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Petrology of High.Ai-Hornblende- and Magmatic-Epidote-Bearing Plutons

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Petrology of High.Ai-Hornblende- and Magmatic-Epidote-Bearing Plutons 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
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