Petrological and structural investigation of rocks around Igarra, southwestern Nigeria

Structural mapping and petrographic study carried out on the exposed Basement rocks of Igarra area, Southwestern Nigeria, have been used to infer the metamorphism, evolution and deformation history prevalent in the Igarra schist belt. The rocks exposed in the Igarra schist belt are mainly metasediments and intrusives. The metasediment comprises of inter-bedded quartz-biotite schist and minor bodies of quartzite, metaconglomerate, quartz-sericite schist and marble. The intrusives which cross-cut the metasediments in places consist of porphyritic-granite, aplite/pegmatite, syenite and subsidiary quartz vein. Petrographic analysis study involved megascopic and microscopic studies of structures in the rocks, while structural analysis entailed field based measurements and stereographic plots of structural elements of the metasediments. Two phases of metamorphism and deformation were inferred from two generations of foliation, S 1 and S 2 in that order. Metamorphic facies defined by index minerals assemblage indicated a low to medium grade metamorphism. Structural analysis of field data reveals a NNW-SSE trending foliation S 2 that cut across an earlier foliation S 1 ,generally in a N-S direction and a tight to isoclinal anticline plunge fold of the metasediments. The evolution of this belt can be related to sedimentary piles that were deformed and metamorphosed as a result of tectonic activities during the Paleoproteozoic and Neoproterozoic times, followed by the intrusion of granitoids and subsequent erosion and weathering of the rocks to produce the present topography. Keywords: Metasediments, petrography, metamorphism, structural mapping, Igarra, southwestern Nigeria