biotites from biotite-rich crusts of enclaves and clots in...

Per. Mineral. (2008), 77, 3, 3-20 doi:10.2451/2008PM0016 http://go.to/permin

PERIODICO di MINERALOGIAestablished in 1930

An International Journal ofMINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY,ORE DEPOSITS, PETROLOGY, VOLCANOLOGYandappliedtopicsonEnvironment,ArchaeometryandCultural Heritage

l

AbstrAct. — Biotite flakes from the biotite-rich crust of three enclaves and a clot, from them the theMonopigadonpluton(Macedonia,northernGreece),areexaminedfromthetexturalandchemicalpointofview,andcomparedwiththoseofhost-rocksandenclaves.Biotitecrystals, forming the biotite-richforming the biotite-rich thebiotite-richcrusts and the clot, are not oriented, but they arelonger than thoseof thehost-rocks. �n particular,-rocks. �n particular,rocks. �n particular,biotitesof theclot are more than 2.5 times longerclotaremore than2.5 times longerthanthoseofthehostrock,andbiotitesofbiotite-richcrustsofenclavesare10to50%longerrelativetothoseofthehostrock.�heir colour is reddish to�heircolourisreddishtoreddish-browndifferingfromthatofthehost-rocks-rocksrockswhere it is reddish-brown to brown and yellow-browntobrown.Generally, contents of �i, �e andGenerally,contentsof�i,�eandMginbiotiteincreaseinthehost-rocktowardstheenclave (or clot), whereas contents ofAl and �icontentsdecreaseinthesamedirection.Biotites ofBiotitesofcrustsandtheclotare�i-richerrelativetothehost-rock biotites.�he main substitutional mechanismsubstitutional mechanismfor �i4+ can be described from the relationship(R2+)V�+2(�i4+)�V=(�i4+)V�+2(Al3+)�V.�heeuhedrallath-likebiotitecrystalsofbiotite-rich crusts and the clotsbiotite-richcrustsand the clotsandtheclotsclotswereformedsimultaneouslywiththecrystallizationofthehost-rock as the result of the surrounding graniticastheresultofthesurroundinggraniticmagmatemperaturedecreasingin a narrow space atinanarrowspaceat

contactwithcolderenclave. �t is suggested that theenclave.�t is suggested that the�tissuggestedthatthebiotitecompositionofbiotite-richcrustsandtheclotdependsonthatofthehost-rockbiotites,whereasitisindependentfromtheenclavecomposition.

riAssunto. — �ono state esaminate lamine dibiotitesiadellacrostariccadibiotiteintornoatreinclusi,duesurmicaceiedunocumulitico,siadiun clot – incluso costituito essenzialmentedabiotite–delplutonediMonopigadon(Macedonia,Greciasettentrionale).L’esameèstatosviluppatosiasottoilprofilostrutturalechechimicoedidatisonostaticonfrontatiancheconquellidellerocceincassanti.�cristallidibiotitechecompongonolecrostericchedibiotiteedilclotnonsonoorientati,masonopiùlunghidiquellidellerocceincassanti.�nparticolare,le biotiti del clot sono lunghe più di 2,5 volterispettoaquelledellarocciaincassante,adifferenzadellebiotitiappartenentiallecrostechehannounalunghezzachevariada1,10a1,50,semprerispettoallecorrispondentidellarocciaincassante.�lcoloredellebiotitidellecrosteedelclotvariadalrossiccioalrosso-marroneedifferiscedaquellodellebiotitidellarocciaincassantechemostranounagammacromaticachevariadalrosso-marronealmarroneedalgiallo-marronealmarrone.�ngenere,icontenutidi�i,�eeMgdellebiotitidellerocceincassantiaumentaversol’incluso (oclot), inparallelo con ladiminuzionedeicontenutidiAle�i.Alcontrario,lebiotitidelle

Biotites from biotite-rich crusts of enclaves and clots in the Monopigadon pluton (Macedonia, northern Greece)

Antonios Koroneos *

DepartmentofMineralogy,PetrologyandEconomicGeology,�choolofGeology,AristotleUniversityof�hessaloniki,GR-54124�hessaloniki,Macedonia,Greece

Submitted, July 2008 - Accepted, October 2008

*E-mail:[email protected]

4 A.Koroneos

crosteedelclotsonopiùricchedi�irispettoaquelledellerocceincassanti.�lprincipalemeccanismodisostituzionedel�i4+puòesseredescrittotramitelarelazione(R2+)�V+2(�i4+)�V=(�i4+)�V+2(Al3+)�V.

Lelamineeuedralideicristalliappartenentiallecrosteedalclotsisarebberoformatisimultaneamenteconlosviluppodellarocciaincassante,inseguitoalladiminuzionedellatemperaturadelmagmagraniticoinglobante, nello spazio ristretto che circondal’incluso.�datistrutturaliechimiciindicanoinoltreche lacomposizionedellebiotitiappartenentiallecroste ed al clot dipende da quella delle biotitidellarocciaincassante,mentretalecomposizioneèindipendentedaquelladell’incluso.

Key words: Mineralogy, biotite composition, biotite-rich crust, surmicaceous enclaves, Monopigadon granite

introduction

Acommoncharacteristicofgranitoidrocksisthepresenceofxenolithsandenclaveswhereassurmicaceous enclaves and crystal clusters ofbiotite,knownasbiotiteclots,arenotsocommon.Micaceouscrusts,biotite-richcrusts,andshellsofbiotiteareusuallydeveloparoundxenolithsandsurmicaceousenclaves,althoughrarely,couldbefoundaroundcumulateenclaves.

�ome information will be given here aboutthe not so common surmicaceous enclavesand clots. �urmicaceous enclaves are presentin many intrusions and they are particularlycommon in anatectic granites associated withmigmatites.Didier(1973)suggestedthattheterm“surmicaceousenclave”mustberestrictedtothosewherethemicacontentformsmorethan50%byvolumeoftheenclave.Latter,DidierandBarbarin(1991) reported thatadistinguishing featureofthesurmicaceousenclaves, in the field, is theirbiotiticcrust.�uchenclavesarereportedbymanyauthors for different granitic rocks e.g. in thein thegraniteintrusionontheVelenceMts.(Hungary;Buda,1993),intheDeddickgranodiorite(Lachlan�oldBelt,�EAustralia;Maaset al.,1997),in thetheperaluminousgraniteof theNelasarea(CentralPortugal;�ilvaet al.,2000)and within the MonteandwithintheMonteCapannepluton(Elba�sland,�taly;Gagnevinet al.,2004).

Didier (1973)accepted that thesurmicaceousenclaves are quite clearly restites whereastheir presence was interpreted as a proof thatcontinental crust participated in the genesis ofgranites(Didier,1987).Manyauthors(e.g.Montelet al.,1991;�chödlbaneret al.,1997;Williamsonet al.,1997;NemchinandPidgeon,1999;BarnesBarneset al.,2001, WaightWaightet al., 2001;Meliand�assi,2000and2003;Clarkeet al., 2005) argued thatargued thatarguedthatsurmicaceousenclavesrepresenteitherpeculiarlithologies from the source area that resistedanatexisorcountry-rockxenolithspickedupbytheplutonatorclosetotheemplacementlevel,orpartiallydigestedxenolithsofcountryrocks.Didier (1973)accepted thatclotsareoriginatedfrom enclaves or xenolithic rocks due to acombinationofprocessescausingdisintegrationmovementswithinthemagma,aidingforthefinaldisruptionanddispersionoftheclotsfromtheirplaceoforigin(theenclave)totheirplaceofrest(thesurroundinggraniticmagma).�heouterbiotiterich-crustoccurringaroundxenolithsandenclavesis interpreted as the result of reaction betweentheferromagnesianmineralsof theenclaveandthesurroundinggranitemagma(Didier, 1973). ADidier,1973). A.AdifferentinterpretationwasproposedbyPeruginiPeruginiet al.(2003)onthebasisoflargebiotitecrystalswithinthehostmagma,disposedmainlyalongtheperipheryofenclavesfoundinthe�ithoniapluton(Greece).�hetabularhabitcharacterizingbiotitesallowedthemtoalignalongpathlinesandhenceremaininthehostrock.

Althoughthesignificanceofxenoliths,enclaves,surmicaceousenclavesandclotsandtheirpossibleorigins have been discussed in detail by manyauthors,littleattentionhasbeenpaidtothetextureand chemistry of biotites forming biotite-richcrust and their relations with biotites of host-rocksandenclaves.AcharacteristicfeatureoftheMonopigadonplutonisthepresenceofnumerousenclaves, xenoliths of different rock types,surmicaceousenclaves, aswell asbiotiteclots.Among enclaves and xenoliths there are manyofferingthepossibilitytostudytheirbiotite-richcrustandtodeciphertheirpeculiarcharacteristics.A detailed study can give, in fact, informationaboutthe factors controlling the composition ofthefactorscontrollingthecomposition ofofthebiotites and the role of the enclaves, with thebiotitesand the role of the enclaves, with theandtheroleoftheenclaves, with thewiththegoalofclarifythegenesisofthebiotites,at leastleastfromaqualitativepointofview.

Biotites from biotite-rich crusts of enclaves and clots in the Monopigadon pluton (Macedonia, northern Greece) 5

the MonopigAdon pluton: regionAl setting, field relAtions, Age

�he Monopigadon pluton is an intrusionoccupyinganareaofabout10km2situatedbetweenthevillagesAg.Antonios,MonopigadonandKriniofChalkidiki(�ig.1)andcomposedofthreebodieselongated NW-�E with the most northwesternbeing the larger.According to Michard et al.(1988aandb)theMonopigadonplutonintrudesboththeChalkidikiophiolitesaswellasasupra-

ophioliticformation(ophicalcites,volcanoclasticflysch,reefallimestonesandconglomerateswithacidic rockpebbles).�heChalkidikiophiolites(Gauthier,1984)belongtotheeasternpartoftheVardar ophiolitic belt (Peonias Zone; Mercier,1968)andintheareashowautochthonouscoverformations (reefal limestones of Kimmeridian-�ithonianage,andclasticdeposits).Ophiolitesandsupra-ophioliticformationsthrustovertheCircumRhodopeandpartofRodopianunits(Michardet al.,1998a,b).�heMonopigadonplutonconsists

�ig.1–GeologicalsketchmapofMonopigadonpluton.W,CandE:westerncentralandeasternbodiesenclosedintheplutonicrocks.Arrowintheinsetmapindicatestheareaofinvestigation.

6 A.Koroneos

ofthreedifferentrock-types:biotitegranodiorite(BGrd), biotite granite (BGr) and leucogranite(LGr).Aplitic dykes (Apl) occur intruding theBGrd, BGr and LGr. Enclosed rocks are veryfrequent inBGrd andBGrwhile they areveryrare inLGr.�hepluton, exhibitsmainly �-typecharacteristics with ASI mostly ≤ 1.1 although theinitial�risotopicratiorangesfrom0.7093to0.7291(Koroneosinpreparation).

�hreeroundedbodiesofcountryrocks,coveringanareaofabout0.25km2eachone,areenclosedin the pluton. �he central (C, �ig. 1) body iscomposed mainly of serpentinized peridotitewhereasthewesternandtheeastern(WandE,�ig.1)arecomposedofhornfelsesandamphibolites.Except the tertiary and neogene sediments,coveringthebroaderarea,theMonopigadonplutonisunconformablyoverlainbytheKimmeridian-�ithonianPetralonaperi-reefallimestones(Ricou,1965; Kockel et al., 1977; Gauthier, 1984;Michardet al.,1998b).�helatterarefollowedbythePrinochoribeds(calcschists,calcarenites,andconglomerateswithslateandquartzpebbles).

�he age of the Monopigadon pluton isconstrainedbytheabovementionedstratigraphicevidenceandradiometricmeasurementsaswell.�heplutonwasdatedbyK/Aronbiotiteseparatesat180Ma(Ricou,1965),149Ma(Mussallam&Jung,1986),and141.5�3.1 Ma (Michard�3.1 Ma (Michard3.1Ma(Michardet al.,1998a, b). Kostopoulos et al. (2001) reportedrecentlyasinglezirconevaporationPb/Pbageat192.5�3.8 Ma. Preliminary zircon U/Pb datinggivesaminimumemplacementageof184.3�0.8Ma(Christofideset al.,unpubl.data).�helasttwoagesconfirmanearlyJurassic intrusionfor theMonopigadonpluton.

petrogrAphy

The host-rocks

BGrd:Biotitegranodioriteisagreytodark-gray,finetomedium-grainedrockwithquartz(25.1to31.4vol%), slightlyzonedplagioclase (35.0 to42.0vol%),oftenalteredtosericite,orthoclase(1.5to4.9vol%),sometimeskaolinized,andbiotiteexisting inhigheramounts relative to theotherrock-types(28.2to34.6vol%).�nthinsections,biotiteisreddish-browntobrownandveryrarelyisdiscolouredoralteredtochlorite.Zirconwith

pleochroichaloesisacommoninclusioninbiotite,and apatite, titanite and opaque minerals arepresentasaccessories.

BGr:Biotitegraniteisagrey-whitishcoarsetomedium-grainedrockintrudingtheBGrd.�hemainmineralconstitutinggranitesare:quartz(20.2to28.9vol%),zonedplagioclase(32.2to40.7vol%),orthoclase(23.8to25.1vol%),andbiotite(12.0to14.6vol%).Orthoclaseisveryrarelykaolinized,occurringboth asphenocrysts andgroundmassconstituents. �t is perthitic and encloses smallcrystals of plagioclase and biotite in poikilitictexture.Biotiteisyellow-browntobrownandoftendiscolouredoralteredtomuscoviteandchlorite.Rarely,rag-shapedamphibolecrystalsarefound.Zirconswithpleochroichaloes,apatite, titaniteandopaquemineralsarepresentasaccessories.

LGr:Leucograniteisafinetomedium-grainedleucocratic rock intrudingBGr. �n someplacesit is altered in coarse-grained granitic sand. �tis richer in quartz (34.8 to 38.1 vol%) and K-feldspar(29.3to49.3vol%),relativelytoBGrdandBGr,andpoorerinbiotite(1.7to6.3vol%).PerthiticK-feldspars(orthoclaseandmicrocline)are kaolinized and slightly zoned plagioclasesare altered to sericite. Biotite is yellow-browntobrownincolourandisveryoftendiscolouredoraltered tochlorite.Muscovite isalsopresentinminoramounts.Zircons,opaquemineralsandrarelyapatitearepresentasaccessories.

Apl: Apliticveinsrichinquartz(55.2vol%)andperthiticK-feldspars(orthoclaseandmicrocline;42.4vol%),withminoramountsofplagioclase(0.9vol%)andbiotite (0.8vol%).Zircons andopaquemineralsarepresentasaccessories.

The enclosed rocks

DifferentkindsofrocksarefoundenclosedintheBGrd andBGr.�hemajority of themhavelenticulartoovoidshapealthoughsomeangularpiecesarealsofound(�ig.2a-c).�heserockscanbefoundalloverthepluton.�heycanbegroupedinthefollowingsix(atof)groups.

a.�erpentinizedperidotite.�heseare10-20cminsizeandoccurmainlyclosetothecontactoftheplutonwiththeenclosedcentral(C,�ig.1)body.

b.Amphibolites.�heyarerichinamphiboleandplagioclase,whereasquartzoccurs invery lowquantities.�heirsizerangesfrom3to10cm.�tis


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8 A.Koroneos

tonoteherethatnomicaceouscrustsaredevelopedaroundthem.

c. Rocks rich in sericitized plagioclase,clinopyroxene,orthopyroxene,hornblende, andsome quartz. �hey probably represent maficgranulites.�heirsizerangesfrom3to10cm.

d.Rocksrichinbiotite(morethan50%),chlorite,somequartz,apatiteandepidote.�heirsizerangesfrom 3 to 8 cm (�igs. 2a-c). �n some of theserocksmorethan40%ofthebiotiteisdiscolouredoraltered tomuscoviteandchlorite. �tmustbestressed that thesealteredbiotite crystals showauniformdistributionandarenotconcentratedclosetothecontactwiththehostrock.�heserocksrepresentsurmicaceousenclaves.�heyshowsharpcontactswiththehost-rocks,andareveryoftensurroundedbybiotite-richcrusts(�igs.2d-e).

e.�mall(2-4cm)coarse-grained enclaves, richcoarse-grained enclaves, rich-grainedenclaves,richinbiotite,sericitizedplagioclaseandmagnetite,withsmallamountofquartzandK-feldspar.�heirmineralogicalassemblageissimilartothatofhost-rocks,althoughtheydifferfromthelatterforthecrystalsizebeingbiggerintheenclaves.�heyalsodifferforthesizeandamountofapatiteandzirconwhicharemoreabundantandformbiggereuhedralcrystalsintheenclaves,relativelytothehost-rock.-rock.rock.�hese enclaves are surrounded by biotite-richcrustssimilartoenclavesd)andentirelymadeofnotorientedbiotite flakes,withwidths ranging

between2and5mm.Basedontheirtextureandmineralogy,these,veryrareenclaves,areprobablyofigneousorigin,andtheycouldbeconsideredascogeneticwiththepluton,representing,hence,cumulateenclaves.Moreover, theycannot representMoreover,theycannotrepresentmaficmicrogranularenclavessincetheircrystalsarebiggerofthoseofthehost-rocksandneedle-likecrystalsarelacking.�imilartypeofenclavesisreportedbyWaightWaightet al.(2001)intheCowraGranodiorite(Lachlan�oldBelt,�EAustralia),amongfivedifferentvarietiesofenclaves,ranginginsizefromafewcentimetresto50cmindiameter,andhavingabiotite-richcrust.

f.Rocksmadechieflyofbiotiteswith diameterdiameterrangingfrom0.7to2cm(�ig.2f),andvery smallerysmallquartz and plagioclase crystals occurring oftenbetween biotite flakes. �hey represent biotiteclots.

Biotitesoffourrepresentativesamplesbelongingtogroupsd),e)andf)werechosen(�able1).�n the�nthefollowingthetermenclave(Enc)includesboththesurmicaceousenclavesandthecumulateenclave.

AnAlyticAl MethodsnAlyticAl Methods

Biotite analyses were carried out at the�canning Microscope Laboratory,�hessalonikiUniversity, using a Jeol J�M-840A �canning

�ample EnclaveGroup Hostrock

Area Length (mm)(mm) Width (mm)(mm)mean std min max mean std min max

MO-23EE d BGrdHost 0.50 0.09 0.31 0.71 0.39 0.14 0.17 0.71Crust 0.68 0.07 0.57 0.86 0.34 0.13 0.17 0.71Enc 0.16 0.06 0.06 0.23 0.07 0.02 0.03 0.09

MO-67Adwith

discolouredbiotite

BGrHost 0.3939 0.12 0.23 0.77 0.23 0.11 0.11 0.43Crust 0.64 0.13 0.29 0.80 0.20 0.05 0.11 0.29Enc 0.26 0.09 0.17 0.34 0.14 0.09 0.06 0.23

MO-89AA e BGrdHost 0.70 0.13 0.51 1.00 0.30 0.11 0.14 0.51Crust 1.03 0.23 0.57 1.43 0.27 0.10 0.17 0.51Enc 1.65 0.80 0.43 3.00 0.56 0.33 0.14 1.29

MO-101 f BGrHost 0.68 0.33 0.29 1.43 0.29 0.07 0.14 0.40Clot 1.78 0.54 0.91 2.86 0.40 0.17 0.14 0.71

tAble 1Dimensions of the investigated biotites


Electron Microscope (�EM) equipped with anEnergyDispersive�pectrometer(ED�)with20kVacceleratingvoltageand0.4mAprobecurrent.Asetofsimpleoxidesandsilicateswasusedasprimarystandards.Basedonrepetitiveanalysesof the standards, the precision of the analyseswasestimatedtobebetterthan2%(relative)forelementswithconcentrationhigherthan1%(�iO�iO2,�iO2,Al2O3,�eO,MgOandK2O)andbetterthan5%(relative)for other elements (MnO, CaO andforotherelements(MnO, CaO andMnO,CaOandNa2O).

�he whole rock samples were analysed byhole rock samples were analysed byX-ray fluorescence for major elements on aon aPhillipsPW2400sequentialX-ray fluorescencespectrometerusinganRh-anodeX-raytube(�aintMary’sUniversityRegionalGeochemicalCentre,Canada). Loss on ignition (LO�) was determinedLoss on ignition (LO�) was determinedLossonignition(LO�)wasdeterminedbytreatingthesamplefor1.5hat1050°Cinanelectric furnace. Major elements were carriedoutonfusedglassdiscs.�nternationalstandardswereusedforcalibration.Analyticalprecision,asdeterminedfromreplicateanalyses,isgenerallybetter than 1% (MgO,Al2O3, �iO2, K2O, CaO,�iO2,MnO),2%(Na2O,P2O5)and3%(�eO).

results

Biotite morphology

Biotite crystals of biotite-rich crusts and theclotshowsomedifferences,concerningsizeandcolour,comparedwithbothbiotitesofenclavesand host-rocks. �or each sample, dimensions(lengthandwidth)weremeasured,inthinsection,forfifteenlath-likebiotitecrystalsfromeachoneofhost-rocks,biotite-richcrustsenclaves,andtheclot;resultsarepresentedintheformofminimum,maximum,meanvalueandstandarddeviationin�able1,andcomparedin�ig.3.�hebiotite-richcrustsandthebiotiteclotcontainbiotitecrystalsranginginlengthfrom0.29mmto1.43mm(�igs.2g-h)and0.91mmto2.86mmrespectively(�ig.2i).Amongthebiotite-richcruststhesampleMO-89Acontainsthelongestcrystalsrelativetotheother twoenclaveswhichisalso thecasewhenthehost-rockbiotitesarecompared.�hebiotitecrystalsofthehost-rocksrangebetween0.23mmand1.43mm,whiletheenclave-biotitesfrom0.06mmto0.34mm.However, longerbiotiteswerefound(upto3.00mm)intheMO-89Aenclave,

andthesearelongerthanthehost-rockbiotites.�tshouldbenotedthatinthecaseofsampleMO-101(biotiteclot)thereisnodifferenceincrystal-sizebetweentheouterandtheinnerpartoftheclot.Moreover, sample MO-101 shows the highestdifference(1.1�0.24mm)betweenthebiotitesinhost-rockandbiotiteinclotinlength.

�hewidthofthebiotitecrystalsrangesfrom0.11mmto0.71mminthecrusts,from0.14mmto0.71mmintheclotandfrom0.11mmto0.71mminthehost-rockbiotites.�hewidthofthecrustsandclotbiotites canbeconsideredcomparable(takingintoaccountthedeviations),whichisalsothecasewhenthehost-rockbiotitesarecompared.Biotitecrystalsaresmaller in thesurmicaceousenclavesandbiggerinthecumulateenclaveMO-89Awherecrystalsupto1.3mmwidewerefound.Concerningcolourofbiotitesinthinsections,itisreddish-browntobrowninBGrd,yellow-browntobrowninBGr,whereasitisreddishtoreddish-browninthebiotite-richcrustsandclot.

Biotite chemistry

Polishedthinsectionswerepreparedinordertocontainatleastpartofthehost-rock,thebiotite-richcrust andpartof the enclaveor thewholeclot. �n each sample analyses were performedapproximatelyonatransect,fromthehost-rockto the enclave through the biotite-rich crust orfromthehost-rocktothecenteroftheclot.�nthehost-rockfivepositionscontainingbiotiteswereselected; thedistancebetween themwas3 to4mm.�hesepositionswerenumberedfromH1toH5withthelatterclosetothebiotite-richcrust.�n thebiotite-richcrust, threepositionsnamelyC1,C2andC3werechosen.�hefirstisclosetotheborderwithhost-rocks, the third isclose totheborderwithenclaves,andthesecondisinthemiddle.Lastly,ineachenclave,twopositionswereselected,E1andE2,oneclosetotheborderwiththebiotite-richcrustandtheother3to4mmfarfromthecenteroftheenclave.E1andE2positionsarelackingforsampleMO-101(biotiteclot).�neachposition,atleast3biotitecrystals(onetotwospotsineachcrystal)wereanalysedandtheresults,intheformofmeanvalues,arepresentedinthe�able2.�henumberofatomsper formulaunit(apfu)wascalculatedonthebasisof22oxygens.Alltheanalysedsamplesareclassifiedasbiotites

10 A.Koroneos

onthebasisof�e/(�e+Mg)ratiovs.Altot(DeerDeeret al.,1966;�ig. 4), inasmuch as they are not very�ig.4),inasmuchastheyarenotveryvariableincomposition.

Compositions of the analysed biotites arecomparedinthediagramsof�ig.5.�orthesakeofsimplicitythedistancebetweenanalysedpositionsH1toE2isreportedequal,evenifthisisnotthecase in thebiotite-richcrust (C1 toC3),wherepositions are closer each others. Using regularspacingfromC1toC3somedetailswillbe,infact,outofsight.�he�icontentincreasesinthehost-rocktowardstheenclave(orclot)inallsampleswhilethe�econtentincreasesalsointhehost-rocktowards thebiotite crust, in all samples except

MO-23E.�heMgcontentincreasesslightly.Onthecontrary theAland�icontentsdecrease inthehost-rocktowardsthebiotitecrust(orclot).Al�VdecreaseswhileAlV�increases.AlthoughthebiotiteswereanalysedalsoforMn,CaandNa,andsomedifferencesbetweenhost-rockandbiotite-richcrustcouldbenoted,theseelementsarenotusedduetotheirlowconcentrationsandpossiblylowanalyticalprecision.

However, taking into account the errorscalculated for the above elements (3.4%) theonly statistically robust variations are those of�icontent;the�icontentsplotwitherrorbarsinfigure5whilesuchaplotoftherestelementswould

�ig.3–Biotite’sdimension(lengthandwidth)inmmofhost-rocks(H),biotite-richcrust(C),enclave(E),andclot(Cl).Diamonds:MO-23E,triangles:MO-67A,squares:MO-89A,andcircles:MO-101.Opensymbols:host-rock;shadedsymbols:biotite-richcrustorclot;solidsymbols:enclave.

Biotites from biotite-rich crusts of enclaves and clots in the Monopigadon pluton (Macedonia, northern Greece) 11�a

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33E1

E2

�iO

236

.03

36.1

136

.37

36.7

337

.44

36.8

936

.23

36.6

036

.77

37.1

737

.05

36.7

237

.838

.21

36.7

736

.44

35.7

535

.94

37.0

037

.91

�iO

23.

483.

493.

183.

093.

123.

334.

003.

454.

294.

063.

343.

373.

283.

252.

863.

173.

363.

073.

323.

21

Al 2O

317

.57

17.5

217

.14

17.4

817

.24

17.5

616

.52

17.8

116

.38

16.5

918

.61

18.5

618

.61

18.4

717

.03

17.5

217

.03

17.9

416

.63

16.7

9

�eO

t18

.64

18.0

317

.67

17.7

318

.56

17.9

418

.36

18.0

518

.24

18.9

917

.76

18.1

117

.95

18.3

219

.40

18.9

119

.32

19.4

118

.13

17.3

6

MnO

0.24

0.28

0.31

0.31

0.24

0.46

0.31

0.31

0.34

0.25

0.19

0.35

0.17

0.47

0.25

0.45

0.37

0.41

0.58

0.42

MgO

10.5

010

.52

10.5

411

.06

11.1

411

.12

10.8

510

.54

10.7

111

.12

9.51

9.47

9.73

9.68

9.47

10.2

39.

839.

6110

.22

11.6

8

CaO

0.00

0.02

0.00

0.00

0.00

0.00

0.00

0.02

0.00

0.00

0.00

0.00

0.00

0.00

0.06

0.00

0.01

0.00

0.00

0.01

Na 2O

0.42

0.40

0.59

0.20

0.49

0.37

0.29

0.38

0.21

0.37

0.76

0.84

0.72

0.47

0.38

0.43

0.14

0.49

0.55

0.46

K2O

8.63

8.30

8.35

8.64

8.81

8.68

8.39

8.60

8.65

8.62

9.38

8.44

8.44

8.5

8.49

7.96

7.88

7.99

8.06

8.35

�ota

l95

.52

94.6

794

.16

95.2

397

.03

96.3

494

.94

95.7

495

.57

97.1

696

.60

95.8

696

.70

97.3

794

.70

95.1

093

.70

94.8

694

.49

96.1

8

�ite

allo

catio

ns(2

2O

)

�i5.

440

5.47

35.

538

5.52

45.

548

5.49

55.

493

5.48

45.

534

5.51

45.

510

5.49

35.

575

5.60

55.

604

5.50

85.

501

5.46

55.

616

5.62

0

Al�V

2.56

02.

527

2.46

22.

476

2.45

22.

505

2.50

72.

516

2.46

62.

486

2.49

02.

507

2.42

52.

395

2.39

62.

492

2.49

92.

535

2.38

42.

380

Z8.

000

8.00

08.

000

8.00

08.

000

8.00

08.

000

8.00

08.

000

8.00

08.

000

8.00

08.

000

8.00

08.

000

8.00

08.

000

8.00

08.

000

8.00

0

AlV

�0.

566

0.60

40.

614

0.62

20.

558

0.57

80.

444

0.63

00.

439

0.41

60.

772

0.76

50.

809

0.79

80.

664

0.62

90.

590

0.68

00.

591

0.55

4

�i0.

395

0.39

70.

364

0.34

90.

347

0.37

30.

456

0.38

90.

485

0.45

30.

374

0.37

90.

364

0.35

80.

328

0.36

00.

389

0.35

10.

378

0.35

7

�e2

2.35

32.

286

2.25

02.

229

2.30

12.

235

2.32

82.

262

2.29

72.

356

2.20

92.

265

2.21

42.

247

2.47

22.

390

2.48

72.

468

2.30

12.

153

Mn

0.03

10.

036

0.04

00.

039

0.03

00.

058

0.04

00.

039

0.04

30.

031

0.02

40.

044

0.02

10.

058

0.03

30.

058

0.04

80.

053

0.07

50.

052

Mg

2.36

42.

378

2.39

32.

480

2.46

22.

469

2.45

22.

355

2.40

32.

458

2.10

82.

112

2.13

92.

117

2.15

12.

305

2.25

52.

178

2.31

22.

582

Y5.

709

5.70

05.

661

5.72

05.

697

5.71

35.

721

5.67

55.

667

5.71

45.

486

5.56

55.

547

5.57

85.

647

5.74

15.

769

5.72

95.

657

5.69

9

Ca

0.00

00.

004

0.00

00.

000

0.00

00.

000

0.00

00.

003

0.00

00.

000

0.00

00.

000

0.00

00.

000

0.00

90.

000

0.00

20.

001

0.00

00.

001

Na

0.12

30.

116

0.17

50.

058

0.13

90.

105

0.08

60.

110

0.06

00.

105

0.21

90.

244

0.20

60.

134

0.11

30.

125

0.04

30.

144

0.16

20.

133

K1.

663

1.60

51.

622

1.65

71.

666

1.64

91.

622

1.64

31.

661

1.63

11.

780

1.61

11.

588

1.59

11.

651

1.53

51.

547

1.55

01.

561

1.57

9

X1.

786

1.72

51.

798

1.71

51.

805

1.75

41.

708

1.75

61.

721

1.73

61.

999

1.85

41.

794

1.72

41.

773

1.66

11.

592

1.69

51.

722

1.71

3

Mg/

(Mg+

�e)0

.501

0.51

00.

515

0.52

70.

517

0.52

50.

513

0.51

00.

511

0.51

10.

488

0.48

20.

491

0.48

50.

465

0.49

10.

476

0.46

90.

501

0.54

5

FeO

t=to

tal�

eas

�eO

tAb

le 2

Biot

ite c

ompo

sitio

ns fr

om h

ost-r

ock

(H),

biot

ite-r

ich

crus

t and

clo

t (C

), an

d en

clav

e (E

) of t

he M

onop

igad

on sa

mpl

es

12 A.Koroneos

�am

ple

MO

-89A

MO

-101

Posi

tion

Hos

t-roc

kB

iotit

e-ric

hcr

ust

Enc

Hos

t-roc

kB

iotit

ecl

ot

Are

aH

1H

2H

3H

4H

5C

11C

22C

33E1

E2H

1H

2H

3H

4H

5C

11C

22C

33

�iO

236

.61

36.2

636

.45

36.1

835

.55

36.3

636

.58

36.7

636

.84

36.8

037

.02

36.7

736

.30

36.5

737

.35

36.6

236

.30

36.3

0

�iO

23.

673.

783.

403.

163.

033.

984.

544.

134.

694.

653.

293.

082.

812.

762.

743.

453.

963.

75

Al 2O

317

.25

17.3

217

.40

16.9

916

.24

16.9

516

.50

17.0

616

.97

16.5

218

.89

19.2

218

.83

19.0

018

.62

18.5

517

.71

18.7

1

�eO

t18

.82

19.2

318

.89

19.2

019

.36

19.0

518

.89

19.4

619

.33

19.6

616

.75

16.5

116

.85

16.1

016

.98

17.4

018

.28

18.2

2

MnO

0.39

0.34

0.13

0.45

0.45

0.34

0.34

0.39

0.21

0.30

0.24

0.32

0.26

0.16

0.13

0.23

0.38

0.38

MgO

9.98

9.85

9.84

9.91

9.70

9.90

9.80

10.0

19.

919.

709.

799.

869.

979.

9010

.03

9.89

9.89

9.83

CaO

0.00

0.01

0.00

0.00

0.11

0.08

0.00

0.00

0.01

0.00

0.09

0.05

0.00

0.02

0.01

0.00

0.00

0.00

Na 2O

0.48

0.52

0.28

0.46

0.28

0.64

0.68

0.57

0.35

0.55

0.44

0.68

0.69

0.63

0.55

0.62

0.47

0.47

K2O

8.41

8.41

8.42

8.06

8.22

8.44

8.50

8.51

8.36

8.39

7.53

7.81

8.07

7.90

8.06

7.87

7.96

7.90

�ota

l95

.62

95.7

294

.80

94.4

192

.94

95.7

595

.83

96.8

896

.68

96.5

894

.04

94.2

993

.78

93.0

594

.47

94.6

494

.94

95.5

5

�ite

allo

catio

ns(2

2O

)

�i5.

516

5.47

35.

530

5.52

95.

546

5.48

95.

514

5.48

65.

493

5.51

35.

558

5.51

85.

504

5.55

25.

602

5.50

55.

480

5.43

2

Al�V

2.48

42.

527

2.47

02.

471

2.45

42.

511

2.48

62.

514

2.50

72.

487

2.44

22.

482

2.49

62.

448

2.39

82.

495

2.52

02.

568

Z8.

000

8.00

08.

000

8.00

08.

000

8.00

08.

000

8.00

08.

000

8.00

08.

000

8.00

08.

000

8.00

08.

000

8.00

08.

000

8.00

0

AlV

�0.

579

0.55

50.

642

0.59

00.

531

0.50

50.

446

0.48

80.

475

0.43

10.

902

0.91

70.

869

0.95

20.

893

0.79

20.

631

0.73

2

�i0.

416

0.42

90.

388

0.36

30.

356

0.45

20.

514

0.46

40.

526

0.52

40.

372

0.34

70.

320

0.31

50.

309

0.39

00.

449

0.42

2

�e2

2.37

22.

427

2.39

72.

454

2.52

62.

405

2.38

22.

429

2.41

12.

463

2.10

32.

072

2.13

62.

044

2.12

92.

188

2.30

82.

280

Mn

0.05

00.

044

0.01

60.

058

0.05

90.

043

0.04

30.

050

0.02

70.

039

0.03

10.

041

0.03

30.

021

0.01

70.

030

0.04

80.

048

Mg

2.24

22.

216

2.22

52.

258

2.25

62.

228

2.20

42.

227

2.20

22.

166

2.19

22.

205

2.25

32.

240

2.24

32.

217

2.22

52.

193

Y5.

658

5.67

15.

669

5.72

35.

727

5.63

35.

589

5.65

75.

642

5.62

25.

599

5.58

15.

612

5.57

25.

591

5.61

75.

661

5.67

5

Ca

0.00

00.

001

0.00

00.

000

0.01

80.

013

0.00

00.

000

0.00

20.

000

0.01

40.

007

0.00

00.

004

0.00

20.

000

0.00

00.

000

Na

0.13

90.

151

0.08

30.

136

0.08

50.

186

0.20

00.

164

0.10

00.

160

0.12

70.

199

0.20

30.

186

0.16

00.

179

0.13

60.

135

K1.

617

1.61

81.

630

1.57

11.

636

1.62

51.

634

1.62

01.

591

1.60

31.

443

1.49

51.

560

1.53

01.

542

1.50

91.

533

1.50

8

X1.

756

1.77

11.

713

1.70

81.

740

1.82

41.

834

1.78

41.

693

1.76

31.

584

1.70

11.

763

1.72

01.

704

1.68

91.

670

1.64

3

Mg/

(Mg+

�e)

0.48

60.

477

0.48

10.

479

0.47

20.

481

0.48

10.

478

0.47

70.

468

0.51

00.

516

0.51

30.

523

0.51

30.

503

0.49

10.

490

FeO

t=to

tal�

eas

�eO

tAb

le 2

— C

ontin

ued.

..


�ig.4–ChemicalcompositionsforanalysedbiotitesplottedonclassificationdiagrambyDeeret al.(1966);seeinsetforzoomedregion.�ymbolsasinfigure3.

�ig.5–Plotofselectedelementsoftheanalysedbiotitesfromdifferentpositions.H1toH5:host-rock;C1toC3(shadedarea):biotite-richcrustorclot;E1toE2:enclave.�ymbolsasinfigure3.

14 A.Koroneos

makethetrendsratherindistinguishable.�he�iincreaseandAldecreaseinallhost-rockbiotitestowardstheenclavesorclotcanbealsoconsideredremarkable,sincethesetrendsareobservedinallsamples,evenifthisisnotwellconstrainedfromthestatisticalpointofview.

�rom�ig.5itisnoticeablethatinallsamples,biotitesfromthebiotite-richcrustsandclotare�i-richerrelativetothehost-rockbiotites.�he�i-,Al-,inthesamepage�e-,Mg-andK-contentsof the biotite-rich crusts and clot do not differsignificantlyfromthatofthehost-rock(exceptKand�eofMO-67A)tackingintoaccounttheerrors.�tmustbeemphasizedthatinallsamples,thecenter(C2)ofthebiotite-richcrust(orclot)is�i-richerrelativetothebordersbothtohost-rockortothe

enclave(C1andC3respectively),whereasotherelementseitherdonotfollowsuchasymmetryortheyhaverelativelysimilarconcentrations.

�everal substitution mechanisms have beensuggestedfor�i4+inbiotites(seeDymek(1983)fora review).�hesecanbedescribed from thefollowingrelationships:(R2+)V�+2(�i4+)�V=(�i4+)V�+2(Al3+)�V(1),(Al3+)V�+(�i4+)�V=(�i4+)V�+(Al3+)�V(2),2(Al3+)V�=(�i4+)V�+(R2+)V�(3),2(R2+)V�=(�i4+)V�+()V�(4)andadehydrogenationreaction(R2+)V�+2(OH)-

=(�i4+)V�+2(O2-)+H2(5).�hefairlygoodnegativecorrelation between Mg+2�i and �i+2Al�V,indicated by the high correlation coefficients(R2=0.961,0.992,0.965and0.991forMO-23E,MO-67A,MO-89AandMO-101biotitesof thehost-rocksrespectively;�ig.6a),confirmsthatthe

�ig.6 – Chemical composition of the analysed biotites from host-rocks, biotite-rich crust, and clot on the: (a) Mg+2�i–Chemical composition of the analysed biotites from host-rocks, biotite-rich crust, and clot on the: (a) Mg+2�iChemicalcompositionoftheanalysedbiotitesfromhost-rocks,biotite-richcrust,andclotonthe:(a)Mg+2�ivs.�i+2Al�Vand(b)�ivs.Al�Vdiagrams.R2:correlationcoefficientforeachline(solidanddashedlines).�heline�i/(Al�V-2)=1/2indicatesthatall �iall�iV�andAl�VarebalancedbysubstitutionMg+2�i=�i+2AlMg+2�i=�i+2Al�V. �ymbols as in figure 3�ymbols as in figure 3�ymbolsasinfigure3


substitutionof�iforMgiscompensatedforbysubstitutionofAl�Vfor�i(type1).�hissubstitutiondecreasesinthebiotitesofthehost-rockstowardsthebiotite-richcrustorclot.�hesamemechanismcanexplainthesubstitutionof�iinthebiotitesofthebiotite-richcrustofthesamplesMO-67AandMO-101(R2=0.921and0.842)butfailstoexplaintheMO-23Ebiotite-richcrustsandtheclotMO-89.�helatterarebetterexplainedbythetype(2)substitution(R2=0.746and0.926respectively, not,notnotshown).

However,ifall�iV�andAl�Vwerebalancedbysubstitution (1), then the data points (�ig. 6b)shouldliealongthelinelabeled�i/(Al�V-2)=1/2,when thestructural formulaofbiotitehasbeencalculatedonthebasisof22O(Dymek,1983).Allanalysedbiotitesplotabove the�i/(Al�V-2)=1/2line indicating that they contain�i impossibleto balance by Al�V, and thereby requiring anadditional�i-substitutionmechanism.Potentially,thissubstitutioncanbe the�ifor2Mg(type4)as indicated by the relatively high correlationcoefficients(R2)rangingfrom0.695to0.884(notshown).

�he relation of Al-content among the host-rock-samples is (taking intoaccount theerrors)MO-101>MO-89A~MO-23E or MO-101>MO-67>MO-89A~MO-23E, if the mean values arecompared,andremainsthesamewhenthebiotite-rich crust is examined (�ig. 5). �he MO-23EbiotitesareMg-richerrelativetotheothersbothinthehost-rockandinthebiotite-richcrust.�heseobservationsimplythatthebiotitecompositionofthebiotite-richcrustdependsmainlyonthatofthehost-rockbiotites.�tistonotethatthebiotitesoftheclotdonotdifferfromthatofthebiotite-richcrusts.

Rock chemistry

�norder toexamine thepossible influenceoftheenclaveschemicalcompositiontothatofthebiotitesof thebiotite-richcrust,major elementchemicalanalysesoftwopairs,host-rock(MO-23A and MO-67) and enclave (MO-23E andMO-67A respectively), are listed in �able 3andcomparedin�ig.7b.At thesametime, thecomposition,intermsofmeanvalues,ofbiotitesofthebiotite-richcrustiscomparedwiththatofthehost-rockbiotites(�ig.7a).Allcomparisons

havebeenmadeonwaterfreebasis.�incebiotiteshavenotbeenanalysedforP2O5thehost-rocksarenotcomparedinfigure7AforP2O5.�orsampleMO-23, thebiotitesof thebiotite-richcrustarericherin�iO2(1.1or10%),MnOandCaO(1.3forbothelements)andpoorerinNa2O(0.82)relativetobiotitesofthehost-rock.However,theenclave(MO-23E)isricherin�iO2(2times),MnOandCaO(1.8forbothelements)andpoorerinNa2O(0.56) relative to the host-rock (MO-23A; �ig.7b).�orsampleMO-67,thebiotitesofthebiotite-richcrustarericherin�eOt(1.07),MgO(1.05)andMnO(1.46),poorerinK2O(0.94),CaO(0.57)andNa2O(0.10),while�iO2,Al2O3and�iO2arethesame,relativetobiotitesofthehost-rock(�ig.7a). However, the enclave (MO-67A) is muchmoreenrichedin�eOt(4.40),MgO(5.16),MnO(3.69),aswellasinAl2O3(1.27),CaO(1.95),�iO2(4.35),andCaO(1.8forbothelements)andpoorerinNa2O(0.54)and�iO2(0.74)relativetothehost-rock(MO-67).

�ig.7 – Biotite composition of biotite-rich crust over that of–Biotite composition of biotite-rich crust over that ofBiotitecompositionofbiotite-rich crust over that ofbiotite-richcrust over that ofoverthatofhost-rockratio(a); whole-rock major element composition(a); whole-rock major element compositionwhole-rockmajorelementcompositionofenclaveoverthatofhost-rockratio(b).Diamonds: sampleDiamonds:sampleMO-23;triangles:sampleMO-67.

16 A.Koroneos

discussion And conclusions

Regarding formation of biotite-rich crustsaroundxenolithsofdifferentkindof rockse.g.amphobolites,gneissesshistsandbasicrocks,Didier(1973)reportsthatcrustsseparatexenolithsfromthehost,pointsoutthattheyarepetrographicallyvery similar to the surmicaceous enclaves, andsuggeststhatthesecrustsaretheresultofreactionbetweenferromagnesianmineralsofxenolithsandsurroundinggraniticmagma.AreactionbetweentheenclavesandthegraniticmeltwassuggestedalsobyMontelet al.(1991),forthedevelopmentofsuchthebiotite-richcrusts.AreactionbetweenbetweenthegranitemeltandthemaficmaterialofenclaveswasacceptedalsobyNemchin and Pidgeon (1999)NemchinandPidgeon(1999)andLonget al.(2005).Biotite-crustssurroundingxenolithshavebeenreportedforotherrocksalso,exceptgranitoids(e.g.Johnsonet al.,2003andLefebvreet al.,2005),andtheywereinterpretedastheresultofreactionwiththemelt.Basedontheabove interpretations, thebiotite-richcrustssurroundingenclavescanbeconsidered,accordingtotheliterature,astheresultofreactionbetween

the former and the melt. Concerning the clots,Concerning the clots,Didier (1973) suggested that during the finalstageintheassimilationofxenolithicrockstheydisintegrateinthemagma.Asaresulttheycrumbleintolittlecrystalclotswhicharedisseminatedintheassimilatingmagma.Clotsareinterpretedasrestiteunmixingproductsorpartiallyassimilatedfragments(xenoliths)ofthesurroundingcountryrocks(Drummondet al.,1988;�peeret al.,1989;�ornelli,1994;�tephens,2001).�heycanbealsoaconsequenceoftheextensivefragmentationofmicrogranularenclaves(MME),butinthiscasetheclotsshouldbecomposedofferromagnesianphasesandAn-richplagioclasesaswell(e.g.Poliand�ommasini,1991).

Biotite-rich crusts and biotite clots fromMonopigadon pluton contain biotite crystalsshowing not strong differences, when theyare compared with those of the host rocks andenclaves. However, some differences do existconcerningtheirchemicalcomposition,sizeandcolouraswell.

�he�iandAldecreaseand�i increase inallhost-rockbiotitestowardsthecrusts(orclot)can

�ample MO23E MO23A Enrichment-depletionfactor MO67A MO67 Enrichment-

depletionfactor

Rock-type Enclave Host-rock Enclave/Host-rock Enclave Host-rock Enclave/

Host-rock�iO2 57.46 64.75 0.89 53.12 71.53 0.74�iO2 1.67 0.82 2.03 1.25 0.29 4.35Al2O3 16.76 15.25 1.10 18.77 14.84 1.27�eOt 6.61 5.91 1.12 8.92 2.03 4.40MgO 2.74 2.52 1.09 4.24 0.82 5.16MnO 4.82 3.63 1.33 4.83 4.26 1.13CaO 0.19 0.10 1.89 0.16 0.04 3.69K2O 4.63 2.45 1.89 3.16 1.62 1.95Na2O 1.79 3.21 0.56 1.80 3.33 0.54P2O5 0.29 0.11 0.34 0.09LO� 3.03 1.25 3.40 1.16�otal 100.00 100.00 100.00 100.00

tAble ��Whole-rock major element composition of host-rocks and enclaves


beconsideredremarkable.Comparing(intermsofmeanvalues)thebiotitesfromthebiotite-richcrustswiththoseoftheenclaves,itisnoticeablethattheformerare�i-poorer,andAl-and�i-richerrelativetothelatter.

�he biotite crystals of the biotite-rich crustsandbiotiteclotsarelongerthanthoseofthehost-rocksandthoseoftheenclaves(excepttheMO-89Asample).�helongestcrystalsexistintheclotwherethebiotitesare2.6�1.5timeslongerthanthoseofthehostrock,whileamongthebiotite-richcrustsoftheenclavesthebiotitesarelongerrelativetothoseofthehostrock(4to12%min.upto64to100%max.).�helongestbiotitecrystalsoccur in the biotite-rich crust of the cumulateenclave.�helengthofthebiotitesfromthebiotite-richcrustsisalso2to4timeslongerthanthoseoftheenclaves(excepttheMO-89Asample).�hewidthofthebiotitecrystalsisslightlybigger(10to20%,intermsofmeanvalues)inthehost-rockbiotitescomparedwiththosefromcrusts but notcrusts but notbutnotfrombiotitesoftheclotwherethebiotitesare20%widerthanthoseofthehost-rock.However,thedifferencesinwidthseemtobeinsignificantwhentherangeistakenintoaccount.Bothdimensionsofbiotitesfromthecrusts and clot are interconnectedcrusts and clot are interconnectedandclotareinterconnectedwiththecorrespondingofhost-rockbiotites.�hebiggerthebiotiteinthehost-rockthebiggerisinthebiotite-richcrust.Concerning thecolourofbiotites,itisreddish-browntobrownandyellow-browntobrowninthehost-rocks(BGrdandBGrrespectively),differingfromthatof thebiotitesconsistingthecrustswhereitisreddishtoreddish-brown.

�akingintoaccountthedifferences,insizeandchemistry, existing between the biotites of thebiotite-richcrustsand thoseof theenclaves,anoriginfromthelattermustbeexcluded.Moreover,thebiotite-richcrustsdonotgrowattheexpenseoftheenclavesbuttheyrepresentovergrowths.�naddition,thedifferencesinchemistryindicatethattheformationoftheselargecrystalsinsolidstatedue toheatingof thebiotitesof theenclaves isratherimpossible.On the other hand, since enclavesOntheotherhand,sinceenclaveswerecompletelysolid,wecannotinvokediffusionor matter exchange because these processesaretooslowfordimensionsofthecrusts(afewcentimeters).�tmustbenotedalsothatenclavesareenrichedinsomeelementsrelativetothehost-rocks(e.g.�i,�e,Mg,Mn,Ca)butbiotitesfrom

crustsarenotenrichedinthoseelementsortheyareslightlyenriched(�i).

�he euhedral lath-like biotite crystals of thebiotite-richcrustandoftheclotalongwiththeirhigher,inrespecttothehost-rock,lengthandwidth(except clot), imply that these crusts and clotswerecrystallized from the magma simultaneouslycrystallizedfromthemagmasimultaneouslysimultaneouslywith the crystallization of the host-rock, whenenough space for their growth was available.�hemineralogical assemblageof thecrusts (orclot) made almost completely by biotites, and made almost completely by biotites, andgiventhatthebiotiteswerenotyetcrystallized,impliesthatmagmaatthecontactwithenclaveshasapeculiarcomposition.�hepartofmagmaatcontactwithcoldenclave decreases its temperatureenclave decreases its temperaturedecreasesitstemperatureinanarrowspace(somemillimetres),hence,atconstant lithostaticpressure,solubilityofwater(fluidingeneral)increases(Yamashita,1999,andreferencestherein);thereforewaterisdrewtowardthepartofmagmaatcontactwithcoldenclave.enclave..�hislocalhighcontentofwaterstabilizesbiotitecrystallization.

However,thebiotitesfromcrustsandclotarericher in �i relative to the host-rock biotites.Probably this local �i-enrichment is the resultof the�i-depletionofhost-rockbiotitesup toaminimumnearthecrust.�iwasconcentratedbydiffusioninthemagmaconsideringthatdiffusionhasreasonabletimeextentinmagmas,inorderto crystallize �i-richer biotites. K and Mg arenotdifferentbecausetheyarepresentalreadyinthe magma in enoughamounts. �i, even if notstatisticallyrobustbutwiththesamebehaviourforallsamples,ishigherwhere�iislower(closetothecrust)anditislowerwhere�iishigher(inthecrust),whereasAl�Vdecreasesfromhosttocrust.�hesearetheresults,in all the host-rock biotites, ofallthehost-rockbiotites, ofofthesubstitutionof�iforMgwhichiscompensatedforbysubstitutionofAl�Vfor�iandthissubstitutiondecreasesinthebiotitesofthehost-rockstowardsthebiotite-richcrustorclot.�hese findings are�hesefindingsareinagreementwiththetwoproposedsubstitutionmechanisms(1)and(2).�tmustbenotedthatthetwosubstitutionsactreverselyinthehostclosetocrustandinthecrust:inthehostclosetocrust�iandAl�Varedepleted,�iandAlV�areenrichedwhereasinthecrust�iandAl�Vareenriched,�iandAlV�aredepleted(e.g.samplesMO-67AandMO-101;�ig6A).AccordinglycationsR2+shouldincrease,

18 A.Koroneos

butconcomitantsubstitutionscoulddecreasesuchcationslikedehydrogenationreaction.

Concerning biotites of the clot, they do notdiffer from that of the biotite-rich crusts. �hesimilarbehaviour of biotites from clots with thebehaviourofbiotitesfromclotswiththebiotitesfromcrustsaroundenclaves, implies that,impliesthatclotsareclearlypiecesofcrustsalreadyformedand disrupted. Based on their euhedral crystal. Based on their euhedral crystalshapearestiteoriginfortheclotsmustberuledout.Moreoverthesebiotitesarelongerandwiderfrom the biotites of the host- rock, having thehighestdifferenceinlengthandwidth.�hismakesunlikelytobetheresultofextensivefractionationofmicrogranularenclaves.

�akingintoaccountthatformostofthecasesthehigherorlowerconcentrationofanelementinthebiotiteofthehost-rockresultsinhigherorlowerconcentrationrespectivelyof theelementinthebiotite-richcrust,itcanbeconcludedthatthebiotite compositionof thebiotite-richcrustdependsonthatofthehost-rockbiotitesanditisindependedfromtheenclavecomposition.�hisrelationbetweenthecompositionalcharacteristicsofhost-rockandbiotite-richcrustbiotitesisalsoanalogouswiththeirsizerelationcorroboratingthefindings.

AcKnowledgeMents

� amverygrateful toananonymous referee forperceptive and helpful reviews that significantlyimproved thepaper.EditorialhandlingandusefulsuggestionsbyG.Poli aregreatelyappreciated. �amalsogratefultoG.Christophides,�.Dimitriadis,and�.�oldatosforhelpfuldiscussions.�hanksareexpressedtoDr.L.Papadopoulou,forheranalyticalassistanceinthebiotiteanalyses.

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biotites from biotite-rich crusts of enclaves and clots in...

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