The geology and geochemistry of some basement rocks in Ilesha schist belt, southwestern Nigeria have been studied and presented here. Petrological studies of hornblende gneiss, (HGN), banded - gneiss (BGN) and grey granite gneiss (GGG) revealed that these rocks contain monazite which indicates a likely sedimentary component in the original magma of the rocks. AFM plots for these rocks showed that they are all calc-alkaline in nature and therefore have originated from a subduction related environment. Discrimination plots based on the Mol. % Al₂O₃/(Na₂O+K₂O+CaO) versus % normative Corundum and histogrammes of Mol. % Al₂O₃/(Na₂O+K₂O+CaO) clearly revealed that these rocks originated from a mixed magma containing both mantle and sedimentary components. REE and spidergram plots also showed that these rocks are from a subduction related tectonic environment. These geology and geochemical characteristics of the Ilesha gneisses implicate evolution of rocks in a back arc tectonic setting in which mantle and sedimentary rocks were mixed together to form magma which gave rise to the precursor of these rocks. Post-magmatic transpressive forces in the back arc basin were probably responsible for metamorphism and deformation of the earlier laid down protorocks, leading to emplacement of gneisses and metasediments in the Ilesha schist belt.