Thermodynamic consideration of equilibria among garnet, biotite, plagioclase, and quartz enables us to estimate pressure-temperature conditions of pelitic and siliceous rocks occurring in a wide range of metamorphic grades, as a difference from the condition of a reference sample. Application of this technique to the Ryoke metamorphic rocks in the Yanai district provides the thermobaric structure of the area in more detail than was available before.
The Gibbs method applied to a low-grade siliceous rock of the area yields a decompressional pressure-temperature path. The validity of the result is still ambiguous and should be evaluated petrographically in terms of constancy of mineral assemblage during garnet growth.
