Metamorphism and Structure of Penfold Creek Area, near Quesnel Lake, British Columbia

The Quesnel Lake area lies within the Omineca Crystalline Belt,and is underlain by the northern extremity of the Shuswap MetamorphicComplex. Closely spaced and steeply dipping isograds mark themargins of the metamorphic belt. In the Penfold Creek area onlyone and one-half miles separate the biotite and sillimaniteisograds. Related to this sharp increase in metamorphic gradethere is a marked change in the fold style. In the chloritezone are similar folds, showing a strong axial-plane cleavage,and tight refolded isoclines dominate in the sillimanite zone.Three periods of deformation and two periods of prograde metamorphismhave been recognized, with the first metamorphic period beingassociated with Phase 2 deformation and the second being post-Phase2. Regression analysis of possible mineral reactions in these rocksshows that they formed under conditions of a gradient in temperatureand possibly H₂O activity, and that most of the assemblages,although containing many phases, have at least two degrees offreedom. Thermodynamic analysis of mineral equilibria indicatesthat equilibrium was closely approached between solid solutionsinvolving garnet, plagioclase, biotite, white mica, and staurolitein the presence of Al₂SiO₅, quartz, and H₂O. The conditionsof metamorphism of rocks containing Al₂SiO₅ are estimated tobe 7000 ± 1500 bars, 680 ± 30 °C, activityof H₂O = 0.80 (approx.). Partial melting did not occur becauseof the reduced activity of H₂O. ^*Present address: Overseas Division, Institute of Geological Sciences, 5 Princes Gate, London SW7, England.