Small scale sampling for Pb–Pb dating of marbles: Example from the Sargur supracrustal rocks,Dharwar Craton,South India

The scale of sampling of marbles could be crucial for their precise Pb–Pb dating, as the range of Pb isotopic homogenization during metamorphism of their sedimentary protoliths may not exceed a few centimeters, unless mediated by a large and pervasive influx of externally derived fluids. We report well defined Pb–Pb isochrons based on the analysis of small subsamples (～200 mg) from each of two hand specimens of a marble band near Bettadabidu village in the type area of the Sargur Group rocks, Dharwar craton, south India. The age and model μ₁ value (2484 ± 71 Ma and 8.73 ± 0.05, respectively) for this marble are in good agreement with those reported earlier for carbonate rocks of the Dharwar Supergroup from the Sandur schist belt to the north. The available carbon and oxygen isotopic data on these marbles give no indication of a difference that is believed in the depositional and metamorphic histories of the Sargur and Sandur carbonates. So a comparable, if not common, crustal history for these two carbonate occurrences cannot be ruled out.