Geothermometry, Geobarometry and T-X(Fe-Mg) Relations in Metapelites, Snow Peak, Northern Idaho

The most recent of two metamorphic events (M2) in the Snow Peak area caused progressive changes in mineral parageneses in pelitic rocks ranging from chlorite-biotite to kyanite grade. Systematic partitioning of elements between coexisting phases indicates a close approach to equilibrium during M2. Temperature estimates for M2 range from 440 °C in the chlorite-biotite zone to 565 °C in the kyanite zone. Coexistence of kyanite, garnet, ilmenite, and quartz places an upper pressure limit of approximately 60 kb, and an upper temperature limit at the kyanite-sillimanite boundary. Equilibrium of garnet, kyanite, plagioclase, and quartz indicates that total pressure of equilibration of kyanite-bearing assemblages was approximately 6 kb. Pressure estimates based on equilibrium of garnet, muscovite, biotite, and plagioclase indicate a pressure gradient between garnet and lower staurolite zone samples, which equilibrated at approximately 3· 5 kb, and upper staurolite to kyanite zone samples, which equilibrated at ∼ 5· 5 kb. Equilibrium of paragonite component of muscovite with plagioclase, kyanite and quartz, distribution of species in C-O-H fluids in equilibrium with graphite, and the presence of zoisite in adjacent calc-silicate rocks indicate that the metamorphic fluid in kyanite-bearing assemblages contained 65-90 mole per cent H₂O. However, the experimentally calibrated equilibrium of staurolite, quartz, garnet, and kyanite can be reconciled with estimated temperature only if X_H2O in the fluid was very low (∼ 33 mole per cent). T-X(Fe-Mg) relations among chlorite, biotite, garnet, staurolite, kyanite, muscovite and quartz are calculated at 6 kb on the basis of 3 independent Fe-Mg exchange equilibria: garnet-biotite, chlorite-biotite (empirical, this study), garnet-staurolite (empirical, this study), and three independent net transfer equilibria. Alternative sets of data for Mg-chlorite and Fe-staurolite are evaluated by comparing observed and calculated changes in mineral paragenesis and mineral composition with grade. Chlorite data from Helgeson et al., 1978 give T-X(Fe-Mg) relations consistent with trends observed in these rocks, whereas data derived from breakdown of clinochlore and clinochlore + dolomite do not. Calculation of T-X(Fe-Mg) relations consistent with observations requires lower values of $ΔHrxno$ and $ΔSrxno$ than those consistent with experiments on the breakdown of staurolite+quartz.