Palaeogene continental to oceanic magmatism on the SE Greenland continental margin at 63°N: A review of the results of Ocean Drilling Program Legs 152 and 163

Drilling along a 63°N transect off SE Greenland during Ocean Drilling 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 from the transect were erupted in a subaerial environment. ⁴⁰Ar-³⁹Ar dating shows that the earliest magmas were erupted at ~61 Ma and has confirmed that the main part of the SDRS was erupted during C24r (56-53 Ma) following continental break-up. Magma represented by the pre break-up lava flows was stored in crustal reservoirs where it evolved by fractional crystallization and assimilation of continental crust. Trace element and radiogenic isotope data show that the contaminant changed, through time, from lower-crustal granulite to a mixture of granulite and amphibolite, suggesting storage of magma at progressively shallower levels in the crust. The degree of contamination declined rapidly as break-up proceeded, and the youngest rocks sampled in the transect are uncontaminated by continental basement. Variation of, for example, Sc/Zr and Sm/Lu through the succession suggests a shallowing of the top of the mantle melting zone, accompanied by an increase in the average degree of melting with time from ~4% to ~12%. These modest degrees of melting imply mantle temperatures only ~100°C hotter than normal upper mantle. Upwelling mantle must therefore 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 flows of 'Icelandic' basalt were erupted in the pre-break-up phase. In contrast, 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.