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 zoisite- 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 850°C to 2·7 GPa at 1025°C, but bends back at higher pressures to 975°C at 3·2 GPa. The melt fraction is always low 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