Depositional age of the Himenoura Group on the Amakusa‐Kamishima area, Kyushu, southwest Japan: Using zircon U–Pb dating of the acidic tuffs

The Upper Cretaceous Himenoura Group in the Amakusa‐Kamishima Island area, southwest Japan is subdivided into the Hinoshima and Amura Formations. In order to determine the numerical depositional age of the formations, zircon U–Pb ages were investigated using laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) for acidic tuff samples from the lower part of the Hinoshima Formation and the upper part of the Amura Formation. Although the two samples contain some accidental zircons, the samples have a definite youngest age cluster and their weighted mean ages are 85.4 ± 1.3 and 81.5 ± 1.1 Ma, respectively (errors are 95 % confidence interval). These age data indicate that the Himenoura Group in the Amakusa‐Kamishima Island area was deposited mainly in the early Santonian to early Campanian which is consistent with biostratigraphic ages. Additionally, zircon age distributions of the two tuff samples from the upper part of the Hinoshima Formation do not show a distinct youngest peak of eruption age but characteristics of detrital zircons suggestive of maximum depositional age of the host sediments. These results demonstrate that the mean age of the youngest zircon age cluster of a tuff sample does not always indicate depositional age of the tuff, and statistical evaluation of age data is effective to determine depositional age of a tuff bed using zircon U–Pb ages. 西南日本九州の天草上島地域に分布する上部白亜系姫浦層群は樋の島層と阿村層とに分けられる. これらの堆積年代を求めるために, 樋の島層下部と阿村層上部の酸性凝灰岩に対してジルコンU–Pb年代測定を行った. これらのサンプルは少量の外来性ジルコンを含んでいたものの, 最も若い年代集団を明確に持ち, それらの加重平均年代はそれぞれ85.4 ± 1.3 Maと81.5 ± 1.1 Maを示した(誤差は95%信頼域を示す). これらの年代データは天草上島地域の姫浦層群が主にサントニアン初期からカンパニアン初期にかけて堆積したことを示し, これは生層序年代とも整合的である. さらに, 樋の島層上部から得られた2つの凝灰岩サンプルのジルコン年代分布は噴出起源の明瞭なピークが見られず, 堆積年代の上限のみを示しうる砕屑性ジルコンの特徴を示した. これらの結果は, 凝灰岩中の最も若いジルコン年代集団の平均値が必ずしも堆積年代を示すとは限らず, 統計的手法による検証が非常に重要であることを示している. Zircon U–Pb age data indicate the depositional age of the tuff layer in the lowermost and uppermost part of Himenoura Group, Amakusa‐Kamishima area, southwest Japan as 85.4 ± 1.3 Ma (95 % conf.; MSWD = 0.83) and 81.5 ± 1.1 Ma (95 % conf.; MSWD = 1.3), respectively. The radiometric ages are consistent with biostratigraphic evidence. Since not all zircons in tuff samples indicate depositional age of the tuff, statistical evaluation of age data are very important to determine depositional age of a tuff bed using zircon U–Pb ages.