THE ECOLGITE CONDUNDRUM AND THE THERMAL EVOLUTION OF MOUNTAIN BELTS
The Eastern Alps in Europe is a classic orogenic belt where the causes of the regional metamorphism have been extensively debated and modelled. Geochronology on the eclogites preserved within the thrust pile on the southern margin of the Tauern window has revealed that these reached peak metamorphic pressures within a few Ma prior (or even after) peak temperatures were attained by the regional Barrovian metamorphism that coincided with or outlasted the later deformations associated with overthrusting. Smye et al., (2011) in Earth and Planetary Science Letters argued that the most plausible (but not necessarily satisfactory) explanation was that the eclogites were formed ~ 10 Ma before the regional Barrovian metamorphism and that their exhumation at velocities associated with plate motions was associated with heating of the basement by overthrusting of hot deeply exhumed material. Subsequent geochronology supporting earlier ages for peak metamorphic conditions at shallower levels in the Barrovian regional metamorphism and the low temperature Alpine assemblages at the base of the overthrust Austroalpine units seem incompatible with this hypothesis. P-T paths and geochronology for the regional Barrovian metamorphism are still best explained by heating of a thickened metamorphic pile over > 10 Ma with exhumation at peak conditions. How the eclogites are emplaced remains mysterious.