Petrogenesis and tectonic setting of the Wulong two-mica monzogranite on Liaodong Peninsula, NE China: Constraints from zircon U-Pb and Hf-O isotopic data

抄録

The petrogenesis and tectonic setting of Middle-Late Jurassic granitoids on Liaodong Peninsula, NE China, are debated. To resolve these uncertainties, this study analyzed geochemistry, zircon U-Pb ages and Hf-O isotopic compositions of the Wulong two-mica monzogranite on the Liaodong peninsula. Zircon U-Pb ages indicate that the Wulong monzogranite was intruded at ca. 168–162 Ma, and included an inherited population of Paleoproterozoic zircon grains (ca. 2.5–2.0 Ga). Wulong monzogranite is characterized by quartz, plagioclase, K-feldspar and minor muscovite and biotite. This S-type granitic intrusion has high SiO₂ (>70 wt.%) contents, is high-K calc-alkaline, shows high zircon δ¹⁸O values (>7.5‰), and is weakly peraluminous (A/CNK = 1.01–1.08). The relatively low Sm/Nd (0.13–0.21), Rb/Sr (0.17–0.53) and FeO^T/MgO (2.54–8.51) ratios suggest that Wulong monzogranite is weakly fractionated. Geochemical characteristics indicate that the monzogranite is probably derived from partial melting of a clay-poor, plagioclase- and biotite-rich psammitic source, such as metasedimentary rocks of the North China Craton. Moreover, the Wulong monzogranite displays high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7099–0.7181), δ¹⁸O values (7.8–9.4‰), low ε_Hf(t) values (–34.7 to –23.0) with two-stage Hf model ages (T_DM2) of 2.67–3.40 Ga and ε_Nd(t) (–24.9 to –21.2) values with two-stage Nd model ages (T_DM2) of 2.67–2.96 Ga, suggesting that they may be the product of the melting of the Neoarchen-Paleoproterozoic metasedimentary basement of the North China Craton. Zircon saturation temperature and Ti-in-zircon thermometry indicate that Wulong monzogranite formed at ~700°C, and high Sr/Y ratio and low HREE imply it was formed at depths within the stability field of garnet in a thickened crust. Taking the regional tectonic and magmatic activities into account, it is proposed that the Wulong monzogranite was generated in an active continental margin that involved the subduction of the Izanagi slab during the Middle-Late Jurassic.