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Sixteen 40 Ar– 39 Ar ages are presented for alkaline intrusions to appraise prolonged post-breakup magmatism of the central East Greenland rifted margin, the chronology of rift-to-drift transition, and the asymmetry of magmatic activity in the Northeast Atlantic Igneous Province. The alkaline intrusions mainly crop out in tectonic and magmatic lineaments orthogonal to the rifted margin and occur up to 100 km inland. The area south of the Kangerlussuaq Fjord includes at least four tectonic lineaments and the intrusions are confined to three time windows at 56–54 Ma, 50–47 Ma and 37–35 Ma. In the Kangerlussuaq Fjord, which coincides with a major tectonic lineament possibly the failed arm of a triple junction, the alkaline plutons span from 56 to 40 Ma. To the north and within the continental flood basalt succession, alkaline intrusions of the north–south trending Wiedemann Fjord–Kronborg Gletscher lineament range from 52 to 36 Ma. We show that post-breakup magmatism of the East Greenland rifted margin can be linked to reconfiguration of spreading ridges in the Northeast Atlantic. Northwards propagation of the proto-Kolbeinsey ridge rifted the Jan Mayen micro-continent away from central East Greenland and resulted in protracted rift-to-drift transition. The intrusions of the Wiedemann Fjord–Kronborg Gletscher lineament are interpreted as a failed continental rift system and the intrusions of the Kangerlussuaq Fjord as off-axis magmatism. The post-breakup intrusions south of Kangerlussuaq Fjord occur landward of the Greenland–Iceland Rise and are explained by mantle melting caused first by the crossing of the central East Greenland rifted margin over the axis of the Iceland mantle plume (50–47 Ma) and later by uplift associated with regional plate-tectonic reorganization (37–35 Ma). The Iceland mantle plume was instrumental in causing protracted rift-to-drift transition and post-breakup tholeiitic and alkaline magmatism on the East Greenland rifted margin, and asymmetry in the magmatic history of the conjugate margins of the central Northeast Atlantic.

We present new mineral and whole-rock compositions and Sr–Nd–Hf–Pb–O–H isotope data on samples from the Kangerlussuaq Alkaline Complex (∼ 1000 km2) in central East Greenland, part of the North Atlantic Igneous Province. This complex mainly consists of the Kangerlussuaq Intrusion but includes at least 13 separate satellite intrusions emplaced in the uppermost crust close to the unconformity between Archaean gneisses and overlying Palaeogene flood basalts. The complex is divided into (i) older satellite intrusions (∼ 55–53 Ma) composed of multiple syenites and granites and minor gabbros and peridotites, (ii) the voluminous Kangerlussuaq Intrusion (∼ 50 Ma), which displays a gradual transition from quartz syenites (nordmarkites) at the margin to nepheline syenites (foyaites) in the centre, and (iii) younger satellite intrusions (∼ 47–45 Ma) of minor syenites, granites and diorites concentrated southeast of the Kangerlussuaq Intrusion. The complex displays a temporal evolution in which SiO2 decreases (74–56 wt.%) and total alkalis (6–16 wt.%), amphibole Na + K content, 206Pb/204Pbmeas, eNdi and eHfi (+ 3 to + 11) increase from the older intrusions through the nepheline syenites. This is followed by a reversal to higher silica (62–73 wt.%) and lower total alkalis (9–12 wt.%), amphibole Na + K content, 206Pb/204Pbmeas, eNdi and eHfi (− 13 to + 2) in the younger satellite intrusions. Temporal changes in the location of magma plumbing systems and in magma production rates played a profound role in controlling silica content, alkalinity and degree of crustal contamination during development of the complex. Phonolitic magma was only generated after prolonged magmatism had shielded the conduits from interaction with country rock. The parental magmas were probably basanitic to alkali olivine basaltic in composition. The older satellite intrusions and the Kangerlussuaq Intrusion have low δ18O magma values (− 1 to + 6‰) compared to the younger, more crustally contaminated, satellite intrusions (+ 4.5 to + 7‰). It appears that the magmas only had sufficient over-pressure to intrude the basalt cover when larger volumes of less contaminated trachyte magma were produced, resulting in the generation of low-δ18O magmas due to dehydration of hydrothermally altered basalt xenoliths.

Primary granophyres are differentiated from olivine tholeiitic magma and occur interstitially throughout the cumulus stratigraphy of the Skaergaard intrusion, East Greenland. Samples from the Lower Zones a–c (LZa–c), the Middle Zone (MZ) and the Sandwich Horizon (SH) are included in the present study together with granophyric accumulations in gabbroic pegmatite from LZa–c. Fluid inclusions in quartz and the mineral assemblage in the granophyres record the pressure under which the Skaergaard intrusion crystallised. Pegmatitic granophyre from LZa–c consists mainly of quartz, plagioclase (An 4–7 ) and alkali feldspar (Or 40–80 ) enclosing an earlier formed assemblage of ferrohastingsitic and ferroedenitic hornblende, fayalite (Fo 4–5 ), titanite, biotite and fluor-apatite. Granophyric quartz, albite and alkali feldspar crystallised from water-saturated granitic melts near eutectic minimum conditions between 680 and 660 °C. The pressure of granophyre crystallisation was modelled by the intercept between fluid inclusion isochores and the minimum melt solidus for granitic compositions. Pressures, recalculated to the roof pendant of the intrusion, are 0.7 ± 0.5 for LZa, 2.0 ± 0.2 for LZb–c, 2.3 ± 0.8 for MZ (the Triple Group level) and 3.3 ± 1.3 kb for SH. Amphibole geobarometry, independently, confirm the pressure estimates for pegmatitic granophyres in LZa–c. The granophyres formed as the intrusion cooled through the minimum melt solidus in LZa, LZb, LZc, MZ and SH, respectively. The pressure increase from LZa to SH granophyres is explained by progressive burial during cooling of the intrusion and contemporaneous outpouring of 5.3–6.3 ± 2.7 km of flood basalts during the initial opening of the Northeast Atlantic Ocean. Accordingly, the Skaergaard intrusion evolved from a subvolcanic magma chamber at emplacement to a more deep-seated igneous system during terminal crystallization when the majority of the intercumulus phases formed. The present cooling history suggests the intrusion cooled through the temperature of granophyre formation in less than 300,000 years. The increase in the pressure of granophyre formation therefore implies that 5.3–6.3 ± 2.7 km of flood basalts were emplaced within this time window.

To constrain the amount and rate of crustal contamination that is possible in basaltic and jotunitic magma, and to gain an insight into the physical and thermal processes of assimilation in crustal magma chambers, we have modelled published Sr and Nd isotopic data from three layered intrusions. Well-exposed sequences of cumulates with no evidence of magma recharge provide direct records of concurrent assimilation and fractional crystallization (AFC). The key to the modelling is that F , the mass fraction of magma remaining in the chamber, can be estimated from the thicknesses of the studied cumulate sequences. This allows AFC model curves to be fitted to the isotopic data by varying r , the ratio of the rate of mass assimilated to the rate of mass crystallized. The results of modelling show that r is nearly constant in 800 to 2000 m thick sequences of cumulates displaying up-section decreases in anorthite content of plagioclase, increases in whole-rock Sr 0 (initial 87 Sr/ 86 Sr) and decreases in whole-rock e Nd 0 (initial e Nd). The r -values of the layered sequences range from ∼0.12 in the Fongen–Hyllingen Intrusion, over 0.20 in the Bjerkreim–Sokndal Intrusion, to 0.27 in the Hasvik Intrusion. The total amount of assimilation, the bulk crust / magma ratio , reaches values of 0.08, 0.19 and 0.28 at the level of the most contaminated samples after 60% to 80% crystallisation, whereas the instantaneous crust / magma ratio of the most contaminated magmas were respectively 0.14, 0.46, and 0.70, for the three intrusions. Innumerable country rock xenoliths occur in the three layered intrusions and played a crucial role in the assimilation process. The xenoliths spalled off the roofs of the magma chambers during magma emplacement and their initial temperature and composition relate to r in the intrusions. In the Hasvik Intrusion ( r = 0.27), the initial temperature of the country rocks was ∼450 °C and the xenoliths were fusible metasediments and therefore produced a high fraction of partial melt that could be assimilated. In the Bjerkreim–Sokndal Intrusion ( r = 0.20), the country rocks were initially at temperatures of 640–880 °C but included both refractory massif-type anorthosite and fusible gneisses. In the Fongen–Hyllingen Intrusion ( r = 0.12), the country rocks were cooler (∼300 °C) and the xenoliths include refractory metabasalt (dominant) and fusible metapelite. We argue that the refractory metabasalt and anorthosite xenoliths acted mainly as heat sinks, resulting in reduced r -values in Fongen–Hyllingen and Bjerkreim–Sokndal Intrusions. Heating of refractory and fusible xenoliths, and melting of fusible xenoliths absorbed sensible and latent heat of the magma. Energy-balanced modelling shows that up to 75% of the heat available was absorbed by xenoliths within the magma chambers, promoting higher rates of cooling and crystallisation than would have resulted from loss of heat to the envelope of country rocks alone. The high r -values reflect the amount of heat absorbed by heating and melting country rock within the magma chambers themselves, and their constancy reflects the ready availability of fusible xenoliths.

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.