Geochemistry and Rb-Sr dating of trondhjemite dykes from the Gula Group, near Snøan, Sør-Trøndelag.
Trondhjemite dykes forming part of a swarm cutting the Gula Group metasediments and tectonically overlying Støren Group metabasalts in the district south of Støren, Sør-Trøndelag, have been investigated in road-cuts in the Snøan area. Geochemically, the fine-grained dykes are high-Al2O3 trondhjemites which show clear trace and rare-earth elements signatures indicative of a continental margin setting. An attempt at Rb-Sr radiometric dating produced a best-fit isochron of 465 +- Ma, interpreted as a minimum age for the dyke swarm. The dyke has also probably been affected by thermal resetting. The dykes do not penetrate the Lower Hovin Group succession which unconformably overlies the Støren Group volcanites, and contains fossils of Late Arenig age. The Rb-Sr minimum age is thus in reasonable accord with the faunal evidence. The trondhjemite dyke swarm was evidently emplacement into a thickened conti- nental margin crust following obduction of the Støren ocean-floor basalts upon the Gula, with coeval deformation, metamorphism and subsequent uplift, probably in earliest Ordovician time.