Journal of Petrology Advance Access originally published online on October 1, 2004
Journal of Petrology 2005 46(2):275-290; doi:10.1093/petrology/egh071
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Journal of Petrology vol. 46 issue 2 © Oxford University Press 2004; all rights reserved
Vapor-Absent Melting of Tonalite at 15–32 kbar
DEPARTMENT OF GEOLOGY, UNIVERSITY OF GEORGIA, ATHENS, GA 30602, USA
The behavior of igneous continental crust during subduction is modeled by means of vapor-absent partial melting experiments on a tonalite, containing equal amounts of biotite and hornblende, at pressures of 15–32 kbar. The experiments produce leucogranitic melts coexisting with garnet + omphacitic clinopyroxene + K-feldspar + kyanite + quartz/coesite ± phengite ± zoisite. Experimental constraints and geometrical analysis of phase equilibria show that the hydrous phases that control dehydration-melting of tonalites in deep thickened continental crust and in the upper mantle are phengite and zoisite. The negatively sloping amphibole + quartz vapor-absent solidus characteristic of amphibolites is largely suppressed in tonalites, because amphibole is eliminated by water-conserving reactions that also consume K-feldspar and kyanite and produce phengite and zoisite. The temperature at which melt first appears in the experiments varies from <900°C at 15 kbar, to 1000°C at 27 kbar, to <925°C at 32 kbar. Moderate degrees of partial melting (20–30%) yield residual assemblages with mantle-like densities but which can still contain minor amounts of hydrous phases. Partial melting of tonalitic crust during continental subduction can thus generate incompatible element-rich residues that would be able to remain in the mantle indefinitely, acting as long-term sources of metasomatic fluids.
KEY WORDS: mantle; melting; metasomatism; tonalite; UHP metamorphism
* E-mail: klingon{at}3rdrock.gly.uga.edu
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