Nd isotope mapping of the Dysart gneiss complex: Evidence for a rifted block within the Central Metasedimentary Belt of the Grenville Province

Precambrian Research 228 (2013) 223–232

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Nd isotope mapping of the Dysart gneiss complex: Evidence for a rifted block within the Central Metasedimentary Belt of the Grenville Province Katherine Moretton, Alan P. Dickin ∗ School of Geography & Earth Sciences, McMaster University, Hamilton, Ontario, Canada

a r t i c l e

i n f o

Article history: Received 11 January 2012 Received in revised form 2 November 2012 Accepted 3 January 2013 Available online 26 January 2013 Keywords: Grenville Province, Dysart complex, Nd isotope mapping

a b s t r a c t More than 65 new Nd isotope analyses are presented for the vicinity of the Central Metasedimentary Belt Boundary Zone (CMBBZ) near Haliburton in the Grenville Province of Ontario, Canada. They fall into two well-defined groups, with distinct ranges of TDM model ages. These two groups comprise the CMBBZ with an average TDM age = 1.46 Ga, and rocks of the Central Metasedimentary Belt (CMB) with an average TDM age of 1.28 Ga. However, a distinct block of crust within the CMB comprising the Dysart gneiss complex has an older model age signature (TDM age = 1.40 Ga) resembling the CMBBZ rather than the rift zone. The Dysart complex is surrounded by marble tectonites interspersed with elongate bodies of orthogneiss that yield young Nd model ages typical of the CMB. However, it is argued that the development of the Dysart complex as a distinct crustal block predates ductile deformation during the Grenville orogeny. Instead, the Dysart Block is thought to have rifted away from the Central Gneiss Belt to the northwest during the formation of the CMB as an ensimatic back-arc rift zone. Isotopic mapping of the Dysart block, along with the previously discovered Elzevir block, provides evidence for the geometry of the rifting process, and also has important implications for the structure of Grenville basement in eastern North America. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The Grenville Province (Fig. 1a) is a Mid-Proterozoic orogenic belt that resulted from a 1.2 to 1.0 Ga collisional orogeny and forms the south-easternmost structural province of the Canadian Shield (Rivers, 1997). However, it is largely composed of older lithotectonic terranes that were accreted to the Archaean cratonic nucleus of Laurentia in multiple orogenic events through the Paleo- and Mesoproterozoic. These multiple high-grade metamorphic events have obscured much of the evidence normally used to reconstruct ancient orogenic belts, but Nd isotope analysis has been shown to be an effective tool in mapping the crustal formation ages of these terranes and the boundaries between them (Dickin, 2000). In Ontario, the Grenville Province is divided into two major belts, the Central Gneiss Belt (CGB) and the Central Metasedimentary Belt (CMB), which have distinct ages of crustal formation (Dickin et al., 2010). The former consists of high-grade gneisses with TDM Nd model ages ranging from ca 1.4 to 2.0 Ga, attributed to crustal formation in a series of accretionary orogenic events from ca. 1.8 to 1.5 Ga, followed by the establishment of a continental margin arc with north-dipping subduction from ca. 1.4 to 1.2 Ga (Rivers,

∗ 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 © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.precamres.2013.01.003

1997; Rivers and Corrigan, 2000), previously termed the Elzevirian orogeny (Moore and Thompson, 1980). The CMB is metamorphosed to much lower grades than the gneiss belt, which should have allowed the preservation of a variety of different kinds of geological evidence to interpret its geological history, but in fact it remains one of the most poorly understood regions of the Province. One model for the origin of the northwesterly part of the CMB interprets it as a collage of small arc fragments, established over subduction zones and amalgamated by collisional sutures (e.g. Easton, 1992). This has led to a well-established terminology (Carr et al., 2000) for this part of the CMB as the Composite Arc Belt (CAB) as shown in Fig. 1b. However, long before this model was popular, Baer (1976) had proposed that the Grenville Supergroup was deposited in an aulacogen, representing a failed continental rift zone similar to the Danokil depression in the southern Red Sea. A continental rift setting of this type was also supported by geochemical fingerprinting of relatively immobile elements in mafic metavolcanics and minor intrusions from the CMB (Holm et al., 1985; Smith and Holm, 1990). However, due to the presence of arc-related volcanics elsewhere in the CMB (Condie and Moore, 1977), Holm et al. (1985) suggested a back-arc basin setting. This interpretation was also followed by Hanmer et al. (2000) and Rivers and Corrigan (2000), who proposed that the CMB was a back arc basin developed behind the Elzevirian continental margin arc. Nd isotope analysis by Dickin and McNutt (2007) showed a cluster of young Nd model ages from 1.15 to 1.35 Ga in units assigned

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2. Regional geology

Fig. 1. Maps of the Grenville Province to show (a) its location in the eastern Canadian Shield, and (b) established subdivisions of the SW Grenville province (Carr et al., 2000; Corriveau and van Breemen, 2000). H, town of Haliburton. CMB, Central Metasedimentary Belt; CAB, Composite Arc Belt, which continues into Quebec as the marble domain; F, Frontenac Terrane, L, Adirondack Lowlands, which continue into Quebec as the quartzite (Q) and Morin (M) domains respectively.

to the Composite Arc Belt of Ontario, but also revealed older Nd model ages in the Quebec segment, as well as an isolated block of older model ages in the vicinity of the Elzevir pluton, in the middle of the zone of younger ages. Based on this geometry, Dickin and McNutt (2007) attributed the CMB in Ontario to a back arc rift zone consisting of a series of en echelon ensimatic grabens, separated by horsts of older crust (such as the proposed Elzevir Block). In addition, they compared the ensialic character of the Quebec segment of the rift zone with the ensialic Gulf of Suez, representing the northward attenuation of the Red Sea ensimatic rift zone. Dickin and McNutt (2007) argued that the overall shape of the rift zone had been largely preserved during the Grenville orogeny, but that the original walls of the rift zone had been reactivated by Grenvillian tectonism, forming the Central Metasedimentary Belt Boundary Zone (CMBBZ) to the northwest, where the CMB is in thrust contact with the gneiss belt (Figs. 1b and 2). One of the key observations in support of this model was that the CMBBZ makes a sharp, nearly ninety degree turn west of Haliburton, Ontario, attributed by Dickin and McNutt (2007) to later tectonic reactivation of the end of an ensimatic rift segment. Hence, one of the principal motivations for the present study was a more detailed isotopic investigation of the CMBBZ in this vicinity.

The significance of the tectonic boundary between the gneiss belt and the CMB was recognised in the early twentieth century (Adams and Barlow, 1910), but more recent reviews of the structure of the SW Grenville Province (Davidson, 1984, 1986) emphasised the existence of a boundary zone. Hence Davidson defined the CMBBZ as a ‘zone of intensely sheared and flattened gneissic rocks, several kilometres in outcrop width’ but also observed that ‘many of the rocks in this zone are indistinguishable from the shear zone rocks that mark the boundaries of domains in the interior of the CGB.’ The latter observation is consistent with the fact that Davidson’s CMBBZ is juxtaposed against the northwestern limit of major marble outcrops within the CMB. In other words, Davidson’s CMBBZ (Fig. 2) has lithological affinities with the gneiss belt rather than the CMB, and does not contain significant areas of marble. Hanmer (1988) made a more detailed study of the area around Haliburton (Fig. 2), in which he identified a lithotectonic unit within Davidson’s CMBBZ consisting of homogenous tonalitic orthogneiss (the Redstone complex) largely surrounded by gneissic tectonites of various types. Hence, he termed this the Redstone thrust sheet. However, Hanmer (1988) observed that two other bodies of plutonic orthogneiss to the south (Dysart and Glamorgan complexes) had similar lithologies, and were surrounded by marble tectonites. Hence he speculated that these three bodies, along with the Grace pluton east of Haliburton, might originally have comprised one lithological unit that was later disrupted by thrusting. On this basis, Hanmer extended the concept of the CMBBZ to encompass a ‘CMB boundary thrust zone’ (CMBbtz) enclosing all of these plutonic bodies as distinct thrust sheets within a deformation zone up to 40 km wide (Fig. 2). This model was developed by Hanmer and McEachern (1992), who proposed that the CMB boundary thrust zone extended 200 km in length, essentially equivalent to the combined CMBBZ and Bancroft Terrane of other workers (Davidson, 1986; Easton, 1992), with a an average dip of 20◦ to the SE and a vertical thickness of ca. 10 km. At the same time that this work was being done, Easton (1992) reviewed the geology of the CMBBZ and presented an accompanying 1:1,000,000 scale compilation map based on several more detailed map sheets (see references in Easton, 1992). However, Easton’s interpretation was closer to the view of Davidson (1986), in which the CMBBZ and Bancoft Terrane were seen as separate lithotectonic units. Shortly afterwards, the regional geology in this area was reviewed again by Davidson (1998a), accompanied by another compilation map (Davidson, 1998b). This map shows the study area to be traversed by a whole series of thrusts, both within the CMB and within the gneiss belt, but without indicating which of these are of regional significance and which are merely of local significance. Approximate U-Pb crystallisation ages for the intrusive rocks that form the major gneiss complexes of the study area were reported by Lumbers et al. (1990). An age of approximately 1350 Ma was estimated for the Dysart complex, which was assigned to an ‘early trondhjemite’ suite, whereas other bodies with similar tonalite-trondhjemite petrology in the CMB were estimated to be more than 50 Myr younger, and assigned to a ‘late trondhjemite’ suite. Lumbers et al. (1990) included the Glamorgan complex as part of this late suite, implying that this body is not co-genetic with the Dysart complex. This cast some doubt on the model of Hanmer and McEachern (1992), but the age difference between the two bodies was only recently confirmed, when McNutt and Dickin (2012) referenced unpublished U-Pb ages of 1337 and 1227 Ma respectively for the Dysart and Glamorgan complexes (Heaman, personal communication). Despite the lack of error estimates, the greater than 100 Myr age difference between these two units clearly refutes the more

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Fig. 2. Map of the Haliburton–Bancroft area showing alternative terminology for the CMB boundary after Davidson (1986) and Hanmer (1988). Domains of the CMB after Easton (1992). Most of the crust within the Bancroft Terrane west of the Grace pluton consists of marble tectonite with variable contents of small elongate plutonic bodies.

extreme aspects of the tectonic model of Hanmer and McEachern (1992). However, this still leaves considerable uncertainty about the nature and extent of the CMBBZ, which may be resolvable using Nd isotope data. For example, previous Nd isotope work in this region (Dickin and McNutt, 2007; Dickin et al., 2010) revealed a significant difference in model ages between the CMB and the Muskoka domain of the gneiss belt (Fig. 2). Therefore, detailed isotopic mapping should be able to reveal useful information about the character of the CMBBZ as a tectonic boundary. For example, there are at least three alternative models for its structure: (1) there may be a sharp age boundary between rocks of the gneiss belt and the CMB, (2) the boundary may be gradational, possibly reflecting tectonic or magmatic mixing across it, or (3) there could be a complex pattern with age reversals due to imbrication of an earlier boundary. Alternatively, some combination of these three models may pertain. 3. Sampling and analytical methods Since the objective of the study is to characterise the protolith age of the crust as an estimate of its crustal formation age, the strategy adopted was to limit sampling to granitoid orthogneisses that are believed to form by anatexis of pre-existing more mafic crust. Previous studies have shown that granitoids of this type, formed in arc systems, have Nd isotope signatures that are consistent and predictable, allowing reliable estimates to be determined of the formation age of the crust using the depleted mantle model of DePaolo (1981). Mafic gneisses were excluded as far as possible, because of the increased likelihood of a younger mantle-derived component in these rock-types. Metasedimentary gneisses were also excluded because of their uncertain sedimentary provenance. The 1:50 000 scale geological maps for the Minden, Haliburton and Wilberforce areas (P.3415, Easton, 2001a; P.3416, Easton, 2001b; P.3526, Lumbers and Vertolli, 2003) show elongated bodies of granitoid orthogneiss of many different sizes within and

adjacent to the marble tectonite unit that surrounds the Dysart and Glamorgan complexes. The largest of the minor elongate bodies is ca. 1 km wide and 10 km long, and although the smallest mapped units are ca. 0.2 km wide and 1 km long, undoubtedly there are smaller bodies of the same suite that were below the threshold for mapping as distinct units. Some of our samples came from such smaller unmapped bodies but none were from floating rafts or blocks below outcrop-scale in size. The extreme complexity of these zones, with interleaved igneous and sedimentary gneisses, cannot be adequately shown on a compilation map such as Fig. 2, whereas the three major gneiss complexes studied (Redstone, Dysart and Glamorgan) represent discrete orthogneissic bodies with clearly defined margins, as shown in Fig. 2. In contrast, the Grace complex of Hanmer and McEachern (1992) is more like a group of minor intrusions than a major plutonic complex, but is shown as a discrete unit in Fig. 2 to indicate its location. The samples chosen for analysis were homogeneous, mediumto coarse-grained gneisses showing textural evidence of a plutonic protolith, with a minimum of migmatization. Samples were selected from what were believed to be the oldest plutonic units in any given outcrop, ideally of tonalitic or granodioritic composition. More granitic/syenitic gneisses were sampled if intermediate rocks were unavailable in a given region, provided they were pretectonic (not late Grenvillian) and representative of a fairly large outcrop area (at least tens of metres across). Syenitic gneisses have in the past proved less reliable for Nd isotope mapping, and these data will be discussed in detail below. On average, 1 kg of rock was crushed, after the removal of any weathered, veined or migmatized material, and careful attention was given to obtain a fine powder that was representative of the whole rock. Major and selected trace element analyses were performed by Activation Laboratories, Ancaster, Ontario, using Liborate fusion ICP analysis. The accuracy of their data was ensured by the inclusion of USGS standards as part of the analytical protocol. These data are included in Appendix 1.

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Sm–Nd analysis followed our established procedures. After a four-day dissolution at 125 ◦ C using HF and HNO3 , samples were converted to the chloride form before splitting and spiking. 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 a 146 Nd/144 Nd ratio of 0.7219. Average within-run precision on the samples was ± 0.000012 (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 analytical uncertainty on each model age of ca. 20 Myr (2 sigma), based on empirical experience over several years of analysing duplicate dissolutions. 4. Results New Nd isotope data for over 65 samples are presented in Table 1, with numbered localities plotted in Fig. 3. Unnumbered localities in Fig. 3 are published data from Dickin and McNutt (2007) and Dickin et al. (2010), which are shown in Table 1 for reference. Symbols in Fig. 3 divide TDM model ages into three groups already defined by Dickin et al. (2010). Model ages above 1.5 Ga (squares) are attributed to Pinwarian-age (ca. 1.5 Ga) crustal formation, possibly representing lateral equivalents of the Quebecia terrane identified by Dickin (2000). Model ages of 1.35–1.5 Ga (triangles) are thought to have been generated by the Elzevirian continental margin arc. This forms most of the Frontenac-Adirondack belt and also the basement to the Quartzite and Morin domains in Quebec (Fig. 1b). Finally, model ages below 1.35 Ga (open circles) are attributed to melting of young rift-related crust formed immediately prior to the 1190–980 Ma Grenville Orogenic Cycle (Dickin et al., 2010). When these age ranges are compared with the extent of the CMBBZ proposed by Davidson (1986), a close correspondence is observed (Fig. 3). With few exceptions, samples within the CMBBZ have model ages over 1.35 Ga, including three samples from the Redstone complex (Table 1, samples 1–3). In contrast, most samples within the main body of the CMB have model ages below

1.35 Ga, including two samples from the Glamorgan complex (# 57–58). In contrast, ten samples from the Dysart complex (# 22–32) yield model ages older than the CMB, but resembling the Redstone complex and CMBBZ. U-Pb chronology for the Redstone complex has yielded a published age of 1344 + 93/−32 Ma (Van Breemen and Hanmer, 1986). For comparison, the unpublished U-Pb data of Heaman cited by McNutt and Dickin (2012) yields an age of 1337 Ma for the Dysart complex, falling within the error limits of the Redstone age. The average TDM model ages for the Redstone and Dysart complexes are respectively 100 Myr and 60 Myr older than their U-Pb ages. However, similar excesses of TDM ages over U-Pb ages have commonly been observed for other granitoid complexes in the Grenville Province. For example, the McKellar gneiss complex in the Parry Sound domain of the Central Gneiss Belt (McNutt and Dickin, 2012) gave an average TDM age of 1.43 Ga, 100 Myr older than the most reliable U-Pb age (Wodicka et al., 1996). However, good agreement between the Nd model age and a Sm–Nd isochron age supported the accuracy of the mantle model. Therefore, McNutt and Dickin (2012) suggested that the Nd model ages and Sm–Nd isochron were not dating igneous crystallisation of the McKellar complex, but the formation age of a (more mafic) crustal protolith. It takes a finite period of time to build mafic arc crust to the point where it undergoes magmatic differentiation to produce granitoid rocks. For example, in the Kohistan arc terrane, this may have taken nearly 50 Myr (Petterson, 2010). However, Nd model ages date the beginning of this process, when mafic crust is first extracted from the mantle, whereas U-Pb ages date the crystallisation of discrete igneous bodies that may be a few tens of Myr younger, even for a juvenile arc terrane. The TDM ages for the Redstone and Dysart complexes are therefore interpreted in this way as formation ages of the crustal protolith. The petrology of the analysed samples is summarised in Fig. 4 on the Q–P plot of Debon and LeFort (1983), using major element atomic proportions (Si/3 − [K + Na + 2/3Ca] = Q and K − [Na + Ca] = P) to construct a petrochemical equivalent of the Streckeisen diagram (and assuming that these elements were relatively immobile during Grenvillian metamorphism). In this diagram, the Redstone, Dysart and Glamorgan complexes are almost entirely restricted to the quartz diorite and tonalite fields, whereas other CMBBZ rocks

Fig. 3. Map showing sample localities relative to the CMBBZ of Davidson (1986). Samples with pink shading denote alkaline chemistry identified on the basis of their distribution in Fig. 5. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

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Table 1 Nd isotope data for orthogneisses from the Haliburton–Bancroft area. Map #

Sample name

Northing NAD83

Easting NAD83

Nd ppm

Redstone 1 2 3 Average

MD26 D17 MD37

5008 766 5004 582 5002 605

17 689 069 17 689 421 17 691 854

19.4 10.8 24.7

CMBBZ 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 pub pub pub Average

MD90 MD45 MD103 MD56 HB01 D14 HB63 HB24 HB26 HB28 CR08 HB29 HB25 WH8 HB16 HB17 WH11 HB56 BA05 BA10 BA54

4981 960 4983 242 4990 640 4993 017 5007 631 5010 510 5006 513 5012 132 5009 990 5005 094 5009 490 5006 979 5011 218 5017 660 5017 560 5016 736 5015 600 5009 300 5007 328 5014 400 5019 600

17 679 450 17 679 421 17 679 700 17 680 438 17 702 173 17 705 495 17 720 857 17 727 436 17 731 085 17 732 411 17 734 550 17 735 064 18 265 895 18 265 894 18 266 000 18 266 200 18 267 700 18 271 950 18 271 316 18 268 700 18 268 900

Dysart 22 23 24 25 26 27 28 29 30 31 32 33 Average excl #33

WG02 HB78 MD94 MD86 HB09 HB70 HB72 HB73 HB07 MD97 MD89 MD87

4997 000 4998 312 4987 460 4989 700 4992 785 4993 424 4995 853 4996 566 4998 666 5004 620 4992 380 4990 720

Rift zone 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 pub pub pub Average

HB81 HB80 MD44 MD92 MD112 MD41 MD38 D10 HB77 MD96 HB75 MD85 HB13 HB11 HB65 HB64 HB15 HB61 HB60 HB59 HB54 HB55 HB21 CR05 CR04 BB20

4977 600 4981 450 4988 533 4988 300 4988 850 4991 925 5000 314 5001 210 5003 400 5004 890 5002 490 4988 540 4996 208 5000 151 5000 400 5000 500 4999 937 5003 373 5001 000 5000 500 5011 800 5009 698 5002 999 4997 300 4989 300 5018 000

Sm ppm

147Sm/144Nd

143Nd/144Nd

E(t) 1.35 Ga

TDM (Ga)

Q

P

4.16 2.10 5.26

0.1294 0.1177 0.1287

0.512238 0.512166 0.512209

3.8 4.4 3.3 3.8

1.45 1.39 1.49 1.44

110 67 147

−202 −249 −172

24.9 27.4 28.3 27.1 20.8 11.6 24.0 40.6 29.7 33.6 32.7 38.1 30.8 37.6 26.4 35.3 46.7 13.2 28.2 15.9 26.5

5.68 4.79 6.05 4.40 1.45 2.01 4.97 9.12 5.69 7.45 7.20 6.42 6.69 8.04 5.46 7.71 9.77 3.03 5.77 3.04 5.15

0.1378 0.1056 0.1292 0.0981 0.1290 0.1053 0.1251 0.1357 0.1160 0.1342 0.1332 0.1018 0.1313 0.1293 0.1257 0.1322 0.1257 0.1390 0.1239 0.1158 0.1174

0.512256 0.512070 0.512222 0.511954 0.512265 0.512045 0.512219 0.512218 0.512124 0.512295 0.512301 0.512000 0.512197 0.512258 0.512170 0.512267 0.512166 0.512341 0.512169 0.512096 0.512093

2.7 4.6 3.5 3.6 4.4 4.2 4.2 2.3 3.9 4.1 4.4 3.9 2.7 4.2 3.1 3.9 3.0 4.1 3.4 3.4 3.0 3.6

1.58 1.37 1.47 1.43 1.40 1.40 1.41 1.61 1.43 1.42 1.40 1.42 1.56 1.42 1.51 1.45 1.52 1.43 1.48 1.47 1.50 1.46

156 148 132 149 62 109 92 123 86 97 66 84 67

−75 −149 −119 −202 −246 −213 −212 155 −181 −233 −198 −165 −219

74 54 33 174 128 47 130

−215 −225 −228 −169 −96 −171 −152

17 689 000 17 693 246 17 690 816 17 694 563 17 695 512 17 697 374 17 700 918 17 701 708 17 702 688 17 704 603 17 705 907 17 703 796

19.3 17.6 31.8 24.5 28.9 34.0 29.7 30.3 9.0 18.4 18.4 26.5

4.32 4.92 5.48 4.90 5.00 5.75 3.89 4.37 1.82 4.56 3.78 6.34

0.1355 0.1688 0.1041 0.1213 0.1046 0.1023 0.0793 0.0870 0.1371 0.1502 0.1239 0.1450

0.512311 0.512626 0.512048 0.512190 0.512021 0.512009 0.511845 0.511879 0.512346 0.512457 0.512220 0.512422

4.2 4.6 4.4 4.2 3.8 4.0 4.7 4.1 4.6 4.5 4.4 4.7 4.3

1.42 1.41 1.38 1.41 1.42 1.41 1.36 1.40 1.38 1.40 1.39 1.37 1.40

212 230 176 156 141 153 214 213 59 211 205 −27

−72 −194 −130 −164 −165 −182 −113 −129 −224 −151 −158 31

17 682 500 17 686 200 17 681 655 17 683 020 17 684 020 17 681 019 17 688 542 17 695 700 17 697 095 17 700 866 17 701 802 17 694 130 17 715 633 17 714 411 17 717 250 17 721 030 17 699 909 17 734 619 17 735 300 18 264 411 18 275 131 18 273 498 18 274 375 17 727 800 17 729 900 18 278 500

31.6 3.6 68.4 26.9 76.5 33.7 52.7 7.3 41.2 60.9 27.4 36.2 35.2 58.4 29.0 23.6 54.8 43.1 6.4 83.6 26.2 37.9 16.7 60.5 34.7 33.1

4.24 0.61 13.33 3.91 11.66 5.01 8.21 1.68 8.07 9.85 4.99 6.81 7.56 10.70 4.72 4.47 10.05 7.59 1.12 14.68 3.94 6.41 4.79 8.94 7.93 7.43

0.0811 0.1034 0.1179 0.0879 0.0922 0.0910 0.0942 0.1391 0.1183 0.0977 0.1102 0.1137 0.1298 0.1107 0.0984 0.1144 0.1110 0.1065 0.1061 0.1062 0.0911 0.1023 0.1732 0.0893 0.1381 0.1357

0.511964 0.512217 0.512272 0.511945 0.512004 0.512023 0.512008 0.512387 0.512201 0.512048 0.512135 0.512208 0.512345 0.512197 0.512081 0.512209 0.512187 0.512099 0.512123 0.512111 0.511964 0.512078 0.512707 0.512063 0.512455 0.512356

6.8 7.9 6.4 5.2 5.6 6.2 5.4 5.0 5.0 5.5 5.1 5.9 5.8 6.2 6.1 5.8 6.0 5.0 5.6 5.3 5.0 5.3 5.4 7.3 6.5 5.0 5.8

1.24 1.14 1.23 1.33 1.30 1.27 1.32 1.34 1.34 1.31 1.33 1.27 1.27 1.25 1.27 1.28 1.27 1.34 1.30 1.32 1.34 1.32 1.26 1.20 1.19 1.34 1.28

143 −11 −2 142 122 137 96 196 81 73 197 −15 −4 −6 19 1 −4 53 198 204 −10 99 186 118 113 80

0 −336 −166 58 −53 −151 −106 −172 −202 −109 −45 −84 −258 −240 −121 −286 −122 −214 −141 −133 −146 15 −165 −145 −37 −203

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Table 1 (Continued) Map #

Sample name

Northing NAD83

Easting NAD83

Nd ppm

Sm ppm

147Sm/144Nd

143Nd/144Nd

E(t) 1.35 Ga

TDM (Ga)

Q

P

Glamorgan 57 GM2 58 GM1 Average

4977 830 4985 140

17 687 170 17 701 520

31.7 27.3

7.35 6.72

0.1404 0.1489

0.512444 0.512515

5.9 5.8 5.9

1.24 1.24 1.24

151 151

−179 −162

CMBBZ alkaline WH9 59 HB18 60 61 HB27 62 HB30 Average

5015 700 5015 667 5007 414 5005 777

18 267 162 18 267 274 17 731 880 18 265 371

158.4 164.3 123.2 141.7

24.96 24.83 20.28 22.88

0.0952 0.0914 0.0995 0.0976

0.512014 0.511968 0.512013 0.512072

5.3 5.1 4.5 6.0 5.2

1.32 1.34 1.37 1.27 1.32

46 22 42 −5

−129 −140 −99 −170

Rift margin alkaline MD40 63 HB10 64 MD98 65 66 HB58 67 HB50 68 HB51 Average

4999 460 4990 308 4988 190 5001 400 5003 836 5010 000

17 687 624 17 699 175 17 708 733 18 265 500 18 276 148 18 275 750

127.5 126.1 116.2 137.8 100.0 107.9

18.57 18.99 17.36 23.08 15.71 17.74

0.0880 0.0910 0.0903 0.1012 0.0950 0.0993

0.511960 0.511977 0.511973 0.512062 0.511997 0.512039

5.5 5.3 5.4 5.2 5.0 5.1 5.2

1.31 1.32 1.32 1.33 1.34 1.34 1.33

72 −10 9 4 1 −9

−32 −64 −96 −129 −41 −141

resemble the Muskoka domain of the gneiss belt in extending from quartz diorite as far as the granodiorite/monzogranite boundary. In contrast to this restricted compositional range, rocks within the proposed rift zone have more variable petrology, tending towards two different suites in Fig. 4. Most samples span the quartz diorite to granite fields seen previously from the Ontario segment of the back-arc rift zone (Dickin and McNutt, 2007), but others extend towards the monzonite and syenite fields, as previously observed for the Mont Laurier domain in Quebec (Dickin et al., 2010). The syenitic samples are believed to be from the 1250 to 1290 Ma pre-tectonic nepheline syenite suite of Lumbers et al. (1990), dated on the basis of cross-cutting relationships.

Fig. 4. Samples from this study plotted on the petrochemical grid of Debon and LeFort (1983), a chemical Streckeisen classification of granitoids. Q is an index of quartz content, and P an index of plagioclase versus K-feldspar content (using atomic proportions of Si/3 − [K + Na + 2/3Ca] = Q and K − [Na + Ca] = P). TN, tonalite, GD, granodiorite, MG, monzogranite, GR, granite, QD, quartz diorite, D, diorite, MD, monzodiorite, M, monzonite, S, Syenite. Samples with pink shading denote alkaline chemistry identified in Fig. 5. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

The concentration of analysed samples in the monzonite–syenite fields is problematical for isotope mapping because previous studies (e.g. Marcantonio et al., 1990) have shown that syenitic rocks have a greater chance than other granitoids of containing a young mantle-derived component. In that case, their Nd isotope signatures would represent a mixture of juvenile mantle derived Nd and old crustal Nd, so that their model ages would not be indicative of any real geological event, as discussed by Arndt and Goldstein (1987). Another characteristic of this type of sample is high concentrations of incompatible trace elements. Therefore, to investigate the relationship between major and trace element chemistry, the Q index from Fig. 4 is plotted against Nd concentration (Table 1). The resulting distribution (Fig. 5) shows that a subset of samples with low quartz indices (Q) also has high Nd contents. Within the field of ten points with Nd concentrations of 100 ppm or above, four points

Fig. 5. Plot of the Q index from Fig. 4 against measured Nd concentration in rocks analysed for this study. Pink shaded symbols denote samples with alkaline chemistry, as discussed in the text. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

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Fig. 6. Plot of epsilon Nd values at 1.35 Ga against Nd concentration, showing consistent distributions of sample suites recognised on the basis of geographical and petrological/geochemical character.

are samples from the CMBBZ with anomalously young TDM ages (# 59–62). In fact, their average TDM age (1.32 Ga) is 140 Myr younger than the average for the CMBBZ as a whole (1.46 Ga). Hence, the alkaline incompatible-rich chemistry of these samples is a flag that their model ages significantly underestimate the formation age of the crust that makes up the CMBBZ. By comparison, it could also be assumed that model ages for rift zone samples with similar alkaline chemistry also underestimate the formation age of the local crust. In contrast to these Nd-rich samples, #33 is the most nephelinenormative syenite analysed, but has a low Nd concentration. This sample is adjacent to the Dysart complex, and yields a model age consistent with the main complex. Therefore it appears that this sample does yield a model age representative of the local crust. This suggests that incompatible trace elements are more diagnostic of the involvement of new mantle-derived material in granitoid magma genesis than petrology and major element chemistry. Hence, a cut-off for less reliable model ages is set at 100 ppm Nd, regardless of the major element composition. To further explore geochemical variations within each sample suite, the isotope data are compared with Nd concentration data in Fig. 6. This plot confirms that each suite defines a consistent distribution of isotope and concentration data, suggesting that the objective of collecting suites representative of the local crust was achieved. To compare the model age distributions within and between the suites, they are shown in histogram form in Fig. 7, where dark red shading denotes alkaline high-Nd samples. In this plot, the sample categories discussed above can also be compared with published samples of the Muskoka domain and the Adirondacks (Dickin et al., 2010). These results show that the Redstone complex, along with other CMBBZ gneisses and the Dysart complex, all have strongly overlapping ranges of TDM model age, except that the CMBBZ has a small tail extending to older model ages. All three groups show very little overlap with the rift zone suite, which includes two samples

Fig. 7. Histograms of TDM model age for sample suites from this study compared with published data sets for the Muskoka and Adirondacks from Dickin et al. (2010). Dark red shading = samples with differentiated alkaline chemistry from Fig. 5.

of the Glamorgan gneiss complex, showing that this is definitely not related to the Dysart and Redstone complexes. The CMBBZ/Redstone/Dysart suites show a strong overlap with the Adirondack model age distribution (data from Daly and McLelland, 1991 and Dickin et al., 2010), consistent with their proposed origin by Elzevirian magmatism on the Laurentian margin. However, the tail of older samples in the ‘other CMBBZ’ probably reflects the presence of some Pinwarian crustal protoliths in this unit, consistent with its formation adjacent to the reworked Pinwarian crust of the Muskoka domain (Slagstad et al., 2009; Dickin et al., 2010). The original Pinwarian–Elzevirian crustal boundary is expected to be a diffuse ‘plutonic front’. However, crustal stacking during thrusting in the CMBBZ has foreshortened the crustal section, cutting out some of the mixed crust, so that the boundary between the Muskoka domain and CMBBZ may correspond approximately to the edge of the Pinwarian continent. The high-Nd (alkaline) samples from the margins of the rift zone have model ages at the upper end of the range for that suite (1.31–1.34 Ga), and also overlap those for anomalously young ages in the CMBBZ (1.27–1.34 Ga). This suggests that the model ages for the rift margin alkaline samples probably underestimate the formation age of the local crust. A majority of these samples lie in a cluster north of Bancroft (Fig. 3), where Davidson’s CMBBZ bulges to the south. This suggests that old crust may extend even further south in this vicinity, in the sub-surface, where it caused crustal contamination of the mantle-derived alkaline magmas. This

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Fig. 8. Map showing areas of volcanic and plutonic rocks that provide evidence for the geometry of the rift zone and its subsequent deformation (see text).

crustal contamination model is supported by the observation of 1.30–1.44 Ga inherited crustal zircons in the Faraday gabbro in this vicinity (Pehrsson et al., 1996). 5. Discussion A significant finding of this work is that a singular isotopic boundary separating the CMB and CMBBZ follows the northwesterly limit of marble outcrops and corresponds to the southeasterly limit of Davidson’s CMBBZ in Figs. 2 and 3. It is notable that marble tectonites are absent from the CMBBZ in this area, suggesting that the tectonites surrounding the Dysart complex have little in common with the CMBBZ. They are in fact typical of much of the Bancroft Terrane (Fig. 3), where the relatively pervasive shear strain was imaged by the Lithoprobe seismic survey (White et al., 1994). Given the lack of marble in the CMBBZ, it seems much more likely that the Dysart complex was separated from the preGrenvillian gneisses presently within the CMBBZ before marble deposition, by extensional tectonic processes in a back-arc rift zone. If this happened, we would expect the intervening crust to consist of juvenile mafic material, quite likely with an alkaline character, since rift margin volcanics are known to be typified by alkaline chemistry (for example, the modern Rio Grand Rift, Gibson et al., 1993). Partial melting of this type of crust could produce felsic material with alkaline petrology similar to many of the samples analysed in the present study (see Leat et al., 1988). Nepheline syenites and associated alkali mafic rocks of ca. 1280 Ma age were specifically noted by Baer (1976) in the Bancroft area, and attributed to continental rifting along a major crustal lineament running from Bancroft to Renfrew. Baer also noted that the thickness of marbles was greatest in the western part of the CMB basin, and hence that his proposed aulacogen was more like a half-graben, in which the Bancroft area, with its notably alkaline chemistry would therefore have been the original locus of rifting. The rifted block model is also supported by Nd isotope analysis of metavolcanic rocks surrounding the Dysart complex (Peck and Smith, 2005) which likewise yield model ages in the

range 1.21–1.33 Ga (excluding an anomalous sample with a nearchondritic Sm/Nd ratio that does not give a meaningful model age). Hence the Nd data for meta-volcanic and plutonic rocks give a consistent picture of juvenile crust surrounding the Dysart complex. We suggest that this juvenile mafic crust was subsequently overlain by carbonate sediments, and the basement and cover were later interleaved by Grenvillian tectonism. The good geometric fit of the Dysart complex to the shape of the CMBBZ also supports the model of a rifted block. The Dysart essentially fills an embayment in the shape of the CMBBZ, except at the eastern end where it appears that the tip of the complex has been folded round a wedge of marble (Fig. 3). If this fold is unwound, the area of old Nd model ages fits almost perfectly into the embayment of the CMBBZ. Baer (1976) proposed the Danakil depression in the southern Red Sea as a modern analogue to the CMB, and specifically, that ‘opening of the aulacogen coincided with volcanic activity along northeasterly trending dikes, that sliced the basement into fault blocks.’ Although the Red Sea rift is not subduction-related, it has a comparable geometry to the CMB, with a 200-km wide ensimatic Red Sea segment similar to the Ontario CMB and a 50 km wide ensialic Gulf of Suez segment similar to the marble domain in Quebec. The Red Sea rift also has separated blocks on its margins, including the large Danakil block in the south (Chorowicz et al., 1999), but also including smaller blocks such as the Gebel Zeit block in the Gulf of Suez section, which is about 60 km long and 10 km wide (Younes and McClay, 2002). Hence, these are comparable to the larger Elzevir Block and the smaller Dysart Block. Subsequent to the Late Elzevirian rifting event, the CMB was involved in the Ottawan phase of the Grenville orogenic cycle. However, Rivers (2008) suggested that much of it escaped high-grade metamorphism because it formed an ‘orogenic lid’ to the collision zone (Rivers, 2008). For example, Schwerdtner et al. (2005) argued that ‘except in the triangular Mazinaw domain, exposed rocks of the Composite Arc Belt seem to be unaffected by 1090–980 Ma metamorphism and ductile deformation of the Grenvillian/Ottawan orogeny.’

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We argue that the contrasting degree of stretching exhibited by granitic rocks on either side of the Elzevir Block provides evidence that granitic bodies do provide an approximate indication of the intensity of regional crustal deformation (Fig. 8). To the east of the Elzevir Block, in the Mazinaw Terrane, stretching is extreme (Schwerdtner et al., 2005), whereas to the west, in the Harvey-Cardiff arch, pretectonic plutons retain the diaper-like sub-spherical outlines of the original intrusive bodies. Therefore, we argue that the pre-Grenvillian geometry of the rift zone was largely preserved in the configuration we see at the present day, except within the Mazinaw domain whose intense deformation was attributed by Dickin and McNutt (2007) to squeezing of this area between the more rigid Frontenac and Elzevir crustal blocks. A final question is whether there is any geometric evidence for the kinematics of the rifting process. Variations of U-Pb ages within the rift zone could potentially chart the rifting process, but much U-Pb evidence remains unpublished. Three of the oldest UPb ages within the rift zone are shown in Fig. 8. Of these, an age of 1280 ± 3 Ma was determined on a mineralised felsite in volcanic rocks of the Grimsthorpe Group at the western boundary of the Elzevir Block (Easton and Kamo (2005). A slightly older age of 1287 + 11/−3 Ma was determined on a dacite from the Cordova Lake Formation of the Belmont Lake metavolcanic complex (Davis and Bartlett, 1988). Finally, an age of ca. 1290 with no reported error was quoted by Burr and Carr (1994) on a migmatitic gneiss from the Anstruther Dome. These limited data therefore cannot resolve any geographical pattern of ages within the rift zone, suggesting that it was all generated in a fairly short time interval around 1290 Ma. An alternative approach to reconstructing the rifting process is based on the geometry of the juvenile crustal zone west of the Elzevir Block. It is observed that the plutons of the Harvey-Cardiff arch form a line approximately down the middle of this ensimatic rift zone segment, with approximately equal widths of marble-bearing crust westwards to the CMBBZ and eastwards to the Belmont and Grimsthorpe volcanics (Fig. 8). Therefore, it is tentatively suggested that the plutons of the Harvey-Cardiff arc were intruded along a line of weakness corresponding to the thinnest crust in the middle of this rift segment. Dickin and McNutt (2007) and Dickin et al. (2010) also argued that this ensimatic segment of the CMB rift zone could be traced southwards into the subsurface of the USA, where it has a NNE trend. This could be tested by Nd isotope analysis of drill cores that penetrate to Grenville basement, since the rift zone should yield juvenile 1.25 Ga model ages that are younger than the surrounding crust. The rift zone model has important implications for the structure of Grenvillian crust in Eastern North America, and its proposed NNE trajectory in the subsurface USA may have caused the Grenville Front to adopt a similar trend, in contrast to its more northeasterly trend in Canada. 6. Conclusions Nd isotope mapping shows that the southeast margin of Davidson’s CMBBZ represents a major discontinuity in crustal formation ages, from an average TDM age of 1.28 Ga in the CMB to an average of 1.46 Ga in the CMBBZ. The sharpness of this boundary is consistent with it representing the reactivated northwestern wall of a back-arc rift zone. The Redstone and Dysart complexes yield Nd model ages within error of other CMBBZ gneisses. When this observation is coupled with the 1337 Ma U-Pb age representative of the Dysart complex, which is older than any other unit in the CMB, it supports a broad correlation with rocks of the Redstone complex, in contrast to the Glamorgan complex, which yields model ages typical of the juvenile rift zone.

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One factor that the Dysart and Glamorgan complexes share is the mantling by marble tectonites. This similarity arises from their presence in the Bancroft domain of the CMB, and is quite different from the lithological setting of the CMBBZ, which contains no significant marble. In view of these factors, we argue that the Dysart complex was not separated from the CMBBZ by Grenvillian thrusting but by rifting away from the northwestern wall of an ensimatic back-arc rift zone. The juvenile ensimatic crust that developed in the rift zone was subsequently mantled by thick carbonate deposits that were partially disrupted by Grenvillian tectonism. A tentative geometric model is proposed for the development of the rift zone, based on the observation that the gneiss domes within the Harvey-Cardiff arch approximately bisect the ensimatic rift zone segment west of the Elzevir block. The plutons that gave rise to these gneiss domes may have been emplaced into thinner crust in the middle of the ensimatic rift segment. The sub-spherical outlines of these bodies suggest that the original geometry of the rift zone was preserved through Grenvillian tectonism on the west side of the Elzevir block. In contrast, extreme elongation of plutonic boundaries in the Mazinaw domain east of the Elzevir block suggests that this was a zone of dutile strain between two more rigid blocks. The CMB rift zone model has major implications for the geology of Precambrian rocks in the subsurface of the north-eastern USA, and can be tested by Nd isotope analysis of drill core samples from Grenville basement. Acknowledgements This work was supported by a Natural Science and Engineering Research Council Discovery Grant to Alan Dickin and a McMaster Graduate Scholarship to Kathy Moretton. We thank Sarah Dickin and Mark Zelek for assistance with fieldwork and acknowledge constructive criticisms by Ken Ashton, David Corrigan and Nick Culshaw that helped to improve this paper. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.precamres.2013.01.003. References Adams, F.D., Barlow, A.E., 1910. Geology of the Haliburton and Bancroft Areas, Province of Ontario. Geological Survey of Canada Memoir 6, 419p. Arndt, N.T., Goldstein, S.L., 1987. Use and abuse of crust-formation ages. Geology 15, 893–895. Baer, A.J., 1976. The Grenville Province in Helikian times: a possible model of evolution. Philosophical Transactions of the Royal Society of London A280, 499–515. Burr, J.L., Carr, S.D., 1994. Structural geometry and U-Pb geochronology near Lithoprobe seismic line 32, western Central Metasedimentary Belt, Grenville province, Ontario. In: Lithoprobe Abtibi-Grenville Project, Report No. 41, 59–62. Carr, S.D., Easton, R.M., Jamieson, R.A., Culshaw, N.G., 2000. Geologic transect across the Grenville orogen of Ontario and New York. Canadian Journal of Earth Sciences 37, 193–216. Chorowicz, J., Colett, B., Bonavia, F., Korme, T., 1999. Left-lateral strike-slip tectonics and gravity induced individualization of wide continental blocks in the western Afar margin. Eclogae Geologicae Helvetiae 1, 149–158. Condie, K.C., Moore, J.M., 1977. Geochemistry of Proterozoic volcanic rocks from the Grenville Province, eastern Ontario. In: Baragar, W.R.A., Coleman, L.C., Hall, J.M. (Eds.), Volcanic Regimes in Canada, Geological Association of Canada Special Paper 16, 149–168. Corriveau, L., van Breemen, O., 2000. Docking of the Central Metasedimentary Belt to Laurentia: evidence from the 1.17–1.16 Ga Chevreuil intrusive suite and host gneisses, Quebec. Canadian Journal of Earth Science 37, 253–269. Daly, J.S., McLelland, J.M., 1991. Juvenile Middle Proterozoic crust in the Adirondack Highlands, Grenville province, northeastern North America. Geology 19, 119–122. Davidson, A., 1984. Identification of ductile shear zones in the southwestern Grenville Province of the Canadian Shield. In: Kroner, A., Greiling, R. (Eds.),

232

K. Moretton, A. P. Dickin / Precambrian Research 228 (2013) 223–232

Precambrian Tectonics Illustrated. Schweizerbart’sche Verlagbuchhandlung, Stuttgart, pp. 263–279. Davidson, A., 1986. New interpretations of the Southwestern Grenville Province, In: Moore, J.M., Davidson, A. and Baer, A.J. (Eds.), The Grenville Pro Geological Association of Canada, Special Paper 31, 61–74. Davidson, A., 1998a. An overview of Grenville Province Geology, Canadian Shield, Chapter 3, in: Lucas, S.B., St-Onge, M.R. co-ord., Geology of the Precambrian Superior and Grenville provinces and Precambrian fossils in North America, vol. 7, Geological Survey of Canada, Geology of Canada, pp. 205–270. Davidson, A., 1998b. Geological map of the Grenville Province, Canada and adjacent parts of the United States of America: Geological Survey of Canada, Map 1947A, scale 1:2,000,000. Davis, D.W., Bartlett, J.R., 1988. Geochronology of the Belmost Lake Metavolcanic Complex and implications for crustal development in the Central Metasedimentary Belt, Grenville Province, Ontario. Canadian Journal of Earth Sciences 25, 1751–1759. Debon, F., LeFort, P., 1983. A chemical-mineralogical classification of common plutonic rocks and associations. Transactions of the Royal Society of Edinburgh: Earth Sciences 73, 135–149. 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. Canadian Journal of Earth Sciences 37, 165–181. Dickin, A.P., McNutt, R.H., 2007. The Central Metasedimentary Belt (Grenville Province) as a failed back-arc rift zone: Nd isotope evidence. Earth and Planetary Sciences Letters 259, 97–106. Dickin, A., McNutt, R.H., Martin, C., Guo, A., 2010. The extent of juvenile crust in the Grenville Province: Nd isotope evidence. Geological Society of America Bulletin 122, 870–883. Easton, R.M., 1992. The Grenville Province and the Proterozoic history of central and southern Ontario, In: Geology of Ontario, Ontario Geological Survey Special Paper 4, part 2, 714–904. Easton, R.M., 2001. Precambrian geology, Minden area. Ontario Geological Survey Preliminary Map P.3415, scale 1:50 000. Easton, R.M., 2001. Precambrian geology, Haliburton area. Ontario Geological Survey Preliminary Map P.3416, scale 1:50 000. Easton, R.M., Kamo, S.L., 2005. Timing of gold mineralization in the western Grimsthorpe domain, Central Metasedimentary Belt, Grenville Province, Ontario. Summary of fieldwork, Ontario geological Survey open file report 6172, 15/115/10. Gibson, S.A., Thompson, R.N., Leat, P.T., Morrison, M.A., Hendry, G.L., Dickin, A.P., Mitchell, J.G., 1993. Ultrapotassic magmas along flanks of the Oligo-Miocene Rio Grande Rift, USA: monitors of the zone of lithospheric mantle extension and thinning beneath a continental rift. Journal of Petrology 34, 187–228. Hanmer, S., 1988. Ductile thrusting at mid-crustal level, southwestern Grenville Province. Canadian Journal of Earth Sciences 25, 1049–1059. Hanmer, S., Corrigan, D., Pehrsson, S., Nadeau, L., 2000. SW Grenville Province, Canada: the case against post-1.4 Ga accretionary tectonics. Tectonophysics 319, 33–51. Hanmer, S., McEachern, S., 1992. Kinematical and rheological evolution of a crustal scale ductile thrust zone, Central Metasedimentary Belt, Grenville orogen, Ontario. Canadian Journal of Earth Sciences 29, 1779–1790. Holm, P.E., Smith, T.E., Grant, B.D., Huang, C.H., 1985. The geochemistry of the Turriff metavolcanics, Grenville Province, southeastern Ontario. Canadian Journal of Earth Sciences 22, 435–441. Leat, P.T., Thompson, R.N., Morrison, M.A., Hendry, G.L., Dickin, A.P., 1988. Silicic magmas derived by fractional crystallization from Miocene minette, Elkhead Mountains, Colorado. Mineralogical Magazine 52, 577–585.

Lumbers, S.B., Heaman, L.M., Vertoli, V.M., Wu, T.-W., 1990. Nature and timing of Middle Proterozoic magmatism in the Central Metasedimentary Belt Ontario, In: Gower, C.F., Rivers, T., Ryan, B., Mid-Proterozoic Laurentia-Baltica, Geological Association of Canada Special Paper 38, 243–276. Lumbers, S.B., Vertolli, V.M., 2003. Precambrian geology, Wilberforce area. Ontario Geological Survey Preliminary Map P.3526, scale 1:50 000. Marcantonio, F., McNutt, R.H., Dickin, A.P., Heaman, L.M., 1990. Isotopic evidence for the crustal evolution of the Frontenac Arch in the Grenville Province of Ontario, Canada. Chemical Geology 83, 297–314. McNutt, R.H., Dickin, A.P., 2012. A comparison of Nd model ages and U-Pb zircon ages of Grenville granitoids: constraints on the evolution of the Laurentian margin from 1.5 to 1.0 Ga. Terra Nova 24, 7–15. Moore, J.M., Thompson, P.H., 1980. The Flinton Group: a late Precambrian metasedimentary succession in the Grenville Province of eastern Ontario. Canadian Journal of Earth Sciences 17, 1685–1707. Peck, W.H., Smith, M.S., 2005. Cordierite-gedrite rocks from the Central Metasedimentary Belt boundary thrust zone (Grenville Province, Ontario): Mesoproterozoic metavolcanic rocks with affinities to the Composite Arc Belt. Canadian Journal of Earth Sciences 42, 1815–1828. Pehrsson, S., Hanmer, S., van Breemen, O., 1996. U-Pb geochronology of the Raglan gabbro belt, Central Metasedimentary Belt, Ontario: implications for an ensialic marginal basin in the Grenville Orogen. Canadian Journal of Earth Sciences 33, 691–702. Petterson, M.G., 2010. A Review of the geology and tectonics of the Kohistan island arc, north Pakistan. In: Kusky, T.M., Zhai, M.G., Xiao, W. (Eds.), The Evolving Continents: Understanding Processes of Continental Growth, 338. Geological Society of London Special Publication, pp. 287–337. Rivers, T., 1997. Lithotectonic elements of the Grenville Province: review and tectonic implications. Journal of Precambrian Research 86, 117–154. Rivers, T., 2008. Assembly and preservation of lower, mid, and upper orogenic crust in the Grenville Province – implications for the evolution of large hot longduration orogens. Precambrian Research 167, 237–259. Rivers, T., Corrigan, D., 2000. Convergent margin on southeastern Laurentia during the Mesoproterozoic: tectonic implications. Canadian Journal of Earth Sciences 37, 359–383. Schwerdtner, W.M., Serafini, G., Yakovenko, A., 2005. Straight-gneiss zones at the northwestern boundary of the Mazinaw domain, Grenvillian Composite Arc Belt, southeastern Ontario. In: GAC-MAC-CSPG-CSSS 2005 Joint Meeting, Abstract volume 20, p. 173. Slagstad, T., Culshaw, N.G., Daly, J.S., Jamieson, R.A., 2009. Midcontinent graniterhyolite provinces revealed. Terra Nova 21, 181–187. Smith., T.E., Holm, P.E., 1990. The geochemistry and tectonic significance of premetamorphic minor intrusions of the Central Metasedimentary Belt, Grenville province, Ontario. Precambrian Research 48, 341–360. Van Breemen, O., Hanmer, S. 1986. Zircon morphology and U-Pb geochronology in active shear zones: studies of syntectonic intrusions along the northwest boundary of the Central Metasedimentary Belt, Grenville Province, Ontario, in: Current Research, Geological Survey of Canada, 86-1B, pp. 775–784. White, D.J., Easton, R.M., Culshaw, N.G., Milkereit, B., Forsyth, D.A., Carr, S., Green, A.G., Davidson, A., 1994. Seismic images of the Grenville Orogen in Ontario. Canadian Journal of Earth Sciences 31, 293–307. Wodicka, N., Parrish, R.R., Jamieson, R.A., 1996. The Parry Sound domain: a fartravelled allochthon? New evidence from U-Pb zircon geochronology. Canadian Journal of Earth Sciences 33, 1087–1104. Younes, A.I., McClay, K., 2002. Development of accommodation zones in the Gulf of Suez – Red Sea Rift. American Association of Petroleum Geologists Bulletin 86, 1003–1026.