DOCUMENTING NEOPROTEROZOIC TO DEVONIAN EVENTS WITHIN INDIA'S SOUTHERN GRANULITE TERRAIN USING IN SITU ION MICROPROBE MONAZITE GEOCHRONOLOGY

India's Southern Granulite Terrain is a key component in reconstructions of Gondwana, but is perhaps the least understood continental fragment in terms of its tectonic evolution. Monazite ages from carbonatites and high-grade metapelitic assemblages exposed along a significant lineament within the terrain termed the Kambam fault were obtained in thin section (in situ) using an ion microprobe. The method has several advantages in that analyses of small grains (~10µm) are possible and interpretations are facilitated by the preservation of textural relationships. Larger monazites were X-ray mapped in Ce and Th to decipher the significance of the ages of individual spots within grains. The Kambam carbonatite contains large (mm-sized) apatite surrounded by ~1µm-thick monazite rims. The age of the rim (715±42 Ma, Th-Pb, ±1s) is consistent with ages of several carbonatite complexes located 50 to 400 km further north, suggesting that extensionally-controlled magmatism occurred along the Kambam fault during Mid-Neoproterozoic. Whole rock bulk and trace geochemical analyses of the Kambam carbonatite links the sample with the other complexes as well, further demonstrating the presence of a ~400 km long rift within South India. Monazite inclusions in garnet from a Southern Granulite Terrain rock found as a lens within a thick unit of charnockite range from 733±15 Ma in the core to 490±8 Ma near the rim, indicating protracted and possible polymetamorphic garnet growth within the region. Another garnet-bearing assemblage nearby contains monazite inclusions consistent with a single population at 502±14 Ma and the Kambam carbonatite contains monazite as young as 405±5 Ma. The younger ages can be linked to movement along other structures within the terrain that folded the rift into its present-day v-shape. The results suggest that the Kambam fault, which records over 300 million years of monazite growth, should be considered a major terrane boundary in reconstructions of Gondwana.