Experimental investigation on low-degree dehydration partial melting of biotite gneiss and phengite-bearing eclogite at 2 GPa

Abstract

The ultrahigh-pressure (UHP) eclogite and gneiss from the Dabie (大别)-Sulu (苏鲁) orogen experienced variable degrees of partial melting during exhumation. We report here dehydration partial melting experiments of biotite gneiss and phengite-bearing eclogite at 2 GPa and 800–950 °C. Our results show that the partial melting of gneiss is associated with the breakdown of biotite into almandine-rich garnet starting at 900 °. About 10% granitic melt can be produced at 950 °C. In contrast, the partial melting of phengite-bearing eclogite exists at slightly lower temperatures (800–850 °C). The melt fraction is in general more in biotite gneiss than in phengite-bearing eclogite under similar pressure and temperature conditions. Both melts are rich in silica and alkali, but poor in FeO, MgO and CaO. These results suggest that low-degree partial melting of gneiss and eclogite is often associated with dehydration of hydrous mineral, such as micas. The dehydration temperature and melt composition can place important constraints on the partial melting phenomena (granitic leucosome and multi-phase mineral inclusions) recorded in UHP rocks.

Keywords

gneiss / eclogite / partial melting / dehydration / phengite / biotite

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