Structure, age and formation of dykes on the island of Smøla, Central Norway

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.