Chemical petrology of some pelitic gneisses and migmatites from the Thor-Odin area, British Columbia

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 reaction[Formula: see text]This reaction is used as a measure of [Formula: see text] conditions. Variations of biotite and garnet composition from rock to rock indicate differences in [Formula: see text] conditions of quenching. Furthermore, according to the ranges in composition, the migmatites from the core zone reflect either a higher [Formula: see text] at constant T or a lower T at constant [Formula: see text] than the mantling gneisses. Assuming that [Formula: see text] is approximately equal to Ptotal during metamorphism of pelitic rocks, the second case is more probable. The [Formula: see text] 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.