Unusual deformation microstructures in garnet, titanite and clinozoisite from an eclogite of the lower schist cover, Tauern Window, Austria

An eclogite sample from the Lower Schist Cover of the Tauern Window, only a few tens of metres distant from the intercalated Eclogite Zone, was studied by transmission electron microscopy. Garnet, titanite and clinozoisite reveal unusual deformation microstructures not reported before from natural occurrences. Garnet shows polygonisation into subgrains of only 0.5 to 3 mu m in size which are separated by low angle grain boundaries. The subgrain cells show a preferred orientation parallel to (110). In titanite, most of the numerous dislocations are organized into low angle grain boundaries with Burgers vectors parallel to <011>. The Burgers vector of free dislocations in [100]. Weak fringes in the wake of dislocations are interpreted as slip traces. Clinozoisite contains submicroscopic mechanical twin lamellae parallel to (100); the twin law is m parallel to (100). The unusual deformation features are explained as the result of imbrication of the Eclogite Zone.