Kadosa Balogh

Timing of low-temperature metamorphism and cooling of the Paleozoic and Mesozoic formations of the Bükkium, innermost Western Carpathians, Hungary

K-Ar ages of illite-muscovite and fission track ages of zircon and apatite were determined from various lithotypes of the Bükkium, which forms the innermost segment of the Western Carpathians. The stratigraphic ages of these Dinaric type formations cover a wide range from the Late Ordovician up to the Late Jurassic. The grade of the orogenic dynamo-thermal metamorphism varies from the late diagenetic zone through the ‘anchizone’ up to the ‘epizone’ (chlorite, maximally biotite isograd of the greenschist facies). The K-Ar system of the illite-muscovite in the < 2 μm grain-size fraction approached equilibrium only in ‘epizonal’ and high-temperature ‘anchizonal’ conditions. The orogenic metamorphism culminated between the eo-Hellenic (160-120 Ma) phase connected to the beginning of the subduction in the Dinarides, and the Austrian (100-95 Ma) phase characterized by compressional crustal thickening. No isotope geochronological evidence was found for proving any Hercynian recrystallization. The stability field of fission tracks in zircon was approached using the thermal histories of the different tectonic units. A temperature less than 250°C and effective heating time of 20–30 Ma had only negligible effects on the tracks, whereas total annealing was reached between 250 and 300°C. Apatite fission track ages from the Paleozoic and Mesozoic formations show that the uplift of the Bükk Mountains occurred only in the Tertiary (not earlier than ca. 40 Ma ago). Thermal modeling based on apatite fission track length spectra and preserved Paleogene sediment thickness data proved that the Late Neogene burial of the recently exhumed plateau of the Bükk Mountains exceeded 1 km.

Contributions to the chronology of the Basal Complex of Fuerteventura, Canary Islands

Abstract K–Ar and Ar–Ar analyses of the Basal Complex of Fuerteventura confirm the early start of magmatism in the Late Cretaceous. Plateau ages of 63.1±0.8 Ma and 64.2±1.0 Ma have been obtained for the oldest syenite intrusions indicating the minimum age of these plutonics. The emplacement of carbonatites and syenites along ductile shear zones took place during a relatively short period of time between 23.2±1.0 and 22.1±0.9 Ma. This episode is synchronous all over the island. Younger plutons were intruded around 21.1±0.8 Ma ago and 20.7±0.9 Ma and are probably related to the building of Miocene subaerial edifices. Almost all the minerals and rocks of the Basal Complex show a clear effect of excess argon. Some was acquired during the hydrothermal stage but also some excess argon was trapped during the early crystallization stage of the magma. Moreover, only a part of the radiogenic Ar which had accumulated in pre-Miocene rocks was released during the Miocene magmatism.

Volcanic architecture, eruption mechanism and landform evolution of a Plio/Pleistocene intracontinental basaltic polycyclic monogenetic volcano from the Bakony-Balaton Highland Volcanic Field, Hungary

Bondoró Volcanic Complex (shortly Bondoró) is one of the most complex eruption centre of Bakony-Balaton Highland Volcanic Field, which made up from basaltic pyroclastics sequences, a capping confined lava field (~4 km2) and an additional scoria cone. Here we document and describe the main evolutional phases of the Bondoró on the basis of facies analysis, drill core descriptions and geomorphic studies and provide a general model for this complex monogenetic volcano. Based on the distinguished 13 individual volcanic facies, we infer that the eruption history of Bondoró contained several stages including initial phreatomagmatic eruptions, Strombolian-type scoria cones forming as well as effusive phases. The existing and newly obtained K-Ar radiometric data have confirmed that the entire formation of the Bondoró volcano finished at about 2.3 Ma ago, and the time of its onset cannot be older than 3.8 Ma. Still K-Ar ages on neighbouring formations (e.g. Kab-hegy, Agár-teto) do not exclude a long-lasting eruptive period with multiple eruptions and potential rejuvenation of volcanic activity in the same place indicating stable melt production beneath this location. The prolonged volcanic activity and the complex volcanic facies architecture of Bondoró suggest that this volcano is a polycyclic volcano, composed of at least two monogenetic volcanoes formed more or less in the same place, each erupted through distinct, but short lived eruption episodes. The total estimated eruption volume, the volcanic facies characteristics and geomorphology also suggests that Bondoró is rather a small-volume polycyclic basaltic volcano than a polygenetic one and can be interpreted as a nested monogenetic volcanic complex with multiple eruption episodes. It seems that Bondoró is rather a “rule” than an “exception” in regard of its polycyclic nature not only among the volcanoes of the Bakony-Balaton Highland Volcanic Field but also in the Neogene basaltic volcanoes of the Pannonian Basin.

Geochemistry and Geochronology of Gneiss Pebbles from Foreland Molasse Conglomerates: Geodynamic and Paleogeographic Implications for the Oligo‐Miocene Evolution of the Eastern Alps

Gneiss pebbles, deposited in foreland fans during the Miocene Epoch, were studied with geochemical and thermochronological methods to reconstruct the paleodrainage evolution of the Eastern Alps and the geodynamic behavior of the source areas. Clear discrimination between upper‐plate Austroalpine and lower‐plate Penninic gneiss pebbles was achieved by single‐pebble paleocooling paths. Austroalpine gneisses showed highly variable and regionally distinctive paleocooling paths, which record Variscan to Cretaceous Rb‐Sr and K‐Ar cooling ages. Penninic gneisses showed Paleogene to lower Miocene high‐temperature (Rb‐Sr, K‐Ar) and Miocene low‐temperature (zircon and apatite fission track) cooling ages. Cooling rates of Penninic gneisses indicate maximum exhumation rates at ∼17 Ma during the climax of lateral tectonic extrusion, recording dominance of tectonic unroofing. The paleodrainage evolution was characterized by a relatively stable catchment configuration in the western part of the Eastern Alps, where northward radial dewatering to fixed foreland entry points was documented throughout molasse sedimentation in Oligo‐Miocene times. The largest river system, represented by the Paleo‐Inn River, displayed a considerably larger catchment area than today and variable entry points into the foreland. The drainage pattern of the eastern part of the Eastern Alps was governed by the fault pattern that developed during lateral extrusion. Penninic basement rocks first became exposed in the eastern part of the Tauern window at ∼13 Ma, some 4 m.yr. after the climax of tectonic unroofing.

The submarine volcanic succession of the basal complex of Fuerteventura, Canary Islands: A model of submarine growth and emergence of tectonic volcanic islands

Three lithostratigraphic units have been distinguished in the volcanic succession of the basal complex of Fuerteventura Island. These units are, from bottom to top: the submarine volcanic group, the transitional volcanic group, and the subaerial volcanic group. These three groups record the submarine growth and emergence of the island. The volcanism is represented by ultra-alkaline and strongly alkaline igneous series. The igneous activity was due to the presence of an anomalous zone in the sublithospheric mantle, the low density of which also caused uplift of the Mesozoic oceanic crust. Two extensional phases and an intervening contractional phase developed coeval to the generation of the volcanic succession. The submarine volcanic group was deposited in the hanging wall basin of a large listric extensional detachment directed toward the SSW. The transitional volcanic group was syntectonic with respect to a late inversion of the listric detachment. Finally, the subaerial volcanic group resulted from a second episode of WNW extension. This study of the evolution of the basal complex of Fuerteventura serves as the basis for a tectonic model of submarine growth and emergence of volcanic islands.

Cenozoic intraplate alkaline volcanism of Western Bohemia

Three independent volcanic suites have been recognised in W Bohemia: (i) the old unimodal alkaline ol. nephelinite-tephrite (29-19 Ma) in the Ohře Rift, (ii) two contemporaneous weakly (trachybasalt/trachyandesite-trachyte/rhyolite; 13-11 Ma) and strongly (ol. nephelinite-tephrite/basanite; 12-8 Ma) alkaline series in the flank of the Cheb-Domažlice Graben formed by the Teplá Highland and (iii) the young unimodal ol. melilitite/ol. nephelinite alkaline suite (2.0-0.12 Ma) at the intersection of the above mentioned structures in the Cheb Basin. The magmas of all the suites are mantle-derived and, in the case of the Cheb-Domažlice Graben series, associated with the AFC process. Two main fault systems: (i) ENE-WSW and (ii) NNW-SSE are developed in W Bohemia, corresponding to the directions of the two prominent taphrogenic structures. The southwesterly continuation of the Ohře Rift across the Mariánské Lázně Fault is marked by volcanics only.

On the age of the Dej Tuff, Transylvanian Basin (Romania)

On the age of the Dej Tuff, Transylvanian Basin (Romania) The Dej Tuff is an important stratigraphic marker in the Transylvanian Basin. However, its Early Badenian age is known only on biostratigraphical grounds so far. A number of radiometric dating techniques including K-Ar, Ar-Ar and fission-track have been used in order to constrain more precisely its age, allowing the calibration of the Transylvanian Basins evolutionary models. Although individual dating methods could not provide a unique, reliable and accurate radiometric age, comparison and evaluation of multiple methods gives 14.8-15.1 Ma as the most likely formation age of the Dej Tuff.

Eoalpine (Cretaceous) very low- to low-grade metamorphism recorded on the illite-muscovite-rich fraction of metasediments from South Tisia (eastern Mt Papuk, Croatia)

Eoalpine (Cretaceous) very low- to low-grade metamorphism recorded on the illite-muscovite-rich fraction of metasediments from South Tisia (eastern Mt Papuk, Croatia) Eoalpine very low- to low-grade metamorphism related to Cretaceous orogenesis has been investigated in the Slavonian Mts, Croatia. Samples belonging to the Psunj metamorphic complex (PMC), the Radlovac metamorphic complex (RMC) and Permian-Triassic and Triassic sedimentary sequences (PTSS) were studied. The Kübler and Árkai indices of all the analysed samples indicate high-anchizonal to epizonal metamorphism. The degree of Eoalpine metamorphism tends to be constant in all samples implying that the different complexes passed through and recorded the same event. Measurements of illite-white K-mica b0-parameter of the RMC samples imply transitional low- to medium-pressure character of the metamorphism. These data together with K-Ar ages (~100-80 Ma) measured on illite-white K-mica rich < 2 μm grain-size fractions point to Late Cretaceous very low- to low-grade regional metamorphism presumably related to the main nappe-forming compressional events in the Pannonian Basin and the Carpathians. The P-T-t (pressure-temperature-time) evolution of the studied area is in good agreement with similar scenarios in the surrounding areas of Tisia, but also in Eastern Alps, Carpathians and Pannonian Basin (ALCAPA).

Loss of 40Ar(rad) from leucite-bearing basanite at low temperature: implications on K/Ar dating.

The Bakony-Balaton Highland Volcanic Field (BBHVF) is located in the central part of Transdanubia, Pannonian Basin, with over 50 alkali basaltic volcanoes. The basanite plug of Hegyestu erupted in the first phase of volcanic activity. K/Ar and Ar/Ar ages were published for the BBHVF. K/Ar and Ar/Ar ages of the leucite-bearing basanite of Hegyestás were conflicting. This is caused by the special Ar retention feature of leucite in this basanite.K/Ar ages measured in the usual way were 25–45% younger, but after HCl treatment of the rock, or after reducing the baking temperature of the argon extraction line from 250°C to 150°C, they became similar to theAr/Ar ages. All Ar/Ar determinations were performed after HF treatment. HCl treatment dissolved olivine, nepheline, leucite, magnetite and from 1-1 sample analcime or calcite. K dissolution studies from different locations of Hegyestü have shown that K content is mostly ≈2%, but it may decrease to ≈0.3%. HCl treatment dissolved 28.0–63.5% of the K content. The calculated K concentration for the dissolved part of samples with ~2%K was 4.02-6.42%: showing that leucite is responsible for the low temperature loss of 40Ar(rad). Ar may release at low temperature from very finegrained mineral, or when the Ar release mechanism changes. A 40Ar(rad) degassing spectrum has been recorded in the 55–295°C range of baking temperature and the data were plotted in the Arrhenius diagram. The diagram shows that a change of the structure in the 145–295°C range caused the loss of 40Ar(rad). On fractions of HCl treated rock 7.56±0.17 Ma isochron K/Ar age has been determined. This is regarded as minimum age of eruption and it is similar to the Ar/Ar isochron age (7.78±0.07 Ma).

Deformation history and nappe stacking in Rudabánya Hills (Inner Western Carpathians) unravelled by structural geological, metamorphic petrological and geochronological studies of Jurassic sediments

Several Jurassic tectonic units of the Rudabánya Hills (Inner Western Carpathians) were studied by structural geological, metamorphic petrological and geochronological methods, in order to unravel its deformation history and nappe stacking. Three series showing pronounced lithostratigraphic similarities (black shale, sandstone turbidite and olisthostrome) are interpreted to be subunits of the Telekesoldal nappe. It is questionable whether a Triassic basement can be associated with these Jurassic rocks. Metamorphic petrological data indicate metamorphism at the transition between the high-temperature anchizone and epizone (300–350°C and 2-2.5 kbar pressure). These series were affected by three events of ductile deformation including in order 1) layer-parallel foliation, 2) folding with axial plane foliation and 3) kink bands. K-Ar ages suggest that the main detected metamorphic event took place about 137–117 Ma ago. In contrast, the uppermost Triassic–Jurassic Telekesvölgy, and the Triassic Bódva series were only affected by diagenetic (partly anchimetamorphic) alteration. The metamorphosed Jurassic complexes thrust upon the Telekesvölgy–Bódva sequence as a separate nappe at 87–94 Ma as suggested by K-Ar data. The metamorphism and deformation of these Jurassic series are suggested to appear simultaneously with the Middle Jurassic–earliest Cretaceous subduction of the Meliata branch of the Neotethys Ocean. Post-metamorphic deformation, including nappe emplacement seems to be coeval with mid-Cretaceous deformations of other Inner Western Carpathian units. The latter is probably related to the closure of the Penninic-Vahic Ocean farther to the north.