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摘要
The Hakusan volcano, central Japan, is located in a region where two subducting plates (the Pacific Plate and the Philippine Sea Plate) overlap near the junction of four plates adjacent to the Japanese Islands (the Pacific Plate, the Philippine Sea Plate, the Eurasia Plate, and the North American Plate). The Hakusan volcano consists of products from four major volcanic episodes: Kagamuro, Ko-hakusan, and Shin-Hakusan I and II. To date the eruption events of the Hakusan volcano we applied thermoluminescence and fission track methods. U-238(U-234)-Th-230 disequilibrium and Pb-206/U-238 methods were applied to date the zircon crystallization ages for estimating the magma residence time before the eruptions. The eruption ages we obtained are ca 250ka for Kagamuro, ca 100ka and ca 60ka for Ko-Hakusan, ca 50ka for Shin-Hakusan I, and <10ka for Shin-Hakusan II. They are concordant with previous reports based on K-Ar dating. Some of the pyroclastic rocks, possibly originating from Shin-Hakusan II activities, are dated to be ca 36ka or 50ka, and belong to the Shin-Hakusan I activity. The zircon crystallization ages show several clusters prior to eruption. The magma residence time was estimated for each volcanic activity by comparing the major crystallization events and eruption ages, and we found a gradual decrease from ca. 500 ky for the Kagamuro activity to ca. 5 ky for the Shin-Hakusan II activity. This decrease in residence time may be responsible for the decrease in volume of erupted material estimated from the current topography of the region. The scale of volcanic activity, which was deduced from the number of crystallized zircons, is more or less constant throughout the Hakusan volcanic activity. Therefore, the decrease in magma residence time is most likely the result of stress field change.
- 出版日期2016-3
