北アルプス,仁科山地における白亜紀後期の高温で水に乏しい珪長質火成活動  [in Japanese] Late Cretaceous hot and dry felsic magmatism in the Nishina Mountains, Northern Japan Alps  [in Japanese]

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Abstract

白亜紀~古第三紀の火成活動で形成された多斑晶質の流紋岩,およびそれと関連した花崗岩は西南日本内帯に広く分布する.本研究ではこれらの多斑晶質の特徴に着目し,その活動地域の一つである北アルプス北東部・仁科山地に分布する白亜紀後期の多斑晶質流紋岩・木崎流紋岩と,関連した花崗岩である花崗斑岩~斑状花崗岩の青木花崗岩を対象に,マグマの性質(温度, 生成初期の含水量)の検討を行った.マグマの情報を残存する急冷周縁相の花崗斑岩を用い,地質温度計とMELTS プログラムによる解析を行った結果,一般的な珪長質マグマよりも高温で水に乏しい(850~900 ℃, H<sub>2</sub>O = 0.4~1.0 wt.%)というマグマの性質が明らかになった.この結果は,西南日本内帯に広く分布する同様の特徴をもつ多斑晶質の流紋岩も同様のマグマから生成され,白亜紀後期のユーラシア大陸縁辺では高温で水に乏しい火成活動が活発に起こっていた可能性を示唆している.

Late Cretaceous crystal-rich rhyolites and related granites are widely distributed in the Inner Zone of southwest Japan. A great deal has been written about these crystal-rich rhyolites; however, little attention has been paid to their crystal-rich character.<br>We focus on the remarkable crystal-rich character of these rocks with the aim of determining the magmatic conditions (i.e., temperature and initial water content) under which the Late Cretaceous Kizaki Rhyolite, and the associated Aoki Granite, formed within the Nishina Mountains, in the northeast of the Northern Japan Alps. The Aoki Granite contains unaltered fayalite and pyroxenes that retain information regarding the nature of the magma from which they formed. The Kizaki Rhyolite consists of a rhyolitic welded tuff, and its essential constituent (fiamme) is characterized by a crystal-rich nature (50%—60% by vol.). The Aoki Granite is remarkable for its lithological variability, but may be classified into three units: (1) fayalite- and pyroxene-bearing porphyritic granite, (2) chilled margin granite porphyry, and (3) mafic magmatic enclave-bearing finegrained biotite granite. These units have similar chemical compositions, characterized by high FeO*/MgO and high contents of CaO, Na<sub>2</sub>O + K<sub>2</sub>O, Ba, Zr, La, Ce, and Nd.<br>We analyzed the granite porphyry of the Aoki Granite using a geothermometer and the MELTS program, and results indicate that it was generated from hot and dry magma (850—900 °C, initial water content = 0.4%—1.0% by wt.). This also suggests that similar crystalrich rhyolites, widely distributed through the Inner Zone of Southwest Japan, were generated from similarly hot and dry magmas, and that such magmatism was vigorous during the Late Cretaceous at the eastern margin of the Eurasian continent.

Journal

  • The Journal of the Geological Society of Japan

    The Journal of the Geological Society of Japan 118(4), 207-219, 2012

    The Geological Society of Japan

Codes

  • NII Article ID (NAID)
    130001915318
  • NII NACSIS-CAT ID (NCID)
    AN00141768
  • Text Lang
    JPN
  • ISSN
    0016-7630
  • NDL Article ID
    023759893
  • NDL Call No.
    Z15-174
  • Data Source
    NDL  J-STAGE 
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