Lithology and Structure of the Auriferous Hutti Schist Belt, Northern Karnataka: Implications for Neoarchaean Oblique Convergence in the Dharwar Craton, South India

The Huttie schist belt N of 16°00 N is characterised by steep, variably deformed, metasedimentary and bimodal metavolcanic rocks that were hornfelsed as the result of HT-LP, low amphibolite facies regional metamorphism. Subsequently, the hornfelses were locally retrogressed to greenschist facies assemblages. Voluminous pillow-Structured metabasalts predominate, divisible into a massive, jointed variety, schistose metabasalts, carbonate-rich schistose metabasalts and chloritic schists. Boundaries between the four varieties are transitional. Felsic volcanic rocks interpreted as the result of rapid resedimentation of freshly erupted pyroclastic or autoclastic grains and fragments occur as volumetrically minor sheets in the metabasalts. The sheets are locally lnterbedded with dolomite, chert and phyllite. The Palkanmardi fining-up sequence of greywackes and polymict conglomerates younging SW dominates the NE of the schist belt in contrast with the Gagalgatta sedimentary and volcanic complex of greywackes and metabasaltic and metagabbroic rocks younging NE in the southern part of the belt. In common with other Neoarchaean schist belts in the Dharwar craton, the metavolcanic and metasedimentary rocks in the Hutti belt were deposited in an unstable marine environment, but in contrast with other belts lithostratigraphic subdivision of the Hutti belt into plausible formations is inhibited by poor exposure and lack of persistent markers.

Chronological relationships between quartz-Carbonate vein systems (locally auriferous) and pervasive schistosity and linear fabrics show that a tectonic continuum of NE-SW shortening, here labelled D_l, gave rise to a large-scale anticline-Syncline pair in the E of the schist belt and an apparent tectonic discontinuity in the NE of the belt that separates the SW younging Palkanmardi fining-Up sequence and metabasalts with a total strike-Normal thickness of ca 3 5 km from the metabasalts, felsic volcanic sheets and the Gagalgatta complex in the central and southern parts of the belt which together young NE and have a total strike-Normal thickness of ca 15 km. This anomalous thickness of ca 15 km is consistent with tectonic thickening in other Neoarchaean schist belts in the craton, but no thrusts were proved in the Hutti belt. Regional HT LP amphibolite facies metamorphism was closely tied to emplacement of granite s 1 within and adjacent to the schist belt contemporaneous with, but also outlasting, the D₁ continuum. Conjugate sets of small-Scale shear zones discordant to D_l schistosity suggest that D_l deformation became progressively brittle and localised. The reason for a major post-D_l swing in D₁ schistosity in the NW of the belt is unclear.

The sheet form of the granites (including granodiorite)withn and adjacent to the schist belt, their chronological spectrum of emplacement relative to deformation, the unstable volcanic and depositionaal setting of the schist belt, the anomalous strike-Normal thickness in the west of the belt, and published geochemical data of the metabasalts indicative of an island-Arc setting are consistent with the position of the Hutti schist belt in the accretionary complex in our published model of Neoproterozoic oblique convergence in the Dharwar craton.