Rb-Sr ages of Proterozoic kimberlites of India: evidence for contemporaneous emplacement

Abstract

Rb---Sr analyses of leached phlogopite macrocrysts from kimberlite pipes 1, 2, 5 and 7 in south India give concordant ages of 1091±20, 1092±15, 1093±20 and 1091±10 Ma, respectively, indicating contemporaneous emplacement of these pipes. The present results are in contradiction with earlier measurements. Groundmass mineral assemblages with 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 derivation of the primary kimberlite magma from a relatively uniform and depleted mantle source. A similar age and initial Sr isotopic ratio of one sample of the Majhgawan lamproite from Central India imply that the Proterozoic kimberlite/lamproite activities in India, though widely separated in space, were nearly contemporaneous.