K-Ar Ages of the Neogene Submarine Volcanic Rocks and Overlying Quaternary subaerial Lavas from the Mt. Karibayama Area, Southwest Hokkaido

Abstract

The Mt.Karibayama area is located at the northwestern part of the Oshima Peninsula, Hokkaido. This area is regarded as the northern extension of the inner belt of the North Japan Arc, and is almost occupied by the Neogene submarine volcanic rocks and associated sedimentary rocks, both of which are mostly covered by Quaternary subaerial lavas (Fig. 1) (YAMAGISHI and KUROSAWA, 1987). For this area, there have been few radiometric age data and their relative ages are presumed in view point of lithologic stratigraphy. In addition, it is difficult to evaluate the length of the time interval if some unconformities are observed. For example, an unconformity has been found between the upper Tertiary formation, such as the Okotsunai Formation, and the Quaternary Anatokomae Lava. The former is identified to be products of submarine effusion, whereas the latter to be those of subaerial one. Thus, it is important to obtain information on the time interval between the submarine and subaerial volcanism. Because the information suggests ane evolution process of a northern part of the North Japan Arc. Hence, we tried to determine K-Ar ages of the volcanic rocks from the representative formations in this area. Five samples from typical formations in this area were selected for the present purpose (Fig.1). The samples of Chihasegawa 16 and Karibayama 65A were taken from Quaternary subaeria lavas, while the samples of Karibayama 63, Karibayama 134A and Oy-103 from Tertiary submarine lavas and hyaloclastites. Their stratigraphical successions are shown in Fig. 2. The sample Chihasegawa 16 is a hornblende dacite lava (the Karibayama Lava), containing phenocrysts of plagioclase, hornblende, biotite, quartz, orthopyroxene and clinopyroxene in the order of abundance. The hornblende and margins of the biotite crystals hav ebeen oxidized. Some of hornblende phenocrysts have been altered into sericite flakes, but no alteration minerals are observed on the biotite phenocrysts. The Karibayama 65A is an olivine basaltic lava (the Anatokomae Lava).