Ultramafic rocks in the Kotaki area, Hida Marginal Belt, central Japan: peridotitites of the Oeyama ophiolite and their metamorphism

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  • Machi Sumiaki
    Graduate School of Natural Science and Technology, Kanazawa University
  • Ishiwatari Akira
    Graduate School of Natural Science and Technology, Kanazawa University Present address: Center for Northeast Asian Studies, Tohoku University

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Other Title
  • 飛騨外縁帯，小滝地域の超苦鉄質岩：大江山オフィオライトかんらん岩とその変成作用について
  • ヒダ ガイエンタイ コタキ チイキ ノ チョウクテツシツガン オオエヤマ オフィオライト カンランガン ト ソノ ヘンセイ サヨウ ニ ツイテ
  • Ultramafic rocks in the Kotaki area, Hida Marginal Belt, central Japan: peridotites of the Oeyama ophiolite and their metamorphism

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Ultramafic bodies in the Kotaki area of the Hida Marginal Belt are composed of Paleozoic ophiolites and Paleozoic-Mesozoic sediments. The ultramafic rocks can be classified into two major types. Type 1 consists of primary ultramafic rocks that preserve primary textures and primary minerals of mantle peridotite. Type 2 consists of regionally metamorphosed peridotite that has a schistose or mylonitic texture. Both types were locally affected by later contact metamorphism.<br>The primary mantle peridotite (Type 1) is subdivided into a high-Al group [spinel Cr#, Cr/ (Cr+Al), of 0.33-0.38] and a high-Cr group [Cr# of 0.48-0.55]. The high-Cr group is pervasive among ultramafic bodies of the Oeyama ophiolite. The high-Al group is identical to lherzolitic peridotite that has been found only in the Oeyama ultramafic body.<br>Regionally metamorphosed peridotite (Type 2) can be subdivided into two subtypes based on mineral assemblage and texture. Type 2A contains olivine (ol) +tremolite (tr) ±antigorite (atg) ±orthopyroxene (opx). Type 2B contains ol+atg+clinopyroxene (cpx). Types 2A and 2B are similar to the peridotite mylonite and serpentinite mylonite, respectively, reported from the Happo ultramafic body to the south. Type 2A metaperidotite contains Na-rich tremolite (up to 2.53 wt.% Na2O) produced by metasomatism, possibly related to the slab-derived fluid that penetrated through the wedge mantle above an early Paleozoic subduction zone. Type 2B metaperidotite may represent the wedge mantle metamorphosed at a lower temperature, and Type 1 peridotite may represent the mantle portion that was unaffected by metamorphism and metasomatism.<br>Serpentinite hornfelses that formed by later contact metamorphism vary in their mineral assemblages, defined by atg, ol+cpx, ol+tr, ol+tr+talc, and ol+tr+opx zones that are arranged from west to east. The zones indicate that metamorphic temperatures generally increased toward the east. This finding suggests that the hornfelsic metamorphism of previously serpentinized peridotites of Type 1 and Type 2 was caused by a concealed plutonic intrusion beneath the Ishizaka rhyolite, which is exposed to the east.

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