Geochemistry of Archean-Proterozoic shales, Dharwar Craton, India: Implications on depositional environment

Shales are significant sedimentary units in the Archean greenstone belts and Proterozoic Cuddapah sequence of Dharwar Craton. We present the geochemical studies of the Archean shales from the Chitradurga greenstone belt and the Proterozoic Vempalle Formation of the Cuddapah Basin to evaluate their chemical composition, weathering, provenance, and depositional conditions. The Archean shales are depleted in transition metals (Ni and Co) but enriched in V, Cr, and Sc relative to upper continental crust, while the Proterozoic shales are depleted in Cr, Co, Ni, Sc, and V compared with Post-Archean Australian shale reflecting on negligible mafic source during their deposition. The REE patterns of these shales are uniform with moderately flat LREE and negative to positive Ce anomalies reflecting on marine conditions of their deposition. The presence of both positive and negative Ce anomalies indicate fluctuating oxic and anoxic conditions. The Archean and Proterozoic shales show negative and positive Eu anomalies, respectively, along with slightly enriched HREE. Compositional variation is observed in the Archean shales, whereas the Proterozoic counterparts were affected by sediment recycling as displayed by the Th/Sc and Zr/Sc relationship. Th/U and Th along with (Gd/Yb)n ratios display significant heavy mineral enrichment in these shales. The chemical index of alteration (CIA) and chemical index of weathering (CIW) of these shales suggest moderate to intense chemical weathering during the Archean and low to moderate weathering during Proterozoic times. The overall geochemical signatures collectively indicate granitic and tonalitic provenance for the Archean and Proterozoic shales, which were deposited in an active and passive continental margins.