Geochemistry of semipelitic schist of Isanlu area, south west Nigeria: implication for the geodynamic evolution of the Egbe-Isanlu schist belt

Abstract

Field and geochemical studies carried out on the semipelitic quartz-mica schist of Isanlu area, SW Nigeria have been used to speculate on the geodynamic activity prevalent in the Egbe-Isanlu palaeobasin at the time its sediment was deposited. Basement rock exposures in the Egbe-Isanlu schist belt are dominated by metasedimentary rocks, chiefly, quartz-mica schist with small occurrences of quartzite, marble and silicate facies iron-formation. These metasedimentary rocks are interbanded with metaigneous rocks such as talc schist and amphibolite, all of which are intruded in places by granitoids. Chemical data on major and trace elements of the sheared and unsheared varieties of quartz – mica schist from the Isanlu area, reveal a composition comparable to that of semipelitic metasediment. However, the sheared variety shows elevated values of Na2O, MnO and Nb, and depleted mean values of FeO and MgO relative to the composition of normal metasediments. This enrichment – depletion signature is probably a consequence of the movement of metamorphic remobilised fluids along these shear zones during the Pan-African or earlier events. A further geochemical characterisation of the rock indicates that its protolith was an immature sub-greywacke to greywacke-type sediment derived from a predominantly granitic provenance. This immature nature of the protolith sediment suggests rapid subsidence of the basin during its genesis, and/or tectonic instability in the surrounding environment from which the sediment was derived. Nonetheless, field observations indicate the absence of typical deep-water sediments and a lack of regional proximal-distal facies variation suggesting a limited depth and width for the basin. The presence of shallow, stable shelf-type sediments such as carbonates and iron-formation (some of which contain carbonaceous material) suggests a change in tectonic regime at some point in the evolution of the basin from a rapidly sinking, to a stable shallow basin. KEY WORDS: Geochemical studies, Semipelitic schist, Geodynamic activities, Rapidly sinking basin [Global Jnl Geol. Sci. Vol.1(2) 2003: 113-128]