An Ordovician magmatic and metamorphic event in the pre‑Mesozoic basement of the Central Alps (Aar Massif and Gotthard Nappe)

Large areas of both the external Helvetic Aar Massif and the internal Helvetic Gotthard Nappe in the pre-Mesozoic basement of the European Alps are composed of felsic gneiss, augen-gneiss and metagranite, traditionally called “orthogneiss”. Based on the scarce existing geochronology, they have been tentatively associated with an active continental margin of late Ordovician age. New zircon LA-ICP-MS U–Pb age determinations confirm the late Ordovician age for the Gärsthorn Gneiss Complex of the southern Aar Massif and the Streifengneis Complex of the Gotthard Nappe, yielding a range of mean ages for individual samples from 441 to 454 Ma. The chemical characteristics of this slightly peraluminous gneiss association, the ubiquitous presence of inherited Proterozoic to Cambrian cores in zircon, and the widely variable Hf isotopic composition of inherited cores as well as of Ordovician growth zones point to a siliciclastic metasedimentary protolith. Our conceptual model involves the accumulation of Gondwana-derived detritus in a massively overthickened fore-arc wedge during the early Ordovician. This latter was molten during the late Ordovician, a cryptic mafic underplating during slab rollback and extension possibly triggering the anatexis. The existence of oscillatory-zoned high-uranium zircon rims around the Ordovician growth zones yielding a Mississippian-age (~ 308–315 Ma) points to renewed melting of these rocks during the Variscan Orogeny and metamorphism.