Some anorthite bearing basic volcanic rocks in Japan

Abstract

In the lavas of Kayodake, Komagatake and Iwate volcanic groups of the Nasu volcanic zone in the north-eastern Japan, there are many basalts and basic andesites which contain much large plagioclase phenocrysts (up to 2cm.) of anorthitic composition (An 95). Particularly the lowest lava of the Kayodake volcanic group contains plagioclase phenocrysts more than 43 per cent in volume; therefore its A1203 and CaO contents attain to 23.74 and 12.34 per cent, respectively. Its chemical composition resembles to that of anorthositic gabbro or kenningite. Regarding to the A1203, this is probably highest value ever reported in the world in the effusive form. It is considered that the origin of such lavas of abnormal composition should be attributed to the floating up of anorthite crystals and to the sinking down of olivine and pyroxene crystals formed in the early stage.