Evaluation of effects of chemical reactions on ductile shear deformation

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We evaluated the effects of chemical reactions on ductile deformation in granite, gabbro and peridotite mylonites formed within ductile shear zones in the middle crust, the lower crust and the uppermost mantle, respectively. The results are summarized below.1) Deformation-induced K-feldspar replacement by myrmekite occurs in the amphibolite-facies granite mylonites along the eastern margin of the Hatakawa shear zone. The fine-grained reaction products compose the main constituent in the mylonite matrix and largely accommodate the strain in the mylonites. Mylonites are preferentially developed in leucocratic domains or aplite veins which are rich in K-feldspar and poor in biotite, where the myrmekite-forming reaction was more favored to produce more amounts of fine-grained reaction products than non-leucocratic domains during deformation. Subsequent reaction weakening would have favored further deformation in leucocratic domains or aplite veins, which in turn would have favored further myrmekite-forming reaction. Such a feedback process is probably responsible for the development of granite mylonites in the Hatakawa shear zone.2) Strain localization into the fine-grained reaction products formed by decomposition of clinopyroxene into amphibole as well as dynamic recrystallization of plagioclase contributed to the development of granulite-facies gabbroic mylonites in the Hidaka metamorphic belt.3) The lehrzolite mylonite in the Uenzaru peridotite body in the Hidaka metamorphic belt developed by strain localization into the symplectite composed of plagioclase, olivine and chromian spinel when spinel lherzolite was transformed into plagioclase lherzolite during ascent of the peridotite body.