ORIGIN OF THE CLASTIC FLOW BANDS IN THE TAKANOOBANE RHYOLITE LAVA

The Takanoobane rhyolite lava in Aso caldera, Japan, was effused at 51+/-5 ka (Matsumoto et al., 1991). The black to dark gray colored flow bands within a few millimeters thick are concentrated in the basal part of the lava. The bands are filled by clastic materials with a diameter of < several hundred μm. The clastic materials are composed of glassy lithics and minerals. Some clastic materials are rounded and fluidal shapes and show different textural occurrences from the surrounding rhyolite. The chemical compositions of the glassy lithics and glassy matrix of the surrounding rhyolite are slightly different. The streak texture, which is defined by difference of clasts and microlite contents, is developed within the bands.

The differences in clast occurrences and chemical compositions between the bands and surrounding rhyolite indicate that the fragmentation was not caused by autobrecciation within the lava. The observation indicates that the clastic bands are likely to be formed by shear fracturing of the high viscous magma within the conduit (Tuffen et al., 2003). The fractures will become pathway of the volcanic gasses, and the clasts are transported and mixed by the gas transport. The streak texture within the bands is interpreted as sedimentary structures by gas transport of clasts through fracture system (Tuffen and Dingwell, 2005). The rounded and fluidal shapes of the clasts will be produced by frictional melting caused by shear fracturing. Gonnermann and Manga (2003) suggest that the clastic materials formed within the conduit can become elongated into flow banding of silicic magmas by flow deformation and reannealing.

It is considered that the clastic flow bands developed in the Takanoobane rhyolite lava are formed within the conduit. Homogenization of the bands by reannealing did not progress effectively; consequently the structure was preserved within the effusive lava. Our results mean that the clastic flow bands developed within silicic lavas is important for understanding of the shallow conduit behavior of silicic magma.