A geochemical, Nd-Sr, and stable Ca isotopic study of siliciclastic and chemical sedimentary rocks, volcanic tuffs, and authigenic glauconites from Proterozoic sedimentary basins of India

Abstract

The Archean-Proterozoic transition is marked by globally synchronous changes in tectonothermal and climatic patterns along with development of widespread epicontinental seas, with extensive shelf-sedimentation. The Indian Shield hosts several Proterozoic sedimentary basins, popularly known as the Purana basins, with mostly undeformed and unmetamorphosed siliciclastic and carbonate sediments. The undeformed and unmetamorphosed nature of these sediments, along with their prolonged sedimentation, makes them excellent geological archives of past tectono-thermal and climatic events as well as paleo-depositional environments, and makes them suitable for geochemical and isotopic studies. In this study, sedimentary rocks were collected from the Paleo-Mesoproterozoic Cuddapah Basin and the Mesoproterozoic Chhattisgarh Basin from the Southern Indian Block (SIB), the Paleo-Mesoproterozoic Vindhyan Basin from the Northern Indian Block (NIB), and the Neoproterozoic Marwar Basin from the Western Indian Block (WIB) or the Marwar Block (MB). A geochemical and Nd isotope study of the siliciclastic sediments from the Cuddapah, Vindhyan, and Chhattisgarh basins show that during periods of tectonic stability in the Indian Craton, these basins derived detritus from evolved sources with unradiogenic Nd isotopic compositions. Periods of juvenile magmatism can be well-traced in the associated sediments across the basins. The volcanic tuffs from Cuddapah (1.86 Ga), Singhora (1.5 Ga), and Chhattisgarh basins (1.0 Ga) show similar geochemical compositions suggesting their derivation from ancient arcs. The Nd isotope evolution of these tuffs, with widely-varying eruption ages, suggest their derivation from a common mantle source. Chemically precipitated Precambrian sediments have been variably used to infer the redox condition of early Earth, to trace the fluxes of crustal and mantle derived cations and anions to the oceans, and to understand the paleo-habitat of ancient lifeforms. However, a significant portion of the preserved Precambrian carbonates are considered to have been deposited in epicontinental environments. To constrain the paleo-depositional environments of the extensive carbonate deposits in the Proterozoic sedimentary basins of India, the samples were first screened for diagenesis using Mn/Sr and Fe/Sr ratios followed by analyses of REY (REE+Y), radiogenic Sr, and stable Ca isotopic compositions. Paleo-Mesoproterozoic carbonates from Cuddapah, Vindhyan, and Chhattisgarh basins do not show prominent seawater-like geochemical signatures. These carbonates also record relatively radiogenic 87Sr/86Sr compositions, which suggests high freshwater input and possibly, deposition under epeiric settings. In contrast, geochemical and isotopic compositions of carbonates from the Neoproterozoic Marwar Basin suggest deposition in an open-ocean setting. Compared to the Paleo-Mesoproterozoic carbonates from Cuddapah, Vindhyan and Chhattisgarh basins, the Neoproterozoic Marwar carbonates record significantly higher δ44/40CaSRM915a values. These high values overlap with isotopic compositions of global Neoproterozoic post-glacial successions. Based on our inferences on provenance and paleo-environmental conditions of the Vindhyan Basin, correlation between the Chambal Valley and the Son Valley sub-basins were investigated based on Nd and Ca isotopic compositions of the Vindhyan carbonates. A significant shift in Nd isotopic composition towards radiogenic values around 1.6 Ga ago is observed in carbonates from both sub-basins, which is consistent with eruption of a juvenile Andean-type arc situated near the Son Valley sub-basin. Similar Ca isotopic composition of the limestone horizons in the two sub-basins suggest deposition from a single water-mass. This similarity in δ44/40CaSRM915a composition suggests that during the deposition of the Lower Vindhyan carbonates, the two sub-basins were connected. In contrast, Upper Vindhyan carbonates from two sub-basins show compositional differences, which suggests that during deposition of the Upper Vindhyan carbonates, the two sub-basins were disconnected. Glauconitic sandstones and limestones from the Semri Group of Son Valley Vindhyans were analysed for their geochemical and Sr isotope compositions. The REE+Y (REY) composition of these glauconites suggest their formation in an enclosed condition without significant interaction with the Proterozoic seawater, while their Sr isotopic compositions suggest that these minerals formed much after the deposition of the ~ 1.6 Ga old Semri Group of sediments. The latter observation is consistent with formation of Precambrian glauconies by pseudomorphic replacement of the substrates.