Multiscale polygenetic mélange in the eclogitic Zermatt-Saas Zone ophiolite, Western Alps

Within the Western Alps, the ophiolitic Zermatt-Saas Zone is a remnant of the Piedmont-Ligurian Ocean, stacked into the Alpine nappe pile. The age of the oceanic lithosphere is assessed at the Middle to Late Jurassic and the Alpine subduction-related UHP to HP metamorphic peak was recorded between the Late Cretaceous and middle Eocene. The Zermatt-Saas Zone mostly consists of serpentinite, metabasite, and carbonatic to quartzitic metasedimentary rocks. These rocks represent a metamorphosed ophiolitic assemblage. The metasedimentary rocks contain metabasite (some with gabbro texture) and minor ultramafite elements. The primary genetic processes envisaged for the protoliths include: tabular basalt flows disrupted within ocean floor sediments; mass transport at the ocean floor, possibly triggered by faults, responsible for the exposure of serpentinite and gabbro structural highs; mass transport from the continental margins and/or interaction with trench sediments. Within these metasedimentary rocks, clear primary structures are not preserved due to intense transposition developed under UHP and HP. This transposition affects also serpentinite and metabasite and is responsible for the pervasive restructuration of the lithostratigraphic architecture. Indeed hectometer-sized meta-gabbro bodies form lenses wrapped by the UHP/HP transposition foliation into the serpentinite. Meter-sized rodingite lenses and layers are variably transposed into the same foliation and represent former gabbro dykes metasomatized at the ocean floor. In serpentinite, millimeter-sized clinopyroxene and micrometer-sized zircon porphyroclasts partially recrystallized during transposition. Trace element geochemistry revealed that clinopyroxene and zircon actually derive from gabbro protoliths and are interpreted as remnants of veinlets that percolated serpentinite. Upon these observations, the Zermatt-Saas Zone can be regarded as an ophiolitic tectonic mélange formed into the Alpine subduction channel. The metasedimentary cover displays features that are consistent with these rocks primarily formed as various types of sedimentary mélange that eventually were overprinted by transposition. Therefore, this ophiolitic assemblage can be regarded as a multiscale polygenetic mélange.