U-PB BADDELEYITE DATING OF DOLERITE DYKES IN THE EASTERN PART OF THE SVECONORWEGIAN OROGEN, SOUTH-CENTRAL SWEDEN

Two dolerite dykes occurring in semi-penetratively deformed Transscand inavian Igneous Belt (TIB) granitoids, east of the traditional 'Protog ine Zone' have a composite U-Pb baddeleyite age of 1568(-8)(+30) Ma. S ince the dolerites are deformed themselves, this age sets a maximum ti me constraint for the ductile deformation in the easternmost part of t he Sveconorwegian orogen, northeast of Lake Vanern, west of the Svecon orwegian Frontal Deformation Zone (SFDZ; Wahlgren et al., 1994). The n ew age agrees, within the error limits, with previously reported but l ess precise U-Pb and Sm-Nd ages for petrographically and geochemically indistinguishable dolerites in the orthogneisses west of the traditio nal 'Protogine Zone', and together provides abundant evidence that the se dolerites belong to the same generation of mafic magmatism. Therefo re, the dolerites straddle the boundary between the orthogneisses and the semi-penetratively deformed TIB, i.e. the traditional 'Protogine Z one'. Apart from intracrustal deformation, the area north of Lake Vane rn from the SFDZ in the east to the Mylonite Zone in the west has cons tituted a coherent segment of crust from the time of the TIB intrusion s (similar to 1850-1650 Ma). The dolerites in this study have intruded as dykes into isotropic host rocks. The present dip and concordance t o the schistosity in the surrounding rocks are due to subsequent progr essive deformation in the eastern part of the Sveconorwegian orogen. I t is suggested that the structural geometry and concordant relationshi ps between the dolerites and the orthogneisses west of the traditional 'Protogine Zone' are also mainly a consequence of this subsequent pro gressive deformation that post-dates the dolerite emplacement.