Petrogenesis for the Yujingshan Pluton in Lushan Area, and Its Constraints on Tectonic Regime Transformation

In this paper, systematic analysis of the Yujingshan monzogranite is conducted to investigate its petrogenesis and source. The results show that the crystallization age of the monzogranite is 125 ± 1 Ma. The ε_Hf (t) values of zircons are between -6.5 and -2.0 and the two-stage model ages (t_DM²(Hf)) are 1.6-1.3 Ga, which suggests that the magma was derived by partial melting of Mesoproterozoic crust. Geochemically, the Yujingshan pluton belongs to peraluminous and high K calc-alkaline series with A/CNK=1.10-1.15 and K₂O/Na₂O=1.01-1.45. The ΣREE of the Yujingshan pluton is relatively low (ΣREE=48.54×10^-6~80.32×10^-6). LREE and HREE fractionation is not significant ((La/Yb)_N=2.91-4.32), but there is a negative Eu anomaly (σ_Eu=0.19-0.28). The samples are enriched with large ion lithophilic elements (Rb, Sr, Th, U, K) and depleted in high field strength elements (Nb and Ti). The enriched Sm-Nd isotopic composition [ε_Nd(t=125) of -10.1 to -8.9] indicates that the magma originated from remelting of ancient crustal materials. Comprehensive analysis shows that the Yujingshan monzogranite was generated by the partial melting of Mesoproterozoic argillaceous rocks and experienced fractional crystallization. The magma belongs to highly differentiated S-type granites, which formed in a tectonic regime transformation process going from compression to extension during the northwest subduction of the paleo-Pacific plate. Subsequently, ductile shearing occurred during the Late Cretaceous, resulting in mylonitization of the Yujingshan pluton. This research, combined with regional magmatic and structural geology data, suggests that the tectonic regime transformation in the South China block occurred at Late Mesozoic, and that regional extension continued until the late Early Cretaceous.