Extracting quartz deformation fabrics from polymineralic rocks

Abstract

Quartz dynamic recrystallization fabrics are the record of deformation temperatures, strain rates, and fluids, and can be qualitatively and quantitatively evaluated using optical microscopy and electron backscatter diffraction (EBSD). Characterization of quartz deformation fabrics have been largely guided by textures in homogenous monomineralic samples. However, in nature such lithological homogeneity is uncommon, and mica and feldspar that interrupt the interconnectedness of quartz inhibit the exchange of material across quartz grain boundaries, effectively pinning these grains and preventing dislocation creep.We present two EBSD methods of extracting quartz domains within polyphase mylonites to investigate the effects of discontinuous quartz distribution and remove isolated quartz grains pinned by other phases. These samples provide textural evidence for grain boundary migration in quartz at temperatures exceeding 500 °C. Quartz domains vary in size from cm thick foliation parallel continuous ribbons, to isolated quartz grains separated by networks of mica and feldspar. We quantify contrasting textural responses to deformation between connected and isolated quartz grains using existing measures of EBSD data analysis, including M-index, grain orientation spread, kernel averaged misorientation, and grain size. We find that removing isolated quartz grains yields pole figures that are more representative of the regional deformation.