Host: The Japanese Association of Mineralogists, Petrologists and Economic Geologists
A small cone Goshikidake situates in inner part of a horse shoe shaped Umanose caldera (1.7 km in diameter), which is located in the central part of the Zao volcano. Crater Lake Okama (350m in diameter) is in the western part of the cone and pre-Okama crater (210m in diameter) is just southeastward of Okama. The past ca. 30-ky eruptive products of the Zao volcano are divided into the Komakusadaira agglutinate and the Goshikidake pyroclastic rock. At about 30ka, Umanose caldera was formed by explosive eruptions. The Komakusadaira agglutinate, which are spattered over along the top of the caldera wall, probably formed this period. Bulk silica contents of juvenile fragments of the agglutinate are 55.1-56.2%. Phenocrystic assemblage is cpx, opx, pl, and mt. The groundmass is mainly composed of glass with various amounts of vesicles. Afterwards (~ca.1ka), Goshikidake (2.2km3), which was composed of the Goshikidake pyroclastic rock, was built in inner part of the caldera. The pyroclastic rock can be divided into five units by angular unconformities. Unit 3 products were the final products from pre-Okama crater and units 4 and 5 products were erupted from Crater Okama. Most of unit 5 products were erupted in AD1895. The vents of units 1 and 2 are unclear. The cone products are composed of numerous pyroclastic layers and most of the constituents are dark gray to reddened colored pyroclastic surge deposits. Climbing duneform, planner bedded deposits with penecontemporaneous slumping, and bomb sags are commonly observed. These deposits thin rapidly away from the vent. Some exceptions can be seen. The lower part of unit 1 is mainly composed of vulcanian fall deposits, and also, the lower part of unit 4 and whole of unit 5 are composed of white to gray colored tuff breccias caused by phreatic eruptions. Phenocrystic assemblage (ol+-, cpx, opx, pl, and mt) and the groundmass (mainly composed of glass with vesicles) of juvenile fragments from the cone are similar among the units. Bulk silica contents of these are 56.2-58.1%, which is higher than those of Komakusadaira agglutinate. In some co-variant diagrams, chemical compositions of the agglutinate are not plotted on the silica poorer extrapolated lines of the trends drawn by the cone products, which is suggesting renewal of magma feeding system under the volcano. Although most of the major elements of the cone products are plotted on the same trends, K2O and some incompatible trace elements show higher values in unit 4 products than in the other units at same silica contents, which may be related to a differentiation of magma under the volcano.
