Dynamic recrystallization in deformed plagioclase during progressive shear deformation

Abstract

Grain size reduction of plagioclase accompanied with progressive ductile deformation was investigated in a centimeter-scale small shear zone, collected from the Hatagawa shear zone, northeast Japan. The angle θ between the foliation and the shear zone boundary decreases from the margin to the center of the shear zone, indicating an increase of shear strain toward the center. The volume fraction of fine plagioclase grains X pl ( diameter <15 μm) increases with progressive shear strain. The value of X pl increases steeply from 30% to 60% between the angles θ=23° and θ=20°, and then it gradually increases from 60% to 90% toward θ≈0°. The fine plagioclase grains have a variable dislocation density and a crystallographic preferred orientation, suggesting that recrystallization accommodated dislocation creep. Transmission electron microscopy (TEM) observations revealed that strain-induced grain boundary bulging occurred along the grain boundaries of plagioclase porphyroclasts. Thus the grain size of plagioclase was reduced by strain-induced grain boundary bulging. The kinetic theory of dynamic recrystallization approximately predicts the formation of the above-mentioned X pl– θ curve, and suggests that the inflection point of X pl at θ=23–20° is probably due to site saturation in which all of the original grain boundaries are covered by the recrystallized grains. The kinetic theory of dynamic recrystallization also gives a physical basis for the classification of mylonite on the basis of the volume fraction of fine grains.