Detrital heavy mineral record in the Bengal Fan constrains the evolution of the Himalaya and the linkage of the Ganga and Brahmaputra Rivers

The Bengal Fan was formed from detritus shed by the Ganges and Brahmaputra Rivers, reflecting India-Asia plate collision. The IODP Expedition 354 Bengal Fan recovered sediments recording the tectonic and climatic history of the Himalayan Orogeny. We examined the heavy mineral assemblage and chemical composition of specific minerals in the sediments deposited from Miocene to present in the deepest hole (U1451A and B) to clarify in detail the denudation history of Himalayan metamorphic rocks. In addition, the heavy minerals in modern river sediments, taken from main tributaries of the Ganges River, the Karnali and Kaligandaki Rivers, in Nepal, and the Brahmaputra River in eastern India, were examined for comparison with the Bengal Fan sediments.

The assemblages of heavy minerals in the sediments of the modern Ganges tributaries are characterized by grains of epidote and garnet. The amphiboles are mainly hornblende, pargasite, and actinolite. The detrital garnets are mainly almandine with high-low pyrope contents. The heavy mineral assemblage of modern Brahmaputra River sands is rich in hornblende, pyroxene, and olivine. Rare chromian spinels occur. The amphiboles include mainly pargasite and kaersutite with hornblende and edenite. Rare winchite and riebeckite were observed. The detrital garnets include a wide variety of almandines with high Mn-Ca contents.

In the Bengal Fan sediments, high-grade metamorphic minerals such as kyanite, sillimanite, and staurolite were found in the early Miocene sediments. However, the heavy mineral assemblage changed with a remarkable increase of amphiboles, including pargasite, winchite, and rare riebeckite, at 13 Ma. Detrital chromian spinels were found in several horizons. Then, the chemical composition of detrital garnets changed to more pyrope-rich compositions at approximately 9 Ma. After 6 Ma, pyroxene and olivine grains were frequently included in the sediments.

The aforementioned result revealed that the Ganges and Brahmaputra River discharges were frequently mixed and then delivered to the Bengal Fan. However, the composition of the Bengal Fan sediments changed, reflecting wide exposure of the High Himalaya crystalline rocks in middle Miocene period and the contribution by the Brahmaputra River increased after the latest Miocene period.