Refining the timing and mechanism of the Triassic partial melting in the Sulu UHP orogen, China: Zircon and garnet evidence from a felsic vein and its host granitic gneiss

Partial melting of high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks in subduction zones has great influence on both the chemical and physical properties of deeply subducted continental crust. Felsic veins in HP-UHP metamorphic rocks are products of partial melting and thus would be a key target to trace partial melting in HP-UHP terranes. This contribution is an integrated study of petrology, zircon and titanite U-Pb ages, as well as zircon Hf-O and major minerals O isotope compositions for a felsic vein and its host granitic gneiss in the Sulu UHP terrane to decipher the timing and mechanism of partial melting. Zircon U-Pb dating for the inherited magmatic zircon cores of the granitic gneiss constrains the protolith age at ca. 750 Ma. The inherited zircons had an average δ18O value of 0.03 ± 0.15‰ and yielded a weighted mean εHf(t) value of −1.16 ± 0.54 and two-stage Hf model ages of 1707 ± 65 Ma. These results suggest that the protolith was derived from reworking of Paleoproterozoic crust in a Neoproterozoic rift. In the two samples, broad garnet core/mantle domains are of metamorphic origin and narrow garnet rims of peritectic origin. There are large differences in δ18O values between the protolith magmatic zircon (δ18O = 0.03 ± 0.15‰) and metamorphic garnet (−4.14 ± 0.21‰ for the felsic vein and −4.13 ± 0.11‰ for the host granitic gneiss), suggesting that the negative δ18O fingerprints might result from hydrothermal alteration or multi-episodes of rift magmatism after the formation of the protolith. Zircon from the felsic vein yielded a U-Pb age of 212 ± 3 Ma. Titanites from the two studied samples show undistinguishable U-Pb ages of 227 ± 14 Ma and 212 ± 6 Ma within the analytical uncertainties. These ages register the timings of the partial melting. The approximately consistent δ18O values of the metamorphic zircon (−4.78 ± 0.15‰) and metamorphic garnet indicate that the melt was sourced from the surrounded gneiss in a closed system, which might be triggered by breakdown of phengite. According to a compilation of partial melting in the Sulu UHP terrane, there exists a clear spatio-temporal variation. The ca. 237-224 Ma partial melting in the northern UHP zone might be due to its relatively high temperature metamorphism; the most extensive partial melting occurred at ca. 220-215 Ma in both the southern and northern UHP zones, which might be triggered by the detachment of the preceding subducted slab; and partial melting of the northern UHP zone continued to ca. 210 Ma due to heat supply from the underneath mantle. Such a variation might be dictated by the tectonic evolution of the different UHP slices. Accordingly, the whole partial melting process of UHP rocks in the Sulu UHP terrane might have lasted for about 27 Myr with two major episodes at ca. 230–224 and 220-215 Ma.