the east kemptville topaz muscovite leucogranite, … · chedabucto fault system (webb 1969, keppie...

Canadian MineralogistVol. 28, pp. 787-825 (1990)

THE EAST KEMPTVILLE TOPAZ_MUSCOVITE LEUCOGRANITE, NOVA SCOTIAI, GEOLOGICAL SETTING AND WHOLE.ROCK GEOCHEMISTRY

DANIEL J. KONTAKNova scotia Departrnent of Mines and Energy, P. o. Box 1087, Halifax, Nova scotia B3J 2xI

ABSTRAC"T

The East Kemptville leucogranite, a co. 370 Ma topaz-muscovite leucogranite occurring at the southwest termination of the Late Devonian South Mountain Batholith ofNova Scotia, is petrologically sigaificant because itrepresents the first documented occurTence of magmatictopaz in granitic rocks of the Meguma Zone and is alsohost to the only primary producing tin deposit in NorthAmerica (56 million tonnes at 0.16590 Sn). Previous radio-metric ages of ca. 300 Ma (40Arl39Ar muscovite) and ca.337 Ma (Rb-Sr whole rock) for the East Kemptvilleleucogranite are herein reinterpreted to reflect updatingrelated to thermotectonic disturbances rather than the ageof crystallization, The leucogranite is medium grained,equigranular and characterized by tlre prcence of magmatictopaz and muscovite; rare, small (<0.1 mm) biotite grainsoccur. The absence of fluid-saturation textures (a.g., miaro-litic cavities, unidirectional solidification textures, pegma-tites) indicate emplacement of the leucogranite as a H2O-undersaturated melt. Although petrographic observationsindicate subsolidus modification of primary magmaticmineralogy and textures, the leucoganite is considered toreflect a magmatic, rather than metasomatic phenomenon.Chemically the leucogranite is characterized by elevated con-centrations of SiO2, A12O3 (1.13 < A/CNK < 1.84), totalFe, P2O5, Rb, Cs, Ga, Li, F, Sn, Nb, Cu,Zn, W, U, Taand depleted levels of TiO2, Cap, MCO, Ba, B, Th, V, Co,Sc, Hf, Zr, Ni, Mo, Sb, Y. Conipared to evolved units (i.e.,leucogranites) of the South Mountain Barholith. the leu-cogranite is enriched in Ta, Sr, Rb, F and Li, whereas R.EEabundances and chondrite-normalized patterns aremarkedly different. Geochemically the leucogranite com-pares favorably to other F-rich felsic suites, particularlytopaz granites. The East Kemptville leucogranite is consi-dered to represent the highly fractionated product of astrongly peraluminous melt, itself derived yra melting ofa fluorine-rich biotite phase. The leucogranite may notrepresent a fractionate of the South Mountain Batholith(or Davis Lake Complex). The location of the East Kempt-ville leucogranite within a major northeast-trending shearzone may indicate tlat structure was involved in bothgeneration and emplacement of the leucogranite.

Keywords: topaz, muscovite, fluorine, tin, leucogranite,East Kemptville, Meguma Zone, Nova Scotia.

Souuarnr

Le massif leucogranitique i topaze + muscovite de EasrKemptville (370 Ma), situg A l,extremitd sud-ouest dubatholite ddvonien de South Mountain, en Nouvelle-Ecosse,est petrologiquement imponant parce qu,il constitue le

premier exemple de topaze primaire dans un granite de lazone de Meguma, et qu'il est le site du seul gisement d'€tainen prodlction en Amdrique du Nord (56 millions de tonnesd 0.16590 de Sn), Les 6ges propos6 antdrieurement, d'envi-ron 300 Ma (+0617:o6t sur muscovite) et 337 Ma (Rb-Srsur roches totales) rdsulteraient d'un rajeunissement dt irdes 6v6nements thermotectoniques posterieurs i l'6ge decristallisation. Le leucogranite possbde une granulomdtriemoyenne; il est 6quigranulaire, et contient I'associationtopzue + muscovite primaires. Les grains de biotite sontrares et petits (<0.1 mm). L'absence de textures indicatives d'une saturation en phase fluide (miaroles, croissan-ces unidirectionnelles, pegmatites) fait penser que le magmaleucogranitique 6tait sous-salurd en eau lors de sa mise enplace. Quoique les observalions p€trographiques indiquentune modification sub-solidus des textures et de la min6ra-logie primaires, le leucogranite reflEte surtout une signa-ture plutdt magmatique que mdtasomatique. Il se distinguepar des concentrations 6lev6es de SiO2, Al2O3 (1.13 <A/CNK < 1.84), fer total, P2O5, Rb, Cs, Ga, Li, F, Sn,}rfb, Cu, Zn, W, U et Ta, et de faibles teneurs en TiO2,CaO, MgO, Ba, B, Th, V, Co, Sc, Hf, Zr, Ni, Mo, Sb etY. Compar6 aux unit6s dvoludes, et donc leucogranitiques,du batholite de South Mountain, le leucogranite de EastKemptville est enrichi en Ta, Sr, Rb, F et Li; leurs teneursrespectives en terres rares sont tros diff6rentes. Le leuco-granite se compare davantage alrx autres suites enrichiesen fluor, et en particulier aux granites i topaze. Ce massifrepr6senterait donc une fraction trds 6volu€e d'un magmafortement hyperalumineux, lui-m6me d6riv6 par fusiond'une source contenant une biotite fluor€e. Il pourrait nepas repr6senter un produit de fractionnement au sein dubatholite de South Mountain ou du complexe de DavisLake. Son emplacement, dans une zone de cisaillementimportante orientde vers le nord-ouest, pourrait vouloirindiquer que cette structure a 6t6 impliqu6e dans la g6n6-ration et la mise en place du magma leucogranitique,

(Traduit par la R6daction)

Mots-clds: topaze, muscovite, fluor, 6tain, leucogranite,East Kemptville, zone de Meguma, Nouvelle-Ecosse.

INTRODUCTIoN

The East Kemptville leucogranite is a term intro-duced by Kontak (1987a) to describe the muscoyite-topaz leucomonzogranite host-rock at the EastKemptville Sn-Cu-Zn-Ag deposit. The depositrepresents North America's only primary producerof tin, with initial reserves estimated at 56 milliontonnes grading 0.16590 Sn (Richardson 1984, Moyle

787

788 THE CANADIAN MINERALOGIST

Ftc. l. Location map for tlte Meguma Terrane in the Canadian Appalachians, showing its relation to otier major ter-ranes and tJre bouridary fault [(Cobequid-Chedabucto Fault System (CCFS)]. Also shown is the simplified geologyof southern Nova Scotia, outline of the South Mountain (SMB) and Musquodoboit (MQB) batholiths, and locationof East Kemptville area. Significance of the Liscomb Complex and Tangier (dykes) is discussed in the text. Abbrevi-ations: MT Meguma Terrane, A Avalon, G Gander, D Dunnage, H Humber.

1985). The deposit, located in southwestern NovaScotia, occurs at the western extremity of the 370Ma South Mountain Batholith, where an elongateprotrusion of granite intrudes metawackes of theLower Paleozoic Meguma Group (Fig. 1).

Previous investigators (Richardson 1983, 1985a,b, Richardson et al. 1982, 1989, Richardson &Spooner 1982, Ford & O'Reilly 1985, Chatterjee &Strong 1984) considered the deposit to representextreme fractionation of this part of the SouthMountain Batlolith, combined with extensive fluid-rock interaction, which resulted in formation of thegreisen-hosted mineralization. Richardson (1988a, b)and Richardson et al. (1988a, 1989) have modifiedearlier models to accommodate rec€nt results of Rb-Sr radiometric dating (Richardson 1988a, Richard-son et al. I 987, 1988b), which indicate a\ age of ca.330 Ma for the Davis Lake Complex (the western-most intrusive phase of the South Mountain Batho-lith) and greisen at East Kempwille. Hence, Richard-son (1988a, b) postulated a distinct petrogeneticevolution for the Davis Lake Complex, comparedto the South Mountain Batholith, which accountedfor its apparently younger age and potential for tinmineralization. Conversely, Kontak (1987a, b, c) andKontak et al. (1986,1988a) recognized that much ofthe East Kemptville deposit area is underlain by a

mineralogically and compositionally distinct, butnevertheless uniform phase of muscovite-topazleucogranite. The presence of topaz is unique to thispart of the South Mountain Batholith, which sug-gests, therefore, that the East Kemptville leucogra-nite has a petrogenesis markedly different from thatof other phases of the South Mountain Batholith.Thus, because of the generally resoenized importanceof tin mineralization and occurrence of topaz-bearinggranites @urt er ql, 1982, Burt & Sheridan 1988,Christiansen et sl. 1986, Kovalenko & Kovalenko1984, Kovalenko et al. 1972, 1979), and the impli-cations that such F-rich igneous suites have for thecomposition of basement protoliths (e.g., Manning& Pichavant 1983, Christiansen et ql. 1983, 1986,1988, Christiansen & Lee 1980, it is important todocument the petrogenesis of the East Kemptvilleleucogranite and to determine its relationship to theSouth Mountain Batholith.

Other F-rich igrreous suites occur within the Cana-dian Appalachians, including topaz - lithium micaexanite in the Fire Tower Zone of the Mount Pleasanttin deposit, New Brunswick (Kooiman et al. 1986,Taylor et al. 1985, Sinclair et al. 1988), the F-richHarvey volcanic suite, also of New Brunswick(Payette & Martin 1986a, b), md minsmlized (Sn,W) parts of the Ackley Granite, southeastern

groni toids ( Devono- Corboniferous)

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TIIE EAST KEMPTVILLE TOPAZ-MUSCOVITE LEUCOGRANITE 789

Flc. 2. (A) $implified geological map of the southwestern part of the South Mountain Batholith (MacDonald e/ a/.1989, O'Reilly 1988a) showing location of tle East Kemptville leucogranite and East Kemptvile and Rushmere LakeShear Zones (EKSZ, RLSZ). The ages represent MAr/3gAr biotite O) and muscovite (m) data from Reynolds ela/. (1981). (B) Outline of rhe area underlain by the East Kemptville leucogranire (EKL) and its approximate contactwith granites of the South Mountain Batholith,/Davis Lake complex @LC). Also shown is tle outline of the areamapped indetail by Kontak el a/. (1986) and locations and results for muscovite dated by Zentilli & Reynolds (1985)using the aoArl3eAr technique. (C) Outline of the northern part of the East Kernptville deposit showing area under-lain by unaltoed versus variably altered East Kemptville leucogranite.

Newfoundlan d (luach et al. I 986, Tuach I 9S7). Allof these suites, including the East Kemptvilleleucogranite, are of Late Devonian - EarlyCarboniferous age and may share commonpetrogenetic histories. Of course the association oflithophile element mineralization and F-rich igneous

rocks is not unique to this area; in fact, it is an asso-ciation that has been noted for some time (e.9.,Tischendorf 1977, Taylor 1979, aad referencestherein). However, the significance of this associa-tion has perhaps only recently been realized as moreocclurences are documented (e.9., Taylor & Strong,

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1985, 1988). Thus, the purpose of this study is to(l) document the petrology of the East Kemptvilleleucogranite, (2) discuss the petrogenesis of the EastKemptville leucogranite in terms of other intrusivecomplexes of the Meguma Terrane, and (3) discussthe implications of this study in terms of potentialfor tin mineralization in the Meguma and adjacentterranes ofthe Canadian Appalachians. The presenceof a topaz-bearing granite within the Meguma Ter-rane also has ramifications in terms of probable base-ment rocks that underlie this part of theAppalachians, based on our understanding of con-ditions prerequisite for the production of such F-richmelts (e.9., Christiansen et al. 1983,1986, Manning& Pichavant 1983, 1988, Pichavant & Manning1984).

In Part I, presented here, the petrology ofthe EastKemptville leucogranite, including whole-rockmajor- and trace-element chemistry, rare-earth-element chemistry and oxygen isotope data, arediscussed in the context of the South MountainBatholith. Part II presents the mineral chemistry ofthe East Kemptville leucogranite.

REcIoNAL Gsot-ocrcer SsrrrNc

The East KempMlle leucogranite is located withinthe Meguma Terrane, the easternmost of severalterranes that collectively constitute the CanadianAppalachians (Williams & Hatcher 1983; Fie. l).This terrane was accreted during the Late DevonianAcadian Orogeny along the east-west Cobequid-Chedabucto Fault System (Webb 1969, Keppie 1982,Mawer & White 1986). Continued transpression ofthis terrane is documented by the presence of numer-ous northeast-to east-trending shear zones that wereactive at ca.370-300 Ma (Keppie & Dallmeyer 1987,Dallmeyer & Keppie 1987, Reynolds et ol. 1987,IJill1988, Muecke et al. 1988). The presence of thesehigh-strain zones is emphasized because of theirimportance for the petrogenesis of the EastKemptville leucogranite (see below).

The Meguma Terrane is underlain by inferredPrecambrian ortho- and paragneiss, as indicated byrecent field and geochemical investigations in theLiscomb and Tangier areas of southern Nova Scotia(Fie. 1; Giles & Chatterjee 1986, 1987, Chatterjee& Giles 1988, Clarke & Chatterjee l988,Ebuz et al.1989). These authors have recognized the presenceof granulite-facies rocks cropping out in the formof a gneiss dome in t}te Liscomb area and as xenolithsin mafic dykes and intrusive bodies in both the Lis-comb and Tangier areas. The rocks are chemicallydistinct from Meguma Group lithologies @berz &Clarke 1988, Eberz et ol. 1989) and are, therefore,interpreted to represent a partial sampling of vari-ous structural levels of the Meguma Terrane infra-structure. Recently acquired 4At/3eAr plateau ages

of ca.370-380 Ma (amphibole, biotite) have beenobtained for the gneissic rocks (Kontak et al. 1989b,1990, Kontak, unpubl. data), indicating thatemplacement of the gneisses may have been coin-cident with later stages of the Acadian Orogeny.

Overlying the inferred basement rocks with pre-sumed unconformity are Lower Paleozoicmetasedimentary and metavolcanic rocks (e.9.,Meguma Group, White Rock Formation). Theserocks were deformed and metamorphosed during theMid- to Late Devonian Acadian Orogeny (Keppie1982, Muecke 1984, O'Brien 1985). Large volumesof meta- to peraluminous granitic rock wereemplaced at ca. 370 Ma (Hill 1988, Reynolds el a/.1981, 1987, Clarke & Halliday 1980, Keppie &Dallmeyer 1987, Krogh & Keppie 1988), with lesseramounts of peraluminous granite intruded at ca. 315Ma (e.9., Wedgeport granite: Cormier et al. 1988).Mafic bodies, in the form of plutons and dyke rocks,also have been recognized in the central part of theMeguma Terrane in recent years (Giles & Chatter-jee 1986); ages of co.370 Ma were obtained basedon 4oArl3eAr dating (Kontak et al. 1989b, 1990,Kempster et al. 1989).

The presence of coarse terrestrial redbeds (i.e.,Hofton Group) of Early Carboniferous age uncon-formably overlying the South Mountain Batholithindicates rapid uplift and erosion during EarlyCarboniferous time. Inherent in the formation of theCarboniferous basins was a regime of widespread,overall extensional tectonics (Fralick & Schenk 1981).

Frcro RnlauoNsHlps oF THE Easr Ksr4prvtll-BLEUCOGRANITE

The East Kemptville leucogranite occurs at thesouthwest termination of the 370 Ma South Moun-tain Batholith (Figs. l, 2A). The batholith (10,000km2) is po$t-tectonic; it comprises compositionallyand texturally variable peraluminous granitic units,from granodiorite to monzogranite. Several intru-sive complexes have been recognized (e.9., the Hali-fax Pluton: MacDonald & Horne 1988) such that thebatholith results from multiple intrusions, each withits own magmatic history [see MacDonald et al.(1988, 1989) and references thereinl. Although end-member units representative of the most evolvedstages of fractionation occur Muecke & Clarke 1981,Kontak et al. L988b, Corcy et a/. 1988), such mus-covite t andalusite leucogranites are barren of topazand are not enriched in fluorine beyond a few thou-sand ppm (Ham et al. L989). Thus, despite theextremely large dimensions of the South MountainBatholith and its degree of fractionation, the occur-rence of topaz as a potential magmatic phase is rare.However, it is worth noting that topaz has beenreported as an accessory phase from a few greisenlocalities in the eastern palt of the South Mountain

Batholith (Logothetis 1985).As illustrated in Figure 2, the East Kemptville

leucogranite occurs as a small, lobate protrusion ofthe South Mountain Batholith at the granite-metasediment contact. It is volnmetrically small, afeature characteristic of topaz-bearing suites ingeneral (Christiansen et al. 1986, Pichavant & Man-ning 1984). Unfortunately, its relationship to the sur-rounding intrusive rocks remains ambiguous owingto lack of outcrop. Regionally the East Kemptvilleleucogranite occurs in a northeast-trending shearzone, referred to as the East Kemptville Shear Zone(EKSZ), which has a widrh of about l-4 km andpresently known strikeJength of about 40 km(O'Reilly 1988a). Subparallel to this shear zone is asecond major structural break, the Rushmere LakeShear Zone (RLSZ: Smirh 1985), which delinearesin part the southern boundary of the South Moun-tain Batholith. Both structures record an intensephase of shearing, with variable amounts ofhydrothermal alteration. The EKSZ is host to several1li11e1alized centers (Sn-W-Cu-Zn-Ag); in contrast,only minor amounts of base-metal mineralizationhave been observed along the RLSZ. The age of thedeformation along these zones is variable over theinterval 330-260 Ma based on 4oArl3eAr age spec-tra on muscovite and feldspar (Zentilli & Reynolds1985, Reynolds et ql. 1981, Soehl et al. 1989) undRb-Sr whole-rock and mineral isochrons (Kontak erol. 1989d.

The East Kemptville leucogranite is exposed in thelarge open-pit mine site east of Yarmouth (Figs. l,2). It occupies a prominent flexure in the regionalgranite-metasediment contact, and its dome orwhale-back configuration is due to its doubly plung-ing attitude (Fig. 2b). It is oriented northeasr-southwest, has a surface outline of ca. 1.5 km by0.5 km, and intrudes quartzites to quartz wackes ofthe Goldenville Formation, lowermost member ofthe Cambro-Ordovician Meguma Group. Thesedimentary rocks define an antiformal structure inthe immediate vicinity of the deposit and are notablyindurated and hornfelsed, with a spotted texture;rafts or xenoliths of this unit are not observed in theleucogranite. However, the sedimentary rocks doform long, n€urow roof-pendants that penetratevertically into the leucogranite, thereby suggestingthat the roof zone is presently exposed. The presenceof local fluidized breccia and granitic dyke materialpenetrating the overlying sedimentary rocks @ig. 3),as observed immediately north of the granite -sedimentary rock contact (Fie. 2C), also suggestsclose proximity to the roof zone of the pluton.

Although only the northern half of the depositarea was mapped as a part of this study (Fig. 2C),the remainder of the deposit has been examined asmining has developed and, therefore, the followingobservations are considered to be generally applica-

791

Ftc. 3. Photograph of outcrop of Meguma Groupmetasedimentary rock cut by subvertical veinlets of fine-erained (i.e., chalcedonic) quartz. In detail, veins areseen to contain angular fragments ofthe wall rock sedi-ments and fine-grained granitic clasts (width ofexposure0.65 m).

ble to the whole pluton. The East Kemptvilleleucogranite is generally homogeneous in its texture,grain size and mineraloCy (FiC. 4) throughout themine site. Moderately fresh leucogranite is estimatedto underlie approximately 3040t/o of the depositarea; the remaining rock consists of variably alteredand greisenized granitic material. The granite istypically medium grained, although finer-grainedtextural varieties do occur. The absence of chilledmargins, K-feldspar megacrysts, miarolitic cavities,breccias and unidirectional solidification textures(USf$ (e.g., Shannon et al. L982, Sinclur et al.1988, Kirkham & Sinclair 1988) is emphasized. Thepresence of local pegmatite segregations @gs. 4A,B) is restricted to a few localities at granite - sedimen-tary rock contacts. In these latter cases, a borderphase of coarse quartz-feldspar intergrowth withoriented quartz crystals is cored by banded apliteFig. aA). Recently, Richardson el a/. (1988b) have

THE EAST KEMPTVILLE TOPAZ-MUSCOVITE LEUCOGRANITE

briefly discussed these localities and inferred thatthey represent examples of USZs. Although the grainsize of the leucogranite may vary from medium- tofine-grained, with the former being more abundant,no intrusive contacts have been observed; the varia-tion may reflect local thermal perturbations and,hence, rate of crystallization. Fine-grained,aphanitic, greenish dyke-rocks ?lso occur, albeit onlyrarely.

The East Kemptville leucogranite is everywhere cutby a heterogeneously developed, northeast-trendingfabric that is variably defined as a spaced or frac-ture cleavage in the least-deformed rocks toultramylonite in the most highly strained areas (Fig.5). C-S fabrics also are present (Fig. 5C) andrepresent a transition between the aforementionedfabrics. Of several hundred thin and polishedsections examined, only a small percentage do notshow textural evidence (i.e., kinked mica, fracturedfeldspar grains, undulose quartz grains with subgraindevelopment) of superimposed deformation.

Acn op rHE EAsr KEMpTvTLLE LsucocRANrre

Based on field relationships, the East Kemptvilleleucogranite is younger than the Mid- to LateDevonian Acadian Orogeny, dated at co. 400 x.10 Ma (Reynolds & Muecke 1978, Muecke et al.1988, Keppie & Dallmeyer 1987), and older than thedeformation alluded to above. Previously published44Ar/3eAr ages of ca. 300 Ma for two samples ofgreisen muscovite (see Fig. 28) were interpreted byZentilli & Reynolds (1985) as representing the ageof the mineralizing event and, therefore, probablya good approximation of the age of intrusion. Morerecently, Richardson et al. (1988b) inferred €m ageof 337 + 5 Ma for the mineralizing event based ona 5-point whole-rock Rb-Sr isochron. Thus, thereappears to be contradictory evidence at presentregarding the timing of mineralization and, hence,intrusion of the leucogranitic magma. Important inlieht of this is a third 4oArl3eAr age spectrum formuscovite reported by Zentilli & Reynolds (1985) fora sample immediately southeast of the East Kempt-ville deposit but within the leucogranite (Fig. 2B).This sample gave a discordant age spectrum, but

793

indicated a possible age of ca,350-360 Ma for thehigh-temperature gas fractions. This latter age is con-sistent wilh a recently obtained ll-point Rb-Srwhole-rock isochron age of 344 a 5 Ma (Kontak elal. 1989a) for relatively fresh samples of leucogranite.

In the studies by Zentilli & Reynolds (1985) andRichardson et al. (1988b), there is no discussion ofthe possible effects the deformation mentioned abovemay have had on the isotopic systems. Both theRb-Sr and K-Ar isotopic systems are sensitive tothermal disturbance (e,9., Jiiger 1979); therefore, thepresence of deformational textures within the studyarea is reason to interpret the age data with caution.The younger Ar-Ar ages are similar to publishedAr-Ar ages from other areas of southwestern NovaScotia @eynolds et ol. 1981, 1987, Dallmeyer & Kep-pie 1987; see Fig. 2A), which have been attributedto thermotectonic disturbances. A similar interpre-tation is offered here for both the Ar-Ar and whole-rock Rb-Sr ages obtained from the East Kemptvillegreisens. The older Rb-Sr whole-rock age mayrepresent a better approximation, albeit still a mini-mum, for the age of the East Kemptville leucogranitebecause of the general unreliability of Rb-Srisochrons in areas where a subsequent thermal dis-turbance has occurred (e.9., Schiirer et al. l988,Wal-rayenet al. 1985, Kontak et ol. 1988c). UnpublishedPb-isotope data for the East Kemptville leucograniteand greisens indicate a Pb-Pb isochron age of 370+ I Ma (Chatterjee & Kontak, in prep.). Hence, theage of intrusion and related hydrothermal activityis considered to be temporally equivalent to the restof the South Mountain Batholith (Le., 370 Ma), withthe younger ages reflecting episodic tectonothermalactivity.

PETRoGRAPHY oFTHE EAST KST4pTVTI-LE LBUcOcRANTTT

As stated earlier, the East Kemptville leucograniteis a medium-grained, equigranular monzogranite onthe basis of point counts on both stained slabs andthin sections (Table I, Fig. 6). It should be noted,however, that the plagioclase present in the rock ispure albite. In the classification scheme ofStreckeisen (1976), the leucogranite would be more


Ftc. 4. Photographs of the East Kemptville leucogranite showing some of the lithological variation in the unit. (A)Sample of pegmatite-aplite from the northern part of the map area (see Fig. 2c). Note the banded nature of theaplitic core and the quartz crystals projecting into the center of the sample. The chemical composition of the aplitein this photo is represented by #099 in Table 2 (bar scale 3 cm). (B) Pegmatite (chemical composition is representedby sample #098 in Table 2) showing coarse intergrowth of quartz and feldspar crystals. Note the deformation tex-tures manifested as color bands in quartz and spaced fracture-cleavage in lower right of the photo @ar scale 2 cm).(C) Typical specimen of East Kemptville leucogranite showing the medium-grained equigranular texture characteris-tic of the unit. Note the absence of megacrysts and miarolitic cavities (bar scale I cm). (D) Photomicrograph ofundeformed specimen of leucogranite showing the idiomorphic to hypidiomorphic granular texture that character-izes undeformed samples (k = alkali feldspar; p = albitic plagioclase; q = quartz) (bar scale 0.50 mm).

THE EAST KEMPTVILLE TOPAZ-MUSCOVITE LEUCOGRANITE 795

correctly described as an alkali feldspar eranite (sensustricto). However, since the albite is interpreted tobe a replacement phase for a calcium-rich precursor(i.e., oligoclase), the term monzogranite is retained.The homogeneous nature of the granite is demon-strated by the fact that point counting ofthin sectionsand stained slabs defines comparable fields in termsof the percentages of quartz, plagioclase andK-feldspar (Fig. O. The essential mineral conqtituentswith their average modal percentages (+ I standarddeviation), as determined from approximately 38,Mpoint counts of 25 thin sections of the leucogranite,are quartz (36.8 r 4.9), plagioclase (30.0 ! 3.7),K-feldspar (19.4 + 5.3), muscovite (7.8 t 4.7) andtopaz (5.4 t 1.5), with accessory biotite, zircon,monazite, apatite and ore phases (cassiterite, sulfides,oxides). Accessory phases account for much less than0.5 modal Vo of the mineral constituents. Theproportions of the major mineral constituents in theleucogranite are illustrated in Figure 6 and demon-strate that, except for a few samples, the mineralproportions are fairly constanl. In thin section, theleucogranite is generally quite variable betweenidiomorphic to xenomorphic, the latter beingattributed to the combined effects of postmagmaticalteration and later deformation.

Quartz is generally anhedral, but may appear asequant grains having the habit of high-temperature(B) quartz (Fig. aD); it forms sharp contacts with allother minerals. Inclusions axe not common, but smallgrains of plagioclase and, rarely, biotite @ig. 7I) havebeen observed.

Plagioclase (Fig. 7A) is invariably subhedral toeuhedral, displays albite and Carlsbad twinning, andis of albitic composition. Sharp contacts with allother mineral phases have been observed. Theplagioclase always contains small inclusions ofapatite @ig. 7B). Plagioclase may also occur as sub-hedral to anhedral inclusions within K-feldspar @igs.7C, D); these are interpreted as representing vestigesof larger plagioclase grains that were replaced byK-feldspar.

K-feldspar forms the coarsest mineral phase, isgenerally subhedral to anhedral, and overgrows otherminerals (Fig. 7C), most notably plagioclase.However, in the least altered and structurally modi-fied leucogranites, K-feldspar is subhedral, rarelyovergrows plagioclase, has straight contacts with

other mineral phases, and may be similar in grainsize to the otler mineral constituents (Fig. 4D). Thus,much ofthe irregular texture and overgrowth ofthismineral is presumed to be related to late magmaticgrowth and subsolidus modification (e.9., Witt1988). Exsolution lamellae are ubiquitous and mostcommonly are flame type, with lesser amounts ofpatch perthite (Figs. 7D, E). The K-feldspar istriclinic, and grid twinning typical of low microclineis common. The structural state of the K-feldsparhas been confirmed by X-ray-diffraction studies,which indicate a triclinicity of I (Kontak, unpubl.data). It is not possible to say if the ordering is anearly feature or related to the superimposed defor-mation.

Muscovite occurs as subhedral, rarely euhedralprains @ig. 7F), with rare inclusions of accessoryminerals. It shares straight contacts with adjacentphases. A magmatic origin is consistent with the tex-tures observed (Ham & Kontak 1988).

Topaz (Figs. 7G, H) occurs in all samples, beinghomogeneously disseminated throughout. It is gener-ally subhedral, but euhedral grains have beenobserved in dyke rocks. The topaz is most commonlyfractured, even in cases where all other mineralphases are apparently pristine in habit (i.e., free ofstructural modification); the reason for this is notpresently known. Inclusions in topaz are rare, butmuscovite and plagioclase do occur. All indicationsare that the topaz represents a magmatic phase andnot a later manifestation of hydrothermal activity(see below).

Trace amounts (i.e., a few grains per thin section)of a biotite-like phase occru as small (< 0.1 mm),subhedral, medium brown to light orange-browngrains (Fig. 7I). They are commonly zoned to alighter shade toward their margin and may bemantled by muscovite. Minute inclusions of opaquephases may occur.

Accessory phases include zircon, monazite,apatite, uraninite, Nb-Ta oxides and ore minerals(sphalerite, cassiterite, pyrite, chalcopyrite, pyrrho-tite, cassiterite).

Examination of thin sections shows extensivedevelopment of deformational textures (e.g., Fig.5B). Details of the structure of the leucogranite isbeyond the scope of this paper and will be reportedelsewhere.

Frc. 5. Photomicrographs of structurally modified East Kemptville leucogranite illustrating the degree of deformationpresent within the deposit area. Protolittrs in all cases represented by samples shown in Figures 4c and d. (A) Blastomylo-nitic texture in cut slab of East Kemptville leucogranite with a few remnant gnins of partially comminuted quartzvisible. The fabric has a field orientation of NE/subvertical Oar scale I cm). @) Photomicrograph of protomylonite(bar scale 0.50 mm). (C) Fietd photograph showing spaced fracture-cleavage (C) cutting pre-existing, penetrativeflattening fabric (S) in high-strain zone of East Kemptville leucogranite. Note that the granite in upper right of thephoto [i.e., beyond the shear zone boundary (SZB)] remains relatively undeformed. Dime is shown for sca]e in lowerleft of photo.


P + A

Frc. 6. Triangular plots showing results of point counting samples of East Kemptville leucogranite (see Table I forsummary of data). Note the comparison of data for thin sections versas slabs in the QAP plot. Abbreviations: Qquafiz, A alkali feldspar, P albiric plagioclase, T topaz, M muscovite).

WTTOLS.ROCT GEOCHEMISTRY

Analytical procedures

Powdered whole-rock samples were analyzed formajor elements using wet-chemical techniques atMemorial University of Newfoundland, St. John's,and the Technical University of Nova Scotia(TIJNS), Halifax. All analyses for Si, Al and Fpresented were obtained at TUNS. Details ofanalytical procedures employed at these twoinstitutions are described by Wilton (1985) andChatterjee et al. (1983), respectively. Trace-elementdata were obtained at St. Mary's University, Halifax,using an automated Philips X-ray-fluorescence(XRF) spectrometer (Rb, Sr, Ba, Zr, Nb, Pb, Ga,Z\, Ay Ni, V), at Bondar-Clegg Ltd., Ottawa, usingneutron activation (U, Th, Cs, W, Mo, Ta, Hf, Sc),

XRF (Sn), DC plasma (B), and atomic absorptionspectroscopy (Li), and at TUNS using titration (S)techniques. Rare-earth-elern ent (REE) data and sometrace-element data (Y, Ba, M, Zr,U,Th, Co, Ta,Hf, Cs, Sc) were obtained by instrumental neutronactivation, wittr data reduction done using facilitiesat Carleton University (Ottawa) and St. Mary'sUniversiry Glalifax), and inductively coupled plasmamass spectrometry (ICP/MS, Memorial University,St. John's).

Orygen isotopes were analyzed at the Universityof Saskatchewan, Saskatoon, employir'g conven-tional procedures for the isotopic analyses of silicates(Clayton & Mayeda 1963). Isotopic data are reportedas 6180 values in per mil (/6s) relative to StandardMean Ocean Water (SMOW) based on analysis ofthe NBS-28 laboratory standard. The overallreproducibility of 6180 values has averaged a

thinsoclions

)"r4-!'t'

r{,\h1

iir#..-*slobs


IABI,E 1. XODAI. UINERAI.og! OF EAST Xn{PIVII.I.E I.EI'COCRAI{ITE

L 9 . 4 3 0 . 0 7 . 8 5 . 41 5 . 3 t 3 . 7 ! 4 . 7 ! L . 5

*Toial ileber of, points co@ted wlth ths Eoe t 1 stsdarddsvlacloa for oach uLneraL.

Nots tlut saEplo EK-85-23A wo poLnt coEtod oa tFo thhaeciloro to 4sesa the hooogEnoity ropresentod bjr thLascale of obseFatiq. The reaultd lndicate that thEgreleg ls comistsnt rLth rEspsct to edal Einoralogyed chat L500 pohts per thln agctLo! l-s repreaontati.ve.

AbbrevlatLoE: kf,s : K-f,elitspar, p!. - plagtocl*e,& - @cwlto.

0.18%0 (2o) in the laboratory (R. Kerrich, pers.gemm., 1987).

During all analyses both international andintralaboratory standards were routinely includedwith the EKL samFles; in addition duplicates of theunknowns were analyzed. The only problemsencountered were for the REEs, where low concen-trations for particular elements resulted in slightlydifferent results (see below).

Who le- rock geo chemist ry

Results of 27 whole-rock analyses, presented inTable 2 (major and trace elements) and Table 3(REE), were obtained from: (l) the medium-grained,equigranular phase, (2) fine-grained dyke rocks(samples 021,091,105), (3) a pegmatite (098), and(4) the aplitic core of a pegmatite (099). Compari-son of the average leucogranite (n = 22) to otherfelsic igneous rosks characterized by enrichment inF and LILE is made in Table 4.

Maj o r-eleme nt chemis t ry

Major-element cons€ntrations in the leucogranite

797

are fairly uniform, and indicate that it is rich inSiOr, with a range of ca. 72-76 wt.Vo; the singlepegmatite sample is sliehtly enriched at77 wr.o/o.Theonly deviation from the apparent homogeneify of thedata is the notable enrichment of the pegoatite sam-ple and the dyke rocks in Na relative to the rest ofthe data sel, with weight 9o Na2O/K2O ratiosexceeding unity. Compared to the average low-calcium granite of Turekian & Wedepohl (1961), theleucogranite is enriched in silica (above), Al (14-16fi.90 Al2O3), Na (3-5 wt.Vo Na2O), and P (0.2-lwt.9o P2O5). Similarly, very low concentrations ofTi (< 0.08 wt.Vo TiO) and Mg (0.16-0.01 wt.9oMgO) characterize the leucogranite compared to theaverage low-calcium granite. The major-elementchemistry of the leucogranite is comparable to thatreported for topaz granites @ichavant & Manning1984), particularly the low Ca, Mg and Ti contents.

Altlough iron contents of topaz granites axe gener-ally low, the slightly elevated concentration of totaliron in the East Kemptville leucogranite (L.41 + 0.42wt.%o) is attributed to finely disseminated opaquephases (i.e., sulfides), and the iron-rich nature ofthemuscovite (see Part II). As is expected, the redoxstate of the iron is mostly Fd*, with FeO/FqO,ratios averaging 3.91 a 2.04. The elevated phos-phorus contents are abnormal for felsic suites ingeneral (e.9., Watson & Capobianco 1981), but notunusual for granites of the South Mountain Batholith(Ham e/ al. 1989) or granitic rocks of the MegumaZone in general (MacDonald & Clarke 1985, Mac-Donald & Horne 1988, O'Reilly 1988b, Ham 1988).The elevated absolute concentrations of Na and thehigh Na/K ratios are not atypical for felsic igteoussuites (e.9., Hughes 1973). Comparable enrichmentof sodium is found in other volatile-enriched suites(e.g., Manning 1982, Kovalenko et al. 1972, 1979,Christiansen el al. 1983,1986, Pichavant et al. 1988,Payette & Martin 1986a), where it is generally con-sidered to reflect the influence of dissolved volatilespecies (i.e., fluorine in this case) on the phase rela-tionships of themelt. For example, 1a[6ning (1981,1982) has shown that the effect of fluorine is toexpand the stability field of quaxtz at the expenseof albite such that residual melts in the quartz - albite- K-feldspar pseudoternary system are relativelyenriched in sodium over potassium.

The strongly peraluminous nature of theleucogranite, consistent with its modal mineralogy,is manifested by elevated values of both normativecorundum (2.88 to 7.884/o; Fig. 8) and the A/CNKvalue (1.13 to 1.84). These strongly peraluminouscompositions are comparable to those reported intopaz granite suites elsewhere lsee Pichavant &Manning (1984) for summary, and also Weidner &Martin (1987) and Stone & Exley (1986)1, but areunusually so compared to topaz rhyolites(Christiansen et al. 1983, l98O which are commonly

saBpls #polnts q@rtz kf,s

L449 40.5 17.01500 32.3 25.L1500 36.2 24.8L829 41.8 L7.01500 34 .7 22 .71500 35 .0 2 r .51500 34 .2 2n .31500 35 .0 24 .61500 31 .2 23 .91500 38 .3 10 .81 5 0 0 3 6 . 8 1 5 . 81500 33 .8 22 .9L594 36.5 2f.71500 31 .1 29 .O1500 36.2 20.8L664 45.2 L4.21500 34 .8 20 .8743 4L .9 7 .2

1418 30.8 L7.41500 47 .6 18 .01500 44 .6 10 .81867 35 .9 20 .81550 33 .2 22 .O1500 27 .7 2X.2L92L 44.6 10.8

2 6 . 0 9 . 6 6 . 53 3 . 0 3 . 8 5 . 82 6 . 6 7 . 3 5 . 02 8 . 6 7 . 9 4 . 53 2 . 6 4 . 5 5 . 03 2 . 0 4 . 9 4 , 72 9 . 8 s . 8 6 . 42 9 . 4 3 . L 6 . 43 L . 5 7 . 4 5 . 62 7 . 7 L 7 . ! . 5 . 83 6 . 2 s . 9 4 . 23 2 . 8 3 . 2 7 . L2 9 . 7 6 . 2 4 . 62 8 . 7 4 . 2 6 . 53 0 . 8 3 . 9 7 . 72 6 . L 8 . 2 6 . : .3 2 . 4 3 . 9 7 . 82 2 . 7 2 4 . 8 3 . 03 6 . 1 1 0 . 7 4 . 62 4 . 3 5 . 2 4 , 82 6 . 7 9 . 8 7 , 62 8 . 4 L 2 . 8 t . 73 2 . 1 8 . 1 3 . 83 8 . 7 7 . 2 3 . 02 6 . 7 9 . 8 7 . 6

*TotaL 38035 35.8t 4 . 9

diopside normative (Fig. 8). The leucogranite dataare comparable to values for restite-rich granites inthe classification of Clemens & Wall (1981), althoughrestite phases have not been identified. Theleucogranites are more strongly peraluminous thanthe average Hercynian two-mica granite (LaRocheet al. 1980), the S-type Violet Town volcanic suite(Clemens & Wall 1984), a suite of biotite - cordieriteI sillimanite monzogranites (Kontak & Clark 1988)and andalusite - sillimanite - muscovite volcanicrocks @ichavaxlt et al. 1988) from southern Peru,and fluorine-rich granites from Mount Pleasant, NewBrunswick (Kooiman el a/. 1980 Fig. 8). Comparedto the field delineated for the average units of theSouth Mountain Batholith in the silica yersas 9o nor-mative corundum plot, the East Kemptville data arealso seen to be markedly more peraluminous over-all. In the same figure, the data also are comparedto some relevant experimental data from Clemens& Wall (1981). The strongly peraluminous nature ofthe East Kemptville leucogranite is evident; by anal-ogy to experimental data, the generation of suchcompositions requires the participation of a stronglyperaluminous phase (i.e., melt, topaz, muscovite).

The average fluorine content of the EastKemptville leucogranite is 0.76 wt.Vo, but there isconsiderable variation (between 0,29 to 2.01 wt.9o).Compared to the abundance of fluorine in felsicigneous rocks in general (Bailey 1977), the leu-cogranite is comparable to topaz rhyolites andgranites (Christiansen et al. 1983,1984, 1986, Picha-vant & fylaaning 1984), and ongonites (Kovalenkoet al. 1972, 1979). A comparison of the fluorinecontent of the leucogranite and granitic rocks of theeastern half of the South Mountain Batholith isshown in Figure 9, where it is demonstrated that theformer are relatively enriched in this element.Samples for the eastern half of the South MountainBatholith ienerally fall in the range 0.03-0.3 wt.9o,with a few samples approaching 0.4O.5 wt.Vo in cassof extreme differentiation. However, MacDonald &Clarke (1989) have recently concluded that much ofthis enrichment is due to late-stage alteration related

799

to deuteric fluids rather than reflecting primarymagmatic enrichment. Also shown in Figure 9 arecompositions of samples from the Davis LakeComplex determined by Richardson (1988a).Althouch these samples show silica enrichment, theredoes not appear to be a comparable enrichment influorine.

The normativs minglalsgy of the leucogranite isdominated by quartz, albite, orthoclase andcorundum; the suite is compositionally similar to thehaplogranite system. Comparison of the normativeproportions of Q(quartz), P(albite) and A(alkalifeldspar) for the leucogranite (Fig. l0A) to variouspolybaric cotectics in the H2O- and F-bearinghaplogranite sytems is shown in Figure l0B. As isevident, the data do not plot near the l-kbar cotecticfor the F-rich system as determined by Manning(1981), but do correspond to the low-pressurecotectic at 0.5-1.0 kbar for the haplogranite system(Luth 1970. How realistic these comparisons areremains questionable though, since the experimentswere run at water-saturated conditions, whereas itis inferred that the East Kemptville leucograniterepresents the product of crystallization of an H2O-undersaturated magma.

In view of the well-documented relationshipbetween normative composition and volatile content(i.e.,F) in felsic igneous rocks, the F contents of theleucogranite samples are shown in Figure l0A. Thereappears to be no obvious correlation between proxi-mity of samples to the albite apex of the triangulardiagram and F content, although the samplescollectively define a trend toward the albite corner.Also shown in Figure l0B are other F-rich suites, viz.the Macusani glass @ichavant et al. 1987b), topazrhyolites (Christiansen et al. 1983), the HarveyGroup volcanic suite (Payette & Martin 1987a, b),ongonites (Kovalenko et al. 1972), cryolite-bearingrhyolite plugs, Texas (Rubin et al. 1987), Mexicanrhyolites Qluspeni el a/. 1984), topaz granites, MountPleasant, New Brunswick (Kooiman e/ ql. L987) andthe lepidolite-topaz Beauvoir granite, France(Pichavant et al. 1987a). There is considerable varia-


Frc. 7. Photomicrographs (all taken in crossed nicols except b and i) of the East Kemptville leucogranite showingmineralogicat features. (A) Relatively undeformed specimen of leucogranite showing euhedral to subhedral texturetypical of albitic plagioclase. Note incipient development of deformation textures in quartz (suberain developmentand undulatory extinction) in lower right of photo Oar scale 0.50 mm). (B) Euhedral apatite crystals t equant sericiteflakes in albitic plagioclase host. Note the uniform distribution of both the size and distribution of the secondaryphases @ar scale 0.50 mm). (C) Alkali feldspar (k) replacing earlier placroclase with highly irregular outline Oarscale 0.50 mm). (D) Alkali feldspar containing albite as exsolution lamellae (irregular, elongate patches orientedWNW in photo) and as remnant plagioclase euhedra (p) that represent trapped, earlier crystal phases overgxownby the alkali feldspar Oar scale 0.50 mm). @) Pothitic alkali feldspar showing edd twinning chamcteristic of microclineand abundant albite exsolution lamellae (a) (bar scale 0.14 mm). (F) Isolated, subhedral muscovite grain interpretedto represent a primary, magmatic phase (bar scale 0.50 mm). (G) Large topaz Crain (gey phase in middle of thephoto) showing subhedral crystal outline. Note tle sharp grain boundary contacts with quartz (qtz) and muscovite(mus) (bar scale 0.50 m-). (H) Subhedral grains of topaz with mild deformational overprint @ar scale 0.50 mm).(I) Isolated grain of biotite in host quartz Oar scale 0.02 mm).

THE CANADIAN MINERALOGIST

TABI,E 2. IIEOII-ROCK GEOCHEUISTRY OF EAST KE}IPWILI.E IjEUCOGRT}IITE

009 012 013A 015 021 o23A o23B 023c 0?5F 027 068

Si02T i QAt2O3Fef3FeOl,lnOl,lgoCaoilafKfPzo5LOIF

73.16 73.11 75.02 74.14 72.72 71.51 74.6 72.35 72.32 73.54 75.700.00 0 .08 0 .08 0 .08 0 .00 0 .00 0 .& 0 .00 0 .00 0 .00 0 .02

15.10 14.52 13.88 14.42 15.60 15.33 14.89 14.37 14.58 16.12'14.690.18 0 .19 0 .24 0 .20 0 .24 0 .18 0 .27 0 .14 0 .16 0 .45 0 .490.87 0 .87 0 .73 0 .87 ' . t .13 0 .47 1 .00 1 .15 1 .03 1 .00 1 .530.06 0 .04 0 .03 0 .04 0 .06 0 .15 0 .03 0 .04 0 .04 0 .06 0 .100.03 0.03 0.03 0.04 0.04 0.03 0.01 0.05 0.03 0.01 0.08o.& 0.46 0.40 0.52 0.36 0.70 0.49 0.50 0.44 0.32 0.573.59 3.81 3.90 3.79 3.44 5.31 3.V 3.49 3.68 5.55 2.U4.08 3.84 3.81 3.96 3.65 2.41 4.03 3.63 5.93 3.&t 3.40.60 0.49 0.53 0.48 0.44 0.97 0.43 0.5'.1 0.49 0.43 0.480.83 0.71 0.79 0.87 0.86 0.62 1.20 0.86 0.76 0.84 1.110.65 1 .15 0 .77 2 .01 0 .69 1 .A 0 .48 0 .86 0 .69 0 .56 0 .4

72.63 72.830.00 0.00

16.03 14.940.35 0.14I .13 ' l .100.05 0.060.16 0.030.37 0.463.72 3.&3.82 3.780.45 0.451.09 0.810.69 1.08

99.$ 99.40 99.52 99.51 99.36 98.96 100.82 97.22 97.58 100.83 99.35 100.47 99.45o.27 0.48 0.32 0.&4 0.28 0.5r 0.20 0.56 0.28 0.2! 0.12 0.28 0.45

99.62 98.92 99.20 98.67 99.08 98.45 100.62 96.& 97.30 100.60 99.24 100.19 99.00

sIL iRbSrBaBarpbGaNbNbrZrZr*Y*t l

u*ThThrSntJ9rZnCu1'lovl icorTaTarHfHfrCscsrScSc*

2117.154 . 5

771411.6

177654

IrlDI0 .3

1412.6<22 . 1

33222 . 12 . 2

4',t0 16 440<10 <10 <10680 216 4179n $ft 963& 5 4 7 79 IID ND

6 6 6 5 3 43 2 3 ! - 3 828 30 31

ao 40 30<10 <15 <10170 234 224&4 892 89460 33 55IID ND ND

20 30 4532 33 3532 33 37

450 1800 690 370<10 NA <10 <10a7 218 728 754958 1010 899 9058 6 & 5 3 3 9t{D 19 U 24- 13.4 18.8

73 332 104 10532 55 37 3850 31 32 26

- 26.5 18.730 31 27 28

- 15.4 16.8- 5 . 1 4 . 3

28 27 27 32

5 5 5 6- 3 . 7 5 . 8

231 890 422 354u 8 3 2 8 4 4

370 193 190 1',1294 333 236 149

6 5 5 31 3 1 N D5 5 6 8

v

42.8

9016

3731 12752

t:

27

:8725

2272

lilD3

?0 90 300<10 <10 <10167 200 218876 905 44777 39 450ND ND 12628.5 13 .5 81 .433 63 5736 37 4234 3t 6313.5 23 .0 37 .728 28 2816.1 16.4 19.54 . 6 3 . 7 1 . 9

't6 16 21' t4 .1

5 64 . 8 4 . 4

101 2391 1 t 59 . 8

27 447 2 7

< 2 2lilD llD2 40 .3

'r5 13 12 ',36- 11.2 10.5 31.9

< 2 < 2 < 2 3- 1 . 7 1 . 5 2 . 4

49 25 33 11- 3 83 . 9 2 . 3 2 . 2 0 . 5- 2 . 5

1 9

:1 1 021

t$134

ND2

t 0

a;

0.339

1 . 8

2a

3 ; ;26.76 54 .9

2't0 2842 1532

222 1AAs6 1674 4

ND IID4 40 . 4

1 1 1 01 0 . 5z < 21 . 9

42 31u? . 2 1 . 82 . 1

1 1 1 2- 12.1

< 2 2- 1 . 1

38 47

2 . 1 2 . 3

1 2 1 19.3

<2 <21 . 2

41 51

1 . 8 2 . 2

t:

2724.855 . 5

972423

?fi113

7lD4o.4

1 1't1 .3<21 . 9

3935

1 . 92 . 3

A. /c lK 1 .31 1 .A 1 .A 1 .26 ' , t .52 1 .22 1 .30 1 .5 1 .31 1 .53 , l .58 1 .47 1 .37'/&o 4.A 4.19 3.37 3.94 6.10 4.08 4.37 4.87 4.44 6.85 6.55 5.&t 4.97Na2O/K2o 0.87 0.99 1.02 0.95 0.94 2.2O 0.93 0.96 0.93 0.92 0.86 0.97 0.96Feo/Fep3 4.A 4.57 3.A 435 4.70 2.61 3.70 8.21 6.43 2.22 3.12 2.30 7.85


TABLE 2. (continued)

801

Sarpte o72 091 098 099 105 109 t t 0 1228 1248 132

72.85 74.OO 76.99 72.52 74.29 73.U 71.85 73.73 7t.48 75.18 72.10 72.97 72.70 73.500.00 0.00 0.07 0.06 0.02 0.02 0.02 0.02 0.02 0.04 0.02 0.02 0.03 0.02

13.51 15 .00 '13 .80 15 .39 14 .28 14 .55 17 .09 '15 .40 15 .81 '13 .42 16 .00 15 .W 14.75 15 .700.41 - 0 .42 0 .99 0 .41 0 .14 0 .27 0 .33 0 .14 0 .44 0 .31 0 .19 0 .11 0 .280.93 - 0.60 1.87 1.03 ' t .03 1.03 0.80 1.60 0.93 0.77 0.93 0.87 0.930.r,6 0.04 0.01 0.11 0.08 0.04 0.04 0.04 0.04 0.04 0.24 0.10 0.04 0.050.05 0.03 0.M 0.16 0.02 0.02 0.02 0.03 0.01 0.06 0.02 0.03 0.01 0.020.66 0.6 0.& 0.60 1.05 1. ' ,12 0.58 0.77 0.42 0.75 0.25 0.31 0.54 0.493.50 4.01 5.05 1.87 3.37 3.89 3.76 3.35 3.62 2.&3 4.12 3.73 4,38 3.883.45 3,50 1.47 3.88 2.96 4.14 3.83 3.85 3.69 4.05 3.55 4.10 3.79 3.860.5't 0.60 0.27 0.29 0.31 0.8 0.41 0.56 0.. |3 0.W 0.46 0.39 0.9 0.451.03 0.61 0.75 1.95 1.14 0.51 0.80 0.33 0.68 0.A 0.55 0.36 0.52 0.351.A 1.19 i lA 0.48 0.67 0.82 0.75 0.62 1.05 0.48 0.43 0.48 0.42 0.46

10758178't61156

Si02Ti02At2O3FeP3FeOl'lnOl,lgOCaOilaPKpPzo5LOIF

F=o98.30 100.87 100.14 100.33 99.7? 100.50 100.56 99.92 100.92 99.32 99.24 98.76 9.23 100.860.51 0.49 0.20 0 .28 0 .34 0 .31 0 .26 0 .44 0 .20 0 . '18 0 .20 0 .17 0 .19

97.79 100.38 100.14 100.13 99.44 100.16 100.25 99.66 't00.48 99.12 99.06 98.56 99.06 100.67

sB

600 1130 1670<10 <10 itA NA545 424 178 811838 7U 32' 11!555 54 68 6128 25 32 395 . 0

270 310IIA NA

454 525714 81422',t 14045 67

26

54 . 4

1071 7

307232

ND!0 .2

1 412.5<21 . 9

30

46 1420IIA NA

728 705895 90172 t314 1414.138 9133 31a a16.831 3019.74 .9

28 26

6 55 . 2

114 36018 21

55 5tr3254 110

2 6i l D 16 7

14 1410.72 21 . 7

% v

150 200iIA NA

82 325659 7$87 706 9 3 l .58.2 21.437 6733 282 2 3 / '10.9 25.5a a14.0 16.75 . 6 3 . 7

28 25

6 65 . 0 4 . 8

270 102

?1244 4188 t36 32 l{D4 40 . 3 0 . 28 1 46.1 12.02 21 . 3 1 . 8

55 26

3 m &[A irA

6m 500858 82252 3327 17

180NA

9701't50

339

56 5931 5130 27

30

25

:?3019

70itA

6191

1 58

373126

27

:27

5

;23

L iRbSrBaBarPbGalbIbr2rZr,YrUurIhThrSnIgr

Znculrlovl iCorTaTarH fHfrCsCgtScsct

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360 73 430 78037 13 10 1932 ',t1.4

226 89 1099 610155 86 249 576< 2 < 2 2 3l{D lilD itD ND7 5 3 60 . 4 0 . 5 1 . 6 0 . 9

1 0 1 5 2 47.5 14 .1 1 .8 3 .2

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t . 6 5 . 7 2 . 1 2 . 61 . 4 5 . 4 2 . 3

2 .O2.42 . 1 2 .6 2 .3 2 .2 2 .12 .4 2 .7

1 . 92 .3

A/C i fK 1 .U l .n 1 .25 1 .84 1 .34 1 .15Ks 4.18 4.65 3.40 7.8 4.46 2.88na}o/w ' l .01 1.14 3.43 0.48 1.13 0.93Feo/Fezor 2.U 1.42 1.88 2.5't 7.35

1.50 1 .39 1 .486.75 5.72 5.45o.flr 0.87 0.98!.81 2.42 11.42

1 . A 1 . 4 4 1 . 3 5 1 . 2 0 1 . s4.42 5.47 4.60 3.50 4.450.69 1 .16 0 .90 1 .15 1 .002.11 2.$ 4.89 7.90 3.32

tdenotes anatysis of trace etement by instruFntal neutron activation technique atong rith REEA/CllK = molecutar ratio (At203/CaO + Na20 +.(r0); ID g not detectd (i.e. betor detection [imit),'7&o . % nonnative (CIPU) corundm catcutated on F-free basis


TS'-E 3. RAIE-EARTE-EI,IO{ENT COtrCBTTf,ATIOtrII BOR TEE EAST TII{FTVIIJ,:B I,:SUCOC8NtrITE

Saspl€ la Dy

r10{ 1.33110S(R) 1.03013Ar 1.1013AS 1.06013A*(R) 1.07015S 0 .79027t 0.2702w L,r2027{ (R) 1 .12068{ 0 .95o72* 1.1072{ 1.010724(R) 1 .00091* 0 .65003r r.4732{ 1.58't"56{ 0.851564(R) 0 .86023A* 0.95023C* 0 .95

0 . 2 3 1 . 6 0 . 6 50 . 2 4 7 . 2 0 . 7 5

0 . 2 0 . 2 0 . 0 90 . 2 0 , 2 0 . 1 0

3 . 9 1 0 . 5 5 1 . 8 4 1 . 0 8 0 . 0 0 0 . 8 6 0 . 2 5 1 . 4 3 0 . 1 5 0 . 3 6 0 . 0 5 0 . 3 1 7 0 . 0 3 32 . 8 9 0 . 3 9 ! . 4 4 0 . 7 r 0 . 0 0 0 . 7 1 0 . 2 2 L . 2 5 0 . 1 5 0 . 3 5 0 . 0 4 0 . 3 1 3 0 . 0 2 82 . 9 - 1 . 0 0 . 7 8 0 . 0 8 - 0 . 2 8 1 . 3 0 . 4 1 - < 0 . 1 0 , 2 0 . 0 92 . 7 7 0 . 3 6 1 . 3 8 0 . 6 5 0 . 0 0 0 . 6 1 0 . 1 8 1 . 0 2 0 . 1 3 0 . 3 0 0 . 0 4 0 . 2 9 7 0 . 0 3 12 , 7 2 0 , 3 7 1 . 3 7 0 . 6 3 0 . 0 0 0 . 6 6 0 . L 9 1 . 1 1 0 . 1 5 0 . 3 1 0 . 0 4 0 , 2 8 0 0 . 0 3 22 . 2 4 0 . 3 L 1 . 1 4 0 . 5 9 0 . 0 0 0 . 5 2 0 . 1 7 0 . 9 3 0 . 1 0 0 . 2 5 0 . 0 3 0 . 2 1 4 0 . 0 2 50.85 0 .12 0 .41 0 .28 0 .00 0 .26 0 .07 0 .43 0 .04 0 .09 0 .01 0 .164 0 .0092 . 9 5 0 . 3 1 1 . 4 5 0 . 7 0 0 . 0 0 0 . 6 9 0 . 2 2 L . 2 2 0 . 1 5 0 . 3 9 0 . 0 4 0 . 3 1 7 0 . 0 3 32.84 0 .39 L .46 0 .69 0 .00 0 .70 0 .19 1 .16 0 .13 0 .31 0 .04 0 .283 0 .0332.62 0 .35 1 .2s 0 .65 0 .00 0 .57 0 .16 1 .00 0 .1"2 0 .29 0 .03 0 .213 0 .0253 . 9 - 0 . 5 0 . 8 3 0 . 0 5 - 0 . 2 8 L . 7 0 . 6 9 - 0 . 0 2 0 . 2 0 0 . 0 82.76 0 .37 L .29 0 .67 0 .00 0 .59 0 .19 1 .29 0 .14 0 .34 0 .04 0 .295 0 .0282 . 7 2 0 . X 5 1 . 0 8 0 . 6 2 0 . 0 0 0 . 6 1 0 . 1 9 1 . 0 8 0 . 1 3 0 . 3 7 0 . 0 4 0 . 2 6 6 0 . 0 2 s

2 . 7 N A 0 . 8 0 . 7 5 0 . 0 73 . 5 l { A 1 . 1 0 . 8 8 0 . 0 5

- 0 . 8 0 . 7 9 0 . 0 4 - 0 . 2 5 1 . 3 0 . 7 6 - 0 . 2 0 , 2 0 . 1 0- 0 . 5 0 - 8 9 0 . 0 5 - 0 . 3 5 1 . 6 0 . 5 2 - < 0 . 1 0 . 3 0 0 . 0 8

3.52 0-4r 1 .51 0 .69 0 .13 0 .69 0 .20 1 .35 0 . r7 0 .47 0 .06 0 .40 0 .0472 . 4 0 0 . 2 9 1 . 2 3 0 . 5 6 0 . 0 0 0 . 5 5 0 . 1 6 0 . 8 5 0 . 1 1 0 . 2 3 0 . 0 2 0 . 2 0 0 . 0 2 32 . 4 7 0 . 3 2 r . 0 4 0 . 6 1 0 . 0 0 0 . 5 8 0 . 1 6 0 . 9 3 0 . 1 1 0 . 2 4 0 . 0 3 0 . 2 0 0 . o 2 2

SaupleE lith en ! denote etu1ys6s p6rlotu6d at Carlolon Univgislcy; thoso elth {, ml-ysoperfoh€d at M€EorlaL Unlverarty.

R lndtcaces a dupLlcate saapls amlysls.

Note that fo! th€ carloton amlyges tho oitots fot Yb and Td ere I 0.2 ppn and for NdapproxlMteLy t 1 ppo.

IA'l]g 4. GEOCESIiISTBY OF SEI.SCTED TLI'OIInE-IICI TEISIC SUITES

(t Lo

slo,

A1&Fe&F6Ol.tnougoca0NaroKrOP&LOlF

72.32 70 ,46 69 .540.04 - 0 .00

15.85 15 .98 16 .600 . 0 5 0 . 1 5 0 . 2 80 . 5 5 0 . 4 9 0 . 3 70 . 0 6 0 . 2 2 0 . 0 3o . o 2 0 . 1 3 0 . 9 9o . 2 L 0 . 5 2 0 . 8 94 . L 3 5 . 8 7 6 . 0 43 . 6 4 3 . 5 3 2 . 2 L0 . 5 3 0 . 0 9 0 . 5 10 . 4 6 - L , 2 41 . 3 3 2 . 0 5 1 . 1 6

7 3 , 7 7 2 . 2 60 . 0 3 0 . 0 3

L2.9 15 .30o , 2 0 , 2 10 . 9 0 , 2 40 . 1 0o , 4 2 0 . 0 7L . 2 7 L . 2 52 . 5 0 3 . 4 85.L2 4 .540 . 0 3 0 . 3 61 . 2 1 0 . 6 71 . 0 5 1 . 1 5

118818

75.9 7L .61 71 .L0 . 1 0 0 . 0 3 0 . 0 / ,

L2 .7 14 .88 L4 .71 . 0 7 0 . 4 3- 0 . 5 3 0 . 60 . 0 6 0 . 0 9 0 . 0 30 . 1 4 0 . 1 1 0 . 0 10 . 8 0 0 . 6 6 0 . 53 . 7 8 4 . 1 5 3 . 54 . 9 2 4 . 3 L 6 . 40 . 0 6 0 . 7 8 0 . 0 1- 1 . 5 40 . 2 8 1 . 5 7 0 . 5

73.27 (O.U)0.02 (0.02)

15.04 (0.86)1.38 (0.26)

0.04 (0.0r)0.03 (0.03)0.49 (0.13)3 .65 (0 .34)3 .78 (0 .20)0.48 (0.11)0 .73 (0 .25)o .71 (O.31)

(e5)( r8 )

(233)(14)

(1) .'vl glss f!@ Uac@el. volceLc floLd, Peru (Plehavdt et a7, L987) | (2) Ongonlto(av6!ago 10 @1yses, Ln Pl.chavant & lt€mlng 1984) ; (3) Boawol.! grdLt€ (averag€ 4@1y6€s, l.! Plcheet e Uamtng 1984) ; (4) Porphrlttc gr6l.t6, Mo6t PL€aset, NerBtllwlck (@lysls 82-71, Kooimn ot at. 1986); (5) flqorlte-tops-@covlte glanito, St.Ast61l, Elgled (!taldn€! C uartta 1987); (6) Tops rhyolltss f!o! T?D@a Rango, Utah(a%rags 11 @Lyaos f,or @Jor eloBonts ed 5 for trace elsnsata, Ctrrl.stl.areea at a7, L984);(7) contsh tope greLt€ (average 5 a$lyssa, ltenlag & Pichavanl 1988); (8) ehsslncLuLoro h q@rtz, tsan€y volcaalc sult€, Nee Breslck (n:30, Payette & ualtta 1986a) i(9) AElage (l I smdard dsvlatlon) East K6BpFt116 loucogrdlto (F22).

rcSrB

Ba

304300 L23 506

- 6 0

LL64 L9751 . 6 2 018503220<10

890t )

<10464

423 L43428 46

504L T4

bility among the various suites, which indicates,therefore, that the deviation of the East Kemptvilleleucogranite data from experimentally determinedcotestics is not unique.

Trace-element chemistry

Trace-element data for the leucogranite areremarkably uniform for most elements Clable 2 and

Fig. I l), altho'eh deviations from this are noted forZn (to > 1000 ppm), Cu (to >500 ppm), Pb (to>300 ppm), Sn and W. The leucogranite is notablyenriched in many of the trace elements consideredindicative of specialized granites @9. l1; c/Tischendorf 1977). For example, the leucogranitecontains elevated contents (average in ppm) of Rb(900), Li (250), U (25), Sn (150), W (40), Nb (40),Cs (35) and Ga (35); conversely, it is depleted in Zr


4

% normotive corundum

normol poroluminousgronties

rsslitc -richgronales

2 mico gronites

803

No6

72o\9t'3

uns€ded

lopoz rhyoll les

(20), Th (5), Ba (<5), Sc Q),V (<2), Ni (4) andHf (<2). Notable are the very low B contents (< l0ppm) considering the evolved nature of theleucogranite (cl, Harder 1975) and the often closeassociation of this element with elevated contents ofboth F and Li e.g., Macusani volcanic suite; Nobleet ol. 1984; Pichavant et ol. 1987b, lg8g). Theevolved nature of the leucogranite is also indicated

:Vlolel Town Volconics

Mounl Ploosont €ronilos

Ftc. 8, Plot of SiO2 yerszs normative corundum for East Kemptville leucogranitecompared to other felsic suites, including Macusani volcanic suite (pichavant ela/. 1988), South Mountain Batholith (Ham et al. 19891' note that only tle aver-ages are shown for the major lithological units , n -_ 456) and San Rafael granite(Kontak & Clark 1988). Also shown are the ranges for experimental glasses (Cle-mens & Wall 1981) and natural metaluminous to peraluminous felsic suites, includ-ing topaz rhyolites (Christiansen et at. 1983), Violet Town volcanic rocks (Cle-mens & Wall 1984), Mount Pleasant granites (Kooiman et al. 1986), and averageHacynian two-mica granite (LaRoche et al. 1980). Classification scheme of granitcis after Clemens & Wall (1981).

I experimenlol

I glossesslllimonito sedod

overoge Hercynion2 mlco gronlte

by very low K,/Rb ratios (average -40), approximat-ing the pegmatite-hydrothermal field of Shaw (1968).

Of particular interest are the elevated Sr contents,considering the otherwise evolved nature of theleucogranite. For example, compared to the Sr versasRb trend for tle South Mountain Batholilh (Fig. l2),the leucogranite is anomalous in Sr; conversely bothZr andBa (plots not shown) have values comparable

Son RotoelGronite


70 72 74 76 78sio2 $r.%)

+ som€ elevoted volues in SMB leucogronitesa SMB overoge unitsr EKLr DLC

FIc. 9. Plot of F versus SiO2 comparing data for EastKempwille lzumgranite to those for the South MountainBatholith (EIon et al. 1989; averages and leucocranites)and Davis Lake Complex (Richardson 1988a).

to the South Mountain Batholith for similar degleesof Rb enrichment. Also shown in Figure 12 is thefield for the Davis Lake Complex (DLC); again, thedata follow the trend defined for the South MountainBatholith, but samples of the East Kemptvilleleucogranite analyzed by Richardson (1988a) alsoindicate an abrupt increase in Sr content.

In Figure 13 the trace elements Li and Rb areplotted against F in the classification diagrams ofTaylor et al. (1985). These plots show that the leu-cogranite compares favorably to the fields outlinedfor topaz granites, whereas relative to topaz rhyo-lites the leucogranite is enriched in Li but has simi-lar F and Rb contents. Compared to the average datafor different lithologies of the South MountainBatlolith, the leucogranite shows relative enrichmentin all the elements, although a few cases ofcomparable enrichment are found (Fig. l3).However, these latter samples reflect variable degreesof late-stage alteration, and the concentrations arenot magmatic in nature (MacDonald & Clarke 1989).

Comparison of the trace-element chemistry of theleucogranite to topaz rhyolites in the spidergramdiagram of Christiansen et al. (1980 Gig. la)indicates the former is enriched in Sr, depleted in Th,Zr, La, Sm and Yb, and has comparable levels ofRb, F, Ba (?), Y, U, Nb and Ta. Compared to thespidergrams and various binary trace-element plotsused by Pearce et al. (L984) to classify granite types,the leucogranite is most similar to the syn- or

post-collisional granite types (diaerams not shown),although the extremely low Ba contents of the EastKemptville leucogranite are more typicd of within-plate granites, such as are found in the Oslo Rift@earce et al. 1984).

Rare-eart h- e I e ment c hem ist ry

Rare-earth-element (REE) data are presented inTable 3, and chondrite-normalized pattems in Figue15. The patterns obtained for the two different ana-lytical techniques (i.e., ICP-MS verszs INAA) agreereasonably well considering the generally low abun-dances, with the greatest discrepancy noted for theHREEs from Ho to Lu. Compared to previous dataobtained by Richardson (1983) for samples from theDavis Lake Complex, the East Kemptville leu-cogranite is more depleted in all the REEs and hasa larger Eu anomaly.

The most notable feature of the REE data is theoverall depletion of all the R.EEs; chondrite-normalized values are less than 10. The average chon-dritic pattern for the leucogranite is unfractionatedt@a/Yb)N = ll, has a distinct negative Eu anomaly(Eu/Eu* = 0.2), andthe HREEs showamarkedlyconcave shape, with an inflection at Tb. A secondinflection occurs at Ho for one of the data sets, butthe absence of this feature in the other data makesit difficult to interpret. However, a similar concaveprofile for the,I{REEs has been noted elsewhere andis considered to reflect the capacity of fluorine tocomplex the REEs (e.9., Taylor & Fryer 1983, Stronget al. lg%). The single aberrant sample (021) shownin Figure 15 is a fine-grained dyke rock that also isanomalous with respect to its relative enrichment inboth Na and Sr (Table 2). Although this sample isdepleted in total REEs relative to the field delineatedby the other leucogranite samples, it has retained thesame chondrite-normalized pattern, suggesting acommon genetic association.

The REE data are compared to other igneoussuites in Figure 15. Attention is drawn to thefollowing observations: (l) Compared to the evolvedmembers of the South Mountain Batholith (i.e.,leucomonzogranites), the data are more depletedwith respect to the ZR.EEs, have a greater Euanomaly, and have a concave HREE chondriticpattern. Kontak et al. (1988b) have shown tiat someof the evolved units of the South Mountain Batholithalso may show a similar concave pattern for theHREEs, but such rocks are markedly more alteredthan the East Kempwille leucogranite owing to exten-sive modification resulting from interaction with apostmagmatic, F-enriched fluid phase. (2) Comparedto other fluorine-rich ganitic and felsic volcanicsuites, the East Kemptville leucogranite has amarkedly different signature than the MountPleasant granites, which are also similar to ttre REE

2.O

t t a '

O 1

a a+ a

r i r r a

.i.*!! *.l^i. -i't '

a | ^ l ' A r

bq;3- 1 . 0

t!

8 0 Q


27(o.zg',tro5(o.67) (o.48)15(o.69) 32 (O.48)

228rc,621721t.23t

23 B(O.86)7rfl08) 1248f l .O5)

3 (0.65)

23A(O.48)23C(0.69)13(2.Ot)t6 l (o.48)

lto(o.75)to75B(0.46)

68(0.69)989rf l .19)9 il.t5)t56(O.43)24u.23',t

r78(O.

FIc. 10. (A) Plot of normative quartz (Q) - alkali feldspar (A) -albite @) for East Kemptville leucogranite samples[numbers in brackets refer to wt.qo F (see Table 2)]. Note tlle trend of the data toward the albite corner of the dia-gram (significance of this discussed in the text). (B) Normative QAP diaexam comparing field for East Kemptvilleleucogranite to otler fluorine-rich felsic suites (sources of data given in text) and experimentally determined cotec-tics for 4, 2 and I wt.9o F at I kbar (r, M4nning l98l) and F-free cotectics at 10, 5, 3, I and 0.5 kbar P(HzO)(+ , Luth 1970.

805

o80

+@ Horvey volconlcs(+volconics, @ melf inclusions)

A Topoz rhyollles,SW Unlted Stoles

o ongonites

{c Mocusqnlle glossr Peru

@ Mt. Pleosonl gronlte

A Merlcon lln rhYolltes

A Cryolife rhyollterTexos

* georvoi, gronlte' Frqnee

806

0 4 0 8 0 0 2 0 + o 6 0FIc. 11. Histograms for selected trace elements in Easr

KempMlle leucogranite. All values in ppm except forF (wt.9o), scale (number of analyses) the same for allelements except U.

patterns for topaz rhyolites (eg., Christiansen et ol.1983, 1986). (3) Depleted abundances approachingthose in the East Kemptville leucogranite are foundin the Macusani glass studied by Pichavant et ol.(1987b).

OxvcBN Isoroprc CouposnroNOF EAST KEMPTYILLE LEUCOCRANITE

Six whole-rock samples have been analyzed foroxygen isotopes (Table 5). Collectively, their 6180composition ranges from + 8.2 to + 12.2/os , with

THE CANADIAN MINERALOGIST

I4 the enriched sample (#105) beine a dyke robk. This.j s latter sample is also peculiar with respect to some

of its trace-element contents (e.81., Rb, Sr). Theremaining five samples define an average dl8O valueof 8.96 + 0.71%o. The reason for 6180 enrichmentin sample 105 remains unresolved, although mechan-isms such as contamination either in situ, along themelt path or near the source region may all be pos-

12 sible candidates. The enrichment is, however, clearly.l 9 not a function of crystal-fractionation processes

(Taylor 1978, Sheppard 1986).The data reported here compare favorably with

previously published data by Chatterjee & Strong(1985) for both granites and greisens from the DavisLake Complex and the East Kemptville leucogranite(greisens). These authors found a uniform 6180value of 9.5 x. 0,2 per o/ss (n = 19) for both freshgranite and its greisenized equivalent at East Kempt-ville.

The mean D18O value of 8.9 for the leucogranitefalls in the spectrum of normal igneous rocks (Iaylor1978). Such values, according to Taylor, requiresome input from the crust via processes of assimila-tion, partial melting, or exchange of either sedimen-tary rocks or modified plutonic or volcanic rocks thatonce resided near the earth's surface. Compared tothe range for peraluminous granites on a world-widescale, the leucogranite data are notably depleted (i.e.,versus 10-14 /06 : Sheppard 1986) and, in the clas-sification of O'Neil et al. (1977), based on theirexamination of granitic rocks of the Lachlan FoldBelt in Australia, the values are more typical of I-type granites.

In Figure 16 the oxygen isotopic data for granitesof the Meguma Terrane are compared to the rangefor the East Kemptville leucogranite. The data areclearly distinguighed from the r8O-enriched granitesof the eastern half of the South Mountain Batholithand the northern granites, whereas there is an over-lap with the range for the southern granites and, asmentioned above, the Davis Lake Complex. It isworthwhile noting that the granites of the southerndomain are composed of a large proportion of rocksof relatively mafic composition compared to theSouth Mountain Batholith and northern granites,including granodiorite, tonalite, trondhjemite anddiorite (Rogers & Barr 1988, de Albuquerque 1977,Douma 1988).

DrscussroN

The petrological data presented above indicateseveral important features of the East Kemptvilleleucogranite that are apparently unique to this partof the South Mountain Batholith and other graniticrocks of the Meguma Terrane. Questions fundamen-tal to the petrogenesis of the East Kenptvilleleucogranite that require consideration are: (l) Are

5r

. Th i s s t udy l -{c Richordson (tggBli Eost Kemplvi l le Leucogronite

A SMB ove roges (Hom e t o l . t 989 )G , M . . . . S M B o v e r o g e s ( C l o r k e B

M u e c k e , 1 9 8 5 )

A

G

TTIE EAST KEMPTVILLE TOPAZ-MUSCOVITE LEUCOCRANITE

300

200

roo

500Rb

r ooo

Ftc. 12. Plot of Sr verlsrrs Rb comparing East Kemptville leucogranite data to fields for Davis Lake Complex (datafrom Richardson 1988a) and South Mountain Batholith. Note tfiat G, M, L, D and Gr represent averages fromClarke & Muecke (1985) for granodiorite, monzogranite, leucomonzonite, dyke rocks and greisens, respectively.

807

a

1 800

1400

Eo-

.9 1000-oE,

600

2AO

topoz gronites

4poz .?\ot.

^fi gronltes

l - - e -O

A

-FI

0 . 8 1 . 2 1 . 6F (wt.%)

t aooo.o.

600J

1200

1 000

200

a

a

fl .'d\. \

p ' \ . b .

n-iN-,

400

0.4 0 .4 0 .8 1 .2 1 .6F (wt.z)

Ftc. 13. Plots of Rb and Li versus F for East Kemptville leucogranite. Fields for Li-mica topaz granites and topazgranites after Taylor et al. (1985) and fields for topaz rhyolites and South Mountain Batholith (average values fordifferent lithologic units) from Christianset et al, (1983) and Ham et al. (1989), respectively. Symbols as in Figure12. Open circles represent the most elevated contents for Rb and F in leucogranites of the South Mountain Batholith(Ham al ol. 1989\,

808 TI{E CANADIAN MINERALOGIST

the mineralogy and chemistry magmatic or metaso-matic? (2) How does the petrology of theleucogranite compare to tlat of the South MountainBatholith? Does it represent a product of differen-

Topoz Rhyolites

w*/\

!{jrl

F Cl Rb Bo Sr Eu Lo Sm Yb Y Th U Zr Nb To

tiation? (3) What is the importance of theleucogranite in terms of the tin potential of theMeguma Terrane? (4) Finally, what constraints doesthe chemistry of the leucogranite impose for the

50

to

5

I

(9E,

c)o.Fo la

o.5

o.l

no. 14. Spidergram plot comparing trace-elemenl chemistry plus F to topaz rhyolites (after Christiansen er a/. 1986).Note that tlere are no Cl data available for the East Kemptville leucogranite.

TI{E EAST KEMPTVILLE TOPAZ-MUSCOVITE LEUCOGRANITE 809

to

CARLETON (N.6)(INAA)

Lo Ce Nd Sm Eu Gd Tb Dy Ho TrnYbLu

infrastructure of the Meguma Terrane? In the fol-lowing sections these points are briefly discussed;although an unequivocal solution may not be possi-ble with the present data-set, working hypotheses arepresented and discussed.

East Kemptville leucogranite: product of mogmaticor metosomatic processes?

The role of magmatic versus metasomaticprocesses in late-stage granite suites is a controver-sial topic; it is important to ascertain whether thefeatures observed in the East Kemptville leucograniteare representative of a magma or a product highlymodified by metasomatism. Factors considered rele-vant to the argument are (l) the petrographic fea-tures and geochemistry of the leucogranite, (2) com-parison with volcanic analogues, and (3)experimental and thermodynamic considerations.

SMB LEUCOMONZOGRANITES(N"6)

MEMORIAL ( N"9)(rcP/Ms)

Lq Co Pr Nd SmEu Gd Tb Dy Ho Er TmYb Lu

IASLS 5. rSO!.g-ROCK OrroK tsorott DAIA rotEAST TEUPTUIIIE I.EUC@IAITIE

srtll?B f 6"0 ('/.. )

MTPLEASANT GRANITES( New Brunswick )

f l N . 3 )

lo.o- ' .oc/oF

to0

to

lrlFEozoIo

Y l oooE,

FIc. 15. Chondrite-normaliz,ed. REE plots for East Kemptville leucogranite ldata from Carleton (INAA) and Memorial(ICPIMS) Universities shown separatelyl compared to data from Mount Pleasant topaz granites (faylor et al. 1985),Macusani volcanic rocks and glass @ichavant el a/. 1987b, 1988) and South Mountain Batholith leucomonzogranites(Muecke & Clarke l98l).

EK:86-023AEK-86-072ER-86-105!K-86-132EK-86-161EK-86-1075B

8 . 6

1 0 . 38 , 29 . 0

The petrographic features have been discussed indetail above, but tle salient point$ may be reiterated,namely (1) lopaz coexists in textural equilibrium withthe assemblage quartz - two feldspars - muscovite;(2) point counting of both thin gsctieas and rockslabs indicates a very uniform modal mineraloCy; (3)the texture of the leucoganite is generally uniform;and (4) miarolitic cavities are absent, and pegmatites

MACUSANI VOLCANICS


Eqst KempMlleleucogronite

8 i 0 1 2 1 4rhole rock d8O ("/*t

Ftc. 16. Histograms of oxygen isotopic data for graniticlithologies of the Meguma Terrane compared to datafor East Kempwille leucogranite. Sources of data:northern granites (O'Reilly 1988b), South MountainBatiolith (Longstaffe et al. 1980, Kontak et al. 1988b,Chatterjee & Strong 1985), Davis Lake Complex (Chat-terjee & Strong 1985), and southern granites (Longstaffeet al. 1980).

are very rare. Also important are the observationsthat whereas melt inclusions in topaz were sought(cl, Naumov et al, 1972), none were observed, andthat fine-grained dyke rocks cut the leucogranile.

The above mentioned features are considered tomilitate against transformation of the EastKemptville leucogranite to its present form by per-vasive metasomatic reactions for the followingreasons:(1) Observations indicate that the leucogranite crys-taUized from a HrO-undersaturated melt; hence afree aqueous phase did not appear until ttre verylatest stages of crystallization. Thus, pervasivemetasomatism in the sense of, for example, Pollard& Taylor (198Q and Taylor & Pollard (1988), is notconsidered to fs imFortant. Infiltration of late-stagefluids, probably via gran boundaries and microfrac-

tures, did occur and was ultimately responsible for,among other things, promoting re-equilibration ofplagioclase to albite (+ apatite) and unmixing ofhomogeneous alkali feldspar to stable pefihiticmicrocline. However, such a scenario, involving verylow fluid/rock ratios, is considered inappropriate ifwhole-scale transformation is to have occurred, sinceexceptionally large volumes of fluid would berequired. In contrast, where fluid focusing did occurand fluid/rock ratios were high, the leucogranite wasconverted to zoned greisens of markedly differentmodal mineralogy (quartz-topaz-cassilerite-sulfides)and geochemistry (Kontak 1988).(2) Assuming that the uniform modal mineralogy ofthe leucogranite represents a terminal stage ofmetasomatism, then one might consider the presenceof pegmatites and dyke rocks. Presumably theserocks also would have been transformed to a similartexture, mineralogy and chemistry as the leucograniteif present prior to the metasomatic overprinting.Since it is unlikely that they represent a separate,much later igteous event, their presence suggests thatthe East Kemptville leucogranite represents theparental melt for these phases and, potentially, aprimary magmatic phenomenon.

Geochemical arguments in favor of a dominantlymagmatic composition are summarized as follows:(1) The bulk major-element composition approxi-mates that of other volatile-rich igneous suites (Iable4), and the normative mineralogy approximatesminimum-melt compositions in the haplogranitesystem (see above). Geochemical comparison of theEast Kemptville leucogranite to ongonites,macusanite and melt inclusions in the Harvey vol-canic suite (Iable 4) is particularly significant sincethese latter rocks are clearly of magmatic origin,whereas the normative compositions of the EastKemptville leucogranite are at least consistent witha magmatic parentage.(2) Clustering of the geochemical data with no devi-ation toward alkali enrichment or depletion is con-sidered to be best explained by magmatic rattrer thanmetasomatic processes (c/ Pollard 1983), consistentwith point l. Thus, even though minor amounts ofre-equilibration occurred, changes are considered tohave been isochemical, consistent with lowfluid/rock ratios.(3) Examination of the behavior of selected com-patible verszs incompatible trace-elements indicatesuniformity of all the data. For example, in plots ofRb/Zr and Rb/Ta versas Rb (Fig. 17), small rangesin the data are observed, the outlying pointsrepresenting aplite, pegmatite and dyke lithologies.In contrast, the range for variably greisenized sam-ples (Kontak 1988) is very broad (Fig. l7), as wouldbe predicted in these rocks of highlyvariable miner-alogy.(4) Rare-earth-element chemistry of t]re leucogranite

1 0 1 2

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NorthsmGronitic rocks

South MountoinBqtholith

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THE EAST KEMPTVILLE TOPAZ-MUSCOVITE LEUCOGRANITE 8 1 1

Rb4

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RbFtc. 17. Plots of Rb/Ta and Rb/Zr versus Rb for leucogranites and greisens at East Kemptville (greisen data from

Kontak 1988).

shows consistent patterns that contrast markedlywith the.LREE-depleted, H&EE-eniched chondriticpatterns characteristic of greisenized lithologies(Chatterjee & Strong 1984). Numerous studies haveshown that metasomatic processes tend to modifythe REE chemistry of felsic rocks (e.g., Taylor &Fryer 1982, 1983). Although the overall depletedpattern of the REE contrasts with that of typicaltopaz rhyolites (Christiansen et al. 1983,1984, l98Oand topaz granites (Taylor et al. 1985, R.P. Taylor,pers. comm., 1988), it is broadly comparable to thatfor the volatile-rich macusanite @ichavafi et al.1987b) and is, therefore, consistent with a magmaticsignature rather than related to metasomaticphenomena (e.g., Higgins et al, 1985, Tuach 1987).In fact, the REEpattern of the macusanite has beensuccessfully modeled by Montel (1986) usingmonazite fractionation; the low Th and REE levelsin the East Kemptville leucogranite would be com-patible with a similar process.(5) Country-rock metasediments of the MegumaGroup irnmediately adjacent (i,e., l-3 m) to theleucogranite rarely show enrichment in elements(e.g., Rb, Cs, Li, Sn, As, Zn, P) commonly consi-

dered diagnostic of lithophile-element mineraliza-tion, whereas the leucogranite is enriched in theseelements (Kontak, in prep.). This apparent lack ofa geochemical halo contrasts with the fildings ofMorgan & London (1987), who documented extrememetasomatism and elemental enrichment (K, Cs, Rb,Li, F, B) of wall-rock amphibolite adjacent to theTanco pegmatite. The absence of a geochemical haloaround the leucogranite is consistent with lowfluid./rock ratios since the aforementioned elementswould be partly transferred into an exsolved fluidphase (e.9., Webster et al. 1989, Kovalenko el a/.1980, V. Kovalenko 1977,Morgan & London 1987)and would presumably migate down thermal andchemical gradients into the wallrock, causing elemen-tal enrichments.

Although the above features are cousidered to beconsistent with a dominantly magmatic origin for theleucogranite, the elevated and erratic abundances ofsome elements (e.g., Cu, Pb, Zn, Sn, W) and theirgenerally good correlation with S content (Table 2)indicate that some postmagmatic, metasomaticenrichment occurred. Althot'gh it is difficult to ascer-tain the proportion of magmatic versrs metasomatic

8t2 THE CANADIAN MINERALOGIST

contributions to such elevated concentrations, it isinteresting to note that enrichment of some of theseelements is found in the Peruvian macusanite glass(i.e., quenched residual melt) by Pichavarfi et al.(1987b: 70 ppm W, 20 ppm Sn, 95 ppm Zn). There-fore, a certain degree of elemental enrichment in theEast Kemptville leucogranite is considered related tomagmatic processes, but at the same time it is clearthat a metasomatic influence also is recorded.

A final point worthy of consideration is whetherthe phase assemblage of the lzucogranite is consistentwith magmatic conditions. Rocks of similar bulk

@lvlocEonife0 Beouvoir gronneo Topoz rhyolltso Violet Torun volconlcax Tuscon rhyolltesJf Eosf K€mptvllle. leucogronlte

Frc. 18. A(AI2O3-Na2O-K2O)-C(CaO)-F@eO,+MgO)and A(AI2O3-Na2O-K2O-CaO)-K(K2O)-F(FeOr +MgO) plots (in moles) for the East Kemptville leu-cogranite, various examples ofcrust-derived melts andexperimental glass compositions in peraluminous sys-tems. Points for the Beauvoir granite, Tuscan rhyolites,Macusani glass, Hercynian two-mica granite, VioletTown volcanic suite and topaz rhyolites from Picha-vant et al. (1988). Note that the two fields in rhe AFKdiagram designated Clemens & Wall (1981) repres€ntsilimanite-seeded and unseeded compositions, respec-tively.

composition have been studied experimentally by N.Kovalenko (1977), Weidner & Martin (1987),Pichavant et al. (1987a).and webster et al. (1987).All of these authors report topaz as a stable mag-matic phase, albeit with different ranges of stabilitydepending on the physical and chemical constraintsof the experimental conditions. In addition, Naumovet al. (1972) reported melt inclusions in topaz fromongonites, which proves a magmatic origin for thehost, whereas Barton (1982) has shown that quartz- topaz - muscovite - alkali feldspar should consti-tute a stable assemblage uuder water-saturatedconditions. The assertion of Barton (1982) that topaacannot coexist with a Ca-bearing phase other thanfluorite (i.e., Ca plagioclase) is not correct; Pichavantet al, (1987a) found plagioclase of up to An ., com-position in experimental products containing topaz.Thus, for the East Kemptville leucogranite, the albitecontaining apatite needles is interpreted to have oncabeen a Ca-bearing plagioclase above the solidus andto have coexisted with topaz.

The foregoing discussion indicates that there isample reason to seriously consider a predominantlymagmatic origin for the leucogranite. However, thisis not to say that some degree of subsolidus modifi-cation did not occur, since pure albite represents thesole plagioclase phase and the K-feldspar clearlyreflects textural re-equilibration. But for the mostpart, the chemistry is considered to closely approxi-mate that of the slightly modified precursor.

Relationship of East Kemptville leucogroniteto South Mountain Batholith - Dqvis Lake Complex:product of fractionol crystallizationor separate source (?)

The spatial association of the East Kemptvilleleucogranite to the Davis Lake Complex, thewesternmost continuation of the South MountainBatholith, implies that there may be a geneticrelationship between tle two, as earlier investigatorshave suggested (e.9., Richardson 1983, 1988a, b,Richardson et al. 1987). However, it is equally pos-sible that there is no such affiliation and that the EastKemptville leucogranite represents a separate,unrelated magmatic event. As reviewed earlier, fieldrelationships do not provide sufficient informationto unambiguously resolve this problem, as nowherecan the leucogranite be observed to grade into a rela-tively less fractionated, potentially parental rock-unit. Thus, the best test at present remains thegeochemical nature of the leucogranite relative todata for the Davis Lake Complex as presented inRichardson (1988a).

The first point to consider is whether the EastKemptville leucogranite represents a product ofdifferentiation or perhaps a minimrrm melt (i.e., inthe sense of White & Chappell 1977) produced by

6 Holloroy

/'o&L Pffi,;,'-=--t

TIIE EAST KEMPTVILLE TOPAZ-MUSCOVITE LEUCOGRANITE 813

batch-melting processes. This point may be assessedby comparing the leucogranite geochemistry toexperimentally produced glass compositions frompartial melting of various bulk compositions. Thegeneral depletion of Ti, Mg and Ca that character-izes the leucogranite contrasts with elevated valuesof these elements reported in experimentallyproduced melts by Clemens & Wall (1981), Green(1970, B6nard et al. (1985) and Vielzeuf & Holloway(1e88).

Projection of the average leucogranite composi-tion into the modified ACF and AKF diagrams @ig:18) indicates the following: (l) the composition ofthe leucogranite is markedly different from that ofexperimentally generated glass, (2) the leucograniteis exceptionally peraluminous [(f.e., compareleucogranite composition to sillimanite-seeded fieldof Clemens & Wall (1981)l; (3) the leucogranite isstrongly depleted in Ca, Fe and Mg compared tosome relatively mafic peraluminous suites (Iuscanrhyolites and Violet Town volcanic suite) andexperimentally generated glasses, and (4) theleucogranite compares favorably to the fields forsome presumed products of extreme fractionationof peraluminous melts (i.e., Macusani gilass andBeauvoir gxanite). Thus, the aforementioned data,in addition to the elevated contents of silica, phos-phorus and LILE, low K,/Rb ratios and depletedvalues of some transition metals, alkali and alkatineearth elements, and REE (particularly Eu), areconsistent with a strongly fractionated origin for theEast Kemptville leucogranite ratler tltan a minimum-melt composition.

In order to properly assess the role of fractionalcrystallization, one must flrst obtain appropriatemineral-melt partition coefficients for the phaseassemblages. These data are not available; consider-ing the large uncertainties that may result from usinginappropriate values (e.9., Christiansen et al. 1984),particularly in highly felsic and volatile-rich suites(Mahood & Hildreth 1983), the following discussionis limited to a qualitative approach using simplebinary-element plots and monomineralic fractiona-tion modeling.

leucogranite and therefore was not an importantphase. Variation of the data wilhin the leucograniteis adequately accommodated by removal of aplagioclase - alkali feldspar mixture.

Binary plots of U versus T)h, Zr versus Ta atdP2O5 versus F in Figures l9A, B and C, respec-tively, also serve to illustrate the unique behavior ofthe leucogranite with respect to both the Davis LakeComplex and the South Mountain Batholith. Thisuniqueness is particularly evident in tlre U yersasThplot, in which the two very divergent trends for theSouth Mountain Batholith and Davis Lake Complex,as determined by Chatterjee & Muecke (1982) andfurther discussed by Ford & O'Reilly (1985), are quiteunlike the field for the East Kemptville leucogranite.Inthe Zr verszrs Ta plot, the relatively fractionatednature of EKL samples is again emphasized, and thegap between data for the Davis Lake Complex andleucogranite is highlighted.

In summary, the East Kemptville leucogranite isinferred to represent a highly differentiated rocksirnilar to the extreme products of peraluminousmagmas represented by, for example, the Macusaniobsidian and the Beauvoir granite. The lack of achemical continuum or zupporting field-relationshipsprecludes at present an obvious link to the DavisLake Complex vla progressive fractional crystalliza-tion. However, this is not to say that protractedfractionation of a common melt did not proceed atdepth, with only the most evolved product presentlyexposed at surface, as documented, for example, byManning & Exley (1984) (see also discussion inWeidner & Martin 1987) for the biotite granite andtopaz - lithium muscovite eranite phases of theSt. Austell granite, Cornwall. Note that N.Kovalenko (1977) suggested that granitic melts of theongonite type containing elevated fluorine contentscaa result from fractionation of a felsic magma giventhe high concentration of acid fluorides. Whereasthe oxygen isotope data are commensurate with deri-vation of East Kemptville leucogranite from theDavis Lake Complex ula a similar process, additionalisotopic data will be required to further substant!ate such a relationship.

Although the East Kemptville leucogranite appearsto form a continuum with Davis Lake Complex Implicotions of the Eost Kemptville leucogranitesamples on a log-log plot of Rb uer,nzs Ba (Fig. l9E), |oi tne fin poiential of the Megumo Zoneand a genetic association is suggested yia fractionalcrystallization of an alkali-feldspar-dominant bio- The tin potential of granitoid rocks of the Megumatite - plagioclase - alkali feldspar mixture, the same Terrane has been addressed by numerous authorsisnotindicatedbyasimilarplotof SrrerslrBa(Fig. (e.g., Ford&O'Reilly 1985, O'Reilly 1988b, Smith19D). In fact, the latter diagram serves to illustrate & Turek 1976, Chatterjee & Muecke 1982, Chatterjeeths uniqueness of the East Kemptville leucogranite et al. 1983) using o variety of geophysical (i.e., ur-(see also Fig. 12) and indicates that it cannot be borne radiometry) and lithogeochemical techniques.related to the Davis Lake Complex by simple frac- In general, comparisons are made between thetional crystallization since (l) there is no continuum geochemical signatures of the granitoid roeks ofin the data and (2) biotite, which is necessary to interest and those from other stanniferous districtsdeplete the melt in Ba, is essentially absent in the (e.g., Cornwall, Nigeria, Malaysia, Erzgebirge), with

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8 1 4 THE CANADIAN MINERALOGIST

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the underlying assumption that common features arepresent, so that extrapolations are valid. However,of utmost importance to this argument is the assump-tion that the progenitor of the mineralization is beingsampled and analyzed, an assumption that is notalways evaluated (Taylor 1979). For example, in theMount Pleasant tin deposit, multiple periods ofmineralization are spatially associated with differenttypes of granite. At this locality, Kooiman et al,(1986) and Taylor et al. (1985) have demonstratedthat the Sn stage is associated with a chemically dis-tinct granite petrologically similar to topaz granites,whereas the W-Mo mineralization shares spatialaffinity with a biotite granite of quite differentcharacter. A similar situation exists in the Peruvianpart of the Central Andean tin belt, where Kontak& Clark (1988) have shown that significant centersof tin - base metal mineralization are generallyhosted by biotite granites with chemical signaturestypical of nonproductive granites (cl, Tischendorf1977).

It is here suggested that a similar situation maybe present in the Meguma Terrane, such that tinmineralization is associated with topaz-bearing (Le.,F-rich) granites. A similar case has been argued byBurt et al. (1982) and Burt & Sheridan (1988) forlithophile mineralization associated with topazrhyolites of the western United States and Mexico.The implication of this proposal is that the SouthMountain Batholith, previously considered to offerpotential for significant stanniferous mineralizationon the basis of conventional trace-element signatures(e.9., Smith & Turek 1976), does not possess theinherent features for producing large deposits of tin.The numerous but volumetrically very small occur-rences of tin and associated lithophile-element miner-aJization in the New Ross - Vaughn area (O'Reillyet ol. 1982, Charest et al. 1985) are thus interpretedas representing the natural termination of the crys-tallization of an S-type magma. In such cases, themagma must evolve a hydrous phase upon crystalli-zation (e.9., Burnham 1979, Burnham & Ohmoto1980) and, consequently, at least a certain amountof mineralization and alteration will result becauseof the tendency of metals to preferentially partitioninto Cl-bearing fluids (Eugster 1985). This scenariocontr€rsts v/ith the East Kemptville leucogranite-typemelt which, because of its different initial composi-

8 1 5

tion and liquidus path, evolves into a volatile-richmelt with the inherent potential to develop into amineralized system (e.9., Strong 1981, 1988).

The East Kemptville leucogranite and implicstionsfor the infrostntcture of the Meguma Terrone

The genesis of fluorine-rich igneous suites in othergeological provinces has been attributed to thepresence of a source reservoir enriched in thiselement. For example, in a recent study of severalhundred samples of granites from the Basin andRange Province of the western United States,Christiansen & Lee (198Q concluded that the elevatedfluorine contents in one of the suites reflected eithermagma generation from or ssaleminsfisn by afluorine-rich reservoir. Similar conclusions werereached by Manning & Pichavant (1983), Pichavant& Manning (1984) and Christiansen et al. (1983,l98O in their petrogenetic modeling of fluorine-richigneous suites. These authors were unanimous intheir conclusions that breakdown of fluorine-bearing, Fe-rich biotite or amphibole (see alsodiscussion in Clemens & Wall l98l) was the mechan-ism responsible for generating a fluorine-rich melt.The source rocks most likely to provide suitableminslalogy and satisfy the intensive parametersinferred from petrological studies of these suites arefelsic granulites (e.9., Christiansen et al. 1983).

The presence of the East Kemptville leucogranitein the Meguma Terrane has importanl implicationsfor the composition of the infrastructure of thisterrane, considering the nature of the protolith gener-ally considered parental to such rocks. Granuliticrocks underlie at least part of the Meguma Terrane,as substantiated by the tsssal findings of Giles &Chatterjee (1980 for the Liscomb Complex, andChatterjee & Giles (1988) and Owen el a/. (1988),who documented the presence of granulite-faciesxenoliths within mafic dykes at Tangier (Fig. l). Inaddition, Clarke & Chatterjee (1988) and Eberz etal. (1989) presented Nd and Sr isotopic evidence thatthe gneissic rocks of the Liscomb Complex representthe protolilh for the South Mountain Batholith. Byextrapolation, therefore, it is inferred that similarrocks underlie the southwestern Meguma Terrane.This same rock would, therefore, be an idealcandidate for the generation of a fluorine-rich melt


Ftc. 19. Element plots comparing data for East Kemptville leucoeranite (EKL) to fields for Davis Lake Complex @LC,Richardson 1988a), average units of South Mountain Batholith (SMB, Ham et al. 1989) and selected leucomon-zogranite samples of the South Mountain Batholith(Hamet al. l9S9). (A) U versusTh diagram showing the trendsfor the South Mountain Batholith and Davis Lake Complex as determined by Chatterjee & Muecke (1982); @) Zrrersa,r Ta plot; (C) P2Os versas F plot; (D) log-log plot of Ba versas Sr; @) log-log plot of Ba yensec Rb. In Figuresl9d and e the vectors shown represent the expected effects on melt composition of Rayleigh crystal fractionationof single mineral phases (bt biotite, kf alkali feldspar, plg plagioclase),


during a melting event by the mechanism of meltgeneration reviewed by Manning & Pichavant (1983)and Christiansen et ol. (1983).

The highly evolved chemistry of the EastKemptville leucogranite and the dominance ofmuscovite to the almost complete exclusion of biotiteindicate that the leucogranite represents an extremeproduct of fractional crystallization. Hence,although it is suggested that the leucogranite wasprobably generated by a mechanism involving break-down of fluorine-rich biotite, subsequent petrologi-cal processes have substantially changed the characterof the original melt.

The reason for the isolated occurrence of atopaz-bearing or extremely fluorine-rich felsic suitein the East Kemptville area remains a problem.However, it is tentatively suggested that the presenceof the East Kemptville Shear Zone (Fig. 2a) may beof fundamental importance in terms of localizing themelt. In a somewhat analogous situation, Strong &Hanmer (1981) have interpreted the role of theArmorican Shear Zone to be important in bothgenerating and localizing the leucogranites ofBritanny, whereas Pitcher (1979) noted that granitescommonly utilize crustal-scale structures to ascendto shallow crustal levels. In addition, Strong (1980)has emphasized that many granites of easternCanada and western Europe owe their origin to a"megashear environment", owing to the lack ofevidence for subduction processes operating at thetime of granite generation. Thus, the East Kemptvilleleucogranite is interpreted to reflect melt ascentwithin an active shear-zone that penetrated a ductileinfrastructure and tapped a zone of partial meltingwithin a granulite source during the continued trans-pression of the Meguma Terrane in the LateDevonian. Similar leucogranites may be found alongsimilar structures where the appropriate geologicalconditions prevailed. In this vein it is interesting tonote that o1hs1 minslalized, fluorine-rich suites inthe Canadian Appalachians show intimate spatialassociations to major structural breaks. Some of themore notable examples include (1) the AckleyGranite, Newfoundland (Tuach et ql. 1986, Tuach1987), which truncates the Dover - Hermitage BayFault, a major terrane boundary between the Avalonand Gander Zones (see Fig. l), and (2) the MountPleasant (Kooiman et al. 1986, Taylor et al. 1985),Burnthill (Taylor et al. 1987, Maclellan & Taylor1989) and Mount Douglas QlIcLeod et al. 1988) com-plexes of New Brunswick, which are all related tozones ofwrench faulting (Ruitenberg & Fyffe 1982).In addition, Whalen (1986) has reinterpreted manyof the peraluminous S-type granites of NewBrunswick to be of A-type affinity, based on theirtrace-element chemistry. Such granites share acommon genetic origin with topaz granites andrhyolites (Collins et al. 1982).

Petrogenesis of the East Kemptville leucogranite

The petrological evidence presented favors thehypothesis that the East Kemptville leucogranite isa product of fractional crystallization of a fluorine-rich melt generated by crustal anatexis. The deriva-tion of the leucogranite from a crustal reservoirrequires an increase in the local isotherms such thatmelting can occur. Mechanisms for creating such asituation might include frictional heating in a shear-zone envtonment, crustal thickening during regionaldeformation concurrent with continental collision,or transport of heat from the mantle via basalticmagmas. The first suggestion is discussed by Strong& Hanmer (1981) in their treatment of the Britannyleucogranites. Whereas it may be appropriate forBritanny, there is not sufficient reason to employ asimilar model here since there are more differencesthan similarities in both regional geology and thegranites between the two areas (Hanmer & Strong1981, J6gouzo 1980, Hanmer et ol. 1982): The secondmodel is analogous to that used to explain the originof the Himalayan leucogranites, described by Le Fort(1981) and Yidal et al. (1984), Although Keppie &Dallmeyer (1987) have interpreted Meguma Terranegranites of ca. 370 Ma age to reflect crustal under-plating of the Meguma Zone, with concomitantupwelling of the isotherms and cmstal melting, thereare presently insufficient geological reasons to fullysupport this concept. For example, reconstructionof the Late Devonian - Early Carboniferous geolog-ical record does not indicate that a crust of appropri-ate thickness existed, as presently observed in theHimalayas, but the recent discussion of this problemby Thompson (1989) offers an alternative interpre-tation. The third model is consistent with the presentstatus of knowledge of MegumaZone geology. Therecent delineation of mafic intrusions in the LiscombComplex dated at ca,370 Ma (Kontak et al. 1989b,1990) and rocks of similar age occurring as dykesalong the Atlantic coastline (Kempster et al, 1989)indicate that the necessary thermal component wastemporally and spatially present to satisfy theinherent requirements of this model, whereas isotopicdata (Clarke & Chatterjee 1988) are consistent witha mantle contribution to the granites of the MegumaTerrane. Although similar isotopic data are not avail-able for the East Kemptville leucogranite, Christian-sen et al. (1983, 1980, Christiansen & Lee (198Q andPichavant & Manning (1984) have considered mantleupwelling and ensuing crustal fusion as the mostlikely mechanism for generation of fluorine-richsuites. Thus, the East Kemptville leucogranite is con-sidered to have been generated via partial meltingof a crustal reservoir as a result of input of heat frombasaltic magma injected into the crust. Ascent to thesurface was facilitated by a crustal-scale structure asalluded to above.

CoNcr-usroNs

A petrological study of the 370 Ma East Kempt-ville leucogranite, a muscovite-topaz leucogranitethat hosts tin mineralization, reveals the followingimportant features:(l) The granite represents the product of crystalli-zation of a highly evolved, volatile-rich (i. e., F-rich)felsic melt. The uniform textures and modal miner-alogy and absence of extreme alkali metasomatismare interpreted to indicate a magmatic origin for bothtopaz and muscovite rather than late-stagemetasomatism. However, a certain amount of ele-ment enrichment (e.9. Cu, Fe, Sn, W, Zn, P, S) isattributed 1o a superimposed metasomaticphenomenon.(2) There is an absence of petrographic and texturalfeatures (e.9., graphic textures, miarolitic cavities,breccias, abundant pegmatites, unidirectionalsolidification textures) that are commonly found influid-saturated melts (e.9., Kirkham & Sinclair 1988),which indicates, therefore, that the East Kemptvilleleucogranite did not reach fluid saturation (c/.Richardson et ol. 1988b).(3) Mineralogically and chemically the East Kempr-ville leucogranite is similar to other fluorine-rich fel-sic suites, including topaz granites. It is character-ized by elevated contents of Cs, F, Li, Nb, Rb andTa, and depletion in B, Ba, Ca, Co, Hf, Mg, Mn,Ni, Sc, Ti and V.(4) The strongly peraluminous character (A/CNKvalues = l.13 to 1.84) and major- and trace-elementchemistry of the leucogranite, including stronglydepleted R.EE contents (particularly Eu), indicatethat the granite represents the product of fractionalcrystallization rather than batch melting. Althoughit is not presently possible to unequivocally eliminatethe Davis Lake Complex as a source of the parentalmagma, the field relationships as presently under-stood suggest that they are not related. Alternatively,magmatic eVolution may have proceeded at depthwith no manifestation of this process at the surface.(5) The chemistry of the East Kemptvilleleucogranite, including whole-rock O isotopic data,indicates a crustal reservoir for the protolith.Analogies with other F-rich suites suggest that themost likely source is a felsic granulite. Hence,the leucogranite is interpreted to reflect the in-congruent breakdown of a fluorine-rich biotite phasedue to crustal anatexis related to injection of basalticmagma into the lower crust. The source rock is con-sidered to be represented by similar lithologiesexposed in the Liscomb Complex (Giles & Chatterjee198O.(6) Future exploration for tin mineralization in theMeguma Terrane should focus on similar fluorine-rich suites since their chemical nature increases thepotential for development of geological conditions

8t7

favorable for mineralization.

ACKNoWLEDGEMENTS

Funding for this project was provided through theCanada - Nova Scotia Mineral Development Agree-ment. Permission to publish was provided by theDirector of the Mineral Resources Division, NovaScotia Department of Mines and Energy. Thefollowing individuals are sincerely acknowledged forassistance in acquiring analytical data presented inthis paper: D.F. Strong, C. Coyle, S. Jackson, H.P.Longerich, R.P. Taylor, J. Dostal, R. Kerrich andK. Cameron. I thank D.F. Strong, B. Murphy, R.F.Martin, R.F. Cormier, J. Dostal, R.P. Taylor andfellow colleagues at the Nova Scotia Department ofMines for illuminating discussions concerning thepetrogenesis of the East Kemptville leucogranite, butall errors, omissions and misinterpretations remainthe responsibility of the author. Constructive reviewsby E. Christiansen, P. Pollard and R.F. Martin ledto substantial improvements of the manuscript andare sincerely acknowledged. L.J. Ham is thanked forhaving provided a preprint of the geochemical data-base for the South Mountain Batholith. C. Poulin,mine geologist at East Kemptville, is acknowledgedfor his continued suppon ofgeological investigationsat the deposit site. Manuscript preparation was onlypossible with the capable assistance of D.R.MacDonald, B. MacDonald, J. Campbell and staff,and R. Morrison.

REFERENCES

BerLry, J.C. (1977): Fluorine in granitic rocks andmelts: a review. Chem. Geol. 19. l-42.

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Received July 17, 1989, revised manuscrtpt acceptedMay 20, 1990.

the east kemptville topaz muscovite leucogranite, … · chedabucto fault system (webb 1969, keppie...

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