Two episodes of partial melting in ultrahigh-pressure migmatites from deeply subducted continental crust in the Sulu orogen, China
Partial melting plays an important role in the geodynamics of continental subduction zones. This is identified from a suite of ultrahigh-pressure (UHP) migmatites in the Sulu orogen through a combined study of zircon U-Pb ages, trace elements, and oxygen isotopes, as well as rock-forming mineral and inclusion compositions. The results indicate two episodes of partial melting in the subducted continental crust during continental collision, providing insights into subduction channel processes. The first episode of anatexis is indicated by the occurrence of nanogranites, not only in zircon, garnet, and monazite from diatexite, but also in zircon cores from leucosome. The anatectic zircon exhibits U-Pb ages of 230−227 Ma and flat rare earth element (REE) patterns with weak or no negative Eu anomalies, and it contains mineral inclusions of coesite and garnet + amphibole. Newly grown zircon grains in the diatexite and zircon cores in the leucosome exhibit high δ 18 O values of 8.3‰−17.3‰, indicating a metasedimentary protolith. The host rocks show high A/CNK (= molar ratio of Al 2 O 3 /[CaO + Na 2 O + K 2 O]) values and the occurrence of peritectic garnet in the diatexite. Thus, the diatexite was produced by partial melting of metasedimentary rocks. The Ti-in-zircon thermometry, the garnet-phengite Fe-Mg partition thermometry for mineral inclusions in the zircon, and the occurrence of coesite inclusions in zircon indicate partial melting at 650−800 °C and 2.5−3.0 GPa, corresponding to high-pressure (HP) to UHP conditions. On the other hand, the second episode of anatexis is recorded by newly grown zircon grains in metatexite and zircon rims in leucosome, which show U-Pb ages of 218−214 Ma, oscillatory zoning, steep heavy (H) REE patterns with negative Eu anomalies, low temperatures of 550−700 °C, and significant variations in Th, U, Nb, and Ta contents. The Zr-in-titanite thermometry for nanogranite-bearing titanite and the garnet-phengite Fe-Mg partition thermometry for mineral inclusions in the leucosome zircon rims indicate anatexis at 800−850 °C and 1.0−1.5 GPa. The zircon in the metatexite exhibits low δ 18 O values of −1.5‰−3.5‰ and Neoproterozoic U-Pb ages for relict magmatic cores, indicating the protolith of a low δ 18 O UHP metagranite. The two episodes of anatexis yield zircon domains with a series of differences not only in U-Pb age, but also in geochemical composition. Thus, protoliths with different origins were involved in the anatexis, with a possible difference in spatial positions. The UHP metasedimentary rocks atop the deeply subducted continental crust would have undergone the first episode of anatexis during the final subduction, whereas their underlying metagranite would have undergone the second episode of anatexis during the exhumation of deeply subducted crust. In either case, the breakdown of UHP hydrous minerals during exhumation is the key for the partial melting of UHP metamorphic rocks in the continental subduction channel.