韓国沃川地域沃川帯の変形・変成史の相対的関係の解析 Time-relationship between deformation and metamorphism of the Ogcheon zone in the Ogcheon district, South Korea

韓国沃川帯の中央部に位置する沃川地域において沃川帯変成岩類の岩石・地質構造と変形・変成史の解析を行った。本地域の1st変形時相は泥質片岩中の十字石・ざくろ石・粗粒黒雲母のSi片理の形成である。この後, これらの鉱物の結晶作用に関わる変成作用(ND1)が非変形条件下で発生する。このND1変成作用後, 巨大なsheath型褶曲(Jiyangri褶曲)とその軸面片理(S1)の形成に関わる変形作用(S1変形時相)があった。このJiyangri褶曲はHabgeumri地質体の上位で一つのnappe(Jiyangri地質体)を形成している。このnappe境界(Ogcheon衝上断層)にそって発達している圧砕岩では右横ずれのextension型crenulation cleavageが見られる。S1変形作用の後, 右横ずれの圧縮型crenulation cleavage (S2)が形成され, このS2形成が進行した段階でOgcheon花崗岩が貫入し, 紅柱石-珪線石型の接触変成作用(ND2)が発生する。そして紅柱石のより後期の結晶作用の組織からは右横ずれのcrenulation cleavage (S3)の形成に関わる変形作用(S3変形時相)があったことが認められる。この後, 左横ずれのshear lensesの形成に関わるCheongmari断層の発生(S4変形時相)によるJiyangri褶曲の大きい欠落が生じる。この後, Geumgang断層にそう左横ずれ運動に伴ってkinkと巨大引きずれ型褶曲(S5変形時相)が形成される。この褶曲はacidic dykesに貫入される。

The rock and geological structures and time-relationship between the deformation history and the metamorphism of the Ogcheon Supergroup (schists) in the Ogcheon district have been analyzed in this paper. The deformation of the first phase is related to the formation of Si schistosity in staurolite, garnet and old biotite. After this deformation occurred the metamorphism (ND1) related to the crystallization of these minerals under non-deformational condition. The ND1 metamorphism was followed by the deformation (S1 phase) related to the formation of a large-scales heath type fold (Jiyangri fold) and associated axial plane schistosity (S1). The Jiyangri fold forms a nappe (Jiyangri nappe=Jiyangri unit) overlying the Habgeumri unit. The mylonitic rocks along the nappe boundary (Ogcheon thrust) show extensional crenulation cleavage of dextral shear sense. The S1 deformation was followed by the deformation (S2 phase) related to the formation of compressional type crenulation cleavage (S2) with dextral shear sense. After this deformation occurred the intrusion of the Ogcheon granite and its associated contact metamorphism (ND2) of andalusite-sillimanite type. The later phase of crystallization of andalusite was accompanied by the deformation (S3 phase) related to the formation of crenulation cleavage (S3) with dextral shear sense. After this deformation, the Jiyangri fold was modified by the formation (S4 phase) of the Cheongmari fault and associated shear lenses with sinistral shear sense. After this deformation, the sinistral displacement occurred along the Geumgang fault cutting across the Jiyangri unit and Habgeumri unit, accompanying the formation (S5 phase) of the giant-scale Geumgang drag-type fold with kink bands (S5). These folds are intruded by acidic dykes.