The continent-ocean (Seve-Köli) boundary in the Sarek-Padjelanta Mts. revisited : Swedish Caledonides

In the Swedish Caledonides, the continent-ocean boundary is represented by the basal thrust of the Iapetus-derived Köli Nappe Complex (KNC) which overlies the Seve Nappe Complex (SNC) derived from the outer rifted margin of Baltica and the continent-ocean transition (COT). Current maps and literature subdivide the Seve teconostratigraphy in northern Sweden (Sarek-Padjelanta region) into three mega-lenses, the lowest of which (Vaimok lens) is composed of quartz-feldspathic gneisses and subordinate marble, all hosting eclogites. It is overlain by the Sarek lens composed of a COT dyke complex and, along its western margin, a narrow belt of phyllites, schists, quartzite, marbles and greenschists. Overlying this belt is Tsäkkok lens of the same composition as the Vaimok lens. The situation with an intermediate-pressure and often excellently preserved sheeted-dyke complex intercalated between two lenses of complexly deformed high-pressure rocks invited to intricate models of accretion, including out-of-sequence stacking of slices of the continental margin during early Ordovician subduction. Based on a detailed study of a select area and examination of the evolution of previous concepts, this study reinterprets the narrow belt along the western margin of the Sarek Lens as typical of the lower KNC. The favoured explanation of the presence of intermediate-pressure Köli rocks below high-pressure Seve rocks is recumbent folding and/or mega-sheath folding of the Seve-Köli nappe sequence during Scandian (Siluro-Devonian) accretion, by analogy with the situation in places in the central Swedish Caledonides. Previous models of early out-of-sequence stacking during subduction of the continental margin need reconsideration

Attempted break-up of Rodinia at 850 Ma: geochronological evidence from the Seve-Kalak Superterrane, Scandinavian Caledonides

Lower thrust sheets of the Scandinavian Caledonides derive from the margin of Baltica, which was imbricated during Early Palaeozoic closure of oceans formed during separation of the Baltica, Laurentia and Siberia cratons. At Vistas (Kebnekaise Mts), the Seve Nappe Complex preserves rare lenses of a dolerite-intruded granite formed by anatexis during emplacement of gabbro into metasedimentary rocks. Ion microprobe U-Pb dating of prismatic zircons from the granite yielded an age of 845 +/- 14 Ma (mean square weighed deviation (MSWD) 1.15), interpreted to date magmatic crystallization. Cores of complex crystals indicate protolith ages of 1778 +/- 11 Ma (MSWD 0.86); one core yielded an Archaean age. From narrow rims, a 605 +/- 42 Ma metamorphic age is obtained, and is interpreted to reflect the emplacement of the extensive Vendian rift magmatic dolerite dyke swarm. The age pattern allows correlation with a previously dated magmatic complex within the equivalent Kalak Nappe Complex 300 km to the north. Bimodal magmatic complexes with this age pattern do not occur within the basement of the Caledonide foreland, nor elsewhere within the Baltic Shield. However, recent reinterpretations of the Knoydartian event in Scotland as rift related invites correlation with the West Highland Granite Gneiss, which intruded Moine metasedimentary rocks at c. 870 Ma. In Central Taimyr, 850-900 Ma granites of continental crustal derivation and with 1800-1900 Ma Sm-Nd model age occur associated with c. 740 Ma plagiogranites. Using recent palaeogeographical reconstructions, we interpret the evidence of 850-900 Ma magmatism, in Scandinavia, Scotland and Taimyr as bimodal rift magmatism in connected arms during an attempted break-up of Rodinia

Subduction-flip during Iapetus Ocean closure and Baltica-Laurentia collision, Scandinavian Caledonides

Evidence is presented here from the northern Scandinavian Caledonides for development of an extensional basin of Ashgill to Mid Llandovery age along the Baltoscandian margin immediately prior to Baltica-Laurentia collision. U/Pb multigrain and ion microprobe zircon dating of plagiogranites in the Halti Igneous Complex complement previous baddeleyite and zircon dating of a dolerite dyke, and zircon dating of anatectic granite; they demonstrate that this dunite, troctolite, gabbro, sheeted-dyke complex ranges in age from c. 445 to 435 Ma. The dolerite dykes intruded and melted arkoses of inferred Neoproterozoic age. This evidence, taken together with previous documentation of ophiolites (Solund-Stavfjord), ophiolite-like associations (Sulitjelma Igneous Complex) and several other mafic suites (e.g. Rana, Artfjallet) of Ashgill to Llandovery age further south in the northern Scandinavian Caledonides, implies that Scandian collisional orogeny along this nearly 2000-km-long mountain belt was immediately preceeded by development of short-lived marginal basins. The latter developed during the final closure of the Iapetus Ocean and are inferred to be of back-arc origin, some (perhaps all) related to E-dipping subduction. Collision of the continents at c. 435 Ma is inferred to have induced a flip in subduction polarity, leading to underthrusting of Laurentia by Baltica