EMPLACEMENT MECHANISM OF THE LITHIC BRECCIA DISTRIBUTED IN THE SW WALL OF ASO CALDERA, JAPAN

We investigated the geological and geochemical characteristics and paleomagnetism of the lithic breccia distributed in SW caldera-rim of Aso caldera in order to clarify the emplacement mechanism. The lithic breccia crops out only at very proximal locations (<1km, maximum thickness of 60m). The clasts occasionally show imbrication, and erosive contacts are often recognized in the breccia. In the massive part, vertical chemical zonation of the matrix is observed. The breccia tends to change laterally into the pumiceous tuff breccia and the stratified pumiceous lapilli tuff, which are obviously ignimbrites. It means that the breccia was deposited from the pyroclastic eruption. The emplacement temperatures (200 and 350°C) revealed by paleomagnetic analyses also support it. 　The breccia contains abundant angular lithic clasts. Most clasts are approximately 20cm in diameter, and the clasts over 2m in diameter are also often occurred. They have no evidence of ballistic impacts. The matrix is composed of lithic ash and finely vesicular glass. Mineral assemblage of the matrix is different from the clasts. It indicates that the matrix was not formed by fragmentation of the clasts. Thus, the breccia is not originated from the autoclastic breccia of lava flows. We interpreted the breccia as the co-ignimbrite lag breccia. The energetic eruption generated the pyroclastic density current including the abundant lithic clasts. In proximal area, large and dense clasts are preferentially deposited from the pyroclastic density current (co-ignimbrite lag breccia), while lighter constituent such as pumice and fine particles remained within the flow and are transported to far. The sedimentary structures, such as imbrication of the clasts, erosive contacts and absence of ballistic impacts, indicate that the sediments were deposited from the density current rather than fallout. Furthermore, the vertical chemical zonation of the matrix glasses in the massive part of the breccia shows that the breccia aggraded progressively from the base upward (Progressive aggradation; Branney and Kokelaar, 1992), rather than en masse freezing.