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

抄録

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 (Fo_82-88) and a highly-zoned narrow rim. Type C is unzoned from core to rim (Fo_64-77). 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.