Variscan Research Articles

Granitoid Metasomatism and Giant Quartz Vein Formation by Mineral Replacement: Insights from the Canigó Massif, Eastern Pyrenees

Abstract Metasomatism is a ubiquitous process in the Earth’s crust, exerting major controls on fluid, heat and mass transfer and rock deformation, and is commonly constituted by mineral replacement reactions. Different types of metasomatism may coexist and/or successively conceal each other in a given area. Deciphering the geochemical behaviour, regional extent, and mineralogical changes of multi-stage metasomatism can be difficult because of the overprinting of successive events and their frequent relationship with deformation. Here we investigate granitoid metasomatism, namely silicification, feldspathisation, and sericitisation, in the Variscan basement rocks of the Canigó Massif (Eastern Pyrenees, SW Europe), which is spatially related to Giant Quartz Veins (GQVs) tens of metres wide and several kilometres long. Unaltered and altered granitic orthogneisses derived from Ordovician intrusives and late-Variscan granitoids, as well as GQV occurrences, are studied across scales through structural and textural characterisation, whole-rock geochemistry, and electron backscatter diffraction. Geochemical analyses are further compared with a new database including more than 600 unaltered granite and orthogneiss samples from the Pyrenees and the Catalan Coastal Ranges (SW European Variscan Belt). Results show that silicification, the dominant metasomatic process, was related to regional-scale shear zones and contributed to form GQVs through mineral replacement. This is confirmed at the macro- (km), meso- (m–cm), and micro-scale (μm) by relict fabrics, mineral phases, and structural features of the precursor rocks within veins, by a progressive depletion of all major and trace elements, except silica, in rocks sampled along decreasing distances from GQV outcrops, and by the localisation of mylonitic deformation along GQVs. Feldspathisation and sericitisation are, in contrast, restricted to specific sectors and exposed as albitite, trondhjemite, and pale green mica-rich outcrops. It is suggested that most of the exposed areas of the studied GQVs are, accordingly, not veins sensu stricto but metasomatic products where the original fabrics and features of precursor rocks were overprinted during coupled deformation and Si-metasomatism. Results presented here have major implications for the scale and geochemical behaviour of multi-metasomatic events, as well as on the kinetics of mineral replacement processes leading to changes in the physicochemical properties of crustal rocks.

Fluid flow in crustal fault zones with varying lengthwise thickness: application to the Margeride fault zone (French Massif Central)

Crustal fault zones, holding promise as potential geothermal reservoirs, remain largely untapped and unexplored. Located in the southern Massif Central, France, the Margeride fault zone (MFZ) varies in thickness (lateral extension perpendicular to the fault plane) from 100 m to over 2500 m. Reactivated several times under different stress regimes since the Variscan orogeny, this zone is characterized by an intense alteration and fracturing. As a result, the multiple reactivation of the fault zone has maintained permeability, leading to favourable conditions for fluid circulation. Structural measurements and geological cross sections were used to precisely constrain thickness and geometry of the fault zone. North of the MFZ, the Coren thermal spring indicates reservoir temperatures of about 200–250 °C, hinting at the possible existence of a temperature anomaly. To investigate this geothermal potential, 3D numerical models simulating fluid circulation within a fault zone were conducted. Various configurations were explored, altering fault zone thickness and permeability for two key geometries. The first geometry, which manipulated the width of the fault zone along its length, demonstrated a direct correlation between fault zone thickness and amplitude of thermal anomaly. Thinner faults (< 500 m) exhibited multiple weak positive thermal anomalies, while thicker faults (> 500 m) tended to develop a single, substantial positive thermal anomaly. In the second examined geometry, where fault zone thickness increased longitudinally, a consistent positive temperature anomaly emerged at the thickest section of the fault zone. Depending on the permeability value, an additional anomaly may develop but will migrate laterally towards the thinnest part of the fault zone. This multi-disciplinary approach, combining numerical modelling and field measurements, presents a predictive methodology applicable to geothermal exploration in analogous basement domains. In our case, it has shown that the northern end of the Margeride fault zone could represent an area that needs to be explored further to assert its high geothermal potential. Our numerical models will increase understanding of how fault width and geometry impact the geothermal potential of the Margeride fault zone and similar areas in crystalline basement.

The Addition of Felsic Sediments to the Lower Continental Crust During the Variscan Orogeny

AbstractLower crustal metasedimentary xenoliths (garnet‐sillimanite granulites) from the Bournac breccia pipe in the Massif Central, France, provide a robust example of sediments transported to depth and incorporated into stable lower continental crust during a collisional orogeny. Dates for detrital igneous zircon range from the Archean (up to 3,300 Ma) to the Devonian and record sedimentation prior to the onset of the collisional phase of the Variscan orogeny. Metamorphic zircon and monazite document the presence of the metasediments in the lower crust by ca. 330 Ma during the later phase of Variscan collision. Zircon and monazite crystallization continued within the lower crust until ca. 265 Ma, corresponding to a period of slow cooling following an episode of ultra‐high temperature (UHT) metamorphism that peaked at 313 Ma. Zr‐in‐rutile thermometry and GASP barometry applied to these samples record conditions of 0.63–0.77 GPa and 830–910°C, which correspond to Ti‐in‐zircon temperatures from the latter part of the Variscan orogeny and geotherms in excess of typical continent‐continent collisions. Rutile in these samples remained open to Pb loss until their eruption at ca. 11.6 Ma, providing an indirect date of the Bournac eruption. These rocks record the incorporation of felsic sedimentary material into the stable deep continental crust during a collisional orogen and their residence there for over 300 Ma. More broadly, the addition of sediments to stable lower crust contributes to changes in crustal composition and has significant implications for the heterogeneity of the deep continental crust, as well as overall crustal heat production and mantle heat flow.

Extensional perpendicular orthogonal quartz veins related to early Variscan Orogeny

Extensional perpendicular orthogonal quartz veins related to early Variscan Orogeny

Scientific, Educational and Geotourism Value of the Ballota Beach that Recorded the Old and Most Recent Geological Histories of the North-Northwest Part of the Iberian Peninsula

One of the enclaves that exhibits the best features to understand the old and most recent geological history of the north-northwest portion of the Iberian Peninsula is the Ballota Beach, located in Asturias, Spain. In the cliff above sea level that borders this beach to the south, a succession of Carboniferous “griotte” limestones crops out, deformed by spectacular ramp and detachment folds, as well as thrusts, backthrusts and duplexes. These structures are testimonies of the shortening produced in the cordillera originated during the Variscan orogeny of Devonian-Permian age, which extended throughout central Europe, northern Africa and eastern North America. Additionally, from a viewpoint located on the cliff, one can recognize a flat surface over the carbonate coastal cliffs and higher flat surfaces developed on quartzites. These flat surfaces, known as “rasas”, are marine abrasion surfaces elevated above current sea level resulting from Cenozoic-Quaternary land uplift and/or sea level fall. This region, already mentioned in the XIX century, has an extraordinary scientific and educational value, from the Structural Geology, Geomorphology, Historical Geology and Stratigraphy points of view, and is a great geotourism attraction due to its beauty. To make this region known, we propose to declare this area a geological interest site in the Global Geosites Spain project, make this article open access, upload a virtual outcrop model we have built in open-access online repositories, make information available to public entities that promote outreach and tourism, propose the realization of a “Geolodía” (Geology day), and make a video to celebrate the Geodiversity International Day.

Multi-stage tectonic record of the Central Sakarya Terrane Basement (NW Turkey): Implications for the Paleozoic evolution of Rheic Ocean and northern Gondwanan margin

In northwestern Turkey, within the Central Sakarya Terrane, the Söğüt Metamorphics is one of the Variscan tectonic units preserved as a pre-Jurassic basement assemblage of the Sakarya Composite Terrane (SCT). It consists of para- and orthogneisses that host bands, lenses, boudins, and tectonic slices of amphibolites, and are altogether intruded by the Sarıcakaya Granitoid. The characterization and timing of the precursor magmatic events for the protoliths of this assemblage and the timing and conditions of metamorphism have been the topics of a long-lasting debate. Here we present new geochemical and geochronological data from the Söğüt orthogneisses, amphibolites, and the crosscutting Sarıcakaya Granitoid to provide further understanding of the geodynamic evolution of the SCT Basement and the larger-scale tectonic events that controlled its formation. Available geological and geochronological data indicate that the generation of the orthogneiss protoliths began in the Late Cambrian (ca. 485 Ma) and continued until the middle Silurian (ca. 430 Ma), accompanied by the subduction of the Iapetus Ocean and its progressive roll-back beneath northern Gondwana. The slab roll-back caused mantle-upwelling and Ordovician (ca. 465 Ma) continental rift-related mafic magmatism, eventually leading to the opening of the Rheic Ocean. Prior to the Variscan orogeny, a short-lived episode of middle Carboniferous (ca. 336−328 Ma) intra-oceanic arc magmatism occurred within the Rheic Ocean north of the SCT. Accretion of all this material and continuous subduction beneath the SCT led to a two-stage metamorphism during the late Carboniferous (ca. 326−324 Ma and ca. 318 Ma) and synchronous generation of granitic (ca. 327−324) and dioritic (ca. 319−317 Ma) rocks of the Sarıcakaya Granitoid during the Variscan orogeny, terminating the life cycle of the Rheic Ocean.

Sun, Sea and Sand; Cretaceous Source to Sink Systems of Senegal, NW Africa

ABSTRACTPaleo source to sink system analysis requires a complete earth systems model approach, utilising regional geology, tectonics, climate and modern‐day source to sink analogues. This study examines the Cretaceous source to sink systems of Senegal, NW Africa, integrating a broad regional dataset using a multidisciplinary mineralogical approach. The most significant regional geological and tectonic events to affect Senegal since the Pan‐African Orogenies (800–520 Ma) are the Hercynian Orogeny (320–290 Ma), Pangea break‐up and rifting between S. America and Africa, with associated Central Atlantic Magmatic Province volcanism (200 Ma) and uplift of the Mauritanide hinterland (113–66 Ma). In addition to tectonic controls, climate is the principal driver for paleo‐drainage reorganisation. During the Cretaceous an antithetical shift in climate from warm and arid (145–115 Ma), to hot and humid (100–88 Ma), increased fluvial catchment and energy. Antecedent paleo‐drainage of the Cretaceous Senegalese Basin is governed by subsurface grabens striking hundreds of kilometres into the continent formed during Atlantic rifting. Early Cretaceous aridity restricted fluvial catchments to recycling pre‐Cretaceous basinal sediments. Climate change triggered expansion of paleo‐drainage catchments during the Aptian caused fluvial incision and erosion of the Gaouâ Group Hercynian to Pan‐African age source rocks along the western flank of the Mauritanides. Exhumation increased significantly throughout the Cretaceous Thermal Maximum during the Cenomanian–Turonian, with exhumation of the Gadel Group Pan‐African source rocks, evidenced from a shift between a garnetiferous to staurolitic basin mineralogy. Inclusion of 200 Ma zircons into the central Senegalese Basin during the Albian is evidence of possible catchment shifts to include CAMP detritus from the Fouta Djallon Plateau. Cretaceous basinal sediments are almost exclusively sourced from the Mauritanide belt which includes Hercynian metamorphic host rocks and Palaeozoic sediments ultimately derived from the erosion of the Pan‐African orogenic belts. During the Maastrichtian, the central fluvial systems breached the southern Mauritanides, sourcing Cambrian zircons from the south.

Timing of post-orogenic silicic volcanism in the eastern part of the European Variscides: constraints from SHRIMP U–Pb zircon study of the Permo-Carboniferous Góry Suche Rhyolitic Tuffs (the Intra-Sudetic Basin)

The Góry Suche Rhyolitic Tuffs in the Intra-Sudetic Basin, in the eastern part of the Variscan Belt of Europe, represent a voluminous (ca. 100 km3), possibly caldera-related, ignimbrite-dominated complex and the Łomnica Rhyolites are associated, post-ignimbrite sills. Zircon separates from nine samples were dated using the U–Pb SHRIMP method. Well-defined concordia ages were determined in four ignimbrite samples (300.5 ± 2.0, 300.5 ± 1.4, 298.0 ± 1.6 and 297.2 ± 0.9 Ma) and in two rhyolite samples (298.4 ± 1.5 and 292.6 ± 1.9 Ma). Clustering of the ignimbrite sample ages between 300.5 ± 2.0 and 297.2 ± 0.9 Ma and geological evidence indicate the eruption and deposition of the tuffs close to the Carboniferous/Permian boundary, in a geologically rapid event at approximately 299 Ma. Zircon assemblages in three tuff specimens are strongly dominated by xenocrysts of various Palaeozoic and Precambrian ages that were incorporated during the eruption through the basin fill. The emplacement of the tuffs was followed (and partly overlapped?) by the emplacement of the Łomnica Rhyolites as sills in two episodes in the early Permian. The Góry Suche Rhyolitic Tuffs may be a few million years older than assumed so far, and this, as well as rather imprecise biostratigraphic constraints from the host sedimentary rocks, suggest a need for revision of the existing lithostratigraphic and evolutionary schemes for the Permo-Carboniferous of the Intra-Sudetic Basin. The studied tuffs and rhyolites together with coeval granitic plutons in vicinity can be linked to the onset of post-Carboniferous lithospheric thinning in Central Europe.Graphical abstract

On the origin of the Azzel-Matti circular structure (southern Algeria): Insights from remote sensing, geological and geophysical data

The Azzel-Matti circular structure (25°51′ N, 0°35’ E) is located on the Tanezrouft plateau at the boundary between the West African Craton and the Bled El Mass compartment. Its morphology exhibits a diameter of about 6.5 km, with a raised rim that stands above the surrounding terrain. This study is based on a combination of remote sensing data (Landsat-8-OLI images and Shuttle Radar Topography Mission (SRTM) images), aeromagnetic and seismic data, and field investigations in and around the Azzel-Matti circular structure to analyze its morphology and ascertain its origin. The investigation of the area surrounding the structure did not reveal any evidence of magmatism, diapirism or impact. However, it did confirm the presence of new faults of a deeper character, as highlighted by remote sensing, aeromagnetic, and seismic analysis. This suggests that tectonic deformation is a probable factor contributing to the formation of this circular structure. The morphology of the structure is explained by the counter-clockwise rotation of the strata, likely resulting from the interplay between movements along the sub-meridian faults marking the eastern and western boundaries and the ESE-WNW oblique reverse fault marking the northern boundary. These faults are coherently arranged to facilitate the counter-clockwise rotation of the strata. The movement of the faults and their depth indicate an NE-SW compressional event corresponding to the Late Paleozoic Variscan (Hercynian) orogeny. This is supported by the presence of horizontal lacustrine carbonate formations, presumably of Jurassic age, which are discordant with the Carboniferous strata, thus providing evidence for the age of the Azzel-Matti circular structure between the Moscovian and the Jurassic.

At the border of peri-Gondwana and Baltica – the structure of the eastern termination of the Variscan belt

The termination of the eastern Variscan belt has been a matter of long-standing debates since it is deeply concealed beneath younger sediments. Its geometry has been addressed by two competing hypotheses of the Variscan orocline and right-lateral strike-slip tectonics. Therefore, a set of new gravity and magnetic maps was compiled for Czechia, Poland and eastern Germany to track Variscan structures in the subsurface. These maps clearly show that the Rheno-Hercynian Suture turns almost 90° east the of the Harz Mts. and continues into Poland toward the ESE. Therefore, our study favours a concept of semi-orocline that terminates in the east against the Brunovistulian Block. The pattern of magnetic and gravity anomalies combined with geological evidence indicates two stages of accretion superimposed in the eastern Variscan belt. The initial W-E convergence led to the formation of NNE-SSW-trending structures that dominate the southern Bohemian Massif. These structures were overprinted by an important N-S shortening event that shaped WNW-ESE oriented features. The latter run parallel to the Baltica margin and dominate the area NE of the Elbe Fault. This is consistent with seismic data showing large-scale underthrusting of the Baltica crust beneath the Variscan belt at a distance of at least 100 km.

A Reappraisal of Nappe Structures and Strain Patterns in the Core of the Variscan Orogen (French Massif Central)

AbstractMajor influences on the architecture of orogens include the overall thermal conditions of orogeny (hot vs. cold) and the angle of collision (orthogonal vs. oblique). In the French Massif Central of the Variscan orogen, a cold‐orogen style crustal nappe architecture was interpreted in the Haut‐Allier, located in the core of the orogen. Based on this model, the Variscan orogenic crust is described as the superposition of three main allochthonous tectonic units juxtaposed along major thrust faults. However, based on a detailed structural analysis, we have found instead that the region is dominated by a network of anastomosing conjugate and coeval dextral and sinistral shear zones striking WNW‐ESE and ENE‐WSW, respectively. The dextral WNW‐trending shear zones are predominant, whereas the sinistral NE shear zones are mainly located in the eastern part of the massif. Between these sub‐vertical shear zones, a sub‐horizontal foliation is observed. Our results indicate that all planar fabrics were partially synchronous during suprasolidus low‐pressure‐high‐temperature conditions. Strain partitioning occurred from high‐temperature suprasolidus conditions to subsolidus retrogression and may represent orogen‐parallel flow, suggesting hot‐orogen style. These results call into question the validity of the crustal nappe model in the Haut‐Allier. Based on new structural data and related observations, we propose a new model in which metamorphic gaps between tectonic units are explained by the juxtaposition of different structural domains by displacement along strike‐slip shear zones.

Fault characterization and its significance on hydrocarbon migration and accumulation of western Tazhong Uplift, Tarim Basin: Insight from seismic, geochemistry, fluid inclusion and in situ U-Pb dating

Understanding the mechanisms of deep hydrocarbon accumulation is of paramount importance in the field of petroleum geology. Studying deep hydrocarbon accumulation mechanisms contributes to the development of effective reservoir management strategies and efficient extraction techniques. In this study, fluid inclusion observation, homogenization temperatures, in situ calcite U-Pb dating, geochemical analyses and seismic data were applied to investigate the hydrocarbon charging history of the western Tazhong Uplift, and the migration pathways were further analyzed to establish the history of oil and gas accumulation in the study area. The results reveal three distinct episodes of oil and gas charging processes in the western Tazhong Uplift. The first episode, determined by in situ calcite U-Pb dating bearing primary oil inclusions, occurred during the late Caledonian to early Hercynian orogeny. This episode is characterized by yellowish fluoresce color oil inclusions. The second episode involved a complex variation of the thermal maturity of oil. Mature oil, with oil inclusions preferentially fluorescing yellow-green color, was charged into the reservoirs near the Tazhong No.1 fault. Subsequently, the high mature oil characterized by blue-fluorescing oil inclusions was charged. At the same time, the mature oil migrated vertically into the reservoir away from the Tazhong No.1 fault along the strike slip faults. The third charging process is dominant by high mature oil in the Southwest of the western Tazhong Uplift, whereas the gas intrusion occurred in the area near the Shuntuoguole Low Uplift. Furthermore, the strike slip faults and reverse faults were distinguished based on the seismic profiles and coherence diagram. The intersections of strike slip faults and Tazhong No.1 fault developing more fractures served as oil and gas charging points. The oil production revealed that the strike slip faults acted as important hydrocarbon migration pathways. This study provides valuable insights into the processes of hydrocarbon accumulation and offers further evidence for deep oil exploration in the region.

Structural controls on hydrothermal fluid flow in a carbonate geothermal reservoir: Insights from giant carbonate veins in western Germany

This study examines the impact of polyphase tectonics on the development of structurally controlled hydrothermal fluid pathways in carbonate geothermal reservoirs. Our case study focuses on hydrothermal carbonate veins and vein-filled faults in Devonian carbonates from the North Rhine-Westphalia region in western Germany, currently being explored as a potential low-enthalpy geothermal reservoir at depths of 4–5 km. These veins, which can be up to 20 m thick, are subvertical and strike NNW, N, or ESE. They exhibit pinch-and-swell undulating geometries, faulted vein-wall contacts, and occasional fillings of hydrothermal breccias. Vein-filled normal faults show hybrid shear-dilatant openings, with fault tips characterized by horsetail vein terminations. The textures, orientations, and age of the veins suggest their formation at low confining pressures and at depths < 2 km during the Post-Variscan East-West extension. Orthogonal North- and East-striking strike slip faults, inherited from the Variscan orogeny, were likely reactivated during post-Variscan extension, with a significant dilatant component that formed the observed veins. In the Ruhr Basin, the dilation tendency analysis indicates that NNW-striking veins or joints are optimally oriented for re-opening under the current strike-slip stress regime, characterized by NNW-trending maximum horizontal stress. The intersections of fractures may currently create moderately SSE-plunging linear zones of enhanced fluid flow. The main uncertainty regards the presence of similar structures at geothermal reservoir depths of ∼4–5 km in the Middle Devonian carbonates underneath the Ruhr Basin. As the study veins are likely to have formed at depths <2 km, the existence of analogous dilatant conduits at greater depths remains speculative. Eventually, we propose that zones characterized by open discontinuities and channelized fluid flow in carbonate geothermal reservoir in strike-slip tectonic settings can be: (a) dilational jogs between overlapping strike-slip faults, (b) bends along faults, and (c) strike-slip fault terminations with horsetail extensional structures. These zones can also be prone to enhanced karstification.

Unravelling the magmatic and hydrothermal evolution of rare-metal granites through apatite geochemistry and geochronology: The Variscan Beauvoir granite (French Massif Central)

Peraluminous rare-metal granites (PRMGs) represent highly differentiated crustal granites characterised by extreme enrichment in metals, such as Li, Sn, Nb, Ta, W, and Be. This geochemical specificity is considered to be the result of a succession of magmatic and hydrothermal processes, the importance and individual impact of which are still debated. In this study, we investigate the magmatic and hydrothermal evolution of the Beauvoir leucogranite, a world-class PRMG from the French Paleozoic Variscan belt, through extensive characterisation of suprasolidus to subsolidus apatite. We employ a multi-tracing approach, combining in-situ elemental compositions (major, trace, and halogen elements), oxygen isotopic systematics, and U-Pb geochronology.Four major magmatic and hydrothermal stages were identified through apatite petrography and U-Pb geochronology. Magmatic apatite crystallised at 314.6 ± 4.7 (2s) Ma. Decrease in the amplitude of the Eu anomaly in magmatic apatite from the deeper to shallower granitic units record an increase in the oxygen fugacity (fO2) of the magma with differentiation, likely contributing to the crystallisation of a first and predominant cassiterite generation. Magmatic apatite REE patterns show significant tetrad effects; they reflect the exsolution of magmatic fluids involved in the precipitation of early hydrothermal apatite replacing igneous minerals or precipitating within veins during greisenisation episode dated at 314.3 ± 5.5 Ma and 311.7 ± 8.1 Ma. Early hydrothermal apatite, characterised by enrichment in Sr or S and Mn-REE depletion along with variable Br/I ratios and δ18O compositions down to negative values, record mixing dynamics between two fluid end-members: (i) magmatic fluids, and (ii) oxidising meteoric fluids partially reequilibrated with country rocks. Meteoric fluids progressively invaded the Beauvoir PRMG at temperatures ≥ 450 °C and triggered precipitation of a second generation of cassiterite during mixing with magmatic fluids. Two late, non-magmatic, hydrothermal events are dated at 268.3 ± 20.4 Ma and 148.5 ± 26.6 Ma. Related apatite is marked by specific mineralogical and geochemical features such as high As contents and heavy oxygen isotope signatures. They are proposed to be linked to extension-related regional hydrothermal fluid circulation that contributed to metal endowment (U, F-Ba-Pb-Zn) in the crystalline basement and overlying sedimentary cover of Western Europe following the Variscan Orogeny.Our results demonstrate that apatite is a key mineral to decipher the role of magmatic and hydrothermal processes leading to ore deposition and remobilisation in PRMGs, in relation with geodynamic evolution. Apatite records of external fluid incursions at near-solidus conditions highlight the open system nature of the Beauvoir PRMG, which is crucial for developing fully integrated metallogenic models applicable to similar deposits.

Metamorphic and chronological records of early Permian continental collision in Beni Bousera granulites (Internal Rif belt): implications for the tectonic evolution of northern Morocco during Pangea construction

This work presents petrological data obtained from shielded mineral inclusions within large garnets from the Beni Bousera metamorphic unit (internal Rif belt, northern Morocco) combined with in situ U‒Th‒Pb dating of monazite inclusions. In the considered Beni Bousera metapelites, the occurrence of mineral inclusions of kyanite + rutile + plagioclase + K-feldspar + quartz ± graphite trapped in garnet cores is diagnostic of high-pressure granulite-facies metamorphism. Geothermobarometry combined with thermodynamic modelling yields minimal P ‒ T conditions of approximately 1.37–1.5 GPa and 780–795°C, consistent with the development of palaeogradients during crustal thickening in collision belts. A concordant U/Pb age of 281 ± 3 Ma is obtained from associated monazite inclusions in the same garnet cores. These new petrochronological data attest to the early Permian continental collision in northern Morocco. The obtained dataset is constrained with petrological and geochronological parameters available on other neighbouring segments of the Variscan belt. Within the framework of recently unified full plate reconstruction models, the presented tectonic scenario suggests that all these orogenic segments were built during the late Carboniferous–early Permian crustal thickening at the northwestern Gondwana margin in response to the convergence of Laurentia and Gondwana during the late stages of Pangaea construction.

Thermal studies on the lower Carboniferous basins of Khenifra and Qasbat-Tadla, Morocco: What do they teach us about the pre-Variscan stages in NW Africa?

Within the Variscan Belt of Morocco, the Central Massif preserves Early Carboniferous rift basins. The Lower Carboniferous Khenifra and Qasbat-Tadla basins are aborted rifts, developed just before the Variscan orogenesis in Morocco that occurred during the Pennsylvanian-Cisuralian in NW Africa. Due to both weak inversion of these basins during the Variscan orogeny and limited burial afterwards, these basins offer the opportunity to study the Early Carboniferous pre-orogenic thermal regimes. In the Khenifra basin, 77 samples collected across the basin and its basement's boundaries, allowed the determination of the maximal temperature reached during the rocks burial by means of Raman spectroscopic analyses. The Ordovician basement shows mean temperatures between 230 and 300 °C whereas the upper Visean/Serpukhovian infill has a wide range, from temperatures <160 °C to 250–260 °C. This thermal variation within the basinal series has been evidenced from west to east and cannot have been acquired during the Variscan events. The acquisition of these maximal temperatures occurred between Late Devonian to Upper Visean/Serpukhovian and is thought to result from the formation of an extended rift. In the Qasbat-Tadla basin, Rock-Eval data from Ordovician to Devonian source-rocks indicate significantly lower maximal temperatures reached by pre-Carboniferous samples that are within the oil window. Our results are in favor of a heterodox model for the Variscan belt in Morocco and NW African in general, suggesting that no pre-Variscan compressional events are needed. Instead, the development of the intraplate Variscan belt in NW Africa was permitted through the development of hot and weak Lower Carboniferous basins, subsequently inverted in a far-field stress field. The striking thermal differences between the Khenifra and Qasbat-Tadla basins suggest that important tectonic segmentation must have shaped the area during the Early Carboniferous extensional phase.

Magnetization related to late-Variscan extensional collapse in the Lugo Dome (NW Iberian Massif): a metamorphic petrology approach

The Eastern Galicia Magnetic Anomaly (EGMA) is one of the most distinctive and best studied magnetic anomalies in Iberia. It is located in NW Spain and overlaps the Variscan gneissic domes of Lugo and Sanabria. The Lugo Dome, in the north of the EGMA, deforms a large thrust sheet, the Mondoñedo Nappe, allowing the outcropping of its relative autochthon in the Xistral Tectonic Window, where extensional detachments resulting from late-Variscan tectonics crop out. A younger major detachment, the Viveiro Fault, developed on the western limb of the dome. Detailed ground-based magnetic mapping and geological charts show a clear spatial relationship between magnetic maxima and the extensional structures. Magnetic and paleomagnetic studies carried out on samples from this region indicate that rocks in the detachments commonly bear magnetite and hematite carrying induced but also remanent magnetizations and a very well-defined anisotropy of the magnetic susceptibility with directions matching those of the late-Variscan extensional fabrics.Three pairs of metasedimentary samples in equivalent structural positions display variously developed medium-pressure Barrovian parageneses attributed to the early compressional phases of the Variscan orogeny, and low-pressure Buchan-type parageneses associated with the late Variscan extension. Our phase diagram-based petrological study suggests that the non-magnetic samples preserve the Barrovian conditions of 560–640 °C, 5–8.7 kbar. The lithologically and structurally equivalent samples, originally non-magnetic, developed magnetite/hematite-bearing mineral assemblages during decompression and cooling to 500–620 °C, 1.5–6 kbar. This recrystallization was probably assisted by metasomatic oxidizing fluids. The induced component of magnetization is essentially carried by magnetite, while hematite seems the main carrier of remanence. These results support the existing cartographic, geophysical and paleomagnetic findings indicating that the magnetization in the Lugo Dome is related to late-Variscan extension.

Comment on: The La Lucette Sb-Au-(W) vein-deposit (Armorican Massif, France): New time and genetic constraints to decipher the 310–295 Ma Sb-Au metallogenic peak in the Variscan Belt by

Recently, Cheval-Garabédian et al. (2023) published a paper in Ore Geology Reviews entitled “The La Lucette Sb-Au-(W) vein-deposit (Armorican Massif, France): new time and genetic constraints to decipher the 310–295 Ma Sb-Au metallogenic peak in the Variscan Belt.” Studies on Sb mineralisation are highly welcomed, especially in France, where there is a lack of updated data hindering a comprehensive understanding of this mineral system. The paper provides new mineralogical, fluid inclusion and geochronological data from historical collection samples of La Lucette mine, a mining site no more accessible since at least 40 years (Serment, 1978). These data, together with structural data from literature, lead the authors to build a metallogenic model related to dextral shear zone during the Carboniferous-Permian transition. The authors further argue for a single and unique 310–295 Ma Sb-Au metallogenic peak in the European Variscan Belt, a late Variscan hydrothermal event coeval with post-thickening tectonics (Munoz et al., 1992; Bouchot et al., 1997, 2005). This assertion is not correct. In this comment, we highlight that (i) several errors and approximations question the validity of their model, and (ii) the authors omit certain information that contradicts their metallogenic model. In our view, their data do not fully support the existence of a generalised and unique Late Carboniferous Sb-Au metallogenic peak in the French Variscan Belt. Furthermore, the idea defended by the authors that ore deposits occurred during dextral strike-slip tectonics that affected the area may better support a possible long-lived history during Palaeozoic times.

Critical analysis between the Ossa-Morena Zone and the Galicia-Trás-os-Montes Zone, Iberian Massif: Geochemical and geochronological comparison and geodynamic implications

Recent studies have suggested the existence of a correlation between the allochthonous units of NW Iberia (Galicia-Trás-os-Montes Zone - GTMZ) with SW Iberia (Ossa-Morena Zone – OMZ) in the European Variscan Belt. Such proposed equivalence is based on lithostratigraphic, tectono-metamorphic, geochronological, and geochemical affinities between these two domains, and implies that both zones had the same Neoproterozoic and early Palaeozoic basins, recording the same magmatic episodes and enduring the same tectono-metamorphic processes related to the Variscan Orogeny. As so, in order to better understand the evolution of these domains during the Ediacaran-Cambrian period, a comprehensive geochemical, isotopic and geochronological analysis of the metasediments that make up the OMZ and the allochthonous units of the GTMZ was carried out. The results show that the metasediments of OMZ and the Lower Allochthonous units of GTMZ present similar geochemical and geochronological features, having been likely deposited near each other, although small, but specific differences between both domains suggest that they do not correspond to the same depocentre. However, no geochemical, isotopic or geochronological correlation can be established with any of these domains and the Upper Allochthonous units of GTMZ. These metasediments display quite exotic features and, as such, were likely deposited in a different narrower basin, located to the W of the basins where OMZ and Lower Allochthonous units of GTMZ were deposited, thus providing precise constraints on the paleogeographic reconstruction of these domains during the Neoproterozoic to Early Ordovician dismembering of Gondwana.

Zircon U‐Pb provenance analysis of impact melt and target rocks from the Rochechouart impact structure, France

AbstractThe Rochechouart impact structure in the northwestern part of the French Massif Central (FMC) has a great diversity of impactites, including monomict impact breccias, suevite, and impact melt rocks (IMRs). The structure is strongly eroded, which allows the study of impactites of the crater fill and the transition into the crater floor. The FMC has had a multistage geological evolution from the late Neoproterozoic to the Ordovician (600–450 Ma) until the later stages of the Variscan orogeny (~300 Ma). Previous geochronological work on Rochechouart has been focused mainly on the impactites and constraining the impact age, and scarce work has been done on the FMC‐related target rocks. Here, U‐Pb isotope analysis by LA‐MC‐ICP‐MS has been conducted on zircon from two IMRs from the Recoudert and Montoume localities, and from a monzodiorite, a paragneiss, and two amphibolite samples of the basement to the impact structure. Zircon from the target rocks yielded mainly Neoproterozoic to Carboniferous ages (~924 to ~301 Ma) that can mostly be correlated to different stages of the geological evolution of the FMC. The monzodiorite also yielded a Permian age of 272 ± 12 Ma. Zircon from the IMRs, and especially from the Montoume sample, gave a comparatively higher diversity of Neoproterozoic to Jurassic ages (~552 to ~195 Ma). Provenance analysis for the zircon age populations of the impactites compared to those of the basement rocks shows overall poor correlation between the two age groups. This suggests that other target lithologies were involved in the formation of these impact melts as well. Post‐Variscan and preimpact ages (281–226 Ma) obtained for both melt rocks probably reflect a previously unconstrained event in the evolution of the regional geological history. Ages similar to the currently most widely accepted impact age of ~204–206 Ma were obtained from both IMR samples. In addition, the Montoume melt rock yielded several post‐204 Ma ages, which might reflect a to date unconstrained, about 194 Ma postimpact thermal/hydrothermal event.