Laser ~(40)Ar/~(39)Ar age constraints on Miocene sequences from the Bengal basin: Implications for middle Miocene denudation of the eastern Himalayas

摘要

Petrographic, mineral-chemistry and subsurface studies reveal that orogenic sedimentation had already begun in the Bengal basin by the early Miocene. Laser ~(40) Ar/~(39)Ar age determinations were made for detrital muscovite grains (145 total, among 4 samples) from the lower-to-middle Miocene Bhuban Formation. The laser fusion ages range from circa 12 Ma to 516 Ma, and thus suggest derivation from a combination of sources: the Himalayas, Indo-Burman ranges and possibly the Indian shield and Tibetan plateau. Modes of circa 16 Ma, 18 Ma, 26 and 40 Ma in the age distributions of these samples are most consistent with unroofing of the Higher Himalayas since the early Miocene. Detrital micas of such an early age (16 Ma) for the Bhuban Formation are interpreted to indicate that little time elapsed between the isotopic closure of ~(40)Ar in the muscovite and its ultimate deposition in middle Miocene strata. The detrital ages of circa 16 and 22 Ma in this study, most prominent in the highest stratigraphic levels sampled in this study, are younger than those previously reported in the western Himalayan foreland basins. These younger detrital ages are consistent with rapid middle Miocene unroofing and erosion as has been proposed for crystalline rocks of the eastern Himalayas. The minimum ~(40)Ar/~(39)Ar ages for muscovite in a particular sample seem proportional to the stratigraphic level sampled, i.e., younger ages tend to occur for samples of higher stratigraphic level. These results support earlier studies indicating that detrital geochronology can be used as an effective tool in evaluating stratigraphic ages.