Petrographic Characteristic and Geochemical features of Basement Rocks in Ikogosi, Southwestern Nigeria.

This study investigates and reports petrographic characteristics, geochemical features and the processes of formation of rocks in Ikogosi, southwestern Nigeria. Eighteen (18) rock samples (six quartzite, six migmatite gneiss and six schists) were collected and subjected to optical study. Ten (10) representative samples (six quartzite, three schist and one migmatite gneiss) were evaluated for major elements composition using Energy Dispersive X-Ray Fluorescence (ED-XRF). Petrographic evaluation revealed quartz, feldspar, muscovite, biotite, hornblende, pyroxene and opaque are dominant mineral constituents in quartzite and schist while migmatite gneiss contains the same set of minerals but without opaque constituents. Quartz forms a dominant constituent in quartzite and migmatite gneiss but as subordinate mineral in the schist. The feldspars are mainly plagioclase with albite twinning, associated microcline has characteristic crosshatched twinning. Chemical investigation reveals the rock units are generally siliceous. Three oxides (SiO2, Al2O3 and Fe2O3) constitute 85–90% of the bulk chemical composition of the rocks. SiO2 constituent as applied in rock classification indicates the quartzite is unequivocally felsic, migmatite is intermediate while the schist is mafic to ultramafic. K2O versus SiO2 variation plot revealed that the quartzite and schist are tholeiitic while the migmatite gneiss is shoshonitic. TiO2 versus SiO2 variation diagram depicts igneous antecedent.