阿武単成火山群のＫ‐Ａｒ年代とマグマ活動史

Abstract

More than 40 monogenetic volcanoes are distributed over an area of 400 km2 in the Abu Monogenetic Volcanic Field, southwest Japan. The eruptive products consist of alkaline basalt and calc-alkaline andesite to dacite lavas and pyroclastics, which are collectively known as the Abu Monogenetic Volcano Group. We have examined the eruption history of the Abu Monogenetic Volcano Group based on 33 new K-Ar ages. Most samples were dated by normal K-Ar method, and yielded ages between 1.9 and 0.1 Ma. The ages of two very young samples were determined using an unspiked method incorporating a mass-fractionation correction procedure. This gave an averaged age of about 40 ka. K-Ar ages from the Abu Monogenetic Volcano Group are concentrated at around 1.7 Ma and 0.2 Ma, and the activity is therefore classified into early and late periods, respectively. The early activity had a duration of about 0.4 mys, during which time 0.3 km3 of alkaline basalt alone was erupted. The late activity produced 0.8 km3 of alkaline basalt and 2.1 km3 calc-alkaline andesite to dacite, and lasted about 0.8 mys. Alkaline basalt was mostly produced in the first half of the late period, with maximum eruption rate at about 0.4 Ma. This activity was followed by more voluminous calc-alkaline andesite-dacite eruptions which climaxed at 0.2 Ma. Although associated alkaline basalt activity continued in this period, eruption volumes steadily decreased. Periodic alkaline basalt activity in southwest Japan is thought to have originated from small mantle diapirs, with intermediate calc-alkaline magmas being produced by mixing between alkaline basalt and felsic crustal melts. Coincidence of alkalic and calc-alkaline magmas and the eruption history in the later period in the Abu Monogenetic Volcano Group suggest that heating induced by repeated intrusion of alkaline basalt resulted in partial melting of the lower crust, thus producing intermediate to felsic calc-alkaline magmas. These crustal melts may then have mixed with alkaline basalt and produced the relatively voluminous intermediate calc-alkaline magmas erupted in the latest stage.