Abstract

Coarse-grained, deformed albite occurs in veins within a blueschist from the Cazadero region, California. In some grains, deformation and recrystallization are concentrated in narrow shear zones less than 50 μm wide. We have examined the substructural progression across these zones by transmission electron microscopy (TEM), in an attempt to determine the details of the dynamic recrystallization mechanism. The misorientation across subgrain and recrystallized grain boundaries has been determined by analysis of electron diffraction patterns.Dynamic recrystallization apparently proceeded by the following stages: 1) the formation of a well-ordered substructure from a more tangled, cell-like array, 2) increasing misorientation between subgrains, 3) rapid growth of subgrains at a misorientation between 3° and 5° to produce new “grains” with straighter grain boundaries and lower internal dislocation densities and 4) continued deformation and rotation of the recrystallized grains with local grain-boundary migration to maintain relatively equiaxed shapes. The ultimate recrystallized structure in the narrow deformation zones consists of grains misoriented by between 5° and at least 30°, most of them containing a well-developed substructure.The combination of subgrain growth and rotation explains a number of features common to dynamically recrystallized minerals. The smaller subgrains present prior to growth and also within recrystallized grains form a population distinct from the larger subgrains and recrystallized grains of approximately equal size, which are those observed in an optical microscope. The smaller subgrains are visible only in TEM. Individual recrystallized grains may remain through substantial straining, rotating in response to dislocation and sub-boundary motion within them, thus preserving and even enhancing the crystallographic fabric (texture). The retention of an initial recrystallized grain population throughout significant continuing deformation may explain the absence of strain softening in some recent experimental studies.