和歌山県三越峠付近の火砕岩岩脈の形成過程

概要

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A NW-SE trending telsic pyroclastic dike exposed at Mikoshi Pass, Wakavama prctecture, is described to infer its process of formation. This dike is composed of three zones I, Ⅱ and Ⅲ. The distinction among three zones is based on joint spacing and grain size. Zones I. Ⅱ and Ⅲ show wide, medium and narrow spacings of cooling joint, respectively, Averagc grain size of each zone decreases from I to Ⅲ Zones I and Ⅱ exist side by side, while zone Ⅲ is contained in the upper part of zone Ⅱ. This causes the upper part of the dike to be wide, and the growth of a horizontal fault consistent with the lower boundary of the zone Ⅲ. Therefore, because the joint pattern of zone I is not symmetrical, and narrow spacing pattern near the dike wall is lacking in its zone Ⅱ-side, zone Ⅱ intruded between preexisting zone I and weakly, consolidated host rock, and zone Ⅰ was eroded partly. After this event, zone Ⅲ was intruded into zone Ⅱ to push zones I and Ⅱ away.