Volcano-stratigraphy and geological development of Akaigawa Caldera from southwest Hokkaido, Japan

Abstract

Late Pliocene-Pleistocene Akaigawa Caldera, 6-7 km in diameter, is located in southwest Hokkaido. It mainly consists of felsic caldera-forming welded tuffs, somma lavas, and lacustrine deposits in order of decreasing age. Caldera-forming pyroclastic flows of the Nagasawa Volcanics were erupted at the southwestern caldera rim and are characterized by high SiO₂ content (76-79%). Contrary to this, the subsequent somma lavas were erupted from multiple vents along ring fractures and have variable SiO₂ contents ranging from basaltic andesite to dacite (56.6-66.7%).
The whole rock K-Ar age of the lower member of the welded tuffs is 2.06±0.57, whereas the age of the middle member 1.62±0.14 Ma. The Maruyama Lava and the Daikokuyama Lava from the somma lavas give ages of 1.65±0.07 Ma and 1.55±0.06 Ma, respectively. The age of the middle member of the welded tuffs coincides within measurement errors with ages determined for the somma lavas. The age and geological structures of the lower member of the welded tuffs suggest that the pre-Akaigawa, outer Yoichigawa Caldera formation began before 2 Ma.
On the basis of petrography and geochemistry, the somma lavas are divided into three units; the eastern somma lavas, the western somma lavas, and the post-caldera central cone. The lithologies of eastern somma lavas range from olivine basaltic andesite to andesite, and the western somma lavas range from quartz andesite to dacite, indicating a similar geochemical trend to that of the eastern somma lavas. The post-caldera central cone is composed of andesite to dacite, and has a different geochemical signature from that of the somma lavas.
The volcanic structure, topography, eruption volume, and petrography indicate that the Akaigawa Caldera-collapse took place after eruptions of both welded tuffs and somma lavas, and that the western side of the caldera is characterized by a higher eruption rate of felsic to intermediate magma and more extensive subsidence compared with the eastern side. This may indicate the existence of a heterogeneous magma chamber beneath the Akaigawa Caldera.