西南日本山陰帯と韓国慶尚盆地の花崗岩類における白亜紀最末期のSr–Nd同位体組成変化とその意義  [in Japanese] Marked change of Sr-Nd isotopic compositions of granitoids in Sanin Belt of SW Japan and Gyeongsang Basin of Korea during the latest Cretaceous, and geologic significance  [in Japanese]

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Abstract

西南日本山陰帯中央部地域の複合バソリスを構成する白亜紀–古第三紀花崗岩類の全岩化学組成とSr–Nd同位体比の時代的変遷について検討し,山陰地域と地質学的関係が深いとされる韓半島の慶尚盆地のそれと比較検討した.<br>因美古期貫入岩類(78–68 Ma)はK<sub>2</sub>O,Rb,Y,Zr,Fに富み,因美新期貫入岩類(68–53 Ma)と田万川期貫入岩類(43–30 Ma)はMgO,P<sub>2</sub>O<sub>5</sub>とSrに富み,高いK/Rb比,Ti/Zr比と低いK<sub>2</sub>O/Na<sub>2</sub>O比を有する.Sr,Nd同位体比は因美新期から田万川期にかけて枯渇する傾向を示し,εNd(T)は68 Ma頃から高い値へと変化する.同位体比にみられる同様の枯渇傾向は慶尚盆地でも認められる.両地域に共通にみられるこのような時代的変化は,アセノスフェアに由来する枯渇した苦鉄質マグマの注入による下部地殻の改変が白亜紀最末期頃から始まった結果と考えられる.

Temporal variations in the whole-rock chemistry and Sr-Nd isotope ratios of Cretaceous-Paleogene granitoids forming composite batholiths in the central Sanin district of SW Japan are examined and compared with correlated granitoids in the Gyeongsang Basin of Korea. The granitoids show marked temporal variations in wholerock chemistry, particularly of some large-ion lithophile and high field-strength element concentrations and Sr-Nd isotope ratios. The Older Inbi intrusive rocks (78-68 Ma) are characterized by having higher K<sub>2</sub>O, Rb, Y, Zr, and F contents, higher K<sub>2</sub>O/Na<sub>2</sub>O ratios, lower MgO, P<sub>2</sub>O<sub>5</sub>, and Sr contents, and lower K/Rb and Ti/Zr ratios than the Younger Inbi intrusive rocks (68-53 Ma) and Tamagawa intrusive rocks (44-30 Ma). Initial <sup>87</sup>Sr/<sup>86</sup>Sr ratios (SrI) and εNd(T) values of the Older Inbi intrusive rocks are 0.7052-0.7055 and –1.5 to –0.1, respectively, and those of the Younger Inbi and Tamagawa intrusive rocks are 0.7037-0.7053 and +0.4 to +6.1, respectively. Comparable changes in Sr-Nd isotope ratios at ca. 68 Ma are also observed in the granitoid suite of the Gyeongsang Basin, Korea. It is hypothesized that the granites were derived from partial melting of the lower crust, and that the temporal changes in the isotopic ratios of the lower crust were imparted during the latest Cretaceous. These changes can be explained by the injection of depleted, mantle-derived mafic magmas from the asthenosphere into the lower crust beneath the Sanin Belt of SW Japan and Gyeongsang Basin of Korea at that time.

Journal

  • The Journal of the Geological Society of Japan

    The Journal of the Geological Society of Japan 119(4), 229-248, 2013

    The Geological Society of Japan

Codes

  • NII Article ID (NAID)
    130003363930
  • NII NACSIS-CAT ID (NCID)
    AN00141768
  • Text Lang
    JPN
  • Article Type
    journal article
  • ISSN
    0016-7630
  • NDL Article ID
    024638545
  • NDL Call No.
    Z15-174
  • Data Source
    NDL  IR  J-STAGE 
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