the Xitieshan terrane, plagioclase feldspar commonly occurs in mafic (garnet)

amphibolites and their country rocks. As shown in Figure 7.9a, feldspar in the biotite schist

09NQ42 is classified as microcline (Ab3Or97), whereas feldspar in the granitic gneiss 09NQ55

forms two groups with generalized compositions of Ab67‐75An23‐32Or1‐2 and Ab5‐8Or91‐95.

Feldspar in the light‐colored and dark‐colored gneisses (09NQ51 and 09NQ52) occurs in two

groups with generalized compositions of Ab70‐84An15‐27Or1‐2 and Ab4‐14Or85‐96 (Figure 7.9a). In

the amphibolites (09NQ43 and 09NQ50) feldspars has compositions of Ab1An99 or

Ab54‐85An2‐42Or2‐41 (Figure 7.9b). Feldspar in garnet amphibolites (09NQ44 and 09NQ45)

occurs in amphibolite‐plagioclase symplectites and is classified as andesine

(Ab53‐56An44‐47Or1), while in the matrix it is albite, oligoclase or anorthite (Ab0‐93An23‐99)

(Figure 7.9b).

1.8

2.1

2.4

2.7

3.0

3.0 3.1 3.2 3.3 3.4 3.5 3.6

Al c

atio

ns p

.f.u

Si cations p.f.u

09NQ51(Light-colored gneiss)

09NQ52(Dark-colored gneiss)

09NQ55(Granitic gneiss)

CHAPTER 7

‐ 101 ‐

Figure 7.9 Ternary plots of feldspar compositions. Data obtained by microprobe analyses.

7.4 Metamorphic evolution

Based on the textural relations and mineral assemblages described above, four main

metamorphic stages have been recognized in the Xitieshan eclogites and their retrogressed

equivalents:

(5) Garnet (core) –clinopyroxene (Cpx‐I, in matrix)–rutile ± quartz (in eclogite, peak

eclogite stage, M1);

(6) garnet (rim)–low jadeite clinopyroxene (Cpx‐II, symplectites)–plagioclase–

symplectites (Di‐Pl) ± amphibole ± quartz (in eclogite, high‐pressure granulite facies

stage, M2);

(7) Garnet–amphibole (Amp‐III)–plagioclase (Pl‐III)–symplectites (Amp‐Pl) (in garnet

amphibolite, upper amphibolite facies stage, M3);

(8) Amphibole–plagioclase (in amphibolite, low amphibolite facies stage, M4).

The mineral assemblages in the eclogite and (garnet) amphibolite samples allow

application of several geothermobarometers to estimate the pressure and temperature of

metamorphism: the garnet‐clinopyroxene Fe‐Mg exchange thermometer (Powell, 1985;

Krogh, 1988), garnet‐clinopyroxene‐plagioclase‐quartz barometer (Newton and Perkins,

1982), amphibole‐plagioclase thermometer (Holland and Blundy, 1994) and

amphibole‐plagioclase‐(quartz) barometer (Bhadra and Bhattacharya, 2007). The

intersection of a geobarometer and a geothermometer provides a P‐T estimate.

The mineral‐assemblages of garnet‐clinopyroxene and amphibole‐plagioclase in the

samples are used for geothermobarometrical study. The chemical compositions of selected

minerals and results of P‐T estimations are presented in Table 7.2

Albite Oligoclase Andesine Labradorite Bytownite AnorthiteCaAl2Si2O8NaAlSi3O8

Anor

thoc

lase

Sani

dine

Micr

oclin

e

Alka

li Fel

dspa

rs

Plagioclase

KAlSi3O8Orthoclase

and Microcline

Albite Oligoclase Andesine Labradorite Bytownite AnorthiteCaAl2Si2O8NaAlSi3O8

Anor

thoc

lase

Sani

dine

Micr

oclin

e

Alka

li Fel

dspa

rs

Plagioclase

KAlSi3O8Orthoclase

and Microcline

09NQ43&09NQ50(Amphibolite matrix)

09NQ44&09NQ45(Garnet Amphibolite matrix)

09NQ44&09NQ45(Amp-Pl Symplectites)

09NQ42(Biotite schist)

09NQ51&09NQ52(Gneiss)

09NQ55(Granitic Gneiss)(a) (b)

‐ 102 ‐

Table 7.2 M

icroprobe analyses of selected

minerals from high/ultra‐high pressure m

etam

orphic rocks in the Xitieshan

terrane (c‐core, r‐rim

)

Sample

09NQ60

09NQ45

Mineral

Grt‐m

Grt‐c

Grt‐r

Grt‐c

Grt‐r

Grt‐r

Grt‐c

Cpx

Cpx‐c

Cpx‐r

Cpx

Cpx‐r

Cpx‐c

Cpx‐c

Grt‐r

Grt‐r

Cpx

Cpx

Mineral

Pl

Pl

Spot

1

3

18

26

28

36

41

4

8

21

31

30

29

46

46

50

38

39

Spot

42

46

SiO2

39.68

39.31

39.91

39.82

39.63

39.74

39.64

54.73

55.10

54.85

54.48

54.74

54.67

54.56

40.01

39.91

51.42 51.60

SiO2

56.01 56.12

TiO2

0.03

0.03

0.02

0.06

0.01

0.03

0.04

0.10

0.11

0.10

0.15

0.10

0.07

0.09

0.07

0.05

0.07

0.09

TiO2

0.00

0.00

Al 2O3

22.53

22.30

22.45

22.37

22.30

22.27

22.33

4.74

5.55

5.51

5.48

5.84

4.90

5.09

22.51

22.72

3.55

3.57

Al 2O3

27.46 27.06

FeO

19.42

19.22

19.49

19.53

19.79

19.15

19.64

3.39

2.75

3.01

3.10

3.30

3.11

2.82

19.79

20.99

6.30

6.26

FeO

0.20

0.05

MnO

0.41

0.40

0.46

0.42

0.46

0.43

0.43

0.02

0.03

0.03

0.02

0.04

0.04

0.03

0.52

0.44

0.22

0.08

MnO

0.00

0.00

MgO

10.35

10.43

9.98

9.98

9.72

10.04

9.87

13.97

13.43

13.17

13.35

13.07

13.60

13.56

7.36

8.20

13.99 13.79

MgO

0.01

0.00

CaO

7.71

7.64

8.20

8.25

8.26

8.30

8.15

20.95

20.48

20.27

20.66

20.35

20.98

20.70

10.36

8.39

22.25 22.62

CaO

9.10

8.95

Na 2O

0.01

0.01

0.01

0.01

0.00

0.01

0.01

1.99

2.64

2.70

2.23

2.62

2.13

2.37

0.01

0.01

1.32

0.95

Na 2O

6.11

6.17

K2O

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.01

0.02

0.00

0.00

0.00

0.00

0.01

0.00

0.00

0.10

0.20

K2O

0.08

0.07

total

100.14

99.35 100.52 100.44 100.17

99.97

100.13

99.90

100.11

99.63

99.46

100.06

99.50

99.23

100.64

100.70

99.21 99.16

total

98.98 98.43

Si

2.98

2.98

3.00

2.99

2.99

3.00

2.99

1.98

1.98

1.98

1.98

1.97

1.99

1.98

3.02

3.01

1.90

1.91

Si

2.54

2.56

AlIV

0.02

0.02

0.00

0.01

0.01

0.00

0.01

0.02

0.02

0.02

0.02

0.03

0.01

0.02

0.00

0.00

0.10

0.09

Al

1.47

1.45

AlVI

1.98

1.97

1.98

1.97

1.98

1.98

1.98

0.19

0.22

0.22

0.22

0.22

0.20

0.20

2.00

2.02

0.05

0.07

Ca

0.44

0.44

Ti

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Na

0.54

0.55

Fe(total)

1.22

1.22

1.22

1.23

1.25

1.21

1.24

0.10

0.08

0.09

0.09

0.10

0.09

0.09

1.25

1.32

0.19

0.19

K

0.00

0.00

Mn

0.03

0.03

0.03

0.03

0.03

0.03

0.03

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.03

0.03

0.01

0.00

XAb

0.45

0.44

Mg

1.16

1.18

1.12

1.12

1.09

1.13

1.11

0.75

0.72

0.71

0.72

0.70

0.74

0.73

0.83

0.92

0.77

0.76

XAn

0.55

0.55

Ca

0.62

0.62

0.66

0.66

0.67

0.67

0.66

0.81

0.79

0.79

0.80

0.79

0.82

0.81

0.84

0.68

0.88

0.90

XOr

0.00

0.00

Na

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.14

0.18

0.19

0.16

0.18

0.15

0.17

0.00

0.00

0.09

0.07

K

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

1.00

Fe3+

0.04

0.06

0.03

0.04

0.05

0.03

0.04

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.14

0.08

Fe2+

1.18

1.15

1.19

1.18

1.20

1.17

1.19

0.10

0.08

0.09

0.09

0.10

0.09

0.09

1.25

1.33

0.06

0.12

XCaG

rt

0.21

0.20

0.22

0.22

0.22

0.22

0.22

0.28

0.23

XJd

0.14

0.18

0.19

0.16

0.18

0.15

0.17

0.05

0.07

‐ 103 ‐

Table 7.2 (continued

)

Sample 09NQ44

Sample

09NQ43

Sample

09NQ50

Mineral

Amp

Amp

Amp M

ineral

Pl

Pl

Pl

Mineral

Amp

Amp

Amp

Amp

Amp M

ineral

Pl

Pl

Pl

Pl

Pl

Mineral

Amp

Mineral

Pl

Spot

29

38

39

Spot

28

41

42

Spot

9

14

30

33

19

Spot

16

2

29

4

25

Spot

58

Spot

53

SiO2

47.20

48.3447.33 SiO2

54.95

55.73

56.01

SiO2

44.26

43.28

44.94

45.84

45.36 SiO2

58.96

58.11

59.82

58.75

58.88

SiO2

45.62

SiO2

59.80

TiO2

0.65

0.59

0.69 TiO

2

0.00

0.00

0.00

TiO2

0.81

0.74

0.77

0.79

0.70 TiO

2

0.03

0.00

0.00

0.01

0.01

TiO2

0.52

TiO2

0.01

Al 2O3

8.82

7.72

9.38 A

l 2O3

27.58

27.63

27.46

Al 2O3

11.35

10.88

10.76

10.20

10.49 Al 2O3

24.88

26.52

25.00

25.40

25.06

Al 2O3

10.37

Al 2O3

27.16

FeO

14.82

14.9714.91 FeO

0.07

0.07

0.20

FeO

15.06

15.95

14.67

14.73

15.16 FeO

0.35

0.33

0.28

0.13

0.07

FeO

14.50

FeO

0.03

MnO

0.15

0.16

0.13 M

nO

0.00

0.02

0.00

MnO

0.32

0.31

0.32

0.32

0.29 M

nO

0.01

0.02

0.00

0.00

0.00

MnO

0.29

MnO

0.00

MgO

12.35

12.6812.38 MgO

0.02

0.00

0.01

MgO

10.41

10.82

10.76

10.96

10.94 MgO

0.45

0.14

0.15

0.04

0.15

MgO

10.78

MgO

0.00

CaO

12.18

12.4912.28 CaO

9.51

9.58

9.10

CaO

11.62

10.42

11.65

11.92

11.48 CaO

2.04

1.15

3.41

5.78

7.03

CaO

12.01

CaO

5.56

Na 2O

0.98

0.73

1.12 N

a 2O

5.75

6.06

6.11

Na 2O

1.11

0.92

1.12

1.02

0.97 N

a 2O

6.50

6.27

6.78

6.92

6.82

Na 2O

0.80

Na 2O

6.29

K2O

0.32

0.28

0.32 K

2O

0.25

0.06

0.08

K2O

1.15

1.03

1.13

1.00

1.13 K

2O

4.07

4.49

3.90

1.36

0.44

K2O

1.15

K2O

0.04

total

97.47

97.9698.55 total

98.12

99.14

98.98

total

96.10

94.34

96.11

96.77

96.52 total

97.29

97.02

99.34

98.39

98.45

total

96.05

total

98.88

Si

6.89

7.01

6.84 Si

2.52

2.52

2.54

Si

6.68

6.60

6.76

6.84

6.80 Si

2.72

2.68

2.69

2.67

2.64

Si

6.85

Si

2.72

AlIV

1.04

0.92

1.09 A

l 1.49

1.48

1.47

AlIV

1.23

1.32

1.15

1.07

1.13 A

l 1.35

1.44

1.33

1.36

1.37

AlIV

1.09

Al

1.45

AlVI

0.48

0.40

0.51 Ca

0.47

0.46

0.44

AlVI

0.79

0.63

0.75

0.72

0.73 Ca

0.10

0.06

0.22

0.16

0.35

AlVI

0.74

Ca

0.27

Ti

0.07

0.06

0.07 N

a 0.51

0.53

0.54

Ti

0.09

0.08

0.09

0.09

0.08 N

a 0.58

0.56

0.51

0.59

0.61

Ti

0.06

Na

0.55

Fe(total)

1.81

1.82

1.80 K

0.01

0.00

0.00

Fe(total)

1.90

2.03

1.84

1.84

1.90 K

0.24

0.26

0.23

0.22

0.03

Fe(total)

1.82

K

0.00

Mn

0.02

0.02

0.02 XAb

0.52

0.54

0.55

Mn

0.04

0.04

0.04

0.04

0.04 XAb

0.63

0.64

0.60

0.63

0.62

Mn

0.04

XAn

0.33

Mg

2.69

2.74

2.67 XAn

0.47

0.46

0.45

Mg

2.34

2.46

2.41

2.44

2.44 XAn

0.11

0.06

0.17

0.29

0.35

Mg

2.41

XAb

0.67

Ca

1.91

1.94

1.90 XOr

0.01

0.00

0.00

Ca

1.88

1.70

1.88

1.90

1.84 XOr

0.26

0.30

0.23

0.08

0.03

Ca

1.93

XOr

0.00

Na

0.28

0.21

0.31

Na

0.33

0.27

0.33

0.29

0.28

Na

0.23

K

0.06

0.05

0.06

K

0.22

0.20

0.22

0.19

0.22

K

0.22

Fe3+

0.43

0.44

0.42

Fe

3+

0.00

0.48

0.00

0.00

0.00

Fe

3+

0.00

Fe2+

1.37

1.38

1.38

Fe

2+

1.90

1.55

1.84

1.84

1.90

Fe

2+

1.82

CHAPTER 7

‐ 104 ‐

7.4.1 Peak stage (M1)

In contrast with garnet from eclogites from the Yuka area, garnets from the Xitieshan

terrane do not preserve prograde zoning or inclusions. Such zoning, if originally present from

the prograde part of the P‐T path was probably obliterated during high temperature

eclogite‐facies recrystallization (Zhang et al., 2005a). Garnet and clinopyroxene pairs in the

cores were used to estimate temperatures for the eclogite facies peak metamorphic

conditions.

7.4.1.1 Garnet‐clinopyroxene pairs

The temperature dependence of iron‐magnesium partitioning between garnet and

clinopyroxene has long been used as a geothermometer (e.g., Raheim and Green, 1974). In

this study, we use the garnet‐clinopyroxene calibrations of Powell (1985) and Krogh (1988)

for comparison. Since a quantitative estimate of the peak pressure of the Xitieshan eclogites

is not possible due to the absence of feldspar in the peak metamorphic assemblages, the

temperatures were calculated for an assumed pressure of 30 kbar, based on the occurrence

of UHP index mineral coesite within zircon from amphibolite from the Xitieshan terrane (Liu

et al., 2012). The results of garnet‐clinopyroxene thermometry are listed in Table 7.3. For

eclogite sample 09NQ60, temperatures of 651 – 733 °C at 30 kbar are estimated from the

garnet‐clinopyroxene core pairs. These P‐T estimates are consistent with those by Zhang et al.

(2005a) and Zhang et al. (2011a).

Table 7.3 Garnet‐clinopyroxene equilibrium temperatures calculated using the Powell (1985) and

Krogh (1988) thermometers for given pressure.

Lithology Sample Grt‐Cpx XCaGrt XJd KD P(kbar) TP85 TK88

Eclogite 09NQ60 1‐8 0.21 0.18 8.83 30 681 651

Eclogite 09NQ60 3‐4 0.20 0.14 7.20 30 733 706

Eclogite 09NQ60 18‐21 0.22 0.19 8.33 30 706 681

Eclogite 09NQ60 26‐31 0.22 0.16 8.13 30 713 689

Eclogite 09NQ60 28‐30 0.22 0.18 7.74 30 727 704

Eclogite 09NQ60 36‐29 0.22 0.15 8.08 30 716 693

Eclogite 09NQ60 41‐46 0.22 0.17 9.23 30 680 653

TP85‐Powell (1985); TK88‐Krogh (1988)

7.4.2 High pressure granulite facies stage (M2)

To identify actual paths in P‐T of high pressure granulite facies overprint rocks, it is

important to evaluate the possible uncertainties caused by re‐equilibrium during heating or

cooling. For garnet‐clinopyroxene thermometer, there are two significant factors may hinder

quantitative P‐T determinations: (1) uncertain Fe3+ content in garnet and clinopyroxene,

given the high sensitivity of Fe3+ to temperature changes and (2) different‐scale chemical


‐ 105 ‐

zoning, possibly due to growth processes and/or diffusion, which prevent the determination

of the equilibrium compositional assemblage at peak conditions.

Table 7.4 Garnet‐clinopyroxene‐plagioclase‐quartz equilibrium temperatures and pressures.

Combined solution of Powell (1985), Krogh (1988) and Newton and Perkins (1982).

Lithology Sample (Grt‐Cpx‐Pl) XCaGrt XJd XAb XAn

TP85 (11.5 kbar)

TP85 (13.2 kbar)

TP88 (11.5 kbar)

TP88 (13.2 kbar)

Garnet amphibolite

09NQ45 46‐38‐42 0.28 0.05 0.55 0.44 805 810 799 804

Garnet amphibolite

09NQ45 50‐39‐46 0.23 0.07 0.55 0.45 774 779 756 762

TP85‐Powell (1985); TP88‐Krogh (1988)

In this study, estimates of Fe3+ in both garnet and clinopyroxene have been made using

charge‐balance constraints (Droop, 1987). The garnets in garnet amphibolite samples usually

have a relatively uniform composition from the core to the rim with a few showing a weak

outward increase in the Fe/(Fe+Mg) ratio (see Figure 7.6b). To minimize the effect of

re‐equilibration that might have affected the mineral compositions at the peak of

metamorphism, the P‐T conditions of high granulite facies were calculated using the rim

composition of garnet and adjacent clinopyroxene (Cpx‐II) in clinopyroxene‐plagioclase

symplectites. Local equilibrium between garnet rims and newly formed clinopyroxene and

plagioclase is hypothesized. The garnet–clinopyroxene thermometers (Powell, 1985; Krogh,

1988) combined with the garnet‐clinopyroxene‐plagioclase‐quartz barometer (Newton and

Perkins, 1982) are used for P‐T calculations. The garnet amphibolite sample 09NQ45 yield a

P‐T range of 11.5 – 13.2 kbar and 760 – 810 °C (Table 7.4), representing high pressure

granulite facies metamorphism conditions.

7.4.3 Upper amphibolite facies stage (M3)

The garnet amphibolite in Xitieshan terrane contains mainly garnet, amphibole,

plagioclase, symplectites of clinopyroxene + plagioclase and amphibole + plagioclase, which

is reflects upper amphibolite facies overprinting (Figure 7.3c and d). Compositions of

amphibole and adjacent plagioclase in the matrix of garnet amphibolite 09NQ44 were

selected for geothermobarometry of P‐T conditions because local equilibrium between

amphibole and plagioclase is assumed. In this study, the Amp‐Pl geothermometer (Holland

and Blundy, 1994) combined with the Amp‐Pl‐(Qz) geobarometer (Bhadra and

Bhattacharya, 2007) reveals P‐T conditions of 8.1 – 9.1 kbar and 630 – 670 °C (Table 7.5),

interpreted to represent upper amphibolite‐facies metamorphic conditions (see Figure

7.10a).

CHAPTER 7

‐ 106 ‐

Figure 7.10 Thermobarometry results for mineral equilibria in garnet amphibolite 09NQ44 (a) and

amphibolites 09NQ43 and 09NQ50 (b). Aluminosilicate stability fields are from (Holdaway, 1971). TH94:

Amp‐Pl thermometry (Holland and Blundy, 1994). PBH07: Amp‐Pl barometer (Bhadra and

Bhattacharya, 2007).

Table 7.5 Amphibole‐plagioclase equilibrium temperatures and pressures. Combined solution of

Holland and Blundy (1994) and Bhadra and Bhattacharya (2007).

Lithology Sample Amp‐Pl XAb XAn TH94(8.1 kbar) TH94(9.1 kbar)

Garnet amphibolite 09NQ44 29‐28 0.51 0.47 662 668



TH94‐Holland and Blundy (1994)

7.4.4 Amphibolite facies stage (M4)

This stage is characterized by the disappearance of garnet and symplectites

(Amp+Pl and Cpx+Pl) in amphibolite samples (09NQ43 and 09NQ50). These rocks consist

dominantly of amphibole and plagioclase. Assuming local equilibrium between

amphibole rims and adjacent plagioclase, the Amp‐Pl geothermometer (Holland and

Blundy, 1994) combined with the Amp‐Pl‐(Qz) geobarometer (Bhadra and Bhattacharya,

2007) reveals P‐T conditions of 5.0 – 7.2 kbar and 530 – 630 °C (see Table 7.6),

representing low‐to‐medium pressure amphibolite facies conditions (see Figure 7.10b).

AndSil

0.0

0.4

0.8

1.2

1.6

2.0

400 500 600 700 800 900

Temperature (°C)

Pres

sure

(GPa

)

TH94

PBH07

Ky

Jd + Q

z

Ab

530-630°C5.0-7.2 kbar

And

Sil

0.0

0.4

0.8

1.2

1.6

2.0

400 500 600 700 800 900

Temperature (°C)

Pre

ssur

e (G

Pa)

Ky

Jd + Q

z

Ab

PBH07

TH94

630-670°C8.1-9.1 kbar

(a) (b)


‐ 107 ‐

Table 7.6 Garnet‐amphibole equilibrium temperatures and pressure. Combined solution of Holland

and Blundy (1994) and Bhadra and Bhattacharya (2007).

Lithology Sample Amp‐Pl XAb XAn TH94(5.0 kbar) TH94(7.2 kbar)

Amphibolite 09NQ43 9‐16 0.63 0.11 529 548

Amphibolite 09NQ43 14‐2 0.64 0.06 554 569

Amphibolite 09NQ43 19‐25 0.62 0.35 611 631

Amphibolite 09NQ43 30‐29 0.60 0.17 549 568

Amphibolite 09NQ43 33‐4 0.63 0.29 567 586

Amphibolite 09NQ50 58‐53 0.65 0.32 535 556

TH94‐Holland and Blundy (1994)

7.5 Discussion

The metamorphic evolution P‐T path of the Xitieshan eclogites and their retrograde

equivalents is shown in Figure 7.11. Information on the pre‐peak stage metamorphism is not

available due to the absence of prograde garnet zoning or associated mineral inclusions in

eclogite samples. According to the petrography, mineralogy and P‐T calculations, four stages

of retrogression can be recognized. The peak UHP stage, however, is dominated by the

development of equilibrated assemblages of garnet and clinopyroxene. The peak

temperature conditions are estimated by the Grt‐Cpx thermobarometry at temperatures of

651 – 733 °C at the assumed pressure of 30 kbar (Figure 7.11, M1). The pressure estimate of

30 kbar of peak metamorphic stage is based on the reported occurrence of coesite within

zircon from mafic rocks in this terrane (Liu et al., 2012). Most eclogites from the Xitieshan

terrane were strongly overprinted by a subsequent granulite facies metamorphism with a

typical mineral assemblage of Grt + Cpx (Jd < 20) + Pl + Hbl. The P‐T conditions of the high

pressure granulite metamorphic stage are estimated by the Grt‐Cpx‐Pl‐Qz thermobarometry

at 11.5 – 13.2 kbar and 760 – 810 °C (Figure 7.11, M2). The temperature is higher than the

peak eclogite stage (651 – 733 °C), suggesting a process of temperature increase during the

decompression stage from UHP eclogite to high pressure granulite. These P‐T data during this

stage also suggest that eclogite in Yuka terrane may have resided at crustal‐mantle

transitional zone for a relative longer period of time, which lead to garnet zonation is

homogenized and prograde mineral assemblages are erased.

The breakdown of clinopyroxene to Cpx + Pl symplectites and then to Amp + Pl

symplectites indicates pressure decompression amphibolite facies metamorphisms. The P‐T

conditions of the upper amphibolite facies metamorphic stage are calculated by the

Amp‐Pl‐Qz thermobarometry at 8.1 – 9.1 kbar and 630 – 670 °C (Figure 7.11, M3) from

strongly retrogressed garnet amphibolite sample. The low amphibolite facies stage is

preserved in the amphibolite rocks. P‐T conditions of this stage are estimated at 5.0 – 7.2

kbar and 530 – 630 °C (Figure 7.11, M4).

CHAPTER 7

‐ 108 ‐

Figure 7.11 Estimated P‐T path for eclogites from the Xitieshan terrane, North Qaidam UHP

metamorphic belt. M1‐a peak stage constrained by eclogite; M2‐a high pressure granulite facies

retrogressed stage constrained by garnet amphibolite; M3‐an upper amphibolite facies retrogressed

stage constrained by garnet amphibolite; M4‐a low amphibolite facies retrogressed stage constrained

by amphibolite. Dia = Gr after Bundy et al. (1961); Coe = Qz after Bohlen and Boettcher (1982); Ab =

Jd + Qz after Holland (1980). Facies boundaries are from Liou et al. (1998).

7.6 Conclusions

The Xitieshan eclogite terrane consists of eclogite, garnet amphibolite, amphibolite,

schist, granitic gneiss and paragneiss. Eclogite occurs as lenses or blocks within both granitic

and pelitic gneisses. Detailed investigation of the metamorphic history of the Xitieshan

terrane has significantly improved constraints on its thermal evolution during exhumation.

The P‐T path shows that the Xitieshan eclogites record a process of high pressure granulite

facies overprint after UHP eclogite facies metamorphism.

0 200 400

1.0

2.0

3.0

4.0

600 800 1000T(°C)

P(G

Pa)

0.0

5°C/

km

HGR

GR

Ep-Ec

GS

BS

AM

Amp-Ec

Dry-Ec

Lw-Ec

EA

GraDia

QzCoe

Jd + Qz = Ab

M4

M1

M2

M3

7 petrography, mineral chemistry and p t conditions … 7.pdfchapter 7 ‐ 91 ‐ 7 petrography,...

Documents