Petrogenesis of Mafic Dykes in the Beishan Orogen and Its Implications for Subduction of Liuyuan Ocean: Constraints From Geochemistry and Sr-Nd-Hf Isotopes

Abstract

Subduction zones are the primary locations of material exchange between the crust and mantle, and thus, arc igneous rocks are extensively examined to explore crust–mantle interactions during oceanic subduction. In this study, the Sr-Nd-Hf isotopic compositions and elemental characteristics of the Yinwaxia mafic dykes from the Shibanshan–Huaniushan arc in the southern Beishan Orogen were investigated to determine their petrogenesis and explore crust–mantle interactions during the subduction of the Liuyuan Ocean. The dykes were subdivided into two groups based on their geochemical composition and spatial distribution. Group I dykes in the northern Yinwaxia area possess depleted light rare earth elements and slightly enriched large-ion lithophile elements, indicated by relatively high ratios of Rb/Nb (1.4–82), Ba/Nb (6–408), and Cs/Nb (0.13–2.74), and low ratios of Th/Yb (0.03–0.39), Th/Nb (0.10–0.83), and (La/Yb) _N (0.33–1.36). Further isotopic composition study revealed that low values of (⁸⁷Sr/⁸⁶Sr) _i (0.7028–0.7052), and high values of ε _Nd(t) (4.9–9.1) and ε _Hf(t) (13.2–24.0), which suggests that the mantle source of Group I dykes was metasomatized by aqueous fluids. In Group II dykes located in the southern Yinwaxia area, enrichment of light rare-earth elements and large-ion lithophile elements was observed, featuring lower ratios of Rb/Nb (0.5–16.7), Ba/Nb (13–51), and Cs/Nb (0.15–1.56), but higher ratios of Th/Yb (0.25–2.64), Th/Nb (0.19–1.49), and (La/Yb) _N (1.09–2.98) compared to Group I. Furthermore, Group II dykes exhibited higher values of (⁸⁷Sr/⁸⁶Sr) _i (0.7051–0.7077) and lower ε _Nd(t) (−1.7 to +3.8) and ε _Hf(t) (3.7–9.4) isotopic compositions, which suggests that the slab fluids involved in their mantle source were dominated by sediment-derived melts. The difference between the mafic dykes from Groups I and II suggests the addition of steadily increasing amounts of sediment-derived melt in their mantle source; moreover, the increased (La/Yb) _N ratio and decreased ε _Nd(t) with increasing distance from the Liuyuan mélange are records of the spatial evolution of the composition of the slab fluids during the oceanic subduction of Liuyuan Ocean in the early Permian.