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.     

Petrology of the Marginal Border Series of the Skaergaard Intrusion

The Marginal Border Series (MBS) of the Skaergaard intrusionconsists of rocks formed by in situ crystallization againstthe walls of the intrusion. Most of these rocks are productsof fractional crystallization, though samples believed to representchilled liquid occur locally at the intrusive contact. The MBScomprises only 5% of the exposed volume of the intrusion, butwithin its thickness, the order of crystallization and the compositionsof fractionated rocks and minerals vary systematically withdistance inward from the intrusive contact in largely the samemanner as rocks and minerals upward through the Layered Series(LS). Earliest differentiates are cumulates of olivine and plagioclase.The most basic compositions of cumulus plagioclase (An₇₂) andolivine (Fo₈₄) in these rocks indicate that the amount of fractionationpreceding formation of the exposed LS was substantially lessthat previously believed. Field and compositional data indicatethat picritic blocks are xenoliths rather than cumulates ofthe Skaergaard magma. Xenoliths of gneiss in all stages of reactionare locally abundant; however, there is no evidence that uppercrustal material contaminated the magma from which the MBS cumulatesformed. Compositions of cumulus minerals in the MBS differ fromthose in comparable LS rocks. Cumulates in the lower marginscontain more calcic plagioclase, more magnesian augite in allbut the late differentiates, and more iron-rich olivine. Thecompositions of cumulus olivine and to a lesser degree thoseof other mafic silicates, were modified to more iron-rich compositionsby re-equilibration with relatively large amounts of interstitialliquid. The lower MBS and LS crystallized from the same magma, but fractionationoccurred at different rates on the walls and floor of the intrusion.The upper margin may have crystallized from a magma of modifiedcomposition and fractionated at rates different from that inthe lower margin and Upper Border Series (UBS). Crystals onthe floor and roof of the intrusion accumulated faster or moreefficiently than on the walls. At any given stage of fractionation,crystals also accumulated against all sides of the magma chamberat about the same rate. Either the rates of cooling, crystallization,and crystal retention affected accumulation rates locally asfunctions of rock type and geometry of the walls, or these rateswere largely independent of wall rock owing to buffering ofconductive heat loss possibly to an envelope of hydrothermalfluid circulating around the crystallizing magma. The appearanceor disappearance of cumulus minerals in the lower MBS occursat higher structural levels than in the LS and at lower structurallevels than in the UBS. These relationships together with cumulusmineral compositions indicate that magma at the margins wasalways somewhat less fractionated than that at the floor androof of the chamber. It is proposed that these relationshipsreflect the combined effects of liquid and crystal fractionationof the magma within largely independent convection systems inthe lower and upper parts of the chamber.     

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.     

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. Received: 1 October 1999 / Accepted: 14 May 2000     

The Skaergaard Layered Series, Part VII: Sr and Nd Isotopes

The initial isotopic ratios of strontium and neodymium in theSkaergaard Layered Series vary both vertically and laterally,on every scale from the intrusion as a whole down to coexistingminerals in a single rock. The magma that filled the Skaergaardchamber was contaminated to various degrees with the metamorphicrocks through which it rose and was never completely homogenizedafter being intruded. The contamination was most pronouncedin contact zones and aureoles around rare xenoliths. The greaterconcentrations of lithophile trace elements in the Upper BorderSeries was previously attributed to assimilation of buoyantfragments of gneiss that collected under the roof, but mostof the rocks of the Upper Border Series are isotopically indistinguishablefrom those of the Layered Series. It is doubtful, therefore,that this part of the intrusion assimilated much more of themetamorphic basement than did the rest of the magma. Similarly,the marked increase in the concentrations of excluded elementsin the upper part of the Layered Series is not matched by achange in the isotopic character of the rocks and cannot beattributed to a later influx of new magma. Analyses of mineralsseparated from rocks with exceptionally mafic or felsic modalcompositions revealed marked inhomogeneities in the isotopiccompositions of their constituent minerals. For example, coexistingplagioclase and pyroxene from closely associated anorthositesand pyroxenites have very different initial isotopic ratiosof both strontium and neodymium. The same is true of mafic andfelsic layers in modally graded gabbros. These differences areunrelated to the low-temperature alteration shown by oxygenisotopes. They must have been introduced when the original gabbrowas largely crystallized and underwent local metasomatic replacementby nearly mono-mineralic mafic and felsic assemblages. KEY WORDS: Nd isotopes; Skaergaard; Sr isotopes     

The Skaergaard Layered Series: I. Structure and Average Compositions

Re-examination of the Skaergaard Layered Series in the lightof more extensive field work and sampling shows that the lithologiczones vary laterally as well as vertically, in both their bulkchemical composition and their mineralogical assemblages. Themargins of the zones differ from both the central part of theLayered Series and Marginal Border Series in being richer inFeO^*, TiO₂, K₂, P₂O₂, and most excluded elements. Mafic mineralstend to be more abundant and more iron-rich, plagioclase ismore albitic and more strongly zoned, and apatite and biotiteare more abundant near the margins. When the average compositions of successive zones are compared,the abundances of most excluded components are seen to declineupward as far as Middle Zone then reverse their trends and increasethrough Upper Zone. P₂O₅ and K₂O are negatively correlated inUpper Zones B and C, owing, perhaps, to separation of immisciblefelsic liquids from the iron-rich magma. No evidence has beenfound for introduction of a new batch of less differentiatedmagma. Layered rocks have an average composition that is more maficthan that of homogenous rocks at the same level. Blocks thatfell from the roof have the opposite relation; they are greatlyenriched in felsic components compared to the original compositionsof the Upper Border Series from which they came. Although some of the compositional variations may be consistentwith differing degrees of fractionation of trapped liquids,no consistent relation has been found between the degree offractionation and rates of crystal accumulation or cooling atthe walls. Contamination with the metamorphic wall rocks, eitherby assimilation or by hydrothermal fluids, seems to have hadonly local effects and cannot account for the large-scale variations.At least some of the compositional differences must have resultedfrom late-stage processes that redistributed certain componentsafter the intrusion reached advanced stages of solidification.     

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 ofits regional setting, combined with new data from explorationdrilling, has resulted in a revised structural model for theintrusion. It is modelled as an irregular box, c. 11 km fromnorth to south, up to 8 km from east to west, and 3·4–4km from the lower to the upper contact. The walls of the intrusionare inferred to follow pre-existing and penecontemporaneoussteep faults, and the floor and roof seem largely controlledby bedding planes in the host sediments and lavas, similar toregional sills. The suggested shape and volume are in agreementwith published gravimetric modelling. Crystallization alongall margins of the intrusion concentrated the evolving meltin the upper, central part of the intrusion, best visualizedas an ‘onion-skin’ structure inside the box. Thetotal volume is estimated to c. 280 ± 23 km³, of which13·7% are referred to the Upper Border Series (UBS),16·4% to the Marginal Border Series (MBS) and 69·9%to the Layered Series (LS). In the LS, the Lower Zone (LZ) isestimated to constitute 66·8%, the Middle Zone (MZ) 13·5%and the Upper Zone (UZ) 19·7%. The new volume relationshipsprovide a mass balance estimate of the major and trace elementbulk composition of the intrusion. The parental magma to theSkaergaard intrusion is similar to high-Ti East Greenland tholeiiticplateau basalts with Mg number c. 0.45. The intrusion representsthe solidification of contemporary plateau basalt magma trappedand crystallized under closed-system conditions in a crustalreservoir at the developing East Greenland continental margin. KEY WORDS: bulk composition; emplacement; mass proportions; Skaergaard intrusion; structure     

A new interpretation of the structure of the Sept Iles Intrusive suite, Canada

The layered mafic intrusion at Sept Iles, Canada, is one of the largest intrusions in the world. A new interpretation of its structure is proposed, based on a review of its geology and a comparison with the Skaergaard intrusion, Greenland. Several different magmatic components are recognized; hence the name Sept Iles Intrusive suite (SIIS) is proposed. Emplacement of the suite may have been preceded by eruption of flood basalts. The first magmas of the suite rose in the crust to accumulate beneath the density filter afforded by the basalts. The largest component is the Sept Iles Mafic intrusion (SIMI). The Lower series of the SIMI is dominated by leucotroctolites and leucogabbros. Above it lie the Layered series, which is largely comprised of gabbro and troctolite. Both these units are unchanged from earlier interpretations. The anorthosites (s.l.), gabbros and monzogabbros, formerly called the Transitional series, are now considered to be the Upper Border series, developed by floatation of plagioclase. Common autoliths in the Layered series are parts of the hydrothermally altered Upper Border series from towards the interior of the intrusion, which have foundered and settled through the magma. The contamination of the magma that accompanied this event oxidised iron in the magma and led to the precipitation of magnetite around the periphery of the intrusion. The subsequent depletion of Fe³⁺ and/or increase in SiO₂, CaO and P₂O₅ may have induced apatite saturation and accumulation to form two layers rich in apatite, near the base and at top of the Layered series. Granitic magma was developed by fractional crystallisation and was emplaced along the roof of the chamber, where it acquired large quantities of xenoliths. These were probably derived from the flood basalts, their evolved members and fragments of mafic dykes chilled by the granitic magma. Accumulations of monzonite pillows in this unit testify to another magmatic event and a floor to the granitic magma chamber, indicating lateral transport of magma. Chemically distinct syenites in the upper part of the intrusion are part of the Point du Criade intrusion, a large, late composite sill. Diabase and leucogabbro components show a close link with the SIMI and all the acidic magmas may have originally formed by differentiation of the main magma in cupolas towards the centre of the intrusion. A series of late gabbro intrusions that cut the SIMI may represent a rejuvenation of magmatism. The Border zone is a mass of fine-grained rocks that occurs along the border of the SIMI: it may be another magmatic component, or just the lateral border series of the SIMI.     

The Origin of Basic-Ultrabasic Sills with S-, D-, and I-shaped Compositional Profiles by in Situ Crystallization of a Single Input of Phenocryst-poor Parental Magma

An attempt is made to develop an in situ crystallization modelbased on the concept of Soret fractionation to explain the originof commonly observed S-, D-, and I-shaped compositional profilesin sills formed from a single pulse of phenocryst-poor parentalmagma. The model envisages that the various compositional profilesobserved in sills can be interpreted in terms of different combinationsof four principal units—Basal Zone and Layered Seriesforming the floor sequence, and Top Zone and Upper Border Seriesconstituting the roof sequence. The Basal and Top Zones representmirror images of the Layered and Upper Border Series, respectively,and therefore are referred to as basal and top reversals. Itis proposed that the formation of basal and top reversals takesplace through the non-equilibrium Soret differentiation of liquidboundary layers which form as a consequence of the temperaturegradient imposed by the cold country rock. In contrast, theLayered and Upper Border Series originate predominantly throughthe crystal–liquid boundary layers developing in equilibriumconditions. The model permits the production of S-, D-, andI-shaped compositional profiles from the same magma composition.All that is necessary to produce a specific shape of compositionalprofile is an appropriate temperature gradient imposed by thecold country rock on the liquid boundary layers of a parentalmagma of a given composition. KEY WORDS: sills; compositional profiles; in situ crystallization; Soret fractionation     

The distribution of trace elements during strong fractionation of basic magma—a further study of the Skaergaard intrusion,East Greenland

A number of trace elements have been determined spectrographically in the rocks and minerals of the Skaergaard intrusion, East Greenland. The original basic magma from which the varied rocks of the complex were developed is shown to have had a normal trace element composition. The sorting out of the trace elements into the various mineral series produced by strong fractional crystallization of the original basic magma is traced in detail by means of analyses of the separated minerals. Certain of the trace elements (Cr, Ni) are shown to be strongly concentrated in the early rocks so that later fractions have little or none of them; other elements (P, V, Cu, Sc, Mn, S) reach maximum values in the middle, or late middle stages represented by certain olivine-free gabbros and ferrogabbros; other elements (Li, Zr, Y, La, Ba, Rb) tend to remain in the residual liquid during fractionation and are thus abundant in the latest granite fraction. Still other trace elements (Co, Sr, Ga, Mo) show only small changes in amount throughout the series. Of these Co is a little more abundant in the early and middle stages, Sr in the middle stages, Ga in the later stages and Mo in the early and later but not in the middle stages. The distribution of the trace elements in the rocks is considered in relation to the varying composition of the minerals produced by the fractional crystallization processes and an attempt is made to discuss the mineral compositions in terms of crystal chemical concepts.The Skaergaard sequence of differentiation from gabbros, through ferrogabbros, to granite is considered to be a common trend of fractionation of basic magma at high levels in the crust, and the observed changes in trace element composition are therefore regarded as having wide geochemical significance. The trace element composition of the intermediate Skaergaard differentiates is significantly different from that of diorites reported by other workers and suggests that diorites have had some other origin than by fractionation of basic magma. On the other hand the trace element composition of many granites resembles that of the granite fraction produced in the Skaergaard intrusion.     

Relationship between the Layered Series and the overlying evolved rocks in the Bjerkreim-Sokndal Intrusion, southern Norway

The Bjerkreim-Sokndal layered intrusion (BKSK) consists of a > 7000-m-thick Layered Series comprising anorthosites, leuconorites, troctolites, norites, gabbronorites and jotunites (hypersthene monzodiorites), overlain by an unknown thickness of massive, evolved rocks: mangerites (hypersthene monzonites; MG), quartz mangerites (QMG) and charnockites (CH). The Layered Series is subdivided into six megacyclic units that represent the crystallisation products of successive major influxes of magma. We have studied a ca. 2000-m-thick section that straddles the sequence from the uppermost part of the Layered Series to the QMG in the northern part of the intrusion. Mineral compositions in 37 samples change continuously in the lower part of the sequence up to the middle of the MG-unit (plagioclase An37-18; olivine Fo40-7; Ca-poor pyroxene Mg#57-15; Ca-rich pyroxene Mg#65-21). Above this compositions are essentially constant in the upper part of the MG-unit and in the QMG (An21-13; Fo6-4; Mg#opx17-13; Mg#cpx25-20). The amount of interstitial quartz and the amount of normative orthoclase, however, both increase systematically upwards through the QMG-unit, implying that these rocks are cumulates. There is no evidence of a compositional break in the MG-QMG sequence that could reflect influx of relatively primitive magma.

Two types of QMG/CH are known in the uppermost part of BKSK. Olivine-bearing types are comagmatic with the underlying Layered Series; the studied stratigraphic sequence belongs to this suite. Two-pyroxene QMG and amphibole CH define a separate compositional lineage related to jotunites. An intrusive unit of dominantly two-pyroxene QMG is discordant to the olivine-bearing jotunite-MG-QMG sequence near Rapstad, confirming the presence of two compositionally distinct suites of QMG and related lithologies in the upper part of BKSK.

A xenolith-rich unit near the olivine-bearing MG-QMG boundary represents a major collapse of the roof of the magma chamber during the final stages of crystallisation.     

A Textural Record of Solidification and Cooling in the Skaergaard Intrusion, East Greenland

The clinopyroxene–plagioclase–plagioclase dihedralangle, _cpp, in gabbroic cumulates records the time-integratedthermal history in the sub-solidus and provides a measure oftextural maturity. Variations in _cpp through the Layered Seriesof the Skaergaard intrusion, East Greenland, demonstrate thatthe onset of crystallization of clinopyroxene (within LZa),Fe–Ti oxides (at the base of LZc) and apatite (at thebase of UZb) as liquidus phases in the bulk magma is recordedby a stepwise increase in textural maturity, related to an increasein the contribution of latent heat to the total heat loss tothe surroundings and a reduction in the specific cooling rateat the crystallization front of the intrusion. The onset ofboth liquidus Fe–Ti oxide and apatite crystallizationis marked by a transient increase in textural maturity, probablylinked to overstepping before nucleation. Textural maturationat pyroxene–plagioclase–plagioclase triple junctionseffectively ceases in the uppermost parts of the Layered Seriesas a result of the entire pluton cooling below the closure temperaturefor dihedral angle change, which is 1075°C. Solidificationof the Layered Series of the Skaergaard intrusion occurred viathe upwards propagation of a mush zone only a few metres thick. KEY WORDS: magma; partial melting; asthenosphere; olivine; mantle     

Isotopic studies of processes in mafic magma chambers: II. The Skaergaard Intrusion,East Greenland

Isotopic ratios of Nd and Sr have been measured in a suite of samples spanning most of the exposed stratigraphy of the Skaergaard intrusion in order to detect and quantify input (such as assimilated wallrock and fresh magma) into the magma chamber during crystallization. Unlike ¹⁸O and D, Nd and Sr isotope ratios do not appear to have been significantly affected by circulation of meteoric waters in the upper part of the intrusion. Variations in initial ⁸⁷Sr/⁸⁶Sr and _Nd suggest that the Skaergaard magma chamber was affected during its crystallization by a small amount (2%–4%) of assimilation of Precambrian gneiss wallrock (high ⁸⁷Sr/⁸⁶Sr, low _Nd) and possibly recharge of uncontaminated magma. Decreases in _Nd and increases in ⁸⁷Sr/⁸⁶Sr during the early stages (0%–30%) of crystallization give way to approximately unchanging isotopic ratios through crystallization of the latest-deposited cumulates. Modelling of assimilation-fractional crystallization-recharge processes using these data as constraints shows that the assimilation rate must have been decreasing throughout crystallization. In addition, the isotope data allow replenishment by an amount of uncontaminated magma equal to 20%–30% of the total intrusion mass, occurring either continuously or in pulses over the first 75% of crystallization. Comparison of the recharge models with published Mg/(Mg+Fe²⁺) data from Skaergaard cumulates shows that the modelled replenishment rates are not inconsistent with available major element data, although significant recharge during the final 25% of crystallization can be ruled out. The isotope data show that the Skaergaard magma could have incorporated only a small amount of the gneiss that it displaced from the floor of the chamber; assimilation appears to have taken place primarily across a partially molten zone that formed at the roof from the wallrock that was dislodged during emplacement. In the latest stages of crystallization (>75% crystallized), the Skaergaard magma may have become stratified into two separately-convecting layers, effectively insulating Layered Series cumulates from further contamination.     

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.     

Petrology of the Proterozoic Potato River Layered Intrusion, Northern Wisconsin, USA

The Potato River intrusion is a Keweenawan (1100 Ma) mafic plutonemplaced in Keweenawan volcanics and earlier Proterozoic metasedimentaryrocks along the southeastern flank of the Lake Superior syncline.It comprises the following lithostratigraphic zones: a thinto absent Border zone of altered olivine gabbro; a Lower zoneof olivine gabbro; a Picritic zone of picrite and troctolite;a Middle zone of olivine gabbro and leucogabbro; an Upper zoneof quartz leucogabbro and ferrogabbro; and a Roof zone of granophyricand granitic rocks. Fractional crystallization is evident fromcompositional changes in the rocks and cumulus minerals withstratigraphic height. Elements concentrated in the cumulus mineralsolivine and plagioclase (Mg, Fe²⁺, Al, Ca, Ni, Co, Cr, Sr) decreasewith height; elements concentrated in the trapped liquid (Na,K, La, Y, Zr, Nb, Rb, Ba) increase with height; and other elements(Ti, Fe³⁺, P, Ga, V, Sc, Cu, Zn) show complicated behavior relatedto the appearance of additional cumulus phases such as clinopyroxene,Fe-Ti oxides, and apatite. Lower zone rocks contain some sulfide,probably from sulfur derived from the country rock, and theUpper zone has sulfides probably precipitated from an immisciblesulfide liquid. The sulfide-bearing rocks have similaritiesto those of other intrusions, such as Bushveld, Stillwater,and Skaergaard. The picritic and troctolitic rocks of the Picritic zone indicatethat the intrusion was open to additional injections of maficmagma. Roof zone granophyric rocks are residual liquids intrudedalong the upper margin of the intrusion during regional tilting,but Roof zone granitic rocks are probably melted country rock.An attempt is made to estimate by reverse stratigraphic summationthe compositional path of the magma that solidified above thePicritic zone. The first compositions are highly aluminous,which suggests that the upper part of the intrusion has beenenriched in plagioclase by convection-aided crystal sorting.A complementary unit of mafic rocks is not exposed, but it couldbe present down dip. Some of the later compositions are similarto typical Keweenawan high-Al tholeiites. The magma did notundergo extreme iron enrichment, probably because of oxygenfugacity buffering.     

Disequilibrium partial melting and rheomorphic layer formation in the contact aureole of the Basistoppen sill,East Greenland

The tholeiitic Basistoppen sill was intruded into the upper part of the Skaergaard complex shortly after the Skaergaard magma had solidified. Heat from the cooling Basistoppen magma caused disequilibrium partial melting in the adjacent Skaergaard ferrogabbros. Olivine, ferrobustamite, and magnetite were selectively melted and removed from the rock as an iron-rich melagabbro magma. Plagioclase acted as a refractory phase during partial melting and was left behind as an anorthositic gabbro restite. Modal and grain-size layering formed rheomorphically in the previously solidified host rocks as a result of partial melting and recrystallization. The rheomorphic layers are distinct from those found elsewhere in the intrusion.The extreme degree of contact metamorphism adjacent to the Basistoppen sill is a consequence of the intrusion of the sill into host rocks that were already near their melting temperature. It is suggested that the slow reaction rates between plagioclase and magma inhibited the dissolution of plagioclase relative to olivine, pyroxene, and opaque oxides and resulted in disequilibrium partial melting. The presence of anorthositic gabbro blocks within the Middle Zone of the Skaergaard intrusion indicates that disequilibrium partial melting may also occur during the assimilation of gabbroic xenoliths by magmas.     

Hydrothermal clinopyroxenes of the Skaergaard intrusion

Magmatic augites reacted with high temperature aqueous solutions to form secondary calcic pyroxenes during the subsolidus cooling of the Skaergaard intrusion. Secondary, hydrothermal clinopyroxenes replace wall rock igneous augites at the margins of veins filled with calcic amphibole. These veins are up to several millimeters wide and tens of meters in length. Hydrothermal clinopyroxenes are a ubiquitous and characteristic phase in the earliest veins throughout the Layered Series of the intrusion, and occur rarely in late veins that, in some places, crosscut the early veins. Associated secondary phases in early veins include amphiboles ranging in composition from actinolite to hornblende, together with biotite, Fe-Ti oxides and calcic plagioclase. Hydrothermal clinopyroxenes in late veins may be associated with actinolite, hornblende, biotite, magnetite and albite.Hydrothermal clinopyroxenes are depleted in Fe, Mg and minor elements, and enriched in Ca and Si relative to igneous augites in the Layered Series gabbros. Secondary vein pyroxenes are similar in composition to calcic pyroxenes from amphibolite facies metamorphic rocks. Clinopyroxene solvus thermometry suggests minimum temperatures of equilibration of between 500° and 750° C. These temperatures, combined with numerical transport models of the intrusion, suggest that vein clinopyroxenes could have formed during 20,000 to 60,000 year time intervals associated with a maximum in the fluid flux through fractures in the Layered Series.     

The differentiation of the Skaergaard intrusion

Conclusions We find no support for the claim that the Skaergaard magma followed the trend of common tholeiitic volcanic magmas, such as those of Iceland and the Scottish Tertiary. The end product of differentiation was not a large mass of rhyolite but an iron-rich, silica-poor liquid not unlike that deduced by Wager in 1960.The proposal that a large mass of rhyolitic liquid occupied the upper levels of the intrusion finds no support in the field. The thick series of ferrogabbos, which became richer in iron and poorer in silica until they reached a field of immiscibility cannot be reconciled with crystallization of a large mass of felsic magma. Mass-balance calculations that indicate otherwise are invalid, because they fail to take into account large volumes of rocks that differ in composition from those assumed in the calculations.While ignoring the existence of major units of the intrusion, Hunter and Sparks propose that lavas in Scotland and Iceland are more relevant to the liquid compositions than rocks that are intimately associated with the intrusion. Their argument that the Skaergaard Intrusion followed a trend of silica enrichment that is universal to tholeiitic magmas is based on an incomplete knowledge of the rocks and faulty calculations of mass-balance relations.We agree that much remains to be learned about the Skaergaard Intrusion and the basic mechanisms of magmatic differentiation. In this case, however, we are ready to hang our case on well-established field relations and a mass of laboratory data for what must be the most intensely studied body of rock on Earth.     

喜马拉雅东段库曲岩体锂、铍和铌钽稀有金属矿物研究及指示意义

稀有金属矿物记录了花岗伟晶岩成岩成矿的重要信息。喜马拉雅是全球著名的淡色花岗岩带,库曲岩体位于喜马拉雅东段的特提斯喜马拉雅岩系中。本文调查了库曲岩体的二云母花岗岩、白云母花岗岩、电气石花岗岩和花岗伟晶岩,其中,花岗伟晶岩涉及花岗岩的伟晶岩相和独立伟晶岩脉。库曲岩体产出的稀有金属矿物包括锂辉石、锂绿泥石、绿柱石、铌铁矿-钽铁矿、钇铀钽烧绿石和细晶石,它们主要赋存于似文象伟晶岩、石英-钠长石-白云母伟晶岩、块体长石-钠质细晶岩、块体长石-电气石钠质细晶岩、锂辉石-块体长石-细晶岩、白云母花岗岩的伟晶岩相以及电气石花岗岩内。显微镜观察、电子探针和LA-ICP-MS测试结果显示锂辉石具有四种产状,包括粗粒锂辉石自形-半自形晶、细粒锂辉石-石英镶嵌晶、中细粒锂辉石-钾长石-钠长石-云母镶嵌晶以及发育锂绿泥石的粗粒锂辉石,揭示了其形成时复杂的熔流体动荡结晶环境。绿柱石背散射电子图像（BSE）下呈均一结构和不均一结构（蚀变边、不规则分带和补丁分带）,元素替代机制包括通道-八面体替代、通道-四面体替代以及通道中碱金属阳离子间的置换。铌铁矿族矿物包括原生、蚀变边和不规则分带结构,部分被钇铀钽烧绿石和细晶石交代。与原生铌铁矿相比,蚀变边和不规则分带铌铁矿族矿物总体上富钽贫锰,显示了结晶分异、过冷却引起的过饱和以及流体作用。根据稀有金属矿物揭示的成因信息,独立伟晶岩脉（似文象伟晶岩）、白云母花岗岩的伟晶岩相和电气石花岗岩在岩浆分异程度、经历的演化过程、以及流体活动方面存在差异,很可能是不同期次岩浆活动的产物。库曲岩体绿柱石的Rb和Zn含量、以及铌铁矿族矿物的Sc₂O₃、SiO₂和PbO含量,与已有指示标志存在相关性,作为潜在指示标志仍需开展更多的研究工作。综合含锂辉石伟晶岩的产出、岩浆分异演化程度、多期花岗质岩浆活动、复杂的流体作用以及所属锂丰度高值区等因素,库曲岩体是喜马拉雅东段找锂的有利地段。