AN EVALUATION OF THE INVERTED METAMORPHIC GRADIENT AT LANGTANG NATIONAL-PARK, CENTRAL NEPAL HIMALAYA

The crystalline core of the Himalayan orogen in the Langtang area of Nepal, located between the Annapurna-Manaslu region and the Everest region, contains middle to upper amphibolite grade pelitic gneisses and schists. These rocks are intimately associated with the Main Central Thrust (MCT), one of the major compressional structures in the northern Indian plate, which forms a 3.7-km-wide zone containing rocks of both footwall and hangingwall affinity. An inverted metamorphic gradient is noticeable from upper footwall through hangingwall rocks, where metamorphic conditions increase from garnet grade near the MCT zone to sillimanite + K-feldspar grade in the upper hangingwall. Petrographic data distinguish two metamorphic episodes that have affected the area: a high-pressure, moderate-temperature episode (Mi) and a moderate-pressure, high-temperature episode (M2). Comparison with appropriate reaction boundaries suggests that conditions for M1 in the hangingwall were approximately 900-1200 MPa and 425-525 degrees C. Thermobarometric results for 24 samples from the footwall, MCT zone and hangingwall reflect P-T conditions during the M2 phase of 400-1200 MPa and 490-660 degrees C. The decrease in estimated palaeopressures from footwall to hangingwall approximate a lithostatic gradient of 27 MPa km(-1), with slight fluctuations in the MCT zone reflecting structural discontinuities. In contrast to the palaeopressures, palaeotemperatures are indistinguishable across the entire area sampled. Although field evidence suggests the presence of the inverted palaeothermal gradient well known in the Himalaya, quantitative thermobarometry indicates that temperatures of final equilibration were all within error of each other across 17 km of section. At Langtang, change in pressure is responsible for the presence of the sequence of index minerals through the section. I interpret these data to reflect diachronous attainment of equilibrium temperature conditions in a lithostatic palaeopressure profile after ductile faulting of the sequence.