petrological and geochemical characteristics of … · pleasant fudge granite has many features in...

895

Canadian MineralogistVol. 30, pp. 895-921 (1992)

PETROLOGICAL AND GEOCHEMICAL CHARACTERISTICS OF THE PLEASANT RIDGEZINNWALDITE _ TOPAZ GRANITE, SOUTHERN NEW BRUNSWICK,

AND COMPARISONS WITH OTHER TOPAZ-BEARING FELSIC ROCKS

RICHARD P. TAYLOROttawa - Carleton Geoscience Cmtre, Department oJ Earth Sciencs, Carleton Univenity, Ottawo, Ontario KIS 586

AssrRAct

The Pleasant Ridge pluton and adjacent tin-mineralized cupola at Bonny fuver, in southern New Brunswick, wereemplaced around 360 Ma, well after the cessation of tectonothermal activity associated with the Acadian Orogeny.Granites from the Pleasant Ridge and Bonny River plutons are fine- to medium-grained, equigranular to seriatetextured, with essential quartz, alkali feldspar, plagioclase, lithian mica, and topaz. Accessory minerals include zhcon,niobian rutile, columbite-tantalite, cassiterite, uraninite, monazite, and bastniisite. The granites contain primaryalbite (An0-3) and axe subsolvus in type. Both the topaz (17.6-19.4 wt.tlo F) and lithian mica (5.9-7.4 wt.qo F) areF-rich. The lithian mica Q84 w.Vo Li2O) resembles zinnwaldite. The granites are weakly peraluminous (A/CNK =1.0-1.2) with normative (Qtz+Kfs+Ab) contents greater than 96t/0, and in the Qtz-Ab-Or system with excess waterthey plot close to the minimum (T : 690'C) at I kbar with I wt.9o added fluorine. The granites are fluorine-rich(F : 0.7 to 1.0 wt.qo, F/Cl>70), high-silica (SiO2 >75 r't.90) types with extremely elevated contents of Li(703-1124ppm), Rb (1444-1752 ppm), Y (79-97 ppm), Sn (9-16 ppm), Cs (28-53 ppm), Ta (ll-28 FFm)' W (3-27 ppm), andU (ll-35 ppm), and depletions of TiOr (<0.03 wt.9o), I{gO (<0.15 wt.Vo), CaO (<0.41 wt.qo), P2O5 (<0.01 wt.9o),V, Cr, Co, Ni, Cu, and Sr (< l0 ppm). Whole-rock 6ruO values are nearly constant (8.1-8.6700), whereas dD valueshave a broad range from -6 to -1060/6n indicating that magmatic degassing likely occurred during emplacement. ThePleasant fudge granite has many features in common with topaz granites from the Proterozoic (e.g,, Eurajoki, Finland)and Phanerozoic (e.g., St. Austell, England); nevertheless it is distinct from some members of this group of intrusivefelsic rocks in: (l) its lower P2O5 ( < 0.1 wt. Vo) and Al2O3 ( < 14.5 wt.9o), and higher SiO2 ( > 73 wt. q0) contents, and(2) its higher abundances of REE, Y, and Th. It is thus representative of a "low-P" subtype of topaz granite, whichis geochemically distinct from its "high-P" (P2Oj >0.4 wt.Vo, Al2O3 > 14.5 wt.Vo, SiO2 <73 wt.Vo, very low REE,Y, and Th) counterparts (ag., St. Austell).

Keywords: granite, topaz, fluorine, phosphorus, geochemistry, Pleasant Ridge pluton, New Brunswick.

SoMMerns

Le pluton de Pleasant Ridge et la coupole min6ralis€e en 6tain de Bonny River, dans le sud du Nouveau-Brunswick,onf 6te mis en place il y a environ 360 million d'ann€es, bien aprbs la fin de I'activit6 tectonothermale de l'orogenbseacadienne. Ces granites prdsentent un grain fin d moyen et une texture 6quante i porphyroide. Ce sont des granitesrenfermant essentiellement quartz, feldspath alcalin, plagioclase, mica lithique et topaze, avec zircon, magn6tite, rutileniobifbre, colombite-tantalite, cassit6rite, uraninite, monazile et bashasite comme accessoires. Ils contiennent del'albite primaire (An6-3) et sont donc de type subsolvus. La topaze (17,6 d 19,4i/o F) et le mica lithique (5,9 d 7,4i/oF) sont riches en fluor. Le mica lithique Q,840/o Li2O,l3,26Vo FeO) est proche du pOle zinnwaldite. Les granites sontmoyennement hyperalumineux (A/CNK entre 1,0 et 1,2), avec un indice de diffdrenciation (Qtz + Kfs + Ab normatifs)sup6rieur d 96. Dans le systdme Qtz-Ab-Or avec excbs d'eau, ils se placent prbs du minimum thermique (T : 690'C)d6termin6 d I kilobar avec ajout de lVo de fluor. G6ochimiquement, les granites sont riches en fluor (0,7 d l,0Vo F,F/Cl >70) et en silice (SiO2 >7590 en poids), avec des teneurs extremement 6levees en Li (703-1124 ppm), Rb(1444-1752 ppm), Y (79-97 ppm), Sn (9-16 ppm), Cs (28-53 ppm), Ta (ll-28 ppm), W (3-27 ppm) et U (ll-35ppm). Ils sont trbs pauvres en TiO2 (<0,03q0), MgO (<0,1590), CaO (<0,4190), P2O5 (<0,0190), V, Cr, Co, Ni,Cu et Sr (< l0 ppm), Les compositions isotopiques drdo des roches totales montrent des variations limit€es (8,1-8,6%0),alors que les rapports isotopiques 6D varient largement, de -66 d -1060/o, caractlristiques interpr6tdes comme r6sultantd'un ddgazage magmatique au cours de la mise en place. Le granite de Pleasant Ridge poss&de plusieurs caractbresen commun avec d'autres granites i topaze mis en place aussi bien au cours du Protdrozoique (par exemple Eurajoki,Finlande) que du Phan€rozoique (par exemple St. Austell, Angleterre). Cependant, il se distingue de certains membresde ce groupe par (l) ses teneurs faibles en P2O5 (<0,190) et A12O3 (<14.5i/o), et 6levdes en SiO2 (>73V0), et (2) sesteneurs plus dlev€es en terres rares, Y et Th. Il est ainsi repr€sentatif d'un sous-type de granites dtopaze "pauvre enphosphore", qui se distingue g€ochimiquement du sous-tlpe "riche en phosphore" @2O5 )0,4V0, N2O3 )14,5s/0,SiO2 <7390, i trbs faibles teneurs en terres rares, Y et Th), par exemple le granite de St. Austell.

(Traduit par la R6daction)

Mots-cl4s: granite, topaze, fluor, phosphore, g6ochimie, pluton de Pleasant Ridge, Nouveau-Brunswick,

896 THE CANADIAN MINERALOGIST

INTRoDUCTIoN

Topaz granite$ are a unique igneous rock typethat occurs only rarely in the geological record;their nature and origin are poorly understood. Thisstudy presents: (l) a comprehensive account of thepetrological and geochemical characteristics of anoccurrence of lithian mica - topaz granite in thePleasant Ridge pluton in southern New Brunswick,and (2) a comparison of the characteristic featuresof the Pleasant Ridge topaz ganite with those oftopaz gmnites from localities in Europe and Asia.

GEoLoGIcAL SERING

A series of small, metaluminous to weaklyperaluminous, granite plutons intrude LowerPaleozoic rocks adjacent to the northwest marginof the St. George batholith in southern NewBrunsvrick (Fie. 1). The four largest (2 to 15 km2)of the bodies are designated, from west to east, theTower Hill, Sorrel Ridge, Pleasant Ridge, andBeech Hill plutons @g. 2). These plutons have beencollectively termed the "Beech Hill Series" byDagger (1972). However, there exists a clear

ffi C"ntr"t Carbonlferous Basin lffil avaton Tenane

Fl uounr Pleasant Caldera

T eo .roy Intrusive Suite

trlffi

FaultsBB - Back Bay FaultL - Letang Harbour FaultB - Belleisle FaultF - Falls Brook Fault

C - Coleman Brook Faull

FIo. l. Location map and tectonic setting of the Pomeroy Intrusive Suite, of whichthe Pleasant Ndge and Bonny River granites are members, and the adjacentSaint George batholith (after Mcleod 1990).

St. Croix Tenane

St. George BatholithDmd = Mount Douglas Granite

l*iiii:ifl wertoro Intrusive suite

H - Honeydale FaultP - Pendar Brook FaultW - Wheaton Brook FaultS - St. George Fault

Legend

THE PLEASANT RIDGE ZINNWALDITE-TOPAZ GRANITE

Flc. 2. Local geology of the Pleasant Ridge - Mount Pleasant area (after Ruitenberg1967, Mcleod 1990), showing the locations of the granites that comprise thePomeroy Intrusive Suite. Legend as in Figure 1.

897

division of ages within this group. The Tower Hillpluton is excluded because of its earlier age ofemplacement, Silurian as opposed to LateDevonian for the more easterly plutons (Butt 1976,Taylor et ol. 1985, Sinclair et al. 1988). Thus theterminology adopted in this paper follows that ofMcleod (1990), who referred to the younger groupofplutons, that includes the Pleasant Ridge granite,as the Pomeroy Intrusive Suite.

The plutons of the Pomeroy Intrusive Suite havebeen interpreted to represent a group of post-Acadian granites of epizonal to mesozonal charac-ter (Butt 1976, Ruitenberg et ol. 1977). They havebeen the focus of Sn-W exploration in recent yearsbecause of their proximity to the large deposits ofW-Mo in the Fire Tower Zone (Kooiman et al,1986) and Sn in the North Zone (Sinclair el a/.

,1988) at Mount Pleasant (Fig. 2). These mineral'deposits are underlain by a suite of geochemicallydistinctive granites (Taylor et al. 1985, 1989,Kooiman et al. 1986). Two partially unroofedtin-mineralized cupolas have been discoveredrecently, as a result of mineral exploration at BonnyRiver and True Hill (Fie. 2). Tin-tungsten-bearingveins and associated greisen alteration are presentin all representatives of the Pomeroy IntrusiveSuite, and also occur in the Mount DouglasIntrusive Suite (Mcleod 1990), which comprises theeastern half of the adjacent Saint George batholith(Fie. l).

On the basis of gravity profiles, derived fromgeophysical data contained in Thomas & Willis(1989), and the results of recent geologicalmapping, Mcleod (1990) concluded that: (l) the

Mount Douglas Granite was emplaced mainly intoLower Paleozoic sedimentary rocks, crystallized atvery high crustal levels, and extends to a maximumdepth of 7 km below the surface, and (2) theroughly contemporaneous Pomeroy IntrusiveSuite, which is located along the northwesternmargin of the main batholith (Figs. l, 2), occupiesa similar position and may represent a direct lateralextension of the batholith below the Paleozoiccountry rocks (Mcleod 1990).

The country rock to the granites in the St.Stephen - Mount Pleasant area consists ofCambro-Ordovician quartzite and graphitic slate ofthe Cookson Formation, which are unconformablyoverlain by a sequence that includes: conglomerateof the Oak Bay Formation, turbiditic greywackeand slate of the Silurian Waweig and Digdeguashformations, and calcareous sandstone, slate andphyllite of the Upper Silurian - Lower DevonianFlume Ridge Formation. All of these lithologieshave been affected by Acadian polyphase deforma-tion (Ruitenberg et ol. 1977, Fyffe et al. l98l).From west to east, there is a gradual change in thelevel of emplacement of the granites; the SorrelRidge Granite (Fig. 2) intrudes Ordovician slate ofthe Cookson Formation and has produced acontact metamorphic aureole of cordierite-biotitegrade, whereas all of the more easterly plutons wereemplaced into phyllite and greywacke of theSilurian Waweig Formation and have a nal'rowaureole of biotite hornfels (Ruitenberg 1967, Butt1976). The intrusive rocks that.host the Sn-W-Modeposits, in the Fire Tower Zone and North Zoneat Mount Pleasant (Figs. l, 2), are subvolcanic in


character (Kooiman et al. 1986) and are locatedclose to the southwestern margin of the MountPleasant caldera (Fig. l), an €uea of gently dippineLate Devonian volcanic and sedimentary rockstermed the Piskahegan Group (McCutcheon 1986).

Although the field relationships permit somebroad constraints to be placed upon the timing ofgranite emplacement (between Late Silurian andMississippian), some uncertainty still exists regard-ing the absolute ages of emplacement and crystal-lization of the individual plutons. Burt (1926)presented an Rb-Sr isochron aee of 422 t 15 Mafor the Tower Hill Stock; Rb-Sr isochron ages of349 t ll Ma(Butt 1976)and337 t tSMa(Fyffeetol. l98l) have been reported for the Beech HillStock. No published geochronological data arecurrently available for the other plutons; however,the unpublished results of a program of {Ar,/3eArincremental heating experiments yield plateau agesof 361 and 362 Ma, respectively, for the PleasantRidge and Bonny River granites (D.A. Archibald,pers. comm.), confirming their emplacement in theLate Devonian. Slightly older 4Ar,/3eAr ages (ca.367 Ma) have been obtained for biotite from theMount Douglas Granire (Mcleod et a/. 1988) (Fig.r) .

Based solely upon petrological criteria, Taylor eta/. (1985) subdivided the plutons ofthe study area,and related granites from the area of MountPleasant (F igs.1,2) , in to four groups: ( l )biotile-muscovite-bearing (Tower Hill), (2) biotite-bearing (Sorrel Ridge, Beech Hill, True Hill), (3)fluorite- and topaz-bearing (Mine North Zoneintrusive bodies), and (4) lithian mica-topaz-bear-ing (Pleasant Ridge and Bonny River). These areconsidered to be the first reported occurrences oftopaz granite in the northern Appalachiaris.Subsequently, more detailed studies of the NorthZone at Mount Pleasant (Sinclair et ql. 1988,Taylor et al. 1989), and the Pleasant Ridge andBonny River plutons (this paper), have clearlydemonstrated: (l) an intimate spatial association ofimportant zones of hydrothermal tin mineralizationwith topaz granite in the North Zone at MounrPleasant, and the Bonny River cupola, and Q) aLate Devonian age (co. 360 Ma) of emplacementfor topaz granite at both localities. A similarrelationship has also been recognized at EastKemptville, in Nova Scotia, where a ca. 370 Matopaz leucogranite body is host to a large depositof tin that contains 56 million tonnes at 0.165goSn (Kontak 1990). In combination, these occurren-ces indicate that an important metallogenic episode,involving the generation of felsic magmas thatcrystallized as topaz granite and of related tindeposits, occurred in the northern Appalachians inthe Late Devonian.

Euhedral

Subhedral

AnhedralMineral inclusionsAb =albite Toz=topaz Ms=mica

Ftc. 3. Schematic sequence of crystallization for thePleasant Ridge granite.

Ftc. 4. Photomicrographs showing characteristic texturalfeatures of the essential and accessory minerals in thePleasant Ridge granite (all taken using crossed nicols).The field of view is 6 mm in Fig. 4A,, and 3 mm inFigs. 48 through 4H: (A) hypidiomorphic-granular toseriate texture; (B) euhedral inclusions of albite inearly, euhedral topaz, and later subhedral lithian mica;(C) early, euhedral topaz; (D) euhedral topaz asinclusions (see arrow) in late, interstitial quartz; (E)topaz grain exhibiting a lateral change in growth habitfrom euhedral (early) to anhedral and embayed (ate);(F) subhedral, interstitial lithian mica grains withseveral minute inclusions of accessory minerals withradioactive haloes (marked by arrows); (G) late,interstitial K-feldspar with inclusions of euhedralalbite; (H) alteration of magmatic top€rz to sericite +fluorite in altered topaz granite from the Bonny Rivercupola. Abbreviations are: Ab albite, Toz topaz, Limlithian mica, Qtz quartz, Kfs K-feldspar, Src sericite,Fl fluorite.

EARLY

fiiiiili*l tiffiil tr] ffit*iiiliiX rffir tffi fffiil rffiia.'-r

fiiffiif fiiffiilry

LATE

JU-

EI

THE PLEASANT RIDGE ZINNWALDITE-TOPAZ GRANITE 899


PETROGRAPHY AND MINERAL CHEMISTRY

From observations of surface outcroD anddiamond drill-core, the Pleasant Ridge pluton (Lat.45o26' N, Long. 66o49' W) and the nearby cupolaat Bonny River @ig. 2) appear to be composed ofessentially one rock type, at the current level oferosion: a medium- to fine-grained, alkali feldspargranite (Streckeisen 1973). The rock is equigranularto seriate in texture, and grey-white in color.Xenoliths of igneous or sedimentary character havenot been observed. The minerals present in thegranite have been identified by optical microscopy,X-ray diffraction (XRD), and scanning electronmicroscopy with energy-dispersion analysis (SEM-ED). All quantitative data concerning mineralcompositions have been produced by electron-microprobe analysis using wavelength-dispersionanalysis (EMPA-WD), with the exception of thedata for lithium, obtained by atomic absorptionspectrometry (AAS), ferrous iron (by titration), andwater (by combustion, with infrared detector,CIRD, or mercury manometry). Details of theoperating conditions and analytical standards usedfor electron-microprobe analysis are contained inthe footnotes to Tables l, 2, and 3.

Essential minerals in the Pleasant Ridge graniteare, in order of decreasing abundance, quartz,plagioclase and K-feldspar, present in subequalamounts, lithian mica, and topaz. On the basis oftextural relationships, the granite contains primaryalbite. A schematic sequence of crystallization forthe Pleasant Ridge granile is illustrated in Figure3, which also serves to summarize the mostimportant textural features. In deducing thesequence of crystallization, use has been made ofinclusion relations, reaction relations, and theshape ofindividual grains. It has been assumed thatsmall euhedral inclusions in the center of a largergrain nucleated no later than the enclosing grain,and that crystals growing suspended in a magmaare likely to grow as euhedra, whereas during thelater stages of magmatic crystallization, crystals willmutually interfere, forming anhedral grains andinterstitial fabrics.

Albite was the first mineral to crystallize (r'.e.,the liquidus phase) @ig. 3). It typically occurs asindividual fine- to medium-grained euhedral crys-tals (Fie. 4A') and as fine-grained euhedralinclusions in all of the other essential minerals (e.8.,in topaz and lithian mica, Fig. 4B). Electron-microprobe analyses of grains of albite from avariety of different textural settings (Table l)indicate that its composition is quite uniform(Ano-:). The presence of albite as a discretemagmatic mineral, together with K-feldspar, in thePleasant Ridge granite, supports its designation asa subsolvus granite s.s. (Tuttle & Bowen 1958). The

TASLE l. oolrPo6rnoN oF ALIqU FEt"oSPAn (Kt ) AXD PI-ACDCT"ASE (F0 N n|EPIEASAI{T RI'GE CRAI{IIE AND THE ST. AUSTEJ. PIUTON

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plagioclase from other topaz granites also ischaracterized by its very low An contents (e.9.,Eurajoki, Vekkiira granite: An6-5, Haapala 1977;St. Austell: Anr, Mannine & Hill 1990; Tregonninggranite: Anr_r, Stone 1975; Meldon aplite: Anr_5,Chaudhry 1971) and its lack of compositionalzoning. Topaz was the next mineral to crystallizein the Pleasant Ridge granite (Fie. 3). Early-formedcrystals of topaz, like those of albite, occur bothas fine- to medium-grained single euhedral crystals(Fig. 4C), and as fine-grained euhedral inclusionsin lithian mica, quartz (Fig. 4D), and K-feldspar.However, topaz commonly also exhibits an an-hedral habit, and it is not uncommon for singlegrains to show a combination of these habits (Fig.4E). In the latter case, optically continuous singlegrains exhibit a lateral change in habit fromeuhedral to anhedral and poikilitic (Fie. 4E). Insuch cases, the contacts typically represent the grainboundaries between topaz and qvartz. Both theeuhedral and anhedral types of topaz in thePleasant Ridge granite have high fluorine contentsthat range from 17.79 to 19.04 wt.9o (Table 2),close to the theoretical maximum for the mineral(i.e., 20.7 wt.9o: Deer et al. 1966). Such high-F


TABLE 2. COiIPOSMON OF TOPAZ IN THE PLEASANT RIDGEGFANITE (PRG) AND IN TOPAZ GRIINITE FROU THE zuRAIOKI

(EP), sT. AUSTELL (SAP), AND uOUm PTEASA|T (UP) PLUTOI{S

sio,Alro.F€-F

Tolal

sirAlxAl

F

ConcenHions in weighl porcant9..V. 9.2 33.65 ?.& e..e. e..7456.05 56,24 65.85 55.S1 55.99 66.041A.42 17.79 18.84 19.04 18.06 18.14-7.76 -7.49 -7.93 €.02 -7.60 -7.U

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PRGIz PRGiS PRG'3 PRG|/6 SAP/1 SAP/2EEU EEU IA I.A Ezu SU(N-3) (N.€) (N-2) (N-4) (N.6) (N=4)

SAPESU(N=4)

s'o,Alro"F€-F

Total

si

dAl

F

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EP/1 EPN MP/1 MPE MP/4Izu Izu SU SU EEU(N-r) (N.2) (N-3) (N-3) . (N=s)

Al+Ti Atom %Fe+Mn+Mg

Frc. 5. Plot of occupancy of the octahedral sites in termsof Li - (Al+Ti) - (Fe+Mn+Mg) (atomic) afterHenderson et al. (1989) for averaged microprobe dataof Pleasant Ridge granite (PRG), non-megacrysticlithian mica granite (NMLC) of St. Austell pluton, andthe Viikkilrii granite of the Eurajoki pluton (Table 3)'with Li determined by AAS and ferrous iron bytitration. Abbreviations are: Poly polylithionite, Tritrilithionite, Zinn zinnwaldite, Musc muscovite, Sidsiderophyllite, Ann annite. Boundaries demarcatingfields of lepidolite, zinnwaldite, and siderophyllite arefrom Stone et al. (1988).

fine-grained subhedral inclusions in quartz andK-feldspar (Fig. G). Later mica is typicallyfine-grained, anhedral, and occurs interstirially toquartz and feldspar. In chemical composition, themica closely resembles zinnwaldite from the typelocality (cl, Deer et al. l97l), and also the lithianmicas of the "non-megacrystic Li-mica granite"(NMLG) phase of the St. Austell pluton (Hender-sor. et ol. 1989). In the Fe-bearing, lithian inicaseries described by Henderson el a/. (1989)' themain mechanism of substitution isvrli + Ivsi : vl(Fd+) + IvAl, with the occupancy ofoctahedral sites being maintained at 6.0 atoms. Thelithian micas from the NMLG phase of the St.Austell pluton thus define an evolutionary trendcharacterized, in terms of the occupancy ofoctahedral sites, by increasing Li and decreasing Feto produce the mineral series zinnwaldite -

lepidolite (Fig. 5). The lithian micas at PleasantRidge plot at the Fe-rich end of the NMLG rend'close to the composition of ideal zinnwaldite (Fig.t .

The non-megacrystic lithian mica granite unit ofthe St. Austell pluton has been termed "topazgranite" by Manning & Hill (1990). It is a medium-to fine-grained rock characterized by the presence

99.60 9920 99.15 99.7'1 99.97

Number of lons on the basb ot 24 (O,OH,F)4.046 3.995 3.994 3.9@ 4.W7

0.0s 0.06 0.@1aia. s.@, 8.G11 8.101 8.1367.Of3 7,A1 7,29 7.87 7.82

t*1o

3.9880.0148.1147.673

Nofe. Analytlcal dFlhod: Eh,PA-WD, 16 Kr', 20 nA, analytbal dandadus€d LE3 fruodopaz (Baion 198a). N . thE number of spot analy86.Abbrwdons a'9: Ezu - @rly, euhsdial topa4 lEt, = brluded,euhsdnl topaz; SU E subh€dtal tcrpaz; LA - he, anhedral toPaz.

topaz is similar to that from other topaz granites,such as the non-megacrystic lithian mica granite(NMLG) phase of the St. Austell pluton, theVnkkiirti granite phase of the Eurajoki pluton, andthose of the North Zone at Mount Pleasant (Table2). The H2O content of topaz in the Pleasant Ridgegranite (measured by mercury manometry duringhydrogen isotope analysis; Taylor, unpubl. data) isvery low, with a mean value of HrO+ (>130'C)of 0.13 wt.9o. Such a value is very similar to therange of values recorded for topaz in topazrhyolites (0.06 to 0.ll wt.9o: Foord el al. l99l).

The mica is a lithium- and iron-bearing (Li2O, -x

: 2.84 wt.Vo, N : 3, analyzed by AAS; FeO, I= 13.40 wt.Vo, N : 8, analyzed by titration),fluorine-rich type (F ap to 7.27 wt.Vo, Table 3). Itexhibits many of the textural features of topaz,which is consistent with their close association inthe sequence of crystallization (Fig. 3), althoughfine-grained euhedral inclusions of lithian micahave only been identified in quartz and K-feldspar.Early lithian mica occurs as individual fine- tomedium-grained subhedral grains (Fig. 4F) or as

' r " \ : ^

-"-'"% $ueropMn€\

TO. o21ALO" 21.86FqOu rlaFeO 13.9:t7J'tg 0.00MnO 1./OMgO 0.mCqO 0.02LLO' zuNa?O 0.19l(.o s.a4Rb.o 0.00F 727H.e 0.55.O.F -3.6

902

TAALE & COUPOSMON OF UICA IN THE PLEASAI{T FIDCE GFAiIITE(PRG) AND IN TOPAZ GFAilIIE FROU IIIE ST. AUSTELL (SAP) AND

EUFA|OK| (EP) PLUTONS

Sarpla PBG/6 SAP/I SAPA SAP/3 Eplt Ept2 EpfsTqtre SU SU tA tA

(N=4) (N-4) (N.5) (N-3) (N-r) (N.r) (N-r)

THE CANADIAN MINERALOGIST

Hc. 6. Back-scattered electron photomicrographs ofaccessory minerals from the Pleasant Ridge granite:(A) monazite, cassiterite, and zoned niobian rutile ina complex mineral intergrowth hosted by a exain ofIithian mica, from polished thin-section C82-5C; (B)large ( > 200 pm) grain of tantalite with very small ( < I 0pm) grains of cassiterite and uraninite adhering to itssurface, from a heavy mineral separate (>3.3 g,/cm3).The circle in (B) shows the location of the cassiteritegrains illustrated in (C), Abbreviations are: Mnzmonazite, Cst cassiterite, Rt rutile, and Tnt tantalite.

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sltAl,AI

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99./tg 99s,l 99.S 1@54 90.67

Nurtor ot bF on th3 bads d 24 (O,OH,F)6.371 6.818 6,47 6.879 5.99 6.01 6.071.e9 1.1& 1.153 1.121 2,O1 .1.99 .t.gt2.114 2.139 L10, e0$ 1.47 tn 1.U0.0at o.wt o.@7 0.028 0.@ O.O7 o.O7

0.64 0.e 0.401.69t 0.885 0.875 0.a71 2.% 2.5 2.@

0.004 0.0Gt 0.@7 0.05 0.05 0.050.172 0.085 0.@ 0.087 0.12 0.13 0.12

o.Gxt 0.@ 0.cxi7 o.Gl o,@, [email protected] 0.(m 0.@ 0.0d! o.cr o.ol o.o11.659 2762 e851 L79B 0.72 0.84 o.OS0.053 0.059 0.69 0.043 0.08 0.07 0.61.@ 1.761 1.76 1.758 1.77 1.@. 1,770.05a 0.@7 0.102 0.101 0.07 0.07 o.(B0.5s2 0.5@ 0544 0.@t 1.88 1.m 1.56

Not . Amlytbal plooodm: EMPA-WD, f E kV,20 nA daJdard usod w FrlctlPhlogqple. N - nw$er of qo analysc. QO - fthlun amtyds ot mLnnlspara!6 by AAS. Abbrevb$om aro: Su - subMrd rd€: lA E hsrst$al,y49 l|||F_q: pt smlfzed. tlab m mba sspanr8 trom Euratold @ftm tlaapala (1S2, TaUo 8, p.91).

of essential euhedral to subhedral topaz in additionto lithian mica, albite, orthoclase, and quartz. plotsof the occupancy of octahedral sites of primarylithian micas from the pleasant Ridge ind St.Austell plutons (Table 3), and the Eurajoki pluton(Haapala 1977), serve to illustrate the broad rangeof compositional variations found in lithian micafrom topaz granites (Fig. 5): from lithiansiderophyllite @urajoki) through zinnwaldite(Pleasant Ridge) to zinnwaldire -- lepidolite (St.Auste[). In addition to their high concentrationsof lithium (up to 6.4 wt.go in lepidolite), mica intopaz gxanites is also notable for having highabundances of Mn, Rb and Cs (Table 3: this study,Chaudhry & Howie 1973, Du Bray gt at. 1988,Haapala 1977, Henderson et al. 1989, Stemprok &Sulcek 1969).

Quartz and K-feldspar occur as medium-sized

grains (up to 5 mm), whose anhedral shape andinterstitial location support their late position in the$equence of crystallization (Fig. 3). Euhedralinclusions of any of the early-formed albite, topaz,and zinnwaldite are common in both minerals (Fig.4). The quartz typically does nol have unduloseextinction or subgrain development or show anyother evidence of strain. The K-feldspar commonlyexhibits simple Carlsbad twinning and the develop-ment of microscopic string perthite (Fig. 4G), andhas an average Or content of 86 mole 9o (Table l).Occasional grains show microcline twinning.

Accessory minerals in the Pleasant Ridge graniteaccount for a very small proportion (<0.01 vol.Vo)

903

of the rock; however, their diversity and unusualcompositions make their distributions highly sig-nificant. Common accessory minerals (identified bySEM-ED analysis) include zircon, a niobium-richvariety of rutile, columbite-tantalite, cassiterite,and monazite; bastn?isite and uraninite occur morerarely. The accessories typically occur as smallinclusions (< 100 pm; Fig. 6) in zinnwaldite, wheretheir presence may be highlighted by the existenceof a radioactive halo (Fig' aF); less commonly, theyare found intergrown with quartz and topaz. Otherthan their diversity, what is notable about many ofthe accessory minerals is the complex nature oftheir intergrowths, and their very fine grain-size

THE PLEASANT RIDGE ZINNWALDITE-:IOP AZ GRANITE

aEEoEEoocoooCDEoaCD

I

sioe/1o Tio, Abo/o MnO

Fro. Z. Histograms of the average major-element abundances (in wt.9o) of topaz granite from the Pleasant Ridgepluton (Tible 4), felsic l- and S+ipe granites (data from Whalen et ol. 1987), and topaz rhyolites (data fromChristiansen et al. 1984).

FeOr Mgo CaO Nqo l(ro Prot


TASLE 4. WHOLE.ROCK GEOCHEI'ICAL DATA ON T}IE PLEASANT RIDGEAND BOlrlNV RMER cFAltms AND OTHER TOPAZ GRAlr|ms

Sanple V4APluton PRG

82.48PRG

a24C 82-5A 82-58 a2-5C +15"t 273 1160 r@1 258t7s 14sn} Mp-05PRG PRG ,PRG PRG BRG SA SA SA EG EG MP

sto? 75.U 75.06Tio2 o.v2. 0.olAtO" 13.rP 13.rt6hro"t 0.79 , 0.79MnO O.@ O.@MgO 0.08 o.OsCaO O.8 0.30LLO o.2o o.2.N4O 4.49 4.25t(ro 4.U 4.24Rb"g 0.18 0.18Fto' o.1 o.2F 0.70 0.98P?O5 o.ot o.ooco, 0.0 0.1s 0.01 0.oo-GF -o29 -O.41

Total 100.14 99.52

A/CNK t* ,-o/oc 0.96 1.69D.l. 96.64 9621

lvlajor elernent corcentations in v{sight percent75.01 75.11 75.U 78.11 74.W 72.71 72.47 71.& 74.s40.01 0.o2 0.01 0.01 o.@ o.o4 o.o4 o.Gr o.o213.77 !3.79 13.m 13.62 13.94 15.51 15.23 16.@ 1s.770.80 0.70 0.82 0.@. 1.@ 0.61 0.65 1.U 1.O70.09 0.o7 0.@ o.07 o.@ o.@ o.@ o.o9 o.o50.11 0.@ 0.07 0.15 0.6 0.19 0.18 0.18 [email protected] 0.32 0.26 0.32 0.41 0.36 0.34 0.36 0.68o.24 0.19 020 0.20 0..15 0.31 0.36 0.51 o.o54.36 4.59 4..14 4.40 4.20 4.U 3.29 S.78 3.654.8 427 4.32 4.8 4.50 4.07 4.98 4.U 4.7A0.19 0.16 0.18 0..t6 0.16 0.15 0.16 02 0..t202 02 0.2 0.2 o.s 0.3 0.6 0.5 0.40.91 0.70 0.76 0.69 0.74 1.05 1.21 1.55 1.060.@ 0.@ 0.01 o.o1 o.@ 0.54 0.61 0.73 0.010.1 0.0 0.1 0.1 o.o o.o o.o o.o o.o0.01 0.01 0.@ o.o.r o.@ o.o1 o.ot o.@ o.o1

99.94 99.9:t 99.73 99.73 [email protected] 1oo..t3 99.76 99.84 [email protected]

, . t * ; t * r " t * r * i " l ] i1.77 '1.05 1.58 1.37 l.zL' 3.53 4.U 5.58 1.3S'96.@ .V 96.29 96.04 94.4 97.62 97.86 97.27 97.62

74.8 75.960.o2 0.€14.06 13.121.17 1.4o.of 0.070.06 0.050.64 0.360.10 0.113.67 [email protected] 4.380.11 0.130.5 0.20.90 0.590.01 [email protected] 0.10.01 0.@

-0.38 -025

1@.@ 1(p.37

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499121021

4aogdl

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1124176263

T|ace element aburdarFeo in Fns per mfllon947 908 7dr 142B .1688 23S5 2401619 14dr 't471 14G] 1507 1997 105849 52 n 1@ 1q7 6S 7.6

<5124

<5IN

15 20 2A91 67 155

5275

5 < 5 7141 98 14

<5 <518 1m

S r < 5 6Ba 169 329

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176 10r

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10214

1 0 1 03 31 8 1 4

1 t315

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taCoPrNdSmEuGdTbDyHoErTmYbLu

85 82 ,2 7 N76 83l ' t t l30 351 1 l t<o.05 <0.o511 122.4 2.a21 204.7 4.617 163.3 3.32s 244.2 3.6

7 9 9 4 & t2 A & t 481 91 9611 12 .t23 4 3 8 3 6'r1 12 12<o.o5 <0.05 <0.0511 12 't22.6 2.9 2.8n 2 . 2 04.3 4.7 4.516 18 163.2 3.4 3.34 2 7 2 43.6 4.0 3.5

97 1218 A86 g212 na36 &!1 2 1 0<0.05 0.613 na3.0 2.724 1952 4.9'18 m3.6 2.22 f 2 14.3 3.1

03 1672 a 5 060 186.6 17

20 535 . t t 5o.05 <o.056.0 151.4 3.1

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na na na '11'1.3 12 0.60 360.30 0.40 020 9.60.16 0.18 0.11 0.20

na na na 9.00.0s 0.10 0.05 2.o0.6 0.41 020 15o.32 4.24 0.18 3.4

n a n a m l 2na m .la 2.'l025 0.a o.u 17o.Gt 0.@t 0.@ 2.a

68 30 114@ 6t 5.1n a m l 036 100 2.1<0.t <0.1 0262 6.0 8.9

4 2 A 1 9na na 7.O2 a 1 9 4 77.6 3.6 30

81 15 !5 1445 45 45 57m n a n a n a1 6 5 9 40.1 0.1 0.1 0.17.6 1.1 1.0 i.r2 a 2 2 1 9 2 53.4 41 s19 2.9 2.7 2.316 13 7.4 72

61 722 6 3 93 61 6 9o.4 0.46.3 5.713 1514 1731 4.t8

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t{ob' Abboddom ato: PRG ' Plesad RHge-grante; BRG - gqryr p1r", gnnits;.sAP E st. Auster ddon; Ep - EmFld pluton; Mp - Mord plesant ,B- no anabze4 reql - tot8l Fs os F€rod-ltqi to0ai uc A/cNta '-

;;t t $9 (CaO + i{a"O + tqo): %c - % nonrdve (clnv) @rudm @buhred ona F-frs besb; D'l' ' Dfrenrddor Ind6l 116 folo$ts ef;otns w* idttury "*ly."a

tor, but rer€ nd prwnt h orrentrdorE grdor than th€€ d theanaly/tbd [mn d dstocdon: Ct (<SO ppn), V (<6 ppm),6r (<ro ppm), flf G ppmt ani C" 1.r.0 pprj.' "-'-'--'

THE PLEASANT RIDGE ZINNWALDITE.TOPAZ GRANITE 905

and mode of occurrence (Fig. 6). Such charac-teristics present a number of problems, mostparticularly recognition by optical microscopy.Nevertheless, similarly diverse assemblages ofaccessory minerals have been described in othertopaz granites: for examplg, zircon, cassiterite,bastnisite and monazite are common, and niobianrutile, columbite, thorite, xenotime, magnetite,ilmenite, and molybdenite are sparse accessories inthe Viikkiirii granite phase of the Eurajoki pluton(Haapala 1977).

Fluorite and sericite, although present in minoramounts, clearly owe their origin to hydrothermalprocesses. Typically they occur together, as replace-ments of primary topaz and lithian mica (Fig. H).

LITHOGEOCHEMISTRY

Major- and- trace-element data for the PleasantRidge and Bonny River granites are presented inTable 4, together with comparative chemical datafor representative topaz-bearing granites from theEurajoki pluton, Finland, the St. Austell pluton,England, and the nearby Mount Pleasant area (Fig.2). Data for the international standard rocks usedas reference materials are given in the Appendix,which also contains full details of the sampling andanalytical methods used in this study. The oxygenand hydrogen isotope data, together with whole-rock H2O concentrations determined by mercurymanometry, are presented in Table 5. All of theisotopic data are reported in 0/oo relative to theStandard Mean Ocean Water (SMOW) referencestandard.

Major elements

The samples of topaz granite from the PleasantRidge and Bonny River granites have a veryrestricted range of compositions: they are allhigh-silica (SiO2 >75 wt.9o), weakly peraluminous(A/CNK : 1.07-1.12), fluorine-rich rocks (F>0.69 fi.qo), with low concentrations of TiO2(<0.03 wt.9o), MgO (<0.15 wt.7o), CaO (<0.41wt.Vo) and P2Oj (<0.01 wt.9o) relative to othertypes of granite (Fig. 7). Low TiO2, MgO, CaOand P2O5 are reflected by the albitic nature of theplagioclase (Table l), the lithium-rich character ofthe mica (i.e., zirnwaldite; Table 3), and theabsence of apatite as a common accessory mineral.Although niobian rutile and monazite are presentas accessory minerals, their abundances are so small(<0.01 vol.7o) as to have a negligible effect uponthe TiO2 and P2Or contents, respectively, of thebulk rock. MnO contents are high (0.07-0.09 wt.9o)in comparison to other granites (Fig. 7), a featurethat results from the Mn-rich nature of the lithianmica present (Table 3). The granite samples haveconcentrations of total iron that are low (total Feas FeOr is less than 0.95 wt.9o) compared to othertypes of granite (Fig. 7), and reflect the leucocraticcharacter of the granite. Alkali contents are high,with Na2O contents ranging from 4.14 to 4.59wt.9o, and (Na2O+KrO) values from 8.46 to 8.86wt.Vo; unlike many common types of granite, thePleasant Ridge granites have Na2O in excess ofK2O, with molar Na/K ratios ranging from 1.42 to1.63.

The low CaO contents, high contents of Na2O,and high Na/K ratios are reflected in the normative

TABLE 5. OX'GEN AND }IYDROGEI{ I8OTOPIC ANALYAES OFTOPAZ GRAT$TES

Sanple tlhobgyIto(wt.%)

Pl@nl Rldgs [email protected] Topaz gntffg snh [email protected] Top@ gnrdte rft dronmklie@-4C Topoz gnnlte sth ,ftnHldie@64 TSoz gEnltewft dflwuno82-68 Topaz gnrffo wih zhnEuno&€C Topaz gruffe xrti! dmHno

Bouty Rlv6 @pola/t-151 Topc gnrdo snt| ztmmHto

(rqk dedelia anerdon)

Mqrt Pl@nt Norlh ZoEMP-6 Top@ gnnlte rfi ddeltptvmo

$. Auslel pMon27E TopazgnniswitrhpHone1160 Topez gBffe Efi l€pklone10gl Topa Snnfrewffi @one

E@lohddouTnE Topaz gEtffe wtfi dd€lodrymeffiIn TAd ganne wnh sideroplylte

tlotq 6130 ard 6D wl@ m rcpon€d h por dl (%) rsldve !o SMOW.FlaO @FrMo6 reaned ry nmryltmmtla me noi€mlyzed

Qt4oAb*Or, QtzrnAbroOr,

Frc. 8. Qtz-Ab-Or plot of Pleasant Ridge and BonnyRiver topaz granites (crosses), showing the minima forthe system with 0 (M6) and I wt.9o added fluorine(Mr) and excess waler at I kbar (data for minimumcompositions are from Manning 1981).

8eo 5D(%) (%)

8 . 6 m m8.1 €6 0.148.r -t6 0.15a2 -72 0.188.4 44 0.188.1 -95 0.19

10.6 .4 02111.1 -74 0,62113 n00 0.30

9.6 -70 0.669.8 -96 o.3A


compositions of the topaz granites. The granitesplot in the Qtz-Or-Ab system between theminimum for fluorine-free conditions and theminimum with I wt.9o fluorine added at I kilobarunder water-saturated conditions (Fig. 8). Theseminima correspond to temperatures of 730oC(Tuttle & Bowen 1958) and 690'C (Manning l98l),respectively. However, care must be taken inassigning such temperatures of crystallization forthe Pleasant Ridge granite. Although there are anumber of geological (see section on GeologicalSetting) and geochemical criteria (see section onStable isotope compositions) that support crystal-lization of the topaz granite magma underwater-saturated conditions at a shallow crustallevel, the confining pressure cannot be preciselyconstrained. In addition, exsolution of an aqueousfluid during granite emplacement will result. in thepartitioning of some of the fluorine into theexsolved fluid phase (cl, Webster & Holloway1990), resulting in an underestimation of theoriginal F content of the magma by analysis of thecrystalline rock.

Allhough they have many features in commonwith the topaz-bearing granite of the Eurajokipluton and Mount Pleasant, samples of PleasantRidge topaz granite differ from topaz granites ofthe St. Austell pluton. In particular, the samplesof topaz granite from the St. Austell pluton arecharacterized by lower SiO2, higher Al2O3, andhigher P2O5 concentrations (Table 4).

Volatiles

Concentrations of total H2O (<0.3 wt.9o), COz(<0.1 wt.9o), chlorine (<l@ ppm), and sulfur(<0.2 fi.90) in the Pleasant Ridge topaz granite€ue very low, and contrast markedly with thefluorine concentrations, which are very hieh (0.69to 0.98 wt.9o). Whether the low abundances ofH2O, CO2, Cl, and S are representative of thechemistry of original magma, or are an artefact ofmagmatic degassing during granite emplacement(cl, Taylor et al. 1983, Taylor 1988), is uncerrain.Although the low solubility of CO, in peraluminousmagmas has been demonstrated experimentally(Holloway 1976), the other volatiles under con-sideration all have moderate to high solubilities inmetaluminous to peraluminous granitic magmas(cl, Burnham 1979, Clemens 1984, Webster &Holloway 1990). Both Cl and S are, however,pafiitioned strongly into the exsolved aqueous fluidduring magmatic degassing at shallow crustal leveis(<2 kbar: Holland 1972, Burnham & Ohmoto1980). In the case of CI, Webster & Holloway(1990) reported a variation in D61 of 9.8 to 46.2(where Ds1 = ppmw of Cl in the fluid,/ ppmw ofCl in the melt) for water-saturated haplogranitic

melt at pressures of approximately 2 kbar and atemperature of approximately 800'C.

In contrast to the other volatile species, fluorinepafiitions selectively into granitic magma ratherthan into the exsolved aqueous fluid phase(Manning 1981, Webster & Holloway 1990),although some F is always lost to the vapor phaseduring degassing; Webster & Holloway (1990)reported a variation in Dp of O.22 to 0.37 forhaplogranitic melt with <2 wt.Vo F at P : 2 kbarand T = 795"C. High concentrations of F are thusto be expected in volatile-rich granites even ifmagmatic degassing has occurred. AlthoughTurekian & Wedepohl (1961) proposed an averagevalue of 850 ppm F for low-Ca granites, Bailey(1977) reported that F contents in pantellerites andalkali granites of varying alkalinity typically rangefrom 1,000 to 3,000 ppm; in individual plutons ofperalkaline albite-riebeckite granite, abundances ofup to 0.94 wt.9o F have been recorded (the KaffoValley granite, Nigeria: Kinnaird et al. 1985), Inaddition to their high F contents, peralkalinegranites and rhyolites also typically have highconcentrations of chlorine: for example, 0.07 to0.18 wt.Vo Cl in granites from the Liruei Complex(Bowden & Turner 1974) and 0.37 to 0.78 wt.9o inpantellerites (Bowden 1974). The onlymetaluminous to peraluminous rocks with Fconcentrations similar to those in the PleasantRidge topaz granite are ongonites (topaz-bearingquartz keratophyres with up to 2.01 wt.9o F:Kovalenko & Kovalenko 1976), topaz rhyolites (upto 1.3 wt.9o F in vitrophyres: Christiansen et al.1984) and, of course, topaz-bearing granites fromother locations, such as St. Austell (Table 4). Allof these rocks are notable in having low to verylow contents of Cl (<0.14 fi.90 for glassy topazrhyolites: Christiansen et al. 1986, <250 ppm fortopaz granites: this study, Manning & Hill 1990)and high to very high F/Cl ratios (3 to l0 for topazrhyolite glasses, 40 to 70 for topaz granites) incontrast to their peralkaline counterparts (F,/Cl<3: Christiansen el a/. 1983).

Trace elements

The trace-element contents of the Pleasant Ridgeand Bonny River granites are unique in manyrespects, regardless of whether they are examinedin a local or a regional context. Even though thecontiguous biotite granites of the Pomeroy In-trusive Suite (Sorrel Ridge, Beech Hill, and TrueHill granites; Fig. 2) and the Mount DouglasGranite (Mcleod et ol. 1988) have unusually high(e.9., Rb) or low (e.9., Sr) concentrations of manytrace elements, which demonstrate their A-typeaffinities (Taylor et al. 1985, Mcleod et al. 1988,Whalen 1986), they do not exhibit levels of


Fro. 9. Enrichment factors for 48 elements in the Pleasant Ridge topaz granite (Table 4) compared to a sample ofbiotite granite from the contiguous Sorrel Ridge pluton (Fig. 2; Taylor, unpubl. data).

907

enrichment or depletion of the magnitude found inthe topaz granite in the Pleasant Ridge and BonnyRiver plutons (Fig. 9).

By comparison with even the most highlyfractionated examples of I- and S-type granite withsimilar contents of SiO2 (cl, Sawka et ol. 1990), thePleasant Ridge topaz granite is enriched in: thealkali metals Li (up to llZ ppm), Rb (up to 1752ppm), and Cs (up to 53 ppm); and the high-valencecations Y (up to 97 ppm), Nb (up to 50 ppm), Sn(up to 16 ppm), Ta (up to 18 ppm), W (up to 26ppm), and U (up to 35 ppm). Conversely, they havelow to very low abundances of the alkaline earthmetals Sr (< l0 ppm) and Ba (< 169 ppm), V, Co,Ni, Cu, Mo (all are < 10 ppm), Cr (<20 ppm), Zn(<52 ppm), and Zr (<72 ppm). Many of theseenrichments and depletions are extreme: forexample, using the mean abundances for the sixsamples of Pleasant Ridge granite in Table 4, thecontents of Rb, Y, and U are from two to seventimes as great as those of aluminous A-typegranites, and the depletions in Sr, Ba, Zn, and Zrare from two to four times lower (comparative datafrom Collins et ql. 1982). The concentrations ofRb, Cs, and Ta are from two to nine times greater

than those of F-rich I-type granite, and from twoto five times greater than those of volatile-richS-type granite (comparative data from Sawka et al.1990). Concentrations of Sc (10 ppm) and Th (30ppm) in the Pleasant Ridge topaz granite are similarto tho$e recorded for aluminous A-type granites.

For the trace elements that show strikingenrichments, the major host minerals are: thelithian mica (i.e., zinnwaldite) for Li, Rb, and Cs(Table 3), niobian rutile for M and Ta, columbite-tantalite for Ta, Nb, W, and Sc, cassiterite for Sn,and uraninite for U. In addition, Sn also isconcentrated in the niobian rutile (EPMA-WDgives 63.4-79 .9 vtt .s/o TiO2, l l.8-7.8 wt.7o Nb2O5,6.7-9.7 wt.9o FeO, 1.5-5.3 wt.9o Ta2O5, 1.3-1.8wt.9o SnO). The analysis of monazite andbastniisite, by SEM-ED, indicates that theseaccessories are the major carriers of Th and LREE.Mica from other topaz granites is typicallylithium-rich, and characteristically also is enrichedin both Rb and Cs (Haapala 1977, Henderson elal, 1989\. Niobian rutile, columbite-tantalite, andcassiterite also have been identified in topaz granitefrom: St. Austell, Eurajoki (Taylor, unpubl. data),Yichun, China (Pollard & Taylor l99l), and


0.05

Pleasant RidgeTopaz Granite

Pleasant RidgeTopaz Granite

F Cl RbBaSrEuLaSmYbY Th U ?NbTa F C l RbBaSrEuLaSmYbY Th U Z NbTa

Ftc. 10. Concentrations of F plus selected trace-elements in topaz granites from Pleasant Ridge (Table 4) comparedto those of representative examples of highly fractionated l-type (WTG 26) and S-type (WTG 9) granites (datafrom Sawka et a/. 190, Table 3: no F and Cl data reported). The values used for normalization pertain to RGM-I,a rhyolite obsidian from Medicine Lake volcano, California, and U.S. Geological Survey geochemical reference(Govindaraju 1984); the elements are listed in order of their relative field-strength, increasing to the right (afterChristiansen et al. 1986).

50

10

T

CI(rlUJ(L

U)

0.5

0.1

Cinovec, Czechoslovakia (Stemprok & Sulcek1969).

Some of the compositional features of thePleasant Ridge granite, other topaz-bearing felsicrocks, and of I- and S-type granites are compared,in terms of their trace element contents, in Figuresl0 and ll. Such illustrations show the relativeconcentrations of many elements in a single sampleor group of closely related samples. All concentra-tions were normalized to those of U.S. GeologicalSurvey geochemical reference sample RhyoliteRGM-I, and the elements in Figures l0 and ll areIisted in order of increasing field-strength (follow-ing the procedure of Christiansen et al. 1986).Thestriking enrichments of Rb, Nb, and Ta and therelative depletions of Sr, Eu, and Zr in the PleasantRidge topaz granite, in comparison to other morewidely distributed types of granite, are readilyapparent from an inspection of Figure 10. It is

noteworthy that these specific enrichments anddepletions are features common to the topaz-bear-ing granites from the Eurajoki and St. Austellplutons (Fig. 1l), and are also distinguishingfeatures of the group of subvolcanic, fluorine-richigneous rocks termed "ongonites" (Fig. l2).

Rare-eorth elements

Total rare-earth-element (El4 REE-) contents forthe individual samples of Pleasant Ridge graniterange from 247 to 281 ppm (Table 4). Such totalabundances are quite typical of those of graniticrocks of the continental crust (cl, Cullers & Graf1984). The Pleasant Ridge topaz granite exhibits aflattened, gullwing-shaped chondrite-normalizedREEpattern (0.67 < La,/Lq1 < l.0l) with a deepnegative europium anomaly @u/Eu' < 0.01) (Fig.l3). The large negative Eu anomaly requires that

Topaz Granites("low-P" subtype)

+ Pleasant Ridge

Topaz Granites("high-P" subtype)n St. Austell

*CI(rIJJJL

a


0.05

F Cl Rb BaSr Eu LaSmYb Y Th U Z NbTa F Cl Rb BaSr Eu LaSmYbY Th U Zr Nb Ta

Frc. lt. Concentrations of F plus selected trace-elements in "low-P" topaz granites from the Pleasant Ridge pluton(sample 82-4C, Table 4) and the Eurajoki pluton (sample 256/73, Table 4), and "high-P" topaz granite from theSt. Austell pluton (sample 1160, Table 4). The values used for normalization pertarn to RGM-I, and the field fortopaz rhyolites is from Christiansen et a/. (t986, Fig. 4l).

0.5

0.1

much feldspar was involved in the mineral-meltequilibria during melting or crystallization (Cullers& Graf 1984). Similarly, the low LalLuN ratiorequires the presence during melting or crystal-lization of one or more LREE-enriched accessoryminerals. Monazite, which occurs in trace amountsin all of samples of the Pleasant Ridge granite, isthe most likely host for the ZR.EE (c/. Charoy 1986,Congdon & Nash 191), and as little as 0.0590monazite is required to account for all of ZREE inthe Pleasant Ridge granite.

Although the depletion of light rare-eafihelements in felsic magmas is not uncommon (Miller& Mittlefehldt 1982), the high concentrations of theheaviest kEE (up to 28 ppm Yb and 4.3 ppm Lu:Table 4), and very low Lall-u1 ratio in the PleasantRidge granite (Fig. l3), are unusual. Uraninite,where present as an accessory mineral in graniticrocks, is known to concentrate the HREE,

specifically Yb and Lu (Fryer & Taylor 1987), andis thus a possible host for these elements. However,the extremely low abundance of uraninite in thePleasant Ridge granite precludes its role as the onlycarrier of the heaviest R.EE. Thus, it is likely thatan as yet unidentified accessory mineral, such asxenotime, thorite, or fergusonite, is the majorreservoir df the nnBn in the Pleasant Ridge topazgranite. Xenotime and thorite both have beenidentified by Haapala (1977) as accessory mineralsin samples of Eurajoki topaz granite, whose REEabundances and distribution are almost identical tothose of the Pleasant Ridge granite (Fig. l3l. Inaddition, thorite and fergusonite both occur asaccessories in topaz rhyolite from the HoneycombHills, Utah (Congdon & Nash l99l).

The REE content and distribution in topazgranite from the St. Austell pluton (Fig. 13) arequite di$tinct from those that characterize the topaz

t Pleasant Ridgeo Eur{okitr St. Austell


Eo-950-oz

20

Egso-oz

20

200

100

200

100

500 1000 2000

Rb(ppm)

granites in the Pleasant Ridge and Eurajoki plutons(Fie. l3), and also that of topaz rhyolire (Fig. l4).The total rare-earth-element content (DI0.R.E@ ofthe St. Austell topaz granite is typically less thanl0 ppm (Table 4), with a La/Lu1q ratio of 3.5 to6.6 (Frg. l3). This very significant depletion in the

total REE content also is accompanied by deple-tions of both Y (<10 ppm) and Th (<5 ppm) inthe St. Austell topaz granite, which again are inmarked contrast to the abundances of theseelements in topaz granite in the Pleasant Ridge andEurajoki plutons (Table 4, Fig. 1l).

+ Pleasant RidgeoEuraiokiuSt. Austell

Ta(ppm)Ftc. 12. Rb-Nb-Ta concentrations in topaz granites from pleasant Ridge, St.

Austell, and Eurajoki (Table 4). Fields for topaz rhyolites and ongonites fromChristiansen et al. (19861.

THE PLEASANT RIDGE ZINNWALDITE-TOPAZ GRANITE 9 1 1

€soEco-cero0)EsE(U(t)

1 0

o.=EEo-cooaE(ua

0.5

La CE PrNd SmEuGdTb DyHo ErTmYb Lu

Frc. 13. Chondrite-normalized rare-earth-element plot ofsamples of topaz granite from the Pleasant Ridge(sample C824C, Table 4), Eurajoki (sample 256/73,Table 4), and St. Austell (sample 1160, Table 4)plutons. The chondritic values of Boynton (1984) wereused for the normalization.

Ratios of oxygen and hydrogen isotopes

Results of oxygen and hydrogen isotope analysesof whole-rock samples of the Pleasant Ridge andBonny River granites are presented in Table 5,together with comparative data for samples oftopaz granite from the Eurajoki pluton in Finland,the St. Austell pluton in England, and the nearbyMount Pleasant area (Fig. 2). The oxygen isotopevalues for the topaz granite samples from thePleasant Ridge pluton are fairly uniform, with 6180values that range from 8.1 to 8.60/*. In contrast,the granite sample from the tin-mineralized BonnyRiver cupola, which has suffered postmagmatichydrothermal alteration (with the partial break-down of magmatic topaz and lithian mica to formsericite and fluorite; Fig. 4H), has a lower 6180value of 6.8/os. Hydrogen isotope compositions forsamples of the Pleasant Ridge granite varymarkedly (-66 to -105o/oi and have a much widerrange than the corresponding 6lEO compositions(Fie. l5).

Studies of the stable isotope characteristics ofclassic I- and S-type granite suites (O'Neil &Chappell 1977, O'Neil et al. 1977) havedemonstrated that a clear distinction can be made

La Ce Pr Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu

Flc. 14. Chondrite-normalized rare-earth-element plot ofthe field of values of the Pleasant Ridge ganite (Table4), and examples of topaz rhyolite (data fromchristiansen el a/' 1986) and Macusani glass (data fromPichavant et ol. 1988). The chondritic values ofBoynton (1984) were used for the normalization.

3 4 5 6 7 A 9 1 0 1 1 U 2 t 3 1 4

6"o f/.)

Frc. 15, Values of 618o (/oo) versus 6D (Y*\ for topazgranites from Pleasant Ridge, St. Austell, andEurajoki (Table 4). The fields for I- and S-tlpegranitesare from O'Neil el al. (1977).

912 THE CANADIAN MINERALOCIST

regarding the nature of the protolith from themeasured oxygen isotope composition. S-typegranites (sedimentary protolith) have consistentlyhigher oxygen isotope values (e.g., 6180 between10.4 and 12.5/oo in the New England Batholith,between 9.9 and 10.5%0 in the Berridale Batholith,Australia) than the spalially related I-type granites(igneous protolith), whose 6180 values range from1.7 to 9.9/oo (New England) and from 7.9 to 9.4/oo(Berridale Batholith). Such a distinction, with adividing line of 0r8O of around 10160 results from.he enrichment in l8O of sediments during processesof suri'ace weathering (Taylor 1968, Savin &Epstein 1970). The drsO values of the pleasanrRidge topaz granite (8.1 to 8.60160) are thusconsistent with partial melting of a sourcedominated by metaigneous rocks. An examinationof the oxygen isotope compositions of other topazgranites (Table 5) indicates that samples fromEurajoki and Mount Pleasant also have dlsO valuesthat are typically less than 100/66, but that samplesfrom St. Austell are characterized by higher 6180values (10.6-1 1.8%J. The higher 6180 values of theSt. Austell topaz granites are consistent, both withtheir more strongly peraluminous character (Table4), and also with a source in which there was animportant component of pelitic sedimentarymaterial (c/ Miller 1985).

In contrast, the variations of 6D values in felsicplutonic rocks reflect the influence of other thansource controls; these include magmatic degassingand exchange with isotopically lieht meteoric water(cf. Taylor 1988). Although the hydrogen isotopiccomposition of "deep-seated igneous rocks" hasbeen estimated to vary between -60 and -85loo(Taylor 1977), it has been clearly demonstrated(Nabelek et al. 1983, Taylor et al. 1983) that sucha range can be extended to values as negative as-ll0%o by magmatic degassing during ascent andemplacement. In their study of the stable isotopesystematics of the Notch Peak granitic stock, Utah,Nabelek et al, (1983) documented a wide range of6D (-55 to -110%0) values in whole-rock sampleswith an almost constant 6180 composition (ca.9.30/oi. They attributed these variations to mag-matic degassing, with the different responses of thetwo isotopic ratios to the operation of this processresulting from the much larger isotopic fractiona-tion for D/H, versus that of 180,/160, betweencoexisting aluminosilicate melt and aqueous vaporphase (c/. Taylor 1988). When taken together withthe petrographic data, which demonstrate the lackof pervasive mineral replacement andmetasomatism (Fig. 4), the oxygen and hydrogenisotope compositions of the whole-rock samples ofthe Pleasant Ridge topaz granite are those thatwould be expected to result from magmatic

degassing during the emplacement of an epizonalpluton.

Comparison of the hydrogen isotopic composi-tions of the Pleasant Ridge granite with those ofF-rich granites in other plutons shows that asimilarly broad range of 6D values (-44 to -100%0)also occurs in the topaz granites of the St. Austellpluton (Table 5) and at Yichun in China (Taylor& Pollard, unpubl. data). Collectively, thehydrogen isotopic data suggest that magmaticdegassing may be an integral feature of theemplacement of these F-rich, topaz-bearinggranites.

DISCUSSIoN

The results of a number of recent experimentalstudies have demonstrated that topaz is a stablemineral in granitic melts over quite a large rangeof P and T conditions (Anfilogov et ol. 1973,Kovalenko 1977, Weidner & Martin 1987, Websteret al. 1989). In particular, experimental datapresented by Weidner & Martin (1987) for a studyof the phase equilibria of a fluorine-richleucogranite (of similar bulk composition to thatof the Pleasant Ridge pluton) provide an excellentmeans of assessing the origin of the variousminerals, interpreted here to be magmatic incharacter, present in the Pleasant Ridge granite.The data of Weidner & Martin (1987) indicate thatfor water-saturated conditions: (l) topaz, mus-covite, and fluorite occur as magmatic minerals inaccessory amounts together with plagioclase, K-feldspar, and quartz; (2) plagioclase (An1) is on theliquidus at low pressure (l kbar), topaz is on theliquidus at higher pressure (4 kbar), and atintermediate pressurg (2 kbar), all crystalline ptrases(albite, K-feldspar, qtJartz, topaz, muscovite, andfluorite) dissolve over a very narrow temperatureinterval of around 25"C; (3) at pressures of I to 4kbar, liquidus temperatures are typically well below750'C; (4) muscovite (exact composition notspecified) is stable up to temperatures of 680-700"Cin the pressure range from I to 4 kbar, a fact thatWeidner and Martin attributed to the F-rich natureof the mica; and (5) highly evolved, F-rich graniticmelts will exhibit subsolvus features unlessemplaced at very shallow depths (<1.5 kbar).

The petrographic and chemical data presentedhere are consistent with a magmatic origin for thebulk of the albite, topaz, and lithian mica presentin the Pleasant Ridge granite. Textural relationssupport the early crystallization of albite (as theliquidus mineral) of homogeneous character (Ans_3:Table 1), and its inclusion in the various other latermagmatic minerals (Figs. 3, 4). Such a mineralparagenesis (Fig. 3) is comparable to the sequenceof crystallization recorded at pressures of I to 2

THE PLEASANT RIDGE ZINNWALDITE-TOPAZ GRANITE 9t3

kbar in the experiments of Weidner & Martin(1987). The lithian mica in the Pleasant Ridgegranite is characterized by its very high F contentand F/(F+OH) ratios (Table 3), which results inits stability at high temperatures (Munoz 1971,1984), and presence as a primary mineral in F-richmagmas (cl, Henderson et ol. 1989). A shallow levelof emplacement for the Pleasant Ridge and BonnyRiver granites (<2 kbar), which is consistent withthe regional geological and geophysical setting ofthe Pomeroy Intrusive Suite (Fig. 2) as a whole(Mcleod 1990, Thomas & Willis 1989), also wouldenhance the possibility of an initially water-under-saturated magma reaching saturation (Burnham1979). The large range of hydrogen isotopecompositions, but almost constant oxygen isotopecomposition of the Pleasant Ridge granite (Table5), provide support for magmatic degassing duringemplacement (c/. Nabelek et al. 1983).

Although the exact timing of degassing isdifficult to assess in terms of the history ofcrystallization of the Pleasant Ridge pluton, theubiquitous occurrence of all of the importantaccessories as mineral inclusions in subhedrallithian mica (Fie. 4F), which crystallized early (Fig.3), is likely significant. As Webster (1990) haspointed out, the timing of exsolution of theaqueous fluid with respect to crystallization iscritical because if (Cl,F)-enriched fluids are notreleased until most of the magma has crystallized,then a large proportion of the rare alkali (Li, Rb,Cs) and lithophile metals (Nb, Ta, Sn, W, U) maybe sequestered into phases such as lithian mica(enriched in Li, Rb, and Cs), niobian rutile(enriched in Nb, Ta, and Sn), columbite-tantalite(enriched in Nb, Ta, and W), cassiterite, anduraninite. These accessories are homogeneouslydistributed in the Pleasant Ridge granite and appearto have crystallized early from the topaz-bearingfelsic magma. As a consequence, the distributionof these elements likely approximates their distribu-tion in the parental F-rich magma.

The unusual mineralogical character of thePleasant Ridge topaz granite is reflected in itschemical composition, which is quite distinct fromthat of the most highly fractionated examples of I-and S-type granite, and aluminous A-type granite.Thus, the Pleasant Ridge topaz granite is hiehlyenriched in fluorine, the alkali metals (Li, Rb, Cs),and the lithophile elements (Nb, Ta, Sn, and W),and has marked depletions of Cl, Ti, Fe, Mg, Ca,P, and Sr (Table 4, Fig. l0), features that arecommon also to the topaz granites from theEurajoki and St. Austell plutons (Table 4). It isclear that topaz granites can, as a group, bedistinguished from all of the other widely dis-tributed types of granite by a variety of geochemical

criteria, the most notable of which is their very highF content.

However, also it is apparent from the results ofthis study that there exists a natural dichotomywithin this unique group of F-rich felsic rocks, suchthat "low-P" and "high-P" subtypes of topazgranite can be distinguished (Fig. 16): topazgranites from the Pleasant Ridge and Eurajokiplutons belong to the "low-P" subtype (P2O5 <0.1wt.Vo, Al2O, (14.5 wt.Vo, SiO2 >73 wt.9o), andthose from the St. Austell pluton belong to the"high-P" subtype (P2O5 >0.4 wt.Vo, Al2O3 > 14.5wt.Vo, SiO2 <73 wt.o/o), Such a division reflectsthe more strongly peraluminous character of"high-P" topaz granites (1.2 < A/CNK < 1.5),in contrast to their "low-P" counterparts, and alsohiehtights the significant difference in the distribu-tion of Y, REE, and Th that exists between the twosubtypes (Table 6). The chemical compositions oftopaz granites from other localities lend support tothe subdivision proposed here, with granites fromFawwarah, Saudi Arabia (Du Bray 1984), MountPleasant, New Brunswick (Taylor et al. 1985,1989), and the.. Erzgebirge. in Germany andCzechoslovakia (Stemprok &Sulcek 1969, Tischen-dorf & Fdrster 1990) as representatives of the"low-P" subtype, and those from Tregonning,Cornwall (Exley & Stone 1982), Meldon, Devon-shire (Chaudhry & Howie 1973), Beauvoir, France(Cuney & Autran 1987), and Yichun, China(Pollard & Taylor l99l) being characteristic of the"high-P" subtype (Table 6).

For the "high-P" subtype of topaz granite(Table 6), the whole-rock concentrations of P2O5that are present (St. Austell: 0.51-0.73 wt.Vo, Table4; Tregonnine: 0.44-0.73 wt.Vo, Exley & Stone1982; Yichun:. 0.51-0.62 wt.Vo, Pollard & Taylor1991) require that phosphorus be present inminerals other than the phosphates. Kontak &Strong (1988) and London et al. (1990) recognizedthat the feldspars may represent a principalreservoir of phosphorus in P-rich peraluminousleucogranites and pegmatites. For example, Kontak& Strong (1988) detected greater than I wt.Vo P2O5in feldspars from the South Mountain batholith inNova Scotia; in a preliminary study of P in alkalifeldspars of rare-element granitic pegmatites,London et al. (1990, p. 771) reported concentra-tions that "..,range from below detection limit to1.20 wt.Vo P2O5, with approximately 60Vo of thedata indicating greater than 0.30 wt.To". Electron-microprobe data for feldspar in samples from theSt. Austell pluton (Table l) indicate that P2O5concentrations in plagioclase (albite) and K-feldspar attain 0.43 and 0.63 wt.Vo, respectively,in this example of a "high-P" type of topaz granite.Given the modal abundance of albite (35-4090) andK-feldspar Q0-250/o) in the St. Austell topaz

9t4 THE CANADIAN MINERALOGIST

66 68 7Q 72 74 76(wl o/^\\ r , L . / v ; /

Frc. 16. Histograms of P2O5, Al2O3, and Si02 concentra-tions (in ft.qo) in topaz granites: "low-P" types fromPleasant Ridge (Table 4), Eurajoki (Table 4), Mt.Pleasant (Table 4) and Fawwarah (Du Bray 1984);"high-P" types from St. Austell (Table 4), Tregonningand Meldon (Exley & Stone 1982), and Yichun (Pollard& Taylor 1991). The vertical scale shows the frequencyof each value.

granite, such levels of phosphorus enrichment inthe two feldspars can account for up to 5070 ofthe total budget of phosphorus in the rock.

The summary presented in Table 6 is intendedonly to aid in the recognition of the magmaticcharacteristics of topaz granite, and its "low-P"and "high-P" subtypes. Although it incorporatesdata from as many of the known occurrences oftopaz granite as possible, Table 6 should not beviewed as a panacea for understanding the natureof this unique group of igneous rocks. As a caveatemptor to the reader, it should be noted that thecurrent petrological data-base for topaz granites isfar from adequate to permit a rigorous analysis oftheir character and petrogenesis. Thus, with theacquisition of new data for topzv granites,particularly for occurrences in Russia, Mongolia,and China, the fabric of Table 6 will undoubtedlybe subject to modification. In addition, althoughevery effort has been made to include only data for"fresh" samples of topaz granite in Table 6, it isevident that high-temperature (deuteric) alterationis a common feature in many other occurrences oftopaz granite. Such postmagmatic alteration typi-cally involves the complete replacement of mag-matic fluortopaz and lithian mica by fluorite andsericite (Fig. 4H), the sericitization of primaryalbite and K-feldspar, and the destruction ofigneous textures (cl, Manning & Exley 1984). Also,it results in the large-scale redistribution of thealkalis (Li, Na, Rb, Cs), the alkaline earths (Ca,Sr), and other elements, and the modification ofoxygen isotope compositions (c/. Fouillac & Rossil99l ) .

Topaz rhyolites share many of the same chemicalcharacteristics as the "low-P" subtype of topazgranite described here (Figs. 7, ll, l4). Althoughmost topaz rhyolites contain only topaz crystallizedduring postmagmatic vapor-phase alteration(Christiansen et al. 1986), the rhyolitic lavas of theHoneycomb Hills, Utah have fluortopaz as aprimary phenocryst phase (Congdon & Nash l99l).These unusual lavas, of Cenozoic age, are theproduct of a single eruptive cycle. In addition totopaz, the Honeycomb Hills lavas contain quartz,sanidine, albite (An6), fluorsiderophyllite, andfluorite as phenocrysts, and zircon, thorite, colum-bite, fergusonite, and monazite as magmaticaccessories (Congdon & Nash 1991). A whole-rockcomposition for the Honeycomb Hills rhyolite(SiO2 = 73.3 wt.0/0, TiO2 : 0.01 wt.Vo, Al2O3 =l4.0wt.Vo, FeOr : 0.8 fi.V0, MnO = 0.07 wt.Vo,MgO < 0.01 wt.Vo, CaO : 0.42wt.4/0, Na2O =4.59wt.V0, K2O : 4.44wt.V0, PzO, ( 0.01 wt.Vo,F : 0.61 wt.%, Cl = 0.10 wt.Vo, and maximumLi : 344 ppm, Rb : 1960 ppm, Cs : 78 ppm,Nb : 8l ppm, Ta : 44 ppm, W = 34 ppm, andSn = 33 ppm: Congdon & Nash 1991) correspondsclosely to that of typical "low-P" topaz granite(Table 6), and provides corroboration for theexistence of natural F-rich magmas of this charac-

17

13

I

()

q)J(to

TL

17

13

I

5

19 21

@17

17

13

I

1513

0.0 0.2 0.4 0.6 0.8 1.0

THE PLEASANT RIDCE ZINNWALDITE-TOPAZ GRANITE

TABLE 6. SUMMARY OF THE C}I,AFACTERISTIC FEATURES OF THETWO SUBTYPES OF TOPAZ GRANITE

915

Low-P subtypo Hlgh.P eubtype

Age andlocation

Essentialminerals

Accessoryminerals

Majorelementg

Volatiles

Minorand traceelements

Rare earthelements,Y, and Th

Stableisotopes

l-ate Probrozoic (ca.5S0 Ma: Fawwarah, Pennsylvanian/Early Permian (ca' 28o Ma:Kingdom of Saudi Arabia). St Austell, Tregonning, and Meldon in S'W.l-ate Devonian (ca.360 Ma: Pleasant Ridge England; B€auvoir, France).Mount Pleasant, New Brunstrvick, Canada). Early Cretaceous (ca, 130 Ma: Yichun' China).Pennsylvanian/Early Permian (ca. 30$295Ma: Cinovec, Czechoslovakia; Altenbergand Znnwald, Germany).l-ate Cretaceous (ca,70 Ma: Kougarok,Alaska, U.S.A.).

QuarE, K-feldspar, alblte (AnrJ, F-dch topaz, and lithian mica (micacompositions range from lithian siderophyllite through lnnwaldite to lepidolite).

Accessories common to both types include lrcon, niobian rutile,columbile-tantalite, cassiterite, monazite, and sulfides.

Middle Proterozoli (a,1570 Ma:Eurajoki, Finland).

Phosphates are very rare:malnly monazite.

Total REE contents of 150 to 350 ppm;fat to gullwing-shaped REE pattems(l-a^rN = 0.6 to 1.5) wih deep negativeEu anomalies (Eu/Eu' <0.1).High Y (6G160 ppm), and moderate tohigh Th contenb (2G50 ppm).

Late Devonian lca.}70 Ma: East KempMlle,Nova Scotia, Canada).

Abundant phosphates: apatite, monazite,am blygonite-montebrasite.

Total REE contents of less than 10 ppm;slight enrichment of LREE ([a/Lu* >1.5);moderate negative Eu anomaly (Eu/Eu'<0.2).Low Y (<15 ppm), and low Th (<10 PPm)abundances.

Common features of both types are: the very low abundances of_TiO, (<0.1 wt'7d'MgO (<0.2 vrt.7o), and CaO (<0.5 w1.74; the low content of FeO' (<1.0 wt'%); and thehigh concentations of NqO (>4.0 wt Td and high Na/K ratios (1.G2.0' but up to 3.6).

Low PrO, (<0.10 wt.74, higher SiO, High P.O. (>0.4O wt.7d, lower SiO,(>73 wt.7d, lower Alp. (12-14.5wl.yoli (6&73 wt.o/o), higher AlrO. (14'$20 wt.7o);weakly peraluminous (Ay'CNK = 1.0-1.2). strongly poraluminous (A/CNK = 1'2-1.5).

Common leafures are the very high concentrations of F {>Q,4 tttl'"/o, up to 2'5 wt.%), andlhe very low abundances of Cl (<250 ppm) and CO, (<0,2 wt'7d'

Common ieatures are the very high concentralions of the alkalis: U (typically 25G26@ ppm'but up to 7500 ppm), Rb (typically 80G240O ppm, but up to 41@ ppm), and Cs (typically2G15O ppm, but up to 70O ppm); and the lithophile elements: Nb (3e120 ppm), Ta (10'60ppm, but up to 250 ppm), W (1G50 ppm), and Sn (1G3@ ppm). Also the low to very lowabundances of V, Cr, Co, Ni, Cu, Sr, ?, Mo, and Ba.

Whole-rock 6D compositions of both types vary over an exbnded range from -40 to -110%.

Whole-rock 6180 compositions are Whole-rock 6180 compositions arelower and range lrom 7.5 to ca. 10.09L. higher and range trom ca. 10.0 to 13.5%.

Note: 'Ages

of boundaries in Time Scale after Palmer (198t1). Sources of dah are: Bray & Spooner (1983);Chaudhry (1971); Darbyshire & Shepherd (1935); Du Bray ef a/. (1988); ftley & Stone (1982); Haapala (197/);Henderson efal. (1989); Kontak (1990); Manning & Hill (1990); Pollard &Taylor (1991); Puchner (1986); Rossi eta/. (1984; Schwartz (1992); Sheppard (197fl; Stemprok & Sulcek (1969); Stone (1975); Stone & George (1988);Stone & Erley (1983); Stone et a/. (19S8); Taylor (this study); Taylor ef a/. (1985, 1989); Tischendorf & Fdrster(1990); von lftoning & Condliffe (1984); and unpublished data of the author.

9t6 THE CANADIAN MINERALOGIST

ter. Although a volcanic analogue of the "high-P"subtype of topaz granite is not as evident, thecompositional similarities between Macusani glass(Pichavant et ol. 1988) and samples of "high-P"topaz granite are striking (Figs. 13, 14).

CoNcr-usroxs

(l) The small plutons at Pleasanr Ridge andBonny River appear, in their upper portions, to becomposed entirely of alkali feldspar granite withessential qlrartz, K-feldspar, albite (An6-), lithianmica (var. zinnwaldite), and topaz. Thus, they areexamples of subsolvus granite (Tuttle & Bowen1958). As such, they represent one of only a verysmall group of known occurrences in the northernAppalachians of the rare igneous rock type, topozgronite, others being located in the North Zorre atMount Pleasant in New Brunswick (Taylor et ol.1985, 1989), and at East Kemptville in Nova Scotia(Kontak et al. 1988, Kontak 1990).

(2) Two features of these occurrences of topazgranite are worthy of note. First, the availablegeochronological data indicate that graniteemplacement occurred between 370 and 360 Ma.Second, significant zones of hydrothermal tinmineralization are closely related, both spatiallyand genetically, to the bodies of topaz granite.Together, these features indicate that an importantmetallogenic episode, involving the emplacement oftopaz-bearing felsic magmas and the genesis of tindeposits, occurred in the northern Appalachians inthe Late Devonian.

(3) The petrographic and chemical data for thePleasant Ridge granite are consistent with amagmatic origin for the bulk of the albite, topaz,and lithian mica. Textural relations support theearly crystallization of albite and topaz, and thesubsequent inclusion of both minerals in laterlithian mica, quartz, and K-feldspar. Such amineral paragenesis is comparable to the sequenceof crystallization recorded at pressures of I to 2kbar in the experiments of Weidner & Martin(1987).

(4) The topaz granites from the Pleasant Ridgeand Bonny River plutons are fluorine-rich, high-silica types with extremely elevated contents of Li,Rb, Cs, Y, Nb, Ta, Sn, W, and U, whose chemicalcompositions are distinct from those of the mosrhighly fractionated examples of I- and S-typegranite, and also from those of aluminous A-typegranite.

(5) Whole-rock 0r8O values of the Pleasant Ridgetopaz granite are nearly constant (8.1-8.6%0),whereas its 6D values have a broad range, from-66 to -105%0, features indicative of a sourcedominated by metaigneous rocks, and also of theoccurrence of magmatic degassing during emplace-

ment. Similarly large ranges of dD value in topazgranite from other localities suggest that degassingmay be, in general, an integral part of theemplacement of these F-rich felsic magmas. Theoxygen isotope compositions of "high-P" topazgranite (e.g., St. Austell) are higher than those oftheir "low-P" counterparts, exemplified here bythe Pleasant Ridge topaz granite. The higher 6180values (typically greater than l0lo), in combinationwith the more strongly peraluminous character,indicate the presence of an important componentof pelitic metasediment in the source of the"high-P" subtype of topaz granite.

(6) The Pleasant Ridge and Bonny River topazgranites have many features in common with topazgranites from Proterozoic (e.9., Eurajoki, Finland)and Phanerozoic (e.9,, St. Austell, England)lithotectonic settings. However, also it is apparentthat there exists a natural dichotomy within thisunique group of F-rich felsic rocks, such that"low-P" and "high-P" subtypes of topaz granitecan be distinguished. Topaz granites from PleasantRidge, Mount Pleasant, and Eurajoki belong to the"low-P" subtype (P2O5 <0.1 wt..Vo, Al2O3 (14.5wt.Vo, SiO2 >73 wt.9o), and those from St. Austelland a number of other localities (e.9., Yichun,China: Pollard & Taylor 1991) are representativesof the "high-P" subtype (P2O5 >0.4 wt.9o, Al2O3> 14.5 wt.Vo, SiO2 <73 wt.9o). Such a subdivisionreflects the more strongly peraluminous characterof "high-P" topaz granite.

(7) Topaz rhyolites share many of the samechemical features as the "low-P" subtype of topazgranite (typified by the Pleasant Ridge granite)described here. In particular, the rhyolitic lavas ofthe Honeycomb Hills, Utah, which have aphenocryst assemblage of quartz, sanidine, albite,fluortopaz, and fluorsiderophyllite (Congdon &Nash 1991) and an almost identical whole-rockcomposition, may represent the eruptive equivalentof "low-P" topaz granite.

ACKNowLEDGEMENTS

This study was funded by NSERC Grant A1877to R.P. Taylor. Thanks go to Jim Atkinson andGlen Lutes for help with sample collection, to PeterJones, John Loop, Ron Hartree, Russ Calow, GregKennedy, and Will Doherty for their technicalassistance, to Derek Thorkelson and Kyle Durocherfor drafting the diagrams, and to Sheila Thayer forthe word processing. The oxygen isotope analyseswere produced at the U.S. Geological Survey,Menlo Park, under the guidance of Jim O'Neil,and the hydrogen isotope analyses were completedat S.U.R.R.C., East Kilbride, under the supervisionof Tony Fallick. I am grateful to Peter Pollard,Dave Sinclair, Dan Kontak, Malcolm Mcleod, Les

Fyffe, Liz Maclellan, and Steve McCutcheon fortheir continuing interest and support, and to DaveManning and Ilmari Haapala for the provision ofsamples from St. Austell and Eurajoki, respective-ly. Reviews of various versions of this manuscriptby Bernard Bonin, Michel Cuney, and AnthonyWilliams-Jones resulted in substantial improve-ments. This paper is dedicated to the memory ofmy dad, Bill Taylor, who died during the courseof its preparation.

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- & HERvlc, R.L. (1989): Partitioning oflithophile trace elements between H2O and H2O +CO2 fluids and topaz rhyolire melt, Econ. Geol. 84,tt6-134.

WrroNEn, J.R. & MenrrN, R.F. (1987): Phase equilibriaof a fluorine-rich leucogranite from the St. Austellpluton, Cornwall, Geochim. Cosmochim. Acta 51.t59l-1597.

WHALEN, J.B. (1986): A-type granites in New Brunswick.Geol. Surv. Can., Pap. E6-1A,297-300.

THE PLEASANT RIDGE ZINNWALDITE-'IOP AZ CRANITE 921

revised manuscript occeptedCuRRrE, K.L. & Cueppell, B.W. (1987): A-typegranites: geochemical characteristics, discriminationand petrogenesis. Contrib. Minerol. Petrol. 95,407-419.

Received August 8, 1991,March 18, 1992.

APPENDIX: ANALYTICAL PROCEDURES

Representative whole-rock samples were collectedfrom diamond drill-core. Samples were selected fromlong sections of core (>20 m) displaying homogeneiry,and a low density of fractures and hydrothermalalteration or mineralization. Sample preparation wasundertaken in such a way as to produce approximately50 g of a homogeneous powder (<150 mesh), and rominimize contamination during crushing and grinding(e.9., use of diamond-impregnated and agateJinedgrinding media; cl, Hickson & Juras 1986); the levels ofcontamination were monitored regularly using quartzblanks.

A variety of analytical techniques were used:wavelength-dispersion X-ray-fluorescence spectrometryusing duplicate glass discs (XRF/GD: major elemenrsplus V, Cr, Ni, Zn, Rb, Sr, Y, Zr, Nb, and Ba;Ottawa-Carleton Geoscience Centre), produced by thefusion of sample with a Li2BaOT flux; instrumentalneutron-aclivation analysis (INAA: Sc, Co, Sb, Cs, l0REE,Hf , Ta, W, Th, and U; Ecole Polytechnique) using-150 mesh rock powder; inductively coupled plasma-source spectrometry (ICP: Y plus 14 REE: GeoscienceLabs, Onlario Geological Survey), and atomic absorptionspectrometry (AAS: Li, Cu, Mo; Ottawa-CarletonGeoscience Centre) after a stepwise acid digestion(HF-HCIO4-HCI); titration (WCT: Omawa-CarleronGeoscience Centre) for the determination of ferrous iron(Wilson's method); ion-selective electrodes (ISE: Bondar-Clegg, Ottawa) for F and Cl; and combustion - infrareddetection (C/IRD: Geological Survey of Canada) fortotal H2O and CO2. Wherever possible, the analysis ofelements contained in minerals that are resistant to acidattack (e.9., cassiterite, tantalite: cl. Potts 1987) wasundertaken in such a way as to avoid the need for aciddigestion; for example, the analysis of samples for tinwas performed by wavelength-dispersion X-ray-fluores-cence spectrometry using 5-g pressed-powder pellets(XRF,zPP: Bondar-Clegg, Ottawa). Dara for inrernation-al reference standard materials used during this study,together with the accepted values for these standards (c/.Abbey 1983), are contained in Table A1.

Some of the elements sought in this study are subjectto significant overlap interferences in instrumentalanalysis. For example, with X-ray-fluorescence analysis,a first-order interference involving the overlap of theYKo peak by Rbfd occurs (Potts 1987). Consequently,other analytical techniques were utilized (for example,ICP spectrometry for Y) for such elements.

Oxygen and hydrogen isotope compositions ofwhole-rock samples were determined using standardprocedures. BrF5 (Clayton & Mayeda 1963) and ClF3(Borthwick & Harmon 1982) were used for the extractionof oxygen, which was subsequently converted to CO2 foranalysis. Hydrogen was released from rock powdermelted in vacuo in a Pt crucible, quantitatively collected

TABLE AI. GEOCHETICAL @UFOAMONS OF INTERMTIONAL REFEnEIiICESTANDARD TATEHALS ANALVZEI' DURINC T}09 STUOY

RGM-I JSCS0! 6RF/GD)Thsshrdy REFI(N-l s)

4?usos[ ERF/GD)Tlds etdy REF1(N-13)

Majd etmst @@ntrdoE h wElght pe@rt

t @80036N9,41

sto,no"Ato"too"MnOMsoCqONorot<ro

Rb

BaZnzNb

73.01 (o.a)0.28 (0.02)13.66 (0.06)1.84 (0.02)0.04 (0.00)0.2s (0.r1)1.17 (0.01)4.12 .0.12)4.31 (0.02)0.03 (0.00

155 (3.3)104 (2.0)846 (33)31 (102 t 4 l 45 (2)

60.52 (0.31)0.50 (0.03)15.g/ (0.07)469 (0.090.m (0.0o)0.7/ (0.08)1.9r (0.02)415 (0.14)4.47 (O.O2l0.r3 (0.01)

172 16l4e6 (14)r€36 (48)e2 (6)326 (48 0 )

T|@ dmad abudtrG In parb per millon

73.47o.2713.801.890.04o.2a1.154.124,350,05?

@.2.0./€15.,1{)2.690.030.751.964.064.480 . 1 3

1704ao19@u3@t3

MRgl &SY.Z@RMPThls study REF1(N=12)

sY-?ccRMP (C/|FD)Thls strdy REFI(N=5)

8.67 (0.20) 8.633.?3 (0.21) 3.62

For@ lmn, lto qnd CO, eMnbdore h weighl perertIto 0.4i1co, 0.15

0.41!0.15

BE-t\Uon-Mc 0NAA)Thbdldy REF20!-12)

BHVOIrusGSil 0CP)T'rb €hrdy-(N-16)

as 0.62 (0.1s)sc 24.4 (051)Co 68.9 (1,3)

ta 81.4 (12)Cs 161:7 14.4,P r EM 88.9 (25)sm 122(oA)Eu 3.8 (0.13)O d B'Ib

r.34 (0.08)Dy d.7 (0.69)l-l't 1.16 (0.11)& mTm 028 (0.6)Yb 1.73 (0.06)Lu 026 (0.03)sb 029 (0.6)Hr 5.4 (0.1e)Ta 6.0 (02)w 31.8 (2.1)Th 10.9 (0.4)u 2.4 (0.14)

Tte deEst abu|da|@ h parts per milbn0 . 8 m -2 E -61 mE 262 (0.8) A282 16.6 (03) 18.7162 38.8 (0.8) 41d 5.88 (0.16) 5.670 25.3 (0.8) 2412 0.38 (021) 6.13.0 2.15 (0.10) 2.0ry 6.3s (0.14 71.3 0.98 (0.04) 1.0tr 5.4{, (020) 4.8F 1.(a(0.Gr) 0.94ru 264(0.11) 2ru 0.3 (0.02) 0.311.8 2.02 (0.06) 2.1o24 029 (0.01) 0.eD M5 . 4 m -5 . 6 m *2 9 m -1 1 mZ 4 m -

Nob. tu @rcenldoB do tu n1@ vdu6 ol a gilon nt@r d analys (N) d lho duo hFreffi@ b h vdue d one l3lM d*idbn. AbrdidioB e: XRF/CD = X-Ey iuoro@msFdroreuy dng gb tui S - Atd ab$eftn spercmdry: lf.l$ = lNm€nld mtunadidion dy$; ICP - lddtuoly 6upld pM lpdomdry: m = ml dtzd: or: rcl rwid.REF1 = trby (19s); REFz - tutuj! 118)i REF3 - Gbdnry eld 119&). ne ICP &b war€po'd@6d by tho Onho G@lqM Surury G@iee bdod6 (Doh6r1y. p€8. @m.).

as H2O, and reacted with a uranium furnace at T= 800'Cto leld hydrogen for isotopic analysis (Bigeleisen er a/.1952, Friedman 1953). All of the isotopic data arereported, in /06 relative to the Standard Mean OceanWater (SMOW) reference standard (O'Neil 1986). Duringthe course of this study, NBS 28 yielded a 6180 of 9.6%0and NBS 30, a 6D of -65o/no.

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