SM-ND AND RB-SR AGES AND P-T HISTORY OF THE ARCHEAN-SITTAMPUNDI AND BHAVANI LAYERED META-ANORTHOSITE COMPLEXES IN CAUVERY SHEAR ZONE, SOUTH-INDIA - EVIDENCE FOR NEOPROTEROZOIC REWORKING OF ARCHEAN CRUST

Sittampundi and Bhavani Archean layered meta-anorthosite complexes occ ur as tectonic lenses within the Cauvery shear zone (CSZ), a crustal s cale shear dividing the Precambrian granulite crust of south India int o late Archean (> 2.5 Ga) and Proterozoic (c. 0.55 Ga) blocks. They an d their host supracrustal-gneiss rocks record at least two stages of t ectonometamorphic history. The first is seen as regional scale refolde d isoclinal folds and granulite metamorphism (D-1-M(1)) while the seco nd stage is associated with dominantly E-W dextral transcurrent sheari ng and metamorphic recrystallisation (D-2-M(CSZ)). Whole rock Sm-Nd is ochrons for several comagmatic rocks of the layered complexes yield co ncordant ages: Sittampundi - 2935+/-60 Ma, epsilon(Nd) +/- 1.85+/-0.16 and Bhavani - 2899+/-28 Ma, epsilon(Nd) + 2.18+/-0.14 (2 sigma errors ). Our Sm-Nd results suggest that: (1) the magmatic protoliths of the Sittampundi and Bhavani layered complexes were extracted from similar uniform and LREE depleted mantle sources; (2) M(1) metamorphism occurr ed soon after emplacement at c.3.0 Ga ago. P-T estimates on garnet gra nulites from the Sittampundi complex characterise the M(CSZ) as a high -P event with metamorphic peak conditions of c. 11.8 kbar and 830 degr ees C (minimum). The M(CSZ) is associated with significant isothermal decompression of the order of 4.5-3.5 kbar followed by static high-tem perature rehydration and retrogression around 600 degrees C. The timin g of M(CSZ) is inferred to be Neoproterozoic at c. 730 Ma based on a w hole rock-garnet-plagioclase-hornblende Sm-Nd isochron age for a garne t granulite from the Sittampundi complex and its agreement with the 80 0-600 Ma published age data on post-kinematic plutonic rocks within th e CSZ. These results demonstrate that the Cauvery shear zone is a zone of Neoproterozoic reworking of Archean crust broadly similar to the i nterface between the Napier and Rayner complexes of the East Antarctic shield in a model Proterozoic Gondwana supercontinent.(1)