Abstract
We have investigated the geology of the Kumano Acidic Rocks (KARs), a major Miocene volcano-plutonic complex in Japan, and in this paper we present a geologic map, geologic profiles, and lithologic and structural descriptions of the KARs in the Owase-Kumano area. The KARs in the mapped area consist of rhyolite lavas (the Konogi Rhyolite), rhyolitic pyroclastics occurring as both ash-flow deposits (the Owase-Shirahama Pyroclastic Rocks) and pyroclastic vent fills, granite porphyry intrusions (the Kumano Granite Porphyry) penetrating the rhyolite and the pyroclastics, and a stock of fine-grained granite (the Fudodani Granite) that intrudes the granite porphyry. Among them, the Kumano Granite Porphyry, comprising most of the KARs, occurs as a ring dike and a subhorizontal sheet intrusion. The ring dike, some 2 km or less wide and approximately 10×15 km in size, is found at the northern end of the mapped area. The subhorizontal sheet intrusion approximately 1 km or more in the maximum thickness, the eastern half of which is hidden in the sea. The roof of the sheet intrusion is observed in the northwestern part of the mapped area. Detailed mapping reveals a cauldron structure and indicates that a part of the sheet intrusion branches off from the ring dike at depth. Our observations argue against the previous interpretation that the Kumano Granite Porphyry formed as a gigantic lava lake body, instead we suggest a new view in which voluminous magma intrusions in the form of a ring dike-subhorizontal sheet interconnection are likely to be an intriguing feature of the volcano-plutonic complex at depth.
