Rapid development of the Late Archaean Hutti schist belt, northern Karnataka: Implications of new field data and SHRIMP U/Pb zircon ages

The Palkanmardi conglomerate is one of many polymict conglomerates interbed ded with greywackes in the NE of the Hutti schist belt. These conglomerates are up to a few metres thick, unsorted, and include rounded to sub-angular clasts of granodiorite, granite, vein quartz and metabasalt in a matrix of coarse- to medium-grained greywacke, Cross-bedding, convolute bedding and grading are well preserved in the interbedded greywackes in spite of deform ation and hornfelsing during Late Archaean regional high temperature - low pressure metamorphism. These primary structures and lack of sorting in the conglomerates are consistent with deposition as turbidites and debrites in an unstable submarine environment. This new interpretation contrasts with p revious views that the Palkanmardi conglomerate is autoclastic, pyroelastic -dettrital or glacio-fluvial. The conglomerate-greywacke sequence occurs lo w in the lithostratigraphy and is overlain by metabasalts. A clast of granodiorite in the conglomerate bed at Palkanmardi village has yielded a SHRIMP weighted mean Pb-207/Pb-206 zircon age of 2576+/-12 Ma whi ch is interpreted as the magmatic age of the erosional provenance. Moreover , SHRIMP zircon geochronology using a sample from the steep elongate wedge of granodiorite that extends for at least 150 km SE of the schist belt has yielded a weighted mean Pb-207/Pb-206 age of 2561+/-24 Ma and a concordia u pper intercept age of 2580+/-31 Ma. These ages are indistinguishable within their errors and are interpreted as the age of magmatic crystallisation. C ombined with the low stratigraphic position of the Palkanmardi conglomerate , the age data imply that basin development (volcanism and sedimentation) i n the Hutti brit was not only rapid, brit began very late in the Archaean h istory of this part of the Dharwar craton.