In situ U–Pb zircon geochronology of Neogene garnet-bearing lavas from Slovakia (Carpatho-Pannonian region, Central Europe)

Abstract

Zircons armored in garnet from four Neogene lavas of Slovakia (three andesites and one dacite) were dated in situ, using excimer laser ablation ICP-MS. For the andesites, the 66 analytical points yield a lower intercept age of 13.3±0.1Ma (MSWD=1.2), which is older than the age of 12.4±0.2Ma recorded for the dacite. U–Pb ages obtained in this study are significantly younger than the Lower Badenian ages (K–Ar and fission-track dating coupled with biostratigraphic correlations) classically assumed for the garnet-bearing lavas of the Western Carpathians, with a minimum discrepancy of 3Ma. Accessorily, a composite zircon with an inherited core reveals that some Pan-African crustal component is present at depth.Garnet-bearing andesites and dacites potentially bear critical information about melt generation in the lower crust and possible recycling of pre-existing crust into igneous rocks. Practically, the problem is to assess the nature of the melt from which the garnet actually grew. For this purpose, zircon grains included into garnet may help to clarify the beginning of the magma history. It seems likely that zircon was not grown from the bulk dacitic or andesitic melt but rather from a specific rhyolitic batch. Later mixing and consecutive change in magma chemistry explain that free zircons in the melt were almost totally dissolved, whereas zircons shielded into garnets were preserved. Finally, owing to their Miocene age, the zircon grains armored into garnet preclude any restitic origin for their host and definitely demonstrate the magmatic origin of garnet.