カルクアルカリ岩系とシソ輝石質岩系およびソレアイト岩系とビジョン輝石質岩系の対応関係に関する再検討
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概要
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There are two main rock series in non-alkalic igneous rocks: the calc-alkalic rock series and tholeiitic rock series. The calc-alkalic rock series is characterized by increasing of SiO_2 contents and gradual decreasing of FeO contents (tatal iron as FeO) with changing from mafic rocks toward felsic ones. On the other hand the tholeiitic rock series shows lower degree of SiO_2 enrichment, and rapid increasing of FeO contents and FeO/MgO ratios with changing from mafic rocks toward intermediate ones. Kuno (1950) considered that there are two different rock series in non-alkalic basalt, andesite, dacite and rhyolite in the Izu-Hakone region, central Japan: the hypersthenic rock series (volcanic rocks with groundmass hypersthene) and the pigeonitic rock series (with only clinopyroxenes in the groundmass). Kuno (1965) considered, moreover, that the variation trend in chemistry of the hypersthenic rock series agrees with that of the calc-alkalic rock series and he proposed to confine his hypersthenic and pigeonitic rock series to the calc-alkalic and the tholeiitic rock series respectively. However, the bulk chemical compositions analyzed by the present writer for Miocene volcanic rocks from the Kitakami river basin and the Ryozen district in northeast Japan can not be divided into the distribution fields of the hypersthenic and pigeonitic rock series in any of the diagrams such as AFM, (FeO+Fe_2O_3×0.9)/MgO-SiO_2 and (FeO+Fe_2O_3×0.9)/Mg-(FeO+Fe_2O_3×0.9). The same feature is also recognized in volcanic rocks of several Quaternary volcanoes in the Japanese Islands, such as Osore-yama, Nekoma-dake and Bandai-san belonging to the Nasu volcanic zone, Chokai-san and Iwaki-san in the Chokai volcanic zone and Aso-san in Kirishima volcanic zone. Pliocene Kitamatsuura basalts in northern Kyusyu give the same evidence. Therefore it may be not valid to regard that the Kuno's hypersthenic and pigeonitic rock series correspond respectively, to the calc-alkalic and tholeiitic rock series. In the present paper, it shuold be emphasized that the calc-alkalic rock series is not clearly distinguishable from the tholeiitic rock series by the above three diagrams and the petrogenetical implications on the distinction between the two rock series are discussed.
- 地学団体研究会の論文
- 1982-07-25
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