Rb-Sr age of kimberlites and lamproites from Eastern Dharwar craton, southIndia

Acid-leached phlogopite Rb-Sr isochron ages of the Kotakonda and Mudalbid k imberlites in the Narayanpet kimberlite field northwest of the Cuddapah bas in, south India are 1085 +/- 14 and 1099 +/- 12 Ma, respectively. These con cordant ages are much younger than the recently reported K-Ar and Ar-40-Ar- 39 ages of 1363 +/- 48 and 1402 +/- 5 Ma, respectively for the former kimbe rlite, and support our earlier conclusion that the south Indian kimberlites erupted episodically close to 1090 Ma ago. Three lamproites - one each fro m Ramannapeta at the northeastern margin and Chelima and Zangamarajupalle w ithin the Cuddapah basin - give distinctly different Rb-Sr ages of 1224 +/- 14, 1354 +/- 17 and 1070 +/- 22 Ma, respectively. While the Rb-Sr age of t he Ramannapeta lamproite is also younger than an earlier reported K-Ar age of 1381 +/- 18 Ma, it is the same as that of many spatially close alkaline complexes like at Elchuru and Kunavaram. The ages for the Chelima and Zanga marajupalle rocks are only tentative, as these rocks contain a large second ary carbonate component with its Sr isotopic composition genetically unrela ted to that of their phlogopites.