| 1. |
- 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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| 2. |
- Faehnrich, Karol, et al.
(författare)
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Geochronological constraints on Caledonian strike-slip displacement in Svalbard, with implications for the evolution of the Arctic
- 2020
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Ingår i: Terra Nova. - : Wiley. - 0954-4879 .- 1365-3121. ; 32:4, s. 290-299
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Tidskriftsartikel (refereegranskat)abstract
- The timing of Svalbard's assembly in relation to the mid-Paleozoic Caledonian collision between Baltica and Laurentia remains contentious. The Svalbard archipelago consists of three basement provinces bounded by N-S-trending strike-slip faults whose displacement histories are poorly understood. Here, we report microstructural and mineral chemistry data integrated with Ar-40/Ar-39 muscovite geochronology from the sinistral Vimsodden-Kosibapasset Shear Zone (VKSZ, southwest Svalbard) and explore its relationship to adjacent structures and regional deformation within the circum-Arctic. Our results indicate that strike-slip displacement along the VKSZ occurred in late Silurian-Early Devonian and was contemporaneous with the beginning of the main phase of continental collision in Greenland and Scandinavia and the onset of syn-orogenic sedimentation in Silurian-Devonian fault-controlled basins in northern Svalbard. These new-age constraints highlight possible links between escape tectonics in the Caledonian orogen and mid-Paleozoic terrane transfer across the northern margin of Laurentia.
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| 3. |
- Kosminska, Karolina, et al.
(författare)
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Blueschist facies metamorphism in Nordenskiold Land of west-central Svalbard
- 2014
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Ingår i: Terra Nova. - : Wiley. - 0954-4879 .- 1365-3121. ; 26:5, s. 377-386
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Tidskriftsartikel (refereegranskat)abstract
- Recent fieldwork in Nordenskiold Land, Svalbard's Southwestern Basement Province, has established the presence of high-pressure (HP) lithologies. They are strongly retrogressed blueschists consisting mainly of garnet and Ca-amphibole with remnants of ferroglaucophane and phengite. The pressure-temperature (P-T) conditions were estimated using phase equilibrium modelling in the NCKFMMnASHTO system. P-T estimates based on the garnet, phengite and ferroglaucophane compositional isopleths and modelled paragenetic assemblage indicate peak metamorphism at 470-490 degrees C and 14-18 kbar. These data fall close to the 7-8 C km(-1) geo-therm, which is similar to that from Motalafjella, the only previously known occurrence of blueschists in Svalbard's Caledonides. The newly discovered blueschists could have formed during the early stage of the Caledonian Orogeny and may represent a vestige of missing marginal basins of the western Iapetus developed at the onset of subduction. The likely counterpart to Svalbard's blueschists is the ophiolitic sequence in the Pearya Terrane of northern Ellesmere Island.
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| 4. |
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| 5. |
- Majka, Jaroslaw, et al.
(författare)
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Neoproterozoic metamorphic evolution of the Isbjornhamna Group rocks from south-western Svalbard
- 2010
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Ingår i: Polar Research. - : Norwegian Polar Institute. - 0800-0395 .- 1751-8369. ; 29:3, s. 250-264
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Tidskriftsartikel (refereegranskat)abstract
- A metamorphosed volcano-sedimentary complex constitutes the Caledonian basement in the south-western part of Wedel Jarlsberg Land, Svalbard. Field, textural and previous thermochronologic data indicate a weak, localized metamorphic Caledonian overprint (M2). Deformed M1 isograds and variation in pressure-temperature estimates indicate a pervasive Neoproterozoic amphibolite-facies metamorphism that pre-dates large-scale Caledonian age folding. Garnet-biotite and garnet-Al silicate-plagioclase (GASP) geothermobarometry of the Isbjornhamna Group mica schists, and their comparison with the K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH) petrogenetic grid, indicates a peak pressure of ca. 11 kbar, and a peak temperature of ca. 670 degrees C during M1 metamorphism. A cooling rate of ca. 5 degrees C My(-1) is estimated on the basis of geothermobarometry and the available U-Th-total Pb and Ar-Ar data.
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| 6. |
- Majka, Jaroslaw, et al.
(författare)
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Pressure-temperature estimates of the blueschists from the Kopina Mt., northern Bohemian Massif, Poland - constraints on subduction of the Saxothuringian continental margin
- 2016
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Ingår i: European journal of mineralogy. - : E SCHWEIZERBARTSCHE VERLAGSBUCHHANDLUNG. - 0935-1221 .- 1617-4011. ; 28:6, s. 1047-1057
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Tidskriftsartikel (refereegranskat)abstract
- The blueschist occurrence at the Kopina Mt. is situated at the eastern margin of the Karkonosze Izera Massif in the West Sudetes of the northern Bohemian Massif. Metabasic rocks with continental crust affinity occur as lenses within the prevailing metasediments. The metabasites consist mainly of garnet, blue amphibole, epidote, chlorite-I, titanite, hematite and quartz forming the high-pressure assemblage. Synkinematic garnet exhibits prograde zoning, a feature allowing for assessment of the prograde course of the pressure temperature (P-T) path. Phase equilibrium modelling has been used to estimate prograde and peak metamorphic conditions. Based on the P-T pseudosections, calculated in the system Na2O-CaO-K2O-FeO-MgO-MnO- Al2O3-SiO2-H2O-TiO2-O-2 (NCKFMMnASHTO), the garnet + glaucophane + epidote + chlorite + white mica(?) + Fe oxide + quartz titanite assemblage was formed between 12-15 kbar and 480-520 degrees C. These results are based on garnet and Na-amphibole compositional isopleths. Garnet shows a prograde zoning expressed by the rapid rim-ward decrease of spessartine, moderate increase of almandine and as light increase of other components. It is inferred here that white mica must have decomposed to secondary albite chlorite-II K-feldspar. The obtained results point to the formation of the Kopina blueschists along a low-temperature P-T gradient of 8-10 degrees C/km, typical of rocks from a subduction exhumation channel. We postulate that the Kopina blueschists were formed in a subduction system developed along the eastern termination of the Saxothuringian Ocean and operating beneath the Tepla Barrandian upper plate. When combined with previous data on continental crust affinity for the protolith, our results demonstrate a derivation of the Kopina blueschists from subducted part of the Saxothuringian margin.
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| 7. |
- Majka, Jaroslaw, et al.
(författare)
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Two garnet growth events in polymetamorphic rocks in southwest Spitsbergen, Norway : insight in the history of Neoproterozoic and early Paleozoic metamorphism in the High Arctic
- 2015
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Ingår i: Canadian journal of earth sciences (Print). - : Canadian Science Publishing. - 0008-4077 .- 1480-3313. ; 52:12, s. 1045-1061
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Tidskriftsartikel (refereegranskat)abstract
- Geochronological studies in northern Wedel Jarlsberg Land, southwestern Svalbard (Norway), showed that the Tonian (c. 950 Ma) igneous rocks were subjected to metamorphism during the Torellian (c. 640 Ma) and early Caledonian (470-460 Ma) events. Predominant augen gneisses, derived from a Tonian protolith, are intercalated in that area, with schists comprising two distinct metamorphic mineral assemblages. The M1 (Torellian) assemblage containing garnet-I + quartz + plagioclase-I + biotite-I + muscovite-I was formed under amphibolite-facies conditions at c. 550-600 degrees C and 5-8 kbar (1 kbar = 100 MPa). The M2 (Caledonian) assemblage comprising garnet-II + quartz + plagioclase-II + biotite-II + muscovite-II + zoisite + chlorite crystallized at c. 500-550 degrees C and 9-12 kbar, corresponding to epidote-amphibolite facies conditions. The M2 mineral assemblage constitutes the pervasive Caledonian fabric of the schists that was subsequently reactivated in a left-lateral strike-slip shear regime. The subsequent c. 70 degrees clockwise rotation of the original structure to its present position was caused by a large-scale passive rotation during the Paleogene Eurekan orogeny. The new pressure-temperature estimates suggest that metamorphic basement in the study area was consolidated during the Torellian middle-grade event and then overprinted by Caledonian moderate-to high-pressure subduction-related metamorphism. A following sinistral shear zone assembled the present structure of basement units. Our results pose a question about the possible extent of Torellian precursor to the Caledonian basement across the High Arctic and the scale of its subsequent involvement in early Caledonian subduction. In conjunction with previous studies, the results suggest a possible correlation between southwestern Spitsbergen and the Pearya Terrane in Ellesmere Island.
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| 8. |
- Mazur, Stanislaw, et al.
(författare)
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Exhumation of the high-pressure Richarddalen Complex in NWSvalbard : Insights from Ar-40/(39)Argeochronology
- 2022
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Ingår i: Terra Nova. - : John Wiley & Sons. - 0954-4879 .- 1365-3121. ; 34:4, s. 330-339
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Tidskriftsartikel (refereegranskat)abstract
- Ar-40/Ar-39 dating of white mica from foliated metamorphic rocks of the Caledonian orogen of NW Svalbard was performed using the single-grain fusion technique. Two samples, an augen orthogneiss and a mica schist, were collected from the high-pressure Richarddalen Complex. Three additional mica schist samples were collected from the tectonically underlying Montblanc Unit. Ar-40/Ar-39 analysis of white mica separates yielded weighted average Ar-40/Ar-39 dates of 446.1 +/- 1.1, 438.4 +/- 1.1, 439.4 +/- 1.2, 439.8 +/- 1.5 and 439.3 +/- 1.0 Ma, with the oldest date provided by the Richarddalen mica schist. The single-grain fusion Ar-40/Ar-39 dating was accompanied by geochemical mapping of white mica using a microprobe. We interpret the oldest date as the age of cooling after a Late Ordovician high-pressure event and the four younger dates to represent the timing of subsequent deformation associated with assembly of a Caledonian nappe stack.
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| 9. |
- Młynarska, Maria, et al.
(författare)
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In situ white mica Rb/Sr geochronology of the Leszczyniec metaigneous complex, West Sudetes: evidence of upper plate deformation at the onset of Variscan collision
- 2024
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Ingår i: International Journal of Earth Sciences. - 1437-3254 .- 1437-3262.
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Tidskriftsartikel (refereegranskat)abstract
- The Karkonosze-Izera Massif in the West Sudetes preserves evidence of subduction of the Saxothuringian Ocean beneath the Teplá-Barrandian Domain. Within the massif, the Leszczyniec metaigneous complex (LMC) is identified as the upper allochthon. It exhibits a unique structural history in comparison to the underlying allochthons, suggesting that the LMC records a distinctive tectonic history. To investigate the timing of this history, two orthogneisses were studied from a single outcrop of the LMC for in situ white mica Rb/Sr geochronology. The outcrop bears a southeast-dipping foliation (S1) and a north/northeast plunging stretching lineation (L1), defined by white mica and quartz. Quartz recrystallization textures, white mica chemistry (celadonite content of 0.23–0.47), preservation of igneous plagioclase phenocrysts, and the metamorphic mineral assemblage, all indicate maximum epidote–amphibolite facies conditions. Single-spot Rb/Sr dates were calculated from white mica using initial 87Sr/86Sr values obtained by titanite and epidote. Results are similar for both rocks, providing weighted averages of 352.4 ± 4.1Ma (MSWD: 0.6; n: 24) and 349.3 ± 2.5Ma (MSWD: 0.4; n: 31). The rocks are interpreted to have the same structural and metamorphic history; thus, a pooled weighted average of 350.1 ± 5.3Ma (2σ) is reported as the timing of white mica (re)crystallization during S1 and L1 development in epidote–amphibolite facies conditions. This event is bracketed by the timing of blueschist-facies metamorphism for the subjacent middle (c. 364Ma) and lower (c. 345–341Ma) allochthons, associated with an east/southeast-plunging L1. Considering the different structural and metamorphic histories of the LMC compared to the subjacent allochthons, it is likely that the complex was extracted from the Teplá-Barrandian upper plate due to subduction erosion prior to collective exhumation and stacking of the Karkonosze-Izera Massif. Graphical abstract: [Figure not available: see fulltext.]
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