Geochemistry of Upper Birimian sediments (major and trace elements and Nd–Sr isotopes) and implications for weathering and tectonic setting of the Late Paleoproterozoic crust

Abstract

The Upper Birimian (Paleoproterozoic) formations consist mainly of a detrital sedimentary pile intercalated with calc-alkaline volcanites that were accreted during the Eburnean orogeny (∼2.1 Ga). The sediments of the Upper Birimian group were analyzed for their major and trace elements concentrations and their Nd–Sr isotopic compositions. The shales are enriched in Al 2O 3 and K 2O relative to PAAS and depleted in the other mobile major elements and LILE whereas the fe-shales show enrichments in CaO and MnO and depletions in Al 2O 3 and K 2O relative to PAAS but have similar depletion in LILE. Together with the subarkose samples, the shales have undergone significant post-depositional K metasomatism as indicated in the A–CN–K diagram. The calculated pre-metasomatism CIA values (83–92) indicates a high degree of source weathering compatible with steady-state weathering under tropical to subtropical conditions. The fe-shales have lower CIA values (46–57) reflecting less intense weathering conditions. Low Eu anomalies and Th/Sc ratios combined with high Cr/Th ratios for certain samples indicate a significant input of basic detritus for the analyzed sediments. Provenance modeling indicates that the shales are best modeled with a mixture having 75% basalt (75B) and 25% granite (25G). The fe-shales have a similar provenance with their compositions being modeled with a 80B 20G mixture. The ɛ Nd values of selected shales, calculated at 2.1 Ga, the age of the deposition, vary from −2.8 to −8.6, which indicate that the sediment received 60–100% of its detritus from a juvenile Birimian volcanic arc. Therefore, the Birimian sediments deposited within an active continental margin setting mostly supplied by the adjacent volcanic arc.