| The eastern syntaxis of the Himalayas is expressed in the crust as a pronounced southward bend in the orogen. The change in strike of geologic features coincides with the high topography of the Namche Barwa region, the exposure of granulite-grade metamorphic rocks, and a 180-degree bend in the Tsangpo river. Surface patterns of ages using high-temperature geochronometers (U-Pb zircon and U-Th-Pb monazite) provide valuable information about deep crustal processes, and are critical in unraveling the tectonic history of this region. Exceptionally young granitic melts (<10 Ma) are documented within the structural core of Namche Barwa, with a geochemical signature characteristic of fluid-absent (decompression) melting. These are not readily understood in the context of conventional Himalayan tectonics, and suggest a mechanism related in part to rapid incision by the Yarlung Tsangpo. Thermobarometry on rocks from Namche Barwa reveal high P-T equilibrium conditions (10--15 kbar, 700--900°C), young metamorphic monazite and titanite ages (3--10 Ma), and decompression P-T paths that are also concentrated around the core of the massif. These features, taken together, support the notion that in a deforming orogen, tectonic-surficial coupling is expressed in the patterns and exposure of metamorphic facies. Moreover, metamorphic ages extending back to the late Miocene suggest that this coupling is not merely a transient feature in the crust, but a long-term interaction between tectonic and surface processes.; In a separate but related study, U-Pb discordance and oxygen isotopic composition of zircon are investigated via ion microprobes delta 18O in both concordant and discordant zircon grains provides an indication of the relationship between fluid interaction and discordance. Results suggest that three characteristics of zircon are interrelated: (1) U-Pb systematics and concomitant age discordance, (2) delta18O and the water-rock interactions implied therein, and (3) zircon texture, as revealed by cathodoluminescence and BSE imaging. From this it is apparent that various stages of water-rock interaction, as evidenced by structured variations in delta 18O, leave their imprint on both the texture and U-Pb systematics of zircon. These delta18O variations are measured in zircons from Namche Barwa and used to draw conclusions regarding the tectonic and metamorphic evolution of this region. |
