GRAIN-BOUNDARY MICROSTRUCTURES IN MICACEOUS QUARTZITE - SIGNIFICANCE FOR FLUID MOVEMENT AND DEFORMATION PROCESSES IN LOW METAMORPHIC GRADE SHEAR ZONES

Observation by Scanning Electron Microscopy of a sericitic quartzite f rom a low-grade 1 cm-wide shear band developed from an originally pure quartzite revealed a nonconnected intergranular porosity. The isolate d pores occur principally on quartz-quartz grain boundaries. Pores sur rounding isolated mica flakes show very irregular geometries, reflecti ng a highly unstable (disequilibrium) crystal-fluid interface, but reg ular (frequently crystallographic) pore shapes predominate in other gr ain boundaries. Statistical analysis reveals that the pores are princi pally concentrated on grain boundaries oriented perpendicular to the t wo foliations developed within the shear band (i.e., grain boundaries at 45-degrees and 90-degrees 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 th e 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 sit es of local quartz dissolution associated with syn-kinematic growth of mica. Conversely, the regular pores are interpreted to be grain bound ary fluid inclusions (i.e., equilibrium crystal-fluid textures) presen t during dynamic recrystallization. Based on the spatial relation betw een pores and serrated-bulged boundaries, a simple mechanism is propos ed where solution transfer of silica across intergranular, isolated fl uid pockets promotes grain boundary migration.