Variscan metamorphism and partial melting of sillimanite-bearing metapelites in the High Tatra Mts. constrained by Th–U–Pb dating of monazite
Abstract: The Tatra Mountains of the Western Carpathians are a key area for the study of the eastern continuation of the Variscan basement within the Alpine–Carpathian orogenic belt in Central Europe. Metamorphic zonation in the Tatra Mts. displays an inverted metamorphic sequence related to Variscan thrusting and emplacement of gneisses, migmatites and granites over micaschists. Here we present new results of Th–U–Pb dating of monazite in sillimanite-bearing metapelitic gneisses, migmatite and granodiorite from the High Tatra along with petrological interpretation based on thermodynamic modelling. The metapelitic gneisses show the peak metamorphic assemblage garnet + sillimanite + plagioclase + biotite + muscovite + ilmenite + quartz; inclusions of rutile, phengitic muscovite and paragonite in the garnet core indicate an earlier metamorphic stage. Thermodynamic modelling suggests a clockwise, prograde P–T path via staurolite-to-sillimanite sequence reactions from above 6 kbar to ca. 5–6 kbar and 650–700 °C, at mid-crustal levels. Migmatites, with dominant K-feldspar, plagioclase (An12–35) and quartz in the leucosome, underwent partial melting involving biotite dehydration reactions and formation of peritectic, Mn-rich garnet and/or Ti-magnetite at ca. 7–8 kbar and 760–770 °C, during decompression from lower-crustal levels. Monazite composition in metapelitic gneisses differs from that in leucosome of migmatite. The latter shows pronounced Eu-negative anomalies interpreted as the result of co-crystallization with feldspars and higher Y contents indicating higher temperature of crystallization. Monazite ages are identical within 2σ errors and indicate that both metamorphic and melting events occurred in Early Carboniferous, between 350–345 Ma as a consequence of continental collision and crustal thickening in the course of the Variscan orogeny.
Permian A-type rhyolites of the Drienok Nappe, Inner Western Carpathians, Slovakia: Tectonic setting from in-situ zircon U–Pb LA–ICP–MS dating
Abstract: Two representative peraluminous A-type rhyolite samples from the Poniky area (the Drienok Nappe) in the Inner Western Carpathians (central Slovakia) were dated using the LA–ICP–MS U–Pb zircon method. These geochronological data represent the first in-situ isotopic dating study undertaken on these volcanic rocks. Oscillatory zoned zircon crystals yielded concordant Permian (Guadalupian) ages of 271.0 ± 1.5 Ma and 267.5 ± 1.6 Ma for the Poniky rhyolites, which supports their genetic link to the analogous mid-Permian (Guadalupian) rhyolites of adjacent Muráň and Vernár nappes. The Ti-in-zircon geothermometry (corrected using the activities of SiO2 and TiO2 using the rhyolite-MELTS thermodynamic software) indicate mean zircon crystallization temperatures of ~910 to 935 °C for the Poniky rhyolites. The results indicate pulses of anorogenic A-type rhyolitic magmatism were coeval with intrusions of granitic rocks associated with an intraplate extensional tectonic regime triggered by asthenospheric upwelling in the Western Carpathian region. The A-type magmatism was most likely related to the break-up of the Pangea supercontinent during the mid-Permian (~270–260 Ma).
Formal definition and description of lithostratigraphic units related to the Miocene silicic pyroclastic rocks outcropping in Northern Hungary: A revision
Abstract: Repeated explosive eruptions of large volume silicic magmas during the early- to mid-Miocene resulted in pyroclastic deposits covering at least 50,000 km2 in the Pannonian Basin. They form extended marker horizons and therefore these pyroclastic formations have a great stratigraphic importance. Lithostratigraphic characterization and classification of these rocks go back for more than a century and have been used widely in geological mapping among other things. In this paper, we outline the former stratigraphical schemes developed for silicic pyroclastic products in Northern Hungary; however, using the new geochronological, volcanological, petrological, and geochemical results, we propose a revision of the lithostratigraphic units, including the unit names as well. Four main units are distinguished, named, and described following the International Stratigraphic Guide. Stratotypes of the revised units were also redefined based on accessibility and representativeness. The four newly-defined lithostratigraphic units are the following: (1) The Tihamér Rhyolite Lapilli Tuff Formation (formerly Gyulakeszi Fm.), 18.2–17.1 Ma (Ottnangian–Karpatian); (2) The Bogács Dacite Lapilli Tuff Formation (formerly classified into the Tar Fm.), 16.8–16.2 Ma (Karpatian); (3) The Tar Dacite Lapilli Tuff Formation, 15.1–14.8 Ma (Badenian) and (4) The Harsány Rhyolite Lapilli Tuff Formation, 14.7–14.4 Ma (Badenian) – four formerly existing formations merged in the latter. Three of these units have corresponding distal volcanic products recognized around Hungary and beyond the Pannonian Basin as well. A correlation of the scattered volcanic products can be made based on lithological characteristics, as well as the chemical composition of glass shards, juvenile clasts, and zircon.
A review of Procházka’s otoliths from Lower Badenian deposits from Moravia, Czech Republic (Langhian, Middle Miocene), primarily from Borač
Abstract: A historic collection of fossil otoliths from V.J. Procházka from the lower Badenian of Moravia, Czech Republic, was recently recovered at the Moravian Museum in Brno. Likely with the intent to publish, Procházka labeled the specimens with original identifications; however, publication never occurred. Procházka’s type-material of his earlier publications, notably that from 1893, remains unrecovered and must be considered lost. Several of the species he described in 1893 are problematic because of his less-than-optimal descriptions and documentation. With this newly recovered material containing his original identifications, it is now possible to select a neotype for one of those problematic species (Otolithus [Berycidarum] kokeni = Diaphus kokeni) and resolve another (Otolithus [Berycidarum] splendidus). Most of the recovered otoliths originated from the deep-water environment of Borač. Although the locality is well studied for otoliths, the recovered collection of Procházka has yielded seven species not previously recorded from the Badenian of the Central Paratethys. One species is described as new: Nezumia prikryli n. sp.
238U/206Pb age of the fossil sinter crust (flowstone) covering fault walls of a Badenian neptunian dyke (Devín quarry, Western Carpathians)
Abstract: Here, we report on the first radiometric dating of pre-Quaternary sinters covering the ENE–WSW striking vertical faults filled with Upper Badenian sands (neptunian dykes) in Mesozoic carbonates of the Devín quarry in the Malé Karpaty Mountains. The 238U/206Pb age of 13.98±0.56 Ma fits well with the former age prediction based on biostratigraphic evidence. Our results provide fault opening age, refine the age of sediments and fossils preserved in the neptunian dykes, and constrain the onset of Badenian sea transgression.
