Intra-granular strains and grain boundary morphologies of dynamically recrystallized quartz aggregates in a mylonite

Dynamic recrystallization process has been investigated by analyzing intra-granular strains and grain boundary morphologies of sheared quartz aggregates deformed under high greenschist facies condition collected from the Hatagawa mylonite zone in northeast Japan. Microstructural features indicate pervasive operation of both subgrain rotation and grain boundary migration. The grains favorably oriented for operation of {101̄1}〈112̄0〉 and {101̄0}〈112̄0〉 slip systems tend to have small mean intra-granular misorientations (less than 3°) and occupy a large volume in the aggregate. Grains favorably oriented for (0001)〈112̄0〉 slip show large intra-granular misorientations, often exceeding 10°, and are often isolated as thin layers and equant small grains between those favorably oriented for prism〈a〉 and rhomb〈a〉 slip. The energies of subgrain boundary developed in the latter grains are estimated to be about 0.28 J m−2, and are 1.5–2 times larger than the former. The morphological features of grain boundaries were well described by means of the curvature distribution along the boundary. The serration of grain boundaries is controlled by the misorientation and spacing of subgrain boundaries developed in the grain. We have demonstrated that serrated grain boundaries have a common asymmetric feature in the curvature distribution and this morphological feature can be used for the determination of direction of the grain boundary migration.