TIMING STRUCTURAL ASSEMBLY, METAMORPHISM, AND COOLING OF CALEDONIAN NAPPES IN THE OFOTEN-EFJORDEN AREA, NORTH NORWAY - TECTONIC INSIGHTS FROM U-PB AND AR-40 AR-39 GEOCHRONOLOGY/

U-Pb and Ar-40/Ar-39 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-E fjorden area of north Norway, the Narvik Nappe Complex [NNC] represent s the basal portion of the composite Caledonian allochthon and consist s of amphibolite-facies oceanic-affinity metasedimentary rocks intrude d by numerous pretectonic felsic dikes. Two of these dikes yield indis tinguishable U-Pb zircon ages of 437 +/- 1 Ma, providing a maximum age for tectonic assembly, metamorphism, and deformation at this structur al level. U-Pb ages of metamorphic monazite (432 +/- 2 Ma) and zircon (431.7 +/- 0.5 Ma) from pelitic schist and migmatite yield robust esti mates of the age of peak metamorphism. Combined with structural eviden ce for transport-parallel stretching and subvertical attenuation of th e rock mass at the time of metamorphism, these ages suggest that local and/or episodic extensional modification of the nappe architecture ma y have begun as early as ca. 432 Ma, as convergence between Baltica an d Laurentia continued. Thermochronologic data indicate cooling at a ti me-integrated rate of 8-10 degrees C/myr from ca. 432 to 400 Ma, follo wed by slower cooling at 2-3 degrees C/myr from ca. 400 Ma to 360 Ma. Slow cooling during early Devonian time suggests gradual exhumation an d is not consistent with significant tectonic unroofing at higher crus tal levels. The late orogenic evolution of this portion of the Caledon ides, therefore, contrasts with that of southwestern Norway where uppe r crustal extension was clearly an important process in unroofing midc rustal rocks during the Devonian period. Variations in the timing, sty le, and magnetude of unroofing in different parts of the Caledonian hi nterland make necessary a re-examination of the ''orogenic collapse'' concept and its widely accepted role in Caledonian tectonics.