Precambrian Research, 59 ( 1992 ) 145-170 145 Elsevier Science Publishers B.V., Amsterdam Rb-Sr ages of Late Archean metavolcanics and granites, Dharwar craton, South India and evidence for Early Proterozoic thermotectonic event (s) Y.J. Bhaskar Rao, T.V. Sivaraman, G.V.C. Pantulu, K. Gopalan and S.M. Naqvi National GeophysicalResearchInstitute (NGRI), UppalRoad, Hyderabad, 500 007, India (Received December 1990; accepted after revision February 5, 1992 ) ABSTRACT Bhaskar Rao, Y.J., Sivaraman, T.V., Pantulu, G.V.C., Gopalan, K. and Naqvi, S.M., 1992. Rb-Sr ages of Late Archean metavolcanics and granites, Dharwar craton, South India and evidence for Early Proterozoic therrnotectonic event (s). Precambrian Res., 59:145-170. The Archean greenstone sequence of the Dharwar craton is intruded by late- and post-kinematic granites like the Chi- tradurga and Toranagallu (Closepet) granites with whole-rock Rb-Sr isochron ages of 2603 +_ 28 Ma (2tr) and 2452 _+ 50 Ma, respectively. The whole-rock Rb-Sr ages of their host metavolcanicrocks 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 perva- sive 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 ToranagaUugranites suggest isotopic resetting on a mineral scale about ~ 500 Ma after their emplace- ment 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. Introduction Geological and geochronological literature on the Precambrian granite-greenstone ter- rains of the Dharwar craton, S. India, (sum- marised by Naqvi and Rogers, 1987 ) indicates that the crustal growth took place mainly in the Middle to Late Archean ( ~ 3.4-2.7 Ga). The supracrustals and the gneisses were multiply deformed, metamorphosed and migmatized during the Archean, the last of these events oc- curring about 2.7-2.6 Ga ago. Widespread em- placement of K-rich granite occurred between Correspondence to: Y.J. Bhaskar Rao, National Geophys- ical Research Institute (NGRI), Uppal Road, Hydera- bad, 500 007, India. 2.6-2.5 Ga. The Archean crust is known to have been reworked during the Proterozoic but the chronology of these events is not estab- lished. The craton is devoid of Early Protero- zoic supracrustal formations and is generally considered to have been tectonically stable during much of the Early Proterozoic. Thus, Drury et al. (1984) suggested that the Archean crust was reworked mainly during the Middle Proterozoic thermotectonic events ( ~ 1.7-1.2 Ga) which included faulting and thrusting in the vicinity of the Chitradurga greenstone belt (Ch, Fig. l ) and development of the Middle- Proterozoic intracratonic basins (Fig. 1). However, Radhakrishna and Naqvi (1986) proposed two major periods of crustal rework- ing: the Early Proterozoic ( ~ 2.6-2.0 Ga) and 0301-9268/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.