Geochronology of the Gloserheia pegmatite, Froland, southern Norway

Relatively unaltered feldspars from the Gloserheia pegmatite yield an Rb/Sr isochron age of 1062 + or - 11 m.y. and a Pb/Pb isochron age of 1085 + or - 28 m.y. The best age of pegmatite crystallization is given by a U/Pb concordia-discordia intersection at 0521060 ⁺⁸_-6 m.y. from euxenite samples. The radiometric ages limit the last major metamorphism in the area to >1060 m.y., although mild later hydration has reset Rb/Sr mineral dates on muscovite and biotite to as low as approx 850 m.y. Isotope data for O, Sr and Pb indicate that the pegmatitic fluids were derived from a mantle source.-G.R.