A multi-geochronological study of the Hakusan volcano, central Japan

Abstract

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. 238U(234U)–230Th disequilibrium and 206Pb/238U 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 250 ka for Kagamuro, ca 100 ka and ca 60 ka for Ko-Hakusan, ca 50 ka for Shin-Hakusan I, and <10 ka 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 36 ka or 50 ka, 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. 白山火山の活動史を明らかにするために,年代学的研究を行った∘噴火年代を調べるために熱ルミネッセンス法およびフィッション▪トラック法を用いた∘結果はK–Ar法による既報と一致したが,いくつかの新データは活動期の時代区分の再考が必要となることを示した∘またジルコンの結晶晶出年代をU–Pb法,およびU–Th放射非平衡法により求めたところ,ジルコンの結晶晶出は活動期を通し概ね定常的に続いていた∘結晶晶出年代と噴火年代の比較により推定した噴火前の滞留期間と噴火規模には相関があった∘白山の噴火規模は時代に伴い小さくなっていることが知られているが,これは火山としての活動度の低下ではなく応力場の変化によるものと推定した∘