PROTOLITH FORMATION ACROSS THE CAUVERY SHEAR ZONE IN SOUTHERN GRANULITE TERRAIN, SOUTHERN INDIA: EVIDENCE FROM U-PB ZIRCON GEOCHRONOLOGY

The Southern Granulite Terrain (SGT) in southern India is widely believed to be a collage of many Late-Archean-Paleoproterozoic to Neoproterozoic terranes now separated by ductile shear zones. One of these is the Cauvery Shear Zone (CSZ) which is an approximately 350 km long E-W trending major ductile shear zone separating mafic-intermediate granulite rocks to the north from amphibole-biotite- migmatitic granitoid gneisses to the south. Existing geochronological data consisting mainly of Nd model ages, Rb-Sr biotite-whole rock ages and a few U-Th-Pb zircon and monazite ages, have been interpreted variedly but in general indicate high-grade metamorphism during late-Archaean-Paleoproterozoic to the north and imprints of Pan-African orogeny on the Proterozoic crust south of CSZ. The time of protolith formation on either side of the CSZ remains debatable. We here present U-Pb zircon ages, from multi-grain measurements by TIMS, across the CSZ to provide new insights on the Precambrian evolution of the SGT.

Elongated as well as rounded zircon fractions from two samples of granitic-gneisses from south of the CSZ yield ²⁰⁷Pb/²⁰⁶Pb ages from 2374.8 to 2431.8 Ma; upper-intercept ages on the concordia diagram are 2492 ± 130 and 2375 ± 10 Ma. Zircons, predominantly elongated in morphology and dark brown to pale brown in color, from a granulite sample from north of CSZ, yield ²⁰⁷Pb/²⁰⁶Pb ages from 2453.6 to 2556.3 Ma. Two fractions of elongated and light gray colored zircons, from a granite intruded within the shear zone, yield ²⁰⁷Pb/²⁰⁶Pb ages of 2461.3 and 2573.2 Ma. Similar zircon ages of ~ 2.5 Ga from the north of CSZ have been earlier interpreted to represent the time of high-grade metamorphism. However, we obtained a Sm-Nd mineral isochron which indicates independently that the granulite facies metamorphism in this region occurred as late as ~880 Ma ago; and thus these zircon ages record the time of protolith formation for these rocks.

These new results, even though preliminary, clearly indicate that the region south of the CSZ does not consist entirely of Neoproterozoic crust, and probably the regions north and south of CSZ have experienced a single protracted event of crust formation ~ 2.37 to 2.57 Ga ago. More detailed data and its implications to the fragmentation and assembly of supercontinents would be discussed during the meeting.