Canadian MineralogistVol. 2Q pp. 307-318 (1982)

GEOTHERMOBAROMETRY OF CORDTERITE.BEARING METAPELITES NEAR THEMORIN ANORTHOSITE COMPLEX, GRENVITLE PROVINCE, OUEBEC

J. MARTIGNOLE aNn S. NANTEL*DCpartement dc g€ologie, IJniversit€ de Montr€al, Case postale 6128,

Succursale "A", Montr€al, Qudbec H3C 317

ABSTRAcT

, Six specimens of cordierite-bearing metapelite havebeen selected from an area of about 10.000 km,in the southern part of the Grenville province,north of Montreal, in order to evaluate temperature,pressure and water fugacity tlat prevailed duringthe 1 billion-year-old episode of high-grade meta-morphism. Iron-magnesium distribution coefficie[tsbetween biotite and garnet, as well as coidieriteand garnet, were used for temperature determina-tion. Pressure estimates were obtained throulh themagnesium content of cordierite using the rialibra-tion method of Martignole & Sisi (1931). Oriing tothe role of channol water in tbe stabilization ofcordierite to higher P, knowledge of the watercontent of cordierite is a prerequisite in geobaro-metry. An approximate maximum hydration wasobtained by subt.raction from the sum of oxidesand an empirical conection was introduced forthe presence of other fluids such as COz. Preszuresobtained by cordierite barometry closely correspondto those obtained using the plagioclase-garnet geo-barometer. The highest temperatuies, near 700oCwith possible peaks around 800"C, are recordedclose to the large Morin anorthosite complex. Thelowest temperatures come from the northeasternpart of the area which is a few km beyond theorthopyroxene isograd (550-600oC). The 600oCisotherm could thus be taken as the limit betweenamphibolite and granulite facies in the area. Pres-sure estimates from 4 to 7 kbar do not supportnor explicitly refute the pressure gradient sug-gested by the restricted occurrence of high-pressuregarnetiferous assemblages in charnockite aroundthe Morin anorthosito. Estimate of water pressureare very crude owing to the imprecision in waterdeterminations of cordierite. The presence of optically positive (COr-bearing) cordierite, in whichthe oxide zum is near ooe hundred, shows thatwater pressuro may locaily have been extremelylow. In tbe cases where cordierite is hydrated,P(H,O) may have attained 0.3 Pbt"r.

Keywords: cordierite, garnet, high-grade metape-lites, goothermobarometry, Grenville province.

*Piesent addres: Minist&re de ltEnergie et desRessourcos, 1620, boulevard de I'Entente, Qu6bec,Qu6. GtS 4N6.

SoMMAIRE

La temp6rature, la pression et la pression d'eaudurant le m6tamorphisme qui a affectd la Provincede Grenville, il y a environ un milliard d'ann6es,ont 6t6 6valu6es au moyen de six 6chantilons dem6tap6lite. Irs estimations de teml€rature ont 6t6obtenues ir partir des coefficients de distribution dufer et du magn6sium entre biotite et gxetrat ainsique cordi6rite et grenat, alors que le conteuu enmagn6sium de la cordi6rite a permis d'estimer lapression (Martignole & Sisi 1981). On doit connal'tre la quantit6 d'eau pr&ente dans la cordi6ritepour pouvoir utiliser ce gdobaromBtre, car I'eaufavorise la stabilisation de la cordi6rite vers despressions plus 6lev6es. En appliquant au reste obte-nu dans la somme des oxydes une corection empi'rique pour tenir compte de la pr&ence de CO,dans la cordi6rite, on obtient des pressions com-parables i celles que donne le baromBtre plagio-clase-grenat. Irs temtr€ratures les plus 6lev&svont de 700 ) 800oC et se localisent au voisinagedu complexe anorthositique de Morin. Irs temp6-ratures les plus basses, de 550 tr 600oC, provien-nent de la partie nord-est de la r6gion, au deli deI'isograde de l'orthopyrox&ne. L'isotherme 600oCmarquerait donc ici la limite entre les facils am-phibolite et granulite. 1ut sstimations de pressionsentre 4 et 7 kbar ne confirment ni a'infirrrent legradient r6gi<inal de pression que suggdre Ia pr6-sence d'assemblages i grenat de haute pressiondans les charnockites entourant le massif d'anortho-site de Morin. Les estimations de la pressioo d'eausont encore trls grossi0res, en partie i cause deI'impr6cision sur la d6termination de l'eau dansla cordi6rite. La pr6sence de cordi6rite optique-ment positive (contenant du COz), dans laquellela somme des oxydes est proche de 100, montreque localement la pression d'eau a pu Stre trBsfaible. Dans le cas de cordi6rite hydrat6e, la pres'sion d'eau a pu atteindre trois dixiimes de lapression totale.

Mots-cl6s: cordidritg grcnat, mdtap6lites, faci0sgranulite, g6othermobarom6trie, province deGrenville.

INTRoDUcrroN

The southern part of the Grenville prorince

307

308 TIIE CANADIAN MINERALOGIST

in the Canadian Shield is underlain by high-grade metamorphic roclts intruded by largeplutons of the anorthositq-farsundite suite(Martignole & Schrijver 1977). Whole-rockRb/Sr data for the three main plutonic massifs,the Morin, Lac-Croche and St-Didace com-plexes (Fig. L), have been published by Barton& Doig (1972, 1973, 1974, 1977) and indicateages of 1030 -+ 4t, 7124 -+- 27 and 1,L63 -r

5l Ma, respectively. Supracrustal rocks aremostly of the marble-quartzite-metapelite-gran-ulite-amphibolite association known as theGrenville Supergroup. These rocks have givenRb/Sr ages of 1094 -f 43 Ma, whereas under-lying granulites have ages ranging from 1600+- 58 to 1576 '+ 19 Ma @arton & Doig 1973).

Detailed analytical work and thermodynamiccalculations are in progress in order to unravelthe physical conditions of both the peak of the1.1 Ga "Grenville evenf' and the post-climaxhistory of cooling and unloading. The presentstudy will report some results concerning cor-dierite-bearing specimens of metapelite of theGrenville Supergroup.

RrcroNer- Mr,rerronpnrc SETTING

In the area north of the St. Lawtence Valleybetween Rivibre Rouge and Rivibre St-Maurice(Fig. 1), orthopyroxene is almost invariablypresent in ovenaturated mafic or intermediaterocks. Sillimanite is the ubiquitous aluminumsilicate in pelitic rocks, although kyanite hasbeen reported from two localities (Martignole& Schrijver 1968). Isograds have been mappedaround the Morin anorthosite complex (Marti-gnole & Schrijver 1971) based on the appear-ance of garnet in saturated and oversaturatedplutonites. Garnet growth is thought to be dueto slow cooling under high pressure. Althoughgarnet-forming reactions are controlled by rockcomposition (Martignole & Schrijver 1973),suitable rocks remote from the Morin anortho-site complex are not garnetiferous, thus sug-gesting a horizontal pressure gradient or a dif-ferent cooling history: Moreover, for a distanceof about 50 km around the Morin anorthosite,wollastonite is very common in impure marble.This mineral invariably is rimmed by quartz

71"

Frc. 1. Location map showing. the Morin, Lac-Croche and St-Didace complexes and tfe sampling sitesmentioned in the text.

73"

73"71"

where it occurs in a calcite matrix and by gros-sular and quartz urhere it oecurs in a feldsparor scapolite matrix. The corresponding reactionscould be due to pressure increase, but theyare best interpreted as due to isobaric coolingunder 'increasing activity of HeO, as they arecorrelated with idocrase-forming reactions(Kerrick et al, 1973, Martignole & Schrijver1977).

On the other hand, 15O km northeast of theMorin anorthosite, in the Shawinigan area,wollastonite occurs sporadically along the con-tacts of small noritic plutons, in contact witheither quartz or calcite, whereas prograde gros-sular-quartz assemblages occur in the same area,suggesting either higher pressure and higherwater activity than in the Morin area or lowertemperature and higher water activity, The sec-ond possibility would be more in agreementwith the absence of pressure-sensitive garnet-forming reactions in charnockitic rocks of theShawinigan area. Finally, pressure estimatesof 7 kbar have been derived from the concen-tration of the Ca-Tschermak's component inclinopyroxene in equilibrium with plagioclasefrom charnockitic rocks that have crystallizedbetween 700 and 800'C (Martignole 1979),but this barometer is not sufficiently preciseto reveal any pressure gradient.

In summary, impure marble and charnockiticrocks seem to bear the record of different meta-morphic evolutions in two areas more than onehundred km apart: 1) isobaric cooling at apressure of approximately 7 kbar from a tem-perature of about 800'C around the Morinanorthosite; 2) "frozen-in" conditions near thepeak of metamorphism at significantly lowertemperature and slightly lower pressure, butprobably higher water fugacity, in the Shawi-nigan area.

In order to evaluate these trends more pr.e-cisely, a search for sordierite-bearing assem-blages was undertaken from more than twohundred metapelite samples throughout the areannder discussion (Nantel 1977), Special atten-tion was paid to the divariant assemblage Cd-Ga-Si-Qz because of its temperature-sensitiveexchange reaction and its pressure-sensitive re-actions involving end-members.

MtNBner- Assstvrsr,ecss IN METApELTTES

Among theliwo hundred specimens of meta-pelite, semipBlite and aluminous quartzite ob-served, only.r seventeerr'- contain cordierite; themost commdn assemblage is biotite-sillimanite-garnet. Cordierite:bearing assemblages are more

309

common in the eastern part of the area, whichcould mean either lower pressure, or higherwater fugacity, or Mg-rich bulk compositions.Around the Morin complexn corundum is wide-spread in metapelite and even in aluminousquartzite. Where it occurs in a quartz matrix,corundum is rimmed by sillimanite and garnet.Spinel is also common in aluminous metapelitearound the Morin anorthosite, and a few occur-rences of a spinel-magnetite intergrowth showthat temperatures probably exceeded the spinelsolvus. In the eastern part of the area, spineland corundum occur only along the contactwith small noritic masses, where garnet-silli-manite gneiss is converted to cordierite-ortho-pyroxene-spinel hornfels. These aluminousphases seem to record very high temperaturesrelated to contact metamorphism superimposedon regional metamorphism. The same high-temperature event is probably responsible forthe regional development of wollastonitearound the Morin anorthosite and its limiteddevelopment around minor intrusive bodies.

Among the seventeen cordierite-beariog sam-ples, six were selected for analysis, all showingthe Cd-Ga-Si-Qz assemblage. In these sections,cordierite comes in globular grains locally al-tered to pinite. Subidiomorphic or elongategrains of garnet constitute up to 3OVo of therock. Sillimanite is prismatic and commonlyassociated with a reddish brown biotite. Texturalevidence for local equilibrium (linear graincontacts, subidiomorphic grains, absence of re-action rims) can be observed in large portionsof each sample, but local disequilibrium alsopersists. In one sample, sapphirine is observedas a relict phase rimmed by cordierite, which inturn is rimmed by garnet. Except for this localreaction, the rest of the sestion shows texturalequilibrium. In another section, sillimanite oc-curs not only in the groundmass, but also assmall inclusions in cordierite. Another sectionshows local signs of textural disequilibrium andcontains orthopyroxene as an additional phase.Garnet is in coiitact with cordierite exceptwhere it is filled'ririth streaks of magnetite. Cor-dierite has broken down to aggregates of fine-grained sillimanite and orthopyroxene. Inanother part of the section, orthopyroxene phe-nocrysts are associated, but not in contact, withcordierite and sillimanite.

Cuevrsrnv oF MINERALS

All analyses were performed with a MACelectfon microprobe. For each sample theminet'ils were analyzed with a wavelength-dis-

GEOTHERMOBAROMBTRY OF CORDIERITE-BEARING METAPELITES

3 1 0 THE CANADIAN MINERALOGIST

persion system with LIF, PET and RAP crystalspectrometers.

Two sets of ten measurements were madeon both cores and rims of mineral phases inoider to check for zoning. Rim analyses wereperformed at a dislance of 80 to 100 pm fromcrystal edges to avoid the effect of late ex-change of cations at grain boundaries (c1., Berg1977), Counting time for each mdasurementwas 1@ seconds; data were processed accord-ing to tle Bence & Albee (1968) correctionprogram. Analytical results (Table 1) havebeen stored in the Depository of UnpublishedData, CISTI, National Research Council ofCanada, Ottawa, Ontario KlA 0S2.

Cordierite is practically unzoned, and its com-position is almost constant throughout a thinsection. The most magnesian cordierite (Xangcd0.91) comes from south of the Morin complex(Fig. 1) and east of the Ias-Croche complex.The least magnesian sordierite comes fromsoutheast of the St-Didace complex (X*u"u0.78-0.80). Grain size does not allow analysesof channel-filling fluids by conventional techni-ques, but the sums of oxides for cordierite aresignificantly low and variable, even at tle scaleof a thin section. As Na has not been detected,these variations reflect changes in the amountof fluids filling cordierite channels (essentiallyCO, and HzO). The proportion COelHsO hasnot been evaluated, but the presence of opti-cally positive cordierite suggests that somegrains contain CO: rather than HrO (Arm-bruster & Bloss 1980).

Garnet, an almandine-rich solid solution, isusually zoned, in contrast with cordierite. Themost conspicuous case of zoning is illustratedby the depletion in the grossular component atgarnet rims in the Shawinigan area. The te-verse pattern, though less pronounced, is ob-served in the Morin area. As the spessartinecontent is very low in both garnet cores andrims, the decrease in grossular content must becompensated by an increase in pyrope-alman-dine components. Fe-Mg zoning in garnet canonly be estimated by comparing almandine-pyrope ratios of cores and rims. This ratio doesnot show systematic regional variations as ingrossular: the garnet in two sections shows amarked increase in almandine towards the rim(6/20.58 and 6/64.56); one section shows thereverse zoning (6/33.24), with a slight increasein pyrope towards the rim. All other sectionsshow minor and erratic zoning or none at all.

Plagioclase is in the range Aneu to Anse, butis not noticeably zoned within a thin section. Asoxide variations from grain to grain are usually

of the same order of magnitude as the anal-ytical error, average values have been calculated.

Biotite contains 4 to 57o TiOz with XMgtt tang-ing from 0.61 to 0.70. The sapphirine-bearingsample contains a TiOr-poor, pblogopite-richmica (TiO, l3A/o) with a Xue" of 0.8O.

Sapphirine included in cordierite (X-c"' O.80)has a composition that plots in the triangledefined by associated cordierite, garnet andsillimanite, suggesting the possible reaction sap-phirine + quartz - cordierite + garnet +sillimanite.

Eeurr,rsnru\4 eNo DrsseurLIBRIUM CRTTERTA INFn-Mc ExcHeNcn Rnecrroxs

Mineral zoning and compositional variationsfor a given mineral at tle scale of a thinsection show that chemical equilibrium maynot necessarily have been attained, e'ven wheretextural equilibrium has been established. There-fore, a set of criteria has to be elaborated inorder to either identify local conditions ofchemical equilibrium or to unravel the trendsin the physical conditions responsible for theobserved disequilibrium. Although disequilib-rium assemblages are not unconditionally suit'able for quantitative thermodynamic calcula-tions, they might provide a better under-standing of metamorphic gradients thando equilibrium assemblages. Equilibrium cri-teria will thus be used to give quantitative esti-mates of intensive parameters, whereas qu4lita-tive estimates of the variations of these para-meters will be obtained through the analysisof disequilibrium processes.

The simplest and most widely used criterionof local equilibrium between two mineral spe-cies concerns composition at grain contacts. Asthis condition is not always rcalized, we shalladmit that grains less than a few tenths ofmillimetres apart and separated by a quartzosematrix or an unzoned Fe-Mg mineral of athird species are probably close to chemicalequilibrium. Fe-Mg distribution coefficientsobtained by applying these criteria will becalled "equilibrium Ko" Yalues.

In order to estimate the conditions of earlierstages of metamorphism, zoned minerals wereconsidered. In the case where a zoned and ahomogeneous mineral are in contact, if theouter rim of the zoned mineral is sufficientlynatrow and if the element participating in thezoning is a major constituent of the unzonedmineral, the unzoned mineral can be consideredas an infinite reservoir for the element parti-cipating in the zoning pattern (c1., Tracy et al.

1976), The distribution coefficient obtainedfrom the core composition of the unzoned min-eral will thus be close to an "early equilibriumKo" value.

Moreover, in order to obtain information interms of intensive parameters about the mag-nitude of disequilibrium, all possible distribu-tion coefficients were calculated according tothe procedure followed by Deb & Saxena(L976). It is clear that this procedure yieldssome extreme values of artifact Ko, but thecomparison between temperatures obtainedfrom extreme Ko, average Kp and "equilibriumKr" values might be infery111iye.

TABLE 2 : TEI.IPERAIURE ESTIMTES FRoM DISIRIBuII0I{ coErrIclEMS*

31.1

ConorsnnB-GenNnr THSnMoMBTRY

The marked Fe-Mg partitioning between cor-dierite and garnet observed both in natural as-semblages (Thompson 1976) and experimentalruns (Hensen & Green 1973, Holdaway & Lee1977) has been used for thermometric calibra-tion (Table 2). The equilibrium constant ofthe exchange reaction:

MCCd * FeGa. i FeCd * MSGa. . . . . . . ( .1)

is represented by the distribution coefficient

be= ' xP"GuKpre_a _ _:

fr"G XlagGu

This coefficient is an exponential functionof T. Among the various published equationsrelating the two variables, we have used theone of Thompson (1976) because it had al-ready been selected as one of the functions inthe calibration of the cordierite-garnet-HrOfields by Martignole & Sisi (1981). This wasnecessary in order to preserve the consistencyof results.

Temperatures obtained lrom "equilibriumKo" valttes

Temperatures calculated from equilibrium K"values range from'542"C northwest of the St-Didace complex to 735oC a few km southof the Morin anorthosite. The number of sam-ples is too small to discuss the regional varia-tion between these two extremes, but one maynotice that the loweSt temperatures tend to berecorded remote from plutonic complexes. Thetwo-hundred-degree difference between theextreme values shows that there is not a singleblocking temperature in high-grade metapelites;thus differences between estimated temperaturesshould be significant. At the scale of the thinsectiono temperatures derived from equilibriumKo values show a significant variation (5O *30'C) near the Morin anorthosite, whereas inthe eastern part of the area variations are witlinthe error limit. This might correspond to re-adjtrstment upon cooling versus o'frozen-in"

equilibria. The possibility of re-adjustment doesnot seem to be related to any special tempera-ture, as it is conspicuoirs from samples record-ing the lowest as well as the highest tempera-tures. The highest T obtained from equilibriumKo values can thus be considered to representthe closest approach to the peak of the lastthermal event.

GEOTHERMOBAROMETRY OF CORDIERITE.BEARING METAPELITES

Typeof''D

BJ-Ga

Graln r1oc1::o ;;" Tfc'30

5/29.58

5/43 .81

6/20.58 EqulEqui

ear]y

6/64.56

765878624

702472602

D-C 673

avermlnmax

Equi N-l{rEqul N-l,lzEqul Gr-62Equl Gz-Gr

avermlnnax

Equl I-CEqul Iz-Ar

671223509

676659646627

696772651

73s683

577

523

591

542

595

6416796 1 9

672636

662

688

704697672

714647

643712590624o t o

605597

678690

averminmax

621693557

I - I 631

averninmx

6/33.24EqulEqui

ear'ly

avermlnmx

Equl6154.47 Equl

EquJ

ear]YearIY

averminmxEquiEqulEqulEqul

earlyearly

DzC ( r )Ar -ArAz-Azor -c (c )

Cr -Fz( r )

L l - i 2 ( c l

Dr-D( r )D, -D( r )Er -F( r )

D" -D(c )r . - r (c i

D r -CA-A(r)Br -FDr-D( r )

A-A(c)Dr -D(c)

678724

701686

640728s80628604

608

660697

A-AD-D(r )

D-D (c )

c-cA-A(r)

D-D( r )

A-A(c)D-D(c)

' The letters (r) and (c) refer to rlm and core conposltions.

respectlvely; aver average, nin n{nimum, mx ruxinum, equiequ i l ib r lum Cd-ca and B l -ca ca l lb ra t ions (Thompson 1976) .

312 THE CANADIAN MINERALOGIST

Temperatures obtained from "early equilibriumKo" values

In all but two samples, Fe-Mg zoning ofgarnet is not sufficiently pronounced to allowderivation of significantly different temperaturesfrom core and rim compositions. These twosamples are from NW and SE of the St-Didacecomplex and do show reliable temperature dif-ferences. Temperatures derived from core andrim compositions in sample 6/20.58 dgfineroughly the same interval as the one recordedby the equilibrium Ko values. In sample 6/64.5.6,the temperature obtained using garnet coreis more than 70oC higher than the one givenby garnet rims and, as such, probably recordsa stage of higher temperature.

Temperature obtained from average and ex-trenxe KD values

The temperatures derived from average Kovalues generally fall in the range defined bytemperatures determined from adjacent cor-dierite and garnet. In cases where garnet i's notzoned, average Ko values give an averageblocking temperature. Where garnet is zoned,the average temperature does not correspond toany specific event. Temperatures obtained fromextreme Kr values show that equilibrium wasattained where they are equivalent to maxi-mum and minimum T obtained from equilib-rium Ko (sample 6/33,24).In this case thereis no artifact Kn. Where their range ismuch larger than the range defined by theequilibrium Ko values, they provide a scale forestimating the magnitude of disequilibrium. How-ever, disequilibrium cannot be described onlyin terms of Mg/ (Mg * Fe) variations. It is alsoinfluenced by the fluid content of cordierite,which in turn controls tle stabilization pressureof this mineral, and thus its Mg/(Mg + Fe),as will be seen in the section dealing withcordierite barometry.

Bronm-GenNBt THSn\4oMETRY

The Fe-Mg exchange reaction between biotiteand garnet is represented by the distributioncoefficient:

KprFu-MgBi-G' :5otl1 5ixFeBt xvr*Gu

Temperatures have been calculated fromThompson's (1976) calibration from KD valuesobtained following the criteria defined above.Owing to the fact that the partition of iron

and magnesium between biotite and garnet isless pronounced than that between cordieriteand garnet, the effect of chemical disequilib-rium in terms of Mg/ (Mg * Fe) will bemore pronounced in terms of temperature de-terminations than it was in the case of cor-dierite-garnet thermometry. This might Ue thereason why, in several cases, average tempera-tures derived from KDBr-Ga are significantlydifferent from those derived from Krcd-c". 1yehave no explanation, though, as to why theyare usually higher. On the other hand, wher-ever biotite is in contact with the same garnetas cordierite, temperatures obtained from thetwo thermometers are identical. When equilib-rium criteria defined above are applied, Bi-Ga thermometry gives temperatures in therange 6O4--710oC, both extremes being fromthe vicinity of the St-Didace complex.

ConprsRnr-GanNnr BenoMETRY

The reaction through which cordiente is con-verted to garnet, sillimanite and quartz is pres-sure-sensitive; the Mg content of cordieriteshould thus be a good geobarometer. Unfortu-nately HrOo like Mg, stabilizes cordieritetowards higher pressure; also water cannot bedirectly determined by electron-microprobeanalysis. Moreover, COz, as well as other gases,enter the cordierite channels, which means that,neglecting the analytical error, the amount ob-tained by subtraction from the total of oxidesin a microprobe analysis represents all gaseousspecies entering the cordierite structure. Pres-sures estimates have been performed accordingto the cordierite-garnet-HrO calibration ofMartignole & Sisi (1981), for which H,O incordierite should be known. Accordingly, asemiquantitative estimate of HzO was obtainedby combining several types of information.These are based on the possible significance ofthe oxide sums and on optical data of analyzedsamples of cordierite.

By subtraction, a very crude estimate of the

TABLE 3. ESTIMATES OF FLUID CONTENT IN CORDIERITE

dr X:Sample

5/29.585/43 .816/20.586/33.246/54.476/64 .56

2 . 7 6 9 8 . 9 02 . 1 0 9 7 . 7 52.08 99 .851 . 6 2 9 7 . 7 80.83 97 .91'|

.45 99.29

Average f lu id z H20 Max

0 . 9 8 1 . 1 r 0 . 9 8 0 . 40 .84 2 .25 ! 0 .84 0 .80 . 8 3 0 . ' 1 5 t 0 . 8 3 00 . 6 5 2 . 2 2 t 0 . 6 5 0 . 70 . 3 3 2 . 0 9 r 0 . 3 3 0 . 70 . 5 4 0 . 7 1 r 0 . 5 4 0 . 2

dt: ranqe in remainder betaeen sum of oxides and ]00.Xl: aveiage sum of oxjdes for all analyzed grains in a sec-

t i o n .

fluid content of cordierite can be obtained.Cordierite totals are usually lower than 100,whereas totals for garnet crystals analyzed

GEOTHERMOBAROMETRY OF CORDIERITE-BEARING METAPELITES 3 1 3

, a

6/20-5 B

+ +

6/64-5 6

a a ? a

5/43-81

a a o l

' 6/33"24- . . - +

under the same conditions tend to be close to100 and even higher (Table 3). At this stage,there are two possibilities for estimating thefluid content. An average fluid content can becalculated from the average oxide sum in asection. In most sections, the number obtainedis higher than the error of oxide sums (= LVo).This method does not give any information onthe nature of the fluid; if it is taken as pureHeO, the degree of hydration of cordierite willusually be overestimated. If the range of fluidcontent is calculated for a single section, itappears to be usually higher than the error inthe oxide sum, and variations in fluid contentcan thus be estimated (Table 3). If combinedwith optical information, a semiquantitativeestimate of HrO and COz content of cordieritecan be obtained. Recent experiments by Arm-bruster & Bloss (1980) have demonstrated thatthere is a relationship among 2V,, mean indexof refrastion and the nature of the channel-filling fluids in Mg-cordierite. Cor-rich cor-dierite is optically positive, whereas water-richcordierite is negative, with a relatively small 2V(= 50"). Fluid-free cordierite has a 2V veryclose to 90o. Combining optical o6servations(wherever possible) with estimated fluid con-tents, three cases of fluid variations have beenfound (Fig. 2).

In the first case, a decrease in totals of mi-croprobe data is correlated with a decrease inthe magnesium content at the scale of the thinsection. If the variation in microprobe totalsis related to fluid content, this shows that theavailability of fluid may not be uniform, evenat a very small scale, and that, for a givenpressure, equilibrium could be attained by in-creasing the magnesium content of cordieritewhere water was noi available. It is interestingto point out that according to the cordierite-garnet calibration of Martignole & Sisi (1981),for a given T, the same reduction of chemicalpotential, resulting in the same AP of stabiliza-tion, is ashieved by either introducing 17o(wt.) H:O in a dry cordierite or by increasingXMs@ by 0.07. This could be applied to sample5/29,58 to show that water could compensatefor the decrease in X*u"u.

In the second case, the fluid content variesfrom 0 to about ZVo, w'rthortt any correlationwith the Mg content (samples 6/20.58, 6/64.56). Keeping in mind the imprecision at-tached to these numbers, tlere is a possibilitythat the corresponding cordierite grains only

B/54-41, +

+ ?- T

0 1 2 3

H20 + C02 Wr .%Frc. 2. Fluid content of cordierite versus Xy"cd.

Optically positive cordierire +, negative -. Dots:unidentified optical sign. Cases I (S/Z9,SB) 2(6/20.58, 6/64.56) and 3 (5/43.81, 6/33.24,6/54.47) are described in the text.

contain cor, a fluid much heavier than HaO,which probably does not interact significantlywith the stability pressure of cordierite (seeMartignole & Sisi 1981). Optically positive cor-dierite has been identified in the correspondingsectionso showing that COe is the main channel-filling fluid (sample 6/20.58).In the pressurecalculations, these cordierite compositions willbe considered as anhydrous (zHzO = 0).

The third case is the one where cordieritealways contains a significant amount of fluid,the variation of which does not show any cor-relation with magnesium content. Two possi-bilities cau account for this situation: 1) thefluid is essentially COr and does not affect theMg content of cordierite, or 2) the fluid is amixture of CO, and HrO, but HzO has a roughlyconstant value in all cordierite grains, corrb-sponding to the minimum fluid content. Asoptically positive cordierite seems to occurpreferentially among those grains that have thehighest flnid content, we favor the decond

0.90

0.80

v cd 0.75^ [,lg

0.05

0.80

0.80

0.75

0.80

0.75

3t4

possibility. In consequencen the minimum fluidcontent will be taken as a maximum HgO valuefor all the grains of the section (e.g., 6/33.24'6/ s4.47).

Turning now to the error resulting from thesubtraction method of estimating HzO, if anerror of lVo on the oxide sum is retained, amaximum error of 0.3 on estimated nHzOwould be obtained. According to the Cd-Ga-IIgO calibration (Martignole & Sisi 1981), thiserror results at 7@ -F 100oC in a 50O to 1500bars uncertainty in estimated pressure, depend-ing on the absolute value of nHeO. Pressuresobtained after application of those criteria arepresented in Table 4.

EsrnuetloN or P(H"O)

Cordierite hydration is a function of T and Pwhen Pnora - P(H,O) (Schreyer & Yoder1964), but it depends on X(H,O) if the fluidphase is a COz-ILO mixture (Johannes &Schreyer 1981). In the first case, water is theunique fluid filling cordierite cavities; its fuga-city should be obtained through experimentalhydration data, given the hydration number of acordierite and the equilibrium P. In the secondcase, water pressure in the mixed fluid in equilib-

TABLE 4. PRESSURE ESTIMTES FROM CORDIERITE BAROMETRY

smpte oraln nHzo *fi| Il:3l p(Kbar) pH"6(bars)ltlln l'lax

5.0-5.85.r-5.9 0- 500

THE CANADIAN MINERALOGIST

5129.58

rium with cordierite cannot be estimated di-rectly from hydration data. Experiments -byJohannes & SclLreyer (1981) have shown thatCO, does not behave as an inert gas; the strongpartition of water in favor of cordierite impliesihat the hydration theory of Martignole & Sisi(1981) should be reformulated with newHenry's constants for various values of X(COz)as soon as more experimental data becomeavailable.

In the meantime, a semiquantitative estimateof P(H,O) can be obtained by using zH:Ofrom the criteria defined above. If all the fluidin cordierite is taken as HzO, the nHrO valuewill be overestimated in most cases. But it canbe seen from the experimental data of Johannes& Schreyer (1981) that Henry's constant forthe adsorption of a CO:*H2O mixture is in-creased compared to that for HsO adsorption.For a given T and nHaO, P(HzO) obtainedfrom hydration isotherms (Martignole & Sisi1981) will lead to an underestimate in thecase of mixed fluids. This effect will partlycounteract the error introduced by consideringall the fluid as HrO. Considering the enormousimprecision attached to the method of estimat-ing tne hydration number and the absence ofcalibration for HIHO2 mixtures, water pres-sures estimated following these procedures canonly be considered as very crude (Table 4).The only conclusion that can 6" lstailed is thatin the metapelites described here, P(HzO) wasconsiderably lower than the lithostatic pressureand, in some cases, the fluid phase was ex-tremely rich in COr. A problem remains con-cerning 1) fluid-absent cordierite (Berg 1977'this study) and 2) possible variations in thefluid content of cordierite grains throughout asingle section. These cases suggest that -latehydration is not inevitable, and that quenchingfiom a situation \rith ste€p gradients in fluidconcentration and composition is not impossible.If such is the case, estimations of regional HrOand COz pres$ures will become very doubtful.

PLAGTocLAsE-Genrqrr BanoMerRY

Where garnet is associated with plagioclase,sillimanite and quartz, its grossular content isrelated to the anorthite sontent of the plagio-clase. The equilibrium constant of the corre-sponding reaction can be derived from the high-pressure breakdown of anorthite:

3 a n i : g r o s s + 2 s i l l * q 2 . . . . . . . . . . . . ( 3 )

K":ffi

5/43.8r , l^

" l 3:i 0.8 !'fi1'l zoo-tsoo

G^ 0.0 0.87 627a( o .o 0 .4 0 .86 646l' l ' 0.0 0.86 676

0.84 6830.84 735

6/20.s8 4 . 8 -4 . 8 -5 . 0 -

0 .00 . 0 0 . 00 . 0

6.rt ssg0.91 5420.90 591

6133.24 0 . 5 n 70 . 5cl

0.800.80

636672 !'fi!'f rooo+ooo

6/54.47 DrD,IL2

lz\2

0 . 6u . o0 . 6

0 . 70 . 60 .6

6 .5-7 .36 ,5-7 .36.3-7.1 2000-2500

o.o- , .oo . r - o t l

0,790.780.81

697704672

714641

0 . 7 80.81

D, 0.0B] 0.0A' 0.0Dl o.o o.2

D1 o.oA ' 0 .0

0.790.780.780t79

0,790.78

624ou5

616597

690678

4.3-4 .54 . l -4 .34.2-4.44 .1-4 .3 0 ,200

5,0-5 .34 .7-5 .0

6/64.56

GEOTHERMOBAROMETRY OF CORDIERITE-BEARING METAPELITES 315

of a thin section in terms of grossular content,disequilibrium in Ca of garnets will be used,following the same general procedure as forFe-Mg disequilibrium, to give qualitative esti-mates of metamorphic trends.

Estimation ol metamorphic pressures

Estimation of metamorphic pressures havebeen performed according to the most recentcalibration of the garnet-plagioclase geobaro-meter (Newton & Haselton 1981), which con-siders the latest refinements on the calculationof the activities of anorthite (Newton & Wood1980) and grossular as well as a correction toaccount for the nonregularity of garnet solu-tions, especially for low concentrations of thegrossular component (Cressey et al. 1978).

Newton & Haselton (1981) based their cal-culations on Goldsmith's (1980) end-membercurve for anorthite breakdown. This is thoughtto carry less uncertainty in pressure calibrationthan previously published experimental data. Inaddition, we have done the same calculationsusing the curve of Hariya & Kennedy (1968),which is closest to an average value of all pub-lished experiments at temperatures correspond-ing to the granulite facies (c1., Newton &

Since the first application of trris reaction ingeobarometry (Ghent 1976), activity modelsfor garnet solutions have been considerablv im-proved (Newton & Haselton l98l); equilildrrmcurves for K, can be drawn with a reasonableprecision. As these curves have a gentle positiveslope in a P-T diagram, derivation of an accu-rate pressure is dependent on a temperatureestimate that represents the same metamorphicevent as the garnetllagioclase equilibrium.Pressure estimates in turn are influenced byintragranular and intergranular chemical in-homogeneities. Garnet zoning, already discussedin terms of almandine-pyrope ratios, is oftenvery pronounced in terms of grossular content.Both increase and decrease in grossular con-tent versus slmladins * pyrope are foundtowards garnet rims. On the contrary, plagio-clase is not zoned; as it constitutes a large re-servoir of Ca compared to the Ca variations ofgarnet rims, we can assume that the plagro-clase in equilibrum with garnet, at the timewhen the garnet had the composition of its core,was not significantly different from the actualplagioclase. Equilibrium constants can thus becalculated for garnet cores and rims using thesame matrix plagioclase. Moreover, as chemicalequilibrium has not been attained at the scale

TABLE 5. PRESSURE ESTIMATES FROM GROSSULAR-ANORTHITE BAROMETRY*

Sample

Rock ca

r l o r ttii-Eil

Plagioclase Garnet . ? Pressure (Kbar)Avr ( cm")- G o l d s m i t h H & KXun dan xg, agr(lo-3)

Cd-Ga(mlnimurn)

659627

735683683

558542591595

636662

704672714647

6?4505616597678690

D(r)6/54.47 5t3

F(c)

5/2s.58 E2

5/43.8I A, ( r )ai (c)

A"6/20.58 iir,

c(c)

6t33.24 Flt.l

0.845 0,02650.847 0.0354

0.682 0.02620.710 0.03680.710 0.0260

0.4v. 0.02690.496 0.02530.456 0.02690.453 0.0366

0.355 0.02750.343 0.0259

0.526 0.03680.547 0.03420.520 0.05640.565 0.0676

0.555 0.03650.679 0.03750.662 0.03810.679 0.03850.614 0.03890.606 0.0568

z . a3 . 3

3 . 9J . J? l

4 .0 4 .83 . 4 4 . 84 . 5 5 . 05 . 9 5 . 0

0.883

0.546

0.272

0.235

0.352

0.41 9

0.06710 .165

0.05880 . 1 l 50.0535

0.09850.o7740.08850,2394

0.05540.0490

0.1250 ,1120.6880.u2

0,'1220.1230.1 350.1 380.1450,440

58.55 U . I

58 .5E 7 a

E A A

58.058.458 .5a I . +

5 U . 558 .5

57.457.8J 5 . 555 .3

57.257 .357 .3E 7 a

t / . J55.2

3.42 . 93 . 95 . J

4 .85 . I

5 . 2,+. 5u . l7 , 0

3 .43 .23 . 4J . l

4 .5o . J

E N

6 . 0t . 55 . 8

1 . 92 A

3 . 32 . 72 . 5

5 .4 5 .65 . 7 5 . 8

r

6t64.56 BIliA(c)D (c )

5 . 8 6 . 55 . 1 6 . 38 .7 6 .87 . 6 6 . 1

4 .0 5 .03 . 8 4 . 14 .0 4 .23 . 7 4 . 35 . l 4 . 77 . ' t 5 . 0

* i l e r t o n t H a s e l t o n ( 1 9 8 l ) c a l i b r a t i o n i l e t t e r s ( " ) . n o

316 THE CANADIAN MINERALOGIST

Haselton 1981). Given the high PrT experi-ments of Hariya & Kennedy, provided &e ex-trapolation to lower T is valid, and after cor-rection for the kyanite-sillimanite inversion us-ing Holdaway's (1971) values for A1$ aad [H,the following equation can be derived for tleend-member reaction used when sillimanite isthe stable aluminum silicate: P (bars) = 113+ 23.8 T ('C). Calculated pressures are re-ported in Table 5. Pressures obtained from cor'dierite barometry are given for comparison. Notethat calculations performed with the end-mem-ber curve of Hariya & Kennedy (1968) moreclosely approach pressures given by cordierite.

Estimation of metarnorphic trends throughc he mic al dise quili b rium

Grossular zoning in garnet, which is mainlypressure-dependent if developed in the presenceof aluminum silicate-plagioclasequartz, can becorrelated with almandine-pyrope zoning, whichis essentially temperature-dependent. Variationsin chemical composition between several garnetgrains throughout a single section can be studiedin the same way. The conelation between Fe*Mg variations and Ca variations is best visual-ized (Fig. 3) by plotting 100M9/(Mg + Fe* Ca * Mn) versas 100Ca/(Ca * Mg * Fe* Mn). In this tlpe of diagram, a verticaltrend [decrease in Mg/(M8 * Fe * 6u 1 Mn)

Ca/Ca+Mg+Fe+Mn

Ftc. 3. Mgl(MC + Fe + Ca + Mn) versusCa/ (Ca * Mg + Fe f Mn) diagram showingthe different trends responsible for chemicaldisequilibrium in garnet compositions.

ar 5/43-81

L..

. 6/20.58

{'a

.

: 6 t 5 4 4 7a

45

xc

T

+q

T

=T

+4

+=

=

6/64-56a

(CalCa +Fe +M g +t\/n) x1[ gFrc. 4. Application of Fig. 3 to analyzed. grains

of garnet (for interpretation see text). The ar-rows go from core to rims.

at constant Ca/ (Ca * Mg 1 Fe * Mn)l cor-responds to a decrease in temperature at constantgrossular content (K constant), namely a tem-perature decrease along a slope of about loCper 24 bars. A horizontal line in thg diagramcorresponds to a pressure variation at constanttemperature; the polarity of this variation isgiven by the position of the points colrespond-ing to garnet cores and rims. A line with anegative slope corresponds to a desrease intemperature correlate$ with an increase ingtossular content, and thus an increase of Ks,suggesting an isobaric path of cooling. Thesethree types of trends are represented in meta-pelites from the area under discussion (Fig. 4).Samples coming from the vicinity of the Morinanorthosite (5/29.58, 5/ 43.8I) have garnet

5/29-58l l

a

6/33-24Ia1C

composition trends indicative of isobaric cool-ing (trend l), whereas cooling accompanied byunloading (trend 2) is exemplified by samplescoming from south and west of the St-Didacecomplex (6/20.58, 6/33.24); an almost iso-thermal unloading (trend 3 ) is suggested bysamples coming from the eastern margin of theSt-Didace complex (6/54.47, 6/64.56). It ap-pears, therefore, that two regions about 2@ kmapart have suffered different cooling*unloadinghistories: rapid cooling after the temperatureclimax in the Shawinigan area and slow coolingwithout significant unloading around the Morinanorthosite. Simultaneous cooling and unloadingbetween these two regions suggest that varia-tions in late metamorphic evolution are pro-gressive from west to east.

CoNcl_ustoNs

Results of careful microprobe analyses ofhigh-grade metapelites have shown that equilib-rium between iron-magnesium solid solutionsis far from being attained (or, perhaps, is rarelypreserved if ever attained) at the scale of athin section (Nantel 1977). This situation ap-pears to be fairly common in granulite-faciesrocks (see Lonker 1981) and is probably largelydue to re-equilibration upon cooling. Keepingin mind the difficulties resulting from tlese re-adjustments, both cordieritFgarnet- and biotite-garnet-derived temperatures show that meta-morphic temp€ratures were higher around theMorin anorthosite complex (over 7O0'C) thanin the Shawinigan area. The coincidence of thehighest temperatures with regional developmentof wollastonite in impure marble is probablydue to a vast contact aureole superimposed ongranulite-facies regional metamorphism.

The minimum temperature (around 550"C)was recorded in a locality immediately north ofthe orthopyroxene isograd (Sshrijver 1973) inthe hornblende zone. The isograd *ould thuscorrespond here to a temperature around 600"C.Finally, near Shawinigan, a sample (6/64.56)containing relics of kyanite and andalusite givestemperatures from 597 to 628'C, a reasonablerange for tbe aluminum silicate triple point.

The interpretation of pressure estimates fromcordierite barometry and plagioclase-garnetbarometry is not as clear, as it does not confirmthe pressure gradient suggested near the Morinanorthosite by the development of garnet incharnockitic rocks. Pressures obtained using thegarnet-cordierite barometer range from 4

'to

nearly 7 kbar, whereas the plagioclase-garnetbarometer gives slightly lower values, especially

317

if Goldsmith's (1980) end-member curve foranorthite breakdown is adopted. According toboth barometers, the highest pressures were notattained near the Morin anorthosite but nearthe St-Didace complex. In this complex, char-nockitic rocks of suitable composition are notgarnetiferous. Kinetic constraints may be re-sponsible for this anomaly in the formation oflate metamorphic assemblages, together withthe variation in cooling-unloading pa&s.

All specimens of metapelite analyzed havesillimanite as the stable aluminum silicate: allthe pressures obtained, when combined withcomputed temperatures, plot in the sillimanitestability field, near the kyanite-sillimanite in-version curve. The sample from which relicsof andalusite and kyanite have been recordedgives pressures in the range of 4.1 to 5.0 kbar(Cd-Ga barometry), showing that kyanite andsillimanite were probably metastable duringandalusite gowth.

Finally, the very crude estimate of P(HzO)obtained by means of the subtraction methodand hydration data on cordierite does not revealany systematic variation in water pressurethroughout the area. The method is not yet suffi-ciently precise to be used as a reliable indicatorof water fugacity but a correction for water isnecessary in order to obtain better results in cor-dierite geobarometry.

AcrNowrnocEMENTs

We express eu1 th4nks to K. Schrijver forhis critical reading of the manuscript.

This research was made possible through apostgraduate fellowship to S. Nantel and agrant (4-7058) to J. Martignole, both from theNatural Sciences and Engineering ResearchCouncil of Canada.

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GEOTHERMOBAROMETRY OF CORDIERITE.BEARING METAPELITES

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