Large scale explosive eruptions of Akan volcano, eastern Hokkaido, Japan: A geological and petrological case study for establishing tephro-stratigraphy and -chronology around a caldera cluster

Abstract

Akan volcano, in eastern Hokkaido in the southern Kurile arc, occupies a rectangular caldera (24 × 13 km) with a long (1.7–0.2 Ma) and complex history. This paper combines tephrostratigraphy, tephrochronology, and petrological and geochemical evidence to elucidate the eruptive history and evolution of the caldera. Pyroclastic deposits from Akan caldera are divided into at least 40 “eruptive units”, separated by paleosols, that constitute 17 “eruptive groups” (Ak1 to Ak17, in descending stratigraphic order) that are petrologically distinct and separated by indicators of longer dormancy periods such as thick (>30 cm) paleosols and angular unconformities. The estimated volumes of most eruptive groups are less than 10 km3 dense rock equivalent, and four groups (Ak2, Ak4, Ak7, and Ak13) are larger. Group Ak2, exceeding 50 km3, is the largest. Pyroclastic deposits from Akan caldera are intercalated with pyroclastic deposits from the adjacent Kutcharo caldera and distal air-fall ash layers from central Hokkaido, suggesting that caldera-forming episodes overlapped in central and eastern Hokkaido. Radiometric ages from these exotic deposits range from 1.46 to 0.21 Ma, indicating that caldera-forming eruptions occurred at Akan volcano for more than 1 million years at an average magma discharge rate of approximately 10−1 km3/kyr. Juvenile materials in Akan caldera pyroclastics consist of dominantly aphyric, two-pyroxene dacite to rhyolite. They are characterized by a wide range of K2O compositions (0.8–2.8 wt.%) within a narrow range of SiO2 compositions (67–73 wt.%). Plots of SiO2 vs. K2O suggest that each eruptive group is the product of a distinct, ephemeral magma system rather than a single long-lived magma system. These magma systems appear to have been generated and erupted successively beneath the caldera for more than 1 million years. Each magma system thus appears to represent a relatively short period of activity, and they were separated by relatively long dormancy periods before the next magma system. In particular, a long dormancy of 400 kyr preceded eruptive group Ak2, which consists of the most voluminous and compositionally varied silicic magmas.