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  • Author or Editor: Tibor Zelenka x
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Abstract

The Tokaj Mts, situated in the northeastern part of the inner arc of the Carpathians, forms a part of a Miocene calc-alkaline andesitic-dacitic-rhyolitic volcanic island arc. The ancient volcanic structures were reconstructed on the basis of the 1:50 000-scale and 22 sheets of the 1:25 000-scale geologicpetrologic maps, as well as the revision of the volcanic facies in 150 boreholes. Multispectral and SAR satellite imagery, aerial photos, data and maps of airborne geophysical surveys (magnetic and radiometric), gravity-filtered anomaly maps, geochemical (soil and stream sediment Au, As, Sb, Hg) concentration distribution maps and the K/Ar dating of 132 samples from 80 paleomagnetic measurements were also used.

The anomalies were only taken into consideration in the interpretation if the coincident results of at least 3 methods indicated the presence of any volcanic structure. In consequence, 91 map-scale volcanic structures were identified by morphology — complex calderas, single lava domes, volcanic fissures, subvolcanic intrusions, diatremes, stratovolcanoes and postvolcanic formations. Conclusions were also drawn regarding the link to the volcanic structures and prospective occurrences of the mineral resources of the Tokaj Mts: andesite, dacite, welded zeolitic tuff, K-metasomatite, perlite, pitchstone, pumice, bentonitic, illitic, kaolinitic, diatom-bearing and silicified lacustrine sediments, hydrothermal Au-Ag and Pb-Zn veins, and Hg stockwerks.

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Abstract

Paleomagnetic measurements were carried out on 163 independently oriented samples from 19 sites of the Bükk Mts and their northern, western and southern forelands. The aim was to correlate the sites with one of three Miocene rhyolite tuff horizons using the combination of paleomagnetic marker horizons (rotational events) and traditional magnetostratigraphy.

In contrast to the results of earlier studies in the southern Bükk foreland, which yielded only reversed polarity magnetizations, nearly half of the presently obtained paleomagnetic directions are of normal polarity. By their declinations they mostly belong to the middle tuff horizon, and only one belongs to the upper.

The paleomagnetic age assignment of the studied sites sometimes supports one or both of the classifications of Balogh (1964) and Pelikán et al. (2005). However, about one-third of the sites classified by these authors as upper or lower tuffs were shown to belong to the middle tuff complex.

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Acta Geologica Hungarica
Authors: Tibor Zelenka, Endre Balázs, Kadosa Balogh, János Kiss and at. al.

Surface Neogene volcanics in Hungary are abundantly documented in the literature, but buried volcanic structures are little known. Early burial of the volcanic centers beneath latest Miocene to Pliocene sediments preserved much of their original relief, permitting their classification into genetic types. More than two-thirds of Hungary is covered by thick Neogene and Quaternary sediments, below which buried volcanic eruptive centers and the extent of their products may only be recognized by complex geologic-geophysical methods. Our study is based on the data of several thousand wells, more than 60,000 km of seismic sections, as well as airborne and surface geophysical (gravimetric, magnetic, electromagnetic, radiometric) data. Results of chemical, mineralogical studies and K/Ar dating of deep cores were also included. The data were evaluated in terms of the regional deep structure of the Carpathian-Balkan region, the Miocene evolution of which was determined by the position, movement and welding of individual microplates. Integration of all available data reveals that the Miocene volcanic centers are concentrated near microplate boundaries. In the volcanic centers the lavas and pyroclastic deposits far exceed 50 m in thickness. The data show that the buried volcanic rocks below the Transdanubian region (Little Hungarian Plain and Somogy-Baranya Hills), the Danube-Tisza Interfluve and the Great Hungarian Plain extend over a much larger area than do the outcropping volcanoes in Northern Hungary (from the Visegrád to the Tokaj Mts). In the southern part of Transdanubia (W. Hungary) a major calcalkaline, rhyolitic, ignimbritic event took place early, in Eggenburgian and Ottnangian (Early Miocene) times. The centers and tuff sheets of this volcanic event can be traced from the Mecsek Mts to the Salgótarján Basin, the southwestern Bükk Basin and the central part of the Great Hungarian Plain. This event was followed by andesitic volcanism. The rhyolite and dacite volcanic centers of Karpatian age are predominantly situated in Transdanubia, whereas the Badenian (Mid-Miocene) andesite and dacite series of large stratovolcanoes are buried below southern Transdanubia, the Danube-Tisza Interfluve and the Great Hungarian Plain. In Sarmatian and early Pannonian (Late Miocene) times, pyroclastic sheets several thousand meters thick and lava domes were formed; they are predominantly rhyolitic, subordinately andesitic and dacitic, and are situated in the eastern part of the Great Hungarian Plain (Nyírség).  With the end of microplate motion, as the plate consolidated in the late Miocene, thick but areally restricted alkali-trachite (Little Hungarian Plain) and alkali-basalt lava domes and tuff craters formed in the Little Hungarian Plain, Transdanubia and the Danube-Tisza Interfluve.

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New K/Ar ages and paleomagnetic data connected with volcano-tectonic observations detected three intermediate (andesitic) and three acidic (dacitic-rhyolitic) magmatic phases. Cserhát magmatic activity occurred between 21-12 Ma. The timing of the initial and final acidic and intermediate phases may be connected with the Mátra volcano situated to the east. During the Badenian (15-14 Ma) the volcano-tectonic evolution was relatively independent in the Cserhát Mts. The third acidic and intermediate volcanic phases, which developed in the Lower Sarmatian, show similar features as the final phases of the Mátra volcano. Based on the major and trace element geochemistry the acidic rocks result from partial melting of the lower crust. Most of the intermediate volcanic rocks were generated from a rather homogeneous fluid-modified source (lithospheric), as triggered by an important heat transfer event. Initial melts sometimes experienced mixing or contamination in the lower or upper crust. This was a period of strong extensive tectonics. The rock of the second and third intermediate phases suggests minor fractional crystallization in the intermediary magma chamber(s).

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