| 1. |
- Bogdanova, Svetlana, et al.
(författare)
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Late Palaeoproterozoic mafic dyking in the Ukrainian Shield of Volgo-Sarmatia caused by rotation during the assembly of supercontinent Columbia (Nuna)
- 2013
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937. ; 174, s. 196-216
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Tidskriftsartikel (refereegranskat)abstract
- The Ukrainian Shield comprises the exposed crust of the large Palaeoproterozoic protocraton Volgo-Sarmatia, which together with the Fennoscandian crustal segment constitutes the East European Craton ("Baltica"). Geological and geophysical data indicate that 1.80 to 1.75 Ga mafic dykes related to anorthosite-mangerite-charnockite-granite (AMCG) plutons are widespread within the Ukrainian Shield. We examined their ages, distribution patterns, orientations and compositions in three different crustal blocks (Volyn, Ingul and Azov), and found close spatial relationships with major strike-slip fault systems developed during two phases of extension. The early, 1.80-1.77 Ga, generation of mafic dykes mostly follows NW (330 +/- 20 degrees) and more rarely N-S- or E-W-trending faults corresponding to major NE-SW extension (the Submoshorino phase). These dykes contain olivine dolerites, picrites, camptonites, lamprophyres, kimberlites and other rocks belonging to tholeiitic and subalkaline jotunitic series. The compositions of these dykes differ between the host blocks, but all feature upper mantle geochemical signatures such as high contents of Ni and Cr, and positive values of epsilon Nd-(1800) up to + 2.8. High degrees of REE fractionation indicate deep levels of mantle melting, which is particularly characteristic of the Ingul block as marked by the most extensive and dense mafic dyke swarms. The later, 1.76-1.75 Ga, dyke swarms occur close to the most voluminous AMCG suites of similar age and were emplaced during the second (Korsun) phase of faulting when all the older strike-slip fault zones were reactivated and partly transformed to tensional faults by E-W extension. These dyke swarms mainly trend 030 +/- 20 degrees. They are jotunitic and their isotopic signatures indicate a greater participation of crustal sources in the parent melts. The overall transtensional tectonic setting of the mafic dyking associated with the AMCG magmatism in Volgo-Sarmatia was created by convergent tectonics and postcollisional collapse of the thickened lithosphere, as well as by mantle delamination coupled with the rotation of Volgo-Sarmatia between 1.80 and 1.75 Ga. This agrees with palaeomagnetic reconstruction suggesting rotation(s) of Volgo-Sarmatia during its protracted oblique docking with Fennoscandian terranes and Laurentia as supercontinent Columbia (Nuna) was assembled. (C) 2012 Elsevier B.V. All rights reserved.
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| 2. |
- Nilsson, Mimmi K.M., et al.
(författare)
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Break-up related 2170–2120 Ma mafic dykes across the North Atlantic craton : Final dismembering of a North Atlantic-Dharwar craton connection?
- 2019
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 329, s. 70-87
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Tidskriftsartikel (refereegranskat)abstract
- Progress with supercontinental reconstructions relies on accurate age determinations for continental geological units such as mafic dyke swarms. Here we present zircon and baddeleyite U-Pb ID-TIMS isotope data of six mafic dykes from the Archean North Atlantic craton in present-day southern West and South-East Greenland. Two dykes in southern West Greenland yield crystallization ages of 2021 ± 4 Ma, for the NE-trending Hamborgersund dyke, and 2125 ± 9 Ma, for the E-trending Angissunguaq dyke. Additional age determinations of mafic dykes from South-East Greenland are 2166 ± 8 Ma and 2158 ± 8 Ma for two E–W trending dykes, herein named the Skjoldungen dykes, and 2137 ± 11 and 2124 ± 11 for two ENE and NE trending dykes, respectively. The name Ruinnæsset dykes is proposed for this slightly younger, ca. 2137–2124 Ma, generation of dykes in SE Greenland, and Nuuk dykes for coeval ca. 2125 Ma dykes in SW Greenland. The Skjoldungen and the Ruinnæsset dykes have primordial mantle geochemical signatures, with only minor LILE-enrichments. These signatures differ from other, more ‘lithospheric’ Proterozoic dykes within the region, and may reflect highly attenuated lithospheric extension during their emplacement. Coeval mafic magmatism in North Atlantic and Dharwar cratons suggests that these two fragments coexisted within a common Paleoproterozoic supercraton. A lack of younger age-matches further argue for ca. 2170–2140 Ma rifting and subsequent break-up of this Dharwar-North Atlantic connection, prior to the 2137–2124 Ma emplacement of the Ruinnæsset dykes. Other global age correlations are discussed and a likely paleogeographic reconstruction of North Atlantic craton together with the Dharwar and Superior cratons within a Paleoproterozoic supercraton is presented.
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| 3. |
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| 4. |
- Nilsson, Mimmi K. M., et al.
(författare)
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Precise U-Pb baddeleyite ages of mafic dykes and intrusions in southern West Greenland and implications for a possible reconstruction with the Superior craton
- 2010
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 183:3, s. 399-415
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Tidskriftsartikel (refereegranskat)abstract
- The Archaean block of southern Greenland constitutes the core of the North Atlantic craton (NAC) and is host to a large number of Precambrian mafic intrusions and dyke swarms, many of which are regionally extensive but poorly dated. For southern West Greenland, we present a U-Pb zircon age of 2990 +/- 13 Ma for the Amikoq mafic-ultramafic layered intrusion (Fiskefjord area) and four baddeleyite U-Pb ages of Precambrian dolerite dykes. Specifically, a dyke located SE of Ameralik Fjord is dated at 2499 +/- 2 Ma, similar to a previously reported Ar-40/Ar-39 age of a dyke in the Kangamiut area. For these and related intrusions of ca. 2.5 Ga age in southern West Greenland, we propose the name Kilaarsarfik dykes. Three WNW-trending dykes of the MD3 swarm yield ages of 2050 +/- 2 Ma, 2041 +/- 3 Ma and 2029 +/- 3 Ma. A similar U-Pb baddeleyite age of 2045 +/- 2 Ma is also presented for a SE-trending dolerite (Iglusuataliksuak dyke) in the Nain Province, the rifted western block of the NAC in Labrador. We speculate that the MD3 dykes and age-equivalent NNE-trending Kangamiut dykes of southern West Greenland, together with the Iglusuataliksuak dyke (after closure of the Labrador Sea) represent components of a single, areally extensive, radiating swarm that signaled the arrival of a mantle plume centred on what is presently the western margin of the North Atlantic craton. Comparison of the magmatic 'barcodes' from the Nain and Greenland portions of the North Atlantic craton with the established record from the north-eastern Superior craton shows matches at 2500 Ma, 2214 Ma, 2050-2030 Ma and 1960-1950 Ma. We use these new age constraints, together with orientations of the dyke swarms, to offer a preliminary reconstruction of the North Atlantic craton near the north-eastern margin of the Superior craton during the latest Archaean and early Palaeoproterozoic, possibly with the Core Zone craton of eastern Canada intervening. (C) 2010 Elsevier B.V. All rights reserved.
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| 5. |
- Petersson, Andreas, et al.
(författare)
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The robustness of the Lu-Hf and Sm-Nd isotopic systems during metamorphism – And case study of the Åker metabasite in southern Sweden
- 2023
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Ingår i: Precambrian Research. - 0301-9268 .- 1872-7433. ; 394
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Tidskriftsartikel (refereegranskat)abstract
- While whole-rock Lu-Hf isotope analysis remains one of the only ways to obtain initial Hf isotope signatures of old mafic rocks, Hf isotope analyses of more robust accessory zircon in intermediate to silicic rocks have largely replaced whole-rock analyses during the last decade. This has led to a discrepancy in the amount of existing data from mafic and felsic lithologies. However, especially in mafic, Si-poor rocks with a metamorphic imprint, Hf isotope data rely on whole-rock analysis since baddeleyite, commonly used for U-Pb age analyses of mafic rocks, is sensitive to alteration and metamorphism. Hence, to accurately evaluate the trace element and isotope signatures of altered mafic rocks, it is important to understand the mechanisms of element mobility during metamorphism. Here, we report whole-rock trace element compositions, Lu-Hf and Sm-Nd isotope data from variably deformed and metamorphosed samples of a mafic intrusion in southern Sweden, the Åker metabasite. These data suggest that trace elements were undisturbed on a whole-rock sample scale during deformation at upper amphibolite facies (at least 1000 MPa and 600 °C) metamorphism under hydrated conditions. Despite redistribution of Zr associated with the breakdown of baddeleyite and other igneous phases, the Åker metabasite has retained its chemical and isotopic integrity since igneous crystallisation at ca. 1565 Ma. This study demonstrates and strengthens the feasibility of whole-rock analyses of (meta-)mafic rocks for determining initial εNd and εHf values, despite deformation and metamorphism under hydrated amphibolite-grade metamorphic conditions. Testing the coherence of the calculated initial Nd and Hf isotope ratios by examining variably deformed and metamorphosed varieties of a rock in a single outcrop, could be used as a model for research on more complex Archean rocks.
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| 6. |
- Wohlgemuth-Uberwasser, Cora C., et al.
(författare)
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Quadrupole LA-ICP-MS U-PB geochronology of baddeleyite single crystals
- 2015
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Ingår i: Journal of Analytical Atomic Spectrometry. - 0267-9477 .- 1364-5544. ; 30:5, s. 1191-1196
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Tidskriftsartikel (refereegranskat)abstract
- We present a straightforward method for accurate and precise U/Pb dating of baddeleyite using quadrupole LA-ICP-MS. To establish the validity of our ICP-MS data we used FC-4b baddeleyite from the Duluth complex as external standard, whereas baddeleyite from the Phalaborwa (South Africa) and the Sorkka (SW Finland) intrusions were treated as unknowns. The analytical protocol includes the use of a matrix matched standard, which allows for an accurate downhole-fractionation and laser-induced interelement fractionation correction. The quadrupole ICP-MS results for the Phalaborwa and Sorkka samples analyzed are accurate within error of the published ID-TIMS U/Pb ages and with a precision of 0.3–0.7% (2σ) from concordia ages of ten to twenty analyses per sample. No orientation effects were observed.
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