RB-SR AGES OF PROTEROZOIC KIMBERLITES OF INDIA - EVIDENCE FOR CONTEMPORANEOUS EMPLACEMENT

Rb-Sr analyses of leached phlogopite macrocrysts from kimberlite pipes 1, 2, 5 and 7 in south India give concordant ages of 1091 +/- 20, 109 2 +/- 15,1093 +/- 20 and 1091 +/- 10 Ma, respectively, indicating cont emporaneous emplacement of these pipes. The present results are in con tradiction with earlier measurements. Groundmass mineral assemblages w ith low Rb/Sr ratios give a tight set of initial Sr ratios at 0.7029 /- 2, 0.7019 +/- 2, 0.7029 +/- 2 and 0.7030 +/- 4 suggesting derivatio n of the primary kimberlite magma from a relatively uniform and deplet ed mantle source. A similar age and initial Sr isotopic ratio of one s ample of the Majhgawan lamproite from Central India imply that the Pro terozoic kimberlite/lamproite activities in India, though widely separ ated in space, were nearly contemporaneous.