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

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The ultrahigh-pressure (UHP) eclogite and gneiss from the Dabie (大别)-Sulu (苏鲁) oro- gen 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 ℃. 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 ℃. In con- trast, the partial melting of phengite-bearing eclogite exists at slightly lower temperatures (800-850 ℃). 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 composi- tion can place important constraints on the partial melting phenomena (granitic leucosome and multi-phase mineral inclusions) recorded in UHP rocks.