Oxygen Fugacity Differences Between the Tungsten‐ and Tin‐Bearing Magmas During Reduced Magmatic Fractionation: Evidence From Apatite in Guidong and Dayishan Plutons in South China

ABSTRACT Magmatic evolution involves oxygen fugacity () changes that affect tungsten (W) and tin (Sn) transport and enrichment, but how varies during magmatic fractionation and differentially impacts W–Sn enrichment remains unclear. We studied mineralogical and geochemical compositions of Dayishan (Sn) and Guidong (W) granites in Nanling Range to clarify difference during the magmatic fractionation of W‐ and Sn‐bearing magmas. Both granites are divided into less‐fractionated (DYS(Sn)‐L and GD(W)‐L) and highly fractionated (DYS(Sn)‐H and GD(W)‐H) groups. The apatites of Dayishan and Guidong granites have low S (4–10 and 4–49 ppm), and the S of apatite from DYS(Sn)‐L are higher than those from DYS(Sn)‐H, but the S of apatite from GD(W)‐L are lower than those from GD(W)‐H. The trace element of zircon shows a variation pattern similar to that of apatite: the of GD(W)‐L (ΔFMQ = −0.42 to 0.33) is lower than that of GD(W)‐H (ΔFMQ = −0.31 to 0.81), but the of DYS(Sn)‐L (ΔFMQ = −1.08 to −0.41) is higher than that of DYS(Sn)‐H (ΔFMQ = −0.67 to −0.37). Therefore, in a reductive condition, decrease in can help the enrichment of Sn, whereas increase in is favoured by the enrichment of W.