PETROTECTONIC AND LITHOSTRUCTURAL STUDIES OF OKEMESI IWARAJA AREA, SOUTHWESTER

Abstract

Okemesi-Iwaraja area lies within the Ilesha Schist belt, and has been described as a polydeformational high strain transcurrent shear belt in the Basement Complex of Southwestern Nigeria. In addition, the structures, important for mineral exploration, associated with this major transcurrent shear zone has not been fully investigated. The aim of this study was to classify the rocks, characterise the structures, and determine their petrotectonic evolution. Remote sensing and Aeromagnetic data were procured and analysed. Band ratios prepared from Landsat-8 were used to delineate structures. Digital Elevation Model and RADAR data were used for identification of surface lineaments. The Aeromagnetic data were processed for subsurface structural features. Geological mapping was undertaken to delineate lithologic units and structures. Trend and plunge of fold axis were determined using stereographic analysis. Rock texture and mineralogical composition was by petrographic studies of thin sections prepared from eighty-four fresh rock samples. Major, trace and Rare Earth Elements (REE), of twenty-four samples were determined using Inductively Coupled Plasma Mass-Spectroscopy technique. Lineament orientation dominantly trend NE-SW (56.9%). Other orientations were NNE-SSW (17.1%), NW-SE (22.3%), E-W (3.8%) and N–S (1.3%). Subsurface structures were also dominantly NE-SW, strongly corresponding to the surface lineaments. Rock types identified in the area were migmatites, amphibole schists, quartz schists, biotite schists, amphibolites, quartzites and the granitoids (grey gneiss, granite gneiss, two-mica granite, medium-grained biotite granite, porphyritic biotite granite). Rock structures were lineations, foliations, folds, joints, shear zones, and faults. Two major deformational phases (D1 and D2) were identified, D1 event recorded mainly on quartz schist were associated with the development of S1 schistosity and a recumbent F1 folding, D2 was well developed in some granite gneisses, with the development of an S2 and tight to isoclinal F2 folds. Fold and foliation axial plane trend in the NNE-SSW and NE-SW directions, while the trend and plunge of the fold axis were dominantly NE, suggesting an E-W compression, which is expressed in the surface lineaments and the subsurface structures. Mineral alteration of perthite to sericite in the granitoids and the calculated Chemical Index of Alteration (49.71 – 62.11) for granites, indicated onset of weathering. The granitoids were relatively potassic K2O (3.34 – 6.23), and classified as high-K calc-alkaline, with high SiO2 composition of 64.83–86.63wt% and molar A/CNK (1.18-2.75) suggesting strong peraluminous, S-type granite. The grey gneisses were however metaluminous. The light REE enrichment and depleted heavy REE with negative Eu anomaly in the granitoids, as revealed by spider plots, confirmed a felsic crustal origin. Plots of Rb/Y+N and Rb/Ta+Yb revealed, the tectonic environments were mainly post collision and within plate granite settings. Okemesi-Iwaraja area is underlain by migmatites and schists intruded by the granites. The rocks have suffered polyphase deformation and are of crustal origin emplaced in a post-collision and within plate tectonic settings.