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

摘要

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 SiO_2 (>70 wt.%) contents, is high-K calc-alkaline, shows high zircon &~(18)O values (>7.5%o), 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 ~(87)Sr/ ~(86)Sr ratios (0.7099-0.7181), &~(18)0 values (7.8-9.4%o), 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 subductio