F Galster

G Manatschal

B Petri
email: bpetri@unistra.fr

G Mohn

E Skrzypek

T Mateeva

U-Pb geochronology of the Sondalo gabbroic complex (Central Alps) and its position within the Permian post-Variscan extension

Keywords: Austroalpine Permian evolution U-Pb dating, Alps, Austroalpine nappes, Gabbro, Permian

U-Pb geochronology of the Sondalo gabbroic complex (Central Alps) and its position within the Permian post-Variscan extension

INTRODUCTION

The end of an orogenic cycle is generally associated with the thinning and extension of a previously thickened continental crust, which is classically referred to as orogenic collapse due to gravitational re-equilibration (e.g. [START_REF] Rey | Gravitational collapse of the continental crust: definition, regimes and modes[END_REF]. This extension (wide rift mode; [START_REF] Royden | Rifting process and thermal evolution of the continental margin of Eastern Canada determined from subsidence curves[END_REF][START_REF] Buck | Modes of continental lithospheric extension[END_REF]) is commonly associated with the formation of extensional detachment faults and core complexes [START_REF] Vanderhaeghe | Partial melting and flow of orogens[END_REF][START_REF] Whitney | Continental and oceanic core complexes[END_REF] exposing in some places partially molten continental crust. Lithospheric thinning associated with an increase of the geothermal gradient causes partial melting of the rising asthenospheric mantle and leads to the production of calc-alkaline to tholeiitic magmas [START_REF] Bonin | Alkali-calcic and alkaline postorogenic (PO) granite magmatism: petrologic constraints and geodynamic settings[END_REF]. Such mafic magmas will likely be pounding at the Moho, forming underplated mafic bodies [START_REF] Fountain | Exposed cross-sections through the continental crust: implications for crustal structure, petrology, and evolution[END_REF][START_REF] Fountain | Growth and modification of lower continental crust in extended terrains: The Role of extension and magmatic underplating[END_REF][START_REF] Rudnick | Nature and composition of the continental crust: a lower crustal perspective[END_REF] which may, in turn, cause the formation of high-temperature (HT) metamorphic rocks [START_REF] Harley | The origins of granulites: a metamorphic perspective[END_REF]. All these processes can interact and overlap in time and space, leading to complex overprinting relationships that characterize the post-orogenic tectono-metamorphic evolution [START_REF] Whitney | Continental and oceanic core complexes[END_REF].

In Western Europe, the late-to post-orogenic extension of the Variscan orogen started in the Late Carboniferous [START_REF] Burg | Syn-to post-thickening extension in the Variscan Belt of Western Europe: Modes and structural consequences[END_REF]) and continued during the Early Permian (e.g. [START_REF] Lorenz | Plate and intraplate processes of Hercynian Europe during the late paleozoic[END_REF]. During the Early and Middle Permian (290-260 Ma), important lithospheric extension was associated with transtensional tectonics [START_REF] Arthaud | Late Paleozoic strike-slip faulting in the southern Europe and northern Africa: Result of a right-lateral shear zone between the Appalachians and the Urals[END_REF], formation of sedimentary basins [START_REF] Cassinis | A General Outline of the Permian Continental Basins in Southwestern Europe[END_REF] controlled locally by extensional detachment faults [START_REF] Froitzheim | Evolution of an Early Permian extensional detachment fault from synintrusive, mylonitic flow to brittle faulting (Grassi Detachment Fault, Orobic Anticline, southern Alps, Italy)[END_REF] as well as intense mafic and felsic magmatic activity (see e.g. [START_REF] Timmerman | Timing, geodynamic setting and character of Permo-Carboniferous magmatism in the foreland of the Variscan Orogen, NW Europe[END_REF][START_REF] Schaltegger | Crustal-scale magmatic systems during intracontinental strikeslip tectonics: U, Pb and Hf isotopic constraints from Permian magmatic rocks of the Southern Alps[END_REF]. This magmatic activity potentially explains the rejuvenation of the lower crust [START_REF] Bois | Dating lower crustal features in France and adjacent areas from deep seismic profiles[END_REF][START_REF] Costa | Lower crustal rejuvenation and growth during post-thickening collapse: Insights from a crustal cross section through a Variscan metamorphic core complex[END_REF]. Heat advected by mantle-derived mafic melts combined with lithospheric extension was responsible for widespread HT-metamorphism, as recorded in several parts of the Alps [START_REF] Schuster | Permian metamorphic event in the Alps[END_REF] and in Corsica [START_REF] Caby | La transition croûte/manteau dans la nappe de Santa-Lucia-di-Mercurio (Corse alpine): les racines d'un rift permien[END_REF]. This event [START_REF] Schuster | Permo-Triassic extension and related HT/LP metamorphism in the Austroalpine-Southalpine realm[END_REF][START_REF] Schuster | Permian metamorphic event in the Alps[END_REF] which occurred between the end of Variscan orogeny and the onset of Mesozoic rifting that ultimately led to the opening of several oceanic domains (e.g. Meliata-Vardar, Alpine Tethys and Southern North Atlantic), remains enigmatic. It is characterized by the complex interaction between extensional tectonics and magmatic and metamorphic processes that significantly modified the continental lithosphere. In particular, two issues remain debated: (1) the spatial and temporal relationship between tectonics, magmatism and metamorphism, and (2) the processes related to extension and the general geodynamic context [START_REF] Lorenz | Plate and intraplate processes of Hercynian Europe during the late paleozoic[END_REF][START_REF] Stampfli | The Intra-Alpine terrain: A Paleotethyan remnant in the Alpine Variscides[END_REF][START_REF] Henk | Did the Variscides collapse or were they torn apart?: A quantitative evaluation of the driving forces for post-convergent extension in central Europe[END_REF][START_REF] Schuster | Permo-Triassic extension and related HT/LP metamorphism in the Austroalpine-Southalpine realm[END_REF][START_REF] Wilson | Permo-Carboniferous magmatism and rifting in Europe: introduction[END_REF][START_REF] De Lamotte | Style of rifting and the stages of Pangea breakup[END_REF].

The Alpine belt and more particularly the Austroalpine domain in SE Switzerland and N Italy represent an important natural laboratory to investigate part of this "Permian event". Indeed the Austroalpine domain allows a direct access to distinct portions of the Permian lithosphere exhumed at or close to the surface during Jurassic rifting and subsequently involved in the Alpine orogeny [START_REF] Mohn | Rift-related inheritance in orogens: a case study from the Austroalpine nappes in Central Alps (SE-Switzerland and N-Italy)[END_REF][START_REF] Ludwig | 3.75: A Geochronological Toolkit for Microsoft Excel[END_REF]. The Austroalpine domain is composed of a pre-Mesozoic metamorphic basement recording a complex pre-syn-and post-Variscan tectono-metamorphic history (e.g. [START_REF] Maggetti | Evolution of the Silvretta Nappe[END_REF][START_REF] Spillmann | The Pre-Alpine Basement of the Lower Austro-Alpine Nappes in the Bernina Massif (Grisons, Switzerland; Valtellina, Italy)[END_REF]. Late Carboniferous to Early Permian plutons emplaced in all crustal levels [START_REF] Spillmann | The Pre-Alpine Basement of the Lower Austro-Alpine Nappes in the Bernina Massif (Grisons, Switzerland; Valtellina, Italy)[END_REF][START_REF] Müntener | Cooling History and Exhumation of Lower-Crustal Granulite and Upper Mantle (Malenco, Eastern Central Alps)[END_REF][START_REF] Braga | The Sondalo Gabbroic Complex (Central Alps, Northern Italy): Evidence for Emplacement of Mantle-Derived Melts into Amphibolite Facies Metapelites[END_REF]) are associated with the deposition of superficial volcano-sedimentary series [START_REF] Quadt | U-Pb zircon ages from igneous rocks of the Bernina nappe system (Grisons, Switzerland)[END_REF]. In particular, the evolution and timing of magmas emplaced at mid-crustal levels during late-to post-Variscan times remain poorly constrained. Therefore, this study focuses on the Austroalpine Campo unit because (1) it escaped significant Jurassic rift-related and later Alpine deformation, and (2) it hosts the Sondalo gabbroic complex, emplaced at mid-crustal levels during the Permian [START_REF] Tribuzio | Petrology, mineral and isotope geochemistry of the Sondalo gabbroic complex (Central Alps, Northern Italy): implications for the origin of post-Variscan magmatism[END_REF][START_REF] Braga | The Sondalo Gabbroic Complex (Central Alps, Northern Italy): Evidence for Emplacement of Mantle-Derived Melts into Amphibolite Facies Metapelites[END_REF][START_REF] Petri | The Sondalo gabbro contact aureole (Campo unit, Eastern Alps): implications for mid-crustal mafic magma emplacement[END_REF]). This study brings new constraints on the emplacement age of the Sondalo gabbroic complex based on U-Pb zircon dating associated with trace-element geochemistry in zircon. Using data from the literature, these results are integrated in the late-to post-Variscan evolution of the Austroalpine and the overall Western European domain. The ultimate aim of the study is to discuss the implications of this "Permian event" for the geodynamic evolution of Western Europe during Permian time.

VARISCAN AND POST-VARISCAN EVOLUTION OF THE AUSTROALPINE DOMAIN

The Austroalpine domain in SE Switzerland and adjacent N Italy (Fig. 1) consists of Late Cretaceous top to the West nappe stack subsequently reactivated during Tertiary N-S compression [START_REF] Froitzheim | Repeated change from crustal shortening to orogen-parallel extension in the Austroalpine units of Graubünden[END_REF]Handy 1996). Due to the effects of minor to moderate Alpine overprint as well as Jurassic rifting [START_REF] Froitzheim | Repeated change from crustal shortening to orogen-parallel extension in the Austroalpine units of Graubünden[END_REF][START_REF] Viola | The Pejo fault system: An example of multiple tectonic activity in the Italian Eastern Alps[END_REF][START_REF] Mohn | Rift-related inheritance in orogens: a case study from the Austroalpine nappes in Central Alps (SE-Switzerland and N-Italy)[END_REF], this domain preserves and document a polyphase pre-Mesozoic tectonic history spanning the Late Carboniferous -Early Permian post-Variscan extension and the Carboniferous Variscan orogeny. Notably, the domain provides access to distinct levels of the Permian lithosphere from the sub-continental mantle to the upper continental crust (e.g. [START_REF] Mohn | Unravelling the interaction between tectonic and sedimentary processes during lithospheric thinning in the Alpine Tethys margins[END_REF]. The post-Variscan evolution is characterized by widespread alkaline and tholeiitic magmatism at different crustal levels and high-grade contact to regional metamorphism locally reaching granulite facies. In contrast, the Variscan evolution of the area is characterized by calc-alkaline magmatism and high-grade metamorphism up to upper amphibolite facies conditions in some metasediments.

Variscan evolution

Variscan metamorphism is reflected by the presence, in the Ulten zone (Upper Tonale unit), of Grt-Ky migmatites recording a pressure peak of 11 kbar attained at 750 °C and 351-343 Ma, followed by a temperature peak of 750-800 °C attained at 8.5-12.5 kbar and 330-326 Ma (U-Pb on monazite; [START_REF] Braga | An early metamorphic stage for the Variscan Ulten Zone gneiss (NE Italy): evidence from mineral inclusions in kyanite[END_REF][START_REF] Braga | Mineralogy of inclusions in zircon from high-pressure crustal rocks from the Ulten Zone, Italian Alps[END_REF][START_REF] Langone | Preservation of old (prograde metamorphic) U-Th-Pb ages in unshielded monazite from the high-pressure paragneisses of the Variscan Ulten Zone (Italy)[END_REF]. Pre-Namurian (Serpukhovian) amphibolite facies metamorphism is documented in the Err and Bernina units [START_REF] Halmes | The transition from passive to active margin tectonics: a case study from the zone of Samedan (eastern Switzerland)[END_REF][START_REF] Spillmann | The Pre-Alpine Basement of the Lower Austro-Alpine Nappes in the Bernina Massif (Grisons, Switzerland; Valtellina, Italy)[END_REF]. Grt-St mica schist of the Campo unit records undated amphibolite-facies metamorphic conditions of 5-9 kbar and 550-600 °C [START_REF] Braga | The Sondalo Gabbroic Complex (Central Alps, Northern Italy): Evidence for Emplacement of Mantle-Derived Melts into Amphibolite Facies Metapelites[END_REF][START_REF] Petri | The Sondalo gabbro contact aureole (Campo unit, Eastern Alps): implications for mid-crustal mafic magma emplacement[END_REF]. Calc-alkaline diorites and granodiorites were emplaced in the Bernina unit at 338-324 Ma (U-Pb on zircon multi-grain dissolution ;[START_REF] Rageth | Intrusiva und Extrusiva der Bernina-Decke zwischen Morteratsch und Berninapass (Graubünden)[END_REF][START_REF] Spillmann | The Pre-Alpine Basement of the Lower Austro-Alpine Nappes in the Bernina Massif (Grisons, Switzerland; Valtellina, Italy)[END_REF][START_REF] Quadt | U-Pb zircon ages from igneous rocks of the Bernina nappe system (Grisons, Switzerland)[END_REF] but do not show evidence for subsequent Variscan deformation [START_REF] Spillmann | The Pre-Alpine Basement of the Lower Austro-Alpine Nappes in the Bernina Massif (Grisons, Switzerland; Valtellina, Italy)[END_REF].

Post-Variscan Permian evolution

The Permian lower crust was intruded between 281 ± 19 and 278 ± 3 Ma by mafic magmas of tholeiitic affinity. Examples are the Fedoz and the Braccia gabbros in the Margna-Malenco units (Figs 1b and 2; U-Pb on zircon; [START_REF] Spillmann | The Pre-Alpine Basement of the Lower Austro-Alpine Nappes in the Bernina Massif (Grisons, Switzerland; Valtellina, Italy)[END_REF][START_REF] Hansmann | U-Pb zircon geochronology of a tholeiitic intrusion and associated migmatites at a continental crust-mantle transition, Val Malenco, Italy[END_REF]. They were emplaced at the crust-mantle transition zone at around 10 kbar [START_REF] Hermann | Fossil crust-tomantle transition, Val Malenco (Italian Alps)[END_REF][START_REF] Hermann | Differentiation of Mafic Magma in a Continental Crust-to-Mantle Transition Zone[END_REF]. The Braccia gabbro is surrounded by HT-MP felsic granulite that consists of Grt-Ky bearing residual rocks equilibrated at 10 kbar and 800-850 °C [START_REF] Hermann | Fossil crust-tomantle transition, Val Malenco (Italian Alps)[END_REF][START_REF] Müntener | Cooling History and Exhumation of Lower-Crustal Granulite and Upper Mantle (Malenco, Eastern Central Alps)[END_REF]. Monazite dating shows that HT conditions lasted ca. 20 Ma between 280 and 257 Ma [START_REF] Hermann | Relating zircon and monazite domains to garnet growth zones: age and duration of granulite facies metamorphism in the Val Malenco lower crust[END_REF]. Crystallization of hydrous phases at around 9 kbar and 600 °C [START_REF] Müntener | Cooling History and Exhumation of Lower-Crustal Granulite and Upper Mantle (Malenco, Eastern Central Alps)[END_REF], dated as Late Triassic to Early Jurassic [START_REF] Villa | 39Ar-40Ar dating of multiply zoned amphibole generations (Malenco, Italian Alps)[END_REF], was related to the onset of rifting and subsequent exhumation of these rocks [START_REF] Müntener | The role of lower crust and continental upper mantle during formation of non-volcanic passive margins: evidence from the Alps[END_REF][START_REF] Trommsdorff | The Geological Map of Valmalenco[END_REF].

The Permian middle crust, represented by the Campo unit, was sporadically intruded by Permian granitoids (Fig. 1b; e.g. [START_REF] Gazzola | Eo-Alpine HP metamorphism in the Permian intrusives from the steep belt of the central Alps (Languard-Campo nappe and Tonale Series)[END_REF] at ca. 285-259 Ma as indicated by Rb-Sr ages on whole rock and mineral separates [START_REF] Moro | Revisione del significato strutturale e geocronologico delle masse intrusive minori dell'Alta Valtellina: risultati preliminari[END_REF][START_REF] Boriani | Il "granito di Brusio" (Val Poschiavo -Svizzera): caratterizzazioni chimiche, petrologiche e radiometriche[END_REF][START_REF] Moro | Rb-Sr Geochemistry of some Hercynian granitoids overprinted by eo-Alpine metamorphism in the Upper Valtellina, Central Alps[END_REF]. Their emplacement at P < 4.5 kbar is inferred from the presence of andalusite and sillimanite in their contact aureoles (e.g. Del [START_REF] Moro | Rb-Sr Geochemistry of some Hercynian granitoids overprinted by eo-Alpine metamorphism in the Upper Valtellina, Central Alps[END_REF]. Tourmaline-bearing pegmatites within the Campo basement yield Sm-Nd garnet-Kfeldspar isochron ages of 255-250 Ma, Rb-Sr muscovite ages of 257-251 Ma and K-Ar muscovite ages of 217-187 Ma [START_REF] Hanson | K-Ar and Rb-Sr ages of pegmatites in the South Central Alps[END_REF][START_REF] Thöni | Degree and evolution of the alpine metamorphism in the Austroalpine unit W of the Hohe Tauern in the light of K/Ar and Rb/Sr age determination on micas[END_REF][START_REF] Sölva | Dating a single garnet crystal with very high Sm/Nd ratios (Campo basement unit, Eastern Alps)[END_REF].

The Permian upper crust documents the emplacement of felsic magmatic rocks during the Early Permian. In the Bernina unit, the Carboniferous calc-alkaline suite was intruded by a 295-292 Ma alkaline syenite-granite emplaced at P < 3 kbar, (Figs 1b and 2;[START_REF] Büchi | Geologie und Petrographie der Bernina IX. Das Gebiet zwischen Pontresina und dem Morteratschgletscher[END_REF][START_REF] Spillmann | The Pre-Alpine Basement of the Lower Austro-Alpine Nappes in the Bernina Massif (Grisons, Switzerland; Valtellina, Italy)[END_REF][START_REF] Quadt | U-Pb zircon ages from igneous rocks of the Bernina nappe system (Grisons, Switzerland)[END_REF]. Both Carboniferous calc-alkaline and Lower Permian alkaline intrusives and their host rocks are unconformably overlain by younger, 288 ± 7 Ma alkaline rhyolite [START_REF] Staub | Fractionation of Nb and Ta by biotite and phengite: Implications for the "missing Nb paradox[END_REF][START_REF] Rageth | Intrusiva und Extrusiva der Bernina-Decke zwischen Morteratsch und Berninapass (Graubünden)[END_REF][START_REF] Quadt | U-Pb zircon ages from igneous rocks of the Bernina nappe system (Grisons, Switzerland)[END_REF].

THE SONDALO GABBROIC COMPLEX AND ITS HOST ROCK

The Sondalo gabbroic complex emplaced in the Campo unit which is delimited to the S by the Insubric line and to the NE and W by Late Cretaceous thrust faults. The Campo unit is separated from the overlying Grosina unit by the Eita shear zone [START_REF] Meier | The Periadriatic Fault System in Valtellina (N-Italy) and the Evolution of the Southwestern Segment of the Eastern Alps[END_REF]. This shear zone is polyphase; it was active during Jurassic rifting, was weakly reactivated during the Alpine orogeny (Fig. 3;[START_REF] Meier | The Periadriatic Fault System in Valtellina (N-Italy) and the Evolution of the Southwestern Segment of the Eastern Alps[END_REF][START_REF] Mohn | Necking of continental crust in magma-poor rifted margins: Evidence from the fossil Alpine Tethys margins[END_REF]) and likely preserves a pre-Jurassic history [START_REF] Petri | Formation and exhumation of Permian granulites: establishing the pre-rift conditions and determining the syn-rift exhumation history[END_REF]. The Campo unit consists of amphibolite-facies mica schist, paragneiss and minor amphibolite and calc-silicate of Ordovician protolith age (U-Pb ages on detrital zircons; [START_REF] Bergomi | Late Neoproterozoic -Early Paleozoic accretion of the Southalpine and Austroalpine basements of Central Alps (Italy)[END_REF]. Alpine overprints remain weak in the Campo unit, being limited to its borders [START_REF] Schmid | Transition from near-surface thrusting to Intrabasement Decollement, Schlinig Thrust, eastern Alps[END_REF][START_REF] Gazzola | Eo-Alpine HP metamorphism in the Permian intrusives from the steep belt of the central Alps (Languard-Campo nappe and Tonale Series)[END_REF].

Among the numerous Permian intrusives in the Campo unit, the Sondalo gabbroic complex is exposed over 40 km² in the N-S oriented Adda Valley (Fig. 3). The pluton is concentrically zoned; it is composed of Ol-gabbro and troctolite surrounded by gabbro and norite in the Central Zone (CZ; Fig. 4ab), and diorite to granodiorite in a Border Zone (BZ) of variable thickness (Fig. 4cde; [START_REF] Koenig | Geologisch-petrographische Untersuchungen im oberen Veltlin[END_REF]. The zone boundaries are rarely exposed. All different facies are thought to be derived from a basaltic parental liquid of tholeiitic affinity, differentiated through fractional crystallization and affected by crustal assimilation [START_REF] Tribuzio | Petrology, mineral and isotope geochemistry of the Sondalo gabbroic complex (Central Alps, Northern Italy): implications for the origin of post-Variscan magmatism[END_REF]. Sm-Nd mineral-isochrons on troctolite and norite samples gave ages of 300 ± 12 and 280 ± 10 Ma, respectively. Rb-Sr isochron ages for the same samples are respectively 266 ± 10 and 269 ± 16 Ma [START_REF] Tribuzio | Petrology, mineral and isotope geochemistry of the Sondalo gabbroic complex (Central Alps, Northern Italy): implications for the origin of post-Variscan magmatism[END_REF]. Zircon grains from a diorite sample collected in the BZ yielded a 270 Ma upper intercept age by U-Pb multigrain dissolution dating [START_REF] Bachmann | Radiometrische Alterbestimmungen des Gabbro von Sondalo. Oberes Velltlin, Italienische Alpen[END_REF].

Petrography of host metasediments

Metasedimentary rocks of the Campo unit essentially consist of Grt-St and Ms-Bt-Sil mica schist and paragneiss. They record a succession of three structures: a relictual NE-SW striking and steeply dipping foliation (S1), a main NW-SE striking and steeply dipping foliation (S2), and a foliation parallel to the pluton margins and dipping moderately away from the pluton center (S3; Fig. 3). Due to a first pre-Permian regional metamorphic event, metasedimentary host rocks reached amphibolite-facies conditions of 6 kbar/650 °C contemporaneously to the development of S1 and S2 [START_REF] Petri | The Sondalo gabbro contact aureole (Campo unit, Eastern Alps): implications for mid-crustal mafic magma emplacement[END_REF]. Subsequently, the Permian intrusion of the Sondalo complex variably affected the metasedimentary unit: far from the intrusion (ca. 10 km), heat advected by the magma caused static growth of cordierite and andalusite, attesting of heating at 3-4 kbar (sample BPA 099-12a from [START_REF] Petri | The Sondalo gabbro contact aureole (Campo unit, Eastern Alps): implications for mid-crustal mafic magma emplacement[END_REF]. Conversely, elongated host rock xenoliths (septa) up to 300 m in thickness and 1 km in length were incorporated within the complex, while migmatization parallel to the S3 fabric occurred in the contact aureole (Fig. 3). From the host rocks to the core of the intrusion, increasing metamorphism led to the transformation of Grt-St-Ms-Bt mica schist into Grt-Sil-Bt paragneiss and Grt-Sil-Crd-Spl granulitic paragneiss which equilibrated at 5.5 kbar/930 °C. In the contact aureole, S2 was transposed into S3 at supra-solidus conditions (Figs 3 and4f) while Grt-Sil-Bt melanosomes in the migmatitic metasediments (Fig. 4f) document similar P-T conditions than granulitic paragneiss between 5.2-6 kbar/500-800 °C, with a last equilibration at P < 4.5 kbar [START_REF] Braga | The Sondalo Gabbroic Complex (Central Alps, Northern Italy): Evidence for Emplacement of Mantle-Derived Melts into Amphibolite Facies Metapelites[END_REF][START_REF] Braga | Origin of prismatine from the Sondalo granulites (Central Alps, northern Italy)[END_REF][START_REF] Petri | The Sondalo gabbro contact aureole (Campo unit, Eastern Alps): implications for mid-crustal mafic magma emplacement[END_REF].

Petrography of the Sondalo gabbroic complex

Ol-gabbro, troctolite gabbro and norite in the CZ of the complex (Fig. 4ab) are mainly composed of euhedral to subhedral plagioclase, olivine, orthopyroxene and clinopyroxene. Plagioclase is labradoritic and often presents a more calcic core. Olivine is often replaced by talc and magnetite. Clinopyroxene is poikilitic, sometimes rimmed by red-brown Ti-pargasite that may also be poikilitic. Both pyroxene and pargasite are sometimes transformed into brown-green hornblende. Interstitial biotite is rare in (olivine-) gabbro and norite. In gabbro, ilmenite is the main opaque mineral while rutile may be found as inclusion in pyroxene. Magmatic rocks are generally medium-grained and show a variably strong fabric defined by the shape-preferred orientation of euhedral to subhedral and elongated plagioclase, pyroxene and amphibole (Fig. 4ab). This E-W fabric is usually steeply dipping to the North or to the South (Fig. 3). The absence of solid-state deformation (dynamic recrystallization) indicates that the mineral fabric is of magmatic origin. Close to the Jurassic Eita extensional shear zone [START_REF] Mohn | Necking of continental crust in magma-poor rifted margins: Evidence from the fossil Alpine Tethys margins[END_REF], magmatic foliations are overprinted by greenschist facies deformation.

Diorites and granodiorite in the BZ show a hypidiomorphic structure and are composed of euhedral to subhedral plagioclase and green-brown hornblende. Plagioclase is zoned with a labradorite core and an andesine rim. Hornblende may contain inclusions of clino-and orthopyroxene. Interstitial quartz is frequent. Opaque minerals are mostly ilmenite. In the absence of sub-solidus deformation, rocks in the BZ locally show a preferred orientation of magmatic minerals. They define a magmatic foliation roughly parallel to the pluton margin and to the S3 foliation in the contact aureole, lying in places at a high angle with respect to the steeply dipping magmatic fabric observed in the CZ (Fig. 3).

Across the pluton, late-stage magma accumulation forms patchy pockets with no apparent foliation (Fig. 4a). These pockets as well as syn-magmatic mafic dykes cross cut the weakly to well foliated gabbro, locally with a sharp contact. Both usually display weaklyfoliated pegmatitic textures with up to 5 cm long subhedral plagioclase and poikilitic hornblende crystals that locally include clinopyroxene. In the BZ, mingling between leucocratic and more mafic magma is indicated by pillow structures with chilled margins (Fig. 4c).

In the entire pluton, the amount of garnet, biotite and rare cordierite in magmatic rocks increases towards the contact with metasedimentary xenoliths (Fig. 4d). This trend is combined with a decrease in the modal amount of amphibole and pyroxene. In some places, centimeter-scale veins form massive garnet-rich mafic rocks along contacts with xenoliths. In such rocks, garnet is typically euhedral, can reach up to 1 cm in diameter and bears numerous plagioclase inclusions. Biotite (1 cm in length) is euhedral and interstitial quartz is frequent.

GEOCHRONOLOGY

In order to constrain the emplacement age of the Sondalo gabbroic complex and the timing of associated contact metamorphism, we performed zircon U-Pb isotopic LA-ICP-MS dating on five samples (location indicated on Fig. 3). Three igneous rock samples (M28B73, GM601 and M1B14) were collected across the pluton. In addition, two samples from the migmatitic metasediments were selected in the contact aureole; one migmatitic paragneiss (BPA 003-12c) and one leucosome (few centimeters in thickness) from the contact aureole (BPA 003-12d).

Zircon U-Pb isotopic measurements and trace element analysis were performed at the Institute of Earth Sciences at the University of Lausanne using a sector field Thermo Scientific Element XR ICP-MS coupled to a NewWave 193nm ArF excimer laser ablation device. Detailed analytical conditions are reported as electronic supplementary material. Spot location was guided by transmitted and reflected light observations as well as backscatter electron (BSE) and cathodoluminescence (CL) images (Fig. 6 and7). Isotopic ratios and ages are reported as supplementary material and in Fig. 5. Results were plotted in conventional and inverse Concordia diagrams (Figs 6 and 7) using Isoplot/Ex 3.75 [START_REF] Ludwig | 3.75: A Geochronological Toolkit for Microsoft Excel[END_REF]) with all errors reported at the 2σ level. For igneous rock and leucosome sample, discordant (concordancy level of 97-103 %) and outlier ages (radiogenic Pb loss, initial Pb incorporation or inherited cores) were discarded for Concordia age calculation. For the migmatitic paragneiss sample, the Discordia line was anchored to an initial Pb ratio of 0.8534 (assumed for an age of 290 Ma following [START_REF] Stacey | Approximation of terrestrial lead isotope evolution by a twostage model[END_REF]. Outlier ages were discarded for age determination.

Sample description

Magmatic rocks from the gabbroic complex

Diorite M28B73 [46.39551°N; 10.34243°E] was collected about 10 m from a Sil-Bt metapelitic xenolith in the dioritic rim in the northern part of the pluton. It contains Bt-Pl-Kfs-Qtz with minor garnet, muscovite and ilmenite. The sample is phaneritic (up to 5 mm in size) and preserves its original magmatic texture with weak solid-state deformation attested by discrete undulose extinction in quartz. Plagioclase (andesine) is locally replaced by secondary muscovite and is preferentially oriented along a weakly developed magmatic foliation that, in the field, is steeply dipping to the North. Interstitial biotite (1 mm) contains ilmenite and zircon inclusions and is occasionally chloritized. Zircon is also included in quartz. K-feldspar corresponds to microcline with patchy exsolution lamellae of albite. Tiny garnet (<1 mm in size) contains rare quartz inclusions. At the outcrop scale it becomes more abundant towards the metapelitic xenolith.

Deformed diorite GM601 [46.36016°N; 10.31110°E] was sampled a few meters away from the contact with the overlying Grosina unit in the vicinity (5 m) of the Eita Shear zone, at the south-western margin of the pluton. This deformed diorite is cutting through the metapelitic host rock at a high angle with respect to the S3 foliation of the migmatitic aureole. The magmatic structure is pervasively overprinted by a mylonitic foliation. The sample contains Ms-Bt-Pl-Kfs-Qtz. Porphyroclastic plagioclase and K-feldspar phenocrysts (up to 2.5 mm in size) are dismembered by numerous brittle cracks. Small porphyroclastic muscovite (0.5 mm) recrystallizes into fine-grained muscovite, whereas biotite occurs only as fine-grained aggregates. Quartz is totally recrystallized into fine grains (few µm) by a bulging mechanism, which brackets the deformation temperature between 300 and 400 °C [START_REF] Stipp | The eastern Tonale fault zone: a "natural laboratory" for crystal plastic deformation of quartz over a temperature range from 250 to 700 °C[END_REF], i.e. at greenschist facies conditions. Quartz aggregates occasionally contain zircon grains.

Leucocratic diorite M1B14 [46.34483°N; 10.32495°E] was sampled close to a large roof pendant in the pluton, in contact with very fine-grained gabbro showing a pillow structure and chilled margins. This sample still preserves a magmatic texture with no subsolidus deformation. It is essentially composed of Pl-Hbl±Kfs. Plagioclase (labradorite, few mm in length) is subhedral while common fine-grained hornblende is interstitial and rare biotite is chloritized. Small garnet (up to 100 µm in diameter) contains inclusions of plagioclase and ilmenite, whereas ilmenite in the matrix is usually associated with hornblende. Quartz bears rare zircon inclusions.

Leucosome and migmatitic paragneiss

Leucosome BPA 003-12d [46.35977°N; 10.33070°E] comes from a metatexite collected in the western part of the contact metamorphic aureole, few tens of meters from the pluton. The rock contains Bt-Grt-Pl-Kfs-Qtz with minor muscovite, sillimanite and ilmenite. The weak shape-preferred orientation of euhedral plagioclase causes a preferential orientation of interstitial biotite, all aligned along S3, without presenting any sub-solidus recrystallization. Biotite is euhedral to subhedral. Atoll to euhedral garnet (up to 3 mm in diameter) is frequent and includes tiny biotite, plagioclase, quartz and ilmenite. Muscovite and partially chloritized biotite form nests with polygonal borders, probably after cordierite. Ilmenite from the matrix contains rare rutile inclusions, whereas zircon is included in biotite. This sample comes from the same outcrop as kinzigite BPA 003-11a for which P-T conditions are estimates at 5.2 kbar at 800 °C [START_REF] Petri | The Sondalo gabbro contact aureole (Campo unit, Eastern Alps): implications for mid-crustal mafic magma emplacement[END_REF].

Migmatitic paragneiss BPA 003-12c [46.35977°N; 10.33070°E] is a metasedimentary metatexite. It presents alternation of leucosome and melanosome bands (few centimeters in thickness) parallel to the moderately NW-dipping S3 foliation. The leucosome is composed of Ms-Grt-Crd-Kfs-Qtz±Bt±Sil±Ilm whereas the melanosome includes Grt-Sil-Crd-Bt-Kfs-Qtz±Spl±Pl. Red-brown biotite from the melanosome contains numerous zircon inclusions. Ilmenite is the only opaque phase in the rock. Zircon is included in biotite from the melanosome, and occasionally in quartz from the leucosome. This sample comes from the same outcrop as kinzigite BPA 003-11a (see comment above).

Zircon morphology and internal structure

Zircon grains from magmatic rocks of the gabbroic complex (samples M28B73, GM 601 and M1B14, Fig. 6 a, b andc) are up to 300 µm in length and 100 µm in width. They present elongated shapes, are prismatic, euhedral to sub-euhedral, with slightly rounded outlines. A few grains from sample M1B14 show irregular outlines pointing to late-magmatic chemical corrosion without or with low recrystallization/overgrowth. CL images reveal frequent oscillatory zoning and rare homogenous cores. Zoning is sometimes disrupted and overgrown by a zoned rim. Sub-solidus recrystallization due to late-to post-magmatic processes is attested by heterogeneous patchy patterns (brecciated) or by the presence of thin homogenous and bright rims (up to 10 µm in thickness) or cracks.

Zircon extracted from migmatitic metasediments (samples BPA 003-12d and BPA 003-12c, Fig. 7a andb) are up to 300 µm in length and 100 µm in width for the leucosome BPA 003-12d and up to 200 µm in length and 80 µm in width for the migmatitic paragneiss BPA 003-12c. They are usually elongated and prismatic, with slightly rounded outlines. Cores present a well-developed oscillatory zoning sometimes disrupted by a bright and homogeneous rim (> 10 µm in thickness) penetrating occasionally deeply into the grain (up to 20 µm from the rim for sample BPA 003-12c). Rare xenocrystic cores were observed.

U-Pb dating results

Diorite M28B73 (Fig. 6a): twenty-nine analyses were carried out on twenty-six zircon grains. Most of the analyses are concordant to sub-concordant, the main population ranging between 284 and 302 Ma, with a maximum at ca. 292 Ma. A rather homogeneous age population defines a Concordia age of 289 ± 4 Ma (MSWD = 0.22) constrained by thirteen spot analyses. Sub-concordant analyses are slightly marginal with respect to the main population, probably due to younger lead loss. No significant age difference could be observed between zircon textures and analyses location.

Deformed diorite GM601 (Fig. 6b): thirty spot analyses were performed on twentyfive zircons. Analyses of zircon cores point to slightly older ages than rim analyses. The main age population spans between 282 and 298 Ma, with a maximum probability at ca. 287 Ma. A homogeneous population of thirteen analyses defines an age of 285 ± 2 Ma (MSWD = 0.33) on a conventional Concordia diagram. The older age population visible on the histogram is mainly composed of discordant ages, probably due to common lead incorporation.

Leucocratic diorite M1B14 (Fig. 6c): nine analyses were performed on eight grains. 206 Pb/ 238 U ages range from 283 to 291 Ma, with a peak at ca. 284 Ma. After discarding discordant and outlier ages, three concordant analyses yields a Concordia age of 285 ± 6 Ma (MSWD = 0.10).

Leucosome BPA 003-12d (Fig. 7a): five spot analyses were carried out on 4 zircon grains. One spot from a zircon core yielded an inherited age of 854 ± 5 Ma. The remaining four spots lies between 282 and 292 Ma showing a poorly constrained peak around 286 Ma. They define a Concordia age of 288 ± 5 Ma (MSWD = 0.037).

Migmatitic paragneiss BPA 003-12c (Fig. 7b): out of four spot analyses performed on 3 zircons, only one is strictly concordant at 288 ± 3 Ma. Three spots lie along a mixing line between initial Pb defining a Discordia line with a lower intercept at 289 ± 4 Ma (MSWD = 0.01) on an inverse Concordia diagram.

U, Th, Hf and trace element geochemistry of zircon

U, Th, Hf and trace element concentrations in zircon were analyzed to constrain the conditions of zircon growth. Raw analytical results and parameters are provided as supplementary material. Parameters are reported in Fig. 8 as well as supplementary material. REE composition diagrams, Eu/Eu* (Eu-anomaly, Eun/((Smn+Gdn)/2)) and Ybn/Gdn (slope of the REE pattern) are normalized to the C1-chondrite composition of [START_REF] Sun | Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes[END_REF].

Magmatic rocks from the gabbroic complex

Typically, most magmatic zircons present a steep HREE pattern (Fig. 8ab), with a positive Ce-anomaly, a negative Eu-anomaly, and Th/U ratios > 0.01, characteristic for magmatic zircons [START_REF] Murali | Trace element characteristics, REE patterns and partition coefficients of zircons from different geological environments-A case study on Indian zircons[END_REF]Schaltegger et al. 1999;[START_REF] Rubatto | Zircon trace element geochemistry: partitioning with garnet and the link between U-Pb ages and metamorphism[END_REF].

Zircons from sample M28B73 present a relatively high Th/U ratio (0.036-0.338). A decrease of the HREE content (from Ybn/Gdn ≈ 40 to Ybn/Gdn ≈ 0-10) is balanced by a reduction of the Eu-anomaly, with Eu/Eu* varying from 0.023 up to 0.160, with younger zircon domains having a low Ybn/Gdn and a high Eu/Eu* values (Fig. 8a). The slope of the HREE diagram is inversely correlated to the Nb/Ta ratio due to a decreasing Ta content while Ybn/Gdn decrease and Nb/Ta increase (Fig. 8a). Similarly, Hf and U-contents weakly decrease with lower Ybn/Gdn values (see supplementary material).

A high Th/U ratio (0.137-0.597) is found for zircons of sample M1B14 (Fig. 8b). They present a relatively low HREE-enrichment (Ybn/Gdn < 20) and a negative Eu-anomaly (Eu/Eu* = 0.063-0.240). No clear trend between Ybn/Gdn or Eu/Eu* is observed. Zircons commonly have a low Ta concentration (around 0.8 ppm) that is not correlated with the Nb/Ta ratio (see supplementary material). Similarly, they are poor in Hf and U, with around 9000 and 600 ppm, respectively. Both element concentrations do not present any peculiar correlation with Ybn/Gdn (see supplementary material).

Leucosome and migmatitic paragneiss

Metasediments sampled in the contact aureole (leucosome BPA 003-12d and migmatitic paragneiss BPA 003-12c, Fig. 8c) present a homogeneous REE pattern, with the exception of inherited cores. They have a high Th/U ratio (0.194-0.624), are depleted in HREE (Ybn/Gdn ≈ 2) and present a medium Eu-anomaly (Eu/Eu* ≈ 0.14, Fig. 8c). The Tacontent commonly varies between 0.9 and 4.0 ppm without being correlated to the Ybn/Gdn and Nb/Ta ratios (Fig. 8c). The Hf and U concentration in zircon is weakly but inversely correlated to the Ybn/Gdn ratio (see supplementary material).

DISCUSSION

Significance of zircon chemistry and interpretation of U-Pb ages

Magmatic rocks from the gabbroic complex

U-Pb zircon ages on magmatic zircons from the Sondalo gabbroic complex are similar within error, ranging from ca. 289 ± 4 Ma to 285 ± 6 Ma. These results are in line with previous Sm-Nd dating of 300 ± 12 and 280 ± 10 Ma [START_REF] Tribuzio | Petrology, mineral and isotope geochemistry of the Sondalo gabbroic complex (Central Alps, Northern Italy): implications for the origin of post-Variscan magmatism[END_REF]) and older than the U-Pb zircon age of 270 Ma [START_REF] Bachmann | Radiometrische Alterbestimmungen des Gabbro von Sondalo. Oberes Velltlin, Italienische Alpen[END_REF].

The oldest concordant age in magmatic rocks is found in a diorite sample (M28B73) at ca. 289 ± 4 Ma. Notably, most of the oldest zircon grains from this sample display steep slopes of HREE and high negative Eu-anomalies, which may indicate the initial crystallization of a garnet-free magma associated with well crystallized plagioclase [START_REF] Rubatto | Zircon trace element geochemistry: partitioning with garnet and the link between U-Pb ages and metamorphism[END_REF]. In contrast, the reduction of the negative Eu-anomaly in younger zircon grains (Fig. 8a) indicates a mixing with a less evolved liquid where plagioclase had not completely crystallized. The associated lower Ybn/Gdn values point to a progressive depletion of the residual liquid, generally interpreted as reflecting a progressive crystallization of garnet [START_REF] Rubatto | Zircon trace element geochemistry: partitioning with garnet and the link between U-Pb ages and metamorphism[END_REF], a mineral which is observed in the sample. Ilmenite and biotite are the main hosts for Nb and Ta, but biotite preferentially incorporates Nb over Ta, in contrast to ilmenite [START_REF] Xiong | Partitioning of Nb and Ta between rutile and felsic melt and the fractionation of Nb/Ta during partial melting of hydrous metabasalt[END_REF]Stepanov and Hermann 2013;[START_REF] Galster | Accessory phases as a tool to investigate magmatic underplating in polymetamorphic metasediments: an example from a fossil lower crust: Ivrea Verbano zone[END_REF]. The Nb/Ta ratio is inversely correlated to the Ta content while the Ta content decreases with decreasing Ybn/Gdn (see supplementary material). This indicates that magmatic ilmenite (or a precursor rutile) is the main phase that fractionates Nb over Ta during crystallization of zircon and before crystallization of magmatic biotite [START_REF] Galster | Accessory phases as a tool to investigate magmatic underplating in polymetamorphic metasediments: an example from a fossil lower crust: Ivrea Verbano zone[END_REF]. This is in agreement with the presence of zircon grains included in interstitial biotite. The weak decrease of Hf content (see supplementary material) is not consistent with its expected increase during magmatic differentiation (Linnen and Keppler 2002;[START_REF] Hoskin | The composition of zircon and igneous and metamorphic petrogenesis[END_REF]. This may be explained by the crystallization of magmatic ilmenite synchronously with zircon crystallization, during or after the crystallization of plagioclase.

Field observations point to increasing garnet, biotite and cordierite contents in magmatic rocks towards the metapelitic xenoliths (Fig. 4d), indicating progressive magma hybridization with melts derived from partially molten metapelites. This scenario may explain the crustal assimilation signature of isotopic data (εNd and 87 Sr/ 86 Sr; [START_REF] Tribuzio | Petrology, mineral and isotope geochemistry of the Sondalo gabbroic complex (Central Alps, Northern Italy): implications for the origin of post-Variscan magmatism[END_REF]. Zircon trace element chemistry of diorite sample M28B73, with decreasing Ybn/Gdn, is coherent with the presence of garnet in the rock, a phase generally enriched in HREE. In the absence of magma mingling structures close to this sample, it is proposed that the mafic magma was contaminated by the liquid derived from the partially molten metapelites. In such case, a progressive enrichment in LREE, Hf and U is expected in the residual liquid and therefore in zircon. The decreasing Hf and U contents (see supplementary material) may be due to the lack of consumption of Hf and U-rich phases, respectively biotite and zircon, apatite and xenotime in migmatites of the BZ, i.e. where the diorite sample was collected. The residual liquid might be progressively depleted in Hf during the crystallization of magmatic ilmenite. Alternatively, magma contamination is recorded in trace element compositions of gabbros and Ol-gabbros from the CZ [START_REF] Tribuzio | Petrology, mineral and isotope geochemistry of the Sondalo gabbroic complex (Central Alps, Northern Italy): implications for the origin of post-Variscan magmatism[END_REF].

The slightly younger concordant ages of the two diorites sampled at the western margin of the pluton lie at 285 ± 2 Ma and 285 ± 6 Ma. The moderately steep to flat HREE profiles (low Ybn/Gdn values) combined with strong to moderate negative Eu-anomalies indicate that zircons crystallized in the presence of garnet and plagioclase. The apparently increasing Hf concentration in zircon indicates fractionation during crystallization (Linnen and Keppler 2002;[START_REF] Hoskin | The composition of zircon and igneous and metamorphic petrogenesis[END_REF].

Leucosome and migmatitic paragneiss

Leucosome and migmatitic paragneiss samples collected at the same outcrop have similar characteristics. The concordant age of 288 ± 4 Ma for leucosome sample BPA 003-12d is similar (within error) to the lower intercept age of 289 ± 4 Ma in the migmatitic paragneiss (BPA 003-12c). The very low Ybn/Gdn coupled to a moderate negative Eu-anomaly attests the presence of garnet and plagioclase during the crystallization of zircon in both samples. The commonly low U value indicates that partial melting of the metapelites did not reach the biotite-out reaction (almost synchronous to apatite and xenotime-out reaction; Bea and Montero 1999) and/or extensive zircon dissolution. This is in agreement with the observed Sil-Bt assemblage in residual parts of the migmatitic metasediments, and the presence of inherited zircon cores (e.g. B4 z10 of sample BPA 003-12d).

Interpretation of U-Pb ages

In conclusion, ages from both magmatic and metamorphic rocks are similar within error, ranging between 289 ± 4 and 285 ± 6 Ma. Zircon chemistry indicates that in plutonic rocks, zircon crystallizes after plagioclase (presence of negative Eu-anomalies). Thus, U-Pb ages record the final crystallization of the magmatic melt in the gabbroic complex and in the migmatitic contact aureole. They do not date older processes associated with magma ascent or younger, post-emplacement alteration. The similar U-Pb ages of the magmatic rocks and the migmatitic metasediments emphasize that the observed partial melting was effectively caused by the emplacement of the Sondalo gabbroic complex in the Campo metasediments. It is also important to note that the age of the deformed diorite (GM601), sampled from a dyke crosscutting at high angle S3-bearing metasediments, indicates that the S3 foliation has to be older than 285 ± 2 Ma.

The Sondalo intrusion in the Variscan to post-Variscan history of the Austroalpine domain

Variscan evolution

The Campo unit records a pre-Permian prograde metamorphism reaching amphibolite to upper-amphibolite facies [START_REF] Petri | The Sondalo gabbro contact aureole (Campo unit, Eastern Alps): implications for mid-crustal mafic magma emplacement[END_REF], in agreement with other units from the Austroalpine domain such as the Err, Bernina [START_REF] Halmes | The transition from passive to active margin tectonics: a case study from the zone of Samedan (eastern Switzerland)[END_REF][START_REF] Büchi | Der variskiscke Magmatismus in der östlichen Bernina[END_REF]) and the Margna units [START_REF] Guntli | Metamorphose in der Magma-Decke im Bereich Piz da la Margna und Piz Fedoz (Oberengadin)[END_REF]. We suggest that the yet undated pre-Permian history of the Campo unit is likely Variscan as: (1) the Campo metasediments documents Ordovician protolith ages (U-Pb on detrital zircons; Bergomi and Boriani 2012), ( 2) Variscan magmatism is documented in the neighbouring Bernina unit [START_REF] Quadt | U-Pb zircon ages from igneous rocks of the Bernina nappe system (Grisons, Switzerland)[END_REF] ,(3) Austroalpine nappes largely document Variscan cooling ages (see below ;[START_REF] Thöni | Degree and evolution of the alpine metamorphism in the Austroalpine unit W of the Hohe Tauern in the light of K/Ar and Rb/Sr age determination on micas[END_REF]) and ( 4) adjacent units shared a similar evolution during Variscan deformation [START_REF] Hoinkes | Evolution of the Ötztal-Stubai, Scarl-Campo and Ulten Basement Units[END_REF]. Indeed, this Variscan prograde metamorphism is dated in the adjacent Ulten area (the Upper Tonale unit) as Lower Carboniferous (351-343 Ma by U-Pb on monazite included in garnet; [START_REF] Langone | Preservation of old (prograde metamorphic) U-Th-Pb ages in unshielded monazite from the high-pressure paragneisses of the Variscan Ulten Zone (Italy)[END_REF]. Similarly, we correlate the D1 deformation phase in the Campo unit with Variscan crustal thickening during the Lower Carboniferous (Fig. 9a). It was followed by the subsequent D2 deformation stage during the regional thermal peak, estimated to be Middle Carboniferous in the Ulten zone (330-326 Ma by U-Pb on matrix monazite; [START_REF] Langone | Preservation of old (prograde metamorphic) U-Th-Pb ages in unshielded monazite from the high-pressure paragneisses of the Variscan Ulten Zone (Italy)[END_REF]). This evolution is fairly consistent with the Variscan history documented elsewhere in the Alpine domain (Schaltegger and Gebauer 1999).

Late-to post-Variscan evolution

Decompression of the Campo unit to P < 4.5 kbar occurred after the Carboniferous burial and exhumation but before the onset of HT-metamorphism related to the emplacement of the Sondalo gabbroic complex at ca. 289-285 Ma. This is indicated by paragenetic relations far from the intrusion, where HT assemblages developed at lower pressure than peak Variscan conditions (see geological setting and discussion in [START_REF] Petri | The Sondalo gabbro contact aureole (Campo unit, Eastern Alps): implications for mid-crustal mafic magma emplacement[END_REF]). However, no evidence of retrograde deformation related to this event could be found in the Campo unit. Therefore, we suggest that the Campo unit was tectonically unroofed during the late-Carboniferous/Permian by shear zones structurally higher in the crust before the emplacement of the Sondalo gabbroic complex (Fig. 9b). These shear zones are possibly located in the overlying Grosina unit where undated deformations are observed. This evolution is comparable to the Late Carboniferous history of the Austroalpine domain characterized by a late-orogenic extensional phase associated with exhumation occurring between 310 and 290 Ma. Such an exhumation is evidenced in the Bernina unit where alkaline intrusive rocks emplaced at P < 3 kbar and ca. 295 Ma are covered by alkaline rhyolites dated at ca. 288 Ma [START_REF] Büchi | Geologie und Petrographie der Bernina IX. Das Gebiet zwischen Pontresina und dem Morteratschgletscher[END_REF][START_REF] Spillmann | The Pre-Alpine Basement of the Lower Austro-Alpine Nappes in the Bernina Massif (Grisons, Switzerland; Valtellina, Italy)[END_REF][START_REF] Quadt | U-Pb zircon ages from igneous rocks of the Bernina nappe system (Grisons, Switzerland)[END_REF]. Accordingly, cooling of basement rocks in the Silvretta unit ranges between 330 and 310 Ma for K-Ar on micas [START_REF] Flisch | Die Hebungsgeschichte der oberostalpinen Silvretta-Decke seit der mittleren Kreide[END_REF]) and ca. 310 Ma for Rb-Sr on micas [START_REF] Thöni | Degree and evolution of the alpine metamorphism in the Austroalpine unit W of the Hohe Tauern in the light of K/Ar and Rb/Sr age determination on micas[END_REF].

From 290 to 270 Ma, the Permian post-Variscan extension is interpreted as characterized by a shallow lithosphere-asthenosphere boundary, responsible for the mantle partial melting and associated emplacement of mafic intrusions at all crustal levels (Fig. 9c; McCarthy and Müntener 2015) during crustal extension. In the Austroalpine domain mafic intrusions are emplaced at the crust-mantle boundary and in the lower crust (e.g. the Braccia gabbro; [START_REF] Hermann | Fossil crust-tomantle transition, Val Malenco (Italian Alps)[END_REF] as well as in the middle crust [START_REF] Tribuzio | Petrology, mineral and isotope geochemistry of the Sondalo gabbroic complex (Central Alps, Northern Italy): implications for the origin of post-Variscan magmatism[END_REF][START_REF] Braga | The Sondalo Gabbroic Complex (Central Alps, Northern Italy): Evidence for Emplacement of Mantle-Derived Melts into Amphibolite Facies Metapelites[END_REF]; this study). In the Campo unit, near-isobaric heating due to contact metamorphism occurred at 5.2-6 kbar during a P-T loop that was distinct from the Variscan one (Fig. 1c; [START_REF] Petri | The Sondalo gabbro contact aureole (Campo unit, Eastern Alps): implications for mid-crustal mafic magma emplacement[END_REF]. Consequently, from the available pressure and temperature data as well as related deformation history, the Variscan orogenic collapse and the Permian thermal event seems to be, at least locally, two distinct events.

In the Austroalpine domain, the Permian granulite facies metamorphism, and therefore the Permian HT-phase ended at ca. 260 Ma (Fig. 9d; [START_REF] Hermann | Relating zircon and monazite domains to garnet growth zones: age and duration of granulite facies metamorphism in the Val Malenco lower crust[END_REF]. This event was followed by the onset of subsidence and the deposition of a thick Triassic succession (> 1 km) passing from fluvial to shallow marine conditions [START_REF] Dössegger | Die Sedimentserien der Engadiner Dolomiten und ihere lithostratigraphische Gliederung (Teil 2)[END_REF][START_REF] Furrer | Stratigraphie und Facies der Trias/Jura-Grenzschichten in den Oberostalpinen Decken Graubündens[END_REF]. The deposition of these sediments is interpreted as related to the thermal re-equilibration post-dating the Permian HT event and possibly to tectonic and thermal subsidence in relation with Triassic rifting in the Eastern Adriatic domain [START_REF] Schuster | Permian metamorphic event in the Alps[END_REF]. Geochronological arguments combined with field relationships throughout the Alps indicate that significant exhumation of middle to lower crustal rocks affected by the Permian HT metamorphism occurred only during Jurassic rifting [START_REF] Villa | 39Ar-40Ar dating of multiply zoned amphibole generations (Malenco, Italian Alps)[END_REF][START_REF] Mohn | Necking of continental crust in magma-poor rifted margins: Evidence from the fossil Alpine Tethys margins[END_REF]Galster et al., submitted).

Geodynamic implications

The Sondalo gabbroic complex is one example of a suite of intrusive bodies that testify the occurrence of an important magmatic and metamorphic Permian event documented across whole Western Europe.

Intra-continental basins filled by volcano-sedimentary sequences are widespread across the former Variscan realm. Some of them were already opened during the Carboniferous (Fig. 10; [START_REF] Capuzzo | Facies and basin architecture of the Late Carboniferous Salvan-Dorénaz continental basin (Western Alps, Switzerland/France)[END_REF][START_REF] Cassinis | A stratigraphic and tectonic review of the Italian Southern Alpine Permian[END_REF]. This attests for a topographic attenuation and a re-equilibration of the crustal thickness during the Late Carboniferous to Permian [START_REF] Lorenz | Plate and intraplate processes of Hercynian Europe during the late paleozoic[END_REF][START_REF] Burg | Syn-to post-thickening extension in the Variscan Belt of Western Europe: Modes and structural consequences[END_REF]) in an extensional to transtensional tectonic regime [START_REF] Henk | Did the Variscides collapse or were they torn apart?: A quantitative evaluation of the driving forces for post-convergent extension in central Europe[END_REF]).

An increasing number of studies presents strong evidences for a Permian magmatic and metamorphic event that can be observed throughout large parts of Western Europe. In the Alpine belt, a thinning of the lithosphere is attested by the isotopic signature of mantle rocks indicating Permian partial melting [START_REF] Rampone | Isotopic contrasts within the Internal Liguride ophiolite (N. Italy): the lack of a genetic mantle-crust link[END_REF][START_REF] Müntener | Refertilization of mantle peridotite in embryonic ocean basins: trace element and Nd isotopic evidence and implications for crust-mantle relationships[END_REF][START_REF] Mccarthy | Ancient depletion and mantle heterogeneity: Revisiting the Permian-Jurassic paradox of Alpine peridotites[END_REF], accretion of underplated magmatic bodies [START_REF] Voshage | A Nd and Sr isotopic study of the Ivrea zone, Southern Alps, N-Italy[END_REF][START_REF] Libourel | Le complexe de Santa-Lucia di Mercurio (Corse): un équivalent possible des complexes de la Zone d'Ivrée[END_REF][START_REF] Vielzeuf | Granulites orthodérivées d'âge tardi-hercynien; exemple de la norite de Treilles, Corbières (Aude, France)[END_REF][START_REF] Hermann | Fossil crust-tomantle transition, Val Malenco (Italian Alps)[END_REF]) and mid-to upper-crustal mafic intrusion [START_REF] Tribuzio | Petrology, mineral and isotope geochemistry of the Sondalo gabbroic complex (Central Alps, Northern Italy): implications for the origin of post-Variscan magmatism[END_REF][START_REF] Monjoie | Contrasting magma types and timing of intrusion in the Permian layered mafic complex of Mont Collon (Western Alps, Valais, Switzerland): evidence from U/Pb zircon and 40Ar/39Ar amphibole dating[END_REF][START_REF] Tribuzio | Petrogenesis of Early Permian olivinebearing cumulates and associated basalt dykes from Bocca di Tenda (Northern Corsica): Implications for post-collisional Variscan evolution[END_REF][START_REF] Petri | The Sondalo gabbro contact aureole (Campo unit, Eastern Alps): implications for mid-crustal mafic magma emplacement[END_REF]. Synchronously, the lithospheric thinning combined with widespread mafic magmatism was responsible for HT metamorphism ranging from greenschist to granulite facies [START_REF] Schuster | Permian metamorphic event in the Alps[END_REF]. Indeed, in the Alps but also in Calabria, in Corsica and in the Pyrenees, most of the high-grade rocks are associated in the field to mafic plutons emplaced at the same pressure conditions (Figs 10 and 11;[START_REF] Vielzeuf | Relations de phases dans le faciès granulite et implications géodynamiques. L'exemple des granulites des Pyrénées[END_REF][START_REF] Libourel | Le complexe de Santa Lucia di Mercurio (Corse): Ultramafites mantelliques, intrusion basique stratifiée, paragneiss granulitique. Un équivalent possibe des complexes de la zone d'Ivrée[END_REF][START_REF] Lardeaux | From granulites to eclogites in the Sesia zone (Italian Western Alps): a record of the opening and closure of the Piedmont ocean[END_REF][START_REF] Barboza | Metamorphism and Anatexis in the Mafic Complex Contact Aureole, Ivrea Zone, Northern Italy[END_REF][START_REF] Müntener | Cooling History and Exhumation of Lower-Crustal Granulite and Upper Mantle (Malenco, Eastern Central Alps)[END_REF][START_REF] Braga | The Sondalo Gabbroic Complex (Central Alps, Northern Italy): Evidence for Emplacement of Mantle-Derived Melts into Amphibolite Facies Metapelites[END_REF][START_REF] Caggianelli | Pressure-temperature evolution of the late Hercynian Calabria continental crust: compatibility with post-collisional extensional tectonics[END_REF]). In addition to mafic magmatism, felsic intrusives and extrusives were emplaced at shallower crustal levels in the Alps (e.g. Von [START_REF] Quadt | U-Pb zircon ages from igneous rocks of the Bernina nappe system (Grisons, Switzerland)[END_REF][START_REF] Galli | U-Pb zircon dating of the Gruf Complex: disclosing the late Variscan granulitic lower crust of Europe stranded in the Central Alps[END_REF], but also elsewhere such as in Sardinia (e.g. [START_REF] Cortesogno | The Variscan post-collisional volcanism in Late Carboniferous-Permian sequences of Ligurian Alps, Southern Alps and Sardinia (Italy): a synthesis[END_REF]) and in the Pyrenees (see Denèle et al. 2012 and references therein). Based on both geochronological and geochemical arguments (e.g. [START_REF] Marquer | Early Permian magmatism in Briançonnais terranes: Truzzo granite and Roffna rhyolite (eastern Penninic nappes, Swiss and Italian Alps)[END_REF][START_REF] Schaltegger | Crustal-scale magmatic systems during intracontinental strikeslip tectonics: U, Pb and Hf isotopic constraints from Permian magmatic rocks of the Southern Alps[END_REF], part of this felsic magmatism originated from mafic intrusives by a combination of magmatic differentiation and mixing with crustalderived melts (e.g. [START_REF] Sinigoi | Production of hybrid granitic magma at the advancing front of basaltic underplating: Inferences from the Sesia Magmatic System (south-western Alps, Italy)[END_REF]. However, evidence for Permian underplated gabbros and HT-metamorphism are not restricted to domains that underwent Mesozoic rifting only. Lower crustal xenoliths in Tertiary to Quaternary lavas are also exhuming mafic magmatic rocks in the French Massif Central (≥ 257 Ma; see references in caption of Fig. 10) as well as felsic granulites (not always associated to mafic rocks; Fig. 11) in the Spanish Central System (ca. 312-277 Ma) and in the French Massif Central (ca. 300-280 Ma). However, available ages indicate that mafic magmatism is slightly older in the Pyrenees (ca. 312-293 Ma) and is progressively younging towards and across the .

Mafic intrusions seem to emplace in segments of the Variscan chain that were still in compression in the Pyrenees (hence syn-to late-Variscan; [START_REF] Aguilar | Late Variscan metamorphic and magmatic evolution in the eastern Pyrenees revealed by U-Pb age zircon dating[END_REF][START_REF] Aguilar | P-T-t-d evolution of orogenic middle crust of the Roc de Frausa Massif (Eastern Pyrenees): a result of horizontal crustal flow and Carboniferous doming?[END_REF]. However, several lines of evidence question the direct relationship between the collapse and the partial melting of the mantle: (1) the main mafic magmatic activity post-dates the crustal re-equilibration in some places (Ivrea zone, [START_REF] Barboza | Regional granulite facies metamorphism in the Ivrea zone: Is the Mafic Complex the smoking gun or a red herring?[END_REF]Campo unit, Petri et al. 2016); (2) the depth of emplacement of Permian pluton do not exceed 11 kbar [START_REF] Spalla | The transition from Variscan collision to continental break-up in the Alps: insights from the comparison between natural data and numerical model predictions[END_REF]); (3) Permian HT-metamorphism is also limited to 11 kbar (e.g. Galli et al. 2010;[START_REF] Manzotti | The pre-Alpine tectonic history of the Austroalpine continental basement in the Valpelline unit (Western Italian Alps)[END_REF][START_REF] Redler | Migmatites in the Ivrea Zone (NW Italy): Constraints on partial melting and melt loss in metasedimentary rocks from Val Strona di Omegna[END_REF][START_REF] Spalla | The transition from Variscan collision to continental break-up in the Alps: insights from the comparison between natural data and numerical model predictions[END_REF]); ( 4) the Permian magmatic system is not restricted to the former orogenic domain (e.g. Northern Germany; Timmerman 2004; [START_REF] Wilson | Permo-Carboniferous magmatism and rifting in Europe: introduction[END_REF]) and finally (5) tectonic extension due to plate divergence is needed to explain the amount of extension in intra-continental Permian basins in the former orogenic domain [START_REF] Henk | Did the Variscides collapse or were they torn apart?: A quantitative evaluation of the driving forces for post-convergent extension in central Europe[END_REF]. Whereas it cannot be excluded for all segments of the Variscan orogeny, the Permian event is not clearly associated with the collapse of the Variscan orogeny. We therefore suggest that the main phase of Late-to Post-Variscan crustal thinning occurred in Late Carboniferous followed during the Permian by an important thinning of the lithospheric mantle while the continental crust was already equilibrated at ca. 30-35 km. Synchronous extension and addition of large volumes of mantle-derived melts may have maintain the crustal thickness to ca. 30-35 km (e.g. Thybo and Artemieva 2013), despite lithospheric and crustal extension during the Permian and hence postponing the marine transgression to Triassic time.

CONCLUSIONS

The following conclusions can be drawn from our study: 1.

The intrusion of the Sondalo gabbroic complex caused partial melting of the host rock Campo unit by contact metamorphism. U-Pb on zircon ages from magmatic and metamorphic rocks are similar and are ranging between 289 ± 4 and 285 ± 6 Ma.

2.

Trace element geochemistry of zircons from magmatic rocks points to an initial crystallization of zircon in absence of garnet and with plagioclase already crystallized (steep HREE slopes and high Eu-anomalies), progressively mixed with a less evolved liquid with plagioclase not completely crystallized and an increasing garnet content (decreasing HREE slopes and decreasing Eu-anomalies). This second melt infiltration is probably derived from the surrounding partially molten metapelites.

3. The Austroalpine units preserve a complex late-to post-Variscan history. From 310 to 290 Ma, late-orogenic extension is associated with emplacement of calc-alkaline to alkaline magmas and exhumation of orogenic crustal units. From 290 to 260 Ma, postorogenic extension is associated with a shallow lithosphere-asthenosphere boundary, responsible for synchronous emplacement of mafic magmas and associated HTmetamorphism. After ~260 Ma, the HT Permian conditions decay, followed by thermal reequilibration and the deposition of Middle to Upper Triassic fluvial to shallow-marine sediments.

4.

We suggest that this Permian event occurred after the termination of post-Variscan crustal reequilibration (i.e. the collapse). Consequently, the Permian mafic magmatism seems to post-date the Variscan collapse in the Austroalpine domain, and probably in Western Europe, although such processes may vary in time and space.

COMPLIANCE WITH ETHICAL STANDARDS

Funding: Field work and analyses have been financed by an ExxonMobil research grant to G. Manatschal.

Conflict of Interest:

The authors declare that they have no conflict of interest. Trümpy and Dösseger (1972), Lorenz andNicholls(1976) , Feys (1989), [START_REF] Châteauneuf | Esquisse paléogéographique du Permien de l'Europe de l'Ouest. Synthèse Géologique des Bassins Permiens[END_REF], [START_REF] Burg | Syn-to post-thickening extension in the Variscan Belt of Western Europe: Modes and structural consequences[END_REF] and [START_REF] Cassinis | Permian continental basins in the Southern Alps (Italy) and peri-mediterranean correlations[END_REF]. Late Carboniferous and Early Permian gabbros and granulites exhumed by tectonic activity or Tertiary to Quaternary volcanoes are indicated with: 1 -Iberian margin (ODP Site 1069; [START_REF] Manatschal | The role of detachment faulting in the formation of an ocean-continent transition: insights from the Iberia Abyssal Plain[END_REF]2 -Spanish Central System (Fernández-Suárez et al. 2006;[START_REF] Villaseca | Metaluminous pyroxene-bearing granulite xenoliths from the lower continental crust in central Spain: their role in the genesis of Hercynian I-type granites[END_REF]); 3 -Gavarnie-Héas dome [START_REF] Kilzi | Geochemistry and zircon U-Pb geochronology of the ultramafic and mafic rocks emplaced within the anatectic series of the Variscan Pyrenees: The example of the Gavarnie-Heas dome (France)[END_REF]);4 -Ursuya massif, Pyrenees [START_REF] Vielzeuf | Relations de phases dans le faciès granulite et implications géodynamiques. L'exemple des granulites des Pyrénées[END_REF]Vielzeuf and Kornprobst 1984); 5 -Quérigut massif, Pyrenees [START_REF] Roberts | Petrogenesis of Mafic to Felsic Plutonic Rock Associations: the Calc-alkaline Quérigut Complex, French Pyrenees[END_REF]; 6 -Treilles norite, Pyrenees [START_REF] Vielzeuf | Granulites orthodérivées d'âge tardi-hercynien; exemple de la norite de Treilles, Corbières (Aude, France)[END_REF]; 7 -Ceret gabbro, Pyrenees [START_REF] Aguilar | Late Variscan metamorphic and magmatic evolution in the eastern Pyrenees revealed by U-Pb age zircon dating[END_REF]); 8 -Calabria [START_REF] Schenk | U-Pb and Rb-Sr radiometric dates and their correlation with metamorphic events in the granulite-facies basement of the serre, Southern Calabria (Italy)[END_REF][START_REF] Caggianelli | From the upper to the lower continental crust exposed in Calabria. 86[END_REF]); 9 -Santa Lucia massif, Corsica [START_REF] Paquette | Episodic and short-lived granitic pulses in a post-collisional setting: evidence from precise U-Pb zircon dating through a crustal cross-section in Corsica[END_REF][START_REF] Rossi | Variscan to eo-Alpine events recorded in European lower-crust zircons sampled from the French Massif Central and Corsica, France[END_REF]; 10 -Bocca di Tenda gabbro, Corsica [START_REF] Cocherie | Comparative use of TIMS and SHRIMP for U-Pb zircon dating of A-type granites and mafic tholeiitic layered complexes and dykes from the Corsican Batholith (France)[END_REF][START_REF] Tribuzio | Petrogenesis of Early Permian olivinebearing cumulates and associated basalt dykes from Bocca di Tenda (Northern Corsica): Implications for post-collisional Variscan evolution[END_REF]11 -Puy Beaunit, French Massif Central (Downes et al. 1991;[START_REF] Costa | Lower crustal rejuvenation and growth during post-thickening collapse: Insights from a crustal cross section through a Variscan metamorphic core complex[END_REF][START_REF] Downes | A Permian underplating event in late-to post-orogenic tectonic setting. Evidence from the mafic-ultramafic layered xenoliths from Beaunit (French Massif Central)[END_REF]; 12 -Bournac, French Massif Central [START_REF] Rossi | Variscan to eo-Alpine events recorded in European lower-crust zircons sampled from the French Massif Central and Corsica, France[END_REF]); 13 -Eiffel massif, Germany (e.g. [START_REF] Loock | Granulite facies lower crustal xenoliths from the Eifel, West Germany: petrological and geochemical aspects[END_REF]); 14 -Monte Ragola, Appenines [START_REF] Meli | Age of mafic granulite blocks from the External Liguride Units (Northern Apennines, Italy)[END_REF]; 15 -Ivrea zone, Southern Alps [START_REF] Pin | Datation U-Pb sur zircons à 285 M.a. du complexe gabbro-dioritique du Val Sesia-Val Mastallone et âge tardi-hercynien du métamorphisme granulitique de la zone Ivrea-Verbano (Italie)[END_REF][START_REF] Vavra | Internal morphology, habit and U-Th-Pb microanalysis of amphibolite-to-granulite facies zircons: geochronology of the Ivrea Zone (Southern Alps)[END_REF][START_REF] Quick | Geologic map of the southern Ivrea-Verbano Zone, northwestern Italy[END_REF][START_REF] Peressini | Duration of a Large Mafic Intrusion and Heat Transfer in the Lower Crust: a SHRIMP U-Pb Zircon Study in the Ivrea-Verbano Zone (Western Alps, Italy)[END_REF]); 16 -Sassa, Sermenza and Mont Collon, Western Alps [START_REF] Bussy | U-Pb ages of magmatic rocks of the western Austroalpine Dent-Blanche-Sesia Unit[END_REF][START_REF] Monjoie | Contrasting magma types and timing of intrusion in the Permian layered mafic complex of Mont Collon (Western Alps, Valais, Switzerland): evidence from U/Pb zircon and 40Ar/39Ar amphibole dating[END_REF][START_REF] Baletti | Structural and petrographic map of the Sassa gabbro complex (Dent Blanche nappe, Austroalpine tectonic system, Western Alps, Italy)[END_REF]): 17 -Gruf complex, Central Alps [START_REF] Galli | U-Pb zircon dating of the Gruf Complex: disclosing the late Variscan granulitic lower crust of Europe stranded in the Central Alps[END_REF]; 18 -Malenco nappe, Eastern Alps [START_REF] Hansmann | U-Pb zircon geochronology of a tholeiitic intrusion and associated migmatites at a continental crust-mantle transition, Val Malenco, Italy[END_REF][START_REF] Hermann | Relating zircon and monazite domains to garnet growth zones: age and duration of granulite facies metamorphism in the Val Malenco lower crust[END_REF]; 19 -Sondalo gabbroic complex [START_REF] Bachmann | Radiometrische Alterbestimmungen des Gabbro von Sondalo. Oberes Velltlin, Italienische Alpen[END_REF][START_REF] Tribuzio | Petrology, mineral and isotope geochemistry of the Sondalo gabbroic complex (Central Alps, Northern Italy): implications for the origin of post-Variscan magmatism[END_REF]this study). See

ELECTRONIC SUPPLEMENTARY

MATERIAL ESM 1 Fig. 2 :

 12 FIGURE CAPTIONS

  Fig. 10: Maps of Early Permian (Autunian, 290-260 Ma) sedimentary basins and volcanics compiled fromTrümpy and Dösseger (1972),[START_REF] Lorenz | The Permocarboniferous Basin and Range Province of Europe. An Application op Plate Tectonics[END_REF] ,[START_REF] Feys | Introduction. Synthèse Géologique des Bassins Permiens Français Mémoire du[END_REF],[START_REF] Châteauneuf | Esquisse paléogéographique du Permien de l'Europe de l'Ouest. Synthèse Géologique des Bassins Permiens[END_REF],[START_REF] Burg | Syn-to post-thickening extension in the Variscan Belt of Western Europe: Modes and structural consequences[END_REF] and Cassinis et al. (2012). Late Carboniferous and Early Permian gabbros and granulites exhumed by tectonic activity or Tertiary to Quaternary volcanoes are indicated with: 1 -Iberian margin (ODP Site 1069; Manatschal et al. 2001); 2 -Spanish Central System (Fernández-Suárez et al. 2006; Villaseca et al. 2007); 3 -Gavarnie-Héas dome (Kilzi et al. 2016);4 -Ursuya massif, Pyrenees (Vielzeuf 1984; Vielzeuf and Kornprobst 1984); 5 -Quérigut massif, Pyrenees (Roberts et al. 2000); 6 -Treilles norite, Pyrenees (Vielzeuf and Pin 1991); 7 -Ceret gabbro, Pyrenees (Aguilar et al. 2014); 8 -Calabria (Schenk 1980; Caggianelli et al. 2013); 9 -Santa Lucia massif, Corsica (Paquette et al. 2003; Rossi et al. 2006); 10 -Bocca di Tenda gabbro, Corsica (Cocherie et al. 2005; Tribuzio et al. 2009); 11 -Puy Beaunit, French Massif Central[START_REF] Downes | A Permian underplating event in late-to post-orogenic tectonic setting. Evidence from the mafic-ultramafic layered xenoliths from Beaunit (French Massif Central)[END_REF][START_REF] Costa | Lower crustal rejuvenation and growth during post-thickening collapse: Insights from a crustal cross section through a Variscan metamorphic core complex[END_REF][START_REF] Downes | A Permian underplating event in late-to post-orogenic tectonic setting. Evidence from the mafic-ultramafic layered xenoliths from Beaunit (French Massif Central)[END_REF]; 12 -Bournac, French Massif Central[START_REF] Rossi | Variscan to eo-Alpine events recorded in European lower-crust zircons sampled from the French Massif Central and Corsica, France[END_REF]); 13 -Eiffel massif, Germany (e.g.[START_REF] Loock | Granulite facies lower crustal xenoliths from the Eifel, West Germany: petrological and geochemical aspects[END_REF]); 14 -Monte Ragola, Appenines[START_REF] Meli | Age of mafic granulite blocks from the External Liguride Units (Northern Apennines, Italy)[END_REF]; 15 -Ivrea zone, Southern Alps[START_REF] Pin | Datation U-Pb sur zircons à 285 M.a. du complexe gabbro-dioritique du Val Sesia-Val Mastallone et âge tardi-hercynien du métamorphisme granulitique de la zone Ivrea-Verbano (Italie)[END_REF][START_REF] Vavra | Internal morphology, habit and U-Th-Pb microanalysis of amphibolite-to-granulite facies zircons: geochronology of the Ivrea Zone (Southern Alps)[END_REF][START_REF] Quick | Geologic map of the southern Ivrea-Verbano Zone, northwestern Italy[END_REF][START_REF] Peressini | Duration of a Large Mafic Intrusion and Heat Transfer in the Lower Crust: a SHRIMP U-Pb Zircon Study in the Ivrea-Verbano Zone (Western Alps, Italy)[END_REF]); 16 -Sassa, Sermenza and Mont Collon, Western Alps[START_REF] Bussy | U-Pb ages of magmatic rocks of the western Austroalpine Dent-Blanche-Sesia Unit[END_REF][START_REF] Monjoie | Contrasting magma types and timing of intrusion in the Permian layered mafic complex of Mont Collon (Western Alps, Valais, Switzerland): evidence from U/Pb zircon and 40Ar/39Ar amphibole dating[END_REF][START_REF] Baletti | Structural and petrographic map of the Sassa gabbro complex (Dent Blanche nappe, Austroalpine tectonic system, Western Alps, Italy)[END_REF]): 17 -Gruf complex, Central Alps[START_REF] Galli | U-Pb zircon dating of the Gruf Complex: disclosing the late Variscan granulitic lower crust of Europe stranded in the Central Alps[END_REF]; 18 -Malenco nappe, Eastern Alps[START_REF] Hansmann | U-Pb zircon geochronology of a tholeiitic intrusion and associated migmatites at a continental crust-mantle transition, Val Malenco, Italy[END_REF][START_REF] Hermann | Relating zircon and monazite domains to garnet growth zones: age and duration of granulite facies metamorphism in the Val Malenco lower crust[END_REF]; 19 -Sondalo gabbroic complex[START_REF] Bachmann | Radiometrische Alterbestimmungen des Gabbro von Sondalo. Oberes Velltlin, Italienische Alpen[END_REF][START_REF] Tribuzio | Petrology, mineral and isotope geochemistry of the Sondalo gabbroic complex (Central Alps, Northern Italy): implications for the origin of post-Variscan magmatism[END_REF]; this study). See Fig.11for cumulative probability plots of ages. Sketch of the Variscan chain is modified from Martínez Catalán (2011) and[START_REF] Ballèvre | Correlation of the nappe stack in the Ibero-Armorican arc across the Bay of Biscay: a joint French-Spanish project[END_REF]

ACKNOWLEDGMENTS B. Petri was supported by the University of Strasbourg. Fieldwork and analysis were financed by an ExxonMobil research grant to G. Manatschal. R. van Elsas and A. Aubert are thanked for the help with mineral separation, and P. Vonlanthen and G. Morvan for the BSE and CL imaging, A. Ulianov for the help with the LA-ICP-MS. A. Bouzeghaia is warmly thanked for her help with figures. O. Müntener is thanked for discussions. All authors acknowledge careful reviews from N. Froitzheim and A. Galli and W.-C. Dullo for editorial handling.