Petrology of a peculiar amphibolite boulder rich in deep vivid green-colored amphibole porphyroblast from Hidaka Town, Hokkaido, Japan

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  • MORISHITA Tomoaki
    Graduate School of Natural Science and Technology, Kanazawa University CNR-Istituto di Geoscienze e Georisorse Sezione di Pavia Present address: Frontier Science Organization, Kanazawa University
  • ARAI Shoji
    Graduate School of Natural Science and Technology, Kanazawa University Institute for Research on Earth Evolution, Japan Agency for Marin-Earth Science and Technology
  • TAMURA Akihiro
    Graduate School of Natural Science and Technology, Kanazawa University
  • ISHIDA Yoshito
    Graduate School of Natural Science and Technology, Kanazawa University

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  • 北海道日高町の濃鮮緑色角閃石斑状変晶を含む角閃岩転石の岩石学
  • ホッカイドウ ヒダカマチ ノ ノウセン リョクショク カクセンセキ ハンジョウ ヘンショウ オ フクム カクセンガン テンセキ ノ ガンセキガク

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Abstract

A peculiar amphibolite rich in deep vivid green-colored amphibole porphyroblast was found as a boulder in the Chiroro River of Hidaka Town, Hokkaido, Japan. The amphibolite mainly consists of deep vivid green-colored amphibole (porphyroblast), colorless amphibole (matrix) and plagioclase with a small amount of chlorite, epidote and chromian spinel. Deep vivid green-colored amphibole porphyroblast is more than 20 modal % in the rock. Chromian spinel rarely occurs as inclusions in deep vivid green-colored amphibole porphyroblast. The Cr2O3 content of the deep vivid green-colored domain is usually ∼1 wt% (up to 2.5 wt% nearby chromian spinel) whereas that of colorless amphibole in the matrix is low, < 0.1 wt%. Deep vivid green-colored amphibole porphyroblast is heterogeneous in chemical compositions within each grain and is divided into three domains as follows: (1) high-Cr2O3 (∼1 wt%, up to 2.5 wt%) and Al2O3 (> 10 wt%) green-colored domain, (2) relatively high-Cr2O3 (∼1 wt%) and low- Al2O3 (< 10 w%) green-colored domain and (3) low-Cr2O3 (< 0.1wt%) and Al2O3 (< 10 wt%) colorless domain. Chondrite-normalized rare earth element (REE) patterns of both deep vivid green-colored amphibole porphyroblast and colorless amphibole in the matrix are characterized by low light REE with positive Eu anomaly. The Cr#[= Cr/(Cr + Al) atomic ratio] and TiO2 content of chromian spinel in the amphibolite are similar to those in a serpentinite complex of the Kamuikotan Zone. The amphibolite was formed by either amphibolitization of a rock containing chromian spinel derived from the surrounding serpentinite complex or extensively metasomatized serpentinite due to interaction with surrounding metamorphic rocks.

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