27. Chemical Composition of Japanese Granites : Part 1. Variation Trends of 400 Analyses

Abstract

About 400 superior-quality analyses of Japanese granitic rocks are plotted in THOKNTON and TUTTLE'S variation diagrams and projections in the Q-Or-Ab-An system. In the variation diagrams, concentration of points is fairly good, forming one broad but well-defined petrographic province. As compared with WASHINGTON'S 5000 igneous rocks, Japanese granites are slightly higher in SiO2 and lower in Fe2O3, Na2O, and K2O. Most of the analyses show considerably high amounts of An when plotted in the Q-Or-Ab-An tetrahedron. Normative compositions of Paleozoic Japanese pelitic sediments are very low in An. It is not possible to form most of the Japanese granitic liquids by partial or total fusion of such pelitic sediments.|服部・野沢(1959)のまとめた日本の花崗岩質岩石の分析値のうち,適当なもの約400個をTHORNTON・TUTTLEの変化図やQ-Or-Ab-An系に投影してみた.変化図においては,点は直線上に集中し,分散は比較的小さく,全休として1つの特徴的な岩石区を示す.WASHINGTONの5000個の火成岩の変化図と比較すると,日本の花崗岩類は,SiO2にやや富み,Fe2O3,FeO,Na2O,K2Oにやや乏しい.花崗岩質成分をよく近似するQ-Or-Ab-Anの4面体に投影すると,点は可成分散するが,An成分に著しく富むものが多い.また点はOr側よりもAn側に偏つて集中する.日本の古生層の粘土質堆積岩の組成を同じ4面体に投影すると,An成分に乏しいのが特徴である.従つて,このような粘土質堆積岩の部分溶融または全溶融によつては,日本の花崗岩質マグマは生じない