Proterozoic and Archaean ages of detrital zircon from the Palaeoproterozoic Västervik Basin, SE Sweden: Implications for provenance and timing of deposition

Abstract

Ages of detrital zircons, derived from Palaeoproterozoic metasedimentary rocks from Finland and Sweden are poorly represented in the presently exposed crust in the Baltic Shield. This study reports U-Pb ages of detrital zircons from the Svecofennian Västervik Basin. 41 spots from 31 zircon crystals were dated using U-Pb geochronology at the NORDSIM ion microprobe in Stockholm. Most analyses are concordant and the zircon grains commonly display well-developed magmatic oscillatory zoning. The ages documented are: ∼3.64 Ga, 3.03-2.95 Ga, 2.72-2.69 Ga, 2.12-1.87 Ga and 1.84 Ga. 75% of the grains are Palaeoproterozoic and 25% are Archaean. Ages gained from Proterozoic metasediments in Sweden, Finland, Svalbard, Greenland and Great Britain also report a large proportion of ∼2.1-1.9 Ga and a smaller proportion of Archaean zircons with ages around 2.7 and 3 Ga. These age groups probably represent major crust forming events. The here presented results provide an estimate of the time of deposition in the Västervik Basin for the time interval of 1882–1850 Ma, constrained by two concordant zircon analyses of 1872±24 and 1870±12 Ma, and the newly presented 1859±9 Ma age for the Loftahammar granite that intrudes the metasedimentary succession in the north. The young detrital zircon age of 1837±22 Ma might suggest that parts of the basin may be younger. As input of detrital grains may occur from several sources simultaneously (e.g. by rivers and by tidal currents from a marine source), the detrital grains were sampled from different depositional environments. Main fluvial sediment transport in the Västervik Basin was from present north whereas the tidal sediment transport was from the present south. The age groups documented in the Västervik Basin are poorly represented in the presently exposed crust in the Baltic Shield, but are represented in Sarmatia.