rajmahal traps petrogenesis

7/24/2019 Rajmahal traps petrogenesis

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E L S E V I E R

Che mical Geo logy 121 ( 1995 ) 73-90

C H E M I C L

G E O L O G Y

I S O T O P E G E O S C I E N C E

Petrogenes is and t imin g o f vo lcanism in

the Rajma hal flood basa lt prov ince northeastern India

A j o y K . B a k s i

D epa r tm ent o f Geology and Geophys ics , Louis iana S ta te U niversi ty , Baton Rouge , LA 70803, U SA

Received 26 April 1994; revision accepted 3 O ctober 1994

Abstract

A s u i t e o f r o ck s f r o m t h e R a j m a h a l - B e n g a l - S y l h e t T r a p s o f n o r t h e a s t e r n I n d i a h a s b e e n a n a l y z e d t o a s c e rt a in

th e t im in g a n d d u r a t io n o f v o l can i sm an d e lu c id a t e t h e i r p e t ro g en es i s . 4 Ar /3 9Ar s t ep - h ea t in g s tu d ie s i d en t i f i ed

sp ec im en s th a t su f f e r ed p o s t - c r y s t a l l iz a t i o n lo ss o f 4Ar an d in d i ca t e t h e Ra jm ah a l P r o v in c e was ex t r u d e d in ~ 2

M a a r o u n d 1 17 Ma ag o . T r ace - an d r a r e - ea r th - e l em e n t d a t a su g g est th e ex i s t en ce o f t h r ee d i f f e r en t t y p es o f m ag -

m as . R a jm a h a l q u a r t z t h o l e i i t e s we r e f o r m ed f r o m p r im ar y m e l t s , f o ll o win g co n s id e r ab l e g ab b r o i c f r ac t io n a t io n .

Ben g a l T r ap o l iv in e t h o l e i it e s r ep r e sen t l av as f o r m ed b y l a r g e p a r t i a l m e l t i n g o f m an t l e m a te r i a l , l e av in g g a r n e t i n

th e r e s id u e . A lk a li b a sa l ts i n t h e Ben g a l T r ap s ap p e a r t o r ep r e sen t p a r t i a l m e l t s o f m an t l e co n ta in in g L I L E - en -

r i ch ed sec t io n s , r a th e r t h an v e r y sm a l l ( < 2 % ) m e l t s o f a g a r n e t lh e r zo l i t e so u r ce . W h o le - r o ck ~ lSO- v a lues f o r

s l ig h t ly a l te r ed th o l e i i t e s fa l l i n t h e r an g e + 5.9 t o + 6 .6 %0 , i n d i ca t in g m an t l e - d e r iv ed m e l t s t h a t h av e su f f e r ed

m in o r c r u s t a l co n tam in a t io n ; two a lk a l i b a sa l t s, f o r m ed f o l lo win g co n s id e r ab l e c r y s t al f r a c t i o n a t io n o f p r im ar y

m ag m as , y i e ld v a lu e s o f ~ + 7 .2%0 . S r - N d i so to p ic an a ly se s sh o w two d i f f e r en t co n ta m in a t io n t r en d s , o v e r l ap p in g

th o se o b se r v ed in an ea r l i e r s t u d y o f su r f ace Ra jm ah a l q u a r t z t h o l e i it e s , w i th t h e m o s t p r im i t i v e m a te r i a l sh o win g

S7Sr/S6Sr ~ 0 .70400 , 143Nd/144Nd ~ 0 .512 80 a t 117 Ma ago . The B engal Tr ap o l iv ine tho le i i tes were fo r m ed fo l -

l o win g a ss im i l a t i o n o f high-S7Sr/S6Sr( g r an u l i t i c ? ) m a te r i a l . T h e m a in co n tam in a t io n t r en d in c lu d es q u a r t z t h o -

l e i it e s f r o m th e Ra jm ah a l T r ap s an d a lk a li b a sa lt s f r o m th e Ben g a l T r ap s . T h o le i i t e s , sh o win g co n s id e r ab l e i so to p ic

m o d i f i c a t i o n , su g g es t i n g es t io n o f a h ig h - Sr co m p o n en t , u n l ik e ly t o b e u p p e r - c r u s t a l m a te r i a l ; f o r th e a lk a l i b a sa l t s,

w i t h hi g h S r ( ~ 1 0 00 p p m ) a n d N d ( ~ 5 5 p p m ) c o n t e n ts , in c o r p o r a t i o n o f a fe w p e rc e n t o f e x o t i c m a t e r i a l ( i n

th e so u r ce r eg io n ? ) i s i n d i ca t ed . Ca r b o n a t i t e i s t h e p r o b ab le co n tam in an t , s t r en g th en in g th e p o s tu l a t ed l i n k b e -

tween f lo o d b a sa l t v o l can i sm an d ca r b o n a t i t e - l am p r o i t e s i n t h is a r ea . T h e o cc u r r en ce o f two l av as w i th r ev e r sed

magnet ic po lar i ty , in assoc ia t ion wi th the 4Ar /39Ar ages repor ted here in , suggests the ISEA reversed even t i s

d i sp l ay ed in t h e l av as o f t h e Ra jm ah a l T r ap s .

1 I n t r o d u c t i o n

T h e R a j m a h a l T r a p s o f n o r t h e a s te r n I n d i a

( F i g . 1 ) c o n s i s t o f a t l e a s t 2 5 f l o w s o f q u a r t z

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

w i t h a t o t a l t h i c k n e s s o f u p t o 6 0 0 m , c o n t a i n i n g

fEW]

i n t e r t r a p p e a n s h a l e s a n d s a n d s t o n e s , a n d o v e r l i e

( U p p e r T r i a s s i c ? ) s e d im e n t s a n d a m e t a m o r -

p h i c b a s e m e n t ; i n t h e e a s t e r n a r e a , t h e l a v a s d i p

g e n tl y b e n e a t h t h e G a n g e t i c a l l u v iu m ( P a s c o e ,

1 9 5 0; K l o o t w i j k , 1 9 7 1; M a h o n e y e t al ., 1 9 8 3 ) .

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

d e p t h s o f ~ 5 k m i n t h e G a n g e t i c a l l u v i u m t o t he

s o u t h e a s t o f t h e s u r fa c e R a j m a h a l e x p o s u r e s ( t h e

0009-2541/95/$09.50 1995 Elsevier Science B.V. All fights reserved

S S D 1 0 0 0 9 - 2 5 4 1

( 9 4 ) 0 0 1 2 4 - 3


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A . K . B a k s i / C h e m i c a l G e o l o g y 1 2 1 ( 1 9 9 5 ) 7 3 - 9 0

86 8 8

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i i i i i i i i i i i i i i : l

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S e d i m e n t s

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odimonts

T r o u g h S a K G ~ t s

Volconics

P r m - C o m l x i o n

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I N D I A

Fig . 1 . Geo log ica l map o f e a s t e rn Ind ia , s how ing expos ure s o f R a jm aha l an d Sy lhe t T raps , t oge the r w i th loca t ions o f d r i l l ho le s

f rom which B enga l T rap s ample s we re recove red ( 1 = B - 1 0 3 8 ; 2 = A - 5 3 1 ; 3 = A - 5 3 8 ; 4 = J a l a n g i ; 5 = B u r d w a n ; 6 = G a l s i ;

7 = D e b a g r a m ; 8 = N a d i a ) . ( M o d i f i e d f r o m B a k s i e t al ., 1 98 7. )

B enga l T raps , s ee F ig . 1 ) ha s b een de t a i l ed e l s e -

w h e r e ( S e n g u p ta , 1 9 6 6 ). B a s e d o n K - A r d a t e s

and geoch em i ca l s i m i la r i ti e s , i t w as sugges t ed t he

R a j m a h a l , B e n g a l a n d S y l h e t T r a p s ( l y in g ~ 4 0 0

k m t o t h e e a s t ) i n i ti a ll y c o v e r e d ~ 2 - 1 0 5 k m 2,

a n d f o r m a s i n g le f lo o d b a s a lt p r o v i n c e ( B a k s i e t

a l ., 1987 ) . A gen e t i c l i nk be t w een t he K e rgue l en

h o t s p o t a c t i v i t y i n t h e s o u t h e a s t I n d i a n O c e a n

a n d t h e f o r m a t i o n o f t h e R a j m a h a l T r a p s h a s

b e e n d e b a t e d i n t h e l i t e r a t u r e ( M a h o n e y e t a l . ,

1983 ; B aks i e t a l . , 1987 ; C u r ray and M una -

s inghe , 1991; Mu l l e r e t a l ., 19 93) .

A s t u d y o f s u r fa c e R a j m a h a l m a t e r i a l s h o w e d

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

s c a t t e r e d K - A r d a t e s ( M c D o u g a l l a n d M c -

E l h i n n y , 1 9 7 0 ) . T h e s e a u t h o r s s ug g e s t ed a r e-

l ia b le m i n i m u m a g e o f ~ 1 0 3 - 1 0 8 M a f o r t h e

o u t p o u r i n g o f t h e l a v a s [ d a t e s r e c a l c u la t e d t o t h e

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

o m m en de d by S t e i ge r an d J ~iger ( 1977 ) ] . A s i n -

g l e w h o l e - r o c k s p e c i m e n , s t u d i e d b y t h e 4Ar/

39Ar

i n c r e m e n t a l h e a t i n g t e c h n i q u e , i n d i c a t e d

pos t - c ry s t a l l i z a t i on l o s s o f 4 A r* , and an age o f

> 1 08 M a ( D a l r y m p l e a n d L a n p h e r e , 1 9 7 4 ).

T h e s e r e s u l t s i n d i c a t e t h e R a j m a h a l T r a p s w e r e

f o r m e d d u r i n g t h e C r e t a c e o u s N o r m a l S u p e r -

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

f i r m e d t h a t t h e R a j m a h a l T r a p s s h o w n o r m a l

m a gne t i c po l a r i ty ; t w o s i te s , ou t o f a t o t a l o f 27 ,

s h o w e d r e v e r s e d p o l a ri ty ; t h e l a v a u n d e r l y i n g o n e

o f th e s e s i te s s h o w e d a r e v e r s e d o v e r p r i n t o n a

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

l a r it y o f th e o v e r l y i n g l a v a i s n o t s p u r i o u s ( K l o o -

t w i jk , 1 9 7 1 ) . A s e c t i o n o f t h e R a j m a h a l T r a p s

d i s p la y s a s h o r t r e v e r s e d e v e n t w i t h i n t h e C r e t a -

c e o u s N o r m a l S u p e r c h r o n .

K - A r d a te s a n d m a j o r - / tr a c e - e le m e n t c h e m -

i s tr y f o r a s u i te o f r o ck s f r o m t h e R a j m a h a l - B e n -

g a l - S y l h e t T r a p s w e r e r e p o r t e d e a r l i e r ( B a k s i e t

a l. , 1987 ) . H e re i n , I p r e se n t t he r e su l t s o f fu r t he r

g e o c h e m i c a l a n d g e o c h r o n o l o g i c a l a n a l y s e s ca r -

r i e d o u t o n t h e s a m e r o c k s .

4Ar/39Ar

n c r e m e n -

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

t he i r c ry s t a l l i z a t i on ages . F o r pe t rogene t i c pu r -

p o s e s , t r a c e - a n d r a r e - e a r t h - e l e m e n t ( R E E ) c o n -

c e n t r a t i o n s a n d S r - N d - O i s o t o p i c c o m p o s i t i o n

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

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


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A.K. Baksi / Chemical Geology 121 (1995) 73-90

75

R a j m a h a l r o c k s ( M a h o n e y e t a l. , 1 9 8 3 ) , a s p li t

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

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

s t ra i n ts o n b o t h t h e t i m i n g o f v o lc a n i s m a n d t h e

p e t r o g e n e s i s o f t h i s p r o v i n c e .

2 A n a l y t i c a l t e c h n i q u e s

T h r e e s u r f a c e R a j m a h a l T r a p q u a r t z t h o l e i -

i te s , tw o a l ka l i ba sa l t s an d t w o o l i v i ne t ho l e i i t e s

f r o m t h e B e n g a l T r a p s , a n d a s i n g le q u a r t z t h o -

l e ii te f r o m e a c h o f t h e S y l h e t a n d B e n g a l T r a p s

w e r e s t u d i e d ( s e e F ig . 1 f o r s a m p l e l o c a t i o n

s i t e s ) . T r a c e e l e m e n t s a n d R E E w e r e d e t e r -

m i n e d b y i n d u c t iv e l y c o u p le d p l a s m a m a s s s p ec -

t r o m e t r y ( I C P - M S ) a t W a s h i n g t o n S ta te U n i -

v e r si ty ; b as a l t s t a n d a r d B H V O - 1 w a s a n a l y z e d t o

v e r i f y t h e a c c u r a c y o f t h e s e r e su l ts . S r - N d i so -

t o p i c c o m p o s i t i o n s w e r e m e a s u r e d a t T h e O p e n

U n i v e r s i t y ( U . K . ) b y C . J. H a w k e s w o r t h fo ll ow -

i n g s t a n d a r d t e c h n i q u e s ( L i g h t f o o t e t a l. , 1 9 9 0 ) ;

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

m i n e d b y J . R . O ' N e i l .

A na l y t i c a l da t a a r e l i s ted i n T ab l e 1 ; e a r l i e r IC P

d e t e r m i n a t i o n o f m a j o r a n d t r a c e e l e m e n t s

(B aks i e t a l. , 1987 ) a r e gene ra l l y i n goo d ag ree -

m e n t w i t h I C P - M S v a lu e s, e x c e p t f o r R b a n d N b

( < 1 0 p p m ) . S o m e o f t h e s p e c i m e n s r e p o r t e d o n

e a r l ie r , a n d f u r t h e r s t u d i e d h e r e i n , w e r e c r u s h e d

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

L S U ( D . K . P a u l , p e r s . c o m m u n . , 1 9 9 2 ) . H e n c e ,

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

du e t o con t am i na t i on [ s ee F rey e t a l. ( 1991 ) fo r

fu r t he r d i s cus s i on ] : C o (B aks i e t a l ., 1987 ) an d

T a ( t h i s s t u d y ) ; t h u s , N b / T a r a t i o s f o r s u r f a c e

R a j m a h a l r o c k s f a l l i n t h e r a n g e 4 - 7 , a s c o m -

p a r e d t o ~ 2 0 f o r th e o t h e r r o c k s ( s e e T a b l e 1 ) .

4Ar/39Ar

a g e d e t e r m i n a t i o n s w e r e c a r r i e d o u t

a t t h e U . S . G e o l o g i c a l S u r v e y a t M e n l o P a r k ,

C a l if o r n ia , U .S .A . ( M P ) a n d a t Q u e e n ' s U n i -

v e r s it y , K i n g s t o n , O n t a r i o , C a n a d a ( Q ) u s i n g

c o a r s e - c r u s h e d w h o l e - r o c k b as a lt s . F o r t h e f o r -

m e r s e t, th e T R I G A r e a c t o r w a s u s e d f o r n e u t r o n

i r r a d i a t io n , u s i n g S B - 3 B io t i te ( a g e = 1 6 2 .9 M a ;

L a n p h e r e e t a l., 1 9 9 0 ) a s t h e m o n i t o r . A l l 4Ar/

39Ar

a g e s r e p o r t e d h e r e i n a r e r e l a t i v e t o t h i s

s t a n d a r d . S t e p - h e a t i n g w a s d o n e b y r a d i o - f r e -

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

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

(S t ace y e t a l. , 1981 ) ; co r r ec t i on s fo r i n t e r f e r i ng

reac t i ons w e re app l i ed fo l low i ng Da l rym p l e e t a l.

( 1 9 81 ) . F o r t h e a n a l y s e s a t Q u e e n ' s U n i v e r s i t y ,

n e u t r o n i r r a d i a t i o n s w e r e a c h i e v e d a t t h e M c -

M a s t e r U n i v e r s i t y R e a c t o r , u s in g F i sh C a n y o n

T u f f - 3 B i o t it e ( a g e = 2 7 . 9 5 M a c a l i b r a te d t o

162.9 Ma for SB-3 Bio t i t e ; Baks i e t a l . , 1992) ;

d e t a il s o f th e t e c h n i q u e s u s e d a r e a v a i l a b l e e l se -

w h e r e ( B a k s i , 1 9 9 4 ) . S o m e o f t h e r o c k s w e r e

a n a l y z e d f o l l o w i n g a c i d w a s h i n g , i n v o l v i n g 2 0

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

i n d i l u t e n i t r i c a c id , f o l lo w e d b y r e p e a t e d w a s h -

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

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

t h e q u a l i t y o f t h e r e s u lt s o b t a i n e d ( B a k s i a n d

F a r r a r , 1 99 1 ) . A s u m m a r y o f t h e g e o c h r o n o l o g -

i ca l r e su l t s is p r e s en t e d i n T ab l e 2 .

In an ea r l i e r r epo r t (B aks i e t a l . , 1987 ) , t h i n -

s e c t i o n d e s c r i p t i o n s w e r e o b t a i n e d f r o m c o -

au t ho r s i n Ind i a , s ince on l y c ru sh ed sam p l e s w e re

ava i l ab l e a t L S U . 4Ar/39Ar d a t i n g a n d g e o -

c h e m i c a l s tu d i e s i n d i c a t e d t h a t s o m e o f t h e

w h o l e - r o c k s p e c i m e n s h a d s u f f e r e d s u f f i c ie n t al -

t e r a t i o n t o b e d i s c e r n i b l e in p e t r o g r a p h i c e x a m -

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

f r o m D . K . P a u l ( I n d i a ) , e x a m i n e d u n d e r t h e

m i c r o s c o p e a t L S U , c o n f i r m e d v is i bl e a l te r a t i o n

i n s o m e o f t h e r o c k s ( s e e t h e A p p e n d i x ) .

3. 4Ar]39Ar da t i ng r e su l t s

T h e d a t a w e r e e v a l u a t e d o n a g e s p e c t r a a n d is -

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

m o r e c o n t i g u o u s s t ep s , c a r r y i n g > 5 0 % o f th e t o -

t a l 39Ar,y i e l d in g a g e s t h a t a g r e e w i t h i n 9 5 % c o n -

f i d e n c e li m i t s u s in g a s t a n d a r d c o n f i d e n c e v a lu e

t e s t ( D a l r y m p l e a n d L a n p h e r e , 1 9 6 9 ) . S t e p s d e -

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

ch ron p l o t s (Y ork , 1969 ) ; s t r a i gh t li ne s y i e l d i ng

i n i t i a l a rgon r a t i o s no t s i gn i f i c an t l y d i f f e r en t

f r o m t h e a t m o s p h e r i c a rg o n v a l ue ( L a n p h e r e a n d

D a l r y m p l e , 1 97 8 ) a n d e x h i b i ti n g M S W D - v a l u e s

o f < 2 . 5 ( B r o o k s e t a l., 1 9 7 2 ) w e r e a c c e p t e d a s

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

p a r t ic u l a r , p l a te a u s e c ti o n s f o r i n t e r m e d i a t e - h i g h


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76 A.K. Baksi / Chemical Geology 121 (1995) 73-90

Table 1

Results of geochemical analyses on basalts from the Rajmahal Province

RM82-8 A-531 A-538 B-1038 Nadia Burdwan Jalangi Debagra m Galsi BHVO-I

Rock type QT QT QT QT QT OT OT AB AB Standard

Major elements

(%):

SiO : 52.0 51.8 51.7 51.3 50.0 49.7 49.8 51.9 51.9 50.2

A1203 14.9 15.1 14.0 14.6 14.3 15.2 15.3 15.6 15.3 13.8

TiO2 1.75 1.62 2.06 1.99 2.59 1.50 1.51 2.94 2.93 2.76

FeO* 10.9 10.7 12.4 13.0 13.7 10.7 9.75 9.1 8.95 12.1

MnO 0.19 0.20 0.20 0.29 0.23 0.17 0.17 0.14 0.12 0.17

CaO 10.7 11.1 10.6 9.2 10.1 10.1 10.5 6.4 5.95 I 1.5

MgO 6.5 6.5 5.6 6.2 5.75 9.9 10.1 5.9 6.8 7.3

K20 0.16 0.17 0.28 0.15 0.23 0.28 0.26 3.55 3.77 0.53

Na20 2.7 2.56 2.69 2.92 2.72 2.15 2.29 3.50 3.28 2.35

P205 0.19 0.23 0.29 0.29 0.38 0.29 0.29 0.92 1.00 0.28

LOI 0.5 0.3 0.4 1.9 1.5 2.5 1.9 2.5 3.5 n,d.

Minor elements (ppm ):

Ni 50 55 45 55 35 210 190 220 200 105

Cr 195 240 1 0 75 45 790 690 360 280 260

V 270 310 330 330 355 215 225 165 160 310

Sr 230 215 225 260 280 350 355 970 1,100 390

Zr 100 95 115 120 215 90 90 390 440 165

Zn 95 95 110 110 115 85 80 120 115 95

Rare, earth and trace elements (ppm):

La 7.2 5.9 9.5 10.5 22.8 10.9 10.5 44.9 54.6 16.0

Ce 16.5 13.7 21.4 23.1 46.2 23.0 22.3 94 111 37.4

Pr 2.4 2.1 2.9 3.2 5.7 3.1 2.9 12.0 13.9 5.3

Nd 12.1 10.6 14.5 15,0 25.8 14.6 13.5 54.0 59.8 24.7

Sm 4.4 4.0 5.1 4,9 7.2 4.3 3.9 12.7 13.2 6.1

Eu 1.6 1.4 1.8 1,7 2.4 1.7 1.5 4.2 4.3 2.1

Gd 5.2 4.8 5.9 5.6 7.6 4.4 4.2 9.5 9.9 5.9

Tb 0.96 0.87 I. 1 1,0 1.3 0.75 0.71 1.4 1.4 0.86

Dy 6.1 5.7 6.7 6,5 8.3 4.6 4.3 7.5 7.4 5.2

Ho 1.3 1.1 1.4 1,3 1.7 0.91 0.83 1.3 1.3 0.95

Er 3.3 3.3 3.7 3.8 4.7 2.4 2.3 3.3 3.3 2.5

Tm 0.45 0.44 0.50 0.50 0.62 0.32 0.30 0.41 0.41 0.35

Yb 2.8 2.7 3.1 3.1 3.9 2.0 1.8 2.4 2.4 2.0

Lu 0.43 0.41 0.45 0.47 0.59 0.30 0.27 0.36 0.34 0.30

Ba 70 60 100 365 165 165 160 625 755 130

Th 0.60 0.39 0.90 0.86 2.2 0.62 0.5 2.8 3,2 1.3

Nb 6.5 5.6 10.1 8.1 23.4 7.9 8.1 31 40 20

Y 31 30 38 36 45 23 22 34 35 28

Hf 2.6 2.1 2.8 2.7 4,7 2.1 2.0 7.5 8.2 4.3

Ta 0.9" 1.3 a 1.5 a 0.4 1.2 0.4 0.4 1.5 2.0 1.2

U 0.13 0.08 0.19 0.12 0.38 0.09 0.10 0.61 0.71 0.44

Pb 1.5 1.2 1.9 3.7 3.9 2.8 2.9 8. I 8.7 3.4

Rb 2.6 0.7 9.3 2.2 0.7 1.9 2.1 33 40 9

Cs 0.03 0.02 0.13 1.13 0.02 0.34 0.85 1.23 0.85 n.d.

(S~Sr/~TSr)r 0.70405 b 0.70403 0.70394 0.70606 0.70543 0.70804 0.70828 0.70439 0.70544 n.d.

~rqd~r~ (%o) +3. 1 b n.d. +2 .3 - 1.8 - 0 . 4 n.d. +0.1 +2.1 +0 .0 n.d.

6180 (%0) n.d. +5.9 +6.6 +4.4 +6.6 +6.6 +6.4 +7.3 +7. 0 n.d.

Major and mi nor elements analyzed by XRF for RM82-8and BHVO- 1, primarily by ICP for other rocks (see Baksi et al., 1987); rare-earth and trace elements

by ICP -MS at Washington State University. Major-element totals no rmalized to 100.0%; LOI = loss of weight on firing at 1000C for 20 rain. n.d. = not

determined. Sr and Nd isotopic co mposition s determin ed by C.J. Hawkesworth, co r ~ e d to 117 Ma. end{77 presumes present-day CHU R 143Nd/~*4Nd = 0.512638.

O-isotopic compositions det ermined by J.R. O'Neil. Rock types: QT = quartz tholeiite; OT = olivine tholeiite; AB = alkali basalt.

"Ta values in error (see text ).

bResults fro m Mahone y et al. ( 1983 ); Sr isotopic value lowered by 0.00010 for inter laborato ry dif ference in analyses reported f or NBS

987

Sr.


5/18

A.K. Baksi / C hem ical Geology 121 (1995) 7 3-90

T a b l e 2

S u m m a r y o f 4 A r / 39 A r s te p - h e a t i n g an a l y s e s o n w h o l e - r o c k b a s a l ts f r o m t h e R a j m a h a l P r o v i n c e

7 7

Debagram, alkali basalt, Bengal Traps:

K = 2 .8 5 % , K - A r d a t e = 1 1 7 _+ 2 M a a

M P , 0 .4 3 g , 8 s t e p s , t o t a l g a s a g e = 1 1 7 .6 _+ 1 .0 M a

p l a t e a u a g e ( s t e p s 4 - 7 , 6 4 0 - 8 9 0 C , 4 8 % g a s ) = 1 1 7 .0 _+ 0 .3 M a

i s o c h r o n r e s u lt : a g e = 1 1 6 .9 _+ 1 .1 M a , ( 4 A r / 3 9 A r ) i = 2 9 7 _+ 1 1, M S W D = 0 . 1 9

Galsi, alkali basalt, Bengal Traps:

K = 3 . 03 % , K - A t d a t e = 1 18 _+ 2 M a

Q , 0 .1 0 g , 9 s t e p s , t o t a l g a s a g e = 1 1 8 .1 _+ 1 1 M a

p l a t e a u a g e ( s t e p s 4-8 , 7 4 0 - 1 0 6 0 C , 8 8 % g a s ) = 1 1 7 .3 _+ 1 .0 M a

Q , 0 .1 2 g , 7 s t e p s , t o t a l g a s a g e = 1 1 8 .0 _+ 0 .6 M a ( d e g a s s e d a t 5 0 0 C f o r 1 5 m i n )

p l a t e a u a g e ( s t e p s

3-6 ,

8 0 0 - 1 0 8 0 C , 7 0 % g a s ) = 1 1 7. 1 _ +0 .3 M a

c o m b i n e d i s o c h r o n r e s u l t : a g e = 1 1 6 .2 _+ 0 .3 M a , ( 4 A r / 3 9 A r ) i = 3 2 9 _+ 3 0 , M S W D = 0 .6 4

A-531, quartz tholei ite , Rajm ah al Traps: K = 0 . 14 % , K - A r d a t e = 1 1 3 -+ 4 M a

M P , 1 .0 4 g, 8 s t e p s , t o t a l g a s a g e = 1 1 4 .4 _+ 0 .9 M a

m a r g i n a l p l a t e a u a g e ( s t e p s 5 - 7 , 7 8 0 - 9 3 0 C , 6 6 % g a s ) = 1 1 5 .5 1 .3 M a

i s o c h r o n r e s u l t : a g e = 1 1 8 .8 - + 1.5 M a , ( 4 A r / 3 9 A r ) L = 23 8 37 , M S W D = 0 . 6 5

Q ( A W ) , 1 .4 5 g , 7 s t e p s , t o t a l g a s a g e = 1 1 7 .3 _+ 1 .3 M a ( d e g a s s e d a t 4 5 0 C f o r 2 5 r a i n )

p l a t e a u a g e ( s t e p s

2-4 ,

6 5 0 - 7 9 0 C , 6 5 % g a s ) = 1 1 7 .5 0 . 5 M a

i s o c h r o n r e s u l t : a g e = 1 1 4 .7 _+ 3 .8 M a , . 7 ( 4 A r / 3 9 A r ) i = 4 2 0 -+ 2 2 0 , M S W D = 0 .0 5

A-538, quartz tholei ite , Ra jma ha l Traps: K = 0 . 2 3 % , K - A r d a t e = 8 8 -+ 2 M a

M P , 1 .0 6 g, 9 s t e p s , t o t a l g a s a g e = 8 5 .2 - + 2 .5 M a

n o p l a te a u a ge : m a x i m u m s t e p a g e 6 4 0 C ~ 1 1 0 M a

i s o c h r o n ( s t e p s

4-8 ,

6 4 0 - 9 5 5 C , 4 0 % g a s ) : a g e = 1 23 _+ 6 M a , ( 4 A r / 3 9 A r ), = 2 6 4 _ + 9 , M S W D = 2 . 5

Q ( A W ) , 1 . 30 g , 7 s t e p s , t o t a l g a s a g e = 9 1 . 7 - + 1 .3 M a ( d e g a s s e d a t 4 0 0 C f o r 2 5 m i n )

n o p l a t e a u , s t e p s

3 - 5 ,

33% gas , age ~ I 11 Ma

i s o c h r o n r e s u l t: a g e = 1 1 1 .3 _+ 2 . 2 M a , ( 4 A r / 3 9 A r ) ~ = 2 9 0 3 5 , M S W D = 0 . 5 6

RM82-8, quartz tholei ite , Rajm ah al Traps: K = 0 .1 2 % , n o K - A r d a t e

Q ( A W ) , 1 .4 0 g , 7 s t e p s , ' t o t a l g a s a g e = 1 0 3 .0 _+ 1 .8 M a ( d e g a s s e d a t 4 5 0 C f o r 2 5 m i n )

n o p l a t e a u , s t e p s

2 - 3

( 7 1 0 - 7 6 0 C , 4 3 % g a s ) , a g e ~ 1 11 M a


2-4 ,

5 3 % g a s ) : a g e = 1 10 .9 -+ 1 .6 M a , ( 4 A r / 3 9 A r ) i = 2 9 0 6 5 , M S W D = 5 .0

Burdwan, olivine tholeiite, Bengal Traps: K = 0 .2 2 % , K - A r d a t e = 1 2 2 _+ 3 M a

Q , 0 .5 5 g , 5 s t e p s , t o t a l g a s a g e = 1 1 8 .9 _+ 1 .5 M a

n o p l a t e a u

Q , 1 .0 2 g , 7 s t e p s , t o t a l g a s a g e = 1 2 1 .5 _+ 1 .2 M a ( d e g a s s e d a t 4 8 0 C f o r 2 5 m i n )

n o p l a t e a u , s t e p s 3 - 5 ( 7 2 0 - 8 9 0 C , 4 6 % g a s ) , a g e ~ 1 22 M a

i s o c h r o n r e s u l t: a g e = 1 1 6 .5 -+ 2 . 2 M a , ( 4 A r / 3 9 A r ) ~ = 4 0 5 -+ 5 0 , M S W D = 1 .0

Jalangi. olivine tholeiite, Beng al Traps:

K = 0 .2 1 % , K - A r d a t e = 1 0 7 _+ 3 M a

Q ( A W ) , 1 .0 5 g , 7 s t e p s , t o t a l g a s a g e = 1 1 3 . 8 _+ 2 . 0 M a ( d e g a s s e d a t 4 5 0 C f o r 2 5 m i n )

p l a t e a u a g e : s t e p s 2 - 4 ( 6 4 0 - 8 1 0 C , 6 3 % g a s ) , a g e = 1 1 6 .9 2 .3 M a

i s o c h r o n r e s u l t : a g e = 1 1 4 .2 -+ 2 .5 M a , ( 4 A r / 3 9 A r ) i

=

430 -+ 140 , M SW D = 1 .8

B- 1038, quartz tholeiite, Sylhe t T raps:

K = 0 . 1 2 % , K - A r d a t e = 1 0 8 4 M a

Q ( A W ) , I . 1 0 g , 6 s t e p s , t o t a l g a s a g e = 1 2 5 -+ 9 M a ( d e g a s s e d a t 5 5 0 C f o r 3 0 m i n )

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


4-6 ,

5 3 % g a s ) : a g e = 1 0 5 -+ 1 5 M a ,

( 4 A r / 3 9 A r ) i =

3 8 6 3 0 , M S W D = 0 . 6 4 .

Nadia, quartz tholeiite, Bengal Traps:

K = 0 .1 8 % , K - A r d a t e = 1 2 8 _+ 7 M a

M P, 1 .06 g , 9 s te ps , t o ta l ga s age = 21 6 .6 _+ 2 . I M a

n o p la t e a u , s a d d l e - s h a p e d ag e s p e c t r u m , m i n i m u m s t e p a g e ( 7 1 0 C ) ~ 1 1 6 M a

Q ( A W ) , 1 . 50 g, 7 s t e p s , t o t a l g a s a g e = 1 5 8 .7 _+ 2 . 5 M a ( d e g a s s e d a t 4 5 0 C f o r 2 5 m i n )

n o p l a te a u , m i n i m u m s t e p a g e ( 7 1 0 C ) ~ 1 2 0 M a

M P = d e t e r m i n e d a t U . S . G e o l o g i c a l S u r v e y , M e n l o P a r k , C a li f o r n ia , U .S . A . ; Q = d e t e r m i n e d a t Q u e e n ' s U n i v e r s it y , K i n g s t o n ,

O n t a r i o , C a n a d a ; A W = r o c k s w a s h e d i n d i lu t e n i t r i c a c i d p r i o r to f a st n e u t r o n i r r a d i a t i o n ( s e e t e x t ) . A l l e r r o r s l is t e d a t t h e 1 6

leve l .

a K - A r d a t a f r o m B a k s i e t al . ( 1 9 8 7 ) .


6/18

7 8


t e m p e r a t u r e s t e p s w e r e t a k e n t o r e fl e ct a r g o n r e -

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

m a i n e d c l o s ed t o K a n d A r lo s s o r g a in s in c e

c r y s t a l l i z a t io n . A l l e r r o r s a r e r e p o r t e d a t t h e 1 tr

l e v e l , i n t e r n a l p r e c i s i o n .

3 .1 . D e b a g r a m - - A l k a l i b a sa lt , B e n g a l T r a p s

R e s u l t s o f a n a n a l y s i s at M P , s h o w n i n F i g. 2 ,

e x h i b i t f e a tu r e s r e s u l t i n g f r o m

9AF

reco i l f o r f i ne -

g r a i n e d w h o l e - r o c k b a s a l t s ( T u r n e r a n d C a d s -

g a s , 1 9 7 4 ; B a k s i , 1 9 9 0 , 1 9 9 4 ) . T h e t o t a l g a s a g e ,

1 1 7 . 6 _ 1 .1 M a , i n g o o d a g r e e m e n t w i t h th e e a r-

l ie r K - A t v a l u e ( 1 1 7 _+ 2 M a ; B a k s i e t a l ., 1 9 8 7 ) ,

i n d i c a t e s n o m e a s u r a b l e a m o u n t o f

9Ar

h a s r e -

c o i l e d o u t o f t h e s a m p l e ; th e 9Ar l o s t f r o m t h e

K - r i c h p h a s e s d e g a s s e d i n s t e p s 2 a n d 3 ( s e e F i g .

2 ) r e c o il e d i n t o c o n t i g u o u s K - p o o r p h a s e s d e-

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

a g e , 1 1 7 . l + 0 . 3 M a , w a s c a l c u l a t e d f r o m s t e p s 4 -

7 o n l y , c a r r y i n g 4 8 % o f t h e t o t a l

9Ar; an

i s o -

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

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

a n d M S W D - v a l u e ( s e e T a b l e 2 ) . T h i s f l ow w a s

e r u p t e d a t 1 1 7 M a .

140

: E

13

a

1 2

C

~ 1 1 0

1 G a fo r t he f i r s t f ew pe rcen t o f ga s , d rop

m o n o t o n i c a l l y t o ~ 1 16 M a a t ~ 7 0 0 C a n d t h e n

i nc rea se s te ad i l y t o ~ 215 M a . S add l e - shap ed age

s p e c t r a a r e c o m m o n f o r i g n e o u s r o c k s c o n t a i n -

i ng excess 4Ar, a n d t h e m i n i m u m s te p a ge m a y

b e c l o s e t o t h e c r y s t a ll iz a t i o n a ge ( L a n p h e r e a n d

Da l rym p l e , 1976 ) . T he ve ry h i gh i n it ia l s tep ages

re f l e c t g ro s s

39Ar

r eco i l lo s s ou t o f c l ay m i ne ra l s .

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

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

spe c t ru m i s unusua l (F i g . 2 ) ; t he l ow es t - s t ep age

is ~ 1 20 M a , a n d m o s t i n t e r m e d i a t e - h i g h t e m -

pe ra t u re s t eps y i e l d ages o f ~ 155 M a . I t i s un -

c l ea r w h e t h e r t h e m i n i m u m s te p a g e s c a r ry g eo -

l og i ca l l y u se fu l i n fo rm a t i on , fo r t he h i ghe r -

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

t h a n ( ~ I 1 7 M a ) f o r o t h e r s p e c i m e n s i n th i s

s t udy . I f a c ry s t a l l iz a t i on age o f ~ 155 M a i s as -

s u m e d , it is n o t e d t h a t t h e 4 At* l o s t f r o m t h e

s e c o n d s t e p is b a l a n c e d b y th e o c c u r r e n c e o f e x -

c e s s 4 A r i n s t ep 4 ( s e e F ig . 2 ) . T h e m i n i m u m

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

n e a r b y R a j m a h a l v o l c a n i s m a t ~ 1 17 M a . T h i s

s p e c i m e n , r e c o v e r e d f r o m a d r i l lh o l e in t h e G a n -

g e t ic a ll u v i u m f r o m a d e p t h o f 4 .5 k m ( s e e A p -

p e n d i x ) , is i n h o m o g e n e o u s , b a s e d o n t h e 4 A r /

39Ar d a t i n g s t u d i e s a n d e a r l i e r K - A r r e s u l t s

(Bak s i e t a l ., 1 987) .

A l l t h r e e s u r f a c e R a j m a h a l q u a r t z t h o l e i i t e s

( A - 5 3 1 , A - 5 3 8 a n d R M 8 2 - 8 ) s h o w v e r y s i m i la r

g e o c h e m i c a l a n d i s o to p i c s i g n a tu r e s ( s e e b e -

l ow ) . F o l an d e t a l . ( 1993 ) c a r r i ed ou t 4 A r /39A r

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

w i t h m ass i ve a rgon l o ss ; the i r r e su l t s sugges t t ha t

t h e h i g h e s t ag e s in i n v e r t e d U - s h a p e d a g e s p e c t ra

f o r m a t r i x a n d p y r o x e n e fr a c t i o n s s h o u l d c lo s e ly

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


9/18

A.K. Baksi / Chemical Geology 121 (1995) 73-90 81

R a j m a h a l T r a p s , i t w o u l d b e h i g h l y f o r t u i to u s i f

r o c k s a s ( g e o c h e m i c a l l y ) s imi l a r a s

A-531, A-538

a n d R M 8 2 - 8 w e r e f o r m e d w i th a n i n t e rv e n i n g

p e r i o d o f ~ 6 M a ( 1 1 7 - 1 1 1 M a a g o ). I s ug g es t

t h e q u a r t z t h o l e i i te s o f th e R a j m a h a l T r a p s a n d

t h e a l k a l i b a s a l ts a n d o l i v i n e t h o l e i i t e s o f t h e

B e n g a l T r a p s w e r e a ll f o r m e d w i t h i n a s h o r t i n -

t e r v a l o f t i m e ( < 2 M a ) a r o u n d 1 17 M a a g o . T h e

S y l h e t T r a p s m a y b e o f s i m i l a r ag e , w h e r e a s t h e

d e e p e s t s a m p l e r e c o v e r e d f r o m t h e B e n g a l T r a p s

( N a d i a ) i s c o n s i d e r a b l y o l d e r. I n t h e a b s e n c e o f

s t r a ti g r a p h i c c o n t r o l f o r th e B e n g a l T r a p m a t e -

r ia l ( s e e A p p e n d i x ) , a n d t h e i r p o s i t i o n re l a ti v e

t o t h e R a j m a h a l T r a p s , i t is n o t p o s s i b l e t o e v a l -

u a t e t h e t e m p o r a l r e l a t i o n s h i p b e tw e e n t h e d i f-

f e r e n t r o c k t y p e s i n t h i s s t u d y . I t h a s b e e n s u g -

g e s t ed t h a t t h e e x t r u s i o n o f t h e R a j m a h a l T r a p s

i s g e n et i ca l ly l in k e d t o t h e f a u n a l e x t i n c t i o n e v e n t

a t t h e A l b i a n - A p t i a n s ta g e b o u n d a r y ( R a m p i n o

a n d S t o t h e r s , 1 9 8 8 ) . T h e l a t t e r e v e n t is d a t e d a t

11 1 + 1 M a ( O b r a d o v i c h , 1 9 9 2 ) a n d a p p e a r s to

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

R a j m a h a l P r o v i n c e .

4 . Trace / rare earth e l ement geochemistry

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

r o c k s s t u d i e d e a r l i e r ( B a k s i e t a l . , 1 9 8 7 ) , t o -

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

p les

( R M 8 2 - 8 )

f r o m t h e c o l l ec t io n o f M a h o n e y

e t a l. ( 1 9 8 3 ) . R E E c o n c e n t r a t i o n s a r e s h o w n

n o r m a l i z e d t o c h o n d r i t i c v a l u e s i n F i g . 3 a n d

t r a c e - e le m e n t d a ta a r e p r e s e n t e d i n t h e f o r m o f

P M N p l o t s [ a b u n d a n c e s n o r m a l i z e d t o t h e p r i -

m o r d i a l m a n t l e v a l u e s o f W o o d e t a l. ( 1 98 1 ) ] ,

i n F i g . 4 . M o d e l l i n g c a l c u la t i o n s w e r e c a r d e d o u t

f o r p a r ti a l ( b a t c h ) m e l t i n g a n d c ry s ta l ( R a y -

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

f ic i e n ts o f H a n s o n ( 1 9 8 0 ) f o r R E E a n d H e n d e r -

s o n ( 1 9 8 2 ) f o r o t h e r e l e m e n t s . T h e e s t i m a t e d

p r e c i s i o n ( _+ 5 %) o f t h e R E E a n a l y s e s r e p o r t e d

h e r e i n i n d i c a t e s t h a t o n l y > 1 0 % c r y s t a l f r a c -

t i o n a t i o n o f p l a g io c l as e w o u l d h a v e l e d t o d e t e c t-

a b le E u a n o m a l i e s in t h e R E E p l o ts .

k.

o

t-

o

t-

O

o

o

t r

100

b.

o

t-

ot-

O

o

IZ

10

1000

100

10

i i i i i i i i i i t i f i i

L a C e P r N d S m E u G d T b D y H o E r T m Y b L u

O T

. . . . . A 5 3 1

t 3 )

1 ' ' i , t i i i l t ~ i i i i

L a C e P r N d S m E u G d T b D y H o E r T m Y b L u

Fig. 3 . Rare-earth-elementc o n c e n t ra t i o n s in R a j m a h a l P r o v -

i n c e b a s a lt s n o r m a l i z e d t o c h o n d r i t i c v a lu e s : ( a ) a l l q u a r t z

t h o l e i i t e s ; a n d ( b ) B e n g a l T r a p a l k a l i b a s a lt s AB= a v e r a g e

o f D c b a g r a m a n d G a l s i ) a n d o l i v i n e t h o l e ii t e s ( O T = a v e ra g e

o f B u r d w a n a n d J a l a n g i ) c o m p a r e d t o l ea s t c o n t a m i n a t e d

R a j m a h a l q u a r t z t h o l e i it e .

4.1. Effects o f alteration

T h i n - s e c t i o n e x a m i n a t i o n ( s ee A p p e n d i x ) i n -

d i c a t e s a ll s a mp l e s h a v e s u f f e r e d a l t e ra t i o n ; t h i s

m a y b e r e f l e c t e d i n R b , K a n d B a c o n t e n t s i n

s o me i n s t a n c e s . I n p a r t i c u l a r , t h e q u a r t z t h o -

l e ii t e f r o m t h e S y l h e t T r a p s

(B-1038)

show s g ross

e n r i c h me n t i n B a ; i t s 8 1 S O - v a l u e i s u n u s u a l

( + 4 .4%0, see Tab le 1 ) a nd sugges t s in te rac t ion

w i t h g r o u n d w a t e r ( s e e b e l o w ) . L o s s o n i g n i t io n

v a l u e s ( s e e T a b l e 1 ) a r e l o w f o r t h e s u r f a c e

R a j ma h a l t h o l e i i t e s a n d g e n e r a l l y h i g h e r f o r a l l

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

T r a p s . T h e e f f e c ts o f a l t e r a t i o n ( o n t h e a l k a l i el e-

m e n t s ) s h o u l d b e m o s t a p p a r e n t i n t h e

4mF/39Ar

a g e s p e c t r a . N e g l e c t i n g t h e a r t i f a c t o f 3 9 A r r e c o i l

r e s u l t i n g f r o m f a s t n e u t r o n i r r a d i a t i o n o f f i n e -

g r a i n e d r o c k s , t h e a l k a l i b a s a lt s a n d A-531 s h o w

m i n i m a l e f f e ct s o f a l t e r a t i o n ( s e e F i g . 2 ) .


10/18

8 2 A.K. Baksi / Chemical Geology 121 (1995) 73-90

1 0 0

c

lO

O

E

el

o

er

i i i i i i i

.1 R b B a T h K N b L a C e S r N d P Z r S m T I Y

1 0 0 0

4~

g

~ ; 1 0 0

1 0

0

. : -

E

. . . . . . A B l

1 J

OT

A 531

b )

i i i i i i i I i i i t i i

. 1 R b

B a T h

K N b L a C a S t N d P Z r S m T I Y

F i g . 4 . I n c o m p a t i b l e e l e m e n t s n o r m a l i z e d t o p r i m o r d i a l

m a n t l e ( W o o d e t al ., 1 98 1 ) f o r b a s a l t s f r o m t h e R a j m a h a l

P r o v i n c e . Symbols as fo r F ig . 3 .

4 .2 . Q u a r tz t h o l e i i t e - R a j m a h a l , S y l h e t a n d

B e n g a l T r a p s

T h e s u r fa c e R a j m a h a l r o c k s

( A - 5 3 1 , A - 5 3 8

a n d R M 8 2 - 8 ) s h o w h u m p e d R E E p a t t e r n s , s li gh t

L a e n r i c h m e n t a n d ( L a / Y b ) N ~ 2 ( s e e F ig . 3 a ) .

B a s e d o n S r - N d i s o t o p i c d a t a , M a h o n e y e t a l .

( 1 9 8 3 ) s u g g e s te d t h a t R M 8 2 - 8 w a s a n u n c o n -

t a m i n a t e d s a m p l e ; s im i l a r P M N p a t t e r n s s u gg e st

A - 5 3 1 a n d A - 5 3 8 h a v e s u f f e r e d li tt le o r n o c o n -

t a m i n a t i o n , a l t h o u g h A - 5 3 8 s h o w s h i g h e r

a m o u n t s o f e le m e n t s R b t h r o u g h K ( F ig . 4 a ) .

U s i n g a m i d - o c e a n ri d g e b a s a lt ( M O R B ) - ty p e

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

a r e s a t i s f a c t o r i l y m o d e l l e d b y a s s u m i n g 2 0 %

b a t c h m e l t i n g f o l l o w e d b y f r a c t i o n a t i o n o f 1 0%

ol + 10% cpx + 5% p l ag . L o w l a rge - i on l i t hoph i l e

e l e m e n t ( L I L E ) c o n c e n t r a t i o n s f o r

A - 5 3 1 ,

a rel -

a t iv e l y u n a l t e r e d s a m p l e b a s e d o n 4 A r / 39 A r da t -

i ng r e su l t s , sugges t de r i va t i on f rom a dep l e t ed

m a n t l e s o u r c e ; b y c o n t r a s t , A - 5 3 8 a n d R M 8 2 - 8 ,

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

t i o n , s h o w s i g n i f i c a n tl y h i g h e r a m o u n t s o f L I L E .

T h e S y l h et T r a p r o c k

( B - 1 0 3 8 )

s h o w s a s i m i l a r

R E E p a t t e r n a t s li g h tl y e l e v a t e d l e ve l s - - ( L a /

Y b ) N ~ 2. 3. T h e q u a r t z t h o l e i i te f r o m t h e B e n -

g a l T r a p s ( N a d i a ) s h o w s h i g h e r a m o u n t s o f R E E

a n d m o d e r a t e t o s t ro n g e n r i c h m e n t o f th e l ig h t

R E E ( L R E E ) - - ( L a / Y b ) N ~ 4 . I t s h ow s a l ow

( ~ 1 0 ) S r / N d r a t i o a n d d i s p la y s a v e r y d i f f e r e n t

P M N p l o t ( F ig . 4 a ) , w i t h lo w le v e ls o f R b a n d

K , s t r o n g e n r i c h m e n t o f e l e m e n t s B a t h r o u g h Y ;

t h e h i g h v a lu e s o f T a a n d N b a p p e a r t o b e p ri -

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

t i o n d u r i n g g r in d i n g , a s t h e N b / T a r a t io i s ~ 2 0 ) .

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

o t h e r q u a r t z t h o l e i i t e s , a n d t o g e t h e r w i t h t h e

geoc h rono l og i ca l r e su l ts , i nd i ca t e N a d i a i s no t

c o m a g m a t i c w i th t h e o t h e r r o c k s in t h i s s t u d y .

4.3 . Ol i v ine tho le i i tes ~ Be ng al Traps

Ba sed o n low Y v a lues , Bak s i e t a l. ( 1987 ) a r -

g u e d t h a t B u r d w a n a n d J a la n g i w e r e f o r m e d b y

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

i n t h e r e s id u e . T h i s i s s u p p o r t e d b y t h e R E E p a t -

t e r n s , s h o w i n g s o m e e n r i c h m e n t i n t h e L R E E

[ ( L a / Y b ) N ~ 4 ] a n d t h e lo w e s t v a lu e s o f t h e

h e a v y R E E ( H R E E ) ( s e e F ig . 3 b ) f o r c o m p a r a -

b l e M g O c o n t e n t s . T h e P M N p l o t ( F i g . 4 b )

s h o w s e n r i c h m e n t b y f a c to r s o f 5 0 - 1 0 0 % o v e r A -

5 3 1

f o r e l e m e n t s R b t h r o u g h P a n d i s s a t is f a c to r -

i ly m o d e l l e d b y 1 0% b a t c h m e l t i n g o f a M O R B -

t y p e s o u r c e , f o l l o w e d b y f r a c t i o n a t i o n o f 1 - 2 %

o l i v in e a n d t i ta n o m a g n e t i t e . M g n u m b e r s ( ~ 6 8 ;

B aks i e t a l ., 1987 ) a l so i nd i ca t e t he se rocks w e re

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

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

L IL E l eve l s w ou l d be s ens i t i ve t o c ru s t a l con -

t a m i n a t i o n ( c f. W e a v e r a n d T a r n e y , 1 98 3 ) .

4 .4 . A l ka l i b a sa l ts - - B en g a l T ra p s

D e b a g r a m a n d G a l s i , w i t h l o w M g O , F e O *

c o n t e n t s a n d l o w C a O / A I z O 3 r a t io , w e r e f o r m e d

f r o m p r i m a r y m e l t s f o l l o w in g f r a c t io n a l c r y s t a l-

l i z a t ion o f 15% cpx + 5% p lag . T hese rocks , w h i ch

c a n b e t e r m e d h i g h - T i b a s a l ts s i n c e T i / Y ~ 5 00

a n d Z r / Y ~ 1 2, a r e h i g h e r t h a n t h e c u t - o f f v a l-

ue s o f ~ 410 a nd ~ 6 , r e spec t i ve l y , fo r such rocks

(H erg t e t a l ., 1991 ) . T h ey exh i b i t s t e ep l y d i p -

p i n g R E E p a t t e r n s [ ( L a / Y b ) N ~ 1 5 ] , a n d s h o w


11/18

A.K. Baksi / Chemical Geology 121 (1995) 73-90 8 3

enrichment of elements K through Sm (see Fig.

4b), indicating derivation as small partial melts;

relative depletion in HREE (see Fig. 3b) sug-

gests garnet was left as a residue. Though many

continental alkali basalts show strong composi-

tional similarities to ocean island basalts (see

Hole et al., 1993 and references therein), Deba-

gram and Galsi show unusual characteristics

(high La/Nb, Ba/Nb, K/ Nb and Ba/Th ratios)

and these continental basalts appear to be de-

rived from different mantle sources. Most o f the

elemental concentrations (e.g., REE) can be

patterned by very small ( ~ 2%) partial melts of

a garnet lherzolite source; serious divergences

occur for K, Ba (and Nb?), which call for ~3

enrichment. An alternate mode of derivation

would be higher ( > 10%) partial melting of me-

tasomatized sections of the mantle (cf. Menzies

and Murthy, 1980). The K/Rb ratios of Deba-

gram and Galsi (~ 750) are unusual for high-K

basalts; they appear to be primary, since undis-

turbed age spectra argue against loss of ~ 50% of

Rb, whilst leaving the K content undisturbed.

This value is different from alkali basalts from

other localities (~ 300-500), such as the Dec-

can Traps (India), Cameroon Line (West Af-

rica), Karoo (South Africa) and South Aus-

tralia. Hole et al. (1993) suggested Antarctic

Peninsula alkali basalts, exhibiting some unu-

sual chemical signatures, were derived as low-

degree melts of sections of the upper mantle that

had undergone a series of melt -extraction epi-

sodes. These rocks show high K/Rb ratios, but

exhibit considerably lower amounts of LILE,

lower Ba/K, U/Nb and Ba/Zr ratios than De-

bagram and Galsi; this may indicate differences

in the mode o f producing depleted upper-mantle

sections below eastern India during early Creta-

ceous time, and below the Antarctic Peninsula

~ 100 Ma later. Alternatively, melts derived from

regions containing amphibole (K-richterite),

which is unlikely to be a residue in the presence

of garnet, could lead to high K/Rb ratios; how-

ever, amphiboles derived from the upper mantle

typically contain < 2% K (Boettcher and O'Neil,

1980; Menzies and Murthy, 1980), which is too

low. Low-degree melts o f depleted sections of the

upper mantle containing metasomatized sec-

tions could have led to the composition exhib-

ited by Debagram and Galsi. If the Rajmahal

Province is genetically linked to hotspot activity,

the plume incubation model for large (continen-

tal) igneous provinces (Kent et al., 1992a) may

have relevance for the chemical diversity of the

lavas noted in this study.

Trace-element ratio plots, including Mahoney

et al.'s ( 1983 ) data, highlight some petrogenetic

features. In general, surface Rajmahal specimens

and quartz and olivine tholeiites from the Bengal

and Sylhet Traps form a coherent group. Ratios

such as Ce/Pb and Nb/ U should be useful petro-

genetic indicators, as they are largely unaffected

by partial melting processes, but are very differ-

ent in oceanic magmas and continental crust. Ce/

Pb ratios for samples herein fall in the range 8-

13, unlike oceanic lavas ( ~ 25 ), and may reflect

addition of continental material. Though the

quartz tholeiites and the alkali basalts show Nb /

U ratios close to oceanic magmas (~ 50; Hof-

mann, 1988 ), the olivine tholeiites show higher

( ~ 85 ) values; this cannot be due to incorpora-

tion of typical continental crust, which shows

Nb/U ratios of ,-, 10-15 (Hofmann, 1988). For

the alkali basalts, chondritic Sr/Nd values

( ~ 18 ) and high levels of Sr and Nd ( ~ 1000 and

,-,55 ppm, respectively) reflect primary fea-

tures. On a Zr/Nb vs. Y/Nb plot (Fig. 5a), all

tholeiitic samples define a trend that could rep-

resent mixing between a depleted source and an

enriched component; however, typical crustal

contaminants would lead to a subparaUel trend.

A source mixing-melting relation for the Rajma-

hal and Bengal Trap rocks is indicated on a La/

Yb vs. Ba/Yb plot (Fig. 5b); the Sylhet Trap rock

(B-1038) shows anomalous enrichment in Ba,

and Nadia departs significantly from the trend.

Evaluation on a Zr/Y vs. Ti/Y plot (Fig. 5c)

permits comparison with rocks from other Me-

sozoic flood basalt provinces. The quartz and ol-

ivine tholeiites lie close to the trend for partial

melting of a primitive mantle, and overlap with

the range for Deccan basalts. The alkali basalts

show Ti/ Y ratios similar to Paran~i high-Ti ba-

salts (Brazil ), Karoo picrites, and some Forma-

tion of the Siberian Traps (Russia), but with

considerably higher Zr/ Y values (see Lightfoot


12/18


10

n

Z ~

>

e j

[ ] .~ , / O

~ A a)

L

1 5

Z r / N b

2 7 1 2

Z r / Y

30

n

11111

B O ~ ~ . O Nb ) . ,

1 0 2 0

L a / Y b

7

1

0.0

: ~ L A M P R C ~ TE

mm

C~

Nh

, B l , - - o

S e d i m e n t

d )

i

0 1

R b / B a

0.

Fig . 5 . Se lec ted t ra ce -e l emen t ra t io p lo t s fo r R a jmaha l P rov ince f lood ba s a l t s . Legend: M83= s a m p l e s f r o m M a h o n e y e t a l .

( 1 9 8 3 ) ; QT=A-531, A-538, B-1038 (B), N a d i a ( N ) ; O T = B u r d w a n , J a la n g i; A B = D e b a g r a m a n d G a l s i ( a ll th i s s t u d y ) ;

P M = p r i m o r d i a l m a n t l e . E s t i m a t e d a n a l y t i c a l e r r o r s s h o w n b y bars i n ( a ) , ( b ) a n d ( c ) . F i e l d f or P o s t - A r c h a ea n T e r r e st r i al

S h a l e o f T a y l o r a n d M c L e n n a n ( 19 85 ) s h o w n i n ( c ) . F i e l d s f o r M O R B , a v e ra g e o c e a n i c b a s a lt , l a m p r o i t e a n d s e d i m e n t s h o w n

in (d ) a f te r Herg t e t a l. ( 1991 ) .

e t al ., 1993, f ig . 6 ) ; t h ey p l o t be t w een M O R B and

t h e l a m p r o i t e f ie l d , t y p i c a l o f p a r ti a l m e l t s l e a v -

i n g g a r n e t i n t h e r e s i d u e . T h r e e r o c k s ( R M 8 2 -

11, -12 a n d R- 20; M ah on ey e t a l. , 1983 ) sugges t

c o n t a m i n a t i o n o f m a n t l e - d e r i v e d m e l ts b y s e di -

m e n t s ; t h i s is s u p p o r t e d b y a R b / B a v s. T i / Y p l o t

( F ig . 5 d ) , w h e r e i n m o s t s a m p l e s s h ow M O R B

l ik e T i / Y r a t io s , b u t w i t h s li g h tl y l o w R b / B a v a l-

u e s . S i n c e m a n t l e m e l t i n g p r o c e s s e s d o n o t r e a d -

i l y a f f ec t R b / B a r a t i o s (H e rg t e t a l ., 1991 ) , th i s

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

reg ions .

5 . S r N d O i so t o p ic d a ta

W h o l e - r o c k ~ 8 0 - v a l u e s f o r t h e s l i g h t l y a l -

t e r e d q u a r t z t h o l e i i te s ( T a b l e 1 ) a r e t y p i c a l f o r

bas i c l avas ; a va l ue o f + 5 .9% 0 fo r A - 5 3 1 i s c lose

t o th a t o f u n c o n t a m i n a t e d m a n t l e - d e r i v e d m e l t s

(K yse r e t al ., 1982 ) . A - 5 3 8 show s a s l i gh t l y

h i gh e r va l ue o f + 6 .6% o; f r e sh g l as s i n t h i s l ava

f l ow , w as p robab l y a l t e r ed t o sm ec t i t e a t l ow

t e m p e r a t u r e , l e a d i n g t o a n i n c r e a s e o f ~ 1 %o i n

t h e 3 ~ 8 0 - v a lu e ( c f. M u e h l e n b a c h s a n d C l a y t o n ,

1 9 7 2 ) . T h e S y l h e t T r a p r o c k s h o w s a v e r y l o w

va l ue ( + 4 .4% o) a nd p oss i b l y re f l e c t s pos t - c ry s -

t a l li z a t io n r e h e a t i n g a n d r e c r y s t a l li z a t io n ( i n t h e

p r e se n c e o f l o w - ~ 8 0 g r o u n d w a te r ? - - s ee A p -

p e n d i x ) ; C a r l s o n ( 1 9 8 4 ) a r g u e d t h a t a C o l u m -

b i a R i v e r b a s a l t ( O R G R N 2 -3 ; W a s h i n g t o n ,

U . S . A . ) show i ng h i gh B a co n t en t and 8~ 80 -va l ue

o f + 4 .9% 0 , r e su l t ed f ro m a l t e r a t i on . T he o l i v i ne

t ho l e i it e s show va l ue s ~ 0 .5% 0 h i ghe r t han A- 531 ,

a n d r e fl e c t a d m i x t u r e o f a c ru s t al c o m p o n e n t t o

m a n t l e -de r i ved m e l ts . R esu l t s o f + 7 . 0 t o + 7 .3% 0

fo r t he a l ka l i ba sa l t s a r e w i t h i n t he r ange ob -

s e r v e d b y K y s e r e t a l. ( 1 9 8 2 ) f o r o c e a n i c al k al i

basa l t s .

F i g . 6 p r e s e n t s t h e S r - N d i s o t o p i c c o m p o s i -

t i o n f o r r o c k s f r o m t h e R a j m a h a l P r o v i n c e . P l o ts

o f B a / Z r v s. S r a n d N d i s o t o p i c r a ti o s ( F ig . 6 a


13/18


85

0.'/~

( a ) 3 e o ( b ) ~ . ( c ) 3

~a , .....

I -

o o

o ' ' ' ' . .. .. .

~ o o ~+ A

(1)

o o

0,706 0 ~ N

O N B ~--

o ~ o

CO

Z

r,. r,~ -3 .3

e ~ oQ

I , n . n ' * J * I i * 6

~ 7 o ~ 2 3 % ~ 2 ~ o . 7 o 3 o . 7 o 6 o . 7 o 9

B a / Z r B a / Z r ( 8 7 S r / 8 8 S r )T

Fig. 6. a and b. Sr and Nd isotopic comp osition for rocks from the Rajmaha l Province vs. Ba/Zr. Mo st samples define mixing

arrays; Nadia and B-1038 show anomalous positions (see text).

c. Sr-Nd isotopic comp osition plot;

straight lines

denote m ain contam ination trends (see text). Isotopic data corrected to 117

Ma.

Symbols as for F ig. 5.

a n d b ) s h o w li n e a r a r ra y s ; s p e c i m e n s N a d i a a n d

B - 1 0 3 8 a r e d i s c o r d a n t . T h e f o r m e r i s n o t c o m -

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

h a s s u f fe r e d g r o ss B a e n r ic h m e n t . T h e S r - N d

i s o t o p i c p l o t ( F i g . 6 c ) f i t s w e l l w i t h t h e r e s u l t s

o f M a h o n e y e t a l. ( 1 9 8 3 ) , a n d s u p p o r t s t h e c o n -

t e n t i o n t h a t t h e R a j m a h a l , B e n g a l a n d S y l h et

T r a p s a r e g e n e t ic a l ly l i n k e d . B a s e d o n t h e s e d a t a ,

A - 5 3 8

( a n d

A - 5 3 1 ? )

r e p r e s e n t s t h e l e a s t c o n -

t a m i n a t e d m a t e r ia l . M o s t o f t h e o t h e r s p e c i m e n s

f a ll o n t h e t r e n d ( s o l i d li n e in F i g . 6 c ) n o t e d b y

M a h o n e y e t a l. ( 1 9 8 3 ) . J a l a ng i , a n d a si n g le

s p e c i m e n f r o m t h e M a h o n e y e t al . ( 1 9 8 3 ) s t u d y ,

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

c o n s i d e r a b l y e n r i c h e d i n 878r/86Sr ( d o t t e d l i n e

i n F i g . 6 c ) . A s n o t e d e a r l i e r ( M a h o n e y e t a l . ,

1 9 8 3 ) , t h e l e a s t c o n t a m i n a t e d m a t e r i a l c o n t a i n s

~ 2 3 0 p p m o f Sr , w i t h

878r/86Sr ,-,

0 . 7 0 4 0 0 a n d

~ N d ( r ) ~ + 2 . 5 % o ; s u r f ace R a j m ah a l r o ck s s h o w -

i n g s i g n i f i c a n t c o n t a m i n a t i o n

( R M 8 2 - 1 1 , - 1 2

a n d R - 2 0 ; M a h o n e y e t a l. , 1 9 8 3 ) c o n t a i n ~ 5 0 %

m o r e S r.

E v a l u a t i o n o f t h e n a t u r e a n d s o u r c e o f c o n -

t a m i n a n t s r e f l e c te d in t h e S r - N d i s o t o p i c p l o t is

r e q u i r e d . T h e h i g h - a T S r / a 6 S r c o n t a m i n a t i o n

t r e n d ( d o t t e d l i n e in F i g. 6 c ) i n c l u d e s t h e B e n g a l

T r a p o l i v i n e t h o l e i i t e ( s ) a n d a si n g le s u rf a c e

R a j m a h a l r o c k

( R M 8 2 - 5 ;

M ah o n ey e t a l. , 1 9 8 3 ) .

A l l t h r e e r o c k s s h o w h i g h e r ( ~ 2 5 ) S r / N d r a ti o s

t h a n u n c o n t a m i n a t e d m a t e r i a l ( ~ 1 8 ) , a t y p i c a l

o f u p p e r - c r u s t a l m a t e r i a l ( g r an i t e s , sh a l e , co n t i -

n e n t a l s h i e l d a r e a s w i t h S r / N d < 1 5; T a y l o r a n d

M c L e n n a n , 1 9 8 5 ) . A g r a n u li ti c c o m p o n e n t i s

p o s si b le b a s e d o n hi gh ( ~ 3 0 ) S r / N d r a ti o

( T a y l o r a n d M c L e n n a n , 1 9 8 5 ) , t o g e t h e r w i th t h e

h i g h ( ~ 8 5 ) N b / U r a t io s o f t h e o l i v i n e t h o l e i it e s

( d e p l e t i o n o f U ? ) , t h e i r l o w R b c o n t e n t s ( s e e

T a b l e 1 ) a n d t h a t

o f R M 8 2 - 5

( 0 . 5 p p m ; M a h o -

n e y e t al ., 1 9 8 3 ) . G r a n u l i t e s f r o m v a r i o u s t e r-

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

o f t e n i n t h e d i r e c t i o n o f h i g h

87Sr/86Sr

r a t i o s

( T a y l o r a n d M c L e n n a n , 1 9 8 5 ) . O - i s o to p i c ra -

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

W es t e r n A u s t r a l i a av e r ag e ~ + 7.2% 00 ( A . F . W i l -

s o n a n d B a k s i, 1 9 8 4 ) ; a s i m i l a r c o n t a m i n a n t i s

i n l i n e w i t h t h e ~ ] S O - v a lu e s fo r B u r d w a n a n d J a -

l a n gi ( s e e T a b l e 1 ) . A s e c t i o n o f t h e R a j m a h a l

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

b a s e m e n t ( s e e Fi g . 1 ) a n d g r a n u l i t e s a r e f o u n d

i n t h e R a n i g a n j c o a l f i e ld s ~ 1 0 0 k m t o t h e s o u t h -

w e s t ( P a s c o e , 1 9 5 0 ) .

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

( s o l i d l i n e i n F i g . 6 c ) r e m a i n s p r o b l e m a t i c .

R o c k s R M l l , - 1 2 a n d R - 2 0 f r o m t h e M a h o n e y

e t a l . ( 1 9 8 3 ) c o l l e c t i o n a p p e a r t o h a v e a s s i m i -

l a t ed u p p e r - c ru s t a l m a t e r ia l ( s e d i m e n t ? - - s e e

F i g . 5 c a n d d ) . T h e s e r o c k s c o n t a i n ~ 5 0 % m o r e

S r ( a n d h i g h e r B a , N d , Z r a n d N b c o n t e n t s ) t h a n


14/18

8 6 A.K. Baksi I Chemical Geology 121 (1995) 73-90

t h e u n c o n t a m i n a t e d t h o le i it e s. I n c o r p o r a t i o n o f

u p p e r - c r u st a l m a t e r i a l a n d c o n s i d e r a b l e ( ~ 3 0 % )

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

t u r e s ; t h i s i s u n l i k e l y , s i n ce t h e m a j o r - e l em en t

c o m p o s i t i o n o f t h e s e r o c k s a r e s im i l a r t o th e u n -

c o n t a m i n a t e d o n e s ( M a h o n e y e t a l. , 19 8 3 , t a b l e

1 ) . A c o n t a m i n a n t w i t h h i g h e r S r c o n t e n t t h a n

u p p e r - c r u s t a l m a t e r i a l ( P o s t - A r c h e a n S h a l e , I -

a n d S - t y p e g r a n it e s; T a y l o r a n d M c L e n n a n ,

1 9 8 5 ) s e e m s t o b e r e q u i r e d . T h e B e n g a l T r a p a l-

k a l i b a s a l t s l ie o n t h e s a m e t r e n d ; t h e S r - N d i so -

t o p i c ra t io s o f D e b a g r a m a n d t h e u n c o n t a m i -

n a t e d q u a r t z t h o l e i i te s a r e s i m i la r , w h e r e a s G a l s i

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

P r e s u m i n g D e b a g r a m w a s si m i la r to t h e p r i m a r y

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

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

u p p e r - c r u s t a l m a t e r i a l, b u t r e q u i r e s > 1 0 % c r y s-

t a l f r a c t io n a t i o n t o p r o d u c e t h e h i g h e r S r a n d N d

co n t en t s o f G a l s i ; th i s i s u n l ik e l y , a s i t w o u l d h av e

l e d t o m e a s u r a b l y d i f f e re n t m a j o r - e l e m e n t c o m -

p o s i t i o n s f o r t h e t w o a l k a l i b a s a l t s ( s ee T ab l e 1 ) .

F u r th e r , G a l si sh o w s h ig h er C e / P b a n d N b / U

r a ti o s ( e n r i c h m e n t i n L R E E a n d N b ) t h a n D e -

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

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

s i t io n o f t h e a l k a li b a s a l t s m a y r e f le c t c o n t a m i -

n a t i o n i n t h e s o u r c e r e g io n , w h e r e i n c o r p o r a t i o n

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

S r a n d N d c o n t e n t s w o u l d a l t er tr a c e - e le m e n t a n d

i s o to p i c c o m p o s i t i o n s o f t h e r e su l ti n g m a g m a

m o r e r e a d il y th a n c o m p a r a t i v e l y l ar g er a m o u n t s

o f m a t e r i a l a t t h e c r u s t a l l ev e l. P l o t s o f 1 / S r v s .

8 7 8 r / 8 6 an d l i N d v s . ~NO ( F i g . 7 ) w e r e u t i l i z ed

t o g a i n i n s ig h t i n t o t h e n a t u r e o f th i s a d d i t i o n a l

i s o to p i c c o m p o n e n t , w h i c h h a d ~ 2 5 0 0 p p m S r,

8 7 S r / 8 6 ~ 0 . 7 1 0 , to g e t h e r w i t h ~ 1 2 0 p p m N d , eNd

~ - 1 1 % 0 . T h i s p l a c e s s e v e r e p e t r o l o g ic c o n -

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

m o s t l ik e l y c a n d i d a t e . G a l s i s h o w s c o n s i d e r a b l e

e n r i c h m e n t i n B a , S r a n d N b o v e r t h e u n c o n -

t a m i n a t e d D e b a g r a m ; t h e s e e l e m e n t a l e n r ic h -

I--

I .

U

r ~

0 . 7 1 3

0 . 7 0 8

0.703

~ ,. . -

' . . . . . . . . . . . . . . . . . . . . . . ' i . . . . . .

- - . . . .. . . .. . . . . . . . . . . . . .

- : . . . . . . . . . .

: 0 a ) . . . . . . . . I

I I

0 . 0 0 2 0 . 0 0 4

l / S r ( p p m 1 )

5 .,-

/ _ i 0

/ ~ Q

~ /

= i i . . .. .. . .. . .. . . . . . .. . .. .

. s i . . . .. . . . .. . i . . . . . . . . . . .

i - : . . . ~ - - - . . . . . . . .

o

Z

0

i i

- 1 5 . . . . . . . I . . . . . . I

0 . 0 0 . 0 4 0 . 0 8

l i N d ( p p m 1 )

Fig. 7. Sr-N d isotop icdata evaluated for mixing rends (circlesdenote Rajmahal Trap rocks;

squares

BengalTrap alkali basalts):

( a ) 1 / S r v s . S T S r/ ~6S r;

and (b) 1/Nd

vs .

end- Mixing lines suggest a c ontam inant with very high Sr and Nd con tents (se e text).


15/18


87

ments, along with the elevated Zr/ Hf and La/

Sm ratios noted for the alkali basalts, would re-

sult from contamination by carbonatitic mate-

rial (cf. Dupuy et al., 1992). Such rocks com-

monly display ~180-values of +6.5 to +7.5%0

(Conway and Taylor, 1969) and the oxygen iso-

topic composition of the contaminated material

would not be radically altered (note ~taO-values

of Debagram and Galsi in Table 1 ). The Sr and

Nd isotopic ratios of the postulated contaminant

is unusual for carbonatites, though values of

0.710 and -8%0, respectively, have been noted

for Proterozoic age material from Algeria (Ber-

nard-Griffi ths et al., 1988). These types of iso-

topic ratios occur dominant ly in continental rift

environments - - the Group II potassic rocks of

M. Wilson (1989). This contaminat ion hypoth-

esis should be evaluated in light of the postulated

link between the Rajmahal volcanism and car-

bonat ite-alkaline suites (Kent et al., 1992b; Sar-

kar et al., 1992) in the rift dominated tectonic

regime existing in northeast India at ~ 120 Ma

ago. A lamproite dyke in the Jharia coalfield,

~ 100 km southwest of the Rajmahal Traps, with

an Rb-Sr age of 110 Ma (Dayal et al., 1993),

postdates Rajmahal volcanism; contaminat ion of

the primary magma by carbonatitic material

would therefore appear to have occurred in the

upper mantle.

6 . C o n s t r a i n t s o n t h e a g e o f I S E A r e v e rs e d e v e n t

The Rajmahal Traps were formed during the

Cretaceous Normal Polarity Superchron ( ~ 119-

83 Ma). The existence of a brief interval of re-

versed polarity (< 100 ka) during this super-

chron, designated as the ISEA interval, has been

postulated to occur during Aptian time

(VandenBerg et al., 1978; Tarduno, 1990). 4Ar/

39Ar

data indicate that the ages for the magnetic

reversals close to the long Cretaceous Normal

Polarity (K-N) Superchron on earlier time-

scales are incorrect. Thus, Pringle et al. (1992)

suggest a min imum age of 121.8 Ma for the M0

anomaly, compared to ~ 119 Ma in an earlier

compila tion (Harland et al., 1982 ), and a mini-

mum age of 115 Ma for the ISEA reversed event.

The data reported herein suggest the Rajmahal

Traps were formed at ~ 117 Ma, and two lavas

exhibiting reversed magnetic polarity (Kloot-

wijk, 1971 ) were apparently erupted during the

ISEA reversed polarity interval.

Paleomagnetic studies place northeastern In-

dia at ~ 50S latitude at the time of eruption of

the Rajmahal lavas (McDougall and Mc-

Elhinny, 1970; Klootwijk, 1971 ). A possible ge-

netic link between the Kerguelen hotspot, the

Ninetyeast Ridge and the Rajmahal flood basalt

province, would involve critical evaluation of

geochronological and geochemical data for rocks

from these areas, and is beyond the scope of this

paper.

A c k n o w l e d g e m e n t s

This work was supported by a grant from the

Ardh endu-Roma Foundation. The suite of sam-

ples from the Rajmahal Province was obtained

through D.K. Paul of the Geological Survey of

India and a split of RM82-8 from Doug Macdou-

gall. Chris Hawkesworth supplied the Sr -Nd iso-

topic data and Jim O'Neil the O-isotopic values.

I am indebted to Brent Dalrymple for providing

access to the argon dating laboratory at Menlo

Park, arranging the fast neutron irradiation at the

TRIGA Reactor, and supplying a split of the SB-

3 Biotite standard; Elliott Kollman provided

considerable help in the laboratory. Edward Far-

rar permitted use o f the facilities at Queen's Uni-

versity (supported by grants from NSERC) and

arranged for the fast neutron irradiations at

McMaster University. Gary Byerly examined

thin sections of the rocks and provided detailed

comments. Fred Frey, Alan Brandon and an

anonymous reviewer made numerous construc-

tive comments on earlier versions of the

manuscript.

A p p e n d i x T h i n s e c t i o n d e s c r i p t i o n s

A-531 andA-538.

Surface exposures of Rajmahal Traps. Me-

dium-grained rocks with fresh feldspar and pyroxene pheno-

erysts set in a groundmass of plagioclase, pyroxene, opaques


16/18

88

A.K, Baksi / Chemical Geology 121 (1995) 73-90

an d g l assy m ate r i a l .

A-538

i s m o re f i n e g ra in ed an d d i sp l ay s

m o re g l as sy m ate r i a l . S m ec t i t e ex t en s iv e ly r ep l aces o l i v in e

an d ap p ear s t o f i ll v es ic l es (A-531) o r f o r m f r o m a l t e r a ti o n

of g lass (A-538). Bo th sam p les d i sp l ay a f ew v ery la rg e ( > 1

m m l r a m ) p h e n o c r y s ts o f p l a g io c l as e f el d s pa r , s h o w i n g

two -s t ag e c ry s t a l l iza t i o n .

B-1038. S u r face ex p o su re , S y lh e t T rap . F in e -g ra in ed ro ck

d i sp l ay in g r ec ry s t a l l i zed la t h s o f f e l d sp ar se t i n g ro u n d m ass

o f op a q u e s , p y r o x e n e , f e ld s p a r a n d a s e c o n d a ry m i n e r a l - -

e p i d o t e ( ? ) .

Nadia.

Ben g a l T rap , t ak en f ro m b o reh o l e d ep th o f ~ 4 4 4 0 m .

F in e-g ra in ed ro ck , w i th o l i v in e g ra in s ex t en s iv e ly a l t e r ed t o

sm ec t i t e . F resh f e ld sp ar l a t h s se t i n g ro u n d m ass o fp y ro x en e ,

a l t e r ed o l i v in e an d o p aq u es . P a t ch es o f c lay m in era l s ap p ea r

to h av e r ec ry s t a l l i zed .

Burdwan and Jalangi. Ben g a l T rap s , t ak en f ro m b o reh o l e

d ep th s o f ~ 2 8 5 0 an d ~ 2 6 9 0 m . Re l a t i v e ly f i n e -g ra in ed ro ck s ,

wi th o l i v in e ex t en s iv e ly a l t e r ed t o sm e c t i t e . F resh f e ld sp ar

p h en o cry s t s se t i n g ro u n d m ass o f au g i t e , o p aq u es an d p l ag io -

c l ase . J a l an g i d i sp l ay s so m e f i n e -g ra in ed seco n d ary m in era l

( ep id o t e ? ) an d a f ew v e in s f il l ed wi th zeo l i t e ( ? ) .

Debagram and Galsi. Ben g a l T rap s , t ak en f ro m b o reh o l e

d ep th s o f ~ 1 05 0 an d ~ 1 2 8 0 m . Med iu m -g ra in ed ro ck s , wi th

o l i v in e p h en o cry s t s ex t en s iv e ly a l t e r ed t o sm ec t i t e . P l ag io -

c l ase p h en o cry s t s ( s l i g h t l y a l t e r ed i n Ga l s i ) se t i n g ro u n d -

m ass r i ch i n g las s, a l t e r ed o l i v in e , p y ro x en e an d o p aq u es . D e-

b a g r a m d i s p l a ys m i n o r ( < 1 % ) a m o u n t o f ( p r i m a r y ) b i o t i t e

i n t h e g ro u n d m ass .

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an d S r i so to p i c ev id en ce . C o n t r i b . M in era l . P e t ro l . , 1 0 0 :

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