Late Paleozoic convergence between Gondwana and Laurussia culminated in terminal collisions that produced the Ouachita-Alleghanian-Mauritanian-Variscan orogen within the interior of Pangea. The evolution and architecture of this orogen was profoundly influenced by a series of ca. 400-300 Ma promontory collisions, which terminated 100 m.y. of subduction and terrane accretion along the Laurussian margin and passive margin sedimentation along the Gondwanan margin. These promontory collisions compartmentalized the orogen into several domains with very different subsequent tectonic evolutions. In Europe, the Variscan belt records coeval collisional (e.g. Iberian massif) and “Mediterranean-style” orogens (e.g. Bohemian massif). The former are characterized by crustal thickening, followed by extensional collapse. The latter occur in re-entrants and are characterized by complex orogenic collages of limited lateral extent produced by the opening and closing of ephemeral oceans. This collage includes the products of subduction of varying polarities within these oceans and accretionary collisions of local significance that preceded terminal collision. Late-stage orogenic processes are characterized by the formation of oroclines, extensional collapse, and the transition to Tethyan tectonics. Because part of the Laurentian-Mauritanide domains were located to the southwest of the promontory collisions, remnants of the Rheic Ocean persisted between them and their respective evolutions, as recorded in the Appalachian belt, are dominated by Andean-style orogenesis that preceded terminal collision.
The geodynamic driver of Pangea amalgamation, by the Appalachian-Mauritanide-Variscan orogen, is consistent with the principles of orthoversion. In other collisional orogens, determining when geological continuity along converging continental margins gives way to compartmentalization may likewise document when promontory collisions have occurred.
How to cite: Murphy, J. B., Nance, R. D., Schulmann, K., Kuiper, Y., and Martínez Catalán, J. R.: Complex morphology of colliding margins in Laurussia-Gondwana supercollision, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5224, https://doi.org/10.5194/egusphere-egu25-5224, 2025.
The termination of the eastern Variscan belt has long been a topic of intense scientific debate due to its burial beneath extensive younger sedimentary cover. Competing hypotheses have sought to explain its geometry: the Variscan orocline model and the right-lateral strike-slip tectonics concept. To address this ambiguity, we compiled high-resolution gravity and magnetic anomaly maps spanning Czechia, Poland, and eastern Germany. These maps, coupled with geological and geophysical evidence, provide a robust framework to reassess the subsurface architecture and tectonic evolution of the region.
Our findings reveal a pronounced eastward deflection of the Rheno-Hercynian Suture. This structural trend takes a nearly 90° turn east of the Harz Mountains and extends south-eastward into Poland. This configuration supports the hypothesis of a semi-orocline that terminates abruptly against the Brunovistulian Block. The observed anomaly patterns, when integrated with geological evidence, point to a two-stage accretionary history in the eastern Variscan belt. The first stage involved W-E convergence during the early phases of Variscan orogeny. This process led to the development of NNE-SSW-trending structures, prominently preserved in the southern Bohemian Massif. These early tectonic fabrics were later overprinted during a subsequent, critical N-S shortening phase. This second stage reoriented the deformation patterns, producing WNW-ESE-trending structures that parallel the Baltica margin and dominate the region northeast of the Elbe Fault. Seismic imaging corroborates this structural interpretation, highlighting significant underthrusting of Baltica's crust beneath the Variscan belt at a distance exceeding 100 km.
The Variscan belt of Europe terminates in western Poland and Moravia, reaching the SW margin of Baltica and the western edge of the Brunovistulian Block. Although elements of the Variscan basement occur much farther east within the Carpathian belt, they cannot currently be correlated with the Variscan zones stretching between the Iberian Peninsula and western Poland. The presence of Variscides farther SE on the eastern side of the Brunovistulian Block is indicated by the direction of the Variscan deformation front running WNW-ESE up to the Ukrainian border. Particularly in SE Poland, Variscan shortening resulted in thin-skinned deformation of the EEC sedimentary cover.
How to cite: Mazur, S. and Schulmann, K.: Interpreting the eastern termination of the Variscan Belt: Insights from gravity, magnetics, and structural evolution, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13424, https://doi.org/10.5194/egusphere-egu25-13424, 2025.
Access to relatively rapid data acquisition techniques has led to detrital zircon geochronology becoming a routine and widely applied tool in provenance studies. These data, and their correlation, are applied both to paleographic reconstructions and the development of tectonic models. Nonetheless, the increasing proliferation of detrital zircon geochronological datasets and their haphazard integration into larger-scale correlations has led to a complex web of competing hypotheses and counter-hypotheses.
In order to formulate and test coherent hypotheses it is important to first establish a consistent framework in which these data can be properly assessed in both a temporal and geographical context. To this end, a database of U-Pb and Lu-Hf zircon isotopic data has been established from late Mesoproterozoic to late Paleozoic strata from Pangea-forming orogenic belts spanning from Atlantic North America through Europe, Northern Africa, the Middle East, and Central Asia to the Pacific Ocean. The original purpose of this database was to test correlations and various paleogeographical reconstructions in these regions during the transition from the Rodinia to Pangea supercontinent and a manuscript exploring these ideas and presenting the database is now published (Collett, 2025).
In this presentation, an extract from this database will be used to test several competing models on the pre-orogenic evolution of European Variscan Belt with specific focus on the Bohemian Massif. The Bohemian Massif is composed of four principal units, Saxothuringia, Teplá-Barrandia, Moldanubia, and Brunovistulia, which have in some tectonic models been considered to represent four distinct crustal blocks separated from one another by oceanic domains. Nonetheless, since oceanic domains should in theory act as barriers to the transportation of detritus and there are superficial similarities in detrital zircon spectra across these units; alternative models discarding one, or even all, of these oceanic domains have subsequently been proposed. However, the significance of these interpretations are hampered by either an incomplete or improper handling of the available data. In this presentation it will be demonstrated that detrital zircon data are actually supportive for, rather than an argument against, potential oceanic separation(s). This will be demonstrated by discussion of three key points: 1) The significance of Mesoproterozoic zircons in Brunovistulia, 2) the widespread occurrence of Stenian-Tonian age zircons in northern Gondwana and their distribution in the units of the Bohemian Massif, and 3) the relative abundance of Early Paleozoic zircons in northern Gondwana and the Bohemian Massif.
Collett, S. (2025). Detrital zircon tales between the Rodinia and Pangaea supercontinents; exploring connections between Avalonia, Cadomia and Central Asia. Journal of the Geological Society, 182(1), jgs2024-026.
How to cite: Collett, S.: Detrital zircon geochronology and the development of tectonic models for the Bohemian Massif, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5614, https://doi.org/10.5194/egusphere-egu25-5614, 2025.
The Lower Silesian Block outcrops in the NE part of the Bohemian Massif with its northeasternmost fragment, the Fore-Sudetic Block, buried under the Cenozoic sediments. The AMINV K-1 borehole drilled in 2013 and located in this area provides a unique insight into geology of the scarcely exposed part of Variscan crystalline basement. The borehole profile exhibits the metamorphosed volcano-sedimentary sequence composed mainly of quartz-rich schists, chlorite-schists and mica schists with garnet-rich layers covered with 100 m of Paleogene sediments.
In this study, we have focused on the metamorphic record of garnet-bearing mica schists. Petrological investigation conducted with use of electron microscopy (SEM, EMPA) reveals the following, interesting features recurring in many studied samples: 1) presence of chloritoid inclusions in garnet; 2) progressive zoning of garnet; 3) diverse composition of white mica ranging from phengite to muscovite. Thermodynamic modeling shows that mineral parageneses including i.e. chloritoid, garnet and phengite crystallized in the conditions corresponding to high-pressure low-temperature (HP-LT) metamorphism, followed by the stage of regional metamorphism, marked by the growth of i.e. muscovite, feldspar and biotite. P-T conditions of HP-LT stage may have reached up to 17 kbar and 550oC, while subsequent regional metamorphism most probably haven’t exceeded 10 kbar and 650oC. QuiG Raman elastobarometry and Zr-in-rutile thermometers has been used to evaluate the modeled P-T conditions. EMPA U-Pb monazite dating determined the average age of crystallization at 339±12 Ma based on 45 point analyses of 18 monazite grains.
How to cite: Bulcewicz, K., Sikora, R., Szczepański, J., Lenik, P., and Zieliński, G.: High-pressure low-temperature metamorphism recorded in mica schists from the central part of the Fore-Sudetic Block (NE margin of Bohemian Massif, SW Poland), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9177, https://doi.org/10.5194/egusphere-egu25-9177, 2025.
The Chamrousse ophiolite in the External Crystalline Massifs (Western Alps) has long been considered one of the oldest and best-preserved Variscan ophiolites (496±6 Ma; Menot et al., 1988). However, new in situ U–Pb zircon geochronology challenges the existence of a Cambro-Ordovician Ocean at Chamrousse.
Zircon from metabasite and metatrondjhemite unit, previously interpreted as the ophiolite volcanic layer, yields Cambro-Ordovician ages (460–520 Ma). The occurrence of few Proterozoic inherited grains and the trace element composition of zircon suggest a continental setting. In contrast, zircon from ultramafic, gabbro, and basaltic dike samples indicates a Devono-Carboniferous magmatic pulse (350–360 Ma). Their oceanic chemical signature suggests this age is that of ophiolite.
The Cambro-Ordovician base of the Chamrousse complex formed in a continental rifting environment within the northern Gondwana margin. The ophiolite itself likely represents a marginal basin developed in a back-arc setting, contemporaneously with other Devono-Carboniferous ophiolites of the Variscan belt.
How to cite: Gunia, M., Cordier, C., Janots, E., Vezinet, A., Milloud, V., Jacob, J.-B., and Guillot, S.: The Chamrousse ophiolite (Western Alps): a window of an oceanic stage at the onset of the Variscan orogeny, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16318, https://doi.org/10.5194/egusphere-egu25-16318, 2025.
In the Alps, numerous mafic, ultramafic, and sedimentary protoliths have been affected by Variscan metamorphism due to processes including subduction, collision, and late-orogenic extension, persisting until Early Permian times. Variscan eclogites derived from oceanic lithosphere are extensively documented, particularly in the External Crystalline Massifs (EMC) of the Western Alps (Helvetic-Dauphinois-Provençal domain). These eclogites are typically found as lenses and boudins enclosed within migmatitic gneiss and offer crucial insights into the location of a Variscan suture zone in the southern segment of the Variscan belt.
The Variscan basement of this massif consists predominantly of migmatitic metaintrusives and paragneisses, interspersed with mafic and ultramafic rocks that have been transposed into the migmatitic foliation. The metabasites are composed of amphibolites, eclogites, and granulites, while the ultramafic rocks occur as variably serpentinized peridotites and pyroxenites. These lithologies are often associated with calc-silicate lenses (clinopyroxene-, epidote-, and garnet-bearing) and marble layers.
This study presents new data from different rocks forming this high-temperature tectonic mélange within the Argentera-Mercantour Massif of the EMC, with a focus on lithostratigraphy, protolith origin, and metamorphic conditions. New lithostratigraphic, structural, geochemical, and petrological data are integrated with LA-ICP-MS U-Pb zircon dating, which reveals REE profiles indicative of crystallization under igneous to high-pressure metamorphic conditions. The eclogite protoliths display distinct geochemical affinities—sometimes oceanic in origin—and emplacement ages ranging from the late Cambrian to Silurian. In some samples, the prograde metamorphic evolution, progressing from prehnite-pumpellyite to eclogite facies, is evidenced by low-grade mineral inclusions within the cores of eclogitic garnet. However, omphacite is only locally preserved.
Petrological modeling and zircon-rutile Ti-Zr thermometry consistently indicate peak eclogite-facies conditions. The prograde path, transitioning from very low- to high-pressure conditions with a temperature-depth ratio of ≤10°C/km, suggests that these rocks were deformed and metamorphosed during oceanic subduction and subsequent continental collision. The occurrence of such a HT-tectonic mélange in the core of the Argentera-Mercantour Massif represents a portion of fragmented Variscan suture zone within the pre-Alpine crystalline basement of the Alps.
How to cite: Roda, M., Filippi, M., Spalla, I., Spaggiari, L., Volante, S., Lardeaux, J.-M., Tiepolo, M., Jouffray, F., Zanoni, D., and Gosso, G.: A high-temperature tectonic mélange marking the Variscan suture in the Argentera-Mercantour Massif of Western Alps, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18726, https://doi.org/10.5194/egusphere-egu25-18726, 2025.
Despite the fact that Earth has numerous high-P belts, relamination remains as a poorly studied process. We describe a continental subduction zone in which exhumed high-P rocks became a relaminant beneath an upper plate. This study focuses on the Variscan Orogeny (SW Iberia). A Variscan lithospheric-scale thrust transported deeper sections of a continental slab onto shallower ones during the Devonian. This piece of exhuming slab reached the base of the upper plate, which locked the high-P rocks' exhumation course through the subduction zone. At this point, the high-P rocks became a relaminant, moving away from the trench and beneath the upper plate. Relamination was achieved by a combination of synthetic and anti-thetic shearing in relation to the subduction polarity. These shear zones were coeval with the constriction of the subduction system as more buoyant lithosphere gradually entered the subduction zone. The combination of these processes produced large-scale recumbent folds, which affected the early thrusts and contributed to the relamination process by channeling subhorizontal flow during fold amplification.
How to cite: Díez Fernández, R., Moreno-Martín, D., Díez Montes, A., Rojo-Pérez, E., Novo-Fernández, I., Martín Parra, L. M., Matas, J., and Rubio Pascual, F. J.: Relamination in a Variscan subduction system: early structural evolution of the Central Unit (SW Iberia), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8199, https://doi.org/10.5194/egusphere-egu25-8199, 2025.
The Variscan orogen marks the late Devonian-Carboniferous collision of Gondwana and Laurrusia, forming Pangea. Throughout this orogen, extensive granitic plutons mark the tectonic and thermal processes behind its origin and development. Their study provides a unique opportunity to reconstruct the tectonic sequences, interpret the stages of crustal evolution, and assess the overall mechanics of complex lithospheric processes.
The Iberian Massif, located at Pangaea’s core, has one of the best exposures of the Variscan orogen in Europe, facilitating the study of deep-to-surface geodynamic phenomena. Ongoing research in the SW Iberian Massif (Ossa-Morena Zone), reveals a close relationship between the deformation, metamorphism, magmatism and sedimentary processes involved in deep to shallow lithospheric dynamics, during both orogenic thickening and gravitational collapse. Multiple magmatic stages mark these events, whose records include multiple granitic plutons (e.g.,[1], [2], [3]).
New data, obtained through the application of modern geological mapping techniques (geochemistry, geochronology and microtectonics) to the Figueira e Barros/Ervedal (307 Ma), and Fronteira (308 Ma) plutons indicate that these are calc-alkaline, aluminous, syn- to post-kinematic granitic intrusions, that cross-cut the Mississippian and Pennsylvanian heterogeneous fabrics formed under low metamorphic conditions (D2-M2 and D3-M3 stages) [4], probably formed in a volcanic arc environment. The surrounding lithologies are mostly composed of Devonian-Silurian(?) schists and slates, with olistoliths and volcanic rocks, probably part of a flysch synorogenic sequence. These units were affected by (i) contact metamorphism in the vicinity of the plutons, locally characterised by spotted mica-schists and cut by late Variscan-early Alpine(?) NW-SE/N-S faults; (ii) a Buchan type HT-LP metamorphism associated to the Ponte de Sor gneiss dome that enabled the blastesis of Mississippian syn-D2-M2 garnet, andalusite and staurolite.
This new data allows for a better definition of the regional sequence of events, and for a comparison between the studied plutons and the ones found in the bibliography (e.g., Benavila [1], Stª Eulália [2], Pavia [3]) which in turn constrains the regional conceptual model for the Variscan Orogeny.
This work is supported by the Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025 and LA/P/0068/2020 https://doi.org/10.54499/LA/P/0068/2020), and by the Spanish Ministerio de Ciencia e Innovación, Fondos Feder, PID2023-149105NA-I00. L.S.H. benefits from the FCT PhD scholarship UI/BD/154616/2023, I.D.S from the FCT research contract DL57/2016/CP1479/CT0030 (https://doi.org/10.54499/DL57/2016/CP1479/CT0030) and J.C.D. from FCT research contract CEECINST/00032/2018/CP1523/CT0002 (https://doi.org/10.54499/CEECINST/00032/2018/CP1523/CT0002).
[1] Canilho, M.H., 1992. Contribuição para o conhecimento petrográfico e geoquímico do maciço ígneo de Benavila (Avis). Ciências da Terra, 11, pp.1004–1018.
[2] Pereira, M.F., et al., C., 2017. Coeval interaction between magmas of contrasting composition (Late Carboniferous-Early Permian Santa Eulália-Monforte massif, Ossa-Morena Zone): field relationships and geochronological constraints. Geologica Acta, 15(4), pp.409–428. 10.1344/GeologicaActa2017.15.4.10
[3] Lima, S.M., et al., 2012. Dissecting complex magmatic processes: An in-depth U–Pb study of the Pavia pluton, Ossa–Morena Zone, Portugal. Journal of Petrology, 53(9), pp.1887–1911. https://doi.org/10.1093/petrology/egs037
[4] Dias da Silva, Í., et al., 2018. Time-space distribution of silicic plutonism in a gneiss dome of the Iberian Variscan Belt: The Évora Massif (Ossa-Morena Zone, Portugal). Tectonophysics, 747-748, 298-317. https://doi.org/10.1016/j.tecto.2018.10.015
How to cite: Steel Hart, L., Dias da Silva, Í., Cambeses, A., and Duarte, J. C.: Variscan plutonism in the Ossa-Morena Zone (SW Iberian Massif): The Mississippian and Pennsylvanian magmatism and its importance for the regional tectonic sequence of events, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-917, https://doi.org/10.5194/egusphere-egu25-917, 2025.
Variscan orogen witnesses in the internal Rif belt (northern Morocco): Metamorphic evolutions, ages and tectonic evolution during Pangaea amalgamation.
Corsini M. 1Lardeaux J.M.1,2, El Bakili1,3, Bosse V. 4, Homonnay E.1, Bosch D. 5 Chalouan A.3, Münch P.5, Ouazzani-Touhami, M.6
1- Observatoire de la Côte d’Azur, Géoazur, Université Côte d’Azur, IRD, CNRS, 250 rue Albert Einstein, Sophia Antipolis, 06560 Valbonne, France.
2- Czech Geological Survey, Centre for Lithospheric Research- Klárov 3, 118 21 Prague 1- Czech Republic.
3- Faculté des sciences Rabat - Université Mohammed V - 4 avenue Ibn Batouta,B.P. 1014 Rabat- Morocco.
4- Université Clermont Auvergne, Observatoire de Physique du Globe de Clermont Ferrand, Laboratoire Magmas et Volcans, 6 Avenue Blaise Pascal, 63178 Aubière, France.
5- Université Montpellier 2, Géosciences Montpellier, UMR 5243, CC 060, place Eugène Bataillon, 34095 Montpellier cedex 5, France.
6- Département de Géologie, Université Abdelmalek Esaadi, 93003 Tetouan, Morocco.
Since the late 1970s it is widely recognized that Variscan orogen witnesses are identifiable in the internal zones of the Rif–Betic orogenic system located in the peri-Mediterranean recent (i.e. Alpine cycle) mountain belts. The Rif belt (northern Morocco) is thus a polyorogenic system involving the superimposition of an Alpine cycle upon a previous Variscan cycle. Deciphering the Variscan palaeo-position and tectonic evolution of northern Morocco is therefore challenging and the link between the southern European Variscan belt and the Rif belt is still matters of debates.
We present and discuss the main results obtained, in the internal Rif, during more than 20 years of cooperation between Moroccan and French geologists. Our database integrates (1) high-resolution lithologic and tectonic mapping of the two zones recognized in the internal Rif (from top to bottom Ghomarides and Sebtides), (2) modern petrologic investigations (i.e. geothermobarometry combined with thermodynamic modelling) performed on relict phases and/or shielded mineral inclusions within large poikiloblasts preserved within alpine metamorphic assemblages in the Sebtides and (3) geochronologic investigations including (a) in situ U‒Th‒Pb dating of both frozen monazite inclusions in large sized garnets and monazites oriented in the main regional foliation from high-grade Beni-Bousera metapelites (Lower Sebtides) and (b) 40Ar–39Ar dating of white mica porphyroclasts from amphibolite facies metagreywackes recognized west of the Beni Bousera peridotite massif (Upper Sebtides) and from greenschist facies of the Ghomarides metapelites.
We document:
- The late Oligocene to early Miocene age of the MP granulite-facies metamorphism, responsible for crustal anatexis and melts production, coeval with the development of the main regional foliation depicted in the Lower Sebtides,
- the discovery, in the Upper Sebtides, of an amphibolite facies metamorphic event coeval with the Beni Bousera peridotite emplacement during the Upper Triassic in relation to the rifting of Pangea,
- the late Carboniferous–early Permian age of a HP granulite facies metamorphism (1.5–2.0 GPa for 760–820°C) recognized in the Beni Bousera metapelites. These conditions indicate a palaeogradient typically developed during crustal thickening in a collision belt.
These new constraints are confronted first with those recently obtained on the Betic orogen and the southern Europe Variscides and second to unified full plate reconstruction models in order to better understand the involvement of northern Morocco in Pangaea amalgamation.
How to cite: Lardeaux, J.-M.: Variscan orogen witnesses in the internal Rif belt (northern Morocco): Metamorphic evolutions, ages and tectonic evolution during Pangaea amalgamation., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6657, https://doi.org/10.5194/egusphere-egu25-6657, 2025.
Ophiolites and eclogite bearing units are important to reveal the tectonostratigraphy of orogenic belts as they define the position of suture zones. Especially an oceanic suture is a significant tectonic element as it separates former continental entities, which might have been far away from each other before oceanic closure and the subsequent continental collision. Conversely, the tectonic subdivision of an orogen should always reflect the former palaeogeographic relationships so that the 1st order tectonic units can be defined in a comprehensible manner.
In the southeastern part of the Bohemian Massif, ophiolite slices and eclogite occurrences are known since several decades. They are likely remnants of the oceanic space (Proto-Rheic and/or Rheic oceanic domain) that separated peri-Baltic (e.g. Avalonia) and peri-African (e.g. Armorican) crustal elements before the Variscan collisional nappe tectonics. However, due to a post nappe-stacking metamorphic overprint reaching granulite facies and anatexis many shear zones responsible for nappe stacking are strongly recrystallized and now difficult to identify.
The tectonic nomenclature most commonly used to this day in geological maps dates from 1927 and divides the area according to a metamorphic discontinuity into the low-grade to amphibolite facies Moravicum and the granulite facies Moldanubicum. The Moravicum is the southwestern continuation of the Brunovistulicum, which also includes parts of the Variscan foreland. In addition, the late to post-deformational South Bohemian Batholith is distinguished and the strongly anatectic Bavaricum in the southwest is optionally separated.
In the course of detailed mapping in the Danube valley between Stein and Spitz, former peri-Baltic and peri-African rock units could be clearly separated based on lithological criteria as well as an intervening oceanic suture zone. This tectonic subdivision can be tentatively extrapolated to the entire southeastern part of the Bohemian Massif. In map view, the oceanic suture is running southwest-northeast. In general, the peri-Baltic derived units form the footwall in the East, whereas the peri-African derived units in the West are overlying the oceanic suture. However, the tectonic style is complex with flower structures and out of sequence thrusts formed in a transpressional environment.
The presently used terminology Moldanubicum vs. Moravicum only partially reflects these palaeogeographical aspects. The peri-Baltic units build up the Moravicum, but also the Drosendorf Nappe System of the Moldanubicum. The ophiolites and sediments of the Variscan oceanic suture zone are part of the Gföhl Nappe System and the eclogites occur in the easternmost part of the Ostrong Nappe System. Both these nappe systems belong to the Moldanubicum according to the classic nomenclature. The remaining Moldanubicum most probably originates from Armorica, i.e., the African sector of Gondwana. In order to create a more logical nomenclature we suggest the following improvements. The term Moldanubian Unit should exclusively be used for rock units derived from the African sector of the Gondwana margin. The oceanic suture zone could be included in a separate 1st order unit (Raabs Unit). Nappes that consist of peri-Baltic rocks like the Drosendorf Nappe System should be affiliated with the Moravian and Brunovistulian units, whereas the Variscan foreland might be treated separately.
How to cite: Schuster, R., Ranftl, E.-M., and Finger, F.: Tectonostratigraphy of the southeastern part of the Bohemian Massif, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10392, https://doi.org/10.5194/egusphere-egu25-10392, 2025.
The western periphery of Baltica has been viewed as a passive continental margin formed during the fragmentation of Rodinia and the opening of the Iapetus and Tornquist Oceans. This view is supported by the Volyn Large Igneous Province (VLIP) of Ediacaran age in Eastern Europe, which may be associated with the opening of the Tornquist Ocean. However, the sedimentary succession overlying the VLIP in western Ukraine contains latest Ediacaran to early Cambrian detrital zircon with mixed ƐHf(t) values that can be interpreted to reflect deposition in a convergent margin setting. To further investigate this issue, we conducted research in the Holy Cross Mts. (HCM), Poland, where tightly folded and slightly metamorphosed middle Cambrian sandstone and slates are unconformably overlain by Lower Ordovician sedimentary rocks. We applied 40Ar/39Ar single grain fusion geochronology on white mica defining cleavage in lower Cambrian rocks and analysed detrital zircons to constrain their age and ƐHf(t) signature. Nine samples of shale and sandstone collected from the HCM showed consistency between stratigraphic age and the calculated maximum depositional age (MDA), ranging between c. 502-538 Ma. All samples have similar Proterozoic and Archean detrital zircon populations with major age peaks at c. 1200, 1500, 1800, and 2100 Ma, suggesting affinity with Baltica-associated sources. New U-Pb detrital zircon ages suggest that the HCM remained a coherent unit throughout the Cambrian and Early Ordovician. Importantly, ƐHf(t) signatures from Ediacaran-Cambrian detrital zircon of the HCM display a wide spread of values from -18 to +12. We interpret these results to reflect a continental magmatic arc setting, where there is significant mixing between mantle derived magmas and evolved crustal material. 40Ar/39Ar geochronology on white mica from one sample yielded two age populations. We interpret a group of ages between 537-640 Ma as detrital, while a younger group of ages yielded a weighted mean age of 510 ± 3 Ma (MSWD = 1.5). Interestingly, this younger age is corroborated by the presence of low-U zircon rims on detrital zircon from the same sample, which have an age of c. 510 Ma. We interpret this c. 510 Ma age population in both muscovite and zircon rims to record docking of a peri-Gondwana terrane, collision with a Baltica-derived terrane or the subduction of a seamount or oceanic plateau and shallowing of the down-going plate triggering deformation. With the present results, none of them can be ruled out. Mixed ƐHf(t) signatures and geochronologic evidence for deformation support the presence of an active margin at the periphery of southwestern Baltica during the Ediacaran and Cambrian. Furthermore, we suggest that this new 40Ar/39Ar geochronologic data may provide a new age constraint for early Gondwanan assembly.
This work was funded by the National Science Centre (Poland) project no. 2019/33/B/ST10/01728 to Majka.
How to cite: Callegari, R., Mazur, S., McClelland, W. C., Barnes, C., Kośmińska, K., and Majka, J.: Records of early Gondwana assembly on the southwestern Baltica margin: Insights from the Holy Cross Mts., Poland, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3726, https://doi.org/10.5194/egusphere-egu25-3726, 2025.
Variscan orogenic evolution was dominated by closure of the Rheic Ocean and its two successors, the Rhenohercynian and Paleotethys oceans. The Rheic subduction started in late Silurian – early Devonian at the margin of Laurussia but also along two Gondwana derived continental ribbons. Rapid mid-Devonian roll-back of peri-Laurentian subduction led to growth of the Rhenohercynian Ocean and migration of remaining subduction systems towards the margin of Gondwana. In the east, late Devonian subduction of the Rheic beneath Gondwana resulted in opening of Paleotethys Ocean and separation of a wide continental ribbon. Spreading of Paleotethys resulted in outboard early Carboniferous collision of three continental ribbons and welding of an elongated continental mass that finally collided with Laurussia in the north. Finally, Gondwana collided with the whole system in the west while ongoing subduction of Paleotethys in the east resulted in massive extension and melting of the Variscan crust.
How to cite: Schulmann, K., Martínez Catalán, J. R., and Schaltegger, U.: Variscan orogeny: a three ocean problem, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18445, https://doi.org/10.5194/egusphere-egu25-18445, 2025.
We investigated paragneisses from the Wyszki and Młynowiec Formations and mica schists of the Stronie Formation from the volcano-sedimentary successions of the Orlica-Śnieżnik Dome (OSD) in the Central Sudetes. Phase equilibria modeling and quartz-in-garnet elastic barometry were employed to directly compare the metamorphic histories of the western and eastern parts of the OSD using consistent methodologies. Our study is based on detailed analyses of nine samples, including three paragneisses and six mica schists.
The studied successions experienced three distinct metamorphic events, identified as M1, M2, and M3. Evidence of the earliest M1 event is fragmentary, including preserved rutile grains, albitic plagioclase, and phengitic white mica. According to pseudosections calculated for unfractionated bulk rock compositions, the mineral assemblage of the M1 event likely occurred under similar pressures in both lithologies, at approximately 13–16 kbar. The associated temperatures ranged from 440 to 470°C in the mica schists and approximately 510–530°C in the paragneisses.
In contrast, the M2 event is better preserved and characterized by garnet, oligoclase, muscovite, biotite, and, in some samples, staurolite, rutile, and ilmenite. The P-T history of this event was reconstructed using thermodynamic modeling of garnet zoning (accounting for variations in rock chemical composition due to garnet fractionation) combined with quartz-in-garnet elastic barometry. For the paragneisses, reconstructed P-T paths indicate an increase in pressure and temperature from approximately 9.5 kbar to 12 kbar and 540°C to 590°C. In contrast, the M2 event in the mica schists is characterized by significantly lower pressures (5–8 kbar) and temperatures of 510–570°C.
According to pseudosections calculated for fractionated bulk rock compositions for the end of garnet growth, the mineral assemblage stable during the final M3 event underwent at approximately 3.5 kbar and 530°C in all analyzed samples. These findings suggest that only the M3 event yielded consistent P-T conditions across the investigated samples.
Our findings suggest that the mica schists and paragneisses of the OSD may represent fragments of distinct tectonic units with contrasting metamorphic histories.
Acknowledgements: The study was financed by the NCN research grant UMO-2022/47/I/ST10/02504.
How to cite: Szczepanski, J. and Zhong, X.: Unraveling the Metamorphic Evolution of the Orlica-Śnieżnik Dome (Sudetes, NE Bohemian Massif) through Phase Diagram Modeling and Quartz-in-Garnet Barometry , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13108, https://doi.org/10.5194/egusphere-egu25-13108, 2025.
We present a new scheme for the tectono-metamorphic architecture of the Erzgebirge. While we follow the scheme of previous studies (Konopásek & Schulmann, 2005; Rötzler & Pleesen, 2010) we made significant changes based on our study of the distribution of high-pressure metamorphism. Our model consists from bottom to top of the following levels: (1) a Lower Gneiss Unit (LGU) that consists of gneisses with both Proterozoic and Ordovician protoliths and experienced amphibolite-facies peak metamorphism; (2) an Upper Gneiss Unit (UGU) that consists of a UHP-metamorphic subunit mainly made from Ordovician protoliths. This UHP unit is sandwiched between a mixed HP gneiss unit, which is dominated by Proterozoic meta-greywackes, but also containins bodies with Ordovician protolith ages. We observe HP eclogites throughout this entire gneiss matrix and do not support models of an eclogite free tectonic level within the UGU; (3) a Mica Schist Unit (MSU) that likewise consists of two subunits: firstly, a MSU sensu stricto made of Ordovician metasediments and rare amphibolites, which experienced only amphibolite-facies peak conditions. Secondly, an internal nappe, that consists of HP-gneisses and eclogites from various Ordovician protoliths. The structural position and the extent of this HP subunit, especially the relation to the HP rocks in the UGU remain enigmatic. We propose fundamental tectonic boundaries between all mentioned units and subunits in the area.
How to cite: Keseberg, M., Görz, I., Weber, S., and Nagel, T.: Tectonic Architecture of the Erzgebirge/Krušné Hory, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19223, https://doi.org/10.5194/egusphere-egu25-19223, 2025.
The Orlica-Snieznik Dome (OSD) is located in the northeastern part of the Bohemian Massif and is interpreted as a fragment of the Moldanubian zone within the Variscan orogen, representing part of the Variscan orogenic root. The dome is composed primarily of orthogneisses interspersed with metamorphosed volcano-sedimentary sequences. In the Snieznik Massif, which forms the eastern segment of the OSD, lenses of high- and ultrahigh-pressure (UHP) rocks, including granulites and eclogites, are embedded within the orthogneisses. This study investigates the metamorphic evolution of eclogites exposed in two specific areas of the Snieznik Massif: Nowa Wies and Bielice.
We distinguish two varieties among the examined eclogites: Ph-bearing and Ph-free eclogite. Both exhibit a typical metamorphic trajectory for UHP rocks, encompassing a UHP metamorphic event followed by isothermal decompression and subsequent retrogression under amphibolite-facies conditions. The samples are characterized by steeply dipping, subvertical foliation, defined by alternating garnet- and omphacite-rich layers and the parallel alignment of elongated grains of kyanite, rutile ± phengite. Evidence of isothermal decompression is observed in the form of small amphibole grains and diopside-amphibole-plagioclase symplectite, which occur locally along grain boundaries. The final metamorphic stage is marked by amphibole+plagioclase+zoisite/clinozoisite±margarite±tytanite, found within fractures that crosscut the primary foliation. This stage is associated with retrogression under amphibolite-facies conditions.
The UHP metamorphic event in the studied samples is reconstructed based on the results of thermodynamic modelling and the presence of coesite, identified as tiny (~10–20 µm) inclusions within omphacite and garnet. The well-preserved mineral assemblage indicative of UHP conditions includes garnet + omphacite + kyanite + rutile + coesite ± phengite. Phase diagram modeling combined with isopleth geothermobarometry indicates peak metamorphic conditions of approximately 3.0 GPa and 750°C. These findings are consistent with results from conventional geothermobarometry (Grt-Cpx-Ph-Ky-Coe geothermobarometer) and Zr-in-rutile thermometry. The onset of isothermal decompression is marked by the formation of small amphibole grains, indicating conditions of around 2.3 GPa at 750°C, within the stability field of amphibole.
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101005611 for Transnational Access conducted at Earth Sciences Department, University of Cambridge. The project was also supported by the Polish National Science Centre (UMO-2022/47/I/ST10/02504) and the Deutsche Forschunggemeinschaft (project Nr. 535198529).
How to cite: Nowak, M., Tajcmanova, L., Dabrowski, M., Buisman, I., Wallis, D., and Szczepanski, J.: The metamorphic history preserved in the UHP Snieznik eclogites (Sudetes, NE Bohemian Massif), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12665, https://doi.org/10.5194/egusphere-egu25-12665, 2025.
Pre-Alpine basement units, that originated along the northern Gondwana margin, were integrated into the Austroalpine Nappe System during Alpine nappe stacking. While many experienced significant Alpine overprinting, some units underwent only low-grade metamorphism, preserving valuable records of their pre-Alpine history. The Seckau Complex, part of the Silvretta-Seckau Nappe System, was subject to greenschist facies metamorphism during Eo-Alpine times and retains mineralogical assemblages related to Variscan or even pre-Variscan processes. To deepen the understanding of the pre-Alpine metamorphic evolution of the Eastern Alps, we apply petrological, geochronological, geochemical and geothermobarometric techniques to analyze in particular the metapelitic sections of this basement unit and reconstruct its tectonic and metamorphic history.
The Seckau Complex features diverse metagranitoids, including the Late Cambrian to Early Ordovician Hochreichart Plutonic Suite and the Late Devonian to Early Carboniferous Hintertal Plutonic Suite. These intrusions are hosted by the Glaneck Metamorphic Suite, which primarily comprises garnet-bearing paragneiss and mica-schist, along with amphibolite and tschermakite-bearing gneiss, the latter of potentially magmatic origin. U-Pb zircon dating of paragneisses reveals a detrital provenance with age clusters spanning in the Neoarchean, Paleoproterozoic and Ediacaran (2.7 Ga to 559 Ma). A migmatized paragneiss provided an age of 505 Ma, suggesting that migmatization was likely induced by the intrusion of the Hochreichart Plutonic Suite and indicating a pre-Variscan metamorphism between 559 Ma and 505 Ma (Mandl et al., 2018). Many metapelite samples exhibit a two-stage garnet growth with significant grossular enrichment towards the rims, pointing to a younger metamorphic event likely related to the Variscan orogeny. Results from geothermobarometry and thermodynamic modeling indicate an initial garnet growth at ~550°C and ~0.4–0.5 GPa and rim formation at ~570°C-620°C and ~1.1–1.2 GPa. Zr-in-rutile thermometry yields consistent temperatures of approx. 600°C. Results from monazite dating by EPMA of garnet-bearing mica-schists from the area of Eisenpass provide a weighted average U-Th-total Pb age of 64 ± 3 Ma, suggesting growth of metamorphic monazite during Alpine metamorphism. Garnet-bearing amphibolites show homogeneous garnet composition with subtle spessartine enrichment towards the core, indicating garnet growth along a pro-grade metamorphic path. Geothermobarometry and thermodynamic modeling indicate peak metamorphic conditions of ~620°C and ~0.7–0.8 GPa, again with consistent Zr-in-rutile temperatures of 600-620°C. Geochemical analyses of the metabasites reveal a tholeiitic differentiation trend derived from basaltic to andesitic protoliths. Trace element compositions suggest affinities to MORB and Within-Plate-Lava signatures.
How to cite: Karner-Ruehl, K., Kurz, W., Hauzenberger, C., and Fritz, H.: Pre-Alpine Metamorphic Evolution of the Seckau Complex: Insights from Alpine low-grade metapelitic and magmatic basement units of the Eastern Alps, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4793, https://doi.org/10.5194/egusphere-egu25-4793, 2025.
The Speik Complex, a meta-ophiolite predominantly exposed in the Gleinalpe region of the Eastern Alps, is part of the Austroalpine Silvretta-Seckau Nappe System and is primarily composed of mafic and ultramafic rocks. By applying petrological, geothermobarometric and geochronological techniques, this study tries to constrain the P-T-t path of the Speik Complex and reconstruct its tectono-metamorphic evolution.
The Speik Complex forms an E-W orientated belt in the Gleinalm and Stubalm mountain ridge and a SE-NW orientated belt at the southern side of the Seckau Mountains. The easternmost appearance of serpentinite is east of the Mur valley but the main ultramafic bodies occur near the village Traföß next to the river Mur, in the vicinity of the village Kraubath and at mountain Hochgrößen. The lithologies of the Speik Complex include serpentinite, abundant (garnet-)amphibolite and some occurrences of eclogite. Eclogites from two different sample localities within the Gleinalpe region are characterized by a mineral assemblage of garnet, clinopyroxene/omphacite, amphibole, zoisite, rutile and quartz. Garnets show homogeneous compositions with an increase in spessartine towards the core. Almandine, pyrope and grossular remain constant with compositional ranges of Alm47-55, Py4-6 and Grs38-41. Results from geothermobarometry and thermodynamic modeling of eclogites suggest peak metamorphic conditions of ~600-620°C and ~1.5–1.7 GPa. Geochemical analyses of eclogites and (garnet-bearing) amphibolites suggest a tholeiitic differentiation trend derived from basaltic to andesitic protoliths. Their trace element compositions show MORB affinities. Sm-Nd whole rock (WR) - garnet ages from garnet-bearing amphibolites sampled near Kraubath in the vicinity of exposed eclogites gave WR-garnet isochron ages of 406 ± 4 Ma and 413 ± 5 Ma. Age calculations including amphibole yield slightly older ages with larger errors. U-Pb zircon dating of a coarse-grained amphibolite vein crosscutting massive serpentinites within the nearby Preg quarry yields a weighted mean age of 395 ± 1.5 Ma, consistent with a previously published 40Ar/39Ar amphibole cooling age (397.3 ± 7.8 Ma) from eclogitic relics at Hochgrößen (Faryad et al., 2002). According to the available data the Speik Complex represents an oceanic suture which initiated in the Early Devonian prior to the Variscan continental collision.
SW Faryad F Melcher G Hoinkes J Puhl T Meisel W Frank (2002) Relics of eclogite-facies metamorphism in the Austroalpine basement, Hochgrössen (Speik Complex), Austria Mineral Petrol 74 49–73
How to cite: Hauzenberger, C., Karner-Rühl, K., Kurz, W., Fritz, H., Gallhofer, D., Schuster, R., and Mali, H.: Eclogites of the Speik Complex, Austria: Indicators of High-Pressure Metamorphism and an Early Variscan Subduction Zone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8913, https://doi.org/10.5194/egusphere-egu25-8913, 2025.
A shallow-mantle and deep crustal gravitational destabilization beneath orogenic zones is evidenced by lithospheric delamination. However, a drip-shaped delamination due to the increasing density has been suggested in the last few years as “dripping”. This dripping process is described at the surface as compressive tectonics associated with mantle-derived plutonic magmatism, followed by extensional tectonics and uplift of the continental crust accompanied by crustal-derived silicic volcanism. Dripping is documented in recent convergent contexts, but also could be considered in old orogenies.
Overall, geology of the Morvan basement (NE Massif Central) is formed by metamorphic rocks intruded by Variscan metaluminous and peraluminous granites emplaced at 350-320 Ma. Some volcanic occurrences are outcropping, especially 1) the basic calk-alkaline Somme Arc sequence related to the Devonian subduction-related magmatic arc, and 2) the “Faisceau Dévono-Dinantien” (FDD) Carboniferous silicic sequences, which are associated with extensional tectonics, but also the Carboniferous Blismes-Montreuillon volcanic complex (BMVC) and Sincey-les-Rouvray faulted zone. Additionally, in the northern part, Pierrot et al. (under review) studied basic plutonism with mantle components (vaugnerites) emplaced at 335-325 Ma in the metamorphic basement.
We conducted new petrographic, geochemical (major and trace elements) and geochronological (U-Pb on zircon) studies on the Carboniferous volcanic rocks of Morvan. U-Pb data on zircon crystals give emplacement ages in the 332-323 Ma range with also inherited Devonian to Proterozoic apparent ages. These silicic volcanic rocks display classical mineralogy (Ab + Qz + Bt + F-K) but advanced petrographic observations revealed the presence of mineralogical aggregates of granitic origin, as well as garnet in FDD, and amphibole in the BMVC. Geochemical data show that silicic rocks are K-rich calk-alkaline to alkaline peraluminous dacites and rhyolites. All these results suggest that basic plutonism with mantle-derived components was immediately followed by a crustal-origin silicic volcanism, all occurring during 10 to 15 Ma. Therefore, a typical lithospheric delamination could not explain the lack of basic volcanism, the extensional tectonics of the FDD at the climax of the Variscan orogeny, and the chronological succession of these processes. On the contrary, dripping process explains that during the drip formation, partial melting of mantle can occur by adiabatic upwelling but does not produce volcanism due to local compressive tectonics triggered at the surface. After the drip detachment, the asthenospheric upwelling would trigger HT conditions beneath the crust, leading to partial crustal melting, which could manifest like both plutonism and volcanism with silicic composition, facilitated by favorable surface extensional tectonics.
How to cite: Pierrot, H., Pallares, C., Poujol, M., Gotti, V., and Barbarand, J.: Lithospheric dripping during Variscan orogeny in the NE French Massif Central: evidence from Visean-Serpukhovian silicic volcanism in the Morvan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20512, https://doi.org/10.5194/egusphere-egu25-20512, 2025.
Post-peak-pressure P–T paths of high-pressure units provide important constraints about the processes of exhumation and orogenic building. An isothermal decompression followed by cooling was proposed for high-pressure unit of the Variscan French Massif Central using secondary assemblage in partially retrogressed eclogite. Yet, this path is poorly constrained due to the localised and partial character of retrograde equilibria. While the peak eclogite facies conditions (20–25 kbar, 850–900 °C) were determined by previous work, the present study of a mafic granulite from the same unit provides further details about the subsequent P–T evolution.
The studied sample consists of pristine high-pressure granulite facies domains of garnet–diopside–plagioclase grading to domains where amphibole is common in replacement textures. Ti-bearing accessory minerals are rutile, titanite or ilmenite. Rutile is included in garnet, plagioclase and titanite whereas titanite and ilmenite occur in the matrix. Titanite is commonly texturally related to amphibole suggesting the introduction of a fluid. Titanite is partially or totally replaced by vermicular ilmenite. The observations constrain the sequential replacement of rutile by titanite followed by the replacement of titanite by ilmenite.
Phase equilibrium modelling indicates that the peak high-pressure granulite facies assemblage, mineral chemistry and proportions are best reproduced around 10–15 kbar and 800–1000 °C. Since zircons was not identified in the rock the result of Zr-in-rutile thermometry only indicates a minimum temperature of ~ 680 °C. Modelling the influence of H2O on the equilibrium assemblage shows that amphibole and titanite were associated to incomplete hydration during an external fluid influx. Titanite stability is modelled at T < 800 °C in a range of pressure of 8–15 kbar, suggesting the replacement of rutile by titanite during cooling and limited decompression. On the other hand, ilmenite is modelled at lower pressure, under 7–8 kbar, suggesting a subsequent decompression along steeper P–T path.
Petrological data and P–T modelling suggest three metamorphic stages during the exhumation: 1) decompression from the eclogite (20–25 kbar, 850–900 °C) to the granulite facies (10–15 kbar, 800–1000 °C); 2) cooling under 800 °C with limited decompression; and 3) steeper decompression below 8 kbar. Contrary to what was suggested previously, this sequence point to at least two main decompression stages separated by cooling. This sequence is compatible with exhumation from mantle to crustal depth followed by partial cooling in the lower orogenic crust and subsequent crustal thinning or redistribution within the crust.
How to cite: de Hoÿm de Marien, L., Štípská, P., and Pitra, P.: Step-by-step exhumation of a high-pressure granulite revealed by sequential replacement of Ti-oxides (Variscan French Massif Central), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10836, https://doi.org/10.5194/egusphere-egu25-10836, 2025.
Paleozoic European crust – i.e., crust formed or reworked before the Permian and the onset of the Alpine orogenic cycle, is exposed in several massifs in the Briançonnais domain of the Western Alps. These Briançonnais crustal rocks (or 'basement', hereafter) show many similarities with other basement rocks accross Europe, particularly those affected by the Variscan orogeny. While the successive Variscan tectono-metamorphic events have been studied in many European basement massifs to unravel the orogenic and paleogreographic organization of Europe during the Paleozoic, not much has been done in comparison on the Briançonnais basement rocks.
Though partly transposed and re-equilibrated during the Meso-Cenozoic Alpine cycle, older fabrics and mineral remnants enable reconstructing part of their pre-Permian history. Our study is focused on three basement massifs of the Briançonnais domain, namely the Ambin, Vanoise and Ruitor massifs. These massifs are largely made of Cambrian to pre-Cambrian metasedimentary units cut by intrusives with variable chemical affinities, interpreted as related to the Cambro-Ordovician bimodal volcanism event. We herein present preliminary results on the (1) lithostratigraphy of this basement in the three different areas, (2) zircon U/Pb geochronology of key formations (e.g., metasediments and intrusives) and (3) the structuration of this basement crust despite the strong Alpine imprint.
How to cite: Ayatti, M., Plunder, A., Agard, P., Poujol, M., Cogné, N., and Bonnet, G.: Pre-Alpine structuration of the Briançonnais basement: lithostratigraphy, geochronology and tectono-metamorphic reconstruction of three Alpine massifs. , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18123, https://doi.org/10.5194/egusphere-egu25-18123, 2025.
The Chamrousse ophiolite is located in the external crystalline massifs of the Western Alps and has long been considered one of the best-preserved Cambro-Ordovician ophiolites of the Variscan belt. However, recent in situ U–Pb zircon dating indicates that the Chamrousse ultramafic-mafic complex consists of a Cambro-Ordovician continental basement unit (peak ages around 490Ma) and a Devono-Carboniferous ophiolitic sequence (350-360 Ma). In this study, we present new in situ U–Pb apatite ages and trace element compositions from the Chamrousse complex. In the Cambro-Ordovician unit, apatite ages (350 Ma), combined with rare earth element (REE) compositions, indicate that apatite crystallization is metamorphic. In contrast, the ages (350 Ma) and REE compositions of apatite in the Devono-Carboniferous mafic and ultramafic rocks suggest a magmatic origin. These results highlight how apatite can serve as an effective petrogenetic tool to constrain and distinguish the magmatic and tectono-metamorphic evolution of mafic and ultramafic units in the Variscan basement.
How to cite: Janots, E., Gunia, M., Cordier, C., Vezinet, A., and Leger, J.: In situ U–Pb dating and trace element composition of apatite in the Chamrousse ophiolite (Western Alps), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18404, https://doi.org/10.5194/egusphere-egu25-18404, 2025.
The Lesser Kabylia massif, situated within the internal zone of the Alpine Algerian Tell in the Maghrebides, contains a metamorphic basement of uncertain Paleozoic age. Despite the negligible tectono-metamorphic Alpine overprint, the pre-Alpine history of the basement is poorly known. Therefore, to fill the gap, we carried a petrological and LA-ICP-MS zircon U-Pb dating study in various rock types of this basement.
The metamorphic basement of Lesser Kabylia is divided into two units: (1) a lower-crustal unit characterized by high-grade metamorphism and (2) an upper-crustal unit composed of low-grade to non-metamorphic rocks, including strata from Cambrian-Ordovician to Silurian-Devonian ages. This structural arrangement is comparable to other internal zones of the Maghrebide belt and Betic cordilleras. However, the Lesser Kabylia metamorphic basement exhibits a more complex structure. It is further subdivided into the Kerekera nappe thrust over the Beni Ferguen nappe.
Near the Texenna village, where the basement is part of the Kerkera nappe, the high-grade metamorphic lower-crustal unit is dominated by felsic migmatites enclosing lenses of mafic to ultramafic granulites. The felsic migmatites are commonly composed of Grt–Pl–Ksp–Qtz–Bt, locally also with sillimanite and spinel. The mafic granulites are composed of Opx–Cpx–Amp–Pl–Qtz–Ilm, locally with spinel and/or biotite. Pseudosection modeling using Perple_X software has been done on a felsic migmatite containing garnet and sillimanite, and for a mafic granulite, yielding peak P-T conditions of 8–6 kbar and ∼725 °C, followed by cooling with slight decompression. Zircon U–Pb dating by LA–ICP–MS revealed predominant Permian dates of 266–295 Ma, interpreted as the age of the high-grade metamorphism. It is not clear whether the Carboniferous dates in the range of 300–320 Ma have geological meaning. One granitic leucosome sample reveals a prominent Permian zircon population, potentially indicating coeval migmatization of the lower crust with the emplacement of the nearby granites (e.g., the Collo granite).
Our findings suggest that the basement of the Lesser Kabylia was affected by Variscan medium-pressure, high-temperature metamorphism, which may have resulted from the closure of the Paleo-Tethys Ocean or its intracontinental propagator near the edge of Gondwana and at the southern part of the European Variscan belt, sealed during Pangea formation.
How to cite: Bouadani, C., Chopin, F., Stipska, P., Bendaoud, A., Fettous, E.-H., Schulmann, K., Miková, J., and Bouzekria, N.: The Lesser Kabylia metamorphic basement: Unraveling pre-Alpine history through petrological and geochronological studies (Texenna, Algeria), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17489, https://doi.org/10.5194/egusphere-egu25-17489, 2025.
