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

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 km or less wide and approximately km in size, is found at the northern end of the mapped area. The subhorizontal sheet intrusion approximately 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. Abstract

Spheroidal weathering of granite porphyry with well‐developed columnar joints by oxidation, iron precipitation, and rindlet exfoliation

“…A large mass of Miocene Kumano Granite Porphyry is distributed in the south‐eastern part of the Kii Peninsula, Japan (Figure ). The granite porphyry crops out over an area of ~240 km 2 ; 85% of the Kumano acidic rocks are intruded into or are partially covered by the Miocene Kumano Group, the Miocene Owase Group, and the Cretaceous‐Paleogene Shimanto Supergroup (Aramaki, ; Miura, ; Kawakami and Hoshi, ). Igneous activity ended at 14.4 ± 0.1 Ma, as determined by biotite K–Ar dating (Sumii et al , ).…”

Instantaneous paleomagnetic record from the Miocene Kozagawa Dike of the Kumano Acidic Rocks, Kii Peninsula, Southwest Japan: cautionary note on tectonic interpretation

“…Igneous activity of the KAR started with the extrusion of rhyolitic magma, which first produced pyroclastic units and, subsequently, an approximately 400‐m‐thick lava flow with a volume of approximately 9 km 3 that is called the Konogi (or Konoki) Rhyolite (Aramaki & Hada ). These rhyolitic eruptions were followed by a caldera‐forming eruption that produced ash flows and intrusions of granite porphyry (Miura ; Kawakami & Hoshi ; Kawakami et al . ).…”

“…). The ash flows produced a series of ignimbrites named the Owase–Shirahama Pyroclastic Rocks (Kawakami & Hoshi ), which reaches a maximum thickness of >300 m. The granite porphyry magma intruded the ignimbrites and underlying strata as dikes and laccolithic bodies (Kumano Granite Porphyry; Kawakami et al . ).…”

Facies architecture, detrital provenance, and tectonic modulation of sedimentation in the Shikoku Basin: Inputs to the Nankai Trough subduction zone

“…Kimura et al, 2014). The near-trench volcano-plutonic complex included I-type granites, S-type granites, rhyolitic lavas, and rhyolitic ash-flow deposits derived from large caldera eruptions (Kawakami and Hoshi, 2007;Miura and Wada, 2007;Shinjoe et al, 2007). Granitic and basaltic magmas were also intruded into accreted sedimentary rocks and forearc-basin deposits along the strike-length of the Shimanto belt.…”