Strain localization due to a positive feedback of deformation and myrmekite-forming reaction in granite and aplite mylonites along the Hatagawa Shear Zone of NE Japan

With increasing mylonitization of aplite veins and their adjacent granite in the Hatagawa Shear Zone of NE Japan, the modal content and grain size of feldspar porphyroclasts decrease, while the modal content of fine-grained (≤50 μm) polymineralic aggregate of plagioclase, K-feldspar, quartz and biotite increases up to 60–80% in the granite and aplite ultramylonites. Deformation-induced myrmekite lobes are ubiquitously developed around K-feldspar porphyroclasts. The occurrence and chemical composition of plagioclase and K-feldspar in fine-grained polymineralic aggregates indicate that the plagioclase is mostly derived from myrmekite, while the interstitial K-feldspar originates from isolated porphyroclast rims between growing myrmekite lobes and/or precipitations from solution. Modal and bulk chemical compositions of granite and aplite indicate that the mylonites are preferentially developed in aplite veins richer in K-feldspar than the surrounding granite. Deformation-induced K-feldspar replacement by myrmekite is then more favored during deformation to produce more abundant fine-grained polymineralic aggregates. Subsequent reaction softening leads to more enhanced deformation, which promotes further myrmekite-forming reaction. Such a positive feedback of deformation and myrmekite-forming reaction in aplite veins and their adjacent granite may be responsible for strain localization in the granite and aplite mylonites along the Hatagawa Shear Zone.