The Versoyen Zone which is situated on the French-Italian border near the Pt.St.Bernard pass is wedged between the Houllière Zone and the external crystalline massives of Mont-Blanc and Belledone, and is structurally related to the Brèches de Tarentaise Zone (Valais Zone). It consists of serpentinite slices in association with gneiss lenses, and a volcano-sedimentary sequence characterized by differenciated basaltic sills intruded into Lower- to Mid-Cretaceous pelitic sediments. Pillow lavas are commonly observed in the upper part of the sequence. Due to the lack of coarse mafic constituants and a sheeted-dike complex (which, though, could possibly be represented by the sills), the classical ophiolite trilogy is not fully exhibited.

Geochemical discrimination diagrams are used to decipher the geotectonic setting in which the ophiolite complex developed. It is interpreted as a small basin generated in a Gulf of California-type transform fault system.

The ophiolites and associated sediments have undergone a complex metamorphic history which has been restored by means of a detailed petrographic study complemented by microprobe analyses of its key minerals. The sequence of events is as follows:

The emplacement of the sills in the sediments resulted in a contact metamorphism which baked and transformed the sediments into porcelain-like rocks, the "adinoles". A sea-floor metamorphism which probably just followed gave rise to a substantial mobilization of volatiles which locally also led to spilitization. The Versoyen Zone was then subjected to multiple phases of Alpine regional metamorphism, of which 3 facies can be recognized. The eclogite facies, already mentioned in previous studies but still considered as an allochtonous metamorphism) was found to have been generated in situ. The P/T conditions of the eclogite assemblage could be estimated due to the discovery of omphacite in several localities, and to the confirmation and extension of the jadeite domain; they were between 7 and 16 Kb, and 300° and 600° C.

The original geodynamic environment is considered to be that of a subduction zone with tectonic overpressure generated by a major overthrust. Then a blueschist facies metamorphism developed under similar high P, low T conditions. The clearly retrograde greenschist facies succeded these major events.

A preliminary K-Ar geochronological study was carried out to date the major metamorphic event. It gave an age of 65 ± 16 m.y ..