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Microstructures in Omphacite and Other Minerals from
Eclogite Near to the Interface Eclogite Zone/Lower Schist
Cover, Tauern Window, Austria
Wolfgang Friedrich Müller and Gerhard Franz 1
1 Institut für Angewandte Geowissenschaften, Technische Universität Berlin
Rapid exhumation of eclogitic rocks from depths between 60 and 100 km to an
intermediate crustal level of about 30 km or less, is frequently reported from high- and
ultra-high pressure terrains. It is unclear, however, how the rocks and their mineral
constituents respond to such high strain rates, which are possibly also associated with
large, crustal earthquakes. It is also an open question, which mechanical and
temperature effects are produced by fast exhumation of high-pressure nappes or
intercalated slices in its neighbourhood and in itself.
It was recently established [1] that the high-pressure terrain of the Pennine in the
Eastern Alps, the Eclogite Zone (EZ) in the Tauern Window, shows very high uplift
rates. Radiometric age determinations (Rb/Sr) from eclogite facies rocks of the EZ and
of post-eclogitic greenschist facies vein assemblage revealed minimum uplift rates of >
36 mm/a. The event of the rapid imbrication of the EZ between other tectonic units
caused unusual deformation microstructures in a sample from the lower tectonic unit
which is the Venediger nappe or Lower Schist Cover (LSC) [2]. In an eclogite sample on
the order of 10 m distant from the EZ polygonisation of garnet into subgrains of 0.5 to 3
µm, high dislocation density in titanite, and deformation twin lamellae on (100) in
clinozoisite were observed [2]. In order to see if there are similar effects in rocks from
the EZ, we studied an eclogite as close as possible to the thrust plane by transmission
electron microscopy (TEM). The distance of the sample locality to the thrust fault
between LSC and EZ is about 50 m. This sample (WP 291) served already Glodny et al.
[1] for the age determination. According to thermobarometry, the EZ eclogites have
experienced peak P-T conditions of 2.0 - 2.5 GPa and 600 ± 30 °C (see [1] and
quotations therein).
Omphacite
The omphacite grains impress by their wealth of microstructures: Antiphase domains
(APDs) separated from each other by antiphase domain boundaries (APBs), chain
multiplicity faults (CMFs), twin lamellae on (100), non-crystallographic faults, free
dislocations, small angle grain boundaries, and recrystallised grains. With exception of
the APDs, all microstructures are due to deformation and to recovery and
recrystallisation as consequence of deformation which may be syn- or
postdeformational.
Antiphase domains (APDs) are present in all grains studied. They arise from the phase
transition C2/c→P2/n due to the diffusion-controlled ordering of Mg and Al. The
displacement vector of the antiphase domains is 1/2[110] [3, 4]. From the peak
metamorphic temperature of the EZ of about 600 °C it is clear that our omphacite
crystallised first in the disordered C-lattice but within the phase regime of the P-phase
[3]. The APDs have mostly a size of 0.2 to 0.4 µm. Electron diffraction patterns show
well-ordered omphacite of space group P2/n with sharp reflections.
CMFs (intercalations of layers with a different chain multiplicity; here double chains like
in amphibole) parallel to (010) are frequenty observed. They have been described in
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