Eo-alpine eclogitisation of Permian MORB-type gabbros in the Koralpe (Eastern Alps, Austria): new geochronological, geochemical and petrological data

Abstract

New petrological, geochemical and isotopic data (SmNd, RbSr) from (meta)-gabbros, eclogites and metapelites from the southern Koralpe are presented. Pl-Cpx gabbros (mostly olivine-free) that have preserved their magmatic minerals to varying extent occur as relics within eclogites. Transformation of gabbro into eclogite is documented in different stages ranging from macro- to microscale. Based on textural characteristics, the eclogites are classified into four groups (types 1–4). Typical eclogite parageneses are Grt + Omp + Zo ± Ky + Qtz ± Amp ± Phe + Rut. The metapelitic eclogite host rocks are composed of Qtz + Ms + Bt + Pl + Grt ± Ky. P-T estimates for both metabasites and metapelites range at 1.8-2.0 GPa/600–650°C. SmNd mineral isochrons (Pl-Cpx) for the gabbroic eclogite precursors give ages of between 247 ± 16 and 255 ± 9 Ma. Their initial Nd and Sr isotopic compositions are clearly within the range of modern depleted mantle values. Minerals from type-1, metagabbroic eclogites are in clear Nd isotopic disequilibrium. Pervasively recrystallized type-3 eclogites yield Grt-WR or Grt-Omp-Zo-Amp-WR ages of 97 ± 5, 101 ± 7 and 109 ± 8 Ma. The metapelitic eclogite host rocks (mica schists) yield Nd DM (depleted mantle) model ages of 1.44-1.63 Ga. Grt-WR isochron ages from these rocks are 86 ± 1, 87 ± 4 and 89 ± 3 Ma. The new data further support the Permian/Early Mesozoic rifting event for this part of the Austroalpine basement, which is related to initial oceanization at the western end of the Alpine Tethys. During Alpine plate convergence pre-Alpine continental basement and oceanic crustal fragments where subducted to depths of c. 60 km. The most likely time span for the high-pressure peak is c. 100 ± 10 Ma. Rapid exhumation in the Upper Cretaceous is inferred from mineral cooling ages between 80 and 60 Ma.