| 1. |
- Buchan, Kenneth L., et al.
(author)
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Rodinia : the evidence from integrated palaeomagnetism and U-Pb geochronology
- 2001
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In: Precambrian Research. - 0301-9268 .- 1872-7433. ; 110:1-4, s. 9-32
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Journal article (peer-reviewed)abstract
- Of many hundreds of well-defined palaeomagnetic poles that have been reported from cratons around the world in the 1700-500 Ma period, only a few are precisely dated. However, such ‘key' palaeopoles are a prerequisite for establishing rigorous palaeomagnetic reconstructions in order to chart the assembly, drift and breakup of the postulated late Precambrian supercontinent of Rodinia. Most key palaeopoles are derived from mafic dykes and sills that have been dated by U-Pb techniques. Most are from Laurentia, the largest and best studied of the continental fragments that are thought to have comprised Rodinia. Thirteen key Laurentia palaeopoles form an incomplete reference set that can be used for comparison with key palaeopoles from other cratons as they become available. Currently, there are four key palaeopoles for Baltica between 1700 and 500 Ma, although only one allows a direct comparison with a similar aged pole from Laurentia. The 1265 Ma match between Baltica and Laurentia is consistent with reconstructions in which Baltica is adjacent to present-day east Greenland, with the ca. 1700-1500 Ma Gothian and Labradorian belts aligned. Few key palaeopoles are yet available from other cratons. However, recent U-Pb dating of dykes, sills, or volcanic rocks in the Siberian, Australian and Kalahari cratons and in Coats Land of Antarctica constrains the ages of individual palaeopoles from each of these areas. Most of these are not key palaeopoles because they have not been conclusively demonstrated to be primary, or local tectonic rotations have not been ruled out. Nevertheless, they are useful in testing Rodinia reconstructions. In this paper, a U-Pb baddeleyite age is reported from the late Gardar magmatic rocks of southwest Greenland. Along with the previously published palaeopole for this unit, this age helps constrain the Mesoproterozoic location of southwest Greenland relative to North America.
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| 2. |
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| 3. |
- Evans, David A.D., et al.
(author)
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An expanding list of reliable paleomagnetic poles for Precambrian tectonic reconstructions
- 2021
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In: Ancient Supercontinents and the Paleogeography of Earth. - : Elsevier. ; , s. 605-639
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Book chapter (peer-reviewed)abstract
- We present a compilation of reliable Precambrian paleomagnetic poles from three successive international workshops (in years 2009, 2014, 2017), comprising paleomagnetists specializing in Precambrian tectonic reconstructions. The working groups compiled lists of two global classes of poles, published through the end of 2017. “Grade-A” results are judged to provide essential constraints on tectonic reconstructions; “Grade-B” poles are judged to be suggestive of high-quality, but not yet demonstrated to be primary, or perhaps lacking precise geochronologic or other constraints. Our catalog documents a resurgence of high-quality data acquisition in recent years, and highlights specific cratons and time intervals that are most lacking in the data needed to reconstruct those blocks through supercontinental cycles.
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| 4. |
- Jensen, Mai-Britt Mose
(author)
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Geophysical tools for prognosis of deformation in hardrock environments
- 2011
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Doctoral thesis (other academic/artistic)abstract
- Underground mining of the Kiirunavaara iron mineralization is causing large-scale deformation of the hangingwall of the orebody. To understand and possibly predict future deformation, a structural model of the hanging wall and a good understanding of the petrophysical and mechanical properties of the constituent rocks is necessary. This thesis presents results from the study of Anisotropy of Magnetic Susceptibility (AMS), magneto-mineralogy, fracture frequency (F/m), rock quality (RQD), rock strength (Point Load) and P-wave velocity and anisotropy of the rocks from the hanging wall, as well as seismic reflection surveying and gravity modelling. The results are combined into a structural model of the hanging wall and further used for analysis of the correlation between petrophysical and rock mechanical parameters.In total, 76 samples from 12 outcrops and 295 samples from three drillcores in the hanging wall have been included in the AMS study. Axial and diametrical P-wave velocity was measured on 25 water-saturated samples and 156 samples were used for the Point Load tests. F/m and RQD data for the drillcores already existed. The drillcores are located along the eastern end of the seismic profiles and consist of both crystalline and sedimentary rocks.A high degree of magnetic anisotropy observed in the crystalline rocks indicates a low degree of metamorphism. AMS data also indicates the presence of a magnetic foliation in the rocks. The dip of the magnetic foliation plane (F) and the degree of magnetic anisotropy (Pj) measured in samples from outcrops was plotted as iso-maps and shows that both F and Pj decreases towards the east, which was confirmed by data from the drillcores. The decrease in both parameters is primarily a reflection of a change in rock type, but is also changing within the crystalline rock sequence.A good correlation (r > 0.6) between Pj, and F and RQD, and F and F/m was observed in one drillcore for both crystalline and sedimentary rocks; and between the shape parameter (T) and F/m in crystalline rock in another drillcore. This suggests the AMS parameter may be used as an indicator of rock mechanical properties. AMS data was also correlated to joint strike orientations and it was concluded that AMS can also be used to predict joint orientation.Two parallel reflection seismic profiles were shot within the town of Kiruna i order to locate deformation zones and lithological boundaries in the hangingwall. No deformation zones were found, but five seismic reflectors corresponding to five lithological boundaries were located, and their strike and dip calculated. The result of the seismic survey was used to constrain the gravity model, as was density measurements of 230 samples from the drillcores. The gravity model has a depth of three km, and indicates that the crystalline rock in the hanging wall can be separated into two parallel N-S trending blocks.
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| 6. |
- Lubnina, Natalia V., et al.
(author)
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A new key pole for the East European Craton at 1452 Ma: Palaeomagnetic and geochronological constraints from mafic rocks in the Lake Ladoga region (Russian Karelia)
- 2010
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In: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 183:3, s. 442-462
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Journal article (peer-reviewed)abstract
- Palaeomagnetic and geochronological studies on mafic rocks in the Lake Ladoga region in South Russian Karelia provide a new, reliably dated Mesoproterozoic key paleopole for the East European Craton (Baltica). U-Pb dating on baddeleyite gives a crystallisation age of 1452 +/- 12 Ma for one of the studied dolerite dykes. A mean palaeomagnetic pole for the Mesoproterozoic dolerite dykes. Valaam sill and Salmi basalts yields a paleopole at 15.2 degrees N, 177.1 degrees E, A(95) = 5.5 degrees. Positive baked contact test for the dolerite dykes and positive reversal test for the Salmi basalts and for the dykes confirm the primary nature of the magnetisation. Comparison of this Baltica palaeopole with coeval paleomagnetic data for Laurentia and Siberia provides a revised palaeoposition of these cratons. The results verify that the East European Craton, Laurentia and Siberia were part of the supercontinent Columbia from the Late Palaeoproterozoic to the Middle Neoproterozoic. (C) 2010 Elsevier B.V. All rights reserved.
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| 7. |
- Salminen, Johanna, et al.
(author)
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Paleomagnetic studies of rapakivi complexes in the Fennoscandian shield – Implications to the origin of Proterozoic massif-type anorthosite magmatism
- 2021
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In: Precambrian Research. - : Elsevier. - 0301-9268 .- 1872-7433. ; 365
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Journal article (peer-reviewed)abstract
- Paleomagnetic studies have been performed on five rapakivi related complexes in Sweden and Finland. Poles of varying quality have been defined and the majority of the similar to 1640-1497 Ma poles are clustering on low latitudinal positions. By combining data from similar to 1500 Ma intrusions a new high-quality pole (Plat: 13 degrees N; Plon: 190 degrees E; A(95): 11 degrees, K: 14) for Baltica has been defined. Tectonic reconstructions, on the basis of the new data and previously published high-quality data, indicate that Baltica experienced stable low latitude to equatorial positions during 1640-1470 Ma, temporally coinciding with globally pronounced rapakivi-anorthosite magmatism. Our study argues against single hotspot source for similar to 1640-1620 Ma, similar to 1590-1520 Ma, and 1470-1410 Ma rapakivianorthosites, but supports a model of large-scale superswell under a stationary low-latitude position of supercontinent Nuna for the origin of rapakivi-anorthosite magmatism. However, a possibility for convergent tectonism as the origin cannot be ruled out.
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| 8. |
- Salminen, Johanna, et al.
(author)
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The Precambrian drift history and paleogeography of Baltica
- 2021
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In: Ancient Supercontinents and the Paleogeography of Earth. - : Elsevier. ; , s. 155-205
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Book chapter (peer-reviewed)abstract
- We review paleomagnetic data and paleoclimatological indicators of Baltica and its subcratons. Between Neoarchean and middle Mesoproterozoic Karelia and Kola, and later the united Baltica were located mostly at the latitudes between 35°N and 35°S. Location of Baltica oscillated between high latitudes and the equator at late Mesoproterozic–Neoproterozoic. Drift velocities of the separate cratons between Neoarchean and middle Proterozoic are lower than the velocities of the united Baltica at late Paleoproterozoic–middle Mesoproterozoic. At Late Mesoproterozoic–Neoproterozoic Baltica shows high velocity peaks, which correlate temporarily with the ones obtained for Laurentia and can be ascribed to true polar wander. Increase in drift rates correlate temporarily with orogenies related to the formation of the supercontinents Nuna and Rodinia and in smaller scale to the crustal growth of Baltica. Based on the results, we review possible nearest neighbors for the Kola and Karelia in the Superia supercraton and for Baltica in the Nuna and Rodinia supercontinents.
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| 9. |
- Sandrin, Alessandro
(author)
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Geophysical targeting of Fe-oxide Cu-Au deposits in the northern fennoscandian sheild
- 2014
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Doctoral thesis (other academic/artistic)abstract
- In the last decades Fe-Oxide Cu-Au deposits have been discovered and developed in Australia, South America and Canada. This class of mineral deposits is also present in northern Sweden and thus represents a potential target for improving the mineral production of the country. The project aims at defining a geophysical signature of IOCG deposits from regional scale to local scale by using available geophysical data and newly measured petrophysical data. The Kiruna district was selected for regional investigations, while the Tjårrojåkka prospect was selected as testing site for the local study. The Tjårrojåkka prospect is located around 50 km west of Kiruna and is constituted by an apatite-iron ore (Kiruna type) and an associated Cu(-Au) deposit. The host rock is an altered andesite of Palaeoproterozoic age. The working hypothesis to be tested was that a spatial relationship exists between IOCG deposits and major tectonic lineaments and between IOCG deposits and alteration zones and that geophysical data can be used to delineate prospective areas. Gravity and magnetic data were used to map faults and associated fracture zones at regional scale, whereas airborne radiometric data were used to map potassic alteration, which appears to be related to the Cu(-Au) mineralising event. The K/Th ratio maps at both regional and local scale seem to be able to define prospective areas for IOCG deposits. Magnetic maps are also used to indicate presence of highly magnetic bodies associated to high concentrations of Fe-minerals. In the Tjårrojåkka area, Anisotropy of Magnetic Susceptibility on oriented samples was used as tectonic indicator for the geological study of the area. Three major tectonic events were identified, one of which is interpreted to be related to the main mineralising event. Density, bulk susceptibility and Natural Remanent Magnetisation were measured for oriented samples and for borehole specimens at the main Cu deposit. Thermal demagnetisation cycles were applied to the specimens to define the main magnetic mineral, which is mostly multidomain magnetite as confirmed by Königsberger ratios lower than one. Haematite is also present and is caused by oxidation of magnetite. The haematitisation of magnetite is contemporaneous to the Cu(-Au) ore deposition and to potassic alteration. At local scale, high magnetic anomalies define magnetite bodies, while intermediate magnetic anomalies mark areas that were affected by haematitisation. These areas also show high K/Th ratios and are associated to Cu-(Au) occurences. The proposed model for IOCG exploration begins with the analysis at regional scale of potential field data to define the major tectonic lineaments and fracture zones that may have acted as pathways for mineralising fluids. The second step is to identify high/intermediate magnetic anomalies caused by magnetite/haematite deposits. High values of K/Th ratio from airborne radiometric data show areas affected by potassic alteration, which often accompanies Cu-(Au) deposition.
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