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At convergent plate margin, part of the continental crust can be subducted and exhumed. During continental subduction a pre-existing crust is reworked. Remnants of an older orogen are recycled and subjected to (ultra)-high-pressure metamorphism. During subduction, polycyclic rocks undertake a second metamorphic cycle, whereas monocyclic rocks are metamorphosed for the first time. In reworked rocks the pre-subduction record is overprinted and partially or completely lost. Despite this difficulty, reconstructing the pre-subduction history of the recycled crust is crucial, because pre-subduction characters (such as H2O content) can strongly influence how rocks respond to reworking during subduction. The Dora-Maira Massif is worldwide renowned as a (ultra)-high-pressure continental terrane. However, its northern part remained essentially unexplored in recent times. In this thesis work the northern Dora-Maira Massif is used as a case study to investigate recycling of continental crust. The aim is to constrain what type of crust is subducted and exhumed and to unravel the role of fluids during subduction of polycyclic material. Field work, petrology, thermodynamic modelling and geochronology are integrated. New field and geochronological evidence indicate that the northern Dora-Maira Massif displays an internal architecture more complex than what previously thought. It is subdivided in several tectonic units likewise the southern Dora-Maira Massif. Chemical and isotopic records of the reworked rocks reveal a pre-Alpine history spanning from the Lower Palaeozoic to the Mesozoic. A polycyclic basement preserves relicts of a pre-Alpine Barrovian metamorphism connected with the Variscan orogenesis. The absence of granulite-facies partially molten pre-Alpine rocks indicates that only the upper crust was reworked in the Dora-Maira Massif. Thermodynamic modelling indicates that polycyclic micaschists were rehydrated between the Variscan and the Alpine peak metamorphism. Polycyclic garnet texture and chemistry and metamorphic zircon record a main episode of fluid infiltration at the end of the Variscan cycle and not during the Alpine cycle. Pre-Alpine re-hydration of the upper crust allowed high-pressure re-equilibration during subduction.

International audience; The distribution of ultrahigh-pressure metamorphism (UHP) at the scale of a mountain belt is of primeimportance for deciphering its past subduction history. In the Western Alps, coesite has been recognized in thesouthern Dora-Maira massif, in the lens-shaped Brossasco-Isasca Unit, but has not been found up to now inthe other parts of the massif. We report the discovery of a new UHP unit in the northern Dora-Maira Massif(Western Alps), named Chasteiran Unit. It is only a few tens of metres thick and consists of garnet-chloritoidmicaschists. Garnet inclusions (chloritoid, rutile) and its growth zoning allow to precisely model the P-Tevolution. Coesite crystals, which are pristine or partially transformed to palisade quartz occur as inclusionsin the garnet outer cores. According to thermodynamic modelling, garnet displays a continuous record ofgrowth during the prograde increase in P and T (25-27 kbar 470-500°C) (stage 1), up to the coesite stabilityfield (27-28 kbar 520-530°C) (stage 2), as well as sub-isothermal decompression of about 10 kbar (down to15 kbar 500-515°C) (stage 3). The main regional, composite, foliation, marked by chloritoid and rutile, beganto develop during this stage, and was then overprinted by chlorite-ilmenite (stage 4). The Chasteiran Unit isdiscontinuously exposed in the immediate hangingwall of the Pinerolo Unit, and it is located far away from,and without physical links to the classic UHP Brossasco-Isasca Unit. Moreover, it records a different, muchcolder, P-T evolution, showing that different slices were detached from the downgoing subduction slab. TheChasteiran Unit is the fourth and the coldest Alpine UHP unit known so far in the entire Alpine belt. Its P-Tconditions are comparable to the ones of the Tian Shan coesite-chloritoid-bearing rocks.

National audience; Si l’histoire alpine du massif Dora-Maira a fait l’objet de nombreuses études, sonhistoire anté-alpine reste encore largement inexplorée. Mettant à profit l’existence dedomaines faiblement déformés durant l’orogenèse alpine, nous nous attacherons àdécrire l’évolution pré-orogénique de ce socle paléozoïque, où figurent deux lithologiesmajeures.La première lithologie est constituée par de rares orthogneiss à foliation pré-alpineconservée. De composition chimique peralumineuse (A/CNK = 1.28), l’orthogneisspréserve de son histoire magmatique (i) de rares enclaves microgrenues sombres, (ii)des porphyroclastes d’orthose, et (iii) des cristaux de zircon ayant fourni un âge U-PbLA-ICP-MS de 442 ± 2 Ma. La foliation de l’orthogneiss, à plagioclase – biotite – grenat– muscovite, témoigne d’une déformation ductile pervasive dans les conditions dufaciès des amphibolites.La seconde lithologie, de loin prédominante, est constituée de micaschistes à grenatcentimétrique et pseudomorphoses prismatiques allongées, à chloritoïde et mica,dérivant de porphyroblastes de staurotide. L’empreinte alpine, dans les échantillonsétudiés, est négligeable (préservation de la muscovite, surcroissances de grenat étroites,voire absentes, absence de foliation alpine). Le grenat contient des inclusions (quartz,ilménite, staurotide pseudomorphosée en bordure des grains) et une zonation chimiqueavec diminution de la teneur en Sps du coeur vers la bordure. Cette zonation autoriseune modélisation de la croissance du grenat anté-alpin, dont trois étapes sontreconstituées en modifiant la composition du volume d’équilibration dans la roche. Legrenat commence à croître vers 510- 530°C, 4-6 kbar, et achève sa croissance dans lechamp de stabilité de la staurotide vers 650- 660°C, 6-8 kbar, caractérisant ainsi unmétamorphisme barrovien.Un effort important a été effectué en vue de caractériser l’âge de ce métamorphismebarrovien. La distribution des inclusions d’apatite, de zircon, et de monazite dans legrenat, et la datation de la monazite et du zircon (U-Th-Pb, LA-ICP-MS) permet, en lienavec leur composition en éléments traces, de déterminer les étapes de l’évolution de cesroches au Carbonifère supérieur.Ainsi se dessine, pour le socle anté-alpin du massif Dora-Maira, une base de donnéesrobuste permettant de discuter la position anté-alpine de ce socle, avec des implicationssur les processus éventuels de ré-hydratation durant la subduction continentale alpine.