U-Pb zircon dating of felsic intrusions, Middle Köli Nappes, central Scandinavian Caledonides

U-Pb zircon data are presented for three metamorphosed trondhjemites from the Middle Köli Nappes
in northern Jämtland and southem Vasterbotten, Sweden. The analysed zircon fractions give concordant
or practically concordant U-Pb ages defining three distinct age groups, one for each sample. The most
precise ages are given by the 206Pb/238U ratios. The average ages are 492 ± 1 Ma and 476 ± 1 Ma for two
metamorphosed trondhjemites in the Stekenjokk Quartz-Keratophyre, and 440 ± 2 Ma for a dyke-like
body in the Blåsjö Phyllite. It is argued that the first of these ages and, due to the possibility of minor
post-crystallization Pb-loss, less confidently even the other two reflect the times of emplacement of the
intrusions. Furthermore, it is considered that the first age lies close to the age of the volcanic activity in
the Stekenjokk Quartz-Keratophyre which is, thus, most likely to be Tremadoc. The pre-Silurian age of
these intrusions and their host rocks confirms earlier suggestions that there exists a major tectonic break
in the lower part of the Köli sequence. The felsic igneous activity is considered to belong to a prolonged
period of rifting. This overlapped in time with and succeeded the waning stages of an island are
development which was active probably during the Tremadoc.