Detrital Chromian Spinel and Garnet From Bangladesh: Constraints on the Uplift History of the Himalayas

Bangladesh is in the northeastern part of the Bengal fan, which is composed of the detritus from the Himalayas. The Cenozoic sediments of the Bengal fan have recorded the uplift history of the Himalayas. This paper treats chemistries of detrital chromian spinels and garnets of the Eocene to Pleistocene sandstones in the Sylhet and Chittagong areas, which are located in northeastern and southeastern Bangladesh, respectively. We studied chemistries of detrital chromian spinels and garnets by EPMA to clarify the host rocks. The detrital chromian spinels were obtained from all the sandstones. Most of the detrital chromian spinels from the Oligocene to Pleistocene sandstones are very similar to those of serpentinites in the Yarlung-Zangbo ophiolite which was a suture zone between the Lhasa Block and Transhimalaya. However, the chemistry of detrital chromian spinels from the Eocene sandstone is somewhat different from that of the Yarlung-Zangbo ophiolite, and some of the detrital chromian spinels have similar chemistry with that of the spinels in the flood basalt in the Deccan Traps. This suggests that the Yarlung-Zangbo ophiolite has supplied detrital chromian spinels since the Oligocene. Garzanti et al. (1987) reported the occurrence of detrital chromian spinels from the lower Eocene in the western part of the Himalayas. Thus there was a time lag in the exhumation of ophiolitic rocks between the western and eastern parts of the Himalayas. The detrital garnets were obtained from most of the sandstones in the Sylhet and Chittagong areas. There are different tendencies of detrital garnet chemistries among sandstones from the Eocene to Oligocene, Early Miocene, and Middle Miocene to Pleistocene. As conclusions, it is inferred that the Tertiary granitic rocks in the Higher Himalaya started to supply detrital garnet to the Bengal fan in the Early Miocene, and the erosion of metamorphic rocks in the Lesser Himalaya seems to have started in the Middle Miocene.