Depositional age of the Himenoura Group on the Amakusa‐Kamishima area,Kyushu, southwest Japan: Using zircon U–Pb dating of the acidic tuffs |
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| Authors: | Yukiyasu Tsutsumi Yuka Miyake Toshifumi Komatsu |
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| Institution: | 1. Department of Geology and Paleontology, National Museum of Nature and Science, Tsukuba 305‐0005, Japan;2. Graduate School of Life and Environment Sciences, University of Tsukuba, Tsukuba 305‐8572, Japan;3. Graduate School of Science and Technology, Kumamoto University, Kumamoto 860‐8555, Japan;4. Faculty of Advanced Science and Technology, Kumamoto University, Kumamoto 860‐8555, Japan |
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| Abstract: | 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. |
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| Keywords: | acidic tuff Amura Formation Hinoshima Formation LA– ICP– MS zircon U– Pb age |
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