Littérature scientifique sur le sujet « Zermatt-Saas Zone »

AbstractThe Zermatt-Saas Zone was part of the Middle to Late Jurassic Tethyan lithosphere that underwent oceanic metamorphism during Mesozoic time and subduction during Eocene time (HP to UHP metamorphism). In upper Valtournanche, serpentinite, metarodingite and eclogite record a dominant S2 foliation that developed under 2.5±0.3 GPa and 600±20°C during Alpine subduction. Serpentinites contain clinopyroxene and rare zircon porphyroclasts. Clinopyroxene porphyroclasts show fringes within S2 with similar compositions to that of grains defining S2. Zircon cores show zoning typical of magmatic growth and thin fringes parallel to the S2 foliation. These features indicate crystallization of clinopyroxene and zircon fringes during HP syn-D2 metamorphism, related to the Alpine subduction. The U–Pb zircon dates for cores and fringes reveal crystallization at 165±3.2 Ma and 65.5±5.6 Ma, respectively. The Middle Jurassic dates are in agreement with the known ages for the oceanic accretion of the Tethyan lithosphere. The Late Cretaceaous - Paleocene dates suggest that the Zermatt-Saas Zone experienced high-pressure to ultra-high-pressure (HP–UHP) metamorphism at c. 16 Ma earlier than previously reported. This result is in agreement with the evidence that in the Western Alps the continental Sesia-Lanzo Zone reached the subduction climax at least from 70 Ma and was exhumed during ongoing oceanic subduction. Our results are further evidence that the Zermatt-Saas ophiolites diachronically recorded heterogeneous HP–UHP metamorphism.

AbstractIn the Western Alps, the ophiolitic Zermatt–Saas Zone (ZSZ) and the Lago di Cignana Unit (LCU) record oceanic lithosphere subduction to high (540°C, 2·3GPa) and ultra-high pressure (600°C, 3·2GPa), respectively. The top of the Zermatt–Saas Zone in contact with the Lago di Cignana Unit consists of olivine + Ti-clinohumite-bearing serpentinites (the Cignana serpentinite) hosting olivine + Ti-clinohumite veins and dykelets of olivine + Ti-chondrodite + Ti-clinohumite. The composition of this serpentinite reveals a refertilized oceanic mantle peridotite protolith that became subsequently enriched in fluid-mobile elements (FME) during oceanic serpentinization. The olivine + Ti-clinohumite veins in the Cignana serpentinite display Rare Earth Element (REE) and FME compositions quite similar to the host-rock, which suggests closed-system dehydration of this serpentinite during subduction. The Ti-chondrodite-bearing dykelets are richer in REE and FME than the host-rock and the dehydration olivine + Ti-clinohumite veins: their Nd composition points to a mafic protolith, successively overprinted by oceanic metasomatism and by subduction zone recrystallization. These dykelets are comparable in composition to eclogites within the ultra-high pressure LCU that derive from subducted oceanic mafic crust. Different from the LCU, serpentinites from the core domains of the ZSZ display REE compositions indicating a depleted mantle protolith. The oceanic serpentinization of these rocks led to an increase in FME and to seawater-like Sr isotope compositions. The serpentinites sampled at increasing distance from the ultra-high pressure LCU reveal different mantle protoliths, still preserve an oceanic geochemical imprint and contain mafic dykelets affected by oceanic metasomatism. The subduction zone history of these rocks thus occurred under relatively closed system conditions, the only possible change during subduction being an enrichment in As and Sb recorded by the serpentinites closer to the crustal LCU. The ZSZ and Cignana serpentinites thus likely evolved in a slab setting and were weakly exposed to interaction with slab-derived fluids characteristic of plate interface settings. Our data suggest two possible scenarios for the evolution of the studied ZSZ and Cignana serpentinites. They are either part of a coherent ophiolite unit whose initial lithospheric mantle was variably affected by depletion and re-fertilization processes, or they belong to separate tectonic slices derived from two different oceanic mantle sections. In the Cignana serpentinite atop the ZSZ, the presence of Ti-chondrodite dykelets similar in composition to the LCU eclogites suggests these two domains were closely associated in the oceanic lithosphere and shared the same evolution to ultra-high pressure conditions during Alpine subduction.

This Ph.D. Thesis is intended as a multidisciplinary research work on the Zermatt-Saas (Western Alps) serpentinites and associated carbonated rocks. The main goal of this thesis is to decipher the carbonation processes involving serpentinites and the concomitant element mobilization, while also attempting at defining the serpentinites’ subduction-to emplacement- evolution. To address these aims, field work activities, spanning from acquisition of macro-structural data to sampling of representative lithologies, and detailed petrography were carried out on a sequence of serpentinites, carbonated serpentinites and listvenites from the Mount Avic Massif (Zermatt-Saas Zone, Aosta Valley, Western Alps) and neighbouring area of Vercoche. On the selected data set a wide range of geochemical analyses were carried out not only to define the bulk rock compositions but also to trace, by means of EMPA, LA-ICP-MS, µ-Raman applied to microstructurally selected sites, variations and chemical gradients of elements relevant for this study (i.e., redox sensitive elements such as C and S, but also major and trace elements). In order to decipher the subduction-to emplacement evolution of serpentinites, detailed petrology and geochemical investigations were applied to unravel the signature of fluid-rock interaction during subduction. Moreover, by integrating structural, petrological and geochemical results with thermodynamic modelling, the P-T-(t)-d paths representative of the evolution of serpentinites were obtained. Serpentinites from the Mount Avic massif retain structural and geochemical fingerprints of a long-lived evolution. Mantle (and oceanic) relics can be easily distinguished from the prograde metamorphic recrystallized phases: clinopyroxene, olivine and Cr-cores within magnetite preserve their mantle texture and, in part, their original composition. These features suggest that the serpentinites derived from mantle peridotites, mainly represented by harzburgite, that were exhumed and exposed at the Mesozoic Tethyan Ocean floor prior to alpine subduction. In this context, mantle peridotites likely interacted with seawater that enhanced the pervasive serpentinization and caused the progressive replacement of olivine and pyroxene by serpentine-lizardite with “mesh” and “bastite” textures, respectively. Bulk rock trace and REE patterns obtained on serpentinites as well as in situ concentrations of trace and rare Earth elements on serpentinite forming minerals suggest that the Mount Avic serpentinite still largely retains its oceanic geochemical signature despite intense Alpine subduction metamorphism. At the onset of the Tethyan ocean closure due to convergence between the Paleo-Europe and Paleo-Adria continental plates, the serpentinites underwent prograde metamorphism during Alpine subduction. Different mineral assemblages marking superposed fabrics are variably retained within the serpentinites. In foliated serpentinites, despite the pervasive overprinting provided by S2 foliation, relics of pre-D2 structures are preserved. In particular, Ti-chondrodite, clinopyroxene, olivine, antigorite and magnetite-bearing assemblage, marking pre-to early D1 metamorphic stage, attest for peak UHP conditions attained at Pmin = 2.8-2.85 GPa for T = 600-645°C. The crystallization of Ti-clinohumite along Ti-Chondrodite rims during D1 deformation stage, suggest that serpentinites, after reaching the peak metamorphic conditions, experienced a quasi-isothermal decompression from pre-to early D1 conditions, still within typical Alpine subduction geothermal gradient of 6.5-8.0°C/km. Detailed LA-ICP-MS analyses carried out on clinopyroxene, humite, olivine and serpentine minerals showed that Zr, Ti, Hf and Sr elements were mobilized during the transition from the mantle to the subduction environment. The lizardite-antigorite transition phase was accompanied by Sr, Cl and alkalis release in an open system which likely occurred in early oceanic stage and was complete at T>390°C. Differently, the partitioning of Ti, Hf, Zr between clinopyroxenes and Ti-clinohumite supports the idea that mantle clinopyroxene breakdown and successive high-pressure clinopyroxene and Ti-clinohumite crystallization took place in a closed space environment. During the retrograde path, the serpentinites re-equilibrated at Pmin = 2.4-1.7 GPa and T = 510-610°C; at these conditions, D2 deformation stage caused the crystallization of Ti-clinohumite and Ti-chondrodite in textural equilibrium along S2 foliation planes together with olivine, clinopyroxene, antigorite and magnetite. Locally, veins and shear zones marked by clinopyroxene, olivine and Ti-clinohumite postdating S2 foliation, attest that during syn-to-post-D2 exhumation hot geothermal gradients (20°C/km), indicative of greenschist facies re-equilibration, were reached. These new results provide unprecedented insights on the geochemistry, petrology and P-T peak conditions at which the Mount Avic serpentinites (Zermatt-Saas Zone) re-equilibrated during the alpine subduction and exhumation and allow for a comparison with the UHP serpentinite slices belonging to the Cignana Lake and Valtournanche (Créton) units of the Zermatt-Saas Zone north of the Aosta-Ranzola fault. Serpentinites of the Mount Avic massif underwent carbonation likely during their retrograde evolution, which caused the formation of carbonate-bearing lithologies comprising carbonated serpentinites and listvenites. These metasomatized rocks represent valuable records of CO2 sequestration enhanced by fluid-rock interaction mainly focused along serpentinite shear zones. Fluid-rock interaction, responsible for the genesis and evolution of carbonated serpentinites and listvenites, is attested not only by macro-scale, but also by micro-scale structural features. Petrographic observations suggest that the Mount Avic serpentinites, after having experienced D2 and post-D2 stages at high-pressure and greenschist facies conditions, respectively, likely reached lower T and P conditions while being progressively infiltrated by CO2-rich fluids. Ti-clinohumite relics within partially carbonated serpentinite, presently pseudomorphically replaced by olivine and ilmenite and the presence of Ti-clinohumite not yet replaced by olivine and ilmenite, but still in equilibrium with olivine and clinopyroxene, constrain the P-T conditions attained by serpentinites prior to carbonation and supports the hypothesis that the initiation of carbonation of serpentinites took place just before, or even simultaneously with, Ti-Chu breakdown, likely during the early retrograde greenschist facies re-equilibration of serpentinites. Based on thermodynamic modelling, the equilibrium P-T conditions for listvenitization are constrained at 0.3GPa and 300°C and the minimum amount of CO2, required for the infiltrating fluid to produce the listvenite mineral assemblage is XCO2 (min) = 0.02. Upon fluxing of CO2-bearing fluids, carbonated serpentinites are progressively transformed into listvenites as evidenced by the crystallization of quartz as a product of talc breakdown, making-up matrices together with aggregates of magnesite and fuchsite fibres, attesting for the presence of K in the infiltrating fluid. The presence of Cr-spinel, magnetite and mesh texture relics in listvenites is regarded as a clear petrogenetic relationship with serpentinites. Electrolytic fluid infiltration modelling suggests that the Mount Avic listvenites attained low T and very high fluid/rock (F/R) ratios: prolonged fluid infiltration at low temperatures causes the solubility of quartz to increases to the point that part of the SiO2 present in the listvenite matrix is dissolved in the fluid and then precipitated in veins which diffusively characterize listvenites. Furthermore, with progressive fluid infiltration, MgO/SiO2 ratio increases up to a point (at F/R between 6 and 7 and ca. 38-42% CO2), where the infiltrating fluid removes CO2 from the system. The dissolution of carbonate and concurrent silicification of the altered rock, once all the talc has reacted with CO2,aq to form magnesite and quartz, is thermodynamically predicted at T ≤ 200°C under prolonged fluid influx. The petrographic investigation on sulfides allowed to depict a chemical gradient: from serpentinites to listvenites the sulfide abundance decreases in accordance with the decrease in S (wt%) abundance observed by measured bulk rock values as well as predicted by thermodynamic models. In serpentinites the sulfur-poor heazlewoodite and pentlandite are associated with awaruite Ni-Fe alloy, in accordance with the observations that Fe-Ni alloys tend to be preferentially found in peridotites that have been only partially serpentinized. Passing to carbonated serpentinites the sulfide abundances progressively decreases and their composition changes. In particular, in carbonated serpentinites, neither Ni-Fe alloys nor pentlandite are identified while sulfur-poor heazlewoodite forms aggregates with sulfur-rich Co-bearing NiS. Finally, the transition from carbonated serpentinites to listvenites is marked by the limited presence of small-grained pentlandite and haezlewoodite, and absence of alloys while magnetite is progressively replaced by hematite displaying martite texture. The pseudomorphic martitic replacement of magnetite by hematite due to magnetite oxidation, observed exclusively within listvenites, and not in carbonated serpentinites where sulfides are still present, suggests that once the sulfides are completely dissolved, the fluid continues to evolve towards high oxygen fugacity (fO2) conditions. Prior to fluid infiltration, serpentinites attained very low fO2 and fS2 as constrained by the presence of Ni-Fe alloys (i.e., awaruite) and sulfur-poor assemblage (heazlewoodite and pentlandite) in magnetite-bearing antigorite-serpentinites. Within the successive steps of fluid infiltration, the redox conditions of the system changed: the passage from serpentinites to carbonated serpentinites is marked by more oxidising conditions, by a marked increase in Fe3+/Fetotal ratios and by the stability of heazlewoodite with sulfur-rich Co-bearing NiS (millerite). The transition from carbonated serpentinites to listvenites is marked by the consumption of sulfides and the concomitant constant increase in fO2. It is likely that sulfides in the Mount Avic carbonated serpentinites and listvenites were dissolved by sulfate-dominant fluids in oxidising rocks. Listvenites are predicted to be stable at high-fO2, above the MH (magnetite-hematite) buffer where hematite is expected to be stable, consistently with petrographic observations of pseudomorphic replacement of magnetite by martite-textured hematite. Interestingly, nickel shows high mobility during the carbonation process, passing from being retained in sulfides and alloys in serpentinites (awaruite, pentlandite and heazlewoodite) to form rare millerite in carbonated serpentinite and listvenites and to be concentrated in trevoritic cores within spinels in listvenites. The comparison between serpentinites and listvenites by means of trace elements allows to evaluate that the compatible elements (V, Sc, Zn, Cr, Ni and Cu) were redistributed between the two rock types suggesting a mainly closed-system behaviour during listvenitization and internal cycling of such components from the host minerals in the serpentinite. Differently, higher incompatible (K, Rb, Sr, Ba Nb) and fluid mobile elements concentrations in listvenites suggest that these elements were sourced externally from the serpentinites. Moreover, Co zoning within sulfides in carbonated serpentinites may reflect the interaction of the host rock with saline fluids, whose provenance could be attributed to dehydration reactions affecting serpentinites or to fluids released upon metasediments devolatilization. Based on stable carbon and oxygen isotope compositions of magnesite, it is proposed that the CO2-rich fluids promoting serpentinite carbonation derived from devolatilization of metasedimentary rocks in the subduction zone while metaperidotites were already facing early exhumation conditions during their early retrograde evolution. During syn-to-post D2 early retrograde path serpentinites experienced strain conditions favourable for the formation of shear zones; such structures served as preferred pathways for CO2-rich fluid circulation and enhanced the carbonation of serpentinites and the formation of listvenites. Only in Oligocene times, when the post-emplacement tectonics was active, the rheological contrast between serpentinites and listvenites worked as weakness surface that facilitated the nucleation of the normal fault along which listvenites and carbonated serpentinite crop out. The combination of field, macro and micro-structural, petrological and geochemical data with thermodynamic modelling, provide new and unprecedented insights 1) on the P-T-(t)-d history of the serpentinites from the Mount Avic: this represents the first attempt at defining a P-T stability field for the Zermatt-Saas serpentinites south of the Aosta-Ranzola fault in the Western Alps; 2) on the interaction of CO2-bearing fluids with serpentinites to form carbonated serpentinites and listvenites and how this process controls element mobilization, variations of redox sensitive elements and carbonation.