We investigated the K-Ar ages and the petrological and geochemical features of lava units from lateral cones and lava distributed around the Aso caldera in central Kyushu, Japan, in order to constrain the spatial range of lateral magma intrusion during the caldera-forming stage. The results of the K-Ar age determination showed that most of the analyzed lava units erupted almost simultaneously with the Aso caldera-forming pyroclastic eruptions (266 to 89. ka). In addition, the petrography, major and trace element compositions, and Sr isotope ratios of these lava units are indistinguishable from the caldera-forming pyroclastic products. The contemporaneous activities of compositionally similar magmas inside and outside of the caldera presumably indicate the occurrence of a lateral intrusion of caldera-forming magma, which had accumulated in a huge magma chamber beneath the caldera system. Our results suggest that a total of 6.3. vol.% of caldera-forming magma in the Aso volcano migrated more than 20. km along the faults from the center of the caldera.
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