Evidence pre-Scandian deformation and metamorphism in the Caledonian margin of Baltica: U-Pb zircon ages from the Tronfjell Gabbro and related mafic intrusives in the Hummelfjell Group, Central Scandinavian Caledonides

Evidence pre-Scandian deformation and metamorphism in the Caledonian margin of Baltica: U-Pb zircon ages from the Tronfjell Gabbro and related mafic intrusives in the Hummelfjell Group, Central Scandinavian Caledonides. Johannes Jakob, Fernando Corfu, Torgeir B. Andersen The Centre for Earth Evolution and Dynamics, University of Oslo, Norway It is commonly accepted that main Scandian thrusting in the Scandinavian Caledonides commenced in the Mid-Silurian, shortly before or during the continental collision of Baltica and Laurentia. Because the Baltican margin was extremely wide (>1000 km) before the collision (Jakob et al. in review), deformation and metamorphism was diachronous and distal parts were deformed earlier than proximal parts and the foreland. As shown by the abundant high and ultra-high-pressure rocks, Baltica constituted the lower plate during the collision. Subduction/collision-related bimodal Late Ordovician to Early Silurian intrusives in the Scandinavian Caledonides are therefore interpreted to occur only in nappes derived from the upper plate, i.e. in the outboard terranes. The Neoproterozoic Hummelfjell Gp. comprises foliated, amphibolite facies metamorphic, metasandstones and schists. These metasediments host a number of foliated mafic plutons and volcanics that have been correlated with the Ottfjället dykes in the Särv Nappe in Jämtland, Sweden. The rocks of the Hummelfjell Gp. are interpreted to be Baltican pre- to syn-rift, proximal, sediments intruded by syn-rift mafic magmas belonging to the Ediacaran Scandinavian Dyke Complex (~615 to 595 Ma). The Tronfjell Gabbro near Alvdal, has previously been interpreted to have intruded the Hummelfjell Gp. between 500 and 425 Ma (Wellings, 1996). In contrast to the deformed and metamorphosed metasediments and mafic intrusives of the Hummelfjell Gp, that show a shared tectono-metamorphic history, the Tronfjell Gabbro is only affected by greenschist facies metamorphism and a weaker deformation than the Hummelfjell rocks. U-Pb zircon geochronology from the Tronfjell Gabbro and from mafic intrusives within the Hummelfjell metasediments in the contact zone immediately below the gabbro yield U-Pb crystallisation ages of 437.0 ± 0.5 Ma and 437.1 ± 1.1 Ma, respectively. Because, the Hummelfjell Group is interpreted to be derived from the margin of Baltica, and because the amphibolite facies fabric in the Hummelfjell group apparently predates the emplacement of the Tronfjell gabbro, the U-Pb ID-TIMS results suggest that the main deformation and metamorphism of the Baltican margin commenced well before 437 Ma; probably already in the Middle to Late Ordovician as indicated by HP metamorphism in the Seve Nappe Complex (446 Ma, Root & Corfu, 2012) and mica by cooling ages and the Middle Silurian unconformity below the Høyvik Gp in the Dalsfjord Nappe Complex of western Norway (449 Ma, Andersen et al., 1998).