西南日本古生代火山岩の組成の問題

概要

論文の詳細を見る
Chemical analyses of the Upper Paleozoic greenstones from the two selected areas of southwest Japan reveal that the parental magmas from which the original igneous rocks were derived were alkali basalt and tholeiite, but not high alumina basalt. Special attention was paid in selecting specimens for analysis so as to get the rocks that were originally holocrystalline or slightly glassy, with the least amygdules and/or veins filled with secondary minerals. But fundamentally, it is questionable to discuss the nature of magma types on the basis of chemical analyses of metamorphosed rocks such as greenstones. The amounts and ratios of alkalis would have been slightly or strongly changed in the period of cooling of glassy material or during diagenesis and burial metamorphism. Accordingly, it would be better to examine relic minerals, particularly clinopyroxene, because the mineral has been preserved commonly in a fresh state if the metamorphic grade did not reach that of the biotite zone. Preliminary optical study shows that, in some cases, the relic clinopyroxenes have the compositions characteristic of the rock series to which the original rocks would have belonged, although there are also some questionable examples.
日本地質学会の論文
1971-07-31