Journal of the Geological Sociefy, London, Vol. 143, 1986, pp. 607-618, 9 figs, 3 tables. Printed in Northern Ireland

High-pressure of the Zermatt-Saas ophiolite zone, Switzerland

A. C. BARNICOAT & N. FRY' Department of Geology, University College of Wales, Aberstwyth SY23 3DB l Department of Geology, University College, Swansea SA2 8PP

Abstract: Within the Zermatt-Saasophiolite zone of the westernAlps a record of polystadial high-pressure metamorphism is well preserved. Early assemblages, retained as inclusions in , were succeeded by eclogitic assemblages which in some rocks contained , kyanite, and chloritoid. These eclogitic assemblages formed by progradereaction from the early blue- schists. Subsequently,reaction of these with a mixed H,O-CO,vapour phase led to the replacement of kyanite by and the partial replacement of omphacite- paragenses by assemblages containing , paragonite and ankerite. High-pressure, eo-alpine metamorph- ism took place under conditions of 550-600°C and 17.5-20 kbar. Values of aH,O between 0.55 and 1 are high enough to accommodateequilibration with a water-rich vapourphase under the highest- grade conditions. The presence of such a fluid phase is locally indicated by the presence of -rich veins containing omphacite and kyanite. This vapour phase was absorbed during retrogression. Later, rehydration is limited to areas close to veins, tectonic contacts and bodies of metasediment. The metamorphic conditions determined for theZermatt-Saas zone are compatible with those suggested for over- and under-lying units. These conditions, and the P-T path inferred by comparing thereaction histories with apetrogenetic grid for the system Na,0-Ca0-Mg0-A1,0,-SiO~-H20, suggest that metamorphismoccurred during subduction to depths of between 60 and 70 km and subsequent exhumation during the Alpine orogeny.

The Zermatt-Saaszone has become the best-known With the exception of the Grand St Bernhard in this area, -bearing region of thewestern Alps dueto the all of these rocks contain,at least locally, evidence of meticulously detailedefforts of Bearth(e.g. 1959,1967, high-pressuremetamorphism (e.g. Dal Piaz et al. 1983; 1973). In these works, Bearth has clearly demonstrated the Chopin & Schreyer 1983). The Zermatt-Saas and associated formation of eclogites froma crustal protolith, and this Combinzone (devoid of evidence of high-pressure alone gives theserocks a crucial importancefor Alpine metamorphism) form the northern portion of the Piemonte geology astheir subduction and re-emergence must be (ophiolite plus schistes lustres) composite nappe or zone of explained in any models of Alpinetectonics. That this the western Alps (Figs 1 & 2). Ophiolitic rocks from other protolith is an ophiolite, albeit dismembered, reinforces its portions of thePiemonte nappe have been described in significance in the geotectonic evolution of the Alps. Valtournanche(Dal Piaz & Ernst 1978), Gran Paradiso Thispaper has three purposes. It summarizes in the (Elter 1971),Val Susa (Pognante 1980),Monviso (Lom- English language key features of the geology and petrology bardo et al. 1978), and western Liguria (Otten & Brouwer of the rocks of the Zermatt-Saas ophiolite zone, drawing 1979; Messiga et al. 1983). togetherinformation from German languagepublications The ophiolitic rocks of the Zermatt-Saas zone appear to and from unpublished British Ph.D. theses, with additional have been emplaced upon the Monte Rosa nappe at an early observationsand analyses of ourown. It then stage in the Alpine orogeny, and to have been subsequently developsand provides new interpretations of the highest- deformedduring backfolding andbackthrusting. Although grade part of the petrogenetic histories of the metabasaltic the exactstructural relationships in thearea still await rocks. Finally, wegive new estimates of high-pressure clarification (Milnes et al. 1981; Martin 1982; Miiller 1983), metamorphic conditions. it is apparent that this structural history has resulted in parts The terms early and late are used throughout this paper of the zone(e.g. south of SaasFee on Egginerand the to refer to the age of high-pressure features relative to peak Mittaghorn) being overturned (Bearth & Schwander1981; eclogite conditions.Specimen numbers refer to material Milnes et al. 1981). Internally, the Zermatt-Saas zone is stored at Aberystwyth. Localities referred to in the text may composed of a series of thrust slices up to 1km thick. be foundon the Swiss 1:25,000 topographic maps 1328 The ophiolites are considered to have formed during the (Randa) and 1348 (Zermatt); grid referencesrefer tothe Jurassicspreading of the Piemonte-Ligurian ocean. The Swiss national grid which is displayed on these maps. high-pressure metamorphism within the Piernonte nappe has beendated at 80-100Maby Hunziker (1974) and 78-100 Ma by Bocquet et al. (1974) using K-Ar isochrons of alkali amphiboles.Delaloye & Desmons (1976) obtained Geological setting K-Ar dates of 62-73 Ma from white , and Chopin & The Zermatt-Saasophiolite zone occurssandwiched Maluski(1980) obtained 40Ar-39Ar plateau ages of between the continentalbasement rocksof theDent 60-75 Ma for paragonites and phengites from boththe Gran Blanche,Monte Rosa and Grand StBernhard nappes. Paradiso basement and the overlying Piemonte nappe. 607

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Constituent rock types unlayeredoutcrops (e.g. Pfulwe,63120963, Bearth 1973; Vorder Wand, 63170991). Ina thrust sheet below the The rock types classically present in an ophiolite suite are all North Mellichgletscher(6318 0988) thereare banded identifiable in the Zermatt-Saas zone, which was mapped at metabasaltic rocks having marked parallel contacts between 1: 25,000 and comprehensively described by Bearth (1967). successive layersor sheets of onlyvery slightly differing Listedbelow aresome of the most notable localities. compositions. In theseoutcrops manyindividual bands Ultrabasicrocks, present as serpentinites, occur both as (

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Sesiazone and Dent Blanche nappe

...... _ . ..__...... ; .: ...... Combin Zone EZerrnatt Saas Fee Zone

m Cornergrat Zone Monte Rosa nappe

DGrand St Bernard ~ppp

Fig. 2. The Zermatt-Saas ophiolite zone.

-schists, containing blocks andpods of metabasic and chalcopyrite. They are distinct from mica-carbonate-rich material,serpentine and ophicalcite, constituting the material filling pillow interstices, interpreted as a mixture of Garten-Riffelbergformation (Bearth 1967). Theother is hyaloclastite and sediment. The -rich zones are represented by widespread,butcomparatively rare interpretedas of hydrothermal fluidflow andmineral manganese-bearing metasediments which occur in units less precipitation, formed during the earliest (oceanic) stage of than 10m thick throughoutthe Zermatt-Saas zone, for the history of these rocks (c.f. Seyfried et al. 1978) and example at Sparrenflue (62970984) andthe Mittaghorn subsequently recrystallized during high-pressure meta- ridge about 2 km SSE of Saas Fee in Switzerland, and at morphism.Similar Ca-rich alterationzones are found in Plan Maison (Bred), Lago di Cignana (Valtournanche) and dredged sea-floor basalts (Humphris & Thompson 1978) and Praborna (Val d’Aosta) in Italy (Bearth1967; Dal Piaz et al. in the Ligurian andCyprus ophiolites (Spooner & Fyfe 1979; Bearth & Schwander 1981). The metacherts are finely 1973;Smewing 1975). Hydrothermal activity alsois bedded withmillimetre-scale quartzand manganese-rich presumed to be the origin of the Cu mineralization and of bandsforming the bulk of the rock.Nodules composed the Mn-rich nature of the overlying metacherts. Syn- largely of braunite, ranging up to an exceptional 0.8m are high-pressure veins are quartz rich, containing some of the distributed throughout these sediments. Spessartine, piemon- phases omphacite, kyanite, glaucophane, ankerite and rutile tite,braunite, pink Mn-bearing phengite, riebeckite and (Fry1972). The latest veins are asuite of near-vertical epidoteoccur together with thedominant quartz in these albite-filled fractures (containing minor clinozoisite, anker- rocks(Bearth & Schwander 1981; Dal Piaz et al. 1979). ite, black calcic-amphibole and chlorite) which cross-cut the Tourmaline is abundantsomein lithologies. Similar high-pressure structuresand fabrics, and mineralogically manganiferous metasediments occur in the Combin unit, but demonstrateareturn to facies metamorphic in places anundisturbed primary contact between the conditions. metachertsand the underlying metabasalts (Bearth & The work of Bearth (1967) has shown that the rocks of Schwander 1981) demonstrates an origin within the ophiolite the Zermatt-Saasophiolite preserve, at least locally, unit.Associated with the Mn-rich metasediments are developed at numerous stages in the evolutionary almandine-bearing, quartz-rich micaschists and calcschists. history. Relics of magmaticclinopyroxene, olivine, plagi- Largenumbers of veinscross-cut the rocks of the oclase are all foundin the Zermatt-Saaszone, most Grmatt-Saas zone; they can be crudely divided into three frequently in gabbros (especially the Allalin gabbro; Bearth suites: pre-, syn- and post-high-pressure metamorphism. Of 1967). The former presence of (hydrothermal?) hornblende the premetamorphic group, the dominant type is now a set has been suggested, though not proven (Bearth 1967, p. 79). of diffuse-margined, oftenirregular patches rich ina pale Mineralscharacteristic of the (polystadial)high-pressure beige clinozoisite, together with minor quantities of garnet metamorphism, such asomphacite, lawsonite (pseudo- and lesser amounts of quartz, carbonate, rutile, glaucophane morphed) and glaucophane are abundant, and these show

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extends across the boundary into the type B field (Fig. 5). Thereis widespread evidence of the reaction ofhigh- pressure assemblages to lower-pressure ones characterized by albite,chlorite, actinolite and minorbiotite. The extensivedevelopment of thesepost-high-pressure as- semblages is particularly marked in three types of locality: areas where albite veins are abundant; where metabasalts arejuxtaposed with largebodies of metasediment; and wheredeformation post-dating the high-pressure meta- morphism isintense. The common feature of these situations may be that they provide readyaccess for aqueous fluids.

Petrography of the meta-igneous rocks Themajor rocktypes are distinct in terms of mineral assemblage and texture.

Ultrabasic rocks Ultrabasic rocks consist largely of serpentine (antigorite), together with minor quantities of magnetite. Relict augite is sometimespresent and olivine can be found in duniticf peridotiticlenses (Bearth 1967, pp. 24-5). Titanian clino- humite (found as orange-brown crystals up to 6cm long), chlorite, diopside, tremolite and forsterite are all found as metamorphic minerals in small quantities. The margins of serpentinite bodies are usually marked by schists containing tremolite-actinolite in a matrix of talc or chlorite. Scattered pods of homogeneousand fine-grained idocrase-chlorite rock, together with -garnet lenses are presumed to be metamorphosed rodingites. Metagabbros Bearth (1967), Chimer & Dixon (1973), and Meyer (1983) have all examined the Zermatt-Saasmetagabbros. The majorityconsist largely of omphacitetogether with paragonite-clinozoisitepseudomorphs after ; minor quantities of chloritoid, garnet and glaucophane are present in someinstances. Compositional layering is expressed primarily by variation in the ratio of omphacite to plagioclasepseudomorphs. Later greenschist-facies altera- tion, producing albite, actinolite and chlorite, is patchy and generally only becomescomplete where post-eclogite deformation has beenintense. Partial alteration of omphacitetoa sodic-calcic amphibole can be seen texturally, and may be evidence of an intermediate stage on the P-T path. Layers so mafic as to contain no plagioclase pseudo- morphs make up a small proportion of metagabbro volume, butcontain extremely varied assemblages. These include Fig. 3. (a) Relict igneous texture in the Allalin gabbro, Allalin both bimineralic eclogites and some glaucophane eclogites gletscher moraine. Dark grey material is omphacite, pale grey in which the is chloromelanite as well as chloritoid pseudomorphed plagioclase and white (left of coin) talc. The latter eclogitesand assemblages with a sub-calcic amphibole. has a dark grey rim of garnet. (b) Sheeted dykes of eclogitic Some layers contain chrome-epidote. The most mafic layers metabasalt below north Mellichgletscher. Note the symmetry and consist of assemblages such as talc-chloritoid-garnet- traceable length of the dykes. (c) Pillows of eclogitic metabasalt, omphacite (high-pressure stage) and chlorite-magnetite-talc with interpillow sediment. From moraine of S Mellichgletscher. (low-pressure stage). A wide variety of complexassemblages occurs in the widespreadevidence of mutualreaction andre- remarkable and almost undeformed Allalin gabbro, a thrust equilibration.The composition of most of thegarnets slice c. 3 X 1km2 situated about 5 km SSW ofSaas formedat this stage placesthe eclogites in type C of Fee.Here, the size of equilibriumdomains during the Coleman et al. (1965), as might beexpected from their high-pressuremetamorphism wasless thanthe original assemblages.However, therange of garnetcompositions (igneous) crystal size (4-5 cm) and so domains of markedly

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differentcomposition have formed. The resulting as- semblages display equilibrium within individual domains, but adjacent domains display assemblages not in equilibrium with each other. Bearth (1967) has illustrated a variety of textures and assemblages. In former augite sites, the igneous pyroxene is partially or completely replaced by omphacite. Pseudomorphsafter plagioclase containclinozoisite, kyan- ite, and one or more of omphacite, paragonite, garnet and glaucophane(Chinner & Dixon 1973). Olivine sites now consist of a garnet rim enclosing talc, chloritoid and kyanite bearing assemblages (Bearth 1967; Chinner & Dixon 1973).

Metabasalts As discussed by Fry (1972) and Bearth (1973), it is possible to subdivide the assemblages present in the metabasalts into threegroups: (i)high-pressure parageneses, (ii) low- pressure parageneses and (iii) mixed assemblages. The first two groups are almost invariably distinguished by seven or fewerminerals in equilibriumalthough eight components (Na,0-Ca0-Mg0-Fe0-A1203-TiOz-Si02-HzO)at leastare needed to describe the minerals, whereas the third group generally contains more than seven phases. All assemblages containan epidote-group mineral, and an early or late amphibole.In groups (i) and (ii) only one of eachpair rutilelsphene and garnetlchlorite is present. The most common high-pressureassemblage as seen now contains omphacite-garnet-clinozoisite-glaucophane- paragonite-quartz-rutile-apatite with orwithout ankerite and phengite (Fig. 4b). Talc, chloritoid and kyanite are the othernotable minerals encountered in omphacite- and garnet-bearingassemblages. Talc and chloritoid occur withoutomphacite in epidote-and garnet-bearing blue- amphibole schists. Talc occurs with omphacite both with and P 2mm withoutglaucophane. Kyanite, where present, occurs surrounded by a reaction rim of paragonite (Bearth 1967, 1973; Fry 1972; Fig. 4c). Mineral assemblages are listed in Table 1. Pseudomorphs of clinozoisiteand paragonite after lawsonite areabundant inmany of therocks of the Zermatt-Saaszone (Fry & Fyfe 1971; Bearth 1973). Fry (1972) recognized the significance of lawsonite as a temporary reservoir for water, a role it plays effectively due to its high watercontent. Fry(1972) alsonoted that the abundance of lawsonitepseudomorphs along (healed) cracks,and their size (generallylarger than the other minerals) implied that a fluid was present during the growth of lawsonite. Fry & Fyfe (1971) attempted to reconcile this with normal crustal pressures (at which eclogite is unstable with water) by suggesting thatlawsonite was anearly mineral,acting as a sink forwater, and givingrise to

Table l. High-pressure mineral assemblages of rnetabasalts from the

4 Zermatt-Saas Fee zone. The minerals were not necessarily all stable 0.5mm together

omphacite-garnet-quartz-clinozoisite-glaucophane-paragonite- rutile-apatite-(1awsonite'-ankerite-phengite)t Fig. 4. (a) Inclusions of talc and sodic-calcic amphibole in early, omphacite-garnet-quartz-clinozoisite-paragonite-kyanite-talc- -poor omphacite from pale eclogite (B84100). (b) A typical, mtile common metabasalt assemblage with paragonite (clear) and omphacite-garnet-clinozoisite-chloritoid-talc-mtile glaucophane (grey) growing overan omphacite-rutile-garnet garnet-epidote-glaucophane-chloritoid-paragonite-talc-sphene fabric. Note the concentration of inclusions towards the coreof the garnet porphyroblast (B83188). (c) Kyanite in pale eclogite * Now pseudomorphed by clinozoisite, paragonite and quartz. (B83171), surrounded by paragonite. t Can all be absent or present in any combination.

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vapour-absent conditions in which eclogites could form. In Pale eclogites contrast, Bearth (1973) considers lawsonite as late relative A few rare, pale metabasicrocks found as moraine blocks in to eclogitemineral growth, citing in evidencethe theground northwest of Pfulwe are more magnesian weakly-deformed nature of the pseudomorphs, and the lack thanthe common metabasalts. These rocks may be of lawsonite inclusions in garnet and omphacite. metabasalticor metagabbroic; coarseearly crystals of Lawsonite pseudomorphs in some eclogites are observed jadeite-poor omphacite suggest a gabbroic protolith. These to cross-cut the fabric defined by the crystals of omphacite rockscontain ragged relict crystals of kyanite,a mineral (e.g. B82112,B82155) andthere are no inclusions of reported only frommetagabbros and veins in thisregion lawsonite within garnet or omphacite. Occasionally, garnet (Bearth 1967; Fry 1972; Chimer & Dixon 1973). Crystals of crystals are present in the pseudomorphs, but these are very pale-bluekyanite, up to 10mm long,are rimmed by rare. We would notexpect to find omphaciteinclusions, paragonite (Fig. 4c). They are set in a matrix of omphacite, becauseomphacite would be consumed in the pseudo- quartz, garnet, and rutile. Talc is present as felted morphing reaction. The lawsonite may have been formed in masses in the matrix of the rock, where some of it may be eclogitescontaining vapour by relativelya late (post- the result of late hydration and silicification of carbonate, as omphacite-garnet) crossing of the reaction that gave rise to in somecases partially replaced ankerite crystals can be the lawsonite breakdown: found.The omphacite is found as large(up to 10mm) lawsonite jadeite = paragonite clinozoisite relatively jadeite-poor crystals recrystallized at the margins + + to omphacite with c. 50 mol % jadeite. The cores of these + quartz + water, omphacitescontain inclusions of talc, of anearlier generationthan seen in thematrix, and a sodic-calcic but at this stage in an up-pressure sense, or they may have amphibole (Fig. 4a). been formed in initially vapour-free rocks by the addition of water.Whatever thelawsonite-forming reaction, we recognize it must have occurred in rockswhich were already Mineral chemistry eclogites after the bulk of their deformation, which imposed Because themetabasalts are derived from relatively a strong fabric on the omphacite matrix of the rocks and homogeneous, fine-grained material, compositional hetero- causedgarnets to becomefringed by omphacite-quartz geneitiescorresponding to igneous grain sue are much filled pressureshadows. The fitting of theseobservations smallerthan the dimensions of metamorphicequilibrium into the overall P-T history will be discussed later. Martin domains. Neighbouring domains are therefore usually close (1982) disagrees with these findings andmakes the in chemical composition, giving littlereason for chemical additional observation that aligned glaucophane needles are interactionbetween domains and reaction-zone develop- sometimescontained within the now pseudomorphed ment. We havetherefore concentrated study on the lawsonite. metabasalts and the pale eclogite (see above) rather than Within thehigh-pressure assemblage, clinozoisite was themore compositionally complex metagabbros.Data on stable early on, as shown by its inclusion in garnet cores, thecomposition of theminerals in these rocks has continued to be stable in the presence of lawsonite in at previously beenpublished by Bearth (1967, 1973), Fry leastsome rocks, and became ubiquitous and greatly (1972) and Oberhhsli (1980). Mineralsfrom rocks increased in amount by the breakdown of lawsonite. The equivalent to the Zermatt-Saas zone have been analysed by status of paragonite is unclear.Some visibly replaces Ernst & Dal Piaz (1978). Representative compositions are lawsonite orkyanite; much of therest occurs as plates presented in Table 2. embedded in and cross-cutting omphacite, implying growth Garnets occur as euhedralcrystals up to 10mm in at the expense of omphacite-bearing assemblages.This does diameter. An inclusion-rich core is usually surrounded by a not rule out the possibility that paragonite might previously clear rim 1-4 mm thick. As pointed out by previous workers havecoexisted with omphaciteand garnet in smaller (Fry 1972; Ernst & Dal Piaz 1978; Oberhbsli 1980), and in quantities. Both glaucophane and ankerite are also seen as common with many other eclogiticterrains (e.g. Tauern porphyroblasts growing across an earlier omphacite-garnet- Window,Holland, 1979a; Syros, Ridley 1984; Sanbagawa rutile fabric. and New Caledonia, Brothers & Yokoyama 1982) they are Inclusions in garnet porphyroblasts are usually mainly of strongly zoned, with Fe, Mn rich cores and Mg richer rims. quartz, but clinozoisite and rutile are quite common, asis Ca decreases slightly from core to rim (Fig. 5). carbonate in somerocks. Omphacites, where present, are Omphacite usually occurs as sub- to anhedral crystals up the outermost inclusions within garnet. Bearth (1973) made to 4mm long, frequentlydefining a planar fabricin the rock. the important observation that glaucophane inclusionsoccur Mineralinclusions areabsent, but small (<3pm) fluid concentrated towards the cores of some garnets, implying inclusions arepresent inmany rocks.In all analysed theirinclusion atan earlier stage than omphacites. The specimens, the contain 40-55 mol % jadeite, and possibility of two stages of glaucophane stability, separated althoughzoning is by no meansregular, most coresare by a glaucophane-free eclogitic stagewill be discussed later. more acmite-rich than the rims, which all tend to jad,, (Fig. It is notable that of the common high-pressure minerals, 6). The pale eclogites referred to previously (e.g. B83171, paragonite and lawsonite are the only silicates not included B83172,B84047) containlarger (c. 10mm) grainsof in garnet, while glaucophane is restricted to the cores; these omphacite which have rims of a similar composition. Their arethe same three silicates which are normallyseen earlycores are markedly less jadeite rich(35-42 mol %) overgrowing or cross-cutting omphacite fabrics. The strong than the smaller omphacites and are notable as they contain implication, though petrography cannot be takenas proof, is inclusions of talc and a sodic-calcic amphibole (Fig. 4a). that paragonite, glaucophane and lawsonite were not stable Rutile is generallypresent as small (

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w3w 333bmd h8?I I W300 000000 iQ

.-E +

8883 13sm- I "?S I Q W-00 00 000 i

NI I

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ubiquitousin the metabasalt assemblages. The other- ...... Field of 41 core compositions chloritoicLis restricted to rocks of peculiarcompositions Field of 30 rim compositions + 883175 cores which, despitea widevariety of mineralcompositions, h 003175 rims representa negligible proportion (

Field of 35 core compositions Fleld of 34 rim composi?ions, S83175 cores B83175 rims B84047 cores 884047 rims Fig. 6. The composition of omphacites from metabasic rocks of the Zermatt- Saas Fee zone. Analyses are plotted in terms of jadeite (calculated first), acmite and augite (including all other end-members). Fe3+ was calculated assuming four cations.

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1 I I Pressure Pressure k bor

25

20

1s

600 700 Temperature 'C Tornperoturo 'C

(4 (b) Fig. 7. The portion of the calculated grid (A. C. Barnicoat & N. Fry, unpubl. data) relevant to the rocks of the Zermatt-Saas Fee zone, plotted in P-T space using the data of Holland (1979b), Newton and Kennedy (1963) and the estimated position of reaction 03 based on the data of Ridley (1984). Reactions in the equivalent jadeite-bearing system are plotted with light lines. The dotted lines represent the position of the equilibria for pyroxene in of composition jad45, calculated assuming molecular mixing (Holland 1983). Reaction 03 is the blueschist to eclogite transition of Ridley (1984), while the high-temperature limit of lawsonite is reaction 01. Reaction J1 marks the upper stability limit of paragonite alone (Holland 19796); in omphacite bearing rocks reaction 02 and lawsonite-bearing analogues limit paragonite stability. The more magnesian nature of the pale metabasites and the lower jadeite content of their pyroxenes means that the reactions crossed by them are displaced as indicated by the dotted lines. 7(n) shows two possible prograde P-T paths for the Zermatt-Saas zone rocks; 7(b) indicates the likely form of the P-T path during return to theEarth's surface. Further discussion of these paths is presented in the text.

assemblages seen are blueschist-faciesinclusions found in Table 3. Metamorphichistory of metabasalts from the Zermatt-Saas garnetcores; no evidence has been found of lawsonite at Fee zone. Themineral assemblages listed at each stage are not this stage, suggesting that the highest-grade conditions may intended to becomplete, butrather to indicateco-existences indicated havebeen reached by a P-T pathremaining tothe bY therocks now high-temperatureside of the lawsonite + omphacite-out reaction (04) and possibly on the high-temperature side of (1) Normal metabasalts (i) glaucophane-clinozoisite-quartz-rutile the lawsonite-out curve (01)too (Fig. 7a). The highest-grade (ii) assemblages found, the kyanite-talc eclogites, are preserved omphacite-garnet-quartz-clinozoisite-rutile (iii) omphacite-garnet-quartz-clinozoisite-rutile-lawsonite only in a very small number of rocks. Many other eclogites (iv) omphacite-garnet-quartz-clinozoisite-rutile-glaucophane- grew lawsonite after eclogitic parageneses were established, paragonite and sometimes ankerite possibly by a drop in temperature, or in temperature and (2) Pale metabasites pressure (Fig. 7b). The subsequent growth of glaucophane (i) omphacite-clinozoisite-quartz-garnet-kyanite-rutile* andparagonite is accompanied in some rocksby the (ii) omphacite-clinozoisite-quartz-garnet-paragonite-rutile development of ankerite. It is not obvious whether this is ~~~ the result of decreasing temperature leading to the crossing * Talc may have been stable with this assemblage. of a CO,-bearing reaction analagous to 03 and J3 (Fig. 7b) in the presence of a mixed H20-C02fluid, or if an influx of CO,-rich fluid gave rise to this late growth of glaucophane, paragonite-bearing assemblages haddeveloped. The P-T paragonite and ankerite. A detailed study of the distribution path plotted in Fig. 7b has been drawn in the light of this. of ankerite-bearing assemblages in pillow lavas suggests that Compatibilityplots of assemblages in sub-systems of the CO2 was derived locally from interpillow sediment. The Na,O-CaO-MgO-FeO-AI,O,-Si0,-H,O (NCMFASH) can exact form of the P-T pathduring thereturn of the be used to examine the relationshipsbetween the Zermatt-Saaszone to the surface is at presentunknown, lawsonite-absent and lawsonite-bearingassemblages de- but consideration of the role of lawsonite suggests that it veloped at and after the highest-grade conditions. If quartz, remainedstable until at leastsome of the glaucophane- clinozoisite anda Fe-phase are assumed ubiquitous, the

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distinction betweenvapour-present and vapour-absent metamorphism. The paragonite- and glaucophane-forming reactions J3 and 03 consumevapour and could each in rocks of appropriatewater content give rise to vapour-deficient conditions, hence arresting further progress of that reaction (c.f. Yardley 1981). It is possible that much of the vapour phase had escaped by this stage, as the P-T path was then crossing the isochores for waterin the direction of increasing volume. The veins containing high-pressureminerals (omphacite, glaucophane, kyanite) could have accompanied fluid escape at this stage of high-pressure metamorphism.

Equilibration conditions A number of previous workers have attempted to estimate N the conditions of equilibration of the Zermatt-Saas eclogites. The earliest was Fry (1972) who guessed 400”C, 17-20 kbar in water-deficient conditions. Chinner & Dixon (1973)suggested thatthe Allalin gabbro crystallized at 10-15 kbarand 500-700°Cwith ‘a sufficiency of water’. Morerecently, Ernst & Dal Piaz(1978) suggested that temperatures were 470 f 50°C and pressures 10 f 2 kbar, with aH,O very low. Oberhiinsli (1980) investigated some of Bearth’s material in detail and postulated equilibration at c. 600°C and about 14 kbar. He pointed outthat equilibration conditions calculated from partitioning of Fe and Mg between garnet and clinopyroxene givehigher temperaturesfor rim compositions than for cores, because of their more Mg-rich nature.Temperatures estimated by Martin (1982)from garnet-pyroxene pairs show a substantial range for the same reasons. He applied the correlations produced by Raheim & Green (1975) to the analyses of Fry (1972). The resulting maximum temperatureline (that for rimcompositions) N passes through 500°C at 10 kbarand 540°C at 19kbar, showing the insensitivity of such methods to pressure. These Fig. 8. Peak eclogite assemblages inNCMFASH, projected from slightly lower temperatures are more comfortably reconciled quartz, clinozoisiteand garnet into the tetrahedron N(CM)AH. with the occurrence of antigorite in the nearby serpentinites. (a) assemblages seen or possible below the breakdownof lawsonite. None of the above authors had reliable geobarometers (b) assemblage stable at temperatures greater than the highest available. Recently,Holland (19796) has experimentally temperature limit of lawsonite. determined the high-pressure stability limit of paragonite, where it breaks down to jadeite, kyanite andH20. The pale eclogites describedabove contain omphacite (jad,,) and system can be reduced to four components (Na,O-CaMg0,- paragonite replacingkyanite. For this assemblage, and A1203-H20).As garnet is by farthe most Fe-rich phase assumptionsabout jadeite mixing in omphacite(Holland present, it is a logical projection point, and furthermore the 1983) a P-aH,O equilibrium line can be calculated for any exact garnet composition used does not significantly affect chosen value of T, or we can consider the equilibrium plane the resultingprojection. Following Holland (1979a),a cuttingthrough P-T-aH,O space.This gives activities of composition representative of a typicat garnet rim has been watergreater than c. 0.4 at 600°C and 0.2 at 500°C for used. The resultingtetrahedra (Fig. 8) show the pressures sufficient to stabilize omphacite as opposed to relationshipsbetween the assemblages listed inTable 3. albitic assemblages. We therefore accept a moderate or high Figure8a is for assemblagescontaining lawsonite + aH,O at the time of rim equilibration. omphacite, equilibrated at temperatures lower than reaction An additional constraint can be placed on aH,O by the 04, while Fig. 8b is for assemblages equilibrated above the coexistence of zoisite, kyanite, garnet and quartz in these stability limit of lawsonite (01). Normalmetabasalts, pale eclogites. This assemblagedefines aH,O by the containing lawsonite pseudomorphs, are represented by the reaction volume omp-lws-vap(-par) in Fig. 8a. The pale metabasites 6Ca2Al3Si3O,,(0H) = 4Ca3Al2Si3OI2 fit in the omp-kya-vap-talc volume of Fig. 8b, and so the talc found in them could be a stable part of the assemblage. The zoisite grossular two important metabasalt assemblages (kyanite-talc eclogite + 5A1,Si05 + SiO, + 3H,O and lawsoniteeclogite) could thus both be fluid present kyanite quartzwater according to the phase rule, although Thompson (1983) has shown that the phase rule cannotconclusively demonstrate a for which thermodynamic data may beobtained from

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Halbach & Chatterjee (1984). Using data for G,,, from this is the case, the removal of a vapour phase cannot by Halbach & Chaterjee (1982) it is again possible to calculate itself reduce aH,O below the value it has when a fluid is P-aH,O relationships at a given temperature. If we further present. assume that the two assemblages equilibrated at the same time at 550°C,we can solve forboth P and aH,O; calculations are quite sensitive to garnet and epidote mixing Regional implications models (Newton & Haselton 1981; Kerrick 1977). The best The data obtained here form aninteresting comparison with estimate of conditions that we obtain in this way for 550°C is Chopin & Moni& (1984) who obtained an estimate of 500°C, 17.5 kbar with aH,O close to 0.55, rising at 600°C to 20 kbar 16 kbarand aH,O of 0.6from a Mg-chloritoid-bearing, and aH,O equal to 1 (Fig. 9). quartz-rich pelite of the Monte Rosa cover sequence, which Thisanswer is interesting in the light of our field structurally immediately underlies the Zermatt-Saas ophiol- observations of widespread veins containingquartz with ite zone. The similarity in conditions for adjacent portions some of omphacite,kyanite and glaucophane, and the of thesetwo units suggests thatthey may havebeen presence of fluid inclusionswithin omphacites.These all juxtaposed by the time of their high-pressure metamorph- indicatethat anaqueous fluid-phasewas present at least ism,dated at 110 f 3Ma in theMonte Rosa rocks by transientlyduring the eclogitic metamorphism of these Chopin & MoniC (1984). Alternatively, the comparability of rocks, which by its presence would buffer aH,O close to 1, the metamorphicconditions may be the result of the contrary to Fry & Fyfe (1971). Thompson (1983) has argued syn-metamorphic juxtaposition of ophiolitic and continental strongly that water is lost during burial and metamorphism, materialin a subduction zone, where downgoing material andthat vapour-present conditions in general onlyoccur wasprogressively underplatedonto the over-riding plate during the course of dehydrationldecarbonation reactions (Rubie 1984). Thistype of model suggests thatthe units when vapourproduction rates exceed escaperates. preserveddeeper in the pile wereunderplated later and However, as pointed out by Norris & Henley (1976), the shouldhence have a younger high-pressure imprint. The molar volume of water under blueschist and eclogite facies highest rocks recording high-pressure metamorphism, those conditions is less than at the Earth's surface, so that unless of the Sesia zone, give aRb-Sr whole-rockisochron of the P-T pathto suchconditions isparticularly tortuous, 129 f 15 Ma (Oberhhsli et al. 1985), and Rb-Sr and K-Ar there will be no driving force for the liberation of water due ages from micas of 80-60 Ma (Hunziker 1974; Oberhansli et to expansion during journey to such conditions. Buoyancy- al. 1985) which have been interpreted as cooling ages. The drivenescape in these rocks may be slowowing totheir rocks of the Zermatt-Saaszone give K-Ar ages from comparatively cool conditions of formation and hence high amphiboles of 100-62 Ma (Hunziker 1974; Bocquet et al. strength.Softening or deformation dueto metamorphic 1974; Delaloye & Desmons 1976). The lowest rocks seen in reaction might enable loss of vapour. Nevertheless, it seems the pile,those of theMonte Rosa nappe, have early far from impossible for an initial vapour phase to remain high-pressure metamorphismdated at 110 f 3 Mausing trapped in such rocks during the whole of the prograde (P 40 Ar-39Ardating of phengites; Ar loss from less retentive increasing) stage of theirmetamorphism. Whether or not sites down to c. 65 Ma is also suggested by the data (Chopin & MoniC 1984). Thus, there is an apparent difference of c. Pressure 20 Ma in the age of high-pressure mineral growth between kbar the highest unit (Sesia zone) and the lowest (Monte Rosa nappe), suggesting that Rubie's (1984) model is correct. The age of the high-pressure imprint in the Zermatt-Saas zone is 24 t notyet clear though, as all K-Armineral ages appear to datethe laststages of open-systembehaviour during high-pressure metamorphism. The similarity in form of the modelled uplift paths of the Zermatt-Saas zone (this work) andthe Sesia zone(Lardeaux et al. 1982; Rubie 1984) 20 corroborates the suggestion that these units were juxtaposed during or prior tothe culmination of high-pressure metamorphism and that they remained together during their 18 subsequent exhumation.

ACB acknowledges research grant GR3/5107 from NERC. Probe 16 work was smoothly performed at Edinburgh with the expert help of P. G. Hill and D. G. Russell. R. Oberhansli, A. B. Thompson and especially John Ridleyhave provided useful andstimulating 14 discussion. Weare grateful for the helpful comments of two anonymous referees.

1 0.9 0.8 0.7 0.6 0.5 0.4 References a H,O BEARTH,P. 1959. Uber Eklogite, Glaucophanschiefer,metamorphe und Pillowlaven. Schweizer Mineralogische und Petrographische Mitteilungen Fig. 9. The zoisite-outand paragonite-out equilibria calculatedat 39,267-86. 550 and 60O0C,allowing the simultaneous estimation of P and aH20 - 1%7. Die Ophiolitheder Zone vonZermatt-Saas Fee. Beitrage (see text for details). Schweiz KarteGeologischer N.F. 132.

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Received 1 July 1985; revised typescript accepted 10 December 1985.

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