BORON DEPLETION ACCOMPANIED BY THE BREAKDOWN OF TOURMALINE IN THE MIGMATITE-ZONE OF THE AOYAMA AREA, RYOKE METAMORPHIC BELT, SW JAPAN: AN IMPLICATION FOR THE FORMATION OF TOURMALINE LEUCOGRANITES

A sharp line delimitating the distribution of Tur (Tur-out isograd) can be defined in the schist-migmatite transitional zone of the Aoyama area, Ryoke metamorphic belt, SW Japan. Pelitic-psammitic schists in this area contain Tur while migmatites do not. The Tur-out isograd is almost parallel to the regional, Grt-Crd isograd, suggesting that it represents the breakdown front of Tur during prograde metamorphism. This is confirmed by the presence of breakdown texture of Tur to Sil and Crd. This texture is contained in the pelitic-psammitic schist with leucosome-filling interboudin partitions found near from the Tur-out isograd. The leucosome in the interboudin partitions contains euhedral Tur crystals while the melanosome in the intervening matrix contains Tur with the breakdown texture. These observations imply that breakdown of Tur is a melting reaction and that the B in the leucosome is derived from the breakdown of Tur in the melanosome.

The whole-rock B contents is determined by PGNAA. The result shows that rocks containing Tur show high B contents (17-79ppm) and rocks without Tur show low B contents (<5ppm). Point counting of Tur and mass balance calculations for the whole-rock B showed that almost all B is contained in Tur. These observations indicate that Tur becomes an important sink of B and its stability controls the behavior of B in Tur-bearing upper amphibolite facies pelitic-psammitic rocks.

Since muscovite (Ms) is an important sink of B in the medium-grade metamorphic rocks, behavior of Ms is also important to understand the behavior of B. The migmatite formation in the Aoyama area may be controlled by dehydration melting of Ms as inferred from the P-T conditions estimated. Ms absence, Tur presence and the schist-dominating nature of the Sil-Kfs zone rocks may be explained by the subsolidus breakdown of Ms accompanied by the Tur formation. On the other hand, Ms absence, Tur absence and the migmatite-dominating nature of the Grt-Crd zone rocks may be explained by the dehydration melting of Ms, followed by the breakdown of Tur.

The very low whole-rock B contents throughout the migmatite-zone suggests that B was effectively removed from this region by the movement of melt and/or fluid. The B extracted form the migmatite-zone may now partly be observed as Tur crystals in the pegmatite veins to the north of the Tur-out isograd.