Search

[sup.40][Ar/.sup.39] dating has identified a succession of middle Miocene (14-13 Ma) basaltic lavas in East Greenland that overlie Eocene flood basalts that were erupted during continental breakup ca. 56-55 Ma. The long postbreakup magmatic history (~40 m.y.) of the East Greenland margin precludes a simple relationship between this later igneous activity and the track of the Iceland hotspot. Chemical and isotopic data suggest that the postbreakup magmas were produced from mantle that had been metasomatized by light rare earth element--enriched, [H.sub.2]O- and C[O.sub.2]-bearing melts originating from the Iceland plume. Episodic melting of recently metasomatized shallow mantle beneath Greenland and the North Atlantic can explain both the composition and the long-lived nature of postbreakup magmatism along the East Greenland margin, as well as lavas on Jan Mayen Island that have enriched, Icelandic-type isotopic signatures. Keywords: Greenland, North Atlantic, Iceland, Jan Mayen, Miocene, continental margin, mantle plumes, flood basalts, metasomatism.

Studies have focused on similarities in the thermal and chemical structure of the Tertiary and current-day plumes, arguing for stability of the mantle anomaly. Some authors assume that the plume was rift-centered throughout its history. Compositional data for flood basalts in east Greenland near to the inferred plume axis, is reported. Limits are placed on the pressures and extents of mantle melting, and show that the mantle was initially moderately hot. This is difficult to reconcile with current plume-head models.

The central part of the 65 000 [km.sup.2] lava plateau in East Greenland between Kangerlussuaq and Scoresby Sund has been investigated by multi-model photogrammetry and profile sampling, and a three-dimensional framework for the lava plateau has been established. Post-volcanic movements have divided the region into several structural blocks with dips varying from 1-2 [degrees] N-NE in the inland areas to 12 [degrees] SE at the coast. A major N-S-trending fracture zone may reflect an underlying Caledonian structure. Early volcanic and sedimentary rocks are overlain by widespread plateau basalts, locally with an angular unconformity of 8 [degrees]. The basalt stratigraphy established in the Scoresby Sund area has been extended over most of the region. The plateau basalts were erupted over a zone more than 150 km wide, from the present coast and inland. The succession attains a vertical thickness of more than 5.5 km in the central coastal areas and thins inland to 2-3 km. A differential sagging of the coastal areas by at least 2.3 km took place during the emplacement of the lavas, accelerating with time. The features suggest increasing focusing of the magmatic production into a developing rift zone close to the present coast. Keywords: Greenland, photogrammetry, basalts, continental margin, rifting.

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 [+ or -] 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). Keywords: Greenland, basalts, continental margin, rifting, volcanism.

Combined electron microprobe and secondary ion mass spectrometry methods were used to study sector-zoned arginine crystals from a peralkaline nepheline syenite in Ilimaussaq, South Greenland, for major and trace elements. Sector enrichment regulation depends on the site properties and the capacity of a site to accommodate a cation.