Tectonic significance of the meta-evaporitic magnesite and scapolite deposits in the Seve Nappes, Sarek Mts., Swedish Caledonides

Abstract

Magnesite and scapolite, hosted by diabase and amphibolite derived from diabase, occur in the Sarektjåkkå Nappe, Seve Nappe Complex in the Caledonides of northern Sweden. The Sarektjåkkå Nappe consists of a sedimentary sequence—the Favoritkammen Group—intruded by vast amounts of diabase, that constitute 70–80% of the nappe. The magnesite and scapolite layers are parts of the Spika Formation in the Favoritkammen Group. The Spika Formation formed in an evaporative setting of alternating influx of continental (magnesian carbonates) and marine water (halite and anhydrite/gypsum altered to scapolite during thermal metamorphism), possibly in rift-related basins in the Late Precambrian. The stratigraphy of the Favoritkammen Group indicates a progressively deepening basin, consistent with deposition in developing rift basins. The diabase dykes that cut the Spika Formation caused regional thermal metamorphism, but are not responsible for the magnesite formation. The lithology of the Sarektjåkkå Nappe thus records the evolution of a rift to the formation of a passive margin. That passive margin was detached and thrust over the Baltic Shield during the Caledonian Orogeny.