Mantle sources and melting dynamics in the British palaeogene igneous province

The North Atlantic igneous province offers an unrivalled opportunity to stu dy mantle sources contributing to flood basalt magmatism, and melting dynam ics 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 Lundy Island in the Sout h) are shown to have concentrations of Nb, Zr and Y consistent with derivat ion from two mantle sources : 'Icelandic' (plume) mantle and hot N-MORB-lik e mantle forming an outer envelope to the plume. These sources were sampled over the period 61-58 Ma (chrons 26R-26N). Values of Delta Nb-an expressio n 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 Br itish Isles (up to 1300 km from the plume axis) increased and then decrease d relative to the contribution from the N-MORB source. Within the British I sles, subsidence data and basalt compositions suggest that melting to gener ate the parent magmas of the bulk of Palaeocene basalts occurred beneath in tact lithosphere. Melting began at depths well in excess of 100km, made pos sible by the high temperature (similar to 1550 degrees C) of the ancestral Iceland plume. In the final stages of magmatism, depleted melt fractions we re generated beneath the Rockall Trough and other basins to the NW of the B ritish Isles, at depths as shallow as 55km. These melt fractions were extra cted rapidly from the mantle, without undergoing significant mixing with me lt generated deeper in the melting column. The result is a distinctive magm a type (Preshal More or Central Mull Tholeiite) not observed in the more so utherly parts of the British Palaeogene igneous province.