Potassium and rubidium values are reported for a suite of 181 metabasites from the amphibolite–granulite transition zone in the Precambrian Bamble sector of south Norway. Both alkalis are more abundant, and the K/Rb ratios lower, in the rocks from the amphibolite facies. Rubidium has been strongly fractionated with respect to potassium.

The K/Rb ratios exhibit a regular and systematic variation and this defines a linear trend which is markedly different from established igneous fractionation trends. The chemical fractionation occurred as a result of the metamorphism, and the chemical changes are in keeping with the concept of a vertical chemical fractionation within the continental crust. Similar linear trends established in other high grade metamorphic terrains may have the same (metasomatic ?) origin.

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