Mineralogical Characteristics and Magmatic Evolution Significance of Garnets in the Late Jurassic Granites in Xingcheng, Eastern North China Craton

A series of Mesozoic granites related to the subduction of the ancient Pacific plate are widely developed in the eastern part of the North China Craton. These granites contain garnet as a minor phase. Garnet records important information such as magma composition, temperature and pressure, and is an important object in the study of petrological evolution of granites. We take the late Jurassic garnet–bearing monzogranite in Xingcheng, Liaoning as the research object, and the magmatic evolution process in the study area is discussed by the in–situ element geochemical variation and the Raman effect of magmatic garnet girdle. It is found that the garnet has a zonation structure in which the contents of MnO and CaO increase and then decrease, while the contents of FeO and MgO decrease and then increase from the core to the edge. Through the simulation of the crystallization sequence of garnet and biotite, it is found that the garnet has “M” type “spessartine bell–shaped profile”, Mn/(Ca + Mg + Fe) value and Mn/Fe value, which reflects that the crystallization pattern of minerals in the magma is dominated by biotite crystallization to garnet crystallization. It is considered that forward and reverse girdle can exist in the same garnet; the separation and crystallization of minerals in the late magmatic evolution is the main reason for the change in the compositional profile of the garnet. This is consistent with the late evolution of granitic magmas in the northeastern of the North China Craton after the late Jurassic.