QUARTZ LATTICE PREFERRED ORIENTATION PATTERNS FROM PORPHYROCLASTS IN MYLONITES OF THE ROCKFISH VALLEY SHEAR ZONE, CENTRAL VIRGINIA

Lattice preferred orientation (LPO) textures reflect the deformation conditions of rocks, including the strain magnitude, shear sense, and temperature of deformation, and are a valuable tool for inferring deformation processes. The evolution of quartz LPO patterns is understood primarily from theoretical models and rock deformation experiments that focus on oriented single crystals or monomineralic polycrystals. To further investigate recrystallization and texture development in quartz, we have characterized LPO from porphyroclasts and associated recrystallized domains from within the Rockfish Valley Shear Zone (RVSF) in the Blue Ridge of Virginia. The RVSF exposes protomylonites and mylonites in a 1-3 km wide anastomosing zone. Some samples are dominated by 250 µm to > 1 mm scale quartz porphyroclasts surrounded by a fine grained micaceous matrix. Isolated quartz porphyroclasts provide a unique opportunity to study microstructural evolution in arbitrarily oriented single crystals of quartz, without the complexities introduced by intergranular interactions.

We observe significant variability in the degree of recrystallization between porphyroclasts within individual samples. LPO of individual recrystallized quartz domains reveals different textures at varying stages of dynamic recrystallization. Possible explanations for the variable recrystallization among these individual porphyroclasts are heterogeneous strain within the sample and/or orientation effects. Despite large imposed shear strains, steady-state quartz textures are rarely observed. As the rheology of shear zones is strongly related to the intrinsic properties of the constituent minerals and the microstructures that evolve during deformation, these observations suggest that steady state deformation was not achieved. We conclude that ductile shear zones with abundant quartz and mica may require extremely large shear strains in order to reach rheological steady state.