Paleoweathering, provenance and tectonic setting of metasedimentary rocks at Ayanfuri area in the Paleoproterozoic Kumasi basin in Ghana: evidence from petrography and geochemistry

This study investigates the provenance, paleoweathering, and tectonic setting of some metasedimentary rocks at Ayanfuri, in the Paleoproterozoic Kumasi basin of Ghana. A total of 20 metasedimentary rock samples were taken from the Chirawewa and Fetish pits and environs for analyses of chemical composition using the XRF and ICP-MS devices. 10 sandstone samples were selected for petrographic studies. Petrographic analysis revealed the presence of quartz (mono-and polycrystalline quartz) with the polycrystalline quartz being dominant. Feldspars were next in abundance, with few sedimentary rocks fragments. Cements were represented by quartz overgrowths and iron oxide. Petrographic constraint indicate weak to moderate weathering at sediment source as the samples plot as feldspathic sandstones. Geochemical and petrographical characteristics indicates that the sandstones show higher compositional maturity compared with the shales. Major elements show significant variation, comparing with early Proterozoic Upper Continental Crust (UCC) and Post Archaean Australian Shale (PAAS). Large Ion Lithophile Elements (LILE) of the metasediment are significantly depleted while High Field Strength Elements (HFSE) are also low relative to PAAS and UCC. The sediments are derived from the adjacent Ashanti and/or Sefwi-Bibiani volcanic belt, which is a predominantly mafic igneous provenance. Major element geochemistry indicates that the sandstone and shales were subjected to minor potassium metasomatism after they were deposited. Pre-metasomatized Chemical Index of Alteration (CIA) shows weak to moderate degree of chemical weathering at the sediment source area. Plagioclase Index of Alteration (PIA) range indicates low-moderate degree of weathering which implies presence of labile minerals in the sediments. Higher average CIA of the sandstone and the sericitization of feldspar grains is consistent with a prevailing subtropical warm to semi-humid climate during sedimentation. The tectonic setting discrimination diagrams support sediment deposition in a predominantly passive tectonic setting.