IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1B-P11

Date/Time: July 24 Poster

Petrological and petrochemical investigations of Iwanoyama-Iyuzan volcanic chain in
Higashi-Izu monogenetic volcano group, Izu-Bonin volcanic arc, central Japan
Hiromichi Kanai1 , Yoji Arakawa2 , Yoshiyuki Tajima1 , Kei Ikehata2
1

Geosciences, Graduate School of Life and Environmental Sciences, University of Tsukuba, Japan,
2
Faculty of Life and Environmental Sciences, University of Tsukuba, JapanE-mail: k2-kanai@geol.tsukuba.ac.jp

Higashi–Izu monogenetic volcano group consists of more than 75 volcanoes in northeastern part of Izu Peninsula
northern Izu–Bonin volcanic arc. These monogenetic volcanoes were interpreted to have formed mainly under
the tensional stress regime due to the several block movement (Koyama and Umino, 1991). Some of thesemonogenetic volcanoes forming volcanic chains (NW–SE in direction) with different space, time are somewhere
distributed (e.g., Hayakawa and Koyama, 1992; Koyama et al., 1995). These eruptions are thought to have occurredin 142.5 ka–2.7 ka (e.g., Hayakawa and Koyama, 1995). Erupted products of these monogenetic volcanoes havewide compositional variations, from basalt to rhyolite, and SiO2 wt.% of 48–73. Some generation processes havebeen provided for those variations. Andesitic magmas were formed by differentiation from basaltic magma, orcrustal contamination, while dacite–rhyolite magmas were by partial melting of granitic crustal materials (e.g.,Hamuro, 1985; Miyajima, 1990; Suzuki, 2000). Recent melt inclusion study proposed a model that the chemicalvariability was attributed to the crystallization within magma chamber (Nichols et al., 2012).
Iwanoyama–Iyuzan volcanic chain is the youngest one in this region (2.7 ka), and also has wide compositionalvariations from basaltic andesite to dacite, even within one monogenetic volcano (e.g., Kurasawa, 1984; Suzuki,2000). These variations in this volcanic chain are not yet fully discussed with enough data. Therefore, the purposeof this study is to clarify their magma genesis, origin and evolution of magma in this volcanic chain based on detailedpetrographic, petrological and geochemical investigations.
The Iwanoyama, Ananoyama, Yahazuyama and Ananokubo in this chain are mostly composed of dacite and havesome different petrography and mineral chemistries. The dacites of former three volcanoes are characterized abimodal plagioclase anorthite content (An). High An group of plagioclases (An=92–96) indicates normal chemicalzoning, but low An group (An=56–70) are dominated, in most cases, by dusty–zoned, or reversely zoned texture.
Clinopyroxene has Mg# of around 80 both in core and rim, while orthopyroxenes are grouped into two: 70–72
and 76–80. Major and trace element characteristics give similar bimodal data distributions, which suggest somemixing including crustal materials as important process. Measured Sr isotope ratios (87Sr/86Sr) of Ananoyama
and Yahazuyama dacites range in 0.70325–0.70349, indicating inhomogeneous magma and exclude simple crystal
fractionation model. They probably reflect the mixing of basaltic magma and crustal material, or mixing of basaltic
magma and crustal–derived felsic magma. We will focus our discussion on the generation mechanism andcompositional variability within single volcano, or within 2.7 ka volcanic chain, and possibly the relationship withregional tectonics.

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