Caledonian terrane analysis in Troms-Torneträsk, northern Scandinavia, utilizing the geochemistry of high-level metabasites

In the Caledonides of the Torneträsk-Troms region, northern Scandinavia several thrust sheets occur above structural units representing the shortened, outermost preserved portions of the Late Precambrian-lower Palaeozoic Baltoscandian continental margin (Seve Nappes). Most are pelitic in character, with sequences of marble and amphibolite occurring at certain levels. On the basis of their position within the regional tectonostratigraphy, they are thought to represent a series of terranes that are suspect and possibly exotic with respect to Baltoscandia. Major- and trace-element characteristics of amphibolite sheets and lenses from most units above the Seve Nappes display remarkably consistent geochemical signatures. These show clear affinities with modern-day basalts from plate-margin settings, notably ocean floor basalts from a back-arc setting. Similarities in lithostratigraphy, tectonometamorphic evolution and metabasite geochemistry suggest that they represent an amalgam of exotic oceanic terranes. These were probably derived from the lower Palaeozoic Iapetus oceanic tract, outboard of their present position along the Baltoscandian continental margin. The Småtinden nappe is quite different, containing mafic dykes (now amphibolite) with a pronounced within-plate basalt geochemistry. This reinforces previously noted differences in lithostratigraphy and tectonothermal evolution, and, on this combined evidence, this unit is defined as part of a separate, interleaved, suspect terrane.