The voluminous Proterozoic continental tholeiites of the Coppermine River province (Northwest Territories), which are coeval with the Mackenzie magmatic ...
Proterozoic flood basalts from the Coppermine River area, Northwest Territories: isotope and trace element geochemistry' C. DUPUY Centre gdologique et gdophysique, Centre national de la recherche scientijique, Universitd Montpellier II, Place E. Bataillon, 34095 Montpellier CEDEX, France
A. MICHARD Centre de recherches pdtrographiques et g6otechniques. Centre national de la recherche scient~)5que,B. P. 20, 54501 Vandoeuvre-les-Nancy, France
J. DOSTAL Department of Geology, Saint Mary's University, Halifax, N.S., Canada B3H 3C3
D. DAUTEL Centre de recherches pHrographiques et gdotechniques, Centre national de la recherche scientljique, B.P. 20, 54501 Vandoeuvre-les-Nancy, France AND
W. R. A. BARAGAR Geological Survey of Canada, 601 Booth Street, Ottawa, Ont., Canada KIA OE8 Received December 13, 1991 Revision accepted May 8, 1992 The voluminous Proterozoic continental tholeiites of the Coppermine River province (Northwest Territories), which are coeval with the Mackenzie magmatic event (1.27 Ga old) and were emplaced over a short period of time ( < 5 Ma), have trace element and Sr-Nd-Pb isotopic characteristics comparable to those of Phanerozoic flood basalts. The variations of the compositional parameters are inferred to be due to a mixing of at least two components: mantle and crust. In addition, the mantle component probably includes two end members. The first is a mantle plume, whereas the other represents the base of the continental lithosphere. The crustal component reflects contamination of the magma by Precambrian basement during its ascent to the surface. Les tholkiites continentales d'bge protCrozoique abondantes dans la province de la rivibre Coppermine (Territoires du NordOuest), lesquelles sont contemporaines de I'tvknement magmatique (il y a 1,27 Ga) et furent mises en place durant une courte ptriode de temps ( < 5 Ma), ont les signatures d'C1Cments en traces et d'isotopes de Sr-Nd-Pb sont comparables celles des effusions de basaltes phanCrozoiques. Les variations des paramktres de la composition chimique sont interprCtCes comme le rtsultat d'un mtlange d'au moins deux composants : le manteau et la croiite. En outre, le composant mantellique inclut probablement deux p8les extrCmes. Le premier correspondrait au panache du manteau, tandis que le deuxikme reprtsenterait la base de la lithosphkre continentale. Le composant crustal reflkte une contamination du magma par le socle prCcambrien durant son ascension vers la surface. [Traduit par la rCdaction] Can. J. Earth SCI.29, 1937- 1943 (1992)
Introduction Geochemical studies of Mesozoic and Cenozoic continental flood basalts have provided many insights into the composition and evolution of the upper mantle. Variations of incompatible trace element and radiogenic isotopic data in continental tholeiites suggest that several source components, including subcontinental lithosphere, asthenosphere, and both upper and lower crust, played a role during their genesis (e.g., Carlson et al. 1981; Hart 1985). However, there is significantly less information on the older flood basalts, particularly those of Precambrian age. Thus, we have investigated Proterozoic basalts related to the Mackenzie igneous event from the northwestern Canadian Shield. Using trace elements and Sr, Nd, and Pb isotopes, this paper attempts to (i) geochemically characterize the Coppermine River flood basalts (about 1270 Ma old) from the Northwest Territories, Canada, and (ii) constrain the petrogenesis of these rocks, particularly the coinposition of their mantle sources. 'Geological Survey of Canada Contribution 41991. Prlntcd In Canada I lmprlrne au Canada
Geological setting Plateau basalts of the Coppermine River Group (Baragar 1969, 1972; Baragar and Donaldson 1973) are part of a gently north-dipping Helikian succession that composes the Coppermine homocline (Kearns et al. 1981) in the northwestern Canadian Shield (Fig. 1). At the base of the homocline is the quartzitic Hornby Bay Group, which rests unconformably on the eroded surface of the Wopmay Orogen (2.2 - 1.8 Ga) and is dated, from a minor volcanic component in its upper levels, at 1663+8 Ma (Bowring and Ross 1985). It is succeeded by the Dismal Lakes Group, which is typically about 1100 m thick and grades upward from quartzites through shales to dolomites, generally recording subsidence and a marine transgression. However, in the upper part of the sequence is a widespread disconformity marking an interval of uplift and karstification followed by shallow submergence just prior to the eruption of the Coppermine River flows (Kearns et al. 1981). The Coppermine River Group comprises a 2000 - 3500 m thick succession of predominantly subaerially erupted basalts interfingering upward into a sequence of fluvial red sandstones and minor intercalated flows at least 1200 m thick. It is overlain
CAN. 1. EARTH SCI. VOL, 29, 1992
70'
MACKENZIE INTRUSIONS
ARCHEAN
a
EARLY PROTEROZO~C
(ILJ COPPERMINE RIVER
DISMAL LAKES GROUP HORNBY BAY GROUP
RAE GROUP
BASALT PLATFORM COVER
FIG. 1. Generalized map of the northwestern Canadian Shield (after Hoffman (1989) and Hildebrand and Baragar (1991)), showing the distribution of the Middle-Late Proterozoic sequences.
unconformably by the Rae Group of Hadrynian age. The Coppermine River basalts are part of the very extensive Mackenzie igneous event (Fahrig et al. 1971), which includes the Muskox layered intrusion and the Mackenzie diabase dykes (Fahrig 1987; Gibson et al. 1987), one of the world's largest continental dyke swarms. The latter extends outward from the locus of the Coppennine River lavas for about 2400 km and radiates through an arc that is at least 1800 km wide at its farthest point. All components of the Mackenzie event were emplaced within an interval of 5 Ma maximum at about 1270 Ma (Pb-U on baddeleyite; LeCheminant and Heaman 1989). The Coppermine River flood basalts crop out continuously over a lateral distance of about 250 km and emerge again from beneath Hadrynian cover at Bathurst Inlet, another 200 km farther east. They are also encountered in drill holes through Paleozoic and Late Proterozoic cover some 250 km to the west (Sevigny et al. 1991) and thus are known to occur over a lateral distance of at least 700 krn. Petrography The petrography of the basalts was described by Dostal et al. (1983). The basalts comprise about 150 lava flows with thicknesses ranging from 4 to 300 m; the flows, some of which can be traced for more than 16 km along the strike, are massive with amygdaloidal tops. The basalts are commonly porphyritic with ubiquitous phenocrysts of augite. Altered olivine
and pseudomorphs of orthopyroxene occur as rare phenocrysts in the lower part of the sequence, whereas phenocrysts of plagioclase are present in the upper part. In addition to altered glass, the groundmass is also composed of augite, plagioclase, and Fe -Ti oxide. The amount of Fe -Ti oxide increases from the bottom to the top of the sequence. Some lavas and sills contain granophyric patches, probably as a result of crustal contamination (Dostal et al. 1983). The Coppermine River basalts were affected by subgreenschist-grade metamorphism. Clinopyroxene and Fe-Ti oxide are fresh, but plagioclase was partially converted to saussurite or clay. Olivine is invariably replaced by chlorite and (or) smectite ( f Fe -Ti oxide). Samples, analytical techniques, and alteration Sr, Nd, and Pb isotopic ratios were determined in eight basalt samples chosen from a collection studied by Baragar (1969) and Dostal et al. (1983). The samples were selected for their relative freshness and are representative of the range of trace elements observed in the basalt suite. The procedure for the analyses of Sr isotopes has been given by Alibert et al. (1983); the Nd and Pb isotopic ratios were determined according to Michard et al. (1986). The 87Sr/86Srand 143Nd/144Nd values were normalized to 86Sr/88Sr = 0.1194 and 146Nd/ lUNd = 0.7219, respectively. Lead isotopic compositions were corrected by 0.1% per amu, a value obtained from
DUPUY ET AL
TABLE1. Selected major and trace element abundances in the Coppermine River basalts Sampleno.
SiO,
K20
Ce
La
Sm
Eu
Tb
Yb
Lu
Li
Ba
Zr
Hf
Nb
Th
[Mg]
2.78 3.08 1.46 1.23 1.77 3.07 2.64 1.15
1.52 1.53 0.83 0.64 0.83 1.35 1.20 0.70
4.01 4.45 2.67 2.07 2.08 4.10 3.50 2.47
0.62 0.69 0.44 0.32 0.29 0.64 0.55 0.41
38 12 11 22 38 17 29 20
837 766 157 209 542 278 643 137
299 301 124 113 171 290 248 89
7.6 7.3 3.1 2.6 4.2 7.0 6.1 2.0
21 27 10 10 14 27 26 8
7.1 3.1 1.1 1.0 4.2 3.3 4.3 0.70
0.32 0.41 0.47 0.52 0.62 0.37 0.43 0.58
-
9-7 18 69-818 69-829 3-66 24-66 42-66 115-66 157-66
58.0 48.4 50.1 49.9 53.7 48.3 52.2 51.0
3.5 1.3 0.4 0.7 1.6 0.8 1.7 0.3
34.1 26.9 10.3 9.2 22.3 27.1 31.6 7.4
78.8 65.2 24.7 21.9 51.6 64.6 72.0 17.2
9.76 9.61 4.09 3.67 5.71 9.08 8.29 3.11
NOTES:Si02 and K 2 0 in wt.%; trace elements in ppm. [Mg] = Mg/(Mg + Fe2+) with F e 3 + / ~ e 2 = + 0.15. Sample locations: Copper Creek Formation. (i) lava 24-66, 140 m (stratigraphic level); 67"14'08"N, 116"21'06"W; 43.2 km W of the northernmost tip of the Muskox intrusion (NMI), 65.7 km SW of estuary of Coppermine River into Coronation Gulf (ECR); (ii) lava 3-66, 1080 m; 67"15'07"N, 116" 12'00"W; 37.8 km W of NMI, 61.2 km SW of ECR; (iii) lava 42-66, 2090 m; 67"18'06"N, 116"04'08"W; 34.2 km W of NMI, 60.9 km SW of ECR; (iv) lava 115-66, 2730 m; 67"21101"N, 116"03'02"W; 35.1 km NW of NMI, 51.3 km SW of ECR; Husky Creek Formation: (v) lava 157-66, 3810 m; 67"32'00"N, 116"08'00"W; 44.1 km NW of NMI, 42.3 km SW of ECR; (vi) Dismal Lakes sill 9-718, 67"12'08"N, 116"30'04"W; 47.7 km W of NMI, 70.2 km SW of ECR; (vii) dyke 69-818, 67"08'OI "N, 115"46'04"W; 22.5 krn W of NMI, 61.2 km S of ECR; (viii) 69-829, 67"09'06"N, 115"40'00"W; 21.6 km W of NMI, 58.5 km S of ECR.
TABLE2. Sr, Nd, and P b isotopic compositions o f the Coppermine River basalts Sample no
Rb Sr (PP~)(PP~)
9-718 69-818 69-829 3-66 24-66 42-66 115-66 157-66
85 28 7.0 15 36 20 37 3.7
215 278 171 173 206 216 332 125
NOTE:Corrected eLd and
87Sr 86Sr
fcT
0.724907f31 0.709664+29 0.705773+32 0.707856f28 0.715903+27 0.707767k33 0.711202540 0.704222+26
13.1 16.6 5.9 1.3 49.9 -4.3 30.4 -7.6
143Nd -
Sm Nd (PP~)(PP~)
9.61 9.47 3.84 3.50 5.67 8.88 7.89 2.77
41.60 39.30 14.25 13.06 25.60 36.40 36.40 10.11
144Nd
TDM eLd
0.512189f27 -0.24 0.512392+34 3.22 3.03 0.512527+21 0.512574k27 4.10 0.511882+38 -4.82 0.512446+_30 3.98 0.512152k26 0.93 0.512577+_35 3.56
U
- zo6pb - 204Pb
zo7pb
208pb
(Ga)
(ppm)
P b (ppm)
1.73 1.47 1.48 1.38 2.17 1.39 1.63 1.54
1.70 0.812 0.322 0.257 0.880 0.878 0.929 0.168
12.03 18.573 15.659 38.268 4.87 19.043 15.623 38.690 1.53 19.203 15.605 39.180 2.27 19.174 15.729 39.085 8.584 18.449 15.677 38.516 5.820 19.097 15.643 39.067 6.707 18.624 15.615 38.853 0.840 19.151 15.583 39.000
2 0 4 ~ b 204Pb
eir age is 1270 Ma.
repeated analyses of NBS 981. The analyses of U and Pb were done by an isotope dilution technique (Michard et al. 1986). The other trace elements were reanalyzed; the atomic absorption technique was used for the determination of Li, X-ray fluorescence for Rb, Sr, Ba, and Nb, and instrumental neutron activation for the analyses of rare-earth elements (REE), Th, and Hf. The analytical error of these trace element data is smaller than 10%.Selected major and trace elements are reported in Table 1, while Table 2 gives isotopic and related data. Dostal et al. (1983) argued that the secondary compositional changes of the basalts mainly affected very mobile elements such as Li, which correlates with H20. The other alkalies and U display only a rough positive correlation with high-fieldstrength elements (HFSE), suggesting that their primary magmatic distribution has also been modified. However, the other analyzed elements were not significantly affected by secondary processes.
Major and trace element geochemistry The geochemistry of major and trace elements in the basalts was described by Baragar (1969) and Dostal et al. (1983). The basalts are quartz normative tholeiites compositionally similar to many Phanerozoic flood basalts. Their span of Mg slumbers (= Mg/Mg + Fe2+ with Fe3+/Fe2+ = 0.15) is large, ranging from 0.62 to 0.32. The distinct variations in the major and compatible trace elements that accompany the changes of Mg number and correlate with the stratigraphic level have been related to fractionation in a high-level magma chamber (Dostal
et al. 1983). The changes include an increase of Fe and Ti and a decrease of Mg number, Mg, Cr, Ni, and K towards the top of the stratigraphic sequence and can be correlated with the petrography (Baragar 1969; Dostal et al. 1983). The variations of immobile incompatible trace elements such as HFSE and REE show overall increase with increasing degree of differentiation. However, the fractional crystallization cannot explain all the observed trace element trends including the shapes of the REE and incompatible trace element patterns in the rocks with similar major and transition elements. Dostal et al. (1983) have argued that these differences are also difficult to reconcile with a variable degree of melting of a homogeneous source. The ratios of the incompatible trace elements (Table 3) vary over a large range and generally display a correlation when plotted against each other (Fig. 2). The variation of these element ratios may be the result of the mixing of two components with contrasting composition. On the Nb/Th versus Th/La plot (Fig. 2), the basalts that have incompatible trace element ratios typical of the upper mantle are close to the end-member component with higher Nb/Th and lower Th/La values. Their mantle-normalized trace element pattern is relatively flat (Fig. 3, sample 69-829) and resembles that of E-type midocean-ridge basalts (MORBs) (Sun et al. 1979). The basalts similar to the second end member with lower Nb/Th and higher Th/La (Fig. 2) have several incompatible element ratios resembling those of the crust (Table 3). Their mantlenormalized patterns display an irregular zigzag shape between Rb and Pb, a distinct enrichment in K, Rb, Ba, and Th, and
CAN. J. EARTH SCI. VOL.
TABLE 3. Selected incompatible trace element ratios of the Coppermine River basalts Sample no.
Th
Nb -
La
Nb La
Th
Ba Nb
Ce Pb
Nb U
3-66 42-66 69-829 69-818 157-66 9-718 115-66 24-66
0.11 0.12 0.11 0.12 0.09 0.21 0.14 0.19
1.1 1.0 1.0 1.0 1.1 0.6 0.8 0.6
10 8 9 9 11 3 6 3
21 10 16 28 17 40 25 39
10 11 16 13 20 7 11 6
39 31 31 33 48 12 28 16
Mantle Crust
0.13 0.22
1.0 0.7
8 3
10 22
9
30 12
4
29. 1992 100:
'
'
' '
-
'
b)
I Y
$
50-
+
0
>
%
. .%
2
-
10:
-
5; BaRbThKNb
LaCePbSrNd
SmZrHf
Tb
Yb
FIG. 3. Primitive mantle-normalized trace element abundance patterns for two Coppermine River basalts. Normalizing values after Hofmann (1988).
NOTES:Mantle and crustal values are from Hofmann (1988). The samples are arranged according the decrease of
&.
FIG.4. Initial (age-corrected to 1.27 Ga) Sr-Nd isotope variations for Coppermine River basalts. FIG.2. Variations of Nb/Th versus Th/La in the Coppermine River basalts. Also shown is the mixing line between basalt 157-66 and average crust given by Hofmann (1988).
206Pb/204Pbmay indicate that these three samples (Fig. 5) contain a component derived either from an ancient crustal source enriched in U, or from a source with at least a twoa negative anomaly of Nb (basalt 24-66 in Fig. 3). The latter stage U-Pb history (an early stage with high p followed by pattern is similar to that found in many continental tholeiites a low p stage). (e.g., Thompson et al. 1982), and in the average continental In the basalts, the Sr-Nd-Pb isotopic systems (except crust. Figure 2 shows that the mixing process can explain the 207Pb/204Pb) are distinctly interrelated; the 143Nd/144Nd values Nb/Th versus ThILa variations in the basalts and suggests, in are positively correlated with 208Pb/206Pb(Fig. 6a), while the agreement with Dostal et al. (1983). that a crustal component 87Sr/86Srvalues are negatively correlated with 206Pb/204Pb played a role in their genesis. and with 208Pb/204Pb.The three isotopic systems are also correlated with Si02 (Fig. 6b) and with several trace element Isotopic data ratios (e.g., Fig. 6c). The Coppermine River basalts display two trends on the Nd and Sr isotopic ratios (corrected age, 1.27 Ga) in the 143Nd/144Nd versus 147Sm/144Nd plot (Fig. 7). The first trend basalts (Table 2) show a large range, as they do in several conti(line I), which includes the four basalts that have high Nb/Th nental flood basalt provinces (e.g., Menzies 1989). A majority and low Th/La values and that are probably devoid of crustal of the basalts have positive values of C N ~ .implying derivation component, has a slope that yields an isochron of 1.2 f from a source depleted in light REE relative to the primitive 0.2 Ga, approximately the age of the Mackenzie igneous event mantle. However. the Sr isotope ratios of only two samples and the emplacement of the basalts. The second trend (line 2) require a mantle source depleted in Rb (Fig. 4). The three basalts may represent a mixing line between the basalts and a crustal with the lowest 143Nd/144Nd have, in several cases, values of component with a composition like the Proterozoic Wopmay incompatible trace element ratios, including Th/La, %/La, and NbITh, close to the crustal average (Table 3). The 206Pb/204Pb belt (Bowring and Podosek 1989). values in the basalts are positively correlated with the Discussion 208Pb/204Pbvalues, but there is no obvious correlation between the 206Pb/204Pb and 207Pb/204Pbvalues (Fig. 5). The The variation trends displayed by the incompatible trace elerelatively high 207Pb/204Pbvalue in the samples with low ment and isotopic ratios (Figs. 2-6) are consistent with the
DUPUY ET AL.
1941
OCFANIC ARRAY D
a
.
B
15.6
p =9.5
D
a
P-
h( O
15.4
MORB (Arlanric + Pacific)
FIG.5. Present-day 206Pb/2"Pb versus (a) 207Pb/206Pb and (b) 208Pb/ '"Pb for Coppermine River basalts. The striped areas represent the oceanic arrays. The fields for Atlantic and Pacific MORB are from White et al. (1987) and Ito et al. (1987). The curved lines are lead growth curves for U -Pb systems having present-day p values of 9.5 and 10.
mixing of two compositionally distinct components: crust and mantle. The crustal component has values of several trace element ratios that are similar to those of the bulk continental crust (Hofmann 1988). It also has low Nd (less than -4), low 206Pb/204Pb and 208Pb/204Pb (respectively < 18.6 and < 38.2), and is enriched in radiogenic Sr (eSr greater than f50). On the basis of these isotopic characteristics, the crustal component might be the equivalent of the Proterozoic rocks from the Wopmay orogenic belt (Bowring and Podosek 1989; Housh et al. 1989). The mantle component is represented by four samples (3-66, 69-829, 42-66, 157-66) with relatively constant 206Pb/204Pb (19.1 - 19.2) and 208Pb/204Pb(39.0 - 39.2) and EN^ ranging from + 3 to +4. Although some trace element ratios in these basalts are similar to those of primitive mantle, the isotopic composition is different. The lead isotopes suggest that the mantle component was generated from a less depleted source than the source of modern MORB. This observation is also supported by the Nd isotopic data. The initial Nd isotope ratios of these four basalts plot below the Nd evolutionary trend (Fig. 6) calculated with a 147Sm/144Nd value of 0.21. This Nd trend, which is significantly different from that of CHUR, is close to the growth rate proposed by Smith and Ludden (1989) for the Precambrian mantle. There are several possible explanations for the suppressed growth rate of the four Coppermine River (CR) basalts (Fig. 8). The basalt composition might reflect a source reservoir consisting of subcontinental lithospheric mantle, as suggested (Fig. 8) by their position on the Nd evolution line of Bell and Blenkinsop (1987). However,
FIG. 6. Present-day 208~b/204Pb versus (a) 143Nd/144Nd, (b) Si02 content, and (c) Ba/Nb in Coppermine River basalts. The continuous line in (a) is a calculated mixing line between a hypothetical mantle component and the Proterozoic rocks from the Wopmay Orogen. The numbers on the line represent the mixing proportions of the crustal component. Parameters for calculations are as follows: Mantle 39.2, Pb 1.5 ppm); (143Nd/144Nd 0.5 126, Nd 14.3 ppm; 208Pb/204~b crust ('43Nd/144Nd 0.51 15, Nd 42.7 ppm; 208Pb/2"Pb 35.3, Pb 24 ppm). For the crustal component, Pb isotopes and Pb content are averaged data, respectively, from Housh et al. (1989) and and Nd content are from Hildebrand et al. (1987). 143Nd/144Nd Bowring and Podosek (1989) (Proterozoic granite L268).
the model age (TDM)around 1.4- 1.5 Ga for the four basalts is close to the age of the CR volcanic activities, suggesting that their source was not Archean lithosphere. On the other hand, Proterozoic lithosphere with EN^ between +2 and + 3 as in two basalts (1.9 Ga old) from the Wopmay Orogen (Bowring and Podosek 1989) may constitute a viable source. A derivation from continental lithosphere, which is typically heterogeneous at various scales (e.g., Menzies 1989), would require that the parental magma was well mixed prior to the eruption in order to generate the relatively uniform trace element and isotopic composition of these four basalts. Such an origin is unlikely because the samples were collected at a distance of a few tens of kilometres from each other. The uniform composition appears to reflect a homogeneous mantle source and might favour generation from an upwelling mantle related
1942
CAN. J. EARTH SCI.
FIG. 7. 147Sm/144Nd versus '43Nd/144Ndratios for the Coppermine River basalts. The empty squares represent the Proterozoic rocks from the Wopmay orogen (Bowring and Podosek 1989). Line (1) is a reference line (MSWD > 2) for four basalt samples without a crustal component (3-66, 69-829, 42-66, 157-66). Line (2) is an estimated trend for the basalts containing a crustal component.
VOL. 29,
1992
four basalts have these geochemical characteristics and support the plume model. A similar interpretation (Nicholson and Shirey, 1990) has been proposed for the 1.1 Ga Midcontinent Rift (MCR) basalts in the Lake Superior region. The MCR basalts share common features (huge volume of igneous rocks, short duration of eruption) with CR basalts but differ in isotopic composition. In particular, the MCR basalts have a less depleted Nd isotope signature, which might reflect either the different composition of the plume or a plume modified in composition by an additional component. The trace and isotopic data do not unequivocally specify which of the two alternatives is most likely. However, the apparently significant erosion of the mantle roof beneath the CR plateau basalts (Hoffman 1990) might suggest that the lithosphere was involved in the generation of the CR basalts. Hence, the mantle component might represent a well-stirred "melange" between the plume material and the influx of a melt from the base of the depleted continental lithosphere (2.0-2.4 Ga, after Bowring and Podosek 1989) thermally eroded by the upwelling plume.
Conclusion The trace element and isotopic characteristics of the Coppermine River tholeiites closely resemble those of many Phanerozoic flood basalts (e.g., Menzies 1989). Their wide variations are inferred to be due to a mixing of at least two components (mantle and crust). However, it is probable that two mantle components were involved, although the magma was rather homogeneous in composition. The first mantle component is probably from a mantle plume, while the other represents the base of the continental lithosphere. The magma was subsequently contaminated by Precambrian basement during its ascent to the surface, and some of the lavas were also affected by upper crustal contamination.
Acknowledgments 1000
2000
3000
Time (Ma)
EL,,
FIG. 8. versus age for samples devoid of crustal component (solid triangles) from the Coppermine River basalts (CR). Data for tholeiitic basalts from the other localities: 1, Abitibi, 2, Alexo, and 4, Cape Smith (Albarede and Brouxel 1987); 3, Circum-Superior belt (Smith and Ludden 1989); 5 - 8, greenstones from southwestern U.S.A. (Nelson and DePaolo 1984); 9, Columbia River basalts with > 0.16 (Carlson et al. 1981; Hart 1985). The open cir147Sm/144Nd cles represent carbonatites from the Canadian Shield (Bell and Blenkinsop 1987) and the broken line is a least-squares fit through the data. The continuous line represents the evolutionary trend calculated = 0.21. with 147Sm/'44~d
to a plume. In this respect, LeCheminant and Heaman (1989) have interpreted the following as evidence for a mantle plume origin: (i) the short time span for the emplacement of the basaltic rocks of the Mackenzie event, (ii) the uplift just prior to the extrusion of the Coppermine River flows (manifested by the Dismal Lakes Group disconformity), and (iii) the radiating aspect of the Mackenzie dyke swarm. In recent intraplate basalts, mantle plume material often displays incompatible trace element ratios with values similar to primitive mantle; EN^ of plume material is lower than that of the depleted mantle, whereas lead isotopes overlap depleted mantle. The
The study was supported by the Centre gkologique et gkophysique, Montpellier, and the Natural Sciences and Engineering Research Council of Canada (operating grant A3782). We thank Drs. K. W. Klewin, K. J. Schulz, and J. H. Sevigny for their critical comments. Albarede, F., and Brouxel, M. 1987. The Sm-Nd secular evolution of the continental crust and the depleted mantle. Earth and Planetary Science Letters, 82: 25-36. Alibert, C., Michard, A., and Albarede, F. 1983. The transition from alkali basalts to kimberlites: isotope and trace element evidences for melililites. Contributions to Mineralogy and Petrology, 82: 176- 186. Baragar, W. R. A. 1969. The geochemistry of Coppermine River basalts. Geological Survey of Canada, Paper 69-44. Baragar, W. R. A. 1972. Coppermine River basalts: geological setting and interpretation. In Rubidium-strontium isochron age studies, report 1. Edited by R. K . Wanless and W. D. Loveridge. Geological Survey of Canada, Paper 72-23, pp. 2 1 - 24. Baragar, W. R. A,, and Donaldson, J. A. 1973. Coppermine and Dismal Lakes map-areas. Geological Survey of Canada, Paper 71-39. Bell, K., and Blenkinsop, J. 1987. Archean depleted mantle: evidence from Nd and Sr initial ratios of carbonatites. Geochimica et Cosmochimica Acta, 51: 291 -298. Bowring, S. A,, and Podosek, F. A. 1989. Nd isotopic evidence from Wopmay orogen for 2.0-2.4 Ga crust in western North America. Earth and Planetary Science Letters, 94: 217-230.
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