Separating Multiple Episodes of Partial Melting in Polyorogenic Crust: AN Example from the Haiyangsuo Complex, Northern Sulu Belt, Eastern China

Determining the timing, mechanism and source of partial melts in polyorogenic crust is challenging. In the Sulu belt, the tectonic affinity of the Haiyangsuo (HYS) complex is controversial due to its polyphase metamorphic history. Here we use detailed field mapping, petrology, microstructural analysis and zircon geochronology to study thin stromatic leucosomes in host granite gneiss, and crosscutting leucogranite dykes to decipher the melting history. Zircon grains from both granite gneiss and thin leucosomes exhibit core-mantle-rim structures. Zircon cores yield protolith ages of 2.86-2.81 Ga, whereas the mantles and rims yield younger metamorphic/melt crystallization ages of ca. 1.82-1.80 Ga. The mantles are characterized by gray luminescence, flat HREE distribution patterns and relatively low Th/U ratios, indicating crystallization during granulite-facies metamorphism. Whereas rims show bright luminescence, steep HREE distribution patterns and higher Th/U ratios, suggesting they crystallized from melt. The mantles and rims have ɛHf (t) of -18.2 to -11.0. Using 176Lu/177Hf = 0.001, these data project back to the array of ɛHf (t) values for the zircon cores. This demonstrates that the thin leucosomes were derived from the gneiss without any mass input from a mantle source. These features are consistent with an origin of the HYS as part of the eastern margin of the NCC prior to juxtaposition with the Sulu belt. Zircons from the leucogranite dykes also show core-mantle-rim structure. Inherited cores yield concordant 206Pb/238U ages of 776-701 Ma consistent with the dominant age range for protoliths of the UHP metamorphic rocks in the Sulu belt. Zircon mantle and rim domains, which both contain multiphase solid inclusions (Kfs + Pl + Qz and Hem + Pl + Qz in mantles and Kfs + Pl + Qz + Bt in rims), yield melt crystallization ages of 226-217 and 169-156 Ma, respectively. High Sr, low Y and Yb contents, high Sr/Y ratios, and the range of ɛNd (t) values (-18.2- -15.0) and initial 87Sr/86Sr ratios (0.7106 - 0.7146) for the leucogranites are consistent with melting of thickened lower continental crust of the Sulu belt. We interpret the dykes to have been emplaced during post-collisional collapse of the orogenic root of this belt in the Middle-Upper Jurassic.