Middle Jurassic Magmatic Mixing in the Southern Qinzhou‐Hangzhou Suture Zone: Evidence From U–Pb Ages and Sr‐Nd‐Hf Isotopes of the Yuanshishan Granitic Pluton in Northern Guangxi

ABSTRACT The extensive Mesozoic magmatic record preserved in South China serves as a vital resource for understanding the region's complex tectonic regime. The Yuanshishan granitic pluton, located in the southern Qinzhou‐Hangzhou suture zone, is mainly composed of porphyritic biotite monzonitic granite with different grain sizes. Mafic microgranular enclaves (MMEs) are widely distributed within the pluton. Here, zircon U–Pb ages, Sr‐Nd‐Hf isotopes, whole‐rock and mineral geochemical data are reported to constrain their petrogenesis and tectonic setting. Our new zircon U–Pb dating constrains that the Yuanshishan granites were emplaced between 160.1 ± 3.4 and 165.2 ± 1.2 Ma, contemporaneous with the MMEs (160.2 ± 2.1 Ma). These rocks show typical characteristics of A‐type granites, including high SiO 2 (71.36–77.28 wt.%), FeO T (1.74–3.26 wt.%), K 2 O (3.93–5.00 wt.%), Zr + Y + Ce + Nb contents (366–465 ppm), high zircon saturation temperatures (an average of 779°C), as well as high 10,000 × Ga/Al (3.76–4.33) and FeO T /MgO ratios (11.39–15.82). In addition, the low Nb/Ta (4.16–8.96) and Zr/Hf ratios (17.28–23.37) suggest that the magma underwent extensive fractional crystallisation. The granites have homogeneous initial 87 Sr/ 86 Sr ratios (0.70772–0.71175) and ε Nd ( t ) values (−5.19 to −4.72), but various ε Hf ( t ) values (up to +9.2), demonstrating a mantle source contribution. The MMEs with relatively low initial 87 Sr/ 86 Sr and high ε Nd ( t ) likely represent the mafic end member involved in magma mixing. Based on previous studies, we propose that the Yuanshishan granitic pluton was generated by mixing of crust‐ and metasomatised lithospheric mantle‐derived magmas in an extensional setting caused by rollback of the subducted Paleo‐Pacific Plate.