Dating synmagmatic folds: a case study of Schlingen structures in the Strona-Ceneri Zone (Southern Alps, northern Italy) |
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| Authors: | R ZURBRIGGEN B S KAMBER M R HANDY & T F NÄGLER |
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| Institution: | Geologisches Institut Universität Bern, Baltzerstrasse 1, 3012 Bern, Switzerland (email:;), Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK,;Institut für Geowissenschaften, Justus-Liebig Universität, Senckenbergstrasse 3, 35390 Giessen, Germany,;Mineralogish-petrographisches Institut, Isotopengeologie, Universität Bern, Erlachstrasse 9a, 3012 Bern, Switzerland |
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| Abstract: | The Strona-Ceneri Zone (Southern Alps) contains folds with moderately to steeply inclined axial planes and fold axes, and amplitudes of up to several kilometres (so-called 'Schlingen'). These amphibolite facies folds deform the main schistosity of Late Ordovician metagranitoids and are discordantly overlain by unmetamorphic Permian sedimentary rocks. Mutually cross-cutting relationships between these folds and garnet-bearing leucotonalitic dykes indicate that these dykes were emplaced during folding. Sm–Nd systematics and the strongly peraluminous composition of these dykes point to an anatectic origin. Pb step leaching of magmatic garnet from a leucotonalitic dyke yielded a 321.3±2.3 Ma intrusive age. Rb–Sr ages on muscovites from leucotonalitic dykes range from 307 to 298 Ma, interpreted as cooling ages during retrograde amphibolite facies metamorphism. Conventional U–Pb data of zircons from an older granodioritic dyke that pre-dates the Schlingen folds yielded discordant U–Pb ages ranging from 371 to 294 Ma. These ages reflect a more complicated multi-episodic growth history which is consistent with the observed polyphase structural overprint of this dyke. Schlingen folding was accompanied by prograde amphibolite facies metamorphism, during the thermal peak of which the leucotonalitic dyke material was generated by partial melting in a deeper source region from where these S-type magmas intruded the presently exposed level. Because partial melting may occur in a relatively late stage of a clockwise P–T–t path, or even during decompression on the retrograde path, we do not exclude the possibility that Schlingen folding had already started in Early Carboniferous time. Schlingen folds also occur in Penninic and Austroalpine basement units with a very similar pre-Alpine history, indicating that Variscan folding affected large segments of the future Alpine realm. |
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| Keywords: | European Variscides Pb step-leaching age Schlingen folds synmagmatic folding syntectonic dykes |
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