The Geochemistry and Petrogenesis of the Agnew Intrusion, … · 2017-09-16 · The 2100 m thick...

JOURNAL OF PETROLOGY VOLUME 40 NUMBER 3 PAGES 423–450 1999

The Geochemistry and Petrogenesis of theAgnew Intrusion, Canada: a Product ofS-undersaturated, high-Al and low-TiTholeiitic Magmas

D. C. VOGEL1∗, R. R. KEAYS1, R. S. JAMES1 AND S. J. REEVES2

1DEPARTMENT OF EARTH SCIENCES, LAURENTIAN UNIVERSITY, SUDBURY, ONT. P3E 2C6, CANADA2SCHOOL OF EARTH SCIENCES, UNIVERSITY OF MELBOURNE, PARKVILLE, VIC. 3052, AUSTRALIA

RECEIVED JULY 29, 1997; REVISED TYPESCRIPT ACCEPTED AUGUST 10, 1998

Partial melting in these mantle sources may have been induced byThe 2100 m thick Agnew Intrusion (50 km2) in central Ontario,‘thermal’ plumes.Canada, is a deformed, Palaeoproterozoic, layered leucogabbronoritic

to gabbronoritic pluton that is believed to have intruded as a sub-volcanic sill between Archaean granitic basement of the SuperiorProvince and overlying Palaeoproterozoic flood basalts. Its em-placement was part of a major magmatic event in the region, which

KEY WORDS: Agnew Intrusion; geochemistry; mantle source; parentalincluded the extensive Hearst–Matachewan dyke swarm, and wasmagma; petrogenesisfollowed by rifting and accumulation of the thick Huronian Super-

group succession in the Southern Province. Litho- and chemo-stratigraphic analyses of the Agnew Intrusion show that it is theproduct of at least three major magma pulses, giving rise sequentiallyto a Marginal, Lower, and Upper Series. The final and largest INTRODUCTIONmagma pulse produced a closed-system differentiated sequence grading The Agnew Intrusion (~50 km2 in outcrop area) is locatedfrom olivine gabbronorites at the base to ferrosyenites and alkali- ~70 km west of Sudbury adjacent to the boundaryfeldspar granites at the top. Parental magmas of the Agnew Intrusion between the Archaean Superior Province and Palaeo-were S-undersaturated, high-Al and low-Ti tholeiites, exhibiting proterozoic Southern Province in central Ontario,some minor and chalcophile element affinities with boninites. These Canada (Fig. 1). It is the best exposed member of severalmagmas have major element compositions that are very similar to leucogabbronoritic to gabbronoritic layered intrusionsthe model parent liquids proposed for the mafic portions of the that belong to the East Bull Lake suite (Bennett et al.,Stillwater and Bushveld Complexes. Other mafic dyke groups that 1991). Other layered intrusions of the suite include theare spatially and temporally associated with the Agnew Intrusion East Bull Lake, River Valley, May Township, Druryhave strong petrological and geochemical similarities with the Hearst– Township, Wisner Township, and Falconbridge Town-Matachewan dyke swarm, but are not comagmatic with the intrusion. ship Intrusions (Fig. 1; James & Harris, 1977; James &Possible mantle sources to the Agnew Intrusion include the mantle Born, 1985; Ashwal & Wooden, 1989; McCrank et al.,residue after partial melting to form the Archaean greenstone se- 1989; Peck et al., 1993a, 1995; Prevec, 1993; Vogel et al.,quences, and plagioclase-bearing mafic or ultramafic intrusions that 1998a). The best estimate of the age of the Agnew

Intrusion is a U–Pb zircon age of 2491 ± 5 Ma from ahave ponded at the crust–mantle boundary during the Archaean.

∗Corresponding author. Present address: 39 Helendale Drive,MS 2131, Toowoomba, Qld. 4352, Australia. Fax:+61-746-976-827.e-mail: [email protected] Oxford University Press 1999

JOURNAL OF PETROLOGY VOLUME 40 NUMBER 3 MARCH 1999

Lake Huron

GrenvilleProvince

Southern

ProvinceSuperiorProvince

82°84°

46°

82° 80°

2480 + 10– 5 2491 ± 5

2477 ± 9

2450 + 25- 10

- 9

Sudbury Igneous Complex and Whitewater Group

Murray and Creighton granite plutons

Huronian Supergroup / Elliot Lake Group volcanic rocks

East Bull Lake layered intrusion suite

THESSALON

SUDBURY

COBALT

Drury Twp.

AgnewEast Bull Lake

Wisner Twp.

May Twp.

Archaean Ramsey-Algoma granitoid suite

40 km

80°

Hearst-Matachewan dyke swarm

Fig. 1

ONTARIO

Gre

nville

Fro

nt

Tectonic Zone

Murray Fault Zone 46°

47°47°

2473 + 16, 2446 ± 3– 9

2441 ± 3Falconbridge Twp.

2475 ± 2River Valley

Fig. 1. Simplified regional geology of central Ontario. All ages are quoted in millions of years (Ma). Geochronology for Agnew, East Bull Lake,and Elliot Lake Group volcanic rocks are taken from Krogh et al. (1984); Falconbridge Twp. (Prevec, 1993); River Valley, Hearst–Matachewandyke swarm (Heaman, 1995); Murray granite pluton (Krogh et al., 1996).

granophyric rock taken from the stratigraphic top of the Keays et al., 1995). Therefore, characterizing these in-trusions may provide a better understanding of the genesisintrusion (Krogh et al., 1984). Available age data for theof the Sudbury Igneous Complex and its ores, as well asother East Bull Lake suite intrusions (Fig. 1) suggest thatof other dynamic sytems that exploit pre-existing crustalall were emplaced coevally, and were part of a majorrocks and their sulphides.magmatic event that included intrusive emplacement of

This paper presents whole-rock major element, tracethe laterally extensive Hearst–Matachewan dyke swarmelement, rare earth element (REE), and platinum-groupand some small granitic batholiths (Murray and Creigh-element (PGE) data for the entire range of exposed rockton plutons), as well as extrusion of the Elliot Laketypes within the Agnew Intrusion. In the absence ofGroup bimodal continental flood basalt sequence. Thissuitable chilled margin exposures, dykes proximal to themagmatic event coincided with incipient rifting in theintrusion are assessed on the basis of field relationshipsregion of the Archaean proto-continent and subsequentand geochemistry as to their possible role as parentaldevelopment of the Palaeoproterozoic Huronian rift zonemagmas to the intrusion. These data are fundamental to( James et al., 1994), which is now manifested in theunderstanding the petrogenetic evolution of the intrusionΖ12 km thick volcanic–sedimentary rock succession ofand the nature of its mantle source. The magmaticthe Huronian Supergroup in the Southern Provincerelationship between the Agnew Intrusion and the tem-(Zolnai et al., 1984).porally associated Hearst–Matachewan dyke swarm isThe Agnew Intrusion and the East Bull Lake Intrusionalso considered.are known to host significant PGE–Cu–Ni mineralization

in marginal rock units (Peck & James, 1990; Peck et al.,1993a, 1993b, 1995; Vogel et al., 1997). It has beenpostulated that some of the massive Cu–Ni sulphide ores

THE GEOLOGY OF THE AGNEWat the base of the 1850 Ma, meteorite impact-inducedINTRUSIONSudbury Igneous Complex may in fact represent proto-

ore associated with older crustal rocks such as the East Comprehensive geological summaries of the central On-Bull Lake suite (Pattison, 1979; Innes & Colvine, 1984; tario region have been given by Card (1978), Zolnai et

al. (1984), Bennett et al. (1991), Dressler et al. (1991) andPeck et al., 1993a; Golightly, 1994; James et al., 1994;

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VOGEL et al. GEOCHEMISTRY AND PETROGENESIS OF THE AGNEW INTRUSION

1

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Savage Lake

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CAMP ELEVEN FAULT

Ramsey-Algoma granitoid suite

Ramsey-Algoma granitoid suite

7a

HuronianSupergroup

46°20'

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56a

4a

4b

6a

4b

0 2km

N

–42 –40

–34–36

–38

–32

25°

(b)

(a)

Fig. 2. (a) Detailed geological map of the Agnew Intrusion. The original mapping was done at a scale of 1:5000. Numerical stratigraphicabbreviations are given in Fig. 3. (b) Location of the synclinal axis (with plunge direction) within the intrusion, and the positive Bouguer gravityanomaly. Indicated gravity values are measured in mGal.

Roscoe & Card (1992). Preliminary descriptions of the of the intrusion is disconformably and locally un-geology of the Agnew Intrusion and its immediate en- conformably overlain at its eastern margin by a thickvirons were provided by Card & Palonen (1976) and Huronian Supergroup sedimentary sequence. At variousJames & Harris (1977). More detailed accounts, based other locations within the Southern Province, East Bullon 8 months of field investigations, have been presented Lake suite layered intrusions are overlain by the thickby Vogel (1996) and Vogel et al. (1998a). Figures 2 and (up to 2600 m), bimodal volcanic succession of the Elliot3 illustrate the results of the last two studies, and a brief Lake Group (Fig. 1; Card et al., 1977; Innes, 1978; Cardsummary is given below. & Jackson, 1995). The lower mafic volcanic component

of the Elliot Lake Group has been intruded by the 2477± 9 Ma (Krogh et al., 1996) Murray granite pluton nearSudbury (Bennett et al., 1991), suggesting that thesemafic volcanic rocks are probably of similar age to theGeological setting2491–2441 Ma East Bull Lake layered intrusion suite. ItThe Agnew Intrusion and the other East Bull Lake suitehas been postulated that the present outcrop distributionlayered intrusions lie at the base of the Palaeoproterozoicof the Elliot Lake Group volcanic rocks is only a remnantHuronian Supergroup in the Southern Province, andof a much more extensive continental flood basalt plainimmediately overlie granitic rocks and orthogneisses ofthat covered a large portion of the southern Superiorthe Archaean Ramsey–Algoma granitoid suite (2710–Province in central Ontario prior to erosion and de-2665 Ma; Prevec, 1993) in the southern part of theposition of the continental rift-related Huronian Super-Superior Province (Fig. 1). Intrusive relationships with thegroup sedimentary sequence (Vogel et al., 1998a).granitic rocks are typically obscured by pseudotachylitic

The distribution of the stratigraphic units and igneousbreccias associated with the Sudbury impact eventlayering attitudes indicate that the Agnew Intrusion forms(Chubb et al., 1994) and by later mafic dyke–sill(?) em-

placement along the intrusion–footwall contact. The top a synclinal structure that plunges shallowly at 25° to the

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Pla

gioc

lase

An77-32

An79-45

An79-61

An64-51

An62-52

An70-47

An63-51

An63-41

An<3

Oliv

ine

Ort

hopy

roxe

ne

Clin

opyr

oxen

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Tita

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rtz

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LOW

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METRES

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disconformity/unconformity

Archaean graniteimpregnated by diabase dykes

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Marginal Gabbronorite Zone

Marginal Leucogabbronorite Zone

Inclusion-bearing Gabbronorite Zone

Massive Unit

Dendrite Unit (lower strat. level)

Upper Layered Unit

Olivine Gabbronorite Zone

Dendrite Unit (higher strat. level)

Mixed Unit

Pod-bearing Unit

Porphyritic Unit

Transition Unit

Leucogabbro Unit

Ferrosyenite Unit

Huronian Supergroup

primocryst mineral phaseinterstitial mineral phaseterminology is not applicable

Fe-Ti Oxide Zone

Lower Layered Unit An65-53

Upper Gabbronorite Zone

Lower Gabbronorite Zone

? ?

Apa

tite

Sul

fide

min

eral

s

100806040200

CIPW-normative Ancomposition of plagioclase

An77-69

MA

RG

INA

LS

ER

IES

Fig. 3. Stratigraphic column and inferred primary mineral distribution within the Agnew Intrusion. Stratigraphic thicknesses are approximateaverage values. The identification and textural distribution of primary mafic silicate phases in the intrusion are based on amphibole petrographyand CIPW-normative data. The stratigraphic variation in calculated whole-rock CIPW-normative plagioclase compositions is shown. Ranges ofprimary An compositional data for plagioclase (electron microprobe) are also indicated from various parts of the intrusion. The An compositionaldata for the Olivine Gabbronorite Zone are taken from the East Bull Lake Intrusion (Peck et al., 1995).

ENE (Fig. 2b). It has been argued that this structural a maximum vertical thickness of ~2100 m. Excellentstratigraphic correlations between the Agnew and neigh-geometry is a product of post-emplacement ductile de-bouring East Bull Lake Intrusions suggest that presentformation associated with the ~1850 Ma Penokean Oro-exposures of the East Bull Lake suite may representgeny, and is not a primary igneous feature (Vogel et al.,erosional remnants of one or more much larger mafic1998a). The regional structure and lithological dis-sills emplaced at the base of the coeval Elliot Lake Grouptribution is best explained by a modified ‘dome-and-continental flood basalt sequence (Vogel et al., 1998a).basin’ structural interference pattern. The predicted pre-Examples of laterally extensive and continuous sills ofdeformational geometry of the Agnew Intrusion is be-this kind (>1000 km2) are common in Antarctica (Marsh,lieved to have been a near-horizontal sill that probably1989).now extends underneath the Huronian Supergroup sed-

iments to the east. This interpretation is supported bythe location of a regional-scale, positive Bouguer gravityanomaly whose centre lies under Agnew Lake im-

Igneous stratigraphymediately east of the intrusion (Fig. 2b; Popelar, 1971;Gupta et al., 1984; Gupta, 1991a). Judging from present The Agnew Intrusion is stratigraphically subdivided intoexposures, the original horizontal dimension of the Agnew three series—Marginal, Lower and Upper Series

(Fig. 3)—each being separated by a break during whichIntrusion layered sill would have exceeded 20 km with

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there was no magmatic injection into the chamber. The the Lower Layered Unit and the underlying homo-geneous Massive Unit is an irregular, non-planar surface.Marginal Series is ~200 m thick and is predominantly

composed of vari-textured leucogabbronorites (Marginal Upper Series rocks constitute over half of the entireAgnew stratigraphic sequence and have a maximumLeucogabbronorite Zone). The zone features a broad-

scale gradation from gabbronorites and lesser melano- thickness of ~1350 m. The Upper Series has been sub-divided into three zones—Olivine Gabbronorite Zone,gabbronorites at the base (constituting ~20% of the zone),

through the main leucogabbronorite sequence (~79%) Upper Gabbronorite Zone, and an uppermost Fe–TiOxide Zone (Fig. 3). The Olivine Gabbronorite Zone atwith minor anorthosite, to local granophyric bands near

its top (<1%). Marginal Leucogabbronorite Zone rocks the base of the Upper Series is a poorly exposed, well-layered, 50 m thick interval separating the texturallycontain many small inclusions of granite, massive quartz,

and ultramafic rocks (generally <50 cm in diameter). similar Lower and Upper Layered Units (Fig. 3). Layeringin the Olivine Gabbronorite Zone is characterized byThe ultramafic inclusions are typically angular, suggesting

that they have not travelled far from their source and alternating, isomodal, ~20 cm thick layers of olivinegabbronorite, leucogabbronorite, and minor olivinemay represent fragments of unexposed ultramafic rock

cogenetically related to the Agnew Intrusion. The granitic melanogabbronorite. This zone has also been recognizedat the equivalent stratigraphic level within the neigh-and massive quartz inclusions were probably derived

from the immediate footwall to the intrusion and com- bouring East Bull Lake Intrusion (Peck et al., 1993a).Within the Upper Gabbronorite Zone, the Uppermonly have corroded and recrystallized margins, in-

dicating that Marginal Series magmas have undergone Layered Unit is overlain locally by the Mixed Unit, whichis a lithologically and texturally chaotic rock intervallocalized in situ crustal assimilation.

The Marginal Leucogabbronorite Zone is locally sep- (locally with mafic dendritic textures; see below) con-taining irregularly distributed granitic, gabbronoritic, andarated from the Archaean granite basement by the Mar-

ginal Gabbronorite Zone, which consists largely of diabase inclusions. The presence of these inclusions sug-gests that the Mixed Unit is the product of a separatemassive, medium-grained gabbronorite in the south-

western part of the intrusion, and 10–20 m wide, contact- magma pulse. The main part of the Upper GabbronoriteZone consists of the Porphyritic Unit (Fig. 3), which isparallel diabase dykes–sills(?) in other locations (Fig. 2a).

These rocks appear to be the crystallized products of characterized by a variable abundance of plagioclasephenocrysts and glomerophenocrysts. Gabbronorites arelater magmas that have intruded along the base of the

intrusion and are therefore younger than the overlying dominant over leucogabbronorites in an approximatevolume ratio of 70:30. Most of the Porphyritic UnitMarginal Leucogabbronorite Zone. Magmatic features

in the granitic footwall that are probably related to features diffuse, macrorhythmic decametre-scale layeringof gabbronorite and leucogabbronorite; centimetre-scalethe emplacement of the Agnew Intrusion include back-

intrusive felsic net-vein textures and rare felsic magmatic layering is prominent in its basal and upper parts. ThePod-bearing Unit occurs within the lower stratigraphicbreccias (Vogel et al., 1998a).

The Lower Series has a maximum thickness of 550 m parts of the Porphyritic Unit (Fig. 3) and is distinguishedby the presence of rounded pods (<1 m in diameter) ofand is dominated by gabbronorites. Lower Series magmas

intruded and disrupted the Marginal Leucogabbronorite porphyritic leucogabbronorite and granophyre set withina porphyritic gabbronorite host rock. The pods are ofZone in the northwest corner of the intrusion where the

projected position of the WNW-striking Streich Dyke local derivation and are characterized by diffuse bound-aries. They are believed to have formed as a result ofwould intersect this zone (Fig. 2a). At this location, large

outcrop-sized remnants of the Marginal Leuco- late-stage slumping and magmatic deformation withinthe crystal pile. The Transition Unit at the contactgabbronorite Zone are preserved within the Inclusion-

bearing Gabbronorite Zone (Fig. 2a), which is a com- between the Porphyritic Unit and the overlying Fe–TiOxide Zone rocks is characterized by large-scale inter-positionally and texturally heterogeneous unit containing

ubiquitous footwall, ultramafic, and leucogabbronoritic mingling and interdigitating gabbronorites and leuco-gabbronorites derived from both the overlying andinclusions. The heterogeneous nature and inclusion

abundance in the zone decrease with increasing strati- underlying stratigraphic units (Vogel, 1996; Vogel et al.,1998a). The unit is interpreted to be the product of late-graphic height, giving rise to the overlying homogeneous

Lower Gabbronorite Zone, which extends laterally away stage, large-scale slumping of the crystal pile.The uppermost rocks of the Agnew Intrusion belongfrom the intrusive site. Mesoscale, modal layering of

gabbronorite and leucogabbronorite in the Agnew In- to the Fe–Ti Oxide Zone (Fig. 3). Field data suggest thatparts of the Fe–Ti Oxide Zone and any pre-existingtrusion is first developed in the Lower Layered Unit of

this zone; layers are discontinuous laterally and often overlying strata were eroded before deposition of theoverlying Matinenda Formation conglomerates of thefade in and out in vertical section. The contact between

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Huronian Supergroup. The Leucogabbro Unit is com- to the textural terminology of McBirney & Hunter (1995)]are shown in Fig. 3.posed of massive, coarse-grained leucogabbro, often con-

taining large, altered titanomagnetite crystals ~2–3 cmin size. The overlying Ferrosyenite Unit features an

Plagioclaseupward lithological gradation from dark ferrosyenite toPlagioclase is the only mineral present as a primocrystlight alkali-feldspar granite. Contact relationships be-phase throughout the stratigraphic sequence and is es-tween the Leucogabbro and Ferrosyenite Units are nottimated to constitute ~60% of the total exposed volumeexposed. All rock types recognized in the Upper Gab-of the intrusion. Vogel (1996) showed that calculatedbronorite and Fe–Ti Oxide Zone succession, except thoseCIPW-normative An compositions provide an excellentfrom the Ferrosyenite Unit, have been observed locallyestimate of the average primary magmatic An values forin gradational vertical sequence on an outcrop-scalerocks from the Agnew Intrusion. As a good stratigraphicwithin the Upper Series, suggesting that they are probablycoverage of microprobe data for unaltered plagioclasea comagmatic differentiation sequence.could not be obtained, we have utilized whole-rockHighly vari-textured and pegmatitic gabbronorites andCIPW-normative An data for 145 surface samples takenleucogabbronorites of the Dendrite Unit occur as largelyfrom throughout the intrusion (Fig. 3). Given that plagio-conformable bands of variable thickness (10–75 m) onclase is the first mineral to crystallize, these data can beeither side of the Lower Series–Upper Series boundaryeffectively used to distinguish whole-rock differentiation(Fig. 3). They are interpreted as the products of latetrends.intrusive, volatile-bearing magma pulses (Peck et al.,

The plagioclase-rich Marginal Series rocks have a wide1993a; Vogel, 1996). The most striking feature withinnormative compositional range from An75 to An50. Inthis unit is the common presence of large, curved andmuch of the Lower Series succession, samples have abranching dendrites that are up to 30 cm long (Vogel etmore uniform normative composition of ~An70 decreasingal., 1998a). Each individual dendrite is a sheath-liketo An60 in the Lower Layered Unit. Dendrite Unit rocksaggregate of smaller amphibole crystals that have re-also have An60 compositions. The Olivine Gabbronoriteplaced original pyroxene. Individual dendrite-bearingZone at the base of the overlying Upper Series is relativelybands commonly contain ultramafic and granitic in-rich in total primocrysts (up to 90% plagioclase+ olivineclusions near their base, and lenses of granophyre along± orthopyroxene) and yields significantly higher valuestheir upper contacts.(An91–69), indicating the influx of a new magma pulse at thisstratigraphic level. Normative plagioclase compositionsdecrease gradationally to ~An60 within the Upper

Mineralogy and order of crystallization Layered Unit and do not generally exceed this com-position throughout the remaining Upper SeriesUpper greenschist to lower amphibolite facies meta-

morphism associated with the Penokean Orogeny has sequence. A marked decrease in An content to albitecompositions occurs above the Porphyritic Unit, con-variably modified the igneous mineralogy of the Agnew

Intrusion, but igneous textures are generally preserved. sistent with closed-system fractionation.Analysed plagioclase compositions (using a CamecaPlagioclase typically preserves igneous compositions

(dominantly labradorite to bytownite), except in upper SX50 electron microprobe) for the Marginal and LowerSeries rocks of the Agnew Intrusion indicate that coresparts of the intrusion where it has often been recrystallized

to fine-grained oligoclase. The primary mafic minerals, of zoned primocryst plagioclase crystals are characterizedby either An79–69 or An64–51 compositions. In examplesolivine and pyroxene, have been replaced pseudo-

morphically by calcic amphibole, and titanomagnetite where the core has an An79–69 composition, the An64–51

composition is most often present as the surroundinghas been altered to biotite, titanite and leucoxene, oftenpreserving a relict herringbone pattern. The meta- rim. Core–rim boundaries are marked by sharp drops

of 5–19% An, and are commonly irregular, indicatingmorphic mineral assemblage and phase compositions inAgnew Intrusion rocks are strongly influenced by the corrosion and/or resorption of the core before rim crys-

tallization. Such features are interpreted as the productoriginal igneous whole-rock compositions.On the basis of detailed petrography, igneous textural of plagioclase crystallization at varying pressures (Bowen,

1913; Carr, 1954; Vance, 1965; Wiebe, 1968; Smith &relationships, CIPW-normative compositions and tetra-hedron projections in the OL–CPX–PLAG–QTZ system Lofgren, 1983), indicating that some parental magmas

to the intrusion entered the Agnew chamber containing(Irvine, 1970), the general primary crystallization orderin the Agnew Intrusion is interpreted to be: small, intratelluric plagioclase crystals. The volumetric

proportion of plagioclase with An79–69 compositions inplagioclase, olivine, orthopyroxene, clinopyroxene, andtitanomagnetite. The distributions and textures of the gabbronorites of the Lower Series is ~10%, but reaches

50% in some leucogabbronorites of the Marginal Series.original mineral phases in the Agnew Intrusion [according

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Limited microprobe data for plagioclase from the Upper of the Agnew Intrusion consists of Ca-rich actinolitichornblende lamellae within a Ca-poor chlorite host,Series suggest that they contain very few plagioclase

primocrysts with compositions of[An70. However, ana- and suggests that some pyroxenes in the intrusion alsocrystallized as pigeonites that later inverted upon cooling.lytical data for plagioclase from the Olivine Gabbronorite

Zone in the neighbouring East Bull Lake Intrusion in- The amphibole minerals that replace pyroxenes increasein Fe content with increasing stratigraphic height in thedicate that higher values (An77–69; Peck et al., 1995) are

fairly common. Preserved igneous plagioclase phenocrysts upper half of the Upper Series from Fe/Mg = 0·4 inactinolite to 1·5 in ferro-tschermakite, consistent with awithin the uppermost Ferrosyenite Unit have albite

(<An3) compositions, consistent with whole-rock CIPW- whole-rock Fe-enrichment trend in the Upper Series.normative data.

Accessory mineralsOlivine Titanomagnetite, quartz, and apatite usually occur as

fine-grained accessory and interstitial igneous mineralPseudomorphs of amphibole after olivine occur as primo-cryst mineral phases in the Marginal and Lower Series phases within the Agnew Intrusion (Fig. 3). In the Leuco-

gabbro Unit of the Fe–Ti Oxide Zone, the uniform grainrocks of the Agnew Intrusion; they are absent from theDendrite Unit and much of the Upper Series (Fig. 3). size (2–3 cm) and high modal abundance (up to 40 vol.

%) of original titanomagnetite suggest that it probablyOlivine rarely accounts for more than ~10% of any givenbecame a primocryst mineral phase at this stratigraphicrock, except in the primocryst-rich Olivine Gabbronoritelevel. Fine-grained magnetite begins to crystallize inZone where it may have reached 25%. Unaltered olivineabundance at the base of the overlying Ferrosyenite Unit.crystals in parts of the East Bull Lake Intrusion, specificallyThe magnetite occurs as parallel curvilinear lamellae thatin the Rhythmically-layered and Olivine Gabbronoriteare interpreted as a post-emplacement deformationalZones (equivalent to the Lower Layered Unit and Olivinefabric. The main surface exposures of the FerrosyeniteGabbronorite Zone in the Agnew Intrusion), have un-Unit on the western shore of Agnew Lake (Fig. 2a) arezoned to weakly zoned Fo65–59 and Fo72–65 compositions,characterized by marked positive magnetic anomaliesrespectively (Peck et al., 1995; Vogel et al., 1998a). This(Gupta, 1991b).upward compositional change in olivine supports the

The intrusion is host to fine-grained, disseminated, butview that the base of the Olivine Gabbronorite Zone isirregularly distributed sulphide minerals in the form ofthe level of a new magma pulse injection that is correlativechalcopyrite with lesser amounts of pyrrhotite. In general,across both intrusions.total sulphide abundances are <1%, but rock units incontact with the footwall during emplacement (i.e. thePyroxeneMarginal Leucogabbronorite and Inclusion-bearing Gab-

Amphibole pseudomorphs after pyroxene are the most bronorite Zones) locally contain coarse-grained sulphidecommon mafic component in Agnew Intrusion rocks, minerals in greater amounts of up to 5%. Many of theseexcept in rocks from the Fe–Ti Oxide Zone at the top latter occurrences are spatially associated with quartzof the intrusion where titanomagnetite and magnetite blebs and felsic footwall inclusions (compare the Eastpredominate. In the Marginal and Lower Series, pyr- Bull Lake Intrusion; Peck et al., 1993a), perhaps indicatingoxenes generally occur interstitially and/or as oikocrysts the influence of local crustal contamination on sulphideenclosing plagioclase and olivine. Within the Upper precipitation along the margins of the intrusion (VogelSeries, pyroxene is a common primocryst mineral phase et al., 1997). In other parts of the intrusion, fine-grained,(Fig. 3). Pyroxene also crystallized as primocrysts in the disseminated sulphide minerals tend to occur within thelate-stage magma pulses that produced the Dendrite Unit interstitial component of the rocks in association withbands, and occasionally within the Olivine Gabbronorite quartz, apatite and sometimes titanomagnetite, suggestingZone. Unaltered pyroxene primocrysts in the Olivine that sulphides crystallized late in the Agnew magmas.Gabbronorite Zone of the East Bull Lake Intrusion arehypersthenes (En70Fs26Wo4–En67Fs29Wo4), whereas inter-stitial pyroxene crystallized as augite (En43Fs14Wo43–En41Fs15Wo44; Vogel et al., 1998a) that exsolved THE GEOCHEMISTRY OF THEhypersthene upon cooling. Hypersthene and augite are

AGNEW INTRUSIONalso recognized within the underlying Rhythmically-Sampling and analytical techniqueslayered Zone of the East Bull Lake Intrusion, but with

slightly more evolved mineral compositions (Vogel et al., Sixty-seven representative samples from the various zonesand units of the Agnew Intrusion, its granitic footwall,1998a), consistent again with an injection of new magma

at this stratigraphic level. An uncommon metamorphic and a variety of spatially associated mafic dykes wereselected from a total suite of ~300 surface samples. Mostmineral assemblage in the Massive Unit (Lower Series)

429


samples collected in the field exceeded a total weight of The PGE are strongly chalcophile (e.g. Peach et al.,2 kg. Weathered surfaces on all samples were removed 1994), and hence, their mobility is influenced by theby a rock saw. Jaw crushers with hardened steel plates extent of sulphide remobilization in the rocks duringwere used to reduce the sample size to small chips, and metamorphism. Although field and petrographic ob-a representative sample aliquot of ~100 g was removed servations identify microscale migration of sulphidesfor milling in an alumina ball mill. within the interstitial component of the rocks, there is

Whole-rock major and trace element compositions no evidence for large-scale sulphide remobilization andwere determined at the University of Melbourne by X- PGE transport in the Agnew Intrusion. However, pre-ray fluorescence (XRF) on fused glass discs using ~2 g cedence does exist for sulphide remobilization adjacentof sample powder. Accuracy and precision are better to major shear zones as documented at the East Bullthan 0·5% for all major elements, and better than 10% Lake Intrusion (Peck et al., 1993a).for all trace elements. Rare earth elements (REE: La–Lu)and low abundance high-field strength elements (HFSE:Nb, Ta, Th, Pb, U) were analysed using ~0·1 g of samplepowder by inductively coupled plasma-mass spectrometry Major element variations(ICP-MS) on a VG PlasmaQuad PQ2+ instrument at Selected major elements for our entire sample suite areMonash University, Melbourne. Accuracy and precision plotted against stratigraphic height in Fig. 5. A notableare better than 5% for the REE, and better than 15% change in rocks with predominantly <50 wt % SiO2 tofor the analysed HFSE. Platinum-group element (PGE: those with >50 wt % SiO2 occurs above the OlivineRu, Pd, Ir, Pt) and Au concentrations were determined Gabbronorite Zone in the Upper Series. The base of thefor several samples by radiochemical neutron activation Porphyritic Unit (at a stratigraphic height of ~1050 m)(RNAA) at the University of Melbourne using the tech- corresponds to a general change from olivine-normativenique described by Hoatson & Keays (1989). The precious to quartz-normative compositions. The lowest SiO2 con-metals were pre-concentrated from ~25 g of sample centrations are in the primocryst-rich Olivine Gab-powder into a 2 g nickel sulphide button by fire assay bronorite Zone (~45 wt %) and are consistent with its(16 h at 1050°C). The buttons were subsequently ir- high modal olivine abundance relative to the rest of theradiated for 24 h at the ‘HIFAR’ reactor in Lucas stratigraphic sequence. In the Ferrosyenite Unit at theHeights, Sydney. The accuracy and precision of the top of the intrusion (>2000 m), SiO2 concentrationsreported precious metal concentrations are better than increase rapidly, but gradationally from 50 to 73 wt %.10% for Ru, Pd, Pt and Au, and better than 20% for High SiO2 contents (51–55 wt %) in the basal MarginalIr. Quoted accuracy and precision levels for all utilized Gabbronorite Zone are confined to late-intrusive fine-analytical methods are based on geological standard and grained diabase dykes or sills whose chemical relationshipreplicate analyses. with the main stratigraphic sequence is uncertain. Sample

438-DV1078 (Table 1), which was collected from a 2 mwide dyke in the Marginal Gabbronorite Zone, has a

Evaluation of element mobility distinct whole-rock chemical composition similar to thatof siliceous high-magnesian basalts or modern boniniticWhole-rock major element, trace element, REE and PGE

data for 40 Agnew rocks are presented in Table 1. As rocks as described, for example, by Crawford et al. (1989).Compositionally similar dykes are locally observed in thethese rocks have undergone at least upper greenschist

facies metamorphism, the effects of element mobility on vicinity of ~2450 Ma layered intrusions in Finland (Vuolloet al., 1995; Vogel et al., 1998b).the geochemistry of the rocks must be considered. The

HFSE, such as Zr, are generally thought to be immobile The TiO2 concentration is controlled by the abundanceof Fe–Ti oxides. Below the Transition Unit (<1700 m),during low grades of metamorphism (Pearce & Norry,

1979; Lesher et al., 1991; Jenner, 1996), and are strongly TiO2 is low, generally <0·5 wt %. The Dendrite Unithas the highest TiO2 contents in the lower half of theconcentrated in the residual liquid during fractionation

of basaltic melts. Strong covariations between Zr and intrusion (excepting marginal dykes or sills). Significantlyhigher TiO2 concentrations in the Fe–Ti Oxide Zoneother incompatible elements, such as Ce (Fig. 4a), Yb

and Y, therefore, provide good evidence that these ele- (>1800 m) reflect the onset of early and abundant titan-omagnetite crystallization. A maximum of 2·25 wt %ments and all REE concentrations have not undergone

significant post-solidus modification, and can be con- TiO2 is reached at the contact between the LeucogabbroUnit and the overlying Ferrosyenite Unit, followed by asidered with relative confidence from an igneous per-

spective. However, the incompatible large-ion lithophile significant drop in TiO2 concentrations to 0·37 wt %.Fe2O3∗ follows a similar path, increasing to almost 23elements (LILE), such as Rb (Fig. 4b), K and Ba, are

widely scattered in plots against Zr and cannot be treated wt % at the same contact and then decreasing to ~4 wt% at the top of the intrusion (Table 1).as immobile elements.

430


Tab

le1:

Who

le-r

ock

chem

ical

data

for

Agn

ewIn

trus

ion

rock

s

Sam

ple

no

.:52

-DV

2155

-DV

2419

5-D

V31

130

3-D

V55

443

8-D

V10

7820

2-D

V32

020

5-D

V32

422

9-D

V35

329

4-D

V52

712

6-D

V19

514

5-D

V21

317

6-D

V27

319

3-D

V30

506

-DV

288

391-

DV

823

392-

DV

823

177-

DV

274

358-

DV

723

375-

DV

758

428-

DV

967

Ro

ckzo

ne/

un

it:

foo

twal

lfo

otw

all

(1)

(1)

(1)

(2)

(2)

(2)

(2)

(3)

(3)

(4a)

(4a)

(4b

)(5

)(5

)(6

a)(6

a)(6

a)(6

a)R

ock

typ

e:g

ran

ite

gra

nit

eg

abb

ro-

gab

bro

-g

abb

ro-

lcg

abb

ro-

lcg

abb

ro-

mg

abb

ro-

gab

bro

-lc

gab

bro

-g

abb

ro-

lcg

abb

ro-

gab

bro

-lc

gab

bro

-o

l.g

abb

ro-

ol.

mg

ab-

gab

bro

-g

abb

ro-

lcg

abb

ro-

mg

abb

ro-

no

rite

no

rite

no

rite

no

r.n

or.

no

r.n

ori

ten

or.

no

rite

no

r.n

ori

ten

or.

no

r.b

ron

or.

no

rite

no

rite

no

r.n

or.

Maj

or(w

t%

)S

iO2

66·0

271

·85

51·6

651

·51

54·5

546

·80

48·4

448

·41

47·0

149

·55

48·7

449

·39

48·1

850

·13

45·2

344

·36

49·3

649

·94

49·7

750

·25

TiO

20·

110·

130·

180·

610·

250·

140·

160·

130·

150·

260·

210·

460·

310·

210·

260·

200·

340·

500·

340·

24A

l 2O

318

·05

14·9

717

·20

14·5

510

·97

21·2

625

·11

17·0

317

·48

20·4

815

·58

21·4

917

·79

20·5

915

·33

9·31

16·3

819

·41

24·8

417

·45

Fe2O

3∗1·

181·

339·

9010

·41

8·44

7·78

4·47

9·55

10·2

87·

7210

·40

6·79

9·54

8·57

11·6

115

·72

10·7

99·

294·

548·

11M

nO

0·01

0·02

0·17

0·19

0·21

0·12

0·07

0·17

0·16

0·13

0·18

0·11

0·15

0·13

0·17

0·23

0·18

0·13

0·06

0·14

Mg

O0·

260·

497·

257·

8410

·73

7·98

4·70

10·7

910

·20

5·93

9·69

5·71

8·92

5·03

12·8

417

·04

8·45

5·34

3·59

8·59

CaO

2·03

2·42

7·83

10·6

110

·62

10·8

912

·71

8·30

10·0

712

·29

10·5

911

·20

10·5

29·

678·

308·

249·

8310

·41

11·8

210

·77

Na 2

O6·

345·

183·

482·

072·

652·

012·

491·

801·

982·

231·

922·

852·

083·

211·

440·

292·

112·

803·

532·

09K

2O4·

372·

120·

670·

540·

200·

480·

480·

550·

180·

280·

490·

560·

240·

720·

580·

180·

500·

820·

280·

31P

2O5

0·07

0·05

0·01

0·05

0·20

0·02

0·02

0·01

0·02

0·04

0·02

0·05

0·03

0·01

0·04

0·02

0·04

0·05

0·06

0·01

LOI

0·72

0·74

1·75

1·15

0·94

2·55

1·19

3·29

2·43

1·34

2·12

1·41

2·05

0·97

3·78

4·14

1·67

1·20

1·43

1·50

Su

m99

·44

99·4

310

0·26

99·7

499

·97

100·

1799

·95

99·8

010

0·09

100·

3910

0·21

100·

1599

·97

99·3

899

·76

99·8

999

·82

100·

0410

0·40

99·6

2Tr

ace

(ppm

)S

<10

<10

4023

813

814

619

4144

5536

551

4638

129

2591

137

8927

Sc

01

2932

3410

1121

1919

2916

2114

1723

2421

723

V12

1412

322

712

755

6681

7811

612

515

015

281

9511

214

713

496

125

Cr

34

209

197

487

9530

450

611

826

448

416

019

839

179

129

166

114

5253

8C

o1

041

4346

5228

6166

3459

3359

3886

107

6041

2343

Ni

42

148

107

108

261

106

314

367

116

305

150

318

6142

972

517

812

411

013

8C

u13

750

103

9142

2741

4152

245

8144

5760

2989

107

4430

Zn

018

8614

011

666

2680

6362

9656

8481

8210

982

7029

69B

a18

0855

415

814

012

311

713

714

762

115

139

160

8927

915

370

179

258

106

112

Rb

5447

1927

612

1221

43

97

619

167

1423

87

Sr

618

534

379

172

444

278

368

203

218

239

208

325

203

286

172

9623

225

139

821

3Y

93

413

112

43

39

410

610

75

810

107

Zr

7976

641

129

125

721

1136

1652

2015

2346

3911

Nb

6·0

4·0

1·2

3·1

1·7

1·1

1·2

1·4

1·1

1·7

1·2

2·4

1·4

2·4

1·1

1·1

1·8

2·8

2·1

1·1

Ta—

—0·

130·

260·

210·

110·

110·

130·

100·

140·

110·

180·

120·

240·

110·

090·

160·

210·

170·

10T

h16

·014

·01·

21·

30·

40·

40·

50·

30·

51·

10·

81·

52·

31·

80·

71·

40·

91·

31·

10·

5P

b16

·720

·62·

95·

45·

22·

72·

62·

52·

04·

93·

48·

9b

·d·

9·0

b·d

·2·

02·

42·

82·

31·

9U

3·0

3·0

0·5

0·4

0·1

0·2

0·2

0·2

0·2

0·3

0·3

0·5

0·4

0·7

b·d

·0·

50·

20·

50·

40·

2R

EE(p

pm)

La29

·63

20·9

33·

806·

796·

902·

923·

422·

772·

955·

103·

156·

604·

097·

303·

901·

895·

328·

196·

843·

47C

e58

·52

39·3

88·

2114

·90

16·8

26·

117·

255·

706·

4511

·22

6·93

14·2

98·

9614

·00

8·30

4·38

11·4

917

·26

14·6

37·

55P

r6·

644·

191·

071·

992·

520·

780·

890·

700·

821·

460·

911·

861·

151·

701·

000·

591·

502·

221·

831·

01N

d22

·60

13·7

04·

117·

9011

·41

2·84

3·33

2·48

3·00

5·91

3·53

7·19

4·46

6·80

4·30

2·49

5·74

8·59

7·22

3·82

Sm

4·32

2·11

0·99

1·99

2·92

0·65

0·73

0·54

0·63

1·23

0·84

1·60

0·98

1·40

0·99

0·74

1·35

1·82

1·59

0·95

Eu

2·09

0·91

0·54

0·79

0·94

0·46

0·56

0·40

0·41

0·63

0·49

0·78

0·55

0·63

0·43

0·27

0·72

0·97

0·72

0·51

Gd

5·34

2·93

1·10

2·35

2·83

0·72

0·85

0·63

0·79

1·59

0·94

1·89

1·15

1·50

1·10

0·76

1·49

2·20

1·87

1·01

Tb

0·48

0·22

0·14

0·39

0·39

0·08

0·10

0·06

0·08

0·24

0·15

0·25

0·16

0·25

0·15

0·16

0·22

0·31

0·24

0·15

Dy

2·24

0·97

0·85

2·57

1·98

0·60

0·68

0·49

0·70

1·55

1·09

1·66

1·16

1·60

1·10

0·96

1·47

1·85

1·46

1·07

Ho

0·36

0·14

0·18

0·58

0·39

0·12

0·15

0·12

0·15

0·33

0·25

0·34

0·26

0·33

0·21

0·22

0·33

0·39

0·30

0·25

Er

0·96

0·44

0·62

1·50

1·08

0·43

0·47

0·45

0·51

0·87

0·66

0·91

0·69

1·00

0·66

0·60

0·92

1·05

0·84

0·70

Tm

0·13

0·06

0·09

0·23

0·16

0·07

0·07

0·07

0·09

0·13

0·11

0·14

0·11

0·15

0·09

0·09

0·14

0·17

0·13

0·11

Yb

0·83

0·42

0·43

1·50

0·81

0·31

0·40

0·33

0·48

0·89

0·75

0·90

0·95

0·98

0·65

0·65

0·96

1·01

0·81

0·63

Lu0·

110·

070·

060·

210·

110·

050·

060·

070·

080·

140·

120·

150·

120·

150·

100·

100·

150·

190·

120·

10PG

E(p

pb)

Pd

——

23·2

016

·80

—32

·30

——

35·4

038

·20

58·3

05·

72—

—17

·20

——

11·9

0—

—P

t—

—26

·20

17·9

0—

20·1

0—

—16

·80

10·4

036

·60

6·16

——

11·5

0—

—15

·20

——

Ir—

—0·

390

0·02

3—

0·47

0—

—0·

400

1·68

01·

090

0·18

0—

—0·

500

——

0·14

0—

—R

u—

—0·

330·

12—

0·73

——

0·09

5·9

1·5

0·09

——

0·18

——

0·01

——

Au

——

2·12

1·38

—1·

07—

—0·

580·

672·

150·

73—

—0·

56—

—1·

07—

—

mg-

nu

mb

er0·

340·

460·

630·

640·

750·

700·

710·

720·

700·

640·

680·

660·

680·

580·

720·

720·

650·

570·

650·

71

431


Tab

le1:

cont

inue

d

Sam

ple

no

.:35

9-D

V72

624

9-D

V38

838

1-D

V78

138

6-D

V79

943

1-D

V10

0843

2-D

V10

2036

3-D

V74

330

6-D

V55

630

7-D

V56

032

3-D

V61

233

2-D

V63

528

3-D

V48

030

1-D

V54

931

4-D

V57

931

6-D

V58

631

3-D

V57

625

8-D

V41

021

3-D

V33

337

4-D

V75

617

-DV

243

Ro

ckzo

ne/

ult

ram

afic

un

it:

(6b

)(6

c)(6

c)(6

c)(6

c)(6

c)(6

d)

(6e)

(6e)

(6e)

(6e)

(7a)

(7a)

(7a)

(7b

)(7

b)

(7b

)(8

)(8

)in

cl.

Ro

ckty

pe:

gab

bro

-lc

gab

bro

-lc

gab

bro

-g

abb

ro-

gab

bro

-lc

gab

bro

-lc

gab

bro

-le

uco

-g

abb

rog

abb

role

uco

-le

uco

-le

uco

-le

uco

-fe

rro

-al

k.fs

p.

alk.

fsp

.g

abb

ro-

gab

bro

-am

ph

i-n

ori

ten

or.

no

.n

ori

ten

ori

ten

or.

no

r.g

abb

rog

abb

rog

abb

rog

abb

rog

abb

rosy

enit

eg

ran

ite

gra

nit

en

ori

ten

ori

teb

olit

e

Maj

or(w

t%

)S

iO2

50·9

353

·61

51·5

351

·73

50·4

850

·28

51·5

553

·27

50·2

651

·41

50·1

851

·01

50·1

251

·06

50·1

062

·84

72·8

850

·98

51·1

348

·56

TiO

20·

480·

360·

420·

440·

370·

730·

580·

661·

120·

790·

820·

680·

770·

862·

250·

970·

370·

560·

370·

25A

l 2O

317

·54

22·0

222

·04

14·9

014

·89

20·0

819

·88

19·9

912

·68

14·0

518

·85

18·5

320

·10

18·5

413

·39

12·9

012

·45

14·4

015

·05

6·76

Fe2O

3∗9·

126·

646·

7311

·18

10·5

410

·07

8·06

8·81

15·8

113

·52

11·5

811

·14

10·8

211

·65

22·6

713

·11

3·93

11·0

210

·94

10·7

6M

nO

0·16

0·09

0·11

0·20

0·20

0·14

0·12

0·12

0·23

0·21

0·19

0·16

0·23

0·25

0·17

0·06

0·02

0·19

0·17

0·21

Mg

O5·

631·

792·

867·

548·

163·

523·

121·

986·

006·

253·

633·

712·

753·

052·

490·

830·

647·

937·

5118

·81

CaO

11·7

69·

8511

·21

10·5

211

·17

9·78

10·9

29·

609·

089·

677·

999·

297·

337·

092·

340·

660·

1911

·55

9·58

10·3

6N

a 2O

2·47

3·70

3·10

2·26

1·91

2·97

3·67

3·46

2·25

2·25

3·09

3·31

4·52

3·54

2·17

2·77

3·09

2·00

2·14

0·22

K2O

0·73

0·82

0·72

0·50

0·66

0·68

0·65

0·92

1·34

0·69

2·27

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432


Fig. 4. Relative mobility of (a) Ce and (b) Rb with respect to the immobile element, Zr, in the Agnew Intrusion. Data are taken from Table 1.

Fig. 5. Variations in selected whole-rock major element compositions (wt %) and mg-number with stratigraphic height in the Agnew Intrusion.Data are taken from Vogel (1996) and Table 1.

Al2O3 concentrations are principally governed by the Leucogabbro Unit near the top of the intrusion containless Al2O3 (18–20 wt %), and this is attributed to theamount of plagioclase within individual samples. A neg-

ative correlation is observed between Fe2O3∗ (or MgO) more sodic composition of the plagioclase at this levelrelative to stratigraphically lower leucogabbronorites.and Al2O3, and reflects an increasing amount of pla-

gioclase at the expense of mafic minerals. It is not Marginal and Lower Series rocks appear to havegradually decreasing whole-rock mg-numbers from ~0·7necessarily an indicator of magmatic differentiation. Val-

ues of Al2O3 generally range between 15 and 20 wt %, to 0·6 (Fig. 5), despite the fact that it is clear from fieldrelationships that Lower Series magmas were intrusivebut show no systematic variation with stratigraphic

height. Rocks defined as leucogabbronorites, as distinct into the plagioclase-rich rocks of the Marginal Series. Amarked shift to higher mg-numbers occurs at the Lowerfrom gabbronorites, typically contain in excess of 20 wt

% Al2O3. Leucogabbros in the Transition Unit and the Series–Upper Series boundary. The mg-numbers of

433


Upper Series rocks then decreases gradationally with produced the Upper Series, rather than as products of astratigraphic height from 0·72 in the Olivine Gab- small, discrete primitive magma pulse as suggested bybronorite Zone to <0·30 in the uppermost Ferrosyenite Peck et al. (1995) for the Olivine Gabbronorite Zone inUnit. Fe2O3∗ concentrations tend to increase with stra- the East Bull Lake Intrusion.tigraphic height in the Upper Series until the top of the Incompatible trace elements include Zr, Y, Nb andLeucogabbro Unit; this is typical of a tholeiitic Fe- La (Fig. 6). Their relative abundances are primarilyenrichment trend. The increase in mg-number within governed by the proportions of primocrysts to interstitialthe Ferrosyenite Unit is due to the large amounts of components in the sample and by their relative degreefractionation of titanomagnetite and magnetite in the of differentiation. Concentration profiles through thelower parts of the unit. The Upper Series data are stratigraphic sequence are crudely asymptotic with veryconsistent with a closed-system fractionation process. The low incompatible trace element abundances in the Mar-highest mg-number in the Agnew Intrusion (0·75) is ginal and Lower Series and gradually increasing abund-in the siliceous high-magnesian basalt-like dyke in the ances in the Upper Series. In detail, there is a recognizableMarginal Gabbronorite Zone. discontinuity at the Lower Series–Upper Series boundary,

manifested by a sudden decrease in incompatible traceelement abundances from the Lower Layered Unit tothe overlying Olivine Gabbronorite Zone. This decreaseTrace element and REE variationsreflects the influx of a new magma pulse, which resulted

Selected trace elements for our entire sample suite (except in a sudden change to a less differentiated bulk rockLa) are plotted against stratigraphic height in Fig. 6.

composition characterized by a lower amount of inter-The analysed samples lack visible sulphide minerals andstitial material. The highest incompatible trace elementchromite, so that variations in Ni and Cr abundancesabundances occur within the Ferrosyenite Unit (e.g. Zr=are principally governed by the different relative amounts422 ppm, Y = 61 ppm, Nb = 23 ppm, La= 34 ppm)of mafic primocryst and interstitial material they contain,and reflect the highly evolved nature of these rocks.as well as the relative degree of fractionation of the

Selected bivariate plots for all samples in Table 1 arecrystallizing liquid.illustrated in Fig. 7. A regression line through Y vs ZrThe Marginal and Lower Series rocks are highly vari-data intercepts very close to the origin (Fig. 7a), con-able in their Ni and Cr abundances (106–367 ppm andfirming their incompatible behaviour during crys-95–506 ppm, respectively; Table 1), reflecting differencestallization of Agnew magmas. Therefore, the Zr/Y valuein the proportions of plagioclase and mafic minerals infor individual samples should be equivalent to that ofthe rocks. These modal differences may in large part betheir source material, where any variation in the ratiodue to variations in the efficiency of separating crystalsreflects heterogeneity in the source, magma mixing, and/from liquid, as well as open-system behaviour involvingor crustal contamination. The excellent linear correlationthe influx of multiple magma pulses into the chamber.between Y and Zr suggests that all Agnew rocks wereIn contrast, the Upper Series exhibits a systematic de-derived from a similar mantle source material. Exceptionscrease in Ni and Cr with increasing stratigraphic heightinclude the Archaean granite, an ultramafic inclusion(Fig. 6), which is more consistent with the effects offrom the Marginal Leucogabbronorite Zone, and theclosed-system fractionation. The Olivine Gabbronoritesiliceous high-magnesian basalt-like dyke in the MarginalZone at the base of the Upper Series contains an-Gabbronorite Zone (438-DV1078), all of which deviateomalously high Ni abundances (up to 725 ppm) thatfrom the linear array defined by the main intrusion. Thecan be attributed to the presence of abundant olivineaverage Zr/Y value of 3·7 ± 1·2 for Agnew samples isprimocrysts (up to 25%). Apart from a single melano-not sufficiently distinctive to categorize its source in termsgabbronorite sample in the Upper Layered Unit (428-of modern mantle reservoirs or their respective tectonicDV967), Cr abundances in the Upper Series are veryenvironments (e.g. Pearce & Norry, 1979; Meschede,low, i.e. Ζ179 ppm Cr in the Olivine Gabbronorite1986; Kerrich & Wyman, 1996). Figure 7b illustrates aZone grading to Ζ33 ppm Cr above the Mixed Unit.continuous variation trend between Y and mg-number,This suggests that the parental magma giving rise to theparticularly in the Upper Series with the primocryst-richUpper Series was probably more evolved than those thatOlivine Gabbronorite Zone samples plotting at high mg-produced the underlying Marginal and Lower Series.number and low Y, and Ferrosyenite Unit samples atTherefore, the primitive composition of the Olivine Gab-low mg-number and high Y, supporting a closed-systembronorite Zone, in terms of whole-rock mg-number, norm-fractionation process. As in Fig. 7a, the ultramafic in-ative An content, and Ni abundances, as well as theirclusion and sample 438-DV1078 fall off the main datahigh primocryst and mafic mineral content, may indicatetrend, indicating that these rocks are not comagmaticthat these rocks formed as cumulates sensu stricto (Wager

et al., 1960) from a much larger magma pulse that with the Agnew Intrusion.

434


Fig. 6. Variations in selected whole-rock trace element compositions (ppm) with stratigraphic height in the Agnew Intrusion. Data are takenfrom Vogel (1996) and Table 1.

Yb)n values from 2·0 to 6·5. Each stratigraphic subdivisionAverage chondrite-normalized REE patterns for eachyields an average REE pattern that is subparallel to theof the major stratigraphic subdivisions are illustrated inothers. The only exceptions are samples 303-DV554 andFig. 8a and b; the salient features of these patterns are438-DV1078 from the Marginal Gabbronorite Zone,given in Table 2. All Agnew rocks have patterns of light

REE (LREE) enrichment with a narrow range in (La/ which have slopes and patterns that are significantly

435


Fig. 7. Selected bivariate plots of (a) Y vs Zr and (b) Y vs mg-number for the Agnew Intrusion. The regression line in (a) is calculated forMarginal, Lower, and Upper Series, and Dendrite Unit samples only, but excludes dyke sample 438-DV1078 from the Marginal GabbronoriteZone. The calculation also excludes alkali-feldspar granite samples, as both Y and Zr no longer behaved incompatibly during crystallization ofthese rocks. Data are taken from Table 1.

Table 2: Salient features of average REE patterns for Agnew Intrusion rocks

N (La/Yb)n (La/Yb)n Eu/Eu∗ Eu/Eu∗ RREE RREE range

range range (× chondrite)

Marginal Series

Marginal Gabbronorite Zone 3 — 3·0–5·9 — 1·01–1·60 — 6–14

Marginal Leucogabbronorite Zone 4 5·5 4·1–6·3 2·04 1·79–2·19 5 4–5

Lower Series

Inclusion-bearing Gabbronorite Zone 2 3·3 2·8–3·8 1·54 1·38–1·70 8 6–9

Lower Gabbronorite Zone

Massive Unit 2 3·9 2·9–4·9 1·49 1·38–1·59 9 7–11

Lower Layered Unit 1 5·0 — 1·34 — 11 —

Upper Series

Olivine Gabbronorite Zone 2 3·0 2·0–4·0 1·19 1·11–1·27 6 4–7

Upper Gabbronorite Zone

Upper Layered Unit 4 4·6 3·7–5·7 1·48 1·28–1·60 10 6–13

Mixed Unit 1 4·3 — 1·33 — 11 —

Porphyritic Unit 5 4·3 3·0–5·4 1·15 1·05–1·24 14 9–20

Pod-bearing Unit 1 6·1 — 1·22 — 18 —

Transition Unit 4 4·5 3·9–5·0 1·20 0·91–1·82 21 17–25

Fe–Ti Oxide Zone

Leucogabbro Unit 3 4·8 4·7–4·9 1·28 1·01–1·51 22 20–23

Ferrosyenite Unit 2 3·8 3·5–4·0 0·79 0·71–0·87 50 44–56

Dendrite Unit 2 4·0 3·3–4·6 1·32 1·29–1·34 13 12–14

Ultramafic inclusion 1 2·1 — 0·39 — 13 —

Archaean footwall granite 2 28·7 23·9–33·4 1·28 1·13–1·43 32 25–38

N, number of samples; Eu∗ = Eun/8Smn.Gdn. Chondrite data are from Nakamura (1974).

different from all other rocks of the intrusion. Slight which is consistent with known REE partitioning in thecommon rock-forming minerals (Henderson, 1982). Thisvariations in the REE slope between samples can gen-

erally be attributed to modal differences, such that is best illustrated by rocks from the Olivine GabbronoriteZone. An olivine melanogabbronorite (392-DV823) com-samples with lower (La/Yb)n values have higher relative

mafic mineral to plagioclase contents and vice versa, prising ~70% mafic minerals has an (La/Yb)n value of 2·0,

436


Fig. 8. Average chondrite-normalized REE patterns for the Agnew Intrusion. (a) Marginal and Lower Series. (b) Upper Series. (c) FerrosyeniteUnit and various felsic rocks from within the region. Data for samples 92-cs-1 and 92-cs-4 are taken from Chai & Eckstrand (1994). (d) Dendriteand Mixed Units, and an ultramafic inclusion. Normalization values are after Nakamura (1974).

whereas an immediately overlying olivine gabbronorite positive Eu anomaly, but there is no systematic changein its magnitude with stratigraphic height, indicating(391-DV823) with a lower mafic mineral content of

~45% has a more LREE-enriched (La/Yb)n value of 4·0. that fractionation involved significant amounts of othermineral phases in addition to plagioclase, probably olivineThe Marginal and Lower Series (excluding Marginal

Gabbronorite Zone samples) exhibit a gradual increase and later orthopyroxene and titanomagnetite.The Ferrosyenite Unit at the top of the Agnew Intrusionin RREE abundances with increasing stratigraphic height

from 5× to 11× chondritic abundances and a con- is the only stratigraphic subdivision with a negative Euanomaly, and is therefore consistent with representingcomitant decrease in their positive Eu anomaly (Eu/Eu∗)

from 2·04 to 1·34 (Fig. 8a; Table 2). Although these data the final product of plagioclase-dominated closed-systemfractional crystallization within the Upper Series. This isare compatible with closed-system magmatic differ-

entiation dominated by plagioclase fractionation, they supported by the three- to four-fold enrichment in REEand most other incompatible trace elements, and severeare not consistent with field data that indicate an intrusive

relationship and temporal break between the Lower and depletion in Cr, Ni, Sr and Co relative to rocks fromthe main underlying Upper Gabbronorite Zone. TheMarginal Series. The large positive Eu anomaly in the

Marginal Leucogabbronorite Zone reflects the high strong Fe enrichment at the base of the FerrosyeniteUnit (~23 wt % Fe2O3∗ at a SiO2 content of ~50 wt %;abundance of primocrysts of calcic plagioclase.

The Upper Series is similarly characterized by a grad- Table 1) is the same as that calculated for late-stage meltsproposed for the Skaergaard and Kiglapait Intrusionsual increase in RREE with increasing stratigraphic height

from 6× to 50× chondritic abundances (Fig. 8b; ([22 wt %; Wager & Brown, 1968; Morse, 1981). Toplis& Carroll (1995) have shown that immediately followingTable 2), lending further support to the hypothesis that

the Upper Series succession is the product of closed- the onset of magnetite crystallization in a magma, theresidual melt may proceed along a trend of Fe depletionsystem fractionation. Most Upper Series rocks have a

437


and SiO2 enrichment. Such a trend accounts very wellfor the rapid, but gradual upward mineralogical andchemical change within the Ferrosyenite Unit from Fe-rich ferrosyenite to Fe-poor and SiO2-rich alkali-feldspargranite. An alternative origin for the Ferrosyenite Unitsequence by increasing contamination of Agnew magmasby felsic material appears unlikely given that all re-cognized felsic rocks in the region are either too low inRREE or have REE slopes that are too steep to accountfor the Ferrosyenite Unit REE pattern (Fig. 8c).

Figure 8d presents average REE patterns for the Dend-rite, Mixed and Lower and Upper Layered Units, tocompare their chemical characteristics and determinethe compositional relationships between them. Markedsimilarities exist between the REE characteristics of theDendrite and Mixed Units (Table 2). Combined withthe presence of various inclusions and mafic dendritictextures in both units, the data suggest that these rockunits may be products of the same volatile-rich magmapulse that intruded at a late magmatic stage. The greaterlithological variability and poorer preservation of dend-ritic textures within the Mixed Unit compared with theDendrite Unit may be related to its emplacement site. TheMixed Unit occurs more centrally within the intrusion(Fig. 2a) and was perhaps emplaced into less crystallizedand consolidated material at the time of intrusion of the

Fig. 9. Average primitive mantle-normalized multi-element diagramDendrite Unit magma. Comparisons of the REE patternsfor Agnew Intrusion rocks. (a) Marginal and Lower Series. (b) Upperwith the adjacent Lower and Upper Layered Units Series. Symbols are the same as those in Fig. 8a and b. Normalization

suggest that the Dendrite Unit rocks either (1) more values are after Taylor & McLennan (1985).closely approach liquid compositions relative to the ad-jacent units or (2) crystallized from a magma that differedin composition only by being slightly more evolved thanthose which formed the Lower and Upper Layered Units. multi-element patterns are negative HFSE anomalies,

The bell-shaped REE pattern of the ultramafic in- particularly for Nb–Ta, P, and Ti. These data are relevantclusion occurring within the Marginal Leucogabbronorite to the petrogenetic evolution of the Agnew Intrusion, andZone (Fig. 8d) is consistent with a rock formed by are considered later. Pronounced positive Sr anomalies,hornblende crystallization (Rollinson, 1993). Coupled principally in the Marginal and Lower Series, correlatewith the observation that it falls off all geochemical trends with high calcic plagioclase abundances, whereas con-defined by other Agnew rocks (Fig. 7), such features versely, the negative Sr anomalies in the upper parts ofsuggest that these inclusions are not cogenetic with the the Upper Series reflect decreasing amounts of calcicintrusion, but are xenoliths entrained in Agnew magmas plagioclase coupled with increasing proportions of sodicduring their ascent through the crust. plagioclase.

Figure 9a and b presents multi-element diagrams that Parallel trace element and REE characteristics arecharacterize and summarize the average incompatible consistent with all Agnew Intrusion rocks (excludingelement distribution for each of the major stratigraphic some parts of the Marginal Gabbronorite Zone) beingsubdivisions. The patterns are subparallel and show the cogenetic. The data do not support differential amounts ofsame general step-wise increases in concentrations with crustal contamination between stratigraphic subdivisions.stratigraphic height within the Marginal–Lower Series Proposed parental magmas of the intrusion must haveand Upper Series as the REE patterns in Fig. 8a and b. trace element and REE patterns that are identical inLILE (Ba, Rb, Th, K, Sr) abundances, which may have slope and anomaly characteristics to those of the Agnewbeen subjected to some remobilization during meta- rocks. The predominance of rocks with positive Eu an-morphism and weathering (Fig. 4b), are typically omalies and absence of volumetrically significant rock10–100× primitive mantle values, whereas the HFSE types with negative Eu anomalies in the intrusion suggests(Nb, Ta, P, Zr, Ti, Y) are generally 1–10× primitive that either: (1) large amounts of residual liquid with

negative Eu anomalies were lost from the chamber duringmantle. As a result, distinct features of all of these

438


crystallization, or (2) the parental magmas already ex- elements behaved sympathetically during magmatic crys-tallization. Pd and Ir show no obvious systematic co-hibited positive Eu anomalies and entered the chambervariation, although both elements are generally elevatedcarrying significant amounts of plagioclase crystals. Thein the Marginal and Lower Series relative to the Upperlatter alternative is supported by previous interpretationsSeries. Ni and Cu show positive and negative correlationsthat the presence of irregular and compositionally dis-with whole-rock mg-number, respectively, suggesting thatcontinuous zonation patterns within many plagioclaseNi partitioned into olivine primocrysts during magmaticprimocrysts of the Marginal and Lower Series reflectsdifferentiation, whereas Cu preferentially partitioned intocrystallization of plagioclase under different pressure re-the residual liquid. Ni abundances are generally highergimes.and Cu abundances lower in the Marginal and LowerSeries than in the Upper Series. An exception is the highNi content in the Olivine Gabbronorite Zone, whichprobably reflects the higher relative abundances of olivine

Chalcophile element (PGE, Cu, Ni) primocrysts.variations in the Agnew Intrusion Most Agnew Intrusion rocks are characterized by a stepFinely disseminated and locally coarse-grained sulphide pattern on a mantle-normalized PGE and chalcophilemineralization occurs within the Agnew Intrusion, par- element diagram and cover the range 0·01–10× mantleticularly near its margins in the Marginal Leuco- values (Fig. 11). For all samples, the Pt–Pd–Au–Cugabbronorite and Inclusion-bearing Gabbronorite Zones. portion of the diagram is elevated with respect to theSimilar sulphide occurrences in marginal rock types of Ni–Ir–Ru portion. Ni tends to be weakly enriched relativethe East Bull Lake Intrusion have been described in to Ir (Nin/Irn is typically 1–7), and Cu is generally depleteddetail by Peck et al. (1993a). Given that the PGE have relative to Pd (Cun/Pdn = 0·9 ± 0·6). Samples fromextremely high sulphide–silicate partition coefficients (e.g. the Inclusion-bearing Gabbronorite Zone have elevatedPeach et al., 1990), they are very sensitive indicators of Ir and Ru abundances relative to the rest of the intrusion,sulphide ore-forming processes. Their abundance and and are less fractionated in terms of Pd/Ir. Both Ir anddistribution in a given rock provide a measure of the S- Ru probably occur as metal alloys that are preferentiallysaturation status of the magma from which the rock incorporated within early crystallizing mineral phasescrystallized (Hamlyn & Keays, 1986; Peck & Keays, (Agiorgitis & Wolf, 1978; Keays, 1982; Crocket & Mac-1990; Vogel & Keays, 1997). This information is relevant Rae, 1986). The increased concentrations of Ir and Ruto establishing the general metallogenic potential of the in the two Inclusion-bearing Gabbronorite Zone samplesAgnew Intrusion in terms of PGE, Cu and Ni, as well may be accounted for by their relatively high modalas further characterizing its petrogenesis. abundances (~20%) of early-crystallizing plagioclase

PGE (Pd, Pt, Ir, Ru) and Au concentrations in 22 primocrysts with An79–69 core compositions. Rocks fromsulphide-poor (p1%) rocks from various zones and units the Marginal Leucogabbronorite and Olivine Gab-within the Agnew Intrusion vary widely, but are relatively bronorite Zones also have large proportions of primo-high (Table 1), namely: Pd (range 0·2–58 ppb; most data crysts and correspondingly elevated Ir concentrationslie between 10 and 40 ppb), Pt (range 0·7–62 ppb; most relative to other samples (Fig. 11). These observationsdata lie between 10 and 30 ppb), Ir (range 0·003–1·68 suggest that the highly variable PGE abundances inppb; most data lie between 0·02 and 0·50 ppb), Ru Agnew rocks and the poorly developed evolutionary PGE(range 0·01–5·9 ppb; most data lie between 0·05 and 0·50 trends through the intrusion as a whole may in part beppb), Au (range 0·17–7·76 ppb; most data lie between 0·5 explained by different proportions of primocrysts vs later-and 3·0 ppb). For example, these concentrations are crystallizing interstitial material in the samples. Multiplegenerally greater than those reported for unmineralized magma pulses and variable degrees of magmatic differ-mafic rocks from below the J-M Reef in the Stillwater entiation have undoubtedly also influenced PGE con-Complex (7 ppb Pd, 13 ppb Pt, 0·17 ppb Ir, 0·4 ppb Au; centrations.Peck & Keays, 1990), but are similar to PGE abundances The chalcophile element data presented above arewithin equivalent rock types of the adjacent East Bull entirely consistent with the view that the stratigraphicLake Intrusion (Peck et al., 1995). Whole-rock S con- sequence crystallized from magmas that were S under-centrations in most Agnew Intrusion rocks are very low saturated when first introduced into the Agnew chamber.(typically <150 ppm; Table 1), and are believed to closely These data include: (1) high PGE concentrations inapproximate the original S tenor of the magma. sulphide-poor rocks; (2) a positive correlation between

Selected bivariate plots of chalcophile elements are Ni and mg-number highlighting the low importance ofpresented in Fig. 10a–d. In general, correlations between sulphides during magmatic crystallization; (3) the in-the chalcophile elements are poor. A good positive cor- compatible behaviour of Cu during crystallization; (4)

the very low S contents of all rocks, i.e. well below therelation exists between Pd and Pt, indicating that these

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Fig. 10. Selected chalcophile element bivariate plots for the Agnew Intrusion. Data are taken from Table 1.

discriminates between rocks that formed from S-under-saturated and S-saturated magmas (Vogel & Keays,1997). Most Agnew samples plot well within the S-undersaturated field, supporting the data presentedabove. Therefore, the parental magmas of the AgnewIntrusion entered the chamber as metal-fertile magmasthat had not previously segregated any sulphides. Fieldevidence suggests that in lower parts of the intrusion(specifically the Marginal Leucogabbronorite and In-clusion-bearing Gabbronorite Zones), S saturation wasinduced locally through assimilation of siliceous countryrock, a mechanism proposed by Irvine (1975) and Nal-drett (1989) to operate in other mafic intrusions. In onelocation, melanogabbronorites that provide both fieldand geochemical evidence for in situ crustal contamination(Vogel et al., 1997), contain abundant blebs of chalcopyritewith highly enriched PGE concentrations (RPGE = 10ppm; BP Resources Canada Ltd, unpublished assay data,

Fig. 11. Average mantle-normalized multi-chalcophile element dia- 1991). However, judging from exposed rock types, crustalgram for the Agnew Intrusion. Normalization values are after Barnescontamination was not pervasive within any part of theet al. (1988).Agnew Intrusion and its magmas appear to have re-mained largely S undersaturated until the very latest

abundances expected for mantle-derived magmas that stages of their crystallization history.have been saturated with S (>800 ppm; Peach et al., The two analysed Ferrosyenite Unit samples from near1990). Figure 12 presents Pd and Cu data for various the top of the intrusion are extremely depleted in Ni, Ir,

and Pd, and yield positively sloping metal patternssulphide-poor mafic intrusive and volcanic rocks, and

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Subdivision, characterization anddistribution of mafic dykesMafic dykes in the Agnew Intrusion area can be sub-divided into four groups on the basis of their petrographicand geochemical characteristics. More than 200 separatedykes are exposed at the present erosional level. Theyintrude into both the Archaean granite footwall and theAgnew Intrusion itself, but do not extend into the over-lying Huronian Supergroup metasediments immediatelyto the east of the intrusion. Dyke density is highest tothe northwest of the intrusion in the vicinity of the StreichDyke (Fig. 2a), which is by far the longest and widest ofthe dykes in the area. All four dyke groups are typicallyoriented along a 110–120° trend parallel to the StreichDyke. Crosscutting relationships between different dykegroups have not been observed. Regional greenschist

Fig. 12. Pd vs Cu discrimination diagram highlighting the S-under- facies metamorphism associated with the ~1850 Masaturated nature of the Agnew Intrusion, except Ferrosyenite Unit

Penokean Orogeny has resulted in pervasive re-samples (313-DV576, 316-DV586) and the ultramafic inclusion. Dataare taken from Table 1. Other data include S-undersaturated ultramafic crystallization of dyke mineralogy. The main physicalrocks from the Munni Munni Intrusion, Western Australia (Hoatson characteristics of each dyke group are outlined below.& Keays, 1989), norites from the Noritic Ring Complex in the VestfoldHills, Antarctica (Seitz & Keays, 1997), average world-wide boninitesand low-Ti basalts (Hamlyn et al., 1985), S-saturated gabbroic rocksfrom the Munni Munni Intrusion (Hoatson & Keays, 1989), basaltsfrom the Newer Volcanic Province, Victoria (Vogel & Keays, 1997), Group I—Streich-type gabbronorite dykesand average world-wide mid-ocean ridge basalt (MORB) (Hamlyn et

This group consists essentially of the Streich Dyke, whichal., 1985). The dashed line has been added arbitrarily.crops out along a prominent 4 km ridge that almost linksthe coeval Agnew and East Bull Lake Intrusions. It variesin width from 50 to 300 m and has well-developed,weakly plagioclase-phyric chilled margins. A rare un-(Fig. 11) that are similar to those of ocean-floor basalts altered plagioclase phenocryst has a composition of An78.(Barnes et al., 1988). Ocean-floor basalts are believed to At its easternmost end, the Streich Dyke is characterized

be the products of S-saturated magmas from which the by a coarse-grained and locally vari-textured interiormetal-bearing sulphides were segregated in the mantle zone. Other dykes with Group I-type chemical com-source or en route to the surface (Hamlyn et al., 1985). If positions are uncommon; one occurs adjacent to thethe Upper Series succession is a product of closed-system contact between the Archaean footwall and the Agnewmagmatic differentiation, then the decrease in chalcophile Intrusion as part of the Marginal Gabbronorite Zone,element abundances in the Ferrosyenite Unit may have and another occurs as a chemically distinct componentcoincided with an S-saturation event that depleted the within a Group III composite dyke.residual melt in these elements. Pd and Cu data inFig. 12 are consistent with the Ferrosyenite Unit rockscrystallizing from S-saturated magmas. S saturation mayhave been induced by the combined effects of progressive Group II—aphyric gabbronorite dykesdifferentiation, decreasing FeO content related to titano- Dykes belonging to Group II are typically fine- tomagnetite and magnetite crystallization, and quartz en- medium-grained and massive, ranging in width fromtering onto the liquidus (Haughton et al., 1974; 50 cm to 50 m. They constitute the most abundantWendlandt, 1982). dyke variety in the Agnew area and are distinguished

geochemically from Group I dykes. Their distribution iswidespread in the Archaean granitic rocks and through-out the intrusion.PARENTAL MAGMAS OF THE AGNEW

INTRUSIONA variety of mafic dykes that are spatially associated with

Group III—plagioclase-phyric gabbronorite dykesthe Agnew Intrusion have been investigated in an effortto identify a suitable parental magma composition(s) for These dykes are common and are distinguished in the

field by differing amounts (2–50%) of small plagioclasethe intrusion.

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glomerophenocrysts set in a fine- to medium-grained continental flood basalts (Fig. 14a and b). This suggeststhat, despite their continental setting, these dykes sharegabbronoritic matrix. The glomerophenocrysts have been

recrystallized to oligoclase compositions. The dykes gen- geochemical characteristics with modern volcanic arctholeiitic basalts. Data for Group I dykes are alwayserally range in width from 2 to 20 m, and some appear

to be composite. Group III dykes are most common to displaced from the other dyke groups and plot withinthe compositional field of boninites, indicating a distinctthe northwest of the Agnew Intrusion, where some are

truncated by the footwall–intrusion contact, but they are minor and chalcophile element chemistry that may rep-resent a fundamental difference in their mantle sources.also abundant with chilled margins within the intrusion,

indicating that some of these dykes post-date the crys- However, although they have some significant chemicaltallization of the Agnew Intrusion. Their abundance is similarities with modern boninites, Group I dykes arelowest within the Upper Series. too depleted in SiO2 and MgO, and too enriched in

Al2O3 to serve as boninitic analogues.Average REE and incompatible element characteristics

for the mafic dykes, including the Hearst–Matachewandyke swarm, are shown in Fig. 15a and b, and importantGroup IV—coarse-grained gabbronorite dykesREE ratios are given in Table 3. All dykes are char-This group is characterized by coarse-grained, ‘spotty’acterized by LREE enrichment relative to chondrite,interiors consisting of 50% equant mafic crystals (ori-with Groups I and II showing steeper patterns [(La/ginally pyroxene?) set within a plagioclase matrix. TheYb)n = 3·7] similar to those of the Agnew Intrusion,dyke interiors grade evenly over ~50 cm into well-compared with Groups III and IV [(La/Yb)n= 2·4–2·6].developed chilled margins. The dykes are generally 5–These slope differences cannot be accounted for by10 m wide. Large megacrysts of saussuritized and locallydiffering degrees of fractionation, and therefore dykes ofchloritized plagioclase (~5 cm) may be present in amountsGroups I and II are probably not comagmatic withof up to 60%. The presence of such large plagioclaseGroups III and IV. Group III and IV dykes may,megacrysts has also been reported from many Hearst–however, be comagmatic with the Hearst–MatachewanMatachewan dykes (Phinney et al., 1988; Halls & Bates,dykes, at least in terms of their REE patterns. The Group1990; Nelson et al., 1990; Ashwal, 1993). Group IV dykesI Streich-type gabbronorite dykes are the only groupare geographically confined to northern areas both withinwith a positive Eu anomaly (Eu/Eu∗ = 1·34). Assumingand outside the Agnew Intrusion, and do not appear tothat normal mantle-derived liquids do not have positiveintrude rocks above the approximate stratigraphic baseEu anomalies, and using partition coefficients for Eu intoof the Upper Series.plagioclase of basaltic liquids (Arth, 1976; Fujimaki et al.,1984), it is calculated that these dykes contain up to 10%intratelluric plagioclase. The other dyke groups haveslightly negative Eu anomalies, indicating that they have

Geochemistry of the mafic dykes probably fractionated small amounts of plagioclase atdeeper crustal levels. All analysed mafic dykes in theTable 3 presents average whole-rock geochemical dataAgnew Intrusion area, as well as the Hearst–Matachewanfor Group I–IV and Hearst–Matachewan mafic dykes.dykes, have negative anomalies in Nb–Ta, P, and TiAverage data for the latter have been calculated fromrelative to LILE and REE (Fig. 15b), identical to those16 analyses taken from Condie et al. (1987) and Nelsonshown by the Agnew Intrusion. A positive Sr anomalyet al. (1990). These were chosen from a larger set of 32in Group I dykes correlates with the positive Eu anomalyanalyses on the basis of their similar REE patterns andand high Al2O3 contents, and is consistent with elevatedare considered to be the best estimate of the compositionplagioclase concentrations.of the dyke swarm. All Group I–IV and Hearst–

Average S concentrations for the four dyke groups areMatachewan dykes are sub-alkaline and are high-Fegiven in Table 3. Although S is considered relativelytholeiitic basalts (Fig. 13). Excepting Group I, whichmobile during metamorphism and weathering, the ob-are olivine-normative, most dykes are quartz-normative;servation that very low S abundances (60 ppm) in a thin,Group IV and Hearst–Matachewan dykes comprise both~1 m wide Group I dyke were preserved immediatelyvarieties. Group I dykes are also distinguished by theiradjacent to high S abundances (800 and 1100 ppm)substantially lower TiO2 (0·45 wt %) and higher Al2O3

within a Group III composite dyke, suggests that the(>17 wt %) and mg-number (0·64). The other dyke groupsreported S values are probably close to their originalhave in excess of 1·0 wt % TiO2, 13–15 wt % Al2O3 andmagmatic concentrations, as both values are typical ofa more compositionally evolved mg-number of 0·32–0·59other dykes in their respective groups. The S data are(typically ~0·45). In various tectonic discrimination dia-plotted on a Poulson–Ohmoto diagram (Fig. 16), whichgrams, the dyke compositions have the geochemical

characteristics of both destructive plate margins and discriminates, on the basis of FeO content, between

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Table 3: Average whole-rock chemical data for Group I–IV and Hearst–Matachewan dykes

Group I Group II Group III Group IV Hearst–Matachewan1

CIPW-normative: ol qtz qtz ol, qtz ol, qtzNo. of samples: 4 9 10 4 16

Major (wt %)SiO2 49·40 ± 0·63 51·25 ± 1·59 49·78 ± 1·22 49·24 ± 0·65 50·26 ± 1·00TiO2 0·45 ± 0·13 1·60 ± 0·25 1·29 ± 0·17 1·10 ± 0·23 1·33 ± 0·30Al2O3 17·13 ± 0·23 13·33 ± 0·34 14·10 ± 0·77 14·70 ± 0·69 14·40 ± 0·73Fe2O3∗ 10·10 ± 0·24 16·31 ± 1·16 15·39 ± 0·56 13·81 ± 1·29 14·28 ± 1·23MnO 0·17 ± 0·01 0·22 ± 0·01 0·22 ± 0·01 0·21 ± 0·01 0·22 ± 0·02MgO 7·69 ± 0·25 4·28 ± 0·76 5·23 ± 0·16 6·46 ± 1·40 6·04 ± 0·81CaO 11·10 ± 0·95 7·95 ± 0·89 9·85 ± 0·52 9·96 ± 0·39 9·73 ± 1·02Na2O 2·32 ± 0·38 2·54 ± 0·68 2·19 ± 0·25 2·23 ± 0·50 2·74 ± 0·37K2O 0·54 ± 0·17 1·23 ± 0·68 0·69 ± 0·30 0·52 ± 0·23 0·82 ± 0·37P2O5 0·05 ± 0·02 0·24 ± 0·11 0·15 ± 0·02 0·12 ± 0·02 0·13 ± 0·05

Trace (ppm)S 77 ± 47 1116 ± 709 908 ± 357 564 ± 428 —Sc 31 ± 2 38 ± 7 42 ± 2 40 ± 4 43 ± 4V 186 ± 17 359 ± 73 348 ± 19 300 ± 36 344 ± 42Cr 227 ± 43 28 ± 40 67 ± 30 100 ± 48 111 ± 53Co 49 ± 5 47 ± 8 44 ± 6 48 ± 5 52 ± 4Ni 166 ± 26 39 ± 16 68 ± 28 78 ± 40 66 ± 29Cu 59 ± 15 148 ± 52 194 ± 38 165 ± 37 —Zn 84 ± 15 134 ± 20 122 ± 10 107 ± 6 —Ba 190 ± 61 405 ± 253 187 ± 75 151 ± 78 206 ± 55Rb 18 ± 8 38 ± 24 20 ± 13 20 ± 11 29 ± 15Sr 225 ± 14 301 ± 104 200 ± 70 157 ± 20 172 ± 17Y 9 ± 2 31 ± 6 28 ± 5 24 ± 5 31 ± 6Zr 35 ± 6 133 ± 28 106 ± 29 79 ± 17 102 ± 23Nb 1·7 ± 0·5 7·6 ± 2·3 5·9 ± 2·4 6·0 8·8 ± 1·1Ta 0·10 ± 0·01 0·42 ± 0·05 0·43 ± 0·22 0·29 ± 0·09 0·42 ± 0·14Th 0·83 ± 0·06 4·27 ± 3·75 2·02 ± 0·97 1·64 ± 0·41 2·11 ± 0·53U 0·19 ± 0·01 0·96 ± 0·76 0·48 ± 0·21 0·42 ± 0·09 0·53 ± 0·25

REE (ppm)La 5·68 ± 0·50 16·63 ± 4·27 11·95 ± 2·90 8·73 ± 1·93 11·95 ± 2·46Ce 11·99 ± 0·78 36·35 ± 9·13 26·35 ± 5·90 19·08 ± 4·36 26·95 ± 5·19Pr 1·53 ± 0·07 4·83 ± 1·15 3·60 ± 0·72 2·54 ± 0·60 —Nd 6·25 ± 0·45 20·58 ± 4·94 15·68 ± 2·95 11·16 ± 2·67 —Sm 1·48 ± 0·09 4·81 ± 0·77 3·94 ± 0·69 2·88 ± 0·64 4·07 ± 0·83Eu 0·68 ± 0·01 1·54 ± 0·26 1·28 ± 0·13 0·93 ± 0·17 1·27 ± 0·23Gd 1·64 ± 0·14 5·51 ± 0·61 4·72 ± 0·78 3·59 ± 0·87 —Tb 0·26 ± 0·03 0·87 ± 0·05 0·80 ± 0·17 0·63 ± 0·16 0·83 ± 0·19Dy 1·73 ± 0·09 5·29 ± 0·35 5·06 ± 1·12 4·09 ± 1·00 —Ho 0·37 ± 0·06 1·14 ± 0·11 1·12 ± 0·26 0·90 ± 0·22 —Er 1·02 ± 0·08 3·05 ± 0·33 3·04 ± 0·72 2·45 ± 0·58 —Tm 0·16 ± 0·03 0·46 ± 0·06 0·48 ± 0·12 0·38 ± 0·09 —Yb 1·04 ± 0·12 2·99 ± 0·40 3·11 ± 0·76 2·45 ± 0·50 3·25 ± 0·61Lu 0·16 ± 0·01 0·46 ± 0·05 0·48 ± 0·12 0·38 ± 0·08 0·50 ± 0·10

PGE (ppb)Pd 18·93 6·51 ± 5·56 11·52 ± 12·09 21·50 —Pt 24·20 6·02 ± 4·25 10·67 ± 8·65 21·90 —Ir 0·300 0·05 ± 0·04 0·08 ± 0·05 0·210 —Ru 0·73 0·11 ± 0·09 0·25 ± 0·25 0·31 —Au 0·62 0·92 ± 0·52 1·61 ± 0·88 0·73 —

mg-number 0·64 ± 0·01 0·38 ± 0·05 0·44 ± 0·01 0·52 ± 0·08 0·49 ± 0·05(La/Yb)n 3·7 3·7 2·6 2·4 2·5Eu/Eu∗ 1·34 0·92 0·91 0·89 0·88RREE (× chondrite) 10 30 23 17 23

mg-number = Mg/(Mg + Fe) in cations assuming 15% of Fe occurs as Fe3+; total Fe expressed as Fe2O3∗; Eu∗ = Eun/8Smn.Gdn. PGE data for Group I and IV dykes are for single samples.1Data sources: Condie et al. (1987)—three samples (36, 107 and 271); Nelson et al. (1990)—13 samples (10B, 17B, 20B, 25A,30C, 33B, 34C, 39C, 40A, 44, 45D, 46B and 49A); chondrite data are from Nakamura (1974).

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II, III and IV crystallized from S-saturated magmas thatmay or may not have segregated sulphide, whereas othersprobably remained S undersaturated.

Relationship between mafic dykes andAgnew IntrusionThere are several factors that indicate that Group I dykesof Streich-type gabbronoritic composition were parentalto the Agnew Intrusion. These include:

(1) field relationships, showing that the trend of theStreich Dyke intersects the intrusion at the site where theInclusion-bearing Gabbronorite Zone magmas intrudedand disrupted the Marginal Leucogabbronorite Zone;

(2) high Al2O3, consistent with plagioclase crystallizingas the earliest and principal liquidus mineral phase withinthe intrusion, and low TiO2, which is consistent withthe absence of ilmenite and the late crystallization of

Fig. 13. Cation diagram of Jensen (1976) for classifying Group I–IV titanomagnetite;and Hearst–Matachewan dyke compositions. Average data are given (3) the presence of positive Eu anomalies, suggestingin Table 3. the presence of up to 10% intratelluric plagioclase in the

parental magma, which supports interpretations basedon plagioclase zonation patterns, as well as accounting formagmas that have S abundances in excess of their Sthe general absence of rocks with negative Eu anomalies;capacity and those that are undersaturated in S (Poulson

(4) olivine-normative compositions, capable of giving& Ohmoto, 1990). Group I dykes are clearly the most Srise to the entirely olivine-normative Marginal and Lowerundersaturated, and given their high PGE concentrationsSeries in which pseudomorphed olivine crystals show no(e.g. Pd = 19 ppb), they were metal-fertile magmas.evidence of having been resorbed;Group II, III and IV dykes have significantly higher S

(5) similar REE and multi-element patterns char-abundances and generally plot near their estimated Sacterized by LREE enrichment and distinctly negativecapacity. Their Pd abundances are highly varied (between

0·45 and 28 ppb), suggesting that some dykes of Groups HFSE anomalies;

Fig. 14. Tectonic discrimination diagrams. (a) TiO2–MnO–P2O5 diagram after Mullen (1983). The fields are MORB; IAT, island-arc tholeiite;CAB, island-arc calc-alkaline basalt; OIT, ocean-island tholeiite; OIA, ocean-island alkali basalt. The boninite field occupies the MnO-richsector of the CAB field. (b) Pd/Ir vs Ni/Cu diagram after Barnes et al. (1988). Symbols are the same as those in Fig. 13. Average data are givenin Table 3.

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Fig. 16. Poulson–Ohmoto diagram used to determine the S capacityof a magma based on its FeO content. Group I–IV dyke samplesplotting below the line crystallized from S-undersaturated magmas,whereas those plotting above the line probably crystallized from S-saturated magmas (Poulson & Ohmoto, 1990). Average data are givenFig. 15. Average trace element patterns for Group I–IV and Hearst–in Table 3.Matachewan dykes. (a) Chondrite-normalized REE patterns with nor-

malization values after Nakamura (1974). (b) Primitive mantle-normalized multi-element diagram with normalization values afterTaylor & McLennan (1985). from the system and may have erupted as part of the

thick Elliot Lake Group volcanic sequence.In the Upper Series, all rocks except those of the(6) PGE-rich and S-undersaturated compositions, in

Olivine Gabbronorite Zone have elevated REE abund-accordance with the high background PGE con-ances relative to Group I dykes. Given that the Uppercentrations and low S abundances within the AgnewSeries succession is consistent with being a product ofIntrusion.closed-system fractionation, Group I dykes are too prim-Can Group I Streich-type gabbronorite dykes be par-itive and too low in their REE abundances to accountental to all three series in the Agnew Intrusion? This canfor the large proportional volume of Upper Series rocksbe evaluated on the comparative basis of REE abundancewith enhanced REE concentrations. Therefore, Group Idata and (La/Yb)n values for the intrusion and Group Idyke compositions were not parental to the Upper Series.dykes (Tables 2 and 3). Average compositions for theIn contrast, Group II dykes are probably too evolved inLower Series and Group I dykes have very similarterms of their REE concentrations to be parental to theREE abundances and similar major and trace elementUpper Series. The data are most consistent with a par-characteristics as a whole (Tables 1 and 3). Some differ-ental magma for the Upper Series that had an inter-entiation from the Inclusion-bearing Gabbronorite Zonemediate whole-rock composition between Group I andto the Lower Layered Unit did occur, but in general,II dykes.the Lower Series appears to have crystallized without

Dykes of Groups III and IV and the Hearst–substantial modification from a single pulse of Group IMatachewan swarm have relatively flat REE char-magma.acteristics that effectively preclude them as importantAverage compositions for the Marginal Series (ex-parental magma compositions to the Agnew Intrusion.cepting those from the Marginal Gabbronorite Zone) areIt is possible that these dykes were feeders to the Elliotdepleted in REE, but have greater positive Eu anomaliesLake Group volcanic sequence at different stages duringrelative to Group I dykes. Therefore, if Group I dykesthe emplacement of the Agnew Intrusion sub-volcanicwere parental to the Marginal Series, then the Marginalsill. This would explain both the local truncation of someSeries rocks must have crystallized under conditions ofdykes at the footwall–intrusion contact and the suddenopen-system fractionation, producing an evolved, maficdecline in dyke numbers at series boundaries, particularlyresidual liquid with relatively elevated REE abundances

and a negative Eu anomaly. This residual liquid was lost the Lower–Upper Series boundary. Group III and

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Table 4: Parental magma compositions proposed for gabbroic portions of selected layered intrusions

1 2 3 4 5 6

Intrusion: Agnew Skaergaard Stillwater Bushveld Bushveld–Stillwater Boninites

Group I dykes marginal gabbro basal chilled zone chilled norite Ao–model liquid world-wide

Major (wt %)

SiO2 49·40 48·08 50·68 51·50 51·49 55·30

TiO2 0·45 1·17 0·45 0·34 0·49 0·21

Al2O3 17·13 17·22 17·64 18·70 17·87 9·47

Fe2O3∗ 10·10 10·70 11·24 10·33 9·82 9·48

MnO 0·17 0·16 0·15 0·47 — 0·18

MgO 7·69 8·62 7·67 6·84 6·37 14·98

CaO 11·10 11·38 10·47 11·00 11·77 8·09

Na2O 2·32 2·37 1·87 1·58 2·59 1·75

K2O 0·54 0·25 0·24 0·14 0·16 0·28

P2O5 0·05 0·10 0·09 0·09 — —

mg-number 0·64 0·65 0·61 0·61 0·60 0·79

Data sources: 1, see Table 3 (this study); 2, Wager & Brown (1968); 3, Hess (1960); 4, Tilley et al. (1967); 5, Irvine et al. (1983);6, Hamlyn & Keays (1986; average of 32 samples).

Hearst–Matachewan dykes are chemically almost ident- element data indicate that the parental magmas proposedical (Fig. 15; Table 3), whereas Group IV dykes may for the mafic portions of the Stillwater and Bushveldbe related comagmatically by lower relative degrees of Complexes have almost identical compositions to Groupfractional crystallization of a basaltic mineral assemblage. I dykes that formed the Agnew Intrusion.Therefore, Group III and IV dykes may represent a An average composition for modern boninites is alsosoutherly extension of the Hearst–Matachewan dyke provided in Table 4. Boninites are thought to be ana-swarm, confirming that the swarm does indeed rotate by logous in composition to the parental magmas that gave~30° from a southeast to ESE direction towards a focal rise to the ultramafic portions of the Stillwater andpoint near Sudbury, as predicted by Halls & Bates (1990). Bushveld Complexes (e.g. Hamlyn & Keays, 1986).It also suggests that the Hearst–Matachewan dyke swarm Clearly, such magmas are very different in compositionis not comagmatic with the Agnew Intrusion, but may and appear very difficult to relate by fractional crys-instead have fed parts of the Elliot Lake Group volcanic tallization to parental magmas of gabbroic portions ofsequence. layered intrusions.

Comparison with parental magmas ofTHE SOURCE OF AGNEWother layered intrusionsINTRUSION PARENTAL MAGMASTable 4 presents major element data for proposed par-

ental magmas of the gabbroic portions of some of the The ~2475–2450 Ma East Bull Lake layered intrusionsuite, Hearst–Matachewan dyke swarm and Elliot Lakebest known layered intrusions in the world. All have

high-Al basaltic compositions (>17 wt % Al2O3) and Group volcanic sequence in the southern Superior Prov-ince are believed to be remnants of a large Palaeo-similar mg-numbers. The proposed parental magmas of

the Agnew, Stillwater and Bushveld Complexes are also proterozoic continental flood basalt province. Vogel etal. (1998a) argued on the basis of the large volume andcharacterized by very low TiO2 concentrations, whereas

the much younger Skaergaard Intrusion was fed by a early appearance of mafic magma that rifting, which ledto the accumulation of the Huronian Supergroup, wasmagma containing almost three times as much TiO2.

Column 5 in Table 4 is a model parent liquid composition caused by mantle plume-induced magmatism rather thantectonically induced adiabatic melting of the sub-(Ao) for the Stillwater and Bushveld Complexes calculated

through the addition of 17% plagioclase (An70) to a chilled continental lithospheric mantle. However, Agnew In-trusion parental magmas, i.e. Group I dykes, do notBushveld rock composition (Irvine et al., 1983). The major

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exhibit typical mantle plume signatures with respect to the form of magma, which would have imparted plume-like chemical signatures to all of the magmatic rocks.major or trace element characteristics. Their low TiO2

Alternatively, Archaean–Palaeoproterozoic mantlecontents, negative HFSE anomalies, and S-under-source compositions may have been significantly differentsaturated nature are geochemical features that are morefrom those giving rise to modern magmas, such that thetypical of modern magmas derived from depletedmodern subduction-like chemical signature in Agnewlithospheric mantle sources that have been modifiedrocks (e.g. depleted HFSE relative to LILE and LREE)by LREE-enriched, subduction-related fluids, or, al-could have been a more pervasive feature in the Ar-ternatively, derived through lower-crustal contaminationchaean–Palaeoproterozoic mantle than at present (Vogelof melts from deeper mantle sources. Given the stronget al., 1998a, 1998b).geochemical similarity between several successive and

discrete magma pulses to the Agnew Intrusion, crustalcontamination of the Group I feeder dykes is consideredunlikely, unless the contamination process was re-

SUMMARY AND CONCLUSIONmarkably uniform in both space and time. Therefore,The Palaeoproterozoic Agnew Intrusion is a 2100 mmagmas associated with the Agnew Intrusion are believedthick leucogabbronoritic to gabbronoritic pluton that isto have inherited their low TiO2, HFSE and S abundancesbelieved to have been emplaced as a sub-volcanic sillfrom a previously melted mantle source enriched bybetween Archaean granitic rocks and a thick, overlyingLREE-bearing fluids. The extensive production of green-sequence of flood basalts. It has been subdivided intostones during the Archaean would have given rise to athree major and distinct stratigraphic series, each ofthick mantle residue that could have provided both thewhich was separated by a time interval in which therevolume and a suitable source composition for the earlywas no magmatic injection into the chamber.Palaeoproterozoic magmatism in the southern Superior

The lowermost Marginal Series is composed mainlyProvince. As Group I dyke compositions are probablyof ~200 m of leucogabbronorites formed predominantlytoo evolved to be primary mantle-derived melts, theirby plagioclase fractionation from a tholeiitic parentalhigh Al2O3 concentrations could have been generatedmagma composition equivalent to that of an S-under-through high-pressure magma fractionation at depth (Leesaturated, high-Al and low-Ti Group I dyke. Group I& Ripley, 1996). More Fe-rich tholeiitic magmas, in thedykes are similar in composition to the model parentform of Group II, III and IV dykes, and the Hearst–liquid proposed for the mafic portions of the Stillwater

Matachewan dyke swarm, may then represent frac- and Bushveld Complexes. An evolved residual liquidtionated partial melt products of deeper, less depleted was removed from the system during Marginal Seriesmantle (Ohta et al., 1996). crystallization, indicating open-system fractionation con-

An alternative, but perhaps less likely source for the ditions. This liquid possibly erupted as part of the ElliotAgnew Intrusion parental magmas is plagioclase-bearing Lake Group volcanic sequence.mafic or ultramafic material that has ponded and crys- Another Group I parental magma subsequently in-tallized at the crust–mantle boundary during the Ar- truded through the semi-consolidated Marginal Series atchaean. A review of chemical compositions of Archaean the southeastern end of the Streich Dyke and extendedmafic–ultramafic rocks [e.g. that by Kerrich & Wyman laterally within the chamber forming the ~550 m thick(1996)] indicates that these are generally low in TiO2 Lower Series sequence. The Lower Series crystallizedand HFSE, and S undersaturated (Lesher & Groves, producing compositions very near those of the original1986). Relative to the Abitibi greenstone belt ~300 km parental magma.to the north, the southern Superior Province in the The ~1350 m thick Upper Series was conformablyvicinity of the Agnew Intrusion lacks substantial amounts emplaced onto the Lower Series succession, crystallizingof exposed Archaean greenstone, such that much of it largely with closed-system fractionation and producing ain this area may have ponded at depth. Partial melting lithologically and geochemically graded sequence fromof this material could produce aluminous melts that primitive and cumulus olivine gabbronorites at the basecrystallize plagioclase upon intrusion at shallow depths to highly evolved ferrosyenites and alkali-feldspar granites(Ashwal & Seifert, 1979). at the top. The parental magma of the Upper Series

Both source models presented above still require the probably had a composition intermediate between Groupinfluence of a plume if our interpretations regarding the I and II dykes. The Agnew sequence was later locallyevolution of the rift zone, and the volume and relative intruded by small-volume, volatile-rich magma pulsestiming of magmatism are correct. Perhaps, the Archaean– that gave rise to conformable Dendrite Unit bands.Palaeoproterozoic era was more typically characterized Group III and IV dykes, which are spatially associatedby ‘thermal’ plumes that induced partial melting in upperwith the Agnew Intrusion, are unrelated to the intrusion

sequence, but show strong petrographic and geochemicalmantle rocks, but did not themselves contribute much in

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In: Prichard, H. M., Potts, P. J., Bowles, J. F. W. & Cribb,similarities with the extensive Hearst–Matachewan dykeS. J. (eds) Geo-Platinum ’87 Symposium Volume. Amsterdam: Elsevier,swarm to the north. This suggests that magmas givingpp. 113–143.rise to the Hearst–Matachewan dyke swarm were not

Bennett, G. B., Dressler, B. O. & Robertson, J. A. (1991). The Huroniancomagmatic with the Agnew Intrusion. Supergroup and associated intrusive rocks. In: Thurston, P. C.,Possible mantle sources to the Agnew Intrusion include: Williams, H. R., Sutcliffe, R. H. & Stott, G. M. (eds) Geology of

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Geological Survey, Report 166, 238 pp.boundary during the Archaean. To satisfy magma vol-Card, K. D. & Jackson, S. L. (1995). Tectonics and metallogeny of the

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Card, K. D. & Palonen, P. A. (1976). Geology of the Dunlop–Shakespeare Area, District of Sudbury. Ontario Division of Mines,

Geoscience Report 139, 52 pp.Card, K. D., Innes, D. G. & Debicki, R. L. (1977). Stratigraphy,ACKNOWLEDGEMENTS

sedimentology, and petrology of the Huronian Supergroup in theThis study formed part of a Ph.D. dissertation undertakenSudbury–Espanola Area. Ontario Division of Mines, Geoscience Study 16,

by D.C.V. at the University of Melbourne, Australia, 99 pp.while in receipt of an Australian Post-graduate Research Carr, J. M. (1954). Zoned plagioclases in layered gabbros of theAward. We are indebted to David Peck, who first in- Skaergaard intrusion, East Greenland. Mineralogical Magazine 30,

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Chubb, P. T., Vogel, D. C., Peck, D. C., James, R. S. & Keays, R. R.comments by P. Golightly and S. Prevec on earlier (1994). Occurrences of pseudotachylyte at the East Bull Lake andversions of this manuscript, and those of the journal Shakespeare–Dunlop Intrusions, Ontario, Canada. Canadian Journalreviewers, D. D. Lambert, A. R. McBirney and D. C. of Earth Sciences 31, 1744–1748.

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