Experimental Constraints on Himalayan Anatexis

Abstract

We have melted metapelitic rocks from the High Himalayan Crystalline Sequence that are likely sources of leucogranite magmas. Starting materials were a muscovite schist and a tourmaline-bearing muscovite–biotite schist. Both are kyanite-zone rocks from the hanging wall of the Main Central Thrust. Experiments were conducted at 6, 8 and 10 kbar and 700–900°C, both without added H2O (dehydration–melting) and with 1–4 wt % added H2O. Dehydration-melting begins at 750–800°C, and produces melts that are virtually identical in composition to the Himalayan leucogranites. Adding H2O lowers the solidus by promoting plagioclase + quartz melting. Melts produced from these starting materials at T ≤ 750°C by H2O-fluxing are trondhjemitic, and different in composition from most Himalayan leucogranites. Leucogranite magmas in the Himalaya formed by dehydration-melting of metapelites during adiabatic decompression, at 6–8 kbar and 750–770°C. The dehydration-melting solidus for muscovite schist has a smaller dP/dT slope than that for biotite schist. In consequence, muscovite schist undergoes decompression-melting more readily than does biotite schist. The two solidi probably cross over at ∼10 kbar, so that muscovite may be a more important deep crustal H2O reservoir than biotite.