GARNET AND CHROME SPINEL CHEMISTRY OF OLIGOCENE AND LOWER MIOCENE SEDIMENTS FROM THE BENGAL BASIN, BANGLADESH: IMPLICATIONS FOR EARLY HIMALAYAN EROSION

The Bengal basin owes its origin to the collision of India with Eurasia and Burma, which built the extensive Himalayan and Indo-Burman Ranges and loaded the lithosphere to form flanking sedimentary basins. Oligocene and Miocene sediments of the Bengal basin show significant change in heavy mineral composition. Microprobe analysis records sediment-input history from multiple sources around the basin.

Detrital garnets analyzed from the Lower Miocene and Oligocene sediments of Bengal basin are almandine rich (average 70%, maximum 80%). Pyrope content generally varies in two groups (average 34% and 10%) and grossular component is subordinate (average 9%). Almandine garnets may have derived from garnetiferous schists formed by regional metamorphism in Himalayas to the north. The higher pyrope content of the garnets from Bengal basin, however, does not necessarily indicate only a Himalayan source, rather suggests the Indian craton as also a possible source. The elemental percent of chromium in chrome spinels of Bengal basin is high, ranging from 20% to 58% (average 42%). Other important cations common in these spinels are Al, Fe³⁺, Mg and Fe²⁺. These spinels show a wide range in TiO₂ wt% (0.3 to 3%). The relatively narrow range of major element compositions of spinels appears to exclude the possibility of a volcanic arc source and thus a distant Himalayan provenance is unlikely. Based on tectonic location of the Bengal basin, paleogeographic considerations, and provenance work on chrome spinels from the Tethyan Himalayas, the Alpine-type ophiolites present in the Indo-Burman ranges and the Rajmahal trap are more logical sources of those spinels.

The basin received sediments mostly from the Indian craton during the Oligocene time with minor input from the Rajmahal trap to the west and distant orogenic belts to the north and the east. At the beginning of the Miocene time, Bengal basin moved proximal to the encroaching Himalayan and the Indo-Burman orogenic belts, and started receiving sediments from the northeast and the east.