Evidence for widespread mid-Permian magmatic activity related to rifting following the Variscan orogeny (Western Carpathians)

Abstract

The Gemeric and Veporic Superunits of the Western Carpathians correlate to the Lower and Middle Austroalpine tectonic units (nappes) of the Eastern Alps. The Gemeric Superunit is characterized by small exposures of rare-metal granites, and their ages impact understanding its tectonic history and how this portion of the Carpathians relates to other Permian age granites exposed throughout Europe and the Western Mediterranean. Here we present new Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Secondary Ion Mass Spectrometry (SIMS) zircon U--Pb ages from northern (Hnilec) and southern Gemeric granite exposures (Betliar, Elisabeth Mine, Poproč) and one from the Veporic Superunit (Klenovec). Geochemical analyses of Betliar and Klenovec samples indicate they are highly differentiated and fractionated from a clay-rich source, consistent with published reports for these granites. Zircon saturation temperatures average 733 ± 27 °C (Betliar) and 756 ± 61 °C (Klenovec). Most Gemeric granitic zircons are Permian (n = 231 spots), with some inherited zircons giving Middle Ordovician to Late Silurian (n = 26), and some Triassic (n = 6) U--Pb ages. The dominant Permian U--Pb yield crystallization ages of at 264 Ma for the Gemeric granite and 265 Ma for the Veporic Superunit's Klenovec granite (n = 26). Klenovec zircons typically show distinct yellow rims in cathodoluminescence, characteristic of lower temperature overprint (<600 °C). These granite zircon ages overlap those from the radiolarite-bearing metasediments from the Meliata Ocean exposed in the Dobšiná region (263.9 ± 2.7 Ma, LA-ICP-MS, ±1σ). The detrital zircon U--Pb ages thus imply a possible beginning of sedimentation in the Meliata Ocean as early as the mid-Permian. The genesis of the Gemeric and the Klenovec granites is related to post-collisional extension. These data link the Western Carpathians to a regional widespread occurrence of Permian magmatism in the European Variscan and Western Mediterranean realms, consistent with regional magmatic re-equilibration of the Moho. This scenario likely occurred in response to a thermal surge that involved significant decompression and extension, lower crustal melting, upward displacement of the Moho, and delamination of the mantle-lithosphere that contributed substantially towards Variscan crustal thinning.