Igneous activity and fractional crystallization of the Abire granodiorite in the Okuizumo area, San'in zone, Southwest Japan

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  • Iwata Chika
    Department of Geoscience, Interdisciplinary Graduate School of Science and Engineering, Shimane University
  • Kamei Atsushi
    Department of Geoscience, Interdisciplinary Graduate School of Science and Engineering, Shimane University
  • Iwata Katsuhiko
    Nippon Koei Co., Ltd.
  • Shibata Tomoyuki
    Institute for Geothermal Sciences, Kyoto University
  • Mitani Asuka
    Nittoc Construction Co., Ltd.

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Other Title
  • 山陰帯島根県奥出雲に分布する阿毘縁花崗閃緑岩の火成活動および結晶作用
  • サンインタイ シマネケン オウイズモ ニ ブンプ スル アビエン ハナオカセンリョクガン ノ カセイカツドウ オヨビ ケッショウ サヨウ

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Abstract

This study investigates the geology, petrology, and geochronology of the Abire granodiorite, which lies in the Okuizumo district, within the San'in zone of the Southwest Japan arc. The Abire intrusion is one of a number of large batholiths in the district, and is mainly composed of hornblende-biotite granodiorite. In its northern part, the Abire intrusion contacts the Yokota intrusion. Both intrusions are locally mingled, with a small quartz-dioritic stock at their boundary. In its southern part, the Abire intrusion is adjacent to the Ryukoma intrusion, but the boundary between these two bodies is gradational. These geological features suggest that all of these plutonic bodies formed at almost the same time. The Abire intrusion yields a Rb-Sr whole-rock isochron age of 60.5 ± 6.3 Ma, with an initial Sr isotopic ratio (SrI) of 0.70531 ± 0.00013. This age is very close to a previously reported age of the Yokota intrusion (59.6 ± 5.5 Ma from RbSr WR; SrI = 0.70484 ± 0.00022). These ages are compatible with the geological evidence. However, the Abire and Yokota intrusions solidified from different magmas, because their SrI values differ. This suggests that several large magma chambers in molten state coexisted in this region during the Paleozoic. SiO2 contents of the Abire intrusion range from 68.1 to 73.4 wt.%. The intrusion belongs to the I-type magnetite series, and is classified geochemically as a volcanic arc granitoid. Major and trace elements generally show marked differentiation trends on Harker diagrams, although chondrite-normalized rare earth element patterns change little with varying SiO2 content. The major cause of this geochemical feature lies in the small degree of differentiation observed, and in the weight proportions of the fractional assemblage (plagioclase: alkali feldspar: hornblende: biotite: apatite: zircon = 28.7: 31.6: 24.5: 14.7: 0.5: 0.1) during fractional crystallization; this assemblage leads to bulk distribution coefficients near unity for the rare earth elements.

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