Geology, Geochemistry and Petrogenesis of Precambrian Rocks of Part of Sheet 244 of Ado-Ekiti, Southwestern Nigeria

摘要

The geology, geochemistry and petrogenesis of Precambrian rocks of part of sheet 244 (Ado-Ekiti), Southwestern Nigeria is reported in this paper. The study area was mapped systematically and the lithologies encountered include migmatite gneiss, granite gneiss, older granite, and charnockite. The various structures mapped on the outcrops are compositional banding, pinch and swell structures, joints, xenolith inclusions among others. The gneisses cover the largest proportion (≈60%) of the study area while the Older granites and charnockites cover the North Eastern and Southwestern part of the area. Six fresh samples of rocks were analysed for petrographic study while another six representative samples (2 per rock type) were also analysed for geochemical composition using Atomic Absorption Spectroscopy. The petrographic study reveals the dominance of quartz, biotite, plagioclase and microcline in the thin sections for the different rock types. The geochemical analyses of the rocks show in percentage average SiO 2 compositionof 67.63 in charnockites, 67.09 in granites and 67.99 in gneisses. Alumina varied from 13.77% in the charnockites, and the lowest in the gneisses with 12.91%.The high percentage composition of SiO 2 obviously indicate siliceous nature of all the rock units in the area with low content of Fe 2 O 3 and MgO as well as various discriminatory diagrams establishing a common petrogenetic origin related to igneous material source.