'l'BE Ab PI NE AGE OF 'l'BE GRAN PARADISO ECJJOG ITES ( •• )

UUSSUSTO. - Nel cristllnino Ilretria!l8ico del ~la!l8iceio del Gran P aradi!lO ~i rinvengono numero!le tlllI lI8e di celogili glaucofaniche, sulla cui età esistono in JcUerlltura pareri controvcrsi. Vengono del!eri tt; alcuni affioramenti dell'alta Valle di Pi ro nlonet.!o (!lettore eerrlrrole del Mro~siccio dd Grro n l)nrIHlIso) nei qua.li le rocce eclogitiehe SOI\O in conlolto primorio con graniti ercinici, che hanno eserci· IIlto una !lellsibile azione termonretnlllorficn sugli scisti inCIISS'lnti, COli IIviluppo di cornubiuniti. Poichè la purtlgenesi eelogitica è instllbile nelle condizioni di P e T del terl1lometamorfilllHO, ne eonllCgue che la formazione delle celogiti in que­III ion6 del'e essere posteriore IIIl' ill tru.IIione dei grallili ercinici e pertanto attri· buibile ;11 ciclo metamorfico alpino.

D 'altra llar le, poichè III pnragene8i celogilica è chiaramente anteriore a quelle iII f"ciCII seiMi .'eTlli che rnJlJlrC!lent~no nella n'gione l'effello della I, rin­cipale fn!le mel'amorfica alpina, ne deriva che le celogil i del Grnn Paradi.110 devono eS8Cre riferite ad un evento nlpino I)TI.~oce, con ~aralleri di alta pre!l8ione e basaa lemperntu rll, de l qlL~lle si Iro\-ano "mpie testimonianze anchc negl i seiali incassanti Il negli ortogneisa grani t ici.

AnSTI!ACT. - The 'lge of the ~Inucophnnic celogites occuring in Ihe pre­'rrin8sic sehists of the Gran Pnrndi .110 MlI!I8it (Western All'a) i8 diseu8!1ed. Eclogite bodiC8, sho ... ing primnry conlaclll ... ilh Hercrnian granites ... hose thermal action has converted the surrounding IIChists illto bornfelscll, ..-cre found in the upper Piantonetto Vallcy (cent rai I,nrt or !he Gran P aradi.110 MII!I8it ). Sinee ee.logitie a.8IICmblnges are unstable in the P·T range of eontaet. metamorl)hillm, the Pianto­nellO eelogilCII llIulIt hll"e fo rmed arler the in trusion of tbe Hercynilln granites alld 'Ire Iherefore to be aseribcd to the Alpine mdllmorl,1oi81l1_ }'urthermore, since the ee.logitie u8emblage (oml)h~lcite-garnet-glaucophane) is elellrly oh1eT than tlre greeusehisl-flleies assemblllgc~ produced by 1he main phaae ot Ihe Alpine meta­morphisnr, it "Hl )" be eoneludc(l tlr"t the Grilli P llradiso eelogi tcK lITe rnost likelr Teluted lo un early Alpine hig" I)re"sure-lo'" tempera ture el'ent, widcspre'ld reHe<! of wh'ch are fOllnd also in Ihe I,re-grunitic schisls nnd us.llOeillted orthogneissell.

(-) Istituto di Pelrografin delt'Ullinrsità d i Toriuo e Centro di Studio sui Problemi dell'Orogeno delte Alpi Oeeidentali del C-N.R. - V ia San Mllsaimo, 24 -10123 Torino.

(--) Lu\-oro e!!egllito con il wntributo finanzia rio del Consiglio Nazionale delle Rieerehe.

224 It01H:UTO COoHPAOl':ONI, BKUt«O I.OMBARDO

Introduction,

Di.fferent opiniolls are fOUJH! ill tile geologica! literature as to tbe age oi tile eclogites occurring in tbe Pell nidic flnd Austrid ic domains of the Westem Alps, An Alpine ('l'ertiary) age is currently assigned to th e glaucophanic eclogites associated with the Mesozoic «Calce­scisti» Complex (see e.g. B &\RTIl, 1959, 1965, 1967). A pre-Alpine age was llitherto Ilseribcd to the widespread lllllsses of gla ucophanic eelo­gites oi the Sesia-T .. anzo Zone (NOVAK(,;S(,;, 1929; BI'\/';CIII & DAL PIAZ, 1963; VFr~;m.lO-BASSAt«1 & BI,.1 CKIlUR", 1968; CAltRAno et al., 1970; CAR­R,\RO & CIIARHI(,;H, 1972), 'l'he pre-Alpine age was inferred from thc assumcd lnte-Paleozoic age of the Canavese voleanic Illld Elvo volcano­delrilal series, both including bouldel';; of Sesia-~Lanm €Clogites, Ho­wevel', according to il )'ecent paleobotllniclii and geochronological work (SclH:um"o et a1., 1974) these volcanics seem to be of Tertial'Y age,

Presently the inter pl'etation givcn by most geologists of the oe­cUl'l'ence of the eelogitie l'ocks in the Sesia-T .. amw ZOlle alld in the II: Calcescisti» Complcx is concordallt: the eelogites would have fo rmed in Il high pressure-low temperature environment at an early stagc of the Alpin e metamorphism (DAL P1AZ, 1971; D AI. PIAZ et. al., 1971, 1972 ;

COMPAG"ONI & ì\LWFEO, 1973). 1'his interpl'etatioll is in good agreement with the polyphase chu rlleler of the Alpine metllll1orphism, which

displays un earIy high prcssure-Iow temperature phase, followed by Il high er-tcm perature phase (BE:ARTll, 1967; NIGGLI, 1970).

Other OCcurrences of eclogitic rocks iII the 'Vestern Alps are fo und in the pre-Triassic scllists of the Monte Rosa-Gran Paradiso nappe (DAI, PI ,\Z & OA'rTo, 1963; Com>,\ONONl & PHATO, J970). Th e age relationships of these rocks in the l\Ionte Rosa Massif were dis­cllssecl by DAL. PI,\Z (1964), who then could not find any elcment lo aseertllin thei)' age; iII 11 snbsequent paper (DAL PIAZ, 1965) ho­wever, the same author is inclincd t.o acccpt an Alpine age, on the basis of the mineralogical Illld textural similflrit;-,' between the Meso­mie eclogites alld tllOse oceurring in the pl'e-Trillssie sehists. More recenti)' D ,\L p ,,\Z ct, al. (1972) stat.ed that these rocks must have an Alpine age since they are illterlaycred in a cha rllctcristie pre-Triassic complex, i,e, the Furgg Zone I.s. , which surely originated later than the H ercynoty pe metamol'phie cycle, A simi la!' interpretation has been

TRE ALPINE AOE 0 1' 't'HE ORAN P,\RAOISO ECl,OOln;s 225

propo.sed by WI:."'t'ZEL (1972) for the Furgg-Zone eclogites oec urring in the Swiss siele or the Monte HOSR Mas,'iif,

In a recent papel' on the eclogiles or the West.e l'n Alps, MOTT,\NA

( 19i2) suggested thnt the cclogit.es included in tlll,! sehists of the !\font.e Rosn-Grnn Pnrndi so 1111ppe could be l'elnt.ed lo Il pre-Alpine Illel.nmorphism, probably of Caledollinn age, Aecording 1.0 his inter­pretlltion, these rocks would be e<lu ivlllent to Ihe eelogites whieh are locnlly foulld in t.ile pl'e-Al pine basemen! of the lI el"etic domaill (Ai­guilles Rouges- and Argentera Massif); during the Alpine melamor­phism (which ollly s lighUy nffected the Hel"etic dOlllilin ) they would h1\"e been subjeeted to glnucophalle-schist-faeies eond itions and partly retrogrnded,

So far tliere has beell ilO fie ld e\'idence us lo !he age of tlie celo­gites occul'J'illg in the Gran Pnradiso Mass'if. 1" I\I,thcI'IIlOre, !he inter­pretatioll put fOJ'\\'llrd by DAI, l'1M et al. (1972) IInd Wb"l'ZEI. (1972) fol' the ~Ionte Hosa Massif calino!. be applied to the Orali Paradiso cclogites sin ce they nre includc<l iII a gn eiss complex older than the HereYllinn granites (see latcr on ), Howe\'er, some rcccnt findillgs in lhe Pinntonetto Valley (centrai pUI't, of lhe Gran Pllradiso Massi!), where eclogites are eut by Uercynian granites, euu bled UI'; to establish thc Alpine age of these eclogites, They will be deseri bed itl this papero

Geological outline of the Gran Paradiso Massif,

The main features or the geology oC the Grilli PIlI'udiso Massi! are rnther sim ilnr to those described for the Monte Hosa Mllssi! by BB..\RTII ( 1952), The p.'e-Triussic base.ment consisted, prior lO the Al, pine metflmorphism, of n complex of meso- to entn1.onnl sehists (I )" cut by Hercynian (2) porphyl'itic granites (BI ,\l'CUI & DAI. PI AZ, 1959; CALI,~.:o ,\RI et aL, 1969) ; the schists were locally nffected by the thermal

(') llilliclI of silli mnllile-bellring s<:hillh were fOlllld nt 80llle 10Clllitie~ (e.g. PilUl della Vallctta, ct. Cololl'AOSOSI &; P RATO, 1969), Nel" oceu rrelleea were re·

cently fOllnd in the northern J1Rrl. ot Ihe Massif (VR leille Rlid BRrdoney Valley); they will be deseribed in n forthcoming J1apcr by Cololl'AlISOSI cl. !LI, ( 1974),

(0) Only the Scalari granite (ul)l}Cr OrfO Valley) h:u~ beon !IO far date<:1 radio­melritnllYi the determinetl \'aluCil lire 301, 340, 350 tu. y. (l';t, PASOAUD et aL, 1957; BUCIIS et al" 1962 ; Cm:SSI:.)( et al" 196-&),

226 ROIU;II'l'O CO.\IP,\ONOSI, BII:Ul'"O LOM B,\IWO

ad ivity or the H ereyn iun granites, IlS shoWIl by the hornfelscs deve­loped IICllr the contacts with the granitic I"Ocks (CAI.U:OARI et al., 1969; COMI"AOl'"ONI & PR,\'I'O, J969),

We woul d like IO stm'iS, hOWCVCI', somc diffCl'cnces betwecn the Gn\ll Paradiso- aneI thc Mon te Rosa baSClllcnt : J ) in Ihe former, lIm­phibolile; 111"e much morc widesprelld tllllll in !he latter; 2) in the Gran Pllrndiso ?lla"''iif we do Ilot fillcl the lithologiclll nssocilltioll mar­bles + IImphibolites Iypical of t ll e P lIrgg Zone of the Monte Rosa Massif, Illnrbles being fllmost abscllt in the Oral i P nra diso 'M assif,

D uring the Alpin e orogeny thc JlI'c-T I'illss ie bflsc men t underwcn t strong mineralogical alHl st!'uelUl'lll chnnges: the Hcrcynian granites \l'ere trnnsformed in to orthogne isses (c gllci~s occJtiadi1li :. a1/ct.) IInd th e pre'gl'llllit.ic rocks iuto ep izollnl schists (c {Jl1 ei,~,~ minuti. (lutt ,), III the Gran P nrad iso l\'lassif th e Alpine metamo l"phism shows a po­lyp hase ehn raeter (BICRTRAl'"D, 1968), as shown by the occurrenee or Alpine minenlls like kyallite, chloritoid alld glnueophane (3), whieh are clcn rly older than greenschists-fac ics mineraI 1l ...... emblages. A high­prc!!Sure phllse (co-A lpine pl!ase) which develo l)cd . ilmong other mill e­mls, ph engite, ehloritoid, kyanite, gl!lllcophunc, omphaeite nnd garnet (Fig. J) ( ~ ) , was followed, and Plll·tly ohscurcd, by 11 later episode characterized by assemblages in which S11bite, biotite, chlorit.e, IIctino­lite ami cpidotes flre t.he major cOlIstituents (A lpine plw.se s.s.), A si­milar sitlllltion \\'llS found by DM, P IM" ( 1!J71 ) in the Montc Rosa Massif.

'rhe b~lSic rocks interlnycred in the pre-grallitie schist !! of tile Gran Pnrndiso l\'l assif are mainly (llbite-epitiote amphibolilcs; celo­gilcs, rirst dese r ibed by COMPAO;':ONt & P Il,\TO (1970), OCClir ns sent­terec1 reI ics, preserved in, ~l1ld gradil'g into, albite nmphibolitcs. 0!11y at few loeali ties (Roe Vnlley, Valcillc, Ribordone Valle)') the eclogites are more widcsprend lhll t the IIlbite am phiboliles. 'l'h ere is s lrong mi-

(') Thu Alpine agO] or Ihese ",iller<lt~ is inferrl~d {rOIl1 IlIeir del'O]!O[III'Cnl ellller 1\1. the el<I,cn!!C or Ihe Hereyninn gn,niles (c_g. ehlorl told, gluueol,hane, kyanlte ) or fil Il,e el'pell8C or Iheir Ioorn'el_ (e.g. kynnile I,seudomoq)h~ afler sillimllnite, CollPAGSOl'l &. l' IU,TO, 1969).

(' ) t-'urther (l1-ideJlee or high-prC!$IIure eomlitiollil in lioe eo_AII,ine 1,1",se is pro"illed by Ihe rcccnt fillding of jlldcile in the sililie gneisses (c gneis8 do Bon· lIm·al., MtCIIF.I" 1953) of Iho }' rene lo ~ido of the Orali Prornd iso Mall.8it (SAJ.lOT, 1973)_

• . ,

D , D, [],

',--,"--,l __ -,I ___ ·,--,' ·m

. ,

M. ColombO

•

* b

t N

.d

f'ig. l. - Oistribution ol the eo·Alpine high prl'88ure . low lell,peralure IIssemblllges or

Uli"er!ll ~ in Ihe 1Iorlilern IlarI oi tbe Gran l'a­rndillO MSlIIli!.

Structural units

c Grte;n Millllth CompIa J Grnn P:mHlixo

2 CCN ri811 Oceliiodini:t Compia: ~ Mnllllif.

3 V:tls.warallehe Ma~ir (Greal St.. Bcrnllrù N"ppe);

4 c C>i1cc~eisti . COlllplex n,,{\ MClKlzoie cOl'e r

or tlle Grall Paradiso- ull.1 V"laum m nehe MIl8~ifs;

5 Se~ia· l,lHLzO ZOlle.

HIgh presure . low temperature assemblages n) omp/w cilc + Darltcl + glollCOp/W llc fI!l/lem­

blllgeB of the eelogites deri"ing from the pregrllnilie metaballites .

Il ) phe1lgile + cllioritoid ::!:: kyoNite ± ganltl

IlMemblages or Il,c pregranilie schist\l.

e) '"yaRilc p8cl<tlomm'phll aller ,mimollile. Thc kyanite psendomorph! replste ei the r tile pri8.rnnt.ie 8il1irnsnite iliwelop·ed in the eon­

taet aureole of the Uereynian grani lell (outerops n_ 32, 33, 35, 39, 40) Or Ibe fi­brolitill lIi ll imllnite ol tbe Ilregrflnilie re­gional rnelarnorphisrn (oulerOl19 n_ 34, 36, 37, 38)_

d ) /llleNgite + cilloritoid ;t Mg-eMorite :;:!:;

:;:!:; kyanile :;:!:; glaucoplwne lu!;,clllblllge8 or the ~.e_ c mieaseisti urgentei ~ (~ i lve ry mi­ea8ehi8ts) tle,-eloped at. the expense or Ille lI ereynian graui !es along pllrtieulll.T "hear plnues.

Non: - The ms p ili base,] Oli peno",,] obsen-a­tiOIlS, Oli the dll.ln ol CA I.U:OARI et al., (1909),

COM I'AONONI & P RATtl ( 1969), PItAT() ( 1970)

Ilud on I,lnpubblisiled data or R. PIlATO .• 'or tbe

exaet. loeation ol tlle oulerop!! see Apl~"dix .

neralogical evidenee, however, thllt mosl albite nmphibolites passed through an eclogitic stage !lS ShO\\'II , for example, by the widespread occurrence in these rocks of the typical albite + actillOlite symplectite developillg from the eclog itc pyl·OXCIlCS (see page 374 ).

The eclogites of the upper Piantonetto Valley.

Primary eclogite-gnmite eonlaets were found in two outcrops (Boc­ehetta di Valsoera and Pinn 'l'eles.'iio Lake) botb located in the upper P iantonetto Valley (Fig. l , outc rops Il. 1 and 2)_ In this arell relics or primary intrusive con!aetii lire well preserved ( BIANCIH & D t.L P ' AZ,

1959 ; CALLEGARI et nl., .1 969): Hercyninn grnnites, probably emplaced IlS multiple intrusions, inject. 1\ complcx of met.nmoq)hic rocks exhi­biting Il clenr pre-g'-Il ll ilic schistosity; fragments of country rocks llre frequcntly fou nd within t.he igneous body nellr the contact zon e. Relics or hornfelsic rocks conta ining K-feldspar, ol1goclase, red bio­tite, sillimallit e, COrUlIdlHll an d green Spil,el, ocetIr nlong the easterll side of the Pian 'r elessio 1.J,llke (CALLI::GARJ et aL, 1969 ; COlJl'AGNO:-;1

& PRATO, 1969). 'l'he grnnites and tlle country rocks were rmbse(IUently affected

by the Alpine metlllllorphism. 'l'he de\"elopment o[ the Alpine schi­stosity is especially e\'ident in the granitic l"ocks, which were con­vcrl.ed into albilc-ph engile·biolil e-cli1!Ozoisite-g(/.r·net (/..1tgen g1/ eisses, wil h rei ics or the primnry K-fcldsp(/)" and biotite.

Bocchetta di Va l$ocm . • 'l' he outcrop Iies on the west.ern slope or the Bocc.hetta di Valsocrn (II I an ele\'lItion of llbout. 2400 III), near the lower limit of the thick balld or pre-granitic sehists which follows the cnstern sidc o[ the UI)PC,- P iautonetto Valley ((,). Helics o[ a dis­co rdllut intrusive contact bet.wecll li sehist eom plex nnd a porphyrit.ie g rullit e lire here recogniznblcj llenr the eontllct zonc relics are fo und of biolite-oligoclase-bea.ring 1-ock.~ with hornfelsic structure. Both the grllil it e all d the country ,·ocks ll!l\'e been deepl'y rellrrllnged by the Al pi/le metlllllorphism which produced albilc- phf'ltgilc-biotilc-clinozoi­sil e .çchisls (at the expense of the pre-grauitic schis,s) Iln d converted the g rnnitic rocks into augen gneisses.

(") l'or the locatioll or the oUltrOtl !!ee t he Appcndix, Il. 2.

228 HO Bt:RTO CO~[ I',\ONON'[, Il nUN'Q [,O~[Il,\R[)Q

Aboul ten mel l'es ft"Om the pri mary grun it e.sc.hist contact t he schists en elose a boudinaged lllyer ( a Cew dm th iek) of metabasites. '{'lte dark green CO I'C or the boud ins is lllllde up of Il fi ne·grni ned rock

which nnde l' t lt e microsco pe a ppelll'S 11S glUllCOplwllitJ ccloyilc (Pig. 2) ; it consisi.s or yar-/ICf lin ci ompllaci/c in nearl.y e(\ UII I amounls ( tolaUing

abollt 80 \'01. 7<:), !J1allcopltallC ;1I1<! IIccessory ridite, lvIIi/c mica and

~·ill. 2. - GlaNcop/wNic eclogile of !lIe Boechetln di VaI8oer: •.

Gnrnet (high relicf), om"hlleitc (light g rcy), glaueophunc (Iight) ruul futile .. re !he mnin constitucnh of this eclogitc, oeeur ring "crr nCllr the contnel wit h tho II creyni:H1 gr:m itCij. [lut;lc slringcrs /lIHrk a poorly·de· \"clOllcd foliatioll , ,,"i t h nl1crnllling Oll lphacitc· ;lIul gltlUeophane • rich laycrs. X 80 - Pt:IIlC potariud lilo':ht.

(jua/"f z. Opt icli i a]\(1 physic111 pro p~ l"ti es or th e nHli n cOlls!Ìtuents are as roU OW!;:

yar'/lct: nD = 1.792 ± 0.002; IlO = 11.665 .t\ ; ì\tn lJl traces only, which should eor respond lo Il gurne!. of cOlllpos itioil A1n1ooGroa8PY:!

(cf. WISC!U:b L, 1958).

p y /"QXCtlC : 11ft = 1.680 ± 0.002; 2V r = 63-65°; c ..... y = 46"; d :!21 = = 2.970 .t\ which correspon d to un omphucit.e o r com position AU!lr.II.1d2l)Ac12 (c f. ESSt:st; & l"v P.:, 1967).

TUE ,\I ,Pl!'."E AOE OF THE GRAN PA RADISO ECLOO1TES :!:!!)

glaucophanc: 2V" = 70-72°; C"'i' = 0-3'>; it is th c \'e ry pale-coloured Vlll"icty known as gastaldite,

AecOI"diug to the molecular composition of the omphacite and the gl\mct this rock cn n be classified lIS 1111 ophiolitie eclogite (S"lULI­

KOWSK! , 1964) or' a C-gr'vrtp eclogite (COLE.\!A!" ct al. , 1965) .

Fig. 3. - Pian T clcuio l.oke, u})})er Pian/onctto Valley.

Sl"up ù iscordallt eOlllnet. betll'ecn Il porphyritic llereynian granite (light.) anù ali amphibolito mass t!o ntaillillg rclie~ of .:!elogitic asscmblages. Tbe prc, A1I)inc fol intion (apl)roxi""Ltcly horizontal in the pictnre) of the me­tal,msi tn i~ cuI. by the gr,mite, whoge AII)i"o seh i$lo~ity is strongl), slan­ting lo the amph ibolitc foliatio n

'l' Il e oute!' portion or the boudins is n COfll"Ser-gl"ain ed Ilm phibolite which in hand specimell a ppears li ghter-coloured than the eclogitie core. 'J'his is due to the progl'essive transformatioll of the eclogite into Hn llibite-epidote amphibolite; undel' the microscope this transforma­tion is marked by the progressive disappea l'anee of the pyroxene (and of the glaucophall e) whieh nre ['eplaced by the ty pieal sym pleetite of

albite + ncti nolite occU1'1"in g in lllost ecJogites of tl1 e Gran Pa radiso l\1assif (CO"[I'AGNO~ I & PIlATO, 1970). l ' he roeks origilHlting from this

230 ROBUtTO CO~II'AGNONI, BRUKO LO~IBARDO

typical transformation of the omphacitic pyroxene shall be called here «Ab-Act-sym,plectite amphibotiles »; t.hey l'epresent the first stage of the ec/ogile ~ albile-a.mpltibolile I rH IlsformatiOIl (ol)_ In the more ad­vanced st,lgCS of !his Il'Illlsfol'lllll!ion the size of !lle neobllls!ic minerals increaSé!S alld the gilme! lot.ally disa ppears, so lhal t.he primlll'y ecJogite is finally conve rte<l into an albite-e pidote ,lmphibolite with accessol'y sphe1w,

Pia,n .7'eless·io Lakc. - 'l'Ile outerop is loea led on the eastern side of the artificial Pian 'l'elessio I.Jll ke, whcI'e the trail to the Rifugio Pon tese stMtS (1).

A large body of eclogite exhibiting a elear pI'e-granitie foliation is cut by a pOl'phYI'i tic granite, now partial1y converted into an augen gn eiss (Fig. 3). Evidell ce of the ill trusive nature of the eontact is pro­\'ided by eclogite in cl usions in the ncighbouring augen gncisses.

Ullder tll e microscopc the P ian 'l'elessi o eclogite is ve!"y similar to the «Ab-Act-symplectite llmphibolite» of t.he Bocchetta di Valsoera, being largely com posed of the typical albile + aclillOlile intergrowths which replaee omphacite. A few rdies or omphacile alld gla1tcophatw are still prescrvcd, the formel· «lIrllloured » by targe zoisite poikilo­blasts, fhe llll\(~ l' in the CO I'e of zoned green amphiboles growillg at the ex pense either of the diablastie illtergrowths or of the gamet. 'l'ypical accessory minenlls include clinozoisile, rntile, a.patite, opaque ores and, in very small HllIounts, quarlz, w/dle mica, alld yellow-browntsh biotite.

A peculiar featu re of the Pian 'l'eles.sio €c logit.es is the seemi ng:y large sil'.e (up to 0.5 cm) of the g(ll"1wt.~, which unde !" the mieroseope a ppenr IIS glomcl'obblSbc aggregntes of slllall idioblastic gamets not larger thall 0.5 mm.

The age ot the eclogitic assemblage,

'l'he mill cnll assemblage or the glaucophanie eelogites of the P ian­tonetto Valley develo pcd 011 metabllsites illtel'lllyered in a prc-gran itic complcx of mcso- to eatazollal schists.

'l'he oceulTence of eclogites in primary eontact with granibe l"Ocks gives strong evidenee that the eclogitie assemblage Illust be younger

(0) J ust frorH tlle Piantonello V"Uey, ZUCCII~::'M'I (1956) rcporled li gnmel nmphiùolite whiclr f rOIil lire microscopical dllscription St~"'~ lo correspoHd lO this type of anll)lIibolitc~.

(') For the loclltion of the outcrop ~ee Ilic Appcndix, Il. l.

TIIE ALPINE ,\01:: O~' TI-IE GRAN l'AIL\Dl SO I::CLOG IT~;S 231

thnn the emplaeement of the grllnites. Tndeed roocnt investigatiolls on the contact aureole or the 'l'el'Iiary Biella and 'l' ravcrsclla plutOllS 8110 \\'ed that the itssembhlges or the glaucoplulIIic cclogil cs 1\1'e highly un8table under the P - 't' l'lIuge or therllllli lIletamorphism; in parti­culll!' om ph llcite and gillucopha ne lire the first minernls to break down wh en t.empemture inc l'ellscs, the eclogitic asscmblngc being replaeed by pingioc lllse, hornblende, diopside un!! biotite (C,\LLfXlAIU, pers, comm.), Consequently, since the Pillntollct.to eclogites are [ound near a primary inirus ive contaet it followl> that the eelogitic parageneses must be yOtlnger than th e granite inll'us ion ; otherwise they woulcl have been converted lo contact u~lllbhlges (8),

011 the other hand thesc cclogit.es ( in COllllnon with most eclogites of the Gran Paradiso Mussif) lire partly rcplaced by Ilsselllbiages t.hat ure typical of the Alpinc g reenschi st-fllcies phase ; they, th el'efo l'e must have t'Mmed between tlte em plitccment of th c ll e l'eyninn granites Ulld tlte development of this i\lpi lle pllllse, It llUly be cOll cluded thll t the Grnll Pnnldiso eelog iles are "cint ed to il high-pressure met nmorphism or Alpine age which preeeded th e greenschist- fllcies metllmorphism.

'rhis e\'eut is nlso reeogniznble in lhe other rock ty pcs of th e Gran Pnradiso Mnssif, the most s ignificilnt. high-p ressul'e nssemblnges or mi­nel'nls being (Fig, I) :

phtmgilc + chloril oid + Mg-chlOl'il c ± kya.llife ± glaucophanc, in gl'llllitie l'ocks;

ph Cllgi le + chlOl'iloid ± kyu.lIilc ± garnet, in pre-gI'lUlit.ic schists;

kyall i. te psclfdomorpll$ after sill imanite, in horufelses ancI pre-gra­nitic schists,

'l'he eo-Alpine llge of these nssemblages is \lIJ(luestionable : firstly beclluse typical high -pressure min er/lls (such as kyanite and glauco­pillme) were de\'eloped II t tlle expense of H creynian grnnites (or COll­tact minerals: e,g, Kynnite pse udolllorphs IIfter sillimnnite); secondly becnuse th e high-pt'essure millentl nssemblages are older thlln those of lIt c gl'ccnsehist flleies wldclt t'CIll'csell t Ihe main minerai nssemblllges or the Alpille metamorphism,

( ' ) E,'c l> making :llIowU llcc (or U oJecpcr ill lrullion lcvcl or t hc l1 ereynian gronile8 or t hc Gran Para oJ itso Mntll!.i r, tbeir sharp discordant eontnet relntionahips a lld the dCI'elopment or bo rn relse. in the Sl.lffounding sehis ts 81.lggeSt the reliability or the eorn JXtriSOIl \l' ith t hc tbermnl errcdi! observed nround the Biella and Tra­l'erselln plulon8.

23l:l ROBERTO COMP,\O!'Q!'!, BRUKQ l,o.\lBARDO

A chronological ill tel'pretation Iike that l'eccntly suggestcd by MQTTANA (1972) seems to us uuacceptable, ]n fuet, if thc Gran Para· diso eclogites \Vere Caledonian in age, then these rocks should bave escnped the H el'cyninn regionlll mctamorphism, the thermal metamor· phislll induced by the granite int.rusiolls alHl ( finally ) thc Al p in e poi)'. phase met.amol'phism. MOl"eover, th e glaucopililllic eclogites or the Gran Pam diso lI"fassif 111I\'e millemlogicnl illld text.ural characters st.l"Ongly diI'· fcring fl"Om those 01' the oldel' (Culedoninn ') ecloghes occUlTing in Ihe pre-Al pine basem ent of the H el\'etic domain , e.g. in tbe Aiguilles Rouges l\L1SSif ( BI!l ~RE, 1920; BELLI~RE, .1958) and in the Argentera l\1assif O"'RANCHI, 1894 ; FA URE-M u RET, 1955; l\IALARODA, 1970; MOT­"fAN,\, 1972; also persollal observations) : th e former belonging lo the ophiolitic eclogites 01' SU U1,IKQWSKI (HI64) or C-group eelogites of COI,E.\lAN et al. ( l!J65), the latter 1.0 the gneiss-migmatite-complex eelo­gites or S~! U LIKOWSIO([ (19(4) or B -group eclogites 01' COLEi\L\N et al.

(1965). In con clusion , field , mine l'alogiclll and petrogruphic evidellee shows

tlH1t the ec!ogites oecUlTing in fhe GrIlli Paradiso basemellt <"Ife co­Alpill e in age jthey are thereforc coeva l with the ophiol itic eelogites associateci witll tllC 'Mesomie «Cnlcescistì» Complex and with the glwlcophallic eclog-ites occul"ring in the Sesia-Lanzo Zone.

Ackno,de,Jgcmcn /8.

'rIU.l !lutllors \\"i811 lo :lckno\\"lellge Profcssor E. Callcg11ri 'a cri!ie islH whieh greatl )" improved tbe llIalluacript. Dr. Hoberto Prato, AGIP Mineraria, San Do'

nato 1\Iil., i8 grHtefully aeknowledged fvr unpublished information on the dislTi· bnliou of high I)TCSsure·lo l\" temperatI,.e ,,~~eu,blng(ls ili the casterll p". t of tlle Gran j>aradiw MHUi f.

Appendix,

T.lOCf<TION" OP '!'In ; OCCURREl\"CES OP Tll E HlGII PReS8 U RE-LQW T EMPERA­

TURE AS SEMBLAO ES 1>18TED 11\' Pig. 1 (. ).

a ) Omphacitc + gurn d + glaucop1tmJe (glaucop1tanic cclogites) .

l ) Pian 'L'elessio lake, uppcr Piantoll etto Valley: 728390 (Fol"l1olosa). 2 ) " ' estern slope or Bocchetta di Valsoerll; uppcr Piantonetto Val­

ley; 732393 (Fomolosa).

(.) Referenees are given to nearest 100 meters in tloe U.T.M. grid, zone 32T, 100.000 meter ~uare .LR ; narn(lS enelosed in braekel~ refef lo Ihe 8heet8 of lini)" l : 25.000 of Ihe 1!111iau Mili!a ry Geographie 1nstitule.

3) 2886 topogrnphic point, lIe8 r B ivacco Carpano in the upper P i8n-tonetto Vnlley ; 713417 ('forrc del Gran Sn u Pietro).

4 ) Alpe Ciusel, Hoc VlIlley : 645358 (Ceresole Rea le). 5 ) POl.es, Hoc Va lley: cf. P RATO ( 1971, p. 460).

6) Ma ison, Hoc V,llley: 665354 (Ceresole Re/de). 7) 2656 tol)ogrnphic poin t south or the Hifugio Sel\l1 , Vaill ontey:

681482 (Gl'ali Paradiso). 8) Valle Grun San P ietro, Vuleille: 734445 ('l'orre del Gran San

Pietro). 9) Bivacco Antoldi, Valeille : 736436 ('f olTC del Grilli Sn n Pietro).

lO) Elistern slope or Colle Coupé di 1\loney, Valeille: 725438 ('forre del Grlln Suu Pietro).

,11 ) Easte rn Ilnd western slope of Vj1leil1e abollt ] Km south of 1684 t.opogruphic poin tj eustel'll slope: 747485 ; western s lope: 742484 (Torre del Gran San P iet ro).

12) East of Punta del Rossin betweell t he 2605 alld 2690 topogl'aphic points, Bllrdoney Vull ey : 766483 (Tor re del Gl'm] Sa n Pietro).

13) Punta di Acque Hosse, B,trdoll ey Valle:--': 78347 6 ( 'l'orre del Gl'1l1l

San P ietro). 14 ) Grange Mulpensatll, southwest or Valprato Soalla: 848415 ( Val.

prato SOllna). ] 5) Rio Arolei, Cam piglia Va lley: 812457 ( Valprato Soana). 16) Rio 'l'ressi, l"'orzo Valley: 8234 14 (Valprato Soana). ] 7) Colle Ciardolley, F orzo Valley: 739421 (Torre del Gran San

Pietro). ] 8) Punta del Vallone, Ribordone Valley: 82537\ (Locana). ]9) Tricet near Talosio, Ribordone Vll lley: 8 L7333 ( l JOO8na). 20) Punta Busiera, Ribordone Val1ey: 805326 (Locana). 21 ) Rio Furà, neal' LOCllna: 795322 ( I~ocann ) .

b) Phe1!gite + clt loriloid ± kyu1!ite ± yar-lId (in lite pre-graJlitic .scllists or lh.e « Gneiss Minuti :t Complex).

22) Rifugio Pontese, upper Pian tonetto Valley: 728398 ( Fornolosa). 23 ) Alpe Breng i, Noaschetta Valley: d. PHATO ( 197], p. 459). 24 ) Alpe Arculà, NoaschcUa Valley: cf . PHATO (1971, p. 459). 25) Pian dell 'Alpe, NO!lschetta VlIlley: cf. PHATO (1971, p. 459). 26) Roley waterfall, llellr Pont Valsuvurallche: 594432 ( Rhemes No·

t re Dame).

R . nd;C<I"U S.l.Y.P .• IG

234 ROBERTO CO:r.rI'AONO~I, BR UNO LOMBAROO

27 ) Bivacco T.Jeoness1t, Valnontey: 673455 (Gran Paradiso).

28) Trai! to the Herbetet hut south of Pon te dell 'Erfaulet, Valnon­tey: 689457 (Gran Paradiso).

29 ) " Testern slope of Vaillontey between Villmiilna IIll d Ponte dell 'Er­faulet: 694466 (Gran P Ul"adiso).

30) Comba COllteleinu, VlIlnontey : 712457 ('l'OI" l"e del Oran San

P ietro).

31 ) Ghiacciaio delle Sellgie, Valeille: 753448 ('1'ol"l"e del Oran Sun Pietro).

c) !(ya11ile pseudolnorphs afle,. sillima·11ite (i11 /wl'nfelses a·nd pre-gra-11itic schists).

32) P;an Telcs,;o Lake, P ;an tonetto V"lley , cf. Cm,,,o,,'" et al. ( 1969, p. 65).

33) 2256 to pogntphic poin t neill" Hifugio Pontese, uppel' Piantonetto Vall ey: 730396 (Fornolosa),

34) 298 1 topogmphic point Ileur the 'l'eleecio Olaeier, llpper Pianto­netto VaUey: 72141i ( '1'ol"l'e del Oran San Pietro).

35) N\V face of Boccia V iva , Valnontey : 688426 (Gran Pilradiso).

36 ) Ensterll slope of Vule ille, about l km SSE or 1684 topograpl!ic point: 747484 ('r orre del Gran San Pietro) .

37) Westem slo pe of Bardoney Valley, southwest of 2272 topograph i:! poillt: 771478 ( '1'OI"1"e del Oran Sa n P ietro).

38) P ian della Vallettit , Forzo Valley: cf. COMI'J\Ol"Ol'l & PRATO (1969, p. 538).

39 ) Western face oi :Monte Colombina, F orzo Valley: cf. COMPAONON I & PR.~TO ( 1969, p. 538).

40) Northem slope of Orco Vall e), between Rio F'unì and Albrella: 791315 (Fol'llolosa),

d) PhC11gite + Alg-chlo /"ile + chlOl-iloid ± kyanil e ± glaflCopha1w (in schisls developed al Ihe expense of lite H CI·cynia.n graniles) .

41 ) Rifugio Vittorio Emilll uele Il , ValSHYllraU ehe: cf. P RATO (1971 , p. 457).

42) Colle di Valmiana, Vuleille: 724464 ('l'orre del G I·an Silll Pietro).

43 ) Between l'hm della "uenetta and 2740 topog,-aph;e po;nt, Val­nontey : 711474 (Torre del Gran San P ietro).

TRE AI .. PINE AGE QF TRE GR,\)' .. PARADISO ECLOGIT ES 235

44 ) Between Pian della Turnetta an d 2057 topographic point, Val­nontcy: 705478 (Tor re del Gran San Pietro).

45 ) Eastern slope of CoUe Coupé di Money, Valeille: 724437 ('l'arre del GnUl San Pietro).

46) Colle Plitrì, Valeille: 721449 ('l'arre del Gran San Pietro). 47 ) 2425 topographie point southeast of Punta Vigeusa, Valeille:

736487 ('f orre del Gran San Pietro). 48) Eastem slope of Ba t'doney Valley near Punta di Acque Rosse:

784483 ('l'ol're del Gran San Pietro). 49 ) Cirque between Monveso di Forzo and Roccia Azzurra, Forzo

Valley: 769433 (Torre del Gran San Pietro). 50) Eastern ridge of Monte Arzola, HibordollC Valley: 801338 (Lo·

canal.

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