Mantle Sources and Melting Dynamics in the British Palaeogene Igneous Province

Abstract

The North Atlantic igneous province offers an unrivalled opportunity to study mantle sources contributing to flood basalt magmatism, and melting dynamics associated with active and passive upwelling of hot mantle beneath the lithosphere. In this study, Palaeogene basalts sampled at localities across the British Isles (from the Hebrides in the north to Lundy Island in the south) are shown to have concentrations of Nb, Zr and Y consistent with derivation from two mantle sources: ‘Icelandic’ (plume) mantle and hot N-MORB-like mantle forming an outer envelope to the plume. These sources were sampled over the period 61–58 Ma (chrons 26R–26N). Values of ΔNb—an expression of the deficiency or excess of Nb relative to the lower bound of the data array for Icelandic basalt in Nb–Zr–Y space—indicate that, with time, the proportion of ‘Icelandic’ material entering the melting regime below the British Isles (up to 1300 km from the plume axis) increased and then decreased relative to the contribution from the N-MORB source. Within the British Isles, subsidence data and basalt compositions suggest that melting to generate the parent magmas of the bulk of Palaeocene basalts occurred beneath intact lithosphere. Melting began at depths well in excess of 100 km, made possible by the high temperature (∼1550°C) of the ancestral Iceland plume. In the final stages of magmatism, depleted melt fractions were generated beneath the Rockall Trough and other basins to the NW of the British Isles, at depths as shallow as 55 km. These melt fractions were extracted rapidly from the mantle, without undergoing significant mixing with melt generated deeper in the melting column. The result is a distinctive magma type (Preshal More or Central Mull Tholeiite) not observed in the more southerly parts of the British Palaeogene igneous province.