On Triassic migmatization, Variscan metamorphism and palaeogeographic origin of the Raabalpen Complex, Eastern Alps (Europe)

The thermal consequences of post-orogenic processes like emplacement of magmatic bodies due slab break-off or slab delamination, ongoing extension due to post-collisional collapse or rifting, are poorly constrained, but all these processes result in high heat flow. In consequence, the heated lithosphere is rheologically weak and is often the site of ongoing rifting reactivating the orogenic crust. In the Alps, the transition from the Carboniferous Variscan orogeny to ongoing Alpine, Permian to Triassic rifting is such a case, and the processes have been well constrained in the last two decades by various methods constraining Permian magmatism and metamorphism and rapid Triassic subsidence due lithospheric extension. Here, we report, for the first time, a concordant U–Pb age of 242.7 ± 1.1 Ma of metamorphic zircons constraining the peak temperature conditions of migmatization in meta-clastic rocks of the Raabalpen basement complex in the northeastern Lower Austroalpine units. The Middle Triassic age of metamorphism is also supported by new concordant U–Pb ages at 245.1 ± 1.8 Ma of metamorphic zircons from granite gneisses, which have Permian protoliths (274.9 ± 1.4 Ma; 274.0 ± 5.4 Ma). Together, the Middle Triassic age of ~245–243 Ma is interpreted by prolonged high heat flow by lower crustal thinning forming the Strallegg Gneiss Dome during rifting. Furthermore, relict zircons from migmatites in the Raabalpen Complex also constrain two stages of the Variscan amphibolite-grade metamorphism. Although overprinted by polymetamorphism (Variscan, Triassic, Cretaceous), detrital zircons are well preserved in the Raabalpen Complex. The detrital zircon age populations of 448, 480, 564, 632 and 688 Ma indicate a Cadomian source. The youngest age is at 412 Ma constraining a maximum Early Devonian depositional age of parts of the Raabalpen Complex.