Nature and Origin of the Amphibolites in the Precambrian Basement Complex of Iseyin and Ilesha Schist Belts, Southwestern Nigeria

Petrographic and geochemical studies of amphibolites from Wonu-Apomu area in Iseyin-Oyan schist belt and Ife-Ilesa area in Ilesha schist belt, southwestern Nigeria, were carried out with a view to unravel their nature and petrogenesis. The amphibolites in Wonu-Apomu are associated with quartzite, quartz schist, talc-schist, granite and pegmatites, while those of Ife-Ilesha area are associated with anthophyllite schist, talc tremolite and talc chlorite schists.

Chemical data of the amphibolites showed high Al2O3 (>14%), Fe2O3 (>12.40%), CaO (>8.10), Na2O (>3.10%) and K2O (>5.0%) contents. Trace element data showed that the amphibolites are enriched in Ba (>137ppm), Sr (>122ppm), Rb (>33ppm), Zr (55pm) and Y (>37ppm), but impoverished in Cr (<62ppm), Ni (<57ppm) and Zn (<83ppm). Petrogenetic indices and discriminant plots of Na2O + K2O vs SiO2, Ga vs Y, Zr/Y vs Zr, Cr vs Ni, Fe2O3 –Na2O + K2O – MgO, indicated tholeiitic basalt affinity of the amphibolites and emplacements of the precursor basaltic rocks within continental crust.