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Abstract
The rock suite from which the 1450±53 Ma Rb-Sr whole rock isochron age (λ= 1.39×10-11 a-1) for the ‘Varberg charnockite’ was derived by Welin & Gorbatschev (1978: Geologiska Föreningens i Stockholm Förhandlingar 100, 225–227) includes examples of both metamorphic and intrusive charnockite. Both types, however, were formed in the same petrogenetic period in the geological history of the region when the cover and basement sequences were both affected by a cycle of igneous activity during which three distinct pulses of mantle-derived gabbroic magma were intruded, accumulations of melt derived from deeper crustal levels were emplaced as differentiated charnockitic adamellite-rapakivi granite complexes, and thermal metamorphism of the metamorphic basement rocks produced localised developments of depleted granulites and charnockites in situ. Tectonism ultimately replaced this period of magmatic activity and the products of the earlier stages were deformed and metamorphosed to a varying extent depending on their location in the cover-basement sequences. The isochron rocks are all from within the basement segment which was that least affected in the late orogenic stage and may, as Welin & Gorbatschev suggested, give the intrusion age. The anorogenic igneous activity is closely similar in style, component composition and age to that of the late Proterozoic of North America and may be its eastern extension to the Baltic Shield.