Structural Evolution and Metasomatism of Subducted Metaophiolites in the Northwestern Alps

Abstract

A subduction complex of the northwestern Alps consists of serpentinites, eclogitic metagabbros, flysch-like metasediments, meta-ophicarbonates, and gneissic slices. Unlike other subduction complexes, it contains unusual hybridized rocks described here for the first time in the northwestern Alps. They are preserved as patches interstitial in the metagabbro and as layers within metagabbros and serpentinites. The hybridized rocks are made of high modal zoisite/clinozoisite + white mica pseudomorphs of lawsonite, garnet, and amphibole associated with an Alpine eclogite-facies fabric. While these eclogitic metagabbros are chemically comparable to oceanic oxide gabbros from the ultraslow Southwest Indian Ridge, the layers are extremely enriched in Al 2 O 3 and CaO and depleted in TiO 2 , MgO, and SiO 2 relative to metagabbros. Patches have a geochemical signature that is intermediate between that of layers and metagabbros. Trace element compositions of hybridized rocks suggest a contribution from a fluid derived from a mixed source made of sediments and serpentinites. Except for Ba, Rb, and K, layers are comparable to the global subducting sediments, indicating a sedimentary contribution, whereas the enrichment in Cr indicates a serpentinite contribution. Metasediment dehydration and chemical exchange of Ca and Sr have resulted in significant lawsonite crystallization in the subduction zone, as reflected by the ubiquitous presence of lawsonite pseudomorphs. In light of the unique textures and geochemical signature of the lawsonite pseudomorph-bearing hybridized rocks, an origin by fluid-rock interaction and Ca-metasomatism in the subduction environment is inferred and considered in the Western Alps context.

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

“…This mélange-like rock assemblage exposed along the southern side of the Urtier valley suffered a tectonic evolution common with that of the underlying Gran Paradiso Nappe dominated by a regional-scale high-pressure foliation marked, in the metaophiolites, by glaucophane, garnet, and omphacite [116,117,129] and a heterogeneous retrogression under blueschist-greenschists facies conditions. Metasediments the eclogite facies imprint is documented by phengite, garnet, and (now pseudomorphed) lawsonite [117].…”

“…For their rock assemblages and metamorphic imprint, the Cogne-Urtier valley metaophiolites have been compared to the Zermatt Saas Zone, although some authors have attributed these metaophiolites to differents units (or sub-units), on the basis of their rock assemblages and metamorphic imprint [79,[114][115][116][117]. For simplicity, the Cogne-Urtier metaophiolites are here referred to "Zermatt Saas metaophiolites".…”

“…For simplicity, the Cogne-Urtier metaophiolites are here referred to "Zermatt Saas metaophiolites". They consist of antigorite-Ti-clinohumite serpentinites, Fe-Ti-metabasalts transformed into glaucophanites with garnet and lawsonite (now pseudomorphed) enclosing eclogite boudins, Mg-and Fe-Ti metagabbros, and serpentinites [54,75,79,[115][116][117][118][119]. Similar to the metaophiolites described in the St. Marcel valley, the Cogne-Urtier metaophiolites are mostly characterized by high-pressure metamorphism variously retrogressed under greenschists facies conditions.…”

“…The Zermatt-Saas metaophiolites extensively crop out along the axial Urtier valley and within the sinistral tributary valleys (Valnontey, Valleille, Bardoney, and Vallon des Eaux Rousses valleys), up to the orogenic divide with the Champorcher valley, to the east. They constitute the so-called Grivola-Urtier unit [79,117] or the Lower Ophiolitic Unit [114], which overlays the Upper Penninic Gran Paradiso Nappe. Similarly to the northern side of the Urtier valley, the Zermatt-Saas metaophiolites exposed in the southern side (Valnontey valley) are interlayered with carbonatic rocks of the Penninic covers.…”

“…The largest ophiolitic slices, up to hundreds of meters wide, consist of serpentinite including chlorite-schist layers (probably representing original dykes) which envelop huge (hectometer scale) gabbro intrusions or mega-boudins (e.g., the Pointe Noire Fe-Ti metagabbro). More fine-grained material is represented by dm-to cm-sized mafic or ultramafic blocks and clasts embedded in a carbonate matrix [116,117]. These metabreccias may be tectonically transposed with calc schists (e.g., Eaus-Rousses valley; Figure 6F), serpentinite (e.g., Loie lake), or metagabbro (e.g., near Champlong).…”