The Sandur Schist Belt and its Adjacent Plutonic Rocks Implications for Late Archaean Crustal Evolution in Karnataka

Six formations are defined in a new lithostratigraphy of the volcanic and sedimentary rocks of the schist belt. The formations (Yeshwantnagar, Deogiri, Raman Mala, Donimalai, Taluru, Vibhuti Gudda) young consistently northeast, except for a part of the Taluru Formation in the northeast of the belt. We propose the new term Sandur Group to include these formations. Their combined thickness of c.35 km is in part an effect of thrust thickening. The Sandur Group was deposited in mainly shallow marine environments in a setting comparable with that of unstable mixed-mode basins. An incomplete, upright, synclinal sheath fold dominates the structure in the east of the schist belt, and another incomplete synclinal sheath fold occurs in the west, its hinge areas and northeastern limb having been cut out by a steepened thrust (the Sandur valley discontinuity). The margins of the belt were intruded by syntectonic, multipulse granites with magmatic and crystal-plastic solid-state linear and planar fabrics and the granite emplacement outlasted deformation. HT/LP metamorphism of the schist belt was contemporaneous with deformation and granite emplacement.

The granites are an integral part of the Late Archaean polyphase granite complex in eastern Karnataka and contiguous parts of Andhra Pradesh and Tamil Nadu, for which we propose the term Dharwar batholith. Published isotopic age data show that its western part includes anatectic granites and relies of Peninsular Gneiss (>2900 Ma), but the eastern part is dominated by juvenile granites. The batholith accreted onto the Archaean foreland to the west comprising Late Archaean marginal basins of the Dharwar Supergroup (Kudremukh. Bababudan-Ranibennur, Chitradurga-Gadag) and their basement of Peninsular Gneiss (>3000 Ma) during Late Archaean plate convergence with NE-SW shortening and sinistral transcurrent displacements. The steep linear belts (Kolar, Ramagiri, Kushtagi. etc.) and irregular tracts (Sandur, Hutti) of volcanic and sedimentary rocks ineastem Karnataka formed as intraarc basins above the evolving Dharwar batholith.