Abstract
The Nakaoku tuffite dike (NTD) is a large, arcuate pyroclastic conduit located along the margin of Odai caldera in central Kii Peninsula, SW Japan. The NTD is one of the most likely candidates as a conduit of an explosive eruption during caldera formation at 13-15 Ma. We reappraised the origins of lithological units at the Hibora-dani section of the NTD and discussed how magma passed through the conduit to form the NTD facies. Our detailed observations and grain-size analyses revealed that three types of lithofacies occur from the margin to the center of rhyolitic pyroclastic dikes: welded lapilli-tuff, welded crystal-tuff, and welded vitric-tuff. In the central part of dikes, the welded vitric-tuff contains spindle-shaped pumice clasts stretched parallel to the dike wall, representing a foliation defined by aligned fiamme in a matrix with eutaxitic texture. The aspect ratio of stringy pumice clasts is 1 : 50, indicating rheomorphic deformation in a viscously flowing body. These findings suggest that the vitric tuff could have agglutinated as a laminar flow unit parallel to the dike wall at the time of “hot” emplacement. The welded foliation of fiamme suggests that the closing pressure of the dike wall compressed a vesiculated magma body. These rheomorphic deformations in hot magma could have resulted from the secondary viscous flow of welded tuffs in the dike interior, as with the drain-back process that occurs during closure of a conduit. Based on the present results and those of previous studies, we conclude that the distinct lithofacies at the Hibora-dani section of the NTD were produced during the final stages of a ring eruption at the Odai caldera.
