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.

This short report presents the investigation results on the zircon from meta-andesitic rock from the Ankarede Volcanite Formation of the Lower Köli Nappe Complex in Scandinavian Caledonides. Previous U-Pb dating revealed a wide span of dates ranging from ca. 520 to ca. 480 Ma, with a mean age of 491 ± 3 Ma for the zircon cores. Using cathodoluminescence and back-scattered electron imaging, along with chemical mapping, we identified distinct zones within the zircon grains; 1) cores of clear magmatic provenance, 2) mantles also of magmatic origin but with a slightly different chemical composition and 3) zircon rims that suffered metamictisation and fluid-induced alterations. These findings highlight a complex growth history and alteration of studied zircon that affect the interpretation of zircon dating results. This research underscores the importance of detailed zircon studies for understanding the intricate processes involved in the magmatic and metamorphic evolution of Virisen terrain in Scandinavian Caledonides.

The Köli Nappe Complex in the Scandinavian Caledonides of Sweden originated as terranes within the Iapetus Ocean, derived from subduction-related magmatic and basin systems. The Krutfjellet Nappe in Västerbotten, Sweden and the Gasak Nappe in Nordland, Norway are both part of the Upper Kӧli Nappes. Siliclastic, carbonate and volcanic protoliths underwent metamorphism up to amphibolite facies, in places involving extensive migmatisation which is not found in other Kӧli Nappe units. Owing to a lack of previous studies, the exact age and origins of these migmatites is currently unknown. Foliations and early folds in the metasediments are cut by c. 445-434 Ma intrusions [1], followed by a regional metamorphic overprint which is assumed to be Scandian. Monazites and zircons from pelitic migmatites in the Norra Storfjället lens of the Krutfjellet Nappe, and monazites from pelitic schists and gneisses in the Sulitjelma area of the Gasak Nappe were dated in-situ using LA-ICP-MS. Monazites yield Th-U-Pb concordia ages of between 428-424 Ma. Individual concordant analyses span between 443-416 Ma. The monazites yield a large proportion of discordant analyses which fall on discordia lines at high angles to the concordia curve, interpreted to be due to the presence of initial Pb in the monazites. Lower intercept ages range between 428-416 Ma. These isotopic monazite ages are younger than the majority of EPMA Th-U-total Pb ages obtained for the Krutfjellet Nappe in the same grains [2]. The monazites often have complex zoning patterns in Y and REEs, however this zoning appears to be decoupled from the Th-U-Pb isotopic systems. We suggest that the monazites from the Krutfjellet and Gasak Nappes underwent metamorphism and/or fluid-related alteration at between 428-416 Ma, associated with assembly of the Caledonian orogenic wedge. This may have involved pervasive resetting of older monazites through dissolution-reprecipitation, leading to contamination with initial Pb and decoupling of the Th-U-Pb system from Y and REE zoning. The actual age of migmatisation still remains uncertain. Zircons yield a spectrum of U-Pb dates which appear to be predominantly detrital. The majority are Mesoproterozoic to earliest Neoproterozoic (1600-900 Ma) and a small number around 600-550 Ma. A significant proportion of dates are discordant. These discordant dates often occur in grains which have a spongey, mottled appearance, indicating that the zircons have also experienced some alteration (metamictisation?). Lower discordia intercepts are poorly constrained, but appear to be Ordovician to Silurian. A zircon included in kyanite produced a single concordant date of 440 Ma, which matches the age for nearby earliest-Silurian intrusions [1]. On the basis of this so-far limited zircon dataset we tentatively propose that these rocks underwent migmatisation at c. 440 Ma, perhaps associated with enhanced heat flow from rift-related magmatism, but the monazite age record has been severely overprinted during subsequent Scandian orogenic wedge assembly and translation and its associated lower-grade metamorphism. Funded by the National Science Centre (Poland) grants no. 2021/41/N/ST10/04298 and 2021/41/B/ST10/03679. [1] Stephens, M.B. 2020. GSL Memoirs, 50, 549–575. [2] Carter, I. S. M., Cuthbert, S. & Walczak, K. 2023. EGU Gen. Assem.

A total of 10 western Norwegian eclogites, whose mineral chemistry records metamorphism of up to 850 °C and 5.5 GPa, were investigated for structural hydroxyl content in nominally anhydrous minerals. Garnet shows pronounced absorption in the wavenumber ranges of 3596–3633, 3651–3694, and 3698–3735 cm−1 and minor absorption centred at about 3560 cm−1. Clinopyroxene with aligned inclusions of either quartz, albite, or quartz + pargasite has major absorption at 3450–3471 and 3521–3538 cm−1 and minor absorption centred at 3350 and approximately 3625 cm−1. The latter band is strongest in a sample with minute lamellar inclusions rich in Al, Fe, and Na and was excluded from hydroxyl quantification. Orthopyroxene has large, narrow absorption peaks centred at 3415 and 3515 cm−1 and smaller peaks at 3555, 3595, and 3625 cm−1. Five orthopyroxene-bearing eclogites exhibit relatively homogeneous amounts of structural hydroxyl in garnet (13–32 µg g−1), clinopyroxene (119–174 µg g−1), and orthopyroxene (4–17 µg g−1). The outer 200 µm wide rims of the orthopyroxene grains illustrate a late hydroxyl loss compared to core values of about 30 %, which is not evident in garnet and clinopyroxene. In contrast, the other five orthopyroxene-free eclogites exhibit variable amounts of hydroxyl in garnet (8–306 µg g−1) and clinopyroxene (58–711 µg g−1). Apart from extreme values, the structural hydroxyl content of clinopyroxene in the eclogites studied is lower than in comparable ultra-high-pressure metamorphic samples, e.g. both metasomatised and pristine eclogite xenoliths from the lithospheric mantle underneath several cratons and coesite- and quartz-eclogites from the Erzgebirge and the Kokchetav massifs, by up to several hundreds of micrograms per gram (µg g−1). The low structural hydroxyl contents, the deficiency of molecular water, and the preservation of diffusion-sensitive evidence from the mineral chemistry for metamorphism well beyond the stability field of amphibole suggest that oriented inclusions of quartz + pargasite were formed isochemically during decompression. In addition, structural hydroxyl content in clinopyroxene is inversely correlated with metamorphic pressure estimates obtained from orthopyroxene of the same samples. Therefore, structural hydroxyl in nominally anhydrous eclogite minerals can serve as an indicator of the effectiveness of retrogression.

Detrital zircon geochronology is reported from the c. 1200 m thick Cambro-Ordovician sedimentary succession recovered in core from the COSC-2 continental drilling project in the Scandinavian Caledonides. Above a regolith marking the sub-Cambrian peneplain, a lower to middle Cambrian(?) succession comprises conglomerate, sandstone and shale overlain by gravity flows fining upwards into the Alum Shale Formation. First results of detrital zircon geochronology from the Cambrian(?) succession show that the basal section of the autochthonous cover is characterized by mainly late Paleoproterozoic - early Mesoproterozoic detrital grains. The middle part of the succession is dominated by late Paleoproterozoic detritus with minor Mesoproterozoic and Archean input. The upper part of lower Cambrian(?) succession is characterized by Archean to Cambrian detritus. The maximum depositional age is calculated to 530.5 ± 4 Ma for the upper part of the lower Cambrian succession. Two samples from the Lower Ordovician(?) succession above the Alum Shale Formation show predominantly Mesoproterozoic to early Neoproterozoic (1.5-0.9 Ga) ages. The autochthonous lower Cambrian(?) passive margin succession in the lower section is dominated by local detritus, sourced exclusively from the Eastern Segment of the Sveconorwegian Orogen, which includes the basement studied in COSC-2. Up-section, the provenance shifts towards the Transscandinavian Igneous Belt and Svecofennian Orogen sources, with the youngest part of the succession showing a notable input of Neoproterozoic -Cambrian active margin detritus. The Ordovician(?) succession is characterized by populations, likely derived from the Sveconorwegian Orogen, and a minor cratonic contribution. Statistical analysis of detrital zircon datasets across Baltica suggests that the Southern Baltica/Sandomirian Arc, rather than the Timanian Orogen, was a significant source of detrital material across the paleocontinent. The influence of Timanian Orogen grains is limited to northernmost Scandinavia, whereas Sandomirian detritus reached central Scandinavia in the lower to middle Cambrian and remained prevalent in southern Scandinavia into the Lower Ordovician.

The Western Gneiss Region (WGR) is a well-known UHP metamorphic terrane in SW Norway, where eclogite bodies crop out among gneisses. These rocks experienced high- and ultrahigh-pressure (HP-UHP) metamorphism during the Caledonian Orogeny and display pressure-temperature (P-T) gradient increasing from the south to the north (e.g., Cuthbert et al., 2000). Eclogites from nine localities along the entire length of WGR, namely Drøsdal, Vårdalsneset, Verpeneset, Halnes, Saltaneset, Grytting, Ulsteinvik, Solholm, and Juvika, have been chosen for P-T condition estimates. Here we present a reevaluation based on the new geothermobarometric techniques in the WGR regional study.Eclogite varies in the amount of amphibole, kyanite, phengite, zoisite, and inclusions of quartz and rutile in garnet. Phengite occurs mainly in the eclogites in the south, whereas those from the north contain orthopyroxene. Garnet in the south is almandine-rich with the average composition of Alm0.41–0.57Grs0.15–0.31Prp0.10–0.37Sps0.01–0.07, with increasing Mg in the rims and often showing complex compositional zoning. Garnet from the other localities is homogenized and dominated by pyrope with the composition of Prp0.41–0.56Alm0.33–0.46Grs0.09–0.13Sps0.01–0.04. Clinopyroxene in the northern part is poorer in jadeite component with XNa=0.20-0.29 and XFe=0.12-0.17 than clinopyroxene in the south with XNa=0.40-0.52 and XFe=0.12-0.19. Silicon in phengite is up to 3.30 atoms per formula unit (apfu) in most localities, with a maximum of 3.48 apfu in the Verpeneset locality.Coesite and polycrystalline quartz inclusions in garnet, evidence of UHP metamorphism, are typical of Saltaneset and Verpeneset localities, which is also validated by thermobarometric calculations. We estimated the peak P-T equilibration condition by Grt-Cpx-Phe thermobarometry, Ti-in-Quartz and Zr-in-Rutile thermometry, along with Quartz-in-Garnet elastic geobarometry. Preliminary estimates give slightly higher P values than previously reported in the two southernmost localities Drøsdal and Vårdalsneset [720-830°C and 1.9-2.1 GPa (Foreman et al., 2005), 635°C and 2.3 GPa (Engvik et al., 2007), accordingly] yielding the peak conditions of 680-740°C and 2.7-2.85 GPa. The obtained results from other localities give P conditions close to the boundary of quartz and coesite stability fields and T in the range of 650-750°C, with the highest P of 3.28 GPa in the Verpeneset locality. The highest T of 875°C has been obtained in Ulsteinvik locality. Those results may help refine previous studies and understand the history of the WGR.This work was funded by the National Science Centre of Poland project no. 2021/43/D/ST10/02305. References:Cuthbert, S.J., Carswell, D.A., Krogh-Ravna, E.J., Wain, A. (2000). Eclogites and eclogites in the Western Gneiss Region, Norwegian Caledonides. Lithos, 52 (1–4), 165-195.Foreman, R., Andersen, T.B., Wheeler, J. (2005). Eclogite-facies polyphase deformation of the Drøsdal eclogite, Western Gneiss Complex, Norway, and implications for exhumation. Tectonophysics, 398, 1-32.Engvik, A.K., Andersen, T.B., Wachmann, M. (2007). Inhomogeneous deformation in deeply buried continental crust, an example from the eclogite facies province of the Western Gneiss Region, Norway. Norwegian Journal of Geology, 87, 373-389.

The Collisional Orogeny in the Scandinavian Caledonides (COSC) project focuses on processes related to the closure of the Iapetus Ocean, causing the Ordovician-Silurian continent- continent collision between Baltica and Laurentia. The rock succession in the second drill core (COSC-2) from the J & auml;mt land County, central Sweden, provides the base for detailed sedi- mentological, stratigraphic, geophysical, geochemical, geothermal and structural studies. The basement, comprising 1.66-1.65 Ga Transscandinavian Igneous Belt porphyries intruded by 1.47 Ga and 1.27-1.26 Ga mafic dykes and sills, is heavily weathered towards the top. Here it grades into typical saprock and saprolite (including immature soil reflecting the sub-Cambrian peneplain). The overlying sedimentary sequence starts with basal conglomerates and heterogeneous sediments with shell fragments, indicating an early Cambrian rather than a Neoproterozoic age for the marine transgression in the area. The developing early Cambrian basin was rapidly filled, initially by mostly coarse-grained sediment gravity flows. These strata are covered by sandstone turbidites that show an upward transition into the Alum Shale Formation, representing a tectonically quieter period (mid-Cambrian/Maolingian to Early Ordovician/Tremadocian). The upper part of the Alum Shale Formation is overlain by a late Early Ordovician turbidite succession. Local sources of sediments below the Alum Shale Formation and the extended deposition period may indicate continuous sedimentation in a pull-apart basin pre served in a window beneath the Caledonian thrust sheets.

The arthropleurid trackway Diplichnites cuithensis has previously been described from Scotland from the Upper Carboniferous Serpukhovian Limestone Coal Formation on the Isle of Arran and the Lower Carboniferous Visean Pittenweem and Anstruther Formations on the East Fife coast. Here we describe a new west coast single trackway from the Serpukhovian Limestone Coal Formation of Glasgow's Linn Park. The trackway occurs associated with simple horizontal burrows assignable to Planolites? , vertical openings of Arenicolites , examples of Taenidium barretti (formerly Beaconites barretti ), and irregular large-scale bioturbation or possibly rootlet casts. The trace fossils and sedimentary structures (including trough cross-bedding and flaser bedding) indicate a fluvial sandbar or plain environment, possibly of estuarine origin, locally colonized by plants. Diplichnites cuithensis (and other Diplichnites species) commonly occur associated with the burrow Taenidium barretti . The latter is known to have been widespread globally throughout the Carboniferous, and is a common component of fluviatile sequences within the Lower Carboniferous succession of NW Ireland. This suggests that previously undocumented older Scottish Carboniferous examples of both Diplichnites ichnospecies and Taenidium barretti may also be present, assuming that suitable environments persisted and are currently adequately exposed.

The Köli Nappe Complex (KNC) of the Scandinavian Caledonide orogen originated as oceanic terranes within the Iapetus Ocean. These terranes have characteristics of magmatic arcs and associated forearc or back-arc basins and underwent several periods of rifting and magmatism prior to their accretion to the Baltican margin. We present new U–Pb zircon ages from the Lower Köli Ankarede Volcanite Formation in Västerbotten, Sweden. U–Pb ages of magmatic zircon grains from metamorphosed dacitic to andesitic rocks show ages of 512 ± 3.5, 497 ± 2, 491 ± 1 and 488 ± 4 Ma. The three younger ages fit with previous ages for Lower Köli volcanic rocks, but the 512 Ma age is older than any previous age for this unit. These dates constrain the age of magmatism in an ensimatic arc system within Iapetus. We compare this evolution with published information from the other Köli nappes. Magmatic ages within the KNC overlap with ages for an early episode of ultrahigh-pressure (UHP) metamorphism within the underlying Seve Nappe Complex (SNC), supporting the hypothesis that attributes UHP metamorphism within the SNC to subduction beneath the island arc now preserved within the Lower Köli Nappes. Supplementary material: Photographs of sampled exposures (S1), BSE images of selected zircons (S2), charts of zircon trace elements (S3) and analyses of zircon (S4, S5) are available at https://doi.org/10.6084/m9.figshare.c.6843787 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

The Köli Nappe Complex (KNC) in the Scandinavian Caledonides of Sweden originated as terranes within the Iapetus Ocean derived from subduction-related magmatic and basin systems. The Krutfjellet Nappe is part of the Upper Kӧli Nappes in Västerbotten, Sweden. Siliclastic, carbonate and volcanic protoliths[3] underwent amphibolite facies metamorphism involving extensive migmatisation, which was of a distinctly higher grade than the other Koli Nappes. No modern P-T-t studies have been made in this nappe. Foliations and early folds in the metasediments (D1 and D2) are cut by latest Ordovician to earliest Silurian metagabbros and metagranites. Regional metamorphism and intrusion were syn-to-post D2. All these predate Scandian thrusting over the middle and lower KNC[3]. A trondhjemitic pebble in a metaconglomerate was dated to c. 489 Ma[4] so the main fabric-forming event is constrained to some time in the Ordovician. The mafic intrusions were partially converted to amphibolite and greenschist[2] and the main greenschist-amphibolite metamorphism in the subjacent KNC was early Silurian, followed by early Devonian thrusting[1], so a Scandian metamorphic imprint in the Krutfjellet Nappe is implied.Four sillimanite and/or kyanite-bearing pelitic migmatite samples from the Norra Storfjället lens of the Krutfjellet Nappe were selected for U-Th-total Pb electron microprobe dating of monazite. Monazites from a variety of fabric elements including matrix, leucosome and inclusions within garnet yielded ages spanning the range 484-390 Ma. The monazites often have complex zoning patterns in Th and Y. However, discrimination of monazite populations based on trace element measurements was not resolvable so zoning appears to be decoupled from ages. There is also no discernable relationship between ages and location of the monazite within fabric elements. Weighted mean specimen ages were found to be 427 ±3.8 Ma, 442.5 ±4.0 Ma, 433.3 ±3.0 Ma and 438.3 ±2.7 Ma.The large span of ages obtained suggests that more than one metamorphic event is recorded, however, some mixing and/or partial resetting of ages has occurred. The oldest ages (474-484 Ma), often outliers, are close to the early Ordovician conglomerate clast age[4] and may have either been inherited from detrital monazite or formed during an early metamorphic event close to the clast age. The youngest ages (c. 430-400 Ma) are likely to be related to final thrusting of the Scandian nappe assemblage. The predominant age population falling around 445-435 Ma is similar to the ages of nearby early Silurian intrusions[3], so monazite may have been generated or reset by the early Silurian intrusions, or by regionally-enhanced thermal regime associated with this magmatism. Funded by the National Science Centre (Poland) grants no. 2021/41/N/ST10/04298 and 2021/41/N/ST10/04298.[1] Bender, H., Glodny, J. and Ring, U. 2019. Lithos, 344–345, 339–359.[2] Senior, A. and Otten, M.T. 1985. In: Gee, D.G. and Sturt, B.A., 953–978.[3] Stephens, M.B. 2020. GSL Memoirs, 50, 549–575.[4] Stephens, M.B., Kullerud, K. and Claesson, S. 1993. GSL, 150, 51–56.

The Western Gneiss Region (WGR) in western Norway exposes ultrahigh-pressure (UHP) eclogites that occur repeatedly, within an area of high-pressure (HP) eclogites, without evidence of being separated by tectonic shear or ductile flow structures. We studied 10 eclogites from two northern UHP areas and the interjacent HP area to evaluate the significance of this pattern. The orthopyroxene in orthopyroxene-bearing samples has low Al2O3 contents (0.17 wt %–0.37 wt %), provided its grain boundaries were unaffected by partial recrystallisation or replacement. Classical geothermobarometry based on element partitioning between coexisting mineral phases suggests metamorphic conditions within the diamond stability field for the samples from both the HP and UHP areas. The primary clinopyroxene in the associated orthopyroxene-free eclogites contains aligned inclusions of either needle-shaped quartz ± pargasite or lamellar albite, which are absent from the secondary (symplectic) clinopyroxene. Reconstructed mineral compositions of the primary clinopyroxene obtained from grain cross-section surfaces using a scanning electron beam or image processing are non-stoichiometric, and they have higher Ca-Eskola and lower Ca-Tschermak components than the inclusion-bearing host clinopyroxene. The molar ratios of these endmembers are consistent with the needles in the primary clinopyroxene being formed from vacancy-bearing precursor clinopyroxene by the exsolution reaction 2 Ca-Eskola = Ca-Tschermak + 3 quartz during early eclogite-facies retrogression. Further retrogression partially transformed the needle-shaped quartz to irregularly shaped albite within the clinopyroxene and partially transformed both clinopyroxene generations to amphibole that occasionally preserves the needles. The similarity of both the maximum metamorphic conditions and the mineral exsolution microstructures in the eclogites from UHP and HP areas indicates a shared metamorphic history within the stability field of diamond, but a history that diverged during retrogression. Consequently, the alternations of UHP and HP areas in the WGR may have formed by a process that allowed for spatial variations in retrogression efficiency, such as the localisation of strain (recrystallisation) or fluid flow (diffusion) or both, rather than by tectonic stacking of UHP and HP units. Evidence for the UHP metamorphism of WGR crustal rocks is now found from NE to SW along the entire coastal section that covers previously recognised UHP and interjacent areas.

<p>The COSC (Collisional Orogeny in the Scandinavian Caledonides) project is an integral of the International Continental Scientific Drilling Program (ICDP), performed by a multidisciplinary and international team of geoscientists. It focuses on processes related to the Early Palaeozoic continent-continent collision between Baltica and Laurentia. The collision resulted in the final closure of the Iapetus Ocean in the Middle-Late Silurian when the Baltoscandian margin was partially subducted beneath Laurentia, forming a Himalayan-type orogen. In west-central Sweden this collisional mountain belt is deeply eroded and COSC-2 successfully recovered a continuously cored succession to a depth of 2276 m..</p> <p>Based on seismic profiling, geophysical models and the resulting interpretations, COSC-2 predicted a continuous Lower Palaeozoic allochthonous sedimentary succession, the main Caledonian décollement in the Cambrian Alum Shale Formation, and a Fennoscandian basement. The unexpected core record therefore perfectly underlines the importance of deep continental drilling. Logging and early studies show that the succession intruded by dolerite dykes involves a thick porphyry sequence instead of Paleoproterozoic granitic basement. Drilling shows that an imbricate zone with Proterozoic and Cambrian sandstones, formed in different settings, covers the basement. The basal sandstones are overlain by deformed Alum Shale comprising the main décollement and by Lower Palaeozoic siliciclastics formed in more outboard and deeper environments. This differs significantly from interpretations based on the preliminary site investigations, which also suggested a main detachment hosted in Alum Shale, but close to the top of the basement, overlain by a zone of imbricates.</p> <p>New detailed core descriptions show that there is a continuous sedimentary succession on top of a weathered basement (saprock and saprolith) covered by regolith (level of the Sub-Cambrian Peneplain?) which is overlain by basal conglomerates and a few meters of heterogeneous sediments (Lower Cambrian?), displaying the unusual development of a basin filled initially by mostly coarse-grained sediment gravity flows grading into finer-grained turbidites. This sedimentation was interrupted by a longer period of Alum Shale deposition (Middle Cambrian through Tremadocian), which transitioned into turbidite sedimentation again. This higher turbidite sequence (Tremadocian and younger) shows fining upward indicating a general deepening and was previously regarded as a much younger foreland basin fill (Föllinge greywackes). However, local sources of the turbiditic sediments below the Alum Shale and the extended time of deposition may rather point to a continuous sedimentation in a long-lived pull-apart basin preserved in a window beneath the Caledonian thrust sheet.</p> <p>After many delays caused by Covid pandemic restrictions, the core was logged in fall 2021 and afterwards by the sampling party at the BGR Core Repository in Berlin/Spandau (summer 2022). Dating of the sedimentary units is the base of a stratigraphic framework for further correlations of geotectonic events, sea-level fluctuations, evolutionary pulses, climate changes, and the re-interpretation of seismic models. The continuous COSC-2 sequence provides various possibilities for interdisciplinary collaborations and studies performed by the COSC science team. The first scientific results are presented in session TS6.4 "The Caledonian Orogen of the North Atlantic region: insights from geological and geophysical studies".</p>

<p>The Western Gneiss Region (WGR) in W Norway exposes ultrahigh pressure (UHP) metamorphic eclogite of Scandian age in domains that are spatially separated from one another for unknown reasons. We studied five eclogites from the two northern UHP domains and the area in between (at the localities <span>Å</span>rsetneset, Fjørtoftvika, Riksheim, Synes, Ulsteinvik) for petrography, mineral chemistry and by Raman spectroscopy. The peak metamorphic mineral assemblages contain garnet, Na-pyroxene (jadeite 0.13–0.46) and – depending on the sample – rutile, ilmenite, quartz, kyanite and/or orthopyroxene. Depending on strain accumulation, the eclogite facies fabric is poikiloblastic or has a foliation formed by elongated grains and grain aggregates of Na-pyroxene and garnet. Secondary processes formed amphibole, biotite and symplectite of plagioclase and diopside. Irrespectively, all samples contain Na-pyroxene with needle-shaped inclusions that are in parallel to the presumed c-axis of the host. These needles are either bi-mineralic (quartz + pargasite) or monomineralic (quartz). Chemically integrated compositions obtained at mineral surfaces with needle exposure using a scanning electron beam yielded lower Ca-Tschermak’s and higher Ca-Eskola components than the host. The molar ratios of these calculated endmembers are consistent with the needles being formed by the reaction: 2 Ca-Eskola = Ca-Tschermak’s + 3 quartz. If Ca-Eskola is regarded to be typical for UHP metamorphism, then the spatial distribution of eclogite with quartz needles does not support a separation of the two northern UHP domains by the interjacent area.</p><p>Garnet has minor compositional zoning with smooth gradients at grain rims. Mineral core compositions of garnet and needle-bearing Na-pyroxene suggest minimum metamorphic conditions after needle formation in the ranges of 700-790 °C and 1.0-1.6 GPa, when the calibrations of the Fe–Mg geothermometer of Krogh Ravna (2000) and the jadeite + quartz geobarometer of Carswell & Harley (1990) are applied. Subsequent retrogression partially transformed quartz needles into albite needles with irregular outline in two of the samples (Riksheim, Ulsteinvik) at the expense of jadeite in the proximal host. Rare associated needles of cristobalite and an unknown phase with albite chemistry in these two southernly samples, perhaps as a result of retrogression, were not observed in the three northernly samples. Hence, the evolution of the pyroxene microstructures after formation allows to investigate spatial differences in the retrogression history.</p><p><span>This work is financially supported by the Norwegian Financial Mechanism 2014-2021 and the Polish National Science Centre, project no. 2020/37/K/ST10/02784.</span></p><p>Carswell, D.A. & Harley, S.L. (1990): Mineral barometry and thermometry. In: Carswell, D.A. (ed.) Eclogite Facies Rocks. Glasgow and London: Blackie, 83-110.</p><p>Krogh Ravna, E. (2000): The garnet–clinopyroxene Fe<sup>2+</sup>–Mg geothermometer: an updated calibration. Journal of Metamorphic Geology 18:211-219.</p>

The Western Gneiss Region (WGR) is dominated by orthogneisses and bounded by normal-sense shear zones against overlying allochthons. This vast mass of granitoid rocks underwent subduction and re-emergence from the throat of the subduction channel, possibly rupturing the overlying orogenic wedge to open a tectonic window in the orogenic hinterland [2]. In this contribution I will explore available information regarding the role of buoyancy in driving tectonics during formation of this huge tectonic window (e.g. [5]) as an additional factor to permissive uprise within an externally-imposed kinematic system (e.g. [1], [8]).The WGR is characterised by foliation domes (culminations) in which orthogneisses emerge from below the Scandian allochthons or UHP domains emerge from below HP rocks [4], [5] [8]. Some are metamorphic core complexes (MCC’s) with solid ductile cores [8] but others, cored by migmatite, resemble gneiss domes [7] such as the eastern part of the WGR, a classic area for the study of gravity tectonics [5]. The domes, ovoidal in plan form, are wrapped by the allochthons; the gneiss cores also over-ride the allochthons to form basement-cored fold-nappes. Ramberg’s analogue models of rising gneiss diapirs generated a similar architecture. A key factor is that the gneisses are initially overlain by a denser lid, which creates gravitational instability; this was possibly represented by the ophiolites and arc rocks of the Trondheim Nappe Complex. The density inversion is enhanced by partial melting in the gneisses. The Oppdal domes area have also been interpreted as giant sheath-folds in a simple-shear field [6]. This may be consistent with a scenario where lateral channel flow is combined with diapiric action [7] where breaching of the lid forms an “aneurism”. MCC’s and gneiss domes are important mechanisms for heat dissipation in orogens; in the eastern WGR metamorphic grade in the nappes flanking the domes increases towards the gneisses and with depth in infolded synformal “keels” [3], [4] suggesting transfer of heat advected by the gneiss into the cover. Inverted metamorphic gradients may be generated where domes over-ride the cover.Understanding the relative roles of buoyancy as a direct driver of exhumation tectonics in the WGR versus permissive uprise controlled by the shear-zone framework will require more detailed mapping-out of Caledonian-age partial melting and metamorphic patterns in the orthogneisses, and new studies of kinematics of the eastern and northern dome systems of the WGR.Financial support from the National Science Centre, Poland (grant 2014/14/E/ST10/00321) and from AGH UST, Krakow, Poland.[1] Bottrill et al. (2014) Geochem. Geophys.Geosyst. doi:10.1002/2014GC005253[2] Brueckner & Cuthbert (2013) Lithosphere doi:10.1130/L256.1[3] Goldschmidt (1915) Skrift. Vid.-Selksk. Kristiana I. Mat.-Naturvid. Klasse, 6: 1-38[4] Krill (1985) In: Gee & Sturt The Caledonide orogen: Scandinavia and Related Areas, pp. 475-483. J. Wiley & Sons Ltd., Chichester.[5] Ramberg (1966) Bull. geol. Instn. Uppsala 43: 72pp.[6] Vollmer (1988) Journal of Structural Geology 10, 735-743[7] Whitney et al. (2004) Geol. Soc. America Special Paper 380: 1-19.[8] Wiest et al. (2020) Journal of the Geological Society, London doi:10.1144/jgs2020-199

<p>The Caledonian Orogeny in the Scandinavian Caledonides (COSC) project aims to investigate the orogenic processes involving Caledonian allochthons together with the underlying sedimentary cover and Proterozoic igneous basement. The basement comprises Transscandinavian Igneous Belt (TIB) rocks with Hallandian and Central Scandinavian Dolerite Group intrusions and is overlain by a regolith (sub-Cambrian peneplain?). A Lower Cambrian(?) sedimentary succession of conglomerate, carbonate and shale covers this immature soil, followed by coarse-grained gravity flows fining upwards and showing a transition into the Alum Shale Formation. The undisturbed middle part of the formation separates the lower sedimentary cover from its overlying turbiditic part and the Lower Ordovician(?) turbidite sequence fining up to the top of the COSC-2 core.</p> <p>First results of detrital zircon geochronology from the Cambrian succession show that the basal section of the autochthonous cover is characterized by mainly late Paleoproterozoic (c. 45% of all grains) – early Mesoproterozoic (c. 52%) detrital grains with age signatures of c. 1.77 Ga, 1.66 Ga and 1.44 Ga and a subordinate 1.25 Ga age peak. The middle part of the succession is dominated by late Paleoproterozoic detritus (c. 62% of all grains) with minor Mesoproterozoic (c. 21%) and Archean (c. 11%) input. The main age signatures are c. 1.80 Ga and 1.90 Ga with subordinate age peaks at c. 2.72 Ga, 2.00 Ga, 1.16 Ga. The upper part of Lower Cambrian(?) succession is characterized by Archean to Cambrian detritus. Archean grains constitute 12% of grains with dominant age signature at c. 2.67 Ga. Paleoproterozoic grains (25%) are grouped in 2.15-1.65 Ga interval with peaks at c. 2.12 Ga, 1.80 Ga, 1.76 Ga and 1.67 Ga. The Mesoproterozoic population (41%) is characterized by major age peaks at c. 1.55 Ga and 1.20 Ga. Neoproterozoic – Cambrian group (17%) contains major populations at c. 0.60 Ga and 0.53 Ga and a significant peak at c. 0.72 Ga. The maximum depositional age calculated via the maximum likelihood age algorithm yielded 530.5±4 Ma for the upper part of the Lower Cambrian succession. Two samples from the Ordovician succession show Mesoproterozoic – Neoproterozoic sources (c. 75% of grains), with more than 38% of grains yielding late Mesoproterozoic – early Neoproterozoic (1.2-0.9 Ga) ages. The dominant population of c. 1.06-1.02 Ga is accompanied by c. 1.50-1.47 Ga, 1.15 Ga and 0.99-0.97 Ga age peaks.</p> <p>The autochthonous Lower to Lower Middle Cambrian passive margin succession in the lower part is dominated by local detritus provided solely from the Eastern Segment of Sveconorwegian Orogen (including the basement investigated by the COSC-2). The provenance shifts up the profile towards TIB-1 and Svecofennian Orogen sources, with the youngest part of the succession characterized by an input of Timanian Orogen detritus, including the uplifted Karelian protocraton. The Ordovician succession is characterized by Meso-Neoproterozoic age populations most likely sourced from the Sveconorwegian Orogen with a minor cratonic contribution. The youngest detritus is early Neoproterozoic, suggesting a passive margin setting with no early Caledonian input present.</p> <p>This work was funded by the National Science Centre (Poland) projects no. 2019/33/B/ST10/01728 and 2018/29/B/ST10/02315.</p>

To better understand the subduction–exhumation cycles of the Baltoscandian margin that reached (U)HP depths during the Caledonian orogeny, we have performed in-situ U-(Th-)Pb dating coupled with REE analysis of zircon and ± monazite in four samples from the supracrustal rocks of the Blåhø Nappe on Gossa island in the Western Gneiss Region (WGR) of Norway. We dated two garnet-plagioclase-biotite gneisses and two garnet-plagioclase-amphibole gneisses. Our research focused on deciphering the early metamorphic evolution of these complex rocks that have been overprinted by exhumation-related structures and pervasive retrogressive metamorphism. The dated zircon grains are spherical or slightly elongated in shape, some of which display clear multi-stage growth features. Only one grain armored by garnet preserved an older detrital core that yielded early Neoproterozoic dates between 1.1-1.0 Ga. This grain does not provide any Caledonian signal. Younger individual 206Pb/238U dates show three distinct populations that yield three concordia ages, each obtained from distinctly different compositional domains, the oldest from cores and the two youngest from overgrowths. The cores are characterized by HREE enrichment (high Lu/Gd ratios ca. 14.5), high Th/U ratios (> 0.1), and large Eu anomalies. They yield a concordia age of 474 ± 6.4 Ma. These cores can be rimmed by two different types of zircon overgrowth. The first overgrowth type (1) displays the same REE pattern as the cores and gives a concordia age of 444± 4.3 Ma. The second overgrowth type (2) shows a very weak Eu anomaly, no HREE enrichment (low Lu/Gd ratios ca. 2.37) and a very low Th/U ratios (<0.1). These yield a concordia age of 416± 3.7 Ma. The two older U–Pb zircon age populations are tentatively interpreted as reflecting two distinct metamorphic events or a prolonged episode of metamorphism. The youngest concordant metamorphic zircon dates a high grade, probably (U)HP, metamorphic overprint at ca. 416 Ma, subsequent to the previous events. Analyses performed on monazite provided complementary age records to those obtained on zircon. Monazite grains are weakly zoned, exhibit wormy shapes and are aligned with the youngest foliation. Th–U–total Pb dating of monazite, coupled with major and trace element mapping of monazite, yielded a very homogeneous age of 382 ± 1.6 Ma (n=65) interpreted to date the late shearing, which possibly accommodated a late stage of exhumation. Funded by the National Science Centre (Poland) project no. 2014/14/E/ST10/00321.

The northern part of the Western Gneiss Region (WGR) has distinctive belts of allochthonous metasediments and mafic rocks lying within tight infolds into the Baltica basement. They outcrop from the Grong-Olden Window to the Norwegian coast, possibly as far SW as Sørøyane, predominantly comprising metapelite and amphibolite with psammite, marble, calc-silicate, local large eclogite (>4km) lenses and ultramafites. These supracrustal lithotectonic units are attributed to the Blåhø Nappe, correlated with the Seve Nappe Complex (SNC) in its main outcrop in Sweden, which is considered to represent the pre-Caledonian continent-ocean transition (COT) of Baltica. They closely resemble the Lower Seve Nappe in northern Sweden where large amphibolite massifs with marbles are common, along with local eclogites. At least some have geochemical characteristics of spilitised extrusive MORB basalt in contrast to the better known, Neoproterozoic Baltoscandian Dyke Swarm in the SNC. In the WGR near Molde a >10km long massif of such “amphibolite” at Tverrfjella commonly exhibits a relict high-P granulite precursor that has, in turn, overprinted eclogite. It encloses marble, scapolite-bearing calc-silicate, garnet peridotite (harzburgite) and Cu ores. Marble and meta-eclogite are intermixed which, along with its high Na spilitic character, suggests that the protolith was extrusive. Limited geochemical data suggest MORB composition. P-T estimates for eclogites in adjacent belts suggest UHP, possibly diamond-stable, conditions; in Sørøyane the well-known Ulsteinvik eclogite contains coesite. In the Molde area some of the mafic rocks and metasediments have partially melted. Eclogite metamorphism was Scandian in the Tverrfjell massif at 418 ± 11 Ma, with similar ages but tighter errors for adjacent belts and Ulsteinvik. These are significantly younger than ages for (U)HP metamorphism in the main SNC outcrop in Sweden, where early Ordovician subduction with a latest Ordovician granulite overprint is recorded. However, metapelites in other Blåhø-like supracrustal belts in the WGR do seem to record this earlier history as does one eclogite, consistent with the “double-dunk” hypothesis in this hinterland region. The protolith age of the metabasalts is unknown; analogy with the BDS suggests Neoproterozoic, but some zircon data from the WGR may suggest magmatic crystallisation during the Ordovician.O-isotopes indicate that the marbles were Palaeozoic, rather than Proterozoic, carbonates. Overall, the available literature data show that some large mafic massifs in the WGR, with associated metasediments and peridotites, are allochthonous with respect to Baltica basement; they represent major additions of extrusive basalt to a far-distal COT or fully oceanic basin that have been subducted at least once during the Caledonian Wilson cycle. Isotopic data hint that at least some of their protoliths are unusually young. These supracrustal belts certainly merit closer attention.