THE P-T-T PATH OF METAPELITES FROM FAR-EASTERN NEPAL HIMALAYA

The channelized flow of weak mid-crustal material is an appealing explanation for the thermal-mechanical evolution of the Tibet-Himalayan system (e.g. Beaumont et al., 2004), but still controversial. Far-eastern Nepal is characterized by the thick sequense (c. 30 km) of the high-grade Higher Himalayan Crystalline Sequences (HHCS), bounded by the Main Central Thrust (MCT) at its base. We examine the spatial distributions of P-T-t paths for metapelites across the MCT and HHCS, using multiple equilibria thermobarometry, growth zoning of garnet with Gibbs method, and Nano-SIMS zircon geochronology with detailed petrography.

From the MCT to the HHCS, the inverted metamorphic zonation occurs with the changes of garnet, staurolite, kyanite, sillimanite+muscovite, sillimaite+K-feldspar, and sillimaite± cordierite isograds. Based on the average P-T method (THERMOCALC: Powell and Holland, 1998), the geothermal gradient at nearly peak T increases towards structurally higher positions from c. 16-18 °C/km in the base of the HHCS to c. 40-46 °C/km in the upper part, through c. 20-35 °C/km in the middle.

The P-T paths from rocks in the footwall of the MCT, where the discontinuity of protoliths across the channel occurs, show the nearly isothermal loading path. In contrast, the paths in the hanging wall show clockwise P-T path with rapid decompression path during heating. These rocks that show different P-T paths were juxtaposed at c. 620 °C and 10 kbar that converged these P-T paths, and then rapidly exhumed together. No isobaric heating path in the footwall that entrained into the channel could be indicative of the rapid extrusion of rocks, but not of the heat conduction from the extruding HHCS.

The metamorphic rim revealed by cathodoluminescence images in zircons from the HHCS migmatic gneisses yielded U-Pb ages ranging between 15 and 18 Ma. These ages constrain the timing of peak sillimanite-grade metamorphism (c. 700-800 °C) during south-directed shearing, consistent with the pervasive high-temperature metamorphism at 12-22 Ma predicted by channel flow models (Jamieson et al, 2004).