NEW U-PB GEOCHRONOLOGY AND A NEW MODEL FOR THE PROTEROZOIC TECTONICS AND SEDIMENTATION IN THE MID-CONTINENT OF INDIA

The Indian Shield is an amalgamation of a northern block (Bundelkhand craton) and a southern block (W and E Dharwar-Bastar-Singhbhum cratons) that are separated by a WSW-ENE tectonic zone, the Central Indian Tectonic Zone (CITZ). The southern block is bordered in the SE by another tectonic zone, the Eastern Ghats Mobile Belt (EGMB) that is a remnant of the East Antarctic-India collision. Recent U-Pb zircon and monazite ages, along with other Sm-Nd, Rb-Sr and Ar-Ar ages, indicate that: (1) batholith-scale granitic rocks of the cratons are about 2.5 Ga, with small enclaves that date back to >3.5 Ga; (2) the southern cratons amalgamated prior to 1.6 Ga; (3) the CITZ closed and likely was uplifted ~1.7 Ga; (4) the EGMB was accreted to the southern block later than 1.1 Ga, and perhaps as late as ~600 Ma; and (5) most of the slightly deformed and virtually unmetamorphosed sedimentary basins, that are devoid of body fossils closed by ~1000 Ma although they had opened at different times. For example, the Cuddapah basin opened ~ 2000 Ma, the Vindhyan basin ~1750 Ma, the Godavari basin ~1700 Ma, and the Chhattisgarh basin ~1400 Ma. New detrital zircon ages from quartz arenites in the southern part of the Chhattisgarh basin are unimodal at ~2.5 Ga, indicating derivation from local granitic batholiths. In contrast, magmatic and detrital zircons in tuffs and volcaniclastic sandstones in the north show major peaks at 1006, 1183, 1226, 1871, 2298, and 2495 Ma. We infer that rock units in CITZ in the north and not EGMB to the south supplied this detritus.

Several previous models for Proterozoic plate reconstruction, regional tectonics, and the origin and evolution of sedimentary basins in India, which were based on pre-2000 geochronology, are invalidated by these new dates. We propose that the sedimentary basins opened as sag-basins in epeiric seas; the Vindhyan in the north block developed into a foreland basin, but the sag basins on the south block filled until about 1.0 Ga. These sag basins were deformed to various degrees and inverted when East Antarctic collided with India preserving, for example, the Godavari basin in a rift. The Pan-African orogeny (~ 550 – 700 Ma?) may have utilized the tectonic zones between and around the cratons to form a major structural feature, the Great Indian Proterozoic Fold Belt through the mid-continent of India.