Rb-Sr whole rock ages from Magerøy, North Norwegian Caledonides

Two Rb-Sr whole rock isochrons from the Gjesvær Migmatite Complex (410 ± 28 m.y., IR:0,75472 ± 0,0015) and the Finnvik Granite (411 ±7 m.y., IR:0,7116 ±0,0010) within the orthotectonic Caledonides of northernmost Norway are presented. The syn-orogenic Finnvik Granite cuts folded Llandoverian strata of the Magerøy Nappe and was intruded coevally with the peak of the Scandinavian Phase regional metamorphism. This granite thus occupies a critical position for the dating of the Scandinavian Phase of the Caledonian Orogeny in the region. The regional metamorphism during this phase resulted in Rb-Sr isotope homogenization in the older Gjersvær Migmatite Complex which
underlies the basal thrust of the Magerøy Nappe. The re-set age of this complex gives the same age as the syn-metamorphic Finnvik granite. A model age calculation for the migmatites gives 1335 m.y., indicating that the provenance rocks of the sedimentary protolith to the Gjesvær Migmatite Complex were probably an old Precambrian terrain. The age of the Scandinavian Phase in the Scandinavian Caledonides is discussed in relation to the chrono- and biostratigraphic evidence presently available.