Was a short-lived Baffin Bay plume active prior to initiation of the present Icelandic plume? Clues from the high-Mg picrites of West Greenland

Abstract

The Tertiary volcanism of the North Atlantic region provides one of the most extensive and best described magmatic records of interaction between a starting plume and continental lithosphere, and the rôle of plumes in continental break-up. Many authors consider the entire volcanic province, from Baffin Island in the north-west to Scotland in the south-east, to be the product of a single “proto-Icelandic” starting-plume head initially centred beneath East Greenland, whose initial eruptions were strongly influenced by the pre-break-up structure of the continental lithosphere. We demonstrate that the presence of Mesozoic-Palaeocene extensional sedimentary basins that developed within older N-S trending mobile belts influenced the location of the earliest on-shore Tertiary basalts. Even taking this into account, however, the prevalence of high-magnesium picrites among volcanics bordering the Davis Strait in the extreme north-west of the province, and their much lower abundance in East Greenland directly above the supposed plume axis, contradict the single-plume model. Best estimates of the most refractory liquid compositions erupted in the Disko area imply potential temperatures beneath West Greenland of 1540–1600°C. Such high temperatures are more consistent with the axial region of a separate, hot plume beneath West Greenland than with the periphery of the Icelandic plume head. West Greenland picrites cannot be attributed to an expanding “hot doughnut” as they are not present in other peripheral areas likely to be swept by such a doughnut. Uniformly high FeO contents and comparison with McKenzie-Bickle melting parameterisations suggest the West Greenland picrites are the product of ∼ 25% melting constrained to depths of 60–90 by a lithospheric lid. Geographical zonation in incompatible element and radiogenic isotope enrichment (Holm et al., 1993) provides support for a separate, somewhat earlier plume centred beneath Baffin Bay. Picrites from Svartenhuk Halvø (north) are more enriched than equivalent picrites on Disko and Baffin Island (south). This pattern, reminiscent of trends along the Reykjanes Ridge, would be radial to a plume axis beneath Baffin Bay, but not to one centred beneath East Greenland.