Petrogenesis of Granite Gneisses Around Igangan-Tapa-Inamere Area, Southwestern Nigeria

Abstract

Precambrian granite gneisses and migmatites constitute the basement complex in some parts of Nigeria, with different modes of origin proposed by different authors based on available data. This study was carried out to determine the origin of granite gneisses around Igangan area, southwestern Nigeria. Geological mapping was carried out on a scale of 1:50000 to delineate the rock types, and thin sections of the identified rocks were studied under the petrologic microscope. Whole Rock analysis of 8 samples of the granite gneisses was carried out using Inductively Coupled Plasma Mass Spectrometry (ICP-MS); results were interpreted using descriptive statistics and major and trace element plots. The granite gneisses are slightly foliated, trend NE-SW, and display a slight easterly dip. They are composed of plagioclase + k-feldspar + quartz + biotite + hornblende and accessory minerals including apatite, zircon and magnetite; perthitic intergrowth was observed in the feldspars. The gneisses are enriched in Na O relative to K O, 2 2 have alumina content below 15% and high SiO content. Average concentration of Ba and Sr are below the crustal 2 average while the average value of Rb is above that of the average crust. The gneisses are also characterized by low mean values of Ni, Cr, Co; high LREE/HREE ratio and a negative Eu anomaly. Geochemical plots and CIPW norm values favor an igneous origin for the gneisses, reveal that they are calc-alkaline and were metamorphosed at 650 700°C. Plots of Ni vs V, Sc vs V and Ce/Sm vs Ce showed that the primary signature of the gneisses have been preseved; with plagioclase fractionation and fractional crystalisation being the dominant features observed in the gneisses. Field, petrogrpahic and geochemical features revealed that the granite gneisses are of igneous origin and their primary features have been preserved due to low grade metamorphism which did not involve partial melting of the protolith.

Publication Info:

Author: Olisa, O.G. , Okunlola, O.A. and Omitogun, A.

Volume: 56

Issue: March

Published By: Journal of Mining and Geology, 2024-03-01