中琉球孤久米島宇江城岳層の玄武岩質安山岩の全岩K-Ar年代

概要

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Whole-rock K-Ar dating for a basaltic andesite lava from the Uegusukudake Formation in Kume-jima, Central Ryukyu Arc, yields age of 2.13±0.19 Ma. The sample dated is from the middle horizon of the Formation. This age is compatible with recently published K-Ar ages of 2.76±0.48 and 2.24±0.10 Ma for a picrite basalt from the lowest part and an andesite from the upper part of the Formation, respectively. The age, however, is inconsistent with the previously reported K-Ar ages of 5.5 and 4.6 Ma. The result is also concordant with ages estimated from Sr isotopic ratios for molluscan fossils (∼3.2-3.1 Ma) for the Aka Formation underlying the Uegusukudake Formation. This result suggests that tholeiitic basalts and basaltic andesites erupted after the eruption of picrite basalts with a short time interval at the late Pliocene.
日本鉱物科学会の論文
2001-09-30