Late Neoproterozoic extended continental margin development recorded by the Seve Nappe Complex of the northern Scandinavian Caledonides

The Scandinavian Caledonides comprise nappe stacks of far-travelled allochthons that record the opening and closure of the Iapetus Ocean, culminating with the collision of Baltica and Laurentia. The Seve Nappe Complex (SNC) of the Scandinavian Caledonides comprises relics of the outermost Baltoscandian passive margin that were subducted to mantle depths during ocean closure. Subduction of these rocks overprinted much of the Neoproterozoic record for Iapetus Ocean formation, and as a result, much of the work conducted in the SNC has focused solely on the Caledonian orogenic history. In this study, we combined petrological and geochronological work to expand the knowledge about the Neoproterozoic metamorphic history of the Baltoscandian margin in the understudied Vaivancohkka-Salmmecohkat region of the northern Scandinavian Caledonides. The work focused on rocks that belong to the upper gneiss unit, which constitutes part of SNC in the region. The unit comprises migmatitic paragneisses and garnet-mica schist containing metamafic bodies. U-Pb zircon and Th-U-total Pb monazite dating of the migmatitic paragneiss yielded consistent age of metamorphism in 602 +/- 5 Ma and 599 +/- 3 Ma, respectively. A similar U-Pb age of 604 +/- 8 Ma was obtained for the zircon from the leucocratic vein transecting the amphibolite within the studied gneiss. Interestingly, no Caledonian ages were identified. Likewise, no evidence for high or ultra-high pressure conditions was found, neither in the gneisses/schists nor in the metamafic rocks. Petrographic observations and calculated metamorphic P-T conditions indicate that rocks belonging to upper gneiss unit underwent upper-amphibolite facies metamorphism in a melt stability field; 8.0-10.5 kbar at 750-790 degrees C. Since the studied rocks underwent high-grade metamorphism in the Ediacaran and lack obvious evidence for Caledonian high-pressure metamorphism, the studied part of the SNC offers an extraordinary insight into the Late Neoproterozoic history of the hot, extended Baltoscandian margin.

Automated summary

This study expands the knowledge about the Neoproterozoic metamorphic history of the Baltoscandian margin in the northern Scandinavian Caledonides. The research reveals that the studied rocks underwent high-grade metamorphism in the Ediacaran and lack obvious evidence for Caledonian high-pressure metamorphism.