Metamorphism of Calcic Pelitic Schists, Strafford Dome, Vermont: Compositional Zoning and Reaction History

Four assemblages from calcic pelitic schists from South Strafford, Vermont, have been studied in detail to determine the relationship between reaction history and compositional zoning of minerals. The lowest-grade assemblage is garnet + biotite + chlorite + plagioclase + epidote + quartz + muscovite + graphite + fluid. Along a path of isobaric heating, the net reaction is Chl + Ms + Ep + Gr = Grt + Bt + Pl + fluid. Garnet grows with decreasing Fe/(Fe + Mg) and XSpa, (from 0⋅2 to 0⋅05), XGra stays nearly constant between 0⋅20 and 0⋅25, and plagioclase grows with XAn increasing from peristerite to 0⋅2–0⋅5. The subsequent evolution depends on whether chlorite or epidote reacts out first. If chlorite is removed from the assemblage first, the net reaction along an isobaric heating path becomes Grt + Ms + Ep + Qtz + Gr = Bt + Pl + fluid. XAn of plagioclase increases to 0⋅20–0⋅70, depending on the bulk-rock composition and changes in pressure and temperature. If epidote is removed first, the assemblage becomes a simple pelite and the net reaction becomes Chl + Pl + Ms + Qtz = Grt + Bt + H2O. Plagioclase is consumed to provide Ca for growing garnet, and XAn, Fe/(Fe + Mg) of garnet, XGra, and XSpa all decrease. After both chlorite and epidote are removed, continued heating up to the metamorphic peak of ˜600°C produces little progress of the reaction Grt + Ms = Bt + Pl; and XAn increases. The four assemblages have been numerically modeled using the Gibbs method starting with measured compositions. The models successfully predict the observed compositional zoning and trends of mineral growth and consumption along the computed P–T paths. The models also predict the compositional mineral zoning that would have resulted from other P–T paths.