Petrologic reconstruction of sulfur transfer processes during mafic magma recharge: Insight from anhydrite-bearing mafic microgranular enclaves

In arc settings, substantial amounts of S are released during volcanic eruptions or sequestered in magmatic–hydrothermal ore deposits, yet the associated silicic melts generally appear S poor. Although mafic magma recharge is considered a major additional S source, the mechanisms by which S is efficiently transferred from recharging mafic magmas into overlying silicic systems remain poorly understood. To address this mechanistic gap, we conducted a detailed petrographic study of anhydrite-bearing mafic microgranular enclaves (MMEs) from the subduction-related Yangxin pluton in the Middle-Lower Yangtze River Metallogenic Belt, China. Two types of plagioclase and amphibole crystals in the MMEs are identified based on textural and compositional characteristics, respectively. Pl A and Amp A exhibit reverse core–mantle–rim zonings (An = 24–50 to 61–81 to 20–46 and Al 2 O 3 = 6.8–7.4 wt% to 8.7–9.6 wt% to 6.7–7.8 wt%, respectively) with patchy cores in both crystals and sieve textures in Pl A , whereas Pl B and Amp B display normal core–rim zonings (An = 60–79 to 28–43 and Al 2 O 3 = 9.3–12.0 wt% to 6.0–8.7 wt%, respectively). Integrating mineral disequilibrium textures, geochemical data, and inferred amphibole equilibrium melt compositions, we propose that the MMEs originated through mingling between the host Yangxin granodioritic magma (characterized by sodic Pl A cores and low-Al Amp A cores) and a recharging, oxidized mafic magma (entraining high-Al Amp B cores). This mingling occurred within a hybrid boundary layer along the mafic–silicic interface and proceeded in two stages: (1) in the early-stage, mixed melts with more affinity to the recharging magma crystallized calcic Pl A mantles, calcic Pl B cores, and medium-Al Amp A mantles; (2) in the late-stage, progressive mingling with the overlying granodioritic magma increased the silicic affinity of the mixed melt, resulting in the development of sodic rims of Pl A and Pl B , and low-Al rims of Amp A and Amp B . Apatite inclusion S–F–Cl and inferred OH variations in the early‐stage assemblage probably record crystallization‐induced melt S enrichment under fluid‐undersaturated conditions. The exclusive occurrence of primary anhydrite inclusions in the late-stage assemblage indicates that anhydrite saturation was achieved during this interval. Late-stage apatite inclusions likely document a transition from fluid-undersaturated to fluid-saturated conditions, with anhydrite saturation preceding fluid saturation. During aqueous fluid exsolution, crystallized anhydrite would decompose to buffer melt S at anhydrite solubility value for the given melt composition and temperature. Accordingly, the S would continuously be transferred from crystallized anhydrite into the exsolving aqueous fluids without altering melt S contents, and the interstitial anhydrite observed in the MMEs may represent the residual phases after partial decomposition. Although these processes most likely occurred near the mafic–silicic interface, crystallization-driven anhydrite and H 2 O saturation, together with anhydrite–fluid interactions are expected to operate throughout the recharging magma. This fluid-mediated S transfer mechanism during oxidized mafic magma recharge holds potential for addressing the “excess S problem” in arc magmatic systems.