Skaergaard Intrusion Research Articles

The Shape and Volume of the Skaergaard Intrusion, Greenland: Implications for Mass Balance and Bulk Composition

Re-examination of the Skaergaard intrusion in the context of its regional setting, combined with new data from exploration drilling, has resulted in a revised structural model for the intrusion. It is modelled as an irregular box, c. 11km from north to south, up to 8km from east to west, and 34---4km from the lower to the upper contact. The walls of the intrusion are inferred to follow pre-existing and penecontemporaneous steep faults, and the floor and roof seem largely controlled by bedding planes in the host sediments and lavas, similar to regional sills. The suggested shape and volume are in agreement with published gravimetric modelling. Crystallization along all margins of the intrusion concentrated the evolving melt in the upper, central part of the intrusion, best visualized as an ‘onion-skin’ structure inside the box. The total volume is estimated to c. 280 23km 3 , of which 137% are referred to the Upper Border Series (UBS), 164% to the Marginal Border Series (MBS) and 699% to the Layered Series (LS). In the LS, the Lower Zone (LZ) is estimated to constitute 668%, the Middle Zone (MZ) 135% and the Upper Zone (UZ) 197%. The new volume relationships provide a mass balance estimate of the major and trace element bulk composition of the intrusion. The parental magma to the Skaergaard intrusion is similar to high-Ti East Greenland tholeiitic plateau basalts with Mg number c. 0.45. The intrusion represents the solidification of contemporary plateau basalt magma trapped and crystallized under closed-system conditions in a crustal reservoir at the developing East Greenland continental margin.

Phosphate Lu–Hf geochronology

Igneous, metamorphic and biogenic apatite contains Lu and Hf in proportions that make this mineral suitable for dating by the 176Lu– 176Hf method. We present a new method for separation of Lu and Hf from apatite that involves a single extraction column step for Hf and a second exchange column step for Lu. This procedure allows rapid sample processing prior to analysis by MC-ICP-MS. Results from the igneous Gardiner, Skaergaard and Khibina Intrusions indicate that the closure temperature for the Lu–Hf isotopic system in apatite is relatively high. The Lu–Hf isochron age of ∼60 Ma for the Skaergaard Intrusion, East Greenland, is older than the generally accepted emplacement age, but does not appear to be a product of mixing. A centimeter-sized apatite from the Otter Lake area, Grenvillian Province, yields high and variable 176Lu/ 177Hf ratios showing that metamorphic apatite may, in some cases, allow for mono-mineral dating. The most important application of the method is undoubtedly its potential use for dating sedimentary formations. Although we find biogenic apatite in porous matrix highly susceptible to post-depositional resetting of the Lu–Hf isotopic system, the preservation of a near-depositional age for a shark tooth from the impermeable London Clay and recent work on crystalline authigenic phosphorites [Earth Planet. Sci. Lett. 201 (2002) 203] show the Lu–Hf system to be a promising tool for direct dating of sedimentary formations.

The compositional evolution of differentiated liquids from the Skaergaard Layered Series as determined by geochemical thermometry

Based on the COMAGMAT-3.65 crystallization model a set of phase equilibria calculations (called geochemical thermometry) have been conducted at P=1 kbar and closed conditions with respect to oxygen for 65 rocks representing the principle units of the Layered Series of the Skaergaard intrusion. It allowed us to define the range of initial temperatures (1145 to 1085 C) and oxygen fugacities (1-1.5 log units above QFM to slightly below QFM) of the original crystal mush from which the rocks from LZa to UZa crystallized. In parallel, average major-element compositions of residual (interstitial) liquid were calculated demonstrating a trend of continual enrichment of FeO (up to 18 wt.%) and TiO2 (up to 5.5 wt.%) with only minor variations in the SiO2 contents (48 to 50 wt.%). Projection of the compositions onto the OLIV -CPX-QTZ diagram provides evidence that most of the Layered Series crystallized on the Ol-Pl-Cpx-oxide cotectic. Systematic dierences between the calculated residual liquid compositions for LZa/LZb and LZc to UZa (which are unlikely to reflect fractional crystallization) are within the accuracy of the COMAGMAT model, but may be also indicative of a late-stage process involving migration and re-equilibration of interstitial liquids. Estimated amounts of interstitial melts trapped in the Skaergaard cumulates range around 50 wt.%. Wager's compositions inferred from simple mass-balance were found to lie too far from the Ol- Pl-Cpx boundary to represent a realistic approximation of the low-pressure Skaergaard magma evolution. The main problem of genetic interpretations of the Skaergaard intrusion is a strong misbalance between the parental compositions followed from contact rocks and the results of geochemical thermometry and that of the whole dierentiated body. It is most apparent for TiO2 and P2O5 which are almost twice as high in the average intrusion composition compared to the proposed parents. Moreover, the intrusion composition has of 2-4 wt.% less SiO2 and much more iron. One possible explanation is to assume the Skaergaard magma came to the chamber with an amount of crystals (Ol +Pl) equilibrated with the calculated parental liquid. However, even if some amount of hidden troctolitic material exists, it is unlikely that crystallization in a closed system could produce large volumes of rocks rich in Fe-Ti oxides without complementary more felsic dierentiates.

Major and trace element variation in ilmenite in the Skaergaard Intrusion: petrologic implications

Ilmenite separates from the floor (LS), roof (UBS), and wall (MBS) sequences of the Skaergaard Intrusion were analyzed for major and trace elements using DCP-AES and ICP-MS techniques. In all three sequences, FeO progressively increases, and MgO and Al2O3 progressively decrease with differentiation. Although trace element abundances are, in general, higher in UBS ilmenite than in MBS and LS ilmenite, all three sequences have similar trends for trace element abundance vs. crystallization. Ba, Cs, Rb, Sr, Th, U, Y, and the REEs are excluded elements in ilmenite, and remained at low abundances during differentiation. Cr, Ni, Sc, and V are included elements in ilmenite and other mafic phases, and decreased during differentiation. V contents in ilmenite, however, do not decrease significantly until the upper part of the middle zone, suggesting that magnetite did not begin to affect the magma differentiation trend until much later than when it first appears in the intrusion. Hf, Nb, Ta, and Zr, which are strongly excluded elements in silicates, are included elements in ilmenite. The element ratios Zr/Hf, Y/Ho, Nb/Ta, and U/Th are relatively constant in Skaergaard ilmenite from different parts of the intrusion, suggesting that fluid transport did not significantly effect these elements during differentiation or post-solidification cooling. Calculated partition coefficients for ilmenite in the Skaergaard Intrusion are similar to those reported from previous studies of lunar and terrestrial basalts and kimberlites, and for most elements are significantly lower than those reported for ilmenite in rhyolitic magma. Similar Di's for Zr, Hf, Nb, and Ta suggest that ilmenite crystallization did not significantly affect Zr/Nb or Hf/Ta in the Skaergaard magma, but the ratios of Zr, Hf, Nb, or Ta to other high field strength elements, such as Th, U, Y, or the REEs, may have been altered by ilmenite fractionation.

The Kraemer Island macrodyke, East Greenland: solidification of a flood basalt conduit

The Kraemer Island macrodyke that is exclusively exposed on Kraemer Island about 7 km west of the Skaergaard Intrusion belongs to a regional dyke swarm termed the ‘Skaergaard-like dykes’ (or FG-1 dykes). Weakly modally layered olivine gabbros dominate the exposed parts of the intrusion that has a width of 650 m to 1000 m. Plagioclase (core An68±2) and Ca-rich pyroxene (core Mg no. 79±1) grains are normally zoned, whereas olivine grains (Fo50–65) are homogeneous. Calculated mineral–magma equilibria, based on experimentally determined Mg–Fe magma–olivine and magma–clinopyroxene partition coefficients, suggest that the observed olivine and clinopyroxene compositions in the gabbros cannot have formed from a common parental magma. The unzoned nature of olivine grains and their iron-rich com-positions relative to clinopyroxene suggest post-cumulus Mg–Fe exchange between olivine and interstitial melt. A gabbroic pegmatite is developed in the centre of the intrusion along its entire exposed 5 km strike length. Here, mineral zonation is limited and compositions are similar to rims of cumulus minerals in the enveloping olivine gabbros. The pegmatite could therefore represent interstitial melt mobilized from gabbroic cumulates that later accumulated and crystallized at its present stratigraphic location. Cumulus olivines in the gabbros are close to equilibrium with Ca-rich pyroxene in the pegmatite. This is interpreted as reflecting interstitial melt mobility during the late stages of solidification of the macrodyke. Chilled margins are well preserved at the intrusion margins and are rich in Fe (14–15.3% FeOTOT), Ti (3–3.3% TiO2) and light rare-earth elements ([La/Sm]N = 1.2–1.3), similar to magmas well represented in the overlying sequence of contemporaneous High-Ti Series flood basalts. It is therefore likely that extensive wall-rock melting adjacent to the macrodyke reflects continuous feeding of the overlying flood basalts through the Kraemer Island macrodyke.

The East Greenland continental margin, the Prinsen af Wales Bjerge and new Skaergaard intrusion initiatives

The rifted volcanic margin of East Greenland has remained a major area for field studies and the development of models for the dynamics of plume-related continental break-up since the start of the Danish Lithosphere Centre (DLC) in 1994. The studies cover a range of disciplines and geological processes from the early development of pre-break-up basin formation and sedimentation over the main phase of basaltic magmatism to the late stages of alkaline magmatism and structural re-equilibration.
 The East Greenland field activities in the summer of 2000, collectively referred to as EG 2000, were facilitated by a logistic platform provided by support from Statens Naturvidenskabelige Forskningsråd (SNF, the Danish Natural Science Research Council) and the Bureau of Minerals and Petroleum (BMP) in Nuuk, Greenland for the retrieval of 6 km of drillcore from the Skaergaard intrusion. During 1989 and 1990 mineral exploration had resulted in drilling of more than 15 km of core through the classic layered gabbros. The logistic platform also provided support for DLC and Geological Survey of Denmark and Greenland (GEUS) field work and projects throughout the Kangerlussuaq region and on the Blosseville Kyst (Fig. 1), as well as mineral exploration and petroleum company activities.

Compositional and Lithological Controls on the PGE-Bearing Sulphide Zones in the Selukwe Subchamber, Great Dyke: a Combined Equilibrium–Rayleigh Fractionation Model

The platinum group element (PGE)-bearing Main and Lower INTRODUCTION Sulphide Zones of the Selukwe Subchamber of the Great Dyke are An increasing number of mafic-layered intrusions are made up of a series of subzones within which the ratios Pd:Pt are reported to have distributions of platinum group elements effectively constant, whereas these ratios vary significantly between (PGE), Au and base metals characterized by vertically the subzones. Fractionation of Pd with respect to Pt varies by a offset profiles. Offset profiles involve the vertical sepfactor of 10 and cannot be modelled using a sulphide collector aration of metal peaks and were first reported in the phase and constant partition coefficients. The link with sulphide is Great Dyke (Prendergast & Keays, 1989; Wilson et al., indisputable and the control is likely to have been the degree of 1989; Naldrett & Wilson, 1990; Wilson & Tredoux, oversaturation of PGE micro-nuggets in the magma. The apparent 1990). The phenomenon has since been recognized in partition coefficients for Pt and Pd between silicate and sulphide the Munni Munni intrusion of Western Australia (Barnes liquid are dependent on the degree of oversaturation and thereby et al., 1990), the Skaergaard intrusion (Bird et al., 1991) exhibit spurious correlation with the PGE content of the sulphide. and the Rincon del Tigre Complex of eastern Bolivia Modelling replicates the Pt and Pd distribution and ratios only by (Prendergast, 2000). PGE are also postulated to be transdramatically changing the effective partition coefficients. Pyroxene ported in fluids emanating from expulsion of intercumulus compositions (including TiO2) are shown to be strongly dependent liquid from the crystal pile, and there is strong evidence on the incompatible element content of the whole rock, and specific to support such processes in several gabbroic sections linear arrays relating these variables can be related to the PGE of layered intrusions such as the Skaergaard intrusion subzones. The overall control is Rayleigh fractionation, but constancy (Andersen et al., 1998) and in the Stillwater Complex of the ratio Pd:Pt and the initial pyroxene composition (before re(Boudreau & McCullum, 1986, 1992). In contrast, the equilibration with trapped liquid) within the subzones is indicative mineralized zones in the Great Dyke and Munni Munni of equilibrium crystallization. This layered structure may have been intrusion are located in pyroxenite, and primary magderived from liquid layers in the magma chamber. matic processes have been invoked to explain the min-

Ferric iron partitioning between plagioclase and silicate liquid: thermodynamics and petrological applications

A series of Fe and Mg partition experiments between plagioclase and silicate liquid were performed in the system SiO2–Al2O3–Fe2O3–FeO–MgO–CaO–Na2O under oxygen fugacities from below the IW buffer up to that of air. A thermodynamic model of plagioclase solid solution for the (CaAl,NaSi,KSi)(Fe3+,Al3+)Si2O8–Ca(Fe2+,Mg)Si3O8 system is proposed and is calibrated by regression analysis based on new and previously reported experimental data of Fe and Mg partitioning between plagioclase and silicate liquid, and reported thermodynamic properties of end members, ternary feldspar and silicate liquid. Using the derived thermodynamic model, FeOt, MgO content and Mg/(Fet+Mg) in plagioclase can be predicted from liquid composition with standard deviations of ±0.34 wt% (relative error =9%) and ±0.08 wt% (14%) and ±0.7 (8%) respectively. Calculated Fe3+–Al exchange chemical potentials of plagioclase, $${\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Pl}} $$ agree with those calculated using reported thermodynamic models for multicomponent spinel, $${\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Sp}} $$ and clinopyroxene, $${\rm \mu }_{{\rm Fe}^{{\rm 3 + }} \left( {{\rm Al}} \right)_{{\rm - 1}} }^{{\rm Cpx}} $$ . The FeOt content of plagioclase coexisting with spinel or clinopyroxene is affected by Fe3+/(Fe3++Al) and Mg/(Fe+Mg) of spinel or clinopyroxene and temperature, while it is independent of the anorthite content of plagioclase. Three oxygen barometers based on the proposed model are investigated. Although the oxygen fugacities predicted by the plagioclase–liquid oxygen barometer are scattered, this study found that plagioclase–spinel–clinopyroxene–oxygen and plagioclase–olivine–oxygen equilibria can be used as practical oxygen barometers. As a petrological application, prediction of plagioclase composition and fO2 are carried out for the Upper Zone of the Skaergaard intrusion. The estimated oxygen fugacities are well below QFM buffer and consistent with the estimation of oxidization states in previous studies.

The differentiation trend of the Skaergaard intrusion and the timing of magnetite crystallization: iron enrichment revisited

Initial studies of the Skaergaard intrusion [L.R. Wager, J. Petrol. 1 (1960) 364–398] and much of the subsequent work [R.J. Williams, Am. J. Sci. 271 (1971) 132–146; S.A. Morse et al., Am. J. Sci. 280A (1980) 159–170; A.R. McBirney, H.R. Naslund, Contrib. Mineral. Petrol. 104 (1990) 235–247; C. Tegner, Contrib. Mineral. Petrol. 128 (1997) 45–51; A.R. McBirney, Contrib. Mineral. Petrol. 132 (1998) 103–105] concluded that the Skaergaard magma followed an iron-enrichment trend with little or no silica enrichment until the final stages of crystallization. Several recent reports [R.H. Hunter, R.S.J. Sparks, Contrib. Mineral. Petrol. 95 (1987) 451–461; R.H. Hunter, R.S.J. Sparks, Contrib. Mineral. Petrol. 104 (1990) 248–254], however, have suggested that the Skaergaard magma began to follow a silica-enrichment trend in Lower Zone c (LZc) of the Layered Series where magnetite first became an abundant mineral. Magnetite in LZc, however, generally occurs in aggregates of magnetite–ulvöspinel and ilmenite–hematite that have undergone extensive subsolidus reequilibration and exsolution [E.A. Vincent, Neues Jahrb. Mineral. Abh. 94 (1960) 993–1016; E.A. Vincent, Geochim. Cosmochim. Acta 6 (1954) 1–26; A.F. Buddington, D.H. Lindsley, J. Petrol. 5 (1964) 310C357; H.R. Naslund, J. Petrol. 25 (1984) 185–212; A.R. McBirney, J. Petrol. 30 (1989) 363–397; Y.D. Jang, Petrological, Geochemical, and Mineralogical Variations in the Skaergaard Intrusion, East Greenland (Ph.D. Dissertation), State University of New York, Binghamton, NY, 1999, 219 pp.]. As a result, it is not clear if magnetite in these samples was an equilibrium, liquidus mineral fractionated from the main magma reservoir, or if magnetite crystallized as a later, interstitial mineral and did not directly affect the differentiation trend of the main Skaergaard magma. The timing of the initial crystallization of abundant magnetite and ilmenite is a key factor in understanding the trend of Skaergaard differentiation. Because V is a strongly included element in oxides, and is not strongly included in silicate minerals, the V content of an evolving magma is generally controlled by the fractionation of oxide minerals, in particular magnetite. The initial crystallization of magnetite should, therefore, be accompanied by a sudden decrease in the V content of the evolving magma, and in all of the coexisting mafic phases in equilibrium with that magma as well. The V content in Skaergaard pyroxene does not decrease significantly until the upper part of the Middle Zone (MZ), suggesting that the onset of extensive magnetite fractionation is much later than has previously been thought, and that the magnetite in LZc and the lower part of the MZ might not have been a liquidus phase at that level. The observed V trend in Skaergaard pyroxene can be modeled almost perfectly using published partition coefficients for the coexisting minerals in the Skaergaard intrusion, assuming that no magnetite fractionation occurred until the upper part of the MZ. Independently calculated trends for fO2 in the Skaergaard magma [R.J. Williams, Am. J. Sci. 271 (1971) 132–146; S.A. Morse et al., Am. J. Sci. 280A (1980) 159–170; A.R. McBirney, H.R. Naslund, Contrib. Mineral. Petrol. 104 (1990) 235–247] change in the upper part of the MZ to more reducing conditions. The onset of magnetite fractionation would remove Fe2O3 from the magma and could initiate such a change. The timing of magnetite fractionation will have a strong effect on whether magma evolves towards iron enrichment or silica enrichment.

Assemblages with titanite (CaTiOSiO4), Ca-Mg-Fe olivine and pyroxenes, Fe-Mg-Ti oxides, and quartz: Part II. Application

In the system CaO-MgO-FeO-Fe 2 O 3 -TiO2--SiO 2 , equilibria among titanite, pyroxene(s), olivine, spinel phase, ilmenite, and quartz constrain f O₂ , T, a SiO₂ , a TiO₂ , a Fe , and the compositional variables, μTiFe 2+ Fe 3+ -2 , μCa(Mg,Fe) -1 , and consequently μMg,Fe -1 among the silicate and oxide phases. These equilibria can provide the framework for a better understanding of relevant titanite-bearing natural assemblages. In addition, such equilibria can constrain the variables that operate during crystallization of igneous and metamorphic rocks. Titanite-bearing assemblages with pyroxene(s), Fe-Mg-Ti ilmenite and spinel, olivine, and quartz are rare in metamorphic rocks but are common in igneous rocks where they are more commonly reported in plutonic than in volcanic rocks. Probably because, at high temperatures, titanite cannot coexist with the relative common in volcanic rocks sub-assemblage augite + orthopyroxene + olivine + spinel phase + ilmenite. Moreover, decreasing pressure and temperature conditions appear to favor titanite. Thus, it is not surprising that titanite is commonly observed in slowly cooled rocks, albeit, most typically in association with amphibole. We argue that the titanite + amphibole association is favored by high a H₂O and, because a H₂O typically increases during crystallization of a pluton, titanite + amphibole (and consequently titanite) is more common in plutonic than in volcanic rocks. We have used a modified version of the program QUILF that includes our thermodynamic data for titanite and appropriate titanite-bearing assemblages to estimate the equilibrium crystallization conditions of samples from: (1) a tonalite from the Myoken-Zan granitic complex in Japan; (2) the Sandwich Horizon in the Skaergaard intrusion in Greenland; (3) rhyodacites, dacites, and andesites from the Sajama, Porquesa, and Parinacota volcanoes in the Andes; (4) the Fish Canyon Tuff in Colorado; and (5) a quartz-rich, calc-silicate gneiss adjacent to the Horse Creek Anorthosite Complex in Wyoming. The calculations suggest that: (1) these titanite-bearing assemblages crystallized at values of f O₂ that ranged from slightly below FMQ to more than 3 logf O₂ units above FMQ at 500- 800 °C; and (2) the stability of titanite is controlled by T, f O₂ , a SiO₂ , and the compositions of the coexisting oxides and silicates.

Major and trace element composition of Skaergaard plagioclase; geochemical evidence for changes in magma dynamics during the final stage of crystallization of the Skaergaard intrusion

Plagioclase separates from the Layered Series (LS), Upper Border Series (UBS), and Marginal Border Series (MBS) of the Skaergaard intrusion were analyzed to examine major and trace element variations. In general, plagioclase from the LS, UBS, and MBS show similar trends in major elements vs. crystallization: SiO2, Na2O, and K2O progressively increase, and CaO and MgO progressively decrease with fractionation. No abrupt changes in the trends of major components of Skaergaard plagioclase during the differentiation of the intrusion are observed. Trace elements in plagioclase reflect changes in the Skaergaard magma and changes in plagioclase distribution coefficients with differentiation. Sr, Ga, and probably Ba are included elements in Skaergaard plagioclase, but were excluded from the other cumulus phases, and as a result systematically increased in the magma and plagioclase during differentiation. Be, Cs, Hf, Rb, Ta, U, and Zr, and the transition metals Co, Cr, Cu, Ni, Sc, V, and Zn were excluded elements in Skaergaard plagioclase, and remained low in plagioclase during differentiation. Changes in the abundances of these elements in plagioclase during differentiation reflect changes in their abundance in the magma. With the exception of the lower zone, which is enriched in the light rare earth elements, rare earth elements in LS plagioclase, in general, increase with differentiation of the Skaergaard intrusion, but decrease dramatically at the UZa/UZb boundary where abundant apatite first appears. Rare earth elements in UBS plagioclase followed a similar trend to LS plagioclase, except during the initial and final stages of differentiation. UBS plagioclase is much more enriched in rare earth elements during the final 20% of crystallization, except for Eu, which is similar in plagioclase from the two series. The observed trends suggest that the floor and roof sequences became isolated from each other and that the floor sequence may have been more reducing and the roof sequence more oxidizing during the final 20% of crystallization. As the Skaergaard magma ceased convection, or convected as isolated cells, during the final stages of differentiation, volatile elements may have accumulated in the UBS magma, resulting in an increase in ƒO2, and a decrease in Eu/Sm in UBS plagioclase. The observed trends of rare earth elements in plagioclase from the LS and UBS fit well with theoretical calculations that assume closed-system crystallization, and would be difficult to reconcile with any model requiring significant discharge of magma from the chamber during the final 20% of crystallization. The enrichment of light rare earth elements in plagioclase, suggests that the lower part of the intrusion re-equilibrated with a late, light rare earth element-rich fluid or melt. The recharge model proposed by earlier workers to explain anomalous Sr and Nd isotopes appears unlikely in light of the two to fourfold enrichment of light rare earth elements in these samples.

Mineral exploration in Greenland in the 1990s

The main exploration and research activities in Greenland in the 1990s fell into the following categories: gold (with palladium); magmatic nickel- copper-cobalt; iron-titanium-vanadium; diamonds; SEDEX- and carbonate-hosted zinc-lead; airborne geophysical projects; geochemistry; and Pb isotopes.Gold was one of the success stories, starting in the late 1980s and lasting into the beginning of the 1990s with the discovery of gold, platinum and palladium in the Tertiary Skaergaard intrusion. It continued in South Greenland, where all the gold occurrences in the Ketilidian Mobile Belt were discovered in the 1990s. Prospecting for diamonds on the west coast of Greenland revealed occurrences of both micro- and macro-diamonds—in particular, in the areas of Maniltsoq and Kangerlussuaq.The increased mineral exploration activity of the past decade has been encouraging. Several projects show promise, such as the Nalunaq gold deposit, the Citronen Fjord Zn-Pb deposit, the Skaergaarden Au-Pd discovery and the diamond showings. Combined with geological knowledge, the large amounts of data from the AEM and AEROMAG Greenland programmes and the drainage surveys will form the cornerstones of future exploration programmes and provide the key to new discoveries.

Re–Os isotope geochemistry of Tertiary picritic and basaltic magmatism of East Greenland: constraints on plume–lithosphere interactions and the genesis of the Platinova reef, Skaergaard intrusion

Re–Os abundance and isotopic studies on a small number of samples from the lowermost part of the East Greenland lava pile and the nearby Skaergaard intrusion show that picrite and ankaramite lavas tend to have high Os abundances and low Re/Os ratios, with most rocks having near-chondritic values for initial 187 Os/ 188 Os . One olivine basaltic sample with very high 187 Os/ 188 Os has most likely been affected by upper crustal contamination. There is no clear evidence for recycled crustal material in the ancestral Iceland plume source or for a significant degree of interaction of the picrites with ancient subcontinental lithospheric mantle of the North Atlantic craton. The Platinova Reef, the precious metal enriched zone in the Skaergaard intrusion, has very high Au and Pd contents but very low Pt and Os contents compared to platiniferous horizons in other layered intrusions. These characteristics are almost certainly due to the very late stage in the evolution of the Skaergaard magma chamber at which the Platinova Reef formed. The Platinova Reef yields a Re–Os T CHUR model age that is younger than the age of the intrusion, which was most likely produced by some form of post depositional disturbance to the Re–Os isotopic system. The high PGE contents of the East Greenland volcanic rocks indicate that they were formed from magmas that had the potential to form major accumulations of Ni–Cu–PGE sulphide deposits. Had similar magmas undergone significant interaction with continental crust elsewhere in East Greenland, they may have formed massive magmatic sulphide deposits. Thus, the potential for the discovery of large magmatic sulphide deposits of the Noril'sk-Talnakh-type in East Greenland must be considered excellent.

Phase equilibria modeling in igneous petrology: use of COMAGMAT model for simulating fractionation of ferro-basaltic magmas and the genesis of high-alumina basalt

A new version of COMAGMAT-3.5 model designed for computer simulations of equilibrium and fractional crystallization of basaltic magmas at low to high pressures is presented. The most important modifications of COMAGMAT include an ability to calculate more accurately the crystallization of magnetite and ilmenite, allowing the user to study numerically the effect of oxygen fugacity on basalt magma fractionation trends. Methodological principles of the use of COMAGMAT were discussed based on its thermodynamical and empirical basis, including specific details of the model calibration. Using COMAGMAT-3.5 a set of phase equilibria calculations (called Geochemical Thermometry) has been conducted for six cumulative rocks from the Marginal Border Series of the Skaergaard intrusion. As a result, initial magma temperature (1165±10°C) and trapped melt composition proposed to be parental magma to the Skaergaard intrusion were determined. Computer simulations of perfect fractionation of this composition as well as another proposed parent produced petrochemical trends opposite to those followed from natural observations. This is interpreted as evidence for an initial Skaergaard magma containing a large amount of olivine and plagioclase crystals (about 40–45%), so that the proposed and calculated parents are related through the melt trapped in the crystal–liquid mixture. This promotes the conclusion that the Skaergaard magma fractionation process was intermediate between equilibrium and fractional crystallization. In this case the classic Wager's trend should be considered an exception rather than a rule for the differentiation of ferro-basaltic magmas. A polybaric version of COMAGMAT has been applied for the genetic interpretation of a volcanic suite from the Klyuchevskoi volcano, Kamchatka, Russia. To identify petrological processes responsible for the observed suite ranging from high-magnesia to high-alumina basalts, we used the model to simulate the Klyuchevskoi suite assuming isobaric crystallization of a parental HMB magma at a variety of pressures and a separate set of simulations assuming fractionation during continuous magma ascent from a depth of 60 km. These results indicate that the Klyuchevskoi trend can be produced by ∼40% fractionation of Ol–Aug–Sp±Opx assemblages during ascent of the parental HMB magma over the pressure range 19–7 kbar with the rate of decompression being 0.33 kbar/% crystallized (at 1350–1110°C), with ∼2 wt.% of H 2O in the initial melt and ∼3 wt.% of H 2O in the resultant high-Al basalt.

Assimilation of Crustal Xenoliths in a Basaltic Magma Chamber: Sr and Nd Isotopic Constraints from the Hasvik Layered Intrusion, Norway

contaminated layered intrusions known. Thousands of recrystallized Strontium and neodymium isotopic data for mafic cumulates, chilled tabular xenoliths of metasedimentary origin enclosed in the cumulates margins, and adjacent crustal rocks of the Hasvik Layered Intrusion, are thought to represent the remnants of the assimilated material. North Norwegian Caledonides, are reported together with new The xenoliths spalled off the roof during magma emplacement, and, mineralogical and whole-rock analytical data to constrain the extent together with the elevated temperatures (400–600°C) of the midand effect of the assimilation of crustal xenoliths in a basaltic crustal country rocks, led to a high degree of assimilation in the magma chamber. Initial Sr/Sr (700 Ma) of 0·7045 and eNd Hasvik magma chamber. (700 Ma) of+3·03 for the chilled margin, which has a tholeiitic composition akin to the chilled rocks of the Skaergaard intrusion, demonstrate that the parental magma was derived from a depleted mantle source. The basal cumulates (0–335 m) show an up-section decrease in Sr/Sr from 0·7045 to 0·7038 and a correlative

The Triple Group and the Platinova gold and palladium reefs in the Skaergaard Intrusion; stratigraphic and petrographic relations

This paper describes the setting, structure, and textural relations of the Platinova reefs in the Skaergaard intrusion. The reefs comprise a strata-bound occurrence of Au + PGE estimated at 91 million tons of rock with 1.8 ppm Au and similar amounts of Pd. The precious metals occur in two major zones, a lower major Pd reef which is concordant with the modal layering and followed by at least ten minor Pd reefs (also concordant), and an Au concentration which fringes the Pd reefs outward and upward through the Triple Group units. The major Pd concentration and the Au concentration are separated in a systematic way across the intrusion. Near the center of the intrusion, the Au concentration occurs some 60 m higher in the stratigraphy than the main Pd reef, but the separation decreases gradually outward, being only a few meters near the margins.The dominant precious metal minerals are small grains of (Cu,Fe)(Au,Pd,Pt) alloys which typically are included in interstitial sulfides (bornite, digenite, and smaller amounts of chalcopyrite). The textural relations suggest a close genetic association between the sulfides and Au + PGE minerals, an association which is confirmed by whole-rock compositional data.Our observations demonstrate that the Platinova reefs formed after significant concentration of the Au + PGE and S in the residual magma during fractional crystallization, before it became saturated with respect to a sulfide (Fe-Cu-S-O) liquid. This sulfide liquid acted as a collector of Au + PGE and settled into the Layered series cumulates. We propose that the structure of the deposit evolved through fractional crystallization of the Au + PGE-bearing sulfide liquid as it was filter-pressed along with the interstitial silicate melt upward into the overlying unconsolidated cumulates. The fractional crystallization resulted in an evolution of the Au + PGE-bearing phases with progressive fractionation from Pd-rich alloys at the base of the deposit to Au-rich alloys at the top.We argue that the sulfides evolved to their present extreme Cu-rich, Fe-poor composition during oxidation-desulfurization reactions in the same event which oxidized the iron-titanium oxides of the intrusion. This oxidation-desulfurization probably had no overall effect on the distribution of Au and PGE.

Iron in plagioclase as a monitor of the differentiation of the Skaergaard intrusion: a discussion of Christian Tegner (Contrib Mineral Petrol 128: 45-51)

Iron in plagioclase as a monitor of the differentiation of the Skaergaard intrusion: a discussion of Christian Tegner (Contrib Mineral Petrol 128: 45-51)

40Ar [sbnd]39Ar geochronology of Tertiary mafic intrusions along the East Greenland rifted margin: Relation to flood basalts and the Iceland hotspot track

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 by 40Ar 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.

INCLUDED BLOCKS (AND BLOCKS WITHIN BLOCKS) IN THE SKAERGAARD INTRUSION : GEOLOGIC RELATIONS AND THE ORIGINS OF RHYTHMIC MODALLY GRADED LAYERS

The early Eocene Skaergaard intrusion of Greenland includes enormous numbers of rocks of both exotic and cognate origins. The lower parts of the Marginal Border Series contain abundant fragments of feldspathic peridotite that are possibly autoliths, intermixed with occasional xenoliths of Precambrian gneiss and metasomatized Cretaceous–Paleocene sediments derived from adjoining country rocks. The Upper Border Series includes one exceptionally large block of gneiss (several hundred meters across), and numerous smaller fragments, these originating from the intrusion9s footwalls, plus a few pieces of peridotite. The Layered Series contains countless autoliths of troctolite, gabbroic anorthosite, and oxide (magnetite-ilmenite) gabbro, broken from parts of the Upper Border Series that have otherwise been lost to erosion; at the upper midlevel of its western half, it contains a few xenoliths of basalt, derived probably from the now-eroded (Eocene) roof of the intrusion. A distinctive postintrusion composite basaltic dike at one place contains 40 or more xenoliths of troctolite, olivine gabbro, and gabbroic anorthosite that may represent parts of the Layered Series still hidden at depth. The Layered Series autoliths range from fragments a few centimeters on a side to blocks more than 400 m across, and they typically are coarser grained than their host cumulates, being in this respect more like Upper Border Series rocks. The autoliths are spread stratigraphically through the lower 70% of the exposed 2500 m thickness of the Layered Series and are generally concentrated in three broad stratigraphic zones. Their physical relationships to their host rocks—particularly the way they indent older layers beneath and are covered by younger layers above—provide abundant evidence that there was generally a sharp, well-defined interface between the top of the cumulate pile and the main body of magma in the intrusion while the Layered Series was forming. The distribution of the autoliths between and through the well-known, rhythmic, thin, modally graded layers shows that these layers were spread by magmatic currents; and their relations to the more extensive macrorhythmic layering suggest that it too was significantly shaped by currents. Many of the larger autoliths are crudely layered internally, and in places it is evident that their stratification existed before they broke loose; therefore, it must have formed in the Upper Border Series. One particularly large block of oxide gabbro exhibits extraordinarily well-developed modal and textural layering and includes small troctolitic autoliths of an earlier generation, and it provides evidence that currents also spread crystalline materials across the top of the magma body. Many of the very small autoliths in the Layered Series are highly anorthositic in composition, apparently because they were leached of mafic minerals, and some of the larger blocks show local patchy internal replacement by anorthosite. Most large blocks show little sign of postaccumulation modification, and some have thin, fine-grained augite-rich rims or rinds, demonstrating that even though they were out of thermal and chemical equilibrium with their host cumulates, they still were effectively armored against extensive chemical change. Also documented is a large block that was cut by several early basaltic dikes before it broke free from the top of the intrusion; these early dikes transgress small anorthositic replacement pipes in the block, showing that the replacement process also occurred in the upper border environment. Two mechanisms are described whereby graded cumulate layers can be sorted and deposited by magmatic crystal-liquid suspension currents. One, involving density surge currents, has been advocated previously; the other is a new concept based on boundary flow separation and reattachment vortex cells. The two mechanisms are used in complementary ways to illustrate the formation of (1) some of the principal Skaergaard structures involving blocks and layers; (2) modally graded layers in the Layered Series that rhythmically alternate with uniform layers; and (3) modally sorted layers in the Upper Border Series featuring “underside draping” beneath small included blocks. Explanations are provided for (1) why plagioclase did not float away from the tops of graded layers even though it was less dense than the liquid, and (2) how the liquid part of a current was fractionated away from the crystalline materials. Modal and grain-size data from Skaergaard intrusion graded layers are shown to be in excellent accord with characteristics predicted for layers sorted by currents; a synthesis diagram is presented illustrating how all the above processes may have functioned in concert in the intrusion.

The Skaergaard Layered Series. Part III. Non-dynamic Layering

Layering in the Skaergaard Intrusion has been divided into two INTRODUCTION general types, one produced by magmatic flow and another by Recent studies of the structural and textural features of processes resulting from variations of rates of nucleation and crysthe Skaergaard Intrusion (McBirney & Nicolas, 1997) tallization, and, in the case of the Layered Series, by compactionhave distinguished two broad types of layering, one related processes. Modal variations caused by shifts of cotectic produced by the dynamic effects of magmatic flow and proportions produce thick layers which, in the Layered and Upper another by processes that operate in situ such as varied Border Series, are diffuse and normally lack strong foliation and nucleation and growth of crystals, recrystallization, or by lineation. In the Marginal Border Series, the layers are thinner and compaction-related mechanisms. We refer to layering sharper; possibly because the rate of accumulation was slower. formed by these latter processes as ‘non-dynamic’ to Oscillatory nucleation may have played a role in producing fineemphasize that it is not the result of fluid dynamic scale cyclic layers, but it was less important than solution and processes. Although most layering combines elements of reprecipitation during slow cooling and Ostwald ripening. Evidence more than one process, the contribution of each mechfor compaction is found in deformed plagioclase laths and a relative anism can usually be recognized from its distinctive form deficiency of incompatible elements in rocks formed on the floor. and setting. Layering related to compaction becomes sharper with increasing The distinguishing features of dynamic magmatic layheight in the Layered Series until it suddenly disappears above the ering have been described in Part II of this series (McBirtrough horizon near the base of Upper Zone b. Mechanical sorting ney & Nicolas, 1997). They are best seen near the steep during compaction may have produced crude layering, but if it did margins of the Layered Series in what Wager & Brown the evidence has long since been destroyed by the superimposed effects (1968) referred to as the ‘cross-bedded zone’ where the of solution and reprecipitation when interstitial liquid rose through layering is disrupted by slumping and channeling and the overlying crystals and re-equilibrated with them. Numerical the rocks have a marked foliation and lineation. simulations illustrate how small differences of surface energy caused Layering not associated with magmatic flow takes a by variations of grain size, textural dependence of solubility, and variety of forms, but we can distinguish two general pressure solution can cause segregation of minerals into layers during end-members, each of which has a distinct origin and solution and reprecipitation. characteristics. The first results from varied rates of