Central Vosges Research Articles

Potassic magmas of the Vosges Mts. (NE France) delimit the areal extent and nature of long-gone Variscan orogenic mantle domains

Highly magnesian and potassic (Mg–K) magmatism is a characteristic feature of the European Variscan Belt. These magmatic rocks have an odd dual geochemical character: they have both high mg# and transition metal contents indicative of being derived from the mantle (and not having lost that character via magmatic differentiation), while simultaneously they are highly enriched in LILE and other fluid-mobile mantle-incompatible elements. There is a growing consensus that these magmas are likely produced by partial melting of metasomatized domains in the subcontinental lithospheric mantle (SCLM) that formed by influx of fluids rich in incompatible elements derived from subducted crust. The Sr and Nd isotopic signature of these fluids determined the isotopic character of the resulting mafic potassic magmas, and can be variable, depending on whether the subducted crust was continental vs. oceanic.The ~340 Ma Mg–K plutonic suites in the Central and Southern Vosges Mts. of northeastern France preserve differences in 87Sr/86Sr340, and to a lesser degree, εNd340 that are consistent with continental underplating producing an isotopically enriched signature near the suture in the Central Vosges (CVMg–K) in contrast to an isotopically depleted/CHUR-like oceanic subduction signature in the Southern Vosges (SVMg–K). The resulting geodynamic model involves (1) pervasive metasomatism of the SCLM wedge by fluids from the leading oceanic part of the subducting slab during Andean-type subduction, (2) underplating of only the near-suture portion of the SCLM by the trailing continental part of the slab during collision, (3) slab break-off and/or partial delamination of the lowermost SCLM accompanied by asthenospheric influx triggering partial melting of the mantle metasomes, (4) generation of primary potassic magmas that sampled the isotopic characteristics of either side of the mantle boundary, reflecting whether the given part of the SCLM was underplated by continental crust or not, and that (5) carried evidence of that boundary into the plutons of the upper plate. In general, whereas some geochemical tracers in these odd Mg–K magmas preserve evidence of long-lost mantle domains, others were rapidly overprinted and instead preserve the complementary record of interaction with the upper plate.

REVEALING LONG-TERM PHYSIOLOGICAL TRAJECTORIES OF GRASSLANDS FROM LEGACY B&W AERIAL PHOTOGRAPHS

Abstract. Landscape reconstruction is crucial to measure the effects of climate change or past land use on current biodiversity. In particular, retracing past phenological changes can serve as a basis for explaining current patterns of plant communities and predict the future extinction of species. Old spatial data are currently used to reconstruct vegetation changes, both morphologically (with landscape metrics) and semantically (grasslands to crops for instance). However, poor radiometric properties (single panchromatic channel, illumination variation, etc.) do not offer the possibility to compute environmental variables (e.g. NDVI and color indices), which strongly limits long-term phenological reconstruction. In this study, we propose a workflow for reconstructing phenological trajectories of grasslands from 1958 to 2011, in the French central Vosges, from old aerial black and white (B&W) photographs. Noise and vignetting corruptions were first corrected in B&W photographs with non-local filtering algorithms. Panchromatic scans were then colorized with a Generative Adversarial Network (GAN). Based on the predicted channels, we finally computed digital greenness metrics (Green Chromatic Coordinate, Excess Greenness) to measure vegetation activity in grasslands. Our results demonstrated the feasibility of reconstructing long-term phenological trajectories from legacy photographs with insights at different levels: (1) the proposed correction methods provided radiometric improvements in old aerial missions; (2) the colorization process led to promising and plausible colorized historical products; (3) digital greenness metrics were useful for describing past vegetation activity.

Interstadial fluvial organic beds with volcanic shards and betula/pinus pollen older than 53 ka (early mis 3) and mis 4 glacial maximum north of the central Vosges mountains

Laterale au bassin intra-vosgien de Saint-Die, la vallee de la Fave a enregistre une sequence sedimentaire continue depuis plus de 53 ka. Le materiel de la nappe grossiere inferieure resulte d’apports torrentiels attribues a un episode de fonte de neves et de masses de glace accumules pendant le stade MIS 4 sur les reliefs au sud de la vallee. La nappe alluviale superieure, a dominante silteuse, indique une sedimentation lente sans episode torrentiel et l’absence d’accumulation significative de neves ou de glace sur le bassin versant pendant le Dernier Maximum Glaciaire et le Tardiglaciaire. Elle inclut une unite basale stratifiee et discontinue a echardes volcaniques, a matiere organique datee a plus de 53 ka et de contenu pollinique a Betula/Pinus suivis d’especes de steppe froide. L’Interstade local est dans la position palynostratigraphique de l’Interstade de Goulotte de la sequence de la Grande Pile, au debut du MIS 3. Cet âge est compatible avec celui du tephra de Saint-Germain. Ces donnees sur la marge nord des Vosges centrales confirment le modele d’histoire glaciaire mis en evidence dans les Vosges du sud, avec un maximum d’enneigement precoce au debut du Wurmien moyen (MIS 4) et une extension glaciaire plus restreinte sous climat froid et sec pendant le Wurmien superieur (MIS 2).

Holocene vegetation history in the Northern Vosges Mountains (NE France): Palynological, geochemical and sedimentological data

Palynological data from the Northern Vosges Mountains (NVM) are very rare, unlike for the Southern and Central Vosges Mountains, where the past vegetation history is relatively well known. As a consequence, the beginning of human activities has never been clearly identified and dated in the NVM. In order to reconstruct the evolution of vegetation in this region, multiproxy studies (pollen, non-pollen palynomorphs, sedimentological and geochemical analyses) were conducted in two peatlands. Overall, the results, extending from about 9500 cal. BP to recent times, show a classical vegetation succession with local particularities resulting from human activities. In the La Horn peatland, a strong human impact related to pastoralism is attested from the late Bronze Age onwards. The second phase of human occupation, mainly characterized by crop cultures, begins during the Hallstatt period. The geochemical results (x-ray fluorescence) also highlight the presence of metallic elements, which, combined with significant quantities of carbonized particles, point to potential metal working. In the Kobert-Haut peatland, human occupation began much later (1500 cal. BP), but lasted from the Gallo-Roman period to the beginning of the Modern Period. Unlike for the vegetation history of the rest of the Vosges, Pinus remains a prevailing taxon throughout the Holocene in the NVM. Another particularity is the early establishment of Picea, long before the 18th to 19th century plantations.

Zircon age of vaugnerite intrusives from the Central and Southern Vosges crystalline massif (E France): contribution to the geodynamics of the European Variscan belt

To provide a better picture of the active geodynamics along the Variscan suture zones during the late collisional stage (particularly regarding the evolution of the orogenic system towards HT conditions), we focused here on vaugnerites, which consist of mafic ultra-potassic magmatic rocks, intrusive into the granite-gneiss sequences of the Variscan Vosges crystalline massif. Those rocks, though subordinate in volume, are frequently associated with late-collisional granites. In the Central-Southern Vosges, they appear either as (1) pluton margin of the Southern Vosges Ballons granite complex or (2) composite dykes intrusive into migmatite and metamorphic sequences classically referred to as granite-gneiss unit (Central Vosges). Both types correspond to melanocratic rocks with prominent, Mg-rich, biotite and hornblende (20–40% vol., 64 < mg# < 78), two-feldspar and quartz. Those Vosges vaugnerites display geochemical signatures characteristic of ultra-potassic mafic to intermediate, metaluminous to slightly peraluminous rocks. Zircon U-Pb ages were obtained by Laser Ablation Inductively Coupled Plasma Mass Spectrometry. Zircon grains were extracted from a sillimanite-bearing gneiss from the granite-gneiss unit hosting the Central Vosges vaugnerites. They yielded an age at 451 ± 9 Ma, indicating a pre-Variscan Upper Ordovician protolith for the host sequence. Zircon from the four vaugnerite intrusives display U-Pb ages (± 2σ) of 340 ± 2.5 Ma (Ballons), 340 ± 25 Ma, 340 ± 7 Ma and 336 ± 10 Ma (Central Vosges). Synchronous within uncertainty, vaugnerite age data suggest a relatively early emplacement during the Late Variscan collisional history (i.e. Middle Visean times). These results are in line with previously published ages from the Southern Vosges volcano-sedimentary sequences (Oderen-Markstein) and the nearby ultra-potassic granite complexes from the Central and Southern Vosges (Ballons, Crêtes) thereby arguing for a magmatic event of regional significance. Recent petrological studies on vaugnerites suggest that they derive from partial melting of a metasomatized mantle contaminated to some different degrees by elements of continental crust. We propose here that the major ultra-potassic magmatic pulse at 340–335 Ma is a consequence of a significant change into the dynamics of the Rhenohercynian subduction system below the Central-Southern Vosges. In the light of recent thermo-mechanical modelling experiments on mature continental collision, magmatism could result from a syn-collisional lithospheric delamination mechanism involving (1) first, continental subduction evolving towards (2) the underthrusting of the Avalonian continental margin lower crust and (3) the initiation of lithospheric delamination within the supra-subduction retro-wedge (Saxothuringian-Moldanubian continental block). This delamination would drive the emplacement of an asthenospheric upwelling, initially localized along the Variscan suture zones, and gradually propagating towards the southern front of the belt during the Late Carboniferous, as the delamination front migrated at the base of the crust.

W and Li-Cs-Ta geochemical signatures in I-type granites – A case study from the Vosges Mountains, NE France

In light of the recent ‘lithium boom’, a screening study for Sn-W and Li-Cs-Ta deposits was carried out and located an early 1980s Bureau de Recherches Géologiques et Minières (BRGM) geochemical survey with W anomalies of >50 ppm in the Sainte-Marie-aux-Mines area, NE France. These anomalous areas, along with historic mining of polymetallic veins in the area, led to the present investigation of the regional potential for Variscan vein-hosted Sn-W-Li-Cs-Ta and Mesozoic-Cenozoic Cu-Pb-Zn-As-Ag-Ni veins in the central Vosges Mountains.A total of 144 stream sediment samples were collected in the central Vosges Mountains covering an area of >200 km2. Multi-element pXRF and ICP-MS assays revealed previously undescribed W-Li-Cs-Ta geochemical anomalies related to major ENE-WSW trending fault zones and splay faults in the I-type Central Vosges Mg-K (‘CVMg-K’) granite. Field mapping evidence, previously published age data along with strong geochemical fractionation trends obtained from K/Rb, Zr/Hf and Nb/Ta ratios, imply that the CVMg-K granites were intruded by younger crustal melts derived from crustal anatexis of metasedimentary and metaigneous source rocks, that potentially exploited pre-existing fault control, and subsequently fractionated and enriched incompatible elements of economic interest.This study demonstrates that Sn-W-Li-Cs-Ta geochemical anomalies can occur in I-type granites, if later orogenic processes leading to crustal anatexis resulted in the production of peraluminous melts, strong magmatic fractionation and emplacement of mineralization along regional fault zones. Therefore, the presence of I-type granites in Variscan and other geological terrains should encourage regional Sn-W-Li-Cs-Ta exploration.

Palaeozoic evolution of the Variscan Vosges Mountains

Abstract A geological synthesis of the Palaeozoic Vosges Mountains (NE France) is presented using existing observations and new data. The geodynamic evolution involves: (1) Early Palaeozoic sedimentation and magmatism; (2) Late Devonian subduction triggering back-arc spreading; (3) early Lower Carboniferous continental subduction, continent–continent collision and polyphase deformation and metamorphism of the orogenic root; and (4) late Lower Carboniferous orogenic collapse driven by thermal weakening of the middle crust. The evolution is integrated within the framework of the European Variscan Belt. The Northern Vosges comprise sediments of Rhenohercynian affinity separated from Teplá-Barrandian metasediments by a Lower Carboniferous magmatic arc. The latter is correlated with the Mid-German Crystalline Rise, and is ascribed to the south-directed subduction of the Rhenohercynian Basin. The Saxothuringian–Moldanubian suture is thought to be obliterated by the magmatic arc, while the Lalaye–Lubine Fault is interpreted as the Teplá-Barrandian–Moldanubian boundary. The Central Vosges are paralleled with the Moldanubian domain of the Bohemian Massif where identical lithologies record the Devonian–Carboniferous SE-directed subduction of the Saxothuringian passive margin below the Moldanubian upper plate. The Southern Vosges represent the upper Moldanubian crust and are linked to the southern Black Forest. The presence of an oceanic domain to the south of the Vosges–Black Forest remains unclear. Supplementary material: List of radiometric ages used for probability plots is available at http://www.geolsoc.org.uk/SUP18734 .

AMS record of brittle dilation, viscous-stretching and gravity-driven magma ascent in area of magma-rich crustal extension (Vosges Mts., NE France)

Orogenic compression-related fabrics (~340–335 Ma) were reworked during regional extensional deformation (~328–325 Ma) in a large anatectic crustal domain of the Central Vosges (NE France). The extension was first accommodated by brittle dilation affecting vertically anisotropic high-grade rocks associated with emplacement of subvertical granitic sheets. The AMS fabric of granitoids is consistent with highly partitioned transtensional deformation marked by alternations of flat and steep foliations and development of orthogonal lineations. This deformation passes to top-to-the-southwest ductile shearing expressed in southerly migmatitic middle crust. The AMS fabric revealed moderately west-dipping foliations bearing subhorizontal NNW–SSE-trending lineations and predominantly plane strain to prolate shapes. This fabric pattern is interpreted as a viscous response of stretched partially molten crust during continuous ductile extension. Vertical ascent of voluminous granites and stoping of the upper crust occurs further south. This gravity ascent triggered by extension leads to development of south-dipping AMS foliations, south-plunging lineations and oblate fabrics in various crustal granites. Vertical shortening related to ascent of these (~325 Ma) granitoids and persistent N–S stretching is responsible for reworking and remelting of originally vertical compression-related fabric in roof supracrustal granites (~340 Ma) and development of highly prolate fabrics in these rocks. This work shows that the finite shape of AMS fabric ellipsoid is highly sensitive to both strain regime and superpositions of orthogonal deformation events.

The South Millevaches Middle Carboniferous crustal melting and its place in the French Variscan belt

AbstractSeveral episodes of crustal melting are now well identified in the Variscan French Massif Central. Middle Devonian (ca 385-375 Ma) migmatites are recognized in the Upper and Lower Gneiss Units involved in the stack of nappes. Late Carboniferous migmatites (ca 300 Ma) are exposed in the Velay Massif only and Middle Carboniferous migmatites crop out in the Para-autochthonous Unit and southern Fold-and-Thrust Belt. In the SW part of the Massif Central, the South Millevaches massif exposes migmatites developed at the expense of ortho- and paragneiss. They form kilometer-sized septa within the foliated Goulles leucogranitic pluton, which is in turn intruded by the non-foliated Glény two micas granite pluton. Monazite grains extracted from these three rock-types have been dated by the EPMA chemical method. Three samples of migmatite yield a late Visean age (ca 337-328 Ma), the Goulles and Glény granitic plutons yield ages at 324-323 Ma and 324-318 Ma, respectively. These new results enlarge the evidence of a Middle Carboniferous crustal melting imprint that up to now was only reported in the eastern part of the French Massif Central, in the northern Cévennes and in the Montagne Noire axial zone. At the scale of the French Variscan massifs, the Visean crustal melting event is conspicuously developed since it is recognized from the Massif Armoricain (Vendée and south coast of Brittany) to the Central Vosges. This episode is synchronous with the huge thermal event responsible for the “Tuffs anthracifères” magmatism of the northern Massif Central and Vosges, and took place immediately after the last thickening phase recorded both in Montagne Noire and Ardennes, that is on the southern and northern outer zones of the Variscan Belt, respectively. However, the geodynamic significance of this major event is not fully understood yet.

Influence of melt induced mechanical anisotropy on the magnetic fabrics and rheology of deforming migmatites, Central Vosges, France

In the Central Vosges (France), we describe and model the structural relationships existing between orthogneiss domains and their surrounding viscous migmatites. These domains show a structural and anisotropy of magnetic susceptibility (AMS) zonation associated with an increasing amount of melt from the core to their margins. A pre-extension oblate to plane strain fabric is preserved in the core of the orthogneiss domains where the granite veins were emplaced along a dilated mechanical anisotropy. The internal margin of the orthogneiss bodies exhibits development of a prolate fabric and active buckling of an orthogneiss/granite multilayer. The external margin of the orthogneiss bodies is characterized by oblate fabrics resulting from a layer perpendicular shortening of a melt-orthogneiss multilayer, pinch-and-swell structures and extensional kink bands along which granite veins have been injected. The surrounding metasedimentary migmatites show a similar structural history but the degree of fabric resetting is significantly higher. This systematic fabric and structural succession is due to contrasting rheological evolution between orthogneiss and metasedimentary migmatites during progressive melting and continuously evolving degree of mechanical anisotropy in the stronger orthogneiss. Numerical modeling confirms the role of the relationship between initial mechanical anisotropy and superimposed deformation overprints on finite AMS fabrics in migmatites.

Model of successive granite sheet emplacement in transtensional setting: Integrated microstructural and anisotropy of magnetic susceptibility study

This study presents a model of successive emplacement of three granite plutons in transtensional deformation regime controlled by the preextensional synconvergent history of the lower and middle crust in the central Vosges Mountains (France). The complex compressional orogenic structure recorded a vertical exhumation‐related fabric in midcrustal levels which was overprinted by the regional extension. Three successively emplaced granite sheets exploited the inherited vertical anisotropy obliquely oriented with respect to the applied tensile stress. This progressive opening of original steep fabric leads to the sequential generation of free spaces toward the south, and emplacement and deformation of granite sheets. Repeated granite intrusions are responsible for reheating of southern margins of the already partially solidified granites, which became reactivated under lower ambient thermal conditions related to overall ∼6 Ma cooling of rapidly exhumed crust. The results of anisotropy of magnetic susceptibility (AMS) fabric modeling suggest a highly partitioned oblique extension divided into pure shear‐dominated deformation close to the central and northern margins of intrusions and wrench‐dominated shear along the southern margins. The AMS modeling suggests the importance of overprinting of the intrusive fabrics by the transtensional deformation.

Tectonics of the Vosges (NE France) and the Schwarzwald (SW Germany): evidence from Devonian-Carboniferous active margin basins and their deformation

Devonian-Carboniferous sedimentary and volcanic rocks are exposed in the Badenweiler-Lenzkirch Zone (BLZ) of the Southern Schwarzwald and in the Southern Vosges (Central Europe). Several lithostratigraphic units are recognized. Sedimentary or tectonic relations between some of the units can be realized by comparing sedimentary and volcanic facies associations as well as petrographic and geochemical datasets. During the Variscan orogenesis the sediments were deposited in different kinds of basins along an active margin setting. Their deformation was studied in the Markstein area of the Southern Vosges and is presented here in detail. Some arguments favour a northward underthrusting of a narrow and oceanic pre-Upper Devonian back-arc basin beneath the crystalline units of the Central Vosges and Schwarzwald. Accretion of continental crust continued from the Upper Devonian to the late Lower Carboniferous along with a characteristical evolution of a deep marine retro-arc foreland basin. It has accumulated about 3500 m of turbidite sandstones, shales and conglomerates of volcano-plutonic origin, assembling now the Markstein Group in the Southern Vosges. To the south of the Markstein Group, deep marine mud-rich turbidite systems and a shallowing-upward succession of fluvio-deltaic sediments filled in the Oderen Basin, which subsided in the late Tournaisian and early Visean. A subduction type volcanism occurred in form of tholeiitic basalts, K-calcalkaline island arc andesites and continental arc sandstones. In contrast to former models, a genetical relationship between the Markstein and the Oderen Group is rejected here. Instead, different subduction systems in space and time have caused the development of the active margin basins. The Variscan deformation of the exposed sedimentary units started in the upper Visean. Structural analysis of kinematic indicators and macroscopic fabrics in the Markstein Group give evidence for thrusting and dextrally transpressive shearing, forming a positive flower structure along the southern border of the Central Vosges crystalline units. This was broadly synchronous with dextrally transpressive deformation along the Lalaye-Lubine/Baden Baden fault at the northern border of the Central Vosges and Schwarzwald and along the Transition Complex between the Central and the Southern Schwarzwald. The high-K-calcalkaline Metzeral granite intruded postkinematically into the principal displacement zone and has a position similar to the dextrally sheared Randgranite association along the northern border of the BLZ. A juxtaposition between the Oderen and the Markstein Groups was achieved in the upper Visean due to the dextral transpressive motion along the ESE-trending Markstein Fault. The exhumation of high grade metamorphic rocks of the Central Vosges and Schwarzwald during the upper Visean is discussed in the context of long lasting accretionary tectonics and the two-sided dextral transpression, favouring crustal extrusion as a main exhumation process. A thorough examination of the supracrustal rocks and their mode of deformation in the Southern Vosges helps us to determine the relationships with the BLZ and contributes also to the paleogeographic reconstruction of the Armorica - Proto-Alpine - Gondwana connection.

Hot contacts of garnet peridotites in middle/upper crustal levels: new constraints on the nature of the late Variscan high‐T/low‐P event in the Moldanubian (Central Vosges/NE France)

P–T paths based on parageneses in the immediate vicinity of former high‐temperature contact zones between mantle peridotites and granulitic country rocks of the Central Vosges (NE France) were derived by applying several conventional thermometers and thermobarometric calculations with an internally consistent dataset. The results indicate that former garnet peridotites and garnet–spinel peridotites were welded together with crustal rocks at depths corresponding to 1–1.2 GPa. The temperature of the crustal rocks was about 650–700 °C at this stage, whereas values of 1100 °C (garnet peridotites) and 800–900 °C (garnet–spinel peridotites) were calculated for the ultramafic rocks. After emplacement of the mantle rocks, exhumation of the lower crust took place to a depth corresponding to 0.2–0.3 GPa. The temperatures of the incorporated peridotite slices were still high (900–1000 °C) at this stage. This is indicated by the presence of high‐T /low‐P parageneses (c. 800 °C, 0.2–0.3 GPa) in a small (1–10 m) contact aureole around a former garnet peridotite. Crustal rocks distant to the peridotites equilibrated in the same pressure range at lower temperature (650–700 °C). High cooling rates (102–103 °C Ma−1) were calculated for a garnet–biotite rock inclusion in the peridotites and for the crustal rocks at the contact by applying garnet–biotite diffusion modelling. Minimum rates of 0.75–7.5 cm a−1 are required for vertical ascent of rock units (30 km vertical distance) derived from the crust–mantle boundary, resulting in a late Variscan (340 Ma) high‐T /low‐P event.

Metamorphic evolution of ultrahigh-pressure garnet peridotites from the Variscan Vosges Mts. (France)

In the Central Vosges Mts. (France) of the Variscan belt, MgCr garnet peridotite bodies occur within the uppermost tectonometamorphic unit (Leptynitic granulites) as lenses in low-pressure/high-temperature metamorphic rocks. Neglecting late-stage serpentinization, the metamorphic evolution of these rocks was characterized by four stages. During stage I, the rocks were equilibrated at high pressures and temperatures (> 4.9 GPa/ > 950°C, in most cases > 1100°C), either within or near to the stability field of diamond. Stage II is documented by the formation of coronas around relict garnet with the assemblage orthopyroxene ± clinopyroxene ± amphibole + spinel ± plagioclase. During stage III, the remaining garnet was transformed to very fine-grained kelyphite consisting of orthopyroxene + amphibole + spinel ± plagioclase. Small relict garnet grains are preserved in one peridotite only. Stage IV corresponds to the late formation of tremolitic hornblende and chlorite which partially replaced the pseudomorphs after garnet or occur along cracks in the matrix of some rocks. Compositional zoning patterns of pyroxene porphyroclasts suggest that initial decompression was either accompanied by a moderate increase in temperature or nearly isothermal. Garnet breakdown textures and compositions of minerals grown during stages II and III also suggest rapid decompression at still elevated temperatures (1000-720°C).

The Late Carboniferous to Early Triassic geodynamic evolution of Variscan Europe in the light of magnetic overprints in Early Permian rhyolites from the northern Vosges (France) and central Black Forest (Germany)

SUMMARY This paper demonstrates how magnetic overprints of a geological series can provide information over a long time period, which can be interpreted in terms of geotectonic evolution. According to these new results, the Late CarboniferousEarly Permian rhyolites from the northern Vosges have recorded the magnetic field over a major part of the Permian. Recent radiometric dating assigns a Late Carboniferous-Early Permian age (298 Ma) to the previously 'Middle Permian' rhyolitic volcanism of the northern Vosges. A palaeomagnetic study was undertaken on rhyolites of the Nideck-Donon massif and on neighbouring Permian basins (VillC, Saint-Die) in order to obtain a new pole for the Early Permian and to detect overprints. Standard palaeomagnetic and rock magnetic experiments demonstrate that over 90 per cent of the characteristic directions of magnetizations are of secondary origin. The high-temperature magnetizations carried by secondary haematite and haematized titano-magnetite fail the fold test, and display directions consistent with the Saxonian-Thuringian directions from Variscan Europe (NideckDonon: 186/-17, k = 167 for 26 sites, VGP: 50 N/178 E; Saint-DiC basin: 184/-20.5 for two sites, VGP: 53 N/180 E; VillC basin: 191.5/-25 for two sites, VGP: 53 N/168 E; Vosges dikes: 197.5/-26.5 for two sites, VGP: 53 N/158 E). Part of the magnetizations with intermediate unblocking temperatures of magnetite consists also of post-tectonic overprints. Two consistent mean directions with lower inclinations, obtained in the Nideck-Donon rhyolites (182/-7, k = 459 for 13 sites, VGP: 45 N/1840 E) and pyroclastic deposits from the VillC basin (178/-9, k = 519 for three sites, VGP: 46 N/190 E), were probably acquired during Late AutunianEarly Saxonian times. In rhyolites the pre- and post-tectonic characteristic mag netizations are carried by magnetite, while in the ash-fall deposits the carrier is allogenic haematite. A few older magnetizations (191/--3, two sites, VGP: 42 N/173 E), consistent with magnetizations carried by magnetite from rhyolites of the central Black Forest, and results from coeval volcanics as well as from Autunian sediments have been observed in tuffs from the Villi basin (188/1.5, two sites, VGP: 40 N/177 E. The oldest magnetizations (207/10, k = 329, VGP: 32 N/155 E), presumably of Stephanian age, were identified in two rhyolitic stocks within the granitic basement from the central Vosges and central Black Forest. The distribution of the palaeomagnetic directions suggests that, from the Late Autunian to the Late Thuringian, overprinting due to low-temperature alteration of titano-magnetite and crystallization of secondary haematite was more or less continuous. The apparent polar wander path (APWP) computed with the new results and the published poles shows a hairpin, which implies a drastic change of the European plate motion during the Permian. The clockwise rotation of Europe initiated in the Late Visean-Namurian stopped in the Late Autunian-Early Saxonian. This event corresponds to the end of the Variscan convergence and of the Appalachian

Retrograde evolution in the Central Vosges mountains (northeastern France): implications for the metamorphic history of high-grade rocks during the Variscan orogeny

Metamorphism in the Vosges massif is characterized by rocks formed under a wide range of pressures and temperatures: high-pressure rocks (garnet peridotites and eclogites facies rocks) are found together with low-pressure rocks (garnetcordierite stability field). Hitherto, metamorphism has been interpreted within the framework of an identical P- T evolution for all rock types corresponding to either two separate geotectonic events or a unique P- T evolution from high-pressure to low-pressure conditions. We studied gneisses from the Vosges massif in order to constrain the P- T- t path they followed. P- T estimation indicates that gneisses equilibrated between 5 and 7 kbar during peak metamorphism time. Rocks later experienced cooling from temperatures above 650°C down to 550°C. It is possible that the entire P- T evolution of the gneisses took place within the lower part of the amphibolite facies; which is inconsistent with the P- T evolution of the high-pressure rocks. The noteworthy absence of muscovite + staurolite assemblages is attributed to conditions of low H 2O and low O 2 activity. The consequences for the geodynamics in the Vosges massif are that at least two separate geotectonic events are required in order to explain the juxtaposition of high-pressure rocks with low-pressure amphibolite facies rocks.

Les massifs anciens de France, segments de la chaîne varisque d'Europe occidentale / The ancient massifs of France, parts of the variscan belt of western Europe

The ancient massifs of France, parts of the variscan belt of western Europe — The French Paleozoic massifs are parts of the Variscan belt. During Early Paleozoic times, two Gondwana-originated platforms were separated by a central domain characterized by a thinned continental or even oceanic crust. Another oceanic axis comprized the Alpine External Crystalline massifs. The convergence and final collision of the two platforms induced the development of a Late Ordovician HP to Late Devonian HT-metamorphism contemporaneous with the emplacement of crystalline nappes upon the southern platform (southern Armorican Massif, Central Massif, Central Vosges). During the Carboniferous, the internal zones were thrusted upon the external zones, the former Gondwanian platforms, where foreland basins developed.

Upper Palaeozoic basins of the British Isles—Caledonide inheritance versus Hercynian plate margin processes

Hypotheses to explain the origin and location of Upper Palaeozoic sedimentary basins are presented. Group A basins were those Old Red Sandstone basins whose location and subsidence were controlled by Caledonide features, including active and remnant fore-arc basins (Midland Valley) and major transcurrent movements (Orcadian basin). Newer Granite plutonic magmas in the Southern Uplands subduction complex post-dated subduction and resulted from post-collision cratonization processes as yet little understood. Group B basins were those whose initial positions in space were controlled by palaeo-plate factors, in particular the location of granitoid plutons, but whose subsidence was controlled by active Hercynian (neoplate) processes. Some Group A basins were regenerated by this process (Midland Valley). A new hypothesis is presented to account for the origin of a Carboniferous rift-to-sag trend similar to that proven in modern continental margin basins such as the North Sea. The hypothesis is an adaptation of McKenzie's lithospheric stretching model, some of whose major features are well shown in the Carboniferous history of the north and central British Isles. Group B processes were controlled by Ligurian/Bretonic plate consumption in the S Brittany–Massif Central–Vosges areas. Group C basins in the Rheno-Hercynian zone also owe their location and origin to back-arc rifting and continental crustal ‘oceanization’ processes caused by Liguro-Bretonic subduction.

Etude des gneiss et granites migmatiques de Kaysersberg, des Trois-Epis et de Ribeauville dans les Vosges

Abstract The migmatitic areas of the Alsatian flank of the central Vosges massif have been mapped geologically. The Kaysersberg facies consists of banded gneisses and gneissic granites, with occasional basic inclusions. The more acid facies of the Trois-Epis, Alspach, and Ribeauville migmatites are described; these appear to be younger.