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 Metamorphic Complex. Closely spaced and steeply dipping isograds mark the margins of the metamorphic belt. In the Penfold Creek area only one and one-half miles separate the biotite and sillimanite isograds. Related to this sharp increase in metamorphic grade there is a marked change in the fold style. In the chlorite zone 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 metamorphism have been recognized, with the first metamorphic period being associated with Phase 2 deformation and the second being post-Phase 2.Regression analysis of possible mineral reactions in these rocks shows that they formed under conditions of a gradient in temperature and possibly H2O activity, and that most of the assemblages, although containing many phases, have at least two degrees of freedom. Thermodynamic analysis of mineral equilibria indicates that equilibrium was closely approached between solid solutions involving garnet, plagioclase, biotite, white mica, and staurolite in the presence of Al2SiO5, quartz, and H2O. The conditions of metamorphism of rocks containing Al2SiO5 are estimated to be 7000 ± 1500 bars, 680 ± 30 °C, activity of H2O = 0.80 (approx.). Partial melting did not occur because of the reduced activity of H2O. © 1979 Oxford University Press.