Himalayan Leucogranites: Field Relationships and Tectonics

Himalayan peraluminous leucogranites were derived from in-situ melting of sillimanite + K-feldspar-bearing pelite-migmatite, and were transported via layer-parallel sill complexes and cross-cutting dykes to feed giant sills up to 5 km thick. Partially melted Himalayan middle crust was extruded southwards between two large-scale, north-dipping, synchronous ductile shear zones: the Main Central Thrust (MCT) below and the low-angle normal fault South Tibetan Detachment (STD) above. U-Th/Pb monazite dating constrains granite melting to ~25–18.5 Ma in Manaslu and ~24–13 Ma in Everest-Makalu. The Manaslu sheeted sill complex was emplaced by progressive underplating with the oldest intrusions structurally above younger intrusions. Heat was dominantly derived by internal radioactive heating from crustal thickening with little or no contribution from shear heating along the MCT or from the mantle.