Garnet-Free Mineral Assemblage at Eclogite-Facies Conditions in the Riffelberg–Garten Unit, Italian Western Alps

The peculiar high-pressure mineral assemblage omphacite, epidote, quartz, calcite, titanite, and opaque minerals, +/- phengite, has been observed in the Riffelberg-Garten Unit (RGU), a heterogeneous metasedimentary rock assemblage of the Zermatt-Saas Zone. Microstructural analysis, mineral chemistry, and petrologic modelling allowed to refine the syn-D2 P-T peak conditions for the Alpine tectono-metamorphic evolution. In the upper Valtournenche, S2 foliation is the dominant fabric at the regional scale of the Zermatt-Saas Zone. Petrologic modelling of the syn-D2 mineral assemblage indicates climax conditions of P = 1.85-2.0 GPa and T = 500-525 degrees C. These estimates are in good agreement with those inferred in the RGU metasedimentary matrix and enclosed eclogite and metagabbro elements. During exhumation, RGU rocks re-equilibrated texturally and mineralogically under blueschist-/epidote-amphibolite (P = 0.4-1.3 GPa and T = 350-500 degrees C during D3) and greenschist (P <= 0.25 GPa and T <= 400 degrees C during) facies conditions. This study highlights the potential of petrologic modelling for constraining the environmental conditions of metamorphism even in anomalous mineral assemblages where conventional thermobarometry is not applicable.