Petrology of the Upper Border Series of the Skaergaard Intrusion

The Upper Border Series of the Skaergaard intrusion consistsof a 960 m thick sequence of rocks that crystallized againstthe roof of the magma chamber. The texture and composition ofthe unit vary systematically from top to bottom as a resultof changes that occurred in the magma during the solidificationof the intrusion. The order of crystallization of primocrystminerals in the Upper Border Series was: olivine; + plagioclase;+ apatite; + ilmenite; + magnetite; + Ca-rich pyroxene;—olivine;+ olivine; + ferrobustamite. The major silicate phases varyfrom high-temperature compositions to low-temperature compositionswith increasing distance from the upper contact. Post-crystallizationre-equilibration has affected the compositions of the oxideminerals and to a lesser extent the compositions of olivineand Ca-rich pyroxene. The Upper Border Series differentiationsequence differs from the Layered Series sequence, in that:(1) apatite appears much earlier; (2) magnetite precipitatedbefore Ca-rich pyroxene rather than after it; (3) orthopyroxeneis much less common; (4) the plagioclase is systematically poorerin K₂O; and (5) the rocks are systematically richer in K₂O andSiO₂. The upper part of the Skaergaard magma appears to havebeen enriched in H₂O, K₂O, SiO₂, and P₂O₅ relative to the partthat was parental to the Layered Series.     

Origin of the picrite blocks in the Marginal Border Group of the Skaergaard Intrusion,East Greenland

New evidence shows that the picrite blocks in the margins of the Skaergaard intrusion, East Greenland are gabbro-contaminated xenoliths of ultramafic rock. Earlier studies suggested that the picrite blocks were cumulates formed in the Marginal Border Group or in the Hidden Zone. However, there are no known occurrences of undisturbed picrite or ultramafic rocks in the Skaergaard intrusion, and an extensive Hidden Zone is not supported by geophysical data. The picrite blocks are most abundant near a body of wehrlite in Precambrian rocks near Watkins Fjord. The wehrlite, which has a composition and mineralogy similar to the most mafic of the picrite blocks, lies structurally below the northern margin of the intrusion. It is possible that the refractory precursors of picrite in the Skaergaard intrusion may have been ultramafic xenoliths and are not representative of the earliest differentiated pan of the intrusion.     

The Chilled Marginal Gabbro and Other Contact Rocks of the Skaergaard Intrusion

Rocks in the outer selvage of the Skaergaard intrusion havea range of textures and compositions, and among these are materialsrepresenting quenched Skaergaard magma. Pristine chilled marginalgabbro (CMG), however, is not ubiquitous at the intrusive contact,because many of the "contact" rocks have been hydrothermallyor metasomatically altered, contaminated with gneiss or olivinexenocrysts, while others contain accumulated minerals. Materialrepresenting quenched magma appears to be restricted to contactrocks that are texturally and mineralogically similar to diabase,and free of accumulated minerals. Where it exists, the CMG isfound within one to three meters of the exposed intrusive contactexcept at the roof of the intrusion where its thickness is greater.CMG was distinguished from the diverse group of contact rocksby petrographic and geochemical screening of over 80 specimens.Samples of CMG from the eastern and western margins and fromthe roof of the intrusion have relatively uniform compositionsimilar to that of ferrobasalt, and are noticeably richer iniron (mg-number=0?51-0?54), TiO₂ K₂O, and P₂O₅ than other unmodifiedcontact rocks. CMG's also have trace element compositions distinctfrom most other rocks in the outer Marginal Border Series (MBS).They have incompatible element contents up to 3–6 timesgreater than in LZa-type cumulates, negligible Eu anomalies,and Ni and Cr contents and Ni/Cr ratios that are among the lowestof rocks in the outer MBS. The results of melting experiments corroborate selection ofthis material as CMG. The composition of glasses obtained frompartial melting experiments of LZa-type cumulates are essentiallyidentical to those of the CMG. The 1-atm. liquidus phase relationsfor one of the CMG samples (KT-39) is largely consistent withthe sequence and composition of cumulus minerals observed withdistance inward through the MBS and upward through the LayeredSeries. Solidification of magma at the outer margin of the intrusionis interpreted to have involved locally efficient quench crystallizationfollowed by initial primocryst growth in an undercooled transitionzone a short distance inward that finally extended into regionsof near equilibrium crystallization. The similarity in composition between samples of chilled marginalgabbro from the exposed roof and sides of the intrusion, andthose of reconstituted trapped liquid from early cumulates inthe outer MBS suggests that a single magma, similar in compositionto ferrobasalt, was parental to the Skaergaard intrusion. Thisinterpretation corroborates geophysical evidence of a significantlysmaller mass for the intrusion than that estimated by Wager,and provides a basis for revision of models of its chemicalevolution. Samples chosen by Wager as chilled marginal gabbrobelong spatially, texturally, and compositionally to the groupof LZa-type cumulates in the MBS, and should no longer be regardedas chilled marginal gabbro.     

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.     

The differentiation of the Skaergaard Intrusion

Previous interpretations of the Skaergaard Intrusion suggested that differentiation involved extreme iron-enrichment but no silica-enrichment until a very late stage. This model is difficult to reconcile with petrological and geochemical evidence, with the behaviour of tholeiitic volcanic suites and with phase equilibria. We propose that the Skaergaard magma evolved on a trend of pronounced silica-enrichment after cumulus magnetite appeared at the top of the Lower Zone. At that stage, the magma was of ferrobasaltic composition with close to 50% SiO₂. The Middle and Upper Zones of the intrusion dominantly represent crystal accumulation during differentiation from ferrobasalt through iron-rich basaltic andesite and icelandite to rhyolite, a fractionation sequence common in tholeiitic volcanic provinces. This interpretation requires re-appraisal of the physical processes responsible for the differentiation. In particular, residual liquids became lower in density with fractionation and would have caused the Skaergaard magma chamber to have become compositionally zoned.     

Precambrian Sequence Bordering the Skaergaard Intrusion

Archean tonalitic-granodioritic orthogneisses bordering theSkaergaard Intrusion contain widespread boudins and lenses ofgarnet-biotite schist, quartzite, amphibolite, and ultramaficrocks. These rocks are similar to and locally gradational withnarrow intact supracrustal belts in the region. We correlateearliest isoclinal folds in supracrustal belt rocks and in earliesttonalitic-trondhjemitic-granitic (TTG1) orthogneisses with regionallydeveloped (D₂) deformation. We also correlate strong foliation(Ssp1) in the supracrustal rocks and banding (Sbgn1) in earliestorthogneisses with D₂ deformation which followed and overlappedearliest M₁ metamorphism. Ssp1 foliation is in part axial planarwith D₂ isoclinal folds transposing compositional and subparallelmetamorphic banding in the hinge areas. Ssp1 assemblages inmetapelites consist of folia of coexisting sillimanite-biotite-quartzand correspond roughly to metamorphism at the second sillimaniteisograd. We correlate syntectonic emplacement of a later generationof orthogneisses (TTG2) with strong D₃ shearing-cataclasis associatedwith tectonic intercalation of supracrustal rocks and earliestorthogneisses. The latest metamorphic assemblages (M₂) consistof granoblastic and porphyroblastic minerals that overprintSsp1 (M₁) foliation and D₃ fabrics. These assemblages formedduring largely static regional metamorphism about 2900 Ma agoand are locally aligned with fabric elements of D₄ folding. Temperatures during M₂ metamorphism equalled or exceeded thestability of biotite-sillimanite-quartz in metapelites and chlorite-orthopyroxene-olivine-spinel-hornblendein ultramafic rocks. Fe-Mg biotite-garnet exchange, and thepressure-dependent garnet-plagioclase-sillimanite-quartz equilibriumassemblage in metapelites yield temperature and pressure estimatesfor M₂ metamorphism of 650–700?C and 3–4 kb. Thesedata suggest that M₂ assemblages formed as results of dehydrationreactions at water partial pressures that were less than thetotal pressure. The temperature-dependent equilibrium assemblagechlorite-orthopyroxene-olivine-spinel-vapor (+hornblende), adjustedfor observed phase compositions, is consistent with the Fe-Mgbiotitegarnet exchange geothermometer. Rare-earth element, Rb-Sr and Pb-Pb isotopic, and other compositionalcharacteristics of the orthogneisses are generally consistentwith a multiple stage magmatic origin of their protoliths. OlderTTG1 orthogneisses have compositions generally consistent withformation of the magmas parental to their protoliths by partialmelting of garnetiferous source rocks such as eclogite, or lowercrust. Younger TTG2 orthogneisses have compositions that areconsistent with their formation as water-saturated second meltsin equilibrium with a hornblende-rich residuum. Their formationoccurred within a few 10⁷ y after crustal emplacement of TTG1orthogneisses. The source of water for the formation of later(TTG2) melts may have been M₂ dehydration reactions deeper withinthe supracrustal pile.     

Mullite-corundum-spinel-cordierite-plagioclase xenoliths in the Skaergaard Marginal Border Group: multi-stage interaction between metasediments and basaltic magma

Metapelitic country rocks were contact- and pyro-metamorphosed by the Tertiary Skaergaard Intrusion, East Greenland. In an initial stage of heating, while they were probably still in place within the host rock contact aureole, they overstepped a range of equilibrium and disequilibrium melting reactions and produced both a granitic melt and very refractory spinel+cordierite+plagioclase±corundum residuals. Parts of these refractory rocks were then subjected to another melting event after being entrained as xenoliths into the Skaergaard Marginal Border Group, where they experienced a temperature of about 1,000°C at a pressure of about 650 bars and at an oxygen fugacity about 0.2–0.5 log units below the FMQ buffer. Here, they underwent bulk melting, but did not mix with the Skaergaard magma, presumably because of the high viscosity contrast. The Al-rich melts crystallized to an assemblage of corundum+mullite+sillimanite+ plagioclase+spinel+rutile±tridymite±cordierite and they reacted with the surrounding basalt producing a strongly cryptically zoned rim of plagioclase (An₅₅ close to the basalt to An₉₀ close to the Al-rich melt). The assemblage in the inner parts of the xenoliths provides textural evidence for disequilibrium growth due to slow diffusivities in the highly viscous, probably water-free Al-rich melt. Later interaction of lower temperature ferrobasaltic to granophyric melts with the xenoliths along their margins and along cracks led to consumption of corundum and mullite and to the stable assemblage of spinel+cordierite+plagioclase+quartz+K-feldspar +magnetite+ilmenite at about 800°C.     

Crystallization and Layering of the Skaergaard Intrusion

Solidification of large slowly cooled intrusions is a complexprocess entailing progressive changes of rheological propertiesas the crystallizing magma passes through successive stagesbetween a viscous Newtonian fluid and a brittle solid rock.Studies of this transition in the Skaergaard intrusion indicatethat most crystallization took place in an advancing front ofsolidification against the floor, walls, and roof where crystalsnucleated and grew in a static boundary layer, much in the mannerproposed by Jackson in 1961. The non-Newtonian properties ofthe crystallizing magma account for the fact that plagioclase,which was lighter than the liquid, is a major component of rockson the floor, while mafic minerals that were heavier than theliquid accumulated under the roof. Crystals that nucleated andgrew in these zones were trapped by an increasingly rigid zonethat advanced more rapidly than the crystals sank or floated.If any crystals escaped entrapment, they were those of the largestsize and density contrast. The rates of accumulation in different parts of the intrusionwere not governed by rates of gravitational accumulation somuch as by the nature of convection and heat transfer. Cumulatetextures, preferred orientations of crystals, and layering,all of which have been taken as evidence of sedimentation, canbe explained in terms of in situ crystallization. Layering cannothave been caused by density currents sweeping across the floor;it is well developed on the walls and under the roof, lacksthe size and density grading and mineralogical compositionsthat would be expected, and shows no evidence of having beenaffected by obstructions in the paths of the currents. We propose an alternative origin of layering that is based onprocesses governed by the relative rates of chemical and thermaldiffusion during cooling. Intermittent layering resulted fromgravitational stratification of the liquid, and cyclic layeringwas produced by an oscillatory process of nucleation and crystalgrowth. The effects of differentiation during in situ crystallizationare strongly dependent on relative rates of diffusion of individualcomponents, and some of the compositional variations in differentparts of the intrusion can be explained in terms of these differences.     

Exsolution in augite from the Skaergaard Intrusion

The exsolution phenomena of augite from Ferrogabbro 4430 of the Skaergaard Intrusion were examined in detail by single crystal X-ray diffraction and heating experiments to study the stepwise exsolution process. In the augite crystals, five different phases were detected: pigeonite (001), pigeonite (100), orthopyroxene (a), orthopyroxene (p) and a small amount of clinoamphibole. The two different pigeonites nearly share the corresponding (001) and (100) planes with the host. Orthopyroxene (a) and orthopyroxene (p) have (100) in common with the host and with exsolved pigeonite (001), respectively. Clinoamphibole was observed in the form of rather weak reflections in many crystals. It has (010) in common with the host.A large number of augite crystals exhibited a pigeonite (001) phase with curved, rotated reflections and diffuse streaks along the a* direction in (h0l) precession photographs. It appears that these streaks are related to orthopyroxene (p). Orthopyroxene (p) seems to be crystallized from pigeonite (001) by nucleation at (100) stacking fault planes (inverted pigeonite). Pigeonite (100) may be formed at growth ledges between augite host and exsolved orthopyroxene (a) at a later stage of exsolution to stabilize the boundaries.From the X-ray diffraction profiles and the results of the heating experiments, a possible exsolution sequence is suggested. Clinoamphibole appears to be a product of alteration at the latest stage of the exsolution process. It seems to be related to particular conditions of partial water pressure.     

Modally-Graded Rhythmic Layering in the Skaergaard Intrusion

The origin of igneous layering in the Skaergaard and other intrusionsis the subject of much debate. Currently held theories for itsorigin include gravitational sorting, rhythmic nucleation, andchemical diffusion models. This paper presents data on small-scale,modally-graded, rhythmic layers, one of several distinct typesof layering found in the Skaergaard. These layers occur onlyin the Layered Series of the intrusion, in which they are awidespread, almost ubiquitous feature. They are characterizedby strong variations in modal composition, with magnetite, ilmenite,and olivine concentrated in the lower part of the layers, pyroxenein the middle of the layers, and plagioclase in the upper partof the layers. In addition, the layers are size graded, withthe largest grain sizes of the different minerals generallyoccurring where the mineral is most abundant. The chemical compositionof both plagioclase and Ca-rich pyroxene do not vary in anyregular manner within the layers studied. The forsterite compositionof olivine is consistently higher in the lower part of the layers.This trend, however, correlates very strongly with the modalvariations of olivine and can be explained by minor amountsof subsolidus reequilibration between cumulus and adcumulusolivine, suggesting that the original composition of cumulusolivine within individual layers was constant. These results are consistent with layers formed by some gravitationalsorting process and, in several cases, are at odds with othertheories that have been proposed for the origin of igneous layers.Based on these data and the widespread evidence for currentinteraction (crossbeds, trough-shaped layers, and internallygraded layers), we propose that these layers result from periodicdisruption of a static zone of crystal growth on the floor ofthe magma chamber by convection or density currents. This mechanism,however, is almost certainly not responsible for the formationof all of the types of layering found in the intrusion.     

Xenoliths of gneisses and the conformable,clot-like granophyres in the Marginal Border Group,Skaergaard intrusion,East Greenland

Results of this research and the earlier work of Wager and his colleagues indicate that contamination influenced the overall character of the Marginal Border Group of the Skaergaard intrusion. Precambrian gneisses were a major source of contamination and are identifiable as xenoliths in the Marginal Border. A variety of other xenoliths occur with a wide compositional range.The range of K _D values for partitioning of Fe and Mg in coexisting pyroxene pairs in xenoliths from various parts of the Marginal Border, and in hornfelses adjacent to it, is consistent with temperatures that rose steeply inward. Temperatures estimated on the basis of compositions of coexisting pyroxenes are also consistent with dehydration that exceeded the stability of amphibole (850°C). The texturally compatible association of some granophyres with gneissic xenoliths suggests that both formed during melting. These observations suggest that there was a similar range of temperature for formation of xenoliths and granophyres.The xenoliths of gneisses have bulk rock compositional features which indicate that lithophilic constituents were removed, causing an increase in basic constituents upon assimilation of the gneissic precursors. If we assume that granophyres and xenoliths of gneisses represent a consanguineous set formed by a fusion process from gneiss, enrichment and depletion factors for excluded trace elements are complementary and generally less than 10. Allowable enrichment in granophyres by crystal fractionation using the average for a large number of chilled marginal gabbro analyses as an initial composition, is also less than 10 for the same elements. The calculated low factors for enrichment of lithophile elements in granophyres by both mechanisms favor the hypothesis of partial melting of gneiss.     

Petrology of the Vandfaldsdalen Macrodike, Skaergaard Region, East Greenland

The Vandfaldsdalen macrodike is a layered and differentiatedgabbroic dike approximately 3?5 km long and from 200 to 500m wide. It appears to cut the eastern margin of the Skaergaardintrusion and may have served as a feeder for the Basistoppensill. The macrodike can be divided into three series of rocks:a marginal series of differentiated gabbros adjacent to thewalls of the dike; a central series of differentiated and subhorizontallylayered gabbros and ferrodiorites in the interior of the dike;and an upper felsic series of granophyric rocks with abundantquartzo-feldspathic xenoliths. The mineral and bulk-rock compositionsthrough both the marginal series and central series show progressiveiron enrichment. The most Ca-rich plagioclase (An₆₉) and mostmagnesian pyroxene (Wo₄₂ En₄₆ Fs₁₂) occur in olivine-bearingrocks of the marginal series about 5 m from the contact withwall rocks. The most Na-rich plagioclase (An₃₉) and Fe-richpyroxene (Wo₃₈ En₂₄ Fs₃₈) are in olivine-free ferrodiorite ofthe central series, about 20 m below the contact with the felsicseries. Evidence from field observations, bulk-rock chemical compositions,and Sr and Nd isotopic data indicate the felsic series formedas a mixture of the initial macrodike magma and granitic countryrock. ⁸⁷Sr/⁸⁶Sr ratios of specimens from the felsic series rangebetween 0?7129 and 0?7294. ¹⁴³Nd/¹⁴⁴Nd ratios vary between 0?51208and 0?51118. Both ratios vary serially with the SiO₂ contentsof the specimens. We suggest that the felsic series evolvedas a separate body of low density liquid which floated on thedenser gabbroic magma of the central series. Heat from crystallizationof the gabbroic magma must have diffused into the felsic layer,enabling extensive assimilation of the granitic xenoliths, butour data indicate there was very little exchange of chemicalcomponents between the two liquids.     

The Zoned Plagioclase of the Skaergaard Intrusion, East Greenland

The variation in zoning and An-content of plagioclases in theSkaergaard intrusion was investigated using microprobe analysisand optical methods on material from the surface of the intrusionas well as from a drill core from the hidden zone of the layeredseries. The plagioclases display cryptic variation and prominentvariations in type of zoning with structural height: oscillatoryzoned and resorbed plagioclase cores are predominant in thehidden zone, unzoned cores are typical of the lower zone, andplagioclases of the upper zone display skeletal zoning. Thereverse, oscillatory and skeletal zoning of plagioclase coresis ascribed to supercooled crystallization, and not to convectionor other petrogenetic factors. The variations in type of zoningand An-content of plagioclases from the hidden zone suggestonset of convection at the same time in the cooling historyof the intrusion as is indicated by the transition between thetranquil division and the banded division of the marginal bordergroup. A crystallization model is suggested, which accountsfor the observed variations. The variation in An-content ofplagioclases from the hidden zone suggests a limited extensionof some hundred metres of the intrusion at depth.     

Further Considerations of Double-Diffusive Stratification and Layering in the Skaergaard Intrusion

Depending on the orientation of thermal and compositional gradientsat the margins of a cooling intrusion, double-diffusive convectionmay take one of two principal forms. If the body is coolingat a steep wall, so that both gradients are horizontal, theresult can be boundary layer flow up or down the wall and segregationof the fractionated liquid under the roof or on the floor. Ifcooling is through the roof or floor of a broad sheet-like body,the gradients will be vertical and if the physical propertiesof the liquid are appropriate, they could lead to broad horizontallystratified convection. The first case has been analysed by Nilsonet al. (1985) and shown to be a physically plausible explanationfor certain types of zoned magmas, but the second case, in whichboth gradients are vertical, appears to be less likely, especiallyin slowly cooled intrusions in which temperatures are controlledby crystal-liquid equilibrium. Depending on whether the magma is losing heat through its roofor floor and on whether its density increases or decreases withcooling and differentiation, four possible conditions are possible.A linear stability analysis based on the experimental and theoreticalcriteria of Baines & Gill (1969) and Shirtcliffe (1969)indicates that neither calc-alkaline liquids that accumulateunder their roof nor tholeiitic liquids ponded on their floorare likely to convect in stratified layers. Even though thegradients and relative diffusivities of heat and compositionmay be in a direction that tends to destabilize the magma, themagnitude of the compositional effect on density is so greatthat the temperature gradient would have to diverge widely fromthe liquidus, and convection would be possible only with largeamounts of under-cooling or superheat. A similar mechanism, based on opposed gradients of two chemicalcomponents of differing diffusivities, may provide an alternativeexplanation for certain types of stratification, but field relationsin the Skaergaard Intrusion do not support any origin that isprimarily dependent on gravity.     

The Isotopic Composition of Strontium in the Skaergaard Intrusion, East Greenland

The isotopic composition of strontium in the different partsof the differentiated Skaergaard intrusion has been determined.The average Sr⁸⁷/Sr⁸⁶ ratio for the basic rocks was found tobe 0?7065?0?002. Higher values, between 0?7101 and 0?7303, wererestricted to the late-stage acid granophyres. The Sr⁸⁷/Sr⁸⁶ratios for the basic Skaergaard rocks are similar to those foundby previous workers. The enrichment in Sr⁸⁷ expressed in theSr⁸⁷/Sr⁸⁶ ratio is taken to indicate contamination of the acidgranophyres by a source enriched in Sr⁸⁷. From considerationsbased upon circumstantial evidence the average country rock,composed of old Precambrian acid to intermediate gneiss, isnot sufficiently enriched in.Sr⁸⁷ to account for the Sr⁸⁷ enrichmentobserved in the acid granophyres by a simple assimilation process.At the present stage of the investigation the enrichment ofthe acid granophyres in Sr⁸⁷ is unexplained.     

Electrochemical measurements bearing on the oxidation state of the Skaergaard Layered Intrusion

The oxygen fugacities (fO₂'s) of magnetically-concentrated fractions (MCF) of three rock samples from the Skaergaard Layered Intrusion were measured between 800–1150° C using oxygen-specific, solid zirconia electrolytes at atmospheric pressure. Two of the bulk rock samples (an oxide cumulate and an oxide-bearing gabbro) are from the Middle Zone (MZ) and the other (an olivine plagioclase orthocumulate) is from the Lower Zone (LZ). All MCF define fO₂ versus T arrays that lie 1.5–0.5 log units above the fayalite-magnetite-quartz (FMQ) buffer. Experiments with different cell-imposed initial redox states (one from a reduced direction and one from an oxidized direction) were run on each sample in an attempt to achieve experimental reversibility. This was accomplished by imposing a known redox memory on the galvanic cell prior to loading each sample. Reversibility for each sample agreed to better than 0.2 of a log unit. Irreversible autoreduction of 0.2 of a log unit was observed on the two MZ samples at temperatures exceeding 1065° C. Scanning electron microscope and electron microprobe study of pre- and post-run products shows that reaction and textural re-equilibration occurred among the oxide phase assemblages under the experimental conditions employed. Careful characterization of pre- and post-run assemblages is clearly necessary before adequate interpretation of the experimental results can be made in these types of electrochemical studies. Different approaches to investigations of the fO₂ of the Skaergaard Intrusion, be it thermodynamic calculations or experimental methods, should yield concordant results or at least understandable discrepancies. Calculated fO₂'s using thermobarometry applied to the ilmenite-magnetite pairs in the post-experimental assemblages agree with the experimentally determined fO₂'s to within one log unit at a given temperature. These results are also consistent with previously calculated fO₂ values (Buddington and Lindsley 1964; Morse et al. 1980), but are considerably more oxidized than a previous electrolyte-based fO₂ study of a different sample suite from the Skaergaard (Sato and Valenza 1980) that include values close to the iron-wustite (IW) buffer from both MZ and LZ oxide separates. Differences between this electrochemical study and that of Sato and Valenza (1980) may be due to variations in the level of indigenous (or curatorially-introduced) carbon in the samples studied. Despite a number of experimental difficulties, electrochemical cells can provide an accurate and precise method of determining the oxygen fugacity of naturally occurring, complex oxide assemblages. Tight experimental reversals and reproducible values obtained in heating and cooling cycles are an indication of the precision and accuracy of the data recoverable with electrochemical cells.     

Petrology of the Partridge River Intrusion, Duluth

Drill core DDH-221 was drilled for the Minnamax Project by AMAX Exploration, Inc., as part of the exploration for Cu-Ni sulfidesin the basal rocks of the northwestern margin of the DuluthComplex. The drill core intersects 525 m of the Partridge RiverIntrusion before passing into the Virginia Formation footwall.The rocks in the drill core comprise plagioclase and olivinecumulates, with troctolite and olivine gabbro as the most commonrock types. Sulfide- and oxide-bearing gabbros are present inthe lowest 100 m of the core where decreases in the crystalsizes of plagioclase and olivine, and the appearance of ophitictextures adjacent to the footwall, indicate that the chilledmargin of the intrusion has been preserved (Chalokwu & Grant,1990). The concentrations of incompatible elements in the wholerocks and the iron contents of olivine and pyroxenes all increasesharply in the lowest 100 m of the drill core (Chalokwu &Grant, 1990), and are interpreted as the downward increase inintercumulus liquid now preserved as intercumulus phases, andthe reaction of this liquid with olivine and pyroxenes. Mass, balance calculations for rocks containing widely differentvolumes of intercumulus phases show that the intercumulus liquidwas a chemically uniform ferrodiorite that can be derived fromKeweenawan high-alumina olivine tholeiite by plagioclase (An₆₃),clinopyroxene (En₅₀Fs₁₀Wo₄₀), and olivine (Fo₇₁) fractionation. Initial ⁸⁷Sr/⁸⁶Sr values for plagioclase range between 0–704764and O-706335, with the highest values occurring adjacent tothe footwall Virginia Formation, and the lowest at intermediatedepths in the core. These variations are similar to ⁸⁷Sr/⁸⁶Srvalues reported earlier by Grant & Moiling (1981) From theadjacent core DDH-295, although the values are all greater thanpublished initial ratios for the least altered Keweenawan lavas.We attribute the isotopic variations in core DDH-221 to isotopicheterogeneities in the Partridge River Intrusion magmas, andto limited assimilation of the Virginia Formation within 50m of the footwall. Rare-earth and other trace elements in the intercumulus liquidfrom core DDH-221 have similar distributions to the same elementsin Keweenawan basic to intermediate lavas. We conclude that the rocks of the Partridge River Intrusionsampled in drill core DDH-221 comprise a mechanical mixtureof cumulus plagioclase and olivine and intercumulus liquid thatwere not in equilibrium with each other, and that the intercumulusliquid was broadly consanguineous with Keweenawan high-aluminaolivine tholeiite lavas, but was modified to a greater extentby assimilative exchange with continental crust. After emplacement,the crystal-liquid mixture was modified by flotation of thecumulus plagioclase out of the basal zone, and by limited —but not ubiquitous — assimilation of footwall VirginiaFormation.     

The Major Element Variation of the Layered Series of the Skaergaard Intrusion and a Re-estimation of the Average Composition of the Hidden Layered Series and of the Successive Residual Magmas

Using variation diagrams for the major elements in the layeredrocks, estimates are made of the average amounts of the variouselements in the total rock separating at successive stages.From the analyses of the chilled marginal gabbro, taken to representthe composition of the initial magma, and with the further likelyassumption that the Skaergaard intrusion is a closed system,at any rate for most of the elements, various hypotheses onthe relative volumes of the different parts of the intrusionare tested to find the one best fitting the known distributionof the elements in the observable rocks. Estimates are thenmade of (1) the overall composition of the hidden part of theintrusion by subtracting the amounts of an element in the observedrocks from the total in the initial magma, and (2) the compositionof the successive residual magmas formed as a result of thecrystal fractionation.     

The Major Element Variation of the Layered Series of the Skaergaard Intrusion and a Re-estimation of the Average Composition of the Hidden Layered Series and of the Successive Residual Magmas

Using variation diagrams for the major elements in the layeredrocks, estimates are made of the average amounts of the variouselements in the total rock separating at successive stages.From the analyses of the chilled marginal gabbro, taken to representthe composition of the initial magma, and with the further likelyassumption that the Skaergaard intrusion is a closed system,at any rate for most of the elements, various hypotheses onthe relative volumes of the different parts of the intrusionare tested to find the one best fitting the known distributionof the elements in the observable rocks. Estimates are thenmade of (1) the overall composition of the hidden part of theintrusion by subtracting the amounts of an element in the observedrocks from the total in the initial magma, and (2) the compositionof the successive residual magmas formed as a result of thecrystal fractionation.     

Pyroxenes from the Late Stages of Fractionation of the Skaergaard Intrusion, East Greenland

The pyroxenes of the upper zone of the Skaergaard layered seriesconsist of an iron-rich series of brown and green clinopyroxenes.Five new analyses are presented, together with a revised trendline which includes the full range of clinopyroxenes believedrepresentative of the Skaergaard fractionation sequence. Therange for the augite-ferroaugite-ferrohedenbergite series isfrom Ca_42?4 Mg_47?9 Fe_9?7 to Ca_42?5 Mg_0?4 Fe_57?1, the most ferriferousvariety coexisting with a pure fayalite. A re-study of the compositionsand textures of certain green ferrohedenbergites supports thebelief that they are the product of sub-solidus inversion offerriferous ß-wollaston?tes, which crystallized asa temporary phase between the periods when brown ferrohedenbergitescrystallized directly from the magma. A consideration of thecompositional and textural relationships between green and brownpyroxenes, and the significance of the mosaic inversion texture,have led to an interpretation of the crystallization and inversionsequence for these minerals. Four new analyses of ferrohedenbergitesfrom the downward-crystallized upper border group rocks provideevidence for a trend which differs slightly from that for thelayered series ferrohedenbergites.