Bakony Balaton Highland Volcanic Field Research Articles

Fossil metasomatized and newly-accreted fertile lithospheric mantle volumes beneath the Bakony-Balaton Highland Volcanic Field (central Carpathian-Pannonian region)

This study focuses on upper mantle xenoliths from Sabar-hill (Bakony-Balaton Highland Volcanic Field), a newly discovered locality of mantle xenoliths, situated in the central Carpathian-Pannonian region (CPR). The investigated seven peridotite and one pyroxenite xenoliths record two episodes in the geochemical evolution of the local subcontinental lithospheric mantle. The moderately deformed Group I xenoliths reveal modal contents (≥9 vol% orthopyroxene), major element characteristics (depletion in Al2O3 and CaO in orthopyroxenes) and trace element features (e.g., enrichment in U, Pb and Sr in clinopyroxenes) typical in supra-subduction zones (i.e., mantle wedge) xenoliths. All these suggest that the studied xenoliths were formed by silica-rich fluid/melt - peridotitic wall rock interactions. The tectonic background to the formation of Group I xenoliths is likely linked to the subduction of an oceanic slab during the Mesozoic-Paleogene. This may have happened far from the current position of Sabar-hill, to where the lithosphere, including the metasomatized mantle volume, was transferred via plate extrusion. The less deformed, dominantly protogranular and porphyroclastic Group II xenoliths are depleted in light rare earth elements (LREE). In contrast, they are enriched in clinopyroxene (≥11 vol%), incompatible major elements (e.g., Al and Na) and heavy rare earth elements (HREE), by a minimum of four times compared to primitive mantle. All these suggest that Group II xenoliths represent a geochemically fertile mantle partition and the degree of partial melting affected this mantle section was limited (≤7% based on trace elements). Group II xenoliths have higher equilibrium temperatures (1088–1168 °C) compared to Group I xenoliths (1018–1089 °C). The lack of petrographic and geochemical evidence pointing to metasomatism-related annealing in Group II xenoliths suggests that they are derived from a greater depth. It follows that Group II xenoliths could represent prior asthenosphere volumes which recently accreted to the lithosphere. The lithospherization of the upper part of the asthenosphere may have initiated after the tectonic inversion of the region, when compressional tectonic regime beneath the CPR gradually overtook the former extensional one in the last ∼8 Ma. The high bulk structural hydroxyl content in both Group I (22–1699 ppm) and II (55–320 ppm) xenoliths is likely linked to the subduction events that took place in the CPR from the Mesozoic onwards. These events transported significant amounts of fluid (H2O) back to the upper mantle, affecting both the mantle wedge and the asthenosphere.

Tectonically-determined distribution of monogenetic volcanoes in a compressive tectonic regime: An example from the Pannonian continental back-arc system (Central Europe)

This paper presents the results from a geographic information systems (GIS) workflow, which was used to analyze the spatial distribution and temporal evolution of volcanoes in the Mio-Pleistocene monogenetic Bakony-Balaton Highland Volcanic Field (BBHVF), located in the Pannonian Basin, Hungary. Volcanism occurred during the tectonic inversion in a back-arc setting and a compressive/transpressive tectonic regime on the hottest and thinnest lithosphere of continental Europe. The main goal of this study is to clarify the effect of the pre-existing structure of the upper lithosphere in the distribution of the volcanic centers across the volcanic field using an innovative GIS methodology. Orientation of the volcanic field was compared to the orientation of the faults in the BBHVF, and in its larger vicinity, which resulted in correspondence, suggesting the dominance of the SW-NE direction. The directions of the volcanic lineaments fit well to the two main fault directions. The fault-volcano proximity analysis suggests that the fault plane of a thrust fault was an important structural feature during the lifespan of the volcanism. All results suggest that the fault plane of a regionally significant Cretaceous thrust fault (Litér Fault) might have served as a temporary pathway for the ascending magma, whereby (similarly to other, smaller faults) redirecting the magmas causing clustering of the volcanoes. This highlights the importance of major upper crustal structural heterogeneities for magma transport in a compressive tectonic system, especially in the case of active, monogenetic volcanic fields from a volcanic hazard perspective. The present GIS workflow can be effective in analyzing the spatial patterns of the volcanism and its connection with crustal structures at monogenetic volcanic fields worldwide.

Deciphering metasomatic events beneath Mindszentkálla (Bakony-Balaton Highland Volcanic Field, western Pannonian Basin) revealed by single-lithology and composite upper mantle xenoliths

Single-lithology and composite xenoliths from Mindszentkálla (Bakony-Balaton Highland Volcanic Field) in the Carpathian-Pannonian region record geochemical evolution of the subcontinental lithospheric mantle. The dominant single-lithology xenoliths are orthopyroxene-rich (22 vol% on average) harzburgites. Three composite xenoliths contain either two or more domains including dunite, olivine-orthopyroxenite, orthopyroxenite, apatite-bearing websterite and amphibole-phlogopite-bearing vein. The presence of different lithologies is a result of at least two metasomatic events that affected the lithospheric mantle. The first event resulted in orthopyroxene enrichment thus formed harzburgitic mantle volumes (Group I xenoliths). Major- and trace element distributions of the bulk harzburgites differ from the geochemical trends expected in residues of mantle melting. In contrast, petrographic and geochemical attributes suggest that the harzburgite was formed by silica-rich melt - peridotitic wall rock interactions in a supra-subduction zone. Within the Group I xenoliths, two subgroups were identified based on the presence or lack of enrichment in U, Pb and Sr. Since these elements are fluid mobile, their enrichment in certain Group I xenoliths indicate reaction with a subduction-related fluid, subsequent to the harzburgite formation. The effect of a second event overprints the features of the Group I xenoliths and is evidenced in all domains of two composite xenoliths (Group II xenoliths). The general geochemical character involves enrichment of basaltic major and minor elements (Fe, Mn, Ti, Ca) in the rock-forming minerals and convex-upward rare earth element (REE) patterns in clinopyroxenes. We suggest that the different domains represent reaction products with variably evolved basaltic melts of a single magmatic event. The tectonic background to the formation of Group I xenoliths is likely linked to the subduction of oceanic crust during the Mesozoic–Paleogene. This happened far from the current position of Mindszentkálla, to where the lithosphere, including the metasomatized mantle volume, was transferred via plate extrusion. The Group II xenoliths appear to bear the geochemical signature of a younger (Neogene) basaltic magmatic event, likely the same that produced the host basalt transporting the xenoliths to the surface.

Melt-rock interaction in the lower crust based on silicate melt inclusions in mafic garnet granulite xenoliths, Bakony–Balaton Highland

Major and trace element composition of silicate melt inclusions (SMI) and their rock-forming minerals were studied in mafic garnet granulite xenoliths from the Bakony–Balaton Highland Volcanic Field (Western-Hungary). Primary SMIs occur in clinopyroxene and plagioclase in the plagioclase-rich domains of mafic garnet granulites and in ilmenite in the vicinity of these domains in the wall rock. Based on major and trace elements, we demonstrated that the SMIs have no connection with the xenolith-hosting alkaline basalt as they have rhyodacitic composition with a distinct REE pattern, negative Sr anomaly, and HFSE depletion. The trace element characteristics suggest that the clinopyroxene hosted SMIs are the closest representation of the original melt percolated in the lower crust. In contrast, the plagioclase and ilmenite hosted SMIs are products of interaction between the silicic melt and the wall rock garnet granulite. A further product of this interaction is the clinopyroxene–ilmenite±plagioclase symplectite. Textural observations and mass ­balance calculations reveal that the reaction between titanite and the silicate melt led to the formation of these assemblages. We propose that a tectonic mélange of metapelites and (MOR-related) metabasalts partially melted at 0.3–0.5 GPa to form a dacitic–rhyodacitic melt leaving behind a garnet-free, plagioclase+clinopyroxene+orthopyroxene+ilmenite residuum. The composition of the SMIs (both major and trace elements) is similar to those from the middle Miocene calc-alkaline magmas, widely known from the northern Pannonian Basin (Börzsöny and Visegrád Mts., Cserhát and Mátra volcanic areas and Central Slovakian VF), but the SMIs are probably the result of a later, local process. The study of these SMIs also highlights how crustal contamination changes magma compositions during asthenospheric Miocene ascent.

Effect of water on the rheology of the lithospheric mantle in young extensional basin systems as shown by xenoliths from the Carpathian-Pannonian region

Incorporation of hydrogen as structural hydroxyl (commonly referred to as water) in nominally anhydrous mantle minerals is known for the ‘hydrolytic weakening’ effect, which decreases the strength and electrical resistivity of the rock. Recent models have provided means of calculating rheological properties from geochemical data of upper mantle xenoliths. In the Carpathian-Pannonian region, upper mantle xenoliths can be found on the surface at five locations, both at marginal and central regions of the basin system. In this study we present a comprehensive overview of water contents in these xenoliths, including previously lacking data from two outcrops (Füzes-tó and Tihany) from the Bakony-Balaton Highland Volcanic Field for the first time. The Tihany xenoliths have significantly higher water contents (< 1–5, 116–353 and 327–1394 ppm in olivine, orthopyroxene and clinopyroxene, respectively) than the Füzes-tó xenoliths (0–2.7, 6.2–114 and 3.1–213 ppm in olivine, orthopyroxene and clinopyroxene, respectively). This can be explained with the older eruption age of their host basalt and greater depth of origin, as they likely represent an asthenospheric layer that became part of the lower lithosphere during thermal relaxation. In contrast, the Füzes-tó xenoliths represent a lot dryer mantle portion and are assumed to have been more affected by decompression-induced water loss resulting from decreased water activity during the extension. In general, the marginal xenolith locations of the Carpathian-Pannonian region, associated to prior supra-subduction environment, contain more water than xenoliths from central locations (with the exception of Tihany locality) which were significantly affected by extension-related lithospheric thinning. To reveal the differences in rheology inferred from the different water contents, we calculated effective viscosities and electrical resistivities for the xenoliths of the Bakony-Balaton Highland and other locationss in the Carpathian-Pannonian region using previously published data for input. Based on the results, the central locations have higher effective viscosities (1.4∙10 20 –2.2∙10 21 Pa s) and electrical resistivities (48–913 Ωm) compared to the marginal locations (9.3∙10 19 –6.8∙10 20 Pa s and 36–182 Ωm, respectively), suggesting that the lithospheric mantle is more rigid in the former than in the latter areas. This may be a common feature for extensional basins, as the extension leads to the ‘drying’ of the upper mantle, whereas subduction zones keep hydrating their overlying mantle wedge. However, for more accurate estimations, other factors such as regional differences in strain rate or the potential presence of melts or fluids in the mantle need to be considered as well. • Water lowers effective viscosity and electrical resistivity in mantle xenoliths • Water contents of xenoliths from different tectonic environments were compared • Central regions of the back-arc basin are drier compared to marginal areas • Rheological differences are present in distinct parts of extension-subduction systems • Decompression related water loss results in strengthening of the lithosphere

Olivine major and trace element compositions coupled with spinel chemistry to unravel the magmatic systems feeding monogenetic basaltic volcanoes

Monogenetic basaltic volcanic systems, despite their considerable smaller size and shorter lifetime compared to polygenetic volcanoes, can have complex pre-eruptive histories and composite volcanic facies architectures. Their source-to-surface investigation is essential for our better understanding of monogenetic volcanism and requires high-resolution mineral-scale analyses.In this study, we focus on diversely zoned olivine crystals and their spinel inclusions from alkaline basaltic volcanics that are the result of mixing of numerous magmas, crystals and fragments of various origins. The Fekete-hegy volcanic complex is one of the largest and most composite eruptive centers in the intracontinental monogenetic Bakony–Balaton Highland Volcanic Field (western Pannonian Basin, Eastern Central Europe). It is a compound multi-vent system built up by multiple eruption episodes: initial maar-forming phreatomagmatic eruptions were followed by massive lava flows and magmatic explosive activity. We performed stratigraphically controlled sampling in order to reveal the history of the successively erupted magma batches represented by the distinct eruptive units, as well as to discover the petrogenetic processes that controlled the evolution of the magmatic system.The juvenile pyroclasts of the phreatomagmatic eruption products (unit 1) contain a remarkably diverse mineral assemblage including five different olivine types and three distinct spinel groups. In addition, they comprise various xenoliths. Based on detailed textural investigations combined with in situ electron microprobe analyses, high-resolution laser ablation ICP-MS trace element mapping and single spot measurements on the variably zoned olivines of unit 1 samples, eight distinct environments are inferred to have been involved in their formation. Four of these environments account for the significant compositional variation of the olivine-hosted spinel inclusions. A complex set of open- and closed-system petrogenetic processes operated during the evolution of the magmatic system: magma stalling, accumulation, storage, fractionation, mixing, replenishments, cumulate remobilization, incorporation of foreign fragments and crystals from the wall rocks. All these diverse environments and processes resulted in the mixed character of the erupted magmas during the initial phreatomagmatic eruptive phase.In contrast, the uniform petrological features and the small variations shown by the olivines and spinels from unit 2–3 indicate that the later magmatic explosive – effusive phase was preceded by a considerable change in the magmatic system; it experienced a simple evolution through olivine + spinel fractional crystallization without any of the complexities seen during the initial phase.The present study emphasizes the importance of high-resolution mineral-scale textural and chemical investigations to unravel the complexity of the sub-volcanic magmatic systems feeding monogenetic basaltic volcanoes. Compared to the application of whole-rock geochemistry alone, this approach enables a direct and more detailed insight into the architecture and evolution of these systems.

Zircon and apatite-bearing pyroxene hornblendite mantle xenolith from Hungary, Carpathian-Pannonian region

A composite xenolith composed of a zircon-bearing, apatite-rich olivine-pyroxene hornblendite and an amphibole harzburgite part was found in an outcrop of Neogene alkali basalt at Szigliget, Bakony-Balaton Highland Volcanic Field, Western Hungary. The hornblendite is composed dominantly of amphibole that is strongly enriched in incompatible trace elements along with apatite, orthopyroxene and olivine. Zircon, rutile, ilmenite and sulphide occur as inclusions in the silicate minerals as accessory phases. On the basis of the chemical composition of the minerals, we estimate that the hornblendite crystallized at ~1000–1015 °C and 1.2–1.4 GPa pressure in the uppermost mantle under oxidising conditions (ΔlogfO2(FMQ)~ + 1.3). The hornblendite precipitated from a volatile and incompatible element-rich silicate melt with trachyte or phonolite composition, which might be derived from an alkali basalt melt by high-degree (>70%) fractional crystallisation.

A Badacsony freatomagmás piroklasztit-sorozata: következtetések a monogenetikus bazaltvulkáni működés folyamataira és formáira

A tanulmányban a Bakony–Balaton-felvidéki Vulkáni Terület (BBVT) legismertebb tanúhegyén, a Badacsonyon kibukkanó freatomagmás piroklasztit-sorozat első kvantitatív leírását és vulkanológiai értelmezését mutatjuk be. A piroklasztitok juvenilis / litikus törmelékeinek arányát pontszámlálásos módszerrel vizsgáltuk vágott kézipéldány-felszíneken. A felület%-ban kapott eredményeket a kőzetminták teljes térfogatára vonatkozónak vettük. E módszer segítségével meghatároztuk a freatomagmás robbanások relatív mélységét a szinvulkáni felszínhez képest, következtetéseket vontunk le a lehetséges szineruptív vulkánmorfológiára, valamint a terepi megfigyelések és korábbi tanulmányok alapján rekonstuáltuk a vulkán működésének teljes fejlődéstörténetét.&#x0D; A képelemzés során megállapítottuk litikus elegyrészek minimális arányát. Ebből, valamint a piroklasztitok térbeli elterjedéséből és rétegtani helyzetéből valószínűsíthető, hogy a Badacsony egy monogenetikus (esetleg policiklikus) tufagyűrű vagy sekély maar vulkán volt, amelynek kialakulásában a víz-magma kölcsönhatás következtében történt sekély mélységű freatomagmás robbanások játszották a fő szerepet. A tufagyűrű vulkáni forma a litoklasztok kis mennyiségével alátámasztható. Viszonylag kevés litoklasztot tartalmazó freatomagmás kitörési termékek nem csak tufagyűrűk, hanem puha, konszolidálatlan kőzeten kialakuló sekély maarok képződményei is lehetnek, amely modell a Badacsony esetében is valószínűsíthető.&#x0D; A terepi megfigyelések, valamint a képelemzés eredménye alapján a vulkanizmus freatomagmás robbanásos, lávaöntő és magmás robbanásos fázisokra osztható. Ezen adatok alapján vulkáni fejlődéstörténeti modellt állítottunk fel. A kevés korábbi, célzott vulkanológiai szemléletű munka miatt e „kezdeti” adatok megfelelő alapot adhatnak a további badacsonyi vulkanológiai kutatásoknak.

Slab-derived metasomatism in the Carpathian-Pannonian mantle revealed by investigations of mantle xenoliths from the Bakony-Balaton Highland Volcanic Field

A suite of fifteen peridotite xenoliths from the Bakony-Balaton Highland Volcanic Field (BBHVF, Pannonian Basin, Central Europe) that show abundant petrographic evidence of fluid and melt percolation were studied in order to decipher the formation of their melt pockets and veins. The suite mainly consists of “fertile” lherzolites (5.8–19.9vol.% clinopyroxene) and a few harzburgites (1.9–5.4vol.% clinopyroxene) from well-known localities (Szentbékkálla, Szigliget) and two previously unreported localities (Füzes-tó and Mindszentkálla). Major and trace element data indicate that most of the peridotites record variable degrees of partial melt extraction, up to >15% for the harzburgites. Subsequently, the xenoliths experienced at least two stages of metasomatic modification. The first stage was associated with percolation of a volatile-bearing silicate melt and resulted in crystallization of amphibole, enrichment in the most incompatible trace elements (Ba, Th, U, Sr), and development of negative Nb-Ta anomalies in clinopyroxene. The second and last metasomatic event, widespread beneath the BBHVF, is associated with the formation of silicate melt pockets, physically connected to a network of melt veins, with large and abundant CO2 vesicles. The glass in these veins has sub-alkaline trachy-andesitic composition and displays an OIB-like trace element signature. Its composition attests to the migration through a supra-subduction zone mantle wedge of silicic melt highly enriched in volatiles (CO2, H2O, Cl, F), LILE, REE and HFSE and consistent with compositions of natural and experimental examples of slab melting-derived magma. In the present case, however, melt was likely derived from melting of oceanic crust and carbonated sediments under conditions where Nb-rich mineral phases were not stable in the residue. A likely scenario for the origin such melts involves melting after subduction ceased as the slab thermally equilibrated with the asthenosphere. Melt-rock reactions due to ascent of hot, CO2-rich, siliceous melt to near-Moho depths triggered destabilization of amphibole and primary clinopyroxene, spinel, and possibly olivine. The resulting andesitic glass in melt pockets evolved to more mafic compositions due to mantle mineral assimilation but has heterogeneous trace element signatures mostly inherited from preexisting amphibole. The present example of melt-rock reactions between highly volatile-enriched siliceous slab-derived melt and peridotite from the upper part of the lithospheric mantle ultimately produced derivative melt with major element composition akin to calc-alkaline basaltic andesite, with generally low trace elements concentrations but selective pronounced enrichments in LILE's such as Ba, Sr, Pb.

Mantle xenoliths from Szentbékálla, Balaton: Geochemical and petrological constraints on the evolution of the lithospheric mantle underneath Pannonian Basin, Hungary

Pliocene alkali basalts from the Bakony-Balaton Highland Volcanic Field (BBHVF) in the western Pannonian Basin carry mantle xenoliths comprising hydrous and anhydrous spinel peridotites. The studied mantle xenoliths from Szentbékálla, near Lake Balaton, Hungary, are fine- and coarse-grained fertile to depleted spinel lherzolites, spinel harzburgites and dunites, with protogranular, porphyroclastic, and secondary protogranular and mosaic equigranular textures. Melt pockets, with shapes resembling amphibole, are common in a number of samples, whereas other samples have thin films of intergranular glass. Bulk-rock major element abundances show that the mantle lithosphere beneath this area experienced variable degrees (up to 20%) of partial melting.The clinopyroxene trace elements systematics retain a record of melt depletion and metasomatic processes attributed to subduction-related melt/fluids or to the infiltration of percolating undersaturated melts in the Pannonian lithospheric mantle. The radiogenic isotopes of Sr, Nd and Hf in clinopyroxene suggest that this metasomatism was a relatively recent event. Textural evidence suggests that the calcite filling up the vesicles in the melt pockets and in veinlets cross-cutting the constituent minerals is of epigenetic nature and not due to carbonatite metasomatism.The non-metasomatized primitive mantle normalized clinopyroxene REE abundances mimic those, but at higher values, of their bulk-rock REE patterns. Bulk-rock and clinopyroxene REE with upward and downward LREE respectively, indicate up to 2.4% host basalt infiltration.The calculated bulk composition of the melt pockets is identical to small amphibole relics found as inclusions in second generation clinopyroxene within the melt pockets, suggesting incongruent melting of amphibole, without the need for additional metasomatic melt/fluids to initiate the amphibole breakdown. The heat for the temperature increase necessary for amphibole breakdown was derived from the host basalt. The estimated time for diffusive Ca exchange between matrix olivine and olivine overgrowth in contact with the melt pockets is very short, ranging between 21 and 200days, indicating that amphibole breakdown took place immediately before or during the xenolith entrainment in the alkali basalt.

Highly CO2-supersaturated melts in the Pannonian lithospheric mantle – A transient carbon reservoir?

Subduction of carbonated crust is widely believed to generate a flux of carbon into the base of the continental lithospheric mantle, which in turn is the likely source of widespread volcanic and non-volcanic CO2 degassing in active tectonic intracontinental settings such as rifts, continental margin arcs and back-arc domains. However, the magnitude of the carbon flux through the lithosphere and the budget of stored carbon held within the lithospheric reservoir are both poorly known. We provide new constraints on the CO2 budget of the lithospheric mantle below the Pannonian Basin (Central Europe) through the study of a suite of xenoliths from the Bakony-Balaton Highland Volcanic Field. Trails of secondary fluid inclusions, silicate melt inclusions, networks of melt veins, and melt pockets with large and abundant vesicles provide numerous lines of evidence that mantle metasomatism affected the lithosphere beneath this region. We obtain a quantitative estimate of the CO2 budget of the mantle below the Pannonian Basin using a combination of innovative analytical and modeling approaches: (1) synchrotron X-ray microtomography, (2) NanoSIMS, Raman spectroscopy and microthermometry, and (3) thermodynamic models (Rhyolite-MELTS). The three-dimensional volumes reconstructed from synchrotron X-ray microtomography allow us to quantify the proportions of all petrographic phases in the samples and to visualize their textural relationships. The concentration of CO2 in glass veins and pockets ranges from 0.27 to 0.96wt.%, higher than in typical arc magmas (0–0.25wt.% CO2), whereas the H2O concentration ranges from 0.54 to 4.25wt.%, on the low end for estimated primitive arc magmas (1.9–6.3wt.% H2O). Trapping pressures for vesicles were determined by comparing CO2 concentrations in glass to CO2 saturation as a function of pressure in silicate melts, suggesting pressures between 0.69 to 1.78GPa. These values are generally higher than trapping pressures for fluid inclusions determined by Raman spectroscopy and microthermometry (0.1–1.1GPa). The CO2/silicate melt mass ratios in the metasomatic agent that percolated through the lithospheric mantle below the Pannonian Basin are estimated to be between 9.0 and 25.4wt.%, values consistent with metasomatism either by (1) silicate melts already supersaturated in CO2 before reaching lithospheric depths or (2) carbonatite melts that interacted with mantle peridotite to generate carbonated silicic melts. Taking the geodynamical context of the Pannonian Basin and our calculations of the CO2/silicate melt mass ratios in the metasomatic agent into account, we suggest that slab-derived melts initially containing up to 25wt.% of CO2 migrated into the lithospheric mantle and exsolved CO2–rich fluid that became trapped in secondary fluid inclusions upon fracturing of the peridotite mineral matrix. We propose a first-order estimate of 2000ppm as the minimal bulk CO2 concentration in the lithospheric mantle below the Pannonian Basin. This transient carbon reservoir is believed to be degassed through the Pannonian Basin due to volcanism and tectonic events, mostly focused along the lithospheric-scale regional Mid-Hungarian shear Zone.

Clinopyroxene with diverse origins in alkaline basalts from the western Pannonian Basin: Implications from trace element characteristics

Clinopyroxene crystals of various origins occur in the unusually crystal- and xenolith-rich alkaline basalts of the Bondoró-hegy and the Füzes-tó scoria cone, which are the youngest eruptive centres in the Bakony-Balaton Highland Volcanic Field, western Pannonian Basin. The clinopyroxenes show diverse textural and zoning features as well as highly variable major and trace element chemistry. Xenocryst, megacryst and phenocryst crystal populations can be distinguished on the basis of their compositional differences.The trace element patterns of green clinopyroxene cores display a large range in composition and indicate that most of them have a metamorphic origin. Most of them were incorporated from lower crustal mafic granulite wall rocks, while only a few of them are of magmatic origin representing pyroxenite (Type II) cumulates. The colourless clinopyroxene xenocrysts reflect the texturally and geochemically diverse nature of the subcontinental lithospheric mantle beneath the studied area, mainly representing regions characterised by various stages of metasomatism.The colourless and green megacrysts are genetically related to each other, having crystallised as early and late crystallisation products, respectively, from petrogenetically related melts as part of a fractional crystallisation sequence. These melts represent earlier alkaline basaltic magmas (as represented by the Type II xenoliths), having stalled and crystallised near the crust–mantle boundary in the uppermost part of the mantle. This serves as evidence that the deep magmatic systems beneath monogenetic volcanic fields are complex, involving several phases of melt generation, accumulation and fractionation at variable depths.We show that in situ trace element analysis is necessary in order to unravel the origins and relationships of the diverse clinopyroxene populations. Such studies significantly contribute to our understanding of the ascent histories of alkaline basaltic magmas and provide information about the characteristics of the rocks that constitute the lithosphere. Additionally, the abundance of foreign crystals incorporated in the ascending basaltic magmas, and their potential for contamination of the host magma, must be taken into account when whole-rock geochemical data are interpreted.

Melting, fluid migration and fluid-rock interactions in the lower crust beneath the Bakony-Balaton Highland volcanic field: a silicate melt and fluid inclusion study

Plio-Pleistocene alkali basalt hosted mafic garnet granulite xenoliths were studied from the Bakony-Balaton Highland Volcanic Field (BBHVF) to trace fluid-melt-rock interactions in the lower crust. Two unique mafic garnet granulite samples were selected for analyses (optical microscopy, microthermometry, electron microprobe, Raman and IR spectroscopy), which contain a clinopyroxene-plagioclase vein and patches with primary silicate melt inclusions (SMI). The samples have non-equilibrium microtexture in contrast with the overwhelming majority of previously studied mafic garnet granulite xenoliths. Primary silicate-melt inclusions were observed in plagioclase, clinopyroxene and ilmenite in both xenoliths. The SMI-bearing minerals located randomly in Mi26 and in a clinopyroxene-plagioclase vein on the edge of Sab38 granulites. Petrography and the fluid and melt inclusion study suggest that at least three fluid events occurred in the deep crust represented by these xenoliths. 1. Primary CO2-dominated ± CO ± H2S fluid inclusions were observed in the wall-rock part of Sab38 xenolith. 2. The crystallization of new clinopyroxene from melt, with CO2 + H2O fluid. 3. The crystallization of new plagioclase occurred in a heterogeneous fluid-melt system with additional N2 and CH4 during crystallization. A local reaction was observed between sphene and acidic melt, which formed ilmenite + clinopyroxene + plagioclase ± orthopyroxene. The ‘water’ content of the rock forming minerals was determined by infrared spectroscopy. The calculated bulk ‘water’ content of the Mi26 xenolith is 171 ± 51 ppm wt. %. The bulk wall rock part of the Sab38 granulite contains 55 ± 17 ppm wt. % of ‘water’, whereas the bulk plagioclase-clinopyroxene vein contains 278 ± 83 ppm wt. %. These results imply a very dry lower crust, locally hydrated by percolating fluids and melts.

Constraints on the thickness and seismic properties of the lithosphere in an extensional setting (Nógrád-Gömör Volcanic Field, Northern Pannonian Basin)

The Nograd-Gomor Volcanic Field (NGVF) is one of the five mantle xenolith bearing alkaline basalt locations in the Carpathian Pannonian Region. This allows us to constrain the structure and properties (e.g. composition, current deformation state, seismic anisotropy, electrical conductivity) of the upper mantle, including the lithosphere-asthenosphere boundary (LAB) using not only geophysical, but also petrologic and geochemical methods. For this pilot study, eight upper mantle xenoliths have been chosen from Barna-Nagykő, the southernmost location of the NGVF. The aim of this study is estimating the average seismic properties of the underlying mantle. Based on these estimations, the thickness of the anisotropic layer causing the observed average SKS delay time in the area was modelled considering five lineation and foliation end-member orientations. We conclude that a 142–333 km thick layer is required to explain the observed SKS anisotropy, assuming seismic properties calculated by averaging the properties of the eight xenoliths. It is larger than the thickness of the lithospheric mantle. Therefore, the majority of the delay time accumulates in the sublithospheric mantle. However, it is still in question whether a single anisotropic layer, represented by the studied xenoliths, is responsible for the observed SKS anisotropy, as it is assumed beneath the Bakony–Balaton Highland Volcanic Field (Kovacs et al. 2012), or the sublithospheric mantle has different layers. In addition, the depths of the Moho and the LAB ( $$25\,\pm \,5, 65\,\pm \,10\,\hbox {km}$$ , respectively) were estimated based on S receiver function analyses of data from three nearby permanent seismological stations.

Neogene calc-alkaline intrusive magmatism of post-collisional origin along the Outer Carpathians: a comparative study of the Pieniny Mountains and adjacent areas

A petrographical and geochemical analysis was carried out on intrusive rocks from the Pieniny and Moravian areas, with special attention to boron content and K-Ar radiometric ages. The intrusions form medium- to high-potassium calc-alkaline andesitic suites, which are compositionally slightly different from each other and from the other calc-alkaline sequences in the Carpathian-Pannonian Region. No significant geochemical differences were observed within the different phase intrusions in the Pieniny areas. However, there is a slight difference in major and trace element composition between the Moravian and Pieniny intrusions. The andesitic rocks in the Pieniny and Moravian area are enriched in large ion lithophile elements and light rare earth elements and depleted in high field strength elements, indicating a metasomatized mantle source of the parent magmas. The low boron concentration of the andesitic rocks in the Pieniny area is in the range measured in back-arc, intraplate basalts of the Bakony-Balaton Highland volcanic field, whereas the higher boron content of the Moravian rocks overlaps with that of the Western Carpathian andesites. This may indicates the heterogeneity of the mantle lithosphere below the areas, or indicates different magma evolution histories. On the basis of the systematic geochronological study, the intrusive rocks along the Outer Carpathians can be divided on three groups, which overlap with each other temporally. The oldest magmatism occurred from 14.8 Ma to 11.0 Ma in the Uhersky Brod area, Moravia, which was followed by the emplacement of andesitic dikes and sills in the Pieniny Mts., south Poland (13.5–10.8 Ma). In the Pieniny area, two intrusive phases were distinguished. Partly overlapping with this area, but generally younger than this magmatism, the emplacement of the youngest intrusions is referable to the Poiana Botizei-Ţibles-Toroiaga-Rodna-Bârgâu intrusive area, Romania, where magmatic activity started at ~11.8 Ma and terminated at 8.0 Ma.

Origin and ascent history of unusually crystal-rich alkaline basaltic magmas from the western Pannonian Basin

The last eruptions of the monogenetic Bakony-Balaton Highland Volcanic Field (western Pannonian Basin, Hungary) produced unusually crystal- and xenolith-rich alkaline basalts which are unique among the alkaline basalts of the Carpathian–Pannonian Region. Similar alkaline basalts are only rarely known in other volcanic fields of the world. These special basaltic magmas fed the eruptions of two closely located volcanic centres: the Bondoro-hegy and the Fuzes-to scoria cone. Their uncommon enrichment in diverse crystals produced unique rock textures and modified original magma compositions (13.1–14.2 wt.% MgO, 459–657 ppm Cr, and 455–564 ppm Ni contents). Detailed mineral-scale textural and chemical analyses revealed that the Bondoro-hegy and Fuzes-to alkaline basaltic magmas have a complex ascent history, and that most of their minerals (∼30 vol.% of the rocks) represent foreign crystals derived from different levels of the underlying lithosphere. The most abundant xenocrysts, olivine, orthopyroxene, clinopyroxene, and spinel, were incorporated from different regions and rock types of the subcontinental lithospheric mantle. Megacrysts of clinopyroxene and spinel could have originated from pegmatitic veins/sills which probably represent magmas crystallized near the crust–mantle boundary. Green clinopyroxene xenocrysts could have been derived from lower crustal mafic granulites. Minerals that crystallized in situ from the alkaline basaltic melts (olivine with Cr-spinel inclusions, clinopyroxene, plagioclase, and Fe–Ti oxides) are only represented by microphenocrysts and overgrowths on the foreign crystals. The vast amount of peridotitic (most common) and mafic granulitic materials indicates a highly effective interaction between the ascending magmas and wall rocks at lithospheric mantle and lower crustal levels. However, fragments from the middle and upper crust are absent from the studied basalts, suggesting a change in the style (and possibly rate) of magma ascent in the crust. These xenocryst- and xenolith-rich basalts yield divers tools for estimating magma ascent rate that is important for hazard forecasting in monogenetic volcanic fields. According to the estimated ascent rates, the Bondoro-hegy and Fuzes-to alkaline basaltic magmas could have reached the surface within hours to few days, similarly to the estimates for other eruptive centres in the Pannonian Basin which were fed by “normal” (crystal and xenoliths poor) alkaline basalts.

The role of CO2-rich fluids in trace element transport and metasomatism in the lithospheric mantle beneath the Central Pannonian Basin, Hungary, based on fluid inclusions in mantle xenoliths

Upper mantle peridotite xenoliths from the Tihany Maar Volcanic Complex, Bakony–Balaton Highland Volcanic Field (Central Pannonian Basin, Hungary) contain abundant pyroxene-hosted negative crystal shaped CO2-rich fluid inclusions. The good correlation between enrichment of the clinopyroxenes in Al2O3, TiO2, Na2O, MREE and Zr, and the presence of fluid inclusions in the xenoliths provide strong evidence for fluid-related cryptic metasomatism of the studied xenoliths. The FIB-SEM (focused ion beam-scanning electron microscopy) exposure technique revealed a thin glass film, covering the wall of the fluid inclusions, which provides direct evidence that the silicate components were formerly dissolved in the CO2-rich fluid phase. This means that at upper mantle conditions CO2-rich fluids are capable of transporting trace and major elements, and are the agents responsible for cryptic metasomatism of the peridotite wall rock.Several daughter phases, including magnesite, quartz and sulfide, were identified in the fluid inclusions. Magnesite and quartz are the products of a post entrapment carbonation reaction, whereby the reactants are the CO2-rich fluid and the host orthopyroxene. It is likely that the thin glass film prevented or arrested further growth of the magnesite and quartz by isolating the fluid from the host orthopyroxene, resulting in the preservation of residual CO2 in the fluid inclusions.

Open-system evolution of the Füzes-tó alkaline basaltic magma, western Pannonian Basin: Constraints from mineral textures and compositions

The crystal-rich alkaline basalt of the Füzes-tó scoria cone in the Bakony-Balaton Highland Volcanic Field, western Pannonian Basin, Hungary has a unique mineral assemblage reflecting a mixture of diverse crystals of various origins. The pheno- and microphenocrysts are represented by olivine with chromian spinel inclusions, clinopyroxene and plagioclase. The basalt contains abundant (20–40vol.% on a vesicle-free basis) xenocrysts of olivine, orthopyroxene, colorless clinopyroxene and spinel derived from different regions of the subcontinental lithospheric mantle. Rare green clinopyroxene xenocrysts are also present. The basalt also includes numerous spinel peridotite xenoliths. In one of them we studied several silicate melt veins which contain fresh vesiculated glass, chromian spinels, clinopyroxenes and olivines. The glass has an evolved subalkaline composition (56.9–64.6wt.% SiO2, 6.6–8.1wt.% Na2O+K2O).Magmatic chromian spinels and clinopyroxenes in the host basalt and in the silicate melt veins of the peridotite xenolith show two compositionally distinct populations (group1 and group2). Group1 chromian spinels and clinopyroxenes both have low Cr contents (18–41 Cr# (100∗Cr/(Cr+Al)) and 0.01–0.86wt.% Cr2O3, respectively) and occur predominantly in the host basalt. Group2 chromian spinels and clinopyroxenes have higher Cr contents (46–54 Cr# and 0.34–1.8wt.% Cr2O3, respectively) and appear mainly in the silicate melt veins. In addition, compared to the group1 clinopyroxenes, group2 clinopyroxenes have higher MgO and SiO2, and lower TiO2 and CaO contents.Based on our detailed mineral-scale study, group1 spinels and clinopyroxenes represent the host alkaline basaltic magma (magma1), while group2 crystals could have been derived from another silicate magma (magma2) that is represented by the silicate melt veins in the studied peridotite xenolith. It is notable that group2 spinels are also present in some olivine phenocrysts, and scarce group1 spinels appear in the silicate melt veins as well. In some cases, group1 and group2 chromian spinels coexist within the same olivine phenocryst, and clinopyroxene zones that have group1 and group2 compositions compose parts of the same clinopyroxene crystal in the host basalt. On the basis of these observations, we infer interaction between magma1 and magma2.The occurrence and interaction of two distinct magma batches suggest a complex magma evolution for this otherwise simple, monogenetic volcano and imply that open-system processes operated during the evolution of the Füzes-tó alkaline basaltic magma. Our results demonstrate the importance of mineral textures and compositions for understanding the complexity of magma processes that can be manifested at small-scale basaltic volcanoes.

On the origin and fluid content of some rare crustal xenoliths and their bearing on the structure and evolution of the crust beneath the Bakony–Balaton Highland Volcanic Field (W-Hungary)

Rarely occurring clinopyroxene-plagioclase bearing, felsic granulite and skarn xenoliths were studied from the mantle and crustal xenolith-bearing alkaline basaltic and pyroclastic localities of the Bakony–Balaton Highland Volcanic Field (W-Hungary). Geobarometry and geothermometry of the xenoliths made it possible to categorise them in three groups according to their depth of formation. The first group formed in the lower crust together with mafic and metasedimentary granulites. The second group represents magmatic intrusions of the middle crust, and the third one comprises contact metamorphic rocks of relatively shallow origin. The calculated pressure difference from the core and rim compositions of plagioclase and clinopyroxene as well as garnet breakdown reactions in some xenoliths show evidence for pressure decrease due to crustal thinning both in lower crustal and middle crustal xenoliths during formation of the Pannonian Basin. Fluid inclusion studies reveal the dominance of the CO2-rich fluids in the whole crustal section in contrast with fluids found in mafic garnet-bearing xenoliths. Crustal stratigraphy was constructed for the periods prior to the extension and after the extension on the basis of geobarometry and geophysical data. On the basis of mineral stabilities and geothermo-barometry, we estimated that the pre-extensional thickness of the lower and upper crust may have been 27–34 and 26–28 km, respectively. Comparison of pre-extensional and present-day thickness of the lower and upper crust indicate that thinning affected both the lower and the upper portion of the crust but on a different scale. The calculated thinning factors are between 2.25 and 3.4 for the lower crust and 1.3–1.56 for the upper crust.

Shallow-seated controls on the evolution of the Upper Pliocene Kopasz-hegy nested monogenetic volcanic chain in the Western Pannonian Basin (Hungary)

Shallow-seated controls on the evolution of the Upper Pliocene Kopasz-hegy nested monogenetic volcanic chain in the Western Pannonian Basin (Hungary)Monogenetic, nested volcanic complexes (e.g. Tihany) are common landforms in the Bakony-Balaton Highland Volcanic Field (BBHVF, Hungary), which was active during the Late Miocene up to the Early Pleistocene. These types of monogenetic volcanoes are usually evolved in a slightly different way than their "simple" counterparts. The Kopasz-hegy Volcanic Complex (KVC) is inferred to be a vent complex, which evolved in a relatively complex way as compared to a classical "sensu stricto" monogenetic volcano. The KVC is located in the central part of the BBHVF and is one of the youngest (2.8-2.5 Ma) volcanic erosion remnants of the field. In this study, we carried out volcanic facies analysis of the eruptive products of the KVC in order to determine the possible role of changing magma fragmentation styles and/or vent migration responsible for the formation of this volcano. The evolution of the KVC started with interaction of water-saturated Late Miocene (Pannonian) mud, sand, sandstone with rising basaltic magma triggering phreatomagmatic explosive maar-diatreme forming eruptions. These explosive eruptions in the northern part of the volcanic complex took place in a N-S aligned paleovalley. As groundwater supply was depleted during volcanic activity the eruption style became dominated by more magmatic explosive-fragmentation leading to the formation of a mostly spatter-dominated scoria cone that is capping the basal maar-diatreme deposits. Subsequent vent migration along a few hundred meters long fissure still within the paleovalley caused the opening of the younger phreatomagmatic southern vent adjacent to the already established northern maar. This paper describes how change in eruption styles together with lateral migration of the volcanism forms an amalgamated vent complex.