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High-Pressure Metamorphism of the Zermatt-Saas Ophiolite Zone, Switzerland

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High-Pressure Metamorphism of the Zermatt-Saas Ophiolite Zone, Switzerland Journal of the Geological Sociefy, London, Vol. 143, 1986, pp. 607-618, 9 figs, 3 tables. Printed in Northern Ireland High-pressure metamorphism 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 blueschist assemblages, retained as inclusions in garnets, were succeeded by eclogitic assemblages which in some rocks contained lawsonite, kyanite, talc and chloritoid. These eclogitic assemblages formed by prograde reaction from the early blue- schists. Subsequently, reaction of these eclogites with a mixed H,O-CO, vapour phase led to the replacement of kyanite by paragonite and the partial replacement of omphacite-garnet paragenses by assemblages containing glaucophane, 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 accommodate equilibration with a water-rich vapour phase under the highest- grade conditions. The presence of such a fluid phase is locally indicated by the presence of quartz-rich veins containing omphacite and kyanite. This vapour phase was absorbed during retrogression. Later, rehydration is limited to areas close to albite veins, tectonic contacts and bodies of metasediment. The metamorphic conditions determined for the Zermatt-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, eclogite-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 mineral 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 micas, 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/4/607/4893026/gsjgs.143.4.0607.pdf by guest on 02 October 2021 608 A. C.BARNICOAT & N. FRY 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 (<l m; wide; Fig. 3b) with internally symmetrical mineral large masses (as onRimpfischhorn, 6347 0966,and Breithorn, distribution can be traced for more than 100m. These are 62480875) andas thin, well-foliated slivers along tectonic most readily interpreted as deformed sheeted dykes. Much contacts (Mellichen, 6311 0983). Gabbros are well displayed of the remaining metabasaltic material of the Zermatt-Saas in structurally intactmasses, for example above Mellichen is of unclear origin, but near Hubeltini (63240974), Pfulwe (6318 0981) and the Allalinhorn (6354 0993). The Mellichen (6312 0961) and on the Rimpfischhorn (6344 0968) there are gabbro consists in its lower portions of alternating mafic and unambiguous pillowlavas (Fig. 3c),some with quartz- leucocraticlayers on a scale of 10m, on which irregular ankeritebearing metasediment between pillows (Bearth compositional layering at a wide range of smaller scales is 1959, 1967). Pillow breccias are present at Pfulwe (Bearth superimposed. This passes gradationally up into the higher 1967). Chalcopyrite isdisseminated throughthe meta- parts of largely homogeneousleucogabbro. In some igneous rocks and is concentrated by mineralization in gabbros, this paler material reaches some hundreds or more interstitialmetasediment. The majorand trace-element metresthick. Despite their tectonic and metamorphic chemistry of the meta-lavas is consistent with the origin of overprint, the metagabbros are recognizable by their coarse these rocks at a constructive plate margin (Bearth & Stern igneoustextures, which arepreserved, withmodification 1979), although extreme care should be taken in interpreting (Fig. 3a), except in zones of high strain. anydiscriminant diagrams involving Tifor these rocks, The originally finer-grained basaltic rocktypes are because metamorphic veins containing rutile crystals up to identifiable, in contrast, by a fine-grained andmore 7 cm long are comparatively abundant. homogeneoustexture. Basic dykesare visible withinmost The meta-lavas are overlain by twocontrasting metagabbros and are seen in considerable numbers in some metasedimentarysequences. One is a suite of ankeritic Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/143/4/607/4893026/gsjgs.143.4.0607.pdf by guest on 02 October 2021 HIGH-PRESSUREMETAMORPHISM,SWITZERLAND 609 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. mica-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 clinozoisite-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
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