Intermontane basins and bimodal volcanism at the onset of the Sveconorwegian Orogeny, southern Norway

Abstract

The greenschist-facies late Mesoproterozoic Bandak succession in southern Norway consists of interlayered quartzites and meta-volcanic rocks with well preserved depositional and structural relations, which when combined provide important information on the late Mesoproterozoic continental margin of Baltica prior to the assembly of the supercontinent Rodinia. The timing of deposition of the Bandak succession is constrained by previously published tuff ages and new detrital zircon analyses reported here. The lower units of the Bandak succession (the Ofte and Røynstaul formations) display diverse detrital zircon age spectra implying derivation from a wide array of sources. The Morgedal and Gjuve formations have unimodal U-Pb age spectra, suggesting input from a single source and probably accumulation in restricted basins. The overlying Eidsborg Formation displays a wide range of detrital zircon age peaks indicative of input from more varied source regions. Hf and O isotopes in detrital zircons of the Bandak succession indicate derivation from typical Fennoscandian basement rocks with the youngest dominant population (∼1150 Ma) having been derived from sources formed by remelting of the pre-Sveconorwegian juvenile Gothian basement. Whole rock geochemistry and Nd isotopic signatures further imply that the rhyolite of the Dalaå Formation was formed from anatexis of c. 1500 Ma crust. In contrast, mafic volcanic rocks indicate a mantle source that had been previously enriched through subduction, and variable contamination from older continental crust, or melts derived from it (i.e. the Dalaå Formation). The mafic lithologies reveal decreasing amounts of crustal contamination higher in the section, compatible with increasing amounts of extension and a thinner crustal column. Age, sedimentological, and geochemical data for the volcanosedimentary units in the Bandak succession record an episode of intracontinental extension, associated with formation of a series of intermontane basins. We hypothesize that these localized basins were formed at different stages of extensional collapse following a pulse of regional orogenesis. The Ofte and Røynstaul formations accumulated during rifting and were followed by bimodal volcanism within the Morgedal, Dalaå, and Gjuve formations. The Eidsborg Formation, at the top of the succession, records post-extension transgressive sedimentation. This tectonic scenario represents a pre-Sveconorwegian stage of orogenesis that can be related to convergent margin processes. The surface expression of crustal growth during this time is relatively small; however, the volume of a postulated mafic underplate that accreted and intruded the lower crust in the Telemark region at this time is unknown. Crustal growth during syn-convergent extension in a retro-arc region may be significant if volumes of accreted mafic underplate are large.