Sr, Nd, and Pb Isotopic Systematics in the Archean Low- to High-Grade Transition Zone of Southern India: Syn-Accretion vs. Post-Accretion Granulites

Abstract

Low-pressure charnockites, their tonalitic precursors, and syn-metamorphic granites in the granulite facies transition zone south of Krishnagiri, Tamil Nadu, India, define single Rb-Sr and Sm-Nd isochrons at, respectively, and . Rb depletion occurred at this time, while Sm/Nd ratios were not significatively changed. Common lead ratios are identical for the granitic and tonalitic gneisses as well as the low-pressure charnockites; they are very unradiogenic (: 14.5 to 15.5), indicating U depletion around 2500 Ma ago. These data suggest (1) that granulites in this part of the transition zone, as well as some high-grade charnockites south of the zone (Nilgiris and Sheravoy Hills) that yield similar results, derive from a 2.5 Ga-old crust and (2) the time elapsed between the crust-forming event and the metamorphism was short. The high-pressure charnockites of the Biligirirangan Hills, Karnataka, are different from those in the transition zone near Krishnagiri since Pb and Nd isotopes indicate an older crustal memory (2.7-2.9 Ga), possibly connected to an earlier U depletion event, presumably a distinct granulite facies metamorphism. The geochronological relationships observed in Tamil Nadu suggest that tonalitic precursors and charnockites may have formed simultaneously in the same geodynamic context (probably involving subduction) during the storage of tonalitic magmas. This category of syn-accretion granulite, often observed in the Archean and Lower Proterozoic, would be genetically different from a second type (possibly illustrated by the Biligirirangan rocks) where the time elapsed between crustal formation and metamorphism is very long. The latter would be explained better by collisional or hot-spot models.