jadeitite in the monviso meta-ophiolite, piemonte zone...

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.


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.


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.


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

Jadeitite in the Monviso meta-ophiolite, Piemonte Zone, Italian western Alps 85tA

ble

1Re

pres

enta

tive

mic

ropr

obe

anal

yses

of c

linop

yrox

ene,

gar

net,

feld

spar

, phl

ogop

ite a

nd c

hlor

ite fr

om th

e Pu

nta

Rasc

iass

a “j

adei

tite”

Stru

ctur

alfo

rmul

aeh

ave

been

cal

cula

ted

onth

eba

siso

f4c

atio

nsa

nd6

oxy

gens

forp

yrox

ene,

8c

atio

nsa

nd1

2ox

ygen

sfor

gar

net,

5ca

tions

and

8o

xy-

gens

forf

elds

par,

8ca

tions

and

11

oxyg

ensa

nd2

OH

forp

hlog

opite

,20

catio

nsa

nd2

8ox

ygen

sand

16

OH

forc

hlor

ite,r

espe

ctiv

ely.

MIN

ER

AL

CLI

NO

PYRO

XEN

EG

ARN

ETFE

LDSP

ARPH

LOG

OPI

TEC

HLO

RITE

AN

ALY

SES

9Cpx

25r

9Cpx

26s

9Jd2

7s10

Cpx

9c11

Jd16

11C

px17

11C

px19

11Jd

207C

px40

3Cpx

71c

10G

rt1c

10G

rt5r

r7P

l39

4Phl

683C

hl77

SAM

PLE

Jd8b

Jd8b

Jd8b

Jd10

Jd10

Jd10

Jd10

Jd10

Jd7

Jd5

Jd10

Jd10

Jd7

Jd5

Jd5

text

ure

rim

exso

lved

co

re (O

mp)

exso

lved

co

re (J

d)cl

oudy

cr

ysta

lsm

atri

x (J

d)m

atri

x (O

mp)

incl

usio

n in

Grt

incl

usio

n in

Grt

met

am.

vein

met

am.

vein

core

rim

met

amor

phic

ve

inm

etam

orph

ic

vein

met

amor

phic

ve

inSi

O2

57.0

256

.39

58.8

954

.39

57.1

355

.58

54.9

955

.87

56.3

754

.12

37.5

237

.15

67.6

040

.57

31.3

1

TiO

20.

000.

530.

000.

520.

000.

000.

000.

000.

000.

480.

000.

000.

000.

000.

00

Cr 2O

30.

000.

000.

000.

000.

000.

000.

000.

000.

004.

420.

000.

000.

000.

000.

00

Al 2O

313

.88

13.4

424

.50

4.72

17.9

69.

629.

5516

.49

12.8

08.

0021

.04

20.7

119

.99

14.1

316

.84

Fe2O

33.

395.

401.

049.

589.

518.

078.

8010

.61

3.55

7.56

2.17

2.83

0.00

0.48

0.00

FeO

0.14

0.00

0.00

2.14

0.00

1.21

1.19

0.57

0.00

0.63

26.3

626

.00

0.00

8.01

9.35

MnO

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

5.37

4.37

0.00

0.00

0.00

MgO

6.85

6.01

0.01

9.13

0.40

7.39

6.35

0.46

7.44

6.37

4.42

4.98

0.00

22.1

029

.38

CaO

9.66

7.81

0.37

13.6

80.

9510

.46

9.06

1.45

10.4

07.

854.

074.

000.

490.

000.

00

Na 2O

9.70

10.7

715

.63

6.99

14.7

28.

989.

2214

.14

9.19

9.97

0.00

0.00

11.7

20.

730.

00

K2O

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

9.04

0.00

H2O

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

4.16

12.3

3

Tota

l10

0.64

100.

3410

0.44

101.

1510

0.67

101.

3199

.16

99.5

899

.75

99.4

100.

9410

0.03

99.8

099

.23

99.1

9

Si1.

976

1.96

31.

975

1.95

71.

977

1.95

71.

983

1.97

11.

975

1.95

32.

958

2.94

72.

954

2.92

16.

093

Ti0.

000

0.01

40.

000

0.01

40.

000

0.00

00.

000

0.00

00.

000

0.01

30.

000

0.00

00.

000

0.00

00.

000

Cr

0.00

00.

000

0.00

00.

000

0.00

00.

000

0.00

00.

000

0.00

00.

126

0.00

00.

000

0.00

00.

000

0.00

0

Al

0.56

70.

552

0.96

80.

200

0.73

20.

399

0.40

60.

685

0.52

90.

340

1.95

51.

937

1.03

01.

199

3.86

2

Fe3

0.09

30.

141

0.02

60.

324

0.24

80.

249

0.27

20.

298

0.09

40.

224

0.12

90.

169

0.00

00.

026

0.00

0

Fe2

0.00

00.

000

0.00

00.

000

0.00

00.

000

0.00

20.

000

0.00

00.

000

1.73

71.

725

0.00

00.

482

1.52

2

Mn

0.00

00.

000

0.00

00.

000

0.00

00.

000

0.00

00.

000

0.00

00.

000

0.35

80.

294

0.00

00.

000

0.00

0

Mg

0.35

40.

312

0.00

10.

490

0.02

10.

388

0.34

10.

024

0.38

90.

343

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


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.


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.


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.

REFERENCES

cAstelli d., rostAgno c. and lombArdo b.(2002)- Jd-Qtz-bearing metaplagiogranite from the Monviso meta-ophiolite. Ofioliti,27, 81-90.

cliFF r.A., bArnicoAt A.c. and inger s. (1998) -Early Tertiary eclogite facies metamorphism in the Monviso Ophiolite. J.Metam.Geol.,16, 447-455.

compAgnoni r.(2003)-HP metamorphic belt of the western Alps. Episodes,26,199-203.

compAgnoni r., giustetto r., ricq-de-bouArd m. and venturino gAmbAri m. (2006)-Studio archeometrico di reperti neolitici e dell’età del Rame in pietra verde levigata: discussione sulle tecniche analitiche ed interpretazione dei risultati. IstitutoItalianodiPreistoriaeProtostoria,XXXIXRiunione Scientifica: Materie Prime e ScambinellaPreistoriaItaliana,Firenze,25-27novembre2004,Vol.Atti, 655-682.Atti,655-682.

compAgnoni r., ricq-de-bouArd m., giustetto r. and colombo F. (1995) - Eclogite and Na-pyroxenite stone axes of southwestern Europe: a preliminary petrologic survey. In:LombardoB.(ed.),StudiesonmetamorphicrocksandmineralsofthewesternAlps. AvolumeinmemoryofUgoPognante.Boll.MuseoReg.Sci.Nat. (Torino),at.(Torino),13,329-359.

compAgnoni r. and rolFo F.(2003)-First finding of jadeite in the serpentinite melange of Monviso meta-ophiolite, western Alps. GeologicalSurveyofNorway,Reportn°2003.055,37-38.

cortesogno l., gAggero l., lucchetti g. and cAbellA r.(2002)–Compositions and miscibility gap in Na-Ca clinopyroxenes through high-pressure metamorphism. Per.Mineral.71, 1-25.

d’Amico c., cAmpAnA r., Felice g. and ghedini m. (1995) -Eclogites and jades as prehistoric implements in Europe. A case of petrology applied to cultural heritage. Eur.J.Miner.,7, 29-41.

d’Amico c., stArnini e., gAspArotto g. and ghedini m. (2004) - Eclogites, jades and other HP-metaophiolites employed for prehistoric polished stone implements in Italy and Europe. Per. Mineral.,SpecialIssue,73,17-42.

dAmour A.(1881)-Nouvelles analyses sur la jadéite et sur quelques roches sodifères. Bull.Soc.Minér.France,4,157-164.

duchêne s., blichert-toFt j., luAis b., télouK p., lArdeAux j.m. and AlbArède F.(1997)-The Lu-Hf dating of garnets and the ages of the Alpine high-pressure metamorphism. Nature,387, 586-589.

giustetto r. and compAgnoni r. (2004) - Studio archeometrico dei manufatti in pietra levigata del Piemonte sud-orientale: valli Curone, Grue e Ossona. In:VenturinoGambariM.(ed.),Allaconquista dell’Appennino. Le prime comunitàdelle valli Curone, Grue e Ossona. CatalogoMostratemp.aBrignanoFrascata(AL),OmegaEd.,Torino,45-59.

hArlow g.e., murphy A.r., hozjAn d.j., de mille c.n. and levinson A.A.(2006)-Pre-Columbian Jadeite Axes from Antigua, West Indies: Description and Possible Sources. Can.Mineral.,44, 305-321.

hArlow g.e., sorensen s.s. and sisson v.b.(2007)-Jade. In.:GroatL.A. (Ed.),GeologyofGemDeposits. Mineral. Ass. Canada, Short CourseMineral.Ass. Canada, Short CourseSeries,37,Chapter9,207-253.

lombArdo b., nervo r., compAgnoni r., messigA b., KienAst j.r., mevel c., FiorA l., piccArdo g.b. and lAnzA r.(1978)-Osservazioni preliminari sulle ofioliti metamorfiche del Monviso (Alpi Occidentali). Rend.Soc.It.Mineral.Petrol.,Petrol.,34,253–305.

lombArdo b., rubAtto d. and cAstelli d.(2002)-Ion microprobe dating of zircon from a Monviso metaplagiogranite: implications for the evolution of the Piedmont-Liguria Tethys in the western Alps. Ofioliti,27,109-117.


mAtsumoto K. and hirAjimA t. (2005) - The coexistence of jadeite and omphacite in an eclogite-facies metaquartz diorite from the southern Sesia Zone, western Alps, Italy. J.Mineral.Petrol.Sci.,100,70-84.

messigA b., KienAst j.r., rebAy g., riccArdi m.p. and tribuzio r.(1999)-Cr-rich magnesiochloritoid eclogites from the Monviso ophiolites (western Alps, Italy). J.Metam.Geol.,17, 287-299.

monié p. and philippot p.(1989)-Mise en évidence de l’âge éocène moyen du métamorphisme de haute pression dans la nappe ophiolitique du Monviso (Alpes occidentales) par la méthode 39Ar-40Ar. C.R.Acad.Sci.Paris,309, 245-251.

morimoto n., FAbries j., Ferguson A.K., ginzburg i.v., ross m., seiFert F.A., zussmAn j., AoKi K. and gottArdi g. (1988) - Nomenclature of Pyroxenes. Schweiz.Mineral.Petrogr.Mitt.,68,95-111.

nAdeAu s., philippot p. and pineAu F.(1993)-Fluid inclusion and mineral isotopic compositions (H-C-O) in eclogitic rocks as tracers of local fluid migration during high-pressure metamorphism. EarthPlanet.Sci.Lett.,114, 431–448.

pétrequin p., pétrequin A.-m., errerA m., cAssen s., croutsch c., KlAssen l., rossy m., gAribAldi p., isetti e., rossi g. and delcAro d. (2006)-Produzione e circolazione delle asce in rocce alpine nel Neolitico dell’Europa occidentale. Verso un approccio pluridisciplinare. IstitutoItaliano di Preistoria e Protostoria, XXXIXRiunione Scientifica: Materie Prime e ScambinellaPreistoriaItaliana,Firenze,25-27novembre2004,Vol.Atti, 629-639.Atti,629-639.

philippot p. (1993) - Fluid-melt-rock interaction in mafic eclogites and coesite-bearing metasediments. Constraints on volatile recycling during subduction. Chem.Geol.,108,93-112.

philippot p. and selverstone j.(1991)-Trace-element rich brines in eclogitic veins: Implications for fluid composition and transport during subduction. Contrib.Mineral.Petrol.,106 , 417-430.

pouchou j.l. and pichoir F.(1988)-Determination

of mass absorption coefficients for soft X-Rays by use of the electron microprobe. MicrobeamAnalysis,SanFranciscoPress,319–324.

pupin j.p.(1992)-Zircon from oceanic and continental granites - coupled study typology-trace element geochemistry. Bull.Soc.Géol.France,163,495-507.

pyle j.m. and speAr F.s.(2000)-An empirical garnet (YAG) - xenotime thermometer. Contrib.Mineral.Petrol.138, 51-58.

ricq-de-bouArd m., compAgnoni r., desmons j. and Fedele F. (1990) - Les roches alpines dans l’outillage poli néolithique de la France méditerranéenne. Classification, origine, circulation. GalliaPréhistoire,32,125-149.

ricq-de-bouArd m. and Fedele F.(1993)-Neolithic rock resources across the western Alps : Circulation data and models. Geoarchaeol.,8,1-22.

rocK n.m.s. (1990) - The International Mineralogical Association (IMA/CNMMN) pyroxene nomenclature scheme: computerization and its consequences. Mineral.Petrol.,43,99-119.

rubAtto d. and hermAnn j.(2003)-Zircon formation during fluid circulation in eclogites (Monviso, western Alps): Implications for Zr and Hf budget in subduction zones. Geochim.Cosmochim.Acta,67,2173–2187.

schwArtz s., lArdeAux j.-m., guillot s. and tricArt p.(2000)-Diversité du métamophism éclogitique dans le massif ophiolitique du Monviso (Alpes occidentales, Italie). Geodin.Acta,Acta,13,169–188.

tsujimori t., liou j.g. and colemAn r.g. (2005)- Coexisting retrograde jadeite and omphacite in a jadeite-bearing lawsonite eclogite from the Motagua Fault Zone, Guatemala. Am.Mineral.,90,836-842.

ulmer p.(1986)-NORM - Program for cation and oxygen mineral norms. ComputerLibrary,InstitutfürMineralogieundPetrographie, ETH-Zentrum,ie,ETH-Zentrum,Zürich,Switzerland.

jadeitite in the monviso meta-ophiolite, piemonte zone...

Documents