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Journal of Petrology Volume 42 Number 8 Pages 1519-1546 2001
© Oxford University Press 2001
Petrogenesis of the Labrieville Alkalic Anorthosite Massif, Grenville Province, Quebec
1DEPARTMENT OF GEOLOGY, PO BOX 8795, COLLEGE OF WILLIAM AND MARY, WILLIAMSBURG, VA 23187, USA
2DEPARTMENT OF EARTH AND PLANETARY SCIENCES, WASHINGTON UNIVERSITY, ST. LOUIS, MO 63130, USA
Received March 21, 2000; Revised typescript accepted February 12, 2001
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONREGIONAL SETTINGFIELD RELATIONS AND DESCRIPTIONS...PETROGRAPHY AND MINERAL...CHEMICAL COMPOSITIONSDISCUSSIONCONCLUDING REMARKSAPPENDIX: ANALYTICAL METHODSREFERENCES |
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The Labrieville massif (
1010 Ma) is an eroded dome consisting of three zones: (1) a foliated inner core of pink anorthosite (An30Or13, 2100 ppm Sr, 1300 ppm Ba) with minor leuconorite; (2) a foliated outer core of pink anorthosite (An35Or10, 1900 ppm Sr, 850 ppm Ba) with more abundant leuconorite; (3) a border of foliated to massive green leucogabbro. Antiperthitic plagioclase, orthopyroxene, hemoilmenite, and small amounts of biotite are present in all rocks. The border zone contains additional clinopyroxene, magnetite, and apatite. Labrieville is more alkalic and Sr- and Ba-rich than almost all other massif anorthosites. The widespread presence of biotite and the high hematite content of hemoilmenite suggest crystallization from a relatively oxidizing and water-bearing magma. Collectively, these features imply that Labrieville originated from a magma that differed considerably from typical tholeiitic liquids. Major- and trace-element variations suggest that all rock types represent mixtures of cumulus minerals, with negligible trapped melt. The compositionally distinct zones imply differentiation during crystallization. Although plagioclase compositions become more evolved inward, other indicators (e.g. XMg in pyroxene) suggest differentiation outward from core to border. We favor the latter interpretation, and suggest that the increase in plagioclase An content reflects a decline in pressure during crystallization. KEY WORDS: anorthosite; Grenville Province; plagioclase; cumulate processes
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONREGIONAL SETTINGFIELD RELATIONS AND DESCRIPTIONS...PETROGRAPHY AND MINERAL...CHEMICAL COMPOSITIONSDISCUSSIONCONCLUDING REMARKSAPPENDIX: ANALYTICAL METHODSREFERENCES |
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Massif anorthosites are an unique type of plutonic igneous rock consisting predominantly of a single mineral—plagioclase feldspar. The circumstances whereby such nearly monomineralic plutons can be produced have puzzled geologists for more than a century, and massif anorthosites remain petrologic enigmas. In addition to this fundamental aspect of the ‘anorthosite problem’ (see Bowen, 1917
; Scoates, 2000), the petrogenesis of massif anorthosites was apparently restricted in time and space. Specifically, virtually all of the world’s supply of massif anorthosite was formed during the Proterozoic, and much of it is found in the Grenville Province of Quebec and Labrador (Kranck, 1961; Anderson, 1969; Ashwal, 1993).
Despite their similar plagioclase-rich nature, it is now apparent that massif anorthosites worldwide are mineralogically and compositionally diverse (see Xue & Morse, 1993), implying involvement of a wide range of parental magma compositions or petrogenetic processes in their formation. However, comprehensive studies on individual anorthosite complexes that can elucidate their petrogenesis—combining field and petrographic observations with mineral-chemical data, and with complete major- and trace-element data on a wide range of rock types—are few in number. Admittedly, the immense size of certain massifs (e.g. Lac St. Jean Complex, Woussen et al., 1988) and the remote or inhospitable locations of others (e.g. Kunene Complex, Angola, Ashwal & Twist, 1994; Nain Complex, Labrador, Ranson, 1981), plus the fact that many (perhaps most) large anorthosite bodies are composite in nature (e.g. Harp Lake, Labrador, Emslie, 1980), make investigations of the sort outlined above a challenging and long-term task. For example, the Laramie anorthosite complex, Wyoming, has been studied by numerous investigators over a period of many years (e.g. Fuhrman et al., 1988; Kolker et al., 1990; Frost et al., 1993; Mitchell et al., 1996; Scoates et al., 1996; Scoates, 2000).
In contrast, the Labrieville, Quebec, massif anorthosite is ideal for detailed study on a somewhat shorter time scale because, at 300 km2, it is one of the smaller massifs in the Grenville Province. Despite its small size, however, Labrieville is highly relevant to massif anorthosite petrogenesis because it contains virtually all of the rock types associated with anorthosite suites that have intrigued investigators for generations. Moreover, the young age for Labrieville (1010 Ma; Owens et al., 1994) sets it apart as one of several late- to post-tectonic anorthosites with respect to the Grenville orogeny. Therefore, interpretation of petrologic data is not complicated by the effects of Grenvillian metamorphism.
This paper has three broad objectives. First, we use spatial variations in mineral and rock compositions to address internal differentiation of the Labrieville massif and show that it is a concentrically zoned pluton. Second, we use variations in whole-rock compositions to evaluate the origin of the principal Labrieville rock types (anorthosite, leuconorite, leucogabbro) and show that they represent mixtures of cumulus crystals with little to no trapped liquid. In doing so, we outline a general strategy for the evaluation of whole-rock geochemical data on plagioclase-rich rocks of seemingly monotonous character. Third, we show by comparison with other plutons that Labrieville represents a compositional extreme among massif anorthosites.
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REGIONAL SETTING |
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The Labrieville region (Fig. 1), situated in south–central Quebec
325 km NE of Quebec City, occurs in the Grenville Structural Province, within what has been termed the Central Granulite Terrain (Wynne-Edwards, 1972), the Core Zone (Woussen et al., 1986), and the Allochthonous Polycyclic Belt (Rivers et al., 1989). These names reflect the fact that Labrieville occurs in a segment of Grenville Province that experienced a protracted tectonothermal history from mid- to late Proterozoic times, highlighted by at least two episodes of anorthositic plutonism: emplacement of the Lac St. Jean massif at 1150 Ma (Higgins & van Breemen, 1992) and the Labrieville massif at 1010 Ma (Owens et al., 1994). Elsewhere in the Allochthonous Polycyclic Belt, a third period of anorthositic magmatism occurred at 1060 Ma (Higgins & van Breemen, 1996). In fact, a semi-continuous belt of andesine anorthosites that post-date emplacement of the Lac St. Jean massif occurs in this region of Quebec, including the Château-Richer, St. Urbain, Lac Chaudiere, Lac à Jack, Mattawa, and Labrieville massifs (Fig. 1; Dymek & Owens, 1998b). Additional information on regional geology and geochronology has been given by Owens et al. (1994) and is not repeated here.
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FIELD RELATIONS AND DESCRIPTIONS OF UNITS |
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Nomenclature
For purposes of description and discussion in this paper, the anorthositic rocks at Labrieville have been named on the basis of the amount of normative feldspar and the dominant ferromagnesian mineral(s) present. As illustrated in Fig. 2, the analyzed samples are all very rich in feldspar, whose normative composition ranges from
An28 to An40. Rocks with >95% normative feldspar are termed ‘anorthosite’ (symbol A). Rocks with <95% feldspar are termed ‘leuconorite’ (symbol N) or ‘oxide-anorthosite’ (symbol O) based on whether the dominant mafic mineral is orthopyroxene or hemoilmenite (an exsolution intergrowth of hematite in ilmenite), respectively. Rocks with <95% feldspar that contain additional clinopyroxene are termed ‘leucogabbro’ (symbol G). We recognize that our usage of the term ‘anorthosite’ is more restrictive than in the IUGS classification scheme, which defines anorthosite as any rock with 
90% plagioclase. However, one of the principal features of Labrieville (and the other Quebec andesine anorthosites noted above) is the abundance and widespread presence of essentially ‘pure-plagioclase’ rocks, and one goal of our work was to investigate compositional changes accompanying the addition of other minerals to such rocks.
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Almost without exception, the rock types correspond to the geographical distribution of the samples, with anorthosite, leuconorite, and oxide-anorthosite being found primarily in the core of the massif and leucogabbro in the border zone (see below). Also included in many figures are data points for plagioclase megacrysts (analyzed as whole-rock samples), both from core anorthosite (symbol a) and from border leucogabbro (symbol g). The locations of all samples for which data appear are shown in the sketch map of Fig. 3b.
Geological overview
As mapped by Anderson (1962, 1963, 1966), the Labrieville massif (Fig. 3a) comprises two structural domains: (1) a northern segment, which represents an eroded dome, consisting of a core of foliated anorthosite and leuconorite (plus a small hemoilmenite ore body), and a continuous border consisting of weakly foliated leucogabbro; (2) a southern segment called the Sault aux Cochons complex, which was interpreted by Anderson (1966) as a funnel-shaped intrusion, consisting of massive leucogabbro to mangerite [nomenclature revised from Anderson (1966)]. The present study deals entirely with the northern, ‘anorthositic’ segment. On the basis of regional correlations and distinctive patterns on aeromagnetic maps, we now believe that the Sault aux Cochons Complex represents a separate intrusion much like the nearby Lac Gouin Complex (see Hocq, 1977).
Core zones
On the basis of chemical compositions, the core zone can be divided into an inner [low An/(An + Ab)] and an outer region (Fig. 2), and an approximate boundary between them is shown in Fig. 3b. This separation of the core into two regions is not manifested in any obvious way through field or petrographic characteristics.
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The predominant rock type in the core is anorthosite as defined above (Fig. 2). This anorthosite displays a seriate porphyritic texture consisting of a few percent blue–gray plagioclase megacrysts (from a few centimeters to
50 cm) set in a matrix of finer-grained (1–10 mm), pink to brown plagioclase. Megacrysts range from tabular to blocky, and many contain Carlsbad and albite twins that can be observed with the naked eye.
Leuconorite is distributed irregularly throughout the core, with the best exposures found in the outer portion. Leuconorite occurs in banded sequences in which layers (from <1 m to several meters) containing plagioclase and medium- to coarse-grained orthopyroxene [in many cases as centimeter-sized megacrysts containing plagioclase exsolution lamellae (see Owens & Dymek, 1995)] alternate with anorthosite (Fig. 4a). Leuconorite also occurs as small ‘patches’ that weather preferentially, forming elongate depressions as a result (Fig. 4b). These patches are massive, range in size from a few centimeters to several decimeters, and appear to be somewhat disk-shaped (flattened) in three dimensions. They contain abundant fine- to medium-grained orthopyroxene crystals, which are possibly oikocrysts.
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Most outcrops of core rocks display a prominent foliation—used here in a general sense for a planar rock fabric (Fig. 4a and b). This foliation is defined by: (1) discontinuous (centimeter- to decimeter-scale) layers of orthopyroxene, including pyroxene megacrysts; (2) thin (millimeter-scale) laminae of orthopyroxene and locally biotite; (3) the disk-shaped leuconorite depressions noted above; (4) the orientation of elongate plagioclase megacrysts. Attitudes on this foliation (Anderson, 1962, 1966; Morin, 1969; this study) show the massif to have a dome shape, in which dips increase from near horizontal in the interior to as much as 60° near the margins. The origin of this foliation is uncertain, but several other Grenville massifs also have dome shapes, and the doming has been attributed to diapiric rise of anorthosite (Martignole & Schrijver, 1970; see also Scoates, 2000).
Small amounts (<1%) of Fe–Ti oxide (hemoilmenite) occur in virtually every sample of anorthosite and leuconorite. Locally, however, certain outcrop areas contain higher proportions (up to 15%) of hemoilmenite, and these have been classified as oxide-anorthosite. The hemoilmenite forms irregularly shaped masses of variable size (from <1 cm to several meters across) and some could represent the recrystallized remnants of much larger crystals. These masses are more abundant in the vicinity of the Lac Brulé ore deposit (Fig. 4c; see below). Some oxide masses contain small (a few centimeters across) inclusions of anorthosite, and possess lobate-shaped reentrants that are suggestive of intrusive ‘blobs’ of an immiscible oxide melt.
The hemoilmenite deposit near Lac Brulé (Morin, 1969) crops out along a discontinuous series of narrow ridges (<30 m wide, oriented N10W) that extend for 1 km. Contacts with enclosing anorthosite are poorly exposed (the deposit has not been mined), but are apparently sharp. Capping the ridges locally is a nelsonite horizon (hemoilmenite–apatite rock) up to 2 m thick (Dymek & Owens, 1996). This nelsonite is medium to coarse grained (
5 mm), crudely banded, and contains up to 5% sulfide (predominantly pyrite) and rare corundum.
Border zone
The border zone of the massif ranges in width from <1 km near the western margin to 2 km near the southeastern margin. It appears to be absent from the northern part of the intrusion, although Anderson (1966) inferred its presence around the entire periphery of the dome. In many places, the transition from core to border coincides with lenses of oxide–apatite gabbronorite (OAGN; Anderson, 1966; Owens & Dymek, 1992).
The border unit is variable in grain size and texture. In many places it is prominently foliated like the core, but locally it is very coarse grained and massive. This massive variety has been observed only near the western margin.
We have assigned the name ‘leucogabbro’ to the border zone to indicate the presence of clinopyroxene in addition to orthopyroxene in the mafic mineral assemblage. In addition, several other features distinguish the border zone from the core. First, although anorthosite is present throughout, the border zone is everywhere more mafic than the core. Second, plagioclase megacrysts (2–10 cm across) in border-zone rocks tend to be equant (Fig. 4d), in contrast to the more tabular shapes found in core-zone rocks. In a few places along the western margin, entire layers (up to 10 m thick) consist almost entirely of these equant megacrysts. Third, the border zone contains magnetite, which is lacking in the core, in addition to hemoilmenite. Fourth, in contrast to the pink to brown color of the core-zone rocks, those in the border zone are generally green or gray–green. The reason for this color difference is not apparent. It may reflect a higher proportion of mafic minerals (some of which are altered) but even samples of anorthosite in the border are green. It is conceivable that this color difference is related to differences in oxidation state during crystallization of the two zones, with the pink core reflecting more oxidizing conditions.
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PETROGRAPHY AND MINERAL COMPOSITIONS |
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The contrasting mineralogy of core and border zones is reflected in modal analyses of representative samples of two core-zone leuconorites (samples 099 and 218) and two border-zone leucogabbros (samples 092 and 152), which are listed in Table 1. The leuconorites and the leucogabbros are highly variable on the outcrop and hand-sample scale, and these modes (based on point counts of four thin sections from each sample) should be considered only as a rough indication of the proportions of minerals. The high cpx/opx ratio of the border zone (Table 1) compared with the core zone is a robust difference. Many rocks of the border zone are more altered than those of the core, as manifested by sericitized plagioclase, interstitial carbonate and quartz, minor chlorite, and uralitization of pyroxene.
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Plagioclase grains in all rocks are anhedral, with highly irregular grain boundaries, and generally show no preferred orientation. Evidence for deformation appears as bent albite twin lamellae and the local development of subgrains. This deformation is not penetrative, and furthermore, there is no indication of extensive static recrystallization in these rocks.
Invariably, the plagioclase is antiperthitic, in which exsolved K-feldspar appears as small (<0·2 mm) blebs irregularly distributed throughout the host. K-feldspar also is found locally as small grains (<0·1 mm) interstitial to plagioclase, which probably represent products of ‘granule’ exsolution. Minute (<0·05 mm) grains of (exsolved?) hemoilmenite also occur in most plagioclase crystals. The abundance of these oxide blebs seems to be greater in plagioclase megacrysts than in matrix plagioclase.
Plagioclase compositions range from An31 to An42, with a peak at An35–36 (Table 2). There is considerable overlap in composition between analyses from the core and border zones. As such, plagioclase An contents determined through spot analysis with the electron microprobe (EMP) do not clearly discriminate between these two main parts of the massif. For the most part, the compositions reported above are of grain interiors, and little zoning was detected at rims adjacent to other plagioclase grains. However, some of the more calcic compositions are of rims adjacent to mafic minerals, i.e. many plagioclase grains are reversely zoned. This feature also has been documented in the St. Urbain and Lac St. Jean anorthosites (Dymek, 1981; Woussen et al., 1988) and in the Kiglapait layered intrusion, Labrador (Morse & Nolan, 1984), and may be a common feature of plagioclase-rich cumulates.
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A few analyses of K-feldspar blebs from antiperthite are also listed in Table 2, and the compositions of these blebs along with corresponding hosts are illustrated on a ternary An–Ab–Or plot in Fig. 5a. The K-feldspar blebs approach pure end-member composition, with only a small amount of Na (<0·5 wt % Na2O) and virtually no Ca, but they contain a high content of Ba (up to 2 wt % BaO), which corresponds to as much as 5 mol % celsian (BaAl2Si2O8).
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Bulk compositions of plagioclase megacrysts from anorthosite and leucogabbro, plus normative compositions of pure-plagioclase anorthosite are illustrated on a ternary An–Ab–Or plot in Fig. 5b. A comparison of these feldspar bulk compositions with the positions of isotherms on the ternary feldspar solvus (see Fuhrman & Lindsley, 1988) suggests a minimum temperature of crystallization >750°C, and some (perhaps all) crystallized at >900°C. Furthermore, comparison of Fig. 5a and b indicates that exsolution occurred at temperatures <750°C.
Orthopyroxene (pink–green pleochroic hypersthene) is the dominant mafic silicate throughout the core zone and typically occurs as equant to slightly elongate grains and grain clusters in the 0·5–2·0 mm size range. Nearly all grains contain abundant small exsolved platelets of hemoilmenite. Some grains contain fine-scale (100) exsolution lamellae of clinopyroxene, but most are free of such lamellae. There is no petrographic evidence for the former existence of pigeonite in any of the Labrieville rocks.
Small amounts of clinopyroxene (pale green salite) are also present in most core-zone rocks as small (0·1 mm) interstitial grains or as thin (exsolved?) rims on orthopyroxene or Fe–Ti oxide; no exsolution features were observed in these grains. Larger (1 mm) grains of clinopyroxene are common in border-zone leucogabbro. These grains typically contain very fine (100) exsolution lamellae of orthopyroxene.
Selected analyses of pyroxene from throughout the massif are listed in Table 3, and additional analyses have been given by Owens & Dymek (1995). Orthopyroxene compositions are in the range En67–55 whereas clinopyroxenes are in the range Ca48Mg40FeT+Mn12 to Ca43Mg37FeT+Mn20. These compositions are relatively magnesian, given the alkalic nature of associated plagioclase. Thus, Labrieville lies at the extreme of the An–En paradox for massif anorthosites, first recognized by Anderson & Morin (1969). The paradox is that Labrieville and other massifs contain orthopyroxene that is much more magnesian at a given plagioclase An content than the compositions of such minerals in layered intrusions [see Fig. 10 below and discussion by Owens et al. (1993)]. Orthopyroxene contains small amounts of Al2O3 (1·0–1·5 wt %), TiO2 (<0·1–0·2 wt %), MnO (0·3–0·7 wt %) and CaO (0·4–0·7 wt %), and negligible Na2O (<0·05 wt %). Clinopyroxene contains higher amounts of Al2O3 (1·8–2·7 wt %), TiO2 (0·2–0·5 wt %) and Na2O (0·4–0·7 wt %), but lower MnO (0·1–0·4 wt %). Both pyroxenes contain negligible Cr2O3.
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Values of XMg in pyroxene are illustrated in Fig. 6, which shows that pyroxenes from the border are slightly more Fe-rich than those from the core. Moreover, pyroxenes from the outer core range to more Fe-rich compositions than those from the inner core. Thus EMP spot analyses of pyroxene suggest a concentric progression in composition in the massif.
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Hemoilmenite is the sole Fe–Ti oxide phase throughout the core zone, although some grains contain reduction lamellae of magnetite (see Anderson, 1966). The proportion of exsolved hematite lamellae is in the range of 20–40 vol. %. In many cases, multiple stages of exsolution are apparent, evidenced by a bimodal size distribution of hematite lamellae, in addition to small lamellae of ilmenite within larger lamellae of hematite. Magnetite in leucogabbro is typically free of exsolution features, but rare grains can be found with one or two ilmenite lamellae.
Representative compositions of Fe–Ti oxides are listed in Table 4. In hemoilmenite, the hosts are ferrian-ilmenite (Ilm65–75) and are enriched in Mg and Mn. The lamellae are titan-hematite (Ilm15–25), and are enriched in Al, V, and Cr. Magnetite in border leucogabbro contains small amounts of Al, V and Mn, and very low amounts of Ti; recalculated compositions indicate that most grains contain <0·5% of the ulvöspinel component.
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The virtual absence of magnetite in the core rocks, together with the nearly end-member Fe3O4 composition of border-zone magnetite precludes the use of Fe–Ti oxide thermometry. However, a semi-quantitative indication of the temperature and oxygen fugacity conditions during crystallization can be inferred from the bulk composition of the hemoilmenite itself. In the system FeO–Fe2O3–TiO2, the occurrence of a single oxide phase, in this case hemoilmenite, implies a specific relationship among composition, temperature, and oxygen fugacity. At a fixed temperature and Fe/Ti ratio, a higher fO2 would result in the production of rutile (or possibly pseudobrookite) whereas a lower fO2 would lead to the formation of magnetite. A higher fO2 could produce a more hematite-rich ilmenite, but only if bulk Fe/Ti were to change.
X-ray fluorescence (XRF) analyses, obtained on several whole-rock samples of massive hemoilmenite (Table 5) were treated as equivalent to bulk-oxide compositions. The bulk composition of the Labrieville hemoilmenite ore is Ilm72, which is constrained to lie on an appropriate ilmenite isopleth in T–fO2 space. This isopleth, as inferred from the results of Spencer & Lindsley (1981), lies between the hematite–magnetite (HM) and quartz–fayalite–magnetite (QFM) buffer curves. Thus, the crystallization conditions for the hemoilmenite deposit (and by inference the anorthosite massif), must have been a few log units above QFM, and thus relatively oxidizing.
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An important feature of the Labrieville rocks is the presence of biotite as an essential, nearly ubiquitous, minor phase. Biotite typically occurs in millimeter-sized, sheaf-like masses in association with orthopyroxene and/or hemoilmenite, and also as isolated grains interstitial to plagioclase. The biotite is typically brown or reddish brown, and a few EMP analyses reveal high contents of TiO2 (4–5 wt %), high XMg (0·70–0·75), small amounts of F and negligible Cl. The common presence of biotite is a feature that Labrieville shares with the St. Urbain and several other Grenville massifs, suggesting that not all anorthosites are the products of exceptionally dry magmas.
Trace amounts of apatite (up to 1·0 mm across) occur in numerous samples of core anorthosite and leuconorite, and apatite is an important constituent of nelsonite in the ore deposit. Apatite is considerably more abundant (up to 2 modal %, Table 1) in border leucogabbro, and occurs in common association with pyroxene and Fe–Ti oxide.
Myrmekite occurs in minor amounts in numerous anorthosite samples along plagioclase–plagioclase grain boundaries or at the junction of several grains. Energy-dispersive analyses and back-scattered electron imaging reveal that they consist of calcic plagioclase (An60–70) plus quartz. As such, they appear similar in most respects to the calcic myrmekite described from the St. Urbain anorthosite (Dymek & Schiffries, 1987), which has been interpreted as the product of late-stage aqueous corrosion of early formed plagioclase.
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CHEMICAL COMPOSITIONS |
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Forty-four samples of anorthosite, leuconorite, oxide-anorthosite, and leucogabbro were analyzed for major and trace elements by XRF methods, and a subset of 23 samples was analyzed for additional trace elements by instrumental neutron activation analysis (INAA). Locations of these analyzed samples are shown in the sketch map of Fig. 3b, and selected analyses are listed in Table 6. The complete dataset is available by request from the authors.
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Major elements
MgO was chosen as an index of variation because this oxide monitors the amount of non-plagioclase components in a given sample. This point is illustrated in Fig. 7, a plot of normative feldspar vs MgO. Here, data points for leuconorites and oxide-anorthosites form roughly linear arrays trending away from plagioclase, whereas leucogabbros define an intermediate array, reflecting the presence of both pyroxene and Fe–Ti oxide. The straightforward interpretation is that oxide-anorthosites, leuconorites, and leucogabbros represent mixtures of different mafic minerals with plagioclase, assembled in variable proportions.
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The MgO-variation diagrams of Fig. 8 also illustrate the effects of mineral–mineral mixing in the Labrieville rocks. For example, although Al2O3 (Fig. 8a) varies essentially as a function of the amount of plagioclase, fields for oxide-anorthosite, leuconorite, and leucogabbro are distinct. In plots for Fe2O3-T and TiO2 (Fig. 8b and c), the effect of adding pyroxene vs Fe–Ti oxide to plagioclase is readily apparent as leuconorites and oxide-anorthosites form distinct arrays, and the leucogabbros fall between these.
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CaO in anorthosite ranges from 6 to 7 wt % (reflecting variable An in constituent plagioclase), but concentrations range to lower values for leuconorites and oxide anorthosites (Fig. 8d). Border leucogabbros, however, show an increase in CaO, which reflects mainly the added presence of clinopyroxene. Plagioclase megacrysts in leucogabbro are also higher in CaO (more anorthitic) than anorthosite in the core.
Na2O (Fig. 8e) in anorthosite ranges from 6·0 to 6·5 wt % and mimics Al2O3. K2O shows a wide range in anorthosite, from 1·5 to 2·3 wt %, and can be correlated crudely with percent normative feldspar (not plotted). K2O decreases with the addition of other minerals, although it is also lower in plagioclase megacrysts from border leucogabbro than from anorthosite.
All core anorthosites and leuconorites contain very low P2O5 (typically <0·1 wt %; Fig. 8f). However, border leucogabbros typically contain higher amounts, up to 0·7 wt %, consistent with more abundant modal apatite.
Trace elements
Much of the variation in the analyzed rocks reflects the proportion of plagioclase in a given sample. For this reason, many of the figures dealing with trace elements contain two plots—one that shows the variation of the individual trace element with MgO, and a companion plot that illustrates how the various trace elements vary with respect to another element or oxide.
Sr, Ba, Rb
Strontium and Ba are the most abundant trace elements in the anorthositic rocks at Labrieville. Sr concentrations (Fig. 9a) are remarkably high, ranging from 1730 to 2270 ppm, apart from two samples with lower amounts (1243 and 1530 ppm), both of which come from near the ore deposit (sample 040 is at the anorthosite–ore contact). The leuconorites and leucogabbros all contain less Sr, although the amounts are still high (1200–1800 ppm). Strontium correlates very well with Na2O (not plotted), reflecting a plagioclase dilution trend, but correlates poorly with CaO (Fig. 9b). The distribution of data points in Fig. 9b suggests mixing among orthopyroxene and ilmenite (which plot near the origin), plagioclase of variable composition, and clinopyroxene at higher CaO and lower Sr.
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Concentrations of Ba are also very high, and span a range in anorthosite from 670 to 1250 ppm. Levels of Ba in leuconorites, oxide-anorthosites, and leucogabbros are also high, but show no correlation with MgO (Fig. 9c), in contrast to Sr. The range of Ba concentrations appears to be controlled primarily by the amount of K2O (Fig. 9d).
Concentrations of Rb (3–12 ppm) are uniformly low, despite the Na- and K-rich nature of this massif. For the sample set as a whole, Rb correlates poorly with MgO but increases with K (not plotted). As a result of low Rb concentrations, K/Rb values are extremely high (all >1000, most >2000), whereas Rb/Sr values are all extremely low (<0·006).
Ga
Ga concentrations in anorthosite are relatively low (19–24 ppm, Fig. 10a). Such low levels of Ga, coupled with high Al, result in values of Ga/Al that are much lower than those of plagioclase from basaltic rocks (see Dymek, 1990). In leuconorites, concentrations of Ga range to slightly lower values, probably reflecting ‘dilution’ by orthopyroxene, but values are slightly higher in leucogabbros, which is probably due to the presence of magnetite in those rocks. Contrary to general expectations (e.g. Goodman, 1972) Ga does not correlate with Al in this sample suite (not plotted).
Transition metals
Nickel shows an increase with MgO (Fig. 10b), and this is also the case for Sc, Cr, Co, and Zn (not plotted). Thus, leuconorites, oxide-anorthosites, and leucogabbros all contain higher amounts than anorthosite, confirming that pyroxene and hemoilmenite are both important reservoirs for these elements. However, the different rock types display contrasting positive correlations, reflecting both the type and modal abundance of the predominant mafic mineral (or minerals) present. For example, for a given MgO content, Ni is higher in leuconorites than in leucogabbros, but the reverse is true for V. The latter element correlates well with Ti (not plotted), indicating that V is harbored primarily in hemoilmenite. Values of Ti/V are high in all samples, in most cases >100, a feature that appears to be a characteristic of the anorthosite–jotunite–mangerite suite (see Owens et al., 1993).
Zr, Hf, Y, Nb, Ta
Zirconium concentrations are low in anorthosite, with most samples having <20 ppm (Fig. 10d). Leuconorites show only slightly higher Zr, but oxide-anorthosites and leucogabbros contain up to 80 ppm. Zirconium and Ti correlate fairly well (not plotted), suggesting either that Zr is harbored in hemoilmenite, or that zircon is associated primarily with Fe–Ti oxide. Although there is some scatter, Zr and Hf are correlated positively (not plotted), as expected. Values of Y, Nb, and Ta are very low in all rocks (typically at or below the detection limit), although higher amounts of Ta (up to 0·4 ppm) are found in oxide-anorthosites and some leucogabbros.
Rare earth elements (REE)
A general indication of how the REE are distributed among the various lithologies can be gained through plots of La and Yb vs MgO. Figure 11a shows that anorthosites and oxide-anorthosites have La concentrations in the 2–4 ppm range. Lanthanum concentrations remain relatively constant as MgO increases in the leuconorites, although the leuconorites have slightly higher amounts than some anorthosites. Leucogabbros, on the other hand, have consistently higher amounts (6–10 ppm). Lanthanum and P2O5 are positively correlated (Fig. 11b), albeit crudely, suggesting that apatite is the primary reservoir for the light REE (LREE), at least in rocks that contain more than 4 ppm La.
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In contrast to La, Yb and MgO are correlated positively in the leuconorites (Fig. 11c), reflecting the ‘compatible’ nature of Yb in orthopyroxene. Values of Yb are generally higher in the leucogabbros and show a crude positive correlation with P2O5 (Fig. 11d), suggesting that apatite (in addition to pyroxene) is an important reservoir for the heavy REE (HREE) in these rocks.
The concentrations of REE are illustrated further in a series of chondrite-normalized diagrams in Fig. 12. The LREE concentrations are low in anorthosite (LaN 7–11x), and overall pattern shapes are steeply fractionated with large positive Eu anomalies (Fig. 12a). Concentrations of HREE are extremely low, and in some samples only maximum values could be estimated for Yb and Lu. Samples from the inner and outer core regions overlap with respect to REE concentrations. The chondrite-normalized patterns for the oxide anorthosites are essentially identical to those of the anorthosites (Fig. 12b). Leuconorites, in contrast, differ from anorthosites in containing slightly (La, Ce, Sm) to significantly (Tb, Yb, Lu) higher amounts of all REE except for Eu. Thus, chondrite-normalized pattern shapes for leuconorites are considerably less fractionated than those for anorthosites, and patterns become flatter as the total amount of REE increases (Fig. 12a). In addition, the magnitude of the positive Eu anomaly is diminished.
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Concentrations of REE in one anorthosite and several leucogabbros from the border unit are illustrated in Fig. 12c. Anorthosite from the border is similar to some core anorthosites with respect to the LREE, but contains a higher concentration of HREE, such that its chondrite-normalized pattern is not as steeply fractionated. Leucogabbros tend to contain higher amounts of all REE than core anorthosites and leuconorites, and their pattern shapes do not show the flattening in the HREE of the leuconorites. Within the leucogabbros, the range in REE concentrations is not well correlated with MgO, but is more closely correlated with P2O5 (Fig. 11). As noted above, this observation suggests that the REE patterns of the leucogabbros do not primarily reflect the isolated effect of additional pyroxene (as is true for leuconorites), but rather show the combined effects of additional pyroxene plus apatite.
Concentrations of REE in two plagioclase megacrysts from core anorthosite and two from border leucogabbro are illustrated in Fig. 12d. Although their chondrite-normalized patterns are similar, megacrysts in leucogabbro have slightly lower La and Ce, but higher Sm, Eu, and HREE than those in core anorthosite. The net effect of these differences is that leucogabbro megacrysts display patterns that are rotated slightly in an anticlockwise direction relative to patterns for megacrysts in anorthosite.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONREGIONAL SETTINGFIELD RELATIONS AND DESCRIPTIONS...PETROGRAPHY AND MINERAL...CHEMICAL COMPOSITIONSDISCUSSIONCONCLUDING REMARKSAPPENDIX: ANALYTICAL METHODSREFERENCES |
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Labrieville compositional extremes
Since the work of Anderson (1966)
, the Labrieville massif has been recognized as an alkalic, oxidized anorthosite, characterized by andesine–antiperthite and hemoilmenite. The results of the present study provide comprehensive and quantitative confirmation of these observations, and reveal unusually high concentrations of Sr and Ba in all rock types as well. To illustrate these compositional extremes, the Labrieville data are compared with results (also obtained in the Washington University laboratory) from two other Grenville anorthosites—the St. Urbain and Morin massifs.
Normative, whole-rock feldspar compositions of these anorthosites are illustrated in Fig. 13. There is a clear progression in normative An from Morin (An54–41) to St. Urbain (An46–36) to Labrieville. Furthermore, Labrieville is more potassic than the other massifs, ranging up to Or14. These results confirm the highly alkalic nature of the Labrieville anorthosite, which is possibly surpassed only by the Roseland, Virginia, massif, for which preliminary results yield An29Or16 (Owens & Dymek, 1999a). Compositions for other anorthosites (e.g. Nain, Adirondacks, Lac St. Jean) range to higher An than those considered here and are not plotted.
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Normative feldspar compositions of whole-rock samples from the Kiglapait layered intrusion, Labrador (Morse, 1981) are included for comparison in Fig. 13. Feldspar compositions as potassic as those at Labrieville are not achieved until the upper part of the Upper Layered Zone, and at an even lower An content. The Kiglapait trend extends to even more evolved feldspars (mesoperthites; An9Or33) at the top of the Upper Layered Zone (Fig. 13). Figure 13 also includes data from whole-rock samples from the Skaergaard layered intrusion, Greenland (McBirney, 1989). It can be seen on this plot that the Labrieville compositions are also more potassic, for a given An content, than late-stage Skaergaard feldspars, and several compositions have even higher normative Or than the most evolved feldspars in the Sandwich Horizon (Wager & Brown, 1967). An additional observation is that the feldspars of all three massifs (at a given An content) are more potassic than those from the Kiglapait and Skaergaard intrusions.
One possible explanation for the alkalic nature of Labrieville feldspars is that they represent highly evolved compositions analogous to those formed at the last stages of differentiation of Skaergaard- or Kiglapait-type liquids. For the Skaergaard or Kiglapait intrusions, the evolved character of their late-stage feldspars can be attributed to extensive and prolonged prior fractionation of plagioclase. At Labrieville, however, there is no field evidence for the existence of the substantial volume of earlier-formed, more calcic plagioclase required by such a model. Furthermore, the extreme levels of Sr in Labrieville plagioclase preclude any significant prior plagioclase fractionation in the history of this massif (unless accompanied by the fractionation of prodigious quantities of low-Sr mafic phases, for which there is also no evidence). For the same reason, the progression in plagioclase compositions from Morin to St. Urbain to Labrieville cannot be interpreted as a fractionation trend, because the absolute amounts of Sr increase in the same sequence. Therefore, the alkalic compositions at Labrieville can be explained most easily by crystallization from an alkalic parental magma, which probably differed considerably from the broadly tholeiitic magmas that gave rise to the Skaergaard and Kiglapait layered intrusions. It has been argued by some investigators (e.g. Gray, 1987) that alkalic compositions in massif anorthosites result from crustal contamination of mantle-derived basalt containing otherwise normal levels of K and Na. At Labrieville, however, the combined Rb–Sr, Sm–Nd, and U–Th–Pb isotopic data contraindicate such contamination (Owens et al., 1994). Our preferred interpretation for the alkalic nature of this massif is that it reflects the composition of the source region, which could be alkali-enriched lithospheric mantle (see Owens, 1998) or lower crust (see Duchesne et al., 1999; Schiellerup et al., 2000).
The Sr and Ba concentrations in Labrieville rocks are compared with those from St. Urbain and Morin anorthosites in Fig. 14. Concentrations of both elements show a crude systematic increase in the sequence Morin 
St. Urbain Labrieville, and correlate with the trend of decreasing normative An (or increasing normative Or) illustrated in Fig. 13. Also shown for comparison in Fig. 14 are data for anorthosites from the Nain complex, Labrador (Xue & Morse, 1993), which are also distinct from Labrieville.
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The extreme levels of these trace elements in the Labrieville massif make it the most Sr- and Ba-rich anorthosite yet reported. Not only is Labrieville distinctive in this respect, but it appears that these feldspar compositions are extreme relative to those reported from virtually all igneous rock types. To illustrate this feature, the Sr and Ba concentrations in anorthosite (rocks with >95% normative feldspar) and plagioclase megacrysts from Labrieville are compared with data on plagioclase compiled by Blundy & Wood (1991) in their studies of Sr and Ba partitioning (Fig. 15). Most samples from Labrieville have Sr and Ba concentrations higher than those reported in other studies, but are most like compositions reported by Berlin & Henderson (1969) for trachyte or phonolite, or by Liotard et al. (1979) for andesite or rhyolite.
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These high levels of Sr and Ba can be accounted for in (at least) two ways: crystallization from liquids with extremely high Sr and Ba assuming ‘typical’ plagioclase–melt partition coefficients, or crystallization of plagioclase having unusually high partition coefficients for these elements. With respect to the latter interpretation, Blundy & Wood (1991) reasoned on thermodynamic grounds that DSr (plag/liq) and DBa (plag/liq) should both increase with decreasing An content. Thus, much higher partition coefficients might apply in the case of the low-An Labrieville plagioclase. Application of the semi-empirical relationships between D and XAn derived by Blundy & Wood (1991) yields DSr = 4·6 and DBa = 0·9 for plagioclase having An/Ab like Labrieville (using T = 1200°C). Use of these calculated partition coefficients implies crystallization from less-extreme liquid compositions (400 ppm Sr and 1100 ppm Ba), although Ba is still relatively high.
On the other hand, Morse (1992) rejected the Blundy & Wood crystal-chemical model for Sr partitioning in plagioclase, and argued instead for a strong control by liquid composition. Values of DSr (plag/liq) derived by Morse (1982a) from data in the Kiglapait intrusion vary from 1·5 to 2 over the course of crystallization. Using a typical Kiglapait value of 1·8, calculated liquid compositions in equilibrium with Labrieville plagioclase (2000 ppm Sr) would contain 1100 ppm Sr, nearly three times the value determined from the Blundy & Wood model.
The effect of pressure on DSr (plag/liq) has recently been evaluated experimentally by Vander Auwera et al. (2000), who concluded that the partition coefficient decreases slightly with increasing pressure. Thus, the elevated levels of Sr in Labrieville plagioclase probably cannot be attributed to higher pressures of crystallization than for other anorthosites. Vander Auwera et al. (2000) also showed that the relationship between DSr (plag/liq) and XAn derived by Blundy & Wood (1991) consistently overestimated DSr relative to the values determined from their experiments, and that the disparity increased with increasing pressure by as much as a factor of three. In contrast, values of DSr based on the Morse (1982a) model slightly underestimated the measured values. Nevertheless, Vander Auwera et al. (2000) concluded from all of their experiments that a DSr (plag/liq) value of 1·8 is probably appropriate for anorthositic magmas crystallizing at a pressure of 10 kbar. A reasonable inference from all of this work is that atypically high values of DSr (plag/liq) are probably not applicable at Labrieville. On the other hand, the elevated levels of Ba in the anorthositic feldspars are entirely consistent with their highly potassic nature, such that DBa (plag/liq) may have been 1 (see below). Therefore, we prefer an interpretation involving high-Sr and high-Ba liquids as the most plausible explanation for Labrieville feldspar compositions.
The nature and implications of chemical variations among Labrieville rocks
The major- and trace-element variation diagrams presented in Figs 7–11 clearly demonstrate the importance of mineral–mineral mixing in controlling the whole-rock compositions of the various Labrieville samples. This result is perhaps not surprising, but it has profound implications for the ‘chemical assembly’ (Dymek, 1989) of the rocks at Labrieville, as well as for the interpretation of variation diagrams for anorthositic rocks in general. Specifically, do whole-rock samples represent cumulus plagioclase plus mafic minerals that crystallized from trapped liquid? Or do the rocks consist primarily of variable mixtures of cumulus minerals, with negligible trapped-liquid component? In this regard, Simmons & Hanson (1978) and Ashwal & Seifert (1980) assumed, largely on the basis of REE contents, that anorthositic rocks from the Adirondacks (and Nain) could be modeled as mixtures of cumulus plagioclase plus trapped (possibly parental) liquid. These investigators inferred furthermore that the mafic fractions of rocks (e.g. in leucogabbro) represent the in situ crystallization products of this trapped-liquid component. The Labrieville data provide a means of testing this hypothesis.
As reasoned by Salpas et al. (1983), anorthositic rocks consisting primarily of cumulus plagioclase plus variable amounts of trapped liquid (of essentially fixed composition) should yield a data array with a positive slope in diagrams such as Figs 8f (P2O5 vs MgO) and 11a (La vs MgO), reflecting plagioclase–liquid mixing. Such trends result from the fact that both elements on each plot are incompatible in plagioclase and, therefore, the levels of both should increase with the amount of trapped liquid in the rock. P2O5 and La are also incompatible in pyroxene, whereas MgO is compatible. Therefore, rocks consisting primarily of variable quantities of cumulus plagioclase + cumulus pyroxene (+Fe–Ti oxide) should yield a broadly horizontal data array, reflecting plagioclase–pyroxene mixing. It can be seen in Figs 8f and 11a that the data for core anorthosites, oxide-anorthosites, and leuconorites yield essentially horizontal trends, consistent with the interpretation that these rocks represent variable mixtures of cumulus plagioclase, orthopyroxene, and hemoilmenite, with negligible trapped-liquid component. Likewise, the good correlation between Yb and MgO shown by the leuconorites (Fig. 11c) indicates that these rocks represent primarily plagioclase–orthopyroxene mixtures.
The amount of trapped liquid component in relatively pure core anorthosite can be evaluated by comparing the compositions of plagioclase megacrysts and their associated anorthosite host rocks. In the two measured cases, the compositions of megacrysts are essentially the same as their hosts (Table 6), even for P2O5 and the REE (in one case the megacryst actually contains higher P2O5 and REE than its host). Thus, these samples of anorthosite appear to be devoid of any trapped liquid component (which would presumably contain a higher concentration of these elements than the cumulus plagioclase).
The horizontal data arrays for P2O5 (Fig. 8f) and La vs MgO (Fig. 11a) clearly demonstrate that there is no correlation between potential trapped liquid components and the amount of mafic material in the leuconorites. However, it is an oversimplification to view the leuconorites as pure orthopyroxene–plagioclase (± ilmenite) cumulates because they tend to contain higher amounts of the LREE than anorthosites (Fig. 12a). Rocks that are simple orthopyroxene–plagioclase mixtures should show somewhat LREE-depleted patterns relative to anorthosite, because partition coefficients for LREE in orthopyroxene are less than those in plagioclase (e.g. Dunn & Sen, 1994). The enhanced LREE in the leuconorites relative to anorthosites may be a trapped liquid effect, which would be consistent with slightly higher P2O5 in leuconorites (Fig. 11b).
In contrast to the core lithologies, the data for border leucogabbros plot away from anorthosite in a direction of increasing MgO, P2O5, La, and Yb (Figs 8f and 11a and c), but the distribution of data points is somewhat scattered and clearly does not define a simple trend. If these rocks represent mixtures of cumulus plagioclase plus variable amounts of trapped liquid, then the composition of the trapped-liquid component must have been different in each case. Alternatively, the leucogabbros might simply represent variable mixtures of cumulus plagioclase + pyroxene + Fe–Ti oxide + apatite, once apatite became a cumulus mineral, but the scatter in the data implies that these minerals do not occur in simple, cotectic proportions. Some mechanical sorting of minerals, especially apatite, is implied. A trapped-liquid component might be present in addition to several cumulus minerals, but in this case it would be more difficult to recognize. We consider the latter interpretation (i.e. the presence of cumulus apatite) to be the one that is most consistent with the chemical data, and with the petrographic observation of coarse apatite grains in many thin sections of leucogabbro. In either case, the REE patterns of the leucogabbros show that they were assembled in a different fashion than the core leuconorites (Fig. 12).
A larger dataset (including REE analyses of individual minerals) would be desirable to evaluate more fully the cause of chemical variation in the Labrieville rocks. However, the above analysis is sufficient to demonstrate that most rocks, even the more mafic varieties, consist primarily of variable mixtures of cumulus minerals, rather than mixtures of plagioclase + trapped liquid (see Simmons & Hanson, 1978; Ashwal & Seifert, 1980). This result implies the efficient expulsion of trapped liquid from all rock types in the massif, not just from relatively pure anorthosite. We infer that the expulsion of liquid occurred during magmatic crystallization or recrystallization accompanying diapiric rise of the anorthosite mass that produced its dome shape (see Dymek, 1999).
The fact that most rocks at Labrieville are made up of variable proportions of cumulus minerals has important implications for the interpretation of whole-rock variation diagrams as applied to anorthositic rocks in general. Regardless of the index of differentiation used (SiO2, MgO, or some combination of oxides), any variation diagram applied to these rocks will normally reflect the effects of mineral–mineral mixing (e.g. plagioclase + orthopyroxene), rather than the liquid line of descent of a magma. Therefore, any suggestion that such variation diagrams illustrate the differentiation trend of anorthosite parental magmas should be viewed with skepticism. For example, Buddington (1972), Ashwal (1978, 1993), and McLelland & Whitney (1990), among others, have argued that rocks of the anorthosite and mangerite (or charnockite) suites in the Adirondacks (and elsewhere) show different and opposing differentiation trends on variation diagrams, and thus originated from separate parental magmas. This assertion has been taken as evidence for ‘bimodal magmatism’ in the evolution of anorthosite complexes. However, if the results from Labrieville have more general application, then this approach has serious limitations. Specifically, the strong effects of accumulation and mineral–mineral mixing are likely to dominate the distribution of data for anorthositic rocks, making the recognition of a true liquid differentiation trend difficult. Therefore, we suggest that variation diagrams, although useful for illustrating whole-rock data, are not the best means of evaluating anorthositic differentiation trends, or possible genetic relationships between anorthosite and the felsic rocks of the mangerite suite.
Differentiation of the Labrieville massif
An unanticipated outcome of our whole-rock compositional dataset was the recognition of an ‘inner core’ consisting of rocks with average normative compositions An30Or13 and an ‘outer core’ averaging An35Or10 (see Fig. 13); in addition, Ba and Sr are higher in anorthosites of the inner core than the outer core (see Fig. 14). We note that normative An contents in border zone rocks overlap entirely with those of the outer core, and that plagioclase megacrysts in the border are more calcic than those in the core (Fig. 2). The existence of these zones with different compositions implies some sort of differentiation during crystallization of the anorthositic portion of the Labrieville massif. If feldspar composition is used as the sole monitor of differentiation, the massif appears to have crystallized inward (or from the top down before doming) because inner core rocks are richer in Na and K (and Ba) than those in the outer core and border.
However, an interpretation of inward differentiation is problematic for several reasons. First, pyroxene XMg decreases from the inner core through the outer core to the border (Fig. 6) implying differentiation outward towards the rim (see Anderson, 1966). Second, inward differentiation is at odds with the higher Sr in inner core rocks, unless plagioclase fractionation was accompanied by vast amounts of mafic material. Such mafic material is certainly not present in the field, and there is no evidence to support arguments for missing, buried mafic cumulates (Owens & Dymek, 1995). Third, the change in mineral assemblage at the pluton scale from core to border is more consistent with outward differentiation. Specifically, the border rocks contain more phases than core rocks, including two pyroxenes, two Fe–Ti oxides, and apatite. The addition of phases to a crystallizing assemblage, rather than their elimination, is much more in keeping with all observational, experimental, and theoretical investigations of crystallizing magmas.
For the above reasons, we consider inward differentiation of the massif to be highly unlikely and favor instead the alternative scenario of outward differentiation from inner core to outer core to border (or from the bottom upward before doming). However, this interpretation is not without difficulty, as it requires explanation of (at least) two additional features. First, Ba concentrations decrease in this proposed fractionation sequence. This finding implies that DBa (plag/liq) > 1, which we believe is a plausible crystal-chemical effect related to high K in Labrieville plagioclase; the fact that Ba correlates well with K (Fig. 6d) supports this interpretation. Second, but more difficult to explain, is that outward differentiation requires plagioclase compositions to become more calcic (and less potassic) from inner core through outer core to border. Such a trend is, of course, opposite to that of plagioclase compositional variation expected during cooling.
One possible explanation for this postulated reverse fractionation trend is that the Labrieville magma was progressively contaminated with a high-Ca, low-K component during crystallization. A convenient contaminant of this type would be marble, but no marble occurs in the vicinity of this massif. Another possible contaminant is labradorite anorthosite, such as represented by the nearby Lac St. Jean massif (Fig. 1). In fact, a few xenoliths of labradorite anorthosite do occur in the Labrieville pluton (see Anderson, 1966; Dymek & Owens, 1998a).
Contamination by calcic plagioclase would have essentially no effect on Fe/Mg in mafic phases, thus accounting for the ‘normal’ cryptic variation recorded by pyroxene, but potentially could alter the An content of crystallizing plagioclase in a profound way. On the basis of theoretical analyses of Bowen (1928), McBirney (1979), and Morse (1980), assimilation of plagioclase that is more calcic than that in equilibrium with a magma occurs by crystallization of additional equilibrium-composition plagioclase, whereas the xenocrystic plagioclase is made over through diffusive reaction into a composition in equilibrium with the melt. As a result, such assimilation is a ‘self-defeating’ process because it consumes melt (Morse, 1980). The net effect of such a process in the case of perfect equilibrium would be to raise the temperature slightly (because the assimilative reaction in this case is slightly exothermic) and promote crystallization of a more calcic plagioclase (Bowen, 1928). Under natural conditions, experiments of Tsuchiyama (1985) suggest that the effects are probably less striking and most labradorite xenoliths would be relatively inert in a Labrieville magma (assuming that the xenoliths were actually incorporated into a liquid rather than a highly crystalline mush). The fact that at least some of the xenoliths can be still recognized as compositionally and physically distinct entities demonstrates that re-equilibration with the melt did not go to completion. Thus, from a consideration of thermal and mass-balance constraints, assimilation seems an ineffective means of producing a reversal of plagioclase compositional evolution at the pluton scale.
Yet another explanation that warrants consideration involves the effects of H2O on plagioclase compositional relationships. Specifically, it has been demonstrated experimentally, first by Yoder (1969) and more recently by Sisson & Grove (1993), that an increase in the amount of water in mafic magmas results in the crystallization of a more calcic plagioclase than in dry magmas. Indeed, Sisson & Grove (1993) documented clear increases in (Ca/Na)plag/(Ca/Na)liquid with increases in the amount of water in their experiments. Thus, it is conceivable that a build-up of H2O during crystallization of the oxidized and water-bearing Labrieville magma resulted in the production of progressively more calcic plagioclase as differentiation proceeded. That the Labrieville magma was fluid bearing (but certainly not vapor saturated) is suggested by the ubiquitous presence of biotite throughout the massif, patches of calcic myrmekite (see Dymek & Schiffries, 1987), and the oxidized nature of the hemoilmenite. In addition, the typically altered character of border leucogabbros could be a deuteric effect, related to this proposed increase in water content.
A final possibility is that the inner and outer core zones crystallized under slightly different conditions, perhaps at different levels in the crust, irrespective of whether the parental magma was water bearing. In this scenario, the inner core zone would commence crystallization at ‘high’ pressure. After sufficient plagioclase accumulation was achieved, the inner core began to rise as a magmatic diapir carrying with it an envelope of residual, more mafic liquid. If our estimate of the size of the inner core in map view is approximately correct (and the units are equidimensional vertically), this zone probably makes up no more than 50% of the massif (Fig. 3). Thus, if the inner core represents a zone of initial crystal accumulation, the proportion of crystals to liquid before doming would appear to lie well below the limit of 65% for movement of plagioclase-rich crystal mushes (Longhi et al., 1993). At a higher crustal level (i.e. slightly lower pressure), the outer core and border zones crystallized. The lack of foliation in parts of the border zone suggests that it crystallized close to the level of final emplacement, i.e. after upward diapiric movement had ceased. We note that jotunites, rocks that we consider to be residual to crystallization of the massif (Owens et al., 1993), also occur in association with border leucogabbros.
Polybaric crystallization is supported by experimental studies, and would be consistent with certain parts of currently popular models for anorthosite petrogenesis. For example, Green (1969) showed that the liquidus plagioclase in several melt compositions (including anorthositic) becomes more sodic and potassic with increasing pressure. More recently, Longhi et al. (1993) documented similar shifts in plagioclase composition with pressure, and observed that the shift is 1% An/kbar (although this value varies somewhat with bulk composition). Thus, the change from a higher-Ab and -Or plagioclase (inner core) to a lower-Ab and -Or plagioclase (outer core) at Labrieville, which involves only a slight difference in composition (Fig. 13), would require a pressure drop of <5 kbar if these experimental results are applied.
Longhi et al. (1993) extended this observation about the pressure effect on plagioclase composition to a more elaborate model involving polybaric fractionation for massif anorthosites. Longhi et al. (1993) puzzled over the apparent lack of reverse zoning at the grain scale in anorthosites and suggested that this was due to plagioclase–melt re-equilibration under very slow cooling conditions (coupled with the fact that plagioclase-rich crystal mushes should show only a small shift in An content with decreasing pressure). Nonetheless, pluton-scale reversals in plagioclase composition are entirely compatible with the experimental evidence, provided that early-formed, high-pressure portions of a pluton are effectively isolated from the liquid at lower pressure.
As a final observation, the presence of compositionally distinct zones is a feature that is apparently not unique to Labrieville. Our recent work on the nearby Mattawa massif (Owens & Dymek, 1998, 1999b) has shown a similar zonal arrangement, and the anorthosites at St. Urbain, Lac à Jack, and Château-Richer appear to have more calcic plagioclase near their borders (R. F. Dymek, unpublished results, 1999). Hence, pluton-scale reverse zoning might be a characteristic feature of these andesine anorthosites, indicating that they evolved in a similar fashion, regardless of the mechanism.
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CONCLUDING REMARKS |
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TOPABSTRACTINTRODUCTIONREGIONAL SETTINGFIELD RELATIONS AND DESCRIPTIONS...PETROGRAPHY AND MINERAL...CHEMICAL COMPOSITIONSDISCUSSIONCONCLUDING REMARKSAPPENDIX: ANALYTICAL METHODSREFERENCES |
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The results presented above indicate that the Labrieville massif is a concentrically zoned pluton that differs significantly in composition from other massif anorthosites. Differentiation involved first the crystallization (and accumulation) of plagioclase, followed by hemoilmenite (which locally accumulated into a sizable ore body), then orthopyroxene (locally as plagioclase-lamellae-bearing megacrysts; see Owens & Dymek, 1995
), then clinopyroxene, then magnetite, and finally apatite. The corresponding rock types range outward from an anorthosite inner core, through a leuconorite–anorthosite outer core, to a leucogabbro–anorthosite rim. Pyroxene and plagioclase compositions (as revealed by XMg, normative percent An–Or, ppm Ba and Sr) change more or less systematically across the pluton but seem to indicate different directions for core-to-rim differentiation. We favor outward (or upward before doming) differentiation under the influence of decreasing pressure, which was possibly accompanied by increasing H2O content in the silicate liquid. The Labrieville rocks (anorthosite–leuconorite–leucogabbro) are interpreted to be cumulates largely devoid of trapped liquid. As such, most trends on variations diagrams are those of accumulation, plagioclase dilution, and mineral mixing, and any attempt to use bulk compositions of anorthositic rocks to reproduce liquid lines of descent is fraught with uncertainty.
Because of their cumulate character, the Labrieville rocks themselves provide little direct evidence for the nature of their parental magma. Nevertheless, we can draw some general and important indirect inferences about that parental magma as follows: (1) it must have been alkalic to produce the extremely Na- and K-rich plagioclase of the massif; (2) similarly, it contained extreme concentrations of Ba and Sr, or concentrations of those elements high enough to yield plagioclase having Ba and Sr higher than virtually any other known feldspar; (3) the magma must have been highly oxidizing, as revealed by the ubiquitous presence of hemoilmenite, while yielding relatively Mg-rich orthopyroxene. Two additional features (Ti/V and Ga/Al ratios) noted here and discussed in detail by us previously (Owens et al., 1993) are very high compared with other rock types. Collectively, the mineralogical and chemical features of Labrieville indicate that this parental magma was unlike the tholeiitic magmas that gave rise to layered intrusions such as the Skaergaard or Kiglapait.
We note that troctolitic or gabbroic rocks are rare to absent from any of the andesine anorthosites of this region. Thus, we find no evidence for the troctolitic or high-Al gabbroic magmas that have been suggested as parental to massif anorthosites elsewhere (e.g. Fram & Longhi, 1992; Mitchell et al., 1996; Markl & Frost, 1999; Scoates & Mitchell, 2000). On the other hand, jotunite dikes in this region probably approach the composition of liquids, but they contain feldspars and pyroxenes that are too evolved for them to be viable as parental magmas for associated anorthosites (Owens et al., 1993). Nevertheless, we find arguments for parental magmas of broadly jotunitic character appealing (seeVander Auwera & Longhi, 1994; Vander Auwera et al., 1998, 2000).
Despite its unusual composition, the radiogenic isotope data for Labrieville indicate derivation from the mantle or juvenile sources in the deep crust at 1·01 Ga, subsequent to the culmination of the Grenville orogeny (Owens et al., 1994). We emphasize that Labrieville is one of several hemoilmenite-bearing, late- to post-orogenic, andesine anorthosites found in a belt cutting across this region of the Grenville Province (Fig. 1; Dymek & Owens, 1998b). We consider it likely that the mineralogical and chemical characteristics of these anorthosites are somehow related to their post-tectonic emplacement (Owens et al., 1994). Thus, their origin may be linked in some way with the Grenville orogeny, and earlier models for massif anorthosites involving aborted continental rifting under anorogenic conditions do not apply in this case (e.g. Emslie, 1978; Morse, 1982b; Windley, 1989; McLelland & Whitney, 1990). In contrast, crustal thickening induced by the final (Ottowan) phase of the Grenville orogeny promoted thermal conditions that resulted in the formation of a distinctive type of magma that might be unique to deep processes in continent–continent collisional orogens.
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APPENDIX: ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONREGIONAL SETTINGFIELD RELATIONS AND DESCRIPTIONS...PETROGRAPHY AND MINERAL...CHEMICAL COMPOSITIONSDISCUSSIONCONCLUDING REMARKSAPPENDIX: ANALYTICAL METHODSREFERENCES |
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All whole-rock chemical compositions presented in this paper represent XRF analyses carried out at Washington University using an automated Siemens SRS-200 instrument. Concentrations of major and minor elements were obtained by analysis of fused glass disks using methods described by Couture et al. (1993)
; trace element concentrations were obtained by analysis of pressed-powder pellets using combined Compton and fundamental parameters methods described by Couture & Dymek (1996). Selected trace elements were determined in a subset of samples by INAA, also at Washington University, using methods described by Korotev (1987a, 1987b). Trace elements determined by XRF include V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Ba, and Pb. Trace elements determined by INAA include Sc, Cr, Co, Ni, As, Br, Sb, Rb, Sr, Zr, Cs, Ba, La, Ce, Sm, Eu, Tb, Yb, Lu, Hf, Ta, W, Th, U, and Au. In the case where two methods determined the same elements, the data reported in the tables correspond to the more precise method in each case: XRF for Sr, Ba, Rb, and Ni; and INAA for Cr and Co.
All mineral compositions reported in this paper represent EMP analyses carried out at Washington University using an automated JEOL-733 instrument. Operating conditions were 15 kV accelerating potential, 20 nA beam current, 1–20 µm spot size, and counting times of 10–100 s (the spot size and counting times were adjusted according to element and mineral being analyzed). Simple silicates and oxides were used as primary standards [Na—albite; Mg,Si—enstatite; Al—kyanite, Ca—wollastonite; Ca,Al—anorthite (synthetic); K—microcline; Ti—rutile; Cr—eskolaite (synthetic); Mn—rhodonite; Fe—fayalite (synthetic); Ba—celsian (synthetic)]. X-ray intensities were converted to oxide weight percent following the methods of Bence & Albee (1968), with correction factors modified from those listed by Albee & Ray (1970). Errors are judged to be 1% relative for most oxides based on repetitive analysis of working standards (Kakanui augite and Nain labradorite) during the course of each run.
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ACKNOWLEDGEMENTS |
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B.E.O. thanks M. S. Smith, J. P. Icenhower, and M. W. Rockow for field assistance, and A.T. Anderson, Jr, for helpful discussions during the early phases of this investigation. We also thank R. A. Couture, D. K. Kremser, and R. L. Korotev for maintenance of the X-ray fluorescence, electron microprobe, and neutron activation facilities, respectively, at Washington University. Much of the work reported here was supported by NSF Grants EAR88-16977 and EAR90-19366 to R.F.D., and by additional grants from the Geological Society of America and Sigma Xi. We thank R. F. Emslie, M. D. Higgins, and J. N. Mitchell for their thoughtful reviews.
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FOOTNOTES |
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*Corresponding author. Telephone: 757-221-1813. e-mail: beowen{at}wm.edu
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REFERENCES |
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TOPABSTRACTINTRODUCTIONREGIONAL SETTINGFIELD RELATIONS AND DESCRIPTIONS...PETROGRAPHY AND MINERAL...CHEMICAL COMPOSITIONSDISCUSSIONCONCLUDING REMARKSAPPENDIX: ANALYTICAL METHODSREFERENCES |
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