岡山市北西部, 日近花崗閃緑岩体の岩石記載および全岩化学組成

概要

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The Hijikai granodioritic complex is a stock-like zoned pluton that has been intruded by Cretaceous Hiroshima granites of the San’yo Belt. The complex is compositionally zoned from a mafic rim to a more felsic core, and is divided into following four rock types: tonalite, granodiorite, hornblende biotite granite and biotite granite. The tonalite changes gradually to hornblende biotite granite via granodiorite, while the biotite granite intrudes into the hornblende biotite granite. Continuous compositional trends of all types of the granitoids on Harker variation diagrams suggest that the granitoids were formed by crystal differentiation from a magma. The variations of Rb and Sr concentrations imply the change in fractionation phases from the assemblage of hornblende and plagioclase to that of K-feldspar and plagioclase during crystallization. The differences in the compositional trends of Na2O, K2O, Rb and Sr between the Hijikai gnanitoids and the Hiroshima granites suggest that both were derived from different magma sources. Two stock-like zoned plutons, Ukan and Myoken-zan, are distributed near the Hijikai complex. The compositional similarity between the Hijikai and the Ukan granitoids suggests that both were derived from similar magma sources, while the Myoken-zan granitoids have different compositional trend of MgO and Na2O compared with them.
日本鉱物科学会の論文
2004-03-30