東北日本弧，稲庭岳地域の火山層序とＫ‐Ａｒ年代 著しくＫ２Ｏに乏しいマグマの活動時期

Abstract

The Inaniwadake volcano, situated on the northern part of the volcanic front of Northeast (NE) Japan arc, comprises two groups of volcanic rocks which overlie the Kiritoshi Formation and the lower part of the Itsukaichi Formation. The lower group is the Joboji Andesite Member belonging to the Itsukaichi Formation, distributed in the northern and eastern flanks of the Inaniwadake volcano, and composed of lavas with pyroclastics. The Inaniwadake Formation, the upper group, is distributed in the most part of the Inaniwadake volcano, and can be divided into the Takamagezawa (lower), Hanakosodemine (middle) and Takamagehara (upper) Basalt Members, all of which are composed of lavas with minor pyroclasts. Three samples were chosen for K-Ar dating. The sample from the Takamagehara Basalt Member, the youngest lava of the study area, shows 2.67±0.23 Ma, while the other two samples from the Joboji Andesite Member show 6.52±0.26 Ma and 6.95±0.26 Ma. All the Inaniwadake volcanic rocks including above samples are characterized by extremely low K2O suites like as the other late Miocene to Pliocene volcanic rocks from Mitaki and Kurohanayama, the southern part of NE Japan arc. These Tertiary volcanic rocks are mainly basalts and basaltic andesites in composition, and distinctively lower in K2O content than those of Quaternary volcanoes on the volcanic front. These extremely low-K volcanism coincide contemporaneously with the formation of a number of late Miocene to Pliocene calderas of NE Japan arc.