U–Pb geochronology of the Sondalo gabbroic complex (Central Alps) and its position within the Permian post-Variscan extension
The end of an orogenic cycle is commonly associated with a general extensional regime, widespread magmatism and complex metamorphic overprints. The Austroalpine domain in SE Switzerland and N Italy preserves a polyphase tectonic history spanning the Carboniferous Variscan orogeny, the Late Carboniferous–Early Permian post-Variscan extension and the Jurassic rifting. In this study, the late- and post-Variscan evolution of the Austroalpine domain is explored by constraining the timing of intrusion of the Sondalo gabbroic complex in the Campo unit. U–Pb zircon dating on magmatic and metamorphic rocks and trace element geochemistry in zircon are performed using laser ablation coupled with mass spectrometry. The ages of both magmatic and metamorphic rocks range between 289 ± 4 and 285 ± 6 Ma. Trace elements in zircon from magmatic rocks indicate that zircon grew in two different chemical systems: an initial one devoid of garnet and with already crystallized plagioclase, and a second one with an increasing modal amount of garnet and lacking (initial) plagioclase. The second system probably reflects mixing of the initial magma with melt derived from the surrounding partially molten metapelites. New results, existing age data and P-T estimates allow to describe the late- to post-Variscan evolution of the Austroalpine domain. These data are compared and integrated in the framework of the Permian geodynamic setting in Western Europe. We present new arguments to consider the “Permian event” in Europe as a process which was temporally distinct from the collapse of the Variscan orogen. Late-Variscan re-equilibration of the crustal thickness occurred between 310 and 290 Ma (i.e. during the collapse phase). Permian mafic magmas emplaced from 290 to 270 Ma formed either during or even after the latest stages of the Variscan collapse.