Seveskollorna i nordöstra Kebnekaise, Kaledoniderna : metabasiter, graniter och ögongnejser

Sammanfattning: The Seve Nappe Complex in the Kebnekaise Mts. is dominated by metabasites (Kebne Dyke Complex) and mylonitic gneisses. This study describes the tectonostratigraphy in two key areas (Nallo and Mårma) and investigates the origin of a granitic gneiss and two varieties of augen gneiss. The Kebne Dyke Complex consists of thin dark bands of amphibolite in a metagabbro, interpreted to represent dolerite dykes, which intruded gabbro during sea floor spreading. An often strongly foliated granitic gneiss (Nallo Gneiss) occurs intercalated with the dyke complex. In major element classification diagrams (R1-R2), samples of the Nallo gneiss fall within the fields of monzogranite, granodiorite and tonalite. Trace element compositions indicate an origin as volcanic arc granite. Two types of augen gneiss occur, the Boginjira mylonite and the Storglaciären Gneiss. The mechanism of augen formation differs between the two types. The Boginjira Mylonite is a mylonitized variety of a granite (Vistas Granite) and megacrysts derive from feldspar phenocrysts. The augens have mantles of albite which formed during the process of mylonitization. Almost all augens are Karlsbad twins, most of which also formed during deformation. The protolith of the Storglaciären Gneiss is a mylonitic psammitic gneiss (Höktoppen Mylonite). Study of thin sections across the transition zone between migmatitic gneiss and fine-grained gneiss without augen suggests two stages of recrystallization. During the first stage, an augen gneiss formed from psammitic gneiss during prograde metamorphism, indicated by myrmekite growth along augens. With increasing temperature, parts of the augen gneiss were migmatized, other parts recrystallized to a homogeneous, strain-free gneiss without augens. The Seve units of the Nallo and Mårma areas can be correlated with upper Seve units occurring to the south in the central Kebnekaise Mts. and to the north in the Abisko and Indre Troms Mts.