中部北海道，滝川地域の中新世―鮮新世玄武岩のＫ‐Ａｒ年代と主成分化学組成 東北日本弧―千島弧，島弧会合部の玄武岩単成火山群

Abstract

Cenozoic volcanism in the Takikawa district of the central Hokkaido, located at the junction of the Kuril and the northeastern Japan arcs, is unique in arc-trench system. It consists of Takikawa monogenetic volcano group (TMV) and four polygenetic volcanoes (Ofuyu-dake, Etai-dake, Shokanbetsu and Irumukeppu volcanoes). TMV are widely distributed in the district and mainly made up of alkali olivine basalt and olivine tholeiitic basalt. Whereas, voluminous polygenetic volcanoes associated with the TMV are mainly composed of calc-alkaline andesite and dacite. New data of K-Ar ages for the rocks show that the volcanic activity of TMV continued from late Miocene to early Pleistocene, with coeval polygenetic volcanism during Pliocene. The presence of TMV and the geological evidence for the development of sedimentary basin suggest that the junction area of the two arcs has been under extensional stress field during late Miocene and Pliocene. The SiO2 contents of nearly primitive basalts (FeO/MgO<1.3) in the Takikawa district exhibit an ellipse shaped zonation in space. More silica-saturated basalts are distributed in the center of the district. This feature is not concordant with across-arc compositional zonation in arc volcanics. Major element chemistry and their spatial zonation of these rocks from Takikawa district suggest that the magmatism should not be explained by the steady state subduction-related one but by mantle upwelling under extensional stress field in the arc-arc junction.