Syn-exhumation partial melting and melt segregation in the Sulu UHP terrane: Evidences from leucosome and pegmatitic vein of migmatite

• We investigate incipient and segregated melts of crust anatexis in the Sulu orogen.
• We date partial melting and decipher partial melt evolution during exhumation of UHP terrane.
• Melt segregation process can also result in low Th/U ratio of zircon.
• Melt segregation could cause zircon Lu–Hf isotopic disequilibrium.

In order to investigate partial melting and melt evolution during exhumation of deeply subducted continental crust, we carried out a combined study on layered leucosomes and pegmatitic veins from the Weihai–Rongcheng migmatites in the Sulu ultra-high pressure (UHP) metamorphic terrane, eastern China. The leucosomes are millimetric to centimetric in thickness and mainly consists of K-feldspar + quartz + plagioclase. The pegmatitic veins emplace along the compositional layers or crosscut them and are mainly composed of K-feldspar + quartz. CL images show that most zircon grains of the leucosomes have a core–rim zoning structure, whereas zircons of the pegmatitic veins are mostly new growth grains with rarely preserved relict domains. The inherited zircon domains (ca. 750 Ma) are of a magmatic origin with zircon εHf(t) values between − 5.7 and 5.3 (mean = − 0.8 ± 1.5), suggesting that the protolith rock is a Mid-Neoproterozoic juvenile crust. The zircon overgrowth rims (223 ± 3 Ma) of the leucosomes and the new zircon grains (217 ± 2 Ma) of the pegmatitic veins formed in equilibrium with melt and are similar to magmatic zircons in terms of their CL images and trace element compositions, e.g., heavy-enriched REE patterns (i.e., very low (Gd/Lu)N ratios) with positive Ce and variably negative Eu anomalies, high Y contents and low Hf/Y ratios. Compared with the inherited protolith zircon domains, the Triassic zircon domains of the leucosomes and the pegmatitic veins have obviously high U but low Th contents, resulting in very low Th/U ratios (< 0.09). The low Th/U ratios (< 0.1) of zircons suggest that Th/U ratio cannot be used to discriminate igneous/metamorphic zircons. The zircons of the pegmatitic veins have remarkably lower Th/U ratios (0.00008–0.01, mostly < 0.006) than those of zircon rims of the leucosomes (0.006–0.09), suggesting that the melt segregation process can also result in low Th/U ratios of zircons. Calculated growth temperature for the zircon overgrowth rims in the leucosomes ranges from 648 to 845 °C with spot ages from 235 ± 3 to 210 ± 3 Ma. The growth temperature for the zircon grains from the pegmatitic veins ranges from 809 to 651 °C with spot ages from 230 ± 3 to 207 ± 3 Ma. The features suggest a process of cooling and segregation of melt. Melt segregation results in dramatic increases of zircon 176Yb/177Hf and 176Lu/177Hf ratios from the leucosomes to the pegmatitic veins, however, it does not affect the zircon εHf(t) values. Combined with previous results of studies on the Sulu orogen, we propose that partial melting occurred at the early exhumation stage and melt segregation lasted until the late exhumation stage. The melt not only lubricated the subducted slices and facilitated the buoyant rise from mantle depths to the lower crustal level but also triggered syn-exhumation magmatism.