Abstract

Rb–Sr whole rock isochron dates of 1780 ± 43 m.y. and 1731 ± 73 m.y. (λ87Rb 1.39 × 10−11yr−1) have been measured on basal granitic gneisses which underlie Caledonian metamorphic rocks in the Glomfjord-Nasafjåll tract of Norway. Rb–Sr data from a part of the region can be interpreted in terms of localized Sr isotope homogenization at around 1200 m.y., but severe Caledonian deformation and metamorphism has had little apparent effect on the whole rock systems on the scale of sampling. The dates are discussed in the context of published Rb–Sr and U–Pb dates from the Baltic Shield, of which a distribution map is given. It is concluded that the gneisses investigated attained their pre-Caledonian form during the Svecofennian orogeny, but may have been affected in part by the Sveconorwegian orogeny.

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