Geochemical, geochronological and isotopic constraints on the origin of members of the allochthonous...

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Precambrian Research 228 (2013) 131– 150

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Precambrian Research

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eochemical, geochronological and isotopic constraints on the origin of membersf the allochthonous Shawanaga and basal Parry Sound domains, Central Gneisselt, Grenville Province, Ontario

.G. Culshawa,∗, T. Slagstadb, M. Raistricka, J. Dostal c

Department of Earth Sciences, Dalhousie University, Halifax NS B3H 4R2, CanadaGeological Survey of Norway, 7491 Trondheim, NorwayDepartment of Geology, Saint Mary’s University, Halifax NS B3H 3C3, Canada

r t i c l e i n f o

rticle history:eceived 17 August 2012eceived in revised form 8 January 2013ccepted 9 January 2013

eywords:renville Provincege of protolithsectonicsentral Gneiss Belt

a b s t r a c t

The Sand Bay and Lighthouse (Lower- and Upper-) and Armer Bay gneiss associations comprise meta-supracrustal rocks within, respectively, the Shawanaga and basal Parry Sound domains – elements ofdeep-crustal thrust sheets of the Central Gneiss Belt of the Grenville Province in Ontario. New and pub-lished geochemical, isotope and detrital zircon data for metavolcanic and metasedimentary rocks of thesegneiss associations suggest origin in one or more late Mesoproterozoic Laurentian margin back-arcs.

Mean εNd values for the Sand Bay, Lower- and Upper Lighthouse gneiss associations and the Armer Baymetavolcanic amphibolites are, with the exception of Sand Bay, close to DM values for their estimatedtime of eruption (+2.8, +5.6, +5.0 and +4.2 respectively). Taken in conjunction with extant and new geo-chemical analyses and field relations these data are compatible with protoliths of all the units originatingin rifted arcs or back-arcs deposited on thin (Sand Bay, Armer Bay) to vanishingly thin (Lighthouse) pre-existing crust. Detrital zircon data indicate the Sand Bay units are younger than ca. 1260 Ma. The LowerLighthouse unit formed after ca. 1234 Ma and metasediments close to the base of the Upper Lighthousegneiss association formed at or after ca. 1350 Ma, whereas deposition age at the top of the unit is esti-mated at less than ca. 1249 Ma. A metasediment layer within the Armer Bay amphibolites has detrital
ages clustered at 1251 Ma, similar to the Upper Lighthouse metasediments. A quartz rich metasedimentfrom the top of Armer Bay gneiss association is estimated to have a maximum age of deposition of ca.1322 Ma. The units represent one or more rifting events of the Andean-type Laurentian margin in a post1.35 Ga episode(s) distinct from that documented in the Muskoka-Ojibway arc at 1.45 Ga. As such theymay be the tectonically reworked equivalents of rifted arcs and back-arcs related to Parry Sound domainarc(s) and related to magmatic units of similar age in the Composite Arc Belt.
. Introduction

In the Grenville Province of Ontario the Central Gneiss BeltCGB) overlies the Grenville Front Tectonic Zone (a Laurentian mar-in derived polycyclic parautochthon), itself tectonically overlyinghe Archean and Paleoproterozoic foreland (Fig. 1). The individual

ajor units of the CGB, ‘domains’, have been interpreted as thrustheets emplaced under upper amphibolite conditions deep within

doubly-thickened orogenic crust (Davidson et al., 1982; Davidson,984; Culshaw et al., 1997; Carr et al., 2000), plausibly during post-hickening nappe-flow (Jamieson et al., 2007). These units present

∗ Corresponding author.E-mail addresses: [email protected], [email protected] (N.G. Culshaw),

[email protected] (T. Slagstad), [email protected]. Raistrick), [email protected] (J. Dostal).

301-9268/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.precamres.2013.01.012

© 2013 Elsevier B.V. All rights reserved.

a fascinating example of deciphering the original tectonic environ-ment of protoliths that have been subjected to extreme tectonicreworking.

An important distinction within the CGB is between thepolycyclic domains (units affected by both Grenvillian and mid-Proterozoic metamorphism and arguably reworked componentsof the ancient Laurentian margin (Carr et al., 2000)) and appar-ently monocyclic domains (units affected only by Grenvilliantectonometamorphism, allochthonous to their polycyclic substratebut which may or may not have been part of the Laurentianmargin). The polycyclic domains, which form a substrate at thelowest structural level of the CGB, were derived from a Paleo- toMesoproterozoic Andean arc on the Laurentian margin, reworked
by ca. 1450 Ma magmatism and high grade metamorphism (Carret al., 2000; Slagstad et al., 2004a, 2009). This polycyclic substrate,itself thrust imbricated, underlies much of the CGB in Ontariowith overlying domains of confidently identified monocyclic
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132 N.G. Culshaw et al. / Precambrian Research 228 (2013) 131– 150

Fig. 1. Location of Central Gneiss Belt (CGB) relative to overlying Composite Arc Belt (CAB) and Archean-Paleoproterozoic foreland. The CGB consists of a large substrateof parautochthonous (relative to Laurentia) gneiss with Paleoproterozoic protolith intruded by ca. 1.45 Ga plutons (polycyclic arcs 1 and 2). These overlie a component ofpolycyclic parautochthonous gneiss with Archean and younger protoliths intruded by 1.24–1.27 Ga plutons. The Parry Sound domain is a thrust sheet dominated by granulitefacies gneiss of predominantly igneous origin; it lies in a thrust stack of amphibolite facies arc-related gneiss (Monocyclic arcs). Geon 12 magmatism is present in thep utochi to the

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arautochthon (1240–1270 Ma pluton); Geon 14 magmatism is present in the paranterpretation of the references to color in this figure legend, the reader is referred

haracter largely restricted to the west of the CGB (Fig. 1) (Carr et al.,000). Domains with a significant Archean component intruded byxtensive 1230–1270 Ma magmatism form a distinct member ofhe polycyclic CGB (Fig. 1) (Easton and Ketchum, 2002).

The CGB is tectonically overridden by the Composite Arcelt (Fig. 1). Plutonic and volcanic magmatism was widespreadhroughout the Composite Arc Belt in the period ca. 1240–1290 Mand amalgamation of the component arcs occurred toward the endf this period when igneous events of similar age are present acrosshe Composite Arc Belt. Younger magmatism (1154–1180 Ma) char-cterizes the Frontenac Terrane lying southeast of the Compositerc Belt (Carr et al., 2000). Detrital zircon ages from the Flintonroup suggest the Composite Arc Belt and the Frontenac Terraneere welded together by ca. 1150 Ma (Sager-Kinsman and Parrish,

993). Likewise the age of earliest deformation along the Compos-te Arc Belt–CGB thrust boundary suggests, in turn, that these units

ere amalgamated tectonically by ca. 1190 Ma (McEachern and vanreemen, 1993).

The Shawanaga and basal Parry Sound domains are allochthonsf clearly monocyclic character lying within the west of the CGBFig. 1). The Sand Bay gneiss association (Shawanaga domain), and
ower and Upper Lighthouse and Armer Bay gneiss associationsbasal Parry Sound domain) are structurally stacked sequences allf which include amphibolites (inferred to be metabasalts) inter-ayered with metasedimentary rocks (Fig. 2). These rock types have
thon and allochthonous polycyclic rocks (brown). See text for further details. (For web version of this article.)

potential to yield key information concerning the pre-orogenictectonic environment of the telescoped Laurentian margin and/orjuvenile material accreted to it. In this paper the composition ofthe Armer Bay amphibolites, metavolcanic members of the ArmerBay gneiss association, are characterized by major, trace elementand isotope (Sm–Nd) geochemistry and the age of their volcanicprotolith is constrained by U–Pb geochronology of detrital zir-cons of associated metasedimentary horizons. A comparable dataset is constructed for the Sand Bay and Lighthouse supracrustalsequences by combining new isotope (Sm–Nd) geochemistry andU–Pb detrital zircon geochronology with published major and traceelement geochemistry (Culshaw and Dostal, 1997, 2002). Synthesisof the data leads to an assessment of the possible role of the units inthe development of the active Laurentian margin before the onsetof the Grenville orogeny.

2. Geological background

2.1. Regional geology – Georgian Bay section

Along the Georgian Bay section of the CGB, the Shawanaga
shear zone, which reworks the Allochthon Boundary Thrust (ABT,Fig. 1) by normal-sense shear, separates foot wall polycyclic- fromhanging wall monocyclic- and polycyclic domains. The ABT car-ries rare, high pressure mafic rocks and more widespread coronitic

N.G. Culshaw et al. / Precambrian Research 228 (2013) 131– 150 133

Fig. 2. Part of southern Parry Sound domain in vicinity of Parry Sound. The internal Parry Sound domain overlies basal Parry Sound domain that has two significants sal Pag tion. S

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upracrustal components: the Armer Bay and Lighthouse gneiss associations. The baneiss association in tectonic contact with the supracrustal Sand Bay gneiss associa

dapted from Culshaw et al. (2004).

abbros in its hanging wall that are absent from foot wall domains.he monocyclic domains, in which metamorphism is restricted tohe interval ca. 1160–1000 Ma (monocyclic arcs in Fig. 1) includehawanaga and Parry Sound domains featured in this study. Withhe exception of the Parry Sound domain the monocyclic domainsomprise predominantly amphibolite facies ortho- and parag-eisses that are interpreted to have originated in ca. 1500–1350 Maontinental magmatic arcs (Carr et al., 2000; Rivers, 1997; Slagstadt al., 2004a) (Fig. 1). The Parry Sound domain, differs in that gran-lite facies gneisses are important components (Fig. 2). The basalarry Sound domain contains a mixture of granitoid orthogneiss,northosite, and rocks of supracrustal origin. The interior Parryound domain is dominantly granulite facies orthogneiss (granitic,hrough intermediate to mafic compositions) with subordinate

etasupracrustal rocks, some of which may have a had a volcani-lastic hydrothermally altered protolith (Marsh et al., 2012). Ages ofranitoids in the basal Parry Sound domain span ca. 1330–1390 MaWodicka et al., 1996). A granitoid age in interior Parry Soundomain is recorded at 1314 Ma (Wodicka et al., 1996) and detri-
al zircons from volcaniclastics lie in the range 1232–1320 MaMarsh et al., 2012). Igneous ages for Parry Island and Whitestonenorthosites are, respectively, 1163 and 1350 Ma (Wodicka et al.,996; van Breeman et al., 1986).
rry Sound domain overlies the Shawanaga domain containing the plutonic Ojibwayample locations are indicated.

Orogenic evolution of the allochthonous monocyclic domainsinvolve building the Parry Sound domain nappe stack at granuliteand amphibolites facies before or in the early stages of northwesttransport of the monocyclic thrust sheets (Wodicka et al., 1996).Building the stack began at ca. 1160 Ma and continued until ca.1120 Ma (Wodicka et al., 2000; Culshaw et al., 2010; Marsh et al.,2011) and was followed by union with subjacent domains, includ-ing Shawanaga and cratonward transport at amphibolite faciescontinuing through the interval 1100–1036 Ma (Tucillo et al., 1992;Heaman and Le Cheminant, 1993; Bussy et al., 1995; Ketchum andKrogh, 1997; Krogh, 1997; Slagstad et al., 2004b; Krogh and Kwok,2005; Marsh et al., 2011).

2.2. Shawanaga domain – Sand Bay gneiss association

The Sand Bay and Ojibway gneiss associations together forma refolded nappe which underlies the Shawanaga domain alongGeorgian Bay. The Ojibway gneiss association is a plutonic mem-ber of the ca. 1.45 Ga Muskoka-Ojibway arc (Slagstad et al., 2004a,
2009). The Sand Bay gneiss association comprises a predominantlypotassic quartzofeldspathic migmatite, plagioclase-quartz-biotiteschistose gneiss (Dillon schist), amphibolite with calc-silicate hori-zons and quartzite. The major- and trace-element geochemistry

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f the potassic migmatite and amphibolite are compatible witholcanic protoliths, respectively, rhyolite and continental tholei-te, interpreted to be members of a bimodal suite erupted on riftedontinental crust (Culshaw and Dostal, 1997). Other members areerived from sedimentary rocks ranging from relatively maturelastic rock (quartz-rich wacke) to locally derived, epiclastic sedi-ent (Dillon schist) (Culshaw and Dostal, 1997).

.3. Basal Parry Sound domain – Lighthouse gneiss association

The Lighthouse gneiss association was originally defined withinhe Shawanaga domain (Culshaw and Dostal, 2002; Fig. 1) butater was assigned to the basal Parry Sound domain together withimilar rocks structurally overlying the studied section (Culshawt al., 2004). This package extends into the interior of Parry Islandhere it includes metasedimentary gneiss, schist and amphibo-

ite of Wodicka et al. (1996) as well as a quartz-rich metasedimentnd three gneiss units of plutonic protolith dated by these authors.hese lithologies continue, much attenuated, into the lower parts ofhe Parry Sound Shear Zone on the mainland northwest and northf the town of Parry Sound (Fig. 2).

Close to its contact with the underlying Shawanaga domain athe west end of Parry Island, in the interval studied by Culshaw andostal (2002), the Lighthouse gneiss association is an amphibolitend metasediment dominated assemblage. In this section an uppernit (upper Lighthouse gneiss association) consists of thick inter-als of amphibolite, lacking relict plutonic igneous textures andhus interpreted as mafic flows, thinly interlayered and laminatedmphibolite and metasediment, interpreted to have originated asafic tuffs interlayered with sediment with a quartzofeldspathic

ource, and thick horizons of semipelite or regularly interlayeredsammites and pelites, interpreted as metamorphosed greywacke-hale turbidites shed from a quartzofeldspathic source. A lower unitlower Lighthouse gneiss association) has a much higher propor-ion of thick amphibolite, interpreted as mafic flows, interlayeredith subordinate semipelitic metasediments. Geochemically, bothpper and lower amphibolites are olivine-normative tholeiites butave distinct trace element characteristics. The mantle-normalizedrace element pattern of amphibolite in the upper unit lacks aubduction imprint and suggests an asthenospheric source. Tracelement patterns in the lower amphibolite have a negative Nbnomaly subduction signature (Culshaw and Dostal, 2002).

.4. Basal Parry Sound domain – Armer Bay gneiss association

The Armer Bay gneiss association (Culshaw et al., 2004; approx-mately equivalent to garnet amphibolite and migmatitic gneiss of

odicka et al., 1996) forms the largest supracrustal unit recognizedn the basal Parry Sound domain. The boundaries of the Armer Bayneiss association are well defined on the mainland where the unities sandwiched between the two principal anorthosite bodies out-ropping in the environs of the town of Parry Sound (Fig. 2). Thermer Bay gneiss association underlies much of Parry Island eastf the Parry Island anorthosite where it is bounded to the southy the Isabella Island granodiorite and the Whitestone anorthosite.ts inferred northern (lower) boundary with the Lighthouse gneissssociation lies in the interior of Parry Island (Fig. 2). On Parry Island

tonalitic orthogneiss layer lies within the Armer Bay gneiss asso-iation and extends southwestward in close proximity to the uppersoutheastern) boundary of the Parry Island anorthosite where itsrystallization age has been determined as 1332 ± 3 Ma (Wodickat al., 1996). Quartz rich metasediment is associated both with the
pper boundary of the Parry Island anorthosite in the southwest,he upper boundary of the tonalitic orthogneiss on Parry Island andhe lower boundary of the Whitestone anorthosite on the mainlandFig. 2).
search 228 (2013) 131– 150

On the mainland, the location of the section sampled in thisstudy, the estimated thickness of the Armer Bay gneiss associa-tion is ca. 0.75–1 km, the same order of magnitude as thicknessesmeasured for the Sand Bay and Upper and Lower Lighthousegneiss associations (Culshaw and Dostal, 1997, 2002). The ArmerBay Amphibolites (ABA) dominate this section. They are inter-preted to be metavolcanic rocks on the basis of the mediumgrained, equigranular texture (i.e. absence of deformed plutonictextures), variably developed compositional layering and the pres-ence of quartzofeldspathic horizons interpreted as metamorphosedpsammitic-sedimentary or epiclastic rocks. However, parallelismof tectonic fabric to compositional and textural layering, and boud-inage of pegmatite dykes indicates substantial structural thinning.On Parry Island outcrop width is greater and perhaps more repre-sentative of original due to lesser structural thinning.

As mentioned, the ABA display a poorly defined but through-going decimeter-scale compositional layering, in places empha-sized by garnet bearing lenses of unknown origin. Medium grainedmetamorphic textures within the layering are formed predomi-nantly by clinopyroxene, hornblende and plagioclase with variableamounts of garnet. White plagioclase-bearing, centimeter thick,cross-cutting or layering-concordant leucosomes form ca 5% orless of the amphibolite. The quartzofeldspathic intervals inter-preted as meta-psammites, epiclastic rocks, are up to 1.5 m thickand make up ca. 10–15% of the section. The meta-psammites aremedium grained and consist of microcline, quartz, biotite and pla-gioclase with accessory opaques, muscovite, titanite, garnet andcarbonate.

3. Geochemistry of Armer Bay amphibolites

3.1. Results

Seventeen samples of ABA were collected from a section alongHighway 69 (now remodeled as Highway 400 with outcrops sam-pled in this study altered or destroyed) at locations shown in Fig. 2.The geochemistry of the rocks is similar to modern volcanic rocksas supported by Zr/TiO2–Nb/Y and SiO2–Zr/TiO2 plots which placethem in the basalt field similar to both Sand Bay and Lighthousegneiss association amphibolites (Fig. 3). The ABA have compositionsranging from continental tholeiites to MORB basalts on a TiO2–Y/Nbplot (Fig. 3) with SiO2 (LOI-free) ranging between 44 and 50 wt.%and Mg# between 33 and 58 (100× MgO/[MgO + FeOtot] in mol%)(Table 1). The similarity to modern volcanic rocks is also shownby the relative uniformity of major element compositions (Table 1)and the values of most Cr/Th and Th/La ratios at >100 and ca. 0.1respectively. Further support is given by the covariance of elementsincluding high field strength and transition elements with Mg #(Fig. 4). This confirms the conclusion reached from the field rela-tions that these rocks are metamorphosed equivalents of basalticrocks and also supports the immobility of these and related ele-ments during metamorphism. Thus our analysis of the geochemicaldata is primarily based upon elements more likely to be relativelyimmobile during metamorphism (e.g. REE and HFSE and selectmajors) but treats with more caution those of the major and traceelements that could be mobilized during metamorphic processes(e.g. Ca, Na, K, Cs, Rb, Ba, P, U).

More specifically, the ABA plot as within plate basalts as shownon a Ti–Cr plot and back arc basalts on a Ti–V plot (Fig. 5). Theamphibolites display a correlation of major and trace elementswith differentiation showing elongate trends which may indicatea cogenetic suite modified by fractional crystallization. The amphi-
bolites show a decrease of Mg, Ca, Cr, Ni, V, Al and Mg withincreasing differentiation but an increase in Fe, Ti, Mn and allconsidered incompatible trace elements and REEs (Table 1 andFig. 4). The Fe and Ti trends show these rocks are tholeiitic, that

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Table 1Major and trace element composition of selected representative samples of amphibolites, Armer Bay gneiss association, Parry Sound highway section.

Sample AB-1 AB-2 AB-3 AB-5 AB-6 AB-8 AB-10 AB-11 AB-12 AB-13 AB-14 AB-15 AB-23 AB-24 AB-16 AB-18 AB-19

SiO[2] 48.94 46.84 44.03 46.88 46.40 46.57 46.36 47.47 46.24 45.90 46.23 46.94 46.08 46.85 48.56 47.79 50.35TiO[2] 2.22 2.41 2.76 2.64 1.34 1.47 1.49 2.06 1.16 2.24 2.13 1.96 1.59 1.53 2.29 2.41 1.86Al[2]O[3] 13.42 13.03 15.15 14.32 16.61 15.08 15.64 13.94 18.07 13.51 14.17 14.42 14.93 15.03 14.02 13.36 14.08Fe[2]O[3] 16.58 17.66 16.87 16.34 12.72 14.92 13.33 16.90 11.52 16.75 16.02 15.04 13.70 13.08 15.80 16.63 13.88MnO 0.24 0.25 0.31 0.26 0.19 0.22 0.19 0.25 0.17 0.25 0.24 0.22 0.21 0.19 0.23 0.23 0.20MgO 4.30 5.40 4.20 5.88 7.94 7.55 8.49 6.77 8.18 6.51 6.84 6.76 8.24 8.24 5.21 6.29 6.29CaO 9.26 9.48 9.57 9.53 8.47 10.41 10.58 9.26 10.84 10.77 10.43 10.15 10.75 10.55 9.44 9.50 9.66Na[2]O 3.36 3.06 3.62 2.61 3.02 2.70 2.85 2.60 3.11 2.68 3.04 3.11 2.79 3.08 3.13 2.82 2.79K[2]O 0.95 1.02 1.33 0.65 2.11 0.62 0.68 0.82 0.58 0.82 0.66 0.64 0.81 0.43 0.74 0.57 0.66P[2]O[5] 0.48 0.40 1.10 0.57 0.25 0.18 0.19 0.34 0.15 0.31 0.23 0.28 0.20 0.19 0.37 0.33 0.20LOI 0.58 0.42 0.61 0.49 0.81 0.46 0.63 0.27 0.56 0.49 0.20 0.61 0.67 0.74 0.77 0.15 0.39Total 100.33 99.97 99.54 100.18 99.87 100.18 100.43 100.67 100.58 100.23 100.19 100.13 99.97 99.91 100.56 100.09 100.37Mg# 33.93 37.72 33.02 41.61 55.28 50.05 55.78 44.24 58.44 43.49 45.82 47.09 54.36 55.51 39.51 42.83 47.30Cr (ppm) 56 63 16 143 203 162 130 81 89 122 121 109 121 109 75 100 174Ni 144 57 21 58 167 109 177 90 118 68 69 82 103 110 52 61 83Co 67 71 54 65 58 69 62 72 64 69 66 66 60 60 67 68 60V 358 377 319 357 204 254 231 323 185 332 317 289 247 239 350 347 309Cu 0 21 20 48 20 35 0 7 29 83 10 41 13 18 50 58 98Zn 179 180 203 268 149 130 123 159 110 156 164 149 142 113 172 153 115Rb 4 4 11 0 42 2 3 5 5 7 4 3 5 4 7 2 2Ba 631 591 673 366 379 386 367 457 300 528 560 181 394 368 535 552 414Sr 290 265 337 237 311 268 316 325 378 322 309 349 309 353 435 279 267Ta 0.75 0.53 0.35 0.00 0.25 0.18 0.15 0.24 0.00 0.00 0.00 0.25 0.17 0.00 0.00 0.00 0.15Nb 10.0 6.7 13.5 8.0 9.5 3.9 3.1 7.3 6.0 9.0 4.0 7.2 4.6 6.0 5.0 11.0 4.1Hf 6.30 5.31 7.31 0.00 2.46 2.67 2.86 4.19 0.00 0.00 0.00 3.75 2.19 0.00 0.00 0.00 4.12Zr 218 173 240 226 83 85 94 146 89 149 144 133 78 101 195 152 142Y 46 40 57 22 27 26 22 35 17 17 17 30 22 17 25 18 34Th 2.01 0.99 3.49 5.00 0.80 0.33 0.78 0.66 1.00 8.00 6.00 0.90 0.46 2.00 8.00 8.00 1.32La 18.71 14.14 26.71 45.00 8.64 5.10 6.02 9.55 21.00 37.00 37.00 10.01 6.52 26.00 44.00 41.00 9.04Ce 45.35 34.59 64.74 0.00 21.25 13.68 15.90 24.92 0.00 0.00 0.00 24.69 16.41 0.00 0.00 0.00 23.17Pr 6.83 5.40 9.91 0.00 3.27 2.30 2.58 4.05 0.00 0.00 0.00 3.83 2.60 0.00 0.00 0.00 3.62Nd 33.52 27.69 49.31 51.00 16.51 12.33 13.34 20.65 31.00 46.00 45.00 19.28 13.41 36.00 49.00 49.00 18.19Sm 8.02 7.06 11.60 0.00 4.26 3.66 3.71 5.65 0.00 0.00 0.00 5.12 3.63 0.00 0.00 0.00 5.12Eu 2.33 2.25 3.70 0.00 1.42 1.29 1.37 1.87 0.00 0.00 0.00 1.76 1.31 0.00 0.00 0.00 1.67Gd 9.73 8.90 13.75 0.00 5.51 4.73 4.65 6.81 0.00 0.00 0.00 6.25 4.58 0.00 0.00 0.00 6.43Tb 1.56 1.44 2.12 0.00 0.93 0.80 0.73 1.11 0.00 0.00 0.00 0.99 0.71 0.00 0.00 0.00 1.07Dy 10.26 9.21 13.37 0.00 6.18 5.27 4.63 7.15 0.00 0.00 0.00 6.40 4.73 0.00 0.00 0.00 7.01Ho 2.40 2.12 3.05 0.00 1.42 1.16 1.00 1.57 0.00 0.00 0.00 1.37 1.02 0.00 0.00 0.00 1.55Er 7.14 6.18 9.03 0.00 4.20 3.38 2.83 4.60 0.00 0.00 0.00 3.97 2.92 0.00 0.00 0.00 4.52Tm 1.03 0.90 1.28 0.00 0.61 0.50 0.42 0.66 0.00 0.00 0.00 0.57 0.41 0.00 0.00 0.00 0.65Yb 6.59 5.65 8.03 0.00 3.90 3.22 2.63 4.32 0.00 0.00 0.00 3.55 2.69 0.00 0.00 0.00 4.21Lu 0.91 0.77 1.11 0.00 0.52 0.45 0.37 0.63 0.00 0.00 0.00 0.50 0.38 0.00 0.00 0.00 0.59Ti/V 37.29 38.40 51.82 44.37 39.53 34.68 38.81 38.25 37.65 40.45 40.32 40.67 38.67 38.51 39.24 41.76 36.17Zr/Y 4.77 4.33 4.19 10.27 3.09 3.29 4.21 4.19 5.24 8.76 8.47 4.45 3.55 5.94 7.80 8.44 4.20Th/La 0.11 0.07 0.13 0.11 0.09 0.06 0.13 0.07 0.05 0.22 0.16 0.09 0.07 0.08 0.18 0.20 0.15

Mg #, Mg/(Mg + Fe); samples collected traversing ca. 2 km SE–NW (left to right in table) along former Highway 69.


Fig. 3. Discrimination diagrams for amphibolites of the Armer Bay gneiss association. (a) Supports basalt protolith; (b) indicates presence of two distinct groups of basalticr e andc

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ocks; fields outlined in green and red are compositions of, respectively, Lighthousolor in this figure legend, the reader is referred to the web version of this article.)

f Al reflects plagioclase crystallization, Ni and Cr marks olivinend pyroxene fractionation and the Zr trend tracks increase ofncompatible elements with differentiation. In summary, theserends are representative of tholeiitic magmas crystallizing olivine,yroxene and plagioclase and lacking crystallization of Fe–Tixides.

Variations of certain element ratios shown by a distinct groupingn some XY plots including Zr, Y, La, Nb and Nd suggest differencesn mantle source or even tectonic environment for the two sub-roups of the ABA. The subgroups are identified on discriminationiagrams as having, respectively, strongly MORB- or within-plate-rc affinities (e.g. Fig. 3b, inset).

The ABA have quite primitive mantle-normalized REE abun-ances with (La/Yb)n between 1 and 2 with most in the mid-range∼1.5) (Fig. 6). The absolute REE abundances vary and those withigher abundances tend to greater (La/Yb)n ratios compatible with

Sand Bay gneiss association amphibolites. (For interpretation of the references to

the increase in REE with differentiation (and correlating withgrouping in Fig. 3b, inset). A modest negative Eu anomaly insome samples also attests to effective low pressure differentia-tion. These patterns are comparable to those of some Cenozoiccontinental tholeiitic basalts (e.g. Lightfoot and Hawkesworth,1988; Govindaraju, 1994) and those of the Sand Bay amphibo-lites (Culshaw and Dostal, 1997). The mantle-normalized traceelement patterns show a minor enrichment in incompatible traceelements. Several samples show a negative Nb anomaly rela-tive to La and Th suggesting a source affected by subductionprocesses.

In comparison with amphibolites from Sand Bay and Lighthouse
gneiss associations it is noteworthy that the ABA cluster with allof them on discrimination diagrams (with the noted exceptions)with a predominant MORB affinity and less marked within-plateand arc affinities (mirrored by the previously mentioned two-fold


phibo

gmLaMchw

4

dami

Fig. 4. XY diagrams for Armer Bay gneiss association am

rouping). They resemble Sand Bay amphibolites most closely onulti-element primitive mantle-normalized plots and, like lower

ighthouse and Sand Bay amphibolites, several have a negative Nbnomaly. Those that do not have a negative Nb anomaly are of theORB-like group. In terms of chondrite-normalized REE plots they

an be described as transitional between Sand Bay and upper Light-ouse amphibolites (i.e. arc or within-plate related contrastingith MORB-like).

. Geochronology

Eight samples were taken for LA-ICP-MS geochronology of
etrital zircons from Armer Bay, Lighthouse and Sand Bay gneissssociations. Those from the Lighthouse gneiss association comple-ent TIMS samples (Raistrick, 2003). Sample locations are shown
n Fig. 2.

lites. Oxides and elements plotted against Mg Number.

4.1. Results

Results are discussed in top-to-bottom tectonostratigraphicorder of samples and units: Armer Bay, upper and lower Lighthouse,Sand Bay.

4.2. Armer Bay: PS07

This sample is a quartz rich rock with lesser amounts of plagio-clase and biotite and accessory garnet. Although its position withina shear zone may suggest a hydrothermal origin it is interpretedto be a quartzite (transitional to a meta-wacke) on account of its
similarity with other quartz-rich units of unequivocal sedimentaryorigin (on account of their composition and those of spatially asso-ciated units) that do not share such tectonic location (e.g. in SandBay gneiss association and elsewhere in bPSD). The Th/U ratios of


lites; fi

tibA

pgmtt(l

Fig. 5. Discrimination diagrams for Armer Bay gneiss association amphibo

he grains and the spread of zircon ages are also suggestive of a sed-mentary origin. It was sampled from an isolated horizon locatedetween the base of the Whitestone anorthosite and the top of thermer Bay Amphibolites.

The zircons from this sample are between 100 and 300 �m,rismatic and euhedral to slightly rounded (Fig. 7e), the latter sug-esting rather limited transport, belying the relative maturity of theetasediment implied by the quartz-rich mineralogy. The grains

ypically have oscillatory-zoned to complexly zoned cores that areruncated by rims of oscillatory or sector-zoned prismatic zirconFig. 7e). In addition there are some oscillatory-zoned grains whichack cores.

elds shown for Lighthouse and Sand Bay gneiss association amphibolites.

Sixty-three LA-ICP-MS analyses were made from 43 grains. 44of 63 analyses are <10% discordant and yield 207Pb/206Pb agesbetween 1175 and 2682 Ma (Fig. 8), with one major peak at ca.1320 and another significant population between 1600 and 1900,in addition to a few older Paleoproterozoic and Archean grains.The analyses yield Th/U ratios between 0.09 and 1.24, consistentwith their magmatic morphology. The analyses can be groupedin two based on zircon morphology, discussed above: analy-
ses of cores yield 207Pb/206Pb ages between 1264 and 2682 Ma(Fig. 8), whereas the oscillatory-zoned, prismatic rims and grainsappear to define a discordia line with an upper intercept ageof 1322 ± 15 Ma, anchored at 0 ± 100 Ma. Two analyses were

N.G. Culshaw et al. / Precambrian Re

Fg

ir

wtttrooB

4

sst

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2r2

ig. 6. Primitive mantle normalized spider diagram and REE plot for Armer Bayneiss association amphibolites; the two groups are distinguished.

dentified as statistical outliers by Isoplot, and excluded from theegression.

The Paleoproterozoic and Archean grains are exclusively coresith 1322 Ma rims, suggesting that cores of these grains represent

he crustal source of the igneous rocks that were eroded to producehe detrital zircon grains. One possible interpretation is deposi-ion near a 1322 Ma arc founded on Laurentian crust. 1322 Ma isegarded as the maximum age of deposition. The relative richnessf the sample in Laurentian grains is consistent with the maturityf the sedimentary protolith compared to most other samples (saveT1, see below).

.3. Armer Bay: AB-37

Sample AB-37 is typical of the ubiquitous but volumetricallyubordinate concordant layers of feldspar and biotite dominatedemi-pelitic gneiss within the Armer Bay amphibolites and inferredo have an epiclastic protolith.

Zircons from sample AB-37 are generally between 100 and50 �m, stubby to prismatic, and from little to moderately roundedFig. 7d). The zircons can be classified in two populations basedn internal characteristics: (1) oscillatory-zoned grains, commonlyut not always overgrown by (2) CL-dark rims of variable thick-ess; some CL dark grains without visible cores are classified withhe second population.

Sixty-one LA-ICP-MS analyses were made from 59 grains (Fig. 8).6 of 46 analyses of oscillatory-zoned cores and grains, with Th/Uatios between 0.026 and 0.575, are <10% discordant and yield07Pb/206Pb ages between 1161 and 1528 Ma, but with a single

search 228 (2013) 131– 150 139

peak at 1285 Ma (Fig. 8). Although the peak is relatively broad, thedata do appear to define a discordia with an upper intercept ageof 1251 ± 17 Ma, anchored at 0 ± 100 Ma. One concordant analy-sis at 1396 Ma was identified as a statistical outlier by Isoplot, andwas excluded from the regression, as was a concordant analysis at1528 Ma, which clearly plots off the discordia; the two analyses areindicated in Fig. 8. We interpret the age of 1251 ± 17 Ma to repre-sent the age of the main sediment source, with minor sedimentaryinput from ca. 1400 and 1500 Ma sources, indicating proximity toLaurentian margin crust or a monocyclic arc such as Muskoka-Ojibway arc (Slagstad et al., 2009). We also regard 1251 Ma as themaximum age of deposition.

15 analyses were made of CL-dark rims and grains, some ofwhich are faintly zoned. These analyses have Th/U ratios mainly<0.05, with three analyses having higher Th/U ratios at 0.13–0.41.These compositional characteristics support the interpretation thatthese CL-dark components are metamorphic. Excluding 4 outliers,the remaining 11 analyses define a discordia line with an upperintercept age of 1156 ± 26 (Fig. 8), interpreted as the age of meta-morphism, consistent with metamorphic ages determined locallywithin PSSZ (van Breeman et al., 1986; Krogh and Kwok, 2005).

4.4. Upper Lighthouse: MR01D

This garnetiferous quartzofeldspathic psammite is from inter-layered psammite-semipelite from the uppermost part of theLighthouse gneiss association section illustrated in Culshaw andDostal (2002). The layer from which the sample was taken isless than 1.5 m thick and is sandwiched between massive, rusty-weathering amphibolite layers. As it is a compositionally immaturefeldspar-biotite-quartz psammite, this layer, like the other Light-house gneiss association metasedimentary rocks, was probably anepiclastic rock derived from a local source.

The zircons from this sample are quite small, generally between50 and 75 �m, elongate and rounded (Fig. 7b). Internally, thezircons are characterized by oscillatory-zoned cores with thick (rel-ative to the cores), 10–20 �m CL-dark rims that truncate the zoningin the cores. The rims are too thin to analyze by LA-ICP-MS, so theanalyses targeted the cores; however, in many cases spots tran-sect both cores and rims, but with a dominant proportion of corematerial.

Seventy-five analyses from 75 grains were made by LA-ICP-MS;60 of these are <10% discordant and yield 207Pb/206Pb ages between1035 and 2369 Ma (Fig. 9). 58 analyses are Mesoproterozoic withages between 1035 and 1511 Ma, with major peaks at ca. 1265 and1340 Ma. Th/U ratios range from 0.03 to 0.76 and there is a weakcorrelation between age and Th/U ratio, with young ages generallyhaving lower Th/U. We interpret this relationship to reflect variableproportions of rim material, of inferred metamorphic origin, in theanalyses. The 3 youngest analyses (1035–1107 Ma, analyses #46, #49 and #50) fall in the lower range of Th/U values (0.03–0.07). CLimages also show that these analyses represent a mix of oscillatory-zoned core and CL-dark, inferred metamorphic rim (Fig. 7b). Weinterpret these analyses as mixed analyses consisting of both coreand rim material, and therefore exclude them from further discus-sion. The 22 youngest, most concordant analyses yield a concordiaage of 1249 ± 10 Ma, which we consider as the maximum age ofdeposition.

Four grains from this sample were analyzed by TIMS (Fig. 9). TheTIMS analyses vary from mildly discordant to concordant and from1847 ± 2.4 Ma (Z1) to 1328 ± 2.3 Ma (Z2). In the sample there is nocorrelation between U content and discordance, U content and age,
or age and grain size (Table in supplementary data). A 1435 ± 7 Magrain with small inclusions (Z3) is concordant and represents a con-tribution from a source of this age. The oldest zircon, a smoky ovoidgrain (Z1) dated at 1847 ± 2.4 Ma is 2.6% discordant and may have


Fig. 7. CL images of zircon grains from metasedimentary gneiss interlayered with amphibolite of Armer Bay, Lighthouse and Sand Bay gneiss associations. Shown are: spotnumbers, 207/206 ages with 1 sigma uncertainties and a 100 �m scale bar. Analyses that are more than 10% discordant are labeled “discordant”. There are a few exceptionsf es, th

fiai

Lttsgs

or samples AB-37 and PS07, in order to highlight core-rim differences; in these cas

ormed at 1882 Ma if ca. 1050 Ma Pb-loss is assumed or at 1892 Maf Pb-loss was at 1160 Ma. The near-concordant analysis Z4 (0.5%)t ca. 1338 Ma and analysis Z2 at ca. 1328 Ma coincide with peaksn the LA-ICP-MS data set.

The data from sample MR01D overlap with those of lowerighthouse gneiss association sample MR01C (see below), althoughhe peak in the probability density diagram is clearly skewed
oward younger ages in MR01D. This indicates similar sedimentources and depositional settings, with sediment input from a sin-le, dominant source in both cases, but not necessarily the sameource.
e discordance is indicated after the age.

4.5. Upper Lighthouse: MR01G

Sample MR01G was collected from the lower part of the sameupper unit as MR01D. Outcropping as a 0.5 m thick, yellowish,leucosome-free homogenous layer, MR01G is a semipelite domi-nated by alkali and plagioclase feldspar, and biotite and interpretedas an epiclastic rock (Culshaw and Dostal, 2002).

The zircons from MR01G are ca. 100 �m, with an oval,rounded shape. CL-images reveal a very characteristic interior,consisting of oscillatory-zoned cores with ca. 10–20 �m, CL-bright rims (Fig. 7c); the CL-bright rims clearly truncate the


Fig. 8. Concordia and probability-age diagrams for detrital zircons from metasediments of the Armer Bay gneiss association. On the probability-age diagrams the ratio ofconcordant to discordant analyses is indicated by light shading (all analyses) and dark shading (all <10% discordant).

iM

ogacs

laartZegaaarmoM

adyegfa

and upper intercept of 1384 +21/−11 Ma. The lower intercept couldreflect Pb-loss during regional amphibolite facies metamorphism

nternal zoning, but are too narrow to analyze with the LA-ICP-S.Fifty analyses from 50 grains were made with LA-ICP-MS;

f these, 42 analyses are <10% discordant and yield a sin-le population at 1350 Ma (Fig. 9). A discordia through all thenalyses and anchored at 0 ± 100 Ma, yields an upper inter-ept age of 1350 ± 7 Ma, considered the age of the sedimentource.

Five high-quality zircon grains from this sample were ana-yzed by TIMS (Fig. 9). They were chosen from the 0◦ magneticnd 0◦ non-magnetic fractions, to reflect morphological vari-bility, size, and variations in internal appearance. The agesange from a slightly discordant grain at 1458 ± 4.2 Ma (Z1)o four concordant to slightly discordant grains (Z2, Z3, Z4,5) that cluster around 1330 Ma (Table 2). Although the old-st grain is the biggest, there is no strong correlation betweenrain size and age (table in supplementary data). Grain Z1t 1458 ± 4.2 Ma and Z2 at 1354 ± 3.1 Ma have similar weightsnd morphologies despite ca. 100 My age difference. Grains Z4nd Z5 have a weighted mean of 1329.5 ± 12.5 Ma which likelyepresents a maximum depositional age. Z2 at 1354 ± 3.1 Maay be a discordant grain of ca. 1385 Ma vintage as it falls

n the 1385–1050 Ma discordia line generated for sampleR01C.The age of 1350 Ma obtained from the LA-ICP-MS analyses,

nd supported by the TIMS data, is similar to age of peaks in theata from samples MR01C (see below) and MR01D, but older andounger grains, as observed in these samples, are missing entirely,xcept for one grain analyzed by TIMS at 1458 Ma. These data sug-est input from a dominant, single source, with very limited input
rom older crustal (Laurentian or Laurentian margin monocyclicrc) sources.
4.6. Lower Lighthouse: MR01C

Sample MR01C, from the central part of the lower unit of theLighthouse gneiss association, is part of a homogenous three-meter thick alkali feldspar-biotite-quartz semipelite sandwichedbetween amphibolite layers. Leucosomes, present elsewhere in thelower unit, are absent from the area of the sample.

Zircons from this sample (Fig. 7a) are rather small, typicallybetween 80 and 120 �m, slightly prismatic to equidimensional androunded. The grains are generally oscillatory zoned, consistent withan igneous origin, and some grains have thin, CL-dark rims that insome cases truncate the internal, oscillatory zoning. The LA-ICP-MSanalyses targeted the oscillatory-zoned zircons.

Seventy-five analyses from 75 detrital grains were made by LA-ICP-MS (Fig. 9). 55 of the analyses are <10% discordant and yield207Pb/206Pb ages between 1113 and 2553 Ma (Fig. 9), although themajority (45) of analyses are Mesoproterozoic with ages between1450 and 1200 Ma. The largest peaks in the LA-ICP-MS data set areat ca. 1310 and 1370 Ma. Measured Th/U ratios range from 0.08 to1.51, and indicate that the grains are of magmatic (detrital) ratherthan metamorphic origin, as suggested by the CL images.

9 TIMS analyses from this sample yield a narrow range of con-cordant to slightly discordant (Table 2) ages between 1384 ± 3.7 Maand 1342 ± 6.4 Ma, overlapping with the broad peak displayed bythe LA-ICP-MS data (Fig. 9). Th/U ratios range from 0.41 to 0.62,suggesting that the detrital grains are of igneous origin. Three dis-cordant grains (Z2, Z6, Z7), and two concordant analyses (Z8, Z9)lie on a discordia with a lower intercept of 1053 +120/−126 Ma

at ca. 1050 Ma (Tuccillo et al., 1992; Bussy et al., 1995; Krogh, 1997;Timmermann et al., 1997; Wodicka et al., 2000; Slagstad et al.,


F from

g the rt

2m1g

atmLvos

ig. 9. Concordia and probability- and frequency-age diagrams for detrital zircons

reen. Shading on probability-age diagrams same as in Fig. 8. (For interpretation ofhis article.)

004b). Four grains (Z1, Z3, Z4, and Z5) appear to have sufferedore recent Pb loss, and with Z8 and Z9, lie on a discordia between

384 +4/−3 Ma and 202 +209/−236 Ma, possibly resulting fromradual diffusive Pb-loss.

The youngest LA-ICP-MS analysis is 1% reversely discordantnd yields a 207Pb/206Pb age of 1114 ± 32 Ma. However, becausehis is a lone analysis we do not consider it a reliable maxi-

um age of deposition. Apart from this analysis, the 14 youngest
A-ICP-MS analyses yield a concordia age of 1234 ± 12 Ma, pro-iding a more reliable maximum age of deposition. The agesbtained from this sample indicate input mainly from juvenileources with ages between ca. 1450 (i.e. Laurentian margin)
metasediments of the Lighthouse gneiss association. TIMS analyses on histograms,eferences to color in this figure legend, the reader is referred to the web version of

and 1200 Ma, but with significant input from Laurentian cratonicsources.

4.7. Sand Bay: BT2

This sample is a plagioclase-biotite rich semi-pelite typical ofthe Dillon Schist member of the Sand Bay gneiss association andinferred to have a locally sourced epiclastic protolith (Culshaw and
Dostal, 1997).
The zircons from sample BT2 are typically between 100 and150 �m, subhedral and stubby to prismatic. Cl images reveal rathercomplex internal zoning patterns, characterized by rounded to


Table 2Sm–Nd data for selected amphibolites of Parry Sound and Shawanaga domains.

Gneiss association Sample Nd (ppm) Sm (ppm) 147Sm/144Nd 143Nd/144Nd 2� Age (then) εNd @ t Tdm (De P)

AB AB 02 22.92 6.524 0.1649 0.512591 5 1.25 4.23 1403AB 03 40.11 10.39 0.1566 0.512489 4 1.25 3.58 1468AB 08 11.80 3.608 0.1849 0.512772 5 1.25 4.53 1418AB 10 12.57 3.643 0.1753 0.512702 5 1.25 4.72 1344AB 11 20.48 5.674 0.1675 0.512605 4 1.25 4.08 1437

Av. 4.23 1414ULH 95-LH-2D 17.71 5.049 0.1723 0.512674 4 1.25 4.62 1350

95-LH-2E 17.04 4.464 0.1584 0.512575 5 1.25 4.92 128495-LH-4A 7.643 2.205 0.1744 0.512739 4 1.25 5.56 117595-LH-4C 8.938 2.649 0.1792 0.512772 4 1.25 5.43 119095-LH-4D 16.38 4.603 0.1699 0.512672 5 1.25 4.97 128595-LH-4E 17.87 4.944 0.1673 0.512619 4 1.25 4.35 1382

Av. 4.98LLH 95-LH-13.2 11.72 3.586 0.1850 0.512855 5 1.23 6.10 1009

95-LH-13.4 11.23 3.485 0.1876 0.512839 4 1.23 5.37 118395-LH-13.7 11.56 3.617 0.1891 0.512841 4 1.23 5.17 124095-LH-13.8 11.84 3.729 0.1903 0.512883 5 1.23 5.81 104795-LH-14.8 12.98 3.919 0.1826 0.512811 5 1.23 5.61 1134

Av. 5.61SB DS 2-14 21.77 6.087 0.1690 0.512573 4 1.26 3.20 1592

DS 2-20 32.16 7.922 0.1489 0.512385 4 1.26 2.80 1540DS 2-29 18.03 4.438 0.1488 0.512375 4 1.26 2.60 1562DS 2-31 16.60 3.978 0.1449 0.512355 5 1.26 2.80 1517DS 2-8 27.43 6.805 0.1500 0.512388 4 1.26 2.70 1560

Av. 2.82DS 2-5a 38.46 13.51 0.2123 0.512368 4 1.26 -7.80 -DS 2-52a 41.41 9.209 0.1344 0.512216 4 1.26 1.80 1793DS 2-53a 52.01 9.883 0.1149 0.512045 5 1.26 1.60 1792

Av. 1.70

A

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4

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tgtUf

atT1

B, Armer Bay; LLH, Lower Lighthouse; ULH, Upper Lighthouse; SB, Sand Bay.a Sand Bay leucogneiss.

ariably embayed cores mantled by thin to thick CL-dark rims thatn many cases constitute a dominant portion of the grain. Theseims are interpreted to be metamorphic. To make matters (i.e.,A-ICP-MS analyses) worse, many of the cores are also complexlyoned (Fig. 7g). Due to the relatively poor spatial resolution of theA-ICP-MS method, obtaining mixed ages was unavoidable.

Fifty analyses from 50 grains were made, yielding 207Pb/206Pbges between 1012 and 1903 Ma. Nearly half the analyses aretrongly discordant, reflecting mixed core-rim analyses (Fig. 10).our young (1012–1162 Ma) analyses that are <10% discordantome from grains with small core/rim ratios and most likely doot reflect the age of their source; these analyses, that most likelyepresent metamorphic ages, have been excluded. Twenty-fivenalyses are <10% discordant. The majority of the grains rangerom 1400 to 1250 Ma, with the largest peak at ca. 1270 Ma;

inor peaks are observed at 1550–1660 (Laurentian margin) and at900 Ma. The peak at 1270 Ma is regarded as the maximum age ofeposition.

.8. Sand Bay: KB1

This Dillon Schist semi-pelite was sampled within the samenit and several kilometers along strike from BT2, which it closelyesembles.

The zircons from sample KB1 resemble those from BT2, buthe CL-dark metamorphic rims are typically even larger, and somerains lack visible cores. In the grains that do have CL-bright cores,hese are generally oscillatory zoned and variably resorbed (Fig. 7h).nfortunately, these features limited the number of good analyses

rom this sample.Fifty-seven analyses from as many grains only resulted in 18

nalyses that are <10% discordant and can confidently be regardedo represent detrital ages (i.e., not mixed detrital/metamorphic).he ages range from 1252 to 1433 Ma, with overlapping peaks at280 and 1360 Ma (Fig. 10), suggesting no or little input from older

Laurentian sources. Unfortunately, the low number of analysesmeans that such sources may easily go undetected.

4.9. Sand Bay: BT1

This is a quartz-alkali feldspar-biotite rock, quartz rich – anexample of a characteristic member of the metasupracrustal SandBay gneiss association. It is comparable to PS07 in the maturity ofits protolith relative to other semi-pelitic and psammitic samplesbut unlike PS07 is not close to a tectonic boundary.

The zircons from this sample are typically between 100 and200 �m, well rounded with well-developed oscillatory zoning(Fig. 7f). Some grains have very thin CL-dark rims.

Seventy-eight LA-ICP-MS analyses were made from 78 grains. Ofthese, 69 are <10% discordant and yield 207Pb/206Pb ages between1211 and 2362 Ma (Fig. 10). Most of the grains are between 1250and 1850 Ma, with a large peak at ca. 1450 Ma. The data suggest bothLaurentian margin as well as younger sources, and the small peakat ca. 1260 Ma is interpreted as the maximum age of deposition.The mixture of distal and proximal sources is consistent with thematurity of its protolith.

5. εNd of Armer Bay, Lighthouse, and Sand Bay Amphibolites

εNd values were determined for selected samples of the fouramphibolite (metabasalt) groups as well a two samples from SandBay leucocratic gneiss (metarhyolite). Calculations were based onLA-ICP-MS estimates of maximum age of sedimentation of inter-layered metasedimentary units.

5.1. Results

εNd values for all units are generally close to DM with someimportant exceptions. εNd values of Armer Bay amphibolites rangefrom +3.5 ± 0.4 (AB 03) to +4.7 ± 0.5 (AB 10) with a mean of +4.2.


F nts ofF

Ttvbae

ig. 10. Concordia and probability diagrams for detrital zircons from metasedimeig. 8.

hese values lie below DM at 1.25 Ga, the estimated age of erup-ion, and are intermediate between Sand Bay and lower Lighthouse
alues (Table 2). Values for εNd from the upper Lighthouse rangeetween +4.4 ± 0.4 (95-LH-2E) and +5.6 ± 0.4 (95-LH-4A), and have
mean of +5.0 (Table 2). All upper Lighthouse analyses are withinrror of each other and are slightly above the mean DM evolution

the Sand Bay gneiss association. Shading on probability-age diagrams same as in

line of DePaolo (1981), but lie within the range of likely DM values at1.25 Ga, the estimated age of eruption. The lower Lighthouse gneiss
association amphibolites have the highest εNd values varying from+5.2 ± 0.4 (95-LH-13.7) to +6.1 ± 0.5 (95-LH-13.2) (Table 2). All thelower Lighthouse samples are within error of each other, and havea weighted mean of +5.6. Comparable to the upper Lighthouse, the

ian Re

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6

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N.G. Culshaw et al. / Precambr

ower unit values are slightly higher than the mean DM value butie within the range of DM at 1.23 Ga. Sand Bay amphibolites haveowest εNd values, between +2.6 ± 0.4 (DS 2-29) and +3.2 ± 0.4 (DS-31) with a mean of +2.8. The analyses lie significantly below DMt 1.26 Ga (Table 2). The leucocratic gneisses have yet lower valuesith a mean of +1.7.

. Discussion

.1. Significance of detrital zircons

The detrital zircons give information concerning potentialource areas of sedimentary detritus. Here we define a Laurentianource as including detritus derived from the parautochthonousaurentian margin within the Grenville orogen (Carr et al., 2000;.g. GFTZ, PA1, PA2 of Fig. 1) as well as from the immediateesoproterozoic to Archean aged Grenvillian foreland and the

hield beyond. Accordingly, Laurentian margin detritus would beominated by ca. 1750–1450 Ma ages, Archean ages would pre-ominate in foreland and shield sourced material. As implicit inarlier remarks, a dominant ca. 1450 Ma signal may mean a mono-yclic arc source (MA in Fig. 1) such as the Muskoka-Ojibway Arc –ocated now in the CGB but pre-orogeny more distal with respecto its present substrate (Slagstad et al., 2009). Younger detritus willerive from sources close to the original location of the metasedi-ents, the PSD itself or the CAB or Southern Adirondacks, candidate

rigins of the PSD thrust sheet (Wodicka et al., 1996).

.1.1. Laurentian and/or CGB-Type Monocyclic Arc (Laurentianargin) Sources

Most units included in this study (samples PS07, LLH MR01C,LH MR01D, BT1, BT2, KB1) have a subordinate detrital zirconopulation with a significant Laurentian component. Samples fromower Lighthouse (MR01C) and Sand Bay (BT1) have ca. 1450 Marains without the older component which may represent Mono-yclic Arc sources completely (Carr et al., 2000) or partially (Dickinnd McNutt, 2007; Hanmer et al., 2000) detached from the Lau-entian margin. Exceptions without Laurentian grains are samplesrom the Sand Bay and Lighthouse gneiss associations (KB1 andLH MR01G respectively). However these are close neighbors to

amples containing Laurentian grains from the same gneiss asso-iations. We infer that the bPSD-Sand Bay protoliths were close tohe Laurentian margin at the time of deposition.

.1.2. Proximal sourcesPeaks in the age distribution of detrital grains identify dominant

ources of detritus (e.g. an active arc). In the samples studied ageeaks are all younger than Laurentian as defined above and sev-ral pairs of samples from different units have age peaks that differy 10 My or less (approximately within error) indicating sharedources. These pairings include: Sand Bay – Armer Bay (KB1-AB-37,280–1285 Ma), upper Lighthouse – Sand Bay (ULH MR01D-BT2,265–1270 Ma; ULH MR01G-KB1, 1350–1360 Ma), lower Light-ouse – Armer Bay (LLH MR01C-PS07, 1310–1320 Ma), and Sanday – lower Lighthouse (KB1-LLH MR01C, 1360–1370 Ma). Theseairings fall into three broad groups that lie within the rangesf, respectively: (i) CAB magmatism, 1224–1287 Ma; (ii) pluton-sm and detrital zircon ages (from interpreted metamorphosedpiclastic sediments) of interior PSD, 1310–1320 Ma. The oldestrouping, (iii) 1350–1370 Ma, also overlaps with younger ages ofidcontinent Granite-Rhyolite Province (Van Schmus et al., 1996;

ulshaw and Dostal, 2002; Slagstad et al., 2009) and ages of gran-
toid gneiss of the bPSD (Wodicka et al., 1996). Notably, all unitsSand Bay, Lighthouse and Armer Bay gneiss associations) haveeaks indicating an important CAB-age source(s) (Fig. 11). The310–1320 Ma interior PSD range is found at the structurally higher
search 228 (2013) 131– 150 145

levels (Lighthouse and Armer Bay gneiss associations). The old-est, 1350–1370 Ma bPSD-Granite-Rhyolite and/or bPSD grouping isstructurally lower (Sand Bay and Lighthouse gneiss associations).

6.1.3. Age of deposition and volcanismThe maximum age of sedimentation is estimated from the age of

the youngest detrital zircon peak (Fig. 11). This age also constrainsthe age of associated volcanism represented by interlayered amphi-bolites. Accordingly a maximum age for volcanism in the ArmerBay gneiss association is 1251 ± 17 Ma (AB 37), upper Lighthousegneiss association, 1249 ± 10 Ma (MR01D), lower Lighthouse gneissassociation, 1234 ± 12 Ma (MR01C) and in the Sand Bay gneiss asso-ciation, 1260 Ma (BT1). This close clustering of ages, most withinerror, is closely similar to that of CAB magmatism (1224–1287 Ma;Carr et al., 2000 and references therein).

6.2. Geochemistry and origin of meta-volcanic units

Major- and trace element data in this study for the Armer Bayamphibolites and those for the Sand Bay and Lighthouse rocks(Culshaw and Dostal, 1997, 2002) are consistent with all the vol-canic protoliths having been deposited on thin to non-existentcontinental crust such as in a rifted arc or back-arc scenario (e.g.Dickin and McNutt, 2007; McLelland et al., 2010). This conclusionis supported by plots of each unit on 19 discrimination diagrams,an example of which is illustrated in Fig. 5 (Armer Bay subdivided,i.e. 5 units in total). All plots grouped analyses into one or moreof MORB, back-arc basalt, within plate, or arc fields. The majorityof analyses plot as MORB-BAB and a minority plot as within-plateor arc-related. The total spectrum of affinities is marked in theSand Bay protoliths which have been previously interpreted as hav-ing formed in a continental back-arc setting (Culshaw and Dostal,1997). A comparable range is present in the Armer Bay plots whichfall into discrete arc/within plate and MORB dominated groups. Thetrends of all units combined may be taken to reflect progressivearc rifting to form a back-arc with evolution to an asthenosphericsource of the basalt protoliths (Fig. 12).

A hypothesis of continental arc rifting and evolution toward aback-arc setting receives support from other geochemical parame-ters. For example, Sand Bay amphibolites have the lowest εNd valuesof all the amphibolites (meta-basalts) in this study (mean +2.8 at1.26 Ga) which together with their intimate association with leu-cocratic gneiss (metarhyolite) with even lower εNd values (mean+1.7) points to involvement of felsic (continental) crust duringpetrogenesis. εNd values, averaging +5.6 in the lower Lighthouseamphibolites, along with geochemical evidence such as negativeNb anomalies and high Th/La (Culshaw and Dostal, 2002), suggestthat the lower Lighthouse magmatic protoliths were DM-derivedmagmas affected by subduction zone processes, but not apprecia-bly contaminated by older continental crust. The amphibolites ofthe upper Lighthouse, like those of the lower unit, have high εNdvalues averaging +5.0, which are within error of the DM at ca.1.25 Ga which suggests that DM-derived parental magmas werenot contaminated by older felsic crust and is compatible withthe conclusions of Culshaw and Dostal (2002) who proposed thatthe protoliths to the upper Lighthouse, which lack a negative Nbanomaly and have low Th/La, were generated from MORB-like DM.We interpret this as indicating the upper Lighthouse magmatic pro-toliths formed at a more advanced stage of back-arc formation thanthe lower Lighthouse protoliths. Although a subset of Armer Baysamples plot in the volcanic arc/alkali basalt fields in some dis-crimination diagrams (e.g. Cr–Y, La–Y–Nb) and consistently form a
separate cluster in several XY plots (Mg# vs. La, Y, Nd, Zr and TiO2)and discrimination diagrams, most samples are placed with theother amphibolites, in back-arc, within plate or MORB fields. How-ever the Armer Bay amphibolites have εNd values at 1.25 Ga (mean


Fig. 11. Probability density plots of zircon grains from metasedimentary gneiss interlayered with amphibolite of Armer Bay, Lighthouse and Sand Bay gneiss associations.LA-ICP-MS data shown in black, TIMS data in white. Included data are <10% discordant. Plots are arranged in tectonostratigraphic order (top to base: Armer Bay, Lighthouseto Sand Bay gneiss associations). Also shown is εNd (range and mean) for related amphibolites, and age of youngest detrital zircon. Age range for Central MetasedimentaryBelt (Composite Arc Belt) volcanism marked by red stripe. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of thisa

+afrcrSfaf

rticle.)

4.2) below DM that are transitional between the upper Lighthousend Sand Bay values. Trace element and REE data, resemble thoserom the Sand Bay amphibolites which compare with continentalift-related basalts and have been interpreted to have formed in aontinental back-arc (Culshaw and Dostal, 1997). We favor an envi-onment with thin continental crust but with limited (compared to
and Bay) contribution from older continental crust i.e. Armer Bayormation in the early stages of rifting of a thin, juvenile continentalrc. Overall, most geochemical indices do not correlate in a simpleashion with tectonostratigraphy. Rather, influence of older crust is
seen at top and base (AB, SB) with a trend to more primitive sourcesat intermediate levels (LH) (Fig. 12).

6.3. Regional isotopic similarities

The similarity of bPSD-Sand Bay depositional and magmatism
ages to those of CAB invites comparison of εNd values of both theareas. Using values calculated from Dickin et al.’s (1990) and Dickinand McNutt (2007) data clear similarities emerge. As expected forrocks from the same domain, Dickin et al.’s (1990) values for PSD,


Fig. 12. Depiction of a plausible scenario in which all units are in position to receive detritus from Laurentia, Laurentian margin, Composite Arc Belt (CAB), interior ParrySound domain (iPSD) and postulated arc basemen of basal Parry Sound domain (bPSD arc; see text): (i) Sand Bay ga is part of a rifted continental (Laurentian? Joined to PSD?)a are prot th. A fi

4tvafcme2BltaO(

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rc; (ii) members of bPSD, lower and upper Lighthouse and Armer Bay, respectively,

o both Laurentia, Laurentian margin (L. margin) and CAB, receiving detritus from bon the observed tectonostratigraphy.

.2 (at 1350 Ma), match those for the Armer Bay gneiss associa-ion. Perhaps more significant is the similarity of CAB juvenile crustalues, 5.1 (at 1250 Ma) with Upper and Lower Lighthouse valuest 5.0 and 5.6 respectively. As already noted trace element dataor the Lighthouse amphibolites are congruent with their juvenileharacter. The Elzevir block and the Sand Bay gneiss associationagmatism are both suspected to have been influenced by pre-

xisting crust and this is reflected in comparable εNd values of.8 (at 1350 and 1260 Ma). The intimate association of the Sanday amphibolites with leucocratic gneiss, with interpreted rhyo-

ite protoliths with the lowest εNd values (mean +1.7) also pointso involvement of felsic (continental) crust during petrogenesisttributed to post 1.45 Ga rifting of the Andean-type Muskoka-jibway arc, developed on the Laurentian margin at ca. 1.45 Ga

Fig. 12; Slagstad et al., 2004a; Culshaw and Dostal, 2002).

.4. Implications for tectonostratigraphy

Armer Bay and Lighthouse gneiss association volcanic eventst ≤1251 ± 17 Ma and 1234 ± 12 Ma respectively are younger thanther ages reported by Wodicka et al. (1996) in the bPSD (exclud-ng the ca 1160 Ma Parry Island anorthosite). These include ca.400–1330 Ma bPSD plutonism, the quartzite locally interlayeredith the plutons (maximum deposition age 1436 ± 17 Ma assum-

ng 1120 Ma Pb loss) (Wodicka et al., 1996) and the 1350 ± 50 Mahitestone anorthosite (van Breeman et al., 1986). The majority of

he bPSD plutons (and the quartzite) sampled by Wodicka et al.1996) lie in the lower part of the bPSD, below the Armer Bayneiss association, and above the dated Lighthouse gneiss associa-ion metasupracrustals (Fig. 2). An exception is the Isabella Islandluton which intrudes the Whitestone anorthosite and overlieshe Armer Bay gneiss association. The Armer Bay rocks are thusandwiched between older packages, consistent with an origin forhe tectonostratigraphy of the bPSD by tectonic interleaving. Thisomports with other details of the tectonostratigraphy such as theosition of the Armer Bay quartzite (PS07) directly at the base of thehitestone anorthosite, and which, at 1322 ± 15 Ma, has an older
aximum deposition age than the underlying metavolcanic rocks
nd younger than the overlying Whitestone. Overall, the impliedcenario of thrusting within the bPSD is a comparable but moreomplex account than that of successive overthrusting of iPSD onto

gressively rifted in back-arc of bPSD arc ± iPSD arc; (iii) these units are in proximityorward-propagating stacking during Grenvillian thickening and nappe-flow results

upper bPSD and lower bPSD at ca. 1169 and 1120 Ma envisaged byWodicka et al. (2000). Complex interleaving is also present withinShawanaga domain where the ca. 1450 Ma Ojibway is interlayeredwithin a fold nappe with the ca. 1260 Ma Sand Bay gneiss associa-tion.

6.5. Origin of bPSD-Sand Bay units

Summarizing, any explanation for the origin of the major unitsmust take into account: (i) all units have more or less Laurentiandetritus (more mature metasedimentary protolith generally hasmore, some immature ones have none) and/or potential CGB-typeMonocyclic Arc detritus; (ii) all units have at least one member withCAB age detritus; (iii) all have maximum age of sedimentation in theCAB range; (iv) other important probable sources include those ofiPSD age (for Lighthouse and Armer Bay) and bPSD and/or Granite-Rhyolite ages (for Lighthouse and Sand Bay); (v) geochemical andisotope data that indicate magmas with a strong DM signature andvariable, but minor amounts of contamination by older continentalcrust and/or subduction affected lithosphere; (vi) tectonic inter-leaving of the units resulting in asymmetry of geochemical indiceswithin the tectonostratigraphy; (vii) isotopic similarities with CAB.

Given the close similarity of the maximum age of depositionof the metasediments associated with the metavolcanic units itis most probable that the units were initially geographically andgenetically closely related despite tectonic transport and potentialdisruption. Accordingly we consider the origin and evolution of thebPSD-Sand Bay units in the context of models for the evolution ofparts of the pre-Grenvillian Laurentian margin.

Recent models of the evolution of the Laurentian margin dur-ing the Palaeoproterozoic and Mesoproterozoic view the margin asone of the oldest examples of a Pacific-rim scale, active Andean- orwestern Pacific-type margin (Kay et al., 1989; Gower and Tucker,1994; Culshaw et al., 2000; Carr et al., 2000; Rivers and Corrigan,2000; Slagstad et al., 2004a, 2009).

A more locally focussed detailed study of the tectono-magmaticevolution of the Central Gneiss Belt Muskoka and Shawanaga
domains (Monocyclic Arcs) adopted this Andean arc theme(Slagstad et al., 2004a, 2009). This study proposed a juvenilecontinental arc (the Muskoka-Ojibway arc) that was affected bycompression and repeated arc-rifting. Slagstad et al. (2004a, 2009)

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uggested that arc growth on the Laurentian margin began before.45 Ga, with arc- and back-arc rifting at 1.45 Ga leading to volu-inous plutonism. Magma sources for the arc-rift rocks may

ave been the pre-1.45 Ga continental arcs, or a basaltic under-late (Slagstad et al., 2004a, 2009). High-grade metamorphismn the continental margin between 1.45–1.43 Ga was followed byenewed arc-extension after 1.4 Ga (based on 1364 Ma estimateor Sand Bay). The Sand Bay and lower Lighthouse units werenterpreted as supracrustal manifestations of the post-1.4 Ga rift-ng of the Muskoka-Ojibway arc. Culshaw and Dostal (2002), andlagstad et al. (2004a, 2009) recognized numerous links betweenhe younger granite-rhyolite provinces and the CGB. The Sanday and lower Lighthouse sequence was considered to fit with aodel of Laurentian margin rifting after 1.38 Ga, probably behind

juvenile continental arc, leading to the creation of the south-rn Granite-Rhyolite Province which formed from melts of juvenileontinental arc rocks. However in this model Sand Bay maximumge of volcanism had been estimated at 1364 Ma on the basis ofIMS detrital zircon geochronology (Krogh, personal communica-ion, in Culshaw and Dostal, 1997). The new data shows this isoo old (1260 Ma Sand Bay, this study) and implies interpretationsf Sand Bay as an integral part of the Granite-Rhyolite Provinceshould be abandoned. Nevertheless the new Sand Bay age data doontain a 1360 Ma peak (KB1) matching the TIMS age allowing aodified version of the post-1.4 Ga rifting of the Muskoka-Ojibway

rc scenario in which Sand Bay sources may have included one oroth bPSD and Granite-Rhyolite Province, as well as CAB sources.

The evidence for tectonic interleaving within bPSD andhawanaga suggests further constraints for sources and refineshe tectonic reconstruction. Asymmetry of geochemical indicatorsithin the bPSD-Shawanaga thrust stack, assuming tectonic stack-

ng was piggy-back, implies a section compatible with thickeningf a rifted, young Andean-type margin. This may have progressedrom a partially rifted Andean back-arc (Sillitoe, 2003) to a moreomplete rift with production of primitive basalts i.e. the restoredection would display continent influenced magmas bracketingore primitive compositions (Fig. 12). Accordingly we speculate

hat the bPSD granitoids are tectonically displaced fragments of anlder arc, basement to the Armer Bay unit and consistent with itsomparatively low εNd values for ABA and the presence of bPSD ageircons in both the Lighthouse and Sand Bay metasediments.

Such a model postulates proximity to Laurentia of the develop-ng back arc cratonward of the 1287–1224 Ma (Carr et al., 2000)AB arcs and nearby, older PSD arc(s). The latter is consistent withhe suggestion of Wodicka et al. (1996) that the PSD formed on the

ost distal part of the Laurentian margin or just offshore within theAB (but excluding the Adirondacks). As such this scenario is a fur-her chapter in ongoing arc-rifting (Slagstad et al., 2004a, 2009). Wepeculate that the Geon 12 magmatism (Easton and Ketchum, 2002)ithin the parautochthon, close to the foreland (Fig. 1) has a role

elated to Geon 12 arcs comparable to that suggested by Slagstadt al. (2009) for Geon 14 parautochthon magmatism with respecto coeval continental arcs.

An intriguing question remains concerning the mode of tectonicuxtaposition of deep crust (e.g. 1160 Ma anorthosite and high pres-ure mafic rocks) with supracrustal rocks. Was the imbrication ofhese relatively young rocks of contrasting deep and surficial origin

ediated by thrusting in the earliest stages of thickening with orithout attempted subduction of back-arc crust?

cknowledgments

NC and JD acknowledge support from NSERC Discovery grants.yvind Skår is thanked for technical support during LA-ICP-MSnalyses at the Norwegian Geological Survey (NGU). We are

search 228 (2013) 131– 150

especially grateful to Louise Corriveau and James McLelland forthoughtful and helpful reviews.

Appendix A.

A.1. Analytic methods: geochemistry

Major and minor element analysis of whole rocks was per-formed on fused glass disks using a Philips PW2400 X-rayfluorescence spectrometer at the University of Ottawa, G.G. HatchLaboratory. Duplicate analyses of the samples yielded total errors of±5% (1�). Trace element analysis was done using a sintering tech-nique and a Perkin Elmer Optima 3000 ICP mass spectrometer atActivation Laboratory, Ancaster, Ontario. Replicate analyses of thereference standard rocks indicate that the errors were better than±10% (1�).

A.2. Analytic methods: U–Pb zircon geochronology

Zircons were separated by standard techniques including watertable, heavy liquids, magnetic separation and final hand pickingunder a binocular microscope. The zircons were mounted in epoxyand polished to approximately half thickness, and cathodolumi-nescence (CL) images were obtained with a scanning electronmicroscope to reveal internal structures such as growth zoning andcore-rim relationships.

Detrital zircon analyses were performed by laser-ablationinductively-coupled mass spectrometry (LA-ICP-MS) at the Geo-logical Survey of Norway (NGU) and the University of Bergen (UiB).Prior to analysis by laser ablation ICP-MS, the sample surfaces werecleaned in 5% HNO3, and rinsed with de-ionized water.

At NGU analyses were made on a Finnigan ELEMENT1 single-collector high-resolution sector ICP-MS, coupled to a UP193-FX193 nm short-pulse Excimer laser ablation system from New WaveResearch. In order to get a stable signal from the reference zircon(the low-uranium 91500) and at the same time be able to analyzesmall zircons, the laser conditions during analysis of unknowns andstandards-as-unknown were not the same as during the analysisof 91500. During analysis of 91500 the laser beam was rastered forca. 1 min using a beam size of 20 �m, a repetition rate of 20 Hz,and a beam energy output of 40% (corresponding to a fluenceof ca 5 J cm−2). During analysis of unknowns and standards-as-unknowns the laser was set to analyze 35 �m single spots witha repetition rate of 12 Hz and an energy output of 60% (fluenceca 7 J cm−2). Laser ablation was done in a He atmosphere, whichtogether with the resultant aerosol was mixed with Ar beforeintroduction into the plasma torch. The data are acquired in a time-resolved counting scanning mode for 60 s, with measurements of202Hg, 204(Hg + Pb), 206Pb, 207Pb, 208Pb, 232Th and 238U. In an ICP-MSanalysis, 204Hg originating from the argon supply heavily contam-inates mass 204. This is corrected by monitoring 202Hg in argonand assuming a 202Hg/204Hg ratio of 4.3611. When necessary, theobserved signals from 206Pb were corrected for common 206Pbusing the observed (202Hg-corrected) 204Pb and the average mod-ern common-lead composition (Stacey and Kramers, 1975).

At UiB isotopic analyses followed the technique described inKosler et al. (2002). A Thermo-Finnigan Element 2 sector fieldICP-MS coupled to a 193 nm Excimer laser (Resonetics RESOlutionM50-L) was used to measure Pb/U and Pb isotopic ratios in zircons.The sample introduction system was modified to enable simulta-neous nebulization of a tracer solution and laser ablation of the
solid sample (Horn et al., 2000). Natural Tl (205Tl/203Tl = 2.3871 –Dunstan et al., 1980), 209Bi and enriched 233U and 237Np (>99%)were used in the tracer solution, which was aspirated to the plasmaby an Apex desolvation nebulizer (Elemental Scientific) and a

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-piece tube attached to the back end of the plasma torch. A heliumas line carrying the sample from the laser cell to the plasma waslso attached to the T-piece tube. Typical acquisitions consisted of

35 second measurement of analytes in the gas blank, followed byeasurement of the ablated material from the zircon, along with

he continuous signal from the aspirated solution, for another 110 s.he data were acquired in time resolved–peak jumping–pulseounting mode with 1 point measured per peak for masses 202flyback), 204 (Pb), 203 and 205 (Tl), 206 and 207 (Pb), 209 (Bi),33 (U), 237 (Np), 238 (U), 249 (233U oxide), 253 (237Np oxide) and54 (238U oxide). Raw data were corrected for dead time of thelectron multiplier and processed off line in a spreadsheet-basedrogram (Lamdate – Kosler et al., 2002). Data reduction includedorrection for gas blank, laser-induced elemental fractionation ofb and U and instrument mass bias. Minor formation of oxides of Und Np was corrected for by adding signal intensities at masses 249,53 and 254 to the intensities at masses 233, 237 and 238, respec-ively. Details of data reduction and corrections are described inosler et al. (2002) and Kosler and Sylvester (2003). Zircon refer-nce materials GJ-1 (Jackson et al., 2004) and Plesovice (Sláma et al.,008) was periodically analyzed during the runs. Laser settingsere: energy 45 mJ, frequency 5 Hz, speed 0.01 mm/s, spot size was

9 um for samples and Plesovice, and 26 �m for 91500 and GJ-1, Heow 0.65 l/min. ICP-MS parameters were: sample gas 0.843 l/min,uxiliary gas 0.69 l/min, cool gas 15.9 l/min, power 1300 W. IsoplotLudwig, 2003) was used to plot and calculate ages from the U–Pbata.

.3. Analytic methods: Sm–Nd

Analyses for 143Nd/144Nd and 147Sm/144Nd isotopic ratios werendertaken at the Atlantic Universities Regional Isotope FacilityAURIF), Memorial University, St Johns, Newfoundland. Samplereparation was done at GEOTOP laboratory of the University ofuébec at Montréal. About 0.1 g of sample was dissolved in HFnd HN03 in screw-top Teflon bombs. A mixed 150Nd/149Sm spikeas added to each sample prior to acid digestion. After five days ofigestion, the solution was evaporated to dryness and then takenp in 6 N HCl acid. The dried solution was taken up in 2.5 N HCl and

oaded on cationic exchange chromatography using an AG50W-X8esin. The REE fraction was purified and Sm and Nd were isolatedsing a secondary column loaded with HDEHP (di2-ethyl-hexyl)reated Teflon powder. All reagents were purified by a sub-boilingrocess in order to ensure a low contamination level.

Measurements were made using a static multicollector configu-ation on a Finnigan MAT 262 mass spectrometer. Data are reportedelative to the La Jolla Nd standard, 143Nd/144Nd = 0.511887 ± 17 (2igma). Using the AURIF system, cumulative analytical errors areess than 0.2% (147Sm/144Nd), thus calculated εNd values have errorsess than ±0.5 units. Analytical errors include uncertainty in blanknd spike composition, systematic error in mass discrimination andonization efficiency, and variation in ion beam intensity resultingrom unstable sample fusion. εNd values (De Paolo and Wasserburg,976) are used throughout, and are calculated assuming CHUR47Sm/144Nd = 0.1967 and zero-age 143Nd/144NdCHUR = 0.512638Zindler and Hart, 1986). The DM evolution model is that of DePaolo1981). εNd values are calculated for a crystallization age estimatedrom U–Pb geochronological and field data.

ppendix B. Supplementary data

Supplementary data associated with this article can be found,n the online version, at http://dx.doi.org/10.1016/j.precamres.013.01.012.

search 228 (2013) 131– 150 149

References

Bussy, F., Krogh, T.E., Klemens, W.P., Schwerdtner, W.M., 1995. Tectonic and meta-morphic events in the westernmost Grenville Province central Ontario; newresults from high-precision U–Pb zircon geochronology. Canadian Journal ofEarth Sciences 32, 660–671.

Carr, S.D., Easton, R.M., Jamieson, R.A., Culshaw, N.G., 2000. Geologic transect acrossthe Grenville Orogen of Ontario and New York. Canadian Journal of Earth Sci-ences 37, 193–216.

Culshaw, N.G., Dostal, J., 1997. Sand Bay gneiss association, Grenville Province,Ontario: a Grenvillian rift (and-drift) assemblage stranded in the Central GneissBelt? Precambrian Research 85, 97–113.

Culshaw, N.G., Dostal, J., 2002. Amphibolites of the Shawanaga domain, CentralGneiss Belt, Grenville Province, Ontario: tectonic setting and implications forrelations between the Central Gneiss Belt and midcontinental USA. PrecambrianResearch 113, 65–85.

Culshaw, N.G., Jamieson, R.A., Ketchum, J.W.F., Wodicka, N., Corrigan, D., Reynolds,P.H., 1997. Transect across the northwestern Grenville orogen, Georgian Bay,Ontario: polystage convergence and extension in the lower orogenic crust. Tec-tonics 16, 966–982.

Culshaw, N.G., Ketchum, J.W.F., Barr, S., 2000. Structural evolution of the MakkovikProvince Labrador, Canada: tectonic processes during 200 m.y at a Paleoprotero-zoic active margin. Tectonics 19, 961–977.

Culshaw, N.G., Corrigan, D.J., Ketchum, W.F., Wallace, P., Wodicka, N., Easton, R.M.,2004. Georgian Bay geological synthesis, Grenville Province; explanatory notesfor preliminary maps P.3548 to P.3552, 28 p.

Culshaw, N., Gerbi, C., Marsh, J., 2010. Softening the lower crust: Modes of syn-transport transposition around and adjacent to a deep crustal granulite nappe,Parry Sound domain, Grenville Province, Ontario, Canada. Tectonics 29, TC5013,http://dx.doi.org/10.1029/2009TC002537.

Davidson, A., Culshaw, N.G., Nadeau, L., 1982. A tectono-metamorphic frameworkfor part of the Grenville Province, Parry Sound region, Ontario. Current Research,Geological Survey of Canada 82-1A, 175–190.

Davidson, A., 1984. Tectonic boundaries within the Grenville Province of the Cana-dian Shield. Journal of Geodynamics 1, 433–444.

De Paolo, D.J., Wasserburg, G.J., 1976. Inferences about magma sources and man-tle structure from 143Nd/144Nd variations. Geophysical Research Letters 3,249–252.

DePaolo, D.J., 1981. Neodymium isotopes in the Colorado Front Range and crust-mantle evolution in the Proterozoic. Nature 291, 193–196.

Dickin, A.P., McNutt, R.H., 2007. The Central Metasedimentary Belt (GrenvilleProvince) as a failed back-arc rift zone: Nd isotope evidence. Earth and PlanetaryScience Letters 259, 97–106.

Dickin, A.P., McNutt, R.H., Clifford, P.M., 1990. A neodymium isotope study of plutonsnear the Grenville Front in Ontario, Canada. Chemical Geology 83, 315–324.

Dunstan, L.P., Gramlich, J.W., Barnes, I.L., Purdy, W.C., 1980. The absolute abundanceand the atomic weight of a reference sample of thallium. Journal of Research ofthe National Bureau of Stan 85, 1–10.

Easton, R.M., Ketchum, J.W.F., 2002. Evidence for a major Mesoproterozoic magmaticprovince within the western Grenville Province near North Bay, Ontario. Abs.Geological Society of America 2002 Denver Annual Meeting (October 27–30,2002).

Govindaraju, K., 1994. 1994 compilation of working values and sample descriptionfor 383 geostandards. Geostandards Newsletter 18, 1–158.

Gower, C.F., Tucker, R.D., 1994. Distribution of pre-1400 Ma crust in the GrenvilleProvince: implications for rifting in Laurentia–Baltica during Geon 14. Geology22, 827–830.

Hanmer, S., Corrigan, D., Pehrsson, S., Nadeau, L., 2000. SW Grenville ProvinceCanada: the case against post-1.4 Ga accretionary tectonics. Tectonophysics 319,33–51.

Heaman, L.M., Le Cheminant, A.N., 1993. Paragenesis and U–Pb systematics of bad-deleyite (ZrO2). Chemical Geology 110, 95–126.

Horn, I., Rudnick, R.L., McDonough, W.F., 2000. Precise elemental and isotope ratiomeasurement by simultaneous solution nebulisation and laser ablation-ICP-MS:application to U–Pb geochronology. Chemical Geology 164, 281–301.

Jamieson, R.A., Beaumont, C., Nguyen, M.H., Culshaw, N.G., 2007. Synconver-gent ductile flow in variable-strength continental crust; numerical modelswith application to the western Grenville Orogon. Tectonics 26, TC5005,http://dx.doi.org/10.1029/2006TC002036.

Jackson, S.E., Pearson, N.J., Griffin, W.L., Belousova, E.A., 2004. The application of laserablation-inductively coupled plasma-mass spectrometry to in situ U–Pb zircongeochronology. Chemical Geology 211, 47–69.

Kay, S.M., Ramos, V.A., Mpodozis, C., 1989. Late Paleozoic to Jurassic silicic mag-matism at the Gondwana margin: analogy to the Middle Proterozoic in NorthAmerica? Geology 17, 324–328.

Ketchum, J.W.F., Krogh, T.E., 1997. U–Pb constraints on high-pressure metamor-phism in the Central Gneiss Belt southwestern Grenville orogen. GeologicalAssociation of Canada–Mineralogical Association of Canada 22, A-78, AbstractVolume.

Kosler, J., Sylvester, P.J., 2003. Present trends and the future of zircon in geochronol-ogy: laser ablation ICPMS. In: Hanchar, J.M., Hoskin, P.W.O. (Eds.), Zircon, vol.
53. Mineralogical Society of America and Geochemical Society Reviews in Min-eralogy and Geochemistry, pp. 243–275.
Kosler, J., Fonneland, H.C., Sylvester, P.J., Tubrett, M.N., Pedersen, R.B., 2002. U–Pbdating of detrital zircons for sediment provenance studies – a comparison oflaser ablation ICPMS and SIMS techniques. Chemical Geology 182, 605–618.

http://dx.doi.org/10.1016/j.precamres.2013.01.012

http://dx.doi.org/10.1016/j.precamres.2013.01.012

dx.doi.org/10.1029/2009TC002537

dx.doi.org/10.1029/2006TC002036

1 ian Re

K

K

L

L

M

M

M

M

R

R

R

S

S

Wodicka, N., Ketchum, J.W.F., Jamieson, R.A., 2000. Grenvillian metamorphism of


rogh, T.E., 1997. Seventy-five million years of convergence recorded in the ParrySound shear zone in the Central Gneiss Belt of the Grenville Province. In: Pro-terozoic Orogenies and Plate Interactions: The North Atlantic Region in Spaceand Time: Abstracts of the Proceedings of the COPENA Conference, NorwegianGeological Survey.

rogh, T.E., Kwok, Y.Y., 2005. The age enigma in the Moon River structure and theParry Sound domain connection, Grenville Province. Geological Association ofCanada Abstracts with Program 30, 106.

ightfoot, P.C., Hawkesworth, C.J., 1988. Origin of Deccan Trap lavas: evidence fromcombined trace element and Sr-, Nd-, and Pb-isotope studies. Earth PlanetaryScience Letters 91, 89–104.

udwig, K.R., 2003. Isoplot 3.00, A Geochronological Toolkit for Microsoft Excel.Berkley Geochronology Center, Special Publication, 4, pp. 1–74.

arsh, J.H., Gerbi, C., Culshaw, N.G., Potter, J., Longstaffe, F., Johnson, S.E., 2011.Initiation and development of the Twelve Mile Bay Shear Zone: the low viscositysole of a Grenvillian thrust sheet. Journal of Metamorphic Geology 29, 167–191.

arsh, J.H., Grew, E.S., Gerbi, C., Yates, M.G., Culshaw, N.G., 2012. The petrogen-esis of the garnet Menzerite-(Y) in granulite facies rocks of the Parry Sounddomain, Grenville Province, Ontario. The Canadian Mineralogist 50, 73–99,http://dx.doi.org/10.3749/canmin.50.1.73.

cLelland, J.M., Selleck, B.W., Bickford, M.E., 2010. Review of the Proterozoic evolu-tion of the Grenville Province, its Adirondack outlier, and the Mesoproterozoicinliers of the Appalachians. In: Tollo, R.P., Bartholomew, M.J., Hibbard, J.P., Kara-binos, P.M. (Eds.), From Rodinia to Pangea: The Lithotectonic Record of theAppalachian Region: Geological Society of America Memoir 206, pp. 21–49,http://dx.doi.org/10.1130/2010.1206(2)

cEachern, S.J., van Breemen, O., 1993. Age of deformation within the CentralMetasedimentary Belt boundary thrust zone, southwest Grenville Orogen: con-straints on the collision of the Mid-Proterozoic Elzevir terrane. Canadian Journalof Earth Sciences 30 (6), 1155–1165, http://dx.doi.org/10.1139/e93-098.

aistrick, M., 2003. Depleted mantle derived magmas and Laurentian detritus in thesupracrustal Lighthouse gneiss association, Grenville Province, Ontario. M.Sc.Thesis. Dalhousie University.

ivers, T., 1997. Lithotectonic elements of the Grenville Province; review and tec-tonic implications. Precambrian Research 86, 117–154.

ivers, T., Corrigan, D., 2000. Convergent margin on southeastern Laurentia duringthe Mesoproterozoic: tectonic implications. Canadian Journal of Earth Sciences37, 359–383.

ager-Kinsman, E.A., Parrish, R.R., 1993. Geochronology of detrital zircons from
the Elzevir and Frontenac terranes, Central Metasedimentary Belt, GrenvilleProvince, Ontario. Canadian Journal of Earth Sciences 30 (3), 465–473,http://dx.doi.org/10.1139/e93-034.
illitoe, R.H., 2003. Iron oxide-copper–gold deposits: an Andean view. MineraliumDeposita 38, 787–812, http://dx.doi.org/10.1007/s00126-003-0379-7.

search 228 (2013) 131– 150

Slagstad, T., Culshaw, N.G., Jamieson, R.A., Ketchum, J.W.F., 2004. Early Meso-proterozoic tectonic history of the southwestern Grenville Province, Ontario:Constraints from geochemistry and geochronology of high-grade gneisses. In:Tollo, R.P., Corriveau, L., McLelland, J., Bartholemew, M.J. (Eds.), Proterozoic tec-tonic evolution of the Grenville orogen in North America. Boulder, Colorado,Geological Society of America, Special Paper, 197, pp. 209–241.

Slagstad, T., Hamilton, M.A., Jamieson, R.A., Culshaw, N.G., 2004b. Timing and dura-tion of melting in the mid orogenic crust; constraints from U–Pb (SHRIMP)data Muskota and Shawanaga Domains, Grenville Province, Ontario. CanadianJournal of Earth Sciences 41, 1339–1365.

Slagstad, T., Culshaw, N.G., Daly, J.S., Jamieson, R.A., 2009. Western Grenville Provinceholds key to midcontinental Granite-Rhyolite Province enigma. Terra Nova,http://dx.doi.org/10.1111/j.1365.3121.2009.00871.x.

Sláma, J., Kosler, J., Condon, D.J., Crowley, J.L., Gerdes, A., Hanchar, J.M., Horstwood,M.S.A., Morris, G.A., Nasdala, L., Norberg, N., Schaltegger, U., Schoene, B., Tubrett,M.N., Whitehouse, M.J., 2008. Plesovice zircon – a new natural reference materialfor U–Pb and Hf isotopic microanalysis. Chemical Geology 249, 1–35.

Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolutionby a two-stage model. Earth and Planetary Science Letters 26, 207–221.

Timmermann, H., Parrish, R.R., Jamieson, R.A., 1997. Time of metamorphismbeneath the Central Metasedimentary Belt boundary thrust zone, Grenvilleorogen Ontario: accretion at 1080 Ma? Canadian Journal of Earth Sciences 34,1023–1029.

Tuccillo, M.E., Mezger, K., Essene, E.J., Van Der Pluijm, B.A., 1992. Thermobarome-try, Geochronology and the Interpretation of P–T–t Data in the Britt Domain,Ontario Grenville Orogen, Canada. Journal of Petrology 33 (6), 1225–1259,http://dx.doi.org/10.1093/petrology/33.6.1225.

van Breeman, O., Davidson, A., Loveridge, W.D., Sullivan, R.D., 1986. U–Pb zir-con geochronology of Grenvillian tectonites, granulites and igneous precursors,Parry Sound, Ontario, in The Grenville Province, Geological Association ofCanada, Special Paper 31, pp. 191–207.

Van Schmus, W.R., Bickford, M.E., Turek, A., 1996. Proterozoic geology of the east-central Midcontinent basement. In: van der Pluijm, B.A., Catacosinos, P.A. (Eds.),Basement and Basins of Eastern North America: Geological Society of AmericaSpecial Paper 308, pp. 7–32.

Wodicka, N., Parrish, R.R., Jamieson, R.A., 1996. The Parry Sound domain; a far-travelled allochthon? New evidence from U–Pb zircon geochronology. CanadianJournal of Earth Sciences 33, 1087–1104.

monocyclic rocks Georgian Bay, Ontario, Canada: implications for convergencehistory. Canadian Mineralogist 38, 471–510.

Zindler, A., Hart, S., 1986. Chemical geodynamics. Annual Reviews in the Earth andPlanetary Science 14, 493–571.

dx.doi.org/10.3749/canmin.50.1.73

http://dx.doi.org/10.1130/2010.1206(2)

dx.doi.org/10.1139/e93-098

dx.doi.org/10.1139/e93-034

dx.doi.org/10.1007/s00126-003-0379-7

dx.doi.org/10.1111/j.1365.3121.2009.00871.x

dx.doi.org/10.1093/petrology/33.6.1225

Geochemical, geochronological and isotopic constraints on the origin of members of the allochthonous...

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