Deformed and metamorphosed quartz-rich sandstones in southern Finland and east-central Sweden were presumed to be deposited in late Svecofennian time (< c. 1.88 Ga). On the basis of this assumption detrital zircons were selected from four localities (Luukkola, Pyhäntaka and Tiirismaa in Finland and Hamrånge in Sweden) for U-Pb SIMS determination.
The obtained ages define three age groups, 2.90 2.40 Ga, 2.15 1.96 Ga and 1.92 1.84 Ga, with no correlation between age and Th/U-ratio. The groups are similar in all four samples, which supports correlation, and they are comparable to previously reported detrital ages in this part of the Fennoscandian Shield. The oldest zircon analysed gave an age of 3.32 Ga (Tiirismaa).
The maximum ages of sedimentation (and of subsequent deformation and metamorphism), indicated by the youngest zircon, from the four localities are 1842±10 Ma (Luukkola), 1865±11 Ma (Pyhäntaka), 1848±13 Ma (Tiirismaa), and 1855±10 Ma (Hamrånge), respectively. Possible source rocks for these zircons are found in and in the vicinity of the vast Ljusdal Batholith in Sweden. Importantly, rocks with such ages are rare in Finland.
It is concluded that one or several sedimentary basins covered an area of at least 400x500 km at c. 1.85 1.82 Ga ago in the Fennoscandian Shield. Basin formation can be constrained between 1.86 and 1.83 Ga, the latter age by onset of regional metamorphism in southern Finland.
Stratigraphically below the metasandstone at Hamrånge is a metadacite with an U-Pb zircon TIMS age of 1888±6 Ma. This indicates affinity to the metavolcanic rocks in Bergslagen to the south, rather than to the younger equivalents at Loos to the northwest. It also suggests a significant time gap of >30 Ma between the volcanism and sand deposition in the Hamrånge area.
2006. 184- p.