| 1. |
- Barnes, Christopher, et al.
(författare)
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High-spatial resolution dating of monazite and zircon reveals the timing of subduction–exhumation of the Vaimok Lens in the SeveNappe Complex (Scandinavian Caledonides)
- 2019
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Berlin/Heidelberg. - 0010-7999 .- 1432-0967. ; 174:1
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Tidskriftsartikel (refereegranskat)abstract
- In-situ monazite Th–U–total Pb dating and zircon LA–ICP–MS depth-profiling was applied to metasedimentary rocks from the Vaimok Lens in the Seve Nappe Complex (SNC), Scandinavian Caledonides. Results of monazite Th–U–total Pb dating, coupled with major and trace element mapping of monazite, revealed 603 ± 16 Ma Neoproterozoic cores surrounded byrims that formed at 498 ± 10 Ma. Monazite rim formation was facilitated via dissolution–reprecipitation of Neoproterozoic monazite. The monazite rims record garnet growth as they are depleted in Y2O3 with respect to the Neoproterozoic cores. Rims are also characterized by relatively high SrO with respect to the cores. Results of the zircon depth-profiling revealed igneous zircon cores with crystallization ages typical for SNC metasediments. Multiple zircon grains also exhibit rims formedby dissolution–reprecipitation that are defined by enrichment of light rare earth elements, U, Th, P, ± Y, and ± Sr. Rims also have subdued Eu anomalies (Eu/Eu* ≈ 0.6–1.2) with respect to the cores. The age of zircon rim formation was calculated from three metasedimentary rocks: 480 ± 22 Ma; 475 ± 26 Ma; and 479 ± 38 Ma. These results show that both monazite and zircon experienced dissolution–reprecipitation under high-pressure conditions. Caledonian monazite formed coeval with garnet growth during subduction of the Vaimok Lens, whereas zircon rim formation coincided with monazite breakdown to apatite, allanite and clinozoisite during initial exhumation.
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| 2. |
- Barnes, Christopher J., et al.
(författare)
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40Ar/39Ar dates controlled by white mica deformation and strain localization : Insights from comparing in situ laser ablation and single-grain fusion techniques
- 2023
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Ingår i: Journal of Metamorphic Geology. - : John Wiley & Sons. - 0263-4929 .- 1525-1314. ; 41:9, s. 1143-1166
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Tidskriftsartikel (refereegranskat)abstract
- In situ laser ablation and single-grain fusion Ar-40/Ar-39 geochronological techniques were directly compared using white mica from nine metasedimentary rocks from the Vaimok Lens of the Seve Nappe Complex (SNC) in the Scandinavian Caledonides. Seven of the rocks are from the eclogite-bearing Grapesvare nappe within the lens that is defined by D2 structures (S2 and F2), which were formed during exhumation following late Cambrian/Early Ordovician ultra-high pressure metamorphism. Two other rocks were obtained from 'Scandian' shear zones that delimit the nappes within the lens. The shear zones were active during terminal collision of Baltica and Laurentia in the Silurian to Devonian. The rocks exhibit variable deformation intensities and degrees of strain localization, expressed in particular by white mica. The in situ laser ablation and single-grain fusion Ar-40/Ar-39 dates both span from the late Cambrian to Middle Devonian. Results of both techniques generally show decreasing dates with increasing bulk deformation intensity and successive structural generations (i.e., D2 then Scandian structures). Furthermore, several discrepancies are evident when comparing the results of the two techniques for the same rocks, indicating the Ar-40/Ar-39 dates are not solely governed by bulk deformation intensities and structural generations. Instead, the discrepancies demonstrate the additional influence of white mica strain localization, which is illuminated by the different analytical volumes of the techniques. Thus, the Ar-40/Ar-39 datasets are altogether deciphered as a function of bulk deformation intensity and degree of strain localization that affected the overall white mica volume. The former controls the gross Ar-40 loss from the overall volume and the latter dictates the variability of Ar-40 loss within the volume. Exploiting the interplay of these two phenomena for the Vaimok Lens rocks with in situ laser ablation allows for the broad span of Ar-40/Ar-39 dates to be contextualized into a sequence of tectonic events: (1) cooling at 474 +/- 3 Ma, (2) pre-collision deformation at 447 +/- 2 Ma and (3) activation of crustal-scale shear zones in the SNC related to continental collision at 431 +/- 3 Ma and 411 +/- 3 Ma.
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| 3. |
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| 4. |
- Barnes, Christopher J., et al.
(författare)
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Detrital zircon U-Pb geochronology of a metasomatic calc-silicate in the Tsäkkok Lens, Scandinavian Caledonides
- 2021
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Ingår i: GEOLOGY GEOPHYSICS AND ENVIRONMENT. - : AGHU University of Science and Technology Press. - 2299-8004 .- 2353-0790. ; 47:1, s. 21-31
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Tidskriftsartikel (refereegranskat)abstract
- The Tsakkok Lens of the Seve Nappe Complex in the Scandinavian Caledonides comprises eclogiie 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 talc-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 talc-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 talc-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 talc-silicate suggests that the Tsakkok Lens may be correlated to Neoproterozoic basins that are preserved in allochthonous positions within the northern extents of the Caledonian Orogen.
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| 5. |
- Barnes, Christopher J., et al.
(författare)
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Garnet-Quartz Inclusion Thermobarometry and Lu-Hf Chronology Detail the Pre-Ultra-High Pressure Metamorphic History of the Grapesvare Nappe, Scandinavian Caledonides
- 2023
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Ingår i: Journal of Petrology. - : Oxford University Press. - 0022-3530 .- 1460-2415. ; 64:12
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Tidskriftsartikel (refereegranskat)abstract
- The subduction–exhumation history of the Grapesvare nappe in the northern Seve Nappe Complex (Scandinavian Caledonides) is recorded by late Cambrian/Early Ordovician ultra-high pressure (UHP) and subsequent amphibolite facies metamorphic events. Records of these events obscured earlier metamorphic episodes that are important for understanding the tectonics of the orogen. To extract the pre–UHP metamorphic records, garnet Lu–Hf geochronology, Titanium-in-Quartz thermobarometry, and Quartz-in-Garnet elastic thermobarometry were applied to garnet porphyroblasts in metasedimentary rocks and eclogite. Metasedimentary rocks contain chemically homogeneous garnet (Grt-M1) with shape-matured quartz inclusions. In some rocks, these garnets are overgrown by garnet with bell-shaped Mn-zoning (Grt-M2) containing irregularly-shaped quartz inclusions. This evolution is interpreted as partial dissolution of Grt-M1 and subsequent growth of Grt-M2. Garnet in the eclogite is volumetrically dominated by eclogite-facies garnet (Grt-E1) that envelope remnants of an older, chemically distinct generation (Grt-E0) with highly irregular and diffuse boundaries. Shape-matured quartz inclusions are present within both garnet generations and define a zoning pattern that is not reflective of the chemical zoning. Collectively, these characteristics are interpreted as replacement of Grt-E0 by Grt-E1 via interface-coupled dissolution–reprecipitation, with the latter inheriting the shape-matured quartz inclusions of the former. Pressure–temperature (P–T) conditions extracted from the quartz inclusions in Grt-M1 and Grt-E0/E1 are 1.08 to 1.21 GPa at 645°C to 695°C and 0.94 to 1.03 GPa at 605°C to 640°C, respectively. These conditions are interpreted as cooling of the rocks from a high temperature metamorphic history, altogether preceding subduction of the Grapesvare nappe. The quartz inclusions in Grt-M2 record 1.04 to 1.21 GPa at 620°C to 675°C, interpreted as prograde metamorphic growth of Grt-M2 during subduction at 495.7 ± 3.2 Ma. Subsequent eclogite-facies metamorphism was responsible for the formation of Grt-E1 at the expense of Grt-E0. The collective results indicate a prolonged polymetamorphic history of the Grapesvare nappe prior to UHP metamorphism that has not been recognized previously.
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| 6. |
- Barnes, Christopher J., et al.
(författare)
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Zircon and monazite reveal late Cambrian/early Ordovician partial melting of the Central Seve Nappe Complex, Scandinavian Caledonides
- 2022
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer. - 0010-7999 .- 1432-0967. ; 177:9
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Tidskriftsartikel (refereegranskat)abstract
- The Seve Nappe Complex (SNC) comprises continental rocks of Baltica that were subducted and exhumed during the Caledonian orogeny prior to collision with Laurentia. The tectonic history of the central SNC is investigated by applying in-situ zircon and monazite (Th-)U-Pb geochronology and trace element analysis to (ultra-)high pressure (UHP) paragneisses in the Avardo and Marsfjallet gneisses. Zircons in the Avardo Gneiss exposed at Sippmikk creek exhibit xenocrystic cores with metamorphic rims. Cores show typical igneous REE profiles and were affected by partial Pb-loss. The rims have flat HREE profiles and are interpreted to have crystallized at 482.5 +/- 3.7 Ma during biotite-dehydration melting and peritectic garnet growth. Monazites in the paragneiss are chemically homogeneous and record metamorphism at 420.6 +/- 2.0 Ma. In the Marsfjallet Gneiss exposed near Kittelfjall, monazites exhibit complex zoning with cores enveloped by mantles and rims. The cores are interpreted to have crystallized at 481.6 +/- 2.1 Ma, possibly during garnet resorption. The mantles and rims provide a dispersion of dates and are interpreted to have formed by melt-driven dissolution-reprecipitation of pre-existing monazites until 463.1 +/- 1.8 Ma. Depletion of Y, HREE, and U in the mantles and rims compared to the cores record peritectic garnet and zircon growth. Altogether, the Avardo and Marsfjallet gneisses show evidence of late Cambrian/early Ordovician partial melting (possibly in (U)HP conditions), Middle Ordovician (U)HP metamorphism, and late Silurian tectonism. These results indicate that the SNC underwent south-to-north oblique subduction in late Cambrian time, followed by progressive north-to-south exhumation to crustal levels prior to late Silurian continental collision.
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| 7. |
- Bukala, Michal, et al.
(författare)
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Brittle deformation during eclogitization of early Paleozoic blueschist
- 2020
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Ingår i: Frontiers in Earth Science. - LAUSANNE SWITZERLAND : Frontiers Media SA. - 2296-6463. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- The Tsakkok Lens of the Scandinavian Caledonides represents the outermost Baltican margin that was subducted in late Cambrian/Early Ordovician time during closure of the Iapetus Ocean. The lens predominantly consists of metasedimentary rocks hosting eclogite bodies that preserve brittle deformation on the mu m-to-m scale. Here, we present a multidisciplinary approach that reveals fracturing related to dehydration and eclogitization of blueschists. Evidence for dehydration is provided by relic glaucophane and polyphase inclusions in garnet consisting of clinozoisite + quartz +/- kyanite +/- paragonite that are interpreted as lawsonite pseudomorphs. X-Ray chemical mapping of garnet shows a network of microchannels that propagate outward from polyphase inclusions. These microchannels are healed by garnet with elevated Mg relative to the surrounding garnet. Electron backscatter diffraction mapping revealed that Mg-rich microchannels are also delimited by low angle (<3 degrees) boundaries. X-ray computed microtomography demonstrates that some garnet is transected by up to 300 mu m wide microfractures that are sealed by omphacite +/- quartz +/- phengite. Locally, mesofractures sealed either by garnet- or omphacite-dominated veins transect through the eclogites. The interstices within the garnet veins are filled with omphacite + quartz + rutile + glaucophane +/- phengite. In contrast, omphacite veins are predominantly composed of omphacite with minor apatite + quartz. Omphacite grains are elongated along [001] crystal axis and are preferably oriented orthogonal to the vein walls, indicating crystallization during fracture dilation. Conventional geothermobarometry using omphacite, phengite and garnet adjacent to fractures, provides pressure-temperature conditions of 2.47 +/- 0.32 GPa and 620 +/- 60 degrees C for eclogites. The same method applied to a mesoscale garnet vein yields 2.42 +/- 0.32 GPa at 635 +/- 60 degrees C. Zirconium-in-rutile thermometry applied to the same garnet vein provides a temperature of similar to 620 degrees C. Altogether, the microchannels, microfractures and mesofractures represent migration pathways for fluids that were produced during glaucophane and lawsonite breakdown. The microfractures are likely precursors of the mesoscale fractures. These dehydration reactions indicate that high pore-fluid pressure was a crucial factor for fracturing. Brittle failure of the eclogites thus represents a mechanism for fluid-escape in high-pressure conditions. These features may be directly associated with seismic events in a cold subduction regime.
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| 8. |
- Bukala, Michal, et al.
(författare)
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U-Pb Zircon Dating of Migmatitic Paragneisses and Garnet Amphibolite from the High Pressure Seve Nappe Complex in Kittelfjäll, Swedish Caledonides
- 2020
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Ingår i: Minerals. - : MDPI AG. - 2075-163X. ; 10:4
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Tidskriftsartikel (refereegranskat)abstract
- The Seve Nappe Complex exposed in the Kittelfjall area of the northern Scandinavian Caledonides comprises a volcano-sedimentary succession representing the Baltica passive margin, which was metamorphosed during the Iapetus Ocean closure. Garnet amphibolites, together with their host migmatitic paragneisses, record a potential (U)HP event followed by decompression-driven migmatization. The garnet amphibolites were originally thought to represent retrogressively altered granulites. The petrological and geochemical features of a studied garnet amphibolite allow for speculation about a peridotitic origin. Zirconium (Zr) content in rutile inclusions hosted in garnet in paragneisses points to near-peak temperatures between 738 degrees C and 780 degrees C, which is in agreement with the c. 774 degrees C obtained from the matrix rutile in the garnet amphibolite. The matrix rutile in multiple paragneiss samples records temperatures below 655 degrees C and 726 degrees C. Whereas the LA-ICP-MS U-Pb dating of zircon cores revealed the age spectrum from Paleoproterozoic to early Paleozoic, suggesting a detrital origin of zircon cores in paragneisses, the metamorphic zircon rims show an Early Ordovician cluster c. 475-469 Ma. Additionally, zircon cores and rims from the garnet amphibolite yielded an age of c. 473 Ma. The REE patterns of the Caledonian zircon rims from the paragneisses show overall low LREE concentrations, different from declining to rising trends in HREE (Lu-N/Gd-N = 0.49-38.76). Despite the textural differences, the cores and rims in zircon from the garnet amphibolite show similar REE patterns of low LREE and flat to rising HREE (Lu-N/Gd-N = 3.96-65.13). All zircon rims in both lithologies display a negative Eu anomaly. Hence, we interpret the reported ages as the growth of metamorphic zircon during migmatization, under granulite facies conditions related to exhumation from (U)HP conditions.
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| 9. |
- Bukała, Michał, et al.
(författare)
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UHP metamorphism recorded by phengite eclogite from the Caledonides of northern Sweden : P-T path and tectonic implications
- 2018
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Ingår i: Journal of Metamorphic Geology. - : Wiley. - 0263-4929 .- 1525-1314. ; 36:5, s. 547-566
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Tidskriftsartikel (refereegranskat)abstract
- The Seve Nappe Complex (SNC) of the Scandinavian Caledonides records a well-documented history of high pressure (HP) and ultra-high pressure (UHP) metamorphism. Eclogites of the SNC occur in two areas in Sweden, namely Jamtland and Norrbotten. The Jamtland eclogites and associated rocks are well-studied and provide evidence for late Ordovician UHP metamorphism, whereas the Norrbotten eclogites, formed during the late Cambrian (Furongian)/Early Ordovician, have not been studied in such detail, especially in terms of the P-T conditions of their formation. Within the studied eclogite, clinopyroxene contains a high-Na core and two rims: inner, medium-Na and outer, low-Na. Garnet consists of a high-Ca euhedral core, low-Ca inner rim and medium-Ca outer rim. A similar pattern occurs within phengite, where high-Si cores are enveloped by medium and low-Si rims. The compositions of the mineral cores, inner rims and outer rims reflect three stages in the metamorphic evolution of the eclogite. Applied Quartz-in-Garnet geobarometry, coupled with Zr-in-rutile geothermometry reveal that garnet nucleation (E0 stage) took place at 1.5-1.6GPa and 620-660 degrees C. The eclogite peak-pressure assemblage developed during the E1 stage, it consists of garnet+omphacite+phengite+rutile+coesite? and yields P-T conditions of 2.8-3.1GPa and 660-780 degrees C as constrained by conventional geothermobarometry and thermodynamic modelling in the NCKFMMnASHT system. Later, lower-pressure stages E2 and E3 record conditions of 2.2-2.8GPa, 680-780 degrees C and 2.1GPa, 735 degrees C, respectively. The prograde metamorphic evolution of the eclogite is inferred from inclusions of epidote, amphibole and clinopyroxene within garnet. The presence of amphibole-quartz-plagioclase symplectites, secondary epidote/zoisite and titanite replacing rutile record the later retrograde changes taking place at <1.5GPa (referred as E4 stage). The obtained P-T conditions indicate that the Norrbotten eclogites underwent a metamorphic evolution characterized by a clockwise P-T path with peak metamorphism reaching up to coesite stability field within a relatively cold subduction regime (7.8 degrees C/km). The obtained results provide the first evidence for UHP metamorphism in the SNC above the Arctic Circle and document cold subduction regime and multistage exhumation of the deeply subducted Baltican margin at early stage of the Caledonian Orogeny.
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| 10. |
- Holmberg, Johanna, et al.
(författare)
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Decompressional equilibration of the Midsund granulite from Otrøy, Western Gneiss Region, Norway
- 2019
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Ingår i: Geologica Carpathica. - Bratislava : Slovak Acdemy of Sciences. - 1335-0552 .- 1336-8052. ; 70:6, s. 471-482
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Tidskriftsartikel (refereegranskat)abstract
- The Western Gneiss Region (WGR) of the Scandinavian Caledonides is an archetypal terrain for high-pressure(HP) and ultrahigh-pressure (UHP) metamorphism. However, the vast majority of lithologies occurring there bear no,or only limited, evidence for HP or UHP metamorphism. The studied Midsund HP granulite occurs on the island of Otrøy,a locality known for the occurrence of the UHP eclogites and mantle-derived, garnet-bearing ultramafics. The Midsundgranulite consists of plagioclase, garnet, clinopyroxene, relict phengitic mica, biotite, rutile, quartz, amphibole, ilmeniteand titanite, among the most prominent phases. Applied thermodynamic modelling in the NCKFMMnASHT systemresulted in a pressure–temperature (P–T) pseudosection that provides an intersection of compositional isopleths ofXMg (Mg/Mg+Fe) in garnet, albite in plagioclase and XNa (Na/Na+Ca) in clinopyroxene in the stability field of melt +plagioclase + garnet + clinopyroxene + amphibole + ilmenite. The obtained thermodynamic model yields P–T conditions of1.32–1.45 GPa and 875–970 °C. The relatively high P–T recorded by the Midsund granulite may be explained as an effectof equilibration due to exhumation from HP (presumably UHP) conditions followed by a period of stagnation under HTat lower-to-medium crustal level. The latter seems to be a more widespread phenomenon in the WGR than previouslythought and may well explain commonly calculated pressure contrasts between neighboring lithologies in the WGR andother HP–UHP terranes worldwide.
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