Orientation-dependent recrystallization and extreme ductility in a sheared quartz vein

Microstructures in quartz are widely used to draw inferences on conditions and history of deformation. In particular quartz recrystallization microstructures are often assumed to provide an interdependent information on e.g., deformation temperatures, strain rates or grain boundary mobility. Piezometric approaches attempt to relate a given recrystallized grain size or low angle boundary (LAB) structure to differential stress during deformation.

Here, a decimeter-scale, sheared quartz vein from the Central Pyrenees hosted in phyllites of the Hospitalet Massif was studied by means of EBSD. The sheared part of the vein consists of a mm-scale, planar, parallel layering representing an estimated shear strain > 5. The layering is defined by domains of different crystal orientations and a pronounced contrast in recrystallization behavior. Domains with the c-axis about normal to the kinematic section consist of almost single grains which stretch to aspect ratios > 40, show a very homogeneous LAB pattern, little dynamic recrystallization (< 20 vol%) and recrystallized grain sizes have a volume weighted mode of ~8 µm. Based on boundary misorientations and microstructure, recrystallization is inferred to proceed predominantly by a combination of subgrain rotation, and geometric recrystallization. The second type of domains consists of non-recrystallized remnants of old grains with low aspect ratios, a very heterogeneous LAB structure and c-axes within the kinematic plane, for example normal to the foliation. Recrystallization at overall > 50 vol% proceeds in localized, often conjugated bands. Bands and c-axes of recrystallized grains within those bands are inclined in mutually opposite directions, unrelated to the host crystal orientation. The grain size varies systematically for different band types, e.g. bands in a C' orientation contain the largest recrystallized grain size (~12 µm). Recrystallization is inferred to proceed, at least partially, by nucleation and local grain boundary migration. The third type of domain shows mixed behavior, whereat more highly sheared parts correlate with a smaller recrystallized grain size and c-axis directions changing towards the normal of the kinematic section.

Neither the dependence of grain size on recrystallization processes, nor the orientation-dependent recrystallized grain sizes and LAB structure are easily compatible with a single controlling factor, i.e. flow stress. The data suggest that especially the orientation of host grains plays a non-negligible role in controlling recrystallization mechanisms, LAB structure and dynamically recrystallized grain sizes. This relationship needs to be considered when investigating highly deformed rocks which usually exhibit a strong degreee of preferred orientation.