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

• Published in: Contributions to Mineralogy and Petrology
• Main Authors: Rao, Y. J. B., Chetty, T. R. K., Janardhan, A. S., Gopalan, K.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 1996
• Subjects: Geology; Antarctic; Napier; Rayner; Antarc*
• Online Access: http://eprints.uni-mysore.ac.in/16549/; https://doi.org/10.1007/s004100050219

Sittampundi and Bhavani Archean layered meta-anorthosite complexes occur as tectonic lenses within the Cauvery shear zone (CSZ), a crustal scale shear dividing the Precambrian granulite crust of south India into late Archean (> 2.5 Ga) and Proterozoic (c. 0.55 Ga) blocks. They and their host supracrustal-gneiss rocks record at least two stages of tectonometamorphic history. The first is seen as regional scale refolded isoclinal folds and granulite metamorphism (D-1-M(1)) while the second stage is associated with dominantly E-W dextral transcurrent shearing and metamorphic recrystallisation (D-2-M(CSZ)). Whole rock Sm-Nd isochrons for several comagmatic rocks of the layered complexes yield concordant 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 occurred soon after emplacement at c.3.0 Ga ago. P-T estimates on garnet granulites from the Sittampundi complex characterise the M(CSZ) as a high-P event with metamorphic peak conditions of c. 11.8 kbar and 830 degrees C (minimum). The M(CSZ) is associated with significant isothermal decompression of the order of 4.5-3.5 kbar followed by static high-temperature rehydration and retrogression around 600 degrees C. The timing of M(CSZ) is inferred to be Neoproterozoic at c. 730 Ma based on a whole rock-garnet-plagioclase-hornblende Sm-Nd isochron age for a garnet granulite from the Sittampundi complex and its agreement with the 800-600 Ma published age data on post-kinematic plutonic rocks within the CSZ. These results demonstrate that the Cauvery shear zone is a zone of Neoproterozoic reworking of Archean crust broadly similar to the interface between the Napier and Rayner complexes of the East Antarctic shield in a model Proterozoic Gondwana supercontinent.(1)