Petrography And Geochemistry Of Schists And Amphibolites From The Paleoproterozoic Birimian Suhum Basin, Southeastern Ghana.

Abstract

The mineralogical composition; as well as, major and trace-element geochemistry of the schists and amphibolites from the northwestern portion of the Suhum basin, were studied to determine the provenance, petrogenesis and tectonic implications of the Basin in relation to the surrounding Birimian rocks. Thirty (30) thin sections were prepared for petrographical studies. Twenty-five (25) samples were analysed for their major and trace elements contents. From the petrography, the rocks are composed of biotite schists, hornblende-biotite schists, muscovite-biotite schists and amphibolites. The hornblende-biotite schists and muscovite biotite schists are metasedimentary in nature. From the major and trace element analysis, however, the schists are classified as greywackes (Fe-shales and wackes) whiles the amphibolites classified as para-amphibolites and ortho-amphibolites. The source rocks of the schists have undergone incipient to intermediate degrees of weathering and low degrees of sediment reworking (recycling). The schists are texturally and mineralogically immature and were derived from mafic to felsic rock sources; possibly from the lower crustal to upper mantle regions. Petrographical and geochemical characteristics suggest the deposition of the schists probably within active continental margin and arc margin settings. Provenance indicators suggest mafic, intermediate and felsic igneous provenances for the schists. Taken together, the overall inferred provenance of the schists could be that of Young Undifferentiated Arc terranes (YUA). Hence, the source area for the sediments of the schists is most likely the adjacent Birimian metavolcanic rocks. The amphibolites from the study area are classified as metaluminous, gabbroic and subalkaline (tholeiitic). Both para-amphibolites and ortho-amphibolites were identified to have been derived from zones typically characteristic of Mid-Ocean Ridge Basalts (MORBs), Island Arc Tholeiites (IATs) and Continental Arc Basalts (CABs). The amphibolites were most probably derived from the adjacent Birimian granite-greenstone belts and show characteristics similar to the Archean greenstone belts. This supports the fact that the Paleoproterozoic terranes of the West African Craton resemble the Archean greenstone terranes.