Nd isotope mapping of Grenvillian crustal terranes in the vicinity of the Manicouagan Impact Structure

Precambrian Research 191 (2011) 184–193

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Nd isotope mapping of Grenvillian crustal terranes in the vicinity of the Manicouagan Impact Structure Stephanie D. Thomson a , Alan P. Dickin a,∗ , John G. Spray b a b

School of Geography & Earth Sciences, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4K1, Canada Planetary and Space Science Centre, University of New Brunswick, 2 Bailey Drive, Fredericton, New Brunswick E3B 5A3, Canada

a r t i c l e

i n f o

Article history: Received 30 March 2011 Received in revised form 21 August 2011 Accepted 26 August 2011 Available online 22 September 2011 Keywords: Manicouagan Grenville Nd isotope Crustal formation

a b s t r a c t Over 70 new Nd isotope analyses are presented for the Manicouagan area of the Grenville Province to estimate the crustal age of target rocks involved in the 214 Ma Manicouagan Impact Structure, and to reconstruct the Precambrian geological evolution of this crustal segment. The rocks fall into two main groups: Samples from the Archean-aged Gagnon Terrane to the north and west of the impact give TDM ages averaging 2.70 Ga. Samples from the Manicouagan Imbricate Zone (MIZ) and other allochthonous lithotectonic domains to the south of the impact yield Paleoproterozoic TDM ages averaging 2.01 Ga for the MIZ and 1.86 Ga for the southern domains. These Paleoproterozoic terranes are correlated with Makkovikage crust in Labrador that was heavily reworked by Labradorian magmatism that increased in intensity southwards. The target rocks involved in the impact event would have consisted almost entirely of the MIZ, which formed a layer several kilometres thick, overlying Archean crust at depth. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The 214 Ma Manicouagan Impact Structure is one of the largest and best preserved of the terrestrial impact structures (Spray et al., 2010). It is currently the subject of renewed geological investigation, and in order to understand the impact process and associated impact-induced crustal melting it is important to improve our knowledge of the age, geological structure and provenance of the target rocks. The target rocks lie within the Grenville Province, representing the exhumed remains of a 1 Ga orogenic event. However, the Grenville Province is largely comprised of an amalgamation of large crustal blocks or accreted terranes formed in earlier orogenic cycles, including the Kenoran (2700–2650 Ma), Makkovik (1900–1750 Ma), Labradorian (1680–1600 Ma), Pinwarian (1550–1450 Ma), and Elzevirian (1350–1200 Ma) events, in addition to the continent–continent collision of the Grenvillian Orogeny itself (1060–970 Ma) (Rivers, 1997; Dickin, 2000). Each of these orogenies was accompanied by magmatic and metamorphic reworking of older crustal units. As a result, much of the geological evidence used to unravel the evolutionary history of the region has been masked or even erased. However, through the use of Nd isotope analysis it is possible to estimate the original formation age of distinct crustal terranes in the Manicouagan area, thereby

∗ Corresponding author. Tel.: +1 905 525 9140; fax: +1 905 546 0463. E-mail address: [email protected] (A.P. Dickin). 0301-9268/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2011.08.006

uncovering important aspects of the geological evolution of this part of the Grenville Province, as well as characterizing the isotopic signature of the target rocks involved in the impact event. Several features make the Manicouagan region one of considerable geological complexity. Firstly, the Manicouagan impact site (M, Fig. 1) straddles an important Grenvillian tectonic boundary, the Allochthon Boundary Thrust (ABT), which represents the limit of major northwestward terrane displacement during the Ottawan phase of the Grenville orogeny. As such, the ABT separates largely in situ crustal units of the Parautochthon to the NW from the laterally transported Allochthonous Polycyclic Belt to the SE (Rivers et al., 1989). The Manicouagan crater also occurs near the meeting point of three or four major crustal blocks, each of which possesses distinct ages of formation (Fig. 1). To the north and west of the impact, the parautochthon consists of the Archean crust of Laurentia, which represents the lateral extension of Superior Province basement into the Grenville Province. To the east and south of the impact, within the allochthonous belt, two major crustal blocks with Nd model ages averaging 1.75 and 1.55 Ga have been identified by Dickin (2000). These were interpreted as juvenile arc terranes (Labradoria and Quebecia) accreted at around 1.65 and 1.45 Ga, respectively, but their extent in the Manicouagan area was not determined. In addition, a possible fourth crustal block in the Manicouagan area is a westerly continuation of Makkovik (ca. 2.0–1.8 Ga) crust. The Makkovik Province itself lies to the north of the Grenville Province in Eastern Labrador, but Dickin (2000) proposed its southerly extension into the Grenville Province, forming the

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Fig. 1. Map showing major blocks and accreted terranes of the Grenville Province with distinct crustal formation ages: Laurentia, 2.7 Ga; Barilia and Makkovikia, 1.9 Ga; Labradoria, 1.7 Ga; Quebecia 1.5 Ga, after Dickin (2000). Fine stipple = metasedimentary terranes of possible Makkovik provenance; coarse stipple = Trans Labrador Batholith; M = Manicouagan Impact Structure; ABT = Allochthon Boundary Thrust; BC = Baie Comeau; BS = Blanc Sablon; MO = Montauban; LJ = Lac Joseph Allochthon.

‘Makkovikia’ arc terrane, as recently confirmed by U–Pb dating in eastern Labrador (Gower et al., 2008). Dickin (2000) also suggested that Paleoproterozoic crust might extend to the west through the metasedimentary terranes of the Lac Joseph Allochthon (LJ, Fig. 1). The further westward extension of Makkovik age crust into the Manicouagan area is implied by recent U–Pb dating (Dunning and Indares, 2010). 2. Geologic terranes in the Manicouagan area Basic geological mapping in the vicinity of the Manicouagan reservoir was carried out by the Ministère de l‘Énergie et des Ressources du Québec (MERQ), summarized on the compilation map of Davidson (1998). The region contains large areas of Proterozoic and Archean orthogneiss, separated by the proposed trajectory of the ABT (Fig. 2). However, the melt sheet generated by the impact covers much of René Levasseur island (Fig. 2), thus obscuring the trace of the ABT through the impact. Paragneiss is widely distributed throughout the Manicouagan region, although its extent is probably less continuous than implied by the compilation map, especially to the SE of the reservoir. For example, sample localities shown in Fig. 2 are all believed to represent orthogneissic rocks (based on hand specimen mineralogy and texture), although several lie within areas mapped as paragneiss. To the NW, on Archean crust, paragneiss units contain a banded

iron formation (Knob Lake Group) deposited unconformably on the basement gneiss. Additionally, there are pockets of anorthosite throughout the Proterozoic terranes, as well as large volumes of mafic gneiss in the Hart Jaune Terrane. The Manicouagan area was the site of Lithoprobe seismic reflection transect Abitibi-Grenville 55 in 1993 (Clowes et al., 1996), and this information, combined with previous mapping, was used by Hynes et al. (2000) to divide the Manicouagan area into several Grenvillian lithotectonic terranes: the Gagnon Terrane, consisting of parautochthonous Archean basement; along with four allochthonous terranes, Hart Jaune Terrane, Berthé Terrane, and the Lelukuau and Tshenukutish Terranes, the latter two constituting the Manicouagan Imbricate Zone (MIZ). Several U–Pb dating studies have been carried out in the area, and these are summarized in the most recent study by Dunning and Indares (2010), wherein some revisions to the lithotectonic terranes of Hynes et al. (2000) were also proposed. The revised boundaries are shown in Fig. 2. 2.1. Gagnon Terrane As noted above, the Gagnon Terrane is the only terrane located on parautochthonous Archean basement. It is composed of strongly foliated, granoblastic quartzofeldspathic gneiss with mafic layers. This is overlain by Paleoproterozoic metasediments of the Knob Lake Group, including quartzofeldspathic schist and gneiss derived

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Fig. 2. Map showing locations of analysed samples relative to principal lithologies and proposed tectonic boundaries and lithotectonic domains in the Manicouagan area. RSZ = Relay Shear Zone; HJF = Harte Jaune Fault; GSZ = Gabriel Shear Zone; Tsh = Tshenukutish Terrane.

S.D. Thomson et al. / Precambrian Research 191 (2011) 184–193

from greywacke, as well as marble, quartzite and a banded iron formation (Rivers and Chown, 1986). In the northern part of the Gagnon Terrane, near the Grenville Front, the supracrustal rock assemblages and underlying Archean basement form imbricated thrust sheets. In the southern part of the terrane, the reworked Archean gneiss and overlying metasedimentary rocks can be indistinguishable from each other in the field. On the eastern side of the reservoir, the Gagnon Terrane is truncated to the south by the Relay Shear Zone (RSZ), which separates Archean basement from the structurally overlying Manicouagan Imbricate Zone, and represents the local manifestation of the ABT. On the western side of the reservoir, the location of the ABT is less clearly defined (see below). U–Pb dating within Gagnon Terrane has yielded Archean ages for zircon from two basement gneisses on the southwest shore of the reservoir (Jordan et al., 2006): 2685 ± 2 Ma for a tonalite and 2693 ± 4 Ma for a diorite. Jordan et al. also determined two late Makkovik ages on monazites from metapelites of the Knob Lake Group on the west and southwest shore. These ages of 1719 ± 30 and 1738 ± 5 Ma were interpreted as inherited metamorphic ages within Knob Lake Group supracrustals. In addition, Dunning and Indares (2010) determined an age of 1741 ± 32 Ma on zircon from a granite dyke on the south shore of the reservoir, near the southeastern edge of the Gagnon Terrane. This area was previously categorized by Hynes et al. (2000) as part of the allochthonous belt, within their ‘Southwest Domain’. Hence, Dunning and Indares have revised the location of the ABT in this area to the position shown in Fig. 2. 2.2. Manicouagan Imbricate Zone The Manicouagan Imbricate Zone (MIZ) is thought to underlie most of the impact crater, surfacing in a large lobe to the north of the impact. It is thrust over the Gagnon Terrane in the north and is overthrust by other Proterozoic allochthonous terranes to the south and east. Based on seismic reflection profiling, the MIZ has been modeled as a shallowly south-dipping crustal wedge, sandwiched between the Relay Shear Zone at the base, and the Hart Jaune Fault at the roof (Hynes et al., 2000). The Lelukuau Terrane is the lower structural unit of the MIZ, forming a large thrust sheet resting on Archean crust to the north of the impact site. It consists of an anorthosite– mangerite–charnockite–granite (AMCG) suite that was imbricated into tectonic slices separated by Grenville shear zones, forming a northwest directed thrust stack (Indares et al., 1998). Extensive U–Pb geochronology was completed within this terrane by Indares et al. (1998), yielding a cluster of high-precision Labradorian ages from 1628 to 1648 Ma for a variety of igneous rock types, and one low-precision age of 1692 ± 85 Ma for an anorthosite. The Tshenukutish Terrane structurally overlies the Lelukuau Terrane and was originally thought to underlie much of the impact structure. It was mapped by Hynes et al. (2000) as a sigmoidalshaped unit, emerging to the NE and SW of the impact. To the southwest, it was divided into two sub-units, the lower Southwest Domain and the upper Island Domain. However, based on new mapping, petrography and geochronology, Dunning and Indares (2010) removed these units from the Tshenukutish Terrane. The Southwest Domain was reassigned to the Gagnon Terrane, while the Island domain was tentatively correlated with the Lelukuau Terrane. Based on this model, the impact is underlain for the most part by the Lelukuau Terrane, while the principal remaining outcrops of the Tshenukutish Terrane lie to the northeast of the impact. This area to the NE is largely composed of 1450 Ma Pinwarian diorite and orthogneiss (Indares et al., 1998, 2000), thrust over the Lelukuau Terrane in a NW-directed thrust movement. The Hart

187

Jaune Terrane, which will be discussed later, was itself thrust over the units of the MIZ at approximately 1015 Ma (Cox et al., 2002). 2.3. Southern domains The Berthé Terrane defined by Hynes et al., 2000 was located to the southeast of the Manicouagan reservoir, structurally lower than the Hart Jaune Terrane, but overlying the MIZ. However, Dunning and Indares (2010) suggested that the units comprising this terrane are not closely related, and recommended abandoning the term. Instead, they subdivided this area into a group of separate lithotectonic domains, which are conveniently referred to as the ‘southern domains’ (Hynes, pers comm). The Gabriel Complex is mainly composed of migmatized felsic gneiss, and its high temperature metamorphism suggests that it acted as the hanging wall during the extrusion of the MIZ. However, the boundary between MIZ and Gabriel Complex was later overthrust by the Hart Jaune Terrane, so the present northern limit of the Gabriel domain is defined by the steeply dipping Gabriel Shear Zone, where the Gabriel Complex rode up against the synformal Hart Jaune Terrane after the latter’s emplacement (Indares and Dunning, 2004). The Banded Complex refers to alternating layers of felsic and mafic layers south of, and structurally higher than, the Gabriel Complex. The Banded Complex is a medium-low pressure metamorphic crustal unit that underwent late Grenvillian metamorphism at approximately 996–971 Ma (Indares and Dunning, 2004). The Canyon Domain was described by Dunning and Indares (2010) as principally a supracrustal package of probable volcaniclastic origin, comprising two strongly layered mafic to intermediate and quartzofeldspathic units. They also drew lithological comparisons between this unit and the southern tip of the Lelukuau Terrane north of the reservoir. A fourth unit in the south of the study area is the Island Domain. Dunning and Indares (2010) described its lithology as augen granitoid gneisses and a gabbro-anorthosite suite, and used lithological comparisons to tentatively correlate it with the Lelukuau Terrane to the north of the reservoir. However, they also noted that, unlike the Lelukuau Terrane, the Island Domain does not exhibit high pressure metamorphism. Therefore, in the present study it will be grouped with the southern domains rather than the MIZ. The Island Domain contains some of the oldest units within the allochthonous terranes, including an age of 1749 +7/−5 Ma determined by Indares and Dunning (2004) for a granite dyke within a unit they interpreted as a klippe that overlies the Gagnon Terrane, but which has affinities with the Island Domain. Additional evidence for pre-Labradorian magmatic activity in the Island Domain is provided by an imprecise upper intercept age of 1694 +52/−45 Ma for a major metagabbro unit. The oldest U–Pb age obtained from the other southern domains is a monazite age of 1478 Ma obtained by Jordan et al. (2006) from the Gabriel Complex. In addition, Dunning and Indares (2010) obtained a monazite age of 1467 ± 5 Ma from the Canyon Domain. The latter age was interpreted as evidence that an older crustal source contributed to the supracrustal sequence of the Canyon Domain, but its age of deposition was estimated at 1410 ± 16 Ma from zircon within an intermediate layer of the sequence, suggested to be a volcaniclastic rock. In addition, Indares and Dunning (2004) established a minimum age of 1403 +23/−25 Ma for the supracrustal sequence from a cross-cutting megacrystic granodiorite from the northern part of the Canyon Domain. 2.4. Hart Jaune Terrane The Hart Jaune Terrane is located on the eastern side of the reservoir. It is somewhat unusual in its evolutionary history as it is an

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erosional remnant of an allochthonous terrane composed of mafic and metasedimentary lithologies that was thrust over the Gabriel Complex. After thrusting was completed, later shortening during the Rigolet phase of the Grenville orogeny caused the Hart Jaune Terrane to develop into an isoclinal syncline whose southern edge was eventually over-ridden by the Gabriel Complex. Mafic granulites from the Hart Jaune Terrane have been dated at 1470 Ma (Hynes et al., 2000). 3. Sampling and analytical methods There is limited road access to the Manicouagan area. However, the Baie Comeau—Fermont highway allows reasonable access to the east of the reservoir, while logging roads allow fairly extensive sampling to the SW of the reservoir. The trajectory of the ABT under the impact structure is obscured by the impact melt itself. However, between 1990 and 2006 approximately 20 km of drill core were acquired as part of exploration initiatives on the island, three holes of which are in excess of 1.5 km deep (Spray and Thompson, 2008; Spray et al., 2010). 10 km of this core is now held by the Planetary and Space Science Centre at the University of New Brunswick. The core penetrates basement lithologies previously concealed beneath the impact melt, thus potentially allowing direct mapping of the isotopic and crustal formation age signatures of the target rocks of the impact, as well as contributing to the understanding of Grenvillian tectonics. Additionally, a small suite of samples collected by boat from the north shore of the reservoir was provided by Aphrodite Indares (Memorial University). As far as possible, samples consisted of granitoid orthogneisses exhibiting minimal migmatization. Three lithologies (anorthosite, mafic gneiss and paragneiss) were largely avoided when sampling for this study, since the first two often contain a mantle-derived component younger than the regional crust, whereas paragneisses may have an uncertain sedimentary provenance. Samples were processed at McMaster University, where they were crushed into a fine powder representative of the whole rock. Analytical techniques followed the established procedures previously described (e.g., Holmden and Dickin, 1995). After dissolution using HF and HNO3 , samples were split and one aliquot spiked with a mixed 150 Nd–149 Sm spike. Analysis by this technique yielded Sm/Nd = 0.2280 ± 2 for BCR-1. Standard cation and reverse phase column separation methods were used. Nd isotope analyses were performed on a VG Isomass 354 mass spectrometer at McMaster University using double filaments and a 4 collector peak switching algorithm, and were normalised to 146 Nd/144 Nd = 0.7219. Average within-run precision on the samples was ±0.000013 (2 sigma), and an average value of 0.51185 ± 2 (2 sigma population) was determined for the La Jolla standard during this work. The reproducibility of 147 Sm/144 Nd and 143 Nd/144 Nd is estimated at 0.1% and 0.002% (1 sigma), respectively, leading to an average uncertainty on each model age of 20 m.y. (2 sigma), based on empirical experience over several years of analysing duplicate dissolutions. All model ages are calculated using the depleted mantle model of DePaolo (1981). 4. Results and discussion New Nd isotope analyses are presented in Table 1, divided by lithotectonic terrane and domain, with samples from the Gagnon Terrane, MIZ and southern domains. Most MIZ samples are probably from the Lelukuau Terrane, but since this is conjectural, the two terranes of the MIZ will henceforth be grouped together. The data were used to calculate depleted mantle model ages, which are shown in the form of histograms in Fig. 3. Samples from the Gagnon Terrane mostly yield Archean Nd model ages with an average of 2.7 Ga, but with a tail down to 2.3 Ga which suggests some

8

Gagnon Terrane 4 0 12

MIZ 8 4 0 12

Southern domains

8 4 0 8

Eastern Labrador

4 0 24

Southern Labrador

16 8 0

1.6

1.8

2.0

2.2 2.4 TDM age, Ga

2.6

2.8

Fig. 3. Histograms of TDM model ages for major terranes of the Manicouagan area, in comparison with suites from eastern and southern Labrador (Schärer, 1991; Dickin, 2000).

magmatic reworking. The southern domains have a fairly restricted range of Paleoproterozoic model ages from 1.7 to 2.0 Ga, whereas the MIZ has a broader range of model ages from 1.7 to 2.2 Ga. The 1.7 Ga lower limit of TDM ages in the Manicouagan area is consistent with published Labradorian U–Pb ages in several rocks, as noted above, and suggests that this was a major period of magmatism in the Manicouagan area. This prompts comparison with Labrador, which was intensely affected by the Labradorian orogeny from 1.68 to 1.60 Ga (Gower and Krogh, 2002). In the Grenville Province of Labrador, Nd isotope data were used by Dickin (2000) to divide the crust into two major crustal blocks or terranes (Fig. 1), consisting of reworked Makkovik crust in eastern Labrador and juvenile Labradorian arc crust in southern Labrador. Comparison of the model age histograms for Labrador and Manicouagan (Fig. 3) shows strong similarities between the MIZ and eastern Labrador distributions. The Nd data for eastern Labrador are from Schärer (1991), who attributed them to Labradorian crustal formation from an abnormal ‘chondritic’ mantle source. However, U–Pb dating by Gower et al. (2008) has confirmed the alternative model proposed by Dickin (2000) that eastern Labrador consists of Makkovikian-age crust that was heavily plutonised during the Labradorian orogeny. The close correspondence between these data and the MIZ distribution (Fig. 3) suggests that the MIZ has a similar origin, representing

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Table 1 Nd isotope analyses of samples from the Manicouagan region. Map # Gagnon 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

Field #

MQ33 MQ34 MQ30 MQ16 MQ3 MQ 4 MQ6 MQ5 MQ18 MQ7 MQ9 MQ11 MQ12 0509-A 0513-11 AD105 AD102 FT17 FT13 FT14 FT15 FT16 Mean Gagnon

MIZ Lelukuau AD85 23 AD66 24 25 0506-A 0506-B 0506-C 0506-D 0506-E 0602-A 26 0602-B 0602-C 0602-D 27 0301-A 0301-B 0301-C 28 0502-9 29 0511-30 0511-A 0511-B 0511-C 30 0302-11 0302-A 0608-D 31 0303-12 32 0501-A 33 Tshenukutish 92AD2 34 92AD98* 35 Mean MIZ Southern domains Island MQ25 36 MQ20 37 MQ27 38 MQ29 39 MQ2 40 MQ1 41 MQ1b Canyon 42 MA228.8 43 MA232.8 MA236.6* 44 MA240.7 45 Banded 46 FT27 47 FT26* Gabriel 48 FT25* FT24* 49 50 FT23

Northing

Nd (ppm)

Sm (ppm)

147

480,321 479,316 488,389 496,834 511,057 510,970 510,327 510,710 500,646 507,770 504,816 495,082 491,449 505,634 496,359 487,000 491,000 561,500 561,000 559,700 559,700 561,300

5,652,265 5,654,045 5,641,594 5,646,102 5,646,151 5,646,752 5,649,818 5,647,591 5,653,412 5,656,604 5,658,291 5,661,754 5,667,444 5,673,483 5,683,606 5,694,000 5,705,000 5,738,500 5,761,600 5,757,500 5,756,800 5,744,400

3.1 10.8 55.0 10.4 40.0 15.8 13.7 14.5 40.2 214.2 5.9 11.8 63.9 36.6 17.7 11.0 10.0 24.9 26.9 41.8 26.5 24.6

0.52 1.95 9.60 2.36 7.08 2.86 2.09 1.86 7.00 26.59 0.85 1.93 12.05 6.75 3.56 1.60 1.40 4.76 4.12 7.54 4.42 4.27

0.0992 0.1091 0.1055 0.1371 0.1070 0.1096 0.0922 0.0774 0.1052 0.0749 0.0860 0.0989 0.1140 0.1115 0.1214 0.0880 0.0851 0.1156 0.0925 0.1089 0.1008 0.1049

495,760 532,823 509,940

5,719,820 5,730,993 5,686,130

512,647

5,692,881

519,588

5,694,978

520,880 522,506

5,694,490 5,694,037

8.15 8.55 7.81 5.83 5.04 4.52 3.12 2.62 5.07 6.12 4.96 0.99 5.25 4.81 4.21 0.37 2.10 0.24 0.28 7.11 6.65 4.84 3.03 0.84

Easting

Sm/144 Nd

143

Nd/144 Nd

TDM (Ga)

Epsilon Nd 1.65 Ga

Q

P

0.511075 0.511087 0.511295 0.511621 0.511391 0.511107 0.510757 0.510649 0.511082 0.510657 0.510835 0.510993 0.511275 0.511503 0.511538 0.510773 0.510684 0.511302 0.510893 0.511207 0.510969 0.511087

2.64 2.88 2.47 2.86 2.37 2.86 2.89 2.69 2.78 2.63 2.65 2.74 2.73 2.30 2.50 2.77 2.82 2.73 2.72 2.69 2.83 2.76 2.70

−10.0 −11.9 −7.0 −7.3 −5.5 −11.6 −14.8 −13.8 −11.1 −13.1 −11.9 −11.6 −9.2 −4.3 −5.6 −13.6 −14.7 −9.0 −12.2 −9.5 −12.4 −11.0

161 159 156 58 37 160 176 150 41 76 149 151 36 113

−190 −209 −13 −249 −217 −203 −182 −108 −99 −179 −189 −195 −172 −111

82 138 58 116 126

−201 −166 −184 −133 −113

0.1026 0.0905 0.1289 0.1056 0.1067 0.1044 0.0963 0.1277 0.1286 0.1038 0.1211 0.0845 0.1201 0.1145 0.1083 0.1187 0.1344 0.1112 0.1081 0.1429 0.1112 0.1457 0.1806 0.1326

0.511641 0.511396 0.511881 0.511603 0.511663 0.511599 0.511589 0.511814 0.511888 0.511588 0.511833 0.511454 0.511718 0.511731 0.511580 0.511789 0.512038 0.511688 0.511758 0.512048 0.511676 0.512103 0.512373 0.512087

1.92 2.04 2.09 2.03 1.96 2.01 1.89 2.20 2.07 2.03 2.01 1.87 2.16 2.02 2.12 2.01 1.95 2.03 1.87 2.15 2.03 2.11

0.3 −1.9 −0.5 −1.0 −0.1 −0.9 0.6 −1.6 −0.3 −1.0 0.2 0.5 −1.8 −0.4 −2.1 −0.2 1.4 −0.6 1.5 −0.2 −0.8 0.3 −1.8 2.7

141 51 104 103 16 73 −21 −2 134 10 176 6 27

−19 −190 −126 −134 −224 −144 −299 −284 −59 −260 −26 −263 −269

−2 7 −1

−224 −345 −353

3 27

−171 −277

19

−295

524,083

5,691,570

522,853 525,402 523,722

5,690,604 5,688,742 5,686,894

48.0 57.1 36.6 33.3 28.5 26.2 19.6 12.4 23.8 35.7 24.8 7.1 26.4 25.4 23.5 1.9 9.4 1.3 1.6 30.1 36.1 20.1 10.2 3.8

553,932 559,359

5,726,568 5,730,154

28.0 73.9

5.40 15.97

0.1164 0.1304

0.511699 0.512111

2.11 1.70 2.01

−1.4 3.7

480,885 491,151 492,459 490,673 513,320 515,211

5,620,599 5,636,244 5,637,894 5,640,213 5,643,250 5,641,038

35.4 58.5 4.5 31.2 61.2 16.2 13.8

7.63 12.56 1.06 4.78 10.41 2.21 1.88

0.1302 0.1297 0.1412 0.0927 0.1028 0.0825 0.0823

0.512018 0.512025 0.512184 0.511490 0.511623 0.511573 0.511559

1.87 1.85 1.80 1.96 1.95 1.71

1.9 2.1 2.8 −0.5 −0.1 3.3 3.0

163 95 −6 42 131 222

−61 −106 −342 −103 −81 −137

523,100 524,300 526,100 526,000

5,621,900 5,625,300 5,628,800 5,632,700

10.5 68.8 35.4 38.4

2.19 13.27 5.38 5.43

0.1256 0.1166 0.0920 0.0842

0.511912 0.511846 0.511622 0.511465

1.96 1.88 1.78 1.86

0.8 1.4 2.2 0.8

59 143 168 214

−255 −169 −127 −83

549,200 553,400

5,666,300 5,674,600

18.5 41.3

3.20 6.85

0.1043 0.1003

0.511722 0.511577

1.84 1.97

1.6 −0.4

180 103

−161 −90

561,600 561,500 560,300

5,683,600 5,685,300 5,687,600

12.3 59.6 29.4

2.06 8.84 5.87

0.1009 0.0897 0.1204

0.511685 0.511576 0.511941

1.83 1.80 1.80

1.6 1.8 2.5

186 109

−50 −151

1.79

190

S.D. Thomson et al. / Precambrian Research 191 (2011) 184–193

Table 1 (Continued) Map # 51

Field #

FT22 Mean Southern domains Harte Jaune FT20 52 FT19 53 FT18 54 Paragneiss Gagnon Terrane FT1 FT2 FT4 MIZ 92AD71 AD244 Canyon MA229.3 MA223 MA209 *

Easting

Northing

Nd (ppm)

Sm (ppm)

147

Sm/144 Nd

143

Nd/144 Nd

TDM (Ga)

Epsilon Nd 1.65 Ga

556,700

5,692,000

19.8

4.04

0.1233

0.511930

1.86 1.86

1.6

553,400 542,217 606,256

5,717,850 5,721,700 5,725,600

5.4 24.5 25.8

1.44 4.06 5.13

0.1604 0.1002 0.1201

0.512314 0.511682 0.511917

2.08 1.83 1.83

1.3 1.7 2.0

623,600 614,200 606,600

5,852,500 5,848,100 5,817,200

14.4 16.6 33.1

1.83 3.45 5.60

0.0771 0.1256 0.1022

0.510573 0.511465 0.511041

2.77 2.76 2.76

−15.2 −8.0 −11.3

548,051 522,335

5,721,104 5,725,530

25.3 56.8

5.15 10.43

0.1233 0.1111

0.511843 0.511668

2.02 2.04

−0.1 −0.9

523,400 522,300 518,800

5,622,200 5,615,800 5,608,900

63.6 23.0

11.41 4.45

0.1086 0.1167 0.1025

0.511744 0.511699 0.511734

1.88 2.11 1.79

1.1 −1.5 2.2

Q 64

P −189

Indicates average of two dissolutions.

pre-Labradorian crust that was heavily plutonised (magmatically reworked) during the Labradorian orogeny. The peak of Nd model ages for the southern domains is somewhat younger that the MIZ, and falls between the distributions from eastern and southern Labrador. This suggests that the southern domains are predominantly Labradorian, but probably also contain a vestige of pre-Labradorian crust. Although the histograms in Fig. 3 allow good comparison between the ranges of Nd model ages for each crustal block, they do not allow examination of the distribution of points within each data set. This involves both geographical and geochemical distributions within each suite. These aspects of the data are examined in Figs. 4–7. Fig. 4 is a detailed map of the area southwest of the reservoir showing the distribution of TDM ages. The distribution of model ages is geographically homogeneous within each identified crustal block, except that the edge of the Gagnon Terrane where it abuts the Island Domain south of the reservoir yields younger model ages in the range 2.3–2.6 Ga, probably due to younger plutonism emplaced into the southeasterly margin of the old craton. The boundary between the Gagnon Terrane and Island Domain is interpreted as the local expression of the Allochthon Boundary Thrust (ABT). The relative sharpness of the model age boundary can be attributed to partial removal of crust with mixed isotopic signatures due to crustal shortening during the Grenville orogeny, which caused the MIZ and southern allochthonous domains to be thrust to the NW over the reworked Archean crust. To examine geochemical variations within each data set, the data are shown first on a Sm–Nd isochron diagram (Fig. 5). On this plot a 2.7 Ga reference line defines a lower bound to the data, with Gagnon Terrane samples clustering fairly closely around this line, but with some evidence of magmatic reworking. At the upper bound, samples from southern Labrador cluster closely around a 1.75 Ga reference line. As argued by Dickin (2000), this suggests that this crustal terrane represents an island arc (‘Labradoria’) that grew over a period of several tens of millions of years shortly before the Labradorian orogeny. The 1.65 Ga Labradorian U–Pb ages of granitoid rocks in this terrane may represent ensialic arc magmatism established on the margin shortly after its accretion, rather than the actual age of formation of the primitive oceanic arc itself. The two intermediate dashed lines in Fig. 5 are best-fit regressions through the MIZ and southern domain data sets, and lie below but sub-parallel with the 1.75 Ga reference line. The slopes of these lines are believed to have age significance, suggesting that much of the material in these crustal blocks was melted during the

Labradorian orogeny. However, because they lie below the juvenile Labradorian arc terrane, they must also contain a significant fraction of older crust. The very close correspondence between the MIZ and eastern Labrador data sets suggests strongly that this earlier crust was Makkovik in age. The rocks of the MIZ could alternatively be attributed to mixing between Archean and Labradorian components, but this would require mixing of ca. 80% Labradorian and 20% Archean components, which would most likely lead to a much more scattered distribution of data points between the 2.70 and 1.75 Ga reference lines. Further examination of isotopic variation within the different terranes can be made using the plot of Nd concentration against Epsilon Nd (t) calculated at 1.65 Ga, the principal age of igneous crystallization obtained from U–Pb dating (Fig. 6). It is important to calculate epsilon values for all samples at the same time, even if they were actually formed over some period, so that the data are fully comparable. Because all crustal samples evolve along sub-parallel Nd isotope evolution lines, slight errors in the actual age of crystallization cause negligible errors in relative epsilon values. Data in Fig. 6 are plotted in the same categories as previous diagrams, except that samples from the southern domains have been subdivided, and a few paragneiss samples from scattered localities are also plotted. Samples from the MIZ show similar behaviour to previous diagrams, strongly overlapping the distribution of gneisses from eastern Labrador attributed to Makkovik crust that was strongly reworked during the Labradorian orogeny. The southern domains show an overlapping range of Nd signatures, apparently falling in a mixed field between the MIZ and juvenile southern Labrador crust. Hence, this suggests that rocks of the southern domains may represent a Labradorian arc established on the very edge of a Makkovikian continental margin, so that some Labradorian magmas contained a component of older crust while others were largely juvenile. Finally, the compositions of a small selection of paragneisses from the Manicouagan region are shown as open circles in Fig. 6. They were collected from the northern Gagnon Terrane near Fermont, from the MIZ north of the impact, and from the Canyon domain south of the impact. Their Nd signatures are consistent with the orthogneisses in each crustal unit, suggesting that they were derived from local sediment sources. In order to obtain a more complete understanding of the petrochemistry of the orthogneissic rocks in each lithotectonic unit, major elemental analysis was performed on the Nd dated samples. This geochemical analysis was completed at Activation

S.D. Thomson et al. / Precambrian Research 191 (2011) 184–193

191

Fig. 4. Detailed map of the area SW of the Manicouagan impact showing TDM ages in relation to proposed boundaries.

Laboratories, and the results were analysed using the Debon and LeFort (1983) Q–P chemical–mineralogical classification scheme. This allows the petrological character of granitoid magma suites to be compared. The results (Fig. 7) show that the majority of basement gneisses from Gagnon Terrane fall within the tonalite or granodiorite fields. In contrast, magmatically reworked samples from Gagnon Terrane, along with most of the Paleoproterozoic gneisses (especially the MIZ) are concentrated in a broad swath across the middle of the diagram from diorite to granite. These two different behaviours were attributed by Dickin (2000) to magmatism in thin crust (oceanic arcs) and thick crust (continental arcs), respectively. The behaviour of the relatively pristine Archean gneisses is similar to that observed in other Archean gneiss terranes within the Grenville Province, typified by the T–T–G (tonalite–trondhjemite–granodiorite) association. In contrast, the distribution of the Paleoproterozoic gneisses across the middle of the Q–P diagram (including southern Labrador) supports the interpretation that the Paleoproterozoic gneiss terranes were all

strongly reworked by a Labradorian ensialic arc, in which magmas were emplaced into crust that had been thickened by arc accretion during the Labradorian orogeny. Overall, the Nd isotope data show consistency, but also some differences with the U–Pb data for the Manicouagan area. For example, the average TDM model age for basement samples from the Gagnon Terrane (2.70 Ga) is close to the U–Pb ages of diorite and tonalite gneisses from this terrane (Jordan et al., 2006). However, TDM ages for the MIZ and the southern domains (averaging 2.01 and 1.86 Ga, respectively) are older than the oldest U–Pb ages measured by Dunning and Indares (2010), who also dated several younger Mesoproterozoic events. This type of behaviour has previously been found in several other areas of the Grenville Province, and should not be a surprise, because the objective of Nd isotope mapping is to estimate the formation age of the crust as a whole, whereas U–Pb dating is normally intended to date a specific igneous crystallization event or metamorphic overprint. Therefore, the observation by Dunning and Indares (2010) that the Canyon and Island Domains have

192

S.D. Thomson et al. / Precambrian Research 191 (2011) 184–193

0.5125

250

TN

200

GD

MG GR

1.75 Ga

0.5120

143Nd/144Nd

150

Q 100 0.5115

QD 50

Gagnon Gagnon reworked

DI

MIZ

0.5110

QSY

MD

S Labrador

-50

E Labrador

-350

-300

-250

-200

0.5105 0.09

0.11

0.13

Gagnon

Fig. 5. Sm–Nd isochron diagram for analysed orthogneissic samples from the Manicouagan area in relation to suites from eastern and southern Labrador.

lithological similarities with parts of the MIZ is consistent with the similar (pre-Labradorian) Nd signatures in both units. It should also be noted that the oldest U–Pb ages from the allochthonous units south of the reservoir (1694 and 1749 Ma) were both obtained from intrusive rocks, which therefore provide only a minimum age for the crust as a whole. Similar Late Makkovik U–Pb ages of 1718, 1735 and 1775 Ma were measured in the Grenville Province of Eastern Labrador by Schärer and Gower (1988) and Philippe et al. (1993), but the crust of Eastern Labrador eventually yielded a group of older Makkovik intrusive ages in the range 1770–1800 (Gower et al., 2008). Considering the younger Mesoproterozoic ages from the Manicouagan area, Dunning and Indares (2010) noted that the

5

0 Gagnon G reworked

-5

-150

-100

-50

0

50

P

0.15

147Sm/144Nd

Epsilon Nd (1.65 Ga)

SY

Southern domains

2.7 Ga

0.07

MZ

0

G reworked

MIZ

Southern

S Labrador

E Labrador

Fig. 7. Petrochemical grid of Debon and LeFort (1983), designed to create a chemical Streckeisen classification of granitoids. Symbols as in Fig. 5. TN = tonalite; GD = granodiorite; MG = monzogranite; GR = granite; QD = quartz diorite; QSY = quartz syenite; DI = diorite; MD = monzodiorite; MZ = monzonite; SY = Syenite.

supracrustal rocks of the Canyon Domain show lithological and age correlations with 1440 Ma volcanic rocks of the Montauban Group west of Quebec (Fig. 1), while the granodiorite of the Canyon Domain can be correlated with 1400–1370 Ma plutonic rocks of La Bostonnais Complex, which intrude the Montauban supracrustal rocks. The Montauban/La Bostonnais rocks have been attributed to the formation and subsequent accretion of an oceanic arc to Laurentia (Corrigan and van Breemen, 1997; Dickin, 2000; Sappin et al., 2009). Hence, Dunning and Indares suggested that the Manicouagan rocks might represent the continuation of the same accretionary margin to the ENE. The new Nd isotope data presented here do not conflict with this overall model, but they show that the Paleoproterozoic rocks of the Manicouagan area represent the Laurentian side of the accretionary suture, since the Montauban rocks have juvenile 1.5 Ga crustal formation ages, whereas the Manicouagan rocks have ca. 2 Ga model ages, suggesting crustal formation during the Makkovik period. Hence a ca. 1450 Ma suture between Paleoproterozoic Laurentia and the accreted Mesoproterozoic arc probably lies near the Manic 5 dam site, since juvenile 1.55 Ga model ages were found 20 km further to the south, within the Quebecia arc terrane (Dickin, 2000).

MIZ Island

5. Conclusions

Canyon

New Nd isotope data for the Manicouagan area, including surface samples and drill-core samples from under the impact melt sheet, allow the crustal formation ages of major crustal blocks to be mapped.

Banded Gabriel

-10

Paragneiss S Labrador E Labrador

-15 0

20

40

60

80

100

Nd ppm Fig. 6. Plot of Nd concentration versus epsilon Nd for analysed samples. Symbols as in Fig. 5, except that southern domains are distinguished.

1. The data confirm the location of the Allochthon Boundary Thrust (ABT), representing the limit of exposed Archean basement, along the western side of the Manicouagan reservoir. 2. The westerly location of the ABT implies that the target rocks of the impact consist almost entirely of the Manicouagan Imbricate Zone, which is part of the Allochthonous Polycyclic Belt of the Grenville orogeny.

S.D. Thomson et al. / Precambrian Research 191 (2011) 184–193

3. These (MIZ) rocks have Nd isotope signatures strongly resembling eastern Labrador, with an average TDM model age of 2 Ga. This crust was reworked by younger magmatic events, notably during the Labradorian and Pinwarian orogenies, but the Nd isotope signatures of younger plutonic rocks largely preserve their Paleoproterozoic provenance. 4. The domains south of the reservoir yield slightly younger Nd model ages, probably representing a greater degree of Labradorian reworking of the Makkovik continental margin southwards. Grenvillian tectonism subsequently shortened the crustal section, so that large sections of crust were cut out between Archean and Paleoproterozoic blocks that were once widely separated. 5. The Manicouagan area was probably located immediately to the north of a ca. 1450 Ma collisional suture between the Paleoproterozoic Laurentian margin and the accreted Mesoproterozoic arc of Quebecia. 6. The target rocks of the impact would have consisted almost entirely of the MIZ, which formed a layer several kilometres thick, overlying Archean crust at depth. This sandwich structure of the crust may allow constraints to be placed on the ‘penetration depth’ of the impact melting event. Acknowledgements ST acknowledges scholarship support from McMaster University, and support for fieldwork from an NSERC Discovery grant to APD. JGS was funded by NSERC, the Canada Research Chairs Program and the Canadian Space Agency in support of the Manicouagan Impact Research Program (MIRP). Aphrodite Indares (Memorial University) kindly supplied samples from the north shore of the reservoir. The authors appreciate constructive reviews by Andrew Hynes and an anonymous reviewer that led to many improvements in this paper PASSC publication 50. References Clowes, R.M., Calvert, A.J., Eaton, D.W., Hajnal, Z., Hall, J., Ross, G.M., 1996. LITHOPROBE reflection studies of Archean and Proterozoic crust in Canada. Tectonophysics 264, 65–88. Corrigan, D., van Breemen, O., 1997. U—Pb age constraints for the lithotectonic evolution of the Grenville Province along the Mauricie transect, Quebec. Can. J. Earth Sci. 34, 299–316. Cox, R.A., Indares, A., Dunning, G.R., 2002. Temperature-time paths in the highP Manicouagan Imbricate zone, eastern Grenville Province: evidence for two metamorphic events. Precamb. Res. 117, 225–250. Davidson, A., 1998. Geological map of the Grenville Province, Canada and adjacent parts of the United States of America. Geol. Surv. Can. Map 1947A, scale 1:2,000,000. Debon, F., LeFort, P., 1983. A chemical-mineralogical classification of common plutonic rocks and associations. Trans. R. Soc. Edinb. Earth Sci. 73, 135–149.

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DePaolo, D.J., 1981. Neodymium isotopes in the Colorado Front Range and crustmantle evolution in the Proterozoic. Nature 291, 193–196. Dickin, A.P., 2000. Crustal formation in the Grenville Province: Nd-isotope evidence. Can. J. Earth Sci. 37, 165–181. Dunning, G., Indares, A., 2010. New insights on the 1.7–1.0 Ga crustal evolution of the central Grenville Province from the Manicouagan-Baie Comeau transect. Precamb. Res. 180, 204–226. Gower, C.F., Kamo, S.L., Kwok, K., Krogh, T.E., 2008. Proterozoic southward accretion and Grenvillian orogenesis in the interior Grenville Province in eastern Labrador: evidence from U–Pb geochronological investigations. Precamb. Res. 165, 61–95. Gower, C.F., Krogh, T.E., 2002. A U–Pb geochronological review of the Proterozoic history of the eastern Grenville Province. Can. J. Earth Sci. 39, 795–829. Holmden, C., Dickin, A.P., 1995. Paleoproterozoic crustal history of the southwestern Grenville Province: evidence from Nd isotopic mapping. Can. J. Earth Sci. 32, 472–485. Hynes, A., Indares, A., Rivers, T., Gobeil, A., 2000. Lithoprobe line 55: integration of out-of phase seismic results with surface structure, metamorphism and geochronology, and the tectonic evolution of the eastern Grenville Province. Can. J. Earth Sci. 37, 341–358. Indares, A., Dunning, G., 2004. Crustal architecture above the high-pressure belt of the Grenville Province in the Manicouagan area: new structural, petrologic and U–Pb age constraints. Precamb. Res. 130, 199–228. Indares, A., Dunning, G., Cox, R., 2000. Tectono-thermal evolution of deep crust in a Mesoproterozoic continental collision setting: the Manicouagan example. Can. J. Earth Sci. 37, 325–340. Indares, A., Dunning, G., Cox, R., Gale, D., Connelly, J., 1998. High P–T rocks from the base of thick continental crust in a Mesoproterozoic continental collision setting: the Manicouagan Imbricate Zone, eastern Grenville Province. Tectonics 17, 426–440. Jordan, S.L., Indares, A., Dunning, G., 2006. Partial melting of metapelites in the Gagnon terrane below the high-pressure belt in the Manicouagan area (Grenville Province): pressure–temperature and U–Pb age constraints and implications. Can. J. Earth Sci. 43, 1309–1329. Philippe, S., Wardle, R.J., Schärer, U., 1993. Labradorian and Grenvillian crustal evolution of the Goose Bay region, Labrador—New U–Pb geochronological constraints. Can. J. Earth Sci. 30, 2315–2327. Rivers, T., 1997. Lithotectonic elements of the Grenville Province: review and tectonic implications. Precamb. Res. 86, 117–154. Rivers, T., Chown, E.H., 1986. The Grenville Orogen in Eastern Quebec and Western Labrador—definition, identification and tectonometamorphic relationships of Autochthonous, Parautochthonous and Allochthonous Terranes. In: Moore, J.M. (Ed.), The Grenville Province. Geol. Assoc. Can. Spec. Pap. 31, 31–50. Rivers, T., Martignole, J., Gower, C.F., Davidson, A., 1989. New tectonic divisions of the Grenville Province, southeastern Canadian Shield. Tectonics 8, 63–84. Sappin, A.A., Constantin, M., Clark, T., van Breemen, O., 2009. Geochemistry, geochronology and geodynamic setting of Ni–Cu ± PGE mineral prospects hosted by mafic and ultramafic intrusions in the Portneuf-Mauricie domain, Grenville Province, Quebec. Can. J. Earth Sci. 46, 331–353. Schärer, U., 1991. Rapid continental crust formation at 1.7 Ga from a reservoir with chondritic isotope signatures, eastern Labrador. Earth Plant. Sci. Lett. 102, 110–133. Schärer, U., Gower, C.F., 1988. Crustal evolution in eastern Labrador: constraints from precise U–Pb ages. Precamb. Res. 38, 405–421. Spray, J.G., Thompson, L.M., 2008. Constraints on central uplift structure from the Manicouagan impact crater. Meteor. Planet. Sci. 43, 2049–2057. Spray, J.G., Thompson, L.M., Biren, M.B., O’Connell-Cooper, C., 2010. The Manicouagan impact structure as a terrestrial analogue site for lunar and martian planetary science. Planet. Space Sci. 58, 538–551.