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  • Author or Editor: Szabolcs Harangi x
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Abstract

Correlation of scattered ignimbrite occurrences is crucial in the context of stratigraphy and the volcanic history of an area. In 2007, two papers were published concerning the classification of the volcanic rocks of the Bükkalja volcanic field. The interpretation of these papers shows an apparent contradiction in the age of the ignimbrite, which crops out at Tibolddaróc and Harsány. This paper attempts to resolve this contradiction. We show that the Harsány ignimbrite defined by Lukács et al. (2007) was indeed formed at 13.5 Ma and is not the same as was described by Márton et al. (2007). We redefine the possible locations of the Harsány and Tibolddaróc samples of Márton et al. (2007). The Tibolddaróc sample could represent the ash flow unit in the middle part of the Tibolddaróc volcanic section, whereas the Harsány sample could be derived from the ‘Harsány-bend’ outcrop. Both rocks have different geochemical character compared to the Harsány ignimbrite. This work emphasizes the usefulness of geochemical correlation of scattered rhyolitic ignimbrites, combined with detailed volcanological field observations.

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Abstract

The alkaline basalt of the Füzes-tó scoria cone is the youngest volcanic product of the Bakony-Balaton Highland Volcanic Field. The bombs and massive lava fragments are rich in various crystals, such as mantle-derived xenocrysts (olivine, orthopyroxene, clinopyroxene, spinel), high-pressure mineral phases (clinopyroxene) and phenocrysts (olivine, clinopyroxene). Peridotite xenoliths are also common. Ratios of incompatible trace elements (Zr/Nb and Nb/Y) suggest that the primary magma was formed in the transitional spinel-garnet stability field, at the uppermost part of the asthenosphere. Magmatic spinel inclusions with low-Cr# (22–35) in olivine phenocrysts can reflect a fertile peridotite source. The olivine, orthopyroxene, colourless clinopyroxene and spinel xenocrysts are derived from different depths of the subcontinental lithospheric mantle and their compositions resemble the mineral phases of the ultramafic xenoliths found in this region. The rarer green clinopyroxene cores of clinopyroxene phenocrysts could represent high-pressure products of crystallization from a more evolved melt than the host magma, or they could be derived from mafic lower crustal rocks. Crystallization of the basaltic magma resulted in olivine and clinopyroxene phenocrysts. Their compositions reflect polybaric crystallization with a final, strongly oxidized stage. The Füzes-tó basalt does not represent a certain magma composition, but a mixture of mineral phases having various origin and mantle-derived basaltic melt.

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In Central Europe, Early Cretaceous alkaline igneous rocks (lamprophyres, basanites, phonolites) occur in the Moravian-Silesian Beskidy area (northern Czech Republic and southern Poland) and in the Mecsek-Alföld Zone (southern Hungary). Presently they are located at about 400 km distance of each other. These alkaline igneous rocks show close similarities in their mineral, chemical, and bulk rock compositional data, implying similar petrogenesis and suggesting that these two regions could have been much closer during the Early Cretaceous; they could belong to the same rift zone in the European continental margin. Their trace element distribution and Sr and Nd isotopic ratios suggest that the parental magmas derived from an enriched, HIMU OIB-like asthenospheric mantle by different degrees (3-6%) of partial melting at the depth of spinel-garnet transitional and garnet stabilization zone (about 60-80 km depth). This mantle source appears to be akin to that thought to have supplied the Tertiary to Quaternary alkaline mafic magmas in Europe (EAR=European Asthenospheric Reservoir). This may imply that this EAR-type mantle reservoir could have been present beneath Europe at least since the Early Cretaceous. It could reside at the base of the upper mantle (670 km discontinuity) supplying upwelling hot mantle fingers, or it may represent a long-lasting, polluted (HIMU+depleted MORB mantle) upper mantle at shallow depth beneath Europe.

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Abstract

The 13.5 Ma Harsány ignimbrite, in the eastern part of the Bükkalja volcanic field, eastern-central Europe, provides a rare example of mingled rhyolite. It consists of two distinct pumice populations (‘A’- and ‘B’-type) that can be recognized only by detailed geochemical work. The pumice and the host ignimbrite have a similar mineral assemblage involving quartz, plagioclase, biotite and sporadic Kfeldspar. Zircon, allanite, apatite and ilmenite occur as accessory minerals. The distinct pumice types are recognized by their different trace element compositions and the different CaO contents of their groundmass glasses. Plagioclase has an overlapping composition; however, biotite shows bimodal composition. Based on trace element and major element modeling, a derivation of ‘A’-type rhyolite magma from the ‘B’-type magma by fractional crystallization is excluded. Thus, the two pumice types represent two isolated rhyolite magma batches, possibly residing in the same crystal mush. Coeval remobilization of the felsic magmas might be initiated by intrusion of hot basaltic magma into the silicic magma reservoir The rapid ascent of the foaming rhyolite magmas enabled only a short-lived interaction and thus, a syn-eruptive mingling between the two magma batches.

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Abstract

The Mid-Hungarian (or Zagreb-Zemplin) Line of WSW-ENE strike divides the Pannonian basement into two mega-units, the Tisia Composite Terrane in the SE and the ALCAPA Composite Terrane in the NW. They became juxtaposed no earlier than the Middle Miocene (Karpathian). Their present adjacent zones show very different Variscan and Alpine evolution and relationships, which are briefly reviewed here and confronted in the light of detailed correlational work published during the last decade. The present contribution summarizes Variscan and Alpine evolution of units/terranes juxtaposed along the Mid-Hungarian Line, the major terrane boundary in the pre-Neogene basement of the Pannonian Basin, as can be seen on the Circum-Pannonian terrane maps.

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Authors: Réka Lukács, György Czuppon, Szabolcs Harangi, Csaba Szabó, Theodor Ntaflos and Friedrich Koller

Silicate melt inclusions are frequent in the phenocryst phases (quartz, plagioclase, orthopyroxene, ilmenite and accessory minerals) of the Miocene silicic pyroclastic rocks of the Bükkalja Volcanic Field, Northern Hungary. These melt inclusions were trapped at different stages of magma evolution; therefore, they provide important information on the petrogenetic processes. The melt inclusions in the Bükkalja pyroclastic rocks show various textures such as (1) wholly enclosed type; (2) hourglass inclusions and (3) reentrant or embayment glass. Among the wholly enclosed type melt inclusions further textural subgroups can be distinguished based on their shape: negative crystal, rounded, elongated and irregular shaped. These various textures reflect differences in the time of entrapment prior to eruption and in the post-entrapment condition in the magma chamber. The largest textural variation was found in the quartz-hosted melt inclusions. However, the major element compositions of these melt inclusions do not differ from one another in the same unit. In general, compositions of the melt inclusions are similar to the chemistry of the glass shards. Comparing the composition of the quartz-hosted melt inclusions from three main ignimbrite units (Lower, Middle and Harsány Ignimbrite Units), slight differences have been recognized, suggesting distinct erupted host magmas. Melt inclusions from the andesitic lithic clast of the Lower Ignimbrite Unit could represent heterogeneous interstitial melt in the crystal mush zone at the magma chamber wall. The largest geochemical variation was found in the melt inclusion of the Middle Ignimbrite Unit, even in single samples. This compositional variation overlaps that of the rhyolitic juvenile clasts, but does not match that of the glasses of scoria clasts. We suggest that syn-eruptive magma mixing (mingling) occurred in a compositionally heterogeneous magma chamber of the Middle Ignimbrite Unit.

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