Partial melting of UHP calc-gneiss from the Dabie Mountains

•The Ganjialing UHP calc-gneisses in the Dabie Mountains experienced a wet melting during early exhumation.•Partial melting is evidenced by microstructures of phengite, feldspars and epidote, etc.•The patchy microstructure of garnet resulted from alteration of melts.•Some polyphase inclusions in garnet were formed by melts infiltration through healed fractures.

Exhumation melting has been proposed for the ultra-high pressure (UHP) metamorphic rocks in the Dabie Mountains based on melting experiments. We document here the first petrological and mineralogical evidence demonstrating that the UHP calc-gneisses from the Ganjialing area in the Dabie Mountains experienced partial melting during early exhumation. The assemblage of garnet, phengite (Si = 3.65 pfu), coesite, rutile and carbonate preserved in the calc-gneisses indicates a peak metamorphic condition of 692–757 °C and 4.0–4.8 GPa. Partial melting is indicated by several lines of evidence: the melting textures of phengite, the feldspar-dominated films, bands, branches, blebs and veins, the euhedral K-feldspars, the intergrowth film of plagioclase and K-feldspar, the plagioclase + biotite intergrowth after garnet and the epidote poikiloblasts. Polyphase inclusions in garnet are characterized with wedge-like offshoots and serrate outlines whereas those in epidote display negative crystal shapes, which can be best interpreted by entrapment of former melts. We propose a wet melting reaction of Phn + Q ± Na-Cpx + H2O = Bt + Pl + Grt + felsic melts, which likely took place at ca.650–800 °C and ca.1.0–2.0 GPa, to interpret the melting event in the calc-gneisses. Chemical exchanges between garnet and melts produced new garnet domains with higher almandine, spessartine, MREE, HREE and Y but lower grossular, pyrope, P, Sc, Ti, V and Zr contents. Zr-in-rutile thermometer reveals a low temperature of 620–643 °C at 5 GPa, indicating a later reset for Zr in rutile. Healed fractures are suggested to be responsible for the formation of some polyphase inclusions in garnet.