Stable Isotope Evidence for Ca. 2.7-Ga-Old Archean Cap Carbonates from the Dharwar Supergroup, Southern India

Carbon isotope compositions of carbonate rocks from ~2.7-Ga-old Neoarchean Vanivilas Formation of the Dharwar Supergroup presented earlier by us are reevaluated in this study, besides oxygen isotope compositions of a few silica-dolomite pairs. The purpose of such a revisit assumes significance in view of recent field evidences that suggest a glaciomarine origin for the matrix-supported conglomerate member, the Talya conglomerate, which underlies the carbonate rocks of the Vanivilas Formation. An in-depth analysis of carbon isotope data reveals preservation of their pristine character despite the rocks having been subjected to metamorphism to different degrees (from lower greenschist to lower amphibolite facies). The dolomitic member of Vanivilas Formation of Marikanive area is characterized by highly depleted δ¹³C value (up to -5‰ VPDB) and merits as the Indian example of ca. 2.7-Ga-old cap carbonate. This inference is further supported by estimated low temperature of equilibration documented by a few silica-dolomite pairs from the Vanivilas Formation collected near Kalche area. These pairs show evidence for oxygen isotopic equilibrium at low temperatures (~0-20°C) with depleted water (δ¹⁸O = -21‰ to -15‰ VSMOW) of glacial origin. We propose that the mineral pairs were deposited during the deglaciation period when the ocean temperature was in its gradual restoration phase. The dolomite of Marikanive area is the first record of cap carbonates from the Indian subcontinent with Neoarchean antiquity.