On the island of Smøla, five different dyke phases are represented in an Ordovician, mature ensialic are
complex. With decreasing relative age these are: granitic dykes and net-veins, composite dykes, porphyritic
microdiorite dykes, dolerite dykes and granophyre dykes. The granophyres have been dated by
the Rb/Sr whole-rock method and yielded a 9 point isochron of 428 ± 10Ma with an initial ratio of
0.70480 ± 0. 0003 and MSWD = 2. 0. From the aspect ratios of the dykes a model is proposed which
suggests that the dykes were formed with a magmatic overpressure of less than 90 MPa. This indicates
that the source of magma was at a maximum depth of 36 km for the basic dykes and up to 15 km for the
granophyres. Theoretical results indicate that a 2 m wide dolerite dyke in the Smøla area solidified within
less than about 70 days and a 10 m wide granophyre within around 5 years. Emplacement of the dyke
swarms resulted in about 35% crustal extension.