Melting of amphibolites and the form of melt-trap in amphibolite-migmatites of the southern Hidaka metamorphic belt, Hokkaido, Japan

Abstract

Physical and chemical processes of partial melting and melt separation have been described based on the petrographical studies of amphibolite-migmatites of the Hidaka metamorphic belt. Melting texture is represented by the crystallization of idiomorphic plagioclase with oscillatory zoned structure. Amphibolites of zones D (low grade part of the granulite facies) and F (high grade part of the granulite facies) had melted and the melt had crystallized as leucosome. Meltpocket and stromatic leucosome are common in basic migmatites of zone D, and leucocratic crosscut veins are developed in zone F. Zone F amphibolites are higher in the volume percent of leucosome than zone D amphibolites, which indicates that the degree of melting of zone F amphibolites was larger than that of zone D amphibolites. Melting reactions took place in a state of disequilibrium, because the restitic plagioclases were heterogeneously dissolved in composition. Melt was trapped in the grain-boundary as a micro-network. As melt-volume had increased, a form of melt-reservoir changed from a chain of melt-pockets through stromatic leucosome to leucocratic vein in basic migmatites.
     Plagioclases which crystallized from melt have two types of zonal structure. One has a core of seed-crystal which was derived from a restitic plagioclase. The other has a complicated growth zoning which is due to the multi-stages of crystallization and a dissolution-texture. The plagioclase composition becomes sodic from core to rim in general. Melt composition had changed many times as often as trapped melts linked each together. Leucosome has experienced the crystallization-differentiation and the segregation of melt in the disequilibrium state.