TECTONIC SIGNIFICANCE OF THE META-EVAPORITIC MAGNESITE AND SCAPOLITE DEPOSITS IN THE SEVE-NAPPES, SAREK MTS, SWEDISH CALEDONIDES

Magnesite and scapolite, hosted by diabase and amphibolite derived fro m diabase, occur in the Sarektjakka Nappe, Seve Nappe Complex in the C aledonides of northern Sweden. The Sarektjakka Nappe consists of a sed imentary sequence-the Favoritkammen Group-intruded by vast amounts of diabase, that constitute 70-80% of the nappe. The magnesite and scapol ite 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 stratig raphy of the Favoritkammen Group indicates a progressively deepening b asin, consistent with deposition in developing rift basins. The diabas e dykes that cut the Spika Formation caused regional thermal metamorph ism, but are not responsible for the magnesite formation. The litholog y of the Sarektjakka Nappe thus records the evolution of a rift to the formation of a passive margin. That passive margin was detached and t hrust over the Baltic Shield during the Caledonian Orogeny.