Subduction of a rifted passive continental margin: the Pohorje case of Eastern Alps-constraints from geochronology and geochemistry

This study presents geochronological and geochemical data from newly dated Permian granitic orthogneisses associated with the Eclogite-Gneiss unit (EGU) from the southernmost part of the Austroalpine nappe stack, exposed within the Pohorje Mountains (Slovenia). LA-ICP-MS zircon U-Pb ages of two samples of the augen-gneisses are 255 +/- 2.2 Ma and 260 +/- 0.81 Ma, which are interpreted as the age of magmatic crystallization of zircon. In contrast, all round zircons from leucogneisses give Cretaceous ages (89.3 +/- 0.7 Ma and 90.8 +/- 1.2 Ma), considered as the age of UHP/HP metamorphism. The round zircons overgrew older euhedral zircons of Permian and rare older ages tentatively indicating that these rocks are of latest Permian age, too. Zircon epsilon Hf(t) values of the four orthogneiss samples are between - 13.7 and - 1.7 with an initial(176)Hf/Hf-177 ratio ranging from 0.282201 to 0.282562; TDMC is Proterozoic. The augen-gneisses show geochemical features, e.g. high (La/Lu)(N)ratios and strong negative Eu anomalies, of an evolved granitic magma derived from continental crust. The leucogneisses are more heterogeneously composed and are granitic to granodioritic in composition and associated with eclogites and ultramafic cumulates of oceanic affinity. We argue that the Permian granitic orthogneisses might be derived from partial melting of lower crust in a rift zone. We consider, therefore, that segment of the EGU is part of the distal Late Permian rift zone, which finally led to the opening of the Meliata Ocean during Middle Triassic times. If true, the new data also imply that the Permian stretched continental crust was potentially not much wider than ca. 100 km, was subducted and then rapidly exhumed during early Late Cretaceous times.