Zircon U-Pb constraints on the timing of Sveconorwegian migmatite formation in the Western and Median Segments of the Idefjorden terrane, SW Sweden

Sammanfattning: The Western and Median Segments of the Idefjorden terrane have been metamorphosed and migmatized during the Sveconorwegian orogeny (ca. 1 Ga), but the timing of these events between the two segments remains unclear. Here I show that Sveconorwegian metamorphism and associated migmatite formation is not coeval between the Western and Median Segments and that there is a contrast between the segments in terms of strain, migmatite abundance and retrograde metamorphic overprint. Magmatic crystallization ages were determined through U-Pb spot dating on zircon using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Zircon cores with Th/U >0.2 yields an age of 1551±6 Ma for the Arsjön migmatite in the Western Segment, 1349±5 Ma for the Tösslanda migmatite within the Göta Älv Shear Zone, and 1601±3 Ma and 1581±8 Ma respectively for the Hälltorp migmatites in the Median Segment. Zircon in leucosome yield dates of 1073±31 Ma for the Arsjön migmatite, 1046±21 Ma for the Tösslanda migmatite and 1019±27 Ma for the Hälltorp migmatites. The dates of the zircon rims are interpreted to be ages of metamorphism and migmatization and are supported by Th/U values around 0.02. The metamorphic ages are coeval with Sveconorwegian metamorphism in the Idefjorden terrane (1.14-0.90 Ga), but the results demonstrate that Sveconorwegian metamorphism and migmatite formation occurred earlier in the Western Segment than in the Median Segment.