Petrogenesis and geodynamic implications of the Cretaceous anorogenic granitoids in east Qingling orogen

Abstract

We report systematical zircon U–Pb datings, Hf isotopic data, major and trace element date on the Zhangshiying (ZSY) and the Taishanmiao (TSM) two plutons at the southern margin of the North China Craton (NCC), in order to investigate their petrogenesis and geological evolution. LA-ICP-MS U–Pb zircon of the ZSY and TSM granites dating yield a weighted mean ²⁰⁶Pb/²³⁸U age of 122.8 ± 1.5 Ma and 112.1 ± 3.2 Ma. The ZSY quartz syenite enriched in SiO₂ (65.59–68.1 wt.%), Al₂O₃ (15.39–15.64 wt.%), high Sr (468–666 ppm), Sr/Y (35–45), (La/Yb)_N (20.1–22.1) ratios, low Y(13.1–16.6 ppm), Yb (1.46–1.77 ppm), depleted in Nb, Ta, P, Ti, Mg, Y and negligible Eu anomalies, which suggesting that the quartz syenite are similar to adakitic rocks. Zircons from this pluton give ε_Hf(t) values from −17.6 to −5.7 with an average of −15.2, and their T_DM2 ages (1.29–1.91 Ga) are much younger than the basement rocks of the south margin of the NCC, which indicate that the ZSY quartz syenite were generated by partial melting of thickened lower crust with minor involvement of mantle-derived materials. The TSM pluton has characteristic of A-type granite, such as, high SiO₂ (69.7–71.87 wt.%), total alkalis (Na₂O + K₂O = 8.5 to 9.41 wt.%), HFSE (Zr, Nb, Ga, Y), low CaO (0.27–1.25 wt.%), Ba, Sr and Eu. Their ε_Hf(T) values ranges from −12.4 to −1.6 with an average of −7.6 and T_DM2 ranges from 1.10 to 1.63 Ga with an average of 1.38 Ma. We propose that the TSM granites results from partial melting of tonalitic crustal sources with a plagioclase-rich residual at high temperatures and low pressure. Materials input from the mantle also is involved for the higher ε_Hf(t) values than tonalitic crustal at the southern margin of NCC and high temperatures. Our results suggested that the thickened LCC at the south margin of the NCC is still existence before the intrusive of the ZSY adakitic rocks (∼123 Ma). During the intrusive of the TSM granites (∼112 Ma) it turns to extensional environment. This extensional event reflects the background of lithospheric thinning and decratonization of NCC.