Geochemistry of the Ilesha granite gneiss in the basement complex of southwestern Nigeria

The Ilesha granite gneiss comprises a varied series ranging from porphyroblastic alkali gneiss and granitic gneiss to banded and strongly foliated melanocratic rocks. Deformation is intense and the dominant structural trend is approximately N-S.Chemical data show essentially a systematic variation reflecting the differences in the petrographic character of the outcrops. SiO₂, Na₂O, K₂O and related trace elements, particularly Rb, are higher in the alkali and granitic varieties, whereas the melanocratic types have lower contents of these elements. The basic rocks are likewise significantly enhanced in TiO₂, Fe, MgO, CaO, Cr and Ni concentrations, with some values being comparable to those of basaltic rocks.