Complex Zoning in Vesuvianite from the Canigou

The Canndint M ine ral.ogi st Vol. 33, pp. 77-84 (1995)

COMPLEXZONING IN VESUVIANITE FROMTHE CANIGOU MASSIF, PYRENEES, FRANCE

ROGERL. GIBSON ANDTHOMAS WALLMACH* Departnentof Geology,University of theWinvatersrand, Private Bag 3, P O WIT$2050, Johantesburg, South Africa

DEONDEBRUIN GeologicalSurvey of SouthAfrica Private BagX112, Prenria000I, SouthAfrica

ABSTRACT

Vesuvianite grains in a calc-silicate layer from the Canigou massif @yrdn6es,France) display oscillatory concentric birefringence-zoningand a locally developd discordant,nearly isotropic marginal zone. The boundariesbetween the zones correspondto sharpchanges in the abundanceof Ti, Al, Mg and Fe, althoughonly Ti displaysa consistentpositive correlation with birefringence.Systematic variation in cation abundancesalso occurswithin individual zonesof equalbirefringence, indi- cating that compositional 2gning is, to some extent, independentof birefringence. Several schemesof cation substitution appeaxto have been operativein the vesuvianite,and different substitutionsseem to have dominatedat different times. The alominantsubstitutionJin tle higher-birefringencezones seem to be Ti + Mg *2Al and Feh:+Al, whereasMg:+Fe2+ may occur in addition to Ti + Mg =2Al in the lower-birefringencezones.

Keywords: calc-silicate, cation substitution,oscillatory birefringencezoning, Ti-Al-Mg-Fe zonation, vesuvianite,Canigou massif,Pyrdndes, France.

Solnl{ens

Les cristaux de v$suvianitedans un niveau i calc-silicatesdu massif du Canigou,dans les Pyr6n6es@ra:rce), font preuve de zonationconcentrique de la bir6fringenceet, localemeut,du d6veloppementd'un liser6 discordantet presqueisotrope. [.es contactsentre ces zonescorrespondent b des changementsabrupts da:rs I'abonda:rce de Ti, Al, Mg et Fe, quolque seul le Ti semblemontrer une corr6lationuniforme et positive avec la bir6fringence.Une variation systdmatiqueavec I'abondancedes cations se voit aussi i I'int6rieur des zonesbir6fringentes, indiquant que la composition,jusqu'i un certain point, est ind6- pendantede la bir6fringence.Plusieurs sch6mas de substitutioncationique semblent tour A tour avoir €tEimportants dans Ia v6suvianite.lrs schEmasles plus importantsdans les zonesconcentriques d bir6fringence6lev6e semblent 6tre Ti + Mg *2Al et Fe34=A1, tandis que le coupleMg = Fe2i pourrait s'ajouter d Ti + Mg =2Al dansles zonasconcentriques h plus faible bir6fringence. (Iraduit par la R6daction)

Mots-cl€s:calc-silicate, substitution cationique, zonation oscillatoire en bir6fringence,zonation en Ti-Al-Mg-Fe, v6suvianite, massifdu Canigou,Pyr6n6es, France.

INfRoDUC"noN Grcat et al. 1992, 1993).Despite this evidence,however, only a few studieshave dealt with compositional Vesuvianite is a relatively common constituentof zoning within single crystals.Groat et al. (1993) have calc-silicaterocks in both regional and contact meta- observedcompositional variation in sector-zoned morphic environments.Studies of samplesfrom a crystalsand variationsin mean atomic numbercorre- variety of localities have shown that it possessesa spondingto multiple concentric optical zones.Apart highly variablecrystal chemis$ @.9.,Hoisch 1985, from this, however,only simple core-to-rim Tsning has been reported:Arem (1973) attributed color zoning in vesuvianitecrystals to variationsin Cr and Mn, Kerrick et al. (L973) linked birefringencezoning to variable Ti content, and Hover Granathet al. (1983) variation of Ti and Fe3+,but * Presentaddress: Department of Geology,University of mentioned core-to-rim Pretoria-hetoria. 0002, SouthAfrica. did not describeany accompanyingvariation in optical 78 TIIE CANADIAN MINERALOGIST

characteristics.In this paper,we presentthe results of side in grossularand vesuvianite show a strong pre- a detailed electron-microprobeinvestigation of vesu- ferred elongationparallel to the banding.Based on vianite from the Canigoumassif (FrenchPyr6n6es) textural relations,diopside is inferred to be the oldest that exhibits unusual, complex birefringence-s6ning. phase,together with the fine-grainedcalcite and We discussbriefly the implications of theseresults for quartz. Vesuvianiteand garnet enclosediopside, sug- modelsof cation substitutionfor this mineral. gestinglater growth, but both are partially replacedin turn by coarse-grainedcalcite, K-feldspar or clino- GEoLocrcALSsr.nNc zoisile. The sampleis cut by fracturesfilled with calcite. Clinozoisite adjacentto the fracturesreplaces The Canigou massif is one of severalmassifs that vesuvianite,and feldsparis commonlysaussuritized. together comprisethe Axial Zone of the Pyr6n6es chain of mountains, situated between France and VssuvIANrE Spain. The massifs form part of a regional, low- pressure,greenschist-amphibolite-facies Hercynian The idioblastic grains of vesuvianitein the sheaf- metamorphicterrane of late Carboniferousage (Zwnt like aggregatediffer from the stubbier,more irregular 1986).In the Canigoumassif, predominantly pelitic grains in the feldspar-diopside band in that they pre-Ordovician metasedimentsand intercalated display concentricoscillatory birefringence-zoning Paleozoicorthogneisses have been metamorphosed up (Frg. 1), whereasthe stubbygrains are optically homo- to the upper amphibolitefacies (675 to 725"C, 4.0 to geneous.Both types are uniaxial negativeand may 4.5 kbar: Gibson& Bickle 1994).Numerous horizons contain inclusions,typically of fine-grainedvermicu- of marble,some up to severaltens of metersthick and lar calcite,quartz and occasionaldiopside. Some of containingsubsidiary calc-silicate layers, occur within the stubby grains are poikiloblastic, but the zoned the metapelites(Guitard 1970). grains contain relatively few inclusions,and theseare Vesuvianite-bearingcalc-silicate gneisses occur as generallyconcenffated in the core. layers within a folded horizon of white calcite marble The vesuvianitein the sheaf-likeaggregate displays approximately 1 km southeastof the Chalet des cuspateto lobate marginsagainst coarse-grained Cortalets,near the summit of the Canigou massif. calcite @ig. 1b), suggestingreplacement of tle vesu- Thermobarometryfrom adjacentgamet-bearing pelitic vianite by calcite. Unstrainedcalcite also occursin schists(Gibson 1989)indicates that the gneissesexpe- ftacturesin the vesuvianite.The relationshipbetween rienced peak metamorphicconditions of at least the vesuvianiteand grossularis less clear, with 600'C, 3.5-4.0 kbar. Vesuvianite occurs in a variety idioblastic vesuvianiteoccasionally totally enclosedin of parageneses$/ith calcite, quartz,grossular, diopside garnet, but displaying straight faces suggestiveof and epidote. In several samplesoindividual grains of textural equilibrium (top, Fig. 1a). In other places, vesuvianiteexhibit core-to-rim compositionalzoning garnet appearsto be partially or totally enclosedin andoin one instance,this is accompaniedby color zon- vesuvianite.Like the vesuvianite,the grossular con- ing. The most unusualocqutence, however,involves tains calcite-filled fractures and has been partially vesuvianitethat exhibits complex oscillatory bire- replacedby calcite. fringence-zoning(Fig. 1). The host rock for this vesu- The elongategrains of vesuvianitedisplay up to vianite is a mineralogicallybanded calc-silicale layer. nine dark (low-birefringence) zones alternating with The margin of the layer in contactwith the surround- lighter zones.Individual zonesmay be less than ing marble comprisesfine-grained polygonal quartz 10 pm in width. Under high magnification,the bound- (90Vo),calcite (5Vo) and,diopside (<57o).This is fol- ariesof the zonesseem diffuse. Although the zoning is lowed by a band gemprising approximately equal generally concentricowith the zone boundariesbeing amountsof grossularand calcite, with accessory straight and parallel to crystal faces,detailed examina- quartz and poikiloblastic vesuvianite. The calcite tion of the grains showsa more complexpicture, with occurs as fine-grained symplectitic intergrowthswith individual zonesvarying in width, bifurcating and ttre garnet, as coarse-grainedaggregates in massive even dying out in places(Frg. 1b). Somezones are garnet, and within fractures in the garnet. Coarse truncated by grain boundariesagainst calcite, garnet calcite also occurs surroundinga sheaf-likeaggregate and even other grains of vesuvianite.In the caseof of radiating grains of vesuvianite tlat are partially vesuvianite-calciteboundaries. this truncationis clear- enclosedin massivegaxnet (Fig. 1a). The vesuvianite ly relaied to replacamentof the vesuvianiteby calcite. grains are idioblastic to subidioblastic and are up to Where the truncationoccurs at vesuvianite-garnetand 4 mm long. Some are slightly bent, and all are frac- vesuvianite-vesuvianiteboundaries, however, the tured. The third band comprisescoarse-grained most likely explanation is that it is causedby inter- feldspar(35Vo) + diopsideQ5Vo) + grossular(25Vo) + ferencebetween adjacent crystals during growth. 'I\e On vesuvianite(LIVo), with late clinozoisite (SEo). the left side of the diamond-shapedgrain in Figure lb, feldspar is predominantlyK-feldspar, but plagioclase a dark, neady isotropic, zone truncatesthe concentric also is present.Optically continuousremnants ofdiop- zones.A similar zone is also found in the adjacent COMPLEXZONING IN VESTIVIANITE

Fro. 1. (a) Radiatingcluster of vesuvianitegrains (ves)showing oscillatory optical zoning. Grain shown in (b) is on the left. Fine-grainedparagenesis (top, righ0 is quartz + calcite + diopside. Calcite (cc) and garnet (grt) are labeled.Crossed polars, scalebar 2 mm. (b) Grain of oscillatory-zonedvesuvianite discussed in detail in text showing locationsof electron-microprobetraverses A-B, A-C and A-D displayedin Figure 2. Crossedpolars, scalebar 0.5mm. 80 THE CANADIAN MINERALOGIST

gfain of vesuyianite. Other grains also show a thin, naturalrhodonite as Ti andMn standards. discontinuousnearly isotropic marginalzone with a A total of 74 analyseswas made in three core-to- sharp boundary truncatitrg the concentric bire- rim traversesacross the diamond-shapedgrain shown fringence-zonesalong parts of their boundarieswith in Figure lb. Additional spot-analyseswere made of garnet and calcite. The margin displaying this zone in the zonesin otler grains to establishthe consistency Figure lb also hasbeen partly resorbedby calcite. of these data. The results for profile A-B are listed No evidencewas found of color zonins in tlese in Table 1, and the variation in cation content along grains,and back-scatteredelectron imageishow no the three traversesis shown in Figure 2. The data in discernablecontrasts in density correspondingto the Table 1 are recordedto two decimalplaces because of birefringencezones. the low standarddeviations on minor elements and low detection-limits.Oxide totals were reducedto a CovposmoNarDara 50-cationunit formula, as originally proposedby Coda et al. (1970),Rucklidge er aL (1975)and Hoisch (1985). Compositionswere determinedusing a IEOL 733 electronmicroprobe at the GeologicalSurvey of South RssuLrs Africa in hetoria. Analyseswere madein wavelength- dispersionmode at an acceleratingpotential of 15 kV Calcium contentvaries in a nturow rangefrom 18.6 and a samFlecurrent of 20 nA. A beam 1-2 pm wide to 19.2 apfu (atomsper formula unit). Sodiumcontent was usedto determinethe fine-scalecompositional is low, up to 0.13 apfu, but it is usually less than variation indicatedby the optical zoning. Counting 0.10 apfu (Table1). This is consistentwith the results times were L0 secondson peaks and2 x 5 secondson obtainedby Groat et al. (1992), who noted that Na backgroundpositions. Concentrations of Si, Al, Na content is less than 0.2 apfu in vesuvianitewith a and Mg were determinedwith TAP, of Ti, Cr and Ca Ti contentless than 0.75 apfu. If the Na and Ca cation with PET, and of Fe and Mn, with LiF crystals.The totals are combined"the resultantmean is 18.96 standardsfor Si, Ca and Mg (diopside,Natural Bridge: (standarddeviation 0.13), statistically identical to the USNM L17733),Al and Fe (Kakanui pyrope: USNM ideal value of 19 X sitesindicated by the 50-cation L43968), Na (Kakanui hornblende: USNM 143965) formula. No consistentpattern was established and Cr (chromite:USNM L17075)were describedby befweenCa and Na content and the variation in bire- Jarosewichet al. (L980). We used syntheticrutile and fringencein the grain.

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Flc. 2. Compositionalprofiles A-8, A-{ and A-D acrossoptically zoned vesuvianiteshown in Figure lb. Concentriclow- birefringence zones are indicated with light stipple and labeled with Roman numerals.Nearly isotropic marginal zone shownin dark stipple. COMPLEX ZONING IN YESUVIANTTE 8l $,s sEH$EX g,$,8S3:rEE E o E

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F: g .E v $s$,sEBfiEf Ec8,$$ssE$s A E o rr.l \e U $B$,sss$E$ E:E sEEsEEE E U E oI& q F g fr $:E,FsI8Et Esn,EsessEsE D qs ut a fisBs*,sEsfi isEsssE,EEE $ E trl t & a -j q g Ei rr{ 8:E,n:rH:X F$E,saccs8:*d q Za |e SoE r-*sKa I E.- -**^oS- N .;t-H€'T = F. HrE 3sEH3n E35,55=3583S E; 6 jd E p 3r-8os<*Fa N 8oo*ooo -S. E C'E F $5EE5Erh3$ il55Es5=,5E3S :EA tnI a oGo 3 H*H *h6 ^F- S EES N g=E,3,5H3 d F f;58,=53,SE=S 3E's Ei,.E & $l$,ngg3-qfi i:8,3gsE$$B$ E;! t gEEaag e E E'EI ciS ee$$sBgsBF ;Lirrphlibui tr {. rJ 82 THE CANADIAN MINERALOGIST

Si contentvaries slightly, ftom 17.7to 18.3 apfu, abundancecan also be discountedas a causeof the with a meanof 17.99(standard deviation 0.01). This variable birefringence, as Al values for the different finding, and the low skewness(-0.09), indicatethat zones are similar (compareAl values for zone I, the thereis little or no Al-for-Si substitutionat the Z sites. marginal zone and the high-birefringencecore zonein The combinedtotal for the remainingcations (Al3+, Fig.2). In contra$t,Ti decreasesprogressively with Mg2*oFe2*o Fe3*, Ti4*, Mn2*) has a mean value of decreasingbirefringence. The cutoff betweenthe high- 13.09(standard deviation 0.11), which is statistically and low-birefringenceconcentric zones appears to be identical to the ideal value of 13 I sites.Al varies approximately0.40 to 0.45 apfu, whereasthe nearly from 9.3to 10.5aptu, Mg from 0.5 to 1.5aptu, and Fe isotropic marginalzone contains little, if any, Ti. from 1.3 to 2.3 apfu. Ti rangesfrom zero,in the neady isotropicmarginal zone, to 0.73 apfu. Mn contentis negligible (0 to 0.04 aptu), but in analysis1 (Iable 1), Implicationsof zoningfor cation substitutions it reaches0.13 apfu. Wavelength-dispersionscans failed to detectthe Models of cation substitutionfor vesuvianitehave presenceof significant F peals abovethe background. beenproposed by severalinvestigators. Hoisch (1985) The vesuvianitealong profile A-B was also analyzed and Groat et al. (1992) proposedthat the main substi- for Cl, which was found only in the nearly isotropic tution involving Ti in vesuvianiteis Ti + Mg ;+ 2Al. marginalzone (0.15-0.23wtVo) and adjacentto a Yalley et al. (1985) suggestedthat in Ti-free vesu- fracture(0.42 wt%o).Boron was not sought,but vianite, Fe3+= AI is important.Groat et al. (1992) indirect evidencesuggests that the vesuvianiteis pointed out tlat involvement of anions in the cation B-free. This includes the optically negativenature of substitutionscould lead to a significant role for substi- the vesuvianite,a low Mg contentand high Al content, tutions such as Ti + 02- = Al + OH- and Mg + OH- and the saturation of the Z cation site by Si (Groat = Al + O2-.They suggestedthat the latter substitution et al. L992). may be particularly significantbecause their study of a wide rangeof vesuvianitecompositions failed to show sufficient variation in Ti to accountfor the observed Patternsof zoning variation n Alvia the Ti-Al substitutions. The cation-cation correlationdiagrams in Figure 3 The variation in abundancesof the main l-site provide an indication of the most likely substitutions cations (Al, Fe, Mg, Ti) in the three core-to-rim responsiblefe1 fls 26ningprofiles in Figure 2. It is traversesacross the vesuvianitegrain (Fig. 2) shows clear from thesediagrams that zonesof different bire- two patterns: (a) cation abundancesdiffer sharply fringence contain different correlationsnindicating a betweenzdnes with different birefringence,and (b) an link betweenths 2ening in birefringenceand various overall decreasein Al, and an increasein Feooccur cation substitutions.The poor inversecorrelation acrossthe grain from core to rim in the high-bire- betweenTi and A1 in the high-birefringencezones fringencezones. (dots,Fig. 3a), indicatesthat significantsubstitution Kerrick et al. (1973) noted a link betweenbire- for Al must occur by a meansother than Ti + Mg = fringence and Ti abundancein vesuvianitecrystals 2Al. The most likely candidateis the homovalent from the Twin Lakes area, California, although tley substitutionFe3* = A1, evidencefor which may be neglectedto stipulatethe exact natureof the relation- provided by the good inverse correlation between ship. More recently, Denr et aI. (1992) suggestedthat Feroral and Al in, the high-birefringencezones an inverse relationship exists between birefringence (Fig. 3d). This Fer+-Al substitutionalso may be and the OH- content of the vesuvianite.The slightly important in the nearly isotropic marginal zone, lower total chargein the nearly isotropic marginal althoughcation correlationsin this zoae areinconclu- zonesofthe.vesuvianite investigated here (Table 1) is sive. In confast, the good inversecorrelation between consistentwith tle relationship proposedby Deer Ti and Al in the low-birefringence concentric zones et al. (1992); however, our inability to measure (squares,Fig. 3a) suggeststhat Ti + Mg = 2Al substi- directly the OH- contentand the valencestate of Fe in tution is stronghere (seealso Figs. 3b, c). The poor the different zonesprecludes a definitive statementon correlation observedbetween Ferorar, and Al this relationship. (Fig. 3d) in thesezones discounts Fe3+ = Al. The data in Figure 2 indicate that Ti is the only Correlation betweenMg and Feroral also is poor cation that shows a consistentrelationship with bire- (Fig. 3e); however,if we assumethat the vesuvianite fringence in all zones.Although Mg and Fe show is saturatedin trivalentcations, such that AI + Fe3*= significant variation acrosszone boundaries, Mg abun- 11 aptu (seeGroat et al. 1992),then the possibility dancedecreases in zoneI relative to the adjacenthigh- of homovalentMg-Fe2+ substitution can be tested. birefringence zones,but increasesin the marginal The resultantplot @ig. 3f) suggeststhat sucha substi- zone. Fe showsthe reciprocalrelationship, increasing tution is plausible in the low-birefringenceconcentric in zone I and decreasingin the marginal zone. Al zones. COMPI.EXZONING IN VESUVIANTTE 83

CL o tll t tt' J t".j{ t .i..;r. F ,f .a .a ttrt o- . t rl o oF !r o.4 r' L .. .t F tat' A tt t:

Al (opfu)

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A 1.2 o.l :t o ,:. o.1.0 .{1. I .,r,st.,, 1 .3. ol I l. . lt I ED0.8 . o l. t! t .;_...'. 0.6 "" .f 't :

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Sunuany Fe2+;than in the high-birefringencezones (Fe3+ = Al, (i) This study has recognizedthat vesuvianitemay Ti+Mg=2Al). contain complex oscillatory zoning expressedin birefringence,which hasimportant implications for under- Aclo{owlmcHvmNTs standingthe crystal chemistry and mechanismsof cation substitutionin this mineral. The samplediscussed here was collected during (ii) The vesuvianiteis zonedwith respectto Ti, Al, RLG's Ph.D.studies at CambridgeUnivenity, England. Mg and Fe. Financial supportwas obtainedfrom the University of (iii) A positive correlationexists between the inten- the WitwatersrandResearch Committee. DdB tha:rks sity ofthe birefringencein the grain and Ti abundance. the Director of the Geological Survey for permission (iv) Cation 2eningoccurs within optically homoge- to publish the data.R. Boer is thankedfor his critical neous birefringent zones and seemsto reflect more review of an early draft of this manuscript,and F.C. than one substitution. Hawthorne,R.F. Martin and two anon)imousreferees (v) Different substitutionsappear to be operativein provided usefirl comments.L. Whitfield producedthe the low-birefringencezones (ti + Mg :- 2A1, Mg = diagrams,and M. Hudson,the photographs. 84 TIIE CANADIAN MINERALOGIST

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