Temporal variations in crustal assimilation of magma suites in the East Greenland flood basalt province: Tracking the evolution of magmatic plumbing systems

Abstract

We review published radiogenic isotope data (> 350 samples in total) on various suites of magmatic rocks within the Palaeogene central East Greenland flood basalt province to evaluate the types of crustal assimilants and the extent of crustal assimilation involved in each suite. We use these observations to build a regional picture of how magmatic plumbing systems changed with time and location during the sequential development of the province as magmatism responded to the development of a volcanic rifted margin and eventual plate separation. The earliest phase of magmatic activity (c. 62–57 Ma) is characterised by highly contaminated magmas that show a temporal change in assimilant type from amphibolite to granulite. This transition has been linked to the effects of an increasing magma supply rate which allows the more refractory granulite lithologies to be melted. The voluminous break-up phase of magmatism (c. 56–54 Ma) saw a significant decrease in the extent of assimilation because of the decreasing availability of assimilant material in the mature feeder systems, and many samples have Sr–Nd–Pb isotope compositions that overlap with those of asthenospheric melts (as represented by recent Icelandic basalts and North Atlantic MORB). Detailed study has allowed us to recognise packets of lavas that ponded at different levels in the crust and assimilated material of different compositions. The later stages of break-up magmatism show more diverse and more contaminated compositions that indicate a shift from a few large robust feeder systems to numerous small new conduits as the rifting continued. The post-break-up magmatism (c. 54–13 Ma) is characterised by a return to more highly contaminated magmas, which reflects a change in the style of magmatism: the eruption of small-volume alkalic lava flows from newly established conduits through the thicker inland crust, and the intrusion of mafic and silicic alkalic magmas at shallow levels in the Archaean basement along the present coast.