Episodic volcanism during break-up of the North Atlantic: evidence from the East Greenland plateau basalts

The lower Tertiary (upper Paleocene/lower Eocene) plateau basalts in the Scoresby Sund region of East Greenland formed during three distinct magmatic episodes. The activity of the first episode was centred in the present inland areas to the northwest and produced a voluminous regional basalt sequence (lower sequence). The activity of the second episode was centred to the southeast, offshore the present Atlantic coast, and this also produced a voluminous regional basalt sequence (upper sequence). The third episode was centred on the Atlantic coast and gave rise to a dense coastal dyke swarm and local lavas which are now only locally preserved by downfaulting. The lavas are all tholeiitic basalts. After an initial phase the two regional lava sequences of the first and second episodes show a similar vertical compositional zonation pattern: a thin lower formation with variable composition is followed by a thick uniform formation which shows systematic compositional change with height, from TiO₂-rich "FeTi" basalts to TiO₂-"poor" basalts, and ending with a reversal to TiO₂-rich basalts. Each of these cyclic patterns apparently resulted from a temporary rifting episode, and much of the compositional variation found can be explained by magma development in open magma chambers. The dykes and lavas of the third episode of activity have chemical affinities to ocean floor basalts, and their TiO₂/P₂O₅ ratios indicate formation at a spreading ridge centre. The pattern of two failed rifting episodes and one event that produced oceanic crust can be correlated with stages in the complicated initiation of seafloor spreading in the region, and the volcanism itself is closely related to this process.