University of Nigeria+++ Research Publications

Aut

hor

B,N Ekwueme

PG/PH.D/81/1230

Title

Petrology Geochmistry And Rb-Sr Geochronology Of

Metamorphosed Rock Of Uwet Area Southeastern Nigeria

Facu

lty

Physical Science

Dep

artm

ent

Geology

Dat

e

November, 1985

Sign

atur

e

PETROIX) GY , GEOCHEMISTRY AND Rb-Sr

GEOCHRONOUIGY OF NETAMORPHOSED

ROCKS OF WIET AREA SOUTHEASTERN NIGERIA

BARTHOUIrnW NWOYE EKIJUEME

(B.SC. HONS., I.P&L.)

(PG/P~ . D ./81/1230)

Thesis submitted to the Department of

Gieology in the Faculty o f Physical

Sciences University of Nigeria

Nsulrka i n partial fulfilment

of the requirements for

the Degree of

DOCTOR OF PH1IX)SOPHY ( P ~ . D . )

JUNE 1985

B E _ _ D I C A T I O _ N - - - - - - - - - -

This t he s i s is dedicated:

( i ) With love:

t o Olunwa and Nem

(my wife and daughter)

f o r t h e i r love and companionship;

( i i ) With gratitude.:

t o A. C. Onyeagocha 1 .

(my most inspir ing teacher) e*

f o r my academic attainment;

( i i i ) With honour:

t o Maria Onuh

( m y l a t e grandmother)

f o r her great care and love f o r me.

iii

Field, petrographic, geochemical and isotopic s tud i e s

on Uwet Area of goutheastern Nigeria indicate t ha t polyphase

deformation and polymetamorphism have transformed pe l i t e s ,

semi-pelites, calcareous pe l i t e s , greywackes and basic t o

intermediate igneous rocks in to phyl l i t es , s ch i s t s , amphibolites,

gneisses and migmatites.

Mesoscopic and microscopic s t ruc tures indicate a t

l e a s t th ree deformational episodes which can be distinguished

i n the f i e l d by s t ruc tu ra l trends i n the N-S t o NE-SU (0-40'

azimuth) ; ENE-WSW (50'-70' azimuth) and NW-SE (1 0 0 ~ - 1 4 0 ~

aeimuth). Thin sect ion study indicates discordant S i d e trails

associated with mineral porphyroblasts, mul t iple fo l ia t ions and

retrograde minerals cu t t ing t he main f ab r i c of high-grade rocks.

~ b / ~ r whole-rock analyses give ages of 922220 Ma f o r

migmatitic gneiss; 784&31 Ma f o r banded amphibolite; 676226 Ma

f o r phyl l i t e , gneisses and some sch i s t s ; and Ca. 527316 Ma f o r .

the migmatitic sch is t s . The l a s t two ages f a l l within the Pan

African thermotectonic event dated a s 600+00+1 50 Ma,

Regional dynamo thermal metamorphism of the kyanite-

s i l l imani te type has been recognized i n t he Uwet area,

Metamorphic isograds, based on the entry of p e l i t i c index

minerals, b io t i t e , garnet, kyanite and s i l l iman i t e have been

napped. The d i s t r i b u t i o n of these isograds ind ica tes a

northwest t o southeast increase i n t h e i n t e n s i t y of

metamorphism f r o m medium greenschist f a c i e s t o uppennost

amphibolite fac ies . The mineralogical assanblage

plagioclase-bio t i te-green hornblende-epidot e ) i n t h e homogenoua

amphibolite ind ica tes t h a t it a t t a ined at l e a s t lower

amphibolite f a c i e s metamorphism whereas t h e banding, associa t ion

with migmatite and t h e abundance of brown hornblende coupled

with t h e absence of s t a b l e epidote i n t h e banded amphibolite #

i n d i c a t e t h a t it a t t a ined an upper amphibolite f a c i e s

regional metamorphism.

Magmatism of calc-alkaline and t h o l e i i t i c bas ic types:

resul ted i n t h e in t rus ion of g ran i t e , granodior i te , t o n a l i t e

and d o l e r i t e i n t h e Uwet area. The calc-alkaline magmatiam

t r iggered o f f K-metasomatism which caused extensive

microcl in isa t ion of these rocks.

Sedimentary rocks (shale, limestone and sandstone)

belonging t o t h e Am River G'roup, t h e Eze-Aku Fomat ion,

the Nkporo Shale and t h e Benin Fomat ion were deposited

unconfomably on the basement rocks between t h e Cretaceous

and Ter t i a ry times.

The inspira t ion f o r t h i s study came from

D r . A. C. Onyeagocha. Not only did he suggest t h i s project ,

he also encouraged and guided me through rigorous field-checking

exercises and careful but blunt cr i t ic ism of the various d ra f t s

of t h i s thes i s , My grea tes t grat i tude goes t o him.

I wish to acknowledge with thanks the receipt of Elenate

Gkants No. 1 54 (1 981 / 82) and No. 0204 (1 983/84) f mm the 1

University of Calabar, Calabar. The f a c i l i t i e s of the

Department of Geology, University of Calabar were used f o r t h i s

study. I thank the e n t i r e member8 of s t a f f of the department

especially D r . R. M, Ramanathan, M r . B. B). Ukpong and

D r , A.A.M.S. Rahman f o r the moral support they gave me during

t h i s investigation,

I have benefit ted from the benevolence of various people

some of whom I had contact with only through correspondence. To t h i s

end, I am very gra te fu l t o Professor S.W. Will iams of the I l l i n o i s

S t a t e University, U.S.A. f o r helping with t he chemical analysis

and t o Dr . M. Caen-Vachette of t he Laboratoire llsssocie No. 10,

C.N.R.S. e t Universite, Clenuont-Ferrand Cedex, France f o r the

isotopic analyses. I n l i k e manner, I appreciate the useful

suggestions from Dr . M.A. Rahaman of the University of I f e ,

v i

I l e - I f e Nigeria which I received a t d i f fe ren t stages of t h i s

study. I thank my friend Mr. Chuma Eze of the University of

Nigeria, Nsukka f o r h i s hosp i ta l i ty during t h i s project.

I am indebted t o Mrs. 9. F. M. I q a n g f o r carefully typing

t h i s thesia.

Last but by no means t he l ea s t , I acknowledge with

love t he patience of my wife Olunwa and the companionship

of my daughter Nena who was born during t h i s study.

v i i

PAGES -

ABSTRACT 0 0 0 0 0 0 0 0 0 o o o o o

.......... ACKNOWLEDGENEIJTS

CONTENTS o o o o o o o o o o o o o o o

I ~ I S T O F F I ~ I R E S ooo.. 0 . 0 . .

LIST OF TABLES o o o o o o o o o o

CHAPTER ONE

INTRODUCTION o o o . o o o o o o

b

1 :l Geologica,l s e t t i n g

1 :2 Na.ture of s tudy

CHAPTER TWO

GENERAL GXOLOGY 0 o o 0 0 0 0 0 0 0

P h y l l i t e s ..... Sch i s t a .....

..... Amphibolites

Gneisses ..... Migmatitea ..... D o l e r i t e s ..... Gra.ni t o i d rocks

Sedimentaxy rocks

i ii

v

v i i

i x

x i i

CONTENTS (CONTD . ) v i i i

CHAPTER !THREE

MINERALOGY AND PETROGRAPHY . * * . * . . . . . 3: 1 Metamorphic rocks .......... 322 I n t r u s i v e rocks ..........

CHAPTER FOUR

STRUCTURAL GEOLOGY ............*.. b t 1 F o l i a t i o n s ............... 4:2 Folds .................... 423 J o i n t s and Fau l t s .......... 4:4 Micros t ruc tures .......... 4:s Deformation and Methorph ic Episodes

CHAPTER FIVE

CHEMICAL COMPOSITION ..... .. .e . . . . . .

5: 1 Analyt ical Method .......... 5: 2 Geochemistry ...............

............... 5: 3 Pe t rogenes is

CHAPTER SIX

GEOCHRONOLOGY e...................

6 : 1 Analyt ica l Method .......... 622 Resul ts ............... 6:3 I n t e r p r e t a t i o n of Resul t s .....

CHAPTER SEVEN

.................... IaTAMORPHISM 149

7:l Metamorphic Type .......... 149 .......... 7:2 Isograds and Zones 152

7:3 Temperature . Pressure condi t ions of Me tamorphism ............... 157

CHAPTEX EIGHT

SUMMARY AND CONCLUSIONS ............... 159

LIST OF FIGURES

Location of Study a r e a on the geologica l

map of Niger ia .......... FIGURE 2t Geological map of Uwet a r e a ( shee t 323)

Oban massif , southeas tern Nigeria

FIGURE 3t FIGURE 4: FIGURE 5s

S c h i s t xeno l i th i n g r a n i t o i d rock

Sample l o c a t i o n map of Uwet a r e a

Contact of d .o ler i te dyke and kyani te g n e i s s ..........

FIGURE 6:

FIGURE 7r FIGURE 8%

FIGURE 9:

FIGURE 10:

Cross-cutting r e l a t i o n of a p l i t e

Pegmatite ve ins i n g n e i s s ..... ..... Folding of a gne i s s outcrop

Meladior i te i n k ranod io r i t e ..... Geological map of Uwet a r e a ( shee t 323) showing the d i s p o s i t i o n of the sedimentary rocks ..........

FIGURE 11: Quartz f o n i n g 120' t r i p l e junct ions and enhancing the f o l i a t i o n of t h e kyanite- s i l l i m a n i t e s c h i s t (XPL; X 10)

Garnet r i d d l e d wi th inc lus ions of qua r t z and p h y l l o s i l i c a t e s i n t h e ga rne t - s i l l i m a n i t e s c h i s t (XPL; X 10)

FIGURE 12r

P h y l l o s i l i c a t e s wrapping around kyani te porphyroblast i n t h e kyani te -s i l l imani te .......... s c h i s t . (XPL; X 10)

FIGURE 13:

FI(=URE 14:

FIGURE 15s

FIGURE 16:

FIGURE 17:

Porphyroblast ic muscovite i n the b i o t i t e hornblende gne i s s (PPL; X 10) ..... Unstable muscovite i n the migmati t ic gne i s s (PPL; X 10) .......... Pleochroic ha loes around z i rcon c r y s t a l ..... i n b i o t i t e (XPL; X 10)

Ghrnet with minor inc lus ions of opaque dus t i n the garnet-mica s c h i s t . . . . . . . . . . . . . . . (PPL; X 10)

FIGURE 18: Kyanite sandwiched i n b i o t i t e c r y s t a l s and

enhancing the g n e i s s o s i t y of the kyan i t e

..... gne i s s ( x P L ; X ~ O ) ..... F i b r o l i t e i n the game$-s i l l imani te s c h i s t

(PPL; X 40) ..... ..... ..... FIGURE 20: Elongate pr i smat ic s i l l i m a n i t e i n the

k y m i t e - s i l l i m a n i t e s c h i s t

(XPL; X 10) .......... ..... FIGURE 21 t F i b r o l i t e coex i s t ing wi th t h e id iob la , s t i c

s i l l i m a n i t e i n the kyani te -s i l l imani te

s c h i s t (PPL; X 40) .......... 6

Fractured epidote enc los ing an FIGURE 22:

allanite c r y s t a l i n t h e biot i te-hornblende ..... gne i s s (XPL; X 10) ----•

P l o t of the modal composition of Uwet

g r a n i t o i d s on QPP diagram ..... FIGURE 23:

P l o t of f o l i a t i o n s i n Uwet a r e a FIGURE 24:

FIGURE 25: Boundinage and a s soc ia t ed f o l d s i n

............... gne i s s

FIGURE 26:

FIGURE 27:

FIGURE 28:

FIGURE 29:

..... Ptygmatic f o l d s i n gne i s s

Rose diagram o f j o i n t s i n Uwet a r e a

F a u l t i n kyani te gne i s s .....

Si02-CaO v a r i a t i o n diagram f o r the rocks ............... of Uwet a r e a

Plo? of Uwet rocks on ............... ACF diagram

FIGURE 30:

FIGURE 31 t

FIGURE 32:

FIGURE 33:

FIGURE 34:

FIGURE 35:

FIGURE 36:

FIGURE 37:

FIGURE 38:

FIGURE 39:

FIGURE 40:

FIGURE 41:

FIGURE 42:

FIGURE 43:

xi

P l o t of W t . % Fe aa Fe203 a g a i n s t W t . % M g O f o r rocks of Dwet a r e a ..... 109

Alka l i - s i l i c a diagram f o r Uwet d o l e r i t e

and banded amphibolite ..... 11 1

AFM va r i a . t i on diagram f o r the rocks of

Uwet a r e a . . . . . . . . . . . . . . . 112

P l o t of Uwet d o l e r i t e and banded amphibolite

on Ti02-K$-P205 diagram .... . P l o t of t h e Uwet metasediments on K20 VS.

Na20 diagram ..... ..... ..... Alka,li - s i l i c d p l o t f o r g r a n i t o i d rocks of

Uwet a r e a . . . . . . . . . . . . . . . Rb-Sr isochron diagram f o r rocks of Uwet

area. . . . . . . . . . . . . . . . P l o t of s c a t t e r e d po in t s of whole-rock

Rb-Sr i s o t o p i c d a t a of Uwet a r e a

Rb-Sr isochron f o r kyani te -s i l l imani te

s c h i s t . . . . . . . . . . . . . . . . Rb-Sr isochron f o r kyanite gne i s s

Rb-Sr isochron f o r banded amphibolite

Metamorphic map of Uwet a r e a .....

TABLE

TABLE

TABLE

TABLE

TABLE

TABLE

TABLE

TABLE

TABLE

TABLE

TABLE

TABLE

TABU

LIST OF TABLES

Modal composition of s c h i s t , amphibolite

and p h y l l i t e i n Uwet a r e a ..... Modal composition of gneisses i n Uwet

a r e a .................... Modal composition of migmatites and

.......... banded amphi bol i t e

Modal averages of minerals i n rocks of

1Jwet a rea , S.E. Nigeria ..... Uodal composition of i n t r u s i v e rocks i n

b

Uwe t a r e a ............... Trends of Geologic s t r u c t u r e s ..... Average chemical composition of rocks o f

Uwe t area, sou theas t e r n Niger ia

Chemical composition of rocks of Uwet a rea ,

sou theas te rn Niger ia .......... Average Nonnative compositions of rocks o f

Uwet a rea , S.E. Nigeria ..... Average major element compositions of rocks

..... similar t o those of Uwet a r e a

Rb-Sr I so top ic da ta of whole-rock ..... determinat ions from U w e t area

Additional I so top ic da.ta from U w e t

.................... area.

Mineral assemblages coexis t i rX with quar tz ,

plagioclase and b i o t i t e , metamorphic

zones and isograds i n Uwet area , .......... Southeas t e r n Nigeria

CHAPTER ONE

GEO LOG1 CAL SE'PTING

The Nigerian topographic sheet 323 wet) is located

i n t h e geological ly unexplored Oban Massif of Nigeria. The

massif which is i n t h e boundary of Cross River S t a t e of Higeria

and Cameroon Republic is approximately 100,000 square kilometers.

The area of inves t igat ion is t h e degree sheet 323 k wet) which

l i e s between longitudes 8°0@1~ and 8 ° 3 0 1 ~ and l a t i t u d e s 5 '00 '~

and 5 ' 3 0 ' ~ ( ~ i g . 1 ).

Uwet sheet is a p a r t of t h e Eastern Nigeria highlands.

These highlands consis t of two giant spurs which a r e t h e

western prolongation of t h e Cameroon Mountains in to t h e Cross

River plains. These spurs a r e known a s t h e Obudu Plateau and

t h e Oban H i l l s . The a rea of inves t iga t ion is an extension of

t h e Oban H i l l s . The Oban H i l l s consis t of a deeply dissected

plateau which i n places a t t a i n a height of 1 ,125m above mean

sea l e v e l ( ~ f o m o t a 1975). The a r e a the re fo re has an undulating

topography. This topography gradually r i s e s towards Benue S t a t e

i n t h e north and towards t h e foo t h i l l s of t h e Cameroon Mountains

i n t h e East. The eas tern e levat ion is about 137m above sea-

l e v e l but general ly appears a s a plain. The area becomes h i l l y

a s one moves towards t h e western pa r t .

160 km.

Basement Complex

Study Areo

Sediments

F IG.1 Locotion of Study Areo on the Geological mop of N iger ia .

3

On t h i s western s ide , t he highest elevation is about 183m

above sea-level. These h i l l s a r e made up, dominantly, of

s ch i s t s whereas t h e eastern par t consis ts mainly of

gneisses, pegmatitic grani tes and granodiori t es . The

extreme south which is basical ly a low land is characterized

by shale, sandstone and limestone lying unconfomably on the

basanent rock.

The Cross River, t he Great Kwa River, t he Calabar River

and the Inyang I t a River dra in t he map area. Their flow appears #

to be s t ruc tu r a l l y controlled s ince they flow from north t o

south which is t h e trend of major s t r u c t u r a l features ( f o r

example, fo l i a t ions , fo ld axes and major jo in t s ) i n t h e area.

Most of these r i ve r s a r e reduced i n volume during the drg season

thereby exposing f resh rock samples. Most of t he rocks i n the

area a r e intensely weathered. The rocks most susceptible to

weathering a r e t h e highly micaceous s ch i s t s t ha t cover most of

the western portion. These s ch i s t s weather in to clayey s o i l s

with a reddish colour. Weathered pegmatites and grani tes a r e

o f ten indicated by scat tered f l akes of shiny muscovite on the

surf ace.

The study area is i n par t r&erd impenetrable by the

equatorial evergreen rainforest . The only major roads traversing

t he area a r e the Calabar - Ikom and t he Calabar - Ekang mads.

Others a r e secondary.

1.2 NATURE OF STUDY

The Oban Massif

Nigeria. This complex

is a par t of the Basanent Complex of

consists mainly of s ch i s t s , gneisses,

mignatit es, metasediments and in t rus ive mcks of g r an i t i c

composition (14cCurry 1976). The s ch i s t s a r e thought t o belong

to t he Newer Ibtasedimentary s e r i e s which represent an in-

folded sedimentary cover in to the gneiss - migmatite - quar tz i te

complex aham am an 1976). Oyaxoye (1 972) s ta ted t ha t s ch i s t s and

metasediments a r e confined to t he western s ide of the country +

along a ~orth-south trending trough. He went fu r ther t o s t a t e

t ha t t h i s contras ts with the disposi t ion of younger volcanics

and intrusiveS on the eastern parts . Metasediments, he said ,

"are v i r t ua l l y absent on the eastern s ide of the country".

Grant (1 971 ) has assigned a Pan-Grican age of 60% 1 50

Ma t o t he s l i gh t ly migmatized t o non-migmatized paraschists and

a minimum Eburnean age (2,000+_200 Ma) and more l i ke ly Liberian

age (2,80&2200 ~ a ) t o the gneiss-rniwatite - quar tz i te complex

of the Ibadan area. A t l e a s t two episodes of deformation

affected the complex and might have occurred p r io r t o o r

cont enpo raneous with the metamorphic events (Oyawoye 1964 3

McCurry 1976; Rahaman 1976; Onyeagocha 1984). The l a t e s t

episode of these deformations is believed to be Pan-African i n

age (60%150 ~ a ) .

5

The purpose of t h i s investigation is to map the Uwet

area i n d e t a i l and t o study the mineralogical and chemical

composition of the mcks. This w i l l make i t possible to:

( i ) d i f f e r en t i a t e the hi ther to undifferentiated basement rocks ,

( i i ) locate metamorphic isograds and zones,

( i i i ) in te rpre t e t he temperature, pressure, degree and timing of the metamorphism of the area,

( i v ) ascer ta in the nature and pecu l i a r i t i e s of the parent mcks and

(v) evaluate the strat igraphy of the area thmugh the geochmnological@studies of the mcks.

Fie ld work consisted of col lect ion of representative

samples of each l i tho log ic un i t mapped and the recognition of

the contact relat ionship between them. Maw of t he samples

were collected a t quarry s i t e s which a r e scat tered a l l over the

area. River beds and channels expose f resh rock samples i n

t h e dry season.

Representative samples were studied using the

petrological microscope. The bulk rock compositions of the

mcks were obtained using X-ray fluorescence and atomic absorp-

t i on spectrometers. Mass spectmmeters were w e d to determine

the Rb and Sr contents of t he rocks f o r geochronological study.

CHAPTER TWO

GE IJER AL GEO I#.) GY

2.1 PHY LLITES

Phyl l i t es crop out i n Camp 1 of t he Calaro Estate.

These p h y l l i t es a r e light-coloured and consis t dominantly of

muscovite, c h l o r i t e and b io t i t e . They a r e highly weathered.

They a r e folded pa ra l l e l t o t h e i r f o l i a t i ons which var ies from

~20'~ to B O ~ E . The fo l i a t i ons d ip 8ONW.

A dark coloured, fine-grained phy l l i t e with a characterc

i s t i c sheen covers the whole stream channel about one kilometer

north of Camp 5 of the C a l a h Estate. It i s weakly fo l i a t ed and

composed dominantly of b i o t i t e and chlor i te . The general trend

of t h e outcrop is northwest. Ketaquar tz i t ic selvages a r e present

i n the phy l l i t e possibly indicat ing a sands tonesha le sequence

of the o r ig ina l ( i .e . pre-metamorphic) sedimentary rock.

2.2 SCHISTS

S c h i s t s a r e r e s t r i c t ed i n t h e i r occurrence. Most outcrops

occur i n t he northwest quadrant of the degree sheet. However,

s c h i s t s occupy t he channels of t he Great Kwa River and a lso occur

around Abbiati i n t he southeastern port ion of t h e sheet.

Megascopically, t he s c h i s t s can be grouped in to three on t he

basis of texture:

( i ) f ine-grained s c h i s t s

( i i ) medium-grained s c h i s t s

( i i i ) coarse-grained sch i s t s .

7.

The fine-grained sch is t s crop out i n Ikot Okpora where

they a r e over l a i n by sandstone beds. They also

cover t he whole of Ikot Ana i n the northwest and extend t o

the Cross River Channel. The fine-grained sch i s t s a r e a

l i t t l e coarser than the phyl l i t es and some of t he minerals,

f o r example, b io t i t e , muscovite and quartz a r e recognizable

i n hand specimen. The s ch i s t s a r e intensely weathered and

i n some places ( f o r example, Ikot h a ) the bedding planes of

the or ig ina l sedimentary sequence a r e s t i l l preserved, They

a r e a l l fo l ia ted and joint&, Schis tos i ty trend i a N-S and

the d ip of the sch is tos i ty decreases from a maximum of

6 3 O ~ to 30°W towards the Cross River Channel. The major

joints a r e generally pa ra l l e l to the sch is tos i ty but they are

occasionally cut by minor jo ints trending east.

A t Ikot Okpora, the fine-grained s c h i s t s a r e intruded

by granodiori t es with which they have sharp contact

relat ionships. The contact of the fine-grained sch i s t s with

unmetamorphosed sediments (limestone, sha le and sandstone)

i n the northwest is not exposed.

The medium-grained sch i s t underlies the whole of

Iwum Obionthan (Figure 2) from where i t extends to t he

Dunlop Rubber &tate. Outcrops of t h i s s ch i s t occur a lso i n

Igbofia, Akwa Ibiami and 0 jo (Figure 2). Characterist ic

of t he medium-grained sch i s t s a r e porphyroblasts of garnet

- _ k e s of muscovite and b i o t i t e which a r e recogni!

ascopically They a lso contain quartzo-f eldspathic

ch rnost3y run p a r a l l e l t o and occasionally cut acrc

i s t o s i t y . Some of these veins are contorted and fc

s ch i s t is intensely deformed. It shows variable

i s t o s i t y trends. For example, i n Igbofia, t h e trend of the

i s t o s i t y defined mostly by b i o t i t e is NNE-SSW which is more

l e s s N-S. Here, the quartzo-feldspathic veins define a

md s e t of f o l i a t i o n i n t h e N W 4 E direct ion. A t Akwa #

mi, the s ch i s to s i t y trends N400W and dips 600NE. Here,

s ch i s t is intensely weathered. A t Ojo, t he medium-grained

ist outcrops i n stream channels. It contains garnet,

nblende and secondary ca lc i t e .

A common fea ture of most of t he mediwn-grained s c h i s t

its extensive invasion by in t rus ive granodior i t ic rocks

compared to o ther rock types i n t he area , However,

n a t i t e bodies and veins occur a s well, I n most of t he

a w n - 4 + n 4 A wnnba f o r example, UWI , t he medium-grained s c h i s t

t h s ( ~ i g u r e 3). These xenoli ths displey

occur as darker patches i n t h e grani toid rock.

t l y composed of b i o t i t e and could have

gmatic stoping which possibly emplaced t he

n t he area.

5

Figure 3 Schis t xenol i th i n g ran i to id rock.

1 1.0

The coarse-grained s c h i s t crops ou t a t t h e Kwa F a l l s

and Abbiati i n t h e southeas tern p a r t o f t h e map a rea

( ~ i ~ u r e 2). Conspicuous segregat ion i n t o l i g h t and dark

por t ions a r e c h a r a c t e r i s t i c of t h e coarse-grained s c h i s t .

~t t h e Kwa F a l l s , t h e l i g h t po r t ion of t h e s c h i s t contains

porphyroblasts o f kyani te and e i l l iman i t e . They a r e less

s c h i s t o s e than t h e dark portion. On t h e o t h e r hand, t h e dark

por t ion of t h e s c h i s t i s highly f o l i a t e d and cons i s t s o f mafic

minerals dominantly t h e p h y l l o s i l i c a t e s . Effervescence was

observed on a p p l i c a t i o n o f hydrochloric ac id t o t h e dark

por t ion of t h e schist. No effervescence was however, observed

when t h e same ac id was applied t o t h e l i g h t po r t ion of t h e

s c h i s t . This i n d i c a t e s t h e calcareous na ture (possibly due

t o secondary c a l c i t e ) of t h e s c h i s t a t t h e Kwa F a l l s . The

coarse-grained s c h i s t i s associa ted with boulders of

pegmati t ic g r a n i t e a t t h e Kwa F a l l s . The s c h i s t is in tense ly

deformed. S c h i s t o s i t y t rends N40°E t o N500E with d i p s

varying from 26' t o ~O*NW. Folds, boudins and f a u l t s were

recognized i n t h e s c h i s t a t Kwa Fa l l s .

A t Abbiati , conspicuous outcrops of t h e coamergrained

s c h i s t occur. Here t h e s c h i s t has a sha rp contact re la tkonship

with t h e Mfamosing Limestone which is Albian i n age. The

s c h i s t h e r e exh ib i t s crude banding i n t o l eucocra t i c p a r t s which

a r e r i c h e r i n qua r t z and f e l d s p a r and melanacrat ic por t ions

1 2'

which a r e r i c h e r i n b i o t i t e and garnet . Poss ib le recrystal-

l i z a t i o n a t high temperatures resul ted i n abundant K-feldspar crys ta l :

which a r e enclosed a s l a r g e aggregate c r y s t a l s i n t h e s c h i s t .

The s c h i s t has been in tensely deformed. Sch i s tos i ty trend is

ROOE and d i p is 2 6 ' ~ ~ . Folds, t runcat ion and f o l i a t i o n

o f f s e t s a r e observed i n t h e coarsegra ined s c h i s t a t Abbiati.

2.3 @?PHIBOLITES

Two types of amphibolites occur i n Uwet area:

( i ) banded and b

( i i ) homogenous amphibolit es . A t h i n l y banded amphibolite has a sharp contact

r e la t ionsh ip with t h e sandstone of A w i Formation i n t h e

c e n t r a l port ion of t h e map area . The rock which is very

dark i n colour, is tough and v a r i e s i n t ex tu re fmm medium=

t o coarse-grained . The f o l i a t i o n trend is general ly NW-SE

(about 140' Azimuth) with a d i p varying from 20' t o 3 0 ' ~ ~ .

This i s contrary to t h e predominantly N-S Pan-African trend

but has been reported i n t h e Akwanga a r e s of north-central

Nigeria where regional f o l i a t i o n trending NE-3W was observed

t o o f f s e t another t rending NW-SH (~nyeagocha and Ekwueme 1982).

I s o c l i n a l fo lds with axes trending 250' Azimuth, boudin

s t ruc tu res and undeformed mafia dykes, ( t rending 146') were

recognised i n t h e banded amphibolite. The f o l i a t i o n is t

13

pa ra l l e l t o t he layering i n t he para l l& - folded rocks and

t h i s indicates the t the fo l i a t i on is older than the folds.

A homogenous amphibolite covers the channels of the

Calabar River and Inyang Ita River a t the Calaro 18tate .

Sample Cal Z ( ~ i ~ u r e 4) was collected a t t he bank of the

Calabar River southwest of Camp 6 i n the es ta te . The amphibolite

i s dark, fo l ia ted and medium-grained. The fo l i a t i on trend is

~ 2 0 ' ~ and the dip is nearly ver t i ca l . I n hand specimen, I

vorphyroblasts of hornblende which make up more than 70 per cent .

of the rock a r e eas i ly recognized. Unlike most of the @chis t b

outcrops, the amphibolite is re la t ive ly f resh and shows l i t t l e

o r no e f f ec t of weathering ac t iv i ty .

2.4 QUARTZO-FELIXSPATHI C GNEISSES'

The eastern (northeast and southeast) portion of the map

e rsa is undcrlaic b~ quartao-feldspathic gneiss (p igwe 2). YWo

major types of quartzo-feldspathic gneiss can be distinguished i n

the area:

( i ) bio tite-hornblende gneiss

( i i ) Kyanite gneiss.

The biotite-hornblende gneiss outcrops a t Okom I t a ,

Obung and the Calam Estate. It is coarse-grained and strongly

fo l ia ted i n t he N-S direction. The rock exhibits variable

colour owing t o d i f fe r ing concentration of b io t i t e , hornblende

and feldspars. Textural var ia t ion i n the gneiss is due to the

15.

changes i n s i z e and amount of K-feldspar. The K-feldspar

forms porphyroblasts which a t t a i n a s i ~ e up t o 2cm across and

is i n places wrapped by b i o t i t e c rys ta l s . The porphyroblasta

form augen s t r u c t u r e s i n t h e gneiss.

The b i o t i t e-hornblende gneiss contains abundant b i o t i t e

and hornblende-rich mafic xenoli ths. A t Okom I ta and Obung

quarry, t h e gneiss is invaded by pegmatitic v e i m ( f o r example, I

outcrop D6 Figure 4 ) which t r ansec t t h e N-S f o l i a t i o n trend of

t h e rock i n a perpendicular d i rec t ion . Rehealed f r a e u r e s a r e

a l s o associated with t h i s gheiss. In - f i l l ing of t h e f r a c t u r e s

( f o r example, OB7, Figure 4) a r e sch i s tose mate r i a l s dominantly

composed of c h l o r i t e and b i o t i t e . Both t h e pegmatitic v e i m and

t h e rehealed f r a c t u r e s could be products of r e c r y s t a l l i z a t i o n

and subsequent segregation of t h e minerals i n t h e host gneiss.

Deposition of such mate r i a l s is f a c i l i t a t e d by v o l a t i l e t ranspor t

( ~ a l t h e r and Orvi l le , 1982),

Generally, t h e biotite-hornblende gneiss is coarse-grained, I

It exhibi ts va r i a t ions i n both t h e trend and d i p of t h e

f o l i a t i o n . For example, i n the Lavori Quarry at Obung t h e trend

i s essen t i a l ly N-S with a d i p of 42-$2OEI whereas i n Calam Bstate,

i t exhibi ts two s e t s of f o l i a t i o n trends. The f i r s t s e t which

is conspicuous has a dominant NE-SW t rend with an a t i t u d e o f

about 20' Azimuth. The second s e t t runca tes t h e f i r s t and

16-

shows a trend i n W d E .di rect ion with an a t t i t u d e of 350'

Azimuth. I n the Calaro Es ta te of Uwet area, the angle between

these two s e t s of f o l i a t i o n is observed t o be about 30'. I f

t he two s e t s represent two dif ferent events separated i n time,

then, the NW4E trending; fo l i a t i on which is considered a s

primary is believed to be of a pre-Pan-African age whereas

the n-SW trending f o l i a t i o n is Pan-African (cf. Onyeagocha

and Ekwueme 1982). Portions of the biotite-hornblende gneiss

have undergone spheroidal weathering. Some outcrops appear

t o enclose pegmatitic g ran i te bodies.

Outcrops of the kyanite gneiss a r e abundant a t t he Crush

Rock Industrg Quarry i n Old Netim. It extends dorm southwards

t o Mbarakom and A w i (Figure 2). The rock is coarse-grained.

The gneissosity which trends N-9 and dips 42% is i n places

deflected by large fe ldspar porphyroblasts which a t t a i n a s i z e

of 2cm x 3cm. The rock has been folded para l le l t o its gneiss-

o s i t y a t Old Netim. Also the kyanite gneiss shows normal

faul t ing here i n the PI-W direct ion and t h i s trend of the f a u l t

is consistent with i ts occurrence i n a period of relaxation

following the main N-S deformation which affected the area.

I n the Old Netim Quarry, the kyanite gneiss is fractured and

baked a t its contact with a do l e r i t e dyke which intrudes i t

(Figure 5). It shows f rac ture - f i l l ings ( fo r example, A7

Figure 5 Contact of do le r i t e qyke (dark) and kyanite gneiss ( l ight) .

F i ~ r e 4) s imi la r t o those of the biotite-hornblende gneiss.

Pre-tectonic pegmatites, ap l i t e s , and quarts veins a r e common

i n the kyanite gneiss (Figures 6 and 7). Sbme of these out-

crops of gneiss have been folded para l le l t o the gneissosity

of the host rock (Figure 8).

2.5 MIGNATITES

The migmatite i n the area assumes a l i t - p a p l i t aspect

a t Mbarakpa, Oban Okomba and Oban. The la rges t outcrops of

banded gneiss were encountered a t the Nigerian Mining +

Corporation Quarry a t Oban (3km east of map area). Here the

rock is coarse-grained and highly foliated. It is also

i soc l ina l l y folded and displays a x i a l plane fo l ia t ion . The

fold axes trend N-3 and they a r e pa ra l l e l to the fo l ia t ion .

There is an evidence of ahear i n t h i s area a s shmn by t h e @

smooth ch lo r i t i c surface tha t separates individual layers of

t he migmatite. The l i t - p a p l i t gneiss is closely associated

with p e ~ a t i t i c granite. A t the contact of the kyanite gneisa

and the migmatite the bands a r e not a s conspicuously developed

a s i n t he Oban quarry ( f o r example, sample Awl2 from Oban

0komba)

A t Ayiebarn and A w i the gneisses i n t he map area s t a r t

t o grade imperceptibly t o migmatite. Outcrop AY 1 (Figure 4)

is highly fo l ia ted and shows e f fec t s of metamorphic segreation.

Figure 6 Cross-cutting relation of aplite.

Figure 8 : Folding of a gneiss outcrop.

.22.

One sect ion of the rock has a higher concentration o f mafic

minerals than the other indicating a trend towards incipient

migmatization. I n addition, the rock is associated with

pegmatitic grani te boulders (AY2). Taken together, outcmps

AY and AY ( ~ i g u r e 4 form agmatite type of migmatite. The 1 2

mck i s highly fol ia ted i n t he N-S to N ~ O E direction. Hajor

0 joints i n t he area trend N20 E whereas minor joints show a

northwest trend.

2.6 DOLERITES: +

A do le r i t e dyke intrudes t he kyanite gneiss a t the Old

Ne th quarry. It is dense, dark, medium grained, not deformed

and urrmetamorphosed. It exhibits spheroidal weathering and

because of numerous joints i n t he outcrop, the top is weathered

t o form l a t e r i t e . It displ.ays chil led borders. Phenocrysts of

olivine, pyroxene and labradori te a r e recognizable i n hand

specimen. Two types of joints were observed i n the db le r i t e

dyke ; tension joints were developed as a resul t of the cooling

and shrinking of the intrusion d i f fe ren t ia l ly producing columnar

and transverse joint s e t s . Compressional joints resulted fmm

the tectonic (compressional) forces which acted on the area.

These compressional joints trend N-S .

2.7 GRANITOIDROCKS:

The following granitoid mcks were mapped :

( i ) granodiori te

( i i ) meladiori te

( i i i ) granite.

UODIORITE :

Granodiorite is the most widespread in t rus ive rock i n

the area. It outcrops mostly a s isola ted and in-si tu boulders,

and shows typ ica l ex- f o l i a t i o n weathering pat terns , The

granodiori te has a sharp contact with the enclosing mcks. b

Xenoliths o f the country rocks, mostly sch i s t s , occur i n the

granodiorite.

Generally, t he granodiori te is coarse t o very coarse-

grained i n texture. Large pink prismatic phenocmsts of

It-feldspar a r e abundant. Other minerals recognized i n hand

specimen are plagioclase, hornblende, b i o t i t e and quartz . The mck d i f f e r s from gran i tes i n t h e area i n having a lower

percentage of s i l i c a and a higher calcium and magnesium content.

The granodiori te is non-foliated and could have been emplaced

post kinematically o r a t t he dying stages of the Pan-African

omgeny (60w50 m.a.). The granodiori te belongs to the Older

Granite ae r i e s recognized throughout Nigeria. ' A light-coloured

dyke (AH2) cuts the granodiori te a t Ahumi ( ~ i ~ u r e 2 ) .

24,

&&J&)IORITE :

This does not occur as a mappable mck uni t i n the area

but a s a xenolith i n granodiorite exposed a t the Igbofia

Rubber plantation. It is thought to be the oldest pluton i n

Uwet area and is n o w represented by a single large mass and

its xenolith t r a in s within the Uwet granodiorite ( ~ i ~ u r e 9 ).

The meladiorite is dark and f ine-grained. Megascopically,

plagioclase, hornblende, m i n o r quartz and b i o t i t e a r e recognized

i n the rock. #e lad lor i te is commonly considered a hybrid rock

( ~ r u s w e l l and Cope 1963)., Direct evidence of the or ig in of the

Igbofia meladiorite is not available. However, it might have

been one of the ea r l i e s t phases of Older Granite intrusions

(car te r e t al. 1963) since i t occurs as xenoliths i n the

granodiorite. It is suggested that both the Igbofia meladio-

r i t e and the Uwet granodiorite seem to have been enplaced by

a react ive stoping mechanism. This was presumably aided by

high f lu id contents which evolved i n the l a t e stages of

c rys ta l l i za t ion as alkali-rich f lu ids (cf. Wyllie 1977) and

localized a s pegmatites, and K-feldspar phenocrysts.

GRANITE :

The gran i te i n the area is pegmatitic. It is closely

associated with gneisses and migmatites of the area. It

occurs as scattered boulders and is generally coarse-grained .

Figure 9 : Meladiorite in granodiorite. (The margin is basified).

26 . Though non fol ia ted, t he gran i te bodies generally trend N-3

which is the fo l i a t i on plane and a plane of weakness of the

country rock. Quarta, feldspar and muscovite a r e the dominant

minerals i n hand specimen. Tourmaline, beryl and garnet a l so

occur i n some outcrops.

2.8 SEDIlBNTARY ROCKS :

Overlying the basement rocks unconfomably i n UweB area

a r e sedimentary rocks of Cretaceous t o Tertiary age. The

sedimentary t e r r a i n consiste of the Asu River Group, the Eze-

&u Foxmation, the lkporo Shale Formation (Figure 10 ) a l l of

Cretaceous age. The Nkporo sha le is overlain by tha Benin B'brm-

a t ion (Figure 10) of l a t e Tertiary-€&at ernary age.

The Asu River Group (~eyment 1965) consists of

( i ) sands tone member

( i i ) shale member and

( i i i ) limestone member.

These th ree members of the A m River Gmup were recognized i n

U w e t area. They belong to the Odukpani Formation of Reyment

(1 965). The basal un i t of the Odukpani Foxmation described as

the A w i Formation (Adeleye and Fayose 1978) has a sharp contact

with the basanent rocks a t A w i , Calaro Estate and Ikot Okpom

(Figure 10 ) . The A w i Fo rmation comprising of f luviodel ta ic

sediments (dip is 8-14ONE) is a folded sandstone, s i l t s t o n e

28.

and carbonaceous shale uni t . Its contact with the basanent

rocks is marked by an unconfomity represented by a

conglomerate bed.

The Awi Fomation is over la in by a sequence of

limestones i n Uwet area. The limestone which is thickly

bedded i n several locations i s best exposed a t the Cmss

River Limestone quarry a t Mfamosing ( ~ i ~ u r e 10 ) . The

limestone exposed a t the quarly is up t o 50 meters i n thick-

ness. The limestone has a sharp contact with the basanent

rocks of Uwet area a t AbMati, Uwet and Ikot Okpora.

The Eze-Aku Formation consisting of lfmestone and

calcareous shale i s well exposed i n B&oko (Figure 10) and

Okongtokong i n the southeast of the map area. Here, it

consists of highly f i s s i l e and gray t o black shale. The dip

of the shale is 8ONB and i t s t r i k e s 160' azimuth. It is

s i l t y , calcareous and r ich i n pelecypod. Thick sections of

yellowish marl (up to 30 meters th ick) belonging t o the Eze-

Aku Fomation outcrop i n fo re s t s on both s ides of the Calaber - Ekang road ( k i l m e t e r s 101 and 102). These marls a r e highly

fractured i n the N43 and E-W directions. Some of the jo in t s

have widened into gorges.

The Nkporo shale marks the top of the Cretaceous

euccession i n the map area. It i s unconformable with the

Eze-Aku Formation and consists of f i s s i l e , gray t o black

29

shale which does not show effervescence on appl icat ion of acid.

This shows, t ha t unlike its Odukpani and Em-Aku counterparts,

i t is carbonaceous ra ther than calcareous. The Nkporo Shale

exposures i n the map area a r e discontinuous and can be observed

i n several road-cuts i n the area. Above the marl band near

Ihebu (kilometers 101 and 102 along the Calabar - Ekang road)

there is an exposure of Nkporo Shale. It is f i s s i l e and dips

2 0 ° ~ ~ . Between kilometers 36 and 43 along C a l a b a ~ I t u road,

about 15 metres of blue - black f i s s i l e shale outcrops. There

a r e t h i n beds of l imestone and bands of gypsum. The top par t

of t h e sha le contains Libycoceraa and Inoceramus. It also

contains f i s h bones and leaf impressions. The Nkpom Shale

is Campanian - Maastrichtian i n age.

The Benin Formation is t h e youngest rock uni t in t he

study area. It i s %ert iary i n age and consists of highly

ferrugineous sandstone, occasional shale and t h in bands of

ironstone. I n some locations, t he sandstone is f i n e g r a i n e d

while i n o thers i t is en t i re ly conglomeritic. This un i t is

highly weathered and non-fossilifemus, although Ophiomorpha

hag been reported i n it outs ide t h e map area (Onyeagocha 1980).

CHAPTER THREE

M I N E R A W GY AND PETROGRAPHY

3 1 I~IETAI,IORPHI C ROCKS

About f i v e hundred t h i n s e c t i o n s of rocks i n Uwet a r e a

were s tudied using t h e pe t ro log ica l microscope. Tables 1,2

and 3 show t h e modal compositions of t h e metamorphic rocks.

Modal averages of minerals i n rocks of Uwet a r e a a r e

presented i n Table 4. b

Quartz is ubiquitous i n a l l t h e rocks. I n a l l , qua r t5

accurs i n two s i z e s , as groundmass c r y s t a l i n p h y l l i t e and

s c h i s t s and as porphyroblasts i n gneisses. I n t h e kyanite-

s i l l i m a n i t e s c h i s t , secondary qua r t z is granoblas t ic and

0 d i sp lays a 120 quartz-quartz t r i p l e juc t ion s t r u c t u r e

( ~ i g u r e 11 .). This i n d i c a t e s at tainment of t e x t u r a l equ i l i -

brium i n those p a r t s of t h e s c h i s t ( ~ ~ r y 1969; Mason 1978).

Sbme of the qua r t z i n t h i s s c h i s t a r e r ec rys ta l l i5ed as

elongate and pdygonised c r y s t a l s e t m n g l y a l i g n e d p a r a l l e l

t o t h e f o l i a t i o n of t h e rocks which is defined mainly by t h e

p h y l l o s i l i c a t es ( ~ i g u r e 11 ) . Quar tz is t h e commonest

inc lus ion i n o t h e r minerals , f o r example, garnet ( ~ i g u r e 12 ).

Plagio c l a s e is abundant and together with qua r t z

comprises 15 t o 40 percent of t h e rocks. Most of t h e

! 'tr '.I i :! -

'-4

! L: c4

--

ill rn 14 - (11 I l l c3 - in , _ '

i 5

C# I , , L, < u <:I .-- U l

i, ( I 1 , , ' . . I < ' > t ,

I

Figure 11 : Quartz forming 1 20° triple junctions and enhancing the foliation of the kyanite-sillimanite schist (XPL; X 10)

Figure 12 : Garnet r iddled with inclusions of quartz and phy l los i l i ca tes i n the garnetcs i l l imani te s ch i s t (WL; X 10)

plagiaclase occurs a s c l ea r elongate to granoblastic crysta ls .

Zoning is r a r e i n t he plagioclase due t o homogenization a t

higher temperature ( ~ r a c ~ 1978). The plagioclase i n t he

phyl l i te , homogenous amphibolite, quartz-mica s ch i s t and

garnet-mica s ch i s t i s typical ly untwinned. The untwinned

nature of these c rys ta l s of plagioclase may be re la ted to the

grade of metamorphism (ph i l i p s 1930; Turner 1951 t Ekwuene and

onyeagocha 1982). Twinning is, however, observed i n the garnet-

s i l l b a n i t e and kyanite-sillimanite sch is t s , gneisses, migmatites b

and banded amphibolites. Among the twins, a l b i t e and per ic l ine

a r e dominant. The composition of the twinned plagico lases

var ies fmm An to +0 i n s ch i s t s and gneisses and from 25 %6

t o Anm i n the banded amphibolites and migmatites. The un-

.bwJxmed plagioclase i n the phyl l i te , homogenous amphibolite,

quartz-mica s ch i s t and garnet-mica sch is t is judged t o be

a lb i te .

K-feldspar is subordinate t o plagioclase where they

occur i n the gneisses and schis ts . Most c rys ta l s a r e typical ly

untwimed but exsolution lamellae a r e abundant i n few twinned

K-feldspar. I n most samples, the &feldspar is i n physical

contact with alumino-silicate and garnet though not

necessarily a t the same location. K-f eldspar i n high grade

pe l i t es e.g. garnet-sillimanite s ch i s t is orthoclase whilst

microcl ine is dominant i n biotite-hornblende gneiss (cf.

Evans and Guidott i 1966). A t some contac ts of K-feldspar

and plagioclaso, mymekite is observed.

P h y l l o s i l i c a t e s make up 50 t o 68 percent of t h e p h y l l i t e ,

quartz-mica and garnet mica s c h i s t s . The abundance of t h e s e

p h y l l o s i l i c a t e s e spec ia l ly c h l o r i t e and muscovite decreases

with increas ing grade . of metamorphism. 14uscovite is

absent i n t h e garnet -s i l l imani te s c h i s t s but b i o t i t e makes

up 67 percent of t h e p h y l l i t e . The p h y l l o s i l i c a t e s occur i n b

two t e x t u r a l hab i t s :

( i ) f o l i a t i o n def in ing groundmass l a t h s and

( i i ) megacrysts.

The f o l i a t i o n dofined by p a r a l l e l alignment of c h l o r i t e ,

b i o t i t e and muscovite is occasional ly disturbed by t h e

porphyroblast ic c r y s t a l l i z a t i o n of garnet , kyan i t e and

s i l l i m a n i t e . Generally, t hese phy l los i l i ca t ea usual ly wrap

around t h e porphymblas ts g iv ing r i s e t o pressure shadows

( ~ i ~ u r e 1 3 ; , Minor f o l d i n g of t h e p h y l l o s i l i c a t e s i a apparent

both i n hand specimen and i n t h i n sec t ion . I n t h e quarta-mica

s c h i s t , b i o t i t e develops on muscovite whereas i t develops

across t h e f o l i a t i o n i n o the r s c h i s t s . This t e x t u r a l evidence

suggests two periods o f development o f both b i o t i t e and

muscovite. yuscovi te c r y s t a l s up t o 2cm across, commonly

Figurn 13 : Phyllosil icates wrapping around kyanite porphymblast in the kyani te-aillimanite schia t (XPL; X 1 0)

l y i n g a t high angles t o the f o l i a t i o n , occur i n some s c h i s t s and

gneisses. These c r y s t a l s which occur a s Porphyroblasts (unusual

t ex tu re ) have o t h e r muscovite c r y s t a l s fonning a t r a i l around

them ( ~ i g u r e 14). This is an evidence of shea r and suggests

t h a t granula t ion and r e c r y s t a l l i z a t i o n occurred a s shown by

t h e r e c r y s t a l l i z e d fine-grained mortar. The porphyroblast ic

muscovite appears t o form by r e c r y s t a l l i z a t i o n and coalescence

of t h e muscovite i n t h e groundmass. Unstable muscovite is

observed i n t h e migmatite ( ~ i ~ u r e 1 5 ) . b

B i o t i t e o f f e r s favourable s i t e f o r t h e nucleat ion of

t h e Fe-Mg minerals , f o r exnmple, hornblende i n gneisses and

t h e a luminos i l ica tes with which they a r e always c lose ly

associated. Pleochroic haloes a r e common around small z i rcon

inc lus ions i n t h e b i o t i t e ind ica t ing radioact ive bombardment

from t h e enclosed Z ~ M O ~ ( ~ i g u r e 16 ) (~qyeagocha 1984, p.44).

Retrograde c h l o r i t e is greenish i n colour and with

b i o t i t e de f ine t h e f o l i a t i o n i n few sec t ions . I n most sec t ions ,

it occurs a s fan-shaped c r y s t a l aggregates c u t t i n g across t h e

f o l i a t i o n and forming intergrowths with and emb.ayments, on

b i o t i t e .

Garnet i n t h e a rea could be grouped under t h e t h r e e types

discussed i n Spry (1 969, p. 270-271 ) . These a r e pre-tectonic

c r y s t a l s ( sha t t e red and c h l o r i t i z e d ) , syntec tonic c r y s t a l s

Figure 14 ; Forphyroblastic muscovite in the biotite hornblende gneias (FPL; X 10)

Figure 15; : Unstable muscovite in the migmatitic gneiss (PPL; X 10)

Figure 16 : Pleochroic haloes a,round zircon crystal in biotite (XPL; X 10)

44.

(rounded with 9-shaped inclusion) and post-tectonic crysta la

(homogenous and does not d i s tu rb the fo l ia t ion) . The garnet

i n the phyl l i t e and garnet-mica s ch i s t (Figure 17) corresponds

t o the f i r s t group whereas those i n the higher grade s ch i s t s

and gneissea f a l l under the second and th i rd types. Pigage

(1 982) observed tha t garnet porphyroblasts usually ou t l ine

stages of growth. The f i r s t s tage garnet forms large, ragged

grains with inclusions of quartz, plagioclase, mica, and opaque

dusting and the garnet i n the garnet-sillimanite sch is t of Uwet b

area corresponds t o t h i s s tage (Figure 12). The garnet i n the

g a r n e t a i c a sch is t occurs as c lear , id ioblas t ic porphyroblasts

with minor inclusion and correeponds t o Pigage's second atage

garnet ( ~ i g u r e 17). Microfolding around garnet porphyroblast

is rampant i n t h in section. The phyl los i l icates form in te rna l

and external g - t r a i l i n and around the garnet porphyroblasts

(Figure 12) . The Bi-Se t r a i l s a r e evidence of polyphase

defamation. In the lower grade sch is t s , garnet is zoned

(Figure 17) posibly due t o continuous garnet-forming reaction

during prograde metamorphism (cf. Tracy e t a l . 1976).

~omogenization a t higher temperature possibly eliminated zoning

( ~ i g u r e 12) i n the garnet of higher grade s ch i s t (~oodsworth

1977)

Figure 17 : & m e t with minor inclusion of opaque dust i n the garnet-mica s ch i s t (PPL; X 10)

46.

@ m i t e forms equant shaped porphyxvblasts i n s c h i s t

and gneiss. It enhances t h e f o l i a t i o n i n kyani te gneiss

( ~ i g u r e 18 ) but d e f l e c t s t h e f o l i a t i o n i n kyani te s i l l i m a n i t e

s c h i s t (Figure 13 ) . Textural evidence indicates t h a t kyani te

seems t o form a t t h e expense of garnet and muscovite hence t h e

garnet appears t o have been almost used up t o form kyani te and

s i l l i m a n i t e i n S m p l e KW a able 1 ). Kyanite does not show 7 any tendency of breakdown o r transformation t o s i l l iman i te .

Both minerals coexist s i d e by s i d e i n t h i n section.

@ Si l l imani te va r ies i n hab i t fxvm radia t ing bundles of

f i b r e s ( ~ i g u r e 19) i n t h e garnet-si l l imanite s c h i s t and

migmatite t o r e l a t i v e l y large , id iob las t i c , elongate, pris-

matic c r y s t a l s i n t h e kyanite-si l l imanite s c h i s t (Figure 20).

F i b r o l i t e and coarse i d i o b l a s t i c s i l l i m a n i t e coexist i n t h e

kyanite-si l l imanite s c h i s t (Figure 21). The t ex tu re shows

t h a t t h e f i b r o l i t e is coarsening to i d i o b l a s t i c s i l l i m a n i t e

probably by g ra in growth (cf . Chinner 1961 ) . Both minerals

a r e int imately associated with b i o t i t e and t h i s corroborates

t h e view of Chinner (1 961 ) t h a t b i o t i t e provides a favourable

s i t e f o r s i l l i m a n i t e nucleation. Furthermore, the re is a marked

reduction i n t h e modal volume of garnet i n the s i t e s where

s i l l i m a n i t e formed. I n KWj where s i l l i m a n i t e makes up 20 per

cent of t h e rock no garnet was observed but i n KW and KW6 5

Figure 18 : Kyanite sandwiched in biotite crystals and enhancing the gneissosity of the kyanite gneiss (XPL; X 10)

Figure 19 : Fibrolite in the garnet-sillimanite schist (PPL; X 40)

Figure 20 : Elongate prismatic sillimanite in the kyani te-sillimanite schist (XPL; X 1 0 )

Figure 21 : Fibrolite coexisting with the idioblastic sillimanite in the kyanite-sillimanite schist (PPL; X 40)

51.

where garnet occurs, only 4 and 2 percent s i l l iman i t e were

observed respectively. Hence the s i l l iman i t e must have used

up the garnet f o r its formation as shown i n the reaction:

garnet + muscovite = b i o t i t e + s i l l iman i t e suggested

by Yardley ( 1 977)

Green hornblende is abundant i n the homogenous

amphibolite and bio tite-hornblende gneiss whereas the

brownish var ie ty f o m s over 70 percent of the banded

amphibolite. I n a l l sect ions, the hornblende is coarse-

grained i n texture. It is associated with epidote i n the

homogenous amphibolite and some gneisses. The banded

amphibolite is epidote-free. Hornblende c rys ta l s o f ten a l i gn

thans elves p a r a l l e l t o b i o t i t e thereby enhancing t he f o l i a t i o n

of the rock.

Epido t e is observed replacing hornblende i n some sect ions

of hornblende-biotite gneiss. I n one section, tens ional ly

fractured epidote encloses a metamict phy l los i l i ca te ( ~ i ~ u r e 22).

Sphene is typical ly wedge-shaped i n form, brownish i n colour

and shows a recognizable cleavage. It encloses some opaque

minerals i n few sections.

Secondary c a l c i t e occurs i n biotite-hornblende gneiss

and the dark port ion of the kyanite-si l l imanite s ch i s t . It is

par t i cu la r ly c m o n i n t he l a t t e r making up 20 t o 30 percent of

the rock ( f o r example, sample Nos. R W I , KU5 and m6, Table 1 ) *

Figure 22 : Frac tu r ed epidote enclosing an allanite crystal i n t h e b i o t i t e -

hornblende gneiss (XPL; X 10)

3.2 INTRUSIVEROCKS

Table 5 shows t h e modal composition of t h e two main

i n t r u s i v e rocks i n Uwet shee t namely d o l e r i t e and g ran i to id

ro cks . p LIERITE :

The only d o l e r i t e outcrop i n t h e a rea is composed of

o l i v i n e (@), pyroxene (44$), p l zg ioc la se (45$) and opaque

oxide (3%) (v ide T a b l q 4 m d 5). The o l i v i n e occurs as equant

aggregates d i s sec t ed by cracks of d i f f e r e n t o r i en ta t ion .

t Zoning is r e r e i n t h e o l i v i n e c rys t a l s . The p l ag ioc la se i s

a l a b r a d o r i t e varying i n composition from An 55 t o An60. They

a r e lath-shaped and some p lag ioc la se l a t h s a r e completely

enclosed i n pyroxene thus d i sp lay ing t h e character is t i t ! o p h i t i c

t e x t u r e of d o l e r i t e s . Some o f t h e p lagioc lase c r y s t a l s a r e

s t rong ly zoned. Gamble (1982) a t t r i b u t e s such zoning i n

d o l e r i t e p l ag ioc la se t o f l u c t u a t i n g P-T condit ions during

c r y s t a l l i z a t i o n . Albite-carlsbad twins predominate i n t h e

pdagioclase. P l ag ioc la se occurs both as phenocryst and matrix

of t h e d o l e r i t e . The pyroxenes a r e co lour less but a few

c r y s t a l s d i sp lay f a i n t pleochroisn under t h e microscope.

Augite has well-marked p a r t i n g s which a r e more prominent than

t h e cleavage. P l a t e s of i r o n oxide a r e d . i s t r ibu ted along t h e

par t ings . Hypersthene is elongated, has p a r a l l e l ext incxion

and is charac ter ized by fewer p a r t i n g s than t h e augi te .

55.

Occasionally, both augi te and hypemthene mant l,e the o l iv ine

c ry s t a l s giving r i s e to corona texture. Emmett (1982)

in te rpre tes coronas i n do l e r i t e within a metamorphic t e r r a i n

s imi la r t o the one under study as pmducts of post- e o l i d i f i -

cation deuterism, possibly ac t i ve a t high pressures and

temperatures, r a ther than pmduct of prograde metamorphism.

According t o him, t he coronas a r e due to a reaction between:

( 1 ) o l iv ine o r clinopyroxene and plagioclase and

(2 ) oxide grains and plagioclase.

H i s in te rpre ta t ion may be valid f o r the b

f a c t t ha t the d o l e r i t e is emplaced a s a

across lcyanite gneiss. However, normal

study area i n view of the

dyke ( ~ i g u r e 5) cu t t ing

igneous reaction during

magmatic c ry s t a l l i z a t i on could have given r i s e t o the

formation of coronas.

The mineralogical evidence f o r the t h o l e i i t i c character

of t he do l e r i t e is provided by the coexistence of Ca-rich and

Ca-poor pymxene. It has both o l iv ine and hypersthene i n t he

mode and according to Camichael e t al. (1 974) there is of ten

a very good corre la t ion between the mode and t h e norm i n such

in t rus ive rocks. The CIPW norm of the d o l e r i t e shows t ha t

o l i v ine and hypersthene occur i n t he norm of t he do l e r i t e '

thereby placing i t i n the group of o l iv ine tho le i i t es . Such

t h o l e i i t i c magma usually lswregate from c r y s t a l mush a t a depth

of 0-15km i n t he mantle r re en and Ringwood 1967) a f t e r X) o r

more percent degree of p a r t i a l melting of t he mantle per ido t i t e

( 0 ' ~ a r a 1968; Ringwood 1975).

GRANITOID ROCKS :

The modal abundances of t he Uwet granitoids were

plotted on the Quartz-Alkali-feldspar-Plagioclase (QAP)

diagram f o r plutonic rocks a f t e r Streckeisen (1 976). The

p lo t s ( ~ i ~ u r e 23) indicate tha t t he rocks range from

-- granodiorite t o g ran i te i n composition,

Quartz plus feldspar a r e the dominant minerals i n the

granodiorite. Single grains and aggregates of quartz appear

t o have been assimilated f Am the metasediments during the

emplacement of the rock. They occur as res i s tan t phases up

t o Icm i n diameter and appear a s phenocrysts peppered through-

out t he rock. K-feldspar (microcline) is subordinate t o

plagioclase and may be square o r rhomb-shaped i n smaller

c rys ta l s and tabular o r rectangular a t ta ining a length of

3 . 5 ~ ~ i n la rger crysta ls . Most of the microcline c rys ta l s

a r e pe r th i t i c and exsolution lamellae a r e common, Often they

give rise t o mymekite. The plagioclase f s an oligoclase-

andesine ranging i n composition from An 28 t o A&j2m Sbme of

the plagioclase c rys ta l s show weak zoning, Twinning is common

and is mostly on the a)bite and albite-carlshad laws.

hl tera t fon of plagioclase to s e r i c i t e is rampant i n t he

granodiorite

Greenish brown hornblende is common i n the rock. It

is int imately associated with b i o t i t e , epidote and apa t i t e .

Unlike some fo l i a ted grani to id rocks i n t he Nigerian basement

complex, these minerals do not show preferred o r ien ta t ion i n

t h i n section. The Uwet granodior i te is unfoliated. Muscovite

occurs a s an addi t ional phase i n few specimens e.g. UWI . The petrography of the granodior i te indicates t h a t the

o r i g ina l rocks might have been t o n a l i t e consis t ing of

ol igoclase, quartl;, hornblende, b io t i t e , sphene and only minor

K-feldspar. However, contamination and/or potash metasomatism

has subs t i an t i a l l y a l t e red t h i s and rendered the pluton

composit iona l ly variable.

Xenoliths and dyke rocks i n the granodiori te of Uwet

a rea show var iable composition. The most conspicuous of t he

mafic xenol i ths is the Igbofia meladiori te which occurs a s

xenoli th t r a i n s within t he Uwet granodiorite. Under t he

microscope, i t is composed, where uncentaminated, of c a l c i c

plagioclase (An ) , hornblende, minor quartz, a p a t i t e and 70

sphene. But d iges t ion o f metasediments, seen a s renocrysta,

h a s introduced addi t ional quartz, sphene, a p a t i t e and a l l a n i t e

changing t h e bulk composition t o melatonalite.

A light-coloured dyke AH^) i n t h e granodiori te at

Ahumi ( ~ i ~ u r e 4 ) is cmposed of quar tz (%), a l t e r ed

plagioclase (1 5%) , b i o t i t e ( 4 ~ 6 ) ~ muscovite (3@) and opaque

oxide (1%)

59 . The rock is deformed and possibly o l d e r than t h e hos t

granodior i te . Petrographic study shows t h a t t h e dyke f 8

very fine-grained and fo l i a t ed . The f o l i a t i o n is dis turbed by

f l a k e s of muscovite.

The me lad io r i t e and t h e light-coloured dyke could be

o l d e r plutons than t h e g ranod io r i t e and i t is poss ib le t h a t

t h e metasediments had already been deformed a t l e a s t once

before t h e i n t r u s i o n of t h e meladior i te and t h e dyke.

I n t h i n sec t ion , t h e g r a n i t e is coarse-grained and +

non-foliated. It is composed dominantly of qua r t z and K-feldspar

a able 5) It a l s o c a r r i e s b i o t i t e with abundant pleochroic

ha loes and muscovite, The p lagioc lase is untwinned a l b i t e a&

i t is in tense ly a l t e r e d t o s e r i c i t e . k s i m i l a t i o n of eome baeement

rocks, f o r example s c h i s t s , has contaminated some of t h e grani te .

Sbme specimens a r e tounuaaine-bearing,

On t h e whole, t h e Igbof ia meladior i te , t h e light-coloured

dyke i n Abumi and t h e granodior i te i n Uwet a r e s seem t o have

been emplaced by a r e a c t i v e s toping mechanism presumably aided

by high f l u i d contents evolved i n t h e l a t e s t a g e s of

c r y s t a l l i z a t i o n as a lka l i - r i ch f l u i d s loca l i zed as pegmatites,

dyke rocks and K-f e ldspa r megacrysts. Metasomatism might

have played a r o l e i n t h e formation o f t h e me lad io r i t e

60.

(cf . Bishop and French 1982). According t o these authors ,

t h e metasomatic process g iv ing r i s e t o the formation of t h e

meladior i tes takes p lace during a period o f slow cooling

between say €300~~ and 6 0 0 ~ ~ a f t e r t h e emplacement of

granodio ri t e . The g ran i t e , (sensu str ict ;o) could be a product o f

regional anat e x i s due t o high grade metamorphism . Evidence

f o r t h i s includes t h e a s soc ia t ion o f t h e g r a n i t e with rocks

of h ighes t grades of regional metamorphism, f o r example, banded

+ amphibolite and migmatite. The p a r t i a l d iges t ion of t h e

basement rocks i n t h e g ran i t e is a l s o an evidence o f p a r t i a l

mel t ing o f t h e surrounding rocks ( c f . Onyeagocha 1984, p.50).

CHAPTER FOUR

JITRUCTURAL GEOLOGY

The Nigerian Basement complex is believed t o have

undergone polyphase deformation during the Precambrian

(oyawoye 1964; McCurry 1976; Rahaman 1976; Oqyeagocha 1984).

The l a t e s t of the deformation episodes occurred during t he

Pan-African omgeny (60%150 ~ a ) . A s a r e su l t of its polyphase nature, complex s t ruc tures

a r e associated with t he d g e r i a n basement. These s t ruc tures

display d i f fe ren t o r ien ta t ions depending on the defomat ional

episode tha t produced them. Because of its high in tensi fy ,

some authors believe t ha t the Pan-African orogeny which was

the last defonnational episode ob l i t emted e a r l i e r s t ruc tures

i n the basement rocks of Nigeria (see Jones and Hockey 1964;

McCurry 1 976 ; Rahaman 1 976). However, Onyeagocha and Ekwueme

( 1 982) showed tha t abundant pre-Pan-African s t ruc tures ex i s t

i n pa r t s of the basement complex. These s t ructures , according

t o them, a r e characterized by or ienta t ions which a r e qu i t e

d i s t i n c t from the N-S s t ruc tures produced by the Pan-African

defomational episode.

Three other thennotectonic events apar t from the Pan-

African have been recorded i n the Nigerian basement complex.

These a r e the Liberian (2800+_200 .), the Eburnean (2200 ~ . a . )

and t h e Kibaran (1 100&200 ~ a ) (oversby 1975; Grant 1971 ;

Grant e t a l . 1972; Ogezi 1977). The pre-Pan-African s t r u c t u r e s

could be imprints of any of these e a r l i e r events.

4.1 FOLIATIONS:

The rocks of Uwet area display var iable f o l i a t i o n trends

a able 6). A p l o t of the f o l i a t i o n readings i s shown i n

Figure 24. The p l o t shows two dominant t rends in the NE-SW

(0'-40 azimuth) and t h e NU-$I (lo()0-1400 aeimuth) d i rec t ions .

Nach of these trends represents imprints of a defomat ional b

episode i n t h e area.

The r e s u l t o f t h e Pan-African deformation i s a f o l i a t i o n

trend largely 0-30' azimuth ( ~ n y e a ~ o c h a and Ekwuane 1982).

This is t h e dominant t rend of f o l i a t i o n i n t h e Uwet a rea and

t h i s f o l i a t i o n d ips i n t h e W, N.W. o r s.E. direct ions. It is

recognized i n t h e phy l l i t e , sch i s t s , gneisses and t h e homogenous

amphibolite. This regional trend shows t h a t t h e rocks i n the

a rea were af fec ted by t h e Pan-African omgew.

0 Fo l i a t ion t rends of 50-80 azimuth occur but a r e l e s s

0 prominent than the 0-30 azimuth trend. Such f o l i a t i o n t rends

(50~-70' azimuth) were in te rp re ted a s ind ica t ive of e a r l i e r

deformation than t h e Pan-African by Onyeagocha and Ekwueme

(1982). A f o l i a t i o n trend of 50-80' azimuth was recorded i n

the s c h i s t a t t h e Kwa Fa l l s . I n t h i a area, however, a f o l i a t i o n

trend o f 40' ctzimuth was a lso recorded and t h i s f o l i a t i o n

Fig. 24 Plot of strikes of foliations of rocks' in Uwet area -

t r a n s e c t s t h e 50-80' azimuth t rend . The s c h i s t s i n t h e Kwa

F a l l s t he re fo re possess impr in ts of a t l e a s t two defo m a t i o n a l

episodes which were separa ted i n time. These episodes may

belong t o t h e same Pan-African orogeny i n which t h e 40' azimuth

t rend r ep resen t s a f o l i a t i o n produced by a l a t e r phase whereas

t h e 50' azimuth t r end ing f o l i a t i o n belonged t o t h e e a r l i e s t

phase of t h e orogeny. I s o t o p i c d a t i n g discussed l a t e r

conf inns t h i s a s se r t ion .

There is a regional t r end of f o l i a t i o n i n Uwet a r e a i n

t h e PJW-SE d i r e c t i o n with a 'low d i p i n t h e NE.

I n t h e rose diagram ( ~ i g u r e 24) t h i s f o l i a t i o n is o f

l e s s magnitude than t h e 0'-40' azimuth t rending f o l i a t i o n of

t h e Pan-African. This f o l i a t i o n is recognized i n many p a r t s

of t he s tudy area . It is observed i n t h e gneisses at Calaro

Es ta t e , i n t h e s c h i s t s a t Akwa Ibiami and i n t h e migmatites a t

Mbarakpa. This f o l i a t i o n is t h e only type recorded i n t h e

0 0 banded amphibolite. The 100 -140 t rending f o l i a t i o n i s a s t r o n g

evidence of a pre-Pan-African d e f o m a t i o n a l episode ( c f .

Onyeagocha and Ekwueme 1982). I n many l o c a l i t i e s where t h i s

f o l i a t i o n t r end was recorded i n Uwet a rea , it is o f t e n observed 0 0 0

t h a t t h e reg ional t rend i n 0-30 off-Sets t h e 100 -1 40 f o l i a t i o n *

For example, i n t h e Calam E s t a t e , t h e biotite-hornblende gne i s s

exh ib i t s two s e t s of f o l i a t i o n t rends. The f i r s t set which is

conspicuous has a dominant (1.30° t rend with an a t t i t u d e of

about 20' azimuth. The second s e t t runcates t h e f i r s t and

ahows a t rend i n NNW-SEtB d i r e c t i o n with an a t t i t u d e of 350'

azimuth. The angle between t h e s e two s e t s of f o l i a t i o n is

JOO. These two s e t s of f o l i a t i o n represent two d i f f e r e n t

de fomat iona l episodes and t h e episode t h a t produced t h e NNW-

SSE t rending f o l i a t i o n is judged t o be pre-Pan-African.

S imi lar ly , i n Igbofia, t h e trend of t h e s c h i s t o s i t y defined

p r inc ipa l ly by b i o t i t e is NNE-SSW which is more o r l e s s N-S.

Here, t h e quartzo-feldspathic veins def ine a second s e t of

f o l i a t i o n i n t h e NW-SE dillection. S imi lar f o l i a t i o n off-sete

were a l s o observed i n t h e m i m a t i t e a t Mbarakpa and t h e e c h i s t s

i n Agbangana ,

4 * 2 FO LD3 :

Rocks i n Uwet a rea have undergone severa l episodes of

folding. Miner f o l d s o f d i f f e r e n t o r i e n t a t i o n were mapped i n

t h e area . The dominant t rend of f o l d axes i s 0-30' azimuth,

a d i r e c t i o n which is p a r a l l e l t o t h e regional f o l i a t i o n o f t h e

rocks. Such fo lds a r e r e l a t ed t o t h e Pan-African omgeny,

They were mapped i n t h e s c h i s t a t Abbiati, Ojo and Kwa Falls;

t h e banded gne i s s a t Mbarakpa, Oban Okomba and t h e Oban quarry

(about 21nn ou t s ide t h e map a r e a i n t h e e a s t ) ; b i o t i t e h o r n -

blende gneiss at t h e quarry i n Obung and t h e kyan i t e gneiss

a t t h e quarry i n Old Netim. I n most of these places, t h e

fo ld ing is best s tudied i n t h e pre tec tonic pegmatites, a p l i t e s

and qua r t z veins. They a r e a l l t i g h t i s o c l i n a l f o l d s with

axes i n t h e N-3 and NE-SW d i rec t ion . of the f o l d s have

v a r i a b l e plunges both i n d i r e c t i o n and mount . Their limbs

usual ly have gen t l e d ips .

A t Abbiat i , f o r example, t h e s c h i s t has been in t ense ly

folded. The fo ld which has an a x i s t rending 32' azimuth is

truncated by quartzo-feldspathic veins i n t h e area. It off-

s e t s t h e f o l i a t i o n o f t h e rock i n some locat ion. The fo ld

axis i n t h e banded gneiss at t h e Oban quarry t r ends 20' azimuth,

again, p a r a l l e l t o t h e banding of t h e rock, Evidence of shea r

i n t h e gne i s s i s t h e development of c h l o r i t i c sur face on each

band of t h e gneiss . This su r face c o n s t i t u t e s a w n e of weakness

o r cleavage along which t h e rock e a s i l y s p l i t s i n bands. These

fo lds a r e associa ted with numerous boudin s t ruc tu res . The

boudins a r e abundant i n both limbs o f t h e f o l d s , an ind ica t ion

t h a t t h e r e was a g rea t tens ion on these p a r t s of t h e f o l d s which

tended t o p u l l t he limbs apar t . The boudins ( ~ i g u r e 25), l i e

p a r a l l e l t o t h e f o l i a t i o n and t h e axes of t h e fo lds . This

i n d i c a t e s t h a t the f o l i a t i o n represents planes of weakness

through which t h e quartzo-feldspathic ma te r i a l W a s ejec ted i n t o t h e

gneiss , S imi la r boudin s t r u c t u r e s were observed i n t h e Kwa Fa l l s .

The banded arnphibolite is i s o c l i n a l l y fo lded, The fo ld

0 axis t rends 70 azimuth. The f o l i a t i o n is p a r a l l e l t o t h e

banding i n t h e paral lel-folded rocks. This i n d i c a t e s t h a t t h e

Figure 25 : Boudimge and associz ted f o l d s i n gneiss.

banding is o l d e r than t h e fo lds . Boudin s t r u c t u r e s a r e

assoc ia ted with t h e fo lds .

A minor fo ld with an a x i s t r end ing %n t h e 1 0 0 ~ - 1 4 0 ~

d i r e c t i o n occurs i n t h e Calam Es ta t e . Folds with axm

0 t rending 70 and 1 d - 1 4 0 ' represent % p r i n t s of t h e pre-Pan-

African f o l d i n g episode.

A common f e a t u r e i n most c r y s t a l l i n e rocks of Uwet a rea

is ptygmatic folds. These a r e h ighly contorted quartzo-

f e ldspa th ic ve ins c u t t i n g accross t h e f o l i a t i o n of t h e rocks.

They were observed i n the'biotite-hornblende gneiss i n Calam

E s t a t e Eprrd in Obung, i n t h e lcyanite gneiss i n Old Netim and t h e

s c h i s t i n Agbangana (Figure 26).

4 .3 JOINTS AND FAULTS:

The r o s e d.iagram o f t h e j o i n t s i n Uwet shee t shows

dominant t r ends i n t h e In{, SE and NE d i r e c t i o n s (Figure 27).

This mul t i -d i rec t ional t rend is an evidence of poyphase

deformation. It is i n t e r e s t i n g t o no te t h a t t h e s e a r e a l s o

t h e dominant t rend d i r e c t i o n s o f f o l i a t i o n s . The j o i n t s i n

t h e a r e a can be grouped i n t o two : t ens ion and compression

types . The t ens ion j o i n t s a r e mostly assoc ia ted with rocks

showing NW-SE t rending f o l i a t i o n s . These j o i n t s were a l s o

observed i n t h e d o l e r i t e dyke. The tens ion j o i n t s i n t h e

liygure 26 : Ptygmatic f o l d s i n gneiss.

Fig.?y Rose diagram of joints in basement rocks of Uwet area-

metamorphic rocks of the area were possibly formed due t o

s t r e s s re lease associated with a dominant E-W tensional

deformation which affected these rocks. I n the d o l e r i t e

outcrop i n Old Netim, t he joints were possibly developed as

a resu l t of the cooling and shrinking of the int rusion

d i f f e r en t i a l l y thereby producing columnar and transverse

joints. Rehealed f rac tures a r e associated with the tension

joints i n Uwet area (Figure 5 ). These f ractures could have

been tension joints before but were l a t e r widened t o larger

fractures. Shear and rec&stal l izat ion of t h e i r host rocks

(kyanit e and b i o t i t e-hornblende gneisses) led to the in - f i l l ing

of these f ractures with materials which gave them a composition

now d i f fe ren t from t h e i r host pocks. These f rac ture - f i l l ing

materials a r e schis tose textural ly and a re dominantly compoaed

of phyl los i l icates of which ch lo r i t e predominates. The

rehealed f ractures a r e generally ve r t i ca l and they occur i n

the kyanit e gneiss a t Old Netim ( ~ 7 ) ; bio tit e-ho rnblende gneiss

i n Obung ( 0 ~ 7 ) and i n biotite-hornblende gneiss a t Okom I t a .

These f rac tures a r e products of tensional s t r e s se s which affected

these rocks. The recrys ta l l i za t ion of materials which now f i l l

up the f ractures were f a c i l i t a t e d by v o l a t i l e transport (cf

Halther and Orvi l le 1982).

Compression joints i n t he Uwet area resulted from the

tectonic (compressional) forces which acted on the rocks.

These j o i n t s have dominant t r ends i n t h e N-S, E-W and NE-SW

d i rec t ions . They a r e t h e prominent j o i n t s i n t h e p h y l l i t e s

and t h e s c h i s t s of t h e a r e a and could have been formed due

t o compression during t h e Pan-African omgeny . A normal f a u l t with an east-west d i r e c t i o n was

observed i n t h e k y a n i t e gne i s s a t t h e Old Netim quarry

(Figure 28 ). The thmw of t h e f a u l t i s about 2 met res and

i t was l a t e r a l l y displaced f o r about 4 metres , The east-

west t r end of t h e f a u l t l i n e is cons i s t en t with t h e f a u l t ' s

occurrence i n a period of "elaxation fol lowing a major

deformation which a f f ec t ed t h e area. I n t h e Obung quarry,

f a u l t i n g of t h e biotite-hornblende gneiss is in fe r r ed from

minor displacements obsemed i n t h e a p l i t e and pegmatite dykes

and ve ins (Figure 6 ). The displacements a r e i n most cases

not more than 4cm. Fau l t ing is a l s o in fe r r ed i n t h e kyanite-

s i l l h a n i t e s c h i s t i n t h e Kwa F a l l s and t h e banded amphibolite

i n Awi. A t t h e Kwa F a l l s , t h e r e is a displacement of about

10 metres a t t h e contac t of t h e kyanite-si l l imanite and garnet-

s i l l j m a n i t e s c h i s t s . The banded amphibolite was h ighly

fragmented as t h e channel o f t h e Erokut River is appreached.

Boulders of t h e rock a r e abundant i n t h e channel and a

displacement of about 1 gcm i n a mafic dyke i n the rock is an

evidence of f a u l t i n g i n t h e area.

Figure 28 : Fault in kyanite gneiss.

4.4 MICROSTRUCTURES :

Thin sec t ion study revealed some microstructures

especia l ly i n the s c h i s t s of t h e area. Prominent among

them a r e f o l i a t i o n , microfolding, S - t r a i l s i n garnet and

mortar texture . These t e x t u r a l f ea tu res could be used t o

d i s t ingu i sh t h e d i f f e r e n t d e f o m a t i o n a l and metamorphic events

i n these rocks and a l s o t o a s c e r t a i n the o rde r of c r y s t a l l i -

za t ion of t h e i r minerals.

A l l t h e metamorphic rocks i n t h e area possess prefer red @

o r i e n t a t i o n defined by s c h i s t o s i t y , gneissos i ty a d bandiw.

The f o l i a t i o n is usual ly defined by t h e p a r a l l e l t o sub-

p a r a l l e l arrangements o f micas i n the metasedimenta and

hornblende i n t h e amphibolites. I n few cases f o r example,

t h e s c h i s t at Agbagans, two s e t s of f o l i a t i o n were dist inguished

i n t h i n sect ion. This is an evidence of polyphase defonnation

which had already been confilmed by t h e t rends of meeoscopic

s t ruc tu res . I n every sec t ion where these two s e t s of

f o l i a t i o n were dist inguished, t h e angle between them is about

30' which is consis tent with t h e f i e l d data.

I n t h i n sec t ion , t h e o rde r of r e c r y s t a l l i z a t i o n was

studied us ing t h e existence of pre-tectonic, syn-tectonic

and post- tectonic c r y s t a l s , f o r example, garnet . As i t was

ehown i n Chapter 3 , a l l these t h r e e types of garnet occur i n

t h e s c h i s t s of Uwet area. The pre-tectonic c r y s t a l s a r e

usually wrapped by the fo l ia t ion defined by b io t i t e . I n

most cases, these b io t i t e s ( ~ i g u r e 17) are kinked and folded,

an evidence tha t they have been deformed. The syn-tectonic

garnet crysta ls ( ~ i g u r e 12) a r e also wrapped by fo l i a t i on i n

some sections but they usually possess an in te rna l 3 - b a i l s

( s t ) that is discordant with fo l i a t i on Qe) . Spry (1969, p.

254) in terpretes such features as indicative of repeated

deformations. These gi-ge t r a i l s of garnet c rys ta l s can be

recognized megascopically i n the garnet-s iJ lhani te sch is t

where large garnet porphyroblasts (up to 3.0an) occur. In

t h in section, the post-tectonic garnet crysta ls i n the sch is t s

develope perfect forms of the crysta l . They a re homogenous

and do not dis turb the fo l ia t ion of the rock. Similarly,

large post-tectonic muscovite f lakes i n the sch is t follow the

general di rect ion of the fol ia t ion. Chlorite, generally grows

across the fol ia t ion.

Spry (1969) is of the opinion that kyanite is generally

post-tectonic and tends to be aligned along the layering.

However, the kyanite and s i l l imani te i n the sch is t a t the Kwa

Fal l s a r e wrapped by a folded fo l ia t ion showing tha t they a re

pre-tectonic to t h i s fo l i a t i on ( ~ i ~ u r e 13).

llortar texture i n which large muscovite porphymblasts

a r e s e t i n a fine-grained muscovite was observed i n th in

77 . sec t ions of bio t i t e-hornblende gneiss i n Okom Ita ( ~ i g u r e 1 4).

The l a r g e r c r y s t a l s a r e possibly o ld r e l i c s now surrounded by

new muscovite. This t ex tu re could have resulted from cata-

c l a s i s and granulat ion o f the rock, events which m i g h t be

responsible f o r t h e development of rehealed f r a c t u r e s i n t h e

bio tite-hornblende gneiss. I n t h i s rock, an o l d e r microfolded

f o l i a t i o n i s observed t o have been cu t across by a l a t e r

f o l i a t i o n . This is a n ind ica t ion of polyphase deformation and

plymetamorphism ( c f . Spry 1969, p. 307 f i g . 63).

#

4.5 DEFORNATION AND METAMORPHIC EPISODES

I n a metamorphic t e r r a i n such a s the Uwet area, the

mount of deformation is estimated i n a very approximate way

from t h e s i z e , number and types of f o l d s o r f o l i a t i o n s produced

(cf . Spry 1969, p. 311). The separa te tec tonic events are

recognized mainly from mesoscopic s t r u c t u r a l evidence such a s

fo lds and fo l i a t ions .

I n Uwet area, t h e p lo t of f o l i a t i o n s i n d i c a t e dominant

t rends i n t h e NE-Sd (0-40' azimuth) and NW-SE (100-140' azimuth).

The jo in t s a l so show dominant t rends i n these d i rec t ions . The

rocks have been i s o c l i n a l l y folded i n these d i rec t ions . These

mesoscopic s t r u c t u r a l t rends a r e s t rong indicat ions , t h a t a t

l e a s t two episodes of deformation took place i n the area.

The deformation episode t h a t gave r i s e t o t h e NW-SE

t r end ing s t r u c t u r e s was pre-Pan-African whereas t h e Pan-

0 African omgeny l e f t impr in ts of s t r u c t u r e s t r end ing 0-40

azimuth. However, Ony eagocha and Ekwueme (1 982) observed

t h a t s t r u c t u r a l t rend of 50'-70' azimuth i n Akwanga a r e a

r ep resen t s impr in ts o f a n e a r l i e r omgeny than t h e Pan-African.

Such s t r u c t u r e s were observed i n t h e Kwa F a l l s a r e a where t h e

s c h i s t o s i t y o f t h e k y a n i t e s i l l i m a n i t e s c h i s t t r ends 50'

azimuth and i n t h e Emkut River a r e a where t h e f o l d axes i n @

t h e banded amphibsl i te show a t r e n d of 70' azimuth. However,

i n t h e Ktrra F a l l s , t h e s c h i s t o s i t y t rending 50' azimuth

0 t r a n s e c t s another t r end ing 40 azimuth. It appears t h a t t h e

s c h i s t has been subjec ted t o a t l e a s t two episodes o f

deformation separated i n time during t h e Pan-African omgeny . I f t h i s is so, t hen t h e l a t e s t episode produced t h e 40'

a i m u t h t rending s c h i s t o s i t y .

Sply (1 969, p. 306) observed t h a t multi-phase

metamorphism (polymetamorphism) is suggested by

( i ) mu l t ip l e f o l i a t i o n s

( i i ) c r y s t a l s which have complex time r e l a t i o n s e.g. have d iscordant and $8,

( i i i ) clystals which a r e post- tectonic t o one f o l i a t i o n but p r e t e c t o n i c t o another , o r by a minera l spec ie s which has d i f f e r e n t forms o r t ime r e l a t i o n s i n d i f f e r e n t c r y s t a l s .

I f t h e above c r i t e r i a a r e applied to t h e rocks of

Uwet area , i t is evident t h a t t h e a r e a was not only subjected

t o mul t ip le deformations but t h a t t h e metamorphism was a l s o

mul t ip le . Evidence of t h i s is t h e mul t ip le f o l i a t i o n s i n t h e

0-40°, 50'-70°, and 100-140° azimuth. The metamorphism

appears t o have occurred contemporaneously with o r p r i o r t o

t h e defomat ion. Evidence f o r t h i s is provided by t h e

migmatitic s c h i s t s . I n these s c h i s t s which occur i n t h e Kwa

F a l l s and Abbiati a rea , t h e l eucocra t i c and melanocratic

+ por t ions were deformed i n t h e same d i r e c t i o n ind ica t ing t h a t

t h e migmati t i z a t i o n occurred p r i o r t o deformation,

I f t h e deformation episode is represented by F', t he

f o l i a t i o n by 2 and t h e metamorphism by 3, then the f i r a t

deformation episode F1 i n t h e a rea gave r i s e t o S1 during o r

p r i o r t o MI. This resul ted i n t h e d is rupt ion of e a r l i e r

beddings o r o r i g i n a l s t r u c t u r e s of t h e rocks and t h e recrys ta l -

l i z a t i o n of such minerals a s mica, quartz and plagioclaee,

During t h e second defonnation episode (3'2) t h i s i n i t i a l

f o l i a t i o n was deformed r e s u l t i n g i n bent micas p a r t i c u l a r l y

b i o t i t e which wrap around porphyroblasts such a s garnet , This

b i o t i t e formed t h e second f o l i a t i o n 52 possibly during o r

p r i o r t o g2. After o r during t h e second deformation unstrained

micas, kyani te , s i l l i m a n i t e and post-tectonic garnet formed

due t o continued r e c r y s t a l l i z a t i o n a f t e r x 2 . The evidence of

a. t h i r d deformation (F3) i n the area is provided by a

f o l i a t i o n which wraps t he post-tectonic kyanite i n t he

kyanite-si l l imanite s c h i s t s ( ~ i g u r e 13). Spry (1 969, p.

271) had argued t h a t syntectonic kyani te is ra re , i f a t a l l

it ex i s t s , and tha t kyani te i s invariably post tec tonic i n

one metamorphic phase but may be pre-tectonic i n multi-phase

s ch i s t s . I t can therefore be argued t ha t t he f o l i a t i o n which

wraps the kyanite i n s ch i s t of Uwet area indicates t h a t t h e

kyanite was post-tectonic t o '2 but pre-tectonic t o FJ which b

produced t h e s. This deformation affected the s ch i s t i n the

Ewa Fa l l s a rea and possibly t he s ch i s t i n Abbiatiand t h e banded

amphibolite i n A w i . It is the youngest Pan-African deformation

episode i n the area.

The p s s i b i l i t y t ha t these prograde events were separated

by retrograde metamorphic events i n between the deformation

episodes is strong. Evidence f o r t h i s is provided by t he

coexistence of cross-cutting c h l o r i t e and aluminosil icates i n

high grade s c h i s t s and gneisses and t h e occurrence of muscovite

i n migmatites. The rehealed f rac tu res examplified by samples

A7 and OB7 a r e a lso pmducts of retrograde metamorphism. The

retrograde event could have occurred during t he period of

quiescence following t h e deformations. It was aidded by

hydration o r a decrease i n temperature o r both (cf . Abbot

1 979; Corbet t and Ph i l l i p s 1 981 ) .

The observat ion t h a t rocks of t h e Oban massif were

subjected t o polyphase deformation had e a r l i e r been made

by Kanp ( 1 981 ) and Onyeagocha and Ekwueme ( 1 982). Kmp

(1981) proposed t h a t a t l e a s t t h r e e phases of deformation/

metamorphism a f fec ted p a r t s of t h e Oban massif , According

t o him, two ea r ly phases a r e represented by an a x i a l plane

f o l i a t i o n (32 3 ) t o t i g h t i s o c l i n a l f o l d s which cross-cut

compositional banding ( ~ 1 ?) with an angular discordance o f up

t o 30'. He observed t h a t t h e r e is a c o r r e l a t i o n between b

f a u l t , j o i n t and f o l d axis o r i e n t a t i o n and outcrop p a t t e r n

r e l a t ed t o a t h i r d , more recent deformation.

Though t h e a r e a s tudied by Kemp (1981 ) l i e s ou t s ide

t h e a r e a of t h e present inves t iga t ion t o t h e nor theas t ,

r e s u l t s obtained a r e i n d i c a t i v e of t h r e e deformation and

metamorphic episodes f o r t h e Uwet sheet.

CHAPTER FIVE

CIIEI4I CAL COMPOSITION

5.1 ANALYTICAL FIETHOD:

Mho le-rock analyses of r ep resen ta t ive samples from

Uwet a r e a were done using a P h i l i p s PW 1540 X-ray

Fluorence Spectrometer and a Perkin - Elmer Model 306

atomic absorpt ion spectrophotometer a t t h e labora tory o f

t h e I l l i n o i s S t a t e Geological Survey, U.S.A.

Major oxides obtained by X-ray Fluorescence a r e

SiOp, A1203, Fef13, CaO, k20, Ti02, and P205. In t h e

ana lys i s by t h i s method, t h e samples were d r i ed a t 105OC

f o r one hour. 0.1 250gm o f each sample was mixed with

1 .Ogm Li2B407 and 0.1250g La 0 i n a g raph i t e c ruc ib le , 2 3

covered and fused a t 1 OOOOC f o r 20 minutes. The fus ion bead

was then cooled, weighed and t h e fus ion l o s s calculated.

Fusion bead was crushed very f i n e with 8 somar mix, spread and

tamped i n a pressure vesse l , covered with 3g somar mix, and

pressed i n t o a p e l l e t a t 40,000 ps i . P e l l e t s were s to red i n

a des i cca to r u n t i l t h e end o f analys is . A s e t o f s u i t a b l e

NBS, USGS and syn the t i c s tandards were run with t h e samples

and t h e concentrat ion ca lcula ted using t h e XRF-4 computer

program.

The determinat ions of Ng, IJa, I1n and Zn were made

using atomic absorpt ion spectrophotometry. Perkin - Elmer

s i n g l e element hollow cathode lamps were used. A f o u r inch

sl i t burner head with an a i r /ace ty lene flame was used f o r Na,

Mn and Zn and a two inch slit burner head with a n i t r o u s

oxide/acetylene flame was used f o r Mg. I n t h e ana lys i s , O.1g

of sample was weighed i n t o 601111 ac id washed l i n e a r polyethylene

b o t t l e s . The sample was then t r e a t e d with 1.5ml of aqua

r e g i a (1:J:l - HN3 :HCh : ~ ~ 0 ) and 2.5rnl I F . The b o t t l e

was then capped t i g h t l y agd placed on a steam bath f o r two

hours. The d iges ted sample was then t r ea ted with 25 m l of

HJ BO3 (50g/l) t o complex the RF and 2 0 h l C s C l s o l u t i o n

(1500,000111g/~ ( s ) a s an ion iza t ion supressant and then d i l u t e d

t o 5 b l with deionized water. This so lu t ion was a sp i ra t ed

i n t o t h e flame. I f t h e concentrat ion of t h e ana la t e i n

t h i s s o l u t i o n was beyond t h e l i m i t of l i n e a r i t y of t h e

c a l i b r a t i o n curve, then t h e so lu t ion was d i lu t ed u n t i l t h e

concentrat ion f e l l wi th in t h e l i m i t of l i n e a r i t y . The

c a l i b r a t i o n curve was constructed us ing syn the t i c s tandards

made from u l t r a pure metals o r compounds o f t h e a n a l a t e and

matching t h e samples i n t h e i r content of reagents. The curve

i t s e l f was ca lcula ted us ing t h e method of l e a s t squares.

For t h e determinat ion of i g n i t i o n l o s s (LOI), 1 .Og

of sample was weighed i n t o a 35ml platinum crucib le . The

s m p l e was f i r s t d r i ed a t l l O ° C f o r two hours and then allowed

t o cool t o ob ta in t h e dry weight of sample. The d r i e d sample

was then placed i n t o a muffle furnance where t h e sample was

heated t o 500°C, uncovered. The temperature o f t h e muffle

furnnnce was then ra ised to 1 OOoOc, and the sample was

igni ted a t t h i s temperature f o r 24 hours with t h e c r u c i b l e

covered. The sample was then allowed to cool and t h e weight #

recorded and t h e weight l o s s on i g n i t i o n was ca lcula ted and

reported as a percentage weight loss .

5 -2.0 GEO CHEHI STRY

The chemical composition of rocks i n Uwet a rea is

presented i n Tables 7 and 8. Table 9 presents the normative

composition of these rocks. For the purpose o f a d iscuss ion

of the geochemistry of t h e rocks they have been grouped as

follows:

(i) Metasediments - p h y l l i t e s s c h i s t s and gneisses

( ii) Meta-igneous rocks - amphibo l i t e s

( i i i ) Basic rocks - d o l e r i t e s

(iv) Grani to i d rocks - granodio rit es , t o n a l i t e s

(v) Sedimentary rock - s h a l e s

mi, (1, ti1 ill C l i '