The Sognefjell volcanic-subvolcanic complex – A late Sveconorwegian arc imbricated in the central Norwegian Caledonides

Abstract

The Sognefjell volcanic-subvolcanic complex is a composite unit underlying the Jotun Nappe Complex in the southern Norwegian Caledonides. The identity and origin of the complex have been highly debated in past decades, one interpretation considering the rocks to be highly deformed orthogneisses of the Jotun Nappe Complex and the other extreme considering them to be partly of volcanic origin and defining a Caledonian suture. In this study some critical components of the complex have been dated by U-Pb yielding ages of 961 ± 11 Ma for gabbro and 949 ± 4 Ma for a tonalite. A tuffite includes euhedral zircon crystals with ages of 985 to 955 Ma, suggesting derivation from the same magmatic complex. Zircon in an orthogneiss points instead to an older age close to 1600 Ma suggesting that this is a sliver of the Jotun Nappe Complex. A thin straight pegmatite cutting mafic gneiss is 949 ± 3 Ma. Titanite grains in the various rocks range in age from about 970 to 890 Ma, demonstrating that the dominant metamorphism was Sveconorwegian with minimal resetting in spite of the likely Caledonian deformation. The new observations, combined with published data, support the notion that these rocks are of volcanic-subvolcanic origin, but show that they are late Sveconorwegian, not Caledonian as originally envisaged. They likely developed in an arc complex at the margin of Baltica and possibly within the realm of the Asgard Sea. Arc magmatism was coeval with lower crustal processes in parts of the Baltic craton that would eventually become Caledonian nappes.