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Earth, Amospheric & Planetary Sciences
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Proterozoic granulite-gneiss complexes of eastern and northeastern India

The Indian continental block, believed to have originated through a collision between the northern and the southern Indian cratonic blocks in the Early Mesoproterozoic, was a crucial component in Precambrian supercontinents. Among these, the Rodinia Supercontinent is thought to have assembled in the Late Mesoproterozoic and dispersed during Mid-Neoproterozoic with the resulting fragments reassembling to form Gondwanaland in Early Paleozoic. The Proterozoic granulite gneiss complexes of the eastern part of the Indian shield, e.g., the Eastern Ghats Belt (EGB) and the Chotanagpur Gneissic Complex (CGC) of eastern India, and the Shillong-Meghalaya Gneissic Complex (SMGC) of northeastern India preserve key evidence of these tectonic episodes. In this collaborative study, we attempt to correlate these important tectonic events with the thermal history of the granulitic complexes through metamorphic characterization, U-Pb zircon geochronology with isotope dilution-thermal emission mass spectrometry (ID-TIMS) and U-Th-Pb monazite chemical dating with the electron microprobe.

Peraluminous granite intrusion related to the collision between the northern and the southern Indian blocks occurred at 1698 ± 17 Ma north of the CGC. This was followed by granulite-facies metamorphism at 1.6 Ga along the northern boundary of the CGC and the western part of the SMGC where 1596 ± 15 Ma granulites are exposed. Emplacement of the Bengal Anorthosite massif in the CGC followed at 1550 ± 2 Ma. Subsequent metamorphism in the CGC is recorded by 947 ± 27 Ma zircon growth in the anorthosite, and 950 ± 20 Ma and 995 ± 24 Ma monazite growths in metapelitic granulite north of the Bengal Anorthosite. These dates coincide with the 1.2-1.0 Ga Grenvilllian collision between the EGB and the Rayner block of eastern Antarctica. Although this collision was coeval with the formation of Rodinia, the Rayner-EGB block was not a part of Rodinia and formed a part of Gondwana only after ~500 Ma. Emplacement of the Balugaon Anorthosite massif at 983.0 ± 2.5 Ma in the Chilka Lake region occurred after the fabric-defining Grenvillian metamorphism in the EGB.

Mid-Neoproterozoic (850-740 Ma) high-grade metamorphism and deformation in the northeastern part of the EGB (the Chilka Lake area) and along the eastern fringes of the CGC, the North Singhbhum Mobile Belt and the Singhbhum Craton are probably related to convergence and possible oblique collision between eastern India and northwestern Australia. Monazite cores (714 ± 11 Ma) and rims (655 ± 12 Ma) in the Balugaon Anorthosite record progressive high-grade metamorphism of the anorthosite during a part of this tectonic episode. The convergence between India and Australia at ~750 Ma is supported through available paleomagnetic data.

Paleomagnetic constraints also suggest that the Indian block collided with the Australo-Antarctic block later than 700 Ma. Pan-African sutures between the EGB and the Archean cratons of India, and south of the Rayner province in eastern Antarctica suggest that the Rayner-EGB block was amalgamated into Gondwana during the Early Paleozoic. Monazite + allanite rims (463 ± 22 Ma) around apatite record a weak Pan-African thermal imprint in the Balugaon Anorthosite.

The eastern part of the SMGC is also overprinted by Early Paleozoic granulite-facies metamorphism. The Sonapahar metapelitic granulites were metamorphosed at 500±14 Ma providing evidence of a Pan-African amalgamation of the Indian plate in East Gondwana. We propose that the northward extension of the Prydz Bay suture of eastern Antarctica passes through the SMGC.

Publications:

• Chatterjee, N., Crowley, J.L., Mukherjee, A. and Das, S. (2008) Geochronology of the 983 Ma Chilka Lake Anorthosite, Eastern Ghats Belt, India: implications for pre-Gondwana tectonics. Journal of Geology, 116, 105-118.doi: 10.1086/528901
• Chatterjee, N., Crowley, J.L. and Ghose, N.C. (2008) Geochronology of the 1.55 Ga Bengal anorthosite and Grenvillian metamorphism in the Chotanagpur gneissic complex, eastern India. Precambrian Research, 161, 303-316.doi:10.1016/j.precamres.2007.09.005
• Chatterjee, N., Mazumdar, A.C., Bhattacharya, A. and Saikia, R.R.(2007) Mesoproterozoic granulites of the Shillong-Meghalaya Plateau: Evidence of westward continuation of the Prydz Bay Pan-African suture into Northeastern India. Precambrian Research, 152/1-2, 1-26. doi:10.1016/j.precamres.2006.08.011

Last Revised 02/25/2008. E-mail: nchat@mit.edu