Preliminary Structural and Petrographic Studies of Oke Ogun Rocks, Southwestern Nigeria

Abstract

This study attempts to analyze structural elements and mineralogical compositions that are present in rocks in parts of Oke Ogun and relates their occurrences to the geology of the area. The main objectives are to better understand the structural history of the rocks and the deformational episodes that pervaded them. Geological field mapping was undertaken to identify and study field occurrences and structural relationships of rock types. Strike and dip directions were measured with compass-clinometer, whereas coordinates of sampling points and locations were taken using Global Positioning System (GPS). In addition, rock samples were collected and thin sections prepared for petrographic analysis. The main lithologic units found in the area are syenite, migmatite-gneiss and porphyroblastic gneiss, mica schist, banded gneiss and biotite hornblende granite. Geologic features such as faults, joints, fractures, foliations, quartz veins, folds, and xenoliths were also observed on the outcrops. The result of petrographic studies shows that most samples have highest concentrations of plagioclase, which range from 29.4% to 26.5%, followed by quartz (24.6% to 24.4%). Few locations recorded highest concentrations of microcline ranging from 41.5% to 19.6%. Biotite values range from 17.5% to 6.7%, whereas hornblende and opaque mineral values range from 7.4% to 5.5% and from 2.0% to 0.9%, respectively. Deformation-induced elongations of minerals were also observed in thin sections. The resultant orientations of macro-structural veins based on structural measurements show a NE-SW and NW-SE trends,                  whereas joints trend N-S and E-W. These structural trends have been interpreted to be analogous to directions of tectonic events responsible for metamorphism and/or fracturing of rocks in the region.