Dislocation creep of fine-grained recrystallized plagioclase under low-temperature conditions

Lattice preferred orientation (LPO) and dislocation substructure of fine-grained plagioclase were investigated in ultramylonite from a shear zone along the Hatagawa fault zone in northeast Japan. In the shear zone, the orthoclase mole fraction of the recrystallized alkali-feldspar is not less than 0.94. This composition suggests that deformation occurred below 500°C. Selected area diffraction by transmission electron microscopy (TEM) for an electron-transparent foil and an X-ray texture goniometer (XTG) were used to determine the LPO of plagioclase. Burgers vectors of dislocations were determined by invisibility criteria. The (100), (010), and (001) pole figures reveal strong preferred orientations. The XTG results are consistent with the LPO determined by TEM. The TEM observations confirmed the presence of at least six Burgers vectors. These Burgers vectors or the net slip directions of multiple systems are almost parallel to the lineation. Thus the present study indicates ductility by dislocation creep for fine-grained plagioclase in ultramylonite deformed at relatively low temperature. The LPO pattern and the Burgers vectors determined in this study are different from those of deformation at much higher temperature. Thus the present study also suggests a switching of the slip systems for plagioclase depending on the temperature.