Grain Boundary Microstructures in Micaceous Quartzite: Significance for Fluid Movement and Deformation Processes in Low Metamorphic Grade Shear Zones

Abstract

Observation by Scanning Electron Microscopy of a sericitic quartzite from a low-grade 1 cm-wide shear band developed from an originally pure quartzite revealed a nonconnected intergranular porosity. The isolated pores occur principally on quartz-quartz grain boundaries. Pores surrounding isolated mica flakes show very irregular geometries, reflecting a highly unstable (disequilibrium) crystal-fluid interface, but regular (frequently crystallographic) pore shapes predominate in other grain boundaries. Statistical analysis reveals that the pores are principally concentrated on grain boundaries oriented perpendicular to the two foliations developed within the shear band (i.e., grain boundaries at 45° and 90° to the shear band boundaries), indicating that the fluid phase occupied extensional sites relative to both the overall and local (scale of one microlithon) strain ellipsoids. There is a strong spatial relationship between interfacial porosity and the presence of mica, with the most porous grains occurring adjacent to the mica-rich folia. An aureole of pore space is commonly present around isolated mica flakes, and small mica crystals are found inside these pores. It is concluded that the irregularly shaped pores represent sites of local quartz dissolution associated with syn-kinematic growth of mica. Conversely, the regular pores are interpreted to be grain boundary fluid inclusions (i.e., equilibrium crystal-fluid textures) present during dynamic recrystallization. Based on the spatial relation between pores and serrated-bulged boundaries, a simple mechanism is proposed where solution transfer of silica across intergranular, isolated fluid pockets promotes grain boundary migration.