Summary
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.
