07 - incipient metamorphism - cmp 1982.pdf

8/10/2019 07 - incipient metamorphism - CMP 1982.pdf

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Co ntrib M ineral Petrol (1982) 79:361-367

ontr ibut ions to

M i n e r a lo g y a n d

P e t r o l o g y

9 Springe r-Verla g 1982

Incipient Blueschist M etamorphism and M etasomatism

in the Tav~anll Re gion Northwest Turkey

A . I . O k a y *

De partm ent of Earth Sciences, Downing Place, Cambridge CB2 3E, England

Abs t r a c t V o l c a n o - s e d i m e n t a r y r o c k s i n a n i m b r i c a t e t e c -

t o n i c z o n e a r o u n d a p e r i d o t i te m a s s i f h a v e b e e n s t u d i e d

n o r t h e a s t o f t h e t o w n o f T a v ~ a n h in N o r t h w e s t T u r k e y .

B a s i c v o l c a n i c r o c k s , w h i c h a r e t h e d o m i n a n t r o c k t y p e

i n t h i s z o n e , s h o w i n c i p i e n t b l u e s c h i s t m e t a m o r p h i s m a n d

a s s o c i a t e d m e t a s o m a t i s m . W h i l e t h e ig n e o u s t e x t u r e s o f t h e

v o l c a n i c r o c k s a r e r e t a i n e d , a u g i t e s a r e p a r t i a l l y t o c o m -

p l e t e ly r e p l a c e d b y s o d i c p y r o x e n e , a n d p l a g i o c l a s e is a l b i t-

i s ed r e s u lt i n g i n r o c k s w i th 6 8 w t . % N a 2 0 . T h e v o l c a n i c

r o c k s a r e c r o s s - c u t b y n u m e r o u s v e i ns o f c a l ci te , a r a g o n i t e ,

q u a r t z , p u m p e l l y i t e , a l b i t e , l a w s o n i t e a n d s o d i c p y r o x e n e .

P e l a g i c l i m e s t o n e s , w h i c h a r e i n t e r b e d d e d w i t h t h e b a s i c

v o l c a n i c r o c k s , c o n s is t o f c o a r s e a r a g o n i t e g r a i n s s h o w i n g

p a r t i a l r e p l a c e m e n t b y c a l ci te . T h e o c c u r r e n c e o f a r a g o n i t e ,

l a w s o n i t e a n d a l b it e i n d i c a te s c o n d i t i o n s o f m e t a m o r p h i s m

f o r t h e w h o l e z o n e i n t h e r a n g e o f 5 8 k b a n d 1 5 0 -2 0 0 ~ C .

M e t a s o m a t i s m , p r o b a b l y r e l a t e d t o h i g h p r e s s u r e s e r p e n t i n -

i z a t i o n , h a s o c c u r r e d c o n t e m p o r a n e o u s l y w i t h t h e i n c i p i e n t

h i gh p r e s s u re m e t a m o r p h i s m .

P r e s e n t a d d r e s s :

Temel Ara~tirmalar, M ineral Research and E x-

p lorat ion Insti tu te (MTA), A nkara, Turkey

I n t r odu c t i o n

Th i s i s t h e t h i rd i n a seq u en ce o f p ap e r s d esc r i b in g t h e

b l u e s c h i s t f a c i e s r o c k s f r o m N o r t h w e s t T u r k e y . T h e t w o

p r e v i o u s p a p e r s ( O k a y 1 98 0 a , b ) h a v e d e s c r i b e d t h e g l a u c o -

p h a n e - l a w s o n i t e a n d l a w s o n i t e z o n e b l u e s c h i s t s i n t h e

T a v ~ a n h r e g i o n . T h i s p a p e r d e a l s w i t h t h e v o l c a n o - s e d i m e n -

t a r y r o c k s , w h i c h a r e t h e p r o t o l i t h s o f t h e b l u e s ch i s ts a n d

s h o w i n c i p i e n t h i g h p r e s s u r e , l o w t e m p e r a t u r e m e t a m o r -

p h i s m a n d a s s o c i a te d m e t a s o m a t i s m .

As sh o wn in F ig . 1 sev era l eas t -wes t o r i en t ed t ec to n i c

z o n e s c a n b e r e c o g n i s e d i n N o r t h w e s t T u r k e y . I n t h e s o u t h

is a d o m e s h a p e d H T / L P m e t a m o r p h i c c o m p l e x ( M e n d e r e s

Mas s i f ) wi th a g n e i s si c co re an d a co v er o f g reen sch i s t f ac i es

r o c k s . T o t h e n o r t h t h e M e n d e r e s M a s s i f i s i n te c t o n i c c o n -

t a c t w i t h a 2 0 k m w i d e p e r i d o t i t e z o n e c o n s i s t in g l a r g e l y

o f h a r z b u r g i te s a n d d u n i t e s w i t h v e r y m i n o r g a b b r o b o d i e s

(L i sen b ee 1 9 7 1 ; Ok ay 1 9 8 0 c) . Th e p e r id o t i t e s a r e cu t b y

a s w a r m o f g a b b r o i c d y k e s , n o w a l l b o u d i n a g e d a n d s a u s -

s u r it i se d , a n d a r e t h r u s t o v e r a t e c t o n i s e d c o m p l e x o f s p i -

l i t e s , ch er t s , i so l a t ed se rp en t i n i t e b o d i es , g r ey wack es an d

sh a l es . P a l a eo n to lo g i ca l (Oz k o g ak 1 9 6 9, p . 7 6 ; L i sen b e e

1 9 7 1 , p . 3 5 5 ) a n d r a d i o m e t r i c ( ~ o ~ u l u a n d K r u m m e n a c h e r

Fig. 1. Generalised geotectonic

map of Northwest Turkey ,

compiled from the geological

maps of Turkey (Maden

Tetkik ve Arama Enst. 1963,

1964) and the author's own

observations

0010-7999/82/0079/0361/$01.40


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36

ZA - W S O N / T E

Z~VE_BLUES_CHISTS ._

~ v

9 - ' ~ / (

9 9 o

9 ~ ~

9 O g e o o . O ~ G R E E N S C H I S T S - - / 9

9 ~ ~

_ _ - ~ - . . . 9

G~m~yenik~y ~ 9 Iv v

v v.~i/VWv ~

Fig. 2. Simplified geological ma p of the Giimii~yenik6y

area, Tav~anll , Northwest Turkey. Only the m ajor shear

zones and large serpentinite outcrops are shown. Blank

areas consist of closely intercalated pelagic sediments, basic

volcanic rocks an d serpentinite, al l o f which are incipiently

metam orphosed and metasomat ised. Circles indicate

specimen loca l i t ie s ; f i l led c irc les augite partially or

completely replaced by sodic pyroxene; open circles no

such replacement. N u m b e r s of the analysed specimens are

also indicated

1 9 6 7 ) d a t a i n d i c a t e a n U p p e r C r e t a c e o u s d e p o s i t i o n a l a g e

f o r s o m e o f th e s e r o c k s . B l u e s ch i s t m e t a m o r p h i s m i s n o t

e v i d e n t i n th i s v o l c a n o - s e d i m e n t a r y se q u e n c e a n d H P / L T

m i n e r a l s , s u c h a s la w s o n i t e a n d a r a g o n i t e , o c c u r in t h e

a m y g d a l e s a n d v e i n s i n t h e v o l c a n i c r o c k s . T h e i n c i p i e n t

n a t u r e o f th i s b lu e s c h is t m e t a m o r p h i s m i n t h e T a v ~ a n h r e -

g i o n i s t h e s u b j e c t o f t h i s p a p e r .

L a w s o n i t e a n d g l a u c o p h a n e - l a w s o n i t e z o n e b l u e s c h i s t s

a r e p r o d u c e d b y t h e p r o g r e ss i v e m e t a m o r p h i s m o f th e s e

v o l c a n o - s e d i m e n t a r y r o c k s . T h e y f o r m a n e x t e n s i v e m e t a -

m o r p h i c b e l t o f v a r y i n g t h i c k n e s s a n d l ie o n a v e r y t h i c k

m a r b l e s e r i e s . T h e i n t e r f o l d e d b l u e s c h i s t - m a r b l e c o n t a c t s ,

t h e p a r a l l e l l i n e a t io n d i r e c t io n s i n m a r b l e s a n d b l u e s c h is t s,

a n d s m a l l i n li e rs o f b lu e s c h i s ts in t h e m a r b l e O k a y 1 9 8 0 b ,

1 98 1 ) a l l s h o w t h a t t h e m a r b l e s h a v e u n d e r g o n e t h e s a m e

H P / L T m e t a m o r p h i s m a s t h e b l u e s c h i s t s . M a s s i v e , w h i t e

m a r b l e i s i n s t e e p t e c t o n i c c o n t a c t , m a r k e d b y e x t e n s i v e

b r e c c i a t i o n , l i n e a r s e r p e n t i n i t e s a n d T e r t i a r y d a c i t e i n t r u -

s io n s, w i t h a h ig h t e m p e r a t u r e / m o d e r a t e p r e s s u r e m e t a m o r -

p h i c b e l t . T h e m a j o r r o c k t y p e s i n t h i s b e l t a r e b a s i c v o l -

c a n i c r o c k s , p e l i t e s a n d l i m e s t o n e s . P r o g r e s s i v e m e t a m o r -

p h i c z o n e s f r o m z e o l i t e t o a m p h i b o l i t e f a c i e s h a v e b e e n

m a p p e d i n th i s b e l t n e a r S 6 g f i t Y i l m a z 1 9 79 ). A s t h e m e t a -

m o r p h i c a g e o f t h e H T / M P m e t a m o r p h i c b e l t i s n o t w e ll

e s t a b l i s h e d , i t i s n o t y e t c l e a r w h e t h e r i t f o r m s a p a i r e d

m e t a m o r p h i c b e l t t o g e t h e r w i th t h e U p p e r C r e t a c e o u s

b l ue s ch i st s. T h e H T / M P m e t a m o r p h i c b e l t is i n t h r u s t c o n -

t a c t t o t h e n o r t h w i t h t h e g r a n i ti c b a s e m e n t o f t h e M i d d l e

S a k a r y a Z o n e . T h e M i d d l e S a k a r y a Z o n e c o n s i s ts o f a n

a u t o c h t h o n o u s e p i c o n ti n e n t a l s e d i m e n t a r y s eq u e n c e r a n g -

i n g i n a g e f r o m P e r m i a n t o E o c e n e A l t in l i 1 9 7 5 ; S a n e r

1980).

T h e r e a r e T e r t i a r y g r a n o d i o r i t e i n t ru s i o n s s u p e r i m p o s e d

o n t h is e a s t - w e s t o r i e n t e d t e c t o n i c p a t t e r n A t a m a n 1 9 7 4)

a n d a n e x t e n s iv e c o v e r o f N e o g e n e l a c u s t ri n e s e d i m e n t s a n d

vo l can i cs .

e o l o g y a n d R o c k T y p e s

V o l c a n o - s e d i m e n t a r y s e q u e n c e s w i t h s m a l l s e r p e n t i n i t e

b o d i e s c h a r a c t e r i s e d b y a c h a o t i c in t e r n a l s t r u c t u r e a r e c o m -

m o n a l o n g t h e m a r g i n s o f th e p e r i d o ti t e m a s s if s in N o r t h -

w e s t T u r k e y . A l t h o u g h s u c h s e q u e n c e s a r e c a l l e d o p h i o l i t e

o r o p h i o l it i c m e l a n g e b y s o m e w o r k e r s L i s e n b e e 1 97 1 ;

K a y a 1 9 72 ), t h e r e p e a t e d i n t e r c a l a t i o n o f s e d i m e n t a r y a n d

v o l c a n ic r o c k s , l a c k o f s h e e te d d y k e c o m p l e x e s a n d g a b b r o s

d o e s n o t m e r i t t h e c u r r e n t u s a g e o f t h e t e r m o p h i o l it e f o r

t h e se r o c k s . T w o m a j o r a r e a s o f s u c h r o c k s h a v e b e e n r e -

c o g n i s e d i n th e T a v ~ a n h a r e a : t h e o n e n o r t h o f th e v i l la g e

o f K e t e n l i k o c c u r s a s a t e c to n i c k l i p p e a m o n g t h e l a w s o n i t e

z o n e b l u e s c h is t s O k a y 1 9 8 0 b , Y e n i k 6 y G r o u p o f F i g . 5 ).

A l a r g e r a r e a l ie s t o t h e s o u t h a r o u n d t h e v i ll a g e o f G f i m f i -

~ y e n i k 6 y . T h i s p a p e r d e a l s m a i n l y w i t h t h e v o l c a n o - s e d i -

m e n t a r y r o c k s i n t h e G f i m i i~ y e n ik 6 y a re a , a l t h o u g h b o t h

a r e a s a r e l i th o l o g i c a l ly a n d s t r u c t u r a l l y v e r y s i m i l a r.

T h e v o l c a n o - s e d i m e n t a r y r o c k s o f t h e G f i m i i~ y e n ik 6 y

a r e a o c c u r d i r e c t l y b e l o w a l a r g e p e r i d o t i te m a s s i f i n a n

i m b r i c a t e t e c t o n i c z o n e o f 2 - 3 k m w i d t h F i g . 2) . T h e d o m i -

n a n t l i t h o lo g i e s in t h i s z o n e a r e r o c k s o f t h e S t e i n m a n n

t r i n i t y : b a s i c v o l c a n i c r o c k s , r a d i o l a r i a n c h e r t s a n d s e r p e n -

t in i t es i n c o n j u n c t i o n w i t h r e d a n d g r e e n s h a l es , g r e y w a c k e s ,

m a n g a n e s e d e p o s i t s , p i n k t h i n l y - b e d d e d l i m e s t o n e s , k e r a t o -

p h y r e s a n d e x o t i c b lo c k s o f w h i t e li m e s t o n e . T h i s g r o u p

o f r o c k s i s c h a r a c t e r i s e d b y a r i c h l i t h o l o g ic a l v a r i e t y ; s e v e r -

a l d i f f e r e n t r o c k t y p e s c a n o f t e n b e s e e n i n t e r b e d d e d o v e r

a d i s t a n c e o f t e n m e t e r s , a n y s i n g l e li t h o l o g y i s r a re l y m o r e

t ha n t en me t e r s t h i ck c f. Oka y 1981 , F i g . 4 ). T he whol e

z o n e i s s t r o n g l y t e c t o n i s e d w i t h l a r g e n u m b e r s o f s h e a r

z o n e s m a r k e d b y s e r p e n t i n i t e a n d t a l c o u t c r o p s ; s l i v e r s o f

g r e e n s c h i s t s o c c u r a s f a u l t - b o u n d e d b l o c k s . O n l y t h e m a j o r

s h e a r z o n e s a n d t h e l a r g e r s e r p e n t i n i t e b o d i e s a r e s h o w n

i n F ig . 2 . T h e s t r o n g t e c t o n i s m a n d l i t h o l o g i c a l h e t e r o g e n e -

i ty m a k e s t h e m a p p i n g o f i n d iv i d u a l r o c k t y p e s i m p o s s i b le

on a 1 : 250 00 sca l e c f . K ay a 1972 , F i g . 10).

I n t h e f i e ld t h e r o c k s o f t h e v o l c a n o - s e d i m e n t a r y s e -

q u e n ce a p p e a r u n m e t a m o r p o s e d a n d d o n o t s h o w p e n e tr a -

t iv e d e f o r m a t i o n . V o l c a n ic r o c k s a r e th e m o s t c o m m o n r o c k

t y p e . T h e y f o r m m a s s i v e g r e e n f l o w s i n t e r b e d d e d w i t h r a -

d i o l a r i a n c h e r t s a n d r e d g r e e n s h a l e s , a n d i n c l u d e d o l e r i t e s ,

s p i li te s , p y r o c l a s t i c s a n d m i n o r a m o u n t s o f k e r a t o p h y r e s ,

t r a c h y b a s a l t s a n d m o r e a c i d i c d i f f e r e n t i a t e s . P i l l o w l a v a s

a r e v e r y r a r e . C l i n o p y r o x e n e c o m p o s i t i o n s f r o m t h e v o l -

c a n i c r o c k s i n d i c a te t h a t m o s t o f t h e m w e r e s u b a l k a l in e ,

a l t h o u g h s o m e k a e r s u t i t e - b e a r i n g s p i l i t e s a n d p y r o c l a s t i c s

h a v e m a r k e d a l k a l i n e a f f in i t ie s O k a y J9 8 0 c ) .

S e r p e n t i n i t e o c c u r s a s b o d i e s o f v a r i o u s s i z e s r a n g i n g


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Fig. 3a--d. Pho tom icrog raph s from the incipiently metam orp hose d zone: a Augite partially replaced by sodic pyroxene px, dark areas)

with leucoxene and albite ab) in a dolerite K48). b Completely metasomatised dolerite K92 0) with sodic pyroxene pseud om orph s

px) after augite, albite

ab)

and interstitial chlorite

chl). c

Lawsonite laths

lw),

fibrous pumpellyite pp) and chlorite

chl)

in an amygdale

of a fine-grained spilite K864). The grou ndm ass shows no signs of any m etam orpism , d Aragonite m arb le; a large aragonite grain

in extinction position) is expitaxically and partially replaced b y elongate lamellae of calcite cc). Aragonite shows the characteristic

110) multiple twinning parallel to the replacing calcite lamellae

f rom a few mete rs to a few k i lomete rs concent ra t ed a long

the shear zones . In the few samples examined, ant igori te

is the main serp ent ini te mineral . Veins of andradi te garnet

occur frequent ly in these serpent ini tes .

Greenschis ts associa ted with serpent ini tes occur in two

wedge shaped a reas . Wel l -marked thrus t p l anes s epa ra t e

them f rom the vo lcano-sed imenta ry rocks . The dominant

rock type is a s t rongly fol ia ted metabas i te wi th a mineral

assemb lage of act inol i te + epidote + chlori te + a lbi te +

quartz . Some greenschis t metabas i tes contain re l ic t lawson-

i te or pumpel lyi te .

Petrography and ineralogy

Fifty-f ive specimens o f bas ic volcanic rocks , including p y-

roclas t ic f lows, doler i tes and spi l i tes , f rom the vic ini ty of

the vi l lage of Gfimfi~yenik6y were inves t igated petr ogra phi-

cal ly Fig. 2). Al l re ta in thei r igneous textures and their

essent ia l mineralogy consis ts of augi te a l ter ing to sodic py-

roxene, a lbi te Ano), chlori te and leucoxene. In ma ny speci-

mens pumpel lyi te occurs as i r regular s t r ingers in large a lbi t -

ised plagioclase grains . Epidote and act inol i te character is t ic

of man y spi l i tes , are absent .

ethod

Minera l compos i t ions were de te rmined for twe lve e lement s

Na, Ca, K, Fe, Mg, Mn, V, Ni , Cr, Ti , A1, Si) us ing an

energy dispers ive e lect ro npro be with a Harw el l Si Li) de-

t ec tor and pul se proces sor . The cor rec t ion procedures a re

g iven by Swea tmann and Lon g 1969) . The me th od for

es t imat ing ferr ic i ron in sodic pyr oxen e is given in an earl ier

pape r Okay 1978). Who le rock ana lys i s were mad e on an

X-ray spectrometer us ing fused discs wi th a l i thium te t ra-

bora t e f lux . Na and K were de te rmined by f l ame photo -

me t ry and FeO by t i t r a t ion .

Topotactic Replacement of Augite by Sodic Pyroxene

The rep lacement o f augi te by green i sh-ye llow sodic pyrox-

ene in the volcanic rocks is variable in extent , ranging from

thin r ims and ve ins F ig. 3a ) to comple te pseudom orphs

Fig. 3 b) . On a region al scale 44 ou t of 55 spil i tes contain

such replacem ent textures Fig. 2). The replacem ent of au-

g i t e by sodic pyroxene has occur red topotac t i ca l ly , on an

ion by ion bas is, leaving the igneous texture intact Carp en-

ter and Okay 1978). This replacement process seems to be

control led by the avai lable mineral s t ructure; in an a lkal i

basa l t kae rsu t i t e has r ims of b lue sodic amphibole , whereas

t i tanaugi te in the same rock has been replaced by sodic


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Table l. Representative mineral analyses

Sodic Pyroxene Pumpellyite Albite Lawsonite Chlorite Garnet

K48 K860 K919/2 K160 K733/1 K733/1 K30t K960 K36 K48 K485/2 K3 01

vein vein

core rim

SiO 2 52.75 53 .46 57. 00 52.54 37.04 37.93 68.17 38.30 37,17 30.31 30. 81 30.67 38.00 35.76

TiOz 0.00 0.28 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0. 00 0.00 0.00 0.00 0.00

A120 3 0.61 2.39 8.01 2.11 21.97 24.17 19.41 27.92 29.72 14.39 13 .5 3 14.37 9.91 1.54

FeO* 26.18 24 .2 4 18 .6 1 22.19 8.59 6.01 0.00 5.25 2.23 24.57 24 .2 9 23.59 16.12 27.29

MgO 1.84 1.14 0.77 2.91 2.26 2,40 0.00 0.00 0.00 16.93 14 .5 6 16.06 0.00 0.00

MnO 0.36 0.24 0.00 0.22 0.17 0.25 0.00 0.00 0.00 0.23 0.25 0.32 0.44 0.23

CaO 6.90 3.53 2.26 7.36 22.31 23.07 0.11 17.32 16.99 0.24 0.80 0.83 34.95 33.69

NazO 10 .13 12 .7 3 13.81 9.92 0.00 0.00 12.23 0.00 0.00 0. 00 0.00 0.00 0.00 0.00

K20 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0. 00 0.24 0.00 0.00 0.00

Total 98 .7 7 98.0 1 100.46 97.25 92.34 93.83 99.92 88.79 86.11 86.67 84. 48 85.84 99.42 98.51

Number of cations per

4 cations 16 cations 8 0 5 cations 28 oxygens 12 cations

Si 2.00 1.99 2.03 2.00 6.07 6.07 2.99 2.03 2.01 6.41 6.70 6.51 3.01 2.97

A14 0 .0 0 0 .0 1 0 .0 0 0 .0 0 0 .0 0 0 .0 0 0 .0 1 0 .0 0 0 .0 0 1 .5 9 1 .3 0 1 .4 9 0 .0 0 0 .0 3

A16 0.03 0.10 0.34 0.10 4.25 4.56 0.99 1.75 1.90 1.99 2.17 2.11 0.92 0.08

Ti 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0,00

Fe 3+ 0.71 0.76 0.55 0.63 0.75 0.44 0.00 0.23 0.10 4.34 4.42 4.19

1 .0 8 1 .8 9

Fe z+ 0.12 0.00 0.00 0.07 0.43 0.37 0.00 0.00 0.00 0.00 0.00

Mg 0.10 0.06 0.04 0.16 0.56 0.57 0.00 0.00 0.00 5. 33 4.72 5.08 0.00 0.00

Mn 0 .0 1 0 .0 1 0 .0 0 0 .0 1 0 .0 2 0 .0 3 0 .0 0 0 .0 0 0 .0 0 0.04 0.05 0.06 0.03 0.03

0 .9 7 0 .9 4 0 .9 3 0 .9 7 6 .0 1 5 .9 7 1 .9 8 2 .0 0

Ca 0.28 0.14 0.08 0.30 3.92 3.96 0.005 0.99 0.99 0.06 0.19 0.19 2.96 3.00

Na 0.74 0.92 0.99 0.73 0.00 0.00 1.04 0.00 0.00 0.00 0.00 0.00 0.00 0.00

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

1 .0 2 1 .0 6 1 .0 7 1 .0 3 3 .9 2 3 .9 6 5 .0 4 0 .9 9 0 .9 9 1 9 .7 6 1 9 .6 2 1 9 .6 3 2 .9 9 3 .0 3

Jd 3 10 34 t0 * Tot ali ron Andr. 54 96

Ac 71 76 55 63 as FeO Gross . 46 4

Au 26 14 11 27

pyroxene. Sodic pyroxene also occurs as acicular crystalls

in monom ineralic veins.

Sodic pyroxene compositions from eight rocks are plot-

ted in the acmite-jade ite-augi te ternary diagr am in Fig. 4

and four sodic pyrox ene analyses are sho wn in Table 1.

Sodic pyroxenes from the incipiently metamorphosed zone

have high acmite compon ents (generally over 50 ) and

are compositionally similar to the sodic pyroxenes from

the lawsonite zone metabasites. Conversely sodic pyroxenes

from the glaucophane-lawsonite zone metabasites show a

wider range of compositi ons (Fig. 4). A lack of omphacit ic

compositions is characteristic of all three metamorphic

zones in the Tav~anh region.

The main chemical changes involved in the constant

volume replacement of augite by sodic pyroxene are an

increase in sodium and ferric iron, and a concomitant de-

crease in calcium and magnesium. For a specific case (sam-

ple K48):

100 g augi te + 9.44 g Na2 0 + 13.52 g Fe20 3 + 2.65 g SiO2

+0.08 g A1203+0.38 g 02

= 103.85 g sodic pyroxene (JdsAcs9Au33)+ 9.90 g CaO

+11.88 g MgO+ 0. 59 g TiO z.

However, as the composition of the replacing sodic pyrox-

ene changes (Fig. 4), the chemical balance in the isovolume

replacement is variable.

c

Au Jd

Fig. 4. Sodic pyroxene compositions from the incipiently metamor-

phosed basic volcanic rocks plotted on the acmite-jadeite-augite

diagram. Subscript v indicates compositions of sodic pyroxenes

from the veins. Compositional fields of sodic pyroxenes from the

lawsonite zone

co n t i n u o u s l i n e s )

and glaucophane-lawsonite zone

d a sh ed l i n e s ) metabasites are also indicated


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365

Table 2. Analyses o f three metasomatised basic volcanic rocks. Ox-

ides are normalised to 100 wt.

K960 K775 K920

SiOz 50.02 50.66 53.82

TiO2 0.97 2.10 1.57

A120 3 16.10 16.22 14.79

F O3 5.63 5.52 5.03

FeO 3.74 3.81 6.05

MgO 5.92 4.89 4.69

MnO 0.15 0.19 0.21

CaO 7.05 5.04 2.90

Na20 6.09 6.19 7.79

KzO 0.20 0.14 0.20

P205 0.11 0.75 0.10

HzO 4.02 4.49 2.85

3 t ~

X t

0.2

0 1 . 0 0 J , 1 0 1 2 0 M . 3 0 , 4 0 1 . 5 0 1 6

Fe/Fe Mg

Fig. 5. Chlorite compositions from the incipiently metamorphosed

basic volcanic rocks plo tted in terms of their F e/Fe + Mg and A1/

AI + Fe + M g ratios. C ompositional fields of chlorites from the law-

sonite zone metabasites are also indicated

T h r e e a n a l y s e s o f s t r o n g l y m e t a s o m a t i s e d b a s i c v o l c a n i c

r o c k s a r e s h o w n i n T a b l e 2 . C o m p a r e d w i t h b a s a l ts o r s p i-

l it e s, t h e a n a ly s e s s h o w i n c r e as e s in N a 2 0 ( f r o m 1 4 .5 w t .

i n b a s a l ts a n d s p il it e s t o 6 - 8 w t . ) , S i O 2 a n d F e 2 0 3 , b u t

d o n o t i n d i c a t e a m a r k e d c h a n g e i n t h e t o t a l i r o n c o n t e n t .

M a g n e t i t e i s a b s e n t i n t h e s e r o c k s , a n d i r o n s e em s t o h a v e

b e e n r e d i s t r ib u t e d t o s o d i c p y r o x e n e . T h e r e a r e c o r r e s p o n d -

i n g d e c r e a s e s i n C a O a n d M g O . A 1 2 0 3 a n d T i O 2 h a v e a p -

p a r e n t l y s t a y e d f a i r l y c o n s t a n t . T i O 2 a n d s o m e o f t h e r e -

l e a se d c a l c i u m i s r e t a i n e d i n l e u c o x e n e , w h i c h f o r m s s m a l l

i n c l u si o n s i n s o d ic p y r o x e n e p s e u d o m o r p h s .

Chlorite i s t h e m a j o r s e c o n d a r y m i n e r a l f o u n d i n t h e

in te rs t i ces o f p lag ioc la se a nd py rox ene c ry s ta l s (F ig . 3 b ) ,

and in a m yg da les (F ig . 3 c ). I t fo rm s f ine -gra ined aggreg a tes

o r b l a d e d c r y s ta l s . S o m e o f t h e c h l o r i t e s s h o w r e l a t i v e ly

h i g h i n t e r f e r e n c e c o l o u r s a p p a r e n t l y r e l a t e d t o t h e i r r e l a -

t i v e ly h i g h p o t a s s i u m a n d c a l c i u m c o n t e n t s ( T a b l e 1 ) . C o m -

p o s i t i o n a l l y , c h l o r i t e s f r o m t h e i n c i p i e n t l y m e t a m o r p h o s e d

b a s i c v o l c a n i c r o c k s a r e s l i g h t l y l e s s a l u m i n o u s a n d m o r e

s i l i c e o u s t h a n l a w s o n i t e a n d g l a u c o p h a n e - l a w s o n i t e z o n e

ch lor i t e s (F ig . 5 , Tab le 1 ) . P hengi te i s absen t in these roc ks

a n d o n l y a p p e a r s i n t h e l a w s o n i t e z o n e m e t a b a s i t e s a s

s lende r c rys ta l s em bedded in ch lor i t e .

Al l p lag ioc la se in the vo lcan ic rocks i s com ple te ly a lb i t-

i s ed . Alb i t i s ed p lag ioc la se i s c loudy and qu i te d i s t inc t ive

f rom the f re sh , inva r iab ly twinned a lb i t e in the ve ins . I t

h a s le ss t h a n 1 a n o r t h i t e c o m p o n e n t a n d m i n o r a m o u n t s

o f F e O ( 0 . 1 t o 0 .5 w t . ) .

Mineralogy of the Veins and mygdales

in the Basic Volcanic Roeks

T h e b a s i c v o l c a n i c r o c k s a r e c r o s s - c u t b y n u m e r o u s v e i n s

o f c a l c i u m c a r b o n a t e ( i n 26 sa m p l e s o u t o f 5 5 ), q u a r t z ( i n

18), pumpellyi te ( in 18), a lbi te ( in 13), lawsonite ( in 6) ,

sod ic pyroxene ( in 4 ) and ga rne t ( in 2 ) . Com pos i t e ve ins

c o n t a i n i n g t w o t o f o u r m i n e r a l s a r e f a i r l y c o m m o n w i t h

q u a r t z - c a r b o n a t e a n d c a r b o n a t e - a l b i t e b e i n g t h e m o s t f r e -

q u e n t c o m b i n a t i o n s . M o r e c o m p l e x v e i n s o f p u m p e l l y i te -

l a w s o n i t e - q u a r tz - c a l c i te ( K 2 6 5 ) , p u m p e l l y i t e - g a r n e t ( K 3 0 1 )

a n d a l b i te - l a w s o n i t e a re a l s o p r e s e n t . C a r b o n a t e m i n e r a l s,

p u m p e l l y i t e , c h l o r i t e a n d l a w s o n i t e a l s o o c c u r f r e q u e n t l y

in a m yg da les (F ig . 3 c ) .

ragonite w a s i d e n t i f i e d a s t h e p r i m a r y C a C O 3 p o l y -

m o r p h i n 1 2 s p e ci m e n s f r o m i t s m u l t i p l e ( 11 0 ) t w i n l a m e l la e

( F ig . 3 d ) ; i t f o r m s l a rg e ( l - 1 0 m m ) c r y s t a ls i n t h e v e in s

and am ygda les , and i s be ing rep laced by sm a l l , i r regu la r

s t ri n g e r s o f c al c it e . In s o m e r o c k s e x t r a o r d i n a r y a n a s t o m o s -

i n g v e i n s c o m p o s e d o f s i n g l e c r y s t a l s o f a r a g o n i t e u p t o

2 6 m m l o n g a n d o n l y 0 .1 m m w i d e a r e p r e s e n t.

Pumpellyite i s p r e s e n t in a l m o s t e v e r y s a m p l e e i t h e r i n

am y gda les (F ig . 3 c ) , in ve ins o r a s pa le ye l low m ic ro gran u-

l a r p a t c h e s o v e r g r o w i n g t h e g l a s s y m a t r i x o f t h e v o l c a n i c

r o c k s ; in s o m e g l as s y v o l c a n i c r o c k s i t m a k e s u p 7 0 o f

t h e m o d e . I n v e i n s a n d a m y g d a l e s i t f o r m s a g g r e g a t e s o f

r a d i a t i n g a c i c u l a r c ry s t a l s s h o w i n g a n o m a l o u s i n t e r f e r e n c e

c o l o u r s . P u m p e l l y i t e s h a v e v i r t u a l l y c o n s t a n t M g c o n t e n t s

and show res t r i c ted A1 for F e 3+ su bs t i tu t ion (Tab le 1 ) .

C o m p o s i t i o n a l l y t h e y a r e s i m i l a r t o a n a l y s e d p u m p e l l y i t e s

f r o m o t h e r l o w - g r a d e m e t a m o r p h i c t e r r a i n s ( c f . C o o m b s

et al. 1976).

lbite

in the ve ins occurs a s f re sh id iob las t i c c rys ta l s

inva r iab le showing s im ple o r m ul t ip le twinn ing . I t has l e s s

t h a n 1 a n o r t h i t e c o m p o n e n t ( T a b l e 1 ).

Lawsonite

a p a r t f r o m i t s u s u a l t a b u l a r s h a p e , d i s p l a y s

unusua l ac icu la r and bow- t i e hab i t s in the ve ins . I t i s found

i n t h e a m y g d a l e s o f a v o l c a n i c r o c k , t h a t o t h e r w i s e a p p e a r s

u n m e t a m o r p o s e d , i n a s s o c i a t i o n w i t h c h l o r i t e a n d p u m p e l -

ly i t e (F ig . 3 c ) . An a lysed l awsoni te s f r om the inc ip ien t ly m e-

t a m o r p h o s e d z o n e h a v e c o n s i s t e n t l y h i g h e r F e 2 0 3 c o n t e n t s

( 1 . 5 -5 . 5 w t . ) t h a n l a w s o n i te s f r o m t h e g l a u c o p h a n e - l a w -

son i te and l awsoni te zones (Tab le 1 ) .

Garnets

o f t h e a n d r a d i t e - g r o s s u l a r s e r i e s o c c u r i n t w o

s t r o n g l y m e t a s o m a t i s e d v o l c a n i c r o c k s i n a s s o c i a t i o n w i t h

p u m p e l l y i t e a n d s o d i c p y r o x e n e . T h e y s h o w d i s c o n t i n u o u s

z o n i n g p r o f i l e s w i th i n t e r m e d i a t e a n d r a d i t e - g r o s s u l a r c o r e s

a n d p i n k a n d r a d i t e r i m s ( T a b l e 1 ) . G a r n e t s o f s i m i l a r c o m -

p o s i t i o n a r e a l s o f o u n d i n t a l c s a n d s e r p e n t in i t e s o f t h e

i n c i p i en t l y m e t a m o r p h o s e d z o n e a n d o f th e l a w s o n i t e z o n e

( O k a y 1 9 8 0 b ) .

ragonitised Limestone

I f t h e a m b i e n t p r e s s u r e s i n t h e v o l c a n o - s e d i m e n t a r y r o c k s

w e r e h i g h e n o u g h t o s t a b i l i z e a r a g o n i t e i n t h e v e i n s a n d

a m y g d a l e s o f b a s ic v o l c a n i c r o c k s , t h e n o n e w o u l d s i m i la r l y

e x p e c t c a l c i t e i n l i m e s t o n e s , w h i c h a r e o f t e n i n t e r b e d d e d

w i t h b a s i c v o l c a n i c r o c k s , t o h a v e t r a n s f o r m e d t o a r a g o n i t e .

I n d e e d , a l l t h e p e t r o g r a p h i c a l l y e x a m i n e d l i m e s t o n e s c o n s i st

o f c o a r s e c r y s t al s o f a r a g o n i t e s h o w i n g v a r i o u s d e g r e e s o f

r e p l a c e m e n t b y c a lc i te .

T w o t y p e s o f a r a g o n i t i s e d l i m e s t o n e h a v e b e e n r e c o g n -

i s ed p e t r o g r a p h i c a l l y . I n t h e f i r s t t y p e t h e s e d i m e n t a r y f a b -

r i c o f t h e l i m e s t o n e w i t h t h e r a d i o l a r i a t e st s, f o s si l fr a g m e n t s

a n d c l a y f r a c t i o n is p e r f e c t ly p r e se r v e d . H o w e v e r , t h e w h o l e

b i o m i c r i t i c g r o u n d m a s s n o w c o n s is t s o f s e v e r al c e n t i m e t e r s

l a r g e g r a in s o f a r a g o n i t e . I n t h e s e c o n d t y p e t h e l i m e s t o n e

i s rec rys ta l l i s ed and cons i s t s o f ve ry l a rge c lea r a ragoni te


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366

g r a i n s p a r t i a l l y a n d e p i t a x i a l l y r e p l a c e d b y s m a l l e r g r a i n s

o f ca l c i t e (F ig . 3 d ) .

i scuss ion

T h e p r e s e n c e o f a r a g o n i t e a s w e l l r e c r y st a l li s e d l a r g e g r a i n s

i n t h e v o l c a n i c r o c k s a n d l i m e s t o n e s o f t h e T a v ~ a n h r e g i o n

a n d i ts a s s o c i a t i o n w i t h l a w s o n i t e a n d s o d i c p y r o x e n e i n d i-

c a t e t h a t t h e v o l c a n o - s e d i m e n t a r y s e q u e n c e h a s u n d e r g o n e

a n i n c ip i e nt H P / L T m e t a m o r p h i s m . T h e a b s e n c e o f j a d e it e

a n d z e o l it e s a n d t h e p r e s e n c e o f a r a g o n i t e , a l b i t e a n d l a w -

s o n i te i n s o m e o f t h e b a s i c v o l c a n i c r o c k s i n d i c a t e s p r e s -

s u r e s o f 5 t o 8 k b a r s a n d t e m p e r a t u r e s 1 5 0 - 20 0 ~ C d u r i n g

t h e i n c i p i e n t m e t a m o r p h i s m ( N e w t o n a n d S m i t h 1 9 6 7 ;

H l a b s e a n d K l e p p a 1 9 6 8 ; J o h a n n e s a n d P u h a n 1 97 1) .

I n t h e i n c i p i e n t l y m e t a m o r p h o s e d v o l c a n i c r o c k s m e t a -

s o m a t i s m i s e v id e n c e d b y t h e s o d ic p y r o x e n e p s e u d o m o r p h s

a f t e r a u g i t e c o e x i s t in g w i t h a l b i t e a n d c h l o ri t e . T h e p r e s e n c e

o f s i m i la r s o d ic p y r o x e n e p s e u d o m o r p h s a f t e r a u g i te i n la w -

s o n i te z o n e m e t a b a s i t e s , w h i c h o c c u r n o r t h o f t h e m a i n

eas t -w es t t r en d in g t h ru s t (F ig . 2 ), p o in t o u t t o t h e c lo se

r e l a t i o n s h i p b e t w e e n H P / L T m e t a m o r p h i s m a n d m e t a s o -

m a t i s m . H o w e v e r , i n t h e l a w s o n i t e z o n e m e t a b a s i t e s , w h e r e

l a w s o n i t e is a b u n d a n t a n d a l b i te a b s e n t , t h e r e i s n o e v i d e n c e

o f s od i u m m e t a s o m a t i s m .

Th e p resen ce o f g reen sch i s t t ec to n i c s l i ces wi th in t h e

v o l c a n o - s e d i m e n t a r y s e q u e n c e ( F i g . 2) i n d ic a t e t h a t t h e i n -

c i p ie n t H P / L T m e t a m o r p h i s m a n d m e t a s o m a t i s m ha s o c -

c u r r e d p r i o r t o t h e s t r o n g t e c t o n i s m . T h e i m b r i c a t i o n o f

t h e w h o l e s e q u e n c e c a n b e r e l a t e d t o t h e e m p l a c e m e n t o f

t h e l a r g e u l t ra m a f i c s l a b ( F ig . 1 ) o n t h e v o l c a n o - s e d i m e n t a -

r y r o c k s . T h e r e i s a n i m p o r t a n t d i f f e r e n c e b e t w e e n t h e m a i n

b o d y o f p e r id o t i t e , w h i c h i s o n l y p a r t i a l ly s e r p e n t i n i se d a n d

h a s c h r y s o t i l e a s t h e m a i n s e r p e n t i n e m i n e r a l , a n d t h e

s m a l l e r c o m p l e t e l y s e r p e n ti n i s e d b o d i e s w i t h i n t h e v o l c a n o -

s e d i m e n t a r y s e q u e n c e , w h i c h c o n t a i n a n t i g o r i t e . S e v e r a l

m e t e r s t h i c k r e a c t i o n z o n e s o f p u m p e l l y i t e , a l b it e , s o d i c

p y r o x e n e , c h l o r i te e t c . h a v e b e e n f o u n d a t t h e c o n t a c t z o n e s

o f s e r p en t i n it e s in N o r t h w e s t T u r k e y ( O k a y 1 9 8 0c ) a n d

i n C a l i f o r n i a ( C h e s t e r m a n 1 9 60 ; L e o n a r d e s a n d F y f e 1 96 7) .

I f t h e s m a l l u l t r a b a s i c b o d i e s i n t h e T a v ~ a n l l re g i o n w e r e

p a r t o f t h e v o l c a n o - s e d i m e n t a r y s e q u e n c e a n d w e r e s e r p e n -

t in i se d d u r i ng t h e H P / L T m e t a m o r p h i s m , t h e n t h e y w o u ld

b e e x p e c t e d t o p r o d u c e s i m i l a r m e t a s o m a t i c z o n e s d u r i n g

t h e i r s e r p e n t i n i z a t i o n . T h i s m i g h t e x p l a i n t h e p r e s e n c e o f

e x t r e m e l y m e t a s o m a t i s e d p u m p e l l y i t e - a n d r a d i t e r o c k t y p e s

o r u n m e t a s o m a t i s e d s p il it es , w h i c h b y a n a l o g y m u s t

h a v e f o r m e d r e s p e c t i ve l y n e a r o r f a r a w a y f r o m t h e s e r p e n -

t i n i t e c o n t a c t s . H o w e v e r , t h e s t r o n g l a t e t e c t o n i s m h a s

m o s t l y o b l i t e r a t ed t h e f i e l d ev id en ce l i n k in g t h ese ro ck s t o

se rp en t i n i t e co n t ac t s .

M e t a s o m a t i s m h a s o f t e n b e e n c i t e d a s a n a c t i v e a g e n t

i n t h e fo rm at io n o f t h e b lu esch i s t s ( e .g . Gresen s 1 96 9 ). T h u s

T a l i a f e r r o ( 1 9 4 3) a s c r i b e d t h e f o r m a t i o n o f b l ue s c h i s ts t o

t h e l o c a l i n t r o d u c t i o n o f s o d a , i r o n a n d m a g n e s i u m i n t o

s a n d s t o n e s , s h a l e s , c h e r t s a n d b a s a l t s n e a r c o n t a c t s w i t h

u l t r a b a s i c i n t r u s i o n s . H o w e v e r , e x p e r i m e n t a l s t u d i e s h a v e

d e m o n s t r a t e d c o n v i n c i n g l y t h e h i g h p r e s s u r e s t a b i li t ie s o f

b lu esch i s t m in era l s , an d i n g en era l i t i s t r u e t h a t b lu esch i s t

m e t a m o r p h i s m i s i s o c h e m i c a l . N e v e r t h e l e s s , a s d o c u m e n t e d

h e r e , v e r y l o w - g r a d e i n c i p i e n t l y h i g h p r e s s u r e m e t a m o r -

p h o s e d r o c k s s h o w e x t e n s i v e m e t a s o m a t i c e f f e c t s . I t s e e m s

t h a t a l t h o u g h b l u e s c h is t s a r e n o t p r o d u c e d b y m e t a s o m a -

t i s m , m e t a s o m a t i s m i s w i d e s p r e a d i n l o w - g r a d e b l u e s c h i s t

f a d e s c o n d i t io n s .

Summary o f the Blueschist Metamorphic Zones

in the Tav~anh Region

I n N o r t h w e s t T u r k e y a t h i c k s e qu e n c e o f i n t e r c a l a t e d b a s i c

v o l c a n i c r o c k s , c h e r t s a n d s h a l e s h a s u n d e r g o n e b l u e s c h i s t

m e t a m o r p h i s m . B a s e d o n t h e m e t a b a s i c m i n e r a l a s s e m b l a g e

a n d t e x t u r e s , o b s e r v e d b o t h i n t h e f i e l d a n d u n d e r t h e m i -

c r o s c o p e , t h r e e b l u e s c h i s t m e t a m o r p h i c z o n e s h a v e b e e n

d i f f e r en t i a t ed i n t h e Tav ~an l l r eg io n .

T h e l o w e s t g r a d e z o n e , d e c r i b e d i n t h i s p a p e r , c o n s i s t s

o f a c l o se l y i n t e r c a l a t e d v o l c a n o - s e d i m e n t a r y s e q u en c e . T h e

h i g h p r e s s u r e m e t a m o r p h i s m w a s i n c i p i e n t a n d i s c h i e f l y

e v i d e n c e d i n t h e v e i n a n d a m y g d a l e m i n e r a l o g y , w h i c h i n -

c lu d es p u m p el ly i t e , a r ag o n i t e , l awso n i t e , a l b i t e an d so d i c

p y r o x e n e . A s s o c i a t e d w i t h t h i s i n c i p i e n t m e t a m o r p h i s m ,

t h e r e h a s b e e n e x t e n s i v e s o d i u m m e t a s o m a t i s m i n v o l v i n g

t o p o t a c t i c r e p l a c e m e n t o f a u g i te b y s o d i c p y r o x e n e .

T h e i n c i p i e n t l y m e t a m o r p h o s e d v o l c a n o - s e d i m e n t a r y

ro ck s t ec to n i ca l l y o v er l i e t h e l awso n i t e zo n e b lu esch i s t s .

R o c k t y p e s i n t h e l a w s o n i t e z o n e a r e s i m i l a r t o t h o s e o f

t h e i n c i p i e n t l y m e t a m o r p h o s e d z o n e a n d a p p e a r s i m i l a r l y

u n m e t a m o r p h o s e d i n t h e f i e l d . H o w e v e r , u n d e r t h e m i c r o -

s c o p e t h e m e t a b a s i c r o c k s c o n s i s t o f s o di c p y r o x e n e p s e u d o -

m o r p h s a f t e r a u g i t e , l a w s o n i t e a n d c h l o r i t e . T h e o r i g i n a l

t ex tu re h as b een l a rg e ly r e t a in ed an d t h e re i s l i t t l e p en e t r a -

t i v e d e f o r m a t i o n ( O k a y 1 9 8 0 b ) .

M e t a b a s i t e s o f th e l a w s o n i te z o n e p a s s a l o n g a m a p p e d

i s o g r a d i n t o t h e r o c k s o f th e g l a u c o p h a n e - l a w s o n i t e z o n e

w i t h t h e d e v e l o p m e n t o f s o d i c a m p h i b o l e f r o m a r e a c t i o n

i n v o l v in g c h l o ri t e, s o d i c p y r o x e n e a n d q u a r t z . M e t a b a s i t e s

o f t h e g l a u c o p h a n e - l a w s o n i t e z o n e h a v e l o s t a ll t h e ir p r i m a -

r y i g n e o u s f e a t u r e s a n d a r e o f t e n s c h i st o se . T h e y a r e c h a r a c -

t e ri s ed b y t h e d o m i n a n c e o f s od i c a m p h i b o l e + l a w s o n i t e

i n t h e m i n e r a l a s s e m b l a g e ( O k a y 1 9 8 0 a ) . T h e c o m m o n o c -

c u r r e n c e o f r e li c t s o d ic p y r o x e n e , o f t e n i n r e a c t i o n r e l a t i o n

t o s o d i c a m p h i b o l e , i n d i c a t e s t h a t t h e g l a u c o p h a n e - l a w s o n -

i t e z o n e r o c k s h a v e p a s s e d t h r o u g h t h e l a w s o n i t e z o n e s t a g e .

A l t h o u g h t h e r e i s a m p l e e v i d e n c e t h a t l a w s o n i t e a n d

g l a u c o p h a n e - l a w s o n i t e z o n e s r e p r e s e n t a p r o g r e s s i v e i n -

c r e as e in m e t a m o r p h i c g r a d e, t h e u b i q u it o u s m e t a s o m a t i s m

i n t he i n c i p ie n t l y m e t a m o r p h o s e d r o c k s i n d ic a t e s t h a t t h e s e

r o c k s u n d e r w e n t a d i f f e r e n t s e t o f p h y s i c a l c o n d it i o n s .

Acknowledgements

I thank R. Colston and J. Spray for reviewing

the manuscript. This work was carried o ut w hile in receipt of a

grant from the Mineral Research and E xploration Institute o f Tur-

key (MTA), which is gratefully acknowledged.

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Accep ted May 13 , 1982

07 - incipient metamorphism - cmp 1982.pdf

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