Geochemistry of Pan-African Granitoids, Southwest Nigeria: Evidence of Magmatic Mixing

This study aimed to determine the major elemental oxides in the exposed rock types of the study area, classify these rocks on this basis and establish the relationships between the various elemental constituents and hence predict the mode of origin of the rocks. The study area falls within the south-western part of the reactivated Basement Complex of Nigeria. Field work to collect samples lasted about 21 days. About 13 fresh rock samples from the igneous suite, which range from fine to coarse grains, were collected. Thin sections were cut and petrographic studies carried out, also geochemical analysis of the samples was done at the Activation Laboratories, Ontario, Canada. Field investigation clearly revealed two distinct suite of rocks, which characterises the study area. These are the migmatite gneiss suite and the igneous suite. The migmatite gneiss suite, which consists of the migmatite gneiss and the banded gneiss, is the basement rock of the terrain. While the igneous suite, which is mainly the rocks of the older granitoids, intrude the older migmatite gneiss suite. Petrographic studies showed these igneous suites as having the major igneous rock forming minerals – that is quartz, biotite, plagioclase, orthoclase, and microcline. Also present was zircon and other accessory minerals. Geochemical studies of these rocks enabled their classification into granites, granodiorites, syenites, syenodiorites, and diorites using the plot of Na2O + K2O versus SiO2. Further geochemical evaluation revealed these felsic and intermediate rocks as peraluminous, non-oceanic in origin, ferroan and magnesian mixed, and alkali rich rocks. They were also grouped as within plate granites. A dual source of origin was finally proposed for the rocks based on their geochemical behaviour.