3:00 PM - 3:15 PM
[SVC47-15] Formation history and magma evolution of Asahidake Volcano of Taisetsu volcanic field, central Hokkaido, Japan
Keywords:Asahidake, Formation history, Transition magma, geology, petrology, Taisetsu volcanic field
Asahidake (2,291 m) consists of a pyroclastic cone formed above the 1600 m altitude and many lava flows on the west side. The horseshoe-shaped explosion crater called Jigokudani crater exists on the west side of the cone and the fumaroles still active. Ushiro-Asahidake lava dome (2,216 m) and Kumagatake pyroclastic cones (2,210 m) locate in the 1 km to the east from the summit of Asahidake and both are covered with the edifice of Asahidake. Based on the difference in volcanic edifice, the volcanic activity can be divided into 3 stages: Kumagatake, Ushiro-Asahidake, and Asahidake stages, in ascending order.
Kumagatake stage is characterized by the formation of pyroclastic cone with multiple craters. The eruptive materials are subdivided into 3 units by the difference in crater. The total eruptive volume of Kumagatake stage is 0.35 km3 DRE.
In Ushiro-Asahidake stage, lava flowed down southward. After that, the lava dome was built on the summit. Total volume of this stage is 0.33km3 DRE.
Asahidake stage is divided into two substage by the difference in eruption style. Early substage is mainly composed of magmatic eruptions. They are subdivided into lower and upper units on the basis of the stratigraphic relationship and magma type. The lower and upper units can be respectively subdivided into L1 - L3 and U1 - U5 subunits. The latest magmatic eruption of Asahidake is ca. 5 ka (Okuno 2005). The total volumes of lower and upper units are 4.50 and 0.99 km3 DRE, respectively.
Asahidake late substage is characterized by phreatic explosions forming Jigokudani crater. The last small phreatic explosion might occur in 250 years ago (Wada et al. 2003).
The rocks of three stages are 2px basaltic andesite to dacite, often contain mafic inclusions. They sometimes include olivine and hornblende phenocrysts. On whole-rock chemistry, the SiO2 contents of host rocks are 54.7-65.4 wt.% . Mafic inclusions show 54.0-59.0 wt.%. Dacitic rocks exhibits little variation in petrological features through all the stages. In contrast, mafic rocks are distinguishable according to stage and unit. Kumagatake stage can be clearly distinguished from the other stages by high Ni and low Cr contents. Ushiro-Asahidake stage and the lower unit of early Asahidake stage show similar, lower Ni and Cr contents. In contrast, the mafic rocks of upper unit of early Asahidake stage exhibit much wider variations
Heterogeneous textures suggest that magma mixing is the main magmatic process in the activities of Asahidake volcano, as mentioned by previous studies. This study revealed that magmatic compositions, especially mafic compositions, has changed in every stages, and units. That is, the mafic end-member magmas has replaced in each volcanic edifice and craters.