Detrital zircon U-Pb geochronology of a metasomatic calc-silicate in the Tsäkkok Lens, Scandinavian Caledonides

The Tsäkkok Lens of the Seve Nappe Complex in the Scandinavian Caledonides comprises eclogite bodies hosted within metasedimentary rocks. These rocks are thought to be derived from the outermost margin of Baltica along the periphery of the Iapetus Ocean, but detrital records from the sedimentary rocks are lacking.Many metasedimentary outcrops within the lens expose both well-foliated metapelitic rocks and massive calc-silicates. The contacts between these two lithologies are irregular and are observed to trend at all angles to the high-pressure foliation in the metapelites. Where folding is present in the metapelites, the calc-silicate rocks are also locally folded. These relationships suggest metasomatism of the metapelites during the Caledonian orogenesis. Zircon U-Pb geochronology was conducted on sixty-one zircon grains from a calc-silicate sample to investigate if they recorded the metasomatic event and to assess the detrital zircon populations. Zircon grains predominantly show oscillatory zoning, sometimes with thin, homogeneous rims that have embayed contacts with the oscillatory-zoned cores. The zircon cores yielded prominent early Stenian, Calymmian, and Statherian populations with a subordinate number of Tonian grains. The zircon rims exhibit dissolution-reprecipitation of the cores or new growth and provide ages that span similar time frames, indicating overprinting of successive tectonic events. Altogether, the zircon record of the calc-silicate suggests that the Tsäkkok Lens may be correlated to Neoproterozoic basins that are preserved in allochthonous positions within the northern extents of the Caledonian Orogen.

Regional study of orogenic ultramafics of the Seve Nappe Complex, Scandinavian Caledonides - preliminary results from northern and central Jämtland, Sweden

<p>The Scandinavian Caledonides comprise numerous ultramafic bodies emplaced within metamorphic nappe complexes. A hypothetical suture between the most distal crustal units representing Baltican margin (Seve Nappe Complex, SNC) with the oceanic Iapetian terranes (Köli Nappe Complex) is abundant in such occurrences. Here we present preliminary data on garnet/spinel peridotites/pyroxenites from SNC in central and northern parts of Swedish Jämtland county. The presented results are a part of a project involving regional study focused on orogenic peridotites (mostly spinel-bearing) of Seve and Köli nappe complexes. </p><p>The ultramafic bodies in the study area range from a meters to kilometer scale and comprise: 1) garnet peridotites, 2) spinel peridotites, 3) spinel pyroxenites and 4) garnet pyroxenites. Individual outcrops often record different levels of serpentinisation. </p><p>The Grt-peridotites are usually harzburgites (sparsely dunites/lherzolites) with an assemblage of Ol+Opx+Cpx+Amph+Grt+Spl.  Minerals within the Grt-peridotites are characterised by Ol Fo=~90-91 and Mg# in pyroxenes 90-92 and 92-96 (enstatite and diopside/Cr-diopside, respectively). Garnet is pyrope with end-members Prp=60-69%, Usp=0-4% and Cr#=0.5-4. Amphibole (pargasite; Mg#=88-92) typically occurs as patches or rims around Grt and often host significant amounts of Spl. The spinel has an intermediate composition between hercynite-spinel and magnesiochromite-chromite (Cr#=41-55, Mg#=40-57). </p><p>The spinel peridotites are formed of Ol+Opx+Amph+Chl+Spl and classify mostly as harzburgites/dunites. Olivine and Opx (enstatite, rarely Cr-enstatite; often as porphyrocrysts) show a high range of Fo/Mg# values (90-95 and 90-94, respectively). Amphibole (tremolite; Mg#=91-96) is usually evenly distributed within the rock, while Chl is often associated with grain boundaries. Spinel has a chromite composition (Cr#=82-100, Mg#=5-10). Within single large (~0.5mm) spinel grains, cores with higher Mg# (~23) and lower Cr# (~82) can be observed.</p><p>The garnet pyroxenites are websterites characterised by lower Mg# (88-90) in enstatite, presence of Al-diopside and lower Cr# (<0.5) in pyrope than in peridotites. The Spl-pyroxenites are orthopyroxenites with Mg# in enstatite (86-88) lower than in peridotitic orthopyroxene.</p><p>The presented preliminary data suggest that lithologies formed under different pressures (i.e. Grt and Spl facies) and must have recorded different evolution paths. Garnet ultramafics mineralogy resembles typical “mantle” assemblage with Prg suggesting possible metamorphic input also for other consisting phases (similarly to M2 paragenesis described in [1]). While the Grt ultramafic rocks and their evolution has been a subject of several studies before, the Spl ultramafics are relatively understudied and can shed new light on the evolution of SNC. The composition of Spl peridotites represents a mixture of typical “magmatic” mantle phases with metamorphic minerals (Amph+Chl). Very high Mg# values and occurrence of 120° triple point junctions in Ol (also described in [2]) suggest complex genesis, which probably includes serpentinisation (+exhumation?) followed by deserpentinisation. This indicates that the Spl ultramafics of SNC might have been subducted after their primary serpentinisation, which can be related either to emplacement and exhumation of ultramafics during Rodinia breakup or derivation from shallow, serpentinised “wet” mantle wedge in the subduction zone. </p><p>Research founded by Polish National Science Centre grant no. 2019/35/N/ST10/00519.</p><p>[1] Gilio et al. (2015). Lithos 230, 1-16.<br>[2] Clos et al. (2014). Lithos 192-195, 8-20.</p>

Late Neoproterozoic granulite facies metamorphism of the Upper Gneiss unit (Seve Nappe Complex) in the Váivančohkka-Salmmečohkat area, northern Scandinavian Caledonides

<p>Here, we present preliminary petrochronological results of paragneisses and schists containing bodies of metamafic rocks belonging the Upper Gneiss unit that occurs within the Seve Nappe Complex (SNC) in the Váivančohkka-Salmmečohkat area, north of the lake Torneträsk in northern Sweden and Norway.</p><p>At the outcrop scale, the paragneiss is pervasively foliated and bears features of migmatization. It hosts garnet amphibolite bodies that are locally transected by leucocratic veins. Thin section observations of the paragneiss reveal a mineral assemblage composed of Q+Grt+Amp+Bi±Pl±Ms±Sil±Ru. The leucocratic vein contains Q+Pl+Ms+Bi+Grt+Kfs±Sil. Importantly, some of the studied gneisses contain quartz, exhibiting lobate boundaries, as well as garnet surrounded by melt rim. The presence of quartz forming pseudomorphs after melt was also identified and observed to host both monophase and fluid inclusions. All of these microtextures are indicative of partial melting.</p><p>Preliminary pressure-temperature estimates derived using conventional geothermobarometry and phase equilibrium modelling corroborated petrographic observations. The peak metamorphic conditions were estimated to 8–10kbar and 800–850°C, i.e., in the stability field of melt.</p><p>Uranium-Pb zircon and Th-U-total Pb monazite dating of the migmatitic paragneiss yielded consistent age estimates of 602±5Ma and 599±3Ma, respectively. Nearly the same U-Pb age of 604±7Ma was obtained for the zircon from the leucocratic vein transecting the amphibolite within the studied gneiss. Interestingly, no Caledonian zircon nor monazite were identified. Considering the textural position of the dated zircon and monazite, as well as their chemical character, we suggest that these minerals date the partial melting event recorded by the rocks.</p><p>Regionally, we interpret that the Upper Gneiss unit of SNC in the Váivančohkka-Salmmečohkat area could be a northern continuation of the Leavasvággi gneiss associated with the Vassačoru Igneous Complex of SNC in the Kebnekaise region. Notably, the latter reveals evidence of high temperature metamorphism at c. 600Ma (Paulsson and Andréasson 2002) and its mafic component (see also Rousku et al. in this session) could be an equivalent to the metamafic rocks enclosed within the Upper Gneiss unit. The Leavasvággi gneiss and the Upper Gneiss unit together with similar rocks farther north in Indre Troms and in Corrovare which also yield a c. 610-600Ma age of high grade overprint (Gee et al. 2016; Kjøll et al. 2019). Altogether, these areas with only localized Caledonian influence diverge from traditional models developed for the SNC farther south and offer an additional insight into the development of the late Neoproterozoic margin of Baltica at the early stages of Iapetus opening.</p><p>This study was supported by the National Science Centre (Poland) grant no. 2019/33/B/ST10/01728 to J. Majka.</p><p>References</p><p>Gee et al. 2016. Baltoscandian margin, Sveconorwegian crust lost by subduction during Caledonian collisional orogeny. GFF 139, 36–51.</p><p>Kjøll et al. 2019. Timing of break-up and thermal evolution of a pre-Caledonian  Neoproterozoic exhumed magma-rich rifted margin. Tectonics 38, 1843-1862.</p><p>Paulsson & Andréasson 2002. Attempted break-up of Rodinia at 850 Ma: geochronological evidence from the Seve–Kalak Superterrane, Scandinavian Caledonides. JGS, 159, 751-761.</p>

The subduction, exhumation, and deformation history of the Vaimok Lens, Seve Nappe Complex, Scandinavian Caledonides

<p>            The Seve Nappe Complex (SNC) of the Scandinavian Caledonides represents portions of the Baltican margin that were subducted to mantle depths. Eclogite-bearing sub-units of the SNC provide a record of this important step in orogen development. One such sub-unit is the Vaimok Lens of the SNC in southern Norrbotten. The Vaimok Lens constitutes eclogites hosted within metasedimentary rocks that reached ultra-high pressure (UHP) conditions in the Cambrian/Early Ordovician period. The metasedimentary rocks are typically composed of quartz, white mica, garnet, plagioclase, biotite, clinozoisite, apatite and titanite, and show a pervasive ‘S2’ foliation that developed during exhumation. Garnet is recognized as a relic of prograde metamorphism during subduction, whereas the other minerals represent retrogressive metamorphism during exhumation. To resolve the timing of prograde metamorphism, Lu-Hf geochronology was conducted on metasediment-hosted garnet that preserves prograde, bell-shaped Mn-zoning with a chemical formula of Alm<sub>69-59</sub>Grs<sub>32-24</sub>Sps<sub>13-2</sub>Prp<sub>5-2</sub>. The results indicate garnet growth at 495.3 ± 2.6 Ma. Quartz-in-garnet (QuiG) elastic geobarometry was also conducted on garnet from the same sample, providing pressures of 0.9-1.3 GPa, calculated at 500-700°C. Six samples were obtained for in-situ <sup>40</sup>Ar/<sup>39</sup>Ar geochronology, targeting white mica defining the S2 foliation. Samples can be classified as: 1) low-strain (n: 3), with large (>400 µm width), undeformed micas that are chemically homogeneous (X<sub>Cel</sub>: 0.24-0.35), which yielded a weighted average <sup>40</sup>Ar/<sup>39</sup>Ar population of 470.5 ± 5.9 Ma; 2) high-strain (n: 3), with small (<300 µm width) mica fish with heterogeneous chemistry (X<sub>Cel</sub>: 0.03-0.27), which provided weighted average <sup>40</sup>Ar/<sup>39</sup>Ar populations of 447.6 ± 2.6 Ma and 431.1 ± 4.1 Ma. An additional sample from the basal thrust of the lens that contains large (>300 µm width), homogeneous (X<sub>Cel</sub>: 0.24-0.34) mica was also dated, yielding a population of 414.1 ± 5.8 Ma. Altogether, the data indicates that the Vaimok Lens was subducting by c. 495 Ma. The lens underwent post-decompression cooling at c. 470 Ma, possibly decompressing to 0.9-1.3 GPa by this time. This would equate to an exhumation rate of 3-9 mm/yr. Imbrication of the SNC in southern Norrbotten is taken to be c. 447 Ma. Scandian deformation was active by c. 431 Ma and led to overthrusting of the SNC onto subjacent nappes by latest c. 414 Ma. Both the timescale for subduction and the rates of exhumation for the Vaimok Lens reflect subduction-exhumation dynamics of large UHP terranes. Furthermore, the timing of imbrication and Scandian deformation in southern Norrbotten is similar to estimates along strike of the SNC. These results indicate that the SNC acted as a large UHP terrane that underwent a ~25 Myr cycle of subduction and exhumation during the late Cambrian/Early Ordovician, before being deformed and partially dismembered in subsequent accretionary and collisional events.</p><p> </p><p>Research funded by National Science Centre (Poland) project no. 2014/14/E/ST10/00321 to J. Majka.</p>

Deciphering the Vássačorru Igneous Complex within the Seve Nappe Complex, Scandinavian Caledonides

<p>The Seve Nappe Complex (SNC) of the Scandinavian Caledonides comprises Neoproterozoic sedimentary and igneous rocks that experienced high-pressure metamorphism and deformation during subduction and exhumation. Fieldwork was conducted in the Kebnekaise region in northern Sweden, focusing on the Aurek metagabbro and the Vistas metaigneous rocks within the Vássačorru Igneous Complex (VIC), hosted within SNC metasediments. Field observations show that the Aurek metagabbro is locally sheared with well-defined foliation and lineation. In contrast, the Vistas metaigneous rocks, consisting of both granite and gabbro bodies, are only locally foliated. Furthermore, the granite is intruded by ENE-WSW striking dolerite and rhyolite dykes that parallel the local foliation, and are weakly deformed, whereas a NNE-SSW striking syenite dyke is observed in a portion of undeformed gabbro.</p><p>The Aurek metagabbro mineral assemblages consist of garnet, amphibole, plagioclase, biotite, chlorite, and pyroxene. The Vistas gabbro and dolerite dyke both consist of plagioclase, pyroxene, and amphibole. The Vistas granites and rhyolite dyke include quartz, feldspar, biotite, muscovite, ± garnet, and the syenite dyke contains feldspar, plagioclase, pyroxene, amphibole, quartz, and biotite. The Vistas metaigneous rocks generally show primary igneous assemblages.</p><p>Bulk rock chemistry shows that the Aurek and Vistas gabbros, and the Vistas dolerite dyke, are classified as tholeiites. For the Aurek gabbros, Th/Yb of 0.06-1.86 and Nb/Yb of 0.11-5.14 indicate that they have N-MORB to E-MORB compositions, with possible crustal input. The Vistas gabbro (Th/Yb of 0.09 and Nb/Yb of 1.15) and the dolerite dyke (Th/Yb of 0.12 and Nb/Yb of 0.66) also suggest such trend. The Vistas granites, rhyolite, and syenite dyke all have calc-alkaline composition. Trace elements confirm volcanic arc affinity for the granites and the syenite dyke (Nb: 3.1-5.9 ppm, Rb: 116.5-177.5 ppm, Y: 12.9-18.0 ppm, Ta: 0.3-0.4 ppm, Yb: 2.04-3.19 ppm), whereas the rhyolite dyke (Nb: 38.2 ppm, Rb: 247.8 ppm, Y: 72.6 ppm, Ta: 2.8 ppm and Yb: 12.62 ppm) reflects a within plate setting.</p><p>Combining the field relationship with geochemistry of the studied metaigneous rocks, we tentatively propose that the VIC is composed of three pulses of magmatism: (1) mafic MORB magmatism represented by the gabbros, emplaced in an extensional regime; (2) felsic calc-alkaline magmatism represented by granites and syenite, emplaced in an active continental margin environment; and (3) bimodal within-plate magmatism or crustal assimilation in a volcanic arc represented by dolerite and rhyolite dykes. However, the only existing age is from U-Pb zircon dating of the Vistas granite, which yielded 845±14 Ma (Paulsson & Andreasson, 2002). Further zircon U-Pb geochronology will be conducted to obtain ages of the various lithologies of the VIC to better understand temporal relationships and to link the VIC with tectonic events in the Scandinavian Caledonides.</p><p>This study was supported by the National Science Centre (Poland) grant no. 2019/33/B/ST10/01728 to J. Majka.</p><p>References</p><p>Paulsson, O., Andreasson, P.-G., 2002. Attempted break-up of Rodinia at 850 Ma: Geochronological evidence from the Seve-Kalak Superterrane, Scandinavian Caledonides. J. Geol. Soc. 159, 751–761. https://doi.org/10.1144/0016-764901-156</p>

Large meta-eclogite massifs within the Western Gneiss Region, Scandinavian Caledonides: Subducted ocean-continent transition?

<p>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.</p><p>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.<sup></sup>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.</p>

Timing of deformation, metamorphism and leucogranite intrusion in the lower part of the Seve Nappe Complex in central Jämtland, Swedish Caledonides

<p>The new LA-ICP-MS zircon isotope age data from paragneiss, amphibolite and two leucogranite intrusions in the Lower Seve Nappe of the Åre synform in the Caledonides of central Jämtland provide evidence of both Silurian and Ordovician tectonothermal histories. Well established concordant c. 468 and c. 470 Ma magmatic ages for the Så quarry leucogranite, which cut earlier foliations and folds in the host-rock amphibolites and paragneisses, imply a tectonothermal history prior to the Middle Ordovician (c. 469 Ma), perhaps synchronous with what has been previously recognized in the Seve Nappe Complex of Norrbotten (e.g. Root & Corfu, 2012), 400 km farther north in the Swedish Caledonides, and very recently also in the Middle Seve Nappe in central Jämtland (Walczak et al. 2020).</p><p>The field relationships and data presented here show that magmatic activity occurred during the early Silurian (c. 443 Ma) and earlier during the Early to Middle Ordovician (c. 469 Ma), and that deformation and metamorphism took place both prior to and after c. 469 Ma. The Lower Seve rocks from the nearby COSC-1 drill core have been metamorphosed in the upper amphibolite facies, however, the remnants of the high-pressure metamorphic history are preserved in the relic minerals, including high-silica white mica, in the garnet-bearing mica schists. The exact age of the high-pressure metamorphism is not known so far; however, it predates the 460-430 Ma amphibolite facies deformation recorded by titanites in the amphibolites (Giuntoli et al. 2020).    </p><p>Zircons in an amphibolite proved to be highly discordant but indicate Early Silurian metamorphism during isoclinal folding. Detrital zircons in a paragneiss are dominated by Sveconorwegian populations, but also include a range of younger Neoproterozoic grains down to the Early Ediacaran (c. 600 Ma).</p><p>This new evidence of early Caledonian deformation and metamorphism indicates that the Seve tectonothermal history in central Jämtland probably started early in the Ordovician, or before. Subduction and accretion along the Baltoscandian outer margin occurred prior to the Scandian continent-continent collision, with Siluro-Devonian emplacement of the Seve Nappe Complex across the foreland basins onto the Baltoscandian platform.</p><p>References:</p><p>Giuntoli, F., Menegon, L., Warren, C.J., Darling, J., Anderson, M.W. 2020. Tectonics, 39, e2020TC006267, https://doi.org/10.1029/2020TC006267.</p><p>Root, D., Corfu, F. 2012. Contributions to Mineralogy and Petrology, 163, 769-788, https://doi.org/10.1007/s00410-011-0698-0.</p><p>Walczak, K., Barnes, C.J., Majka, J., Gee, D.G. Klonowska, I., 2020. Geoscience Frontiers (in press), https://doi.org/10.1016/j.gsf.2020.11.009.</p><p>This work is financially supported by the National Science Centre (Poland) research project no. 2018/29/B/ST10/02315 and is part of the ICDP project “Collisional Orogeny of the Scandinavian Caledonides.”</p>

Core-log-seismic integration in metamorphic rocks at the ICDP drilling project COSC-1, Sweden

<p>Continental collision causes deformation in the crust along shear zones. However, the physical and chemical conditions at which these zones operate and the deformation processes that enable up to hundreds of km of tectonic transport are still unclear because of the depth at which they occur and the challenges in imaging them. Ancient exhumed collision zones allow us to investigate these processes much better, for example at the COSC-1 borehole in the central Scandinavian Caledonides. This study combines data from the COSC-1 borehole, such as downhole logging and zero-offset vertical seismic profile data, with 2D and 3D seismic measurements to provide constraints on the spatial lithological and textural configuration of the Seve Nappe Complex. This is one of the few studies that shows that core-log-seismic integration in metamorphic rocks allows to identify the spatial distribution of major lithological units, even though the methodology was originally developed for sedimentary basins in the hydrocarbon industry. Especially gamma ray logs in combination with density data are powerful tools to distinguish between mafic and felsic lithologies in log-core correlation. Reflections in the Seve Nappe Complex are not as distinct as in greater depths but continuous, and our results indicate that they are primarily caused by compositional rather than textural changes. Several of the reflections can be linked to magmatic intrusions, which have been metamorphically overprinted. Their setting indicates that the Seve Nappe Complex consists of the remnants of a volcanic continental margin. It appears that in spite of the metamorphic overprint around 417+/-9 Ma, the original configuration of the volcanic passive margin is partly preserved in the Seve Nappe Complex and that it outlasted continent-continent collision, including the nappe emplacement. Thus, an integration of borehole and three-dimensional geophysical data can image lithological changes that can then be extrapolated in three dimensions to arrive at a better understanding of the composition and geometry at mid-crustal levels. Furthermore, our results suggest that ductile-deformed middle crustal reflectivity is primarily a function of pre-orogenic lithological variations which has to be considered when deciphering mountain building processes.</p>