Search

Preferred and modelled bulk composition of the Skaergaard intrusion are compared to coeval basaltic compositions in East Greenland and found to relate to the second evolved cycle of Geikie Plateau Formation lavas and coeval Skaergaard-like dikes in major and trace element (Mg# ∼45, Ce/Nb ∼2·5, (Dy/Yb)N ∼1·35), and precious metal composition (Pd/Pt ∼3, Au/Pt ∼2) as well as in age (∼56 Ma). Successful comparisons of precious metal compositions only occur with Skaergaard models based on mass balance. The bulk liquid of the intrusion evolved along the liquid line of descent to immiscibility between Si- and Fe-rich silicate liquids after ∼90% of crystallization (F = ∼0·10) in agreement with experimental constraints. Immiscibility led to accumulation and fractionation of the Fe-rich silicate melt in the mushy floor of the intrusion and continued accumulation of granophyre component in the remaining bulk liquid. The composition of plagioclase in the precious metal mineralized gabbro and modelling of Pd/Pt and Au/Pt in first formed droplets of sulphide melt suggest that sulphide saturation was reached in interstitial melts in crystal mushes in the floor and roof and in bulk liquid with a composition equivalent to that of the bulk liquid at lower UZa times and after crystallization of 82–85% of the bulk liquid (F = 0·19–0·16). Prior to sulphide saturation in UZa type melt, the precious metals ratios of the bulk liquid were controlled by the loss of Pt relative to Pd and Au in agreement with the low empirical and experimental solubility of Pt of ∼9ppb compared to a much higher value for Pd and Au. The relative timing between sulphide saturation (F = ∼0·18) and immiscibility between silicate melts (F = ∼0·10) and modelled precious metal ratios underpin the proposed multi-stage model for the mineralization, advocating initial accumulation in the mushy floor of the magma chamber controlled by sulphide saturation in mush melts rather than bulk melt, followed by redistribution of precious metals in a macro-rhythmic succession of gabbroic layers of the upward migrating crystallization zone.

The Palaeogene Kangerlussuaq Intrusion (∼50 Ma) of East Greenland displays concentric zonation from quartz-rich nordmarkite (quartz syenite) at the margin, through pulaskite, to foyaite (nepheline syenite) in the centre; modal layering and igneous lamination are locally developed but there are no internal intrusive contacts. This is an apparent violation of the phase relations in Petrogeny's Residua System. We propose that this intrusion is layered, grading from quartz syenite at the bottom to nepheline syenite at the top. Mineral and whole-rock major and trace element data and Sr–Nd–Hf–Pb isotope data are presented that provide constraints on the petrogenesis of the intrusion. Radiogenic isotope data indicate a continuously decreasing crustal component from the quartz nordmarkites ( 87 Sr/ 86 Sr = 0·7061; ɛ Ndi = 2·3; ɛ Hfi = 5·2; 206 Pb/ 204 Pb meas = 16·98) to the foyaites ( 87 Sr/ 86 Sr = 0·7043–0·7044; ɛ Ndi = 3·8–4·9; ɛ Hfi = 10·7–11·1; 206 Pb/ 204 Pb meas = 17·78–17·88); the foyaites are dominated by a mantle isotopic signature. The average Mg-number of amphibole cores becomes increasingly primitive, varying from 26·4 in the nordmarkites to 57·4 in the pulaskites. Modal layering, feldspar lamination and the presence of huge basaltic xenoliths derived from the chamber roof, now resting on the transient chamber floor, demonstrate bottom-upwards crystallization. The intrusion cannot, therefore, have formed in a system closed to magmatic recharge. The lack of gneissic xenoliths in the nordmarkites suggests that most contamination took place deeper in the crust. In the proposed model, the nordmarkitic magma formed during crustal assimilation in the roof zone of a large, silica-undersaturated alkali basaltic/basanitic, stratified magma chamber, prior to emplacement in the uppermost crust. The more primitive syenites, terminating with foyaite at the top of the intrusion, formed as a consequence of repeated recharge of the Kangerlussuaq Intrusion magma chamber by tapping less contaminated, more primitive phonolitic melt from deeper parts of the underlying chamber during progressive armouring of the plumbing system.

The Kangerlussuaq Intrusion in East Greenland is concentrically zoned from quartz nordmarkite (quartz syenite) at the margin, through pulaskite, to foyaite (nepheline syenite) in the centre, with no apparent intrusive contacts. The δ18O values of coexisting minerals are consistent with oxygen isotope equilibrium at magmatic temperatures. Most of the intrusion formed from low-δ18O magma; magma δ18O values generally increased upwards from about 3.3 ‰ in the quartz nordmarkites to 5.6 ‰ in the foyaites. The lowest magma δ18O value of about −1.0 ‰ is from the upper part of the nordmarkites, where there is a high concentration of foundered basaltic xenoliths (stoped from the roof of the intrusion). The amphiboles in the syenites have δD values that range from those typical of hydrous mantle-derived minerals to much lower values (−86 to −157 ‰), as do whole-rock samples of xenolith and country rock (−125 to −148 ‰). The low magma δ18O and δD values are consistent with continuous incorporation, exchange and upward escape of low-δ18O and δD fluids released from stoped basaltic roof material. Mass balance suggests that the integrated amount of water involved was 7 wt% of the volume of the magma, but locally reached 30 wt% water. The requirement for large amounts of water with low δ18O value is satisfied only if the foundered basalt contained most of its water in cavities as opposed to hydrous minerals. Even with this requirement, the volume of stoped basalt would have been equal to the volume of the magma. Repeated recharge of the residual magma with progressively less contaminated silica undersaturated melt resulted in a gradual shift across the low-pressure thermal divide. Crystallisation was suppressed by the depression of the liquidus due to water saturation of the residual magma (pH2O ~1 kbar).

Dunitic xenoliths from late Palaeogene, alkaline basalt flows on Ubekendt Ejland, West Greenland contain olivine with 100 × Mg/(Mg + Fe), or Mg#, between 92.0 and 93.7. Orthopyroxene has very low Al2O3 and CaO contents (0.024–1.639 and 0.062–0.275 wt%, respectively). Spinel has 100 × Cr/(Cr + Al), or Cr#, between 46.98 and 95.67. Clinopyroxene is absent. The osmium isotopic composition of olivine and spinel mineral separates shows a considerable span of 187Os/188Os values. The most unradiogenic 187Os/188Os value of 0.1046 corresponds to a Re-depletion age of ca. 3.3 Gy, while the most radiogenic value of 0.1336 is higher than present-day chondrite. The Os isotopic composition of the xenoliths is consistent with their origin as restites from a melt extraction event in the Archaean, followed by one or more subsequent metasomatic event(s). The high Cr# in spinel and low modal pyroxene of the Ubekendt Ejland xenoliths are similar to values of some highly depleted mantle peridotites from arc settings. However, highly depleted, arc-related peridotites have higher Cr# in spinel for a given proportion of modal olivine, compared to cratonic xenolith suites from Greenland, which instead form coherent trends with abyssal peridotites, dredged from modern mid-ocean ridges. This suggests that depleted cratonic harzburgites and dunites from shallow lithospheric mantle represent the residue from dry melting in the Archaean.

We present compositional data on a 1,250-m-thick sequence of sparsely porphyritic lavas that comprise the Geikie Plateau Formation, part of the ~55-Ma break-up-related flood basalts in East Greenland. Major element compositions are relatively restricted (6.3–7.6 wt% MgO; 2.2–2.4 wt% TiO2), with two excursions to more evolved compositions (2.4–3.4 wt% TiO2) that are similar to the inferred parental magma of the nearby Skaergaard Intrusion. Major and trace element calculations show that fractional crystallisation is the principal control on magma compositions, and the cyclical sequential variations imply regular magma chamber replenishment events. Isotopic data indicate minor crustal assimilation, but with different contaminants for the main group (amphibolitic gneiss) and evolved cycles (granulitic gneiss). Rifting episodes may have allowed more primitive magmas to ascend to shallow crustal levels and subsequently fractionate to more evolved compositions in a separate chamber, which was perhaps similar to the source of the Skaergaard Intrusion.

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. © 1998 Elsevier Science B.V. All rights reserved.

Polymineralic aggregates composed of clinopyroxene, Fe-Ti oxide minerals, apatite and accessory K-feldspar, biotite, and amphibole are enclosed in cumulus plagioclase grains in the Middle Zone of the Skaergaard intrusion. The chemistry of the minerals in the aggregates, and the textural relations between the aggregates and the host plagioclase grains indicate that they represent inclusions of the contemporaneous melt of the Skaergaard intrusion. Through mass balance calculations a quantitative estimate of the melt composition for this level in the intrusion can be obtained, and this estimate confirms that the silica content in the Middle Zone melt was similar to, or possibly even lower than, the silica content in the initial Skaergaard melt, and relatively enriched in iron.

The presence of Fe- and Si-rich liquids found as melt inclusions in apatite and olivine in the Upper Zone of the Skaergaard intrusion, East Greenland, demonstrates the occurrence of liquid immiscibility in the late-stage evolution of tholeiitic magmas in a plutonic setting. However, it remains unclear at which stage of crystallization unmixing began. To constrain the onset and the petrological importance of liquid immiscibility in the Skaergaard and tholeiitic magmas in general, we have studied crystallized melt inclusions entrapped in early primocryst plagioclase. Such melt inclusions become abundant from the top of the Lower Zone and upwards in the Layered Series, in primocryst plagioclase of composition An54^26. The daughter phase assemblage is the same in all the inclusions, although the modal proportions of the daughter phases are highly variable: plagioclase (42^59%), clinopyroxene (28^41%), ilmenite (4^9%), magnetite (3^10%), apatite (1^9%) and accessory phases (51%). Accordingly, the bulk compositions of reheated and homogenized melt inclusions show large variations in SiO2 (40^54 wt %), FeO t (7^23 wt %), P 2O5 (0^1 ·9 wt %) and K2 O( 0^2·8 wt %), and have variable CaO/Al2O3 ratios.These variations are best explained by trapping of varying proportions of immiscible iron- and silica-rich melts and demonstrate that liquid immiscibility started in the upper part of the Lower Zone. We conclude that a significant part of the Skaergaard intrusion crystallized from an emulsion of Fe- and Si-rich immiscible melts.The heterogeneous trapping of a mixture of Fe- and Si-rich immiscible liquids in primocryst plagioclase indicates that the dispersed droplets in the Lower and Middle Zones were smaller than the size of the inclusion (5500mm). In the Upper Zone, most of the inclusions in apatite are composed of the conjugate end-member liquids, indicating a larger size for the dispersed droplets. Metre-sized pods and layers of melanogranophyre in the upper part of the intrusion are believed to represent pooled bodies of the immiscible Si-rich liquid. Differentiation of an emulsified magma must be considered in petrogenetic models for the Skaergaard intrusion.

Troctolite blocks with compositions akin to the Hidden Zone are exposed in a tholeiitic dyke cutting across the Skaergaard intrusion, East Greenland. Plagioclase in these blocks contains finely crystallised melt inclusions that we have homogenised to constrain the parental magma to 47.4–49.0 wt.% SiO2, 13.4–14.9 wt.% Al2O3 and 10.7–14.1 wt.% FeOT. These compositions are lower in FeOT and higher in SiO2 than previous estimates and have distinct La/SmN and Dy/YbN ratios that link them to the lowermost volcanic succession (Milne Land Formation) of the regional East Greenland flood basalt province. New major- and trace element compositions for the FG-1 dyke swarm, previously taken to represent Skaergaard magmas, overlap with the entire range of the regional flood basalt succession and do not form a coherent suite of Skaergaard like melts. These dykes are therefore re-interpreted as feeder dykes throughout the main phase of flood basalt volcanism.

Solutions of H2O−NaCl−CH4 occur in fluid inclusions enclosed by quartz, apatite and feldspar from gabbroic pegmatitites, anorthositic structures and intercumulus minerals within the Skaergaard intrusion. The majority of the fluid inclusions resemble 10 μm diameter sub-to euhedral negative crystals. A vapour phase and a liquid phase are visible at room temperature, solids are normally absent. The salinity of the fluids ranges from 17.5 to 22.8 wt.% NaCl. CH4, which comprises less than six mole percent of the solution, was detected in the vapour phase of the fluid inclusions with Raman microprobe analysis. Homogenization of the fluid inclusions occurred in the liquid phase in the majority of the fluid inclusions, though 10% of the inclusions homogenized in the gas phase. Thermodynamic consideration of the stability of feldspars + quartz, and the C−O−H system, indicates that the solutions were trapped at temperatures between 655 and 770°C, at oxygen fugacities between 1.5 and 2.0 log units below the QFM oxygen buffer. Textural evidence and the composition of the solutions suggest that the fluids coexisted with late-magmatic intercumulus melts and the melts which formed gabbroic pegmatites. These solutions are thought to have contributed to late-magmatic metasomatism of the primocryst assemblages of the Skaergaard intrusion.

Acid-basic net-veined or pillowed dykes and intrusive bodies are small but important components in the felsic complexes of the Tertiary magmatic province in East Greenland. In Astrophyllite bay in the Snout Series syenite intrusion at the margin of the Kangerdlugssuaq syenite intrusion basic trachyandesite melis invade and melt the host of Archaean granitic gneisses. Pillows of trachyandesite are chilled to the surrounding felsic anatectic melt. Tue felsic melt is not granitic but syenitic in composition. Element distributions between the chilled basic and felsic melts suggest that the anatectic melt was transformed to a syenitic composition by diffusion between the two contacting melts.

Volcanic rifted margins have voluminous magmatic constructions at continent-ocean transitions including thick sequences of flood basalts, dike swarms, and mafic plutons developed in crustal-scale flexures. In the Tertiary East Greenland volcanic rifted margin, upper crustal dilation and flexure were accommodated by extensive faulting, locally pervasive cataclastic deformation, and pseudotachylyte generation. The widespread distribution and large volumes of pseudotachylyte suggest that regional seismogenic faulting was vigorous during rapid crustal subsidence, dike intrusion, and the start of sea-floor spreading.

When a tholeiitic liquid differentiates, it may give rise to either iron-poor, silica-rich (rhyolitic) differentiates or to iron-rich, silica-poor differentiates, as found in the Skaergaard intrusion. Iron-rich differentiated liquids are rare among erupted rocks, but are found in small quantities from several localities at divergent plate margins. Among the Tertiary basalts and intrusions in East Greenland, normal erupted basalts may be mixtures of primitive liquid and differentiated iron-rich liquid, which exists at depth but normally does not reach the surface because of its high density. The evolved liquids of the Skaergaard intrusion were of this kind. Data from mid-oceanic ridges confirm this view. We believe that iron-rich differentiated liquids, despite their scarcity on the surface, are much more voluminous at depth, as picrites, at the other side of the density minimum attained during liquid evolution, are thought to be. The trend toward iron enrichment develops when the tholeiitic magma differentiates in a closed system at a relatively low oxidation state, whereas the trend toward silica enrichment and iron depletion occurs when the magma has interacted with the oxidized and hydrated surroundings in the crust.

The whole-rock geochemistry and mineral chemistry of six samples of Lower Cretaceous tholeiitic basalt from Franz Josef Land, U.S.S.R., have been studied. Geochemical criteria indicate that the basalts are initial rifting tholeiites characterised by low contents of Ti and other H-elements, suggesting derivation from a depleted mantle source. These tholeiites formed during a Lower Cretaceous rifting stage in the formation of the Arctic Ocean basin, most likely the opening of the Canada Basin.

SummaryThe Kangerdlugssuaq late dike swarm, which strikes at a high angle to the well-known East Greenland coastal swarm, is described and chemical analyses presented. The basic members are characterized by a high potassium content. A variety of kaersutite-bearing gabbroic inclusions in one member of this swarm is described in detail and microprobe analyses of clinopyroxenes, amphiboles, plagioclases, sheet silicates, and spinel minerals are presented. On the basis of this evidence it is deduced that these inclusions derive from a cumulate sequence formed at depths of between 5 and 10 km beneath the Lower Tertiary land surface. It is likely that fractionation of such assemblages causes a transition, at relatively low pressure, from undersaturated to oversaturated compositions, but the products appear to be quantitatively minor.

The fission track method has become acceptable in geochronology, not only in straightforward dating of igneous and metamorphic events but also in stratigraphic studies by the dating of ash layers1,2, the elucidation of crustal uplift history3–7, and in archaeology8. Such studies have mostly used accessory minerals such as apatite, sphene and zircon, whose track-retentive properties are well documented. However, fission tracks can be developed and observed under the optical microscope in most minerals9–11. We report here on reconnaissance studies of fission track dating of zeolites, a complex group of tectosilicates, which is, for example, widely developed in basaltic rocks after their emplacement by later stage processes. We describe the etching conditions for some zeolites, in particular the minerals chabazite, stilbite and heulandite12 and detail the track recording and retention characteristics for chabazite, with particular reference to fossil hydrothermal systems in the Faeroe Islands.

Astrophyllite has been found in alkalic pegmatite dykes on the two sides of Kangerdlugssuaq t~ord. At the eastern locality, Kraemers Island, the dyke is silica oversaturated and the astrophyllite occurs in a broad tabular crystal habit. The dyke at the western locality, Bagnaesset, is silica undersaturated and the astrophyllite occurs in elongated prisms of acicular habit. Electron probe analyses of the crystals of the two habits have shown systematic chemical differences which not onty relate to Mn and Ti compositional variations, but also reveal departures from ideal stoichiometry in varying degrees. Based on this latter feature a model involving polysomatism has been constructed which may explain the differences in crystal habit. Two Tertiary alkalic pegmatite astrophyllite- bearing dykes in the Kangerdlugssuaq region of East Greenland were identified in the 1977 field season. A small number of samples were collected at this time, but a much more comprehensive collection was made when the localities were re- visited in 1979. At the second visit, dynamite was used to assist in obtaining adequate amounts of fresh material. The dykes are located at Bagnaesset and Kraemers Island, on the west and east sides, respectively of Kangerdlugssuaq Fjord. Fig. 1 contains maps indicating the precise locations of these dykes which are separated by a distance of about 10 km. The Kraemers Island dyke cuts a quartz-bearing syenite and is believed to be genetically related to this body. The dyke on Bagnaesset cuts both gneisses and a quartz-bearing syenite and its genetic relationships are unclear. In the silica-saturated Kraemers Island dyke astrophyllite has a broad tabular crystal habit with a reddish coppery lustre, whereas in the under- saturated Bagnaesset dyke it shows an elongated prismatic to acicular habit and a darker and more brassy colour. The variations in the crystal habit have been considered in relation to the mineral

Lamprophyric dikes, mainly of camptonite/monchiquite affinities occur in the Wiedemanns Fjord area. One example contains a complex assemblage of olivine-orthopyroxene-chrome spinel nodules, megacrysts of kaersutite, diopside, strongly reverse-zoned green salite and various spinel phases. Microprobe analyses are presented for all these phases and for the lilac-coloured titansalites and strongly-coloured kaersutites of the groundmass. It is concluded that these minerals record evolution under various P, T and oxidation regimes during the formation of a lamprophyric parent magma. The nodules provide evidence for deep fractures in this area supposedly associated with early rifting in the North Atlantic.