Figure - available from: Contributions to Mineralogy and Petrology
This content is subject to copyright. Terms and conditions apply.
Garnet composition for different samples and domains (core–rim) expressed in mole per cent of almandine, pyrope, grossular and spessartine. Data for each sample are a summary of analyses from different garnet grains and thus are not plotted according to distance from rim. Note that in most samples core and rim are relatively homogeneous when compared to core–rim variations
Source publication
Fluids are considered a fundamental agent for chemical exchanges between different rock types in the subduction system. Constraints on the sources and pathways of subduction fluids thus provide crucial information to reconstruct subduction processes. The Monviso ophiolitic sequence is composed of mafic, ultramafic and minor sediments that have been...
Similar publications
We constructed thermodynamic models of the formation of two types of gold-ore mineralization at the Kagan ultramafic massif in the Southern Urals (Russia). The first type of gold-mineralization is widely spread at the massif in the tectonic zones of schistose serpentinites containing typically ≤ 0.1 ppm Au. The second type of gold-ore mineralizatio...
Citations
... Since first documented in Franciscan blueschists (Dudley, 1969), oscillatory chemical zoning (short-wavelength interface-parallel peaks and troughs in elemental concentration) in HP-LT garnet has been widely recognized (e.g., García-Casco et al., 2002;Kabir and Takasu, 2010;Li et al., 2016;Tual et al., 2022). Though it is clear that not every garnet from HP-LT settings carries oscillatory elemental zoning patterns (e.g., see Rubatto and Angiboust, 2015), empirical evidence seems to suggest that most HP-LT settings contain rocks with these features. Even in cases where major element oscillations are seemingly absent (e.g., due to diffusional homogenization or lack of analytical resolution), trace element mapping reveals that oscillations are often preserved in rare earth elements (REEs; e.g., Rubatto et al., 2020;George et al., 2021;Konrad-Schmolke et al., 2023). ...
... Slow oxygen diffusivities (e.g., Vielzeuf et al., 2005;Higashino et al., 2019;Scicchitano et al., 2021) ensure that δ 18 O variations at scales of <10-100 µm in garnet are rarely modified by diffusion at HP-LT conditions, offering a record of prograde processes in eclogites and blueschists that cannot be accessed with the elemental record alone (e.g., Russell et al., 2013;Page et al., 2014;Cruz-Uribe et al., 2021;Bovay et al., 2021). Long wavelength core-to-rim δ 18 O variation in garnet from metasomatized HP-LT rocks has been associated with influx of external fluids with distinct isotopic signatures, with positive shifts (2.5-4 ‰) attributed to channelized influx of isotopically-heavy, sediment-derived fluid (e.g., Russell et al., 2013;Rubatto and Angiboust, 2015;Page et al., 2019). Shifts to lower δ 18 O values (by ≤8 ‰) have been attributed to infiltration of isotopically-light, serpentinite-or altered gabbro-derived fluids on either the prograde (e.g., Errico et al., 2013;Martin et al., 2014;Bovay et al., 2021) or retrograde path (Page et al., 2014;Cruz-Uribe et al., 2021). ...
... Shifts to lower δ 18 O values (by ≤8 ‰) have been attributed to infiltration of isotopically-light, serpentinite-or altered gabbro-derived fluids on either the prograde (e.g., Errico et al., 2013;Martin et al., 2014;Bovay et al., 2021) or retrograde path (Page et al., 2014;Cruz-Uribe et al., 2021). Oxygen isotope zonation has been documented in garnets displaying oscillatory elemental zoning (Bovay et al., 2021;Cruz-Uribe et al., 2021), in addition to garnets showing smooth prograde elemental zoning (e.g., Russell et al., 2013) or patchy metasomatic zoning (e.g., Errico et al., 2013;Rubatto and Angiboust, 2015). ...
... The more common Ph-bearing eclogites record dehydration as well, but also evidence for the stability of phengite at (near-)peak-P conditions. The abundance of phengite in a HP assemblage could result from a protolith enriched in LILE due to seafloor alteration (e.g., Angiboust & Agard, 2010) or HP fluid infiltration from external reservoir (Rubatto & Angiboust, 2015). While the phengite inclusions attest to its presence during prograde metamorphism (hence, supporting seafloor alteration), phengite infilling garnet microfractures suggest that post-fracturing crystallization was assisted by LILE-bearing fluid, either produced internally by the Ph-bearing eclogite or derived from an external source. ...
... Nevertheless, the provided evidence shows that in-situ dehydration has led directly to microfracturing of eclogites at HP conditions. Unlike other studies (e.g., Angiboust et al., 2012;Rubatto & Angiboust, 2015) the Ph-free Tsäkkok eclogites show that no external fluid was necessary to trigger the brittle response. Each eclogite dehydrates at the single-crystal scale, and only when internally produced fluids collectively exceeded a volume that allowed them to propagate through eclogite, the network of brittle fractures/veins could be formed (Connolly, 2010;Miller et al., 2003;Zhang & Green, 2007). ...
Aqueous fluids released during dehydration of a subducting slab have a large effect on the rheology of the subduction interface. While high-pressure experiments and natural-case studies link deformation with critical dehydration reactions during eclogitization, the exact interplay between these processes remains ambiguous. To investigate fluid–rock interaction and associated deformation at high-pressure, we studied a suite of eclogites from the Tsäkkok Lens of the Scandinavian Caledonides that record prograde metamorphism within an Early Palaeozoic cold subduction zone. Our results show that in-situ dehydration during the blueschist to eclogite facies transition produces fluid fluxes leading to rheological weakening and densification, consequently promoting ductile-brittle deformation. Petrographic evidence, supported by thermodynamic modelling and thermobarometry, attest to a prograde passage from lawsonite-blueschist to peak eclogite facies of ~2.5 GPa and ~620°C. Phengite-bearing eclogites imply interaction with an externally-derived fluid, whereas rare phengite-free, kyanite-eclogites only record internally-derived fluid production. Models predict that prograde breakdown of chlorite, lawsonite and amphibole between 500 and 610°C lead to progressive dehydration and release of up to 4.6 wt.% of aqueous fluid. Microstructural data reveal elongated shapes of highly strained omphacite porphyroblasts, displaying minor yet gradual changes in misorientation towards the grain boundaries. Occasionally, these intragranular structures form subgrain cells that have similar sizes to those of neoblasts in the rock matrix. These observations point to the potential onset of dynamic recrystallization processes via dislocation creep. Moreover, the omphacite neoblasts and rutile show non-random crystallographic preferred orientations (CPOs), which are characterized by the subparallel alignment of point-like maxima in rutile [001] and [100] axes to those of [001] and (010) of omphacite neoblasts, respectively. Additionally, the [001] axes of these minerals are also subparallel to the weak stretching mineral lineation, and the (100) of rutile and the (010) of omphacite neoblasts are distributed in the plane of the foliation. This suggests that the development of their CPOs was coeval and structurally controlled. Garnet microfractures normal to the foliation are dilated and sealed predominantly by omphacite. The lack of obliquity between CPO and foliation plane, as well as the systematic orientation of garnet microfracture orientations, are consistent with coaxial deformation at peak-pressure conditions. Unlike other studies, we show that neither an external fluid source nor channelized fluid flow is needed to facilitate a ductile-brittle deformation of eclogite in a subduction setting.
... Since first documented in Franciscan blueschists (Dudley, 1969), oscillatory chemical zoning (short-wavelength interface-parallel peaks and troughs in elemental concentration) in HP-LT garnet has been widely recognized (e.g., García-Casco et al., 2002;Kabir and Takasu, 2010;Li et al., 2016;Tual et al., 2022). Though it is clear that not every garnet from HP-LT settings carries oscillatory elemental zoning patterns (e.g., see Rubatto and Angiboust, 2015), empirical evidence seems to suggest that most HP-LT settings contain rocks with these features. Even in cases where major element oscillations are seemingly absent (e.g., due to diffusional homogenization or lack of analytical resolution), trace element mapping reveals that oscillations are often preserved in rare earth elements (REEs; e.g., Rubatto et al., 2020;George et al., 2021;Konrad-Schmolke et al., 2023). ...
... Slow oxygen diffusivities (e.g., Vielzeuf et al., 2005;Higashino et al., 2019;Scicchitano et al., 2021) ensure that δ 18 O variations at scales of <10-100 µm in garnet are rarely modified by diffusion at HP-LT conditions, offering a record of prograde processes in eclogites and blueschists that cannot be accessed with the elemental record alone (e.g., Russell et al., 2013;Page et al., 2014;Cruz-Uribe et al., 2021;Bovay et al., 2021). Long wavelength core-to-rim δ 18 O variation in garnet from metasomatized HP-LT rocks has been associated with influx of external fluids with distinct isotopic signatures, with positive shifts (2.5-4 ‰) attributed to channelized influx of isotopically-heavy, sediment-derived fluid (e.g., Russell et al., 2013;Rubatto and Angiboust, 2015;Page et al., 2019). Shifts to lower δ 18 O values (by ≤8 ‰) have been attributed to infiltration of isotopically-light, serpentinite-or altered gabbro-derived fluids on either the prograde (e.g., Errico et al., 2013;Martin et al., 2014;Bovay et al., 2021) or retrograde path (Page et al., 2014;Cruz-Uribe et al., 2021). ...
... Shifts to lower δ 18 O values (by ≤8 ‰) have been attributed to infiltration of isotopically-light, serpentinite-or altered gabbro-derived fluids on either the prograde (e.g., Errico et al., 2013;Martin et al., 2014;Bovay et al., 2021) or retrograde path (Page et al., 2014;Cruz-Uribe et al., 2021). Oxygen isotope zonation has been documented in garnets displaying oscillatory elemental zoning (Bovay et al., 2021;Cruz-Uribe et al., 2021), in addition to garnets showing smooth prograde elemental zoning (e.g., Russell et al., 2013) or patchy metasomatic zoning (e.g., Errico et al., 2013;Rubatto and Angiboust, 2015). ...
Subduction facilitates the transfer of volatiles from the Earth’s surface to its interior. However, the rock-scale processes that govern the efficiency of deep volatile transfer are not fully understood. Garnets from subduction zone rocks commonly have fine-scale, oscillatory elemental zoning that is typically considered to record external fluid ingress/transfer. Elemental and oxygen-isotope zoning in garnets from five exhumed subduction zone complexes show that in subduction zone rocks these records are not necessarily coupled; oxygen isotope evidence of ingress of buffering fluids, obvious only in rare cases, is decoupled from shorter length scale elemental and oxygen isotope zonings (which also show no coupling with each other). This finding suggests multiple mechanisms of internal chemical transfer operate at the grain and rock scale during subduction, and that rocks may commonly experience only limited interaction with external fluids. The results presented are consistent with a picture of volatile transfer in subduction that is spasmodic, highly localized, and variably efficient at evacuating fluids inherited from the surface then released by metamorphic dehydration.
... Raimondo et al. 2012Raimondo et al. , 2017Russell et al. 2013). For example, garnet can preserve variation of fluid compositions as it grows, particularly in eclogites or skarns (D'Errico et al. 2012;Rubatto and Angiboust 2015). Oxygen in garnet is very similar to the bulk rock δ 18 O value, but deviations can occur following temperature variations affecting equilibration factors between minerals (i.e. ...
The study of magmatic and metamorphic processes is challenged by geological complexities like geochemical variations, geochronological uncertainties, and the presence/absence of fluids and/or melts. However, by integrating petrographic and microstructural studies with geochronology, geochemistry, and phase equilibrium diagrams investigations of different key mineral phases, it is possible to reconstruct pressure-temperature-deformation-time histories. Using multiple geochronometers in a rock can provide a detailed temporal account of its evolution, as these geological clocks have different closure temperatures. Given the continuous improvement of existing and new in-situ analytical techniques, this contribution provides an overview of frequently utilised petrochronometers such as garnet, zircon, titanite, allanite, rutile, monazite/xenotime, and apatite, by describing the geological record that each mineral can retain, and explaining how to retrieve this information. These key minerals were chosen as they provide reliable age information in a variety of rock types and, when coupled with their trace element composition, form powerful tools to investigate crustal processes at different scales. This review recommends best applications for each petrochronometer, highlights limitations to be aware of, and discusses future perspectives. Finally, this contribution highlights the importance of integrating information retrieved by multi-petrochronometer studies to gain an in-depth understanding of complex thermal and deformation crustal processes.
... The available data from the literature point to a Middle-Upper Eocene age for the peak metamorphic conditions in both the continental and oceanic units of the working area. The eclogites from the Monviso complex equilibrated at ~45 Ma (Rubatto and Hermann, 2003;Rubatto and Angiboust, 2015) or ~51 Ma (Garber et al., 2020), while the overlying metasedimentary unit yielded 40 Ar/ 39 Ar ages from ~60 to ~50-45 Ma (Agard et al., 2002). The age of UHP metamorphism in the Brossasco-Isasca unit is now established at ~35 Ma (Gebauer et al., 1997;Rubatto and Hermann, 2001;Gauthier-Putallaz et al., 2016;Xiong et al., 2021). ...
Here we describe the structure, the high-pressure, low-temperature (HP-LT) metamorphism and tectonic evolution of the Briançonnais distal margin units from the south Western Alps. The studied area extends southwest of the Dora-Maira (U)HP basement units and east-southeast of the classical Briançonnais nappes. A new structural map accompanied by geological profiles shows the thrusting of the oceanic nappes (Monviso and Queyras units) onto the distal Briançonnais units (D1 and D2 late Eocene deformation phases) under blueschist-facies conditions. Subsequent deformation during the Early Oligocene (D3 deformation phase) took place under greenschist-facies conditions and was associated with back-folding and -thrusting in the units overlying the Dora-Maira massif and with exhumation related to normal reactivation of former thrusts within the latter massif. Two large cover units, detached from their former distal Briançonnais basement, are redefined as the Maira-Sampeyre and Val Grana Allochthons (shortly: Maira-Grana Allochthons = MGA) including, (i) the Val Maira-Sampeyre unit involving Lower and Middle Triassic formations, seemingly detached from the Dora-Maira units during the subduction process, and (ii) the Val Grana unit with Middle-Upper Triassic and Early-Middle Jurassic formations, which was probably detached from the Maira-Sampeyre unit and correlates with the “Prepiemonte units” known from the Ligurian Alps to the Swiss Prealps. Three major shear zones involving tectonic mélanges of oceanic and continental rocks at the base of the Val Grana, Maira-Sampeyre and Dronero units testify to an early phase of exhumation within the subduction channel in front of the Adria plate. We present a new metamorphic map based on published and new petrological data, including new thermometric data obtained by Raman spectroscopy of carbonaceous material (RSCM). The T RSCM values range from ~ 400 °C to > 500 °C, going from the most external Val Grana unit and overlying Queyras schists to the uppermost Dora-Maira unit. During the Late Triassic, the width of the Briançonnais s.l. domain can be restored at ~ 100 km, whereas it reached ~ 150 km after the Jurassic rifting. A significant, second rifting event affected the Briançonnais domain during the Late Cretaceous-Paleocene, forming the Longet-Alpet chaotic breccias, which deserve further investigations.
... Meteoric water values from central British Columbia are mean values from streamflow between 2013-2019 (Gibson et al., 2020). Also plotted are previous work on oceanic (Wenner and Taylor, 1973;Agrinier and Cannat, 1997;Ribeiro Da Costa et al., 2008;Rubatto and Angiboust, 2015), ophiolite (Magaritz and Taylor, 1974;Früh-Green et al., 2001 and references therein;Cluzel et al., 2020), and continental (Wenner and Taylor, 1973 and references therein;Kyser et al., 1999), serpentines that encompass lizardite, chrysotile and antigorite. b) δ 13 C and δ 18 O of selected carbonate-altered rocks from this study (maroon square: ophicarbonate, orange triangle: soapstone, yellow triangle: listvenite). ...
The serpentinized and tectonized mantle in the Decar area in central British Columbia, including rocks which host the Baptiste Ni Deposit, consists of several ultramafic protolith lithologies that are variably altered to serpentinite, ophicarbonate, soapstone and listvenite. Alteration minerals include brucite (Mg(OH)2), which can be used to sequester atmospheric CO2 and awaruite (Ni3Fe), which is an economically attractive nickel alloy. This study examines the formation and preservation of brucite (up to 13 wt.%) and awaruite (up to 0.12 wt.%) in the Decar area and demonstrates that both minerals are formed during serpentinization and destroyed during carbonate alteration of mantle rocks.
We distinguish five alteration stages that occurred primarily in a continental environment: 1) low-temperature lizardite serpentinization from meteoric fluids at <300 °C, 2) high-temperature antigorite (±metamorphic olivine) serpentinization from metamorphic fluids at >300 °C, 3) carbonate alteration, 4) chrysotile veining (±antigorite) serpentinization, and 5) later carbonate alteration from crustal fluids. Brucite formed primarily during late lizardite serpentinization and is most abundant in rocks that originally had high olivine-pyroxene ratios. Awaruite formed during both late lizardite serpentinization and during antigorite serpentinization and is most abundant in serpentinized olivine-rich harzburgite. The stability and abundance of brucite and awaruite are controlled by both the host rock composition and degree of serpentinization. The coexistence of brucite and awaruite reflects formation in serpentinized olivine-rich peridotite, and creates an opportunity for carbon-neutral nickel mining.
... It plays a crucial role in revealing thermal and mechanical processes controlling the evolution of Earth's crust at plate boundaries (Caddick and Kohn 2013) through its chemical and isotopic zoning. In particular, oxygen isotope heterogeneities in natural garnet crystals can record the infiltration of external fluids in metamorphic or hydrothermal systems, allowing for the determination of timing and rates of rock-fluid interactions in the crust (e.g., Kohn et al. 1993;Crowe et al. 2001;Skelton et al. 2002;Vielzeuf et al. 2005;Page et al. 2010Page et al. , 2014Sobolev et al. 2011;D'Errico et al. 2012;Errico et al. 2013;Russell et al. 2013;Martin et al. 2014;Rubatto and Angiboust 2015;He et al. 2019;Higashino et al. 2019;Gauthiez-Putallaz et al. 2020;Vho et al. 2020). Successful geothermobarometry and retrieval of accurate P-T-t-X fluids paths relies on the assumption that mineral assemblages were formed at equilibrium. ...
Knowledge of oxygen diffusion in garnet is crucial for a correct interpretation of oxygen isotope signatures in natural samples. A series of experiments was undertaken to determine the diffusivity of oxygen in garnet, which remains poorly constrained. The first suite included high-pressure (HP), nominally dry experiments performed in piston-cylinder apparatus at: (1) T = 1050–1600 °C and P = 1.5 GPa and (2) T = 1500 °C and P = 2.5 GPa using yttrium aluminum garnet (YAG; Y3Al5O12) cubes. Second, HP H2O-saturated experiments were conducted at T = 900 °C and P = 1.0–1.5 GPa, wherein YAG crystals were packed into a YAG + Corundum powder, along with 18O-enriched H2O. Third, 1 atm experiments with YAG cubes were performed in a gas-mixing furnace at T = 1500–1600 °C under Ar flux. Finally, an experiment at T = 900 °C and P = 1.0 GPa was done using a pyrope cube embedded into pyrope powder and 18O-enriched H2O. Experiments using grossular were not successful.
Profiles of 18O/(18O+16O) in the experimental charges were analyzed with three different secondary ion mass spectrometers (SIMS): sensitive high-resolution ion microprobe (SHRIMP II and SI), CAMECA IMS-1280, and NanoSIMS. Considering only the measured length of 18O diffusion profiles, similar results were obtained for YAG and pyrope annealed at 900 °C, suggesting limited effects of chemical composition on oxygen diffusivity. However, in both garnet types, several profiles deviate from the error function geometry, suggesting that the behavior of O in garnet cannot be fully described as simple concentration-independent diffusion, certainly in YAG and likely in natural pyrope as well. The experimental results are better described by invoking O diffusion via two distinct pathways with an inter-site reaction allowing O to move between these pathways. Modeling this process yields two diffusion coefficients (D values) for O, one of which is approximately two orders of magnitude higher than the other. Taken together, Arrhenius relationships are:logDm2s-1=-7.2(±1.3)+(-321(±32)kJmol-12.303RT)
for the slow pathway, andlogDm2s-1=-5.4(±0.7)+(-321(±20)kJmol-12.303RT)
for the fast pathway. We interpret the two pathways as representing diffusion following vacancy and inter-stitial mechanisms, respectively. Regardless, our new data suggest that the slow mechanism is prevalent in garnet with natural compositions, and thus is likely to control the retentivity of oxygen isotopic signatures in natural samples.
The diffusivity of oxygen is similar to Fe-Mn diffusivity in garnet at 1000–1100 °C and Ca diffusivity at 850 °C. However, the activation energy for O diffusion is larger, leading to lower diffusivities at P-T conditions characterizing crustal metamorphism. Therefore, original O isotopic signatures can be retained in garnets showing major element zoning partially re-equilibrated by diffusion, with the uncertainty caveat of extrapolating the experimental data to lower temperature conditions.
... The available data from the literature point to a Middle-Upper Eocene age for the peak metamorphic conditions in both the continental and oceanic units of the working area. The eclogites from the Monviso complex equilibrated at ~45 Ma (Rubatto and Hermann, 2003;Rubatto and Angiboust, 2015) or ~51 Ma (Garber et al., 2020), while the overlying metasedimentary unit yielded 40 Ar/ 39 Ar ages from ~60 to ~50-45 Ma (Agard et al., 2002). The age of UHP metamorphism in the Brossasco-Isasca unit is now established at ~35 Ma (Gebauer et al., 1997;Rubatto and Hermann, 2001;Gauthier-Putallaz et al., 2016;Xiong et al., 2021). ...
Here we describe the structure, the high-pressure, low-temperature (HP-LT) metamorphism and tectonic evolution of the Briançonnais distal margin units from the south Western Alps. The studied area extends southwest of the Dora-Maira (U)HP basement units and east-southeast of the classical Briançonnais nappes. A new structural map accompanied by geological profiles shows the thrusting of the oceanic nappes (Monviso and Queyras units) onto the distal Briançonnais units (D1 and D2 late Eocene deformation phases) under blueschist-facies conditions. Subsequent deformation during the Early Oligocene (D3 deformation phase) took place under greenschist-facies conditions and was associated with back-folding and-thrusting in the units overlying the Dora-Maira massif and with exhumation related to normal reactivation of former thrusts within the latter massif. Two large cover units, detached from their former distal Briançonnais basement, are redefined as the Maira-Sampeyre and Val Grana Allochthons (shortly: Maira-Grana Allochthons = MGA) including, (i) the Val Maira-Sampeyre unit involving Lower and Middle Triassic formations, seemingly detached from the Dora-Maira units during the subduction process, and (ii) the Val Grana unit with Middle-Upper Triassic and Early-Middle Jurassic formations, which was probably detached from the Maira-Sampeyre unit and correlates with the "Prepiemonte units" known from the Ligurian Alps to the Swiss Prealps. Three major shear zones involving tectonic mélanges of oceanic and continental rocks at the base of the Val Grana, Maira-Sampeyre and Dron-ero units testify to an early phase of exhumation within the subduction channel in front of the Adria plate. We present a new metamorphic map based on published and new petrological data, including new thermometric data obtained by Raman spectroscopy of carbonaceous material (RSCM). The T RSCM values range from ~ 400 °C to > 500 °C, going from the most external Val Grana unit and overlying Queyras schists to the uppermost Dora-Maira unit. During the Late Triassic, the width of the Briançonnais s.l. domain can be restored at ~ 100 km, whereas it reached ~ 150 km after the Jurassic rifting. A significant, second rifting event affected the Briançonnais domain during the Late Cretaceous-Paleocene, forming the Longet-Alpet chaotic breccias, which deserve further investigations.
... GPa were reached at c. 45 Ma (e.g. Rubatto & Angiboust, 2015;Locatelli et al., 2018). Fig. S4. ...
We herein report the finding of rare eclogite-facies vugs forming millimeter- to centimeter-sized pockets in meta-ophiolites from the western Alps. Euhedral garnet crystals covering the vug walls display oscillatory chemical zoning for a wide range of major and trace elements including Cr, Mn and rare earth elements. Thermodynamic modelling reveals that closed-system fluid production through the breakdown of prograde glaucophane, lawsonite and chlorite between 505‒525 °C can successfully explain porosity creation of ~4 % and the mineralogical properties of the vugs. Available geological and geochronological constrains indicate that the eclogitization of the downgoing mafic crust spanned a window of at least 1 Myr. These observations can only be explained by the presence of extremely low permeability values (< 10-22 m²) to keep the fluid confined at the meter scale within vugs on such timescales. Our field-based report of eclogite porosity provides the first in situ confirmation of previous experimental data and geophysical estimates on active margins. A substantial amount of fluid trapped in this porosity may be carried deeper than expected into the Earth’s mantle, with implications for volatile recycling budgets.
... The Monviso complex is interpreted to be a former oceanic core complex in a slow-spreading ocean (Lagabrielle and Cannat, 1990;Festa et al., 2015). Its main unit (Lago Superiore unit), just above the Dora-Maira complex, was metamorphosed at 2.7 GPa and 580°C (Groppo and Castelli, 2010;Angiboust et al., 2011Angiboust et al., , 2012Locatelli et al., 2019); it reached its peak between 53 and 45 Ma (Monié and Philippot, 1989;Duchêne et al., 1997;Garber et al., 2020;Rubatto and Hermann, 2003;Rubatto and Angiboust, 2015) and was retrogressed at lower crustal depths at 42-36 Ma (Cliff et al., 1998;Angiboust and Glodny, 2020). The timing of peak metamorphism of the overlying Monviso unit, metamorphosed at 2.2 GPa and 490°C (Schwartz et al., 2000a;Angiboust et al., 2012) is unknown, but retrogression occurred at the same time as in the Lago Superiore unit (Angiboust and Glodny, 2020). ...
The Dora‐Maira massif is an archetypal nappe stack of subducted and exhumed upper crust. Slices of continental crust experienced metamorphism at upper blueschist to ultrahigh‐pressure (UHP) eclogite‐facies conditions. While the timing of peak metamorphism in the UHP unit has been extensively studied, little is known about the other units. In order to constrain the timing and conditions of high‐pressure metamorphism, U‐Pb‐trace element analyses of rutile and titanite were carried out across the nappe stack. The data reveal Alpine peak metamorphic ages younging downwards in the stack, from ∼40 to ∼33 Ma. Greenschist‐facies retrogression of the whole massif occurred at ∼32–31 Ma, after high‐pressure metamorphism of the lowermost unit (Sanfront‐Pinerolo Unit). Tectonic implications include (a) continuous and fast exhumation of subducted continental crust, (b) long‐lived subduction from ∼60 to ∼33 Ma of the distal European margin, reconstructed to be a hyperextended margin spread over ∼130 km for the Dora‐Maira massif alone, and (c) the initiation of continental collision synchronous with the end of high‐pressure metamorphism.
