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Keywords: magmatic system, crustal contamination, diffusion, hybridization, partial meltingMagmatic differentiation requires a variable combination of fractional crystallization and/or crustal contamination that influences the liquid line of descent, as well as the composition and the final paragenesis of resulting magmatic rocks. However, the vectors of crustal contamination and how they influence the magmatic differentiation remain poorly constrained, notably because the depth at which they are active are usually hardly accessible. Several processes have been invoked in the literature: (1) small-scale diffusion; (2) energetically costly partial melting of crustal material coupled with magma hybridization; (3) The dissolution of crustal rocks by reactive bulk assimilation. Instead of focusing on the deepest crustal levels, we here explore crustal contamination processes active in the intermediate continental crust. We use the example of the Sondalo gabbroic complex that intruded the metasedimentary Campo unit, both exposed in the Central Alps.The Sondalo gabbroic complex is a Permian intrusion of tholeiitic affinity (troctolite and norite, 300±12 and 280±10 Ma by Sm-Nd) that evolved towards calc-alkaline intermediate bodies (diorite and granodiorite, 289±4 - 285±6 Ma by U-Pb on Zrn). Mafic melts intruded the Campo unit composed of fertile amphibolite-facies micaschist and paragneiss (Ms-Bt-St-Grt-Pl stable), attesting of a (supposed) Carboniferous prograde P-T paths (5.5 - 6 kbar/600°C-650°C). The emplacement of this intrusion caused a HT-contact metamorphism reaching partial melting of host rocks at 289±4 – 288±5 (U-Pb on Zrn) Ma and in-situ formation of Crd-Grt-Sil-Spl granulite-facies restite composing large septa. Field and petrological observations coupled with geochemical bulk rock major and trace element analyses show the contribution of host-rock contamination, by: (1) mafic magmas of tholeiitic affinity becoming progressively calc-alkaline; (2) the increase in modal amount of garnet, biotite and cordierite in magmatic rocks around metasedimentary septa, stabilized by the influx of some major elements (e.g., SiO2, K2O, Al2O3 and H2O) in the noritic mush; (3) liquid line of descent departs from theoretically predicted compositions (with both equilibrium and fractional crystallization) with enrichment in elements typical for crustal rocks (i.e., K2O and Al2O3 at high Mg#).Field observations and bulk rock major and trace elements composition highlight that crustal contamination is achieved through a combination of vectors having a variable spatial extent. Their respective weight is, however, still difficult to constrain. The middle crust seems to be the ideal location for crustal assimilation because host-rocks are fertile and the mafic magmas benefit from a high and durable thermal regime that appears to favor physical and chemical interactions. Further constraints will be brought by in-situ trace element analyses and Sr-Nd isotopes to estimate their respective influence on hybridization.

Keywords: Central Alps, magmatic system, thermodynamic modelling, crystallization, crustal contaminationUnderstanding which processes are active and quantifying their relative influence during the differentiation of intracontinental magmatic systems remains a major challenge, as these processes can either (1) involve magmas and their crystallization products (fractional crystallization, reactive melt flow...) and/or (2) crustal contamination through various vectors (bulk assimilation, reactive assimilation, host-rock partial melting…). Whereas the influence of some of these processes can be inferred from field evidence, it needs to be constrained and quantified. This question can be addressed in the Central Alps (N Italy, SE Switzerland), where a complete, crustal-scale post-Variscan (Permian) magmatic system has been documented from lower crustal (Braccia gabbro, Malenco unit) and mid-crustal intrusives (Sondalo gabbro, Campo unit) to upper crustal intrusives and extrusives (Bernina unit). We present preliminary results, combining field work to petrological and geochemical characterization and modelling. Petrological investigations on major element bulk-rock composition shows a complete differentiation trend from the less differentiated lower crust intrusive mafic rocks (Ol-gabbro, gabbro: 40-50 wt.% SiO₂, Mg# 45-75, 0.1-0.8 wt.% K₂O; and diorite: 45-60 wt.% SiO₂, Mg# 45-55, 0.15-0.5 wt.% K₂O), to upper crust felsic rocks (granite/rhyolite: 55-85 wt. % SiO₂, Mg# 5-50, 1-6 wt.% K₂O). By contrast, middle crust intrusive rocks encompass the full compositional range from Ol-gabbro and gabbro (45-50 wt.% SiO₂, Mg# 35-90, 0-3 wt.% K₂O), to alkali-rich diorite (50-60 wt.% SiO₂, Mg#: 40-55, 0.5-2 wt.% K₂O) and granite (50-85 wt.% SiO₂, Mg#: 5-50, 1-6 wt.% K₂O). To test the role of equilibrium and fractional crystallization, thermodynamic models were run using Rhyolite-MELTS software, and compared to experimental results in the 0-1 GPa pressure range from the literature. Some correlations between our samples compositions and the models (e.g., for CaO contents and Mg#) can be seen, but the latter fails at reproducing SiO2 and K2O differentiation trends. Bulk-rock compositions indicate that magmas follow a composite differentiation trend between tholeiitic and calc-alkaline series, and the low abundance of olivine, even for the most primitive rocks indicates that before reaching the lower crust, magma was already fractionated during it ascent through the mantle. However, major differentiation does not seem to occur in the lower crust, being set fertile by previous tectono-metamorphic events. Instead, most of differentiation occurs in the fertile middle crust, since a wide major elements compositional range is observed. Both experimental and modelling results show that the observed diversity of composition cannot be attributed to fractional crystallization solely, notably by the high K₂O content at high Mg#. This suggest a potential role on crustal contamination; although evidence for contamination can be documented in the field (e.g., garnet, cordierite-bearing gabbro surrounding xenoliths), the extent of this contamination and its vectors remains to be constrained.

Modelling of geophysical data is often subject to choices made by the researcher undertaking the work. The level of structural complexity in the model, the bounds on parameters imposed by a priori knowledge, the thoroughness and efficiency in exploring the parameter space may all lead to bias in determining what the best fitting models can be.To avoid bias from our own ideas in constraining the subsurface shape of a given density anomaly, we hereby invite anyone interested to create their own models. This is planned by sharing the same gravity data measured in the field, the same digital elevation model, the main features of the local geological maps, and bounds on the encountered rock density values. These data will be shared openly, in the form of a modelling challenge: each participating researcher or group is expected to submit their solution(s). All these will be compared during a dedicated workshop, ultimately resulting in a joint publication.The target of this modelling challenge is the world-famous Balmuccia peridotite body (45.84°N, 8.16°E) in the Ivrea-Verbano Zone (IVZ). Here mantle rocks are naturally exposed at the surface, in the broader context of the IVZ, a middle- to lower crustal terrain along the Europe-Adria plate boundary’s eastern side. The surface exposure of the Balmuccia peridotite is ~ 4.4 km N-S by 0.6 km E-W, with outcrop elevation changes exceeding 1000 m. About 150 new gravity data points have been measured within a radius of 3 km from the centre of the peridotite body, along more or less accessible paths and slopes. The measurements have been carried out with a Scintrex CG-5 relative gravimeter, tied to a reference point, and all points located via differential GPS with typical vertical precision of a few cm. Farther away regional gravity data is available at few km spacing.Beyond the modelling challenge, the interest in constraining the subsurface shape of the Balmuccia peridotite body is its future target role in the ICDP DIVE continental drilling project (www.dive2ivrea.org).

The fieldwork was supported by the DIPS project (grant no. 240467) and the MIMES project (grant no. 244155) funded by the Norwegian Research Council awarded to O.G. O.P.'s position was funded from Y-TEC. Anisotropy of magnetic susceptibility (AMS) and anisotropy of magnetic remanence (AARM and AIRM) are efficient and versatile techniques to indirectly determine rock fabrics. Yet, deciphering the source of a magnetic fabric remains a crucial and challenging step, notably in the presence of ferrimagnetic phases. Here we use X-ray micro-computed tomography to directly compare mineral shape-preferred orientation and spatial distribution fabrics to AMS, AARM and AIRM fabrics from five hypabyssal trachyandesite samples. Magnetite grains in the trachyandesite are euhedral with a mean aspect ratio of 1.44 (0.24 s.d., long/short axis), and > 50% of the magnetite grains occur in clusters, and they are therefore prone to interact magnetically. Amphibole grains are prolate with magnetite in breakdown rims. We identified three components of the petrofabric that influence the AMS of the analyzed samples: the magnetite and the amphibole shape fabrics and the magnetite spatial distribution. Depending on their relative strength, orientation and shape, these three components interfere either constructively or destructively to produce the AMS fabric. If the three components are coaxial, the result is a relatively strongly anisotropic AMS fabric (P’ = 1.079). If shape fabrics and/or magnetite distribution are non-coaxial, the resulting AMS is weakly anisotropic (P’ = 1.012). This study thus reports quantitative petrofabric data that show the effect of magnetite distribution anisotropy on magnetic fabrics in igneous rocks, which has so far only been predicted by experimental and theoretical models. Our results have first-order implications for the interpretation of petrofabrics using magnetic methods. Publisher PDF

SUMMARY We provide a high-resolution image of the Ivrea Geophysical Body (IGB) in the Western Alps with new gravity data and 3-D density modelling, integrated with surface geological observations and laboratory analyses of rock properties. The IGB is a sliver of Adriatic lower lithosphere that is located at shallow depths along the inner arc of the Western Alps, and associated with dense rocks that are exposed in the Ivrea-Verbano Zone (IVZ). The IGB is known for its high seismic velocity anomaly at shallow crustal depths and a pronounced positive gravity anomaly. Here, we investigate the IGB at a finer spatial scale, merging geophysical and geological observations. We compile existing gravity data and we add 207 new relative gravity measurements, approaching an optimal spatial coverage of 1 data point per 4–9 km2 across the IVZ. A compilation of tectonic maps and rock laboratory analyses together with a mineral properties database is used to produce a novel surface rock-density map of the IVZ. The density map is incorporated into the gravity anomaly computation routine, from which we defined the Niggli gravity anomaly. This accounts for Bouguer Plate and terrain correction, both considering the in situ surface rock densities, deviating from the 2670 kg m–3 value commonly used in such computations. We then develop a 3-D single-interface crustal density model, which represents the density distribution of the IGB, including the above Niggli-correction. We retrieve an optimal fit to the observations by using a 400 kg m–3 density contrast across the model interface, which reaches as shallow as 1 km depth below sea level. The model sensitivity tests suggest that the ∼300–500 kg m–3 density contrast range is still plausible, and consequently locates the shallowest parts of the interface at 0 km and at 2 km depth below sea level, for the lowest and the highest density contrast, respectively. The former model requires a sharp density discontinuity, the latter may feature a vertical transition of densities on the order of few kilometres. Compared with previous studies, the model geometry reaches shallower depths and suggests that the width of the anomaly is larger, ∼20 km in west–east direction and steeply E–SE dipping. Regarding the possible rock types composing the IGB, both regional geology and standard background crustal structure considerations are taken into account. These exclude both felsic rocks and high-pressure metamorphic rocks as suitable candidates, and point towards ultramafic or mantle peridotite type rocks composing the bulk of the IGB.

International audience; The study of rock fabric properties (orientation, planar, linear, anisotropy) is key to unravelling the geological processes that generated them. With advancements in data acquisition and treatment, X-ray micro-computed tomography (μXCT) represents a powerful method to analyse the shape preferred orientation (SPO) of rock-forming elements, including minerals, aggregates, and pores, in the three-dimensional space. After reconstruction and segmentation of μXCT images, we developed a novel protocol to construct and analyse the fabric tensor, a second-rank symmetric tensor constructed using the orientation and the length of the three characteristic axes of each grain (simplified to a best fit ellipsoid). The analysis of the fabric tensor permits calculation of mean principal directions and associated confidence ellipses, and quantifies the degree of anisotropy (P 0) and the shape (T) of the fabric ellipsoid by eigenvalue and eigenvector analysis. We implement this method in the TomoFab open-source MATLAB package. The code integrates a graphical user interface (GUI) that allows the visualisation of the full set of ellipsoid orientation, shape, and size. Density plots and contouring can be utilised to identify fabrics graphically, and a full set of fabric parameters can be calculated based on the analysis of the fabric tensor and/or the analysis of each principal direction orientation tensor. We demonstrate the versatility of TomoFab with synthetic datasets and a field-and laboratory-based investigation of a sample presenting a magmatic foliation and lineation, collected in the Mafic Complex within the lower crustal section of the Ivrea-Verbano Zone (North Italy). In the light of these developments, we stress that μXCT represents a pertinent tool for rock fabric analysis to characterise the SPO of rock components. This approach can be performed parallel or complementary to other rock fabric quantification methods (e.g., AMS, EBSD) and applied to various rock types. TomoFab is freely available for download at https://github. com/benpetri/tomofab.

The deformation pattern in active orogens is in general diffuse and distributed, and is expressed by spatially scattered seismicity and fault network. We select two relating datasets in the region encompassing Switzerland and analyse how they compare with each other. The datasets are not complete but are the best datasets currently available which fully cover the investigated area at a uniform scale. The distribution of distances from each earthquake to the nearest fault suggests that about two-thirds of the seismicity occurs near faults, yet about 10% occurs far from known faults. These numbers are stable for various selections of earthquakes and even when considering location uncertainties. Earthquake magnitudes in the catalogue are smaller than what could be expected from faults lengths. This suggests that the deep fracture pattern is more segmented than the superficial one, or mostly partial rupture during earthquakes, and (partly) the impropriety of the scaling law. Statistics on the distances from each fault to the nearest earthquake reveal that all supposedly-active faults in Switzerland have experienced a typically felt (magnitude 2.5 or larger) event, and only one out of six has not done so in the past four decades. Future applications of the presented approach to more complete or comprehensive fault databases may result in revised numbers regarding the connection between deep and superficial fracture patterns, representative of the stress regime of the region. The public and educational message: (1) in the region of Switzerland, earthquakes can happen in areas without known or mapped faults; (2) not all faults produce earthquakes within a human lifetime, but they seem to do so over long times.

International audience; Magma transfer between lower and upper continental crust is a fundamental process linking the dominantly mafic composition of the lower crust with the more felsic composition of the upper crust. We explore the mechanisms of mafic magma ascent and emplacement in the middle crust by studying a mid-crustal gabbroic to dioritic magmatic system (Sondalo complex, Eastern Central Alps, N Italy). We characterize the structure and Anisotropy of Magnetic Susceptibility (AMS) fabric of concentric gabbroic to dioritic intrusions. The significance of AMS fabrics is discussed using Anisotropy of Anhysteretic Remanent Magnetization (AARM) and Crystallographic Preferred Orientation (CPO) data acquired on different test sites. The magmatic and magnetic fabrics of the pluton were acquired during its emplacement in the Permian and were not subsequently tilted: the fabrics are essentially vertical, indicating vertical magma transfer through the crust with a two-phase intrusion history. (1) The concordant orientation between the magmatic foliation and the host-rock xenoliths in the center of the pluton suggest that the first magma ascent phase occurred along pathways subparallel to the vertical fabric of the host metasedimentary rocks. (2) The second magma ascent phase was controlled by a change in the rheology of the host-rock and the mafic magma. Heat dissipation to the contact aureole induced partial melting, thereby lowering the mechanical strength of the host-rocks, whereas the viscosity of the mafic magma increased due to cooling and associated fractional crystallization. This caused an en-masse rise of the pluton resulting in the formation of a structural aureole, i.e. a vertical foliation in the contact aureole and a weaker but concordant magmatic foliation at the rim of the pluton. This ascent phase accounts for the P−T evolution recorded by metasedimentary rocks in the contact aureole of the pluton.

Magma-poor ocean-continent transitions at distal rifted margins record complex stratigraphic interactions engendered by extreme crustal thinning and mantle exhumation. The Tasna ocean-continent transition, exposed in the Middle Penninic Tasna nappe in eastern Switzerland, is so far the only known example where the lateral transition from continental crust to exhumed serpentinized mantle lithosphere is exposed and not overprinted by later Alpine deformation. This paper presents sedimentological, structural, and petrographical observations and detrital zircon provenance data to document: (1) the processes controlling continental hyperextension and mantle exhumation; and (2) the facies, depositional systems, sediment sources, delivery pathways, and depositional stacking patterns associated with magma-poor ocean-continent transitions. Our results show that the basement units of the Tasna ocean-continent transition are composed of prerift upper and lower crust and subcontinental mantle rocks juxtaposed as part of the continental crustal thinning process. The absence of pervasive, synrift deformation in the lower-crustal rocks indicates that the thinning was likely achieved by deformation along localized shear zones before being exhumed at the seafloor by brittle, late extensional detachment faulting and not by any form of lower-crustal flow.The age of the first sediments deposited on the continental crust and exhumed mantle, the so-called Tonschiefer Formation, is considered to be Late Jurassic. A key observation is that the restored morpho-tectonic and sedimentary evolution of the Tasna ocean-continent transition shows the intercalation of downdip, transported platform-derived sediments and along-axis–derived siliciclastic sediments originating from the recycling of prerift sediments, local basement, and/or extra-Alpine sources.

International audience; The mechanisms responsible for the formation of extremely thinned continental crust (

International audience; The end of an orogenic cycle is commonly associated with a general extensional regime, widespread magmatism and complex metamorphic overprints. The Austroalpine domain in SE Switzerland and N Italy preserves a polyphase tectonic history spanning the Carboniferous Variscan orogeny, the Late Carboniferous-Early Permian post-Variscan extension and the Jurassic rifting. In this study, the late-and post-Variscan evolution of the Austroalpine domain is explored by constraining the timing of intrusion of the Sondalo gabbroic complex in the Campo unit. U-Pb zircon dating on magmatic and metamorphic rocks and trace element geochemistry in zircon are performed using laser ablation coupled with mass spectrometry. The ages of both magmatic and metamorphic rocks range between 289 ± 4 and 285 ± 6 Ma. Trace elements in zircon from magmatic rocks indicate that zircon grew in two different chemical systems: an initial one devoid of garnet and with already crystallized plagioclase, and a second one with an increasing modal amount of garnet and lacking (initial) plagioclase. The second system probably reflects mixing of the initial magma Manuscript submitted to International Journal of Earth Sciences 2 with melt derived from the surrounding partially molten metapelites. New results, existing age data and P-T estimates allow to describe the late-to post-Variscan evolution of the Austroalpine domain. These data are compared and integrated in the framework of the Permian geodynamic setting in Western Europe. We present new arguments to consider the "Permian event" in Europe as a process which was temporally distinct from the collapse of the Variscan orogen. Late-Variscan re-equilibration of the crustal thickness occurred between 310 and 290 Ma (i.e. during the collapse phase). Permian mafic magmas emplaced from 290 to 270 Ma formed either during or even after the latest stages of the Variscan collapse.

International audience; Contact aureoles rimming plutonic rocks are the locus of metamorphism and deformations witnessing magma emplacement mechanisms in the crust. In this study, structural and petrological observations are combined to mineral equilibria modelling to unravel the polyphase tectono-metamorphic history of the Permian Sondalo gabbro and its host rock, the Campo unit (Eastern Alps). The Campo unit consists of Grt–St–Ms–Bt–Pl–Qtz±Sil±And±Crd micaschists attesting of a Carboniferous prograde P–T path, reaching 6 kbar/600°C and subsequently 5.6 kbar/650°C. This metamorphism is coeval with the formation of a sub-vertical NE-SW trending foliation (S1) and its overprint by a sub-vertical NW-SE trending foliation (S2). The heat brought by the Permian intrusives subsequently caused heating of the Campo unit at around 3–4 kbar/540°C reflected by regional static crystallization of cordierite and andalusite porphyroblasts. During the intrusion of the Sondalo gabbro, thermal peak conditions are recorded by Grt–Sil–Spl–Crd–Ilm granulitic xenoliths at ~5.5 kbar/930°C, subsequently exhumed at ~4 kbar during the development of a new foliation (S3). This foliation is localized around the pluton and moderately dips away from the centre of the pluton. In the migmatitic contact aureole, Grt–Sil–Bt–Pl–Qtz–Ilm and Grt–Sil–Crd–Spl–Bt–Kfs–Ilm residual rocks bear the new foliation (S3) and document a decompression from 6 kbar/750°C to 5 kbar/725°C and from 5.2 kbar/800°C to reach 4.8 kbar/770°C, respectively. The P–T–d paths recorded by the host rock and the xenoliths point to a two-step magma emplacement. First the Sondalo gabbro intruded the Campo unit causing heating of the host rock without deformation at 3–4 kbar. Second, the ductile flow along the pluton margins developed a new foliation (S3) during exhumation of the pluton and its immediate contact aureole from 6 kbar to 4 kbar. Altogether, it indicates a progressive increase of mechanical coupling between the pluton and the host rock during magma emplacement.

International audience; Lithospheric extension can generate passive margins that bound oceans worldwide. Detailed geological and geophysical studies in present and fossil passive margins have highlighted the complexity of their architecture and their multi-stage deformation history. Previous modeling studies have shown the significant impact of coarse mechanical layering of the lithosphere (2 to 4 layer crust and mantle) on passive margin formation. We built upon these studies and design high-resolution (~100–300 m) thermo-mechanical numerical models that incorporate finer mechanical layering (kilometer scale) mimicking tectonically inherited heterogeneities. During lithospheric extension a variety of extensional structures arises naturally due to (1) structural softening caused by necking of mechanically strong layers and (2) the establishment of a network of weak layers across the deforming multi-layered lithosphere. We argue that structural softening in a multi-layered lithosphere is the main cause for the observed multi-stage evolution and architecture of magma-poor passive margins. Passive continental margins are the focus of intense research activity since decades 1–3 , enhanced by the inten-sification of both industrial and academic geophysical surveys associated with ocean drilling programs (ODP). These research programs brought evidence of hyper-extended domains associated with severely thinned crust (< 10 km thick), exhumed subcontinental mantle and crustal allochthons 4. These observations led to new conceptual and mechanical models for the formation of rift basins and magma-poor margins 5,6. Rifting eventually leading to continental break-up develops in lithospheres that exhibit a complex pre-rift evolution characterized especially by orogenic to post-orogenic processes 7. For example, Mesozoic rifting in Europe occurred in a litho-sphere affected by the Variscan orogeny 8 , while Northern Atlantic rifting took place in a lithosphere affected by the Caledonian orogeny 9 and South China Sea rifting succeeded the Yenshanian orogeny 10. During these pre-rift events, interaction of magmatic, metamorphic and tectonic processes generates a heterogeneous continental lith-osphere (Fig. 1a). The mechanical heterogeneity derived from (i) mechanical anisotropies 11,12 , (ii) a layered crust with a variably metamorphosed upper crust, a migmatitic middle crust and a granulitic lower crust 13 sometimes hosting underplated mafic plutons 14,15 occasionally associated with shallower granitic plutons 16 , and (iii) a layered mantle with structural (e.g. shear zones) or compositional remnants (e.g. pyroxenite) from ancient collisional events 17. Such tectonic inheritance results in an overall layered continental lithosphere 18,19 where mechanical heterogeneities range from a few meters to several kilometers in thickness (Fig. 1a). The extension of small-scale power-law viscous or visco-plastic layered materials can result in shear localiza-tion and asymmetric extension 20. This behavior is the result of structural softening due to necking of strong layers and formation of a network of interconnected weak layers. Structural softening is hence caused by internal reorganizations within heterogeneous materials in order to minimize deformational work 21. Structural softening differs from material softening that requires an a priori defined alteration of material properties with increasing strain and which has been implemented in numerical models to explain the structures of rifted margins 22. The role of coarse mechanical layering of the lithosphere, inherited dipping heterogeneities, and the role of bimineralic crust and mantle on rifting processes has been addressed in previous studies 5,23–31. Here we present high-resolution numerical models that incorporate layering at finer scale, which show that structural softening arises during the extension of lithosphere and controls the development of the key structures observed in passive margins.

A microstructural and metamorphic study of a naturally deformed medium- to high-pressure granitic orthogneiss (OrlicaSnieznik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction-type apparent thermal gradient (similar to 10 degrees C km-1). The granitic orthogneisses develops three distinct microstructural types, as follows: type I augen orthogneiss, type II banded orthogneiss and type III mylonitic orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I orthogneiss is composed of partially recrystallized K-feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II orthogneiss is defined by plagioclase- and K-feldspar-rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III orthogneiss shows relicts of quartz and K-feldspar ribbons preserved in a fine-grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K-feldspar + muscovite + biotite + garnet + sphene +/- ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a prograde P-T path ranging from 700 degrees C (types II and III orthogneisses). The deformation types thus do not represent evolutionary stages of a highly partitioned deformation at constant P-T conditions, but reflect progressive formation during the burial of the continental crust. The microstructures of the type I and type II orthogneisses result from the dislocation creep of quartz and K-feldspar whereas a grain boundary sliding-dominated diffusion creep regime is the characteristic of the type III orthogneiss. Strain weakening related to the transition from type I to type II microstructures was enhanced by the recrystallization of wide myrmekite fronts, and plagioclase and quartz, and further weakening and strain localization in type III orthogneiss occurred via grain boundary sliding-enhanced diffusion creep. The potential role of incipient melting in strain localization is discussed.

International audience; We combine structural observations, petrological data and 40Ar-39Ar ages for a stack of amphibolite-facies metasedimentary units that rims high-pressure (HP) granulite-facies felsic bodies exposed in the southern Bohemian Massif. The partly migmatitic Varied and Monotonous units, and the underlying Kaplice unit show a continuity of structures that are also observed in the adjacent Blanský les HP granulite body. They all exhibit an earlier NE−SW striking and steeply NW-dipping foliation (S3) which is transposed into a moderately NW-dipping foliation (S4). In both the Varied and Monotonous units, the S3 and S4 foliations are characterized by a Sil-Bt-Pl-Kfs-Qtz-Ilm±Grt assemblage, with occurrences of post-D4 andalusite, cordierite and muscovite. In the Monotonous unit, minute inclusions of garnet, kyanite, sillimanite and biotite are additionally found in plagioclase from a probable leucosome parallel to S3. The Kaplice unit shows rare staurolite and kyanite relicts, a Sil-Ms-Bt-Pl-Qtz±Grt assemblage associated with S3, retrogressed garnet−staurolite aggregates during the development of S4, and post-D4 andalusite, cordierite and secondary muscovite. Mineral equilibria modelling for representative samples indicates that the Varied unit records conditions higher than ~7 kbar at 725 °C during the transition from S3 to S4, followed by a P−T decrease from ~5.5 kbar/750 °C to ~4.5 kbar/700 °C. The Monotonous unit shows evidence of partial melting in the S3 fabric at P−T above ~8 kbar at 740-830 °C and a subsequent P−T decrease to 4.5-5 kbar/700°C. The Kaplice unit preserves an initial medium-pressure prograde path associated with the development of S3 reaching peak P−T of ~6.5 kbar/640 °C. The subsequent retrograde path records 4.5 kbar/660 °C during the development of S4. 40Ar-39Ar geochronology shows that amphibole and biotite ages cluster at c. 340 Ma close to the HP granulite, whereas adjacent metasedimentary rocks preserve c. 340 Ma amphibole ages, but biotite and muscovite ages range between c. 318 and c. 300 Ma. The P−T conditions associated with S3 imply an overturned section of the orogenic middle crust. The shared structural evolution indicates that all mid-crustal units are involved in the large-scale folding cored by HP granulites. The retrograde P-T paths associated with S4 are interpreted as a result of a ductile thinning of the orogenic crust at a mid-crustal level. The 40Ar-39Ar ages overlap with U-Pb zircon ages in and around the HP granulite bodies, suggesting a short duration for the ductile thinning event. The post-ductile thinning late-orogenic emplacement of the South Bohemian plutonic complex is responsible for a re-heating of the stacked units, reopening of argon system in micas and a tilting of the S4 foliation to its present-day orientation.