Abstract:

The Miaoxi pluton is situated in the eastern Jiangnan orogen along the Anhui-Zhejiang border, consists of monzogranite and syenogranite, with several iron ore deposits developed along its northern margin. In order to investigate the Late Mesozoic magmatic evolution and metallogenesis of the Jiangnan orogen, this study presents systematic petrological and petrographic investigations of the Miaoxi pluton, including analyses of the biotite compositions, LA-ICP-MS zircon U-Pb dating and Lu-Hf isotope geochemistry, to constrain its mineralogical characteristics, crystallization age, and petrogenesis. The results indicate that biotite is relatively iron-rich, with w(FeO) ranging from 18.88% to 25.39% (mean 21.29%), and magnesium-poor, with w(MgO) ranging from 5.91% to 9.17% (mean 7.57%), classifying it as iron-rich biotite. A negative correlation between w(MgO) and w(Al2O3) in the biotite chemical composition suggests that cation exchange between Mg2+ and Al3+ likely occurred during magma crystallization. Combined with previously published whole-rock major, trace, and rare earth element data for the Miaoxi monzogranite and syenogranite, the pluton is identified as an A-type highly fractionated granite. The zircon U-Pb age of the syenogranite is (127.13±0.71) Ma, representing its emplacement age in the Early Cretaceous, consistent with the timing of late-stage A-type granite emplacement in the Jiangnan orogen. Petrogenesis is attributed to an extensional environment following a tectonic regime transition. Calculated crystallization temperatures and pressures for the Miaoxi pluton are 753-788  ℃ and (2.73-3.08)×108 Pa, respectively. Previous studies have demonstrated that two episodes of different granite types in the eastern Jiangnan orogen exhibit consistent crystallization temperatures, reflecting partial melting of deep crustal materials and continuity of the regional tectonic stress field. Biotite compositions indicate relatively high oxygen fugacity during crystallization, which may be related to volatile release or oxide precipitation in the magma; The magma underwent multiple episodes of mixing and fractionation. High-oxygen-fugacity biotite typically indicates favorable metallogenic potential. Both biotite mineral chemistry and zircon Lu-Hf isotopes suggest that the Miaoxi magma was likely derived from partial melting of ancient lower crust, closely associated with the evolutionary history of Mesoproterozoic crust. This ancient lower crustal partial melting may have resulted from crustal thermal events, such as the tectonic regime transition during the Late Jurassic to Early Cretaceous, which led to partial melting of crustal materials and the generation of new magma.

Key words: Miaoxi pluton, biotite, mineralogy, A-type highly differentiated granite, zircon U-Pb age, Lu-Hf isotope, eastern section of Jiangnan orogenic belt