1

CHAPTER ONE

1.0 INTRODUCTION

1.1 Statement of the Problem

- The Basement Complex of Nigeria consists of migmatites and migmatitic gneisses, slightly

migmatized to unmigmatized paraschists with interbeds of meta- and non-meta-igneous

rocks, also referred to as the younger metasediments or the Schist belts and the Older

Granite suite comprising mainly different varieties of granitic rocks/granitoids, including

charnockites (hypersthene granites), syenites, as well as minor gabbroic and dioritic rocks.

Unmetamorphosed dolerite and rhyolite porphyry dykes, pegmatite dykes and numerous

veins of quartzo- feldspathic materials are intrusions commonly found in the Basement

Complex.

- In Keffi areas, there are not much records of detailed work carried out on the various units

of the Basement Complex. Most parts of the study area had been mapped as underlain by

only the migmatites and migmatitic gneisses. Occurrence of a Schist belt was only shown

in parts of the area in the Geological Map of Nigeria produced by the Geological Survey of

Nigeria in 1994 as well as the most recent Geological Map of Nigeria (Malomo, 2004).

This work was therefore necessitated by the need for the delineation of the various rock-

types of the different units of the Basement Complex in the area, as well as the need for

detailed petrographic and geochemical studies on the rocks for assessment of their

petrogenesis.

1.2 Location, Accessibility and Communication

The study area lies within Latitudes 80 30’ N and 9

0 07’N and Longitudes 7

0 37’E and 8

0 18’E.

The area is within the north-central Nigeria Precambrian Basement Complex (Fig. 1). It is

bordered by Asokoro and Galadima to the North West, Gitata and Gidan Mutum Daia to the

2

Fig.1 Location of the study area on a Geological Map of Nigeria showing the Precambrian

Basement Complex (Modified from Obiora, 2005)

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North; Gudi, Anzo and Mada to the East; Agwada, Onda and Ragwa to the south; and Waru,

Karshi and Sabe to the west (Fig. 2).

The study area covers the following Local Government Areas: Keffi, Karshi and Kokona and

cuts large parts of others, namely: Nassarawa, Karshi, Nassarawa-Eggon and Toto, all of which

are within Nassarawa State.

The area is accessible by network of major and minor roads, as well as foot paths and

partly dry river channels (Fig.3). Most of the outcrops of schists were studied and sampled along

river channels which run for tens of kilometers (generally north to south) across the study area.

This posed serious accessibility problems as the researcher had to trek (8 to 15) km up or down

the stream channels where fresh outcrops are exposed. The area of study is about 5,141.14 km2.

Motorcycles and bicycle were used to traverse the interior parts of the study area. Some

villagers were very helpful as field assistants.

4

Fig. 2 Map of parts of north-central Nigeria showing the study area and some

important adjoining areas (Muro, Gudi, Abuja, Nassarawa, Nassarawa- Eggon

and Akwanga).

5

Fig. 3 Map of Keffi and its environ showing access to the study area

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1.3 Topography and drainage

Keffi and its environs are situated in a highly undulating terrain. The Granitic Basement

Complex rocks constitute the highlands while the schistose rocks occupy the valleys. The highest

elevation within the area is the Maloni Hill (Plate 1). This hill is part of a NE-SW trending range

of hills which flanks Keffi main town, especially around the Emirs palace, to the west, standing

at an elevation of about 1340ft (402m).

A plateau/hill range system which runs E-W from the western margin of Keffi Local

Government Area through Kokona Local Government Area forms part of the Kokona- Nasarawa

Eggon plateau/hill ranges. It underlies the following villages; Akora, Gandun Sarki, Dari, Gurku

Toni, Angoro, Anzo, Kudaru, Mada and ends in Wana where the elevation is highest. The study

area is drained by major rivers which form a dendritic drainage pattern (Fig.4).

The rivers include Apo, Zimbabwe, Old barracks, Keffi , Nassarawa , and Mada rivers.

All these rivers with the exception of the Mada river, empty into the Nasarawa river which is the

main water course in the study area. They all form tributaries to the Benue River to the south.

1.4 Climate, Vegetation and Soil. The study area is under the influence of two major

weather conditions, namely: the rainy season which starts in April and ends in September with its

peak in July/August; and the dry season which begins in October and ends in March. The annual

rainfall recorded ranges between 1250mm to 1500 mm while the annual temperature is 90-950

F

or 32-350 C, with February to April being the hottest periods of the year (Barber and Depreez,

1965). By February and March, the river channels are almost totally dry, providing easy access

to outcrops exposed by the rivers.

The study area lies within the Savanna belt of Nigeria characterized by preponderance of

tall grasses dotted by few giant trees, except for the river channels which support rain-forest

vegetation characterized by deciduous trees.

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Plate1. Keffi town as seen from the sloping flank of the Maloni hills

8

FIG. 4 Drainage Map of Keffi and its environs, including the study area.

9

The forest intensity increases from Northern (source) points to the southern points of the

rivers as observed in the field. Like the rivers, vegetation is dry at the peak of the dry season; this

facilitates geological field work. Silty sand and silty clay units form the dominant soil types

within the study area. These soil types are a consequence of the weathering of the granitic

Basement Complex rocks and schistose rocks, respectively. Combined with the climatic

conditions, these soil types typically support mostly cereals farming and animal grazing, which is

the main occupation of the natives of the area studied.

1.5 Literature Review

The Basement Complex rocks in the study area have received very little attention from

researchers as evidenced in literature. Much of the information on the area is contained in

Geological Maps of Nigeria produced by the Geological Survey of Nigeria (Okezie, 1974,

Okezie, 1984, Ojo, 1994 and Malomo, 2004). Of all these Maps, it was only the ones of Ojo

(1994) and Malomo (2004) that showed that the area is underlain by the Schist belt in addition to

migmatites/migmatitic gneisses and granitoids. A few workers have done some work in areas

around the study area.

The earliest work done in parts of the study area was by Onyeagocha (1984). This

author’s work covered areas like Kubare, Angoro, Kudaru, Gurku, Dari, Sabon-gida, Andu,

Garaku, Yelwa and Ninkoro which form the east-central part of the study area. These areas are

bounded by Latitudes 8º 45´ N - 9º 00´ N and Longitudes 8º 00´ E - 8º 15´ E as against Latitudes

8º 30´ N - 9º 07´ N and Longitudes 7º 37´ E - 8º 18´ E of the study area.

The author’s study area is underlain by pelitic to semi-pelitic schists, gneisses,

migmatites, meta-quartzites and ultramafic schists. Prismatic tourmaline, plagioclase (An26-32)

and staurolite (up to 10%) were observable in hand specimen in the pelitic to semi-pelitic schists

whereas the ultramafic schists consist of anthophyllite, tremolite, talc, chlorite, chromite and

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olivine. The pelitic to semi-pelitic schists were named the river Audu schist and considered to be

the oldest rock in his study area.

According to the author, the Sabon-gida augen gneiss is conspicously very dominant in

the western areas but contains the xenoliths of the River Andu schist and is therefore younger. It

contains abundant biotite- and hornblende- rich mafic xenoliths. These rocks have been

metamorphosed to the oligoclase- epidote amphibolite facies of the regional metamorphism.

Modal mineralogy of the augen gneiss consists of quartz, plagioclase (An26-33), microcline,

biotite and hornblende ± garnet and epidote with accessory tourmaline, sphene and opaque iron

oxides.

The Garaku granite gneiss is thought to be coeval with the Sabon gida augen gneiss,

containing xenoliths of the River Andu schists but lacking the augen structures, and is further

invaded by granitic pegmatites and aplite dykes (Onyeagocha, 1984).

Granites crop out in Yelwa, Garaku and Ninkoro areas and were observed to be medium-

grained and contain both biotite and muscovite. They also contain xenoliths of the older schist,

and show similarity to other granitic bodies in the area in terms of their mineralogy. Pegmatites,

aplite dykes and dolerites also occur in the area. Using their geochemistry and rare-earth element

analysis, Onyeagocha (1986), pointed out that the granites are younger than the granite-gneiss;

that Al/ (Na + K + Ca) ranges from 0.92 to 1.09, indicating that the rocks are aluminiun-rich to

aluminium excess.

Chondrite-normalised rare-earth patterns indicated normal magmatic abundances and

point to derivation by partial melting of schists and gneisses. Also the high Th content (151-313)

ppm suggests a crustal origin for these rocks. The geochemical data the author used was,

however on only the granites and granite gneisses. Anike et. al. (1990) studied the Basement

Complex rocks in Muro in Nassarawa area, which is about 78 km south west of Nassarawa Local

Government Area headquarters (the southernmost part of the study area).

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The petrography of the principal rock units in Muro areas as presented by Anike et. al.(1990)

shows that the gneiss which underlies the schist is composed of Quartz 40%, K-feldspar- mainly

microcline 25%, plagioclase (An25-44)(19%), biotite (8%), muscovite (4.5%), with hornblende,

garnet, opaques, sphene, zircon, tourmaline, and apatite as accessories.

According to them, quartz-mica-schists overlie the gneiss with basal sections being either

micaceous quartzite with up to 70% quartz, or muscovite-biotite-quartz schist containing an

occasional garnet crystal. Microcline, albite and chlorite are occasionally present. Other schistose

sub-units include the muscovite- quartz schist, chlorite- muscovite- quartz schist and graphite-

bearing schist; all of the later contain modal chlorite and muscovite (Anike et. al., 1990).

Umeji and Caen-Vachette (1991) reported that the Basement Complex in the vicinity of

Nassarawa-Eggon which is about 52 km east of the study area contains granitic gneisses,

gneissic granites and occasional lenses of amphibolites within dominant migmatites. They

reported that the Pan-African tectonics imposed NE-SW to ENE-WSW trends on all the rocks

and that the Basement Complex is locally sheared with a mylonitic shear zone (340 m wide).

Within this mylonitized basement lies a rhyolite dyke with outcrop width of up to 50 m, which

forms a ridge rising up to 12 m above the surrounding basement rocks.

Chemically, the granitic rocks in Nassarawa-Eggon contains SiO2 (61 - 74.5%, average

69.8%) as against average of 75.9% for the Nassarawa-Eggon rhyolite dyke. When compared

with the leuco-granites of the Basement Complex, the rhyolite dyke generally has lower TiO2,

Al2O3, Fe2O3, CaO and Na2O contents. Also, the Nassarawa-Eggon rhyolite dyke has more SiO2

and CaO, and significantly less TiO2, Al2O3, Fe2O3, Na2O and P2O5 when compared with Mada

Younger Granite Complex -rhyolite feeders of Abaa (1985b). Unlike the Mada rhyolites, the

Nassarawa-Eggon rhyolite dyke is not alkalic but calc-alkalic and is chemically distinct from the

Younger Granite suite. However, Umeji and Caen-Vachette (1991) did not present data on the

trace-element geochemistry of the Basement Complex rocks in Nassarawa-Eggon.

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The Geological Map of Nigeria by Okezie (1974) shows that the Precambrian Basement

Complex can be divided into two distinct zones, namely: a western zone in which the N-S

trending elongate Schist belts are separated from one another by migmatites/migmatitic gneisses

and granites, and an eastern zone, comprising mainly migmatites/migmatitic gneisses and

granites, in which the Schist belts are scarcely present. These Precambrian rocks of Nigeria are

found within the Pan-African (Neo-Proterozoic, ca-0-6 Ga) province east of the Archaean to

Meso- Proterozoic West African craton. They are often grouped into three major sub-units

namely: The ancient Gneiss-Migmatite complex, the schist belts and the Pan-African granitic

complex plus affiliated minor rocks (Elueze, 1999).

The first review of works done on the rocks in the Precambrian Basement Complex of

Nigeria was carried out by Oyawoye (1965). The author succeeded in subdividing the Basement

Complex rocks into three major groups which he described as (a) the older metasediments,

consisting of calc-silicate rocks, arkosic quartzite and high grade schists which are present as

lensoid relicts in regional gneisses or as paleosomes of the migmatites. He considered this group

as the oldest rocks of the Basement Complex. (b) the gneisses, migmatites and the older granites.

In this group, the author recognized two major types of gneisses which include: the biotite gneiss

and the banded gneiss. He also grouped the migmatites into two types, namely; the lit-par-lit

gneiss and the migmatitic gneiss. In the lit-par-lit gneiss, according to the author, the paleosome

(a granulite or high grade schist of the ancient metasediment) occurs with quartz-feldspar veins

and dykes in parallel orientation. In the migmatitic gneiss, the metasome is also quartz-

microcline veins but the paleosome, which is biotite or banded gneiss, is dissected into irregular

blocks. On the basis of petrography, Oyawoye (1965) suggested that the gneisses and migmatites

originated through silica-potash metasomatism. For the ‘Older Granites’, the author used field

and petrographic evidence to suggest that they probably have a metasomatic origin and

associated them with three other types of rocks namely; the coarse-grained, greenish fayalite-

bearing rocks, pyroxene-quartz-diorite and pyroxene-amphibole syenite. The very coarse-grained

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fayalite-bearing rocks, otherwise called the Bauchites were first described from Bauchi town in

Nigeria (Oyawoye 1962). At Toro in Jos Plateau, Oyawoye (1964) reported the presence of the

pyroxene-quartz-diorite within the Basement Complex, typified by the blue-black

‘charnockitic’quartz-diorite while he described the pyroxene-amphibole-syenite (a typically

purplish rock) as zoned intrusion at Shaki, southwestern Nigeria (Oyawoye 1972). (c) The

younger meta-sediments, to which he ascribed Paleozoic age (Oyawoye, 1965).

Cooray (1974) in further review added another family of rocks (the intrusives) to the

works of Oyawoye (1965). He further effected some changes in the conclusions of Oyawoye

(1965) such as (a) that the Older granites and related charnockitic rocks are of intrusive rather

than metasomatic origin. (b) the Older granites and granodiorites based on the relative time of

emplacements and deformation are subdivided into; syn-tectonic microcline-megacrystic, partly

foliated granites and late- tectonic, less richly megacrystic, weakly foliated xenolithic granites

and granodiorites with cross-cutting contacts and occasional thermal aureoles (McCurry and

Wright, 1977; Jones and Hockey, 1964). (c) the author pin- pointed the generally north-south to

northeast-southwest structural pattern in the Basement Complex and suggested a polyphase

metamorphism to have affected the Basement Complex rocks, concluding that the Basement

Complex bears the imprints of, at least, three plutonic events during the Eburnean, Kibaran and

the Pan-African orogenic episodes (Grant, 1978).

Rahaman (1976, 1988) and Rahaman and Ocan (1978) presented a more classical

description of the rocks of the Basement Complex. The three rock groups described by Oyawoye

(1965) and Cooray (1974), was now subdivided into six major lithologic groups. These include

(a) the Migmatite-Gneiss Quartzite Complex described as a heterogeneous group of rocks

composed. It has sub-groups namely; (i) early (grey) gneiss, the oldest recognisable member of

the migmatite-gneiss-quartzite complex, grey, foliated biotite and / or hornblende gneiss of

granodioritic to tonalitic composition, (ii) mafic to ultramafic components which are usually

amphiboles, biotite and biotite-hornblende schists and (iii) felsic components, usually of granitic

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composition and aplitic, granitic to pegmatitic. The granite gneisses are interpreted as deformed

intrusive granites and the other felsic components are either products of partial melting of the

older basement or magmatic injection. Other subgroups are; (iv) bodies of meta-sediments which

are quartzites, calc-silicates and garnet-sillimanite-cordierite gneiss (b) Slightly Migmatized to

Unmigmatized Paraschists and Metaigneous rocks also referred to as Newer Meta-sediments

(Oyawoye, 1964), younger metasediments (McCurry, 1976) or rocks of the Schist belts (Ajibade,

1976 and Turner, 1983); (c) the Charnockitic, gabbroic and dioritic rocks which were subdivided

by Rahaman (1988) into (i) gneissic charnockitic rocks that possess a planar penetrative fabric,

(ii) foliated charnockites which show a migmatic foliation due to the platy parallelism of the

feldspar megacrysts and the concentration of mafic minerals into discrete planes; and (iii) coarse-

grained, massive, often porphyritic charnockitic rocks. They all have granulite facies mineral

assemblages. The rocks are thought to be of igneous origin because of the presence of contact

relationship with granitic rocks. Some are thought to be younger than the granites. (d) members

of the Older granite suite within which Rahaman (1988) recognized the following sub-members

based on textural characteristics; (i) migmatitic granite, (ii) granite gneiss, (iii) early pegmatite

and fine-grained granite, (iv) slightly deformed pegmatites, aplites and vein quartz,, (v)

homogenous to coarse porphyritic granite, and (vi) undeformed pegmatites, two mica granites

and vein quartz.

(e) Metamorphosed and un-metamorphosed calc-alkaline volcanics and hypabyssal rocks.

(f) Unmetamorphosed dolerite dykes; syenite dykes (Rahaman 1988).

According to Obiora (2005), the schistose components of the migmatitic terrain were

designated, “the older metasediments” while the distinct N-S trending schist belts, which are

clearly younger than the gneisses and migmatites were mapped as “the younger meta-

sediments”. The Younger meta-sediments or the schist belts consist of low- to high-grade mica-

schists, quartz schists, quartzites, and concordant amphibolites, talc schists (meta-basites, meta-

mafites/meta-volcanics). In places, especially in the south-western Basement Complex, the

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Schist belts are associated with marbles, dolomites and calc-silicate rocks (e.g. calc- silicate

schists) and meta-conglomerates. These are the products of metamorphism of limestone, marls,

calcareous sediments and conglomerates, respectively. Banded Iron Formation (BIF) is also

sometimes associated with the Schist belts for example, the Muro Schist belt, Anike et al. (1989,

1990). In Ilesha, Sokoto, Minna and Birnin-Gwari areas, the schists and amphibolites of the

Schist belts are the host rocks of the Nigerian gold deposits which occur mainly as alluvial

deposits in some parts of the Basement Complex.

Rahaman (1988) had described the rocks of the Schist belt as composed of predominantly

metamorphosed pelitic to semi-pelitic rocks, granites, sandstones, polymict conglomerates,

calcareous rocks, mafic to ultramafic rocks with minor amounts of greywacke and acid to

intermediate volcanic rocks.

The schist belts occupy N-S trending synformal troughs and 17 such troughs have been identified

and described (Ekwueme, 2003). The Schist belts in the Precambrian Basement Complex of

Nigeria have been compared to the Archaean greenstone belts (Turner, 1983, Attoh and

Ekwueme, 1997) which are known to harbor important economic mineral deposits. They differ

from the Archean Greenstone belts by their contents of more clastic sediments than volcanic

rocks (Turner, 1983). They are thought to be relicts, now preserved in synclinal keels, of a once

widespread cover deposited in a single basin. Ajibade (1988), however opposed the idea of

deposition in a single basin.

Geochemical data confirm that the rocks in the Nigerian Schist belts are pelites, semi-

pelites and greywackes. The mafic rocks now believed to be largely of igneous origin, are

amphibolites with variable tectonic settings including Island arcs (Fitches et. al., 1985), Island

arcs and ocean floor (Ekwueme, 2003), within-plate to Mid- ocean ridge (Obiora, 2008).

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1.6 Objectives of the Study

This study was undertaken to achieve the following:

(1) To produce a Geologic Map of the area at the scale of 1: 644,000

(2) To carry out Petrographic descriptions of the rock types in the area and determine their

geochemical (major-, trace- and rare-earth elements-REE) characteristics.

(3) To assess the petrogenesis of the rocks and evaluate the economic mineral resources in

the rocks

1.7 Methodology

The study was carried out in four phases, namely:

(1) Desk study

This involved the examination of geologic reports on different parts of the Precambrian

Basement Complex of Nigeria, especially the north-central Basement Complex.

(2) Field Mapping

This was carried out on three occasions and lasted for a total of forty one (41) days. The

reconnaissance survey was undertaken from 3rd to 12th

February, 2007. The preliminary

fieldwork lasted from 2nd

to 13th

March, 2007 while the detailed investigation was from 4th

to

14th

April, 2007. Follow-up study/supervision by my supervisor and other staff of the

Department to cross-check some of the field observations lasted from the 21st to 23rd August,

2007. Exposures of the different rock types occur along river valleys, quarries, road-cuts, as well

as on hill sides. Access to the remotest outcrops was gained by walking through the dry to semi-

dry river channels and use of a bicycle, which was purchased by the researcher, as well as

motorcycles.

Field instruments used include:

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Brunton Compass used for measuring attitude (dip and strike) of foliation and bearing of

outcrops.

Global Positioning System (GPS) used for quantitative determination of the grids and

elevations of the outcrop locations.

Field Note books were used for recording of observations/descriptions of the rocks and

measurements

Measuring tape was used for measuring dimensions of rock bodies and thicknesses of foliae.

A Monday Harmer and chissel were for the breaking and collection of rock samples

respctively.

Masking tape, maker pen and biro pen for the labeling of collected rock samples and field

note taking.

A clinometer for mesurement of angles of elevation, depression and height of outcrops and

objects.

A field bag was used to carry field equipments, rock samples and lunch packs.

The map of the study area was used to locate access routes and for direct plotting of

measured data in the field.

(3) Laboratory work

This was carried out in two parts, namely: petrographic and geochemical analysis. Thin

sections for the petrographic studies were prepared at the Thin section Laboratory of the

Department of Geology and Mining, University of Jos, Jos, Plateau State. Modern Petrographic

microscopes purchased by the Petroleum Trust Development Fund and housed in the Department

of Geology, University of Nigeria, Nsukka were used for the petrographic studies.

Photomicrographs were taken by the combined use of the petrographic microscope and Konica

digital camera.

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Representative samples selected from the petrographic studies were pulverized and sent to the

Activation Laboratories (ACT LABS), Canada for geochemical analysis, using the Inductively

Coupled Plasma, Fusion Method (FUS- ICP). This method was used to obtain the major element

oxides (weight percentages), trace elements and rare earth elements proportions in parts per

million.

(4) Data analysis

This involved the plotting of the data obtained from the Field and Laboratory work and

the writing and interpretation of the results presented in this final form.

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CHAPTER TWO

2.0 REGIONAL GEOLOGIC SETTING OF THE PRECAMBRIAN BASEMENT

COMPLEX OF NIGERIA.

2.1 Regional Tectonic Setting:

The Precambrian Basement Complex of Nigeria lies within the Pan-African mobile belt,

east of the West African craton and northwest of the Congo-Gabon craton. Evidence from the

eastern and northern margins of the West African craton indicates that the Pan-African belt

evolved by plate tectonic processes which involved the collision of the passive continental

margin of the West-African craton and the active margin of the Pharusian belt (Tuareg shield),

about 600 Ma (Burke and Dewey, 1972; Leblanc, 1981; Black et. al., 1979; Caby et. al., 1981).

This evidence includes the presence of basic to ultrabasic rocks believed to be either remnants of

mantle diapirs or a paleo-oceanic crust. These rocks are characteristic of an ophiolitic

complex.Another evidence is a high positive gravity anomaly which occur in a narrow zone

within the Dahomeyide orogen located at the southeastern margin of the West African Craton in

Togo and Benin Republic (Schluter and Trauth, 2006). The collision at this plate margin is

believed to have led to the reactivation of the internal region of the Pan- African belt. The

Nigerian Basement Complex lies within the reactivated part of the belt (Rahaman,1976).

Radiometric ages indicate that the Nigerian Basement Complex is polycyclic and includes rocks

of Liberian (2700±200Ma), Eburnean (2000±200Ma), Kibaran (1100±200Ma), and Pan-African

(600±150Ma) (Black et. al., 1979; Caby et. al., 1981). In recent times, using the International

Geological Time Scale (2002) in Gunter and Mensing (2005), these ages can be referred to as,

“Paleoarchean to Mesoproterozoic (3600 to 1600 Ma)” for Liberian and Eburnean,

“Mesoproterozoic to Neoproterozoic (1600 to 1000 Ma)” for Kibaran and “Neoproterozoic to

Early Paleozoic (1000 to 545 Ma)” for Pan-African (Obiora, 2008). Two distinct provinces can

be recognised in the Nigerian Basement Complex, namely: the western province, approximately

west of Longitude 80

E, characterized by narrow, sediment-dominated, N-S trending, low-grade

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schist belts in a predominantly migmatite-gneiss ‘older’ basement, and the whole is intruded by

Pan-African granitic plutons; and the eastern province which is composed mainly of migmatite-

gneiss complex intruded by larger volumes of Pan-African granites and by the Mesozoic ring

complexes (Younger Granites) of central Nigeria (Ajibade et. al., 1987; Kogbe, 1989). The

evolution of the Nigerian Basement complex during the Pan-African can best be discussed in the

regional context of the Pan-African orogenic belt of West Africa. Geological and geophysical

evidence from the western province of the belt and the cratonic margin has been used to erect

geophysical evidence for the evolution of the belt (Grant, 1970; Burke and Dewey, 1972;

Bertrand and Caby, 1978; Black et. al., 1979; Caby et. al., 1981). Essentially, the evolution of

the belt is seen as a collision type orogeny with an eastward dipping subduction zone. Initial

crustal extension and continental rifting at the West African cratonic margin, about 1000 Ma, led

to the formation of graben-like structures in the western Nigeria and the subsequent deposition of

the rocks of the Schist belts. Closure of the ocean at the cratonic margin, about 600 Ma and

crustal thickening in the Dahomeyan led to the deformation of the sediments, reactivation of the

pre-existing rocks and the emplacement of the rocks of the Pan-African granites (McCurry,

1976). The granitoids have been emplaced within both the migmatite-gneiss complex and the

Schist belts, and they occur in all parts of the Nigerian Basement Complex, though the extent of

the Pan-African plutonism had not been fully understood (Rahaman 1976). Fitches et. al. (1985)

have shown that the Older Granites are high level intrusions emplaced by stoping and diapiric

processes in different parts of the country. The migmatite-gneiss complex is considered to

contain rocks of the Archaean age which have been deformed and modified several times prior to

the Pan-African orogeny (Rahaman 1976). The Precambrian ages are mostly related to the Pan-

African (900-450 Ma) thermotectonic event, with few imprints of the older events such as the

Liberian, Eburnean and the Kibaran (Obiora, 2005). Evidence from the Ibadan area indicates

that Archaean rocks included meta-sedimentary and meta-volcanic rocks which were deformed

prior to the emplacement of the Eburnean granite-gneiess (Burke et. al., 1976). The early

21

Proterozoic (Eburnean) event was probably accompanied by sedimentation, deformation,

metamorphism and syn-tectonic igneous activity (Burke et. al., 1976). Since the recognition of

the suture along the eastern margin of the West African craton, attempts have been made to

relate the Schist belts to the subduction processes at the cratonic margin (McCurry, 1976).

Vaniman (1976), Holt (1982) and Turner (1983) consider that the Schist belts have been

deposited in a back-arc basin developed after the onset of subduction at the cratonic margin.

2.2 Regional Stratigraphic Setting.

The Precambrian Basement Complex of Nigeria is exposed in five major locations,

namely: Northeastern zone, Southwestern zone, Southeastern zone (extension of the Bamenda

Massif into Nigeria), Northeastern zone (the Hawal massif) and South-southeastern (the Oban

Massif) (Obiora, 2005). Oyawoye (1965) subdivided the rocks of the Basement Complex into

three major groups, namely:

1. The Older Meta-sediments consisting of calc silicate silicate rocks, arkosic quartzite and

high grade schists

2. The gneisses, migmatites and the Older granites

3. The Younger Meta-sediments.

Cooray (1974) recognized three major lithologic units similar to those identified by Oyawoye

(1965) as presented below:

1. Metasediments (older and younger)

2. Gneisses and migmatites and

3. Intrusives.

Rahaman (1976) subdivided the Basement Complex rocks into six major petrologic groups as

follows:

1. Migmatite-Gneiss-Quatzite Complex

2. Schist belt

22

3. Charnockitic, gabbroic and diortic rocks

4. Rocks of the Older granite suite

5. Metamorphosed and unmetamorphosed calc-alkaline volcanics and hypabyssal rocks and

6. Unmetamorphosed dolerite dykes, syenite dykes.

Obiora (2005) subdivided the Precambrian Basement Complex rocks into three main groups,

namely:

1. Gneisses and Migmatites / Migmatitic gneisses.

2. Schist belts.

3. Pan-African (Older) granites.

He also described basic and acid intrusive and extrusive rocks emplaced within the Basement

Complex rocks.

Dada (2006) in his review of the previous works on the Basement Complex presented four

distinct lithologies, namely:

1. Migmatite-Gneiss complex (GMC)

2. Schist belts

3. Pan A frican granitoids and

4. Undeformed acid and basic dykes.

On the whole, the different lithologic types can be conveniently grouped into three and described

as follows:

2.2.1 Gneisses and migmatites/migmatitic gneisses (Migmatite-Gneiss Complex, MGC)

These rocks are the Basement (sensu stricto). They are the oldest rocks of the Nigerian

Basement Complex. They exhibit great variations in composition because of the differences in

their protoliths (pelitic, psammitic or igneous) and the metamorphic (P-T) conditions under

which they were formed. They consist essentially of gneisses and migmatites with supra-crustal

relicts referred to as older meta-sediments (Obiora, 2005). This unit is also referred to as a

23

heterogeneous assemblage including migmatites, ortho-gneisses, para-gneisses and a series of

basic and ultra-basic metamorphosed rocks (Rahaman, 1988). The general trend of the foliation

in these basement rocks is NE – SW, and occasional E-W. The rocks possess gradational

contacts with high grade metamorphic rocks suggesting origin by granitization (Obiora, 2005).

Petrographic evidence has also shown that the Pan-African reworking has led to

recrystallization of many of the constituent minerals of the MGC by partial melting. Most of the

rocks display medium to upper amphibolite facies metamorphism. The rocks are generally

medium to coarse grained and felsic, showing alternating layers or bands of light and dark

coloured minerals. The minerals in the light coloured layers are commonly quartz, orthoclase

and/or microcline and plagioclase (An23 to An33) while biotite with or without hornblende

dominate the dark layers. The rocks show characteristic gneissose foliation due to a preferred

orientation of platy minerals like biotite. Most of the gneisses are meta-sedimentary rocks as

shown by their mineral constituents of the aluminosilicates (kyanite, silimanite, andalusite,

cordierite, staurolite and garnets). The migmatites are composed of a metamorphic host

(commonly schists and gneisses) which is streaked and veined with granitic (quartzo-feldspathic)

materials, typical of anatexis.

Radiometric age data show that rocks of this unit are truly the oldest of the Basement

Complex. For instance, Kaduna areas, both multiple and single zircon U-Pb, as well as Rb-Sr

studies have confirmed metamorphic events at 3.5 Ga (Paleoarchean), 3.1-3.0 (Mesoarchean),

2.8-2.7 (Neoarchean), 2.3 – 1.8 (Paleoproterozoic) and 0.6 Ga (Neoproterozoic) ( Grant, 1970;

Rahaman, 1988; Caen- Vachette and Umeji, 1988; Oversby, 1975; Pidgeon et. al., 1976; Dada,

1989; Bruguier et. al., 1994; Dada et. al., 1998).

24

2.2.2 Schist belts

The Schist belts constitute the most remarkable structural parttern in the Precambrian

Basement Complex of Nigeria. They are essentially NE-SW trending belts which are more

conspicuous in the western parts of the country. They were formerly referred to as the “younger

meta-sediments” to distinguish them from the migmatitic gneisses which were designated the

“older meta-sediments”. They constitute distinct N-S trending belts of schists which are clearly

younger than the gneisses and migmatites. These meta-sediments consist of low- to medium-

grade mica schists, quartz schist, quartzites and concordant amphibolites. In the southwestern

Basement Complex, the belt is associated with marbles, dolomites and calc-silicate rocks

(Obiora, 2005). In the NW, there is a dominance of schists of greywacke origin comprising meta-

pelites and quartzites, phyllites, mica-schists, quartzo-feldspathic schists, paragneiss, Fe-Mn rich

quartzites and garnet amphibolites (Dada, 2006). Acid and intermediate volcanic rocks are

interbedded in the metamorphosed pelitic to semi-pelitic rocks of Anka, Birnin Gwari and

Zungeru schist belts. In these areas, as well as, other schist belts, particularly in northwest

Nigeria, the rock units form recognizable discrete belts with distinct and contrasted lithologies,

separated by either the migmatite gneiss complex or the Pan-African granitods (Dada, 2006).

Apart from metamorphic Rb-Sr and K-Ar cooling ages of between 700 and 450 Ma

(Ogezi, 1988) there are no reliable ages of formation of the Nigerian schist belt. The only

indirect evidence for a minimum Palaeoproterozoic age of 2100 Ma is the U-Pb zircon data on

Kabba-Okene gneiss (Annor, 1995). This gneiss hosts meta-sedimentary xenoliths and show the

same early phase metamorphic fabric, with the Okene-Igarra schist (Annor, 1998). The Rb-Sr

isotopic data on the same rocks (Annor, 1998) show a disturbed systematics, reflecting Pan-

African rehomogenisation. The works of several authors(Ogezi, 1988; Holt, 1982; Fitches et. al.,

1985; Caen-Vachette and Umeji, 1987; Caen-Vachette and Ekwueme, 1988) on the Rb-Sr

systematics of meta-sediments of Nigeria show a high degree of elemental and isotopic

fractionation in these rocks that have been reworked during the Pan-African thermtectonic event.

25

In all cases, the rehomogenisation gives disturbed experimental points whose errochrons suggest

mixed ages, between the pre-Pan-African basement and the 0.6 Ga event that are usually

wrongly interpreted as Kibaran (Dada, 2006).

2.2.3 Pan-African (Older) granites

The Pan-African granites are syn-to late-tectonic intrusions into the “MGC” and the Schist

belts. They are otherwise referred to as the ‘Older granites’ to distinguish them from the Jurassic

granites which are also found alongside their outcrops in the Jos Plateau and adjoining localities.

The Pan- African granites suite consists mainly of granites and grandiorites (Rahaman, 1976).

Members of the suite include porphyritic/porphyroblastic muscovite granites, aplites,

granodiorites, diorites and quartz-hypersthene diorites (Obiora, 2005). They range in size from

small sub-circular cross-cutting stocks to large elongate concordant batholiths. They are often

weakly foliated and described as foliated granites and gneissic granites. Contacts of these Older

granites with the Basement Complex rocks are characteristically gradational suggesting a non-

magmatic origin, possibly emplaced during the last of the reactivation events to affect the

Basement Complex, i.e. during deformation and metamorphism of the supracrustals (Pan-

African orogeny) (Rahaman, 1976). The Pan-African granites are medium to coarse-grained,

containing both muscovite and biotite, plagioclase (An6 to An15) and microcline.

Petrographically and geochemically, the older granites are calc-alkaline; thus, they represent

products of subduction/collision or mountain building events, characteristic of

convergent/compressional tectonic events, and are therefore orogenic granites (Obiora, 2005).

Neoproterozoic to Early Paleozoic (638 - 510 Ma) U-Pb, Rb-Sr, K-Ar ages have been

reported from granitoids within the Nigerian Schist belt areas (Grant, 1978; Caen- Vachette and

Umeji, 1983; Matheis and Caen-Vachette, 1983; Tubosun et. al. 1984; Umeji and Caen-

Vachette, 1984; Fitches et. al., 1985; Ogezi, 1988; Rahaman, 1988; Rahaman et. al., 1991).

According to Dada (2006), Pan- African ages show an order of emplacement of the granitoids in

26

relation to the tectonometamorphic phases discussed below and tentatively suggests the

following:

a. Early deformational phase D1 with migmatisation and local anatexes at between 640 and

620 Ma.

b. Main deformational phase D2 with the emplacement of syntectonic granitoids at the peak

of the Pan African magmatism (620-600 Ma).

c. Emplacement of late to post-tectonic granitoids during the late second phase D2

deformation (600-580 Ma).

Annor and Freeth (1985) had, however, warned that Pan-African tectonometamorphism

was heterogeneous in style, degree and grade. In addition, increasing evidence suggests that

deformation may not be synchronous with magmatism (Grant, 1978; Rahaman et. al., 1991).

27

CHAPTER THREE

3.0 DESCRIPTION OF LITHOLOGIC UNITS AND THEIR FIELD

CHARACTERISTICS

The rocks types encountered within the study area include:

Porphyroblastic /Augen Gneiss (PAG).

Migmatitic Banded Gneiss (MBG)

Banded Hornblende Gneiss (BHG)

Garnet Mica Schist (GMS).

Kyanite Schist (KS).

Quartzofeldspathic Schist (QS)

Staurolite Mica Schist (SMS).

Micaceous Quartzite (MQ)

Garnetiferous Biotite-Muscovite Granite (GBMG).

Simple Pegmatites (SP).

Complex Pegmatites (CP)

These rock types have been systematically lumped into eight different mapable units as

shown in the geologic map (Fig.6). These mapable units include;

The Porphyroblastic/Augen Gneiss unit (PAG)

The Migmatitic Banded Gneiss (MBG)

The Garnet Mica Schist (GMS)

The Staurolite Mica Schist (SMS)

The Kyanite-bearing Schist (KbS)

The Kyanite Schist (KS)

The Garnetiferous Granite (GG)

The Pegmatites (CP/SP)

28

The gneisses constitute the Basement sensu stricto, and are either overlain or intruded by

the other units. They generally have NNE-SSW trending gneissose foliations with few ENE-

WSW and NNW-SSE trends and dip values which vary from as low as 150 to as high as 75

0 in

dominantly both ESE and WNW directions. Gradational contacts exist between gneisses and

schists as observed at the outcrops along the northern end of the Apo River channel (Fig. 5,

Location 21), while sharp contacts also exist between gneisses and schists as observed at the

quarry pit within Keffi town (Location 12).

Gneisses are the most dominant suite of rocks in the study area. They enclose the schists

within the schist dominanted areas. All elevated areas are conspicuously underlain by the

gneisses, which are subdivided into Porphyroblastic /Augen Gneiss and the Migmatitic Banded

Gneiss.

The schists occupy the central parts of the study area and are second most dominant rocks

in the area. They are notably found within the low-lying regions and valleys, especially along the

river valleys.

29

Fig. 5 Outcrop map of Keffi and its environ, north-central Nigeria.

21

23

20

142

16(PAG)

12(KS)

7

1

3 4

18b

8

6

10

19

22

13

27

18a

25

11

9

510

26

24

(PAG)

(BHG)

(GMS)

(GMS)

( )MBG

(MBG)

(MBG)

(GG)

( )SGMS

(PAG)

(PAG)

(GMS)

(GMS)

(GMS)

(MBG)

(PAG)

(PAG)

(GMS)

(MQ)

(GMS)

(SP)

(CP)(MBG)

(CP)

90

80 30

’

80

70

30’

Formatted: Font: 13 pt, Superscript

Formatted: Superscript

Formatted: Superscript

Formatted: Superscript

30

3.1 Porphyroblastic /Augen gneisses (PAG)

This rock unit is characterized by porphyroblasts of feldspars, which occasionally exhibit

eye-like (augen) structures. They occur over an extensive area stretching from about 20 km to

Keffi town along Akwanga - Keffi road, to peripheries of the study area through the northeast to

the northwest and down to the southwestern corner of the study area.(Fig 6). At 20 km to Keffi,

the rock unit appears mesocratic and porphyroblastic with numerous euhedral porphyroblasts of

plagioclase feldspar with zoned structures. The rock essentially lacks banding, but closer

observation will reveal stretching and flow structures occasionally creating augen structures

constituted by the feldspar porphyroblasts. In this section also the porphyroblasts are

inequidimensional. Some of the large feldspar crystals measure over 5 cm in length and 1.5 cm

across. While some are spherical and small with diameters measuring 1 - 2 mm, others range

between these two extremes (Plates 2 a & b).

At approximately 12-15 km up the Old Barracks River this rock unit is observed

surrounding the outcrops of schists around Gidan Mutum, Gitata, Keffi, Angwan Alura, Angwan

Koyo and Gunduma (See locations 23 and 24 in the outcrop map). This lithologic unit becomes

more leucocratic within the regions of Gitata, Angwan Alura, Angwan Koyo and Gunduma

(Locations 23 and 24). At Gitata, precisely 20 km north of Keffi town, the rock is very

leucocratic but the grain sizes become much smaller and the rock gradually loses its

porphyroblastic fabric (Location 23). The grain sizes are medium with a dominance of quartz

and feldspars and tiny specks of biotite which still define the foliation. Muscovite specks

(colourless with strong vitreous luster) also dot the rock randomly.

At about 5km from Nasarawa/Toto boundary post (towards Toto), the unit is again

mesocratic, porphyroblastic and weakly foliated. Its highly resistant nature to weathering makes

it to form highly undulating terrain as one move towards Toto.

A special variety of this unit is exposed around the centre of Keffi town (Locations 12, 16 & 25).

It is unique at these points because it contains lensoid/cylindrically shaped schistose enclaves

31

which are mostly aligned in the NE-SW direction. The schistose enclaves contain numerous

flakes of biotite showing preferred orientation. These enclaves have variable dimensions,

measuring on the average 2cm in length 4 mm across (Plates 3a, b & c). This variety of the

porphyroblastic gneiss could be called a hybrid rock (Olarewaju, 1987) and it constitutes the

central Maloni Hills which form a range of four hills running essentially NE-SW through Keffi

town. It has a sharp contact with a melanocratic homogeneous micaceous-schist (the Kyanite

Schist (KS)) exposed in a quarry site (Location 12). The schist trends concordantly with the

gneissic foliation and has a total vertical height of about 10m (the base of the schistose rock is

now exposed by the rock-quarrying activities).

At location 25 however, this unit is intercalated with the Micaceous Quartzite (MQ),

which is a very fine grained, micro-porphyroblastic and leucocratic rock. It is exposed as a

concordant layer within the migmatitic banded gneiss towards the top of the Maloni hills at

location 25. It has average thickness of about 0.5m with length of over 20 m. The rock is very

weakly foliated such that the alignment of the tiny biotite flakes which define the foliation can

only be detected using a magnifying lens.

32

Fig. 6 The Geological map of Keffi and its environs, North-central Nigeria.

90

80 30

’

80 7

0 30

’

Formatted: Superscript

Formatted: Superscript

Formatted: Superscript

Formatted: Superscript

33

Plat 2 (a and b). The porphyroblasts of feldspars in the porphyroblastic /augen gneiss unit.

Plates (3a and 3b) showing the hybrid rocks at the Kyanite schist/ prphyroblastic gneiss contact with

lenses and pods of biotite nodules.

Plate(3c);Cavities formed by the differential weathering and erosion of the biotite nodules relative to

the porphyroblastic gneiss. (3c) also shows sharp contact between two hybrid rocks.

AA BB

CC

AA BB

34

3.2 Migmatitic banded gneiss (MBG).

This is also very extensive, but restricted to the central parts and southern boundary of the

study area. The long ridge locally known as ‘the Mada hills’ is continuous with this unit; it is

generally granitic in areas around the Mada Station. It possesses some qualities of migmatites

such as the occurrence of mixtures of different rock types and cross-cutting veins of quartzo-

feldspathic materials and grades into the granitic Gneiss towards the Western end of the ridge.

The ridge trends mainly East-West. Climbing the ridge from the Mada Station from the

northwestern end, a much more melanocratic rock (melanosome) is observed embedded within

the more leucocratic migmatitic banded gneiss host. One of such melanosome measures about 10

m long and 4 m across. The melanosome is also banded with thinner layers of light coloured

minerals alternating with thicker layers of dark coloured ones. It is generally medium-grained

and consists of both biotite and hornblende as the mafic minerals. The melanosome is banded

hornblende gneiss (BHG). At Gurku village, an outcrop containing the melanosome has strike of

400 NNE – 220

0SSW, with dip of 40

0 in 130

0 ESE direction.

Occasionally this unit (MBG) is cut across by complex pegmatite dykes with thicknesses

varying from about 0.5 to 2m.

At location 15, about 7.5 km to Keffi and 1km south of the Keffi –Akwanga road, this

rock unit is mesocratic, fairly porphyroblastic and weakly foliated. At Location (14) in Awta-

Balefi village, about 27.7 km to Keffi, along Keffi-Abuja road, a local quarry reveals an

interesting mixture of mesocratic and melanocratic gneisses in a sort of suture-like sharp contact.

The two materials are equigranular with similar foliation trends. The melanocratic portion is

biotite-rich, containing over 50% biotite by megascopic observation.

35

3.3 Garnet-mica-schist (GMS).This unit crops out at locations 1, 2, 3, 4, 7, 8, 11and 18b. The

rock unit is the most dominant facies of the schistose rocks in the study area. It is medium-

grained with a characteristic schistose fabric and consists mainly of micas (biotite and

muscovite), quartz and garnet. At locations 1 and 18b, the rock is greenish-grey in colour,

indurated and irregularly impregnated with leucocratic quartzo-feldspatic materials. The foliation

has high angle dips of 450 - 75

0. This green coloured variety of the unit becomes less indurated

downstream where it is gradational into staurolite schist. A whitish coloured variety of the unit

occurs at the boundary between the green variety and the staurolite-bearing schist facies

(Location 6). The rock is mesocratic to leucocratic, containing mainly white feldspar, biotite,

muscovite and quartz. It occurs in a layer exposed under a bridge with thickness varying from

10m – 15m. It is NNE-SSW trending with dip of 350

to the 1000

ESE direction.

The schist is melanocratic to mesocratic and more indurated at location 11. At location 3,

the rock is leucocratic, very fine-grained, and rich in quartz and feldspars (Quartzofeldsparthic

Schist (QS)). It is highly weathered, hence very loose.

3.4 Staurolite-Mica-Schist (SMS) Exposures of this unit are most prominent at location 10. It is

exposed over a distance of 1km in a river valley. The surface of the long ourcrop is dotted with

long and slender, euhedral, staurolite crystals which are not observed inside the rocks in hand

specimens (Plates 4 a, b and c). The staurolite crystals have average length of 2.5 cm with width

of 0.3 cm. The rock weathers into silty-sand, with specks of shiny mica flakes.

36

Plate 4a: Steeply-inclined layers of the porphyroblastic Staurolite-mica schist (Location 10)

Plates 4b and 4c: Staurolite-mica schist (magnified to show the rough surface created by the presence

of the Staurolite porphyroblasts)

37

3.5 Kyanite-bearing Schist (KbS)

At locations (9 & 5), the unit is well exposed though its colour becomes more of grayish

black. The rock is more indurated than most of the other schistose rocks in the study area but

also more weathered than the Kyanite schist. This Kyanite-bearing Schist underlies the whole of

Nassarawa town and is bound to the southwest by granitic materials. Sampling of this unit was

done only on the Nassarawa River channel where it is well exposed as it runs along the river

channel.

3.6 The Kyanite Schist (KS)

This unit, as mentioned in section 3.1 above, is typically exposed in a quarry site

(Location 12) where the exposure reveals a sharp boundary between the schist and

Porphyroblastic/Augen Gneiss (Plates 5a, b). This dominantly micaceous schist is medium to

coarse-grained and unusually melanocratic with a greenish black ting. It consists dominantly of

biotite, with minor amounts of muscovite and quartz. It is occasionally cross-cut by veins of

clear subhedral quartz crystals.

3.7 The Garnetiferous Granite (GG).

At location 22, about 38 km to Keffi along Akwanga-Keffi road, to the southern side,

this unit is exposed. It is a weakly foliated granitic rock which is leucocratic with few

porphyroblasts of feldspars in a medium to coarse-grained, equigranular matrix. The matrix

consists of interlocking crystals of biotite, muscovite, quartz, garnet, and feldspar. The outcrop

rises to a height of about 100 m and over 150 m in diameter. It is flanked by another hill, which

is constituted by the same rock type to the West towards Keffi on which a Water Reservoir tank

is sited. The second hill is about 200 m high and approximately 200 m wide.

38

Plate 5a. Exposure of the Kyanite schist at the quarry site(Location 12) showing sharp contact with

the porphyroblastic gneiss. (5b). Closer observation showing the homogeneous nature of the schist,

fissility planes created by strong foliation and the dark-bluish grey surface colour as a result of

weathering; fresh surface is dark greenish black.

BB

AA

39

3.8 Simple pegmatite (SP)

This unit is exposed within Toto L.G.A. at about 1.4 km from Nasarawa/Toto

boundary post, (location 26). The rock is highly leucocratic and extremely coarse grained. Its

constituent minerals are mainly K-feldspar and Quartz, with traces of prismatic tantalite. The

outcrop is almost circular but a little elongated in the (WNW-ESE) direction. Its total length

is about 0.5 km with a width of about 100 m. It has been mined locally in some places for

tantalite. On the Keffi-Akwanga road, and very close to the Garnetiferous Granite unit of

location 22, a belt of weathered simple pegmatite runs in the North-South direction. It has

been deeply weathered leaving behind breccia-size crystals of mainly quartz and a few

feldspars in a matrix of deep reddish lateritic soil.

3.9 Complex pegmatites (CP)

These pegmatites are exposed at locations 5, 13, 16 and 23. They occur mainly as

dykes emplaced within the migmatitic banded gneiss. They have trends which are discordant

to the foliation in the host rock. They are generally extremely coarse-grained and leucocratic

and constituted by quartz, feldspars, micas, hornblende, aquamarine and tantalite. The

exposure at location 16 on the 2nd

Maloni hill otherwise known as Angwan Toni hill has a

width of 1.5 m with a trend of 850 ENE - 265

0 WSW.

At about 200m from Nasarawa/Toto boundary post, sharp contact between schist and

micropegmatite is exposed by a road cut. The pegmatite unit exposed here is very

leucocratic, coarse grained, and equigranular. It contains minerals such as, quartz, feldspars,

and muscovite almost in equal proportions. The road cut exposure at location 5 is about 0.6m

to 1m thick and 10-15m long. The strike direction was measured as 370 NE/217

0SW. Dip

direction is 3070 NW and Dip amount is 19

0.

Another outcrop of this unit, located at about 500m from Nasarawa boundary post

within Toto, is faulted. The exposure which is about 1.3m thick is a sill that is cut with a

throw of 1.1m and the materials here are more micropegmatitic.

40

This unit is also found at location 27, which is about 25 km off Angwan waje junction

in Kokona, southwards of the Akwanga-Keffi road. The exposure is very extensive on a ridge

which runs through Agwada and Igwo Local Government Areas to Erigo and Onda. The host

rock is mesocratic, medium- to coarse-grained and equigranular. The unit is deeply

weathered into a dark-grayish, sandy-clay soil which supports thicker vegetation. This flat

lying depression on top of the hill trends in the NNE-SSW direction. Cassiterite and topaz are

mined in large quantities from the areas covered by this soil.

41

CHAPTER FOUR

4.0 PETROGRAPHY

Representative hand specimens of the various rock types were selected and described

megascopically. Also, thin sections of the specimens were prepared for microscopic

descriptions using petrographic microscopes.

4.1 Porphyroblastic/Augen gneiss (PAG)

This is a medium- to coarse-grained, porphyroblastic, mesocratic, weakly foliated

rock which contains almost equal amounts of dark- and light-coloured minerals. Biotite is the

commonest dark-coloured mineral. The light coloured minerals are quartz, feldspar and

occasionally muscovite. The porphyroblasts are commonly euhedral to subhedral crystals of

feldspars and sometimes quartz in a medium to coarse-grained groundmass of mainly quartz,

feldspars and biotite. Tiny platy minerals (biotite and muscovite) define the weak foliation,

being aligned in a parallel to sub-parallel manner. The porphyroblasts and the groundmass

commonly show random orientation and interlocking relationship.Some of the

porphyroblasts of the feldspars are up to 5 cm long and 1.5 cm wide while those of quartz are

as high as 3 cm long and 1 cm wide. Occasionally elongate crystals of K-feldspar and quartz,

which are up to 2 cm long and 1 cm wide, are aligned along the foliation trend. Augen

structures constituted by quartz and feldspars are sometimes observed in this rock.

In thin section, the PAG consists of layers of micas arranged in a parallel to sub-

parallel manner alternating with layers of large crystals of quartz and feldspars. The quartz

crystals occur in mosaic form and measure up to 8 mm long. Single porphyroblasts of

microcline measure up to 1 cm long. The rock is generally nematoblastic (dominance of

prismatic minerals) in texture.

The quartz crystals are colourless and clear, lacking any form of cleavages. They

show first order polarization colours of grey to white. Some of the quartz crystals show

straight extinction while fractured and dislocated ones show undulose extinction. Conversely

42

microcline is colourless and cloudy, showing alterations to sericite. It is recognized by its

characteristic grid (cross-hatched) twinning. Plagioclase is also colourless and cloudy,

showing a high degree of alteration to the extent that its two directional cleavages and

characteristic albite twinning are almost obliterated. The alteration is mainly sericitization

and chloritization.

Biotite occurs as brown to green brown slender crystals (laths) which are strongly

pleochroic from brown to greenish brown. It contains some inclusions of light coloured

minerals and has perfect one directional cleavage. It has moderate birefringence with reddish

to yellowish brown maximum interference colour of the second order and goes into parallel

to near-parallel extinction with maximum extinction angle of about 30.

Various forms of myrmekitic and perthitic intergrowths are commonly associated

with feldspars in this rock and these include:

(1) Both bulbous myrmekite and myrmekite perthite (Plates 6 a, b, & c ).

(2) Normal myrmekite is a microstructure comprising intergrowth of plagioclase and

vermicular quartz. Bulbous myrmekite is ribbon-shaped myrmekite while myrmekite perthite

is a myrmekite-like (worm-like) intergrowth of microcline (K-feldspar) and vermicular

plagioclase.

43

S

Bm

A.

Mp

Ms.

Mc

B.

Mp

C

Mp

C

P

Mc

D

P

Mc

D

Plate 6(c) Myrmekite Perthite (Mp) in

porphyroblast gneiss from location 17.

Crossed polars (x20)

Plate 6(d) Microcline porphyroblast (Mc) with

a core of plagioclase (P) and muscovitic

alteration product in porphyroblastic gneiss

from location 17. Crossed polars (x20)

Plate 6(a) Bulbous myrmekite (Bm) and a

ribbon of sericite (S) in a microcline

porphyroblast in porphyroblastic gneiss

from location 24. Crossed polars (x20).

Plate 6(b) Myrmekite (Mp) and muscovitic

alteration (Ms) of microcline porphyroblast

(Mc), showing crossed-hatched twinning in the

background in porphyroblastic gneiss from

location 24. Crossed polars (x20)

44

Some specimens of the rock reveal evidence of fine-grained, crushed matrix of quartz,

feldspars and biotite within which the porphyroblasts of quartz and feldspars are embedded.

Others show alteration of the porphyroblasts (Plate 6d). Some sections of the matrix

represent a formerly large single grain that has been sheared and drawn-out .i.e. crushed and

distributed linearly along a preferred direction. These result in a mortar (core and mantle)

structure, and ribbon quartz microstructures.

Accessory minerals in this rock include zircon, cordierite and ilmenite (opaque).

Zircon occurs is colorless and occurs in prismatic, short, subhedral to euhedral crystals with

high relief. Some of the crystals have rhombohedral while others have pseudo-hexagonal

outlines. The crystals show partings close to the edges, occurring on its own and as

inclusions in some platy minerals. The mineral has moderate birefringence with interference

colour which is mostly mantled or obscured by the thickness of the crystals but along some

fractures, high 3rd

to 4th

order blue and yellowish green interference colours are observed. It

goes into parallel extinction.

Cordierite occurs as porphyroblasts. The modal compositions of this rock and the

others in this study are presented in Table 1.

A subunit of the porphyroblastic/Augen gneiss is the Micaceous Quartzite (MQ). It is

a fine-grained, leucocratic and foliated rock containing few porphyroblasts of feldspars and

quartz. The foliation in the rock is defined by parallel to sub-parallel alignment of biotite

(Plates 7a and 7b). The feldspars are microcline and plagioclase. The quartz crystals which

constitute 60% of the rock mostly show undulose extinction (See Table 1).

1

TABLE 1. Modal Analysis of the Rocks.

GMS KBS SMS KS BHG MBG PAG GG MQ SP CP

Min/sample no 1 2 3 4 6 7 8 11 9 10 12 13 14 15 16 17 18 19 20 21 23 24 22 25 26 5

Quartz 30 50 35 55 45 30 40 45 40 25 10 10 40 30 35 25 10 30 30 35 30 40 15 60 20 30

Plagioclase 15 40 15 5 5 10 25 20 15 25 10 30 35 40 30 10 25 10 40

Kfp/Microcline 5 15 20 30 10 10 30 20 25 15 35

Biotite 15 25 20 30 38 30 20 35 10 40 20 15 10 10 30 20 10 15 5 15 5 15

Muscovite 30 20 10 15 5 25 5 15 10 5 10 3 5 10 30

Cordierite 15 5 30 10 5

Staurolite 10 25 10 25

Sphene 2

Garnet 5 3 5 13 10 3 10 2 5 10 5 5 5 5 5 5 5

Chlorite 10 10 10 15

Sericite 10 2 5 3

Hornblende 70 15 5

Epidote 2 10

2

Zircon 5 5

Kyanite 10 25 10

Opaques 2 5 2 3 5 5 2

Total 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

3

1

4.2 Migmatitic Banded Gneiss (MBG). This medium- to coarse-grained, mesocratic to

leucocratic rock is characterized by alternating layers of dark- and light-coloured minerals,

otherwise described as a banded structure. The dark-coloured layers consist mainly of biotite

and occasional hornblende which are arranged in a parallel to sub-parallel manner. The light-

coloured layers, on the other hand contain feldspars, and quartz, with tiny specks of biotite.

The mesocratic rocks commonly contain quartz, biotite and garnet while the leucocratic ones

are constituted by quartz, microcline, plagioclase, biotite and heamatite (opaque). In the

mesocratic type, the growth of the porphyroblasts of garnet which is accompanied by volume

increase pushes away the crystals of quartz and biotite while most of the biotite crystals

around the porphyroblasts seem to have been eaten up by formation of the garnet. Some

varieties of the mesocratic type contain hornblende and sphene (See Plates 8a & 8b); they are

also characterized by muscovitic/sericitic and chloritic alterations of their feldspar

constituents. Porphyroblasts of microcline and myrmekitic intergrowths are also common in

these varieties.

The Banded Hornblende Gneiss (BHG) subunit. This is a melanocratic, medium to

coarse grained rock showing preferred orientation of the constituent minerals in parallel to

sub-parallel manner. The mineral constituents include hornblende, plagioclase, biotite and

epidote. The rock has a dominance of mafic minerals. The plagioclase feldspar has bold

albite twin laminae and maximum extinction angle of 240 corresponding to An35 (andesine).

Epidote is colourless and occurs in granular to columnar aggregates but with high

relief and fractures that makes it distinct (Plates 9a and 9b). It has moderate to strong

birefringence and the interference colors range from middle first order to upper second order

and some even show up to third order colors. The middle first order colors are anomalous

blue interference colors as shown in the plate below. Elongate sections show parallel

extinction. Epidote can be seen mantling brown hornblende and surrounded by greenish and

brown hornblende in the sections.

2

A B

Sp

A

Sp

B

Plate 7(a) Micaceous quartzite showing

alignment of biotite (Location 26). Crossed

polars (×20).

Plate 7(b) Alignment of biotite in

micaceous quartzite in plate 3(a). Plane

polarized light (x 20)

Plate 8(a) Euhedral crystal of sphene showing

parting fractures and inclusions in migmatitic

banded gneiss (Location 20). CPL(x20)

Plate 8(b) Euhedral crystal of sphene in the

migmatitic banded gneiss (location 20). PPL

(x20).

Ep

Ep

Hb

Hb

Ep

Hb

Ep

Ep

Hb

Hb

Ep

Hb

A

Hb

Hb

Hb

Ep

EP

EP

Hb

Hb

Hb

Ep

EP

EP

B

Plate 9(a) Epidote (Ep) showing both anomalous blue interference color (top right and base)

and low second order red where it mantles brown hornblende(Hb) (center) CPL. Plate 9(b) is

the PPL. (×16)

3

Hornblende is strongly pleochroic from brown to greenish brown. It also occurs in

sections showing two directional cleavages meeting at obtuse and acute angles, i.e. 1240/56

0

(Plate 10 a & b). Elongate sections showing one directional cleavage traces are present

(Plates 10c and 10d). The crystals are largely poikiloblastic and some show alteration to a

colourless or neutral mineral with high relief. It has moderate birefringence with 2nd

-3rd

order

interference colours. It goes into oblique extinction of (20-22)0 angle.

4.3 Garnet-Mica-Schist (GMS)

This is a fine to medium-grained melanocratic rock showing a well-developed

schistose foliation, which is defined by the parallel to sub-parallel arrangements of tiny flakes

of biotite and muscovite (micas), enclosing quartz and feldspars. Reddish brown garnet

(almandine) is a common mafic constituent enclosed by the phyllosilicates. As observed in

thin section, the well-developed schistose foliation is defined by platy minerals, namely:

chlorite and biotite. Porphyroblasts of garnet (almandine), staurolite and quartz are randomly

distributed within the rock. Some varieties consist of muscovite and large crystals of

feldspars which are deeply sericitzed (Plates 11a & b). Poikiloblastic kyanite crystals and

opaques are sometimes present.

Chlorite occurs mostly as elongate thin scaly aggregates with few sections which are

tabular and almost six-sided. The elongate sections show perfect one directional cleavage

traces. It is pleochroic from light greenish-brown to greenish-brown. The relief is fairly high.

It has a characteristic Berlin-blue anomalous interference colour (Plates 11c and d) which is

common in a variety known as penninite. The extinction is parallel to the cleavage traces.

4

A

Hb

AA

Hb

B

Hb

Ms

B

Hb

Ms

C

Hb

Hb

BtBt

C

Hb

Hb

Bt

C

Hb

Hb

BtBt

Hb

D

Bt

Hb

Hb

D

Bt

Hb

Plate 10(a & b) Basal sections of hornblende (Hb) and Muscovite (Ms) in Banded

Hornblende Gneiss (Location 13). Crossed polars (×10)

Plate 10(d) plane polarized light for the

hornblende in plate 10(c) (x8).

Plate 10(c) Exinction position of longitudinal

section of hornblende and its Basal section

(Hb), and Biototite (Bt). Crossed polars (x8)

5

A B

Pn

Q

C

Pn

D

Q

Pn

Q

E

Plate 11(c) Berlin-blue

interference colour of pennine

(chlorite) (Pn) in garnet-mica-

schist from location 1. CPL (x24)

Plate 11(d) Pennine

(chlorite) PPL (x24)

Plate 11(e) Normal view of

the thin section in plate

11c. Crossed polars (x4).

Plate 11(a and b). Alteration of feldspars into sericite in Garnet-mica-schist. (Location 1 and 2).

Crossed polars (×12)

6

Sericite is a silky-tread-like mineral which is colourless with low relief. It has moderate

to high birefringence and interference colours of the middle to upper second order. It

commonly occurs in irregular matted, felt-like shreds within and/or in place of coarse

feldspar crystals. Within these matted, felt-like shreds occur very tiny crystals showing

maximum 2nd

order interference colours, much like tiny muscovite. The recognition of relicts

of partially altered feldspar and muscovite crystals closely associated with sericite is clear

evidence that it is an alteration (replacement) product from these minerals.

Garnet has neutral colour and high relief. It occurs in subhedral to euhedral hexagonal

crystals. Some crystals are variously fractured and riddled with inclusions of light and dark-

coloured minerals. It has low birefringence and is isotropic (that is, becomes totally dark

under crossed nicols) (See Plates 11f and 11g). Porphyroblasts of garnet appear to have

forced their way through a maze of platy minerals (micas) which seem to be giving way to

the growing porphyroblasts.

Tiny recrystallized muscovitic minerals also appear to be aligned in one direction.

The Quartzo-feldspathic Schist (QS) subunit is a very fine-grained mesocratic to leucocratic

rock in which the crystals, seem to have been crushed or mylonitized, so that the only very

small, anhedral crystals of quartz are recognizable, even in thin section. It shows weak

alignment of dark-coloured minerals. An unidentifiable mineral has been totally crushed and

distributed in-between the quartz crystals. Under crossed polars, the mineral looks more like

altered feldspar with numerous tiny inclusions of sericitic minerals; it is cloudy and

colourless to neutral in plane polarized light.

7

StGt

MsF

Bt

Plate 11(f) Poikiloblastic porphyroblasts of Staurolite (St) and garnet (Gt)enclosed by an aggregate of

muscovite (Ms) and biotite (Bt) crystals). (×60) (Location 6).

GtSt

Bt

Ms

G

Plate 11(g) Poikiloblastic porphyroblasts of Staurolite (St) and garnet (Gt) in Plate 11f. Plane

polarized light (x60).

8

4.4 Staurolite-Mica-Schist (SMS). This rock is dense, very fine- to fine-grained,

melanocratic and schistose. The schistose foliation is imparted by parallel to sub-parallel

arrangements of the micas (biotite and muscovite) which are not segregated from the felsic

components (quartz and feldspars). The rock occasionally contains large porphyroblasts of

quartz which are irregularly arranged while the flaky minerals (muscovite and biotite) cluster

in some roughly parallel bands which are in association with a denser, very fine to glassy

melanocratic material resembling iron ore.

In thin section, this dominantly schistose rock contains bands of light coloured

minerals which are not completely segregated from the dark-coloured schistose fabrics. The

minerals include biotite, muscovite, quartz, staurolite, kyanite, garnet and cordierite.

Staurolite crystals are mostly poikiloblastic, with inclusions of mainly quartz. (Plates 12a &

12b). The basal sections are mostly isotropic although some show low interference colours.

Skeletal staurolite crystals form bands in the schistose layers (Plates 12 c & d).

4. 5 Kyanite Schist (KS)

This is a medium grained, melanocratic rock, with well-developed schistose foliation

constituted by a dark, greenish coloured micacous mineral (biotite). Biotite, quartz and

kyanite make up over 70% of the rock. In thin section, the rock is a coarse-grained

melanocratic rock, which has pronounced schistose foliation. The rock contains 80% mafic

platy minerals giving it a lepidoblastic texture. The mineral constituents include biotite,

muscovite, kyanite, quartz, microcline, plagioclase and garnet.

Kyanite is light brownish to pale blue in colour and weakly pleochroic. It occurs in

elongate plates that are tabular. The sections have high relief and show perfect to imperfect

one directional cleavages. The birefringence is moderate to low, giving the mineral first

order interference colours which range up to a maximum of red (Plates 13a & 13b). Most of

the sections show extinction angles of about 300 or more but some others show even parallel

extinction.

9

St

A

St

B

St

St

C

St

D

St

Ky

A

Ms

Ky

B

Ms

Plate 12(c) Bands of skeletal crystals of

Staurolite(St) in Staurolite-kyanite-garnet-

cordierite-mica-schist (location 5). Crossed

polars (x 20)

Plate 12(d) Skeletal Staurolite(St) in plate

12(c). Plane polarized light (x 20).

Plate 12(a) Poikiloblastic Staurolite (St)

Crystals in Staurolite-garnet-mica-schist

(location 10). Crossed polars (x20)

Plate 12(b) Staurolite (St) in Plate 12(a)

showing inclusions of quartz. Plane

polarized light (x20)

Plate 13(a) Broad, columnar kyanite crystal

(Ky) in Kyanite-schist (location 12). Crossed

polars (x 20)

Plate 13(b) Kyanite (Ky) in plate 13(a).Plane

polarized light (x 20)

10

4.6 The kyanite-beaing Schist (KbS).

This unit is mesocratic in colour and medium-grained. It is highly indurated relative

to other schistose units so that it provides good surfaces for laundry purposes for the local

porpulace. The mineral constituents include Biotite, Muscovite and quartz. The unit contains

fewer amounts of kyanite (10%), and more quartz, biotite and muscovite relative to the

Kyanite Schist above. (See table 1 no. 9)

4.7 Garnetiferous Granite.

This is a leucocratic, coarse-grained rock containing a large proportion of quartz and

feldspar (plagioclase and microcline) with subordinate specks of biotite, muscovite, tiny

crystals of garnet and cordierite. The rock is generally inequigranular and shows interlocking

of the constituent minerals.

Cordierite occurs in large somewhat rounded hexagonal crystals which are colourless

and show high relief. It lacks visible cleavage traces but possesses numerous inclusions.

Some of the inclusions are surrounded by pleochroic haloes. It has low birefringence with

maximum interference colour of first order grey. Zoning and twinning are also present

(Plates 14a & 14b). Some portions of the mineral has been altered to silky-tread-like mineral

which is colourless with low relief and whose extinction angle could not be determined due

to its irregular-coiling nature its birefringence is moderate to high and it has a maximum

interference colour of the middle to upper second order. This alteration product is sericite.

Plagioclase occurs in colourless and cloudy, rectangular subhedral crystals. The

crystals are twinned according to the albite law and have low birefringence with first order

grey interference colour. The maximum extinction angle of the crystals is 50

corresponding

to An10-30 (oligoclase)

11

Cd

A

Cd

B

Plate 14(b) The Zoned and twinned

cordierite (Cd) in 14(a) plane polarized light

(x16)

Plate 14(a) Zonning and twinning in

cordierite (Cd) in garnetiferous cordierite-

biotite-muscovite granite (location 22).

Crossed polars (x16)

12

The plagioclase is partly sericitized. It possesses few inclusions of sericitic minerals with

a cloudy surface portions, it has cleavage traces which is faint but in one direction. It has a low

birefringence which gives it first order grey interference colour.

Muscovite differs from biotite in its colourless nature and low relief. It is platy in form

and subhedral to euhedral in shape. It shows cleavage traces in one direction; some crystals

contain inclusions, while some others are show some degree of alteration. It has moderate to

high birefringence giving it maximum interference colours of the upper second order. It goes

into parallel to near-parallel extinction with maximum angle of 30. Twinning according to the

mica law is observed in some crystals.

4.8 Pegmatites (Simple and complex).

These are leucocratic, extremely coarse-grained rocks showing random orientation and

interlocking relationship of the constituent minerals, which are generally very large crystals

of quartz, orthoclase and plagioclase feldspars and biotite. Some varieties (complex

pegmatites) contain muscovite, lepidolite, aquamarine, cassiterite and topaz. Most of the

cyrstals are 1.5 to 2 cm long while some are as large as 4 cm long and as small as 0.5 cm

long. Most of the quartz and feldspar crystals do not have smooth surfaces; that is to say,

they are subhedral to anhedral. The simple pegmatites contain mainly quartz and feldspars.

The minerals show a very high degree of alteration.

13

CHAPTER FIVE

5.0 GEOCHEMISTRY OF THE ROCKS

5.1 Analytical Procedure

Twenty (20) representative samples of the different lithologic units and subunits were

analyzed. Samples numbered 1, 2, 3, 6, 7, 8 and 11 are representative of garnet-mica schists; 10

is staurolite-mica schist while 12 is kyanite schist. Samples 14 and 15 are representative of the

migmatitic banded gneiss while 13 is banded hornblende gneiss. Samples 16, 20, 21, 23 and 24

are representative of the porphyroblastic/augen gneiss while sample 22 is granite; Sample 25 is

micaceous quartzite; 26 is simple pegmatite while samples 27 and 28 are complex pegmatites.

The sample numbering follows the outcrop numbering in Figs. 6 and 7.

The major element oxides, the trace- and rare-earth element (REE) compositions were analyzed

using the Inductively Coupled Plasma (ICP) methods at the Activation Laboratories (ACT

LABS) in Canada. Powdered samples of the rocks were sent to the ACTS LABS by courier

service.

5.2 Results

5.2.1 Major elements geochemistry

The major element oxide compositions of the rocks are presented in Table 2. The samples

of the migmatitic banded gneiss and porphyroblastic/augen gneiss (14, 15, 16, 20, 24 and 25)

have SiO2 values that range from 67.11 to 74.21% with a mean of 71.12%. They show moderate

Al2O3 (13.24-15.20%), low TiO2 (0.176- 0.935%), MgO (0.24- 1.28%), MnO (0.011- 0.052%)

and high K2O (3.15- 5.56%), Na2O (2.6- 4.21%) and CaO (0.9- 3.59%).

These values with the mean values of the garnet-mica schists, the pegmatites (samples

26, 27, 28), the hornblende banded gneiss (sample 13), the kyanite schist (sample 12) and the

granite (sample 22) are presented in Table 3.

14

Table 2: Major element oxide (wt %) compositions of the rocks from Keffi and its environ.

1 2 3 6 7 8 10 11 12 13 14 15 16 20 22 24 25 26 27 28

SiO2 72.7 74.1 63.7 75.1 61.1 73.8 69.56 65 56.8 52.7 69.16 68.72 67.4 74.2 74.2 71.9 72.26 70.6 78.78 67.21

Al2O3 12.7 12.1 16.1 10.9 16.9 12 14.99 15.1 8.45 13.5 14.43 14.47 14.5 13.2 14.4 15.2 15.14 15.6 11.57 17.2

Fe2O3t 5.02 4.41 7.07 3.97 7.37 4.08 4.82 6.11 12.4 10.7 4.09 3.72 4.26 2.69 1.43 1.88 1.16 0.12 0.93 0.85

MnO 0.14 0.08 0.11 0.17 0.11 0.13 0.048 0.13 0.12 0.19 0.052 0.034 0.03 0.04 0.03 0.05 0.011 0 0.017 0.04

MgO 1.94 1.84 3.37 1.39 3.12 1.46 1.31 2.57 8.61 6.85 1.05 1.13 1.28 0.65 0.24 0.53 0.46 0.02 0.01 <0.01

CaO 1.66 1.06 1.68 1.55 1.66 3.3 1.19 2.03 6.52 10.6 1.69 3.26 3.59 1.59 0.91 1.68 1.31 0.09 0.08 0.11

Na2O 3.52 3.02 3.66 3.45 2.26 2.16 0.51 4.1 0.59 2.75 3.34 3.21 3.96 2.6 3.44 3.58 4.21 1.03 2.95 2.76

K2O 1.67 2.09 3.06 2.13 4.1 2.2 2.77 1.76 4.19 0.67 5.55 4.54 3.15 4.63 5.56 5.26 5.01 11.5 4.11 10.19

TiO2 0.73 0.63 0.94 0.8 0.96 0.65 0.61 0.87 0.67 0.95 1.875 0.935 0.66 0.36 0.18 0.3 0.249 0 0.079 0.023

P2O5 0.17 0.13 0.19 0.26 0.22 0.18 0.85 0.18 0.21 0.1 0.33 0.23 0.2 0.14 0.14 0.17 0.15 0.63 0.02 0.31

LoI 0.73 0.56 0.85 1.22 1.33 1.03 1.83 1.38 1.17 0.61 0.31 0.48 0.79 0.4 0.43 0.4 0.38 0.31 1.22 0.51

Total 101 100 101 101 99 101 98.5 99.4 99.7 99.7 100.9 100.7 99.8 101 101 101 100.3 99.9 99.76 99.22

Fe203t/K20=a 3.01 2.11 2.31 1.86 1.88 1.85 1.74 3.47 2.96 16 0.737 0.82 1.35 0.58 0.26 0.36 0.23 0.01 0.23 0.08

SiO2/Al2O3=b 5.72 6.12 3.96 6.89 3.62 6.15 4.64 4.3 6.72 3.9 4.8 4.75 4.65 5.62 5.15 4.73 4.79 4.53 6.8 3.91

Na2O/K2O=c 2.11 1.45 1.2 1.62 0.55 1.03 0.18 2.33 0.14 4.1 0.6 0.71 1.26 0.56 0.62 0.68 0.84 0.09 0.72 0.27

log(a) 0.48 0.32 0.36 0.27 0.27 0.27 0.24 0.54 0.47 1.2 -0.13 -0.08 0.13 -0.25 -0.58 -0.44 -0.64 -2 -0.64 -1.09

log(b) 0.76 0.78 0.6 0.84 0.56 0.79 0.67 0.63 0.83 0.59 0.68 0.68 0.67 0.75 0.71 0.67 0.68 0.66 0.83 0.59

log( c) 0.32 0.16 0.08 0.21 -0.25 0.01 -0.75 0.37 -0.85 0.61 -0.22 -0.15 0.1 -0.25 -0.21 -0.2 -0.07 -1.04 -0.14 -0.57

Fe2O3t/Al2O3 0.4 0.36 0.44 0.36 0.44 0.34 0.32 0.41 1.47 0.79 0.28 0.26 0.29 0.2 0.1 0.12 0.08 0.01 0.08 0.05

Table 3. Comparison of the mean values of major element oxide compositions of the migmatitic

banded gneiss (MBG), porphyroblastic/Augen gneiss (PAG), garnet-mica schist (GMS) and

pegmatities (PG) with those of the banded hornblende gneiss (BHG), Staurolite mica schist (SMS),

kyanite schist (KS) and garnetiferous granite (GG).

MBG(mean of 2) PAG(mean of 3) GMS (mean of 7) PG (mean of 3) BHG SMS KS GG

SiO2 68.94 71.2 69.35 72.21 52.65 65.04 56.81 74.15

Al2O3 14.45 14.32 13.68 14.78 13.54 15.12 8.45 14.44

Fe2O3(t) 3.91 2.94 5.43 0.63 10.73 6.11 12.34 1.43

MnO 0.043 0.037 0.12 0.02 0.191 0.128 0.117 0.027

MgO 1.09 0.82 2.24 0.013 6.85 2.57 8.61 0.24

CaO 2.48 2.29 1.85 0.09 10.62 2.03 6.52 0.91

Na2O 3.28 3.38 3.16 2.24 2.75 4.1 0.59 3.44

K2O 5.05 4.35 2.43 8.61 0.67 1.76 4.19 5.56

TiO2 1.41 0.44 0.796 0.034 0.95 0.867 0.673 0.176

P2O5 0.28 0.17 0.166 0.32 0.1 0.18 0.21 0.14

Loi 0.395 0.53 1.014 0.68 0.61 1.38 1.17 0.43

Total 100.8 100.48 100.27 99.63 99.66 99.37 99.7 100.9

15

From their mean values, the garnet-mica schist, migmatitic banded gneiss, granite and

pegmatites all show saturation with respect to silica (i.e. 69.37%, 71.12%, 74.15%, and 72.21%,

respectively).Their K2O-content increases from the garnet-mica schist to the pegmatites (i.e.

2.47%, 4.69%, 5.56% and 8.61%, respectively). The total iron oxide values show a reverse order

(i.e. 5.36%, 2.96%, 1.43% and 0.63%, respectively). MgO also decreases in that order (i.e.

2.13%, 0.85%, 0.24% and 0.013%, respectively).

The banded hornblende gneiss and the kyanite schist show much less saturation with

respect to silica content (i.e. 52.65 and 56.81%), with much higher total iron oxide (10.73 and

12.34%), MgO (6.85 and 8.61% ) and CaO (10.62 and 6.52% ) relative to the garnet-mica schist,

migmatitic banded gneiss, granite and pegmatites. Expectedly, these trends show that the rocks

attained different levels of fractionation, with the banded hornblende gneiss and kyanite schist

being most unfractionated while the pegmatites are the most fractionated.

On the TiO2 versus SiO2 diagram of Rahesh and Santosh (2004) (Fig. 7), the rocks show

a wide spread on both the igneous and sedimentary fields. The banded hornblende gneiss, the

kyanite schist and pegmatites 26 & 28 are clearly of igneous protolith. The schist samples 3, 7,

10 and 11, and the gneisses 16, 24 & 25, that plot close to the boundary line, give ambiguous

results because their petrography depicts a pelitic origin. The migmatitic banded gneiss samples

14 & 15 expectedly plot in the sedimentary field while pegmatite sample 27 is also indcated as

having a sedimentary protolith.

In the K2O-Na2O-CaO diagram (Fig. 8), the rocks generally show moderate to high alkali

values. Whereas the schists except the staurolite mica schist (sample 10) and the kyanite schist

(sample 12) show a modest tendency towards Na2O enrichment; the gneisses with the exception

of the banded hornblende gneiss(sample 13), the granite and the pegmatites have a greater K2O

enrichment; the pegmatites(samples 26 and 28) have absolute K2O enrichment. Conversely, the

kyanite schist (samples 12) and the banded hornblende gneiss (sample 13) show enrichment in

CaO-content relative to K2O and Na2O, which suggests a basic nature of these two rocks.

16

Sedimentary

Igneous

13

73

11

12 1610

24

2528

15

14

16

32

20

2227

50 60 70 80

0

1

2

3

TiO

2

50 60 70 80SiO2

0

1

2

3

Fig 7. TiO2 versus SiO2 (binary) diagram after Rajesh and Santosh (2004) for the rocks in Keffi and its

environ.

25

28

2722

10

14 207

2425

15

23 12

156 8

1

11

0 20 40 60 80 100

CaO

100

80

60

40

20

0

K2O

100

80

60

40

20

0

Na2O0 20 40 60 80 100

100

80

60

40

20

0100

80

60

40

20

0

Fig. 8. K2O-Na2O-CaO (ternary) diagram of Olade and Elueze(1979) for the rocks in Keffi and its environs.

17

In Mgo-CaO-Al2O3 (ternary) diagram of Leyreloup et. al. (1977) used for the

determination of the protoliths of metamorphic rocks (Fig. 9), 85% of the rocks plot in the

sedimentary field. The garnet-mica schist (sample 8), the migmatitic banded gneiss (15) and the

porpyroblastic/augen gneiss (16) plot almost on the magmatic/sedimentary boundary within the

magmatic rocks field while the kyanite schist (12) and the banded hornblende gneiss (13) plot

within the basalt field. This ternary diagram proves further that samples 12 and 13 have basic

igneous protoliths. Samples 3, 7 and 11 (GMS) which ploted in the igneous field on the TiO2 vs

SiO2 diagram, now plot high in the sedimentary field of the MgO-CaO-Al2O3 ternary. Only

sample 16 has persisted on the igneous side of the igneous/sedimentary boundary.

Coincidentally, this sample 16 is the porphyroblastic/augen gneiss hosting the kyanite schist

(sample 12) (see Fig.6)

In the plot of log (Fe2O3 (t)/K2O) vs log (SiO2/Al2O3) (Fig. 10), the migmatitic banded

gneisses and porphyroblastic/augen gneisses with the exception of sample 20 plot in the

greywacke region. The banded hornblende gneiss (13) plots in the Fe-shale region. The garnet-

mica schist (3, 7, and 11) and the staurolite-mica schist (10) plot in the shale region while the

other samples of the garnet-mica schist (1, 2, 6, and 8) and the kyanite schist (12) plot in the

greywacke region. It is shown in this diagram that the sedimentary protoliths of the rocks of the

study area are dominantly greywacke followed by arkose and then shale.

The major chemical difference between the meta-pelites (meta-shales) and the meta

greywackes as observed in this study is shown in their major element geochemistry where the

greywackes (samples 1, 2, 6 and 8) show higher silica content relative to the shales (samples 3,

7, 10 and 11) and lower Al2O3, Fe2O3(t), and Mg relative to the shales.

18

100

80

60

40

20

0

MgO

0 20 40 60 80 100

Al2O3

100

80

60

40

20

0

CaO

12

13

8 16

15

3711 2

10

2225

24

1461

20

27/28

Magmatic

Rocks Field

Basalt

Field

Sedimentary

Field

Fig.9. MgO-CaO-Al2O3 (ternary) diagram of the rocks in Keffi and its environs (after Leyreloup et. al. 1977)

0 0.5 1 1.5 2 2.5L og (SiO2/Al2O3 )

-2

-1

0

1

2

LOg

(Fe2

O3/

K2O

)

13

11

3

7

1 12

28

316

10

15

14

20

24 22

25

26

28

Fe-shale

Fe-sand

ShaleLitharenite Sub-Litharenite

Quartz-Arenite

A rkose Sub-Arkose

Greywacke

Fig. 10 Plot of Log [Fe2O

3 (t)/K

2O] versus Log (SiO

2/Al

2O

3) (after Herron 1988).

19

5.2.2. Trace- and rare-earth-element geochemistry

The trace-element compositions of the rocks are presented in Table 4 while rare-earth

element (REE) compositions are presented in Table 5. Table 6 has been included for comparison

of the trace-elements and REE contents of the rocks in this study with those of geological

materials including; chondrite, crust, ultrabasic, basalt, granite, greywacke and shale.

Ba is high in almost all the rocks, with the gneisses containing the highest mean value

(782.5 ppm). This is followed by concentration of Ba in the kyanite schist (sample 12). Be is

extremely high (2097 ppm) in the complex pegmatites, but is almost absent in the rest of the

samples. Cr, Co, Zn and Ni are present in considerable amounts in the kyanite schist with

concentrations of 680, 38, 600 and 300 respectively, as well as in the banded hornblende gneiss

(sample 13) with concentrations of 170, 42, 100 and 90 respectively. These suggest high basicity

of the samples (Carr and Turekian, 1961). Conversely, Cr, Co and Ni are generally low in the

gneisses, granite and pegmatites. Snelling (1958) ascribed a sedimentary origin to an amphibolite

which contained 10ppm Cr, 5ppm Ni, and 20ppm Co. These values are very low in comparison

with basic rocks, and are consistent for the three elements. Taylor (1955) concluded from the Cr,

Ni, Co and V content of greenschist bands in quartz-albite-biotite schists, that they were

originally basic sills intruded into greywacke sediments

The rare-earth-element concentrations in rocks of Keffi and its environment compare

closely with those of the crustal and shale concentrations. All the schists with the exception of

the kyanite schist show good correlation with crustal concentrations. The kyanite schist shows

twice the concentration of the light rare-earth- elements (LREE) and half the concentration of the

heavy rare-earth-element (HREE), compared to the crustal concentrations, suggesting that the

Kyanite schist is more fractionated and therefore of a different and more basic protolith. The

REE data are normalized to the chondrite composition given by Nakamura (1974) with additions

from Haskin et. al. (1968) and used to plot REE patterns (Figs. 11 and 12).

20

Table 4. Trace element compositions (ppm) of rocks in Keffi and its environ.

S. No 1 2 3 6 7 8 10 11 12 13 14 15 16 20 22 24 25 26 27 28

Sc 12 11 18 10 20 10 11 15 8 43 5 2 3 6 2 2 <1 <1 <1 4

Be 2 1 1 1 2 2 5 2 7 1 4 2 6 2 5 6 7 7 7 2097

Ba 329 400 359 508 802 418 426 392 619 200 503 1055 598 834 415 953 752 145 23 75

Sr 321 134 213 116 128 158 89 222 237 241 205 326 376 203 84 328 380 40 4 37

Y 32 25 34 36 35 29 26 30 17 29 31 7 4 50 17 14 2 <2 33 <2

Zr 221 174 207 342 219 219 199 221 145 47 263 212 232 289 128 140 142 11 267 11

Cr 100 70 100 120 100 40 50 110 680 170 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20

Co 13 10 22 10 18 11 11 17 38 42 9 9 9 4 1 3 2 <1 <1 <1

Ni 30 30 60 20 60 <20 20 40 300 90 <20 <20 <20 <20 <20 <20 <20 <20 <20 <20

Zn 90 80 130 70 120 70 90 110 600 100 80 110 240 50 60 70 50 <30 50 160

Rb 67 64 126 83 124 117 135 64 175 8 199 141 114 180 300 293 196 >1000 455 >1000

Sn 2 2 3 2 3 3 100 3 6 <1 7 3 5 3 9 3 3 11 >1000 5

Hf 6.9 5.2 6.4 9.9 6.9 6.7 6.3 7 4.4 1.9 8.2 6.5 7.8 17 4.7 5.1 4.9 0.3 19 0.3

Ta 0.7 0.6 1 0.6 0.8 0.9 1.3 0.7 0.2 0.2 2.5 0.8 0.6 0.8 1.9 1.8 0.5 0.9 29 1

Pb 9 29 24 13 12 18 24 15 30 8 32 26 25 45 39 39 35 20 49 38

Bi <0.4 0.5 <0.4 <0.4 <0.4 1.8 0.7 <0.4 0.9 0.6 <0.4 <0.4 1.6 <0.4 <0.4 2.3 1.2 <0.4 <0.4 0.4

Cs 3.9 4.1 7.9 2.7 7 7.4 36.8 3.9 25.8 0.9 4.9 6.5 7.2 6.7 10.1 36 7.9 259 9.1 265

Th 8.7 6.8 9.9 7.9 9.6 10.5 11.4 7.4 13.8 2.3 19.1 9.8 13 22.4 23.6 16.5 17 <0.1 65.1 0.2

U 2.2 2 2.9 1.8 2.8 7.3 2.7 2 3 0.9 3.2 2 3 6 5.3 2.9 5.4 0.1 8.2 3

Th/U 3.95 3.4 3.4 4.39 3.43 1.44 4.22 3.7 4.6 2.56 5.97 4.9 4.33 3.73 4.45 5.69 3.15 1 7.94 0.067

K% 1.39 1.74 2.54 1.77 3.4 1.83 2.29 1.46 3.48 0.56 4.61 3.77 2.62 3.84 4.62 4.37 4.16 9.56 3.41 8.46

K/Rb 206 271 202 213 275 156 169 228 198 695 232 267 229 214 154 149 212 95 75 84

Table 5. Rare-earth element compositions (ppm) of rocks from Keffi and its environs.

S. No 1 2 3 6 7 8 10 11 12 13 14 15 16 20 22 24 25 26 27 28

La 28 22.9 34.1 30.4 30.8 32.3 30.5 27.8 52 8.4 55.2 40.8 42.2 35.3 40.9 33 29.3 0.5 28 0.2

Ce 58.6 46.8 70 59.4 63.8 65.8 57.6 57.3 121 18.2 108 67.8 61.6 75.1 83.8 70.4 75.4 0.8 44.7 0.4

Pr 6.76 5.43 8.25 7.03 7.41 7.32 6.84 6.78 14.8 2.26 11.8 8.49 9.11 8.54 9.09 6.45 6.81 0.1 6.39 <0.05

Nd 26.9 20.4 31.5 26.5 28.2 27.3 24.1 25.2 56.1 11 43.6 29.8 32.1 32.8 31.3 21.3 22.6 0.4 23.3 <0.1

Sm 5.9 4.5 7.1 5.8 6.5 5.9 5.1 5.8 12.5 3.2 9.1 6.4 6.3 7.3 6.5 4.4 4.3 <0.1 5.1 <0.1

Eu 1.37 1.21 1.66 1.34 1.5 1.4 0.97 1.53 2.42 1.17 1.4 1.44 1.48 1.04 0.54 0.95 0.97 <0.05 0.31 <0.1

Gd 5.6 4.3 6.6 5.6 63 5.3 4.7 5.5 9.8 4 7.9 4.7 4.1 7.3 4.7 3.2 2.1 0.1 4.4 <0.1

Dy 5.7 4.3 6.1 5.7 6.3 5.4 4.7 5.5 4.9 5.2 6.7 2.3 1.4 8.5 3.5 2.4 0.8 <0.1 5.1 <0.1

Ho 1.2 0.9 1.2 1.2 1.3 1.1 0.9 1.1 0.7 1.1 1.2 0.3 0.2 1.8 0.6 0.4 0.1 <0.1 1.1 <0.1

Er 3.5 2.5 3.6 3.8 3.8 3.3 2.9 3.4 1.4 3.4 3.2 0.6 0.3 5.4 1.7 1.3 0.3 <0.1 3.5 <0.1

Yb 3.5 2.4 3.4 4.3 3.5 3.4 2.9 3.2 0.7 3.2 2.6 0.3 0.1 4.9 1.4 1.1 0.3 <0.1 4.7 <0.1

Lu 0.54 0.35 0.48 0.65 0.52 0.51 0.41 0.48 0.1 0.47 0.38 <0.04 <0.04 0.7 0.2 0.16 <0.04 <0.04 0.72 <0.04

21

Table 6. The abundances of trace-elements and REE contents in geological materials

(chondrite, crust, ultrabasic, basalt, granite, greywacke and shale), after Taylor (1962). For

comparison with those of the rocks from Keffi and its environs.

Chondrite Crust Ultrabasic Basalt Granite Greywacke Shale

Li 2.7 20 - 10 30

Rb 2.3 90 - 30 150

Cs 0.18 3 - 1 5

Ti 0.15 0.45 0.05 0.1 0.75

Be 1 2.8 - 0.5 5

Be 5 10 5 5 15

Sr 10 375 1 465 285 Chondrite Crust Ultrabasic Basalt Granite Greywacke Shale

V 70 135 50 250 20 70 130

Cr 2500 100 2000 200 4 140 100

Co 800 25 150 50 1 20 20

Ni 1.34% 75 2000 150 0.5 50 70

Th 0.03 10 0.003 2.2 17 10 12

U 0.01 2.7 0.001 0.6 4.8 3 4

Zr 35 165 50 150 180 140 160

Hf 0.2 3 0.5 2 4 2 3

Nb 0.5 20 15 20 20 20 20

Ta 0.04 2 1 0.5 3.5 2 2

Mo 1.5 1.5 0.3 1 2 - 3

W 0.14 1.5 0.5 1 2 2 2

Sc 11 16 10 38 5 10 15

Y 2 30 - 25 40 10 25Chondrite Crust Ultrabasic Basalt Granite Greywacke Shale

La 0.39 30 3.3 10.5 25 - 20

Ce 1.05 60 8 35 46 - 50

Pr 0.15 8.2 1.02 3.9 4.6 - 6

Nd 0.62 28 3.4 17.8 18 - 24

Sm 0.21 6 0.57 4.2 3 - 6

Eu 0.072 1.2 0.16 1.27 - - 1

Gd 0.27 5.4 0.65 4.7 2 - 6

Tb 0.049 0.9 0.088 0.63 0.05 - 1

Dy 0.32 3 0.59 3 0.5 - 5

Ho 0.079 1.2 0.14 0.64 0.07 - 1

Er 0.22 2.38 0.36 1.69 0.2 - 2

Tm 0.037 0.48 0.053 0.21 - - 0.2

Yb 0.19 3 0.43 1.11 0.06 - 3

Lu 0.03 0.5 0.064 0.2 1.01 - 0.5Chondrite Crust Ultrabasic Basalt Granite Greywacke Shale

Cu 90 55 10 100 10 40 50

Ag 0.09 0.07 0.06 0.1 0.04 0.05 0.05

Au 0.1 0 0.005 0 0.004 0.004 0

Zn 35 70 50 100 40 50 100

Cd 0.5 0.2 - 0.2 0.2 0.2 0.2

Hg 8 0.08 - 0.08 0.08 0.08 0.5

22

.

Fig. 11. Chondrite-normalised rare –earth-element patterns for the schists (1-12) and the

banded hornblende gneiss (13).

Fig 12. Chondrite-normalized rare-earth-element patterns for the migmatitic banded

gneiss and porphyroblastic/augen gneiss (14-25), granite (22) and pegmatites (26-28) from

Keffi and its environs.

23

All the gneisses and the granite except the banded hornblende gneiss (sample 13), have

values (slightly greater than or equal to) crustal values of the LREE and slightly less than or

equal to crustal values of the HREE. The REE compositions of the banded hornblende gneiss

compares well only with those of ultrabasic compositions. This also suggests that the banded

hornblende gneiss has ultrabasic protolith but may have undergone little fractionation to the

basic composition as shown by the MgO-CaO-Al2O3 ternary diagram.

The pegmatites (samples 26 and 28) show absolutely low concentrations in both the

LREE and HREE relative to crustal values while sample 27, shows conformity with crustal

abundances of the REE. In terms of Cr, Co and Ni, the kyanite schist (sample12) shows basic to

ultrabasic chemistry while the banded hornblende gneiss has basic chemistry Taylor (1955).

The chondrite-normalized REE patterns of all the schists, gneisses, granite and the

pegmatite (27) (Figs. 11 and 12) are characterized by enrichment in the light rare-earth-elements

relative to the heavy rare-earth-elements, with over 70 times chondrite value for (La) and over 50

times chondrite value for (Ce). This indicates a high level of fractionation, hence high

fractionation indices. They also show negative (Eu) anomaly which is more pronounced in the

gneisses and granite relative to the schists (compare Fig. 11 with Fig. 12). The negative Eu-

anomaly indicates plagioclase fractionation with the highest fractionation in the gneisses and the

granite. They all show flat HREE trends, with the values for the schists between 10 and 20 times

chondrite and those for the gneisses between 0 and 25.

The banded hornblende gneiss (sample 13) (Fig. 11) shows an almost flat-lying REE pattern with

slight LREE enrichment and flat HREE trend which is essentially 15 times chondrite. The

kyanite schist (sample 12) shows the highest LREE enrichment of all the samples analyzed. Its

HREE content is the lowest of all the schists, with a sloping trend.

Pegmatites 26 and 28 also show flat-lying REE trends. Their REE contents are

approximately equal to chondrite values so that sample/chondrite ~1 for all the REE.

24

CHAPTER SIX

6.0 DISCUSSION

The study area is a typical example of the Precambrian Basement Complex of Nigeria as

it is underlain by all the rocks of the Basement Complex including migmatitic banded gneisses,

porphyroblastic/augen gneiss and banded hornblende gneiss, representing the migmatite-gneiss

complex or Basement sensu stricto, garnet-mica-schist, staurolite-mica schist and kyanite schist,

representing the Schist belt and granite, representing the Older or Pan-African granite suite, as

well as simple and complex pegmatites.

The general trend of foliations is in the N-S to NNE-SSW directions. This foliation trend

coincides with the structural trend imprinted by the Pan-African ~ (600Ma) orogeny according to

McCurry (1971).

The study area is dominantly underlain by gneisses; the porphyroblastic/augen gneiss

covers the northern and southwestern parts while the migmatitic banded gneiss underlies the

central and the southeastern parts. The schists on the other hand are enclosed within the gneisses

in the central portions of the study area.

Outcrops of the schists all lie in synformal troughs which run generally in the NNE-SSW

orientation, which is the general trend of the Nigerian Schist belts as recorded by earlier workers,

namely: Russ (1957), Truswell and Cope (1963), Oyawoye (1964), McCurry (1971,1973, 1976),

Ogezi (1988), Grant (1978), Ajibade et. al.(1979), Ajibade (1980), Rahaman (1976, 1978, 1998),

Annor et al. (1996), Annor, (1998) and Caby (1989), Caby and Boesse, (2001).

Outcrops of the pegmatites encountered in the study area trend mainly in the E-W

directions, and occationally N-S and NE-SW. The predominant cross-cutting trend indicates that

the pegmatites occur mainly as dykes and that they may have multiple origins. Similar features

are known in pegmatites from Wamba area according to Matheis (1987) and Kuster (1990). In

terms of rare-metals enrichment, the barren varieties of pegmatites from all the investigated

25

localities in Nigeria are found to have K/Rb ratios above 100 and Rb values below 500 ppm

while the mineralized ones have K/Rb <100 and Rb values >500 ppm ( Stavrov et. al., 1969,).

Based on this, the pegmatites in this study can be classified as mineralized as seen in samples 26,

27 and 28 ,which have K/Rb ratios of 95, 75 and 84 and Rb values (>1000, 455 and >1000 ppm),

respsctively. They are mineralized with Sn (>1000 ppm in sample 27) and Be (2097 ppm in

sample 28).

From the petrography, the minerals that are grouped to make up the different assemblages

include; quartz, plagioclase, K-feldspar/microcline, biotite, muscovite, chlorite, garnet, staurolite,

cordierite, kyanite, sphene, zircon and opaques (see Table 1).

The minerals, namely; chlorite, biotite, garnet, staurolite, cordierite and kyanite are

typical of meta-pelites (meta-shales) whereas plagioclase and microcline are common in meta-

arkoses and meta-greywackes. Quartz is ubiquitos in metamorphic rocks. The major difference

between the meta- pelites and the meta-greywackes as observed in this study is shown in their

major element geochemistry. The meta-greywackes (samples 1, 2, 6 and 8) have higher silica

content (72.7-75.1) relative to the meta-pelites (samples 3, 7, 10 and 11) whioch have values

ranging between (61.6-69.56)and lower Al2O3, Fe2O3(t), and Mg relative to the shales.

This metasedimentary (shale-greywacke-arkose) gneisses and schists in the study are also

confirmed by the plots of log (Fe2O3/K2O) vs log (SiO2/Al2O3) in Fig.10.

The mineral assemblage of quartz + biotite + garnet (almandine) + K-feldspar + plagioclase +

chlorite + zircon + opaque in the migmatitic banded gneiss represents medium-grade (lower

amphibolite facies) condition. The index assemblage (garnet + biotite + plagioclase) has stability

ranges of 550-600 0C and pressures of 3-10 Kb (Bucher and Frey, 1994). The presence of K-

feldspar in the absence of muscovite is expected at temperarures ≥ 700-750 °C if Al2SiO5

(kyanite/sillimanite) was present; otherwise K-feldspar could be suspected to be detrital. The

presence of chlorite suggests a retrograde condition. The assemblages of porphyroblastic/augen

26

gneiss represent middle to upper amphibolite facies conditions of metamorphism with

temperatures of 600-700 0C and pressures of 3-10 Kb based on the stability of the index

assemblage, namely: garnet (almandine)+ biotite + cordierite. However, the presence of K-

feldspar and plagioclase might be indicative of the fact that the conditions reached the upper

amphibolite facies metamorphism (670-700)0C (Massone and Schreyer, 1987).

The assemblage of chlorite + biotite + garnet (almandine) + staurolite + quartz +

muscovite in the garnet-mica-schists represents middle amphibolite facies conditions based on

the index assemblage; garnet + staurolite + biotite. These are stable at temperatures of 600-670

0C and pressures of 3-10 Kb. The assemblage staurolite + quartz + muscovite + cordierite +

biotite + chlorite in the staurolite-mica-schist could also be formed at temperatures of about 600-

670 0C and pressures of 3-10 Kb in the mid-amphibolite facies metamorphism while the

assemblage biotite + kyanite + muscovite + quartz + garnet (almandine) + K-feldspar +

plagioclase in the kyanite schist represents upper-amphibolite facies metamorphism with

temperatures of 670-700 0C and pressures of 4 -7 Kb because of the index minerals, biotite +

garnet + kyanite (Massone and Schreyer, 1987; Barker 1998 ). The presence of kyanite suggests

higher pressure condition than in the rest of the assemblages.

The metamorphic conditions attained by the rocks in the study are similar to those of rocks in

other parts of the Nigerian Basement Complex. According to Dada (2006), the metamorphic

imprints in the rocks of the Nigerian schist belts are reflected in their mineralogy and to a lesser

extent in the granitoids, and this shows heterogeneity of metamorphic activity. The author further

stated that the major Pan-African deformations took place under medium to high amphibolite

facies metamorphism, producing in general a bimodal distribution of metamorphic facies. These

are the greenschist to almandine amphibolite facies in the meta-sediments on one hand and the

upper amphibolite to granulite facies in the gneisses. The greenschist facies is indicated by the

presence of chlorite while biotite + garnet + plagioclase ± staurolite define the almandine

amphibolite facies in the meta-sediments (Dada, 2006). In the Tilde-Fulani areas (Jos Plateau),

27

the garnet+ biotite pairs of the meta-pelites gave temperatures ~ 620 °C and 700 °C for cores and

rims respectively for pressure fixed at 5Kbar (Ferry and Spear, 1978) while the garnet–cordierite

thermometry indicates an equilibrium temperature around 650 °C (Harley et. al., 2002). Elueze

(1981) recorded assemblages of quartz + muscovite + chlorite + biotite, indicative of the upper

greenschist facies and biotite + garnet + staurolite, indicative of middle amphibolite facies in the

rocks of the Ilesha Schist belt. According to Dada (2006), the greenschist facies is indicated by

the presence of chlorite while biotite + garnet + plagioclase ± staurolite, define the almandine

amphibolite facies in the meta-sediments of the Precambrian Basement Complex of Nigeria. For

the schists to have reached amphibolite facies metamorphism, might be indicative of a drier

condition of metamorphism relative to the gneisses and a strong pointer to different episodes of

metamorphism.

The Migmatitic Gneiss Complex, shows higher metamorphic grades with sillimanite and

sometimes kyanite in the mineralogy (McCurry, 1976), with most assemblages reflecting

staurolite- almandine sub-facies of the amphibolite facies (Rahaman, 1976). This is typical for

the rocks of the study area. In the Northwest Akwanga areas, however, the petrography of the

pelitic sections revealed kyanite and sillimanite assemblages for the Sabon-Gida augen gneisses

indicating that the highest grade of the Barrovian type metamorphism has been attained

(Onyeagocha, 1983).

The mineral assemblage of hornblende, epidote, plagioclase and quartz in the banded

hornblende gneiss suggests that it is a metabasic rock formed at the amphibolite facies. This

basic nature of the rock is confirmed by its major and trace element geochemistry. The

migmatite-gneiss complex of the Nigerian Basement Complex is known to have mafic to

ultramafic components; including amphibolites, biotite and biotite-hornblende schists (Oyawoye,

1965; Cooray, 1974; Rahaman, 1976, 1988; Rahaman and Ocan, 1978).

There seems to be agreement in the progressive nature of the metamorphism from the works of

McCurry (1976) and Rahaman (1976). Whereas Annor et al. (1996) and Annor, (1998) recorded

28

retrograde metamorphism in the Egbe-Isanlu and the Okene-Igarra Schist belts, both prograde

and retrograde metamorphic assemblages and microstructures have been recorded for the rocks

in this study. For instance, the presence of myrmekitic intergrowths, which are commonly

associated with the breakdown of feldspars in the gneisses is also evidence that the rocks were

affected by a retrograde metamorphism. The occurrence of augen structures, paleosomes, and

abundant quartzo-feldspathic veins and dykes suggests that the gneisses were affected by partial

melting (anatexis), which commonly occurs at the highest grade of metamorphism. The growth

of garnet with inclusions of muscovite, and staurolite with the inclusions of quartz (Plate 11f)

and the growth of staurolite (Plates 12 a & b) are some of the microstructures indicating prograde

metamorphism while the sericitization of plagioclase crystals (Plates 11 a & b), the mantling of

hornblende by epidote (Plates 9 a & b) and the myrmeckitic structures (Plates 6 a, b and c) are

indicative of retrograde metamorphism. The myrmekitic intergrowths, together with chloritic and

muscovitic alterations and sericitization in the gneisses suggest that retrograde metamorphism

was superimposed on these high amphibolite facies grade rocks.

The similarity in the major element oxides, trace- and rare-earth-elements compositions

of most of the rocks in this study with crustal compositions is strong indication that they are

crustally-derived. The ubiquitos LREE enrichment, suggesting high fractionation in the schists

and gneisses, coupled with pronounced negative Eu-anomalies suggests that their sedimentary

progenitors were derived from a cratonic source (Taylor and McLennan, 1985; Das et. al., 2008).

In this case, the cratonic source is the West African craton.

29

CHAPTER SEVEN

7.0 SUMMARY AND CONCLUSION

Gneisses are the most dominant suite of rocks in the study area and all elevated parts of

the area are conspicuously underlain by these gneisses, which are subdivided into

porphyroblastic/augen gneiss and the migmatitic banded gneiss units.

The schists occupy the central parts of the study area and are second most dominant rocks

in the area. They are notably found within the low-lying regions and valleys, especially along the

river valleys and the entire schist outcrops lay in a NNE-SSW trending trough. This is also true

for the general foliation trend and also the fold axes.

The schists are mostly fine to medium grained while the gneisses are medium to coarse grained.

The varieties of the schists are garnet-mica-schists, staurolite-mica-schist and kyanite schist.

From their mean values, the garnet-mica schist, migmatitic gneiss, granite and

pegmatites all show saturation with respect to silica (i.e. 69.37%, 71.12%, 74.15%, and 72.21%,

respectively). Their K2O-content increases from the garnet-mica schist to the pegmatites (i.e.

2.47%, 4.69%, 5.56% and 8.61%, respectively).

The total iron oxide values show a reverse order (i.e. 5.36%, 2.96%, 1.43% and 0.63%,

respectively). Magnesia also decreases in that order (i.e. 2.13%, 0.85%, 0.24% and 0.013%,

respectively). The banded hornblende gneiss and the kyanite schist show much less saturation

with respect to silica content (i.e. 52.65 and 56.81%), with much higher total iron oxide (10.73

and 12.34%), MgO (6.85 and 8.61% ) and CaO (10.62 and 6.52% ) relative to the garnet-mica

schist, migmatitic banded gneiss, granite and pegmatites.

The metamorphic Basement Complex rocks in this study originated from sedimentary

protoliths of shale, greywacke and arkose compositions. The salient differences between the

meta-pelites (meta-shales) and the meta-greywackes as observed in this study is shown in their

major element geochemistry where the meta-greywackes (GMS no. 1, 2, 6 and 8, KS no. 12 and

30

PAG no. 20) show higher silica content relative to the meta-shales (BHG no.13, GMS no.3, 7

and 11, and SMS no.10) and lower Al2O3, Fe2O3 (t), and Mg relative to the meta-shales. The lack

of igneous trend both in the ternary and binary diagrams of the rocks in this study is also a strong

pointer to their sedimentary origin.

Mineral assemblages recorded in the rocks of Keffi area and its environ indicate that the

conditions of metamorphism range from the lower, through middle, to upper amphibolite facies.

Features that suggest partial melting are also present. Retrograde conditions were imposed on

these high-grade rocks. These conditions of metamorphism have also been recorded in similar

rocks of the Precambrian Basement Complex in other parts of Nigeria.

The major element oxides, trace- and rare-earth-elements compositions of the rocks suggest that

their progenitors were derived from a cratonic/crustal source; in this case, the West African

craton.

31

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