Origin and tectono-metamorphic history of the Riffelberg-Garten meta-sedimentary ophiolitic unit, Western Alps

Within the Zermatt-Saas Zone (Western Alps), the Riffelberg-Garten Unit is part of the ophiolitic meta-sedimentary cover, which in the uppermost Valtournenche valley contains metabasite (some with a gabbroic texture) and minor ultramafite elements. Seven Riffelberg-Garten Unit types are distinguished based on the composition of the rock matrix and the amount, shape, composition and size of the enclosed elements. The rock matrix varies between carbonate-rich and quartz-rich, with white mica in most types. These rocks record four groups of Alpine syn-metamorphic ductile structures. The first two, developed during oceanic subduction, are responsible for the present-day lithological multilayers. The genetic processes include: (1) formation of tabular basalt bodies into the sediments, subsequently boudinaged during Alpine subduction; (2) mass transport at the ocean floor, possibly triggered by active faults, responsible for the exposure of serpentinite and gabbro; and (3) mass transport from the continental margins and/or interaction with trench sediments. Clear primary structures are not preserved due to transposition during subduction (at 515 ± 50°C and 2.1 ± 0.2 GPa) and subsequent exhumation under epidote–amphibolite and greenschist facies conditions. The dominant S2 transposition foliation, developed at a thermal state of 6–8.5°C km−1, supports the polygenetic origin of the Riffelberg-Garten Unit, which is mostly regarded as a sedimentary mélange restructured in a subduction system.