International audienceA structural and petrochronological study was carried out in the southern part of the Belledonne crystalline massif. A first tectonometamorphic event, Dx, corresponds to the eastward thrusting of the Chamrousse ophiolitic complex characterized by a low-temperature–moderate-pressure metamorphism reaching 0.535 ± 0.045 GPa and 427.5 ± 17.5 °C. A subsequent D1 deformation is defined by a penetrative S1 foliation that mostly dips toward the west and displays an E–W- to NE–SW-trending mineral and stretching lineation L1. D1 is associated with a top-to-the east shearing and is responsible for the crustal thickening accommodated by the eastward nappe stacking and the emplacement of the Chamrousse ophiolitic complex upon the Rioupéroux-Livet unit. This event is characterized by an amphibolite facies metamorphism (0.58 GPa ± 0.06; 608 ± 14 °C) that attains partial melting at the base of the nappe pile (0.78 ± 0.07 GPa; 680.5 ± 11.5 °C). LA-ICP-MS U-Pb dating of monazite grains from the mica schists of the Rioupéroux-Livet unit constrain the age of D1 to 337 ± 7 Ma. The D2 tectono-metamorphic event is characterized by NE–SW trending, upright to NE-verging synfolial folding. Folding associated with D2 is pervasively developed in all lithotectonic units with the development of a steeply-dipping S2 foliation. In particular, D2 involves the uppermost weakly metamorphosed Taillefer unit. LA-ICP-MS U-Pb dating performed on detrital zircon grains shows that the Taillefer conglomerates was deposited during the Visean. A zircon SIMS U-Pb age of 352 ± 1 Ma from a plagioglase-rich leucocratic sill of the Rioupéroux-Livet unit is interpreted as the age of magmatic emplacement. Our results suggest that the D2 event took place between 330 Ma and 310 Ma. We propose a new interpretation of the tectonometamorphic evolution of the southern part of the Belledonne massif, focusing on the Middle Carboniferous stages of the Variscan orogeny

We decipher late-orogenic crustal flow characterized by feedback relations between partial melting and deformation in the Variscan Montagne Noire gneiss dome. The dome shape and finite strain pattern of the Montagne Noire Axial Zone (MNAZ) result from the superimposition of three deformations (D1, D2 and D3). The early flat-lying S1 foliation is folded by D2 upright ENE-WSW folds and transposed in the central and southern part of the MNAZ into steep D2 high-strain zones consistent with D2 NW-SE horizontal shortening, in bulk contractional coaxial deformation regime that progressively evolved to noncoaxial dextral transpression. The D2 event occurred under metamorphic conditions that culminated at 0.65 ± 0.05 GPa and 720 ± 20°C. Along the anatectic front S1 and S2 foliations are transposed into a flat-lying S3 foliation with top-to-NE and top-to-SW shearing in the NE and SW dome terminations, respectively. These structures define a D3 transition zone related to vertical shortening during coaxial thinning with a preferential NE-SW to E-W directed stretching. Depending on structural level, the metamorphic conditions associated with D3 deformation range from partial melting conditions in the dome core to subsolidus conditions above the D3 transition zone. We suggest that D2 and D3 deformation events were active at the same time and resulted from strain partitioning on both sides of the anatectic front that may correspond to a major rheological boundary within the crust.

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This paper aims to decipher the thermal evolution of the Montagne Noire Axial Zone (MNAZ, southern French Massif Central) gneiss core and its metasedimentary cover through determination of P-T paths and temperature gradients. Migmatitic gneiss from the core of the dome record a clockwise evolution culminating at 725 AE 25°C and 0.8 AE 0.1 GPa with partial melting, followed by a decompression path with only minor cooling to 690 AE 25°C and 0.4 AE 0.1 GPa. Field structural analyses as well as detailed petrological observations indicate that the cover sequence experienced LP-HT metamorphism. Apparent thermal gradients within the cover were determined with garnet-biotite thermometry and Raman Spectroscopy on Carbonaceous Matter. High-temperature apparent gradients (e.g. $ 530°C km À1 along one transect) are explained by late brittle-ductile extensional shearing evidenced by phyllonites that post-date peak metamorphism. In areas where normal faults are less abundant and closely spaced, gradients of $ 20 to 50°C km À1 are calculated. These gradients can be accounted for by a combination of dome emplacement and ductile shearing (collapse of isotherms), without additional heat input. Finally, the thermal evolution of the MNAZ is typical for many gneiss domes worldwide as well as with other LP-HT terranes in the Variscides.

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Based on new structural, petrological and U-Th-Pb data, a reappraisal of the Variscan tectono-metamorphic history of the SW Belledonne-Pelvoux (Eastern Cristalline Massifs, French Alps) mid-lower crust is proposed. These results are keys to better understand the flow pattern of the variscan partially molten crust and the geodynamic evolution of the sub-meridional branch of the variscan belt. The SW Belledonne area exposes the suprastructure where the medium grade metamorphic rocks record a middle Carboniferous evolution, with a westward gently dipping S1 foliation refolded by east-verging inclined folds (F2) with a subvertical to west-dipping axial plane cleavage S2. Further east, in the Pelvoux area, the high-grade metamorphic rocks of the infrastructure, mostly migmatites, record a prominent D2 deformation with a penetrative NE-SW steeply dipping S2 foliation and a network of NS and NW-SE trending shear zones the kinematics of which indicate a bulk sinistral transpression. D1 and D2 features are interpreted as reflecting a NW–SE contraction, first marked by dominant nappe stacking and subsequently overprinted by transpressional shearing. The supra/infrastructure boundary is typified by a D3 deformation zone that consists of a flat laying S3 foliation with a NW-SE stretching lineation along which top to NW sense of shear is observed. The D1 event corresponds to mid-carboniferous crustal thickening. Exhumation of deep-seated rocks during the transpressional D2 event followed a near isothermal (ca. 700 °C) evolution down to pressure of ca. 5kbar in the period 325-306 Ma. From ca. 306 Ma, D2 and D3 acted synchronously while the mid-lower crust recorded a near isobaric temperature increase up to 850 °C before final cooling and retrogression. The P-T-t history of the Pelvoux area is very similar to the nearby Velay dome and suggest that the Belledonne-Pelvoux area may belong to the Western Moldanubian zone of the Variscan Belt. In that vision, we suggest that coupling between D2 and D3 deformation enable the south-eastward migration, i.e. from the orogenic plateau to the foreland, of the hot and low-viscosity partially-molten crust.

<p>                The Aiguilles-Rouge Massif (ARM) is one of the Western External Crystallin Massifs (ECM) of the French Alps. Similarly to the other ECMs, the ARM exposes a Variscan basement made of migmatitic ortho- and paragneisses and micaschists that hold metric boudins of retrograded eclogites, amphibolites and serpentinites. Upward, low-grade and weakly metamorphosed Late-Carboniferous terrigenous sediments overly the Variscan basement. Deformation and metamorphism occurred between 330 and 300 Ma. The whole ARM is structured by a main N-S to NE-SW trending and vertical foliation formed in response to a regional dextral transpression. The tectonic significance of the ARM’s high-pressure rocks in the Variscan belt realm as relics of a subduction zone, pieces of crustal root of an orogenic plateau or overpressure phenomenon along a high-strain zone is still highly debated. A question that also remains is how eclogite Pressure–Temperature–time-Deformation history (P–T–t-D path) relates to the metamorphic paths recorded in the surrounding migmatitic rocks. In this contribution we present new structural and microstructural (EBSD data) observations that give us a detailed vision of the partitioning of the crustal scale deformation during Late-Variscan time. Three main deformations, named D1, D2 and D3, have been recognized in the gneissic core of the ARM. D1 is relictual and corresponds to a flat-lying S1 foliation that is only visible in the high grade metasedimentary rocks and preserved in low-D2 strain domains. D1 is associated with a partial melting metamorphic event M1. D2 is characterized by three main orientations of planar fabrics that are oriented in directions N160, N0 and N20. These planar fabrics are interpreted as S2-C2-C2’ related to anastomosed system developed under a bulk dextral transpression. D2 shearing becomes more penetrative toward the NE, where it is associated to local partial melting. D3 corresponds to the development of a flat-lying S3 cleavage together with the folding of vertical D2 foliations. The D3 is linked to a regional vertical shortening, associated to few liquid injections. These partial melting conditions occurring during D1, D2 and D3 deformations may unravel a continuum of these three deformations during a short period of time. Processing of new thermobarometric and LA-ICP-MS U-Pb geochronological data on eclogites, surrounding rocks and migmatites are currently in progress. The new obtained results will be presented in addition to the structural and metamorphic data in order to discuss the P-T-t-D path of the deeply buried metasedimentary rocks, migmatites and preserved eclogites.</p>

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