VARIATION IN HEAVY MINERAL ASSEMBLAGES IN OLIGOCENE SUCCESSIONS OF THE ASSAM BASIN, NORTHEAST INDIA, IN RESPONSE TO EARLY UPLIFT STAGES OF THE EASTERN HIMALAYAS

Sandstone modal analysis and heavy mineral distributions of Oligocene siliciclastic sediments of Digboi-Margherita area in Assam, India records the early erosional history of the eastern Himalayas. The Oligocene Barail Group is thick (~5 km), deposited mainly in brackish water to deltaic environments, which consists of three formations from old to young: Naogaon (Qt₆₉F₆L₂₅), Baragolai (Qt₆₆F₁₂L₂₂), and Tikak Parbat (Qt₈₂F₄L₁₄). Heavy minerals were separated from sixteen sieved samples from these formations using a heavy liquid, and again separated into different sets of heavy minerals fractions based on magnetic intensity using a Franz magnetic separator. Each set of separated fractions were identified and counted under petrographic microscope using a modified Fleet method. Total heavy mineral concentrations vary from negligible to up to 1.5 wt. percent in entire succession with the minimum value in lowermost Naogaon Formation and the maximum value in uppermost Tikak Parbat Formation. The concentration of heavy mineral and diversity increases up section. The assemblages of heavy minerals are mainly dominated by opaque minerals (nearly around 80%) along with non-opaque varieties including ultra-stable heavy minerals (zircon, tourmaline and rutile), garnet, apatite, epidote, hornblende, muscovite, chlorite, and topaz. Of the ultra-stable heavy minerals zircon is more abundant which was followed by tourmaline and rutile, and shows significant variation in morphology (euhedral non-corroded to anhedral corroded crystals), size (50 µm to 200 µm), zoning and inclusion pattern (oriented or randomly-oriented) in entire succession which probably indicates the change in type of source and/or sediment dispersal paths. The ZTR index varies from 20% in Naogaon to 3.65% in Tikak Parbat Formation showing decreasing compositional maturity of heavy mineral assemblages with time. There is also an increase in the concentration of garnet, staurolite, andalusite and decrease in chlorite, chloritoid, prehnite, epidote in up-section, suggesting change in the source-type from lower to intermediate grade metamorphic rocks. Ongoing research in mineral chemistry and detrital geochronology may provide better resolution in detrital history of these Oligocene clastic wedges of the eastern Himalayas.