Origin and emplacement of ultramafic bodies in the Köli nappe and the lower- and upper belt of the Seve Nappe Complex, Central Scandinavian Caledonides

Abstract

The Scandinavian Caledonides is composed out of a westward dipping and north-south running nappe stack with multiple outcrops of orogenic peridotite bodies. Earlier research has revealed different mechanisms by which the ultramafic mantle material is incorporated into the country rock, either as obducted ophiolites or as mantle wedge peridotites (MWP). The Seve Nappe Complex (SNC) which is build out of the high grade middle belt (granulite facies), bounded by the medium grade upper- and lower belt (amphibolite facies), contains abundant ultramafic bodies. Recently the ultramafic bodies in the middle belt have been classified as MWP, however the mechanism of peridotite emplacement in the upper- and lower belt remains unknown. This thesis investigates the deformational, mineralogical and chemical evidence of the country rocks and the peridotites of the upper- and lower belt plus the overlying Köli belt in order to determine their provenance and mechanism of emplacement. This research has found that peak metamorphism of the country rocks and peridotite of the same unit are in correspondence. However prior to their mutual metamorphic history the peridotites have experienced (intense) serpentinization and exhumation. The provenance of the upper and lower Seve belt peridotites have been interpreted as sub-cratonic. We propose that the peridotites of the upper- and lower belt are of the Ocean-Continent transition zone (OCT) type exposed and serpentinized during hyper extension of the western margin of Baltica. Subsequently they have become metamorphosed under amphibolite facies during the Finnmarkian metamorphic overprint (500Ma). The Köli peridotite is also of ophiolitic type (type undetermined) that has progressively metamorphosed, but during a later event, the Scandian metamorphic overprint (425 Ma). Geodynamical models of the development of the Caledonides have to be reconsidered by including the crustal emplacement of various types of peridotites from the hyperextended margins of Baltica.