Formation of pure quartz domains: role of fluid-rock reaction and syn-tectonic veining in granitic mylonite

Pure quartz domains (PQD), also called pure quartz layer (Stunitz & Fitz Gerald, 1993) or pure quartz band (Fliervoet, et al., 1997 Hippett, 1998), are commonly developed in quartzo-feldspathic mylonites. Generally, these foliation-concordant quartz domains are believed to be formed by extreme elongation of large quartz grains or quartz aggregates into ribbons. Here. I present micro-structural and textural evidences to demonstrate that these PQDs are actually foliation-concordant quartz. Oriented samples collected from a metre-scale shear zone cutting across granitoids in the Elbe Zone, Saxony, Germany, show distinct microstructural features when viewed on different sections. On the XZ section (cut parallel to stretching lineation), PQDs appear as polycrystalline foliation-concordant ribbons and extend across the entire thin section. On the YZ section (cut normal to stretching lineation), PQDs can only be described as crack-sealing quartz veinlet. Fibrous quartz crystals show a growing-direction almost perpendicular to vein boundaries, suggesting that foliation-parallel tensile fractures, in which quartz veinlets precipitated, have an opening direction parallel to Z axis of the sample reference system. Boundaries of quartz veinlets show an obvious convergence along the foliation, e.g., the Y axis, which suggests a propagation direction of tensile fractures parallel to the Y axis. Crystallographic data of fibrous quartz are characterised by a strong c-axis concentration around the Y axis, indicating an abnormal growing direction for quartz to be normal to their c-axes. Formation of pure quartz domains in granitic mylonites is due to repeatedly operation of foliation-normal dilatation event induced by fluctuation of fluid pressure in a rock system with high anisotropy during a bulk ductile shearing. Fibrous quartz crystals are growing under a local stress configuration being different from regional stress condition. A fracture model has been proposed.