Gneisses, granites and migmatites of the Central Alpine basement (Aar Massif and Gotthard Nappe, Helvetic Zone of the Swiss Alps) record a long-lasting geological history over several hundred million years. This complex history is resolved through detailed zircon U-Pb geochronology and Hf isotope analysis:

(1) Inherited cores in zircon record a 750-550 Ma old orogenic and magmatic history during Rodinia disintegration and Gondwana amalgamation. The cores reflect zircon crystallization during the Pan-African and Cadomian orogenies with the involvement of cratonic and oceanic materials, leading to scattering initial epsilon Hf values of +10 to -15. (2) The Cenerian orogeny caused widespread melting of sedimentary wedge material consisting of this Pan-African and Cadomian siliciclastic detritus during a phase of lithospheric thinning along an accretionary continental margin. Large volumes of anatectic melts formed at ca. 460-440 Ma, which occur today as gneisses, migmatites and metagranites. The Hf isotope systematics of the detrital precursor zircon was recycled into the new magmatic zircon and homogenized. (3) Some of the quartz-feldspar rich Ordovician-age migmatites and granites were remelted at a late stage of the Variscan orogeny at around 315 Ma, facilitated by addition of several volume percent of water to a near-minimum melt quartz-feldspar composition. The resulting anatectic melts formed heterogeneous granite bodies with diffuse borders. Newly grown U-rich zircon rims around older zircon again recycled and somewhat homogenized the initial epsilon Hf composition of partially consumed previous zircon generations. This generation of anatectic granites is coeval with more deeply sourced intrusive suites at 335 and 300 Ma. High-temperature metamorphism and magmatism are explained by late-orogenic lithospheric thinning in the back-arc area of the retreating Paleotethys subduction. (4) Alpine deformation in greenschist facies at around 25 Ma partly reactivated existing structures and led to low-temperature hydrous alteration of previous mineral assemblages.

The new data confirm existing hypotheses that the Variscan orogeny mainly recycled fertile igneous protoliths of early to late Ordovician age, which ultimately originated to a overwhelming extent from the melting of Neoproterozoic and Cambrian siliciclastic orogenic detritus. The Variscan orogeny is thus characterized by abundant crustal recycling and little juvenile addition.

How to cite: Schaltegger, U., Berger, A., Noroña Muñoz, E., Gerdes, A., Abrecht, J., and Wiederkehr, M.: Repeated recycling of sedimentary continental margin sequences during extensional and contractional orogenic episodes (Cenerian and Variscan orogenic cycles, Central Alps), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4548, https://doi.org/10.5194/egusphere-egu26-4548, 2026.

The Rheic ocean is one of the many oceanic basins inferred to form part of the intricate puzzle of continents and microcontinents in the Paleozoic. It opened in the Early Ordovician, separating the microcontinent Avalonia from Gondwana, and subsequently closed in the late Paleozoic with the amalgamation of the supercontinent Pangea, playing a major role in the Variscan orogeny. The existence of the Rheic ocean is accepted and required in plate reconstructions. However, its actual width, along-strike length, and relationships with other oceans and seaways (e.g., the Rhenohercynian, Galicia-Moldanubian, Saxo-Thuringian oceans) are unconstrained and controversial.

To address these issues, we perform a detailed review of available data on lithostratigraphy, magmatism, geochronology, geochemistry, structural geology, and metamorphism of tectonostratigraphic units in Iberia and the British Isles, where the Variscan belt comprises accreted units of Gondwana, Avalonia and their intervening ocean(s). We compile these datasets in orogenic architecture diagrams, with the aim of objectively assessing the current state of knowledge on the paleogeographic limits and evolution of the Rheic ocean and on the nature and continuity of its suture. Through our preliminary compilation, we identify what data constitutes solid evidence for the existence of the Rheic ocean and whether gaps in the current knowledge exist that have been filled by interpretative work, and discuss tectonic implications and potential paths forward.

How to cite: Maremmani, A., Pastor-Galán, D., and Negredo, A.: Solving the western European Rheic Puzzle Through Orogenic Architecture Diagrams, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-489, https://doi.org/10.5194/egusphere-egu26-489, 2026.

Lithogeochemical and Sr-Nd-Pb isotope data obtained on gabbro, metagabbro, amphibolite, garnet amphibolite and diorite from the Palaeozoic Odenwald basement, Mid-German Crystalline Rise, Germany, show that their protolithic melts formed from different mantle source regions and were emplaced in different tectonic environments. Four geochemically different rock groups can be distinguished. The calc-alkaline Group I (mostly gabbro and amphibolite) and Group IV rocks (diorite) have low TiO₂ and high to intermediate Mg#, whereas the tholeiitic Group II and III rocks (predominantly garnet amphibolite) have intermediate to very high TiO₂ and low to intermediate Mg#. The Group I and II rocks have N- to E-MORB affinities, with the N-MORB type rocks having depleted Nd isotope compositions of e_{Nd, initial} =4.5-7.7. The precursor melts of all Group I and II rocks formed by partial melting in the shallow depleted mid-ocean ridge mantle and were emplaced in a divergent setting, possibly in a back-arc environment. Group III garnet amphibolite is strongly enriched in TiO₂, FeO^total and V (TiO₂ of up to 4 wt. % and FeO^total ranging from 14.4-17.6 wt. %). The parental melts of these high Ti-Fe rocks formed most likely by low-degree melting from a deep-seated, fractionated magma source. We propose that the melts were generated in an extensional setting, possibly in a continental rift environment during incipient rifting. The protolithic melts of the Group IV diorite formed by partial melting in the subcontinental lithospheric mantle in a supra-subduction setting (mature volcanic arc). The chemical features of the diorite are virtually identical to those of 340 Ma old western Odenwald and Spessart diorite. Thus, we propose that all diorite from the Spessart-Odenwald basement are part of one coherent intrusion that underlies the whole area. We think it likely that diorite formation was related to the presence of a mantle plume, which was also responsible for the widespread late Carboniferous magmatism and the associated high-temperature metamorphism in the Mid-German Crystalline Rise and other areas of the Variscan orogen. Most likely, this marks the beginning of lithospheric extension in the central European Variscides and may correlate with the incipient break-up of Pangaea.

How to cite: Will, T. and Schmädicke, E.: Mantle sources of Palaeozoic mafic rocks from the eastern Odenwald basement, Mid-German Crystalline Rise, Germany, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3387, https://doi.org/10.5194/egusphere-egu26-3387, 2026.

The Austroalpine nappe stack includes basement units with a partly polyphase pre-Alpine history that were part of the northern Gondwana margin before the Variscan orogeny. Although Alpine metamorphism strongly overprinted many of these units, parts of the Silvretta-Seckau Nappe System preserve a complex record of earlier metamorphic events. This study reconstructs the pressure-temperature-time (P-T-t) evolution of the Seckau and Speik complexes, two key elements of the Silvretta-Seckau Nappe System, to refine models for the tectonic evolution of the Eastern Alps.

Metapelitic rocks of the Seckau Complex (Glaneck Metamorphic Suite) document a polyphase metamorphic evolution. Garnet textures and compositions record two distinct growth stages. Early garnet nucleation occurred at approximately 550°C and 0.4-0.5 GPa, followed by rim growth at higher pressures (1.1-1.4 GPa) and temperatures of 570-620°C, the latter being characteristic for conditions of Eo-Alpine metamorphism. Zr-in-rutile thermometry consistently yields temperatures around 600°C, corroborating these estimates. Locally, EPMA monazite ages of metapelites in the range of ~68-64 Ma indicate Late Cretaceous metamorphic overprinting, suggesting a tectonic affinity of parts of the Seckau Complex with the adjacent Koralpe-Wölz Nappe System. The Glaneck Metamorphic Suite is associated with plutonic suites, that are related to magmatic episodes from the late Cambrian-Early Ordovician (Mandl et al., 2018) through early Carboniferous to the late Permian, as constrained by U-Pb zircon ages from calc-alkaline and predominantly peraluminous metagranitoids with I- to S-type characteristics.

In contrast, the Speik Complex preserves evidence of high-pressure metamorphism related to Early Devonian oceanic subduction. This ophiolitic unit comprises serpentinized ultramafics, (garnet-) amphibolites, rare eclogites, and subordinate gneisses and marbles. Eclogites contain garnet, omphacite/clinopyroxene, amphibole and zoisite, typical of high-pressure metamorphism. Garnet textures show homogeneous compositions with spessartine-rich cores, while others display two-stage growth with rims having higher grossular and pyrope contents. Geothermobarometry and thermodynamic modelling indicate peak conditions of 600-650°C at 1.3-2.0 GPa. Whole-rock geochemistry shows a tholeiitic trend, with dominantly MORB, but also arc-related affinities, confirming an oceanic protolith. U-Pb zircon ages from metabasaltic dikes within serpentinite (403-395 Ma), together with Sm-Nd garnet-whole rock isochrons on amphibolite (413-406 Ma) and ⁴⁰Ar/³⁹Ar amphibole cooling ages of ~397 Ma (Faryad et al., 2002) constrain high-pressure metamorphism in the Early Devonian, preceding Variscan continental collision. Metagranitoids of the Speik Complex yield late Cambrian (503-493 Ma) ages, consistent with recently published ages from Guan et al. (2025). However, a metagranitoid sample from the same area yields a middle Permian age of 272.3 ± 3.2 Ma.

Together, these results indicate that the Seckau Complex preserves a polyphase metamorphic history from pre-Variscan to Alpine times, whereas the Speik Complex represents remnants of oceanic lithosphere as part of an Early Devonian suture zone, related to subduction of an oceanic basin that formed along the northern Gondwana margin (Neubauer et al., 2022; Finger & Riegler, 2023). Their combined P-T-t paths highlight a complex mosaic of continental and oceanic domains later assembled during Alpine orogeny.

How to cite: Karner-Ruehl, K., Kurz, W., Christoph A., H., Harald, F., Daniela, G., Ralf, S., and Heinrich, M.: Polyphase metamorphism of Austroalpine basement units in the Eastern Alps: hints to Early Devonian subduction, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6494, https://doi.org/10.5194/egusphere-egu26-6494, 2026.

The main Variscan phase in Northwest Africa occurred in the late Carboniferous-Permian, which is a late event compared to Western Europe. Yet, an early tectono-metamorphic event is recorded in so-called “Eovariscan” outcrops, mainly in Morocco. In spite of the debate that exists on the tectonic meaning of the Eovariscan event, its P-T conditions and timing are still poorly precised. In only one case are LP-HT conditions recognized and estimated (High Moulouya, Morocco) at 2–4 kbar and 450–650 °C (Filali et al., 1999).

The Mekkam inlier (Northeast Morocco) shows Eovariscan deformation affecting Upper Devonian rocks, sealed below unconformable late Visean sedimentary rocks. This deformation overprints inherited metamorphic cordierite and biotite, originally formed during contact metamorphism due to a granodiorite intrusion. The P-T conditions of the deformation have been evaluated through the use of classical metamorphic petrology in addition with Raman Spectroscopy on Carbonaceous Matter for independent temperature estimates. These conditions were then compared to the P-T conditions of emplacement of the granodiorite, determined using the Al-in-amphibole geobarometer (Mutch et al., 2016) and the Holland & Blundy (1994) amphibole-plagioclase geothermometer. P-T conditions for both the granodiorite emplacement and the cordierite-bearing mica schists largely overlap those recorded in the High Moulouya inlier. At last, zircon U-Pb dating on the granodiorite and a late leucogranite have been carried out, whose results are used in order to precise the chronology of events in the Mekkam inlier.

The P-T conditions do not support a compressive tectonic context and are more consistent with an extensional one. Our new data confirm the peculiarity of the Eovariscan event in Northwest Africa, which is significantly distinct from the late Carboniferous-Cisuralian Variscan phase. The classical Eovariscan compressional context must be significantly modified because it cannot account for our results and suggest that Northwest Africa behaved in a different manner than Western Europe at the same time. A generalized early Carboniferous rifting context is more suitable to explain our results and data from literature. This could be related to the opening of the Paleotethys, whose influence would be effective as far as northern Morocco and northern Algeria.

How to cite: Leprêtre, R., El Houicha, M., and Chopin, F.: The Eovariscan in the Northwest Africa Variscan belt, a key to Paleozoic Africa-Europe connexions: Example of the Mekkam inlier, Morocco, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10151, https://doi.org/10.5194/egusphere-egu26-10151, 2026.

The Scandinavian Caledonides consist of a stack of thrust nappes emplaced during the Caledonian Orogeny. The Upper Allochthon of the Caledonides in Norway and Sweden is dominated by Iapetus derived rocks of the Köli Nappe Complex (KNC), which is traditionally separated into the Lower, Middle, and Upper KNC. In the Hammaren-Stáddátjåhkkå region, located to the North of the Sulitjelma ophiolite, the Middle KNC is composed of metasedimentary rocks of Cryogenian to early Ordovician age (Stephens et al. 1985), intruded by various igneous rocks including gabbros, trondhjemites and diabase dikes of unknown age.

Hereby we report new geochemical and geochronological results from three adakite samples, previously believed to be trondhjemites, from the region, and reveal unusually old magmatic ages within zircon grains. Collected samples were originally mapped as trondhjemite (Thelander 2009). However, bulk-rock geochemical data suggests that two of the samples are high-silica adakites related to a supra-subduction environment, which formed on an active continental margin or intra-oceanic arc, and the third is an adakite-like trachyandesite with the geochemical signature of a subduction-related environment. 

The absence of an Eu anomaly in zircon trace element patterns indicates that the source of melt was feldspar-free, while the low Ce anomaly suggests reducing conditions during melt formation. Such features also corroborate the thesis that the melt was derived from eclogitized oceanic crust in a subduction environment. In each sample, 14 zircons were analysed for 206Pb/U238 dating, and the calculated concordia ages are 549.3 ± 2.4 Ma (n=8), 551.9 ± 1.7 (n=13), and 559.8 ± 2.8 Ma (n=5), respectively. 

Both the geochemical signatures and the age of the adakites are quite rare in the Caledonides. Similar ages were only reported from the Seiland Igneous Province, however, they are believed to have formed in extensional settings. Regarding the age of the Northern branch of Iapetus opening (starting c. 590 Ma), it is highly improbable to develop a subduction zone in such a short time.  Thus, we claim the Middle KNC of the Hammaren-Stáddátjåhkkå area to be of exotic, possibly Timanian origin. However, the possibility that Iapetus was “infected” with early subduction, by a process similar to that described by Waldron et al. (2014), cannot be excluded.

This study underlines the importance of geochronological work on igneous and sedimentary rocks from the Hammaren area, which is emerging as a key locality to yield novel insights about the origin of the Iapetus terranes of the Northern Caledonides. 

Stephens, M.B., Furnes, H., Robins, B. and Sturt, B.A. 1985a. Igneous activity within the Scandinavian Caledonides. In: Gee, D. G. and Sturt, B. A. (eds) The Caledonide Orogen – Scandinavia and Related Areas, pp. 623–656.

Thelander, T., 2009: Berggrundskartan Kaledoniderna i norra Sverige, skala 1:250 000. Södra delen. Sveriges geologiska undersökning K 222:2.

Waldron J.W.F., Schofield D.I., Murphy J.B., Thomas C.W., 2014. How was the Iapetus Ocean infected with subduction? Geology  42 (12): 1095–1098.

How to cite: Gitter-Dentz, G., Walczak, K., Cuthbert, S., Greczyński, K., Carter, I., and Sláma, J.:  Late Ediacaran adakites from Middle Köli Nappe Complex in Northern Caledonides of Sweden, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13688, https://doi.org/10.5194/egusphere-egu26-13688, 2026.

The Scandinavian Caledonides formed as a result of plate convergence, closing of the Iapetus Ocean, and continental collision between Baltica and Laurentia. The orogen is composed of allochthonous units, situated on top of the autochthonous basement of Baltica. The Köli Nappe Complex (KNC) of the Upper Allochthon and Seve Nappe Complex (SNC) of the Middle Allochthon of the Scandinavian Caledonides represent a transition between the oceanic terranes of the Iapetus Ocean and Baltica’s outer margin, respectively. Located at the interface between the KNC and the SNC is the Bunnerviken soapstone quarry in Handöl (west-central Jämtland, Sweden), interpreted as mélange-like lithology (Bergman, 1993) containing abundant lithic fragments. The suite of fragments is commonly deformed and reworked together with the host rock, showing a range of textural and compositional characteristics. Up to five groups are currently recognized, possibly reflecting different origins.

This preliminary study focuses on the characterization of the lithic fragments within the soapstone. Special emphasis is put on a single sample representing heavily altered amphibolite with an albite + amphibole + chlorite + calcite + titanite + Fe-Ti phase assemblage. Amphibole and albite are widespread throughout all zones of this lithology, suggesting they are a primary mineral assemblage. However, chemical zoning in amphibole and euhedral to subhedral titanite, dominantly associated with chlorite + calcite, indicates alteration and metamorphic record. U–Pb geochronology of titanite reveals a young, post-Caledonian lower-intercept age of 382 ±10 Ma.

The obtained age is younger than the Scandian collisional phase of the Caledonian orogeny. However, extensional, post-orogenic collapse of the orogen offers an alternative explanation. Normal faulting, thinning of the crust and the development of a post-orogenic metamorphic core complexes (Fossen et al. 2024) in the area could explain intense, prolonged heating, resulting in re-opening of the U-Pb system and the recorded post-Caledonian, Middle to Late Devonian age. The record from Bunnerviken quarry is consistent with earlier local observations by Sjöström et al. (1991) on the Röragen Detachment and may offer additional evidence for post-collisional evolution of the Caledonian allochthons.

References

Bergman, S. (1993). Geology and geochemistry of mafic-ultramafic rocks (Köli) in the Handöl area, central Scandinavian Caledonides. Norsk Geologisk Tidskrift, 73(1), 21-42.

Fossen, H., Polonio, I., Bauck, M.S., Cavalcante, C. (2024). The North Sea rift basement records extensional collapse of the Caledonian orogen. Commun Earth Environ, 5, 206. https://doi.org/10.1038/s43247-024-01374-y

Sjöström, H., Bergman, S., & Sokoutis, D. (1991). Nappe geometry, basement structure and normal faulting in the central Scandinavian Caledonides; kinematic implications. Geologiska Föreningen i Stockholm Förhandlingar, 113(2–3), 265–269. https://doi.org/10.1080/11035899109453877

How to cite: Nilsson, C., Klonowska, I., Buczko, D., and Majka, J.: Petrology and geochronology of the Handöl mélange lithologies, Köli Nappe Complex, Scandinavian Caledonides: deciphering orogenic and post-orogenic signatures , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22775, https://doi.org/10.5194/egusphere-egu26-22775, 2026.

The Seve Nappe Complex (SNC) of the Scandinavian Caledonides preserves a record of HP-UHP metamorphism related to continental subduction along the Baltican margin. Geochronological studies traditionally identified a late Cambrian (c. 490–480 Ma) (U)HP event in the northern SNC, whereas the southern SNC was interpreted to record younger Ordovician metamorphism at c. 470–455 Ma (Gee et al. 2020, and references therein), leading to models of localized late Cambrian subduction restricted to the north.

In this contribution, we present a new approach using in situ Lu-Hf geochronology on polymetamorphic garnets to further constrain the subduction histories in the SNC. It is a powerful tool to directly date garnet growth associated with (U)HP conditions and allows recovery of early metamorphic histories in the Scandinavian Caledonides.

Preliminary results show ages of 495-480 Ma for a paragneiss in Marsfjället, a garnet schist in Avardo, two garnet schists in Lillfjället, two eclogites in Sjouten, and a schist in EASU. This extends the late Cambrian-early Ordovician subduction record to the central SNC. Furthermore, three Avardo eclogites yield ages of 460-450 Ma, indicating that the central SNC was affected by two metamorphic events, both possibly (U)HP.

In the ongoing project, in situ Lu-Hf dating will be applied on garnets farther south in the SNC to constrain the spatial extent of late Cambrian subduction of Baltica.

References:

Gee, D.G., Klonowska, I., Andréasson, P.G. and Stephens, M.B. 2020. Middle thrust sheets in the Caledonide orogen, Sweden: the outer margin of Baltica, the continent–ocean transition zone and late Cambrian–Ordovician subduction–accretion. Geological Society Memoir, 50, 517–548, https://doi.org/10. 1144/M50-2018-73

How to cite: Roos, A., Barnes, C. J., Callegari, R., Klonowska, I., and Majka, J.: In situ Lu-Hf dating of garnets: reconstructing subduction zone histories in the Seve Nappe Complex, Scandinavian Caledonides, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21025, https://doi.org/10.5194/egusphere-egu26-21025, 2026.

Metamorphic rocks exposed on Svalbard document a complex tectonothermal history associated with the early stages of the Caledonian orogeny. Particularly high-pressure (HP) rocks are key-targets for reconstructing the geodynamic evolution of the Arctic. Two HP units crop out on Svalbard, namely the Richarddalen and Vestgötabreen complexes. Although the rocks have been recognized since the 1960s, the age of the HP metamorphism was not unequivocally resolved. Here, we present new petrochronological data for both units.

The Richarddalen Complex comprises HP orthogneisses, eclogites, and metagabbros. Peak conditions for the eclogite reached 2.4–2.5 GPa and 720–740°C, followed by decompression to ~1.2 GPa (Elvevold et al. 2013, GSL, Spec Pub). The prograde conditions estimated using quartz in garnet and Zr in rutile thermometry yield 1.7–1.8 GPa at 700°C for eclogite and 1.2–1.4 GPa at 700°C for orthogneiss. In-situ Lu-Hf dating of garnet from augen gneiss and mylonitic orthogneiss yields Tonian ages of 967±44 Ma and 959±28, respectively. Smaller, II-generation garnet yields a poorly constrained age of 477±98 Ma. In-situ Rb-Sr dating provides Early Ordovician ages of 470±12 Ma for white mica from mylonitic orthogneiss, and 473±4 Ma for biotite from augen gneiss. A recent geochronological study constrained Neoproterozoic age of HP metamorphism based on U-Pb zircon dating (Koglin et al., 2022, JGSL), while Mazur et al. (2022, Terra Nova) presented Ar-Ar dating of white mica interpreted as cooling after HP event and further deformation and tectonic assembly with lower-P units at ca. 440–438 Ma. The latter ages together with the new geochronological data presented here, rule out the Neoproterozoic age of HP metamorphism proposed by Koglin et al. (2022). Additionally, Lu-Hf data further confirm the Tonian age of the protoliths (e.g. Pettersson et al. 2009, JGSL; Gromet&Gee 1998, GFF).

The Vestgötabreen Complex represents HP low-temperature units composed of eclogites, blueschists, schists, and serpentinites. Geothermobarometry defines three stages for eclogite: prograde at 1.6±0.3 GPa and 460±60°C, peak-P at 2.3±0.3 GPa and 507±60°C, and peak-temperature at 2.1±0.3 GPa and 553±60°C (Kośmińska et al. 2023, ConMinPet). U-Pb zircon age of 482±10 Ma records prograde growth, whereas U-Pb monazite age of 471±6 Ma is interpreted as post-peak P growth. Peak-P conditions of 2.0±0.03 GPa and 500±30 °C were estimated for blueschist. Lu-Hf garnet dating yields 471±4 Ma for blueschist. Barnes et al. (2021, Minerals) presented an extended dataset of Ar-Ar ages and interpreted age populations as: cooling after HP metamorphism at 476±2 Ma, assembling the Upper and Lower units at 454±6 Ma, and late deformation in the Lower Unit at c. 430–400 Ma. This data provides further support for an early Ordovician subduction system along the Baltican margin in the High Arctic sector of the orogen.

The recent studies are extending our understanding of the geological evolution of this part of the Arctic during the early stages of the Caledonian orogeny. However, further integrated field and analytical studies are needed to help develop the geodynamic reconstructions for the Arctic. This study was supported by the NCN projects 2021/43/D/ST10/02305 (KK) and 2019/33/B/ST10/01728 (JM).

How to cite: Kośmińska, K., Majka, J., Barnes, C., and Gilio, M.: Recent insights into the metamorphic evolution of high-pressure rocks from Svalbard, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16159, https://doi.org/10.5194/egusphere-egu26-16159, 2026.

Maps of the Appalachian–Caledonide Orogen have sought to identify a unique Iapetus suture marking either a collision between Laurentian and Gondwanan crust, or final closure of the Iapetus Ocean. However, orogen syntheses based in Britain and Ireland show the Iapetus suture as Silurian; those in Newfoundland show a Late Ordovician suture; those in Cape Breton Island show no Iapetus suture, and those in southern New England show closure in the Early Ordovician. The provenance and the timing of accretion can be examined using detrital zircon distributions and stratigraphic relationships. For example, the approach of a Ganderian terrane to the Laurentian margin is typically marked by an influx of ~1 Ga zircon from the Grenville Orogen. The end of accretion is typically bracketed by an angular unconformity, above which forearc basin sedimentary and volcanic rocks contain both Laurentian and non-Laurentian zircon. This approach allows identification of terrane assembages separated by multiple anastomosing sutures, ranging in age from Early Ordovician to Devonian. Terranes derived from peri-Gondwanan Ganderia arrived diachronously, such that the Laurentia–Gondwana boundary is marked by sutures of different age along the orogen. We therefore argue that efforts to identify a single Appalachian–Caledonide "Iapetus suture" are not worthwhile.

How to cite: Waldron, J., Barr, S., McCausland, P., Schofield, D., Wang, C., Schwangler, M., van Rooyen, D., White, C., and White, S.: Deconstructing the Iapetus Suture: Terrane assemblage map of the northern Appalachians and western Caledonides, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22526, https://doi.org/10.5194/egusphere-egu26-22526, 2026.

Closure of the Iapetus and Rheic oceans occurred diachronously along the Appalachian orogen, and documentation of sutures is complicated by post-collisional deformation and by irregularities in the original Laurentian margin along which accretion of terranes occurred. In Nova Scotia, at least four cryptic terrane boundaries involved ocean closures but do not show the typical geological assemblages associated with subduction-related accretion of terranes. Three collisions discussed here are (i) between two Ganderian terranes (Aspy and Bras d’Or), (ii) between the Ganderian Bras d’Or terrane and Avalonian Mira terrane, and (iii) between the Meguma terrane and Avalonia in northern mainland Nova Scotia. In Cape Breton Island arc magmatism spanned the Ediacaran to Cambrian (620 Ma – 530 Ma) in both Aspy and Bras d’Or terranes, but only the Aspy terrane records arc magmatism in the Ordovician to Silurian. The Eastern Highlands shear zone (EHSZ) juxtaposed the Ganderian Aspy and Bras d’Or terranes at ca. 420-390 Ma. Minor magmatism at ca. 402 Ma likely occurred in a syn-collisional pull-apart basin that formed in an overall transpressional environment. No evidence is preserved in Nova Scotia of a magmatic or metamorphic event associated with collision of the Ganderian Bras d’Or terrane with the Avalonian Mira terrane, and the suture is not exposed at the surface. Geophysical data and clasts in a conglomerate overlying the suture constrain the location and age of the boundary, but its nature is not well understood. However, in Newfoundland this collision is marked by extensive subduction-related Silurian to Devonian magmatism and metamorphism, suggesting that in the Nova Scotian segment the collision was mainly transpressional. The accretion of the Meguma terrane to the southern Avalonian margin in Nova Scotia is also a well- documented transpressional collision. No subduction-related magmatism has been associated with the collision, but it was coeval with voluminous S-type magmatism throughout the Meguma terrane.  The transpressional character of these three accretionary events in Nova Scotia, in contrast to the equivalent events elsewhere in the northern Appalachians, suggests that the Nova Scotian segments of each collision may have repeatedly developed as transform boundaries.

How to cite: Barr, S., van Rooyen, D., and White, C.: A tale of three collisions: terrane accretions and cryptic ocean closures in the Nova Scotia segment of the Appalachian orogen, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22522, https://doi.org/10.5194/egusphere-egu26-22522, 2026.

The western-central European Variscan orogen developed during the Upper Devonian to Carboniferous continental collision between Gondwana and Laurussia, culminating in the assembly of Pangea. This orogen records a complex sequence of tectono-thermal events providing insights into continental crustal evolution and lithospheric deformation mechanisms. Late Devonian D₁ contractional deformation is associated with crustal thickening, Mississippian D₂ extensional deformation was probably caused by gravitational collapse of the mountain range, and Pennsylvanian D₃ contractional deformation represents subsequent crustal shortening. The Iberian Massif, located at the core of the Variscan Orogen, offers exceptional conditions for detailed analysis of deep-to-shallow crustal Variscan tectono-thermal processes, preserving these three superimposed deformation events.

In the Ossa-Morena Zone (SW Iberian Massif), the youngest Variscan orogenic activity is associated with the emplacement of syn- to late-D₃ plutons. We present new geological mapping, whole-rock geochemistry, and zircon U-Pb geochronology from the Pennsylvanian Figueira e Barros-Ervedal and Fronteira plutons (west-central Ossa-Morena Zone). These shallow-crustal, calc-alkaline, peraluminous granodioritic to granitic intrusions are syn- to late-D₃ because they crosscut D₂-D₃. SHRIMP U-Pb zircon dating indicates crystallisation ages of 307 ± 3 Ma and 308 ± 2 Ma for the Figueira e Barros-Ervedal and Fronteira plutons, respectively.

Their host metamorphic succession consists of Silurian-Devonian siliciclastic flysch, containing olistostromes and olistoliths, overlying a bimodal volcanic-sedimentary complex assigned to the Cambrian-Ordovician (?). Both stratigraphic units underwent post-kinematic contact metamorphism associated with the emplacement of these Pennsylvanian plutons, producing pelitic hornfels, dominated by spotted mica schists with post-kinematic porphyroblasts. Prior to this contact metamorphism, regional M₂ Buchan-type metamorphism produced pre- to syn-kinematic garnet porphyroblasts and syn-kinematic andalusite and staurolite porphyroblasts. These mineral assemblages are associated with the development of a flat-lying pervasive S₂ foliation and mineral lineation, defined by biotite and muscovite (after sillimanite?), which is comparable to that observed in the hanging-wall blocks of Mississippian gneiss domes in the Iberian Massif, including in nearby sectors of the Ossa-Morena Zone. It should also be noted that locally, pre-early-kinematic garnets preserved as cores or as isolated minerals, together with possible high-pressure/low-temperature mineral assemblages in the kyanite zone, were also identified, pointing to a pre-D2 process of regional pressurisation (Barrovian metamorphism), which possibly represents D₁-M₁(?). About 20 km northwest of the Pennsylvanian Figueira e Barros-Ervedal and Fronteira plutons, the Mississippian Ponte-de-Sôr gneiss dome exhibits a pervasive S₂ foliation and top-to-the-SE tectonic transport synchronous with M₂ Buchan-type metamorphism. We propose that a comparable, though cryptic, D₂ gneiss dome developed in the study area prior to the emplacement of the syn- to late-D₃ Figueira e Barros-Ervedal and Fronteira plutons.

Work supported by FCT, I.P./MCTES through national funds (PIDDAC): LA/P/0068/2020- https://doi.org/10.54499/LA/P/0068/2020, UID/50019/2025, https://doi.org/10.54499/UID/PRR/50019/2025, UID/PRR2/50019/2025, 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), J.C.D. from FCT contract CEECINST/00032/2018/CP1523/CT0002  (https://doi.org/10.54499/CEECINST/00032/2018/CP1523/CT0002), and M.F.P. from grant Nº. FCT/UIDB/06107-Center for Sci-Tech Research in Earth System and Energy-CREATE.

How to cite: Steel Hart, L., Cambeses, A., Pereira, M. F., García Casco, A., C. Duarte, J., and Dias da Silva, Í.: New mapping and geochronology constraints on the Variscan plutonism, metamorphism and deformation in the Ossa-Morena Zone (SW Iberian Massif), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15562, https://doi.org/10.5194/egusphere-egu26-15562, 2026.