Rifting and Mafic Magmatism in the Hebridean Basins

Basalt dykes from the regional dyke swarm of the British Palaeogene Igneous Province (BPIP) were emplaced parallel to structural lineaments linking onshore and offshore volcanic edifices. Basalts which underwent minimal interaction with the crust, have Mg# 60-75, εNd58 c. 8, 206Pb/204Pb c. 17.5, δ18O 5.9±0.3‰, and 87Sr/86Sr <0.7040. Basalts with convex- upwards REE profiles ([La/Sm]N <1; [Sm/Yb]N >1) were generated by limited extents of melting (<10%) in the garnet-spinel transition of the upper mantle. Basalts with LREE depleted ([La/Sm]N <1) or flat REE profiles require substantial (up to 20%) melting of spinel lherzolite. Modelling of major element compositions and olivine equilibration temperatures indicates that the mantle potential temperature was a maximum of 1530°C beneath the BPIP at 58-60 Ma. Magmatism occurred at the periphery of a mantle thermal anomaly (proto-Iceland plume; TP ≤ 1560°C) centred beneath western Greenland. The distribution of BPIP magmas was controlled by extensional tectonism driven by plate boundary forces resulting from plate reorganizations in the northern hemisphere starting at c. 62 Ma. The well-known mildly alkaline lava piles of Skye and Mull represent volcanoes on the flanks of the resulting rift system.