Decompression and anatexis of Himalayan metapelites

The Miocene leucogranites of the High Himalayas have been emplaced within a metasedimentary wedge, defined by the Main Central Thrust (MCT) and the South Tibetan Detachment System (STDS), a low‐angle detachment fault. Isotopic and tectonic constraints indicate that orogenic collapse along the South Tibetan Detachment System occurred at ∼20 Ma, synchronous with anatexis and emplacement of the leucogranites, thus suggesting that exhumation and anatexis were related. The isotope geochemistry of the Himalayan leucogranites indicates that their source lies within the metapelites of the metasedimentary wedge. Pelitic assemblages exhibit an inverted metamorphic geotherm consistent with wedge corner flow which stacked sillimanite‐grade thrust sheets onto kyanite‐grade rocks. However, the leucogranite protolith is not the sillimanite migmatites into which the melts have been emplaced but may be correlated with kyanite schists from near the base of the wedge. In the Langtang section of northern Nepal, leucogranite melts, formed from vapor absent incongruent melting of muscovite, were extracted from their source and migrated over distances >10 km before emplacement close to the STDS. A consequence of fractional melting in the hanging wall of the MCT is instability of the metasedimentary wedge. Decompression melting from depths >40 km can generate a melt fraction of ∼7%, depending on the initial temperature and muscovite content of the protolith. Enhancement of the available melt fraction during exhumation may have been critical in allowing the melt to migrate from its source under an extensional régime.