Constraints on the metamorphic evolution of the eastern Himalayan syntaxis from geochronologic and petrologic studies of Namche Barwa

The eastern Himalayan syntaxis is host to the actively deforming metamorphic massif, Namche Barwa. This massif has experienced a complex history of uplift and deformation, influenced by intense fluvial erosion associated with the Yarlung Tsangpo. Here we present new thermobarometric and geochronologic information on metamorphic rocks from the Namche Barwa-Gyala Peri region. Pressure-temperature data are combined with U-Th-Pb ages of monazite and titanite in an effort to trace the metamorphic evolution of the eastern Himalaya. Metapelitic rocks containing garnet-biotite-plagioclase assemblages yield peak metamorphic pressures and temperatures of 10-14 kbar and 700-900 degrees C in the structural core of the massif. There is a distinct metamorphic break across the Namula thrust, separating high-grade rocks to the north from lower grade rocks to the south. Ion microprobe monazite and titanite ages of 3-10 Ma indicate that timing of metamorphism is roughly coincident with the age of granitic melt production (<10 Ma) as well as the onset of rapid denudation. In-situ ages determined from monazites included in garnet show that they grew over a period of several million years (6.4 +/- 0.3 Ma to 11.3 +/- 0.2 Ma) and during a pressure decrease of similar to 5 kbar. These data suggest that high-grade metamorphism and anatexis is a phenomenon that has been operative at Namche Barwa since at least the mid-Miocene. Geodynamic models for the evolution of Namche Barwa must therefore account for these features. We conclude that our data most closely fit the tectonic aneurysm model, based on distinct spatial correlations between pressure-temperature (P-T) conditions, age of metamorphism, and erosion by the Tsangpo.