t237

The C an a.dian Mine ralo gi stVol. 33, pp. 1237 -1253 (199s)

MINERAL CHEMISTRY AND oArFAr DATING OF MUSGOVITEFROM THE EAST KEMPWILLE LEUGOGMNITE, SOUTHERN NOVA SCOTIA:

EVIDENCE FOR LOCALIZED RESETflNG OF oArFAr SYSTEMAilCS N A SHEAR ZONE

DANIELJ. KONTAKNwa Scotia Depanment of Naaral Resources, P.O. Box 698, Halifa.n Nova Scotia B3J 279

EDWARD FARRAR El.p SANDRA MCBRIDE

Department of Geolagical Sciences, Queen's Ilniversity, Kingston, Ontario K7L jN6

ROBERTF. MARTINDepamnent of Eanh and. Pl,anztary Sciences, McGilI tlniversity, Montreal, Quebec HjA 247

ABSTRACT

The East Kemptville topaz-muscovite leucogranite, part of the 370 Ma peraluminous South Mountain Batholith of southern

Nova Scotig exfrenced icomplex thermal evolution. e variety of geocbionological studies (Pb-Pb, U-Pb, Rb/Sr, 40Ar/3eAr)

indicate thai after initial emplacement at ca,3?0 Ma" variable resetting of radiometric systems occurred .wtil ca.-24o Ma.

The presence of mylonitic zones, CJ fabrics and spaced cleavage within the leucogranite indicate that it is located within a

high-straln zone, the East Kemptville - East Dalhousie Fault, one of several regionally extensive fault zones transecting^-the

South Mountain Batholith. The iresent study combines results of electron-miooprobe arialyses, X-ray diffraction and 4ArfeAr

step-heating experiments on seven grain-size fractions (550 to 175 Um) of muscovite from a sample of leucogranite to determine

ttre influenJe ofchemistry and grain size on argon diffusivity. The similar composition ofthe size fractions, pret"o"" 69s 5inglepolytype QMr) and discordani but compamble qlul3e/lr age spectra indicate tbat argon ditfrrsion was not controlled by

differences in-mineral chemistry, a mixture of polytypes or differences in apparent grain-size. Although a regional thermal

overprinting event and mixing of grain-size fractions of different ages are considered as potential explanations for_the.discor-

dances in tie age spectra the mosilikd reason is dilfrrsive loss of argon controlled by an effective grain-size smaller than the

finest size-fracion-prepared (<175 FmI tne effective grain-size was controlled by development ofmicrostructures and subtle

chemical heterogeoiltl. fnus, ttr" variably discordant 4ArF9Ar age spectra of muscovite from fhis area are considered to reflect

structural focusiig of-heated fluids thal was facilitated by the inherent anisotropy of the host leucogranite (;.e., shear zones).

Keyvords:4oArleAr dating, tin mineralization, muscovite, chemical composition, shear zones' leucogranite' East Kemptl'ille'

Nova Scotia.

Somaens

lp leucoganite d wpaze et muscovite de East Kemptl'ille, qui fait partie du batholite de South Mountain, au sud de la

Nouvelle-foisse, a subi une 6volution thermique complexe. ks r6sultats d'6tudes gdocbronologiques (Pb-Pb' U-Pb, Rb/Sr'4oArfeAr) temoignent d'un remaniement variabie des systimes radiom6triques apGs la'mise en place il y a environ 370 million

d'ann6es,'pendant un intervalle jusqu'i environ 24AMa. La prdsence de zones mylonitiques, des microtextures G'!' et de

clivages eipac€s dans le leucograrnite tdmoignent de l'importance d'une zone de d6formation locale relativement intense, la faille

ae Uist fonptvilte - East Dihousi", *" pu.mi plusieurs failles r6gionales recoupant le batholite de South Mountain. Nous

pr6sentons t". reroUts d'une 6tude de la muscovite par microsonde dlectronique et par ditfraction X' ei de sept^concentres

ilass6s selon la aille des grains (de 550 I 175 Um), ei chauffds par 6tapes afin &assurer la lib6ration de 4Ar et de 3eAr. Notre

but 6tait de caractdriser l'influence qu'ont la composition chimique et la granulom6rie sur la diffirsivit6 de I'argon. La composi-

tion chinique i peu prds constante, ia pr6sence d'un seul polytype (2Mr), et les spectres 4Ar/3eAr discordants mais tout de m6me

comparables Oemonnent que la diffusivit6 de I'argon n'i piJetf influenc6e par les variations chimiques, par la pr6sence d'un

m6lange de polytypes, ou par des variations dans ia granulom6trie apparente. Quoiqu'un 6v6nement de rdchauffement r6gional

"t * iemg" a! gpaurulom6trias pourraient expliquei les spectres d'6ge, nous atfibuons la perte de I'argon par_diffirsion d partir

tte rl,omaineJd'une granulorrdnie effective inf6rieure b la taille la pluspetite que nous avons isol6e (<175 Fm)' la granulom6trio

effective a 6t6 influenc6e par le d6veloppement de microstructures et dh6t6rog6n6itds subtiles en composition. Irs discordances

relev6es dans les .p""tt"i ddge 4arftAr'de la muscovite de cette r6gion r6sulteraient de la focalisation d'une phase fluide

chauff€e, dont la circulafion-a 6tl facilit6e par I'anisotopie du leucogranite h6te, c'est-h-dire, la pr6sence des zones de

cisaillement. (Iraduit par la R6daction)

Mots-cl€s: datation a()ArFeAr min6ralisation en 6tain, composition chimique, muscovite, zones de cisaillemen! leucogranite,

East KempMlle, Nouvelle-foosse.

1238 TIIE CANADIAN MINERALOGIST

INTRouuquoN

The application of a0Ar/3eAr incremental step-heating studies has successfully resolved the timing bftectonotherrnal events in orogenic belts (Dallmeyer1975, Dallmeyer & Keppie 1987, Muecke et al. L988.Ha:rison et al, 1989, Wright & Daltneyer 1991,Haggafi et al. 1993), thermal overprinting in contactaureoles (Berger 1975, Hanson et al.1975. Harrison &McDougall 1980), and deformation in shear zones(Costa & Maluski 1988, Goodwin & Renne 1991,West & Lux 1993). This is possible because of rhedependence of Ar diffirsivity on physical and chemicalparameters [McDougall & Harrison (1983) andreferences thereinl. For example, in a slowly cooledterrane, minerals with high activation energies of Ardiffusivity (e.9., amphibole: Harrison 1980, Baldwinet al. 1990) will give older ages than minflals withlower activation energies [e.g., K-feldspar @oland1974) orbiotite Qlarison et al. l9B5)1. Consequently,

it is possible to determine a cooling curve for athermally disturbed area by dating several coexistingK-bearing phases (e.9., Berger & York 1981).Howevero one paftuneter, grain size, is known toinfluence the extent of Ar loss by diffusion, but israrely considered in 4oArFeAr studies, despite the factthat it has been known to be important since Dodson(1973) formulated equations relating diffrrsivity andblocking temperatures. A few cases illushate thispoint (l) Berger & York (1981) analyzed two sizefractions of biotite from an infiusive rock (Glamorgancomplex, Haliburton, Ontario) and obtained apparentplateau ages of 905 Ma (5@ pm grains) and 955 Ma(1500 p"m grains); (2) in fhe case ofa Pliocene granitefrom the Caucasus, Russi4 Hess et a/. (1993) foundthat a time-temperature path could be determinedexclusively by +041$4. dating of different grain-sizefractions of a single mineral (biotite in that case); and(3) Zweng et al. (L993) analyzed different size-fractions of hydrothermal muscovite and biotite in

A rundr

* / * * * * * * * ]

+ . tv+ + + ++ + + + + +

+ + + + + ++ + + + + + + t +

+ + \ + + + + + + ++ + + \ + + + + + +

I + + + + \ + + + + ++ + + + + l + + + +

i + + + + + + - ++ + + + + +

+ + + + + - +

+ + + + + + *+ + + + + + +

' * /** ** ** **.*)

+ + + + { + f+ + + + + r + + +

+ + + + + t + + ++ + + + + + r + + + +

+ + + + + + + + + + +, :. :. :.!2 :.:. :.w| ; . ){+ + ' + ) h + + J l r + + + . +

+ ' + - \ - \ - ^ + + ( \ + + + + ++ + \ - - f \ + + \ l r + + + + +

+ / { V \ + { , l + + + + . + . ++ - + + + +

- v /-\ rocks (clostics, corbonotes)

{f> -fj TRlAsslc: sedimentory &

- \-,1- volconic rocks

\J ffi caneontreRous: sedimento.ry

ee,sr iE"rrvrr* W sourH MoUNTAIN BATHoUTIILEUcooRANtra (370 Mo)

f] cauannl TO DEVoNIAN:sedimentory & volconic rocks m

Ftc.-I. limnlified geological map of southern Nova Scotia showing fhe extent of the Soutl Mountain Batholith, outline of theDavis lake pluton @LP),-loc$* o-f q" E*t Kemptville leucogradte and tin deposit, and terranes of the afpAacr'ianQrogen in the inset (after Williams & Ilatcher 1983). Abbreviations: A, Avaloq M, Mego*a: G, Gander; o, burmage;H, Humber; CCFS, Cobequid{hedabucto Fault System.

RSSETTING OF MUSCOITIE AGE, EAST KEMFTYILLE, N'S. 1239

their thermocbronological study of gold-bearing quartzveins of the Camflo deposig Quebec, and with thesedata deduced rateg 6f sp6ling and uplift.

The East KempMlle leucogranite, $outhwestemNova Scotia (Figs. 1, 2), forms part of the 370 MaSouth Mountain Batholilh (MacDonald et al. 1992).The leucogranite is important economically because ithosts a major tin deposit which, until its recent closure(in January, 1992), was the only primary producer oftin in North America. In order to establish the ageof the host leucogranite and related mineralization,several geocbronological studies (U/Pb, Pb-Pb, Rb/Sr,40tupefu) have been compleled (Iable 1). Collec-tively, the age data indicate that the leucograniteexperienced a complex therrnal history in response toreactivation of a fault zone @ast Kemptville - EastDalhousie Faulq EKEDFZ in Fig. 2) within which itresides (Kontak & Cormier 1991). Four previous40ArFeAr age spectra for muscovite indicate markedlydifferent profiles, with apparent plateau ages of. ca.295to 350 M4 despite the fact that these samples comefrom a small area (<l km2; Fig.2).To assess the reasonfor these variably discordant age spectra, aoAr/3eAr

step-heating experiments and geochemical analysesof grain-size fractions of muscovite from a single,

relatively undeformed sample of leucogtanite wereundertaken. These data should permit assessment ofthe influence of grain size and chemical compositionon rate of Ar diffirsion, and of the importance of theseparameters in affecting the apparent 4Ar/3eAr agesobserved. In a similar study, West & Lux (1993)focused on a determination of the age of mylonitizationwithin a fault zone in Maine by analyzing differentsize-fractions of muscovite.

Gnoloclcar Ssrrnqc

The study area is located within the MegumaTerrane of the Canadian Appalachians @g. 1). Twolithotectonic units dominate the geology of this terrane,na-ely metaturbidites of the Cambro-OrdovicianMeguma Group and bodies of late Devonian peralnmi-nous granite, such as the South Mountain Batholith.The area was penetratively deformed during the mid-Devonian Acadian Orogeny, which records thedocking of the Meguma Terrane with ancestral NorthAmerica along the Cobequid-Chedabucto FaultSystem (CCFS in Fig. l); an age of 400 t 1^0 Ma isestimated for this event based on whole-rock {ArfvAr

dating (Reynolds & Muecke 1978, Muecke et al. 1988,

Ftc. 2. Geology of the Davis lake pluton lafter MacDonald et aL (1989,Fig. 1) and Ham & MacDonald (-1991)] and inset map

of the Easikemptville deposit area [after Richarclson et al. (1982),FiE 4.2)]. The age data shown are aAr/3eAr plateau ages

for mica 1t, biotite; rn, muscovite) inm the Davis Lake pluton (from Reynolds et aL l98l) and 4ArfeAr plateau ages for

muscovite from the East Kempwille deposit area fleucogranite and gpisen; Zennllu & Reynolds (t98S) a{ Kontak &

Corrnier (1991)1. The fault zones shown are the Rushmere Iake - Tobeatic Fault Zone (RLTM) and the East Kemptville -

East Dalhousie Fault Zone @KEDFZ). EKL and DLP in the inset map refer to East Kemptville leucogranite and Davis Lakepluton, respectively.

1240 TTIE CANADIAN MINERALOGIST

TABLE I. GE@}IRONOLOGICAL DATA FOR TIIE EAST KEMPTVILLE IJUCOGRAMIE AND GREISENS

CommenfAge(M!)

{ArFeAr

{Arl3eAr

4A/eAr

RbiSr

Rbisr

Rb/Sr

Rb/Sr

RbiSr

Rb/Sr

Rb/Sr

Pb/Pb

UiPb

Musovile (N = 2, greisen)

Muscodte @KL)

Musmvite (EKL)

Whole rock (EKL)

Whole msk minus musovile(EKL)

Mucoviies @KL)

WRMMS-QP-KF (EKL)

WRMM$.WR-MS

Whole rock (greisen)

Whole mk @KL)

whole mcks @KL and greircn)and mineral separate,r (MS,KF)

whole mck @KL and greirene)

3O0 Deformed greisens; dating oftectonothermal event

-350 Sample of EKL marginal to d€posir (Fig, 2); dirturted agesFtrum, btft ffiVo of gas indicates an age of oa. 350 Ma

338 Sample of EKL from d€,po8it ar€a; dieturbcd age 0p€otrum, but6% of gas gives plafeau age of 338 Ma

344X5 Isoohfon(MSWD = 1.26; N = 11); Sri =0.7245 *0.C056;repr€{ents time of Rb/Sr rehomogmizalion due io teatono-thermal evenl.

354 ! 13 Errcrohron (MSWD = 9.0; N = 9); Sr, = 0.72f5 X O.OLO7;minimum age for leucognnite md time of rehomogenization ofRb/Sr

320t4 Isochron(MSWD =3.0;N =7);Sr i =0.?519 *0.0320ioxplanation remains ambiguous

254 f I Average of seven 3-point isochmno; low 0emp€rature re,J€fringgvent

361-3l l Range of ages (N = 7) relleting variable rese$ing of WR-MSRb/Sr iro0opic system

337 +5 Errorchmn(MsWD =2.87,N =5);Sr, =0.?29 10.(Xl1;time of leolonothermal activity and raetting (cf. Richardson s,a/ . ,1988)

333 ! 27 Errorchron (MSWD - 12, N = 6); Sti = 9.72n t 0.020;re[rHents time of rchomogenizafion (cf. Richudson, 1988)

366 I 4 Age of EKL intrusion and xsmiated mineralization

367 X 10 Age of EKL intrusion and assmiated mineralization

NoTEs: source6: 1 Zentilli & Roynolds (1985); 2 Kontak & Cormier (1991); 3 Riohardson et aL (lggg)i 4 Riohardson (1988); 5 Kontar &chanerje (1992); srt' initiat msrAsJ ratio; EKL, East Kemptville leucogranile; wRMMs, whole mck minus muscovile; ep, quartz-plagioclasefraotion; KF, K-feldspar; WR, whole mcki MS, musvite.'Reprqents interprstation of the date by the authors.

Keppie & Dallmeyer 1987). Continued tectonothermalactivity within the Meguma Terrane, albeit morelocalized in nature, is recorded by: (1) zones ofbrittleto ductile faulting that cross-cut the peralumin6uggranites (Fie.2; Smirh 1985, MacDonald et at. lgg2.Home et al. 1,992), and (2) variably reset isotopicsysterns (Keppie & Dallmeyer 1987, Dallneyer &Keppie 1987, Reynolds et al. 1987, Cormier et at.1988, Kontak & Cormier 1991).

At East Kemptville, a topaz-muscovite leucograniteof <1 kmz infiudes fine- to medium-gained, grey toblack psammites of the Meguma Group. This graniterepresents the late-stage differentiate of the zonedDavis Lake pluton (MacDoiald et at. 1992). Thepluton has been dated at ca.370 Ma based on a whole-rock Rb/Sr isochron (374 t 5 M4 n = 32; &atterte,e &Cormier 1991) and 4oArFeAr mica dating of mica(Reynolds et al. L981., J.D. Keppie, pers. comm.,

1993). The leucogranite is fine- to medium-grained andvaries from relatively fresh, with variable.rnxounts ofdeuteric alteration, to intensely altered equivalents inarcas of greisen and vein mineralization (Kontak 1990,1991). However, all rocls record variable amounts ofdeformation, and one observes in outcrop the presenceof a well-developed northeast-trending (generally040'/45-75'NE) .l fabric and, more rarely, myloniticzones (L-2 meters in width) of similar srisatati.t(Kontak et al. 1986, Kontak & Cormier 1991).Examination of deformed leucogranite in thin sectionreveals the following feahues: (l) recrystallization ofquartz and development of undulatory extinction,(2) fracturing ofgrains offeldspar and displacement ofalbite twin planes, (3) inversion of orthoclase rcmaximum microcline, and (4) alignment and kinkingof mica flakes and formation of through-going zones ofdeformation defined by recrystallized textures and

RSSSTTN{G OF MUSCOWIE ACE, EAST KEMPIVILLE, N.S. 1.241

fine-grained while mica. Kontak (1990) and Konuk &Cormier (1991) interpreted the geological features ofthe area and the complex lectono&ermal evolution toreflect emplacement of the leucogranite into an activefaull-zone, represented today by the EKEDFZ (Fig. 2),which initially localized the intrusion and subsequentlyfocused strain during continued deformation within theMeguma Terrane.

Prsuous Gsoct*.oNor.ocrcAI, Sruonsem IvpucenoNs ron rns TmRMAL [IrsroRy

or rHs EAsr Knnnpfirtrlx Lrucocnaffrs

Previous geocbronological work at East KempwilleClable 1) indicates that both emplacement of theleucogranite and associated hydrothermal activityoccurred at 366 Ma (Pb/Pb, U/Pb). However, finalclosure of the whole-rock RblSr system did not occuruntil 344 Ma- at about the same time as the oldestof four 40ArFeAr dates for muscovite (350 Ma). Aminimum temperature of ca 350'C is estimated forthe latler thermal event, on the basis of Ar diffrtsivity

in muscovite (McDougall & Harrison 1988). Thesedata do not permit one to distinguish befween a short-hved versas a protracled thermal event; however, thegeological setting of the batholift (MacDonald et al.1992) and additional geochronological data in this area@eynolds et al. 1981, 1987) sugge{ a short-livedevent. Concordant to discordant 40{rPe/a age spectafor the three remaining muscovite separates from EastKemptville, with plateau ages forpart of the spectra of338 Ma n 295 Mu indicate variable thermal historiesor steep thermal gradienls in the region. A similarscenario is suggested on the basis of variable whole-rock - mineral Rb/Sr ages, as seven whole-rock -

muscovite ages vary from 361 to 311 Mq and sevenrvhole-rock - plagioclase - K-feldspar ages vary from276to24SMa(Iable 1). Although precise estimales ofblocking temperature$ for Rb/Sr systems are lacking,the highly diffsrent rales of diffrrsion ofthese elementsin mica versas feldspar (Giletti 1991) would corre-spondingly tanslate into different blocking tempera-tures. For our purposes, it suffices to note that the largespread in Rb/Sr ages for similar whole-rock - mineral

Frc, 3. Phoomicrograph (crossed nicols) of sample EK-9G-A showing muscovite grains (outlined in black ink). Muscovitegrains are characterized by inegular terminations, internal recrystallizalion (i.e., subgrain development) and kink zones.ihe large central grain consists mostly of a single, anhedral flake of muscovite, but the remaining areas consist ofvariably recrystallized and ileformed muscovite and also aggregates of fine-grained muscovite. Width of photo isapproximately 6 mm.

t242 TIIE CANADIAN MINERALOGIST

pairs must reflect variable thermal histories of thesamples. In summary, previous work at EastKempMlle indicates that separate thermal events havevariably reset isotopic sy$tems over a protracted time-interval and that the temperature accompanying theseevents must in some areas have approached ca,350.C,the blocking temperature of muscovite.

Sewm Sm-eciloN, DrscnnrroN aNoANALvTTcAL Trcrnrrqurs

A single sample @K-90-A) of barren, medium-grained, equigranular leucogranite (approximately10 kg), characterized by a poorly developed spacedfracture-cleavage (M0'/45'NE), was collected from theEast Kemptville open pit (94 level, south end, summer1990). In thin section, the rock contains no tlrough-going fracfures and has a hypidiomorphic-granulartexture (Fig. 3). The petrography of the sample issimilar to that of the leucogranite described by Kontak(1990), but we emphasize here the lack of obviousdeformation. There is, however, subtle development ofundulose extinction and sutured grain-boundaries inquartz, fracturing of the sodic plagioclase, and kinkingof muscovite. Muscovite flakes are anhedral to sub-hedral, commonly with feathered or ragged termina-tions; kink zones transect grains, and subgraindevelopment is not uncommon. In terms of grain size,muscovite occurs .rs coarse (500 to 1500 pm) to fine(<d00 pm) grains; the coarser grain-size dominatesvolumetrically (Frg. 4).The muscovite is interpreted toreflect both a primary magmatic generation (with laterchemical equilibration); however, there is no evidencein this sample of a later tectonic stage (i.e., neomorphicmuscovite), which would have led to aligned grains ofmuscovite (e.9., West & Lux 1993).

The leucogranite sample was pulverized in a jawcrusher, and the material sieved to recover several size-fractions (-40l+50, -50/+60, -601+7O, -70l+80,

-80/+100, -LOOI+120, -1201+L40; Fig. 5). The amountof crushing done was minimized so as to preserve asmuch as possible the original dimensions of themuscovite grains in the rock High-purity mineralseparates were prepared using conventional techniques(Franz isodynamic separator, heavy liquids, handpicking); petrographic examination of the separatesindicates an approximate purity of 99.9Vo. Measwe-ments (r, in the range 50 to 55) of the size of individualgrains within the various size-fractions indicate thefollowing results (avg. + lo): -40l+50 (547 I 109 pm),-50/+60 (473 ! LL3 pm.), -70l+80 (342 + 68 ttrr'),-80/+100 (310 t 53 pm), -100/+120 (251 i 5l pm),-7201+l4O (173 130 pm). Small aliquots of each meshsize were used for electron-microprobe analysis,X-ray-diffraction studies and 4Ar/3eAr dating.

Electron-microprobe analyses were done using aJEOL 733 Superprobe equipped with an Oxford LinkEXL Energy Dispersion system at DalhousieUniversity using the following operating conditions:accelerating voltage 15 kV, sample current 10 nA,beam diameter 1 pm, and counting time 40 seconds.Instrument calibration was done on pure metalstandards, and data reduction ofraw counts was doneusing Link's ZAF matrix-correction progtam.Intralaboratory standards were run to check for preci-sion and accuracy. Representative compositions arepresented in Table 2.

For the X-ray-diffraction study, a small amount(-10 mg) of the finest and of the coarsest fraction ofthe muscovite separated from sample EK-90-A wasground briefly in a mortar and pestle, so as not toinduce structural damage. An internal standard(synthetic spinel, MgAlrO) was mixed in with themuscovite. A paste was prepared with this mixture andsilicone-based stopcock grease. The paste was spreadon a piece of tape on the sarnple mount intended for thetransmission-mode Guinier-Hiigg focussing powder-diffraction canera (CuKcrl radiation). The paste was

Groln Slze(lm)

Flc. 4. Histogram plot of maximum dimensions of muscovile grains measured in thinsection of leucogranite sample EK-9G-A.

RFJETTING OF MUSCOWTE AGE, EAST KEMPTVILLE, N.S. 1243

Flc. 5. Photomioographs (plane-polarized light) of various size-fractions of muscovite; all photos have the same scale(ong dimension 2.6 rnm). Note that there are xsentially rwo types of grains present pitted grains full of inclusions and cleargrains (see text for discussion). The dark spots in some grains reflect radiation haloes related to uraninite and zirconmicro-inclusions.

spread with n minimum 9f manipulation, so as tominimiz€ the preferred orientatioir of flakes in thepaste. The diffraction patterns were indexed byreference to paftems for the polytypes of muscovite.The cell parameters of the two subsamples, refinedwith the program of Appleman & Evans (1973) asmodified by Garvie (1986), are listed in Table 3.

Samples for sArfeAr analysis and several fluxmonitors were irradiated with fast neutrons at the

McMaster University Nuclear Reactor (Hamilton,Onario) for 10 hours. The flux monitor used was thehornblende standard MMHb-1, which has an apparentK-Ar age of 519 Ma (Alexander et al. 1978), and eachmonitor wrs analyzed in two to four splits to check theuncertainty (0.l%o) or reproducibility of the standard.J values derived from the analysis of the flux monitorsformed a linear array when plotted as a function ofposition in the canister, indicating no detectable

1244 TTIE CANADIAN MINERALOGIST

TABLB 2" RPRESE$TATTYE COMSINOI{S OF THE VARIOUS SZE-FRACTIOI{S OF MI'S(DVIrBTAKEN FROM SAMPLE EK-90-A EAST xHvTP-TvIujE I.EUCqIRANITE

rmn4o8 l

Mesh 4ry50 40/50 50i60 50/60 7040 ?0nOf t 1 3 2 5 2 9 3 9 4 5

80nm tonm 10/120@ 6 s E t

tm.[m tmn&101 76

sio,Tio,Alro.FeOMnOMsoC{ONa"Or90Fcl

47.X 4E.@ 47.Vt 49.9t 48.490.m 0.35 0.u2 0.26 0.A

N.fi U.y2 29.E5 23.73 26.705.9E 8.49 5.E7 E.46 7.650.49 0.15 0.n 0.m 0.250.0 0.36 0.n 0.38 0.370.m 0.m 0.m 0.00 0.000.30 a.n 0.17 0.21 0.109.67 r0.t1 9.59 9.E3 9.69r.E6 3.45 t.75 4.05 2.80.m 0.m 0.m 0.m 0.0

4.75 47.29 4E.090.28 0.m 0.07

25.53 3t.n A.948.33 4.98 6.210.29 0.39 0.340.32 o.lt 0.210.m 0.05 0.r00.16 0.18 0.r29.62 9.41 9.7A3.29 1.67 2.X0.05 0.00 0.03

4.E9 47.26 45.E4 *.n0.11 0.m 0.a 0.3E

31.6 .@ 28.53 25.614.74 7.61 7.74 1.940.1t 0.27 0.43 0.200.21 0.31 0.10 0.350.m 0.0 0.m 0.060.14 0.16 0.33 0.06

10.96 10.6t 10.57 rc.n1.@ 3.U 2.4 3.390.m 0.07 0.@ 0.03

.lE .240.78 '1.44 95.69 %.86 96.41

0.n 1.70 1.21%.67 .U 96.10

r.3E 0.70 0.9995.94 .030.67 1.37

.v 97,17l.ol \aCl,F=O

JI

Ti

M!MgCsNaKFcl

95./10 94.E0 94.96 95.16 %.2A 95.29 9s.14 95.11 .6 95.30 %.75

STRUqfiIRAL FORMUI.AE BASED ON 1I ATOMS OF OXYGEN

3.245 3.491 3.2E3 3.507 3.Tt5 3.4t0 3.2t9 3.315 X.2r4 3.354 3.nO 3.9t20.?55 0.593 0.717 .4% 0.a5 0.590 0.781 0.6E5 0.786 o.ffi 0.?80 0.61E1.709 1.488 1.696 t.472 1.566 1.515 1.765 r.61 r.724 1.530 1.582 t.4950.m0 0.019 0.001 0.014 0.012 0.015 o.m 0.m4 o.ffi 0.011 0.015 0.92.0.341 0.503 0.331 0.4n 0.44 0.491 0.& 0.35E O.tn 0.456 0.455 0.,t650.033 0.009 0.016 0.m 0.015 0.01? 0.u23 0.020 0.011 0.017 0.v26 0.0120.m 0.039 0.q2E 0.040 0.(n9 0.oJ4 0.011 o.gn o.w 0.m3 0.011 0.0370.m 0.m 0.00 0.m 0.m 0.m o.qx o.m 0.m 0.m o.m 0.m40.044 0.08E 0.@3 0.030 0.015 0.v22 0.924 0.016 0.020 0.922 0.045 0.0120.847 0.914 0.839 0.882 0.861 0.E59 o.ElE 0.853 0.959 0.y79 0.94E 0.962o.q7 0.n5 0.3E0 0.9@ 0.637 0.7A 0.6 0.515 o.y1 0.72A 0.59 0.7500.m0 0.m 0.m 0.m 0.m 0.w 0.m 0.m4 0.m o.ffi o.m 0.04

C,!rt'ad@ of oxids in wt% sd€t@hsdby el€.dceis[E{6e elyds[shgeefotbeiIgopqlti|g&dii@t@ls@mltlge 15 kV, srnplc mt l0 !A bem vidlh 1 lrn, s@g d@ 40 gods. Redd6 of d@-d@ uiag f.Wr Zaf naUr_ffie{ci@p@g@

gradient in the neutron flux for this position. ForaoArl3eAr incremental step-heating bxperiments,samples were loaded into a quartz tube connected to astainless-steel extraction system and heated usinga Lindberg furnace. The extraction system wasoperated on-line to a substantially modified AEIMS-10 mass specfiometer run in the static mode.Measured mass-spectrometric ratios were exftapolatedto zero-time and correcled to an 4ArF6Ar atrnospheric

TABI,E 3. IJNIT.(3T.L PARAMEIERS OF MUSCOVTIBIN THB FINEST AND@ARSES'T SXZB.FRAC'NONS,

SAMPLE B(-.9O-4" EAI'T ASMPTVILLB LBUOO@ANITB

-()/150Eesh

-r2',hr40nelh

* @bq of hdqedD@ts 6od h6e l€ar(-squaB n6!@st tglgrao ofAld@r & Ee@(l5rB), d adcplod by Gd€y (198611. Udts: a ,, d h A" @d V (dt-dr FI@) h A3;irbrarial ql$ hdhd @d!€dtroal 3!86 h deg6 Eid-@bsil,@vib-2Mi GDP6-263) hB t@ fo[oeilS @it-€[ pmeE$ a 5,19, b 9.$, c n6 It P :n6,

ratio of 295.5. The 4ArfeAr values were correcled forneutron-induced 4Ar from potassium and 3eAr and36Ar from calcium. Ages and erronl were calculatedusing formulae grven by Dalrymple et al. (1981) andthe constants recommended by Steiger & Jiiger (1977).The errors represenl the analytical precision at 2o,assuming that the error in the J-value is zero. Theresults are summarized in Tables 4 and 5.

PE"rR.ocRApHy AND MNERAL CtmrnsrRvor fiIE St7.p FRAC'1ONS OF MUSCOVffE

Photomicrographs of the size fractions of muscoviteare shown in Figure 5; the representative compositionsof the size fractions (Table 2) are summarized inFigure 6. The muscovite grains can be crudely sub-divided into two sa[egories on the basis oftheir lexture,namely pitted and clear t,?es. The pitted types axecbaracterized by the presence of a variety of mineralinclusions ofvarious sizes, shapes and abundance. Onthe basis of back-scatlered elecfron (BSE) imaging *6semiquantitative energy-dispersion analysis @DS),these include pyrite, chalcopyrite, galena, sphalerite,native bismuth, uraninite, cassiterite, Nb-Ta oxides,

5216 9.06@ r9J965 90 95.8m n S$,62 33o.wr 0.0015 0.m64 0.018 agt

52145 9.05$ 20.S6 $ 95X& 90 940.18 X0.mD 0.m19 0.0103 o.ml 0.4i

RESE.TNNG OF MUSCOVIIE AGE, EAST KEMPTVILTE, N.S. L245

ci @ ilc613 a09n675 S.&n O0rlTX 16.8,1 0.063m n.n4 0.m488t0 F3l 0tr,re 81.& 0.@6e0 84.118 0.69

410 sJ6! 0.s12w n.me o,m',

.tN vx 0.0t01lm &.o79 0Jll2

c, @ t1!,010 0.G39m ?9.847 0.0Lt715 75.98 0.m78ft 76.ffi 0.m813 79.8 0.m&5 81.9 0,m

a5 84S O.Nt.96 86.631 0.@0s0 86.m 0.@/

10J0 ata 0.01mum uJq 0.01ro

c€ @ 7r.9s 0.01J2ffi fs4 0.mm T.$t 0.643n5 B.tq 0.m8tJ 81Jt 0.m44845 84.735 0.640

s r.qg 0.M2s r,o3 0.w,50 g7 2t2 0.s9lm &6 0.0171120 t8.189 o.dDl

c4 m 88.13 0.0rt3& Bn 0.0123ns ns$ 0.{n66m 79.4t o.rmJ8tt 81.631 o.mts g.s 0.&1

.8't5 86J1t 0.m905 S.7J9 0.@s &76 0.@

l0J0 n,99 0.(n6l& 89.8t A.Al1

cio @ s.ls 0.ffi1m &6 uotna5 n.N 0.Grfr B.la 0.661815 ata 0.Bl95 &310 0.m

.6'tt 85,746 0,m5q s.t4 0.ml950 $.Bt 0.0s12

lm $.8 O0t6lm s.a w

cil 6m 65.4@ (L066$ n.l$ 0.087u %.4 0.MTE ?&mt 0.6t!815 81.C13 0.m84J 83.588 o.mJO

w5 8t.883 0.@r.{J 8rJ65 0.@s &.6 u@

tm 9.93 0@tn s.'s 0.w

Gr2 @ 86.942 0.04f,I@ n355 o.mla5 75.912 0.@n5 ngn 0.w815 6,42lt 0,@48845 81.995 0.@45

.r5 srJE 0,016s &.6 0116{J0 s.8!8 0.M

.1(r0 9.448 0.0llltm s.m o.w

0.m 0ts1 0.m1 753 93qo(n o.m 0.6f 94J0 Ni0.m 0,63' 0.181 9, 98 AtS0.m o.gt 0.(F6 98.16 294.80,m l3xJ 0.108 98.63 ,'.r.70,m 1.630 0.\9 s.?t 3I.00.m l.g9 0.149 gr.9 1t7.90.m ,.8r4 0.3u 9J9 t21.90.m c:753 0.rb1 97Jr tn20.m 0 0.018 96Jt %.60.m 061 0.6 96.14 3193

0.m 09, 0,017 75,' pL4om 0.693 0.0J1 9493 N4.6oqb uM 0.0t2 5.98 6.90.@ 0.@ o.w s.o? m.3m Lm 0.14 ffi s.60.m LID olJ7 98.ro 310.50,m 3J9 o.ffi 9.4 il.8o.m 1.99e 0.149 ufi n430.m 0at6 0.61 t8.7J ra.30.m om 0.m 96.46 tt120.m o.& 0.m g.o2 306J

0.128 8&18 30t3o.r!t 95.4t aJJ0!t5 cr.47 89.90.@ %,m 8.4o.tm *.9 309.30.19 98J3 318.2o8 nn gts0.101 98.9 9&60.6 99.01 tn.20.@ 935 W30.019 9L'6 9r.6

TABIE 4. 4Af9AI ETOBJImTTA], DATA FoR THB vARIoUs suE-FRAc[o{sOF MUSWIB TAKN ROM SAMPB K-gGA. BST MUU.B LBT' TBANTTE

Tq 'e'k tArPtu tArFtu vot. tuata %.a'C D& D& 8alt0€ cd

A S t hk zdP

otherwise seem ttre same with respect to theirmorphology and birefringence. Both types contain kinkbands, undulatory extinction and some intragranularrecrystallization, suggesting that they have experiencedtle same complex evolution.

The various size-fractions of muscovite have a largerange in composition, mostly in the proportions of Fe,Si, Al and F; histograms in Figure 6 illustate the broadoverlap of the composition for each of these sizefractions. The elemental variations in muscovite fromthis single sample are the same as documented inmuscovite from the leucogmnite and greisens (Kontak1991,1994). The broad variation in chemistry can alsobe documented within singls grains; this is particularlynotable with respect to Fe conlents, as illustrated by aBSE image of a muscovite gain in Figure 7. In thisi-age, tJre light portions, which represent areas withhigher average atomic numbero contain 7 to 9 wt.VoFeO, whereas the darker areas have less than 5 wt.VoFeO. Thus, the chemical variability occurs on an intra-grain scale and is not simply a function of grain size.The results of the mineral chemistry, in conjunctionwith petrographic observationso suggest that althoughmuscovite is of variable morphology and chemistry, itmay have formed in one of several manners: (1) initialcrystallization of the leucogranitic magm4 (2) crystal-lization as in (1) but viith subsequent interaction witha hydrothermal fluid during deuteric alteration, or(3) formation during the hydrothermal stage, f.e., themuscovite is of postmagmatic origin.

Rrsurrs oF X-RAY-DmRAcfloN SruDrEs

The X-ray-diffraction QRD) studies of the coarsestand finest sizes-fractions Clable 3) showed that boththe 40-50 and 120-140 mesh samples containmuscovite-2M'. The two sets of cell dimensions differslightly, and are within 2o of each other. There is noevidence of significant compositional differencesbetween the two size-fractions. The background of thefilms is darker than expected, which is consistent withthe presence of Fe in the octahedral sites (phengiticcomponent). In addition, the cell dimensions a md bare slightly larger than in end-member muscoviteClable 3, footnote), presumably reflecting the presenceof Fe, whereas c is slightly smaller than expected,presumably because of the presence of significantfluorine at the hydroxyl site. Finally, and mostimportantly, the data do not indicate the presence of apolytype other than 2Mrnthe samples examined.

Our results can be compared to the findingsof Palmer (1991), who studied twelve samples ofmuscovite from East Kempwille. Palmer determined,with the use of a Debye-Scherrer camera, that alltwelve samples consist of the muscovite-2Mtpolytype.He atfiibuted the differences between the two popula-tions (eucogranite versus gteisen) to variations in theoccupancy of the octahedral sites.

35IJl.o t1 2

o.7

OJL9L9L4

951.4

l.EI.5uI ta

2-13.0

st 2xl31.6Ll

t.5

3.1

0.m 0n4 0.69 9.9 ntiom rru 0.G3 srJ 218.90.@ 0.910 0.@ 98J4 8.40.m o.qb 0.@ v1.19 a50,m t.8o 0.1t 9.4 310.40.m m ofi s.6t tul0.@ 2.w 0.183 98J6 t,9.900 1,483 0.il3 s5, ?4.90.m 0,566 0.93 s.$ s.90,(I! 0rlt 0.015 M ws0.m on o.@t 96 317.4

0.@ 03@0,m 0.7580.m 0.7J30.m 0.8,0.m lJn0.m 2.glo@ 3M0.@ Bnom t60,m o.4t70.m 024

0.m 03fl 0.028 85.(B 313.00.m 0.8 0.m 95s N.20m on6 0.061 9t.68 ,'4.r0.m 0.m 0.6, 91.t2 m.l0.m LB' 0,19 98J2 3@.60.@ 2s o.82 n32 tm30.@ 2JS 0.1r 6r %.10.m Lw 0,14 98.9 ttt-30m 0.s 0.m gs 9.90.m 03@ 0.44 96.43 tzl.o0.m 0.1( 00ll tr.B 3164

0.m 0-J66 0.u3 s.J9 atJ0.m L94 0.o4 9.01 aLo0.m 0.m 0.060 98JA ttt.20.m 0.918 0.m m.x E.\0.m 1.620 0.t21 x?5 3m.20.m 26 0.218 BA 3ln90.G) 234 o.tn nff .30.m 1.660 0.r2b w4t tus0.m 0,9b 0.m s.49 %.40.@ 03s 0.m 69 9.70.m o.t2 0.@ s.4 s.0

0.m 024s 0.017 E3.6 2'L80.m l.o2 0,fn 96.91 ,,130.m ottt 0.60 98,6 49.60.m 0.M 0.63 nfit 294.80.m l.s5 0.135 *9. 305.90.m m 0.1& R5 311.E0.m 3& 0a $.r fi,60.m 28 o.lt 9r.18 ss0.m 0.s0 0.m ,8.43 #30.m 0.4 0.018 *% u,20.m 0.0D 0.fi1 a|.t6 m3

4.t1.8L3u

t9

IA1.9u83.6

al2-6

L4L Lt 27.4OJ3.63S

t2u

1,01.60.9l.EOJl . l3.J

t3

IJ

IJL5t .1

L29.O

. Ddlr rtsF lrcd b alo!@ plM !S6 h F&@ 8 @d TaSt 5, 6 ds@€d tn & @

niobian rutile, apatite, niplite, monazite and zircon.The pitted grains commonly have a lexture similar tothe sagenitic texture seen in biotite; it possibly is due toa fine-grained, lamellar intergrowth of rutile. Incontrast, the clear grains are devoid of inclusionsn but

1246 TIIE CANADIAN MINERALOGIST

TABLE 5. SUMMARY OF 4ofufgAf SIEP-HEATNG SPR,IMENTS ON lItE VARIOUS SZE-FRACTIOI{SOF MUSCOWTE TAKEN FROM SAMPIJ EK-90-A" EAST KB{PTVII.I,E I"BU@GRANITB

Sample SizeFraotim

V.K Minimm MaximmAF@a) Ace(Ma)

Inlegmed Maxinmz Temp ('C) at PlateauAge(Ma) tAr Max. eAr Age

VoeAr #SbpsPlaFau Plsteau

6

n

CE

c9

cl0

c l l

ct2

315.9

313.0

J I J . J

316.9

J I / . J

J I I . U

5 LZ.+

J .D

3.53

z.7Q

2.58

3.92

2.K

3.25

910

E75

845

845

875

845

m5

tl0l50 1.5

50/60 8.t

@n0 7.9

70/30 7.7

80/1m 8.0

rmn20 7.9

lmn$ 8.5

az.8 3y.3

?xs 329.3

u8.9 329.9

2%.t n0.2

793.9 330.3

nt.1 328.4

2n2.8 n53

327.7 39.0 3

324.0 41.t 2

329.9 33.9 3

w.2 33.2 2

327.1 33.2 2

325.5 NA 2

324.1 6.5 4

I% K (expresd tn wt. %) is approximaed, based on tbe volume eAr released and appropdab coDsiants.h4uim volre sAr (10{ m1 al$labd fm individual sE6.

Fe No SiAIFMeshSize 0.1 0,3 0.5 0.2 0.6 1.0

r--l--r---r-T-----l f-T-r-T--r-]

_fll L -

J |J_ l L l t " lE t I

- t 10-1-1 6 n

t ^f "

(4o/50)

(s0160)

(70/80)

(60/1o0)

(1oo/120)

(120/140)

r--L- J {\E - r ' L - o J r -

_rL[ l I

- f _ _ lf l l L -

- f-1-t - l

l r t t r l

0.1 0.3 0.5 0.2 0.6 1.0

2.0 2.2 2.4 2.6 0 .02 .O+ .06 3.1 J.2 3.3 3.4 3.5r-r-r-r-I--T--r-l t--T-t--r-t-T-l t-----T-----F---r---l

f.tl L _

*[ut!

- l [ =

. l - l

[J{s

2.0 2.2 2.4 2.6 0 .02 .04 .06 3.1 J.2 J.3 3.4 3.5

FIc. 6. Histograms srrmmarizing the chemistry of muscovite for the various size-fractions. A11 data are given in atomicproportions based on 1 I atoms of oxygen per formula unit

t-\|

*-1-

J-tJI

"j-Il - l

-lL,_=

|-]t ll L _

r - l

l----l-

dtLJ

ut_L

tl_.J

-l

tl-

J-t- - L -L:_-J

t l- t

nr J L r n

|-.] .-r1- - t

RSSULTS OT $ATF9AT INCNNT,MNIALSrm-IlrarNc E:<prnmnqrs

The age spectra for seven incremental step-heatingexperiments are presented in Table 4 and Figure 8,and the important results are srrmmarized in Table 5.For each sample, outgassing was performed using thesarne heating schedule (n = Il steps) to ensure validcomparison among the size fractions. The results areremarkably uniform for all size fractions, with thefollowing points noted: (1) similar values f61 minimum(with one exception), maximum and integrated ages,(2) similar gas-release patterns, as indicated in the plotsof volume of gas liberated (represented as 3e Ar) versustemperature, and (3) comparable age spectra reflecting,therefore, similar distibution of 4Ar within the sizefractions.

Examination of the age spectra in more detailindicales that in all cases, samples are characterized byyoung ages for the low-temperature gas fractions, after

t247

which ages increase monotonically for successivesteps, such that maximum ages are recorded for thehigh-temperature gas fractions. There occurs, however,in all samples a tailing off of the ages for the finatl-3%o of the gas released, presumably reflectingoutgassing ofAr from a relatively more retentive site.The low-temperature ages are generally uniform at300 t 20 Ma, except where either anomalouslyyounger (C-11) or older (C-6) apparcnt ages wereobtained. For sample C-l1, the first step at 600'C hasan anomalously low age of 23IMu but the subsequenttwo steps (680" and 735'C) have identical apparentages of 29I Ma. Similarly, for sample C-6, the firststep (600'C), with an age of 337 M4 is succeeded bythree steps with apparent ages of.292 to 296Ma.

The results for the seven size-fractions are internallyconsistent, but differ from results for the four samplesof muscovite previously analyzed Q.en61h & Reynolds1985, Kontak & Cormier 1991) by step-heating exper-iments in terms of their profiles, plateau ages and

RESETTING OF MUSCOWIE AGE. EAST KEMPTVILLE, N.S.

Hc. 7. Back-scattered electron image @SE) of a muscovite gra.in from the -40l+50 meshfraction. The variation in color (kght versus dark patches) reflects the variationin average atomic number in the mineral, in this case the variation in proportion ofFein the light Q-9 wt.Vo FeO) aud dark (<5 ,ut.To FeO) areas. Note the iregul,ar natureof ttre disuibution of these zones, suggestive of a secondary (Le., postmagmatic)origln.

r . A . . s r o . g t 1 . 2 &P . 4 . . 9 . 7 : 1 . 0 &

1248 TIIE CANADIAN MINERALOGIST

Fto. 8. Summary of €Arf9Ar age sp€ctra for stepwise heating experiments conducted on the seven size-ftactions of muscovite.The samples are ordered from the coarsest (C6, -40l+50) to the finest (Cl2; -1201+1,40 grain size. Abbreviations:I.A. = in16g'41s6 age; P.A. = plateau age. For plateau segments (P.A.), the steps used are indicated by an asterisk in Table 4,but note that these are not strictly plateau ages, since in these segments less than 50Vo of the total gas released is includedin the calculations. In the adjacent box diagrams, the dashed line shows the volume of 3eAr (lO3 cm3 tfi?) for each step,whereas the solid line shows the volume 3eArfC for each steo.

I

! a3 g

q t

9 S

I. E t3 a

a

II

c

integrated ages. The profiles obtained are most similarto the single age spectrum reported by Kontak &Cormier (199L), for which the low-temperaturefractions of gas indicated apparent ages of'269 to286 Mu and the high-temperature steps indicated aplateau (62Vo gas released) age of 338 t 2 Ma. The

age of this sample, 326 Mu is older thanthe integrated ages for the fractions dated in this studyCIable 5).

DIscussIoN

The results of our crystal-chemical and step-heatingexperiments on muscovi!9 from a singls samIils q1leucogranite from East Kemptville provide informationthat can be used to interpret the complex geochrono-logical evolution of this part of the Meguma Terrane.However, first it is necessary to address the role ofvarious parameters that have influenced the age spectra

of the muscovite samples in order to discern whichfactors are the most important and to evaluate thesignificance of the apparent ages.

Pattems of regional cooling

On the basis of 40ArFeAt dating (Reynolds et aI.1981, Keppie et al. 1993, J.D. Keppie, pen. comm.,1993), whole-rock Rb/Sr (Chatterjee & Cormier 1991)and Pb-Pb (Chatterjee & Ham 1991) methods, regionalsooling of the Davis Lake pluton to below ca.350"Coccurred at 370 Ma. For example, outside the faultzones, but within the pluton @g. 2), muscovitesepaates yield undisturbed onAr/teAr release specrathat are identical to ca.370 Ma whole-rock Rb/Sr andPb-Pb isochron ages. If the resetting observed atEast Kemptville was part of a regional thermal eventanalogous to differential postmetamorphis cooling,then a systematic pat[ern of varying mica ages should

1 . A . . 9 1 7 . 3 ! e . 2 hP . A . . 9 . 1 : 1 . 8 S

l . A . . 3 1 3 . O ! 2 . 1 &P . 4 . . 9 4 . 0 : 1 . 8 &

cl1 MTJSCOVITE

r . A . . 3 1 r . 0 : 1 . 5 &P . A . . G . 5 t l . 3 &

! . A . . A 1 2 . 4 ! 1 . 9 kP . A . . e 4 . t : 2 . 1 &

Cumlaffw o/osAr

l . A . " 9 1 6 . 9 ! 1 . 9 &P . 4 . . S . 2 ! 1 . 3 8

CumuHve 7osfu

RESETTING OF MUSCOWTE AGE, EAST KEMPTVILLE, N.S. 1249

be observed (e.9., Dallmeyer 1975, Harison et aI.1989, West & Lux 1993); such is not the case. Instead,a broad area of consistent mineral ages is observed inthe Davis Lake pluton, with younger ages observedwithin fault zones transecting the granite, as at EastKemptville. Thus, we conclude that neitler slowcooling nor a regional reheating event can account forthe discordant age spectra in the muscovite samples.

Cawe of discordance in the muscovite samples

. There are several aspects of the discordant agespectra for muscovite from East Kemptville thatrequire consideration: (l) the origin of the apparentdiffirsive loss profiles in the various size-fractionsanalyzed, (2) the significance of the apparent agesobtained for the size fractions, and (3) the reason forthe variability in specta for the various samplesanalyzed within the deposit area. These aspects areevaluated in turn.

Origin of apparent diffusiveJoss profi.ks

The origin of the diffrrsion-type profiles for themuscovite age specha may reflectl (1) partial diffirsiveloss ofradiogenic argon during a thermal event, (2) amixture of different grain-sizes or presence of contam-inants, (3) a mixture of muscovite polytypes, and(4) intragranular deformation during formation of theEKEDFz.

In the fust case, data discussed above suggest thatthe discordant age specta are not related to partialdifhrsive loss of 4Ar related to a regional thermalevent after the initial cooling of the area tbroughthe argon blocking temperature, ca.350"C (McDougall& Harrison 1988). Instead, tle observation thatdiscordant age specta characterize samples collectedfrom a fault zone @KEDFZ, Fig. 2) suggests possibleloss of radiogenic {Ar from the muscovite concurrentwith the same tectonothermal event responsible forformation and re-activation of this structure.

A second possible explanation for the apparentdiffirsive-loss profile for the muscovite samples isrelated to mixed or contaminating phases of differentages. Wijbrans & McDougall (1986) and West & Lux(1993) have inferred such a model to account for theapparent diffirsive profiles obtained in age studies ofmetamorphic rocks containing two distinct generationsofmuscovite, a coarse and fine fraction. In the case ofthe East Kempwille samples, there is no correlationof any measured parameter with grain size, as might beexpected if paragenetically and temporally differentgrains of muscovite of various sizes were mixed. Forexample, West & Lux (1993) showed that both theshape of the age specta and lower integrated agescharacteize the finer grain-size fractions of muscovitein deformed rocks from a fault zone. These authorscould relate this feature to the fact that fine-grained

neomorphic muscovite grew late during formation ofthe fault zone. On the basis of petrographic observa-tions that a distinct, later-generation muscovite oftectonic origin does not exist in sample EK-9G-A, weconclude that the presence of a mixture of two distinctpopulations of muscovite of different age and differentgrain-size is not a plausible explanation of the agespecfta.

As a third possible explanation, there could be amixture of polytypes of muscovite, as found byWijbrans & McDougall (1986, 1988), and Scailletet al. (1992). It is not possible to model the age spectaby mixing the more retentive 3Tpolytype with the lessreientive 2Mt polytype, as documented in studies ofpolythermal metamorphic terranes (e.9., Wijbrans &McDougall 1986, 1988), as we have shown that asingle polytype occurs (2M). T\e composition of themuscovite in the size fractions is consistent withthe cell parameters determined from refinement of theX-ray-diffraction data.

The fourth and final explanation, that of intra-granular deformation of muscovite resulting in partialdiffirsive loss ofargon, is difficult to assess; the samplewas selected for its lack of deformational features.However, there is certainly petrographic evidencesuggesting deformation of muscovite and partialdevelopment of subgrains within the coarser grains.Other samples within the deposit area" albeit ftommore stongly deformed areas, clearly demonsftate moreextensive development of such features. The chemicalheterogeneity within muscovite (Fig. 7) indicates thepresence of distinct domains that would not be readilynoticed in a routine petographic examination.

Thus we conclude that the apparent djffirsive-lossprofiles for the muscovite samples are confrolled bysubgrain development due to a combination of bothphysical and chemical phenomena. The physicalprocesses reflects the influence ofthe fault zone and itsperiodic reactivation over time, whereas the chemicaleffect reflects the inherent heterogeneity developedearly owing to a combination of magmatic andhydrothermal processes. Since there is no notabledifference in the age specta for different grain-sizes,the effective grain-size confiolling argon diffrrsivitymust have been smaller than the smallest size-fractionaualyzed (i.e., <175 W).

Significance of apparent ages fromeAr/igAr age spectra

Both theoretical modeling (Turner 1968) and field-based studies (Hanson et al. 1975, Harrison &McDougall 1980, West & Lux 1993) show thal theinitial low-temperature gas increments in diffrrsive-lossprofiles in 4ArFeAr age specfia are coincident with theime of the event that produced loss of radiogenic 4Ar.

Thus the minimum ages of ca.300 Ma for the low-lemperature gas fractions (Tables 4, 5) approximate

L250 TIIE CANADIAN MINERALOGIST

the time of the tectonothermal event. These ages aresimilar to the nearly concordant age specha of295 to300 Ma obtained for two muscovite samples by Zentilli& Reynolds (1985). By analogy, the ages ofthe high-temperature gas fractions should give a minimumestimate of the original age of the sample. However, inthis case, the ages of ca. 330 Ma are well below thebferred age of crystallization of the leucogranite,370 Ma (Table 1). The potential significance of rhe330 Ma age is discussed below.

Origin of varinble aqAr/3eAr age spectrawithin thz East Kempnille area

Results of the present study, combined withprevious work in the East Kemptville area, indicate amarked variation in the 4oArFeAr age spectra formuscovite. We have slaminsd the most importantfactors generally considered to affect argon diffrrsivityand conclude that they have not been significant interms of accounting for the observations. In light ofthese conclusions and additional geocbronological datafor the southwestern Meguma Terrane that documentthe presence of episodic tectonothermal events ofCarboniferous and Permian age @luecke et al. 1988,Dallmeyer & Keppie 1987, Cormier et al. L988), wesuggest that the variable age spectra reflect veryleealized structural focusing of heated fluids. Thismechanism would result in the generation of steepthermal gradients that could account for the markeddifferences in age spectra over short lateral distances.The strong anisotropy within the rocks (i.e., deforma-tion fabric) may be significant in this respect because itwould provide a means of concentrating heated fluidsalong na:row structures or conduits (i.e., perhaps onlytens of meters) within which partial resetting ofmuscovite phases occurred. An additional factor, butone that is diffrcult to quantify, is the extent ofdevelopment of the subdomains within the muscovitegrains, which will control the effective grain-size ofmgon diffirsion.

Implicartons for the thermal historyof the southwest Meguma Terrane

The new data presented here, in conjunction withresults of previous geochronological studies of theEast Kemptville (fable 1) and the sunounding area,invite some comments with regards to the thermalevolution of this pafi of the Meguma Terrane. As notedin Table 1, the emplacement of the leucogranite andrelated mineralization both occurred at ca. 370 Ma.However, less robust isotopic systerns than U-Pb andPb-Pb indicate considerably younger ages, with themaximum Rb-Sr whole-rock ages coinciding withtle maximum +o6tfe6t muscovite age at about350 Ma. Although a single temperature of closure doesnot exist as such for the whole-rock Rb-Sr system, a

temperature of ca. 350'C is sufficient to allow argonditfrrsion in muscovite (McDougall & Harrison 1988),and a slightly lower temperature (ca, 325"C) issuggested by Snee et al. (1988) if fhe muscoviteconsists of the 2M, polytype. It is not possible,howevero to distinguish between a protracted thermalevent of about 20 Ma and a single tectonothermal eventof short duration that would have had 16 sompletelyhomogenize the Rb-Sr systematics and reset the K-Arsystem. The various Rb.Sr whole-rock - mineral andofrAil3eAr muscovioe ages indicate that later thermalevents rvere clearly of variable magnitude, eausing insome cases complete resetting of muscovite (e.9.,300 Ma age spectra), whereas in otherr, only partialresetting (Frg. 8 of this study). Thus, temperaturesassociated with these episodic events must havefluctuated from close to tlat required to reset themuscovite K-Ar syslem to that causing only patialresetting.

CoNcLUsIoNs

Detailed nful3eAr step-heating experiments com-bined with crystal-chemical and )(RD studies of sevengrain-size fractions of muscovite from the EastKemptville leucogranite (366 Ma) indicate: (l) asimilar mineral chemistry for all size fractions, (2) thepresence of a single polytype of muscovite, 2Mp aad(3) similar age spectr4 with maximum and minimumapparent ages of about 330 Ma and 300 Ma respec-tively. Thus, we consider it unlikely that eitherapparent grain-size, differences in mineral chemistry ora mixture of polytypes was responsibls fe1 selftellingapparent diffusion-loss profiles in the muscoviteanalyzed.Instead, it is more likely that the presence ofmicrostructures, intragranular recrystallization andchemical heterogeneity within the muscovite (i.e.,suffi6mnins) resulted in an effective grain-size withrespect to argon diffrrsivity that is smaller than thefinest size-fraction analyzed (<175 pm).

AcKtIowuDGHvml"Is

Field work at East Kemptville (D. Kontak) wascarried out through the Canada - Nova Scotia MineralDevelopment Agreements. The assistance over theyears of the staff of East Kemptville Tin Corporationis acknowledged. The argon laboratory at Queen'sUniversity is funded tbrough an NSERC operatinggrant to E. Fa:rar; X-ray-diffraction work by R.F.Martin also was funded through NSERC. Manuscriptpreparation was done rvith the capable assistance ofpersonnel of the Nova Scotia Departuent of NaturalResources; they are thanked for their continued$upport. We acknowledge the comments of L. Snee oncertain aspects of the paper. This paper is publishedwith permission of the Director, Nova ScotiaDeparbaent of Natural Resources.

RESETTING OF MUSCOWIE AGE, EAST KEMPTVILLE, N.S. I25L

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Received August 3, 1994, revised m.anuscript acceptedJuly 6, 1995.