NJG79-2-02
1999
Emplacement ages and metamorphic overprinting of granitoids in the Sveconorwegian Province in Varmland,Sweden - an ion probe study
79
2
pp. 87-96

North of Lake Viinern in the Sveconorwegian province of Sweden, veined and banded grey to red granitoids characterize the Western Segment in western Värmland. Two gneiss samples from Rackstad, north of Arvika, yielded concordant U-Pb emplacement ages of 1596 ±Il Ma and 1590 ± 14 Ma, respectively from zircon cores, while metamorphic zircon rims in one of the samples gave a weighted mean Pb-Pb age of 97 1 ± 8 Ma. Patches of metamorphic zircon overgrowth pre-dating the metamorphic rims indicate that an even older metamorphic event influenced this rock. East of the Mylonite Zone, a N -S-trending Sveconorwegian shear zone, more massive reddish granitoids occupy the segment between this zone and another N-S-trending Sveconorwegian shear zone, the Protogine Zone. A sample from Hänsjön, north of Torsby, yielded concordant ages with a weighted mean of 1689 ± 12 Ma. One grain has a Pb-Pb age of 1770 ± 29 Ma, which is inherited from the source of the granite, and is similar to the age of early magmatism in the Transscandinavian Igneous Belt, east of the Torsby Segment. These results confirm that north of Lake Vänern, the Torsby Segment was formed ca. 100 Ma earlier than the adjoining segment to the west, in accordance with previous results from further south.

Sven Åke Larson & David H. Cornell, Department of Geology, Goteborg University, Earth Sciences Centre, Box 460, S-405 30 Göteborg, Sweden;
Richard A. Armstrong, Research School of Earth Sciences, Australian National University, Canberra, ACT0200, Australia.

0029-196X