Beach placers of southwestern India: An archive of Precambrian supercontinent growth histories

Abstract

The southwestern coast of India is known for its rich beach placer deposits. So far, there have been no substantial attempts to determine the provenance of these deposits on a regional scale. Here we present for the first time geochronology, trace element chemistry and Hf isotope systematics of zircon and monazite from representative beach placer deposits along the southwestern coast of India to constrain provenance. Detrital zircon grains display a wide variety of ages, with modes in the ranges of 650–450 Ma, 1100–650 Ma, 2300–1600 Ma, 2800–2300 Ma and 3500–2800 Ma. A high proportion of ages fall within the Neoproterozoic–Late Cambrian range (1066–490 Ma). Growth zoning in zircon grains shows magmatic growth of many generations with ages affected by Neoproterozoic Pb loss, along with overgrowths typical of those formed during high-T metamorphism. Grains of metamorphic zircon, along with grains of monazite from the same samples of sediment, have strong modes between 570 and 470 Ma, indicating derivation from sources in the Southern Granulite Terrane affected by a major Late Neoproterozoic to Cambrian tectonothermal event. Mineral geochemistry of zircon grains indicates continental derivation, especially with older zircon from TTG-type sources. The geochemistry of late Neoproterozoic–Cambrian zircon and monazite are consistent with derivation from amphibolite to granulite-facies source rocks. Zircon ages and Hf isotope systematics can be ascribed to provenance from various source terranes exposed along the Western Ghats. This young rift-flank escarpment formed along the coast at a high angle to the distribution of different Precambrian terranes in the Indian Peninsula. Erosion promoted by a tropical climate and a steep coastal geomorphology, in combination with strong southward along-shore ocean currents, led to the concentration and distribution of heavy minerals from multiple sources along southwestern coast of India. The relative intensity of age modes in detrital zircon differs between localities, demonstrating differing inputs from proximal sources (from rivers dissecting steep terrain) versus distal sources from more northerly cratons and terranes brought to deposition sites by along-shore drift.