Proterozoic Granulite-Gneiss Complexes of Eastern India: Rodinia and Gondwana Reconstructions

The Indian continental block, believed to have originated through a collision between the northern and the southern Indian cratonic blocks beginning in Early Mesoproterozoic and culminating with terminal collision in Late Mesoproterozoic, was a crucial component in Precambrian supercontinents. Among these, the Rodinia Supercontinent is thought to have assembled in Late Mesoproterozoic-Early Neoproterozoic and dispersed in Mid-Neoproterozoic with the resulting fragments reassembling to form Gondwana in Early Paleozoic. Recent geodynamic models suggest that the relative positions of India and East Antarctica were approximately the same in Rodinia and Gondwana. The Proterozoic granulite gneiss complexes in the eastern part of the Indian shield, e.g., Eastern Ghats Belt (EGB) and Chotanagpur Gneissic Complex (CGC) in Eastern India, and Shillong-Meghalaya Gneissic Complex (SMGC) in 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 coeval with the early collision between the northern and the southern Indian blocks occurred at 1697 ± 17 Ma north of the CGC. This was followed by granulite-facies metamorphism at 1.6 Ga along the northern boundaries of CGC and western SMGC where 1596 ± 15 Ma granulites are exposed. Emplacement of the Bengal Anorthosite massif in the southeastern part of the CGC followed at 1550 ± 2 Ma. Subsequent high-grade Grenvillian metamorphism (peak T=788-861oC, P=7-10.5 kb) in the CGC is recorded by 975 ± 67 Ma xenotime growth in granitic gneiss in the northwestern sector, 947 ± 27 Ma zircon growth in the Bengal Anorthosite, and 995-950 Ma monazite growths in metapelitic granulite north of the Bengal Anorthosite. These dates correspond with a Grenvillian collision between the EGB and the Rayner Complex of East Antarctica, and may be related to the assembly of Rodinia. Emplacement of the Balugaon Anorthosite massif at 983.0 ± 2.5 Ma in the Chilka Lake region of EGB occurred after the fabric-defining Grenvillian metamorphism in the EGB.

Mid-Neoproterozoic (876-784 Ma) high-grade metamorphism, ductile deformation and sinistral faulting along a north-south elongated belt known as the Eastern Indian Tectonic Zone (EITZ) along the eastern fringes of CGC, North Singhbhum Mobile Belt (NSMB) and Singhbhum Craton (SC), and in the northeastern part of EGB (Chilka Lake area) are probably related to a distinct post-Grenvillian southward movement of the Indian block after its collision with East Antarctica. Therefore, contrary to consensus geodynamic models, rifting related to Rodinia break-up between India and Australia-Antarctica did not start until c. 780 Ma. High-grade metamorphism and anatexis along the EITZ occurred along a "counterclockwise" P-T path with peak conditions of 11.3 kb and 800oC, followed by an isothermal pressure increase to 12.7 kb and retrogression to 9 kb and 730oC.

The EITZ can be traced southward within the submarine Kerguelen Plateau in southern Indian Ocean and perhaps within East Antrarctica. In a reconstructed configuration of continents (both Rodinia and Gondwana), the EITZ in eastern India, the eastern margin of EGB, the western salient of Kerguelen Plateau known as Elan Bank, and the coast of Rayner Complex are co-linear. Metapelitic clasts in conglomerate recovered from an Elan Bank drill site followed a P-T path of retrograde metamorphism (10.2kb/760oC to 6.2kb/560oC) that overlaps with the P-T path of the EITZ metapelites. Monazite chemical dates (785 ± 12 Ma to 694 ± 18 Ma) and U-Pb isotope TIMS dates (824-675 Ma) from the Elan Bank metapelitic clasts are remarkably similar to the monazite chemical dates from metapelites in the EITZ and the eastern margin of EGB. Similar metamorphic conditions and dates are also known from the Oygarden Group and Mt. Vechernyaya along the coast of the Rayner Complex. Therefore the EITZ was probably a major, ~1800 km long tectono-metamorphic belt in Rodinia stretching from eastern India through the Eastern Ghats to the basement of Elan Bank and probably to the Rayner Complex.

High-grade Pan-African metamorphism and thrusting at the boundary between the EGB and the Archean cratons of India, and south of the Rayner Complex in East Antarctica suggest that the Rayner-EGB block was perhaps amalgamated in Gondwana during the Early Paleozoic.

The central and eastern parts of the SMGC were intensely reworked in Late Cambrian by granulite-facies metamorphism. This high-grade metamorphism occurred at 494 ± 6 Ma, and is characterized by a "clockwise" P-T path with near isobaric heating (prograde) between 635oC and 730oC at ~5.7-5.5 kb, peak PT conditions of approximately 730oC/5.5 kb, and cooling between 650oC and 595oC at 3.4-3.2 kb. Metamorphism along a "clockwise" P-T path is known from many continental collision zones, and may be correlated with a Pan-African collision between India and Australia-Antarctica during the assembly of East Gondwana. The retrograde PT segment of the Late Cambrian metamorphic PT path in the SMGC is similar to those estimated in the Prydz Bay area of East Antarctica. The Late Cambrian metamorphism evidently overprinted a Paleo-Mesoproterozoic terrane with high-grade metapelites metamorphosed along a "counterclockwise" P-T path at 1609 ± 9 Ma. This study strongly indicates that the western boundary of the Prydz Bay Pan-African suture of eastern Antarctica passes through the SMGC that may be used as evidence of a Pan-African assembly of the Indian block in East Gondwana at 494 ± 6 Ma.