Carbon, Oxygen and Strontium Isotope Geochemistry of Carbonate Rocks from Kurnool Group, Southern India

Carbon isotopic compositions of carbonate rocks from Narji and Koilkuntla Formations belonging to the Kurnool Group range frorn -1.5 to +2.2%0 and +3.1 to +4.3%0 respectively. 13C values on coexisting organic carbon are around -26%0. ⁸⁷Sr/⁸⁶Sr ratios of limestones from Koilkuntla Formation (0.7086 - 0.7104) exhibit less variation than those belonging to Narji limestones (0.7073 - 0.7132). Bivariant plots of ⁸⁷Sr/⁸⁶Sr and δ¹³C against Mn/Sr and δ¹⁸O show open system diagenetic trends for the carbonate rocks of Narji Formation. Similar plots for Koilkuntla Formation do not reveal any definite alteration trends. Comparison of carbon isotope data coupled with the least altered ⁸⁷Sr/⁸⁶Sr values of Narji and Koilkuntla limestones shows similarity in the isotopic ratios exhibited by other worldwide Late Neoproterozoic carbonate successions. This finding is corroborated by available age information on the underlying Cuddapah Supergroup of rocks as well as palaeontological evidences on the rock formations of Kurnool Group.