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

Abstract

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.

An eclogite-bearing continental tectonic slice in the Zermatt–Saas high-pressure ophiolites at Trockener Steg (Zermatt, Swiss Western Alps)

“…Thus, the serpentinites represent hydrated mantle peridotites probably from the subducted lithosphere and not from the overriding mantle wedge. Similarly to other sectors of the ZSZ (e.g., Li et al, 2004;Rebay et al, 2012;Zanoni et al, 2011) the schistosity observed in the ZSZ serpentinites at Trockener Steg location may be ascribed to the eclogitic S 2 fabric, where metamorphic olivine is distributed along the pervasive foliation plane (Fig. 4d & e).…”

“…However, variations in the rate of subduction have been reported from the Piemonte-Liguria oceanic slab, which result from an interplay between density, geometry of the slab and mantle viscosity (Groppo et al, 2009;Peacock and Wang, 1999;Royden and Husson, 2006). The ZSZ rocks preserve different deformation stages, in which one or more eclogite facies foliations related to Alpine subduction (S 1 and S 2 ) are overprinted by a late stage greenschist facies foliation (S 3 ) (Pleuger et al, 2007;Rebay et al, 2012;Zanoni et al, 2012).…”

“…The eclogite facies Zermatt-Saas ophiolites include ultramafic rocks, mostly serpentinites (Li et al, 2004;Rebay et al, 2012;Zanoni et al, 2012), metagabbros (Bucher and Grapes, 2009), metabasalts with locally preserved pillow structures (Bearth, 1967;Bucher et al, 2005), meta-rodingite (Li et al, 2008;Zanoni et al, 2012) and thin bands of oceanic meta-sediments locally with Mn-rich meta-cherts (Bearth and Schwander, 1981;Fazies, 2002;Reinecke, 1991). The composition of eclogite facies pillow basalts of the ZSZ is characteristic of mid ocean Fig.…”

“…This work demonstrates that oceanic lithosphere, which underwent subduction and exhumation as a relatively coherent unit, may contain fragments of exotic rocks, which in turn show differences in the tectonic evolution compared to the surrounding ophiolite rocks (e.g. Rebay et al, 2012).…”

Deeply subducted continental fragments – Part 1: Fracturing, dissolution–precipitation, and diffusion processes recorded by garnet textures of the central Sesia Zone (western Italian Alps)

“…It is thought to derive from the distal portion of the Adriatic passive margin that was involved in the Alpine subduction (Dal Piaz, 1999;Beltrando et al, 2014). The Sesia Zone is bounded to the west by blueschist to eclogite facies subunits derived from the Piemonte-Liguria Ocean (Bearth, 1967;Dal Piaz and Ernst, 1978;Martin et al, 1994;Cartwright and Barnicoat, 2002;Bucher et al, 2005;Groppo et al, 2009;Rebay et al, 2012;Negro et al, 2013) and to the east by the Insubric Line. This dextral brittle fault separates the Sesia Zone from the southern Alps, which show a weak Alpine imprint at sub-greenschist facies (Bertolani, 1959;Zingg, 1983).…”