Retrograde garnet zoning at low water pressure in metapelitic rocks from Kemiö, SW Finland

The compositional zoning of almandine-spessartine garnet has been studied in pelitic gneisses from the Kemiö region, SW Finland. Garnets with inverse zoning (outward increase in Mn) abound in the low-grade musc+qz zone. Garnets with inverse zoning, normal zoning (outward decrease in Mn), and with a homogeneous composition occur in the higher-grade sill+ Kfs zone. The formation of garnet with inverse zoning is explained by continuous retrograde reactions in the model system K₂O-Al₂O₃-FeO-MnO-MgO-SiO₂-H₂O at strongly reduced partial water pressures.A comparison of temperatures based on critical metamorphic reactions and on garnet-biotite thermometry shows the usefulness of geothermometry in rocks with spessartine-rich garnet and Mn-bearing biotite.