Partial melting time of the Sulu UHP terrane: Constraints from zircon U-Pb age, trace element and Lu-Hf isotope composition of leucosome in Rongcheng granitic gneiss

In order to decipher the partial melting time of the Sulu ultrahigh pressure (UHP) terrane during exhumation, we carried out a combined study of zircon internal structure, U-Pb age, trace element and Lu-Hf isotopic compositions coupled with fieldwork on leucosome of the Baocun grantic gneiss in Rongcheng region. Our works suggest that the leucosome, mainly composed of quartz, plagioclase, K-feldspar, and biotite, was the product of partial melting. Zircon from the leucosome display core-rim structure in cathodoluminescence (CL) images : inherited magmatic core and new growth magmatic rim. Most age data of the inherited cores are concordant with apparent Pb-206/U-238 ages ranging from 620 +/- 8Ma to 784 +/- 7Ma with a weighted mean age of 701 +/- 33Ma. The inherited cores have the characteristics of typical magmatic zircons, e. g., high HREE and Y contents, strong HREE enrichment on chondrite-normalized REE plot, very large positive Ce and negative Eu anomalies, and very low (Gd/Lu)(N) and Hf/Y ratios. The inherited cores have epsilon(Hf)(t =700Ma) values of -13. 0 to -8. 0 (mean = -10. 9 +/- 0. 8) and t(DM2) of 2120Ma to 2429Ma (mean =2303 +/- 51Ma). The new growth rims have concordant Pb-206/U-238 ages ranging from 211 +/- 6Ma to 244 +/- 5Ma with a weighted mean age of 226 +/- 10Ma. Although the new growth rims are greatly different from the inherited cores in trace element compositions, they still display features of the magmatic zircon, e. g., strong HREE enrichment on chondrite-normalized REE plot, large positive Ce and negative Eu anomalies, and very low (Gd/Lu)(N) and Hf/Y ratios. The new growth rims have epsilon(Hf)(t = 225Ma) values of -17. 3 to -14. 2 (mean = -15. 2 +/- 0. 6) and t(DM2) between 2152Ma and 2347Ma (mean = 2214 +/- 33Ma). These results suggest that protolith of the leucosome are Mid-Neoproterozoic (701 +/- 33Ma) magmatic rocks which were from Paleoproterozoic (2.2 similar to 2. 3Ga) continental crust The partial melting is expected under eclogite-facies to granulite-facies conditions during the early stage hot exhumation of the Rongcheng UHF felsic grieisses, and the partial melting time might be 226 10Ma. The K-feldspar- and quartz-rich pegmatitic veins (ca. 220 similar to 210Ma) in the Weihai-Rongcheng region might be from fractional crystallization of the partial melts during late stage cool exhumation under amphibolite facies conditions.