Polyphase tectonics and late Variscan extension in Austria (Moldanubian Zone, Strudengau area)

Abstract

New data suggest syn-convergent extrusion and polyphase tectonics followed by late Variscan extension in the Strudengau area of the southern Moldanubian zone in Austria. The tectonic history can be summarized as follows: (1) The oldest ductile event is observed in HT/LP metamorphic pelitic gneisses, which preserve E-dipping foliation planes (D₁-fabric) with NW–SE-trending lineations. (2) The overlying gneisses record HT/HP conditions with decompression-induced anatexis in the central part of the domain. These gneisses exhibit N–S trending, horizontal lineations along steep-dipping foliation planes (D₂-fabric) crosscutting the D₁-fabric of the pelitic gneisses. Along the margin, these rocks have been strongly mylonitized under amphibolite facies conditions (D₂). D₂ is interpreted as a significant vertical shear zone, which juxtaposes the HT/LP rocks against the orogenic lower crust. (3) Lastly, the whole area is overprinted by localized shear zones (D₃-fabric) with top-to-the-NW kinematics. This newly discovered Strudengau shearing event is associated with isoclinal folding that possesses axial planes parallel to the mylonitic foliation and fold axes parallel to the stretching lineations. Initial mylonitization occurred under greenschist facies, representing the latest ductile event of the Strudengau area. The new geochronological data presented here indicate a narrow time frame (c. 323–318 Ma) for the D₃ deformation. Therefore, this event is contemporaneous with the intrusion of the granites of the South Bohemian Batholith (330–310 Ma). The nearby South Bohemian Batholith and generally steep dyke swarms in the Strudengau area and to the north trend in a NE–SW preferred orientation, interpreted to be D₃-synkinematic magmatism. In a regional context, the NW–SE stretching during D₃ together with the synkinematic intrusion of dykes is associated with late orogenic extension in the Austrian Moldanubian Zone. Kinematic data of brittle normal faults and tension gashes are consistent with NW–SE-oriented extension under cooler conditions.

Appendix

Methods

Mineral chemical analyses

Mineral compositions were obtained on carbon-coated polished thin sections with a Camera SX-100 EPMA (electron-probe microanalyzer, Department of Lithospheric Research, University of Vienna, Austria) equipped with energy and wavelength dispersive spectrometers. All measurements were performed against natural standards using an acceleration voltage of 15 kV as well as a beam current of 20 nA (Tschegg et al. 2011). Feldspar and amphibole were performed using a defocused beam (5 μm feldspar, 3 μm amphibole), while garnet and micas were performed by a focused beam.

Sm–Nd analyses

The sample (VT 20/09) used for Sm–Nd analyses is a peraluminous and Grt-bearing dyke, which is located south of the village Viehtrift (Lower Austria). The mineral composition of the sample is as follows: Tur + And + Grt + Kfs + Pl + Ms + Bt + Qtz. This locally ductile-deformed dyke is an important crosscutting element, constraining the maximum formation age of the Strudengau Shear Zone (SSZ). The sample was taken from an undeformed domain of the dyke. Garnets (0.16–0.45 mm grain size) were used for the Sm–Nd isotopic analytical work. They were carefully handpicked and investigated optically for inclusion-free crystals. To be sure that no inclusions affect the age, one measuring procedure was performed with the normal garnets, and one was performed with preceding leaching experiment garnets. The analyses were performed at the Laboratory of Geochronology, Department of Lithospheric Research (University of Vienna, Austria). Analytical procedure has been performed after Thöni et al. (2008). The handpicked garnets were rinsed, using acetone and deionized water, and accessorily washed for 60 min in warm (70 °C) 2.5 N HCl. The pure garnet mineral separates used for Sm–Nd analysis weight 100.95 mg. Sample digestion for Sm–Nd analysis was performed in Savillex™ beakers using an ultrapure 5:1 mixture of HF and HClO₄ for 10 days at 110 °C on a hot plate. The H₂SO₄ leaching experiment of garnet is performed after Anczkiewicz and Thirlwall (2003), expatiated on 53.02 mg handpicked garnet sample and a minimum dissolution time of 2 weeks. For the whole rock powder, a minimum dissolution time of 3 weeks was applied to ensure complete leaching of the REEs from refractory material such as zircon. After evaporation the acids, repeated treatment of the residue using 5.8 N HCl resulted in clear solutions for all samples. Upon cooling, between 7 and 20 % of the sample solution was split off and spiked for Sm and Nd concentration determination by isotope dilution (ID) using a mixed REE tracer (¹⁴⁷Sm–¹⁵⁰Nd spike). The REE fraction was extracted using AG™ 50 W-X8 (200–400 mesh; Bio-Rad) resin and 4.0 N HCl. Nd and Sm were separated from the REE fraction using teflon-coated HdEHP, and 0.24 and 0.8 N HCl. Maximum total procedural blanks were >50 pg for both Sm and Nd. Sm and Nd were run as metals from a Re double filament, using a Finnigan™ MAT262 (for ID) and a ThermoFinnigan TM Triton TI TIMS (for IC). Determined ¹⁴³Nd/¹⁴⁴Nd and ¹⁴⁷Sm/¹⁴⁴Nd isotope ratios and uncertainties on the Nd isotope ratio (quoted as 2σ) are shown in Table 3. A mean error of ±1 % is estimated, including blank contribution, uncertainties on spike composition, and machine drift; regression calculation is based on these uncertainties, and the isochron calculations follows Ludwig (2003).

Table 3 Sm-Nd analytical data for dyke sample VT 20/09

⁴⁰Ar/³⁹Ar Analyses

The undeformed dyke sample VT 20/09 was also used for ⁴⁰Ar/³⁹Ar muscovite analyses to resolve a cooling age of the dyke, with the prescribed accepted closure temperatures of 350–400 °C (Harrison et al. 2009). The magmatic muscovite phase (0.355–0.600 mm grain size) in the sample was observed in thin section and has been carefully separated. The sample was washed, crushed, and sieved to isolate target minerals. Individual mineral separates of mica were loaded into aluminum foil packets along with a single grain of Fish Canyon Tuff Sanidine (FCT-SAN) to act as a flux monitor (apparent age: 28.03 Ma; Renne et al. (1998)). The sample packets were arranged radially inside aluminum can and underwent a 240 MWH irradiation at the research reactor of McMaster University in Hamilton, Canada.

Laser ⁴⁰Ar/³⁹Ar step-heating analysis was carried out at the Geological Survey of Canada geochronology laboratories in Ottawa, Canada. Upon return from the reactor, samples were split into two aliquots each and loaded into individual 1.5-mm-diameter holes in a copper planchet. The planchet was then placed in the extraction line, and the system evacuated. Heating of individual sample aliquots in steps of increasing temperature was achieved using a Merchantek MIR10 10 W CO₂ laser equipped with a 2 mm × 2 mm flat-field lens. The released Ar gas was cleaned over getters and then analyzed isotopically using a Nu Instruments Noblesse multi-collector mass spectrometer, equipped with a Faraday detector and three ion counters. For the analyses, a single ion counter peak-hopping mode was used for small signals, and in cases where the ⁴⁰Ar signal exceeded ion counting tolerance, a Faraday plus single ion counter peak-hopping routine was employed. Baselines were measured prior to each analysis, and blank measurements were made throughout the analytical sessions. Mass fractionation and detector efficiencies were determined from repeated measurements of air aliquots, whereby ⁴⁰Ar and ³⁶Ar signals were measured on all collectors. ⁴⁰Ar/³⁶Ar ratios were then determined for each collector individually, and for each combination of collectors (e.g., ⁴⁰Ar on the Faraday/³⁶Ar on each ion counter). Raw data from the mass spectrometer were imported and processed using a spreadsheet programmed with data-handling macros written by M.E. Villeneuve that employ average, linear or nonlinear regression protocols based on the equations of Koppers (2002).

Error analysis on individual steps follows numerical error analysis routines outlined in Scaillet (2000); error analysis on grouped data follows algebraic methods of Roddick (1988). Corrected argon isotopic data are listed in Table 4 and presented as age spectra of gas release (Roddick et al. 1980). Each gas-release spectrum contains step-heating data shaded and normalized to the total volume of ³⁹Ar released. Such plots provide a visual image of replicated heating profiles, evidence for Ar-loss in the low-temperature steps, and the error and apparent age of each step.

Table 4 CO₂ laser step-heating ⁴⁰Ar/³⁹Ar data from Strudengau granitic dyke

Neutron flux gradients throughout the sample canister were evaluated by analyzing the sanidine flux monitors included with each sample packet and interpolating a linear fit against calculated J-factor and sample position. The error on individual J-factor values is conservatively estimated at ± 0.6 % (2σ). Because the error associated with the J-factor is systematic and not related to individual analyses, correction for this uncertainty is not applied until calculation of dates from isotopic correlation diagrams (Roddick 1988). Nucleogenic interference corrections were (⁴⁰Ar/³⁹Ar)K = 0.025 ± 0.005, (³⁸Ar/³⁹Ar)K = 0.011 ± 0.010, (⁴⁰Ar/³⁷Ar)Ca = 0.002 ± 0.002, (³⁹Ar/³⁷Ar)Ca = 0.00068 ± 0.00004, (³⁸Ar/³⁷Ar)Ca = 0.00003 ± 0.00003, (³⁶Ar/³⁷Ar)Ca = 0.00028 ± 0.00016. All errors are quoted at the 2σ level of uncertainty.