西南日本外帯, 熊野酸性岩類に含まれる赤色・無色・灰濁色ジルコンのU-Pb年代および微量元素組成と珪長質マグマの成因  [in Japanese] U-Pb ages and trace element compositions of reddish, colorless, and detrital zircons in the Kumano Acidic Rocks in the Outer Zone of southwest Japan and origin of the host magma  [in Japanese]

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Abstract

Sタイプ花こう岩類に含まれるジルコンの組成的特徴とその珪長質マグマの成因との関連性を明らかにするために西南日本外帯,熊野酸性岩から抽出した無色・赤色ジルコンについてLA-ICPMSによるU-Pb年代および微量元素組成を新たに測定した.ジルコン結晶内のクラックや微細包有物の効果を取り除いたU-Pb年代は15.5-16.7Maであり,同一試料から得られたFT年代に比べ若干(約1 Ma)古い年代を示した.無色・赤色ジルコンのREEパターンは右上がりで顕著なCeの正異常・Euの負異常を示し,これまで報告された花こう岩類のジルコンのREEパターンと比べ顕著な違いは見出されなかった.しかし,同ジルコンと平衡共存する珪長質メルトのREE組成を見積もると,無色ジルコンの共存珪長質メルト組成は花崗斑岩の全岩化学組成のより分化したものと類似し,赤色ジルコンの共存珪長質メルト組成はアダカイト的な特徴を示した.このことは熊野酸性岩の起源として異なる2種類の珪長質マグマの存在を示唆し,この組成差は付加体堆積物の部分溶融度の差を反映していると思われる.<br>

In order to understand compositional characteristics of zircon crystals with various appearance and to relate them with evolution of S-type, felsic magmatism of the Kumano Acidic Rocks in the Outer Zone of southwest Japan, U-Pb ages and trace element compositions of colorless and reddish zircons were measured using LA-ICPMS technique. Sorted U-Pb ages for these zircons range from 15.5 Ma to 16.7 Ma and ca. 1 m.y. older than fission track ages, implying slightly longer time elapsed between generation and cooling of the felsic melt. The REE patterns for both colorless and reddish zircons have a steep increase in LREE contents from La to Lu with significant positive Ce and negative Eu anomalies and show a common feature with those in granitoids. Scrutiny in the obtained data revealed that the slopes of HREE region of the reddish zircons are gentler than those of colorless zircons. REE composition of coexisting felsic melt estimated from the colorless zircons are relatively similar to the bulk rock composition of the evolved host rocks. Whereas the melt composition estimated from the reddish zircons has an adakitic signature. These suggest existence of roughly two different felsic melts for the origin of the Kumano Acidic Rocks. Large amounts of plagioclase fractionation from the parental magmas are required due to deep Eu troughs in the host melts. The production rate of the adakitic melts might be significantly smaller due to low population of reddish zircon. This compositional variation may reflect variable degree of partial melting of accretionary prism sediments that comprise the basement of the Kumano area. The partial melting of the sediments would have been triggered either by underplating of tholeiitic basalt magma derived from Shikoku basin or felsic magma derived from slab melting of the subducted plate, or melting directly of the subducted sediment.<br>

Journal

  • The Journal of the Geological Society of Japan

    The Journal of the Geological Society of Japan 113(7), 366-383, 2007-07-15

    The Geological Society of Japan

References:  62

Cited by:  4

Codes

  • NII Article ID (NAID)
    110006368280
  • NII NACSIS-CAT ID (NCID)
    AN00141768
  • Text Lang
    JPN
  • Article Type
    Journal Article
  • ISSN
    00167630
  • NDL Article ID
    8914832
  • NDL Source Classification
    ZM49(科学技術--地球科学--地質)
  • NDL Call No.
    Z15-174
  • Data Source
    CJP  CJPref  NDL  NII-ELS  J-STAGE 
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