Note on the Graphite-Free Corundum-K feldspar Gneiss in the Hida Metamorphic Belt, Central Japan

The association of corundum-K feldspar in a graphite-free rock is newly found in the western portion of the Hida Metamorphic Belt, central Japan. The equilibrium assemblage of the rock is corundum-K feldspar-plagioclase-biotite-apatite-zircon-ore with no graphite. Ore minerals are characterized by the abundance of rutile and by the absence of ilmenite and/or magnetite. Corundum and K feldspar had been in direct contact with each other before the formation of kelyphitic rims of muscovite aggregates around corundum crystals. The association of corundum-K feldspar in the graphite-free system must suggest the formation by the breakdown of muscovite under the condition when PH2O is not so low. Therefore, the breakdown must occur more or less near the maximum temperature expected. The metamorphic conditions have been estimated by the stable associations of zincian spinel-garnet-biotite-sillimanite-K feldspar-plagioclase and clinopyroxene-garnet in the metamorphites from the area to be around 700°C and 6 kb, which are not inconsistent with the condition deduced from the association now discussed.