Compositional comparison between river sand and deep fan sediments using heavy mineral assemblage and detrital garnet composition: the reconstruction of the erosion history in the Himalayas

Abstract:

The Bengal Fan deposits exist from the formation of the Himalayan and Tibetan regions caused by the Asian and Indian continental collision that occurred in the early Cenozoic era. Many studies have attempted to reconstruct the erosional history of the Himalayas using detrital mineral assemblage, chemical composition and detrital age data. However, there is a scarcity in reference datasets on the composition, mineral assemblage, and specific mineral chemistry. This study examined the heavy mineral assemblage and chemical composition of specific minerals (i.e., detrital garnets), in the sediments of Himalayan rivers. These were compared with deposits of the Bengal Fan. The heavy minerals in modern river sediments used for comparison, were sampled from the Kari Gandaki River, in central Nepal, and the Sian, Lohit, Dibang and Noa Dihing rivers in the upper tributaries of the Brahmaputra river, eastern India.

The heavy mineral assemblage in the Kari Gandaki and Sian rivers clearly reflected the rock assemblage distribution in these catchments. In particular, in the detritus shed from the catchment area where the High Himalaya Crystalline Sequence (HHCS) are distributed, high temperature metamorphic minerals were predominant. Mafic minerals (i.e., olivine and orthopyroxene), were present in the eastern tributaries (the Lohit, Dibang and Noa Dihing rivers) of the Brahmaputra River, within the catchment area occupied by the Indus Yarlung Tsangpo suture zone.

Detrital garnet compositions were also dependent on geological units distributed in the catchment area; the area occupied by HHSC derives mainly almandine (Mg>20 mol %) garnets, whilst the Lesser Himalayan rocks produce Mg-poor (Mg<20 mol %) almandine garnets.

When compared with Bengal Fan deposits, the results indicate that detrital amphiboles increased from the mid-Miocene (13 Ma) onward; this suggests that the initial major supply from the HHCS had commenced during this period. In addition, Mg-rich garnets were a major component from the Late Miocene period (9 Ma) onward.