上野玄武岩類Ｉ： ２つの単成火山における不均質なマグマ

Abstract

Sakashita and Suzuran are two largest monogenetic volcanoes belonging to the Ueno basaltic rocks of Pliocene to Pleistocene age in Gifu Prefecture, central Japan. Bulk and mineral composi-tions of lavas from these two volcanoes are determined. They show petrographically and chemically remarkable heterogeneities. They are composed of (augite-) olivine basalt to basaltic andesite with 50.3-55.2 and 48.4-53.9 wt% SiO2, respectively. Their compositions construct nearly straight trend lines in variation diagrams. Three Fe-Mg zoning types of olivine are recognized. Type A is characterized by a nearly homogeneous broad core (Fo82-88) and a highly-zoned narrow rim. Type C is unzoned from core to rim (Fo64-77). Type B is normally zoned, an intermediate type between A and C. Core compositions of olivine phenocrysts with type A zoning are interpreted to be not affected by crystal-melt diffusion. Those with type C zoning result from obliteration of all zoning by diffusion. Chemical compositions of phenocrystic minerals indicate that olivine began to crystal-lize first in a stable magmatic environment, while existing phenocrysts of augite and plagioclase nucleated and grew later at lower temperatures than olivine. Mixing calculations using the major-element compositions reveal that the chemical heter-ogeneities are caused by fractional crystallization of olivine, augite, plagioclase and Ti-magnetite, with a significant amount of accompanying assimilation of low-potassium acidic rocks. Based on the trace-element chemistry, however, the assimilant is requested to have peculiar chemical compositions enriched in Th, Sr, Ba, Zr, Nb etc., but highly depleted in K and Rb compared with normal acidic rocks distributed in the vicinity of the Ueno basaltic rocks.