THE ECOLGITE CONDUNDRUM AND THE THERMAL EVOLUTION OF MOUNTAIN BELTS

Eclogites metamorphosed at mantle pressures and exhumed sufficiently rapidly to preserve high and very high pressure mineral assemblages challenge our understanding of tectonic processes. The mechanisms by which they are exhumed so rapidly are speculative. Likewise the tectonic and chronological relationships between eclogite exhumation, continental collision and regional metamorphism in orogenic belts are problematic; not least because eclogites appear to be crystallised and exhumed from the mantle at, or even after, continental collision and emplaced in thickened regional metamorphic crust close to the time of peak metamorphic conditions. Conventional conductive thermal models imply the regional metamorphism would need more than 10 Ma to reach the observed metamorphic temperatures which makes the intercalation of eclogites within regional metamorphic terrains with only a few Ma difference in ages hard to explain.

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.