六甲山地東南部の白亜紀火砕岩火道

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Two volcanic vents presumed Cretaceous age, which would feed ash-flows and lava flows, are found in southeast part of Mt. Rokko, western Honshu, Japan. The Konan vent breccia, the older body with NW-SE trend, 5 km in length and 1 km in width, intruded into so-called Paleozoic sedimentary formation, and was intruded by Okamoto and Rokko granites (Fig.1). It consists of accidental lithic fragments up to 30 cm in diameter, elongated lens-shaped essential fragments, crystal fragments, and matrix materials (Plate 1). Accessory lithic fragments consist of andesite, dacite, and welded tuff, and those of accidental lithic fragments consist of sandstone and mudstone. The Konan vent breccia was metamorphosed and silicified due to later intrusion of Okamoto and Rokko granites (Fig.5). Plastic deformation and convolute folding of elongated essential fragments and matrix materials are common and most of the deformed essential fragments are parallel to the outline of lithic fragments (Plate 1, Fig. 4). The existence of such structures suggests the welding and the secondary flow of the Konan vent breccia. Deformation of the essential fragments and abration of the lithic fragments are intense as compared with usual subaerial welded and secondary-flowed ash-flow, especilly near the contact with surrounding Paleozoic sedimentary formation. Consequently, the Konan vent breccia is considered to be welded and secondary-flowed vent-filling tuff breccia. The Sumiyoshigawa acidic rock body, the younger one, consists of four members namely Uchikoshiyama vent lappili tuff, Higashitani vent tuff, Habudani vent lava, and Kuroiwadani granite porphyry, in order of the eruption. This body extends towards N30°W, has 4.5km in length, and spreads upwards (Fig.8). This body intruded into Paleozoic sedimentary formation, Konan vent breccia, and Okamoto and Rokko granites (Fig.1). The Uchikoshiyama body consists of accessory and accidental lithic fragments, crystal fragments, and glass shards (Plate 2-a). Eutaxitic texture is often obserbed. Accessory and accidental lithic fragments consists of sedimentary rocks, granitic rocks, and volcanic rocks, especially welded tuff. The Higashitani body is well sorted crystal tuff with welded texture (Plate 2-c). Crystals in this body are less fragmented than those of the Uchikoshiyama body (Plate 2-a, 2-b). This suggests the possibility of less explosive eruption. The Habudani body shows relatively coares-grained and porphyritic texture with pinkish potash-feldspar (Plate 2-d). The absence of glassy chilled contact between this body and the Higashitani body suggests that the former intruded immediately after the formation of the latter. The Kuroiwadani body shows scarcely-phyric and microgranitic texture (Plate 2-e). The groundmass becomes finer-grained towards the margin and the shallower portion of the body (Fig. 10). Tilting towards 15°ESE of this body since the intrusion is measured from the analysis of the cooling joints (Fig. 11). The absense of glassy chilled contact between this body and the Higashitani body suggests that the former intruded immediately after the formation of the latter (Fig. 7). Consequently, present-day exposure of the Sumiyoshigawa acidic rock body is considered to be the shallower part of the feeder of an acidic volcanic complex. The activity, which started with explosive eruptions, progressively changed to more effusive ones.
特定非営利活動法人日本火山学会の論文
1979-07-01

六甲山地東南部の白亜紀火砕岩火道

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