MICROSTRUCTURES IN THE ROCKFISH VALLEY SHEAR ZONE OF THE CENTRAL VIRGINIA BLUE RIDGE

The Rockfish Valley shear zone is a 1 – 3 km wide zone of ductile deformation that extends through the Blue Ridge of central Virginia. This zone separates granulite-facies rocks of the Pedlar and Lovingston massifs, primarily Grenville-age rocks dominated by coarse-grained, green charnokite and granulite gneiss, respectively. Near Big Island, VA, the ductile deformation zone exposes augen gneisses, protomylonites, and mylonites, with fabrics that anastomose around lenses of less deformed gneiss on both macro- and meso- scales. In augen gneiss outcrops, feldspar porphyroclasts and an oblique foliation suggest top-northwest shear. Microstructurally, the protomylonites are characterized by a quartz-rich matrix recrystallized by subgrain rotation, quartz ribbons, feldspar porphyroclasts, and thin quartz veins parallel to the main foliation. The porphyroclasts are mostly perthite, some with lamellae. Many exhibit signs of cataclastic deformation; the grains are fractured and often extended. The protomylonites contain indications of pressure solution; porphyroclasts are surrounded by mica mantles and strain shadows that separate the shards of the fractured feldspars. Shear sense indicators include both extensional and contractional shear bands, sigma and delta porphyroclasts, and an oblique foliation in the recrystallized quartz. The mylonites are dominated by recrystallized quartz and display a well-developed oblique foliation, mica fish, subgrain development in the quartz ribbons, and recrystallized quartz mantling larger and more coherent quartz grains. Quartz c-axis pole figures display a single girdle, indicating non-coaxial deformation at low temperatures, and rhomb <a> slip. Collectively, these microstructural observations are consistent with non-coaxial deformation in the Rockfish Valley shear zone at temperatures of about 400°C.