Christian Tegner

40Ar39Ar geochronology of Tertiary mafic intrusions along the East Greenland rifted margin: Relation to flood basalts and the Iceland hotspot track

Abstract The East Greeland Tertiary Igneous Province includes the largest exposed continental flood basalt sequence within the North Atlantic borderlands. More than ten layered gabbro complexes, including the ∼55 Ma Skaergaard intrusion, and a large dolerite sill complex are the plutonic equivalents of flood basalts; both lavas and intrusions have been regarded as synchronous with continental breakup at 57-54 Ma. We report ten new ages of the mafic intrusions, determined by40Ar 39Ar incremental heating experiments, demonstrating that the mafic intrusions formed in two distinct time windows. Only Intrusion II of the Imilik Gabbro Complex, the Skaergaard intrusion, and the Sorgenfri Glestcher Sill Complex formed at 57-55 Ma coeval with the eruption of regional flood basalts and continental breakup. Other layered gabbro intrusions at Imilik (Intrusion III), Kruuse Fjord, Igtutarajik, Nordre Aputiteˆq, Kap Edvard Holm, and Lilloise are distinctly younger and formed between 50 and 47 Ma. Plate-kinematic models indicate the axis of the ancestral Iceland mantle plume was located under Central Greenland at ∼60 Ma and subsequently crossed the East Greenland rifted continental margin. We propose that tholeiitic magmatism along the East Greenland rifted margin largely occurred in three distinct pulses at 62-59 Ma (lavas and dykes), 57-54 Ma (lavas, dykes, sills, and some gabbros) and 50-47 Ma (gabbros, dykes and rare lavas), related to discrete mantle melting episodes triggered by plume impact, continental breakup, and passage of the plume axis, respectively. This model implies northwestward continental drift of Greenland relative to the plume axis by ∼3.9-5.0 cm/yr between ∼60 and ∼49 Ma, consistent with estimates from seismic studies of submerged flood basalts.

Immiscible iron- and silica-rich melts in basalt petrogenesis documented in the Skaergaard intrusion

Silicate liquid immiscibility in basalt petrogenesis is a contentious issue. Immiscible iron- and silica-rich liquids were reported in melt inclusions of lunar basalt and in groundmass glasses of terrestrial volcanics. In fully crystallized plutonic rocks, however, silicate liquid immiscibility has yet to be proven. Here we report the first finding of natural, immiscible iron- and silica-rich melts in a plutonic environment documented in the Skaergaard intrusion, East Greenland. Primary melt inclusions (now finely crystallized) in apatite are either dark or light colored. The predominant dark colored type contains 30.9 ± 4.2 wt% FeO t and 40.7 ± 3.6 wt% SiO 2 , whereas the light colored type contains 8.6 ± 5.9 wt% FeO t and 65.6 ± 7.3 wt% SiO 2 . Similar light colored melt inclusions in olivine and fine-grained dark and light colored interstitial pockets also give evidence of crystallization from emulsion of silica and iron-rich liquids. On the outcrop scale, silica-rich (melanogranophyre) pods and layers in iron-rich ferrodiorite of the Upper Zone of the Skaergaard intrusion witness segregation of the two liquids. These findings demand that silicate immiscibility is considered in basalt petrogenesis. Some granitic rocks may represent unmixed silica-rich melt, whereas the dense, iron-rich melt is likely to sink in the crust and could mix with hot mantle-derived magma to form unusual rocks, like ferropicrites, otherwise interpreted as products of heterogeneous mantle sources.

Evidence from the rare-earth-element record of mantle melting for cooling of the Tertiary Iceland plume

Widespread flood basalt volcanism and continental rifting in the northeast Atlantic in the early Tertiary period (∼55 Myr ago) have been linked to the mantle plume now residing beneath Iceland. Although much is known about the present-day Iceland plume, its thermal structure, composition and position in the early Tertiary period remain unresolved. Estimates of its temperature, for example, range from >1,600 °C in some plume models to ∼1,500 °C based on the volume and composition of basaltic crust. Several recent studies have emphasized similarities in the thermal and chemical structure of the Tertiary and present-day plumes to argue for stability of the mantle anomaly, whereas others, relate variations in basalt volumes and compositions to changes in plume flux. Moreover, some authors,, have assumed that the plume was rift-centred for its entire history, whereas others argue that it became ridge-centred only after plate separation,. Here we report compositional data for ∼6,000 metres of flood basalts erupted in east Greenland, close to the inferred plume axis, that we use to constrain the Tertiary plume structure. Rare-earth-element systematics place limits on the pressures and extents of mantle melting and show that the mantle was initially moderately hot (∼1,500 °C), but that its temperature declined during flood volcanism. These observations are difficult to reconcile with current plume-head models, and call for important lithospheric control,, on actively upwelling mantle along the rifted margin.

Post-breakup basaltic magmatism along the East Greenland Tertiary rifted margin

Mafic and ultramafic intrusions in East Greenland adjacent to the offshore Greenland‐Iceland ridge were emplaced 5‐9 My after continental breakup at 55 Ma [1]. Rare earth element (REE) concentrations determined by secondary ion mass spectrometry are reported for cumulus clinopyroxene from these intrusions, and the data are used to estimate REE abundance in equilibrium melts using available partitioning data. Estimated equilibrium melts from intrusions have strongly fractionated REE patterns with Nd=Dy(N) in the range 2 to 5.6 and Yb=Dy(N) 0.55 to 0.92, similar to values for coeval basalts. These melts have markedly higher Nd=Dy(N) than earlier breakup related flood basalts. The moderately low Yb=Dy(N) for the post-breakup volcanism is indicative of residual garnet in the source, while their high Nd=Dy(N) ratios can best be explained by aggregating low degree melts from a light-REE-enriched garnet- and spinel-bearing mantle source. We also report He, Sr, and Nd isotopic data for the intrusions. The highest 3 He= 4 He ratios (>10 R=Ra) are found in the samples whose REE data reflect the largest proportion of melts from a garnet-bearing source, and having Sr and Nd isotopic compositions identical with the radiogenic Sr and unradiogenic Nd isotope end of the Iceland compositional field. There is no indication of a MORB-type mantle in the source of the intrusions. We postulate that post-breakup volcanism along the East Greenland coastline reflects the increasing proximity of the mantle plume to the East Greenland continental margin. The low degree of melting at high mean pressure inferred for the parental melts for the intrusions may reflect re-thickening of the lithosphere, which in turn was caused by the vigorous volcanism during breakup, with accompanying depletion of upper mantle and underplating of the crust at the continental margin.

The ocean-continent transition in the mid-Norwegian margin: Insight from seismic data and an onshore Caledonian field analogue

Understanding the structure of the ocean-continent transition (OCT) in passive margins is greatly enhanced by comparison with onshore analogues. The North Atlantic margins and the “fossil” system in the Scandinavian Caledonides show variations along strike between magma-rich and magma-poor margins, but are different in terms of exposure and degree of maturity. They both display the early stages of the Wilson cycle. Seismic reflection data from the mid-Norwegian margin combined with results from Ocean Drilling Program Leg 104 drill core 642E allow for improved subbasalt imaging of the OCT. Below the Seaward-Dipping Reflector (SDR) sequences, vertical and inclined reflections are interpreted as dike feeder systems. High-amplitude reflections with abrupt termination and saucer-shaped geometries are interpreted as sill intrusions, implying the presence of sediments in the transition zone beneath the volcanic sequences. The transitional crust located below the SDR of the mid-Norwegian margin has a well-exposed analogue in the Seve Nappe Complex (SNC). At Sarek (Sweden), hornfelsed sediments are truncated by mafic dike swarms with densities of 70%–80% or more. The magmatic domain extends for at least 800 km along the Caledonides, and probably reached the size of a large igneous province. It developed at ca. 600 Ma on the margin of the Iapetus Ocean, and was probably linked to the magma-poor hyperextended segment in the southern Scandinavian Caledonides. These parts of the SNC represent an onshore analogue to the deeper level of the mid-Norwegian margin, permitting direct observation and sampling and providing an improved understanding, particularly of the deeper levels, of present-day magma-rich margins.

Late Cretaceous–Palaeocene continental rifting in the High Arctic: U–Pb geochronology of the Kap Washington Group volcanic sequence, North Greenland

Abstract: The Kap Washington Group volcanic sequence is exposed on the north coast of Greenland. The sequence was erupted in a continental rift setting during the opening of the Arctic Ocean and provides important geological constraints on the timing of this event. In this study we present the first isotope dilution thermal ionization mass spectrometry U–Pb zircon ages from 10 samples of trachytic to rhyolitic composition. Samples from Lockwood Island (n = 4) gave concordant ages between 67.2 ± 0.5 and 61.0 ± 0.3 Ma (2σ). Lavas from Kap Kane (n = 5) yielded concordant ages between 70.5 ± 0.3 and 69.7 ± 0.2 Ma. An ignimbrite from the Kap Washington peninsula yielded a concordant age of 68.5 ± 0.3 Ma. The Kap Washington Group sequence and similar occurrences on Ellesmere Island record a prolonged period of continental rift volcanism lasting from 92 to 58 Ma. We propose that this volcanism occurred along the margins of a nascent Eurasia Basin undergoing east–west extension linked to rifting in the Labrador Sea–Baffin Bay system.

Eocene to Miocene igneous activity in NE Greenland: northward younging of magmatism along the East Greenland margin

Radiometric dating by the 40Ar–39Ar incremental heating method was carried out on lavas, sills, dykes, and a central intrusion from NE Greenland. Eighteen samples gave acceptable crystallization ages. Lavas of both Lower and Upper Plateau Lava Series gave ages in the range 55.5–53.5 Ma and cannot be constrained to better than 56–53 Ma. Sills and dykes from Traill Ø to Shannon, with compositions fairly similar to those of the lavas, gave ages of 55.1–51.3 Ma, contemporaneous with and slightly younger than the lavas. Alkaline lavas on inland nunataks have ages of 53–50 Ma, and the Kap Broer Ruys intrusive centre has an age of 48.7 ± 0.5 Ma. An alkaline sill on Hvalrosø is much younger at 20.3 ± 0.1 Ma. There are no pre-breakup lavas onshore NE Greenland. We surmise that the hot mantle of the Iceland plume arrived and melted extensively beneath the northern basins only at the time of breakup around 55 Ma. Post-breakup intrusive events in NE Greenland coincided with plate-tectonic events such as reorganization, uplift and opening in the north. The Hvalrosø sill represents a local small melting event that may be related to coeval opening of the Lena Trough. Supplementary materials: Details of the source data, results, and the compositions and locations of the dated samples, are available at www.geolsoc.org.uk/SUP18738.

Crystallization sequence of the Upper Border Series of the Skaergaard Intrusion: revised subdivision and implications for chamber-scale magma homogeneity

Although it is one of the best-studied layered mafic intrusions in the world, the crystallization sequence of the Skaergaard Intrusion, east Greenland, remains in debate. In particular, it has been argued that the crystallization sequence in the Upper Border Series, which crystallized downwards from the roof of the magma chamber, differs from that in the Layered Series formed at the floor. The proposed deviation would require chemical stratification of the magma, and a reexamination of the crystallization sequence therefore has important implications for understanding the dynamics of the system. Here, we examine a new sample set from the Upper Border Series, combining field observations, petrography and anorthite contents of plagioclase with bulk rock Ti, V, P, Cu and Mn concentrations. We demonstrate that the first phases on the liquidus were plagioclase and olivine followed by augite, then ilmenite and magnetite (simultaneously), sulfides, apatite and finally ferrobustamite (now inverted to hedenbergite). This crystallization sequence represents extreme differentiation along the tholeiitic trend, and it mirrors those at the floor (Layered Series) and walls (Marginal Border Series). We therefore propose a revised subdivision of the Upper Border Series into equivalents of the subzones in the Layered Series denoted by apostrophes (LZa′, LZb′, etc.). Moreover, the first appearance of each of the cumulus phases occurs at similar plagioclase core anorthite contents. The mirror images of the crystallization sequences and the anorthite contents of plagioclase cores in the three series imply that the Skaergaard magma chamber solidified by in situ crystallization along the floor, walls and roof from one, largely homogenous, convecting magma body.

Toward an understanding of disequilibrium dihedral angles in mafic rocks

The median dihedral angle at clinopyroxene-plagioclase-plagioclase junctions in mafic rocks, Θcpp, is generally lower than equilibrium (109˚ {plus minus} 2˚). Observation of a wide range of mafic bodies demonstrates that previous work on systematic variations of Θcpp is incorrect in several important respects. Firstly, the spatial distribution of plagioclase compositional zoning demonstrates that the final geometry of three-grain junctions, and hence Θcpp, is formed during solidification (the igneous process): sub-solidus textural modification in most dolerites and gabbros, previously thought to be the dominant control on Θcpp, is insignificant. Θcpp is governed by mass transport constraints, the inhibiting effects of small pore size on crystallization, and variation in relative growth rates of pyroxene and plagioclase. During rapid cooling, pyroxene preferentially fills wider pores while the narrower pores remain melt-filled, resulting in an initial value of Θcpp of 78˚, rather than 60˚ which would be expected if all melt-filled pores were filled with pyroxene. Lower cooling rates create a higher initial Θcpp due to changes in relative growth rates of the two minerals at the nascent three-grain junction. Low Θcpp (associated with cuspate clinopyroxene grains at triple junctions) can also be diagnostic of infiltration of previously melt-free rocks by late-stage evolved liquids (the metasomatic process). Modification of Θcpp by sub-solidus textural equilibration (the metamorphic process) is only important for fine-grained mafic rocks such as chilled margins and intra-plutonic chill zones. In coarse-grained gabbros from shallow crustal intrusions the metamorphic process occurs only in the centres of oikocrysts, associated with rounding of chadacrysts.

Intense and widespread seismicity during the end-Triassic mass extinction due to emplacement of a large igneous province

Multiple levels of earthquake-induced soft-sediment deformations (seismites) are concentrated in the end-Triassic mass extinction interval across Europe. The repetitive nature of the seismites rules out an origin by an extraterrestrial impact. Instead, this intense seismic activity is linked to the formation of the Central Atlantic magmatic province (CAMP). By the earliest Jurassic the seismic activity had ceased, while extrusive volcanism still continued and biotic recovery was on its way. This suggests that magmatic intrusions into sedimentary strata during early stages of CAMP formation caused emission of gases (SO 2 , halocarbons, polycyclic aromatic hydrocarbons) that may have played a major part in the biotic crisis.