Petrology of the oceanic island tholeiite-origin greenstones in the Shimamori Formation, North Kitakami belt, northeastern Japan.

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  • MIURA Ryo
    Department of Earth Sciences, Faculty of Science, Kanazawa University (Present address; Ocean Research Institute, University of Tokyo)
  • ISHIWATARI Akira
    Department of Earth Sciences, Faculty of Science, Kanazawa University

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  • 北部北上帯，島守層に産する海洋島ソレイアイト起源緑色岩の岩石学
  • ホクブ キタカミタイ シマモリソウ ニ サンスル カイヨウトウ ソレイアイト キゲン リョクショクガン ノ ガンセキガク

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Abstract

The North Kitakami belt in northeastern Japan is a Jurassic accretionary complex with abundant greenstones. The Shimamori Formation greenstones in the Hachinohe area, Aomori Prefecture, comprise basaltic massive lava, pillow lava and hyaloclastite associated with limestone as well as dykes of dolerite and gabbro. Association of these greenstones with limestone, without any chert, suggests their formation at a depth shallower than the carbonate compensation depth. Bulk rock major and trace element chemistry of these greenstones reveals that they are within-plate tholeiitic basalt enriched in Nb, Ti and LREE. The relic mineral chemistry such as Ti-poor (TiO2<1.3 wt.%) clinopyroxene and Cr-rich (Cr#=60) chromian spinel also supports “tholeiite” identification. These chemical and mineralogical characteristics indicate that the Shimamori Formation represents a seamount fragment mainly formed by within-plate tholeiitic (ocean island tholeiitic) magmatism, and accreted to the Eurasian continental margin through subduction process. The tholeiitic magmatism, however, is contrasting to the alkali basalt magmatism, which dominates the accreted greenstones in the main part of the North Kitakami belt to the south.

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