Chapter 1 The Basement Complex

The basement complex is one of the three major litho-petrological components that make up the geology of (Fig. 1.1). The Nigerian basement complex forms a part of the Pan-African mobile belt and lies between the West African and Congo Cratons (Fig. 1.2) and south of the Tuareg Shield (Black, 1980). It is intruded by the calc-alkaline ring complexes (Younger Granites) of the Jos Plateau and is unconformably overlain by and younger sediments. The Nigerian basement (Fig.1.3) was affected by the 600 Ma Pan-African and it occu- pies the reactivated region which resulted from plate collision between the passive continental margin of the West African craton and the active Pharusian continental margin (Burke and Dewey, 1972; Dada, 2006). The basement rocks are believed to be the results of at least four major orogenic cycles of deformation, metamor- phism and remobilization corresponding to the Liberian (2,700 Ma), the Eburnean (2,000 Ma), the Kibaran (1,100 Ma), and the Pan-African cycles (600 Ma). The first three cycles were characterized by intense deformation and isoclinal folding accompanied by regional metamorphism, which was further followed by extensive migmatization. The Pan-African deformation was accompanied by a regional meta- morphism, migmatization and extensive granitization and gneissification which pro- duced syntectonic granites and homogeneous (Abaa, 1983). Late tectonic emplacement of granites and and associated contact metamorphism accompanied the end stages of this last deformation. The end of the orogeny was marked by faulting and fracturing (Gandu et al., 1986; Olayinka, 1992) Within the basement complex of Nigeria four major petro-lithological units (Fig.1.4; Explanatory Note 2) are distinguishable, namely:

1. The – Complex (MGC) 2. The Belt (Metasedimentary and Metavolcanic rocks) 3. The Older Granites (Pan African granitoids) 4. Undeformed Acid and Basic Dykes

N.G. Obaje, Geology and Mineral Resources of Nigeria, Lecture Notes 13 in Earth Sciences 120, DOI 10.1007/978-3-540-92685-6 2, C Springer-Verlag Berlin Heidelberg 2009 14 1 The Basement Complex

° 5 E 10°

C N I G E R Lake Chad H A D Sokoto IN o S A B TO O K Kano CHAD (BORNU) BASIN SO

Maiduguri n i

s

a

b Kaduna b Bauchi su

a l 10° Minna o E Jos g U n N M E B ID B Go R ID -N E A I Abuja P la subbasin N G P Yo B E U I A R / SI N Ilorin N Lafia E NU E E B LE B DD ue MI en Lokoja . B R. R N a Ibadan iger

br Makurdi

n

i

m N Akure s a LOWER a O BF n Lagos B BENUE O A Benin N R IG Enugu C E E O R o M DE Abakaliki M Warri PL LT A EX A ATLANTIC C ° OCEAN CF 5 N Calabar Portharcourt 200 km

Tertiary - Recent sediments Younger Grantes Tertiary volcanics Basement Cretaceous Major (reference) town BF Benin Flank CF Calabar Flank

Fig. 1.1 Geological sketch map of Nigeria showing the major geological components; Basement, Younger Granites, and Sedimentary Basins

The Migmatite – Gneiss Complex (MGC)

The Migmatite – Gneiss Complex is generally considered as the basement complex sensu stricto (Rahaman, 1988; Dada, 2006) and it is the most widespread of the component units in the Nigerian basement. It has a heterogeneous assemblage com- prising , orthogneises, paragneisses, and a series of basic and ultrabasic metamorphosed rocks. Petrographic evidence indicates that the Pan-African rework- ing led to recrystallization of many of the constituent minerals of the Migmatite – Gneiss Complex by partial melting with majority of the rock types displaying medium to upper facies metamorphism. The Migmatite – Gneiss Com- plex has ages ranging from Pan-African to Eburnean. The Migmatite – Gneiss Complex (MGC) 15

10°W 0° ADRA DES 10°E MAURITANIDE IFORAS AIR

BELT T A O U D E N I SENGAL IULLMEDDEN Dakar BASIN B A S I N GOURMA BASIN Niamey Banjul

Bamako Ouagadougou BOVE Bissau BASIN SWELL

NIGERIA

Conakry BELT 10°N CRATION 10°N GUINEA VOLTA BIDA BENIN Freetown BASIN BASIN AFRICAN Rokelide belt TOGO TOGO WEST RISE TROUGH Monrovia Lagos BENUE

Accra NIGER 0 500 km Abidjan DELTA Dahomey Basin 10°W Ivory Coast Basin 0° 10°E Key volcanics Pan African (c. 550 Ma) Mesozoic-Quaternary sedimentary sediments rocks of the basins (mainly ) metamorphic rocks anorogenic Infracambrain-Palaeozoic (c. 2000 Ma) of the swells Mesozoic granites igneous rocks sediments

Atchaean (c. 2500 Ma) Mesozoic dolerites

Fig. 1.2 Generalized geological map of Nigeria within the framework of the geology of West- Africa (Adapted from Wright, 1985)

° 5 E 10°

C N I G E R H A D

Gusau Kano NORTH CENTRAL o NIGERIA BASEMENT

Zaria o Kaduna Bauchi

10° Minna Jos ADAMAWA Abuja HIGHLAND N R. N I ig er N Ilorin

E e Lafia u en B B R. Lokoja Ibadan Makurdi EASTERN NIGERIA Takun Akure Gembu BASEMENT WESTERN NIGERIA N O BASEMENT O R ATLANTIC E OCEAN M 5°N Calabar A 200 km C OBAN MASIF

Fig. 1.3 Basement Geology of Nigeria 16 1 The Basement Complex

Fig. 1.4 Basement Geology of Nigeria: The Migmatite-Gneiss Complex (mgn), the Schist Belts (sb) and the Older Granites (og) (Modified from Wright, 1985)

The Migmatitie-Gneiss Complex also termed by some workers as the “migmatite-gneiss-quartzite complex” makes up about 60% of the surface area of the Nigerian basement (Rahaman and Ocan, 1978). These rocks record three major geological events (Rahaman and Lancelot, 1984); the earliest, at 2,500 Ma, involved initiation of crust forming proceses (e.g. the banded Ibadan grey gneiss of mantle origin) and of crustal growth by sedimentation and orogeny; next came the Eburnean, 2,000 ± 200 Ma, marked by the Ibadan type granite gneisses; this was followed by ages in the range from 900 to 450 Ma which represent the imprint

A. Migmatite-Gneiss Complex (Migmatites, Gneisses, Granite -Gneisses)

B. Schist Belts (Metasedimentary and Metavolcanic rocks) (Phylites, , , Quartzites, Marbles, )

C. Older Granites (Pan African Granitoids) (Granites, Granodiorites, , Monzonites, Gabbro, )

D. Undeformed Acid and Basic Dykes Explanatory Note 2: (-, tourmaline- and beryl-bearing pegmatites, Components of the Basement applites and dykes; basaltic, doleritic and lampropyric complex dykes) The Migmatite – Gneiss Complex (MGC) 17 of the Pan-African event which not only structurally overprinted and re-set many geochronological clocks in the older rocks, but also gave rise to granite gneisses, migmatites and other similar lithological units. The close analogy in time with the development of the Birrimian of the West African Craton is striking. However, although gold, manganese and iron mineral deposits are associated with Birrimian rocks, the same age rocks in Nigeria are very sparsely, if at all, mineralized. The extent of Eburnean and older rocks in Nigeria is not known. Definite geochemi- cal evidence for the existence of these rocks exists for the area south of latitude 9◦N (Rahaman and Lancelot, 1984). Lithologically similar rocks in other parts of Nigeria, especially in the northeast and southeast, have given only Pan-African ages (Tubosun, 1983). Many areas in northern, western and eastern Nigeria are covered by rocks of the Migmatite – Gneiss Complex (Figs. 1.3, 1.5, 1.6, and 1.7). These areas include, but not limited to: Abuja, Keffi, Akwanga, Bauchi, Kaduna, Kano, Funtua, Okenne, Egbe, Ajaokuta (in northern Nigeria); Ibadan, Ile-Ife, Akure, Ikerre, (in western Nigeria) and Obudu and the Oban Massif areas in eastern Nigeria.

Fig. 1.5 Baement Complex outcrops: 30 km north of Bauchi on the Bauchi – kari road. Basement Complex rocks are generally low lying and concordant in contrast to the cross-cutting, discordant, and steeply occurring hilly outcrops in the form of ring dykes and cone sheets in the Younger Granites 18 1 The Basement Complex

Fig. 1.6 Abuja, the capital city of Nigeria built on Basement complex rocks: Northern view from the 6th floor of Nicon Luxury Hotel, Garki

Fig. 1.7 Keffic in Nasarawa State; an ancient town on the Basement complex (Road from NRDC to Emir’s Palace). Inset: A migmatitic gneiss sample collected from Basement complex rocks in Keffi The Schist Belt (Metasedimentary and Metavolcanic Rocks) 19

The Schist Belt (Metasedimentary and Metavolcanic Rocks)

The Schist Belts comprise low grade, metasediment-dominated belts trending N–S which are best developed in the western half of Nigeria (Fig. 1.8). These belts are considered to be Upper Proterozoic supracrustal rocks which have been infolded into the migmatite-gneiss-quartzite complex. The lithological variations of the schist belts include coarse to fine grained clastics, pelitic schists, phyllites, banded iron formation, carbonate rocks (marbles / dolomitic marbles) and mafic metavolcanics (amphibolites). Some may include fragments of ocean floor material from small back-arc basins. Rahaman (1976) and Grant (1978) for example suggest that there were several basins of deposition whereas Oyawoye (1972) and McCurry (1976) consider the schists belts as relicts of a single supracrustal cover. Olade and Elueze (1979) consider the schist belts to be fault-controlled rift-like structures. Grant (1978), Holt (1982) and Turner (1983), based on structural and lithological asso- ciations, suggest that there are different ages of sediments. However, Ajibade et al. (1979) disagree with this conclusion and show that both series contained identical deformational histories. The structural relationships between the schist belts and the basement were considered by Truswell and Cope (1963) to be conformable meta- morphic fronts and it was Ajibade et al. (1979) who first mapped a structural break.

Fig. 1.8 Schist belt localities within the context of the Geology of Nigeria (After Woakes et al., 1987) 20 1 The Basement Complex

The geochronology of the schist belts remains problematical although the ages of the intrusive cross-cutting Older Granites provide a lower limit of ca 750 Ma. A Rb/Sr age of 1,040 ± 25 Ma for the Maru Belt phyllites has been accepted as a metamorphic age by Ogezi (1977). The schist belt rocks are generally considered to be Upper Proterozoic. The geochemistry of the amphibolite complexes within the schist belts has also led to controversy. Klemm et al. (1984) have concluded that the Ilesha belt may be an Archaean greenstone belt. Olade and Elueze (1979), Ogezi (1977) and Ajibade (1980) have favoured dominantly ensialic processes in the evolution of the schist belts while Ajayi (1980), Rahaman (1981) and Egbuniwe (1982) have stressed that some include oceanic materials with tholeiitic affinities. Some metallogenetic fea- tures of the schist belts are relevant to these problems; the apparent absence of sub- duction related mineral deposits may be indicative of a limited role for the ensimatic processes; the distribution of primary gold occurrences in some belts but its marked absence in others may indicate that they do not represent a single supracrustal sequence. The schist belts are best developed in the western part of Nigeria, west of 8◦E longitude, though smaller occurrences are found to the east but only spo- radically. The belts are confined to a NNE-trending zone of about 300 km wide (Fig. 1.9). The area to the west of this zone is made up of gneisses and migmatites that constitute the Dahomeyan of Burke and Dewey (1972). Similarly to the east, no schist belts are known for a distance of 700 km until in Cameroun where a num- ber of schist belts, considered to be Upper Proterozoic, occur in the Pan-African granite-migmatite terrain north of the Congo Craton. The schist belts have been mapped and studied in detail in the following localities: Maru, Anka, Zuru, Kazaure, Kusheriki, Zungeru, Kushaka, Isheyin Oyan, Iwo, and Ilesha where they are known to be generally associated with gold mineralization.

Case Studies on Schist Belts (Derived Mainly from Turner, 1983)

The Kusheriki Schist Group A key for the interpretation of the Nigerian schist belts is the district around Kusheriki in northern Nigeria. Detailed mapping was carried out by Truswell and Cope (1963) and extended southwards to the Zungeru area by Ajibade (1980). Four formations were recognized for the Kusheriki Schist Group, namely:

1. The Kusheriki Psammite Formation at the base of the succession, 2. The Kushaka Schist Formation, 3. The Zungeru Formation and 4. The Birnin Gwari Schist Formation at the top.

The Birnin Gwari Schist Formation and the underlying quartzo-feldspathic rocks of the Zungeru Granulite Formation together form a single structural unit, termed the The Schist Belt (Metasedimentary and Metavolcanic Rocks) 21

5°

7 6 4 5 o Kano 8 3 2

10° 10°

R Niger 12

Benue R 9 10 11

Lagos

0 500 km

5° West African Craton Pan African Province VOLTAIAN BUEM DAHOMIDES BIRRIMIAN GREENSTONE BELTS ATACORAN

GNEISS, MIGMATITE, GRANITE SCHIST BELTS

GNEISS, MIGMATITE, GRANITE

Fig. 1.9 Schist Belt localities in Nigeria within the context of the regional geology of parts of West Africa (After Wright, 1985). 1. Zungeru-Birnin Gwari, 2. Kusheriki-Kushaka, 3. Karaukarau, 4. Kazaure, 5. Wonaka, 6. Maru, 7. Anka, 8. Zuru, 9. Iseyin-Oyan River, 10. IIesha, 11. Igara, and 12. Muro Hills

Zungeru-Birnin Gwari Schist Belt. This is a simple N–S syncline, 150 km long, with the northern part displaced dextrally by a NE–SW transcurrent fault. The Zungeru Granulite Formation outcrops on both flanks of the schist belt. It is largely made up of fine-medium grained quartzo-feldspathic rocks which are interbedded with amphibolites and some quartzites. 22 1 The Basement Complex

The Birnin Gwari Schist Formation occupies the synclinal axis of the schist belt. The lower part consists of finely banded phyllites in the west and higher grade -muscovite schists in the east. They are overlain by the Durimi pebbly schist, a metamorphosed mudstone conglomerate containing some beds of impure quartzite or metagreywacke. The Kushaka Schist Formation forms a number of curving schist belts, separated by domes and anticlines of gneiss. They contrast with the Zungeru-Birnin Gwari belt in lithology, structure and igneous associations. The main rock type is semi-pelitic biotite-muscovite schist, in places containing and . Other rocks are phyllites, metasiltstones and graphitic schists. Several thick units of banded garnet- grunerite iron formation are interbedded with the schists. A variety of amphibo- lites and amphibole, epidote, chlorite and talc-bearing schists correspond at least partly to tholeiitic (Elueze, 1981). Amphibolites are locally very thick, sug- gesting large volcanic accumulations. The curving Kushaka Schist Belt contrasts with the straight Zungeru-Birnin Gwari Belt. Grant (1978) described smaller scale structures in the Kushaka Schist Formation which demonstrate its longer and more complex history. Another important difference between the Zungeru-Birnin Gwari and Kushaka Schist Formations is in their relationship with Pan-African granite plutons. Although granites intrude both the Zungeru and Birnin Gwari Formations, they penetrate into the marginal part of the Birnin Gwari schist belt only; the cen- tre is free from granite intrusions. In contrast the Kushaka schist belts are invaded extensively by plutons of granite, and syenite, which often penetrate the axial zone of the belts. Ages in the Kusheriki Schist Group have been estimated to be in the Kibaran (1,159 ± 70 Ma)

The Karaukarau Schist Belt East of the Kushaka schist belts, migmatites and gneisses form a zone nearly 50 km wide, bounded in the east by the Karaukarau Schist Belt. This consists mainly of muscovite and muscovite-biotite schists and phyllites interbedded with thin quartzites (McCurry, 1976). The politic rocks include minor graphitic and felds- pathic schists, and contain frequent quartz and quartz-tourmaline veins. Interbedded quartzites are generally thin, but may be grouped to give sections which are dom- inantly quartzite, and units of well-bedded quartzite, several tens of metres thick, also occur. Minor rocks are spessartite quartzite, calc-silicate rocks and anthophyl- lite cordierite schist. Amphibolites form occasional discontinuous bands, but the largest occurrence is only approx. 12 m thick. The Karaukarau belt has been com- pared lithologically with the Kushaka schist belts (Grant, 1978), but there are impor- tant differences: the quartzites are detrital and not chemically precipitated iron-silica formations; and rocks of basic igneous composition are much less important. The rocks represent a fairly well differentiated sequence of muds and fine-grained sands. Structurally, the Karaukarau belt appears similar to the Zungeru-Birnin Gwari Schist Belt, matching it closely in size and form. Ages in the Karaukarau schists have been estimated to be Pan-African. The Schist Belt (Metasedimentary and Metavolcanic Rocks) 23

The Kazaure Schist Belt The Kazaure Schist Belt is situated northeast of the Karaukarau belt. It is dominated by massive quartzites which can be traced for 90 km south from the Niger border. Associated rocks, exposed in the north, are schists and metaconglomerates, the lat- ter containing rounded deformed pebbles and cobbles of quartzite and schist in a matrix of ferruginous schist. A possible extension to the Kazaure Schist Belt is seen about 100 km to the northeast in the Damagaram area of southern Niger Republic. Here, thick quartzites with associated schists occur as long curving ridges, showing a more open style of folding than the quartzites of the Kazaure belt which have lin- ear outcrops with some moderately tight fold closures. Correlation with the Kazaure belt is uncertain. The presence of conglomerates and thick quartzites suggests a con- tinental or littoral environment, which is consistent with its position at the eastern margin of the region containing Upper Proterozoic metasediments. It seems struc- turally simple, free from central granite intrusions and from mafic igneous rocks. Therefore, it is provisionally correlated with the Birnin Gwari Schist Formation and the Karaukarau belt rather than the Kushaka belt. Pan African ages have been esti- mated for the Kazaure schists by Turner (1983).

The Maru Schist Belt The Maru Schist Belt lies 200 km NE of the Kushaka schists, with which it cor- responds quite closely in lithological assemblages. In the Maru belt, pelitic rocks are dominant, mainly as phyllites and slates interlaminated with siltstones. Banded iron formation, containing magnetite, hematite and garnet is also present. Impure micaceous quartzites occur near the eastern margin of the belt. Mafic volcanic rocks are represented by the amphibolites at several localities. The fine-grained laminated sediments, both pelites and iron formation, indicate quiet water conditions; the pre- dominance of iron oxides suggests oxygenated waters, although sometimes pyrite occurs, indicating anoxic conditions. Metasandstones were deposited in a higher energy environment, reflecting shallowing water or increased sediment supply. The lithological similarities between the Maru and Kushaka schists suggest that the two could be correlated. Like the Kushaka belts, the Maru Schist Belt also contains internal plutons of granite, granodiorite and syenite. There is, however, an important difference in structure: the Maru belt shows little of the complexicity and variable fold trends shown by the Kushaka belt. It is a straight NNE-trending belt with a steeply dipping foliation which is axial planar to tight folds and deformed by later crenulation cleavages. Kibaran ages have been estimated for the Maru Schist Belt.

The Anka Schist Belt This belt lies west of the Maru belt, the two being separated by the Pan-African Mai- inchi granodiorite and by a probably older gabbro-granite-pegmatite complex. The rocks contrast with those of the Maru belt and include metaconglomerates, sand- stones, slates, phyllites and acid volcanic rocks (Holt, 1982). Metaconglomerates 24 1 The Basement Complex form several units with thicknesses reaching 150–250 m, but which die out later- ally and are interbedded with feldspathic metasandstones. They contain rounded to angular boulders and pebbles composed of granite, quartzite, quartz, phyllite and volcanic rocks. In a partly faulted outlier near Sado, east of the main belt, green and purple grits are interbedded with and siltstones which show fine cross bed- ding and ripple marks (Turner, 1983). In the western part of the belt, phyllites are dominant with some metasiltstones and metasandstones, and rhyolitic to dacitic vol- canic rocks. The coarse clastic sediments were deposited in a much higher energy environment than the pelites and iron formation of the Maru belt and belong to a more active tectonic setting. The laminated and ripple-marked sediments in the east are shallow-water deposits; purple colouring suggests drying and oxidation. A pos- sible environment is an intermontane or rifted basin. The Anka belt includes both Kibaran and Pan-African elements. A possible Kibaran igneous feature is the meta- morphosed complex of mafic and ultramafic rocks and very extensive epidotised granite and pegrnatite which extends for 100 km along the east side of the Anka belt. Ogezi (1977) discussed the possibility that an ophiolite relic may be represented in the ultramafic rocks, which form a very minor outcrop near Sado, together with the amphibolites which have a tholeiitic composition. The Anka belt also includes a post-orogenic element of Pan-African age; the unmetamorphosed volcanic and sed- imentary rocks of the Maradun and Kiserni areas which rest unconformably on the schists and granites and are dated provisionally at 516 ± 20 Ma (McCurry, 1976).

The Zuru Schist Belt This is the largest of the northern schist belts with a length of 280 km and a maximum width of 40 km. Its geology is little known, except for work in the NE sector (McCurry, 1976). The main rock type is quartzite, locally feldspathic, interbedded with quartz – muscovite schists. Structures are complex, varying widely in dip and strike. Low angle dips of bedding relative to open E–W folds, upon which steep N–S structures have been superimposed. There is a marked discontinuity across the boundary with the Anka belt with its straight steep structures, although at the boundary the two belts show parallel trends and the rela- tions between the Zuru quartzitas and the Anka pelites are not known. There is little evidence on the age of the Zuru schist belt although the complex structures suggest correlation with the Kushaka schist belts and therefore possibly of a Kibaran age.

The Iseyin-Oyan River Schist Belt This large arcuate schist belt is wrapped around the nucleus of Archaean and Lower Proterozoic rocks centred on Ibadan. It is composed mainly of mica schists. Quartzites occur near the margins of the belt in the Iseyin district. Banded amphibole schists are widespread, mainly as thin discontinuous sheets, and have been inter- preted as metamorphosed calcareous sediments; more massive amphibolites may represent mafic igneous rocks (Jones and Hockey, 1964). Metamorphism is higher The Schist Belt (Metasedimentary and Metavolcanic Rocks) 25 in grade than in most northern schist belts, with pelitic rocks containing biotite, garnet, staurolite and locally sillimanite (Rahaman, 1976). The outstanding feature of the Iseyin-Oyan River Schist Belt is the remarkable crowding of granite plutons within the schist belt and at its margins, in contrast to their virtual absence from the surrounding migmatite-gneiss terrain. The most abundant granite type, occurring as well defined intrusions, is the porphyritic potassic granite which is the major variety of Pan-African granite throughout Nigeria. Associated with these are two large plu- tons of mafic potassic syenite (Oyawoye, 1972; Rahaman, 1976). The Iseyin-Oyan River belt is generally poorly exposed, and structural data are sparse. Kibaran ages have been obtained in rocks of the Isheyin-Oyan River Schist Belt.

The llesha Schist Belt The Ilesha Schist Belt lies east of the Ibadan Archaean to Lower Proterozoic mas- sif. It has a N–S length of over 200 km and reaches its maximum width of 60 km in the south. Here it consists of two structural units with contrasting lithology, sepa- rated by the NNE-trending Ife fault zone (Hubbard, 1975). The western unit consists of amphibolite, amphibole schists and pelitic schists with much intimately associ- ated trondhjemitic granite, gneiss and pegmatite. It shows a moderately open style of folding with N–S axes. Metamorphism is mainly in the amphibolite facies, but locally in the greenschist facies. East of the fault, quartzite is dominant, occurring together with quartz schist, quartzo-feldspathic gneiss and minor iron-rich schists and quartzites. This assemblage, named the Effon Psammite Forrnation, shows amphibolite facies metamorphism and tight isoclinal folds. About 30 km NE of Ile- sha it apparently overlies amphibole schists of western type, although it is not known whether this is a stratigraphic superposition or an overthrust relationship. Geochem- ically, the western amphibolites resemble low potassium tholeiites although some show evidence of minor crustal contamination or metasomatic alteration (Olade and Elueze, 1979). Associated talc-tremolite rocks represent metamorphosed ultra- mafic minor intrusions or lavas. A deformed and altered mafic to ultramafic com- plex adjacent to the fault zone may be an ophiolite fragment. This association of mafic and ultramafic rocks with metamorphosed granitic rocks and extensive peg- matites is similar to that found in the Anka meta-igneous complex in north-western Nigeria. Granite-gneiss at Ife, west of the fault zone, gave a Rb/Sr isochron age of 1,190±140 Ma (Grant et al., 1972).being therefore Kibaran, correlating with the Kushaka and Maru belts in the north.

Igarra Schist Belt The most easterly schist belts in south-western Nigeria are distributed around the Okene migmatitic nucleus. It trends NNW with a length of only 50 km and in the west is joined to the NW-trending Owo belt and to the Itobe belt in the east which may have extended into the Muro Hills in the north. The presence of both calcare- ous rocks and conglomerates sets it apart from the other schist belts described. These 26 1 The Basement Complex rock types, together with quartzites, occur as bands in the dominant biotite schists. Gneisses at the margins of the belt may be a highly metamorphosed basal part of the sequence; they are equivalent to the non-migmatitic gneisses described by Furon (1960) from the margins of other schist belts in this eastern region. The main struc- ture is an open synform, but this re-folds earlier E–W folds. Porphyritic granites intrude the centre and margins of the belt. Similar lithologies occur in schist belts in the Kabba, Jakura and Lokoja areas which also have associated granite plutons. They represent well-sorted shelf and littoral deposits. Their diverse structural trends and association with Pan African granites had indicated a Kibaran age.

Comments on the Kibaran and Pan African

The Kibaran Orogeny The Kibaran Orogeny is recognised in several regions of Africa. It is named from the Kibara Mountains of eastern Zaire (now Democratic Reublic of Congo), and the age of the orogeny ranges from 1,300 Ma for the main tectonic phase to 900 Ma for the emplacement of late granites and pegmatites. Together with the parallel and proba- bly contemporaneous Irumide belt of Zambia, the Kibarides have been interpreted as ensialic, floored by and terminated within an older continental basement. Also of Kibaran age is the Namaqua-Natal belt of southern Africa; although this shows evidence for ocean floor obduction and continental convergence (Turner, 1983). In contrast to these well-defined orogenic belts, Kibaran relics in West Africa are scattered, discontinuous and generally overprinted by Pan-African structures and magmatism. Dated and inferred Kibaran rocks occur in widely separated areas of the Pan-African Province; in the Hoggar, in NW and SW Nigeria, and north of the Congo Craton in Cameroun. There is thus, in part, a coincidence between the Kibaran and Pan-African orogenic belts in West Africa. At Ibadan, 60 km to the southwest schist belts, Archaean and Lower rocks contain no isotopic record of a Kibaran event; this area, therefore, remained as a stable craton. West of the stable Ibadan block is the Kibaran sequence of mainly clastic sediments and minor mafic volcanics in the large Iseyin-Oyan River Schist Belt. Thus, the Kibaran Orogeny in NW Nigeria was a significant thermal and tectonic event, apparently of ensialic type. The Kibaran schist belts of Cameroun and the Hoggar show similar- ities with the Nigerian belts which indicate an essential unity to Kibaran events in this large region. The Poli Schist Belt in Cameroun resembles the Nigerian belts in its volcanic to clastic assemblage, and its association with Pan African granites. Hubbard (1975) has suggested that the NNE-trending zone of Kibaran schist belts in Nigeria developed as an extensional feature parallel to the Pan-African geosyncline to the west. The Kibaran schist belt zones of Nigeria and Cameroun give a bilateral symmetry to the Pan-African Province between the West African and Congo Cra- tons, the central zone between them being dominated by voluminous Pan-African granites and migmatites. The Older Granites (Pan African Granitoids) 27

The Pan-African Orogeny Deposits of Pan-African age are probably represented in the Northern Nigerian schist belts only. They consist almost entirely of clastic sediments; Pan African vol- canics are absent or of very minor importance, and so are the iron formations found in the Kibaran belts. An aspect of the Nigerian schist belts deserves a further com- ment, being the coincidence of schist belts of Kibaran and Pan African ages in the same zone and their absence in areas to the east and west. In the Hoggar region to the north, great mylonite zones divide the crust into compartments with contrasting geology. No such mylonites are known in Nigeria, but the well-defined schist belt zone does appear to be a region with a different crustal structure, composition and history from the areas on its flanks. Also is the tendency for the Pan-African granites to be concentrated in the Kibaran schist belts and not in the Pan-African belts. Part of the explanation for this may lie in the relative ease with which granite magma was able to rise into the already metamorphosed Kibaran schist belts compared with the cooler water-bearing sediments of the Pan-African belts.

The Older Granites (Pan African Granitoids)

The term “Older Granite” was introduced by Falconer (1911) to distinguish the deep-seated, often concordant or semi-concordant granites of the Basement Com- plex from the high-level, highly discordant tin-bearing granites of Northern Nigeria. The Older Granites are believed to be pre-, syn- and post-tectonic rocks which cut both the migmatite-gneiss-quartzite complex and the schist belts. They range widely in age (750–450 Ma) and composition. They represent a varied and long lasting (750–450 Ma) magmatic cycle associated with the Pan-African orogeny. The rocks of this suite range in composition from and through granodiorites to true granites and syenites. Charnockites form an important rock group emplaced during this period. They are generally high level intrusions and anataxis has played an important role (Rahaman, 1981). The Older Granites suite is notable for its gen- eral lack of associated mineralization although the thermal effects may play a role in the remobilization of mineralizing fluids. The Older Granites are the most obvious manifestation of the Pan-African orogeny and represent significan additions of materials (up to 70% in some places) to the crust (Rahaman, 1988). Attempt to classify the Older Granites with respect to timing during an orogenic event are valid over only short distances. Contact fea- tures between members of the Older Granites suite suggest the coexistence of sev- eral magmas. Compositionally, the granites plot in the field of calc-alkaline rocks on the AFM diagram and although they contain significant amount of alkalis, are also often slightly corundum normative. Dada (2006) was of the opinion that the term “Pan African Granitoids” be used for the Older Granites not only on the merit of age which was not available at the time they were named Older Granites, but because it covers several important petrologic groups formed at the same time. 28 1 The Basement Complex

The granitoids which outcrop with the schist belts in northwestern and southwest- ern Nigeria include biotite granites, biotite muscovite granites, syenites, chanock- ites, serpentinites and anorthosites. Rahaman (1988) discarded the earlier classification of members of the Older Granites suite on the basis of their texture, mineraological composition and the rel- ative timing of their emplacement. In its place, members of the Older Granite suite were classified as follows, based mainly on the textural characteristics:

1. Migmatitic granite; 2. Granite gneiss; 3. Early pegmatites and fine-grained granite; 4. Homogeneous to coarse porphyritic granite; 5. Slightly deformed pegmatite and vein quarz; and 6. Undeformed pegmatites, two-mica granites and vein quartz.

In northern Nigeria, the abundance of Pan-African granites appears to increase eastward. In the area west of Zaria these occur as isolated intrusions (McCurry, 1973), whereas in the region between Rahama and the Mesozoic-Cenozoic cover the intrusive granites and related rocks envelope remnants of Migmatites. McCurry (1973) working mainly west of Zaria divided the granites into two main groups according to their field relationships. The first “syntectonic” group comprised elon- gate batholithic sheets that are partly concordant, and foliated. The second group “late tectonic” are made up of poorly foliated discordant bodies, rich in mafic xeno- liths and having a lower proportion of potash feldspar. The late granites are consid- ered to be the products of widespread mobilisation and reactivation of older base- ment rocks during the Pan-African orogeny. The Older Granites occur intricately associated with the Migmatite-Gneiss Complex and the Schist Belts into which they generally intruded. Older Granite rocks therefore occur in most places where rocks of the Migmatite-Gneiss Complex or of the Schist Belt occur. However, Older Granites are particularly noteworthy in and around Wusasa (Zaria), Abuja, Bauchi, Akwanga, Ado-Ekiti and Obudu areas. In Bauchi area and some parts of southwest- ern Nigeria, most of the Older Granite rocks occur as dark, greenish-grey granites with significant quantities of olivine (fayalite) and pyroxene occurring with quartz, feldspars and micas. For this unusual composition, the Older Granites in these areas are termed Bauchite (in Bauchi area) and Oyawoyite (After Professor Oyawoye who first mapped them) in southwestern Nigeria. For uniformity of terminology, both the Bauchites and Oyawoyites constitute the charnockitic rocks (Charnockites)ofthe Basement Complex.

Charnockites

According to Dada (1989), it was at Toro that was first described within the Nigerian basement by Falconer (1911) where it was then referred to as a “quartz porphyrite”. It was assumed to present a certain affinity with the basic mem- bers of the charnockitic series of the Ivory Coast. Wright (1970) described it as an Undeformed Acid and Basic Dykes 29 annular complex of hypersthene diorite at the centre of three circular, concentric granites. He considered the hypersthene diorite as older than the granites from con- tact relations. Cooray (1975) in his review of charnockitic rocks of Nigeria came to the same conclusion, using for argument the presence of granitic veins In the diorite, of dioritic xenoliths in granites and microcline porphyroblasts in the diorite. A field study of the same hybrid rocks led Rahaman (1981) to consider both the granites and the charnockites as either contemporaneous or the latter emplaced shortly after the former. The basement in Toro area consists of gneisses and migmatites into which the Toro Charnockitic Complex intruded (Dada et al., 1989); Older Granites and charnockites which constitute the complex proper; and undeformed basic (doleritic) dykes considered to be later than the Pan-African Granites. The Toro charnockite was described by Dada (1989) to be typically greenish black, fine to medium-grained, equigranular and massive, sometimes porphyritic. The granites consist from the periphery of the complex (in contact with the migmatitic gneiss) towards the centre (in contact with the hypersthene diorite) of: a fine to medium grained biotite-muscovite granite, an equigranular biotite- hornblende granite and a porphyritic biotite-hornblende granite. In general, these granites as well as the diorites are not affected by a penetrative deformation, but undulatory extinction of the quartz and fractures in feldspar crystals indicate local brittle deformation. The Older Granites are recognized as of Pan-African age. Despite the close asso- ciation and field relations with the Older Granites, older ages have been suggested for the charnockites. These ages are in contradiction to the evidence of mix reac- tions at the contact between the charnockites and the Older Granites advanced by Rahaman (1981). Van Breemen et al. (1977) obtained the imprecise ages of 663 ± 164 Ma and 668 ± 128 Ma. on the bauchites in the areas around Bauchi. Tubosun et al. (1984) using U-Pb method on zircons attributed precise Pan-African ages of 620 ± 20 Ma and 634 ± 21 Ma to the charnockites of Ikerre and Akure respec- tively. For the Idanre Complex they obtained late Pan-African ages of 580 ± 10 Ma and 593 ± 11 Ma thereby confirming definite Pan-African ages for the charnockitic complexes of SW Nigeria. Charnockitic rocks constitute one of the important petrological units within the Precambrian Basement Complex of Nigeria. They are generally characterized by their dark greenish to greenish grey appearance which makes them easily recognis- able in hand specimen. They usally contain quartz + plagioclase + alkali feldspar + orthopyroxene + clinopyroxene + hornblende ± biotite ± fayalite. Accessory min- erals are usally zircon, apatite, and iron ores (Olarewaju, 2006). Apart from Toro, other localities of charnockite occurrence include Bauchi, Ado-Ekiti, Ikere (Ekiti), Akure, Idanre, and in the Obudu Plateau.

Undeformed Acid and Basic Dykes

The undeformed acid and basic dykes are late to post-tectonic Pan African. They cross-cut the Migmatite-Gneiss Complex, the Schist Belts and the Older Granites. The undeformed acid and basic dykes incude: 30 1 The Basement Complex a. Felsic dykes that are associated with Pan African granitoids on the terrain such as the muscovite, tourmaline and beryl bearing pegmatites, microgranites, aplites and syenite dykes (Dada, 2006) b. Basic dykes that are generally regarded as the youngest units in the Nigerian basement such as dolerite and the less common basaltic, felsite and lamprophyric dykes.

The age of the felsite dykes has been put at between 580 and 535 Ma from Rb-Sr studies on whole rocks (Matheis and Caen-Vachette, 1983; Dada, 2006), while the basic dykes have a much lower suggested age of ca. 500 Ma (Grant, 1970). The structural and geochronological importances of this suite of rocks, which have been put to immense chronological use elsewhere (Dada, 2006) are often overlooked in Nigeria. When they cross-cut basement, they could be used to infer relative age of metamorphic structures and rock suites and could also suggest the existence of older basement windows in the Nigerian schist belts, apart from the immense guide they provide in sampling for isotope geochemistry, analysis and interpretation (Dada, 2006).