Neodymium isotope composition of Palaeoproterozoic Birimian shales from the Wa‐Lawra Belt, north‐west Ghana: Constraints on provenance

Abstract

An Nd‐isotopic study was carried out on Palaeoproterozoic Birimian shales from the Wa‐Lawra volcanic belt, located in the southeastern margin of the West African Craton. The objective of this study was to assess the nature and average crustal residence ages of the source areas, and deduce the provenance of these shales. Nd‐model ages range from 2.12 to 2.58 Ga, which are within the range of Nd‐model ages reported for the Birimian volcanic rocks, suggesting that the Birimian volcanic rocks are the main sources of detritus for the Birimian shales. The εNd values of the Birimian shales, calculated at an age of 2.1 Ga, which represents the time of the deposition of the Birimian shales, ranges from −1.65 to +3.01. The range of observed εNd values (i.e., −1.65 to +3.01) suggest that the Birimian shales were predominantly derived from the juvenile Palaeoproterozoic crust, with minor contribution of older crustal material. εNd(T) versus Th/Sc, and ƒSm/Nd versus εNd(T) systematics indicate that the provenance can be explained by mixing between Palaeoproterozoic volcanic arc rocks and variable amounts of Archean continental crustal rocks. Provenance modelling shows that the Birimian shales received at least 80% detritus from a juvenile Birimian volcanic arc. The mean Archean component is 8%, suggesting that the continental crust of the Wa‐Lawra Belt is clearly dominated by material derived directly from the mantle.