Limestone xenolith in Siddanpalli kimberlite, Gadwal granite-greenstone terrain, Eastern Dharwar craton, southern India: Remnant of Proterozoic platformal cover sequence of Bhima/Kurnool age?

We report an interesting occurrence of a limestone xenolith in the Siddanpalli kimberlite that intrudes the Gadwal granite-greenstone terrain, Eastern Dharwar craton, southern India. The xenolith is unmetamorphosed, is extremely fine grained (micritic), and displays laminations characteristic of sedimentary carbonate rocks. Its mineralogy, major- and trace-element (including rare earth element) geochemistry, and stable isotopes (C and O) are also consistent with a sedimentary origin. However, no sedimentary rocks have ever been found in the immediate vicinity of the Siddanpalli kimberlite field. The Bhima and Kurnool basins that are widely considered to be homotaxial, having a terminal Neoproterozoic age, based on fossil evidences and lithostratigraphic correlation, are the closest (similar to 50 km) known sedimentary rocks containing similar carbonate lithounits. Therefore, the Siddanpalli carbonate xenolith is inferred to be their preserved remnant. This implies that (i) the platformal sediments of the Bhima/Kurnool basins extended at least up to the Siddanpalli kimberlite cluster; (ii) these two basins were possibly interconnected at the time of kimberlite eruption; and (iii) in contrast to present consensus favoring a terminal Neoproterozoic age for the two basins, the carbonate xenolith and its source unit is of at least late Mesoproterozoic age because the kimberlite emplacement was at 1090 Ma. A recent proposal for an adjustment of similar to 500 Ma in the age of the Neoproterozoic Chattisgarh basin, elsewhere in central India, which was also long considered to be homotaxial with Kurnool/Bhima basins, finds support from our study in that many of the Proterozoic (Purana) sedimentary basins of peninsular India could be, in fact, not younger than the Mesoproterozoic.