Petrography and Petrogenesis of Pre-Mesozoic rocks, Ago-Iwoye NE, SW Nigeria

K.O. Omosanya, R. Adebowale Sanni, T.A. Laniyan, G. Mosuro, H.O. Omosanya, and L. Falana

Detailed geological mapping was carried out in a part of Ago-Iwoye NE with a view to identify the rock types and determine their origin. Petrographic and whole rock analysis of seven rock samples collected during the initial mapping exercise was done through thin sectioning, and X-ray fluorescence. Minerals were studied in cross and plane polarized light; their contact angle and boundary interaction were inferred in thin section. Normative mineralogy of the rocks was also determined using CIPW Norm.

The study area is underlain by both igneous and metamorphic rocks; the gneisses are migmatized and intruded by granitic rocks and pegmatite. The dominant minerals in thin section include quartz, biotite, muscovite and hornblende. Estimated mineral contact angle is ~56° and ~26° for hornblende-biotite and hornblende-muscovite, respectively. Mineral grain boundaries are planar and undulating in thin section.

The geochemical study revealed major oxides such as SiO2, Al2O3, Fe2O3, CaO, MgO, K2O and Na2O. The highest estimated liquid temperature occurred in the gneisses with a value of ~1014°C, mean of ~862°C, range of ~281°C and water content of 1.6% which shows that the rocks are metamorphic in origin. The volume composition shows that quartz has the highest volume in all the seven samples with a percentage value of ~68%. Discriminant plots show that the granites are quartz-rich granitoid and granodiorite while the gneisses have high liquidus temperature; all the rocks samples record high alteration indices and are derived from passive and continental margins.

Rocks of the study area are Pan African granitoids emplaced into the polycyclic Pre-Cambrian migmatized complex host rock.