40Ar/39 Ar geochronological constraints on timing of deformation and metamorphism of the Central Norrbotten Caledonides, Sweden

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Abstract
40Ar/39Ar geothermochronology is especially useful within the Scandinavian Caledonides, which experienced deformation and metamorphism as a result of two major tectonothermal pulses during the Early and Middle Palaeozoic. This detailed 40Ar/39Ar geochronological study of the Central Norrbotten Caledonides allows examination of the timing of tectonothermal activity within the Lower Köli Nappe, the Seve–Köli shear zone, the Seve Nappe and the shear zone rocks of the Middle Allochthon. The results obtained from a suite of 19 samples from the study area can be incorporated with structural and metamorphic constraints to establish the geological history of the region. Results from this study include: (1) the high grade metamorphism and associated deformation of the Seve units was a Late Cambrian to Early Ordovician event (Finnmarkian) in which the rocks cooled below the respective closure temperatures for hornblende at ≈ 490 Ma and muscovite at ≈ 454 Ma; (2) assuming a simple linear cooling model a cooling rate of 3–6°C/Ma was obtained for the older tectonothermal event; (3) there is evidence for late stage Finnmarkian (450 Ma), relatively high grade shear zones separating different tectonostratigraphic elements within the Seve; (4) the Scandian phase of deformation and metamorphism partially reset some of the Seve hornblendes and most of the muscovites, which indicates that the rocks affected by the Finnmarkian event felt a second tectonothermal pulse of more than 350°C beginning at ≈ 430 Ma; and (5) during the Scandian event all of the far travelled allochthonous tectonic units were juxtaposed and the Middle Allochthon mylonites were formed as these nappes were emplaced above the Baltic Shield. The tectonic units of the Singis‐Nikkaluokta transect were assembled before regional cooling through the closure temperature of muscovite.