U-PB DETRITAL ZIRCON GEOCHRONOLOGY AND PETROGRAPHY OF THE MIDDLE NEOPROTEROZOIC VISINGSÖ GROUP, SOUTHERN SWEDEN

The Neoproterozoic Visingsö Group is exposed along the shores of Lake Vättern, south-central Sweden. It is estimated to be ca. 800-640 Ma, an age range based upon acritarch and vase-shaped microfossil biostratigraphy, and a diagenetic Rb-Sr date on whole-rock shale. To further constrain the age of the Visingsö strata and characterize sedimentary provenance, petrographic analyses and LA-ICPMS U-Pb detrital zircon analyses were conducted on 6 sandstone samples.

The Visingsö is a mixed siliciclastic carbonate group divided into three units. At the bottom, quartz arenite predominates. The middle strata is marked by a feldspathic wacke and granite boulder conglomerate that grades upward into marine shale and stromatolitic dolomite. The succession is indicative of a tidal environment shifting to a retrograding fan delta system.

Detrital zircon ages from all three units range from 0.919 to 2.84 Ga with significant populations at 1.02, 1.32, 1.8, and 1.87 Ga. Zircon populations found within each unit display unique trends. In the lower unit, zircon ages are skewed toward 1.02 Ga. In the middle unit ages are skewed toward 1.8-1.87 Ga. Zircons from the upper unit display a more bell-curve shaped distribution, with age peaks at 1.55 Ga and 1.42 Ga. The maximum depositional age is 919.4 ± 25.1 Ma, which is likely 100-150 My older than the true age of the strata. The disparity between detrital zircon populations, petrography, and sedimentology across the lower-middle unit contact is indicative of an unconformity or an intra-group tectonic episode. The distribution of detrital zircon ages between the lower and middle unit (from 1.02 to 1.86 Ga) may indicate an unroofing of a Proterozoic basin in the region. 

Detrital zircon populations were likely derived directly from igneous rocks in southern and western Fennoscandia or from recycled sedimentary cover. Specifically, the 1.02 Ga peak comes from the Sveconorwegian orogeny from 1.14-0.95 Ga. The main 1.8 and 1.87 Ga peaks were derived from accretionary events ranging from 1.95-1.75 Ga, including the Svecofennian Orogeny. The 1.55 peak is indicative of the 1.65-1.55 Ga accretion in westernmost Fennoscandia, and the 1.42 peak is from the Danopolonian Orogeny from 1.5-1.4 Ga. The collective age populations suggest there was no Laurentian connection at the time of deposition.