DATING SYNMAGMATIC FOLDS - A CASE-STUDY OF SCHLINGEN STRUCTURES IN THE STRONA-CENERI ZONE (SOUTHERN ALPS, NORTHERN ITALY)

The Strona-Ceneri Zone (Southern Alps) contains folds with moderately to steeply inclined axial planes and fold axes, and amplitudes of up t o several kilometres (so-called 'Schlingen'). These amphibolite facies folds deform the main schistosity of Late Ordovician metagranitoids a nd are discordantly overlain by unmetamorphic Permian sedimentary rock s. 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 compos ition 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 ran ge from 307 to 298 Ma, interpreted as cooling ages during retrograde a mphibolite facies metamorphism. Conventional U-Pb data of zircons from an older granodioritic dyke that pre-dates the Schlingen folds yielde d discordant U-Pb ages ranging from 371 to 294 Ma. These ages reflect a more complicated multi-episodic growth history which is consistent w ith the observed polyphase structural overprint of this dyke. Schlinge n 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 the se S-type magmas intruded the presently exposed level. Because partial melting may occur in a relatively late stage of a clockwise P-T-t pat h, or even during decompression on the retrograde path, we do not excl ude the possibility that Schlingen folding had already started in Earl y Carboniferous time. Schlingen folds also occur in Penninic and Austr oalpine basement units with a very similar pre-Alpine history, indicat ing that Variscan folding affected large segments of the future Alpine realm.