Orthopyroxene-cordierite mafic gneiss from the Nomamisaki metamorphic rocks, Southern Kyushu, Japan: Implication for western continuation of the Usuki-Yatsushiro Tectonic Line

We describe an orthopyroxene-cordierite mafic gneiss from the Nomamisaki metamorphic rocks in the Noma Peninsula, southern Kyushu, Japan. The mineral assemblage of the gneiss is orthopyroxene, cordierite, biotite, plagioclase, and ilmenite. Thermometry based on the Fe-Mg exchange reaction between orthopyroxene and biotite yields a peak metamorphic temperature of 680°C. The stability of cordierite relative to garnet, quartz, and sillimanite defines the upper limit of the peak metamorphic pressure as 4.4 kbar. These features indicate that the Nomamisaki metamorphic rocks underwent low-pressure high-temperature type metamorphism. Although a chronological problem still remains, the Nomamisaki metamorphic rocks can be regarded as a western continuation of the Higo Belt. The Usuki-Yatsushiro Tectonic Line, which delineates the southern border of the Higo Belt, is therefore located on the east of the Nomamisaki metamorphic rocks in southern Kyushu.