Primary and tectonic mélange in the eclogitic Zermatt-Saas Zone, Western Alps

Within the Alpine nappe stack of the Western European Alps, the Zermatt-Saas Zone (ZSZ) is a remnant of the Piedmont-Ligurian Ocean. The oceanic lithosphere formed in Middle-to-Late Jurassic times and was subducted during the Alpine convergence up to UHP-to-HP metamorphic conditions, between the Late Cretaceous and middle Eocene. The ZSZ consists of serpentinite, metabasite, and metasedimentary rocks that are interpreted as a metamorphosed ophiolitic assemblage. This rock assemblage records multiple stages of ductile deformation that developed during the Alpine subduction, collision, and exhumation. In this assemblage, chaotic complexes consist of metasediments with a matrix containing variable amount of carbonatic and quarzitic components, including metabasite (some with gabbro texture) and ultramafite elements. In this kind of rocks, clear primary structures are hardly preserved due to intense transposition developed under UHP and HP conditions. However, we detected poorly preserved primary features that allow to define this metasedimentary matrix as a former sedimentary mélange. Additionally, we envisaged some primary genetic processes for the protoliths formation such as: tabular basalt flows disrupted within ocean floor sediments; mass transport at the ocean floor, possibly from serpentinite and gabbro exposed at structural highs; mass transport from the continental margins and/or interaction with trench sediments. Close to serpentinite, some portions of this metasedimentary matrix are interpreted as metasomatic products. Alpine transposition affected also serpentinite and metabasite and determined pervasive reorganization of the lithostratigraphy, with the formation of hectometer-sized meta-gabbro bodies forming lenses wrapped by the UHP/HP transposition foliation of serpentinite. Serpentinite also includes meter-sized rodingite lenses and layers that are variably folded and disrupted into the dominant foliation, deriving from former gabbro dykes metasomatized at the ocean floor. Serpentinite contains millimeter-sized clinopyroxene and micrometer-sized zircon porphyroclasts that are partially recrystallized during transposition. Clinopyroxene and zircon porphyroclasts show trace element composition consistent with gabbro protoliths and therefore are actually interpreted as remnants of mafic veinlets that percolated serpentinite during ocean floor evolution. On the other hand, serpentinite also preserves Ti-condrodite porphyroclasts that formed during UHP metamorphism predating the pervasive foliation development. In addition, the tectono-metamorphic history predating the dominant fabric at the regional scale is composite and variable in adjacent portions of the ZSZ. These lithostratigraphic, structural and metamorphic data are compatible with the existence of a tectonic mélange, in which the metasedimentary cover displays preserved original features that are consistent with various types of deformed and transformed primary sedimentary mélanges. Thus, the ZSZ can be regarded as an ophiolitic polygenetic mélange formed by HP pressure transposition, which almost completely obliterated primary and tectono-metamorphic features.