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 °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 °C during D3) and greenschist (P ≤ 0.25 GPa and T ≤ 400 °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.
Rebay et al. (Wed,) studied this question.
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