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Geochemical and radiogenic isotope (Sr–Nd)characteristics of Paleoproterozoic anorthositicand granitoid rocks in the Umiakoviarusek Lakeregion, Labrador, Canada.

Stephen J. Piercey and Derek H.C. Wilton

Abstract: Recent work in the north-central Labrador has identified Paleoproterozoic anorthositic and granitoid rocksthat are spatially associated with, yet temporally distinct from, younger Mesoproterozoic intrusions of the Nain PlutonicSuite. The Umiakoviarusek Lake (UL) region of Labrador contains several of these Paleoproterozoic intrusions andprovides an opportunity to study their geochemical and radiogenic isotope (Sr–Nd) characteristics. Geochemically, theanorthositic and granitoid rocks have features consistent with contemporary anorthositic and granitoid rocks from otheranorthosite–mangerite–charnockite–granite complexes. The anorthositic rocks contain elevated contents of Al2O3, CaO,Sr, and Eu with low Ba, Rb, K, Zr, total rare earth elements (REE), and light REE. The granitoid rocks, on the otherhand, contain lower concentrations of these elements along with elevated SiO2 and K2O. Isotopic data at 2050 Ma forthe anorthositic rocks (ISr = 0.7048–0.7082;eNd = –4.1 to –15.9) and granitoid rocks (ISr = 0.7036–0.7094,eNd = –5.1to –9.7) are consistent with variable crustal and mantle contributions to their genesis. The relatively unradiogenic Srand slightly evolved Nd isotopic data from the UL granitoid rocks is consistent with a significant juvenile mantlecomponent, possibly derived from an underplating mantle plume; this component may also be present in theanorthositic rocks. The Nd and Sr isotopic data are also consistent with crustal contamination from Archean sourcematerials; however, based on the existing isotopic database for the Nain Province gneisses, it is not possible todelineate a specific gneiss component. Furthermore, it is also quite possible that an Archean source, unlike anydescribed at present, was a crustal source component in the UL intrusive rocks.

Résumé: Les études récentes de la région nord-centrale du Labrador ont révélé la présence de roches anorthositiqueset granitoïdes datant du Paléoprotérozoïque, lesquelles sont associées spatialement aux intrusions plus jeunes, duMésoprotérozoïque, de la suite plutonique de Nain, avec la réserve qu’elles sont considérées temporairement distinctes.La région du lac Umiakoviarusek du Labrador renferme plusieurs de ces intrusions paléoprotérozoïques propices àl’étude de leurs caractères géochimiques et de leurs isotopes radiogéniques (Sr–Nd). Géochimiquement, les rochesanorthositiques et granitoïdes montrent des particularités compatibles avec les roches anorthositiques et granitoïdes ded’autres complexes d’anorthosite–mangérite–charnockite–granite datant de la même époque. Les roches anorthositiquescontiennent des proportions élevées de Al2O3, CaO, Sr et Eu et de faibles teneurs de Ba, Rb, K, Zr, terres rares totaleset terres rares légères. D’autre part, les roches granitoïdes affichent des concentrations plus faibles de ces éléments etde fortes proportions de SiO2 et K2O. Les données isotopiques à 2050 Ma des roches anorthositiques (ISr = 0,7048–0,7082;eNd = –4,1 à –15,9) et des roches granitoïdes (ISr = 0,7036–0,7094;eNd = –5,1 à –9,7) sont en accord avecl’hypothèse évoquant que leur formation implique des contributions variables de la croûte et du manteau. Les donnéesisotopiques du Sr relativement nonradiogénique et du Nd peu évolué des granitoïdes de type lac Umiakoviarusekcadrent bien avec la présence d’un composant mantellique significativement juvénile, dérivé probablement d’un panachemantellique sous-plaque; ce composant est probablement présent aussi dans les roches anorthositiques. Les donnéesisotopiques du Nd et Sr sont en accord également avec une contamination crustale par des matériaux d’originearchéenne, cependant l’examen de la base de données des isotopes dans les gneiss de la Province de Nain ne peutconfirmer l’identification d’un tel composant gneissique spécifique. De plus, il est aussi fort probable que la sourcearchéenne, contrairement à toutes celles décrites jusqu’à présent, correspondait à un composant de source crustale dansles intrusions de type lac Umiakoviarusek.

[Traduit par la Rédaction] Piercey and Wilton 1972

Can. J. Earth Sci.36: 1957–1972 (1999) © 1999 NRC Canada

1957

Received October 31, 1998. Accepted July 27, 1999.

S.J. Piercey1 and D.H.C. Wilton.2 Department of Earth Sciences, Memorial University of Newfoundland, St. John’s,NF A1B 3X5, Canada.

1Present address: Mineral Deposit Research Unit (MDRU) Geochronology Laboratory, Department of Earth and Ocean Sciences,University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4, Canada.

2Corresponding author (e-mail: [email protected]).

IntroductionRecent research, including field mapping (e.g., Ryan et al.

1997, 1998; Piercey 1998) and geochronological studies(Emslie and Loveridge 1992; Ryan and Connelly 1996), innorth-central Labrador have shown that anorthositic andgranitoid rocks previously assigned to the MesoproterozoicNain Plutonic Suite (NPS; Ryan 1990) are, in fact, not mem-bers of this suite, as they have Paleoproterozoic emplace-ment ages (2137–2032 Ma; Hamilton et al. 1988). This suiteof intrusions is proximal to the younger NPS (e.g., Ryan etal. 1998), but display postemplacement metamorphic andstructural overprinting due to the 1.86–1.74 Ga. TorngatOrogeny (e.g., Bertrand et al. 1993; Van Kranendonk andWardle 1997; Van Kranendonk 1996); such features are notobserved in the NPS (e.g., Emslie et al. 1994; Ryan et al.1997, 1998).

Anorthositic and granitoid rocks in the UmiakoviarisekLake (UL) region of Labrador have field and geochronologi-cal features similar to those of Paleoproterozoic intrusionselsewhere in the Okak Bay region (e.g., Ryan et al. 1997,1998; Hamilton et al. 1998; Piercey 1998) and lie on thenorthern extremity of the 15 × 30 km belt of Paleo-proterozoic intrusions that have thus far been identified andsubdivided from the Mesoproterozoic massifs by Ryan et al.(1998) (Fig. 1). This region is particularly interesting be-cause it hosts three granitoid and two anorthositic (sensulato) intrusions, presumably belonging to this older suite(e.g., Ryan et al. 1998; Piercey 1998). In this paper, we pro-vide geochemical and isotopic data from these Paleo-proterozoic anorthositic and granitoid rocks to documenttheir geochemical and isotopic characteristics and to providean understanding of the petrogenetic controls and the role ofbasement in the genesis of this older suite. Whether these in-trusions have similar geochemical and isotopic characteris-tics, basement contamination influences, and petrogenesis ascompared to the Mesoproterozoic suite is at present un-known. Thus the aim of this new dataset is to provide insightinto the nature of the Paleoproterozoic anorthositic–granitoidmagmatism and the Paleoproteorzoic crustal evolution ofthis part of North America.

Regional settingThe geology of the UL area consists of an eastern and

northern portion, underlain by quartzofeldspathic and maficgranulite–amphibolitic basement gneiss of the Archean NainProvince, and a western portion, underlain by a group ofMesoproterozoic (1350–1290 Ma; Ryan et al. 1991; Ryanand Emslie 1994; Emslie et al. 1994), bulbous-shapedanorthositic and granitoid intrusive rocks forming the north-ernmost component of the Nain Plutonic Suite (Ryan 1990;Fig. 1). The NPS is a typical multiphase anorthosite–mangerite–charnockite–granite (AMCG) intrusive complexthat intruded episodically from ca. 1350 to 1290 Ma (Ryanand Emslie 1994). The igneous phases within the NPS gen-erally lack any structural fabric.

In contrast to the NPS intrusive rocks is a northwest-striking linear swath of variably foliated, Paleoproterozoic,metamorphosed and deformed anorthositic, granitoid and ba-sic dyke intrusive rocks (Fig. 1; Emslie and Loveridge 1992;Emslie et al. 1997; Ryan and Connelly 1996; Ryan et al.

1997, 1998; Hamilton et al. 1998). The granitoid magmatismis bracketed between ca. 2137 and 2032 Ma (WheelerMountain, Sheet Hill, Loon Island, Halbach, Illulik, AlligerLake plutons; Emslie and Loveridge 1992; Ryan andConnelly 1996; Hamilton et al. 1998; Fig. 1), andanorthositic and basic dyke magmatism is between 2121 and2045 Ma (Aupalukitak Mountain intrusive and two meta-morphosed basic dykes; Hamilton et al. 1998; Fig. 1). Al-though, of all the intrusive rocks in the UL area, only theIllulik granite at 2124 Ma (Hamilton et al. 1998) has actu-ally been dated, the proximity to dated Paleoproterozoic in-trusions and common field characteristics (Piercey 1998;Ryan et al. 1998) make them likely members of this oldersuite.

Geology and petrography of anorthositicand granitoid rocks in the UmiakoviarusekLake region

The geology of the UL region has been the focus of re-gional work by Ryan et al. (1998) and a detailed scale studyby Piercey (1998). The geology of the area consists of atleast three granitoid plutons and two anorthositic plutons(sensu lato), with lesser felsic and mafic dykes (Fig. 2); asmapped to date, none of these plutons contains supracrustalenclaves. The oldest granitoid rocks comprise foliated faya-lite–orthopyroxene granitoid rocks that are the northerly ex-tension of the Paleoproterozoic Illulik pluton (Fig. 2). Theseeastern granitoid rocks have a wide range in compositionfrom quartz monzonite to granite and K-feldspar granitewith variable amounts of mafic minerals. Clinopyroxene andorthopyroxene dominate the mafic mineral mode, with lesserbiotite, hornblende, and fayalite. Typically, green–brownhornblende, and lesser red biotite, form coronas which par-tially, and in some cases fully, replace the pyroxenes; inother cases, actinolite is present on the fringes of thepyroxene grains. Accessory zircon, apatite, and titanite arecommon inclusions within the hornblende and biotite,whereas myrmekitic plagioclase and perthitic and micro-clinic K-feldspar often contain sericite and epidote dustings.The foliated (defined by K-feldspar and hornblende clots)granitoid intrusive lies in sinistral strike-slip fault contactwith the Archean Nain Province amphibolites, mafic granu-lites, and quartzofeldspathic gneisses, and exhibits the samestructural grain as the gneisses (Fig. 2).

The Illulik granitoid rocks were intruded by lesser foliatedto unfoliated, buff-weathering leuconorite to gabbronorite ofthe Pripet Marshes pluton (Fig. 2; Ryan et al. 1998). Withinthe study area, the Pripet Marshes pluton consists predomi-nantly of an oxide-rich leuconorite. It differs from theanorthositic rocks of the Goudie Lake pluton to the westwith its buff colouration, greater modal orthopyroxene oxideminerals, and lesser developed greenschist facies mineral as-semblages. This being said, the euhedral to subhedralplagioclase in the leuconorites have sericite and epidotedustings and variable replacement of the mafic minerals byactinolite–chlorite assemblages.

The Goudie Lake pluton (Ryan et al. 1998) in the westernportion of the study area is dominated by anorthosite (sensustricto) and leuconorite (Fig. 2). North of UL, the GoudieLake pluton consists predominantly of dark black to grey an-orthosite (sensu stricto; e.g., P96-3A, P96-19, P96-33),

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1958 Can. J. Earth Sci. Vol. 36, 1999

which appears to grade into leuconorite south of UL. Theanorthositic rocks are variably metamorphosed with partialto complete replacement of orthopyroxene and plagioclaseby actinolite–chlorite and epidote–sericite assemblages, re-spectively. Some samples contain crosscutting polymineralicveinlets (ca. 1–2 mm) of calcite, dolomite, sericite, orchlorite. Local pockets of unmetamorphosed leuconorite ex-ist south of UL, which are characterized by pristine cumulusplagioclase with unaltered intercumulus orthopyroxene con-taining rod-like exsolution lamellae of rutile.

North of UL, the anorthositic rocks of the Goudie Lakepluton are intruded by mafic dykes and granular, aplitic to

granitic dykes (P96-3B, P96-28B). The granitic dykes aredominated by recrystallized quartz, sericite, carbonate, andlesser epidote. Both the Goudie Lake pluton and the felsicdykes have been deformed by ca. 2–5 m wide zones ofsouth-southwesterly directed ductile (mylonitic) shearing,and the dykes themselves exhibit south-southwesterly ver-gent folding.

Mafic dykes also intrude the Goudie Lake pluton south ofUL. These dykes exhibit variable strikes (e.g., northwest,east-northeast, north) and range from diabase to lesser gab-bro. Shearing along dyke margins was not observed in thedykes documented in this study, but is typical of other dykes

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Piercey and Wilton 1959

Fig. 1. Regional geologic setting of the Umiakoviarusek Lake intrusives and the spatial relationships between Paleoproterozoic andMesoproterozoic intrusions. UL, Umiakoviarusek Lake; GL, Goudie Lake; SL, Saputit Lake; TTL, Tasiuyak–Tasialua Lake; NL, NormaLake; FL, Frozen Lake; TL, Talifer Lake; SHL, Staghorn Lake. Figure modified after Ryan et al. (1998), geological subdivisions oninset map from Taylor (1971).

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Fig. 2. Geology of the Umiakoviarusek Lake area and related intrusions. Mapping inside study area from Piercey (1998), outside studyarea is modified from Ryan et al. (1998). GLP, Goudie Lake pluton; PMP, Pripet Marshes pluton; ORVP, Owl River Valley pluton;QMG, quartz monzonite group.

in the region (e.g., Ryan et al. 1997, 1998). Widespread ret-rogression of mafic minerals to greenschist facies assem-blages of chlorite–actinolite–tremolite–sericite–epidote iscommon in all Paleoproterozoic dykes (Ryan et al. 1998) in-cluding the UL ones. Anorthositic rocks in contact with thedykes have a distinctive bright white bleaching with brightgreen patches of altered mafic minerals.

Leuconoritic rocks of the Goudie Lake pluton south of ULwere intruded by the Owl River Valley pluton (Ryan et al.1998; Piercey 1998; Fig. 2), a variably foliated, brown torusty coloured, hornblende-, biotite-, clinopyroxene-,fayalite-bearing intrusive of monzonite to syenite. Highertemperature mafic minerals (fayalite, clinopyroxene) exhibitsignificant hydration as coronitic overgrowths ofhornblende ± biotite, which lend a speckled appearance tothe granitoid rocks. Complete replacement of fayalite andclinopyroxene by hornblende is common, and in other caseshornblende and biotite occur as small patches along thecrystallographic axes of these primary minerals. Accessorytitanite, zircon, and particularly apatite are common withinthe hornblende and biotite grains. Of particular note is thatquartz is absent in the Owl River Valley pluton (cf. Ryan etal. 1998), while plagioclase and perthitic K-feldspar arepresent in roughly subequal proportions. Foliations in theOwl River Valley pluton granitoid rocks, defined by felsparsand hornblende, are strongest near their contact with theGoudie Lake anorthositic rocks, have variable orientationand dip, and appear to be present not only along their mar-gins, but also sporadically within the intrusion (Fig. 2).

In the western edge of the study area, the Owl River Val-ley pluton is intruded by a regional unit of ovoidal-feldsparquartz–monzonite (Ryan et al. 1998; QMG, quartz mon-zonite group). These granitoid rocks are volumetrically mi-nor inside the study area and are texturally variable; someintrusive rocks are granular with abundant quartz andrecrystallized plagioclase and K-feldspar. The granular intru-sive rocks contain lesser mafic minerals, which are restrictedto hornblende and biotite that exhibit a distinctive metamor-phic parallelism on the hand-specimen scale. In contrast, acoarser variant observed in one locality contained coarse-grained clinopyroxene, hornblende, and biotite, along withovoidal feldspars and recrystallized feldspars and quartz.

Geochemistry

Sampling and analytical protocolA total of 22 samples were collected from the different

plutons and petrographic groups in the UmiakoviarusekLake region. Anorthositic rocks (n = 10) include (1) threeanorthosites (sensu stricto) and five leuconorites from theGoudie Lake pluton, and (2) two leuconorites from thePripet Marshes pluton. Granitoid rocks (n = 12) include(1) two felsic (quartz monzonite) dykes that intrude anortho-site north of Umiakoviarusek Lake; (2) four fayalite–hornblende (± biotite) monzonites and one syenite from theOwl River Valley pluton; (3) monzonite, quartz syenite, andquartz monzonite (n = 3) from the QMG; and (4) onemonzonite and K-feldspar granite from the foliated granitoidrocks in the eastern sector of the property. Trace elements(Ba, Rb, Sr, Zr, Y, Nb, K, Ti) and rare earth elements(REE) + Th geochemistry were determined by pressed-powder-pellet X-ray fluorescence (XRF) and Na2O2 sinter

inductively coupled plasma mass spectrometry (ICP–MS)techniques following the methods, respectively, ofLongerich (1995) and Longerich et al. (1990), and Jenner etal. (1990). Major element determinations were also under-taken on the pellets by XRF; the light element (SiO2, MgO,Al2O3, Na2O) determinations are semiquantitative (Longerich1995); however, given the monomineralic character of therocks, these results are sufficient for the purposes of thisstudy. Precision in the trace element analyses by XRF andICP–MS, measured as percent relative standard deviation(%RSD), is 0–3‰ for all elements (Longerich 1995;Longerich et al. 1990; Jenner et al. 1990).

All anorthositic and granitoid rocks, with the exception ofthe foliated granitiod rocks, were analysed for Rb–Sr andSm–Nd isotopic composition by thermal ionization massspectrometry (TI–MS) using the isotope-dilution preparationtechnique outlined by MacLachlan and Dunning (1998) andHoran (1998).

The sample powder was weighed to the fifth decimal andattacked with 8N nitric and 2× hydrofluoric acids, with twoto seven days refluxing. Following redissolution, the samplewas split into isotopic concentration (IC) and isotopic dilu-tion (ID) fractions. The ID fraction was spiked with ORNL150Nd–147Sm mixed spike based on Nd concentration. InitialRb and Sr separation was completed with EICHROM TRU-Spec ion exchange resin; the Rb and Sr were collected using2B 3N nitric acid, evaporated to dryness, and loaded ontoEICHROM SR-Spec ion exchange resin, from which Rb (2B3N nitric acid) and Sr (2B water) were collected. For the Ndand Sm separation, the fraction was loaded into Teflon col-umns with 2× 0.15N HCl; the Nd fraction was collectedwith 2× 0.17N HCl, and the Sm ID by 2× 0.50N HCl. Twodrops of 1N phosphoric acid were added to all three frac-tions prior to evaporation.

Analyses were completed using a Finnigan MAT 262VTI–MS on outgassed Re double filaments. The Rb fractionwas analysed via single-peak jumping routine on either Dalyor Faraday collectors. The Sr IC fraction was analysed usinga simultaneous Faraday multicollector routine. The Sr IDfraction was analysed using a simultaneous Faraday multi-collector routine for84Sr/86Sr and87Sr/86Sr ratios. Each rou-tine analysed five data blocks of 20 scans for a total of 100scans.87Sr/86Sr ratios were calculated via a LOTUS 123 97spreadsheet, based on measured87Rb/85Rb and 84Sr/86Srspiked ratios (87Rb/85Rb errors are absolute at 95% confi-dence levels (2σ), quadratically added using 2σ errors for87Rb/85Rb) and84Sr/86Sr measured ratios (87Sr/86Sr errors arereported at 2σ and are based solely on spectrometry mea-surements).

Nd and Sm IC and ID fractions were analysed using staticFaraday multicollector routines consisting of five blocks of20 scans for a total of 100 scans with online drift and massfractionation correction and statistical analysis. The147Sm/144Nd ratios were calculated via a LOTUS 123 97spreadsheet, based on measured147Sm/149Sm and150Nd/144Ndspiked ratios (147Sm/144Nd errors are absolute2σ quad-ratically added using 2σ errors for 147Sm/149Sm) and150Nd/144Nd measured ratios (143Nd/144Nd errors are reported2σ levels and are based solely on spectrometry measure-ments). Values ofeNd are calculated using143Nd/144Nd(CHUR) of 0.512638 and147Sm/144Nd (CHUR) of 0.19659.

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The average of 51 analyses of NBS 987 standard was0.710251 (errors as two sigma (2σ) = 12); accepted is0.710250 (2σ = 15). Average of three analyses of NBS 984Rb standard was 2.59265 (2σ = 400); accepted is 2.59265(500). Average of 37 analyses of La Jolla standard was0.511855 (2σ = 12); accepted is 0.511850 (15). Average of23 analyses of AMES Sm standard was 1.084953 (2σ = 89);accepted is 1.084955 (150).

Initial eNd and fSm/Nd were calculated using the presentday chondrite uniform reservoir values of147Sm/144Nd =0.1966 and143Nd/144Nd = 0.512638; while depleted mantlemodel ages (TDM) were calculated using depleted mantle val-ues of147Sm/144Nd = 0.21144 and143Nd/144Nd = 0.513113,and a decay constant ofλ = 6.54 × 10–12 a–1 (Faure 1986).The 87Sr/86Sr ratios were fractionation corrected by normal-ization to 86Sr/88Sr = 0.1194.

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Sample Name: P96-3A P96-19 P96-33 P96-44A P96-55B P96-83 P96-174 P96-182 P96-199 P96-203

Rock Type: An An An Ln Ln Ln Ln Ln Ln Ln

Pluton: GLP GLP GLP GLP GLP GLP GLP GLP PMP PMP

SiO2 (wt.%) 54.20 54.95 55.17 51.07 55.61 54.48 52.98 55.85 54.42 53.09TiO2 0.15 0.07 0.11 3.86 0.13 0.40 0.43 0.21 1.26 1.26Al 2O3 23.48 26.89 25.91 21.85 26.88 16.07 19.31 25.23 18.79 20.80Fe2O3T 4.38 1.57 2.09 7.46 1.27 9.75 9.24 2.21 10.36 8.25MnO 0.07 0.02 0.03 0.06 0.02 0.14 0.15 0.03 0.13 0.12MgO 3.27 1.14 1.77 1.61 0.31 6.59 5.38 1.50 1.49 2.26CaO 9.84 9.99 9.84 8.93 9.48 8.85 8.12 9.21 7.10 8.89Na2O 4.18 4.54 4.53 4.42 5.55 3.10 3.65 4.93 4.29 4.09K2O 0.44 0.83 0.55 0.56 0.74 0.56 0.59 0.74 1.65 0.64P2O5 0.01 0.01 0.01 0.18 0.02 0.06 0.15 0.09 0.52 0.60

Sc (ppm) <LD <LD <LD 13 <LD 21 16 <LD 16 <LDV 27 <LD 10 341 8 71 43 9 113 94Cr 56 28 27 40 <LD 105 141 17 22 23Ni 15 <LD 10 18 <LD 21 42 <LD <LD <LDGa 21 24 23 24 23 18 20 20 25 24Rb <LD 5 2 7 7 13 3 9 15 1Sr 664 813 735 755 987 564 665 820 660 746Y <LD <LD <LD 2.2 1.5 3.9 4.5 5.1 24 13Zr <LD <LD <LD 41 <LD 6.0 15 17 50 15Nb <LD <LD <LD 8.2 3.0 1.3 1.7 9.7 4.4 4.4Hf 0.1 0.0 0.1 1.4 1.0 0.4 0.7 0.7 2.0 0.8Ta 1.0 0.7 0.9 0.5 1.4 0.6 0.4 1.1 0.4 0.5Ba 266 529 344 358 540 314 529 538 1766 561Th <LD <LD <LD 0.02 0.30 0.02 0.15 1.69 0.51 0.08Pb 4 5 5 <LD 19 6 <LD 6 12 5U <LD <LD <LD <LD <LD <LD <LD <LD <LD <LDLa 1.60 4.47 2.58 4.80 6.25 4.19 6.77 10.7 31.9 12.2Ce 2.81 6.88 4.39 9.75 10.8 8.80 12.9 21.2 69.4 27.8Pr 0.32 0.66 0.49 1.22 1.22 1.14 1.60 2.38 9.13 3.77Nd 1.29 2.09 1.75 5.42 4.32 4.67 6.30 8.32 36.7 16.9Sm 0.33 0.36 0.31 0.98 0.80 1.19 1.33 1.48 6.69 3.42Eu 0.54 0.78 0.59 0.86 0.80 0.74 0.86 0.85 3.27 1.83Gd 0.21 0.11 0.12 0.77 0.53 0.98 0.96 1.09 5.80 2.91Tb 0.03 0.01 0.01 0.10 0.07 0.14 0.15 0.15 0.81 0.40Dy 0.19 0.05 0.10 0.55 0.51 0.91 0.89 1.01 4.62 2.36Ho 0.04 0.01 0.02 0.10 0.10 0.17 0.18 0.19 0.93 0.50Er 0.12 0.03 0.05 0.24 0.29 0.55 0.53 0.58 2.60 1.37Tm 0.02 0.00 0.01 0.04 0.04 0.07 0.09 0.09 0.36 0.18Yb 0.14 0.02 0.04 0.21 0.27 0.50 0.48 0.67 2.32 1.19Lu 0.02 0.00 0.01 0.03 0.05 0.07 0.09 0.10 0.36 0.18(La/Yb)n

a 7.87 149 47 15.4 15.4 5.71 9.59 10.7 9.30 6.94Eu/Eu* 6.24 12.00 9.55 3.02 3.75 2.09 2.32 2.06 1.61 1.78

Notes: <LD, below limit of detection. Rock type: An, Anorthosite; Ln, Leuconorite; Monz., monzonite; Q monz., quartz monzonite; Sy, syenite. Pluton:GLP, Goudie Lake pluton; PMP, Pripet Marshes pluton; Dyke, felsic dykes; ORVP, Owl River Valley pluton; QMG, Quartz monzonite group.

aNormalized to chondritic values of Taylor and McLennan (1985).

Table 1. Major and trace element data for the Umiakoviarusek Lake plutons.

Initial 143Nd/144Nd, 87Sr/86Sr (ISr), and eNd values werecalculated at 2050 Ma, the published lower limit of Paleo-proterozoic magmatism in this region (ca. 2045 Ma; Hamil-ton et al. 1998). To account for potential age variations, theeNd andISr values were calculated at 100 Ma year intervalsof 2050 Ma (within the known range of Paleoproterozoicmagmatism; cf. Hamilton et al. 1998) and yielded variationsof 0.75–1.73 and 0.88–1.27eNd units, with averages of 1.17and 1.06 for the anorthositic and granitoid rocks, respec-

tively. Variations in 87Sr/86Sr ratios range from 0.00001 to0.00009 and 0.00071 to 0.00305 for the anorthositic andgranitoid rocks, respectively.

Results

GeochemistryGeochemical data are presented in Table 1 and Figs. 3 and

4. Given the cumulate nature of the anorthositic rocks from

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P96-86B P96-75B P96-76 P96-87 P96-131A P96-131B P96-3B P96-28B P96-39 P96-60 P96-66A P96-85

Monz. Q monz. Q monz. Q monz. Monz. Monz. Q monz. Q monz. Monz. Monz. Monz. Sy

ORVP QMG QMG QMG Illulik Illulik Dyke Dyke ORPV ORPV ORPV ORPV

62.74 56.39 67.62 73.64 59.00 73.09 71.77 73.82 53.37 56.24 62.40 64.110.56 1.12 0.79 0.35 0.57 0.33 0.37 0.35 1.12 1.04 0.53 0.45

15.99 13.58 13.15 13.11 17.30 13.57 13.87 14.22 12.56 15.07 18.60 16.267.45 14.72 7.20 3.09 4.53 3.08 4.02 2.91 19.25 13.51 5.00 6.160.16 0.30 0.14 0.04 0.13 0.06 0.08 0.05 0.41 0.28 0.10 0.120.53 0.91 0.68 0.33 2.39 0.25 1.03 0.75 0.76 0.83 0.33 0.362.45 4.58 2.53 1.44 10.44 1.75 1.71 1.88 4.92 4.06 3.08 2.004.64 3.65 3.41 3.20 4.31 3.32 2.50 3.62 3.78 4.23 4.83 4.035.34 4.39 4.22 4.74 0.72 4.44 4.57 2.33 3.52 4.48 5.01 6.410.14 0.35 0.25 0.07 0.61 0.10 0.08 0.06 0.32 0.26 0.13 0.11

17 26 15 6 16 10 14 4 21 19 8 7<LD <LD 10 11 83 <LD <LD <LD <LD <LD <LD <LD<LD <LD <LD 26 270 27 14 <LD <LD <LD <LD <LD<LD <LD <LD <LD 29 <LD <LD <LD <LD <LD <LD <LD

22 18 19 23 25 19 8 13 19 23 20 21137 115 101 142 4 119 79 49 83 116 98 188295 324 243 197 375 178 306 244 209 329 561 28958 61 35 24 33 20 32 27 50 60 17 60

123 329 615 459 128 501 369 796 230 281 67 38576 100 27 18 15.5 22.4 18 37 38 118 33 79

5.1 9.6 12.9 10.8 3.3 13.5 8.3 19.0 7.2 8.4 0.2 10.42.8 1.5 1.6 1.5 1.1 1.3 2.5 3.0 1.8 3.3 0.6 2.8

4466 5830 2426 1636 171 1587 10 554 4558 3027 6070 8695 52854.88 0.47 0.41 0.65 6.40 0.19 0.68 2.77 3.00 3.14 3.16 3.75

15.6 15.4 25.1 18.2 13.5 14.9 <LD 4.6 16.7 20.4 10.0 19.4<LD <LD <LD <LD <LD <LD <LD <LD <LD <LD <LD <LD

71.6 60.0 48.0 46.1 69.2 30.8 33.9 46.7 58.9 72.6 4.4 44.9169 125 108 96.6 157 69.0 62.0 80.2 126 166 8.35 12620.7 16.4 13.9 12.4 19.5 9.42 8.02 8.92 15.4 21.3 1.04 17.7780.72 64.92 58.54 51.68 75.51 39.85 35.1 35.2 64.5 87.6 4.18 74.714.6 11.2 10.6 8.90 12.9 7.53 7.19 6.00 12.0 16.3 0.90 14.9

5.41 3.54 3.20 2.20 2.14 1.90 7.04 5.06 5.65 6.97 0.72 5.7911.8 9.12 8.76 7.01 8.89 6.12 6.92 5.56 10.7 14.4 0.78 12.9

1.81 1.27 1.23 0.93 1.14 0.77 0.95 0.76 1.48 2.03 0.10 2.1011.2 7.67 7.06 5.10 6.62 4.37 6.24 4.88 9.10 12.6 0.67 11.8

2.35 1.50 1.44 1.04 1.24 0.91 1.27 1.07 1.81 2.43 0.13 2.516.81 4.01 4.07 2.77 3.46 2.27 3.49 3.11 5.59 6.62 0.38 7.340.97 0.55 0.55 0.37 0.45 0.29 0.54 0.48 0.88 1.00 0.06 1.016.53 3.55 3.42 2.20 2.91 1.77 3.20 3.21 6.20 6.26 0.37 7.030.96 0.55 0.55 0.31 0.42 0.26 0.54 0.57 1.18 1.15 0.06 1.067.41 11.4 9.47 14.2 16.1 11.8 7.16 9.83 6.42 7.84 8.10 4.321.26 1.07 1.02 0.85 0.61 0.86 3.05 2.68 1.52 1.39 2.62 1.27

the UL region, it is not surprising that the major elementand, to a lesser extent, the trace element geochemical signa-tures of these rocks strongly reflect their cumulate mineral-ogy. In particular, all of the anorthositic rocks have elevatedAl 2O3, CaO, Sr, and to a lesser extent Ba (Table 1). Othernotable features of the anorthositic rocks can be observed inthe form of chondrite-normalized REE and primitive mantle-normalized trace element plots in Figs. 3 and 4. Notably, allthe anorthositic rocks are characterized by downward-sloping profiles from light REE (LREE) to heavy REE(HREE) and variable fractionation ((La/Yb)n = 5.7–149),with variably developed positive Eu anomalies (Eu/Eu* =1.6–12) (Figs. 3a–3c; Table 1). Within the anorthositicrocks, there are subtle differences in REE chemistry that ap-pear to be related to their mineralogy. For instance, theanorthosites (sensu stricto) from the Goudie Lake plutonhave the steepest REE patterns and largest Eu anomalies(Fig. 3a; Table 1), whereas the leuconorites (15–25%orthopyroxene) from the same pluton have a less steep pat-tern with a smaller Eu anomaly (Fig. 3b), and the leuco-norites from the Pripet Marshes pluton (>25%orthopyroxene) exhibit the flattest pattern and smallest Euanomaly (Fig. 3c). A similar relationship is observed for Srin the primitive mantle normalized plots (Figs. 3a–3c; Ta-ble 1). The mafic intrusive rocks are also characterized byrelatively high Rb and Ba relative to Th and U (Figs. 4a–4c;Table 1). Similarly, the Nb contents are variable, but gener-ally show negative anomalies (Figs. 4a–4c) and exhibit arough positive correlation with Th contents (Table 1). Theother high field strength elements (HFSE) have variableconcentrations with broad increases, for the most part, in Hf,Zr, Y, TiO2, and P2O5, with increasing pyroxene content inthe anorthositic rocks (Table 1; Figs. 4a–4c).

The granitoid rocks from the UL region, for the most part,are characterized by elevated K2O, SiO2, and Fe2O3T con-tents, and have variable trace element signatures (Table 1;Figs. 3e–3h, 4d–4g). The REE profiles for the granitoidrocks are broadly similar, with slightly fractionated REEpatterns ((La/Yb)n = 4.3–16) and generally flat to weaklypositive or negative Eu anomalies (Eu/Eu* = 0.61–2.6), withthe exception of the felsic dykes (Eu/Eu* = 2.68, 3.05;Figs. 3e–3h). Similar to the anorthositic rocks, the primitivemantle normalized patterns of the granitoid rocks exhibitlow Th and U relative to Ba and Rb, but variable Nb con-tents ranging from flat, to negative, to slightly positiveanomalies (Figs. 4d–4g). All the granitoid rocks are charac-terized by elevated Zr and depletion in Sr and Ti relative tothe primitive mantle (Figs. 4d–4g; Table 1).

Neodymium and Strontium isotope geochemistryNeodymium and strontium isotopic data are presented in

Table 1 and Fig. 5. Anorthositic and granitoid rocks from theUL area display distinctive vertical and horizontal trends, re-spectively, ineNd–ISr space (Fig. 5), typical of AMCG com-plexes (e.g., Geist et al. 1989, 1990; Emslie et al. 1994).Anorthositic rocks from the UL region have very restrictedISr (at 2050 Ma) values, ranging from 0.7048 to 0.7082, withmost between 0.7048 and 0.7060 (Fig. 5; Table 2). The Ndisotopic data are more variable witheNd (at 2050 Ma) rang-ing from –4.11 to –15.88,fSm/Nd from –0.30 to –0.68, andTDM from 2.54 to 3.80 Ga (Fig. 5; Table 1).

The TDM ages for the granitoid rocks from the UL regionare broadly similar (2.82–3.26 Ga). Even though theTDM ex-tends to lower values in the granitoid rocks relative to theanorthosites, both groups have average values within0.13 Ga of each other (3.03 Ga for granitoid rocks, 3.16 Gafor anorthositic rocks). TheeNd (at 2050 Ma) values for thegranitoid rocks range from –5.14 to –9.70, whereasfSm/Ndvalues are from –0.34 to –0.50 (Fig. 5; Table 2). Initialstrontium ratios for the granitoid rocks range from 0.6901 to0.7094, however, the lower values (<0.700) may have beenreset by the Torngat Orogen-related metamorphism (as evi-denced by sericitic alteration of feldspars and hornblende re-placement of pyroxene), and if removed from the dataset, thevalues range from 0.7036 to 0.7095 (Fig. 5; Table 2)

Discussion

Traditionally, AMCG complexes in north-central Labradorhave been considered to be Mesoproterozoic (e.g., Emslie1978, 1980, 1985; Ryan 1990; Emslie et al. 1994). Recentwork, however, has also identified a PaleoproterozoicAMCG magmatic event (Emslie and Loveridge 1992; Ryanand Connelly 1996; Hamilton et al. 1998; Ryan et al. 1997,1998). Geological features of the anorthositic and granitoidrocks described in this paper and their proximity to datedPaleoproterozoic intrusions (e.g., Hamilton et al. 1998) indi-cate that the UL intrusive rocks are part of this Paleo-proterozoic suite, and Ryan et al. (1998) have explicitlymapped the UL intrusive rocks as Paleoproterozoic. Some ofthe diagnostic physical features used by Ryan et al. (1998)to define the intrusive rocks as Paleoproterozoic includegreenschist facies assemblages, faulting, and localized shear-ing and folding. Hence, the geochemical and isotopic char-acteristics as defined above for the UL intrusive rocksprovide new information on the origin of the whole Paleo-proterozoic AMCG suite.

Geochemical attributesThe high SiO2, K2O, Rb, Zr, and LREE, coupled with low

Al 2O3, CaO, TiO2, and Sr contents (e.g., Figs. 4d–4g; Ta-ble 1) of the Paleoproterozoic granitoid rocks are typical ofAMCG-related granitoid rocks (e.g., Emslie 1991; Emslieand Stirling 1993; Emslie et al. 1994, 1997). Similarly, theanorthosite geochemical character, with elevated Al2O3,CaO, TiO2, Sr, Eu, and Ba, and depletion in K2O, Rb, and Zrcontents (e.g., Figs. 4a–4c; Table 1), is also typical ofAMCG-related anorthositic and basic rocks (e.g., Xue andMorse 1993; Emslie et al. 1994, 1997). Various interpreta-tions have been proposed to explain the geochemical varia-tions between the granitoid and anorthositic suites, and mostrecent models for the NPS advocate a petrogenetic link be-tween the two. For instance, Emslie and Stirling (1993);Emslie and Hegner (1993), and Emslie et al. (1994) pro-posed that basaltic underplating of the lower continentalcrust by a mantle plume could induce crustal melting andgeneration of granitoid magmas. Such a melting episodewould result in a hot, pyroxene–plagioclase residue enrichedin Al, Ca, Ti, Sr, and Eu that, when assimilated by the ba-saltic plume, would place plagioclase on the liquidus for ex-tended periods of time, forming anorthositic magmas andultimately giving rise to the anorthositic rocks of AMCG

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Fig. 3. Chondrite-normalized REE plots for the intrusive bodies from the Umiakoviarusek Lake region, including (a) Goudie Lakepluton anorthosites, (b) Goudie Lake pluton leuconorites, (c) Pripet Marshes pluton leuconorites, (d) all Paleoproterozoic intrusives inUL area, (e) Owl River Valley pluton granitoid rocks, (f) quartz monzonite group granitoid rocks, (g) Illulik foliated granitoid rocks,and (h) felsic dykes. Chondrite values from Taylor and McLennan (1985).

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Fig. 4. Primitive mantle normalized multielement plots for the intrusive bodies from the Umiakoviarusek Lake region, including(a) Goudie Lake pluton anorthosites, (b) Goudie Lake pluton leuconorites, (c) Pripet Marshes pluton leuconorites, (d) Owl River Valleypluton granitoid rocks, (e) QMG granitoid rocks, (f) Illulik foliated granitoid rocks, and (g) felsic dykes. Primitive mantle values fromHoffman (1988).

suites (Emslie and Stirling 1993; Emslie and Hegner 1993;Emslie et al. 1994; Xue and Morse 1993).

Alternatively, the granitic (or jotunite–charnockite) phasesof AMCGs have been modelled as fractionates from a moreprimitive jotunite suite (e.g., Auwera et al. 1998). Duchesneand Wilmart (1997) indicated that mixing of a fractionatingolivine-bearing residual magma with replenishing magmas,coupled with extensive crustal contamination of jotunite–charnockite phases, was involved in the generation ofAMCG granitoid rocks.

To summarize the geochemical features, the data providedfor the Paleoproterozoic UL anorthositic and granitoid rocksare consistent with current models for AMCG genesis. Ourisotopic data were generated with a view to determining ifcontamination of the magmatic suite was also important inthe generation of the Paleoproterozoic AMCG suite and in-deed if the potential contaminant could be identified.

Crust–mantle interaction: inferences from isotopegeochemistry

The distribution of isotopic data for the UL intrusiverocks is very similar to that in younger MesoproterozoicAMCG suites (e.g., NPS, Emslie et al. 1994; Laramie, Geistet al. 1989, 1990; Scoates and Frost 1996), with horizontal

and vertical trends ineNd–ISr space for the granitoid rocksand anorthositic rocks, respectively (Fig. 5). The data formost of the granitoid rocks of the UL region (Fig. 5) havelow ISr values when compared to signatures in the youngerNPS granitoid rocks at their time of formation. For instance,with the exception of the QMG, all of the granitoid rockshave less radiogenicISr values than the NPS granitoid rocks(Fig. 5). In contrast, the QMG granitoid rocks have moreevolved signatures, but still have a strong juvenile compo-nent when compared to the range of possible crustal contam-inants (e.g., Fig. 6a). The Nd isotopic data mimic the Srisotopic data and are consistent with an Archean crustalcomponent (e.g.,TDM ages, loweNd), but a less radiogeniccomponent must also be present given the highereNd values(Fig. 5; Table 1).

Based on Nd isotopic data, Emslie et al. (1994) showedthat even with up to 90% crustal influence, granitoid rocksfrom the NPS needed a juvenile mantle component to ex-plain their observed Nd isotopic signatures. The Paleo-proterozoic Wheeler Mountain granites, which are probablycorrelative to the UL intrusive rocks, also have a significantdocumented juvenile component, and akin to the NPS, thecomponent was likely derived from a basaltic underplate(plume) that drove crustal anatexis and granitoid genesis(Emslie and Loveridge 1992). We hypothesize that the juve-

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Fig. 5. Epsilon Nd vs.ISr (at t = 2050 Ma) for the Umiakoviarusek Lake granitiod rocks and anorthositic rocks in comparison to otheranorthositic and granitoid rocks from north-central Labrador. KI, Kiglapait intrusion (DePaolo 1985); AW, NPS anorthositic rocks westof Nain–Churchill Province suture; GW, NPS granitoid rocks west of Nain–Churchill Province suture; AE, NPS anorthositic rocks eastof Nain–Churchill Province suture; GE, NPS granitoid rocks east of Nain–Churchill Province suture (Emslie et al. 1994); WM,Paleoproterozoic Wheeler Mountain granite (Emslie and Loveridge 1992). All of the Mesoproterozoic units (KI, AE, AW, GE, GW)are calculated at 1300 Ma, all Paleoproterozoic units (this study, WM) are calculated at 2050 Ma. The granitoid and anorthositic rocksfrom the UL intrusions have distinctive horizontal and vertical trends similar to rocks from other anorthositic–granitoid complexes(e.g., Geist et al. 1990; Emslie et al. 1994). Granitoid rocks from the UL region have very radiogenic signatures, akin to thetemporally equivalent (?) Wheeler Mountain granites, suggesting a possible mantle component in their genesis. Other details arediscussed in the text.

nile isotopic component, and possibly HFSE characteristicsof the granitoid rocks, may originate from variable mantlecontributions from an underplating mantle plume, similar tothe model proposed by Emslie et al. (1994).

The evidence for crustal contamination in AMCG suites iswell documented (e.g., Ashwal and Wooden 1983, 1985;Ashwal et al. 1986; Auwera et al. 1998; DePaolo 1985;Duchesne and Wilmart 1997; Geist et al. 1989, 1990; Kolkeret al. 1991; Emslie and Loveridge 1992; Emslie et al. 1994;Scoates and Frost 1996) and is also supported by recent ex-perimental work on anorthosite genesis (Longhi et al. 1999).The crustal component in the UL intrusive rocks can be hy-pothesized, based on their Paleoproterozoic age and intrusiverelationships, as part of the Nain Province, pointing to thelatter as potential crustal contributor. To test this hypothesis,mixing lines (Langmuir et al. 1978) were calculated usingvalues for middle and early Archean gneisses from the OkakBay (Schiøtte et al. 1993; Fig. 1) and Saglek Bay areas(Collerson et al. 1989), respectively (approximately 100 kmnorth of Okak Bay on Fig. 1). These crustal end-memberswere mixed with a mantle end-member akin to Mitchell etal.’s (1995) high-alumina gabbro (eNd = 0.7, ISr = 0.70360,Sr = 443 ppm), a possible parent to AMCG suites; the re-sults of these calculations are presented in Fig. 6a. In Fig. 6athe results indicate that ineNd–ISr space, mixing lines forboth gneiss suites partially envelope the UL intrusive rocks,but do not form direct mixing lines to either crustal contami-nant. Changing the initial mantle componenteNd–ISr charac-teristics will not drastically change the shapes of thesecurves. For instance, a starting material at 2050 Ma withchondritic eNd = 0 and ISr = 0.7021 and elevated Sr =300 ppm (e.g., Emslie and Loveridge 1992) would not sig-nificantly change the results in Fig. 6a. In contrast, by keep-

ing all things constant and only reducing the lower endISr(at 2050 Ma) for the early Archean gneiss mixing line to0.7040 would encompass the entire dataset. However, such alow value for the Nain Province gneisses seems unrealisticgiven their Archean age, but may be possible given the iso-topic heterogeneity exhibited by the Nain Province gneisses(e.g., Fig. 6a; Schiøtte et al. 1990, 1993). An added com-plexity is the possibility that the Sr isotopic system has notremained closed in the UL intrusive rocks (samples P96-85and P96-75B show evidence for isotopic resetting (Ta-ble 2)).

To remedy the possibility of Sr isotopic resetting, the datafor the Nain Province gneisses and the UL intrusive rockswere plotted ineNd–fSm/Nd space (Fig. 6b). In this diagram,the samples are proximal to, and slightly overlap with, theNd isotopic signatures of the Nain Province gneisses; how-ever, delineation of the specific age of the gneisses, be thatmiddle or early Archean, is not possible.TDM ages for theUL intrusive rocks also exhibit characteristics consistentwith either age group as a potential contaminant (Table 2).In an attempt to quantify the possible contributions from thesetwo crustal sources, we have employed neodymium crustal in-dex (NCI) calculations, whereby the NCI = (eNd (rock) –eNd(mantle component))/(eNd (crustal component) –eNd (mantlecomponent)) (DePaolo et al. 1992). The results of the NCIcalculations for the crustal end-members and potential man-tle contributors are presented in Table 3. In all of these cal-culations, there is evidence for a significant crustalcontribution given any starting mantle end-member (Ta-ble 2). By using CHUR, depleted mantle (DM at 2050 Ma),high-alumina gabbroic Nd-isotopic composition (e.g., Mitch-ell et al. 1995) and the Kiglapait intrusion (KIG) (e.g.,DePaolo 1985; Emslie et al. 1994), mantle end-members

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Sample Pluton

143

144

NdNd

147

144

SmNd eNdT1 eNd fSm/Nd TDM

87

86

SrSr

87

86

RbSr ISr

aRbb Srb Ndb Smb

P96-3B Dykes 0.511437 0.12865 –5.57 –23.43 –0.35 3064 0.72849 ND NA ND 306 37.5 7.82P96-28B Dykes 0.511103 0.10860 –6.81 –29.94 –0.45 2960 0.72256 0.5661 0.70583 51.2 256 35.6 6.26P96-39 ORVP 0.511293 0.11638 –5.14 –26.24 –0.41 2900 0.73798 1.1650 0.70357 82.6 201 66.6 12.6P96-66A ORVP 0.511250 0.11813 –6.45 –27.08 –0.40 3023 0.71892 0.4854 0.70458 96.4 562 23.8 4.55P96-85 ORVP 0.511341 0.12883 –7.50 –25.30 –0.34 3245 0.75977 2.0930 0.69795 214 291 66.4 13.9P96-86B ORVP 0.511191 0.11280 –6.20 –28.23 –0.43 2951 0.74411 1.3289 0.70485 136 290 79.5 14.5P96-75B QMG 0.511013 0.09858 –5.92 –31.70 –0.50 2819 0.71684 0.9069 0.69005 101 316 71.3 11.4P96-76 QMG 0.511055 0.11596 –9.70 –30.88 –0.41 3261 0.74370 1.1625 0.70936 101 244 57.9 10.8P96-87 QMG 0.511002 0.10735 –8.46 –31.91 –0.45 3070 0.77113 2.0869 0.70948 145 197 54.4 9.46P96-3A GLP 0.511252 0.13834 –11.76 –27.04 –0.30 3844 0.70629 0.0076 0.70606 1.78 660 1.25 0.28P96-19 GLP 0.510634 0.06365 –4.11 –39.09 –0.68 2544 0.70869 0.0162 0.70821 4.75 818 2.35 0.24P96-33 GLP 0.510364 0.07297 –11.87 –44.36 –0.63 3006 0.70512 0.0106 0.70481 2.77 738 1.44 0.17P96-44A GLP 0.510650 0.10931 –15.88 –38.78 –0.44 3644 0.70641 0.0282 0.70558 7.51 753 3.55 0.63P96-55B GLP 0.511018 0.11307 –9.66 –31.60 –0.42 3222 0.70567 0.0222 0.70501 7.75 987 3.16 0.58P96-83 GLP 0.511260 0.11649 –5.82 –26.88 –0.41 2955 0.70759 0.0511 0.70608 9.85 545 4.76 0.90P96-174 GLP 0.511350 0.12572 –6.50 –25.12 –0.36 3113 0.70616 0.0134 0.70577 3.14 655 6.39 1.30P96-182 GLP 0.510949 0.09530 –6.31 –32.95 –0.52 2823 0.70618 0.0301 0.70529 8.75 822 8.33 1.29P96-199 PMP 0.511029 0.11059 –8.79 –31.39 –0.44 3127 0.70767 0.0612 0.70587 14.2 655 36.2 6.48P96-203 PMP 0.511157 0.12198 –9.29 –28.89 –0.38 3307 0.70584 0.0038 0.70572 0.99 738 16.6 3.29

Notes: ND, not determined; NA, not applicable.aCalculated at 2050 Ma.bgiven in ppm.

Table 2. Neodymium and Strontium isotope data for the Umiakoviarusek Lake intrusions.

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Fig. 6. (a) eNd–ISr data for the Umiakoviarusek Lake intrusive rocks and potential basement contributors. Fields for the middleArchean and early Archean Nain Province gneisses are from Schiøtte et al. (1993) and Collerson et al. (1989), respectively. Mixinglines were calculated using the equations of Langmuir et al. (1989). Subdivisions on each curve represent 10% mixing of thecomponents. HALG, isotopic composition of high-alumina gabbro from the Laramie Complex, Wyoming (Mitchell et al. 1995).(b) eNd–fSm/Nd for the UL relative to possible middle (Schiøtte et al. 1993) and early Archean (Collerson et al. 1989) Nain Provincegneisses. CHUR, chondrite uniform reservoir. Details are provided in the text.

yield NCI averages requiring 26–39% and 33–61% Nd inputfor early Archean (Collerson et al. 1989) and middleArchean (Schiøtte et al. 1993) Nain Province gneiss crustalend-members, respectively (Table 3). Similarly, theanorthositic rocks, using the same end-members, yield NCIaverages requiring 41–62% and 53–72% crustal Nd input forearly Archean (Collerson et al. 1989) and middle Archean(Schiøtte et al. 1993) Nain Province gneiss crustal end-members, respectively (Table 3). In both cases, it is permis-sive that these end-members were contamination sourcesduring their genesis. However, it should be noted that somevalues using the mantle end-members and crustal end-members yield unrealistic degrees of Nd contamination us-ing both of these end-members. For instance, samples P96-3A, 96-33 and 96-44A would require unrealistic Nd input, insome cases greater that 100% crustal Nd input, a thermaland petrological impossibility (Table 3).

Given this problem, it can be concluded that the NainProvince gneisses could have been crustal contaminators,and, given their heterogeneity (e.g., Collerson et al. 1989;Schiøtte et al. 1990, 1993), it is possible that the isotopicdata used for the end-members are not fully representative,thus throwing the calculations off. Alternatively, there maybe other crustal contaminants not readily visible on the sur-face in this region of Labrador. Recent work at Voisey’s Bayhas shown that many of the intrusions in this region are con-taminated by Archean crustal material that is similar to theNain Province, but is U-depleted (Amelin et al. 1998). It ispossible that such a lower crustal block may lie beneath theUL area, and given the requirement for Archean source ma-

terials (e.g.,TDM ages) involved in UL intrusion genesis, thistype of contaminant cannot be ruled out as a contributor tothis intrusion.

In summary, the geochemistry of the UL intrusive rocks isconsistent with the influence from Archean crustal materials,but given the current database for potential Nain Provincegneisses, a specific contaminant cannot be fingerprinted. It ispossible that this region is underlain by other Archeansource materials. There is also a strong mantle component inmost of the UL intrusive rocks.

Conclusions

(1) Anorthositic rocks from the Umiakoviarusek Lake re-gion have lower Ba, Rb, K, K/Ti, Zr, REE, and LREE con-tents, but higher Sr and Eu relative to the granitoid rocks inthis region. Both groups contain low Nb and Th contents,and somewhat similar Ti contents. The REE contents of theanorthositic rocks appear to be controlled by plagioclase :orthopyroxene ratios, with decreases in both the total REEand the size of the Eu anomaly with increasing ortho-pyroxene relative to plagioclase from the Goudie Lakepluton anorthosites and leuconorites (with 15–25% ortho-pyroxene) through Pripet Marshes pluton leuconorites (with25–35% orthopyroxene).

(2) Strontium and Nd isotopic data for anorthositic andgranitoid rocks of the Umiakoviarusek Lake region are con-sistent with crustal contamination. Initial Sr ratios at2050 Ma for the granitoid rocks (0.6901–0.7094, average =0.7032) are consistent with crustal influence, but the lower

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Early Archean gneissesa Middle Archean gneissesa

Sample Group eNdT CHUR DM HALG KIG CHUR DM HALG KIG

P96-3B Dyke –5.57 0.31 0.47 0.34 0.17 0.38 0.55 0.41 0.22P96-28B Dyke –6.81 0.38 0.53 0.41 0.26 0.47 0.61 0.49 0.33P96-39 ORVP –5.14 0.29 0.45 0.32 0.14 0.35 0.53 0.38 0.19P96-66A ORVP –6.45 0.36 0.51 0.39 0.23 0.44 0.59 0.47 0.30P96-85 ORVP –7.50 0.42 0.56 0.44 0.30 0.52 0.65 0.54 0.39P96-86B ORVP –6.20 0.35 0.50 0.37 0.22 0.43 0.58 0.45 0.28P96-75B QMG –5.92 0.33 0.49 0.36 0.20 0.41 0.57 0.43 0.25P96-76 QMG –9.70 0.54 0.65 0.56 0.45 0.67 0.76 0.68 0.58P96-87 QMG –8.46 0.48 0.60 0.50 0.37 0.58 0.69 0.60 0.47Average –6.86 0.39 0.53 0.41 0.26 0.47 0.61 0.50 0.33P96-3A GLP-An –11.76 0.66 0.74 0.67 0.59 0.81 0.86 0.82 0.76P96-19 GLP-An –4.11 0.23 0.41 0.26 0.08 0.28 0.48 0.32 0.10P96-33 GLP-An –11.87 0.67 0.74 0.68 0.60 0.82 0.87 0.82 0.77P96-44A GLP-Ln –15.88 0.89 0.92 0.90 0.87 1.09 1.07 1.09 1.12P96-55B GLP-Ln –9.66 0.54 0.65 0.56 0.45 0.66 0.75 0.68 0.58P96-83 GLP-Ln –5.82 0.33 0.48 0.35 0.19 0.40 0.56 0.43 0.24P96-174 GLP-Ln –6.50 0.37 0.51 0.39 0.24 0.45 0.60 0.47 0.30P96-182 GLP-Ln –6.31 0.35 0.50 0.38 0.22 0.43 0.59 0.46 0.29P96-199 PMP –8.79 0.49 0.61 0.51 0.39 0.60 0.71 0.62 0.50P96-203 PMP –9.29 0.52 0.63 0.54 0.43 0.64 0.74 0.66 0.66Average –9.00 0.51 0.62 0.52 0.41 0.62 0.72 0.64 0.53

Notes: CHUR, chondrite uniform reservoir (eNdT = 0); DM, depleted mantle (eNdT = 5.38); HALG, high-alumina gabbro (eNdT = 0.7); KIG, Kiglapaitintrusion (eNdT = –3.0). An, Anorthosite; Ln, Leuconorite. Dyke, felsic dykes; ORVP, Owl River Valley pluton; QMG, Quartz monzonite group; GLP,Goudie Lake pluton; PMP, Pripet Marshes pluton..

aMiddle Archean Nain Province gneisses (eNdT = –14.55); early Archean Nain Province gneisses (eNdT = –17.8).

Table 3. Neodymium crustal index calculations for the Umiakoviarusek Lake intrusions using different mantle end-members andvarious middle Archean (Schiøtte et al. 1993) and early Archean (Collerson et al. 1989) Nain Province gneisses.

values may be the result of isotopic resetting associated withTorngat Orogen-related greenschist facies metamorphism. Incontrast, the anorthositic rocks retained crustally influencedhigh initial Sr ratios at 2050 Ma (0.7048–0.7082, average =0.7058). The highISr values imply a source with significantcrustal residence time.

(3) Epsilon Nd values (eNd (at 2050 Ma) in theanorthosites (–4.11 to –15.88) and granitoid rocks (–5.14 to–9.70); fractionation factors (fSm/Nd, anorthosite = –0.30 to−0.68, granitoid = –0.34 to –0.50); and depleted mantlemodel ages (TDM, anorthosite = 2.54–3.80 Ga, granitoid2.82–3.26 Ga) are all consistent with the Sr isotope data andrequire a LREE-enriched source with significant crustal resi-dence time.

(4) The similarities in radiogenic isotope systematics withthe younger Nain Plutonic Suite and other AMCG com-plexes, suggest that the Paleoproterozoic UL intrusive rockslikely formed in a similar manner to other MesoproterozoicAMCG complexes.

Acknowledgments

Numerous people have made significant contributions tothe completion of this paper. We would like to thank NelsonBaker, Don Sheldon, Eugene Buekman, Terry Ryan, andJohn O’Sullivan of Castle Rock Exploration Corporation forlogistical and financial support. Jeff Morgan and Mike Reesprovided capable field assistance. Lakmali Hewa, Pam King,and Mike Tubrett assisted in sample preparation and (or)analyses of trace and rare earth elements; Pat Horan pre-pared samples and collected the Nd–Sr isotope data. CoreyFitzgerald and Ken Smith assisted in sample cutting, crush-ing, and preparation of samples for XRF analyses. Discus-sions with George Jenner, Andy Kerr, Bruce Ryan, andMark Wilson were invaluable. Richard Cox reviewed an ear-lier draft of this manuscript, and detailed reviews by LouiseCorriveau, Jonathon Patchett, and Carol Frost led to a sub-stantial improvement in the paper. This contribution repre-sents part of the senior author’s M.Sc. thesis undertaken atthe Department of Earth Sciences, Memorial University ofNewfoundland. Funding for this project has been providedby a Natural Sciences and Engineering Research Council ofCanada (NSERC) operating grant and a grant from CastleRock Exploration Corporation to D.H.C.W.; while S.J.P. ac-knowledges the kind support of an NSERC Post-GraduateStudent award and the Buchans’ Scholarship of ASARCOIncorporated (MUN).

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