Palaeogene continental to oceanic magmatism on the SE Greenland continental margin at 63 degrees N: a review of the results of ocean drilling programlegs 152 and 163

Drilling along a 63 degrees N transect off SE Greenland during Ocean Drilli ng Program (ODP) Legs 152 and 163 recovered a succession of volcanic rocks representing all stages in the break-up of the volcanic rifted margin. The rocks range from pre-break-up continental tholeiitic flood basalt, through syn-break-up picrite, to truly oceanic basalt forming the main part of the seaward-dipping reflector sequence (SDRS). All the lava flows recovered fro m the transect were erupted in a subaerial environment. Ar-40 Ar-39 dating shows that the earliest magmas were erupted at similar to 61 Ma and has con firmed that the main part of the SDRS was erupted during C24r (56 53 Ma) fo llowing continental break-up. Magma represented by the pre-break-up lava fl ows was stored in crustal reservoirs where it evolved by fractional crystal lization and assimilation of continental crust. Trace element and radiogeni c isotope data show that the contaminant changed, through time, from lower- crustal granulite to a mixture of granulite and amphibolite, suggesting sto rage of magma at progressively shallower levels in the crust. The degree of contamination declined rapidly as break-up proceeded, and the youngest roc ks sampled in the transect are uncontaminated by continental basement. Vari ation of, for example, Sc/Zr and Sm/Lu through the succession suggests a sh allowing of the top of the mantle melting zone, accompanied by an increase in the average degree of melting with time from similar to 4% to similar to 12%. These modest degrees of melting imply mantle temperatures only simila r to 100 degrees C hotter than normal upper mantle. Upwelling mantle must t herefore have been fed dynamically to the melt zone to generate the igneous crust of 18 km thickness deduced from seismic and gravity studies. N-MORB- like magmas dominated the earliest part of the succession although a few fl ows of 'Icelandic' basalt were erupted in the pre-break-up phase. In contra st, the post-break-up magmas had an Icelandic mantle source. This suggests that the developing head of the ancestral Iceland plume was compositionally zoned, with a core of Icelandic mantle surrounded by a thick outer zone of hot, depleted upper mantle.