European Variscan Research Articles

Insights on Variscan geodynamics from the structural and geochemical characterization of a Devonian-Carboniferous gabbro from the Austroalpine Domain (Western Alps)

Ivozio kyanite-bearing eclogites represent one of the main pre-Alpine mafic complexes of the Sesia Lanzo Zone (Austroalpine Domain) whose protoliths have been dated at Late Devonian to Early Carboniferous. The complex consists of various HP mafic-ultramafic rocks such as amphibole-bearing eclogites, zoisite-bearing eclogites, amphibole-epidote-bearing eclogites, quartz-bearing eclogites, chlorite-bearing amphibolites and ultramafites surrounded by eclogitic micaschists and jadeite-bearing metagranitoids. Multiscale structural analyses, integrated with geochemical investigation, allowed to distinguish heterogeneities of eclogite types consequent to strain partitioning and different degrees of metamorphic re-equilibration during Alpine polyphase deformation history or to pristine magmatic differences. Five groups of Alpine structures (D 1 to D 5 ) have been detected: they consist of folds, foliations and shear zones developed during the Late Cretaceous to Eocene subduction-exhumation path. The metamorphic mineral assemblages related to the deformation stages indicate a polyphase deformation (pre-D 1 to D 3 ) under eclogite-facies conditions followed by a blueschist re-equilibration (during D 4 ), localized shearing and successive folding and discrete shearing (D 5 ) under greenschist-facies conditions. Four veining stages, developed under the eclogite- and blueschist-facies conditions have been documented. Whole rock major and trace element composition of main igneous lithotypes suggest a subduction-related context for the emplaced of the Ivozio mafic Complex. It also suggests that all rocks are not co-genetic and that they likely emplaced in the continental crust. Igneous products with arc and back-arc geochemical affinity (from 380 to 345 Ma) are described in the European Variscan Belt from French Central Massif to Bohemian Massif, supporting the interpretation that the Ivozio mafic Complex protoliths are the witness of such Devonian-Carboniferous arc magmatism in the Alpine chain. TO DOWNLOAD THE GEOLOGICAL MAP SEE THE IN THE MENu SUPPLEMENTARY FILE (ON THE RIGHT COLUMN)

Lithospheric mantle beneath NE part of Bohemian Massif and its relation to overlying crust: new insights from Pilchowice xenolith suite, Sudetes, SW Poland

The Oligocene/Miocene basanite from Pilchowice (Sudetes Mts., SW Poland) carries numerous small xenoliths of mantle peridotite, mostly harzburgite. The Pilchowice xenolith suite is dominated by harzburgites and dunites containing olivine Fo 90.2–91.5 (group A). The peridotites of group B (olivine Fo 88.6–89.4), and C (olivine Fo 83.2–86.5) are subordinate. The peridotites suffered from significant melt extraction (20–30%) and were subsequently subjected to metasomatism. Three different trace element compositional patterns of group A clinopyroxene occur, which are typical of silicate melt, carbonatite melt and silicate–carbonatite melt metasomatism, whereas groups B and C were affected by silicate melt metasomatism only. The Pilchowice basanite occurs at the contact of Karkonosze–Izera Block and Kaczawa Complex, two major geological units of Sudetes. The Pilchowice xenolith suite documents underlying lithospheric mantle of composition and depletion degree similar to those described in the whole Lower Silesian mantle domain, which forms NE termination of Saxo–Thuringian zone of the European Variscan orogen. The crustal structure of the orogen is, therefore, not mirrored in the mantellic root.

Episodic melting and magmatic recycling along 50 Ma in the Variscan belt linked to the orogenic evolution in NW Iberia

The advent of a large amount of more precise U-Pb age data on Variscan granitoids from NW Iberia in recent years has provided a more focused picture of the magmatic history of the Western European Variscan belt (WEVB). Based on these data, three main pulses of magmatic activity seem to be well established.

Timing of K-alkaline magmatism in the Balkan segment of southeast European Variscan edifice: ID-TIMS and LA-ICP-MS study

The Variscan orogen in southeast Europe is exposed in isolated remnants, affected by a subsequent Alpine tectono-magmatic overprint. Unlike the central European Variscides, in SE Europe the juxtaposition and correlation of the events and products are impeded by the scarcity of Variscan domains with preserved magmatic, metamorphic, sedimentological and structural characteristics. To reveal the particular evolution of the Variscan orogen in Balkan Mts, we present the results of ID-TIMS and LA-ICP-MS dating of three potassic-alkaline intrusions: Svidnya, Buhovo–Seslavtsi and Shipka. The age determinations from the plutons do not permit to establish their unequivocal ages, but they bracket the time interval of emplacements. Based on geochronological, tectonic and stratigraphic evidence the emplacement interval for plutons could be: 317–310 Ma for Svidnya, 330–310 Ma for Buhovo–Seslavtsi and 320–303 Ma for Shipka. These results show that the generation of potassic-alkaline magmas was post-Visean and is contemporaneous with the adjacent numerous calc-alkaline granitoid plutons. Thus, the Variscan orogen in the Balkan Mts is not characterized by a time-dependent geochemical evolution of magma generation. Hence, the observed differences in the rocks’ compositions can be interpreted solely by distinction between the magma sources. The available data for both potassic-alkaline and calc-alkaline rocks indicate that the major episodes of crustal stacking and shearing in the Balkan part of the Variscan edifice are pre-Visean (~330 Ma). The present study reveals that the potassic-alkaline rocks from the Balkan Mts are younger than the central European potassic granitoids (durbachites). It suggests that melting of enriched mantle source took place at different times throughout the Variscan orogen. In spite of the alkaline character of the magmas, the studied zircons show a complex nature, with inherited cores and magmatic overgrowths. The observed heterogeneities in the zircons imply the presence of much older materials in the petrogenesis of the rocks from the potassic-alkaline plutons.

How far did the Cadomian ʽterranesʼ travel from Gondwana during early Palaeozoic? A critical reappraisal based on detrital zircon geochronology

ABSTRACTIn this paper, laser ablation ICP-MS U–Pb detrital zircon ages are used to discuss provenance and early Palaeozoic palaeogeography of continental fragments that originated in the Cadomian–Avalonian active margin of Gondwana at the end of Precambrian, were subsequently extended during late Cambrian to Early Ordovician opening of the Rheic Ocean, and finally were incorporated into and reworked within the European Variscan belt. The U–Pb detrital zircon age spectra in the analysed samples, taken across a late Neproterozoic (Ediacaran) to Early/Middle Devonian metasedimentary succession of the southeastern Teplá–Barrandian unit, Bohemian Massif, are almost identical and exhibit a bimodal age distribution with significant peaks at about 2.1–1.9 Ga and 650–550 Ma. We interpret the source area as an active margin comprising a cratonic (Eburnean) hinterland rimmed by Cadomian volcanic arcs and we suggest that this source was available at all times during deposition. The new detrital zircon ages also corroborate the West African provenance of the Teplá–Barrandian and correlative Saxothuringian and Moldanubian units, questioned in some palaeogeographic reconstructions. Finally, at variance with the still popular concept of the Cadomian basement units as far-travelled terranes, we propose that early Palaeozoic basins, developed upon the Cadomian active margin, were always part of a wide Gondwana shelf and drifted northwards together before involvement in the Variscan collisional belt.

Uraninite, Coffinite and Brannerite from Shear-Zone Hosted Uranium Deposits of the Bohemian Massif (Central European Variscan Belt)

New mineralogical data are presented for shear-zone hosted uranium mineralisation from selected uranium deposits that occur in the Bohemian Massif. The uranium mineralisation is in high-grade metamorphic rocks of the Moldanubian Zone and/or in granitic rocks of the Moldanubian batholith and Bor pluton as complex uraninite–coffinite and uraninite–coffinite–brannerite assemblages. For analysed coffinites and brannerites, anomalous enrichment of Y (up to 3.4 wt % Y2O3) and Zr (up to 13.8 wt % ZrO2) is significant. The microprobe data indicate that coffinites from the Rožná and Okrouhlá Radouň uranium deposits contain variable PbO (0–4.3 wt %), FeO (0–2.5 wt %), Al2O3 (0–3.5 wt %), P2O5 (0–1.8 wt %), and CaO (0.7–3.5 wt %). Brannerite is present in unaltered and altered grains with variable concentrations of U4+ (0–0.5 apfu), U6+ (0.06–0.49 apfu), Ti (0.90–2.63 apfu), Ca (0.09–0.41 apfu), and low concentrations of Al (0–0.19 apfu), Th (0–0.04 apfu), Y (0–0.08 apfu), Zr (0–0.13 apfu) and REE (0–0.14 apfu).

Computational study of deformation mechanisms and grain size evolution in granulites – Implications for the rheology of the lower crust

Most of granulite terrains worldwide are characterized by large mean grain sizes of 1 mm or more. An important exception are the high-pressure felsic granulites in the Bohemian Massif, the European Variscan belt. There, recrystallization of original coarse-grained ternary feldspar led to formation of a fine-grained (∼100 μm) mixed matrix dominated by plagioclase and K-feldspar. This change occurred at temperatures of ∼850 °C and was probably caused by chemically induced decomposition related to slight cooling and enhanced by deformation during continental collision. The resulting microstructure shows indications of diffusion creep assisted by melt-enhanced grain-boundary sliding. Further on, minor coarsening occurred associated with deformation by dislocation creep and aggregation of mineral phases. Using a thermodynamics-based model of grain size evolution we show that stability of the fine-grained microstructure crucially depends on Zener pinning in the two-phase mineral matrix. Pinning efficiently hinders grain growth, and the small grain size that resulted from the ternary feldspar decomposition can be stable even at high temperatures. The late switch from the grain-size-sensitive creep to dislocation creep is rather difficult to explain by temperature and strain rate (or stress) changes only. However, a simple incorporation of melt solidification can successfully simulate this behavior. Alternatively, the switch and the associated grain size growth can be related to mineral phase aggregation at lower pressure–temperature conditions resulting into a decrease of pinning efficiency. This study suggests that the fine grain size of the Bohemian granulites, in contrast to the common coarse-grained type, stems from abrupt recrystallization during the high-pressure high-temperature conditions, and pinning in the fine-grained matrix. Such a process may in some cases significantly and suddenly reduce the strength of the lower continental crust and allow for its efficient redistribution.

Geochemical Signature of Magmatic-Hydrothermal Fluids Exsolved from the Beauvoir Rare-Metal Granite (Massif Central, France): Insights from LA-ICPMS Analysis of Primary Fluid Inclusions

The Beauvoir granite (Massif Central, France) represents an exceptional case in the European Variscan belt of a peraluminous rare-metal granite crosscutting an early W stockwork. The latter was strongly overprinted by rare-metal magmatic-hydrothermal fluids derived from the Beauvoir granite, resulting in a massive topazification of the quartz-ferberite vein system. This work presents a complete study of primary fluid inclusions hosted in quartz and topaz from the Beauvoir granite and the metasomatized stockwork, in order to characterize the geochemical composition of the magmatic fluids exsolved during the crystallization of this evolved rare-metal peraluminous granite. Microthermometric and Raman spectrometry data show that the earliest fluid (L1) is of high temperature (500 to >600°C), high salinity (17–28 wt.% NaCl eq), and Li-rich (Te<−70°C) with Na/Li ratios ~5. LA-ICPMS analyses of L1-type fluid inclusions reveal that the chemical composition of this magmatic-hydrothermal fluid is dominated by Na, K, Cs, and Rb, with significant concentrations (101–104 ppm) in rare-metals (W, Nb, Ta, Sn, and Li). This study demonstrates that primary fluid inclusions preserved the pristine signature of the magmatic-hydrothermal fluids in the Beauvoir granite but also in the metasomatized W stockwork, despite the distance from the granitic cupola (>100 m) and interaction with external fluids.

Crustal exhumation during ongoing compression in the Variscan Maures-Tanneron Massif, France-Geological and thermo-mechanical aspects

The Maures-Tanneron Massif (MTM), together with Corsica and Sardinia, represent the Southeastern branch of the huge European Variscan belt. A continuous evolution from continental collision to exhumation is described from ca. 350Ma to ca. 320Ma, based on an extended compilation of available geological and geochronological data. This geological compilation is complemented with thermo-mechanical modelling that tests the conditions under which deep-seated HP units can melt and exhume massively during continued convergence. We simulate in two-dimensions the development of gravitational instabilities in a partially melting crust, depending on rheological layering and heat sources, and accounting for temperature dependent elasto-visco-plastic rheologies. In order to reproduce PT exhumation patterns in the MTM, over the appropriate time-scale (>20My) and spatial extent (>100km), a best fit was obtained with a convergence rate of 0.5cm/yr and moderate diffusive surface processes. The contribution of radiogenic heating in the pre-stacked felsic crustal units is crucial. A lateral alternation with mafic units is also required in order to prevent lateral spreading of the orogen. A low viscosity partially molten crust (1020Pa·s, thus rather felsic) accompanies crustal exhumation in a decoupled mode from the deeper mantle. A shallow asthenosphere below the orogen (LAB at ~70km depth, e.g. shallow slab break-off) produces too warm and sudden exhumation as opposed to a competent mantle lithosphere (120km depth LAB) that restrains it. The MTM witnesses the typical competition between far-field plate convergence and internal body forces, and our study pleads for a progressive evolution of transpression towards perpendicular extension from ca. 320Ma.

Composition of zircons from the Cornubian Batholith of SW England and comparison with zircons from other European Variscan rare-metal granites

AbstractZircon from 14 representative granite samples of the late-Variscan Cornubian Batholith in SW England was analysed for W, P, As, Nb, Ta, Si, Ti, Zr, Hf, Th, U, Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb, Al, Sc, Bi, Mn, Fe, Ca, Pb, Cu, S and F using electron probe microanalyses. Zircons from the biotite and tourmaline granites are poor in minor and trace elements, usually containing 1.0–1.5 wt.% HfO2, <0.5 wt.% UO2 and P2O5, <0.25 wt.% Y2O3, <0.2 wt.% Sc2O3 and Bi2O3 and <0.1 wt.% ThO2. Zircon from topaz granites from the St. Austell Pluton, Meldon Aplite and Megiliggar Rocks are slightly enriched in Hf (up to 4 wt.% HfO2), U (1– 3.5 wt.% UO2) and Sc (0.5–1 wt.% Sc2O3). Scarce metamictized zircon grains are somewhat enriched in Al, Ca, Fe and Mn. The decrease of the zircon Zr/Hf ratio, a reliable magma fractionation index, from 110–60 in the biotite granites to 30–10 in the most evolved topaz granites (Meldon Aplite and Megiliggar Rocks), supports a comagmatic origin of the biotite and topaz granites via long-lasting fractionation of common peraluminous crustal magma. In comparison with other European rare-metal provinces, the overall contents of trace elements in Cornubian zircons are low and the Zr/Hf and U/Th ratios show lower degrees of fractionation of the parental melt.

Transient allanite replacement and prograde to retrograde monazite (re)crystallization in medium-grade metasedimentary rocks from the Orlica-Śnieżnik Dome (Czech Republic/Poland): Textural and geochronological arguments

We use textural and geochronological arguments to constrain monazite petrogenesis and the age of metamorphic events in medium-grade metasedimentary rocks from the European Variscan Belt (Orlica-Śnieżnik Dome, Czech Republic/Poland). The relationships between monazite, allanite and other major and accessory phases are characterized for staurolite-, sillimanite- and kyanite-grade, garnet-bearing mica schist samples. Garnet porphyroblasts with prograde compositional zoning include allanite in the core and monazite at the rim. Allanite is virtually absent from the matrix; it shows replacement textures involving a cluster of Th±U±Y-rich silicates or phosphates. Monazite, with frequent micro-cracks or trimmed grain boundaries, abounds in the mica-rich matrix; it occurs as lone grains close to partially resorbed garnet, apatite or staurolite, and as subhedral and compositionally zoned aggregates associated with allanite and xenotime. Electron microprobe analyses of monazite define Pb vs. Th* isochrons (ages=363–343Ma) with positive intercepts (Pb0=40–120ppm) which suggest the presence of initial Pb and age overestimation. Isotopic analyses obtained by laser ablation coupled with mass spectrometry confirm the minor presence of initial Pb which leads to variable discordance in the 206Pb/238U–207Pb/235U system. Conversely, the 206Pb/238U–208Pb/232Th system is not visibly affected; analyses are concordant and 208Pb/232Th ages range from 358 to 299Ma (±7–8Ma) regardless of monazite texture or composition. They define a main group at 330–310Ma, with few older outliers. All observations suggest that a first monazite generation (MnzI) formed via allanite breakdown at prograde to peak P–T conditions (5–7kbar/575–640°C). A comparison with previous geochronological results reveals that the age of this prograde reaction is, however, preserved by only few domains yielding 360–340Ma. Instead, our results emphasize the importance of retrograde processes which built the dominant age population of 330–310Ma. The latter ages are ascribed to a combination of (1) transient monazite growth after allanite and controlled by P availability, (2) variable recrystallization/replacement of older monazite grains, and (3) minor monazite neoformation due to the resorption of garnet or apatite. The young monazite grains or domains (MnzII) mostly formed during fluid influx and chloritization; they constrain the end of the orogenic history when mid-crustal rocks were exhumed and re-hydrated at temperatures below ~300°C.

Interference folding and orocline implications: A structural study of the Ponga Unit, Cantabrian orocline, northern Spain

The origin of the Cantabrian orocline of the Variscan orogen in NW Iberia remains a topic of debate. We present a structural study of the Ponga Unit, a Cambrian to Carboniferous tectonostratigraphic package within the West European Variscan belt foreland fold-and-thrust belt that lies within the core region of the orocline. Our primary goal was to determine if W-plunging folds of the fold-and-thrust belt are attributable to formation of the Cantabrian orocline, or if they reflect lateral ramps in the underlying Variscan thrust faults. The major lithologic units of the Ponga Unit are the rheologically competent Lower Ordovician Barrios quartzite, and the less-competent, Carboniferous Barcaliente limestone and Beleno shale and sandstone formation. Our mapping and structural analysis within the Ponga Unit focused on the Laviana, Rioseco, and Campo de Caso thrust sheets, and associated bounding thrusts. Over 800 structural orientation measurements were collected across the study area. These data, coupled with data compiled from regional geological maps, allow for analysis of the crustal structure. West-plunging folds of the Laviana, Rioseco, and Campo de Caso thrust sheets form kilometer-scale anticline-syncline pairs, producing a complex fold interference pattern that is characteristic of the Ponga Unit. Our analysis shows that: (1) the geometry of the W-plunging folds is inconsistent with a lateral ramp model; (2) the map pattern defines a mushroom-type fold interference pattern, indicating two distinct deformational events characterized by principal compressive stresses oriented at a high angle (perpendicular) to one another; and (3) paleomagnetic data from the study area are consistent with the secondary model of orocline formation and indicate that there was a short window of time between the end of Variscan orogenesis and the onset of oroclinal buckling. Our results indicate that early N-S–trending folds, which resulted from Variscan orogenesis, were refolded during a post-Variscan orogen-parallel compression event attributable to formation of the Cantabrian orocline.

Oroclinal buckling of the Armorican ribbon continent: An alternative tectonic model for Pangean amalgamation and Variscan orogenesis

The coupled Iberian oroclines of the western European Variscan orogen accommodated >1100 km of post-Variscan orogen-parallel shortening at translation rates in excess of 5 cm⋅yr –1 . Palinspastic restoration of the Iberian coupled oroclines reveals a north-south–trending 2300-km-long Variscan ribbon bound by ophiolite-bearing allochthons. The requirements for orocline formation, including continuing subduction and the consumption of vast tracks of oceanic lithosphere, cannot be reconciled within traditional models that view the Variscan as a record of the closure of a single (Rheic) ocean resulting in terminal Gondwana-Laurussia collision and a stable Pangea supercontinent. Paleomagnetic data from the Gondwana-derived Variscan autochthon indicate its mid-Paleozoic separation (as the Armorican microplate) and the growth of a second mid-Paleozoic (Paleotethys) ocean. The Gondwanan stratigraphic and faunal character of the Variscan autochthon can be reconciled by a model within which counterclockwise separation results in an Armorican ribbon continent that remains close or connected to Gondwana to the south and extends north toward Laurasia between an older (Rheic) ocean to the west and a newly opened (Paleotethys) ocean to the east. Geologic and paleomagnetic data further indicate that, despite translating steadily northward, Gondwana remained separated from Armorica throughout the various stages of Variscan orogenesis. We explain Pangean amalgamation as being coincident with buckling of a linear Armorican ribbon continent caught between Laurussia to the north and the northward-migrating Gondwana to the south. In this model, Variscan orogenesis is explained in terms of individual accretionary events generated through continuous, west-dipping subduction along the margins of the Armorican ribbon continent.

Present and future status of the underground space use in Poland

The exploitation of solid and liquid mineral deposits is a basic way to use rock mass in Poland. The degree of utilization of shallow rock mass areas—for any purpose other than exploitation of minerals—is low. Deeper areas are used to store natural gas, oil, and fuel; another option is underground waste storage. The use of rock mass in Poland in the future will be associated with the exploitation of minerals (including thermal waters), waste disposal, and storage of fuels and natural gas in the pore space of aquifers, hydrocarbon reservoirs, and salt deposits. Poland is characterized by complex geological structure, composed of three main geological units: the East European Craton, Western European Variscan Belt, and the Alpine-Carpathian chain. The aforementioned structures differ in geological structure, which affects the possibilities of underground space use. Most of the Polish Lowlands is prospective for the presence of thermal waters and raw mineral deposits. This area offers the best prospects for the development of waste disposal and storage of substances in the pore space, which can interfere with the other ways of underground space use. In order to properly exploit the potential of the rock mass and to avoid conflicts of interest, the appropriate land use has been suggested. Recommendations for future legislation in this area have been determined. They should—in case of a conflict of interest, associated with a number of ways of utilization (exploitation of mineral resources, thermal waters, underground storage, etc.)—determine which activities should be a priority.

Tectonometamorphic evolution of an intracontinental orogeny inferred from P–T–t–d paths of the metapelites from the Rehamna massif (Morocco)

AbstractNew petrographic and microstructural observations, mineral equilibria modelling and U/Pb (monazite) geochronological studies were carried out to investigate the relationships between deformation and metamorphism across the Rehamna massif (Moroccan Variscan belt). In this area, typical Barrovian (muscovite to staurolite) zones developed in Cambrian to Carboniferous metasedimentary rocks that are distributed around a dome‐like structure. First assemblages are characterized by the presence of locally preserved andalusite, followed by prograde evolution culminating at 6 kbar and 620 °C in the structurally deepest staurolite zone rocks. This Barrovian sequence was subsequently uplifted to supracrustal levels, heterogeneously reworked at greenschist facies conditions, which was followed locally by static growth of andalusite, indicating heating to 2.5–4 kbar and 530–570 °C. The 206Pb/238U monazite age of 298.3 ± 4.1 Ma is interpreted as minimum age of peak metamorphic conditions, whereas the ages of 275.8 ± 1.7 Ma and 277.0 ± 1.1 Ma date decompression and heating at low pressure, in agreement with previous dating of Permian granitoids intruding the Rehamna massif. The prograde metamorphism occurred during thickening and associated horizontal flow in the deeper crust (S1 horizontal schistosity). The horizontally disposed metamorphic zones were subsequently uplifted by a regional scale antiform during ongoing N–S compression. The re‐heating of the massif follows late massive E–W shortening, refolding and retrograde shearing of all previous fabrics coevally with regionally important intrusions of Permian granitoids. We argue that metamorphic evolution of the Rehamna massif occurred several hundred kilometres from the convergent plate boundaries in the interior of continental Gondwanan plate. The tectonometamorphic history of the Rehamna massif is put into Palaeozoic plate tectonic perspective and Late Carboniferous reactivation of (Devonian)–Early Carboniferous basins formed during stretching of the north Gondwana margin and formation of the Palaeotethys Ocean. The inherited heat budget of these magma‐rich basins plays a role in the preferential location of this intracontinental orogen. It is shown that rapid transition from lithospheric stretching to compression is characterized by specific HT type of Barrovian metamorphism, which markedly differs from similar Barrovian sequences along Palaeozoic plate boundaries reported from Variscan Europe.

Ordovician magmatism in the Lévézou massif (French Massif Central): tectonic and geodynamic implications

New U–Pb dating on zircon yielded ca. 470 Ma ages for the granitoids from the Levezou massif in the southern French Massif Central. These new ages do not support the previous interpretation of these granitoids as syn-tectonic intrusions emplaced during the Late Devonian-Early Carboniferous thrusting. The geochemical and isotopic nature of this magmatism is linked to a major magmatic Ordovician event recorded throughout the European Variscan belt and related to extreme thinning of continental margins during a rifting event or a back-arc extension. The comparable isotopic signatures of these granitoids on each side of the eclogite-bearing leptyno-amphibolitic complex in the Levezou massif, together with the fact that they were emplaced at the same time, strongly suggest that these granitoids were originally part of a single unit, tectonically duplicated by either isoclinal folding or thrusting during the Variscan tectonics.

Late Permian (Tatarian) fluvio-lacustrine successions in NW Anatolia (Zonguldak Terrane, Turkey): palaeogeographic implications

AbstractLate Permian fluvio-lacustrine successions of the Çakraz Formation in the Zonguldak Terrane between the regions of Akçakoca and Ereğli were investigated in order to describe the litho- and biostratigraphic properties and explain the depositional environment. The studied succession with black, dark-grey to greenish-grey shales, siltstones and limestones is named the Alaplı Member to distinguish it from the classical red clastic successions, which are tentatively named the Ereğli Member of the Çakraz Formation. The organic-rich black shales, mudstones and limestones of the Alaplı Member yielded palynological assemblages suggesting a Lopingian (Tatarian) age. The lack of any marine macro- or microfossils, the fine-grained character of the lithofacies with abundant plant material and the association of poorly sorted conglomerates in the middle part of the succession indicate possible deposition in a broad range of fluvial and lacustrine environments. Successions of similar age and depositional environment are known from the East European Variscan Belt in Bulgaria and Romania. Common successions were also developed in actively extending shallow-marine platforms on the NW Palaeotethyan margin at the end of the Permian Period.

European Variscan orogenic evolution as an analogue of Tibetan‐Himalayan orogen: Insights from petrology and numerical modeling

AbstractThe European Variscan orogeny can be compared to the Tibetan‐Himalayan system for three main reasons: (1) The Variscan belt originated through progressive amalgamation of Gondwanan blocks that were subsequently squeezed between the Laurussia and Gondwana continents. Similarly, the Tibetan‐Himalayan orogen results from amalgamated Gondwanan blocks squeezed between Asia and India. (2) The duration of the collisional period and the scale of the two orogens are comparable. (3) In both cases the collisional process resulted in formation of a thick crustal root and long lasting high‐pressure granulite facies metamorphism. Recent petrological data allow a more detailed comparison pointing to similarities also in the midcrustal re‐equilibration of the granulites and their association with specific (ultra)potassic magmatic rocks. In both orogens, the origin of the granulites was attributed to relamination and thermal maturation of lower crustal allochthon below upper plate crust. Subsequent evolution was explained by midcrustal flow eventually leading to extrusion of the high‐grade rocks. We propose that the lower and middle crustal processes in hot orogens are connected by gravity overturns. Such laterally forced gravity‐driven exchanges of material in the orogenic root were already documented in the Variscides, but the recent data from Tibet and Himalaya show that this process may have occurred also elsewhere. Using numerical models, we demonstrate that the exchange of the lower and middle crust can be efficient even for a minor density inversion and various characteristics of the crustal layers. The modeled pressure‐temperature paths are compatible with two‐stage metamorphism documented in Tibet and Himalaya.

Antimony deposits in the Variscan Armorican belt, a link with mafic intrusives?

AbstractThe Variscan Armorican belt was the World leading producer of antimony at the beginning of the twentieth century. However, geological controls on the deposits remain unconstrained. Here, we illustrate the field setting of mineralisations and perform a statistical analysis using Geographical Information System software, geophysical and geological data. The analysis shows that Sb deposits are spatially correlated with high‐density and magnetic zones. This may reflect the presence of mafic/ultramafic bodies at depth – a suggestion that is further supported by numerous occurrences of doleritic dykes throughout the region. Future antimony prospecting should focus on the high‐density and high‐magnetic zones which comprise about 18% of the Armorican massif. Links between Sb deposits and mafic intrusions are recognised elsewhere in the West European Variscan belt, emphasising that these may be key for antimony prospecting in deformation belts.

The Kulm Facies of the Montagne Noire (Mississippian, southern France)

1. Introduction With the beginning of geological and stratigraphical research in Western Europe in the early 19th century, two main marine facies realms have been identified in the lower Carboniferous (Mississippian) strata based on specific lithologies and organic content. The Carboniferous Limestone or Kohlenkalk represents the shallow-water platform facies and was first identified in the British Isles and Southern Belgium. The Kulm is equivalent of the deeper water, basinal facies of the German Rhenohercynian Basin (Fig. 1). Both names are still widely in use and especially the Kulm has been exported outside its regional and stratigraphical context. Figure 1. Localisation of the main outcrops of Kulm Facies in the framework of Variscan Europe. The attribution of the regions 5-6 in the southern Variscides is disputable. Kulm deposits in Variscan massifs included in Alpine mountain chains (e.g. Betic Cordillera) have been omitted except for the Pyrenees. The division into large tectonic units is from Franke (2014). One of these examples is found in southwest France. Here, south of the Massif Central, the remnants of a Mississippian foreland basin are exposed in the Montagne Noire, the Mouthoumet Massif and the Pyrenees (Fig. 1). The best-exposed and most complete succession is found in the Montagne Noire (Figs 1, 2). It is the aim of the paper to summarize the currently available data on the Kulm Facies of that region, to discuss a depositional model, to place the successio