Sandstone Composition and Detrital Heavy Minerals from Sylhet Area, Bangladesh: Constraints on the Uplift History of the Himalayas

The Sylhet area of NE Bangladesh is located in the northernmost 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 of framework compositions and chemistries of detrital heavy minerals of the Oligocene to Pleistocene sandstones in the Sylhet area. As a result, it reveals that the Cenozoic beds yield detrital chromian spinels and they might have been provided from the Yarlung-Zangbo ophiolite along the Indus suture. This suggests that it has been eroded continuously since 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 may be a time lag in the exhumation of ophiolitic rocks between the western and eastern parts of the Himalayas.\According to Uddin and Lundberg (1998), the Cenozoic beds in the Sylhet area are divided into seven units; Eocene Sylhet Limestone and Kopili Shale, Oligocene Barail Formation, Miocene Boka Bil and Bhuban Formations, upper Miocene to lower Pliocene Tipam Group and upper Pliocene to Pleistocene Dupi Tila Formation. The sandstones treated here were collected from the Barail, Bhuban and Dupi Tila Formations, and the Tipam Group. Two groups are recognized on the basis of the framework compositions of sandstones except for the Bhuban Formation which consists of indistinguishable finer grains under the microscope. The sandstones from the Barail Formation and part of the Dupi Tila Formation are extremely quartzose. Also the Tipam Group sandstone and the rest of the Dupi Tila Formation sandstone are relatively poor in quartz. All the sandstones contain a small amount of feldspars and fragments of metamorphic and felsic volcanic rocks. It is inferred that most of those rock fragments originated mainly from metamorphic rocks in the Himalayas, though there are no suitable source rocks in the Himalayas concerning felsic volcanic rock fragments. The felsic volcanic rock fragments may originate from the Lhasa Block on the northern side of the Indus suture.\We studied chemistry of detrital chromian spinel and garnet by EPMA to clarify the host rocks. The detrital chromian spinels were obtained from all the sandstones. Most of the detrital chromian spinels are very similar to those of serpentinites in the Yarlung-Zangbo ophiolite. This means that the host rocks of detrital chromian spinels have the same or similar setting origin as the Yarlung-Zangbo ophiolite, and they have been exposed continuously since Oligocene. The detrital garnets were obtained from the Barail and Dupi Tila Formations and the Tipam Group. There is a different tendency of detrital garnet chemistries between the Barail Formation and the others. It is considerable that the provenances of the detrital spessartine-rich almandine and pyrope-rich almandine correspond to the Lower Himalaya and the Higher Himalaya, respectively.