Journal of Petrology, Volume 39, Issue 4: April 1 1998.

Experimental Constraints on Himalayan Anatexis

ALBERTO E. PATIO DOUCE¹ AND NIGEL HARRIS²

¹DEPARTMENT OF GEOLOGY, UNIVERSITY OF GEORGIA, ATHENS, GA 30602, USA ²DEPARTMENT OF EARTH SCIENCES, THE OPEN UNIVERSITY, MILTON KEYNES MK7 6AA, UK

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.

Keywords: leucogranites;trondhjemites; Himalaya; anatexis; muscovite

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