98s

Th.e C anadian M ine rala g i s tVol. 33, pp. 985-1010 (1995)

A STUDY OF FELDSPAR PHASES IN THE HIGH.SILICA,HIGH.LEVEL ACKLEY GRANITE, SOUTHEASTERN NEWFOUNDLAND

DANIELJ. KONTAK

Nwa Scotia Departurent of Natural Resources, P.O. Box 698, Halifax, Nova Scotin B3J 279

ASSTRACT

Mineralogical and chemical studies of feldspar phases within ttre highly evolve4 volatile-rich, polyphase Devono-Carboniferous Ackley Granite, southeastem Newfoundland reveal a complex physicochemical history. The two oldest units(X90 Ma) are characterized by triclinic alkali feldspar (0.8 < A < 0.93), with coarse film perthite and zoned sodic plagioclase(Anrz+J. In contrast, younger units (374 Ma) are characterized by orthoclase, in places characterized by film perfrite, withonly local, incipient development of triclinic alkali feldspar. Plagioclase compositions within the younger units show acontinuum from An* to end-member albite. Albite is considered to have formed by interaction ofprimary sodic plagioclasewitl late-stage, higb-temperature (>450'C), 56[i1s magmafic fluids generated dwing resurgent boiling. Whereas ordered,6iglinis alkrli feldspar within the older units is consistent with the presence of deformational t€xfires, the occurrence oforthoclase within the younger units saturated with volatiles is consistent with the widespread development of postmagmaticalbite formed within the stabiJity field of orthoclase (i.e., >450oC). Subsequent cooling through the microcline-stable regimewas rapid owing to the shallow level of emplacement" ard led to the development of "stranded" orthoclase. Bulk compositionsof tlre alkali feldspars reflect decreasing mol.Vo Or toward more evolved compositions, consistent with phase equifibria(i.e., indicates lower T of formation), and togetler with coexisting plagioclase phases, indicate minimum T of 55G-700'C forcrystallization. In contras! subsolidus re-equilibration continued to 350" to 400'C based on the composition of coexisting albitelamellae and host orthoclase. Trace-element chemistry ofthe alkali feldspar samples reflec8 progressive geochemical evolutionof mineral phases, in parallel with tle whole-rock geochemistry. However, local fluctuations of elemental concentrations(to several l@ ppm) of Ba, Rb and Sr are considered to reflect modification of the melt chemistry by local saturation in fluid.Although the itEE contents and chondrite-normalized patterns are generally typical of alkali feldspar, the emichment of somesamples in IIREE [Gb/Lu)N less than 1] is considered to reflect the influence of exsolved fluids on,REE partitioning such thatalkali feldspar with similar bulk compositions have highly vmiable concentrations of HREE.

Keywords: alkali feldspar, Al-Si order, geochemistry, granite, Ackley Granite, Newfoundland.

Somvlann

Une 6tude min6ralogique et chimique des felclspaths dans le massif granitique relativement 6volud et enrichi en phasevolatile de AclJey, dans le secteur sud-est de Terreneuve, d'ige ddvono-carbonifbre, r6vdle une dvolution physico-chimiquecompliqu6e. l,es deux unit6s les plus anciennes G390 Ma) possbdent un feldspath alcalin triclinique (0.8 < A < 0.93), danslequel la texture perthitique est grossibre et en pellicules, et un plagioclase sodique zon6 (Anr"-rd. Par contre, les phases plustardives (374 Ma) contiennent de I'orthose, dont la texture perthitique est en pellicules, avec d6veloppement local seulementd'un feldspar friclinique. la composition du plagioclase coexistant ddfinit un continuum entre An4 et de l'albite pure. L'albiter6sulterait de llnteraction du plagioclase sodique primaire avec lme saumure orthomagmatique i une temp€rature 6lev6e(>450'C) suite son exsolution du magma- Tandis que la pr6sence d'un feldspath alcalin 6lsliniqus ordonn6 dans les phasespr&oces concorde avec les signes de d6formation, la pr6sence d'orthose dans les phases tardives saturdes en phase volatileconcorde avec le d6veloppement r6pandu d'albite postmagmatique i l'int6rieur du champ de stabilit€ de I'orthose, c'est-i-dire audessus de 450oC. Un refroidissement rapide par la suite I travers le champ de stabilit6 du microcline a conservd I'orthose sousforme m6tastable. La composition globale des 6chantillons de feldspath alcalin indique une diminution dans la proportion d'Orvers les compositions de plus en plus 6volu6es, selon les pr6dictions des 6tudes de l'dquilibre des phases, indiquant une plusfaible temp6rature de formation. L'6quilibre avec le plagioclase coexistant indique une tempdrafire minimale de cristallisationentre 550o et 700"C. Par contre, le rd-6quilibrage subsolidus a continu6 jusqu'i 350' ou 4(EoC, d'ap€s la composition deslamelles d'exsolution d'albite coexistantes avec I'orthose, l,a g6ochimie des 6chantillons de feldspath alcalin change selon1'6volution progressive des phases min6rales, en parallOle avec la g6ochimie des roches totales. Toutefois, les flucfirations enconcentrations de Ba, Rb, et Sr (jusqu'a plusieurs centaines de ppm) r6sulteraient d'une modification de la composition dumapa suite i une saturation locale en phase fluide. Quoique les teneurs en teres rares et les spectre.g normalisds en fonctionde teneurs chondritiques sont en gdneral typiques de feldspath alsalin, l'snrighissement des terres rares lourdes dans certains6chantillons [(Tb/Lu)r < 1] t6moignerait de I'influence d'une phase volatile exsolv6e sur le coeffrcient de dishibution des terresrares, de sorte que les dchantillons de feldspath alcalin de composition globale i peu prbs constante monaent une grandevariabilitf dans leurs teneurs.

(Traduit par la R6daction)Mots-cl6s: feldspath alcalin, degr6 d'ordre Al-Si, g6ochimie, granite, Ackley, Tene-Neuve.

TIIE CANADIAN MINERALOGIST

Inrnopuc"noN

The Ackley Granite, southeastern Newfoundland(Fig. 1), is a post-tectonic, predominantly high-silica

biotite t hornblende granite complex that representsthe product of crystallization of a zoned,2700-ffibody of magma (Dickson 1983, Whalen 1983, Tuachet al.1986, Tuach 1987). Geochemical gradients from

MOLLYGUA.JECKMOUNTSYLVESTER

KEPENKECK

MOUNTSYLVESTER

z453?'aJ4s

/'| 4168

GANDER

HUNGRY GRovE 4 Oeg, 'Tu' o*'\

?

a' z O r r sAVALONZONE

MEIA

ioNi F6."^*qffi ,.i( -

fV;:t*-o'*'i'; ,,3ryKosKAEcoDDE- /l=r".;-.# '':rx

a

ffi

l".i: ilE.-,h_'l

@

/ a56:t \aa,,l <.,'\\. OOO+- \- V"'cJ.- \

ogen e E '\

aros or*a,.

:*:"ft,rt"-r."^)&ti;l^';*'-^68 8Ol

zslo eosan 1 q

kmRENCONTREI.AKE

SAGE POND

plogioc lose somple

olko l i fe ldspor somple

foul t

roof pendont

Flc. 1. Location of the Ackley Granite in southeastern Newfoundland at the boundary between the Gander and Avalontectonic zones. Map of the Ackley Granite shows the outline of tle different units (after Tuach 1987) and sample locationsfor the study of alkali feldspar and plagioclase.

sla osoro566

6aoo 4508 ,

t 1 @ tkm

FELDSPARS IN TTIE ACKT.EY GRANIIE. NEWFOIJNDI.AND 987

north to south culainate in the development of highlyevolved granites spatially associated with extensivezones of greisen with Sn-W and Mo mineralization(Whalen 1980, 1983, Tuach 1984, 1987). Tuach (1987)has suggested affinities with the A-type of granite(Whalen et a\.1987).

The geometry of the pluton and excellent density ofsampling (i.e., one sample per 4 ffi, 357 stationlocalities), in addition to the presence of compositionalzonation, make the Ackley Granite an excellent site forstudies of igneous petrology. Whereas whole-rockchemistry (Diclison 1983, Whalen L983, Tuach et al.1986) and some isotopic (O, Sr; Ttach et al. 1985,Tuach 1987) work has been done, the mineralogicalphases remain essentially unstudied. This studyfocuses on the chemical and physical variations in thefeldspars, and the insight that they provide concemingthe magmatic trends and subsolidus adjustraents thathave affected the Ackley Granite.

GsolocrcAl SBrrnIc AND SUBDIVIsIoNsOFTHEACKLEY GNANTTB

The Ackley Granite is one of several Devoniangranitoid bodies innuded into the Gander Znne andwestern side of the Avalon Tane rn southeasternNew'foundland (Frg. 1). In the Gander Zone, thecountry rocks include mid-Ordovician and oldermetamorphosed and deformed sedimentary rocls ofthe Gander Group and volcano-sedimentary rocksof the Baie d'Espoir Group. In the Avalon 7ane, thecountry rocks include Late Precambrian volcanic andsedimentary rocks of the Long Harbour and Love Covegroups, sedimentary rocks of the Late PrecambrianMusgravetown Group, and Cambrian or older YoungsCove Group (Williams 1971). Although mid-Paleozoicdeformation and metamorphism were broadly contem-poraneous (ca.360400 Ma) within, respectively, boththe westem Avalon Zone and the eastem Gander Zone,they were more intense and of higher grade in thelatter. The Ackley Granite is largely undeformed and isinfened to be younger than the regional deformationand metamorphism. Undeformed, fossiliferous LateDevonian sedimentary rocks of the Cinq Isles andGreat Baie de L'Eau formations outcrop south of theAckley Granite.

The Ackley Granite was originally subdivided byDickson (1983) into 1s1 major lithological units on thebasis of texoral and mineralogical features, with con-tacts interpreted to be gradational. Dickson recognizedthe granite as being epizonal, with a narrow cotrtactaureole containing cordierite and andalusite.Subsequently, Tuach (1987), Ttac,h et al. (1986) andKontak et al. (1988) moffied the subdivisions of thegranite on the basis of more detailed petrographic,isotopic and geochronological studies (Fig. 1).4oArFeAr dating indicates that the intrusion ispolyphase, with distinct magmatic events recognized at

>390 Ma (Koskaecodde and Mollyguajeck units),378-374 Ma (Kepenkeck - Mount Sylvester, Meta,Hungry Grove, Sage Pond and Rencontre Lake units)and 355 Ma (Tolt uni$. Mineralization (Sn, W, Mo),concentrated along the southem marym of the granite,is dated at372 Ma (Kontak er aL 1988), thus coincidentwith the 374Ma plutonic event.

MNRALocY oF TIIE ACKLBY GneNrrs

The granite (Iable 1) contains biotite throughout,with both hornblende and muscovite of restricteddistribution, and accessory titanite, allanite, apatite,zircon and magnetite. In general, the Koskaecodde unitis a coarse-grained, K-feldspar-phyric biotite-horn-blende granodiorite, whereas the Mollyguajeck unit isa medium- to coarse-grained, K-feldspar-phyric biotitemonzogranite with minor hornblende granodiorite. Theadjacent Mount Sylvester - Kepenkeck unit is amedium- to coarse-grained, biotite t muscovite,K-feldspar-phyric monzogranite. The bulk of theAckley Granite underlies the area to the south ofthe Hermitage Bay Fault and is dominated by medium-to coarse-grained, equigranular to K-feldspar-megacqystic biotite monzogranite to alkali feldspargranite. There is a general southward decrease in gxainsize and increasing abundance of miarolitic cavities inthe southern part of the Hungry Grove unit" culminat-ing in highly variable textures in the Rencontre Lakeand Sage Pond units. These two units contain fine- tomedium-grained, equigranular to K-feldspar-megacrystic biotite monzogranite and alkali feldspargranite, with some pegmatitic segregations, miaroliticcavities, breccias and granophyre. These features, inconjunction rvith the general textural and chemicalzonation, and the presence of roof pendants, reflectclose proximity to the original top of the magrnacharnber. Of particular relevance to this study are theoccrurence of deformational fabrics in quartz, plagio-clase and biotite, and the dominance of grid-twinnedmicrocline in the two older units; such twinning israrely seen in the rest of the Ackley Grardte.

T\e alkalifeldsparin all units of the Ackley Graniteconsists of microcline and ortloclase perthite, andthere is a notable reddening of the feldspar grainstoward the southem part of the intrusive complex. Inthe older units, coarse flame and patch perthitedominate, almost to the exclusion of other types. In theyounger unit, the most common types of perthite arefla-e and patch, and these generally increase inproportion toward the southern part of the intrusivecomplex. Film perthite occurs within isolated areas orbetween flame and patch types; this phenomenon ismosfly confined to less evolved sarnples and is rarelyseen in soutlern, most evolved part of the AckleyGranite.

The distribution of perthite within the AckleyGranite shows a general relationship with geochemical

988 THB CANADIAN MINERALOGIST

TABLE 1. SUMMARY OF RELEVANT FEATURES FOR UNITS OF THE ACKLEY GRANITE

&remures

Mlar Grarlciap Pegn Brec AplPhase

Koskaecodole

MoUyguajeck

Mt. SylYestsr

Icpenksck

Meta

Toll 355 @t

3HrmgryGmve 367@9

Rencode Irke

Sage Pond

IAge (Ma) Gratn Slze aMneralogr

>410 (Bt) coaf,se Qz-P1-Kf-BttIIb

>390 (Bt) coan}e Qz-Pl-Kf-BttI{b

374./l,t') mediumio @-Pl-Kf-BttMsctxlrsg

c@rse Qz-Pl-Kf-BtjMs

@arse Qz-Pl-Kf-BttHb

medium to Qz-PLKf-Btc&trse

coarse to Qz-Pl-Kf-Btfins

medimro Qz-Pl-Kf-BttI{bfine

medium b Qz-n-Kf-BtfiDe

367 @037E (Ms)

D€tu Rap

372 (Ms)368 (80

374 (Hb)372 (80374 (Ms)

rAge: Toal fusion €ArPAr agas (Konrak et al. l9B8); Ms from Rencontre kke and Saga Pond represn! mineralized greisens.zl'exores: Summarized fron Tuach (19E7), Dickon (1983) and orrn obs€rvatiom.3}Iugry Grwe: Texnres vary ftom north b soulh srch thal more evolved facies (texnrally) occu in the sou6.'Mineralogy: Kf here indicaes dominanily microcline pertlite in older phasesAbbreviatioDs: Defu = deftrmation; nap = raplCvi; Miar : miamlitio; pegm : pegmatitici Breo : breccia (fluialized): Apl = aplitic; gran =gaDophyric; *p = gaphic

evolution such that more evolved units are chaxacter-ized by volumenically more perthite that is coarser andof a gxeater vaiety (i.e., several types). This tend issimilar to observations made by Weiss & Troll (1989)with respect to perthite within the zoned BallachulishComplex of Scotland. Coarsening of the pertlitictexture in parallel with geochemical evolution is, assuggested by Parsons (1978), consistent with anincreasingly more active role of fluids in the moreevolved units.

In Ught of the following section on geochemisty, itis noted that rare micro-inclusions of biotite and quartzaxe encountered.

Aner,rncar Trcrnueurs

High-quality separates of K-feldspar were preparedby pulveri"ing and sieving (-20 to +65 mesh) foUowedby hand picking utrder a binocular microscope andcleaning with deionized water and drying. The sepa-rates were prepared for chemical analysis and X-raydiffraction Q(RD) by cmshing to -200 mesh. TheX-ray work was done at Memorial University using aPhilips diffractometer and the following operatingconditions: CuKa radiation, goniometer speed of1' 2Qlminute, chaxt recorder speed of I cm/minule. A

total of 61 separates wsre scrmned over the intervals10o to 60o 2e, with several diffractograms prepared foreach sample to check for homogeneity. Where triclinicdomains arc present, the apparent degree of ootriclin-

icity" (A) was calculated using the relationship ofGoldsmith & Laves (1954): A,= 12.5ld(131) - dQTDl.

Major and minor element chemistry of theK-feldspar separates was detennined at MemorialUniversity using atomic absorption spectrometry(AAS) and the methods of Langhyr & Paus (1968),except for PrO5, which utilized a modification of thelechnique ofShapiro & Brannock (1962). In all cases,total iron is reported as FqO3. Concenfrations of thetrace elements, including the rare-earth elements(REE), were determined using inductively coupledplasma - mass $pecftometry GCP-MS) at MemorialUniversity using procedures outlined in Jenner et al.(1990). In order to determine the potential variationthat may occur as a result of heterogeneity with respectto sample size in perthitic feldspm, two separates wereanalyzed several times (NS-85-18 and NS-85-44A).For sample NS-85-18, grains (z = 1 to 18) of differingsize (<1 mm to 5 mm) and total weight (0.093 to0.944 $ were analyzed, whereas for sampleNS-85-44A, powdered material of O.0204 to 1.0023 gwas analyzed. Analytical results (Iable 2, Fig. 2)

FELDSPARS IN TTIB ACKI.EY GRANIIE. NEWFOTJNDLAND 989

. . 1f

C n

T

qa

F tI

Y

Flc. 2. Chondrite-nonnalized plots ofREE data for K-feldsparsamples used as intralaboratory standards during thecourse ofthe present sfirdy. Analyses by ICP-MS (Jenneret al. 1990). Where a small box deviates from the largerbox, there is but a single sample represented. In the casesof sample NS-{5-18, the same analytical data apply in allcases.

indicate good reproducibility of the data" with thevalues ofrelative $tandard deviation (7o RSD) relatedto the absolute concentration of the element.

Electron-microprobe analyses of plagioclase andK-feldspar were carried out at Memorial Universityusing a three-spectometer JEOL IXA-50A automatedelechon microprobe with Krisel programming and atDalhousie University, Halifax, 6ing a IEOL 733Superprobe. At Memorial University, natural silicateminerals were used as standards, and operatingconditions were as follows: accelerating voltage 15 kV,sample current 0.02 pA, and a beam diameter of 5 pm.The data were corrected on-line by the Bence & Albee(1968) metlod. At Dalhousie University, an energy-dispersion spectrometer (EDS) was used with anaccelerating voltage of 15 kV, abeam current of 10 nA,

IABLE 2. GEOCHEMICAL DATA FOR K.FELDSPAR STANDARDS

a counting time of 40 seconds and a beam diameler of1-3 pm. Data were processed using the TracorNorthern ZAF manix-correction progftrm. At bothinstitutions, accuracy and precision were modtored byanalyzing well-characterized standards.

Srr.ucruner, Sruprs oF ALKAU FE-DSPAR

The degree of AVSi order in the nlkali feldsparphases has been determined by a combination of XRDstudies (n = 6l) and petrographic observations(z = 110); results are summarized in Figure 3. Asshown in the studies of Jirdnek (1982) and Cemf &Chapman (1984, 1986), inferences on the presence oftriclinic domains can be made from the shape of the(131) reflection; a similar methodology is used herein.

In Figure 3A,, the distribution of the dominant poly-morph is shown, whereas in Figure 38, representativediffractograms are presented. Microcline is ttredominant phase in the older tnits (Kosl<aecodde andMotyguejeck), whereas orthoclase is present in the restof the Ackley Granite (Frg. 3A). For two of the fivesamples studied by )(RD from the Koskaecodde phase,only microcline is present (A = 0.80, 0.85). Theremaining three samples from this phase consist ofvariable mixtures of microcline and orthoclase, withapparent A values of 0.63, 0.65 and 0.70. Severalsamples from along the eastern part of this unitproximal to its contact with the Hungry Groveunit contain orthoclase only (snmFle 492 in Fig. 3B).For the Mollyguajeck unit" samples studied by XRDindicate that microcline (A = 0.87 + 0.05, n = 4)dominates. For the samples examined peuographically,either microcline, orthoclase, or mixtures of the twooccur, with orthoclase dominating along the northernpart of this phase near the contact with the youngerMount Sylvester unit.

T\e Mount Sylvester - Kepenkeck wtit is dominatedby orthoclasg from both petographic observations andthe eight diffractograms. The broadening of the (131)peak in the diffractogram for sample 510 @9. 3B),indicative o1 u 6islinic component, is an exception forsamples from this unit, as other samples do not exhibitthis phenomenon.

Ortloclase dominates in the Meta anit, wth mlnoramounts of riclinic phases manifested by (l) a slightbroadening of tle (131) peak, and (2) the weakdevelopment of microcline twinning in petrographicstudies. Where peaks could be distinguished in alkalifeldspar phases, apparent A values of 0.60 t 0.06(n = 3) are calculated. All samples studied by )(RDfrom the Tolt anit consist oforthoclase; however, a fewsamples oriented along a northeastern trend areobserved to contain variable proportions of microclinein rhin section. Of the 12 samples studied by XRD, allhave diffracto$ams typical of orthoclase, with onlyvery slight broadening of the (131) peak observed(apparent A value of 0.71 in one sample).

Ns-85-aaA (n=8)

[ n IU N u n un - [ [ [ [ tr

n oU N

U

NS{tlE (n = 9)

lo % RSD

NS{5-44A (u : E)

lo %RSD

Rb DNSr 5.6Ba l8.lPb 6.3Cs 16.0Th 0.,v o,74Y N DZ{ 4.8

0.8 14.5 6.7 0.6 9.8a8 10.5 N 7.4 2,6

0.5 9.5 196 4.r 2.13.1 t1.t l?90 67.0 3,72,2 36.1 48.8 8.9 r8.2t.4 9.1 2.8 0.(n 3.70.3 81.0 1.10 0.34 30.90.23 31.9 0.53 0.()? t3.2

0.r 0.43 493.4 1t.6 8.3 2.6 31.5

ND = mtdeffil@d. Comltratiotlppm.

990 THE CANADIAN MINERALOGIST

aooc

orthoclose .t---'P "

microcrine -6":5f.S5!(,

orthoclose ,$;l;

orthoclqse ) microcline n{'i jpti, t "

'

l^1"" !ri.."o 9microcline r orthoclose /.t"" \ lr" .orthociose = microctine / l r o/ j-

.-r."tt*i,y'- I"ry,l " ")o - i ' l ^ o

Iil

o r s l l

rllll]\llil/lj I

i' \, l

-__.,

ll r"rrlw\

/ \ /!\t"l\

km

Frc. 3. A. Srrmmary of XRDand petrographic observa-tions of samples from theAckley Granite. Note thatthe two older units aredominated by a triclinicK-feldspar, whereas theyounger units mosfly con-tain monoclinic K-feldspar.B. Diffoactogram traces ofrepresentative samples ofK-feldspar samples fromdifferent units of the AckleyGranite. The (i@ lineslabeled for two of thesamples, a monoclinic andtriclinic phase, pertaia toK-feldsoar.

A frTrh/ o o ^ . o . / c - (

i,/i" ".'i'": 9-., .r"'" t$:;.".\.""{* ":,o " o " i 0 oo o

o o o \ : A ^

o od"r 'a-.

.n"o o \t o7 1o

I *til.." " *t/ "'"\i" :./

-t?;i+o -o- o7 \_9- o 15

( ! 3 r )

/t. i l11 ili l t l

ll/\l \'"/ ' lr 1

sirte 20 degrees (Cu Ka)

FELDSPARS IN TTM ACKLEY GRANTTE. NEWFOTJNDI.AND 991

T\e Hungry Grove unit is dominated by orthoclase,except in a few ca$es, where partial to complete domi-nance of triclinic alkali feldspar occurs @g. 3A).However, from north to south there is a clear changefrom a sharp (131) reflection, indicative of a singlemonoclinic phase, to the broad (131) reflectionscharacteristic of mixed monoclinic - triclinic domains@ig. 3B, compare samples 456 and 695).

Tlte Rencontre Iake unit is similar to the HungryGrove. in that orthoclase dominates. However. theslight broadening of the (131) peak noted in two ofthe four diffractograms obtained indicate trace amountsof triclinic domains (sample 622 in Fig. 3B).Petrographic observations of an additional 18 samplesindicate that orthoclase occurs, apparently withoutmicrocline.

In the Sage Pond unit, no samples of alkali feldsparseparates have been studied by )(RD because the fine-grained nature ofthe granite precluded preparation ofclean separates. However, petrographic study of sixsamples indicates that orthoclase is here also thedominant polymorph.

In summary, the distribution of alkali feldspar poly-morphs in the Ackley Granite appears to reflect the agedifferences of the units. The two oldest units, theKoskaecodde and Mollyguajeck, are dominated bytriclinic feldspar, whereas the younger units aredominated by orthoclase, almost to the exclusion ofmicrocline. However, there is local development oftriclinic domains, but more common is the mixtureof the two phaseso with the monoclinic varietydominant.

GnocnsmsrRv oF Tr{E ALKALT Fb-mpan

lulk separates of alkali feldspar (n = 6l) have beenanalyzed for major, minor and trace elements. The data

are summarized diagrammatically; the complete data-set is available from the Depository of UnpublishedData, CISTI, National Research Council, OttawaoOntario KlA 0S2.

M aj or- e lemcnt chemistry

Representative bulk compositions of perthiticK-feldspar from the different units are presented inTable 3, and the bulk compositions are summarizedgraphically in Figures 4 (Or-AFAn plot) and 5(mol.Vo Or). Compositional variability within a singleunit is limited, with variation typically <lFli mol.VoOr. In the two older units (Koskaecodde andMollyguajeck), the alkali feldspar has the mostpotassic bulk composition (Orzo*o), except for threesamples of Orrr-u, composition. The more potassicsamples of these two units are located toward themargins of the units.

Samples from the younger units of the AckleyGranite fall in the range Or52-7so but for individual unitsthe spread is much smaller. For example, in the Metaunit, the perthite has a range of Or6s-67, whereas in theHungry Grove unit, it is 0155-63, except for a singlesample of &zo. h addition, there is a systematicchange within the Hungry Grove unit, with lesspotassic compositions in the southern part. In confiast,a pattem of elevated Or content occurs in tlte Tolt unittoward the southeast. Other oxides are generally low,with maximum values of 0.05 wt.Io P2O 5, 0I8Vo TiO2,0.027oMnO ard0.09Vo MgO, whereas up to 0.5 wt.7oFe2O3 is noted. The ferromagnesian elements areattributed to trace amounts of biotite inclusions thatare zonally arr"anged along the inner margins of alkalifeldspar crystals. The low P contents confrast withvalues of up to 2 wt.Io P2O5 in alkali feldspar reportedin peraluminous granite suites (Kontak et al. 199Lb,

TABLE 3. REPRESENTATIVE COMPOSITIONS OF K-FELDSPAR SEPARATES,ACKLEY GRANITE

SAMPLE 9l W2 096 510 188 545 413 6DUNIT Koqk Mollv Ken Mrsvl Tolt HuGr Meta ReDL

SiO2wt%oTio,Alro3Fqo.MnOMgoCaONaro&o, 9.29 98.40 S8.63 1m-20 9.95 9.9 100.9 1m.01

68.60 63.80 63.60 6.20 65.30 6.m 6.m 66.000.06 0.16 0.04 0.08 0.04 0.06 0.06 0.04

16.40 18.10 19.70 18.40 l8.m 18.70 18.60 18.600.n 0.65 0.50 0.4E 0.17 0.38 0.m 0.D0.01 0.01 0.a 0.a2 0.01 0.01 0.01 0.000.06 0.m O.ffi 0.10 0.a 0.m 0.00 0.030.16 0.@ 0.48 0.48 0.32 0.26 0.16 0.322.9 1.% 4.25 3.54 3.47 4.44 4.14 4.22

u..32 12.C2 936 10,90 11.72 9.94 10.67 10.50

END-MEMBER PROPORTIONS (Mol %)

Or 73.2 78.8 58.7 65.3 67.9 58.8 62.4 61.'tAb 2s.9 1E.2 3E.9 32.2 30.06 39.9 36.8 37.3An 0.S 3.'l 2.4 2.4 1.6 1,.3 0.8 1.6

Kosk: Kosbecoddei MoUy: Molyguajech IGp: Kepentpck; MtSyl: Mt. SylvesEri Hurcr: I{mgryGrove: RenL: Rencontre IJke

992 TTIE CANADIAN MINERALOGIST

Flc. 4. Triangular plots and histograms compositions ofplagioclase and bulk alkali feldspar for each unit ofthe AclJey Granite.

London et al. 1990, London 1992),but are in keepingwith the A-type nature of the Ackley Granite (Tuachet aL 1986). Although Fe can readily substitute into thefeldspar stucture (Smith i974), high Fe contents arecorrelated with a distinct reddening or turbidify, whichsuggests frace amounts of hematite either as free grainsor as a phase within fluid inclusions (e.g., Martin &Lalonde 1979, Lalonde & Maxtin 1983,Worden et al.1990). There is no systematic disftibution of the Fecontents that might reflect, for example, a build-up offluid in the southem, more evolved parts of the granite.

Trac e - element chernistry

A detailed examination of the trace-element contentof alkali feldspar is given by Smith (1974, 1983),Smith & Brown (1988) and Cernf et at. (1985). T\issection focuses upon the relative enrichment anddepletion of elements within the Ackley Granite.Because of the potential effect of contaminants on thetace-element chemistry, detailed petrographic studiesaddressed the presence of accessory mineral phases.On the basis of microscopic observations and back-

KOSKAECODDE MOLLGUAJECK KEPENKECK

N=28

AbHUNGRY

ROVE

60 s loomol % Ab

80 too

RENCONTRE SAGE

FELDSPARS IN TIIE ACKT.EY GRANITE, NEWFOT]NDI.AND

m

Grove unit. The trace elements follow general trendsdefined by whole-rock analyses in the Ackley Granite,as outlined by previous investigators.

Barium is most strongly enriched in samples fromthe Koskaecodde unit and the central part of the MountSylvester - Kepenkeck unit with values of 1500 to4000 ppm (Fig. 64). A fiend ofdepletion is seen frommafic to felsic rocks in the Rencontre Lake unit, andthere is a crude decrease from nortl (<2800 ppm) tosouth (400 ppm) in the Hungry Grove unit (Ftg. 9).Comparison of the histograms for Ba and Sr(Figs. 64, B) and a binary plot @g. 9) indicate thatthese fwo elements are geochemically coupled. InFigure 88, a plot of Ba verszs Rb/Sr, a single trend isdefined by the data with a strong inverse correlation.

The distribution of strontium is generally a converseof the Rb trends, with the highest values for therelatively primitive samples; the overall range is from40 to ca.600 ppm (Fig. 6B). The highest concentra-tions are found in samples ftom the cenfral part of theMount Sylvester - Kepenkeck unit, which containmore Sr than hornblende-bearing samples from theKoskaecodde and Rencontre Lake units, and aregenerally enriched three to four times over the rest ofthe samples. tn the Hungry Grove unit, there is a trendof decreasing Sr toward the southem, more evolvedpart ofthis unit (Fig. 9).

Rubidium shows a large overall range from less than200 ppm to greater than 700 ppm, with most sampleshaving befween 300 and 500 ppm (Fig. 6C). Severalunits display Rb enrichment or depletion that is inaccordance with whole-rock fractionation trends withinthe granite (Iuach 1987), notably the Mount Sylvester- Kepenkeck unit, the Rencontre Lake unit and theHungry Grove uniq the latter unit shows increasing Rbcontents toward the south @gs. 8A, 9).

T\e Rb/Sr ratro (Fig. 6D) of the samples magnifiesthe trends for Rb and Sr such that clear patlems emergefor the data from the Mount Sylvester - Kepenkeck,Hungry Grove and Rencontre Lake units, as the Rb/Srratio increases toward more fractionated parts of theunits (eft to right in Fig. 6D). Also, the more evolvednature of the latter two units, as well as the Tolt unit" isevident compared to the rest of the Ackley Granite(Frg. 8). It is also evident in Figure 8 that samples withelevated Rb/Sr values fall on the extensions defined bvdata with lower Rb/Sr values (e.9., Figs. 8A, C).

There is an overall rarge of. 2 to 12 ppm for cesiutnin K-feldspar, with the most enriched samples comingfrom a variety of units, but mostly the Hungry Grove(Fig. 6E), in samples with the highest Rb/Sr values(Frg. 8C). For the Hungry Grove and Rencontre Lakeunits. the trend of Cs enrichment is consistent withother indices of fractionation (whole-rock and mineralchemistry).

The abundance of lithium tbroughout the variousunits is generally erratic (Fig. 6D, but there is somecorrelation with Rb. Lithium is narkedly depleted in

993

KOSKAECODDE

MOLLYGUAJECK

MT. SYLVESTERKEPENKECK

12

MTTA

HUNGRYGROVE

RENCONTRELAKE

m mffi

n%

% r--

TOLT mm50 60 70 60

mol % orthoclcse

Ftc. 5. Summary of end-member, bulk composition (in mol.7oOr) for alkali feldspar samples from different units of theAckley Granite.

scattered imaging with the electron microprobe,accessory phases were not observed in samplesexamined. Therefore, the trace-element chemisty,unless noted otherwise, is considered to reflectstructurally bound elemental concentrations andcrystal-melt partitioning rather than putativecontaminants.

Trace-element data for alkali feldspar separates arcsummarized in Figures 6,7 atd 8. In Figure 9, resultsfor Rb, Sr and Ba are plotted separately for the Hungry

El 'gO oO - 9 3

= {> 6 _E , F "to

1

0

trtr

EE En | tr

994 T}IE CANADIAN MINERALOGIST

ftc. 6. Histograms summarizing the trace-element chemistry of alkali feldspm sepaxates from different units of the AckleyGranite. For each unit, the samFles are arranged in order of increasing fractionation based on whole-rock geochemistry@ickson 1983, Tuach 1987). For the Hungry Grove unit" samples are arranged from north to south in the unit (labeled andsee arrow). The amowhead on the sides of the diagrams indicate detection limits. Numbers refer to units: (1) Koskaecodde- Mollyguajeck, (2) Mount Sylvester - Kepenkeck, (3) Meta (4) Tolt, (5) Hungry Grove, (6) Rencontre Lake.

the two oldest units (<4 ppm) compared to the rest ofthe Ackley Granite (4 to 28 ppm), in contrast to otherLILE @:b, Cs), which are not relatively depleted in theolder units.

The concentration of tharium (Fig. 7A) is betweenI and L4 ppm, and there is apparently no consistentpattern of endshment or depletion. Although notshown, U is generally below 1 ppm (1e., the detectionlimit) except for a few samples that have up to3 ppm U.

Elevated contents of. nalybdznum (10 to 55 ppm)are characteristic of n1ll samFles from the AckleyGranite (Fig. 7B). However, there is no obviousincrease in Mo concentration proximal to mineralizedzones. Although the enrichment of Mo might be

considered to reflect mechanical contamination"several points militate against this: (1) about 78Vo ofthe samples have values of 10 to 30 ppm, thus a verynarrow spread, (2) other samples of K-feldsparprocessed with the Ackley samples (Kontak & Strong1988) do not show such elevated values (i.e., <5 ppm),thus ruling out contaminationo and (3) none of thegranite samples selected show signs of mineralization.Similafly, elevated Mo contents for K-feldspar havebeen reported by Tauson et al. (L970), as reported bySmith (1974).

Zirconiurn varies from 5 to 35 ppm and is enaticallydistributed throughout the phases, but the relativedepletion of 7r tn the oldest units is noted (Fig. 7C).The large range and variance of values, plus the

l-Jeoorglzo.S, loo=trzO 2ooEFo

EIgo-g=Do6fg,

000

8{[)

@

400

m

El*m

oF< 1 6E

&Q 1 0.oE

0

@

tr--l rc

El-to . 1 0e,=

E,o

E'=a 2 0g,== . "=

FELDSPARS IN TI{E ACKLEY GRANTTE. NEWFOTINDLAND 995

Fl-l rxa-go- r0c=lc,$ sF

tr tr l x 1 8 |

I tLI

r

E 3 0o-

== azooE 1 0N

0

El'l E a

d,ullrjE ar

o

0

! lI

I

I - r -

F r - & I r. J [ - - . l . t l E r r

l l

2 3Thousandg

BARIUM (PPM)

E'! aAg.t, I!uIJJt t

=

0

a t t r

. l t '. t t

- f

.*'T,;h'ljiT+' '

a

t at lT I

4)

zrRcoNluM (PPM)

Frc. 7. Histograms and binary plots summarizing the trace-element chemistry of alkali feldspar separates from different units ofthe Ackley Granite. For each unit, the samples me arranged in order of increasing degree of fractionation on the basisof whole-rock geochemistry @ickson 1983, Tuach 1987). For the Hungry Grove unig samples are arranged from north tosouth. The arrowhead on the sides of the diagrams indicafe detection limic. Nunbers refer to units: (1) Koskaecodde -Mollyguajeck, (2) Mount Sylvester- Kepenkeck, (3) Meta (4) Tolt, (5) Hungry Grove, (6) Rencontre Lake.

absence of clearly defined trends (Frg. 8E), are sugges-tive of the presence of micro-inclusions of zircon; apositive correlation between Zr ud Hf (not shown)also suggests contamination. However, recently Smith& Brown (1988) have noted the presence of up to100 ppm in alkali feldspar and pointed out thatconsistency of analysis suggests the potential forincorporation oftlfs element in the feldspar stxucture.Thus, the present data-base does not permit a moredefinitive statement conceming the nature of Zr withinthe feldspar samples.

Levels of lead (25 to 35 ppm) and gallium (30 to

50 ppm) are very consislsnl rtvithin the Ackley Granite,and there is no systematic change in the abundance ofthese elementg. gimilarly, thallium (1-3 ppm) showsno systematic enrichment or depletion in samples fromdifferent units.

The spatial variation of Ba, Sr and Rb are examinedin more detail for the Hungry Grove unit (Fig. 9).There is a general north-to-south trend of enrichmentor depletion for all three elements, and a binary plot ofBa versus Sr concentrations indicates that theseelements are chemically coupled. However, there are"rapid" local variations of Ba Sr and Rb superimposed

996 TIIE CANADIAN MINERALOGIST

El*@0

ECLo.400

l!tr2@

0

Rb/Sr

* r l a

t r . ai

a * a ^a ^ l S . - * a

.iIf 'rstr

E5

1

6

E P s&Bf i E'

a

Rb/Sr

'.

I

l

t* *rt

II

k.*" r t . A 4 a D

tcll-I t4

12

10

f , ,CL

8o4

2

0

Rb/Sr

t *

X L

r r I ; ^ ^5 : - U

A

A

i a

Fo*

El''

E* t *IJJuJd

j *ct,

Rb/Sr

I* r ^A r a

ffi1.'n 4 ^ ^trF A . t

I ^ a

l .S l

I

El*30

ECLcL 20

N

't0

0

Rb/Sr

t 1 af a l

^*- * !^d * ^ a

F ^ r f . .! - lq i . l ^ *^ ^

r t lr a r

on the broad trends ofenrichment or depletion definedby Tuach et al. (1986). These data suggest, therefore,that "rapid" spatial variations of either a chemical(i.e., bulk chemistry 9f the magma) or physical (i.e.,T)nature rnay have occured, assuming no postnagmatic

modification. Smith & Brown (1988), in discussinglarge variations in Sr and Ba contents sf alkali feldsparfrom the same infusive suite, also suggested that suchprocesses could alter the Ke of these elements,resulting in the observed tends.

KosKaecoddeMollyquajeck

Meta-Tolt-Mt. Sylvester-' .Kepenkeck

Hungry Grove. .

Rencontrs Lake .

t

*

A

tr

FIc. 8. Binary plots summarizing trace-element chemistry of alkali feldspar as a function of Rb/Sr ratio.

FELDSPARS IN TTIE ACKLEY GRANTTE. NEWFOUNDLAND

Frc. 9. Selected trace-element data (Rb, Sr, Ba) for alkali feldspar separales ftomalso are summarized in a binary plot. All data are in ppm.

997

Bo

200

the Hungry Grove unit. Data for Ba and Sr

Rare - earth- elemcnt chernistry

Rare-earfl-element (REE) datafor the suite of alkalifeldspar are summarized in Figures 7, 8, 10 and 11. Ingeneral, the data resemble patterns 1e1 alkali feldsparfrom intrusive (e.9., Shearer et al. 1985, Smith &Brown, 1988, Kontak et al. I99ta) and extrusive(keman & Phelps 1981) felsic rocks, but there aresystematic changes that occur within the differentunits. There is a trend of increasing IREE toward themore evolved parts of the granite @gs. 10, 11), whichis best illusfrated in a binary plot of Ba concentationversus ZIIKEE (Frg. 7E). Howevel ttre lack of apositive correlation of DREE or LIIREE with Zr(e.g.,Fig.7F) suggests that this enrichment is not duesolely to micro-inclusions of REE-bearing phases. Inaddition, using REE analysis of zircon from graniticrocks (e.9., Gromet & Silver 1983) and Z contents ofalkali feldspar, mass-balance calculations indicate thatzircon contamination cannot account for the HREEcontents given the levels of Z.

In the Koskaecodde unil the alkali feldspar ischara{rw-tlzel by a strongly fractionated patlern (Layfrom 11 to 32, LalYbp from 2 to 47), a stonglypositive Eu anomaly except for sample 521, which isnotably depleted n the LREE Q,REE ot 4.8 ppmcompaxed to a range of 15 to 31 ppm). Sample 521 alsois depleted in Ba (270 ppm) compared to the rest of thesamples, and has a markedly lower @u/Eu*)* value.All the feldspar separates from this unit are distinctfrom those of the other units of the Ackley Granite inthat they have (Tb/Yb)N greater rhan 1, compared tovalues of less than I for the rest of the samples. Thesingle sample from the Mollyguajeck unit has a REEpattern similar to those for the Koskaecodde unit, withLa" = 13, @u/Eu*)y = 10.6 and (LalYb)* = 11.

Samples fro.m the Mount Sylvester - Kepenkeckunit have similar, sffongly fractionated ZREE patterns,witl (LalYb)N in the range 4 to 27, and similar@ulEu*)p values. Tbree of the samples (497 ,345,532)show relative enricbment n the HREE and have YbNvalues of 4.6 to 9. 1, compared to 1.4 to 1.8 for the other

. t * r - : {u " ' . ,

12r .34 \' .40 .46

'tos.l"' '+eo

1 0

KOsIGECODDE

\l

Y/ fI

\ru\&

. fcldrpor somplc

MOLLYGT'A'ECK

100

1 0

/;

7,a

'( U"\..'. O '

\: .4,: *.-yw

%

I

KM

O

1 w+,Loc"PtNd StErGtbDy"oEt trYoLu

998 TIIE CANADIAN MINERALOGIST

ftc. 10. Chondrite-normaltzed kEE plots for alkali feldspar separates from different units of the Ackley Granite; data for theHungry Grove unit are given in Figure 1 1. The numbers refer to specific samples, which are discussed in the text.

'100

o l o

a

oL

Y l

samples. In addition, these tbree samples have strongconcave-upward HREE pattems, with (Yb/Tb)1rgreater than 1 rather than equal to 1 as for the othersamples. T\e H&EE-eniched samples show enrich-ment or depletion compared to the other samples withrespect to Ba (609-840 versus 1458-2402 ppm), Rb(432-681 1)ersus 274450 ppm), Sr (118-130 versas471-593 ppm), and Th (8-11 versus 2-5 ppm). Thechemical distinction of samples from the MountSylvester unil gem.Fared to the Kepenkeck unit wasalso noted by Tuach (1987), who interpreted it toindicate a genetic affiliation with rocks of the northern

part of the Hungry Grove unit. A similar conclusion issuggested by tl'rc REE and associated trace-elementdata presented here.

Samples from the Meta unit all have broadly similarREE pattems, although one sample (1 11) is relativelydepleted in XREE compared to the other two (17 versus40 and 50 ppm >REE); in fact, this sample has thelowest IllREE for the entire Ackley Granite. Al1 threesamples have strongly fractionated.I,REE pattems and(Lalfb)N in the range l0 to 31, have positive@ulEu*)* values, and unfractionated HfuEE pattems,with (Ib/Yb)" in the range of l.

FELDSPARS IN TI{E ACKLEY GRANTTE. NEWFOTJNDI.AND 999

100

HUNGRYGROVE

0 15l l

km

r fe ldspor somple

Frc. I l. Chondrite-normalized REE plots for alkali feldspar separates from the Hungry Grove unit of the Ackley Granite. Thesamples have been arbitrarily ananged into tbree subgroups to show the gradual north-to-south change in the REE pat0sms,as discussed in the text.

10

1

q,

.=L.tf

oE()L

oo-og,o)u-

I

vLcauPr Nd SrkucdTbDyHoErTmYbLu

r000 TIIE CANADIAN MINERALOGIST

12

In the Tolt unit, samples generally have similar REEpattems with strongly fractionated LREE patterns,positive Eu anomalies, and unfractionated HREEpatterns, with (Tb/Yb)* equal to about 1, except forthree samples in which this value is less than 1(samples 413, 407,565). There is no geographicalassociation among these lrse samFles (Fig. 1) or

80 90 100

n l t rv t J

km

associated pattern of enrichment or depletion for theother trace elements. For the grouping of samples fromthe Tolt unit (Fig. 10), there is a complete continuumfor REE patterns such that the HREE enricbmentillustrated by the three samples (413,407,565) reflectsa gradual process.

Samples from the Rencontre Lake unit give a pattem

100

16

12

(s-ta)' Caiol \

1ta-zz1' \, - l \-<6

"-._ /t .(o-") \-

-\'

, a . z o \ . ( 0 - 6 ) \ 1 2,/

t .(o-")

/"G>AE). .95V7,^ff 'a-(*:'};:j9,7-, 8

on80

, r t ;

- \ '

16

312(tc)L*8

80 90 100

. p log ioc lose somple 4(10 -20 ) p log ioc lose compos i t i on

(mol % An)

80 90 100

mol % Abft6. 1f,. $nnmary of compositional daa on plagioclase for samples from the Hungry Grovq Rencontre lake and Sage Pond

units of the Ackley Granite. Note the gradual change from oligoclase to albite compositions from north to south. Thesubdivisions of the Hungry Grove unit are arbitrary, as discussed with respect to Figure I 1.

FELDSPARS IN TIIB ACKI;EY GRANITE. NEWFOTNDTAND 1001

similar to the other feldspar patterns in the unitsdiscussed above, but have smaller Eu anomalies.However, these data are wholly consistent with whole-rock REE data for this unit (Tuach 1987).

Samples from the Hungry Grove unit are subdividedinto three groups @g. 11) in order to evaluate thechanges in levels of the REE that accompany fraction-ation, as defined by Tuach et aI. (1986) and Tuach(1987). There is a southward increase in the HREE and,a decrease in @u/Eu*)* and (Ib/Yb)p, consistentwith tle pervasive development of albitic plagioclase(see below) and pattems of enrichment or depletion ofassociated ftace elements (e.9., Rb, Sr, Ba; Fig. 9). TheREE patterns for the southemmost part of the HungryGrove unit are similar in all respects to those from theRencontre Lake unit.

The REE data in K-feldspar from the Ackley Granitecompare favorably with whole-rock data from the samegranite presented by Tuach (1987); both data setsdistinguish the older suites with their more fractionatedpatlerns fi.e., greater (LalLu)N valuesl and distinguishthe Mount Sylvester unit based on its I/REE enrich-

ment. The data presented here are considered to reflectREE variations within the magma at the time ofK-feldspar crystallization, rather than later processes.Thus" the f/REE enrichment observed in some units isinterpreted to reflect similar enrichment within themelt. As discussed above. the enrichment of the HREEcannot be satisfactorily accounted for by putativecontamination by zircon.

Ptectoclase Covposmoxs

Plagioclase compositions (core and rim; 326 analy-ses in 38 samples) have been determined by electronmicroprobe for samples from all units of the AckleyGranite (Figs. 1, 4 alld 12; representative compositionsin Table 4). The spatial coverage emphasizes thesouthem part of the intrusion, within which mineral-ization occurs.

Plagioclase in the older units is the most calcic, andranges from Ant, to Ana6, with the maximum intra-sample variability bang ca.20 mol.Vo An. The mostcalcic sample is from the western part of the

TABLE 4. REPRESENTATIVE RESULTS OF ELECTRON.MICROPROBE ANALYSES OF PLAGIOCLASEAND ALKALI FELDSPAR, ACKLEY GRANITE

SAMPLEUNITFEI.DSPAR

252 542RmL HmGr

PI PI

523 6v7 I 99Kck Kmk Molly MollyPI PI PI PI

89 89 M 473 W2Kep Kep Tolt Tolt RdLPI PI PI PI PI

58.81 @.m26.34 23.947.75 4.916.73 8.n

67.85 6?.54m30 22.N0.74 2.69

10.61 10.180.15 0.n

57,5s2.f.249.V26.60.300.31

62.K23.V74.169.1,40. t2

61.05 $.n 6r,33 68.79n.n 22.10 u,34 19.885.24 3.t2 5.87 0,&8.53 9.58 7.79 11.1E

0.36 0.1E

62.8723.284.t78.810.45

9.58 9.659,6799.94 98.19 99.n 99.15

siqAlro,CaONa.Oqo

x

AnAbOr

END-MEMBER PROPoRTIoN Mol %)

t.9 m.2 3,5 12.597.2 n3 95.7 86.10.9 2.5 0.7 r,4

38.2 24.3 56.2 2t.7 24,9 15.860.1 74.2 42.1 77.7 73,7 82.91.8 1.5 1,7 0.6 1.4 1.3

23.E69.12.1

SAMPLEUNITFELDSPAR

542 542HrmGr RuncrAb/B] A TB)

268 717 3?2InmGr Sage Sage

PI PI PI

542Huocr

Kfr

268 443 447Ilulcr ftnGr llmGrAb/BI AbTB) Ab(B)

332 372Sage Sago

Kfs Ab (B)

67.6219.440.m

10.la0,18

&,t2 63.39 @.89 65,M 65.7522.30 22.81 18.62 m.8t 20.872.59 ?.49 0.00 1.62 1.54

10.00 9.56 1.13 10.53 10.630.21 0.19 15.33 0.\7 025

67.?A @.619.86 18.340,52 0.m

11.54 0.730.11 16.11

67.N 67.73 67.3419.88 t9.79 20.160.13 0.11 1.08

11.22 tO.Tt 10.300.15 0.18 0.34

91.7499.98 98.1998.78 98.58 9.n

siqAlqCaONqoKrO

t

AnAltOr

END-MEMBER PRoPoRTIoN (Mol %)

2.3 0.096.8 93.80.9 6.2

r2.1 i6.l 0.0 8.086.9 82.4 10.3 91.21.0 0.8 89.7 0.8

0.5 0.5 5.398.8 98.5 92.80,7 1.0 1.9

7.29L.Z

l . o

0.@99.10.9

Kosk Kmhcodde; MoIy: Mo[ygujeolg Kep: Kepak*k; I{uuGr: Hmgry Gmve; RaL: Rmcrrtre Iiko; Sago: Sago Pmd; Pl plagielas graiaAb: albite oroolutim; B: bleb pertffio; Kfr: K-feldspar.

r002 THE CANADIAN MINERALOGIST

Koskaecodde unit (sample 701, An ou)" whereasthe least calcic samFle is from a nearby location(sample 697, Anp.). Petrographic observationsindicate that delicate zonation is present (1e., normaland reverse types) in plagioclase from these units.

Plagioclase compositions for the northern part oftheAckley Granite proper (Kepenkeck and MountSylvester units) range from An,o to Anru, withintrasample variability similqr In the two samplesanalyzed.

The southern part of the Ackley Granite incorpo-rates five units, for which data are summarized inFigures 4 and 12. The Tolt unit (n = 8) has an overallrange of plagioclase composition from Anr to An r, but16 eingle sample covers this broad spectrum, and theinfrasample range varies from An, to Antr. There is apronounced concentration of data in the range Ani6_25,with only a few compositions extending to more calcicvalues. For the Hungry Grove unit, the data have beensubdivided into three populations (Frg. 12), as dis-

AA A

r A A A A r A A

90

AA

A A A

@100

aaa a o. . . . t r t r u t r t r

5 1 0 1 5

HUNGRYGROVE

Baaatr

- t A A Itr 90 100

o t r o. o o u o. t r o t r t r o. . t r . u E t r o o

q 1 n 1 i

Boo o

EEE

K-feldspor hostl ome l l oe I ^ . . . .UfeU I

Albite Exsolution

@

@

r A A A tqo 100- - m o l

% O r 'o t r

. t r o . t r o o t r

5 1 0mol % An

1 5

Ftc. I 3 . Histogram plots of compositions (derived from electron-microprobe data) of K- and Na-rich phases of perthitic feldsparin samples from the Hungry Grove and Sage Pond units.

FELDSPARS IN TIIE ACKTEY GRANITE. NEWFOI'NDLAND 1003

cussed above for the REE The extreme northern pargwhich corresponds to Unlt 12 in Dickson's (1983)classification, has plagioclase of An3_23 with peaks atAn.5 and Anro-ro; the albitic compositions are found insample 196 only. The middle part has a range inplagioclase composition of Anaaa, but only a singleanalysis indicates a composition more sodic than Anr.In marked contrast, plagioclase of the southernmostpart of the Hungry Grove unit is, except for a singleanalyses of An14, more sodic than Aq.

The Sage Pond unit contains plagioclase moresodic than An5, except for two analyses of Aqftom the same sample (332). Despite the sodic compo-sition of the plagioclase, it is emphasized that there is aconsistently small amount of calcium (Anr_:) in mostsamples. In the southernmost unit (Rencontre Lake), thecomposition of plagioclase ranges from Ani to An28.Howevero the albite component progressively increasestoward the southeast (1e., toward the area of mineraliza-tion), such that three zones can be recognized, contain-ing An rp, furo_zo and An ls, respectively (Frg. 12).

Cowosrnox orPnasrsN rns PExtrur[c FEDSpAR

Compositions of the K- and Na-rich phases in alkalifeldspar have been determined for sarnples from theHungry Grove (z = 4) and Sage Pond (a = 1) units toassess the degree of low-temperature equilibration@ig. 13, representative compositions in Table 4).Albite lamellae range from Anrr to An6 composition,and there are two fends: (1) coarse blebs are relativelyenriched in Ca compared to flame and filrn perthite,and.Q) the coarse lamellae or blebs in alkali feldsparfrom samples in the less evolved part of the granite(i.e., northern part) are the most enriched in Ca. Thereis also a trend observed in terms of the composition ofthe K-rich phase, with increasing mol.Vo Or occurringin samples from the more evolved part of the granite.The data indicate that the alkali feldspar from the moreevolved part ofthe Ackley Granite has equilibrated tolower temperatures.

Figure 14, a photomicrograph of perthitic ortho-clase, illustrates the nature of the Ca-rich blebs. Notethat the clear parts of the blebs have compositions ofAn11_17, whereas the pitled wing areas are An., incomposition. In this particular sample, most of theperthite lamellae are characterized by such a pitted orporous texture. Similar pits or micropores i1 alkalifeldspar have also been discussed by Worden et al.(1990) and Waldron et al. (1990; these authors favor asecondary origin for the poreso which reflect fluidinfiltation that promoted microperthite developmentand subgrain growth. With respect to the Ca-rich natureof the blebs, it is noted that similar tlpes of perthiteblebs of Ca-rich nature (to htr_:r) have beendescribed in both granitic and metamorphic (i.e.,granulitic) rocls by Yund & Ackermand (1979) and

Frc. 14. Photomicrographs of perthitic feldspar (sample 542)from the Hungry Grove unit Figure l4B is an enlarge-ment of the central part of Figure 14A and shows thedevelopment of a porous texture in parts of the albitedomains. Numbers in Figure 14B refer to mol.7a An, asdetermined from electron-microprobe analysis.

Yund e/ al. (198O), respectively. These authorsattribute the formation of these Ca-rich blebs toearly heterogeneous nucleation in the high-temperaturepart of the ternary miscibility gap (Seck 1971). Theextended t'me at high temperatures, compared to thecase of the Ca-poor filrn perthite, resulted in theexsolution-related domains becoming blebby inresponse to minimization of the interfacial energy.

i004 TIIE CANADIAN MINERALOGIST

Dtscusslox

Al-Si order relationships in the alkalifeld*par

The degree of Al-Si order in the alkali feldspar ofthe Ackley Granite indicates fwo types of granite,dominated by microcline and orthoclase, respectively.In units dominated by microcline, tle inversion to a6islinic phase is considered to have developed inconcert with deformation. Samples from theKoskaecodde unit proximal to its intrusive contact withthe Hungry Grove unit indicate that reheating led todevelopment of orthoclase. For the remainder of theAckley Granite, orthoclase dominates, with minordomains of triclinic feldspar, including areas proximalto mineralization.

The problem of partially ordered (1e., standed)feldspars in large granitic intrusions has receivedconsiderable discussion (e.9., Parsons & Boyd 1971,Cherry & Trembath 1978, Panons 1978, Ferguson1979, Parsons & Brown 1984, Stevenson & Martin1988, Brown & Parsons 1989). It is generally agreedthat several physical and chemical parametersinfluence rates of Al-Si ordering, including rate ofcooling, stress, bulk-rock composition, fluid chemistryand fluid-rock interaction (also see Guidotti et al. L973,Stewart & Wright 1974, Martln 1969, 1974, 1984,Wordet et al. 1990). For the Ackley Granite, several ofthese factors may have been important. The inversionof stranded orthoclase (Martin L974) to ordered micro-cline requires disruption of the crystal structure and is,therefore, potentially inhibited because of an energybarrier. In situations where energy is contributed to thelocal environment (e.g., deformation, strain aboutperttrite lamellae) and the temperature is within thestability field of microcline (i.e., 350oC; Bambauer &Bernotat 1982, Bernotat & Bambauer 1982), fullyordered K-feldspar forms, as in deformation regimes(e.9., Blasi et al. 1980, Kontak et aI. 1984) and meta-morphic terranes (Guidotti et al. 1973, Bernotat &Bambauer 1982, Bambauer & Bernotat 1982.Bambauer et al. 1984). Within the microcline-dominant units of the Ackley Granite, deformationtextures indicate post-intrusion stress, and the presenceof triclinic feldspar indicates that much of this occurredbelow the orthoclase-microcline inversion tempera-ture. These data suggest that the older units ofthe granite had different thermal histories, consistenrwith their older aArFeAr ages (Kontak et al. 19881.The redevelopment of orthoclase in parts of theKoskaecodde unit indicates that a thermal aureoleinvolving temperatures greater than 450"C is asso-ciated with the Hungry Grove uniq this hypothesissupports the geochronological work cited above.Earlier studies have documented similar situations incontact aureoles (Wright 1967, Steiger &Haft 1967,Tilling 1968, Bonin & Mafiin L974, Ihtnek 1982,Bambauer et al. 1984\.

The most intiguing aspect of this study is thedominance of orthoclase even though part of theintrusion has been pervasively infilhated in part byhigh-temperature, saline-rich fluids (Tuach 1987).Parsons (1980), Parsons & Brown (1984) and Brown &Parsons (1989) emphasized the imFortance of late-stage fluids in the ordering process. Observations fromboth laboratory studies (Martin 1969) and nature$fhite & Mafiin 1980, Martin & Bowden 1981,Lalonde & Martin 1983) indicate that an alkaline fluidenhances Al-Si ordering, just as a peraluminous bulkcomposition of a rock inhibits it (e.g., Gurdoti et al.1973, Kontak et al. 1984). In the case of the AckleyGranite, both the bulk composition of the graniteand the presence of a late-stage fluid should havefacilitated ordering, yet apparently they did not. Thehigh level of emplacement and rapid cooling ofthe granite, as evidenced by the petrology of the rocksand concordant hornblende, muscovite and biotiteAr-Ar ages, respectively, suggest that the cooling rateof the granite was faster than the kinetics of theordering process.

Sodic plagioclase within the Ackley Granite

Albitization of calcic plagioclase is a common andpervasive feature within highly evolved felsic suites(White & Martin 1980, Van de Pijpekamp 1982, Wift1988, Taylor & Pollard 1988, Kontak L990, L993,Schwartz & Surjono 1990). However, there has beenlittle investigation of this phenomenon, and il is notclear if some of the albite may in fact represent aliquidus phase. Based on experimental study of ahighly evolved, F-rich, subsolvus granite fromCornwall, Weidner & Mafiin (1987) proposed amagmatic origin for the albite. In contrasg Pichavantet al. (1987) interpreted the albite in the Li- and F-richBeauvoir granite to represent a metasomatic featurebased on experimental studies.

Observations of albitic plagioclase in the AckleyGranite that are relevant to understanding its origin are:(1) the lack of associated alteration, such as pervasivechloritization of biotite or formation of epidote, thelatter being predicted from experimental observations(Moody et al. 1985) over the interval 300-500'C;(2) the absence of porosity, since the reaction of calcicplagioclase to albite involves a change in volume,unless a constant-volume reaction is involved @oles1982), and (3) lack of a compositional gap in theperisterite region @gs. 4, 12).

These features are consistent with a high-temperature origin for the albitization, consistent $/ithexperimental data (Orville 1963,Lagache & Weisbrod1977, Moody et al. 1985). Tuach (1987) documentedthe presence of a high-temperature, saline (to 6G-70 wt.Vo eq. NaCl) fluid occurring in secondary fluidinclusions within magmatic quartz in the evolvedunits of the Ackley Granite. The forrnation of albite

FELDSPARS IN TTIE ACKLEY GRANTTE. NEWFOT]NDLAND 1005

is here considered to reflect high-temperature exchangeequilibria between a more calcic plagioclase and anexsolved, saline fluid coincident with resurgent boilingof the Ackley Granite, which may have occunedsimultaneously with the onset of mineralization in thesouilremmost part of the inrusion. The continuum inthe plagioclase compositions suggests that the temper-ature remained above the peristerite solws at ca.40H50"C (Smith & Brown 1988), which is alsosupported by the presence of orthoclase over micro-cline.

Geothermometry

The compositions of coexisting plagioclase andalkali feldspar have been used to estimate temperaturesfor some of the units (Frg. 15). In order to estimatetemperature, the bulk composition of an alkali feldspar

has been paired with a range of compositionsdetermined for plagioclase in each sample. Since alkalifeldspar appears to have been a late primary phase, rimcompositions of plagioclase have been used forgeothermometry. In Figure 15, some of the data falloutside the range of equilibrium conditions (e.9.,Hungry Grove unit), which results from the continuedre-equilibration of plagioclase at subsolidus conditionsto an albitic composition. The maximum temperatures(up to 700'C) are preserved in the Tolt unit, whereast€mperatures of ca, 650'C and 550'C are es.:mated forthe Renconfre Lake and Hungry Grove units. Thesetemperatures are interpreted as follows: (1) the Toltunit was spatially outside the main hydrothermalsystem and, therefore, did not equilibrate to a lowertemperature, and (2) the Renconffe Lake and HungryGrove units re-equilibrated to lower a lemperatureduring the infiltration of late-stage fluids, which pro-

LOW-TEMPEMTURE EQUILIBMTIONy'

" -,---<? HUNGRYGROVE

RENCONTRE. I-AKE

X:ttrend ofolbi t izot ion

t ' t it l i tt rtii,I l1 l,'

!!',,'it t y ' ,t!r"

V kfsAob

Frc. 15. Partitioning of albite between coexisting alkali feldspar and plagioclase in sam-ples from various rmits of the Ackley Granite. Higber temperatues arc det€rminedusing bulk compositions of alkali feldspar coexisting with plagioclase, whereas a low-temperatue equilibration is determined for perthitic orthoclase using EMPA data forbotfi K- and Na-rich phases. Contou$ of temperafires are taken from Brown &Parsons (1981). The low-temperature data for the Hungry Grove and Sage Pond unitsare based on perthite and show a north-to-south progression in terms of decreasingtemperaqre.

1006 TIIE CANADIAN MINERAI-OGIST

moted the development of albite. Thus the increasingdevelopment of sodic plagioclase within the HungryGrove unit from north to south probably reflectspervasive infiltration of fluid within this part of theintrusion. Similarly, the trend in the RencontreLake unit Grg. 12) also reflects re-equilibration ofthe plagioclase vrith a fluid phase, thus shifting theequilibria in Figure 15.

The low-temperature equilibration of alkali feldspar,as reflected petrographically by the presence of albitelamellae, has been estimated for the Hungry Grove unitby using the composition of the host alkali feldsparphase and albite exsolution lamellae. These dataindicate arange of temperatures thatreflects the trend ofchanging perthite composition (Figs. 13, 15). Since therange ofdata lies below the calibration for the binary inFigne 15, the lowest temperatures of re-equilibrationare based instead on the alkali feldspar solvus (Smith &Brown 1988) and are estimated at 350'to 400oC: lowertemperatures would require near pure end-memberalbite and K-feldspar. Similarly, low temperatures arerecorded in other granites using the same technique(e.9., Ferry 1978, 1985). Thus, the inferred range ofpostmagmatic temperafures based on chemicalequilibria of the feldspar phases supports the overalllack of triclinic feldspar; the intrusive units cooledquickly after emplacement, and there was insufficienttime for development of ordered K-feldspar to form.

Irnplications of chemistry of the alkali feldspar

The trace-element chemistry of the alkali feldsparwithin the different units shows systematic variationsthat are consistent with previous whole-rock data andthe generally inferred evolution of the Ackley Granite.However, tle data for Rb, Sr and Ba for the HungryGrove unit (Fig. 9) indicate that the smooth trend-surfaces generated by the work of Whalen (1983) andTuacb et al. (L986) may be too simplistic; instead,more complicated distributions of elements areinferred. This latter assumption depends, of course, onthe premise that the various distribution-coefficientsfor these elements remained constant tbro.ghout thisunit; the bulk compositions of the samples supportthis assumption (Fig. 5). The abrupt variations inconcentrations observed may reflect the role of fluidsin the late-stage evolution of the Ackley Granite, forwhich there is abundant corroboration from petro-graphic observations.

T\e REE show large, but syslematic, variationswithin all units of the Ackley Granite. Enricbment inHKEE occurs in all units of the main Ackley Graniteexcept the two older ones (Koskaecodde andMollyguajeck). The strongest enrichment in the HREEshow a systematic negative correlation with Ba anddoes not seem to be due to micro-inclusions ofzircon.Emichment of. the HKEE rnay occur by complexingwith Cl (Flynn & Burnham 1978), and is commonly

observed where granitic rocls have interacted withsaline, magmatic fluids, as in granite-hosted mineraldeposits (Taylor & Fryer 1983). Thus, i/i?EEenrichment in alkali feldspar of the Ackley Granite isconsidered to reflect localized enrichment due toseparation of a fluid phase. Since the major- and trace-element chemistry of the alkali feldspar is consideredto reflect dominantly magmatic processes, this fluid isinferred to have been late magmatic in origin, Le., itcoexisted with the mell and enhanced enrichment oftbe HREE. Fluids of similar composition wereinvolved in greisen formation, as there is a similarHREE-ertrrched signafure for these rocks in the AckleyGranite (tuach 1987).

The systematic decrease in the positive Eu anomalyofthe alkali feldspar in the Ackley Granite is correlatedwith tle geochemical evolution; the anomaly issmallest in the Hungry Grove and Rencontre Lakeunits, As there is no correlation between bulk compo-sition and the magnitude of the positive Eu anomaly,the relative depletion of Eu cannot be attributed tosubsequent re-equilibration,

'as might be the case if

albitlzation ofthe K-feldspar had occurred (cl Fowler& Doig 1983). However, the positive correlationbetween Eu/Eu* and Ba suggests that decreasing EuNwas probably a primary feature of the magma.

CoNCLUSIoNS

Feldspar phases wifhin the higbly evolved AckleyGranite reflect a complex physical, thermal andchemical history. The two oldest units ()390 Ma) arecharacterized by triclinic alkali feldspar and relativelycalcic plagioclase (An1._ao). In contrast, the youngerunits (<374 Ma) comprising the pluton arecharacterized by "stranded" orthoclase, with only local,incipient development of triclinic alkali feldspar.Whereas the formation of the triclinic polymorph in theolder units is related to deformation, the stranding ofthe monoclinic polymorph in the younger units thatwere once saturated with volatiles is related to rapidpostmagmatic cooling of the granite. Plagioclasecompositions within 1trs younger units show a con-tinuum from An,' to end-member albite, with the latterreflecting interaction of original plagioclase with alate-stage, saline magmatic fluid, preserved assecondary fluid inclusions in magmatic qrar1u,, at atemperafure above the peristerite solvus. Thusthe environment of albitization is consistent with thestranding of monoclinic alkali feldspar.

The trace-element chemistry of the alkali feldsparreflects chemical evolution toward more evolvedphases in the southern part of the granite, parallelingtrends defined by whole-rock chemisffy. However, theelemental concenfrations of Ba" Rb and Sr are con-sidered to reflect modification of the melt chemisty bylocal saturation of the magna in a fluid phase and,therefore, legalized pattems of enrichment or depletion

FELDSPARS IN TIIE ACKLEY GRANTTE. NEWFOUNDI.AND 1007

of elements. The distribution of. HREE elementssimilarly reflects the influence of exsolved fluids onthe partitioning, such that alkali feldspar samples withsimilar bulk compositions have highly variableconcentrations of the HREE.

ACKNOWLEDGEMEI{TS

This study was undertaken while the author was inresidence at Memorial University. The work wasfinanced by an NSERC operating grant to Dr. D.F.Strong. Work on the Ackley Granite would not havebeen possible without access to the suite collected byW.L. Dickson and archived by the New'foundlandDepartment of Mines and Energy. Permission to usematerial from the suite is sincerely acknowledged.Discussions with W.L. Diclson and J. Tuach aboutaspects of the geology of the Ackley Granite provedinvaluable and are sincerely appreciated. Severalindividuals assisted with the acquisition of analyticaldata namely S.J. Jackson, G. Andrews, B.J. Fryer andH. Longerich. Discussions over the yems with R.F.Martin concerning many aspects of granite peffology,specifically feldspar phases, have been inspiring andbeneficial. Reviews of the paper by R.F. Martin,A. Kerr and R. Mason led to significant changes thatgreatly improved the text.

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