A nine point Rb-Sr total-rock isochron of gneisses from the Tingvoll Group, dose to the contact with schists of supposedly Paleozoic age in the Surnadal district of central Norway, defines an age of 1707 ± 63 m. y. and an initial B7Sr/B6Sr of 0.7026 ± 0.0007. This age is the same as that obtained earlier from the Kristiansund and Frei Groups in the Kristiansund area. Rb, Sr totalrock samples from the Røros Group do not define a unique isochron age, but suggest that the rocks were originally older (1700 m. y.) and were subsequently partially reequilibrated during a younger metamorphism (the Sveconorwegian and/or the Caledonian). This possible old initial age for the Røros Group agrees with field studies where no structural and/or metamorphic beaks between the gneisses and the metasediments of the Surnadal syncline have been observed.
A RB, SR study of the rocks of the Surnadal syncline