The application of single zircon evaporation and model Nd ages to the interpretation of polymetamorphic terrains: an example from the Proterozoic mobile belt of south India
The technique of single zircon dating from the thermal evaporation of 207Pb/206Pb (Kober 1986, 1987) provides a means of dating successive periods of growth and nucleation of zircons in polymetamorphic assemblages. In contrast Nd model ages may provide a measure of the period of crustal residency for the sample or its protolith. These two techniques have been combined to elucidate the tectonic history of the Proterozoic mobile belt of southern India, exposed south of the Palghat-Cauvery Shear Zone that marks the southern boundary of the Archaean craton of Karnataka. The two main tectonic units of this mobile belt comprise the Madurai and Trivandrum Blocks, both of which are characterised by massive charnockite uplands and low-lying polymetamorphic metasedimentary belts that have undergone a complex tectonic history throughout the Proterozoic. Evidence for early Palaeoproterozoic magmatism is restricted to the Madurai Block where single zircon evaporation ages from a metagranite (2436+/-4 Ma) are similar to model Nd ages from a range of lithologies suggesting crustal growth at that time. The Trivandrum Block, to the south of the Achankovil shear zone, is comprised of the Kerala Khondalite Belt, the Nagercoil charnockites and the Achankovil metasediments. Single zircon evaporation ages, together with conventional zircon and garnet chronometry, suggest that all three units underwent upper-amphibolite facies metamorphism at 1800 Ma, an event unrecorded in the metagranite from the Madurai Block. This implies that the Madurai and Trivandrum blocks represent distinct terrains throughout the Palaeoproterozoic. Model Nd ages from the Achankovil metasediments are much younger (1500-1200 Ma) than those from the adjacent Kerala Khondalite Belt and Madurai Blocks (3000-2100 Ma), but there is no evidence for zircon growth in these metasediments during the Mesoproterozoic. Hence the comparatively young model Nd ages of the metasediments 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 that all tectonic units of the Proterozoic mobile belt of South India shared the same metamorphic history from the early Palaeozoic. This event has been recognised in the basement lithologies of Sri Lanka and East Antarctica, confirming that the constituent terrains of East Gondwana had assembled by this time.