Radiometric ages of the Akashima Formation, Oga Peninsula, NE Japan

概要

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U-Pb and fission-track dating of zircons and Ar-Ar dating of plagioclase were conducted to estimate the eruption age of the welded dacite lapilli-tuff of the Akashima Formation, Oga Peninsula, NE Japan, yielding ages of 72 Ma, 65–63 Ma and 34 Ma, respectively. The zircon grains are mainly euhedral with zonal structures concordant with their external form, and are set in a welded matrix and in dense lenses of pumice; thus, they are likely to be juvenile. The Standard deviation of U-Pb ages of different zircon grains is small (1–2 Ma) and their age likely represens the eruption age of the magma. Fission-track ages are significantly younger than U-Pb ages, and are thought to represent thermally rejuvenation as suggested by shortening of fission tracks. The Ar-Ar age of the plagioclase remains fairly stable at almost all stages of heating; the plateau age, however, falls within the time range of the Late Eocene Monzen Formation and could represent almost complete albitizion ages by associated volcanic activity. The U-Pb ages provide reliable evidence for presence of late Late Cretaceous acidic volcanism in NE Japan.