Geochemical and Nd-isotopic compositions of juvenile-type Paleoproterozoic Birimian sedimentary rocks from southeastern West African Craton (Ghana): Constraints on provenance and tectonic setting

The Paleoproterozic Birimian Supergroup consists of sedimentary basins composed predominantly of wackes and argillites, and volcanic belts composed predominantly of basaltic rocks. Whole rock geochemical and Nd-isotopic studies were carried out on fine-grained sedimentary rocks (shales and fine-grained sandstones) from one of the Birimian sedimentary basins, the Kumasi basin. The sedimentary rocks are immature in nature, with very little recycled materials. Relative to average early Proterozoic upper continental crust, the Birimian sedimentary rocks are depleted in the alkalis and alkaline earth elements and the high field strength elements, and enriched in transition metals. Chondrite-normalized rare earth element (REE) plots for both the shales and sandstones show fractionated LREE patterns (average LaN/SmN = 3.45), with slight negative Eu-anomalies (average Eu/Eu∗ = 0.79) and a slightly fractionated HREE patterns (average GdN/YbN = 1.84). Low to moderate degree of chemical weathering in the sediment source area is indicated by Chemical Index of Alteration values of between 52 and 80. Elemental ratios such as La/Co, Th/Co, Th/Cr, and Eu/Eu∗ that are critical to provenance suggests that the sediments were derived from rocks of mixed felsic and mafic compositions. Mixing calculations based on the REEs indicate that, the shale is best modeled with a mixture having 62% TTG, 16% granite and 22% basalt whereas the sandstone is almost 100% TTG. The εNd values for the analyzed samples, calculated at the age of deposition (i.e., 2.1 Ga), range from −1.69 to 1.07, suggesting derivation from an undifferentiated source. The Nd-isotopic compositions, together with Eu/Eu∗ and Th/U values and other trace element characteristics suggest Young Undifferentiated Arc provenance, and this is in agreement with previous studies that show that the Birmian crust is juvenile and little amount of Archean crust is present in the sedimentary rocks. The geochemical and Nd-isotopic systematics further suggest that the bulk of detritus was supplied by adjacent Paleoproterozoic Birimian greenstone belts with only 10% or less contribution from the Archean crust. Trace element characteristics further suggest that deposition took place in an island arc setting.