C-Axis fabrics and microstructures in a recrystallized quartz vein deformed under fluid-rich greenschist conditions

Abstract

The quartz v-axis orientations and microstructures have been analyzed in an originally single-crystal buckled quartz vein of greenschist metamorphic grade from the Sambagawa metamorphic belt, southwest Japan. The basal (0001) plane was inclined by ca 10–20 to the quartz vein surface before deformation, and the flexure (essentially kink) which produced the fold was accomplished by basal (0001) slip alone in the hinges (i.e. flexural-slip folding). In addition to crystal plasticity, dissolution microstructures are ubiquitous in the quartz vein. Dynamic recrystallization resulted from both subgrain rotation and grain-boundary migration. Rotation recrystallization associated with basal (0001) slip resulted in host-controlled v-axis orientation distribution along a great circle, whereas migration recrystallization, perhaps assisted by intergranular fluid, resulted in a penetrative development of a weak X (elongation)-maximum v-axis fabric nowhere rotated by the folding. The migration recrystallization is evidenced by large v-axis misorientations across the grain boundaries, as well as irregular grain shape. Based on the weak v-axis fabric development and lack of grain shape fabric, solution-precipitation creep may have dominated in the fluid-assisted grain-boundary migration regime, and hence it is inferred that the transition from rotation to migration recrystallization was caused by a local decrease of differential stress in the buckled quartz vein.