Rare earth element geochemistry and Rb-Sr geochronology of Archaean stromatolitic cherts of the Dharwar craton, south India

Abstract

Stromatolites associated with cherty dolomites of the Vanivilaspura Formation of the Archaean Dharwar Supergroup show a morphology indicative of the deposition of the latter in a intertidal to subtidal environment. The cherts are moderately high in their Al/Al + Fe ratios but depleted in Fe₂C₃ and also most trace elements. Unlike most other Archaean cherts, the Vanivilaspur cherts exhibit significant negative Ce anomaly, which is interpreted to have resulted from contemporary manganese deposition. The Rb/Sr ratios in the cherts show a sufficient spread to define a linear correlation line in the Rb-Sr evolution diagram corresponding to an age of 2512 ± 159 Ma and initial Sr ratio of 0.7128 ± 0.0012 (2σ ). While this age is strikingly close to that of regional metamorphism in the Dharwar craton, the initial ratio is distinctly higher than that of the associated volcanics. Acid leaching experiments on the cherts suggest that they may have been isotopically equilibrated on a mm to cm scale about 500 Ma later than the time of regional metamorphism.