Late Neoproterozoic basin evolution of the magma rich Iapetus margin of Baltica

Abstract

Kjøll, H.J. 2020: Late Neoproterozoic basin evolution of the magma-rich Iapetus margin of Baltica. Norwegian Journal of Geology 100, 202005, https://dx.doi.org/10.17850/njg100-1-6. © Copyright the authors. This work is licensed under a Creative Commons Attribution 4.0 International License. The Särv and Seve nappe complexes (NC) in the central and northern Scandinavian Caledonides locally display well-preserved, mafic dyke-intruded sedimentary successions commonly interpreted to have been deposited in sagto rift basins along the margin of the Iapetus Ocean. The sedimentary successions are generally interpreted to have been deposited prior to or during the Late Neoproterozoic opening of the Iapetus Ocean. They were later incorporated into the Scandinavian Caledonides during the Silurian-aged Scandian orogeny. Whereas the minimum depositional age is constrained by the dated mafic dyke swarm at c. 596‒608 Ma, a maximum depositional age for the sedimentary successions is poorly constrained. No fossils or diamictite units have hitherto been reported from the sedimentary successions found in the Seve NC. This contribution presents new geological observations and geochronological data from allochthonous, dyke-intruded, rift-related basins. Key elements in the sedimentary succession, such as carbonates with meta-evaporitic domains, diamictite and stromatolites are described from the Särv and Seve NC. Evidence presented here suggests that the diamictite is of glaciogenic origin. It is cut by the dykes and is therefore older than 608 Ma and could be related to the Marinoan or Sturtian glaciations. The stromatolite resembles Eleonora laponica and is found below the diamictite. Two field areas have young detrital zircons of c. 700‒750 Ma providing possible upper maximum depositional ages and thus bracket the deposition within a c. 100 M.yr. time interval between 700 and 608. This is corroborated by a Palaeoproterozoic orthogneiss with a 631 Ma Pb-loss event, possibly reflecting the development of a top-west, ductile extensional shear fabric and frictional co-seismic deformation. This extensional event generated accommodation space at the surface and thus basin formation. Similarities between the separate basins, like the highly dyke-intruded nature, the similarities in contact and regional metamorphism as well as the detrital zircon age distributions, suggest that the basins are related.