U-PB ZIRCON CONSTRAINTS ON PROVENANCE OF CENOZOIC SEDIMENTS OF THE ASSAM-BENGAL SYSTEM, EASTERN HIMALAYAS

The confluence of eastern limits of the Himalayas and the northern tip of the north-south trending Indo-Burman ranges forms the eastern Himalayan syntaxis. To the southwest of this syntaxis is the Assam-Bengal basin, which has accumulated synorogenic sediments in an evolving foreland basin. Kilometers of Tertiary sediments are exposed and have been penetrated by drilling. U-Pb dates on detrital zircons from Cenozoic sequences of Assam and Bengal basins provide a complex provenance history.

Zircons from Eocene Disang sandstones of Assam yield dates ranging from Early Proterozoic to Cretaceous, suggesting that orogen-fed rivers drained multiple terranes prior to debauching into the Tethyan Sea. Paleogene zircons are conspicuously absent from Paleogene sandstones of Assam suggesting that any early Tertiary magmatism is not recorded in Eocene sandstones of Assam.

Upper Miocene sandstones from the Bengal basin have zircons of Cambrian to lower Eocene age. Possible derivation of these rocks is mostly from lower Paleozoic granitic rocks of the Higher Himalayas, confirming our compositional results that drainage systems draining to the Bengal basin from the Himalayas were established by the Miocene.

Younger Plio-Pleistocene sandstones from Assam contain zircons predominantly of Cretaceous age; the most likely contributor is the Gangdese batholiths near the Indus-Tsangpo suture belt. The Brahmaputra river, draining through this magmatic arc consisting of intermediate to calc alkaline volcanic rocks, may have brought the zircons to Assam basin during the Plio-Pleistocene time, suggesting possible constraints on the likely swing of the Brahmaputra from being an eastward-draining to a southern and westward directed river. Tectonism at the eastern syntaxis was intense during the Plio-Pleistocene time.