DETRITAL U/PB AND 40AR/39AR CONSTRAINTS ON EXHUMATION HISTORY OF THE EASTERN HIMALAYAS AT THE ASSAM-BENGAL SYSTEM

The denudation history of the Himalayas is recorded in part within thick clastic wedges deposited in subsiding foreland troughs of upper Assam (in Shuppen belt), lower Assam (near Shillong plateau), the Bengal basin (Sylhet trough) and the Bengal Deep Sea Fan. A greater thickness of Paleogene strata is observed at the core of upper Assam. U/Pb ages of zircons (obtained at the geochronology laboratory of MIT by late Sam Bowring) from Eocene marine lithic phyllarenites of upper Assam provide dates ranging from Early Proterozoic to Cretaceous, suggesting that orogen-fed rivers drained multiple terranes prior to their deposition. In addition to contributions from lower Paleozoic granites in the Higher Himalayas, continental crustal rocks may have also provided zircons to these sandstones. Zircons from Eocene quartzose sandstones of the Bengal basin yield dominantly a Proterozoic age and are most likely derived from the Indian continental crust. ⁴⁰Ar/³⁹Ar study of muscovites from the Oligocene strata (mostly non-marine quartzolithic) of upper Assam provide cooling ages ranging from Late Triassic to Eocene with dominance of Late Cretaceous grains. In the samples (turbidite to fluvial) of lower Assam, muscovite grains show cooling ages from Paleoproterozoic to Early Oligocene with conspicuous presence of Late Cambrian-Early Ordovician, Late Cretaceous, and Eocene assemblages. Muscovites from Miocene strata in the upper Assam range from Late Cretaceous to Oligocene, whereas the muscovites from the Miocene strata from lower Assam range from Early Cambrian to Miocene. Muscovite grains from Lower Miocene strata of the Bengal basin also provide a similar distribution (Early Cambrian to Miocene) of cooling age. Possible derivation of these rocks is from lower Paleozoic granitic rocks of the Higher Himalayas, confirming our earlier compositional results that drainage systems had developed by the Miocene to the Bengal basin from the Himalayas. The arkosic Plio-Pleostocene sandstones from Assam-Bengal system contain high-grade metamorphic detritus, including zircons and muscovites of predominantly Cretaceous age; the most likely contributor was the Gangdese batholiths near the Indus-Tsangpo suture belt.