APPLICATION OF EPMA OF ALMANDINE GARNETS TO REGIONAL GEOLOGICAL PROBLEMS IN THE LOWER PALEOZOIC BASEMENT OF THE NW-ARGENTINE ANDES

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APPLICATION OF EPMA OF ALMANDINE GARNETS TO REGIONAL GEOLOGICAL PROBLEMS IN THE LOWER PALEOZOIC BASEMENT OF THE NW-ARGENTINE ANDES

G. Lissner, A. Willner

To cite this version:

G. Lissner, A. Willner. APPLICATION OF EPMA OF ALMANDINE GARNETS TO RE- GIONAL GEOLOGICAL PROBLEMS IN THE LOWER PALEOZOIC BASEMENT OF THE NW-ARGENTINE ANDES. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-745-C2-746.

�10.1051/jphyscol:19842171�. �jpa-00223845�

JOURNAL DE PHYSIQUE

Colloque C2, suppl6ment au n02, Tome

45,

f6vrier 1984 page

C2-745

APPLICATION OF EPMA OF ALMANDINE GARNETS T O REGIONAL GEOLOGICAL PROBLEMS IN T H E LOWER PALEOZOIC BASEMENT OF T H E NW-ARGENTINE A N D E S

G. Lissner and A.P. Willner

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MineraZogie, GeoZogisch-PaZCZont. I n s t i t u t , Mtlnster, F.R.G.

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A l m a n d i n e g a r n e t s w e r e s e p a r a t e d f r o m 22 s a m p l e s o f m e t a g r a y w a c k e s o f a u n i f o r m m o n o t o n o u s p e l i t i c - p s a m m i t i c s e q u e n c e . T h e s e m e t a s e d i m e n t a r y r o c k s c o n s t i t u t e t h e S i e r r a d e A n c a s t i ( A n c a s t i a n d S i e r r a B r a v a C o m p l e x ) , w h i c h f o r m s p a r t o f t h e s o c a l l e d m e t a m o r p h i c b a s e m e n t o f t h e NW-Argentine A n d e s . T h r e e m e t a m o r p h i c e v e n t s w e r e o b s e r v e d w h i c h a c t e d u p o n t h e s e m e t a s e d i m e n t a r y r o c k s d u r i n g t h e Lower P a l e o z o i c ,

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Ca s h o w s r a t h e r i r r e g u l a r c u r v e s p r o b a b l y i n f l u e n c e d by t h e o n l y o t h e r C a - b e a r i n g c o m p o n e n t o f t h e r o c k , p l a g i o c l a s e . The CaO c o n t e n t d i m i - n i s h e s f r o m c o r e t o m a r g i n i n a l l m e t a m o r p h i c g r a d e s . S t r o n g a s y m m e t r i c

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842171

C2-746

JOURNAL DE PHYSIQUE

d e p l e t i o n o f CaO i s e s s e n t i a l l y c o m p e n s a t e d by MnO.

I n t h e h i g h g r a d e a r e a g a r n e t s a r e unzoned ( f i g . 1 , t y p e B , No.30) o r a t l e a s t u n z o n e d i n t h e c r y s t a l c o r e . T h i s e f f e c t i s g e n e r a l l y i n t e r p r e t e d by r a p i d c r y s t a l g r o w t h o r a l t e r n a t i v e l y by a volume d i f f u s i o n p r o c e s s c a u s i n g a h o m o g e n i s a t i o n o f a n o r i g i n a l l y z o n e d i n d i v i d u a l ( 3 ) .

R e v e r s e z o n i n g a t t h e c r y s t a l m a r g i n s i s i n t e r p r e t e d a s a n e f f e c t of e l e m e n t a l e x c h a n g e w i t h t h e m a t r i x d u r i n g r e t r o g r a d e metamorphism ( 3 ) . It i s p r e d o m i n a n t l y f o u n d i n h i g h g r a d e a r e a s ( f i g . 1 , No.47), w h e r e Mn and Mg show s t r o n g e s t r e v e r s e z o n i n g . Q u a n t i t a t i v e a n a l y s i s o f s a m p l e 47 ( f i g . 1 ) d o c u m e n t s a d i r e c t c o r r e l a t i o n b e t w e e n b o t h e l e m e n t s . T h i s i s e x p l a i n e d by a b s o r p t i o n o f big by s u r r o u n d i n g b i o t i t e a n d b a c k d i f f u s i o n o f Mn i n t o t h e g a r n e t d u r i n g r e t r o g r e s s i o n .

I n medium a n d low g r a d e g a r n e t s t h e i n f l u e n c e of r e t r o g r a d e metamor- phism i s r e s t r i c t e d o n l y t o p u r e b o r d e r e f f e c t s , by w h i c h e s s e n t i a l l y a s l i g h t f l a t t e n i n g of t h e o t h e r w i s e s t e e p m a r g i n a l f o r m o f t h e c u r v e s r e s u l t s . An e x c e p t i o n i s s a m p l e 278 ( f i g . 1 , t y p e D) w i t h a n o r m a l b e l l s h a p e d c o r e and s t r o n g r e v e r s e z o n i n g a t t h e m a r g i n . T h i s g a r n e t

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( 1 ) WILLNER, A.P. ( 1 9 8 3 ) : T h e s i s , M a t . - n a t . Fak. Univ. N u n s t e r , 203p.

( 2 ) HOLLISTER, L . S . ( 1 9 6 6 ) : S c i e n c e m, ^1647-1651

( 3 ) WOODSWORTH,

G . J .

( 1 9 7 7 ) : Canad. M i n e r a l o g i s t 2 , 230-242

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