Superposed Neoproterozoic and Silurian magmatic arcs in central Cape Breton Island, Canada: geochemical and geochronological constraints

1. Introduction

Neoproterozoic subduction-related magmatism spans800–580 Ma in the Pan-African orogens of northwestAfrica (Caby & Andreopoulis-Renaud, 1987; Ducrot &Lancelot, 1978) and 700–550 Ma in the Brasiliano oro-gens of South America (Teixiera et al. 1989). Suchmagmatism in the Avalon Composite Terrane ofAtlantic Canada (Fig. 1a) has been correlated bothwith northwest Africa (O’Brien, Wardle & King, 1983)and northern South America (Dostal et al. 1996).This igneous activity has been related to the amalga-mation of Gondwana, and precedes a major, latestPrecambrian–Early Cambrian plate reorganization(Keppie et al. 1996). In this context, the ~ 540–585 Macalc-alkaline igneous rocks in the eastern CreignishHills of central Cape Breton Island represent the termi-nal stage of subduction-related magmatism in the

Avalon Composite Terrane of Nova Scotia. Accuratedating and geochemistry of this magmatic activity iscrucial to the formulation of tectonic models for thetermination of subduction. Similarly, the extent ofSilurian magmatic arc rocks in Cape Breton Island,and their transition to rift tholeiites in mainland NovaScotia, are critical to the understanding of the EarlyPalaeozoic evolution of the Appalachians.

There have been two schools of thought regardingthe geology of Cape Breton Island. First, it has beenproposed that Cape Breton Island represents anoblique section through the Neoproterozoic arc, over-stepped in southern and central Cape Breton Island bythe Avalonian Cambrian sequence, and underlain by~ 1 Ga basement exposed in northwestern Cape BretonIsland (Keppie, 1985). Second, it has been suggestedthat Cape Breton Island is made up of distinct terranes.South to north, these are Mira, Bras d’Or, Aspy andBlair River (the latter is inferred to represent theLaurentian margin), which were not amalgamated

Geol. Mag. 137 (2), 2000, pp. 137–153. Printed in the United Kingdom © 2000 Cambridge University Press 137

Superposed Neoproterozoic and Silurian magmatic arcs incentral Cape Breton Island, Canada: geochemical and

geochronological constraints

J. D. KEPPIE*, J. DOSTAL†, R. D. DALLMEYER‡ & R. DOIG§

*Instituto de Geologia, Universidad Nacional Autonoma de Mexico, 04510 Mexico DF, Mexico†Department of Geology, St. Mary’s University, Halifax, Nova Scotia B3H 3C3, Canada

‡Department of Geology, University of Georgia, Athens, Georgia 30602, USA §Department of Geology, McGill University, Montreal, Quebec H3A 2A7, Canada

(Received 4 January 1999; accepted 2 November 1999)

Abstract – Isotopic and geochemical data indicate that intrusions in the eastern Creignish Hills of cen-tral Cape Breton Island, Canada represent the roots of arcs active at ~ 540–585 Ma and ~ 440 Ma.Times of intrusion are closely dated by (1) a nearly concordant U–Pb zircon age of 553 ± 2 Ma in dior-ites of the Creignish Hills pluton; (2) a lower intercept U–Pb zircon age of 540 ± 3 Ma that is withinanalytical error of 40Ar/39Ar hornblende plateau isotope-correlation ages of 545 and 550 ± 7 Ma in theRiver Denys diorite; and (3) an upper intercept U–Pb zircon age of 586 ± 2 Ma in the Melford graniticstock. On the other hand, ~ 441–455 Ma 40Ar/39Ar muscovite plateau ages in the host rock adjacent tothe Skye Mountain granite provide the best estimate of the time of intrusion, and are consistent withthe presence of granitic dykes cutting the Skye Mountain gabbro–diorite previously dated at438 ± 2 Ma. All the intrusions are calc-alkaline; the Skye Mountain granite is peraluminous. Trace ele-ment abundances and Nb and Ti depletions of the intrusive rocks are characteristic of subduction-related rocks. The ~ 540–585 Ma intrusions form part of an extensive belt running across central CapeBreton Island, and represent the youngest Neoproterozoic arc magmas in this part of Avalonia.Nearby, they are overlain by Middle Cambrian units containing rift-related volcanic rocks, whichbracket the transition from convergence to extension between ~ 540 and 505/520 Ma. This transitionvaries along the Avalon arc: 590 Ma in southern New England, 560–538 Ma in southern NewBrunswick, and 570 Ma in eastern Newfoundland. The bi-directional diachronism in this transition isattributed to northwestward subduction of two mid-ocean ridges bordering an oceanic plate, and themigration of two ridge–trench–transform triple points. Following complete subduction of the ridges,remnant mantle upwelling along the subducted ridges produced uplift, gravitational collapse and thehigh-temperature/low-pressure metamorphism in the arc in both southern New Brunswick and centralCape Breton Island. The ~ 440 Ma arc magmatism in the Creignish Hills extends through the CapeBreton Highlands and into southern Newfoundland, and has recently been attributed to northwesterly subduction along the northern margin of the Rheic Ocean.

*Author for correspondence: [email protected]

until the Devonian (Barr & Raeside, 1986). Over thepast decade, these two views have converged. Thus,White et al. (1994) have concluded that the Avalonianoverstep sequence of Cambrian age does indeedunconformably overlie their Bras d’Or and Mira ter-ranes. However, they still believe that the two terraneswere separate, because Middle Cambrian volcanic

rocks in the Bras d’Or terrane are nearly absent in theMira terrane.

On the other hand, Hutchinson (1952) has shownthat the Middle Cambrian rocks change rapidly frommainly volcanic rocks to mainly sedimentary rocks,and so facies changes are a reasonable explanation forthe differences between Mira and Bras d’Or Cambrian

138 J. D. K E P P I E A N D OT H E R S

ZZZ

ZZZ

ZZZ

287287287

222 264

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196196196

N EC R E I G N I S H H I L L S

46° 00'61° 10'

61° 15'

SKYE MOUNTAIN

GABBRO-DIORITE

438 ±2438 ±2438 ±2

688-694688-694688-694637-676637-676637-676

WHYCOCOMAGH

CAPEBRETONISLAND

2 km

544 ±5 (H)

CREIGNISH HILLSPLUTON

RIVER DENYSPLUTON

100,101,110,325,332

31,33,63,64,66,72,74,128

SKYE MOUNTAIN

GRANITE

449 ±7 (M)449 ±7 (M)449 ±7 (M)

180,181,184,270,271a,271b,275,277a,282

551 ±1551 ±1551 ±1

CREIGNISHHILLS

PLUTON

BLUE’S MILL

111 222

333444

555

666

MELFORD STOCK

20 km

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N

M E G U M AAVA L O N

BURGEO

TERRANE

M E G U M A

HU

MB E R

200 km

N O T R E D A ME

E X P L OIT

S

A T L A N T I C

BR

AB

M

AH

COB

NOVA

SCOTI

NEWFOUNDLAND

O C E A N

(a)

(b)

(c)

G A ND

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G A ND

ER

Mafic Metavolcanic,Marble, Schist

Felsic Metavolcanic,Semipelitic Schist

Quartzite, Schist,Marble, Slate

~550-580 Ma Pluton

NeoproterozoicGeorge RiverMetamorphic

Suite

Bras d'OrGneiss

Early Silurian Gabbro

Carboniferous

550-580 Ma Igneous rocks

Neoproterozoic Metasedimentary rocks

~ 440 Ma igneous/sedimentary rocks

40Ar / 39Ar

Dated Sample

Geochemical Sample196196196

333ZirconZZZ

Figure 1. (a) Terrane map of the northern Appalachians modified after Keppie, Davis & Krogh (1998). M = Mira terrane;B = Bras d’Or terrane; A Aspy terrane; BR = Blair River terrane; AH = Antigonish Highlands; COB = Cobequid Highlands.The terranes in Cape Breton Island are after Barr & Raeside (1986). (b) The distribution of ~ 550 Ma igneous rocks in CapeBreton Island (modified after Keppie, Davis & Krogh, 1998). (c) Simplified geological map of the eastern Creignish Hills, show-ing published ages and sample locations.

stratigraphy. Lynch, Tremblay & Rose (1993) havedemonstrated that the boundary between the Bras d’Orand Aspy terranes is an unconformity, and so does notqualify as a terrane boundary. Dostal et al. (1996),Keppie & Dostal (1998) and Murphy et al. (in press)have demonstrated that the 700–550 Ma igneous activ-ity in Cape Breton Island (Mira, Bras d’Or and Aspy)and southern New Brunswick (Caledonia andBrookville terranes) can be explained as supra-subduc-tion zone magmatism above a single northwest-dipping(present co-ordinates) Benioff zone. Ayuso, Barr &Longstaffe (1996) have shown that the Pb isotope sig-natures in Neoproterozoic igneous rocks in CapeBreton Island represent mixing of two end members:Blair River and Mira. Murphy et al. (1998) havededuced that the model Nd ages of igneous rocks in theAvalon composite terrane indicate melting of a ~ 1 Gabasement.

The MacIntosh Brook Fault that separates the Brasd’Or and Mira terranes is a brittle structure with minordisplacement, not a major shear zone with large lateralmovement as required by the model proposed by vanStaal, Sullivan & Whalen (1996). These authors placethe Mira and Bras d’Or terranes several thousand kilo-metres apart along strike in the Neoproterozoic, juxta-posing them in the Siluro-Devonian by large dextraldisplacements. On the other hand, the same boundaryin southern New Brunswick (Bellisle–Kennebacasisfault zone) has undergone both dextral and sinistralSiluro-Devonian ductile deformation followed byCarboniferous dextral and vertical brittle movements(Schreckengost & Nance, 1996). Thus, in Cape BretonIsland at least, the Avalon composite terrane appears torepresent a relatively intact Neoproterozoicforearc–arc–backarc resting on a ~ 1 Ga basement andoverlain by a Lower Palaeozoic overstep sequence.

2. Geological setting

The Creignish Hills represent one of several basementblocks that project through the Carboniferous cover ofcentral Cape Breton Island (Fig. 1b). Igneous rocksdated at ~ 540–580 Ma are present in all of these base-ment blocks, and are also represented in the coastal blockof southern Cape Breton Island (Fig. 1b).Contemporaneous magmatism elsewhere in Nova Scotiais limited to isolated plutons in the Antigonish andCobequid highlands. In the Creignish Hills, plutonsdated to ~ 540–580 Ma intrude metasedimentary andmetavolcanic rocks deposited between ~ 977 Ma and~ 637 Ma, which were multiply deformed and metamor-phosed to greenschist–amphibolite facies at low pres-sures and high temperatures at ~ 553–550 Ma, andintruded by syn-tectonic granitic sheets containing zirconand monazite dated at 551 ± 1 Ma (Keppie, Davis &Krogh, 1998). Gravitational collapse of the arc is inferredto have placed low- over high-grade rocks along low-angle decollements (Keppie, Davis & Krogh, 1998).

The synchroneity of intrusion and tectonism isreflected in the igneous rocks that show fabrics rangingfrom strongly foliated to massive, and reflect the syn-tectonic to post-tectonic nature of the intrusion rela-tive to local deformation. Thus, the contact of theMelford stock (Fig. 1c) cuts nearly at right anglesacross the multiply deformed metamorphic layers ofthe Bras d’Or Gneiss, and yet is internally weakly foli-ated parallel to the composite foliation in the hostrocks. This suggests that intrusion was late syn-tec-tonic. The River Denys pluton is generally massive, butalong its northern margin it exhibits ductile–brittledeformation, indicating that it is also late syn-tectonic.On the other hand, the Creignish Hills and SkyeMountain granitic plutons are both massive, and theircontacts cut across the composite foliation in the coun-try rocks, indicating that they are post-tectonic.Furthermore, the Skye Mountain granite cuts acrossthe sheared contact between the high- and low-graderocks, and andalusite in the contact aureole overgrowsall the fabrics in the host rocks.

The Creignish Hills pluton has yielded a Rb–Srwhole-rock isochron age of 446 ± 13 Ma (White, Barr& Campbell, 1990). However, diorite in the easternpart of the Creignish Hills pluton yielded two 40Ar/39Arhornblende plateau ages of 544 ± 5 Ma and 535 ± 3Ma, interpreted closely to post-date intrusion (Keppie,Dallmeyer & Murphy, 1990). This led White, Barr &Campbell (1990) to exclude the diorite analyses fromthe Rb–Sr whole-rock isochron, which then gave an ageof 441 ± 8 Ma. Although White, Barr & Campbell(1990) state that the intermediate felsic rocks couldhave been fractionated from the mafic rocks, they pre-fer a correlation with the Kellys Mountain and CapeSmoky granites dated at ~ 495 Ma (Dunning et al.1990a) based upon geochemical data. To allow thisconclusion, White, Barr & Campbell (1990) excludedtwo more analyses of aplite from the Rb–Sr whole-rock isochron, which then produced an age of473 ± 102 Ma.

40Ar/39Ar analyses of muscovite in the gneiss justeast of the Skye Mountain granite yielded a plateauage of 449 ± 7 Ma, whereas muscovite from a peg-matite produced a discordant spectrum that decreasedfrom ~ 485 to 450 Ma (Dallmeyer & Keppie, 1993). TheBras d’Or Gneiss was subsequently intruded by thearc-related Skye Mountain gabbro–diorite, which hasyielded a U–Pb concordant zircon age of 438 ± 2 Ma(Fig. 1c; Keppie et al. 1998). The contact between theSkye Mountain gabbro and the Skye Mountain graniteis not exposed; however, granitic dykes extending sev-eral metres into the gabbro perpendicular to theinferred contact suggest that the granite is younger.The possibility that the granitic dykes represent backveining is considered unlikely, because the graniticdykes form discrete intrusions rather than a vein net-work, and the gabbro is a small, high-level pluton thatdoes not appear to have been a conduit for mafic

Superposed magmatic arcs, Cape Breton Island 139

magma (Keppie et al. 1998). Carboniferous rocksunconformably overlie the eastern Creignish Hillsaround their northern margin, and are in fault contactalong most of its southern margin (Fig. 1c).

Igneous lithologies vary from diorite through gran-odiorite, and granite to pegmatitic granite. The RiverDenys pluton is predominantly massive diorite, theeastern Creignish Hills pluton varies from diorite togranodiorite and the Skye Mountain granite consistsof granite and pegmatitic granite. The diorite consistsmainly of plagioclase, amphibole and biotite variablyaltered to actinolite and chlorite with minor amountsof microcline and quartz, and accessory apatite, titan-ite and zircon. The granodiorite is composed of plagio-clase, alkali feldspar, quartz, amphibole and biotitemore or less altered to chlorite, epidote and calcite withaccessory apatite, zircon and titanite. The granite ismade up of plagioclase, alkali feldspar and quartz withminor biotite and muscovite and accessory apatite, zir-con and opaque minerals. Pegmatitic phases containalkali feldspar and quartz with minor biotite.

3. Analytical methods

A total of 40 samples were collected from several smallintrusions for U–Pb, 40Ar/39Ar isotopic, major andtrace element analyses (Fig. 1c). Samples weighing 20kg were collected for U–Pb zircon analyses from theeastern part of the Creignish Hills pluton, Melfordstock, River Denys pluton and Skye Mountain granite.Zircon was processed by a method similar to that ofKrogh (1973), but using a 0.3 ml resin volume and amixed 205Pb –233U–235U spike. Magnetic and non-mag-netic fractions were separated using a Franz separator,

and some were abraded. Isotope ratios were measuredon a VG sector mass spectrometer at the Université duQuebec à Montreal. The blank for the entire analyticalprocedure ranged from 5 to 20 pg. Dimensions of theerror ellipses in the concordia diagrams and errors inages are at the 95 % confidence level, and include mea-surement error, confidence in the fractionation factors,error in the U–Pb ratio of the spike and the effect ofthe common lead correction. Table 1 lists the U–Pbdata.

Concentrates of hornblende and muscovite wereanalysed using incremental-release 40Ar/39Ar analyses(Tables 2, 3). The techniques used generally followedthose described in detail by Dallmeyer & Takasu (1992).Optically pure (> 99 %) mineral concentrates werewrapped in aluminium foil packets, encapsulated insealed quartz vials, and irradiated in the United StatesGeological Survey TRIGA reactor in Denver.Variations in the flux of neutrons along the length ofthe irradiation assembly were monitored with severalmineral standards, including Mmhb-1 hornblende(Samson & Alexander, 1987). The samples were incre-mentally heated until fusion in a double-vacuum, resis-tance-heated furnace following methods described byDallmeyer & Gil-Ibarguchi (1990). Measured isotopicratios were corrected for total system blanks, the effectsof mass discrimination and interfering isotopes pro-duced during irradiation. 40Ar/39Ar ages were calculatedfrom corrected isotope ratios using the decay constantsand isotopic abundance ratios listed by Steiger & Jäger(1977).

Intralaboratory uncertainties have been calculatedby statistical propagation of uncertainties with mea-surements of each isotopic ratio (at two standard devi-


Table 1. U–Pb isotopic analyses of intrusive bodies in the eastern Creignish Hills

Weight U Pbrad207Pb/206Pb

Zircon fractiona (mg) (ppm) (ppm) 206Pb/204Pbb 208Pb/206Pbc 206Pb/ 238Uc 207Pb/235Uc 207Pb/206Pbc age (Ma)

Creignish Hills pluton. Regression through zero = 553 ± 2 Ma

2NM, Ab, 75 ×250 0.300 301 29.8 3966 0.234 0.08873 0.7167 0.05859 5522M, Ab, 75 ×250 0.256 337 33.5 4999 0.230 0.08961 0.7246 0.05865 554

Melford Stock. Regression through zero = 586 ± 2 Ma

2NM, Ab, 75 ×250 0.102 233 22.2 1254 0.139 0.09221 0.7571 0.05955 5872M, 100 ×300 0.061 257 22.9 1459 0.121 0.08782 0.7203 0.05949 585

River Denys pluton. Regression: lower = 540 ± 3 Ma; upper = 2095 ± 13 Ma

2NM, 50 ×150 0.052 194 27.5 1501 0.223 0.12599 1.5056 0.08667 13532M, 50 ×150 0.030 185 18.3 1651 0.226 0.08922 0.7408 0.06022 612

Skye Mountain granite.

2NM, Ab, 50 ×175 0.027 351 35.4 1300 0.199 0.09290 0.8183 0.06389 7382M, 50 ×175 0.021 404 38.9 1724 0.222 0.08705 0.7662 0.06384 736

a N(M) are nonmagnetic (magnetic) fractions and the numeral 2 is degrees tilt on the Frantz LB-1 Separator; 75 ×250 etc. are zircondimensions in microns. Ab, abraded.

b Atomic ratios corrected for fractionation and spike.c Atomic ratios corrected for fractionation, spike, blanks and common Pb from the model of Stacey and Kramers (1975).

ations of the mean) through the age equation.Interlaboratory uncertainties are c. 1.25–1.5 % of thequoted age. Total-gas ages have been computed foreach sample by appropriate weighting of the age andthe percentage of 39Ar released within each tempera-ture increment. A ‘plateau’ is defined when the agesrecorded by two or more contiguous gas fractionsadding up to > 50 % of the total 39Ar evolved are mutu-ally similar within ± 1 % intralaboratory uncertainty,each fraction having similar apparent K/Ca ratios andrepresenting > 4 % of the total 39Ar evolved. Plateauportions of the analyses have been plotted on 36Ar/40Arisotopic correlation diagrams. Regression techniquesfollowed the methods of York (1969). A mean squareof the weighted deviates (MSWD) has been used toevaluate isotopic correlations. Analyses of the Mmhb-1 monitor indicate that apparent Ca ratios may be cal-culated through the relationship 0.518(100.005) ×(39Ar/37Ar)corrected.

Geochemical samples were collected from the mainintrusions and also from isolated small stocks indicatedby a sample location on the map (Fig. 1c). They wereanalysed by X-ray fluorescence for major and sometrace elements (Rb, Sr, Ba, Zr, Nb, Y, Cr, Ni) (Table 4).Precision and accuracy are discussed in Dostal, Dupuy& Caby (1994) and Dostal et al. (1994). In general, pre-cision is better than ± 5 % for major elements and2–10 % for trace elements.

4. Geochronology

4.a. U–Pb zircon data

Two abraded fractions of clear, euhedral zircon grainsfrom diorite at the eastern edge of the Creignish Hillspluton yielded two nearly concordant U–Pb analysesthat plot on a chord through zero, with an upper inter-cept at 553 ± 2 Ma (Fig. 2, Table 1). Two fractions ofclear euhedral zircon grains from the Melford granitic


Table 2. 40Ar/39Ar analytical data for incremental heating experiments on hornblende concentrates

Release (37Ar/39Ar)c %40Ar 36ArCa Apparenttemp (°C) (40Ar/39Ar)* (36Ar/39Ar)* % of total 39Ar non-atmos. + % age (Ma)**

Sample 3: J = 0.010302

610 118.30 0.29820 9.404 1.35 26.15 0.86 501.5 ± 5.3710 51.42 0.05469 10.560 1.11 70.21 5.25 573.7 ± 4.4 750 46.92 0.05194 18.035 1.26 70.37 9.45 533.4 ± 2.2 780 41.51 0.02867 15.995 1.62 82.68 15.18 550.9 ± 0.7 800 39.52 0.01397 8.565 2.94 91.28 16.68 572.6 ± 1.3 820 37.38 0.00849 7.236 8.66 94.83 23.18 563.6 ± 1.0 835 36.28 0.00691 6.971 12.95 95.90 27.44 554.6 ± 0.4 845 35.71 0.00558 7.197 15.87 96.99 35.09 552.6 ± 0.7 855 35.17 0.00429 6.975 9.95 97.98 44.27 550.0 ± 0.5 870 35.13 0.00308 7.353 8.10 99.08 65.02 554.9 ± 0.4 885 34.73 0.00343 7.897 8.89 98.89 62.55 548.8 ± 0.9 905 35.12 0.00512 8.241 8.99 97.56 43.82 547.7 ± 0.9 930 34.61 0.00337 8.083 9.64 98.98 65.28 547.6 ± 0.4

Fusion 34.85 0.00804 8.137 8.65 95.04 27.54 531.9 ± 1.1 Total 37.14 0.01114 7.871 100.00 95.18 39.91 550.5 ± 0.8 Total without 610–820 °C 74.40 551.1 ± 0.6and fusion

Sample 4: J = 0.010322

620 110.87 0.28396 7.310 2.044 24.84 0.701 453.1 ± 8.6 720 42.26 0.04077 9.440 2.02 73.27 6.30 502.9 ± 3.6 765 45.74 0.03698 17.741 3.76 79.21 13.05 578.6 ± 6.4 785 40.68 0.01839 11.127 1.87 88.83 16.46 575.3 ± 2.5 805 38.73 0.01352 8.002 4.09 91.33 16.10 563.8 ± 2.1 820 36.52 0.01021 6.744 5.46 93.21 17.97 545.1 ± 0.5 830 35.73 0.00633 6.590 15.07 96.23 28.30 549.9 ± 0.8 840 35.64 0.00538 6.570 12.64 97.00 33.21 552.4 ± 0.4 850 35.04 0.00364 6.321 13.14 98.36 47.17 550.9 ± 0.4 865 34.57 0.00182 6.390 12.78 99.92 95.70 551.9 ± 0.2 880 34.60 0.00363 6.674 9.01 98.44 50.06 545.4 ± 0.9 900 34.73 0.00339 7.280 8.21 98.78 58.34 549.0 ± 0.4 925 34.97 0.00307 7.320 7.00 99.08 64.96 553.7 ± 1.4

Fusion 34.34 0.00422 7.611 2.9300 98.13 49.02 540.7 ± 1.3 Total 37.50 0.01274 7.314 100.00 94.68 44.75 549.1 ± 1.1 Total without 620–805 °C 83.30 550.2 ± 0.6

and fusion

* measured; ** two sigma, intralaboratory errors.c corrected for post-irradiation decay of 37Ar (35.1 day !s life).+ [40Artot. – (36Aratmost.) (295.5)] / 40Artot.Data from River Denys pluton, Creignish Hills, Nova Scotia


Table 3. 40Ar/39Ar analytical data for incremental heating experiments on muscovite concentrates

Release 39Ar %40Ar 36ArCa Apparenttemp (°C) (40Ar/39Ar)* (36Ar/39Ar)* (37Ar/39Ar)c % of total non-atmos. + % age (Ma)**

Sample 1: J = 0.009182Bras d’Or Gneiss: contact zone of Melford Stock

550 35.4200 0.00978 0.177 4.35 91.86 0.49 471.6 ± 1.8600 35.13 0.00325 0.242 4.56 97.30 2.02 492.4 ± 0.4 640 34.66 0.00180 0.362 4.72 98.54 5.49 492.1 ± 0.9 675 35.19 0.00268 0.191 5.94 97.78 1.94 495.3 ± 0.7 710 34.76 0.00289 0.131 9.07 97.55 1.23 488.9 ± 0.3 745 33.96 0.00188 0.050 14.15 98.36 0.73 482.5 ± 0.3 775 33.87 0.00203 0.471 7.46 98.32 6.31 481.3 ± 0.2 800 33.79 0.00171 0.186 7.39 98.53 2.95 481.1 ± 0.3 830 33.9979 0.00182 0.059 6.66 98.41 0.88 483.1 ± 0.4 870 33.62 0.00095 0.343 4.64 99.23 9.77 482.1 ± 0.3 900 33.92 0.00148 0.103 14.44 98.71 1.88 483.6 ± 0.2 930 34.12 0.00117 0.113 13.55 98.99 2.62 487.3 ± 0.3

Fusion 35.17 0.00049 0.222 3.0700 99.63 12.41 503.2 ± 1.7 Total 34.28 0.00218 0.172 100.00 98.16 2.91 485.5 ± 0.4 Total without 550–710 °C 54.74 482.1 ± 0.3 and fusion

Sample 2: J = 0.010235Melford Stock

470 32.30 0.02446 3.207 0.34 78.40 3.57 416.5 ± 2.2 530 30.87 0.00815 0.241 1.67 92.24 0.80 461.5 ± 0.7 565 29.48 0.00154 0.519 2.02 98.58 9.18 469.9 ± 0.4 600 29.36 0.00212 0.106 1.87 97.87 1.35 465.1 ± 0.4 635 29.47 0.00237 0.086 3.60 97.63 0.99 465.6 ± 0.4 670 30.20 0.00262 0.032 5.87 97.43 0.33 474.9 ± 0.2 705 30.17 0.00209 0.026 10.52 97.94 0.34 476.6 ± 0.3 740 29.66 0.00140 0.168 11.85 98.63 3.25 472.6 ± 0.2 775 29.53 0.00171 0.075 12.18 98.29 1.19 469.3 ± 0.2 810 29.58 0.00226 0.080 12.58 97.75 0.97 467.7 ± 0.3 845 29.59 0.00212 0.045 10.77 97.88 0.58 468.3 ± 0.3 885 29.96 0.00196 0.040 9.92 98.06 0.56 474.3 ± 0.2 930 30.24 0.00182 0.141 8.88 98.24 2.10 478.9 ± 0.2

Fusion 30.50 0.00123 0.072 7.92 98.81 1.60 485.0 ± 0.2 Total 29.877 0.00209 0.101 100.00 97.95 1.41 472.6 ± 0.2 Total without 470–635 °C 73.70 471.6 ± 0.3and fusion Data from Creignish Hills.

Sample 5: J = 0.009745Bras d’Or Gneiss

490 21.02 0.01477 3.625 0.51 80.60 6.68 276.3 ± 3.8 550 30.65 0.00492 0.362 2.11 95.33 2.00 452.0 ± 0.6 585 29.58 0.00257 0.801 2.74 97.63 8.46 447.5 ± 0.4 620 29.47 0.00077 0.259 3.65 99.28 9.16 452.5 ± 0.5 655 29.46 0.00087 0.435 3.47 99.22 13.52 452.3 ± 0.2 690 29.93 0.00165 0.100 8.09 98.38 1.65 455.1 ± 0.6 725 29.78 0.00085 0.174 13.64 99.18 5.58 456.3 ± 0.1 755 29.58 0.00107 0.064 12.33 98.93 1.62 452.7 ± 0.2 765 29.800 0.00171 0.196 7.92 98.34 3.13 453.3 ± 0.3 785 29.65 0.00100 0.071 11.26 99.00 1.95 453.8 ± 0.2 820 29.69 0.00100 0.141 10.03 99.02 3.83 454.5 ± 0.2 855 29.77 0.00088 0.148 8.94 99.14 4.54 456.1 ± 0.1 940 30.04 0.00065 0.106 11.61 99.37 4.44 460.6 ± 0.2


Sample 6: J = 0.009492Skye Mountain granite

490 29.70 0.01050 2.202 0.79 90.12 5.70 408.8 ± 3.8 550 31.15 0.00523 0.314 3.20 95.10 1.60 447.0 ± 0.4 585 29.78 0.002436 0.353 3.40 97.66 3.95 439.8 ± 0.8 620 29.69 0.00248 0.174 3.82 97.5681 1.91 438.2 ± 0.5 655 29.605 0.00225 0.294 8.45 97.81781 3.55 438.0 ± 0.4 690 29.650 0.00229 0.204 8.76 97.7598.93 2.43 438.4 ± 0.4 725 29.601 0.00112 0.237 9.08 98.9397.91 5.76 442.4 ± 0.7

stock yielded discordant data that plot on a chordthrough zero, with an upper intercept of 586 ± 2 Ma(Fig. 2b). The non-magnetic fraction was abraded, andis nearly concordant with a 207Pb/206Pb age of ~ 587 Ma(Table 1). Two zircon fractions (a magnetic fractionwith clear and euhedral grains, and a non-magneticfraction with zoned euhedral grains) from the RiverDenys pluton yielded discordant data, and a chordconstructed through these two points gives lower and

upper intercepts at 540 ± 3 Ma and 2095 ± 13 Ma (Fig.2c). The magnetic fraction plots close to concordia,and yielded a 207Pb/206Pb age of 612 Ma (Table 1).Zircon is relatively rare in the Skye Mountain granite;however, two small fractions of euhedral–anhedral,zoned zircon grains were separated, and gave highlydiscordant data that plot on a chord through zero withan upper intercept at ~ 737 Ma (Fig. 2d).


Table 3. (Cont.)

Release 39Ar %40Ar 36ArCa Apparenttemp (°C) (40Ar/39Ar)* (36Ar/39Ar)* (37Ar/39Ar)c % of total non-atmos. + % age (Ma)**

755 29.611 0.00211 0.156 8.89 97.918.26 2.01 438.5 ± 0.4 785 29.5170 0.00179 0.290 9.62 98.267.63 4.40 438.6 ± 0.3 815 29.703 0.00243 0.254 9.07 97.638.58 2.84 438.6 ± 0.3 850 29.733 0.00144 0.128 7.40 98.587.81 2.41 442.7 ± 0.4 885 29.839 0.00222 0.094 9.23 97.818.13 1.16 441.0 ± 0.3 925 29.89 0.00192 0.194 8.78 98.1320 2.74 443.0 ± 0.3


* measured; ** two sigma, intralaboratory errors.C corrected for post-irradiation decay of 39Ar (35.1 day !s life).+ [40Artot. – (36Aratmos.) (295.1)] / 40Artot.

Figure 2. U–Pb isotopic analyses plotted on concordia diagrams of zircon from (a) the easternmost Creignish Hills pluton, (b)the Melford stock, (c) the River Denys pluton and (d) the Skye Mountain granite.

Tab

le 4

.R

epre

sent

ativ

e ch

emic

al a

naly

ses

ofin

trus

ive

rock

s fr

om th

e ea

ster

n C

reig

nish

Hill

s

Riv

er D

enys

plu

ton

Cre

igni

sh p

luto

nSm

all s

tock

sSk

ye M

ount

ain

gran

ite

Sam

ple

6364

6612

810

010

111

032

515

022

226

418

018

427

1A27

7A28

2

SiO

2(%

)57

.47

58.9

356

.70

57.0

060

.63

62.0

850

.78

56.9

560

.82

62.5

953

.82

72.5

669

.26

76.1

965

.43

76.5

5T

iO2

0.66

0.64

0.71

0.72

0.62

0.56

0.89

0.71

0.53

0.63

0.67

0.32

0.44

0.08

0.56

0.07

Al 2O

317

.12

17.1

315

.77

16.7

514

.55

14.2

718

.58

17.4

016

.70

16.0

010

.68

14.3

115

.69

13.0

816

.22

12.8

1F

e 2O3*

6.95

6.54

7.94

7.24

6.53

5.55

9.37

7.34

5.97

5.87

9.84

2.36

3.25

0.87

4.30

0.63

MnO

0.14

0.14

0.16

0.15

0.12

0.11

0.16

0.16

0.14

0.19

0.24

0.10

0.12

0.03

0.06

0.03

MgO

3.79

3.53

5.26

3.99

4.14

3.51

4.74

3.30

3.08

2.46

10.4

90.

630.

990.

202.

430.

15C

aO6.

475.

696.

096.

325.

534.

726.

145.

404.

873.

848.

761.

481.

790.

491.

490.

40N

a 2O2.

673.

042.

613.

072.

622.

385.

063.

163.

383.

181.

403.

513.

883.

413.

713.

12K

2O1.

941.

672.

131.

663.

114.

321.

191.

902.

251.

881.

463.

462.

934.

753.

194.

57P

2O5

0.16

0.15

0.14

0.15

0.15

0.15

0.20

0.16

0.11

0.19

0.10

0.08

0.11

0.02

0.19

0.02

LO

I1.

912.

112.

121.

961.

631.

281.

912.

031.

431.

921.

390.

300.

900.

401.

700.

20To

tal

99.2

899

.56

99.6

198

.99

99.6

398

.94

99.0

398

.52

99.2

898

.75

98.8

699

.11

99.3

699

.52

99.2

898

.55

Mg

no.

51.9

51.7

56.7

52.2

55.7

55.6

50.0

47.1

50.5

45.3

67.9

34.6

37.6

31.3

52.8

32.0

Cr

(ppm

)34

3110

044

8077

2527

1414

406

117

5912

Ni

1819

2828

2519

112

119

535

33C

o20

1931

2418

1931

2217

1638

Sc5

1412

810

1212

105

810

V16

015

219

216

616

214

825

315

213

513

625

629

398

89C

u38

1849

3467

4639

144

2558

6921

Zn

8077

8488

7366

9170

122

123

9640

6310

429

Rb

6462

7959

112

142

3489

6164

5911

591

126

108

114

Ba

717

391

475

365

620

772

425

592

506

582

463

420

429

247

589

166

Sr29

231

326

630

824

721

827

631

727

941

216

916

025

158

484

42G

a17

2017

1917

1718

1718

1814

1315

1218

10N

b10

1110

1113

138

108

138

1212

89

8Z

r19

514

316

615

117

223

415

718

817

518

116

813

717

166

158

57Y

2527

2631

2625

3233

1826

2431

3236

2437

Th

55

55

78

45

96

5L

a19

2819

3342

4240

3526

3435

Ce

5371

5180

6462

5969

5465

42N

d24

3644

3025

Riv

er D

enys

plu

ton:

sam

ples

63,

64,6

6,12

8;C

reig

nish

plu

ton:

sam

ples

100

,101

,110

,325

;sm

all s

tock

s:sa

mpl

es 1

50,2

22,2

64;S

kye

Mou

ntai

n gr

anit

e:sa

mpl

es 1

80,1

84,2

71A

,277

A,2

82.

Mg

no.=

100

×M

gO/M

go+

FeO

tot(m

ole

%).


4.b. 40Ar/39Ar data

Multigrain hornblende and muscovite concentrateswere prepared from samples collected within variousgeological units exposed in the Creignish Hills (Fig. 1).The 40Ar/39Ar analytical data are provided in Tables 2and 3, and are displayed as apparent age spectra inFigures 3, 4 and 5.

4.b.1. Hornblende

Hornblende separated from two samples of diorite atlocations 3 and 4 in the River Denys pluton displaysvariably discordant apparent age spectra (Fig. 3). Therelatively small volume low-temperature gas fractionsrecord considerable variation in apparent ages. Theseare matched by fluctuations in apparent K/Ca ratios,which suggest that experimental evolution of argonoccurred from compositionally distinct, relatively non-retentive phases. These could have been represented by(1) very minor, optically undetectable mineralogicalcontaminants in the hornblende concentrates; (2) pet-rographically unresolvable exsolution or compositionalzonation within constituent hornblende grains; (3)minor chloritic replacement of hornblende; and/or (4)intracrystalline inclusions. Most intermediate- andhigh-temperature gas fractions display little intra-sample variation in apparent K/Ca ratios, suggestingthat experimental evolution of gas occurred from com-

positionally uniform sites. The intermediate- and high-temperature gas fractions experimentally evolvedfrom the two hornblende concentrates record similarintrasample apparent 40Ar/39Ar ages, which defineplateau dates of 551 ± 0.6 Ma (sample 3) and550.2 ± 0.6 Ma (sample 4). 36Ar/40Ar v. 39Ar/40Ar iso-tope correlations of the plateau data are well defined(MSWD < 2.0), and define inverse ordinate intercepts(40Ar/36Ar ratios) of 406 ± 10 (sample 3) and 293.4 ± 12(sample 4). These are generally similar to that of pre-sent-day atmosphere, and suggest no significantintracrystalline contamination with extraneous(‘excess’) argon components. Using the inverse abscissaintercepts (40Ar/39Ar ratios) in the age equation yieldsplateau isotope-correlation ages of 545.3 ± 0.9 Ma(sample 3) and 549.8 ± 0.5 Ma (sample 4) (Fig. 3).Because calculation of isotope-correlation ages doesnot require assumption of a present-day 40Ar/36Arratio, they are considered more significant than thosedirectly calculated from the analytical data. The 545and 550 Ma isotope-correlation ages recorded by thetwo hornblende concentrates from the River Denyspluton are considered geologically significant, and areinterpreted to date the last cooling through tempera-tures required for intracrystalline retention of argon inconstituent grains.

4.b.2. Muscovite

Muscovite concentrates were prepared from four sam-ples collected in the Melford stock and Skye Mountaingranite and their contact aureoles (Figs 4, 5). The mus-covite concentrates are characterized by very largeapparent K/Ca ratios, which are marked by consider-able analytical uncertainties. The apparent K/Ca ratiosdisplay no significant and/or systematic intrasamplevariations, and therefore are not presented with theapparent age spectra.

The four muscovite concentrates display variableintrasample age variations. Those from samples 5 and 6(Fig. 4) are characterized by only limited age varia-tions, and record well-defined plateau ages of455.4 ± 0.2 Ma (sample 5: 690–940 °C; 83.82 % of thetotal gas evolved) and 440.6 ± 0.4 Ma (sample 6: 655°C-fusion; 88.79 % of the gas evolved). These are con-sidered geologically significant, and are interpreted todate the last cooling through temperatures required forintracrystalline argon retention.

The muscovite concentrates prepared from samples1 and 2 (Fig. 5) are marked by more internally discor-dant 40Ar/39Ar spectra, in which apparent ages system-atically decrease throughout low-temperature portionsof the analysis to define intermediate-temperatureplateaux that are followed by an increase in apparentage in the highest-temperature increments. The745–900 °C increments evolved from sample 1 recordsimilar apparent ages that define a plateau of482.1 ± 0.3 Ma. These comprise 54.74 % of the total


Figure 3. 40Ar/39Ar incremental-release spectra for horn-blende from the River Denys pluton (samples 3 and 4).

analysis. A 471.5 ± 0.3 Ma plateau is defined by the670–885 °C increments evolved from sample 2 (repre-senting 73.70 % of the total analysis). The geologicalsignificance of these plateau ages is uncertain in view ofthe internal complexity of the two analyses.

4.c. Interpretation of geochronological data

The nearly concordant zircon age of 553 ± 2 Ma fromdiorite at the eastern margin of the Creignish Hills plu-tonic complex is inferred to date the time of intrusion(Fig. 2a). Hornblende from the same location pro-duced a plateau age of 544 ± 5 Ma (Keppie, Dallmeyer& Murphy, 1990), and indicates that the pluton cooledrelatively quickly through closure temperatures forargon in hornblende estimated to be ~ 500 °C(Harrison, 1981). Similarly, the U–Pb zircon lowerintercept age and 40Ar/39Ar on hornblende isotope-cor-relation ages from the River Denys dioritic pluton arethe same within analytical errors (Figs 2c, 3), and over-lap in the range 540–550 Ma, which is interpreted to

post-date closely the time of intrusion. The similarityof the zircon and hornblende ages indicates that thispluton also cooled quickly through ~ 500 °C. The simi-larity of the Creignish Hills and River Denys dioritessuggests that they are part of the same magmatic event(see below).

Interpretation of the isotopic data from within andadjacent to the Melford granitic stock is more complex.The similarity of the U–Pb upper intercept age of586 ± 2 Ma and the 207Pb/206Pb age of ~ 587 Ma age ofthe nearly concordant, abraded, clear, euhedral frac-tion suggests that this represents the time of intrusion.This is consistent with the observation that the stockwas foliated during a deformational event dated at 550Ma (Keppie, Davis & Krogh, 1998). On the other hand,the muscovite plateau ages of ~ 470–484 Ma suggesteither slow cooling between 500 and 400 °C (i.e. theclosure temperatures for Ar in hornblende and mus-covite; Harrison, 1981; G. A. Robins, unpub. M.Sc.thesis, Brown Univ. 1972) or reheating. The latter isconsistent with the saddle-shaped spectra, which sug-gest the presence of excess argon. Such reheating maybe associated with intrusion of the 438 ± 2 Ma SkyeMountain gabbro–diorite (Keppie et al. 1998).

Interpretation of the isotopic data from the SkyeMountain granite is more problematic. Inheritanceand recent lead loss in the U–Pb zircon data are indi-cated by the zoned nature of the zircons and the zerolower intercept, respectively. Thus, the ~ 737 Ma upperintercept age represents a maximum for the time ofintrusion. On the other hand, the ~ 441–455 Ma mus-covite plateau ages from north and west of the SkyeMountain granite (Fig. 5) are similar to the 449 ± 7 Mamuscovite plateau age from just east of the pluton(Dallmeyer & Keppie, 1993). The adjacent SkyeMountain gabbro–diorite has been dated at 438 ± 2 Maby U–Pb on zircon (Keppie et al. 1998), and it is possi-ble that the Skye Mountain granite was penecontem-poraneous, a conclusion consistent with the presenceof granitic dykes in the gabbro near their mutual con-tact. This suggests that the muscovite ages in the adja-cent host rocks represent relatively complete thermalre-equilibration of argon at ~ 438 Ma.

5. Geochemistry

5.a. Results

The intrusive rocks of the Creignish Hills were affectedto varying degrees by secondary processes includinglow-grade metamorphism, which might have modifiedthe chemical composition of these rocks. However, theconcentrations of most major and trace elements,including alkali and alkali-earth elements, are thoughtto reflect the primary magmatic distribution. Whenthese elements are plotted against SiO2, which is con-sidered to be a good indicator of the fractionation andis apparently immobile under most metamorphic con-ditions (e.g. Winchester & Floyd, 1977), they display


Figure 4. 40Ar/39Ar incremental-release spectrum for mus-covite from gneiss west of the Skye Mountain granite (sam-ple 5), and from the contact aureole adjacent to the northernboundary of the Skye Mountain granite (sample 6).

Figure 5. 40Ar/39Ar incremental-release spectra for mus-covite from gneiss adjacent to (sample 1), and within (sample 2), the Melford stock, respectively.

distinct correlations (Fig. 6). Remobilization duringmetamorphism is unlikely to produce such a consistentresult. The consistency of these trends, and their simi-larities to those of modern igneous rocks, suggest thatthe distribution of these elements was not significantlymodified.

The intrusive rocks of the eastern Creignish Hillsmay be subdivided into two groups. First, the~ 540–585 Ma Creignish Hills diorite and associatedmafic–intermediate intrusive bodies, and second, the~ 438 Ma Skye Mountain granite. The Neoproterozoicgroup has SiO2 in the range 50–63 % (LOI-free) withthe majority falling between 56–62 % (Fig. 6, Table 4).The Skye Mountain granite has SiO2 contents from 65

to 77 % (Fig. 6, Table 4). Both groups are subalkalineand display calc-alkaline fractionation trends (Figs 6c,6d, 7). Chemical evolution, as revealed by the relativelyconstant Na2O/K2O ratio while CaO decreases (Fig.6f), resembles the calc-alkaline trend of Nockolds &Allen (1953), but departs significantly from the trond-hjemitic trend of Barker & Arth (1976) and the low-calcium granite trend of Breaks & Moore (1992). TheNeoproterozoic rocks are relatively diverse, althoughthey have compositions that correspond mainly to thatof medium-K andesites (Fig. 6e).

The rocks of the Skye Mountain granite are not onlyyounger, but are also geochemically distinct from theNeoproterozoic rocks. In contrast to most calc-alkaline


Figure 6. Plots of various major elements against SiO2 from intrusions in the eastern Creignish Hills. (a) FeOtot; (b) TiO2; (c)FeOtot/MgO (fields after Miyashiro, 1974). (d) Na2O + K2O (fields after Le Bas et al. 1986). (e) K2O (fields after Le Maitre,1989). (f) CaO. For explanation of symbols see Figure 7.

rocks from volcanic arcs and continental margins, theSkye Mountain granites are peraluminous, with CIPWnormative compositions containing corundum andwith molecular Al2O3/(CaO + Na2O + K2O) ratios > 1.Due to alteration processes, we cannot be certain thatCaO, Na2O and K2O concentrations represent truemagmatic values. However, as noted above, we believethat in overall terms the rocks have not been signifi-cantly modified and that the Ca, Na and K values areclose to their original levels.

All the intrusive rocks of the eastern Creignish Hillsreported here show trace element abundances charac-teristic of volcanic arc granitoids (Table 4). For exam-ple, the felsic rocks all fall within the volcanic arcgranitoid field in the Y + Nb v. Rb, and the Y v. Nb tec-tonic discriminant diagrams (Fig. 8; Pearce, Harris &Tindle, 1984). The mantle-normalized trace elementpatterns of the analysed rocks are characterized by Nband small Ti depletion (Fig. 9), a feature typical of sub-duction-related rocks. These data are similar to thosereported by White, Barr & Campbell (1990) from themain Creignish Hills pluton. These authors also pre-sented rare earth element (REE) analyses that showlight-REE enrichment and unfractionated heavy-REE.In particular, the Neoproterozoic rocks are comparableto their tonalite–diorite unit, whereas the SkyeMountain granites are compositionally similar to thegranitic rocks of the Creignish Hills pluton.

5.b. Petrogenesis

The geochemistry of both the Neoproterozoic andSilurian intrusions in the eastern Creignish Hills rocksare typical of arc-related rocks. However, the limitedamount of data does not allow discrimination betweenthe various models invoked for the origin of such rocks,

including partial melting of subducted oceanic crust(Defant & Drummond, 1990) and mantle wedge (Gill,1981). Similar Neoproterozoic intrusions elsewhere incentral Cape Breton Island originated above a north-west-dipping (present co-ordinates) subduction zone(Dostal et al. 1996).

The peraluminous nature of the Skye Mountaingranitic rocks distinguishes them from the SkyeMountain gabbro–diorite. Two processes can account


Figure 7. Intrusive rocks of the eastern Creignish Hills plot-ted on the AFM diagram (Na2O + K2O:FeOtot:MgO). SeeFigure 1c for sample locations. Fields from Irvine & Baragar(1971).

Figure 8. Intrusive rocks of the eastern Creignish Hills(shaded field) plotted on (a) Nb v. Y, and (b) Rb v. Y + Nb(fields after Pearce, Harris & Tindle, 1984). VAG = volcanic arcgranite; syn-COLG = syn-collisional granite; WPG = within-plate granite; ORG = ocean ridge granite.

for the origin of these granitic rocks. Hornblende frac-tionation can generate trends towards peraluminouscomposition (Cawthorn & Brown, 1976; Cawthorn &O’Hara, 1976; Cawthorn, Strong & Brown, 1976).However, the absence of hornblende in the SkyeMountain granite indicates that this mechanism hasnot been important in its genesis. Alternatively, modelshave been proposed for the generation of peralumi-nous melts by partial melting of pelitic rocks (Grant,1985; Vielzeuf & Holloway, 1988), and these appear tobe applicable to the Skye Mountain granite. It is sug-gested that intrusion of mafic and intermediate magma of the Skye Mountain gabbro–diorite (Keppieet al. 1998) into the crust caused crustal melting. Inparticular, the heat for the melting may have been sup-plied by mafic mantle-derived magmas. If these rocks

are subduction related, the tectonic setting is likely tohave been an active continental margin rather than anoceanic arc.

6. Tectonic implications

6.a. Neoproterozoic

The 540–585 Ma supra-subduction zone magmaticrocks in the eastern Creignish Hills form part of anextensive belt running across central Cape BretonIsland (Fig. 1b). These magmatic arc rocks are overlainby Middle Cambrian–Lower Ordovician rocks thatinclude bimodal, within-plate, rift-related volcanicrocks dated at 505 ± 3 Ma (White et al. 1994; Keppie etal. 1997). This brackets the change from subduction torifting between 545 and 505 Ma. This may be moretightly constrained if the interbedded MiddleCambrian rocks are part of the rift sequence. Okulitch(1995) places the base of the Middle Cambrian at520 ± 10 Ma.

Northeastwards along strike in the volcanic arc to the Avalon Composite Terrane of Newfoundland, thetransition from subduction-related to extensionaligneous activity is dated at ~ 570 Ma (O’Brien et al.1996). Southwestwards in the Avalon CompositeTerrane of southern New Brunswick, 630–600 Ma vol-canic arc rocks are succeeded by voluminous exten-sional volcanic rocks dated at 560–550 Ma overlain byCambrian rocks in the Caledonia assemblage (Barr &White, 1996). In the Brookville assemblage of southernNew Brunswick, correlated with central Cape BretonIsland, the youngest calc-alkaline plutons are 538 ± 1Ma (White et al. 1990; White & Barr, 1991). Fault-bounded units of the Cambro-Ordovician Saint JohnGroup crop out between the Caledonia andKennebecasis faults that border the Brookville assem-blage (Nance & Dallmeyer, 1994). In the AvalonComposite Terrane of southern New England,the transition from arc to extensional magmatism probably occurred at ~ 590 Ma (Thompson et al. 1996;Mancuso, Gates & Puffer, 1996). This indicates that thechange from subduction to extension occurred at dif-ferent times along the Avalon arc (from southwest tonortheast): 590–538–540–570 Ma. However, althoughthe transition is diachronous, it is bi-directional (i.e. itbecomes progressively younger from New England toNew Brunswick, and from Newfoundland to CapeBreton Island). The end of subduction has generallybeen related to plate reorganization as Iapetus began toopen; however, this does not explain the apparentdiachronism. Dostal et al. (1996) and Murphy et al. (inpress) have documented that the 580–550 Ma subduc-tion was toward the (present-day) northwest, beneathboth Cape Breton Island and southern New Brunswick. Polarity has not been determined in NewEngland or Newfoundland, but it is assumed also tohave been to the northwest in the plate tectonic modelpresent below.


Figure 9. Mantle-normalized trace element data from intru-sive rocks of the eastern Creignish Hills. Sample numberscorrespond with those in Table 4.

The termination of subduction generally appears tohave taken place without a major orogenic event,although polyphase deformation accompanied byhigh-temperature/low-pressure metamorphism is doc-umented in southern New Brunswick and central Cape Breton Island (Bevier, Barr & White, 1990;Keppie, Davis & Krogh, 1998). A younger limit for theage of this metamorphism in southern New Brunswickis given by a U–Pb 565 ± 6 Ma titanite age (Bevier,Barr & White, 1990) and 40Ar/39Ar hornblende iso-tope-correlation ages of ~ 540 Ma (Dallmeyer et al.1990; Nance & Dallmeyer, 1994). In central CapeBreton Island the metamorphism has been dated byconcordant U–Pb zircon analyses at ~ 553–550 Ma(Keppie, Davis & Krogh, 1998), who related thistectonothermal event to gravitational collapse of thearc. The nature of the Early Cambrian extension isgenerally unknown; however in the AntigonishHighlands, the Early Cambrian succession is inferredto have been deposited in a dextral pull-apart basin(Keppie & Murphy, 1988).

It is proposed that the plate tectonic setting of west-ern Mexico and Central America may provide a mod-ern analogue (Protti, Güendal & McNally, 1995).Here, the Cocos Plate is being subducted beneathMexico and Central America. The northern margin ofthe Cocos Plate is the East Pacific Rise, which is being subducted beneath Mexico along the Middle AmericaTrench. At the triple point intersection of the ridgewith the trench, the plate margin changes from sub-

duction along the Acapulco–Middle America Trenchto an oblique transform rift along the Gulf ofCalifornia. As the triple point migrates, the associatedvolcanism switches from arc to extensional. A similar,but opposite, pattern may be observed where the south-ern margin of the Cocos Plate (Galapagos– Costa RicaRift) is being subducted. Subduction of the CocosRidge also produced indentation, arching and inver-sion in the forearc region, and backarc basin inversion(Kolarsky, Mann & Montero, 1995).

This model may be applied to the termination of sub-duction and the switch from calc-alkaline to extensionalmagmatism in the Avalon Composite Terrane, in whichtwo ridges were subducted beneath Avalonia (Fig. 10).The term ‘Merlin Plate’ is introduced for theNeoproterozoic equivalent of the Cocos Plate. Anotherconsequence of this model occurred when the MerlinPlate was completely subducted. The subducted platemay become stationary beneath Avalonia, and remnantmantle upwelling along the subducted ridge axes couldheat up the overlying lithosphere. This mechanism mayexplain the high-temperature/low-pressure metamor-phism that occurred in southern New Brunswick andcentral Cape Breton Island at the end of all arc-relatedmagmatism. The absence of such metamorphism in the~ 540–585 Ma forearc regions of southern Cape BretonIsland and coastal southern New Brunswick accordswith observations where the Cocos Ridge has been sub-ducted beneath Costa Rica (Kolarsky, Mann &Montero, 1995).


Figure 10. Plate tectonic model for West Avalonia during the latest Neoproterozoic (590–550 Ma). CBI = Cape Breton Island;NE = New England; NB = New Brunswick; NEWF = Newfoundland.

Detrital zircon geochronology indicates that Avaloniaoriginated off the Amazon Craton, and geochemistry ofthe volcanic rocks shows that subduction was consis-tently towards the present northwest over the period700–550 Ma (e.g. Keppie & Dostal, 1998; Keppie, Davis& Krogh, 1998). The plate tectonic setting proposed inthis paper implies that following subduction of the mid-ocean ridges and cessation of subduction, oceanic lithosphere continued to exist to the present south ofAvalonia. As there was ~ 150 million years of northwest-dipping subduction, a large ocean must have existed tothe present south of Avalonia in the Neoproterozoic.This scenario suggests that the Amazon Craton may have lain to the present north of Avalonia. The separa-tion of Avalonia from the Amazon Craton, its rotationand its accretion to Laurentia in Cambro-Ordoviciantimes have been modelled by Keppie & Ramos (in press).

6.b. Silurian

The Silurian rocks of the Skye Mountain gabbro–dior-ite and granite form part of a belt of similar rocksextending northwards along the axis of the CapeBreton Highlands (Sarach Brook and Jumping Brookmetamorphic suites, Money Point Group; 439 ± 7 Mato 428 ± 4 Ma; Currie, Loveridge & Sullivan, 1982;Dunning et al. 1990a; Keppie, Dallmeyer & Krogh,1992) and into southern Newfoundland (La PoileGroup; 428 ± 6 Ma to 420 + 8/– 2 Ma; Dunning et al.1990b). The 441 ± 8 Ma Rb–Sr whole-rock isochronobtained from the calc-alkaline granitoid rocks in thewestern Creignish Hills (White, Barr & Campbell,1990) suggests that the magmatic arc extends to thewestern side of Cape Breton Island. Synchronous vol-canism in the Antigonish Highlands is tholeiitic and rift related (Keppie et al. 1997). The Silurian magmaticarc has previously been attributed to the terminal clo-sure of remnant basins within Iapetus by obliquesoutherly subduction beneath Avalonia (e.g. Keppie etal. 1998). However, Murphy, van Staal & Keppie (1999)have recently proposed that this magmatic arc is relatedto the closure of the Rheic Ocean, with northerly sub-duction beneath Avalonia. These authors relate thetransition from calc-alkaline to tholeiitic volcanism to a change in the dip of the subduction zone.

Acknowledgements. Funds for this project were provided bythe Canada–Nova Scotia Mineral Development Agreement,CONACYT Project 0255P-T9506 (El complejo Oaxaqueno y el bloque Chortis en las reconstruciones paleogeograficasde Laurencia y Gondwana anteriores a Pangea), the Instituto de Geologia at the Universidad NacionalAutonoma de Mexico (Project number IN101095), and theNatural Sciences and Engineering Research Council(NSERC). We are grateful to Dr R. D. Nance and an anony-mous reviewer for their constructive comments on an earlyversion of the paper. We would like to thank John Lord, JoseLuis Arce Saldaña and Gabriel Valdez Moreno for draftingthe figures.

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Superposed Neoproterozoic and Silurian magmatic arcs in central Cape Breton Island, Canada: geochemical and geochronological constraints