Petrogenesis and Chemical Alteration of Basement Complex Rocks of Ago-Iwoye, Southwest Nigeria

This chapter examines the petrogenesis and chemical alteration of the basement complex rocks of Ago-Iwoye, Southwestern Nigeria using geochemical analysis and chemical weathering indices such as the Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Ruxton ratio (R), and Vogt’s Residual Index (V). Seventeen rock exposures consisting of biotite-hornblende banded gneiss, migmatite gneiss, granite gneiss, and pegmatite were studied for their textural, structural, and mineralogical characteristics. Ten samples consisting of weathered and unweathered couples for five rock exposures were subjected to petrographic and geochemical analyses. The normative mineralogical composition for these samples shows a preponderance of quartz, plagioclase, orthoclase, hypersthene, biotite, hornblende, and opaque minerals. The banded gneiss, migmatite, granite gneisses, and pegmatite of the area are S-type, peraluminous, and calcalkaline granitoids emplaced in a continental margin setting. These granitoids have undergone a high degree of chemical weathering, as revealed by the high weathering indices for both fresh and weathered samples. Our proposed ratio for evaluating changes in elemental oxide values, R_wf, revealed enrichment in Fe₂O₃, FeO, and MgO. This is due to the chemical alteration of ferromagnesian minerals in the weathered samples of migmatite gneiss and some granite gneiss, compared to their fresh equivalents. However, other oxides such as SiO₂, Al₂O₃, CaO, Na₂O, K₂O, and SO₃ have a relatively constant proportion in both weathered and fresh samples. The quantitative approach used here showed that rocks visually categorized as unweathered are also chemically altered, showing intense chemical alteration of the basement rocks since the Pan-African orogeny.