Tectonic regimes and terrane boundaries in the high-grade Sveconorwegian belt of SW Norway, inferred from U–Pb zircon geochronology and geochemical signature of augen gneiss suites

Six units from three suites of gneissic megacrystic granitoids in the Sveconorwegian Province of SW Norway were dated by the U–Pb zircon method. The Gjerstad suite crystallized between 1.19 and 1.15 Ga in the Bamble, Telemark and Rogaland–Vest Agder terranes; it has an A-type geochemical signature suggesting intrusion in a tensional setting. The Vennesla unit of this suite yields an age of 1166+⁶¹₋₂₁ Ma. The Feda suite intruded in Rogaland–Vest Agder during a short period at 1.05 Ga (four units between 1051⁺²₋₈ and 1049⁺²₋₈ Ma) and has a high-K calc-alkaline trend suggesting a subduction- related setting. The Fennefoss augen gneiss of Telemark (1035⁺²₋₃ Ma) possesses a geochemical signature transitional between high-K calc-alkaline and A-type.

The spatial association of tensional plutonism with the Kristiansand–Porsgrunn shear zone between Bamble and Telemark suggests that crustal thinning occurred along this axis between 1.19 and 1.13 Ga, preceding Sveconorwegian compressive shearing. The boundary between Rogaland–Vest Agder and Telemark, the Mandal line, probably separated, at 1.05 Ga, a mobile belt (Rogaland–Vest Agder) and a colder, more rigid terrane (Telemark). Major strike-slip shearing along the southern section of the Mandal line was probably associated with intrusion of an elongate pluton of the Feda suite and ductile amphibolite-facies deformation along a banded gneiss unit.