Polyorogenic (super 40) Ar/ (super 39) Ar mineral age record within the Kalak Nappe Complex, northern Scandinavian Caledonides

The Kalak nappe complex is composed of variably retrogressed, middle Proterozoic basement and imbricated, polydeformed, metasedimentary cover successions. Mafic-alkaline plutons (Seiland igneous suite) occur within the uppermost structural units of the nappe complex. Post-metamorphic cooling dates of approximately 490 m.y. are locally recorded by hornblende within the structurally highest tectonic unit, suggesting significant orogenic activity just before or during the Arenig. A distinctly later metamorphic overprint occurred in the Silurian and rejuvenated Ar systems in muscovite and nepheline (and locally in hornblende). This was followed by relatively rapid post-metamorphic cooling through hornblende and muscovite Ar closure T between approximately 425 and 415 m.y. Together, (super 40) Ar/ (super 39) Ar mineral ages from both the Kalak nappe complex and the central Swedish Caledonides indicate that significant tectonothermal activity occurred along the Baltoscandian margin prior to the Silurian, associated with the formation of the late Cambrian-early Ordovician accretionary wedge over a westward-dipping subduction zone during initial closure of the Iapetus Ocean.