GEOCHEMISTRY AND PETROLOGY OF BASEMENT COMPLEX ROCKS AROUND AKUNGBA-AKOKO, SOUTHWESTERN NIGERIA

Field, petrographic and petrochemical studies of the Basement Complex rocks around Akungba-Akoko were carried out with the aim of determining their petrology, petrochemical characteristics and origin.

The petrology of Akungba-Akoko area in the Southwestern Nigerian Basement Complex comprises of migmatite, granite gneiss and biotite gneiss intruded by biotite granite, charnockite and minor felsic and basic rocks. Seven rock samples comprising of two granite gneiss, two biotite gneiss, two biotite granite and one charnockite were collected during detailed geological mapping for petrographic and geochemical analyses.

Petrographic analysis shows that the gneisses and granitoids of the area are enriched in quartz, feldspars and mica and are granitic. Petrochemical data of the rocks reveal that they are enriched in SiO₂, Al₂O₃ and Fe₂O₃ compared to CaO, Na₂O, K₂O and MgO. Using petrochemical variation diagrams, it was revealed that Akungba-Akoko granite gneiss and biotite gneiss are orthogneisses derived from metamorphism of igneous protolith(s) of granitic composition. The biotite granite and charnockite are of igneous/magmatic origin. The igneous protolith(s) of the granite gneiss and biotite gneiss are S-type granitoids probably derived from shallow crustal or sedimentary rocks; the biotite granite is I-type granitoid derived from crustal igneous rocks and the charnockite is M-type granitoid derived from mantle or deeper crustal source.