Tektonostratigrafi och deformationsrelaterad metamorfos i norra Kebnekaisefjällen, Skandinaviska Kaledoniderna

Sammanfattning: The tectonostratigraphic units in northern Kebnekaise Mountains mainly belong to the Seve Nappe Complex. These rocks were originally formed at the continental margin of Baltica in Late-Proterozoic time and mainly consist of amphibolite, granitic gneisses and metasediments. The metamorphic grade is usually amphibolite facies. The areas investigated in this work, Nallo and Mårma, consist of four different tectonostratigraphic units that belong to the Seve Nappe Complex. The lowermost of these units is a mylonitic granite, Boginjiramyloniten (Mårma area). Overlying Boginjiramyloniten in Mårma and as the lowest unit in Nallo is a 150 m thick sequence of mylonitic metasediment, Höktoppsmyloniten. Between the Höktoppsmyloniten and the uppermost unit is another gneiss of granitic origin, Nallognejsen. This granitic gneiss is tonalitic and monzogranitic in composition and also exists as imbricated sheets in the overlying amphibolite. The amphibolite belongs to the Kebne sheeted dyke complex and is the uppermost unit in the Seve Nappe Complex in this area as well as the most common. Two main phases of deformation have affected the area and are related to different stages in the orogeny. During the transportation of the nappes the rocks were subject to progressive deformation which resulted in both open folds formed N-S, perpendicular to σ1, and isoclinal folds formed parallel to σ1. Other kinematic indicators such as rotated clasts and s-c fabric also indicate E-W movement during the transportation of the nappes. The second phase of deformation was a result of extensional forces during de gravity caused collapse of the mountain range. Structures related to this event are mainly normal faults and flexures. Locally higher metamorphic grade exists in the Seve Nappe Complex as partial melting of the Höktoppsmyloniten. Mineral assemblages indicate that the mylonite was migmatized according to the reaction biotite+sillimanite+plagioclase+quartz→garnet+K-feldspar+melt and in thin section evidence was found proving that a younger generation of garnet has been growing in a biotite and sillimantite rich groundmass. These younger garnets are different from other garnets in Höktoppsmyloniten both structurally and chemically. Due to a pressure gradient caused by deformation, the melt migrated to areas of low pressure before it crystalized. Leucosome has been found in both boudin necks and cracks in these boudins together with stromatic migmatite.