[SGL34-P05] Zircon U-Pb age and stratigraphy of some Quaternary tephras around Kagoshima Bay
Keywords:Quaternary, tephra, zircon, U-Pb dating, Kagoshima
We observed Komiyaji pyroclastic flow deposits (pfl) and Oda pfls I and II, in ascending order at the outcrops near Yamada River (Yamada Outcrop). Based on the fact that these tephras are covered by the ~0.34 Ma Kakuto pfl and some biostratigraphic evidence, their eruption ages were assumed to be middle Pleistocene (Kagawa and Otsuka, 2000). We obtained zircon U-Pb ages of 0.67±0.34 Ma (n=5, MSWD=2.8) for Komiyaji pfl and 0.71±0.12 Ma (n=5, MSWD=0.86), 0.61±0.11 Ma (n=8, MSWD=3.2) and 0.40±0.17 Ma (n=6, MSWD=3.6) at three localities of Oda pfls I and II. We also obtained ages of 0.57±0.09 Ma (n=9, MSWD=1.3) for Nabekura pfl and 0.48±0.14 Ma (n=8, MSWD=2.9) for Hegawa pfl, both of which crop out near the Yamada Outcrop. A zircon U-Pb age of 0.65±0.12 Ma (n=12, MSWD=2.1) was also obtained from Kakuto pfl that cover all the tephras mentioned above.
Therefore, our results indicate that the three lower-most tephras (Komiyaji, Nabekura and Hegawa plfs) were erupted at 0.5–0.6 Ma, which is consistent with the stratigraphy. The 0.4–0.7 Ma Oda pfls I and II and the 0.65 Ma Kakuto pfl ages are older than the stratigraphy, which may indicate that these are not eruption ages but the time when magma crystallized zircons or the tephras may have incorporated older zircons prior to final deposition. We plan to investigate zircon morphology and composition to further examine magmatic conditions that erupted these tephras.
References: Ito, 2014, J. Volcanol. Geotherm. Res., 289, 210–223; Kagawa and Otsuka, 2000, J. Geol. Soc. Japan, 106, 762–782; Sakata, 2018, Geochem. J. 52, 281–286.