Development of shape- and lattice-preferred orientations of amphibole grains during initial cataclastic deformation and subsequent deformation by dissolution-precipitation creep in amphibolites from the Ryoke metamorphic belt, SW Japan

摘要

Amphibolites from the Ryoke metamorphic belt, SW Japan were deformed initially by cataclasis and subsequently by dissolution -precipitation creep. Initial cataclastic deformation produced a rather weak shape-preferred orientation (SPO) of brown amphibole grains with small aspect ratios as well as a poorly developed amphibole lattice-preferred orientation (LPO) with nα (≈ α[100]) axes scattered subnormal to the foliation and nγ or c[001] axes scattered around the lineation. During later deformation by dissolution-precipitation creep, preferential dissolution at grain boundaries subparallel to the foliation and simultaneous compaction normal to the foliation have likely produced a distinct SPO of elongate brown amphibole grains subparallel to the foliation as well as their LPO such that their nγ or c axes are scattered around the lineation, while nα (≈ α) and nβ ( = b[010]) are spread along a girdle normal to the lineation. Also during this deformation green amphibole precipitated as isolated grains or in pressure shadow regions around brown amphibole grains. Nucleation and anisotropic growth of isolated green amphibole grains according to the orientations of the principal stress directions produced an LPO of these grains such that their nα (≈ α) are oriented normal to foliation, nβ ( = b) within the foliation normal to the lineation and nγ (or c) axes are parallel to the lineation. In addition, there is an associated SPO. Growth of green amphibole in pressure shadow regions around brown amphibole grains occurs either syntaxially or anisotropically according to the orientations of the principal stress directions.