Timing structural assembly, metamorphism, and cooling of caledonian nappes in the ofoten-efjorden area, north Norway: Tectonic insights from U-Pb and ⁴⁰Ar/³⁹Ar geochronology

U-Pb and ⁴⁰Ar/³9Ar geochronology from part of the northern Norwegian Caledonides indicates a cooling and unroofing history different from that inferred for the Caledonides in southwest Norway. In the Ofoten-Efjorden area of north Norway, the Narvik Nappe Complex (NNC) represents the basal portion of the composite Caledonian allochthon and consists of amphibolite-facies oceanic-affinity metasedimentary rocks intruded by numerous pretectonic felsic dikes. Two of these dikes yield indistinguishable U-Pb zircon ages of 437 ± 1 Ma, providing a maximum age for tectonic assembly, metamorphism, and deformation at this structural level. U-Pb ages of metamorphic monazite (432 ± 2 Ma) and zircon (431.7 ± 0.5 Ma) from pelitic schist and migmatite yield robust estimates of the age of peak metamorphism. Combined with structural evidence for transport-parallel stretching and subvertical attenuation of the rock mass at the time of metamorphism, these ages suggest that local and/or episodic extensional modification of the nappe architecture may have begun as early as ca. 432 Ma, as convergence between Baltica and Laurentia continued. Thermochronologic data indicate cooling at a time-integrated rate of 8-10°C/myr from ca. 432 to 400 Ma, followed by slower cooling at 2-3°C/myr from ca. 400 Ma to 360 Ma. Slow cooling during early Devonian time suggests gradual exhumation and is not consistent with significant tectonic unroofing at higher crustal levels. The late orogenic evolution of this portion of the Caledonides, therefore, contrasts with that of southwestern Norway where upper crustal extension was clearly an important process in unroofing midcrustal rocks during the Devonian period. Variations in the timing, style, and magnetude of unroofing in different parts of the Caledonian hinterland make necessary a re-examination of the "orogenic collapse" concept and its widely accepted role in Caledonian tectonics.