04 - lawsonite zone - CMP 1980.pdf

8/11/2019 04 - lawsonite zone - CMP 1980.pdf

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C o n t r i b M i n e ra l Pe t ro l 7 5 , 1 7 9 -1 8 6 (1 9 8 0 )

ontributions to

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

P e t r o l o g y

9 b y Sp r i n g e r -V e r l a g 19 8 0

Lawsonite Zone Blueschists and a Sodic Amphibole Producing Reaction

in the Tav~anll Region Nor thwe st Turk ey

A I

Okay*

D e p a r t m e n t o f E a r t h S c ie nc es , D o w n i n g P l a c e, C a m b r i d g e C B 2 3 E W , E n g l a n d

A bst rac t

The petrology and mineralogy of lawsonite

zone metabasites have been studied northeast of town

of Tav~anh, NW Turkey. In the field the metabasites

are characteristically green and lack folia tion; the es-

sential mineral assemblage being sodic pyroxene+

lawsonite + chlorite + quartz_+ sodic amphibole. Sodic

pyroxene of aegirine-jadeite composition occurs as

pseudomorphs after magmatic augite. Lawsonite and

chlorite are the other two dominant minerals. Sodic

amphibole forms progressively from a reaction be-

tween sodic pyroxene, chlorite and quartz, and an

isograd representing the first abundant occurrence of

sodic amphibole in basic rocks has been mapped.

The widespread occurrence of sodic pyroxene pseudo-

morphs in other blueschist terrains indicates that the

inferred sodic amphibole producing reaction is of gen-

eral significance for blueschist metabasites.

The conversion of greenstones with the assem-

blage albite + chlorite + actinolite directly into glauco-

phane-lawsonite blueschists without any intervening

lawsonite zone illustrates the influence of the initial

mineral assemblage on the reaction path.

Int roduct i on

The Northwest Turkish blueschist belt extends

250 km in an east-west direction flanking the northern

margins of the ultrabasic massifs in the Izmir-Ankara

Zone (Okay 1980a, Fig. 1). The dominant rock types

throughout the belt are basic volcanic rocks, pyroclas-

tics, cherts and shales. The volcanics and tufts are

repeatedly intercalated with pelagic sediments indi-

cating that volcanism was contemporaneous with sed-

imentation. The metamorphic grade shows a general

increase northwards away from the peridotite massifs

from incipient high pressure metamorphism to glau-

Present address: T e m e l A r a ~ t l r m a l a r , M i n e r a l R e s e a r c h a n d

E x p l o r a t i o n I n s t i tu t e (M T A ) , A n k a r a , T u r k e y

cophane-lawsonite or glaucophane-epidote zone

blueschists. The increase in metamorphic grade takes

place over a distance of I0-20 kin. A section across

this peridotite/blueschist belt has been mapped north-

east of the district of Tav~anh (Okay 1980a, Fig. 1).

Here an imbricate tectonic zone of spilite-chert-serpen-

tinite association with occasional slices of green-

schists, forms a 2 3 km wide rim arou nd the large harz-

burgite massifs. The spilites of this zone show effects

of an incipient high-pressure metamorphi sm and me-

tasomatism (Okay, in preparation). This schuppen

zone is succeeded along major thrust faults by the

lawsonite zone blueschists, which pass with the devel-

opment of sodic amphibole to fully recrystallised and

often schistose glaucophane-lawsonite zone blue-

schists (Okay 1980a). This paper deals with the law-

sonite zone blueschists around the village of Ketenlik,

south of the area shown in Fig. 1 of Okay (1980a).

R o c k T y p e s a n d P e t r o g r a p h y

Th e ro c k t y p e s i n t h e a re a i n c l u d e b a s i c v o l c a n i c s , c h e r t s , sh a l e s ,

se rp e n t i n i t e , g a b b ro a n d g re y w a c k e . M e t a b a s i t e s , w h i c h a re t h e

mo s t c o mmo n l i t h o l o g y , fo rm ma ss i v e g re e n d o l e r i t e s , p y ro c l a s t i c s

a n d l a v a f l o w s . In t h e f i e l d t h e b a s i c v o l c a n i c ro c k s a re g re e n ,

a p p e a r u n m e t a m o r p h o s e d a n d s h o w n o p e n e t r a t i v e d e f o r m a t i o n .

Th e o n l y s i g n o f b l u e sc h i s t me t a mo rp h i sm i s r a re b l u e s t r i n g e rs

o f so d i c a mp h i b o l e a l o n g sh e a r p l a n e s . H o w e v e r , u n d e r t h e mi c ro -

sc o p e t h e i g n e o u s mi n e ra l o g y i s s e e n t o b e c o m p l e t e Iy re c o n s t i t u t e d

w h i l e t h e i g n e o u s t e x t u re i s p re se rv e d . A u g i t e i s p se u d o m o rp h e d

b y so d i c p y ro x e n e (c f . , C a rp e n t e r a n d O k a y 1 9 7 8 ) , w h i l e t h e re s t

o f t h e ro c k c o n s i s t s o f a n a s se mb l a g e o f l a w so n i t e -q u a r t z -c h l o r i t e -

l e u c o x e n e (F i g . I ) . In so me ro c k s l a rg e p l a g i o c l a se c ry s t a l s h a v e

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

r i t e (c f. , C o ~ u l u 1 9 6 7 , Ph o t o 2 0 ). Ex c e p t fo r t h e g e n e ra l a b se n c e

o f p u mp e l l y i te , t h e l a w so n i t e z o n e me t a b a s i t e s o f t h e Ta v s~ an h

a re a c l o se ly re se mb l e t h e Ty p e I i m e t a b a s i t e s o f t h e Fra n c i sc a n

( C o l e m a n a n d L e e 1 9 6 3 ) . T h e c o m m o n m i n e r a l a s s e m b l a g e s in

t h e l a w so n i t e z o n e me t a b a s i t e s a re :

l a w so n i t e + so d i c p y ro x e n e + c h l o r i t e + q u a r t z + l e u c o x e n e

+ p h e n g i t e + so d i c a m p h i b o l e .

0010/7999/80/0075/0179/ 01.60


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180 A. I . Oka y : A Sodi c Am phi b o l e P rodu c i ng R e a c t i on

F i g. l a a n d b . P h o t o m i c r o g r a p h s o f l a w s o n it e z o n e m e t a b a s i te s f r o m t h e T a v ~ a n ll r e g io n : a S o d ic p y r o x e n e p s e u d o m o r p h s ( p x ) a f t er

augi te se t on a chlori te chl)+lawsonite lw)+quartz qtz)+matrix; l e uc oxe ne is a f t e r t i t a noma gne t i t e , b C o a rse g ra i ne d m e t a dol e r i t e

( K 5 4 ) w i th s o d i c p y r o x e n e p s e u d o m o r p h s ( p x ) r i m m e d a n d p a r t i a ll y r e p l a c ed b y s o d i c a m p h i b o l e amp). T h e g r o u n d m a s s c o n s i s t s

of la wsoni t e a nd qua r t z . Sodi c pyroxe ne ha s i r r e gu l a r qua r t z i nc l us i ons

Sodi x pyroxe ne , l a wsoni t e a nd c h l or i t e oc c ur i n roughl y e qua l

p r o p o r t i o n s , a n d t o g e t h e r c o n s t i t u te a p p r o x i m a t e l y 8 0 m o d a l p e r

c e n t o f t he roc k (Ta b l e 1 ). Le uc oxe n e a nd qua r t z usua l l y ma k e

up l e s s t ha n 10% . Sodi c a mphi b o l e oc c urs e i t he r a s t h i n r i ms

a rou nd sod i c pyroxe ne pse udo mo rph s (F ig . l b ) o r a s i d i ob l a s t ic

c rys t a l s g row i ng ove r a f i ne -gra i ne d sod i c pyroxe ne -c h l or i t e ma t r i x .

Sodi c a mphi bo l e , whe n pre se n t , ge ne ra l l y doe s no t c ons t i t u t e more

t ha n 10% of t he me t a ba s i t e i n t he l a wsoni t e z one . A fe w gra i ns

o f p y ri t e o r m a g n e t i t e w er e n o t e d f r o m a b o u t o n e q u a r t e r o f t h e

spe c i me ns . In ge ne ra l c a rbona t e m i ne ra l s a re r a re, b u t sma l l

a m oun t s o f a ra goni t e , pa r t i a l l y i nve r t e d t o c a l ci t e, oc c u r i n t he

g r o u n d m a s s o f f iv e s p ec i m e n s. A l b i t e is a b s en t . M e a s u r e d m o d e s

of s ix a na l yse d me t a b a s i t e s ba se d on ove r 1 ,000 po i n t c oun t s a re

given in Table 1.

V e i n s ar e n o t v e r y a b u n d a n t 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 ;

t he ve i n mi ne ra l s i nc l ude qua r t z ( i n 17 sa mpl e s o u t o f 40) , c a l ci t e

( in 4) , a ragoni te ( in 2) , chlori te ( in 2) , l awsoni te ( in 2) , and sodic

pyroxe ne ( i n I ) .

R e d c he r t s , whi c h a re i n t e rc a l a t e d wi t h t he me t a ba s i t e s , a re

poor l y r e c rys t a l l i s e d a nd re t a i n t he i r s e d i me nt a ry fe a t ure s . U l t r a -

ba s i c roc ks oc c ur a s sma l l l e nse s wi t h i n t he l a wsoni t e z one se -

que nc e . Th e y a re c ompl e t e l y s e rpe n t i n i s e d a nd c o ns i s t o f a n t igor i t e

( a s d e t e r m i n e d b y i ts X R D p a t t e r n s cf . W h i t t a k e r a n d Z u s s m a n

1956) wi t h mi n or c h l or i t e , b ruc it e , t r e mol i t e , d i s se mi na t e d ma g ne -

t i te a nd re l i c t sp i ne l. I r r e gu l a r ve i ns a nd gra nul e s o f a ndra d i t e

garnet (Ca2.gsMn0.ozFel .84Alo. l lTlo.ogS13012)3+ are commonly

pre se n t i n t he se s e rpe n t i n i t e s .

R a re vo l c a noge ni c g re ywa c ke s re t a i n t he i r s e d i me nt a ry t e x-

t u re s . Howe ve r , a l b i t e c l a s t s ha ve be e n re p l a c e d by pr i sm a t i c bun-

d l e s o f j a de i t e a nd q ua r t z ; t he qua r t z g ra i ns sho w i nc i p i e n t r e c rys -

ta l l i sa t ion.

M e t h o d

M i ne ra l c omp os i t i on s we re de t e rmi ne d for t we l ve e l e me nt s (Na ,

C a , K , Fe , M g, M n, V, Ni , C r , T i , A1, S i) us i ng a n e ne rgy d i spe r s i ve

e l e c t ronprobe w i t h a Ha rwe l l S i (L i) de t e c t or a nd pu l se p roc e s sor .

T h e c o r r e c t io n p r o c e d u r e s a r e g i ve n b y S w e a t m a n a n d L o n g ( 1 96 9 ).

The me t h od for e s t i ma t i ng fe rr i c i on i n sod ic pyroxe ne a n d sod i c

a mph i bo l e i s g i ve n i n e a r li e r pa pe rs (Oka y 1978, 1980 b

respect ive ly) .

Table 1 M e a s u r e d m o d e s o f a n a l y s ed l a w s o n i te z o n e m e t a b a s i t e s

K 5 4 K 6 2 5 K 6 2 7 K 7 6 1 K 7 6 9 K 7 8 2 K 8 5 6

Law son i te 32.1 20.2 44.6 23.8 29.7 15.0 x

Chlo ri te 8.9 27.5 1.1 18.6 25.6 26.7 x

Sodic pyr oxe ne 26.1 33.3 26.2 42.0 22.1 45 .2 x

Sodic am ph ibo le 13.7 - 20.4 - - 0.9 -

Qu ar tz 13.6 14.1 0.2 4.1 4.0 0.1 -

Le uc ox en e 3.1 4.4 5.4 11.2 11.1 10.1 x

Ph eng ite 2.5 - 1.7 - 7.5 1.0 -

Pyr ite - - - 0.3 - tr x

M a g n e t i t e - 0 .5 . . . . .

Relic t aug ite - - 0.4 - - 1.0 x

100.0 100.0 100,0 100.0 100.0 100.0

* T o o f in e - g r a in e d f o r m o d a l co u n t i n g

Ja de i t i c pyroxe ne

M ine r a logy

Sodic Pyroxene

A l l s o d ic p y r o x e n e s i n t h e m e t a b a s i t es a r e p s e u d o m o r p h s a f te r

a ug i t e. In t he 40 sa mpl e s t ha t we re e xa mi ne d , sod i c pyrox e ne wa s

ne ve r found t o ha ve nuc l e a t e d i nde pe nde nt l y . In t he g re a t ma j or i t y

of t he spe c i me ns sod i c pyrox e ne fo rm s a pp l e -gre e n c rys t a l s , whi c h

a re o f t e n t u rb i d a s a r e su l t o f sm a l l l e uc oxe ne i nc l us i ons (F i g . 1 ).

Suc h sod i c pyroxe ne s a re a e g i r i ne -j a de i t e s wi t h h i gh a c m i t e c ompo -

ne n t s (> 40% , F i g . 2 , Ta b l e 2) . In four spe c i m e ns a ug i t e s a re pa r -

t i al l y o r c ompl e t e l y r e p l a c e d by j ade i t i c pyroxe ne . In t wo of t he se

c a se s j a de i t e a nd a c mi t i c pyrox e ne ha ve re p l a c e d d i f fe re n t g ra i ns

i n t he s a me se c t i on , o r e ve n d i f fe re n t pa r t s o f t he s a me gra i n .

T h e c o m p o s i t i o n s o f t h e j a d e it i c p y r o x e n e s f r o m o n e o f t h e s e m e t a -

ba s i t e s a re p l o t t e d i n F i g . 2 . Ompha c i t e , w hi c h oc c urs i n some

T y p e II m e t a b a s i t e s f r o m t h e F r a n c i s c a n ( E s s e n e a n d F y f e I 9 6 7,

F i gs . 12 a nd 13 ; B rown a nd B ra dsh a w 1980, F i g . 3 ) i s no t foun d

i n t he Ta v~ a nh a re a .


3/8

A.I. Okay: A Sodic Amphibo le Producing Reaction 181

,C

Au Jd

F ig . 2 . So d ic p y ro xe n e co mp o s i t i o n s f ro m th e ] a wso n i t e zo n e me ta

basites plotted on the acmite-jadeite-augite diagram

X s i

0.7

0 . 6

0 . 5

O . t +

) A t 0 .3

0 . 2

0.0

o

~ = o ,~, , e

i 0 3 .

0 . 1 0 . 2 0 . ~ 0 . 5 0 . 6

F e / F e + M g

Fig. 3. Chlorite compositions from the lawsonite zone metabasites

plotted in terms of their Fe/Fe+Mg, Si/Si+A1 and AI/AI+Fe+

Mg ratios. Symbols as in Fig. 2

Chlori te

In contras t to the glaucophane-lawsonite zone, chlorite is abund ant

in the lawson ite zone metabasffes. However, it is less well recrystall-

ised and often has appreciable K20 and CaO contents (Table 2).

Chlorite compositions frmn six metabasites are plotted in Fig. 3

and four chlorite analyses are shown in Table 2. Chlorites show

a wide range of ferromagn esian ratios (Xre = 0.2-0.5) and a restrict-

ed A1/A1+ Fe + Mg ratio (0.30 0.34). There is no appreciable com-

positional difference between the glaucophane-lawsonite and law-

sonite zone chlorites.

La w so n i t e

Lawsonite invariably forms small tabular crystals with prominent

(101) faces. Compositionally it is very uniform and closely approxi-

mates the ideal s tructural formula CaAIzSi2OvH20(OH)2.

S o d i c m p h i b o l e

Sodic amphiboles, which are texturally replacing sodic pyroxenes

(cf., Fig. 1 b) tend to be compositionally inhom ogeneo us with Ca-

rich patches. This apparently has arisen from the difficulty of

diffusing calcium atoms away from the sodic pyroxene through

the mantling sodic amphiboIe. Idiobtastic sodic amphiboles grow-

ing over the chlorite-sodic pyroxene matrix are unifo rmly homoge-

neous and sodium-rich (Na /Na +C a>9 0 ). Sodic amphibole

composi tions from three metabasites are plotted on the Miy ashiro

diagram in Fig. 4 an d two of the analyses are given in Table 2.

Ph e n g i t e

Phengite is much rarer in the lawsonite zone metabasites than

in the correspondin g higher grade glaucophane-l awsonite zone me-

tabasites. It fo rms feathery elongate crystals closely as sociated and

often intergrown with chlorite. Its forma tion is related to the recrys-

tallisation of relatively potassium-rich microgranu lar chlorite to

potassium-poor coarser chlorite. The few analysed phengites do

F G R

V

O

v

f i d R

Fig. 4. Sodic amphibole compositi ons from the lawsonite zone me-

tabasites plotted on the Miyashiro diagram. Symbols as in Fig. 2.

G gtaucophane;

F G

ferroglaucophane; R riebeckite;

M R

magne-

sioriebeckite

not have any measurable paragonite or margarite components (Ta-

ble 2).

Le u c o x e n e

Leucoxene is a constant accessory mineral in the metabasites. It

forms diffuse semiopaque pseudomorphous after titanomagnetite

or ilmenite. It has a nonstoich iometric and variable comp osition

with sphene as the dominant mineral (Table 2).

Sodic mphibole Isograd

The passage from green massive lawsonite-chlorite-

sodic pyroxene rocks to blue often schistose glauco-

phane-lawsonite metabasites is gradual. The more


4/8

182

Table 2. Re presentative mineral analyses

A.I. Okay: A Sodic Amphibole Producing Reaction

Sodic pyroxene Chlorite Sodic amphibol e Phengite Leucoxene

K625 K761 K627 K782 K856 K625 K769 K54 K627 K782 K761 K54

SiO2 53.92 53.67 55.67 54.26 58.40 29.55 29.19 28.68 29.05 59,48 56.75 52.43 30.00 29.78

TiO2 0.25 0.40 1.05 0.14 0.14 0.00 0.00 0.00 0.00 0.43 0.17 0.00 25.95 39.05

AI203 5.02 5.16 9.66 6.60 18.08 16.70 16 .7 9 17.87 18.05 10.23 5.99 21.73 6.42 2.42

FeO 21.19 18,47 15.77 13.25 5.94 21.64 22.51 18.77 16.75 9.82 19.47 4.51 7.79 2.23

MgO 1.41 3.08 1.46 4.82 1.43 13.95 14.60 20.48 17.81 10.80 7.69 4.63 6.12 1.42

MnO 0.00 0.00 0.00 0.00 0.00 0.49 0.77 0.56 0.54 0.19 0.19 0.00 0.15 0.00

CaO 2.99 6.05 2.37 9.10 2.67 0.39 0.22 0.13 0.28 0.44 0.69 0.00 19.45 23.60

NazO 12.99 10 .6 5 12 .73 9.46 13.32 0.00 0.00 0.00 0.00 7,78 7.02 0.00 0,00 0.00

K2 0 0.00 0.00 0.00 0.00 0.00 1.47 0.49 0.09 0.98 0,00 0.00 10.46 0,00 0.00

Tota l 97.77 97,48 98.71 97.63 99.98 84.19 84.57 86.58 83.46 99.17 97.97 93.76 95,88 98.50

Totaliron as FeO

Number of cations per

6 oxygens 28 oxyge ns 23 O 22 O 5 oxygens

Jd 22 23 41 29 74

Ac 65 54 48 39 I5

Au 13 23 11 32 11

Si 1.9 9 2.00 2 . 01 2.00 2.02 6.40 6.29 5.91 6.16 7.97 8.01 7.18 1.0 2 0.98

A14 0.01 0.00 0,00 0,00 0.00 1.60 1.7 1 2.09 1,84 0.03 0.00 0.82

A16 0.21 0.23 0 . 41 0.29 0.74 2.66 2.56 2.25 2.68 1.59 1.00 2.6--9 0.26 0.09

Ti 0.01 0. 01 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.04 0.02 0.00 0.66 0.97

Fe 3+ 0.65 0.54 0.48 0.39 0.15 0.28 0.91

3.92 4.06 3.24 2.97 0.52 0.22 0.06

Fe 2+ 0.00 0.03 0.00 0.02 0.02 0.82 1.39

Mg 0.08 0.17 0.08 0.26 0.07 4.50 4.69 6.29 5.63 2.16 1.62 0.95 0. 31 0.07

M n 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 9 0 . 1 4 0 . 1 0 0 . 1 0 0 . 0 2 0 . 0 2 0 .0 0 0 . 01 0 . 01

0.95 0. 9 8 1. 00 0. 96 0.98 4.91 4.96 4.1--~

Ca 0 .12 0 . 24 0 . 09 0 .3 6 0 .10 0 .09 0 .0 5 0 . 03 0 .06 0 .06 0 .10 0.0---0 0 .71 0 .83

Na 0 .93 0 . 77 0 . 89 0 . 68 0 .89 0 .00 0 . 00 0 . 00 0 .00 2 .02 1 .92 0 .00 0 .00 0 .00

K 0 . 00 0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0 0 .4 1 0 . 1 3 0 . 0 2 0 . 2 7 0 . 0 0 0 . 0 0 1.83 0 . 0 0 0 . 0 0

1.05 t.0 1 0. 98 1.04 0. 99 2.0 8 2. 02 1.8---3 3.19 3.01

9.67 19. 63 19. 93 19,71

f is s il e v o l c a n i c s h a l e s a n d t u f t s o b t a i n a b l u e c o l o u r

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

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

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

b o l e i s o g r a d i s m a p p e d i n th e s e m a s s i v e b a s a l t s a n d

d o l e r it e s . ( I s o g r a d i s u s e d h e r e p u r e l y in t h e d e -

s c r ip t i v e s e n s e - e . g ., t h e f ir s t a b u n d a n t a p p e a r a n c e

o f so d i c a m p h i b o l e i n m a s s i v e m e t a b a s i t e s ) .

T h e p a s s a g e f r o m l a w s o n it e z o n e t o g l a u c o p h a n e -

l a w s o n i t e z o n e o c c u r s w i t h i n 5 0 m i n th e v i l l a g e o f

K e t e n l i k . T h e c h a n g e i n m i n e r a l a s s e m b l a g e w a s o b -

s e rv e d in t w o b a n d e d t u f f s p e c im e n s c o l l ec t e d f r o m

w i t h i n 5 0 m o f e a c h o t h e r . K 5 0 7 / 2 i s g r e e n a n d s h o w s

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

y e l lo w i s h - g re e n s o d ic p y r o x e n e , a n d c h l o r i t e + l a w -

s o n i te b a n d s w i t h m i n o r q u a r t z b u t n o s o d i c a m p h i -

b o l e . K 5 0 7 / 1 c o l l e c t e d 5 0 m t o t h e s o u t h i s b l u e a n d

s c h i st o s e , a n d c o n s i s t s o f o v e r 4 0 m o d a l p e r c e n t s o d i c

a m p h i b o l e a n d a b u n d a n t l a w s o n it e . S o d ic p y r o x e n e

h a s la r g e l y r e s o r b e d o r c o n c e n t r a t e d a l o n g c e r t a i n

b a n d s .

F i g u r e 5 is a g e o l o g i c a l m a p o f t h e K e t e n l i k a r e a ,

N W T u r k e y s h o w i n g th e s a m p l e l o c a t i o n s o f t h e b a s ic

i g n e o u s r o c k s . L o c a t i o n s o f m e t a b a s i t e s w i t h t h e e s -

s e n t ia l m i n e r a l a s s e m b l a g e l a w s o n i t e + c h l o r it e + s o d -

i c p y r o x e n e ( a f t e r a u g i t e ) a r e s h o w n b y f i l l ed c i r c le s .

H a l f - f i ll e d c ir c le s i n d i c a te s p e c i m e n s w h i c h a l s o c o n -

t a in m i n o r a m o u n t s o f s o d ic a m p h i b o l e . M e t a b a s i t e

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

s o d ic a m p h i b o l e + t a w s o n i t e a r e sh o w n b y o p e n c i r-

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

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

s e r p e n t i n i t e c o m p l e x ( Y e n i k 6 y G r o u p ) .

T h e f o r m a t i o n o f s o d ic a m p h i b o l e i s g e n e ra l ly

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

i n t h e b l u e s c h is t s . I n t h e l a w s o n i t e z o n e , s o d i c a m p h i -

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

m a s s i v e m e t a b a s i t e s , w h e r e i t f o r m s t h i n b l u e s c a l es .


5/8

A . I . O k a y : A S o d ic A m p h i b o l e P r o d u c i n g R e a c t i o n 1 83

[ [ ~ B l u e s c h is f I U l f r a b a s i t

p_~==:~6r eenschis ~ Ma rble

F~7 ~ Y e n i ko yQroup

f Dip and strike of schistosity

/ Faul t ~

Landslide

/ - '" Sadie

amphibolesograd

9 No-Px + Laws.+ChL

No-Px * Laws.+Chl.Na-Amph.(30~ -*Na-Px.

Ikm

o

i

0

1S2

0 _ _ _

. . . . . . - 6 2 7 . . . . . . . . . . . . . . . .

9

.,_. (I,

e s6 9 9 9

< ~ v ~ , \

ZEZ

: : ~ : 7

- i e . > ; ? .

70 ,, .~: .::

, , , , , : :

o o

Ketenl

Fi g . 5 . Ge o l o g i c a l ma p o f t h e

Ke t e n l i k a re a sh o wi n g t h e

sa mp l e l o c a t i o n s a n d t h e so d i c

a m p h i b o l e i s o g r a d . Y e n i k 6 y

Gro u p c o n s i s t s o f a n i n c i p i e n t l y

m e t a m o r p h o s e d a n d

me t a so ma t i se d sp i l i t e -c h e r t -

se rp e n t i n i t e c o mp l e x 9 Th e inset

sh o ws t h e ma j o r p e r i d o t i t e

o c c u r r en c e s i n W e s t e r n T u r k e y

With increasing deformation sodic amphibole grows

parallel to the schistosity wrapping around rotated

sodic pyroxene and lawsonite crystals. The typical

mineral assemblage in the glaucophane-lawsonite

zone metabasites is "sodic amphibole+lawsonite_+

sodic pyroxene + chlorite + sphene +phe ngi te" . The

ghost igneous texture, which is so well preserved in

the lawsonite zone metabasites, is completely obliter-

ated with the development of a schistose metamorphic

fabric. Cherts which are clearly recognizable as such

in the lawsonite zone, lose their distinct red colour,

and form banded, schistose metaquartzites. These

rocks are described in detail in the previous paper

(Okay 1980a).

Sod ic mph ibo le Producing React ion

in the Metabas i tes

The reaction textures between sodic pyroxene and

sodic amphibole, as well as the drastic decrease in

the modal amounts of chlorite and sodic pyroxene

in the glaucophane-lawsonite zone metabasites, indi-

cate that the formation o f sodic amphibole is related

to a reaction involving chlorite and sodic pyroxene.

In quartz-bearing rocks, the minerals sodic pyrox-

ene, sodic amphibole, chlorite and lawsonite can be

described in the six component system Na20 - Ca O-

Fe O- Mg O- A1 20 3- Fe 20 > The relationship be-

tween sodic pyroxene and sodic amphibole in this

system .can be shown in a t ernary phase projection

with Na, A1 and Fe 3 + at its apices (NAF3), involving

A(

/~ +L f lwsomfe

/ \ *Chtor l te

/ ~

Fi g . 6 . NA F3 p ro j e c t i o n o f so d i c p y ro x e n e a n d so d i c a mp h i b o l e

c o m p o s i t i o n s f ro m l a wso n i t e , c h l o r it e , q u a r tz , a n d H 2 0 . Th e j a -

d e i te a n d a u g i te c o n t e n t s o f s o d ic p y r o x e n es , a n d t h e N a / N a + C a

ra t i o s o f so d i c a mp h i b o l e s thkk dashed l ines) a re i n d i c a t e d . Th e

o b se rv e d so d i c p y ro x e n e c o mp o s i t i o n a l f i e l d f ro m t h e l a wso n i t e

z o n e m e t a b a s i t e s i s s t i p p l e d

projections from lawsonite for Ca and from chlorite

for Fe 2+, Mg (Fig. 6). Both lawsonite and chlorite

are stable and ubiquitous on both sides of the isograd.

The composition of chlorite used in the projection

is (Fe 2+, Mg)4.8oAl>2oSi2.95Olo(OH)8, typical of

blueschist chlorites9 The variation of A1/Fe2 + + Mg

of chlorites in different rocks is slight (Fig. 3) and


6/8

1 8 4 A . I . O k a y : A S o d i c A m p h i b o l e P r o d u c i n g R e a c t i o n

t h e r e i s n o a p p re c i a b l e d i f f e r e n c e in t h e A 1 / A I+ F e +

M g r a t i o s o f c hl o r it e s f ro m 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 . Th e a l g e b ra i c p ro c e d u re u s e d

fo r p ro j e c t i n g p h a s e s i s t h a t g i v e n b y G re e n w o o d

(1975).

O n l y C a -p o o r s o d i c p y ro x e n e s c a n b e r e p re s e n t e d

(a p p ro x . < A u 3 5 ) i n t h e N A F 3 d i a g ra m a s th e m o re

c a l ci c m e m b e r s h a v e n e g a t iv e p ro j e c t i o n p o i n t s . H o w -

ever , th i s is no t a p rob le m as a l l ana lysed sod ic py rox-

e n e s f ro m m e t a b a s i t e s h a v e l o w a u g i t e c o n t e n t s

(< A u 3 o ) . Th e c o m p o s i t i o n a l ra n g e o f so d i c p y ro x e -

n e s f ro m m e t a b a s i t e s i s s h o w n s t i p p l e d i n t h e N A F 3

d i a g ra m i n F i g . 6 . S o d i c a m p h i b o l e c o m p o s i t i o n s

d o w n t o N a / N a + C a = 0 .8 ( a s o b s e rv e d i n m e t a b a s i t es )

a re ind ica ted by dashed l ines .

Th e m o s t i m p o r t a n t f e a t u r e r e v e a l e d b y t h e N A F 3

diag ram in F ig . 6 i s the su perpos i t ion o f the sod ic

a m p h i b o l e a n d s o d i c p y ro x e n e f i e l d s . Th i s i n d i c a t e s

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

i n g to t h e s o d i c a m p h i b o l e f i e l d i n th e N A F 3 d i a g ra m ,

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

o f q u a r t z , l a w s o n i t e a n d c h l o r i t e u n d e r d i v a r i a n t c o n -

d i ti o n s. F o r s o d i c p y ro x e n e s w i th c o m p o s i t i o n s l y in g

o u t s i d e t h e s o d i c a m p h i b o l e f i e l d i n t h e N A F 3 d i a -

g r a m (A u < 0 .2 o r A u > 0.3) , t h e s o d i c a m p h i b o l e p ro -

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

i ng s o d ic p y ro x e n e s fu r t h e r e n r i c h e d (> A u o .3 ) , o r

dep le ted (< Au0 .2) in aug i te com pon en t th an the in i-

t ia l c o m p o s i t i o n (d e p e n d i n g o n w h i c h s id e o f t h e s o d i c

a m p h i b o l e f i el d t h e y li e) . A s p o i n t e d o u t b y B ro w n

and Bradshaw (1980) , th i s wi l l resu l t in the po lar iza-

t i o n o f so d i c p y ro x e n e c o m p o s i t i o n s a n d m a y e x p l ai n

the app aren t so lvus in the sod ic py ro xene f i e ld . Such

a g ro u p i n g i s h o w e v e r n o t h p p a re n t i n s o d i c p y ro x e n e

c o m p o s i t i o n s f ro m 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

(cf. , Okay 1980a, Fig . 4).

Th e s o d i c a m p h i b o l e p ro d u c i n g r e a c t i o n i n t h e

Ta v ~ a n h a r e a c a n b e b a l a n c e d fo r t h e o b s e rv e d c o m -

pos i t ions o f the par t i c ipa t ing minera l s :

2 . 5 N a o . s o C a o , 2 0 A l o . s o F e 3 + o ,s o M g o . 1 6 F e 2

o . o 4 8 i 2 0 6

s o d i c p y r o x e n e

+ 0 . 5 M g a F e 2 + 2 A 1 2 S i 3 O s O H 8 + 2 . 5 S i O 2 =

c h l o r i t e q u a r t z

0 . 5 C a A 1 2 S i 2 0 6 O H ) 4

l a w s o n i t e

+ 1 N a a M g l . 9 0 F e 2 + x . lo A l o . 75 F e a+ 1 . 2 5S i s O 2 a O H ) z .

s o d i c a m p h i b o l e

I t i s i m p o r t a n t t o n o t e t h a t t h is r e a c t i o n d o e s

no t invo lve

H 2 0 ,

a n d i s t h u s i n d e p e n d e n t o f c h a n g e s

in f lu id p ressu re .

The ch lo r i t e compos i t ion in the reac t ion i s s l igh t ly

idea l i sed [ (Mg, F e)sAl2 . . . . in s tead o f (M g, Fe)4 .s -

A12 .z . . . ] and the smal l Ca-componen t in sod ic am-

p h i b o l e i s n e g l e c t e d i n o rd e r t o u s e t h e t a b u l a t e d

t h e rm o d y n a m i c v a l u e s . En t ro p i e s a n d m o l a l v o l u m e s

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

o b t a i n e d f ro m t h e i r r e s p e c t i v e e n d -m e m b e r s (H e l g e -

son et al . 1978) by ass um ing ideal m ixing o n s i tes.

O rd e r i n g o f i o n s in t h e M 1 a n d M 2 s it es h a s b e e n

d e m o n s t r a t e d fo r P l a t ti c e o m p h a c i t e s (C l a rk a n d P a -

p ike 1968) . Aeg i r ine- jade i tes and c h lo romelan i tes , o n

t h e o t h e r h a n d , s h o w n o o rd e r i n g a n d h a v e t h e h ig h e r

s y m m e t ry s p a c e g ro u p C 2 / c w i t h e q u i v al e n t M 2 s i t es

(C a rp e n t e r 1 9 8 0 ). Th e re fo re e n t ro p i e s o f m i x i n g fo r

s o d i c p y ro x e n e s a n d s o d i c a m p h i b o l e s h a v e b e e n c a l -

cu la ted assuming no coup l ing on s i t es . The en t ropy

data fe r rog laucophane i s no t ava i lab le . I t i s as sumed

t h a t i n c r e a s e i n e n t ro p y f ro m m a g n e s i o r i e b e c k i t e t o

r i e b e c ki t e ( d u e t o M g < - - = > F e z + s u b s t i tu t i o n ) is

t h e s a m e (1 8 .6 c a l / m o l .K ) a s f ro m g l a u c o p h a n e t o

f e r ro g l a u c o p h a n e , a n d a c o r r e s p o n d i n g v a l u e fo r f er -

rog lau coph ane has been ob ta ined (148 .6 ca l /mol . K) .

There i s a very smal l decrease in vo lum e (A V =

-1 .4 5 c ra B ; a d e c re as e o f 0 . 5 % i n v o l u m e c o m p a re d

w i t h 1 7% f o r t h e r e a c ti o n a l b i t e - > j a d e i t e + q u a r t z )

a n d i n e n t ro p y (A S = ~ - 9 c a l / d e g re e ; a d e c r e a s e o f

4 .7% ) to the r igh t s ide o f the reac t ion . Such sm al l

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

t a i n t y o f t a b u l a t e d t h e rm o d y n a m i c v a l u e s a n d t h e

a s s u m p t i o n s i n v o l v e d , m a k e t h e c a l c u l a t e d s l o p e o f

t h e r e a c t i o n dP/dT) mean ing less .

Importance of Topotaxy in Low-Grade Metamorphism

Such smal l d i f fe rences in A V and

AS

i n l o w -g ra d e

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

m i n i n g f a c t o r s in t h e fo rm a t i o n o f a n y n e w m i n e ra l

assemblages . In low-gra de (200~ ~ C) s ta t i c meta-

morph ism, where there i s no d i f fe ren t ia l s t res s , nu -

c lea t ion o f new m inera l s i s d i f f icu l t . One way o f over-

c o m i n g t h i s p ro b l e m o f n u c l e a t i o n i s b y t o p o t a c t i c

rep lacem en t o f a p reex i s t ing minera l s t ruc tu re . In

s u c h c i rc u m s t a n c e s a m i n e ra l a s s e m b l a g e c o u l d fo rm ,

which , a l though no t rep resen t ing the lowes t poss ib le

f ree energy o f the sys tem a t the par t i cu la r p ressu re

and tempera tu re , i s k ine t i ca l ly favoured . Equ i l ib r ium

ma y on ly be s lowly a t t a ined ( i f a t a ll ) by a se ries

o f me tas tab le m inera l as sem blages invo lv ing a se -

q u e n c e o f O s w a l d s te p s .

Th e i m p o r t a n t t o p o t a c t i c r e a c t i o n i n t h e Ta v ~ a n h

a re a i s s o d i c p y ro x e n e p s e u d o m o rp h i n g a u g i t e . I t

i s c lear tha t a l l sod ic py roxene in metabas i t es , bo th

i n t h e 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 ,

has fo rmed in i t i a l ly as pseudomorphs a f te r aug i te .

R o c k s w i t h a n a l r e a d y e x i s ti n g c a lc i c a m p h i b o l e ( fo r

e x a m p l e s o m e r a r e b a s i c a lk a l i r o c k s w i t h m a g m a t i c

kaersu t i t e ) have deve lo ped a b lue amph ibo le in a re -


7/8

A.I. Okay: A Sodic Amphibole Producing Reaction 185

placement texture. In such rocks a sodic pyroxene-

chlorite assemblage was never formed.

In the formation of sodic amphibole, penetrative

deformation has played an importan t role in facilitat-

ing the nucleation of new minerals. In fact the sodic

amphibole isograd in Fig. 5 corresponds roughly to

the limits of penetrative deforma tion in the metabasic

rocks. In the more massive metadolerites of the glau-

cophane-lawsonite zone, sodic pyroxene pseudo-

morphs are clearly discernable, whereas in the sur-

rounding more fissile tufts they have been completely

obliterated. This, and the common presence of the

reaction mineral assemblage sodic amphibole +so dic

pyroxe ne+chlor ite +lawsonite + quartz in many

glaucophane-lawsonite metabas ites cf., Okay 1980a

Tables 1 and 2), indicates that all metabasites in the

glaucophane-lawsonite zone have passed through a

lawsonite zone stage.

The Type II and III metabasites of the Franc iscan

Coleman and Lee 1963) are closely analogous both

in mineralogy and texture to the lawsonite and glau-

cophane-lawsonite zone blueschists of Northwest

Turkey. The presence of minor amounts of sodic am-

phibole in many Franciscan Type II metabasites Cole-

man and Lee 1963; Brown and Bradshaw 1980, Ta-

ble 2) indicates that most of the mineral assemblages

are in fact reaction assemblages. The completion of

the sodic amphibole producing reaction would be

greatly facilitated by de formation and the characteris-

tic feature of Type III metabasites is a strong penetra-

tive fabric. The gradational contacts betweeen Type II

and III metabasites in the Ward Creek area Coleman

and Lee 1963) may represent, like those in the Tav~an-

11 area, the limits o f penetrative deformation.

The extensive presence of sodic pyroxene pseudo-

morphs after augite in many blueschist terrains of

the world [in Franciscan Type II metabasites Cole-

manandLee 1963,p. 270 ;Ernst 1965, p. 888 ; Esseneand

Fyfe 1967, p. 3; Ernst et al. 1970, p. 49) ; in the lawson-

ite zone metabasites of New Caledonia Black 1974,

p. 282); in the Celebes, Indonesia de Roever 1950,

p. 1457); in the lawsonite-albite facies rocks of the

Fuscaldo area in Italy de Roever 1972, p. 63); in

the Western Alps Bearth 1965, p. 181) and in Corsi-

can blueschist terrain Brouwer and Egeler 1952,

p. 32)] indicates that the reaction between sodic pyr-

oxene and chlorite is an important sodic amphibole

producing reaction in such rocks.

Several other reactions have been proposed for

the forma tion of sodic amphibole in low-grade rocks.

The often quoted sodic amphibole producing reaction

Miyashiro and Banno 1958, p. 101; de Roever and

Beunk 1976, p. 224) of

albite+a ntigori te mol. in chlorite

= glaucophane + H 20

is not realistic as there is no appreciable composition-

al difference between the lawsonite and glaucophane-

lawsonite zone chlorites, and furthermore the actual

sodic amphibole produced is generally a crossite

rather than glaucophane. Sodic amphibole forming

reactions involving calcite Ernst 1963, p. 23) imply

regional decarbonat ion for which there is no evidence

at these low temperatures o f metamorphism. As para-

gonite and stilpnomelane are both rare minerals in

low-grade blueschists, sodic amphibo le producing re-

actions involving these minerals Ernst 1963, p. 23;

Brown 1974, p. 339) would at the very best be o f

localised importance.

However, in the case of greenstones with the initial

mineral assemblage of chlorite+albite+actinolite,

sodic amphibole production can involve a one step

reaction such as:

actinolite + chlorite + albite

= sodic amphibole + lawsonite,

This reaction de Roever 1955, p. 241) is observed

in the metabasites of the Panoche Pass, California

Ernst 1965, Fig. 2) and also in uralitised metagabbros

in the Tav~anll area Okay 1980c).

It is not clear what niche in the P-T-X space the

assemblage sodic pyroxene+chlorite+lawsonite+

quartz occupies. It is quite plausible that in the Tav-

~anh metabasites this assemblage may have formed

metastably within the sodic amphibole+lawsonite

stability field through the seeding effect of a preexist-

ing pyroxene structure. If the initial spilitisation pro-

cess had produced an assemblage of actinolite+al-

bite+chlorite, this intermediate sodic pyroxene stage

probably would not have been observed. This strong

influence of the initial mineral assemblage on the reac-

tion path suggests that reaction kinetics rather than

the drive of the system towards the lowest free energy

may be the dominant factor in the formation of new

mineral assemblages in such low-grade metamorphic

rocks.

Acknowledgements

I thank T.J.B. Holland, R. Muir Wood, and

I. RossoI for reviewing the manuscript. This work was carried

out while in receipt of a grant from the Mineral Research and

Exploration Institute of Turkey M.T.A.), which is gratefullyac-

knowledged.

e f e r e n c e s

Bearth P 1965) Zur Entstehung alpinotyper Eklogite. Schweiz

Mineral Petrogr Mitt 45:179-188

Black PM 1974) Mineralogy of New Caledonian metamorphic

rocks III. Pyroxenes. Contrib Mineral Petrol 45:281-288

Brouwer HA, Egeler CG 1952) The glaucophane facies metamor-

phism in the Schistes Lustres Nappe of Corsica. Verh K Ned

Akad Wet 48:1 71


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