Composition and age of tertiary sills and dykes, Jameson Land Basin, East Greenland: relation to regional flood volcanism

The Paleozoic-Mesozoic Jameson Land Basin (East Greenland) is intruded by a sill complex and by a swarm of ESE trending dykes. Together with dykes of the inner Scoresby Sund fjord, they form a regional Early Tertiary intrusiv e complex located similar to 200-400 km inland of the East Greenland rifted continental margin. Most of the intrusive rocks in the Jameson Land Basin are geochemically coherent and consist of evolved plagioclase-augite-olivin e saturated, uncontaminated high-Ti basalt with 48.5-50.2 wt.% SiO2, 2.2-3. 2 wt.% TiO2, 5.1-7.4 wt.% MgO, 9-17 ppm Nb and La/Yb-N = 2.8-3.6, Minor tho leiitic rock types are: (a) low-Ti basalt (similar to 49.7 wt.% SiO2, simil ar to 1.7 wt.% TiO2, similar to 6.8 wt.% MgO, similar to 2.6 ppm Nb and La/ Yb-N = similar to 0.5) akin to oceanic basalts; (b) very-high Ti basalt (si milar to 48.6 wt.% SiO2, similar to 4.1 wt.% TiO2, similar to 5.1 wt.% MgO and similar to 21 ppm Nh); and (c) plagioclase ultraphyric basalt. The thol eiitic dolerites are cut by all;ali basalt (43.7-47.3 wt.% SiO2, 4.1-5.1 wt .% TiO2, 4.9-6.2 wt.% MgO, 29-46 ppm Nb and La/Yb-N = 16-17) sills and dyke s. Modelling of high-field-strength and rare-earth elements indicate that the high-Ti basalts formed from similar to 6-10% melting of approximately equal proportions of garnet- and spinel-bearing mantle of slightly depleted comp osition beneath thick continental lithosphere. Conversely, dolerite intrusi ons and flood basalts of similar compositional kindred from adjacent but mo re rift-proximal occurrences in Northeast Greenland formed from shallower m elting of dominantly spinel-bearing mantle beneath extended and thinned con tinental lithosphere. These variations in lithospheric thickness suggest th e continent-ocean transition of the East Greenland rifted volcanic margin i s sharp and narrow. Ar-40-Ar-39 dating and paleomagnetism show that the high-Ti dolerites were emplaced at similar to 53-52 Ma (most Likely during C23r) and hence surpris ingly postdate the main flood volcanism by similar to 2-5 Ma and the incept ion of seafloor spreading between Greenland and Europe bg similar to 1-2 Ma . The formation of tholeiitic and alkaline magmas emplaced into the Jameson Land Basin corroborates to the importance of post-breakup magmatism along the East Greenland volcanic rifted margin. Upwelling of the ancestral Icela nd mantle plume under central Greenland at 53-52 Ma (rather than under the active rift), perhaps accompanied by a failed attempt to shift the rift zon e westward towards the plume axis, may have triggered post-breakup continen tal magmatism of the Jameson Land Basin and the inner Scoresby Sund region, along preexisting structural lineaments. (C) 2000 Elsevier Science B.V. FL U rights reserved.