Ion probe zircon dating of metasedimentary units across the Skagerrak: new constraints for early Mesoproterozoic growth of the Baltic Shield

Abstract

Cathodoluminescence images were used to select zircons for ion probe U–Pb dating of detrital grains and overgrowths in metasedimentary samples from three Proterozoic supracrustal units; the Bamble gneiss in southern Norway, the Stora Le-Marstrand (SLM) and Skagerrak formations in SW Sweden. Because of their position on either side of the Permian Oslo Rift and Skagerrak, they could provide a link between the eastern and western halves of the Sveconorwegian Province. Detrital grains from an SLM metapsammite gave Pb–Pb ages in the 1.66–1.57 Ga interval. Volcanic zircon morphology and lack of rounding point to an immature sedimentary environment, consistent with the inferred island arc setting. Multiple-spot analyses of three young, originally igneous grains gave near-concordant U–Pb data with weighted mean Pb–Pb ages at 1613±11, 1598±37 and 1579±19 Ma. The third age provides a maximum age for the SLM Formation at 1598 Ma, superseding a previous ca. 1.76 Ga Sm–Nd age. A Bamble metaquartzite from the Kragerö area, shows a broad distribution of Pb–Pb zircon ages at 2.62, 1.95–1.76, 1.61, 1.50–1.48 Ga. Thick metamorphic rims, one dated at 1.16±0.03 Ga, are attributed to high-grade Sveconorwegian metamorphism. Multiple-spot analyses on two young igneous grains yielded Pb–Pb ages at 1495±45 and 1484±15 Ma, which imply a maximum depositional age of 1499 Ma and invite re-examination of previous 1.6–1.54 Ga age constraints for the Bamble metasupracrustal gneisses. The 2.62 Ga zircon confirms the presence of a late Archean component in SW Norway whereas the broad age spectrum of detrital zircons is consistent with derivation from continental crust. The Skagerrak Formation is a recently recognized volcano-clastic unit, occurring as megaxenoliths in western parts of the 0.92 Ga Bohus granite. Most zircons from a dacitic sample gave concordant to near-concordant U–Pb ages in the 1.56–1.44 Ga range, yielding a maximum depositional of 1.48 Ga. One ca. 1.88 Ga old grain was found, which indicates a pre-Gothian provenance not known today in the Gothian orogen of SW Sweden. A broad correlation between the Bamble and lowermost Telemark supracrustals in southern Norway and the Skagerrak Formation in SW Sweden is suggested. Distinct ages and tectonic settings for the SLM and Bamble metasupracrustal rocks point to an early Mesoproterozoic lithotectonic boundary in the Oslo area, re-activated by the Sveconorwegian Orogeny and ultimately obscured by Permian activity in the Oslo Rift. It is argued that this boundary represents a N–S trending suture along which the active Baltica and passive SW Norwegian continental margins were joined ca. 1.58 Ga ago, and also controlled a subsequent 1.46 Ga rift zone.