Nature and origin of volcanite slices in the Cadillac break, Abitibi Greenstone belt, Canada

Pontiac metavolcanic rocks within the Cadillac tectonic zone, McWatters, Abitibi Belt, Quebec'

DAVID M O R I N , ~ MICHEL JBBRAK, MARC BARDOUX, AND NORMAND GOULET De'papartement des sciences de la Terre, Universite' du Que'bec a Montre'al, P. 0. Box 8888, Station A,

Montre'al, QC H3C 3P8, Canada

Received April 23, 1992 Revision accepted June 30, 1993

The McWatters metavolcanic rocks are structurally bounded lenses within the Cadillac tectonic zone on the southern boundary of the Abitibi greenstone belt. They comprise komatiite, tholeiitic basalt and gabbro, and calc-alkaline andesitic lavas and volcaniclastic rocks cut by calc-alkaline dioritic and lamprophyric dykes. The McWatters basalts are mid-ocean-ridge basalt type tholeiites exhibiting low incompatible trace element contents and [LalYb], < 1. They may have formed via relatively high degree partial melting of a rare-earth element depleted mantle source. The andesites exhibit chondrite-normalized trace- element patterns with light-rare-earth and large-ion lithophile element enrichments and negative Nb and Ti anomalies, comparable to those of subduction-related calc-alkaline andesites. McWatters units are distinct from nearby Blake River Group rocks, despite comparable lithological assemblages and some common geochemical characteristics. The McWatters basalts exhibit lower TiIY, ZrlY, and LalYb than the Blake River tholeiites, whereas the McWatters andesites display lower Ti/Zr and higher ZrIY than the Blake River calc-alkaline units. The McWatters tholeiites can be correlated with northern Pontiac Group tholeiitic units based on similar trace-element ratios and parallel rare-earth-element patterns. Thus, the McWatters tholeiites represent Pontiac rocks, underthrust beneath the southern Abitibi belt and appearing as isolated and retrograded lenses in the Cadillac tectonic zone. They may represent the remnants of an ocean basin that once separated the southern Abitibi greenstone belt from the Pontiac Subprovince.

Les mCtavolcanites de McWatters forment des lentilles tectoniques dans la zone de faille de Cadillac, a la limite sud de la ceinture de roches vertes de 1'Abitibi. Elles comprennent des komatiites, des basaltes et des gabbros tholkiitiques, des laves et des volcanoclastites andksitiques calco-alcalines recoupes par des dykes dioritiques et lamprophyriques calco-alcalins. Les basaltes de McWatters sont similaires a des basaltes des dorsales mCdio-ocCaniques et montrent des teneurs faibles en ClCments traces incompatibles et des [LaIYb], < 1. 11s pourraient rksulter de taux relativement ClevCs de fusion partielle d'une source mantellique appauvrie en terres rares 1Cgkres. Le patron d'C1Cments traces normalisCs aux chondrites des andCsites montre un enrichissement en terres rares 1Cgkres et en gros ions lithophiles ainsi que des anomalies negatives en Nb et Ti, comparables a ce que I'on observe dans les andCsites calco-alcalines des zones de subduction. Les unitCs de McWatters sont distinctes des roches du Groupe de Blake River bien qu'elles prksentent un assemblage lithologique et certains caractkres gCochimiques comparables. Les basaltes de McWatters prCsentent des rapports Zr/Y, TiIY et LalYb plus faibles que ceux des tholeiites du Blake River alors que les andCsites montrent des rapports TiIZr plus faibles et ZrIY plus Cleves que ceux des unitts calco- alcalines du Blake River. Les basaltes de McWatters peuvent &tre corrClCs avec les tholkiites de la partie nord du Groupe de Pontiac, comme le montrent des rapports d'tlCments traces similaires et des patrons de terres rares parallkles. Ainsi, les tholkiites de McWatters reprCsentent des roches du Pontiac, charrikes sous la partie sud de la ceinture de roches vertes de 1'Abitibi et apparaissant en lentilles isolCes et rCtrogradCes dans la zone tectonique de Cadillac. Elles pourraient representer les restes d'un bassin ocCanique ayant sCpare la partie sud de la ceinture de roches vertes de l'Abitibi, de la sous-province du Pontiac.

Can. J . Earth Sci. 30, 1521 - 1531 (1993)

Introduction Correlation across fault zones presents a major difficulty in

attempts to reconstruct the evolution of the southern Archean Superior Province. In the McWatters area, the Cadillac tectonic zone (CTZ) is a major geologic discontinuity separating the metavolcanic Blake River Group from the metasedimentary Pontiac Group (Dimroth et al. 1982; Card 1990) (Fig. 1). These rock assemblages were interpreted to have formed, respectively, in an island-arc setting and a n accretionary complex (Dimroth et al. 1983b; Ludden et al. 1986; Card 1990). There is structural and metamorphic evidence to support the Kenorean underthrusting of the Pontiac Group below the Blake River Group (Dimroth et al. 1983a; Feng and Kerrich 1990; Jackson and Sutcliffe 1990; CamirC et Burg 1993).

The McWatters metavolcanic rocks are tectonic lenses within

'Ministkre de l l~nerg ie et des Ressources du QuCbec Contribution 92-5 130-21.

2Present address: Centre gtoscientifique de QuCbec, 2700 Einstein Street, P.O. Box 7500, Sainte-Foy, QC G1V 4C7, Canada. Printed in Canada I Imprimt au Canada

the C T Z (Figs. 1 and 2). The origin of these rock slivers has been debated, as it is not possible to establish, by simple stratigraphic means, their position relative to adjacent lithologic assemblages. O n petrographic and structural grounds, Wilson (1962) suggested that they are rocks of the Blake River Group (Abitibi Group), reworked within the fault zone. The geochemical study of the metavolcanic rocks of the C T Z can thus help t o unravel the tectonic evolution of the fault zone and the relationships of adjacent lithologic assemblages.

We report the petrography and geochemistry of the McWatters metavolcanic rocks. Relatively fresh samples can be obtained away from shear zones so that komatiitic ultramafic, tholeiitic mafic, and calc-alkaline intermediate units can be identified (Morin et al. 1990; Jkbrak e t al. 1 9 9 1 ~ ) . Emphasis is placed o n the trace elements such as Zr, Ti, and Y and the rare- earth elements which are relatively immobile during secon- dary processes. Metavolcanic rocks of the nearby Blake River and Pontiac groups in the McWatters area (Fig. 1) were also analyzed to investigate their relationship with the McWatters units. Some possible implications of the results on a regional scale a re also raised.

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1522 CAN. 1. EARTH SCI. VOL. 30, 1993

V V V V V

V V V V V V

V V V V V V

V V V

v v v

l v v v v

Sedfrnentary rocks

VoIcanlc rocks 0 5 lOkrn - --- Fault zone

FIG. 1. Location map. BR, Blake River Group; CA, Cadillac Group; CO, Cobalt Group; CTZ, Cadillac tectonic zone; KE, Kewagama Group; KI, Kinojkvis Group; MA, Malartic Group; PI, PichC Group; PO, Pontiac Group; PTZ, Porcupine-Destor tectonic zone; TI, Timiskaming Group.

Geological setting The Cadillac tectonic zone (CTZ) is an east -west-trending

structure over 200 km long, 30-200 m in width, with a steep dip, commonly to the north (Goulet 1978; Dirnroth et al. 1982) (Fig. 1). Seismic reflections related to the CTZ were observed to depths of 15 km (Jackson et al. 1990). The tectonic history of the fault zone is complex, and several lines of evidence indicate that the CTZ acted as a thrust fault reactivating an older normal fault system (Dimroth et al. 1983a; Stone 1990; Jtbrak et al. 1991~). Early to late transcurrent displacements along the fault zone have also been documented (Hubert et al. 1984; Gauthier et al. 1990; Thurston and Chivers 1990; CamirC et Burg 1993).

Regional metamorphism increases southward, passing from low-pressure prehnite -pumpellyite or greenschist facies in the Blake River Group to medium-pressure amphibolite facies in the Pontiac Group (Jolly 1978; Dirnroth et al. 19836; Feng and Kerrich 1990). The CTZ is marked by retrograde metarnor- phism, which is manifested by an abundance of hydrated and carbonate minerals and disequilibrium assemblages (Dimroth et al. 1983b; GClinas et al. 1984; Morin et al. 1993).

In northwestern Quebec, the volcanic rocks of the southern Abitibi belt have been divided into two broad sequences (Laflbche et al. 1992): a tholeiitic - komatiitic lower sequence (PichC, Malartic, KinojCvis groups), and a tholeiitic - calc- alkaline upper sequence (Blake River Group) (Fig. 1). The 2700 f 3 Ma (Corfu et al. 1989) Blake River Group consists of interlayered tholeiitic and calc-alkaline lavas and volcaniclastic rocks confined between two shear zones, the Porcupine - Destor tectonic zone to the north and the CTZ to the south (Dimroth et al. 1982; Gtlinas et al. 1984; Ptloquin et al. 1990; Laflbche et al. 1992) (Fig. 1). The volcanic pile has been intruded by synvolcanic to postkinematic dioritic to granitic intrusives (Dimroth et al. 1982, 19838; Paradis et al. 1988; Rive et al. 1990).

The northern Pontiac Group comprises clastic metasedimen-

tary rocks in tectonic contact with ultramafic to intermediate metavolcanics appearing in the core of anticlinal structures (Wilson 1962; Goulet 1978; Dimroth et al. 1982) (Fig. 2). It is not known if these volcanic units are scattered flows or tectonic slices. Detrital zircon dating indicates a maximum age of 2695 f 4 Ma for the deposition of the sedimentary sequence (GariCpy et al. 1984; Feng et al. 1993).

The 2680 f 5 Ma (Corfu et al. 1991) Timiskaming metasedimentary rocks straddle the CTZ. They overlie the Blake River Group unconformably and are in faulted contact with the underlying Pontiac Group (Goulet 1978; Dimroth et al. 1982; Hubert 1990). At McWatters, the Timiskaming sequence comprises mainly coarse clastics preserved in an east- west-trending syncline (Wilson 1962; Goulet 1978; Jtbrak et al. 1991a) (Fig. 2). In the Kirkland Lake area, alkaline volcanic~ are associated with Timiskaming sediments (Cook and Moorhouse 1969).

The Picht Group contains an assemblage of ultramafic to intermediate igneous rocks restricted to the CTZ, farther east (Latulippe 1976; Stone 1990) (Fig. 1). The stratigraphic position of these rocks is debated but they possibly occupy the highest part of the Malartic Group of komatiites and tholeiitic basalts, intruded by granitoid plutons (Gunning 1941; Latulippe 1976; Bourne and Danis 1987; Rive et al. 1990).

The McWatters metavolcanic units occur within the CTZ, between the town of McWatters and the Kinojtvis River, east of Rouyn-Noranda (Fig. 2). They are in fault contact with the Timiskaming metasediments to the south and are separated from the metavolcanic rocks of the Blake River Group and the metasedimentary rocks of the La Brubre Formation to the north by deformation zones within the CTZ (Wilson 1962; Goulet 1978; Gauthier 1985; JCbrak et al. 1991~). Lithologies include komatiite, tholeiitic basalt and gabbro, and calc-alkaline andesitic flows and volcaniclastic and intrusive rocks which are cut by several generations of alkaline to tholeiitic dykes of Late Archean to Proterozoic age (Wilson 1962; Gauthier 1985;

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MORIN ET AL.

Proterozoic diabase dyke

TlMlSKAMfNG GROUP AND RELATED UNITS

. * . . . . BLAKE RIVER GROUP

' ' : ' : Andari t lc volcanoclas!ic

Cadillac tectonic zone Ultramafic and mafic schist

PONTIAC GROUP * SchlstosIty

McWatters mine shaf t Andssite, basalt, and

rpx=~ Shear zone, fault = Road 117 komatiite

FIG. 2. Simplified geological map of the McWatters area. BR, Blake River Group; LB, La Brukre Formation; PO, Pontiac Group; TI, Tirniskam- ing Group; 1 , Bowes fault; 2, McWatters fault; 3, Ruisseau Davidson fault. Modified after Wilson (1962), Goulet (1978), Gauthier (1985), and JCbrak et al. (1991~).

Morin et al. 1990; JCbrak et al. 199 la). The contacts between the units are in most cases faulted. The rocks have been affected by three major deformation episodes (Bardoux et al. 1990; JCbrak et al. 199 la) : D 1 was an extensional episode that created a stretching lineation, mylonitic fabric, and asymmetrical rotation of clasts and vein fragments indicating a north - side- down movement. D2 was a north - south compressional episode forming east -west isoclinal folds with an axial-plane schistosity. D3 was a late southeast-northwest compressional episode that created minor Z-shaped folds and developed a crenulation schistosity (Gauthier et al. 1990; Barboux et al.

1990; JCbrak et al. 1991~). Late brittle faults cut older structures. The McWatters metavolcanic rocks were affected by medium-pressure metamorphic conditions at about 5 kbar (1 kbar = 100 MPa) and 475"C, followed by a retrograde metamorphic episode with estimated conditions of 4 kbar and 400°C (JCbrak et al. 1991b; Morin et al. 1993).

Analytical methods The samples studied were collected in the McWatters mine

workings, from drill holes, and from outcrops in the vicinity

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1524 CAN. J . EARTH SCI. VOL. 30, 1993 I TABLE 1. Representative analyses of McWatters, Blake River, and Pontiac units

McWatters Blake River Pontiac

Sample No.: 236 202 199 175 248 190 6 1 27 173 105 260 198 157 Unit:" Kom Gab Bas Bas Bas And And Volc Dior Lamp Bas And And

SiO,(wt.%) 44.82 49.56 51.50 51.53 52.22 55.83 57.18 58.17 61.84 50.56 51.46 58.53 55.63 TiO, 0.32 0.95 0.81 0.88 0.99 0.94 0.88 0.68 1.44 1.18 1.69 1.14 2.08 A1z03 5.81 12.44 15.04 14.58 14.01 17.43 16.56 14.86 15.28 12.13 13.43 13.89 14.64

3.88 4.38 4.06 13.04 5.27 3.07 2.60 1.91 3.29 10.50 4.16 8.90 2.73 FeO 7.04 10.60 8.51 na 8.34 5.61 4.94 5.14 4.78 na 8.89 na 8.38 MnO 0.21 0.22 0.25 0.22 0.22 0.13 0.12 0.12 0.10 0.16 0.25 0.13 0.22 MgO 28.42 9.14 6.46 7.61 6.63 6.12 5.53 8.92 2.31 8.94 6.45 5.22 3.29 CaO 9.25 9.75 9.47 7.19 8.64 6.30 5.30 5.55 3.69 8.59 10.25 6.10 7.32 Na,O <0.1 2.61 3.65 4.77 3.42 3.71 5.76 4.10 6.69 5.02 3.23 5.89 5.16 K20 0.24 0.30 0.21 0.14 0.21 0.66 0.97 0.45 0.21 2.36 0.05 0.08 0.39 Pz05 0.01 0.05 0.05 0.05 0.06 0.18 0.15 0.10 0.38 0.56 0.15 0.11 0.16 LO1 14.20 2.29 2.31 2.53 2.36 3.72 2.52 4.71 2.39 11.50 2.22 1.58 1.42 Hz0 4.58 2.01 1.92 2.26 2.02 2.79 2.48 3.32 1.22 0.30 2.04 na 0.41 co2 9.61 0.13 0.37 0.27 0.29 0.93 0.40 1.39 1.13 11.20 0.18 na 0.88 S 0.01 0.15 0.02 <0.01 0.05 <0.01 <0.01 <0.01 0.04 <0.01 na na 0.13

Sc (PPm) 22 50 48 5 1 53 26 26 2 1 19 27 45 43 48 Co 96 54 39 na 45 27 26 32 na na 54 na 46 Cr 2599 255 70 na 23 106 116 439 na na 29 na 20 Rb 14 9 7 4 4 20 3 1 8 5 110 < 3 4 9 Cs ~ 0 . 2 0.5 <0.2 0.4 <0.2 ~ 0 . 2 <0.2 <0.2 0.5 18.1 <0.2 1.7 <0.2 Sr 200 133 97 113 145 374 319 230 123 272 185 80 212 Ga 9 17 15 14 18 22 19 17 23 19 23 19 2 1 Th <0.1 0.1 <0.1 0.2 0.1 1.5 1.4 1.9 5.2 4.5 0.2 1.2 0.3 U <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 1.4 2.0 <0.5 0.7 <0.5 Ta <0.1 <0.1 <0.1 <0.1 0.1 0.3 0.3 0.2 0.6 0.4 0.3 0.7 <0.1 Nb 3 4 4 4 < 3 6 5 4 10 9 6 10 7 H f 0.4 1.1 1.1 1.4 1.5 3.3 3.2 3.0 6.4 4.6 3.0 4.2 3.4 Zr 32 57 58 59 65 145 144 126 267 193 113 194 14 1 Y 8 21 23 22 25 22 21 17 35 40 3 8 73 46 La 0.84 2.75 2.13 2.46 2.38 13.9 15.1 12.2 33.8 40.8 6.02 12.3 Ce 3.74 6.42 6.18 6.88 7.44 28.9 29.0 22.9 77.9 95.2 13.3 31.7 14.1 Sm 0.79 2.04 1.72 2.05 2.07 3.95 3.71 3.08 8.61 12.5 3.71 6.13 4.78 Eu

6'30 ~ 0.28 0.61 0.62 0.72 0.82 1.11 1.11 0.85 2.05 2.95 1.20 1.63 1.71

Tb 0.12 0.51 0.44 0.51 0.49 0.61 0.50 0.43 1.03 1.25 0.89 1.23 1.05 I

Yb 0.61 1.94 2.27 2.16 2.61 1.93 1.58 1.60 2.56 2.83 3.64 5.01 4.40 Lu 0.11 0.32 0.36 0.35 0.38 0.29 0.27 0.22 0.44 0.44 0.56 0.84 0.70

NOTE: na, not analyzed. "Korn, komatiite; gab, gabbro; bas, basalt; and, andesite; volc, volcaniclastic rock; dior, diorite; lamp, lamprophyre.

of McWatters and Rouyn. The McWatters rocks show various degrees of alteration. The most altered rocks are restricted to highly strained shear zones where amphibole and epidote are replaced by chlorite, carbonates, and quartz (Morin 1992; Morin et al. 1993). Therefore, only samples with modal amphibole and epidote were considered for the geochemical analysis of the mafic and intermediate units. Samples containing more than 5 % loss on ignition (LOI) have been rejected except for komatiites. Major-, minor-, and trace-element (Ga, Rb, Sr, Nb, Y, Zr) concentrations were determinied by X-ray fluores- cence spectroscopy (XRF) at Centre de Recherche MinCrale in QuCbec City. C02 and S contents were measured by infrared spectroscopy at the same laboratory. The standard deviation is less than 5% for SO2, A1203, Fe203, LOI, Sr, Y, and Zr; less than 10% for MgO, CaO, MnO, Na20, K20, P2O5, Ti02, Ga, and Rb; and less than 15% for Nb. The abundances of the rare-earth elements (REE), Ta, Hf, Cs, U, Th, Sc, Cr, Co were determined by instrumental neutron activation analysis (INAA) at Centre de Recherche MinCrale and at UniversitC du QuCbec h Montrtal. The standard deviation is less than 10%

for Ta, Hf, Th, Sc, Co, and the REE except Sm and less than 20% for Cs, U, and Sm. For comparative purposes, the major- and trace-element concentrations have been normalized on a volatile-free basis. For brevity, the prefix meta will be omitted from the names of the different lithologies. Representative analyses of the McWatters units are presented in Table 1.

Petrography and geochemistry of the McWatters volcanic units

Komatiite The ultramafic units are confined to the high-strain zone of

the CTZ (Fig. 2). They have been intensely foliated, brecciated, and altered, and are recrystallized to talc, carbonates, serpentine, chlorite, tremolite, quartz, magnetite, and pyrite. Spinifex textures can be preserved (Gauthier et al. 1990) but have not been observed at McWatters.

These units have low abundances of Si02 (44.8-47.3%), TiOz (0.3 1 -0.41 %), (5.8 -7.2 %), and incompatible trace elements. The elements S, P, Nb, and Th are close to or

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MORIN ET AL.

I I I I I

Tholeiitic - -

* * . m A

- -

Calc-alkaline

I I I I I 0 42 0 0.5 1 .O 1.5 2.0 2.5 3.0

0.001 0.01 0.10 1 .o FeO* I MgO Zr I TiO, FIG. 4. FeO*lMgO vs. FeO* diagram of McWatters basalts and

F ~ ~ , 3. Zr/TiO, vs. SiO, diagram of the McWatters units (after gabbro (a) and andesites and volcaniclastic rocks (A). The dividing

Winchester and Floyd 1977). line between the tholeiitic and calc-alkaline field is from Miyashiro (1974).

below the detection limits. The komatiites show high concentrations of MgO (24.5 -28.9%), Co, Cr, K, Rb, Sr, and volatiles. Their Mg# (Mg/(Mg + total Fe)) is high (82) as well as their CaO/A1203 (0.9 - 1.9). The REE, Y, and Sc are about three times the chondritic abundance.

The chemistry of the McWatters ultramafic rocks is comparable with that of komatiites from Ontario, Australia, or South Africa, in terms of most major- and trace-element contents (Nesbitt and Sun 1976). The differences include higher concentrations of volatiles, K20, Sr, Rb, and Zr and lower concentrations of Na20 that could be the result of intense alteration. This is compatible with the fact that the fluid circu- lation has been concentrated in the most highly deformed rocks of the CTZ (Morin et al. 1993).

Basalt and gabbro The basalts lie on the south side of the CTZ as lens-shaped

bodies bounded by shear zones (Fig. 2). Gabbro appears in a domal structure near the KinojCvis River (Gauthier 1985) (Fig. 2). The basaltic and gabbroic units present the same mineralogy and chemistry and will be considered together. The rock is dark green, massive or slightly schistose, fine to medium grained, and consists of actinolite, hornblende, albite, chlorite, epidote, titanite with minor amounts of calcite, biotite, magnetite, ilmenite, and sulfides. No primary structures such as flow contacts, flow breccia, or pillows have been observed but primary textures such as ophitic textures with microphenocrysts of amphibole after pyroxene and albite after plagioclase are locally preserved (Morin et al. 1993).

An original basaltic composition is suggested by Si02 abundances of 48.1 -53.7% and supported by low Zr/Ti02 ratios (Fig. 3). Some units are Mg rich (MgO = 6.5-13.1%; Mg# = 45 -63). The rocks plot in the tholeiitic field on a FeO*/MgO versus FeO* diagram (Fig. 4) as well as on the AFM diagram of Irvine and Baragar (1971) (Jtbrak et al. 1991~). GClinas et al. (1984) have shown that the tholeiitic or calc-alkaline affinity of the Blake River units can be distin- guished using Zr/Y and Ti/Zr. These ratios (mean values) are, respectively, lower than 4 and higher than 70 in tholeiitic rocks, whereas they are higher than 4 and lower than 70 in calc-alkaline rocks. The McWatters basalts and gabbro display Zr/Y < 3 and Ti/Zr > 83 which supports a tholeiitic affinity. The data define a linear trend on binary diagrams (Fig. 5). The incompatible trace-element concentrations of the mafic units

42 0 5 10 15 20 25 30

MgO (wt.%)

0 0 50 100 150 200 250 300

Zr ( P P ~ )

FIG. 5. MgO vs. Si0, diagram (a) and Zr vs. TiO, diagram (b) of the McWatters komatiite, tholeiitic basalt and gabbro, calc-alkaline andesite, volcaniclastic rock, and diorite.

are low (Table I), being about 10x chondritic abundances. The REE pattern is slightly depleted in the light-rare-earth elements (LREE), giving [LaISm], = 0.8 and [La/YbIN = 0.8 (Fig. 6a). Europium can show a very slight negative anomaly (Fig. 6a).

The McWatters basalts and gabbros are less affected by the

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CAN. 1. EARTH SCI. VOL. 30, 1993

L \ l l l t l l l l l l l l l J

: (a)

-

L

--c McWatters basalt 1- 1 1 1 1 1 1 1 1 1 1 1 1 l l

- McWatters andesite 1 I SmEu Tb Yb Lu

FIG. 6. Chondrite-normalized rare-earth-element diagram of McWatters (a) tholeiitic basalts and gabbro and (b) calc-alkaline andesitic lavas and volcaniclastic rocks. Normalizing values are given in Fig. 7a.

alteration processes than the komatiites. This is indicated by the lower volatile contents, by the constancy of the Ti, Zr, Y, and Ce concentrations (Morin 1992), and by the regularity of the REE patterns (Fig. 6a). Figure 7a is a chondrite-normalized extended trace-element diagram, including REE, Sr, K, Rb, Nb, Zr, Ti, Y, and Sc. The elements are arranged in order of increasing compatibility with the mantle mafic phases from La to Sc, for the elements with high ionic potential (known to be relatively immobile in a fluid phase), and from Rb to Sr for elements with low ionic potential (mobile in a fluid phase) (Pearce 1982; Bienvenu et al. 1990). The elements Nb, Zr, Ti, and Y have similar chemical behavior in the mantle as La, Sm, Eu, and Tb, respectively, so unaltered samples should display equivalent chondrite-normalized REE and non-REE abundances (Sun et al. 1979; Bougault and Treuil 1980; Ludden et al. 1982; Bienvenu et al. 1990). The unfractionated chondrite- normalized trace-element pattern of Fig. 7a indicates only minor disturbance to the chemistry of these rocks.

The constancy of the trace-element ratios (ZrIY = 2.5 -2.9; TiIZr = 83 -99; LaISm = 1.2 - 1.4) and the linear trend on both MgO versus Si02 and Ti versus Zr diagrams (Fig. 5) indicate a genetic link between the McWatters mafic units. These rocks probably originated from a relatively high degree of partial melting of a depleted mantle source as indicated by their low concentrations in the incompatible trace elements and their [LaISm], < 1 (Table 1; Fig. 6a). The trends on the variation diagrams of Fig. 5 and the variable Mg# and Cr contents of these rocks can be accounted for by the fractional

I I I I I I I I I I I I I I I "

- (a) .......... - ,---.. . . . . -175 i

. . . . . . . I : ' . '. ... . . _ . . ,.. . .

l l l l l l l l l l k l l l l l l l Sr K RbLaNbCe ZrSrnTi EuTb Y YbLu Sc

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Sr K RbLaNbCeZrSrnTi EuTb Y YbLuSc

1 I I I I I I I I I I I I I I I

Sr K RbLaNbCe ZrSrnTi EuTb Y YbLu Sc

FIG. 7. Chondrite-normalized multielement diagrams of McWatters units. (a) Tholeiitic basalt (175); the MORB pattern is the mean of 9 analyses from various localities (Sun et al. 1979); IATB is the mean of 12 analyses of tholeiitic basalts from the Sunda Arc (Basaltic Vol- canism Study Project 1981). The normalizing values are from Sun and Nesbitt (1978) for the REE, Sun and Nesbitt (1977) for Sr, Ti, Y, Zr, Nb, and Sc, and Sun et al. (1979) for K and Rb. (b) Calc- alkaline andesite (190); Taos is a representative calc-alkaline andesite from the Taos Plateau (McMillan and Dungan 1988); Papua is a representative calc-alkaline andesite from Papua (Johnson 1982). (c) Dioritic dyke (173); the pattern of a McWatters andesite (190) is also shown for comparison. See text for further explanations.

crystallization of minerals such as olivine, pyroxene, and plagioclase (Cox et al. 1979; Pearce and Norry 1979; Morin 1992). The McWatters tholeiitic basalts, with their flat patterns on chondrite-normalized trace-element diagrams (Fig. 7a) are comparable to recent mid-ocean-ridge basalts (MORB). They lack the negative Nb and Ti anomalies and light ion lithophile elements (LILE) enrichment of the island-arc tholeiites.

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Andesite and volcaniclastic rocks These units are well exposed south of the McWatters fault,

on the mine property (Fig. 2). The andesitic lavas are massive, pillowed, or brecciated flows at places featuring amygdules and (or) rounded and stretched andesitic clasts. The lavas are conformably overlain by epiclastic breccia and lapilli tuff with 20 - 60 % clasts of andesitic composition. The close association of amygdaloidal and pillowed lavas with volcaniclastic units indicates a shallow-water setting, not far from a volcanic centre (JCbrak et al. 1991~).

The andesitic lavas contained 1-20% euhedral phenocrysts of plagioclase (now replaced by albite, An < 1) and pyroxene or amphibole (now actinolite). The groundmass is fine grained and composed of albite, chlorite, biotite, quartz, calcite, and titanite with minor amounts of pyrite, chalcopyrite, magnetite, hematite, and tourmaline. The andesitic volcaniclastic rocks tend to be more altered than the lavas, probably as a conse- quence of their higher permeability

The analyzed andesitic volcaniclastic rocks (whole rock or single clast) are chemically comparable to the andesitic lavas and thus all are treated together. It should be noted, however, that the volcaniclastic rocks show slightly higher MgO and Cr contents than the andesitic lavas, suggesting that they could contain a proportion of more mafic material, possibly within the rock matrix. Compared with the basalts, the andesitic units (55.1 - 60.6 % SO2) have higher Zr/Ti02 (Fig. 3). They plot in the calc-alkaline field of both the AFM diagram of Irvine and Baragar (1971) (JCbrak et al. 1991a) and &e FeO- MgO diagram (Fig. 4). The calc-alkaline affinity is confirmed by the ratios Zr/Y > 6 and TiIZr < 43 (Gtlinas et al. 1984). The andesitic units have similar Ti and Y concentrations to the mafic rocks but higher Zr and other incompatible trace elements (Table 1). They form a distinct trend on the variation diagrams of Fig. 5. The LREE are about 40x chondritic abundance, whereas heavy rare-earth elements (HREE) are lower than 10 x chondritic abundance, giving [LaISm], = 2.2 and [La/YbIN = 5.2 (Fig. 6b). Europium shows a slight negative anomaly (Fig. 6b).

The consistency of trace-element ratios (ZrIY = 6.7 - 7.9, TiIZr = 32 -41, La/Sm = 3.2 -3.9) and REE patterns (Fig. 6b) suggests that the andesitic units may be cogenetic. This is also indicated by the distinctive trend on the Zr versus Ti diagram (Fig. 6b), which can be the result of fractional crystallization. The slight negative Eu anomaly (Fig. 6b) suggests that plagioclase was fractionated.

In addition to high LREE contents and high [LaIYb],, the McWatters andesites exhibit high LILE concentrations and negative Nb and Ti anomalies on a multielement diagram (Fig. 7b). These characteristics are typical of the calc-alkaline andesitic lavas from destructive plate boundaries. However, comparable geochemical features can also characterize the andesites of some intracontinental extensional settings, such as the Taos Plateau of the Rio Grande rift (Fig. 7a), which result from the contamination of a tholeiitic magma by the continental crust (McMillan and Dungan 1988; Foley and Wheller 1990). Hence, the McWatters calc-alkaline andesites can have been generated in an island-arc setting and (or) by interaction of a mafic magma with continental material.

Diorite Deformed centimetre- to metre-scale dioritic dykes appear

locally cutting the basalts. They are composed of fine-grained albite crystals in a groundmass of albite, chlorite, quartz, epi-

dote, ilmenite, carbonate f magnetite, hematite, pyrite, and chalcopyrite.

The diorites have Si02 contents of 57.5 - 6 1.8 % and plot in the andesitic field of Fig. 3. They are calc-alkaline as indicated by a high Zr/Y ( = 8) and low TiIZr ( = 35) (GClinas et al. 1984) and plot on the same trend as the andesites on a Zr versus Ti diagram (Fig. 5b). The REE are fractionated with [La/Sm], = 2.4 and [La/YbIN = 8.3. They have a chondrite- normalized trace-element pattern almost parallel to that of the andesites, though enriched in all elements but the LILE (Fig. 7c).

Lamprophyre Centimetre- to metre-scale deformed lamprophyric dykes

cut the andesitic lavas and the komatiites (JCbrak et al. 1991~). They contain various amounts of biotite, albite, carbonates, actinolite, chlorite, quartz, epidote, apatite f tourmaline, magnetite, rutile, titanite, pyrite, and chalcopyrite. The coarse- grained unoriented (primary) biotite phenocrysts and the groundmass plagioclase lead to their classification as kersantite (Rock 1977). The Si02 content of the lamprophyres is relatively low (= 50%), whereas the incompatible trace-element concentrations are high (Table 1). They plot in the alkali basalt field of Fig. 3. Their major- and trace-element content are comparable with those of other calc-alkaline (shoshonitic) lamprophyres (Rock 1987) showing a LILE and LREE enrichment, with [LaIYb], = 10.

Volcanic rocks from the Blake River and Pontiac groups

Blake River Group The Blake River volcanic rocks have been sampled north of

the CTZ in the McWatters and Rouyn areas. They consist of pillowed or massive basalts and andesites. The rocks are fine grained and appear undeformed microscopically, and primary textures, such as microlites, amygdules, and spherulites, are commonly preserved. This is in marked contrast with the McWatters samples. The rocks are metamorphosed to the upper greenschist facies as indicated by the presence of chloritoid in the graywacke of the La Brukre Formation (Fig. 2).

The collected samples are from the Pelletier, Rouyn-Noranda, and Duprat-Montbray geochemical units of GClinas et al. (1984). Both basaltic and andesitic rocks are of tholeiitic affinity, with ZrIY < 3.7 (GClinas et al. 1984). The basalts exhibit high TiIZr ratios (> 90) but the andesites have lower TiIZr, probably due to magnetite fractionation (Winchester and Floyd 1977). The Ti depletion in the more evolved units is evident from Fig. 8b. The REE contents vary from 5 x chondritic abundance in the most mafic rocks (Si02 = 48.2%) to about 40 x chondritic abundance in andesite (Si02 = 58.5 %) (Fig. 9a). The [La/YbIN ratio varies from 1.0 to 1.8. Plagioclase fractionation in some andesitic units is indicated by the negative Eu anomaly (Fig. 9a).

Although the Blake River tholeiites exhibit some geochemical similarities with the McWatters basalts (Table 2), their distinct trend on variation diagrams (Fig. 8) and higher [LaIYb], indicate that these two rock assemblages evolved from different sources and (or) processes. Although calc-alkaline magmatism is abundant north of the CTZ (GClinas et al. 1984; PCloquin et al. 1990), no calc-alkaline rocks of the Blake River Group have been recognized in the McWatters area. The McWatters andesites exhibit comparable LaIYb, but lower TiIZr and higher ZrIY compared with the Blake River calc-alkaline units (Table 2).

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/ ' l " ' l " ' l ' ~ p l ' ~ ' -

- (a) - E { Blake River

- -

! ~ i -

- 0 -

- K l -

~ , , , 1 , , , l , ~ , l , , , l , , ~

8 10

MgO (wt.%)

FIG. 8. Variation diagrams of the McWatters, Blake River, and Pontiac tholeiites, showing the Pontiac rocks plotting on the same trend as the McWatters units. (a) MgO vs. SiO, diagram; the correlation coefficient for the McWatters with the Pontiac tholeiites is 0.85. (b) Zr vs. Ti diagram; the correlation coefficient for the McWatters with the Pontiac tholeiites is 0.99.

Pontiac Group Mafic to intermediate volcanic rocks of the Pontiac Group

have been sampled from outcrops located 5 krn south of McWatters (Fig. 1). These units appear as massive and pillowed lava flows a few metres in thickness. The volcanic rocks are medium- to coarse-grained amphibolites with a mineral assemblage dominated by hornblende, plagioclase, quartz, biotite, pyrrhotite, and ilmenite (Morin 1992). The primary textures have been obliterated by metamorphism which outlasted deformation. The Pontiac ultramafic rocks have not been sampled.

The amphibolites correspond to basalts and andesites (SiOz = 51 -55%) with high TiO, (1.62-2.08%) and low MgO (3.3 -4.4%) contents, and Mg# ranging from 27 to 43. Both lithologies are of tholeiitic affinity, with Zr/Y < 3.1 and TiIZr > 85 (Gtlinas et al. 1984). The REE abundances are 2 0 ~ chondritic values with slightly depleted LREE yielding [LatSm], = 0.8 and [La/Yb], = 0.8 (Fig. 9b).

The Pontiac tholeiites are geochemically comparable to the McWatters basalts as shown by their similar trace-element ratios (Table 2) and REE patterns (Fig. 9). Furthermore, the data points for the Pontiac volcanic rocks plot along the McWatters basalts trend (Fig. 8). These lines of evidence suggest a common source and processes for the McWatters and the Pontiac tholeiitic units (Morin 1992). The latter are more evolved, as indicated

1 1 1 ' 1 1 1 1 1 1 1 1 1 4 1 1 1

La Ce SrnEu Tb Yb Lu

FIG. 9. Chondrite-normalized REE diagram of the Blake River (a) and Pontiac (b) tholeiitic units. The McWatters tholeiitic basalt 175 is also plotted for comparison. Normalizing values are given in Fig. 7a.

by their higher incompatible minor- and trace-element contents, and their lower Mg# and Cr content (Tables 1 and 2). As opposed to the Blake River tholeiites, the more evolved units of the McWatters-Pontiac trend (Fig. 8b) do not exhibit a Ti depletion.

Discussion and concluding remarks Since most contacts between the McWatters volcanic units

are tectonic in nature, the stratigraphic relationships are poorly constrained. The andesitic lavas conformably underlie geochemically comparable volcaniclastic rocks. The deformed lamprophyric dykes cut the andesites and the komatiites and thus are younger than these two units. The tholeiitic basalts are cut by deformed dioritic dykes, the geochemistry of which presents similarities with the calc-alkaline andesites. This indicates that the McWatters tholeiitic -calc-alkaline association was present before deformation and suggests that the andesites overlie the basalts.

The chondrite-normalized trace-element patterns of the McWatters tholeiitic and calc-alkaline units are comparable to those of MORB and subduction-related andesites, respectively. The geochemical relationship between the McWatters tholeiitic basalts and calc-alkaline andesites is unknown. Their distinct REE (Fig. 6 ) and trace-element (Fig. 7) patterns suggest that they originate from a different source and (or) processes. Petro- genetic modelling using trace-element concentrations fails to link the McWatters basalts and andesites by partial melting and fractional crystallization processes (Morin 1992). The McWatters andesites are geochemically comparable, but not similar, to the calc-alkaline andesites from the Blake River such as the Dufault and Renault units (Gtlinas et al. 1984)

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TABLE 2. Geochemical characteristics of the McWatters, Pontiac, Blake River, and Pontiac units

McWatters Pontiac Blake River PichC

1 2 3 4 5 6 7 8 9 10 11

SiO, (wt. %) TiO, p205 Rb ( P P ~ ) Sr Zr Y Nb La Yb

TilZr Zr/Y TiIY

La/Sma La/Yba

NOTES: Numbers in parentheses are standard deviation. 1, McWatters tholeiitic basalts (this study, mean of 16 analyses); 2, McWatters calc-alkaline andesites (this study, mean of 13 analyses); 3, Pontiac tholeiitic basalts and andesites (mean of 5 analyses); 4, Blake River tholeiitic basalts (this study, mean of 7 analyses); 5, Blake River tholeiitic basalt (this study, sample 260); 6, Rouyn-Noranda unit tholeiitic basalts (GClinas et al. 1984, mean of 4 analyses); 7, Pelletier unit tholeiitic basalts (GBlinas et al. 1984, mean of 5 analyses); 8, Dufault unit calc-alkaline andesites (Gklinas et al. 1984, mean of 14 analyses); 9, Renault unit calc-alkaline andesites (GClinas et al. 1984, mean of 6 analyses); 10, PichB tholeiitic basalts (Stone 1990, mean of 3 analyses); 11, PichC calc-alkaline andesites (Stone 1990, mean of 4 analyses).

"Chondrite-normalized ratios.

(Table 2). The origin of the Blake River Group calc-alkaline andesites has been ascribed to the contamination of a tholeiitic magma by a felsic crust or melt (Gtlinas and Ludden 1984) or to the partial melting of a mafic pyroxene -garnet-bearing deep crustal source (Laflbche et al. 1992). Compared with the Blake River Group tholeiites, the McWatters basalt and gabbro exhibit lower TiIY, ZrIY, and LaIYb (Fig. 9; Table 2) and define a distinct trend on binary diagrams (Fig. 8). Wilson (1962) considered the McWatters units as Blake River rocks reworked within the CTZ. However, the present data set rather indicates that the McWatters and the Blake River units are tectonically juxtaposed, geochemically unrelated volcanic rock assemblages (Figs. 8 and 9; Table 2).

The McWatters basalts are cogenetic with the tholeiitic units of the northern part of the Pontiac Group. This is indicated by the similarities in the trace-element ratios (TiIZr, ZrIY, TiIY (Table 2) and the REE patterns (Fig. 9b). The McWatters and the Pontiac tholeiites define a common trend on variation diagrams, with the Pontiac rocks occupying the evolved end of the trend (Fig. 8). Petrogenetic modelling, based on trace- element concentrations, shows that both the McWatters and Pontiac tholeiites could originate from a similar partial melt from a common depleted mantle source, with different degrees of fractional crystallization of mafic minerals and plagioclase (Morin 1992). The bringing together of the two rock assemblages is also compatible with structural evidence showing that the Pontiac metavolcanic rocks lie on the south limb of a syncline, whereas McWatters volcanic rocks lie on the north limb (Goulet 1978). Moreover, both the McWatters and the Pontiac tholeiites were affected by medium-pressure metamorphism,

I even though the former were subseauentlv retrograded in the

I u

CTZ oily 1978; Morin et al. 1993). -

The PichC Group possibly represents the equivalent of the

1990) (Table 2) and occupies the same tectonic position in the CTZ, farther east (Fig. 1). The PichC Group has been correlated with the Malartic and Pontiac groups by Latulippe (1976). The PichC and Malartic groups belong to the lower volcanic sequence of the southern Abitibi belt which is distin- guished from the upper sequence by the presence of komatiitic flows and the presence of tholeiitic units exhibiting low ZrIY and LaIYb (Laflbche et al. 1992). The McWatters and northern Pontiac volcanic rocks also display these characteristics and can be regarded as part of this mafic -ultramafic sequence.

The Pontiac and Malartic groups expose deeper crustal levels compared to the surrounding tectonic blocks (Jolly 1978; Feng and Kerrich 1990). The Malartic Group is part of the Lacorne Block, which is considered by Feng and Kerrich (1990) as a possible tectonic window through which the Pontiac Group can be seen in the southern Abitibi belt. Thus, the relatively high metamorphic grade of the Pontiac, McWatters, and Malartic, the presence of the lower sequence ultrarnafic - mafic assemblage in the Pontiac, McWatters, PichC, and Malartic, and the lenses of Pontiac volcanic rocks in the CTZ suggest that the Pontiac and Malartic groups could be linked by the McWatters (and possibly the PichC) volcanic rocks. The McWatters units, which appear as deformed and retrograded lenses in the CTZ, may represent the remnants of an ocean basin that once separated the southern part of the Abitibi belt from the Pontiac Subprovince, as proposed by Feng et al. (1993). In the later compressional tectonics, the McWatters volcanic rocks, and particularly the ultramafic units, may have served as a dCcolle- ment zone during the underthrusting of the Pontiac under the southern Abitibi belt.

Acknowledgments McWatters units, as it comprises a similar lithologic assem- This paper presents part of the results of a M.Sc. thesis at blage of komatiites, tholeiitic basalts and gabbros, and calc- UniversitC du QuCbec i Montreal. Earlier versions of the alkaline lavas and volcaniclastic rocks (Latulippe 1976; Stone manuscript greatly benefitted from the critical comments of

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1530 CAN. J . EARTH SCI. VOL. 30, 1993

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Nature and origin of volcanite slices in the Cadillac break, Abitibi Greenstone belt, Canada