The polydeformed basement area in SE Norway is situated within the southwest border of the Baltic Shield. The investigated area covers the northem part of the Østfold segment (the Østfold Complex) and adjoining parts of the Median segment (the Romerike Complex). New Rb-Sr whole-rock age determinations are integrated with the structural evolution. The oldest rock unit is the Østfold-St. Le-Marstrand formation. During successive stages the supracrustal unit was intruded by major granitic bodies now forming the biotite gneiss ( oldest), the augen gneiss and metatonalite/granite complexes (youngest). The emplacement of the granitic units was separated by orogenic deformation events, F0-F3, recognized by the development of large-scale regional folding accompanied by amphibolite facies metamorphism. Rb-Sr whole-rock age determinations indicate a maximum age for the biotite gneiss (pre-F0) at 1 .71 Ga; this age also marks the maximum age of the Gothian orogeny. Intrusion of the Nordstrand-Sørmarka granite complex, 1 .58 ± 0.13 Ga, and the Østmarka metatonalite complex, 1 .56 ± 0.04 Ga, around the F2 deformation metatonalite/marks the end of the Gothian orogeny at ca. 1.57 Ga. A resetting age of the augen gneiss, 1 .06 ± 0.17 Ga, is ascribed to the F3 deformation; this age indicates a minimum age for the onset of the Sveconorwegian orogeny.
O. Graversen & S. Pedersen, Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark