Prograde metamorphism, decompressional partial melting and subsequent melt fractional crystallization in the Weihai migmatitic gneisses, Sulu UHP terrane, eastern China

In order to decipher fluid/melt activity during subduction and subsequent exhumation of felsic continental crust, we carried out a combined study of zircon internal structure, zircon U–Pb age, zircon trace element composition and whole-rock geochemistry on migmatitic gneisses from the Weihai region in the Sulu ultra-high pressure (UHP) metamorphic terrane, eastern China. The Weihai migmatitic gneisses are composed of intercalated compositional layers of melanosome and plagioclase (Pl)-rich leucosome and K-feldspar (Kfs)-rich pegmatite veins. Whole-rock geochemistry suggests that MgO, Fe2O3T, CaO, TiO2, P2O5, rare earth elements (REE), and large ion lithophile elements (HFSE) are nearly completely retained in the melanosome, whereas significant amounts of LILE (e.g., Rb, Ba, K, Sr, Pb) and U are partitioned into the Pl-rich leucosome and the Kfs-rich pegmatite veins. The Kfs-rich pegmatite vein has much higher K2O, U, Pb and Rb, but lower CaO and Na2O contents than those of the Pl-rich leucosome layer. Protolith magmatic zircon domains, with an upper intercept age of about 780 Ma, are common in the cores of zircon grains from the melanosome and the Pl-rich leucosome layers, indicating that the protolith of the migmatitic gneiss is Mid-Neoproterozoic magmatic rock. Metamorphic zircons with concordant ages ranging from 243 to 256 Ma occur as overgrowth mantles on the protolith magmatic zircon cores. They are characterized by remarkably flat heavy rare earth element (HREE) patterns without obviously negative Eu anomalies, and have low Y and Th contents, low Th/U and high Hf/Y ratios, indicating a prograde metamorphism in the HP eclogite-facies during subduction. Recrystallized rims of zircons (228 ± 2 Ma) in the Pl-rich leucosome layer are similar to magmatic zircons in terms of their HREE enriched patterns with negative Eu anomalies, high Y and low Th contents, and low Hf/Y and Th/U ratios. This indicates that the gneiss has experienced decompressional partial melting and the Pl-rich leucosome layers were formed by fractional crystallization of the primary melt during the early stage hot exhumation. Zircon grains (219 ± 2 Ma) in the Kfs-rich veins are characterized by aqueous fluid/melt-related signatures with strongly HREE enriched patterns, high U, Pb and low Th contents, and low Th/U and high Lu/Hf ratios, indicating that the pegmatite vein was formed by crystallization of residual aqueous melt after the crystallization of Pl-rich leucosome layers during later stage cooling during exhumation. Our results suggest that the migmatitic gneiss successively recorded information concerning the protolith, prograde HP metamorphism during subduction, fractional crystallization of primary melt and crystallization of the residual melt during exhumation.