DETRITAL MINERAL CHEMISTRY AND AGE CONSTRAINTS ON TERTIARY SOURCE TERRAIN EVOLUTION FOR THE BENGAL BASIN

The Bengal basin has built a thick sequence (as deep as 20 km) of Cenozoic deposits, and holds considerable potential for recording erosional history from regional topographic highs (including the Himalayas, the Indo-Burman ranges, and the Indian craton). Petrographic and subsurface studies reveal that orogenic sedimentation had already begun in the Bengal basin by the Lower Miocene. Chemical data and single-crystal laser ⁴⁰Ar/³⁹Ar ages for detrital minerals from Lower Miocene strata provide new constraints and insights on the early sedimentary provenances in the eastern syntaxis of the Himalaya.

Detrital garnet chemical data for the Bhuban Formation are indicative of mixed sources: Approximately two thirds of the garnet we have observed have chemical compositions consistent with amphibolite facies metamorphic conditions, whereas about one third have X_pyr~0.34, X_alm~0.60, X_grs~0.04 and seem most compatible with granulite facies metamorphism of a pelitic or granitic protolith. Such a high-grade source may have been within the gneisses of the Indian craton.

Single-crystal laser ⁴⁰Ar/³⁹Ar age determinations were made for the Bhuban Formation detrital muscovite in the Auburn Noble Isotope Mass Analysis Laboratory (ANIMAL). The laser fusion ages range from ca. 25 Ma to 350 Ma, and thus also suggest derivation from a combination of sources. The age distribution has a mode at ca. 25 Ma and an overall distribution very similar to that reported by Najman et al. (1997) for the Oligocene-Miocene units in the western syntaxis of the Himalayas. We interpret the age range in the Bengal basin to reflect derivation of minerals from early stages of the evolving Himalayan orogenic system and mixed with a lesser proportion of minerals from the Indian craton. The similar age distributions from the eastern and western syntaxes can be taken to suggest an overall magnitude of diachroneity for initial erosion of Himalayan source terrains that was within a few million years.