Ishizuchi collapse caldera and Tengudake pyroclastic flow, Shikoku Island

Abstract

Mt. Ishizuchi, northwestern Shikoku Island, is composed of late Miocene Takano pyroclastic flow, Yoake-toge altered andesites, Tengu-dake pyroclastic flow, and Omogo acidic intrusive complex, in the order of activity. The last three form a circular complex from 7 to 8km in diameter consisting of a caldera and central plutonic intrusions and ring dykes along the caldera wall. At the first, more than 300m thick dacitic to rhyolitic welded tuff erupted (Takano pyroclastic flow), then andesitic lavas and their pyroclastics (Yoake-toge altered andesites) followed this activity which formed arch-shaped and inclined block in the northern part of the complex. Next, the tremendous amounts of andesitic welded tuff which formed the main part of the circular complex erupted and built up an ash-flow field (Tengudake pyroclastic flow), cut off by ring fault. In the final stage, porphyritic granodiorites and fine-grained adamellites (Omogo acidic intrusive complex) intruded in the central part of the complex associating with ring dyke intrusions.