CHRONOLOGY OF PROTEROZOIC OROGENIC PROCESSES AT THE ARCHEAN CONTINENTAL-MARGIN IN NORTHERN SWEDEN

The Proterozoic Svecofennian orogeny was characterized by the rapid fo rmation of great amounts of juvenile continental crust and extensive r emobilization of older crust. This paper considers U-Pb zircon ages an d some geochemical and lithostratigraphic features of the Svecofennian in the continental-margin area between the Skellefte ore district and the boundary of the Archaean craton farther north in Sweden. New U-Pb zircon ages constrain the igneous activity of Svecofennian crust form ation in northern Sweden to between 1930 and 1870 Ma ago. The same tim e constraints apply also to crust formation farther east in Finland. O rogeny along the entire Archaean-Proterozoic boundary zone was thus si multaneous and does not represent an east-to-west event succession. It is argued that rocks with similar major element compositions but dist inctly different trace element characteristics were formed simultaneou sly but in different plate-tectonic environments. By similar to 1875 M a ago, the Svecofennian volcanic arc had matured and a variety of syn- to late-orogenic igneous rocks appeared in both tensional and compres sional settings. Shortly thereafter, the Svecofennian magmatic activit y ceased altogether, probably as a result of collision between the are and the Archaean continent in the north. It is also suggested that pr e-Svecofennian rifting of the Archaean craton had created a passive co ntinental margin and that the transition to an active margin with subs equent island-are magmatism and subduction beneath the Archaean craton commenced prior to 1930 Ma ago. It may well have been initiated by a relative drift of the Archaean craton towards the present southwest as a consequence of the formation of the collisional Proterozoic Lapland Granulite Belt in the northernmost Baltic Shield.