THE APPLICATION OF SINGLE ZIRCON EVAPORATION AND MODEL ND AGES TO THEINTERPRETATION OF POLYMETAMORPHIC TERRAINS - AN EXAMPLE FROM THE PROTEROZOIC MOBILE BELT OF SOUTH-INDIA

The technique of single zircon dating from the thermal evaporation of Pb-207/Pb-206 (Kober 1986, 1987) provides a means of dating successive periods of growth and nucleation of zircons in polymetamorphic assemb lages. In contrast Nd model ages may provide a measure of the period o f crustal residency for the sample or its protolith. These two techniq ues have been combined to elucidate the tectonic history of the Proter ozoic mobile belt of southern India, exposed south of the Palghat-Cauv ery Shear Zone that marks the southern boundary of the Archaean craton of Karnataka. The two main tectonic units of this mobile belt compris e the Madurai and Trivandrum Blocks, both of which are characterised b y massive charnockite uplands and low-lying polymetamorphic metasedime ntary belts that have undergone a complex tectonic history throughout the Proterozoic. Evidence for early Palaeoproterozoic magmatism is res tricted to the Madurai Block where single zircon evaporation ages from a metagranite (2436 +/- 4 Ma) are similar to model Nd ages from a ran ge of lithologies suggesting crustal growth at that time. The Trivandr um Block, to the south of the Achankovil shear zone, is comprised of t he Kerala Khondalite Belt, the Nagercoil charnockites and the Achankov il metasediments. Single zircon evaporation ages, together with conven tional zircon and garnet chronometry, suggest that all three units und erwent upper-amphibolite facies metamorphism at similar to 1800 Ma, an event unrecorded in the metagranite from the Madurai Block. This impl ies that the Madurai and Trivandrum blocks represent distinct terrains throughout the Palaeoproterozoic. Model Nd ages from the Achankovil m etasediments are much younger (1500-1200 Ma) than those from the adjac ent Kerala Khondalite Belt and Madurai Blocks (3000-2100 Ma), but ther e is no evidence for zircon growth in these metasediments during the M esoproterozoic. Hence the comparatively young model Nd ages of the met asediments are indicative of a mixed provenance rather than a discrete period of crustal growth. Zircon overgrowths from the Madurai Block ( 547 +/- 17 Ma) and Achankovil metasediments (530 +/- 21 Ma) suggest th at all tectonic units of the Proterozoic mobile belt of South India sh ared the same metamorphic history from the early Palaeozoic. This even t has been recognised in the basement lithologies of Sri Lanka and Eas t Antarctica, confirming that the constituent terrains of East Gondwan a had assembled by this time.