PETROLOGY AND GEOCHEMISTRY OF BASEMENT GNEISSIC ROCKS AROUND OKA-AKOKO, SOUTHWESTERN, NIGERIA

The gneissic rocks of Oka-Akoko area form part of the Migmatite-Gneiss-Quartzite Complex of the southwestern Nigerian Basement Complex. The petrologic units in the study area include grey gneiss, granite gneiss, Older granite, charnockite and minor felsic and basic rocks. Twelve gneissic rock samples comprising six granite gneiss and six grey gneiss from the area were collected for petrographic and geochemical analyses. Petrographic analysis revealed that the granite gneiss is more enriched in quartz and alkali feldspar than the grey gneiss. The grey gneiss is richer in plagioclase, hornblende and opaques compared to the granite gneiss. Interpretation of petrographic and geochemical analyses results revealed that Oka-Akoko granite gneiss and grey gneiss were derived from igneous protoliths of granitic and granodioritic compositions respectively. The grey gneiss is ferroan, alkalic to alkali-calc and metaluminous suggesting that its igneous protolith(s) is a M-type granitoid derived from melting of rocks from upper mantle or lower crustal region under conditions of limited availability of H2O and low oxygen fugacity while the granite gneiss is magnesian, alkali-calc and slightly peraluminous suggesting that its igneous protolith(s) is an I-type granitoid derived from the partial melting of crustal igneous rocks.