Early Permian Magmatism in Briançonnais Terranes : Truzzo Granite and Roffna Rhyolite (Eastern Penninic Nappes, Swiss and Italian Alps)
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Early Permian magmatism in Briançonnais terranes : Truzzo granite and Roffna rhyolite (eastern Penninic nappes, Swiss and Italian Alps) Autor(en): Marquer, Didier / Challandes, Nathalie / Schaltegger, Urs Objekttyp: Article Zeitschrift: Schweizerische mineralogische und petrographische Mitteilungen = Bulletin suisse de minéralogie et pétrographie Band (Jahr): 78 (1998) Heft 3 PDF erstellt am: 10.10.2021 Persistenter Link: http://doi.org/10.5169/seals-59297 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. 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MITT. 78,397-414,1998 Early Permian magmatism in Briançonnais terranes: Truzzo granite and Roffna rhyolite (eastern Penninic nappes, Swiss and Italian Alps) by Didier Marquer1, Nathalie Challandes' and Urs Schaltegger2 Abstract In the internal part of the Central Alps, heterogeneous Tertiary deformation of basement nappes led to large weakly deformed domains. This is where studies of pre-Alpine magmatism can be performed. The Truzzo granite and the Roffna rhyolite, which are part of the Penninic Tambo and Suretta nappes, respectively, are shallow-seated intrusions in old polycyclic basement rocks. Petrographical, geochemical and isotopic similarities suggest that Truzzo granite and Roffna rhyolite are part of the same post-orogenic magmatic event. Furthermore, U-Pb systematics on single zircon give a 206Pb/238U age of 268.0 ± 0.4 Ma for the Truzzo granite and a 206Pb/238U age of 268.3 ± 0.6 Ma for the Roffna rhyolite. These intrusions were derived from the mixing of crustal and mantle sources, as indicated by the isotopic results and the occurrence of different types of xenoliths. The proximity of these intrusions becomes apparent in an Early Permian reconstruction of the Briançonnais ter- rane. They might have had their origin in upwelling asthenospheric mantle, related to lithospheric thinning at the site of the future Jurassic rifting and opening of the Liguro-Piemontais ocean. Keywords: granites, U-Pb ages, zircons, Early Permian magmatism, Penninic nappes, Central Alps. Introduction The Swiss Alps are usually divided into three broad domains from north to south: (i) an external, Orogens normally comprise imbricated Helvetic domain, (ii) an internal part, the crystalline terranes which have to be integrated in Penninic zone and (iii) the South-Alpine and Aus- pre-collisional paleogeographical reconstructions. troalpine units, the latter exposed on top of the The main problem for studying such strongly Alpine edifice (Coward and Dietrich, 1989). deformed basement is to distinguish between Basement rocks in the Helvetic domain and the structures that pre-date the most recent orogenic South- and Austro-Alpine units did not undergo cycle, in order to get information about pre-colli- strong penetrative Alpine deformation, facilitating sional structures, petrology and geochemistry of the petrological and geochemical studies of these rocks. Very heterogeneous deformation, in pre-Alpine structures and magmatism in these some places within the Alpine nappe pile, has areas. On the contrary, the Penninic zone represents allowed the preservation of metamorphic and the strongly deformed internal parts of the Alpine magmatic relics (Marquer, 1991; Biino et al., 1997), mountain belt, the result of collision between the which can be used to reconstruct the lithological European plate in the north and the Adriatic plate composition, geochemistry and geochronology of in the south, with a series of microcontinents in pre-Alpine basement and intrusive rocks. In the between, one of them being the Briançonnais ter- case of intrusive rocks, it is necessary to determine rane. The Penninic domain consists of imbricate primary mineralogical compositions, magmatic stacks of sedimentary cover and basement slices textures and geochemical characteristics in order with a complex nappe geometry as the result of to constrain their geodynamic setting in the both thrust tectonics and post-nappe refolding earlier, pre-Alpine (Variscan) orogenic cycle. (Fig. 1) (Schmid et al., 1990; Schreurs, 1993; 1 Geological Institute, 11, rue E. Argand, CH-2007 Neuchâtel, Switzerland. <[email protected]> 2 Department of Earth Sciences, ETH-Zentrum, Sonneggstrasse 5, CH-8092 Zürich, Switzerland. 398 D. MARQUER,N. CHAI.I.ANDES AND U. SCHALTEGGER Pfiffner et al., 1990; Marquer et al., 1996). Our Marquer et al., 1996) and the Truzzo granite in investigations focus on the basement rocks of the the southern part of the Tambo nappe (Weber, upper Penninic Tambo and Suretta nappes in the 1966; Marquer, 1991) were investigated (Fig. 2). eastern internal Swiss Alps, which belong to the Previously published Variscan ages for these Briançonnais paleogeographic domain (Fig. 1) granitoids show strong discrepancies: for the Truzzo (Trümpy, 1980). The tectonic framework of the granite, a 339 ± 70 Ma U-Pb age for zircon frontal part of the Suretta nappe and of the Tambo (Grünenfelder in Weber, 1966) and a 293 ± 14 Ma nappe has been recently described by Marquer Rb-Sr whole rock age (Gulson, 1973) were et al. (1996) and Baudin et al. (1993) (Fig. measured. For the Roffna rhyolite an intrusion age 2). One of the main results from these recent studies around 350 Ma (U-Pb zircon) was suggested by is that Tertiary deformation of the basement Hanson et al. (1969). Based on new mapping, rocks was strongly heterogeneous, leading to the petrological and isotopic studies, we propose a preservation of undeformed lenticular domains close genetic link between these two intrusives. surrounded by mylonites at a broad range of Precise intrusion ages and geochemical characteristics scales. Petrological and geochronological studies permit their integration in reconstructions of pre-Alpine intrusives can therefore be concentrated of the Pre-Alpine setting of the Briançonnais ter- on these areas of weak Alpine deformation. rane. The role and significance of this type of mag- Two granitoid bodies, the mostly effusive to matic activity will be discussed with respect to late subvolcanic Roffna rhyolite in the northern part and post-Variscan tectonics prior to the opening of the Suretta nappe (Grünenfelder, 1956; of the Tethys ocean. Fig. 1 Location of the Tambo and Suretta nappes on a simplified geological map of the eastern part of the Central Alps. EARLY PERMIAN MAGMATISM: TRUZZO GRANITE AND ROFFNA RHYOLITE 399 Geological setting Peucat, 1994). To avoid textural and chemical perturbations related to Alpine deformations in The Tambo and Suretta nappes form two ca. high-strain zones, only samples from weakly 3.5 km thick crystalline slivers, mainly composed deformed zones were collected for the following of old crystalline basement (Fig. 2). The poly- study. metamorphic basement is essentially composed of metapelites and metagreywackes including lenses of mafic rocks (amphibolites), porphyritic Petrology and intrusive structures of granitoids orthogneisses and local migmatites (Staub, 1916; Wilhelm, 1921; Gansser, 1937; Grünenfelder, TRUZZO GRANITE 1956; Zurflüh, 1961; Strohbach, 1965; Schae- ren, 1974). The study of metamorphism and The Truzzo granite is a E-W-elongated intrusive deformation in the malic lenses has led to the recognition body located in the southern part of the Tambo of two distinct metamorphic events, a pre- nappe (Fig. 2). The granite cross-cuts different Alpine HP-HT and an Alpine HP-LT one (Biino lithologie units such as older orthogneisses, banded et al., 1997; Nussbaum et al., 1998). In the migmatites and amphibolites, which were Suretta nappe, a late Variscan subvolcanic intrusion previously deformed under high grade conditions is located in the frontal part, the so-called (Fig. 2). Primary magmatic contacts with the Roffna rhyolite (Roffna porphyry; Grünenfelder, surrounding gneissic rocks are sub-vertical and show 1956; Streiff et al., 1976; Marquer et characteristics of syn-intrusive deformation of the al., 1996; Fig. 2). The Tambo basement was granitoid, such as preferred orientation of large intruded by theTruzzo granite in the south (Weber, K-feldspar porphyroclasts and enclaves (Marquer, 1966; Blanc, 1965; Gulson, 1973; Marquer, 1991). Mafic magmatic enclaves and basement 1991) (Fig. 2). xenoliths (orthogneisses, migmatitic gneisses) The autochthonous cover of the Tambo and are present in the whole body of the Suretta nappes is composed