5:15 PM - 6:30 PM
[SVC48-P08] Geology and Petrology of Torikabutoyama-Yokokurayama, Old Kumanodake, and Nakamaruyama volcanic edifices in Zao volcano
Keywords:Zao volcano, andestic lava, calc-alkaline series
The eruption products of these three volcanic edifices can be divided into eleven units. These units can be grouped into three (the early period, the middle period, the late period). The early period, the middle period, and the late period eruption products are composed of thick andesitic to dacitic lava flows (Yokokurayama-Torikabutoyama lavas, Gorodake lava, Naganohoppo lava, Sanpokouzinsan lava, Oiwake lavas), thin basaltic andesite to andesitic lava flows (Zaosawatyuryu lavas and pyroclasts, Zaosawajoryu lavas and pyroclasts, Senninsawa lavas and pyroclasts, Zaoseibu lava), thin basaltic andesite lava flows (Nakamaruyama-lower lavas, Nakamaruyama lavas), respectively. The early period is mainly equivalent to StageⅡ. The middle period and the late period are equicalent to Stage Ⅲ.
Most of rocks are ol bg. cpx-opx andesites. Most of plagioclase phenocrysts have dissolution texture such as dusty zone and/or patchy zoning. All rocks of the early period and some of the late period contain dissolutive quartz. Quartz, orthopyroxene, and olivine sometimes have a reaction rim. Mafic inclusions are observed in all andesitic to dacitic rocks. These are several millimeters to tens of centimeter in length. The groundmass texture of most of rocks is hyalo-ophitic, while that of mafic inclusions is dikty-taxitic. We note that troctlite inclusions are characteristically observed in Senninsawa lavas and pyroclasts.
All products belong to medium-K calk-alkaline series. SiO2 contents of the early period, the middle period, and the late period are 57-62wt. % (some are >65wt. %), 57-62wt. %, 58-63wt. % (some are 55wt. %), respectively. The early period eruption products and the late period show relatively low trends in K2O-SiO2 diagram, while the middle period eruption products show relatively high trend. The compositional trends of the volcanic units in these three edifices are slightly different each other.