Evolution Of The Suhum Basin Of The Rhyacian Birimian Terrane In Ghana: Insight From Paraschist And Orthoamphibolite

Abstract

We present petrographical and geochemical data of paraschists and orthoamphibolite from the Suhum-Akwadum area of the Suhum Basin, Ghana, to determine their provenance, petrogenesis, and tectonic setting, and discuss the crustal evolution of the Rhyacian Birimian terrane of the West African Craton. The amphibolite exhibits trace element signatures similar to those of subalkaline basalts. Two main geochemical groups are present: (1) dominant arc-rocks with negative Nb-Ta, Ti, low Nb/Th and high La/Nb, and (2) subordinate enriched mid ocean ridge basalts characterised by moderate to high TiO2, positive Ta and high Nb/Th value. These compo sitions are interpreted as formation in a supra-subduction zone setting. The schists are geochemically classified as shales and wackes and have <60 CIA, <70 CIW, moderate to high Al2O3/TiO2 and Na2O/K2O, low SiO2/Al2O3, La/Sc, and Ti/Zr values. These geochemical signatures are consistent with low chemical weathering intensity, immature to moderately mature sediments and deposition in an active continental margin setting with sediments sourced from greenstone and granitoid-gneiss complexes of the Rhyacian Birimian terrane. Therefore, the current and previous geochemical data confirm the presence of an arc tectonic setting in the formation of the meta sedimentary basins of the Birimian terrane. The overall geochemical data suggest that the greenstones and metasedimentary rocks formed during an orogenic event related to an arc environment where subduction zone components contributed to the generation of their parental magmas and sedimentation. This finding is consistent with the onset of “modern-style” subduction-related processes during the Archean-Paleoproterozoic transitional period.