火山-深成複合岩体にみられる環状岩脈とシート状貫入岩 : 紀伊半島,尾鷲-熊野地域の熊野酸性火成岩類の地質(<特集>紀伊半島における中新世火成作用とテクトニクス)  [in Japanese] A ring dike and a subhorizontal sheet intrusion in a volcano-plutonic complex : geology of the Kumano Acidic Rocks in the Owase-Kumano area, Kii Peninsula, Japan(<Special Issue>Miocene igneous activity and tectonics in the Kii Peninsula)  [in Japanese]

紀伊半島の尾鷲-熊野地域に分布する熊野酸性火成岩類(熊野酸性岩)について詳細な地質調査を実施し,花崗斑岩岩体の産状と定置様式を検討した.調査地の熊野酸性岩は流紋岩溶岩(神ノ木流紋岩),流紋岩質火砕岩類(火砕流起源と貫入起源),花崗斑岩(熊野花崗斑岩),細粒花崗岩(不動谷花崗岩)よりなる.調査地の熊野酸性岩の大部分を占める熊野花崗斑岩は,その一部がコールドロン構造の中で環状岩脈(幅2km以内)とそこから派生するシート状貫入岩(厚さ最大1km以上)として産することが新たに判明した.シート状貫入岩の天井を調査地北西部の複数の地点で確認した.熊野花崗斑岩は従来言われてきたような溶岩湖が冷却固結した岩体ではなく,巨大な貫入岩体である可能性が高い.環状岩脈とそこから派生したシート状貫入岩という組み合わせは,火山-深成複合岩体の深部における特徴的な構造である可能性がある.

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.