Variscan potassic dyke magmatism of durbachitic affinity at the southern end of the Bohemian Massif (Lower Austria)

Abstract

Dykes in the Strudengau area (SW Moldanubian Zone, Austria) can be mineralogically divided into lamprophyres (spessartites and kersantites) and felsic dykes (granite porphyries, granitic dykes and pegmatoid dykes). Geochemical analyses of 11 lamprophyres and 7 felsic dykes show evidence of fractional crystallization. The lamprophyres are characterized by metaluminous compositions, intermediate SiO₂ contents and high amounts of MgO and K₂O; these rocks have high Ba (800–3000 ppm) and Sr (250–1000 ppm) contents as well as an enrichment of large-ion lithophile elements over high field strength elements, typical for enriched mantle sources with variable modifications due to fractionation and crustal contamination. This geochemical signature has been reported from durbachites (biotite- and K feldspar-rich mela-syenites particularly characteristic of the Variscan orogen in Central Europe). For most major elements, calculated fractionation trends from crystallization experiments of durbachites give an excellent match with the data from the Strudengau dykes. This suggests that the lamprophyres and felsic dykes were both products of fractional crystallization and subsequent magma mixing of durbachitic and leucogranitic melts. Rb–Sr geochronological data on biotite from five undeformed kersantites and a locally deformed granite porphyry gave cooling ages of c. 334–318 Ma, indicating synchronous intrusion of the dykes with the nearby outcropping Weinsberger granite (part of the South Bohemian Batholith, c. 330–310 Ma). Oriented matrix biotite separated from the locally deformed granite porphyry gave an Rb–Sr age of c. 318 Ma, interpreted as a deformation age during extensional tectonics. We propose a large-scale extensional regime at c. 320 Ma in the Strudengau area, accompanied by plutonism of fractionated magmas of syncollisional mantle-derived sources, mixed with crustal components. This geodynamic setting is comparable to other areas in the Variscan belt documenting an orogenic wide extension by the end of the Carboniferous.

Appendix

Methods

Whole-rock geochemistry

Major-element and trace-element whole-rock analyses were performed at the ACME Analytical Laboratories, Vancouver, Canada. Major oxides were analyzed by pressed pellet XRF, and trace elements were determined by ICP-MS. Details about the analytical methods can be found at acmelab.com.

Mineral chemical analyses

Mineral compositions were obtained on carbon-coated polished thin sections with a Cameca 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. 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.

Methodology applied for Rb–Sr TIMS isotope analysis

The Rb–Sr analytical work was performed at the Laboratory of Geochronology, Department of Lithospheric Research, Center for Earth Sciences, University of Vienna. Results are based on ID-TIMS analytical procedure. Details of technique and data precision are given by Thöni et al. (2008).

Sample preparation/separation

Pure mica (biotite) separates for Rb–Sr analysis were prepared from the bulk crushates by sieving, magnetic concentration (using splits of the 0.16–0.45 mm grain size = sieve fraction for kersantite samples, 0.02–0.12 mm and 0.2–1.6 mm grain size for the granite porphyry sample), repeated grinding in an agate mill (using alcohol), sieving, drying and, finally, careful magnetic purification.

Pure mineral separates used for Rb–Sr analysis weighed between 150 and (maximum) 200 mg. For whole-rock analyses, c. 50–(maximum) 200 mg of ultrafine, well-homogenized sample powder was used.

Sample digestion, element separation and isotope dilution (ID)

Sample dissolution for Rb–Sr analysis was performed in Savillex^® beakers using HF/HNO₃ (4:1), and element separation followed conventional procedures. Samples were spiked in solid form, using a highly concentrated mixed ⁸⁷Rb-⁸⁴Sr tracer, and subsequently dissolved in ultrapure HF-HNO₃ (4:1). Element separation followed conventional techniques, using AG^® 50W-X8 (200–400 mesh, Bio-Rad) resin and 2.5 and 1.0 N HCl as eluants. Maximum total procedural blanks were <500 pg for both Sr and Rb and were taken as negligible in all cases.

Mass spectrometry and data processing/evaluation

Sr was loaded on and evaporated from a Re double filament assembly, using a ThermoFinnigan^® Triton TI TIMS, while Rb fractions were loaded on Ta single filaments using H₃PO₄ and run on a Finnigan^® MAT262 mass spectrometer. A ⁸⁷Sr/⁸⁶Sr ratio of 0.710241 ± 0.000002 (n = 18) was determined for the NBS987 Sr international standard during the period of investigation. Within-run mass fractionation for Sr isotopes was corrected for relative to ⁸⁶Sr/⁸⁸Sr = 0.1194. Uncertainties on the Sr isotope ratios (IC) are quoted as 2σ _m. For the ⁸⁷Rb/⁸⁶Sr ratio, a mean error of ±1 % is applied, including blank contribution, uncertainties on spike composition and machine drift; regression calculation is based on these uncertainties and the isochron calculations follow Ludwig (2003; isoplot). Age calculations are based on a decay constant of 1.393 × 10⁻¹¹ a⁻¹ for ⁸⁷Rb; age errors are given at the 2σ level.