EXPERIMENTAL CONSTRAINTS ON HIMALAYAN ANATEXIS

We have melted metapelitic rocks from the High Himalayan Crystalline S equence that are likely sources of leucogranite magmas. Starting mater ials were a muscovite schist and a tourmaline-bearing muscovite-biotit e schist. Both are kyanite-zone rocks from the hanging wall of the Mai n Central Thrust. Experiments were conducted at 6, 8 and 10 kbar and 7 00-900 degrees C, both without added H2O (dehydration-melting) and wit h 1-4 wt % added H2O. Dehydration-melting begins at 750-800 degrees C, and produces melts that are virtually identical in composition to the Himalayan leucogranites. Adding H2O lowers the solidus by promoting p lagioclase + quartz melting. Melts produced from these starting materi als at T less than or equal to 750 degrees C by H2O-fluxing are irondh jemitic, and different in composition from most Himalayan leucogranite s. Leucogranite magmas in the Himalaya formed by dehydration-melting o f metapelites during adiabatic decompression, at 6-8 kbar and 750-770 degrees C. The dehydration-melting solidus for muscovite schist has a smaller dP/dT slope undergoes decompression-melting more readily than does biotite schist. The two solidi probably cross over at similar to 10 kbar, so that muscovite may be a more important deep crustal H2O re servoir than biotite.