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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)
3
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).
6
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.
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
10
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).
11
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.
12
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
13
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
14
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
15
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).
16
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:
17
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.
18
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.
19
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
20
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
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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