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An Archaean geotransect is well preserved within the West Troms Basement Complex, a basement outlier west of the Caledonides in North Norway, whose structural architecture is the finite result of both Archaean and Palaeoproterozoic (Svecofennian) tectonic events. Geochronology by U-Pb TIMS and SIMS of nine samples of magmatic and migmatitic rocks collected in a transect perpendicular to the NW-SE strike of the complex record three main stages of Archaean magmatism and one superimposed, Neoarchaean, metamorphic high-grade event. The oldest Mesoarchaean ages of 2.92-2.80 Ga are found within a paired tonalitic complex and greenstone belt in the northeast. These rocks are separated by a high-grade, ductile shear zone from younger Neoarchaean rocks with only local, inherited, Mesoarchaean zircons. The period from 2.75-2.70 Ga was particularly active in the rest of the complex, with intrusion of diorite-granodiorite plutons on Kvaloya and Senja, followed by local migmatisation and another pulse of diorite-granite magmatism between 2.70 and 2.67 Ga. This period was concluded by the intrusion of mafic dykes at 2.671 Ga, documented in one locality on Kvaloya. A latest Neoarchaean age for stromatic migmatite or metamorphism is indicated in two localities by secondary zircon in neosome, with restitic zircon derived from the 2.70-2.67 Ga event. The Archaean rocks were variably reworked, metamorphosed and intruded by felsic and mafic plutons during the Svecofennian (1.8-1.7 Ga) orogeny, and locally also formed the substrate to Palaeoproterozoic supracrustal rocks. Possible correlatives are found in the Lofoten and Vesteralen area to the south and in the Karelian and Norrbotten provinces in the Fennoscandian Shield. Given the tectonostratigraphic position of the West Troms Basement Complex, correlations with Archaean provinces elsewhere in the North Atlantic region are also possible. [ABSTRACT FROM AUTHOR]

The archipelago of Svalbard records several stages of the Caledonian Wilson cycle: Late Precambrian to Ordovician sediments of the Laurentian passive margin, high-pressure metamorphic nappe complexes that record Ordovician continent-ocean collision, and a major phase of continent-continent collision which led to regional metamorphism. Eventually this gave way to orogenic collapse and formation of Old Red Sandstone basins, and rifting in the Carboniferous as well. The Svalbard outcrops provide a link between the Atlantic and the Arctic Caledonides and the key to understand what happened along strike at the intersection of the mountain belt with the Arctic Ocean realm. Concerning the timing of subduction, continent-continent collision and orogenic collapse, the Svalbard records of these events should be accounted for in models of the Caledonian Wilson cycle as a whole.We present new CA-ID-TIMS U-Pb-ages for orthogneiss and migmatite from Nordaustlandet in north-eastern Svalbard that demonstrate a three-stage orogenic evolution of these rocks: Intrusion of granite in the Mid-Silurian, followed by a phase of deformation and final anatexis in the Early Devonian. While bracketing Late Silurian deformation directly, these ages also demonstrate a temporal correlation between the Caledonian thermal phase and extension. The migmatites appear to have formed by partial melting of granitic augen gneisses in the presence of a H2O-bearing fluid phase. Anatexis occurred at relatively low temperatures at or near the solidus, and field evidence such as lack of garnet and medium grade mineral assemblages in the pelitic anatectic units speak towards a relatively shallow crustal level for the migmatisation. Recently, a major shear zone in north-west Svalbard, separating the Old Red Sandstone and migmatites, was re-interpreted as a Late Silurian to Mid-Late Devonian extensional detachment (Braathen et al., 2018). If such a tectonic process was active across a larger area, extension-related exhumation might provide a mechanism for the anatexis dated here and correlative migmatite regions in the archipelago.The new data and interpretation highlight some unresolved questions pertaining the across-strike metamorphic gradients of the Caledonides of Svalbard. While several large shear zones are mapped, the knowledge on the dynamics in the waning stages of the Caledonian orogeny and subsequent rifting here is still lacking.Braathen, A., Osmundsen, P. T., Maher, H., & Ganerød, M. (2018). The Keisarhjelmen detachment records Silurian–Devonian extensional collapse in Northern Svalbard. Terra Nova, 30(1), 34–39. https://doi.org/10.1111/ter.12305 [ABSTRACT FROM AUTHOR]