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Experimental Constraints on Himalayan Anatexis

Experimental Constraints on Himalayan Anatexis 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. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Petrology Oxford University Press

Experimental Constraints on Himalayan Anatexis

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References (60)

Publisher
Oxford University Press
Copyright
© Published by Oxford University Press.
ISSN
0022-3530
eISSN
1460-2415
DOI
10.1093/petroj/39.4.689
Publisher site
See Article on Publisher Site

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.

Journal

Journal of PetrologyOxford University Press

Published: Apr 1, 1998

Keywords: leucogranites trondhjemites Himalaya anatexis muscovite

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