Lotte Melchior Larsen

40Ar/39Ar geochronology of the West Greenland Tertiary volcanic province

Abstract Paleocene volcanic rocks in West Greenland and Baffin Island were among the first products of the Iceland mantle plume, forming part of a larger igneous province that is now submerged beneath the northern Labrador Sea. A 40Ar/39Ar dating study shows that volcanism commenced in West Greenland between 60.9 and 61.3 Ma and that ∼80% of the Paleocene lava pile was erupted in 1 million years or less (weighted mean age of 60.5±0.4 Ma). Minimum estimates of magma production rates (1.3×10−4 km3 year−1 km−1) are similar to the present Iceland rift, except for the uppermost part of the Paleocene volcanic succession where the rate decreases to 1 m/year) lateral spreading of the Iceland plume head at the base of the Greenland lithosphere at ∼62 Ma. We suggest that the arrival, or at least a major increase in the flux, of the Iceland mantle plume beneath Greenland was a contributing factor in the initiation of seafloor spreading in the northern Labrador Sea. Our study has also revealed a previously unrecognised Early Eocene volcanic episode in West Greenland. This magmatism may be related to movement on the transform Ungava Fault System which transferred drifting from the Labrador Sea to Baffin Bay. A regional change in plate kinematics at ∼55 Ma, associated with the opening of the North Atlantic, would have caused net extension along parts of this fault. This would have resulted in decompression and partial melting of the underlying asthenosphere. The source of the melts for the Eocene magmatism may have been remnants of still anomalously hot Iceland plume mantle which were left stranded beneath the West Greenland lithosphere in the Early Paleocene.

Distribution of REE and other trace elements between phenocrysts and peralkaline undersaturated magmas, exemplified by rocks from the Gardar igneous province, south Greenland

Abstract Partition coefficients for iron-rich olivine and pyroxene, sanidine, nepheline and apatite are reported from peralkaline trachytic to phonolitic dyke rocks and the agpaitic Ilimaussaq intrusion. Partition coefficients for many elements in olivine and pyroxene decrease with increasing peralkalinity and undersaturation of the magma, i.e. with decreasing polymerisation. The REE partition coefficients for olivine and pyroxene also show dependence on the mineral chemistry, i.e. the iron content. Probably due to the larger lattice sites in the iron end-members the heavy REEs enter the small six-coordinated lattice sites with increasing ease as the iron content of the mineral increases. La and Ce partition coefficients for apatite increase with increasing peralkalinity; this condition seems to stabilise a Na-REE-phosphate component in the mineral.

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.

A review of the 2500 Ma span of alkaline-ultramafic, potassic and carbonatitic magmatism in West Greenland

Abstract Kimberlites, carbonatites and ultramafic, mafic and potassic lamprophyres have been produced in West Greenland in recurrent events since the Archaean. Five distinct age groups are recognised: Archaean (>2500 Ma). Early Proterozoic (1700–1900 Ma), Middle Proterozoic (Gardar, c. 1100–1300 Ma), Late Proterozoic (600 Ma) and Mesozoic-Tertiary (200-30 Ma) The rocks comprise two large and four small carbonatite occurrences, four kimberlite dyke swarms, one lamproite dyke swarm and one lamproite pipe, one dyke swarm of potassic lamprophyre (shonkinite) and some ten dyke swarms of ultramafic lamprophyre and monchiquite. Geochemical data for the various rock groups are presented. Some of the carbonatites may represent relatively unmodified mantle-derived melts. The kimberlites range from primitive to differentiated compositions, and there are regional differences between kimberlites within Archaean and Proterozoic basement. The ultrapotassic lamproites and shonkinites have strong negative Nb spikes in their trace element spectra. The ultramafic and monchiquitic lamprophyres encompass a large compositional variation; however, several of the dyke swarms have individual chemical characters. The rocks are very unevenly distributed in West Greenland, indicating a lithospheric control, probably by old weakness zones providing access to the surface. The kimberlites are considered to be mainly of asthenospheric derivation. The regional differences are interpreted in terms of melting with phlogopite as a residual phase, with smaller degrees of melting at deeper levels beneath the Archaean lithosphere than beneath the Proterozoic. The ultrapotassic lamproites and shonkinites occur almost exclusively within a continental collision zone with possible two-way subduction and they are interpreted as mainly of lithospheric derivation, with a contribution from a subducted slab. Data for the other rock types are equivocal. Except for the Archaean rocks, the age groups can be related to major geotectonic events. The Early Proterozoic group is related to continental collision at 1850 Ma and subsequent rifting; the Middle Proterozoic group is related to continental rifting (Gardar) and the Mesozoic group is likewise related to continental rifting prior to continental break-up in the Tertiary. The 600 Ma kimberlites and carbonatite are envisaged as cratonic, extra-rift activity in relation to continental break-up and formation of the Iapetus ocean further south, perhaps with a common cause in a broad, impinging mantle plume.

Structure and stratigraphy of the Early Tertiary basalts of the Blosseville Kyst, East Greenland

The central part of the 65 000 km2 lava plateau in East Greenland between Kangerlussuaq and Scoresby Sund has been investigated by multi-model photogrammetry and profile sampling, and a three-dimensional framework for the lava plateau has been established. Post-volcanic movements have divided the region into several structural blocks with dips varying from 1–2° N-NE in the inland areas to 12°SE at the coast. A major N-S-trending fracture zone may reflect an underlying Caledonian structure. Early volcanic and sedimentary rocks are overlain by widespread plateau basalts, locally with an angular unconformity of 8°. The basalt stratigraphy established in the Scoresby Sund area has been extended over most of the region. The plateau basalts were erupted over a zone more than 150 km wide, from the present coast and inland. The succession attains a vertical thickness of more than 5.5 km in the central coastal areas and thins inland to 2–3 km. A differential sagging of the coastal areas by at least 2.3 km took place during the emplacement of the lavas, accelerating with time. The features suggest increasing focusing of the magmatic production into a developing rift zone close to the present coast.

The Ilímaussaq intrusion—progressive crystallization and formation of layering in an agpaitic magma

Summary Agpaitic rocks form the major part of the Ilímaussaq intrusion in S Greenland. The agpaitic magma developed as the narrow top zone in a large stratified basalt-syenite magma chamber at depth. The extreme composition of the agpaitic magma is related to an unusually high crustal position of the cupola. After emplacement, the agpaitic magma developed in an essentially closed system. The magma chamber was shallow and the volatile-rich alkaline magma was light and fluid. Heat loss was mainly through the roof, and the earliest agpaitic rocks crystallized successively downwards from the roof. The magma was probably well mixed in the early stage, but there is evidence for accumulation of residual components in a layer below the roof. This accumulation of low-melting components eventually suppressed the downwards crystallization of the roof rocks. The exposed floor rocks, kakortokites and lujavrites, are younger than the roof rocks, and at this stage the magma had probably developed repeated layering. The layering in the kakortokites, with density-graded units 7 m thick repeated continuously over the whole exposed floor, can be simply explained if they formed from a layered magma by successive upwards crystallization of individual layers. The magma at this stage was nearly volatile saturated, and each layer crystallized in response to the upward loss of a certain amount of volatiles. The lujavrites conformably overlie the kakortokites and formed after a roof collapse which caused the rate of heat loss from the remaining magma to increase. The upward crystallization became faster than the upward transport of residual components, and the successive lujavrites contain more and more of these components which finally gave rise to potentially economic concentrations of U, Be and other rare elements. Finally, a hydrothermal phase was lost from the system.

Trans-Atlantic correlation of the Palaeogene volcanic successions in the Faeroe Islands and East Greenland

Before continental break-up in the NE Atlantic, the Faeroe Islands and central East Greenland were within a distance of 100–120 km. Chemical and lithological data for complete sections through the 5 km thick piles of contemporaneous Palaeogene flood basalts in the Faeroe Islands and in the Nansen Fjord area in East Greenland show very similar basalt compositions and evolution patterns with time. The Faeroes lower basalt formation and the equivalent Nansen Fjord Formation in East Greenland form a pre-break-up succession overlain by a sediment horizon. A syn-break-up succession consists of the Faeroes middle and upper basalt formations and the equivalent Milne Land Formation in East Greenland in which five intervals can be correlated with a compositional evolution from Ti-rich magnesian basalts and picrites at the base to a dominance of MORB-like low-Ti basalts at the top. The successions were generated in the same mantle melting column beneath a thinning continent with a rift zone that eventually ruptured the continent. The evolution pattern is very similar to that seen on the SE Greenland margin, but spreading according to the Palmason model of 1973 was not yet established. The pre- and syn-break-up successions formed volcanic megasystems stretching across the rift zone with areal extents of 70 000 and 220 000 km2 and volumes of 120 000 and 250 000 km3. Rocks from the pre- and syn-break-up successions can be discriminated based on a simple major-element plot. The overlying succession was 3–3.5 km thick in E Greenland but was thin or absent in the Faeroes; the energy source for the melting appears to have been concentrated on the Greenland side.

The water content of olivines from the North Atlantic Volcanic Province

The water content of olivine crystals from picrites and basalts from the North Atlantic Volcanic Province has been estimated using non-polarised Fourier transform infrared spectra. H2O concentrations vary from about 18 ppm by weight to below the detection limit of ca. 0.5 ppm. The most H2O-poor olivines (≤0.5 ppm H2O) probably reflect olivine crystallisation or re-equilibration at shallow crustal levels, possibly after magma degassing. However, some of the Mg-rich olivines (Fo87–91.5) have elevated H2O contents (3–7 ppm) and indicate the presence of mantle source regions with >300 ppm H2O for any reasonable crystallisation depth. The elevated H2O content may be sufficiently high to significantly affect the mantle viscosity and melting behaviour and thus contribute to the simultaneous production of melt over a vast area from Baffin Island to the British Isles at 58–62 Ma.

Geology and mineralogy of the Sarfartôq carbonatite complex, southern West Greenland

Abstract The Sarfartoq carbonatite complex was emplaced in lower Palaeozoic time in a weakness zone within the Precambrian shield. Dolomitic magma intruded in two major stages of activity. In the first stage a steeply dipping conical body of concentric sheets of rauhaugite was formed, while in the second stage several batches of magma were emplaced into the surrounding marginal shock-zone as concentric and radial beforsite dykes and agglomerates. Hydrothermal activity gave rise to several phases of mineralisation in veins and shear zones. The accompanying fenitisation was of the Na-type. The whole complex covers about 90 km 2 . The main rock-forming minerals are dolomite-ankerite, apatite, orange reversely pleochroic phlogopite, richterite-arfvedsonite and magnetite. Important accessories are pyrochlore, zircon and niobian rutile. A complete mineral list is given, together with microprobe data on mineral chemistry. The dolomitic magmas were poor in SiO 2 , Al 2 O 3 and K 2 O in relation to other carbonatites. Nb, U and LREEs are strongly enriched in pyrochloremineralised zones where the Nb content may be up to 40%. Some shear zones are strongly enriched in Th and HREEs (specifically Eu) and lesser Pb and Zn. Niobium, uranium, rare earth elements and phosphorus occur in economically interesting concentrations.

Episodic volcanism during break-up of the North Atlantic: evidence from the East Greenland plateau basalts

Abstract The lower Tertiary (upper Paleocene/lower Eocene) plateau basalts in the Scoresby Sund region of East Greenland formed during three distinct magmatic episodes. The activity of the first episode was centred in the present inland areas to the northwest and produced a voluminous regional basalt sequence (lower sequence). The activity of the second episode was centred to the southeast, offshore the present Atlantic coast, and this also produced a voluminous regional basalt sequence (upper sequence). The third episode was centred on the Atlantic coast and gave rise to a dense coastal dyke swarm and local lavas which are now only locally preserved by downfaulting. The lavas are all tholeiitic basalts. After an initial phase the two regional lava sequences of the first and second episodes show a similar vertical compositional zonation pattern: a thin lower formation with variable composition is followed by a thick uniform formation which shows systematic compositional change with height, from TiO2-rich “FeTi” basalts to TiO2-“poor” basalts, and ending with a reversal to TiO2-rich basalts. Each of these cyclic patterns apparently resulted from a temporary rifting episode, and much of the compositional variation found can be explained by magma development in open magma chambers. The dykes and lavas of the third episode of activity have chemical affinities to ocean floor basalts, and their TiO2/P2O5 ratios indicate formation at a spreading ridge centre. The pattern of two failed rifting episodes and one event that produced oceanic crust can be correlated with stages in the complicated initiation of seafloor spreading in the region, and the volcanism itself is closely related to this process.