U–Pb geochronology and geochemistry of early Proterozoic rocks of the tectonic basement windows in central Nordland, Caledonides of north-central Norway

U–Pb dating of Proterozoic rocks of the tectonic basement windows in the Caledonides of north-central Norway has provided new information of the westernmost margin of the Fennoscandian Shield. Four granitic gneisses dated along a 70-km profile in central Nordland have yielded ages in the interval 1800–1795 Ma. These ages are similar to U–Pb ages from the Lofoten–Vesterålen area and the Rombak Window in northern Norway. The granitoid rocks are also coeval with the oldest group of granitoids in the Trans-Scandinavian-Igneous Belt (TIB) in southern Sweden (1.81–1.76 Ga) and the late Svecofennian granitoids in central Sweden (1.80–1.77 Ga), confirming the importance of the 1.80 Ga magmatic period at the western parts of the Fennoscandian Shield. The geochemistry of the dated granitoids is consistent with formation in a subduction-related regime. The granitoid rocks are interpreted to have been generated by mixing of mafic melt and a large proportion of melts derived from early Svecofennian granitoids (1.91–1.87 Ga), probably with minor contribution from Archaean crustal material. The new data support a model where the rocks of the basement windows in central Nordland represent the western continuation of the Fennoscandian Shield under the Caledonian nappes. The Sm–Nd isotope data of the granitoid rocks implies that they were generated to the south and west of the Archaean–Proterozoic boundary.