Original paper

TEM-microstructures in omphacite and other minerals from eclogite near to a thrust zone; the Eclogite Zone Venediger nappe area, Tauern Window, Austria

Müller, Wolfgang Friedrich; Franz, Gerhard

Abstract

The event of the rapid imbrication of the Eclogite Zone (Tauern Window, Austria) between its lower and upper tectonic units had caused unusual deformation microstructures in garnet, clinozoisite and titanite of a sample from the lower tectonic unit about 10 m distant from the shear zone. In order to see if there are similar effects in rocks from the Eclogite Zone itself, we studied an eclogite at a distance to the thrust plane of about 200 m, as close as possible, by transmission electron microscopy. Omphacite impresses by its wealth of microstructures: Antiphase domains, chain multiplicity faults parallel to (010), deformation twin lamellae on (100), non-crystallographic faults, free dislocations, small angle grain boundaries, and recrystallised grains have been observed. The non-crystallographic faults are faults that are not oriented parallel to one distinct lattice plane, they have a displacement vector, which is not a low rational fraction of a lattice vector. The recrystallisation is due to the movement of large-angle grain boundaries into the deformed omphacite. With exception of the antiphase domains, all microstructures are due to deformation and to recovery as consequence of deformation. The other minerals investigated were garnet, clinozoisite, and amphibole. Garnet is largely free of defects and rarely contains dislocations and small angle grain boundaries. Clinozoisite contains deformation twins on (100). Special deformation effects unequivocally due to the rapid imbrication and not detectable in eclogites further away from the thrust plane were not detected.

Keywords

omphacitegarnetclinozoisitetemmicrostructuresdeformation twinsnon-crystallographic faultsrecrystallising grains