Mesozoic to Palaeogene intrusive igneous rocks in West Greenland range from a large, coast-parallel dyke swarm to small, poorly defined dyke swarms or single intrusions. New age and geochemical data indicate that intrusion forms and melt compositions changed with time, dependent on changing stress fields and increasing lithospheric attenuation. During the period c. 220–150 Ma (Late Triassic to Late Jurassic) incipient stretching is reflected in the production of highly alkaline, volatile-rich melts formed in small volumes in the deep lithosphere. Around 150 Ma (Kimmeridgian), increased extension took place and melts were intruded in a 60 km long swarm of scattered alkaline dykes. In the Early Cretaceous, 140–133 Ma, the regional stress field was intense, upwelling asthenospheric mantle started to melt, and alkali basaltic magmas were emplaced in a 400 km long coastal dyke swarm parallel to large linear faults offshore. In the Palaeocene, continental break-up took place and flood basalts (62–60 Ma) were extruded in the Nuussuaq Basin. Large basalt sills and dykes extend the region with Palaeocene activity 150 km southwards and form a link between the Nuussuaq Basin and the Sisimiut Basin offshore. Dykes with ages of 57–51 Ma indicate widespread younger volcanic activity.
Sample details, Ar/Ar data and plots, and Rb–Sr isochrons are available at http://www.geolsoc.org.uk/SUP18374.