Precambrian Continental Crust of India and Its Evolution

Abstract

The Indian Precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes. The bulk of this crust was formed prior to 2600 m.y. ago and remobilized at least twice between 2600-2000 m.y. ago (early Proterozoic Mobile Belt, EPMB) and 2000-1500 m.y. ago (middle Proterozoic Mobile Belt MPMB). Three early Precambrian nucleii: Karnataka (KN), Jeypore-Bastar (JBN), and Singhbhum (SN) appear to have survived in the craton and are characterized by low-grade supracrustals and tonalitic trondhjemite gneisses, formed 3800-2600 m.y. ago. The EPMB event involved sedimentation, amphibolite-granulite facies metamorphism, and metasomatism and produced amphibolite facies rocks and K-granites in the north, and charnockite and other granulite facies rocks in the south. Gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the EPMB event. K-granites form a garland around the central Dharwar craton, suggesting some type of collision between two blocks. The compressional stress directions in the craton and the surrounding mobile belts were EW, producing almost identical structures in all the regions. The supracrustals of the Indian Archean are broadly divisible into an older and a younger sequence. Older belts are characterized by argillites and chemogenic sediments of high Mg, Fe, Al, Cr, and Ni abundances, while younger belts are characterized by graywacke shale suites with abundant Na, K, Rb, and Sr. The REE, U, and Th abundance patterns of the two groups show significant differences. The small amount of ultramafic rocks in the Indian Precambrian necessitates alternative sources for the high Ni and Cr contents in the supracrustals. Cr and Ni contents are high even in gneisses of this region. The available data provide constraints for a model which suggests that older schist belts were developed in shallow water basins on a simatic crust. On the other hand, the platformal components of the younger greenstone belts were laid down in rifted basins on a sialic basement. Crustal deformation and thickening gave rise to the EPMB. At 2000-1500 m.y. ago, another intensive mobile belt event occurred in which subduction and flexure at the eastern and northern margins of the Dharwar-Singhbhum Protocontinent gave rise to Proterozoic sedimentary basins, rift valleys, and igneous and metamorphic suites. Plate tectonic regimes had clearly set in by 2000 m.y. ago; the middle Proterozoic orogeny shows clear evidence of modern-style collision tectonics.