STRUCTURAL EVOLUTION OF THE BASEMENT COMPLEX ROCKS IN IWARAJA AREA, SOUTHWESTERN NIGERIA

Abstract

A study on the structural evolution of the Basement Complex rocks in Iwaraja area, southwestern Nigeria had been carried out. The study was done in order to produce a detailed geological map of the area as well as understanding the deformation conditions and the chemical changes associated with the mylonitisation of the affected rocks across the Iwaraja Shear Zone. Detailed field mapping was carried out and thin sections obtained from samples collected were analysed. Samples were also selected across the Iwaraja Shear Zone for geochemical analysis. The results of this study show that the basement rocks of the Iwaraja area comprising quartzite and quartz-schist of the Effon Psammite Formation, quartz-mica schist, biotite schist, migmatitic gneiss, granitic gneiss, late and post-tectonic granitic rocks including pegmatites have been subjected to two episodes of deformation, D1 and D2. D1 structures include schistosity (S1) in the metasediments, gneissic foliation (S1) including banding in the migmatitic gneiss and the granitic gneiss as well as moderately plunging mineral lineation (L1). D2 gave rise to the formation and evolution of the Iwaraja Shear Zone, which took place in several stages. D2 structures include steeply dipping, subvertical (S2) shear fabrics; sub-horizontal, shallow plunging mineral lineations (L2); as well as shear zone-related minor folds (F2) of the pegmatite dyke in the mylonite which vary from early to late folds. Late deformation characterised by ptygmatic folds of the pegmatites dyke in the migmatitic gneiss, which also locally deform and crenulated foliation (S1) in the quartz-mica schist as well as the development of major folds were recognized. Two phases of metamorphism M1 and M2 were recognized in the Iwaraja area. The first phase was associated with prograde metamorphism that affected all the rocks in the amphibolite facies. M2 was associated with retrograde metamorphism from amphibolite facies in the unsheared rock to greenschist facies in mylonite indicating retrograde mineralogical changes. Kinematic indicators such as -type porphyroclasts and fractured feldspar porphyroclasts indicate a dextral sense of shear with a dominantly strike-slip, transcurrent displacement along the northeast direction of the mineral lineation (L2). Chemical changes across the Iwaraja Shear Zone indicate relatively high enrichment of Fe2O3, TiO2, MgO, CaO, P2O5, MnO, Eu, Zr and Sc major and trace elements and with relatively high depletion of Na2O and Rb in the mylonitic-ultramylonitic rock. This enrichment and depletion was influenced by fluid influx and tectonic mixing of the granitic gneiss and the more rheologically stiffer pegmatite dyke. The Iwaraja Shear Zone had a transcurrent displacement and is part of the major shear system traceable from SW to NW Nigeria which is traceable to Central Hoggar Belt in the Trans-Saharan Mobile Belt that played a major role in the bringing together of the different terranes during the closing stages of the Pan-African Orogeny.