Per. Mineral. (2007), 76, 2-3, 79-89 doi:10.2451/2007PM0017SPECIAL ISSUE: In honour of Ezio Callegari on his retirement http://go.to/permin
l PERIODICO di MINERALOGIAestablished in 1930
An International Journal ofMINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY,ORE DEPOSITS, PETROLOGY, VOLCANOLOGYandappliedtopicsonEnvironment,ArchaeometryandCultural Heritage
AbstrAct. — Followingthesearchofjadeititeinthealluvialdepositsatthemouthsofthemainalpinevalleys,asystematicfieldsurveyaroundMonviso,italianwesternAlps,ledtothediscoveryofaprimaryjadeititeoutcrop.Thejadeititeoccursasatectonicblockabout1m3involumenearPuntaRasciassa,andisembeddedinaserpentinisedlherzoliteofthebasalserpentiniteunitoftheMonvisometa-ophiolite.
On hand specimen, the jadeitite is a very palegrassgreen,fine-grainedmassiverocksurroundedby a darker retrogression margin and crosscut bycoarsergrainedphlogopite-richpocketsandirregulardiscontinuousveins.Aportionoftheboudinconsistsofapeculiarrockwithpegmatoidgrain-size.
Microscopically,thefinegrainedjadeititeappearsmoreheterogeneouswithclinopyroxenecompositionranging from jadeite to omphacite. Three mainportionsmaybedistinguished,whichconsistofclearclinopyroxene,clinopyroxenewithaccessoryrutile,anddustyclinopyroxene,respectively.Inallportionsthe matrix clinopyroxene occurs as aggregates ofinterlocked stumpy prismatic crystals with sharpcompositionalzoning,whichlocallycontainlargercloudy clinopyroxene relics. Zircon is a typicalubiquitousaccessorymineral.Thecoarser-grainedveins and pockets consist of randomly oriented
clinopyroxene idioblasts included in xenoblasticalbiteoraggregatesofphlogopiteandMg-chlorite.Inboththerockmatrixandtheveinsapoikiloblasticallanitic epidote locally occurs. The pegmatoidportionconsistsofCr-bearingomphaciteandchlorite+phlogopitedomains.Veryraregarnetporphyroblastslocallyoccur,whichcontaininclusionsofilmeniteandminorapatite.
The occurrence of rutile and the equilibrationtemperature deduced by the Y content of garnetsuggesteclogite-faciesconditions.Theubiquitouspresenceofzirconsuggeststhatthejadeititederivedfromaformerfelsicdyke(plagiogranite?),originallyintrusive into upper mantle peridotite, whichexperienced a significant metasomatism duringperidotiteserpentinisation.
riAssunto. — L’occasionale ritrovamento digiadeititineidepositialluvionaliallosboccodelleprincipalivallatealpinehaportatoadunalororicercasistematicanelmassicciodelMonvisoeallascopertadelprimoaffioramentoneipressidiPuntaRasciassa.Sitrattadiunbloccodicirca1m3,inclusoinlherzolitiserpentinizzate appartenenti all’unità basale delcomplessometa-ofioliticodelMonviso.
Macroscopicamente,lagiadeititeapparedicoloreverdepallidoegranamoltofine,orlatadaunaporzionescuradiretrocessione.Nellagiadeititeagranafine
Jadeitite in the Monviso meta-ophiolite, Piemonte Zone,Italian western Alps
roberto compAgnoni 1*, FrAnco rolFo 1, 2, FrAnco mAnAvellA 3 and FrAnco sAlusso 4
1DipartimentodiScienzeMineralogicheePetrologiche,UniversitàdegliStudidiTorino,ViaValpergaCaluso,35,10125Torino,Italy.
2CNR–IGG,ViaValpergaCaluso,35,10125,Torino,Italy3ViaPinerolo,39,10060Macello(TO),Italy
4ViaPapaGiovanni,11,10060Macello(TO),Italy
*Correspondingauthor,E-mail:[email protected]
80 r. compAgnoni, F. rolFo, F. mAnAvellAandF. sAlusso
sono localmente presenti sacche e vene ricche inflogopiteedunaporzioneagranapegmatoide.
La giadeitite è costituita principalmente daclinopirossenolacuicomposizionevariadagiadeiteaonfacite,erisultamicrostrutturalmenteeterogeneacon porzioni limpide, porzioni torbide e porzionirelativamente ricche in rutilo. Il clinopirossenocresce principalmente in aggregati di individuiprismatici tozzi, neiquali è evidenteunamarcatazonaturacomposizionale.Localmentesonopresenticristallirelittidimaggioridimensioni,caratterizzatidaunaspettotorbido.Leveneelesacchecontengonoclinopirosseno,albite,flogopiteeclorite,mentrelaporzionepegmatoideconsistedidominiaprevalenteclinopirossenoedominiaprevalentifillosilicati,consubordinataonfaciteaciculare.Raramentesonostatiosservatiporfiroblastidigranatoedepidotoallanitico.Tragliaccessoriècaratteristicolozircone.
Le caratteristiche geologiche, petrografichee minerochimiche della giadeitite suggerisconoun’originelegataaprocessimetasomaticiassociatialla serpentinizzazionedellaperidotite incassante,a partire da un protolite filoniano di tipo acido,probabilmenteunplagiogranito.
Key words: jadeitite, ophiolite, metamorphism, Monviso, western Alps.
introduction
From the early Neolithic to the BronzeAge,i.e.fromthesixthtothesecondmillenniumBCinwesternEurope,prehistoricpeasantsusedaxesandchiselsmadeofpolishedstones,collectivelyknownas“greenstones”(seee.g. Ricq-de-Bouardand Fedele, 1993). The “green stones” mainlyconsistofserpentinite,fine-grainedeclogiteandNa-pyroxenite(mostlyomphacititeandjadeitite).Long since it has been established that “greenstones” derive from the meta-ophiolites of thePiemonte Zone of the western Alps. Damour(1881), who analysed a sample from a privatecollectionlabelled“GreenJasperfromMonviso,Piemonte”,concludeditwasmadeofjadeiteandinvited geologists and mineralogists to checkcarefully the Monviso area and search for thismineral.However,thoughprimaryoutcropsoftheotherlithologiesusedasarchaeologicimplements(i.e., serpentinite, fine-grained eclogite andomphacitite)arewellknowninthePiemonteZone,
sofarjadeititeshavebeenfoundonlyaspebblesorbouldersfrom“secondary”deposits,suchasthealluvialpost-orogenicOligocene toQuaternaryconglomerates.Asearchofjadeititebouldersinthealluvialdepositsatthemouthsofthemainalpinevalleys(DinoDelCaro,personal communication,2001) showed that the most promising areawas thataround theMonvisometa-ophiolite.Asystematicfieldsurveyoftheareainsummer2002ledtothediscoveryofajadeititeboudinonthenorthernsideofPuntaRasciassa,Povalley(Fig.1):this new occurrence was preliminary reportedat the International Eclogite Field Symposium(CompagnoniandRolfo,2003).
Inthispaper,petrographyandmineralchemistryof this jadeitite is described and its origin andfeaturescomparedwithsimilar lithologiesfromprehistoricimplementsarediscussed.
the ophiolitic belt oF the western Alps
The Monviso meta-ophiolite composite unit(inthefollowingreferredtoas“Monvisometa-ophiolite”) belongs to the Piemonte Zone ofCentral and western Alps, which consists offragmentsofformerTethyanoceaniclithosphere(meta-ophiolites)and itsMesozoicsedimentarycover(calcschistsor“schisteslustrés”).DuringtheAlpineorogeny,therocksoftheInternalPiemonteZone experienced an early high-pressure (HP)eclogite-facies metamorphism, whereas thoseoftheExternalPiemonteZoneexperiencedonlyblueschistfaciesconditions(Compagnoni,2003withref.therein).
In the Monviso meta-ophiolite the Alpinemetamorphismreachedeclogite-faciesconditions,withtemperaturesofabout600°C(580±40°C:Schwartzet al.,2000;620±50°C:Messigaet al.,1999)andpressuresofabout20kbar(19±2kbar:Schwartzet al., 2000;24kbar:Messigaet al.,1999).Locally,theeclogitesoftheMonvisometa-ophiolitearecutbyomphacitemetamorphicveins,whichhavebeenthesubjectofdetailedstructural,petrographic,fluidinclusions,andstableisotopestudiessuggestingtheyformedbylocalcirculationoffluidsateclogite-faciesconditions(PhilippotandSelverstone,1991;Nadeauet al.,1993;Philippot,1993).Tworetrogrademetamorphicstageshavebeeninferred,withequilibrationunderblueschist-
Jadeitite in the Monviso meta-ophiolite, Piemonte Zone, Italian western Alps 81
Fig.1 -GeologicmapoftheMonvisometaophiolitecomplex(fromLombardoet al.,1978;metaplagiograniteoccurrencereportedbyCastelliet al.,2002),showingthelocationofthestudiedjadeitite.InsetshowsthelocationoftheMonvisometaophiolitecomplex,withintheframeworkoftheSouthwesternAlps.InthesmallinsetthepoliticalboundariesamongFrance(F);Italy(I)andSwitzerland(CH)arereported.TO:Torino.
82 r. compAgnoni, F. rolFo, F. mAnAvellAandF. sAlusso
and greenschist-facies conditions, respectively.(Lombardoet al.,1978).
Geochronologic data on the Monviso meta-ophiolite suggest aTertiary age for theAlpineHPmetamorphism(MoniéandPhilippot,1989;Duchêneet al.,1997;Cliffet al.,1998),whichisinlinewiththeradiometricagesobtainedfromotherportionsoftheInternalPiemonteZone.ThemostrecentgeochronologicstudyoftheMonvisometa-ophioliteisaSHRIMPU-Pbdeterminationon zircons from a syn-eclogitic metamorphicvein,whichyieldedanEoceneageof45±1Ma(RubattoandHermann,2003).
the jAdeitite
Thejadeititewasfoundatabout2400ma.s.l.(coordinates:LAT_UTM:4944144;LONG_UTM:357530)inasmallcirqueonthenorthernsideofPuntaRasciassa,Povalley(Fig.1).Itoccursasalooseblock,aboutonecubicmetreinsize,withinthe massive or sheared antigorite serpentinitebelonging to the “basal serpentinite”unit (Fig.1)oftheMonvisometa-ophiolite(cf.:Lombardoet al.,2002).Thoughnotinitsprimarysetting,itisevidentthatthejadeititeisaboudinwhichwasoriginally locatedatmost a few tensofmetersupstreamwithinthesameserpentinite(Fig.2a).
Theverypalegrassgreen,massive, and finegrained jadeitite, locallycontains irregularcm-sizedcoarsegrainedphlogopite-richpocketsandveins(Fig.2c).Ononesideoftheblock,acoarsegrainedportionwithpeculiarpegmatoidgrain-sizeoccurs,whichconsistsofcm-sizedcrystalsofbrightgreenclinopyroxeneandlightgreychloriteflakes(Fig.2b).Thejadeititeblockismantledbyadarkerretrogressionmarginfrom10to20cmthick(Fig.2b).
petrogrAphy
Under themicroscope, even the finegrainedjadeitite is heterogeneous, being formed byvariouslydistributed:1)portionsmainlyconsistingofclear,inclusionfree,clinopyoxene;2)portionswhere clinopyroxene is associated with up toabout10vol.%ofrutile;3)portionswithdustyclinopyroxene,densewithmicron-sizedmineral
inclusions,and4)portionswithcoarsergrainedveins and pockets of phlogopite + Mg-chlorite(Fig.2c).Inallportions,theclinopyroxeneoccursasaggregatesofinterlockedblockycrystalsless
Fig. 2 - Field appearance of the Monviso “jadeitite” atPuntaRasciassa.A)Panoramicviewofthejadeititeboudin(jd)amongthecoarseserpentinitedebris(srp)inthesmallcirqueNofPuntaRasciassa,Monvisometa-ophiolite.B)Close-upviewofthejadeititeboudin.Thelighterjadeititeinthecentre(jd)isincontactwithacoarse-grainedpegmatoidportion(gb)ontheright.Noteontheupper-leftthedarkerretrogressedmargin.C)Jadeititesliver,inwhichmm-thickveinsandpocketsofMg-chlorite±phlogopite(arrows)areevident.
Jadeitite in the Monviso meta-ophiolite, Piemonte Zone, Italian western Alps 83
than0.5mmacross,whichsystematicallyshowfromcoretorimastrongandsharpcompositionalzoning(Fig.3c).Locallyinthefinegrainedmatrix,coarserclinopyroxenesuptoseveralmmacrossoccurwithacloudycore,crowdedwithveryfinegrainedoriented inclusions,whichappear toberelictportions,mostprobablyderivedfromformerigneouscrystals(Fig.3b).Distinctiveaccessoryminerals are zircons, few micrometers across,commonly showing a blocky prismatic shape.Veinsandintergranularpocketsconsistofrandomlyoriented clusters of idioblastic clinopyroxene
surroundedbyxenoblasticalbiteorphlogopiteandMg-chlorite(Fig.3d).Alightpurpleallanitelocallyoccurs as poikiloblasts in both the rockmatrixandtheveins.Theportionwithpegmatoidgrain-sizeconsistsofgreenishclinopyroxenedomainsand chlorite ± phlogopite, which may includeclinopyroxeneneedles.Veryfewsmallgarnets(<1mm)locallyoccur,whicharepartiallyretrogressedtochlorite;asinglegarnetporphyroblastabout6mmacrosswasobserved(Fig.3a),whichcontainsinclusionsofilmeniteandminorapatiteinboththecoreandtherim.
Fig.3 –MicrostructuresofthePuntaRasciassa“jadeitite”boudin.A)MicrophotographofagarnetbearingNa-pyroxenite(sampleJd10).Garnet(Grt)rimiscrowdedwithinclusionsofilmeniteandapatite,minorjadeiteandomphacite,andispartiallyreplacedbychlorite(Chl).Noteinthematrixtheoccurrenceof“cloudy”crystalsofNa-pyroxene(Cpx).Planepolarizedlight(PPL).B)Microphotographoftheportionwithpegmatoidgrain-size(sampleJd3).A“cloudy”porphyroclastofclinopyroxene(Cpx)isincontactwithchlorite(Chl)andisrimmedwithneoblastictopotacticNa-pyroxene(NaPx).PPL.C)MicrophotographofthematrixNa-pyroxene(sampleJd7)whichissharplyzonedwithadustycore“C”andaclearrim“R”.Thesuperimposeddottedlinedefinesanindividualcrystal.PPL.D)Microphotographofametamorphicpocketinarutile-bearingomphacitite(sampleJd5),whichconsistsofCr-bearingomphacite(Omp),chromite(Chr),chlorite(Chl)andphlogopite(Phl).PPL.
84 r. compAgnoni, F. rolFo, F. mAnAvellAandF. sAlusso
Thehost serpentinite still preserves relics ofboththeoriginaluppermantleclinopyroxeneandagranoblasticmicrostructureduetotheprevalentdevelopment of pseudomorphic replacementreactionsduringserpentinisation.
minerAl chemistry
MineralsfromthePuntaRasciassajadeititewereanalysedattheDepartmentofMineralogicalandPetrologicalSciences,UniversityofTorino,ItalywithaCambridgeSEM-EDS,operatingat15kVacceleratingvoltage,50scountingtimeand2.70-2.80Abeamcurrent.SEM-EDSquantitativedata(spotsize=2µm)wereacquiredandprocessedusing the Microanalysis Suite Issue 12, INCASuiteversion4.01;therawdatawerecalibratedonnaturalandsyntheticmineralandoxidestandardsand theΦρΖ correction (Pouchou and Pichoir,1988) was applied.Analyses of minerals wereprocessedbyusingthesoftwareofUlmer(1986).Representative mineral analyses are reportedin Table 1, while compositional variations ofclinopyroxenesandgarnetarereportedinFig.5.
Theclinopyroxenecompositionvariesdependingonthemicrostructuralsite(Fig.5a).Thecloudyclinopyroxene porphyroclasts (Fig. 3b, 4c),whichincludeabundantapatiteandarecrowdedwith ilmenite segregations, range from ferrianomphacite(XJd=0.33-0.34;XAe=0.26-0.27)toaluminianaegirine-augite(XJd=0.10-0.16;XAe=0.32-0.34)(Rock,1990)andarepartlyreplacedbyretrogradephlogopiteandMg-chlorite(XMg<0.80).Matrixclinopyroxene, themostabundantmineral, is compositionally heterogeneous andits composition varies from calcian to calcian-ferrianjadeite(XJd ≤ 0.83; XAe ≥ 0.05) to ferrian omphacite(XJd ≥ 0.20; XAe ≤ 0.27). Compositional heterogeneities of clinopyroxene in the matrixareclearlyevidentalsofromback-scatteredSEMimages(Fig.4a)showingthattheclinopyroxenecore is made of tiny (≤ 10 µm)blebsofferrianomphacite(XJd ≤ 0.56; XAe ≥ 0.14) in an almost pure jadeite(XJd ≤ 0.97; XAe ≥ 0.03), which we interpretasexsolution,whiletherimconsistsofferrianomphacite(XJd ≥ 0.49; XAe ≤ 0.17) (Fig. 5a).SimilarmicrostructureshavebeenreportedbyHarlowet al. (2006)injadeiteaxesfromAntigua;coexistingjadeiteandomphacitehavebeenalso
foundinaneclogite-faciesmeta-quartzdioritefromthesouthernSesiaZone,westernAlps(Matsumotoand Hirajima, 2005) and in a jadeite-bearinglawsonite eclogite from Guatemala (Tsujimoriet al, 2005), as well as in various metabasitesfromtheLigurianAlps(Cortesognoet al.,2002);the coexistence of two Na-pyroxenes suggestsa metamorphic crystallization at temperatureslowerthanthesolvus(MatsumotoandHirajima,2005withref.therein).Dustyportionsofmatrixjadeititicclinopyroxenesdifferfromcleargrainsinthattheycontaintiny(1-2µm)titanite,includedduringcrystallization.Clinopyroxenesincludedingarnetarebothferrianjadeite(XJd ≤ 0.66; XAe ≤ 0.30)andferrianomphacite(XJd ≥ 0.37; XAe ≥ 0.27) suggestingthecoexistenceoftwoNa-pyroxenesevenifthecompositionalgapisnotaspronouncedasinthematrix.Clinopyroxeneinmetamorphicveinsandpocketsiscompositionallyvariablefromaegirine-pooromphacite toaegirine-richferrianomphacite (XJd = 0.18-0.50; XAe = 0.09-0.26),beinglocallyrichineitherCr(Cr2O3 ≤ 4.42 wt%) orTi (TiO2 ≤ 0.63 wt%). In some metamorphic pockets,Cr-bearingomphaciteisassociatedwithrelictchromite,chloriteandphlogopite(Fig.3d).
The garnet poikiloblasts (Fig. 3a, 4b) showavery littlepyrope increase fromcore (Grs03-05Alm58-59Prp16-18)torim(Grs03-06Alm59-60Prp17-21)(Fig.5b).Thislackofzoningisalsosupportedbythehomogeneouscontentofyttrium(about1100ppm),whichindicatesmetamorphicT(YinGrt,PyleandSpear(2000)calibration)ofabout500°C(GloriaVaggelli,personal communication, 2007).The presence of single inclusions of ilmenite,jadeiteandomphacite(Fig.4b)indicatesthatthegarnetdidovergrowamatrixalreadyconsistingoftheseminerals.GarnetsarelocallysurroundedbyaretrogressionrimofMg-chlorite(XMg<0.74).
Themetamorphicveinsconsist,inadditiontoclinopyroxene,ofalmostpurealbite (Ab98)andallanite (La=2.14wt%;Ce=8.00wt%;Nd=4.42wt%).Distinctivedomainsandpockets intheportionwithpegmatoidgrain-size (Fig.2c,3d)containminorCr-bearingomphaciteandrelictchromite together with abundant Mg-chlorite(clinochlore topenninite:XMg<0.90)andverypale Ti-poor (TiO2 ≤ 0.73 wt%) homogeneous phlogopite(XMg=0.80-0.82).
In conclusion, the petrographic andminerochemicalstudyofthejadeititerecognizes
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0.51
90.
589
0.00
02.
372
8.52
3
Ca
0.35
90.
291
0.01
30.
527
0.03
50.
395
0.35
00.
055
0.39
00.
304
0.34
40.
340
0.02
30.
000
0.00
0
Na
0.65
20.
727
1.01
60.
488
0.98
80.
613
0.64
50.
967
0.62
40.
698
0.00
00.
000
0.99
30.
103
0.00
0
K0.
000
0.00
00.
000
0.00
00.
000
0.00
00.
000
0.00
00.
000
0.00
00.
000
0.00
00.
000
0.83
00.
000
OH
00
00
00
00
00
00
02
16
86 r. compAgnoni, F. rolFo, F. mAnAvellAandF. sAlusso
adecompressionalevolutioncharacterizedbyanearliereclogite-faciesmetamorphiceventfollowedby a blueschist and a later greenschist faciesretrogression,similartothatreportedfromallovertheMonvisometa-ophioliteslices(Lombardoet al.,1978).
the origin oF jAdeitite
ThediscoveryofamassivejadeititewithintheserpentiniteoftheMonvisometa-ophioliteisveryimportantfortheunderstandingoftheoriginofanunusualrocktype,whichhasbeenusedbythePrehistoricpeopleasarawmaterialtomakestoneimplements.
Aswiththejadeititefromotherfamousandwellstudiedlocalities(e.g.BurmaandGuatemala:cf.Harlowet al.,2007withref.therein),thePuntaRasciassajadeititeoccursasatectonicblockinaserpentiniseduppermantleperidotite,whichstillpreservesraremineralogicalandstructuralrelicsofthehigh-Tultramaficprotolith.Theoccurrenceof jadeititewithinserpentinitesuggests that thetworockswerecloselyassociatedatleastasearlyastheAlpineeclogite-faciesmetamorphism.
Thejadeititecompositiondoesnotfitthatofanyfreshigneousprotolith:therefore,itscompositionmust have been acquired later through ametasomaticprocess.Thisprocess shouldhaveoccurredinthepresenceofanabundanthydrousfluid phase, such as that present during theperidotite serpentinisation.Theoccurrenceofametasomatisingfluidduringthejadeititeformationis supported by: i) the “pegmatitic” grain sizein someportions; ii) the irregularmetamorphicpocketsenriched inchloriteandphlogopite (anhydratedandK-bearingmetamorphicphase);iii)thestrongandoscillatorycompositionalzoningofmanyNa-pyroxenes.Onthisbase, thePuntaRasciassa jadeitite is therefore interpreted as ametasomaticrockderivedfromanigneousprotolith
Fig.4 -Back-scatteredSEMimagesofthePuntaRasciassa“jadeitite”.A)TypicalappearanceofaNa-pyroxenite(sampleJd7),whichmainlyconsistsofstumpyprismaticcrystalsofsharplyzonedNa-pyroxenewithaeuhedraljadeitecore(C-darkportions)showingveryfine-grainedexsolutionblebsofomphacite(lighterportions),andanomphaciterim(R).CrystalsofNa-poorerjadeite(Jd)alsooccur.Thewhitegrainsarerutile.B)Garnet-bearingNa-pyroxenite(sampleJd10).Thegarnetpoikiloblastincludessinglecrystalsofilmenite(Ilm),jadeite(Jd)andomphacite(Omp),andthematrixmainlyconsistsofinterlockedstumpyprismaticcrystalsofjadeite(Jd)andomphacite(Omp).C)InthematrixofthegarnetbearingNa-pyroxenite(sampleJd10),the“cloudy”porphyroclastsofNa-pyroxene(Cpx)arecrowdedwithilmenitesegregations(Ilm)andareinhomogeneouswithNa-richer(darker)andNa-poorer(lighter)portions.
Jadeitite in the Monviso meta-ophiolite, Piemonte Zone, Italian western Alps 87
during theperidotiteserpentinisation,aprocessthatmayhaveoccurredduringeitherocean-floorororogenic(subduction)serpentinisation:sinceitiswellknownthatserpentinisationisadesilicationprocess,inthefirstcaseanalcimeshouldhavefirstformedattheexpenseofasodicplagioclaseandthenconvertedtojadeite,whereasinthesecondcasejadeiteshouldhaveformeddirectlyfromthealbitebreakdown.
Astothenatureofthejadeititeprotolith,boththe dusty clinopyroxene porphyroclasts withoriented ilmenite segregations and the smallubiquitousstumpyprismaticzirconssupportanigneous protolith. In particular, these types ofzirconswerereportedbyPupin(1992)fromlatemagmaticdifferentiatesofbasalticmelts,suchasNa-granite,Na-syeniteorplagiogranite,andbyCastelliet al. (2002)fromameta-plagiogranitefromtheMonvisometa-ophiolite.
compArison with neolithic implements
Asystematicpetrographicstudyofthepolishedstone implements from Mediterranean France(Ricq-de-Bouardet al.,1990)andNorthernItaly(Compagnoniet al.,1995;D’Amicoet al.,1995,
2004; Giustetto and Compagnoni, 2004) hasshown that, especially in the Neolithic period,jadeititewasatleast15-20%oftherawmaterialusedbyPrehistoricpeople.Thesejadeititesshowawidespectrumofmicrostructures,Na-pyroxenecomposition,andaccessoryminerals.Followingthe Na-pyroxene nomenclature suggested byRock(1990),whichismoredetailedwithrespecttothatofMorimotoet al.(1988),jadeititesonlycomposed of pure jadeite are relatively rare,mostlycontainingoneormoreotherNa-pyroxeneswithhigherdiopsideand/oraegirinecomponents.Even pure jadeite usually contains omphaciteblebssuggestinganoriginbyexsolution.As totheaccessoryminerals,theubiquitouszirconandtheTi-bearingminerals(rutile,titaniteorilmenite)are very important, the former for the geneticinterpretationand the latter for thegeotectonicprovenance.As for the zircon significance, werefertotheformersectiononthejadeititeorigin,whereas the local occurrence of rutile togetherwiththatofantigoriteinthehostserpentiniteandthegarnetequilibrationtemperatureof500°C,areconsistentwiththeeclogite-faciesconditionsoftheInternalPiemonteZone(Compagnoniet al.,1995;2006).However,giventhegreatpetrographic(andmost probably also genetic) variability shown
Fig.5–A)CompositionsofclinopyroxenesfromthePuntaRasciassa“jadeitite”inthejadeite-aegirine-(wollastonite+enstatite+ferrosilite)diagramofMorimotoet al.(1988);dashedlinesboundtheadditionalcompositionalfieldsproposedbyRock(1990).Analysedsamples:Na-pyroxenitewithdarkandclearportions(Jd8b);garnet-bearingNa-pyroxenite(Jd10);rutile-bearingomphacitite(Jd5);Na-pyroxenitewithpegmatoidgrain-size(Jd3).Symbols:1)matrixcrystals(exsolvedcore);2)matrixcrystals(rim);3)inclusionsingarnet;4)reliccloudyporphyroclasts;5)inmetamorphicveins.B)Garnetcompositionsofthegarnet-bearingNa-pyroxenite(Jd10)areshowninthe(Fe2++Mn)–Ca-Mgdiagram.C:garnetcore;R:garnetrim.
88 r. compAgnoni, F. rolFo, F. mAnAvellAandF. sAlusso
by archaeological jadeitites, data are still toofewtodisserton theoriginof the rawmaterialofprehistoric implements(seee.g. Pétrequinet al.,2006)anditisjustpossibletoconcludethatthe Monviso meta-ophiolite is one of the mostpromising source areas from the westernAlps.Therefore,amoredetailedpetrologicandgeologicworkonnewjadeititeoutcropsisneededbeforedrawingmorereliableconclusions.
AcKnowledgements
ThispaperisdedicatedtothememoryofGiorgioPeyronel, former mineral curator at the RegionalMuseum of Natural Sciences (Museo Regionaledi Scienze Naturali), Piemonte, Italy, who firmlybelieved on the primary occurrence of jadeititedepositsintheMonvisometa-ophiolite.CarefulandconstructivereviewsofL.GaggeroandG.Harlow,which improved the manuscript, are gratefullyappreciated.Financial support fromM.I.U.R. andItalianNationalResearchCouncil(C.N.R.–I.G.G.)isacknowledged.
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