POTASSIUM-ARGON AGES OF DOLERITE DYKES IN LATE ARCHEAN EARLY PROTEROZOIC CLOSEPET GRANITE

Emplacement of the Closepet granite around 2.6 to 2.5 Ga age was the last major event in the crustal formation process in the Dharwar Craton. This granitic body has been intruded by a number of dykes mainly trending E-W or NE-SW between Madhugiri and Tumkur. Potassium-argon ages on these dykes range from 2.5 Ga to 1.2 Ga and can be classified into three different phases. The first phase (2.5 -2.4 Ga) dyke activity might be correlated with emplacement of the Closepet Granite. The second phase (1.8-1.6 Ga) of dyke activity could be associated with hypabyssal magmatism connected with the Proterozoic Cuddapah Basin formation and volcanism. The last phase (1.4-1.2 Ga) represents the middle Proterozoic dyke activity during which emplacement of Rapakivi granites and alkaline magmatism were reported.