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STRONTIUM IN FELDSPARS OF HIGH-K PROTEROZOIC IGNEOUS ROCKS OF THE ROBITAILLE SUITE, BUCKINGHAM, QUEBEC

Donald D. Hogarth and Michel J.L. Robin

Department of Earth Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada

E-mail address: dhogarth{at}uottawa.ca

	ABSTRACT

Top Abstract Introduction The Robitaille Suite Other High-Strontium Igneous... Geochemistry of Sr in... Whole-Rock and Mineral... Mineralogy Development of the Robitaille... Conclusions Aknowledgements References

 
Near Buckingham, Quebec, Sr-, K-rich rocks exposed in small, Mesoproterozoic, volcano-plutonic bodies of the Robitaille suite overlie or intrude the Grenville metasedimentary sequence and are cut by Neoproterozoic diabase dykes. They may represent near-surface equivalents of the Mont Laurier – Marmora alkaline plutons. The rocks includes latite and alkali-feldspar trachyte, lamprophyres (kersantite and minette), monzodiorite and alkali-feldspar syenite. Ultrapotassic compositions are common. The complexes, up to 2.7 km long and 1.0 km wide, are partly fringed with aureoles of Sr-rich, dolomitized calcitic marble. In 20 samples from three igneous complexes, Sr (up to 3960 ppm) is present in celestine (very rare), barite (rare), fluorapatite and calcite, but feldspars are the main hosts. K-feldspar without plagioclase occurs in samples with K₂O/Na₂O > 15 (weight ratio), and K-feldspar + plagioclase in samples with a lower ratio. Individual grains of plagioclase in the groundmass of lamprophyre are unzoned, but distinct ranges of An content are found in grains from restricted areas within a single polished thin section. The amount of Sr increases linearly with An content, from near zero at end-member albite to 1.0–2.6 wt% SrO at An₄₀. In one sample of latite, Sr peaked at An₃₀. Variation of Sr within a single sample is due to both differentiation (mainly by crystallization of sanidine) and equilibrium factors (such as temperature and An content). Clustering of point compositions on An – wt% diagrams for individual samples may reflect periods of volcanicity, separated by quiescent intervals, as differentiation evolved continuously in the parent magma-chamber. Barium is low, but can be correlated with Sr in one kersantite sample. The distribution of Sr between contiguous K-feldspar and plagioclase grains is governed by disequilibrium factors at low An content. For feldspar pairs, Sr preferentially enters more calcic plagioclase (An > ca. 10), but K-feldspar where plagioclase is more sodic (An < ca. 10).

Keywords: strontium, barium, lamprophyre, alkali-feldspar trachyte, ultrapotassic rock, plagioclase, sanidine, microcline, Robitaille suite, Buckingham, Quebec.

	INTRODUCTION

Top Abstract Introduction The Robitaille Suite Other High-Strontium Igneous... Geochemistry of Sr in... Whole-Rock and Mineral... Mineralogy Development of the Robitaille... Conclusions Aknowledgements References

 
Alkaline igneous rocks of the Robitaille Suite, of Mesoproterozoic age, exposed near Buckingham, Quebec, 35 km northeast of Ottawa, have been found to contain unusually high levels of strontium. Whole-rock compositions commonly contain between 1500 and 4000 ppm Sr, mostly held in the feldspar-group minerals. We report here a description of the rocks and their constituent minerals. We document the trends of Sr buildup during feldspar growth, investigate the relationships of Sr with other elements, seek to explain the seemingly contradictory data reported elsewhere for Sr distribution between plagioclase and K-feldspar, and shed light on the complex development of the Robitaille Igneous Suite in terms of the tectonic setting of its emplacement.

	THE ROBITAILLE SUITE

Top Abstract Introduction The Robitaille Suite Other High-Strontium Igneous... Geochemistry of Sr in... Whole-Rock and Mineral... Mineralogy Development of the Robitaille... Conclusions Aknowledgements References

 
The alkaline igneous rocks exposed near Buckingham (now sector of Buckingham, municipality of Gatineau), Quebec, were first called lamprophyre by Wilson (1914, 1920), but later reclassified as volcanic rocks associated with stocks and dykes by Doyle (1969), Lafleur & Hogarth (1981), Hébert (1988), Hogarth & Dumas (1998), Cava (1999), Khan (2003), and Hogarth (2004, in press). Their age is Mesoproterozoic, as shown by three U–Pb determinations (on baddeleyite from monzodiorite, and on zircon from kersantite and alkali feldspar trachyte), each giving 1060 Ma (T. Krogh, pers. commun., 1998; precision not specified).

These rocks occur in a zone 2 km wide that extends from Leitrim on the eastern outskirts of Ottawa, 40 km north–northeasterly to Mayo, Quebec. South of the Ottawa River, the rocks are covered with Paleozoic sedimentary strata, but the zone can be traced on an aeromagnetic map (Geological Survey of Canada, Aeromagnetic Series, Sheet 31G5) as a positive magnetic anomaly. Northward, in Quebec, the rocks crop out as 19 distinct volcano–plutonic complexes, up to 2.7 km long and 1.0 km wide, and a number of consanguineous narrow dykes, all included in the Robitaille Suite (Hogarth, in press). These shallow intrusive bodies and volcanic rocks seem to have been preserved in down-dropped fault-blocks (Lafleur & Hogarth 1981). To the north and south, the zone is terminated by west– northwest-trending faults.

Steeply dipping, medium-grade strata of Grenville marble, biotite gneiss and feldspathic quartzite unconformably underlie or are intruded by rocks of the Robitaille Suite, which comprise alkali-feldspar syenite to monzodiorite, alkali-feldspar trachyte to latite, and minette to kersantite (IUGS modal terminology; Le Maitre 2002). The volcanic rocks are dominant in the suite. Both Robitaille and Grenville rocks are cut by east–west-trending diabase dykes, with U–Pb baddeleyite and zircon ages of 590 ⁺²_–1 Ma (Kamo et al. 1995).

The mineralogy of the Robitaille Suite is simple. In most samples, phlogopite and feldspars are the sole major constituents, but clinopyroxene is common in some lamprophyre samples (e.g., M·35 and R·354). Clinopyroxene is also common in latite, monzodiorite and monzonite (e.g., R·152 and R·173), calcic amphibole in others (e.g. BD·65 and BD·158). Minor amounts of fluorapatite, titanite and hematite are ubiquitous.

Characteristically, rocks of the Robitaille Suite have an exceptionally high content of potassium (commonly >10 wt% K₂O) but little sodium (commonly <0.4 wt% Na₂O). Half of the analyzed samples (146 out of 293) are ultrapotassic according to the criteria of Foley et al.(1987): K₂O/Na₂O (wt) > 2, K₂O > 3 wt%, MgO > 3 wt%. In some of the ultrapotassic specimens, the atomic level of K approaches, but never attains the level of Al, or the lower limit of perpotassic rocks set by Rock et al. (1991, p. 222). The highest K/Al (atomic) value [K^#] of any whole-rock composition in the suite is 0.92. The Robitaille rocks display surprisingly few effects of low-temperature alteration, which include widely separated veinlets of green or yellow clinochlore after phlogopite and rare patches of muscovite as a product of feldspar alteration. Effects of prograde metamorphism appear to be entirely absent. In the present paper, we focus on three volcano-plutonic complexes (1, 2 and 4 in Fig. 1), exposed over a length of 8 km.

View larger version (52K): [in this window] [in a new window]   FIG. 1. Geology of the Robitaille Igneous Suite and its aureole, Buckingham area, Quebec.

Aureoles of metasomatized quartzite, gneiss and marble fringe the complexes (Miller 2004, Hogarth, in press). Complex 1 has a blue-amphibole fenite on its west side and a very small exposure of a gray calcite–apatite rock on the east. Amphibole fenite also is exposed on the southeastern side of Complex 4. Elsewhere, the metasomatic rock is a coarse-grained dolostone marble, averaging 3900 ppm Sr in the aureole of Complex 2, and 1700 ppm Sr in the aureoles of each of Complexes 3 and 4 (Hogarth, in press).

	OTHER HIGH-STRONTIUM IGNEOUS ROCKS IN THE GRENVILLE PROVINCE

Top Abstract Introduction The Robitaille Suite Other High-Strontium Igneous... Geochemistry of Sr in... Whole-Rock and Mineral... Mineralogy Development of the Robitaille... Conclusions Aknowledgements References

 
The Robitaille Suite is not the sole high-Sr igneous group situated within the Grenville Province. Samples from Mesoproterozoic, alkaline plutons of the Mont Laurier – Marmora belt, of southwestern Quebec and southeastern Ontario, contain up to 2.64 wt% SrO (alkali-feldspar syenite at Sainte-Véronique, Quebec; Rive 1976). On a SiO₂–K₂O diagram, most plot in the high-potassium ("leucitite") field, but some in the "shoshonite" field (Corriveau & Gorton 1993). Corriveau (1990) ascribed the absence of leucite to depth of emplacement, but the presence of H₂O (shown by biotite in the mode) may also play a part. Corriveau et al.(1990) noted that younging of plutons takes place from west (1089 Ma) to east (1076 Ma) in the belt. Later work (e.g., Corriveau et al. 1998) showed a few exceptions to this generalization, with some eastern plutons having ages of 1,170 Ma.

Elsewhere in the Grenville terrane, 325 km northeast of Quebec City, narrow dykes of Mesoproterozoic "biotite lamprophyre" intrude the Labrieville anorthosite (Owens & Tomascak 2002). Like the stocks described by Corriveau et al.(1990), the Labrieville dykes have high contents of Sr and K₂O (up to 0.31 and 6.40 wt%, respectively). Owens & Tomascak (2002) considered these rocks to be the product of an inhomogeneous, subcontinental mantle, enriched in LILE components. The level of Sr in feldspar-group minerals was not established in either the Sainte-Véronique pluton nor the Labrieville dykes.

	GEOCHEMISTRY OF SR IN SIMILAR ROCKS

Top Abstract Introduction The Robitaille Suite Other High-Strontium Igneous... Geochemistry of Sr in... Whole-Rock and Mineral... Mineralogy Development of the Robitaille... Conclusions Aknowledgements References

 
Judging from the published record, examples of plagioclase with Sr at the percent level in igneous rocks are not abundant. Such cases fall into two categories: 1) plagioclase in highly fractionated shoshonitic intrusive bodies in Russian Karelia (Eklund et al. 1998), and 2) plagioclase in the groundmass of certain alkaline volcanic rocks, such as the phonolite at Monte Vulture, Italy (Melluso et al. 1996). Rocks of the Robitaille Suite fit Category 2 better, although petrological and mineralogical similarities also do exist with rocks of category 1.

The fundamental rules for ionic substitution have been discussed for feldspars by Heier & Taylor (1959), Virgo (1968), and Berlin & Henderson (1968), among others. For a fixed supply of Sr, the Sr of a plagioclase should depend on its An content, at least in an equilibrium assemblage. Of course, the effects of temperature and possibly pressure, as well as the presence of other minerals competing for Sr, must also be considered.

The content of Sr in plagioclase of igneous rocks as a function of An content seems to follow a Gaussian distribution. On the calcic flank of this curve (An_50–90), the level of Sr increases as An decreases (Wager & Mitchell 1951, Turekian & Kulp 1956, Iida 1961, Butler & Skiba 1962). The progressive increase in Sr was attributed to the cocrystallization of Sr-depleted phases, like olivine and pyroxene-group minerals, from the magma (Berlin & Henderson 1968, 1969). On the sodic flank of the distribution (An_0–40), Sr increases as An content increases (Sen et al. 1959, Hall 1967). Hall (1967) attributed this trend to cocrystallization of a Sr-bearing phase (microcline), which would progressively remove Sr from the melt. One should keep in mind that these inferences were made on the basis of plagioclase with an order of magnitude less Sr than in the Robitaille Suite, and from more "normal" rocks.

Examples of K-feldspar with a high content of Sr have also been described, especially in K-rich volcanic rocks lacking plagioclase. In the Roman magmatic province, K-feldspar is reported with up to 3.0 wt% SrO at Monte Sabatini (Della Ventura et al. 1992) and 4.4 wt% SrO at Monte Vulture (Melluso et al. 1996). Sanidine associated with Sr-rich plagioclase in monzogabbro of Vuoksi, Russia, also contains considerable Sr (up to 3.2 wt% SrO; Konopelko 1997).

The plagioclase in the Italian rocks is much more calcic than in the Robitaille Suite, commonly with An_60–80 in zoned phenocrysts and An_40–60 in the groundmass. Samples from the Ladoga area, Russia, also are similar to those of Buckingham, having high-Sr and high-Ba whole-rock contents, two-feldspar associations, high-Sr feldspars, plagioclase in the An_20–30 range and abundant fluorapatite (ca. 5 modal % at Elizenvaara, 3% at Buckingham), but a relatively low K* (ca. 2 at Elisenvaara versus 28 for average ultrapotassic rock at Buckingham; Konopelko 1997, Hogarth 2004). The origin of both the Italian and Ladoga rocks has been ascribed to subduction-related, polyphase metasomatic events (Peccerillo 2001, Eklund et al. 1998). Plots of high-Sr (>2 wt% SrO) plagioclase are shown in terms of the anorthite – albite diagram in Figure 2.

View larger version (12K): [in this window] [in a new window]   FIG. 2. Major components of high-Sr (2 wt% SrO) plagioclase from igneous and metamorphic rocks) in worldwide localities.

View larger version (11K): [in this window] [in a new window]   FIG. 3. Major components of high-Sr (>2 wt% SrO) plagioclase from igneous and metamorphic rocks in worldwide localities. Symbols: +: Bjorkedalen, Norway (Brastad 1981, 1985), : Podolska Complex, Czech Republic (Krenn & Finger 2004), : Vuoski Massif, Russia (Konopelko 1997), Elisenvaara Massif, Russia (Konopelko 1997), : Colima, Mexico (Luhr & Carmichael 1981), : Roman Province, Italy (Cundari 1979, Baldridge et al. 1981, Melluso et al. 1996).

	WHOLE-ROCK AND MINERAL COMPOSITIONS

Top Abstract Introduction The Robitaille Suite Other High-Strontium Igneous... Geochemistry of Sr in... Whole-Rock and Mineral... Mineralogy Development of the Robitaille... Conclusions Aknowledgements References

Whole-rock compositions were determined at the University of Ottawa by X-ray fluorescence (XRF) spectrometry on discs fused with LiBO₂ and Li₂B₄O₇. Some were repeated at Actlabs (Ancaster, Ontario), using inductively coupled plasma – optical emission spectrometry (ICP–OES) for major constituents, and inductively coupled plasma – mass spectrometry (ICP–MS) for trace elements. Rocks with low Na₂O content (0.15% by XRF) were re-analyzed for Na at Actlabs by delayed neutron counting (DNC), after activation in the Hamilton reactor.

Minerals were analyzed by wavelength-dispersion spectrometry with an electron-probe microanalyzer (WDS–EPMA) at McGill University, using a JEOL microprobe (JXA–8900L). Accelerating voltage was 15 kV, beam current 20 nA, and beam diameter, 5 µm. Strontium and Ba were determined using synthetic feldspar glasses as standards, with 3.625 wt% SrO ("plagioclase" An₅₁) and 4.086 wt% BaO ("orthoclase"); counting time was 50 s on peaks, 25 s on background.

Standard deviations (in wt% of the recorded concentration) for elements in individual points (shown by error bars below) were obtained from a program supplied by JEOL, and involved X-ray intensities at peak and background levels, as well as counting time. Standard deviations of oxides in tabulated compositions (Tables 2, 4, 5) represent root-mean squared deviation from the average of compositional clusters, i.e., variability between the point analyses. In both cases, two standard deviations (2) are used.

Unconventional symbols and abbreviations include: d; element distribution (atomic) between two associated minerals, not necessarily in an equilibrium assemblage, K^#: K/Al (atomic) and K*: K₂O/ Na₂O (wt ratio). R²: confidence level, e.g., 0.9517 indicates 95.17% confidence, Sf: strontium feldspar (SrAl₂Si₂O₈), and X^z: Z/ A ions, where A ions are Ca²⁺, Sr²⁺, Ba²⁺, Na⁺, K⁺.

Whole-rock compositions

Twenty representative samples comprise the research suite. Specific petrological and chemical features are listed in Table 1. Many samples are highly fractionated (Mg# < 45), and rather few have strictly primitive Mg signatures (Mg# > 65). The potassic nature is summarized in columns K* and K^#. Note the relatively high K* and K^# for BD·78A, BD·226, R·3 and R·142A, which have K₂O contents of 13.55, 9.57, 11.33 and 11.83 wt%, respectively. Strontium averages 1660 ppm in the 20 samples, nearly five times the level of concentration in the Earth’s upper continental crust, estimated by Taylor & McLennan (1985). Especially noteworthy are the ten lamprophyres of Table 1, with high contents of Sr similar to that in lamprophyres and potassium-rich rocks described by Foley et al.(1987), Rock et al.(1991), and others. However, plotting Sr against Mg#, K* and K^# did not show clear relationships.

Barium exceeds Sr in nearly all of the research samples and the vast majority of the remaining 273 analyzed samples of the suite. The 20 research samples average 3020 ppm Ba, a 5.5-fold enrichment in the estimated composition of the upper crust of the Earth, according to Taylor & McLennan (1985). Like Sr, it correlates poorly with K, but does show a general positive trend. The reason for this different behavior is that Sr, besides its presence in K-feldspar, is found in non-potassic minerals such as plagioclase, fluorapatite and calcite. In contrast, Ba is contained mainly in the potassium minerals K-feldspar and biotite, in trace amounts in plagioclase and is below the detection limits in fluorapatite and carbonates. In some samples, scattered tiny grains of barite may have contributed to the Ba budget, but they were probably insignificant in the whole-rock content of Sr.

	MINERALOGY

Top Abstract Introduction The Robitaille Suite Other High-Strontium Igneous... Geochemistry of Sr in... Whole-Rock and Mineral... Mineralogy Development of the Robitaille... Conclusions Aknowledgements References

 
K-feldspar without associated plagioclase

Feldspars are the most abundant minerals of the research suite. For K* > 15, K-feldspar only is found; for K* < 15, K-feldspar is associated with plagioclase. It forms anhedral crystals, particularly in the finer-grained portions of the samples, or tabular crystals, some as Carlsbad twins, and is either low sanidine (2V_x 5°, distinct horizontal dispersion, v > r), as in M·35, or tartan-twinned microcline, as in R·3.

Table 2 shows representative compositions of sanidine and microcline within each sample of the "K-feldspar-only" group. Back-scattered-electron images (Fig. 4), followed by WDS microprobe analysis, indicate that BD·85, R·22A and R·22B are systematically zoned, with Ba and Sr concentrated on the crystal rim. This is shown in Figure 5, where d_r/c represents the distribution of Sr or Ba (with respect to all A atoms of the feldspar) between the rim and core of the crystal. Note that four pairs (for core and rim) lie close to an origin-based linear trend and that all five have Sr and Ba concentrated at the rim. None of the other samples show significant zonation of feldspar crystals but, for these, there is a vague suggestion of Sr concentration in the core of crystals, with patches of Ba enrichment distributed irregularly throughout.

View larger version (141K): [in this window] [in a new window]   FIG. 4. Back-scattered electron image of a phenocryst of sanidine with Ba, Sr-enriched tip (light). Small, white dots are specular hematite. Alkali-feldspar syenite; sample BD·78A.

View larger version (12K): [in this window] [in a new window]   FIG. 5. Distribution of Sr and Ba between rim (r) and core (c) of sanidine crystals, Robitaille Igneous Suite.

Composite dyke R·22 is a special case. Its fractions are clasts of coarse-grained alkali-feldspar melasyenite (sample R·22B) cemented by fine-grained minette (sample R·22A). In each sample, the content of feldspar (entirely sanidine) is low (<20% modal), but it is the most abundant host for Na and Sr. The whole-rock compositions are almost identical, the principal difference being the relative Na contents (Table 1). The net result is higher K* in the whole-rock composition of R·22A, and high Na and Sr in the feldspars of each.

Further details of K-feldspar composition in K-feldspar ± plagioclase associations are given in Table 3.

We applied the highest Na in K-feldspar to the geothermometer of Barth (1969), to determine the temperature of equilibration, but most samples gave unrealistically low temperatures (<450°%, Table 3), perhaps due to non-equilibrium.

For the investigation of plagioclase we found that analyses of feldspar taken close to clinochlore, as in sample BD·226, consistently produced unusable (low-Na) data and, therefore, analyses were taken from another part of the section. Similarly, results of analyses made near calcite ovoids, in samples such as R·22A, were avoided. It was found that the best grains for this work were provided from the relatively unaltered groundmass in kersantite samples BD·96A, BD·145 and BD·163 from a small lens in Complex 4. The grains appeared to be unzoned, but compositions with similar An content occurred together in restricted areas of the section. Average compositions of contiguous grains are shown in Tables 4 and 5. Nine compositions, with SrO (wt%) plotted against An content (mol %), are shown for BD·163 in Figure 6. The distribution of compositions characterizes all three of the above-mentioned lamprophyre samples. Taken together, the following features summarize their compositions: 1) SrO increases linearly from zero wt% at An₀ to ca. 1.0 wt% at An₄₀, 2) An values are clustered at An₀ to An₁₅ and An₂₅ to An₄₀, with a definite compositional gap at An₁₅ to An₂₅.

View larger version (14K): [in this window] [in a new window]   FIG. 6. Relationship between An content and SrO (lower) and between An content and dSrPl/Kfs (upper) in groundmass plagioclase. Kersantite, sample BD·163.

Sample BD·158 (Fig. 7), from porphyritic latite, shows a wide range of plagioclase composition (An_0.2 to An₅₂). Its Sr content peaks at about An₃₀ and declines through An₄₂, to An₅₂ but, as no Sr determination is appreciably above the mdl and some analyses were made on zoned phenocrysts, the analytical data are not considered comparable with those of lamprophyre. Note that the fitted polynomial curve has no physical significance. It is given only to summarize the data.

View larger version (16K): [in this window] [in a new window]   FIG. 7. Relationship between An content and SrO in groundmass plagioclase. Kersantite, sample BD·158.

View larger version (12K): [in this window] [in a new window]   FIG. 8. Relationship between An content and SrO in groundmass plagioclase. Kersantite, sample R·354.

View larger version (15K): [in this window] [in a new window]   FIG. 9. Strontium–Ba relationship of plagioclase in groundmass of kersantite, sample R·354. XBa = Ba/A ions, and XSr = Sr/A ions.

View larger version (167K): [in this window] [in a new window]   FIG. 10. Sanidine crystal (gray) replaced on edges and in cleavages by albite (black). White grains are fluorapatite, except large ovoid at lower right, which is celestine. Backscattered electron image. Kersantite; sample R·354.

	DEVELOPMENT OF THE ROBITAILLE IGNEOUS SUITE

Top Abstract Introduction The Robitaille Suite Other High-Strontium Igneous... Geochemistry of Sr in... Whole-Rock and Mineral... Mineralogy Development of the Robitaille... Conclusions Aknowledgements References

One may speculate on the ultimate source of LILE, Sr, Ba and K. The general eastward younging of alkaline complexes in the Central Metasedimentary Belt (Corriveau et al. 1998) is consistent with a hot spot emanating from an inhomogeneous, subcontinental mantle, with the overlying Laurentia plate transgressing from southeast to northwest at a rate of approximately cm/year [using ages of 1060 and 1089 as extremes, and the distribution of complexes as given in Corriveau (1989), Corriveau et al.(1990), and in this paper]. In the Buckingham area, volcanic rocks were preserved in a downdropped fault-block related to the Ottawa graben, formerly with protective Paleozoic cover such as the Cambro-Ordovician sandstone, exposed as two small outliers, immediately south of the area of Figure 1 (Hogarth, in press).

Forerunners of the main Robitaille suite were Cl-, Cr-bearing, low-K, low-Sr monzonite and monzodiorite dykes (here represented by samples BD·65 and R·152), which intruded Grenville metamorphic rocks immediately outside the complexes. In general, early latite on the eastern side of Complex 4 is low in K but high in Sr (e.g., sample BD·158), but later alkali-feldspar trachyte on the western side (e.g., sample BD·226) is high in both K and Sr. A later-still alkali-feldspar syenite dyke (e.g., sample BD·78A) is high in K but low in Sr (except on the rim of sanidine phenocrysts). However, intra-complex kersantite stocks (e.g., BD·163) are also late but with intermediate K and high Sr. Nothing further can be added to these generalizations, and no further details can be given on the relative ages of the various igneous units.

During emplacement, the magma dismembered layers of Grenville marble, which now appear as attenuated dolostone lenses in the volcanic rock. Similar rock is seen in the aureoles surrounding Complexes 2, 3 and 4, the marble having been transformed to a dolomite – calcite – fluorapatite rock. Transitions to normal calcitic marble are evident in all three aureoles: first with interlayering of the two rock types, then on further retreat from the complex, disappearance of the dolomite – calcite – fluorapatite rock altogether.

Returning to Sr-bearing plagioclase, we emphasize the groupings of SrO–An and d^Sr_Pl/Kfs–An plots. These suggest that igneous activity was not one long continuous process, but one of a differentiating magma in a chamber or several chambers at depth, that periodically injected new material toward the Earth’s surface. Thus kersantite at Complex 1 (sample R·354) had active stages at An₁₅ and An₂₇, with a notable quiescent stage between. Later Pl (An₀₅) is not in equilibrium with contiguous sanidine. At Complex 4, kersantite (sample BD·163) had active stages at ca. An₁₀ and An₃₀, separated by inactivity.

	CONCLUSIONS

Top Abstract Introduction The Robitaille Suite Other High-Strontium Igneous... Geochemistry of Sr in... Whole-Rock and Mineral... Mineralogy Development of the Robitaille... Conclusions Aknowledgements References

 
The Sr content of Buckingham plagioclase commonly shows considerable variation across a single thin section. The An–Sr relationship in high-grade metamorphic gneiss (An_26–48, Fig. 2) reappears in the Robitaille Igneous Suite. It is an equilibrium Sr–Ca–Na effect. However, at Buckingham, equilibration with K-feldspar was never completely attained. Instead, the residual melt was continuously depleted in Sr by precipitation of K-feldspar and other Sr receptors. As the liquid cooled, new crystals of plagioclase became progressively more sodic, with a lower Sr content. After the final precipitation of K-feldspar, sanidine crystals were replaced by essentially end-member albite. The process was a dynamic hybrid of equilibration and differentiation.

Compared to plagioclase, Sr in K-feldspar changes little within an individual sample. Its content depends on the amount of Sr available, temperature, and the presence of plagioclase and other Sr-competitors. Sodium and Ba may have minor effects.

The linear, positively sloping distribution d^Sr_Pl/Kfs versus An for feldspar pairs within a single specimen, will pass through d = 1 (at Buckingham, commonly near An₁₀). For more calcic albite, Sr is preferred in plagioclase, for more sodic plagioclase, in K-feldspar.

The fact that at Buckingham, the Sr content of seemingly unzoned plagioclase varies even within a single thin section warns petrologists to exercise caution in interpreting "K_D" of "coexisting minerals" in igneous rocks. However, that compositions of contiguous grains in the groundmass of kersantite from our Complex 4 can be assigned to linear distributions on d^Sr_Pl/Kfs versus An plots suggests that here, equilibrium was approached. Finally, plots clustered on SrO–An diagrams, possibly indicate periods of eruption and volcanic inactivity. Tentative paths of petrogenetic development of Sr-rich plagioclase (point analyses plotted on Fig. 1) are shown on Figure 11.

View larger version (13K): [in this window] [in a new window]   FIG. 11. Possible paths of mineral development of the high-Sr plagioclase illustrated in Figure 1. Symbols: N: Norwegian, B: Bohemian, R: Russian, I: Italian, and C: Canadian.

	AKNOWLEDGEMENTS

Top Abstract Introduction The Robitaille Suite Other High-Strontium Igneous... Geochemistry of Sr in... Whole-Rock and Mineral... Mineralogy Development of the Robitaille... Conclusions Aknowledgements References

 
Crushed and grouped specimens were first analyzed using X-ray fluorescence by Ron Hartree at University of Ottawa. Polished thin sections were prepared at University of Ottawa by George Mrazek and analyzed by EPMA at McGill, with the assistance of Lang Shi. D.D.H profited from many helpful discussions with colleagues Keith Bell, Tony Fowler, Ralph Kretz and André Lalonde. We are grateful for constructive and helpful comments by Anton Chakhmouradian, an anonymous referee, and for indispensable comments used in redrafting an early version of this paper by Robert F. Martin.

	References

Top Abstract Introduction The Robitaille Suite Other High-Strontium Igneous... Geochemistry of Sr in... Whole-Rock and Mineral... Mineralogy Development of the Robitaille... Conclusions Aknowledgements References

 
BALDRIDGE, W.S., CARMICHAEL, I.S.E. & ALBEE, A.L. (1981): Crystallization paths of leucite-bearing lavas: examples from Italy. Contrib. Mineral. Petrol. 76, 321–335.[CrossRef][ISI][GeoRef]

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Received April 4, 2005 ,revised manuscript accepted March 27, 2007.

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