[MIS08-P01] The Paleo-Brahmaputra signature found from the Lower Miocene of Bengal Fan deposits, IODP Exp. 354
Keywords:Himalaya, Orogeny, provenance
It is already reported that the heavy mineral assemblage of the Early Miocene silt-sands mainly was characterized by the predominance of garnet and amphibole grains with a small amount of kyanite, sillimanite and staurolite by Yoshida et al. (2016). The most sand layers in the lower Miocene contain tourmaline, apatite, rutile and hornblende grains with high Mg garnets correlatable to the metamorphic garnets in the High Himalaya Crystalines. However, several sand layers contain sodic amphiboles and chromian spinels, which show very low TiO2 content(<0.05wt%) suggestive of depleted ultra-mafic rock origin. The detrital garnet grains, which are included in these sand layers with chromian spinels, mainly consisted of low-Mg almandine garnets. In the Middle Miocene sequence, most of the sand layers are scarce of sodic amphibole and chromian spinels, though the Pliocene and Pleistocene sands includ both sodic amphiboles and chromian spinels.
The sand layers including both sodic amphibole and chromian spinels were possibly derived from Yarlung Tsangpo suture zone and the ophiolite zone in the Burman mountain ranges. Garzanti et al. (2010, 2011) reported the predominance of chromian spinels in the upper stream of the Brahmaputra river, which is indicative that the sand layers including sodic amphiboles and chromian spinels were carried by "paleo-Brahmaputra river" and supplied to Bengal Fan. Though the Gangs river, which carried the most of detritus from central and western Himalayas, constantly supplied a large amount of detritus into Bengal Fan, the paleo-Brahmaputra river could supply the detritus from eastern Himalaya and Burman mountain ranges in the early Miocene period. The detritus originated from the paleo-Brahmaputra river's discharge in the early Miocene was considered to be derived from the collision orogen among the eastern edge of Indian continent, southeastern part of Tibet block and Burman block in late Oligocene to early Miocene period. The predominance of the sodic amphiboles and chromian spinels in Pliocene and Pleistocene sands is thought to be caused by strong uplift around Eastern Himalaya syntaxis.
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