Geochronology of gneisses adjacent to the Mylonite Zone in southwestern Sweden: evidence of a tectonic window?

Sammanfattning: The relationship between the Eastern Segment and Idefjorden terrane prior to the Sveconorwegian orogeny has long been debated. Evidence of a common history is minimal and localised; at the same time, the notion that the Sveconorwegian orogeny formed as a result of a collisional event is also disputed. In this thesis, I analyse a region of the Idefjorden terrane, colloquially known as ‘B-Bulan’, close to the boundary with the Eastern Segment, which shows evidence of Eastern Segment-age (1.81-1.65 Ga) signatures and possibly Hallandian-age (1.47-1.38 Ga) signatures previously unseen in the Idefjorden terrane. Ages were determined using U-Pb spot dating of zircon using Laser Ablation Inductively-Coupled Mass Spectrometry (LA-ICP-MS). The 207Pb/206Pb ages of samples analysed shows the presence of Idefjorden and Eastern Segment signatures, as well as a potential Hallandian signature in a small number of samples. Idefjorden signatures are recorded in three samples at three locations (4f-i, 4g-i and 3m-ii). They show weighted mean 207Pb/206Pb ages of 1625 ± 5 Ma (4f-i), 1603 ± 4 Ma (4gi) and 1641 ± 11 Ma (3m-ii). Eastern Segment protolith ages are recorded in five samples at two locations (2.5j and 3a). At location 3a, samples show weighted mean 207Pb/206Pb ages of 1693 ± 12 Ma (3a-v), 1692 ± 3 Ma (3a-iii) and 1705 ± 3 Ma (3a-i). At location 2.5j, zircon populations of Eastern Segment age are observed between 1850 and 1650 Ma (2.5j-i) and around 1685 Ma (2.5j-iv). Both samples show multiple zircon populations (including Hallandian signatures) and complex field relationships which require closer examination. This study shows the presence of Eastern Segment-aged units and Hallandian signatures within B-Bulan, further west than previously acknowledged.