Origin of basalts from Sambosan accretionary complex, Shikoku and Kyushu

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  • Onoue Tetsuji
    Department of Earth & Planetary Sciences, Graduate School of Science, Kyushu University
  • Nagai Katsuya
    Kiso-Jiban Consultants Limited Company
  • Kamishima Aya
    Pasco Corporation
  • Seno Mamoru
    Department of Liberal Arts and Science, College of Liberal Arts and Science, Kurashiki University of Science and the Arts
  • Sano Hiroyoshi
    Department of Earth & Planetary Sciences, Faculty of Science, Kyushu University

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Other Title
  • 九州・四国の三宝山付加コンプレックスの玄武岩類の起源
  • 九州・四国三宝山付加コンプレックスの玄武岩類の起源
  • キュウシュウ シコク サンボウサン フカ コンプレックス ノ ゲンブガンルイ ノ キゲン

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Abstract

Sambosan accretionary complex comprises a Late Jurassic to Early Cretaceous subduction-generated accretionary complex of southwest Japan. In order to understand the origin of basalts of the Sambosan accretionary complex, this paper describes their modes of occurrence, petrographic features, and geochemical properties.<BR>Sambosan basaltic rocks crop out as, (1) exotic blocks in a mudstone matrix of melange units; (2) displaced blocks in basaltic volcaniclastic rocks; (3) pillow fragment breccias intercalated in Middle Triassic ribbon chert. In the former two cases, the blocks of basaltic rocks, ranging in size from a few meters to several tens of meters, are mainly composed of vesiculated pillow lavas with interpillow limestone and subordinate massive lavas, pillow fragment breccias, and hyaloclastites. The basalts are described mostly as aphyric basalt, clinopyroxene basalt, and olivine basalt. Dolerite occurs locally.<BR>Major and trace elements of the Sambosan basalts were analyzed by using the X-ray fluorescence spectrometry. Interpretation of geochemical data focuses on the relatively alteration-insensitive elements (Ti, Zr, Nb, Y). On the basis of the discrimination diagrams and the N-MORB normalized patterns, it is revealed that most of the Sambosan basalts have a geochemical affinity with an oceanic island basalt (OIB) enriched in Ti and Nb.<BR>Field and petrographic properties indicate that the Sambosan basalts were formed at shallower depth than the carbonate compensation depth in mid-oceanic realm. The predominance of OIB-type basalt means that the Sambosan basalts were erupted to form seamounts by an intraplate volcanism that most presumably took place in a mid-oceanic realm.

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