ION PROBE ZIRCON DATING OF METASEDIMENTARY UNITS ACROSS THE SKAGERRAK- NEW CONSTRAINTS FOR EARLY MESOPROTEROZOIC GROWTH OF THE BALTIC SHIELD

Cathodoluminescence images were used to select zircons for ion probe U -Pb dating of detrital grains and overgrowths in metasedimentary sampl es from three Proterozoic supracrustal units; the Bamble gneiss in sou thern Norway, the Stora Le-Marstrand (SLM) and Skagerrak formations in SW Sweden. Because of their position on either side of the Permian Os lo Rift and Skagerrak, they could provide a link between the eastern a nd western halves of the Sveconorwegian Province. Detrital grains from an SLM metapsammite gave Pb-Pb ages in the 1.66-1.57 Ga interval. Vol canic zircon morphology and lack of rounding point to an immature sedi mentary environment, consistent with the inferred island are 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 +/- 137 and 1579 +/- 19 Ma. The third age provides a maximum ag e for the SLM Formation at 1598 Ma, superseding a previous ca. 1.76 Ga Sm-Nd age. A Bamble metaquartzite from the Kragero area, shows a broa d distribution of Pb-Pb zircon ages at 2.62, 1.95-1.76, 1.61 and 1.50- 1.48 Ga. Thick metamorphic rims, one dated at 1.16 +/- 0.03 Ga, are at tributed to high-grade Sveconorwegian metamorphism. Multiple-spot anal yses 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 pre sence of a late Archean component in SW Norway whereas the broad age s pectrum of detrital zircons is consistent with derivation from contine ntal crust. The Skagerrak Formation is a recently recognized volcano-e lastic unit, occurring as megaxenoliths in western parts of the 0.92 G a 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 maxim um 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 oro gen 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 th e SLM and Bamble metasupracrustal rocks point to an early Mesoproteroz oic lithotectonic boundary in the Oslo area, re-activated by the Sveco norwegian Orogeny and ultimately obscured by Permian activity in the O slo Rift. It is argued that this boundary represents a N-S trending su ture along which the active Baltica and passive SW Norwegian continent al margins mere joined ca. 1.58 Ga ago, and also controlled a subseque nt 1.46 Ga rift zone. (C) 1998 Elsevier Science B.V.