Timing of Gothian structural evolution in SE Norway: A Rb-Sr whole-rock age study

The polydeformed basement area in SE Norway is situated within the southwes t border of the Baltic Shield. The investigated area covers the northern pa rt of the Ostfold segment (the Ostfold Complex) and adjoining parts of the Median segment (the Romerike Complex). New Rb-Sr whole-rock age determinati ons are integrated with the structural evolution. The oldest rock unit is t he Ostfold-St. Le-Marstrand formation. During successive stages the supracr ustal unit was intruded by major granitic bodies now forming the biotite gn eiss (oldest), the augen gneiss and metatonalite/granite complexes (younges t). The emplacement of the granitic units was separated by orogenic deforma tion events, F-0-F-3, recognized by the development of large-scale regional folding accompanied by amphibolite facies metamorphism. Rb-Sr whole-rock a ge determinations indicate a maximum age for the biotite gneiss (pre-F-0) a t 1.71 Ga; this age also marks the maximum age of the Gothian orogeny. Intr usion of the Nordstrand-Sormarka metatonalite/granite complex, 1.58 +/- 0.1 3 Ga, and the Ostmarka metatonalite complex, 1.56 +/- 0.04 Ga, around the F -2 deformation marks the end of the Gothian orogeny at ca. 1.57 Ga. A reset ting age of the augen gneiss, 1.06 +/- 0.17 Ga, is ascribed to the F-3 defo rmation; this age indicates a minimum age for the onset of the Sveconorwegi an orogeny.