福岡県小呂島及び烏帽子島における新第三紀玄武岩類の全岩化学組成とK-Ar年代

抄録

Cenozoic alkali basalts are distributed from the northern Kyushu to Chugoku regions. I present new K-Ar ages and geological and geochemical data for the basalts on Oronoshima and Eboshijima, northern Kyushu, southwest Japan. The volcanic rocks on Oronoshima comprise basaltic andesite, trachybasalt to basalt, and basaltic andesite in ascending stratigraphic order, intercalated with volcaniclastic and clastic rocks. The K-Ar age of the plagioclase in the lowest basaltic andesite is 10.78±0.25 Ma (Late Miocene). On Eboshijima, the volcanic rocks comprise hornblende basalt to trachybasalt. The K-Ar age of the basalt is 4.27±0.10 Ma (Early Pliocene), which is within the age range of hornblende basalts distributed across northern Kyushu (4.50-3.51 Ma).