FRONTIERS IN THE BALTIC SHIELD

Recent work in the Baltic Shield and its continuation in the basement of the East European Platform has created a new image of the Precambrian in the northeastern half of Europe. Towards the southeast, a continuation of the crust in the Shield can be traced to a Proterozoic system of palaeorifts in the East European Platform. These rifts apparently reproduce an earlier Precambrian crustal boundary that separates Fennoscandia, the northwestern crustal segment of the East European Craton, from its other two segments, Sarmatia and Volgo-Uralia. In the Archaean of Fennoscandia, the granite-greenstone province in Karelia is the only part that has ages in excess of 3 Ga. Interpretations of the Belomorian Belt along the White Sea and part of the Kola Peninsula as Early Archaean are therefore untenable. Rifting and break-up between 2.5 and 2.0 Ga strongly affected and partly dispersed the Archaean Domain. In the north, the resultant basins were closed by collisional orogeny between similar to 1.95 and 1.82 Ga ago. Semi-simultaneously, the Svecofennian orogeny created the continental crust in the central Baltic Shield. The Svecofennian Orogen continues into the area southeast of the Baltic Sea where a system of beltiform structures extends to the fault boundary with central Europe, featuring westward younging from similar to 2.0 to 1.8 Ga. In the west, the Svecofennian Orogen is truncated by the ensialic Transscandinavian Igneous Belt of similar to 1.8-1.65 Ga age. The final stage of extensive crust formation in the Baltic Shield occurred in western Scandinavia between similar to 1.75 and 1.55 Ga ago. Many of the granites in the interior of Fennoscandia are better explained as intracratonic manifestations of this process than as late- or post-orogenic products of the orogenies that created the immediately surrounding crust. After its formation, the continental crust of Fennoscandia underwent major reworking during the Svecononvegian-Grenvillian and Caledonian orogenies similar to 1.2-0.9 and 0.5-0.4 Ga ago. A previously unknown realm of crustal stacking and high-pressure Sveconorwegian metamorphism has recently been discovered in southwestern Sweden. While the formation of new and the reworking of preexisting continental crust in Fennoscandia was a consequence predominantly of processes at destructive plate margins, the geodynamics cannot be interpreted in terms of a succession of more or less uniform cyclic orogenies of similar duration. Although there were Late Archaean and Palaeoproterozoic activity maxima in the formation of new continental crust in the Baltic Shield, the definition of individual orogenies requires a new look.