Archaean and Proterozoic crustal evolution in Lofoten–Vesterålen, N Norway

Abstract

The region exposes an unusually deep section through the continental crust. Gravity and seismic surveys show a ridge-like NE-SW up-warping of the Moho to within 25 km of the surface. The oldest rocks are migmatitic gneisses of generally intermediate composition, probably largely of supracrustal origin; these were intruded by a granodiorite/granite pluton at about 2600 Ma. Pb isotope data indicate that the migmatites formed c. 2700 Ma ago, and the isotopic systems in some rocks have been disturbed by Proterozoic granulite-facies metamorphism. A Proterozoic supracrustal series is composed dominantly of felsic metavolcanic gneisses but includes marble, quartzite, graphite schist and iron formations. An Rb-Sr whole-rock isochron age of 1830 ±35 Ma is interpreted as the age of a metamorphism which reached intermediate-pressure granulite facies in the western part of the area. An early low-P, high-T event was followed by PT conditions near 900°C and 10 kbar at the metamorphic maximum. This thermal peak was essentially post-tectonic and coincided with the intrusion of gabbro, anorthosite and huge volumes of mangeritic to charnockitic magma between c. 1800 and c. 1700 Ma ago. A swarm of alkali-olivine dolerites was intruded shortly after the mangerites. The Lefdingen granitic batholith gives an Rb-Sr isochron age of 1380 ±80 Ma and may represent remobilized basement rocks. Pelitic schists of the Leknes Group were tectonically emplaced against the mangerites and gneisses, and metamorphosed in amphibolite facies 1140 ±135 Ma ago. Rb-Sr and K-Ar mineral ages suggest that the extensive retrograde metamorphism observed in the older rocks occurred at this time. Small granitic pegmatites were also emplaced during this episode. The Caledonian orogeny is recorded by some K-Ar mineral ages, the development of local W-dipping thrust zones, and the local intrusion of large pegmatites. Lofoten-Vesteralen was probably part of the Baltic plate during the Caledonian orogeny, but escaped deformation because it lay at a high tectonic level and consisted largely of massive granulite-facies rocks.