AGE OF YOUNGER TONALITIC MAGMATISM AND GRANULITIC METAMORPHISM IN THESOUTH INDIAN TRANSITION ZONE (KRISHNAGIRI AREA) - COMPARISON WITH OLDER PENINSULAR GNEISSES FROM THE GORUR-HASSAN AREA

A major episode of continental crust formation, associated with granul ite facies metamorphism, occurred at 2.55-2.51 Ga and was related to a ccretional processes of juvenile crust. Dating of tonalitic-trondhjemi tic, granitic gneisses and charnockites from the Krishnagiri area of S outh India indicates that magmatic protoliths are 2550-2530+/-5Ma, as shown by both U-Pb and Pb-207/Pb-206 single zircon methods. Monazite a ges indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510+/-10 Ma. These data pro vide precise time constraints and Sr-Nd isotopes confirm the existence of late tonalitic-granodioritic juvenile gneisses at 2550 Ma. Pb sing le zircon ages from the older Peninsular gneisses (Gorur-Hassan area) are in agreement with some previous Sr ages and range between 3200+/-2 0 and 3328+/-10 Ma. These gneisses were derived from a 3.3-3.5-Ga mant le source as indicated from Nd isotopes. They did not participate sign ificantly in the genesis of the 2.55-Ga juvenile magmas. All these dat a, together with previous work, suggest that the 2.51-Ga granulite fac ies metamorphism occurred near the contact of the ancient Peninsular g neisses and the 2.55-2.52-Ga 'juvenile' tonalitic-trondhjemitic terran es during synaccretional processes (subduction, mantle plume?). Rb-Sr biotite ages between 2060 and 2340 Ma indicate late cooling probably r elated to the dextral major east-west shearing which displaced the 2.5 -Ga juvenile terranes toward the west.