Abstract:
The necessity for fluids during the transformation of a dry mafic protolith into an eclogite is likely inevitable but the pathways by which fluids can infiltrate are variable. One of the best studied examples showing the relation between fluid infiltration and mineral replacement can be found in the Bergen Arc. Here lower-crustal fractures in granulite are interpreted as initial fluid pathways allowing for localized eclogitization. But what if the protolith does not want to fracture?
In this presentation I will discuss data from a different geological setting, the Koralpe range in the Easter Alps (Austria), which is part of the eclogite type-locality. There, large eclogite bodies are surrounded by highly foliated micaschist and paragneiss. Detailed microstructural investigations on a set of eclogite specimens collected along a strain gradient show that the formation of eclogite at close to static conditions resulted in the development of a porosity at high pressures. Interconnected pores and open grain boundaries allowed for dispersed fluid infiltration. Continuous supply of a H2O dominated fluid phase from the surrounding metasediments assisted eclogitization by the activation of dissolution-precipitation processes. Subsequent deformation of eclogite was however accompanied by a reduction in porosity and apparent strain hardening. The observed negative feedback between deformation and fluid pathways is contrary to previous studies where deformation is usually known to introduce porosity.
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