The age of the Lac-Saint-Jean Anorthosite Complex and associated mafic rocks, Grenville Province, Canada1

MICHAEL D. HIGGINS Sciences de la Terre, Universite' du Que'bec ci Chicoutirni, Chicoutimi, Que., Canada G7H 2BI

AND

OTTO VAN BREEMEN Geochronology Laboratory, Geological Survey of Canada, 601 Booth Street, Ottawa, Ont., Canada KIA OE8

Received November 25, 1991 Revision accepted February 13, 1992

U-Pb analyses of zircon and baddeleyite from the south-central and southeastern parts of the Lac-Saint-Jean Anorthosite Complex (LSJA) give an igneous crystallization age of 1157 * 3 Ma. Parts of the anorthosite were deformed in the solid state and subsequently intruded by a diorite megadyke, which also gives a crystallization age of 1157 f 3 Ma, indicating that crystallization and deformation of the anorthosite were essentially synchronous. The diorite megadyke was intruded into a north-northeast-trending shear zone and deformed by sinistral strike-slip movements. Emplacement was followed by intru- sion of a subparallel leucotroctolite megadyke that again gives the same crystallization age and hence dates movement of the shear zone at 1157 + 3 Ma. This short history of crystallization and synchronous deformation rules out slow diapiric rise as the emplacement mechanism for the anorthosite. Instead, anorthosite parental magmas probably rose up offsets in subverti- cal strike-slip shear zones to their present level.

In the southwestern part of the LSJA an age of 1142 * 3 Ma is interpreted to represent igneous crystallization. Contem- porary thermal metamorphic effects recorded in the southeastern sector by growth of new zircon in granophyric segregations and zircon coronas on baddeleyite suggest this event was more widespread at slightly deeper levels. Evidence has not been found for a separate Grenville regional metamorphism.

The emplacement into the LSJA at 1076 f 3 Ma of two small leucogabbro intrusions was part of a widespread magmatic event similar to the main event at 1157 - 1142 Ma.

Les analyses U -Pb sur zircon et baddeleyite des parties centre-sud et sud-est du Complexe d'anorthosite du Lac-Saint-Jean fournissent un Ige de cristallisation ignCe de 1157 & 3 Ma. Des portions du massif d'anorthosite furent dCformtes B 1'Ctat solide, et subsCquemment pCnCtr6es par un mtgadyke de diorite, qui donne aussi un Ige de cristallisation de 1157 f 3 Ma, ce qui indique que la cristallisation et la dCformation de l'anorthosite sont essentiellement synchrones. Le mCgadyke de diorite s'est infiltrk dans une zone de cisaillement de direction nord-nord-est qui Ctait dCformCe par des mouvements de dkcrochement senestre. La mise en place fut suivie par une intrusion d'un m6gadyke de leucotroctolite sub-paralltle, lequel fournit aussi un Ige identique de cristallistaion, par consCquent il date les mouvements de la zone de cisaillement de 1157 f 3 Ma. Cette histoire qui dCcrypte une trts courte durCe de la cristallisation et de la dkformation synchrone, Climine l'hypothtse d'une ascension diapirique lente comme mCcanisme de mise en place de l'anorthosite. Au contraire, les magmas parentaux de l'anorthosite ont probablement migrC rapidement vers le haut, en continuitk, dans les zones de cisaillement decrochant subver- ticales jusqu'B leur niveau actuel.

Dans la partie sud-ouest du Complexe d'anorthosite du Lac-Saint-Jean, un age obtenu de 1142 f 3 Ma est considCrC comme 1'Ige de cristallisation ign6e. Les effets du mCtamorphisme thermique contemporain, tels que r6vClks dans le secteur sud-est par la croissance de nouveaux zircons dans des sCgrCgations granophyriques, et la formation d'aurColes de zircon sur la bad- deleyite, suggkrent que cet Cvknement fut plus rCpandu B des niveaux 1Cgtrement plus profonds. Aucune indication n'a CtC trouvke d'une activitt skparCe de mCtamorphisme grenvillien rkgional.

La mise en place dans le Complexe d'anorthosite du Lac-Saint-Jean de deux petites intrusions de leucogabbro, il y a 1076 f 3 Ma, Ctait associte 51 un Cvbnement magmatique Ctendu, analogue B l'tvtnement principal apparu il y a 1157 - 1142 Ma.

[Traduit par la rdaction] Can. J . Earth Sci. 29, 1412-1423 (1992)

Introduction One of the major problems in understanding the Grenville

Province is the tectonic significance of the anorthosite com- plexes, a problem which can be attacked only when their ages are better known (Martignole 1986; Moore 1986; Emslie and Hunt 1990; McLelland and Chiarenzelli 1990). Early attempts at dating these intrusions were based on Rb-Sr determina- tions of associated, more acidic intrusions or supposed contact aureoles (e.g., Frith and Doig 1973). However, the geological evidence of contemporaneity of the anorthosite and associated rocks is generally poor, and the ages themselves are imprecise.

Although Sm - Nd isochrons of anorthosite bodies can give direct ages of initial crystallization, uncertainty in the original

'Geological Survey of Canada Contribution 20991. Printed in Canada I lmprimd au Canada

isotopic homogeneity of the samples has limited the reliability of the method. Nevertheless, Ashwal and Wooden (1983a, 1983b) used this method to establish approximate crystalliza- tion ages: 1.6 Ga for the Harp Lake and Mealy Mountains massifs, 1.4 Ga for several other mafic bodies in Labrador, 1288 f 36 Ma for the Adirondack massif, and 1079 f 22 Ma for the St. Urbain intrusion. However, these Sm-Nd isotopic ages generally lack the precision necessary for detailed geo- logical reconstructions in the Precambrian and may have been disturbed by granulite-facies metamorphism in the Grenville Province (McLelland and Chiarenzelli 1990). U - Pb mineral ages can supply much more precise data but have only recently been applied to this problem.

Using U - Pb zircon methods, Emslie and Hunt (1 990) dated pyroxene monzonites and other intrusions that cut several massif anorthosites in the Grenville Province. They found

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1414 CAN. J. EARTH SCI. VOL. 29, 1992

CHARNOCKITE-MANGERITE SEDIMENTARY ROCKS INTRUSIVE SUITE

*,+;,-, LATE- TO POST-GRENVILLIAN GRANITIC PLUTONS :;G$g;:$ ANORTHOSITE

BEGIN MEGADYKE OLDER GNEISS COMPLEX

LAC CHABOT MEGADYKE - BRITTLE FAULT DUCTILE SHEAR BELT

FIG. 2. Geology of the southern part of the Lac-Saint-Jean Anorthosite Complex and adjacent area.

cote anorthosite intrusion. Doig (1991) determined an age of Doig 1973, 1975). All these units are cut by several swarms 1155 f 3 Ma for the Morin anorthosite. of tholeiitic metabasite dykes.

Here we present U -Pb age data for anorthosite and mafic Paragneisses occur just east of the LSJA, and Woussen et al. igneous rocks of the Saguenay - Lac Saint-Jean region (Figs. (1981) suggested that the lack of melting in these rocks indi- 1, 2). Data from the associated intermediate and acid rocks cates that the anorthosite could not have cooled in situ. How- form a companion study currently in progress (Hervet et al. ever, detailed studies of the contact zone of the LSJA at Lake 1990). Kenogami (Fig. 2) (Hervet 1987) and elsewhere (Roy et al.

1986) indicate that it is a complex zone of variably deformed

Geology of the Saguenay - Lac Saint-Jean region

The Lac-Saint-Jean Anorthosite Complex (LSJA) is located in the central part of the Grenville Province, which has been termed the allochthonous polycyclic belt (Rivers et al. 1989) or the core zone (Woussen et al. 1986, 1988). The gneiss com- plex in this region comprises three major units (Woussen et al. 1981, 1988; Roy et al. 1986): (i) rafts of Aphebian paragneiss that are enclosed in (ii) multiply deformed and migmatized quartzofeldspathic gneisses of unknown age, and (iii) deformed and migmatised late to post-Hudsonian granites (Frith and

mafic and intermediate intrusions which developed by repeated shearing and injection. Therefore, there is no evidence that the paragneisses were in contact with the LSJA during its emplacement. This lack of evidence is important also because RbISr data on nearby paragneisses (Frith and Doig 1973) have been used to suggest that these rocks were metamorphosed at 1482 f 72 Ma by the emplacement of the LSJA (Woussen et al. 1981).

Although many authors (BerrangC 1962; Hocq 1977; Dirnroth et al. 1981; Woussen et al. 1981; Roy et al. 1986) have sug- gested that the LSJA is a multiple intrusion, it has not yet been

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HIGGINS AND VAN BREEMEN

EUCOTROCTOLITE

LAC CHABOT DlORlTE

CANTON TACH^ LEUCOGABBRO - SHEARZONE

a MC-STJEAN ANORTHOSITE FOLIATION

FIG. 3. Geology of the Saint Nazaire region.

possible to determine the geographical limits of these intrusive units. The complex as a whole is variable in composition, from true anorthosite to leucogabbro and leucotroctolite, but almost all of the rocks are plagioclase cumulates. Large-scale modal layering occurs in some sectors of the complex, but cryptic layering has not been documented (Hocq 1977; Woussen et al. 1988).

The LSJA consists mostly of plagioclase ( A I ~ ~ - A ~ ~ , , ) , which is black when fresh. It ranges in grain size from milli- metric granoblastic grains in deformed rocks to megacrysts up to 70 cm long in less deformed rocks. Mafic minerals are vari- able in abundance and include orthopyroxene, clinopyroxene, and olivine. Al-rich orthopyroxene megacrysts occur sporadi- cally throughout the anorthosite as isolated crystals or masses.

Solid-state deformation of the LSJA was extremely hetero- geneous and has not been studied in detail: some areas are almost pristine, whereas elsewhere the anorthosite has been transformed into a grey gneiss. The foliation plane is com- monly subvertical, although shallow angles have been observed locally (G. Woussen and E. Sawyer, personal communication, 1990). Where orthopyroxene - amphibole coronas developed between olivine and plagioclase they postdate the deformation (Woussen et al. 1981).

The central part of the LSJA is cut by two large dyke-like intrusions (megadykes), each about 1 krn wide and 25 km long (Figs. 2, 3) (Jooste 1958; CBtt 1986). The Lac Chabot mega- dyke is the older of the two and is generally straight, except south of the Saguenay River, where it bends to the west. It was emplaced by multiple injections of magma into an existing,

subvertical, north-northeast, ductile shear zone, which is part of an important regional set (Woussen et al. 1988; Du Berger et al. 199 1). Kinematic indicators and subhorizontal lineations are consistent with sinistral strike-slip motion.

All components of the dyke comprise plagioclase, pyrox- enes, and minor opaque minerals (Jooste 1958). The earliest component of the dyke is represented by xenoliths of diorite, which were deformed in the solid state before they were incor- porated into later dioritic magmas. Mingling of at least two such magmas in the dyke produced sinuous elongated enclaves of diorite slightly different in composition from the bulk of the intrusion. These enclaves are aligned parallel to the walls of the dyke but are not fractured or boudined, implying that they were deformed and aligned while they were still plastic. Fol- lowing solidification there was pervasive solid-state deforma- tion of the dyke, generally parallel to the earlier fabrics (Woussen et al. 1981). Fabrics in an associated thinner dyke indicate that this movement was sinistral and subvertical. All these features imply that emplacement and deformation were synchronous.

The Btgin megadyke cuts the Lac Chabot megadyke at its southern end (Woussen et al. 1981). The bulk of the dyke is parallel to the Lac Chabot megadyke, but the overall shape is sigmoidal. It is a leucotroctolite with relatively little composi- tional variation and is composed mainly of plagioclase and oli- vine, with minor biotite and opaque minerals. Lamination defined by plagioclase crystals is poorly developed in the cen- tral and northern parts of the megadyke but is well developed, with a subhorizontal orientation, in the southern sector. Defor-

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HIGGINS AND VAN BREEMEN 1417

mation is relatively minor, as compared to the Lac Chabot megadyke. The earliest deformation consists of rotation of previously laminated plagioclase crystals. The lack of fractur- ing and the absence of changes in shape of the crystals imply that this deformation occurred during the magmatic stage. Later deformation, which is locally parallel to the earlier deformation, fractured and changed the shape of the crystals and clearly occurred after the rock had cooled and solidified. These features imply that deformation commenced during the magmatic stage and continued after solidification with approx- imately similar orientation. Subsequent metamorphism pro- duced in some areas coronas of orthopyroxene and amphibole between olivine and plagioclase as well as sparse crystals of garnet, the latter commonly associated with minor fractures (Woussen et al. 1981).

The southern end of the BCgin megadyke is cut by the small Canton TachC North (or St. Nazaire) intrusion (Fig. 3) (Arpin 1984; CBt6 1986; Woussen et al. 1986). An adjacent intrusion (Canton TachC South) is very similar, and both are probably the uppermost parts of a single, dyke-like intrusion, with an orientation similar to that of the two megadykes. Both intru- sions are composed of a homogeneous, medium-grained leucogabbro that is extensively exploited for dimensional stone. Major minerals are plagioclase, orthopyroxene, clino- pyroxene, and biotite. Locally the plagioclase is poorly lami- nated, and pegmatitic areas, some with granophyric cores, occur towards the edge of the intrusion. These two intrusions are almost completely undeformed, but their contacts with the host rock are not chilled. This implies that they postdate the main phase of regional deformation but were emplaced into hot rocks.

The whole Saguenay - Lac Saint-Jean region is cut by numerous late intermediate to granitic plutons (Fig. 2). The position of some of these plutons appears to have been guided by shear zones with north-northeast orientations that parallel those that controlled the earlier intrusions (G. Woussen, per- sonal communication, 1990). Minor magmatism continued with the emplacement of sparse but widespread diabase dykes of unknown age.

Geochronology Analytical methods

The concordance of zircon and baddelevite data has been enhanced by processes of selection and strong air abrasion (Krogh 1982). Analytical techniques are summarized by Parrish et al. (1987). Isotopic data are presented in Table 1. Treatment of analytical errors follows Roddick (1987), and regression analysis follows York (1969). Uncertainties for ages are quoted at the 20 level. Where possible, the minerals used for dating were studied in thin section (Fig. 4).

Lac-Saint-Jean Anorthosite Complex The generally low Zr content of the anorthosite suggested

that a sample selection strategy based on less deformed, coarse-grained areas (anorthositic pegmatite) might be more successful than one based on sampling of more typical material. Pegmatitic areas in mafic intrusions form late in the process of solidification from residual magmas that are enriched in many elements, including Zr. In addition, all crys- tals in anorthositic pegmatites, including the Zr minerals, are larger than in other rocks. Therefore, the greater size and abundance of zircon and (or) baddeleyite in these areas should ease the problems of separating sufficient material for dating.

Such anorthosite pegmatites have been recognized in many of the less deformed parts of the LSJA. It should be noted that these anorthositic pegmatites are quite different from late, relatively rare dykes of granitic pegmatite which cut some parts of the anorthosite and probably formed during metamor- phism or much later deformation.

Small quantities of both baddeleyite and zircon were found in pgmatitic leucotroctolite in the south-central sector (sample I), although neither mineral was observed in thin sec- tion. Zircon occurs as small (less than 0.1 mm), clear, color- less fragments with no euhedral surfaces or visible cores. Baddeleyite occurs as small, mid-brown, irregular to prismatic crystals without zircon rims. Both fractions of baddeleyite from this sample are concordant or almost concordant and give 207Pb/206Pb model ages of 1152 f 2 and 1156 f 2 Ma (Fig. 5a). Zircon yielding the younger model age has probably suffered minor lead loss during a later thermal event (see below). Therefore, the crystallization age of this rock is inter- preted as the greater of these two ages, 1156 f 2 Ma.

Some anorthositic pegmatites in the LSJA have cores of granitic granophyre set between euhedral to subhedral mega- crysts of plagioclase and pyroxene. This relationship indicates that the granophyre must have been liquid during the final stages of solidification of the rock. Similar structures in anor- thosites of the Sept Iles mafic intrusion were formed by frac- tional crystallization during the last stages of consolidation of the rock (Higgins and Doig 1981). Granophyric segregations in other intrusions, such as Skaergaard, have formed by fusion of crustal xenoliths (Kays et al. 1981). During this process the high temperature of the mafic magmas and their typical under- saturation in Zr cause some or all of the zircon in the xenoliths to go into solution (Watson and Harrison 1983). New zircon crystals are reprecipitated on cooling.

The lack of xenolith remnants in the segregations and of crustal xenoliths elsewhere in the LSJA, as well as the absence of cores and other evidence of inheritance in the zircons from the segregations, argue against an origin by digestion of xeno- liths. However, from a geochronological point of view, the origin of the segregations is unimportant, only that new zircon crystals were produced. Baddeleyite was not observed in any of the granophyric segregations.

Zircon crystals from a granophyric segregation in almost undeformed anorthosite near the southwestern contact of the LSJA (sample 2) have flat to irregular shapes and are clear, colorless, and lack visible cores (Fig. 4a). The irregular mor- phology of these crystals suggests near-solidus growth in frac- tures or interstices between other crystals, which favours an igneous over a metamorphic origin (Poldervaart 1956). Also, if the crystals formed during a later metamorphic event, then there is no evidence of original zircon or baddeleyite, either as cores or separate populations of crystals, as is found in other similar segregations studied here. Therefore, it is most likely that these zircons crystallized during the formation of the granophyric segregation, although this interpretation requires independent verification. The two fractions are close to con- cordia and have identical 207Pb/206Pb model ages of 1142 k 2 Ma (Fig. 5a). This age is interpreted to be that of crystalliza- tion of the segregation and the enclosing anorthosite.

Two different granophyric segregations in the anorthosite from the southeastern sector were sampled. Zircon crystals are extremely abundant in one of the segregations (sample 3) and are clearly visible in the field. These crystals are clear, color- less, commonly euhedral prisms up to 1 mm across and 10 rnrn

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1418 CAN. J . EARTH SCI. VOL. 29, 1992

FIG. 4. (a) Zircon crystal from the western part of the Lac-Saint-Jean anorthosite (sample 2). The crystal is 1 mm long. Field of view is 5 rnrn. (b) Cross sections of highly elongated euhedral zircon crystals from a granophyric segregation in the eastern part of the Lac-Saint-Jean anorthosite (sample 3). Field of view is 5 mm. (c) Deformed crystal of zircon from sample 3. The crystal is 2 mm long. (d) Small rounded zircon crystals formed following deformation of the larger crystal seen in c. Field of view is 1 mm. (e) Crystal of baddeleyite with corona of zircon from a pegmatitic area of the Btgin megadyke (sample 6). The whole composite grain has a diameter of 0.7 mm. (f) Skeletal crystal of zircon from a pegmatitic segregation in the Canton Tacht intrusion (sample 8). The crystal is 1 mm long. I

long without any visible cores (Fig. 4b). The morphology and solution and recrystallization into small, rounded "meta- textural setting of these crystals indicates that they are igne- morphic" zircons (Fig. 4d). ous. Many zircon crystals that occur within or pass across Two fractions of large undeformed segments of prismatic grain boundaries or are adjacent to coronas are bent with vari- crystals gave concordant analyses with 207Pb/206Pb ages of able extinction angles of up to 50' (Fig. 4c) or are broken into 1154 f 2 and 1155 + 2 Ma (Fig. 5b). This age is taken to short segments. In some crystals deformation has produced be that of initial crystallization of the granophyre. A third frac-

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HIGGINS AND VAN BREEMEN 1419

LAC CHABOT DlORlTE

0.195

d % P

8

0.W1

0.191 2.05 2.09 2.13

207pb/235"

FIG. 5. Concordia diagram for data from the Lac-Saint-Jean anor- FIG. 6. (a) Concordia diagram for data from Lac Chabot megadyke thosite intrusion. All fractions are zircon except fractions l a and l b (sample 5). Fractions 5a and 5b are zircon. (b) Concordia diagram from sample 1, which are baddeleyite (shaded). (a) Samples 1 and 2. for data from BCgin megadyke (sample 6). Fraction 6a is pale bad- (6) Samples 3 and 4. The discordia line on each concordia diagram deleyite (shaded), and fraction 6b is dark baddeleyite (shaded). Frac- is for recent lead loss and is for reference only. tions 6c, 6d, and 6e are coarse zircon. Fractions 6f and 6g are

aggregates of fine-grained zircon.

tion comprised four flat crystals with some transverse frac- tures. It was slightly discordant and gave a 207Pb/206Pb model age of 1142 f 2 Ma. This age may record growth of new zircon during reheating, perhaps enhanced by the passage of fluids through the segregations. Alternatively, radiogenic lead may have been lost from some zircon crystals as a result of migration of defects through the lattice during bending of the crystals.

Sample 4 was taken from a much smaller segregation about 200 m away from sample 3. Zircon was much less abundant in this sample and lacked the euhedral form of most zircons from sample 3. Separated zircon consisted almost entirely of clear, colorless, unzoned fragments. Two fractions of several grains were almost concordant and gave overlapping 207Pbl 206Pb model ages of 1 15 1 f 2 and 1 154 f 2 Ma (Fig. 5b). A single large fragment gave an age of 1142 f 5 Ma. These data parallel those from sample 3 and can be interpreted in the same way.

Lac Chabot megadyke I Although zircon was not observed in thin section, it is suffi-

ciently abundant in this unit that a representative sample could be used for dating purposes. The sample was taken from the center of the dyke, away from zones of solid-state defor- mation. Zircon occurs as clear, prismatic, pale-brown crystals up to 0.3 mm long. One fraction from sample 5 falls on the concordia, and the other fraction falls on a recent lead-loss

trajectory for the same age (Fig. 6a). Therefore, the age of crystallization of the intrusion is well constrained at 1157 f 3 Ma.

Bbgin megadyke The low Zr content of this leucotroctolite intrusion sug-

gested a similar sampling strategy to that of the LSJA, i.e., analysis of pegmatitic areas. Such rocks occur at the southern end of the intrusion near the contact with the host LSJA. The pegmatitic zones comprise megacrysts of plagioclase, olivine, and biotite up to 20 cm long, with coronas of amphibole between olivine and plagioclase. Although Zr minerals were only rarely observed in thin section, both baddeleyite and zir- con were abundant in the mineral separates, testifying again to the extremely heterogeneous distribution of Zr in pegmatites.

Baddeleyite occurs as euhedral, equant grains up to 0.5 mm in diameter which are strongly zoned from dark cores to pale brown rims. Some grains are rimmed or completely replaced by aggregates of fine-grained zircon (Fig. 4e). Similar fea- tures have been seen in coronitic metagabbros from the western Grenville Province (Davidson and van Breemen 1988). Zircon also occurs as large, clear, pinkish subhedral grains without visible cores.

Three fractions of coarse zircon fragments define a recent lead-loss trajectory that intersects the concordia at 1157 f 2 Ma (Fig. 6b). The two baddeleyite fractions are almost coin- cident with two of the zircon fractions and hence fall on the

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1420 CAN. I. EARTH S( 31. VOL. 29. 1992

CANTON TACHC

FIG. 7 . Concordia diagram for data from the Canton Tach6 leuco- gabbro intrusions. Fraction 7a from sample 7 is zircon. Fractions 8a and 8b from sample 8 are baddeleyite (shaded). Fractions 8c and 8d are coarse zircon.

same lead-loss trajectory. These data indicate that the bad- deleyite and the coarse zircon crystallized at the same time, both indicating an igneous crystallization age of 1157 f 2 Ma.

Flakes of "sugary" zircon aggregates that spalled off the rims of baddeleyite crystals have yielded two slightly discor- dant data points with 207Pb/206Pb model ages of 1141 f 3 and 1143 f 3 Ma. The mean of these ages, 1142 f 3 Ma is taken to be that of formation of the corona.

Canton Tach6 intrusions The Canton Tach6 leucogabbro intrusions are also low in

Zr, hence it was decided to utilize pegmatitic areas of the intrusion for dating purposes. These areas occur towards the edge of the intrusions and vary from zones of coarse plagio- clase, pyroxene, and biotite (with crystals to 10 cm long) to well-zoned pegmatites with granophyric cores and rims of epitaxial plagioclase megacrysts.

Both samples contain skeletal zircon crystals up to 1 rnrn long (Fig. 4 f ) . These crystals appear as diagonally fractured thin plates in the mineral separates. Baddeleyite occurs sparsely in mineral separates from the pegmatite that lacks granophyre (sample 8) as small mid-brown crystals with thin coronas or "frostings" of tiny zircon crystals.

Two fractions of platy zircon crystals from sample 8 and similar material from sample 7 all lie close to concordia and give 207Pb/206Pb model ages of 1076 f 2 Ma (Fig. 7 ) . This age is taken to be that of crystallization of the intrusions. Two baddeleyite fractions from sample 8 are slightly discordant, with 207Pb/206Pb model ages of 1066 and 1068 Ma. These crystals may have been partially reset by the event that pro- duced the zircon coronas. Unfortunately it was not possible to analyse the zircon coronas, as they were too thin to provide sufficient material.

Evolution of the Saguenay - Lac Saint-Jean region

Early phase (ca. 1157-1 142 Ma) The best defined age of the magmatic activity associated

with the emplacement of the LSJA comes from the two mega- dykes, which give crystallization ages of 1157 f 3 Ma. The

part of the LSJA that hosts the megadykes must be slightly older than the dykes but is highly deformed, and features used for sampling elsewhere could not be identified. Both adjacent sectors of the anorthosite give similar ages, implying that the intervening sector is coeval. Baddeleyite and zircon data from the south-central and southeastern parts of the LSJA are dis- tributed along the concordia from 1156 to 1150 Ma, indicating that they were partly reset from a slightly older crystallization age. Therefore, the south-central and southeastern parts of the LSJA were emplaced and started to crystallize at or slightly before 1157 f 3 Ma. These rocks must have cooled relatively rapidly, so the anorthosite could be deformed in the solid state before the emplacement of the Lac Chabot megadyke at 1157 + 3 Ma.

The Lac Chabot megadyke was deformed in the solid state, but the deformed rocks are cut by the B6gin megadyke. Since these two intrusions have the same U - Pb age, within experi- mental error, the sinistral strike-slip movements along the Lac Chabot megadyke (Du Berger et al. 1991) must have occurred at 1157 k 3 Ma.

The data presented here indicate a very short history of crys- tallization, emplacement, and deformation for the south- central and southeastern LSJA and the two megadykes and rule out emplacement models based on diapirism of solid masses over a period of hundreds of millions of years (e.g., Woussen et al. 1981; Roy et al. 1986). Moreover, the exis- tence of large-scale igneous structures (Woussen et al. 1988) implies that magmas, rather than crystal mushes, were emplaced. Therefore, models based on emplacement of magma by rapid diapirism (Mahon et al. 1988) or fracture mechanisms (Shaw 1980) must be considered.

It is not possible to distinguish between these theories without clear observations of the surrounding rocks, including those above and below the intrusion. However, it has been shown that emplacement of the two megadykes was related to major shear zones, and it is probable that these shear zones existed slightly earlier, at the time of emplacement of the anor- thosite. Hence, the magmas that crystallized to produce the LSJA may have risen up a subvertical strike-slip shear zone, similar to that used by the subsequent megadykes.

Shortly after the emplacement of this part of the LSJA, dioritic magma rose up an offset or offsets in a north- northeast-trending sinistral, strike-slip shear zone and crystal- lized to form the Lac Chabot megadyke. The orientation of the B6gin megadyke implies that it too rose up a parallel shear zone but that movements had almost ceased by this time.

The next event was the emplacement of a minor pyroxene monzonite intrusion at 1148 f 4 Ma in the south-central part of the LSJA (Emslie and Hunt 1990). Activity apparently resumed in 1142 Ma with anorthosite emplacement and crys- tallization in the southwestern part of the LSJA. Thermal

I

events are also recorded at this time in the southeastern part of the LSJA by zircons in granophyric segregations (samples 3 and 4 ) and zircon coronas on baddeleyite (sample 6 ) . These sample localities are too far away for contact metamorphism from younger anorthosite intrusions to the southwest but may be related to eroded, hidden, or unidentified intrusions in this area. There is no geochronological evidence for a separate Grenville regional metamorphic event in the LSJA.

Late phase (ca. 1076 Ma) The Canton Tach6 leucogabbro intrusions have crystalliza-

tion ages of 1076 f 3 Ma. The hiatus between these rocks and those of the LSJA shows that they are unrelated. However, the

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HIGGINS AND VAN BREEMEN 142 1

absence of chilled margins indicates that the region had not yet completely cooled when intrusion occurred. The orientation of these intrusions implies that older shear zones were reactivated at this time in response to regional stress fields.

Hervet et al. (1990) obtained a U-Pb crystallization age of 1082 f 2 Ma for the Chicoutimi Mangerite intrusion. Similar rocks occur in a much larger mass to the south that extends all the way to QuCbec City. The almost undeformed Chicoutimi Granite intrusion was dated at 1057 f 27 Ma using a RbISr whole-rock isochron (Frith and Doig 1973). Many other late granitoid intrusions in the Saguenay - Lac Saint-Jean region may also be synchronous with the magmatism described above, and dating of these intrusions is currently underway.

Correlations with intrusive activity elsewhere Early studies noted the association of anorthosite with

pyroxene-bearing granitoid rocks, leading to the term anorthosite - mangerite -charnockite - granite suite (AMCG, Emslie 1978). A number of AMCG suites in the Grenville Province are broadly contemporaneous with the early phase of magmatism in the Saguenay - Lac Saint-Jean area.

U-Pb zircon dating of the Morin anorthosite gave an age of 1155 f 3 Ma (Doig 1991), close to that of the LSJA. This age also agrees with an age of 1160+: Ma determined from an intrusion of quartz monzonite spatially associated with the anorthosite (Emslie and Hunt 1990). Doig (1991) found other spatially associated rocks, a monzodiorite and quartz monzonite, to be slightly younger at 1146 f 4 and 1135 + 3 Ma, respectively. Recent work on the Marcy Massif in the Adirondack Mountains indicates an igneous emplacement age between 1138 and 11 13 Ma, with the most probable age lying between 1135 and 1125 Ma (McLelland and Chiarenzelli 1990). Other components of the AMCG suite in this area are slightly older, 1160 - 1145 Ma (Chiarenzelli and McLelland 1991). Granitic to gabbroic plutons in the Frontenac terrane gave U-Pb ages of 1177 - 1162 Ma (Marcantonio et al. 1988; van Breemen and Davidson 1988).

The late magmatism at 1080- 1075 Ma in the Saguenay - Lac Saint-Jean Region, although not part of the earlier (about 1157 Ma) AMCG suite, was approximately co-temporal with the anorthosite massif of Saint Urbain, i.e., 1079 f 22 Ma (Sm-Nd isochron, Ashwal et al. 1983b). In the Central Metasedimentary Belt there was K-rich alkaline plutonism during the period 1089-1076 Ma (Corriveau et al. 1990). These data suggest that this magmatic event was of regional extent, but further discussion must await new data. However, it should be noted that there are many similarities between this event and the earlier event discussed above, such as rock types and tectonic settings.

The AMCG complexes of the Nain and Grenville provinces have been also commonly ascribed to an extensional environ- ment (Wynne-Edwards 1976; Emslie 1978). Late Proterozoic igneous activity occurred in the Gardar Province of southern Greenland in what was clearly a rift-related setting (Blaxland et al. 1978). There magmatism began around 1300 Ma but reached a peak around 1160 Ma. Both acidic and mafic magmas were emplaced, and some of the intrusions contain abundant xenoliths of anorthosite that are thought to be cog- nate (Bridgwater and Henry 1968).

A dyke from the Abitibi dyke swarm, parallel to the Gren- ville Front but just north of it, has been dated at 1141 f 2 Ma (Krogh et al. 1987). Minor intrusions of coronitic metagabbro are, furthermore, widespread in the western part of the

Grenville Province, and some have been dated at 1170 f 30 Ma (Davidson and van Breemen 1988). Emplacement of these gabbros was, however, coeval with ductile thrusting in the same region (van Breemen et al. 1986). Similarly, the later magmatism in the Saguenay - Lac Saint-Jean region and else- where is coeval with thrusting in the southwestern Grenville Province dated at 1060 f 6 Ma (van Breemen and Hanmer 1986).

The classic anorogenic model for the tectonic setting of anorthosite massifs allows only a single structural setting, namely rifting (Emslie 1978). However, the LSJA was emplaced in a sinistral strike-slip regime that appears to have been coeval with convergent tectonics to the southwest. Recent models, such as the one of Gordon and Hempton (1986), linking Grenville thrusting to Keeweenawan volcan- ism permit a variety of structural settings within a single tectonic environment. Such models could account for the evi- dence for synchronous thrusting, rifting, and strike-slip fault- ing within the same orogen and throw light on the tectonic setting of the magmatism. However, without further mapping and structural control, speculation about possible tectonic models is at this time premature.

Conclusions (1) The south-central and southeastern part of the Lac-Saint-

Jean Anorthosite Complex was emplaced at 1157 + 3 Ma. Deformation was synchronous with emplacement and cooling. The parent magma of the anorthosite may have risen up a sub- vertical shear zone, similar to that associated with the mega- dykes.

(2) The Lac Chabot diorite megadyke was emplaced soon afterward. The magma rose up a north-northeast-trending, subvertical shear zone, possibly along an offset(s) opened up by sinistral strike-slip movements. Since deformation was synchronous with crystallization, the age of this intrusion, 1157 f 3 Ma, also dates movement on the shear zone.

(3) The BCgin leucotroctolite megadyke was emplaced shortly afterward (1 157 f 2 Ma). It rose up a parallel north- northeast-trending fracture, but movement during emplace- ment and cooling was relatively minor.

(4) The short history of emplacement, crystallization, and deformation of the anorthosite rules out emplacement models based on slow diapiric rise of solid masses. Instead, some or possibly all of the magmas were associated with strike-slip movements along shear zones.

(5) There is strong evidence that anorthosite was emplaced at 1142 f 3 Ma in the southwestern part of the complex. Thermal effects on rocks farther east at the same time may be related to contemporary magmatism, but the intrusions responsible for this have not been identified. Evidence has not been found in the igneous complex for a separate Grenville regional metamorphic event in this area.

(6) The Lac-Saint-Jean Anorthosite Complex is part of a broad zone of tectonism and magmatism that stretches from southern Greenland to the Adirondack Mountains of New York and dates to 1170 - 1125 Ma. This zone comprises anor- thosite and related rocks, as well as gabbros, granites, and syenites. The peak of magmatic activity was around 1160- 1155 Ma and was associated with thrusting, rifting, and strike- slip movements in different parts of southeast Laurentia.

(7) A separate magmatic event occurred at about 1076 Ma. Two small intrusions are ~arallel to the direction of the megadykes, suggesting rejuvenation of older zones of weak-

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1422 CAN. J. EARTH SCI. VOL. 29, 1992

ness. This event had both mafic and acidic components and shows many similarities with the early event (1 157 - 1142 Ma). It may be contemporary with anorthosite emplacement at Saint Urbain and elsewhere in the Grenville Province, as well as with alkali magmatism.

Acknowledgments Colleagues in the Geochronology Laboratory of the Geo-

logical Survey of Canada are thanked for their help in the generation of the U-Pb isotopic data. We especially thank G. Woussen, D. CBtC, and M. Hervet for their help in the field and many discussions on anorthosites. We also thank N. Salomon and S. Brassard for their help in the laboratory. The manuscript has benefitted from critical readings by L. Corriveau, J. K. Mortensen, and J. M. McLelland. This research was partly funded by an operating grant from the Natural Sciences and Engineering Research Council of Canada to M.D.H.

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Appendix: Sample locations and descriptions

Sample localities have been given a latitude and longitude and a universal transverse Mercator (UTM) grid reference. Grid data are taken from 1 : 50 000 NTS topographic maps.

Sample I , Lac-Saint-Jean Anorthosite Complex; 48"39'N, 71 "47'W; UTM 19UBD 942927 A 3 m high, 100 m long road cut to the north of Route 169, 2 km east of Saint Henri de Taillon, was sampled.

The sample of leucotroctolite was selected from the coarsest part of the outcrop, where plagioclase crystals are up to 20 cm long. Coronas of amphibole and other minerals up to 1 cm thick are present between the plagioclase crystals and the mafic phases.

Sample 2, Lac-Saint-Jean Anorthosite Complex intrusion; 48 "49'N, 72 "01 ' W; UTM 18UYK 1881 14 The sample was taken from a small outcrop on the north side of the road to Saint Augustin, 7 km southeast

of Sainte Jeanne d'Arc. This outcrop has been blasted for the insertion of a utility pole, and the sample was taken from the loosened material. This leucogabbro is undeformed and has a color index of 15. It comprises black plagioclase (1 -2 cm), pyroxene, and oxide minerals. The sample was taken from the core of a granophyric segregation in the leucogabbro. This segregation has a rim of megacrystic plagioclase, which is generally black, except toward the interior of the segregation, where the plagioclase is pink.

Samples 3 and 4, Lac-Saint-Jean Anorthosite Complex; 48"27'N, 71 "15'W, UTM I9UCD 342677 Outcrops in the bed of Saguenay River, near the confluence with Aux Sables River, immediately north of

Jonquibre, were sampled. The anorthosite in the outcrop is little deformed. Samples were taken from the cores of two granophyric segregations (50- 100 cm in diameter) in pegmatitic areas of the anorthosite. The two samples were separated by about 200 m. There are no dykes of pegmatite in the outcrop that could have fed or drained these patches. The outcrop is cut by several metre-scale rnigmatized mafic and felsic dykes.

Sample 5, Lac Chabot megadyke; 4g032'N, 71 "29'W, UTM I9UCD 163784 A large outcrop underneath high-tension power lines, 12 krn west of Saint Ambroise, was sampled. The dyke

here is homogeneous and is composed of the coarser grained component of this composite intrusion. At this loca- tion there is little evidence of solid-state deformation.

Sample 6 , Bt!gin megadyke; 48"33'N, 71 "32'W, UTM I9UCD 134798 The sample is from material excavated from a road cut beneath high-tension power lines, 5 km south of Tacht.

It is from a pegmatitic patch at the southern end of the megadyke. The mineral assemblages in the pegmatitic areas are the same as the rest of the dyke (i.e., plagioclase, biotite, and olivine), but the grain size is much larger, up to 25 cm.

Samples 7 and 8, Canton Tacht! intrusions; 48 "32'N, 71 "31 ' W, UTM 19UCD 141 785 These samples were taken from a quarry 5 km south of Tacht. The host leucogabbro is massive and homogene-

ous, with a grain size of 1-2 cm. These samples were taken from pegmatitic areas near the edge of the intrusion. Sample 7 is from the granitic core of a different pegmatitic area. It comprises quartz, orthoclase, and minor bio- tite. Sample 8 is from a pegmatite that lacks a granitic core. It comprises black plagioclase crystals up to 15 cm long, biotite, some pyroxene, and minor quartz.

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