An appraisal of Archaean supracrustal sequences in Chitradurga Schist Belt, Western Dharwar Craton, Southern India

Archaean metasedimentary sequences of the Chitradurga Schist Belt in Western Dharwar Craton were reassessed in terms of their protoliths, potential timing of deposition and metamorphic events along with metamorphic conditions. Existing geological and stratigraphic constraints and our field observations suggest that the oldest supracrustal sequence of the Sargur Group, which occurs as enclaves in Peninsular Gneiss, is overlain by the Dharwar Supergroup consisting of lower Bababudan and upper Chitradurga Groups. In the Chitradurga Schist Belt, the metamorphic conditions of each stratigraphic unit are distinct, with most prominent difference observed in the Chitradurga Group. The lower unit of this group are metamorphosed under biotite-muscovite grade, whereas the upper unit at chlorite-muscovite grade. Detrital zircon U-Pb dates obtained by ion microprobe analyses suggest 3.14 Ga and 3.22-2.92 Ga for the youngest protolith's magmatic ages, which constrain the oldest age limit of deposition of Bababudan Group and Lower Unit of Chitradurga Group, respectively. The depositional age of Upper Unit of Chitradurga Schist Belt can be bracketed by 2.68 Ga magmatic zircon ages from meta-rhyolite and 2.63 Ga magmatic detrital zircon age of the overlying sandstone that are significantly younger than the Lower Unit. Monazite U-Th-Pb ages are similar for both Lower and Upper Units of the Chitradurga Group (similar to c. 2.4 Ga) but are significantly younger than those of the underlying Bababudan and Sargur Groups (similar to c. 3.1 Ga). Combined zircon and monazite data imply that the continuous or composite Mesoarchaean-Neoarchaean supracrustal sequences of the Chitradurga Group are metamorphosed together at the latest Archaean-early Proterozoic. These regional metamorphic ages are not detected in the underlying Bababudan and Sargur Groups, as well as the basement Peninsular Gneiss. Contrasting depositional and metamorphic history in coherent Mesoarchaean and Neoarchaean strata in the Chitradurga Schist Belt have important implications for the comparison with the well-established sedimentary sequences of similar ages in the Pilbara and Kaapvaal cratons. The protolith and metamorphic ages of the Dharwar Supergroup, when compared with available similar information from the surrounding Archaean cratons in India, Madagascar, and East Antarctica, testify the probable Archaean connection, and give clues to the formation of deep to shallow Archaean crust. (C) 2012 Elsevier B.V. All rights reserved.