Tourmaline and boron as indicators of the presence, segregation and extraction of melt in pelitic migmatites: examples from the Ryoke metamorphic belt, SW Japan

Abstract

The mode of occurrence of borosilicates and the breakdown fronts of prograde tourmaline (tourmaline-out isograd) in three anatectic migmatite regions of the Ryoke metamorphic belt, SW Japan, are reported. The breakdown of tourmaline in the migmatite zones and release of boron into the melts, followed by the extraction of the boron-bearing melts from the migmatite zones occurred throughout the Ryoke metamorphic belt. Retrograde, magmatic tourmaline in interboudin partitions filled with leucosome is useful for calculating the degree of partial melting in the migmatites. Using boron contents in the leucosomes and pelitic schists, the degree of partial melting at the migmatite front of the Aoyama area is estimated to be 12 wt.%. Extraction of the boron-bearing melt is suggested by the boron-depleted nature of the migmatites. Connection of boudinage structures probably supplied the vertical pathways of the segregated melts, and major transport of the melts was accomplished by dyking. Irregularly shaped, amoeboid tourmaline locally occurs on the high-temperature side of the tourmaline-out isograds in the Yanai and Komagane areas, implying incomplete extraction of boron-bearing melts from those areas. Discriminating retrograde from prograde tourmaline enables correct recognition of the tourmaline-out isograd. The amount of retrograde tourmaline in migmatites can potentially be used as an indicator of the degree of melt extraction from them.