Int J Earth Sci (Geol Rundsch) (2014) 103:61–82 DOI 10.1007/s00531-013-0941-1 ORIGINAL PAPER Basement lithostratigraphy of the Adula nappe: implications for Palaeozoic evolution and Alpine kinematics Mattia Cavargna-Sani • Jean-Luc Epard • Franc¸ois Bussy • Alex Ulianov Received: 6 March 2013 / Accepted: 8 July 2013 / Published online: 20 August 2013 Ó Springer-Verlag Berlin Heidelberg 2013 Abstract The Adula nappe belongs to the Lower Penni- are pre-Alpine in age. Consequently, me´lange models for nic domain of the Central Swiss Alps. It consists mostly of the Tertiary emplacement of the Adula nappe are not pre-Triassic basement lithologies occurring as strongly consistent and must be rejected. The present-day structural folded and sheared gneisses of various types with mafic complexity of the Adula nappe is the result of the intense boudins. We propose a new lithostratigraphy for the Alpine ductile deformation of a pre-structured entity. northern Adula nappe basement that is supported by detailed field investigations, U–Pb zircon geochronology, Keywords Adula nappe Á Central Alps Á Palaeozoic and whole-rock geochemistry. The following units have basement Á U–Pb dating Á Zircon been identified: Cambrian clastic metasediments with abundant carbonate lenses and minor bimodal magmatism (Salahorn Formation); Ordovician metapelites associated Introduction with amphibolite boudins with abundant eclogite relicts representing oceanic metabasalts (Trescolmen Formation); Palaeozoic basement rocks in the Alpine realm experienced Ordovician peraluminous metagranites of calc-alkaline a complex pre-Mesozoic geological evolution, which has affinity ascribed to subduction-related magmatism (Ga- generated major lithostratigraphic unconformities. These renstock Augengneiss); Ordovician metamorphic volcanic– structures are relatively easy to distinguish from those sedimentary deposits (Heinisch Stafel Formation); Early resulting from the Alpine orogeny in the external parts of Permian post-collisional granites recording only Alpine the Alpine belt because pre-Mesozoic structures were orogenic events (Zervreila orthogneiss). All basement mostly formed under high-grade metamorphic conditions, lithologies except the Permian granites record a Vari- whereas Alpine structures developed under low-grade scan ? Alpine polyorogenic metamorphic history. They metamorphic conditions. The situation is much more con- document a complex Paleozoic geotectonic evolution fusing in the internal parts of the Alpine belt because both consistent with the broader picture given by the pre- pre- and post-Mesozoic tectonic structures formed under Mesozoic basement framework in the Alps. The internal high-grade conditions. There is thus a potential risk in consistency of the Adula basement lithologies and the ascribing pre-Mesozoic features to Alpine events, which stratigraphic coherence of the overlying Triassic sediments may lead to erroneous tectonic interpretations. The best suggest that most tectonic contacts within the Adula nappe way to avoid such cases is to identify internally consistent lithologic formations within the Alpine basement, which will document its Paleozoic geodynamic evolution. Electronic supplementary material The online version of this article (doi:10.1007/s00531-013-0941-1) contains supplementary The Alpine domain is composed in part by nappes made material, which is available to authorized users. of Palaeozoic basement. Nappes of this type occur in the Lepontine dome. The nappes of the Lepontine dome are & M. Cavargna-Sani ( ) Á J.-L. Epard Á F. Bussy Á A. Ulianov mainly assigned to Lower Penninic structural domain. Institut des Sciences de la Terre, Baˆtiment Ge´opolis, Universite´ de Lausanne, 1015 Lausanne, Switzerland Their Paleogeographic position prior to the Alpine oro- e-mail: [email protected] genesis is the distal European margin. The Lepontine dome 123 62 Int J Earth Sci (Geol Rundsch) (2014) 103:61–82 represents a structural and metamorphic dome and includes Alpine age) in eclogites (Herwartz et al. 2011). Alpine the deepest nappes of the Alpine edifice. Consequently decompression and a general Barrovian metamorphism in these nappes are mainly formed by Palaeozoic basement the Lepontine dome resulted in amphibolite facies condi- rocks affected by a strong Alpine metamorphism and tions in the south and greenschist facies conditions in the deformation. northern part of the Adula nappe (Nagel et al. 2002b). The Adula nappe, in the Central Alps, is a good example Eclogite facies assemblages are restricted to eclogites, of a complexly structured polyorogenic unit and is a key garnet–phengite–metapelite, and garnet peridotite (only in tectonic unit located at the transition between the Helvetic the southern part). They are more abundant in the eastern and Brianc¸onnais domains. It is mostly composed of part of the northern Adula nappe. Paleozoic basement rocks and has been strongly deformed The pre-Alpine paleogeographic position of the Adula by Alpine orogenic events. Various interpretations have led nappe is at the limit of the distal Helvetic margin (Stein- to contrasting kinematic models of its Alpine evolution in mann 1994; Galster et al. 2012), close to the North Pen- the literature. ninic basin. The Triassic and Jurassic stratigraphy of the The Adula nappe is located on the border of the Swiss- Adula nappe and its surrounding nappes show a transition Italian Alps on the eastern Lepontine dome and is one of between the Brianc¸onnais domain and the Helvetic domain the highest units of the Lower Penninic basement nappe (Galster et al. 2012). The limited occurrence of eclogite stack (Fig. 1). It is overthrust by the Bu¨ndnerschiefer relicts in a few formations and the strong internal defor- (Gansser 1937; Schmid et al. 1996), a sedimentary series mation are the main arguments that led several authors to deposited in the North Penninic basin (Steinmann 1994) introduce a ‘‘lithospheric me´lange’’ model to explain the and followed upward by the easternmost Middle Penninic geometry and kinematics of the Adula nappe (Trom- Tambo and Suretta basement nappes with their associated msdorff 1990; Berger et al. 2005). This hypothesis implies sediments of Brianc¸onnais affinity. The Adula nappe has that the Adula nappe is a me´lange representing the suture been the subject of numerous recent publications and a between Lower and Middle Penninic (Engi et al. 2001; recent review paper by Nagel (2008); it can be considered Berger et al. 2005). This hypothesis remains controversial one of the best-studied nappes of the Central Alps. The (see discussion in Herwartz et al. 2011). overall structure of the Adula nappe is a north-facing The petrographic subdivisions of the Adula nappe were recumbent fold, but its internal structure, which records defined in a few classical papers (i.e. Jenny et al. 1923; several deformation phases, is much more complex than Ku¨ndig 1926; Van der Plas 1959;Egli1966). A clear that of a simple fold-nappe and contains several slices of lithostratigraphic sequence, however, was never estab- Mesozoic rocks folded within the basement (Jenny et al. lished, and the granitic rocks are still undated. Hence, a 1923; Nagel et al. 2002a). The oldest Alpine deformation consistent picture of the Adula basement architecture, also phases are only locally observable. The dominant Alpine essential to unravel the Alpine structures, was never structure in the northern part of the nappe is called the proposed. Zapport phase (Lo¨w 1987; Nagel et al. 2002a); it is also the This work thus aims to: oldest recorded pervasive phase. This phase is associated • determine the protolith of the northern Adula basement with the regional foliation, isoclinal folding with approxi- rocks; mately N–S fold axes, and a well-marked N–S-stretching • date the magmatic protoliths of the northern Adula lineation associated with a top-to-the-North sense of shear. nappe; The Leis and Carassino deformation phases (Lo¨w 1987) are • propose a lithostratigraphy for the Alpine basement younger and more pronounced in the western part of the rocks; northern Adula nappe. The Leis phase is principally • suggest a paleogeographic scenario for the protolith expressed by folds, which are locally associated with a formation; cleavage. Large open folds are ascribed to the Carassino • identify information regarding the pre-Alpine orogenic phase. cycles; Eclogites and eclogitic facies metapelites are well • discuss the implications of the pre-Alpine unconformi- known in numerous areas of the Adula nappe (Heinrich ties in the Alpine kinematics. 1983; Meyre et al. 1999; Dale and Holland 2003; Zulbati 2010). They have been studied mostly in the polymeta- Obtaining an accurate lithostratigraphy is essential to morphic basements. The Adula nappe was subject to high- conduct a reliable geological and structural map and is of pressure metamorphic conditions during the Alpine orog- primary importance for the understanding of the nappe eny as demonstrated by the metamorphic overprint kinematics and its metamorphic evolution. This work observed in Mesozoic cover rocks (Zulbati 2008) and the combines analytical and precise field investigations to dating of two metamorphic events (the last one being of constrain the nature of the protoliths of the northern Adula 123 Int J Earth Sci (Geol Rundsch) (2014) 103:61–82 63 Fig. 1 Tectonic map of the eastern Lepontine Alps. Grey and white (2006), Steck (2008), and Galster et al. (2012). The square indicates tones are basement nappes, and sediments