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

Abstract

The Theodul Glacier Unit (TGU) at “Trockener Steg” represents a continental slice, embedded within the ophiolitic Zermatt–Saas Zone. The Zermatt–Saas Zone is the remnant of the Piemonte–Liguria oceanic lithosphere, formed in the middle Jurassic and subducted up to eclogite facies conditions in the Early Tertiary. The close spatial association of the TGU to the Zermatt–Saas Zone permits a comparison of the metamorphic evolution of the units by detailed field mapping and a petrological investigation of eclogites. The eclogites from both tectono-metamorphic units can be clearly distinguished by their textures, mineral assemblages and by mineral and bulk-rock composition. Geothermobarometry and computed assemblage stability diagrams for the TGU eclogites indicate P–T conditions of 2.2±0.1GPa and 580±50°C. These derived P–T conditions must be considered as minimum peak metamorphic conditions the rocks achieved during subduction. The P–T data are different from those derived for eclogites of Zermatt–Saas Zone adjacent to the Theodul Glacier Unit, that reached maximal burial depths at 2.3–2.4GPa and 500±50°C. While the estimates of the eclogites of Zermatt–Saas Zone are in good agreement with some of the previous studies, the contrasting P–T estimates for the TGU eclogites suggest that the Zermatt–Saas complex must be subdivided into several tectonic subunits. The non-uniform peak conditions over the “Trockener Steg” area and the maximum pressures conditions reported from ultra-high pressure localities within Zermatt–Saas Zone suggest, that individual tectonic slices have been assembled after detachment from the slab at the return-point, i.e. along the exhumation path. Detached packages of rocks may range from small tectonic slices up to several kilometer-sized fragments.The TGU is separated from the surrounding rocks of the ophiolite unit by two major tectonic contacts. In addition, the formation of biotite-rich crusts along the basal contact of the TGU is evidence of prolonged fluid channeling along the basal thrust. The presence of hydrous decompression assemblages replacing earlier formed high-pressure mineral assemblages within the studied eclogite suggests that fluids were present throughout most of the TGU exhumation history.