Interaction of deformation and metamorphism during subduction and exhumation of hydrated oceanic mantle: Insights from the Western Alps

Petrological investigations supported by multi-scale structural analysis of eclogitized serpentinite in the Zermatt-Saas Zone of the Western Alps allows for the determination of mineral assemblages related to successive fabrics, upon which the P-T-d-t path of these hydrated mantle rocks can be inferred. Serpentinites of the upper Valtournanche, with lenses and dykes of metagabbro and meta-rodingite, display an Alpine polyphase metamorphic evolution from eclogite to epidote-amphibolite facies conditions associated with three successive foliations having different parageneses in these rocks. Serpentinite mainly consists of serpentine with minor magnetite; however, where S1 and S2 foliations are pervasive, metamorphic olivine, together with Ti-clinohumite and clinopyroxene, are also found. The mineral assemblage associated with D1 includes serpentine1, clinopyroxene1, opaque minerals, titanite ± olivine1, Ti-clinohumite1 and ilmenite; the D2 assemblage is the same (±chlorite) but minerals have different compositions. The assemblage associated with D3 comprises serpentine3, opaque minerals, ±chlorite3, ilmenite and amphibole3. Ti-clinohumite is associated with veins that are older than D2 and pre-date D3. Veins that post-date D3 are characterized by amphibole + chlorite or by serpentine. P-T conditions for S2 parageneses evaluated using two pseudosections for different bulk compositions suggest that these rocks experienced pressures >2.5 ± 0.3 GPa at temperatures slightly higher than 600 °C. The late epidote-amphibolite facies re-equilibration associated with D3 and D4 developed during late syn-exhumation deformation related to folding and testifies to a small temperature decrease. These results, which were integrated in the regional framework, suggest that different portions of the Zermatt-Saas Zone registered different P-T peak conditions and underwent different exhumation paths. In addition, the inferred P-T-d-t path suggests that the Valtournanche serpentinites re-equilibrated close to the UHP conditions registered by the Cignana meta-cherts. These results imply that tectonic slices exhumed after UHP metamorphism might be wider than previously reported or that small-size UHP units, tectonically sampled during the Alpine convergence, are more abundant than those that have been detected to date.