Geochemical Study of Precambrian Metavolcanic Rock Around Bunu Area, Part of Kabba-Lokoja-Igarra Schist Belt, SW Nigeria

Abstract

In Bunu area, Precambrian meta-sedimentary rocks are extensively exposed and interbanded with amphibolitic rocks. These dark green foliated rocks, known as amphibolites, are mostly made of green hornblende, plagioclase, and quartz, with lesser amounts of pyroxene, quartz, and epidote. The amphibolitic rocks are calcalkaline Island Arc tholeiites by nature, but their initial chemistry has been extensively altered both before and after their emplacement, according to a geochemical investigation. Geochemical analysis also revealed that Ba is enriched over Rb signifying that the rocks that were formed from K feldspar-rich protoliths and relatively high Cr and, occasionally, Ni concentrations suggest the existence of considerable amounts of mafic elements. Very low K2O, high Al2O3, low TiO2, Nb, and Y contents, and low Al2O3 concentration are all characteristics of backarc or island-arc settings in Bunu amphibolites. The protoliths of the amphibolites were igneous tholeiitic basalts that were extruded in a back-arc tectonic setting, close to a subduction zone, according to geochemical characterization. The work supports the orogenic nature of the Pan-African event, which occurred in the region around 600 Ma and resulted in regional metamorphism and the origin of the amphibolites magmas from upper mantle melting. The original properties of the fabric of the parent basaltic rocks may have changed as a result of later metasomatic and metamorphic processes. The study also reveals similarities in petrological and geochemical properties between metavolcanic rocks studied in this area and those in Zuru Schist Belt to the north and the Ilesha Schist Belt to the west