The Prinsen af Wales Bjerge Formation Lavas, East Greenland: the Transition from Tholeiitic to Alkalic Magmatism during Palaeogene Continental Break-up

thick lithospheric cap, with 3 He contributed from volatile-rich fluids We present elemental and isotopic (Sr-Nd-Pb-Hf-Os-He) data from elsewhere in the Icelandic plume. The exact nature of the recycled on primitive alkalic lavas from the Prinsen af Wales Bjerge, component is not yet resolved, although Hf isotope compositions rule East Greenland. Stratigraphical, compositional and 40 Ar-39 Ar data out any significant role for recycled pelagic sediment, and the low indicate that this inland alkalic activity was contemporaneous with 187 Os/ 188 Os limits the participation of recycled basaltic material the upper parts of the main tholeiitic plateau basalts and also postand argues instead for a contribution from the mantle section of the dated them. The alkalic rocks show a marked crustal influence, recycled slab. indicating establishment of new magmatic plumbing systems distinct from the long-lived coastal systems that fed the relatively uncontaminated plateau basalts. The least contaminated lavas have high 3 He/ 4 He isotope ratios (R/R A 124-185), sub-chondritic 187 Os/ 188 Os i (0120-0126), low Nd i (>+4) and Hf i KEY WORDS: alkalic lavas; flood basalts; high 3 He/ 4 He; East Greenland; (>+6) that plot below the 'Nd-Hf mantle array', and trace recycled lithosphere; Iceland plume element characteristics similar to HIMU ocean island basalt (OIB). The uncontaminated magma is inferred to have more radiogenic 206 Pb/ 204 Pb values (>192) than the plateau basalts and Icelandic basalts, and thus represents a possible 'enriched' component to