A structural and petrological study of Saxothuringian eclogites in the central Erzgebirge, Czech Republic, revealed their polyphase thermal and tectonic evolution. The first stage of deformation is related to eclogite-facies metamorphism of mafic rocks at pressures of 26 kbar and temperatures of 650–700°C. The clinopyroxene crystallographic preferred orientation is consistent with high-temperature flow.

The second stage of deformation is characterized by transport of eclogitic rocks at the base of a crustal nappe and their large-scale folding. During this stage, HP–HT metamorphic assemblages developed in the surrounding metapelites (14.5 kbar/600°C) while eclogite assemblages were preserved in passively transported eclogitic boudins. Heat advection and similar P–T conditions in the surrounding crustal material are responsible for the stability of mineral assemblages and preclude closure of isotopic systems in the eclogites. Therefore the published isotopic ages of the eclogites do not reflect the subduction stage but the age of cooling under crustal conditions. Subsequent large-scale (D3) folding is responsible for reorientation of previously flat-lying earlier fabrics (D1–D2).

The final stage of deformation is marked by the development of a heterogeneous network of ductile retrograde shear zones. The thermometry and geochemical study of amphibolitized eclogites in the cores of shear zones indicates thermal conditions not exceeding those in surrounding metapelites and fluid influx. The fluid influx is interpreted to be the result of water-producing prograde reactions in the footwall metasediments and fluid chanelling into the base of the nappe during brittle deformation of the eclogites.

Scientific editing by Kate Brodie.
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