Petrography and bulk rock composition of the Wakurayama Dacite from Matsue City, Shimane Prefecture, Southwest Japan

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  • Sato Daisuke
    Institute of Geology and Geoinformation, AIST
  • Matsumoto Ichiro
    Department of Elementary Education (Science Course), Faculty of Education, Shimane University
  • Kamei Atsushi
    Department of Geoscience, Interdisciplinary Faculty of Science and Engineering, Shimane University

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  • 島根県松江市，和久羅山デイサイトの岩石記載と全岩化学組成
  • シマネケン マツエシ ワクラヤマ デイサイト ノ ガンセキ キサイ ト ゼンガン カガク ソセイ

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Late Miocene to Pliocene volcanic rocks occur in the Wakurayama and Dakesan areas of Matsue city, San-in district, Southwest Japan. We present new petrographic and geochemical data for the volcanic rocks. Previous studies refer to the volcanic rocks as the “Wakurayama Andesite.” However, on the basis of bulk rock compositions (SiO2: 64.47–65.84 wt.%) determined in this study, we propose to rename the volcanic package the “Wakurayama Dacite.” The Wakurayama Dacite is divided petrographycally into three types: Type I, Type II, and Type III lavas, reflecting the order of eruption. The Wakurayama Dacite lavas have adakitic compositions (i.e., higher Al2O3 and Sr, and lower Y). This adakitic magma may have been generated by partial melting of the subducting Philippine Sea Plate. The Wakurayama Dacite was erupted at ∼5 Ma, the earliest Tertiary to Quaternary adakitic activity along the southwestern portion of the Japanese arc. The eruption of the Wakurayama Dacite suggests that melting of the Philippine Sea Plate occurred at ∼5 Ma.

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