サヌカイト溶岩と共存する無斑晶質安山岩のマグマ沸騰現象 : 二上層群,春日山安山岩溶岩の産状

抄録

The Sanukite lava is found in the Kasugayama andesite mass middle Miocene in the Nijyo Group, southeast of Osaka Prefecture. The Sanukite lava of irregular shaped block is included in aphyric andesite lava near the eruption vent. The aphyric andesite lava contains many light gray small spherical grains in 0.2-3mm diameter. Silica minerals and micas are found in the grains. The aphyric andesite magma may have reached second boiling point by increasing gaseous pressure during crystallization process. In contrast, the Sanukite lava blocks show homogeneous glassy texture free from the grains. It is supposed that the Sanukite was formed by magma with low volatile content, whereas the aphyric andesite magma hosted larger amount of volatiles. These two magmas were mingling while eruption of the host aphyric andesite magma.