難波山堆積・構造区における火成活動 : 特にhypabyssal-extrusive associationについて

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The Nanbayama Nepto-Tectonic Province is characterized by the wide developmment of the Neogene Tertiary nepton which is more than 6,000 m thick, the flysch-type sedimentary facies (Nanbayama facies) in the lower part of it, abundant sheet-form intrusives of porphyrite, and poor developmment of green tuff facies in general, except in the area along R. Hime, and the Pliocene hypabyssal-extrusive association. The Nanbayama facies is developed with in the wide area extending about 50 km in the N-S direction and about 70 km in the E-W direction, its eastern boundary being defined by the inferred Kashiwazaki-Choshi Tectonic Line. Pre-Tertiary basement rocks of the province are inferred to be the northern extension of the Hida Marginal Tectonic Zone. The folding pattern of the Nanbayama Tectonic Province dispalys two distinct anticlines with the axes running in the NNE trend, which are slightly oblique to the general trend of the adjacent Central Folding Zone running in the NE trend. The Neogene System of the Nanbayama Nepton is divided into the following formations in ascending order, the Hiuchiyama, the Nishihiyama, the Nohdani, the Kawazume, the Nadachi and the Tanihama Formations. The foraminiferal fossils indicate that these Formations are correlative with that of the standard strtigraphic units of the Niigata Tertiary Basin as follows. Lower Hiuchiyama=Upper Tsugawa and Nanatani F.(N. 10); Upper and Middle Hiuchiyama to Nishihiyama F.=Lower Teradomari F.; Lower Nohdani F.:=Lower Teradomari F.=Upper Nohdani F. to Kawazume F.=Shiiya F; Nadachi F.=Nishiyama F.; Tanihama F.=Haizume to Tsukayama F. The Amakazariyama igneous massif is not composed of a single intrusive body, but of composite intrusives consisting of porphyrite, rhyolite, andesite, basalt and dacite in order of emplacement. Some andesite intrusives occurring around Mt. Amakazari contain large hornblende megacrysts. In connection with these intrusives the reoccur dikes of porphyrite, two-pyroxene andesite, hornblende-two pyroxene andesite, biotite-two pyroxene andesite, basalt, garnet dacite and rhyolite in the upper Miocene Nochi Formation and the lower part of Umidani andesite extrusive formation distributed to northeast of the Mt. Amakazariyama. These dike rocks are very similar to each other in lithology and petrochemistry. The Umikawa Pliocene volcanic Formation consists of thick calc-alkaline andesitic pyroclastics, intercalating thin gray mudstone. Those andesitic extrusives display distinctive cyclic changes of lithology. Each final andesite is characterized by the ocurrence of hornblende megacryst and hornblende gabbro inclusions. Petrochemically each cycle shows regular changes of principal chemical compositions. The Umikawa extrusives, the Amakazariyama intrusives and allied dike rocks are comagmatic, lying on the same trend of chemical variation (F-M-A diagram) and having high contents of potash. These extrusives and intrusives show quite different mode of occurrence, but belong to the same magmatism. The authors propose to call as hypabyssal-extrusive association. The Hokodake intrusives and the Eboshiyama extrusives occurring to the north of Mt. Amakazariyama possibly belong to the association.
日本地質学会の論文
1984-11-30

難波山堆積・構造区における火成活動 : 特にhypabyssal-extrusive associationについて

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