Central European Geology

First report on dendrochronological and radiocarbon studies of subfossil driftwood recovered across the Mureş/Maros Alluvial Fan

Mon, 25 Jul 2022 00:00:00 +0200

Visiting three gravel pits and three natural outcrops across the Mureş/Maros Alluvial Fan 58 samples were collected from subfossil driftwood recovered from coarse grained fluvial sediment layers. While no subfossil wood was found at three additional gravel pits. Dendrochronological and radiocarbon analysis of these relict wood can promote the temporal extension of the regional dendrochronological reference datasets and their dating can provide useful contribution to the reconstruction of the landscape evolution of the Mureş/Maros Alluvial Fan. The tree-ring widths of the subfossil samples were measured. Dendrochronological synchronization resulted in two oak chronologies which encompassed five and two reliably cross-dated series covering 191 years (MURchr1) and covering 127 years (MURchr2), respectively. Based on the ¹⁴C ages the subfossil driftwood material represents Middle and Late Holocene ages. The thick, occasionally up to 6 m, fluvial sediment covering relatively young, < 1000-yr-old, wood indicate intense accumulation at the apex of the Mureş/Maros Alluvial Fan, which explains the documented rapid and significant Holocene avulsions.

Petrographic evidences of open system magmatic processes in the felsic rocks of the northern part of the Ditrău Alkaline Massif (Eastern Carpathians, Romania)

Luca Kiri; Máté Szemerédi, Elemér Pál-Molnár — Mon, 25 Jul 2022 00:00:00 +0200

Over the almost 190 years-long research of the Ditrău Alkaline Massif (Eastern Carpathians, Romania), felsic rocks have been regarded as homogeneous, uniform units of the igneous complex. Nevertheless, our detailed textural study revealed that the felsic suite (diorite–alkaline feldspar syenite and nepheline-bearing syenite–granite series) cropping out north of the Jolotca creek valley is more heterogenous on micro-scale than previously thought. This heterogeneity partly derives from abundant mafic mineral-rich clusters; nevertheless, felsic minerals also exhibit various, remarkable textural features. Outcrop to micro-scale traits of felsic crystal settling, mafic mineral aggregates and flow fabrics along with metamorphic country rock xenoliths suggest that the studied rocks crystallised under dynamic magmatic conditions. Cumulate formation, shear flow, convection currents as well as various open system magmatic processes (e.g., magma recharge, magma mixing and mingling, crystal or mush transfer and recycling, country rock assimilation) played a significant role in the petrogenesis of the examined felsic suite.

Based on field observations as well as on the microtextural relationship of the minerals, two major groups of felsic rocks were distinguished: (1) felsic rocks (lacking or containing sparse mafic minerals) spatially associated with mafic rocks and (2) felsic rocks (with mafic minerals and clots) spatially unassociated with mafic rocks. Rocks of the former group are dominated by plagioclase, accompanied by minor alkaline feldspar, biotite and accessory titanite. Distinct structural and textural features suggest the physical accumulation of the rock-forming phases. Such textural properties can also be observed in some rocks of the second group. Isolated mafic minerals are rather scarce in the latter; nevertheless, different types of aggregates made up of either identical or various mafic phases are more common. Clustered minerals are either intact or show different stages of alteration.

A detailed petrographic study of the above-mentioned peculiarities has been implemented in order to define their potential origin(s) and petrogenetic significance.

CAVE MONITORING IN HUNGARY: AN OVERVIEW

György Czuppon — Mon, 25 Jul 2022 00:00:00 +0200

In this study published and new monitoring data are compiled from the Baradla and Béke caves in the Aggtelek Karst, the Ajándék-Ariadne cave system in Pilis Mountain as well as in the Szemlőhegy and Pálvölgy caves in Buda Mountains. Recent investigations (2019-2020) include monitoring of climatological parameters (e.g., temperature, CO₂) measured inside and outside the caves, the chemical, trace element and stable isotopic compositions of drip waters, and the stable isotope composition of modern carbonate precipitates. In the Baradla Cave, the main focus of the investigation was on the stable isotope composition and the temperature measurements of drip water, as well as on the stable isotope composition of carbonates formed on glass plates at the Nehéz-út site. Scratched carbonate samples from speleothem surfaces were collected at several sites in the Ajándék-Ariadne cave system. In addition, in the Vacska Cave, which belongs to this system, CO₂ measurements and drip water collection were conducted in order to perform chemical and stable isotope measurements. Carbonates formed on glass plate were also harvested twice in the Vacska Cave. In the Szemlőhegy and Pálvölgy caves, the chemical and stable isotope compositions of drip waters at six sites were determined. These data sets helped to reveal the characteristics of the studied caves and the hydrological processes taking place in the karst, and to trace anthropogenic influences. In addition, they provide a basis for the interpretation of speleothem-based proxies.

Complex deformation history of the Keszthely Hills, Transdanubian Range, Hungary

Gábor Héja — Mon, 25 Jul 2022 00:00:00 +0200

We have investigated the deformation history of the Keszthely Hills (Transdanubian Range, W Hungary), which belongs to the uppermost slice of the Austroalpine nappe-system. This Upper Triassic to upper Miocene sedimentary rock sequence documented the deformation of the upper crust during repeated rifting and inversion events. We investigated the structural pattern and stress-field evolution of this multistage deformation history by structural data collection and evaluation from surface outcrops. Regarding the Mesozoic deformations, we present additional arguments for pre-orogenic (Triassic and Jurassic) extension (D1 and D2 phases), which is mainly characterised by NE-SW extensional structures, such as syn-sedimentary faults, slump-folds, pre-tilt conjugate normal fault pairs. NW-SE-striking map-scale normal faults were also connected to these phases.

The inversion of these pre-orogenic structures took place during the middle of the Cretaceous, however minor contractional deformation possibly reoccurred until the Early Miocene (D3 to D5 phases). The related mezo- and map-scale structures are gentle to open folds, thrusts and strike-slip faults. We measured various orientations, which were classified into three stress states or fields on the basis of structural criterion, such as tilt-test, superimposed striae on the same fault planes. For this multi-directional shortening we presented three different scenarios. Our preferred suggestion would be the oblique inversion of pre-orogenic faults, which highly influenced the orientation of compressional structures, and resulted in an inhomogeneous stess field with local stress-states in the vicinity of inherited older structures.

The measured post-orogenic extensional structures are related to a new extensional event, the opening of the Pannonian Basin during the Miocene. We classified these structures to the following groups: immediate pre-rift phase with NE-SW extension (D6), syn-rift phase with E-W extension (D7a) and N-S transpression (D7b), and post-rift phase with NNW-SSE extension (D8).

Development and dolomitization of Anisian isolated carbonate platforms in the Transdanubian Range, Hungary

György Czuppon; János Haas; Tamás Budai, Orsolya Győri — Mon, 25 Jul 2022 00:00:00 +0200

In the Middle Anisian, extensional tectonic movements led to the development of isolated carbonate platforms in the area of the SW part of the Transdanubian Range. The platforms are made up of metre-scale peritidal–lagoonal cycles bounded by subaerial exposure surfaces. One of the platform successions (Tagyon Platform) consists predominantly of limestone that contains partially and completely dolomitized intervals, whereas the other one (Kádárta Platform) is completely dolomitized. Drowning of the platforms took place by the end of Pelsonian when submarine highs came into existence and then condensed pelagic carbonate successions with volcanic tuff interbeds were deposited on the top of the drowned platforms from the late Illyrian up to the late Ladinian. Comparative study of dolomitization of the coeval platforms affected by different diagenetic history are discussed in the current paper. Traces of probably microbially mediated early dolomitization were preserved in the slightly dolomitized successions of the Tagyon Platform. This might have been present also in the successions of the Kádárta Platform, but it was overprinted by geothermal dolomitization along the basinward platform margin and by pervasive reflux dolmitization in the internal parts of the platform. The Carnian evolution of the two submarine highs was different, and this may have significantly infuenced the grade of dolomitization.

Nb-Ta mineralization in Ti-oxide minerals from the Bagolyhegy Metarhyolite Formation (Bükk Mountains, NE Hungary)

Mon, 25 Jul 2022 00:00:00 +0200

The foliated low-grade metamorphic rocks of the Triassic Bagolyhegy Metarhyolite Formation, mainly of pyroclastics origin, host postmetamorphic quartz-albite veins containing abundant tourmaline and rutile/ilmenite occasionally. The study of the rutile-mineralized veins with SEM-EDX revealed an unusual mineral assemblage comprising fine-grained Nb-Ta-bearing oxides (columbite-tantalite series, fluorcalciomicrolite and further Nb-Ti-Y-Fe-REE-oxide minerals) intergrown with niobian rutile or ilmenite, brookite and zircon enriched in Hf. A paragenesis like this is common in pegmatite enriched in high field strength elements but unexpected in this lithology, and it was not described from any formation in the Palaeozoic-Mesozoic rock suite of the Bükk Mountains (NE Hungary) before. The host metavolcanics are significantly depleted in all HFSE compared to the typical concentrations in felsic volcanics, which can be attributed to crystallization from extremely fractionated magma. The mineralized quartz-albite veins have even lower Ti-Nb-Ta concentration than the host rock, so the mineralization does not mean any enrichment. From proximal outcrops of the Triassic Szentistvánhegy Metavolcanics potassic metasomatized lenses with albite-quartz vein fillings containing rutile/ilmenite are known. We studied them for comparison, but they contain only REE mineralization (allanite-monazite-xenotime), the Nb-Ta-content of Ti-oxide minerals is undetectably low. LA-ICP-MS measurements for U-Pb dating of Hf-rich zircon of the Nb-Ta-rich mineral assemblage gave ca. 70 Ma as lower intercept age while dating of allanite of the REE mineralized quartz-albite veins gave ca. 110 Ma as lower intercept age. The REE-bearing vein fillings formed during a separate mineralization phase in the Early Cretaceous while the Nb-Ta mineralization was formed by the migration of alkaline, probably magmatism-related fluids in the Late Cretaceous, controlled by fault zones and fractures.