Evidence for partial melting of eclogite from the Moldanubian Zone of the Bohemian Massif, Czech Republic

摘要

Eclogites from the Moldanubian Zone of the Bohemian Massif (Czech Republic) have experienced ultrahigh–pressure (UHP) and ultrahigh–temperature (UHT) metamorphic conditions (P > 4.0 GPa, T > 1000 °C). One eclogite sample collected from Nové Dvory, eastern part of Czech Republic, contains leucocratic pockets. The mineral assemblage of the melanocratic part of the eclogite is garnet + omphacite + rutile + later–stage minerals (biotite + amphibole + plagioclase). The leucocratic pockets mainly consist of plagioclase and include moderate amounts of garnet and clinopyroxene + small amounts of biotite and amphibole. Garnet and clinopyroxene grains in the leucocratic pockets display faceted shapes whereas those in the melanocratic part show irregular shapes. These are consistent with the idea that garnet and clinopyroxene grains in the leucocratic pockets had been surrounded by melt, as melt can provide free space for the formation of such faceted shapes. The major and trace element composition of garnet grains in the melanocratic part are the same as those in the leucocratic pockets. Clinopyroxene also shows the same major and trace element composition for irregular shape grains in the melanocratic part as faceted grains in the leucocratic pockets. If the melt had originated from the gneiss surrounding the peridotite body with eclogite lenses or layers, the composition of clinopyroxene in the leucocratic pockets would have been completely different from that in the melanocratic part. In addition, the trace element composition of melt estimated from clinopyroxene composition resembles that of Tonalite – Trondjhemite – Granodiorite (TTG), which can be produced by partial melting of eclogite. Thus, these observational and analytical data suggest that the leucocratic pockets were originally melt that was internally produced by partial melting of the eclogite.