Neoarchaean Stromatolitic Carbonates of the Aleshpur Formation, Shimoga Schist Belt, India: Geochemical Insights
S. Chethan Kumar
*
Department of Geology, Bangalore University, Bangalore 560056, India.
Nallamuthu Malarkodi
Department of Geology, Bangalore University, Bangalore 560056, India.
*Author to whom correspondence should be addressed.
Abstract
Neoarchaean stromatolitic carbonates preserve valuable records of early microbial ecosystems, seawater chemistry, and environmental conditions on the primordial Earth. This study investigates the stromatolitic carbonates of the Aleshpur Formation, Shimoga Schist Belt, Western Dharwar Craton, using integrated sedimentological observations, major- and trace-element geochemistry, rare-earth-element and yttrium (REE+Y) systematics, and stable carbon and oxygen isotope analyses to constrain their depositional environment and post-depositional evolution. The carbonates are characterised by high CaO and low Al₂O₃ and TiO₂ contents, indicating minimal siliciclastic contamination and preservation of primary carbonate chemistry. PAAS-normalised REE+Y patterns display positive La and Y anomalies, weak Ce anomalies, and variable Eu anomalies ranging from moderately negative to weakly positive, together with elevated Fe₂O₃ and MnO concentrations, suggesting precipitation from ferruginous Neoarchaean seawater with localised and intermittent hydrothermal influence. Stable isotope compositions exhibit a narrow range of δ¹³C values (−1.60 to +0.18‰ VPDB) and relatively depleted δ¹⁸O values (−16.82 to −13.26‰ VPDB). The coupled variation between δ¹³C and δ¹⁸O, together with the relatively narrow range of δ¹³C values and preservation of primary REE+Y signatures, indicates that carbon isotopes largely retained their primary marine dissolved inorganic carbon (DIC) signature, whereas oxygen isotopes experienced greater post-depositional modification. Integrated sedimentological and geochemical evidence indicates that the Aleshpur stromatolitic carbonates were deposited on a shallow, low-energy carbonate platform under ferruginous marine conditions, with minimal terrigenous input and episodic hydrothermal activity, thereby providing optimum conditions for microbial carbonate growth. This study provides the first comprehensive geochemical framework for the Aleshpur stromatolitic carbonates. It establishes them as a well-preserved archive of Neoarchaean ferruginous shallow-marine environments in the Western Dharwar Craton, thereby improving our understanding of early microbial carbonate formation and seawater evolution.
Keywords: Neoarchaean stromatolites, Aleshpur Formation, Shimoga Schist Belt, Western Dharwar Craton, rare earth elements and yttrium, stable carbon isotopes, stable oxygen isotopes, ferruginous seawater, hydrothermal influence, palaeoenvironmental reconstruction.