| 1. |
- Bispo-Santos, Franklin, et al.
(författare)
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Tectonic implications of the 1419 Ma Nova Guarita mafic intrusives paleomagnetic pole (Amazonian Craton) on the longevity of Nuna
- 2012
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 196-197, s. 1-22
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Tidskriftsartikel (refereegranskat)abstract
- The Nuna supercontinent was probably assembled in the Paleoproterozoic, but its paleogeography and the timing for its demise are still a matter of debate. A paleomagnetic and geochronological study carried out on the Mesoproterozoic Nova Guarita dyke swarm (northern Mato Grosso State, SW Amazonian Craton) provides additional constraints on the duration of this supercontinent. Paleomagnetic AF and thermal treatment revealed south/southwest (northeast) magnetic directions with downward (upward) inclinations for nineteen analyzed sites. These directions are carried by PSD magnetite with high unblocking temperatures as indicated by additional magnetic tests, including thermomagnetic curves, hysteresis loops and the progressive acquisition of isothermal remanence in selected samples. A positive contact test with the host granite in one of the studied dykes further attests to the primary origin of the characteristic magnetic component. A mean site direction was calculated at Dm = 220.5°, Im = 45.9° (α95 = 6.5°, K = 27.7), which yielded a paleomagnetic pole located at 245.9°E, 47.9°S (A95 = 7.0°). 40Ar/39Ar dating carried out on biotites from four analyzed dykes yielded well-defined plateau ages with a mean of 1418.5 ± 3.5 Ma. The Nova Guarita pole precludes a long-lived Nuna configuration in which Laurentia, Baltica, North China, and Amazonia formed a long and continuous block as previously proposed for the Paleoproterozoic. It is nevertheless fully compatible with a SAMBA (Amazonia-Baltica) link at Mesoproterozoic times.
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| 2. |
- D’Agrella-Filho, Manoel S., et al.
(författare)
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A new 1.42 GA paleomagnetic pole from the Amazonian craton : implications for the Columbia supercontinent configuration
- 2011
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Ingår i: Proceedings of Second Biennial Meeting of Latinmag.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Paleomagnetic studies carried out on the 1.42 Ga Indiavaí mafic intrusive rocks, that crop out in the SW Mato Grosso State provides a great opportunity to test the proposed paleogeographic models for Columbia. Paleomagnetic AF and thermal treatment revealed south/southwest magnetic directions with downward inclinations for sixteen analyzed sites. These directions are probably carried by SD/PSD magnetite with high coercivities and high unblocking temperatures as indicated by additional magnetic tests, including thermomagnetic curves, hysteresis loops and the IRM acquisition curves. A different magnetization obtained for host mafic rocks from the basement ca. 10 km NW away from the Indiavaí intrusive, further attests to the primary origin of the characteristic magnetic component. A mean site direction was calculated at Dm=209.8°, Im=50.7° (α95=8.0°, K=22.1), which yielded a paleomagnetic pole located at 249.7°E, -57.0°N (A95=8.6°). Comparison of this pole with other paleomagnetic poles of similar age from Baltica and Laurentia provides evidence for a link of north-northeastern Amazonian craton, southwestern Baltica and Laurentia, as previously suggested from the similar Mesoproterozoic geological evolution of their margins.
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| 3. |
- D'Agrella-Filho, Manoel S., et al.
(författare)
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The 1420 Ma Indiavaí Mafic Intrusion (SW Amazonian Craton): Paleomagnetic results and implications for the Columbia supercontinent
- 2012
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Ingår i: Gondwana Research. - : Elsevier BV. - 1342-937X .- 1878-0571. ; 22:3-4, s. 956-973
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Tidskriftsartikel (refereegranskat)abstract
- The configuration and the timing of assembly and break-up of Columbia are still matter of debate. In order to improve our knowledge about the Mesoproterozoic evolution of Columbia, a paleomagnetic study was carried out on the 1420 Ma Indiavaí mafic intrusive rocks that crosscut the polycyclic Proterozoic basement of the SW Amazonian Craton, in southwestern Mato Grosso State (Brazil). Alternating field and thermal demagnetization revealed south/southwest ChRM directions with downward inclinations for sixteen analyzed sites. These directions are probably carried by SD/PSD magnetite with high coercivities and high unblocking temperatures as indicated by additional rock magnetic tests, including thermomagnetic data, hysteresis data and the progressive acquisition of isothermal remanent magnetization. Different stable magnetization components isolated in host rocks from the basement 10 km NW away to the Indiavaí intrusion, further support the primary origin of the ChRM. A mean of the site mean directions was calculated at Dm = 209.8°, Im = 50.7° (α95 = 8.0°, K = 22.1), which yielded a paleomagnetic pole located at 249.7°E, -57.0°N (A95 = 8.6°). The similarity of this pole with the recently published 1420 Ma pole from the Nova Guarita dykes in northern Mato Grosso State suggests a similar tectonic framework for these two sites located 600 km apart, implying the bulk rigidity of the Rondonian-San Ignacio crust at that time. Furthermore these data provide new insights on the tectonic significance of the 1100-1000 Ma Nova Brasilândia belt –a major EW feature that cuts across the basement rocks of this province, which can now be interpreted as intracratonic, in contrast to previous interpretation. From a global perspective, a new Mesoproterozoic paleogeography of Columbia has been proposed based on comparison of these 1420 Ma poles and a 1780 Ma pole from Amazonia with other paleomagnetic poles of similar age from Baltica and Laurentia, a reconstruction in agreement with geological correlations.
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| 4. |
- Donadini, Fabio, et al.
(författare)
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Paleointensity determination on a 1.786 Ga old gabbro from Hoting, Central Sweden
- 2011
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Ingår i: Earth and Planetary Science Letters. - : Elsevier BV. - 0012-821X .- 1385-013X. ; 309:3-4, s. 234-248
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Tidskriftsartikel (refereegranskat)abstract
- Paleointensities from Precambrian rocks are rare and might be biased by remagnetization processes. Here we present new analyses of samples from a 1.786 Ga gabbro near Hoting, Central Sweden. Rock magnetic and mineralogical analyses indicate that one of the sites (site 5) may be pristine, whereas the others exhibit evidence of alteration. Characteristic remanent magnetization was determined using principal component analysis for each sample and was compared with results obtained in a previous study of Elming et al. (2009). Intensity measurements from site 5 show higher values compared to those of the other sites, suggesting that alteration processes may lead to underestimation of the field intensity. After cooling rate and anisotropy correction, the field moment at 1.786 Ga was estimated to be 25.6 ± 3.3 ZAm2 and 15.2 ± 6.1 ZAm2 from site 5 only and from all sites respectively. We consider the result from site 5 to be more accurate owing to the lack of evidence for alteration; our estimates agree well with the Proterozoic VDM values suggested by Biggin et al. (2009).
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| 5. |
- Elming, Sten-åke, et al.
(författare)
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A palaeomagnetic and Ar-40/Ar-39 study of mafic dykes in southern Sweden: A new Early Neoproterozoic key-pole for the Baltic Shield and implications for Sveconorwegian and Grenville loops
- 2014
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 244, s. 192-206
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of palaeomagnetic and Ar-40/Ar-39 studies of the Proterozoic mafic dykes in the Norrkoping and Falun areas of the southern Sweden. The primary remanence of two 939 +/- 3 Ma dykes is supported by the rigorous baked contact test. The remanence direction of two other dykes, one of which was previously U-Pb dated at 946 +/- 1 Ma is close to the reverse direction of 939 Ma dykes. Using these results together with previously published 935 +/- 5 Ma palaeomagnetic data from the Goteborg-Slussen mafic dykes and some dykes from the Falun area we calculated the mean 946-935 Ma palaeopole for Baltica (0.9 degrees S, 240.7 degrees E, A(95) = 6.7), which can be qualified as the key pole. Using this pole together with other date we conclude that the Grenville and Sveconorwegian loops of Laurentian and Baltican Apparent Polar Wander Paths are temporary displaced by 100-150 m.y. We propose new palaeogeographic reconstructions of Baltica and Laurentia at ca. 940 Ma and ca. 850 Ma. We also present two new Mesoproterozoic non-key poles from 1410 Ma and 1595 Ma dykes. 2014 Elsevier B.V. All rights reserved.
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| 6. |
- Elming, Sten-åke, et al.
(författare)
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Proterozoic Basic dykes in the Ukrainian Shield: A palaeomagnetic, geochronologic and geochemical study-The accretion of the Ukrainian Shield to Fennoscandia
- 2010
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 178:1-4, s. 119-135
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Tidskriftsartikel (refereegranskat)abstract
- A palaeomagnetic, geochronologic and geochemical study of basic dykes in the Ukrainian Shield has been undertaken with the purpose of testing a hypothesis for timing of accretion of the Ukrainian Shield with Fennoscandia. On the basis of geological data it has been suggested that the two shields amalgamated at ca. 1900-1800 Ma. With this study of 11 basic dykes the palaeomagnetic database for the Ukrainian Shield is extended. The palaeomagnetic data indicate three generations of dykes. This is supported by the age data and the difference in geochemical compositions. Ages of two ca. 25 m wide dykes (Susly) in the North-Western Block of the shield have been determined, one for which a positive palaeomagnetic field test is demonstrated. The lower of the two Ar-40/Ar-39 hornblende ages of this dyke, 2061 +/- 20 Ma, is similar to the U-Pb, zircon age of the granitic host rock (ca. 2067 Ma, U-Pb, zircon), which suggests that the dyke intruded shortly after crystallization of the granite. Combined with the magnetization of the baked contact and that of another dyke a new ca 2060 Ma pole is presented (Plat. = 15.7 degrees, Plon. = 182.9 degrees, A(95) = 13.7 degrees). which may be regarded as a VGP. An U-Pb age of 1722 +/- 12 Ma of baddeleyite from the Khmelnik dyke, which yields a well-defined VGP, is similar to the age of nearby anorthosite complexes. A new 1740-1770 Ma key pole for the Ukranian Shield is also presented (Plat. = 26.5 degrees, Plon. = 169.1 degrees, A(95) = 3.9 degrees). On the basis of palaeomagnetic and geochronological data a 2060-1720 Ma apparent polar wander for the Ukrainian Shield is defined. This apparent polar wander is significantly different from that of Fennoscandia during the same time interval and indicates that the two shields were not in their present relative positions at ca. 1770 Ma. A scenario is here proposed where the Ukrainian Shield (and Sarmatia) collided with Fennoscandia at ca. 1900-1800 Ma and after that event, at ca. 1720-1660 Ma, it rotated approximately 43 degrees into its present position. (C) 2010 Elsevier B.V. All rights reserved.
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| 7. |
- Eriksson, Per, et al.
(författare)
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Magma flow directions inferred from field evidence and magnetic fabric studies of the Streitishvarf composite dike in east Iceland
- 2011
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 206:1-2, s. 30-45
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Tidskriftsartikel (refereegranskat)abstract
- Anisotropy of magnetic susceptibility (AMS) and rock magnetic studies have been made on three outcrops separated by 12 km along strike (NNE–SSW) on the Streitishvarf composite dike in east Iceland. Samples for this study have been collected from the inner quartz-porphyry part of the dike, which show clear field evidence of a lateral flow component from north to south at one of the sites. This flow component is consistent with margin AMS results from all three sites. The quartz-porphyry has a substantial bulk magnetic susceptibility (10-2 SI) mainly carried by magnetically soft titanium-poor titanomagnetite (MDF 15 mT). The ferrimagnetic grains yield a characteristic remanent magnetization in all three sites which gives a virtual geomagnetic pole at latitude 52:6°S and longitude 319:6°E. The degree of anisotropy is low (PJ = 1:033) and the magnetic fabrics shifts from oblate to prolate shapes depending on dike margin and outcrop. The magnetic fabric has been interpreted according to the imbrication model, using the minor susceptibility axis as shear plane indicator. The absolute directions given by the minor susceptibility are then quantified using vector algebra. The magma flow is indicated as an upward directed flow, flowing from north to south with an inclination between 30° – 64°, with a 95% confidence ellipse of 3° – 9°. A model for the intrusion of the Streitishvarf dike has been constructed where a magma pocket with felsic magma is punctured by a mafic dike, enabling the felsic magma to rise and extend to the south within the pathway created. The results of this study confirm the applicability of AMS in studies of magma flow directions in igneous dikes of felsic composition.
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| 8. |
- Ingri, Johan, et al.
(författare)
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Changes in trace metal sedimentation during freshening of a coastal basin
- 2014
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Ingår i: Marine Chemistry. - : Elsevier BV. - 0304-4203 .- 1872-7581. ; 167, s. 2-12
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Tidskriftsartikel (refereegranskat)abstract
- Holocene freshening has turned the Bothnian Bay, northern Baltic Sea into an oligotrophic basin. Sequestering of trace elements has changed significantly during the oligotrophication process. In principle, trace metals have been transferred from permanently buried sulfides to Fe–Mn-oxyhydroxides in the top layers of the sediment. The oxyhydroxide layers restrict the flux of trace metals from the sediment to the oxic bottom water. Hence, Fe–Mn cycling in the suboxic sediment enriches a number of trace metals in the surface sediment. Arsenic, Sn, Ge and Bi show enrichment in the Fe-oxyhydroxide layer, whereas Mo, Cd, Ni, Co, Cu, and Sb are enriched in the uppermost Mn-oxyhydroxide layer. This natural redox cycling in the sediment obscures pollution effects.The oligotrophication process started approximately 3500 years ago, reflected in decreasing deposition of Zn, a proxy for phytoplankton production, and formation of Mn oxyhydroxide layers. Similarly, Ba/Al data indicate a decrease in the pelagic input of plankton. Barium data also suggest that dissolved sulfide in the sediment never reached high concentrations. Germanium is closely related to Ba, suggesting that Ge can be used as a proxy for phytoplankton production. Vanadium, U, Re, and Mo all indicate that the bottom water never has been significantly sulfidic during the last 5500 years. Rhenium data indicate that the organic carbon oxidation rate has decreased during the last 5500 years. Cadmium follows the organic matter distribution, but started to increase 1000 YBP (years before present). The reason for this enhanced input of Cd is unclear.
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| 9. |
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| 10. |
- Jensen, Mai-Britt Mose, et al.
(författare)
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A correlation of AMS and rock quality in crystalline and sedimentary rocks in the Kiirunavaara hanging wall
- 2012
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 86, s. 54-69
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Tidskriftsartikel (refereegranskat)abstract
- A study of the magnetic fabric and rock quality has been performed on crystalline and sedimentary rocks from the hanging wall of the Kiirunavaara iron ore deposit, northern Sweden. The rock samples were taken from 12 outcrops and three oriented drillcores and consist of quartzite, siltstone, conglomerate, porphyritic basalts, quartz-bearing porphyry and mylonite.Anisotropy of magnetic susceptibility (AMS) was used to determine the texture of the rocks by defining the shape of the magnetic susceptibility ellipsoids, the orientation of the magnetic foliation planes and the degree of magnetic anisotropy. AMS data from the outcrops as well as data from the uppermost part of two of the drillcores was used to produce a surface contour map which indicates that both the dip of the magnetic foliation plane and the degree of magnetic anisotropy decreases towards the east. These two parameters thus generally follow the major structural features of the Kiruna region.Two classical rock mechanical parameters, RQD (Rock Quality Designation) and F/m (fractures pr. meter), were used to quantify the amount of discontinuities and fractures in the drill cores. RQD and F/m was subsequently correlated with the AMS data, and a correlation was observed, with the correlation coefficient r varying between 0.6142-0.7557. The correlation was limited to just one of the drillcores however it could be traced across rock type boundaries. The correlation indicates that AMS may be used as an indicator tool of the rock mechanical properties in both crystalline and sedimentary rocks.
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