Provenance of sandstones from the Neoproterozoic Bombouaka Group of the Volta Basin, northeastern Ghana
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ORIGINAL PAPER
Provenance of sandstones from the Neoproterozoic Bombouaka Group of the Volta Basin, northeastern Ghana C. Y. Anani 1 & A. Mahamuda 2 & D. Kwayisi 1 & D. K. Asiedu 1
Received: 6 December 2016 / Accepted: 11 October 2017 / Published online: 30 October 2017 # Saudi Society for Geosciences 2017
Abstract The provenance and tectonic setting of sandstones from the Bombouaka Group of the Voltaian Supergroup, in the northeastern part of Ghana, have been constrained from their petrography and whole-rock geochemistry. Modal analysis carried out by point-counting sandstone samples indicates that they are quartz arenites. The index of compositional variability values and SiO2/Al2O3, Zr/Sc, and Th/Sc values indicates that the sediments are mature. The sandstones are depleted in CaO and Na2O. They are, however, enriched in K2O, Ba, and Rb relative to average Neoproterozoic upper crust. These characteristics reflect intense chemical weathering in the source region as proven by high weathering indices (i.e., CIA, PIA, and CIW). In comparison with average Neoproterozoic upper crust, the sandstones show depletion by transition metals and enrichment by high field strength elements. They generally show chondrite-normalized fractionated light rare-earth element (LREE) patterns (average LaN/SmN = 4.40), negative Eu anomalies (average Eu/ Eu* = 0.61), and generally flat heavy rare-earth elements (HREE) (average GdN/YbN = 1.13). The sandstones have La/Sc, Th/Sc, La/Co, Th/Co, Th/Cr, and Eu/Eu* ratios similar to those of sandstones derived from felsic source. Mixing calculations using the rare-earth elements (REE) suggests 48% tonalite–trondhjemite–granodiorite and 52% granite as possible proportions for the source of the sandstones. Both the petrographic and whole-rock geochemical data point to a passive margin setting for the sandstones from the Bombouaka Group.
* C. Y. Anani [email protected]
1
Department of Earth Science, University of Ghana, Legon, Accra, Ghana
2
Department of Earth and Environmental Sciences, University for Development Studies, Navrongo, Ghana
Keywords Geochemistry . Siliciclastic . Tectonic setting . Source rock . Modal analysis . Mature sediments
Introduction The mineralogical and chemical compositions of siliciclastic sedimentary rocks are controlled by the composition of their source rock. Processes such as weathering, transportation, climatic conditions, and relief features affect the source rock’s compositions (Armstrong-Altrin et al. 2015, 2017). These subsequently control the mineralogical and chemical compositions of siliciclastic sedimentary rocks (McLennan et al. 1990; Adel et al. 2010; Anani et al. 2013). These may be inferred through an examination of the constituents of clastic sedimentary rocks (e.g., Condie 1993; Fedo et al. 1996; Anani et al. 2013). Studies have shown that immobile trace elements of siliciclastic sedimentary rocks such as Nb, V, Ni, Co, Sc, Y, La, and Zr are least influenced by weathering, diagenesis, and low degree of metamorphism and as such are preserved i
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