ERUPTION, TRANSPORT AND DEPOSITION OF COARSE LITHIC BRECCIAS OF ASO VOLCANO, JAPAN

We investigated lithofacies, petrological and paleomagnetic characteristics of coarse lithic breccias (0.67+/-0.09Ma, NEDO, 1991) in the caldera-rim of Aso volcano in southwestern Japan in order to clarify their emplacement mechanism. .

The lithic breccias crops out at very proximal locations (<1km, maximum thickness of > 60m) and tend to change laterally into the pumiceous tuff breccia and the stratified pumiceous lapilli tuff. The variation of lithofacies obviously shows these deposits are ignimbrite. The deposits include abundant angular to subangular dense lithic clasts with about 20cm in diameter. The clasts are grayish or reddish in color. Hornbrende phenocrysts in the clasts show various degree of opacitisation. These characteristics of the clasts are common in the constituents of the lava dome.

The paleomagnetic estimation indicates that the deposition temperatures of the ignimbrite are between 200 to 300 degrees C, and some clasts show > 400 degrees C. The temperature ranges were not very different from that recorded at 8 sites regardless of the different lithic sizes and contents. Since cold lithic clasts carried by a pyroclastic density current (PDC) potentially decrease the deposition temperatures (Bardot, 2000), this implies the clasts were still hot when the clasts were embedded to the PDC. These geological and paleomagnetic observations show that the clasts were originated from the pre-existed active lava dome.

The breccias occasionally show imbrication of the clasts and have erosive contacts. The ballistic impacts are not recognized below the large clasts with over 1m in diameter. These lithofacies indicate that the clasts were transported by the energetic lateral current rather than fallout.

Although the massive part (15m thick) of the breccia shows no significant stratification and grain-size variation, vertical chemical zonation of the volcanic glasses is clearly observed. Silica contents of the glasses gradationally decrease from lower part (about 75wt.%) to upper part (about 68wt.%). This compositional change probably indicates that the breccia aggraded progressively from the base upward (Progressive aggradation; Branney and Kokelaar, 1992), rather than en masse freezing.