Geology of Iwate volcanic group, northeastern Japan

Abstract

Iwate volcanic group is situated at the southeastern portion of Hachimantai and surrounding volcanic clusters, northeastern Japan. It consists of several volcanic bodies extending in E-W direction, parallel to the maximum horizontal compressional axis of the tectonic stress in this region. The basement rocks of Iwate volcanic group are composed mainly of the Tertiary rocks and the early Quaternary rhyolitic welded tuff which is a member of the Tamagawa welded tuffs. The activity of Iwate volcanic group is summarized as follows. Matsukawa andesite : this andesite consists of altered lavas and pyroclastics and are exposed at the western portion of the volcanic group. Their K-Ar ages have been reported to be 1.62 and 1.36 Ma. Amihari volcanic group: it is divided into two substages, older and younger, according to preservation of their volcanic primary topography. Older stage: in early to middle Pleistocene, several volcanic bodies, composed of an alternation of lavas and pyroclastics, were built up at the nearly same time. They are arranged in E-W direction 12km in length. Younger stage: after the formation of the volcanic chain, two cinder cones and a lava dome were formed, and lava flows were effused from the western portion of the volcanic chain. It is possible that they were active during the growth of Iwate volcano. Iwate volcano: in middle or late Pleistocene, the activity of Iwate volcano started at the eastern portion of Amihari volcanic group. Its history consists of four substages, and involves collapse of the volcanic body. Older Iwate 1st stage: voluminous pyroclastics and subordinate amounts of lavas formed two stratovolcanoes, Kurokura and Older-Iwate volcanos, with a net height of about 2, 000m. Older Iwate 2nd stage: at about 0.15Ma, the southeastern portion of the Older-Iwate volcano collapsed to form Onimata caldera. After the collapse, lava flows and pyroclastics were erupted from the central and flank vents to fill the Onimata caldera. Older Iwate 3rd stage: west-Iwate caldera was formed at the top of Older-Iwate volcano. From the vent in the caldera, lavas and pumice were effused to built up a composite central cone, Onashiro cone. It seems that these activities started at about 0.05Ma and ceased before 0.03Ma. Younger Iwate stage: after more than 25, 000 years of pause, about 5, 000 years ago, East-Iwate caldera was formed at the eastern portion of Older-Iwate volcano. Pyroclastics and lava flows were eruped from the vents mainly within the caldera, resulting in the formation of the composite Younger-Iwate volcano, Yakushidake volcano (2, 040.5m). In 1719, Yakehashiri lava flow was extruded from the northeastern flank of the volcano. The eruptive rocks are mainly basalt and basaltic andesite and associated with andesite. Two types of mafic phenocryst assemblage in basaltic rocks, clinopyroxene+olivine and orthopyroxene+olivine, are found. Although quartz phenocryst often occur, no hornblende phenocryst is found in the rocks from Iwate volcanic group.