The fluid-absent partial melting of a zoisite-bearing quartz eclogite from 1•0 to 3•2 GPa;: Implications for melting in thickened continental crust and for subduction-zone processes

Fluid-absent melting experiments on a zolsite- and phengite-bearing eclogite (omphacite, garnet, quartz, kyanite, zoisite, phengite and rutile) were performed to constrain the melting relations of these hydrous phases in natural assemblages, as well as the melt and mineral compositions produced by their breakdown. From 1.0 to 3.2 GPa the solidus slopes positively from 1.5 GPa at 850degreesC to 2.7 GPa at 1025degreesC, but bends back at higher pressures to 975degreesC at 3.2 GPa. ne melt fraction is always loud,, and the melt compositions always felsic and become increasingly, so with increasing pressure. The normative Ab-An-Or compositions of the initial melts vary from tonalites at 1.0 GPa to tonalite-trondhjemites at 1.5 GPa, adamellites at 2.1 and 2.7 GPa, and to true granites at 3.2 GPa. At pressures < similar to 2.5 GPa zoisite and phengite break down more or less simultaneously. At 3.2 GPa and 1000 degrees C zoisite is unreacted whereas phengite is absent, so that the first formed melt at these conditions is granitic. Our experiments show that if sufficiently high temperatures (of the order of 1000 degrees C) are attained, zoisite- and phengite-bearing eclogites can produce small fractions of silicic melts of a wide range of compositions. These melts are rich in water and, probably, in Sr and other incompatible elements, so that they, can act as metasomatic agents in the mantle wedge.