Regional Geology
Rahaman (1971), Oyawoye (1972), Cooray (1972), Elueze (1981), Caby (1981), Dada (1998, 1999) previously studied the general geology and geochronology of the basement rocks of Nigeria. The crystalline rocks in Nigeria are distributed in a circular area in the north central, a triangular area in the west which runs into the Benin Republic, and a rectangular area broken into three parts by sedimentary rocks on the eastern border of Nigeria with Cameroun Republic. The crystalline rocks are divided into three main groups: Basement complex, Younger granites and Tertiary to Recent volcanic. The Precambrian rocks of Nigeria, collectively known as the basement complex, occupy nearly half the total area of the country. The other half is covered by the Cretaceous and younger sedimentary rocks (Fig. 1).
Polycyclic Migmatite-Gneiss Complex (MGC)
Oyawoye (1965) recognized the following rock types as part of the MGC: banded gneisses, augen gneisses and pegmatites. Recent petrological division of the MGC includes: A grey foliated biotite acid or biotite hornblende quartz feldspathic gneiss of tonalitic to granodioritic composition, which is also known as the "grey gneiss" (Rahaman, 1981). Mafic and ultramafic components which when present, often outcrop as discontinuous boudinaged lenses or concordant sheets of amphibolites with minor amounts of biotite-rich ultramafic. Except where it constitutes the palaeosome to the migmatite, these are present in grossly subordinate amounts to the grey gneisses. Felsic components are a varied group of rocks consisting essentially of pegmatites, aplite, quartz-oligoclase veins, fine-grained granite gneiss, porphyritic granites etc. The three components may or may not be present together on a single outcrop. The migmatite-gneiss complexes are Archean (Dada et al., 1989). Another important type of migmatite common in the Basement complex is the Agmatite, in which schistose or gneissic rocks (palaeosome) are dissected into irregular blocks by quartzo-feldspathic dykes and pegmatites (metasome) (Oyawoye, 1970).
Pan African Granitoid
The term "Pan African granitoid" covers group of rocks that include biotite and biotite-muscovite granites, syenites, charnokites, diorites, monzonites (Bauchite), serpentines, anorthosites, etc. Pan African as a name was introduced by Falconer (1911) to distinguish this rock type based on morphology and texture from Jurassic, anorogenic, peralkaline “Younger Granites” in north central Nigeria. The older granites, e.g. the coarse-grained biotite-hornblende granites, have concordant foliation with the MGC or schists (Oyawoye 1965) though they vary in composition, texture and colour.