Crustal anatexis and melt migrations in the Higo metamorphic terrane, west-central Kyushu, Kumamoto, Japan

Abstract

This paper deals with the anatectic migmatites and leucogranites developed in a high- T, low- P metamorphic terrane, Kyushu, Japan. The migmatites in this terrane are divided into metatexites and diatexites and they occur at and above the garnet-cordierite grade. Layers, lenses and pods of leucogranite (a few centimeters up to 6 m in thickness) are wide-spread in the metamorphic terrane and are most abundant, not in the highest grade zone, but in intermediate grade zones, in which migmatites do not develop. The migmatite leucosomes are typically depleted in orthoclase component, while many leucogranites contain abundant orthoclase. It is suggested that the leucogranites are crystal cumulates precipitated from peraluminous melts, that had been generated and ascended from deeper levels in the crust and that the zone of metatectic migmatites represents both the source region and passage region for the anatectic melts. It is thus clear that the anatectic melts had segregated and ascended on the order of 5 km in the crust of this region. Diatexites on the other hand represent solid-melt mixtures at more advanced stages of partial melting, dominated by biotite breakdown. Implications of the observations made in the Higo terrane are discussed in reference to an evolutionary scheme for crustal anatexis and granite magma genesis.