Petrological investigation of long-term evolution of magma chambers and preparing processes of caldera-forming eruption, Numazawa volcano, NE Japan

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  • Masubuchi Yoshiko
    Graduate School of Science and Engineering for Education, University of Toyama Toyama Science Museum
  • Ishizaki Yasuo
    Graduate School of Science and Engineering for Research, University of Toyama
  • Shirai Tomohito
    Department of Earth Sciences, Faculty of Science, University of Toyama
  • Matsumoto Akiko
    Department of Earth and Planetary Sciences, Faculty of Science, Hokkaido University
  • Amma-Miyasaka Mizuho
    Department of Earth and Planetary Sciences, Faculty of Science, Hokkaido University
  • Nakagawa Mitsuhiro
    Department of Earth and Planetary Sciences, Faculty of Science, Hokkaido University

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Other Title
  • 岩石学的にみた沼沢火山におけるマグマ溜りの長期的進化とカルデラ形成噴火の準備過程
  • ガンセキガクテキ ニ ミタ ショウタク カザン ニ オケル マグマ タマリ ノ チョウキテキ シンカ ト カルデラ ケイセイフン ヒ ノ ジュンビ カテイ

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Abstract

<p>Numazawa volcano in northeastern Japan has produced six eruptions of silicic magma (rhyolite and dacite) since volcanic activity began at 110 ka. The volcano formed the Shirifukitoge Pyroclastic Deposit (110 ka; >72 wt.% SiO2), the Mukurezawa Lava (71 ka; 69.1-71.7 wt.% SiO2), the Mizunuma Pyroclastic Deposit (53 ka; 68.1-69.0 wt.% SiO2), the Sozan Lava (43 ka; 66.9-67.7 wt.% SiO2), the Maeyama Lava (24 ka; 62.7-65.4 wt.% SiO2), and the Numazawako Pyroclastic Deposit (5.4 ka; 63.5-67.0 wt.% SiO2). The youngest eruption (5.4 ka) was the most voluminous, erupting ~2 km3 of magma and forming the Numazawa caldera. The distinct major and trace element chemistries, phenocryst assemblages and compositions, and 87Sr/86Sr ratios of the erupted silicic magmas imply that each eruption was fed by an independent and transitory silicic magma chamber. Relatively homogeneous silicic magmas were tapped between 110 and 53 ka. In contrast, evidence of interaction between silicic (dacitic) and mafic magmas is recorded in eruptions that are 43 ka in age and younger, including mafic enclaves and banded pyroclasts. This may have resulted from an increase in the rate of mafic magma input into the roots of the Numazawa system beginning at 43 ka. The silicic and mafic magmas erupted at 43 ka and 24 ka have similar 87Sr/86Sr ratios of 0.70385-0.70386 and 0.70396-0.70398, respectively. This suggests a common source material for the silicic and mafic magmas in each eruption. In contrast, during the 5.4 ka caldera-forming eruption, concurrent eruptions of silicic magma and two mafic magmas have different 87Sr/86Sr ratios. It is therefore likely that the 5.4 ka caldera-forming eruption resulted from the nearly simultaneous generation of distinct magmas from three different sources and, consequently, the rapid formation of a large magma chamber.</p>

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