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 Fermoscandian 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-Vesteralen 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 Fermoscandian 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 Fermoscandian 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. (C) 2002 Elsevier Science B.V. All rights reserved.