Geology of the Saint-Marcel valley metaophiolites (Northwestern Alps, Italy)

Abstract

The geological map of the Saint-Marcel valley at the scale of 1:20,000 illustrates the tectonic setting of metaophiolites from the southern Aosta Valley, in the Italian side of the Western Alpine belt. The map highlights the sharp contact between the metaophiolitic basement and its metasedimentary cover, which mainly consists of quartzites, marbles, and calcschists. In spite of the Alpine tectonics, this contact is regarded as deriving from the original oceanic crust/sediments interface. Metaophiolites mostly consist of metabasalts hosting Fe-Cu sulphide mineralisations, characterised by high-pressure metamorphic imprint. These rocks likely represent the shallowest portion of the Tethyan oceanic lithosphere created near the axis of the slow-spreading ridge where hydrothermal fluid circulation was active. Selected key-sections through metasediments reveal a consistent internal lithostratigraphy, in spite of the pervasive metamorphic and tectonic reworking acting during the Alpine evolution. Metasediments reflect various sedimentation episodes starting from pelagic and proximal settings to the onset of the orogenic stage. The Saint-Marcel valley metasediments thus reflect a changing in the sedimentation environments through time and space during the overall geologic evolution. ARTICLE HISTORY

Structural Evolution and Metasomatism of Subducted Metaophiolites in the Northwestern Alps

“…The HP metaophiolites exposed in the Urtier and nearby valleys belong to two units, namely, the Zermatt‐Saas and the Grivola‐Urtier units (Bocchio et al, ; Dal Piaz et al, ) (Figure a). The Zermatt‐Saas unit consists of serpentinites, metagabbros, metarodingites, and mafic rocks including Fe‐Cu ore bodies, calcschists, marbles, and metaradiolarites with Mn mineralization (Martin & Tartarotti, ; Tartarotti & Caucia, ; Fontana et al, ; Martin et al, ; Panseri et al, ; Tumiati et al, ; Tartarotti, Festa, et al, , Tartarotti, Martin, et al, ). The Grivola‐Urtier unit consists of flysch‐type and chaotic calcschists (including blocks and slices of mafic and gneissic rocks), metabasites, metagabbros, serpentinites, and meta‐ophicarbonates, enveloping metric to hectometric bodies of eclogite‐facies metagabbros, metarodingites, and chloriteschists.…”

“…Further evidence of S 1 is provided by mafic metabreccias exposed in the southern Vallon des Eaux Rousses (not shown in Figure ), where clasts still retain an internal foliation oriented at high angle to S 2 (Figure d). Finally, within flysch‐type calcschists, more widely exposed in the nearby Champorcher and Soana Valleys (e.g., Battiston et al, ; Tartarotti, Festa, et al, , Tartarotti, Martin, et al, , ), the S 1 foliation corresponds to the compositional layering typically defined by alternating metapelites and marbles and interpreted as being the relict primary bedding.…”

A mapping approach for the investigation of Ti–OH relationships in metamorphic garnet

“…The first sample ServRod is a 83 garnet from a rodingite from the Servette area (St Marcel, Aosta valley, Italy). The various rock types 84 in this area are interpreted as both oceanic and continental margin units derived from the Mesozoic 85 Tethyan ocean and the Palaezoic crust of the African margin (Tartarotti et al 2017). The area of St. 86…”

Metasediments Covering Ophiolites in the HP Internal Belt of the Western Alps: Review of Tectono-Stratigraphic Successions and Constraints for the Alpine Evolution

“…Two main types of those successions are recognizable. The first type comprises quartzite (probably from radiolarite), marble, and calc schists [68,69,104,105]. These metasediments directly cover the ophiolitic metabasalts or metagabbro, often associated with manganeseor sulphide-ore deposits.…”

“…In the St. Marcel valley, the metaophiolite basement is almost unique, being composed of crustal lithologies of the oceanic lithosphere, and not of mantle serpentinites as in most sectors of the Aosta valley. Metaophiolites consist of eclogite facies Fe-Ti-and Mg-metagabbros, metabasalts represented by glaucophanites with garnet and lawsonite (now pseudomorphed) and enclosing eclogite boudins, garnet-chloritoid-glaucophane-chlorite-bearing schists, and minor talc schists [69,104,105,108]. These rocks include a well-known Cu-Fe sulfide mining district and a Mn-rich deposit interpreted as palaeo-hyrothermal fields referable to the Jurassic oceanic environment [108][109][110].…”