U–Pb electron probe geochronology of the Nagercoil granulites, Southern India: Implications for Gondwana amalgamation

The Nagercoil granulites form the most southerly basement rocks in peninsular India and have considerable significance in correlating the now-dispersed continents that made-up Gondwana. In this study, we date monazites and zircons in granulite-facies rocks from the Nagercoil region using the electron microprobe technique and identify a broad range of zircon ages between 2800 and 590 Ma from undisputed metasedimentary gneisses. A similar range of zircon apparent ages (2500–500 Ma) was obtained from massive charnockites. The majority of zircons from all analysed rocks yielded Palaeoproterozoic and early Mesoproterozoic ages between 2200 and 1530 Ma. These data suggest that either: (1) the charnockite protoliths were metasedimentary; (2) were emplaced in the Neoproterozoic and contain many zircon xenocrysts; or (3) were intruded in the Palaeoproterozoic with many zircon experiencing variable Pb-loss during Neoproterozoic metamorphism. All analysed monazites yielded late Neoproterozoic/Early Palaeozoic Th–U–Pb ages between 605 and 438 Ma with an age spectra maxima at 565 Ma. These are interpreted to have grown during high-grade metamorphism related to the final amalgamation of Gondwana. Zircons from undisputed metasedimentary rocks preserve age profiles that share a number of similarities with the Itremo and Molo Groups in central Madagascar and may have been derived from similar-aged source rocks.