Geochronological constraints on the last stages of terrane assembly in the central part of the Grenville Province

Precambrian Research 92 (1998) 145–164

Geochronological constraints on the last stages of terrane assembly in the central part of the Grenville Province Jacques Martignole a,*, Richard Friedman b a De´partement de Ge´ologie, Universite´ de Montre´al, Montre´al, Que´., Canada b Department of Geological Sciences, University of British Columbia, Vancouver, B.C., Canada Received 2 July 1997; accepted 29 April 1998

Abstract New U–Pb zircon and monazite ages for syn- and post-kinematic pegmatites that cut major shear zones within the western Quebec portion of the Grenville Province constrain the timing of deformation and final terrane assembly within this region. From the Grenville Front SE along the present section, the Grenville Province consists of a reworked Archean promontory partly covered by Proterozoic allochthons and two Proterozoic metasedimentary and metaigneous terranes, which are probably allochthonous. The age of the Grenville Front is still poorly constrained in this area as pegmatites intersecting high-grade rocks exhumed some 30 km south of the Front have upper intercept U–Pb ages of 2642+6/−5 Ma. Field and geochronological data in this paper, however, lend support to the possibility of late-Archean exhumation of high-grade terranes along the Front. South of the Front, two types of ductile shear zone are responsible for the geometry of terranes in this part of the Grenville Province. The first type corresponds to the boundary of allochthons transported onto the Archean parautochthon. The corresponding structures are not directly dated, but reheating of the parautochthon at about 1020 Ma ( U–Pb ages on monazite) sets a minimum age for thrusting in this area. Late movement along an east-dipping shear zone located within the parautochthon, the Cabonga thrust (CT; 998+9/−5 Ma), is rather unique in that it involves rocks of the parautochthon and is characterized by a westerly directed transport. The shear zones of the second type are subvertical, NE-trending, generally sinistral and intersect the parautochthon, the allochthon or both. From the Front towards the SE, these shear zones are: the Cadgecrib shear zone (998+16/−6 Ma) intersecting the parautochthon, the Renzy shear zone (1003+4/−5 Ma) involving allochthonous metasediments, the Baskatong shear zone (1020+2/−1 Ma) which marks the NW limit of Proterozoic rocks of the Mont-Laurier terrane, the Labelle shear zone (1078±6 Ma) separating the Mont-Laurier and the Morin terrane, and the Taureau shear zone (1074±4 Ma) bounding the Morin terrane to the east. All the above timing constraints are from U–Pb ages on syn- to post-kinematic pegmatites emplaced within the respective shear zones. This age distribution suggests a propagation of transcurrent deformation towards the NW, which is responsible for the final configuration of terranes. © 1998 Elsevier Science B.V. Keywords: U–Pb geochronology; Shear zones; Grenville Province; Terrane assembly

* Corresponding author. Fax: +1 514 343 5782; e-mail: [email protected] 0301-9268/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S0 3 0 1 -9 2 6 8 ( 9 8 ) 0 0 07 2 - 2

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1. Introduction During the last two decades, the recognition of crustal-scale shear zones along which major displacements took place has contributed to the understanding of ancient collisional belts like the Grenville Province of the Canadian Shield (Baer, 1977; Davidson, 1984; Rivers et al., 1989). Such shear zones, together with geological and aeromagnetic maps, have been used to subdivide the Grenville Province into first-order longitudinal belts (Rivers et al., 1989). Each of the above belts in turn can be divided into terranes on the basis of common lithological, metamorphic, tectonic and/or isotopic characteristics. The Parautochthonous belt includes reworked equivalents of adjacent structural provinces that constitute the Laurentian craton (mostly Archean and Paleoproterozoic). The allochthonous belts are comprised of transported monocyclic and polycyclic rock packages that cannot be directly correlated with the nearby Laurentian craton. The above structural units are generally limited by zones of high strain, including high-temperature mylonites, although the sole presence of a shear zone does not warrant the identification of a terrane boundary. Most shear zones dip to the SE and have accommodated NW-directed transport during the Grenvillian orogenic events between 1180 Ma and 1000 Ma (Davidson, 1984; Hanmer, 1988; Rivers et al., 1989; McEachern and van Breemen, 1993; Rivers, 1997). In fact, the lobate shape of large scale allochthons shows that contractional tectonics was a primary process in the development of the Grenville orogeny (Rivers et al., 1989). However, several shear zones have been described along which displacement is essentially transcurrent and the question arises as to the relative importance and the timing of these structures. For instance, in the Ontario segment of the Monocyclic Belt boundary zone, SE-plunging minor folds, with S-shaped asymmetry when viewed down the plunge, have been interpreted as resulting from the superposition of a NE-directed sinistral wrench on the dominant 1060 Ma-old NW direction of tectonic transport (Hanmer, 1988). In the Mauricie area, 200 km east of the Morin terrane (MT ), N-trending sinistral extensional

deformation dated at 1090–1040 Ma post-dates high-grade metamorphism and NW-directed stacking of crustal slices (Corrigan and van Breemen, 1997). In the Lac St. Jean area, a NE-trending transcurrent shear zone, with a sinistral sense of motion, is dated at 1154 Ma (Higgins and van Breemen, 1992). The SE-dipping oblique, sinistral, shear zone of the Rivie`re Pentecoˆte Anorthosite, some 300 km to the NE, has yielded a U–Pb titanite age of 1004±6 Ma (Martignole et al., 1993). The low metamorphic grade Wakeham terrane (Martignole et al., 1994) is limited to the NW by an extensional sinistral shear zone dipping to the SE which postdates the intrusion of charnockites of the Rivie`re Romaine dated at 1079±5 Ma (Loveridge, 1986). Finally. in the easternmost part of the Grenville Province, reactivation of the Gilbert River shear belt occurred under greenshistfacies conditions between 1113 and 1062 Ma (Scott et al., 1993). Transcurrent shear zones are thus widespread throughout the Grenville orogen, most of them being NNE-trending and sinistral, although their individual characteristics and their ages vary from one region to another. Purely transcurrent and high-grade shear zones dominate in the southwestern Grenville Province, whereas in the NE part of the province sinistral shear zones are mainly transtensional and occur at higher crustal levels. In the central part of the Grenville Province, north of Montre´al, several such structural zones can be observed along a transect orthogonal to the Grenville Front ( Fig. 1). The aim of this paper is to constrain the timing of displacement along these shear zones that seem to dominate the last stages of terrane assembly in western Que´bec. This will be achieved through U–Pb dating of zircon and monazite, with the ultimate goal of determining how and when final terrane assembly took place in the central part of the Grenville Province.

2. Geological setting The 400 km-long transect across the Grenville Province in western Que´bec sector (Fig. 1) displays most of the lithological and tectonic characteristics which typify this orogenic belt. Intersected terranes

J. Martignole, R. Friedman / Precambrian Research 92 (1998) 145–164

147

Fig. 1. Geological framework of the Grenville Province in western Quebec and location of analysed samples.

either belong to the Archean parautochthon ( X terrane ( XT ), Lac Timiskamingue terrane (LTT ), Re´servoir Dozois terrane (RDT )), or to the Proterozoic allochthons (Re´servoir Cabonga terrane (RCT ), Renzy shear belt ( RSB), Lac Dumoine terrane (LDT ), Mont-Laurier terrane (MLT ) and MT ). More than half a dozen mappable ductile shear

zones have been identified from the Grenville Front to the St. Lawrence Lowlands. From the NW to the SE, these are: the Grenville Front, the Bouchette shear zones (corresponding to the Allochthon boundary thrust of Rivers et al., 1989); the CT; the Cadgecrib shear zone; the Renzy shear zone; the Baskatong shear zone (BSZ); the Labelle shear zone (LSZ); the Morin shear zone (MSZ);

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the Taureau shear zone ( TSZ ). Most of these shear zones represent terrane boundaries in the sense of Rivers et al. (1989), but some occur within terranes (e.g. Cadgecrib shear zone, Renzy shear zone, MSZ, Fig. 1). Based upon their geometric and kinematic characteristics, most of these shear zones are of two types. The first type is preserved along the leading edges of allochthons and consists of SE-dipping cuspate surfaces with evidence of top-to-the-NW tectonic transport, as observed along the NW margin of the RCT (minor shear zones intersecting the Bouchette anorthosite). The same may prove to be true for the lobe-shaped leading edge of the LDT ( Fig. 1). This type of shear zone also developed in other parts of the Grenville Province where NW thrusting has been documented (Ontario: Davidson, 1984; Hanmer, 1988; Hanmer and McEachern, 1992; eastern Que´bec: Rivers and Chown, 1986). The second type consists of subvertical shear zones along which displacement is most commonly sinistral and transcurrent. None of these shear zones has a NW orientation, which excludes their being lateral ramps of NW-directed thrusts. They are N- to NE-trending structures along which the last increments of strain appear to have been transcurrent. At the regional scale, this strike–slip regime altered the geometry of previously accreted terranes. Whereas unequivocal evidence for NW-directed thrusting is observed at the leading edges of the allochthons (the Allochthonous Belt boundary thrust of Rivers et al. (1989)) close to the Grenville Front, NE-trending sinistral transcurrent shear becomes more conspicuous southeastward, where such motion appears to accommodate a major and late component of terrane displacement. This late-stage strike–slip regime appears to be less developed in the Ontario segment of the Grenville Province, where NW-directed thrusting is the principal process which has been documented (e.g. Davidson, 1984).

3. Sampling Sampling of pegmatites was undertaken in the main shear zones in order to constrain the timing

of displacement of major terranes whose metamorphic/cooling ages have previously been determined (Garie´py et al., 1990; Childe et al., 1993; Friedman and Martignole, 1995; Martignole and Reynolds, 1997). In the selection of samples, priority was given to pegmatite dykes crosscutting mylonitic fabrics but themselves affected by the last increments of strain. Where such dykes were not found, samples were collected from dykes intersecting the mylonitic foliation at angles appropriate to extensional fractures generated during the last increments of shear strain. Finally, in the least favourable cases, undeformed pegmatites randomly intersecting mylonitic foliation were sampled. According to these criteria, pegmatites selected for this study can reasonably be described as syn-, late- or post-kinematic and serve to set time constraints, at least on the lower limit of the age of deformation.

4. Geochronology 4.1. The Grenville Front The Grenville Front is a complex zone of ductile and semi-brittle deformation. Some 2 km south of the Front, along highway 117, outcrop-scale, NW-directed ductile thrust duplexes are intersected by low-grade, semi-brittle, SE-dipping extensional faults. Metamorphic studies have shown that, in the same area, biotite-garnet and charnockitic gneisses of the XT override muscovite schists along what is probably a major reverse fault zone (Indares and Martignole, 1989, Fig. 1). On the other hand, the existence in this area of a major, crustal-scale, extensional shear zone is suggested by the Lithoprobe seismic profile (Martignole and Calvert, 1996). Several attempts have previously been made to understand the nature and timing of metamorphism and associated deformation that took place along the Grenville Front south of Val-d’Or. Metamorphic studies (Indares and Martignole, 1989) suggest that a short-lived thermal perturbation caused growth of Ca-rich garnet coronas around inherited, most likely Archean, Ca-poor garnet in high-grade assemblages of the XT. This

J. Martignole, R. Friedman / Precambrian Research 92 (1998) 145–164

interpretation was in part confirmed by Pb/Pb studies on mineral separates, including garnet, which yielded an age of 2596±5 Ma, whereas apatites from the same rock grew or were reset during a Grenvillian event at about 1030 Ma (Garie´py et al., 1990). On the other hand, monazite fractions from a pegmatite located about 30 km southeast of the Front along highway 117 are 3 to 6% discordant and give an age of 2643±5 Ma, if the lower intercept is fixed at 1000 Ma ( Krogh, 1994). These results tend to confirm the mild character of the Grenvillian metamorphic overprint, i.e. melting did not occur and lead loss in Archean monazites was only minor. Metamorphism is more intense at the Grenville Front some 250 km along strike to the SW, in the Sudbury-North-Bay area, where quasi-concordant zircon and titanite from deformed pegmatites yield ages of 987±2 Ma and 991±4 Ma respectively ( Krogh, 1994). Given this discrepancy in ages along strike, another attempt was made in order to date ductile deformation within the Grenville Front zone south of Val-d’Or. Quartz-K-feldspar-biotite pegmatites cut SE-dipping biotite-garnet-orthopyroxene gneisses within the southeastern part of the

149

Grenville Front zone (35 km southeast of the Front). Some of these pegmatites have been rotated into concordance with the regional foliation and were converted into coarse-grained gneiss with discrete SE-dipping shear planes and sigmoidal augen, indicating top to the NW-direction of displacement. Owing to its parallelism with the Front this fabric was considered as potentially resulting from NW-directed Grenvillian thrusting. Consequently, this gneiss has been sampled from a 60 cm-thick vein in an attempt to date deformation in the Grenville Front zone. Primary zircon and monazite were recovered from the deformed pegmatite. Zircon is of gem quality, euhedral, clear, and pale pink in colour. Crystal morphologies vary from elongate prismatic to equant multifaceted. Monazite is of high quality, clear, yellow and equant to tabular with poorly developed crystal faces. Single grain analyses of four zircons and one monazite ( Table 1) define a quasi-linear array ( Fig. 2) which is attributed to post-crystallization Pb loss. These analyses are 1.0–7.7% discordant relative to their respective 207Pb/206Pb ages ( Table 1). A five-point regression through these data gives an upper intercept age of 2642.6+6.1/−5.3 Ma (MSWD=7.0), which is

Fig. 2. Concordia diagram for a syn-kinematic pegmatite of the Grenville Front zone.

1050 1325 1218 1334 967 1343 1563

59 72 96 112

1065 1165 1031 1093 2232 878 1064 500 707 444

2—Cadgecrib pegmatite A cc,N1,p 0.222 D c,N1,p 0.040 E cc,N1,p 0.061 F cc,N1,p,b 0.037 G cc,N1,p,b 0.061 H cc,N1,p,b 0.024 I cc,N1,p 0.012 3—Cadgecrib gabbro T1 c,N7,t,b 0.016 T2 c,N7,t,b 0.081 T3 c,N7,t,b 0.046 T4 c,N7,t,b 0.379 4—Cabonga pegmatite A cc,N1,a 0.265 B cc,N1,a 0.273 C cc,N1,p,b 0.115 D cc,N1,p,b 0.148 E cc,N1,p 0.019 F cc,N1,p,b 0.083 G c,N1,p 0.074 I m,N1,p 0.052 J m,N1,p 0.076 K f,N1,p 0.067

Ub (ppm)

85 64 119 60 400

Weight (mg)

1—Grenville Front zone A cc,N1,p,e 0.063 B cc,N1,p 0.090 C cc,N1,p,e 0.039 D cc,N1,p,eq 0.048 M2 c,N20 0.030

Fractiona

168 183 161 171 339 136 166 94 133 89

11 14 18 22

163 201 185 205 148 207 243

48 40 68 38 5101

Pb*c (ppm)

89365 59346 31335 39911 14911 26148 34954 3867 8602 8606

353 993 1132 2235

81708 8224 14665 18962 20507 4116 6450

8640 21810 30930 17180 14500

206Pb/ 204Pbd

32 54 38 41 28 29 23 77 73 43

29 60 40 192

29 63 50 26 29 79 29

18 8 5 5 24

Pbe (pg)

Table 1 U–Pb analytical data for the Grenville Province, western Quebec

2.4 2.4 2.4 2.4 2.0 2.2 2.4 7.2 5.5 6.2

16.7 20.2 20.8 23.1

2.4 2.9 2.4 2.2 2.2 2.3 2.7

18.9 20.9 13.2 18.9 96.3

208Pbf (%)

0.16682 0.16634 0.16479 0.16505 0.16138 0.16496 0.16549 0.18754 0.19106 0.20182

0.16455 0.16492 0.16351 0.16333

0.16418 0.15951 0.16096 0.16293 0.16233 0.16358 0.16388

0.46250 0.48924 0.48717 0.49980 0.46001

(0.28) (0.14) (0.22) (0.33) (0.28) (0.15) (0.11) (0.10) (0.16) (0.07)

(0.12) (0.11) (0.16) (0.43)

(0.16) (0.13) (0.19) (0.15) (0.12) (0.13) (0.13)

(0.13) (0.09) (0.09) (0.09) (0.60)

1.6704 1.6651 1.6570 1.6558 1.6035 1.6474 1.6502 2.0634 2.1232 2.3339

1.6326 1.6358 1.6221 1.6213

1.6336 1.5837 1.5958 1.6178 1.6160 1.6275 1.6302

(0.18) (0.15) (0.15) (0.15) (0.61)

(0.28) (0.21) (0.27) (0.36) (0.28) (0.22) (0.19) (0.19) (0.22) (0.09)

(0.64) (0.29) (0.30) (0.47)

(0.22) (0.20) (0.25) (0.21) (0.19) (0.21) (0.21)

11.1413 11.9910 11.8964 12.2762 11.1100

0.07262 0.07260 0.07293 0.07276 0.07206 0.07243 0.07232 0.07980 0.08060 0.08387

0.07196 0.07194 0.07195 0.07199

0.07216 0.07201 0.07190 0.07202 0.07220 0.07216 0.07215

0.17471 0.17776 0.17711 0.17814 0.17516

(0.02) (0.09) (0.09) (0.09) (0.04) (0.09) (0.09) (0.10) (0.09) (0.04)

(0.56) (0.21) (0.20) (0.14)

(0.09) (0.09) (0.09) (0.09) (0.09) (0.10) (0.10)

(0.08) (0.07) (0.07) (0.07) (0.03)

(2.3) (2.1) (2.9) (7.8)

(3.0) (2.2) (3.5) (2.7) (2.2) (2.3) (2.4)

994.5 (5.1) 991.9 (2.6) 983.3 (4.1) 984.8 (6.0) 964.4 (5.0) 984.3 (2.8) 987.2 (2.1) 1108.1 (2.1) 1127.1 (3.3) 1185.1 (1.6)

982.0 984.1 976.3 975.2

980.0 954.1 962.1 973.0 969.7 976.6 978.3

2450.6 (5.5) 2567.4 (3.7) 2558.4 (3.8) 2612.9 (3.7) 2439 (24)

(3.5) (3.8) (3.7) (3.6) (3.7) (4.2) (4.0)

(2.6) (2.4) (2.4) (2.4) (0.9)

1003.4 (1.0) 1002.8 (3.5) 1011.9 (3.7) 1007.3 (3.7) 987.8 (1.6) 998.1 (3.6) 995.0 (3.6) 1192.0 (4.1) 1211.6 (3.7) 1289.6 (1.5)

985 (23) 984.2 (8.6) 984.5 (8.1) 985.8 (5.8)

990.6 986.2 983.2 986.4 991.6 990.4 990.1

2603.3 2632.1 2626.0 2635.7 2607.6

207Pb/206Pb

206Pb/238U

207Pb/206Pb

206Pb/238U

207Pb/235U

Apparent ages (±2s, Ma)g

Isotopic ratios (±1s, %)g

150 J. Martignole, R. Friedman / Precambrian Research 92 (1998) 145–164

363 323 373 676 478 928 361 733 815 387

7—Macaza aplite D f,N1,p,e,n E f,N1,p,s G c,N3,M1,p,s H c,N3,M1,p I c,N3,M1,p,s J m,N3,M1,p K f,N1,p,e L f,N1,p,e,ti M f,N3,M1,n N f,N3,p,e,ti

dyke 0.035 0.028 0.113 0.087 0.053 0.101 0.010 0.011 0.011 0.010

292 245 711 452 276 304

6—Baskatong pegmatite A1 g,cc,N1,p,b 0.044 A2 g,cc,N1,p,b 0.043 B2 g,cc,N1,p,e 0.008 C cc,N1,p,b 0.105 D cc,N1,p,b 0.105 E cc,N1,p,e 0.045

Ub (ppm)

680 921 671 686 719 789 755 593 1060

Weight (mg)

5—Renzy pegmatite A cc,N1,p,e 0.030 B cc,N1,p,s 0.110 C cc,N1,p,s 0.032 D cc,N1,p,s 0.093 E cc,N1,p,s 0.020 F cc,N1,p,s 0.016 M1 cc,M10,a 0.036 M3 cc,M10,a 0.009 M4 cc,M10,a 0.017

Fractiona

66 59 64 96 81 131 61 116 124 59

54 46 141 85 51 56

114 163 111 114 121 133 2122 2261 2343

Pb*c (ppm)

3179 3788 9257 2797 3078 2470 2322 3070 2888 2134

10510 8103 3265 19778 10688 6268

7810 42532 9674 23247 2469 5564 3234 1242 4012

206Pb/ 204Pbd

43 26 47 183 84 334 16 25 30 17

13 14 20 25 28 22

27 24 23 28 59 23 84 43 45

Pbe (pg)

Table 1 (continued ) U–Pb analytical data for the Grenville Province, western Quebec

11.0 8.9 8.0 7.8 8.5 6.2 9.1 8.2 5.5 7.6

15.2 15.1 20.5 16.6 15.7 16.1

7.5 12.3 7.3 6.6 8.5 9.2 94.6 96.1 93.1

208Pbf (%)

0.17414 0.17871 0.16933 0.14236 0.16712 0.14386 0.16521 0.15778 0.15521 0.15203

0.17136 0.17127 0.17093 0.16960 0.16985 0.16757

0.16784 0.16819 0.16623 0.16847 0.16649 0.16577 0.16287 0.16204 0.16345

(0.10) (0.06) (0.15) (0.14) (0.12) (0.11) (0.12) (0.19) (0.10) (0.11)

(0.08) (0.09) (0.10) (0.14) (0.14) (0.11)

(0.13) (0.22) (0.13) (0.18) (0.13) (0.14) (0.14) (0.16) (0.09)

1.8036 1.8771 1.7623 1.4524 1.7322 1.4608 1.6984 1.6216 1.5901 1.5522

1.7292 1.7305 1.7255 1.7105 1.7138 1.6922

1.6825 1.6830 1.6635 1.6868 1.6668 1.6627 1.6145 1.6060 1.6201

(0.20) (0.09) (0.15) (0.15) (0.13) (0.13) (0.22) (0.25) (0.20) (0.22)

(0.10) (0.10) (0.12) (0.20) (0.21) (0.20)

(0.21) (0.26) (0.20) (0.24) (0.22) (0.21) (0.22) (0.36) (0.19)

0.07512 0.07618 0.07548 0.07399 0.07517 0.07364 0.07456 0.07454 0.07430 0.07405

0.07319 0.07328 0.07321 0.07315 0.07318 0.07324

0.07271 0.07257 0.07258 0.07262 0.07261 0.07275 0.07190 0.07188 0.07189

(0.11) (0.06) (0.03) (0.05) (0.05) (0.05) (0.13) (0.14) (0.11) (0.13)

(0.03) (0.04) (0.06) (0.09) (0.09) (0.10)

(0.10) (0.09) (0.09) (0.09) (0.12) (0.11) (0.12) (0.28) (0.10)

1034.9 1059.9 1008.4 858.0 996.2 866.5 985.6 944.4 930.1 912.3

1019.6 1019.1 1017.2 1009.9 1011.3 998.7

1002.2 1002.2 991.3 1003.7 992.7 988.7 972.7 968.1 975.9

(1.9) (1.1) (2.9) (2.2) (2.2) (1.8) (1.8) (3.3) (1.7) (1.9)

(1.6) (1.7) (1.8) (2.6) (2.6) (2.1)

(2.4) (4.0) (2.4) (3.4) (2.4) (2.6) (2.5) (2.9) (1.6)

1071.6 1099.8 1081.4 1041.3 1073.2 1031.7 1056.7 1056.2 1049.7 1042.8

1019.1 1021.8 1019.9 1018.0 1019.0 1020.6

(4.5) (2.2) (1.3) (1.9) (1.9) (2.1) (5.1) (5.8) (4.5) (5.2)

(1.3) (1.6) (2.3) (3.6) (3.7) (4.0)

1005.7 (3.9) 1002.0 (3.6) 1002.2 (3.8) 1003.3 (3.7) 1003.1 (4.7) 1006.9 (4.3) 983.0 (4.8) 983 (12) 982.8 (4.2)

207Pb/206Pb

206Pb/238U

207Pb/206Pb

206Pb/238U

207Pb/235U

Apparent ages (±2s, Ma)g

Isotopic ratios (±1s, %)g

J. Martignole, R. Friedman / Precambrian Research 92 (1998) 145–164 151

Weight (mg)

Ub (ppm)

24 46 34 105 44 51 95 91 106 51

Pb*c (ppm)

4213 6037 5046 9692 10828 3456 9433 8668 10434 5500

206Pb/ 204Pbd

27 26 33 32 17 51 26 14 17 9

Pbe (pg)

7.9 9.2 8.7 10.2 8.0 8.1 10.2 7.0 9.5 6.9

208Pbf (%)

0.18337 0.18328 0.18607 0.17708 0.18647 0.18966 0.18195 0.18310 0.17567 0.18446

(0.10) (0.09) (0.13) (0.17) (0.10) (0.10) (0.11) (0.11) (0.10) (0.10)

1.9252 1.9368 1.9858 1.8360 1.9741 2.0588 1.9184 1.9404 1.8218 1.9520

(0.20) (0.18) (0.20) (0.23) (0.11) (0.20) (0.19) (0.19) (0.18) (0.19)

0.07614 0.07664 0.07740 0.07520 0.07678 0.07873 0.07647 0.07686 0.07522 0.07675

(0.10) (0.10) (0.10) (0.09) (0.04) (0.11) (0.09) (0.09) (0.09) (0.10)

1085.4 1084.9 1100.1 1051.0 1102.2 1119.5 1077.6 1083.9 1043.3 1091.3

(2.1) (1.8) (2.5) (3.4) (2.1) (2.1) (2.1) (2.1) (2.0) (1.9)

1098.9 1111.9 1131.5 1073.7 1115.5 1165.4 1107.4 1117.6 1074.3 1114.7

(4.2) (3.9) (4.0) (3.7) (1.6) (4.2) (3.8) (3.8) (3.7) (4.1)

207Pb/206Pb

206Pb/238U

207Pb/206Pb

206Pb/238U

207Pb/235U

Apparent ages (±2s, Ma)g

Isotopic ratios (±1s, %)g

Notes: analytical techniques are listed in Journeay and Friedman (1993) ( UBC ), Parrish et al. (1987) (GSC, zircon, monazite). All analytical errors were numerically propagated through the entire age calculation using the technique of Roddick (1987). Concordia intercept ages and associated errors were calculated using a modified version of the York-II regression model (wherein the York-II errors are multiplied by the square root of the MSWD) and the algorithm of Ludwig (1980). Zircon fractions A1, A2, and B2 of Baskatong pegmatite (RF91-18A) from GSC Geochronology Laboratory, all other analyses from UBC Geochronology Laboratory. All zircon fractions are air abraded except J of Macaza aplite dyke (RF92-1) and I of Taureau quartz monzonite dyke (RF92-2). a Upper case letter±numeral=fraction identifier; M+numeral=monazite, T+numeral=titanite, otherwise zircon; g=GSC Geochronology Laboratory; otherwise UBC Geochronology Laboratory. Grain size, intermediate dimension: cc=>149 mm, c=<149 mm and >134 mm, m=<134 mm and >74mm, f=<74 mm. Magnetic codes: Franz magnetic separator sideslope (in degrees) at which grains are nonmagnetic (N ) or magnetic (M ); field strength for all zircon and titanite fractions=1.8 A; monazite magnetic at 10°/0.3a; front slope for all fractions=20°; grain character codes: a=anhedral, b=broken fragments, e=elongate, eq=equant, n=needles, o= ovoid, p=prismatic, s=stubby, t=tabular, ti=tips. b UBC analyses: U blank correction of 1–3 pg±20%; U fractionation corrections were measured for each run with a double 233U–235U spike (about 0.005/amu), except for Cabonga pegmatite which was corrected for 0.0044/amu±10% fractionation, based on NBS U standard U500. GSC analyses: U blank correction of <1 pg; U fractionation corrections were measured for each run with a double 233U–235U spike (about 0.0009/amu). c Radiogenic Pb. d Measured ratio corrected for spike and Pb fractionation of 0.0043/amu±20% and 0.0012/amu±7% for Daly and Faraday runs respectively ( UBC ) and 0.0009/amu (GSC ), based on replicate analyses of the NBS-981 Pb standard and the values recommended by Todt et al. (1984); laboratory blank Pb concentrations of 10±5 pg to 20±10 pg ( UBC ), 12 pg (GSC ). Laboratory blank Pb concentrations and isotopic compositions based on total procedural blanks analysed throughout the duration of this study. e Total common Pb in analysis based on blank isotopic composition. f Radiogenic Pb. g Corrected for blank Pb, U and common Pb. Common Pb corrections based on the Stacey–Kramers model (Stacey and Kramers, 1975) at the age of the rock or the 207Pb/206Pb age of the fraction.

8—Taureau quartz monzonite dyke A c,N2,p 0.079 129 B m,N2,p,e 0.057 244 C m,N2,p,eq 0.082 179 D f,N2,p,n 0.049 578 E m,N2,p,s 0.068 237 F m,N2,p,t 0.057 268 G m,N2,p,e 0.043 504 H m,N3,p,e,ti 0.022 501 I f,N3,p,e,ti 0.028 591 J m,N3,p,t 0.015 279

Fractiona

Table 1 (continued ) U–Pb analytical data for the Grenville Province, western Quebec

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interpreted as the best estimate for the igneous age of this rock ( Fig. 2). A poorly defined lower intercept of 827±151 Ma, which may be interpreted as the time during which Pb loss occurred, is significantly younger than the ca. 987 Ma age obtained in the Ontario section of the Grenville Front. This is in keeping with previous determinations (e.g. Garie´py et al., 1990) showing that the Grenvillian metamorphism represents a short-lived burial event, probably too short and not hot enough to cause remobilization of Archean pegmatites. A less plausible interpretation of the data set would consider the pegmatite as Grenvillian, with zircons inherited from the wall-rocks. However, the fact that several attempts at dating Grenvillian thrusting along the Grenville Front in this area have given Archean ages (Garie´py et al., 1990; Krogh, 1994) raises the question of how exhumation of high-grade rocks occurred in the Front zone. The preservation of Archean monazite in pegmatites crosscutting SE-dipping fabrics is compatible with uplift along a structural zone whose geometry was not fundamentally different from that of the Grenville Front but whose age, still unconstrained, may be as old as Archean. Pending more definitive determinations, the 987±2 Ma zircon age from the Sudbury-North-Bay area will be provisionally taken as the age of the last main tectonic activity along the Grenville Front in western Quebec. 4.2. The Allochthon boundary Some 60 km south of the Grenville Front, the southern edge of XT is overlain by recycled Archean migmatites of the RDT (Indares and Martignole, 1990a; Archean Sm/Nd model ages, Dickin et al., 1989). These migmatites, which are exposed for more than 120 km along the western Quebec Grenville transect (Fig. 1), are thought to comprise what was a prong in the Paleoproterozoic Laurentian continental margin (Baskatong promontory, Martignole and Calvert, 1996). Overlying the RDT is the RCT, a Proterozoic allochthon (Indares and Martignole, 1990b; Martignole and Pouget, 1993, 1994). The main phase of tectonic activity along the SE-dipping leading edge of the Cabonga allochthon ( Fig. 1) involved a minimum of several tens of kilometres of NW-directed

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thrusting if the root zone of the RCT is located in the eastern part of the MLT (Martignole and Calvert, 1996). The leading edge of this allochthon (Bouchette gabbro-anorthosite, BA) is dissected by several SE-dipping shear zones with top to the NW kinematic indicators. Appropriate samples have not been found to date the basal shear zone along which the allochthon was transported towards the NW. However, granulite-facies metapelites from this tectonic unit have given monazite U–Pb ages ranging from 1180 to 1145 Ma and rutile ages of 955 Ma (Friedman and Martignole, 1995). In contrast, deformed migmatites of the RDT, underlying the RCT, contain monazite dated between 1021 and 995 Ma (Childe et al., 1993) and 40Ar/39Ar ages on hornblende indicating cooling of these rocks through about 480°C at about 963 Ma (Martignole and Reynolds, 1997). As dated rutile from the RCT indicates cooling through 500°C at about 960 Ma, the parautochthon and the allochthons were at the same temperature 960 m.y. ago, whereas the high temperature portion of their thermal history is discordant by at least 150 m.y. A plausible scenario is that highgrade metamorphism, and likely partial cooling of the RCT took place before its transport onto the parautochthon. Also, the emplacement of allochthons so far appears to be the most likely mechanism for the metamorphism of underlying migmatites, and 1021 Ma can be taken as a minimum age for the thrusting event in the area. We note that this event might be somewhat older than the 991 Ma-old deformation at the Grenville Front if this age is taken as representative. 4.3. The CT The RCT is limited on its western side by a long east-dipping splay of the Allochthon boundary that may be at first sight taken as a lateral ramp of the main thrust. In fact, this shear zone is rather interpreted as a late westward thrust: the CT (Martignole and Pouget, 1994). The RCT is deformed by large wavelength NE-trending buckle folds. The CT is a mappable zone a few tens of metres thick, comprising straight gneisses and discrete bands of anastomosing mylonites. An outcrop-sized pegmatite cutting straight gneisses,

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Fig. 3. Concordia diagrams for a syn-kinematic pegmatite of the Cabonga shear zone: (A) upper intercept; (B) all data with inherited zircons).

from within the east-dipping segment of the CT, was sampled. This pegmatite has undergone minor deformation, based on the presence of small pegmatitic clots and isolated feldspar megacrysts apparently pulled from the main mass of pegmatite, around which foliation has been deflected. It is thus interpreted as late syn-deformational with

respect to the west-directed transport along the CT. Two distinct zircon types have been recognized in this sample. In order of decreasing abundance they are: relatively large, very clear, colourless to light grey, irregularly shaped to vaguely prismatic, crack- and inclusion-free crystals, and cloudy to rarely clear, smaller, stubby to moderately elon-

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gate, doubly-terminated prisms. Fractions A–G from the clear, irregularly shaped population exhibit slight discordance at about 1.0 Ga and are interpreted as primary igneous zircon from the pegmatite ( Table 1; Fig. 3(A)). Fractions I, J, and K from the finer cloudy population are 7–9% discordant, give significantly older Pb/U and Pb/Pb ages and define a chord with an upper intercept of ca. 1.6 Ga and a lower intercept of ca. 1.0 Ga ( Fig. 3(B)). These latter grains are interpreted as xenocrystic and are likely to have been derived from the enclosing gneisses. Two alternative explanations for the discordance of these grains are: profound Pb loss at ca. 1.0 Ga (the age of the pegmatite), or, the presence of pegmatite aged rims with older cores. The former explanation is favoured, as clear rims have not been observed. However, the presence of minor rims the same age as the pegmatite cannot be ruled out. The interpreted age of this pegmatite is based on zircon fractions A–G only. The dispersion of these analyses indicates that their discordance is the result of Pb loss and/or minor inheritance (Fig. 3(A)). Fractions G and E bound the left side of the array and are interpreted to contain little or no inherited zircon. The upper intercept of a two-point chord through these fractions, 998.4+9/−5 Ma, provides the best estimate for the age of this sample and thus for west-directed transport along the CT. This chord has a lower intercept of 365+170/−169 Ma which may not have any geological significance. 4.4. The Cadgecrib shear zone Structurally beneath the RCT, migmatites of the parautochthon are intersected by subvertical, NE-trending straight gneisses and high-grade mylonites of the Cadgecrib shear zone. This zone is several hundred metres in width and is thought to have accommodated dominantly transcurrent sinistral displacement. It comprises mylonitic equivalents of a medium-grained gabbro, the Cadgecrib gabbro and strongly deformed migmatites. Locally, it cuts foliations associated with the CT, but NE-trending mylonites are also found isolated between two splays of the CT (Martignole and Pouget, 1994).

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Dating of a small pegmatite body which intersects ductile fabrics within the Cadgecrib shear zone, but is itself slightly deformed by the shear zone ( late-kinematic pegmatite), places an age on the last ductile deformation along this subvertical structure. Zircons from this rock vary from colourless, crack- and inclusion-free with perfect clarity, to pale amber with minor cracks and cloudy cores. Grains with visible cores were not selected for analysis in an attempt to minimize or exclude inherited zircons. The coarsest zircon grains with the lowest magnetic susceptibility, from which analysed fractions were selected, include intact stubby prisms and angular slabs interpreted as broken fragments of larger grains. Seven analysed zircon fractions define a quasi-linear array indicative of Pb loss, with possible minor inherited zircons in fractions D and G ( Table 1; Fig. 4). A five-point regression (excluding fractions D and G) with an upper intercept of 998.1+ 15.8/−6.5 Ma (MSWD=0.36) provides the best estimate for the age of this rock. This chord has a lower intercept of 475±215 Ma. We note that the 998 Ma age, which records the last increments of strain recorded by U–Pb systematics, in the above pegmatite is not significantly different from the ca. 1 Ga ages of monazites from the tectonic basement of the RCT (Childe et al., 1993). The Cadgecrib gabbro was sampled a few hundred of metres away from the Cadgecrib shear zone to constrain cooling ages within the footwall of the CT. This coarse grained, foliated metamorphosed (presence of sparse garnet coronas) gabbro predates deformation along the Cadgecrib shear zone and gives an Sm/Nd whole-rock model age of ca. 1.3 Ga (Dickin, 1991). The sample of Cadgecrib gabbro collected for U–Pb dating yielded only titanite (probably of metamorphic origin), which is clear, light brown to amber in colour and in general free of inclusions. Grains are roughly tabular in shape, but at least 50% of the recovered titanite occurs as broken fragments of larger grains. Four analysed titanite fractions are concordant to very slightly discordant, the latter having undergone minor Pb loss (Fig. 5). The mean 206Pb/238U age and combined precisions for concordant overlapping fractions T1 and T2, 983.1+3.1/−3.4 Ma, provides the best estimate

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Fig. 4. Concordia diagram for the syn-kinematic Cadgecrib pegmatite.

Fig. 5. Concordia diagram for the Cadgecrib gabbro.

for the titanite age of this rock. This age is interpreted to correspond with the time that the Cadgecrib gabbro cooled through the titanite closure temperature of ca. 660°C (Scott and St-Onge, 1995). We note that the temperature–time coordi-

nates of this sample fit on the curve defining a cooling rate of 6°C Ma−1 between 725 and 480°C obtained from U–Pb and 40Ar/39Ar age determinations for the parautochthon (Martignole and Reynolds, 1997).

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4.5. The Renzy shear zone This NE-trending shear zone is located within an inlier of Mesoproterozoic rocks located in the SE sector of the parautochthon. It is subvertical, from 1 to 3 km in width, and is the locus of transcurrent sinistral displacement. This shear zone probably also affects the underlying Archean parautochthon. Moreover, it is located some 50 km to the SW of and along strike with the NE-trending sinistral shear zone that delineates the southern limit of the RCT (Martignole and Pouget, 1994). Sinistral wrenching thus appears to have been a dominant feature of deformation in the parautochthon, although it also locally affects the Proterozoic allochthons. Metasedimentary rocks affected by this shear zone are lithologically similar to those of the RCT. Both appear to record high-pressure metamorphic histories with mineral compositions suggesting relicts of relatively high pressure (Indares and Martignole, 1990b) and yield Mesoproterozoic Sm–Nd model ages (Dickin et al., 1989). However, monazite from metasedimentary rocks deformed in the RSZ has been dated at 1072±12 Ma (Childe et al., 1993) which is about 100 m.y. younger that

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the dates obtained for the monazites of the RCT ( Friedman and Martignole, 1995) and 70 m.y. older than monazites of the underlying parautochthon (Childe et al., 1993). We failed to find in this zone an unequivocal syn-kinematic pegmatite, but zircon and monazite were recovered from a postkinematic pegmatite that cuts mafic and ultramafic mylonites. The igneous age of this pegmatite therefore places a minimum timing constraint on deformation within this steeply dipping ductile fault zone. Zircons from this sample are clear, inclusionfree, and stubby to elongate (often cracked) prisms. Six analysed zircon fractions are concordant or slightly discordant, the latter due to a combination of minor inheritance and Pb loss ( Table 1; Fig. 6). The average 206Pb/238U age and combined precision for concordant overlapping fractions B and D provide the best estimate for the igneous age of this pegmatite at 1003.0+4.1/−4.8 Ma. Monazite is clear, yellow and commonly anhedral, although we note the occurrence of rare grains exhibiting tetrahedral parting. Three single grain monazite analyses are slightly discordant and define a quasi-linear array between about 970 Ma

Fig. 6. Concordia diagram for the Renzy post-kinematic pegmatite.

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Fig. 7. Concordia diagram for the Baskatong post-kinematic pegmatite.

and 980 Ma. The weighted 207Pb/206Pb age for this array of grains, 982.9±3.0 Ma, being younger than the igneous age, can be interpreted as a cooling age. As regional cooling through 480°C occurred around 963 Ma (Martignole and Reynolds, 1997), the age obtained for the monazite would correspond to a closure temperature intermediate between the igneous temperature of about 700°C and the regional cooling through 480°C. The above U–Pb data thus suggest that transcurrent displacement ceased prior to 1003 Ma along the sampled part of the Renzy shear zone and that cooling of the pegmatite through the closure temperature of monazite took place around 983 Ma, some 20 m.y. thereafter. 4.6. The BSZ The BSZ is a 10 km-wide zone of highly deformed grey gneisses located some 170 km southeast of the Grenville Front (Fig. 1). Its NW boundary corresponds to the southernmost limit of Archean Sm–Nd model ages (Dickin et al., 1989), whereas its SE limit is marked by the first outcrops of metasedimentary rocks of the MLT. This SE limit corresponds to the Central

Metasedimentary Belt boundary zone (CMBBZ; Davidson, 1984), also known as the Monocyclic Belt boundary zone (MBBZ; Rivers et al., 1989). Rocks of the BSZ consist of straight gneisses and local porphyroclastic mylonites with NNE-trending, subvertical foliations and subhorizontal stretching lineations. Within this zone, mesoscale evidence (C/s fabrics, rotated porphyroclasts) indicates dextral shear, whereas the deflection of foliations from NW to W and then to NE-trending (counterclockwise rotation) indicates sinistral displacement. This can be better understood by considering that the trailing edge of the LDT underwent counterclockwise bulk rotation in the sinistral BSZ (Martignole, 1995). Mesoscopic evidence for dextral shear, in turn, could be interpreted as strain markers of an earlier deformation during which the LDT (Fig. 1) was transported towards the NW: dextral shear is consistent with what is expected in a lateral ramp along the eastern side of such an allochthon. The sampled Baskatong pegmatite cuts steeply dipping foliations within the BSZ and is therefore interpreted to post-date ductile deformation at this locality. Zircons from this sample are clear, light pink in colour and free of visible inclusions. They

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occur as elongate prisms and angular fragments of larger grains. Six analysed zircon fractions are concordant, or slightly discordant due to Pb loss and minor inheritance in some fractions ( Table 1; Fig. 7). The best estimate for the age of this rock, 1020.1+1.8/−1.3 Ma, is based on the upper intercept of a chord through all six fractions (MSWD=2.08). Exclusion of data most likely to contain minor inheritance (fractions E and A2) reduces the upper intercept age by less than 1 Ma. The upper intercept strongly overlaps with the 206Pb/238U age of concordant fraction A1 at 1019.1±1.6 Ma. We note that the end of deformation in this zone coincides with the inferred time of peak metamorphism in the parautochthonous RDT (ca. 1020 Ma; see above). Deformation along the BSZ was thus over by 1020 Ma, whereas 20 m.y. later, syn-kinematic pegmatites where formed along the Cadgecrib shear zone. 4.7. The LSZ Some 100 km ESE of the BSZ is the LSZ, a major discontinuity that has been considered as the limit between the MLT and the MT (Indares and Martignole, 1990b). It is subvertical, several kilometres thick and trends towards the N or NNE, commonly with a shallowly south-plunging stretching lineation and apparently conflicting shear sense indicators (Martignole and Corriveau, 1991; Zhao et al., 1997). Deformation is concentrated within narrow, metre-scale bands of straight gneisses and mylonites. Along the eastern margin of the LSZ, hectometre-scale south-dipping mylonite zones with top to the north kinematic indicators suggest northward thrusting along the western margin of the MT. If these mylonites are related to the LSZ, the latter forms the western lateral ramp along which the MT was transported towards the north. Some of the deformation along the LSZ post-dates the 1076 Ma-old Loranger pluton (Corriveau et al., 1990), whose eastern margin is slightly deformed. Subvertical, WNW–ESE-trending, 15 cm-wide aplitic dykes intersecting NS-trending straight gneisses of the LSZ at high angle were sampled near Lac Macaza, some 15 km north of Labelle (Fig. 1). These dykes bear some petrographic and

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structural resemblance to post-kinematic dykes of aplitic two-mica granites dated at 1054+14/−29 Ma in the central part of the MLT ( Friedman and Martignole, 1995). Moreover, their orientation and ‘en e´chelon’ distribution is compatible with the filling of extension cracks developed during the last increments of sinistral shear within the straight gneisses. Dating of this aplite dyke places a minimum, local timing constraint on ductile deformation along this structural zone. Zircons recovered from this sample are clear (rare cloudy cores), colourless to pale pink and, in general, crack- and inclusion-free. Grain shapes vary from stubby prismatic to elongate prismatic and acicular. Grains with cloudy cores were avoided during sample selection. Ten analysed zircon fractions are disposed in a quasi-linear fashion indicative of Pb loss (Fig. 8). Six fractions composed of relatively fine elongate to acicular grains (including fractions L and N, which are tips of elongate grains) define a linear array which bounds the left-hand side of the data set ( Table 1; Fig. 8). Fractions which are comprised of stubby and coarse grains (H, I, G, E) lie to the right of the latter array and are interpreted to contain minor inherited zircon. The upper intercept of the six-point regression that bounds the left-hand side of the data set, 1078.8+6.0/−5.6 Ma (MSWD= 1.55), is considered to be the best estimate for the igneous age of the Lac Macaza aplite. This regression yields a lower intercept of 244±22 Ma. 4.8. The MSZ and TSZ The southernmost shear zones along this transect dip to the west or the southwest and occur in the eastern part of the MT. These include the MSZ, which affects the eastern lobe of the Morin Anorthosite (Martignole and Schrijver, 1970), and the TSZ along the boundary between the MT and the Re´servoir Taureau terrane (RTT ). Both sets of shear zones have gentle west to southwest dips with subhorizontal stretching lineations and a dextral sense of shear (Zhao et al., 1997). These shear zones are several hundred metres thick and have a fairly wide surface exposure due to their gentle dips. The easternmost, southwest-dipping splay of the TSZ is approximately coincident with

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Fig. 8. Concordia diagram for the Macaza late- to post-kinematic aplitic dyke.

the orthopyroxene (−) isograd (Schrijver, 1973). As higher grade rocks occur on the hanging wall, the shear zone is a thrust responsible for the metamorphic inversion and compatible with northward displacement of the MT (see also Zhao et al., 1997). The highest degree of strain is attained in the MSZ, where several hundred metres of mylonitic anorthosite and charnockite of the Morin complex occur. The Morin complex is a composite anorthosite–charnockite suite with U–Pb zircon ages ranging from 1135 Ma (charnockites, Doig, 1991; Emslie and Hunt, 1990; Friedman and Martignole, 1995) to 1155 Ma (undeformed anorthosite of the main massif (Doig, 1991)). As the sinistral LSZ and the dextral Morin–Re´servoir Taureau shear zones seem to merge north of the Morin Anorthosite, the intervening MT is interpreted to have been transported towards the north. Late- to post-kinematic dykes similar to the one dated at Lac Macaza occur NE of the Morin Anorthosite in the dextral transcurrent TSZ. A sample was collected from a >10 m-wide quartz monzonite dyke which cuts mylonitic foliation west of Re´servoir Taureau. It is locally brittly deformed and places a minimum age on the timing of the ductile deformation at this locality.

Zircons recovered from this sample are clear (some grains have cloudy cores), pale pink, commonly contain tiny black inclusions and are generally crack-free. Grain shapes vary from stubby to elongate prismatic and acicular. Rare tabular grains have also been observed. Grains with cloudy cores were avoided during sample selection. Ten analyses indicate that most or all fractions contain inherited zircon and have undergone post-crystallization Pb loss ( Table 1; Fig. 9). In order to minimize or eliminate inherited zircon components, very fine needles and tips from slightly coarser elongate grains were analysed (fractions D and I respectively). These analyses give similar 207Pb/206Pb ages of ca. 1074 Ma. The average 207Pb/206Pb age and the total range of associated errors for fractions D and I, at 1074.0±4.0 Ma, are considered to give a likely minimum age estimate for this rock. The MT thus appears to be limited by a horseshoe-shaped shear zone that had ceased its main transcurrent activity by 1074 Ma. The nature and amplitude of displacement along the LSZ–TSZ is not yet fully understood, but it is clear that by 1074 Ma the MT and MLT were more or less stitched together and probably behaved as a coher-

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Fig. 9. Concordia diagram for the late- to post-kinematic Taureau quartz monzonite.

ent unit during the last deformations of the Grenvillian cycle.

5. Timing of thrust and wrench tectonics in western Quebec According to the available geochronological data base, age estimates of peak metamorphism along the western Quebec Grenville transect range from 1180 to 1000 Ma. Younger ages are found in reworked rocks of the parautochthon and along the Grenville Front ( Fig. 10). Older ages characterize the inner parts of the orogen, as evidenced by the ages of the probably allochthonous MLT and MT. The 1180 and 1072 Ma monazite ages obtained from the Cabonga and Renzy allochthonous metasedimentary rocks imply pre-1180 Ma and pre-1072 Ma episodes of crustal thickening within the inner parts of the orogen where the allochthons were probably rooted. The growth of 1000–1020 Ma-old monazite within migmatites of the RDT constrains a re-heating episode of the Archean parautochthon, probably due to the emplacement of metamorphic allochthons (LDT and RCT ). Less than 200 km to the SW, in the

Ontario segment of the Grenville Province, NW-directed transport is related to closure of a marginal basin at about 1190 Ma ( Elzevirian orogeny) with a rejuvenation of the NW-directed imbrication at 1060 Ma (McEachern and van Breemen, 1993). Similarly, the emplacement of the RCT around 1020 Ma could be correlated with imbrication of incompletely cooled slabs of metasedimentary rocks originating from the SE, which accounts for the thermal perturbation in the Archean parautochthon of the RDT. In this scenario, the 1180 Ma-old metamorphic event in the RCT is tentatively correlated with the Elzevirian orogeny to the SW. Considering the northwestward younging of metamorphic ages from 1180 Ma to 1000 Ma and the fact that thrusting along the Grenville Front probably took place around 990 Ma, the sequence of NW thrusting in western Quebec is piggy-back in style as in other parts of the Grenville Province (e.g. Rivers et al., 1989). On the other hand, the NE–SW wrench tectonics dated in this study range in age from 1078 Ma (end of deformation in the LSZ–TSZ ) to 996 Ma (age of deformation in the Cadgecrib–Cabonga area). In this case, strike–slip deformation ended at least 70 Ma earlier in the core of the orogen

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Fig. 10. U–Pb ages versus distance diagram (open symbols: accessory minerals from rocks of the parautochthon; full symbols, accessory minerals from rocks of the allochthons; shaded areas correspond to high-grade metamorphism in the allochthons; double arrows indicate sense of displacement in dated shear zones).

than in the Front zone ( Fig. 10). This sequence of deformation could be referred to as normal (equivalent to piggy-back propagation in fold-and-thrust belts). Along the transect studied, petrographic data show that transcurrent displacements, whether in the parautochthon or in the allochthons, post-date the peak of metamorphism and took place under declining P–T conditions. As an example, the Cadgecrib mylonite recrystallized under declining P–T conditions of 620°C and 510 MPa (Martignole and Pouget, 1993) and crosscuts a gabbro that cooled through 660°C at about 983 Ma (titanite age, Fig. 5) some 20 Ma after cooling through the closure temperature of

monazite in the surrounding migmatites (Childe et al., 1993). Consequently, transcurrent displacement large enough in magnitude to produce mylonites, under declining P–T conditions, is thus considered as responsible for the final jostling, and hence the actual configuration of terranes in this part of the Grenville Province.

6. Conclusions Systematic U–Pb dating of syn- to post-kinematic pegmatites intersecting major transcurrent ductile shear zones along a western transect of the

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Grenville Province reveals a decreasing age trend from SE to NW towards the Grenville Front (Fig. 10). Ages obtained for these shear zones range between 1074 and 998 Ma, whereas monazite cooling ages combined with field observations suggest that final docking of crustal slices responsible for the reworking of parautochthonous terranes occurred before 1020 Ma. The geometry of allochthons lying on the parautochthon and their local involvement in NE wrenching, combined with P–T estimates in the transcurrent shear zones, show that in the central part of the Grenville Province, the Front-directed thrusting (orthogonal collision) predates sinistral wrenching (oblique collision). Precise age determinations on syn- to post-kinematic pegmatites of the main shear zones accord with the field and petrographic evidence and demonstrate that sinistral wrench tectonics postdates thrusting in this part of the Grenville Province. This succession of events allows us to rule out a partitioning of the movement resulting from oblique convergence into Front-directed thrusting and Front-oblique sinistral (and locally dextral ) wrenching. Rather, the observed succession of displacements can be viewed as belonging to a protracted collisional event evolving from orthogonal to oblique convergence.

Acknowledgment The manuscript has been improved significantly by constructive reviews by S. Hanmer, M. van Kranendonk and H. Wasteneys. This is Lithoprobe contribution 914.

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