Seve terranes of the Kebnekaise Mts., Swedish Caledonides, and their amalgamation, accretion and affinity

摘要

A major allochthon of the Scandinavian Caledonides, the Seve belt has traditionally been considered to be derived from the rifted margin and continent-ocean transition (COT) of Baltica. However, geochronological results obtained from its inferred northern equivalent, the Kalak Nappe Complex (KNC), have been taken to indicate an exotic affinity of this complex and of also Seve terranes, an interpretation adopted in recent palaeogeographic models. In the Kebnekaise Mts., the COT is represented by the Kebnekaise terrane composed of amphibolitized dykes of gabbro and dolerite of depleted magma source and rare felsic and ultramafic rocks. Coronitic dolerite and gabbro with abundant rutile suggest high pressures before or during amalgamation with the underlying Marma terrane, composed of quartzofeldspathic gneisses intruded by mafic and granitic rocks, the latter including a previously dated c. 845 Ma-old granite. The granite mingled with mildly alkaline dolerites chemically similar to transitional basalts of continental rifts. Following emplacement at shallow (andalusite stability) crustal levels, the igneous complex and host rocks underwent extensive deformation, metamorphism within the sillimanite-kyanite stability field and local migmatization. The Kebnekaise and Marma terranes amalgamated in early Ordovician as indicated by the U-Pb age of 487 +/- 7 Ma obtained from titanite fabrics of deformed granite in thrust vicinity, and by Ar-40-Ar-39 results. The pressure increase in both terranes suggests that amalgamation occurred during initial subduction and imbrication. Correlation of the Marma terrane with the KNC is discussed. Results obtained in this study give no reason to ascribe an exotic affinity to the Seve terranes of the Kebnekaise Mts.