Repeated recycling of sedimentary continental margin sequences during extensional and contractional orogenic episodes (Cenerian and Variscan orogenic cycles, Central Alps)

Gneisses, granites and migmatites of the Central Alpine basement (Aar Massif and Gotthard Nappe, Helvetic Zone of the Swiss Alps) record a long-lasting geological history over several hundred million years. This complex history is resolved through detailed zircon U-Pb geochronology and Hf isotope analysis:(1) Inherited cores in zircon record a 750-550 Ma old orogenic and magmatic history during Rodinia disintegration and Gondwana amalgamation. The cores reflect zircon crystallization during the Pan-African and Cadomian orogenies with the involvement of cratonic and oceanic materials, leading to scattering initial epsilon Hf values of +10 to -15. (2) The Cenerian orogeny caused widespread melting of sedimentary wedge material consisting of this Pan-African and Cadomian siliciclastic detritus during a phase of lithospheric thinning along an accretionary continental margin. Large volumes of anatectic melts formed at ca. 460-440 Ma, which occur today as gneisses, migmatites and metagranites. The Hf isotope systematics of the detrital precursor zircon was recycled into the new magmatic zircon and homogenized. (3) Some of the quartz-feldspar rich Ordovician-age migmatites and granites were remelted at a late stage of the Variscan orogeny at around 315 Ma, facilitated by addition of several volume percent of water to a near-minimum melt quartz-feldspar composition. The resulting anatectic melts formed heterogeneous granite bodies with diffuse borders. Newly grown U-rich zircon rims around older zircon again recycled and somewhat homogenized the initial epsilon Hf composition of partially consumed previous zircon generations. This generation of anatectic granites is coeval with more deeply sourced intrusive suites at 335 and 300 Ma. High-temperature metamorphism and magmatism are explained by late-orogenic lithospheric thinning in the back-arc area of the retreating Paleotethys subduction. (4) Alpine deformation in greenschist facies at around 25 Ma partly reactivated existing structures and led to low-temperature hydrous alteration of previous mineral assemblages.The new data confirm existing hypotheses that the Variscan orogeny mainly recycled fertile igneous protoliths of early to late Ordovician age, which ultimately originated to a overwhelming extent from the melting of Neoproterozoic and Cambrian siliciclastic orogenic detritus. The Variscan orogeny is thus characterized by abundant crustal recycling and little juvenile addition.