| 1. |
- Kielman, Ross, 1991-, et al.
(författare)
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U-Pb age distribution recorded in zircons from Archean quartzites in the Mt. Alfred area, Yilgarn Craton, Western Australia
- 2018
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Ingår i: Chemical Geology. - : Elsevier. - 0009-2541 .- 1872-6836. ; 310, s. 278-290
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Tidskriftsartikel (refereegranskat)abstract
- The U-Th-Pb isotopic data from detrital zircon grains from five samples of Archean quartzite from the Mt. Alfred area of the Illaara greenstone belt in the Yilgarn Craton of Western Australia are presented in this study. The zircon grains are typically fractured and contain both irregular and oscillatory zoned internal structures as revealed by cathodoluminescence imaging. Concordant 207Pb/206Pb ages range between 3109 ± 17 and 3918 ± 16 Ma (2σ), with three main age peaks at ca. 3640, 3690 and 3760 Ma. Older 207Pb/206Pb ages up to 4067 ± 5 Ma are strongly affected by at least one recent disturbance event, however one single-grain discordia yields an upper intercept age of 4107 ± 12 (MSWD = 1.2). A further sixteen zircon grains with multiple analyses define discordia that suggest U-Pb disturbance events in the Neoarchean and the Mesozoic, the latter as a result of invasive low temperature weathering solutions. The notable lack of grains with ages less than ∼3.6 Ga in the Mt. Alfred detrital zircon population differentiates it from other quartzite samples from both the Illaara Formation and the Eoarchean zircon-bearing metasedimentary rocks of the Narryer Terrane. Also, the limited spread of zircon ages between 3640 and 3760 Ma suggests a relatively uniform and possibly local source region. However, no rocks of this age have been found in the Youanmi Terrane. This implies either the distal transport of similarly aged clastic sediments at 3.1 Ga from the Narryer Gneiss Complex (NGC) to the Mt. Alfred area, or the previous existence of NGC-like rocks near the Illaara greenstone belt that are either not currently recognised or have since been destroyed.
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| 2. |
- Billström, Kjell, et al.
(författare)
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Regional variations in the Pb isotopic compositions of ore galena across the Archaean-Proterozoic border in northern Sweden
- 1997
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Ingår i: Precambrian Research. - 0301-9268 .- 1872-7433. ; 81:1-2, s. 83-99
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Tidskriftsartikel (refereegranskat)abstract
- Galena is a rare component in the Palaeoproterozic mineralizations in northernmost Sweden. In the present work Pb isotope data from galena from nineteen deposits tend to vary as a function of geographical location. Basically, the zone dividing radiogenic and non-radiogenic lead isotopic compositions is consistent with the position of the inferred Proterozoic-Archaean border in northern Sweden. Radiogenic lead is generally found in galena of the 2.0-2.7 Ga old Greenstone group in the northeast along the Finnish border, and in some Cu occurrences in the ∼ 1.9 Ga old Porphyry group near Malmberget. Non-radiogenic lead is found in the Huornaisenvuoma Zn-Pb ore in the Greenstone group, in epigenetic Cu-Zn-Pb ores in the Porphyry group in southern Norrbotten, and in similar deposits near the Caledonides. The lead isotopic compositions of galena in the Cu-Zn-Pb ores of southern Norrbotten are fairly homogeneous with 206Pb/204Pb = 15.34 to 15.50, 207Pb/204Pb = 15.16 to 15.30, similar to those of the sulphide deposits associated with the Central Finnish batholith and to data from the North-central Skellefte district. The coincidence of data for the areas of southern Norrbotten and the North-central Skellefte district is interpreted such that these terrains formed one crustal unit at the time of ore deposition. Tentatively, this event took place at ∼1.87 Ga or slightly thereafter, and was related to the suturing of the Skellefte arc to the northerly inferred continent. Although Pb model ages should be treated with caution when applied in Proterozoic terrains in Sweden, the present data suggest an age difference between the deposits (and between source rocks) at Tjåmotis and Huornaisenvouma (∼2.0 Ga old) on one hand, and the remaining Cu-Zn-Pb occurrences (∼1.9 Ga old) on the other hand. Possibly, the indicated depositional event at ∼2.0 Ga is an expression of simultaneous, large-scale crust-mantle interactions, in accordance with the hypothesis of magma-forming processes at that time.In the Pb-Pb diagrams, data for galena in northern Sweden indicate linear relationships. The radiogenic lead is the result of leaching processes in Phanerozoic (Caledonian) times, which released rock lead evolved in the time interval ∼2.0-1.9 Ga to 0.4 Ga. The data furthermore suggest that radiogenic galena was formed by mobilizing lead from U-rich rocks (238U/204Pb ratios ca. 25-32) of the Greenstone group which are likely to have a similar age, possibly close to 2.0 Ga. Since the deposits with radiogenic lead are distributed over a very large area, and still define linear trends, it is likely that the ∼2.0 Ga age is typical for the bulk of Greenstone rocks in Norrbotten. From the slopes of straight-line relations for the Pb isotope data, it is inferred that only Proterozoic rock lead was mobilized at ∼0.4 Ga and that no significant Archaean lead was involved in such processes. Accordingly, lead mobilization was quite local and only involved the upper portions of the crust. As the radiogenic lead is found at localities far away from the Caledonian front, the indicated Pb mobilization processes were most probably connected with reactivation of old Proterozoic fracture zones.
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| 3. |
- Buchan, Kenneth L., et al.
(författare)
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Rodinia : the evidence from integrated palaeomagnetism and U-Pb geochronology
- 2001
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Ingår i: Precambrian Research. - 0301-9268 .- 1872-7433. ; 110:1-4, s. 9-32
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Tidskriftsartikel (refereegranskat)abstract
- Of many hundreds of well-defined palaeomagnetic poles that have been reported from cratons around the world in the 1700-500 Ma period, only a few are precisely dated. However, such ‘key' palaeopoles are a prerequisite for establishing rigorous palaeomagnetic reconstructions in order to chart the assembly, drift and breakup of the postulated late Precambrian supercontinent of Rodinia. Most key palaeopoles are derived from mafic dykes and sills that have been dated by U-Pb techniques. Most are from Laurentia, the largest and best studied of the continental fragments that are thought to have comprised Rodinia. Thirteen key Laurentia palaeopoles form an incomplete reference set that can be used for comparison with key palaeopoles from other cratons as they become available. Currently, there are four key palaeopoles for Baltica between 1700 and 500 Ma, although only one allows a direct comparison with a similar aged pole from Laurentia. The 1265 Ma match between Baltica and Laurentia is consistent with reconstructions in which Baltica is adjacent to present-day east Greenland, with the ca. 1700-1500 Ma Gothian and Labradorian belts aligned. Few key palaeopoles are yet available from other cratons. However, recent U-Pb dating of dykes, sills, or volcanic rocks in the Siberian, Australian and Kalahari cratons and in Coats Land of Antarctica constrains the ages of individual palaeopoles from each of these areas. Most of these are not key palaeopoles because they have not been conclusively demonstrated to be primary, or local tectonic rotations have not been ruled out. Nevertheless, they are useful in testing Rodinia reconstructions. In this paper, a U-Pb baddeleyite age is reported from the late Gardar magmatic rocks of southwest Greenland. Along with the previously published palaeopole for this unit, this age helps constrain the Mesoproterozoic location of southwest Greenland relative to North America.
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| 4. |
- Elming, Sten-åke
(författare)
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Palaeomagnetism of Precambrian rocks in northern Sweden and its correlation to radiometric data
- 1994
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 69:1-4, s. 61-79
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Tidskriftsartikel (refereegranskat)abstract
- 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
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| 5. |
- Elming, Sten-Åke, et al.
(författare)
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Post Jotnian basic Intrusions in the Fennoscandian Shield, and the break up of Baltica from Laurentia: a palaeomagnetic and AMS study
- 2001
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Ingår i: Precambrian Research. - 0301-9268 .- 1872-7433. ; 108:3-4, s. 215-236
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Mattsson, Håkan J., et al.
(författare)
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Magnetic fabrics and paleomagnetism of the Storsjön-Edsbyn deformation zone, central Sweden
- 2001
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Ingår i: Precambrian Research. - 0301-9268 .- 1872-7433. ; 107:3-4, s. 265-281
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Tidskriftsartikel (refereegranskat)abstract
- 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
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| 7. |
- Mellqvist, C., et al.
(författare)
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The Archaean–Proterozoic Palaeoboundary in the Luleå area, northern Sweden: field and isotope geochemical evidence for a sharp terrane boundary
- 1999
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Ingår i: Precambrian Research. - 0301-9268 .- 1872-7433. ; 96:3-4, s. 225-243
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 8. |
- Perdahl, Jan-Anders, et al.
(författare)
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Petrochemical and petrological characteristics of 1.9 Ga old volcanics in northern Sweden
- 1993
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Ingår i: Precambrian Research. - 0301-9268 .- 1872-7433. ; 64:1-4, s. 239-252
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 9. |
- Romer, Rolf, et al.
(författare)
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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 Shield
- 1994
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 67:1-2, s. 141-158
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Tidskriftsartikel (refereegranskat)abstract
- 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
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| 10. |
- Romer, Rolf L., et al.
(författare)
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Svecofennian crustal deformation of the Baltic Shield and U-Pb age of late-kinematic tonalitic intrusions in the Burtrask shear zone, northern Sweden
- 1995
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Ingår i: Precambrian Research. - 0301-9268 .- 1872-7433. ; 75:1-2, s. 17-29
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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