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 ophiolite meta-sedimentary cover, which in uppermost Valtournanche contains metabasite (some with gabbro texture) and minor ultramafite elements. Seven Riffelberg-Garten Unit types are distinguished based on rock matrix composition and amount, shape, composition, and size of 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 the major responsible for the present lithological multilayer. Genetic processes include: a) tabular basalt bodies into sediments, subsequently boudinaged during the Alpine subduction; b) mass transport at ocean floor possibly triggered by active faults, in place responsible for serpentinite and gabbro exposure; c) mass transport from continental margins and/or interaction with trench sediments. Clear primary structures are not preserved due to the transposition during subduction (at 515 ± 50 °C and 2.1 ± 0.2 GPa) and subsequent exhumation, to epidote-amphibolite (D3) and greenschist facies conditions (D4). The dominant S2 transposition foliation, developed under thermal state of 6-8.5°Ckm-1, supports the polygenetic origin of the Riffelberg-Garten Unit, which is mostly regarded as a sedimentary mélange restructured in the subduction system.