Regional Variation of Biotite Composition of Granitoid and Its Significance to Deep Crust Component and Mineralization: A Case Study on Biotite from Early Mesozoic Granitoid in the Qinling Orogen

The Qinling orogen, which is well development of granitic magmas and mineralization in the Early Mesozoic, is one of the important areas to study the relationship of deep crust composition and mineralization. Many studies have been carried out from the petrology, ore deposit and geochemistry, but the understanding of mineralization and composition of granite and deep crust component is still unclear. Biotite is a common mineral in medium-acid igneous rocks and one of the important minerals indicating the possibility of mineralization. Therefore, biotite composition in granitic rocks of this period in the Qinling orogen is studied in this paper to explore a new way to solve this problem. The study of biotite composition in the Early Mesozoic granitic rocks of the Qinling orogen shows that there are systematic differences in biotite composition and formation conditions in the North Qinling and South Qinling, indicating that their magmatic sources are different, thus restricting the characteristics of ore-forming types. The composition and formation condition variation of biotite in the Early Mesozoic granitic rocks in the North Qinling are small, characterized by the high contents of MgO (8%similar to 13%), TiO2 (3%similar to 5%) and Cl (0.02%similar to 0.6%) and low content of F (0.2%similar to 0.4%). And high oxygen fugaticity (logfO(2) value of -16.96 similar to-14.62) and temperature (682 similar to 771 degrees C). While these of the South Qinling vary widely, with low contents of MgO (3%similar to 15%), TiO2 (2%similar to 4.5%) and Cl (0.01%similar to 0.18%) and high content of F (0.1%similar to 1.6%). And low oxygen fugacity (logfO(2) value of -20.88 similar to-15.08) and temperature (536 similar to 754 degrees C). In addition, the study also shows that the pressure of biotite is related to the degree of magma evolution and mineral association, and the pressure is higher when the biotite+muscovite +/- garnet association occurs in the rock. The difference of biotite composition and formation conditions in the two tectonic units of the Qinling orogen, especially the obvious difference of oxygen fugacity and Cl content, reveals the different mineralization types and ore-forming potential of the two units. For example, the higher oxygen fugacity and Cl content in the North Qinling, the potential of copper ore formation is greater than that in the South Qinling. At the same time, the difference of biotite composition in the two tectonic units also reveals the difference of magmatic source and deep composition, that is, the North Qinling has a younger crust than the South Qinling, which is basically consistent with the difference of deep component in this region by isotopic mapping. Therefore, the study of biotite composition in contemporaneous granitic rocks in a region can not only reveal the magmatic evolution and physical and chemical conditions of magmatic crystallization process, but can also trace the spatial changes and differences of deep composition as well as the types and potential of mineralization. It is expected to become a new method for deep component exploration, and a new way to obtain the component background and potential of regional ore-forming