Geochemistry and petrography of beach sands along the western coast of Ghana: implications for provenance and tectonic settings

Authors: MAHAMUDA ABU, EMMANUEL DAANOBA SUNKARI

Abstract: The paleoweathering, provenance, and tectonic setting of sediments of the western coast of Ghana were unraveled using the geochemistry of 29 beach sands, which are characterized by coarse-, medium-, and fine-grained sizes. The coarse-grained beach sands contain higher SiO₂ content (2.9-96 wt.%) than the medium-grained (4.9-94 wt.%) and fine-grained (16.1-90.7 wt.%) sands, implying that the increase in grain size is related to the increase in SiO₂ content. Al₂O₃ and CaO concentrations are higher in the fine- and medium-grained sands than the coarse-grained sands. The beach sands are compositionally immature based on the index of compositional variable values (1.17-141) and Th/Sc versus Zr/Sc diagram although they have high SiO₂/Al₂O₃ values. The high SiO₂/ Al₂O₃ is not indicative of the weathering conditions of the coastal sediments in the area. The sands are chemically unaltered clastic materials of first cycle regime that still have their labile minerals retained in them very close to the sediment source based on the chemical index of alteration values, plagioclase index of alteration values, and A-CN-K and A-CNK-FM ternary diagrams. The total rare earth elements (ΣREE) content increases with decreasing grain size. However, there are some discrepancies where some coarsegrained samples have high ΣREE content. This suggests that apart from the grain size, the provenance of the sediments has direct control over their geochemical composition. The correlation of the rare earth elements patterns of the beach sands with those of adjacent source rocks points to felsic sources for their derivation. The felsic (granodioritic composition) igneous suites of the Paleoproterozoic Birimian highland rocks adjacent to the beach sediments in the western coast are possibly the source rocks of the beach sands. Rifting in the continents is the tectonic regime probably during the Paleoproterozoic times responsible for deposition of the beach sands in the western coast of Ghana.

Keywords: Beach sands, geochemistry, provenance, tectonic setting, western coast, Ghana

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