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Two volcanic phases took place within the Styrian basin: an older one during the Miocene and a younger one during the Pliocene. While the alkaline basalts of the Pliocene phase have been studied in detail in the recent years (e.g. Ali et al. 2013), the Miocene volcanism around Bad Gleichenberg has not received much attention. The hills of Gleichenberg and Bschaidkogel are composed of trachyandesites and rhyolites with a calc-alkaline to shoshonitic affinity. A crystallization age of 13 ± 1 Ma was determined by K/Ar dating (Balogh et al., 1990), which clearly differentiates the Gleichenberg formation from the younger Pliocene alkaline basaltic magmatic formations that characterize the Styrian Basin. To further characterize and determine the age of the Gleichenberg volcanic formation, samples from different locations around Bad Gleichenberg were analyzed by petrological and geochemical methods. The majority of the samples are trachyandesites that follow typical calc-alkaline differentiation trends with SiO2 varying from 57 to 62 wt%; CaO, FeO and MgO decrease with increasing SiO2, while Al2O3, K2O, Na2O and P2O5 increase with increasing SiO2. The second main lithology – although much less voluminous - are rhyolites with SiO2 contents >71 wt%. Biotite and apatite are commonly found in both rock types. The fluorine content is high in both biotite and apatite with values up to 2 and 3 wt%, respectively. TiO2 contents in biotite can be as high as 8 wt%. U-Pb zircon dating by LA-MC-ICP-MS was carried out on zircons from two samples. Idiomorphic zircons, partly with abundant apatite inclusions or visible zircon cores and zonation, could be found in almost all samples. No age difference between zircon cores and rims or between zoned and homogeneous grains was observed. The analyzed samples yielded homogeneous crystallization ages of ~14 Ma for both volcanic rock types. Therefore, the Gleichenberg formation is likely related to the Balatonmária and Bükkalja volcanic fields in the Western Pannonian Basin System (Harangi et al. 1995). Based on similar geochemical characteristics, both occurrences can be related to the opening of the Styrian/Pannonian Basin, which resulted from slab retreat after collision between the Adriatic plate and Europe. ALI, S., NTAFLOS, T., UPTON, B. (2013). Petrogenesis and mantle source characteristics of Quaternary alkaline mafic lavas in the western Carpathian–Pannonian Region, Styria, Austria. Chemical Geology. 337. 10.1016/j.chemgeo.2012.12.001. BALOGH, K., HARALD, L., PÉCSKAY, Z., RAVASZ, CS., SOLTI, G. (1990). K/Ar radiometric dating of the Tertiary volcanic rocks of East-Styria and Burgenland. MÁFÍ Évi Jel. 1988-ról, 451-468. HARANGI, S., VASELLI, O., TONARINI, S., SZABÓ, C., HARANGI, R., CORADOSSI, N. (1995). Petrogenesis of Neogene extension-related alkaline volcanic rocks of the Little Hungarian Plain volcanic field (Western Hungary). Acta Vulcanologica, 7, 173-188. |
