Thermodynamic modeling applied to metamorphic processes in migmatites: An example from the Nason Ridge Migmatitic Gneiss (Cascades Core, Washington)

Thermodynamic pseudosection modeling is used to understand metamorphism in migmatitic metapelites. First, research focuses on evaluation of changes in effective composition on pseudosection topology and on the use of pseudosections for refinement of metamorphic peak P-T estimates. Potential changes in effective composition originating from prograde growth of zoned garnet were investigated to assess the application of pseudosections for predicting peak assemblages. For Nason terrane pelites, changes in effective rock composition originating from growth of up to 5% modal zoned garnet would have insignificant effects on pseudosection topology. Therefore, pseudosections constructed with bulk rock compositions for Nason terrane metapelites can be used to refine P-T estimates and to construct meaningful P-T paths. Second, research investigates the origin of quartz-plagioclase lenses and associated selvages in migmatites. Pseudosection modeling is used to investigate the origin of leucosomes and associated selvages through wet partial melting in pelitic rocks from the Nason Ridge Migmatitic Gneiss. Modeling addresses water content at temperatures close to the wet solidus, wet melting temperatures, melting reactions, melt compositions, and potential retrograde back reactions. Wet partial melting is likely a common process in pelites similar to the study samples, but preservation of textures indicative of wet partial melting depends on the amount of water present. In an open system, all free water escapes, wet partial melting is limited, no melt segregation occurs, and textures are not likely preserved because of back reactions between melt and restite and retrograde re-equilibration. In a closed system, enough water remains in the system such that wet partial melting is extensive and partial melts segregate in pockets. After melt segregation, partial melts back react with restite producing biotite-rich selvages and modifying melt composition. Third, the origin of Nason Ridge Migmatitic Gneiss is addressed. Presence of leucosome-selvages pairs, and discordant injected leucosomes with variable deformation state indicate that the migmatite has multiple origins: melt injection before or during the last deformation event, partial melting, and a late intrusive event. Strongly deformed leucosomes are evidence for the first melt injection event, while weakly deformed leucosomes are the evidence for the last melt injection event.