The source volcano for the Murou pyroclastic-flow deposit in the Southwest Japan

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  • 室生火砕流堆積物の給源火山
  • ムロオ カサイリュウ タイセキブツ ノ キュウゲン カザン
  • The source volcano for the Murou pyroclastic‐flow deposit in the Southwest Japan

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Abstract

The Murou pyroclastic-flow deposit, middle Miocene, covers an area of about 1.9×10^2 km^2 of the central part of Kii peninsula, Kinki district, Japan. It is divided into two parts, the basal and the main parts. The basal part (less than 50 m thick) is composed of accidental rock fragments-bearing welded ash flow tuff, accretionary lapilli bearing fall- and surge- tuff layers. The main part is further divided into lower thin (less than 30 m thick) orthopyroxene bearing dacitic lapilli tuff, and upper voluminous biotite rhyolitic lapilli tuff attaining 400m in maximum thickness. Most of the matrix in the main part is composed of welded crystal tuff. The accidental rock fragments contained in the basal part are composed of chert, sand and mudstone as sand to pebble in size, and must have been derived from the basement of the source area of the Murou pyroclastic-flow deposit. The source area has been estimated to south of the distributed area of the deposits, because voluminous felsic igneous bodies such as the Kumano and the Omine acidic rocks of middle Miocene age are distributed. The fact that the chert of accidental rock fragments contains the Permian to Jurassic radiolarian fossils, strongly suggests that the part of source volcano was located in the Chichibu Terrane. We propose that one of the source volcanoes for the Murou pyroclastic-flow deposit was the Odai cauldron found in the Chichibu Terrane, where semicircular fractures intruded by pyroclastic dyke swarm formed a cauldron structure. The eruption of voluminous pyroclastic-flow is probable cause for the cauldron subsidence.

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