A drastic drop of whole-rock boron contents is observed across the tourmaline-out isograd located near the schist-migmatite boundary of the Aoyama area, Ryoke metamorphic belt, southwestern Japan. The variation in whole-rock boron contents corresponds well with the modal proportion of tourmaline; most of the boron is contained in tourmaline in the upper-amphibolitefacies metasedimentary rocks. The drastic drop in whole-rock boron contents, from 10-79 to <5 ppm across the tourmaline-out isograd, may be caused by the breakdown of tourmaline accompanied by the extraction of a small amount of boron-bearing melts or fluids from the migmatite zone. Tourmaline-bearing pegmatite veins observed to the north of the tourmaline-out isograd are one of the important sinks of boron considered to have been extracted from the migmatite zone. The behavior of boron in muscovite and tourmaline is modeled on the P-T path of the Aoyama area. Near the P-T conditions of its breakdown, the X ⅪAlNa-1 Mg-1 exchange is insignificant in tourmaline, whereas the Al 2 Mg-1 Si-1 , AlOMg-1 (OH)-1 and CaMg 2 X Ⅺ-1 Y Al-2 or CaMgO X Ⅺ-1 Y Al-1 (OH)-1 exchanges are important. The amount of the X-site vacancy is controlled by the incorporation of Ca into tourmaline.