Rb-Sr age of granitoid gneisses in the “Pregothian” area of south-western Sweden

Abstract

Grey granodioritic to tonalitic gneisses in the migmatite terrain of south-western Sweden, sampled along a traverse between Vara and Vänersborg, yield a six-point isochron Rb-Sr age of 1735 ± 85 Ma at an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7027 ± 0.0008. This large-sample whole-rock age fails to support the concept of an early, Presvecokarelian development of the migmatites in this part of the “Pregothian”. It suggests a major geological event of age and, in comparison, only subordinate influence in this area of later events (900 Ma, 1250 Ma, etc.), which do not reset the isotope system on a regional scale. The low initial Sr-isotope ratio argues against a long prehistory of upper crustal evolution of the dated rocks.