Abstract

ODP Leg 152 drilled a transect across the SE Greenland seaward-dipping reflector sequence. The earliest, and most landward, lava flows (Hole 917A, Lower and Middle Series) represent a pre-break-up continental sequence showing an overall variation from olivine basalt to dacite. Evolution in deep crustal magma reservoirs, with a dwindling supply of primitive magma and an increasing role of crustal contamination, can account for the variation. Eruption of picrite and olivine basalt magmas (917A, Upper Series) marked a dramatic change in the style of magmatism to one of unrestrained passage of primitive magma from the mantle to the surface during the final phase of break-up. The younger parts of the seaward-dipping reflector sequence (915A and 918D) comprise compositionally uniform basalt (7.6 ± 0.8% MgO), suggesting that an effective magmatic filtering system was established, soon after break-up, in magma chambers associated with a spreading axis. The continental to oceanic magmatic transition is marked by clear changes in composition. The continental magmas had a garnet lherzolite source (indicated by low Y/Zr) and were contaminated with continental lithospheric mantle and crust (high Ba/Zr). The majority of the break-up and oceanic volcanic rocks show no evidence for lithospheric contamination (low Ba/Zr) and appear to have had a source in progressively shallower, depleted (?MORB-source) asthenosphere (increasing Y/Zr, low Nb/Zr).

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