CRUSTAL ANATEXIS AND MELT MIGRATIONS IN THE HIGO METAMORPHIC TERRANE,WEST-CENTRAL KYUSHU, KUMAMOTO, JAPAN

This paper deals with the anatectic migmatites and leucogranites devel oped in a high-T, low-P metamorphic terrane, Kyushu, Japan. The migmat ites in this terrane are divided into metatexites and diatexites and t hey occur at and above the garnet-cordierite grade. Layers, lenses and pods of leucogranite (a few centimeters up to 6 m in thickness) are w ide-spread in the metamorphic terrane and are most abundant, not in th e highest grade zone, but in intermediate grade zones, in which migmat ites do not develop. The migmatite leucosomes are typically depleted i n orthoclase component, while many leucogranites contain abundant orth oclase. It is suggested that the leucogranites are crystal cumulates p recipitated from peraluminous melts, that had been generated and ascen ded from deeper levels in the crust and that the zone of metatectic mi gmatites represents both the source region and passage region for the anatectic melts. It is thus clear that the anatectic melts had segrega ted and ascended on the order of 5 km in the crust of this region. Dia texites on the other hand represent solid-melt mixtures at more advanc ed stages of partial melting, dominated by biotite breakdown. Implicat ions of the observations made in the Higo terrane are discussed in ref erence to an evolutionary scheme for crustal anatexis and granite magm a genesis.