The Shuswap metamorphic terrain of British Columbia is marked by the occurrence of gneiss domes, one of these in the Thor-Odin area. In this case, a core of migmatites and granitoid gneisses is surrounded by a mantle of unmigmatized gneisses. Rocks of both structural zones belong to the sillimanite – almandine – orthoclase subfacies of the almandine–amphibolite facies. A common mineral assemblage in pelitic gneisses and migmatites is quartz + plagioclase + K feldspar + biotite + garnet + sillimanite. These minerals are related by the reactionThis reaction is used as a measure of conditions. Variations of biotite and garnet composition from rock to rock indicate differences in conditions of quenching. Furthermore, according to the ranges in composition, the migmatites from the core zone reflect either a higher at constant T or a lower T at constant than the mantling gneisses. Assuming that is approximately equal to Ptotal during metamorphism of pelitic rocks, the second case is more probable. The values, rather than representing peak metamorphic conditions, are attributed to quenching during cooling when no more water is available. A higher water content in the core rocks might account for migmatization as well as for a somewhat lower quench temperature.