K-Ar dating on age-unknown volcanic rocks in the northwestern part of the 'Geological map of Japan 1: 200,000 Kagoshima'

摘要

Upon compiling the "Geological Map of Japan 1:200,000 Kagoshima", K-Ar ages were determined for ten volcanic rocks to clarify their ages of formation. Three ages for older-stage andesites of the Hokusatu Volcanic Rocks are from 4.2-4.4 Ma, suggesting that the extensive volcanism occurred over the wide area associated with hydrothemal gold mineralization. Five ages for the middle-stage andesites and a rhyolite of the Hokusatu Volcanic Rocks range from 2.7-3.3 Ma, about million years younger than those of the older stage. Two ages for the pre-Aira Caldera volcanic rocks suggest that volcanism occurred at around 0.8 Ma in the western rim of the Aira Caldera and that the activity had ended not later than about 0.1 Ma.