| 1. |
- Brander, Linus, 1978, et al.
(author)
-
Mesoproterozoic (1.47-1.44 Ga) orogenic magmatism in Fennoscandia; Baddeleyite U-Pb dating of a suite of massif-type anorthosite in S. Sweden
- 2009
-
In: International Journal of Earth Sciences. - : Springer Science and Business Media LLC. - 1437-3254 .- 1437-3262. ; 98:3, s. 499-516
-
Journal article (peer-reviewed)abstract
- The Jo¨nko¨ping Anorthositic Suite (JAS) in S. Sweden has characteristics typical for (Proterozoic) massiftype anorthosites. The interstitial liquid of these plagioclase- porphyritic rocks solidified at 1,455 ± 6 Ma, as determined by U–Pb isotope analysis of baddeleyite. The JAS developed during a regional 1.47–1.44 event in Fennoscandia that generated widespread mafic magmatism (basalts, and diabase dykes and sills) in the north and emplacement of felsic plutons in the south. The event of 1.47–1.44 Ga magmatism in Fennoscandia largely coincides in age with dynamic high-grade metamorphism in SW Sweden and was probably related to convergent activemargin processes during the Danopolonian orogeny.
|
|
| 2. |
- Söderlund, Ulf, et al.
(author)
-
Geochronology of high-pressure mafic granulite dykes in SW Sweden; tracking the P-T-t path of metamorphism using Hf isotopes in zircon and baddeleyite
- 2008
-
In: Journal of Metamorphic Geology. - : Wiley. - 0263-4929 .- 1525-1314. ; 26:5, s. 539-560
-
Journal article (peer-reviewed)abstract
- Although the U-Pb zircon chronometer has been widely used for dating metamorphism in moderate to high-grade rocks, it is generally difficult to link the U-Pb age of zircon to specific metamorphic reactions. In this study, the initial Hf isotopic composition of secondary zircon is compared with the evolution of Hf isotopic composition of the bulk sample, back-projected from the measured value through time. This approach may enhance the interpretation of radiometric ages performed on metamorphic mineral assemblages. Here, U-Pb, Sm-Nd and Lu-Hf geochronology and thermobarometry have been integrated and applied to two metamorphosed diabase dykes in the Sveconorwegian orogen, SW Sweden. The dykes are located ~ 5 km east of the NNE-trending Göta Älv deformation zone in the Idefjorden terrane, and trend parallel to this zone. The Lunden dyke is recrystallized into a coronitic granulite-facies assemblage. U-Pb isotopic analyses of baddeleyite in this dyke indicate an emplacement age of ~1300 Ma. Thermobarometric techniques applied to garnet and omphacitic clinopyroxene coronas indicate high-pressure metamorphism at c. 15 kbar and c. 740 °C. Growth of polycrystalline zircon at the expense of baddeleyite occurred at 1046 ± 6 Ma. Identical Hf isotopic composition of polycrystalline zircon and baddeleyite shows that the baddeleyite-to-zircon transition took place before Hf equilibration among the other metamorphic minerals and, hence the c. 1046 Ma age of polycrystalline zircon sets an upper age limit of metamorphism of this sample. The Haregården dyke is recrystallized into a granoblastic transitional upper amphibolite to granulite-facies assemblage. The estimated P-T conditions are c. 10 kbar and c. 700 °C. Analyses of small (~30 µm), clear and round zircon in this sample yield a Concordia U-Pb age of 1026 ± 4 Ma, which is indistinguishable from the Lu-Hf and Sm-Nd mineral isochron ages of 1027 ± 9 Ma and 1022 ± 34 Ma. This type of secondary zircon plots at the lower end of the Lu-Hf isochron and indicates simultaneous growth with garnet at c. 1026 Ma, a time when Hf isotopic equilibrium among minerals must have been reached.
|
|
| 3. |
- Elming, Sten-åke, et al.
(author)
-
Cooling history and age of magnetization of a deep intrusion : A new 1.7 Ga key pole and Svecofennian-post Svecofennian APWP for Baltica
- 2019
-
In: Precambrian Research. - : Elsevier. - 0301-9268 .- 1872-7433. ; 329, s. 182-194
-
Journal article (peer-reviewed)abstract
- 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.
|
|
| 4. |
- Elming, Sten-åke, et al.
(author)
-
Proterozoic Basic dykes in the Ukrainian Shield: A palaeomagnetic, geochronologic and geochemical study-The accretion of the Ukrainian Shield to Fennoscandia
- 2010
-
In: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 178:1-4, s. 119-135
-
Journal article (peer-reviewed)abstract
- A palaeomagnetic, geochronologic and geochemical study of basic dykes in the Ukrainian Shield has been undertaken with the purpose of testing a hypothesis for timing of accretion of the Ukrainian Shield with Fennoscandia. On the basis of geological data it has been suggested that the two shields amalgamated at ca. 1900-1800 Ma. With this study of 11 basic dykes the palaeomagnetic database for the Ukrainian Shield is extended. The palaeomagnetic data indicate three generations of dykes. This is supported by the age data and the difference in geochemical compositions. Ages of two ca. 25 m wide dykes (Susly) in the North-Western Block of the shield have been determined, one for which a positive palaeomagnetic field test is demonstrated. The lower of the two Ar-40/Ar-39 hornblende ages of this dyke, 2061 +/- 20 Ma, is similar to the U-Pb, zircon age of the granitic host rock (ca. 2067 Ma, U-Pb, zircon), which suggests that the dyke intruded shortly after crystallization of the granite. Combined with the magnetization of the baked contact and that of another dyke a new ca 2060 Ma pole is presented (Plat. = 15.7 degrees, Plon. = 182.9 degrees, A(95) = 13.7 degrees). which may be regarded as a VGP. An U-Pb age of 1722 +/- 12 Ma of baddeleyite from the Khmelnik dyke, which yields a well-defined VGP, is similar to the age of nearby anorthosite complexes. A new 1740-1770 Ma key pole for the Ukranian Shield is also presented (Plat. = 26.5 degrees, Plon. = 169.1 degrees, A(95) = 3.9 degrees). On the basis of palaeomagnetic and geochronological data a 2060-1720 Ma apparent polar wander for the Ukrainian Shield is defined. This apparent polar wander is significantly different from that of Fennoscandia during the same time interval and indicates that the two shields were not in their present relative positions at ca. 1770 Ma. A scenario is here proposed where the Ukrainian Shield (and Sarmatia) collided with Fennoscandia at ca. 1900-1800 Ma and after that event, at ca. 1720-1660 Ma, it rotated approximately 43 degrees into its present position. (C) 2010 Elsevier B.V. All rights reserved.
|
|
| 5. |
- Gong, Zheng, et al.
(author)
-
Paleomagnetism, magnetic anisotropy and U-Pb baddeleyite geochronology of the early Neoproterozoic Blekinge-Dalarna dolerite dykes, Sweden
- 2018
-
In: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 317, s. 14-32
-
Journal article (peer-reviewed)abstract
- 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 (Plat = −2.6°N, Plon = 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 (Plat = −27.0°N, Plon = 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.
|
|
| 6. |
- Gumsley, Ashley P., et al.
(author)
-
Timing and tempo of the Great Oxidation Event
- 2017
-
In: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424. ; 114:8, s. 1811-1816
-
Journal article (peer-reviewed)abstract
- The first significant buildup in atmospheric oxygen, the Great Oxidation Event (GOE), began in the early Paleoproterozoic in association with global glaciations and continued until the end of the Lomagundi carbon isotope excursion ca. 2,060 Ma. The exact timing of and relationships among these events are debated because of poor age constraints and contradictory stratigraphic correlations. Here, we show that the first Paleoproterozoic global glaciation and the onset of the GOE occurred between ca. 2,460 and 2,426 Ma, ∼100 My earlier than previously estimated, based on an age of 2,426 ± 3 Ma for Ongeluk Formation magmatism from the Kaapvaal Craton of southern Africa. This age helps define a key paleomagnetic pole that positions the Kaapvaal Craton at equatorial latitudes of 11° ± 6° at this time. Furthermore, the rise of atmospheric oxygen was not monotonic, but was instead characterized by oscillations, which together with climatic instabilities may have continued over the next ∼200 My until ≤2,250-2,240 Ma. Ongeluk Formation volcanism at ca. 2,426 Ma was part of a large igneous province (LIP) and represents a waning stage in the emplacement of several temporally discrete LIPs across a large low-latitude continental landmass. These LIPs played critical, albeit complex, roles in the rise of oxygen and in both initiating and terminating global glaciations. This series of events invites comparison with the Neoproterozoic oxygen increase and Sturtian Snowball Earth glaciation, which accompanied emplacement of LIPs across supercontinent Rodinia, also positioned at low latitude.
|
|
| 7. |
- Petersson, Andreas, et al.
(author)
-
The robustness of the Lu-Hf and Sm-Nd isotopic systems during metamorphism – And case study of the Åker metabasite in southern Sweden
- 2023
-
In: Precambrian Research. - 0301-9268 .- 1872-7433. ; 394
-
Journal article (peer-reviewed)abstract
- While whole-rock Lu-Hf isotope analysis remains one of the only ways to obtain initial Hf isotope signatures of old mafic rocks, Hf isotope analyses of more robust accessory zircon in intermediate to silicic rocks have largely replaced whole-rock analyses during the last decade. This has led to a discrepancy in the amount of existing data from mafic and felsic lithologies. However, especially in mafic, Si-poor rocks with a metamorphic imprint, Hf isotope data rely on whole-rock analysis since baddeleyite, commonly used for U-Pb age analyses of mafic rocks, is sensitive to alteration and metamorphism. Hence, to accurately evaluate the trace element and isotope signatures of altered mafic rocks, it is important to understand the mechanisms of element mobility during metamorphism. Here, we report whole-rock trace element compositions, Lu-Hf and Sm-Nd isotope data from variably deformed and metamorphosed samples of a mafic intrusion in southern Sweden, the Åker metabasite. These data suggest that trace elements were undisturbed on a whole-rock sample scale during deformation at upper amphibolite facies (at least 1000 MPa and 600 °C) metamorphism under hydrated conditions. Despite redistribution of Zr associated with the breakdown of baddeleyite and other igneous phases, the Åker metabasite has retained its chemical and isotopic integrity since igneous crystallisation at ca. 1565 Ma. This study demonstrates and strengthens the feasibility of whole-rock analyses of (meta-)mafic rocks for determining initial εNd and εHf values, despite deformation and metamorphism under hydrated amphibolite-grade metamorphic conditions. Testing the coherence of the calculated initial Nd and Hf isotope ratios by examining variably deformed and metamorphosed varieties of a rock in a single outcrop, could be used as a model for research on more complex Archean rocks.
|
|
| 8. |
- Plan, Anders, et al.
(author)
-
Exceptional preservation of reidite in the Rochechouart impact structure, France: New insights into shock deformation and phase transition of zircon
- 2021
-
In: Meteoritics and Planetary Science. - : Wiley. - 1086-9379 .- 1945-5100. ; 56:10, s. 1795-1828
-
Journal article (peer-reviewed)abstract
- Reidite, the high-pressure zircon (ZrSiO4) polymorph, is a diagnostic indicator of impact events. Natural records of reidite are, however, scarce, occurring mainly as micrometer-sized lamellae, granules, and dendrites. Here, we present a unique sequence of shocked zircon grains found within a clast from the Chassenon suevitic breccia (shock stageIII) from the ~200 Ma, 20–50 km wide Rochechouart impact structure in France. Our study comprises detailed characterization with scanning electron microscopy coupled with electron backscatter diffraction with the goal of investigating the stability and response of ZrSiO4 under extreme P–T conditions. The shocked zircon grains have preserved various amounts of reidite ranging from 4% up to complete conversion. The grains contain various variants of reidite, including the common habits: lamellae and granular reidite. In addition, three novel variants have been identified: blade, wedge, and massive domains. Several of these crosscut and offset each other, revealing that reidite can form at multiple stages during an impact event. Our data provide evidence that reidite can be preserved in impactites to a much greater extent than previously documented. We have further characterized reversion products of reidite in the form of fully recrystallized granular zircon grains and minute domains of granular zircon in reidite-bearing grains that occur in close relationship to reidite. Neoblasts in these grains have a distinct crystallography that is the result of systematic inheritance of reidite. We interpret that the fully granular grains have formed from prolonged exposure of temperatures in excess of 1200 °C. Reidite-bearing grains with granular domains might signify swift quenching from temperatures close to 1200 °C. Grains subjected to these specific conditions therefore underwent partial zircon-to-reidite reversion, instead of full grain recrystallization. Based on our ZrSiO4 microstructural constraints, we decipher the grains evolution at specific P–T conditions related to different impact stages, offering further understanding of the behavior of ZrSiO4 during shock.
|
|
| 9. |
- Söderlund, Ulf, et al.
(author)
-
The Central Scandinavian Dolerite Group - Protracted hotspot activity or back-arc magmatism? Constraints from U-Pb baddeleyite geochronology and Hf isotopic data
- 2006
-
In: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 150:3-4, s. 136-152
-
Journal article (peer-reviewed)abstract
- 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 Vdsterbotten-Ulvo-Satakunta complexes) and similar to 1247 Ma (the Jamtland 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 Fermoscandia relative to the hotspot, in agreement with palaeomagnetic data [Elming, S.-angstrom., Mattsson, H., 2001. Post Jotnian basic intrusion in the Fermoscandian 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 similar to 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 Fermoscandia. 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 Hf-176/Hf-177 value of 0.282303 +/- 0.000003 (2 sigma). (c) 2006 Elsevier B.V. All rights reserved.
|
|
| 10. |
- Brander, Linus, 1978, et al.
(author)
-
Tracing the 1271–1246 Ma Central Scandinavian Dolerite Group mafic magmatism in Fennoscandia: U–Pb baddeleyite and Hf isotope data on the Moslätt and Børgefjell dolerites
- 2011
-
In: Geological Magazine. - 0016-7568. ; 148:4, s. 632-643
-
Journal article (peer-reviewed)abstract
- Between 1271 and 1246 Ma, dolerite dykes and sills of the Central Scandinavian Dolerite Group intruded into the Fennoscandian Shield during three distinct magmatic pulses. They are distributed around five large magmatic complexes extending from Sweden to western Finland and record large-scale intracratonic tensional stress. Coeval plutonism is observed in the westernmost terrane of the Sveconorwegian orogen in southern Norway, but differs in the sense of a bimodal character and uncertain Fennoscandian ancestry of the host terrane.We report a U–Pb baddeleyite age of 1269±12 Mafor a gabbronoritic member of an E-trending set of dykes, called the MoslättDolerites, near Lake Vättern in southern Sweden, much farther to the south than any previously known Central Scandinavian Dolerite Group rock. A similar age of approximately 1275 Ma is obtained for a metadolerite sheet in the Børgefjell basement window in the Scandinavian Caledonides in Mid-Norway. The initial epsilon-Hf values for these two dykes are +3.9 and +10.1, respectively, and correspond to the range of values for other occurrences of the Central Scandinavian Dolerite Group (+4.7 to +10.3). They add to the evidence that the Central Scandinavian Dolerite Group is characterized by more positive epsilon values (depleted source) than other mafic Proterozoic suites in Fennoscandia. These results extend the distribution of c. 1270–1245 Ma mafic magmatism in Fennoscandia, particularly when accounting for significant Caledonian shortening. The Central Scandinavian Dolerite Group and coeval bimodal magmatism in S Norway may represent distal magmatic events related to a Mesoproterozoic subduction along the western margin of Fennoscandia rather than hotspot (mantle plume) activity as previously suggested.
|
|
