Introduction
The pre Mesozoic rocks of the Nigerian Shield reside in the mobile belts that separate the West African craton from the Congo Craton (Kennedy, 1964, Caby & Boesse, 2001, Black, 1980, Rahaman, 1976). The shield is composed of a polycyclic basement and remnants of metamorphic cover sequences that represent a former ensialic domain, which was buried underneath pericratonic sediments, and subsequently reworked during the multistage Pan-African orogeny (Grant, 1978, Mullan, 1979, Caby, 1989).
The Pan African orogeny is multistage in nature as it involves the rejuvenation of older polyorogenic domain (Rahaman, 1988), and tectonometamorphic episodes in the whole of the Nigerian province. Ages include 640-620 Ma (Dada et al., 1989, Dada & Respaut, 1989, Ferre et al., 1998), 585 Ma, and 580 Ma (Tubosun et al., 1984, Rahaman et al., 1991). Structurally, the Nigerian shield is a southern continuation of the Touareg shield of central Sahara at the scale of Gondwana (Black et al., 1994., Caby, 1989).
The rocks of the study area have been erroneously described as Precambrian to Pan African rocks from a polycyclic basement, because these rocks petrologically have some similarity with those previously described as the migmatized gneiss complex and other Pan African rocks. The understanding of the geology of the area has been based on regional studies and published maps by the Geological Survey of Nigeria. It is, however, important to provide information on the geology of rocks at a local scale.
The rationale for this research is to provide information on the petrology and genesis of the rocks of the area with a viewing to creating an archive of published information locally. In this paper, we aim to describe the composition of the rocks from field, thin section analyses, and also use chemistry to tie the genesis of the rock. We start by discussing the regional geology, petrographic description of the rocks, geochemical analysis and discriminants plots for genesis of the rock.
Figure 1. Geological Map of North-West-Africa
Geological Map of North-West Africa, with index map of the study area in square, inset of Africa and Nigeria in the left uppermost part of the map. (Modified after Black & Liegeois, 1993).