Effect of mica on the grain size of dynamically recrystallized quartz in a quartz–muscovite mylonite

Quartz ribbons in a quartz–muscovite mylonite from the Sunchang shear zone of South Korea show a variation in the average size (25−107 μm) of dynamically recrystallized quartz depending on the fraction and dispersion of muscovite within the ribbons. Micas within the quartz ribbons occur both inside quartz grains and along their boundaries, having a strong preferred orientation with the basal planes parallel to the mylonitic foliation. Micas (or mica aggregates) inside the quartz show a size range of 1–25 μm, whereas those along the quartz grain boundaries range from 2 to 98 μm. The average quartz grain size (D) calculated from three-dimensional grain size distribution decreases drastically with increasing mica volume fraction (Fm) up to Fm = 3% in the ribbons, and further increases in the mica fractions above 3% result in only a little decrease (with some fluctuation) in the quartz grain size, generating the equation of D = −20.11 × lnFm − 9.64. With almost the same fraction of mica however, the quartz ribbons with dispersed mica show a smaller quartz grain size than those with clustered mica, indicating the importance of dispersion as well as fraction of second-phase particles to the grain size of dynamically recrystallized quartz. Quartz c-axes tend to have a random orientation with an Fm > 3%, presumably due to the greater contribution to the total deformation of grain boundary sliding for quartz with a smaller grain size (D < 47 μm).