High-precision U-Pb and 40Ar/39Ar dating of an Alpine ophiolite (Gets nappe, French Alps) Autor(en): Bill, Markus / Bussy, François / Cosca, Michael Objekttyp: Article Zeitschrift: Eclogae Geologicae Helvetiae Band (Jahr): 90 (1997) Heft 1 PDF erstellt am: 04.10.2021 Persistenter Link: http://doi.org/10.5169/seals-168144 Nutzungsbedingungen Die ETH-Bibliothek ist Anbieterin der digitalisierten Zeitschriften. Sie besitzt keine Urheberrechte an den Inhalten der Zeitschriften. Die Rechte liegen in der Regel bei den Herausgebern. Die auf der Plattform e-periodica veröffentlichten Dokumente stehen für nicht-kommerzielle Zwecke in Lehre und Forschung sowie für die private Nutzung frei zur Verfügung. Einzelne Dateien oder Ausdrucke aus diesem Angebot können zusammen mit diesen Nutzungsbedingungen und den korrekten Herkunftsbezeichnungen weitergegeben werden. Das Veröffentlichen von Bildern in Print- und Online-Publikationen ist nur mit vorheriger Genehmigung der Rechteinhaber erlaubt. 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Tethys ABSTRACT RESUME Coarse-grained gabbros from two different localities in the Gets nappe (Upper Des gabbros de deux localités différentes de la nappe des Gets (Préalpes Prealps) have been dated by U-Pb and ""'Ar/'^Ar isotopie analyses. Zircons supérieures) ont été datés par les méthodes isotopiques U-Pb sur zircon et from both gabbros gave identical concordant U-Pb ages of 166 ± 1 Ma (Fig. 4). "Ar/'^Ar sur amphibole. Les zircons des deux gabbros ont donné des âges U- Amphibole from one of them gave an 4"Ar/wAr plateau age of 165.9 ± 2.2 Ma Pb identiques ct concordants de 166 ± 1 Ma (fig. 4). L'amphibole de l'un d'eux (Fig. 5). This concordance implies that 166 ± 1 Ma is the age of magmatic a donné un âge plateau 4"Ar/î,Ar de 165.9 + 2.2 Ma (fig. 5). Cette concordance crystallization of these gabbros. démontre que 166 ± 1 Ma est l'âge de leur cristallisation magmatique. The Gets wildflysch with its mafic and ultramafic lenses is an ophiolitic Nous interprétons le wildflysch à lentilles mafiques et ultramafiques de la mélange, that we infer to come from a proximal part of the accretionary prism nappe des Gets comme un mélange ophiolitique qui provient d'une partie at the loot of the active SE margin of the Piémont ocean. In this position we proximale du prisme d'accrétion. au pied de la marge active SE de l'océan can expect to find remnants of the oldest parts of the Piémont oceanic crust. Piémontais. On peut donc s'attendre à y trouver des témoins des roches les These are the first high-precision dates using modern techniques from an plus anciennes de la croûte océanique Piémontaise. Alpine ophiolite and are in excellent agreement with the following: Cette première datation d'une ophiolite alpine par des méthodes 1 The few. somewhat younger, reliable ages on ophiolites from the probable modernes est en excellent accord avec: continuation of the Piémont basin into the Apennines and Corsica: 1) Les rares âges fiables (un peu plus jeunes) obtenus à ce jour sur des 2) Recent data on the age of the first supra-ophiolitic sediments (Late ophiolites de la continuation probable du bassin piémontais dans les Apennins Bathonian to Early Callovian radiolarites); et en Corse: 3) The structural and stratigraphie evolution of the Briançonnais (s.S.) 2) Les données récentes sur l'âge des premiers sédiments supra-ophioli- domain, the future NW margin of the Piémont ocean. We note a remarkable tiques (radiolarites du Bathonien sup. - Callovien inf). coincidence, in Late Bajocian time, between: (A) the end of tensile fracturing in 3) L'évolution structurale et stratigraphique du domaine Briançonnais s. the Briançonnais continental crust: (B) the beginning of its subsidence: (C) str.. future marge NW de l'océan Piémontais. On note une coïncidence the age of the Gets ophiolites. This coincidence is consistent with an ocean remarquable, au Bajocien supérieur, entre: (A) la fin de la tectonique d'extension opening mechanism based on a combination of subhorizontal extension and dans la croûte continentale Briançonnaise: (B) le début de sa subsidence: (C) thermally driven vertical movements of the lithosphère. l'âge des ophiolites de la nappe des Gets. Cette coïncidence est en accord avec un mécanisme d'ouverture océanique basé sur une combinaison d'extension subhorizontale et de mouvements lithosphériques verticaux d'origine thermique. 1. Introduction Ophiolites. relies of oceanic lithosphère, are keystones for complete resetting of the radiogenic clocks. In many orogenic reconstructing the location and history of former oceans in belts, precise dating of ophiolites remains a challenge. orogenic belts. They provide the only direct evidence for The aim of this paper is to provide the first high precision dating the formation of old oceanic crust. geochronologic age of an ophiolite in the Alps. We present But dating the crystallization of ophiolitic rocks is a notoriously U/Pb zircon and 40Ar/39Ar hornblende data from difficult task, because of their very low content in radioactive coarsegrained ophiolitic gabbros sampled in a nappe that was tectonically elements, and because their tectonic position usually expelled out of the closing ocean at an early stage of the predestines them to alteration, metamorphism and partial or orogeny and therefore escaped any significant metamorphism. 1 Institut de Géologie et Paléontologie. Université de Lausanne. BFSH 2. CH-1015 Lausanne 2 Geochronology lab. Royal Ontario Museum. 100 Queen's Park. Toronto. Ontario. M5S 2C6. Canada 3 Institut de Minéralogie et Pétrographie. Université de Lausanne. BFSH 2. CH-1015 Lausanne Precision dating of an Alpine ophiolite 43 10° BASEMENT COVER OPHIOLITES « ASSOCIATED PALEOZOIC MESOZOIC 4 CENOZOIC OCEANIC SEDIMENTS FORE LAND JURA 4 PROVENCE 46 ' HELVETIC LOWER PENNINIC MIDDLE & UPPER * PENNINIC AUSTRO AND mm SOUTH-ALPINE 44" c. CORSICA i LIGURIAN & SUB-LIGURIAN UNITS PROVENCE ^ ELBA » TUSCAN UNITS MEDITERRANEAN SEA ft ALPINE GRANITOIDS CENOZOIC BASINS Be, Bern; Br, Briançon; Gr, Grenoble; La, Lausanne; 3 Km 50 Km CORSICA Mi, Milano; Ni, Nice; Pi, Pisa; To, Torino; Zu, Zürich Fig. 1. Simplified tectonic map of the western Alps. Corsica and northern Appennmes (modified from Escher et al. 1988 and Bigi et al. 1990). 2. Ophiolites in the western Alps this basin was a major branch of the Tethys ocean in the In the Alps, ophiolites are present in several distinct tectonic Alpine system during the Mesozoic (e.g. Elter 1971. Lemoine zones (Fig. 1 and it remains a matter of controversy if all these 1972 and 1983. Dal Piaz 1974a. Bernoulli & Lemoine 1980. zones originated from a unique ocean disrupted by tectonics, Trümpy 1980. Beccaluva et al. 1984. Lagabrielle 1987. Stampfli or if there were two or more oceans perhaps of different age 1993. Martin et al. 1994). and history. Various models of Alpine paleogeography coexist On a cross-section through the western Swiss Alps and in the recent literature (e.g. depending on the assumed status neighbouring regions of Italy and France (Escher et al. 1988 of the Valais domain, the Monte Rosa nappe or the Canavese and 1993). ophiolites of presumed Piémont origin occur in four zone). We will not discuss them here, as a general evaluation tectonic positions (Fig. 2): of all these models would be beyond of the scope of this paper, 1 and 2. - The Zermatt - Saas and Antrona zones: These and we shall base the interpretation of our data on one contain the whole range of ophiolitic rocks (ultramafic and possible reconstruction whose main features are reasonably mafic plutonic rocks, dykes and volcanic rocks) associated with founded and generally accepted. In particular most palinspastic minor amounts of oceanic sediments. They have been affected restorations agree that at least a majority of the ophiolites during eoalpine subduction by a high-P (eclogite facies) from the western Alps belong to the Piémont basin and that metamorphism. followed by a greenschist or amphibolite grade 44 M. Bill et al. NW Rhone Valley Matterftom SE SESIA ZONE W IVREA ZONE >¦ Jen! Blanche Molasse -™ Hs; c;„s >-<<>,. Mischabel iSS s'.s -.>•¦" <rg„_ ""H «£"<* C»UST L°*e« 'JPp v.*,«Tic cfi Cf.Us Pü"Op T-., c*usr «DR,. rie lCW »ANT Zur. Op Eto CRosr eo*ope^ MANTI 40 Km Fig. 2. Tectonic cross-section through the western Swiss Alps Black ophiolitic nappes (modified from Escher et al. 1993 and in press) metamorphism of Tertiary age (e.g. Bearth 1974. Dal Pia/ <md partly chaotic basal formation (wildflysch) with abundant 1974b. Hunziker 1974. Dal Piaz & Ernst 1978. Colombi 1989, blocks and lenses of ophiolites (basalts and diabases, serpentinites Pfeifer et al. 1989. Steck 1989. Vannay & Allemann 1990). and more rarely gabbros). deep-water sediments 3. - The Tsaté nappe (middle and upper parts of the former (radiolarites. pelagic limestones, manganesiferous shales) and Combin /one of Argand 1909: Escher et al. 1988. Sartori 1987): Paleozoic granites imbedded in a black pelitic matrix (Fig. 3). It Here the ophiolites form slices and lenses imbricated with is affected by a very weak metamorphism (illite "cristallinity" calcschists (the "schistes lustrés").