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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 - 156 -
detail by [5]. CMFs are apparently quite typical for omphacites from the EZ and the LSC from the Tauern Window, and are seldom described from other occurrences. An astonishing feature observed in several omphacite grains are faults which are not oriented parallel to a distinct crystallographic plane but have preferential orientations subparallel to (100); they are terminated by dislocations. These ‘non-crystallographic faults’ were apparently produced by moving dislocations. Attempts to analyze their displacement vector failed. While free dislocations are rare, small-angle grain boundaries formed by one or two sets of dislocations are relatively frequent. In one case, even a recrystallizing grain was observed (Fig. 1a) forming a large angle grain boundary with the matrix grain. Polysynthetic twin lamellae on (100) (Fig.1b) are interpreted as deformation twins because growth twins of this type are unknown and the grain is obviously deformed as evidenced by free dislocations. Clinozoisite shows deformation twin lamellae on (100) with widths which vary between about 3 and 150 nm. They have been first observed in the LSC sample close to the interface to the EZ [2]. Garnet contained rarely dislocations and low angle grain boundaries. Barroisitic amphibole displayed a small-angle grain boundary which consisted of segments parallel to (110), but otherwise no specific deformation effects were apparent. In conclusion: The TEM results show strong deformation of omphacite and clinozoisite, but garnet is almost free of dislocations. Comparison of these results with those by Barnert [6] on samples farther away from the interface to the LSC shows no significant differences of the effects of ‘normal’ deformation in the EZ omphacites and the deformation associated with the rapid exhumation. The only exception are the noncrystallographic faults in omphacite mentioned above which may indicate locally elevated temperatures. Fig.6: TEM electron micrographs of omphacite. (a) Recrystallising grain (bright grain in the left corner) growing into the omphacite matrix in which APBs are visible; bright-field image. (b) Omphacite with deformation twin lamellae on (100); dark-field image. [1] Glodny J. et al. (2005) Contr. Mineral. Petrol. 149, 699-712 [2] Müller W.F., Franz G. (2004) Eur. J. Min. 16, 939-944 [3] Champness P.E. (1973) Am. Mineral. 58, 540-542 [4] Phakey P.P., Ghose S. (1973): Contr. Mineral. Petrol. 39, 239-245 [5] Müller, W.F. et al. (2004) Eur. J. Min., 16: 37-48 [6] Barnert, E.B. (2003) Doctoral thesis, TU Darmstadt, Fb 11 - 157 -
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ANNUAL REPORT 2 0 0 6 FACULTY OF MA
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