Abstract:The strain localization process and accompanied microstructures during ductile deformation, especially influence of the second phase on main phase mineral microstructure evolution in multiphase mylonite, has always been a difficulty in microstructural geology research. Previous studies have shown that grain boundary sliding is a potential mechanism to achieve multiphase mineral mixing and form multiphase aggregation in mylonite. Actually, natural mylonite is normally composed by multiphase minerals. Generally, in the multiphase mylonite, the second phase exerts a zener resistance at the boundary of the matrix phase grains, which restrains the migration rate of the grain boundaries in the matrix phase. This can destroy the dynamic equilibrium process of the matrix phase grains, and leading to the matrix phase grains below the grain size corresponding to the paleopiezometer. Furthermore, after the matrix phase grain size decreases, the overall surface area of the matrix phase increases that will contribute to the diffusion exchange process. In addition, due to the contribution of diffusion creep and the less efficiency of dislocation creep, the deformation mechanism is intending to change from grain size insensitive creep mechanism (GSI) to grain size sensitive creep mechanism (GSS). Moreover, the second phase in the multiphase mylonite has the effect on strain localization initiation, which probably changes the material strength, the rock deformation process and as well as rheological behavior. In this paper, based on the summary of previous research results, granitic mylonite from the Qinling Group is selected for the quantitative study. The preliminary results show that grain size of quartz and its strength of CPO is significantly reduced where increased mica content and the degree of mixing of the mineral phases within the granitic mylonite. Thus, we concluded that the microstructure of the matrix phase is gradually controlled by the second phase during mylonitic deformation in the natural mylonite.