Mineral Chemistry of Biotite and Its Petrogenesis Implications in ca. 2.5 Ga Wangjiazhuang Granitic Pluton, North China Craton

The Wangjiazhuang granitic pluton is located in the central Zanhuang Domain, the central part of the North China Craton, which is mainly composed of biotite monzogranite with few mafic microgranular enclaves. Biotite is an important ferromagnesian mineral in most of the intermediatefelsic igneous rocks, and its mineral chemistry can record the properties of magma and the petrogenetic physicochemical conditions. In this study, we carried out a detailed petrographic study by electric probe microanalysis on biotite for the biotite monzogranite and mafic microgranular enclaves, to discuss the source, physicochemical conditions, and the magma mingling/mixing processes of the Wangjiazhuang granite. The results show significantly different chemical compositions from the biotite monzogranite and mafic microgranular enclaves. The crystallization of these biotite grains from the biotite monzogranite and mafic microgranular enclaves all occurred in low oxygen fugacity. The biotite grains in biotite monzogranite are rich in Fe, poor in Mg, which belong to siderophyllite. The ratios of [(Fe3+ + Fe2+)/(Fe3+ + Fe2+ + Mg2+)] are between 0.78 and 0.86. The average of FeOT (total FeO)/MgO of biotite grains in biotite monzogranite is 9.02. The MF values [2 × Mg/(Fe2+ + Mg + Mn)] of biotite monzogranite are between 0.31 and 0.47, suggesting biotite monzogranite derived from crustal source rocks (metasedimentary rocks). The formation of granitic rocks including the Wangjiazhuang granite was related to the subduction event at ca. 2.5 Ga which resulted in the melting event, and then induced the early partial melting of TTGs and metasedimentary rocks. The biotite in mafic microgranular enclaves varies from siderophyllite to ferrobiotite, and MF values range from 0.63 to 1.06, suggesting that magma of mafic microgranular enclaves had experienced magma mixing/mingling in various degrees. Biotite monzogranite and parts of mafic microgranular enclaves have a similar crystallized condition, while other mafic microgranular enclaves are different from biotite monzogranite. The differences between biotite monzogranite and mafic microgranular enclaves may be a consequence of continuous interaction between granitic and mafic magmas.