斜長石累帯構造が示すマグマ溜まりの分化過程

抄録

Relationships between whole rock composition of magma and plagioclase zoning were investigated on volcanic rocks from the Shirahama Group, Izu Peninsula, Japan, which have the various chemical compositions of tholeiitic series produced only by crystallization differentiation. The plagioclase zoning can be divided into the following three zoned regions, (1) oscillatory-zoned, (2) patchy-zoned, and (3) unzoned regions, based on Anderson(1984)’s classification. We described the flatness of zones and the spatial distribution of zoned regions from core to rim of the phenocrysts, and found the relations between the characteristics of plagioclase zoning and the whole rock compositions. As the whole rock SiO₂ increases, (1) the number of resorption zones, defined as zones with resorbed track, increases; (2) the frequency of plagioclase with patchy-zoned region decreases; (3) the frequency of plagioclase with the patchy-zoned region in central parts of crystals increases; (4) the diversity in the zoning pattern becomes smaller. In basalts, various zoning patterns can be observed whereas in dacite, oscillatory zoned regions are dominant. Our observation can be explained by a simple model involving the homogenization processes of heterogeneity in a magma chamber, associating with the crystal growth or dissolution processes. According to the model and the observations of natural plagioclase zoning, we can give a constraint on the characteristics of homogenization process developed in an evolving magma chamber: parameters of the characteristics φ, the volume ratio of relatively differentiated part in the magma to homogenize and r. chemical contrast of the magma. At the initial stage (basalt magma), the homogenization process with large φand large γ is dominant. At the later stage (dacite magma), the homogenization process with largeφ or smallγ is dominant.