METAMORPHIC ROCKS OF THE GREATER HIMALAYAN SEQUENCE (BHUTAN) RECORD CHANNEL FLOW

Pseudomorphs of sillimanite after kyanite and of muscovite after staurolite (Guidotti, 1974), high resolution composition maps of garnet, arrested metamorphic reaction textures, and thermobarometry support the model of flow of low viscosity rock in a channel from under the Tibetan Plateau to the Himalayan front of Bhutan. The viscosity was lowered by anatectic melt. The depth of the channel corresponds to pressures of 10 – 12 kbar (kyanite-bearing granite). At the Himalayan front, the channel decompressed to 5 - 6 kbar (sillimanite after kyanite) before cooling below 700oC (thermobarometry). An arrested reaction that defines the decompression was garnet + muscovite = sillimanite + biotite. Deformed kyanite and truncated growth zoning of garnet attest to ductile flow following initial growth of garnet and kyanite.