Water-deficient Calc-alkaline Plutonic Rocks of Northeastern Superior Province, Canada

34 Pages·2002·1.36 MB·English

JOURNALOFPETROLOGY VOLUME43 NUMBER9 PAGES1617–1650 2002
Water-deficient Calc-alkaline Plutonic Rocks
of Northeastern Superior Province, Canada:
Significance of Charnockitic Magmatism
JOHN A. PERCIVAL1∗ AND JAMES K. MORTENSEN2
1GEOLOGICALSURVEYOFCANADA,601BOOTHSTREET,OTTAWA,ONTARIO,K1A0E8,CANADA
2DEPARTMENTOFEARTHANDOCEANSCIENCES,UNIVERSITYOFBRITISHCOLUMBIA,6339STORESROAD,
VANCOUVER,B.C.,V6T1Z4,CANADA
RECEIVED JULY 24, 2001; REVISED TYPESCRIPT ACCEPTED MARCH 1, 2002
Calc-alkalinebatholithsoftheArchaeanMintoblock,northeastern INTRODUCTION
SuperiorProvince,Canada,havepyroxene-andhornblende-bearing
SinceHolland’s(1900)initialdescriptionofsouthIndian
mineral assemblages inferred to have crystallized from hot, water-
charnockites as orthopyroxene-bearing rocks of granitic
undersaturated magmas at 2·729–2·724 Ga. A regional am-
composition, the term charnockite has been applied to
phibolite- to granulite-facies tectonothermal event at 2·70 Ga
rocks of widely divergent origin: granitic rocks meta-
resultedinmildtonegligiblemetamorphiceffectsonthedominantly
morphosed to the granulite facies (metamorphic char-
granodioritic units. Geochemical, textural and thermobarometric
nockites); and rocks whose pyroxene crystallized
studies define the crystallization history in compositions ranging
directly from a magma (igneous charnockites). Unmeta-
fromcumulatepyroxenitethroughquartzdiorite,granodiorite,granite,
morphosed,discreteplutonssuchastheBarringtonTops
and syn-magmatic gabbroic dykes. Early magmatic assemblages
batholith (Eggins & Hensen, 1987), Kleivan granite (Pe-
include orthopyroxene, clinopyroxene, plagioclase, biotite, Fe–Ti
tersen,1980),Mawsoncharnockite(Young&Ellis,1991;
oxidesandternaryfeldspar,indicatingcrystallizationfrommagmas
Young et al., 1997; Zhao et al., 1997) and Ballachulish
containing <2 wt % HO at 1100–900°C. Water enrichment
2 complex (Weiss & Troll, 1989) contain pyroxene of
in the residual melt induced hornblende crystallization at 5 ± undisputedigneousorigin.However,inhigh-gradeterra-
1kbar, 800–600°C. Characterized by a continuum of large ion nesthatcontainbothgranulite-faciesmetamorphicrocks
lithophile element (LILE)-enriched, high field strength element andpyroxene-bearinggranites,theoriginofpyroxenein
(HFSE)-depleted compositions, the I-type suite resembles modern granites may be less evident (Newton, 1992; Percival,
continental arc batholiths in composition and size but not primary 1994). By virtue of their large abundance, these units
mineralogy. Magmatic arcs produced between 2·75 and 1·85 Ga can be inferred to have played a significant role in the
commonly have charnockitic components, possibly because slab- evolution of cratonic crust.
derivedfluidsinteractedwithmantlewedgesatambienttemperatures Several complementary approaches can be used to
higher by >100°C than at present, producing large volumes of distinguishigneousandmetamorphicoriginsofpyroxene-
water-deficient magma. bearing granitoids. Field observations such as dykes of
pyroxene-bearinggranitecuttingamphibolite-faciesrocks
(Frost & Frost, 1987; Bohlender et al., 1992) argue for
hot, dry magmas. Similarly, textures that indicate the
relative crystallization order of pyroxenes and hydrous
ferromagnesian phases serve to distinguish prograde
KEY WORDS: granitoid rocks; igneous pyroxenes; water-undersaturated metamorphic from igneous histories. Thermobarometry
magma;charnockite can provide clues to origin, for example where relict
∗Correspondingauthor.E-mail:[email protected] OxfordUniversityPress2002
JOURNALOFPETROLOGY VOLUME43 NUMBER9 SEPTEMBER2002
phases preserve temperatures >1000°C and must be datedat2775Ma(Skulskietal.,1996),andthecomposite
consideredinthelightofpossibleigneousprocesses(e.g. 2·84–2·77 Ga basement overlain unconformably by
Bohlen&Essene,1978;Rollinson,1982).Geochronology <2748Ma sedimentary rocks including conglomerate,
may establish different generations of mineral growth ironformationandgreywacke(Percival&Skulski,2000).
related to igneous and metamorphic events. Themetasedimentaryunitsareprobablycorrelativewith
Pyroxene-bearing granitoid rocks and their hydrous high-grade schists and paragneisses of the Lake Minto
equivalents constitute a large part of the 500km × domain to the west (Fig. 1). Calc-alkaline plutonic rocks
500km Minto block of the northeastern Superior Prov- of 2730–2720Ma age are widespread throughout the
inceofCanada(Fig.1).Pyroxenewasinitiallyconsidered Utsalik and Lake Minto domains (Percival et al., 2001)
to be of metamorphic origin based on reconnaissance andvolcanicrocksofsimilarageoccurintheVizienbelt
investigations (Stevenson, 1968). Herd (1978) supported (Percivaletal.,1994),representingvoluminouscontinental
this conclusion, regarding pyroxenes to have formed arcmagmatism(Sternetal.,1994).In>2725Magrano-
during a granulite-facies M metamorphism and horn- diorites of the Utsalik domain, (cid:1) values of +1·1 to
1 Nd
blende and biotite during a retrogressive M event. In −0·5 indicate significant involvement of older crust,
2
contrast, recent studies have concluded that many of whereas in the Lake Minto domain to the west, rocks
theserocksareessentiallyunmetamorphosed,containing of similar age and composition are relatively juvenile
earlyigneouspyroxeneandlateigneousamphibole(Per- (2725(cid:1) = +0·1 to +1·3). The youngest supracrustal
Nd
cival et al., 1990, 1992, 2001; Shore, 1991; Be´gin & rocksareconglomerateandgreywackeoftheVizienbelt,
Pattison, 1994; Stern et al., 1994). In this paper we with clasts <2718Ma (Percival & Card, 1994; Lin et al.,
document the age, geochemical and mineral chemical 1995; Skulski & Percival, 1996). Calc-alkaline plutonic
characteristicsofpyroxene-bearinggranitoidsuitesofthe rocksoftheTikkerutukdomain(2712–2702Ma;Percival
Mintoblockanddiscussimplicationsformagmagenesis. etal.,2001),representrenewedarcmagmatism.Eastward
thrusting of this active arc over the Lake Minto domain
resulted in burial, deformation, and metamorphism to
the amphibolite and granulite facies (3·5–10kbar,
REGIONAL GEOLOGICAL SETTING 575–900°C, 2702Ma; Be´gin & Pattison, 1994; Percival
The Superior Province consists of two distinct regions &Skulski,2000),aswellasproductionandemplacement
(Fig. 1): a southern block made up of alternating, east- of crustally derived plutons including diatexites
trending, relatively low-grade greenstone and meta- (2696–2693Ma). Still younger magmatism included
sedimentary subprovinces; and a northeastern (Minto) 2688Ma orthopyroxene granite (Stern et al., 1994; Per-
block, consisting essentially of granitoid and high-grade cival et al., 2001), 2675Ma granite (Percival & Skulski,
metamorphic rocks with northerly structural and aero- 2000),and2660Mapegmatites(Percival&Card,1994).
magnetic trends (Card, 1990;Percival et al., 1992, 1996, Late-stage growth of monazite in supracrustal belts
2001; Labbe´ et al., 1998; Pilkington & Percival, 1999). (2688–2628Ma; Percival & Skulski, 2000) and zircon
Inthesouth,thedistributionofbeltshasbeenattributed overgrowths(Percivaletal.,2001)mayreflectcirculation
tosuccessivelateralaccretionofjuvenileoceanicterranes, of metamorphic, magmatic and hydrothermal fluids.
microcontinents and collisional sedimentary prisms in
the interval 2·72–2·70 Ga (Card, 1990; Corfu & Davis,
1992; Williams et al., 1992). (All quoted ages are U–Pb
zircon dates unless otherwise indicated.)
ANALYTICAL TECHNIQUES
RecentworkintheMintoblockhasrecognizedaseries
oforogeniceventsbetween2·81and2·70Ga,providing All data were acquired in laboratories of the Geological
linkages to areas to the south and west (Percival & Survey of Canada (Ottawa). Geochemical data were
Skulski, 2000). Scattered remnants of ancient (2·9–3·0 obtainedfromwhole-rockpowders.Majorelementswere
Ga; Percival et al., 2001) crust occur in the east, in analysed on fused discs by X-ray fluorescence (XRF),
the Goudalie and Douglas Harbour domains, possibly and trace elements, including rare earth elements, by
representing an orogenic foreland during subsequent inductivelycoupledplasmamassspectrometry(ICP-MS).
tectonicevents.Volcanicandassociatedrocks(2·84–2·83 Errors are estimated at ±5% for major elements and
Ga) in the Qalluviartuuq domain have juvenile Nd iso- ±10% for trace elements.
topic signatures (Skulski et al., 1996), suggesting ocean- Zirconconcentrateswerepreparedfrom20kgsamples
floor and arc settings. Early (>2·81 Ga) shear zones using conventional crushing, grinding, Wilfley table,
boundingdistincttectonostratigraphicpackagesmayrep- heavyliquidsandFrantzmagneticseparatortechniques.
resent intraoceanic accretionary structures (Percival & The techniques for zircon grain selection, abrasion, dis-
Skulski, 2000). The early collage was cut by a suite of solution,geochemicalpreparationandmassspectrometry
calc-alkalineplutonsassociatedwithminorvolcanicrocks, have been described by Parrish et al. (1987). All zircon
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PERCIVALANDMORTENSEN WATER-DEFICIENTCALC-ALKALINEMAGMAS
Fig.1. GeneralizedgeologicalmapshowingdistributionofmajorrockunitsanddomainsinthecentralMintoblock.Zircondates(Percivalet
al.,1992;Percival&Card,1994;Skulski&Percival,1996)arefromigneousgrainswitherrorsgenerally<±3Ma.Insetmap(lowerright)shows
SuperiorProvincewithsubprovinceboundariesandlocationofstudyareaintheNE.
fractions were air abraded (Krogh, 1982) before dis- MINERALOGY AND GEOCHEMISTRY
solutiontominimizetheeffectsofpost-crystallizationPb
OF PYROXENE-BEARING PLUTONS
loss. Procedural blanks were 10–22 pg for Pb and from
1 to 3 pg for U. Four suites of coarse-grained, pyroxene-bearing grano-
Errors assigned to individual analyses were calculated dioritic rocks have been identified in the Minto block,
usingthenumericalerrorpropagationmethodofRoddick through reconnaissance and detailed petrological and
(1987). Decay constants used are those recommended geochronological studies (Leclair et al., 2001; Percival et
by Steiger & Ja¨ger (1975), and compositions for initial al., 2001). Many bodies vary internally from pyroxene-
common Pb were taken from the model of Cumming & dominated to hornblende-, biotite-dominated mafic
Richards (1975). Regressions of discordia arrays were mineral assemblages and therefore compositional
carried out using the model of Davis (1982). All errors terms with mineral modifiers are used (e.g. ortho-
are given at the 2(cid:2) level. pyroxene–clinopyroxene–biotite granodiorite), rather
Mineral chemical analyses were obtained on a Came- than nomenclature specific to pyroxene-bearing rocks
bax electron microprobe equipped with wavelength-dis- (e.g. charnockite, mangerite, opdalite, etc.). The term
persive spectrometers. Accelerating voltage was 15kV, ‘charnockitic’ is used in this paper for general reference
with specimen current varying from 10 to 30 nA, de- to igneous pyroxene-bearing granitic rocks.
pending on mineral type. Counting times varied from Asuiteof>2·78Gacalc-alkalineplutonsinnorthern
10 to 30 s per element, for total count times of >100 s Lake Minto domain represents the oldest pyroxene-
per spot. Standards include a variety of minerals, oxides bearing granodiorites recognized to date (Skulski et al.,
and metals, and data reduction was performed using 1996). A younger group of calc-alkaline, pyroxene- and
routines provided by Pouchou & Pichoir (1984). Ana- hornblende-bearinggranodioritesoftheLakeMintoand
lytical reproducibility is of the order of ±5% for most Utsalikdomainsformsthemainfocusofthisstudy.Based
elements.Structuralformulaewerecalculatedusingpro- on common ages of 2725 ± 5Ma, these rocks were
grams developed by G. J. Pringle. grouped as the ‘Leaf River suite’ (Stern et al., 1994).
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JOURNALOFPETROLOGY VOLUME43 NUMBER9 SEPTEMBER2002
ThelinearTikkerutukdomainofpyroxene-bearingcalc- and ilmenite form the principal exsolved phases (Fig.
alkaline granodiorite and granite has ages in the range 3f). With increasing development of hydrous matrix
2710–2693Ma (Percival et al., 2001). Orthopyroxene- phases, patches of hornblende and biotite develop as
bearingperaluminousgranodiorite(diatexite)intheLake overgrowths and along 100 planes in pyroxenes. The
Minto domain (2696Ma) may have originated through granodioritesgradethroughzoneswithpyroxenecoresin
crustal melting resulting from collisional tectonism at hornblende(Fig.4a–d)tocommonhornblende-,perthite-
2700Ma (Percival & Skulski 2000). bearingassemblagesingranodioritesandgranites.Mag-
netite is generally abundant, giving these rocks high
magneticsusceptibilityandinturnproducinganintense
regional aeromagnetic anomaly (Pilkington & Percival,
Utsalik domain plutons
1999, 2001).
The Utsalik domain is characterized by north-striking NewchemicalanalysesofplutonicrocksoftheUtsalik
sheetsofgranodioriteandmonzograniteonthe1–10km domain (Fig. 5) are reported in Table 1 and presented
scale (Percival & Card 1994). Pods and enclaves include withcomplementarydatafromSternetal.(1994)inFigs
pyroxenite, gabbro and diorite in bodies of 10cm to 6–10. Sample locations and mineral assemblages are
30m scale. Pyroxenites consist of assemblages of ortho- listed in the Appendix. The suite can be divided into
pyroxene, clinopyroxene and biotite, with accessory eightsub-unitsbasedonmodeofoccurrenceandmineral
olivine, spinel, Fe–Ti oxides, hornblende and plagio- assemblages. Mafic rocks include gabbroic and dioritic
clase. Gabbro and diorite have common clino- enclaves, as well as two chemically distinct types of syn-
pyroxene–biotite–plagioclase ± orthopyroxene assem- plutonic dykes. Five suites of main-phase plutonic rock
blages. are distinguished on the basis of their mafic mineral
Granodiorite is medium to coarse grained, homo- assemblage: (1) orthopyroxene–clinopyroxene–biotite ±
geneous and massive to weakly foliated, with several hornblende diorite to granodiorite; (2) clinopyroxene–
mineral facies (Fig. 2a–c). Mafic mineral assemblages of biotite diorite to granodiorite; (3) clinopyroxene–
orthopyroxene, clinopyroxene and biotite (Fig. 3a), all hornblende–biotite diorite to granodiorite; (4)
of apparent igneous origin, grade through facies with hornblende–biotite granodiorite; (5) biotite granodiorite
clinopyroxene cores in hornblende (Fig. 2b), to massive, to granite. Together, the Utsalik units represent silica
homogeneous rocks containing hornblende and biotite contentsinthe45–73wt%SiO rangeandformcoherent
2
(Fig. 2c). Hornblende-bearing enclaves commonly have majorandtraceelementtrendsonHarkerdiagrams(Fig.
centimetre-scale orthopyroxene-rich margins where in 6). They are dominantly calc-alkalic, with a few calcic
contact with pyroxene-bearing granodiorite (Fig. 2d). and alkali–calcic compositions using the classification of
Pyroxene-rich selvages are also observed where dykes of Frost et al. (2001).
pyroxene-bearinggranodioritetransectmaficrocks(Fig. Gabbroic enclavesare characterizedby variablecom-
2e; Ross, 1991). Sheets and plutons of granite with patible-element (Mg, Cr, Ni, V) and alumina contents
similarmineralassemblagestothoseingranodioritehave within a narrow (49–52%) silica range (Fig. 6). Some
common accessory allanite. Gabbro and diorite occur rockshavelowAlO andhighCaOcontentssuggesting
2 3
as 1–5m sheets or dykes in granodiorite and granite. the presence of cumulate clinopyroxene. A second suite
The dykes generally consist of assemblages of of enclaves, of gabbroic to dioritic composition (49·1–
clinopyroxene–biotite–plagioclase, are straight walled, 58·6 wt % SiO), forms collinear and continuous trends
2
massive to weakly foliated and boudinaged (Percival et with more evolved rocks on variation diagrams (Fig.
al., 1992). Some dykes have cuspate margins (Fig. 2f) 6). The enclaves have orthopyroxene–clinopyroxene–
and are back-veined by granodiorite, suggesting syn- biotite–plagioclase assemblages in common with grano-
magmatic emplacement (Shore, 1991; see Blundy & diorites and may represent early crystallized units ap-
Sparks, 1992). proaching parental compositions.
In quartz diorite, granodiorite and quartz mon- Mafic dykes also plot on compositional trends defined
zodiorite, pyroxenes occur as randomly oriented sub- by the more evolved rocks. Some group 1 dykes display
hedral crystals up to 5mm, along with plagioclase widely variable compatible-element contents, suggesting
(An ), quartz, alkali feldspar, ilmenite, magnetite and fractionation before emplacement. The second group of
11–48
accessoryapatite,zirconandraremonazite;biotiteoccurs dykes has relatively low MgO contents (<5 wt %) and
as fox-red grains (Fig. 3a and b). Clinopyroxene rarely high KO, PO, TiO, Zr and Ba (Table 1; Fig. 6).
2 2 5 2
has 001 lamellae of possible pigeonite (Fig. 3c) (see Three of five dykes have slight negative Eu anomalies.
Ollilaetal.,1988).Orthopyroxenerarelyexhibitsinternal On a primitive-mantle-normalized extended-element
exsolution whereas clinopyroxene commonly has fine profile (Fig. 7), the mafic rocks have patterns char-
(5–20(cid:3)m) 100 exsolution lamellae in the cores of some acterized by enrichment in light rare earth elements
grains,withunexsolvedrims(Fig.3d–f).Orthopyroxene (LREE) (La/Yb = 1·2–43) and large ion lithophile
n
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PERCIVALANDMORTENSEN WATER-DEFICIENTCALC-ALKALINEMAGMAS
Fig.2. Fieldphotographsofpyroxene-bearingandassociatedrocks:(a)orthopyroxene–clinopyroxene–biotitegranodioriteshowingcoarsegrain
size and homogeneous, massive character; (b) transitional granodiorite: pale patches are rich in perthite, with dark hornblende; matrix is
orthopyroxene–clinopyroxene–plagioclase–quartz;(c)hornblende–biotitegranitewithcoresofclinopyroxene(pale)inhornblende;(d)enclaves
of hornblende-bearing gabbro in coarse orthopyroxene–clinopyroxene granodiorite have orthopyroxene-rich selvages; (e) dyke of pyroxene-
bearinggranitecuttinghornblende-bearingmaficgneiss(noteorthopyroxene-richreactionselvage);(f)syn-plutonicclinopyroxene–biotitediorite
dykeinorthopyroxene–clinopyroxene–biotitegranodiorite(noteboudinagedgeometry).
elements (LILE), as well as negative Nb, Zr and Ti moreevolvedcompositions,suggestthatthetraceelement
anomalies. The compositions correspond well to those patterns may reflect a mantle wedge signature.
of calc-alkaline basalt and andesite formed in modern TheUtsalikplutonicdomainisdominatedbyrocksof
arc environments (see Rollinson, 1993). The high LILE granodioriticandgraniticcomposition.Theseformlinear
contentsandelementdepletionsintheserelativelyprim- trends on Harker plots of compatible elements (MgO,
itive members of the suite, which also characterize the FeO, V,Ni, Cr, etc.), thatdecrease systematically with
2 3
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JOURNALOFPETROLOGY VOLUME43 NUMBER9 SEPTEMBER2002
Fig.3. Photomicrographsofpyroxene-bearingplutonicrocks:(a)medium-graineddioriteshowingrandomorientationoforthopyroxene(O),
clinopyroxene (C), magnetite (M) and plagioclase (P); minor biotite (B) is not associated with pyroxenes (sample U-12); (b) medium-grained
quartzdioriteshowingrandomlyorientedpyroxenes(O,C),plagioclase(P)andbiotite(B)intexturalequilibium(PBA91-58);(c)clinopyroxene
(C)ingranodioriteshowing001lamellaeofpossiblepigeonite(indicatedbyarrows)andfaint100lamellae(orthopyroxene?);matrixphasesare
biotite(B),plagioclase(P)andquartz(Q)(PBA91-6);(d)clinopyroxene(C)ingranodioriteshowing100exsolutionlamellaeoforthopyroxenein
grain core (formerly augite) and unexsolved rims (diopside) (U-18); (e) granodiorite showing clinopyroxene (C) overgrown by biotite (B) (note
concentrationofexsolutionlamellaeincoreofgrainandunexsolvedrims)(U-3);(f)detailofrelationshipbetweenexsolved(E)regioninaugitic
coreofclinopyroxenegrainandclear(C)diopsidicrim(U-1).High-temperatureigneouscrystallizationfollowedbyorthopyroxeneexsolutionin
sub-calcic portions is implied from textures illustrated in (d), (e) and (f). All photomicrographs are taken in plane-polarized light except (d)
(crossednicols).
silica(Fig.6).Plotsofincompatibleelementsshowmore inthe60%SiO range.Aluminacontentsdeclineslightly
2
scatter (e.g. KO, Rb, Th, Ba) but generally increase withsilicaabove>60%SiO,whereaspotashlevelsrise.
2 2
with silica. Both KO and AlO have inflection points These compositional changes appear to correspond to
2 2 3
1622
PERCIVALANDMORTENSEN WATER-DEFICIENTCALC-ALKALINEMAGMAS
Fig. 4. Photomicrographs of hornblende-bearing plutonic rocks: (a) partial hornblende (H) rims around clinopyroxene; orthopyroxene (O) is
unaltered(PBA91-143);(b)completerimsofhornblende(H)aroundclinopyroxene(C)andorthopyroxene(O)inaplagioclase-rich(P)matrix
(U-13); (c) large twinned hornblende (H) grain with inclusions of clinopyroxene (C), plagioclase (P) and magneite (M) (U-14); (d) hornblende
(H)–perthite (Pe) association in granite and granodiorite; quartz (Q) and plagioclase (P) are also present (GS90-1). All photomicrographs are
takeninplane-polarizedlightexcept(d)(crossednicols).
the appearance of hornblende in the mafic mineral have negative Nb and Ti anomalies and slight Zr en-
assemblage,inadditiontoorthopyroxene,clinopyroxene richment. Thorium is systematically higher in the horn-
and biotite. blende-bearingandbiotite-onlyrocks,consistentwithits
Rare earth patterns (averages shown in Fig. 8) vary partitioning into hydrous magma (e.g. Wood & Blundy,
withbulkcompositionandmineralassemblage.Allhave 2001).
sinusoidal patterns with concave-down LREE, concave- Thegeochemicalfeaturesareconsistentwithfractional
upheavyrareearthelement(HREE)profiles(La/Yb = crystallization of a basaltic calc-alkaline magma re-
n
13–197)andnegligibleEuanomalies.However,thelevels sembling the mafic end-members of the suite. Some
of LREE enrichment and HREE depletion increase crustalcontaminationisindicatedby2725(cid:1) valuesinthe
Nd
systematically from orthopyroxene–clinopyroxene, range −0·5 to +1·1 (Stern et al., 1994). Using known
through clinopyroxene–hornblende and hornblende– ages of potential contaminants (2·8–3·0 Ga; Stern et al.,
biotite,tobiotite-onlyassemblages(Fig.8),probablyalso 1994; Percival et al., 2001), the volume of older crust
reflecting higher silica contents (respectively 61·2, 65·4, assimilated before fractionation is in the range 10–20%.
67·4 and 71·5 wt % SiO). Mass-balance considerations would predict a large mass
2
On primitive-mantle-normalized extended-element of complementary cumulates.
plots, the intermediate and felsic rocks show strong en-
richment in LILE as well as in LREE (Figs 7 and 9).
Pyroxene-bearingrockshavecomparableconcentrations Lake Minto domain plutons
of both compatible and incompatible trace elements to Plutons of Lake Minto domain occur as kilometre-scale
hornblende-, biotite-bearing units (Fig. 9). Both suites sheets, separated by screens of supracrustal rock and
1623
JOURNALOFPETROLOGY VOLUME43 NUMBER9 SEPTEMBER2002
2 272 90 3 5 988
U-52grntmainB 71·590·313·62·230·050·901·52·855·330·198·415141·627331609404005·93·41206·1560·6110190605·30·81·61·170·10·350·00·4
39761257924 7 22 26409746
U-51gdimainHB 63·40·15·6·80·11·83·4·22·40·99·<10<101223351101400470134·270278·21·4386407·2·6·5·20·2·90·2·
U-4gdimainOCHB 69·10·514·63·90·00·72·14·23·60·199·1<10<107·41118842300380123·7140111·80·42037173·12·62·72·10·41·00·11·1
1 623 48 9 3125
U-45gdimainCHB 66·80·515·54·50·01·93·94·21·40·199·040214·43315404605404·25·12109·31·50·73152203·10·82·41·70·30·80·10·6
2 143 80 5 9
n 87391191427 1 1 83436727
U-7gdimaiCB 60·0·16·6·0·3·4·4·2·0·99·3924158925120380420145·20019321·71130496·1·4·3·0·1·0·1·
7 760 65 8 8
U-44gdimainCHB 66·40·414·55·00·01·93·34·32·60·198·829187·7562488500470106·429026270·880150629·41·56·04·40·82·20·32·3
3 408 41 2 18921
U-43gdimainCHB 69·10·414·64·50·01·63·54·31·40·199·724154·17028812204606·93·81206·1680·652100272·60·51·00·80·10·40·10·7
6 557 85 3 3 7
43210604419 9 58 53 19924010
8 n 6·0·5·4·0·1·3·3·4·0·8·537·810003·3·024·0·3664·0·2·2·0·1·0·1·
U-3gdimaiCB 6 1 921 521213048 151 362
U-37gdimainOCHB 56·11·5414·012·50·193·086·673·90·821·0799·9<10<1020891216250460173110432·80·48781608315·02·112·08·11·74·00·663·4
5 843 76 1 1 3
U-11gdimainOCHB 62·50·915·86·30·01·33·83·63·70·298·410<10126321731400310147·2320172·40·83768325·51·74·53·10·61·50·21·3
U-36gdimainOCHB 56·80·8216·16·90·094·185·764·31·550·1596·76229201101667340490123·8100246·80·973262295·90·825·64·40·92·40·42·1
U-35gdimainOCHB 57·11·4816·610·00·142·426·274·51·030·51100·11212179215174105802010530292·80·51481105610·02·68·45·61·12·60·392·1
6 554 98 8 7259
U-32gdimainOCHB 62·50·417·05·30·01·83·54·23·50·198·732175·4812212010004606·55·2220118·70·54277284·11·12·71·80·30·80·10·7
0 962 51 6 8 6
U-31gdimainOCB 58·00·716·67·30·03·55·74·41·50·298·152331512015674606209·44·1160171·70·23979335·41·13·82·90·51·40·21·6
rocks U-30dteenclH 58·60·6516·08·50·113·865·564·21·790·1599·43648138215110350410123·911018102·92855234·01·13·42·80·61·60·261·7
7 655 45 93 3 4
utonic U-12dteenclOCB 50·51·317·111·90·14·57·83·71·60·599·32223211808·2521000770125·4260220·60·24590457·22·26·14·10·82·10·31·8
pl 6 631 79 2 5 5 x.
salik U-29gbrenclCB 49·11·016·511·20·16·39·43·61·60·299·39252292001175580570103·2110221·30·33477355·61·64·53·70·72·00·32·1 pendi
ofUt U-28gbrenclOCB 49·11·1918·612·00·134·077·654·01·850·4799·12032181801293630970133·3120332·80·8640985811·02·38·55·91·13·00·532·9 heAp
sitions U-27dteenclOCB 53·80·8417·59·60·133·866·653·92·410·2799·021251617021758907809·43·2120237·80·363983407·11·45·54·00·832·10·352·0 asint
compo U-23gbrdyke1OCH 50·90·4016·48·60·148·8610·902·40·540·0799·2210180261105·3171603702·21·928112·10·271122112·10·722·11·70·381·00·171·1 ations
Chemical 8U-19gbre1dyke1OCH 5·649·81·492·377·615·86·014·20·230·156·904·706·897·400·63·82·281·230·100·567·7100·003504411601504·89·3027047056802273·75·622408221·90·230·570·116373817467·09·01·12·54·07·54·04·64·90·852·02·02·00·319·71·5 alabbrevi
1: U-1gbrdykB 4 1 1 913081419 162163 99 1541 22 1 ner
e 5 346 87 6 85 3 mi
Tabl U-15gbrenclH 49·40·716·712·10·17·08·33·11·40·199·27485162108·2595109102·91·9528·42·81·21737173·30·92·51·50·20·60·10·6 and
Sample:∗Lithology:Setting:Min.ass.: SiO2TiO2OAl23OFe23MnOMgOCaOONa2OK2OP25TotalCrNiScVPbRbBaSrNbHfZrYThULaCeNdSmEuGdDyHoErTmYb Lithology
1624
PERCIVALANDMORTENSEN WATER-DEFICIENTCALC-ALKALINEMAGMAS
Fig.5. Locationsofsamplesanalysedinthisstudy,alongwithpyroxeneoccurrencesbasedonfoottraversesandspot-checkedpetrographically.
Owingtothecoarsegrainsizeofallunits,fieldidentificationofassemblagesisgenerallyreliable.
younger plutonic sheets. Pyroxenite, hornblendite, gab- Martin, 1994; Schiano et al., 1995). In contrast, Utsalik
bro, and diorite occur as pods up to 40m in size. plutonicrocksfallmainlywithinfieldsdefinedby‘normal’
Boudinageddykesofdioriteandgabbroarealsocommon. arcs (see Castillo et al., 1999; Fig. 10b). In view of the
Raretonalitegneissenclaveshaveyieldedages>3·1Ga. gradation from adakitic to normal arc signatures (Fig.
Granodiorite, the main plutonic phase, is medium to 10b),itispossiblethattheLakeMintoandUtsaliksuites
coarse grained, homogeneous and weakly foliated to represent magmas derived from a single mantle wedge,
massive. Mafic mineral assemblages range from ortho- fluxed respectively by slab melts and slab-derived fluids.
pyroxene–clinopyroxene–biotite with or without late
hornblende, to hornblende–biotite.
No new geochemical results are reported here; how-
ever, analyses of Lake Minto domain rocks reported by GEOCHRONOLOGY
Stern et al. (1994) are shown for comparative purposes U–Pb data are listed in Table 2 and plotted on con-
in Fig. 10. In comparison with the coeval (2725Ma) ventional U–Pb concordia diagrams in Fig. 11.
Utsalik domain plutonic suite, granodiorites of the Lake
Minto domain have systematically lower Th and U
contents,andaremoredepletedintheHREE(Fig.10a).
Utsalik domain
The fractionated HREE profile suggests the presence of
Sample U-34 (orthopyroxene–clinopyroxene–biotite
residual garnet in the source (Stern et al., 1994). Lake
Minto granodiorites show several geochemical features granodiorite)
comparablewiththoseofmodernadakites,suchashigh Four fractions of zircon and two of monazite were
Sr/Y,lowY(Fig.10b)andLa/Ybupto260,whichare analysed (Fig. 11a). The zircons form stubby euhedral
commonly considered to result from the presence of a prisms with faint internal growth zoning and no evid-
slab melt component (Drummond & Defant, 1990; ence of metamorphic overgrowth. One fraction (B) is
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JOURNALOFPETROLOGY VOLUME43 NUMBER9 SEPTEMBER2002
Fig.6. Harkerdiagramsshowingvariationofcompatibleandincompatibleelementcontentswithsilica.Naneygranodioriteandgranitefrom
Naney’s(1983)experimentalruns;averageI-type(n=991)andS-type(n=578)fromWhalenetal.(1987):C-type(n=12)fromKilpatrick
&Ellis(1992);M-type(n=8)fromEggins&Hensen(1987).
concordant with a 207Pb/206Pb age of 2724·3 ± 4·5Ma, this unit, or partial and variable Pb loss from igneous
and a regression line through all four zircon analyses monazite, or some combination of the two.
givescalculatedupperandlowerinterceptagesof2725·6
+7·4/–3·8Ma and 1472Ma, respectively. The crys-
tallizationageoftherockisgivenbythe207Pb/206Pbage Sample U-54 (clinopyroxene–biotite granodiorite)
of fraction B, and the lower intercept indicates that Zircons in this sample form relatively coarse, medium
mainlyMesoproterozoicPblosshasaffectedthezircons. brown, weakly zoned, stubby euhedral prisms. Four
The two monazite fractions give considerably younger fractions were analysed (Table 2). Three of these are
ages, at 2704 and 2695Ma. These results may indicate 2·1–9·1%discordant,anddefinealineardiscordiaarray
either prolonged growth of metamorphic monazite in witharelativelyimpreciseupperinterceptageof2737·0
1626