Microscale Controls on Garnet Textures: The Role of Fluids and Local Equilibrium in Fjørtoft Metapelites (WGR, Norway)

Garnet textures in metamorphic rocks provide valuable insights into fluid–rock interactions, metamorphic reactions, and the mechanisms driving mineral transformations during retrogression. This study focuses on metapelitic samples from the Blåhø Nappe on Fjørtoft Island (Western Gneiss Region, Norway), part of the Caledonian allochthon. The aim of this work is to investigate how local mineralogical composition and fluid availability during retrogression influence garnet textures. Chemical mapping reveals a systematic association between grossular-enriched garnet rims and the presence of plagioclase and biotite, suggesting a coupling between garnet zoning and the surrounding mineralogy. Geothermobarometric and thermodynamic constraints, support amphibolite overprinting over older eclogite facies. Thermodynamic calculations based on Gibbs free energy minimization were performed to better understand the reactions involved. In particular, the breakdown of phengite in the presence of quartz and fluid is considered a key process leading to the formation of biotite and plagioclase, with implications for garnet zonation. This case study highlights how microscale equilibrium conditions and mineral interfaces can interact with high-grade metamorphism garnets.