The “intraorogenic” Svecofennian Herräng mafic dyke swarm in east-central Sweden: age, geochemistry and tectonic significance

ABSTRACT

The Herräng mafic dykes form an E-W-trending dyke swarm within the Bergslagen lithotectonic unit of the Svecofennian orogen in east-central Sweden. They intrude the Svecofennian supracrustal rocks and early-orogenic granitoids, but are themselves cut by late Svecofennian pegmatites, and have undergone Svecofennian amphibolite-facies metamorphism, leading to their classification as “intraorogenic” Svecofennian dykes. They can be assigned an age between 1870 and 1850 Ma, with metamorphism of the dykes dated at 1848 ± 13 Ma by U-Pb in titanite. Their current mineralogy is dominated by metamorphic plagioclase and amphibole, with variable amounts of quartz and biotite, and minor to accessory titanite, apatite, epidote, pyrite, magnetite, ilmenite and zircon. Textures range from massive to strongly foliated. Twenty samples of dyke rocks from three subareas in the Roslagen region, including the Herräng type area, range in composition from basaltic to andesitic with 47 to 60 wt% SiO₂, broadly similar to the Dannemora dykes and the Avesta-Östhammar gabbros and diorites. Initial ⁸⁷Sr/⁸⁶Sr ratios (at 1870 Ma) varies between 0.7026 and 0.7038, corresponding to initial ε_Sr between +5 and +21, and initial ε_Nd between −0.4 and +1.3, suggesting a slightly enriched to mildly depleted mantle source, similar to other Svecofennian mafic rocks. The dykes dominantly show a calc-alkaline volcanic arc signature related to subduction. They formed during an extensional episode, possibly related to incipient back-arc spreading or subduction roll-back following the main early-orogenic subduction-related Svecofennian magmatism, but penecontemporaneous with amphibolite-facies metamorphism in the area.

KEYWORDS:

• Herräng dykes
• mafic dyke swarms
• Svecofennian
• intraorogenic dykes
• Roslagen
• east-central Sweden

Acknowledgments

Torsten Persson is thanked for mass spectrometer maintenance, Hans Schöberg for assistance with Sr and Nd isotope data processing, Per-Olof Persson for assistance with titanite separation, and the Nordsim staff (Martin Whitehouse, Lev Ilyinsky and Kerstin Lindén) for assistance with the ion microprobe analyses, including preparation of mount, the actual analysis, and data processing. The Nordsim facility was funded by the Swedish Museum of Natural History, the Swedish Research Council (VR infrastructure grant 2014-06375), University of Iceland and the Consortium of Danish geoscience institutions. This is Nordsim contribution no. 625. Discussions with Thomas Lundqvist and reviews by Dick Claeson and George Morris are much appreciated.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplemental appendices for this article can be accessed online here: https://doi.org/10.1080/11035897.2019.1708450.

Correction Statement

This article has been republished with minor changes. These changes do not impact the academic content of the article.

Additional information

Funding

This work was supported by the Magnus Bergvalls Stiftelse to ÅJ.