U-Pb detrital zircon geochronology, petrography, and synthesis of the middle Neoproterozoic Visingso Group, Southern Sweden

The middle Neoproterozoic Visingso Group, Southern Sweden, is a fossiliferous mixed carbonate-siliciclastic unit that provides an environmental and biological record of southeast (present day coordinates) Baltica. Extensive work on microfossil assemblages has allowed correlation with geochronologically constrained middle Neoproterozoic units elsewhere and a proposed depositional age between similar to 780 and 730 Ma. The stratigraphic succession comprises three informal formations that record a shift in depositional environment from fluviomarine to a retrograding marine fan delta system. Petrographic and U-Pb LA-ICPMS detrital zircon (DZ) analyses are reported on six sandstone samples spanning the three formations. DZ ages range from 2840 to 919 Ma, with significant populations at 1870, 1800, 1320, and 1020 Ma. Each formation is characterized by a distinct age distribution. In the lower formation, ages are dominated by a peak at 1020 Ma, in the middle formation, however, ages are skewed toward 1800-1870 Ma. DZs from the upper formation display a more bell-curve shaped distribution, with dominant age peaks at 1550 and 1420 Ma. Provenance analysis indicates that the lower formation was likely sourced from the Feda-Sirdal magmatic belt in the distal Telemarkia lithotectonic unit to the west. DZ populations in the middle formation, combined with sedimentology, indicate tectonic activity and sourcing of the older Transscandinavian Igneous Belt east of the Sveconorwegian Front. DZs from the upper member show a predominance of Gothian and Hallandian magmatic rocks sourced from the west and, coupled with sedimentology, indicate tectonic quiescence. The maximum depositional age (919 +/- 25 Ma, youngest single grain; 941 +/- 12 Ma, average of five youngest grains) is more than 100 m.y. older than the purported age of the strata based on previous paleontological studies, and therefore necessitates that other dating methods be utilized to generate an age model for the Visingso Group.