Skip to main content

Lund University Publications

LUND UNIVERSITY LIBRARIES

Paleomagnetism, magnetic anisotropy and U-Pb baddeleyite geochronology of the early Neoproterozoic Blekinge-Dalarna dolerite dykes, Sweden

Gong, Zheng ; Evans, David A.D. ; Elming, Sten Åke ; Söderlund, Ulf LU and Salminen, Johanna M. (2018) In Precambrian Research 317. p.14-32
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... (More)

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.

(Less)
Please use this url to cite or link to this publication:
author
; ; ; and
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Baltica, Blekinge-Dalarna dolerite (BDD) dykes, Magnetic anisotropy, Paleomagnetism, Sveconorwegian loop, U-Pb baddeleyite geochronology
in
Precambrian Research
volume
317
pages
19 pages
publisher
Elsevier
external identifiers
  • scopus:85052622402
ISSN
0301-9268
DOI
10.1016/j.precamres.2018.08.019
language
English
LU publication?
yes
id
2e40835e-1df7-4c14-a407-7cc03440c9d4
date added to LUP
2018-09-25 10:56:49
date last changed
2021-10-06 04:44:09
@article{2e40835e-1df7-4c14-a407-7cc03440c9d4,
  abstract     = {<p>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, A<sub>95</sub> = 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, A<sub>95</sub> = 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.</p>},
  author       = {Gong, Zheng and Evans, David A.D. and Elming, Sten Åke and Söderlund, Ulf and Salminen, Johanna M.},
  issn         = {0301-9268},
  language     = {eng},
  month        = {10},
  pages        = {14--32},
  publisher    = {Elsevier},
  series       = {Precambrian Research},
  title        = {Paleomagnetism, magnetic anisotropy and U-Pb baddeleyite geochronology of the early Neoproterozoic Blekinge-Dalarna dolerite dykes, Sweden},
  url          = {http://dx.doi.org/10.1016/j.precamres.2018.08.019},
  doi          = {10.1016/j.precamres.2018.08.019},
  volume       = {317},
  year         = {2018},
}