DYNAMIC RECRYSTALLIZATION OF QUARTZ UNDER GREENSCHIST CONDITIONS

The SEM electron channelling technique has been used to investigate th e dynamic recrystallization of three quartz grains representative of o ptical textures observed in quartzite deformed under greenschist facie s conditions. The degree of recrystallization has been determined at 1 0-20, approximately 50 and 100%, respectively. We demonstrate that dyn amic recrystallization is a sequential process involving subgrain-grai n rotation and grain-boundary migration. It initiates via subgrain pol ygonization on a single crystal slip system, and continues via subgrai n and neoblast rotation on several slip systems. In the early stages o f recrystallization (< 50%), the orientation relationships between sub grains-neoblasts and parent grains are systematic and are related to t he dominantly active crystal slip systems. Beyond approximately 50% re crystallization, orientation relationships are less systematic and thi s is attributed to an increase in the activity of grain-boundary migra tion. However, dynamic recrystallization is potentially a cyclical pro cess with new subgrains forming within migrational neoblasts due to co ntinued deformation. The three sequences represent different stages in the same continuous, time- or strain-dependent, dynamic recrystalliza tion history. They therefore combine to give a generalized dynamic rec rystallization history for greenschist facies quartz grains at low to moderate strains. Porphyroclasts have similar recrystallization histor ies because the original bimodal grain size distribution leads to stra in partitioning, allowing the porphyroclasts to rotate into weak orien tations with the subsequent activation of weak crystal slip systems.