北海道日高変成帯,幌満かんらん岩体における延性剪断運動の反転 Inverted ductile shear movement of the Horoman peridotite complex in the Hidaka metamorphic belt, Hokkaido, Japan

日高変成帯の最南端に分布する幌満かんらん岩体の上部層と下部層最上部のかんらん岩はイクイグラニュラー組織で特徴づけられ, そのかんらん石の非対称格子ファブリックは, 上盤が南に移動する剪断センスを示す。一方, 最上部を除く下部層は主にポーフィロクラスティック組織で特徴づけられ, 上盤が北に移動するセンスが優勢であるが, いくつかの試料は剪断センスを決定できない。最下部かんらん岩マイロナイトは周辺のテクトナイトと同様に上盤が北に移動するセンスが得られ, 両センスを示す変形斜方輝石が混在する。以上の組織と運動像の解析から, もともと上盤が南に移動する剪断センスのファブリックを持っていた岩体の下位の部分に上盤が北に移動するセンスの変形が重複した, と結論される。また, 最下部かんらん岩マイロナイトはその上盤が北に移動するセンスの変形時の相対的に低温条件である最終ステージに形成されたと考えられるが, 周囲の変成岩および, これまでに報告された日高変成帯の運動像とはセンスが逆であることが明らかになった。

Plastic deformation microstructures and kinematics of the Horoman peridotite complex were investigated to infer the process of upwelling and emplacement of Alpine-type peridotite massif into the high-grade Hidaka metamorphic belt in Hokkaido, Japan. The complex has a conspicuous layered structure, ca. 3000 min thickness, and is divisible into an upper zone and a lower zone based on its constituent rock types and thickness of layering. The peridotite from the upper zone and the uppermost part of the lower zone are characterized by equigranular texture which indicate a top-to-the-south sense of shear determined by asymmetric lattice fabrics of olivine. Getting closen to the lower zone, average olivine neoblast sizes decreases from about 640μm to 170μm, accompanied by a change of olivine lattice fabrics from (010) [100] through {Okl}[100]to weak (001)[100] type fabrics, suggesting the decreasing of temperature in descending order. The lower zone is mainly porphyro-clastic in texture and a dominantly deformed in a top-to-the-north sense of shear. The porphyroclastic peridotite tectonites in the lower zone grade into peridotite mylonites rather close to the basal thrust plane near the southern margin of the complex. The basal peridotite mylonites have (001)[100] type olivine fabrics and S-C fabrics indicating a top-to-the-north sense of shear. However, they also contain highly stretched orthopyroxene porphyroclasts whose obliquity of the (100) slip plane to the elongated direction suggests preservation of a top-to-the-south sense of shear. In this paper it is herein concluded that a top-to-the-north shear deformation has overprinted the lower part of the complex which has an original asymmetric fabric bearing a top-to-the-south sense of shear. The basal peridotite mylonite was probably formed at the latest stage of the top-to-the-north shear deformation event wherein a relatively lower temperature condition prevailed. The surrounding metamorphic rocks, however, were sheared in a ductile manner with a top-to-the-south sense.