Multiphase solid inclusions in zoisite-bearing eclogite: evidence for partial melting of ultrahigh-pressure metamorphic rocks during continental collision

Multiphase solid (MS) inclusions in both garnet and omphacite were investigated for zoisite-bearing eclogite from the ultrahigh-pressure (UHP) metamorphic zone in the Sulu orogen. The results provide petrological evidence for local anatexis during exhumation of deeply subducted continental crust. There are three types of MS inclusions: (1) plagioclase + quartz, (2) plagioclase + quartz + K-feldspar, and (3) barite + plagioclase + K-feldspar ± zoisite/epidote. The host minerals mostly exhibit radial fractures surrounding the MS inclusions. The first and second types of MS inclusions were analyzed for their bulk compositions, yielding high SiO2 and Na2O but very low FeO + MgO + TiO2 with variable K2O for the second type. Trace element analyses of representative MS inclusions yield generally very low concentrations except such large ion lithophile elements as Sr, Ba and Rb. These features suggest different origins for the three types of MS inclusions. The first type of MS inclusions would be primarily derived from dehydration melting of paragonite, whereas the second type of MS inclusions would be derived from dehydration melting of both paragonite and phengite. The third type of MS inclusion may result from the interaction between metamorphic fluid and host mineral in view of the occurrence of barite as a filling phase in the fractures of host minerals. Zoisite breakdown is also indicated by its highly cuspate shape in coexistence with quartz, providing components for growth of garnet during the exhumation. Therefore the anatexis of zoisite-bearing UHP eclogite is primarily driven by the breakdown of hydrous minerals such as phengite and paragonite. The major and trace element compositions of MS inclusions in the eclogites mainly depend on the species of hydrous minerals involved in the mineralogical reactions of dehydration melting in subduction channel.