Petrology and Geochemical Features of Crystalline Rocks in Ora-Ekiti, Southwestern Nigeria

Abstract

This research investigates and reports on the petrology and geochemical characteristics of crystalline basement rocks in Ora-Ekiti, Southwestern Nigeria. Exhaustive geological investigation reveals migmatite, banded gneiss, granite gneiss and biotite gneiss underlie the area. In reducing order of abundance, petrographic examination reveals that migmatite contains quartz, muscovite and opaque minerals. Banded geniuses contain quartz, biotite, plagioclase, and opaque minerals. Granite geniuses contain quartz, plagioclase, biotite, microcline and opaque; while biotite geniuses contain biotite, plagioclase, opaque minerals, and quartz. Silica contents in migmatite (69.50%-72.66%; ca. 71.23%), banded gneiss (71.66%-77.1%; ca. 75.23%), biotite gneiss (72.32%-76.18%; ca. 73.83%) and granite gneiss (69.82%-73.15%; ca. 71.95%) indicate the rocks are siliceous. High alumina contents in migmatite (12.18%), banded gneiss (10.28%), biotite gneiss (11.46%) and granite gneiss (9.97%) are comparable to similar rocks in the basement complex. All the rocks show Ba, Sr and Rb enrichment. Harker diagrams of Al2O3 versus SiO2 and CaO versus SiO2 show negative trends while Na2O versus SiO2, K2O versus SiO2 and TiO2 versus SiO2 plots showed positive trends. This variation probably depicts extensive crystal fractionation in the magmatic systems that produced the rocks prior to metamorphism or partial melting of the precursor rock. SiO2 versus (Na2O + K2O) classifies the rocks as granite to granodiorite. The rocks are high K-calc-alkaline and calc-alkalic on SiO2-K2O plot. This shows the rocks are potassic meaningthat they are formed from a potassium-rich source. The plot of Al2O3/(Na2O + K2O) versus Al2O3/(CaO +Na2O + K2O) reveals the crystalline rocks are orogenic and originated from granitoid with meta luminous affinity. The rocks consist of gneisses of no economic minerals, but the petrology reveals them as common rocks typical of metamorphic terrains and geochemical features of the rocks reveal they are felsic and of granitic composition.