Potassium-argon ages for basic dykes from East Finnmark: stratigraphical and structural implications

Abstract

Synopsis K–Ar age determinations for basic dykes from both sides of the Trollfjord-Komagelv fault zone of eastern Finnmark fall into three groups. Group A dykes occur on both sides of the fault zone, in the autochthon and in the Raggo and Barents Sea Groups. They are post-tectonic and give an age of 340 m.y. (Upper Devonian). Group B dykes occur in the Båtsfjord area of the Barents Sea region. They cut the thrust between the Raggo and Barents Sea Groups and give ages of about 640 m.y. One dyke (B2) from the Raggo Group, is cleaved and gives an age of 542 m.y. Group C dykes are syntectonic within the Barents Sea Group and give ages of more than 1,000 m.y. but the low potassium content of all but one (1,950 m.y.) dyke reduces the significance of these ages. The age of the Group B dykes shows that the folding and thrusting of the Barents Sea and Raggo Groups of the Båtsfjord area is significantly older than deposition of apparently similarly deformed Cambro-Ordovician of the autochthon. Therefore the Trollfjord-Komagelv fault zone separates two regions, with distinct early geological histories, possibly within different plates. Later, early Ordovician deformation, Silurian sedimentation and late Silurian/early Devonian deformation progressively affected both regions as movement along the fault zone from Ordovician to Devonian times brought the two regions together.