Dawn of Phanerozoic orogeny in the North Atlantic tract; Evidence from theSeve-Kalak Superterrane, Scandinavian Caledonides

Despite their Early Phanerozoic age, the Scandinavian Caledonides provide a remarkably rich record of the continental break-up and development of the continent-ocean transition as well as the ensuing subduction and imbricatio n of the pristine plate margin, all emplaced on land and accessible. We fir st organize the evidence for Baltoscandian rift basin formation and magmati sm, now scattered in two major nappe complexes (and by semantics) in terms of a superterrane, the Seve-Kalak Superterrane. Extensive (1000 km) mafic d yke swarms and partly sheeted dyke complexes are interpreted as fragments o f a Large Igneous Province. While attempted break-up and some tholeiitic ma gmatism took place already at c. 800 Ma, successful rifting occurred first in connection with intense, partly alkaline dyking and emplacement of ultra mafic complexes between c. 620 and 550 Ma. This late magmatism is markedly enriched as compared to MORE, interpreted to reflect mixing between an enri ched mantle source component and depleted mantle. The evidence for Early Or dovician imbrication and subduction of the rifted and intruded margin is th en reviewed. In order to explain the preservation of parts of the continent -ocean transition and rift basins, we suggest early detachment and thrustin g towards the foreland, by analogy with the emplacement of the Semail Ophio lite. Intercalated garnet peridotites require incorporation of subcontinent al mantle fragments during imbrication. Structures and fabrics of eclogites and their host rocks suggest that extensional tectonics was important duri ng their Early Ordovician exhumation. If so, how much of Scandian extension al tectonics is instead inherited Finnmarkian?