40Ar/39Ar stepwise degassing age spectra have been obtained from phlogopite phenocrysts from two lamprophyre dykes in the Trondheimsfjord district. Phenocrysts of phlogopite from a previously investigated, unmetamorphosed, post-Scandian lamprophyre on Ytterøy have yielded a complex age spectrum with no clear plateau; however, the calculated 'total fusion' age is 370 Ma, supporting earlier
indications of a probable late-Caledonian (Devonian) age for this dyke. The 'saddle-shaped' release curve for phlogopite from a weakly metamorphosed lamprophyre from the Leksdalsvatn Nappe
near Steinkjer suggests that the rock is younger than c.688 Ma and probably Late Precambrian, although a Cambrian age cannot be excluded. An upper constraint is set by the initial regional metamorphism in this nappe which, based on data from elsewhere in the orogen, may be of Early Ordovician age.