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Journal of Petrology | Volume 44 | Number 4 | Pages 757-771 | 2003
© Oxford University Press 2003
The Skaergaard Layered Series, Part VII: Sr and Nd Isotopes
1 DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF OREGON, EUGENE, OR 97403-1272, USA
2 DEPARTMENT OF EARTH AND ATMOSPHERIC SCIENCES, UNIVERSITY OF ALBERTA, EDMONTON, ALTA.,T6G 2E3 CANADA
Telephone: (541) 344-2539. Fax: (541) 346-4692. E-mail: McBirney{at}darkwing.uoregon.edu
RECEIVED APRIL 10, 2002; ACCEPTED OCTOBER 30, 2002
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ABSTRACT |
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The initial isotopic ratios of strontium and neodymium in the Skaergaard Layered Series vary both vertically and laterally, on every scale from the intrusion as a whole down to coexisting minerals in a single rock. The magma that filled the Skaergaard chamber was contaminated to various degrees with the metamorphic rocks through which it rose and was never completely homogenized after being intruded. The contamination was most pronounced in contact zones and aureoles around rare xenoliths. The greater concentrations of lithophile trace elements in the Upper Border Series was previously attributed to assimilation of buoyant fragments of gneiss that collected under the roof, but most of the rocks of the Upper Border Series are isotopically indistinguishable from those of the Layered Series. It is doubtful, therefore, that this part of the intrusion assimilated much more of the metamorphic basement than did the rest of the magma. Similarly, the marked increase in the concentrations of excluded elements in the upper part of the Layered Series is not matched by a change in the isotopic character of the rocks and cannot be attributed to a later influx of new magma. Analyses of minerals separated from rocks with exceptionally mafic or felsic modal compositions revealed marked inhomogeneities in the isotopic compositions of their constituent minerals. For example, coexisting plagioclase and pyroxene from closely associated anorthosites and pyroxenites have very different initial isotopic ratios of both strontium and neodymium. The same is true of mafic and felsic layers in modally graded gabbros. These differences are unrelated to the low-temperature alteration shown by oxygen isotopes. They must have been introduced when the original gabbro was largely crystallized and underwent local metasomatic replacement by nearly mono-mineralic mafic and felsic assemblages.
KEY WORDS: Nd isotopes; Skaergaard; Sr isotopes
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INTRODUCTION |
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Any attempt to explain the compositional variations of differentiated igneous rocks must address the question of how and to what degree the magmas have been altered by assimilation of crustal rocks or by late-stage metasomatic effects. In the long history of petrologic work on the Skaergaard Intrusion this question has been discussed repeatedly without being firmly resolved. It would seem that the excellent exposures and propitious geological setting of the intrusion would make it a simple matter to assess the role of assimilation during magmatic differentiation. Thanks to the antiquity of the Archean gneisses through which the magma rose, the isotopic ratios of the gabbros would have been very sensitive to even minor amounts of contamination.
The early work of Wager and his colleagues (Wager & Deer, 1939
; Wager, 1960; Wager & Brown, 1968) was based on an explicit assumption that, despite the presence of xenoliths in the Marginal and Upper Border Series, contamination of the main magma was minimal. This view seemed to find support when Hamilton (1963) carried out the first studies of radiogenic isotopes and concluded that only the granophyric dikes and sills had sufficient radiogenic strontium to require a substantial contribution from the basement series.
Taking advantage of the greater precision of improved analytical techniques, Leeman & Dasch (1978) confirmed that the amount of assimilation in the Layered Series rocks was indeed small, even though they found clear evidence that metasomatic exchange had raised the Sr isotopic ratios of the outermost parts of the Marginal Border Series. Stewart & DePaolo (1990) undertook a much more comprehensive study of the strontium isotopes and provided the first determinations of neodymium isotopes for samples representing all major units of the intrusion. They concluded that, although the contribution of assimilated gneiss was only 2–4%, it had a measurable effect on both the Layered and Upper Border Series.
Julius Dasch carried out additional analyses of the same units, and found that the variations within a single lithologic zone could be almost as great as that of the entire intrusion. The present study was undertaken to explore these variations by obtaining new analyses to supplement the unpublished data that Dasch has generously made available.
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GEOLOGICAL RELATIONS |
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The geologic setting, internal structure, and basic petrologic relations of the intrusion were first described by Wager and his coworkers (Wager & Deer, 1939
; Wager & Brown, 1968) and have since been described in greater detail by numerous subsequent studies [summarized by McBirney (1996)]. The intrusion was emplaced during a major magmatic episode associated with opening of the North Atlantic 55·5 Myr ago (Hirschmann et al., 1997). The main host rocks are felsic gneisses and amphibolites with ages of at least 3000 Ma (Kays et al., 1989) and a thick series of overlying Eocene basalts and hyaloclastites that were erupted from nearby fissures during an earlier stage of the same magmatic episode that produced the intrusion (Wager, 1947; Nielsen, 1978; Brooks & Nielsen, 1982).
The body has been divided into three major units: the Layered Series, which crystallized on the floor, and the Marginal and Upper Border Series, which crystallized on the walls and under the roof, respectively (Fig. 1). Although these series followed roughly parallel trends of differentiation converging on the Sandwich Horizon they have subtle differences that reflect the different physical conditions under which they crystallized. The general trend is one of steady iron enrichment, but the Upper Border Series is more felsic and richer in excluded trace elements than corresponding rocks of the Layered Series (Naslund, 1984; McBirney, 2002), and the latter tends to be slightly more evolved than contemporary rocks of the Marginal Border Series (Hoover, 1989). Layering in the Layered Series shows that denser mafic components were segregated from the Upper Border Series and deposited by density current that descended along the walls and across the floor (McBirney & Nicolas, 1997). Other types of layering in the interior of the Layered Series are thought to have resulted from recrystallization in thermal and compositional gradients near the front of crystallization (Boudreau & McBirney, 1997). Similar processes of re-equilibration resulted in extensive textural and compositional changes throughout the intrusion but most conspicuously in blocks that fell from the Upper Border Series and settled in the Layered Series (Sonnenthal & McBirney, 1998).
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Prolonged hydrothermal activity affected parts of the intrusion, particularly in the upper levels where the circulation of meteoric water was enhanced by the highly permeable character of the basaltic wall rocks (Taylor & Forester, 1979
; Manning & Bird, 1987, 1991; Bird et al., 1988). Metasomatic alteration is locally conspicuous in parts of the Layered and Marginal Border Series (McBirney & Sonnenthal, 1990). Irregularly shaped pods and schlieren of anorthosite and closely associated olivine pyroxenite are common in the lower parts of the Layered Series, where they transect layering and clearly formed later than the gabbros they replace (Fig. 2a). In parts of the Marginal Border Series, particularly on the western side of the intrusion, the strong vertical layering is replaced by amoeboid masses of olivine pyroxenite in a matrix of unlayered anorthosite (Fig. 2b).
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ISOTOPIC CHARACTER OF WALL ROCKS AND RELATED INTRUSIONS |
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Metamorphic rocks
As one would expect for crustal rocks of great age, many of the Archean metamorphic rocks are exceptionally rich in radiogenic strontium, but this is true only of the Rb-rich quartzofeldspathic gneisses; amphibolites interlayered with these gneisses contain very little Rb and have much lower Sr isotopic ratios. Although Hamilton (1963)
obtained present-day ratios of only 0·716 and 0·712 for mafic and felsic facies of the basement series, Pankhurst et al. (1976) reported values ranging from 0·71102 to 1·33405, and Kays et al. (1989) cited an even greater range of 0·70784–1·3997. Not all the low values are initial ratios, however; some come from metamorphic rocks that have been affected by Tertiary intrusions. Similar effects were noted by Leeman & Dasch (1978), who found a marked decrease of 87Sr/86Sr in samples of gneiss taken close to the contact of the Skaergaard Intrusion.
Data on the neodymium isotopic ratios of the basement rocks are very sparse. Stewart & DePaolo (1990) reported two analyses, one of quartzofeldspathic gneiss, the other of an amphibolitic gneiss. The 
Nd values at the time of the intrusion (t = 55 Ma) are, respectively, -39·7 ± 0·5 and +3·4 ± 0·4. Blichert-Toft et al. (1992) reported a Nd value of -37·5 ± 0·4 for a gneiss with about 72% SiO2 (hence quartzofeldspathic) and another of -3·6 ± 0·3 for an amphibolite, both collected from near the Södalen segment of the Miki Fjord macrodike. They also reported a value of -30·6 ± 0·2 for a sample of basement gneiss collected from a hematized shear zone, probably of Proterozoic age, about 4 km east of the eastern end of Torrsukátak (Watkins Fjord). Thus, it is evident that basement rocks exposed near the Skaergaard Intrusion have a very wide range of Sr and Nd isotopic ratios, and although it would be difficult to estimate their average composition, even small amounts of contamination should be readily detectable.
Basaltic lavas
Most of the upper parts of the intrusion are walled by early Tertiary basaltic lavas, hyaloclastites, and tuffs of the Miki and Vandfalsdalen Formations [the ‘Lower Basalts’ of Brooks & Nielsen (1978, 1982)]. In most places these wall rocks have been visibly altered by circulation of hydrothermal fluids (Taylor & Forester, 1979; Fehlhaber & Bird, 1991). Holm (1988) and Larsen et al. (1989) found initial 87Sr/86Sr ranging from about 0·7030 to 0·7045, and Nd values of about +9·0 to +3·2 for East Greenland plateau basalts, mostly from the Scoresby Sund area NE of Skaergaard. No isotopic analyses have been obtained for the Tertiary lavas and agglomerates closer to the Skaergaard Intrusion, because all of the available samples are too altered to warrant analysis.
Basaltic dikes
The numerous basaltic dikes that cut the intrusion may represent magmas that followed a course of differentiation similar to that of the main Skaergaard magma (Brooks & Nielsen, 1978), but few could have any direct relation to the Skaergaard gabbro. A possible exception is a north-trending dike of 4 m width that cuts the Lower Zone and coarsens upward before disappearing into the Middle Zone. Its trace-element character relative to that of the evolving Skaergaard magma indicates that the dike could have introduced a substantial new pulse of liquid about the time that the Middle Zone was crystallizing (McBirney, 2002), but the isotopic ratios of a representative sample (SK-756 in Table 1) clearly rule this out. The initial 87Sr/86Sr of the dike (0·70641) is significantly greater than that of the Skaergaard gabbros, and the latter show no corresponding change that can be correlated with a major addition of magma at this stage. Neodymium isotopes show a similar difference; the dike's initial 143Nd/144Nd of 0·512440 (Nd = –2·5) is well below the range of values for the Middle Zone (0·512772 to 0·512816 and Nd = +4·0 to 4·8). The isotopic character of the dike is much closer to that of the Basistoppen sill, which intruded the Upper Border Series when the latter was almost completely crystallized (Naslund, 1984, 1989; White et al., 1989). Although it is unlikely that this particular dike contributed significantly to the main body of magma from which the Layered Series crystallized, one cannot rule out the possibility that an unexposed dike elsewhere in the intrusion could have done so.
Granophyric dikes
Hamilton (1963) was the first to demonstrate the elevated Sr isotopic ratios of the granophyric dikes and Tinden Sill. In keeping with the early view that these were differentiates of the Skaergaard magma, he reasoned that the late-stage liquids assimilated large amounts of the basement gneiss. Additional analyses performed by Leeman & Dasch (1978), Stewart & DePaolo (1990) and Hirschmann (1992) confirmed the overwhelming contribution of crustal rocks, and established the granophyres as an independent suite formed from a differentiated post-Skaergaard magma that was highly contaminated with Archean gneiss.
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ISOTOPIC VARIATIONS OF THE SKAERGAARD GABBROS |
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Sr–Nd correlation
Figure 3 shows the initial 87Sr/86Sr and
Nd values for all samples in Table 1 except those of dikes and sills (BAS-1, SK-756 and SK-48). The 87Sr/86Sr value for the chilled margin, CM-636 determined by Stewart & DePaolo (1990), was referenced to a ratio of 0·71026 for SRM987 to be consistent with the new data. Comparisons of values of initial 87Sr/86Sr and Nd values for 21 specimens analyzed both by Stewart & DePaolo and for this study show that differences in the values for Sr in the two sets of data are in all cases much less than typical analytical uncertainties. In almost all instances, differences in Nd values are similar to cited uncertainties and do not seem to be systematic.
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) are distinguished from Upper Border Series rocks (x). UMZ-298 is a sample taken close to a basement xenolith in the unit of the Upper Border Series equivalent to the Middle Zone of the Layered Series. Uncertainties are given in Comparison of the Layered and Border Series
Scattered xenoliths in all stages of assimilation, together with the greater concentrations of lithophile elements in the Upper Border Series and a few early analyses giving more radiogenic Sr ratios, seemed to confirm the long-held view that the Upper Border Series was grossly contaminated with Archean gneiss. As more analyses became available, however, it has become apparent that, if atypical samples are omitted, the average value for 87Sr/86Sr in the Upper Border Series is statistically indistinguishable from that of the Layered Series. Indeed, as shown below, there are markedly greater variations in initial Sr isotopic ratios between mineral separates extracted from a single rock than between the averages for the Upper Border Series and Layered Series as a whole.
A few samples, such as UMZ-298 and UMZ-77, were collected to determine the effect of their proximity to xenoliths and are not representative of the Upper Border Series as a whole. When these samples are omitted, the initial Sr ratios for 12 samples average 0·70445 ± 19 (single standard deviation), with little statistical difference from those of the Layered Series. Sixteen Lower Zone rocks average 0·70434 ± 20, 11 Middle Zone rocks average 0·70438 ± 14, and 14 Upper Zone rocks average 0·70442 ± 8. The average for the Upper Border Series is slightly more radiogenic, so one cannot rule out assimilation entirely, at least on a local scale, but the small difference and the wide ranges of values found in each unit make it difficult to postulate enough assimilation to account for the elevated contents of lithophile elements.
Using a simple mass balance, one can easily estimate the amount of Archean gneiss and amphibolite that would be required to account for the average initial 87Sr/86Sr of the Upper Border Series. Assuming 0·6 parts gneiss with an average Sr content of 225 ppm and an initial isotopic ratio of 1·3997 and 0·4 parts amphibolite with 78 ppm Sr with a ratio of 0·70784, <1·0% contamination would be sufficient to raise the average ratio of the Layered Series (0·70439) to that of the Upper Border Series (0·70445). This is in contrast to 16·0% that would be required to raise the Ba content of the Layered Series (73·5 ppm) to the average concentration in the Upper Border Series (139·2 ppm) by adding the same average wall rock with 487 ppm Ba. Calculations of this kind are subject to large errors when the isotopic differences between the Layered and Upper Border Series are so small, but the amount of contamination required by the Sr isotopes is clearly much smaller than indicated by trace elements. The greater concentrations of excluded trace elements in the Upper Border Series are now thought to be mainly the result of upward infiltration of late-stage liquids derived from the compacting Layered Series (McBirney, 1995, 2002).
There is no conspicuous difference between the Layered Series and most of the Marginal Border Series, but the outermost parts of the latter have exchanged Sr across the contact with Archean gneisses (Leeman & Dasch, 1978). Analyses of samples along a profile on Kraemers Island revealed a gradient of elevated Sr ratios extending about 10 m into the gabbros with corresponding lowered ratios in the adjacent gneiss. A typical Skaergaard ratio of 0·7040 found in a gabbro (DS-10) collected 300 m from the gneiss led Leeman & Dasch (1978) to conclude that ‘high-level contamination was confined largely to the contact aureole’.
Variations within the Layered Series
As Stewart & DePaolo (1990) correctly observed, the isotopic ratios of Sr and Nd are not constant but vary sympathetically in almost all major units of the intrusion. With the additional analyses now available, one can contour these ratios in two-dimensional projections (Fig. 4). When this is done, the Sr and Nd ratios are seen to vary laterally as well as vertically in very similar patterns. The most radiogenic Sr values in the Layered Series are near the center of Lower Zone C and the lower part of the Middle Zone where the rocks have been most strongly affected by a late-stage influx of liquid that was rich in lithophile elements, iron, and radiogenic Sr (McBirney, 1995). Because these elevated ratios are found in rocks directly over the root of the intrusion and are closely associated with granophyric dikes containing large amounts of radiogenic Sr, Hirschmann (1992) reasoned that the walls of the feeder were partly melted and contributed Sr and other lithophile elements to liquids along the margins of the feeder. Petrographic evidence of extensive replacement seen in the rocks in this area (McBirney, 1995, fig. 11) indicates that the anomaly was not a primary magmatic feature but the result of a late-stage influx of iron, titanium, and excluded trace elements.
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Variations with modal composition
In the course of examining the spatial relations of initial Sr isotopic ratios, E. J. Dasch (personal communication, 1987) noted erratic variations that seemed to be related to the mineralogical character of the gabbros. Subsequent work has revealed several examples of isotopic differences between mafic and felsic rocks, the most striking example being a pair of samples, one of anorthosite and the other of olivine pyroxenite, from part of the Marginal Border Series (Fig. 2a). Although the samples were taken within 10 cm of each other, the anorthosite (SI-84.1) has an initial Sr ratio of 0·70483, whereas that of the pyroxenite (SI-84.7) is 0·70412 (Table 2). In other instances, however, this relationship is reversed, the mafic component being more radiogenic than the felsic.
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To determine how these differences might have come about, two sets of closely associated felsic and mafic rocks were selected for closer study. The first is a graded layer in UZa for which there were already extensive analytical data (McBirney & Noyes, 1979
). The second is a pair of anorthositic and ultramafic schlieren from LZa (Fig. 2a). For both examples, analyses were obtained for mineral separates of the constituent plagioclase and pyroxene of the mafic and felsic units and adjacent gabbro. Results are shown in Table 2. In the mafic part of the graded layer the pyroxene is nominally more radiogenic (0·70440) than the plagioclase (0·70437), and the same is true to a greater degree of the felsic part (pyroxene 0·70442; plagioclase 0·70434), but the isotopic ratios of the minerals of the unlayered host gabbro UA-362A (pyroxene 0·70441; plagioclase 0·70440) are the same within the precision of the analyses (±0·00004). In the latter case, however, the whole-rock ratio for the host gabbro is slightly less radiogenic (0·70435) than either the plagioclase or pyroxene. This may be the effect of apatite, small amounts of which are present in the rock but not in the mineral separates. Analyses of the plagioclase and pyroxene in the anorthosite, pyroxenite, and adjacent gabbro (SK-609 and SK-608) of Lower Zone A (Fig. 2a) reveal isotopic differences that are even greater than those in the mafic and felsic layers (Fig. 5a). In both rocks the dominant mineral has the lowest Sr isotopic ratio. The initial Sr ratio of the plagioclase of the anorthosite (0·70408) is substantially less than that in the coexisting pyroxene (0·70422), but the opposite is true of the minerals in its mafic counterpart, where the plagioclase has a ratio of 0·70414 as opposed to a ratio of 0·70402 for the pyroxene. The initial Sr ratios of the plagioclase and pyroxene of the host gabbro, however, are identical. The initial bulk-rock Sr ratio of the gabbro (0·70413) is the same as the average gabbro in this part of the Layered Series (0·70413 ± 0·00015). The fact that the dominant minerals in both the mafic and felsic rocks have lower Sr ratios than the respective residual minerals (i.e. the pyroxene in the anorthosite and the plagioclase in the pyroxenite) suggests that the dominant minerals grew from a liquid having a lower ratio than that of the original rock.
The Nd isotopes exhibit a similar pattern of disequilibrium (Fig. 5b). The Nd values of the stable minerals, i.e. the plagioclase in the anorthosite and the pyroxene in the pyroxenite, appear to be nominally higher than those of the subordinate, unstable minerals, but both have values close to the bulk-rock values for the normal gabbro. These relations suggest that the stable minerals grew from a liquid having an Nd value of 4·5 or more. Although the bulk-rock Nd values for the anorthosite and pyroxenite are nominally greater than that of the normal gabbro, the latter is almost identical to the residual, unstable minerals. This relationship suggests that all three rocks originally had Nd values close to 3·9 or 4·0. The difference between the plagioclase and pyroxene of the gabbro indicates that the rock did not escape alteration. The same liquid responsible for the formation of the anorthosite and pyroxenite must have infiltrated through the gabbroic host as well. The effect on the plagioclase was greater than on the pyroxene, because the rare-earth content of the latter was greater.
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DISCUSSION |
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Virtually every large layered intrusion that has been investigated in detail has proved to be isotopically inhomogeneous. For example, Kruger (1990)
found that the Sr ratios in the Bushveld Intrusion increase upward in the lower part of the intrusion and suggested that the variations are due to gradual additions of a magma having more radiogenic Sr than the initial liquid. Poitrasson et al. (1994) found the opposite situation in a mafic intrusion in Corsica. They interpreted an up-section decrease of Sr and increase of Nd ratios as the result of dilution of an early contaminated magma by later, more pristine liquids. It is reasonable to conclude that gradational variations on the scale of an entire intrusion are inherited from inhomogeneities in the original magma, but to interpret the manner in which they were produced, one must look beyond the one-dimensional variations in a single stratigraphic section. As we have seen in the Skaergaard case, lateral variations can be of the same magnitude as vertical ones. More localized isotopic differences are usually attributed to contamination with the crustal rocks through which a magma rose or picked up as xenoliths, but again, the extent of this effect can be determined only if the contaminated samples are shown to be representative of the entire unit from which they were taken. Unusually radiogenic samples from the Skaergaard Upper Border Series initially gave the impression that the entire upper part of the intrusion had assimilated large amounts of gneiss, but analyses of a larger, more representative set of samples eventually showed that this was not the case.
Some of the isotopic variations in the Skaergaard rocks seem to be related to the compositions and modal proportions of minerals, particularly of plagioclase. Similar relations have been reported for other intrusions. Sorensen & Wilson (1995) and Wilson et al. (1996) observed that Sr isotopic ratios in the Bjerkreim–Sokndal Intrusion were positively correlated with the albite content of plagioclase and iron content of mafic minerals, and Morse (1983) noted a similar relation in the Kiglapait Intrusion. Palacz & Tait (1985) found an abrupt increase in Sr and decrease in Nd ratios where plagioclase becomes abundant in the Rum Intrusion. Changes of this kind can logically be explained as the result of a new pulse of magma having more radiogenic Sr and greater proportions of plagioclase. In the Skaergaard case, however, modally related variations are found in a variety of situations, including nearly mono-mineralic rocks that transect layering. The disequilibrium between minerals in these same rocks must have been introduced after the rocks were largely crystalline. The Skaergaard rocks are not unique in this regard. When Weis & Morse (1993, 1995) analyzed mineral separates from the Kiglapait Intrusion, they found a marked disequilibrium between the Pb isotope compositions of mafic minerals and plagioclase. All that can be said about these small-scale inhomogeneities is that they do not seem to be related to hydrothermal alteration but to some other late-stage magmatic process. As others have already pointed out (Taylor & Forester, 1979; Fehlhaber & Bird, 1991), the variations of Sr and Nd isotopes have no apparent spatial correlation with those of oxygen or hydrogen. The isotopic changes must be related in some way to migration of the same late-stage liquids that were responsible for the anomalous distribution of excluded trace elements observed in many of the same rocks (McBirney, 2002).
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CONCLUSIONS |
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The Sr and Nd isotopic character of the intrusion varies on every scale from kilometers to millimeters. Assimilation of xenoliths was locally important, but the large-scale variations must reflect inhomogeneities resulting from differing degrees of contamination with crustal material prior to intrusion. The isotopic relations offer no conclusive evidence for injections of new magma after the initial intrusion. Differences in the initial isotopic ratios of Sr in minerals coexisting in an individual sample are best explained as the result of metasomatic alteration after the magma was largely crystalline. The exact nature of this process remains uncertain, but it seems to be related to late-stage liquids or fluids that permeated the entire intrusion.
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APPENDIX A: SELECTION AND ANALYSES OF SAMPLES |
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Samples from locations shown in Fig. 1 have been selected for analysis in order to obtain a better perspective of vertical and horizontal variations of radiogenic isotope compositions within both the Layered Series and Upper Border Series. Each sample has been analyzed for major elements and an extensive set of trace elements to ensure that it is representative of the average rock in a given part of the intrusion. Further analytical data for the same samples are given in Parts V and VI of this series (McBirney, 1998
, 2002). |
APPENDIX B: ANALYTICAL METHODS |
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 TOPABSTRACTINTRODUCTIONGEOLOGICAL RELATIONSISOTOPIC CHARACTER OF WALL...ISOTOPIC VARIATIONS OF THE...DISCUSSIONCONCLUSIONSAPPENDIX A: SELECTION AND... APPENDIX B: ANALYTICAL METHODS REFERENCES |
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Sr and Nd isotopic ratios presented in Table 1 were determined at the University of Alberta, Edmonton, by thermal ionization mass spectrometry using MM30 and VG354 mass spectrometers unless otherwise noted in Table 1 and were performed using methods outlined by Goles & Lambert (1990)
and by Holmden et al. (1996). All Sr isotopic data in Table 2 were determined using methods outlined by Holmden et al. (1997), and all Sr isotopic analyses in Tables 1 and 2 are presented relative to a value of 0·71026 for SRM987. For Nd isotopic analyses, all data were determined using methods described by Creaser et al. (1997), and are presented relative to a value of 0·511850 for the La Jolla standard. Measured uncertainties in Sr and Nd isotopic ratios and estimates of imprecisions in abundances of Rb, Sr, Sm, and Nd have been propagated to yield the cited uncertainties (all at the two-sigma level) in the final three columns of Table 1. In calculating Sr and Nd initial ratios, an age for the intrusion of 55 Ma was assumed (Brooks & Gleadow, 1977; Hirschmann et al., 1997). |
ACKNOWLEDGEMENTS |
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Many friends and colleagues have provided valuable assistance and encouragement during the long course of this study, but we are particularly indebted to Gordon Goles, Richard Lambert, and Julius E. Dasch, not only for the many excellent analyses they contributed, but also for their useful advice and stimulating discussions. The field work was carried out with the generous support of the National Science Foundation.
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REFERENCES |
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