南部北上山地におけるスレートへき開の形成と花崗岩体の上昇・貫入過程

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書誌事項

タイトル別名
  • ナンブ キタカミサンチ ニ オケル スレート ヘキカイ ノ ケイセイ ト カコ
  • Development of Slaty Cleavage and Emplacement of Granitic Bodies in the South Kitakami Mountains, Japan

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説明

In the South Kitakami Mountains, the Silurian to Lower Cretaceous formations had been folded, cleaved and intruded by granitic bodies during the Early Cretaceous. The relation between the development of slaty cleavage and granite intrusion has been studied by investigating (1) the relation between illite crystallinity (an indicator of metamorphic grade) and the degree of preferred orientation of chlorite and (2) the regional variations of microstructure of argillaceous rocks. Around the Senmaya granitic body, metamorphic grade increases toward the granitic body. Metamorphic grade is lower in an area between Kesennuma and Utatsu than in other areas. Such regional variations of metamorphic grade are concordant with a zonal arrangement of granitic rocks. Slaty cleavage is characterized by microstructure with mica-rich seams and preferred orientation of chlorite and illite. This microstructure has developed in association with crystallization and recrystallization of chlorite and illite. Metamorphic grade higher than an IC (illite crystallinity index) value of 0.29 is a requisite for a high degree of preferred orientation of chlorite. The microstructure of argillaceous rock varies around the Senmaya granitic body. In an outer zone of the contact aureole, chlorite and illite show preferred orientation parallel to the slaty cleavage. However, metamorphic biotite shows no preferred orientation and has inclusions of mica-rich seams, indicating the generation of slaty cleavage prior to the crystallization of metamorphic biotite. In an inner zone of the contact aureole, metamorphic biotite shows preferred orientation parallel to the contact with the Senmaya granitic body. Consequently, slaty cleavage had been developed during regional heating by the ascent of granitic bodies. After the emplacement of the granitic bodies in the present situation, the deformation of country rocks weakened except in a narrow zone within the contact aureole.

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