紀伊半島中央部に分布する岩脈のK-Ar年代

概要

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Six new K-Ar ages for Tertiary dikes in central Kii Peninsula, Southwest Japan, were determined. The four K-Ar ages from quartz porphyry, dacite and andesite dikes are between 14.5 to 15.2 Ma, showing a good agreement with the ages both of the Outer Zone Granitoids including Kumano and Omine AcidiC Rocks, and of the Setouchi Volcanic Rocks previously reported. A K-Ar age indicating ll.0 Ma from an andesite dike is the yougest one in ages previously reported in this area. This might suggest that the duration of Miocene igneous activity in this area was slightly longer than that known before. Although a sample from a rhyolitic tuff dike shows 37.3 Ma in age, abundance of sandstone. mudstone and shale xenoliths within the dike indicates that this age is probably of a host rock. The preferred orientation of these five Miocene dikes is WNW-ESE. This direction in central Kii Peninsula is different σHmax direction from that in southern Kii Peninsula showing N-S direction after 15 Ma.