Recrystallization and Fabric Development in Plagioclase

The microstructures and fabrics of naturally deformed intermediate composition plagioclase feldspars from the amphibolite-granulitic gneisses from the lower crust have been studied. Special interest in the effects of dynamic recrystallization and deformation fabrics has been taken. The plagioclase feldspars (An40-50) have undergone extensive ductile deformation by intracrystalline slip and twinning. They show strong crystallographic preferred orientation which was produced by dominant (010) 001 slip. Dynamic recrystallization is indicated by fine-grained new grains (neoblasts) interspersed with deformed relicts (porphyroclasts). The neoblasts have formed preferentially along grain boundaries, kink band boundaries (KBBs), intracrystalline shear zones, and around solid inclusions, such as garnet and pyroxene. Optical microstructural evidence suggests that dynamic recrystallization occurred primarily by grain boundary migration with subsidiary subgrain rotation which appears to have occurred only at low strain. The orientation of neoblasts is closely related to the kinematic framework of flow (X-Y-Z), not to the original orientation of their host grains. The stable preferred orientation developed by dynamic recrystallization favours the easy glide of dislocations on the (010) 001 system. We suggest that recrystallization-accommodated dislocation creep is an important process in the development of preferred orientations in plagioclase feldspars, which will result in geometric softening.