The Fluid-absent Partial Melting of a Zoisite-bearing Quartz Eclogite from 1middle dot0 to 3middle dot2 GPa; Implications for Melting in Thickened Continental Crust and for Subduction-zone Processes

Abstract

Fluid-absent melting experiments on a zoisiteand phengite-bearing INTRODUCTION eclogite (omphacite, garnet, quartz, kyanite, zoisite, phengite and Zoisite or clinozoisite is present in many high-pressure rutile) were performed to constrain the melting relations of these eclogites, and many occurrences are known from the hydrous phases in natural assemblages, as well as the melt and Scandinavian Caledonides (e.g. Holsnoy: Austrheim & mineral compositions produced by their breakdown. From 1·0 to Mork, 1988; Jamtveit et al., 1990; Western gneiss region: 3·2 GPa the solidus slopes positively from 1·5 GPa at 850°C to Griffin et al., 1985; Seve Nappe: Kullerud et al., 1990). 2·7 GPa at 1025°C, but bends back at higher pressures to 975°C Indeed, most eclogites worldwide contain minor amounts at 3·2 GPa. The melt fraction is always low and the melt of zoisite or other hydrous phases, according to the compositions always felsic and become increasingly so with increasing compilations of Smith (1988) and Carswell (1990). During pressure. The normative Ab–An–Or compositions of the initial prograde eclogitization, zoisite forms by breakdown of melts vary from tonalites at 1·0 GPa to tonalite–trondhjemites at the anorthite component of plagioclase in the presence 1·5 GPa, adamellites at 2·1 and 2·7 GPa, and to true granites of a hydrous fluid phase. The origin of these hydrous at 3·2 GPa. At pressures 2·5 GPa zoisite and phengite break fluids is controversial. One possibility is that they are down more or less simultaneously. At 3·2 GPa and 1000°C zoisite introduced from below during continental collision, for is unreacted whereas phengite is absent, so that the first formed melt example, if wet continental sedimentary rocks are deeply at these conditions is granitic. Our experiments show that if subducted. During subduction of oceanic crust zoisite sufficiently high temperatures (of the order of 1000°C) are attained, forms by prograde metamorphism of hydrothermally zoisiteand phengite-bearing eclogites can produce small fractions altered oceanic crust, which in its upper levels contains of silicic melts of a wide range of compositions. These melts are low-temperature hydrous Ca-rich phases (e.g. Poli & rich in water and, probably, in Sr and other incompatible elements, Schmidt, 1997). At the gabbro to dyke transition zone so that they can act as metasomatic agents in the mantle wedge. the temperature and pressure are high enough to allow