SHRIMP U PB ZIRCON AGES OF ACID VOLCANIC-ROCKS IN THE CHITRADURGA ANDSANDUR GROUPS, AND GRANITES ADJACENT TO THE SAHDUR SCHIST BELT, KARNATAKA/

SHRIMP U/Pb dating of zircon grains from a schistose acid volcanic roc k from the Daginkatte Formation in the Chitradurga Group of the Late A rchaean Dharwar Supergroup in western Karnataka has yielded a precise concordant age of 2614 +/- 8 Ma which indicates the time of melt cryst allisation. In contrast, zircons from similar schistose acid volcanic rocks in the Vibhuti Gudda Formation of the Sandur Group in the Sandur schist belt of eastern Kamataka yielded less precise concordia interc ept ages of 2658 +/- 14 and 2691 +/- 18 Ma. Zircons from high-strain g rey gneisses which appear to be the oldest recognisable component of t he Late Archaean granite complex adjacent to the Sandur schist belt ha ve an imprecise age of 2719 +/- 40 Ma. Zircons from the youngest recog nisable granite adjacent to the belt have also yielded an imprecise, b ut younger, age of 2570 +/- 62 Ma. The ages show that granite emplacem ent and deformation adjacent to the Sandur schist belt took place in a period of c.150 Ma. The imprecise ages of zircon in the acid volcanic rocks in the Sandur Group and the adjacent granites are related to Ne oproterozoic loss of lead which may have been an effect of either weat hering or a regional thermal event. We favour the latter in the light of the record of Pan-African thermal effects in the east and south of southern Peninsular India.