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  • 1.
    Bauer, Tobias
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Skyttä, Pietari
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Allen, Rodney
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Weihed, Pär
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Syn-extensional faulting controlling structural inversion: Insights from the Palaeoproterozoic Vargfors syncline, Skellefte mining district, Sweden2011In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 191, no 3-4, p. 166-183Article in journal (Refereed)
    Abstract [en]

    The Vargfors basin in the central Skellefte district, Sweden, is an inverted sedimentary sub-basin within a Palaeoproterozoic (1.89 Ga) marine volcanic arc. The sub-basin formed from upper-crustal extension and subsequent compression, following a period of intense marine volcanism and VMS ore formation. Detailed mapping and structural analysis reveals a pattern of SE–NW-striking normal faults and interlinked NE–SW-striking transfer faults, which define distinct fault-bound compartments, each with an individual structural geometry and stratigraphy. Constraints on the deformation style and mechanisms achieved by 2D forward modelling are in agreement with the previously inferred inversion of the early normal faults during a regional crustal shortening event. A rheologically weak carbonate-rich layer at the base of the sedimentary sequence favoured the fault inversion over more distributed shortening as the controlling deformation mechanism. Transposition of sedimentary strata into the approximately SE–NW faults led to formation of asymmetric synclines that were tightened during progressive shortening. Structural analysis infers a progressive opening of the basin towards SE and NW with time. Furthermore, it is inferred that a displacement gradient was developed along the main structural grain, with decreasing dip-slip displacements towards SE and NW, both during the extension and the structural inversion.VMS deposits in the vicinity of the contact between the volcanic and the overlying sedimentary rocks were formed along early normal faults, which reacted as fluid conduits. Subsequently, the deposits were transposed into the inverted faults during crustal shortening. Consequently, the inverted faults provide a useful tool for mineral exploration in the district.

  • 2.
    Billström, K.
    et al.
    Dep. of Geological Sciences, Swedish Museum of Natural History, Stockholm, Sweden.
    Evins, P.
    WPS Consulting Group, Stockholm, Sweden.
    Martinsson, Olof
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Jeon, H.
    Dep. of Geological Sciences, Swedish Museum of Natural History, Stockholm, Sweden.
    Weihed, Pär
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Conflicting zircon vs. titanite U-Pb age systematics and the deposition of the host volcanic sequence to Kiruna-type and IOCG deposits in northern Sweden, Fennoscandian shield2019In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 321, p. 123-133Article in journal (Refereed)
    Abstract [en]

    The Northern Norrbotten region, and in particular the Kiruna area, hosts a number of large apatite iron oxide deposits (e.g. the huge Kiirunavaara ore) of significant economic importance. Age data from rock lithologies hosting these ores, represented by metamorphosed rocks of the Porphyrite and Kiirunavaara Groups, are complex to interpret. This is illustrated by (LA-ICP-MS) data for titanite, and to some extent for rutile, which scatter considerably yielding ages within a span from ca. 2.1 Ga to 1.7 Ga. These analysed hydrothermal minerals, characterized by complex BSE images revealing darker and brighter zones, are located in ore zones and associated with e.g. strong scapolitisation, albitisation and actinolitisation. Previous (TIMS) zircon ages of host rocks, on the other hand, define a more narrower age interval between ca. 1900 and 1870 Ma, and this is supported by new U-Pb zircon results presented here. Furthermore, one coherent set of SIMS data for titanite from the Luossavaara ore favour that crystallization took place at ca 1.88 Ga, although laser ICP data from the same locality are much more complex. An implication arising from published pre-1.9 Ga laser ablation ages for titanites is that the emplacement of host rocks started already at around 2.1 Ga. As the depositional time of these rocks is crucial for the understanding of the overall crustal formation in northern Norrbotten, additional rocks were selected for age dating. New zircon age data (LA-ICP-MS and SIMS) give support to a scenario where host rocks to ores started to develop at around 1900 Ma and this calls for a re-evaluation of published LA-ICP-MS data of hydrothermal mineral phases.

    Here, we present four models that aim to explain how pre-1.9 Ga titanite ages, believed to have a questionable geological significance, may develop. The principal idea is that ≤2.1 Ga alteration events were not responsible for the crystallization of the hydrothermal minerals, instead it is believed that apparent old age domains carry excess radiogenic lead due to the effect of ≤1.9 Ga hydrothermal processes. Currently, the interpretation of U-Pb isotope data in the study area remains enigmatic, and further radiometric analyses are required.

  • 3.
    Billström, Kjell
    et al.
    Swedish Museum of Natural History.
    Frietsch, Rudyard
    Luleå tekniska universitet.
    Perdahl, Jan-Anders
    Luleå tekniska universitet.
    Regional variations in the Pb isotopic compositions of ore galena across the Archaean-Proterozoic border in northern Sweden1997In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 81, no 1-2, p. 83-99Article in journal (Refereed)
  • 4.
    Bispo-Santos, Franklin
    et al.
    Universidade de São Paulo.
    D’Agrella-Filho, Manoel S.
    Universidade de São Paulo.
    Pacca, Igor I.G.
    Universidade de São Paulo.
    Janikian, Liliane
    Universidade de São Paulo.
    Trindade, Ricardo I.F.
    Universidade de São Paulo.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Silva, Jesué A.
    METAMAT.
    Barros, Márcia A.S.
    Department of Mineral Resources, UFMT.
    Pinho, Francisco E.C.
    Department of Mineral Resources, UFMT.
    Columbia revisited: paleomagnetic results from the 1790 Ma colider volcanics (SW Amazonian Craton, Brazil)2008In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 164, no 1-2, p. 40-49Article in journal (Refereed)
    Abstract [en]

    In an attempt to improve our understanding of the Paleoproterozoic geodynamic evolution, a paleomagnetic study was performed on 10 sites of acid volcanic rocks of the Colider Suite, southwestern Amazonian Craton. These rocks have a well-dated zircon U-Pb mean age of 1789 +/- 7 Ma. Alternating field and thermal demagnetization revealed northern (southern) directions with moderate to high upward (downward) inclinations. Rock magnetism experiments and magnetic mineralogy show that this characteristic magnetization is carried by Ti-poor magnetite or by hematite that replaces magnetite by late-magmatic cleuteric alteration. Both magnetite and hematite carry the same characteristic component. The mean direction (Dm = 183.0 degrees, Im = 53.5 degrees, N = 10, alpha(95) = 9.8 degrees, K = 25.2) yielded a paleomagnetic pole located at 298.8 degrees E, 63.3 degrees S (alpha(95) = 10.2 degrees, K = 23.6), which is classified with a quality factor Q = 5. Paleogeographic reconstructions using this pole and other reliable Paleoproterozoic poles suggest that Laurentia, Baltica, North China Craton and Amazonian Craton were located in laterally contiguous positions forming a large continental mass at 1790 Ma ago. This is reinforced by geological evidence which support the existence of the supercontinent Columbia in Paleoproterozoic times.

  • 5.
    Bispo-Santos, Franklin
    et al.
    Universidade de São Paulo.
    D’Agrella-Filho, Manoel S.
    Universidade de São Paulo.
    Trindade, Ricardo I.F.
    Universidade de São Paulo.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Janikian, Liliane
    Universidade de São Paulo.
    Vasconcelos, Paulo M.
    University of Queensland.
    Perillo, Bruno M.
    Universidade de São Paulo.
    Pacca, Igor I.G.
    Universidade de São Paulo.
    Silva, Jesué A. da
    Companhia Matogrossense de Mineração–METAMAT.
    Barros, Márcia A.S.
    Universidade Federal de Mato Grosso.
    Tectonic implications of the 1419 Ma Nova Guarita mafic intrusives paleomagnetic pole (Amazonian Craton) on the longevity of Nuna2012In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 196-197, p. 1-22Article in journal (Refereed)
    Abstract [en]

    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.

  • 6. Buchan, Kenneth L.
    et al.
    Ernst, Richard E.
    Hamilton, Michael A.
    Mertanen, Satu
    Pesonen, Lauri J.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Rodinia: the evidence from integrated palaeomagnetism and U-Pb geochronology2001In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 110, no 1-4, p. 9-32Article in journal (Refereed)
    Abstract [en]

    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.

  • 7.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Palaeomagnetism of Precambrian rocks in northern Sweden and its correlation to radiometric data1994In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 69, no 1-4, p. 61-79Article in journal (Refereed)
    Abstract [en]

    A palaeomagnetic study has been performed on Palaeo- to Mesoproterozoic basic intrusions and volcanic rocks from the Fennoscandian shield in northern Sweden. Three, possibly four, different generations of magnetizations were identified, the oldest assigned to a Svecofennian age (1.86-1.89 Ga). A second generation is related to the intrusion of granitoids of 1.80-1.76 Ga. In this geological event probably also the third group of directions has its origin. These different magnetizations may indicate that there are at least two different generations of basic intrusions in northern Sweden. A fourth group of directions is isolated as overprints. This magnetization is interpreted to be a Subjotnian magnetization, reflecting a previously unrecognized regional Subjotnian metamorphic event in the northwestern part of the Fennoscandian shield. The drift history for the Fennoscandian shield during the period 1.88-1.50 Ga has been defined based on these new palaeomagnetic data

  • 8.
    Elming, Sten-åke
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Layer, Paul
    Geophysical Institute, University of Alaska Fairbanks.
    Söderlund, Ulf
    Department of Geology, Lund University.
    Cooling history and age of magnetization of a deep intrusion: A new 1.7 Ga key pole and Svecofennian-post Svecofennian APWP for Baltica2019In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 329, p. 182-194Article in journal (Refereed)
    Abstract [en]

    A paleomagnetic and chronogical study has been performed on the Turinge gabbro-diabase formation and on a cross cutting basic dyke in central Sweden and on the Joulovaara gabbro intrusion in northern Sweden in the Fennoscandian Shield. U-Pb age of baddeleyite and 40Ar/ 39Ar ages of hornblende and biotite reveal a cooling history of the deep gabbro-diabase intrusion in Turinge. The cooling is suggested to have taken place in two stages, one related to the time of intrusion in temperature down to ca 500 °C with a cooling rate up to 46 - 59°/Ma and another at a lower rate of ca 2.9 °C/Ma, which is suggested to be related with uplift. From this cooling history it can be concluded that the magnetization age of the diabase, ca 1695 – 1700 Ma is close to the crystallization age and the 40Ar/39Ar age of hornblende. Applying a similar cooling history for the other studied deep intrusion, the ca 1800 Ma gabbro of Joulovaara gabbro, it is estimated that the magnetization age of the gabbro should be close to that of the U-Pb age of the formation, although the pole of the Joulovaara gabbro is less reliable.

    The cooling history presented here for the Turinge gabbro-diabase has implications for estimations of magnetization ages also for other deep intrusions.

    The new pole (Plat. = 51.6°, Plon. = 220.2°; A95= 4.8°) of the Turinge gabbro-diabase passes most of the reliability criteria and is considered a new key pole for Fennoscandia.

    The Basic dyke that cuts the Turinge gabbro-diabase was here dated at ca 1200 Ma (whole rock, 40Ar/ 39Ar) and the virtual geomagnetic pole calculated from its primary magnetization falls into the expected trend of APWP for Baltica.

    The new Turinge key pole prolong the time of overlapping poles for Fennoscandia, indicating only small movements of the shield between ca. 1870 to 1700 Ma.

  • 9.
    Elming, Sten-åke
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Mattsson, Håkan
    Luleå tekniska universitet.
    Post Jotnian basic intrusions in the Fennoscandian Shield, and the break up of baltica from Laurentia: a palaeomagnetic and AMS study2001In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 108, no 3-4, p. 215-236Article in journal (Refereed)
    Abstract [en]

    A palaeomagnetic and anisotropy of magnetic susceptibility (AMS) study has been performed on dolerite sills of the Central Scandinavian Dolerite Group (CSDG) in the Fennoscandian Shield. The dolerites occur in four previously known complexes in central Sweden and Finland and from the results of this palaeomagnetic study another complex has been identified in northern Sweden. These complexes cover an area of at least 100 000 km2 and the palaeomagnetic data suggest a small difference in time between the intrusion of the dolerites. The measurements of anisotropy of magnetic susceptibility reveal a magnetic fabric with almost horizontal foliation planes and lineations that indicate fairly uniform ca NW or SE directed magma flows. The dolerites of the CSDG are geochemically rather uniform and have compositions typical of mantle derived melts formed in continental tensional settings. In a palaeomagnetic reconstruction of Baltica versus Laurentia at ca 1.27 Ga the two continents were joined, with NE Greenland attached to NW Baltica. AMS data from a few dolerites and a basalt in NE Greenland indicate magma flow directions that in the tectonic reconstruction are more or less parallel to the flow of the dolerites in Sweden. This may suggest a common magma source located at the reconstructed contact between Baltica and Laurentia. Both the dolerites in Greenland and those in Sweden are of tholeitic composition indicating an intraplate origin, which supports the interpretation of joined continents at that time. The tensional regime, that is reflected by the huge sill complexes, is in our interpretation related to the break up of Baltica from Laurentia at ca 1.27 Ga ago.

  • 10.
    Elming, Sten-åke
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Pisarevsky, Sergei A.
    Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (CCFS) and the Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtain University.
    Layer, Paul
    Geophysical Institute, University of Alaska, Fairbanks.
    Bylund, Göran
    Department of Geology, GeoBiosphere Science Centre, Lund University.
    A palaeomagnetic and 40Ar/39Ar study of mafic dykes in southern Sweden: A new Early Neoproterozoic key-pole for the Baltic Shield and implications for Sveconorwegian and Grenville loops2014In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 244, p. 192-206Article in journal (Refereed)
    Abstract [en]

    We present the results of palaeomagnetic and 40Ar/39Ar studies of the Proterozoic mafic dykes in the Norrköping 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 Göteborg-Slussen mafic dykes and some dykes from the Falun area we calculated the mean 946 -935 Ma palaeopole for Baltica (0.9°S, 240.7°E, A95 = 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.

  • 11.
    Elming, Sten-åke
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Shumlyanskyy, Leonid
    Institute of Geochemistry, Mineralogy and Ore Formation, Kiev.
    Kravchenko, Svetlana
    Institute of Geophysics, National Academy of Sciences of Ukraine.
    Layer, Paul
    Geophysical Institute, University of Alaska, Fairbanks.
    Söderlund, Ulf
    Department of Geology, GeoBiosphere Science Centre, Lund University.
    Proterozoic basic dykes in the Ukrainian shield: a palaeomagnetic, geochronologic and geochemical study – the accretion of the Ukrainian Shield to Fennoscandia2010In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 178, no 1-4, p. 119-135Article in journal (Refereed)
    Abstract [en]

    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 to 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 40Ar/39Ar 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. = 15.7o, Plon. = 182.9o, A95 = 13.7o), 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. = 26.5o, Plon. = 169.1o, A95 = 3.9o). On the basis of palaeomagnetic and geochronological data a 2060 Ma to 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 43o into its present position.

  • 12.
    Gong, Zheng
    et al.
    Department of Geology and Geophysics, Yale University.
    Evans, David A.D.
    Department of Geology and Geophysics, Yale University.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Söderlund, Ulf
    Department of Geology, Lund University.
    Salminen, Johanna M.
    Department of Geosciences and Geography, University of Helsinki.
    Corrigendum to "Paleomagnetism, magnetic anisotropy and U-Pb baddeleyite geochronology of the early Neoproterozoic Blekinge-Dalarna dolerite dykes, Sweden" [Precambrian Res. 317 (2018) 14-32]2019In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 320, p. 484-485Article in journal (Refereed)
    Abstract [en]

    The authors regret that the incorrect version of Table 3 Fig. 12 appeared in the paper. The corrected Table 3 and Fig. 12 are presented below. In Table 2, Group A mean direction should be Dec = 128.8° Inc = 39.6° α95 = 6.5° and Group B mean direction should be Dec = 127.6° Inc = 65.4° α95 = 9.7°. These changes do not affect any main conclusion of the paper. The authors would like to apologize for any inconvenience caused.

  • 13.
    Gong, Zheng
    et al.
    Department of Geology and Geophysics, Yale University.
    Evans, David A.D.
    Department of Geology and Geophysics, Yale University.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Söderlund, Ulf
    Department of Geology, Lund University.
    Salminen, Johanna M.
    Department of Geosciences and Geography, University of Helsinki.
    Paleomagnetism, magnetic anisotropy and U-Pb baddeleyite geochronology of the early Neoproterozoic Blekinge-Dalarna dolerite dykes, Sweden2018In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 317, p. 14-32Article in journal (Refereed)
    Abstract [en]

    Paleogeographic proximity of Baltica and Laurentia in the supercontinent Rodinia has been widely accepted. However, robust paleomagnetic poles are still scarce, hampering quantitative tests of proposed relative positions of the two cratons. A recent paleomagnetic study of the early Neoproterozoic Blekinge-Dalarna dolerite (BDD) dykes in Sweden provided a 946-935 Ma key pole for Baltica, but earlier studies on other BDD dykes discerned large variances in paleomagnetic directions that appeared to indicate more complicated motion of Baltica, or alternatively, unusual geodynamo behavior in early Neoproterozoic time. We present combined paleomagnetic, rock magnetic, magnetic fabric and geochronological studies on BDD dykes in the Dalarna region, southern Sweden. Positive baked-contact and paleosecular variation tests support the reliability of the 951-935 Ma key pole (Paleolatitude = -2.6°N, Paleolongitude = 239.6°E, A95 = 5.8°, N = 12 dykes); and the ancient magnetic field was likely a stable geocentric axial dipole at that time, based on a positive reversal test. Detailed analysis of the 947 Ma Nornäs dyke, one of the dykes previously showing anomalous directions, suggests a partial viscous remagnetization. Therefore, the observed large variances in nearly coeval BDD dykes are suspected to result from present-day overprints that were not adequately removed in earlier studies. In addition, we obtained a 971 Ma virtual geomagnetic pole (Paleolatitude = -27.0°N, Paleolongitude = 230.4°E, A95 = 14.9°, N = 4 dykes) for Baltica. Comparing similar-aged poles from Laurentia, we suggest that Baltica and Laurentia drifted together from high to low latitude between 970-960 Ma and 950-935 Ma, and returned back to high latitude by 920-870 Ma. In this scenario, the apparent polar wander paths of Baltica and Laurentia may be more complicated than the previously proposed, solitary Sveconorwegian and Grenville loops. The new U-Pb baddeleyite ages do not support BDD dykes as a giant circumferential swarm generated by a mantle plume, and the prolonged timespan of dyke intrusion is likely associated with the plate boundary forces as causing gravitational extension at the waning stage of the Sveconorwegian orogeny.

  • 14.
    Kampmann, Tobias Christoph
    et al.
    Lund University.
    Gumsley, Ashley Paul
    Lund University.
    Kock, Michiel Olivier de
    University of Johannesburg.
    Söderlund, Ulf
    Department of Geology, GeoBiosphere Science Centre, Lund University, Lund University.
    U-Pb geochronology and paleomagnetism of the Westerberg Sill Suite, Kaapvaal Craton – support for a coherent Kaapvaal–Pilbara Block (Vaalbara) into the Paleoproterozoic?2015In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 269, p. 58-72Article in journal (Refereed)
    Abstract [en]

    Precise geochronology, combined with paleomagnetism on mafic intrusions, provides first-order information for paleoreconstruction of crustal blocks, revealing the history of supercontinental formation and break-up. These techniques are used here to further constrain the apparent polar wander path of the Kaapvaal Craton across the Neoarchean–Paleoproterozoic boundary. U–Pb baddeleyite ages of 2441 ± 6 Ma and 2426 ± 1 Ma for a suite of mafic sills located on the western Kaapvaal Craton in South Africa (herein named the Westerberg Sill Suite), manifests a new event of magmatism within the Kaapvaal Craton of southern Africa. These ages fall within a ca. 450 Myr temporal gap in the paleomagnetic record between 2.66 and 2.22 Ga on the craton. Our older Westerberg Suite age is broadly coeval with the Woongarra magmatic event on the Pilbara Craton in Western Australia. In addition, the Westerberg Suite on the Kaapvaal Craton intrudes a remarkably similar Archean-Proterozoic sedimentary succession to that on the Pilbara Craton, supporting a stratigraphic correlation between Kaapvaal and Pilbara (i.e., Vaalbara). The broadly coeval Westerberg–Woongarra igneous event may represent a Large Igneous Province. The paleomagnetic results are more ambiguous, with several different possibilities existing. A Virtual Geomagnetic Pole obtained from four sites on the Westerberg sills is 18.9°N, 285.0°E, A95 = 14.1°, K = 43.4 (Sample based VGP, n = 34: 16.8°N, 2879.9°E, dp = 4.4°, dm = 7.7°). If primary (i.e., 2441–2426 Ma), it would provide a further magmatic event within a large temporal gap in the Kaapvaal Craton's Paleoproterozoic apparent polar wander path. It would suggest a relatively stationary Kaapvaal Craton between 2.44 Ga and 2.22 Ga, and ca. 35° of latitudinal drift of the craton between ca. 2.66 Ga and 2.44 Ga. This is not observed for the Pilbara Craton, suggesting breakup of Vaalbara before ca. 2.44 Ga. However, it is likely that the Woongarra paleopole represents a magnetic overprint acquired during the Ophtalmian or Capricorn Orogeny, invalidating a paleomagnetic comparison with the Westerberg Sill Suite. Alternatively, our Westerberg Virtual Geographic Pole manifests a 2.22 Ga magnetic overprint related to Ongeluk volcanism. The similarity between Ongeluk and Westerberg paleopoles however may also infer magmatic connections if both are primary directions, despite the apparent 200 million year age this difference.

  • 15.
    Kampmann, Tobias
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Stephens, Michael
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering. Sveriges Geologiska Undersökning.
    Ripa, Magnus
    Sveriges Geologiska Undersökning.
    Hellström, Fredrik
    Sveriges Geologiska Undersökning.
    Majka, Jarosław
    Uppsala universitet.
    Time constraints on magmatism, mineralisation and metamorphism at the Falun base metal sulphide deposit, Sweden, using U-Pb geochronology on zircon and monazite2016In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 278, p. 52-68Article in journal (Refereed)
    Abstract [en]

    U–Th–Pb (zircon and monazite) ion probe data have provided constraints on the timing of emplacement and metamorphism of magmatic rocks close to the Palaeoproterozoic, Falun base metal sulphide deposit in the Bergslagen lithotectonic unit, Fennoscandian Shield, Sweden, and, thereby the timing of mineralisation. Hydrothermal alteration and mineralisation at Falun are constrained to a short interval of several million years between a 207Pb/206Pb weighted average age of 1894 ± 3 Ma for a rhyolitic sub-volcanic rock in the felsic volcanic to sub-volcanic host rock suite, and a 207Pb/206Pb weighted average age of 1891 ± 3 Ma for a post-sulphide, porphyritic dacite dyke. Magmatism also included the emplacement of granite plutons with igneous crystallization ages of 1894 ± 3, 1894 ± 2 Ma and 1893 ± 3 Ma. The felsicsub-volcanic to volcanic activity and the emplacement of dacite dykes and granite plutons overlap in age within their respective analytical uncertainties, indicating hydrothermal alteration and sulphide mineralisation inside a narrow time span of intense magmatic activity, and burial of the supracrustal rocks.Two distinct patchy and homogeneous metamorphic monazite types in a felsic volcanic rock around and hydrothermally altered rocks at the Falun deposit yield 207Pb/206Pb weighted average ages of 1831 ± 8 Ma and 1822 ± 5 Ma, respectively. These ages fall well within the temporal range of a younger1.84–1.81 Ga (M2) metamorphic episode during the 2.0–1.8 Ga Svecokarelian orogeny, with the older episode (M1) inside the Bergslagen lithotectonic unit at around 1.86 Ga. This shows the major influence of the M2 event in the north-western part of this unit, leading to a complete resetting of the U–Th–Pb isotopesystem in monazite.

  • 16.
    Malehmir, Alireza
    et al.
    Department of Earth Sciences, Uppsala University.
    Tryggvason, Ari
    Department of Earth Sciences, Uppsala University.
    Lickorish, Henry
    Weihed, Pär
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Regional structural profiles in the western part of the Palaeoproterozoic Skellefte ore district, northern Sweden2007In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 159, no 1-2, p. 1-18Article in journal (Refereed)
    Abstract [en]

    The Kristineberg mining area is situated in the western part of the Palaeoproterozoic Skellefte Ore District, northern Sweden, and is well known for its VHMS base-metal and gold deposits. This paper presents five upper crustal geological cross sections that have been constructed and mainly constrained by seismic reflection data, potential field modeling as well as geological field observations. These profiles are visualized in 3D to highlight the three dimensionality and internal consistency of structures across the region. The resulting structural model for the Skellefte volcanics and overlying metasediments comprises two thrust-sheets that expose the Skellefte volcanics in the cores of hanging-wall anticlinal structures. A shear-zone is imaged as a band of seismic reflectivity terminated by the southern Revsund granite unit. Another shear-zone, possibly a continuation of the Skellefte Shear Zone (SSZ) runs through the centre of the region and accounts for some of the structural complexity and shearing observed between the two anticlinal exposures of the volcanics. Additional smaller scale shear-zones have been identified from geological and geophysical mapping within the main structural blocks of the Skellefte volcanics. The Mala volcanic rocks in the north are separated from the Skellefte volcanics by a fault that cuts discordant to the strike of the Mala volcanics. A structural basement has been proposed to the Skellefte volcanics, constrained by seismic reflection data. Exposures of Bothnian Basin rocks south of the Revsund granite outcrops, suggest that the domain beneath the north dipping reflectivity is associated with Bothnian Basin stratigraphy. The preferred interpretation for the contact between the Skellefte volcanics and the Bothnian Basin rocks is a thrust fault that brings the felsic volcanic rocks over the metasedimentary domain. The Revsund granites are divided into two major groups based on their present day thickness and shapes. Although parts of the Viterliden intrusion are almost undeformed, it is cut by a series of shear-zones, causing the magnetic lineations seen within these rocks. The structural profiles presented demonstrate that the Kristineberg ore is situated in the northern limb of a local synformal structure. The new crustal-scale structural model demonstrates the potential of integrating geophysical and geological data when modelling structures hosting mineralization in a complex region like the Skellefte District. The structural profiles presented in this paper, have greatly improved our understanding of the 3D tectonostratigraphy and architecture of the poly-deformed ca. 1.9 Ga the Skellefte Ore District.

  • 17.
    Mattsson, Håkan
    et al.
    Luleå tekniska universitet.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Magnetic fabrics and paleomagnetism of the Storsjön-Edsbyn deformation zone, central Sweden2001In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 107, no 3-4, p. 265-281Article in journal (Refereed)
    Abstract [en]

    A regional deformation zone, separating tectonomagmatic blocks in the central part of the Fennoscandian Shield, has been studied using anisotropy of magnetic susceptibility and paleomagnetism in order to define the tectonic character and age of the zone and to put the deformation into a plate tectonic context. The deformation zone (the Storsjön-Edsbyn Deformation Zone; SEDZ) separates the Rätan granite of the Trans Scandinavian Igneous Belt (TIB) and the southern Svecofennian subprovince in the southwest from older Svecofennian granitoids of the central Svecofennian subprovince in the northeast. The SEDZ is the most prominent deformation zone in a regional pattern of NW-SE trending shear zones. On basis of AMS data the width of the zone, which cuts through 1.7-1.9 Ga old rocks, is estimated at 10-12 km in its central-southern part and 20-25 km in its northern part. The orientations of the susceptibility axes vary systematically across the zone, in a way that suggests that the SEDZ is a pure shear dominated transpression zone caused by a compressive stress from the present SSW. Paleomagnetic data from a dolerite sill (Plat=10°, Plon=166°) and from a dyke (Plat=25°, Plon=190°) that cut the SEDZ indicate that no plastic deformation has occurred along the zone during the past 1.25 Ga, and probably not for the past 1.5-1.6 Ga. AMS data do not point to any deformation of the 1.70 Ga old Rätan granite, which further limits the time span of deformation. The main phase of deformation most likely took place between ≈1.80 and 1.70 Ga ago. The geological evolution of this part of the Fennoscandian Shield at the time between 1.80 and 1.70 Ga ago is very complex and in part still enigmatic. This time period involves magmatism of the TIB, which is related to eastward subduction. It also comprises the beginning of the Gothian orogeny and the development of a regional system of transpressive deformation zones. The compressive stress field from the present SSW indicated in this study is interpreted to be related to an initial stage of plate collision between the southwestern Scandinavian subprovince and older parts of the Fennoscandian Shield. This collision resulted in a system of regional NW-SE trending intra-plate deformation zones, one of which is the SEDZ

  • 18.
    Mellqvist, C.
    et al.
    Luleå tekniska universitet.
    Öhlander, Björn
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Skiöld, Torbjörn
    Laboratoriet för Isotopgeologi, Naturhistoriska Riksmuseet, Stockholm.
    Wikström, A.
    Geological Survey of Sweden.
    The Archaean-Proterozoic Palaeoboundary in the Luleå area, northern Sweden: field and isotope geochemical evidence for a sharp terrane boundary1999In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 96, no 3-4, p. 225-243Article in journal (Refereed)
    Abstract [en]

    Extensive Sm-Nd isotope work has been carried out in northern Sweden with the intention of studying crustal reactivation and assimilation processes relative to juvenile arc accretion at an Archaean cratonic margin. Previous regional work has shown a gradual change from negative εNd(t) values in northernmost Sweden to positive values towards the Skellefte sulphide ore district farther south. Isotopic variation and geophysical data show a break in the character of the crust along a WNW-trending zone. It is suggested that this zone, which passes close to the towns of Luleå and Jokkmokk, represents the transition from the Archaean craton in the northeast to Proterozoic juvenile crust in the southwest. A profile across the eastern part of this transition zone has been studied in detail in the Luleå area, where Archaean rocks recently have been discovered. The distribution and character of the Archaean rocks have been studied. These occur as fragments, ranging in size from several km2 to dm in scale, and consist of ca 2.7 Ga orthogneisses of granodioritic to dioritic composition and porphyritic granitoids of quartz monzodioritic to granodioritic composition. Plutonic rocks of ca 1.9 Ga age and Archaean rocks were analysed for major and trace elements, and for Sm-Nd isotopes. The calc-alkaline ca 1.9 Ga granitoids are dominated by granodiorites, tonalites, quartz diorites and quartz monzodiorites, and there is a distinct boundary rather than a transition zone between intrusions with postive εNd(t) values in the southwest and intrusions with negative εNd(t) values in the northeast. The locations of the outcropping Archaean fragments coincides with this distinct boundary, which is interpreted as a terrane boundary separating a ca 1.9 Ga juvenile arc terrane in the southwest from the Archaean continent in the northeast. A possible explanation for this is that the Proterozoic juvenile terrane was obducted onto the Archaean craton during the final stages of the Svecofennian orogeny.

  • 19.
    Pease, Victoria
    et al.
    Department of Geology and Geochemistry, Stockholm University.
    Daly, J.S.
    School of Geological Sciences, University College Dublin.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Kumpulainen, R.
    Department of Geology and Geochemistry, Stockholm University.
    Moczydlowska, M.
    Department of Earth Sciences, Uppsala University.
    Puchkov, V.
    Institute of Geology, Ufimian Science Centre RAS, Ufa.
    Roberts, D.
    Geological Survey of Norway.
    Saintot, A.
    Faculty of Life and Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam.
    Stephenson, R.
    Faculty of Life and Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam.
    Baltica in the Cryogenian 850-630 Ma2008In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 160, no 1-2, p. 46-65Article in journal (Refereed)
    Abstract [en]

    This new tectonic synthesis provides a framework for understanding the dynamic evolution of Baltica and for constraining tectonic correlations within the context of the Neoproterozoic break-up of Rodinia-Pannotia. Cryogenian Baltica is described with respect to five geographic regions: the northwest, norBaltica; Rodinia; Neoproterozoic; Orogeny; Paleogeographyheast, east, south, and southwest (modern coordinates). These geographic regions define three principal Cryogenian tectonic margins: a rifting northwestern margin, a passive northeastern margin, and a poorly understood southern margin.The northwest region is characterized by Neoproterozoic to lower Ordovician sedimentary successions deposited on Archean to late Mesoproterozoic crystalline complexes, reworked during Caledonian orogenesis. Lare Neoproterozoic to lower Ordovician sedimentary strata record the change from an alluvial setting to a marine environment, and eventually to a partially starved (?) turbidite basin. They document rifting from the Rodinian-Pannotian supercontinent, which was unsuccessful until ca. 620-550 Ma when voluminous dikes and mafic/ultramafic complexes were intruded.Baltica's northeastern and eastern regions document episodic intracratonic rifting throughout the Mesoproterozoic, followed by pericontinental passive margin deposition throughout the Cryogenian. In the northeast platformal and deeper-water basin deposits are preserved, whereas the eastern region was later affected by Paleozoic rifting and preserves only shelf deposits. The northeastern and eastern regions define Baltica's Cryogenian northeastern tectonic margin, which was an ocean-facing passive margin of the Rodinia-Pannotia supercontinent. It remained a passivemargin until the onset of Timanian orogenesis at ca. 615 Ma, approximately synchronous with the time of Rodinia-Pannotia rifting.Baltica's southern and southwestern regions remain enigmatic and controversial. Precambrian basement is generally hidden beneath thick successions of Ediacaran and younger platform sediments. Similarities between these regions exist, however, and suggest that they may share a similar tectonic evolution in the Cryogenian and therefore define the southern tectonic margin of Baltica at this time. Paleo- to Mesoproterozic basement was affected by Neoproterozoic and younger tectonism, including Cryogenian (?) and Ediacaran rifting. This was followed by Ediacaran(ca. 550 Ma) passive margin sediment deposition at the time of Rodinia-Pannotia break-up, until Early Paleozoic accretion of allochthonous terranes record the transition from rifting to a compressional regime.Paleomagnetic and paleontological data are consistent with Baltica and Laurentia drifting together between ca. 750 and 550 Ma, when they had similar apparent polar wander paths. Microfossil assemblages along the eastern margin of Laurentia and the western margin of Baltica (modern coordinates), suggest proximity between these two margins at this time. At ca. 550 Ma, Laurentia and Baltica separated, consistent with paleomagnetic, paleontological, and geological data, and a late break-up for Rodinia-Pannotia.

  • 20.
    Perdahl, Jan-Anders
    et al.
    Luleå tekniska universitet.
    Frietsch, Rudyard
    Luleå tekniska universitet.
    Petrochemical and petrological characteristics of 1.9 Ga old volcanics in northern Sweden1993In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 64, no 1-4, p. 239-252Article in journal (Refereed)
    Abstract [en]

    Palaeoproterozoic, 1.9 Ga old, subaerial felsic to mafic volcanics of the Kiruna-Arvidsjaur Porphyry Group (KAPG) are abundant in northernmost Sweden. The felsic-dominated KAPG is situated north of a proposed north-dipping subduction zone along the boundary between the Archaean craton in the northeastern part of the Baltic Shield and juvenile Proterozoic crust in the south. The geochemistry of the volcanics indicates that plate-tectonic and volcanic-arc processes similar to recent ones occurred in northern Sweden 1.9 Ga ago. The volcanics were formed in extensional as well compressional environments. Calc-alkaline volcanics mark compression in the south and in the east. Mildly alkaline volcanics occurring in the west were formed in an extensional setting. The same rift zone gave rise to a basin filled with a thick sequence of younger sediments. On the basis of regional differences in geochemistry and petrology, the KAPG has been divided into the Kiruna, Arjeplog. Arvidsjaur and Luleå subprovinces. The volcanics in the Skellefte ore district south of the KAPG occurrence area have a volcanic-arc character and can be related to the volcanics of the Arvidsjaur subprovince immediately to the north; however, the latter are subaerial whereas the Skellefte volcanics represent a marine facies. The northerly Kiruna subprovince represents a different tectonic environment with a mafic to intermediate, trachytic volcanism that indicates extensional environments. The pattern of the different geotectonic settings indicates that the development of the 1.9 Ga old volcanics cannot be assigned solely to the action of a subduction zone dipping northwards beneath the Skellefte district.

  • 21.
    Pisarevsky, Sergei A.
    et al.
    Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (CCFS) and the Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtain University.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Pesonen, Lauri J.
    Division of Geophysics and Astronomy, Department of Physics, University of Helsinki.
    Li, Zheng-Xiang
    Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (CCFS) and the Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtain University.
    Mesoproterozoic paleogeography: Supercontinent and beyond2014In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 244, p. 207-225Article in journal (Refereed)
    Abstract [en]

    A set of global paleogeographic reconstructions for the 1770–1270 Ma time interval is presented here through a compilation of reliable paleomagnetic data (at the 2009 Nordic Paleomagnetic Workshop in Luleå, Sweden) and geological constraints. Although currently available paleomagnetic results do not rule out the possibility of the formation of a supercontinent as early as ca. 1750 Ma, our synthesis suggests that the supercontinent Nuna/Columbia was assembled by at least ca. 1650–1580 Ma through joining at least two stable continental landmasses formed by ca. 1.7 Ga: West Nuna (Laurentia, Baltica and possibly India) and East Nuna (North, West and South Australia, Mawson craton of Antarctica and North China). It is possible, but not convincingly proven, that Siberia and Congo/São Francisco were combined as a third rigid continental entity and collided with Nuna at ca.1500 Ma. Nuna is suggested to have broken up at ca. 1450–1380 Ma. West Nuna, Siberia and possibly Congo/São Francisco were rigidly connected until after 1270 Ma. East Nuna was deformed during the breakup, and North China separated from it. There is currently no strong evidence indicating that Amazonia, West Africa and Kalahari were parts of Nuna.

  • 22.
    Romer, Rolf L.
    Luleå tekniska universitet.
    The late archaean to early proterozoic lead isotopic evolution of the northern Baltic shield of Norway, Sweden and Finland1991In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 49, no 1-2, p. 73-95Article in journal (Refereed)
    Abstract [en]

    The Late Archaean rocks of the Northern Baltic Shield comprise high-grade gneiss terrains and granite-greenstone terrains. The high-grade gneiss terrains consist mainly of older upper-crustal material which became metamorphosed during the Lopian orogeny (ca. 2.9-2.6 Ga). In contrast, the ≈2.7 Ga-old granite-greenstone terrains comprise mainly mantle-derived materials, At the southern and western edge, the Archaean craton became partially remobilized during the Svecofennian orogeny (2.0-1.75 Ga). This orogeny represents a major crust-forming event in the Baltic Shield, during which mantle material was added to the crust and older material was remobilized.Whole-rock, galena and sulfide trace lead are used to outline lead provinces on the Northern Baltic Shield. The Archaean lead from the Baltic Shield defines three secondary lead isochrons at ≈ 2.65 Ga with different initial lead compositions which reflect average pre-2.65 Ga μ1 ratios of 8.0, 8.1 and 8.3, respectively. The variation of the initial lead composition of the high-grade gneisses and tonalites (Lofoten and Koitelainen area) probably indicates (1) variable crustal residence time and/or (2) different degrees of crustal contamination/assimilation before the 2.65 Ga metamorphic event, and the lead from the Eastern Finland greenstone belts indicates a heterogeneous mantle with respect to U/Pb before 2.65 Ga.Lead in Proterozoic rocks of the different lead provinces defines sets of subparallel mixing fields which can be modelled by a ≈2.65 Ga lead component and a ≈ 1.8 Ga lead component. The parallel offset of these fields reflects the pre-2.65 Ga μ1 ratio of these terrains, i.e. it reveals the pre-2.65 Ga μ1 of the involved Archaean rocks. Proterozoic terrains with involvement of Archaean lower-crustal lead show a poor correlation in the 206Pb/204Pb-208Pb/204Pb diagram, whereas the other Proterozoic terrains show a good correlation in the 206Pb/204Pb-208Pb/204Pb diagram.

  • 23.
    Romer, Rolf L.
    et al.
    Luleå tekniska universitet.
    Nisca, Dan. H.
    Luleå tekniska universitet.
    Svecofennian crustal deformation of the Baltic Shield and U-Pb age of late-kinematic tonalitic intrusions in the Burtrask shear zone, northern Sweden1995In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 75, no 1-2, p. 17-29Article in journal (Refereed)
    Abstract [en]

    Post-collisional crustal deformation of the Svecofennian region involved 1) lateral escape movements along crustal-scale dominantly dextral shear zones that roughly correspond to boundaries between volcanic and sedimentary terranes, and 2) crustal thickening eventually resulting in regional migmatite-formation and granitic magmatism. U-Pb dating of titanite from skarnified limestone in the contact-metamorphic aureole of late-kinematic tonalites in the Burtrask Shear Zone, one of these major dextral shear zones in northern Sweden, yields 1825 +4-3 Ma (2σ). This age corresponds to the end of ductile deformation along the Burtrask Shear Zone, but it also coincides with important migmatite formation and granite emplacement in the southern part of the Svecofennian region.

  • 24.
    Romer, Rolf L.
    et al.
    Luleå tekniska universitet.
    Smeds, Sten-Anders
    Institute of Earth Sciences, Mineralogy-Petrology, Uppsala University.
    U-Pb columbite ages of pegmatites from Sveconorwegian terranes in southwestern Sweden1996In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 76, no 1/2, p. 15-30Article in journal (Refereed)
    Abstract [en]

    Post-kinematic and post-metamorphic rare-mineral pegmatites in the Sveconorwegian Province of southwestern Sweden yield U-Pb columbite ages at 1041.4+ or -1.6 Ma (2 ), 1038.7+ or -3.4 Ma, 1029.7+ or -1.4 Ma, and 984.3+ or -6.4 Ma. The contrasting age of spatially close pegmatites with equivalent rare-mineral assemblages demonstrates that spatial and mineralogical coincidence does not imply genetic kinship, but rather reflects the repeated prevalence of comparable pressure and temperature conditions favourable for the formation of rare-mineral pegmatites. Furthermore, our data demonstrate that the pegmatites are more than 50 Ma older than the nearby Bohus Granite that earlier had been considered to be genetically related to the pegmatites. These new U-Pb columbite age data together with earlier U-Pb data from southern Norway and Sweden define a zonal pattern with increasingly younger ages to the east. These pegmatites are interpreted to have formed in relation to the thermal peak in the host or underlying terranes as they were stacked on each other in a piggy-back style during eastward thrusting onto the Baltic Shield. The youngest of these pegmatites (980 Ma) gives a minimum age for the cessation of thrusting. We interpret the 941.6+ or -1.4 Ma old Riddaho pegmatite of the Eastern Gneiss region to be related to the post-collisional extension of the Sveconorwegian Province. This event exposed high-grade gneisses in the Eastern Gneiss region and eventually resulted in the emplacement of voluminous melts in the internal allochthonous belt of the Sveconorwegian orogen.

  • 25.
    Romer, Rolf L.
    et al.
    Luleå tekniska universitet.
    Smeds, Sten-Anders
    Institute of Earth Sciences, Mineralogy-Petrology, Uppsala University.
    U-Pb columbite chronology of post-kinematic Palaeoproterozoic pegmatites in Sweden1997In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 82, no 1-2, p. 85-99Article in journal (Refereed)
  • 26.
    Romer, Rolf
    et al.
    Luleå tekniska universitet.
    Smeds, Sten-Anders
    Uppsala University.
    Implications of U-Pb ages of columbite-tantalites from granitic pegmatites for the Palaeoproterozoic accretion of 1.90-1.85 Ga magmatic arcs to the Baltic Shield1994In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 67, no 1-2, p. 141-158Article in journal (Refereed)
    Abstract [en]

    The Palaeoproterozoic growth of the Baltic Shield involved the accretion of several ~1.90-1.85 Ga old magmatic arcs to the southwest of the Archaean craton and the deformation and migmatization of sedimentary basins located between the arcs. During crustal thickening after the collision of the arcs, the sedimentary basin fill became intruded by peraluminous two-mica granites. Locally, columbite-bearing pegmatites are genetically associated with these granites. Columbites from lithium-cesium-tantalum-type (LCT-type) pegmatites from the Stockholm area (Sormland gneisses) yield U-Pb ages at 1815-1820 Ma, while niobium-yttrium-fluorine-type (MYF-type) pegmatites from the same area are younger (1795±2 Ma, 2σ). Farther to the north, LCT-type pegmatites from the central Bothnian Basin, that correspond geochemically and mineralogically to those of the Stockholm area, yield U-Pb columbite ages at 1795-1800 Ma, while LCT-type pegmatites in the sedimentary basin between Skelleftea and Lulea yield less well constrained U-Pb columbite ages at 1765-1775 Ma. LCT-type pegmatites are mainly associated with crustal melts that form during postcollisional thickening of continental crust. They represent markers for the time when the Palaeoproterozoic Baltic Shield suffered sufficient thickening to yield voluminous anatectic melts. The U-Pb columbite ages from the LCT-type pegmatites indicate that comparable phases of postcollisional crustal thickening of the Svecofennian area of the Baltic Shield occurred later to the north

  • 27.
    Skiöld, Torbjörn
    et al.
    Laboratoriet för Isotopgeologi, Naturhistoriska Riksmuseet, Stockholm.
    Öhlander, Björn
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Early Proterozoic crust-mantle interaction at a continental margin in northern Sweden1989In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 45, no 1-3, p. 19-26Article in journal (Refereed)
    Abstract [en]

    On the basis of recent geochemical and Sm-Nd isotopic data, alternative models for the early Proterozoic evolution of the Archaean craton of the Baltic Shield and its marginal areas in northern Sweden are discussed. Before the deposition of a craton cover and the subsequent Svecofennian orogenic activity, the Archaean areas probably formed a continuous domain. The 1.89-1.86 Ga old early Svecofennian granitoids found close to exposed Archaean rocks were created largely by the remobilization of Archaean crust. To the south of the Archaean Domain, however, chronologically equivalent granitoids are made up of matter with short crustal residence times. That area, the Skellefte district, represents a new addition to the continental crust of the Baltic Shield ≈ 1.90 Ga ago. The particulars of the crustal accretion process are still controversial. However, recent data indicate different tectonic developments in the Skellefte district and in the vicinity of the Archaean-Proterozoic boundary farther north. Apparently, the one-time edge of the Archaean craton was situated somewhere between these two areas, but appears to have had a rather more northerly position than previously assumed.

  • 28.
    Skiöld, Torbjörn
    et al.
    Laboratory for Isotope Geology. Swedish Museum of Natural History, Stockholm.
    Öhlander, Björn
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Markkula, Heikki
    Sveriges Geologiska AB, Luleå.
    Widenfalk, Lennart
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering.
    Claesson, Lars-Åke
    Sveriges Geologiska AB, Luleå.
    Chronology of Proterozoic orogenic processes at the Archaean continental margin in northern Sweden1993In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 64, no 1-4, p. 225-238Article in journal (Refereed)
    Abstract [en]

    The Proterozoic Svecofennian orogeny was characterized by the rapid formation of great amounts of juvenile continental crust and extensive remobilization of older crust. This paper considers U---Pb zircon ages and some geochemical and lithostratigraphic features of the Svecofennian in the continental-margin area between the Skellefte ore district and the boundary of the Archaean craton farther north in Sweden. New U---Pb zircon ages constrain the igneous activity of Svecofennian crust formation in northern Sweden to between 1930 and 1870 Ma ago. The same time constraints apply also to crust formation farther east in Finland. Orogeny along the entire Archaean-Proterozoic boundary zone was thus simultaneous and does not represent an east-to-west event succession. It is argued that rocks with similar major element compositions but distinctly different trace element characteristics were formed simultaneously but in different plate-tectonic environments. By ≈ 1875 Ma ago, the Svecofennian volcanic arc had matured and a variety of syn- to late-orogenic igneous rocks appeared in both tensional and compressional settings. Shortly thereafter, the Svecofennian magmatic activity ceased altogether, probably as a result of collision between the arc and the Archaean continent in the north. It is also suggested that pre-Svecofennian rifting of the Archaean craton had created a passive continental margin and that the transition to an active margin with subsequent island-arc magmatism and subduction beneath the Archaean craton commenced prior to 1930 Ma ago. It may well have been initiated by a relative drift of the Archaean craton towards the present southwest as a consequence of the formation of the collisional Proterozoic Lapland Granulite Belt in the northernmost Baltic Shield.

  • 29.
    Skyttä, Pietari
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Bauer, Tobias
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Tavakoli, Saman
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Hermansson, Tobias
    Boliden Mineral AB.
    Andersson, Jenny
    Geological Survey of Sweden.
    Weihed, Pär
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Pre-1.87 Ga development of crustal domains overprinted by 1.87 Ga transpression in the Palaeoproterozoic Skellefte district, Sweden2012In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 206–207, p. 109-136Article in journal (Refereed)
    Abstract [en]

    The complex structural evolution within the VMS-hosting Skellefte district, Sweden, has been investigated to provide a solid structural framework for the known mineral deposits in the area. The area occurs in a transition zone between dominantly N-S to NNE-WSW striking structures in the north and approximately WNW-ESE oriented structural trends in the south. The presence of high-strain zones with both the above orientations in the Skellefte district allows constraining their mutual relationship, as well as their significance for the build-up of the Svecokarelian orogen at around 1.89 Ga and for the following tectonic overprint between 1.87-1.80 Ga. The methods used in this study include structural analysis complemented by potential field modelling and SIMS U-Pb geochronology on zircon. Based on the results of this study, the earliest deformation (D1) is constrained at 1.89–1.88 (1.87) Ga and tentatively attributed to crustal extension occurring synchronously with volcanism. Deposition of the Skellefte Group metavolcanic rocks is inferred to have occurred in a pull-apart basin developed due to dextral strike-slip shearing along approximately N-S striking regional-scale shear zones. Variations in the development of deformation fabric across the district indicate that the crust was divided into an upper, un-metamorphosed domain and a lower, strongly metamorphosed domain during D1. We further infer that the transition from the upper to lower crust was locally coupled with development of low-angle crustal-scale detachment zones during D1. The heterogenous crust was subsequently overprinted by transpressional deformation which may be explained by two alternative models. According to the first model, one single SSE-NNW transpressional event with distinct strain partitioning between the coaxially deformed upper crust and the non-coaxially deformed lower crust is largely responsible for the present-day structural geometry. A post-folding rhyolite dyke, here dated at 1871 ± 4 Ma, constrains the minimum age of this event (D2). The alternative model includes two separate transpressional events: a SW-NE one at (1.88-) 1.87 Ga, followed by SSE-NNW transpression at 1.86 Ga. Recognition of the early-orogenic detachment zones allow us to suggest that many of the major crustal-scale shear zones in the central Fennoscandian Shield have originated as 1.89-1.87 Ga crustal detachment zones, i.e. earlier than typically considered.

  • 30. Skyttä, Pietari
    et al.
    Mänttäri, Irmeli
    Geological Survey of Finland.
    Structural setting of late Svecofennian granites and pegmatites in Uusimaa Belt, SW Finland: age constraints and implications for crustal evolution2008In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 164, no 1-2, p. 86-109Article in journal (Refereed)
    Abstract [en]

    Complex late Svecofennian deformation patterns and voluminous granitic magmatism at ˜1.85-1.80 Ga characterise the Palaeoproterozoic Uusimaa Belt in SW Finland. Detailed structural analysis and ion microprobe U-Pb zircon geochronology were used to (i) evaluate the evidence for crustal extension that was assumed to pre-date the predominant contractional deformation, and (ii) integrate tectonic evolution with granitic magmatism.The early Svecofennian structures (D1 and D2) are only locally preserved in specific parts of the Uusimaa Belt, while the late Svecofennian structures occur throughout the area. Coaxially deformed granite dykes and weak S3 foliations in granite most likely indicate that crustal extension (D3) took place concurrent with the initial stages of crustal melting during ˜1835-1825 Ma. Subsequent evolution involved continued granitic magmatism and a shift to contraction at ˜1820 Ma. Contraction started under horizontal ˜N-S bulk compression and resulted in intense upright as well as N-NW vergent asymmetric folds (D4). Continued deformation with bulk compression shifting towards E-W at <1820 Ma caused strain localisation into sub-vertical transpressive shear zones with east-side-up shear sense (D5). Another expression of sub-vertical stretching during D5 is the generation of sheath and doubly plunging F5 folds.The volume of granite magmatism decreased from D3 to D5. Also the mode of granite emplacement shifted from large flat-lying sheets during D3 towards thinner migmatising sheets and veins during D4, while D5 involved intrusion of cross-cutting pegmatite dykes along sub-vertical crustal weakness zones.

  • 31.
    Stephens, Michael
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Andersson, Jenny
    Sveriges Geologiska Undersökning, Geological Survey of Sweden.
    Migmatization related to mafic underplating and intra- or back-arc spreading above a subduction boundary in a 2.0−1.8 Ga accretionary orogen, Sweden2015In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 264, p. 235-257Article in journal (Refereed)
    Abstract [en]

    Absolute ages of migmatization and protolith formation, and constraints on the timing of ductile deformation in two major lithotectonic units in the south-western part of a 2.0–1.8 Ga orogenic belt in the Fennoscandian Shield, south-eastern Sweden, have been determined using U-Pb ion probe analysis of different generations of zircon in veined gneisses and leucocratic granite. Detrital and xenocrystic zircon in paragneiss and garnet-bearing leucogranite, respectively, in the Bergslagen lithotectonic unit show ages of 2.1–2.0 Ga and 1.9 Ga. Deposition of the sedimentary material occurred during or after a subduction-related magmatic event at 1.91–1.87 Ga. Two orthogneiss protoliths formed during this magmatic event around 1.88 Ga while most zircon in the leucosome in a third migmatitic orthogneiss was inherited from a 1.85 Ga igneous protolith. A polyphase tectonothermal evolution with anatexis under low-P metamorphic conditions around 1.86 Ga (M1) and 1.84–1.81 Ga (M2) is inferred for the migmatitic gneisses in the Bergslagen unit; garnet-bearing leucogranite crystallized around 1.84–1.83 Ga, close in time to major folding of the M1 gneissic fabric. A previously unrecognised 1.86–1.85 Ga ductile deformational event under medium-grade metamorphic conditions has been identified in the adjacent lithotectonic unit to the south (Småland lithotectonic unit), close in time to the M1 event in the Bergslagen unit to the north.By constraining the timing of anatexis and comparing with information bearing on crustal thickness, excess mass at depth and the character and age of magmatic activity, it is inferred that anatexis in the Bergslagen lithotectonic unit is related to pulses of mafic underplating, during the early stages of two separate, subduction-related magmatic episodes after the 1.91–1.87 Ga magmatic event. It is suggested that each pulse was related to intra- or back-arc spreading above a subduction boundary, which had entered a retreating mode with separate, long periods (20–50 Ma) of extension or transtension. This study challenges the need to invoke crustal thickening related to plate collision at 1.9–1.8 Ga as a mechanism to explain high-grade metamorphism in the southern part of the 2.0–1.8 Ga orogen. Instead, a solely accretionary tectonic model involving an overriding plate along an active continental margin with significant extensional or transtensional crustal deformation is preferred.

  • 32.
    Söderlund, Ulf
    et al.
    Lund Universitet.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Ernst, Richard E.
    Geological Survey of Canada.
    Schissel, Don
    BHP Billiton Minerals Exploration, Moskva.
    The Central Scandinavian Dolerite Group: Protracted hotspot activity or back-arc magmatism? Constraints from U-Pb baddeleyite geochronology and Hf isotopic data2006In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 150, no 3-4, p. 136-152Article in journal (Refereed)
    Abstract [en]

    The Central Scandinavian Dolerite Group (CSDG) occurs in five separate complexes in central Sweden and SW Finland. U-Pb baddeleyite ages of dolerite dikes and sills fall into three age intervals: 1264-1271 (the Dalarna complex), 1256-1259 (the Västerbotten-Ulvö-Satakunta complexes) and ≈1247 Ma (the Jämtland complex). Timing and spatial distribution of CSDG are unlike expressions of the voluminous and short-lived magmatism which characterises plume-associated large igneous provinces (LIPs). Protracted mafic magmatism in association with mantle plume tail (hotspot) activity beneath the Fennoscandian lithosphere or discrete events of extension behind an active margin (subduction) are considered more plausible tectonic settings. Both settings are consistent with timing, relative magma volumes between complexes and vertical ascent of individual magma pulses through the crust, as inferred from seismic sections [Korja, A., Heikkinen, P., Aaro, S., 2001. Crustal structure of the northern Baltic Sea palaeorift. Teconophysics 331, 341-358]. In the hotspot model, the lack of a linear track of intrusions can be explained by an almost stationary position of Fennoscandia relative to the hotspot, in agreement with palaeomagnetic data [Elming, S.-Å., Mattsson, H., 2001. Post Jotnian basic intrusion in the Fennoscandian Shield, and the break up of Baltica from Laurentia: a palaeomagnetic and AMS study. Precambrian Res. 108, 215-236]). Together with geological evidence, dolerite sill complexes and dike swarms in Labrador (Canada), S Greenland and central Scandinavia in the range 1234-1284 Ma are best explained by long-lived subduction along a continuous Laurentia-Baltica margin preceding Rodinia formation. There is no support for the hypothesis that CSDG was fed by magma derived from a distal mantle plume located between Baltica and Greenland and, hence, for rifting between the cratons at ≈1.26 Ga. The epsilon-Hf in various members of the CSDG varies between 4.7 and 10.3, which are overall higher than both older and younger Mesoproterozoic mafic intrusions in central Fennoscandia. Magma generated from a hotspot mantle source that was mixed to highly variable degrees with an enriched subcontinental lithospheric mantle could account for the wide range in Hf isotope composition. In the course of Hf isotope development work during this project we have analysed four fragments of the Geostandard 91500 reference zircon and after evaluating the existing ICPMS and TIMS data we calculate a mean 176Hf/177Hf value of 0.282303 ± 0.000003 (2σ).

  • 33. Weihed, Pär
    et al.
    Bergman, J.
    Uppsala University.
    Bergström, U.
    University of Gothenburg.
    Métallogeny and tectonic evolution of the early proterozoic Skellefte district, Northern Sweden1992In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 58, no 1-4, p. 143-167Article in journal (Refereed)
    Abstract [en]

    The various mineralizations in the Early Proterozoic Skellefte district are discussed and placed in a palaeotectonic context. The metallogeny, together with volcanic stratigraphy, geochemistry, isotope studies, and structural geology, favour a former destructive plate margin. At 1.89 Ga the area resembled a modern island arc environment where volcanogenic massive sulphide and gold-rich porphyry-type deposits formed. Mafic to ultramafic sills and dykes hosting Ni mineralizations intruded a greywacke sequence immediately south of the Skellefte district as a result of large scale rifting at the end of the volcanic period. Gold was emplaced 20-60 Ma later during peak metamorphism at the end of the Svecofennian orogeny. Finally at 1.80 Ga, after accretion of the island arc to the Archaean continent in the northeast, large granitoids intruded. Some of these have associated W, U, Mo, and Li mineralizations-an evidence for a crustal or cratonic influence

  • 34.
    Weihed, Pär
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Rutland, R.W.R.
    Research School of Earth Sciences, Australian National University.
    Skiöld, T.
    Laboratory for Isotope Geology. Swedish Museum of Natural History, Stockholm.
    Kero, L.
    Geological Survey of Sweden.
    A discussion on papers “Nature of a major tectonic discontinuity in the Svecofennian province of northern Sweden” by Rutland et al. (PR 112, 211-237, 2001) and “age of deformation episodes in the Palaeoproterozoic domain of northern Sweden, and evidence for a pre-1.9 Ga crustal layer” by Rutland et al. (PR 112, 239-259, 2001)2003In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 121, no 1-2, p. 141-147Article in journal (Refereed)
  • 35.
    Wolde, Begashaw
    Luleå tekniska universitet.
    Tonalite-trondhjemite-granite genesis by partial melting of newly underplated basaltic crust: an example from the Neoproterozoic Birbir magmatic arc, western Ethiopia1996In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 76, no 1-2, p. 3-14Article in journal (Refereed)
    Abstract [en]

    Partial melting of young (i.e., hot) subducted oceanic crust and of newly underplated basaltic crust are regarded as alternative processes capable of generating high-Altonalite-trondhjemite-granodiorite suites. High Sr/Y ratio (> 40) is the most distinctive trace element characteristic of such rocks and has been modelled as a product of variable degrees of partial melting of subducted oceanic crust transformed into eclogite or garnet amphibolite. By contrast, other arc-related rocks with low Sr/Y ratios (< 40) are generally thought to be generated at low pressure either by fractionation of mantle-derived magmas or by partial melting of basaltic rocks. Tonalite-trondhjemite-granite (TTG) rock samples from two bodies with ages of > 800 Ma intruding the Neoproterozoic Birbir magmatic arc in western Ethiopia, however, form a curved array on the Sr/Y versus Y plot, which extends from high to low Sr/Y ratios. In these rocks, the abundance of Sr varies inversely with those of other incompatible elements, including Rb, Ba and K, but is positively correlated with CaO and Al2O3. These compositional variations provide strong evidence for the formation of the Birbir TTG in a granulitic residue. Experimental studies on basalt melting indicate that the Birbir TTG may have been generated by water-undersaturated and dehydration partial melting of garnet amphibolite in the pressure and temperature ranges of 8-12 kbar and 800-1000°C, respectively. The depth range of partial melting suggests that the source of the Birbir TTG was newly underplated basaltic crust.

  • 36.
    Wolde, Begashaw
    et al.
    Department of Geology, Addis Ababa University.
    Asres, Zemene
    Lege Dembi-Sakarro Primary Gold Exploration and Development Project, P.O. Box 17, Shakisso, Sidamo, Ethiopia.
    Desta, Zerihun
    Lege Dembi-Sakarro Primary Gold Exploration and Development Project, P.O. Box 17, Shakisso, Sidamo, Ethiopia.
    Gonzalez, Julio J.
    Luleå tekniska universitet.
    Neoproterozoic zirconium-depleted boninite and tholeiitic series rocks from Adola, southern Ethiopia1996In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 80, no 3-4, p. 261-279Article in journal (Refereed)
    Abstract [en]

    In Adola, southern Ethiopia, mafic and ultramafic igneous rocks occur in narrow, 4-10 km wide, north-south-trending belts bounded by high-grade gneisses and migmatites. The mafic/ultramafic rocks are complexly deformed and metamorphosed in greenschist to lower amphibolite facies and are thought to be tectonically dismembered parts of an ophiolite complex. Preliminary geochemical and geochronological data highlight that the high-grade rocks in southern Ethiopia and northern Kenya include a significant portion of juvenile rocks that were accreted at the same time as ophiolitic rocks at 885-765 Ma. This is also the time of widespread oceanic magmatism and closure in the Arabian-Nubian Shield to the north. The Adola mafic rocks were previously described as island arc tholeiites and mid-ocean ridge basalts (MORB). New chemical analyses on the Megado Belt rocks reveal the presence of boninites and related dacites interspersed with tholeiitic rocks. The Adola boninites are similar to the Cambrian boninites in western Tasmania in having relatively low Zr/Sm (< or =32). Boninites with similarly low ratios have not been reported from elsewhere. The Adola tholeiites have high Ti/Zr (150-300). Mixing between tholeiite and boninite magmas may have resulted in elevated Ti/Zr (80-126) in some Adola boninites. Otherwise, Ti/Zr in the latter is low (20-40). Low Ti/Zr is characteristic of Tertiary boninites in the West Pacific. The fact that both Ti/Eu and Zr/Sm increase from the Adola and Tasmania type to the Tertiary boninites at constant Ti/Zr suggests that Ti might be an element that is also metasomatically added to the source of boninites and raises doubts about the role of amphibole in boninite petrogenesis.

  • 37.
    Öhlander, Björn
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Hamilton, P. Joseph
    Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow.
    Fallick, Anthony F.
    Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow.
    Wilson, Michael R.
    Sveriges Geologiska AB, Luleå.
    Crustal reactivation in northern Sweden: the Vettasjärvi granite1987In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 35, no C, p. 277-293Article in journal (Refereed)
    Abstract [en]

    The 1.8 Ga old Vettasjärvi granite belongs to a widespread granite type in northern Sweden and northern Finland. It is a leucocratic, generally massive, pink granite covering a large area but possesses a small vertical extent. Foliated varieties occur and gneissic ghost structures are common. Gradual transitions from biotite-rich gneiss to granite have also been observed. Mineralogy and major and trace element geochemistry demonstrate the minimum-melt character of the Vettasjärvi granite. The low contents of elements such as Nb, Y, Sn and F indicate that it is also undifferentiated. Initial εNd values between −6.2 and −8.2 suggest that the Vettasjärvi granite was generated by crustal anatexis with a major Archaean component as source material. REE contents support this conclusion. Molar proportion ratios Al2O3/Na2O+K2O+CaO lower than 1.1, combined with δ18O values between 8 and 5 suggest that little or no pelitic sedimentary material was involved in the genesis. The Vettasjärvi granite is not a single intrusion, but more likely a zone of remobilisation where granite was generated and gently and irregularly emplaced.

  • 38.
    Öhlander, Björn
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Mellqvist, C.
    Skiöld, T.
    Swedish Museum of Natural History.
    Sm-Nd isotope evidence of a collisional event in the Precambrian of northern Sweden1999In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 93, no 1, p. 105-117Article in journal (Refereed)
    Abstract [en]

    The Sm-Nd isotope characteristics of c. 1.9 and c. 1.8 Ga granitoids occurring close to the edge of the Archaean craton in northern Sweden have been studied. Strongly negative εNd(t) values occur farther southwest in the potassic 1.8 Ga granitoids than in the calc-alkaline 1.9 Ga granitoids. Intrusions of the younger granitoids with distinctly negative εNd(t) values are surrounded by granitoids of the older group with positive values. Northeast of the 1.8 Ga granitoids discussed here, the c. 1.9 Ga calc-alkaline intrusions formed within the Archaean continent have the same Sm-Nd isotope characteristics as the 1.8 Ga granitoids to the southwest. Our interpretation is that a juvenile volcanic-arc terrane to the south collided with and was thrust onto the Archaean continent after the formation of the 1.9 Ga granitoids. The outcome was that these juvenile rock masses were obducted onto the reworked Archaean continent. The 1.8 Ga granitoids were formed by remobilisation of continental crust. Partial melting at 1.8 Ga resulted in the intrusion of granitoids carrying the Sm-Nd isotopic signature of the Archaean continent into juvenile rocks. It is probable that the collision discussed here was part of a major accretion and amalgamation to the Archaean craton after the formation of the calc-alkaline 1.9 Ga granitoids, but before the formation of the 1.8 Ga granitoids.

  • 39.
    Öhlander, Björn
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Skiöld, T.
    Elming, Sten-åke
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Geosciences and Environmental Engineering.
    Claesson, S.
    Nisca, D.H.
    Luleå tekniska universitet.
    Delineation and character of the Archaean-Proterozoic boundary in northern Sweden1993In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 64, no 1-4, p. 67-84Article in journal (Refereed)
    Abstract [en]

    Before the deposition of a Proterozoic cover and the repeated Proterozoic reworking of the older rocks, the presently exposed Archaean areas in northern Sweden formed part of a coherent craton. In the present study, we have used Sm---Nd isotopic analyses of Proterozoic granitoids and metavolcanics to delineate the Archaean palaeoboundary. In a regional context, the transition from strongly negative εNd(t) values in the northeast to positive values in the southwest is distinct, and approximately defines the border of the old craton. The Archaean palaeoboundary extends in a WNW direction, and is subparallel to the longitudinal axis of the Skellefte sulphide ore district but it is situated ≈ 100 km farther to the north. The ≈ 1.9 Ga old granitoids on the two sides of the palaeoboundary were all formed in compressional environments, but those situated to the north have higher contents of LILE and LREE at similar contents of Si. This indicates that they were generated in an area with thicker crust and supports the location of the Archaean-Proterozoic palaeoboundary. There is no simple correlation between the Archaean palaeoboundary, as defined by the isotopic results, and any of the major fracture systems as interpreted from regional geophysical measurements. Reflection seismic work indicates that juvenile volcanic-arc terrains to the south have been thrust onto the Archaean craton. Possible thrust faults have been identified from aeromagnetic measurements. Rifting of the Archaean craton created a passive margin ≈ 2.0 Ga ago. Spreading shifted to convergence with subduction beneath the Archaean continent ≈ 1.9 Ga ago. Subsequently, the resulting juvenile volcanic arc collided with the old continent, and the Archaean palaeoboundary as existing today was formed by a collision characterized by overthrusting. The boundary then was disturbed by later deformation predominantly along NNE-trending fracture systems.

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