American Mineralogist, Volume 66, pages 443-472, l98I

Petrogenesis of eclogites and peridotites from the Western and Ligurian Alps

W. G. EnNsr

Department of Earth and Space SciencesInstitute of Geophysics and Space Physics

University of California, Los Angeles, California 90024

Abstract

Aluminous peridotites and eclogites of the Western and Ligurian Alps have been investi-gated in terms of their mineral assemblages, major element phase compositions, and bulk-rock REE and major element chemistries. Except for the garnetif€rous ultramafic lens ex-posed at Alpe Arami, all studied peridotites are spinel lherzolites. The peridotites show lightREE depletions due to low-temperature hydrous alteration, to the hypothesized incipient fu-sion of a now completely obliterated garnet-bearing prototth, or-more probably-to the ex-istence of mantle already fractionated relative to primordial REE abundances. Eclogiticrocks are tholeiitic in their major element affinities, but show apparent metasomatic efects ofiron, titanium and REE loss, and alkali enrichment during pervase and ubiquitous late-stageretrogression to greenschist-facies assemblages. Alternatively, these differences may reflectthe former existence of two chemically distinct protoliths. Employing various geothermome-ters and geobarometers, the following P- T conditions for the last reequilibration of ultrama-fic and mafic units have been computed:

SOUTH ALPINE PLATE NORTH ALPINE PLATEPeridotites

FineroBalrnucciaBaldisscroLanzow. Liguriae. Liguria

Eclogites (none)

Peridotites900'C -10 kbar Alpe Arami925'C -15 kbar Z,ermatt925oC -15 kbar w. Liguria

l000oC <12 kbar EclogitesI100'C -15 kbar Alpe Aramil000oC <12 kbar Zeffiatt

w. Lieuria

950oC 40 kbar<600"C (l0kbar)<600"C (l0kbar)

900"C (40kbar)525"C >l0kbar435"C >l0kbar

knowledge concerning relationships among the vari-ous lithologic units and the manifest structures. Morerecently, the concepts of global tectonics have beenapplied to the Alps (Laubscher, l97la,b; -Smith,l97l; Ernst, l97l;DalPiaz et al.,1972;Dewey et al.,1973; Hunziker, 1974). That petrologic features canbe attributed to associated lithospheric plate motionsis now becoming clear. The goal of the present paper

Retrograde assemblages are interpreted in terms of a nearly adiabatic ascent towards pres-ent crustal levels for all investigated rock types. In western Liguria and the vicinity of Zermatt-and in general for the North Alpine plate-this P-T trajectory may reflect buoyant re-turn towards the surface of southward and eastward subducted crustal sections plusserpentinized periodotite underpinnings during late Mesozoic closure of Mesozoic Tethys.The crustal emplacement of large masses of relatively dense lherzolite of the South Alpineplate apparently is a consequence of tectonic imbrication accompanying overthrusting andthe transportation of deeper portions of the more southerly plate northward and westwardduring convergence.

Lithotectonic framework

Introduction

The Alpine chain represents the most intenselystudied orogenic belt in the world. Over the past cen-tury, careful investigations of this magnificently ex-posed terrain by more than a thousand Europeanearth scientists have provided us with detailed0003-004X/8 l/0506-0443$02.00 441

-?Ultramafic Rocks

NSouthern Plate

Fi ,.'liii,.I':,ii:iril

Northern Plate

#

Thrust Fault

Fig. l. Wcstern Alpine-Ligurian tectonic setting and geography, largely after Niggli er al. (1973). Abbreviations are as follows: G :

Geneva; L : Lausanne; DB : Dent Blanche nappe; S-L : Sesia-Lanzo realm; IV: Ivrea zone. Parts ofthe Dent Blanche and Sesia-Lanzo zones decoupled from the Southern Alpine plate during Early Alpine convergence and began descending with the Tethyan-European (Northern Alpine) plate.

is to summarize the major element and REE chem-istries and mineral parageneses of selected lherzoliticand eclogitic rocks of the Western and Ligurian Alpsalready investigated by the author, and to provide anew analysis of the operating P-Tconditions; a sec-ondary purpose involves the correlation ofthese oc-cunences to inferred Alpine plate tectonic processes.

Regional geology

Major tectonic realms of the Western Alps and Li-guria (the northernmost Appenines and the regionbordering the Ligurian Sea) are illustrated in Figure1. This strongly arcuate mountain belt consists ofthree principal domains (Triimpy, 1960, 1973, 197 5):(l) the structurally high Southern Alps-and itsbasal portion, the lvrea zone, Austroalpine nappesand Appenines bordering the Po Plain; (2) the struc-turally low continental margin of Europe, including

the European Foreland, Jura, Swiss Plain and Hel-vetic nappe system; and (3) an interleaved, compositePennine terrain. Structural entities of Southern Al-pine derivation, such as the Sesia-Lanzo zone, andthe Dent Blanche and related thrust slices, as well asmore northerly European continental basement (e.9.,the Lepontine gneisses and the internal massifs) havebeen tectonically juxtaposed and are intimately asso-ciated with rocks of the Pennine realm. The terrainadjacent to the Po Plain is referrred to as "internal"by Alpine geologists, whereas the outer portions ofthe arc, lying farther away from the Po Plain, aretermed "external."

Both European and Southern Alpine realms con-sist of Hercynian and pre-Hercynian sialic basementrocks, plus a well-ordered superjacent series of Meso-zoic and younger strata. In contrast, the Pennine ter-rain, although ssllaining Hercynian and pre-Hercy-

ERNS?1. ECLOGITES, PERIDOTITES, WESTERN LIGURIAN ALPS

High-angle Fault

ERNSN ECLOGITES, PERIDOTITES. WESTERN LIGURIAN ALPS

nian allochthonous complexes, is characterized byMesozoic continental margin and deep-sea sedimentswhich were deposited at least in part on oceanicbasement, especially in the internal portions of thiszone. Serpentinized peridotitic rocks are scatteredthroughout and adjacent to the Pennine + Sesia-Lanzo realn; high-pressure metamorphic rocks arealso best developed in these complexes. The presentsummary, therefore, focuses on portions of the Pen-nine and Sesia-Lanzo terrains.

The post-Hercynian tectonic history of the Alpsand Appenines involves several principal contrastingstages. (A) In Late Triassic time, a major riftingevent initiated the separation of European andSouthern Alpine sialic crust-capped plates. Sea-floorspreading produced a narrow inlsrvsning sea, Meso-zoic Tethys, situated between the fragmented conti-nental margins. Newly generated oceanic crust evi-dently provided the basement for more southerly andeasterly Pennine sediments of Jurassic and Cre-taceous age, accounting for the abundance of ophio-litic material in this realm. (B) During mid- and LateCretaceous ti-rre, the narrow seaway began to close asa consequence of consumption of the Tethyan platesouthward and eastward beneath the Southern Alps.Continental impaction seems to have ensued duringLate Cretaceous time along a complex, imbricate su-ture zone. Portions of the European basement weresubducted during this event, as evidenced by theirparticipation in the Pennine-style deformation andhigh-pressure metamorphic overprint. Many of theAlpine nappe structures and d€collements, whichdemonstrate that structurally higher units were trans-ported away from the Po Plain relative to underlyingunits. seem to bear witness to the underflow and lossof basement (as long ago described and pondered byAmpherer, 1906). This latest Cretaceous event istermed Early Alpine in terms of the orogenic history.The collisional zone included not onlv the Pennineophiolitic realm, but the Sesia-Lanzo ztne and lowerparts of the Dent Blanche nappe system as well. Im-bricate thrust movement apparently continued into,or was reactivated in, the Early Tertiary. (C) Thecounterclockwise rotation of the Italian Peninsulacommenced at the end of Cretaceous time in re-sponse to renewed rifting and localized sea-floorspreading between microplates in the Mediterraneanregion (Zijderveld et al.,1970). By the Early Tertiary,referred to as Late Alpine in the mountain-buildingcycle, compressional forces had given way to majorstrike-slip faulting along breaks such as the Insubricand Sestri-Voltaggio lines (Gansser, 1968; Scholle,

1970; Laubscher, 1975), probably accompanying theformation of the intense arcuate curvature of theWestern and Ligurian Alps. It should be noted thatthe sense ofvergence changes across the Sestri-Volt-aggio Line; hence rocks of the northern Appeninesconsitute the stable, nonsubducted plate and do notexhibit the blueschist metamorphic paragenesescharacteristic of that produced in western Liguriaand the Alps. Perhaps the Sestri-Voltaggio Line is alate Mesozoic structural feature which was reacti-vated under contrasting dynamic conditions duringthe more recent 70o counterclockwise rotation ofItaly (Lowrie and Alvarez, 1975).In summary, post-Hercynian plate tectonics in the western part of theAlpine chain seem to have involved first divergence,then convergence (perhaps several stages), followedby periods of rotation and conservative (transform?)motion.

Petrologic relations

The Alpine metamorphic zonation, general struc-tural features, distribution ofperidotites and both pe-ridotite and eclogite localities referred to in the textare presented in Figure 2.

Serpentinized peridotites constitute a commonrock type in the Western and Ligurian Alps. Vol-umetrically, they are most important in the westernAppenines, the southerly and easterly, more internalsectors of the Pennine terrane, associated with theSesia-Lanzo realm, and near the base of the Ivreazone. lgnoring late hydration effects, at least four dif-ferent types of ultramafic protolith appear to be rep-resented: (l) pre-Mesozoic basement ultramafic com-plexes, such as the strongly foliated and progrademetamorphosed Val Malenco body (Trommsdorffand Evans, 1972, 1974); (2) small garnet lherzolitepods with associated eclogite, confined to the south-ern Lepontine region, such as the enigmatic occur-rence at Alpe Arami (O'Hara and Mercy, 1966;Mbckel, 1969; Rost et al., 1974\; (3) completely ser-pentinized, post-Hercynian harzburgite and eclogi-tized gabbro-pillow lava complexes such as presentin the vicinity of Zermatt-Saas Fee (Bearth, 1967;Dietrich et al.,1974); and (4) pre-Mesozoic lherzoliticmasses associated with the Southern Alps, such asthe Ivrea zone bodies of Lanzo, Balmuccia, Bald-iserro and Finero (Vogt, 1962; Lensch, 1968; Nicolas,1968,1974; Boudier, 1978; Shervais, 1979). Althoughmafic rocks are associated with all these ultramafics,only type (3), which characterizes more oceanic partsof the Pennine and Appenine realms, can be consid-ered as disrupted ophiolite in the strict sense (Anony-

ERI{SN ECLOGITES, PERIDOTITES. II/ESTERN LIGURIAN ALPS

\H[h-aryle Fault

Fig. 2. Alpine age metamorphism of the Western and Ligurian Alps, largely after Niggli et al. (1973). Abbreviations are: G : Geneva;

L : Lausanne. Although the Dcnt Blanche and related nappes as wcll as the Sesia-Lanzo zone were derived from the Southern Alps,

they were subjected to the Early Alpine high-pressure metamorphism. Because of the difficulty in distinguishing between similar

metamorphic products, some of the greenschists assigned on this map to Early Alpine crystallization--+specially in the Lepontine

region-are probably of Late Alpine metamorphic age.

_4

6J

4

ag

mous, 1972). All four petrologic types seem to havereached their present structural positions throughtectonic prooesses involving largely solid-state em-placement, although incipient partial melting effectsare thought to characterize Liguria, Lanzo and Fin-ero occurrences (Bezzi and Piccardo, l97l; Boudierand Nicolas, 1972; Cawthorn, 1975). Nicolas andJackson (1972) called attention to the fact thatlherzolite tectonites are commonest in the WesternAlps, whereas the Dinarides and Turkey are charac-terized by harzburgitic peridotites; these authors as-signed the former group to subcontinental litho-sphere of the Southern Alpine plate, and the lattergroup to the Mesozoic Tethyan suboceanic plate. Inthe Gruppo di Voltri Complex of western Liguria,Southern Alpine lherzolites of the Erro-Tobbio unithave been thrust over the structurally lower Beigua

serpentinite, of presumed Tethyan affinities (Messigaand Piccardo,1974; Chiesa et aI.,1975; Mottana andBocchio, 1975).

High-pressufe metamorphic assemblages (Bearth,1959, 1962,1974; Niggli, 1960, 1970; Bocquet, l97l;Compagnoni, 1977; Compagnoni et al-, 1977) wereproduced in rocks of the more northerly terrain, es-pecially the Pennine and Sesia-Lanzo zones duringEarly Alpine descent of the Mesozoic sea floor andadjacent European foreland beneath the SouthernAlpine plate (Ernst, l97l; Hunziket,1974)- The moreinternal sections were subjected to the most profoundsubduction, hence bear relics of fts highest pressureassemblages (Ernst, 1973; Dal Piaz, l974a,b). Sub-sequent to continental collision-which evidentlytook place in stages and led to pronounced tectonicimbrication and recrystallization (Dal Piaz et al.,

Thrurt fauh

ER]VST ECLOGITES, PERIDOTITES, WESTERN LIGURIAN ALPS

l9T2Fthese sections rose buoyantly towards thesurface, probably producing the pervasive green-schist facies (prasinitic) metamorphic overprint andback-folding characteristic of the more internal por-tions of the Western Alps. Regions such as the Le-pontine gneiss tenain which evidently experienced adelayed return to the surface, were subjected to sub-stantial heating before buoyant, plastic, diapiric up-ward motion was initiated; this more complicatedthermal history may aocount for the obliteration ofthe hypothesized Early Alpine high-pressure phasecompatibilities, the production of high-rank am-phibolites and the incipient melting of quartzo-feldspathic units during the Late Alpine metamor-phic culmination in the Lepontine zone (Wenk, 1962,1970; Wenk and Keller, 1969; Jiiger et al.,1967).

Upper mantle fragments in the Western Alps andLiguria

P erido tite p et ro grap hy

Mineralogic and bulk-rock chemical data havebeen obtained by the author and his coworkers(Ernst, 1978; Ernst and Piccardo, 1979; Ottonello etal., 1979) for five Western Alpine peridotites-AlpeArami, Finero, Balnuccia, Baldissero and Lanzo-for the structurally high Eno-Tobbio mass of west-ern Liguria, and for single samples of ultramaficsfrom eastern Liguria (northern Appenines)--MonteAiona, Monte Nero and Suvero. With the exgeptionof Alpe Arami, which is a garnet peridotite, all inves-tigated masses are spinel lherzoliteq evidently derivedfrom the South Alpine lithospheric plate. Incipientfusion phenomena and/or partial recrystallization toplagioclase peridotites on decompression are wide-spread atLanzo,less so at Finero and in the Ligurianspecimens. Average modes are summarized in Tablel. The investigated peridotite suites contain essential

clinopyroxene and are appropriately classified aslherzolites (Streckeisen, 1974, 1976). All have pene-trative (tectonite) fabrics and show evidence of plas-tic defonnation (e.g., sectoral strain in olivines, bentlamellae in pyroxenes, minel granulation and grada-tional extinction among primary phases). Some ofthe crystallographic orientation may reflect differen-tial flow within the upper mantle, whereas late, su-perimposed strain effects probably were producedduring tectonic emplacement in the present continen-tal setting (e.9., see Nicolas et al., l97l).

The Erro-Tobbio and eastern Ligurian rocks con-tain greater proportions of late hydration products,but otherwise appear to be mineralogically similar tothe other spinel lherzolites examined. The garnet-bearing peridotite of Alpe Arami is distinctly lowerin orthopyroxene proportions, and carries sub-stantially more late-stage calcic amphibole, but isotherwise quite comparable to the other studied sam-ples.

P erido t it e bul k-r o c k chemist ry

Major elements. Bulk-rock XRF analyses for 35lherzolites have been published previously by the au-thor. Averages for each occurrence are presented inTable 2. Analyses appear to be remarkably similarfrom one sample to another, and the bulk-rockchemistr ies of these rocks are sensibly in-distinguishable. Individual analyses as well as meansare quite comparable to data presented by otherworkers (O'Hara and Mercy, 1966; Nicolas, 1966;Lensch, l97l; Rost et al.,1974; Boudier, 1976,1978)for these same localities. The lherzottes studied cor-respond well to estimates of relatively primitivemantle composition proposed by Wyllie (1970) andMaal{e and Aoki (1977) based on natural oocur-rences, but are slightly depleted i-n CaO and Al'O,

Table l. Average estimated modes and one standard deviation for petrographically investigated lherzolites from Liguria and the WesternAlps (original data from: Ernst, 1978; Ernst and Piccardo, 1979).

Local i ty -

N o . o f S a n p l e s OI iv ineO r t h o - C l l n o -

Pyroxene Pyroxene

S p lnel( I n c l . P l a g i o -

Garnet opaques) claseCa- Phlogo- A-l terat{on

anphibole pi te Produits*

A l p e A r a m i - 1 4 4 7 . O ! I 4 . 4

F i n e r o - 7

Balnuccia - 18

1 3 . 0 r 5 . 9 1 6 . I r 1 1 , 0

2 3 . 6 ! 6 . 3 9 . 9 ! 7 . 8

2 2 . 8 ! 8 . L 1 6 . 5 1 1 0 . 8

2 4 . 9 ! 8 . 8 1 6 . 9 1 1 1 , 4

2 6 . I ! 6 . 7 1 6 . 9 i 9 . 9

2 3 . 4 ! 8 . I 1 3 . 3 1 5 . 0

2 8 . 0 1 6 . 1 8 . 3 ! 2 . 9

- 4 . 4 1 0 . 8

- 5 . 1 1 0 . 9

5 . 6 1 5 . 8 2 . 3 ! 2 . 7 0 . 5 1 1 . 9 6 . 9 r 8 . 3 8 . 6 ! 7 . 0

5 5 . 0 1 9 , 1

52.9 ! r4 . r 0 . 2 ! o . 4 2 . 4 ! 5 . 9

4 . 4 ! 7 . 2

1 2 . t ! L 2 . 2

r . 0 1 1 . 9 2 . 7 ! 5 . 6 3 . 4 ! 4 . 5

Ba ld lssero - 20 48 .3=14.4

La\zo - 27

- 4 . 4 ! t . 6 0 . 1 1 0 . 4 1 . 0 1 2 . 8

- 2 . 4 ! 0 . 4 2 . 6 ! 5 . 5 1 . I 1 2 , 5

- 3 . 5 1 r . 1

- 5 . 0 1 1 . 0

3 8 , 8 r 1 4 . 2

E r r o - T o b b i o - 3 3 5 . 9 i 1 2 , 1

Eastern L igur ia - 3 44 .0 ! 6 .6

2 3 . 9 1 1 0 . 0

t 4 . 7 ! 7 . 2

*chlo"i Le, serpentine ninenals, tall

48 ERNST: ECLOGIT:ES, PERIDOTITES, WESTERN LIGURIAN ALPS

Table 2. Average whole-rock XRF compositions and one standard deviation for chemically analyzed lherzolites from Liguria and the

Western Alps (original data from: Ernst, 1978; Ernst and Piccardo, 1979).

L o c a l t t y -

No o f Samples s i02 4 1 . O , Cr203 Fe2O3" N i 0 N a 2 o KzoI g n i t i o n

L o s s

Alpe Aran i - 6

F i n e r o - 2

Ba lnucc ia - 7

B a l d i s s e r o - 6

Lanzo - 3

Er ro Tobb io - 8

E a s t e r n L i g u r l a - 3

4 4 4 5 1 1 3 9 I 5 0 1 0 2 7 0 1 5 1 0 0 7

4 2 t 6 ! 1 2 0 0 6 5 1 0 5 6 0 0 5 1 0 0 5

4 7 t 4 ! I 5 8 I 4 9 i 0 1 6 0 1 4 1 0 0 4

47 66!1 94 | 62!0 44 0 1610 07

4 7 t 7 ! 2 5 9 2 1 3 ! 1 5 3 0 2 9 1 0 r 2

4 1 4 3 ! 0 7 1 2 6 5 1 0 4 1 0 1 3 1 0 0 3

4 2 3 8 1 0 3 4 2 9 0 1 1 l 0 0 2 2 ! O 0 2

0 3 8 1 0 0 5 8 2 3 1 0 1 0

o 2 4 ! 0 2 t 8 8 4 1 0 6 0

0 3 6 1 0 0 4 8 0 1 1 0 4 8

0 4 0 1 0 0 9 7 8 2 1 1 0 3

0 1 1 1 0 l 0 7 3 7 ! t 2 0

0 3 4 1 0 0 3 8 0 6 1 0 3 2

0 3 0 1 0 0 8 7 8 8 1 0 . 2 5

0 2 5 1 0 0 2 3 9 8 4 1 1 2 3

0 3 3 1 0 . 0 4 4 6 5 4 ! 2 4 5

0 2 3 1 0 0 l 3 9 2 3 1 1 . 8 5

0 2 3 J 0 0 2 3 8 5 8 1 2 8 7

0 2 1 1 0 , 0 4 3 6 0 9 1 4 4 3

0 - 2 3 ! O 0 2 3 7 6 9 1 0 . 9 0

0 2 0 1 0 0 2 3 7 9 9 1 0 8 6

0 , 1 2 J 0 0 0 2 l 8 ! 0 5 7

0 1 3 1 0 0 1 0 . 5 1 1 0 . 5 4

c . 1 2 1 0 0 t 2 9 2 ! 0 8 4

0 1 2 1 0 0 1 2 9 8 ! l 2 8

0 1 2 1 0 0 1 3 8 1 ! 2 1 1

0 . 1 2 1 0 . 0 0 2 0 6 ! 0 . 4 2

0 1 2 1 0 0 0 2 4 5 ! 0 2 8

0 . 4 9 1 0 1 7 0 0 5 1 0 0 5 2 6 l ! 0 8 4

0 . 3 8 J 0 . 0 0 0 2 4 ! 0 3 3 0 0 7 1 0 0 8

nd 0 0010 00 0 44!0 70

n d 0 0 0 i 0 0 0 0 6 1 1 0 4 6

0 1 5 1 0 . 0 0 0 0 0 1 0 0 0 1 7 4 ! 0 . 9 9

0 0 8 i 0 0 9 0 , 0 1 1 0 0 1 6 , 9 1 ! L 2 5

0 1 8 1 0 0 5 0 0 5 1 0 0 8 5 6 1 1 1 0 2

tota'L iran as F?ZOS; nd = not deterfrined

compared to the model pyrolite of Ringwood (1975,Table 5-2).

The high ignition loss for the Ligurian samples(Erro-Tobbio, Monte Aiona, Monte Nero and Su-vero) reflects extensive late-stage serpentinization; incontrast, most of the Western Alpine samples containonly small amounts of amphibole, phlogopite and/orserpentine. The Finero peridotite appears to be espe-cially low in CaO, AlrO, and TiOr, and rich in MgO,NiO and KrO. The rest are chemically rather homo-geneous.

Rare earth elements. Bulk-rock radiochemical neu-tron activation analyses of rare earth elements (REE)for some Ligurian periodotites---+ight Erro-Tobbiolherzolites and a single sample from Monte Aiona-were published by OttonelTo et al. (1979). Prelimi-nary REE contents of 7 specimens from Balmucciaand 6 from Baldissero have been obtained more re-cently (Ottonello and Ernst, unpublished). The chon-drite-normalized rute earth element patterns are pre-sented in Figure 3 for all 22 rocks. These peridotitesexhibit light rare earth element (LREE) depletionrelative to chondrites.

Assuming an initial chondrite-like pattern for theLigurian specimens, Ottonello et al. (1979) ascribedthe LREE depletion to the combined effects of lessthan five percent partial fusion of the original mantlematerial, (spinel lherzolite) and of hydrous alterationunder oxiding conditions. lnasmuch as the Erro-Tobbio peridotites contain an average of nearly 25volume percent alteration products as indicated inTable l, such conclusions seem reasonable. However,analyzed Balmuccia and Baldissero peridotites con-tain only very minor serpentine and other secondaryphases, yet display light REE depletions analogous topatterns for the Ligurian rocks (compare Figs. 3A, Band C). Accordingly, if the low LREE concentrationsat Balnuccia and Baldissero reflect leaching whichattended an hypothesized near-surface hydration, thealteration minerals so produced must have been sub-sequently obliterated later during high-temperature

recrystallization and annealing, because volatile-bearing phases are rare in these "fresh" samples.

The Western Alpine lherzolites in general containslightly greater abundances of each REE, so theirdistribution patterns are shifted up compared to theinvestigated Ligurian samples. Higher concentrationsof REE in many of the Balmuccia and (especially)the Baldissero peridotite samples may reflect eitherlower degrees of incipient fusion, or the existence ofan initial protolith enriched in rare earth elementsrelative to the Erro-Tobbio protolith. In general,higher CaO samples tend to possess higher modalproportions of clinopyroxene, and greater concentra-tions of REE as well; such peridotites may be re-garded as more nearly primitive.

An alternative hypothesis to explain the LREE de-pletion and the slight heavy rare earth element(HREE) enrichment relative to chondrites displayedby all the analyzed spinel peridotite samples involvesthe incipient melting of an hypothesized garnetlherzolite precursor. Distribution coefficients (thefractionation of elements between crystal and meltphases) are appropriate to realtze this effect (e.9., seeHanson, 1980, Table 2), but no mineralogic or tex-tural relics are present which would support the priorexistence of a garnet-bearing protolith. Bulk-rockREE analyses of garnet peridotites from Alpe Armainow underway may shed light on this problem, butas discussed below, this body appears to be very dif-ferent from all other lherzolites investigated in thisstudy.

Yet a third explanation is that upper mantle mate-rial in general-and in the western Mediterraneanarea in particular-was slightly fractionated relativeto chondrites at an earlier stage in its history. Hence,prior to the crystallization event recorded in the in-vestigated peridotites, the mantle protolith wouldhave possessed a LREE depletion and a slight HREEenrichment relative to the primordial earth. Suchconclusions are suggested by examination of rareearth element patterns from similar lherzolitic masses

ERt{STj ECLOGITES, PERIDOTITES, WESTERN LIGURIAN ALPS 449

0.01[LSm Eu Gd Tb Er Tm Yb [u

or 1.0.=tB(J

J

IE

I

=E 0.1

o, l'oL

o(.)

JIo

:E o.r

Er Tm Yb Lu

q, l'O

L

o(J

J(J

oI

J

co 0.1

Nd Sm Eu Gd Tb Er Tm Yb Lu

Fig. 3. Chondrite-normalized rare earth element concentrations in lherzolitic samples from Liguria and the Western Alps. Data arefrom: (A) Ottonello el al., 1979, modified; (B) and (C) Ottonello and Ernst, unpublished and preliminary. Analyzed Ligurian specimensare from the Erro-Tobbio unit except for one sample from Monte Aiona (triangles and dashed lines).

0.01;

(A) LTGURTA

ERffSZ. ECLOGITES, PERIDOTITES, WESTERN LIGURIAN ALPS

OLIVINES

8.5 9.0 9.5 ro.0 r0.5100 Fd*/(l\49+ Fe2*) in atomic ProPortions

Fig. 4. Mole proportions of Fa in olivines from Western Alpine and Ligurian peridotites. Filled triangles indicate analysesfrom eastern Liguria, filled circles are ol from the Erro-Tobbio unit. Data from: Ernst (1978); Ernst and Piccardo (1979).

(e.g., see: Frey, 1969, 1970; Menzies, 1976; Menzies etal.,1977).

P e rido tit e mineral c hemistry

Electron microprobe analyses of coexisting phasesfrom 37 peridotites have been published previouslyby the author. Analyzed minerals include olivhe, or-thopyroxene, clinopyroxene, garnet, spinel, plagio-clase and calcic amphibole. The data are comparableto analyses presented by O'Hara and Mercy (1963),Nicolas (1966), Lensch (1971, 1975), Lensch andRost (1972), Boudier (1972\, Boudier and Nicolas(1972), Rost et al. (1974, Cawthorn (1975), Boudier(1979) and Shervais (1979).

Similar to bulk rock compositions, the mineralchemistries vary little among the analyzed samples.Olivines range from Fa.r-,,, with the following aver-ages: Finero, 8.6; Alpe Arami,9.8; Baldissero and Li-guria, 9.9; Balamuccia, 10.0; and Latzo,l0.3. Olivineconpositions are illustrated in Figure 4. Manganeseand calcium contents are small but measurable. As

evident from Figure 5, orthopyroxenes are slightlymore magnesian than the coexisting olivines (exceptfor a few pairs from eastern Liguria), and show simi-lar Fel(Fe+Mg) trends. Alurrina contents of ortho-pyroxenes (Fig. 5) are also relatively consistent, withsignificant contrasts in measured average weight per-cents of AlrOr: Alpe Arami, 0.87; Finero, 1.08;Lanzo,3.l0; Balmuccia, 3.80; Liguria, 4.ll; andBaldissero, 4.57. Calcic pyroxenes, although chem-ically more complex than orthopyroxene, never-theless exhibit analogous systematic compositionaltrends, being a trifle more magnesian than the associ-ated orthopyroxenes, and, ofcourse, are considerablyenriched in cations such as Na and especially Ca.Average ternary compositions are: Alpe Arami,Woo, En o, Fsrr; Finero, WoorrEno"rFsr.; Batmuc-cia, Woo, ,En.".rFso ,; Baldissero, Woor rEn " rFs, niLanzo, Woo, .Eno, oFsr.o; and Liguria, Woo, oEnr' Fso.n.Calcic pyroxene compositions are shown in Figure 6.Probably one of the most remarkable aspects of thesemineral data is their internal consistency. Although

LIGURIA OA O OOA O

IANZO X x x xx

BALDISSERO O O OO

BALMUCCIA + +}F +

A I\ FINERO

ALPEARAMI tr tr tr tr EIII

ER]TSZ. ECLOGITES, PERIDOTITES, WESTERN LIGURIAN ALPS 451

0.t4

0.r2atl

q)ooXo.xraLq)o-z

9.08.58.07.5 r0.59.5 r0.0 il.0

IOO Fe'*/(Mg + Fe2*)in atomic proPortions

Fig. 5. Computed octahedrally and tefiahedrally coordinated aluminum (A), and mole proportions ofFs (B) for orthopyroxenes fromWestern Alpine and Ligurian peridotites. Filled triangles indicate analyses from eastern Liguria, filled circles are orthopyroxene fromthe Erro-Tobbio unit. Data from: Ernst (1978); Ernst and Piccardo (1979).

ORTHOPYROXENES

0.10

0.08

0.06

0.04

0.02

0.c01 | | | I | |0.00 0.02 0.04 0.06 0.08 0.10 0.t2

Alvr per six oxygens

well within the analytical uncertainties, small butsystematic chemical differences exist among the ma-jor phases from the different occurrences.

For the investigated samples, garnet is restricted tothe Alpe Arami body. The composition of this phaseis quite uniform, averaging 7l percent pyrope endmember. Among the analyzed spinels, at least twodifferent types are present-a primary translucentbrown Mg + Al-rich variety which averages about 70percent MgAlrOo end member, and a secondaryopaque phase rich in chromium and iron whichprobably was produced during alteration and recrys-tallization. A few plagioclase grains have been ana-lyzed from Liguria (k-) and Lanzo (An,r-r.). Sev-eral different types of pn4ary(?) calcic amphiboleoccur in various Western Alpine peridotites (but notin either Lanzo or Liguria). Titaniferous hornblendesfrom Balmuccia are very low in silicon and corre-spondingly rich in aluminum and sodium; they ap-pear to be primary upper mantle phases. Finerohornblendes probably formed during reequilibrationof the peridotite under basal crustaI P-T conditionsin the presence of an aqueous fluid. Alpe Arami am-phiboles include an analogous aluminous horn-blende, overgrown and partly replaced by actinolite

which crytallized subsequent to the emplacement ofthe ultramafic body in the present sialic environment.

P-T conditions of peridotite crystallization

The generalized phase relations for aluminouslherzolites under physical conditions extant in theupper mantle and crust are well known. A diagram-matic petrogenetic grid for such lithologies, largelyafter Wyllie (1970) is presented as Figure 7. Due tosolid solution among the various minerals, "uni-variant" curves illustrated in this diagram which sep-arate the fields for plagioclase-, spinel- and garnet-peridotite are actually zones of finite P-I width. Arecent study by Jenkins and Newton (1979) suggeststhat the transition between garnet- and spinel-bear-ing lherzolitic assemblages may lie at lower pressuresthan illustrated here, but the topology is unchanged.

Judging from Figure 7, garnet lherzolites form atdepths exceeding about 60-80 km, with plagioclase-bearing equivalents confined to depths shallowerthan approximately 25-35 km. Most of the peri-dotites studied in the present investigation containspinel as the chief Al-rich phase, hence must havebeen derived from mantle depths on the order of50+20 km. Those which contain plagioclase replac-

B

LIGURIA - o-o o .a a

LANZO x xx x x(

BALDISSERO o cEcl @

BALMUCCIA + {+ ++

A A, FINERO

tr tr EIt tr gALPEARAMI

o

t , *& oD

* x I

+ A

ooo

oxfu*

a

trA A

452 ERNSZ. ECLOGITES, PERIDOTITES, WEST:ERN LIGURIAN ALPS

CLINOPYROXENES

tt,

a)ooXoXatl

a)o-

0.20

0.16

0.r2

0.08

0.04

0.000.00 0.08

)er six

468

0.r6ns

42

0.200.12

\Fe

Fig. 6. Computed octahedrally and tetrahedrally coordinated aluminum (A), and Ca/ME/Fe proportions (B) for clinopyroxenes fromWestern Alpine and Ligurian peridotites. Filled triangles indicate analyses from eastern Liguria, filled circles are clinopyroxene from theErro-Tobbio unit. Data from: Ernsr (1978); Ernst and Piccardo (1979).

ing and surrounding spinel, such as Lanzo and Ligu- cipient partial fusion during their ascent towards therian units, have recrystalhzed during decompression. surface. The Finero mass contains large tracts ofThese bodies contain locally-derived gabbroic'dikes coarsely crystallized hornblende gabbro, and seemsand stringers, and appear to have undergone in- to have undergone extensive reequilibration in the

,0246M6 Atomic proportions

A

A tr ALPE ARAMI

Alvl per six'oxygens

Ca/ os

ERNSI\. ECLOGITES, PERIDOTITES, WESTERN LIGURIAN ALPS

40

30

20

n

r000 r500 2000Temperature in oC

Fig. 7. Phase equilibria for pyrolite compositions (Wyllie, 1970). The dry solidus is after Kushiro et al. (1968), the wet solidus from

Mysen and Boettcher (1975a). Subsolidus assemblages are generalized after Green and Ringwood (1967a,b), Ito and Kennedy (1967)'

O'Hara (1967, 1968) and MacGregor (1968). For general reference, steady-state intraplate geothermal gradients (Clark and Ringwood,

1964) are also illustrated. Estimated P- Z conditions of equilibration for the analyzed lherzolites are shown as follows: A : Alpe Arami;

B : Balmuccia and Baldissero; ET : Erro-Tobio (western Liguria); F : Finero; L : Lar:.l:o and eastern Liguria' Arrows show inferred

P-Ztrajectories as thc studied peridotites partly reequilibrated during transit towards their present crustal environments.

presence of an aqueous fluid under deep crustal con- mochemical determinations of chemical variations

ditiottr. The Alpe Arami body, on the other hand, for the constituent phases as a function of ?i P and

contains pyropic garnet and evidently was derived composition. Usually, the partitioning of com-

from a more profound level in the mantle (it also car- ponents, such as Ca and Mg between coexisting or-

ries plagioclase, but as a subsolidus symplectic reac- thopyroxene and clinopyroxene, Fe and Mg between

tion product formed during later metamorphism ac- clinopyroxene or olivine and garnet, or between

companying emplacement in the sialic Lepontine spinel + olivine pairs, and the solubility of AlrO, in

terrain). Thus all the masses reported on here may be orthopyroxene are used. The theory of element frac-

assigned provisional P- T sites of origin, and their de- tionation has been treated by numerous authors and

compression recrystallization paths roughly deline- will not be repeated here (for a recent review, see

ated from consideration of the framework of Figure Saxena, 1973).7. tmplicit in all such studies, in which geother-

More quantitative evaluations of the operating mometers and geobarometers are applied to naturalphysical conditions attending formation of the perid- rocks, is the assumption that chemical equitbriumotites depend on accurate experimental or ther- has been attained for the analyzed phases under a

453

-o>ze-

a ;

LrlJatlat,a)t-a-

ERNS11 ECLOGITES, PERIDOT:ITES, WESTERN LIGURIAN ALPS

specific set of conditions, and that subsequent trans-portation of the material to the present site of ex-posure has not disturbed the partitioning. If in factpartial reequilibration has been imposed on the ana-lyzed assemblage, a smearing out of apparent P and7 values will be obtained at best. whereas in themore general case, the derived set of physical condi-tions may be practically meaningless due to non-equilbrium chemical readjustment of the minerals.With these provisos in mind, let us consider the ana-lytical data for coexisting phases in the investigatedperidotites, and the nominal physical conditions soindicated. A program devised by A. A. Finnerty ofthe Jet Propulsion Laboratory allowed machinetreatment of the microprobe data (Ernst, 1978; Ernstand Piccardo, 1979) to provide applicable geother-mometers and geobarometers. For the purpose ofcalculation, all iron was treated as ferrous in olivine,garnet and orthopyroxene, as half ferric in clinopy-roxene, and the Fe2*/Fe3* ratio in spinels was ob-tained by recasting cations to 3.00 assuming no anionomissions. These assumptions result in sensible cat-ion proportions for the analyzed mineralogic sam-ples, as discussed previously by Ernst and Piccardo.The presence of minor amounts of additional com-ponents in natural phases required that correctionsbe applied to allow use of the synthetic experimentaldata (Wood,1974; Wells, 1977).

Alpe Arami is the most confidently addressed ofthe lherzolite localities, for many studies have dealtwith elerrent partitioning in garnet peridotites, nu-merous relevant mineralogic geothermometers areavailable and the pressure effect on alumina concen-tration in orthopyroxene is known accurately, henceestimates of operating P and ? are readily obtained.Moreover, as will be described later. conditions oforigin for the associated eclogitic lenses at AlpeArami have also been quantified.

Apparent conditions of equilibration for six AlpeArami garnet peridotites are listed in Table 3. Thenew computer-generated values presented are basedon AlrOle. isopleths as determined by MacGregor(1974). P-Z values were obtained utilizing the fol-lowing geothermometers: orthopyroxene, Mercier(1976); orthopyroxene * clinopyroxene pairs, Woodand Banno (1973) and Wells (1977); garnet + olivinepairs, O'Neill and Wood (1979) and Kawasaki(1979). As evident from the table, the Mercier andWells methods yield slightly lower temperatures andpressures (-900oC,37 kbar) than the formulations ofWood and Banno, O'Neill and Wood, and Kawasaki(-1000"C, zl4 kbar). Summation of all the P-Z esti-

mates of Table 3 provides a grand average of9491-75"C,40.9+5.7 kbar, considered by the presentwriter to be the "best" assignment. These values arenearly identical to a previous grand average (Ernst,1978) of 966t78"C obtained using the Wood andBanno technique plus several different geother-mometers (Mysen and Boettcher, l975a,b; Mysen,1976; and, Lindsley and Dixon, 1976), and an attend-ing pressure of about 4344kbar employing both thedata of MacGregor (1974) and Akella (1976). Theyare slightly higher than P-Testimates determined forAlpe Arami lherzolites by Carswell and Gibb (1980),but lower than thermal equilibration values pre-sented by Mori and Green (1978). As will be dis-cussed below, these values for physical conditions arecompatible with terrperatures obtained by assuminga lithostatic pressure of 40 kbar for garnet + clinopy-roxene pairs in both lherzolite and associated eclo-gitic lenses employing yet another method (Ellis andGreen, 1979). However, Evans and Trommsdorf(1978) and O'Neill and Wood (1979) obtained lessextreme conditions of crystallization, on the order of25 kbar and 800"C, for two samples of garnet perido-tite associated with eclogite nearby (as well as anAlpe Arami sample).

Disparites in apparent physical conditions ofequilibration probably reflect differential chemicalreadjustment of the coexisting minerals duringdepressurization and cooling. Possibly Fe-Mg ex-change between olivine and garnet diminishedsharply or actually ceased at a higher temperaturethan characteristic of the Ca-Mg fractionation be-tween coexisting pyroxenes. Shervais (1979) hasdemonstrated that at Balmuccia, clinopyroxene + or-thopyroxene pairs continued to reequilibrate at lowertemperatures than did spinel + olivine + pyroxeneassemblages.

Alumina isopleths for orthopyroxenes are nearlyindependent of pressure in the spinel lherzolite field,and are as yet poorly constrained in the plagioclaselherzolite field (PresnalT, 1976; Obata, 1976, 1980;Danckwerth and Newton, 1978), thus only the tem-peratures of equilibration may be obtained with rea-sonable accuracy for the other Western Alpine andLigurian peridotites studied in this report. For thisreason, only geothermometers whose formulation isindependent ofpressure can be used. Average valuesare presented in Table 4, where they are comparedwith earlier estimates for the same samples by Ernst(1978) and Ernst and Piccardo (1979).

As in the case of the Alpe Arami complex, thereseems to be no obvious indication as to which of the

ERilST: ECLOGITES, PERIDATITES, WESTERN LIGUNAN ALPS 455

Table 3. Apparent temperatures and pressures of equilibration for analyzed Alpe Arami garnet lherzolite samples employing Al2O3

solubilities in orthopyroxene after MacGregor (1974).

Reference (11

Sanp le No. T(oC) P(kb)

( 2 ) ( 3 )

r ( " c ) P (kb ) r ( ' c ) P ( kb )

( 4 )

r ( oc ) P (kb ) r ( ' c ) P(kb)

( 5 )

A-2at - r o a! - I O D

F-l6ct - ) z c! - )o

Average 3-6

907 41 .09 7 0 3 8 . 0867 36 .8869 29 .O9 1 1 3 8 . 9924 38 .6

908138 37 .L !4 .2

854 34.2 9808s0 34 .4 973

957 43.2 10648 7 1 3 6 . 8 9 8 9

gsg a i .o1130 52_ .6

985 44 .4

LO25L92 46.0!6.0

915 38.2r .054 o: . '

944 41,7

975 !79 42 .7 !5 .1

4 2 . 44 2 . 4

5 0 . 4

883 t50 37 .2L4 .2 1001142 45 .013 .8

(L) opr: Mez,ctez' (i976)(2) op& + cpt : WeLLs (1977)(3) opx + cpx: Ilood anl Banno (1973)(4) gar + oL: ?tNei l t and Vood (1979)(5) gar + oL: Y'ouaeaki (1979)

methods employed provides more accurate results.Apparent temperatures of Ca-Mg equilibration be-tween orthopyroxene and clinopyroxene are approxi-mately as follows: Finero, 900'C; Balmuccia andBaldissero, 925"C; Lanzo and eastern Liguria,1000"C; and western Liguria, ll00'C. Attendingpressure must have been on the order of 7-20 kbar,with the hotter Ligurian and Lanzo masses under-going partial melting to produce the observed maficdikes and plagioclase-bearing lherzolites during as-cent towards the surface. Hypothesized P-I pathsare illustrated in Figure 7. Similar histories havebeen documented recently for the Ronda peridotiteby Obata (1980). Study of Figure 7 yields several im-portant tentative conclusions.

(l) The Alpe Arani garnet lherzolite contains aprimary assemblage indicative of very high pressuresand relatively modest temperatures. Such values areappropriate to deep portions of a thick subcon-tinental lithosphere, where geothermal gradients typ-ically are lower than those capped by continentalmargins and island arcs. Estimated physical condi-tions are si-milar to those obtained for kimberlites(e.9., see Boyd, 1973). Another, perhaps more reason-able, site of origin for the Alpe Arami peridotite andassociated eclogite involves formation within the de-scending lithospheric slab of a subduction zonewhere relatively elevated P/T ratios would have pre-vailed (Toksdz et al., l97l; Turcotte and Oxburgh,1972). Both the very high pressures and temperaturesestimated in the present work, and the more moder-ate values proposed by Evans and Trommsdorff(1978) are several hundred degrees and several tensof kbar in excess of the P-T conditions attendingmetamorphic culmination in the surrounding Lepon-

tine quartzo-feldspathic gneisses. The latter havebeen estimated (Ernst, 1977) as approximately 600-650'C and 6-'l kbar, at a moderately high activity ofHrO; in corroboration, Hoernes and Friedrichsen(1980) obtained a maximum temperature in the Le-pontine gneiss region of 650oC employing oxygenisotopes. The presence of plagioclase rather than ja-deitic pyroxene in these country rocks limits the at-tending pressures to a maximum of l0-ll kbar(Newton and Smith, 1967; Green and Ringwood,r967b).

(2) Provided the temperatures of equilibration areaccurate, the hornblende-bearing Finero complexmust have recrystallized at pressures on the order of10 kbar near the base of the crust under high activi-ties of HrO (Lensch, l97l; Nicolas et al., l97l; Caw-thorn, 1975). As evident from Table 4, estimatedtemperatures of crystallization employing the Wells(1977) method would seem to be subsolidus no mat-ter how high the d",o, unless a total fluid pressureconsiderably exceeding l0 kbar is assumed.

Table 4. Average apparent temperaturcs of equilibration foranalyzed spinel lherzolite samples of the Westem and Ligurian

Alps employing pressure-independent geothermometers.

Refereoce

Loca l lcy - No. o f Sa i lp les

( 4 )( 2 )(r)

Fhero - 2

Ba lNcc la - 5

Ba ld isseEo _ 5-7

Lanzo - b

Er ro-Tobb lo - I

Eas tern L igur ia - 3

8 5 8 r 5 8 ' c a 8 3 l 5 2 o c 8 9 3 1 9 4 " c

a32!44'c 948l4ooc 973150oc

828137'c 946!33"c Loo2l37"c

918151oc ro22!44"c lo69t85'c

1o12l6r 'c uoo152"c - l15ot5ooc

8911500C 997!37'C - r0oot500c

( 1 ) w e L L B ( 1 9 7 ? )(2) ltood and Bmo (1973)(3) Emet (1978)(4) Enst @td Pi'cc@do (1979)

456 ERITSZ. ECLOGITES, PERIDOTITES, WESTERN LIGURIAN ALPS

ALPE ARAMI

Fig. 8. Schematic paragenetic sequence developed in mafic lenses associated with garnet lherzolite at Alpe Arami (Ernst, 1977). Earlystages are shown on the left. Solid line indicates essential presence ofa phase, dashed line symbolizes minor or sporadic presence.

(3) Baldissero and Balmuccia lherzolitic massescontain traces of a Ti-rich hornblende, indi@fing atleast a low to moderate aH,o, but show little or notransformation to plagioclase-bearing rocks. Appar-ently their transportation towards the surface did notresult in back-reaction, nor did wholesale partial fu-sion ensue. Pressures on the order of 15 kbar, wellwithin the spinel peridotite field, seem to be in-dicated.

(a) The occurrences of Lanzo and Liguira, in-dicated as circles in Figure 7, exhibit incomplete con-version to plagioclase-bearing assemblages and moreextensive fractional melting phenomena, hence theirP-T trajectories must have intersected the solidusduring decompression. Therefore, although thehigher temperature western Ligurian lherzolitesprobably equilibrated at lithostatic pressures ap-proaching 15 kbar, the lower temperature eastern Li-gurian and Lanzo masses must have intersected thesolidus at pressures equal to or less than 12 kbar.None of these peridotites contain primary horn-blende, although late-stage, colorless amphibole oc-curs at Lanzo; therefore, these rocks probably recrys-tallized under conditions involving o",o insufficient

to allow the formation of hornblende in the mantle,but high enough to permit incipient fusion. Of thetwo geothermometers employed in Table 4, apparenttemperatures by the Wood and Banno (1973) compu-tation are more compatible with the inferred condi-tions of origin.

(5) With the possible exception of the Finero com-plex, all the peridotites studied in this report recrys-talhzed at relatively shallow upper mantle depthscharacteiaed by high temperatures, subsequentlywere transported to their present locale and were tec-tonically juxtaposed against crustal rocks under con-ditions of much lower P and Z.

(6) Employing Al,O, isopleths for orthopyroxeneas presented by Obata (1976), Danckwerth and New-ton (1978) and Lane and Ganguly (1980) and the as-signments of pressures in the range 10-15 kbar asdiscussed above in points 2-4, approximate temper-atures of equilibration for the investigated peridotitescontaining MgAlrOo-rich spinel may be obtainedwhich are independent of the Ca/Mg partitionmethod. Although values for Balmuccia and Bal-dissero bodies are roughly compatible with geo-thermometric data based on Ca/Mg partitioning

ECLOGITIC AMPHIBOLITIC GREENSCHISTIC

GarnetClinopyroxeneCa-amphiboleBiotiteEpidoteKyanite

PlagioclaseQuartzSymplectite

RutileOpaques

Pargasitic

t Chlorite

I I I I I I I I

I I I I - -

Oligoclase

I - I I I I I I

I I T I I - I I

ERNSN ECLOGITES, PERIDOTITES, WESTERN LIGURIAN ALPS 457

listed in Table 4, nominal crystallization temper-atures averaging 100 to more than 200oC lower thanTable 4 values are indicated for Finero, Lanzo andLiguria. In part, this reflects small amounts of ironand chromium in the MgAlrO*-rich spinels, butchiefly it may be a consequence of the fact that thedescribed partial melting in these latter masses dur-ing decompression has promoted reequilibration ofthe pyroxenes at shallower depths. Although Ca/Mefractionation would be virtually unaffected by thenearly isothermal pressure drop, alumina content oforthopyroxene would be expected to decrease sub-stantially due to the steep negative dP/d.T slope forAlrO:P- calculated for the plagioclase lherzolite fieldby Obata (19'76). The alumina contents of these or-thopyroxenes are therefore probably more appropri-ate for the plagioclase lherzotte field than the spineltherzolite field.

High-pressure metamorphic rocks in the WesternAlps and Liguria

Eclogile petrography

Mineralogic and bulk-rock chemical data havebeen published by the author and his coworkers(Ernst, 1976,1977; Cortesogno et al,1977; Ernst andDal Piaz, 1978; Dal Piaz and Ernst, 1978) for eclo-gites from the Alpe Arami and the Zermatt-Saas Feeareas of the Western Alps and from the structurallylow Beigua serpentinite of western Liguria.

At Alpe Arami, ecologitic lenses occur as mafic sel-vages along the margins of the garnet lherzolite de-scribed in previous sections; although discontinuous,the eclogitic layers tend to separate the peridotitefrom the surrounding Lepontine gneiss (Mbckel,1969). The contact between eclogite + peridotite andencompassing quartzofeldspathic gneiss is myloniticaccording to Buiskool Toxopeus (1976). Similar tothe associated ultramafic body which shows the ef-fects of chloritization of original garnet and growthof Ca-amphiboles at the expense of clinopyroxene(see Table l), the primary eclogitic assemblage ex-hibits extensive conversion to later stage symplecticamphibolite and lower-grade assemblages. Early-formed garnet and omphacitic pyroxene + rutilehave been partly replaced by hornblende, kyanite,clinozisite and plagioclase + biotite + chlorite. Aschematic paragenetic diagram is presented as Figure8.

Eclogitic metabasaltic and metagabbroic rocks inthe Zermatt-Saas Fee (Breuil-St. Jacques) area nearthe Matterhorn have developed in a typical ophiolitic

suite representing the inner, more oceanic portion ofthe Pennine realn (Bearth, 1959, 1974;Dal Piaz andErnst, 1978). Similar assemblages have been pro-duced in mafic layers within the Beigua serpentiniteof the Gruppo di Voltri, western Liguria (Cortesognoet al.,1975; Mottana and Bocchio,1975).In both re-gions, the early-formed garnet + omphacite + rutileassociation and less abundant metarodingitic rocktypes have been progressively replaced by glauco-phane-, barroisitic hornblende-, and actinolite-bear-ing assemblages. Although a continuum of phaseproportions is present, average modes of the some-what arbitrarily distinguished stages are presentedfor the Gruppo di Voltri parageneses in Table 5 toprovide the reader with a feeling for the sequence.Margins of the bodies tend to be greenschistic, suc-cessively more interior portions barroisitic andglaucophanic, with eclogitic cores; however, abun-dant relict phases in all lithologies demonstrate thatthe rocks share a common recrystallization history.The paragenetic diagram for the Gruppo di Vottri il-lustrated as Figure 9 is atnost identical to the se-quence of phase compatibilities observed at Zermatt,and is remarkably sinilar to reported paragenesesfrom the Sesia-Lanzo eclogites (Compagnoni, 1977),as well as those developed in Pennine ophiolites ofthe Tauern Fenster, Eastern Alps (Richter, 1973;Miller, 1971).In contrast to the production of parga-sitic hornblende at Alpe Arami, the ophiolitic eclo-gites of the inner Pennine realm show importantstages characterized by blue amphibole and by blue-green subcalcic hornblende (:63rroiti

"r.

Eclogite bulk-rock chemistry

Major elements. Bulk-rock XRF analyses for l0leclogitic rocks from the Western and Ligurian Alpshave been previously pubtished by the author and hiscolleagues, 5 from Alpe Arami, 13 from theZermatt-Saas Fee area, and 83 from the Gruppo di Voltri.Averages for Alpe Arami and Zermatt analyses arepresented in.Table 6. Although the Alpe Arami eclo-gites appear to be more magnesian than those of theZermatt ophiolitic terrain, both groups have tho-leiitic affinities, being very low in KrO, relatively lowin TiO2, and reasonably rich in SiOr.

Metagabbros of the Gruppo di Voltri in westernLiguria are enriched in iron, titania and soda, andare relatively siliceous and K-poor (Table 6)' henceare regarded as the products of somewhat differenti-ated tholeiitic magmas. An early metasomatic eventwhich preceded the high-pressure metamorphism is

ENNTSZ. ECLOGITES, PERIDOTITES, WESTERN LIGURIAN ALPS

Table 5. Average modes for the paragenetic sequence developed in mafic layers within the Beigua serpentinite; Gruppi di Voltri, westernLiguria (original data from Cortesogno el al.,1977\.

St age

Minera lM e t a r o d i n g i t i c(p re- ec I og i t i c ) E c l o g i t i c G 1 aucophani c B a r r o i s i t i c G r e e n s c h i s t i c

p l ag ioc 1 as egaTnetNa-c1 inopyroxener u t i l esphene

O I E S

Ca-anph ibo leNa- anphibo 1 eep ido t e - c1 . ino zo i s i t ech1 or i t e

w h i t e m i c a sb i o t i t equart zapat i t ecarbonate

I I

5

05

3t 70

387

0 . 63 4 . 93 7 . 85 , 81 . 0

3 . 07 . 26 . 61 . 01 0

0 . 20 . 00 . 40 . 50 . 0

3 . 6r 3 . 02 6 . 52 . 43 . 1

3 . 15 . 0

2 4 . 81 3 . 5 *

? a

0 . 90 . 00 . 40 . 70 . t

9 . 99 . 7

r t . 72 . r3 . 3

5 . 14 2 . 2

1 . 69 . 3

o . 40 . 40 . 00 . 60 . 5

2 3 . 1 * *2 . 48 . 80 . 63 . 8

2 . 52 3 . 5r . 2

2 0 . 08 . 8

1 . 91 . 80 . 80 . 10 . 7

00000

* Including Laasonite in one sarnple; x* Including oligocLase in one sample,

attested to by the Gruppo di Voltri (meta)rodingites.Such rocks, thought to reflect original mafic units as-sociated with peridotites, have been converted tohigh Ca + Al, low Si + alkali compositions duringserpentinization of the enclosing ultramafic body(Coleman, 1966, 1967; Dal Piaz, 1969). The roding-ites of western Liguria were subsequently convertedto garnet + clinopyroxene assemblages during eclogi-taation of less altered-hence more gabbroic-layerswithin the Beigua serpentinite (Piccardo et al.,lgg}).A somewhat similar metarodingitic sequence nearAlpe Arami has been reported by Evans et al. (1979).Still later recrystallization gave rise to the glauco-phanic, barroisitic and actinolitic (i.e., greenschistic)temporal gradation referred to in the previous sec-tion. That metasomatic effects evidently accom-panied this latter process may be seen from study ofTable 6. In particular, greenschists seem to be de-pleted in TiO, and iron and enriched in I(rO relativeto the higher pressure lithologies. However, if someof the greenschistic lithologies reflect a magnesianleucogabbro precursor, as mentioned by Cortesognoet al. (1977) and Messiga and Piccardo (1980), someof the metasomatism may be more apparent thanreal.

Rare earth elements. Preliminary bulk-rock instru-mental neutron activation analyses of rare earth ele-ments for 29 samples of Gruppo di Voltri eclogiticparageneses have been obtained by Ernst and Ram-baldi (supported by NSF EAR78-03336). Specimensinclude 2 metarodingites, 8 eclogites, 5 glaucophaneschists, 7 barroisitic amphibolites and 7 greenschists.

Chondrite-normalized REE patterns are presented inFigure l0 for all 29 rocks.

In aggregate, the analyses show broad dispersal ofthe REE trends, reflecting the metasomatism whichoperated to a variable extent during one or morestages in the metamorphism. Many of the chondrite-normalized patterns are nearly horizontal from Tb toLu, with a systematic light rare earth depletion; suchpatterns are characteristic of most oceanic tholeiites(Frey et al., 1968; Kay et al., l97O; Schilling, 1975).However, the impoverishment in LREE accom-panying the hypothesized late metasomatic eventprobably is a bonsequence ofthe abundance ofgar-net (and clinopyroxene) in the precursor eclogitic as-semblage; these phases preferentially concentrateHREE over LREE during crystal-aqueous (andlorCO'-rich) fluid reequilibration (Hellman et al.,1979;Mysen, 1979; Wendlandt and Harrison, 1979).Some-but by no means all-analyses of a particularrock type (especially the greenschists) include a posi-tive Eu anomaly, but this does not seem to be corre-lated with the presence and modal amount, or ab-sence of sodic plagioclase. During the late-stagemetasomatism, rocks rich in feldspar-such as thegreenschists-would be expected to preferentiallyconcentrate Eu (see also Sun and Nesbitt, 1978).

The two metarodingites (Fig. l0A) exhibit rela-tively flat patterns, but differ in rare earth propor-tions. This suggests that REE concentrations havebeen afected by the early metasomatism accom-panying serpentinization which preceded the high-pressure metamorphism. Di-ffering abundances of

ERNTSTj ECLOGITES, PERIDOTITES, WESTERN LIGURIAN ALPS

WESTERN LIGURIA

Nry ECLOGITICGLAUCO -

PHANICBARRorsrnc gT..:!T;

PlagioclmeLansoniteEpidoteNa-amphiboleCa-amphiboleClinopyroxeneGarnetBiotiteWhite micaChloriteSpheneRutileCalcite

-Brroisite Na+Al-actinolite

- -a

Ca-rich Na+Al-richrl--rNF6{+rich MsrFe-rich-

Pnmsrte .nil6?-fii?gon',-

I I - I I I I

I

I I

I I - I I - I I - I I !

459

Fig. 9. Schematic paragenetic sequenoe developed in mafic lenses of the Beigua serpentinite, Gruppo di Voltri (Cortesogno el a/.,

Sllj. ealy stag€s ar€ shown on the left. Very similar parageneses occur in dismembered ophiolites exposed in the vicinity of Zermatt-

Saas Fee (Dal Piaz and Ernst, 1978). Solid line indicates essential presence of a phase, dashed line symbolizes minor or sporadic

pr€senc€.

Table 6. Average whole-rock XRF compositions and one standard deviation for chemically analyzed eclogitic rocks frorn Liguria and

the Western Alps (original data from: Cortesogno et al., 19711. Err,st, 19'17; Dal Piaz and Ernst, 1978).

Gruppo d i Vo l t r i , wes tern L lgur raWestern A lpsLoca l i t y

Oxid e A l p e A r a m i Z e r m a t t M e t a r o d i n g i t i c E c l o g i t i c G l a u c o p h a n i c B a r r o i s i t i c G r e e n s c h i s t i c

s i 0 2

A1 203

T iO2

F e203*

FeO

Mn0

Mgo

Ca0

Na20

Kzo

Pzos

H2o+ or ign i t ion loss

Nunber of sanples

4 7 . 3 2 ! 2 9 6

1 3 . 7 7 t t . 2 0

r . 3 3 ! 0 . 2 2

1 0 9 5 1 0 . 7 7

0 . 1 8 ! 0 . 0 2

11.14 !2 62

t l . 3 2 ! 1 . 2 I

2 7 3 t 0 . 4 2

0 . 2 6 1 0 . 3 0

0 . 1 8 1 0 0 4

0 5 8 1 0 . 5 9

5

48.39 !2 87

1 4 4 5 1 1 . 5 8

1 . 9 2 ! 0 . 8 9

4 . 0 0 1 1 . 1 7

6 . 5 I 1 2 . 3 1

0 . 2 0 1 0 . 0 9

6.43 !7 . 39

IO .20 !2 . 39

4 . 4 6 1 0 9 9

0 . 1 8 1 0 t 2

0 . 1 9 r 0 . 1 3

2 . 6 5 1 1 . 8 4

1 3

4 3 2 5 1 0 . 5 7 4 6 - 0 2 ! 2 . 0 2

1 1 . 8 7 1 2 0 9 9 . 8 1 l l . 0 5

1 . 2 0 1 0 . 7 0 4 6 r ! 1 . 2 7

9 . 0 6 ! 2 . 0 4 1 8 . 2 1 1 1 . 6 9

0 . 1 8 1 0 0 2 0 2 5 1 0 . 0 6

1 1 . 4 6 J 1 . 0 0 6 . 8 1 1 1 . 4 1

1 9 0 4 1 0 . 4 9 I 3 2 ! 1 . 6 7

0 . 9 2 1 0 . 1 8 5 . 8 5 1 0 . 8 5

0 . 0 2 1 0 . 0 3 0 . 0 5 1 0 . 0 5

0 6 6 1 0 r 0 0 . 4 3 1 0 . 3 3

2 . 2 2 ! 0 - 6 4 0 7 8 1 0 8 8

2 3 7

4 7 . 9 9 ! 0 . 4 9

1 2 . 3 6 ! 2 . 0 4

3 . I A l t 4 7

14 -90 !4 .70

0 . 1 8 1 0 . 0 8

6 . 4 8 i 1 . 5 4

7 . 5 8 ! 2 6 0

4 . 6 0 1 1 . 1 0

0 . 1 6 1 0 . 1 7

0 . 6 1 1 0 . 5 3

2 . 0 4 ! 1 . 7 5

6

4 5 . 0 5 1 3 8 5

1 1 . 2 3 1 1 . 3 0

4 1 5 r r . 8 0

1 7 . 2 2 ! 3 7 8

48.64 !6 .46

1 3 . 8 0 1 2 1 2

r . 7 3 ! r . 7 2

l 0 . I 7 ! 4 . 7 2

0 . 1 6 1 0 . 0 9

8 63!4 27

9 . 1 6 r 3 . 4 5

4 1 3 1 1 4 4

0 . 5 2 1 1 . 0 3

0 . 3 7 1 0 . 1 9

2 - 3 6 ! 1 4 r

2 3

+ Total iron as Fe20Z ercept for anaLgses of Zematt eelogites md qssoeiated roeks.

(A) METARODINGITES,W. LIGURIA

a,'tr!

oU\ 10.J

I

J00

q)'trEo(J

>10ooI

J

co

ER]VSZ. ECLOGITES, PERIDoTITES, WESTERN LIGIJRIAN ALPS

(c) GrAucoPHANlcECLOGITES,W. LIGURIA

a)-L

ET(J

E

*t

t0

o

Eo(J

J

I

=3oo

1.0

r00

r.0

(D) BARROTSIilCAMPHIBOLITES,

W. LIGURIA

t.0La Ce Nd Sm EuLa Ce Nd Sm Eu

'R]VS?1. ECLOGITES, PERIDOTITES, WESTERN LIGURIAN ALPS 461

(u

€o(J

\ 10J(Jo

IJ

00

La Ce Nd 5m Eu Tb Yb LuFig. 10. Chondrite-normalized rare earth el€ment

concentrations in eclogites and related rocks from the Beiguaserp€ntinite, Gruppo di Voltri, western Liguria. Data are fromErnst and Rambaldi, unpublished and preliminary. Although acontinuum of transitional assemblages was produced by therecrystallization of earlier metarodingites (A), and eclogites (B),three principal stages in this later metamorphism have beendelineated, glaucophane-rich (C), barroisite-rich (D), andactinolite-rich (E).

layer silicates, presumably impoverished in REE,may account for the disparity in REE concentrationsin the two samples.

Eclogite analyses displaj, uniformly flat, sub-parallel HREE trends, and consistent LREE deple-tions (Fig. l0B). Samples with high concentrations ofrare earth elements generally contain less modal epi-dote and more garnet t apatite than samples withlow REE proportions. All analyzed rocks carry 4l+6volume percent omphacite, so although clinopyrox-ene is expected to be a major host of lanthanide se-ries elements, the observed dispersal of the data can-not be ascribed to variable proportions of thismineral.

Glaucophane schists possess flat patterns, althoughLa contents tend to be relatively low in all analyzedrocks (Fig. lOC). Abundances of rare earth elementsrange to higher values than those'of parental eclo-

gites. Similar to the eclogitic protolith, higher con-centrations of REE more or less reflect the abun-dance of apatite, and perhaps also are inverselyproportional to the modal layer sitcates formed dur-ing the late-stage greenschistic metasomatism.

Fairly flat HREE patterns and marked LREE de-pletions, especially of La, charaderue the barroisiticamphibolites (Fig. l0D). An indistinct tendency forapatite-bearing amphibolites to be enriched in rareearths, and chloritic amphibolites to be impoverishedin rare earths may be recognized.

Greenschists have lower REE contents relative tosodic and calcic amphibole-bearing precursors (Fig.l0E). From this we may infer that interaction be-tween metasomatic fluid and chlorite does not con-centrate REE in the layer silicate as strongly as be-tween the aqueous phase and chain silicates. Becausemost silicates experimentally investigated thus farconcentrate REE relative to an associated aqueousphase (Hanson, 1980), it appears that large amountsof such a metasomatic fluid must have coursedthrough the pre-existing high-prQssure Gruppo diVoltri mineral assemblages to account for the sys-tematically lower rare earth concentrations in thegreenschists. An alternative explanation is that mostgreenschists studied had an Mg-rich leucogabbroparent, as advocated by Cortesogno et al. (1917) andMessiga and Piccardo (1980), and hence reflect theREE abundances of a source rock different from theeclogitized ferrogabbros.

E c I o git e miner al c hemis t ry

Electron microprobe analyses for coexisting miner-als from Western Alpine and western Ligurian eclo-gitic parageneses have been previously published bythe author. Phases from 6 rocks from Alpe Arami, 13from the Zermatt area and 25 from the Gruppo diVoltri were investigated in these studies. Chemicaldata were provided for garnet, omphacite, sodic am-phibole, calcic amphibole, epidote, plagioclase, layersilicate minerals (chlorite, biotite, paragonite, phen-gite), kyanite and sphene. For the purpose of calcu-lating cation proportions, iron was considered as halfferric in sodic amphiboles and omphacites, as exclu-sively ferrous in calcic amphiboles. Similar mineralanalyses have been obtained for the general area byBearth (1967, 1973), Chinner and Dixon (1973), Kie-nast (1976), Compagnoni (19'17), Compagnoni et al.(1977), and Desmons and Ghent (1977).

Proportions of iron, magnesium and Ca * Mn foranalyzed garnets from Alpe Arami, Zermatt andwestern Liguria are illustrated in Figure I l. Distinctcompositional clustering is evident. Gruppo di Voltri

(E) GREENSCHTSTS,W. LIGURIA

Ce Sm Eu

ERdSri ECLOGITES. PERIDOTITES. WESTERN LIGURIAN ALPS

protoliths are more femrginous than the ophioliticparents from the Zermatt region, hence western Li-gurian garnets are more alnandine-rich than thosefrom the Western Alps. Both groups, however, arefar less pyropic and somewhat more manganiferousthan eclogitic layers associated with the garnetlherzolite at Alpe Arami. Average compositions are:Alpe Arami, AlmrnPyrrGrossrrSpesso, ; Zermafi , Alm""PyorGrossrrSpessor; and Gruppo di Voltr i ,AhnroPy*Gross,. Spess*.

Cations inferred to be occupying the larger M(2)site in analyzed clinopyroxenes are shown in Figure12. Departure of the analytical data points from thedashed line, along which I Na * Ca) : 1.00, reflectsanalytical error and,/or the minor presence of addi-tional cations such as Mn, Fe'* and Mg (below line),or the very unlikely actual presence in M(l) of minorCa * Na (above line). At Alpe Arami, the early stageretrograde metamorphism of the eclogite producedsymplectite, consisting of intergrown clinoamphibole,clinopyroxene and intermediate plagioclase; the sym-plectic clinopyroxgne is very diopsidic, as can be seenfrom Figure 12. In the Gruppo di Voltri, the clinopy-roxene of a metarodingite reflects the sodium-poornature of the host rock, hence also is very diopsidic.All other 3l analyses are of intermediate Na-Casolid solutions properly termed omphacites (*chloromelanites). Although extensive compositionaloverlap is evident, the western Ligurian specinenstend to be slightly more sodic than analogues fromthe Zermatt area, with the Alpe Arami primary eclo-gitic clinopyroxenes more calcic. Based on inferredM(1) cation proportions, average compositions forthe omphacites are: Alpe Arami, Jdo.Ac-DLrHdor',Zermatt, JdorAc,rDiroHd,r; and Gruppo di Voltri,JdrrAc,.DirrHd,u. Comparison of these proportionswith M(2) proportions illustrated in Figure 12 sug-gests that the actual Fel*/Fe'* ratios in Ligurian om-phacites may exceed the unit value assigned; higherratios would increase the computed Ac content at theexpense of Hd and would be more reasonable in thelight of the observed highly sodic compositions ob-tained by direct analysis.

1r1a-amphiboles are not present in the Alpe Aramiparagenesis. Analyses of blue amphiboles from theZermatt and Gruppo di Voltri areas are plotted inFigure 13. The inferred Al"'and Fe'* occupancies ofthe M(2) site are shown along the abscissa, that of

I Assumption of all ferrous iron in the Alpe Arami clinopyrox-ene provides Di + Hd proportions equivalent to the analyzed Cal(Na + Ca).

Mg * Fe'?* in M(1) and M(3) along the ordinate. Zer-matt phases tend to be more magnesian than sodicamphiboles from western Liguria, undoubtedly re-flecting the iron-rich nature of the latter rocks. Aver-age compositions are approximately: ZermatqGlrrRiebrr; Gruppo di Voltri, Gl"nRieb''.

Inferred cation occupancies of the large M(4) andA sites in analyzed calcic amphiboles from AlpeArami, Zermatt and western Liguria are shown inFigure 14. At all three localities, early hornblendesare armored and partly replaced by more actinoliticclinoamphiboles. At Alpe Arami the precursor horn-blendes are magnesian, Ti-rich pargasites which con-tain substantially more tetrahedrally than octahe-drally coordinated aluminum, averaging 1.64 Al'"and0.74 Al'' per formula units. The early-stage cal-cic amphibioles in.both Zermatt and Gruppo diVoltri parageneses are barroisitic; they are inter-mediate iron-magnesium solid solutions, carry onlyminor amounts of Ti, and have subequal amounts ofAl'" and Alvr. The calcic amphiboles in these meta-ophiolites also exhibit a gradational and continuoustransition from barroisitic compositions to late-stageactinolites.

The eclogites and related rocks studied in this re-port contain epidote group minerals, plagioclase,layer silicates and sphene as major retrograde phases,and in addition, some of the Alpe Arami assemblagescontain very minor amounts of kyanite (see Figs. 8and 9). Averages of Fe3*/(Al"r + Fe3*) for epidotefrom Alpe Arami, Zermatt-Saas Fee and the Gruppodi Voltri are 0.03, 0.15 and 0.17, respectively. In bothlatter areas, indistinct zoning occurs in the epidotes,and the late-stage greenschistic compatibilities in-clude a more aluminous clinozoisite compared to themore pistacitic composition which accompanies bar-roisitic hornblende. Analyzed plagioclase grains arehomogeneous in all three areas. It is chiefly oligo-clase at Alpe Arami, where three analyses range fromAn,, to Anrr. In contrast, in the ophiolitic eclogites,greenschistic stages are characterved by pure albite,Ano, at Zermatt, Anoo to Ano, in the Gruppo diVoltri. A single analysis of plagioclase from a meta-rodingite at the latter locality is Anrr, reflecting thehigh calcium and low sodium contents of the hostrock. Only a few layer silicates have been analyzed,so conclusions regarding compositional ranges forthe eclogitic parageneses remain provisional. A singleanalyzed Alpe Arami chlorite is compositionally in-termediate between sheridanite and iron-poor ripido-lite; more extensive chemical data for Zermatt andGruppo di Voltri chlorites indicate that they range

FeF

" ALPE ARAMI90 'i ZERMATT

GARNETS o \[/. LIGURIA

EX.itrSTi ECLOGITES, PERIDOTITES, WESTERN LIGURIAN ALPS

Mg Ca+MnAtomic proportions

Fig. I l. Atomic proportions of Fe2+, Mg and Ca + Mn in analyzed garnets from W€stern Alpine and Ligurian eclogites. Data from:Ernst (1976, 1977), Ernst and Dal Piaz (1978). All iron has been considered as ferrous for the computation ofcation proportions. Notethat only the almandine-rich portion ofthe ternary diagram is shown.

from ripidolite towards iron-rich clinochlore. Whitemicas in the latter two areas include both paragoniteand phengite. Late-stage biotite and sphene occur inall three parageneses, but were not studied in detail.

P-T conditions of eclogite crystallization

In contrast to aluminous peridotite petrogeneses,where numerous theoretically derived and experi-mentally tested geothermometers and several inde-pendent geobarometers may 6s ulilized, only a veryfew suitable methods are available for the assign-ment of physical conditions of origin to eclogites.This is a consequence of the fact that lherzolites rep-resent well-constrained mineralogical associations(typically 4 or 5 phases in a 5-component system),whereas eclogitic assemblages possess high variance(2 or 3 phases in a 9-or l0-component system). More-over, due to the chemical complexities, petrogeneticgrids for basaltic compositions are as yet impossibleto calculate from fragmentary thermochemical' data

on end member minerals, and have proven difficultto experimentally determine at subsolidus temper-atures (Yoder and Tilley, 1962; Green and Ring-wood,1967b; Cohen et al.,1967; Essene et al., 1970).Other than '8O/'5O fractionation studies (e.9., Taylorand Coleman, 1968; Desmons and O'Neil, 1978),which provide an indication of the temperature ofoxygen isotopic equilibration, the most useful esti-mates of P-T conditions appear to be based on thepartitioning of iron and magnesium between coexist-ing garnet and clinopyroxene. Ko values, (Fe'*/Mg)"*"/(Fe'*/Mg)*, are a function of the physicalvariables @nheim and Green, 1974, 1975), and tosome extent reflect compositional variables as well,especially Ca content (Ellis and Green, 1979).

Employing the eclogitic garnet * clinopyroxenemicroprobe data previously summarized, and the ex-perimentally calibrated iron-magnesirm fractiona-tions elucidated by Ellis and Green, the computerprogram devised by A. A. Finnerty (see section on

CLINOPYROXENES

otto

* \* *

o ALPE ARAMI* ZERMATTo LIGURIA

,rrot).rir.,' s . t a

metaro?ingite

ERNS?1 ECLOGITES, PERIDOTITES, WESTERN LIGURIAN ALPS

0.8

E o.sg

.o- 0.4q,)arh

oGI

o n 1z- ' '

o'oo:

Ca atoms per formula unit

Fig. 12. Sodium and calcium atoms per six oxygens in analyzedclinopyroxenes from Western Alpine and Ligurian eclogites. Datafrom: Ernst (1976, 1977); Ernst and Dal Piaz (1978). Equalproportions ofFe3* and Fe2* have been assumed for the purposeof calculating cation proportions, except for Alpe Arami, wheremore reasonable cation totals were obtained by assuming that alliron is divalent. Note that the low Ca, high Na portion of thediagram is not shown.

peridotite conditions of crystallization) providednominal temperatures of eclogite equilibration as aunivariant function of pressure. For Alpe Arami, anoperating pressure of40 kbar was chosen to conformwith the value obtained for the associated garnetlherzolite. For Zermatt-Saas Fee and the Gruppo diVoltri, a pressure of l0 kbar was arbitrarily selected.Ten kbar appears to represent a reasonable minimalvalue because of the sporadic occurrence of nearlypure jadeitic pyroxene in associated quartzose rocks(Lorenzoni, 1963; Bocquet, l97l; Compagnoni andMaffeo, 1973); moreover, at lower pressures, morehydrous, lower grade assemblages would have beengenerated. If the actual pressures did in fact exceedl0 kbar, as appears likely, the calculated temper-atures of eclogite formation would also be slightlyhigher. Results of the computations are presented inTable 7.

Quite clearly the Alpe Arami eclogites, character-ized by a Ko of about 6, cfystallized at much highertemperatures than the ophiolitic eclogites of Zer-matt-Saas Fee and western Liguria, where Ko valuesaverage approximately 2l and 29, respectively. As-

signing a pressure of 40 kbar at Alpe Arami yields anaverage temperature for eclogite equilibration of898+43oC, ia surprisingly good agreement with thegrand average value of 949+75"C arrived at pre-viously for the associated garnet lherzolite (compareTables 3 and 7).

Assuming a minimal 10 kbar total pressure, theless extreme conditions of origin, 433+62"C for theGruppo di Voltri arfi, 52'7+52oC for Zernatt, havebeen obtained (Table 7). Support for the Zermatt P-Z assignments may be found in petrologic investiga-tions by other workers for more internal portions ofthe Pennine and the Sesia-Lanzo zones (presumablymore deeply subducted), where the following phys-ical conditions have been estimated for similar eclo-gitic assemblages: Kienast (1976), 500-600oC, P > 10kbar; Campagnoni (1977),500-600oC, P > 15-17kbar; Desmons and Ghent (1977),500-530oC, P >l2kbar; Reinsch (1979) 450-550oC, P : 12-16 kbar;and Desmons and O'Neil (1978), 540'C. For thekyanite-bearing eclogites of the Tauern Fenster ofthe Eastern Alps, Holland (1979) has suggested theattendance of even more extreme physical condi-tions, approaching 620+30"C and 19.5+2.5 kbar. AtZeml^att and in western Liguria, the association of

SODIC AMPHIBOLES* ZERMATT

o W. LIGURTAferro- riebeckite

glaucophane

_l crossite t-----------

,,ru.oon.n"-].j-' magnesio-riebeckitef *?l'

*

Fe3*/(Alvr+ Fel*)

Fig. 13. Compositions of analyzed sodic amphiboles fromWestern Alpine and Ligurian eclogitic rocks. Data from: Ernst(1976, 1977); Ernst and Dal Piaz (1978). Equal proportions ofFe3* and Fe2+ have been assumed for the purpose of calculatingthe cation ratios for M(2) along the abscissa, M(l) + M(3) alongthe ordinate.

0.4

t.0

0.8

..P o.o+ooE+\

e 0.4

0.0Lo.o 0.4

ERNSfi ECLOGITES, PERIDOTITES, WESTERN LIGURIAN ALPS K5

tr*

ot.8

.= l'6

; t4

E r.2o

b l'oo.I 0.8o(€ 0.6(!z o.+

0.2

0.0 r-0.0

eclogitic rocks with antigoritic serpentinite suggestthermal maxima less than about 600oC (Evans,r977).

Retrograde paths followed by the eclogitic rockson their transit to the present crustal levels for allthree occurrences seemingly involved rapid decom-pression relative to the rate of temperature decline.As a matter of fact, a small increase in Z during theinitial stages of decompression is theoretically pos-sible, as discussed by England (1978) and Richard-son and England (1979), but such heating is not re-quired by the observed parageneses. The stability.relations of glaucophane (Maresch, 1977), barroisiteand actinolite (Ernst, 1979), the muscovite-parago-nite solvus.(Eugster et al., 1972), the greenschist-am-phibolite facies transition for rocks of basaltic com-position (Liou et a1.,.1974), and reactions involvingamphibolites (Lambert and Wyllie, 1972; Holloway

and Burnham, 1972; Allen and Boettcher, 1978;Spear, l98l), all provide important constraints on themineral assemblages of interest. A previously dis-cussed petrogenetic grid, consistent with availableoxygen isotopic and phase equilibrium data for re-gional metamorphic rocks of roughly tholeiitic com-position is presented as Figure 15. The decompres-sion P- 7 paths indicated for the retrogrademetamorphism in all three cases appear to be nearlyadiabatic, reflecting geologically rapid ascent to thepresent locus. The manner in which this was accom-plished is a serious tectonic problem.

Petrology and plate tectonic setting

Mineral assemblages and inferred conditions ofcrystallization described here for eclogites and perid-otites from the Western and Ligurian Alps appear tobe consistent with data obtained bv other workers.

CALCIC AMPHIBOLES1.0

ALPE ARAMIZERMATTW. LIGURIA

0.2 0.4 0.6 0.8 t.0 t.2 t.4 t.6 t.8 2.0Ca atoms per formula unit

Fig. 14. Sodium and calcium atoms per 23 oxygens (assumhg'one HzO) in analyzed calcic amphiboles from Western Alpine andLigurian eclogitic rocks. Data from: Ernst (1976, 1977); Ernst and Dal Piaz (1978). All iron has been considered as ferrous for thecomputation of cation proportions.

* t * t r tB '

\t \ *

.EnrVSZ' ECLOGITES, PERIDOTITES, IYESTERN LIGURIAN ALPS

Table 7. Apparent temperatures of equilibration for analyzedeclogite samples employing the iron-magnesium partitioning data

of Ellis and Green (1979).

son, 1972). This accounts for their general pre-Meso-zoic crustal setting, except for those which have un-dergone tectonic transport, such as Lanzo and theErro-Tobbio peridotite of western Liguria.

In contrast, the eclogitized mafic and serpentinizedultramafic associations of the North Alpine platepossess ophiolitic affinities, and were probably gener-ated during the mid-Mesozoic sea-floor spreadingevent which produced the Tethyan oceanic realm.Although the original peridotites have been totallyserpentinized(e.9., see Dietrich et a|.,, 1974), nonna-tive anyhydrous equivalent assemblages apparentlywould be harzburgitic. The accompanying maficrocks include gabbros and pillow basalts (Bearth,1959,1967), now variably converted to eclogitic com-patibilities. Such assemblages have equilibratedunder the high-pressure, low-temperature conditionsinferred to characterize subduction zones. Tempera-tures and pressures of 45G-600'C,lFl2 kbar wouldalso explain the association with antigoritic serpenti-nite (Evans, 1977). Ttle direction of North Alpinelithospheric underflow--to the south and east be-neath the South Alpine plate-has been deduced onthe basis of fold vergences, d€collements, and the ex-ternally decreasing grade of high-pressure meta-morphism (Ernst, 1971, 1973; Dal Piaz et al., 1972;Frey et al., 1974). Buoyant ascent toward the surfaceof these subducted ophiolitic sections was probably atectonic response to the gravitative instability gener-ated by the downward conduction of relatively low-density serpentinized and hydrated oceanic crust anduppermost mantle and sialic massifs with surround-ing sediments which characterue the Tethyan realm.

The striking anomaly in an otherwise self-consis-tent petrologic and plate tectonic scenario is pre-sented by Alpe Arami. Although various authoritieshave calculated different nominal P-Tconditions oforigin for this body, all agree that the pressures andtemperatures greatly exceed those under which thesurrounding Lepontine gneisses recrystallized. Forinstance, based on methods described in previoussections of this repoft, Alpe Arami garnet lherzolitesand marginal eclogites are computed to have equili-brated at 900-950"C and about 40 kbar, whereas theadjaccnt gneisses evidently crystallized at approxi-mately 650"C and 6-7 kbar. The sheared, myloni-tized contact bounding the Alpe Arami lens (Buis-kool Toxopeus, 1976) is evidence that it wastectonically inserted into its present site. Although anoverprint of lower grade, hydrous assemblages is evi-dent from the described parageneses (Table l, Fig.8), such phenomena occur in the Lepontine gneisses

Sanpl eNunber

Tenperature(oc)

As sumedPres sure

(kb)

Alpe Arami

F - I 6 dF . I 6 fF - 5 3F - 5 4

average

Zemat t

MRO- 85 7MRO- 1 I 05MRO- r I 09MRO- 1 609MRO- 1 61 1DBL- 379DBL- 409DBL- 4 97

average

Gruppo di Voltri

EE- 3EE- 31EE-43E E - 6 3

. E E - 6 4EE- 66E E - 7 2E E - 7 7E E - 7 9 *EE_79E E . 8 2EE - 85*EE- 85

average

8 5 59 1 09 5 3874

898143

4 7 24994485 9 55 7 65 1 35 6 35 5 r

5 2 7 ! 5 2

5 5 63965 3 94 9 54383 9 13663 7 44 1 3395L \ )

4 5 I378

433!62

4 04 04 04 0

4 0

1 01 01 01 01 01 0i 01 0

1 0

t 01 01 01 0I Ol 01 01 01 01 01 01 01 0

1 0

* core oaluesl mstarred are homogeneous or rin xaLues.

Of considerable importance is the fact that the con-trasting parageneses seem to be related to the platetectonic settings of the northern and southern Alpineterrains.

Gabbroic rocks of the nonsubducted South Alpineplate are devoid of high-pressure phase assemblages,indicating that they were produced at relatively shal-low depths in the upper mantle, or deep within thesialic crust. Those associated with peridotites wereprobably formed during decompression melting ac-companying tectonic ascent of the parental lherzo-lites. Conditions of last equilibration of the ultrama-fics appear to be 900-1 100'C at l0-15 kbar; for thosesubjected to incipient partial melting, somewhatlower pressures of pyroxene annealing seem to be in-dicated. The fact that most of the South Alpine ultra-mafics are clinopyroxene-bearing rather than beingharzburgitic in composition supports the hypothesisthat these rocks are samples of subcontinental ratherthan shallow suboceanic mantle (Nicolas and Jack-

ER.ITS?.. ECLOGITES, PERIDOTITES, WESTERN LIGURIAN ALPS

500 r000

Temperature in tFig. 15. Petrogenetic grid for rocks ofbasaltic compositions based on available experimental and oxygen isotopic data (see t€xt and

Ernst, 1973). Metamorphic zone boundaries are shown as univariant lines for simplicity, but are actually multivariant P- T zones of finitewidth; their locations and widths are sensitive functions of numerous chemical variables. Facies abbreviations are: a : amphibolite; bs :blueschist; gs : greenschisl pp : prehnite-pumpellyite; z: zeolite. Where aqueous fluid pressure is equal to P,o,.1, amphibole-dominantassemblages are stable over most reasonable crustal- and uppermost mantle-type conditions. The approximate ficlds of formation fortype A, B and C eclogites of Coleman et al. (1965) are indicated. Symbols denote the initial conditions of eclogite formation as listed inTable 7 for Alpe Arami (: box), Zermatt (: star) and the Gruppo di Voltri (: circle).

467

-o>z-

a ;

t- l0Jataat,q)l-

o-

as well. It seems unlikely that this small mafic and ul-tramafic body could have been emplaced in such aquartzofeldspathic terrain prior to the penetrative,high-1a1ft amphibolite facies metamorphism withoutundergoing extensive metasomatic exchange, and re-sulting in the obliteration of the granulated contacts.Evidently the Alpe Arami lens was a late metamor-

phic interloper in the Lepontine gneiss realm. How itgot there is unclear and must await investigations forfuture workers. As an added incentive, it should benoted that P-T conditions suggested for crystalliza-tion of this body are not far removed from those esti-mated for diamond-bearing kimberlites (Boyd,r973\.

- tr.,C

-r f,-garnet a

b,"**,,f,-ffi(;liii*,n"aUa px-

hornfels

tfl00I

crystals* melt

granulite

68

AcknowledgmentsThe University of California, Los Angeles, and the National

Science Foundation through grant EAR 79-23615, provided sup-port which made the present synthesis possible. A. A. Finnerty,the Jet Propulsion Laboratory, Pasadena, undertook the computertrcatment of mineral data which allowed a comprehensive analysisof geothermometry and geobarometry of the investigatcd perido-tites and eclogites. Giulio Ottonello, the University of Pisa, andErmanno Rambaldi, the University of California, Los Angeles,supplied me with preliminary REE data from yet unfinished col-laborative projects dealing with lherzolites and eclogitic rocks.Constructive reviews of the draft manuscript were obtained fromB. W. Evans, the University of Washingtoq and F. A. Frey, theMassachusetts Institute of Technology. I am very grateful to allthe above institutions and peoplc for their help.

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Manuscrtpt received, February 11, l98I;acceptedfor publication, February 12, 1981.