Geochemical Composition and Petrogenesis of Schists and Amphibolites of Parts of Sheets 203 (Lafiagi) SW and 224 (Osi) NW, Southwestern Nigeria

Abstract

Precambrian meta-sedimentary rocks are widely exposed and are interbanded with amphibolitic rocks in Sheets 203 (Lafiagi) SW and 224 (Osi) NW, southwestern Nigeria. They are underlain by migmatitic and granitic gneisses and intruded by Late- to Post- Pan African granitoids. Petrographic and geochemical studies show that the meta-sediments are para-schists of arkosic to greywacke parentage which were sourced from a moderately weathered feldspathic igneous rock(s) while the amphibolitic rocks are sub alkaline Island Arc tholeiites whose original chemistry have been significantly altered during and after the emplacement. Derivation of the schists from K feldspar-rich protoliths is indicated by the enhancement of Ba over Rb while the presence of significant amount of mafic constituents is signified by relatively high amount of Cr and, sometimes, Ni. Low level of weathering of the protolith is suggested, based on the calculated CIA values. The geochemical characteristics of the amphibolites indicate a possible origin from a basaltic magma under an oxidizing condition. The study area shows similar petrological, structural and geochemical characteristics with the Ilesha Schist Belt to the south and to which it is considered to be the northern extension.