ポロシリオフィオライト中に見出された左横ずれ剪断帯における変成作用とマイロナイト化 Metamorphism and mylonitization in sinistral shear zones found in the Poroshiri ophiolite

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Abstract

<p>日高変成帯と接合後衝上時に広域的に右横ずれ剪断変形を受けたとされているポロシリオフィオライト中に,NNW走向の小規模な左横ずれ剪断帯を見出した.ポロシリオフィオライト中の変はんれい岩は角閃岩化と同時に右横ずれ剪断変形を受けており,左横ずれ剪断帯では,その角閃岩が斜長石の動的再結晶とホルンブレンド+斜長石の細粒粒子集合体を生成する変成反応により,マイロナイト化している.面構造の方位変化や非対称微細構造などは,全て左横ずれの剪断センスを示す.ホルンブレンド-斜長石の地質温度計より,角閃岩とマイロナイトの変成温度はそれぞれ,約708±7°Cおよび678±6°Cと推定され,角閃岩の左横ずれ剪断変形とマイロナイト化は後退変成時に起こったと考えられる.先立つ右横ずれ剪断変形が衝上成分を持つのに対し,左横ずれ剪断変形は正断層成分を持っており,左横ずれ剪断帯は後退変成時の局所的な後退運動によって形成された可能性がある.</p>

<p>We found meter-wide, NNW-trending sinistral shear zones in the Poroshiri ophiolite which has been dextrally sheared during thrusting with the Hidaka metamorphic rocks. Metagabbro in the Poroshiri ophiolite was transformed into porphyroclastic amphibolite and dextrally sheared during the amphibolitization, the latter of which in turn was mylonitized in the sinistral shear zones through dynamic recrystallization of plagioclase and metamorphic reactions to produce fine-grained hornblende and plagioclase. Foliation deflection, σ- and d-type mantled plagioclase porphyroclasts, shear bands, and shape-preferred orientations of hornblende porphyroclasts as well as of dynamically recrystallized plagioclase grains oblique to mylonitic foliation all indicate a sinistral sense of shear. Hornblende-plagioclase geothermometry suggests the metamorphic temperatures of porphyroclastic amphibolite and amphibolite mylonite to be 708±7℃ and 678±6℃, respectively. Thus the mylonitization occurred during the retrograde metamorphism. In contrast to the preceding dextral shearing with a thrust component, the sinistral shearing has a normal slip component, which possibly occurred by local backward movement during the retrograde metamorphism.</p>

Journal

  • The Journal of the Geological Society of Japan

    The Journal of the Geological Society of Japan 122(9), 461-475, 2016

    The Geological Society of Japan

Codes

  • NII Article ID (NAID)
    130005250149
  • NII NACSIS-CAT ID (NCID)
    AN00141768
  • Text Lang
    ENG
  • ISSN
    0016-7630
  • NDL Article ID
    027700125
  • NDL Call No.
    Z15-174
  • Data Source
    NDL  J-STAGE 
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