NEW PROTEROZOIC K-AR AGES FOR SOME KIMBERLITES AND LAMPROITES FROM THE CUDDAPAH BASIN AND DHARWAR CRATON, SOUTH-INDIA - EVIDENCE FOR NONCONTEMPORANEOUS EMPLACEMENT

We report new K-Ar age determinations carried out on mica separates fr om kimberlites and lamproites that occur within and adjacent to the in tra-cratonic Cuddapah basin in southern India. This work includes the first ages obtained from the Ramannapeta lamproite (at the northeaster n margin of the Cuddapah basin) and from one of the kimberlites in the Mahbubnagar district. The Ramannapeta lamproite and the Kotakonda kim berlite were both emplaced in the Proterozoic and have ages of 1384 +/ - 18 and 1363 +/- 48 Ma, respectively. These dates are similar to the 1350 +/- 52 Ma age obtained here for the Chelima lamproite (within the Cuddapah basin). The Muligiripalle kimberlite (Pipe 5 of the Anantapu r district) on the other hand yielded a much lower age of 1153 +/- 17 Ma. Our results demonstrate that the emplacement of the Indian kimberl ites and lamproites during the Proterozoic did not take place within t he span of 30 Ma around 1090 Ma as some workers have recently advocate d, but occurred over a period of > 300 Ma. The Chelima (within the Cud dapah basin) and Ramannapeta lamproites represent some of the oldest l amproites world-wide and, together with the Kotakonda kimberlite, they might constitute a hitherto unrecognised Proterozoic episode of potas sic magmatism at similar to 1350-1380 Ma.