Staurolite-bearing sillimanite schist cobble from the Upper Jurassic Tetori Group in the Kuzuryu area, Hida Mountains, central Japan


A cobble of staurolite-bearing sillimanite schist is found from a conglomerate boulder on the Itoshiro River floor along which the middle part of the Upper Jurassic-Cretaceous Tetori Group is exposed. This is the first report of a staurolite schist cobble from the Tetori Group, and the rock assemblage of the conglomerate indicates that the staurolite schist has · been exposed with quartzite, sandstone, granite, gneiss, and marble in the source area of the Tetori Group in the late Jurassic time. The cobble is chemically different from the other staurolite-bearing rocks in the Hida Mountains in view of its extremely high Al2O3 (25 wt.%), low FeO* (total Fe as FeO) (1.7 wt.%) and low MgO (0.5 wt.%) contents in the bulk rock composition. The absence of cordierite and primary chlorite, and stable sillimanite-staurolite-biotite-quartz-muscovite assemblage in this high-Al metapelite indicate the condition of the upper amphibolite facies (P < 8.5 kbar, T < 720°C), which corresponds to the high-grade part of the Unazuki metamorphic belt rather than that of the Hida metamorphic belt.

地質学雑誌 (Journal of the Geological Society of Japan), v. 101, no. 12, p. 971-977, doi:10.5575/geosoc.101.971