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A persistent problem in the study of garnet geochemistry is that the consideration of major elements alone excludes a wealth of information preserved by trace elements, particularly the rare-earth elements (REEs). This is despite the fact that trace elements are generally less vulnerable to diffusive resetting, and are sensitive to a broader spectrum of geochemical interactions involving the entire mineral assemblage, including the growth and/or dissolution of accessory minerals. We outline a technique for the routine acquisition of high-resolution 2D trace element maps by LA-ICP-MS, and introduce an extension of the software package XMapTools for rapid processing of LA-ICP-MS data to visualise and interpret compositional zoning patterns. These methods form the basis for investigating the mechanisms controlling geochemical mobility in garnet, which are argued to be largely dependent on the interplay between element fractionation, mineral reactions and partitioning, and the length scales of intergranular transport. Samples from the Peaked Hill shear zone, Reynolds Range, central Australia, exhibit contrasting trace element distributions that can be linked to a detailed sequence of growth and dissolution events. Trace element mapping is thus employed to place garnet evolution in a specific paragenetic context and derive absolute age information by integration with existing U–Pb monazite and Sm–Nd garnet geochronology. Ultimately, the remarkable preservation of original growth zoning and its subtle modification by subsequent re-equilibration is used to ‘see through’ multiple superimposed events, thereby revealing a previously obscure petrological and temporal record of metamorphism, metasomatism, and deformation.

Subduction zones represent one of the most critical settings for fluid recycling as a consequence of dehydration of the subducting lithosphere. A better understanding of fluid flows within and out of the subducting slab is fundamental to unravel the role of fluids during burial. In this study, major and trace element geochemistry combined with oxygen isotopes were used to investigate metasediments and eclogites from the Sesia Zone in order to reconstruct the effect of internal and external fluid pulses in a subducted continental margin. Garnet shows a variety of textures requiring dissolution–precipitation processes in presence of fluids. In polycyclic metasediments, garnet preserves a partly resorbed core, related to pre-Alpine high-temperature/low-pressure metamorphism, and one or multiple rim generations, associated with Alpine subduction metamorphism. In eclogites, garnet chemical zoning indicates monocyclic growth with no shift in oxygen isotopes from core to rim. In metasediments, pre-Alpine garnet relics show δ18O values up to 5.3 ‰ higher than the Alpine rims, while no significant variation is observed among different Alpine garnet generations within each sample. This suggests that an extensive re-equilibration with an externally-derived fluid of distinct lower δ18O occurred before, or in correspondence to, the first Alpine garnet growth, while subsequent influxes of fluid had δ18O close to equilibrium. The observed shift in garnet δ18O is attributed to a possible combination of (1) interaction with sea-water derived fluids during pre-Alpine crustal extension and (2) fluids from dehydration reactions occurring during subduction of previously hydrated rocks, such as the serpentinised lithospheric mantle or hydrated portions of the basement.

This study investigates the P–T–t–D evolution of two metapelitic samples from the middle crust exposed in the Aiguilles Rouges Massif. Garnet compositional mapping, phase equilibrium modelling, zirconium-in-rutile thermometry, trace element geochemistry of garnet and monazite, and U-Pb LA-ICP-MS dating on monazite were used to better understand the tectonic and thermal history of the variscan External Crystalline Massifs. In the sample representing the upper-middle crust (AR736, southwestern part of the massif), using the preserved mineral assemblage in garnet inclusion (Grt + St + Bt + Ms + Qz + Pl + Rt) and garnet compositions, the prograde P–T path was constrained from ∼0.5–0.6 GPa and 550–625 °C to ∼0.76–0.82 GPa and 600–640 °C. The P–T conditions at the onset of this prograde evolution suggest a high geothermal gradient (∼30–35 °C/km) prior to the onset of crustal thickening. In the sample representing the lower-middle crust (AR14, central part of the massif), using the preserved mineral assemblage in garnet inclusion (Grt + Bt + Ms + Qz + Pl + Rt), the occurrence of sillimanite and ilmenite in the matrix and garnet compositions, a β-shaped P–T path characterised by a late temperature increase during exhumation was identified. Both samples recorded a retrograde P–T stage at ∼0.4 GPa and 545 °C, dated at 315–305 Ma. Microstructural analysis indicates dextral transcurrent deformation from the late crustal thickening stage to the exhumation phase. Comparison with previously published P–T paths from eclogitic lenses highlights the juxtaposition of middle and lower crustal domains during dextral transcurrent deformation. We propose a tectonic model in which the formation of supra-subduction volcano-sedimentary basins (∼350 Ma) is followed by crustal thickening between 350 and 340 Ma under a thermal gradient of ∼5 – 15 °C/km. The exhumation of the lower and middle crust took place in a transcurrent regime between 340 and 305 Ma. This prolonged transcurrent tectonic activity suggests that the numerous transcurrent shear zones in the Variscan belt are not merely late orogenic structures but played a significant role in the geodynamic evolution, particularly in the exhumation of the orogenic crust, from the end of continental collision to the closure of the Variscan orogeny. Cette étude examine l’évolution P–T–t–D de deux échantillons de métapélites provenant de la croûte moyenne du massif des Aiguilles Rouges. Des cartes compositionnelles de grenat, la modélisation des équilibres de phases, le thermomètre zirconium-in-rutile, la géochimie des éléments traces dans le grenat et la monazite, ainsi que la datation U-Pb par LA-ICP-MS sur monazite ont été utilisés pour mieux comprendre l’histoire thermique et tectonique des Massifs Cristallins Externes varisques. Dans l’échantillon représentant la croûte moyenne supérieure (AR736, partie sud-ouest du massif), à l’aide de l’assemblage minéralogique préservé dans les inclusions du grenat (Grt + St + Bt + Ms + Qz + Pl + Rt) et des compositions chimiques des grenats, le trajet P–T prograde a été contraint entre ∼0,5–0,6 GPa et 550–625 °C jusqu’à ∼0,76–0,82 GPa et 600–640 °C. Les conditions P–T au début de ce trajet suggèrent un gradient géothermique élevé (∼30–35 °C/km) avant le début de l’épaississement crustal. Dans l’échantillon représentant la croûte moyenne inférieure (AR14, partie centrale du massif), à l’aide de l’assemblage minéralogique préservé dans les inclusions de grenat (Grt + Bt + Ms + Qz + Pl + Rt), de la présence de sillimanite et d’ilménite dans la matrice et des compositions chimiques des grenats, un trajet P–T en forme de β, caractérisé par une augmentation tardive de la température durant l’exhumation, a été identifié. Les deux échantillons ont enregistré une phase rétrograde à ∼0,4 GPa et 545 °C et, datée entre 315 et 305 Ma. L’analyse microstructurale indique une déformation transcurrente dextre depuis la phase tardive d’épaississement crustal jusqu’à la fin de l’exhumation. La comparaison avec les trajectoires P–T publiées pour des lentilles éclogitiques met en évidence la juxtaposition de domaines de croûte moyenne et inférieure durant la déformation transcurrente dextre. Nous proposons un modèle tectonique dans lequel la formation de bassins volcano-sédimentaires suprasubduction (∼350 Ma) est suivie par l’épaississement crustal entre 350 et 340 Ma sous un gradient thermique d’environ ∼5 – 15 °C/km. L’exhumation de la croûte moyenne et inférieure s’est déroulée dans un régime transcurrent dextre entre 340 et 305 Ma. Cette activité tectonique transcurrente prolongée suggère que les nombreuses zones de cisaillements transcurrentes de la chaîne Varisque ne sont pas seulement des structures tardi-orogéniques mais ont joué un rôle significatif dans l’évolution géodynamique, en particulier dans l’exhumation de la croûte orogénique, depuis la fin de la collision continentale jusqu’à la fin de l’orogène Varisque.

Oscillatory compositional zoning in minerals has been observed in hydrothermal, magmatic, and metamorphic environments and is commonly attributed to chemical or physical cyclical changes during crystal growth. Chemical zoning is a common feature of solid solutions, which has been rarely reported in phyllosilicates. In this study, oscillatory zoning in chlorite is described in samples from the Pic-de-Port-Vieux thrust, a minor thrust fault associated to the major Gavarnie thrust fault zone (Central Pyrenees, Spain). The Pic-de-Port-Vieux thrust sheet comprises a 1-20 m thick layer of Triassic red pelite and sandstone thrust over mylonitized Cretaceous dolomitic limestone. The thrust fault zone deformation comprises secondary faults and cleavage affecting the Triassic pelite and sandstone. An important feature responsible to this deformation is a set of veins filled by quartz and chlorite. Chlorite is present in crack-seal extension veins and in shear veins; both structures opened under the same stress conditions. In some shear veins, chlorite occurs as pseudo-uniaxial plates arranged in rosette-shaped aggregates. These aggregates appear to have developed as a result of radial growth of the chlorite platelets. Oscillatory zoning has been imaged by backscattered scanning electron microscopy and by X-ray quantitative micro-mapping. These oscillations correspond to chemical zoning with alternating iron-rich and magnesium-rich bands. The chlorite composition ranges from a Fe-rich pole to a Mg-rich pole. Fe3+/ΣFe values were measured in chlorite using µ-XANES spot analyses and vary from 0.23 to 0.44. The highest values are in the Fe-rich area. Temperature maps, built from standardized microprobe X-ray images and redox state using the program XMapTools, indicate oscillatory variations from about 310 to 400±50°C during chlorite crystallization. These temperature variations are correlated with a Fe3+/ΣFe variation by Al3+Fe3+-1 and di-trioctahedral substitutions highlighted by Mg and FeTot contents (Fe-Mg zoning). Chemical variations could be then explained by alternation of cooling times and cyclical pulses of a fluid hotter than the host rock. It is however not excluded that kinetic effects influence the incorporation of Mg or Fe during chlorite crystallization.

To provide a better picture of the active geodynamics along the Variscan suture zones during the late collisional stage (particularly regarding the evolution of the orogenic system towards HT conditions), we focused here on vaugnerites, which consist of mafic ultra-potassic magmatic rocks, intrusive into the granite-gneiss sequences of the Variscan Vosges crystalline massif. Those rocks, though subordinate in volume, are frequently associated with late-collisional granites. In the Central-Southern Vosges, they appear either as (1) pluton margin of the Southern Vosges Ballons granite complex or (2) composite dykes intrusive into migmatite and metamorphic sequences classically referred to as granite-gneiss unit (Central Vosges). Both types correspond to melanocratic rocks with prominent, Mg-rich, biotite and hornblende (20–40% vol., 64 < mg# < 78), two-feldspar and quartz. Those Vosges vaugnerites display geochemical signatures characteristic of ultra-potassic mafic to intermediate, metaluminous to slightly peraluminous rocks. Zircon U-Pb ages were obtained by Laser Ablation Inductively Coupled Plasma Mass Spectrometry. Zircon grains were extracted from a sillimanite-bearing gneiss from the granite-gneiss unit hosting the Central Vosges vaugnerites. They yielded an age at 451 ± 9 Ma, indicating a pre-Variscan Upper Ordovician protolith for the host sequence. Zircon from the four vaugnerite intrusives display U-Pb ages (± 2σ) of 340 ± 2.5 Ma (Ballons), 340 ± 25 Ma, 340 ± 7 Ma and 336 ± 10 Ma (Central Vosges). Synchronous within uncertainty, vaugnerite age data suggest a relatively early emplacement during the Late Variscan collisional history ( i.e. Middle Visean times). These results are in line with previously published ages from the Southern Vosges volcano-sedimentary sequences (Oderen-Markstein) and the nearby ultra-potassic granite complexes from the Central and Southern Vosges (Ballons, Crêtes) thereby arguing for a magmatic event of regional significance. Recent petrological studies on vaugnerites suggest that they derive from partial melting of a metasomatized mantle contaminated to some different degrees by elements of continental crust. We propose here that the major ultra-potassic magmatic pulse at 340–335 Ma is a consequence of a significant change into the dynamics of the Rhenohercynian subduction system below the Central-Southern Vosges. In the light of recent thermo-mechanical modelling experiments on mature continental collision, magmatism could result from a syn-collisional lithospheric delamination mechanism involving (1) first, continental subduction evolving towards (2) the underthrusting of the Avalonian continental margin lower crust and (3) the initiation of lithospheric delamination within the supra-subduction retro-wedge (Saxothuringian-Moldanubian continental block). This delamination would drive the emplacement of an asthenospheric upwelling, initially localized along the Variscan suture zones, and gradually propagating towards the southern front of the belt during the Late Carboniferous, as the delamination front migrated at the base of the crust. Afin d’avancer dans la compréhension de la géodynamique des zones de suture varisque au stade tardif de la collision (en particulier au regard de l’évolution du système orogénique en contexte de HT), nous nous sommes intéressés à la mise en place des vaugnérites, roches basiques ultra-potassiques intrusives au sein des ensembles granito-gneissiques de la croûte varisque des Vosges. Ces roches, souvent associées aux ensembles granitiques tardi-collisionnels, sont de faible extension mais quasiment toujours présentes au sein de ces systèmes. Elles affleurent (1) dans les Vosges méridionales, en masses plutoniques marginales du Granite des Ballons et (2) dans les Vosges centrales (région de Plainfaing), en filons composites, intrusifs dans un complexe de migmatites et de roches métamorphiques appelées granite-gneiss. Les vaugnérites sont des roches mélanocrates à biotite et hornblende magnésiennes (20 à 40 % vol, 64 < mg# < 78), présentant des signatures géochimiques caractéristiques de roches ultra-potassiques mafiques à intermédiaires, métalumineuses à légèrement peralumineuses. L’âge U-Pb obtenu par ablation laser ICP-MS du zircon d’un gneiss à sillimanite du granite-gneiss encaissant des vaugnérites est de 451 ± 9 Ma, révélant un bâti pré-varisque à protolithe ordovicien supérieur. Les grains de zircon extraits de quatre vaugnérites donnent des âges U-Pb (± 2σ) de 340 ± 2,5 Ma (Ballons), 340 ± 25 Ma, 340 ± 7 Ma et 336 ± 10 Ma (Vosges centrales). Les données de datation des vaugnérites, identiques aux incertitudes de mesure près, apparaissent donc cohérentes et révèlent un âge assez précoce dans l’histoire tardi-collisionnelle de la chaîne. Ces données, en accord avec les âges publiés préalablement sur ce secteur, montrent que les vaugnérites se mettent en place au Viséen moyen, au cours d’un événement magmatique majeur, exprimé tant dans les séries volcano-sédimentaires (Séries Oderen-Markstein) que dans les granitoïdes ultra-potassiques des Vosges méridionales (Ballons) et centrales (Crêtes). Les études pétrologiques récentes sur les vaugnérites suggèrent qu’elles dérivent de la fusion partielle d’un manteau métasomatisé et contaminé à différents degrés par des éléments de croûte continentale fondue. Nous proposons ici que ce « pulse » magmatique ultra-potassique d’ampleur à 340–335 Ma soit le signe une évolution majeure dans la dynamique de la subduction de la lithosphère rhénohercynienne sous les Vosges centrales et méridionales. Sur le modèle d’expériences thermo-mécaniques récentes simulant le déroulement d’une collision continentale mature, il pourrait traduire les premiers effets d’un phénomène de délamination lithosphérique syn-collisionelle impliquant (1) une subduction continentale relayée (2) par le sous-charriage d’une lame de croûte inférieure de la marge continentale avalonienne et (3) l’initiation de la délamination lithosphérique au sein du prisme orogénique supra-subduction qu’était le bloc continental saxothüringien-moldanubien. Ce processus conduirait à la mise en place d’un « upwelling » asthénosphérique, initialement localisé aux zones de suture varisque et se propageant au cours de la fin du Carbonifère vers le front sud de la chaîne à mesure de la propagation du front de délamination à la base de la croûte.

We present a new thermodynamic activity-composition model for di-trioctahedral chlorite in the system FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O that is based on the Holland–Powell internally consistent thermodynamic data set. The model is formulated in terms of four linearly independent end-members, which are amesite, clinochlore, daphnite and sudoite. These account for the most important crystal-chemical substitutions in chlorite, the Fe–Mg, Tschermak and di-trioctahedral substitution. The ideal part of end-member activities is modeled with a mixing-on-site formalism, and non-ideality is described by a macroscopic symmetric (regular) formalism. The symmetric interaction parameters were calibrated using a set of 271 published chlorite analyses for which robust independent temperature estimates are available. In addition, adjustment of the standard state thermodynamic properties of sudoite was required to accurately reproduce experimental brackets involving sudoite. This new model was tested by calculating representative P – T sections for metasediments at low temperatures (<400 °C), in particular sudoite and chlorite bearing metapelites from Crete. Comparison between the calculated mineral assemblages and field data shows that the new model is able to predict the coexistence of chlorite and sudoite at low metamorphic temperatures. The predicted lower limit of the chloritoid stability field is also in better agreement with petrological observations. For practical applications to metamorphic and hydrothermal environments, two new semi-empirical chlorite geothermometers named Chl(1) and Chl(2) were calibrated based on the chlorite + quartz + water equilibrium (2 clinochlore + 3 sudoite = 4 amesite + 4 H 2 O + 7 quartz). The Chl(1) thermometer requires knowledge of the (Fe 3+ /ΣFe) ratio in chlorite and predicts correct temperatures for a range of redox conditions. The Chl(2) geothermometer which assumes that all iron in chlorite is ferrous has been applied to partially recrystallized detrital chlorite from the Zone houillère in the French Western Alps.

Understanding mass transfer associated with fluids circulation and deformation in the Alpine orogeny is often complex due to common multistage crystallization. For example, in two emblematic and historic Pb-Ag deposits of the French Alps, Macôt-la Plagne (MP) and Peisey-Nancroix (PN), a sedimentary or orogenic origin is still debated. To discriminate between the metallogenic models of the two deposits, an integrative methodology combining field, microstructural, mineralogical, thermobarometrical, and geochronological data was here applied for establishing detailed Pressure–Temperature–Time–Deformation (P-T-t-d) mineralization conditions. Both deposits are located in Permo-Triassic quartzite of the External Briançonnais domain along the Internal Briançonnais Front (Internal Western Alps). The ore mainly occurs as veins and disseminated textures containing galena, pyrite, and variable content of tetrahedrite–tennantite and chalcopyrite. Quartz porphyroclasts and sulfide microstructures indicate a dynamic recrystallization of the quartzite during the main fluid mineralization episode. Chlorites and K-white micas (phengite) chemical analysis and thermodynamic modeling from compositional maps indicate an onset of the mineralization at 280 °C, with a main precipitation stage at 315 ± 35 °C and 6.25 ± 0.75 kbar. In situ U-Pb dating on monazite, cogenetic with sulfides, gives ages around 35 Ma for both deposits. The integrative dataset converges for a cogenetic MP-PN Alpine Pb-Ag mineralization during deformation in relation to the thrusting of the “Nappe des Gypses” and the Internal Briançonnais at the metamorphic peak.