HEAVY MINERALS FROM NEOGENE DEPOSITS OF ASSAM, INDIA: UNROOFING HISTORY OF THE EASTERN HIMALAYAS

The western Himalayan mountains have been studied in relative detail, and much is known about the tectonic history of that area. Many of the same questions remain unanswered about the eastern Himalayas. This complex region includes both the Himalayan and Indo-Burman ranges. These two systems wrap around the Bengal and Assam basins, which record the geologic history of the area in the form of trapped orogenic sediments. Analysis of sediments from these two sites sheds light on the history of this poorly known area.

This study is focused on identification, description, and abundance of the heavy-mineral fraction from the Neogene Surma and Tipam Groups near the Dibrugarh area of Assam, India. Heavy minerals account for between <1% and 3.5% of total sample weight. The most common particle size is 2-3f (fine sand), with 41-66% of the heavy fraction falling into this interval. Most of the remaining heavy fraction is roughly evenly split between the 1-2f and 3-4f intervals. The 0-1f fraction generally contains less than 1% of total heavy minerals. Preliminary mineral identification has yielded angular to subrounded garnet, topaz, zircon, tourmaline, apatite, zoisite, augite, rutile, pyroxenes, amphiboles, chlorite, and opaque minerals. Opaque minerals dominate, followed by garnets.

The presence of angular grains indicates close proximity to the sediment-source area. Preliminary mineral identification suggests derivation of the minerals from mostly medium-grade metamorphic rocks. Unlike the Neogene sections of the adjacent Bengal basin, heavy-mineral contents in the Dibrugarh area do not show an obvious trend in heavy-mineral distributions that would indicate progressive unroofing of an orogenic belt. High-grade and low-grade minerals are subequally distributed in all Neogene sections studied. Ongoing analysis is directed toward constraining orogenic timing, metamorphic grades of the orogenic sources, erosional patterns, and basin-wide stratigraphy.