ACCRETIONARY GROWTH OF THE SVECONORWEGIAN PROVINCE OF THE BALTIC SHIELD BETWEEN 1.7-1.5 GA AND LINKS TO INTRACONTINENTAL MAGMATISM

Accretionary growth between at least 1.8-1.2 Ga along a contiguous margin has been inferred for a mid-Proterozoic supercontinent that included Baltica and Laurentia. In Baltica, growth attributed to convergent-margin processes was coeval with widespread intracontinental, largely bimodal magmatism in belts that were offset from, but generally migrated sympathetically with, the evolving supercontinent margin. In SW Baltica, recent studies have unveiled a complex sequence of juvenile crust-forming events between 1.70-1.55 Ga (Gothian) that involved successive stages of subduction-related magmatism and intervening deformation. The consequent rock associations and deformational fabrics are best modeled by westward-stepping subduction and island arc accretion, a model that accounts for the present proximity of continental and oceanic arc rocks in SW Sweden. Mid-Proterozoic intracontinental magmatism in Baltica has been considered unrelated to events along the active margin, but new geochronological data demonstrate strong temporal links between the end of each subduction stage and the onset of intracontinental magmatic pulses (at 1.65-1.62, 1.58-1.56, 1.55-1.50 Ga). Changes in regional lithospheric stresses related to the termination of each episode of subduction apparently triggered bimodal, rapakivi magmatism between 500 to 1500 km inboard of the active continental margin. By 1.50 Ga, intracontinental bimodal magmatism (albeit less voluminous) swept westwards to intrude the recently formed Gothian arc rocks at 1.50, 1.38, 1.33 and 1.25-1.20 Ga, apparently continuing to follow the westward migrating active margin. This pattern of bimodal magmatism was interrupted at least once by large-scale rifting at 1.46 Ga and by high-grade metamorphism and granite magmatism at 1.45-1.40 Ga. Despite minor temporal and compositional differences, the prolonged record of convergent-margin activity and associated inboard magmatism in Baltica have recognizable correlatives in Laurentia. These correlations strengthen plate reconstructions that place Baltica and northeastern Laurentia along strike on a common mid-Proterozoic supercontinent, a configuration that must be accounted for in Rodinian reconstructions.