University of Nigeria Research Publications

OBIORA, Smart. Chika A

utho

r

PG/M.Sc/91/12828

Title

Petrology of Rocks West Anyim River, Lower Benue Trough

/Fac

ulty

Physical Sciences

Dep

artm

ent

Geology

Dat

e

June, 1994

Sign

atur

e

Petro logy o f Magm~t ic Rocks

West of Anyim R i v e r ,

Lower Renue Trough

0910RA, SMART CtI IKA

,'B..Cic. Irons. ( N i g . )

A D i e ~ e r t s t i o n presented to t h e Department o f Geology

U n i v e r s i t y o f ~ i g e r i a , ~ s u k k a i n p e r t i a l

f u l f i l m e n t o f t h e requirements f o r

t h e n w ~ r d o f d e ~ r e e o f MASTER

OF S7 TENCE (M .SC. ) I N GEOLOGY

UNTVERSTTY OF NIGERIA, NSUKKA

JUNE, 1991+

Mr. O b i o r a , Smart q h i k a , a p o s t g r a d u a t e s t u d e n t

i n the Department o f Geology, U n i v e r s i t y o f N i g e r i a ,

N ~ u k k n , h a s S A t isfac t o r i l y comple ted t h e r e q u i r e m e n t s

f o r t h e r e f i e a r c h p r o j e c t and c o u r s e work f o r the degree

o f lilnp t.er o f & c i c n c e CM.Sc. Minera logy and ~ e t r o l a g y )

The work embodied i n t h i s d i ~ t 3 e r t a t i o n i f 3 o r i g i n a l

end h n s no t been e u b m i t t e d i n p a r t o r i n f u l l f o r any

D t h e r diploma o r d e g r e e o f t h i s o r any o t h e r u n i v e r s i t y .

P r o f . (Dr . ) A . u m i j i (Superv imor ) T'epnrtment o f G e o l o g y Univer f i i ty o f N i g e r i a Nfiukke

P r o f . ( ~ r . ) C .O. Okogbue Head n e p ~ r t m e n t o f Geology U n i v e r s i t y o f N i g e r i a N s u k k ~

J U N E , 1992 -

ABSTRACT

R e s u l t 8 o f f i e l d , p e t r o g r ~ p h i c , and majo r element

~ ~ o c h ~ m i c e l ~ i n v e s t i g a t i o n s o f magmatic r o c k s west o f

nnyim R i v e r e r e p r e s e n t e d , The magmatic r o c k s i n v a d e d a

c ~ l c a r e o u s s e d i m e n t a r y Sequence made up o f e h a l e , a l t e r n a t - \

i n g 8 h a l e - s a n d y s h a l e , and sil t s t p n e u n i t e . The s e d i m e n t a r y

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

w i t h n ~ e n e r a l NE-SW t r e n d and an a v e r e g e d i p v a l u e o f 30'.

The shn1.e u n i t i g c h a r a c t e r i s e d by m i n e r a l i s e d j o i n t s w i t h

j 3 0 ° ~ ~ ~ ; - - 1 1 j 0 0 ~ ~ ~ and ~ O ~ E N E - ~ L C O ~ W S W t r e n d * , f i l l e d by

c~ rbon -1 t : e m i n e r ~ l n ( c n l c i t e and n j d e r i t e ) , q u a r t z , and

somet i m e n by p y r i t e , ~ a l e n a and s p h a l e r i t e .

o f twenty-one (21 ) o u t c r o p s o f t h e magmatic r o c k s

s t r ~ d i c d , t h r e e ( 3 ) e r e o l i v i n e - b a s a l t s , s e v e n (7) d o l e r i t e s ,

f i ix ( 6 ) t u f f s nnd l n p i l l i t u f f s , and f i v e ( 5 ) are d i o r i t e e .

O n e o f t h e d i o r i t e b o d i e s t h ~ t i n t r u d e d t h e f i h a l e u n i t is

s u r r o u n d e d by a 2 1 m-wide c o n t a c t n u r e o l e c o n s t i t u t e d by

v e r y low ~ r a d e rocks; bnked u h n l e and s p o t t e d s l a t e .

The r nag rn~ t i c r o c k s n r e g e n e r ~ l l y f i n e - t o medium-

g r ~ i n e d (0.51-3.60mm, g r a i n - s i z e ) . The m i n e r a l c o m p o s i t i o n s

o f t h . e d i o r i t e s a r e e s s e n t i a l l y c l i n o p y r o x e n e ( a u ~ i t e ) ,

p l a ~ i . o c l . n s e ( ~ n ), b i o t i t e , ' end r a r e l y h o r n b l e n d e w i t h I 35-38

ii.

n c c e s s o r y mngnet i t e , p y r i t e , a n d a p a t i t e . The b a s a l t i c

s j 11s nnd d o l e r i t e s c o n t ~ i n e s f i e n t i n l l y a u g i t e , c a l c i c

p l ~ g i o c l a s e (An 1 , and o l i v i n e , w i t h a c c e s s o r y p y r i t e and 5 7

m a g n e t i t e . M ~ J o r i t y o f these i g n e o u s r o c k s show p e r v a s i v e

d e u t e r i c ~ l t e r n t i o n i n d i c a t e d by t h e p r e s e n c e o f a low-

t e m p e r a t u r e aAsemblage: C a l c i t e . c h l o r i t e , s e r i c i t e ,

f 4 ne-6:r:qined q u a r t z and opftque o x i d e s / s u l p h i d e s . T h i s

nl.tev.nticnn i~ an i n d i c n t i o n o f t h e nbundance o f wa te r i n

t h c In ;I Q IR FI . ~ : ~ P T F ~ C R ~ ~ n ~ l y 6 e ~ o f t e n r e p r e s e n t a t i v e sarnples g a v e

S i O v a l u e s o f 48.149% and 46.38% f o r t h e two a n a l y s e d 2

b n s a l t ~ , 46.06 t g 49.32% f a r t h e d o l e r i t e s , 42.45 t o

411.34% f o r t h e p y r o c l ~ ~ t i c s , ~ n d 50.34 t o 52.26% f o r t h e

d l o r i tw. T o t e l v l k n l i (Na20 + Kp) in a l l t h e r o c k s

rpngps from I t o 9.82%. w i t h N~~O(2.62-6.55%)>

K 0(0-(10-3.7696). ~ i p ; h t o f t h e ana l .yses h a v e n o r m a t i v e 2

nephaline v ~ l u e f i af 0.39 t o 22.69% (9.18%, a v e r a g e ) and

~ b u n f i a n t n o r m a t i v e o l i v i n e o f 2.28 t o 22.33% ( 15.38%~

a v e r a g e ) . Only two samples have n o r m ~ t i v e h y p e r s t h e n e

vnlueo o f 8.93% ~ n d 12.80%. w i t h q u a r t z , 3.36%. The

m i n e r n l o g i c ~ l c h p I - 9 ~ t e r i s t i c s and t h e c h ~ r a c t e r i s t i c a l l ~

h j gh n l . k n l i - f i i l i c e r a t i o s i n d i c ~ t e t h a t t h e r o c k s are

olknlinc r e k ~ t i v e t o c a l c - a l k a l i n e and t h o l e i i t i c va r i e t i e s . '

They h ~ v c , i n a d d i t i o n , low a l k a l i - a l u m i n a r a t i o s .

iii.

The predomjnant ly n l k ~ l i n e , s t r o n e l y sodic with

I-PS~FC t t o po t ~ s h nnd t h e nlumi noun chnrec ter is consis-

tent w i th thnt known in most igneous rocks from other

P P T ' ~ , , P : o f t h e Benue Trough. This n l . ~ o compare8 favourably

wi t.h t h n c h n r n c t e r of' rocks i n o t h e r c o n t i n e n t a l r i f t s such I

P S t h e B e i k a l r i f t , i n Siberia. The r o c k 6 d i f f e r from

t h o s e o f t h e East Africnn r i f t eystem o n l y i n t h e

nlurninoufi c h n r ~ c ter.

Page

I . .I r j n c k ~ r ' ~ ~ ~ i n r l t o t h e problem . . . 1.3 ~ ~ c p t i n n , Pccpns ~ n d 7ommunicqt ion I . -T T o p o ~ r n p h y nnd d r ~ i n ~ g e . . . . . . 1 .I1 T l i r n n t r , V e g c t n t i o n , and S o i l s .. . I. 5 F r c v i o u s i n v c n t i p ~ t i o n s . . . . . . I.( Nnturc o f p r c s c n t s t u d y . . .-. 1-7 M e t h o d of i n v e n t i g n t i o n s . . .

R E G T O N bL, GEOLOGSCA~, SETTING 0 . .

i i v v i

v i i vii i

i x

P e F r o g r n p h y o f t h e m n g m a t i c B n s n l t , i c s i l l s .. . ... Pa 1 er i t, e s ..- ... P i o r i t c r ; . . . ... T u f ' f ~ ... ... ... 1 , n p i l l i t u f f s ... .. .

r o c k s ... 0 . . ... ... ...

l ' e t r o w c 7 p h y o f t h e c o n t n c t m e t a m o r p h o s e d shnlcn ... .. . ... ... S p o t - t ~ d s l . n t c ... . . . ... R n k e d n h p l e . . . ... . . .

A n - L y t - i c n l p r o c e d u r e s .. . ... r j e ~ u l t ~ ... . . . ... . . . Mngmnt ic rocks ... 0 . . ... 7 o n t n c t r n e t ~ m o r p h o n e d s h n . l e s . . .

Ilnp o f p n r t o o f t h e Lower H ~ n u e Trough .... showi np t h e s t u d y a r e s . . - a

Mnp o f EnuQu S t ~ t e ~ n d e n v i r o n s showing n c c e s s t.13 t h e 6 t u d y n r e n .... ....

0 u f ? c r o P m ~ p o f nrens West ~ f Anyim R i v e r

G e o l o p i c n l mnp o f nrc)as West o f Anyim .... .... R i v e r .... .... I l l u s t r ~ t i v e s k e t c h o f t h e r r l a t i o n s h i p h e t u e ~ n t h e c o m p o s i t e d i o r i t e and t h e h o s t s h a l e 8 t t h e metamorph ic ~ u r e o l e l o c n t c d n t "Ngele Opoo" nnd " N g e l e ~ , c b n r v " - Locs. 62 nnd 6 2 0 . .... S k c t c h o f t h e m i a r o g r n p h s o f t h e .... p y 1 ~ c l n . r t . i ~ r o c k s .... ....

b L k n 1 . i - s i l i c n d i a g r a m f o r t h e magmatic r o c k s .... .... .... .... T l i s t r i b u t . i o n o f t h e m:?t!:mntic r o c k s Wert o f 4nyim R i v e r o n t h c n o r m ~ t i v e d i a ~ r n r n o f T r v i n e nnrl R n r n g n r .... I ' o t n s h - s o d a rl jagram f o r t h e rnngmatic r o c k s ..... .... .... .... A l k ~ l i -nlumi.na d i e g r a m f o r t h e magma t i c r o c k s .... .... .... .... f : o m p - r i w n o f t h e r o c k s w i t h o t h e r t i i n t h e 9r.nue 'Prough o n t h e a l k n l i - s i l i c a rl j 8p:rnrn .... .... .... .... n ,omp,orison w i t h r o c k s f rom o t h e r c o n t i n e n t 8 1 r i f t s o n t h e a l k a l i - a l u m i n a r l j ~ p r n r n .... .... .... ....

l l o d n l compon i t ioriw o f t h e mngma tic 1 . 7 ~ 1c.c: . . . . . - . I . * . a 23

N n j o r - c - L w n e n t oxide d n t e , w i t h CTI'W norms f o r t h e r n a g r n n t i c rocks 40

F l n j o r - e l e m e n t oxides d ~ t n f o r t h e c o n t n c t metarnorphoced ~ h ~ l e ~ . . . LC2

0 u t ; c r o ~ ) o f 8 f i r ~ e L y I n m i n n t e d s i l fiil t s t o n c b e n e a t h t h e s u s p e n s i o n b r i d g e a c r o s s R i v e r o d i d i . . . . ....

A s r n s s - n ~ c t i q n o f t h e rn icn?eous s h a l y s n n d ~ t o n c lcnnc n t ? ~ w e l e d e o h n - 1,oc. 31 - n h o w i n g b e d d i n g .. . . ....

v i i i

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19

19

2 1

2 1

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ix.

ACKNOWLEDGEMENT

T wish t o thenk my d e ~ r s u p e r v i s o r and immedia te b o s s ,

I ' r o f e s s o r A . C . U m e j i f o r s u g p e s t i n g t h i ~ work and t a k i n g

rill t h e p a i n s a t n 1 1 t h e fitage8 t o s e e t h a t i t is

s u c c e s s f u l . Hie f e t h e r l y c a r e , i n v a l u a b l e s u g g e s t i o n s , \

m o r n 1 and f i n n n c j n l / m ~ t e r i a l . s u p p o r t were h i g h l y

indi.spensab1.e i n t h e comple t ion o f t h e work. My g r a t i t u d e

is ~ l s o d u e t o t h e p r e s e n t Head o f Department o f Geology,

P r o f . C. 0 . O k o ~ b u e ~ n d h i s s t a f f f o r t h e i r c o - o p e r a t i o n a t

t h e v n r i o u s s t n g e s o f t h i s work.

It i s n l s o p e r t i n e n t t o ~ c k n o w l e d g e t h e a s s i s t a l i h e o f

t h e f o l low in^ i n d i v i d u ~ l s ~t t h e v ~ r i o u s s t a g e s o f t h e work.

They i n c l u d e my s e n i o r c o l l e a g u e s , D r . K . 0. Uma, and Mr.

1'. 0- Anike ( n p o s t g r n d u n t e s t u d e n t i n t h e depa r tmen t and a

l e c t u r e r R t t h e G e o l o g i c e l S c i e n c e s Depar tment , Nnamdi

Azikiwe U n i v e r s i t y , A w k ~ ) who p rov ided t h e i r v e h i c l e s i n

two o c c ~ s i o n s o f f i e l d v i e i t s . The t h e n s e c r e t a r y t o t h e

' z x i L o c ~ l Gov~rnrnent Area, Mr. Krian ~ z i k e p rov ided

nccommod~t ion f o r me, R R well ~ f i a l e t t e r o f i n t r o d u c t i o n

to t h e v i l l a p e c h i e f e / c o u n c i l . l , o r s i n t h e a r e a who were

v w y h o ~ p i t a b l e t o me duri.np; t h e f i e l d work i n t h e i r

. l o r n 1 i t i e s . Mr. Alphonso Ugwii nnd h i s c o l l e a g u e s i n o u r

d e p ~ r t m e n t e l Workshop p r e p ~ r e d t h e t h i n - s e c t i o n s used f o r

p e t r o g r ~ p h i c fi t u d i es. The samples f o r chemica l a n a l y s e s

X

were p r e p ~ r e d u n d e r t h e d i r e c t i v e f i ~ n d ~ u p e r v i s i o n o f

Mr. S . E. Ani o f o u r G e o c h e m i s t r y L a b o r a t o r y . H e a l s o

c ~ r r i ed o u t t h e U V - v i s i b l e s p e c t r o p h o t o m e t r y a n a l y s e s a t

t h e Food S c i e n c e and T e c h n o l o g y L ~ b o r a t o r y o f this

i n s t i t u t i o n . The AAS a n e l y e a s were d o n e at t h e Soil

Sc i r % n c e ( ~ i n k n ~ c ) Wet C h e m i s t r y L n b o r n t o r y , a l s o o f t h i s

I l n i u c l r ~ i t y . q h i c f V i n c e n t 0 f " l e b e , D i r e c t o r , BENCOV

C o n ~ t r u c t j o n C o m p ~ n y , Abe p r o v i d e d some f u n d i n a i d o f t h e

n n n l y s e s .

T A m hfph1.y i n d e b t e d t o t h e U n i v e r ~ i t y o f N i g e r i a

f o r p i v i n ~ m e t h e o p p o r t u n i t g t o s e r v e and do t h i s s t u d y

n s n C I r n d u ~ t e Aeei s t e n t i n t h e Geology Depar tment t h r o u g h

t h e recommendnt jon o f P r o f . I l m e j i .

Above ell, t h e g u i d ~ n c e nnd p r o t e c t i o n by my good

God t h r o u p h o u t my l f f e , e R p e c i a l l y d u r i n g t h e p e r i o d o f

t h i s work i a Rapreme.

CHAPTER ONE

IN TRO DU C T ION - ?. 1 -...-- Ilnckground to- the problem.

The Benue Trough ( F i g . 1 i n s e t ) is f i l l e d by a e e q u e n c e

of C r e t n c e o u s sedimentary and magmatic r o c k s . Only r e c e n t l y

h ~ f i i n t e r e s t been shown i n t h e m a g m ~ t i s m and i t s s i g n i f i c a n c e

on t h c o r i ~ i n o f t h e Benue r i f t . I I f t h e r t o , t h e magmatic

r o c k s i n t h e w e s t e r n f i i d e e of Anyjm R i v e r were r e l a t i v e l y

~ ~ n k r ~ o w n compnred t o t h e o n e s n t t h e e ~ s t e r n s i d e s - Workum

H i 11 n nnd Wanakmde.

T h i s s t u d y was t h e r e f o r e u n d e r t ~ k e n t o f i n d o u t t h e

s p a t i n 1 d i s t r i b u t i o n ond p e t r o l o g i c c h ~ r a c t e r i f 3 t i C ~ o f t h e .

rnagmn t i c ( i E n p o u ~ ) r o c k s .

I? L o c a t i o n , Access and C o m m u n i c ~ t i o n

The r o c k s p r e s e n t e d i n t h i s r e p o r t c r o p o u t w i t h i n

Lat i turies 6 ° 3 0 ' ~ - 6 0 4 5 ' ~ and L o n g i t u d e s 8 ° ~ 3 ' ~ - 8 0 2 6 ' ~

(Fip. 2 ) . The l o c n l i t i e s i n which t h e y o c c u r a r e i n I z z i

L o c ~ l Government aren i n t h e A b n k a l i k i D i v i s i o n o f Enugu

S t n kc. T h e s e loanli t i e s i n c l u d e Mkpumeakwaokuku, Otam,

N d i n p e l e , Omingi , Opankwa, Ndiogogo, Ndinwikwe, N d i - e z i o k e ,

~ ~ e f i , Of erekpe, Opbnru, Okpongs-ozze , Ndi-ogbagn o b a s h i ,

and O n u ~ n y i m Acbajn.

I 3

Triter-connec t i n p minor r o a d s and foo t p ~ t h s i n t h e s e

form1 nnd-dominn t.pd l o c a l i t j e~ p r o v i d e e a s y a c c e s s t o t h e

o u t r ~ ~ o p s . T I o w e v ~ r , rnovemcnt f r ~ m o n e l o c a l i t y t o a n o t h e r

i ~ , i n moqt cpses , not easy b e c n u s e o f h i g h d e n n i t y o f

r J r p i n n K p whicp do n o t h ~ v e c u l v e r t s b u i l t a c r o s s them.

'L'hr, o r ~ ? c r q p s , mnny o f which R r c . .Lor -1 t e a a l o n g t h e Anyim R i v e r

PI.F 71')k v e r y e ~ s i 1 y n c c e s e i b l e h c ? c n ~ ~ n e o f t h i c k v e p ; e t n t i o n

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

r i v e r b a n k s n l o ~ ~ f o o t p a t h s t h r o u g h which t h e v i l l a g e r s g e t t o

t h e n t r e ~ r n s tr, f e t c h w a t e r f o r d o m e s t i c p u r p o s e s .

Tn t e r v i pws w i t h t h e v i 1 l n p ; e C h i efe ~ n d c o u n c i l l o r s

who n r r n n g e d f o r f'i e l d g u i d e s r n ~ d e i n v e q t i g ~ t i o n s e a s i e r .

'Chir rrnn n e q e s s n r y , e s p e c i ~ l l y i n t h e s o u t h e r n p a r t s of t h e

s t u d y a r e a w h e r e t h e o u t c r o p s n r e h j ~ h l y r e v e r e d by v i l l a g e r s

who cotnmonly h ; r v ~ t h e i r e h r i n e s and p l f l y ~ r o u n d 6 at t h e

o u t c r o p s o f t h e mncmat ic r o c k c .

O i ~ t , s t n n d i n ~ t r b n i t s t o f i e l d w o r k i n t h e area i n c l u d e :

( 1 ) r i o n - a v ~ i l n h i 1 i t y o f m e n n s o f movine f rom o n e l o c a l i t y

ko noth her d i f i t n n t one. M o t o r / r n o t o r c y c l e s are n o t a v a i l a b l e

f o r t r U : 1 n ~ p o r t n t i o n . Comrnerci a 1 v e h i c l e s which c o n v e y f a r m

prorlu-tr , o r e a v n i l ~ b l e o n l y on mnjor m a r k e t days .

5

( i i ) non-ava i l ab i1 . i t y o f nccommodation and d r i n k i n g w e t e r

e s p c c j n l l y i n t h e c o u t h e r n p:,rts.

7.3 Topography and d r ina age

The a r e a i~ g e n e r a l l y a p l a i n , w i t h s p o r a d i c h i g h l a n d s

c o n s t i t u t e q by t h e magmatic r o c k s and s a n d y s h a l e s . The

mnx.imum e l e v ~ t i o n is 90m above f i e R l e v e l -

The Anyim R i v e r , f l o w i n g i n a n e f i s e n t i e l l y NW-SE t r e n d ,

f o r m s R majo r d r a i n a g e i n t h e e r e e . It h a s numerous t r i b u -

t.nr i es which d r a i n t h e en t i r e l o c a l i t i e f i . These i n c l u d e

Oleme, Tukuruma, Nwangel e , Ako , l'Abefq N d i - ~ z i o k e , and llAbe"

O f s r c k p e .

1.4 C l i m a t e , V e g e t a t i o n , and S o i l s

Ra iny s e a s o n i n t h e s t u d y Rrea l n s t ~ be tween t h e months

o f March and O c t o b e r w h i l e t h e p e r i o d be tween November and

mid-Mnrch i s d ry . The are^ h n s Rn a v e r a g e a n n u a l t e m p e r a t u r e

0 o f ,?9 C . B e c a u s e of t h e s e c l i m ~ t i c c o n d i t i o n s and t h e

fcrti1.e c l a y e y s o i l d e r i v e d f rom the w e a t h e r i n g o f t h e

cn rbonnceous shn le, a s w e l l n x t h e m a g m ~ t i c r o c k s , v e g e t a t i o n

i n the a r e a is t h i c k . The c ~ r b o n ~ c e o u s s h a l e a l s o w e a t h e r s

t o a p i n k i s h s o i l . T h i s i n d i c a t e s h i g h e r r a t e o f o x i d a t i o n , ,

common i n h i l l y Rreas .

6.

1 , n t e r i t i . c r n a t e r i ~ l ~ d e r i v e d f rom t h e d i s a g g r e g a t i o n

o f t h ~ magmat ic bodies and t h e h n r d c n r b o n a c e o u s f 3 h ~ l e a r e

conrent , rnted a l o n g some o f t h e ~ t r e ~ m s ' v a l l e y s i n t h e

s o u t h ~ t ' n p a r t s o f t h e n r e s . T h e s e l a t e r i t i c m a t e r i a l s

c h n r n c t ; c r i s t i ~ a l l y o c c u r i n t h e fo rm o f c o n g l a m e r a t i c

n g p - q t a t e s r e f e r r e d t o a s tfekwu"' by t h e v i l l a g e r s . The

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

o f t h e l e u c o c r a t i c p o r t i o n o f a compof i i t e d i o r i t e a t ~ g e f i .

S i l t y to s a n d y f i o i l ~ were d e r i v e d f rom t h e d i s a g g r e g a t i o n

o f s j l t s t o n e s and s e n d y s h a l e s .

1.5 - P r e v i o u s i n v e ~ t i g a t i o n s - E:vidcnc e f rom l i t e r a t u r e rjhows t h a t t h e e a r l i e s t mapping

o f t h e r o c k s i n t h e a r e a was o n a r e g i o n ~ l s c a l e . It was

c a r r i cd o u t by t h e SHELL-UP who p r o d u c e d t h e G e o l o g i c a l map

o f O p o j n ( s h e e t No. 73; 1 : 2 5 0 , 0 0 0 ) i n 1957. Most o f t h e

mngrn;.tic r o c k s w e s t o f t h e Anyim R i v e r were, however , n o t

r e p r w e n t e d o n t h a t mnp. O k e z i e (1961) p r o d u c e d a d e t a i l e d

map o f t h e magmat,ic r o c k s i n Ogo j a , Renue, ~ n d A b ~ k a l i k i

p r o v i n c e s . The work i n c l u d e d o n l y t h e s o u t h e r n p a r t s o f

t h e a r e a c o v e r e d i n t h i ~ fi tudy. H i s r e p o r t W R R p u b l i s h e d

b y t h e G e o l o ~ i c ~ l S u r v e y o f N i g e r i a ( G S N ) o n a g e o l o g i c a l

7

m n p of l e e k w e , Wanikande ( ~ e f i n ) , O j u , and Nkpumeakwaokuku

d i s t r i c t s , on a f icn le o f 1: 100,000. T h i s map a n d h i s e a r l i e r

description o f t h e r o c k s ( ~ k e z i e , 1957) c o n s t i t u t e t h e o n l y

docuni t~n ted a c c o u n t o f t h e magmat ic r o c k s i n p a r t s of t h e

s t u d y area. The magmat ic r o c k s e a s t o f t h e Anyim R i v e r - I

Workl~m Hi.1.l.s and W n n ~ k a n d e a r e 8 6 -.- were e l s o s t u d i e d by

Renkhri1.il ( 1986) d u r ' i n a h i s i n v e n t i a n t i o n s o f s t r u c t u r e s i n

the a r e a ,

1.6 - N a t u r e --.-- o f p r e s e n t s t u d x -- T h i s work i n v o l v c f i s u r f a c e gcalogdc m ~ p p i n g , p e t r o -

p;rnpli ic r n d p;rochrmicrl l s t u d i e s o f t h e m n g r n ~ t i c r o c k s , The

s t u d y s e e k s t o u r i r n v e l t h e f o l l o w i n g :

( i ) t h e e x t e n t ond s p n t i a l d i s t r i b u t i o n o f t h e

m8gmptic r o c k s i n t h e a r e a .

(ii) t h e i r r e l . n t i o n e t o t h e h o s t rock^ end a n y

a s s o c i ~ t e d structures/miner~lis~tion.

( i i i ) t h e i r i n f l u e n c e o n t h e c o u n t r y ( h o s t ) r o c k s ; and

( i v ) c l n s f i i f i c ~ t i o n / n o m e n c l a t u r e o f t h e m ~ g m a t i c r o c k s

throup;h p e t r o ~ r o p h i c and ~ e o c h e m i c ~ l d a t a .

1.7 !lfli!~od o f i n ~ e w t ~ i ~ a t i o n s

Il ' ic>ld mappiny i n v o l v i n g t h e r?x?rninat ion o f t h e r o c k s

~t t l ~ (< i r C ) I I ~ C ~ O ~ E W R F c ~ r r i e d o.ut i n two s t ~ p e s b e t w e e n

P ~ c c r n b e r 7 and 13, 1992, a n d March 1 t o 16, 1993. The

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

w h i l e t h e s e c o n d s t ~ g e i n v o l v e d t h e mapping o f t h e n o r t h e r n

p n r t ~ nnd c r o n a - c h e c k i n g o f t h e work done i n t h e s o u t h e r n

p ~ r t ~ . T h e r e were e l ~ o f i e l d t r i p s o n two o t h e r o c c a s i o n s

be twccn 2 0 t h and 32nd J u l y , 1993, ond f rom 14-16 th J a n u a r y ,

193/+. 'I!he f i r ' s t t r i p waR i n thc compnny o f my ~ u p e r v i s o r

and D r . K.O. urn^ - h y d r o g e o l o g i o t i n t e r e f i t e d i n s a l i n e

w a t e r s i n t h i s b e l t w i t h i n t h e Trouph. The s econd t r i p

was w i t h my s u p e r v i s o r And Mr. O.L. Anike - a p o s t g r a d u a t e

f i t uden t who i s s t u d y i n g t h e s t r u c t u r e 6 w i t h i n t h e Trough.

P r w h ~ n d r e p r e s e n t n t i v e R R ~ ~ ~ P R we re c o l l e c t e d f rom

t h e o~r t ;cropr ; f o r p ~ t r o p - ~ p h l c ~ t u d i e~ end ma j o r - e l e m e n t

E e o c h r ~ m i c n l . nnnl yncs. J ' e t r o p r n p h i c n n ~ l g f i e f i were d o n e by

t h e 1 r s 0 o f p r t r o l o g i a m i c r o s c o p e e . A n n l y s i s o f Na 0 , K20, 2

? n o , f1~0, FeO( t ), and P 0 w n s by t h e u s e o f t h e a t o m i c 2 5

ab.c;or.ption s p e c t r o p h o t o m e t e r ( P A S ) whe rea s s i l i c a ( s ~ O ) 2

and n l ~ l m i n a ( ~ l O ) were ~ n a l g f i e d by t h e ufie o f t h e UV- 3

v i s i h l c s p e c t r o p h o t o m ~ t e r .

' L % P A F . S a n a l y s i s g n v e c o n c e n t r e t i o n s o f t h e ma jo r e l e m e n t s

which w e r e c o n v e r t e d t o t h e i r r e s p e c t i v e o x i d e s . F e r r o u s

i r o n ox ide ( F ~ O ) was o b t a i n e d f rom t h e t i t r a t i o n o f a

s t a n d n r d r o c k sample . Norme t ive mine r81 c o m p o s i t i o n s o f

t h e rocks were c n l c u l p t e d from' t h e o x i d e s u s i n g t h e ClPW

method.

CHAPTER TWO

3.0 WEGLONAL GEOLOGTCAL SETTING

T h e Anyim ( 0 k p a u k u ) R i v e r i a s t r i b u t a r y o f t h e

? ros r ; -River . It f l o w s i n a n e s s e n t i ~ l l y NW-SE t r e n d ,

t r n v c r s j n g tFc A b n k e l i k i a n t i c l i n o r i u m which is a ma jo r

~ t r u c t u l ' ~ 1 f ~ n t u r e i n t h e 1,ower. Renue Trough ( F i g . 1 ) .

The S c n u e T r o u ~ h i t s e l f i s A n i n t r a - c o n t i n e n t a l

t e c t o n i c e l emon t w i t h Rn e s s e n t i ~ l l y NE-SW t r e n d , v a r i o u s l y

r e f c r r e d t o n s Benue r i f t o r ~ u l a c o g e n lade, 1975) . I ts

cvol.11t ion rnipht h a v e s t a r t e d w i t h m n ~ m n t j s m i n t h e L ~ t e

V n l n n p : i n i n n ( J U ~ R E ~ S ~ C ) t o E a r l y C r e t ~ c e o u s times (Umeji and

Caen -Vache t t e , 1983, 1991) . The Trough i s f i l l e d by m a i n l y '

R C r r t n c e o u s s e d i m e n t a r y sequence ( T a b l e 1 ). The o l d e s t

s e d i m m t n r y f o r r n n t i o n s - Asu R i v e r Group i n t h e Lower Benue

't'roli~:l1 c r o p o u t i n t h e c o r e o f t h e A b n k ~ l i k i u p l i f t o r

~ n t i c l i no r jum. The Asu R i v e r Group wsa d e p o s i t e d i n modera-

t e l y d e e p m n r i n e w n t e r s d u r i n g t h e e ~ r l i e ~ t documented

m n r i n r t r n n ~ g r e s c i o n i n N i ~ e r i s i n t h e mid-Albien times

( ~ e ~ m ~ n t , 1965 ) . I ts m ~ i n l i t h o - f ~ c i e s is c o n s t i t u t e d by

s a n d y s h a l e s , m icaceous f i i l t s t o n e ~ , and f i n e - g r a i n e d micnceoue

f i onds tones ( ~ r a t c h l e ~ ~ n d J o n e s , 1965). The N i g e r i a n Albdan

s e a - w n s r i c h i n ammoni tes , FIR, w e l l 8 s f o r a m i n i f e r a ,

10

T n b l c 1: S t r a t j g r a p h i c succesfi ion in t h e Lower Benue ' C r o u ~ h (Comp.iled from C r n t c h l e y end J o n e s , 1965, R e n k h e l i l , 7986 1.

---- LOWER R E N U E ( S o u t h e r n Benue

STAGES Anembra b ~ s i n end Afikpo s y n c l i n e )

NSUKKn FM. (Upper Coal ~ e a s u r e s ) -I---- I A J A L T FM. (Fe laebedded s a n d s t o n e s )

I

MAASTR?CHTIhN M A M I J FM. ( ~ o w e r C q n 1 ~ e ~ s u r e n ) .. - -- J

CAMI'AN IAN ENUGU-NKPORO SHALES

CON l AC TAN AWGU-NDEAROH SHALES - EZE-AKU SHALE GROUP

G ENOMAN IAN ODlJKPANT FM . ASU RIVER GROUP

3 W 0"- " " U APTlAN ( 2 ) r 7 C > t) OGOJA SANDSTONE

. FRE-APT!AN CRYSTALLINE BASEMENT

r a d i o l n r i s , ~ t n d pollens (Reyment , 1965). The t h i c k n e s ~ --- o f i t s mnin s e d i m e n t a r y s e q u e n c e was e ~ t i m a t e d t o b e u p t o

750(!m h y SHEI,L ( 195.7) A t the Workum h i l l s , B e n k h e l i l

( 1 ? W c t e sc r i bed t h e r o c k s i n t h i s s e q u e n c e as s h d y silt.8-

t o n c c w i t h f i n e l n m i n ~ t i o n s , g r e y f i n e - l a m i n a t e d c a r b o n a t e I

si I t n t . o n e ~ : a f fec t e d by eyn-sed iment n r y s l u m p i n g , f i n e - g r a i n e d

o i l t s t o n e s r i c h i n ~ n ~ u l n r v o l c e n i c c l a ~ t s and f o r a m i n i f e r a

tes ts , and f i n e - t o medium-grained s ~ n d s t o n e s .

I n t h e A b a k a l i k i ~ n t i c l i n o r i u m , t h e A l b i a n Asu R i v e r

Group i~ o v e r l a i n uncon fo rmab ly by t h e T u r o n i a n ~ z e - A k u

s h n l e Group d e p o s i t e d d u r i n g a eecond m a r i n e t r a n s g r e s s i o n

which mnrked t h e t e r m i n k t i o n o f t h e d o m i n a n t l y c o n t i n e n t a l

c o n d i t i o n s t h ~ t e x i s t e d d u r i n g t h e Cenomanian i n N i g e r i a

( ~ e y m e n t , 1965). The main l i t h o f a c i e s o f t h e Eze-Aku s h a l e

Group i r i much s i m i l ~ r t o t h a t o f t h e Asu R i v e r Group. The

o c c l i r r e n c e o f p u r e l i m e s t o n e b e d s common i n t h e ~ z e - A k u

St l : l l~ Group, c o u l d b e a d i s t i n g u i f s h i n g c r i t e r i o n

( ~ r v k h r . l i 1, 1986 ).

'Yhc s e d i m ~ n t a r y eequenc e i n th e A b a k a l i k i a n t i c l i -

nor ium i s c h ~ r a c t e r i s e d by a series o f NE-SW t r e n d i n g

a n t i c l i n e s end R y n c l i n e s which d e v e l o p e d d u r i n g t h e

~ n r i t o n j nn f o l d i ng ~ n d u p l i f t ( Nw~chukwu , 1972). ~ i k e mos t

p n r t ~ o f t h e Trough , t h e e r e a is ~ l s o c h a r a c t e r i s e d by

numrToiis f r n c t u r c o wi t h N-S, NNE-SSW, ENE-WSW, and WNW-ESE

12

t r p n d t i . Many of t h e s e Fractures o c c u r a8 v e i n s f i l l e d by

c r l c i t e (?a70 1, s i d e r i t e ( F ~ c o 1, q u a r t z ( S ~ O 1, and 3 3 2

~ o r n ~ ~ t i m ~ . ~ ~ 1 s o h J p y r i t e (FPS ) c h ~ l c o p y r i t e (CUF~S~), 2

b a r y t , ~ ~ ( R R s o ~ ~ ) , p;nlenn ( F ~ s ) , and s p h n l e r l t e ( z ~ s ) . T h i s

mi t ~ t t r . : l \ i s n t i o n 0 c c u r . s i n t h e snme bc?l t w i t h m a g m a t i c r o c k n l

a n d n n l t s p r i n ~ s (brines). T h e - b e l t o f m a p n a t i s m and

m i n e r e l i f i n t i o n e x t e n d t i f r o m t h e Lower Benue ( I s h i a g u

d i s t r i c t s ) , t h r o u ~ h t h e M i d d l e Renue , t o a s f a r as t h e

R o u t h e r n m o s t p a r t s o f t h e Upper Benue. The b r i n e s , l i k e t h e

magmnt i l : r o c k s , tend t o b e r e s t , r i c t e d t o t h e o l d e s t f o r m a t i o n e

( ~ 6 1 1 Rivcr G r o u p i n t h e 1,ower PIKI M i d d l e Renue , end t h e Bima

s n n 8 s t o r l r s i n t h e Upper Denue) .

'L'hc m n g m n t i o r o c k n in t h e [,nwer Henue a r e o f b a f i i c t o

i n t e r r n e d i n t e ( d i o r i t i c / t i y e n i t i c ) c o m p o t i i t i o n s , They o c c u r

i n t h c form o f i n t r u s i v e o ( p l u t o n s a n d h y p ~ b y s 6 a l s ) and

e x t r u x i v e s ( l a v a t i n n d p y r o c l a s t i c s ) . The a r e a s w h e r e t h e y

h n v ~ h c e n s t u d i e d i n c l u d e ~ z i ~ t o r H i l l ( ~ u n t h e r t and R i c h a r d s ,

1960) , h f u t Eso i n llp,ep ( I I o s s a i n , 1 9 8 1 ) ~ T s h i a g u ( ~ z e p u e ,

1 ° 9 ? ) , Oyioba-Uganga ( ~ k w u e m e , l 9 9 2 ) , Abakaliki ( U z u a k p n w a ,

7 C ) i ' ) I ; Ol.ride, 1979, I loque , 19814, orn jaka and U m e n w a l i r i , 1 9 8 9 ) ,

Ah,-kal ikj ~ n d ~ z i l l o ( ~ f o e g b u ~ n d Amnjor , 1987) . They h a v e

r71m h e e n s t u d i e d o n a r e g i o n p l ~ c d e by U m e j i ( 1 9 8 5 , 1992).

With t h e e x c e p t i o n o f t h o t i e s t u d i e d b y H o s s a i n ( 1 9 8 1 ) w h i c h

13

hnve t h o l e i i t i c a f f i n i t y t h e r e s t a r e p r e d o m i n a n t l y a l k a l i n e ,

w i t h s t r o n g n o d i c and a luminous c h s r a c t e r -

Rndjomekr ic ~ g c s i n d i c a t e t h a t the magmatic r o c k s are

n o t o n l y p o s t e r i o r to, b u t a r e ~ l s o con t emporaneous with t h e

scd i rncn tnky units jn whjch t h e y a r e empleced. F o r i n s t a n c e ,

t h r ~ y e n i t e r n 8 ~ s s o c i p t e d b a s e l t s n t Wanakande, and Rome'

b n ~ i c d y k ~ a 2nd r ; j l l s o t p a r t s o f t h e Anyim R i v e r emplaced

w i t h i n t h e P l h i n n A G U R i v a r Group n r e , r e f i p e c t i v e l y 101-1- -., -6 3Ma

n ~ ~ c ! 102 - t 3Ma ( ~ l h i n n ) o f nlr;c, 'Phis i s opposed t o some b a s i c

~ . o c k r n l e o ernplnced w i t h i n I.he Ar;u R j v e r Group a t t h e Workum

I l i J . l f i which a r e 80.7 - + 2Ms ( ~ n m p ~ n i ~ n ) o f Age ( ~ e n k h e l i l , 1986) .

Mild c o n t ~ c t metnmorphic e f f e c t i s common i n t h e s e d i m e n t s ,

~ s p e c i n l l y R ~ P L F P , which b o r d e r l a r g e r magmatic r o c k b o d i e s

( C r ~ t c h l e y ~ n d J o n e s , 1965) - Very low g r a d e t h e r m n l

mc~tnrnorphism w n G n l e o r e c o r d e d i n t h e s e d i m e n t s which a r e n o t

i n close ~ ~ s o c i n t i o n w i th mspmatic r o c k s a t t h e Workum H i l l s

( R m k h e l i l , 1986, 1987). Rnsed on aRe d e t e r m i n a t i o n s o n

~ n m p l e ~ of t h c s e d i m ~ n t s this metnmorphism wns t h o u g h t t o b e

r c l p t e d t o t h e R ~ n t o n i n n d e f o r m ~ t i o n i n t h e a r e a ( ~ e n k h e l i l ,

1986, 1987) .

'rhr s e q u p n c e i n c o n s t j . t u t e d ( f rom o l d e s t t o y o u n g e s t ) by a

n h n l e uni t , ~ n n ~ t e r n r t i n g shn1.e-sandy s h a l e u n i t , and a

i 1 t - 1 . I t i I;. L ' ' i , > n i l t .eC"nes , e x p o ~ e d o n l y i n t h e n o r t h e r n -

rest end o f t h e s t u d y Rren , mnke up less t h n n o n e - s i x t h o f t h e

cnt, i T'e spquence . A micaceous shaly s a n d s t o n e l e n s e WAS a l s o

t i . Althntrph no tracts o f l i m e s t q n e were e n c o u n t e r e d

i n t h e s t u d y , n] 1 t h c s e d i m e n t a r y r o c k s , and even t h e magmetic

r q c k s , exhihi t c d ~ t r q n a e f f e r v e s c e n c e r e a c t i o n w i t h d i l u t e

I I y ( 4 r o ~ h l o r i c nci d . The e n t i r e s e d i m e n t n r y s e q u e n c e is f o l d e d '

i n t o n scrip6 of n n t i c l i n e s nnd s y n c l i n e s w i t h 8 ME-SW t r e n d .

0 'I'hr -vnrV,-rpe d i p v n l u r f q r t h e r t ~ r k ~ ; is 30 (rnnf;e: 3-49'].

l ' h ~ ~ rni1qmnt.i o r o c k s o c c u r n r s m n l l p111 tonr:, h y p o b y s s a l s ,

a n d r y r . r ~ c l a s t j c bod i cn which f ' r e q u t r ~ t l y c o n n t i . t u t e t o p o g r a p h i c

h j ~ h s wi t h i n t h c p e n e r n 1 1 y f l e t s e d i m e n t a r y t e r r a i n . V i r t u a l l y

n l l t h e o u t c r o p s of' t h e mnpmntic r o c k s o c c u r w i t h i n t h e s h n l e

u n i t . M n n y nppen r t,o h a v e been emplaced a l o n g t h e g e n e r a l

IlE-fk s t r u c t u r ~ l t r e n d i n t h e p r e a . An a u r e o l e (21m w i d e )

ronr;.t i t11 tcr! b y b-kcd s h ~ l e nnd 6r'o t t e d s l n t e d e v e l o p e d a round

t h r c o n t ~ c t o f thc . l ~ r g e s t pl .uton w l t h t h e h o s t s h a l e .

D m i m p s

Trend of Pb-Zn v r l n s

3. I S e d i m e n t a r y u n i t s

3.1.1 S h e l e u n i t

T h i s l i t h o l o ~ i c u n i t is t h e mos t a b u n d a n t i n t h e area.

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

It6 o u t c r o p s were s t u d i e d ~t l o c a t i o n s 2 , 1 0 , 1 5 , 2 4 , 2 5 , a n d 51. I

The u n i t i s c h a r a c t e r i s e d by j o i n t s w i t h ~ ~ o ~ ( N N w ) - I ~ O ~ ( S S E )

~ n d 6 0 ° ( ~ ~ ~ ) - 2 4 ~ 0 ( ~ ~ t r e n d s . The j o i n t s a r e f i l l e d b y

c n I c i t , e , q u ~ r t z ; s o m e t i m e s by p y r i t e , g a l e n a , and s p h a l e r i t e .

V e i n s c n r r y i n g p n l e n n nnd t i p h n l e r i t e were r e c o r d e d a t t h r e e

1oc ; l t l o t i n : 2, 2 5 , 30-

3.1.2 --.- S h a l e - s a n d y - a u n i t

T h i s u n i t i s c o n s t i t u t e d by a f i n e - g r a i n e d , d a r k - g r e y ,

m i c p c e o u s s h a l e , c o n t a i n i n g s a n d - s i z e d m a t e r i a l s w h i c h make

i t m r ~ ~ e i v e . It is ~ l t e r n n t e d w i t h R d i s t i n c t l y f i s s i l e s h a l e

which i l l r much ~ i r n i l a r t o t h e s h n l e u n i t i n t h e a r e a . T y p i c a l

~ ; e c t i o n s o f t h i s a l t e r n a t i n g sha1.e-sandy s h a l e u n i t c r o p o u t

a t 1 o r n t . i o n s 59, 6 0 , nnd 63. The o u t c r o p a t l o c a t i o n 63

( p l n t e 1 ) i s t r a c e a b l e f o r 350m. The v e r t i c a l t h i c k n e s s is

y r e n t e r t h n n lorn. ' C h i c k n e s s e s o f t h e i n d i v i d u a l l a y e r s ( b e d s )

r.nnp;c b e t w e e n 8cm ~ r l d 1m f o r t h e s a n d y - e h a l e , a n d 18cm t o Im

f o r t h e u n d e r l y i n f ~ . s h a l e . I n p l a c e s ( l o c a t i o n s 33, 38) t h e

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

i n the f ipnces b e t w e e n them. F r a c t u r e s w i t h s i m i l a r t r e n d s

a s t h ~ o n e s i n t h e s h ~ l e u n i t a r e common.

3.1.3 S i l t c t o n e u n i t

O u t c r o p o of t h i s u n i t o c c u r a t l o c ~ t i o n s 34 and 35.

T h e r o c k is f i n e - g r a i n e d , w i t h a l i g h t b r o w n i s h c o l o u r , It

e x h i b i t s a very d i s t i n c t l ~ m i n n t i o n ( P l a t e 2 ) . It is

u n d e r l o i n by R c o n r s e r - g r e i n e d , d a r k - g r e y s i l t s t o n e w h i c h

l n c k s t h e l n m i n s t i o n , a t l o c a t i o n 35.

3.1.4 M i c a c e o u s s h a l y s a n d f i t o n e -- T h i s g r e y - l o o k i n g r o c k o c c u r f l a s a n i s o l a t e d r i d g e

( S O O m l o n ~ b y 80m w i d e , r i s i n g r o u g h l y 7 m a b o v e t h e s u r r o u n d -

i n g s ) w i t h i n t h e p r e d o m i n n n t l y n h ~ l y t e r r a i n ( l o c a t i o n 31,

p l a t e 3 ) . T h e o u t c r o p m a i n t a i n s t h e g e n e r a l NE-SW s t r i k e f o r

Lhe r o c k s i n t h e n r e a , w i t h a N W c l ip d i r e c t i o n a n d d i p v a l u e

o f 3'. T t i n d i s t i n c t l y s t r a t i f i e d ; d n d i v i d u a l bedsr 0.5m

thick h a v e d i s t i n c t b e d d i n p p l o n e s .

3.3 M n m n t i c rocks .---

: . ; 1 I I y p a b y s s a l r o c k s - -- T h e ~ e m i n o r i n t r u s i v e r o c k s are g e n e r a l l y f i n e r t h a n the

p l u t o n i c r o c k s . They ere g p e y t o d a r k - g r e y i n c o l o ~ r . They

a c c r ~ r a s frills o r s i l l - l i k e b o d i e s w h i c h are wel l - expooed .

a l o n g t h e b a n k s o f the Anyj.m River. Two v a r i e t i e s are

r a r c : o ~ n i s e d o n t h e b s s i s o f , t e x t u r e a n d mode o f o c c u r r e n c e . They

1 L9t . e 1 : 0ilt:r:rop of the h n l a (a)-sandy : ; h ~ l ~ (b uni t 31-ong khe hnnk of the b n y i m R i v e r n t Ominy i .

-3.2. I . 1 R a s a l t i c s i l l s

T h e u e t h i n i n t . r u a i v e r o c k b o d i e a c r o p p i n g o u t at l o c a t i o n s

3'1, 9, 51, 54, 55, and 56 ( F i g . 3) are g e n e r e i l l y g l a s s y ,

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

t h e r ? o l o r i t e ~ . Ln th-shaped p l ~ g i o c l ~ s e c r y s t a l s a r e

c o n s p i c l ~ o r l s i n t h e f i n e - a r ~ i n e d meso.

In R d i s t ~ n c e o f bout 2.5km, l o c ~ t i o n s 51 t o 56, t h e r e

8re t r n ( 1 0 ) sills which we re empleced i n t h e b e d d i n e p l a n e s

o f t h e ~ e d j m e n t s . T h e i r t r u e t h i c k n e s s e s r n n g e be tween 3 and

l ? m , w i t h v e r t i c n l t h i c k n e s s up t o 4.2m. They fo rm c a t a r a c t s

n c r o s c t h c Anyim R i v e r . The o u t c r o p s n r e c h a r a c t e r i s e d by

i n c l i n ~ d t o v e r t i c ~ l , c l o s e l y - s p n c e r l (0.'~-1.0m) j o i n t s e t s

w i t h 3?O0 ( N N W ) - ? ~ O ~ (SSE) t r e n d . A t L o c a t i o n o 51, 54, 55, 56',

t h e . j n i n t i n ~ i s R c r u d e c o l u m n ~ r t y p o ( p l a t e 5 ) . The j o i n t s

a t l o r n t i o n 214 n r c f i 1 l e d by w e l l - d e v e l o p e d , e l o n g a t e ,

h c x n ~ o n n l c r y s t n l s o f c l e a r q u a r t z , s i d e r i t e , a n d c a l c i t e ,

e x h j b i t i n r a c o m b - s t r u c t u r e . The veinf i m e a s u r e up t o 8cm

ncroF;n. The r o c k ~ t . t h i s l o c a k i o n w e a t h e r s i n t o a l i g h t

E r e y i s h m 3 t e r i a l which i s n o t e e s i l y d i s t i n g u i s h a b l e f r om t h e

h o ~ t s h a l e u n i t when c o m p l e t e l y w e a t h e r e d .

3.7. I . ? D o l e r i t e s ----- These r o c k ~ n r c c o ~ r s e r - ~ r e i n e d t h ~ n t h e b a s a l t i c sills.

' r h q n r e p r e y - l o o k i n g and composed o f randomly o r i e n t e d

. l n t h - ~ ' 1 2 ~ c d c r y s t n l ? of p l n ~ i o c l ~ s e r e c o g n i a a b l e i n hand

s p r c i m e n . They ?re exposed a t l o c n t i o n s 10, 15, 21, 23 , 45,

h I , ~ n d 62c . The o u t c r o p s (generrr l ly l ~ c k c l e a r l y d e f i n e d

o r i e n t a t i o n 6 u n l i k e t h e b ~ ~ ~ l l t i ~ 6ills. 'They, however , a p p e a r

to b e exposed a l o n g t h e f i t r i k e o f t h e h o s t s h a l e . S p h e r o i d a l

c7nr3 r c c t ~ n p u l q r block^ r e s u l t from f i p h e r o i d a l w e a t h e r i n g and

intense j o i n t i n g . The o u t c r o p ~t 1 . o c ~ t b o n 2 1 is p e c u l i a r i n

its c o n t e n t o f we l l - deve loped c r y s t n l a o f c a l c i t e . The

g r o u n d r n ~ s s is composcti o f g r e y i s h nnd d i r t y , m i l k y , d u s t y

f ~ 1 d r : p ~ r s .

3 . 7 l ' l u t o n i c r o c k s -..- -- - ---.- -- 't'hcse r o c k s n r e much c o a r s e r - g r a i n e d t h a n o t h e r r o c k s

e n c o u n t e r e d i n t h i s ~ t u d y . They a r e g e n e r a l l y emplaced as

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

o ~ r t c r o p ~ c o v e r n r e n n which R r e p e n e r a l l y up t o 250m by 20m.

3.7.7.1 D i o r i t e s ---- P i o r i t e s o c c u r a t l o c a t i o r l s I , 7 , 9, 28, 29/48 - f i v e

o u t c r o r ~ s i n e l l . The o u t c r o p s a t 1 . o c e t i o n s 9 , 2 8 , and 29/48

pre c l o n ~ a t e i n a NE-SW t r e n d . The d i o r i t e s a re g e n e r a l l y

m e s o c r n t i c , w i t h I i g h t - t o g r e e n i s h g r e y c o l o u r . The minerals

r e c o ~ n i s a b l e i n hnnd s p e c i m e n s i n c l u d e d u s t y - w h i t e and , g r een i sh

grPy f e l d f i p a r ~ , b l ~ c k m n f i c m i n e r a l s w i t h a d u l l l u s t r e and

two d - i r e c t j o n ~ l c l e n v n ~ e s . P y r i t e is c o n ~ p i c u o u s l y p r e s e n t

i n t h ~ r o c k f rom l o c n t i o n 28. The r o c k a t l o c a t i o n 7 is

p ~ c u l i e r j n i t s c q n t e n t o f a randomly o r i e n t e d , n e e d l e - l i k e , '

e u h e d r ~ 1 c r y s t ~ l s o f o b l a c k m r f'ic m i n e r a l (ho rnb lende? ) .

P o c k c t ~ n o f l a r g e q u n r t z g r a i n n , un t o 20cm long b y 5cm wide c

are confipicuoufi ftt l o c a t i o n 9.

Two t y p e s o f o c c u r r e n c e , o n t h e b e s i s o f t e x t u r a l and

m i n e r e l o g i c a l , v a r i f t t i o n s w i t h i n o n e rock-body, a r e r e a o g n i s e d .

They o r e s i n p l e and c o m p o s i t e t y p e s . Of t h e f i v e o u t c r o p s ,

o n l y o n e ( l o c a t i o n 29/1+8) is a c o m p o s i t e type . The c o m p o s i t e

d i a r i t e is o v e r 2.0km l o n g by O.8km wide. It is c o n s t i t u t e d

mninly by two t e x t u r n l l y ~ n d m i n e r e l o g i c a l l y d i f f e r e n t r o c k

v a r i e t i e s . The f i r s t v n r i e t y is a l e u c o c r e t i c , c o a r s e r - g r a i n e d

t y p e ( l e u c o d i o r i t e ) w h i l e t h e s eoond ie a melanocratio, f iner-

g r ~ i n e d t y p e ( m e l m d i o r i t e ) . The f i r s t vnriety makee up more t h a n

3/4 o f t h e l e n g t h o f t h e e n t i r e o u t c r o p . It is composed

p rcdominnn t ly o f a n h e d r e l e l o n g n t e c r y e t a l s o f p i n k f e l d s p a r s ,

nnd c o l o u r l e s s c ~ l c i t e c r y s t a l s . It w e a t h e r s more e a s i l y t h a n

t h e s econd v a r i e t y . The second v a r i e t y is t e x t u s a l l y and .

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

It is r e s t r i c t e d t o t h e p e r i p h e r i e ~ o f t h e e n t i r e rock-body.

3.2.3 P y r o c l a s t i c r o c k s

Loce t jonf i 16, 17, 18, 19, 20, and 47 a r e o u t c r o p s o f t h e

p g r o c l a s t i c r o c k s . The f i r u t f i v e a r e c o n c e n t r a t e d w i t h i n t h e '

~ o u t h e r n pnl ts o f t h e s t u d y a r e a . They are commonly c o v e r e d

by t h i c k v e g e t a t i o n s . They g e n e r a l l y s t i c k o u t as e i t h e r

r e s i ~ t a n t cone- o r dome-shaped bod ies . The p y r o c l a e t i c r o c k s

a r e composed p r e d o m i n ~ n t l y of f r a g m e n t s o f l i g h t t o dark-

c o l o u r e d l a v a , and r a r e l y t h o s e o f c r y s t a l s and t h e h o s t

s h ~ l e . Base: on t h e s i z e s o f t h e f r a g m e n t s t h e r o c k s h a v e

been c l n s s i f i e d , ~ c c o r d i n g t o the scheme o f Moorhouse (19641,

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

t h e p e r i p h e r i e s . They a r e l i t h i c - t y p e s s i n c e t h e i r predominant 6

c o n s t i t u e n t 6 Rre rock f r agmen t s .

3 - 2 - 3 0 7 T u f f s

The p y r o c l ~ s t i c r o c k s a t l o c a t i o n s 16, 17, 20, and 47 are

t u f f s . They o c c u r g e n e r a l l y R B v e r y s m a l l b o d i e s w i t h c i r c u l a r

t o ovrtl /dome-shaped o u t l i n e s . The h e i g h t and d i a m e t e r of t h e

o u t c r o p o are hp t o 7m and 12m, r a a p a o t i v e l y - The lava

f r ~ g m e n t e h ~ v a s i z e s whiah r a n g e from l m m t o 15mm (average, 3mm). I

The f r a g m e n t s are g e n e r a l l y a n g u l a r i n shape* They are set i n

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

t h e f i r ~ t t h r e e a r e composed o f l i g h t g r e y i s h c o l o u r e d

m a t e r i a l s w h e r e ~ a t h e l a s t ( l o c a t i o n 47 i e e x c e p t i o n a l l y

de rk -co loured ( g r e y i s h g r e e n ) . T h i s da rk -co loured v a r i e t y

c o n t a i n s c a v i t i e s which a r e f i l l e d by a n h e d r a l , w h i t e c a l c i t e

crystal^. The f r agmen t s o f t h i s v a r i e t y a r e much similar t o

t h e r n ~ t r i x .

2 L a p i l l i tuffs v

These o c c u r a t l o c a t i o n s q8 end 79- They c h a r a c t e r i s t i c a l l y

s t i c k o u t s s r e s i e t q n t cones, up t o 70m h igh . M a t e r i a l s

from t h e v e n t s , c o v e r i n g a n w e n o f more t h a n 250m by ISOm, a r e '

d e p o n i t e d t'o o n e e i d e o f t h e conea. The l ap i l .LS t u f f s +

FIT'&> ~ e n e r ~ l l y f e l s i c look in^. They a r e composed of finer-

g r n i n e d , g l a s s y , l i g h t g r e y i s h brown l a v a f r a g m e n t s , set in

a v e r y f i n e - g r n i n e d , g r e y i s h b l a c k m a t r i x ns in t h e l i g h t

c o l o u r e d t u f f s . T h e f r agmen t8 which a r e a n g u l a r t o s q u a r e i n

s h e p e r a n g e i n s i z e from 22mm t o 50mm. ~ i t h i c f ragmentn o f

h e r d , g r e y i s h , f i s s i k e s h a l e , up t o 220mm by 20mm are also

n o t n b l e c o n s t i t u e n t s .

A v e r y f i n e - g r ~ i n e d , l i ~ h t g r e y i s h c r y s t a l l i n e r o c k

c o v e r i n e a n Rrea o f 50m by 25Om c r o p s o u t a t the n o r t h e r n end

o f t h e o u t c r o p ~t l o c a t i o n 19. The r o c k i s much similar t o I

t h e l a v a f r agmen t s .

3 . 3 . 4 C o n t a c t metamorphosed a h a l e s

These rock6 were s t u d i e d a t t h e c o n t a c t s o f t h e

compor;ite d i o r i t e w i th t h e h o s t a h s l e a t l o c a t i o n s 62 ("Ngele

0poo") and 628 ("Ngele o g b a r u f f ) . The a u r e o l e is made up o f

two t e x t u r a l l y d i f f e r e n t r o c k s . One is a v e r y f i n e - g r a i n e d

daik-.Looking, mnsnive, s p o t t e d ~ l a t y r o c k , w i t h a c o n c h o i d a l

f r ~ c t u r e c u r f ~ c e . T t c o v e r s a w i d t h o f l 3 m c l o s e s t t o t h e

i n t r u s i o n . The o t h e r is a f i s s i l e , baked s h a l e c o v e r i n g a

w i d t h o f 9m f ~ r t h e r aweg f rom t h e i n t r u s i o n . T h i s s i z e o f

t h e n u r e o l e s u g p e s t s t h a t t h e i n t r u s i o n was a much l a r g e r

body which bedame d e e p l y e roded . IIence t h e metamorphosed

h o ~ t n p p e n r f i t o b e o u t o f p r o p o r x i o n w i t h t h e r e m n a n t s o f

t h e i - n t r u s i v e body.

CHAPTER FOUR

PETROGRAPHY

T h i n - s e c t i o n 6 were s t u d i e d by t h e u s e o f p e t r o g r a p h i o

m i c r o ~ ; c o p e ~ . The samples e r e a l t o g e t h e r , f i v e d i o r i t e s , I

f i v e d o l e r i t e n , nnd two f ine-g-rained r o c k s from t h e b a a n l t i c

gi l l s . R e p r e s e n t a t i v e samples of t h e p y r o c l a s t i c r o c k s and

c o n t ~ c t me t~morphosed s h a l e s (baked s h a l e and s p o t t e d s l a t e )

were a l s o examined. Modal c o m p o s i t i o n s o f t h e samples o f t h e

mRKrnntic r o c k 8 a r e p r e o e n t e d i n T e b l e 2.

4.1 P e t r o g r s p h y o f t h e magmatic r o c k s

4. I. .I B a e ~ l t i c sills

The samples of t h e s e r o c k s s t u d i e d were c o l l e c t e d from

The r o c k s a r e f i n e - g r a i n e d , wilth a v e r a g e g r a i n - s i z e s i n

t h e r o n g e o f 0 .51 t o 0.79mm. The m i n e r a l c o n s t i t u e n t s o f t h e

r o c k s i n c l u d e p l a g i o c l ~ s e , py roxene , o l i v i n e , and a c c e s s o r y

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

py roxene , end a s amygdales . The s a m p l e from l o c a t i o n 55 is

p o r p h y r i t i c , w i t h s i z e s o f t h e p h e n o c r y s t s i n t h e r a n g e o f

0.9mm t o 2.2mm. P l a g i o c l s a e and py roxene form t h e p h e n o c r y s t s

which mpke up bout 13% o f t h e rock . The dark-brownish

T a b l e 2: Modal compos i t i ons o f t h e magmatic r o c k s

I D IOR TTES I DOLERITES BASALTIC SILL

P l a g i o c l a s e

Py roxene ( c l i n o - 1 - .-

B i o t i t e

,%qkibcle ( h o r n b l e n d e ) - F e p h e l i a e 1 - Q u a r t z ( i n t e r s t i t i a l ) -

C a l c i t e ( s e c o n d a r y ) - A p a t i t e q.8

F h e r o c r y s t s ( o i i v i n e a c d nvroxene 1 Groundmas ( o l i v i n e , py roxene , p l a g i o c l a ~ e )

( s u r o f c c i o u r e f I I I

c.;rAerzis, i n c l u d i n g I 42.4 156.54 57.6

t

g r o ? ~ n d r n ~ ~ f i i~ c o m o o ~ e d of t h e nome m i n e r a l s which o c c u r a s

phenocrya ta .

P l a g i o c l a s e o c c u r s i n e u h e d r a l l a th-shaped t o r e c t a n g u l a r

c r y s t e l s which a r e f a i r l y randomly o r i e n t e d . The c r y s t a l s

fihow R h i g h d e g r e e o f a l t e r a t i o n which o b l i t e r a t e s t h e

~ l b i t e p o l y s g n t h e t i c twinning . The pyroxene i s c o l o u r l e s s t o

brown, And commonly zoned. I t i 6 . e s s e n t i a l l y i n t e r s e r t a l *

L o n p i t u d i n d s e c t i o n n show i n c l i n e d e x t i n c t i o n , w i t h maximum

a n p l e ( C A W ) o f 37O, c o r r e s p o n d i n g t o a u g i t e . The o l i v i n e i n

t h e ~ n m p l e from l o c n t i o n 55 i f f c h l o r i t i z e d . The c h l o r i t i z e d

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

rounded o u t l i n e ~ n d p n r ~ l l e l e x t i n c t i o n o f o l i v i n e . M a g n e t i t e

occur^ i n s m a l l - e i z e d a n h e d r ~ l , opRque g r a i n s , showing b l a c k

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

4.1.2 D o l e r i t e s

'Phc d o l e r i t e s a r e g e n e r a l l y h o l o c r y s t a l l i n e , f i n e - t o

medium-proined (0.51-2.28mm, g r a i n - s i z e ) ~ n d e q u i g r a n u l a r .

They nre compoced mpinly o f randomly o r i e n t e d , e u h e d r a l and

r e c t a n g u l a r p l ~ g i o c l e s e c r y s t a l s , a s w e l l a s l a t h e . The

p l e f l i o c l a s e is i n R s u b - o p h i t i c r e l ~ t i o n s h i p w i t h s u b h e d r a l

c r y c t ~ l s of pyroxene. Q u ~ r t z is a common, b u t i n t e r s t i t i a l

c o n s t i t u e n t - The pyroxene shews a h i g h e r d e g r e e o f a l t e r a t i o n

t hen t h e p l e a i o c l a s e f e l d s p e r s . It a l e o shows s y m p l e k t i c

i n t e r g r o w t h s . The ~ l t e r a t i o n p r o d u c t s o f t h e pyroxene

inc1,ude c a l c i t e and opaques . C a l c i t e a l s o o c c u r s as an

s l t e r ~ t . ion p r o d u c t o f t h e p l a g i o c l a s e . The opaque m i n e r a l s

n r e commonly f i y r i t e , end r a r e l y magnetite.

The sarnplefi from l o c a t i o n s 45 and 6 2 ~ show a s l i g h t

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

t h e s e s a m p l e s , t h e py roxene i s c l e a r l y i n t e r s e r t a l . Idd ing-

s i t i z e d o l i v i n e w i t h dark-brownish r i m i.s p r e s e n t i n t h e

finmple from l o c ~ t i o n 45. Meximum e x t i n c t i o n a n g l e of t h e

pyroxene , de t e rmined i n t h e snmple from l o c a t i o n 6 2 ~ i s 3y0,'

c o r r e s p o n d i n g t o auq j te.

The semple from l o c a t i o n 2 1 i s p e c u l i a r i n i t s h i g h

c o n t e n t o f l e r g e , wel l -deve loped c r y s t n l s of c a l c i t e , most o f '

which o c c u r ss ~mypdEIle6 which f i l l c a v i t i e s . These amygdales

g i v e khe r o c k R pseudq-porphyr i t i c t e x t u r e . The maximum

d e x t . j n c t i o n ~ n g l e for t h e p l ~ g i o c l n s e i n t h i s s ample i s 3 2 , c o r r e n n o n d i n f f t o An ( l a b r a d o r i t e ) .

57 4.1.3 D i o r i t e s

The d i o r i t e sampl e s a r e h o l o c r y s t a l l i n e and medium-grained !

(2.53-3.60mm, g r e i n - s i z e ) . The samples from f o u r l o c a t i o n s :

1, 9 , 28, and 29 show randomly o r i e n t e d p l a g i o c l a s e c r y s t a l s ,

i n R c u b - o p h i t i c r e l e t i o n e h i p w i t h t h e c l inopyroxene . The

c l r ~ r p l e from l o c a t i o n 9, however, ~ R B t h e c l i n o p y r o x e n e i n

i n t c r s e r t a l p o s i t i o n s . The rest l a c k random o r i e n t a t i o n

o f t h e p l u g i o c l ~ s e c r y s t a l s . Vug- l ike f e a t u r e s t r u n c a t i n g

t h e m ~ f i c m i n e r d s are common i n t h e samples from l o c a t i o n s

1, 28, ~ n d 29. The common m i n e r a l c o n s t i t u e n t s o f t h e

d i o r i t e s i n a l u d e p l p g i o c l e s e (An 35-38 ' s o d i c a n d e s i n e ) and

c l i n o p y r o x e n e . Hornblende i s p r e s e n t i n o n l y t h e sample

from l o c e t i o n 7 . B i o t i t e is a n e s e e n t i a l c o n s t i t u e n t o f t h e

a m p l e 6 from l o c a t i o n s 7 and 9. It i s a c c e s s o r y i n t h e

sample from l o c a t i o n 29. Othe r a c c e s s o r i e s i n t h e d i o r i t e s

n r e m n ~ n e t i t e , p y r i t e , a p a t i t e , ~ n d i n t e r s t i t i a l q u a r t z .

Cel c j t e i s ~ e c o n d n r y .

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

c r y c t . 0 1 ~ . They e r e c o l o u r l e s o , b u t c loudy. They h a v e a low

r e l i e f , w i t h v e r y f a i n t o u t l l n e n . The c r y s t a l s a r e much

l a r g e r t h e n t h o s e of t h e c l i n o p y r o x e n e i n a l l t h e samples ,

excep t t h e o n e from l o c a t i o n 1. The p l a g i o c l a s e is sometimed

p a r t l y e e r i c i t i z e d . The c l i n o p y r o x e n e is l i g h t brown i n

c o l o u r , ~ n d locks p leochro i sm. ~ o n g i t u d i n a l s e c t i o n s o f t h e I

pyroxene show p e r f e c t u n i d i r e c t i o n e l c l e a r a g e s w i t h maximum

e x t ' i n c t i o n a n g l e i n t h e r a n g 4 of 37' t o 4 8 O , c o r r e s p o n d i n g t o

t h e rEtnp;e f o r aup ; i t e . The c r y s t a l 6 c how m o d e r a t e b i r e f r i n g e n c e ,

w i t h rn:?xi mum p o l n r i e n t i o n c o l o u r ~ o f t h e s econd o r d e r .

Hornblende occurf i i n e u h e d r a l c r y f i t a l s which e x h i b i t

s t r o n g pleochroif im from deep r e d d i s h brown ( 9 ) t o red-brown

( 20. The maximum e x t i n c t i o n a n g l e f o r t h e l o n g i t u d i n a l s e c t ' i o n e I

is 16'. B i o t i t e c r y s t a l s a r e reddish brown and s t r o n g l y

p l e o c h r o i c from r e d d i s h brown t o brown. They show p e r f e c t

u n i + i r w t i o n a l c l e ~ v a g e s , and e x t i n c t i o n . The c h a r a c t e -

r i s t 5 . q b i r d ' s - e y e s t r u c t u r e a t n e a r - e x t i n c t i o n p o s i t i o n is .

shown b y t h e r e d d i s h brown v a r i e t y . I n t h e s amples from

l o c n t ~ o n s 7 m d 9 , t h e b i o t i t e c r y n t a l ~ o c c u r as pseudomorphs o f

h o r n b l e n d e which n r e p r i s m a t i c ( e l o n g a t e ) , e u h e d r a l , and

d n r k - g r e e n i e h i n c o l o u r . The pseudomorphs are s t r o n g l y

p l e o c h r o i c , f rom ye l lowi f ih g r e e n t o deep g r e e n , w i t h a h i g h

r e l i e f . The marg ins o f t h e s e c r y s t a l s a r e d a r k ( b l a c k )

compnred t o t h e d e r k - g r e e n i s h c o r e . L o n g i t u d i n a l s e c t i o n s

fihow p a r a l l e l e x t i n c t i o n . In r e f l e c t e d l i g h t , t h e d a r k m a r g i n s

6how t h e c h ~ r o c t ~ r i s t i c s p p e r a n c e o f m a g n e t i t e . M a g n e t i t e

o c c u r 6 i n e u h e d r a l c r y s t a l s , w i t h rhombus t o s q u a r e o u t l i n e s .

P y r i t e o c c u r s i n e u h e d r a l c r y s t a l s w i t h s q u a r e o u t l i n e . It h a s

n l i ~ h t y e l l o w c o l o u r , w i t h g r i d t w i n n i n g i n r e f l e c t e d l i g h t . ,

A pclnd omount o f m ~ g n e t i t e end p y r i t e comes from t h e

n l t t,r.ra t i o n o f t h e c l i n o p y r o x e n e e n d b i o t i t e .

A p a t i t e o c c u r s i n c o l o u r l e s s , e u h e d r a l , r e l a t i v e l y s m a l l +

e l q n r n t e ~ n d n i x - ~ i d e d c r y e t n l ~ showing f i r s t o r d e r grey .

p o l n r i s a t i o n o o l o u r . C a l c i t e o c c u r s f rom the a l t e r a t i o n o f

b o t h t , h ~ p l ~ e i o c l e s e ~ n d c l i n o p y r o x e n e . It o c c u r s i n well-

deve loped c r y s t a l s , n s w e l l a s i n m i c r o c r y s t a l l i n e form* I t

i s c h v r n c t e r i s e d by i t s twinkl i -ng h ~ b i t and v e r y h i g h

b i ref r .1 ngence. The c r y s t a l s show mu1 t i p l e c o l o u r s o f t h e ,

h i p h e n t o r d e r . ' I n t e r s t i t i a l g r a i n s o f q u a r t z show a po ly -

c r y ~ t ~ l l i n e s t r u c t u r e , and wavy e x t i n c t i o n .

4.1.4 T u f f s

Of a l l t h e t u f f s , o n l y two - t h e s amples f rom l o c a t i o n s

16 and 47 - R r e made up o f v o l c a n i c f r a c m e n t s which e r e n o t

e n t i r e l y g l a s s y ( i e . , v i t r i c ) . The f r a g m e n t s o f t h e two samples

Are , however , d i f f e r e n t i n t h e i r t e x t u r e and t y p e o f c o n s t i t u e n t e .

The sample from l o c a t i o n 16 is made up of f i n e - g r a i n e d

f r a p n e n t s which a r e d i s t i n c t l y c r y s t a l l i n e . The m i n e r a l

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

o f pl n p i o c l n s e c r , v s t o l s , w i t h l i g h t g r e e n i s h brown, s m a l l - s i k e d

( 0 . ?mm, d i e m e t e r ) , g e n e r s l l y rounded g r a i n s o f pyroxene.

Qunrt:x o c c u r 8 ~ F I a l i k e l y p r o d u c t o f d e v i t r j f j . c e t i o n . T h i s

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

do1't.r-i t e s . On t h e b ~ e i s o f t e x t u r e and m i n e r a l c o n s t i t u e n t e ,

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

hpv ing 8 t r e c h y b ~ n ~ l t i c compos i t i on .

f r ~ ~ r n e n t f i o f h i g h l y g l a s s y v o l c a n i c m a t e r i a l s . The fragmentis

P r e c o n f i t i t u i p d by v e r y f i n e l o t h s (0 .8 x 0.1rnm) o f

p l ~ g i o c l a s e f e l d f i p n r s l ~ c k i n g p r e f e r r e d o r i e n t a t i o n , i n a

p r o u n d m ~ s s which hne been ~ l t e r e d t o m i c r o c r y s t a l l i n e c a l c i t e .

C ~ l c i t e c r y s t a l s a r e a l s o ebundant a s c a v i t y f i l l i n g s . The

c a v i t i e f i which t h e y f i l l t end t o b e e l o n g a t e , and measure up

t o ?.?mm. A second e ~ s e n t i ~ l r o c k f r agmen t is g r e e n i s h and .

i s o t r o p i c i n t h i n-sec t i o n . No t a b l e c r y s t a l c o n s t i t u e n t s

i n c l u d e 8 c o l o u r l e s s m i n e r e l ~ h o w i n g t h e o u t l i n e o f s o d a l i t e

( h ~ u y n e ) , c o l o u r l e s s c r y s t ~ l s o f o l i v i n e showing p a r a l l e l

e x t i n c t i o n mnd p reen R R rnnximum p o l ~ r i s a t i o n c o l o u r , and a

pyroxrbnp showing t h e b ~ f i n l s e c t i o n ( e i g h t - s i d e d c r y s t a l ) . The

text11t.c of t h e volcor l ic f r a g m e n t s nnd t h e t y p e o f c r y s t a l s i n

t h i ~ t u r f sugy.efit t h ~ t t h e f r m p m m t ~ h ~ v e a b n ~ d c composi.t ion.

4.1.5 L e p i l l i t u f f 8

Only t h e l ~ p i l l i t u f f from l o c ~ t i o n 18 p o s s e s s e s non-

g l a s s y v o l c a n i c frogrnents . The f r a g m e n t s h a v e t h e same 'L

t e x t u r e , f l ow s t r u c t u r e , and sirnil~r m i n e r a l s as t h e t u f f from

l o c ~ t i o n 16. The d i f f e r e n c e between t h e two is t h a t t h e

m i n e r a l s which mnke up t h e f r a g m e n t s o f t h e t u f f a r e l a r g e r ,

nnd t h e p l n g i o c l a s e tw inn ing i s b e t t e r developed i n some o f

t h e c r g s t ~ l e . Q u n r t z , t o g e t h e r wi th c a l c i t e , o c c u r s as

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

Q. 2 Petroaraphgrf t h e c o n t a c t metamorphosed s h a l e s

4.2.1 S p o t t e d s l a t 6

T h i s i~ a v e r y f ine -g ra ined- . rock showing a pronounced

f i ~ s i l i t y , and R p o o r l y developed s l a t y c l e a v a g e . It is

m ~ d e up , p redominan t ly , of a m a t r i x c o n s t i t u t e d by submicros-

c o p i c c l a y ~ n d m i c r o e r y s t a l l i n e c a l c i t e . G r a i n s o f c a l c i t e and

few g r ~ i n s o f q u ~ r t z whose f i i z e ~ r a n g e from O.lmm t o 0 . 5 m m

n r e e n s e n t i a l c o n s t i t u e n t s . These make up about 443% by volume

o f t h e rock . Euhedrnl crystal^ o f p y r i t e w i t h n q u a r e o u t l i n e

end si:!e~: up t o 0.6mm a r e notnb1.e c o n s t i t u e n t s . The g r a i n s o f

c e l c i t , e , p y r i t e , nnd q u a r t z g i v e t h e r o c k a e p o t t e d appearance .

4.2.2 Baked S h d e

The f i s s i l i t y i n t h i s r o c k is less pronounced t h a n i n t h e

t ipot ted s l a t e . Its m a t r i x is s i m i l a r t o t h a t o f t h e s la te .

It i s n l s o composed o f g r ~ i n s of c a l c i t e , o f however, smaller

s i z e s i n t h e rPnge o f 0.05rnm t o O.4mm. The g r a i n s make up

about 30% by volume o f t h e rock . The g r a i n s of p y r i t e i n the!

~ p o t t ~ e d s l a t e a r e n o t e b l y a b s e n t .

CHAFTER FIVE

5 0 MAJOR-ELEMENT GEOCHEMTSTRY OF THE ROCKS

5.1 -- A n a l y t i c a l p r o c e d u r e s

The r o c k a ~ n ~ l . y f i e d i n t h i ~ s t u d y a r e r e p r e s e n t a t i v e

6nml.l-a f rom t h e j n d i v i d u ~ l o u t d ' l ' o p ~ o f t h e d i f f e r e n t r o c k -

t y y w . 130th t h e v o l c a n i c bombs ( l e r g e v o l c a n i c f r a g m e n t e l and

whole-rock ~ a m p l e s were a n a l y ~ e d f o r t h e p y r o c l a s t i c r o c k s .

I p r e p n r e d t h e rock-cemplefi from rock-powder o f s i z e less t h a n

200 r n c ~ h , p u t i n t h e f i n a l form o f a c i d s o l u t i o n s .

The a l k a l i n e - e a r t h m e t a l s (Ca, Mg), a l k a l i metals (Na, K),

t o t a l i r o n ( F e ) end phosphorus (P) were a n a l y s e d by t h e u s e

o f t h e a t o m i c a b s o r p t i o n s p e c t r o p h o t o m e t e r (AAS) w h i l e s i l i c a

( s ~ O ) nnd ~ l u r n i n n (A1 0 ) were a n a l y s e d by t h e UV-v i s ib l e 2 2 3

spec t r o p h o t o m e t e r . The two ana lyse t t were done a t t h e S o i l

S c i e n c e ( L i n k ~ g e ) Wet Chemis t ry L ~ b o r n t o r y and t h e Food S c i e n c e

and Technology L a b o r a t o r y , r e s p e c t i v e l y , b o t h o f t h e

University o f N i p c r i . ~ , Nsukka. I c o n v e r t e d t h e AAS a n a l y ~ e s

t o t h c o x i d e s o f t h e r e ~ p e c t i v e e l e m e n t s . S t a n d a r d s , ba sc l l t n

o f known c o n c e n t r ~ t i o r l s and b l a n k s ~ m p l e s were ueed as contro1.s .

The ~ ~ p n r a t i o n o f t h e t o t a l i r o n i n t o f e r r i c and f e r r o u s

components was done by u s i n g f - e r r o u s i r o n o x i d e (Fe0) o b t a i n e d

from t h e t i t r a t i o n o f a s t ~ n d a r d s ample .

5.2 t tef :ul ts

The mnjor -P lement o x i d e d a t a , w i t h t h e CIPW n o r m a t i v e

c o m p n s i t i o n s , o f t h e mngmatic r o c k s is p r e s e n t e d i n T a b l e 3a.

The d n t ~ f o r t h e c o n t ~ c t m e t ~ m o r p h o s e d s h a l e s a r e i n T a b l e 3b-

The a v e r a R e y ~ l u e s f o r ~ R G ~ C nnd i n t e r m e d i a t e r o c k s (~ynd rnan2

19711, nnd s h n l e s , 1 i m e ~ t o n e s (Mason, 1966) h a v e b e e n

i n c 1 u f l e d f o r compnri son .

5 . 2 . 1 M A ~ ~ R t i c r o c k s -- T h e v n l u e n f o r S i 0 2 r n n g e from 46.06 t o 49.32% i n t h e

b n ~ n l t l c sill^ and d o l e r i t e s , 50.34 t o 52.26% i n t h e d i o r i t e s ,

snd 4?.45 t o 4/+.3/+% i n t h e t u f f s and l n p i l l i t u f f s .

G e n ~ r ~ ~ l l y , t h e r e ifi R n o v e r a l l i n c r e a s e i n S iO f rom t h e 2

b ~ s n l t i c s i l l s ~ n d d o l e r i t e s , t h e p y r o c l a s t i c s , t o t h e

d i o r i t ~ n . The v n l u c s f o r p y r o c l a a t i c r o c k s are, hhwever ,

rn t :h (? r ].ow. T h e s e low v a l u e s c o u l d b e d u e t o t h e i r f r a g m e n t a l

n n t u r p nnd i n c o r p n r r t i o n o f s e d i m e n t a r y m a t e r i a l s . Alumina is

fnir1 .y h i g h i n t h e b a n a l t i c ~ i l l s , d o l e r i t e s , and p y r o c l a s t i c

r o c k s c o m p n r ~ d t o e v e r e g e ~ l k n l i n e b a s a l t . A 1 2 0 3 i s i n t h e

r a n e e o f 12.96 t o 16.36% (15.74%, a v e r a g e ) i n t h e b a s a l t i c

s i l l6 a n d d o l e r i t e s , and 1 4 - 5 2 t o 19.95% ( 1 7 . 6 7 % ~ a v e r a g e ) i n

t h e p y r o c l a s t i c s . T o t a l i r o n o x i d e (FeO+Fe 0 ) is g e n e r a l l y z 3 a b u n 3 e n t ; t h i s tor-relates w i t h t h e abundance o f ma f i c m i n e r a l s

V X Z ~ ~ * Z J - I ~ ~ ~ U C N N pl L a ID m r r t p 3 O O N O O O N N O C a ut o o O N N

w w

and mi7Rneti t e i n t h e modal c o m p o s i t i o n s o f t h e romks. The

n l k n ] * i n r . - e ~ r t h o x i d e s ( c ~ O ~ n d MgO) show a t r e n d which is, .

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

i n d r x - s i l i c ~ . ~ r r a t i c Ve lues i n t h e two o x i d e s a re l i k e l y

d u e t o t h e e 8 f e c ts o f w e a t h e r i n g ( s e e G~r re l s and Mackenzie ,

1971) . These v n l u p s i n c l u d e M&; 2.89% and 4.72% i n t h e

d o l e r i t e s , 3.38 t o 6.76% i n t h e p y r o c l a s t i c s , and C a O ; 6.20% *

i n o n e of t h e d o l e r i t e s , 1.3% end 4.26% i n t h e p y r o c l a s t i c s .

The r n ~ g m a t i c r o c k s n r e g e n e r a l l y e n r i c h e d i n t h e a l k a l i o x i d e s

(1a30 + K20). T o t a l a l k a l i r a n g e s from 3 - 2 1 t o 9.81%, w i t h

t h e v a l u e s o f Na20 (2.62-6.55%) 7 K20 (0.00-3.76%). Phosphorus

p e n t n - o x i d e (F' 0 ) is r ~ t h e r c o n s t a n t a t 0.2% i n a l l t h e 3 5

maprnn t ic r o c k s .

Tt i~ n o t c d t h a t t h e G u m t o t a l o f t h e o x i d e s i n most o f

the) r o c k s a r e lower t h a n 100%. T h i s c o u l d b e a c c o u n t e d f o r by

t h e vp l l l es o f $0 nnd H 0 ( e q u i v r l l ~ ~ n t o f l o s s o n i g n i t i o n ) which 3 2

were riot a n ~ l y s e d . The d e v i a t i o n s may c o r r e s p o n d t o t h e

nb~lndnnc-e of c ~ l r : ~ t e R S e l t e r n t . i o n p r o d u c t s i n t h e magmatic

r o c k s . Va lues o f SO2 i n t h e r a n g e o f 3 t o 12% and H20 (3-11%)

p r e common i n t h e a n ~ l y s e s o f s i m i l a r r o c k s from t h e Lower

Bmue 'Crough ( O ~ a d e , 1979, Hossa in , 198 I , R e n k h e l i l , 1986,

Umej i , 1992 )

Tnbl c 5h: M a j o r - ~ l e m e n t oxide d n t ~ for t h e c o n t a c t metnmorphosed ~ h n l e s

Rock- t y p e

S c r i n l No.

S p o t t e d S t n t e AVERAGE SHALE (Mason,

AVERAGE LIMESTONE 1966

Nor-1nnf.i v e (:omposi tion? ---- .-..--*-

'Ylle re-cnlc111 n t i o n o f t h e o x i d e s i n t o norms shows f r e q u e n t

ocql~r . r r .nce o f c l i n o p y r o x e n e , n c p h e l i n e , and a b u n d a n t o l i v i n e

i n most o f t h e r o c k s . E i g h t o f t h e a n a l y s e s h a v e n o r m a t i v e

n e p h c l i n e v ~ l l u c ~ s o f 0.39 t o 22.69% (9.18~4, a v e r a g e ) a n d

n o r m v t i v c q l i v i n ~ o f 2.28 t o 22.33% (15.3876, a v e r a g e ) . Only

two c n m p l e ~ ; h ~ v e n o r m ~ t i v e h y p c r o t h e n e v a l u e s o f 8.93% and

I?. RO?', w i t h q u n r t , , 3.36%. Normnt ive o l b i t e i s g e n e r a l l y

h i p h i n ~ 1 1 t h e r q c k ~ ; t h i s agreec wi th t h e mode which p a v e

nnor t .h i te composi t ~ o n c o f t h e p l a ~ j o k l ~ s e , a mnjor c o n s t i t u e n t

i n t h e r o c k s , i n t h e r a n g e o f s o d i c a n d e s i n e .

The h i g h e s t v n l u e s o f n o r m ~ t i v e o l i v i n e which o c c u r i n .

o n e o f t h e b e s a l t i c fiilts (b-551, and o n e o f t h e p y r o c l a s t i c a

(p -?17) i s v e r y cons i r : t en t w i t h t h e petr .oy;rnphic o b s e r v a t i o n a .

O l i v i n ~ i s n not .nble model c o n s t i t u e n t o f t h e b a s a l t i c s i l ls ,

d o l c r + i t e ( d - 4 5 ) , ~ n r ' t h i s p y r o c l a s t i c (p -47) . The a luminous .

n<q tu r*c> o f t h e p y x o ~ l ~ s t i c r o c k s is r c f l e c t e d i n n o r m a t i v e

P ~ ~ I I T I , ! I I ~ (R.?'l?', m,~ximum).

9.7-3 T o n t e c t -A*--- metamorphosed s h a l e s

' h e c o n t n a t rnctnmorphoserf s h a l e s g e n e r a l l y h a v e l o w e r

SiO? (27.613% a n d ~ 7 . 4 7 % ) compared t o a v e r a g e s h a l e , b u t h i g h e r a

t,h,vn n v e r n ~ e l i m c n t o n e . The v ~ l u e f i o f C a O (37.50% e n d 32.14%)

nrc c l n n ~ j d ~ e r ~ b l y h i q , h e r t h ~ n t h n t f o r e v e r a g e s h a l e , but

c i o n r ? ~ to t h ~ t f o r Rverage limestone. MgO is f a i r l y higher

t han f o r a v e r n p e s h n l e . Tote1 i r o n oxide and a l k a l i a r e

c o m p ~ r a b l e t o t h o s e i n t h e a v e r a g e s h a l e . T h i s chemica l I

compos i t ion o f t h e meta-sedimentary rock6 s u p p o r t t h e

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

was c ~ l c ~ r e o u s s h n l e .

'JHPPTER S?X - D T S C U S S r O N S -- ---

R e s u l t s o f t h e p e t r o g r a p h i c nnd ma jo r - e l emen t g e o c h e m i c a l

~ t u i i i e s h a v e c o l l e c t i v e l y shown t h e m ~ g m a t i c r o c k s t o b e o f

bnfiic t o i n t e r m e d i R t e c o m p o s i t i o n s , namely: o l i v i n e - b a s a l t I

s i l l o , d o l e r i t e s , t u f f s m d l a p i l l i t u f f s , and a l k a l i - d i o r i t e s .

The f i -ne- to medium-grni ned n a t u r e o f t h e p l u t o n s - d i o r i t e ,

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

e v i d e n c e o f crnplncement ~t s h a l l o w e r d e p t h s . A t s u c h d e p t h s ,

t h e r n t e o f c o o l i n g o f t h e mPgmes i~ h i g h , and i t is a l s o

p o s s i b l e f o r t h e gR8es c o n t ~ i n e d i n them t o b u b b l e - o u t t o '

g j v e r i s e t o t h e c n v i t i e ~ , s i m i l n r t o v e s i c l e s i n ex t rus ive

rock:;, which c o u l d b e f i l l e d by s e c o n d a r y m i n e r a l s s u c h as

qnLl-it.e i n t h e dole rite^ ( e e e Hyndman, 1972, p. 715). The

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

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

p l n ~ i o c l a s e ) i n most o f t h e d i o r i t e s e r e c h a r a c t e r i s t i c s o f

b n r i c d i o r i t e ~ : i n t h e diorite-rnonzonite-syenite g r o u p

( t4oorhouee, 1964, p. 2 9 6 ) .

The o c c u r r e n c e o f o n l y c l i n o p y r o x e n e ( w i t h r e s p e c t t o

o t h o r p y r o x e n e ) i n t h e mode o f t h e m ~ g r n ~ t i c r o c k s , h i g h

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

r n t i o s a s shown on t h e a l k a l i - s i l i c a d i e g r a m (rig. 7 ) are

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

to ai licn. The rock^ pre t h e r e f o r e p r e d o m i n a n t l y a l k a l i n e .

The n n r m o t i v e c ~ m p o s i t i o n s o f t h e r o c k s compare f a v o u r a b l y

wit.1) t h o ~ ; e o f t h e n . l k s l i n e ~ l e r i e s (F'ig. 8).

The c h n r p c t e r o f t h e ~ l k ~ l i n i t y o f t h e s u i t e is s o d i c ,

w i t h r e s p e c t t o p o t e f i s i c ( F i g . 9 ) . The r o c k s are a l s o

elurnirlous (F'ip;. 1 0 ) . T h i s i s a p o s s i b l e e f f e c t o f c o n t a -

m i n n t i g n by t h e c r u s t e l r o c k s . T h e s e geochemica l c h a r a c t e r i s -

t i c [ : n r e c o n s i s t e n t w i t h t h o s e shown b y t h e r o c k s s t u d i e d i n

t h e o t h e r p n r t s o f t h e Renue Trough by O l a d e ( 1 9 7 9 ) ~ U r n e j i

( 198'7 1, R e n k h e l i l ( 1 9 8 6 ) ~ U m e j i ( 1 9 9 2 ) ~ and Ezepue (1992 ) . -. T h e y , however , d i f f e r . f rom t h o ~ e o f t h e t h o l e i i t e s o f Hoeae in

( 1 ) f rom Ugep ; ~ I ' C R S ( F i g . 1 1 ) . The c h a r a c t e r i s t i c s compare

f n v n r ~ r n b l y w i t h t h ? s e o f r o c k e i n known c o n t i n e n t a l r i f t s

srich n s t h e Rni kg1 r i f t , i n S i b e r i a . The a l u m i n o u s

c h ~ r ~ c t e r i ~ , however a b s e n t i n t h e r o c k s o f t h e E a s t ~ f r i c a n

r i f t ~ y s t e m ( F i g . 12 ) .

Pllthough t h e rnejor-e lement g e o c h e m i s t r y o f t h e r o c k s

i n t h i s s t u d y shows t h a t t h e y a r e o f a l k a l i n e magma o r i g i n ,

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

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

bccfirlse t h e major- clement^ a r e commonly m o b i l e d u r i n g

~ l t r r n t i o n s ( F l o y d 3nd W i n c h e s t e r , 1975, 1978; L e a t and

'Chor.pc, 19%). l b w e v e r , t h e a l .knl i.ne affinity as shown by

. -

A n x l O O / ( A n + ~ b - t ~ / ~ ~ e ) 1

* _ .__._ _ ". . . - . , . , ̂ l i ."-- __._..l_ _ ..,. ).____ __r.,

I ! '{ ,"

T..i(j.B ~ i s l ; r i b u t i o n of the r n a g m a t i c r o c k s West o f A n y i m ~ i v e r : $. 0. t h e n o r m a t i v e d i a g r a m of I r v i n e a n d B a r a g a r (1971),?,

.\. Ifos 1-10, a s i n F i g . 7 - i

I. 111 '

4 -I +

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

i ~ n c o u s r o c k s n u ~ c ~ ~ t c a n ~ l k ~ l i n e R I R ~ ~ R p a r e n t a g e .

The o c c u r r e n c e o f t h e p y r o c l a n t i r : r o c k s is i n d i c a -

t i v e o f e x p l o s i v e v o l c e n i s m , c h a r a c t e r i s t i c o f t h e more

v i s c o u s l a v n s .

An a l k a l i n e o l i v i n e - b a s a l t . magma is g e n e r a t e d u n d e r

8 c o n d j t i o n o f low p R r t i a l m e l t i n g ( a b o u t 20%) i n t h e

uppe r m a n t l e w i t h i n t h e d e p t h s o f 35 t o 70km reen en and

R i n ~ w o o d , 1967) . Tts generation is commonly a s s o c i a t e d

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

t n l c n v i r o n m e n t o r ~ n o r o c e n i c r;ettj ngs . A c o n t r a r y no t i o n

o f s l l b d u c t l o n ( c o n v r r g e n t s e t t i r ~ ~ ) i n p n r t s o f t h e Lower

R c n r ~ a - 'llx'oudl;h w n f ; , he)wever, t r i f 5pc r . d o f f by t h e r e c o e n i t i o n

o f rnckr ; o f i n t e r m c d j a t e ( d j o r i t i a ) compos t i on . 'Phis was

h o o s t e d by e wronff c l ~ s e i f i c n t i o n , m e r e l y b a s e d o n p h y s i c a l

appeRrnnce o f some l ~ v e s and t u f f s i n t e r b e d d e d w i t h A l b i a n

s e d i m ~ n t s i n A b n k a l i k i a r e a s a s l I a n d e s i t i c t f ( B u r k e et a l l

1971 , 1972) . T h e s e p y r o c l a s t i c s we re l a t e r p r o v e d , t h r o u g h

r e t r o ~ r n p h i c and ~ e o c h e m i c e l d n t a , t o b e a l t e r e d . a l k a l i

basnl t : : ( ~ l e d e , 1 9 7 9 ) . I n a d d i t i o n t o t h e s e , a m a j o r i t y

o f t h e ~ n ~ l y s e s o f t h e d i o r i t i c r o c k s i n t h e Lower Benue

Trorr'gh show e l k a l i ne a f f i n i t y , . They i n c l u d e t h e monzon i t e s /

, Fig . 9 : P o t a s h - s o d 8 d i a g r a m f o r t h e ~ a ~ n l n t i c ' .

r o c k s . Nos 1-10, a s i n Fi[;. 7. I I

Fig. 10: A lk a l i - a l umi n a diagram f o r the magmat ic , r o c k s . Nos 1-10, as in F i g . 7.

rnoni- ,o<ior i tes i n t h e work o f A e n k h e l i l (19861 , m o n z o d i o r i t e s ,

I J m f a , j i ( 1 9 9 2 ) , nn? m i c r o f i i o r i t e s ~ n d d i o r i t e s , Ekwueme

( 1992). Tn t h e l i g h t of t h e d e t a f r om t h e p r e s e n t s t u d y

which i s v e r y c o n r ; i s t e n t w i t h t h e oneu f rom p r e v i o u s

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

anfl nn j o r - e l emen t a e o c h e m i c a l G t u d i e s , a d i v e r g e n t

( c n r l t i n e n t a l r i f t ) s e t t i n g i~ w e l l f a v o u r e d f o r t h e

mogmatic r o c k s o f t h e Lower Benue Trough.

The a l t e r a t i o n o b s e r v e d i n t h e magmgtic r o c k s i n t h i s

~ t ~ r d y i~ o f d e u t e r i c o r i a i n . D e u t e r i c a l t e r a t i o n r e s u l t s

f rom h o t arid c o r r o s i v e s o l u t i o n s g e n e r a t e d f rom t h e

c r y s t a l l i s a t i o n o f magma. T t it3 common a t t h e l a t e magmatic

s t a ~ e , b u t more p ronounced a t t h e h y d r o t h e r m a l s t a g e which

is u c u n l l y e n r i c h e d i n t h e f l u i d phflue (ngndmm, 1972 ,

p . 84; Moorhouse, 1964, p. 179) . C h a r a c t e r i s t i c d e u t e r i c

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

s t i ~ d y i n c l u d e c a l c i t e , s e r i c i t e , m g n e t i t e , p y r i t e , and

f i r r \ - p:rnined q11nrt.7,. C h l o r i t e wns o b s e r v e d i n t h e

hn:-n l t i c s i l l s . T h i s m i ne rn l . n snemblage is c o m p a r a b l e t o

t h ~ t r e p o r t e d by Q e n k h e l i l ( 1986 ) n t t h e Workum H i l l s .

S j m i l ~ r a l t e r a t i o n p r o d u c t s e r e common i n t h e r o c k s

d ~ c c r i b e d by H o s s ~ i n ( 1981) . * D e u t e r i c a l t e r a t i o n t e s t i f i e s

t o t h e ebundnrice o f w n t e r i n t h e o r i g i n a l magma f rom which

thr. rocks w e r e d e r i v e d ( ~ e e T u r n e r and Verhoogen , 1960,

r. 167)).

T h r pronounced f issil i t y nnd t h e a p p e a r a n c e of "spo t o " ,

o f c ~ l r i t e , q u n r t z , p y r i t e i n t h e c o n t a c t metamorphosed

c n l c a r c o u s s h k l es -spo t t e d s l n t e and b ~ k e d s h a l e , are f i r s t

i n d i r n t , i n n s o f r r c r y s t n l l i ~ n t i o n i n t h e c o n t a c t metemorphiem

o f t h c h o s t s h ~ l e ( ~ e e Nockolds e t a l , 1978, p. 351)- T h i s

c l~pr~nt~t . . . : t h e o b s ~ r v n t i no by rnnng p r e v i o u s w o r k e r s t h a t

t h e l rrgcr i n t r u e i v e b o d i e s i n t h e Renue Trough t r ~ n s f o r m e d

thcs h ~ : ; t s c d i m c n t n r y r o c k s ( q r n k c h l e y and J o n e s , 1965,

R e n k h e l . i l , 1986, I l m e j i , 1992) .

rift- system) --

Rocks from Baikal rift,Siberia

/ ROC& from- presen~ study

A L U M I N A ( w t O / o ) -

~ i g , 12 Comparison with rocks from other continental rifts on the alkali-alumina diagram. N O S 1-70,-as ~ n Fig.'?, Dashed l i n e marks the l i m i t f o r p e r a l k a l i n e r o c k s ( ~ u n o , 1960). 1 ~ a t . a u s e d i n

protting o b t a i n e d frdm ~ i n k , 1965.~ and KifiePer et a l , 1978)-

CHAPTER SEVEN --- 7.0 -- SUMMARY A N D CONCLUSIONS

? t hnfi been pot io ib le t o c a r r y o u t A d e t a i l e d mapping

o f the r n ~ g m ~ t i c r o c k s i n t h e h i t h e r t o , s c a r c e l y mapped

nl'c'n? W F F ; ~ o f t h e Arlyirn R i v e r , S o u t h e a s t e r n N i g e r i a . The

mpprnntir: r o c k s invntled a m a r i n s s c ~ I i m e n t a r y sequence of

probably Alhinn p n d Turonion ages . The ~ e d i m e n t a r y r o c k s

a r e c a l c a r e o u s . They a r e f o l d e d i n t o a e e r i e 8 of a n t i c l i n e s

and s y n c l i n e s wi th FI g e n e r a l NE-SW t r end . They a r e c u t by

f r n c t w e s which n r e f i l l e d by c e l a i t e , s i d e r i t e , q u a r t z ,

n n d c n r n e t i m e ~ , by p y r j t e , Ra lene , ~ n d s p h e l e r i t e . The

s c d i r n e n t ~ r y r o c k i n c o n t a c t w i th t h e compos i t e d i o r i t e waa

The mngmntic r o c k s e r e g e n e r a l l y o f b a s i c t o in t e rme-

d in t c cornposi t i q n ~ . They i n c l u d e o l i v i n e - b a s a l t i c sills,

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

t u f f s . The mnRmntic b o d i e s a r e p e n e r a l l y o f s m a l l - s c a l e .

I ' e t roprqphi c s t u d i e s r e v e ~ l . t h e predominance of t h e

c h ~ ~ r n ~ : t r r i f i t i c s o f t h e a l k a l i n e b a s ~ l t magma, namely, t h e

presence o f nephel i n e and c l i n o p y r o x e n e , w i t h o thorpyroxene .

T h r mnjor-elcment geochemica l s t u d i e s a l s o s u p p o r t

t h i n n l k n l i n e ~ f f i n i t y , w i t h ~ d d i t i o n a l a luminous c h a r a c t e r .

'rhi s rc su l . t cou ld not, however, b e r e l i a b l y employed i n

p c t r o p e n e t i c n r p u m f - n ts b e c f l u ~ e of t h e p e r v a s i v e

n l t c r n t i o n i n t h r r o c k 8 which c o u l d b e ~ L t r i b u t e d t o t h e

~f f e c t e o f d e u t c r i c f l u i d s . The m a j o r - e l e m e n t s are

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

concen t r n t i on ' mag, t h e r e f o r e , h ~ v e changed c o n s i d e r a b l y .

Tn t h e l i ~ h t o f t h e p e t r o g r a p h i c d a t a , t h e f o l l o w i n g

c o n c l u s i o n s mny b e p o s s i b l e , namely:

( a ) t h e mngmnti? rockf i r e p r e s e n t members o f a n

n l k a l i n o s u i t e ;

( h ) t h e r o c k 6 ware emplaced i n e d i v e r g e n t

( c o n t i n c ~ t n l r i f t ) s t - t t i n p ;

( c ) t h e m n p r was most l i kc1.y e n r i c h e d i n water as

i n d i cn t e d by t h e p e r v ~ a i v c d e u t e r i c a l t e r a t i o n

i n t h e r o c k s .

R E F E R E N C E S

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