徳島県南東部に分布する四万十帯玄武岩類の変質作用 Alteration of basalts from the Shimanto belt in southeastern Tokushima Prefecture, Southwest Japan

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著者

    • 麻木 孝郎 ASAKI Takao
    • 東北大学大学院理学研究科地圏環境科学 Institute of Geology and Paleontology, Graduate School of Science, Tohoku University
    • 吉田 武義 YOSHIDA Takeyoshi
    • 東北大学大学院理学研究科地球物質科学 Institute of Mineralogy, Petrology and Economic Geology, Graduate School of Science, Tohoku University

抄録

The mineralogical and chemical changes by alteration of the <i>in-situ </i>basalts from the Shimanto belt are studied using 16 weakly- to strongly-altered rocks from an outcrop of lava flows. The samples show variolitic to intersertal-intergranular texture, and are mainly composed of plagioclase, clinopyroxene, chlorite, sericite, hematite and titanite. No glass is preserved. Plagioclase is mostly albitized, as its anorthite component is less than 10%. Particularly in the samples cut by prehnite veins of high-temperature hydrothermal origin (ca. 250°C), plagioclase is severely albitized, clinopyroxene and sericite are much less, and the other secondary minerals are more abundant than in the remaining samples. These petrographical variations observed in a single flow suggest that the alteration occurred in at least two stages: an earlier one where sericite and the other secondary minerals appear at temperatures higher than 250°C, and a later one in which clinopyroxene and sericite disappear, accompanied by additional formation of the other secondary minerals, at ca. 250°C.<br>     Remarkable compositional variations are observed in LILEs (K<sub>2</sub>O, Ba, Rb, and Sr), MnO, Ce, and Pb. Although MnO, Ce, and Pb show no consistent trends with modal proportions of any minerals, LILEs show positive correlations with the sericite abundance. In consideration of previous works on interactions between seawater and basaltic rocks (Thompson, 1991), that show LILEs to be lost from rocks at high temperature (>100°C), secondary enrichment of LILEs is unlikely to occur in such alteration stages. LILEs are preserved by sericite replacing feldspar and glass in the first stage of alteration, and are depleted by decomposition of sericite in the later stage. Original abundance of total alkalis (Na<sub>2</sub>O+K<sub>2</sub>O) is retained by formation of albite after the disappearance of sericite.<br>     Except for a sample having the signature of intraplate type volcanics, the <i>in-situ</i> basalts originally have much higher abundance of total alkalis and LILE/HFSE ratios than N-MORB, and negative Nb anomaly on MORB-normalized spidergrams, which are characteristics of subduction-related volcanics.

収録刊行物

  • 岩鉱 : 岩石鉱物鉱床学会誌 : journal of mineralogy, petrology and economic geology  

    岩鉱 : 岩石鉱物鉱床学会誌 : journal of mineralogy, petrology and economic geology 94(1), 11-36, 1999-01-05 

    Japan Association of Mineralogical Sciences

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各種コード

  • NII論文ID(NAID)
    10002090812
  • NII書誌ID(NCID)
    AN10047992
  • 本文言語コード
    JPN
  • 資料種別
    ART
  • ISSN
    09149783
  • NDL 記事登録ID
    4681166
  • NDL 雑誌分類
    ZM46(科学技術--地球科学--岩石・鉱物・鉱床)
  • NDL 請求記号
    Z15-71
  • データ提供元
    CJP書誌  CJP引用  NDL  J-STAGE 
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