Penetrative ductile deformation and amphibolite-facies metamorphism prior to 1851 Ma in the western part of the Svecofennian orogen, Fennoscandian Shield

The Svecofennian orogen in the Fennosandian Shield consists of several Palaeoproterozoic meta-igneous provinces that generally young progressively southwards and westwards. The understanding of the tectonic evolution of these different provinces is linked to the ability to constrain the timing of ductile deformation and metamorphism within them. In the western part of the orogen, in central Sweden, ductile deformation and metamorphism is conventionally thought to have occurred after 1.85 Ga and mainly around 1.82–1.80 Ga. At Forsmark in central Sweden, we have dated two undeformed granite dykes that cross-cut the tectonic fabric in their host rocks to 1851 ± 5 and 1855 ± 6 Ma, respectively. Since the former shows an unequivocal field relationship with respect to the tectonic fabric, we establish that penetrative ductile deformation under amphibolite-facies metamorphic conditions occurred prior to 1851 ± 5 Ma. This U–Pb zircon minimum age for the timing of ductile deformation is supported by a U–Pb titanite age of 1844 ± 4 Ma. These data as well as the field relationships at Forsmark indicate that an early phase of penetrative deformation and metamorphism affected the Svecofennian bedrock in the western part of the Fennoscandian Shield and was overprinted, after 1851 ± 5 Ma, by spatially more confined deformational events. In conjunction with earlier results, our data help to establish the presence of tectonic domains with contrasting tectonothermal histories in the western part of the Svecofennian orogen.