Dating eclogite-facies metamorphism in the Eastern Alps – approaches, results, interpretations: a review

Abstract

In the Eastern Alps, different tectonic units contain eclogites of Carboniferous, Cretaceous (eo-Alpine), and Cenozoic age: (i) In the structurally lower Penninic unit, which is exposed in the Tauern window, eclogites formed at peak P–T conditions of 2.4 GPa and ≤630 °C as a result of subduction of the former European continental margin at the base of an imbricate stack of Mesozoic oceanic rocks below the Austroalpine domain. Amphibole and phengite 40Ar–39Ar and Rb–Sr multimineral isochron ages suggest an Eocene-Early Oligocene (<45–31 Ma) age for the stage of deepest subduction and rapid exhumation/cooling. (ii) Variscan MORB-type eclogites are present in the central Otztal basement (2.7 GPa/730 °C; mean Sm–Nd age: 347 O 9 Ma), the eastern Silvretta (ca. 350 Ma), and the Ulten high-grade crystalline (336 ± 4 Ma). These domains pertain to the N Apulian (Austroalpine) crust. (iii) In the southern Austroalpine units, eclogites and high-P metapelites are known from the Texel complex (1.3 GPa/520–600 °C), the Schober area (1.8 GPa/≤690 °C), and the Saualpe-Koralpe-Pohorje domain (2.2–≤2.7 GPa/≤750 °C). Their tectonic position and near-identical Cretaceous ages suggest a common subduction-exhumation history. This “eo-Alpine high-pressure belt” (EHB) resulted from burial of the strongly thinned, distal passive Neotethyan margin of the Meliata back-arc basin and, further west, pre-Alpine Austroalpine crust along an intracontinental subduction zone during the Late Mesozoic convergence of Apulia and Europe. Peak metamorphism and initial decompression/exhumation is dated by the Sm–Nd, Lu–Hf, U–Pb and Rb–Sr systems as close to 90 Ma, with mean exhumation rates in the range of 5–≤10 km/Ma for the time between 90 and 85 Ma B.P.