Metamorphic rocks of Lofoten, Norway, represent a leading edge of the Baltic craton that was partly subducted beneath Laurentia during Caledonian orogeny. We find that muscovite porphyroblasts in Lofoten record a remarkably complete history of cooling and unroofing and yield data about Lofoten's tectonic evolution in the interval from 425 to 265 Ma. These data confirm that culminating metamorphism of basement and allochthonous rocks in Lofoten occurred during the Scandian phase of orogeny. Permian ages for muscovite are the youngest obtained for regional metamorphic rocks anywhere in Scandinavia and appear to record significant basement unroofing during extensional evolution of the Norwegian-Greenland seaway. Regional age variations of muscovite in Lofoten can be related to present crustal thickness, Devonian to Permian sedimentation in the Norwegian seaway, and extensional structures, and they indicate the timing of multiple extension events in the evolution of Norway's passive margin. The results of this study show that 40Ar closure profiles in porphyroblasts can form even in active tectonic settings characterized by rapid and episodic unroofing.

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