Archean seawater composition and depositional environment - Geochemical and isotopic signatures from the stromatolitic carbonates of Dharwar Craton, India

Abstract

The well-preserved stromatolitic carbonates from Neoarchean Sandur, Chitradurga and Shimoga greenstone belts of Dharwar Craton were studied to constrain the depositional conditions, geochemical, isotopic characteristics and the Archean seawater temperatures. These rocks display limited diversity in their morphological features such as columnar (occasionally branched), pseudo-columnar, domal, crinkly laminated, and oval/elliptical shapes. The stromatolites are associated with Fe-Mn formations along with arenites, argillites, limestones and carbonaceous shales. These stromatolites are cherty dolomites having variable SiO sub(2), MgO and CaO contents. The stromatolitic carbonates of the three greenstone belts possess low total Rare earth elements plus Ytterium (sigma REY) along with positive La, Eu, Gd, Y anomalies and Y/Ho ratios reflecting their deposition in marine conditions with variable hydrothermal and terrigenous inputs. These stromatolites display predominantly negative delta sup(13) C values ranging from -1.61 percent 0 to 0.01 percent 0 VPDB of Sandur, - 2.53 percenrt 0 to 1.35 percent 0 VPDB of Chitradurga and positive delta sup(13) C of Shimoga varying from -0.30 percent 0 to 1.29 percent 0 VPDB combined with the negative delat sup(18) O values (-20.95 percent 0 to - 7.72 percent 0 VPDB). Our studies indicate fluctuating oxic to anoxic; acidic to alkaline environment responsible for the deposition of these stromatolites with locally varying Archean ocean temperatures from 25 to 75 degrees C. These conditions favoured the deposition of cherty dolomitic stromatolites in a shallow marine environment ranging from supra to subtidal zones with the minor influence of hydrothermal activity which is evident for the existence of Archean smaller plate tectonics.