Geochemistry of Paleoproterozoic black shales from the Intracontinental Cuddapah basin, India: implications for provenance, tectonic setting, and weathering intensity

The Cuddapah Supergroup is one of the series of thick Proterozoic sedimentary sequences developed on Archean cratonic peninsular India. The stratigraphically higher Cumbum Formation includes well-preserved and unweathered black shales, that include marine facies, recording lower greenschist facies metamorphism. In multielement normalized diagrams, conservative elements (Ti, Al, REE, Sc, Co) plot parallel, and close to, post-Archean upper continental crust (PA-UCC). Small variations in absolute concentrations of conservative elements covary with Fe2O3 content, which ranges from 1.6–11 wt.%; accordingly, those concentrations are controlled by hydrothermal leaching or additions of Fe, consistent with reduced fluids reflected in variable Eu anomalies. In Th/Sc vs. Sc co-ordinates, these shales trend to slightly lower Sc but greater Th/Sc than either PA-UCC or average Proterozoic shale. Cratonic lithologies, such as tonalite clasts or detrital gold, are not represented in basal conglomerates, and REE systematics and low Al2O3/Yb, Th/Yb, and Zr/Sm ratios rule out a significant tonalite component in the source; consequently, both sedimentary and geochemical evidences endorse an extra-cratonic provenance. The best compositional fit is to average Paleoproterozoic, Andean-type continental margin, granite, with ∼10% of Closepet-type granite contributing to the excess Zr and REE. An active continental margin provenance is also consistent with the Th–Sc–Zr discriminant diagram of siliciclastic sedimentary rocks. The chemical index of alteration (CIA), corrected for diagenetic addition of K, is 85–100 indicative of intense weathering of the catchment, compared to a range of mean values of 70–88 at ∼1.8 Ga. Shales are variably enriched in V–Cr–Ni relative to PA-UCC consistent with the established kerogen–transition metal association.The Cuddapah Supergroup may have evolved as a foreland basin to the 2.1–1.8 Ga Central Indian Tectonic Zone, with its associated granites, evolving into an intracontinental basin connected to the ocean given the marine kerogen–V–Ni association. This scenario is similar to the Paleoproterozoic McArther River, Australia, and Athabasca, Canada, basins which evolved from foreland basins respectively to the Barrumundi and Trans Hudson orogens, collectively as part of assembly of the proposed Paleoproterozoic supercontinent Columbia.