JOURNAL OF PETROLOGY VOLUME 57 NUMBER 3 PAGES 497-519 1996

STEFAN BERNSTEIN1*, PETER B. KELEMEN1 AND C. KENT BROOKS1-3

'DANISH LITHOSPHERE CENTRE, 0STER VOLDGADE 10, DK-1350 COPENHAGEN K, DENMARK"WOODS HOLE OCEANOGRAPHIC INSTITUTION, WOODS HOLE, MA 0254J, USA'GEOLOGICAL INSTITUTE, UNIVERSITY OF COPENHAGEN, 0STER VOLDGADE 10, DK-1350 COPENHAGEN K, DENMARK

Evolution of the Kap Edvard HolmComplex: a Mafic Intrusion at a RiftedContinental Margin

The Kap Edvard Holm Layered Series forms part of the EastGreenland Tertiary Province, and was emplaced at shallowcrustal level (at depths corresponding to a pressure of 1-2kbar) during continental break-up. It consists of two suites: agabbro suite comprising olivine and oxide gabbros, leucocraticolivine gabbros and anorthosites, and a suite of wehrlites that

formed from the intrusion of the gabbros during their solidifi-cation by a hydrous, high-MgO magma. Ion microprobe ana-lyses of dinopyroxene reveal chemical contrasts between theparental melt of the wehrlite suite and that of the gabbro suite.Thin sills (1-2 m thick) of the wehrlite suite, however, havedinopyroxene compositions similar to the gabbro suite, andwere formed by interaction with interstitial melts from the hostlayered gabbros. All evolved members of the gabbro suite haveelevated Nd, Zr and $T concentrations and Nd/Yb ratios,relative to the melt parental to the gabbro suite. These char-acteristics are attributed to establishment of a magma chamberat depths corresponding to a pressure of 10 kbar, where meltsevolved before injection into the low-pressure magma chamber.Anorthosites of the gabbro suite are believed to have crystal-lized from such injections. The mdts became supersaturated inplagiodase by the pressure release that followed transportationto the low-pressure magma chamber after initial fractionationat ~10 kbar. The most evolved gabbros formed by subsequent

fractionation within the low-pressure magma chamber. Ourresults indicate that high-pressure fractionation may be impor-tant in generating some of the lithological variations in layeredintrusions.

KEY WORDS: fractionation; ion micwprobe; layered intrusions; rift

processes; trace elements

INTRODUCTIONThe classical approach to estimating the compositionof the melts parental to mafic intrusions has been thesampling of chilled margins from the contact zones(Wager, 1960; Hoover, 1978). As discussed by Ford(1981) and Hoover (1989), this method, although itmay be pertinent to the investigation of intrusionsthat crystallized from a single batch of magma (suchas the Skaergaard intrusion), is not appropriate forintrusive complexes that appear to have crystallizedfrom multiple replenishment episodes. In this case,the chilled margin method provides no constraintson how representative the chosen sample is of thebulk of the intrusive complex. Another problem isencountered when characterizing the nature of thecrystallization processes that were active during theformation of mafic intrusions. Crystallization pro-cesses are typically addressed by studying the major-element variations in cumulus phases. Althoughmajor-element variations of cumulus phases providea qualitative measure of the degree of fractionationand of replenishment events, calculation of meltcompositions from major-element mineral data isseverely hampered by the continuous re-equili-bration of major elements in ferro-magnesianminerals during post-cumulus melt migration, andthe subsequent subsolidus equilibration of intra-crystalline chemical gradients (e.g. Scowen et al.,1991). In contrast, silicate phases potentially retaintheir concentration gradients in high valence trace-elements, owing to their considerably lower diffusiv-ities in minerals compared with diffusivities of mostmajor elements (Freer, 1981; Watson & Baker,1992). Furthermore, partition coefficients for several

* Corresponding author. I Oxford University Prea 1996

JOURNAL OF PETROLOGY VOLUME 37 NUMBER 3 JUNE 1996

trace-elements deviate substantially from unity andtheir incorporation in cumulus phases thereforerecords the characteristics of the crystallization pro-cesses. The ion microprobe is ideal for determiningthe trace-element concentrations in silicates. On thebasis of these concentrations, equilibrium melt con-centrations may be calculated, using published par-tition coefficients. Clinopyroxene is present in allmafic and ultramafic lithologies from the KapEdvard Holm Layered Series, and the partitioncoefficients between clinopyroxene and basaltic meltsare fairly well constrained. This phase thereforeforms the basis for a trace-element study of the crys-tallization history of this intrusive complex.

GEOLOGICAL SETTINGThe Kap Edvard Holm Layered Series (KEHLS)was emplaced into the volcanic margin of the centralGreenland Archaean craton during the opening ofthe North Atlantic Ocean (Fig. 1) (Wager, 1934;Abbott & Deer, 1972; Myers, 1980; Brooks &Nielsen, 1982a, 19826). The earliest recorded vol-canic event in the East Greenland Tertiary Provincewas the outpouring of a series of heterogeneousbasalts, including picrites, now mainly exposedaround the mouth of the Kangerlussuaq Fjord(Soper etal, 1976; Nielsen etal., 1981). The majorityof the overlying plateau basalts are chronologically

N

ICE CAP

SkaergaardKangerlussuaq Fjord

Kap Edvard Holm

Kruuse Fjord ^ ^

Imilik ^

Kap Gustav Holm

, 200 km ,

32°

| gabbro| basalt| syenite| sedimentsI basement

66°

28°

Fig. 1. Regional map of the East Greenland Tertiary Province.The eroiional level south of the Kangerluuuaq Fjord is deeper,

exposing plutoru that have been covered with flood basalts.

confined to magnetic anomaly 24R, i.e. 57-55 m.y.,with intermittent activity well into the mid-Eocene(Brooks et al., 1976; Soper et al., 1976; Larsen &Watt, 1985; Larsen et al., 1989). In addition to floodbasalts and dyke swarms, several tholeiitic intrusiveslike the KEHLS are exposed along the coast (Wager,1934; Brooks & Nielsen, 1982a). The layered maficintrusions have generally been intruded immediatelybefore the flexure that affected the coastal area atthe latest stages of flood basalt formation (Wager,1934; Myers, 1980; Brooks & Nielsen, 19824; Bott,1987). Some of the large tholeiitic intrusions(including the Skaergaard intrusion and theKEHLS) appear to be emplaced at the contactbetween the Archaean basement and the overlyingbasalt-sediment pile (Wager, 1947; Irvine, 1987;Bird etal., 1988; Myers etal., 1993).

Previous studies on the KEHLS (Abbott & Deer,1972; Bernstein et al, 1992; Tegner et al., 1993) haveconcluded that the complex crystallized from a drytholeiitic magma, and that the restricted range inmodal composition of the gabbros and in the com-positional variation of the cumulus minerals was dueto frequent replenishments of less differentiatedmelts. In this respect, the KEHLS shares chemicaland structural features with layer three in theoceanic crust, as revealed by the large gabbroicsequences in ophiolites (e.g. Pallister & Hopson,1981; Brooks & Nielsen, 1982A). On these groundsand on the basis of field evidence, Bernstein et al.(1992) suggested that the KEHLS served as ashallow-level reservoir for basaltic flows during thewaning stages of the flood basalt magmatism.

Bernstein et al. (1992) also documented the exis-tence of two distinct magmatic suites in the KEHLS.The most voluminous is termed the 'gabbro suite'and constitutes the layered gabbros, which mostlyare plagioclase-clinopyroxene—olivine or plagio-clase-clinopyroxene-(Fe-Ti)-oxide cumulates (seebelow). The second magmatic suite occurs as ultra-mafic or melanocratic sills and small plugs within thelayered gabbros, and is termed the 'wehrlite suite'(Bernstein et al., 1992; Tegner & Wilson, 1993), andis a further similarity to classic ophiolite occurrences(e.g. Benn & Laurent, 1987; Juteau et al., 1988).

PETROLOGY OF KEHLSDescriptions of the KEHLS have been given by Deer& Abbott (1965), Wager & Brown (1968), Elsdon(1969, 1971), Abbott & Deer (1972), Bernstein et al.(1992), Tegner et al. (1993) and Tegner & Wilson,(1993). Below is a brief account of the gabbro suitethat makes up the bulk of the intrusion, and of thevolumetrically subordinate wehrlite suite.

498

BERNSTEIN et al. EVOLUTION OF KAP EDVARD HOLM COMPLEX

The gabbro suite

The bulk KEHLS comprises olivine (ol) gabbros andFe—Ti oxide (ox) gabbros, with modal cumulusassemblages of plagioclase-clinopyroxene-olivine(plag-cpx-ol) and plag-cpx-(Fe-Ti) oxide (±ol), inorder of decreasing modal abundance. On the basisof cumulus assemblages and major changes in com-position of cumulus minerals, the KEHLS has beendivided into a Lower Layered Series (LLS), aMiddle Layered Series (MLS) and an UpperLayered Series (ULS) (Deer & Abbott, 1965;Elsdon, 1969; Abbott & Deer, 1972; Bernstein et al.1992) (Figs 1 and 2). The ULS may represent theroof-zone of the magma chamber, and compriseslayered oxide gabbros and anorthosite lenses. TheULS lithologies are heavily affected by hydro-thermal alteration, possibly caused by the emplace-ment of nearby syenite bodies to the south. The LLSis dominated by ol gabbros that in places are suc-ceeded by ox gabbros (Bernstein et al., 1992). A 2000m stratigraphic package reveals several reversals tomore primitive mineral compositions and modalassemblages, suggesting repeated influxes of fresh,less differentiated magma (Bernstein et al., 1992; Birdet al., 1995). The MLS is made up by a number of

Kap Edvard Holm

Fig. 2. Map of the Kap Edv»rd Holm Layered Series, lhowing thelocation of samples referred to in the text. LLS, MLS and ULS—Lower, Middle and Upper Layered Seriei, respectively. Averageorientation of layering ij indicated. The wehrlite mite is not volu-

minoui enough to make mappable units.

thick, cyclic units (average thickness of 80 m; Tegneret al., 1993) that appear to cut the LLS, although thecontact to the LLS is not exposed (Fehlhaber &Bird, 1991). The base of the MLS is composed ofanorthosites grading into poikilitic ol gabbros, sug-gesting the crystallization sequence: plagioclase-olivine/clinopyroxene. In general for all three series,it appears that when reversals to less differentiatedcompositions are found in the cumulus minerals,suggesting new pulses of magma, the gabbros aremore leucocratic relative to the more evolvedsamples, reflecting higher modal proportions of pla-gioclase. This is especially evident in the basal anor-thositic part of the MLS. Only in one instance has anew pulse of magma given rise to a rather thin (3-4m thick), sill-like, melanocratic gabbroic unit(sample 61, see below) which grades into a 'normal'ol gabbro within 5—10 m. The above-described rocksare believed to be cogenetic, because there is agradual transition from all primitive 'end-members'(anorthosites and ultramafic layers) to the olgabbros and ox gabbros. In addition, all rocks of thegabbro suite have very low contents of primarymagmatic hydrous minerals, contrasting with thewehrlite suite described below.

The wehrlite suiteThe wehrlite suite (< 1 % of the total volume ofintrusive rocks) is typically manifested as sills of 50cm to 1 m thickness in the layered gabbros, althougha considerable span in thicknesses is observed. Thethinnest (about 2-3 cm thick) are hardly distin-guishable from melanocratic layers in the layeredgabbroic sequence, whereas the thickest are > 10 mthick and can be traced to the limit of outcrop (up to3 km). Discordant pipes of wehrlite in the layeredgabbros and slump-like structures at the contactbetween wehrlite sill and host layered gabbro arecommonly observed. Replacement structuresresulting from interaction between the intrusiveultramafic melt and the host gabbro also occur,especially in the MLS (Tegner & Wilson, 1993).Wehrlite sills occur throughout the entire intrusionwith an average abundance of one sill per 50 m ofstratigraphic section. A large, irregular body(minimum 30 m x 40 m x 100 m) of wehrlite (here-after the wehrlite plug) is emplaced into the LLS(sample 1610 in Fig. 2).

As described by Bernstein et al. (1992) and Tegner& Wilson (1993), field evidence suggests that theprimitive magma, which formed the sills of thewehrlite suite, intruded at a stage where the hostlayered gabbros were not completely solidified. Therocks of the wehrlite suite have magmatic amphibole

499

JOURNAL OF PETROLOGY VOLUME 37 NUMBER 3 JUNE 1996

and phlogopite crystals, whereas the host layeredgabbros are virtually devoid of magmatic hydratedphases. Phlogopite crystals show evidence of defor-mation, suggesting that the mica crystallized beforeemplacement. The rocks of the wehrlite suite are alsogenerally coarser grained and have higher modalcontents of olivine (30-70%) compared with the hostgabbros (0-20%).

PETROGRAPHYA total of 12 samples were selected for trace-elementanalysis. Six samples are from the KEHLS gabbros,six are from the wehrlite suite including five sills andthe wehrlite plug sample 1610 (see Table 1 and Figs1 and 2). The following is a brief description of theindividual samples, starting with the gabbro suite(see Fig. 2 for sample locations). Representativemajor element analyses for minerals from some of thesamples have been given by Bernstein et at. (1992)and Tegner & Wilson (1993) (sample number inparentheses).

Sample 61 (61xx), gabbro suite, is the most primitive rep-resentative of thij suite. It is a melanocratic olivine gabbro, with asharp lower contact to some 'normally' layered, mesocratic olivinegabbros of the LLS. The upper contact is las distinct, and there isevidence of disturbance in the surrounding gabbros, with slumpingand recumbent folds. The contact relations suggest that themelanocratic gabbro formed by injection of primitive magma intoa layered gabbro mush, thus resembling the sills of the wehrlite

suite. The estimated thickness of this unit is ~ 3 m, and the samplecomes from ~ 0 5 m above the lower contact. Olivine and somedinopyroxenei are coarse (up to 5 mm), whereas plagioclase andpart of the clinopyroxenes form a fine-grained intergrowth, whereclinopyroxene typically displays poilrilitic texture. Olivine, whichis somewhat altered, has inclusions of anhedral chromium spinel.The large clinopyroxenes are strongly zoned (optically andchemically), some are cuhedral to subhedral, and some have skel-etal morphology, with evidence of resorbed cores (Fig. 3). Thefine-grained poikilitic clinopyroxenes usually appear as an over-growth on the euhedral and skeletal clinopyroxene crystals, to thatthe original euhedral shape is preserved in the zoning pattern.There is a fine-grained matrix consisting of talc and actinolite,some of which could result from hydrothermal alteration of fine--grained olivine. Therefore the proportions of phases in the sampleare difficult to estimate, but roughly it consists of 30% olivine,30% clinopyroxene, 20% plagioclase and 20% altered material.As the plagioclase crystals are fresh, the altered material probablyoriginated as mafic phases, i.e. olivine or possibly orthopyroxene.With mf-number [molar Mg/(Mg + Fe)] of up to 90 for the clino-pyroxene cores, this sample represents the most primitive melt yetfound in the KEHLS.

Sample 64, gabbro sui te, comes from some of the- lowermostexposures of the LLS. The sample is representative of a package offairly uniform, well-layered olivine gabbros, some 300-500 m inthickness, that is interrupted only by a few melanocratic sills of0-5—1 m thickness belonging to the wehrlite suite (see sample 44).The texture is granular, and fine to medium grained, except forthe occasional occurrence of elongated olivine crystals. The samplecontains ~ 50-60% plagioclase, 25—30% clinopyroxene and 15—20% olivine. With m^-number ~ 79 for clinopyroxenes, the sampleis typical for the primitive end of 'normal' gabbros from theKEHLS.

Table 1: Sample collection of the present study

Sample no.

Gabbro suite

312361

352164

312380

312383

312337

312338

WehrHtB suite

1610

PCT75

WW

SD

312357

352144

Sample no.'

61

64

80

83

37

38

1610

75

WW

SD

57

44

Location

LLS

LLS

MLS

MLS

LLS

LLS

LLS

MLS

ULS

ULS

LLS

LLS

Rock typo

ol gabbro ml

ol gabbro ms

ol gabbro Ic

anorthcaite-lc gabbro

ox gabbro rra

ox gabbro ms

wehrlite

wehrlrte

wohrlrto

ol gabbro ml

ol gabbro ml

ol gabbro ml

cpxmg-no.

75-90

79-80

76-78

78-80

73-76

73-76

85-88

79-81

76-77

73-76

74-77

78-81

No. analysest

12

6

4

6

2

7

12

5

3

3

7

10

LLS, MLS and ULS—Lower, Middle and Upper Layered Series, respectively, ml, ms and Ic—melanocratic, mesocratic and leucocratic,respectively.'Abbreviated, as presented in figures.tTotal number of ion microprobe spot analyses of cpx in each sample.

500

BERNSTEIN et al. EVOLUTION OF KAP EDVARD HOLM COMPLEX

Fig. 3. Clinopyroxene crystal in the melanocratic sill sample 61.The crystal is strongly zoned—lighter areas are Al, Mg and Crrich. The darker areas, which are the rims and poikilitic over-growth (upper left comer), are compositionally identical to ch'nc-

pyroxene from the layered gabbro series (see Fig. 5).

Samples 37 and 38, gabbro smite, are medium-grained, well-laminated orthocumulates. They are oxide gabbros, with verylittle olivine (0—5%, always extensively altered), and their generalmodal compositions are 55% plagioclase, 30-35% dinopyroxeneand 5—10% Fe-Ti oxides, with average grain-size of ~2 mm.They represent the most evolved gabbros in the sample collection,as reflected in their relatively low m -̂number (~75; Table 1). Theoxide gabbros come from a section of well-laminated and layeredgabbros, which give an impression of a quiet state in the crystal-lization of the KEHLS magma chamber in the stratigraphic in-terval 900-1300 m [as described by Bernstein et al. (1992); thesamples are from the 1200 m level], with unusual continuouslayering and absence of both deformation features and intrusivewehrlite sills. The two samples are spaced ~10 m apart.

Simple 80, gabbro suite, is a leucocratic olivine gabbro.This, and sample 83 (see below), represent the lowermost exposureof the MLS, which is very heterogeneous, with discontinuousbandj and layers, slump-structures and other signs of an unstablemagma chamber. Sample 80 is granular and medium grained,with few signs of hydrothermal alteration. The modal compositionis 60-65% plagioclase, 15-20% clinopyroxene and 10-15% oliv-ine, plus minor amount of Fe—Ti oxides. Clinopyroxene crystalsare 2—3 mm in size and slightly poikilitic, enclosing small (01-0-5mm) plagioclase crystals.

Sample 83, gabbro suite, is a fine-grained anorthosite, withan average grain-size of 02 mm, and a few larger plagioclasegrains of up to 4 mm. It has sparse pyroxene and olivine crystalsto ~5 mm. Both mafic phases are poikilitic arid occur almost en-tirely in bands with thicknesses of a few centimetres. In thesebands, olivine and pyroxene, are equally abundant, and make up~50% of the mode. Scarce dinopyroxene oikocrysu are seen inthe anorthositic bands. Olivine is even more rare outside the maficbands. On outcrop-scale, the mafic bands constitute ~ 20% of thevolume, the rest being anorthosite. The analysed sample rep-resents one of the mafic bands.

Sample 1610 (16xx), wehrlite suite, is from a feldspathicwehrlite plug, emplaced into the LLS (see Bernstein et al., 1992).

This plug (at least 30mx40mxl00min size) is the most volu-minous expression of the wehrlite suite found within the KEHLS.No internal structures have been observed, and the rock has amassive appearance, with ~ 55-60% olivine, 30-40% greendinopyroxene, 5-15% plagiodase and 5% chrome-spind.Additionally, ~ 1—2% anhedral phlogopite crystals are present.The phlogopite usually forms dusters, so that the abundance inthin-section varies widdy (1-5%). Olivine, chrome-spind andphlogopite are fine grained (05—1 mm), whereas the pyroxene andsubordinate plagiodase oikocrysts are coarser (10—40 mm). ThechadacryjU are subhedral olivine and chromite, and the latterphase also occurs as indusions in olivine. The pyroxene oikocrystsare sometimes zoned, but not as strongly as in sample 61. Withmj-number in dinopyroxene up to 88, the plug is the most prim-itive representative of the wehrlite suite. The high modal contentof olivine, and the bulk-rock chemistry (>32 wt% MgO) suggestthat the wehrlite is olivine accumulative.

Sample 75 (PCT 75), wehrlite suite, is the only re-presentative of the wehrlite suite from the MLS in this study. Itintrudes medium-grained ol gabbros, and forms a large (up to 2km long, and 2-20 m thick) sill-like body [see description byTegner & Wilson (1993)]. Contact relations to the host gabbrosare complex, with remarkable replacement structures bearingevidence of reaction between the host gabbro and mdt emergingfrom the wehrlite sill (Tegner & WiUon, 1993). The analysedsample belongs to the intrusive part of the sill and has a modalcomposition of 50% olivine, 30% dinopyroxene, 15% plagiodaseand 5% oxide. Like all other members of the wehrlite suite, it hasphlogopite crystals (~1%) and poikilitic dinopyroxene, enclosingolivine, chromite and plagioclase grains. With dinopyroxene mg-number of ~80, the sample is among the most primitive of thewehrlite sills.

Sample SD, wehrlite suite, comes from the western part ofthe KEHLS, and is a 1 m thick sill, with sharp contacts to the hostlayered ol gabbros. It is coarse grained, with typical grain-size of5-8 mm. Poikilitic texture is not so wdl devdoped as in most otherexamples of the suite, and this sample has rdativdy evolved min-eral compositions (m^-number is 73—76 in dinopyroxene). Themode is 45% dinopyroxene, 30% olivine, 25% plagiodase andsparse,mica. This sample is a mdanocratic gabbro, but to avoidconfusion about the distinction between the gabbro and thewehrlite suites, all sills belonging to the latter suite will be referredto as 'wehrlites'.

Sample WW, wehrlite suite, ~800 m west of sample SD,represents a thick (4 m) sill which can be followed for almost 1 kmalong strike, with little change in thickness and in stratigraphicposition. It intrudes some well-layered and laminated ol and oxidegabbros, which resemble the oxide gabbros of samples 37 and 38.Large (10-20 mm) dinopyroxene oikocrysts dominate the texture,which in genera] is very similar to that of the wehrlite plug sample1610. The modal proportions are 50% olivine, 35% clinopyroxeneand 15% plagiodase. In addition to a few percent phlogopitecrystals, euhedral amphibole crystals to 2 mm are sporadicallypresent. The m^-number of dinopyroxene is 76—77.

Sample 57 (57zx), wehrlite suite, which is a coarse-gTainedmelanogabbro, represents a sill of 1—2 m thickness at the 1320 mstratigraphic levd in the LLS (see Bernstein et al., 1992). The sillhas poorly defined borders, and along strike the sill seems to dis-appear into a mafic mixing—mingling zone. Compared with othermembers of the wehrlite suite, and with many of the host gabbros,this sill has rather evolved mineral compositions, with mg-numbcrof clinopyroxene of between 74 and 77. The texture of sample 57resembles that of sample WW.

501

JOURNAL OF PETROLOGY VOLUME 37 NUMBER 3 JUNE 1996

Sample 44, wehrlite suite, coma from the IXS, ~20 mitratigraphically above sample 64 (from the gabbro suite). It is amelanocratic lUl of 2 m thickness, whose upper contact grades intothe host gabbros over a few tens of centimetres. The texture re-sembles that of most ol gabbros from the gabbro luitc, with sub-hedral clinopyroxenes and olivine crystals of 2-3 mm andplagioclaie mostly forming elongated crystali in the interjticcs. Afew of the dinopyroxenc grains are optically zoned, in a sector-likefashion. The modal proportions are 40% clinopyroxene, 30%olivine and 25-30% plagiodaje. Phlogopite and amphiboletogether make up ~5% of the mode. Clinopyroxene m£-number is78—81, which makes sample 44 among the more primitivememben of the wehrlite sills.

ANALYTICAL TECHNIQUESTo evaluate the composition of the liquids fromwhich the gabbro and the wehrlite suite crystallized,an ion microprobe study was performed on clino-pyroxenes from the sample suite described above.The ion microprobe allows high-precision analyses ofindividual clinopyroxene domains, and is thereforeideal for the study of zoned mineral grains. Theanalyses were conducted on gold-coated thin-sectionswith the Cameca IMS-3f ion microprobe at theWoods Hole Oceanographic Institution, using abeam of negatively charged oxygen ions. The traceelements with relatively low atomic numbers—Ti,Cr, V, Sr, Y and Zr—were measured in onesequence, with the following primary beam char-acteristics: current of 0-2 nA, energy of ~12-6 keVand beam diameter of 5—8 fim. The rare earth ele-ments (REE) were measured in a separate sequence,with a primary beam current of ~ 1-2 nA and abeam diameter of 12—15 fim. The secondary ionaccelerating voltage was reduced by -90 eV for thefirst sequence of elements, and by -40 to -60 eV forthe REE (see Shimizu et al., 1978). The intensities ofthe following isotopes were measured: 28Si, 47Ti, V,52Cr, "Sr, %9Y, *Zr, 139La, 140Ce, 146Nd, 147Sm,151Eu, 163Dy, 166Er, 174Yb. These secondary ionintensities were then related to concentrations byempirical working curves. The precision andaccuracy of the analyses are usually within ±5% forthe more abundant trace-elements, and ± 10% forthe REEs (see also Table 2). The accuracy wasmonitored by repeated routine analyses of the clino-pyroxene standard KH-1.

As the data are acquired in two sequences, it is notpossible to analyse all elements in exactly the samespot. This is because the primary beam erodes a deephole in the sample material (typically 20 (im deep,when measuring the REEs over a 30 min period),which can affect the ionization of the samplematerial, besides the fact that most thin-sections areonly ~25 //m thick. Therefore the two sequencesrequire two separate analysis areas. This obviously

presents a problem when analysing heterogeneousmaterial such as strongly zoned clinopyroxenecrystals. To circumvent this problem, variations intrace-element data are generally only comparedwhen the elements of interest were acquired from thesame analysis sequence. However, some clino-pyroxene cores appear homogeneous so that REEand other trace elements can be compared. Thenumber of analyses per sample ranges from two to12, dependent on degree of zoning.

Major-element concentrations were obtained byelectron microprobe analyses at the GeologicalInstitute, University of Copenhagen.

TRACE-ELEMENTDISTRIBUTION INCLINOPYROXENERepresentative trace-element analyses of clino-pyroxenes are given in Table 2. The variation intrace-element concentrations in clinopyroxene fromindividual samples is very large. This is particularlythe case with sample 61. Before calculated liquidcompositions are evaluated, the trace-element vari-ation in this sample will therefore be described.

In Fig. 4, analyses of sample 61 clinopyroxenes arepresented. The overall patterns show positive corre-lations between Nd and Nd/Yb, Zr and Sr, andnegative correlation between Zr and mg-number[molar Mg/(Mg + Fe)], and between Zr and Cr.Individual crystals of sample 61 have variationtrends similar to the overall pattern. Cores of clino-pyroxene crystals from sample 61 appear to define agroup with restricted ranges in trace-element con-tents. The cores with the highest m^-number and Crcontent also have the lowest concentrations ofincompatible elements (e.g. Zr and Sr) and low Nd/Yb ratios. The distinct chemistry of the clino-pyroxene cores versus the rims in sample 61 is alsodisplayed by the variation in major and minorelement chemistry. In Fig. 5, clinopyroxene com-positions of sample 61 are shown, together withclinopyroxenes from the KEHLS rocks (Bernstein etal., 1992). The clinopyroxene cores of sample 61form a distinct group with significantly higher Al(and Cr) compared with the rims. The bulk of theKEHLS pyroxenes have lower contents of Al, andoverlap with the compositions of clinopyroxene rimsof sample 61. In terms of trace-elements, the vari-ation patterns expressed by the sample 61 analysesare typical for all members of the gabbro suite,although the remaining samples show smaller rangein the parameters described.

The plug and the sills belonging to the wehrlite

502

BERNSTEIN et d. EVOLUTION OF KAP EDVARD HOLM COMPLEX

Table 2: Representative trace-element analyses of clinopyroxene cores, obtained by ion microprobe

Sample:

La

Cs

Nd

Sm

Eu

Dy

Er

Yb

Tl

V

Cr

Sr

Y

Zr

37 p2

1-91

685

622

2-54

0-85

3-15

1-72

1-66

6080

436

338

24-8

15-1

24-4

±reL%

226

0-54

5-37

2-27

1-98

4-89

1-22

0-68

0-17

0-85

2-4

1-27

3-61

tabs.

004

004

0-33

0-O6

0-02

ooa0-02

41-59

0-75

2-89

0-6

0-19

0-88

38 p4

1 92

6-92

8-68

3 21

1-15

4-12

2-43

2-22

7894

548

718

26-4

36-8

±rel.%

7-64

6-7

113

3-31

1-76

333

3-12

079

1-73

1-91

2-31

1-16

labs.

0-15

0-46

0-1

0-11

002

008

007

62-6

9-46

13-68

0-61

0-43

83 p1

1-56

6-25

5-20

2-14

0-78

2-36

1-27

1-38

6626

432

1431-7

27-7

30-7

±nH.%

5-4

3-13

6-91

556

6-14

3-04

1-52

065

0-93

0-67

1-8

2-78

±abs.

008

0-16

036

0-12

005

004

002

4287

401

9-55

0-5

0-85

80 pi

1-22

408

409

1-73

0-57

1-91

107

0-94

4364

264

2424

21-3

10-0

17-1

±reL%

1505

11-16

10-4

7-4

6-75

11-29

5-33

0-39

1-9

1-5

1-26

1-96

4-86

labs.

0-18

0-46

0-18

0-04

0-13

0-12

0-05

17-02

6-02

36-36

0-27

0-2

0-83

Calculttad melt compositions

Nd/Yb

Eu/Eu'

P-P-m.

La

Ce

Nd

Sm

Eu

Dy

Er

Yb

Tl

V

Cr

Sr

Y

Zr

3-11

0-79

64 p1

1-47

4-36

4-45

2-10

0-71

201

1-23

1-20

4393

378

2931

19-5

10-5

16-3

5-44

9-36

±rel.%

8-43

9-14

6-97

16-7

11-41

11-23

17-1

0-81

1-82

101

1-42

1-62

1-61

0-17

007

tabs.

0-12

0-4

0-15

0-12

0-23

0-14

0-2

35-58

6-88

29-6

0-28

0-17

0-26

3-24

0-84

61 p5

0-40

1-48

1-53

0-67

0-31

0-86

0-50

0-51

1851

217

7538

14-8

4-50

7-10

3-36

7-42

±re\.%

8-13

4-1

1-18

203

11-13

6-37

11-92

0-4

2-65

0-66

5-98

5-86

3-5

on006

±abs.

003

006

002

001

003

003

006

7-48

5-76

50-05

0-89

0-26

0-25

3-12

0-88

44 p2

1-46

5-20

4-41

2-38

0-65

2-19

1-21

0-91

4824

365

3700

20-4

13-3

20-9

707

20-1

±rel.%

6-36

2-08

2-55

14-78

3-43

8-49

12-82

0-69

0-7

0-96

2-76

2-48

2-92

0-22

0-18

±abs.

009

0-11

on0-35

002

0-1

0-12

33-29

2-56

35-52

0-56

0-33

0-61

3-61

0-80

57 p2

2-56

10-15

10-14

3-92

099

4-69

2-56

1-98

6612

346

127

28

226

32-8

6-32

28-55

±rel.%

6-81

307

5-33

7-03

3-95

5-79

3-54

0-48

0-6

4-7

1-81

1-27

1-03

0-19

0-23

tabs.

0-17

0-31

0-21

0-07

0-18

0-15

0-07

31-54

2-08

5-97

0-51

0-29

0-34

Calculated melt compositions

Nd/Yb 307 18-67

Eu/Eu' 0-84 52-95

0-51

0-44

2-49

1-08

11-84

29-99

0-29

0-33

4-02

0-68

13-42

30-6

0-54

0-21

4-24

0-60

3-54

23-75

0-15

0-14

(confined on next page)

503

JOURNAL OF PETROLOGY VOLUME 37 NUMBER 3 JUNE 1996

Table 2: continued

p.pjn.

La

Ce

Nd

Sm

Eu

Dy

Er

Yb

T1

V

Cr

Sr

Y

Zr

SDpi

1-82

623

604

2-61

0-80

288

1-57

1-25

5686

361

2354

23-2

14-8

25

±rel.%

5 86

5-35

3-56

704

7-77

5 89

3-38

0-66

107

0-25

2-36

386

6-11

Calculated mett compositions

Nd/Yb

Eu/Eu"

4 0 0

0-74

4-85

2508

tabs.

0-11

0-33

0-22

0-18

0-06

0-09

004

37-53

3-86

5-89

0-55

0-57

1-53

0-19

0-19

WWp1

282

10-62

11-3

3-99

1-21

4-94

2-74

2-30

6369

329

895

23

44

4-07

0-72

±rel.%

7-13

8-03

7-14

3-54

3-45

3-97

2-69

0-95

0-87

1-17

109

201

7-63

11-79

tabs.

0-2

0-85

0-81

0-14

004

0-11

006

60-19

2-86

10-47

0-25

0 88

0-31

008

1610p6

0-81

3-11

288

1-01

0 39

0-89

0-47

0-38

2810

227

7981

36

7-1

9-2

628

0-97

±rel.%

8-9

3 8

11

10

8-3

15

4-4

2-16

305

0-29

4-13

303

3-97

12-29

29-2

±abs.

007

0-12

0-32

0-1

003

007

002

60-7

6-92

23-14

1-49

0-22

0-37

0-77

0-28

75 p2

1-50

5-26

5-26

2-15

0-76

2-57

1 -21

109

5368

355

5310

25

11-9

22

4-00

0 84

±rel.%

2

5-7

3-6

10

2-4

4-1

7-8

1-9

4-5

2-4

4-2

4-3

11

9-01

15-26

tabs.

0-O3

0 3

0-19

0-22

0-02

0-06

009

102

15-98

127-4

1-05

0-61

2-42

036

0-13

Nd/Yb and Eu/Eu* are chondrite-normalized, calculated melt compositions in equilibrium with clinopyroxene. Eu/Eu* = Eu/[2(Sm+Dy)/3]-

suite exhibit variation patterns different from thoseof the gabbro suite. The most primitive repre-sentative of the wehrlite suite, the wehrlite plugsample 1610, shows a complex pattern of variation(Fig. 6). Some of the trends are opposite to those ofthe gabbro suite. This is the case with the Nd/Yb vsNd diagram, where a weak, negative correlationexists. Negative correlations are also evident in theEu/Eu* vs Nd and Cr vs Zr diagrams. The wehrlitesills roughly mimic the trends defined by the wehrliteplug. This is especially the case in the REE variationdiagrams. The wehrlite sill samples 44 and 75 showlarge variations in Nd/Yb within a narrow range inNd concentration. If any correlation can bedetected, it would be a tendency for lower values ofNd/Yb with increasing Nd content (sample 57).Similarly, for samples 44 and 75 the variation in Eu/Eu* is large and correlates poorly with Nd. As istrue for the Nd/Yb vs Nd plot, samples 57, WW andSD define weak negative trends between Eu/Eu* andNd. In the plot of Sr vs Zr, a similar pattern is seen,with a more marked tendency for Sr to decrease withincreasing Zr (samples WW, SD and 57). Comparedwith the wehrlite plug, all sill members of the

wehrlite suite have low Nd/Yb, lower concentrationsof Sr and Cr, and more pronounced negative Euanomalies.

DISCUSSIONIn general, the analysed clinopyroxene crystals fromthe gabbro suite show normal zoning patterns, withnegative correlation between compatible andincompatible elements. Similar zoning of major ele-ments in clinopyroxene from the MLS has beendemonstrated by Tegner & Wilson (1995). This typeof zoning is in contrast with the studies of clinopyr-oxene phenocrysts in basalts by Shimizu (1981),Shimizu & LeRoex (1986) and Vannucci et al.(1993), and suggests that disequilibrium crystal-lization played only a minor role in developing thetrace-element patterns described above. For thegabbro suite samples, as exemplified by sample 61(Fig. 4), the trace-element patterns can be explainedin terms of fractional crystallization (see Fig. 12,below, for other gabbro suite samples). The increasein LREE to HREE ratio (Nd/Yb) and strongdecrease in chromium can be caused by clinopyr-

504

BERNSTEIN it al. EVOLUTION OF KAP EDVARD HOLM COMPLEX

10 20 30Zr p.p.m.

50

0.7620 30Zr p.p.m.

Sri.

X

u

20 30Zr p.p.m.

Fig. 4. Chemical variation diagrams for compositions of clinopyroxenes from mdanocratic sill sample 61 (gabbro suite). Nd/Yb is chon-drite normalized (Sun & McDonough, 1989). Analyses from individual crystals are linked, and (c) is crystal core analysis. Crystal (a)

referred to in text is marked with a dashed line. Average analytical error is given by the bars in the corner of each diagram.

0.20KEHLS clinopyroxene

0.05

o KEH layered series• 312361 -rims• 312361 -cores

0.01 0.02 0.03

Cr atomic0.04

Fig. 5. The compositions of KEHLS clinopyroxenes in terms ofaluminium and chromium, expressed as atoms per six oxygen inunit formula. [Note the distinctly higher Al in cores of clinopyrox-

ene from sample 61 (Fig. 3).]

oxene fractionation, which, together with olivinefractionation, also accounts for the decrease in mg-number with increasing Zr content. The generalincrease of Sr with increasing Zr points towardssuppressed plagioclase fractionation, because thehigh proportion of plagioclase (~50%) in the frac-tionating solid assemblage along the low-pressurecotectics will result in a bulk partitioning coefficientclose to unity for Sr (given the high partition coeffi-cient for Sr in plagioclase; Table 3). Crystal 'a' inFig. 4 may thus record the early crystallization ofclinopyroxene (+olivine) which results in increasingNd/Yb ratios, strong depletion in Cr, and a positivecorrelation between Zr and Sr. The break in the Sr—Zr curve (at ~12 p.p.m. Zr) would then record theonset of plagioclase fractionation.

Similarly, for the wehrlite plug (sample 1610; Fig.6), the positive trend in Sr-Zr, followed by a

505

JOURNAL OF PETROLOGY VOLUME 37 NUMBER 3 JUNE 1996

3.0

2.5

2.0

1.5

i n

o

• %

+s.d.

• o

D

1

wehrliteI i

suite

• 1610a 44a 57o WW+SD» 75

D

clinopyroxene

0 5 10 15 20 25 30 35Nd p.p.m.

40 60 80 100 120Zr p.p.m.

wehrlite suite clinopyroxene

10 15 20 25

Nd p.p.m.

wehrlite suite clinopyroxene

U

40 60 80Zr p.p.m.

Fig. 6. Variation in trace-element chemistry of clinopyroxene from the wehrlite suite. The clinopyroxene from the wehrlite sills haiconsistently lower Nd/Yb ratios, Sr and Cr contents compared with the clinopyToxene of the wehrlite plug sample 1610. Average error

bars are given in each diagram.

Table 3: Partition coefficients for solid phases—melt

La

Ce

Nd

Sm

Eu

Dy

Er

Yb

Tl

Cr

Sr

Y

Zr

Ca-pyrcocene

0-05

0-09

0-19

0-29

0-3

0-44

0-39

0-43

0-38

10(3-8)

0-13

0-47

0-12

Ref.

1

1

1

1

2

1

1

1

1

3(1)

1

1

1

Plegioclase

0-16

0-15

0-14

0-12

068

0-O8

0-08

0-07

0-05

0-02

2

0-03

0-O1

Ref.

4

4

4

4

5

4

4

4

6

7

4

4

8

Olivirte

0-00001

0-00001000007

00007

00009

0004

0009

0023

0016

1

0003

002

001

Ref.

9

9

9

9

9

9

9

9

9

10

6

11

8

Fe-Tl oxide

0-006

0-006

0-008

0-O1

0-02

0-O3

0-05

12

B0

0-7

0-04

8

Ref.

8

8

8

8

8

8

8

12

12

12

11

12

References: 1, Hart & Dunn (1993); 2, McKay (1989); 3, Barnes (1986); 4, Drake & Weill (1975); 5, Paster etal.(1974); 6, Irving (1978) and references therein; 7, Sun et al. (1979); 8, Fujimaki et al. (1984); 9, Kelemenefa/.(1992); 10, Lemarchand era/. (1987); 11, by analogy with Er and Yb; 12, Villemant et al. (1981).

506

BERNSTEIN et al. EVOLUTION OF KAP EDVARD HOLM COMPLEX

negative trend, may be explained by clinopyroxene-olivine fractionation followed by plagioclase frac-tionation. The decrease in Eu/Eu* and Sr withincreasing Nd and Zr observed for the samples 57,WW and SD may likewise reflect crystallization of aplagioclase-rich solid assemblage. It should be notedthat these three samples also are the most evolved interms of m^-number and Cr content (Table 1 andFig. 6). In contrast, the large variations in Nd/Yb,Eu/Eu* and Sr within very restricted Nd and Zrcontents, especially in samples 44 and 75, suggestthat the pyroxenes crystallized from heterogeneousmelts, which are not related by simple fractionalcrystallization. The heterogeneity may rise frommagma mixing or metasomatic processes, and will bediscussed below.

For the studied samples, it appears that most ofthe variations in trace-elements can be qualita-tively related to crystal fractionation, and that thefractionating assemblage, according to the vari-ation patterns, is olivine/clinopyroxene followed byolivine/plagioclase-clinopyroxene. Even though theoikocrystic state of many of the analysed clino-pyroxene crystals conventionally is taken as evi-dence of late growth of clinopyroxene, the positivecorrelation in the Sr—Zr diagram argues againstsignificant plagioclase fractionation before orduring the early growth of the clinopyroxene.Early crystallization of clinopyroxene is also sug-gested by the increase in Nd/Yb at low contents ofNd for most samples (see also Fig. 12, below). Thisis because clinopyroxene is the only plausible phaseduring crystallization of tholeiitic melt (at pressuresbelow garnet stability) that has sufficiently highpartition coefficients for the heavy REE to frac-tionate Nd from Yb. The increase in Nd/Yb seenin, for example, Fig. 4 would therefore be difficultto obtain with low-pressure 'gabbro' fractionationbecause of its low proportion of clinopyroxene inthe fractionate (-35%). A similar conclusion ofearly clinopyroxene growth in cumulates was alsoreached by Tegner & Wilson (1995) on the basis ofmajor element variations in clinopyroxene oiko-crysts from the MLS. The sequential crystallization(olivine/clinopyroxene joined later by plagioclase)suggested by the trace-element distributionsdescribed above is in agreement with the work ofKirkpatrick (1977, 1983) and Brandeis et al.(1984). They demonstrated that plagioclase,because of its higher polymerization state, hashigher nucleation energy, and therefore requireshigher degrees of undercooling to reach maximumgrowth rate than do the ferro-magnesian mineralsolivine and clinopyroxene, which have simplerstructures.

Composition of equilibrium melts

Because it appears that clinopyroxene preceded sub-stantial plagioclase growth in all samples considered,only olivine and chrome-spinel fractionation mayhave affected the melt composition before saturationof clinopyroxene was reached. Owing to their verylow partition coefficients for the trace-elements con-sidered, any early fractionation of olivine andchrome-spinel must have played a negligible role(except for Cr) in the characteristics of the trace-element composition of the parental melt, con-sidering the range of variability observed. Therefore,for a given sample, the calculated equilibrium meltcomposition based on the 'least evolved' clino-pyroxene analysis in each sample is believed to berepresentative of the parental melt. The 'leastevolved' analysis refers to the lowest recorded con-centration of the incompatible elements REE and Zrin each sample. The selection is based on theassumption that none of the observed cumulus phaseassemblages has a bulk partition coefficient higherthan unity for the REEs or Zr. The 'least evolved'analysis is taken as the closest approximation to thevalues in equilibrium with the melt from which thefirst clinopyroxene crystallized. Trace-element con-centrations in equilibrium liquids are then calculatedusing published partition coefficients (Table 3). Thepartition coefficients used arc determined in basalticsystems with limited mjf-number, and are assumedinsensitive to variations in intensive parametersduring the crystallization processes considered in thisstudy. This is substantiated by the high-pressureexperimental work of Hart & Dunn (1993), whosepartition coefficients are adopted here, and whoseresults do not differ markedly from those obtained atlow pressures by, for example, Gallahan & Nielsen(1992).

The melt compositions calculated in this way areshown in the chemical variation diagrams of Fig. 7.The gabbro and the wehrlite suites define twocoherent groups with the following characteristics.The gabbro suite shows positive correlation betweenNd/Yb and Nd, and a strong positive correlationbetween Sr and Zr. Sample 61 has a weak positiveEu anomaly, whereas the remaining samples of thegabbro suite cluster at a negative Eu anomaly of~0-8. Finally, Cr shows strong negative correlationwith Zr. As for sample 61 from the gabbro suite,sample 1610 defines the primitive end of the morecomplex trends of the wehrlite suite. Apart fromsample 1610 from the wehrlite plug, the wehrlite sillsamples define trends sub-parallel to those of theevolved gabbro suite samples. Generally, thewehrlite suite has higher Nd/Yb ratios and higher Zr

507

JOURNAL OF PETROLOGY VOLUME 37 NUMBER 3 JUNE 1996

I4

+ s-d.

D61

• 1610

.44 •D80

CP6483

75.SD

37

• Wehrlite suite• Gabbro suite

• 57• WW

D O38

melt compositions

40

35

£30

25

20

• 1610 melt compositions

61

8 3# 75 37D38

• 57

D 80

D»4464

'SDWW

+ s.d.

20 40 60 80 100 120 140Nd (N)

10 20 30 40 50 60 70 80 90Zr (N)

1.1

1

0.9

0.8

0.7

" ( 5 1

+ s.d.

• 1610

44• 64#

D80

7 5 nD37

• S D

mdt compositions

38

• WW.

. 5 7

20 40 60 80 100 120 140

Nd(N)

1000

800

E 600a.a.6 400

200

0

1610

• 61

• s.d.

44

D64

75

• 83

melt compositions

SD

WW

P 3 ? D 3 8 j ? 7 . #

10 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0

Zr (N)

Fig. 7. Compositions of melts in equilibrium with the analysed samples, computed from 'least evolved analyses' (see text) using thepartition coefficients listed in Table 3. The two primitive end-member melt compositions 61 and 1610 for the gabbro and wehrlitesuite, respectively, have strongly contrasting Nd/Yb ratios and Sr contents. The sills of the wehrlite luite show compositions roughlyparallel to those of the gabbro suite melts, although somewhat displaced towards higher Nd/Yb. Average error bars are given in each

diagram.

contents than the gabbro suite for a given Nd or Crconcentration. The overall range in trace-elementconcentrations is larger for the wehrlite suite. Thisbecomes especially evident when comparing thewehrlite sills with the evolved gabbros.

Crystallization modelsThe evaluation of crystallization processes belowshould be seen in the light of the sample collectionthat forms the basis of this work. There is littleknowledge of the time difference between the for-mation of individual lithologies, although they wereall formed during the supersolidus stage of theKEHLS magma chamber. Furthermore, because ofthe relatively few samples in this study, comparedwith traditional basalt studies, our modelling shouldbe considered in mainly qualitative terms.

The primitive end of the gabbro suite is defined bysample 61, which is chosen as the closest approx-imation to the melt parental to the gabbro suite.This seems justified by the observations that thesample 61 calculated liquid has low chondrite-nor-malized abundances of incompatible elements,unfractionated REE patterns, Cr concentrations inthe range of primitive MORB (Wilson, 1989), andthat the clinopyroxene cores have m^-number of~90, which is in the range expected for clino-pyroxene in equilibrium with a primary, mantle-derived melt. Various crystallization models havebeen conducted to evaluate how the evolved liquidmembers of the gabbro suite relate to the primitiveliquid sample 61. The modelling has been performedon the elements listed in Table 2, with the solid-liquid partition coefficients in Table 3, using theRayleigh equation Q = C(yF<-D~l\ where C\ and Co are

508

BERNSTEIN el al. EVOLUTION OF KAP EDVARD HOLM COMPLEX

concentrations of an element in fractionated andinitial melt respectively, F is residual melt fractionand D is bulk partition coefficient.

Rayleighfractionation at low pressures

For low-pressure fractional crystallization of a tho-leiitic basalt, the proportions of plagioclase—clino-pyroxene—olivine are approximately 0-50—0-30—0-20,respectively (Biggar, 1983; Komor«/u/., 1987; Groveetal., 1992). These proportions are broadly consistentwith the cumulus assemblage and modal abundanceobserved in the KEHLS gabbros (Bernstein et al.,1992). To produce the Nd concentration of the inter-mediate melts (e.g. sample 83) from sample 61, 65-70% fractional crystallization is required, using thelow-pressure fractionation assemblage in the propor-tions listed above. The resulting liquids (see frac-tionation paths 'LP frac' in Fig. 8) develop negativeEu anomalies (low Eu/Eu* ratios), that correspondwell to the values of the intermediate gabbros. Also,Zr and Cr concentrations are modelled satisfactorily(Fig. 8). However, Nd/Yb ratios and Sr concentra-tions are not in accordance with observed values.During low-pressure fractionation, Nd/Yb ratios andSr concentrations of residual liquids are nearly con-stant, reflecting the high proportion of plagioclase inthe cumulus assemblage [with a high partition coeffi-cient (Z>) for Sr and high Z)Nd/Z)Yb]- The positivecorrelation between Sr and Zr may be explained interms of early olivine fractionation, but the inter-mediate samples 64, 80 and 83 have m^-numbers thatare too low (clinopyroxene trig-number 78-80) tohave only olivine on their liquidi. More importantly,olivine is not capable of fractionating the REE suffi-ciently, and it is not likely that olivine fractionationalone can dominate for up to 65% crystallization,which is needed to account for the variations in Ndand Zr concentrations by fractionation. Applicationof a more complex fractionation model involvingreplenishments, tapping and continuous fractionalcrystallization (RTF; O'Hara & Mathews, 1981; seebelow) does not change the overall pattern outlinedby simple fractional crystallization. It can thereforebe stated that fractional crystallization involving thephase proportions appropriate for crystallization of atholeiitic melt at low pressure cannot produce thetrace-element pattern observed for the gabbro suite.Crystallization at higher pressures is therefore evalu-ated below.

The effect of pressure on the proportions of

crystallizing phasesThe cotectic proportions of the phases plagioclase,clinopyroxene and olivine change progressively with

increasing pressure towards higher clinopyroxeneproportions relative to the other two phases (e.g.Bender et al., 1978; Presnall et al., 1978; Fujii &Bougault, 1983). In high-pressure experiments onmildly alkaline basalts from Surtsey, Iceland (10-125 kbar), the phase proportions of plagioclase,clinopyroxene and olivine are roughly 0-18-0-47-0-35 (Thy, 1991), and 10 kbar experiments on asynthetic high magnesium tholeiite yielded the plag-cpx-ol proportions of 013-0-55-032 (Grove et al.,1992), highlighting the importance of clinopyroxenein the crystallizing assemblage at higher pressures.In addition, Furman et al. (1991) successfully mod-elled a suite of Surtsey lavas using an inferred 10kbar crystallizing assemblage plagioclase, clino-pyroxene and olivine of roughly 0-20-0-51-0-29. Inthe following, the proportions plag-cpx-ol 0-20-0-50-0-30 have therefore been employed indescribing the effects of fractional crystallization atdeep crustal levels, i.e. at pressures of ~ 10-12 kbar.

Rayleighfractionation at high pressures

Simple fractional crystallization of the high-pressurecumulus assemblage yields a somewhat better matchto some of the observed trends (trend 'HP frac' inFig. 8). The amount of fractionation to produce theNd concentration of sample 83 from the primitivesample 61 melt is again 65—70%, and the high pro-portion of clinopyroxene in the fractionate has a sig-nificant effect on the Nd/Yb ratio, compared withthe low-pressure fractionation. The suppressed pla-gioclase fractionation results in strong positive cor-relation between Sr and Zr, duplicating the observedgabbro suite fairly well. However, the low plagio-clase proportion also results in much less depletion inEu with fractionation, so that the model Eu/Eu*trend differs markedly from the observed gabbrosuite. Another problem arises from the strongdepletion of Cr in the 10 kbar model, which is due tothe high clinopyroxene proportion in the fractionate.Rayleigh fractionation of 65—70% leaves only a fewp.p.m. Cr in the residual melt. Similarly, calculatedmelts of the samples 83, 80 and 64 have m^-numberof 48-50, in apparent contradiction to the high frac-tionation degrees inferred from the incompatibleelement enrichment. If the initial sample 61 meltwith m£-number of 69 is assumed to haveFeO + MgO of ~21 wt%, which is typical for Ter-tiary basalts from Greenland (Pedersen, 1985;Larsen et al., 1989), the evolved melt, after some 65—70% Rayleigh fractionation, will have mg-number of~32 (Fig. 9). Although it is noted that some of thepatterns of the chemical variations in the gabbroicsuite point towards the characteristics of high-

509

JOURNAL OF PETROLOGY VOLUME 37 NUMBER 3 JUNE 19%

3.8

3.6

3 4

3.2

3.0

2.8

2.6

2.4

melt compositions

HP RTF ss-

x x x

x * X * K

61

•

in"JI

—»••

x x x

80

LP

X *

V

frac-

x x ^g

^ • H P frac.

P free.

30

28

26

24

22

20

18

16

14

melt compositions KX |

HP freer—^x

X

** "80X

x "64X

X

XX

X

^ LP frae*.

LP Trac.14?̂ + + a 3g ^v

\ ' 3 ? * * * 'N / /F RTF ss

10 20 30 40 50 60 70 80

Nd(N)10 20 30 40 50 60

Zr(N)70

1.1

1.0

!= 0.9

0.8

0.7

• X* x

+

LP f

x x x

+

racr +

LP

x x

tm

+ .

fra

melt compositions

^ H P frac.< *^

x x

" X x x

^•HP RTF ss

. 6 4 3>8

- 3 7

10 20 30 40 50 60 70 80

800

700

600

500ea. 400o.

U 300

200

100

• 6 1

+ Mx +

HX

X

HP f r a c ^

^LP frac.

/• 64

• • 80

+ + + +

melt compositions

•

HP RTF ss

83/ / ^ L P f r a c-

10 20 30 40 50 60 70

Nd (N) Zr (N)Fig. 8. CompositionaJ variations in melts from the gabbro suite. The two trends 'LP frac' show low-pressure Rayleigh fractionation ofthe assemblage 0-5plag-0-3cpx-0-2ol with sample 61 as parental liquid, and the assemblage 0-5plag-0-3cpx-018ol-002FeTi oxide withsample 83 as parental liquid. 'HP frac' stands for high-pressure fractionation of the assemblage 0-2plag-0-5cpx-03ol with sample 61 asparental liquid. 'HP RTF ss' is the steady-state product of high-pressure RTF processes (see text for details). Crones depict Rayleigh

fractionation of the phase assemblage stated above in 5% increments for 10>F>0-5 , and 2% increments for 0-5>F.

pressure fractionation (Nd/Yb vs Nd, and Sr vs Zr),it must be concluded that a simple Rayleigh frac-tionation model is insufficient to describe theobserved trends. Two other possibilities exist, whichwill be evaluated below: (1) mixing processesbetween a melt of sample 61 composition and meltswith contrasting chemical characteristics, or (2)involvement of RTF (replenishment, tapping andfractionation) processes.

Thegabbroic suite as a product of mixing processes

With the evidence of co-magmatism of the wehrliteand the gabbro suites (Bernstein et al., 1992; Tegneret al., 1993), mixing between the two suites is anobvious possibility. The wehrlite suite mixing end-member is taken as the sample 1610, because the

wehrlite sills are regarded as being mixing productsthemselves (see the section below) and not frac-tionation products of the primitive sample 1610melt. The field evidence suggests that the wehrlitesills intruded the solidifying gabbros, i.e. during thecooling stage of the magma chamber, but thisobservation does not rule out that the two liquidsuites could have coexisted throughout the lifetime ofthe KEHLS magma chamber. Two mixing processescan be envisaged; the first is injection of sample 1610melt into the magma chamber, where it hybridizeswith the residing magmas undergoing low-pressurefractionation. This process, however, is incapable ofaffecting the REE budget sufficiently because thewehrlite suite volumetrically makes up < 1 % .

The second process is mixing between the sample61 and 1610 melts followed by injection of the

510

BERNSTEIN et aL EVOLUTION OF KAP EDVARD HOLM COMPLEX

k.

-cEaeI

5

70

65

60

55

50

45

40

"61

\

\HPfrac

melt compositions

1 a83 HPRTFss

80 " ^ 37 ?

10 20 30 40 50 60

Zr(N)Fig. 9. The m^-numbcr and Zr concentrations in meltj calculatedin equilibrium with dinopyroxene cores of the gabbro suite, basedon KA (n^-number liquid/m£-number cpx)=025 (ice text). SimpleRayleigh fractionation leadi to a depletion in mj-number outsidethat observed in the KEHLS gabbros (arrow labelled HP 'gabbro'fractionation), whereai RTF process ('HP RTF ss'), with theparameters estimated in the text, accounts more satisfactorily forthe observed values. The Rayleigh fractionation model used thebulk partition coefficients and itarting liquid composition

described in the text.

resulting hybrid magma into the KEHLS magmachamber, where the magma subsequently underwentlow-pressure fractionation. The chemical con-sequence of this mixing process is outlined in Fig. 10.It is evident that large amounts of 'wehrlite' magmaare required to mix with the sample 61 magma toapproach the values of the gabbro suite in terms ofNd/Yb ratios and Sr concentrations. Apart fromsample 80, the combined mixing and fractionationcurves fit the Sr concentrations in the gabbro suitepoorly (Fig. 10). It is thus unlikely that the chemicaltrend of the gabbroic suite is solely due to mixingprocesses, and possibility (2) involving RTF pro-cesses will be considered next.

RTFmodel

In the RTF model (O'Hara & Mathews, 1981), theperiodical replenishment, tapping and continuousfractionation results in a steady-state melt compo-sition after a certain number of cycles, provided thatthe parameters x (amount of fractional crystal-lization), y (amount of tapping) and z (amount ofreplenishments) are held constant. The success of theRTF model, which in general is more realistic indescribing magma chamber processes than is simpleRayleigh fractionation, has been demonstrated by,for example, Cox (1988), Hogg et al. (1988) andLarsen et al. (1989). An effect of RTF processes isthat incompatible elements with slightly differentbulk partition coefficients are fractionated from eachother to a greater extent than expected from exam-ining the variations in absolute concentrations. Forthe major elements that have bulk partition coeffi-cients around unity the RTF process results in verylimited variations, so that qualitatively the RTFmodel may be able to explain the large variation inincompatible elements along with the small range in777£-number in clinopyroxenes.

The anorthosite-leucogabbro sample 83 at thebasal part of the MLS was chosen to represent theintermediate gabbros, because the field relations andcompositional variations suggest that this rock bodyrepresents a new injection of fresh magma (Abbott &Deer, 1972; Fehlhaber & Bird, 1991). To assesswhether the sample 83 melt composition can berelated to the parental melt 61 through RTF pro-cesses, an enrichment factor (r = C\jCQ, where Co isinitial melt concentration, sample 61, and Q is con-centration in evolved melt, sample 83) was deter-mined for the elements listed in Table 4. Also listedare the bulk partition coefficients for the high-pressure assemblage 02plag-0-5cpx-0-3ol. The rela-

4.0

3.5

3.0

61 mixing with

4 + + "*

********

1610 (melts)

. 8 0

******** * *

83 64 I1 38• • •

********* +"

z*****>* * * * * *10 20 30 40 50

Nd(N)60 70 80

28

26

24

20

18

16

14

****** ,* * * * *

l%tt****** I61

++++ + .(^^

64

• + + • * * * +

•83

)

* * *• *

•3 7

4 4

4 4

"38

10 20 30 40

Zr(N)50 60

Fig. 10. The result of bulk mixing of sample 61 liquid with 1-35% of sample 1610 liquid, followed by low-pressure Rayleigh frac-tionation of the assemblage 0-5plag-0-3cpx-0-2ol. Fractionation increments as in Fig. 8.

511

JOURNAL OF PETROLOGY VOLUME 37 NUMBER 3 JUNE 1996

Table 4: Enrichment factors r, and bulk

partition coefficients D, for calculating

RTF parameters, assuming that sample 83

is an RTF product of sample 61

Nd Yb Cr Sr

i

D

2-72

0-122

2-23

0-23

0-16

5-3

1-72

0-474

2-84

0067

Bulk partition coefficients from Table 3, based on the assemblage0-2 plagioclase-0'5 clinopyroxene-0-3 olivine.

tionship between the concentrations of an element tin the initial melt, C,o, and in the steady stateerupted melt, C.-j, can be expressed by the equation

Cifl(1)

where x and_> are defined above (assumed constant),and where D,- is bulk partition coefficient for elementi (O'Hara & Mathews, 1981). Solving equation (1)for_y yields

y= \x-xr, + n--— x

(2)

The result of equation (2) for the elements inTable 4 is shown in Fig. 11. The near parallelism ofthe lines for the incompatible elements in Fig. 11implies that the sample 83 melt may indeed be aproduct of RTF processes with sample 61 as theparental melt. If the sample 83 melt composition isto be modelled by the RTF model with reasonableaccuracy with respect to chromium, x and_>> must bein the range of 0-16-0-22 and 005-0-10, respect-ively. Furthermore, x\y must be in the range 2-8-3-2,the lower limit arbitrarily set by Nd/Yb in residualmelts to be higher than 3-0 and the upper limitdefined by contents of Nd and Zr in residual melts tobe lower than 55 and 50 times chondrite, respec-tively (Fig. 8). These values for x and j are similar toresults of numerical modelling by Cox (1988) of asequence of basalts from the Deccan Traps. Hefound that if RTF were to produce the observedchemical variation, x and y had to attain valueslarger than 005 and 004, respectively. The xjy ratioof around three is higher than Cox's estimate(0-b<xjy< 1-3) and results from the higher degree offractionation required in the model, because of therather large r values in the present study (Table 4).The erupted steady-state composition resulting from

0 30

•J 0.25a.5 0.20a% 0.15

a o.io

^ 0.050

0.0

o Nd• Yb• Cr« Sr* Zr

k l h parameters

•

• 1VoO°x*><*

•

0.1 0.2 0.3 0.4x - mass fraction crystallized

0.5

Fig. 11. The RTF parameters x andjs which in addition to theparameters in Table 4 determine the composition of steady-statemelt, x and^ are calculated from equation (2) in the text. [Notethe restricted ranges in * andji to allow the chemical characteristicof sample 83 melt (given by bars on * and_y axes).] The paralle-lism of the curves for the incompatible elements suggests a good fitbetween the RTF model and the observed elemental distribution

in melt 83.

x = 0-2 a n d j = 0-08 is presented in Figs 8 and 9 ('HPRTF ss'). As expected, the composition of the 'HPRTF ss' is close to that of sample 83, but not only interms of the elements considered in Table 4; Eu/Eu*(Fig. 8) and mg-number also are better accounted forby the RTF model. In the high-pressure assemblageof 0-2plag—0-5cpx—0-3ol, the bulk partition coeffi-cients of Mg and Fe are 2-2 and 0-7, respectively,calculated on the basis of JLT>lO\(Mg,F<:) —0-33 (Roeder& Emslie, 1970) and A"D)Cp^MgFFc) = 0-25 (Grove &Bryan, 1983). These bulk partition coefficients arepractically identical to those determined by O'Hara(1977). The RTF steady-state melt calculated abovewill have an mg-number of ~48 , which is within therange of the melt calculated for the sample 83 clin-opyroxene (mg-number 48-50). No information onthe size of z (amount of incoming parental melt) isgiven by the above calculations, as the steady-statecomposition is independent of z- In contrast tostudies of basalt series, there are poor constraints onthe relative age of individual sequences in intrusions,rendering z impossible to estimate.

For sample 64, it is possible that fluctuations in thex zn&y parameters and subsequent mixing with low-pressure fractionates residing in the KEHLS magmachamber could give rise to the observed low Sr andZr, and high Cr concentrations compared withsample 83. For example, when x and y are verysmall, chromium is depleted very slowly by RTFprocesses. For sample 80, however, no possible frac-tionation process (at pressures <15 kbar, i.e.

512

BERNSTEIN et al. EVOLUTION OF KAP EDVARD HOLM COMPLEX

without garnet fractionation) can account for itshigh Nd/Yb. The chemistry of this sample may beaccounted for by other processes, and it should benoted that from the mixing calculations presented inFig. 10, it appears that sample 80 may indeedrepresent mixing between the gabbro and wehrlitesuite melts.

Evidence of high-pressure fractionation can befound in the composition of the clinopyroxenes fromsample 61. As noted above, some of the large clino-pyroxenes have euhedral cores, sometimes partlyresorbed, with high m^-numbers and Cr contents.These clinopyroxene cores are interpreted as pheno-crysts, which formed at the deep-seated RTF magmachamber before emplacement of the sill. As demon-strated in Fig. 5, the rims of the sample 61 clinopyr-oxenes are indistinguishable from the bulk of theKEHLS gabbro clinopyroxenes, whereas the sample61 clinopyroxene cores are displaced towards con-siderably higher aluminium and chromium contents.Numerous experiments and calculations have shownthat the solubility of the Ca-tschermak component inclinopyroxene increases with pressure (e.g. Wood,1976; Herzberg, 1978), so on this basis the high Alcontent of the euhedral clinopyroxene cores lendsfurther credit to the high-pressure fractionationmodel.

The liquid, from which the evolved oxidegabbros 37 and 38 crystallized, cannot be directlyproduced from sample 61 by crystallization ateither high or low pressure. The evolved oxidegabbros may be low-pressure fractionates of sample83 melt, with the assemblage 0-5plag-0-3cpx-0-18ol-002FeTi oxide (see Fig. 8). In this way, the'oxide gabbro1 melts may represent some 35%fractionation of melt 83. The Eu contents of theoxide gabbro melts are not readily explained byfractionation of the above assemblage. The highplagioclase content in the fractionate is expected toresult in a strong negative Eu anomaly. However,the Eu/Eu* values in the sample 37 and 38 meltsare similar to those of the parent melt. A possibleexplanation could be higher / o , m t n e low-pressureKEHLS magma chamber, relative to the high-pressure fractionation magma chamber (s). Thepartitioning of Eu between plagioclase and liquid isa strong function of the oxygen fugacity, with Z>EU

varying from ~ TO at the iron—wtistite buffer to~0"25 at the quartz—magnetite-fayalite buffer at~1200°C (McKay et al., 1989). The relativeincompatibility of Eu in the low-pressure systemcould also be due to a negative Eu anomaly inclinopyroxene, as suggested by the study of Morse& Nolan (1985) on the REE systematics of theKiglapait intrusion. In the present study, the high

errors in the determination of Eu/Eu* (Table 2)render these values difficult to constrain.

To summarize, all gabbros more evolved thansample 61 have significantly higher Nd/Yb thansample 61. Also, these gabbros display a strongpositive correlation between Zr and Sr. These fea-tures are indicative of high proportions of clino-pyroxene in the fractionate, and coupled with themild depletion of chromium when considering thehigh contents of incompatible elements, suggest thatRTF processes at ~10 kbar are responsible for theobserved trends. The independent calculation of mg-number of the HP RTF ss product using x and yvalues derived from the concentrations of trace-elements, and the high Al content of sample 61clinopyroxene cores, provide arguments in favour ofthe high-pressure RTF model. Field relations suggestthat the leucogabbro—anorthosite sample 83 repre-sents a new influx of melt into the KEHLS magmachamber. The high modal proportions of plagioclasein this rock can hence be explained as a result ofpreceding fractionation of primitive sample 61 meltat high pressures. In response to the pressure releasefollowing transport of the evolved magma from thedeep-seated magma chamber to the KEHLS magmachamber at 1-2 kbar, the melt was saturated withplagioclase, and fractionated large amounts of thisphase before reaching the low-pressure eutectic pla-gioclase-clinopyroxene-olivine. The preferred modelfor the evolution of the KEHLS magma chamber istherefore that it was predominantly fed by magmasevolved at high pressure, which upon injection intothe KEHLS magma chamber underwent low-pressure fractionation to form the bulk of the layeredolivine and oxide gabbros. Only rarely was a pri-mitive, essentially unfractionated, Mg-rich meltallowed through the plumbing system, in this caseforming the melanocratic sill sample 61.

Origin of the wehrlite sillsAs mentioned above, and as stressed by Bernstein etal. (1992), there is compelling evidence for a geneticlink between the wehrlite plug and the wehrlite sills.One of the strongest arguments is the presence ofphlogopite crystals in all members of the wehrlitesuite, in contrast to the gabbro suite, which is vir-tually devoid of this phase. The phlogopite crystals inthe wehrlite suite are commonly strongly deformed,as described by Bernstein et al. (1992). The latterobservation suggests that this phase crystallized in the'wehrlite' melt before its emplacement in the KEHLSmagma chamber as sills or plugs. However, accordingto Tegner & Wilson (1993) and Tegner et al. (1993),the wehrlite suite can be divided into two groups, one

513

JOURNAL OF PETROLOGY VOLUME 37 NUMBER 3 JUNE 1996

as primary intrusives into the host layered gabbros,the other as secondary replacement bodies. The sec-ondary group is abundant at Taco Point (see map,Fig. 2), where it occurs as complicated replacementstructures around the primary (intrusive) wehrlitesills. Phlogopite is also present in the replacementwehrlites, but is undeformed and interstitial. In thepresent study, all samples of the wehrlite suite are ofprimary, intrusive nature.

Calculated liquid compositions of the sills and thewehrlite plug are presented in Fig. 12. The sills ofthe wehrlite suite (i.e. excluding the wehrlitc plugsample 1610) display large variations in traceelement contents. They do, however, form acoherent array. For example, whereas the Ndcontent ranges from 45 to 123 times chondrite, theNd/Yb ratio only ranges from 39 to 43 . Between Srand Zr diere is a marked positive correlation. It isalso evident that the wehrlite sills have geochemicalcharacteristics more in common with the cumulus

and intercumulus melts from die gabbro suite (seebelow), than with the wehrlite plug sample 1610.Because of the primitive nature of sample 1610, fea-turing high m£-number in the mafic phases, highchromium in clinopyroxenes, high modal propor-tions of clinopyroxene and olivine, and low con-centrations of incompatible elements, the meltcomposition of sample 1610 is regarded as repre-sentative of the melt parental to the wehrlite sills.Considering die genetic relationship between sample1610 and the wehrlite sills, some process clearlymodified the compositions of the sills.

Simple fractional crystallization of melt 1610 isunable to account for the discontinuous trends of thewehrlite suite (see Nd/Yb vs Nd and Sr vs Zr in Fig.12). The field observations of soft sediment defor-mation around many of the wehrlite sills (Bernsteinet al., 1992), and die occurrence of replacive wehrlitebodies (Tegner & Wilson, 1993; Tegner et al., 1993)point towards interaction between the intruding

(N)

. 0

Z

6.5

5.5

4.5

3.5

7 S

* 1610

44 7 5

D•

dB <

• •

* +°

• Wehrlite suiteo 83° 80• 64• 37+38

57* .WW

D

melt compositions

20 40 60 80 100 120 140 160 180

Nd(N)

35

30

25

•>n

• 1610

o»-

D

Bf44

melt compositions

-

• °,57«

+a° t " *

••SD . • •WW

20 40 60

Zr(N)80 100

1.0

0.9

0.8

0.7

0.6

0.5

0.4

• 1610

• 44

D

•

°o *• 75 ,°\ -

D

SD

o

melt compositions

• WW

• 57

D

20 40 60 80 100 120

Nd(N)140 160 180

55

50

45

40

35

30

25

20

15

•

o44,35,

.1,610

ooo •

••75

•

•

+ •

SD+

melt compositions

-

57*WW

20 40 60 80 100 120

Nd(N)140 160 180

Fig. 12. Calculated liquid compositions of samples of the wehrlitc suite, compared with equilibrium liquid compositions of all clino-pyroxenej from the gabbro suite (excluding sample 61). For the gabbro suite, large symbols denote 'least evolved' compositions. "Least

evolved' compoiitioru of wehrlite suite memben are labelled with sample numbers.

514

BERNSTEIN ct al. EVOLUTION OF KAP EDVARD HOLM COMPLEX

wehrlite magma and the host layered gabbro, whileinterstitial liquid was still present in the gabbros. InFig. 12 the compositions of all analysed clino-pyroxenes from the gabbro suite (except sample 61,which is volumetrically unimportant) are included.The fractionated clinopyroxene analyses from thegabbro suite are taken as an approximation to thecomposition of intercumulus melt from the gabbrosuite (small symbols). The wehrlite sills show pat-terns that are strikingly parallel to those defined bythe cumulus and intercumulus melts from thegabbros, although displaced slightly towards thesample 1610 composition in the Nd/Yb vs Nd plot.The preliminary interpretation is that the wehrlitemagma mixed or interacted with the gabbro mushduring emplacement of the sills. The composition ofthe melt from which the first clinopyroxene crystal-lized appears to reflect the composition of the mixingcomponent. For example, sill samples 44, 75 andperhaps SD mixed with relatively unfractionatedmelts from the gabbro suite, whereas the sills 57 andWW mixed with strongly evolved intercumulusmelts. This observation corresponds well to the mg-number of the clinopyroxenes (Table 1), wheresamples 44 and 75 have highest m^-number, andsamples SD, 57 and WW have lower m^-number.

Support for a mixing hypothesis for the chemicalcharacteristics of the wehrlite sills comes from thecomposition of the deformed phlogopite crystals inthe sills. From Fig. 13 it is noticed that there is littlecorrelation between mg-numbex in mica and theaverage mg-number in coexisting clinopyroxene. Infact, the two sills, samples 57 and WW, that havesome of the lowest m^-numbers in clinopyroxene andhave the highest concentrations of incompatibleelements among the wehrlite sills, have phlogopitecrystals with some of the highest m^-numbersrecorded, i.e. resembling the composition of thephlogopites from the wehrlite plug sample 1610. Thetendency for phlogopites from the wehrlite sills toapproach compositions of sample 1610 phlogopites isdemonstrated in Fig. 14. To account for the range inphlogopite compositions, it is inferred that the phlo-gopite crystals, after or during emplacement of thesills, partly equilibrated with the infiltrating meltfrom the gabbro suite. Alternatively, low-tem-perature alteration may have resulted in loss of Mg,but the important point is that the wehrlite sillphlogopites are not in equilibrium with their coex-isting clinopyroxenes, and originated as high mg-number phlogopites.

Additional information on possible mixing pro-cesses comes from the closely spaced sample set oflayered gabbro and a wehrlite sill from the LLS.Sample 64 (gabbro suite) comes from a thick ( > 150

!

I

90

85

80

75

70

wehrlite suite mica and clinopyroxene

65

1

#ww

11

1

1i

63

1610

577544

45'2

• sills

o plug 1610

76 78 80 82 84 86 88 90

mg-number in clinopyroxene (average)

Fig. 13. Ranges in m^-number for phlogopite crystals from silli ofthe wehrlite suite, as a function of average m£-number of clino-pyroxene from the same sample (solid lymbols). [Note that clino-pyroxenes from the samples 45, 63 and 72 have not been analysedby ion microprobe. The wehrlite plug sample 1610 (primitive end-

member of the wehrlite suite) is represented by open symbols.

TiO230 fr .30

Wehrlite pluR 1610

Wehriite sills

\J V/ \J

MgO so FeOFig. 14. Composition of phlogopite crystals from wehrlite sills(open circles) and wehrlite plug sample 1610 (solid squares).[Note the large variations in compositions of phlogopite from thesills, forming a continuous band from the primitive compositions

of sample 1610 towards more Fe- and Ti-rich types.]

m), uniform sequence of well-layered ol gabbros.Some 20 m above sample 64 is the 2 m thick wehrlitesill, sample 44. Clinopyroxene analyses of the twosamples together with the least evolved analyses ofwehrlite plug sample 1610 are presented in Fig. 15.The clinopyroxene compositions of sample 44 followthe trend of sample 64 closely, apart from the Nd/Ybratios, which are higher for the most primitivesample 44 pyroxenes. The trace element pattern forthe ol gabbro sample 64 is typical for the gabbrosuite samples, reflecting early crystallization of maficphases. It is considered unlikely that the chemistry ofsample 64 was affected by the emplacement of thewehrlite sill, as that would require that large quan-tities of melt from the wehrlite sill migrateddownward for tens of metres. Because of the uni-

515

JOURNAL OF PETROLOGY VOLUME 37 NUMBER 3 JUNE 1996

2.8

2.4

2.0

1.6

1 1

W '

•

B

• 1610 wehrllte plug• 44 wehrlite silla 64 gabbro

cllnopyroxene

•• • m

B B

5 10 15 20 25 30 35 40 452 3 4 5 6 7 8

Nd p.p.m.

Fig. 15. All dinopyroxene analyses of the two neighbouring samples 64 (ol gabbro) and 44 (wehrlite sill). The 'least evolved' analyses ofsample 1610 are given for comparison. The 1610 composition is believed to be parental to the 44 wehrlite sill. (Note the strong paral-

lelism of the two trends.)

formity of the layered gabbro package in whichsample 44 is found, sample 64 is representative of thegabbros hosting the wehrlite sill. The composition ofthe magma parental to the wehrlite sill (sample 44),may have approached that of the primitive gabbroduring emplacement of the sill through mixing orreaction between incoming 'wehrlite' magma andthe host gabbro mush. To account for the Nd/Ybratios of the primitive sample 44 clinopyroxenes,about 1:1 mixing between sample 1610 and 64 meltsis required. The high degree of mixing is supportedby the very similar m^-numbers in clinopyroxenesfrom samples 44 and 64, and the higher modal pro-portions of plagioclase in the sill compared with thewehrlite plug sample 1610. Given that the variationsin dinopyroxene composition occur within thin-section scale, it seems logical to preclude magmaheterogeneities rising from the syn-emplacementmixing to be solely responsible for the trace-elementpatterns. Rather, the chemical patterns of thewehrlite sill clinopyroxenes appear to originate frominfiltration of melts migrating from the gabbromush. To acquire fractionation patterns practicallyidentical to those of the host gabbros, mixingbetween liquid in the wehrlite sill and melts from thecumulus gabbro pile must have occurred duringdinopyroxene crystallization, and this in turnrequires a high flux of interstitial melt through thegabbro mush. The migration of melts through thewehrlite sill was allowed because the sill, byreaction—assimilation of maybe some 50% materialfrom the host gabbro, had reached temperaturessimilar to the surroundings.

Similar processes are envisaged to be responsiblefor the chemical diversity of the wehrlite sills ingeneral, and for their deviation from the chemistryof the wehrlite plug sample 1610. Migration of

interstitial melt through a porous cumulus pile haslong been known to be an important process inaffecting the composition of cumulus rocks (e.g.Irvine, 1980; Tait et al, 1984; Tait, 1985; Bedard etal., 1988; Tegner & Wilson, 1993; Tegner & Robins,1996). The recognition of extensive melt migrationduring the crystallization of the wehrlite sills con-firms that they intruded while the KEHLS magmachamber was still hot enough for the gabbros to beabove solidus. It also demonstrates that intercumulusmelt migration must operate at least on the scale ofseveral metres.

CONCLUSIONS(1) Clinopyroxenes from the KEHLS mafic andultramafic lithologies show large variations in trace-element concentrations and ratios, as revealed by ionmicroprobe analyses. The variations can largely beexplained as a product of sequential fractional crys-tallization, with substantial crystallization of dino-pyroxene (and possibly olivine) before plagioclase.

(2) The trace-element characteristics of the ana-lysed clinopyroxenes confirm field observations thatthe KEHLS formed from at least two distinctmagma types. One, termed the gabbro suite, fromwhich the bulk of the intrusion crystallized, gave riseto anorthosites and olivine and oxide gabbros. Theother, termed the wehrlite suite, was less abundant,comprising ultramafic and melanocratic sills andplugs intruding the solidifying gabbro suite.

(3) The composition of the liquids in equilibriumwith dinopyroxene cores from the intermediatemembers of the gabbro suite can be related to theprimitive end-member of the suite through RTFprocesses at elevated pressures (10—12 kbar). Themore evolved members of the gabbro suite can be

516

BERNSTEIN et aL EVOLUTION OF KAP EDVARD HOLM COMPLEX

related to the intermediate members through low-pressure (~ 2 kbar) RTF processes.

(4) Injection into the KEHLS of melts thatevolved through RTF processes at elevated pressurebefore injection resulted in the formation of poikiliticanorthosites, owing to oversaturation in plagioclase.After precipitation of plagioclase, these melts evolvedinto crystallizing the 'normal' olivine and oxidegabbros.

(5) In terms of trace-elements, sills of the wehrlitesuite show liquid compositions that are closelysimilar to estimated intercumulus liquids from thegabbro suite. Coupled with the field evidence, thissuggests that interstitial liquids in large, slowlycooled layered intrusions migrate at a length scale ofat least several metres.

(6) To accommodate the large quantities offractionates inferred from the RTF model above,the deep-seated RTF-type magma chamber mustbe of considerable volume. If the pressure of frac-tionation refers to the transition between lowercrust and upper mantle, then the crustal thicknessat the time of emplacement (50 m.a.; Nevle et al.,1993) of the KEHLS can be estimated to be ~30km. Given that the formation of oceanic crust atthe proto-mid-Atlantic Ridge was well establishedby that time (Larsen, 1988), this implies that thetransition between continental and oceanic crust isvery abrupt around the Kangerlussuaq Fjord. Withthe evidence for the involvement of garnet in high-pressure ( > 15 kbar; equivalent to depth of 40—45km) fractionation, for basalts of the Faeroe LowerSeries (Bernstein, 1994, 1995) that were formed at60-58 m.a. (Waagstein, 1988), it may be proposedthat the thickness of the continental crust wasreduced some 10-15 km during 8-10 m.y. ofrifting.

ACKNOWLEDGEMENTSWe thank Nobu Shimizu for access to the ionmicroprobe facilities at WHOI, and for manyfruitful discussions. Dennis K. Bird, Geoff Radford,Guy DellaValle, Karen Hanghoj and Andy Tuthillare thanked for assistance and company in the field,Bob Gannicott for logistical field support, andChristian Tegner for providing sample PCT 75.Assistance with the electron microprobe facilities atthe Geological Institute by Jorn Rensbo and JohnFleng are much appreciated. Minik Rosing andHeidi Sjelin Thomsen are thanked for their supportduring the course of this work. Finally, we aregrateful for critical comments on the manuscript byChristian Tegner and Ole Stecher, and for con-structive reviews by S. A. Morse and K. G. Cox. S.B.

acknowledges Danish Science Foundation Grant 11-8610, under which all the analytical work wascarried out.

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