Continental Trondhjemites

8/13/2019 Continental Trondhjemites

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F. F. AB U EL ELA

diagramme s de Harker (par exemple, T iO 2, AI203, FeO *, M gO, Ca O, P205, Cr, Ni, V,Sr, Zr et Y) . La plupart de ces 616ments mo ntrent deu x tendances g6o chimique ssugg 6rant deux so urces magmat iques.Les roch es gabbro = quesso nt relativem ent enrichis en LILE (K, Rb, Sr et Ba) et appau vrisen HFSE (Nb, Zr , Ti et Y) ce qu i sugg~re un m agma de s ubd uct ion. Les donn6es deterres rares m ontre nt que les roches gabro iques ont un faible e nr ichissem ent enterres rares 16g~res [ (La/Y b)N =2.6 7-3 .91] et un appa uvr isseme nt en terres raresIourdes [ (Tb/Yb) N= 1 .42 -1 .47] , ce qu i sugg6re un m agma parent issu d une sourcemante l l ique re la t iveme nt pr im i t ive .Les diorites et les tona lites son t clairement calco-alcalines et pr~sentent des anoma liesn6ga t ives en Nb, Zr , Y, ce q ui sugg6re au ssi un magma de sub duct ion . El les son treli6es aux thron dhj6m ites cont inentales en termes de Rb-Sr, K-Na-Ca et aux grani tesd arc vo lcaniqu es en termes de Rb-(Y + Nb) et Nb-Y.Le com plexe int ru s i f de Wadi Dabr est analogue aux in t rus ion s mises en place dansdes arcs insulaires ensimat iques imm atures et repr6sente un m61ange des pro duitsde fus ion de roche s m antel l iques ( roches gabbro iques) et c rustales (dior i tes ettonal i tes) , modif6s par des processus de f ract ionnement .

(Received 14 M ay 199 6: rev ised vers ion received 8 February 1997 )

I N T R O D U T I O NA l t h o u g h t h e r e a re m a n y r e c o n n a i s s a n c e s tu d i e so f i s l and -a r c i n t r us i ve r ocks ( e . g . K es le r e t a l . ,1 9 7 7 ; M a s o n an d M c D o n a l d , 1 9 7 8 ; H i n e a n dM a s o n , 1 9 7 8 ) , d e t a i l e d s t u d i e s o f i n d i v i d u a li n t r us ion s ( e .g . Ch ivas , 19 78 ; P e r f it e t a l . , 1 9 8 0 ;K ay e t a l . , 1 9 8 3 ; W h a l e n , 1 9 8 5 ) a re r e l a t iv e l yu n c o m m o n . A n u n d e r s t a n d in g o f th e s e r o c k s h a si m p l i c a t i o n s f o r t h e g e n e s i s o f c a l c - a l k a l i n ev o l c a n i c r o c k s , w h i c h m a y r e p r e s e n t e x t r u s i v ee q u i v a l e n t s . A s i s t h e c a s e f o r c a l c - a l k a l i n ev o l c a n i c s u it e s , k n o w l e d g e o f t h e c h a r a c t e r i s ti c so f i s land -a r c in t r us i ve r ocks has g rea t r am i f ica t i onsf o r t e c t o n i c i n t e r p r e t a t i o n s in o l d e r o r o g e n i c b e l ts(e . g . Wha len and Cur r i e , 1982 ) .

I s land-arc vo lcan o-sed ime ntary rocks , w h ich arecom posed o f we ak ly m e t am orphosed ca lc -a lka l inein termedia te vo lcan ics (m ain ly o f and es i tes , dac i tes ,v o l c a n i c l a s t i c s o f c o m p a r a b l e c o m p o s i t i o n a n dsubord inate basa l t s and rhyodac i tes ) , a re a l readyes tab l ished and ident i f ied in the Egypt ian basem ent(e .g . S tern , 19 81; A bu EI-E la , 199 0, 199 2; Ab u EI-E la and Hassan , 1992 ) . How eve r , t he p lu t on i sm o fthe is land-arc in the Egypt ian base me nt is mu ch lesss t ud ied ( e . g . A bde I -Rahm an , 1990 ; A bu E I -E la ,1996 ) . Th i s pape r add resses t he ques t i on o f h owplu ton ic rocks in the Egypt ian base me nt are re la tedto the is land-arc assoc ia t ion us ing geo logy , m inera land wh o le - rock chem is t r y .

G E O L O G YT h e i n v e s t i g a t e d W a d i D a b r in t r u s i v e c o m p l e xi s a g a b b r o - d i o r i t e - t o n a l i t e s u i t e ( F i g . 1 ) w h i c hw a s m a p p e d a s a n e p i d i o r i t e - c o m p l e x (E l R a m l ya n d A k a a d , 1 9 6 0 ) , a s m e t a g a b b r o s a n d d i o r it e s( A k a a d a n d E s s a w y , 1 9 6 4 ; E l R a m l y , 1 9 7 2 ) a n d

a s a m e t a g a b b r o - d i o r i t e c o m p l e x ( H a s s a n a n dE s s a w y , 1 9 7 7 ) . T h e g a b b r o - d i o ri t e ro c k s h a v eb e e n a f fe c t e d b y t h r e e p r o c e s s e s , v i z .hyb r id i sa t i on , pos t -m agm at i c ( deu t e r i c ) a l t e ra t i ona n d r e g i o n a l m e t a m o r p h i s m a n d w e r e c l o s e l yf o l l o w e d b y e m p l a c e m e n t o f s y n o r o g e n i cg r a n i t e s ( H a s s a n a n d E s s a w y , o p . c i r . .

T h e p r e s e n t s t u d y r e v e a l s t h a t t h e g a b b r o i cr o c k s i n c lu d e p y r o x e n e - h o r n b l e n d e g a b b r o ,h o r n b l e n d e g a b b r o , q u a r t z - h o r n b l e n d e g a b b r o ,m e t a g a b b r o a n d a m p h i b o l it e . T h e y a r e d a rkg r e e n , s h o w m a r k e d v a r i a t i o n i n g r a i n s iz e , fr o mf i n e t o c o a r s e g r a i n e d , a n d v a r y i n t h e r a t i o o fm a f i c t o f e l s i c m i n e r a l s . T h e y a r e m a s s i v e a n dd e v o i d o f a n y t r a c e o f i g n e o u s l a y e r i n g . T h eg a b b r o i c r o c k s i n s o m e p l a c e s a r e h i g h l yt e c t o n i s e d w i t h d e v e l o p e d m i n o r f o l d s a n ds t r e t c h i n g l i n e a t i o n s .

T h e d i o r i t e a n d q u a r t z d i o r i t e a r e l o c a l l yf o l i a t e d , c a t a c l a s e d a n d h a v e a g n e i s s i c t e x t u r ea l o n g W a d i D a b r , W a d i U m M a r k h a a n d W a d iTham i l i El Za rka . The y con t a in r e l i c s o f pe gm at i t ea n d q u a r t z v e i n s w h i c h o c c u r a s p i n c h a n d s w e l lt e x t u r e s . T h e c o a r s e g r a i n e d d i o r i t e , q u a r t zd i o r i t e a n d a p p i n i t i c d i o r i t e a re i n t r u d e d i n t o t h eg a b b r o i c r o c k s . X e n o l i t h s o f g a b b r o a n dam ph ibo l i t e a re enc losed w i t h in t he d io r i t i c rocks .T h e d i o r i t e a n d q u a r t z d i o r i t e a r e d i s s e c t e d b yv e i n s a n d d y k e - l ik e b o d i e s o f m u s c o v i t e g r a n i t e s( p o s t - o r o g e n i c g r a n i t e s ) .

T h e t o n a l i t e s f o rm s m a l l m a s s e s a n d v e i n st h a t i n v a d e d b o t h g a b b r o i c a n d d i o r i ti c r o c k s .T h e y s h o w a g m a t i t e s t r u c t u r e a n d e n c lo s ed i f f e r e n t s h a p e s a n d s i z e s o f f i n e g a b b r o ,c o a r s e g r a i n e d g a b b r o a n d q u a r t z d o l e r i t ex e n o l i t h s . S o m e t o n a l i te v e i n s a re c a t a c l a s e da n d d i s p l a y a u g e n t e x t u r e .

474 Journal of African Earth Sciences


3/24

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

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Journal of African Earth Sciences 75


4/24

F. F. ABU EL ELA

T h e o p h i o l i t ic m e l a n g e , a l o n g W a d i D a b rc e n t ra l a n d s o u t h w e s t e r n p a r t s o f t h e m a p a r e a)

a n d a l o n g W a d i T h a m i li E I- H a m r a w e s t e r n p a r to f th e m a p a r e a ) , i s c o m p o s e d o f f o l ia t e d s l a t e sa n d p e b b l y s l a t e s t o p e b b l y m e t a m u d s t o n e s ,w i t h f r a g m e n t s o f d i ff e r e n t s i ze s o f u l t r a m a f i cs c h i s t s , s e r p e n t i n i t e l e n s e s , f i n e m e t a g a b b r o s ,m e t a b a s a l ts , q u a r t z it e s a n d m e t a g r e y w a c k e s . T h ec o n t a c t b e t w e e n t h e o p h i o l i t i c m e l a n g e s a n d t h eW a d i D a b r i n t ru s i v e c o m p l e x i s i n tr u s iv e , w h e r ethe l a t t e r i n t ruded a l ong t he f o l i a t i on p l anes o rt h e t r a c e o f b e d d i n g o f t h e m a t r i x o f t h e o p h i o l it icme l ange , w i t h i n j ec t i on o f ve i ns and ve i n l e t s o fg a b b r o i c a n d d i o r i t i c r o c k s . A l o n g t h e c o n t a c t sb o t h t h e o p h i o l i t i c m e l a n g e a n d t h e W a d i D a b ri n t rus i ve comp l ex a re f o l ded i n to isoc l i na l m i no rfo l ds .

T h e c o n t a c t b e t w e e n t h e i s la n d - a r cme tavo l can i cs o f Gaba l I g la E l I sw i d an d t he W ad iD a b r i n t r u s iv e c o m p l e x i s a l s o in t r u s i v e , w h e r et h e la t t e r e n c l o s e s x e n o l it h s o f th e f o r m e r A b uEI -E la , 1992) .

T h e W a d i D a b r in t r u s i v e c o m p l e x in t u r n i si n va d e d by y o u n g g a b b r o s p o s t - t e c t o n i ci n tr u s iv e p h a s e s ) a lo n g W a d i D a b r . T h e y o u n gg a b b r o s c o m p r i s e o l i v i n e g a b b r o , n o r i te ,g a b b r o a n d a n o r t h o s i t ic g a b b r o . T h e y c a u s e da t h e r m a l m e t a m o r p h i c e f f e c t u p t o h o r n b l e n d eh o r n f e l s - f a c i e s .

I n add i t i on , t he W ad i D a b r i n t rus i ve com p l ex ,y o u n g g a b b r o s a n d o p h i o l i t i c m e l a n g e a r e a l s oi n v a d e d b y l a t e t o p o s t - o r o g e n i c g r a n i t e s . T h ec o n t a c t s a r e s h a r p w i t h n o i n t e r a c t io n w i t h t h e s er o c k s , w h i c h p r o b a b l y i n d i c a te s t h a t t h e s e g r a n i t icm a s s e s w e r e e m p l a c e d a t s h a l l o w d e p t h s .

P E T R O G R P H YA b r i e f p e t r o g r a p h i c d e s c r i p t i o n o f W a d i D a b ri n tr u s i v e c o m p l e x is s u m m a r i s e d u n d e r th ef o l l o w i n g h e a d i n g s .Gabbroic rocksPyroxene hornblende g abb rosT h e s e r o c k s a r e f i n e t o c o a r s e g r a i n e d , d a r kg r e en i n c o l o u r a n d c o m p o s e d m a i n l y o fp l a g i o c l a s e , c l i n o p y r o x e n e a n d b r o w nh o r n b l e n d e . A c t i n o l i t e , c h l o r i t e , e p i d o t e , z o i s i t ea n d se r i c it e ar e s e c o n d a r y c o m p o n e n t s . I lm e n i t ea n d a p a t i t e a re c o m m o n a c c e s s o r i e s .U r a l it is a t io n a n d s a u s s u r i t is a t i o n a re c o m m o n .P o i k i o li ti c t e x t u r e i s c h a r a c t e r i s t i c .Hornb lende gabbrosT h e s e a r e m e d i u m t o c o a r s e g r a i n e d , d a r k g r e y -g r e e n in c o l o u r a n d c o m p o s e d m a i n l y o f

p l a g i o c l a s e a n d b r o w n to b r o w n - g r e e nh o r n b l e n d e . C h l o r i t e , e p i d o t e , z o i s i t e , s e r i c i t e ,c a l c i te a n d q u a r t z a r e s e c o n d a r y m i n e r a ls .A p a t i te , s p h e n e a n d il m e n i t e a r e c o m m o nacce sso r i es . C l i no py ro xen e i s l ack i ng . P o i k i o l i ti ca n d e q u i g r a n u l a r t e x t u r e s a r e c h a r a c t e r i s t i c .Quar tz hornblende gabbrosT h e s e a r e s i m i l a r t o h o r n b l e n d e g a b b r o s , e x c e p tf o r t h e p r e s e n c e o f s u b o r d i n a t e a m o u n t s o f q u a r t z ,a n d t h e y a r e c o a r s e t o v e r y c o a r s e g r a i n e d .MetagabbrosT h e s e r o c k s r e p r e s e n t t h e p r o d u c t o f l o w - g r a d er e g io n a l m e t a m o r p h i s m o f t h e p r e v io u s l ym e n t i o n e d r o c k t y p e s a n d a r e r e p r e s e n t e d b yt h e n e w m i n e r a l a s s e m b l a g e o f a l b i t e , e p i d o t e ,a c t i n o l i t e / c h l o r i t e , q u a r t z a n d s p h e n e . T h e f in eg r a i n e d m e t a g a b b r o s a re s c h i s t o s e a n d t h eo r ig i n a l t e x t u r e c a n n o t b e t r a c e d , w h e r e a s t h ec o a r s e g r a i n e d m e t a g a b b r o s s h o w t r a c e s o f t h eo r i g i na l po i k i l i t i c t ex tu re .Amph ibo l i tesT h e a m p h i b o l i te s o c c u r e i t h e r a s x e n o l i t h s w i t h i nd i o r i t e s a n d q u a r t z d i o r i t e s o r a s a l o c a l o u t c r o pa l o n g W a d i T h a m i li E I - H a m r a. T w o m a i n v a r i e ti e sa r e r e c o r d e d , viz. h o r n b l e n d e a m p h i b o l i t e s a n db i o t i te - h o r n b l e n d e a m p h i b o l i t e s . H o r n b l e n d ea m p h i b o l i t e s a r e f in e g r a i n e d , s c h i s t o s e a n dc o m p o s e d m a i n ly o f h o r n b l e n d e a n d p l a g io c l a s eA n 3 4 ). C h l o r i t e , s e r i c i t e a n d e p i d o t e a r es e c o n d a r y m i n e ra l s . S p h e n e a p a t i te a n d Fe

o x i d e s a re c o m m o n a c c e s s o r i e s . H o r n b l e n d e -b i o t i t e a m R h i b o l i te s a r e s i m i la r t o t h e p r e v i o u s l ym e n t i o n e d a m p h i b o l i te s e x c e p t f o r t h e p r e s e n c eo f m i n o r a m o u n t s o f b i o t i t e a n d q u a r t z .Dior i t ic rocksCoarse grained dior itesT h e y a r e c o m p o s e d m a i n l y o f p l a g io c l a s e a n dh o r n b l e n d e . L o c a l l y , c h l o r i t e p a r t i a l l y r e p l a c e sh o r n b l e n d e a n d q u a r t z i s a m i n o r c o n s t i t u e n t .H y p i d i o m o r p h i c t e x t u r e i s c h a r a c t e r i s t ic , A p a t i t e ,z i rc o n a n d F e o x i d e s a r e c o m m o n a c c e s s o r ie s .Gneissose dior itesT h e y a r e c o a r s e g r a i n e d a n d c o m p o s e d m a i n l yo f p l a g i o c l a s e , h o r n b l e n d e a n d m i n o r b i o t i t e .G n e i s s i c t e x t u r e is c h a r a c t e r i s t ic . A p a t i t e , z i r c o na n d i ro n o x i d e s a r e c o m m o n a c c e s s o r i e s .Qu ar tz d ior itesT e x t u r a l l y , t h r e e v a r i e t i e s o f q u a r t z d i o r i t e s a r ed i s t i n g u i s h e d , viz. h y p i d i o m o r p h i c q u a r t z d i o r i t e ,c a t a c l a s e d q u a r t z d i o r i t e a n d g n e i s s o s e q u a r t z

76 Journal of Af rican Earth Sciences


5/24

Geochemistry of an island arc plutonic suite

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78 Journal o f African Earth Sciences


7/24

Geochemistry of an island arc plutonic suite

d i o ri te . T h e y a r e c o a r s e g r a i n e d a n d c o m p o s e dm a i n l y o f v a r i a b l e a m o u n t s o f p l a g i o c l a s e a n dh o r n b l e n d e w i t h s u b o r d i n a t e b i o t it e a n d q u a r tz .A p a t i t e , z i r c o n , s p h e n e a n d F e o x i d e s a r ec o m m o n a c c e s s o r i e s .T o n a l i t i c r o c k sC a t a c la s e d m i c r o t o n a l i t e sT h e s e r o c k s o c c u r a s q u a r t z o - f e l d s p a t h i c v e in sc r o s s - c u t t i n g t h e g a b b r o i c a n d d i o r i t i c r o c k s .T h e y a r e f in e g r a i n e d a n d c o m p o s e d m a i n l y o fp l a g i o c l a s e a n d q u a r t z w i t h m i n o r a m o u n t s o fh o r n b l e n d e , c h l o r i t e a n d b i o t it e . Z i r c o n a n da p a t i t e a re a c c e s s o r i e s . A u g e n t e x t u r e isc h a r a c t e r i s t i c o f t h e p l a g i o c l a s e p o r p h y r o c l a s t s .C o a r s e g r a i n e d t o n a l i t e sT h e c o a r s e g r a i n e d t o n a l i t e s a re m a i n lyc o m p o s e d o f p la g i o c la s e a n d q u a r t z w i t hs u b o r d i n a t e a m o u n t s o f h o r n b l e n d e . C h l o r i t e ,b i o t i t e a n d m i c r o c l i n e a r e m i n o r c o n s t i t u e n t s .H y p i d i o m o r p h i c t e x t u r e i s c o m m o n . L o c a ll y,s o m e o f t h e s e r o c k s a r e f o l i a t e d . Z i r c o n a n da p a t i t e a r e a c c e s s o r i e s .

M I N E R L O G Y N D M I N E R L C H E M I S T R Yn a l y t i c a l m e t h o d s

M i c r o p r o b e a n a l y s e s o f th e s t u d i e d m i n e ra l s w e r ep e r f o r m e d a t t h e I n s t i t u t e o f E a r t h S c ie n c e ,U t r e c h t U n i v e r s i t y , T h e N e t h e r l a n d s , w i t h a J e o lJ X A 8 6 0 0 s u p e r p r o b e a n d T r a c t o r 5 5 0 0 E D,u s i n g w a v e l e n g t h d i s p e r s i v e t e c h n i q u e f o r N a ,C r , M n a n d F e , a n d e n e r g y d i s p e r s i v es p e c t r o m e t e r f o r M g , A I , S i, K , C a a n d T i.O p e r a t i n g c o n d i t i o n s w e r e 2 0 k V a c c e l e r a t i o nv o l t a g e a n d 1 0 n A s a m p l e c u r r e n t . M a t r i xc o r r e c t i o n s w e r e a p p l i e d u s i n g Z A F p r o g r a m .

P l ag ioc l ase i s an i mp or tan t phase i n t he gabb ro i cr o c k s o f t h e W a d i D a b r in t r u s i v e c o m p l e x .C h e m i c a l c o m p o s i t i o n s a n d s t r u c t u r a l f o r m u l a eo f p l a g i o c l a s e a r e l i s t e d i n T a b l e 1 . M o s tp l a g i o c l a s e s h o w s n o s i g n i f i c a n t o p t i c a l z o n i n g ;f o r t h i s r e a s o n o n l y c o r e s o f p l a g i o c l a s e w e r ea n a l y s e d . T h e c o r e c o m p o s i t i o n i s p r e s u m a b l yrep resen ta t i ve o f t he o r i g i na l c r ys ta l l i sa t i on ands h o u l d p r o v i d e t h e b e s t i n d i c a t i o n o f m a g m af rac t i ona t i on . P l ag ioc l ase co re comp os i t i ons in t heg a b b r o i c r o c k s r a n g e f r o m A n ~ s. 9 [ 1 0 0 C a /( Ca + N a + K )] t o A n 5 o fo r p y r o x e n e - h o r n b l e n d egabb ro , f r om A n~ . 4 t o A n384 f o r h o rnb l ende g abb roand f rom A n380 t o A n34 .s f o r qua r t z -ho rn b l end eg a b b r o . M e t a g a b b r o s h a v e A n c o n t e n t r a n g i n gfrom An18.3 to Anlo.9.C l i n o p y r o x e n e s o c c u r a s r e l i c t c o r e s w i t h i n t h ec a l c i c a m p h i b o l e s , o r a s s u b h e d r a l p l a t e s t h a t

enc l ose po i k i o l i t i ca l l y subhed ra l p r i sma t i c c r ys ta l so f p l ag i oc l ase , and have t he i r pe r i phe r i es a l t e redto ac t i no li t e . C hem i ca l com pos i t i ons and s t ruc tu ra lf o rmu l ae o f r e l i c t co res o f c l i nopy roxen e a re l is t edi n Tab l e 2 . The c l i nopy roxenes a re c l ass i f i ed asd iop s ide and aug i te (Wo41.o : En438:Fs73 to Wo47 s :En4~ s: Fs12.8)acco rd i ng t o t he c l ass i fi ca t ion schem eo f P o l de rvaa r t and H ess (195 1 ). The con ven t i ona lS i02-A I203 (F ig . 2 ) method o f Le Bas (1962) i sl ik e l y t o b e u s e f u l f o r s l o w l y c o o l e d i g n e o u s r o c k s( C o is h a n d T a y lo r , 1 9 7 9 ) . T h e c l in o p y r o x e n e s a r el oca ted i n t he suba l ka i ne f i e l d o f Le B as (1962 ) .I n t h e T i + C r v e r s u s T i d i a g r a m ( n o t s h o w n ) , m o s to f t he c l i nopy roxene s p l o t in t he vo l can i c a rc basa l tf i e ld o f Leter r ie r e t a l . ( 1 9 8 2 ) . I n a d d i t io n , m o s to f t he c t inopy rox enes o ccu py t he ca l c -a lka l i basa l tf ie ld (Fig. 3) on the Ti -AlCt~d i s c r im i n a t i o n d i a g ra mof Leter r ie r et a l . op . c i t . ) .A m p h i b o l e s a r e t h e d o m i n a n t f e r ro m a g n e s i a nm i n e r a l s i n t h e g a b b r o i c r o c k s . T h e y g e n e r a l l yd i s p la y w e l l - f o r m e d c r y s t a l m a r g i n s , g o o dc l ea v a g e s a n d u n i f o r m b r o w n a n d b r o w n i s hg r e e n c o l o u r . T h e b r o w n h o r n b l e n d e o c c u r s a ss u b h e d r a l p l a te s t h a t p o i k i o l i t i c a l l y e n c l o s ep l a g i o c l a s e c r y s t a l s , o r a s s u b h e d r a l c r y s t a l sc o n t a i n i n g r e l ic c o r e s o f c l i n o p y r o x e n e . T h eb r o w n h o r n b l e n d e i s p a r t i a l l y a l t e r e d t o g r e e nh o r n b l e n d e a n d c h l o r i t e .

T h e ch e m i c a l c o m p o s i t io n s a n d s t r u c t u r a lf o rmu l ae o f amph i bo l es a re l i s t ed i n Tab l e 3 . Thea m p h i b o l e s , a c c o r d i n g t o t h e c l a s s i f i c a t i o n o fL e a ke ( 1 9 7 8 ) a n d H a m m a r s t r o m a n d Z e n ( 1 9 8 6 ) ,a re ca l c i c amph i bo l es (F i g . 4 ) be l ong i ng t o t het s c h e r m a k i t e - a c t i n o l i t e s e r ie s . T h e b r o w nh o r n b l e n d e i n t h e p y r o x e n e - h o r n b l e n d e g a b b r o sr a n g e s f ro m t s c h e r m a k i t i c h o r n b l e n d e t om a g n e s i o h o r n b l e n d e , w h e r e a s th e b r o w nh o r n b l e n d e i n h o r n b l e n d e a n d q u a r t z - h o r n b l e n d eg a b b r o s i s c la s s i fi e d a s m a g n e s i o h o r n b l e n d e . T h eg a b b r o i c r o c k s c o n t a i n s u b s o l i d u s f i b r e s o f p a l eg reen ac t i no l i t e rep l ac i ng ea r l i e r c l i nopy roxene .B o t h s u b s o l i d u s a c t in o l i te a n d m e t a m o r p h i ca c t i n o l i t e , a s s o c i a t e d w i t h t h e m e t a g a b b r o s , f a l li n the ac t ino l i te f ie ld o f the amphibo le c lass i f i ca t iond iagram (F ig . 4 ) .

A c c o r d i n g t o L e a k e s ( 1 9 6 5 ) S i- Ti d i a g r a m , tod i f f e r e n t i a t e b e t w e e n i g n e o u s a n d m e t a m o r p h i ca m p h i b o l e s ( F ig . 5 ) , t h e b r o w n h o r n b l e n d e i n t h egabb ro i c rocks f a l ls w i t h i n t h e i gneous am ph i bo l esf ie l d , w h e r e a s t h e g r e e n v a r i e t i e s ( g r e e nho rnb l ende and ac t i no l i t e ) f a l l w i t h i n t he f i e l d o fm e tam orph i c am ph i bo l es . I n add i t i on , t he S i-T id i ag ram (F ig . 5 ) sh ow s t ha t t he ca l c ic ho rnb l end esd i sp l ay dec reas i ng T i w i t h i nc reas i ng S i , w h i chm a y s u g g e s t c o n t i n u a l c h e m i c a l c h a n g e w i t hm a g m a t i c d i f fe r e n t i a ti o n .



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The ho rnb lende ba romet ry i s based on thep r e ss u r e s e n s i t i v i t y o f t o ta l A I c o n t e n t inh o r n b le n d e , w h e r e A I c o n t e n t i n cr e a s e s w i t hpressure. Hornb lende c rys ta l l i sa t ion pressures forthe gabbro i c rocks have been ca l cu l a ted us i ngthe ca li b ra ti on o f Hamma rs t rom and Zen (1986) ,J o h n s o n a n d R u t h e r f o r d ( 1 9 8 9 ) a n d S c h m i d t(1992) . The p ressu re es t ima tes fo r t he gabbro i cr o c k s r a n g e f r o m 2 - 5 k b ( T a b l e 4 ) . Ageo the rmomete r has been dev i sed (B lundy andHo l l and , 1990) based on the A l ( i v ) con ten t o fh o r n b l e n d e c o e x i s t i n g w i t h p l a g i o c l a s e . O n l yt h o s e h o r n b l e n d e s t h a t s a t i s f i e d t h e i g n e o u s

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