Articles

Whole-rock chemical and Sm-Nd isotopic composition of a Late Proterozoic metasedimentary sequence in Ganderia:

Kellys Mountain, Bras d’Or terrane, Nova Scotia, Canada

Sandra M. Barr
Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada
Christian Pin
Département de Géologie, UMR 6524 CNRS, Université Blaise Pascal, 5 rue Kessler 63 038 Clermont-Ferrand Cedex, France.
David W.A. McMullin
Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6, Canada
Chris E. White
Nova Scotia Department of Natural Resources, P.O. Box 698, Halifax, Nova Scotia B3J 2T9, Canada
Date received 23 October 2012 ¶ Date accepted 26 February 2013 ¶ Date digitally published April 15
doi:10.4138/atlgeol.2013.002

ABSTRACT

The relationship between low-grade metasedimentary and high-grade paragneissic Precambrian rock units in the Bras d’Or terrane of central Cape Breton Island is a long-standing geological problem. Whole-rock geochemical and Nd isotopic data from representative components of these units in the Kellys Mountain area, the Glen Tosh formation and Kellys Mountain Gneiss, show strong similarity. Major and trace element characteristics suggest that protolith sediments for both units were wackes derived from felsic igneous sources and deposited at an active continental margin. Samarium-Nd isotopic data in combination with previously published detrital zircon ages show that the sediments contained an ancient (ca. 2 Ga) end-member of recycled continental crust mixed with a juvenile component of Late Neoproterozoic age. The similarities suggest that the rocks represent the same sedimentary unit at different grades of metamorphism, implying a major crustal break under the area of mixed dioritic and granitic rocks that separates the gneissic and lower grade rocks.

RÉSUMÉ

Le lien entre les unités lithologiques précambriennes métasédimentaires faiblement métamorphisées et celles paragneissiques fortement métamorphisées du terrane Bras d’Or et du centre de l’île du Cap Breton est un problème géologique de longue date. Les données isotopiques du Nd et géochimiques sur roche totale d’éléments représentatifs de ces unités dans le secteur du mont Kellys, la formation de Glen Tosh et le gneiss du mont Kellys révèlent une similarité prononcée. Les caractéristiques des éléments majeurs et des éléments traces laissent supposer que les sédiments protolithiques des deux unités étaient des wackes provenant de sources ignées felsiques qui se sont déposés le long d’une marge continentale active. Les données isotopiques du samarium-néodyme combinées avec les datations sur zircon détritique précédemment publiées révèlent que les sédiments comportaient un membre extrême ancien (environ 2 Ga) de croûte continentale recyclée mélangée avec un jeune élément de l’époque du Néoprotérozoïque tardif. Les similarités notées permettent de penser que les roches constituent la même unité sédimentaire à des niveaux différents de métamorphisme, ce qui suppose une rupture crustale majeure sous le secteur des roches dioritiques et granitiques mixtes séparant les roches gneissiques et moins métamorphisées.

[Traduit par la redaction]

INTRODUCTION

1 Kellys Mountain is a northeast-trending ridge of crystalline rocks surrounded by unconformably overlying Carboniferous sedimentary rocks. It is one of several such blocks which characterize the Bras d’Or terrane of central Cape Breton Island (e.g., Raeside and Barr 1990; Keppie et al. 1998). All of these blocks are dominated by metasedimentary, in some areas gneissic, rocks, intruded by varied granitic and dioritic rocks with ages of ca. 580–550 Ma (Raeside and Barr 1990; Keppie et al. 1998). Based on tectonic history, age, and isotopic data, these characteristic components of the Bras d’Or terrane, like similar units in the Brookville terrane of southern New Brunswick (White and Barr 1996), are now interpreted to be part of Ganderia (Fig. 1 inset), a peri-Gondwanan continental fragment of probable Amazonian provenance (e.g., Hibbard et al. 2007; van Staal et al. 2009; van Staal and Barr 2012). Only southernmost Cape Breton Island (Mira terrane, Fig. 1) and northern mainland Nova Scotia are interpreted to be part of Avalonia (Fig. 1 inset). Northernmost Cape Breton Island, known as the Blair River Inlier (Fig. 1), is interpreted to be part of Laurentia (Fig. 1 inset).

Figure 1. Simplified geological map of the Bras d’Or terrane after Raeside and Barr (1990) showing the location of the Kellys Mountain area. Black box outlines the area shown in Figure 2. Inset map shows components of the northern Appalachian orogen after Hibbard et al. (2006). Abbreviation: BRMS, Barachois River Metamorphic Suite.
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2 In addition to the larger questions such as the relationship of these rocks to units of similar age in adjacent Avalonia and their original position within Rodinia (e.g., Barr and White 1996; Keppie et al. 1998; Keppie et al. 2000; Barr et al. 2003), a long-standing uncertainty has been the relationship between the lower grade units, collectively termed the George River Metamorphic Suite in the Bras d’Or terrane and the Green Head Group in southern New Brunswick, and the gneissic units, collectively known as the Bras d’Or Gneiss and Brookville Gneiss in the Bras d’Or and Brookville terranes, respectively. Where observed, contacts between these units in both Bras d’Or and Brookville terranes are Carboniferous and older faults that range from ductile to brittle, and hence several interpretations of their relative depositional ages have been proposed: (i) the gneissic rocks form the basement beneath the low-grade metasedimentary units (e.g., Nance 1987); (ii) the gneissic rocks are the high-grade metamorphic equivalent of the lower grade units (e.g., Milligan 1970); and (iii) the gneissic rocks are younger than the low-grade units (Wardle 1978; Bevier et al. 1990; White and Barr 1996).

3 The present study was undertaken to try to resolve the fundamental question of the relationship between the metasedimentary and gneissic rocks by comparing whole-rock major-element, trace-element, and Sm-Nd isotopic composition of clastic units in the George River Metamorphic Suite to that of paragneissic components of the Bras d’Or Gneiss.Kellys Mountain was selected for this study because both the gneissic and lower grade metasedimentary units are well exposed in that area, and field relations are relatively well understood (Barr et al. 1982; Jamieson 1984; Raeside and Barr 1990).

GEOLOGICAL BACKGROUND

Kellys Mountain Gneiss

4 Gneissic rocks form the central core of Kellys Mountain, surrounded and intruded by dioritic and granitic rocks (Fig. 1). As described by Jamieson (1984), the gneiss is mainly medium- to coarse-grained cordierite-bearing migmatitic paragneiss consisting of cordierite–biotite–K-feldspar– plagioclase–quartz with accessory tourmaline, apatite, and opaque minerals (Fe-Ti oxides). K-feldspar is stable, muscovite is present only as a retrograde mineral, garnet is very rare, and cordierite occurs in both mesosome and leucosome. The paragneiss locally contains areas of tonalitic to granodioritic orthogneiss, as well as amphibolite sheets, likely originally mafic dykes. The foliation in the gneissic rocks generally trends north-northwest with steep dip (Fig. 2; Jamieson 1984).

Figure 2. Geological map of the southwestern part of the Kellys Mountain peninsula after Barr et al. (1992) showing sample locations. Representative foliation orientations are shown in the Kellys Mountain Gneiss and Glen Tosh formation (this study).
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5 The degree of migmatization increases toward the centre of the gneiss, away from the exposed contacts with the surrounding plutonic rocks. Based on mineral assemblages and absence of prograde muscovite, Jamieson (1984) inferred that peak metamorphic conditions occurred at 100–350 MPa and 580–700°C. The age of metamorphism is constrained by U-Pb dating of titanite from amphibolite in the gneiss, which gave an age of 496 ± 5 Ma (Dunning et al. 1990), and by similar ca. 495 Ma 40Ar/39Ar dates from hornblende, muscovite, and biotite in both the gneiss and amphibolite sheets in the gneiss (Keppie and Dallmeyer 1989; Reynolds et al. 1989). Metamorphic monazite separated from the gneiss yielded nearly concordant data, with a 206Pb/238U age of 515 ± 1 Ma, interpreted to date the time of migmatization (Keppie et al. 1998). Attempts to constrain the protolith age of the gneiss were unsuccessful, as single- and multi-grain zircon analyses are moderately to highly discordant, with 207Pb/206Pb ages ranging between 843 and 1700 Ma (Keppie et al. 1998). The data scatter about a chord between 657 and 1840 Ma, but the discordant nature of the data and their scatter indicate that the zircons have several generations of growth and (or) Pb loss, making it difficult to interpret the significance, if any, of the chord.

Glen Tosh Formation

6 Lower grade metasedimentary rocks (described below) form the southwestern part of Kellys Mountain, separated from the gneissic rocks by dioritic and granitic rocks (Fig. 2). Similar rocks occur north of Big Hill and in scattered outcrops farther north, and all of these rocks, including those in the southwestern part of Kellys Mountain, were assigned by Raeside and Barr (1990, 1992) to the Barachois River Metamorphic Suite, a component of the George River Metamorphic Suite (Figs. 1, 2). According to Raeside and Barr (1992), the Barachois River Metamorphic Suite is composed mainly of semipelitic and mafic lithologies, with metamorphic grade that ranges from upper-amphibolite to subgreenschist facies. The highest grade rocks occur in the north where the suite is surrounded by the 555–565 Ma plutons of the southeastern Cape Breton Highlands. Although Raeside and Barr (1990, 1992) interpreted the gneissic rocks of the northern outcrop areas of the Barachois River suite as being correlative with lower grade phyllite and greenschist in the southern Cape Breton Highlands and with metasiltstone in the Big Hill and Kellys Mountain areas, that relationship is uncertain. Hence the new informal name Glen Tosh formation is introduced here for the low-grade, mainly metasedimentary, rocks in the Kellys Mountain-Big Hill area (Fig. 2).

7 In contrast to other areas of the George River Metamorphic Suite which include carbonate as well as clastic rocks, the rocks in the Glen Tosh formation are psammitic and semi-pelitic, consisting dominantly of sutured quartz and feldspar, with variable but generally small (<10%) amounts of biotite and muscovite, with accessory apatite, tourmaline, and opaque minerals. Grain size is variable and may represent variation in clast size in the original wacke protolith. Cordierite appears to be common in many of the rocks, though in most cases is completely retrograded to pinite or sericite making it difficult to be certain in rocks that have been completely retrograded. Extremely large cordierite porphyroblasts up to 10 cm in length now pseudomorphed by pinite/sericite are present in muscovite-rich rocks from the extreme southwestern part end of the study area. Ragged andalusite grains were noted in a few samples, where it is at least partly altered to sericite, and garnet also is present in a few samples. In rocks showing the greatest level of alteration, chlorite and epidote are common.

8 Bedding is difficult to distinguish in most places but the rocks have a well-defined foliation that trends uniformly north-northeast with moderate to steep dips to west-northwest. In thin section the foliation is defined by aligned muscovite and biotite that wraps around cordierite porphyroblasts. Furthermore, within cordierite porphyroblasts, muscovite and biotite grains have a random or reticulated alignment. The discordance between fabrics internal and external to the porphyroblasts indicates that cordierite growth predated the last phase of foliation development. Where the psammitic/pelitic rocks of the Glen Tosh formation are intruded by the dioritic and granitic rocks, they are contact metamorphosed and less foliated. In these contact metamorphic rocks cordierite has overgrown early ragged andalusite porphyroblasts.

9 These various relationships suggest the following series of events. Early somewhat random muscovite and biotite growth (with garnet and andalusite in rocks of suitable composition) followed by peak regional metamorphism produced cordierite porphyroblasts up to 10 cm in suitable (though rare) compositions. Peak deformation (and foliation development) postdated peak regional metamorphism. Subsequent intrusion of diorite and granite produced pronounced contact metamorphic effects including renewed cordierite growth (over early andalusite) and recrystallization of the matrix to produce a more granoblastic texture. The ages of these events are uncertain but Dallmeyer and Keppie (1993) reported a metamorphic muscovite cooling age of ca. 550 Ma from metapsammitic rocks in the Big Hill area, likely providing a minimum age for the metamorphism.

10 Mafic dykes occur rarely in the Glen Tosh formation, and are of dioritic and lamprophyric composition (Gates 2011). Their age is uncertain but they may be related to Cambrian extensional igneous activity in the Boisdale Hills to the south (Fig. 1; Gates 2011).

Plutonic Rocks

11 Barr et al. (1982) divided plutonic rocks of the Kellys Mountain area into two main units, leucogranite (now termed Kellys Mountain Granite; Fig. 2), which forms most of the northeastern part of the mountain, and a more varied but less extensive dioritic unit that includes quartz diorite, tonalite, diorite, granodiorite, quartz monzonite, and hornblendite. The dioritic suite occurs mainly in the area between the gneissic and psammitic units of the Glen Tosh formation. The dioritic suite is intruded by the Kellys Mountain Granite, which yielded a U-Pb (zircon) age of 498 ± 2 Ma (Barr et al. 1990). The dioritic suite may be significantly older, based on its similarity to varied dioritic, tonalitic, and granitic units dated at ca. 560 Ma elsewhere in the Bras d’Or terrane (e.g., Raeside and Barr 1990; Dunning et al. 1990). However, the close physical association between the dioritic suite and leucogranite is also suggestive of a comagmatic relationship. More geochronology is needed to resolve this uncertainty.

GEOCHEMISTRY

12 Seventeen samples were selected for whole-rock chemical analysis, 10 from the Glen Tosh formation and 7 from the Kellys Mountain Gneiss (Tables 1, 2, 3). Analyticial methods are described in Appendix A. The Glen Tosh formation samples include cordierite phyllite, quartz metawacke, and 8 more typical metawacke samples (Table 1). The gneissic samples include 6 typical paragneiss samples and one sample with higher plagioclase and biotite contents (Table 1). The metawacke and typical paragneiss samples show considerable similarity in whole-rock chemical characteristics (Fig. 3a-g). The metawacke samples range in SiO2 from 67.4 to 72.0 weight % (69.1–73.4 weight % calculated volatile-free) and the paragneissic samples from 67.1 to 72.8 % (68.4–73.7% calculated volatile-free). Other major element components overlap in both units, although the gneissic rocks tend to be lower in Fe and higher in CaO and K2O (Fig. 3b, d, f). The gneissic samples also tend to have lower volatile content (Fig. 3g). The quartz metawacke sample has higher SiO2 and lower Al2O3. The cordierite phyllite and plagioclase-biotite paragneiss samples have similar low SiO 2 (about 60%) and both have high Al2O3, reflecting their more pelitic compositions. Although element mobility during metamorphism may have affected the chemical compositions of these rocks, such consistency between petrography and chemistry suggests that the changes have not been substantial.

Table 1 . Petrographic descriptions of analyzed samples*.
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Figure 3. Major element oxides and loss-on-ignition (LOI) plotted against SiO2 to illustrate chemical characteristics of samples from the Kellys Mountain Gneiss and Glen Tosh formation. Data are in weight % from Table 2.
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Table 2 . Whole-rock chemical analyses* of samples from Kellys Mountain area.
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Table 3. Rare-earth element analyses* of samples from Kellys Mountain area.
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13 Trace element compositions are also similar between the Kellys Mountain Gneiss and Glen Tosh formation metawacke samples, as illustrated in Fig. 4. Data from the majority of samples are closely similar to the average North American Shale Composite (Gromet et al. 1984), as shown in Fig. 4a, further evidence for limited post-depositional chemical changes. However, the typically more mobile elements such as Cs, Rb, Ba, and Sr show more variability, suggesting that some changes may have occurred. The plagioclase-biotite paragneiss sample has higher Rb, Ba, Th, and Nb than any of the other samples, metawacke sample DM91-40 has high Sr, and DM91-34 has low Cs, Rb, and Ba compared to most other samples (Fig. 4a). Sample DM91-40 has abundant accessory apatite compared to the other samples but is otherwise similar (Table 1). Rare-earth element patterns in the majority of samples, both gneiss and metawacke, also show patterns similar to the North American Shale Composite (Fig. 4b). Exceptions are cordierite phyllite sample DM91-19 and metawacke sample DM91-34 which have low and erratic light REE, sample DM91-40 which has high heavy REE, and plagioclase-biotite paragneiss sample DM91-01 which has high light REE (Fig. 4b). On the more commonly used chondrite-normalized REE plot (Fig. 4c) all of the samples show elevated light REE, relatively flat heavy REE, and a strong negative Eu anomaly, a pattern typical of many felsic igneous rocks. The differences in a few samples as seen in Fig. 4b are also apparent, but more muted due to the relatively compressed scale of the diagram.

Figure 4. Trace element characteristics of samples from the Kellys Mountain Gneiss and Glen Tosh formation (data from Tables 2 and 3). (a) Selected elements normalized against the North American Shale Composite from Gromet et al. (1984). (b) Rare-earth elements normalized against the North American Shale Composite from Gromet et al. (1984). (c) Rare-earth elements normalized against chondrite data from Sun and McDonough (1989).
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14 On a chemical classification diagram of Herron (1988), all of the samples plot in or near the wacke field (Fig. 5a). They plot in or near the active continental margin field on the depositional setting diagram (Fig. 5b) of Roser and Korsch (1986). Both the TiO 2-Zr and TiO2-Ni diagrams indicate that the sediments are derived from felsic igneous sources (Fig. 5c and d), and the sediments are relatively immature (Fig. 5d). The low Ni that characterizes all of the samples compared to the North American Shale Composite (Fig. 4a) is consistent with derivation from felsic sources (Fig. 5d).

Figure 5. Chemical data for samples from the Kellys Mountain Gneiss and Glen Tosh formation plotted on chemical discrimination diagrams for sedimentary rocks. (a) Chemical classification diagram using Fe2O3/K2O vs SiO2/Al2O3 with fields from Herron (1988). (b) Depositional tectonic setting discrimination diagram using K2O/Na2O vs SiO2 (in weight %) with fields from Roser and Korsch (1986). (c) Provenance discrimination diagram using TiO2 (in weight %) vs Zr (in ppm) with fields from Hayashi et al. (1997). (d) Provenance discrimination diagram using TiO2 (in weight %) vs Ni (in ppm) with fields from Floyd et al. (1989). Trends in (d) are for (1) magmatogenic greywacke, (2) mature recycled mudstone, and (3) mature recycled sandstone.
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15 Sm and Nd isotopic data were obtained from 4 samples from the Kellys Mountain Gneiss and 5 samples from the Glen Tosh Formation (Table 4). The Nd isotope data are expressed following the ε-notation, after correction for the effect of in situ decay of 147Sm, assuming a 650 Ma depositional age. Epsilon values calculated at 500 Ma, the minimum estimate for the age of metamorphism, are also given for comparison (Table 4). The ε Nd(650) show a range from -9.0 to -1.5 but no consistent differences between the two sets of samples; the range of values for the Glen Tosh formation (-9.0/-1.5) is broadly similar to that of the Kellys Mountain Gneiss samples (-7.1/-1.6) (Fig. 6). All but two samples (DM-91-03 and DM-91-11) provide consistent TDM model ages (DePaolo 1981) of ca. 1.4 Ga, interpreted to reflect the average provenance age of the detrital components, without any specific geological significance Two samples, DM-91-03 (Glen Tosh formation) and DM-91-11 (Kellys Mountain Gneiss) depart markedly from the rest of the samples in having distinctly less radiogenic Nd isotope composition (ε Nd(650) = -9.0 and -7.1) and older model ages (1.88 Ga and 1.75 Ga, respectively) that point to a much higher contribution of Early Proterozoic ultimate (bearing in mind the possibility of multiple, “cannibalistic” recycling) sources of the sediments. In contrast, the other samples are all characterized by relatively radiogenic Nd isotope signatures (mildly negative epsilon values) pointing to the relatively significant involvement of “juvenile” components in the source(s) of the detritus.

Figure 6. Plot of εNd against time, with εNd values calculated at 650 Ma (Table 4). The dashed lines are εNd evolution lines, grey for metawacke samples and red for paragneiss samples. Grey shaded area indicates isotopic evolution of Neoproterozoic igneous samples from Avalonia (estimated from Murphy 2002) and overlapping yellow shaded area encloses the field for Neoproterozoic igneous samples in Ganderia estimated from Samson et al. (2000).
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Table 4. Sm-Nd isotopic data*.
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DISCUSSION

16 The chemical and Sm-Nd isotopic similarity between the gneissic (Kellys Mountain Gneiss) and metawacke (Glen Tosh formation) samples is consistent with the interpretation that they represent the same sedimentary unit at different grades of metamorphism. If they represent two unrelated units, or units of different age, then the sediment sources in both cases were similar.

17 The subordinate presence, in both units, of samples containing a major component with an old crustal residence age (TDM = 1.75–1.88 Ga) is an important similarity. It is consistent with the minimum age of ca. 1.84 Ga indicated by the upper intercept age provided by detrital zircons analysed by Keppie et al. (1998) in three samples of Kellys Mountain Gneiss. Because these U-Pb data were based on multigrain analyses (their Table 1, p. 230), it is tentatively suggested that the chord drawn in the concordia plot might represent both a lead loss trajectory and a two-component mixing line between an old (>1.8 Ga) component and a Late Proterozoic (ca. 0.65–0.70 Ga) component. The apparent scarcity of young subconcordant zircons of ca. 0.70–0.65 Ga age (their Fig. 7; p. 228) could reflect the fact that the juvenile component implied by the mildly negative εNd isotope signature corresponds to zircon-poor material (e.g., relatively mafic or intermediate volcanic rocks). This interpretation is supported pelitic paragneiss of inferred volcanogenic origin (Keppie et al. 1998, p. 228) in the Creignish Hills (Fig. 1). Such high Th/U ratios are typical of zircon derived from either mafic or alkaline igneous rocks. However, most chemical characteristics are consistent with a dominantly felsic source (Fig. 5c, d).

18 In summary, Sm-Nd isotope data are consistent with a composite derivation of detrital components having fed the sedimentary basin or basins in which the protoliths of both the Kellys Mountain Gneiss and Glen Tosh formation were deposited. These sediments contained an ancient (ca. 2 Ga) end-member of recycled continental crust mixed with a juvenile component of Late Neoproterozoic age. However, major and trace element data indicate that the sedimentary protoliths were relatively immature and dominated by felsic igneous material, likely derived from an eroding active continental margin of still uncertain age.

19 The tectonic events that led to the near-juxtaposition of the Glen Tosh formation and Kellys Mountain Gneiss, parts of originally the same or similar sedimentary basins, remain enigmatic. On Kellys Mountain, the gneissic and lower grade rocks occur a few kilometres from one another, separated by plutonic rocks that intruded both. The U-Pb ages of 496 ± 5 Ma from titanite in an amphibolite layer in the paragneiss (Dunning et al. 1990), and ca. 495 Ma40Ar/39Ar (cooling) dates from hornblende, muscovite, and biotite in both the amphibolite and gneiss (Keppie and Dallmeyer 1989; Reynolds et al. 1989) indicate that post-metamorphic uplift was occurring in the Late Cambrian. Early to mid-Cambrian metamorphism is suggested by a nearly concordant 206Pb/238U age of 515 ± 1 Ma for monazite from the Kellys Mountain Gneiss reported by Keppie et al. (1998), although they preferred the interpretation (consistent with their data from Creignish Hills and North Mountain) that the metamorphic age was 550 Ma and partially reset during granite emplacement. If so, then the metamorphic muscovite cooling age of ca. 550 Ma from the Glen Tosh formation at Big Hill reported by Dallmeyer and Keppie (1993) supports the interpretation that low- and high-grade metamorphism was synchronous in the Late Neoproterozoic.

20 The high-temperature and low-pressure metamorphic events in the Bras d’Or terrane have been attributed to a within-plate extensional regime that affected a pre-existing active continental margin, but the reason for the scattered distribution of the high-grade rocks and their tectonic juxtaposition with more widespread low-grade rocks such as the Glen Tosh formation remains enigmatic. They could represent ca. 500 Ma core complexes disrupted by subsequent tectonic episodes. Extension in the Bras d’Or terrane, as indicated by within-plate volcanic rocks in the Boisdale Hills, has been attributed to the detachment of the Ganderian microcontinental block from Gondwana (e.g., van Staal and Barr 2012).

CONCLUSIONS

21 The petrographic, major and trace element, and Sm-Nd isotopic data presented here strongly support the interpretation that the Glen Tosh formation and Kellys Mountain Gneiss represent the same sedimentary protoliths, now at different metamorphic grades. Given the similarities among equivalent units elsewhere in the Bras d’Or terrane, it is likely that this is the case everywhere. However, the reason for the contrasting metamorphic histories and subsequent tectonic juxtaposition remains uncertain.

APPENDIX A: Analytical methods

Major and trace elements were analyzed at Acme Analytical Laboratories Ltd., Vancouver, British Columbia. Major elements were determined by X-ray fluorescence after LiBO2 fusion. Rare earth and refractory elements were determined by ICP Mass Spectrometry following lithium metaborate / tetraborate fusion and nitric acid digestion of a 0.2 g sample. A separate 0.5 g split was digested in Aqua Regia and analysed by ICP Mass Spectrometry to obtain precious and base metal data.

Sm-Nd isotope analyses were done by Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) techniques. First, ca. 30 mg of powder samples were decomposed by fusion in an induction furnace with LiBO2 as a fluxing agent, as described by Le Fèvre and Pin (2005), and the resulting melt was dissolved in 1.25M HCl. Then, Sm and Nd were separated from matrix elements and from each other by a procedure adapted from Pin and Santos Zalduegui (1997) combining cation-exchange and extraction chromatography techniques. Sm concentrations were measured with a 149Sm-enriched tracer and an upgraded VG54E mass spectrometer in the single collector mode (Clermont-Ferrand), while Nd concentrations and 143Nd/144Nd isotope ratios were determined concomitantly with a 150Nd-enriched tracer and a Triton TIMS machine operated in the static multicollection mode (GIS Laboratory, Nîmes). Two measurements of the standard of the Japan Geological Survey JNdi-1 made along with the samples provided 143Nd/144Nd ratios of 0.512102 +/- 2 and 0.512099 +/- 3, respectively, the mean of which corresponds to a value of 0.511843 for the La Jolla standard (Tanaka et al. 2000).

ACKNOWLEDGEMENTS

We acknowledge R. Raeside for his major contributions to the understanding of the Bras d’Or terrane. This work was funded in part by a Discovery Grant to S. M. Barr from the Natural Sciences and Engineering Research Council of Canada (NSERC). C. Pin is grateful to Prof. J. Lancelot and Dr P. Verdoux for generous access to the Triton mass spectrometer of GIS Laboratory (University of Nîmes). We thank J. Kim and an anonymous reviewer, as well as editor D. West, for their helpful comments and suggestions which led to significant improvements in the manuscript.

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Editorial responsibility: David P. West