Margarite.Bearing Pelites from the Western Rocky Mountains, Northwest of Golden, British Columbia

Canadian Mineralogist Vol.29, pp. 1l-19(1991)

MARGARITE.BEARINGPELITES FROM THE WESTERNROCKY MOUNTAINS,NORTHWEST OF GOLDEN,BRITISH COLUMBIA

LEN P. GAL ANNEDWARD D. GHENT Departmentof Geologyand Geophysics,University of Calgary,Galgary, Alberta T2N 1N4

ABSTRACT INTRODUCTION

Margarite [CaAl2(Al2SirOto(OH)21has been found in Winkler (1979)noted the common occurrenceof five pelitic and calcicschist samples in the SolitudeRange margarite in some greenschist-faciesrocks. This of the western Rocky Mountains, British Columbia. lt mineral is widespreadin the Alps (e.9., Frey et ql. occurswith muscovite,chlorite and biotite in a lepidoblastic 1982),but very few occurrenceshave been repofied garnet textureand is apparentlystable in the zonethrough from the CanadianCordillera (e.g., Jones1971). (XRD) of the kyanite zone. X-ray-diffraction analysis interest becauseit participates in margarite. Geother- Margarite is of lower-grade slates failed to detect minerals such as mobarometry of muscovite-plagioclase-biotite-garnet equilibria with many common assemblagessuggests temperatures near 580-600oCand plagioclase, calcite, the aluminum silicates and pressuresnear 7 kbar. Margarite + calcite + quartz assem- clinozoisitein a range of bulk compositions.This blagespersist to temperaturesthought to be at or near the note discussesthe petrology of some margarite- stability limit of the assemblage.In the highest-gradeoccur- bearing pelites and calc-pelitesfrom the western rences,margarite + plagioclaseis present rather than Rocky Mountains. kyanite + clinozoisite, indicating that pressureswere prob- In the Solitude Range, about 100 km northwest Fluids that equilibratedwith ably lessthan about 7.5 kbar. of Golden, British Columbia, margaritewas identi- margarite + calcite + plagioclase+ clinozoisitewere of five samplesof two Cambrian lithostrati- mixed H2O-CO2 composition. fied in graphicunits (Fig. l). Margarite occursin schistsof Keywords: margarite, geothermobarometry, phase the Mahto Formation (greenschist to middle equilibria, calc-silicateminerals, metamorphosed pelites, amphibolite facies),which is the uppermostunit of Rocky Mounta.rns,British Columbia. the Gog Group, and in the overlying Tsar Creekunit' which is the lowermost unit of the ChancellorGroup. The structural geologyand stratigraphyof the area Sontnetnr are discussedin Gal el al. (1989)and Gal (1989). r6ussi identifier la margarite CaAl2 Nous avons d PETROGRAPHY (Al2SitOt0(OH)2 dans cinq 6chantillons de schiste p6litique provenantde la chainede la Solitude, et calcique schistsare listed partie occidentaledes montagnes Rocheuses, en Colombie- Assemblagesof margarite-bearing Britannique. Ce mica coexisteavec muscovite, chlorite et in Table 1, and samplelocations are shown in Figure biotite dansune lexture ldpidoblastique,et serait stabledans l. In all but onesample (88LG426)' margarite coex- les zonesd grenat et a kyanite. Selonune analysepar dif- ists with muscovite. Margarite was differentiated fraction X, la margaritesemble Otre absente des ardoises, from muscoviteby its slightly higher relief and lower i faible degr€de mdtamorphisme,Une analyseg6othermo- birefringence(Fig. 2). Subsequentanalyses of four - - barom6triquede I'assernblagemuscovite plagioclase bio- of the samplesby electronmicroprobe confirmed the - pression tite grenat indique une temp6ratureet une de presenceof a Ca-rich mica. No coexistingparagonite 7 kbar. L'assemblage formation d'environ 580-600'C et by eitherX-ray diffraction or electron- margarite + calcite + quartz persistejusqu'd environ la was detected temp6raturelimite de sa stabilit6,Dans les r6gionssoumi- microprobe analysis. sesaux plus degr6sde m6tamorphisme,I'assemblage mar- The margariteoccurrences range in metamorphic garite + plagioclaseest stable plut6t que kyanite + clino- grade from the garnet zone to the kyanite zone, zoi'site,indication que la pression6tait inf6rieure d environ although kyanite is not presentin any of the sam- 7.5 kbar. La phasefluide en 6quilibre avecmargarite + ples. X-ray-diffraction studies of sub-garnet-zone calcite + plagioclase+ clinozoisite6tait un m6langede slatesand phyllites failed to detectmargarite. H2O et de CO2. Margarite occursas fine plates,aligned in the early schistosity,and folded by a later (Traduit par la R6daction) bedding-parallel crenulationfabric. It appearsto be havecrystallized with muscovite, Mots-clds:margarite, g€othermobarom6trie, dquilibre des early in the metamorphichistory, phases,min6raux calco-silicat6s,p6lites mdtamorphi- biotite, and chlorite. The peak conditions of s6es,montagnes Rocheuses, Colombie-Britannique. metamorphismare considered to haveoccurred syn- ll t2 THE CANADIAN MINERALOGIST

SophistMtn.^ ffh:'

117050,w

FIc. 1. Samplelocation map. Garnet, stauroliteand kyanite zonesare indicated.The dashedgarnet isograd is approx- imately located. On inset map of British Columbia, 49 representsthe forty-ninth parallel, 60, the sixtieth parallel, C, Calgary,G, Goldenand V, Vancouver.Dot pattem is the OminecaCrystalline Belt, and the diagonalline pattern is the Rocky Mountain Fold and Thrust Belt. Sampleswere collected by LPC during 1987and 1988,and are curated at the University of Calgary.

chronouslyto just after the formation of the crenu- Except for fine-grained graphite, there are no lation fabric (CaI et ol. 1989), The margarire, inclusions in the margarite. Margarite was nor however, exhibits sweepingextinction across the observedas an inclusion in other minerals, except microfolds, with no evidenceof recrystallization. in 88LG426, where margarite needles occur in The margariteis generallyfiner grainedthan the plagioclasexenoblasts. These needles have an orien- coexistingmuscovite, though both phasesmay be tation that is continuouswith the externalcrenula- intergrown on a fine scale (Fig. 2). This intimate tions. In addition,small resorbed crystals of clinozoi- €rssociationhampered microprobe analyses. In sam- site suggestthe following equilibrium: ple 87LG062c,late-formed (post-crenulation)muscovite appearsto havepartially replacedthe earlier- mrg + 2czo + 2qtz = 5an + HrO (l) crystallized,finer-grained muscovite + margarite. in 88LG426,where abbreviations with lower-caselet- tersrefer to the components,and abbreviations thar

TABIJE 1. UINERAI' ASSE!,IBIJAGES OF I{AREARITE-BEARING are capitalizedrefer to thephase. Mineral and phase ROCKS, SOtiIgUDE RANGE, BRITISI{ COLU!,tBIA abbreviationsare listed in Table l, except for an (: anorthite). S@pLe otz Ug Mrg Cal CtrL Cld E! crt gt pL Czo All samplesfrom the Tsar Creek unit, except STTG061XXXXXEXXXX 88LG44l, are characterizedby calcite(ferrous dolo- 87IJG062eX X X X X+XE X X X X mite or ankerite also is present).Sample 88LG44l 88IG44lXXXXgXlXX is also the only sample lacking plagioclaseand clinozoisite.In all samples,garnet is ubiquitous,and 8AIG403aX X X X XE X X X X X either chloritoid or biotite may be present.Chlorite aar_G426xXXXXXX is generallya retrogradephase after garnetand biotite, but there are samplesthat haveapparently sta- X = preEenti Xs = sacondaryi X1 = lnc1uslon ble progradechlorire (e.g., 88LG426).The amphi- AbbrevLatlonE froB Kretz (1983): bole in 87LG062c is a ferrotschermakite (cal, Qtz = quartzi l{E = auscovite; l.trg - nargaritei unpubl. data, 1989). cal = caLcltei Ch1 = chlorltei Cld = chl_orltoid; Bt = bio!'ltet crt = garneti 9t = EtauroLlte; Conaposrnox Pl E pLagloclasei Czo E cLlnozolElte. AlphlboLe oF THEMnncanrrs ptesent 1n 87IGO62C, lLnenlte preeent in all Emples exceFt 87LG06Li rutlLe present In 88LG426; plElte Analytical data on the margarite in samples preEat ln 88LG441 and 88IJc403a. 88LG426and 88LG403aare presentedin Table 2. MARGARITE-BEARING PELITES FROM THE ROCKY MOUNTAINS l3

Frc. 2. Photomicrographof margarite-muscoviteschist 88LG403a. Margarite (Mrg), muscovite(Ms), biotite (Bt) and ilmenite (opaque) lie in the plane of schisrosity. Garnet (Grt), plagioclase (cloudy, low-relief grains above gamet), clinozoisite (small, high-relief grains) and carbonate (upper left corner) are present.Note the higher relief and smaller grain-sizeof the margarite comparedto muscovite.

Other sampleseither were not analyzed, or the fine TABLE 2. C!!'IPOSITION OF iTARCARITE FRO!{ TBE SO''ITUDE grain-sizeof the muscovite-margariteintergrowths RANGE, BRITISE COIi'IIBIA yieldedpoor results.Compositional data for other a918426 88I,GAo3a major phasesin samples88LG426 and 88LG4O3aare Empl€ a9lc,426 aAIG4o3a No. of, No. of, presentedin Tables 3 and 4. Electron-microprobe aallees l7\ atom atom data were reducedaccording to the principlesout- Si.O2 32.L4 32.26 9i 2.1S3 2.148 lined by Bence& Albee (1968).Details of the stan- TiO2 0.17 0.10 Al 1.817 t.aL2 available A12O3 46.74 47.40 s@ 4. 000 4. OO0 dards used and operating conditions are FeO 1".26 0.38 Al 1.925 L.977 from EDG. !{no bd1 bdl Ti 0.009 0.005 ugo 0.93 0.20 Fe O.072 o.o22 There is considerably more sodium in crystalline CaO 9.72 LO.72 llg 0.094 0.020 Na2O L.75 1.26 sm 2.100 2.O24 solution in the margaritethan in muscovite.This is K2o 0.38 0.64 ca 0.707 o.779 analogousto the limited solution of the anorthite BaO bdt bdl Na 0.230 0.166 bdl bdl ( 0,033 0.055 componentin K-feldsparand is likely dueto the size 0.06 0.05 Su 0.970 1.O00 difference in the cations, as well as the difference ItZO 4.70 4.74 in valenceof the cations (Guidotti 1984). 97.46 97.76 Substitutions involving octahedrally coordinated -O-F .O2 .02 Fe and Mg also occur, although they are somewhat sm 97.44 97.74 lessimportant in margaritethan in muscovite.This All Fe trsted aB f,enoue. F20 f,lon EtoichXoDetry' is because such substitutions enlarge the bdl = b€lm detection LinLt. NwberE ln parentheEeg l2-coordinated site and, as a result, the highly are ttr€ n@b€rE of, Epot analyseg Ln the avsagg. No' charged, though relatively small Ca ion is unstable of atoE = nuber of atmE calolated on tbe ba8lE in sucha structure. Substitution invohing Fe and Mg of, - 22 anlontc charge. l4 THE CANADIAN MINERALOCIST

TABT,E 3. COUPOSITIONS OF BTOTTTE, I{USCOVITE, CHIORTTE TABI,E 4. COI.IPOSITION OF PI,AGTOCXNSE AND CI.INOZOISITE AND GARNEtr F?O}T TEA SOI,ITUDE RANGE, BRITISII COLUI.TBIA rROU T1IE SOTITUDE RANGE, BRITIAE COil'T,IBIA

praga0craEe cranozotElla Bt Bt l.tE Chl Crt crt Eamle r-G426 r,G403a I&426 r,Gi403a Eeple I&426 tG4O3a LCt4O3a LG426 I&426 Ic4O3a anaiyEeB ( 10) (r2') ( 10) analyEeE (9) (15) (13) (12) (x5) (11)

S1O2 36.43 35.55 46.03 25.L2 37.81 37.25 59.L9 51.60 39.42 39.43 TiO2 L.A2 1.58 0.30 0.08 o.o2 0.06 ilt, n/a n/a o.L2 0.09 A12o3 18.62 L9.L4 34.26 22.9A 2L.L2 20.42 25.83 31.16 30.23 XO.22 F€O 16.13 L7.a4 L.20 19.4S 30.80 3L.76 iis6, 0.19 0. 07 5.834 6.024 MnO bdl bdl" bdl 0.07 r.46 0.57 l.tnO \/a n/a bdl 0.05 l.lgto L3.26 rL.97 0.87 19.18 3.89 2.8r ugo bdL 0.06 0.04 0.03 0.06 0.01 n/a \/a n/a 7 .63 13.14 23.96 23.95 Na2O o.22 0.08 0.84 n/a n/a nla 7 .42 3.92 nla nla Kzo 8.33 8.91 9.83 bdl \la n/a Iia" 0. 04 0.06 nla nla Bao bdL o.27 0.58 n/a \la nla n/a \la 0.96 0.s7 bdL bdl bd1 n/a n/a nla 0.35 0.49 0.06 n/a \la \la Hzo 4.03 4.00 4.43 rr.64 t 00.29 99.94 100.58 100.67 Nunb€r of atons 99.22 99.a6 98.38 98.56 100.34 100. 19 -O=F 0.r5 O.2L 0.03 n/a 2.636 2.34L 2.99 1.356 1.668 0.o1 0. 01 Em 99.07 99.65 98.38 98.56 100.34 100. 19 At 0. 000 0. 000 2.70 Fe 0.007 0.003 0.33 0. 34 o.364 0. 639 1.95 L.94 NA 0. 640 0.344 nla n/a sl 2.7!O 2.667 3.114 2.5a7 2.997 3.005 K 0.004 nla n/a A1 L.290 1.333 0.886 1.413 0.003

9U 5.005 4.999 7.99 8. 00 AL 0.343 0.359 L.447 1.378 L.97L L.942 Tt^. 0.102 0.089 0.015 0.006 0.001 0.004 o.649 Fe'- 1. oo4 1.120 0.068 L.674 2.042 2.L43 I"l 0.636 o.347 Ms L.47O 1.338 0.087 2.945 0.459 0.33S T:: 0. 002 0.004 ltn bd1 bdL bdl 0.006 0.098 0.039 x;; 0.11 Ba bd1 0.008 0.015 nla nla nla tizo 0.67 o.67 ca 0.002 o.004 0.001 \la O.445 0.550 Na 0.032 0.011 0,110 \la nla nla K. 0.790 0.852 0.848 H2O fron gtolchtonetryi < totaL - bdt n/a nla a Fe aa Fe,O"i-Nuqbers n/a not analyz€di bdl - beLow dEtection llnlt.- In parentheses AlL F€ treated aa f,errouE. ti2o f,ron etolchlonetry. ale the nunbe! of spot analyBes. Nuabe! of atons calculated on the basle of -16 anlonlc chalge hL = belov detectlon Unit. n/a =not analyzed. fo! pLagloclaEg and -25 anlontc charge fot apldote. - - Blollte, nuEcovlte, and chLorLte numb€r of atong I; nole fraction of anorthltei L" nole fracLion of albllet - nole fractlon of o-rthoclaBei = baeed upon -22 alonlc chargei garnet based upon I"r X* nole fractloii of plBtaciLei q- - acttvtty of-&Ilno- -24 anionlc pasthesgs chalge. Nunbars in beLov zoiaite coalE!€n!. See text for explanatlon. Cllno- suple nunber iE nurber of, spot analyEeE, zolsite analys€s carrled out by M. Z. gtout.

is very minor in paragonite(Guidotti 1984).Thus, ing on octahedrally coordinated sitesin daphnite has although there is considerablesolution between beenused for the clinochlore end-member.The equa- paragoniteand margarite, thesetwo end members tion is a..6 : l(Me/6)/(5/6)J5.[(A1/6)/(1/6)],where show considerably less substitution in the octa- a..6is the activity of clinochlorecomponent in chlo- hedrally coordinatedsites compared to muscovite. rite, and Mg and Al are the number of Mg and Al atoms, respectively,per formula unit. The solution THERMODYNAMIcDATA ANDACTIVITY MoDELS model for clinozoisite-pistacitesolutions is the ideal solutionmodel of Skippen& Carmichael(1977); see Exceptwhere otherwise noted we havecalculated also Jenkins et al. (1985).The equation is aoo : all phaseequilibria using the program PTg, of Brown (l-3XoJ, wherec.ro is the ideal activity of clinozoi- et al. (1989)and the thermodynamicdata-base of site componentin epidote,md Xo, is the mole frac- Berman(1988). Thermodynamic data for anniteare tion of pistacitein the epidote.A simpleactivity- given by Berman (1990). model for margarite is presentedbelow. There is significant crystalline solution in mar- The major substitution in margariteis Ca + Al garite, chlorite, biotite, plagioclase and garnet. + Na * Si, which reflectsthe crystallinesolution Activity modelsused for garnet and plagioclasesolu- betweenparagonite and margarite.Muscovite solutions are those of Berman (1990)and Fuhrman & tion and Fe-Mg substitution in octahedrallycoor- Lindsley(1988), respectively. The solutionmodel for dinatedsites are minor and can be ignored(Guidotti biotite is that of Indares& Martignole (1985),and 1984;also seeTable 3). An ideal coupled subsritu- that for muscovite-paragonitesolutions is taken tion expressionfor the activity of margaritewould from Eugsteret ol. (1972)and Chatterjee& Froese be a.ru = Ca/(Ca * Na) = X.r*, where Ca and (1975).Chlorite solutionis treatedas being ideal; the Na are the number of Ca and Na at6ms, respectively, activity model of Chernoskyet al, (1988)for mix- and Xms is the mole fraction of margarite. Solution MARGARITE-BEARING PELITES FROM THE ROCKY MOUNTAINS 15 betweenmargarite and paragoniteis not ideal, and has a composition (0.71 I Xwe < 0.78) far the solvushas beenstudied using synthesisexperi- removedfrom the estimatedlimb of the solvus,we menrs by Franz et al. (L976). They stated (1976, will usea regular solution-model,which requiresonly prob- p. 308), "Altogether our experimentsdo not allow one Margulesparameter. This assumptionwill unequivocalconclusions for the positionofthe bino- ably overestimate the nonideal behavior of dal curve for the paragonite-margaritesolws." They margarite-richcrystalline solutions. Consequently, estimatedthat the critical point was near 600oCat we have also performed calculationsin which we a compositionnear firrgro.They suggestedthat the haveassumed ideal behaviorof margarite-richcrys- solvusshould be rather insensitiveto pressure,over talline solutions.These calculations should bracket their experimentalrange of l-6 kbar, sincethe molar the "true" solution model. If the critical tempera- volumesof the two mica end-membersare very simi ture Z. of the solvusis known, then W6 can be esti- lar. An activity model including two Margules mated from: parameters( t/o) is required to describesolution involving a nonsymmetricalsohus (e.9.,Nordstrom T" = Wc/2R (Nordstrom & Munoz 1985) (2) & Munoz 1985).Because the solvusis poorly con- strainedand the margaritefrom the SolitudeRange where lZ6 is the Margules parameterand R is the

*/o .ro/f- +l+9 */ld-/8-: 8000 3 /6I ^,o P zooo c] fj.u."* lrJ o. 6000 f (t) o ld I sooo

4000

3000

2000 400 450 500 550 6@ 650 700 750 800 TEMPERATURE"C Frc. 3. P-I diagram showing representativeequilibria for the system CaO- A12O3-SiO2-H2O-CO2. See text for methodsof calculationand activity models used.-Abbriviations(after Kretz 1983)are listed in Table 1; in addition' Prl = pyrophyllite, Ky : kyanite, and Sil = sillimanite. t6 THE CANADIAN MINERALOCIST

gasconstant. The critical temperatureof the solvus set one another, and calculationverifies that the tem- can be conservativelyestimated from the data of peraturechanges very little when we apply the solu- Franz et al. (1976)at about 600oC,over a pressure tion modelsof margariteand plagioclase.Margarite range of 1-6 kbar. Substitution into equation (2) + quartzin 88LG426,88LG2141 and 88LG403awas yieldsa W6 of 14,5kJlmol. The expressionfor G." probably at or nearits upper limit of thermal stabil- (excessGibbs free energyof mixing) of a binary solu- ity in the Solitudef,.ange pelites. Thermobarometric tion is: estimatesfrom kyanite- and staurolite-zonepelites G"* = W6X1X2 (3) yield maximum temperaturesnear 570oC,depend- ing upon which calibration of the garnet-biotite (Nordstrom& Munoz 1985,p. 159),where X, and geothennometeris used,and pressuresnear 6-7 kbar X2 are the mole fractions of componentsI and 2, (Gal & Ghenl, in prep.). For 88LG403awe have a respectively.Then for a binary solution, in which muscovite-biotite-garnet-plagioclaseassemblage. Xz : l-Xr, we have Applymg the garnet-biotite geothermometerand the muscovite-biotite-garnet-plagioclasegeobarometer RT1n71: Wc(l-X)2 (4) (e.g., Ghent & Stout l98l), we estimatea temperature near 580'C and a pressurenear 7 kbar. For where.1, is the activity coefficient of component I 88LG426,the garnet-biotitegeothermometer yields (Nordstrom& Munoz 1985,p. 136).With our esti- about 605oCat about 7 kbar. mate of Wb the activity coefficient for margarite Bucher-Nurmi\e\ et al. (1983)identified critical with a compositionX,,* of 0.75 can be estimated low-pressure(margarite + anorthite) and high- from (4). From the data of Franz et ot. (1976),at pressure(kyanite + clinozoisite)stability fields based temperaturesof 873, 823, and 773 K, the activity on the topology of the CASH + CO2system. The coefficientsare, respectively,1.13, 1.14, and 1.15. shrinkingof the plagioclase+ margaritefield of sta- The activity coefficient can then be approximated bility with respectto that of kyanite + clinozoisite asbeing 1.14 over the temperaturerange 500-600oC. was estimatedto occur at about 6 kbar, regardless The activity of margaritein crystallinesolution can of temperatures,but only for calcic plagioclase then be calculatedfrom: (> 50 mol9o An). Sample88LG403a has an aver- age plagioclase composition of Anu, and lacks a^ru= X^r". 1.74 (5) kyanite.For the pure end-members,the equilibrium,

PTnsT EQuTI-IBRIAINvoLvING MARGARITE 2zo*2kY=mrg+3an (8)

Margarite wasnot found in pelitesbelow the gar- is near6.7 kbar at 550oC,where zo is zoisite.The net zonein the SolitudeRange. In the Alps, Bucher- activiry of clinozoisite is 0.67 (Table 4), and the Nurminen et al. (1983)suggested that the incoming equilibriumbetween clinozoisite of this composition of margarite in the lower greenschistfacies occurs and purezoisite lies at about 400oCat7,6kbar. Zoi- through the reaction: siteis favoredby highertemperature; consequently, we haveused the thermodynamicdata on zoisitein 2Pyr + Cal: subsequent calculations. Crystalline solution in Mrg + 6Qtz + CO2 + H2O (6) plagioclase,margarite and clinozoisite(zoisite) would raise the mqximum possiblepressure at which this No pyrophyllite was found in the low-gradesamples, latter assemblageis stableto about 7.5 kbar at 550oC though an occurrencewas describedby Ghentet ol. (Fig. 3), which is consistentwith the estimatesmade (1989)in rocks similar to the lowestgrade rocks in above. the Solitude Range(biotite zone), about l0 km to We next considerother equilibria in the system the southeast.Using the solution modelsdescribed CaO-AIrO3-SiO2-H2O-CO2. For 88LG426 and above for margarite, the presenceof margarite -r 88LG403awe havethe assemblageplagioclase - cal- calcite + quartz assemblagessuggests minimum tem- cite - clinozoisite- margarite- qtrafiz. This assem- peraturesin the range of 415-425'C at 7 kbar and blage would be in equilibrium with an H,O-CO, X(Co) = 0.5 (Fie. 3). fluid with X(H2O) near 0.57 at 531"C and The upper stability limit of margarite + quartz 7 kbar (88LG426),and the equilibrium would be near is constrainedby the equilibrium, 541"Cwith X(H2O)near 0.46 (88LG403a,also see Fig. 4). Theseresults were calculatedusing an ideal mrg+etz=ky+an+H2O A) solution model for margarite; the effects of using the non-ideal solution model for margarite are presented This equilibrium (7) lies at approximarely 580.C and in Figure 4 for sample88LG426 as dashedlines. In 5.9 kbar (Fig. 3). Crystallinesolution of paragonite this examplethere is very little differencebetween in margarite and albite in plagioclasewill tend to off- the resultsusing the different models. MARGARITE.BEARING PELITES FROM THE ROCKY MOUNTAINS t7

P ?o?ol' PCO2 PXZO . TOOOBARS "

C) o

lrJ (l 54n. l ^ F 550 \1-!N.e ..;. E UJ & ) - k., tr, F /?-2fr:";>\ ,"{f [; o

4@ r- o.o0.0 0 | o2 0.3 0.4 0 5 0.6 0.7 0.8 0.9 l.o x co2

Frc.4. T-X(CO) diagrarn at P (total) = 7ffi bars for equilibria in the systemCaO- AI2O3-SiO2-H2O-CO2.Abbreviations after Kretz (1983); also, see Table l. Methods of calculation and activity models are discussedin text. Solid curvescal- culated with a-rg = 0.71 (ideal solution model); dashedcurves are calculatedwith dme = 0.81 (non-idealsolution model).

In sample88LG403a there is a large number of gram. This scattercould be due to a number of fac- phasespresent in a thin section(Table l). Clearly, tors; for example, (a) all of the phasesdid not not all of thesephases can be touching and indeed equilibrate at a unique P(H2O) = P(total) and Z; not all of the phasesare presentin a singlefield of (b) there €ueerrors in the solution models.Adding view (about 500 pm) on the electron microprobe calcite to the systemand examining the equilibria on (e.g., Fig. 2). Examination of the P-Z-X relations a T - X(CO) diagram at 7 kbar also results in wide in the KFCMASH (K2O-FeO-CaO-MgO- scatter of intersectionsof the equilibrium curves. AI2O3-SiO2-H2O)system among the phasesgarnel, Sample88LG426 also containsa largenumber of muscovite, plagioclase,biotite, margarite, zoisite phases;equilibria involving chlorite, garnet' mar- (clinozoisite), quartz, and a fluid phaseconsisting garite,plagioclase, quartz, clinozoisite(zoisite)' and of H2O yields a number of intersections of an H2O phase producesintersections near 640oC equilibrium curves on the P(H2O) : P(total) dia- and 9 kbar. This estimateis in disagreementwith esti- 18 THE CANADIAN MINERALOGIST matesbased upon other geolhermobarometersdis- Bucsen-NununEN,K., Fnam, E. & FnEv,M. (1983): cussedabove. A modelfor the progressiveregional metamorphism of margarite-bearingrocks of the CentralAlps. .4rn. CoNcr-usroNs "L Sci. 283A. 370-395. Margarite-bearingpelites of the Solitude Rangein Cuerren.lrr, N. D. & Ftoesn, E. (1975): A ther- the westernRocky Mountains equilibratedin mid- modynamicstudy of the pseudobinaryjoin musco- dle greenschistto middle amphibolite faciescondi- vite - paragonitein the system KAlSi3O8- tions, in calcicand carbonate-absentrocks. Though NaAlSi3Os- AlzO: - SiO2- }J2O.Am. Mineral. the margarite-forming reaction is unknown, the mar- 60. 985-993. garite + calcite + quartz assemblagepersisted into kyanite zone, probably in equilibrium with Csrwosrv, J. V., Jn., BenueN,R. G. & Bnwoaa, L. plagioclaseand clinozoisite.P-Iestimates basedon T. (1988): Stability, phase relations, and ther- phaseequilibria in simple subsystemsyielded plau- modynamicproperties of chlorite and serpentine sible results,even with a simple solution-modelfor groupminerals. In HydrousPhyllosilicates @xclu- margarite. Inferred compositionsof the fluid indi- siveof Mica) (S.W. Bailey,ed.). Rev. Mineral. 19, cate that margarite ( + calcite + plagioclase -r 29s-346. clinozoisite) equilibrated with a mixed H'O-CO, fluid. Theseoccurrences ofmargarite are ofinterest Cnaw, D. (1978): Metamorphism,structure and because of the abundance of other aluminous stratigraphyin the southernPark Ranges,British minerals in Cambrian metapelitesin the Rocky Columbia. Can. J. Earth Sci.15. 8G98. Mountains (Gal 1989,Ghentet al, 1989,Craw 1978, Cruickshank & Ghent 1978). Cnurcrsuaxr,R.D. & GueNt,E.D. (1978):Chloritoid- bearingpelitic rocksof the HorsethiefCreek Group, Contrib. Mineral. AcrNowt-SocEMENTS southeastBritish Columbia. Petrol.65,333-339. This paper representspart of the researchin the senior author's Masters' thesis. We thank Phil Eucsrrn,H. P., Alsrs, A. L,, BeNcr,A. E., THolrar- soN,J. B., Jn.& Wer-osauu,D.R. (1972):The two Simony for his help with the field aspectsof this phase reglon and excessmixing properties of work. A. Silas,C. Velu, J. Machacek,D. Johnston, paragonite-muscovitecrystalline solutions. J. Petrol, J. Resultay, and M. Horvath helped with various t3. 141-179. aspectsof this work. Mavis Stout provided analytical data on the clinozoisite.The authors thank M. Fnnxz, G., HrNnrcHsrN,T., & WerrnteuacHen,E. Frey for his constructive comments. Ghent (1976):Determination of the miscibilitygap on the acknowledges financial support from NSERC solid solution seriesparagonite-margarite by means Operating Grant A4379,Field work was supported of infrared spectroscopy.Contrib. Mineral. Petrol. by EMR Contract 69-4981to Phil Simony. 59, 307-316. Frev, M., BucuEn,K., FnaNr, E. & ScHwaNoen,H. REFERENCES (1982):Margarite in the Central Nps. Schweiz. Mineral. Petrogr.Mitt. 62,2l-45. BeNcr,A.E. & Alnrr, A.L. (1968):Empirical corre- FuHnnaaN,M. L. & LtNostEv,D. H. (1988):Ternary lation factorsfor electronmicroanalysis of silicates feldsparmodeling and thermometry.Am. Mineral. and oxides.J. Geol. 6,382-403. 73,201-215.

BenuaN,R.G. (1988): Internally-consistentther- Gar, L. P. (1989):Metamorphic and StructuralGeol- modynamicdata for stoichiometricminerals in the ogy of theNorthern SolitudeRange, Western Rocky systemNa2O - KzO - CaO - MgO - FeO - Fe2O3 Mountains,British ColumDia.M.Sc. thesis,Univ. - A12O3- SiO2- TiO2- H2O- CO2.J. Petrol.29, Calgary,Calgary, Alberta. M5-522. Gnrxr, E. D. & SIuoNy,P. S. (1989):Ceol- northernSolitude Range, Western Rocky (1990): Mixing properties of Ca-Mg-Fe-Mn ogy of the Mountains,British Columbia. Geol. Surv. Con., garnets. Am. Mineral. 75,328-344. Pap. 89-lD, 55-60. Bnowu,T. H., BrnuaN, R. G. & PemrNs,E. H (1989): GHeNr,E. D. & Srour, M.Z. (1981):Geobarometry PTA-SYSTEM: A GeO-Calc software package for and geothermometryof plagioclase- biotite - gar- the calculation and display of activity-temperature- net - muscoviteassemblages. Contrib. Mineral. pressure phase dtagrarns, Am. Mineral, 74,485487 , Petrol. 76. 92-97. MARGARITE.BEARINC PELITES FROM THE ROCKY MOUNTAINS l9

-, & Fennr, F. (1989): Chloritoid- Knrrz, R. (1983):Symbols for rock-formingminerals. paragonite-pyrophylliteand stilpnomelane-bearing Am. Mineral. 6E,n7-279. rocks near BlackwaterMountain, westernRocky Mountains,British Columbia. Can. Mineral. 21, Nonosrnou,D. K. & MuNoz,J. L. (1985):Geochemi' 59-66. cal Thermodynamics.Blackwell Scientific Publ., Palo Alto, California. Guroorn, C. V. (1984):Micas in metamorphicrocks. /r Micas (S.W. Bailey, ed.). JRev.Mineral. 13, SrpeEN,G. B. & CanutcuanL,D. M. (1977):Mixed- 3s7-461. volatile equilibria. .In Applications of Ther- modynamicsto Petrologyand Ore Deposits(H.J. INoanns,A. & MenrrcNou, J. (1985):Biotite-garnet Greenwood,ed.). Mineral. Assoc. Can., Short geothennometryin the granulitefacies: the inlluence CourseHandbook 2, 109-124. of Ti and Al in biotite. Am, Mineral. 70.272-278. Wnrren, H. G. F. (1979):Petrogenesis of Meta' Jrwnxs, D.M., Nrwror, R.C. & Gorosnarrn,J.R. morphic Roc&s.(fifth edition). Springer-Verlag, (1985): Relative stability of Fe-free zoisite and New York. clinozoisite.J. Geol. 93, 663-672. JoNes,J. W. (1971):Zoned margarite from the Bad- shot Formation (Cambrian)near Kaslo, British Receivedlanuary 25, 1990,revised manwcript accepted Columbia.Can, l, Eorth Sci.8. 1145-1141. September10, 1990,