Rapid formation and exhumation of the youngest Alpine eclogites: A thermal conundrum to Barrovian metamorphism

Eclogite facies metamorphic rocks provide critical information pertaining to the timing of continental collision in zones of plate convergence. Despite being amongst Earth's best studied orogens, little is understood about the rates of Alpine metamorphism within the Eastern Alps. We present LA–MC–ICPMS and ID–TIMS U–Pb ages of metamorphic allanite from the Eclogite Zone, Tauern Window, which when coupled with rare earth element analysis and thermobarometric modelling, demonstrate that the European continental margin was subducted to between 8 and 13 kbar (30–45 km) by 34.2 ± 3.6 Ma. These data define: (i.) an upper limit on the timing of eclogite facies metamorphism at 26.2 ± 1.8 kbar (70–80 km) and 553 ± 12 °C, (ii.) plate velocity (1–6 cm·a−1) exhumation of the Eclogite Zone from mantle to mid-crustal depths, and (iii.) a maximum duration of 10 Ma (28–38 Ma) for juxtaposition of Alpine upper-plate and European basement units and subsequent conductive heating thought to have driven regional Barrovian (re)crystallisation at ca. 30 Ma. One-dimensional thermal modelling of tectonically thickened crust shows that conductive heating is too slow to account for Tauern Barrovian conditions (550 °C at 9–13 kbar) within the maximum 10 Ma interval between eclogite exhumation and the thermal peak (28–32 Ma). Given that the Tauern Window is a classic locality for understanding rates of conductive thermal relaxation in tectonically thickened crust, this work raises questions of fundamental importance concerning the length scales of the mechanisms responsible for heat transfer within orogenic crust.

Research highlights
► U–Pb geochronology shows European margin was subducted to 8–13 kbar by 34.2 ± 3.6 Ma.
► Maximum 10 Ma duration for juxtaposition of Alpine upper- and lower-plate units.
► Thermal modelling shows conductive heating requires advective component.
► Syn-thrusting heating provides possible explanation for rapid heating.
► May be significant in other overthrust terranes.