Phase Equilibria in Three Assemblages of Kyanite-Zone Pelitic Schists, Lincoln Mountain Quadrangle, Central Vermont

Petrologic and geologic arguments suggested that a close approach to chemical equilibrium at a single temperature and pressure might be found in the rocks near Mt. Grant in central Vermont. An area 2,800 feet by 4,000 feet was selected and studied in detail. The major assemblages are: kyanite-chloritoid-chlorite-quartz-muscovite-paragonite-rutile; garnet-chloritoid-chlorite-quartz-muscovite-paragonite-ilmenite; and garnet-chlorite-biotite-quartz-muscovite-albite-ilmenite. All minerals were separated from a primary sample for each of these assemblages and purified, complete gravimetric and spectrographic analyses performed, and optical properties, X-ray properties, and densities measured. Chlorite, garnet, and chloritoid or biotite were separated from an additional nine samples of these assemblages and spectrographic and partial gravimetric analyses performed. Distribution coefficients of each mineral pair for Mg/Fe and Mn/Fe+ Mg and for the minor elements are similar in nearly every sample for a given assemblage. Distribution coefficients for garnet-chlorite differ in the two assemblages which contain this pair; this difference is attributed to the difference in Al-content of the chlorite from the two assemblages. The small range of distribution coefficients, despite a wide range in the relative proportions of the ferromagnesian phases, is convincing evidence that an equilibrium partition of the major and minor cations between the phases in each sample had been attained and that the equilibration temperature was the same for each sample. The coexistence of kyanite+muscovite+quartz and comparison of the composition of coexistent Ca-free muscovite and paragonite and of O18/O10 ratios for coexistent quartz and magnetite with experimental values indicate that these rocks formed at Ps ≥ 11 kb, T≈ 550° C, and aH2o low enough (Pe(H2o) sufficiently less than Ps) to depress the pyrophyllite breakdown temperature by about 40° C. The cations and the ao2 are equilibrated locally (within each sample), but all the samples have equilibrated to a common O18/O16 ratio. A possible explanation is that a fluid medium had sufficient oxygen in H2O to control the O18/O16 ratio of the rock and its phases, but that the rock system had sufficient buffer capacity to control the ao2 of the fluid.