Geochemistry and provenance of Neoproterozoic metasedimentary rocks from the Togo structural unit, Southeastern Ghana.

Abstract

Neoproterozoic metasedimentary rocks of greenshist facies, consisting predominantly of quartzites, phyllites, and phyllonites, occur in the Akwapim range of the Togo Structural Unit (TSU) in Ghana. The geochemistry of the phyllites were studied to determine their provenance and tectonic setting. The major element analysis was carried out by the Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) and trace elements, including REEs, by the Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) method. The studied metasedimentary rocks have SiO2 and Al2O3 contents comparable to that of average Neoproterozoic upper crust. The metasedimentary rocks are strongly depleted in CaO, Na2O, and Sr and enriched in K2O, Ba and Rb with respect to average Neoproterozoic upper crust, reflecting K addition during diagenesis. Zr and Hf concentrations are significantly above Neoproterozoic upper crustal values. Chondrite-normalized rare earth element (REE) pat terns are characterised by fractionated light-REE (LREE) (average LaN/SmN = 3.68), significant negative europium anomaly (average Eu/Eu* = 0.61) and fairly flat heavy-REE (HREE) (average GdN/YbN = 1.3). The geochemical data, particularly the high La/Sc, Th/Sc, La/Co, Th/Co, Zr/Sc, La/Th ratios, Eu/Eu* values, and high Zr and Hf concentrations, suggest that the metasedimentary rocks of the Togo Structural Units in the Akwapim range were derived mainly from recycled sedimentary sources. Comparison of geochemical signatures of the studied metasedimentary rocks with those of the Paleoproterozoic Birimian rocks suggests that the felsic materials in the Togo Structural Units (TSU) could not have been derived predominantly from the Birimian rocks, implying more distal sources. The studied metasedimentary rocks exhibit provenance characteristics si milar to that of the Kwahu/Bombouaka Group of the Voltaian Basin, suggesting derivation from the same source, probably in the Amazon Craton. The metasedimentary rocks exhibit geochemical characteristics indicative of sediments derived from a passive continental margin. This inference supports previous studies that indicate that prior to the Pan-African Dahomeyide orogenic event, the southern margin of the West African Craton (WAC) was under passive margin settings.