Precambrian rocks of the Himalaya

Abstract

The Himadri (Great Himalaya) ranges are made up of Precambrian high-grade metamorphic rocks evolved at temperatures of 600-650°C and pressures of 5-7 Kb. The early Precambrian rocks which formed the basement of the Tethys basin to the north, have been uplifted from great depths (15-30 km) by the Main Central Thrust. The metamorphic rocks of Himadri are overlain in the discontinuous Tethys basin by a great succession of sediments of the later Proterozoic and Phanerozoic time span. The lower part is uniformly argillo-arenaceous flysch, characterized locally by intraformational lenses of diamictites. These are followed upwards by an argillo-calcareous succession with large lenticular horizons of stromatolitic dolomites. Significantly, the Cambrian strata are altogether missing in the central and eastern sectors. South of the Himadri, the larger part of the Lesser Himalaya comprises Riphean to Vendian sedimentary rocks, lithologically similar to and homotaxial with the Proterozoic succession of the Tethys province. The flysch formation possibly rests upon a basement of 1900 ± 100 Ma. old porphyritic granite of middle crustal origin, now found tectonically implanted in the sedimen-taries in some places. The lower part of the succession is of argillo-arenaceous facies, characterized locally by turbidites, diamictite lenses and very prominent penecontemporaneous basic volcanic rocks of Lower Riphean (1350 ± 50 Ma.) age. This assemblage grades upwards into a calc-argillaceous succession, characterized by stromatolite-bearing dolomites, lenticular deposits of coarsely crystalline magnesite associated with talc and base-metals and carbonaceous shale-marble rhythmites in the upper part. While the columnar-branching stromatolites indicate an age from Middle Riphean to Vendian for the dolomites of the inner sedimentary belt, the conodonts, primitive brachiopods and trilobites in a horizon of the youngest (terminal) formation of the outer sedimentary belt point to a Lower Cambrian age.