UPb baddeleyite dating of dolerite dykes in the eastern part of the Sveconorwegian orogen, south-central Sweden

Abstract

Two dolerite dykes occurring in semi-penetratively deformed Transscandinavian Igneous Belt (TIB) granitoids, east of the traditional ‘Protogine Zone’ have a composite UPb baddeleyite age of 1568 +30 −8 Ma. Since the dolerites are deformed themselves, this age sets a maximum time constraint for the ductile deformation in the easternmost part of the Sveconorwegian orogen, northeast of Lake Vänern, west of the Sveconorwegian Frontal Deformation Zone (SFDZ; Wahlgren et al., 1994). The new age agrees, within the error limits, with previously reported but less precise UPb and SmNd ages for petrographically and geochemically indistinguishable dolerites in the orthogneisses west of the traditional ‘Protogine Zone’, and together provides abundant evidence that these dolerites belong to the same generation of mafic magmatism. Therefore, the dolerites straddle the boundary between the orthogneisses and the semi-penetratively deformed TIB, i.e. the traditional ‘Protogine Zone’. Apart from intracrustal deformation, the area north of Lake Vänern from the SFDZ in the east to the Mylonite Zone in the west has constituted a coherent segment of crust from the time of the TIB intrusions (∼ 1850-1650 Ma). The dolerites in this study have intruded as dykes into isotropic host rocks. The present dip and concordance to the schistosity in the surrounding rocks are due to subsequent progressive deformation in the eastern part of the Sveconorwegian orogen. It is suggested that the structural geometry and concordant relationships between the dolerites and the orthogneisses west of the traditional ‘Protogine Zone’ are also mainly a consequence of this subsequent progressive deformation that post-dates the dolerite emplacement.