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The study of rock chemistry is a milestone in understanding fluid–rock interactions and fluid migration in subduction zones. When combined with thermodynamic models, it can provide direct insight into fluid composition, metasomatic reactions, and pressure–temperature (P – T) conditions, as well as their role in rock deformation. Here, a shear zone – located in the Mont Avic area of the Zermatt-Saas zone (Western Alps) – is analyzed. This shear zone consists of several blocks of different lithotypes, including a Ca-rich metasomatite block embedded in a serpentinite mylonitic matrix, and structurally underlies a coherent eclogitic mafic unit. This work aims to estimate the pressure–temperature conditions of the Ca-rich metasomatism and the amount of fluid involved. The brecciation exhibits mosaic breccia textures with clasts comprising ∼80 vol % of garnet, together with omphacite, epidote, titanite, rutile, and apatite hosted in an omphacite matrix. Quantitative chemical mapping of the garnet reveals primary garnet cores with embayment and lobate edges with a chemical composition similar to unaltered reference eclogite garnet. These primary garnet cores are overlain by Ca-rich metasomatic garnet rims with oscillatory chemical zoning. The oscillatory chemical zoning, together with the morphology of the primary garnet cores, suggests repeated influxes of external Ca-rich fluid that destabilized the primary garnet cores and promoted the growth of Ca-rich rims. Mass balance calculations between precursor metabasite and Ca-metasomatite indicate multiple fluid sources involving dehydrated serpentinite, calcic metasediments, and metabasites with time-integrated fluid fluxes calculated between 11.5×103 and 5.5×104 m fluid3 m rock-2 , consistent with channelized fluid flow in an open system. Thermodynamic modeling of garnet from unbrecciated and non-metasomatized metabasites – from the Savoney eclogitic mafic unit – indicates peak metamorphic conditions of 2.5±0.1 GPa and 535±40 °C, consistent with regional estimates. Pressure–temperature conditions of metasomatism were constrained using P – X and T – X phase modeling (where X represents changes in bulk CaO and Na 2 O composition) between 2.6–2.2 GPa and 570–500 °C, showing that Ca-rich fluid percolation occurred close to the metamorphic peak (i.e., prograde to the peak or early exhumation path). [ABSTRACT FROM AUTHOR]