Apatite geochemical records of oxygen fugacity and water content of the Fanshan ultrapotassic magmas, Hebei Province

The Late Triassic Fanshan ultrapotassic alkaline complex in the northern margin of the North China Craton is mainly composed of clinopyroxene and pyroxene syenite, and is famous for the development of endogenetic apatite-magnetite deposits. Apatites in the Fanshan complex were selected as the research object in this paper to clarify the oxygen fugacity and water content of the parent magmas by measuring their major-and trace-elements and in-situ Sr-Nd isotopic compositions. The study shows that the Fanshan apatites are characterized by enrichment of light rare earth elements and depletion of heavy rare earth elements. The distribution patterns of rare earth elements exhibit the characteristics of right-deviation with low Ga levels and high 6Eu values. These characteristics indicate that the complex has high magmatic oxygen fugacity, which is also supported by apatite Mn oxygen fugacity meter. All apatites are characterized by high Sr and low Y, and their high ratios of Sr/Y (>30) and La/Yb (>90) exhibit an obvious positive correlation, suggesting a high water content for the Fanshan ultrapotassic magmas. The in-situ Sr-Nd isotopic compositions of the Fanshan apatites plot in the mantle field, obviously deviating from the crustal trend. The characteristics of "oxidation and water-rich" of the parent magmas is an important factor for the apatite-magnetite mineralization in the Fanshan complex, which is probably related to the metasomatism of the altered oceanic crust on the lithospheric mantle during the plate subduction. Comparing the composition data of apatites in the Fanshan complex with those in other geological settings worldwide, it is considered that the apatites with the characteristics of "high Sr, low F and little Cl" can serve as a characteristic mineral for identifying ultrapotassic alkaline rock complexes (and their related metal deposits).