Central European Variscan Belt Research Articles

Episodic Precipitation of Wolframite during An Orogen: The Echassières District, Variscan Belt of France

Monazite and rutile occurring in hydrothermally altered W mineralizations, in the Echassières district of the French Massif Central (FMC), were dated by U-Pb isotopic systematics using in-situ Laser ablation-inductively coupled plasma–quadrupole mass spectrometry (LA-ICP-MS). The resulting dates record superimposed evidence for multiple percolation of mineralizing fluids in the same area. Cross-referencing these ages with cross-cutting relationships and published geochronological data reveals a long history of more than 50 Ma of W mineralization in the district. These data, integrated in the context of the Variscan belt evolution and compared to other major W provinces in the world, point to an original geodynamic-metallogenic scenario. The formation, probably during the Devonian, of a quartz-vein stockwork (1st generation of wolframite, called wolframite “a”; >360 Ma) of porphyry magmatic arc affinity is analogous to the Sn-W belts of the Andes and the Nanling range in China. This stockwork was affected by Barrovian metamorphism, induced by tectonic accretion and crustal thickening, during the middle Carboniferous (360 to 350 Ma). Intrusion of a concealed post-collisional peraluminous Visean granite, at 333 Ma, was closely followed by precipitation of a second generation of wolframite (termed “b”), from greisen fluids in the stockwork and host schist. This W-fertile magmatic episode has been widely recorded in the Variscan belt of central Europe, e.g. in the Erzgebirge, but with a time lag of 10–15 Ma. During orogenic collapse, a third magmatic episode was characterized by the intrusion of numerous rare-metal granites (RMG), which crystallized at ~310 Ma in the FMC and in Iberia. One of these, the Beauvoir granite in the Echassières district, led to the formation of the wolframite “c” generation during greisen alteration.

Paleofluid Fingerprint as an Independent Paleogeographic Correlation Tool: An Example from Pennsylvanian Sandstones and Neighboring Crystalline Rocks (Tisia Composite Terrane, S Hungary)

In the basement areas of the southern Pannonian Basin, Central Europe (Tisia Composite Terrane, Hungary), Variscan blocks are essential components. The existing paleogeographic reconstructions, however, are often unclear and contradictory. This paper attempts to give a contribution for paleogeographic correlation of the Tisia using paleohydrological features (e.g., vein mineralization types, inclusion fluid composition and origin) of the Pennsylvanian continental succession and neighboring crystalline complexes. Vein-type mineralization in the studied samples dominantly forms blocky morphological types with inclusion-rich quartz and carbonate crystals. The evolution of hydrothermal mineralization and host rock alteration in the study area comprises three major stages. The first one is characterized by chloritization, epidotization, and sericitization of metamorphic rocks together with subsequent formation of Ca-Al-silicate and quartz-sulfide veins (clinopyroxene-dominant and epidote-dominant mineralization). The related fluid inclusion record consists of high-temperature and low-salinity aqueous inclusions, corresponding to a reduced retrograde-metamorphic fluid phase during the Late Westphalian (~310 Ma). The next mineralization stage can be related to a generally oxidized alkaline fluid phase with a cross-formational character (hematite-rich alkali feldspar-dominant and quartz-dolomite veins). High-salinity primary aqueous inclusions probably were originated from the Upper Permian playa fluids of the region. The parent fluid of the third event (ankerite-hosted inclusions) was derived from a more reductive and low-salinity environment and can represent a post-Variscan fluid system. Fluid evolution data presented in this paper support that the W Tisia (Mecsek–Villány area) belonged to the Central European Variscan belt close to the Bohemian Massif up to the Early Alpine orogenic phases. Its original position is presumably to the northeast from the Bohemian Massif at the Late Paleozoic, north to the Moravo-Silesian Zone. The presented paleofluid evolution refines previous models of the paleogeographic position of the Tisia and puts constraints on the evolution of the Variscan Europe.

Late Palaeozoic strike-slip tectonics versus oroclinal bending at the SW outskirts of Baltica: case of the Variscan belt\u2019s eastern end in Poland

Geophysical and geological data from the eastern sector of the Central European Variscan belt are presented and reviewed in the regional tectonic context. Matched filtering of isostatic gravity, guided by results of spectral analysis, along with other derivatives of gravity and magnetic fields reveal a dominant WNW–ESE-trending pre-Permian structural grain in the external zones of the Variscan belt in Poland. This trend is confirmed by regional distribution of dips in Carboniferous and Devonian strata that were penetrated by boreholes beneath Permian-Mesozoic sediments. Based on these data, two alternative concepts explaining the connection of the Variscan belt and its NE foreland, those of strike-slip tectonics versus oroclinal bending, are discussed. The WNW–ESE structural trend in the Variscan foreland is parallel to a set of major strike-slip fault zones in the area, including those of Upper Elbe, Intra-Sudetic, Odra, Dolsk and Kraków-Lubliniec. These faults are considered to convey a significant dextral displacement between Laurussia and Gondwana. The revised position of the Variscan deformation front shows a similar, uninterrupted, generally WNW–ESE trend, up to the SE border of Poland, which indicates an initial continuation of the Variscan belt into the area of the present-day Western Carpathians. The geometry of the Variscan deformation front along with the pattern of the Variscan structural grain are inconsistent with the idea of an oroclinal loop affecting the external, non-metamorphic Variscan belt. However, the data presented do not entirely rule out an oroclinal loop within the Variscan internides. The still possible options are (1) a semi-oroclinal model postulating ~ 90° bending of the Variscan tectonostratigraphic zones into parallelism with the WNW–ESE strike-slip faults or (2) an orocline limited only to the belt linking the Wolsztyn High and Moravo-Silesian non- to weakly-metamorphic fold-and-thrust belt. Regardless of the kinematic model preferred, our data indicate that structural evolution of the Polish Variscides was concluded with the end-Carboniferous NNE–SSW shortening that resulted in the present-day extent of the Variscan deformation front.

Post-Variscan thermal history of the Intra-Sudetic Basin (Sudetes, Bohemian Massif) based on apatite fission track analysis

The Intra-Sudetic Basin, a ~ 12 km deep Variscan intramontane basin, has the best preserved post-orogenic sedimentary record available at the NE margin of the Bohemian Massif. Apatite fission track (AFT) analyses have been performed on 16 sedimentary and volcanic samples of Carboniferous to Cretaceous age from the Intra-Sudetic Basin to improve understanding of the post-Variscan thermal evolution. AFT central ages range from 50.1 ± 8.8 to 89.1 ± 7.1 Ma (Early Eocene to Coniacian), with 13 of them being Late Cretaceous. The mean track length values range from 12.5 ± 0.4 to 13.8 ± 0.5 (except for one sample 14.4 ± 0.2) µm. This relatively short mean track length together with the unimodal track length distributions and rather low standard deviation (0.8 to 1.7 µm) in most samples indicate a long stay in the partial annealing zone during slow cooling. However, in the northern part of the Intra-Sudetic Basin, samples show a wider track length distribution (standard deviation of 1.8 to 2.1 µm) that could indicate a more complex thermal evolution possibly related to Mesozoic reheating. Vitrinite reflectance data combined with thermal models based on the AFT results indicate that the Carboniferous strata reached maximum palaeotemperatures in the latest Carboniferous to Early Permian time, corresponding to a major coalification event. The second phase of temperature rise occurred due to Late Mesozoic sedimentary burial, but it had no influence on maturation of the Carboniferous organic matter. Final cooling phase in the Late Cretaceous–Paleogene was related to tectonic inversion of the Intra-Sudetic Basin, which occurred after deposition of a significant thickness of Cenomanian–Turonian sediments. Thermal modelling demonstrates that ~ 4 km thick cover of Upper Cretaceous sediments is required to obtain a good match between our AFT data and modelled time–temperature paths. This outcome supports a significant amount of Late Cretaceous–Paleogene inversion within the Variscan belt of Central Europe.

Geology, sulfide mineralogy and petrogenesis of the Angstberg Ni-Cu-(PGE) sulfide mineralization (Lausitz Block, Bohemian Massif, Germany): A potential Ni-Cu exploration target in Central Europe?

Germany’s largest known magmatic Ni-Cu-(PGE) sulfide enrichment is associated with the gabbroic Angstberg intrusion that is one of several small-scaled, Ni-Cu sulfide-bearing intrusions in the northern Bohemian Massif (Central European Variscan Belt). The intrusions are part of a Middle Devonian to Early Carboniferous, mafic-(ultramafic) magmatic system that emplaced within Cadomian granodiorites of the Lausitz Block. The Angstberg intrusion represents an exposed sub-vertical, dike-like body (320 × 60 m in plan view) and comprises olivine-gabbronorite, olivine-gabbro, gabbro, olivine-hornblende-gabbro, and hornblende-pyroxene-gabbro. The hornblende-bearing and hornblende-free lithologies originate from at least two variably fractionated basaltic parental magma batches which differ in their Ti, LREE, HREE and volatile contents. Fractionation probably took place in deeper-seated sections of the magmatic system, possibly within staging magma chamber(s). Overall enriched incompatible trace-element patterns suggest that the Angstberg lithologies are linked with intra-plate magmatism, likely associated with a transtensional setting in the course of the Variscan orogeny. The previously suggested Devonian age of the intrusion is confirmed by an 40Ar-39Ar biotite age of 373.4 ± 0.9 Ma for the olivine-gabbro. The magmatic sulfide mineralization is predominated by disseminated sulfides that are hosted in the olivine-richest parts of the olivine-gabbronorite and olivine-hornblende-gabbro lithologies. In addition, some sulfide-matrix breccias (up to 70 modal % sulfides) occur directly on the footwall contact of the intrusion to granodiorite. Both mineralization types are characterized by a predominance of pyrrhotite over pentlandite and chalcopyrite with minor secondary violarite and pyrite, and accessory PGE, Au, and Ag phases. Nickel-rich bismuthotellurides of the melonite–merenskyite solid-solution series and froodite are the most common platinum-group minerals. Disseminated sulfides are characterized by high Ni, moderate Cu, and low Pt and Pd tenors (∼4–9 wt% Ni, ∼2–6 wt% Cu, ∼200–800 ppb Pt, ∼200–1000 ppb Pd). Sulfide-matrix breccias feature comparable Ni, but lower Cu, Pt and Pd tenors (∼4–6 wt% Ni, ∼0.3–1 wt% Cu, ∼10–100 ppb Pt, ∼40–200 ppb Pd). Fractional crystallization and wall-rock assimilation of granodiorite are regarded as key factors for sulfide saturation in the magmas associated with the Angstberg intrusion. Sulfur isotope compositions of pyrrhotite-pentlandite-chalcopyrite assemblages (−1.4 to −0.2‰ VCDT) propose a mantle-origin of S, whereas addition of crustal S cannot be completely ruled out as the S isotope values of the sulfides overlap with those of the granodiorite (−7 to +4‰ VCDT). Olivine- and sulfide-textures suggest that disseminated sulfides likely originate from sulfide melts that were entrained in an olivine-rich crystal mush during magma rise to upper levels. The sulfide-matrix breccias also resulted from entrained sulfide melts, or from gravity-driven downward percolation of sulfide melts from stratigraphically higher, now-eroded levels. In both cases, the formation of sulfide-matrix breccias most likely was amplified by in-situ thermomechanical erosion of the granodioritic wall-rock, caused by flash-boiling of thermally dehydrating biotite. Despite spatial limitations of the mineralized zones and limited information about the architecture of the intrusions in the northern Bohemian Massif, the Angstberg intrusion, as well as other neighboring magmatic sulfide occurrences, may represent a new exploration target for Ni and Cu in Central Europe.

The rise of the Brunovistulicum: age, geological, petrological and geochemical character of the Neoproterozoic magmatic rocks of the Central Basic Belt of the Brno Massif

The Brno Massif is the largest exposed part of the Brunovistulicum (eastern Bohemian Massif) representing Precambrian basement incorporated into the Central European Variscan Belt. Two well-known Cadomian granodiorite complexes of magmatic-arc origin are separated by N–S-trending belt of mafic rocks previously compared to ophiolite. This so-called Central Basic Belt is formed by a slightly metamorphosed volcanic part (Metabasite Zone) in the east and dominantly plutonic Diorite Zone in the west. Our new geological, geochemical and isotopic data including U–Pb zircon dating reveal two distinct Precambrian magmatic events within the Central Basic Belt preceding the Cadomian arc. The geochemical signatures of the dominant late Tonian (c. 730 Ma) tholeiitic basalts ( $$\varepsilon_{\text{Nd}}^{725}$$ = + 7.8 to + 6.7) in the Metabasite Zone suggest a direct derivation from a mantle source in an extensional setting. Also, the associated sporadic rhyolitic lavas and tuffs are primitive, showing a short mean crustal residence ( $$\varepsilon_{\text{Nd}}^{ 7 2 5}$$ = + 6.0 and + 5.7; $$T_{\text{DM}}^{\text{Nd}}$$ .2stg ~ 0.9 Ga). By contrast, the Cryogenian (c. 650 Ma) magmatism of the Diorite Zone clearly demonstrates features of a magmatic-arc origin. Rather primitive whole-rock geochemistry and radiogenic Nd isotopic signature ( $$\varepsilon_{\text{Nd}}^{ 6 5 5}$$ values typically falling between + 7 and + 6) show that this arc was either intraoceanic, or developed on recently accreted, immature mafic crust. Based on all the available data, three successive tectono-magmatic stages have been identified in the Brno Massif in the Neoproterozoic times (c. 730–600 Ma), as products of a single long-lived, multi-stage subduction system spanning nearly full Neoproterozoic supercontinent cycle from the break-up of Rodinia to the assembly of Pannotia.

Uraninite, Coffinite and Ningyoite from Vein-Type Uranium Deposits of the Bohemian Massif (Central European Variscan Belt)

Uraninite-coffinite vein-type mineralisation with significant predominance of uraninite over coffinite occurs in the Příbram, Jáchymov and Horní Slavkov ore districts and the Potůčky, Zálesí and Předbořice uranium deposits. These uranium deposits are hosted by faults that are mostly developed in low- to high-grade metamorphic rocks of the basement of the Bohemian Massif. Textural features and the chemical composition of uraninite, coffinite and ningyoite were studied using an electron microprobe. Collomorphic uraninite was the only primary uranium mineral in all deposits studied. The uraninites contained variable and elevated concentrations of PbO (1.5 wt %–5.4 wt %), CaO (0.7 wt %–8.3 wt %), and SiO2 (up to 10.0 wt %), whereas the contents of Th, Zr, REE and Y were usually below the detection limits of the electron microprobe. Coffinite usually forms by gradual coffinitization of uraninite in ore deposits and the concentration of CaO was lower than that in uraninites, varying from 0.6 wt % to 6.5 wt %. Coffinite from the Jáchymov ore district was partly enriched in Zr (up to 3.3 wt % ZrO2) and Y (up to 5.5 wt % Y2O3), and from the Potůčky uranium deposit, was distinctly enriched in P (up to 8.8 wt % P2O5), occurring in association with ningyoite. The chemical composition of ningyoite was similar to that from type locality; however, ningyoite from Potůčky was distinctly enriched in REE, containing up to 22.3 wt % REE2O3.

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).

Intracrystalline microstructures in alkali feldspars from fluid-deficient felsic granulites: a mineral chemical and TEM study

Samples of essentially “dry” high-pressure felsic granulites from the Bohemian Massif (Variscan belt of Central Europe) contain up to 2-mm-large perthitic alkali feldspars with several generations of plagioclase precipitates in an orthoclase-rich host. The first generation takes the form of lenses homogeneous in size, whereas the size of a second generation of very thin albite-rich precipitates is more variable with comparatively high aspect ratios. In the vicinity of large kyanite, garnet or quartz inclusions, the first generation of plagioclase precipitates is significantly less abundant, the microstructure is coarser than in the remainder of the perthitic grain and the host is a tweed orthoclase. The first generation of precipitates formed at around 850 °C during the high-pressure stage (16–18 kbar) of metamorphism. Primary exsolution was followed by primary coarsening of the plagioclase precipitates, which still took place at high temperatures (850–700 °C). The coarsening was pronounced due to the access of fluids in the outer portions of the perthitic alkali feldspar and in more internal regions around large inclusions. The second generation of albite-rich precipitates was formed at around 570 °C. TEM investigations revealed that the interfaces between the second-generation plagioclase lamellae and the orthoclase-rich host are coherent or semi-coherent. During late evolutionary stages of the perthite, albite linings were formed at phase boundaries, and the perthitic microstructure was partially replaced by irregularly shaped precipitates of pure albite with incoherent interfaces. The albitization occurred below 400 °C and was linked to fluid infiltration in the course of deuteric alteration. Based on size-distribution analysis, it is inferred that the precipitates of the first generation were most probably formed by spinodal decomposition, whereas the precipitates of the second generation rather were formed by nucleation and growth.

Growth of plagioclase rims around metastable kyanite during decompression of high-pressure felsic granulites (Bohemian Massif)

Samples of high-pressure felsic granulites from the Bohemian Massif (Variscan belt of Central Europe) characterized by a peak metamorphic (high-pressure) mineral assemblage of garnet-kyanite-plagio- clase-K-feldspar-quartz ± biotite show well-developed plagioclase reaction rims around kyanite grains in two microstructural settings. In one setting, kyanite is randomly distributed in the polyphase matrix, whereas in the other setting, it is enclosed within large perthitic K-feldspar. Kyanite is regarded as a relict of the high-pressure metamorphic assemblage that became metastable during transition to a low- pressure overprint. Plagioclase rims from both microstructural settings show continuous outwards decrease of the anorthite content from An32-25 at the contact with kyanite to An20-19 at the contact with the matrix or to the perthitic K-feldspar respectively. Based on mass balance considerations, it is shown that in some cases, a small amount of kyanite was consumed in the rim-forming reaction to provide the Al2O3 component for the growth of plagioclase, whereas in other cases no Al2O3 from kyanite was necessary. In a majority of examples, the necessary Al2O3 was supplied with CaO and Na2O from the surrounding matrix material. For kyanite in perthite, a thermodynamic analysis reveals that the kyanite became metastable at the interface with the host perthite at the peak metamorphic pressure, and therefore the plagioclase rim started to grow at � 18 kbar. In contrast, kyanite in the polyphase matrix remained stable down to pressures of � 16 kbar, and the plagioclase rim only started to grow at a later stage during the decompression. Plagioclase rims around kyanite inclusions within large perthite have a radial thickness of up to 50 lm. In contrast, the radial thickness of plagioclase rims around kyanite in the polycrystalline matrix is significantly larger, up to 200 lm. Another peculiarity is that the plagioclase rims around kyanite in the matrix are polycrystalline, whereas the plagioclase rims around kyanite inclusions in perthitic hosts are single crystals with the same crystallographic orientation as the host perthite. The difference in rim thickness for the two microstructural settings is ascribed to the differences in the efficiency of chemical mass transfer next to the reaction site. The comparatively large thickness of the plagioclase rims grown around kyanite in the matrix is probably due to efficient material transport along the grain and phase boundaries in the matrix. In contrast, chemical mass transfer was comparatively slow in the large perthitic K-feldspar grains.

Distribution of zinc and its role in the stabilization of spinel in high-grade felsic rocks of the Moldanubian domain (Bohemian Massif)

(Fe–Zn–Mg)-spinel-bearing assemblages in the high-grade rocks from three localities in the Moldanubian domain of the Bohemian Massif (Variscan belt of Central Europe) have been studied to monitor the partition behaviour of zinc along the pressure–temperature path followed by the host rock and to discuss the role of Zn in spinel stabilization. Spinels in two studied samples are characterized by low gahnite content (max. 0.09 Zn p.f.u), they crystallized only under silica-undersaturated conditions and the presence of zinc is thus assumed to be insignificant for their stabilization. One sample contains spinels that are gahnite-rich (up to 0.58 Zn p.f.u) leading to their stabilization within a quartz-bearing matrix. All spinel-bearing domains reflect late re-equilibration corresponding to decompression. However, in samples with preserved relics of a high-pressure mineral assemblage, changes in the zinc content attributed to prograde garnet growth were observed in the garnet cores. In order to monitor the distribution of zinc along the pressure-temperature path, textural observations have been combined with electron-microprobe and laser ablation ICP-MS data. The results point to preferential partitioning of zinc into biotite during the spinel-absent early stages of decompression and suggest that biotite was the main source of zinc for spinel crystallizing at low pressures.

Geothermometry of the ultrahigh-temperature Saxon granulites revisited. Part II: Thermal peak conditions and cooling rates inferred from oxygen-isotope fractionations

Anhydrous mineral assemblages in felsic and mafic rocks from the Saxon Granulite Massif, part of the central European Variscan Belt, preserve O-isotope equilibrium fractionations recording temperatures of 1000–1020 °C. Although ultrahigh-temperature (UHT) metamorphism has been reported from Precambrian shields and Phanerozoic fold belts, this is the first time that an independent confirmation of UHT conditions based on both O-isotopic and chemical equilibria in the very same set of samples is presented. The evidence comes from feldspathic prismatine granulite and garnet-two-pyroxene granulite, where decompression-related minerals and the whole rocks are in isotopic equilibrium. The rocks thus behaved as closed systems with respect to oxygen at least during early stages of exhumation. Based on isotopic fractionations between the whole rock and minerals that cease to exchange oxygen at high temperatures, the closed-system behaviour enabled us to estimate near-peak temperatures, even if the predominant minerals were reset during cooling. This has been demonstrated in several samples mainly for whole-rock/garnet fractionations, which indicate temperatures similar to or slightly higher than the multiphase equilibria. Average cooling rates calculated from O-isotopic disequilibrium assemblages using the program COOL (Jenkin et al. , 1991) are in the range of 50–80 °C/Ma and correspond to an average exhumation rate of 10 mm/a. The preservation of the UHT isotopic and chemical equilibria is due partly to rapid cooling, partly to anhydrous conditions during peak metamorphism and early cooling. Partial chemical and isotopic homogenisation of peak-metamorphic mineral compositions under anhydrous conditions suggests that the chemical and isotopic exchange reactions were essentially controlled by volume diffusion.

Lithospheric structure of the Bohemian Massif and adjacent Variscan belt in central Europe based on profile S01 from the SUDETES 2003 experiment

The SUDETES 2003 seismic experiment investigated the lithospheric structure of the eastern part of the Variscan belt of central Europe. The key profile of this experiment (S01) was 630 km long and extended southwestward from the margin of the East European craton, across the Trans‐European suture zone (TESZ) and Sudetes, and across the Bohemian Massif that contains the active Eger (Ohře) rift, which is an element of the European Cenozoic rift system. Good quality first arrivals and later phases of refracted/reflected P and S waves were interpreted using 2‐D ray‐tracing techniques. The derived seismic model shows large variations in the internal structure of the crust, while the depth to the Moho varies in the relatively narrow depth interval of 28–35 km. Except for the Polish basin on the northeast end of the profile, the sedimentary cover is thin. The crystalline upper and middle crust with velocities of 5.9–6.4 km s−1 is about 20 km thick, and the 7–10 km thick lower crust can be divided into three regions based on P wave velocities: a low‐velocity region (6.5–6.6 km s−1 beneath Eger rift and Sudetes) that is bounded on the southwest and northeast by regions of significantly higher velocity (6.8–7.1 km s−1 beneath the Saxothuringian and Moldanubian in the southwest and Fore‐Sudetic Monocline and Polish Basin in the northeast). High‐velocity bodies (Vp > 6.5 km s−1) were delineated in the upper crust of the Eger rift region. The seismic structure along the S01 profile images a Variscan orogenic wedge resting on the down warped margin of the plate margin containing the TESZ. This situation implies the northerly directed subduction of the Rheic Ocean that existed between the southern margin of the Old Red Continent and the Armorican terranes presently accreted into the Variscan belt. Closure of this ocean produced the Rheic suture between low‐velocity crust of the Variscan orogenic wedge and higher‐velocity crust of the TESZ.

Uplift and late orogenic deformation of the Central European Variscan belt as revealed by sediment provenance and structural record in the Carboniferous foreland basin of western Poland

The Carboniferous foreland basin of western Poland contains a coherent succession of late Visean through Westphalian turbidites derived from a uniform group of sources located within a continental magmatic arc. Detrital zircon geochronology indicates that two main crustal components were present in the source area of Namurian A sediments. They represent Late Devonian and Early Carboniferous ages, respectively. The detritus from Westphalian D beds is much more diversified and contains admixture of Late Carboniferous zircons suggesting rapid unroofing of Variscan igneous intrusions in the hinterland between Namurian A and Westphalian D times. Tectonic repetitions of tens of metres thick fault-bounded stratigraphic intervals, recorded in several wells, provide evidence for compressional regime that occurred in the SW part of the Carboniferous basin not earlier than during the Westphalian C and produced NW-SE trending folds, concordant with the structural grain of the adjacent, NE part of the Bohemian Massif.

Seismic evidence for whole lithosphere separation between Saxothuringian and Moldanubian tectonic units in central Europe

The Bohemian Massif is part of the Variscan belt of central Europe. We carried out a high resolution mapping of lithospheric thickness beneath central Europe by investigating 264 teleseismic events recorded at 80 broad band stations in the western Bohemian Massif with the method of S receiver function analysis. A negative phase beneath the Saxothuringian and northeastern Teplá‐Barrandian units at about 9–10 s before the S onset is interpreted as caused by the lithosphere‐asthenosphere boundary (LAB) at 80–90 km depth. In the Moldanubian unit, the negative phase occurs at 13–15 s before the S onset, corresponding to lithospheric thickness of 120–130 km. The boundary between the domains is oriented E‐W and probably marks the northern extension of Moldanubian lithosphere. The Moho also deepens from the Saxothuringian to the Moldanubian unit. The observed crustal/lithospheric domains could represent two distinct microplates with a relatively sharp boundary cutting through the whole lithosphere.

Age constraints for the thermal evolution and erosional history of the central European Variscan belt: new data from the sediments and basement of the Carboniferous foreland basin in western Poland

Three Carboniferous-age detrital muscovites from the Variscan foreland basin of SW Poland and two muscovites from phyllites underlying the basement have been dated by the 40 Ar/ 39 Ar step-heating and single-crystal laser fusion method. 40 Ar/ 39 Ar analysis of the detrital micas defines step-heating preferred ages of 370.7 ± 1.4, 363.0 ± 1.9 and 355.0 ± 1.3 Ma. Single-crystal laser fusion data indicate little dispersion for the first of three samples, with an integrated age that closely matches the step-heating data, but the latter two describe inhomogeneous populations. The white mica concentrate from one phyllite yields a step-heating preferred age of 358.6 ± 1.8 Ma. The second phyllite sample displays an incremental discordant apparent age spectrum representing a mixture of white mica grains of varying ages. Our most important finding is that the Variscan foreland basin was supplied by source rocks that were exhumed and cooled in the Late Devonian, probably as a result of an early Variscan collisional event, previously largely undocumented. Although accessible exposures of the Variscan basement in SW Poland exhibit only a minor component of rocks exhumed before the Carboniferous, our work suggests that large tracts of rocks with a Devonian cooling signature are preserved at depth beneath the foreland basin.

Diffusion-controlled development of silica-undersaturated domains in felsic granulites of the Bohemian Massif (Variscan belt of Central Europe)

Plagioclase rims around metastable kyanite crystals appear during decompression of high-pressure felsic granulites from the high-grade internal zone of the Bohemian Massif (Variscan belt of Central Europe). The development of the plagioclase corona is a manifestation of diffusion-driven transfer of CaO and Na2O from the surrounding matrix and results in isolation of kyanite grains from the quartz- and K-feldspar-bearing matrix. This process establishes Si-undersaturated conditions along the plagioclase- kyanite interface, which allow crystallization of spinel during low-pressure metamorphism. The process of the plagioclase rim development is modeled thermody- namically assuming local equilibrium. The results combined with textural observations enable estimation of equilibration volume and diffusion length for Na and Ca that extends ~400-450 and ~450-550 lm, respectively, around each kyanite crystal. Low esti- mated bulk diffusion coefficients suggest that the diffusion rate of Ca and Na is controlled by low diffusivity of Al across the plagioclase rim.

Thermal evolution of the orogenic lower crust during exhumation within a thickened Moldanubian root of the Variscan belt of Central Europe

AbstractAt the eastern margin of the Bohemian Massif (Variscan belt of Central Europe), large bodies of felsic granulite preserve mineral assemblages and structures developed during the early stages of exhumation of the orogenic lower continental crust within the Moldanubian orogenic root. The development of an early steep fabric is associated with east–west‐oriented compression and vertical extrusion of the high‐grade rocks into higher crustal levels. The high‐pressure mineral assemblage Grt‐Ky‐Kfs‐Pl‐Qtz‐Liq corresponds to metamorphic pressures of ∼18 kbar at ∼850 °C, which are minimum estimates, whereas crystallization of biotite occurred at 13 kbar and ∼790 °C during decompression with slight cooling. The late stages of the granulite exhumation were associated with lateral spreading of associated high‐grade rocks over a middle crustal unit at ∼4 kbar and ∼700 °C, as estimated from accompanying cordierite‐bearing gneisses. The internal structure of a contemporaneously intruded syenite is coherent with late structures developed in felsic granulites and surrounding gneisses, and the magma only locally explored the early subvertical fabric of the felsic granulite during emplacement. Consequently, the emplacement age of the syenite provides an independent constraint on the timing of the final stages of exhumation and allows calculation of exhumation and cooling rates, which for this part of the Variscan orogenic root are 2.9–3.5 mm yr−1 and 7–9.4 °C Myr−1, respectively. The final part of the temperature evolution shows very rapid cooling, which is interpreted as the result of juxtaposition of hot high‐grade rocks with a cold upper‐crustal lid.

Cadomian and Early Paleozoic Magmatic Events: Insights from Zircon Morphology and Geochemical Signatures of I- and S-Type Granitoids (Saxo-Thuringia/Bohemian Massif/Central Europe)

This paper presents studies of zircon morphologies and geochemical signatures of Neoproterozoic and Early Paleozoic metagranitoids and metarhyolites of the Saxo-Thuringian Zone (Central Europe). The Saxo-Thuringian Zone was derived from the northern periphery of Gondwana and became part of the Variscan belt of Central Europe during Paleozoic orogenic processes. Five tectonomagmatic events are preserved in the Saxo-Thuringian Neoproterozoic and Early Paleozoic igneous rocks. These different geotectonic scenarios are each characterized by distinctive geochemical signatures and zircon morphologies. Samples of felsic lithologies with age ranges of ~580-560 Ma, ~500 Ma, and ~380-360 Ma show whole-rock compositions of dry I-type with a tendency to A-type granites and zircon crystals with strong dominance of {100} prisms and {101} pyramids. The ~580-560 Ma and ~500 Ma periods of predominantly juvenile magmatism both were followed by tectonothermal events producing S-type granitoids at ~550-530 Ma and 495-480 Ma. Associated zircon populations are dominated by {110} prisms and {211} pyramids. Inasmuch as the errors of individual geochronological analyses may overlap two different geotectonic episodes, the relationship between geochemical features and zircon morphologies of the Saxo-Thuringian Neoproterozoic and Early Paleozoic igneous rocks can aid in assigning the dated sample to the proper stage of formation.

U-Pb SHRIMP and Nd isotopic data from the western Bohemian Massif (Bayerischer Wald, Germany): Implications for Upper Vendian and Lower Ordovician magmatism

U-Pb SHRIMP dating of zircons of metamagmatites from the Bayerischer Wald (Germany) reveals a complex evolution of this section of the Moldanubian Zone exposed in the western Bohemian Massif of the central European Variscan belt. In the south-western part of the Bayerischer Wald Upper Vendian magmatism is constrained by pooled 206Pb/238U mean ages of 555±12, 549±7 and 549±6 Ma from metarhyolites and a metabasite. Inherited zircon cores were not observed. Zircon overgrowths, yielding pooled 206Pb/238U ages of 316±10 and 319±5 Ma, provide evidence for Variscan metamorphic zircon growth; cathodoluminescence imaging reveals a two-stage metamorphic overprint. In contrast, Lower Ordovician magmatism and anatexis are documented in the north-eastern parts of the Bayerischer Wald by metagranitoids (480±6, 486±7 Ma), an eclogitic metabasite (481±8 Ma) and a leucosome (491 to 457 Ma). Inherited zircon cores are found in Lower Ordovician metagranitoids and the leucosome, indicating a Palaeoproterozoic-Archaean (ca. 2.7, 2.0 Ga) source region, presumably of Gondwana affinity (West African craton), and documenting Cadomian magmatism (ca. 640 Ma). Post-Cadomian metamorphism is inferred from concordant ages of 433±4 and 431±7 Ma. Upper Vendian magmatism is assumed at an active continental margin with ensialic back-arc development (eNd(t) –3.01 to +1.22); the lack of inherited zircon is due to either derivation from juvenile (?volcanic arc) material or complete isotopic resetting of pre-existing zircon. An active continental margin setting, possibly with some lateral variation (accretion/collision) is envisaged for the Lower Ordovician, producing granitoids, rhyolites and leucosomes (eNd(t) -0.5 to -6.27); MORB-type metabasites may be related to ZEV or Marianske Lazně Complex metabasites. A tentative palaeogeographic reconstruction puts the “Bayerischer Wald” in close relationship with the Habach terrane (proto-Alps), as the “eastern” extension of terranes of the northern Gondwana margin.