AbstractThe paragenetic relations of epidote‐glaucophane schists are described in terms of the system Al2O3‐Fe2O3‐Fe2O3‐MgO‐CaO with excess of quartz, albite and epidote. If alkali‐amphibole is free from Ca and AlIV, its composition when associated with epidote is invariant, univariant or divariant at a given pressure and temperature on Miyashiro's (1957) diagram of alkali‐amphibole solid solution if it is also associated, respectively, with three, two or one additional minerals in the system.Using a group of epidote‐glaucophane schists from the Kotu area of the Sanbagawa metamor‐phic belt in Shlkoku, Japan (isophysical compositional),univariant boundary lines were determined for the assemblages that, in addition to the ubiquitous quartz + albite + phengitic mica, contain hematite + chlorite, garnet + chlorite and actinolite + chlorite, respectively. The slopes of the univariant boundary lines obtained from petrographical data are in good agreement with those calculated in a model system.The positions of isophysical univariant boundary lines on the amphibole compositional diagram serve to distinguish the grade of metamorphism among the rocks of the same mineral facies. The hematite‐chlorite univariant boundary line can be used to divide the zone of epidote‐glaucophane schists of the Sanbagawa metamorphic belt into three, and the garnet‐chlorite‐paragonite invariant equilibrium can be used to divide the epidote zone of