09:15 〜 09:30
[MIS23-02] 重鉱物組み合わせを用いたヒマラヤ河川砂とベンガル深海扇状地の比較
キーワード：ヒマラヤ、風化、ベンガル海底扇状地
The Asian and Indian continental collision resulted in the Bengal Fan due to the formation of the Himalayan and Tibet regions during the Cenozoic era[1]. Numerous studies have elucidated Bengal Fan deposits to obtain accurate data on the detrital mineral assemblage, chemical composition, and detrital zircon U-Pb age[2]. However, the reference datasets of the composition, mineral assemblage, and specific mineral chemistry in each catchment basin of the Himalayan and Tibet areas are inadequate to reconstruct the erosional history of the Himalayas. Therefore, this study examined the heavy mineral assemblage and chemical composition of detrital garnets found in the sediments of Himalayan rivers, and compared them with those of the Bengal Fan deposits. The sands and heavy minerals in modern river sediments were sourced from the Kari Gandaki River in central Nepal as well as the Siang, Lohit, Diban, and Noa Dihing rivers in the upper tributaries of the Brahmaputra River in eastern India, and subsequently compared with the Bengal Fan deposits.
The assemblage of heavy minerals in the Kari Gandaki River, Nepal and Sian River, India, clearly shows the bed rock assemblage distributed in the catchment area. In particular, the detritus supplied from the catchment area, in which the High Himalayan Crystalline Sequence (HHCS) is distributed, show a predominance of high-temperature metamorphic minerals, such as amphibole, kyanite, and sillimanite. However, mafic minerals (chromian spinels, olivine and orthopyroxene) are found in the eastern tributaries of the Brahmaputra River, including the Lohit, Diban, and Noa Dhing rivers; they are sourced from the catchment area occupied by the Trans-Himalayan rocks along with the Yarlung-Zangbo suture zone that comprises mylonite and ophiolitic rocks.
Detrital garnet compositions are also dependent on the geological units distributed in the catchment area; the area occupied by the HHSC mainly comprises almandine (Mg>20 mol %) garnets, whereas the Lesser Himalayan rocks and low-grade parts of the HHCS produce Mg-poor (Mg<20 mol %) almandine garnets. Detrital ilmenite is common in the catchment of the Tethys Himalayan and Lesser Himalayan rocks, whereas it is rare in the HHCS area.
A comparison with the Bengal Fan deposits indicated that detrital amphiboles increased since the Middle Miocene (13 Ma), suggesting that the initial major supply from the HHCS started during this period. However, Mg-rich garnets became a major component since the Late Miocene (9 Ma). The chroman spinels were observed since the Early Miocene (17-16Ma) and sporadically found in the Late Miocene sediments, indicating a supply from the eastern tributaries of the Brahmaputra River.
Reference
[1] Yin, A. & Harrison, M.T. (2000). Geologic Evolution of the Himalayan-Tibetan Orogen. Annual Review of Earth and Planetary Sciences, 28, 211-218.
[2] France-Lanord, C., Michard, A., Bouquillon, A. & Tiercelin, J. J. (1990). Isotopic chemistry and sedimentology of the Bengal fan sediments: The denudation of the Himalaya. Chemical Geology, 84, 368–370.
The assemblage of heavy minerals in the Kari Gandaki River, Nepal and Sian River, India, clearly shows the bed rock assemblage distributed in the catchment area. In particular, the detritus supplied from the catchment area, in which the High Himalayan Crystalline Sequence (HHCS) is distributed, show a predominance of high-temperature metamorphic minerals, such as amphibole, kyanite, and sillimanite. However, mafic minerals (chromian spinels, olivine and orthopyroxene) are found in the eastern tributaries of the Brahmaputra River, including the Lohit, Diban, and Noa Dhing rivers; they are sourced from the catchment area occupied by the Trans-Himalayan rocks along with the Yarlung-Zangbo suture zone that comprises mylonite and ophiolitic rocks.
Detrital garnet compositions are also dependent on the geological units distributed in the catchment area; the area occupied by the HHSC mainly comprises almandine (Mg>20 mol %) garnets, whereas the Lesser Himalayan rocks and low-grade parts of the HHCS produce Mg-poor (Mg<20 mol %) almandine garnets. Detrital ilmenite is common in the catchment of the Tethys Himalayan and Lesser Himalayan rocks, whereas it is rare in the HHCS area.
A comparison with the Bengal Fan deposits indicated that detrital amphiboles increased since the Middle Miocene (13 Ma), suggesting that the initial major supply from the HHCS started during this period. However, Mg-rich garnets became a major component since the Late Miocene (9 Ma). The chroman spinels were observed since the Early Miocene (17-16Ma) and sporadically found in the Late Miocene sediments, indicating a supply from the eastern tributaries of the Brahmaputra River.
Reference
[1] Yin, A. & Harrison, M.T. (2000). Geologic Evolution of the Himalayan-Tibetan Orogen. Annual Review of Earth and Planetary Sciences, 28, 211-218.
[2] France-Lanord, C., Michard, A., Bouquillon, A. & Tiercelin, J. J. (1990). Isotopic chemistry and sedimentology of the Bengal fan sediments: The denudation of the Himalaya. Chemical Geology, 84, 368–370.