RETROGRADE BACK REACTIONS IN MIGMATITES: INSIGHTS FROM PSEUDOSECTION MODELING OF THE NASON RIDGE MIGMATITIC GNEISS, WA

Pseudosection modeling of migmatitic gneisses supports retrograde back reaction between quartz-plagioclase leucosomes and country rocks for development of biotite selvages. Leucosomes and associated biotite-rich selvages in migmatites were investigated using MnNaCaKFeMgAlSiH pseudosection modeling of wet partial melting. Pelitic rocks of the Nason Ridge Migmatitic Gneiss are predicted to undergo wet partial melting at temperatures between 655 ºC (10 kbar) and 703 ºC (3 kbar). P-T paths, based on thermobarometry and P-T pseudosections, indicate peak metamorphism at 650-720 ºC, 6-9 kbar, with a pressure increase of < 2 kbar. Textures, predicted metamorphic conditions, and pseudosections are compatible with partial melting and local segregation of melt into quartz-plagioclase leucosome lenses parallel to or at a low angle to schistosity. Fluid-present melting would be dominated by consumption of quartz and plagioclase with lesser consumption of biotite and muscovite, and peritectic garnet growth. Melt reactions would produce leucocratic melts, low in iron and magnesium (FeO+MgO < 1%) and high in silica (SiO₂ > 75%). Only slight discrepancies are found between SiO₂, Al₂O₃, and Na₂O concentrations predicted for melts and observed in leucosome-lenses; however, significant discrepancies are evident for K₂O and CaO. Modeling using T-X pseudosections was used to explore the possibility that discrepancies result from back-reactions between the melt and country rock. T-X pseudosections were calculated between bulk composition prior to melt segregation, composition after melt removal, and composition after back-reactions. Chemical gradients between country rock and melt, during crystallization, would cause compositional changes in segregated melts. Modeling suggests that K, Al, and Na diffuse from melt lenses to country rock while Si and Ca diffuse from country rock to melt lenses. These processes would drive retrograde back reactions that are inferred to explain the formation of biotite selvages and discrepancies between predicted and observed leucosome compositions. Formation of this migmatitic texture would be favored by retention of water in the system during prograde metamorphism, partial melting at the wet solidus, and melt segregation into leucosome lenses.