岩石鉱物鉱床学会誌

広島県東南部における万成型花崗岩と流紋岩類の岩石学的特徴とその相互関係

友成才

1976 年 71 巻 2 号 p. 37-43
発行日: 1976/02/05
公開日: 2008/08/07

DOIhttps://doi.org/10.2465/ganko1941.71.37

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On the southeast of Hiroshima Prefecture, several kinds of granitic rocks and rhyolites belonging to the Upper Mesozoic are widely distributed.
The writer has classified the granities into four types according to their petrological features and geological relations, and has distinguished three bodies of rhyolites, namely,
granites: (1) the Mannari type (2) the Ashida type (3) the Hirotani type (4) the Mikawa type
rhyolites: (1) the Toyosaka rhyolite (2) the Joge rhyolite (3) the porphylitic rhyolite
Among these bodies, it has been made clear that the Mannari type granite is the oldest one and older than the rhyolites as well. This fact indicates the presence of granitic activity before the eruption of at least some members of the so-called “Takata rhyolite”.

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阿蘇カルデラ内の偽枕状溶岩

渡辺一徳, 勝井義雄

1976 年 71 巻 2 号 p. 44-49
発行日: 1976/02/05
公開日: 2008/08/07

DOIhttps://doi.org/10.2465/ganko1941.71.44

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阿蘇カルデラ内北部の本塚・北塚・灰塚はいずれも石英安山岩の溶岩からなる古い中央火口丘で,それらの下底部には海抜約510mのレベルまで水冷により破砕された溶岩が発達している。故にこの水中溶岩が噴出した時期にはカルデラはこのレベルまで湖であったと推定される。この水中溶岩は大小の岩塊と同質の細片からなり,その中には枕状溶岩に類似した柱状節理をもつ岩塊が含まれている。この種の岩塊は多くの点で通常の枕状溶岩とは異なっているので,偽枕状溶岩と呼ぶことにする。その産状の詳細な観察によれば,偽枕状溶岩は,水中を前進中の粘性の高い溶岩に湾曲した割れ目がはいり,これに沿って侵入した水で急冷され,分離したものである。このため偽枕状溶岩は表面に垂直な柱状節理を生じているが,通常の枕状溶岩とちがって外形はもとの流理構造を明瞭に切っている。

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群馬県南西部の秋間層,相間川層および剣ケ峰層中の火山岩類の岩石学的研究

磯貝基一, 周藤賢治, 富樫敬吾

1976 年 71 巻 2 号 p. 50-59
発行日: 1976/02/05
公開日: 2008/08/07

DOIhttps://doi.org/10.2465/ganko1941.71.50

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The Akima, Aima-gawa and Kennomine formations of the Neogene system are distributed between the Quaternary Asama and Haruna volcanoes, which occupy the south-western part of Gunma Prefecture. The Pliocene Kennomine formation overlies unconformably the Akima and Aima-gawa formations of the Upper Miocene time (Akima Collaborative Research Group, 1975).
These formations mainly consist of pyroclastic rocks and volcanic rocks which occur in the form of lava flows and intrusive masses. The main part of these volcanic rocks is made of two pyroxene andesites of hypersthenic rock series, while a little amount of siliceous basaltic rocks of the same rock series are also observed in the early stage of volcanism in this district.
Twenty representative andesites and basalts are chemically analysed and ⁸⁷Sr/⁸⁶Sr ratios are determined for five andesites. They are characterized by low alkali contents, especially low potassium contents. The AFM diagram, (FeO+0.9×Fe₂O₃)/MgO-SiO₂ diagram and (Na₂O+ K₂O)-SiO₂ diagram show that andesites and basalts correspond to those of calc-alkalic type with low alkali contents and those of low alkali tholeiitic type respectively. These volcanic rocks have low K₂O/Na₂O ratios resembling to those of volcanics of pigeonitic rock series from Izu-Hakone region.
Five andesites have ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7041 to 0.7043 and the average is 0.7042. These ratios are similar to those of volcanic rocks of the L zone recently proposed by Shuto (1974a, 1974b) and Shuto and Kagami (1975).
Considering from the results of the present work, it may be concluded that the genesis of volcanic rocks in the above formations can not be ascribed to the contamination with granitic material in magma and that andesites are derived by fractional crystallization of certain basic andesitic (basaltic andesitic) magma probably originated from the upper mantle material presumably of peridotitic composition.

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