Staurolite-bearing gneiss and re-examination of metamorphic zonal mapping of the Higo metamorphic terrane in the Kosa area, central Kyushu, Japan

抄録

This paper describes the finding of staurolite-bearing gneiss from the Higo metamorphic terrane and proposes new mineral zones in the Kosa area, Kumamoto Prefecture. Previous mineral zones of the Higo metamorphic terrane proposed by Obata et al. (1994) consist of five zones: Zone A characterized by Chl + Ms, Zone B by Bt + Ms + And, Zone C by Kfs + Sil + Bt, Zone D by Grt + Crd + Bt, and Zone E by Opx in metapelites. They identified three zones from B to D in the Kosa area. However, we found that sillimanite appears together with Grt + Crd, hence this paper shows that Zone C is absent in the Kosa area. New finding of Opx in the southern-most part of the area made it possible to define three mineral zones; Bt zone, Grt-Crd zone, and Opx zone, in the order of increasing grade from north to south in the Kosa area. Analysis of staurolite-bearing assemblage in a KFMASH system together with textural evidence reveals that the following reactions occurred in the staurolite-bearing gneiss in the excess of Kfs, Qtz and H₂O:
[Sil] Str + Bt = Grt + Crd
[Bt] Str = Grt + Crd + Sil
    Chemographic analysis of these reactions together with Grt-Bt geothermometers shows the metamorphic condition of P = 200 MPa and T = 600-620°C, which is much lower in pressure than that estimated by Osanai et al. (1996) in the Toyono area, about 10 km west of Kosa.