U-Pb zircon dating of the Blåfjellhatten granite in the Olden Nappe, Grong-Olden Culmination, Central Norwegian Caledonides, has yielded a discordia upper intercept of 1633.2 ± 2.9 Ma, interpreted as the crystallisation age of the pluton. The precise age is comparable to an approximate age of emplacement, 1650 Ma, reported for the neighbouring Olden granite but contrasts with an earlier, Rb-Sr whole-rock, isochron age of 1356 ± 29 Ma for the Blåfjellhatten body. Geochemical data indicate that the Blåfjellhatten granite was derived from a fairly evolved crustal source in a continental, extensional, and probably anorogenic setting. Both the Blåfjellhatten and the Olden granite are representative of I-type granitoids (though with some A-type features) of the youngest magmatic episode of the Palaeoproterowic, Transscandinavian Igneous Belt (TIB). The Blåfjellhatten granite thus appears to be one of the youngest plutons in the TIB domain of the Fennoscandian Shield, extending the terminal stages of TIB magmatism by some 1 5-20 million years.
David Roberts & August L Nissen, Norges Geologiske Undersøkelse, 7491-Trondheim, Norway;
Nicholas Walker, Institute of Geological and Nuclear Sciences, 764 Cumberland Street, Private Bag 1930, Dunedin, New Zealand