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Complex Zoning in Vesuvianite from the Canigou

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Complex Zoning in Vesuvianite from the Canigou 77 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,how- ever, 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 zon- ing 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 cal- cite. 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)show- ing 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
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