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\u2013Saas Zone of the Western Alps allows for the determination of mineral assemblages related to successive fabrics, upon which the P\u2013T\u2013d\u2013t 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 \ub1 olivine1, Ti-clinohumite1 and ilmenite; the D2 assemblage is the same (\ub1chlorite) but minerals have different compositions. The assemblage associated with D3 comprises serpentine3, opaque minerals, \ub1chlorite3, 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\u2013T conditions for S2 parageneses evaluated using two pseudosections for different bulk compositions suggest that these rocks experienced pressures >2.5 \ub1 0.3 GPa at temperatures slightly higher than 600 \ub0C. The late epidote\u2013amphibolite 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\u2013Saas Zone registered different P\u2013T peak conditions and underwent different exhumation paths. In addition, the inferred P\u2013T\u2013d\u2013t 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