Control and Implications of Crust–Mantle Interaction on Fayalite‐Bearing Granite Porphyry in the Xishan Pluton

ABSTRACT The fayalite‐bearing granite porphyry of the Xishan pluton in the Nanling Range is one of the few A‐type granites in China with petrographic evidence, and there remain differing views on its source composition and genetic mechanism. Based on field investigations, this study conducted a systematic LA‐ICP‐MS zircon U–Pb geochronological study on it. The results show that four samples from different locations were formed between 154.2 ± 1.0 Ma and 156.1 ± 1.7 Ma, belonging to the Early Yanshanian period. Lithological and mineralogical analyses indicate that the granite porphyry locally contains single crystals and micro‐inclusions of dark minerals (including iron‐rich olivine, pyroxene and biotite). The latter formed in the middle crust at 24–26 km depth and 757°C–857°C. The matrix mineral assemblages developed near the surface at 3.3–9.9 km depth and 650°C–750°C, which provides constraints on the mineralogical and physicochemical conditions of this type of rock. Geochemical data confirm that this lithology is characterised by high silica and alkali contents, low calcium and magnesium contents, metaluminous properties (A/CNK = 0.82–1.00), and high FeO*/MgO ratios (5.69–9.59, average 7.11). The rare earth element (REE) contents range from 308.3 × 10 −6 to 441.5 × 10 −6 , with right‐dipping LREE‐enriched distribution patterns and negative Eu anomalies (δEu = 0.08–0.25). The rocks are enriched in large‐ion high‐field‐strength elements (e.g., Ga, Y, Nb, Zr, Hf) and depleted in Ba, Nd, Sr, P and Ti. The Ga/Al ratios range from 3.27 × 10 −4 to 5.00 × 10 −4 (average 3.99 × 10 −4 ), and Zr + Nb + Ce + Y contents are 367.1 × 10 −6 to 656.6 × 10 −6 (average 538.6 × 10 −6 ). Zircon saturation temperatures (Tzr) range from 757.1°C to 821.1°C (average 793.0°C), and the rock exhibits the geochemical characteristics of a typical A‐type granite. Systematic Sr, Nd and Hf isotope studies show high initial 87 Sr/ 86 Sr ratios (Isr = 0.71720–0.71851), low ε Nd ( t ) (−7.04 to −7.72) and ε Hf ( t ) (3.28 to −11.26) values, indicating that its source region is determined to be partial melting of granulite‐facies greywacke/argillite in varying proportions, with minor contributions from juvenile crustal materials (< 10%) during magmatic petrogenesis. Combined with existing data, the Xishan pluton is inferred to be located in the Qin‐Hang Joint Belt, which is characterised by relatively weak tectonic zones, past island arc magmatism and the presence of juvenile crust. Sustained lateral subduction of the Paleo‐Pacific Plate reactivated this belt, triggering mantle convection and asthenosphere upwelling, which caused partial melting of crustal materials to form A 2 ‐type granites. Therefore, the authors conclude that the fayalite‐bearing granite porphyry records the “high‐temperature transient” of the crust–mantle magma mixing process, and was rapidly emplaced and consolidated under low oxygen fugacity (logƒO 2 = −15.05 to −17.28) and favourable tectonic conditions, thus being preserved.