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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 intrusive complex located ∼ 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–olivine 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/YbN = 2.8–3.6. Minor tholeiitic rock types are: (a) low-Ti basalt (∼ 49.7 wt.% SiO2, ∼ 1.7 wt.% TiO2, ∼ 6.8 wt.% MgO, ∼ 2.6 ppm Nb and La/YbN = ∼0.5) akin to oceanic basalts; (b) very-high-Ti basalt (∼ 48.6 wt.% SiO2, ∼ 4.1 wt.% TiO2, ∼ 5.1 wt.% MgO and ∼ 21 ppm Nb); and (c) plagioclase ultraphyric basalt. The tholeiitic dolerites are cut by alkali 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/YbN = 16–17) sills and dykes. Modelling of high-field-strength and rare-earth elements indicate that the high-Ti basalts formed from ∼ 6–10% melting of approximately equal proportions of garnet-and spinel-bearing mantle of slightly depleted composition beneath thick continental lithosphere. Conversely, dolerite intrusions and flood basalts of similar compositional kindred from adjacent but more rift-proximal occurrences in Northeast Greenland formed from shallower melting of dominantly spinel-bearing mantle beneath extended and thinned continental lithosphere. These variations in lithospheric thickness suggest the continent–ocean transition of the East Greenland rifted volcanic margin is sharp and narrow. 40Ar–39Ar dating and paleomagnetism show that the high-Ti dolerites were emplaced at ∼53–52 Ma (most likely during C23r) and hence surprisingly postdate the main flood volcanism by ∼ 2–5 Ma and the inception of seafloor spreading between Greenland and Europe by ∼ 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 Iceland 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 zone westward towards the plume axis, may have triggered post-breakup continental magmatism of the Jameson Land Basin and the inner Scoresby Sund region, along preexisting structural lineaments. © 2000 Elsevier Science B.V. All rights reserved. |
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