Rb-Sr ages of late Archean metavolcanics and granites, Dharwar craton, South India and evidence for Early Proterozoic thermotectonic event (s)

Abstract

The Archean greenstone sequence of the Dharwar craton is intruded by late- and post-kinematic granites like the Chitradurga and Toranagallu (Closepet) granites with whole-rock Rb---Sr isochron ages of 2603 ± 28 Ma (2σ) and 2452 ± 50 Ma, respectively. The whole-rock Rb---Sr ages of their host metavolcanic rocks show a wide variation; Bababudan, 2728 ± 102 Ma; Shimoga, 2520 ± 62 Ma; Ingaldhal, 2240 ± 50 Ma; Mardihalli, 2160 ± 174 Ma; Gadag, 2456 ± 76 Ma and Sandur, 2377 ± 646 Ma. While the ages of the Bababudan and the Shimoga volcanic suites can be related to the time of the pervasive regional folding and metamorphic events (~ 2.7-2.6 Ga), those of the other volcanic suites from the Chitradurga and Sandur supracrustal belts indicate effects of post-Archean thermal event(s). The internal isochron ages of 2.0 ± 0.1 Ga for the Chitradurga and Toranagallu granites suggest isotopic resetting on a mineral scale about ~500 Ma after their emplacement and support the evidence from the metavolcanic suites for an Early Proterozoic thermal event at ~ 2.0 Ga in the Dharwar greenstone-granite/gneiss terrain.