火山 第 57 巻 ( 2012)
第 4 号 177-197 頁

論 説

大雪火山群，御鉢平カルデラ形成期における珪長質マグマ溜まりの進化過程
佐 藤 鋭 一＊,＊＊・和 田 恵 治＊
（2012 年 2 月 24 日受付，2012 年 10 月 29 日受理）

Evolution of Silicic Magma Chamber for Caldera-forming Eruption of Ohachidairain the Taisetsu Volcanic Group, Central Hokkaido, Japan
Eiichi SATO＊,＊＊ and Keiji WADA＊
The 30 ka caldera-forming eruption of Ohachidaira started with plinian pumice fall and pyroclastic flows. The
deposits contain pumice (SiO2 =64.9-68.4 wt.%), scoria (SiO2 =56.6-59.0 wt.%), and banded pumice. This study
examined the evolution processes of silicic magma chamber through mineralogical and petrological analyses of theeruption products. Three types of plagioclase phenocrysts such as An-rich (type A : An70-90), An-poor (type B :An36-56), and intermediate (type C: An56-70) were observed. Type-A plagioclase phenocrysts were further classified into
two sub-types on the basis of MgO content in the cores; type A1 (MgO>0.05 wt.%) and type A2 (MgO<0.05 wt.%).
Type-A1 and type-A2 plagioclase phenocrysts were derived from mafic magma, type-B plagioclase phenocrysts werederived from silicic magma, and type-C plagioclase phenocrysts were derived from hybrid magma formed by the mixingof mafic and silicic magmas. The pumice mainly contains type-B plagioclase phenocrysts with rare type-A2 and type-C
plagioclase phenocrysts. The scoria contains type-A1, type-A2, and type-B plagioclase phenocrysts with rare type-C
plagioclase phenocrysts. These assemblages in the products can be explained by the mixing of magmas. Initially, maficmagma including the type-A1 plagioclase phenocrysts was injected into the bottom of the silicic magma chamber, and adensity-stratified magma chamber was formed. The first mixing occurred at the interface of mafic and silicic magmas,
and a hybrid magma was formed at the interface of the two magmas. During the period from the mixing to the eruption,
type-A2 plagioclase phenocrysts were formed due to the diffusion of MgO in type-A1 plagioclase phenocrysts.
Whereas, type-C plagioclase phenocrysts were derived from hybrid magmas. During the eruptions, the lower-layermagmas (hybrid and mafic magmas) were sucked into the conduit due to the viscous force of the upper-layer silicicmagma. Outer part of the conduit, silicic and hybrid magmas mixed. The mixed magma contained type-B, type-A2, andtype-C plagioclase phenocrysts. Whereas, in the center of the conduit, the mixing of the three magmas (mafic, hybrid,and silicic magmas) occurred, and the mixed magma containing the type-A1, type-A2, type-B, and type-C plagioclase
phenocrysts was formed.
Key words : Taisetsu volcanic group, Ohachidaira caldera, magma mixing, magma chamber, plagioclase

1．は じ め にする．また，マグマ混合は噴火のトリガーになり得るば島弧火山では噴火前にマグマ混合が生じている例が多かりでなく (Sparks et al., 1977; Blake, 1981; Pallister et al.,く (Eichelberger, 1978, 1980; Sakuyama, 1979, 1981)，その1992)，マグマ供給系の進化過程に影響を与える (Tomiya

場合，マグマは均質または不均質に混合した状態で噴出

and Takahashi, 1995; Chertkoff and Gardner, 2004; Toya et＊〒070-8621 旭川市北門町 9
北海道教育大学旭川校地学教室
Earth Science Laboratory, Hokkaido University of Education at Asahikawa, Hokumon-cho 9, Asahikawa 0708621, Japan＊＊
現所属：〒 657-8501 神戸市灘区鶴甲 1-2-1

神戸大学大学教育推進機構
Institute for Promotion of Higher Education, Kobe University, Tsurukabuto 1-2-1, Nada, Kobe 657-8501, Japan
Corresponding author: Eiichi Satoe-mail: esato@people.kobe-u.ac.jp