Mechanisms of subgrain boundary development and grain size decrease in mylonite

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The deformation structures of orthopyroxene(Opx) in mylonite rock from Hidaka Metamorphic belt have been investigated mainly using transmission electron microscopy(TEM). Low-angle grain boundaries with the b-axis as the rotation axes and almost parallel to the(001) plane of Opx are common and their atomic structure are varied from coherent to semi-coherent and finally incoherent, depending on the misorientation angle of the boundaries. Asymmetric misfit dislocation densities along the two(100) interfaces between exsolved augite lamella and host Opx were also found, which is probably the origin of the folding of the Opx crystal.