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E L S E V I E R Tectonophys ic s 278 (1997) 31-45

TE TONOPHYSI S

Nd Sr and P b i s o top i c ev i den ce for d i vers e l ithos pher i c mant l e

sources o f East Afr ican Ri f t carbonat i tes

A . K a l t a , , E . H e g n e r b , M .

a t i r b

a Mineralogisches lnstitut, Universitiit Heidelberg, lm Neuenh eimer Feld 236 , D-69120 Heidelberg, Germ any

h lns titu tfiir Mineralogie, Petrologie und Geoc hemie, UniversiRit Tiibingen, W ilhelmstr. 56, D-72074 Tiibingen, Germany

Accepted 25 Ap ril 1997

Abstract

C a r b o n a t i t e s m a y p r o v i d e v a l u a b l e i n f o r m a t i o n o n m a n t l e s o u r c e c o m p o s i t i o n s a s t h e i r i s o t o p i c r a t io s a r e i n s e n s i t iv e to

c r u s t a l c o n t a m i n a t i o n . I n o r d e r t o p l a c e c o n s t ra i n t s o n m a n t l e s o u r c e s , n i n e t e e n s a m p l e s f r o m t h r e e M i o c e n e t o Q u a t e r n a r y

c a r b o n a t i t e a r e a s i n t h e E a s t A f r i c a n R i f t w e r e a n a l y s e d f o r t h e i r S r , N d , a n d P b i s o t o p i c c o m p o s i t i o n s . T h e s a m p l e s

f r o m K e r i m a s i ( n o rt h e r n T a n z a n ia ) , H o m a M o u n t a i n , a n d W a s a k i P e n i n s u l a ( b o t h L a k e V i c t o ri a , K e n y a ) a s a w h o l e

s h o w c o n s i d e r a b l e v a r ia t i o n s i n th e i r i s o t o p e r a t i o s ( 0 . 7 0 3 2 7 - 0 . 7 0 5 0 2 f o r 878r 86Sr~ 0.51249-0.51283 for 143Nd]144Nd,

1 8 . 7 2 - 2 0 . 4 1 f o r 2 6 p b ] 2 4 p b 1 5 . 5 2 - 1 5 . 7 8 f o r 2 7 p b / 2 4 p b and 39 .22- -40 .47 fo r 2spb /24pb) t ha t l i e be tween the i n fe r red

c o m p o s i t i o n s f o r H I M U ( h i g h 2 38 U /2 4 pb m a n t l e ) a n d E M I ( e n r i c h e d m a n t l e I ) c o m p o n e n t s i n m o s t i s o t o p e p l o t s . T h e

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

A l t h o u g h t h e i s o to p e d a ta d e f in e l in e a r ar r a y s i n S r - N d a n d P b - P b d i a g r am s , w h i c h s u g g e s t b i n a r y m i x i n g b e tw e e n H I M U

a n d E M I m a n t l e c o m p o n e n t s , n e i t h e r th e i s o t o p i c c o m p o s i t i o n s o f th e c a r b o n a t i t e s a s a w h o l e n o r t h e c o m p o s i t i o n a l r a n g e s

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

d a t a tr e n d s i n S r - P b a n d N d - P b i s o t o p e p lo t s a n d f r o m t h e l a c k o f c o n s i s t e n t e n d m e m b e r c o m p o s i t i o n s . T h e s e f e a t u r e s a r e

a l s o d i s p l a y e d b y p r e v i o u s l y p u b l i s h e d i s o t o p e d a t a f o r E a s t A f r i c a n c a r b o n a t i te s . I t i s t h e re f o r e s u g g e s t e d t h a t c a r b o n a t i te

c o m p l e x e s w i t h i n t h e E a s t A f r i c a n R i f t h a v e i s o t o p i c a l l y d i s t i n c t a n d s m a l l m a n t l e s o u r c e s t h a t a r e p r o b a b l y n o t a d e q u a t e l y

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

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

K e y w o r d s :

c a r b o n a t it e ; E a s t A f r i c a n R i f t ; N d - S r - P b i s o t o p e ra t i o s ; m a n t l e r e s e rv o i r s

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

C a r b o n a t it e s a r e m a n t l e - d e r iv e d m a g m a t i c r o c k s

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

a s s o c i a t e d w i t h i j o li t e s a n d m e l i l i t i t e s . T h e y c a n p r o -

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

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

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

* Corresponding author. Fax: +49-6221-564805.

c o n t a m i n a t i o n . T h i s i s d u e to : ( 1) v e r y h i g h S r a n d N d

c o n c e n t r a t i o n s ( B e l l a n d B l e n k i n s o p , 1 9 89 ) ; ( 2 ) l o w

s o l i d u s a n d l i q u i d u s t e m p e r a t u r e s c o m p a r e d t o m o s t

m a n t l e - a n d c r u s t a l - d e r i v e d r o c k s ( f o r a r e v ie w s e e

W y l l i e , 1 9 8 9 ) ; ( 3 ) l o w v i s c o s i t i e s ( T r e i m a n , 1 9 8 9 );

a n d ( 4 ) r a p i d a s c e n t to t h e s u r f a c e ( W i l l i a m s e t a l .,

1 9 8 6 ) .

A p p r o x i m a t e l y h a l f o f th e k n o w n c a r b o n a ti t e o c-

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

m a j o r i t y b e in g y o u n g e r t ha n 2 0 0 M a a n d a s s o c i a t e d

0040-1951/97/ 17.00 1997 Elsevier Science B.V. A ll rights reserved.

P I I S 0 0 4 0 - 1 9 5 1 9 7 ) 0 0 0 9 3 - 0


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3 A. Kalt et al./Tectonophysics 278 1997) 31- 45

w i t h t h e E a s t A f r i c a n R i f t s y s t e m ( W o o l l e y , 1 9 8 9 ) .

I s o t o p e s t u d i e s o f E a s t A f r i c a n c a r b o n a t i t e s i n d i c a t e

v e r y h e t e r o g e n e o u s m a n t l e s o u r c e s ( L a n c e l o t a n d

A l lbg re , 1974 ; Be l l and B lenk ins o p , 1987 ; N e l s o n e t

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

E a s t A f r i c a n c a r b o n a t i t e s w i t h a g e s b e t w e e n 0 a n d

113 M a to co r r e l a t e in a l inea r w ay on an ENd ve r s us

E Sr d i a g r a m ( E A C L : E a s t A f r i c a n C a r b o n a t i t e L i n e ,

F i g . 2 ) , s u g g e s ti n g a m i x i n g p r o c e s s . F o r o c e a n i c

i s l a n d b a s a lt s ( O I B ) , m i x i n g p r o c e s s e s a r e g e n e r a l l y

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

( D M M : d e p l e te d M O R B ( m i d - o c e a n ri d g e b a s al t)

ma n t l e ; H IM U : h igh 23S u/Z4pb ma n t l e ; EM I : en -

r i c h e d m a n t l e I; E M I I : e n r i c h e d m a n t l e I I; Z i n d l e r

and H ar t , 1986) . D ue to s imi la r va r i a t ions in S r ,

N d , a n d P b i s o t o p i c c o m p o s i t i o n s b e t w e e n c a r b o n -

a t i te s a n d b a s a l t s f r o m o c e a n i c i s l a n d s ( L a n c e l o t a n d

A l lbg re , 1974 ; Be l l and B lenk in s op , 1987 ; N e l s o n

e t a l . , 1988 ; D aw s on e t a l . , 1995) , man t l e s ou rces

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

c o n t e x t o f th e s e m a n t l e c o m p o n e n t s ( B e l l a n d B l e n k -

in s op , 1987 ; N e l s o n e t a l ., 1988 ; H o ern le and T i l ton ,

1991 ; S imo ne t t i e t a l ., 1995) . A s the EA CL i s s imi la r

i n s l o p e t o th e L o - N d l i n e o f H a r t e t a l. ( 1 9 8 6 ) i n

a S r - N d i s o t o p e d i a g r a m , t h e f o r m e r w a s i n t e r p r e t e d

a s a b i n a r y m i x i n g l i n e b e t w e e n H I M U a n d E M I

m a n t l e c o m p o n e n t s ( B e l l , 1 9 9 4 ) . F u r t h e r S r - N d a n d

a d d i t i o n a l P b i s o t o p e s t u d i e s o f E a s t A f r i c a n c a r -

b o n a t i t e s a n d r e l a t e d r o c k s w i t h a g e s u p t o 1 2 6 M a

w e r e i n l i n e w i t h t h i s l i n e a r c o r r e l a t i o n ( N e l s o n e t

a l . , 1988 ; K w o n e t a l ., 1989 , c i t ing unpub l i s h ed da ta

o f B e l l a n d T i l to n ; K e l l e r a n d K r a f f t, 1 9 9 0; B e l l a n d

P e te r s on , 1991 ; S imon e t t i and Be l l , 1993 , 1994a ,b ;

T i l t o n a n d B e l l , 1 9 9 4 , c i t i n g u n p u b l i s h e d d a t a o f

B e l l a n d T i l to n ; B e l l a n d D a w s o n , 1 9 9 5 ; D a w s o n e t

a l . , 1995) . O n thes e g rounds , i t w as conc luded tha t

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

2 0 0 M a , i n c l u d i n g t h o s e f r o m E a s t A f r i c a , a r e b e s t

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

n e n t s w i t h i s o t o p e c h a r a c t e r i s t i c s s i m i l a r t o H I M U

and EM I (Be l l , 1994 ; T i l ton and Be l l , 1994 ; Be l l

a n d D a w s o n , 1 9 9 5) .

T h e i n t e r n a l S r - N d - P b i s o t o p i c v a ri a t io n s o f i n -

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

A f r i c a n R i f t , h o w e v e r , h a v e b e e n d o c u m e n t e d i n

o n l y a f e w c a s e s ( B e l l a n d P e t e r s o n , 1 9 9 1 ; S i m o n e t t i

and Be l l , 1993 , 1994a ,b ; Be l l and S imone t t i , 1996) ,

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

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

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

a t i t e s . M o r e o v e r , t h e a s s u m p t i o n o f a m i x i n g m o d e l

t o e x p l a i n t h e i s o t o p i c v a r i a ti o n s i n E a s t A f r i c a n c a r -

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

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

p l i e s tha t the l a t t e r a r e con t inuous ly ava i l ab le in

a phys ica l o r chemica l s ens e . I t i s ques t ionab le

w he the r th i s app l i e s to con t inen ta l s e t t ings (P e r ry

et a l ., 1987; Pas l i ck e t a l ., 1995; Volk er e t a l ., 19 95) ,

s uch as the Eas t A f r i can R i f t . F o r nephe l in i t e s , a l -

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

par t o f the Eas t A f r i can R i f t , P as l i ck e t a l . ( 1995)

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

the i r S r , N d , and P b in the l i thos pher i c man t l e .

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

b e e x p l a i n e d b y m i x i n g o f t h e m a n t le c o m p o n e n t s

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

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

t ions (Wal lace and G reen , 1988 ; Egg le r , 1989 ; Wyl -

l i e , 1989 ; S w e eny , 1994) . The ve ry s ma l l vo lum es

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

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

m a n t l e c o m p o s i t i o n s a r e n o t c o m m o n l y a v a i l ab l e .

W e h a v e a n a l y s e d N d , S r , a n d P b i s o t o p i c c o m -

p o s i t io n s o f M i o c e n e t o Q u a t e r n a r y c a r b o n a t it e s a n d

g e n e t i c a l l y r e l a t e d r o c k s f r o m t h e K e n y a R i f t in o r -

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

s o u r c e s . W e h a v e f u r t h e r c o n f i n e d t h e s t u d y t o c a r -

bona t i t e s and to ve ry f ew s i l i ca te rocks ( i jo l i t e s and

mel i l i t i t e s ) f o r w h ich a gene t i c r e l a t ions h ip w i th ca r -

b o n a t i t e s i s i n d i c a t e d b y f i el d e v i d e n c e . N e p h e l i n i t i c

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

a t i t e s (Le Bas , 1987) a r e no t cons id e red he re a s the

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

m a g m a s i n g e n e r a l i s n o t c l e a r ( f o r a r e v i e w s e e B a i -

l ey , 1993) . M o reove r , f i e ld r e l a t ions in the Was ak i

P e n i n s u l a a n d i n t h e H o m a M o u n t a i n a r e a ( s e e S e c -

t i o n 2 ) i n d i c a t e a s p a t ia l a n d t e m p o r a l c o i n c i d e n c e o f

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

s a r il y a g e n e t i c l i n k b e t w e e n t h e t w o . C o n s e q u e n t ly ,

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

par i s on in the d i s cus s ion .

2 Geologica l s e tt ing and samples

T h e E a s t A f r i c a n R i f t s y s t e m e x t e n d s f r o m t h e

A f a r t r i p l e j u n c t i o n i n t h e n o r t h t o t h e Z a m b e s i


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A. Kalt et al. / Tectonophysics 278 1997) 31- 45 33

River i n sou the rn Afr i ca . There i s a d i s t i nc t b i -

f u r c a t i o n n o r t h o f t h e T a n z a n i a C r a t o n i n U g a n d a ,

sp l i tt i ng t he r i f t sys t e m in to a wes t e rn b ranch Albe r t

R i f t , T a n g a n y i k a R i f t ) a n d a n e a s te r n b r a n c h K e n y a

R i f t) . T h e e a s t e r n b r a nc h , c u t t i n g t h r o u g h K e n y a a n d

nor the rn Tanzan ia , rep resen ts an i n i t i a l s t age o f con -

t i nen ta l b reakup . Vo lcan ic ac t i v i t y s t a r t ed i n no r th -

e r n K e n y a i n t h e L a t e O l i g o c e n e M c D o u g a l l a n d

W atk ins , 1988) and p ropaga ted so u thw ard wi th time .

Geo phys i ca l s t ud i es Eb inger e t a l ., 1989 ; Ac haue r

et a l . , 1994) as wel l as s t ra t igraphic and s t ructural

cons t ra in t s Smi th , 1994) suppor t a ma in ly ac t i ve

m o d e o f r i f t i n g d r i v e n b y a m a n t l e p l u m e . C h e m i -

ca l and i so top ic com pos i t i ons o f r i f t- re l a t ed basa l ti c

rock s and basa ni tes Class e t a l . , 1994; Volker e t a l . ,

1995) a re cons i s t en t w i th t he p resence o f a man-

t l e p l u m e u n d e r n e a t h n o r t h e r n K e n y a . T h e y f u r t h e r

revea l i n t e rac t ion o f t he p lume mate r i a l w i th l i t ho -

spher i c ma n t l e C lass e t a l ., 1994) , w i th a s theno-

s p h e r ic M O R B - t y p e m a n t l e m i d - o c e a n r i d g e b a s a lt )

a n d w i t h l o w - t e m p e r a t u r e m e l t i n g d o m a i n s o f E M

I- type in t he t he rm al boun dary l aye r Vo lke r e t a l .,

1995).

The samples u sed in t h i s s t udy were co l l ec t ed

f rom two reg ions : t he sho res o f Lake Vic to r i a ,

K e n y a , a n d K e r i m a s i v o l c a n o , n o r t h e r n T a n z a n i a

F ig . 1 ) . The Lake Vic to r i a reg ion fo rms pa r t o f

the Kav i rondo b ranch o f t he Ken ya Ri f t F ig . 1 ).

M a g m a t i s m i s o f M i o c e n e t o P l i o c e n e a g e B a k e r ,

1987). I t p roduced p re dom inan t ly nephe l in i t e s a long

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

l i ti c nephe l in i t e s and phono l i t e s Mc Cal l , 1958).

T h e T e r t ia r y m a g m a t i c r o c k s i n t ru d e d a n d e x t r u d e d

o n t o P r e c a m b r i a n b a s e m e n t o f t h e T a n z a n i a C r a -

ton Shack le ton , 1993) . The nephe l in i t e s K i s ing i r i

Vo lcan ic Group , M cCal l , 1958) fo rm dee p ly e roded

f lows and minor agg lomera t es , whereas t he l a t e -

s t age phono l i t i c nephe l in i t e s and phono l i t e s fo rm

iso l a t ed p lugs . W i th in t he Lake Vic to r i a reg ion , t he re

a re two sepa ra t e a reas con ta in ing ca rbona t i t e com-

p l e x e s . T h e a r e a n o r t h e a s t o f H o m a B a y , r e f e r r e d

t o a s H o m a M o u n t a i n a r e a , c o m p r i s e s t h e H o m a

Mounta in ca rbona t i t e cen t re a s we l l a s seve ra l o the r

d i s t i nc t ca rbona t i t e p lugs F ig . 1 ). The a rea wes t

a n d n o r t h w e s t o f H o m a B a y , r e f e r r e d t o a s W a s a k i

P e n i n s u l a , c o m p r i s e s t h e c o m p l e x e s o f O k u g e , U s -

ak i , Soko lo , Sou th Rur i and Nor th Rur i F ig . 1 )

a n d o t h e r s m a l l e r c a r b o n a ti t e p lu g s . T h e c o m p l e x e s

cons i s t o f i n t rus ive ca rbona t i t e s , ca rbona t i t e b recc i a

o r agg lomera t es , me l i l i t i t e s and i j o l i t e s . Carbona t i t e

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

ou tc rops F ig . l ) bu t age de t e rm ina t ions po in t t o a

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

v o l c a n ic s e r ie s p r o b a b l y f o r m e d b e t w e e n 2 2 a n d 1 6

M a a s i n f e r r e d f r o m L o w e r M i o c e n e f o s s i l a s s e m -

b lages i n t e rca l a t ed wi th Ki s ing i r i po rphyroc l as t i c s

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

and b io t i t e fo r a rev i ew see Baker e t a l. , 1970). An

a g e o f 1 3 M a f o r a p h l o g o p i t e f r o m a c a r b o n a t i t e

s a m p l e o f H o m a M o u n t a i n B e l l a n d B l e n k in s o p ,

1987) p roba b ly m arks t he o nse t o f ca rbona t i t i c ac t iv -

i t y in t he K av i rond o Ri f t .

The Ker imas i vo l cano in no r the rn Tanzan ia s t rad -

d l es t he wes t e rn fa u l t sca rp o f t he K eny a Ri f t . I t re st s

on Neog ene l avas f l oo r ing the r i f t Daw son , 1992).

T h e v o l c a n o i s y o u n g e r t h a n t h e m a j o r f a u l t in g p h a s e

a t 1 .2 Ma and has bee n d i sp l ace d by l a t e r fau l t ing a t

0 .37 M a B.R Daw son , 1992). I t cons i s t s o f ca rbon-

a t i t e and nephe l in i t e py roc l as t i c s and f lows Ke l l e r

and Spe t t e l, 1995). An ag e o f 0 .5 M a i s g iven fo r the

Ker im as i by Pas l i ck e t a l . 1995) .

A l t o g e th e r , n i n e t e e n s a m p l e s w e r e c o l l e c te d a n d

ana lys ed Tab le 1 ) . In t he La ke Vic to r i a reg ion , sam-

p l e s w e r e t a k e n f r o m t h e H o m a M o u n t a i n a r e a a n d

f r o m t h e c o m p l e x e s o f O k u g e , U s a k i , S o k o l o , S o u t h

Rur i and Nor th Ruff a l l W asak i Pen insu l a , F ig . 1 ) .

F r o m e a c h d i s t in c t c a rb o n a t i te c o m p l e x a t l e a s t tw o

samples excep t fo r Nor th Ruff , W asak i Pen insu l a )

were se l ec t ed fo r S t, Nd , and Pb i so top ic ana lys i s .

The s tudy i s re s t r i c t ed t o ve ry young ca rbona t i t e s


4/15

34 A. Kalt e t a l. / Tec tonophys ics 278 1997) 31-4 5

r~ Archean

I ~

crust

ajor rift

boundary

DDIS

faults

BEB

Albert

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Nyaaa Rift

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centres

approximate border

of Tanzania Craton (TC)

and MozambiqueBelt (MB

/

0 0 K U G E

~ o

NORTH

RURI

) O

MB

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oso nlk ~ RURI

Oldolnyo Lengal::

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RECENTCOVER

Fig . 1 . ( a ) S implif ied tec ton ic map of the Eas t Af r ican Rif t sys tem, modif ied a f te r Smith and Mos ley (1993) and Shackle ton (1993) ,

with approxim ate loca t ions of the ca rbona t i te occur rences cons idered in th is s tudy. 1 = Napak (E-Uganda) , 2 - - Ka lyango, Rusekere

(W-Uganda) , 3 = Bukusu , Tororo, Sukulu (E-Ugan da) , 4 = Hom a Mounta in a rea and Wasaki Peninsu la (Lake Vic tor ia, Kenya) , 5 =

Kanga nkunde , Chi lwa (Malawi) , 6 = Rufu nsa (Zambia) . (b ) S implified geologica l map of the Ken ya Rif t and ad jacent a reas in wes te rn

Kenya , eas te rn Uganda , and nor thern Tanzania , modif ied a f te r Dawson and Smi th (1988) and Pe te r son (1989) , with ca rbona t i te loca t ions .

(c ) Geologica l ske tch map of the Homa Mounta in a rea and Wasaki Peninsu la , Lake Vic tor ia , modif ied a f te r McCall (1958) and Le Bas

(1977).

Hegn er e t a l . 1995) . I so tope r a tios were de te rmined

on a F inn igan-MAT 262 m ul t i co l lec to r mass spec-

t romete r employ ing a s t a t i c co l l ec t ion mode fo r S r

and Pb and a dynamic mul t ip l e mass co l l ec t ion rou-

t ine for Nd. Values for Nd, Sr , and Pb s tandards and

analyt ical precis ion are included in Table 1 .

Me as ure d 878r 86Sr

rat ios for the nineteen sam-

p les r ange f rom 0 .70327 to 0 .70502 and mea-

sured 143Nd/144Nd ratios fr om 0 .51249 to 0.51283

Table 1) . As al l samples are between 0.37 Ma and

13 Ma old, age correct ions are negl igible . Calculated

init ial ratios for 13 Ma , a ssum ing 87Rb/S6Sr ratios be-

twe en 0 .0004 and 0.07 and 147Sm/144Nd ratios from

0.04 to 0.15 Ne lson et al . , 1988; Ziegler, 1992),

do no t dev ia t e f rom measured r a t ios wi th in ex te rna l

prec ision Table 1).

In

a

143Nd]144Nd

vers us 878r 86Sr

diagram, the

measured r a t ios o f s amples f rom Lake Vic to r i a and

Ker imas i fo rm an inver se ly cor re l a t ed a r ray F igs . 2

and 5 ). Excep t fo r one s am ple f rom the Ho ma Mou n-

tain area 1434, Tab le 1), all data points plot within the

uppe r lef t depleted) quadrant of the diag ram Fig. 2) .


5/15

A. Kalt et al. / Tectonophysics 278 1997) 31 -45

Tab le t

S r, N d , a n d P b i s o t o p i c d a ta f o r K e r i m a s i , H o m a M o u n t a i n a r e a a n d W a s a k i P e n i n s u l a

35

S a m p l e O c c u r r e n c e R o c k t y p e S 7 S r/ 86 S r 1 4 3N d /1 4 4N d 2 6 p b / 2 4 p b 2 7 P b / 2 4 p b 2 s p b / 2 4 p b

Tanzania

T K - 1 2 K e r i m a s i c a r b o n a ti t e 0 . 7 0 3 9 1 3 0 . 5 1 2 7 2 9 2 0 . 8 1 7 1 5 . 7 8 4 4 0 . 4 7 4

T K - 1 4 K e r i m a s i c a r b o n a ti t e 0 . 7 0 3 9 1 1 0 . 5 1 2 7 2 5 2 0 . 8 4 8 1 5 . 7 77 4 0 . 4 6 2

T K - 2 6 K e r i m a s i c a r b o n a ti t e 0 . 7 0 4 0 5 9 0 . 5 1 2 7 1 7 2 0 . 6 4 8 1 5 . 7 4 0 4 0 . 3 1 8

Homa Mountain area

H B - 1 9 H o m a M t . o l - m e l i l i t i te 0 . 7 0 3 2 6 6 0 . 5 1 2 8 2 7 2 0 . 1 6 5 1 5 . 68 8 3 9 . 8 2 5

H O - 1 H o m a M t . c a r b o n a ti t e 0 . 7 0 3 4 8 6 0 . 5 1 2 7 6 3 2 0 . 4 1 4 1 5 . 7 19 4 0 . 2 5 3

1 4 3 4 N d i r u M b i l i c a r b o n a t i t e 0 . 7 0 5 0 1 5 0 . 5 1 2 4 8 6 1 8 . 7 1 8 1 5 . 5 1 6 3 9 . 0 2 6

1 4 2 3 B a l a G u l l y c a r b o n a t i t e 0 . 7 0 4 3 6 2 0 . 5 1 2 6 4 2 2 0 . 2 5 6 1 5 . 7 2 0 3 9 . 9 6 1

1 4 2 6 B a l a G u l l y c a r b o n a ti t e 0 . 7 0 4 1 5 6 0 . 5 1 2 7 0 5 2 0 ~2 1 6 1 5 . 7 13 3 9 . 9 8 9

Wasaki Peninsula

H O - 4 O k u g e c a r b o n a ti t e 0 . 7 0 4 0 7 3 0 . 5 1 2 6 7 9 2 0 . 0 5 2 1 5 . 6 9 4 3 9 . 6 8 9

1 4 7 2 O k u g e c a r b o n a ti t e 0 . 7 0 3 8 9 5 0 . 5 1 2 7 2 9 2 0 . 1 7 4 1 5 . 7 16 3 9 . 8 4 5

1 4 73 O k u g e c a r b o n a ti t e 0 . 7 0 3 9 5 9 0 . 5 1 2 7 2 1 2 0 . 2 1 8 1 5 . 7 0 9 3 9 . 8 7 0

1 4 3 8 U s a k i i j o l i te 0 . 7 0 3 9 3 0 0 . 5 1 2 6 6 3 1 9 . 7 2 0 1 5 . 6 5 3 3 9 . 4 5 7

1 4 4 3 U s a k i i j o l it e 0 . 7 0 3 9 8 7 0 . 5 1 2 6 6 6 1 9 . 8 7 9 1 5 . 6 5 7 3 9 . 5 1 1

1 4 4 8 S o k o l o P o i n t c a r b o n a ti t e 0 . 7 0 3 8 0 6 0 . 5 1 2 7 0 8 2 0 . 2 5 7 1 5 . 6 9 4 3 9 . 9 5 7

1 4 5 3 S o k o l o P o i n t c a r b o n a t i t e 0 . 7 0 3 8 4 5 0 . 5 1 2 6 7 5 2 0 . 3 0 3 1 5 . 7 3 2 4 0 . 0 7 0

1 4 6 4 S o u t h R u r i c a r b o n a ti t e 0 . 7 0 3 9 5 8 0 . 5 1 2 7 4 3 2 0 . 3 4 3 1 5 . 7 l l 3 9 . 8 9 5

1 4 65 S o u t h R u r i c a r b o n a t it e 0 . 7 0 3 9 7 9 0 . 5 1 2 7 3 7 2 0 . 2 21 1 5 . 7 08 3 9 . 9 1 0

H B - 3 0 S o u t h R u r i c a r b o n a t it e 0 . 7 0 3 8 7 2 0 . 5 1 2 6 7 9 1 9 . 27 2 1 5 . 6 07 3 9 . 2 2 0

1 5 1 3 N o r t h R u r i c a r b o n a t it e 0 . 7 0 4 1 7 3 0 . 5 1 2 6 8 9 2 0 . 0 5 8 1 5 . 7 48 3 9 . 7 7 0

87Sr/86Sr ra t io s a re no r m a l iz ed to 86Sr/88Sr = 0 .119 4 , 143Nd /144Nd ra t io s a re no rm a l iz ed to 146Nd /144Nd = 0 .721 9 . An a ly s i s o f NB S

S R M 9 8 7 y i e l d e d 8 7 S r/ 86 S r = 0 . 7 1 0 2 3 7 1 2 ( 2 s .d . , n = 5 0 ) . A n a l y s i s o f L a J o l la N d s t a n d a r d y i e l d e d 1 4 3N d /1 44 N d -- - 0 . 5 1 1 8 4 9 1 0

( 2 s .d . , n = 1 6 ) . T w o a n a l y s e s o f r o c k s t a n d a r d B H V O - I g a v e a n a v e r a g e 8 7 S r/ 86 S r o f 0 . 7 0 3 4 7 4 a n d 1 4 3N d /1 4 4N d o f 0 . 5 1 2 9 7 3 . T w e n t y

ana ly s e s o f NB S S R M 981 gav e 2 pb /294pb = 16 .93 5 9 , 207pb /24Pb = 15 .486 12 , Zo~Pb /2~4pb = 36 . 689 35 co r rec te d fo r 0 .13 o

f r a c t io n a t i o n p e r m a s s u n i t, a n d h a v e a n e s t i m a t e d a c c u r a c y o f 0 . 0 3 ( 2 s .d . ) p e r m a s s un i t .

T h e K e r i m a s i d a t a i n d i c at e a n o v e r a ll h o m o g e n e o u s

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

t i ons 0 .70391-0 .70406 , 0 .51272-0 .51273) . Samples

f r o m W a s a k i P e n i n s u l a d i s p l a y a s m a l l v a r ia t i o n in

1 4 3 N d ] t 4 4 N d r a t io s r ang ing f rom 0 .51265 to 0 .51275

Table 1 , F ig . 2) a nd a lso a sm al l var ia t ion in 87Sr/a6Sr

rat ios be tw ee n 0 .70381 and 0 .70417 Table 1 , F ig . 2) .

T h e s a m p l e s f r o m t he H o m a M o u n t a i n a re a s h o w a n

ex t r eme va r i a t ion in

1 4 3 N d / 1 4 4 N d

r a t io s f r om 0 .51249

to 0 .51283 Table 1 , F ig . 2) and in 87Sr/86Sr ra t ios

0 .70327 to 0 .70502, T able 1 , F ig . 2) .

M easu red 26pb/24pb r a t io s r ange be twe en 18 .72

and 20.41, 27pb/24pb ra t ios between 15.52 and

15.78, and 2spb/24pb ra t ios between 39.22 and

40.47 Table 1) . A ge cor rect ions are neg l ig ib le as-

sum ing ave rage 238U/24pb r a t io s a s g iven by Ne l so n

e t a l . 1988) and S imo ne t t i and Be l l 1994b). O n ly

calcu la t ion s wi th e xt rem e 238U/24pb ra t ios o f > 100

lead to s igni f icant sh i f t s in 26pb/24pb ra t ios , and

ca lcu la t ions wi th h igh Th /U r a t io s r e su l t i n s ign i f i -

cant sh i f t s in 2spb/24pb ra t ios when cor rect ing for

in-si tu dec ay Ne lson e t a l . , 1988).

I n the Pb i so tope p lo t s , the m easu red r a t io s o f sam -

p l e s f r o m L a k e V i c t o r i a a n d K e r i m a s i f o r m a r r a y s

wi t h posi t ive cor re la t io ns F ig . 3) . In a 27pb/24pb

ver sus 26pb /e4pb d i ag ram the da t a p lo t on and pa r -

a l l e l t o t h e N o r t h e r n H e m i s p h e r e R e f e r e n c e L i n e

NH RL ; Har t , 1984) , w i th 2Tpb/24pb ra t ios shi f ted

towa rds s l ight ly h ighe r value s for a g iven 26pb/Z4pb

ratio Fig. 3a) . In the 2spb/e4pb ve rsus 26pb/Z4pb

d i a g r a m a t r e n d o b l iq u e t o t h e N H R L i s o b s er v e d w i t h

A8/4 va lues Har t , 1984) i nc r eas ing wi th dec r eas ing

26pb/24pb r a t io s . A na log ous to t he S r and N d i so tope

sys t ema t i c s , t he l a rges t va r i a t i on in Pb i so tope r a t io s

i s s h o w n b y t h e s a m p l e s f r o m t h e H o m a M o u n t a i n

area 26pb/24pb = 18.71 8-20.41 4, Tab le 1, F ig . 3) .

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

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


6/15

36

A. K alt et a l./ Tectonophysics278 (1997) 31-4 5

0 .5 12 9 - - ' - - ' - - ~ r - - - - ' - - - - ~

T - - - - - - - - -

H I M U

~

/ 1 4 6 4

'0 0 5127 HB-30~ ~ ~ g , BE

EACL

0.5125 -

E M I l

0.5t23 Kerimasi

0 .7 0 30 0 . 7 0 3 5 0 . 7 0 4 0 0 . 7 0 4 5 0 . 70 5 0

8 7 S r / 8 6 S r

Fi g . 2 . D i a g r a m o f 143N d]144N d ve r s us 87Sr/ 86Sr s how i ng m e a -

s u r e d r a t i o s o f c a r bona t i t e , i j o li t e , a nd m e l i l i t i te s a m p l e s f r om t he

H o m a M o u n t a i n a r e a , W a s a k i P e n i n s u l a ( L a k e V i c to r i a , K e n y a ),

a nd K e r i m a s i ( T a nz a n i a ) . T he E a s t A f r i c a n C a r bona t i t e L i ne

( E A C L ) o f B e l l a n d B l e n k i n s o p ( 1 9 8 7 ) i s al s o s h o w n .

B E =

b u l k e a r t h v a l u e . F i e l d s f o r H I M U a n d E M I c o m p o n e n t s s h o w n

a c c o r d i ng t o H a r t e t a l . (1986) .

K er im as i 2 6pb /24pb = 2 0 .648 -20 .8 17 , Tab le 1 ,

F i g . 3) , w h e r e a s t h e s a m p l e s f r o m W a s a k i P e n i n s u l a

have in te rm ed ia te P b is o tope r a t io s 2 6pb /24pb =

19 .27 2 -20 .3 43 , T ab le 1 , F ig . 3 ).

4 D i s c u s s i o n a n d c o n c l u s io n s

4 1 C o m p o s i t io n s o f t h e m a n t l e s o u rc e s

T h e N d - S r a n d t h e P b - P b d a t a a r r a y s i n d i c a t e

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

V i c t o r i a a n d K e r i m a s i w e r e g e n e r a t e d f r o m a n i s o -

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

a r r a y s i n t h e S r - N d a n d P b - P b i s o t o p e p l o t s s u g -

g e s t s b i n a r y m i x i n g p r o c e s s e s . A s s e t o u t i n S e c -

t i o n 1 , c a r b o n a t i t e s o u r c e s a r e c o m m o n l y i n t e r p r e t e d

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

o c e a n i c b a s al ts H I M U , D M M , E M I , E M I I ) . S i m i -

l a rl y , 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 s o f t h e s tu d i e d

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

m i x i n g o f t w o e n d m e m b e r c o m p o n e n t s : a H I M U -

type c om po nen t w i th h igh 143N d/144N d r a t io s and

low 87S r/86S r r a t io s and a n EM 1 - type com po nen t

with low 143Nd/144Nd ra t ios and h ig h 87Sr/86Sr ra t ios

F i g s. 2 a n d 5 ). T h e e n r i c h e d E M I ) e n d m e m b e r is

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

15.9 1 H ~ ~

Homa Mt.

a r e a

O W a s a k i

P e n i n s u l a

15.8 Kerimasi i ~/~q.N~

1 ~ ~ k ~ . 1 4 6 4

o 15.7

Q ~ ~--- HB-19

oe/

HBjo

15.6

D M M

55

41.0 ~ . , j l ~,~

/

HIIVIU

40.5

~ 40.0 ~ 1 4 6 4

39.5

ttB-30 ~ ~

1434

3 9. 0 ~M i ~ D M M / / / ~

38 .5 ,v ,q , z , I

1 8 1 9 2 0 2 1 2 2

2 o 6 p b / 2 O 4 p b

Fig. 3 . Diag ram s of 27pb/24pb ve r su s 26pb/24pb ( a ) and

28pb/24pb ve r sus 26pb/Z4pb (b) showing measured r a t ios of

c a r bona t i t e , i j o l i t e , a nd m e l i l i t i t e s a m pl e s f r om t he H om a M oun-

t a in a r e a , W a s a k i Pe n i ns u l a ( L a ke V i c t o r i a , K e nya ) , a nd K e r i m a s i

( T a n za n ia ) . A l s o s h o w n i s t h e N o r t h e r n H e m i s p h e r e R e f e r e n c e

L i n e

(NHRL)

o f H a r t ( I 9 8 4 ). F i e l d s f o r H I M U , D M M a n d E M I

c om pone n t s s how n a c c o r d i ng t o H a r t e t a l . ( 1986) .

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

S r - N d i s o to p e di a g ra m . H I M U - E M I b i n a ry m i x i n g

fo r the s amples a s a w ho le i s a l s o s ugges ted by a

da ta t r end ob l ique to the N H RL in the 2s pb /Z4pb

versu s 26pb/24pb d iag ram Fig . 3b) .

H o w e v e r , a c l o s e r e x a m i n a t i o n o f t he d a t a r e v e a l s

tha t the obs e rve d va r i a t ions in S r, N d and P b i s o to p ic

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

H I M U a n d E M I c o m p o n e n t s . I n t h e S r - N d i s o t o p e

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

a r e a s a r e d iv e r s e . O n l y t h e d a t a f o r H o m a M o u n t a i n

a r e a a re c o n s i s te n t w i t h b i n a r y m i x i n g o f H I M U a n d

E M I c o m p o n e n t s . T h e s p r e a d d i s p l a y e d b y s a m p l e s


7/15

A . K a l t et a l . / T e c to n o p h y s i cs 2 7 8 1 9 9 7 ) 3 1 - 4 5 3 7

f r o m K e r i m a s i i s t o o s m a l l f o r a n y c o r r e l a t i o n t o

b e r e c o g n i s e d a n d t h e d a t a f i e l d f o r W a s a k i P e n i n -

s u la s how s no l inea r tr end . A dd i t iona l ly , the pos i t ion

of data poin ts in the 27pb/24pb versus 26pb/24pb

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

D M M - t y p e c o m p o n e n t ra t h e r th a n E M I a s t h e sa m -

p l e w i t h th e l e a st r a d i o g e n i c P b 1 4 3 4 ) p l o ts o n t h e

N H R L c l o s e t o D M M F ig . 3 a ).

A f u r t h e r f e a t u r e n o t i n l i n e w i t h b i n a r y

H I M U - E M I m i x in g i s th e l a c k o f c o n si s te n t e n d-

m e m b e r c o m p o s i t i o n s . I f t h i s p r o c e s s w e r e a r e a l i st i c

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

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

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

s i m i l a r t o H I M U c o m p o s i t i o n s h o u l d h a v e th e l o w e s t

S7Sr]86Sr

ra t ios , th e h ig he s t 143Nd/144Nd ra t ios , a nd

t h e m o s t r a d i o g e n i c P b i s o t o p e r a t i o s . O n t h e c o n -

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

s h o u l d d i s p l a y t h e h i g h e s t 878r 86Sr r a t io s , the low es t

143N d/144N d r a t io s , and the l eas t r ad iogen ic P b i s o -

t o p e r a t io s . T h i s i s n o t t h e c a s e f o r t h e c a r b o n a t i t e s

s t u d i e d h e r e . S a m p l e s f r o m K e r i m a s i , f o r e x a m p l e ,

have an in te rme d ia te com pos i t ion in the 143N d/144N d

ver s us 87Sr ]86Sr p lo t w h i le th ey d i s p lay th e m os t

r a d i o g e n i c P b i s o t o p e r a t i o s . A s i m i l a r o b s e r v a t i o n

c a n b e m a d e f o r th e s a m p l e s a p p r o x i m a t i n g H I M U

H B - 1 9 ) a n d th e E M I 1 4 3 4 ) c o m p o s i t i o n s i n t h e

S r - N d i s o t o p e p l o t F i g . 2 ) . S a m p l e 1 4 3 4 a l s o p l o t s

c l o s e t o E M I i n b o t h P b i s o t o p e d i a g r a m s F i g . 3 ) ,

b u t s a m p l e H B - 1 9 d i s p l a y s a n i n t e r m e d i a t e P b i s o -

t o p e c o m p o s i t i o n u n l i k e H I M U . T h i s i n c o n s i s t e n t

beh av io u r i s a l s o ev ide n t f ro m the 87S r/86S r ve r -

sus 26pb/24pb F ig . 4a) and the 143N d la 4N d ve r su s

26pb/24pb d iag ram F ig . 4b ) . The da ta po in t s fo rm

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

t r e n d i s d e v e l o p e d .

W h e n a p p l i e d t o t h e i n d i v i d u a l c a r b o n a t i t e a r e a s

K e r i m a s i , H o m a M o u n t a i n a r e a , W a s a k i P e n i n s u l a ) ,

a b i n a r y H I M U - E M I m i x i n g m o d e l a l s o f a i l s t o

e x p l a i n t h e r e s p e c t iv e d a t a a rr a y s . T h e s a m p l e s f r o m

K e r i m a s i a r e s i m i l a r i n a l l is o t o p e d i a g r a m s , s o t h a t

n o tr e n d is r e c o g n i s a b l e F i g s. 2 - 4 ) . A d d i t i o n a l d a ta

f r o m K e r i m a s i P a s l i c k e t al ., 1 9 9 5; se e S e c t i o n 4 . 2 ,

F i g s . 5 - 7 ) s h o w t h a t t hi s is n o t d u e t o o u r s m a l l d a t a

b a s e . T h e d a t a f o r W a s a k i P e n i n s u l a s h o w t r e n d s

c o n s i st e n t w i t h b i n a r y H I M U - E M I m i x i n g in t h e

P b - P b d i a g r a m s , b u t n o t i n th e S r - N d i s o t o p e p l o t

c o m p a r e s a m p l e s H B - 3 0 a n d 1 4 6 4 i n F i g s . 2 - 4 ) .

0.7055

0.7050

0.7045

0.7040

0.7035

0.7030

0.7025

0.5129

0.5128

0.5127

~ 0 . 5 1 2 6

0.5125

~ E M 1 1 4 34

HB 30 0

M M

M M

H8 3o 0

i

Hom a Mt. area

W asaki Peninsula

[] Kerimasi

^ 1 4 6 4

o j Kmm ]

HB-19

H I M U

i i

HB-19

H1M U

1434

~M I

0.5124

I I I

18 19 20 21 22

2 0 6 p b / 2 0 4 p b

F i g . 4 . D i a g r a m s o f 87 S r/ 86 S r v e r s u s 2 6p b / 2 4 p b a ) a n d

1 43 N d /1 4 4N d v e r s u s 2 6 p b/ 2 4 p b b ) s h o w i n g m e a s u r e d r a t io s

o f c a r b o n a t it e , i j o li te , a n d m e l i l it i te s a m p l e s f r o m t h e H o m a

M o u n t a i n a r e a , W a s a k i P e n i n s u l a L a k e V i c to r i a , K e n y a ) , a n d

K e r i m a s i T a n z an i a) . F i e l d s f o r H I M U , D M M a n d E M I c o m p o -

n e n t s s h o w n a c c o r d i n g t o H a r t et a l. 1 9 8 6 ) .

T h e s a m p l e s f r o m H o m a M o u n t a i n a r e a su g g e s t

a H I M U - E M I t re n d o n t h e 1 43 N d/1 44 N d v e r s u s

S V Sr /86S r p lo t F ig . 2 ) . N ever the les s , a s a rgued above ,

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

s a m p l e s 1 4 3 4 a n d H B - 1 9 i n F i g s . 2 - 4 ) . A d d i t i o n a l l y ,

t h e v e r t i c al tr e n d s d i s p la y e d b y t h e H o m a M o u n t a i n

d a t a i n t h e S r - P b a n d N d - P b d i a g r am s F i g. 4 )

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

S m - N d s y s te m s r a t h e r t h a n b i n a r y m i x i n g .

I n s u m m a r y , n e i t h e r th e d a t a a s a w h o l e n o r t h o s e

f o r i n d i v i d u a l c o m p l e x e s c a n b e e x p l a i n e d b y b i -

n a r y m i x i n g b e t w e e n H I M U a n d E M I o r a n y o t h e r

c o m p o n e n t s i d e n t i f i e d i n o c e a n i c i s l a n d b a s a l t s . I n -

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

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


8/15

38

A. Kalt et al./Tectonophysics 278 (1997) 31-45

0.5129 ' ' '

ItlMU ~sL~ Kerimasi W-Uganda

~ ~ . Oldoinyo

D E - U g a n d a

_ ~Q,~f f~,~ Lengai

Z~10 5127 . ~ / ~ / ~ ~ k ~ ~ ~ ,B E + H o m a B a y

O N-Tanzania

Z '.~ S-Ta nzani a

0.5125 ~ ~ . ~

_ _

EACL /k Malawi

Zam bia

1 7 .

0.5123 , , , EM I i- '~ A, ' ~

0.703 0.7035 0.7 04 0.7045 0.705

87Sr 86Sr

Fig. 5. D iagram of 143Nd/144Nd versus 87Sr/86Sr showing data fro m previous studies on E ast Afr ican carbonatites as listed in Table 2.

For samples younger than 40 Ma (W-Uganda, E-Uganda, Homa Bay, N-Tanzania), measured ratios are plotted. For samples older than

40 Ma (S-Tanzania, Malawi, Zambia), initial ratios are shown. Two samples from Zambia (filled triangles) plot outside the diagram to

the lower right side. Also shown for comparison are the data fields of this study: Kerimasi (stippled), Wasaki Peninsula (checkered), and

Hom a Bay (grey). The East African Carbonatite Line (EACL)of Bell and Blenkinsop (1987) and the bulk earth value (BE)are also

shown. Fo r further explanation see text. Fields for HIM U and EM I components shown according to Hart et al. (1986).

to different degrees. Along with the fact that carbon-

a t i te magmat ism in genera l i s vo lumetr ica l ly very

restr ic ted, these observations suggest the existence

of small-scale mantle sources and mixing processes

be tween components tha t vary in composi t ion and

space.

4 2 Comparison with other East African

carbonatites

Numerous Sr, Nd, and Pb isotope data on East

Afr ican carbona t i tes and re la ted rocks have been

published. Although there is a considerable range

in age and tectonic set t ing between individual com-

plexes (Fig. 1 , Table 2) i t has been conc luded that the

Sr, Nd, and Pb isotope system atics of East African

carbonati tes younger than 200 Ma studied so far are

cons is ten t w i th mix ing of HIMU and EM I man-

tle components (Bell and Blenkinsop, 1987; Bell ,

1994; Til ton and Bell , 1994; Bell and Dawson,

1995). Therefo re, da ta for carbonati tes o lder than

those studied here, but younger than 200 Ma, and

data for carbonati tes from other parts of the East

Afr ican Rif t sys tem have been inc luded in the fo l -

low in g d iscuss ion . A l l da ta used for com par ison

have been compiled in Table 2 and are shown in

Figs. 5 -7 . Compar ison of the c i ted i so tope s tandard

values shows these to coincide within error l imits

(see footnote s to Tables 1 and 2) .

Carbon ati tes have been grouped according to their

tectonic and geographic si tuations (Figs. 1 , 5-7,

Table 2) . This gross oversimplif icat ion is adapted

for the sake of c lari ty in Figs. 5-7. For carbonati tes

youn ger than 40 M a, age correction s for Sr, Nd, and

Pb isotope rat ios are negligible assuming average

Rb/Sr, Sm/Nd, U/Pb, and Th /U rat ios (Nelson et a l .,

1988; Ziegler, 1992; Simonetti and Bell, 1994a,b;

Paslick et a l . , 1995). For these samples, measured

ratios are shown in Figs. 5-7. For carbonati tes older

than 40 Ma, ini t ia l isotope rat ios are plotted in the

respective diagrams.

The carbonati tes of Kerimasi , Oldoinyo Lengai,

and Shomb ole vo lcanoes are re fe r red to as N-Tan-

zania . Th ey are s i tuated next to the western fau lt

scarp within the Kenya Rift (eastern branch of the

East African Rift) and are of Neogene age (Daw-

son, 1992). The carbonati te complexes of Bukusu,

Tororo, Sukulu, and Napak (E-Uganda) are located

on the eastern sh oulder of the Ken ya Rift and display

mineral ages ranging between 7 and 32 Ma (Si-

monett i and Bell , 1993, 1994a). Carbonati tes from

Kalyango and Rusekere c rop ou t in the For t Por -

tal area (W-Uganda) on the eastern shoulder of the

Albert Rif t . Volcanic activi ty occurred about 4-6


9/15

A. Kah et al . / Tectonophysics 278 (1997) 31-4 5

39

15.9

15.8

5.7

15.6

15.5

[] r T r HIMU

Kerimasi [] ~ /

A

, - - a ~

. . . . : :W: [ [] E-Uganda

~

N-Tanzania

EM I

~(:::::::::::::::::::::.... Malawi

~ Malawi

41.0

40.5

HIMU

40.0

Oldoinyo

Lengai

3 9 . 5 ~ . d ~

[] Kerimasi

39.0 ~iii~i~i~:

D M l ~ / % - %'~ I [] E-Uganda

~ M I ~ . 0 N-Tanzania

38.5 1 .

18 19 20 21 22

206pb 2O4pb

Fig. 6. Diagrams of 27pb/24pb versus 26pb/24pb (a) and

2spb/24pb versus 26pb/24pb (b) showing data from previous

studies on East African carbonatites as l isted in Table 2. For

samples younger than 40 Ma (E-Uganda , N-Tanzania) , measured

ratios are plotted. For samples older than 40 Ma (Malawi), initial

ratios are shown. One sample from Malawi plots outside both

diagrams to the left side. Also shown for comparison are the

data fields of this study: Kerimasi (stippled), Wasaki Peninsula

(checkered), and Homa Bay (grey).

NHRL

i s the Northern Hemi-

sphere Reference Line of Hart (1984). For further explanation

see text . Fie lds for HIMU, DMM and EM I components shown

according to Hart et al. (1986).

ka ago (Barker and Nixon, 1989). Panda Hill and

Sengeri (S-Tanzania) are located at the junction of

the eastern and the western r if t branch south of

the Tanzania Cra ton . A K-Ar age of 113 Ma has

been reported by Snell ing (1965) for Panda Hill .

The carbonati tes from Chilwa Island, Lake Chilwa,

and f rom Kangankunde a re par t o f the Chi lwa Alka-

l ine Prov ince (Malawi) and re la ted to the eas t -w es t

t rend ing fau l t sys tem of the Zam bes i R if t (S imonet t i

0.7055 [ ] r r

[] E-Uganda

0.7050 , ~ - +:: ~:::: + HomaBay

EM I ........ ........


10/15

4 0 A . K a l t e t a l. / T e c t o n o p h y s i c s 2 7 8 1 9 9 7 ) 3 1 - 4 5

T a b l e 2

C o m p i l a t i o n o f S r - N d - P b i s o t op i c d a t a fo r E a s t A f r i c a n c a r b on a t i t e s

Re g io n Co m ple x A ge M a) Re f . 87Sr/86Sr 143Nd /144Nd 2 6pb /204pb 2 7pb /2 4pb 2 spb /Z 4pb

W - U g a n d a K a l y a n g o 0 B B 8 7 0 . 70 4 1 7 0 . 5 12 5 7

R u s e k e r e 0 B B 8 7 0 . 7 0 4 1 7 0 . 5 1 2 5 8

E - U g a n d a B u k u s u 2 6 - 3 2 B B 8 7 0 . 70 3 1 5 0 . 5 1 27 9

B u k u s u 2 6 - 3 2 B B 8 7 0 . 70 3 0 7 0 . 51 2 8 3

S u k u l u 2 6 - 3 2 B B 8 7 0 . 7 0 3 1 2 0 . 51 2 8 1

T o r o r o 2 6 - 3 2 B B 8 7 0 . 7 0 3 6 0 0 . 5 1 2 7 3

T o r o r o 2 6 - 3 2 N 8 8 0 . 7 0 3 6 0 0 . 5 1 2 6 7 2 1 . 0 6 8 1 5 .8 6 5

S u k u l u 2 6 - 3 2 N 8 8 0 . 7 03 1 1 0 . 5 1 2 8 0 2 1 . 1 5 4 1 5 .7 8 1

N a p a k 7 - 3 0 N 8 8 0 . 7 0 3 2 0 0 . 5 1 2 7 8 2 0 . 7 1 9 1 5 . 7 8 0

N a p a k 7 - 3 0 S B 9 4 a 0 . 7 0 3 2 3 0 . 5 1 2 8 3 2 0 . 4 0 1 5.8 1

N a p a k 7 - 3 1 ) S B 9 4 a 0 . 7 0 3 2 3 0 . 5 1 2 8 3 2 0 . 4 7 1 5 . 7 5

K e n y a H o m a B a y 1 3 B B 8 7 0 . 7 0 5 0 2 0 . 5 1 2 4 4 1 8 .5 0

N - T a n z a n i a O l d L e n g 0 B B 8 7 0 .7 0 4 4 2 0 . 5 1 2 61

O l d L e n g 0 B B 8 7 0 . 7 0 4 4 5 0 . 5 1 2 5 9 1 9 .2 5

O l d L e n g 0 K K 9 0 0 . 70 4 3 7 0 .5 1 2 6 2

O l d L e n g 0 K K 9 0 0 . 7 04 3 9 0 .5 1 2 6 3

O l d L e n g 0 D 9 5 1 9 . 24 5 1 5 . 67 6

O l d L e n g 0 D 9 5 1 9 .2 61 1 5 . 6 3 7

O l d L e n g 0 D 9 5 1 9 . 2 00 1 5 . 5 5 2

K e r i m a s i 0 . 5 P 9 5 0 . 7 0 3 9 7 5 0 . 5 1 2 7 4 4 2 0 . 8 0 9 1 5 . 7 5 4

K e r i m a s i 0 . 5 P 9 5 0 . 7 0 3 9 6 7 0 . 5 1 2 7 3 0 2 0 . 7 1 2 1 5 . 7 2 2

K e r i m a s i 0 . 5 P 9 5 0 . 7 0 4 0 0 3 0 . 5 1 2 7 0 6 2 0 . 9 0 7 1 5 . 8 01

S h o m b o l e 2 B P 9 1 0 . 7 0 3 9 5 0 . 5 1 2 6 3

S h o m b o l e 2 B P 9 1 0 . 7 0 4 3 0 0 . 5 1 2 5 5

S h o m b o l e 2 B P 9 1 0 . 7 0 4 3 6 0 . 5 1 2 6 6

S h o m b o l e 2 B P 9 1 0 . 7 0 4 4 5 0 . 5 1 2 6 0

S - T a n z a n i a P a n d a H i l l I 1 3 B B 8 7 0 . 7 0 4 4 2 0 . 5 1 2 4 3 1 8 .8 5

S e n g e r i 1 13 B B 8 7 0 . 7 0 4 2 3 0 . 5 1 2 4 3 1 8 . 6 5

P a n d a H i l l 1 13 Z 9 2 0 . 7 0 3 8 7 4 0 . 5 1 2 4 5 8

P a n d a H i l l l 1 3 Z 9 2 0 . 7 0 3 4 0 6 0 . 5 1 2 4 4 2

M a l a w i K a n g a n k 1 2 6 N 8 8 0 . 7 0 3 1 0 . 5 1 2 6 4 2 0 . 11 1 5 . 78

K a n g a n k 1 2 6 Z 9 2 0 . 7 0 3 0 1 6 0 . 5 1 2 6 5 0

K a n g a n k 1 2 6 Z 9 2 0 . 7 0 3 0 6 7 0 . 5 1 2 6 6 0

K a n g a n k 1 2 6 Z 9 2 0 . 7 0 3 0 3 3 0 . 5 1 2 6 3 7

C h i l w a 1 2 6 S B 9 4 b 0 . 7 0 3 2 8 0 . 5 1 2 6 5 1 9 .7 8 1 5 . 7 0

C h i l w a 1 2 6 S B 9 4 b 0 . 7 0 3 3 4 0 . 5 1 2 6 7 1 9 . 5 4 1 5 . 6 6

C h i l w a 1 2 6 S B 9 4 b 0 . 7 0 3 4 9 0 . 5 1 2 6 8 1 9 . 1 3 1 5 . 6 7

C h i l w a 1 2 6 S B 9 4 b 0 . 7 0 3 2 2 0 . 5 1 2 6 8 1 9 . 1 8 1 5 . 6 8

C h i l w a 1 2 6 S B 9 4 b 0 . 7 0 3 1 9 0 . 5 1 2 6 7 1 7. 31 1 5 . 5 9

C h i l w a 1 2 6 S B 9 4 b 0 . 7 0 3 3 4 0 . 5 1 2 6 8 1 9 . 5 7 1 5 .7 1

C h i l w a 1 2 6 S B 9 4 b 0 . 7 0 3 4 2 0 . 5 1 2 6 6 1 9 .5 3 1 5 . 65

C h i l w a 1 2 6 S B 9 4 b 0 . 7 0 3 6 1 0 . 5 1 2 6 6 1 9 . 15 1 5 . 6 3

Z a m b i a K a l u w e 1 13 Z 9 2 0 . 7 0 3 2 8 4 0 . 5 1 2 6 6 8

K a l u w e 1 13 Z 9 2 0. 7 0 4 12 1 0 . 5 1 2 6 3 0

K a l u w e 1 13 Z 9 2 0 .7 0 3 7 4 0 0 . 5 1 2 6 8 3

C h a s w e t a 1 1 3 Z 9 2 0 . 7 0 7 7 3 8 0 . 5 1 2 3 9 2

C h a s w e t a 1 1 3 Z 9 2 0 . 7 0 5 7 1 7

C h a s w e t a 1 1 3 Z 9 2 0 . 7 0 6 3 3 7 0 . 5 1 2 2 2 8

C h a s w e t a 1 13 Z 9 2 0 . 7 0 3 8 1 0 0 . 5 1 2 7 81

4 1 . 1 1 2

4 0 . 6 4 6

4 0 . 3 7 9

4 0 . 4 6

40 .21

3 9 . 3 6 5

3 9 . 3 9 6

3 9 . 1 2 5

40 .361

4 0 . 2 3 6

4 0 . 5 1 6


11/15

A. Kalt et al./Tectonophysics 278 1997) 31-4 5 41

S r - N d d a t a a re a v a i l a b l e f o r a ll th e c a r b o n a t i t e o c -

c u r re n c e s m e n t i o n e d a b o v e . S a m p l e s o f 4 0 M a a n d

y o u n g e r a s a w h o l e f o r m a n a r r a y o n t h e 143Nd/144Nd

v e r s u s S 7 S r/ 86 S r d i a g r a m t h a t i s s i m i l a r t o t h e o n e

d e f i n e d b y o u r s a m p l e s F i g . 5 ). T h i s c o r r e l a t i o n

i n c l u d i n g o l d e r s a m p l e s f r o m S - T a n z a n i a ) h a s b e e n

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

s o u r ce r e s ul t i n g f ro m m i x i n g o f a H I M U a n d a n E M

I c o m p o n e n t B e l l a n d B l e n k i n s o p , 1 9 8 7 ; B e l l , 19 9 4 ;

B e l l a n d D a w s o n , 1 9 95 ). H o w e v e r , t h e i s o t o p e d a t a

f o r i n d i v id u a l c a r b o n a t i te a r e a s y o u n g e r t h a n 4 0 M a

d o n o t f o l l o w th e E A C L o r an y o t h er H I M U - E M

I m i x i n g l i n e . A n e x c e p t i o n m a y b e t h e d a t a a r -

r a y f o r E - U g a n d a . H o w e v e r , t h is f i el d c o n t a i n s d a t a

f r o m f o u r i n d i v i d u a l c a rb o n a t i t e c e n tr e s a n d t h u s t h e

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

n o t c o n s t r a i n e d . O n t h e o t h e r h a n d , i t i s n o t e w o r -

t h y t h a t d a t a p o i n ts f o r th e n e i g h b o u r i n g O l d o i n y o

L e n g a i a n d K e r i m a s i v o l c a n o e s N - T a n z a n i a ) d e fi n e

s e p a r a t e f i e l d s o n t h e 1 43 N d/1 44 N d v e r s u s 8 7 S r/ S 6S r

d i a g ra m . T h u s, b i n a r y m i x i n g o f a H I M U a n d a n E M

I c o m p o n e n t a s a p r o c e s s c o m m o n t o t h e s o u r c es o f

E a s t A f r i c a n c a r b o n a t i t e s y o u n g e r t h a n 4 0 M a i s n o t

c l e a r l y e v i d e n t fr o m t h e S r - N d d a t a . T h e d a t a a r ra y s

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

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

h a v e n o c o m m o n m i x i n g e n d m e m b e r s .

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

P b d a t a . I n a 2 7 p b / 2 4 p b v e r s u s 2 6 p b / 2 4 p b d i a g r a m ,

d a t a fo r c a r b o n a t i te s y o u n g e r t h a n 4 0 M a s h o w a

t r e n d p a r a l l e l t o a n d a b o v e th e N H R L , s i m i l a r t o

o u r d a t a F i g . 6 a ) . A s f o r o u r s t u d y , t h e tr e n d s d i s -

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

E - U g a n d a a n d N - T a n z a n i a ) a r e n o t in l i n e w i th t h e

g e n e r a l t r en d . A g a i n , i t i s n o t e w o r t h y t h a t K e r i m a s i

a n d O l d o i n y o L e n g a i N - T a n z a n i a ) d i s p l a y e n t i r e l y

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

s h o w n i n t h e 2 8p b /Z 4p b v e r s u s 2 6 p b /z 4p b d i a g r a m

F i g . 6 b ). A t r e n d o b l i q u e t o t h e N H R L , a s d i s -

p l a y e d b y o u r d a t a , is a ls o s h o w n b y t h e s a m p l e s

f r o m E - U g a n d a a n d N - T a n z a n i a a s a w h o l e , b u t t h e

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

l at io n . T h e l a c k o f c o n s i st e n t H I M U - E M I m i x i n g

t r e n d s c a n a l s o b e d e p i c t e d f r o m t h e 8 7 Sr /8 6 Sr a n d

1 43 N d/1 44 N d v e r s u s 2 6 p b/ 2 4 p b d i a g r a m s F i g . 7 ) .

A g a i n , i n d i v i d u a l c a r b o n a t i t e a r e a s d e f in e d i s t i n c t

t r e n d s .

T h e o l d e r E a s t A f r i c a n c a r b o n a ti te s 4 0 - 1 2 6 M a )

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

< 4 0 M a ) i n t e r m s o f m a n t l e s o u r c e s . M a n t l e s o u r c e

c o m p o s i t i o n s f o r c a r b o n a t i t e s y o u n g e r t h an 4 0 M a

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

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

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

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

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

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

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

s o u r ce . C a l c u l a t i o n s o f th i s k i n d b e a r v a s t u n c e r t a i n -

t i e s a s t h e a c t u a l R b / S r , S r r d N d , U / P b , a n d T h / U

r a t io s o f th e m a n t l e s o u r c e s a r e n o t k n o w n . T h e r e -

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

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

g r a m s a r e no t k n o w n a n d c a n n o t b e c o m p a r e d w i th

t h o s e o f c a r b o n a t i t e s y o u n g e r t h a n 4 0 M a . I t i s o n l y

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

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

f o r m e d b y i n i t i a l S r - N d i s o t o p e r a ti o s o f i n d i v i d u a l

a r ea s S - T a n z an i a , M a l a w i , Z a m b i a ) in t h e S r - N d

i s o t o p e p l o t F i g . 5 ) .

Footnote to T able 2

Isotope ratios given as measured ratios for sam ples younger than 40 M a and as init ial ratios for older samples. BB: B ell and Blenkinsop

1987), init ial Sr and N d isotope ratios for Panda Hill and Sengeri calculated with averages of aTRb/86Sr and

147Snaj144Nd

ratios as

given by Ziegler 1992) for Panda Hill , 26pb/24pb ratios taken from Fig. 6 of Tilton and Bell 199 4); N88: N elson et al. 1988),

143Nd/ 144Nd calculated with ENd values given in Table 2; SB9 4a: Simonetti and Bell 1994a); K K9 0: Keller and Krafft 1990); D95:

Dawson et al . 1995); P95: Paslick et al . 1995); BP 91: Bell and Peterson 1991); Z92: Ziegler 1992); init ial Sr and Nd isotope ratios

calculated using 87Rb/86Sr and

1478m/144Nd

ratios as given in Tables 2-3; S B9 4b: Simonetti and Bell 1994b), initial isotope ratios as

given by the authors.

)~Rb =

1.42 x 10 11, )~Sm = 6.54 x 10 -12. In all cited stud ies, 87Sr/86Sr ratios are norm alized to 86Sr/88Sr

0.1194, 143Nd]144Ndratios to 146Nd/144Nd = 0.7219, ex cept for N 88 146Nd]142Nd = 0.63151). S tandard values given by the authors: La

Jolla: 0.51186 -t- 2 BB87, BP91, SB94a,b), 0.511845 4- 9 P95); BCR I: 0.511833 4- 20 N88), 0.51266 4- 2 SB94a,b); NBS SR M 987:

0.71023 4- 3 BB87), 0.71022 4- 4 N88), 0.71025 4- 2 SB94a,b), 0.710247 4- 15 P95); Eim er and Am end: 0.70808 4- 3 BB87, BP91),

0.70800 4- 3 N88); NBS SRM 981 26pbfl4pb): 16.927 4- 9 N88), NBS S RM 981 27pb/24pb): 15.486 4- 13 N88), NBS SRM 981

Cspb/24pb): 36.668 44 N88). For further explanations see Section 4.2.


12/15

42 A . K a l t e t a l . / T e c t o n o p h y s i c s 2 7 8 1 9 9 7 ) 3 1 4 5

I n s u m m a r y , S r , N d , a n d P b i s o t o p e c o m p o s i t i o n s

o f E a s t A f r i c a n c a r b o n a t i t e s d i s p l a y a w i d e r a n g e

i n S r , N d , a n d P b i s o t o p e c o m p o s i t i o n s . A l t h o u g h

a c ru d e H I M U - E M I c o r r e l a ti o n m a y b e d e p i c te d

f r o m th e S r - N d F ig s . 2 a n d 5 ) a n d t h e P b - P b

i s o tope p lo t s F ig s . 3 and 6 ) , s uch a t r end i s s een ne i -

t h e r i n th e S r - P b a n d N d - P b i s o to p e r el a ti o n s hi p s

F igs . 4 and 7 ) no r in the t r ends d i s p layed by s ing le

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

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

E a s t A f r i c a n c a r b o n a t i t e s t h a t a r e n o t p r o d u c e d b y

m i x in g o f c o m m o n e n d m e m b e r s.

4 3 Location of the mantle sources of East African

carbonatites

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

g e n e i t i e s i n t h e m a n t l e s o u r c e s f o r E a s t A f r i c a n

R i f t c a r b o n a t it e s , a c o n c l u s i o n t h a t h a s a l r e a d y b e e n

d r a w n i n o t h e r s t u d i e s o n E a s t A f r i c a n c a r b o n -

a t i te s B e l l a n d B l e n k i n s o p , 1 9 8 7; B e ll a n d D a w s o n ,

1 9 9 5 ) a n d h a s a l s o b e e n p r o p o s e d f o r E a s t A f r i c a n

n e p h e l i n i t e s o n t h e b a si s o f is o t o p e d a t a S i m o n e t t i

and Be l l , 1993 , 1994a , 1995 ; Be l l and S imone t t i ,

1 9 9 6 ) . S e v e r a l m i x i n g s c e n a r i o s w e r e p r o p o s e d t o

e x p l a i n t h e i s o t o p i c v a r i a t i o n s i n t h e c a r b o n a t i t e

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

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

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

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

B e l l a n d D a w s o n , 1 9 9 5 ) , o r p l u m e - l i t h o s p h e r e i n -

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

m e l t i n g B e l l a n d S i m o n e t t i , 1 9 9 6 ) . W e s u g g e s t th a t

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

t i r e ly i n t h e l it h o s p h e r i c m a n t l e . A s i m i l a r c o n c l u s i o n

h a s b e e n d r a w n b y P a s l i c k e t al . 1 9 9 5 ) f o r y o u n g

v o l c a n i c s o f n o r t h e r n T a n z a n i a , i n c l u d i n g c a r b o n -

a t i te s f r o m K e r i m a s i . A m a j o r r o l e o f th e l i t h o s p h e r e

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

a l s o b e e n i n v o k e d b y B e l l a n d S i m o n e t t i 1 9 9 6 ) .

I s o top ic ev idence a rgues aga ins t s ign i f i can t in -

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

I f t h is w e r e t h e c a s e , a t l e a st o n e m i x i n g e n d m e m -

b e r w i t h c o n s i s t e n t S r - N d - P b i s o t o p i c c o m p o s i t i o n

a s f o r e x a m p l e D M M s h o u l d b e r e c o g n i s a b l e . T h i s

i s c l e a r l y n o t t h e c a s e f o r E a s t A f r i c a n R i f t c a r -

b o n a ti t es . I n v o l v e m e n t o f p l u m e m a t e r ia l , c o m m o n l y

r e l a t e d t o a H I M U - t y p e c o m p o n e n t , a l s o a p p e a r s

u n l i k e l y . I n c a s e s w h e r e a m a n t l e p l u m e a n d / o r a s -

t h e n o s p h e r e h a v e s i g n i fi c a n t l y c o n tr i b u t e d to a n E a s t

A f r i c a n R i f t m a n t l e s o u r c e , c o n s i s t e n t m i x i n g t r en d s

a n d e n d m e m b e r s i n i s o to p e a n d t r a c e -e l e m e n t c om -

pos i t ions w ere found C las s e t a l ., 1994 ; V o lke r e t

a l . , 1995) . F o r Eas t A f r i can R i f t ca rbona t i t e s , t he

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

i n v e s t i g a t i o n a r g u e a g a i n s t c o n t r i b u t i o n s f r o m a n y

m a n t l e r e s e r v o i rs t h a t a r e c o n t i n u o u s l y a v a i la b l e .

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

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

r e g i o n . T h e d a t a a r r a y s f o r E a s t A f r i c a n R i f t c a r -

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

1) In the 2spb/24pb versu s 26pb/Z4pb d iag ram

F ig . 3b and F ig . 6b ) , a t r end tow ards compos i t ions

with h ig h 2spb/Z4pb and low 26pb/Z4pb ra t ios , d ue

t o h i g h T h / U r a t i o s , c a n b e r e c o g n i s e d . R e s e r v o i r s

w i t h h i g h T h / U r a t io s a r e k n o w n t o e x i st in t h e l i t h o -

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

T h e n r i c h m e n t b y m e t a s o m a t i s m W i l li a m s e t a l. ,

1992 ; Car l s on and I rv ing , 1994) . 2 ) I n the S r -P b

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

t o w a r d s a c o m p o s i t i o n t h a t sh o w s H I M U f e a t u r e s f o r

P b , b u t i n t e r m e d i a t e 87Sr 86Sr a n d m N d / 1 4 4 N d r a -

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

i s l and bas a l t s . S imi la r i s o top ic cha rac te r i s t i c s have

p r e v i o u s l y b e e n d e s c r i b e d b y P a s l ic k e t a l. 1 9 9 5 )

f o r y o u n g v o l c a n i c s f r o m T a n z a n i a . P a s l i c k e t a l .

1 9 9 5 ) w e r e a b l e t o m o d e l t h e i s o t o p e a n d t r a c e -

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

s u m i n g a l i t h o s p h e r i c m a n t l e e n r i c h e d w i t h a 2 G a

O I B s o u r c e . I s o t o p ic h e t e r o g e n e i t i e s i n E a s t A f r i c a n

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

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

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

d i f f e r en t t imes to d i f f e r en t deg rees .

T h i s c o n c l u s i o n i s i n l i n e w i t h x e n o l i t h e v i d e n c e

f r o m t h e E a s t A f r i c a n R i f t . G e o c h e m i c a l a n d m i n -

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

H i l l , O l m a n i , a n d L a s h a i n e a l l n o r t h e r n T a n z a n i a )

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

m en t in the l i thos pher i c man t l e D aw s on e t a l ., 1970;

Re id e t a l . , 1975 ; J ones e t a l . , 1982 ; D aw s on and

S mi th , 1988 ; Rudn ick e t a l . , 1993) . I s o tope da ta

fo r thes e xeno l i th s r evea l s t rong he te rogene i ty , i nd i -

c a t i n g a n c i e n t e n r i c h m e n t p r o c e s s e s C o h e n e t a l. ,

1 9 8 4 ), a l t h o u g h s o m e o f t h e l i t h o s p h e r i c m a n t l e h e t -

e r o g e n e i t y s e e m s t o r e s u l t f r o m y o u n g e r , s h o r t- l iv e d


13/15

A. Kalt et al./Tectonophysics 278 1997) 31-4 5 43

a n d s m a l l - s c a le d m e t a s o m a t i s m w i th a s t h en o s p h e r ic

c h e m i c a l i m p r i n t ( D a w s o n a n d S m i th , 1 9 88 ; R u d -

n i c k e t a l . , 1 9 9 3 ) .

A m a j o r r o l e o f t h e l it h o s p h e r e i s fu r t h e r s u p -

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

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

a r e a s . T h e d i s t i n c t is o t o p i c t re n d s d i s p l a y e d b y e a c h

o f t h e s e a r e a s m o s t l i k e l y r e fl e c t th e d i f f e r e n t t e c -

t o n i c s e t t i n g s ( s e e S e c t i o n 4 . 2 ) a n d t h u s c o n t r i -

b u t i o n s f r o m d i f f e re n t p o r t io n s o f a h e t e r o g e n e o u s

l i t h o s p h e r i c m a n t l e .

A l i t h o s p h e r i c o r i g i n i s a l s o c o n s i s t e n t w i t h e x -

p e r i m e n t a l r es u l ts , re q u i r i n g u n u s u a l m a n t l e c o m p o -

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

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

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

( W a l l a c e a n d G r e e n , 1 9 8 8 ; E g g l e r , 1 9 8 9 ; W y l l i e ,

1 9 8 9 ; D a l t o n a n d W o o d , 1 9 9 3 ; S w e e n y , 1 9 9 4 ) . T h e

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

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

A f r i c a n R i f t ( a n d i n g e n e r a l) i n d i c a t e t h es e m a n -

t le c o m p o s i t i o n s t o o c c u r o n l y i n s m a l l p o r t io n s o f

t h e m a n t l e . C a r b o n a t e d p h l o g o p i t e a n d / o r a m p h i -

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

c a r b o n a t i t e m e l t s a t d e p t h s b e t w e e n 6 0 a n d 1 2 0 k m

( W a l l a c e a n d G r e e n , 1 9 8 8 ; T h i b a u t e t a l ., 1 9 9 2 ;

W h i t e a n d W y l l i e , 1 9 92 ; S w e e n y , 1 9 9 4 ) , i n d i c a t -

i n g t h a t g e n e r a t i o n o f c a r b o n a t i t e s e n t i r e l y i n th e

l i t h o s p h e r e i s p o s s i b l e i n r e g i o n s o f v e r y t h i c k l i th o -

s p h e r e . E s t i m a t e s o f l i t h o s p h e r i c th i c k n e s s , d e d u c e d

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

l i th s ( H e n j e s - K u n s t a n d A l t h er r , 1 9 9 2 ) r a n g e f r o m

7 5 k m b e n e a t h M a r s a b i t ( N - K e n y a ) t o 1 1 5 k m b e -

n e a t h C h y u l u H i l l s ( S - K e n y a ) to > 1 45 k m b e n e a t h

L a s h a i n e ( N - T a n z a n i a ) .

cknowledgements

T h e a u t h o rs w i s h t o t h a n k J 6 rg K e l l e r f o r p r o v i d -

i n g s am p l e s T K - 1 2 , T K - 1 4 , T K - 2 6 , H B - 1 9 , H O - 1 ,

H O - 4 , a n d H B - 3 0 a n d f o r c r i t ic a l c o m m e n t s o n a

f i rs t d r a f t o f t h e m a n u s c r i p t . W e w o u l d a l s o l i k e t o

t h a n k R a i n e r A l t h e r r fo r d i s c u s s i o n a n d h e l p f u l c o m -

m e n t s . A . S i m o n e t t i a n d H . G . S t o s c h a r e t h a n k e d f o r

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

b y t h e D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t w i t h in t h e

S p e c i a l R e s e a r c h P r o g r a m S t r e s s an d s t re s s r e l e a s e

i n t h e l i t h o s p h e r e ( U n i v e r s i t y o f K a r l s ru h e , S F B

1 0 8, C 2 ) i s g r a t e f u l ly a c k n o w l e d g e d . T h i s i s S F B

1 0 8 c o n t r i b u t i o n N o . 5 8 3 .

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