Dehydration melting of ultrahigh-pressure eclogite in the Dabie orogen: evidence from multiphase solid inclusions in garnet

Several types of multiphase solid (MS) inclusions are identified in garnet from ultrahigh-pressure (UHP) eclogite in the Dabie orogen. The mineralogy of MS inclusions ranges from pure K-feldspar to pure quartz, with predominance of intermediate types consisting of K-feldspar + quartz +/- silicate (plagioclase or epidote) +/- barite. The typical MS inclusions are usually surrounded with radial cracks in the host garnet, similar to where garnet contains relict coesite. Barite aggregates display significant heterogeneity in major element composition, with total contents of only 5773% and highly variable SiO2 contents of 0.3225.85% that are positively correlated with BaO and SO3 contents. The occurrence of MS inclusions provides petrographic evidence for partial melting in the UHP metamorphic rock. The occurrence of barite aggregates with variably high SiO2 contents suggests the coexistence of aqueous fluid with hydrous melt under HP eclogite facies conditions. Thus, local dehydration melting is inferred to take place inside the UHP metamorphic slice during continental collision. This is ascribed to phengite breakdown during hot exhumation of the deeply subducted continental crust. As a consequence, the aqueous fluid is internally buffered in chemical composition and its local sink is a basic trigger to the partial melting during the continental subduction-zone metamorphism.