Petrography and structural features of the Precambrian basement rocks in the Benin-Nigerian Shield, NW Nigeria: Implications for their correlation with South Atlantic Precambrian terranes

Abstract

This study investigated the petrographic and structural features of the Precambrian (Neoproterozoic) basement rocks of the Benin-Nigerian Shield that crop out in northwestern Nigeria within Kanoma and its environs to give an insight into the evolution and deformational episodes that pervaded them. The major rock types in the area are schists and quartzites, which have been intruded by granitic rocks that appear to be metamorphosed. The origin of these rocks is attributed to the Eburnean Precambrian orogenic episode and the Pan-African orogeny, which started and ended with the intrusion of the granite suites. The dominant mineralogy associated with the rock types includes quartz, orthoclase, plagioclase, microcline, biotite, chlorite, and very few accessory minerals. The schist shows the dominance of quartz, feldspars (alkali and plagioclase), biotite, muscovite, chlorite, and opaque minerals. The quartzite is typically dominated by quartz that appears recrystallized in places, whereas the meta-granite contains quartz, feldspars (alkali and plagioclase), biotite, and opaque minerals. Structural features such as joints, quartz veins with minor folds, and faults observed in the lithological units have a predominant N-S trend and are the imprints of the last tectonic event (Pan-African orogeny). The level of deformation in Kanoma led to the development of N to NNE trending moderately (S1) to steeply (S2) dipping foliations in the schist. The evolution of these deformational mechanisms from moderately dipping foliations to steeply dipping foliations along the N to NNE- trend is associated with late orogenic uplift and exhumation following oblique convergence during the Pan-African orogeny. Structural overprinting relations recognized within Kanoma and its environs allow us to decipher the geologic structures into three successive Pan-African deformational events (D1–D3). D1 fabrics are manifested by simple anticline micro folds in the schist. The D2 structures are the predominant ones in the area comprising the N-S directional joints and faults. The D3 phase of deformation is a progressive one, which started as N-S high angle thrusts and thrust-related folds that resulted from the NE–SW contraction during the orogenic episodes. The studied rocks can be correlated with the Pan-African and Brasiliano belts based on their overlapping features.

Keywords

• petrography
• structural evolution
• precambrian basement rocks
• benin-nigerian shield