Some distinctive trends in the evolution of the early Precambrian (Archaean) Dharwar Craton, South India

Abstract

The early Precambrian (Archaean) Dharwar sequence in south India provides evidence for a distinct trend of evolution which differs from trends exhibited in many other early Precambrian regions of the world. The supracrustal rock associations preserved in the Dharwar schist belts suggest the evolution of the depositional environment from a stable to a mobile regime. Layered igneous complexes, mature to supermature quartz-pebble conglomerates and quartzites deposited on platforms, ortho-quartzite-carbonate shelf assemblages and deposition of persistent beds of iron and manganese formations which serve as stratigraphic markers-all these developed in the lower sections of the Dharwar supracrustal sequence may be cited as evidence for early stability. The mobile regime which can be shown on sedimentological grounds to have succeeded the stable regime witnessed accumulation of turbidite greywackes in a volcanically active environment. The tectonic environment of sedimentation and magmatism in the supracrustal belts seems to have evolved from a rifted continental margin, through a stable shelf to a geosynclinal type of setting. Sialic crustal differentiation about 3400 Ma. ago may have been responsible for the early stable conditions. The supracrustal rocks show evidence of three phases of deformation (DF¹ to DF₃). A structural unity is evident amongst the supracrustal sequence, the Peninsular Gneiss and granulites of the Dharwar tectonic province. This structural unity suggests the evolution of the Peninsular Gneiss and granulites synkinematically with the deformation of the Dharwar supracrustal sequence. There is a wide spread of ages from 3400 Ma. to 2600 Ma. in the Dharwar craton. The structural unity amongst rocks developed over such a wide range of ages implies long-term stability of the stress regimes in the Archaean lithosphere. Dharwar schist belts are a rich repository of iron, manganese, and limestone deposits, indicating substantial biological activity during the early Precambrian times. Stromatolites and _o¹³C values for carbon in graphitic schists and preservation of syngenetic cyanobacterial microfossils have been recognised in widely separated regions in the Dharwar schist belts. The morphological diversity as exhibited by Dharwar stromatolites are rare in pre-Riphean sequences. The foregoing observations indicate that, as compared to other Archaean terranes, the Archaean Dharwar craton appears to have more evolved characteristics.