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

Abstract

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 imbrication of the pristine plate margin, all emplaced on land and accessible. We first organize the evidence for Baltoscandian rift basin formation and magmatism, now scattered in two major nappe complexes (and by semantics) in terms of a superterrane, the Seve-Kalak Superterrane. Extensive (1000 km) mafic dyke swarms and partly sheeted dyke complexes are interpreted as fragments of a Large Igneous Province. While attempted break-up and some tholeiitic magmatism took place already at c. 800 Ma, successful rifting occurred first in connection with intense, partly alkaline dyking and emplacement of ultramafic complexes between c. 620 and 550 Ma. This late magmatism is markedly enriched as compared to MORB, interpreted to reflect mixing between an enriched mantle source component and depleted mantle. The evidence for Early Ordovician imbrication and subduction of the rifted and intruded margin is then reviewed. In order to explain the preservation of parts of the continent-ocean transition and rift basins, we suggest early detachment and thrusting towards the foreland, by analogy with the emplacement of the Semail Ophiolite. Intercalated garnet peridotites require incorporation of subcontinental mantle fragments during imbrication. Structures and fabrics of eclogites and their host rocks suggest that extensional tectonics was important during their Early Ordovician exhumation. If so, how much of Scandian extensional tectonics is instead inherited Finnmarkian?