Manganese-nitrogen-carbon KFMASH metapelite phase equilibria, garnet activity modeling, and garnet samarium-neodymium chronology: Applications to the Waterville Formation, Maine, and south-central Nason Terrane, Washington

Results from thermodynamic modeling of metapelite bulk rock compositions, investigation of major-element garnet chemistry, thermobarometry, and garnet Sm-Nd chronology, are presented that illustrate the use of modern techniques to constrain metamorphic histories from metapelitic rocks. Phase diagrams constructed for metapelite compositions from the Waterville Formation, Maine, illustrate the dependence of mineral assemblage stability on bulk rock Al ₂O₃ and MgO/(MgO + FeO). Comparisons of KFMASH and MnNCKFMASH pseudosections with reported Waterville Formation assemblages clearly indicate the inadequacies of the KFMASH system for modeling the stability of garnet-bearing assemblages in metapelites.; A linear regression analysis of garnet-aluminum silicate-plagioclase-quartz experimental phase equilibria was used to derive four garnet activity models. The derived regular solution models are more consistent than the sub-regular solution models with the assumption that spessartine mixes ideally with the other garnet end-members, and use of a grossular-almandine interaction parameter does not significantly improve the regression statistics. A regular solution model is proposed that utilizes a grossular-pyrope interaction parameter of 44.2–0.01651 T (kJ, mol, K−1), and an almandine-pyrope interaction parameter from Holland & Powell (1998) of 2.4 kJ/mol.; Garnet thermobarometry and MnNCKFMASH phase equilibria diagrams constructed for pelite bulk rock compositions using the derived garnet activity model were compared with measured garnet core compositions to calculate Cretaceous aged garnet growth P-T paths in the south-central Nason Terrane of the Cascades Core. Garnet growth 11 kilometers northeast of the Mt. Stuart batholith occurred during isobaric heating after attainment of maximum metamorphic pressures, while garnet growth approximately 2 kilometers northeast of the batholith occurred after or during the latter stages of crustal loading. Precise garnet Sm-Nd isochron ages from samples 1.5 to 4 kilometers north of the batholith indicate garnet growth occurred at 88 to 86 Ma. A Mt. Stuart batholith zircon U-Pb isotopic age of 93.5 ± 1.4 Ma. suggests the batholith was not the only source of heat that led to garnet growth at 88–86 Ma. The derived garnet growth P-T paths and garnet Sm-Nd ages indicate exhumation of this particular portion of the south-central Nason terrane must have begun after 86 Ma. garnet growth.