Tertiary picrites in West Greenland: melting at the periphery of a plume?

Abstract

Abstract The West Greenland/Baffin Island Tertiary volcanic province differs from other CFB provinces in containing an unusually high proportion (30–50% by volume) of magnesian picritic lavas and hyaloclastites. Olivine-liquid equilibrium considerations suggest the presence during the earlier stages of eruption of picritic melts with MgO contents as high as 20%. Calculations based on McKenzie-Bickle melting models point to high degrees of melting (24–30%) at depths of 60–90 km in the underlying mantle, and require potential temperatures of 1540–1600°C. Such high potential temperatures are inconsistent with reconstructions that attribute the West Greenland volcanism to melting on the margins of the incipient Iceland plume-head. The distribution of Tertiary volcanic activity in Greenland, in particular its relation to Mesozoic-Tertiary extensional basins, indicates that lithospheric structure plays a part in determining where the plume-head can undergo melting. But to explain the restriction of high-temperature picrites to West Greenland, together with their distinctive trace element geochemistry, it is necessary to invoke (a) either an elongated Icelandic plume initially extending as far as West Greenland or a short-lived precursory plume head that developed directly beneath West Greenland, and (b) an active extensional regime that allowed rapid access of picritic melts to the surface.