CRUSTAL EVOLUTION IN SOUTH-INDIA - CONSTRAINTS FROM ND ISOTOPES

Nd-isotope analyses from a range of lithologies, including charnockite s, metasediments, and granitoids that comprise the high-grade terrain of South India define contrasting age provinces. North of the Palghat- Cauvery shear zone, the Karnataka Craton and the granulite blocks of N ilgiri and Madras are characterized by model Nd ages from 3.4 to 2.4 G a with significant crustal growth during the Late Archean. South of th e Palghat-Cauvery shear zone a Pan-African granulite-facies event resu lted in Sm/Nd fractionation, as observed in several incipient charnock ite localities. Model Nd age calculations that allow for Sm/Nd fractio nation at 550 Ma yield ages in the range of 2.9 to 1.3 Ga. This range compares closely with published model Nd ages from the high-grade terr ain of Sri Lanka. Mid-Proterozoic crustal growth (approximately 1.1 Ga ) observed in the Vijayan Complex of Sri Lanka may have provided sourc e material for metasediments characterized by young model Nd ages in s outhern Peninsular india. The Palghat-Cauvery shear zone therefore rep resents a boundary between southern terranes that are characterized by crustal reworking and high-grade metamorphism during the Pan-African tectonothermal event and an Archean craton to the north where no Pan-A frican overprint has been detected. Charnockite formation is identifie d both with the period of Late Archean crustal growth and with the per iod of Pan-African metamorphism, as can be observed not only in southe rn India, but also in the proximal Gondwanan fragments of East Antarct ica and Madagascar.