Canadion Mineralogist Vol. 30, pp.377-392(1992)

PETROLOGYOF HIGH.AI-HORNBLENDE-AND MAGMATIC-EPIDOTE-BEARINGPLUTONS IN THE SOUTHEASTERNCAPE BRETON HIGHLANDS, NOVA SCOTIA

CATHARINE E.G. FARROW" ENOSANDRA M. BARR Departmentof Geology,Acadia University,Wolfuille, Nova ScotiaBOP IXO

ABSTRACT

Six large late Precambrian plutons in the southeasternCape Breton Highlands of Nova Scotia are composed dominantly of diorite, quartz diorite, tonalite, and granodiorite.Their petrochemicalcharacteristics indicate that they are the plutonic equivalentsof moderate-to high-K orogenic andesites,formed in a continental margin subduction zone. The occurrenceof high-Al hornblende and magmatic epidote in the plutons in the northwest suggeststhat crusal levels of ca. 20 km are exposedin that area. Calculated pressuresof hornblende crystallization decrease systematicallyto the southeast,indicating that granodioritesexposed in that sector crystallizedin the epizone.The magmas may have formed by variable amounts of partial melting of mafic granulitic source-rocks,followed by fractionation of mafic minerals and some plagioclase,to producethe internal variation within eachpluton. Keywords:. diorite, quartz diorite, tonalite, granodiorite, I-type granite, calc-alkaline, subduction, catazonal, mesozonal,epizonal, Cape Breton Island.

SoMMarnE,

Nous d6crivons six plutons volumineux, d'6ge prdcambrien, situ€s dans la partie sud-ouestdu Cap Breton (Nouvelle-Ecosse);ils contiennentsurtout diorite, diorite quartzifCre,tonalite et granodiorite. Leurs caract6ristiques p6trochimiquesen font les 6quivalentsplutoniques d'une suite anddsitiquei teneur en potassiummoyenne i 6lev6e, typique d'un milieu de subductionprbs d'une margecontinentale. La pr6sencede hornblenderiche en Al et d'dpidote magmatiquedans les plutons du secteurnord-ouest fait penserque des niveaux d'environ 20 km de profondeur y affleurent. Les pressionsde cristallisation de la hornblende diminuent progressivementvers le sud-est, et les granodioritesde ce secteursemblent dpizonales. Les magmaspourraient r6sulterd'un taux variablede fusion partielle d'une suite granulitiquemafique; un fractionnementde minerauxmafiques (surtout) et de plagioclaseest responsable de la variation interne de chaquepluton.

(Iraduit par la R6daction) Mots-cl4s: diorite, diorite quartzifdre, tonalite, granodiorite, granite de tlpe I, suite calco-alcaline,subduction, catazone,m6sozone, 6pizone, lle du Cap-Breton.

INTRODUcTIoN cording to the criteria of Zen & Hammarstrom (1984a,b, 1988),and all have mineral assemblages appropriate for the application of the hornblende Six largeplutons composedof hornblende-bear- geobarometer(Hammarstrom & Zen 1986, Hol- ing diorite, quartz diorite, tonalite, and lister el al. 1987, Johnson & Rutherford 1989, granodiorite are major components of the Rutter et al. 1989D. southeastern Cape Breton Highlands of Nova The purpose of this paper is to document the Scotia(Fig. 1). U-Pb dating hasindicated that these mineral chemistry and petrological characteristics plutons crystallized in the time interval between of these plutons, and to use the occurrence of about 565 and 555 Ma (Dunning et al, 1990),and magmatic epidote, combined with the hornblende henceare approximately contemporaneous.Three geobarometer,to interpret their depths of crystal- of the plutons locally contain epidote having lization. This study is particularly significant given characteristicsindicative of magmatic origin, ac- the continuing controversyabout the applicability of hornblende compositions as geobarometers "Presentaddress: Department ofEarth Sciences,Carleton versus Eeolhermometers(e.g., Blundy & Holland University, Ottawa, Ontario KlS 586. 1990,Vyhnal et al. l99l). 377 378 THE CANADIAN MINERALOGIST

Ftc. l. Simplified geologicalmap of the southeasternCape Breton Highlands showing plutons that are the focus of this study and associatedunits (after Barr et al, 1985, Farrow 1989, Raeside& Barr, in press). Hadrynian metamorphic suites are Bateman Brook Metamorphic Suite (BB), McMillan Flowage Formation (MF), and Barachois River Metamorphic Suite (BR). Inset map shows location of the study area in easternCape Breton Island. HORNBLENDE. AND EPIDOTE-BEARING PLUTONS, CAPE BRETON 379

GsoLocIceL SBrrtNc et at. (1985,1987)and Raeside& Barr (in press). Together,they compriseabout half the areaof the Plutons included in this study are the Kathy Bras d'Or terrane (Barr & Raeside 1989) of the Road Dioritic Suite, Timber Lake Dioritic Suite, southeasternCape Breton Highlands (Figs. l, 2, more felsic Gisborne Flowage Quartz Diorite, Wreck Cove inset). Also present in the area are Dioritic Suite, Ingonish River Tonalite, and Indian hornblende-freepluton$ that range in age from Brook Granodiorite, asmapped and namedby Barr Hadrynian to Devonian; theseare not included in

Frc. 2. Simplifiedgeological map of the southeasternCape Breton Highlands (legend as in Fig. l), showinglocations for samplesin which mineral compositionswere analyzed;stars indicate locations of samplesin which magmatic epidote was observed:circles indicate locations of other samples.Adjacent numbers are averagepressures (in MPa) of hornblende crystallization calculatedusing the equation of Johnson& Rutherford (1989).The data are from Table 2' in which samplesare listed from north 10 south within each pluton. Inset map shows proposedteranes in Cape Breton trslandafter Barr & Raeside(1989). 380 THE CANADIAN MINERALOGIST the present study. Metamorphic rocks of the of plagioclaseor hornblende. Compositional and Bateman Brook Metamorphic Suite, McMillan textural variations appearto be gradational within FlowageFormation, BarachoisRiver Metamorphic eachsuite; Wreck Cove is the most varied pluton, Suite, and Price Point Formation occur in associa- and in some areas, changesin both composition tion with the plutonic units (Fig. l) (Barr et al, (from diorite to quartz diorite or tonalite) and 1985, Raeside& Barr 1990,in press).Although texture (from fine- to coarse-grainedand locally contacts are not well exposed and faulting is pegmatitic and from porphyritic to equigranular) widespread,all of theseunits appearto have been occur within a single large outcrop. In some intruded by one or more of the 565-555 Ma outcrops of the Kathy Road and Timber Lake plutons. plutons, compositional banding suggestive of Becausethey are generally separatedby other igneous layering is present. Foliation is present units, contact relations among the plutons are locally in all four dioritic plutons. Generally, it known in only some cases,and relative ages are appearsto have been the result of shearingnear mainly based on radiometric ages.U-Pb (zircon) the margins; however, in places flow foliation data indicate that the Kathy Road Dioritic Suite definedby alignmentof hornblendeand plagioclase (560 a 2 Ma) and the Gisborne Flowage Quartz has beenpreserved. Diorite (564 + 2 Ma) are of similar age (Dunning Accessory minerals include epidote, titanite, et ol. 1990).Although undated, the Timber Lake apatite, allanite, zircon, ilmenite, and magnetite. Dioritic Suite is similar in petrological featuresto Secondary (alteration) products include epidote, the Kathy Road Dioritic Suite, and hence is chlorite, actinolitic amphibole, and "saussurite". assumedto be of essentiallythe same age. The Although secondaryepidote is abundant, some of Ingonish River Tonalite is somewhatyounger, with the epidote in most samples from the Gisborne a U-Pb (zircon) (Dunning el age of 555 x. 2 Ma FlowageQuartz Diorite and in many samplesfrom ol. 1990).The tonalite contains dioritic xenoliths the central and northern parts of the Kathy Road near contactswith the Wreck Cove Dioritic Suite, Dioritic Suite is inferred to be of magmatic origin indicating that the Wreck Cove unit is older than (Fig. 2) because it displays the characteristic the tonalite, consistent with the maximum aoAr/3eAr features described by Zer & Hammarstrom hornblendeplateau ageof about 561 Ma (l98 a). These include epidote with euhedral for the Wreck Cove diorite (Reynoldset al. 1989). contactswith biotite, epidote forming overgrowths U-Pb dating of zircon and titanite has on partially resorbed,embayed hornblende, epidote demonstrated minimum and maximum ages of with zoning and with zones of inclusions, and crystallizationof 564 t 5 Ma and 575 Ma for the epidote displaying wormy intergrowths with Indian Brook Granodiorite (Dunning et al, 1990). plagioclase. In both plutons, magmatic epidote occursonly in samplesthat contain modal biotite. PernocnapHv

Dioritic plutons Ingonish River Tonolite

Rocksof the Kathy Road Dioritic Suite, Timber Lake Dioritic Suite,Wreck CoveDioritic Suite,and The Ingonish River Tonalite is similar in both GisborneFlowage Quartz Diorite vary from diorite major and accessorymineralogy to the dioritic through quartz diorite to tonalite (using the plutons but is more leucocratic, with more classification of Streckeisen 1976), with quartz abundant plagioclaseand quartz relative to mafic diorite being the most abundant lithology. The minerals. It also contains more biotite relative to dominantmajor mineralsare plagioclase(andesine) hornblendethan the dioritic plutons, with thesetwo and hornblende, with much less abundant quartz mafic minerals occurring in approximately equal and biotite. K-feldspar is a minor interstitial abundance.The amount of K-feldspar (microper- component in many samples,and clinopyroxene, thitic microcline)increases in abundancetoward the partially replaced by amphibole, occurs rarely in south, where the pluton is locally granodioritic to the Kathy Road, Timber Lake, and Wreck Cove monzogranitic in composition. The texture is plutons. Locally, dioritesofthese threeplutons lack generallymedium- to coarse-grainedhypidiomor- biotite and are very hornblende-rich.The Gisborne phic granular, with mafic mineralstending to occur Flowagepluton is characterizedby a much greater in clusters. Magmatic epidote is a prominent abundanceof biotite comparedto the other three accessorymineral in many samples,and displays dioritic plutons. featureslike those describedin the dioritic plutons. Texturesare typically medium-grainedhypidio- Other accessoryminerals and secondaryminerals morphic granular to inequigranular, and rarely in the tonalite alsoare similar to thosein the dioritic porphyritic. In the latter case,the phenocrystsare plutons. HORNBLENDE. AND EPIDOTE-BEARING PLUTONS, CAPE BRETON 381

Indion Brook Granodiorite hornblende, edenitic hornblende, ferrotscher- makite, or ferroan pargasitic hornblende. How- The Indian Brook Granodiorite consists ever,for simplicity in subsequentdiscussion, all the dominantly of zoned plagioclase (oligoclase-an- amphibole is referred to as hornblende. desine),perthitic microcline, quartz, hornblende, The suite of hornblende compositions display and biotite. The texture is medium grained, variation in aluminum content, both from one hypidiomorphic granular. Abundant large intersti- pluton to another and within individual plutons. tial grains of titanite are typically present; other On average, hornblende from the Kathy Road accessoryminerals include zircon, apatite, and Dioritic Suite containsthe highesttotal aluminum, magnetite.Alteration tendsto be more intensethan and hornblende in the Wreck Cove and Indian in the other plutons, and includes moderate to Brook plutons has the lowest aluminum content intense saussuritizationof plagioclase,chloritiza- (e.g., Table l). The differences in aluminum tion of mafic minerals,replacement of hornblende content appear to be reflected in color differences by actinolite, and pervasive hematitization. Al- in thin section.Hornblende in the Wreck Cove and though secondaryepidote is abundant, no epidote Indian Brook plutons is pleochroic,with Z = dark with features indicative of magmatic origin was green, I : medium green, and X = beigeto pale observed. yellow, whereas higher-aluminum hornblende in the Kathy Road, GisborneFlowage, and Ingonish : AMPHIBoLE CHEMISTRY River plutons has Z dark blue or blue-green,Y = medium blue-green, and X = beige to pale yellow. Chemical compositions geobarometry geothermometry Representativecompositions of the amphibole Hornblende and are presentedin Table l. The amphibole is calcic, asdefined by Leake(1978), with [Ca+Na]s greater Geobarometersbased on the aluminum content than 1.34 and Nas lessthan 0.67. In most cases, of hornblende equilibrated with quartz have been the amphibole has (Na + K)6 lessthan 0.5, and is proposedby Hammarstrom&Zen (1986),Hollister mainly tschermakiteto tschermakitichornblende in et at. (1987),and Johnson& Rutherford (1989)for the Kathy Road and Timber Lake suites, tscher- rocks containing the mineral assemblagequartz + makitic hornblendeto magnesio-hornblendein the plagioclase+ K-feldspar + hornblende + biotite Gisborne Flowage, Wreck Cove, and Ingonish + titanite + an oxide phase (magnetite or River plutons, and magnesio-hornblendein the ilmenite). The equationsproposed by theseinves- Indian Brook Granodiorite. However,some grains, tigators give somewhatdifferent results, with the mostly in the Gisborne Flowage Quartz Diorite, lowest calculated pressures obtained from the have (Na + K)o slightly more than 0.5 and consist equationof Johnson& Rutherford(1989). For the of magnesianhastingsite, magnesianhastingsitic presentstudy, pressureswere calculatedusing all

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TABLE2. CATCULATEDPRESSURES AND TEMPERATURESBASED ON HORNBLENDE

Ptsute Temp. (MPal fc)

H&Z H st J&F B&H al

SB-8/r-49 2,40 4.26 el E 878 669 855 SB€/t-42 1 2.94 6.23 0.61 1013 771 843 sB-84-41 J 2.37 6.27 0.61 800 eal 657 863 sB-84-134 o 2.13 6.32 o.5l 679 726 TAt AEA sB-84-124 1.41 6.90 0.51 317 319 260 769

Tl"DS AM-84-53 589 624 479 837

GFOD K10-521 3 8.27 0.64 694 742 668 816 cF€8-14 2 2.29 6.20 0.64 760 816 A8 823 RR{4-8 4 2.19 0.64 710 759 680 425

IRT K09-S103 2.O1 6.40 0.50 619 958 604 862 KOg-S89 2,31 6.33 0.60 770 827 631 847 K10-S8 0.50 589 424 479 818 sB-86-3184 1.81 0.50 518 646 420 830 AM-84-52 1.81 6.49 0.60 618 546 420 844 'r.80 K10-S14 4 0.52 513 539 416 420 cF-88409 6.67 o.62 403 477 322 809

IBG AM-8+7 6 1.46 6.7r 342 347 272 763 Westl RR-87-5503 4 1.25 7.O4 o.65 237 229 183 703 AM-84-38A 7 1.30 6.91 262 267 204 729 AM-84-35 7 1.30 6.90 262 267 204 732 AM-44-74 , t.ou o.ou 0.66 413 426 331 761 AM€4.75 4 1.40 6.82 0.65 312 305 246 743

IBG AM-84-86 7 0.87 7.18 0.66 48 16 22 697 Gast) AM-84-r6 6 0.89 7.10 0.65 56 26 30 710 IBG.1 4 1.09 6.91 u.oo 156 r39 116 742 rPluton abbr*iations as in Table 1. Prsssuraswere ulculated 6ing th€ eq@tio6 of Hammarstrom& Zen 1986 (H&Zl, Hollistsr6t al. 1987 (H et al.), and Johmon and Ruthsrtord1989 (J&F). Calculatedprssures ot homblsnd€crystalllzation ars multipli€dby 100 to givg vsluesIn MPa.Tomparaturos wg.g €lculated sing tho sqmtions ot Bludy & Hollend(19901 with th€ calcutatodprossuros from thB eqution of Johmon & Rutherford(19891. n = numberof homblsndsrim amlys€s usBdto €lculate averag€total alminum (Af) and avetaoesillcon (Sil per iomula unit. Ab ls tho ccsxisting plagislase composition. three equations for comparison (Table 2); the Diorite, crystallized at mesozonal to catazonal patterns displayedby the data are the samein all depths,whereas the southernmostpart of the Kathy three cases,but the pressurescalculated using the Road Dioritic Suite formed at a much shallower equation of Johnson & Rutherford (1989) seem depth. This variation is broadly consistentwith a most compatible with adjacent metamorphicunits changein metamorphic grade documentedin the and are cited in the following discussionand shown adjacent McMillan Flowage Formation from am- on Figure 2. phibolite faciesin the north to greenschistfacies in The application of the hornblendegeobarometer the south, although the pressuresindicated by has been questioned, on the basis that the Al mineral assemblagesin the northern part of the contentof hornblendeis more sensitiveto tempera- McMillan Flowage Formation are not as high as ture than to pressure,and a geothermometerhas thoseindicated by the hornblendegeobarometer for been proposed for hornblende coexisting with adjacentparts of the Kathy Road Dioritic Suiteand plagioclase in silica-saturatedrocks (Blundy & GisborneFlowage Quartz Diorite (Raeside& Barr Holland 1990).Temperatures have beencalculated 1986,1990). for the units of this study (Table 2) using the The Timber Lake, Ingonish River, and Wreck equation of Blundy & Holland (1990)and urilizing Cove units yield intermediatepressures of crystal- the pressurescalculated by the equationof Johnson lization, and the lowest pressuresare from the & Rutherford(1989). southeastern part of the Indian Brook The calculatedpressures are highest for samples Granodiorite; these results indicate crystallization from the northern part of the Kathy Road Dioritic at a pressureless than 100 MPa (epizonal). The Suite, and they generallydecrease to the east and latter is consistentwith the preservationin that arca south (Table2,Fig.2), The pressuressuggest that of subgreenschist-faciesvolcanic rocks of the Price the nofihern part of the Kathy Road Dioritic Suite, Point Formation (Fig. 1) (Raeside& Barr 1990). as well as the adjacent Gisborne Flowage Quartz In contrast to pressures, the temperatures HORNBLENDE. AND EPIDOTE.BEARING PLUTONS, CAPE BRETON 383

(e.g. replace- calculatedusing the equation of Blundy & Holland of epidotewith textural relationships , (1990) show a relatively small range within each ment of plagioclase) that indicate a secondary 26 pluton that doesnot exceedthe reported uncertain- origin; such epidote generallycontains between of epidote ty for their calibration (t 75"C). As would be andnn/o (Table 3). However, both types 25 to expected,higher temperaturesare generally indi- have pistacite componentswithin the range cated for the dioritic plutons compared to the 29 th^.has beenreported to be typical of magmatic Indian Brook Granodiorite. epidote (Tulloch 1979,1986,Vyhnal et ol. l99l). The epidote compositions documented in this by EPIDOTE CHEMISTRY study differ markedly from those analyzed Vyhnal et at. (1991)from monzograniticplutons in the southern Appalachian Orogen. The latter compositions Chemicol typically have a higher proportion of the pistacite component, lower Si, Al, and Ca, and higher Fe' Epidote grainsof inferred magmaticorigin from the Kathy Road, GisborneFlowage, and Ingonish PressureimPlicotions River plutons commonly show a decreasein Al and increasein Fe contentsfrom core to rim (Table 3). is stable in This is attributed to replacementof Al by Fe3+near Naney (1983) showed that epidote granodioritic the margins of magmatic epidote crystals and syntheticH2O-saturated tonalitic and subsequentgrowth of more Fe-richepidote. Similar melts at /(Or; valuesbetween the nickel-bunsenite (HM) buffers, at compositional differences are apparent between (NB) and himatite-magnetite (600'C) and magmatic and secondaryepidote, with composi- i.mperatutes between the solidus pressures least in tions of secondaryepidote similar to rim composi- approximately 700oC, and at at tions on magmatic epidote grains. Secondary the 600 to 800 MPa range. Zen & Hammarstrom near epidote also tends to be lower in Ti; Cr, Mg, Mn, (1984a)proposed that magmaticepidote forms to and Na contentsare variable, whereasCa content the solidusof the crystallizingmagma according is relatively constant (Table 3). the following schematicreaction: + I magnetite The average proportion of the pistacite com- 2 hornblende + 2 alkali feldspar 6 ponent ranges from 24.50/oit the Kathy Road + nHrO + mO2 : 3 epidote + 3 biotite + Dioritic Suite to 26.8t/o irt the Ingonish River qvarlz. Tonalite. Thesevalues are slightly lower than those Minimum pressuresof crystallizationlower than

TABI-E 3. REPRESENTATIVECOMPOSMONS OF MAGMAIC AND SECONDARY EPIDOTE

Magmatic Epidote SecondaryEpidote

KRDS GFOD IRT KRDS IRT WCDS Core Rim Core Rim Core Rim

sio2 38.02 37.68 57.74 38.24 37.80 37.61 37.84 37.69 37.36 Tloz 0.16 0.15 0.11 0.10 0.37 0.12 0.o5 0.12 0.o7 Alro" 26.18 23.77 24.89 24.64 24.11 22.93 22.92 22.68 23.19 Cf2o! o.04 0.08 0 0.03 0.07 0.06 0.06 0.o2 0.08 Feol 10.67 12.63 11.53 12.23 11 .36 13.09 13.23 12.97 12.11 MnO o.32 0.08 0.23 0.24 0.30 0.23 0.32 0.16 0.48 Mso 0.07 0.03 0 0 0.05 0 0.05 0 0.19 CaO 23.31 23.54 25.55 23.71 23.44 23.46 23.25 23.88 23.25 Na20 0.07 0.09 0.01 0 0.09 0.05 0.03 0.09 0.08 Total 97.74 98.05 98.05 99.12 97.65 97.55 97.74 97.49 96.80

NUMBEFOF CA'NONS ON THE BASIS OF 25 OXYGENATOMS

st 3.06 3.06 3.05 3.07 3.07 3.09 3.10 3.10 3.08 At 2.39 2.2A 237 2.32 2.31 2.22 2.21 2.19 2.25 Fo' 0.76 0.86 0.78 0.82 o.77 0.90 0.91 0.89 0.83 Cr 0.003 0.01 0 0.002 0.004 0.004 0.003 0.001 0.01 -ri o.01 0.01 0.01 0.01 0.02 0.01 0.003 0.01 0.004 Mg 0.01 0.004 0 0 0.01 0 o.o1 0 0.02 Mn O.O2 0.01 0.o2 0.02 0.o2 0.o2 0.02 0.01 0.03 Ca 2.O1 2.05 2.O4 2.O4 2.O4 2.06 2.04 2.10 2.O5 Na 0.01 0.01 0.002 0 0.01 0.01 0.005 0.01 0.01

Ps% 22.96 27.37 24.73 26.12 25.05 2a.A2 29.05 28.97 27.03

Epidote analysesby slectron microprobeas in Table 1 Ps% = pistacits componefi. Pluton abbrgviationsas in Table l. 384 THE CANADIAN MINERALOGIST

600 MPa were suggestedfor more silica-rich rocks rocks of quartz dioritic and granodioritic composi- such as calc-alkaline granite, whereas pressures tion. higher than 800 MPa were proposed for less silica-rich rocks, including quartz diorite and Wrrorr-Rocr CHEMISTRY diorite (Zen & Hammarstrom 1986). Magmatic epidote in granite two-mica commonly is more General characteristics pistacite-rich and formed at considerably lower pressure than is indicated by the hornblende- epidote association(Zen & Hammarstrom 1988). Means and standard deviations for major and trace elementconcentrations are compiledin Table More recently, the high-pressure origin of 4. Resultsof the analysesand samplelocations for magmaticepidote in hornblende-bearingrocks has the dioritic and tonalitic units are presentedby questioned, been becauseepidote with featuresthat Farrow (1989). Data for the Indian Brook suggestmagmatic origin occur in rocks for which Granodiorite are given in Table 5. the calculated pressures of crystallization for The Timber Lake, Wreck Cove, and Kathy Road coexistinghornblende are as low as 280 t 50 MPa dioritic plutons have averagesilica contentstypical (Vyhnal et al. 1990).However, the bulk composi- of mafic rocks (less thar 52t/o), with the lowest tions of the rocks are monzogranitic rather than averagein the Timber Lake Dioritic Suite. The tonalitic and granodioritic, and a high-pressure lowest silica values are from samples with high origin for epidote in rocks of the latter composi- concentrations of mafic minerals (mainly tions appears still to be valid. hornblende), and each unit has a range in silica The occurrenceof magmaticepidote in the Kathy contents and corresponding variations in other Road Dioritic Suite, Gisborne Flowage Quartz elements,as illustrated in Figure 3, consistentwith Diorite, and Ingonish River Tonalite is consistent the variation in modal compositionof the samples with the high calculated pressuresof hornblende that range from diorite through to quartz diorite crystallization (Fig. 2). The Timber Lake and and tonalite. The GisborneFlowage Dioritic Suite Wreck Cove dioritic suites and the Indian Brook has generally higher silica content than the other Granodiorite lack magmatic epidote, and in these three dioritic suites, averaging about 5590, al- plutons, hornblende geobarometry has yielded though some sampleshave SiO2values as low as pressuresof crystallization too low to be com- 5090(Fie. 3). patible with crystallization of magmatic epidote in The Ingonish River Tonalite hasa higheraverage

TABLE4. MEANSAND STANDARDDEVIATIONS OF CHEMICALCOMPOSMONS OF PLUTONS' OFTHE STUDY AREA

PLUTON TLDS wcDs KRDS GFOD IRT IBG (n) ln=51 (n=91 (n=161 (n=101 (n=201 (n=111

sio2 47.31 * 4.45 51.49r 6.19 51.91r 4.41 55.15r 2.58 69,2'l x 2.72 64.10 * 4.37 ro" 0.76 o.24 0.81 0.25 0.82 0.27 0.81 0.15 0.53 0.09 0.48 0.16 Ar2q 18.34 2.97 16.16 1.43 17.47 1.43 17.76 0.62 17,08 0.67 16.27 1.60 Fe2O.t10.42 1.62 10.09 2.6',1 9.70 1.55 8.03 1.12 6.94 1.11 4.47 1.64 MnO O.19 0.02 o.17 0.03 0.18 0.03 o.17 0.o2 o.14 0.o2 0.10 0.04 MsO 6.64 2.04 5.89 1.81 5.34 0.77 3.63 0.52 3.39 0,57 2,10 0.98 CaO 10.11 1.77 9.O2 2.40 9.26 1.45 7,48 1.40 4,O7 1.67 Na2O 2.1O 0.79 2.24 0.67 2.62 't.43 0.39 3.27 0.31 2.83 0.2',1 3.33 0.42 KrO O.21 1.45 0.46 0.88 0.43 2.16 0.77 1.71 0.42 3.24 't.O2 PrOr 0.35 0.38 0.18 0.10 0.16 0.05 o.25 0.09 0.16 0.02 0.16 0.06 LOr 2.22 0.65 2.32 0.95 1.24 0.60 o.97 0.44 1.38 0.43 1.8r 0.86 Total 99.86 0.42 99.82 0.59 99.58 0.42 100.04 0.62 99.63 0.83

Ba 224 57 216 97 166 76 491 132 326 S6 439 106 Rb 55 I 43 20 2S 17 62 26 49 ,18 95 37 sr 439 97 385 70 368 61 588 1,16 426 5? 343 132 Y217217226253182224 Zt 62 16 93 66 82 30 i73 56 93 18 164 27 Nb5172619272102 Cu 7a 23 132 71 91 38 44 30 61 24 39 3g Pb16169210<110183103 ,t7 ,t2 zn 1O4 20 94 24 91 90 79 I 64 17 .10 Ni 31 25 30 21 26 12 2 6 3 I 3 cr 72 77 45 45 55 61 14 9 24 9 39 14 v 297 124 302 125 301 63 209 40 198 39 123 66 Ga183183182192182162 Th100839393431513 tPluton abbreviatioGas in Table1. Amlytical datafor TLDS,wcDs, KRDS,GFoD and IRTfrom Farow (19991, and tor IBGfrom Table5. Amlyses by X-ray fluorescenc€as describ€dby Fanow (19g91. HORNBLENDE. AND EPIDOTE-BEARING PLUTONS. CAPE BRETON 385

TAAIE5. CHEMICALCOMPOSMON OF INDIANBROOK GFANODIORIrE

129450t9

so. 7r.08 87.6E 65,16 66.66 63.4'l 57.65 81.32 e4.19 67.01 €3.S6 58.73 To. 0.37 0.42 0.43 0.r0 0.54 0.6t 0.53 0.54 0.40 0,64 0.73 &q 11.88 t3.54 16.98 $.42 16.47 16,63 1€.13 15,18 10.rO 15.49 18.97 Fqq' 3.50 3.41 4.47 2,42 4.98 7.04 6.06 5.0a 7.sl Mro 0.00 o.o7 o.o8 0.05 0.08 0,15 0,13 o.t2 0.10 0,1't 0,18 Mgo 1.14 l.t6 1.09 1.83 1.62 3,81 2.79 2.47 1.42 L& 3.58 CaO 2.O7 2.90 3.72 2.82 3.28 7.12 5.65 3.A2 3.50 4,14 6.07 NaP 2.49 3.4it 3.46 s.21 3.05 3.24 3,34 3.24 3.88 3.23 4.10 Kp 4.42 4.0S 3.34 3.S0 4.14 'f.83 2.73 3.90 2.45 3.67 1.32 PP! 0.r0 0.13 0.r4 o.o8 0.r3 o.17 0.17 0.r€ 0.16 o,l7 0.t2 LOI 3.O8 2.91 1.70 1.66 3.26 1.50 0.99 1.21 0.97 1.10 1.60 To@l 100.64 99.87 99.59 97.1S 99.84 99.55 99.84 9S.86 99,49 99.?6 99.51

Ba 2e7 670 459 487 308 624 553 514 399 '123 647 329 & r38 rO8 143 135 46 74 109 60 110 4g Sr 60 242 335 216 288 462 350 360 40S 408 5m 26"26212618152A172622 Zr 217 . 167 113 177 1e7 142 161 188 187 155 M 10 " 12 11 l0 8 13 lO I 10 5 5 a 78 5 5 38 rO5 58 1r 51 20 tu 1091010105101412" 2n 62 68 49 36 71 7A 72 69 64 57 90 M 7 " 14 0 I I 8 rr 6 6 10 Cr 67-374151583035372314 48 - 94 43 137 222 146 140 62 110 224 Ga 14"141416r81817101817 n 43-11271021012612

Amlts by l-q R@t!@ @ d@ibed by Frr@ 119991. Smple lcathB qtoonIn mil|& ot ladnde 46'N 8d mlnd6 ol lomMo @Ht 1,20.76 &32.4t 2,2O.7 &32.6i 3.20,76 & 33.1;4, lg.3 & 33.7; 5,20.55 & 33.8; 6, 30.8 & 2S.85;7, 30,85 & 28.s; S,26.95 & 29.95; 9, 25.95 & 32.7:10,20.8 932.2t 11,28,5 & 33.4.

SiO2content of about 5990, but overlaps in SiOt Petrogenesis content with more silicic samplesfrom the dioritic units, and with the more mafic samplesfrom the Overall, the chemical variations within each Indian Brook Granodiorite (Fie. 3). The Indian pluton and among the plutons as a group suggest Brook Granodiorite shows a wide range in silica a major role for crystal fractionation in magma content from about 57 to 72t/0, with an average evolution. Plots of Rb versatsSr and Ba versasSr value of 64V0.This range in chemicalcomposition (Fig. ) show large increases.inRb and Ba with correspondswith variations in the proportions of little change in Sr in going from more mafic to feldspar and mafic minerals in the unit; these more felsic samples,consistent with fractionation with some variations seemto be gradational. of dominantly hornblende combined plagioclase; the latter appears to become more group, plutons Taken as a the rangemore or less significant in the Indian Brook Granodiorite. The continuously from mafic to felsic (Fig. 3). Most very low Rb and Ba values in some mafic dioritic major elementoxides display linear trends on silica samplesmay reflect hornblendeaccumulation. variation diagrams (e.9., Fig. 3); TiO2, Al2Or, Although both Fe and Mg show negative Fe2O3t,MnO, MgO, and CaO display definite correlation with silica content (Fig. 3, Table 4), the negativecorrelations (correlation coefficientsmore FeOt/MgO ratio staysapproximately constant (Fig. -0.7) with whereas K2O and Nap than silica, 5), indicative of calc-alkaline affinity (Miyashiro positive (correlation display definite correlations 1974). However, the wide variation in Cr with coefficients more than +0.7) with silica. P2O5 approximately constant V in the dioritic and shows only a moderate correlation (correlation tonalitic plutons (Fig. 6) is not typical of calc- coefficient -0.55), mainly because of scatter in alkaline suites. Electron-microprobe analysesof mafic samples that appears to correspond to hornblende in these plutons indicate that minor varying apatite content. amounts of Cr (up to 0.4v/oCr2O3) are present Among the trace elements,Zn, Y, and Ga show (Farrow 1989), and hence Cr variation may have from hornblende fractionation in these strongnegative correlation with silica (e.g., Fig. 3). resulted plutons. In addition, relict pyroxene cores €ue Ni and Cu show only moderate correlation, with presentin somehornblende grains, suggestingthat most of variation in mafic plutons. the the Sr shows earlier fractionation of Cr-bearing pyroxene also moderate negative correlation (correlation coeffi- may have contributed to the range in Cr values. -0.5) cient about with SiO2, and both Ba and Rb The lack of changein V suggeststhat magnetite showpositive correlation (+0.5 and +0.7, respec- frilctionation was not significant in the dioritic- tively) with SiO2. Nb contents are low, between tonalitic plutons; in contrast, decreasein V with about 5 and 10 ppm. Zr values are high in the lesschange in Cr indicatesthat magnetitemay have GisborneFlowage Quartz Diorite comparedto the played a significant role in producing variation in other dioritic plutons (Fie. 3). the Indian Brook Granodiorite (Fig. 6). 386 THE CANADIAN MINERALOGIST

KRDS x h -lR 23 TLDS D OFOD + n:10 IRT A n.20 'E wcDs I n.9 "1' IBG O n:11 AlrO.tz q:Sle oof t-. ;oo t \to pt s\ ,/ Rb ppm 10 61" *-\ \ 20 \ KRDS X n=15 ), TLDS D n.5 Fe.of to 6FoD * n.10 "ol WrR&ftt IRT A n:4 WCOS I n=9 IBG O n.11 o

It

CoO o F'T1"*,m:*- "

Kzo \1 Bo ppm 1oo xXx o

0vu t x{ o o t50 xr tr 10 100 1000 Sr ppm Zn xcl ts,.'T4msfu s Frc. 4. Plots of (a) Rb-Sr and (b) Ba-Sr for plutons of o this study. Vectorsshow trends for Rayleighfractiona- tion of 3090plagioclase (Pl), K-feldspar (Kfs), biotite 250 (Bt), and hornblende (Hbl), calculated using the following KD values(from McCarthy & Hasty 1976, Tindle & Pearce 1981) for plagioclase,K-feldspar, biotite, and hornblende,respectively: Bal. 0.4, 6, 6.36, Zr 0.35;Rb: 0.04,0.8, 3.26,0.011; Sr: 3.35,3.6, 0.12, 0.058.

o 45 50 55 60 65 70 75 among the plutons; they are unlikely to be related si02 to one another by differentiation processes. In- stead, each pluton may represent an individual Ftc. 3. Silica variation diagrams for selected major evolving batch of magma, perhaps generated from elementoxides and trace elementsin plutons of this partial study. Data from Farrow (1989)and Table 5. similar source-rocks by varying degrees of melting and then subject to fractional crystal- lization. The similarity in chemical trends displayed Distribution in large separate intrusions that in by the plutons (Fig. 3) probably represents some casesappear to have crystallized over a range similarity in both source rocks and subsequent of depths does not suggest direct genetic links evolution of the batches of magma. HORNBLENDE. AND EPIDOTE.BEARING PLUTONS, CAPE BRETON 387

KRDS X n=15 THOLEIITIC TLDS tr nE5 OFOD + n 310 IRT A n=20 wc05 I n=9 IBG n =11

3 FeoTMso trt + ,+ lx a/ ms tbx6 tr xl I

CALC-ALKALINE oL 40 50 60 70 80 sio2wlTo Frc. 5. Plot of FeOr/MgO againstSiO2, with dividing line betweentholeiitic and calc-alkalinerocks after Miyashiro (1974).

Granodiorite (Table 6). Tolal REE abundanceis low in the Kathy Road and IngonishRiver samples, and much higher (especiallyin the caseof the light REE) in samples from the Gisborne Flowage Quartz Diorite and Indian Brook Granodiorite (Fig. 7). This difference is consistent with the abundanceof accessoryphases (epidote, allanite, zircon, titanite) in the latter two units. The Kathy Road and Ingonish River samples TH*CA have essentially identical chondrite-normalized patterns, slightly lower heavyR.EE in the KRDS X n.15 REE with TLDS tr n.5 Ingonish River Tonalite (Fig. 8). The three samples OFOD * n=10 from Kathy Road Dioritic Suite representa range IFI A n'20 from low to moderate to high. WCDS I n'9 of silica contents IBG O n=10 The low-silicasample has the lowestconcentrations of LREE, but HREE patterns are similar in all 1 10 't@ t000 three samples,and very flat. Cr ppm The Gisborne Flowage and Indian Brook samplesalso havesimilar R.EEpatterns,with a high Frc. 6. Plot of V against Cr, with ttroleiitic (TH) and abundanceof heavy REE comparedto the Kathy calc-alkaline (CA) fields after Miyashiro & Shido Ingonish River samples(Fie. 8). (r97s). Road and Overall, theflat HREE patternsin all theseunits aremost consistentwith an origin by partial melting of source rocks in which neither garnet nor Rare-earth elements hornblendewas presentin the residue.The presence ofgarnet in the residuegenerally leads to depletion Rare-eafih element (REE) dala are available for irt HREE, whereas the presenceof hornblende a total of nine samplesrepresenting the Kathy Road tends to result in concave-upwardHREE patterns Dioritic Suite, Gisborne Flowage Quartz Diorite, (Hanson 1980), neither of which is indicated by Ingonish River Tonalite, and Indian Brook thesedata (Fig. 8). A mafic granulitic source,with 388 THE CANADIAN MINERALOGIST

TABT"E6. FAFE.EAATHELEMENT DATAI 't234567A9 KRDS KRDS KRDS IRT IBT GFOD GFOD IBG IBG

ta 9.94 10,74 7.94 10.70 9.08 26.59 24,79 31.26 27.67 Co 23.22 24.78 77.2A 26.03 17.63 54.68 56.13 69.18 62.54 Pt 9.12 3.00 2.38 3.38 6.75 6.89 Nd 13.26 11.70 10.15 14.36 11.30 25.86 27.35 30.87 24.85 sm 3.23 2.72 2.73 3.48 2,79 5.19 6.83 6.16 4.13 Eu 0.81 0.63 0.79 d 7q 1.21 1.38 1.44 1.54 Gd 2.96 2.37 2.67 2.77 2.25 3.55 4.34 Tb 0.60 o.42 0.48 0.45 0.39 o.a7 0.69 o,71 0.47 Dy 3.36 2.69 2.94 2.37 3.46 3.93 Ho 0.67 0.57 0.65 0.54 o,47 0.64 0.80 E! 2.OA 1.60 1.84 1.64 t.J6 1.82 2.22 Tm 0.28 o,24 0.26 0.23 o.21 0.26 0.3r Yb 1.99 1.61 1,42 1.47 1.82 2.17 2.49 1.74 Lu 0.31 0.26 0.28 o.24 o.28 0.32 u.50 u.z6 I Amlysd 1-7 by ICP-MSat Memo.ialUniversity, St John's, Noffoundtand. Amlysos I & 9 by lnstrumental NautronActivation at St. Mary's Univorsity,Halifax, Nova Scotia. Samplelocations giv€n in minutosof fatftuds46oNandminutesotlongituda6Olil:1,16.5&45.5;2.23.7&42.4:3,25.3&43.6;4,33.1 & 30.8;5,23.3& 39.1;6,32.1& 37.4;7,31.6& 38.0;S, 20.8& 32.2;9,30.86 & 28.9.Samptes 8 andI aro numbers10 and 7, rospectively,in Table 5.

Tectonic setting

Overall, the plutons of the southeasternCape Breton Highlands are typical of compositionally expanded calc-alkaline I-type suites formed in ?o- association with continental-margin subduction g zones(e.9., Pitcher 1982,1987, Brown et al. 1984). lrJ 100 They are compositionallyequivalent to subalkaline H basaltic to dacitic volcanic suites formed in such settings (Fig. 9). On diagrams suited for the t discriminationof tectonicsettings of mafic volcanic E, rocks, the mafic dioritic samples(SiO, lessthan p 52Vo) generally plot in the volcanic arc fields, although with considerableoverlap with ocean- floor fields (Figs. l0a, b). to 50 60 70 Further support for origin of these units in a Si0a subduction-relatedsetting is their hieh Al contents; the GisborneFlowage Quartz Diorite is very similar Ftc. 7. Total REE againstSiO2 for samplesfrom Kathy in average major-element composition to the Road Dioritic Suite, IngonishRiver Tonalite,Gis- averagehigh-K, low-SiO2orogenic andesite of Gill borne Flowage Diorite, Brook Quartz and Indian (1981), Tonaliteis like Granodiorite.Total includes La, Ce,Nd, Sm,Eu, Tb, and the IngonishRiver the Yb. and Lu from Table6. averagemoderate-K, high-SiO2orogenic andesite (Table 7). These similarities to moderate- and high-K arc rocks indicate the presenceof thick plagioclase and pyroxene in the residue, could continentalcrust. We interpret theseunits to be the generate melts with REE patterns like those plutonic equivalents of volcanic-arc basalts and displayed by these units. The lack of strong Eu andesites and their differentiation products, anomalies in most samples suggestseither that formed in a late Precambriansubduction zone. The feldspar fractionation was not a major processor, calc-alkaline volcanic rocks of the Price Point more likely, given the evidencefrom other major Formation (Macdonald & Barr 1985)may be relics and trace element data for fractionation of of the cogeneticvolcanic arc suprastructure,most plagioclaseand mafic minerals within each unit, of which has been removed by erosion farther to that feldspar fractionation was balanced by the northwest, where deeper crustal levels are hornblende fractionation so as to minimize Eu exposed. anomalies (Hanson 1980). Fractionation of both The apparent difference in age may be related feldspar and mafic minerals. probably led to to more rapid cooling and crystallization of the enhancementof LREE and depletion in HREE. high-level magmas. However, the Ingonish River HORNBLENDE- AND EPIDOTE.BEARING PLUTONS, CAPE BRETON 389

:o o !ro o, o E (n KRDS X acnn + IRT A IBG

Lo Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Ftc. 8. Chondrite-normalizedREE patternsfor samplesfrom Kathy Road Dioritic Suite, Ingonish River Tonalite, Gisborne Flowage Quartz Diodte, and Indian Brook Granodiorite. Normalization to the chondrite values of Evensenel a/. (1978).

KRDS x n=15 TLDS E ft=5 GFOD + n ='10 Rhyotite Cornendite IRT A n =20 Pontellerite wcDs I n=9 IBG o .n=10 Rhyodocite Docite OTrochyondesite Zr/TiO" Andesite

Andesite/Bosolt Alkoline Bosolt

Subolkoline Bosolt

SUBALKALINESERIES ALKALINESERIES '-0.0't 0.10 1.0 10 Nb/Y

FIG. 9. Plot of Zr/TiO2 againstM/Y. Names of volcanic fields after Winchester & Floyd (1977). 390 THE CANADIAN MINERALOGIST

CONTINENTAL

tl ISLAND 1l l-l 500 /^Rc irqnsitionqi Atkoline Ti/Y 300 OCEAN FLOOR 200

100 o.2 0.3 0.5 't.0 Nb/Y

2xNb KRDS X TLDS tr OFOD + wcDs I

frn7 M.RB

Zr/4 Y Frc. 10. Mafic samples(SiO2 less than 52Vo)plotted on the (a) TilY againstNb/Y diagramof Pearce(1982) and (b) the Nb-Zr-Y triangular diagramof Meschede (1986).

TABII 7. COMPARISON wlTH HIGH.K AND MEDIUM.K OROGENTC Tonalite appearsto be significantly younger, ANDESITES' and may representa later pulse of magmatism. High-K

si02 55.87 55.7 80.01 60.5 nor 0.82 0.93 0.54 o,71 CoNcr-usroNs Al203 r7.99 18.1 17.31 17.3 FeO! 8.13 7.6 7.03 6.4 MnO o.17 o.18 0.14 o.12 Dioritic, tonalitic, and granodioritic plutons of Mso J.O6 4.0 3,44 7.64 7.4 8.78 6.7 the southeastern Cape Breton Highlands are NaP t?l 3.4 2.87 K2o 2.19 1.73 interpretedto be the roots of a volcanicarc, formed Prog 0.25 0.31 0.15 0.19 by late Precambriansubduction. The occurrenceof 'Hloh-K baslc ard6tto and medlm"K acid andshe arc trom Gill (198i 1, high-Al hornblende and magmatic epidote in the @lculated to total I 00% volatilefreo. Avs€ge Gtsbore Flowage Owrtz northwestern part of the area, the systematic Dlorlte (GFODI and Ingonish Fivq ToElite llRTl 8re t,om Table 4, r@lcqlsGd to 100%, vol€dlelreo. decreasein calculated pressures of hornblende HORNBLENDE- AND EPIDOTE.BEARING PLUTONS, CAPE BRETON 391 crystallization toward the southeast, and the DuNNrNc, G.R., Benn, S.M., Rarstoe, R.P. & preservationof volcanic rocks in the southeastern JavresoN, R.A. (1990): U-Pb zircon, titanite, and part of the area are consistentwith the interpreta- monazite ages in the Bras d'Or and Aspy terranes implications tion that progressively deeper crustal levels are of Cape Breton Island, Nova Scotia: igneous and metamorphic history. Geol, Soc, preservedfrom southeastto northwest acrossthe for Am. Bull. 702,322-330. area. The hornblende geobarometermay not be of sufficiently precise to determine the level Evrmell, N.M., HavrlroN, P.J. & O'NtoNs, R.K. expo$urein the northwest, but the co-occurrence (1978): Rare-earth abundances in chondritic of high-Al hornblende and magmatic epidote, meteorites. Geochim. Cosmochim. Acta 42, ll99' combined with the nature of the mineral as- 12t2. semblagesin adjacentmetamorphic units, suggest that the rocks formed at depths equivalentto ca. FARnow, C.E.G. (1989): Petrography, Mineral 600 MPa (ca, 20 km), More detailedstudies of the Chemistry, and Geochemistry of Dioritic and associatedmetamorphic units are in progress to Tonalitic Plutons of the Southeastern Cape Breton further constrain the P-T conditions and tectonic Highlands, Nova Scotia, M.Sc. thesis, Acadia Wolfville, Nova Scotia. evolution of this area. Univ., Grrr, J.B. (1981): Orogenic Andesites and Plate AcrNowI-SncEMENTS Tectonics. Springer-Verlag, Berlin.

Field and petrological studies for this project Helrvensrnolt, J.M. & ZsN, E-An (1986): Aluminum were funded by an NSERC Operating Grant to in hornblende: an empirical igneous geobarometer. S.M. Barr. C.E.G. Farrow was supportedby an Am. Mineral. 77, 1297-1313. NSERC 1967Science and EngineeringScholarship (1980): Rare earth elements in during her graduate studies at Acadia University. HeNsoN, G.N. petrogenetic studies of igneous systems.Ann, Rev. Raesideand A.S. Macdonald for We thank R.P. Earth Planet. Sci. E, 371-406. their major role in the geologicalmapping of the southeasternCape Breton Highlands, which made HoLLrsrEn, L.S., Gntssotu, G.C., PsrEns, 8.K., the presentstudy possible.We thank C.G. Barnes SroweLL,H.H. & SIssoN,V.B. (1987): Confirma- and an anonymousreviewer for their constructive tion of the empirical correlation of Al in comments that led to significant changes and hornblende with pressure of solidification of improvementsin the concept of this manuscript. calc-alkaline plutons. Am. Mineral. 72, 231-239.

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SrnEcrcrsEN,A. (1976): To each plutonic rock its Received October 5, 1990, revised manuscript accepted proper name. Earth-Sci. Rev.72, l-33. August 27, 1991.