40 Ar / 39 Ar geochronology of Burdigalian paleobotanical localities in the central Paratethys ( South Slovakia )

 K. Šarinová, S. Rybár, F. Jourdan, A. Frew, C. Mayers, M. Kováčová, B. Lichtman, P. Nováková, M. Kováčs, 2021 CC BY-SA K . Š a r i n o v á e t a l . G e o l o g i c a A c t a , 1 9 . 5 , 1 1 9 , I I V ( 2 0 2 1 ) D O I : 1 0 . 1 3 4 4 / G e o l o g i c a A c t a 2 0 2 1 . 1 9 . 5 Geochronology of Burdigalian paleobotanical localities 2 2007; Hably, 1985; Márton et al., 2007; Pálfy et al., 2007; Sitár and Kvaček, 1997; Vass et al., 2006). The area in the vicinity of Lipovany-Mučín-Ipolytarnóc belongs to the Ipolytarnoc Fossils Nature Conservation Area (Fig. 1). These three localities include rich fossil plant assemblages consisting of about 41 genera and 65 species of leaf remains (e.g. Hably, 1985; Kučerová, 2009; Němejc and Knobloch, 1969; Sitár and Kvaček, 1997). The importance of the mentioned localities follows from the very good preservation of leaf impressions that enabled interpretation of morphological characteristics and from the numerous remains sufficient for statistical evaluation (Hably, 1985). Mentioned localities also contain silicified tree trunks (e.g. Hably, 1985; Sitár and Kvaček, 1997) and Ipolytarnóc locality contains mammal and bird footprints localized immediately under the tuff (see Kordos, 1985). The assemblage of taxa is dominated by laurophyllous plants, indicative of a subtropical rainforest developed in a warm and humid climate (e.g. Hably, 1985; Kučerová, 2009; Sitár and Kvaček, 1997). Vegetation from the Lipovany section was last described as a multi storeyed forest with higher canopy occupied by Platanus neptuni, Engelhardia and admixture of Pinus; lower tree storey with Lauraceae, Tetraclinis, Magnolia, Cyclocarya and Cassia; and the shrub storey with palms, Lauraceae, enigmatic Pungiphyllum, Theaceae and “Celastrus” (Sitár and Kvaček, 1997). From the taphonomic point of view, almost no cuticles were preserved due to fusinisation. The Mučín locality was last studied by Kučerová (2009), who documented dominance of Celastrus genus supplemented by Platanus neptuni, Engelhardia orsbergensis, Cassia berenices, Podocarpium podocarpum, Dalbergia nostratum and Leguminosites sp. The flora from the Lipovany section was previously described as parastratotype for the Ottnangian regional stage (Němejc and Knobloch, 1973). The first dating of the tuffs using the Fission Track method (FT; biotite) indicated an age of 20.1±0.3Ma (Repčok, 1987), thus it was considered Eggenburgian (lower Burdigalian) in age (Vass, 2002; Vass et al., 2006). Additionally, similar and rather imprecise K/Ar radioisotopic ages of 20.0±2.0Ma (biotite) and 19.8±3.0Ma (plagioclase; Hámor et al., 1979 in Pálfy et al., 2007) were obtained from the neighboring Ipolytarnóc area. However, subsequent paleomagnetic results (Márton, 2007; Márton et al., 2007; Vass et al., 2006) suggested, that the ignimbrite together with footprints containing sandstone from the Ipolytarnóc area are younger than expected. Finally, a younger date was supplemented by new radioisotopic age of 17.42±0.04Ma by U-Th and 17.02±0.14Ma by 40Ar/39Ar (Pálfy et al., 2007). This 40Ar/39Ar date was recalculated to Mučín ave c and outside sections Lipovany sandpit state border river, road village 1 2 Localities Ipolytarnóc Miocene park Mučín


INTRODUCTION
The Lipovany and Mučín paleobotanical localities contain important floral associations within the tuff horizons, which were used for determination of subtropical to tropical climatic conditions during the Early Miocene. Based on the combination of results from plagioclase and biotite 40 Ar/ 39 Ar dating, the age of the tuff deposition is around 17.3Ma. For the Lipovany locality, single-grain 40 Ar/ 39 Ar convergent ages of 17.49±0.54Ma and 17.28±0.06Ma, for plagioclase and biotite were obtained, respectively. The Mučín locality only provide an imprecise convergent age of 16.5±1.4Ma due to the small size of the analyzed plagioclase crystals. The results thus allowed to include the fossil subtropical flora of the studied localities in the late Ottnangian regional stage (upper part of the Burdigalian). Additionally, these age data indicate that deposition of the overlaying Salgótarján Formation starts much later than originally thought (during Ottnangian-Karpatian boundary).

Slovakia Hungary
Quaternary FIGURE 1. A) Location of the study area (B) in the Alpine-Carpathian-Pannonian system (compiled from Fusán et al., 1987;Hók et al., 2014;Horváth et al., 2015;Nováková et al., 2020). B) Geologic map of the study area showing location of studied sections (compiled by Gyalog and Síkhegyi, 2005 and by Vass, 1992).  Table 1) which are fully calibrated against the U-Pb system . These data shift the studied ignimbrites toward the Ottnangian/Karpatian boundary (ca. to mid/upper part of the Burdigalian stage). However, magnetostratigraphy of the fosilliferous Lipovany section (NE Lipovany) revealed a reverse polarity opposite to normal polarity in the Mučín and Ipolytarnóc sections (Vass et al., 2006;Márton et al., 2007). Therefore, the mentioned authors erroneously decided to leave the Lipovany section assigned to the Eggenburgian.
The main aim of this paper is to present new 40 Ar/ 39 Ar radioisotopic data from key paleobotanical Lipovany and Mučín sections. The new data will also contribute to the lithostratigraphic and paleogeographic framework of the area, as well as to the paleovegetation and paleoclimate evolution model.

METHODOLOGY Sedimentology and petrography
The outcrops were manually excavated to expose the section, located in old quarries and in forest scours, and cleaned by palette knifes and brushes. The lithofacies abbreviations were adopted and modified from Németh and Martin (2007) and Miall (2006).
The mineralogy of specified lithotypes were studied under polarizing microscope. Samples from the fine grained tuff and lapilli tuff were analyzed under the Cameca SX 100 microprobe (State Geological Institute of Dionýz Štúr). Minerals were identified using WDS analysis with accelerating voltage 15keV, probe current 20nA, with a beam width of 10μm. These conditions were also used for some glass shards. Second group of vitroclasts were analysed under 2 conditions: probe current 3nA (Na, K, Si) and 10nA (other elements) for elimination of mobile element loss. Raw analyses were recalculated to weight percent of oxide using the ZAF correction. Other minerals were determined by EDAX analyses.  Six whole rocks samples plus one reference sample from Ipolytarnóc were crushed and send to Bureau Veritas mineral laboratories (Canada, Vancouver). Samples were pulverized and processed by Lithium Borate Fusion. Major elements were analyzed by ICP-ES, and trace elements by ICP-MS. One sample from the Mučín mudstones was selected for Rock-Eval pyrolysis (done in Montanuniversität Leoben). 40 Ar/ 39 Ar dating method Two whole rock tuff samples from Lipovany and Mučín sections (GRTF, Fig. 2) were sent to Western Australian Argon Isotope Facility of Curtin University for separation of minerals (plagioclase, biotite) and 40 Ar/ 39 Ar dating.
Plagioclase and biotite crystals were separated from 150-215μm and 215-315μm fractions using a Frantz isodynamic magnetic separator and then hand-picked grain-by-grain under a binocular stereomicroscope. Plagioclase crystals were further leached using diluted HF (2N) for 5 minutes and thoroughly rinsed in distilled water to remove any adhering alteration.
The samples were loaded into two 1.9cm-diameter and 0.3cm-depth Al disks that contain multiple smaller sample wells; all sample wells containing the separated crystals were surrounded by sample wells that carried the Fish Canyon sanidine neutron fluence monitor (28.294 [±0.13%]Ma; Jourdan and Renne, 2007;Renne et al., 2011). The sample disks were Cd-shielded (to minimize undesirable nuclear interference reactions) and irradiated for 40h in the TRIGA reactor (Oregon State University, USA), in a central position. The J-value and mass discrimination factor are given in Annex 1. The correction factors for interfering isotopes were ( 39 Ar/ 37 Ar) Ca = 6.95·10 -4 (±1.3%), ( 36 Ar/ 37 Ar) Ca = 2.65 • 10 -4 (± 0.83%) measured on CaF 2 and ( 40 Ar/ 39 Ar) K = 7.02 • 10 -4 (± 12%) determined on K-Fe glass (Renne et al., 2013). Ar isotopic data are corrected for blank, mass discrimination, and radioactive decay. Individual uncertainties are reported in Appendix I at the 1σ level unless otherwise indicated.
For each sample, a series of single crystals were fused in a single step using a continuous 100 W PhotonMachine© CO2 (IR, 10.6µm) laser fired on the aliquot material for 60 seconds. All standard crystals were fused in a single step. The gas was purified in an extra low-volume stainless steel extraction line of 240cm 3 , set up to run with two SAES AP10 and one GP50 getter. Ar isotopes were measured in static mode using a low-volume (600cm 3 ) ARGUS VI mass spectrometer from Thermo Fisher© set with a permanent resolution of ~200. Measurements were carried out in multi-collection mode using three Faraday cups equipped with three 10 12 ohm (masses 40; 38; and 37) and one 10 13 ohm (mass 39) resistor amplifiers and a low background Compact Discrete Dynode (CDD) ion counter to measure mass 36. We measured the relative abundance of each mass simultaneously during 10 cycles of peak-hopping and 16 seconds of integration time for each mass. Detectors were calibrated to each other through air shot beam signals. Blanks were analyzed for every three to four incremental heating steps and typical 40 Ar blanks range from 1·10 -16 to 2·10 -16 mol. Mass discrimination was monitored using an automatic air pipette and values are provided in Appendix I in per Dalton (atomic mass unit).
Criteria for the determination of a convergent age are as follows: an age must include at least 3 consecutive single crystal ages agreeing at 95% confidence level and satisfying a probability of fit (P) of at least 0.05. Convergent ages are given at the 2 level and are calculated using the mean of all the plateau steps, each weighted by the inverse variance of their individual analytical error. The raw data (Appendix I) were processed using the ArArCALC software (Koppers, 2002), and the ages have been calculated using the decay constants recommended by Renne et al. (2011). All analytical parameters and relative abundance values are provided in Table 1 and Appendix I and have been corrected for blanks, mass discrimination and radioactive decay. Individual errors in Appendix I are given at the 1σ level. Convergent ages include uncertainties on the decay constants and standard age, and were calculated using the Monte Carlo approach of Renne et al. (2010).

Facies analysis
Mučín three mould cave locality (GPS: N 48.23322º, E 19.67651º) is an outcrop accessible by a forest trail from the Mučín village (Fig. 1). The sections are exposed in a creek valley and bounded by a forest scour. Outcrop includes a small cave enclosed within the basal part of a lapilli tuff and a minor section outside the cave, approximately 20-25m before the cave entrance (Fig. 3). Several layers can be described in both partial sections ( Fig. 2B; 3C-D). In the lowermost part of both sections, dark mudstones (Fm.; for lithofacies explanation see Figure 2) are present. At the outside section a 4cm thick brown clay (Fm.; Fig. 2D) is present above the dark mudstone. Higher up in both sections, fine grained tuff (Ft) with some gradation follow. Thickness of the fine tuff is between 14cm in the cave and 21cm outside the cave. The maximum grain-size of clasts is circa 1mm; samples show moderate sorting with recognizable normal gradation. The sample for 40 Ar/ 39 Ar dating was taken from the fine tuff of the outside section ( Fig. 2D; 3E, H). The fine tuff is overlain by 10cm of coarsegrained tuff(Ct) with well-rounded sandstone extraclasts Geochronology of Burdigalian paleobotanical localities 6 (ca. 3cm) at the base (Fig. 3E). The coarse-grained tuff is well sorted and dominantly formed of 2mm large clasts. Especially biotite shows preferential orientation of clasts (Fig. 3I). The top of the sections are characterized by a lapilli tuff layer (Lt) and a recent soil layer. The lapilli tuff layer shows no sorting but inverse gradation (Fig. 3C).
It contains a high amount of approximately 10mm large pumice fragments and carbonized plant fragments (Fig.  3I). The observed thickness of this layer is only 20-40cm in the outside section and about 400cm in the cave section (Fig. 2B). Its total thickness could not have been measured because in both cases the upper boundary is erosive. The observable structure of the lapilli tuff documents its origin in an ash and pumice flow (ignimbrite). Pedogenesis and weathering of the outside sections is accompanied with lateral changes in coloration from light gray to yellow and presence of ferric oxides. Samples were taken from the outside section, with a single exception of the lapilli tuff, which was sampled from an exfoliated part of the ignimbrite inside the cave.
Well preserved fossil leaves occur mainly at the boundary between fine-grained and coarse-grained tuff in the outside sections. Other plant remains were found close to the cave bottom and within the tuff. . Grain size increases upward, but the tuffs are visibly finer than in the Mučín locality. Tuff is divided into several parallel layers, probably due to sheet jointing (Fig. 4C). Observable total thickness of tuffs is approximately 140cm, but their upper boundary is formed by recent soil. Unweathered tuff occurs in the central part of the second level. The samples for petrographical analysis and 40 Ar/ 39 Ar dating were taken from the fresh, Lt tuff (Figs. 2B; 4B, D). Thin section were made from a tuff affected by pedogenesis, and from the underlying, well cemented sandstones which are present at the base of the sandpit.

Petrographic description
Based on the sedimentological results, three different tuff lithotypes were described: fine grained tuff (Ft), sandy grained tuff (Ct), both only in Mučín locality and lapilli tuff (Lt; Mučín and Lipovany; Fig. 2). Petrographic composition of tuff from both localities is very similar.
The texture is crystallovitroclastic, composed of glass shards, pumice fragments and crystalloclasts of plagioclase, quartz and biotite (Fig. 5). Apatite, allanite, zircon and ilmenite are rare. Plagioclase crystalloclasts often contains adhering glass (Fig. 5B). Pumice fragments often have flattened vesicles and rarely contain phenocrysts of plagioclase or biotite (Fig. 5A). Accidental clasts are mainly made of mudstones; cognate recrystallized volcanic glass and vitrophyric volcanic lithoclasts are rare (Fig.  5G, H). Some muscovite is also present. Main differences between lithotypes and localities are grain-size and degree of alteration. Dated, fine tuff in the Mučín section is significantly altered to clay minerals (Figs. 2C-D; 3E; Appendix II). In the sandy tuff the amount of clay minerals is negligible due to good sorting. The content of quartz is higher, where a part of the grains is well rounded. Biotite crystalloclasts are often bended around dense grains. Lapilli tuffs shows larger admixture of accidental mudstone clasts. In the Mučín locality, all primary biotites are deficient at interlayer position due to the alteration into clay mineral (  Table 3; Fig. 7). Additionally, large crystalloclasts from the lapilli tuff show zonation with more basic central part (An 78-72 , Figs. 5C-D; 7). One plagioclase crystalloclast from the Mučín lapilli tuff contains sieve texture with An 42 core overgrown by An 78 to An 29 in rim (Fig. 5C), that documents input of a more basic magma in the magma chamber. Phenocrysts of sanidine were found only in rare cognate, vitrophyric volcanic lithoclasts (Figs. 5H; 7; Table 3). Mudstone lithoclasts contain quartz, albite, K-feldspar, muscovite, biotite/chlorite and sphene in clay matrix (Fig. 5G).
The amount of volatile components in the tuff is relatively high, especially in the markedly altered fine grained tuff from the Mučín section (16.5wt%; Table 4). Less altered fine tuff from the same layer consist of 13.0-10.7% volatiles. Ignimbrite tuffs contain only 6.6-8.1% of volatiles on both localities. The content of total carbon varies between 0.1-1% in all samples, which is influenced by the presence of carbonized plant fragments and leaves. However, the content of volatiles and total carbon questions their classification in the Total Alkali-Silica (TAS) diagram (Le Bas et al., 1986) and other diagrams based on major elements (e.g. Peccerillo and Taylor, 1976). Therefore, the diagrams using trace elements are preferred for chemical classifications (Hastie et al., 2007;Pearce, 1996). Based on whole rock chemical composition, studied samples belong to rhyodacitic volcanic rocks of high-K calc-alkaline series (Fig. 8). However, the tuff samples contain large amount of glass shards and pumice fragments. Thus, parental lava could have been more basic. The samples show medium Eu anomaly (0.53-0.68; Table 4; Fig. 8). The trace elements pattern (La N /Yb N , Zr/Y, Ba, Rb, Sr) indicate an origin within continental arc volcanism on a thick continental margin (e.g. Bailey, 1981).
Additionally, there are well observable trend that show loss of mobile, major elements in the TAS diagram (Fig. 8), Geochronology of Burdigalian paleobotanical localities 8 which reflect the alteration degree of the studied samples. In more altered, yellowish-ocher colored parts, the content of ferric oxide increases and the content of SiO 2 , K 2 O and Na 2 O decreases. The slightly different trend is observed in chemical composition of vitroclasts, where alkali loss leads to higher relative content of SiO 2 (Table 5; Fig. 8). Although this trend is general, the position of samples in the TAS diagram is also affected by process of the probe analysis (see measurement condition in the Methods chapter). Data obtained with respect to elimination of mobile element loss during measurement provide more reliable result.
For better interpretation of non-volcanic admixture, the two samples from the underlying formation were    Geochronology of Burdigalian paleobotanical localities 10 also analyzed. In the Mučín locality the underlying dark mudstones (silty claystones) are unsorted and composed of mono-polycrystalline quartz, feldspar, muscovite, biotite, felzite/silicite, and glauconite grains in a clay matrix. Mudstone contains 1.6% of Total Organic Carbon (TOC) and show kerogen type IV (HI 23.7mg HC/gTOC; Tmax 429ºC; S1 0.06mg HC/g rock, S2 0.38mg HC/g rock), which support terrestrial deposition with severe oxidation of organic matter. In the Lipovany section only the cemented sandstones from the base of the outcrop were analyzed. The sandstone is sorted and composed of subangular grains of quartz, K-feldspar, plagioclase, mica, felzite/silicite, schist, carbonate, glauconite and rare fossils cemented by calcite. Monocrystalline quartz dominate the mineral assemblage, but polycrystalline quartz is also present. K-feldspar show various degree of sericitization. Mica is represented by muscovite, chlorite and biotite. 40

Ar/ 39 Ar results
A sample from the Lipovany ignimbrite was selected for radioisotopic dating due to its low degree of alteration. Based on the petrological observations, biotite and plagioclase were analyzed. In both cases, 15 measurements    (Pearce, 1996), B) TAS diagram (Le Bas et al., 1986), black arrows show weathering trend of whole rock samples. Gray arrows show weathering trend of volcanic glass. Note, glass shards were measured in two different condition (see methods). C) Classification diagram of volcanic series based on trace elements (Hastie et al., 2007), D) Classification diagram of volcanic series based on major oxides (Peccerillo and Taylor, 1976), black arrows show weathering trend of whole rock samples. E) REE patterns of studied tuff (chondrite normalization value after Sun and McDonough, 1989), F) Trace element patterns (normalized after Sun and McDonough, 1989) o l o g i c a A c t a , 1 9 . 5 , 1 -1 Rieder et al., 1998)    Geochronology of Burdigalian paleobotanical localities 13 were made (Appendix I.I-II). Generally, due to the low potassium content (ca. 0.05-0.1wt%; Verati and Jourdan, 2014), plagioclase is less suitable for single crystal total fusion 40 Ar/ 39 Ar dating but the analysis single crystal is necessary due to the possibility of crystal inheritance in tuff rocks. In this case no sanidine crystals were present, and as indicated by low K/Ca values, only plagioclase could be analyzed. For the age calculation, only the nine youngest plagioclase grains were used whereas the oldest crystals were interpreted as inherited from previous eruptions. The obtained converging age of 17.49±0.54Ma (n= 9; P= 0.96) is supported by an inverse isochron age of 17.3±1.1Ma with a trapped ratio of 305±28, indistinguishable from atmospheric value, and a P-value of 0.95 ( Fig. 9A-B, Appendix I.I). Omitted analyses with older ages have similar K/Ca and 40 Ar(r) values and fail to align on a common inverse ischron mixing line (Fig. 9B). It suggests their source in older deposits incorporated into pyroclastic flow. On the other hand, only six oldest biotites were used for the age calculation (Appendix I.II). Other biotite crystal have likely been affected by alteration, which is indicated by the their low radiogenic 40 Ar* values and therefore, high content in atmospheric Ar. The biotite converge toward an age of 17.28±0.06Ma with probability of fit 0.11. Inverse isochron give an age of 17.29±0.10Ma with probability 0.07 (Fig. 9C-D; Appendix I.II).
Second sample was taken from lower fine grained tuff of the Mučín section. As is mentioned above, the Mučín locality yields high degree of alteration. From this point of view, biotites are not suitable for measurements. However, this fine tuff contains the majority of the fossil leaves in its upper boundary and therefore it was selected for 40 Ar/ 39 Ar dating. From 15 plagioclase analyses, only nine with 40 Ar(r)> 35% were used for age calculation (Appendix I.III) as the other younger crystals have likely been altered. The calculated a convergent age of 16.5±1.4Ma with a probability of fit of 0.32 ( Fig. 9E-F; Appendix I.III) shows the relatively large uncertainty being due to the small crystal sizes and therefore the small (close-to-background level) 40 Ar signal generated by each crystal.

DISCUSSION
All studied tuffs are crystallovitroclastic and rhyodacitic in composition. They have similar mineralogical and geochemical properties. Small differences can be explained by different grain size and degree of alteration. From this point of view, tuffs from both localities represent probably a single event. The three different lithotypes observed at the Mučín section (Ft, Ct, Lt; Fig. 2C, D) can be interpreted as follows. The basal fine grained tuff (Ft) with indistinct gradation represents an ash fall deposit. The origin of the sandy grained tuff (Ct) is more difficult to interpret because of its poor exposure. The presence of high amount of quartz grains, good sorting and the occurrence of few sandstone pebbles (Fig. 3E) indicate transport and deposition by a flash flood (reworking) or by a pyroclastic surge. In both cases, part of the quartz grains and the rare pebbles were sourced from the underlying sediments. If the reworking by a flash flood is true, the correct petrographic term for this lithotype is volcanic sandstone. However, deposits of pyroclastic surges are common at the base of dense pyroclastic flows. Such pumice pyroclastic flow is interpreted in the overlying lapilli tuff layer (Lt) with inverse gradation. Regardless of the sandy layer origin, these sediments were deposited immediately after each other. The lapilli tuff (Lt) from Lipovany also shows structural signs of a pyroclastic flow (absence of sorting, inverse grading, and carbonized plant fragments). Considering the similar mineralogical and geochemical composition of these deposits, it probably represents lateral continuation of the Mučín ignimbrite layer. However, the boundary between the ignimbrite  and the underlying terrestrial sediments does not crop out now in the Lipovany section. Previous paleontological works did not contain a lithological column and detailed description of the fossiliferous Lipovany tuff is absent (Němejc andKnobloch, 1969, 1973;Sitár and Kvaček, 1997). Other regional works described some vertical and lateral changes (Kuthan ed., 1963;Vass and Elečko, 1992).
Two 40 Ar/ 39 Ar ages of 17.49±0.54Ma (plagioclase) and 17.28±0.06Ma (biotite) obtained from the Lipovany tuff are indistinguishable within uncertainties (Fig. 9A-D; Appendix I.I-II) and therefore, the most probable eruption age is best represented by the more precise biotite age of 17.28±0.06Ma. The 40 Ar/ 39 Ar convergent age of 16.5±1.4Ma from the fine grained Mučín tuff, which underlies the   ignimbrite, shows a large error due to the low K-content and small crystal size of plagioclase (and consequently low 40 Ar yield during measurement). Although the ages of both tuffs overlap, due to the large uncertainty of the Mučín tuff age, it cannot be unequivocally defined if all tuffs are of the same age or if they come from several consecutive volcanic events. Although the data cannot be clearly linked, they strictly indicate deposition of these tuffs close to the Ottnangian/Karpatian boundary according to Harzhauser et al. (2019). Additionally, our new 40 Ar/ 39 Ar data fit well with a single-crystal laser-fusion plagioclase 40 Ar/ 39 Ar age of 17.02±0.14Ma (Pálfy et al., 2007;Fig. 10). Especially if the 40 Ar/ 39 Ar age of Pálfy et al. (2007) is recalculated to 17.19±0.14Ma using the constants of Renne et al. (2011) adopted in this study, and which are fully calibrated against the U-Pb system . Our age is further supported by the single-crystal zircon U-Pb isochron age of 17.42±0.04Ma from Ipolytarnóc (Pálfy et al., 2007). Because of extremely high closure temperature of zircon, this U-Pb age probably document zircon crystallization in magma chamber and therefore it is possible that this age include data point from antecrysts zircon crystals (Schaltegger and Davies, 2017).
Based on these results, the previous stratigraphic interpretations of the Lipovany tuff based on magnetostratigraphy may be rejected Vass et al., 2006). These authors ranked the Mučín section with normal polarity chron C5Dn (Hilgen et al., 2012) to the Ottnangian and Lipovany section (marked as NE Lipovany in Vass et al., 2006 andMárton et al., 2007) with reversed polarity (C5Er) to the Eggenburgian. However, the new 40 Ar/ 39 Ar data from NE Lipovany section, produced by this study, date the studied deposits as late Ottnangian in age. Moreover similar result was also presented by Pálfy et al. (2007) from the Ipolytarnóc (normal polarity) and Nemti tuff (reversal polarity; Márton et al., 2007;Vass et al., 2006). Pálfy et al. (2007) concluded that both mentioned localities are indistinguishable in age and petrography. Furthermore, such units usually have wide distribution, therefore these authors concluded that both tuffs represent a single ignimbrite eruption. It must be mentioned here, that only first two ignimbrite sheets from the Ipolytarnóc show normal polarity and the third displays reverse polarity . It can be noted, that the assumed age of 17.28±0.06Ma (this study) fit with C5D-C5C magnetochron boundary (Fig. 10).  (Pálfy et al., 2007) (Vass et al., 2003;Kocsis et al., 2009)  ? Pálfy et al. 2007 17.235 Mučín FIGURE 10. Correlation of age results. Note, that the gained ages is the same as the normal and reverse chron boundary described by Márton et al. (2007); For abbreviations see Figure 2. o l o g i c a A c t a , 1 9 . 5 , 1 -1  Geochronology of Burdigalian paleobotanical localities 16 Implication for paleoclimatology and stratigraphy context As mentioned above, the majority of fossil leaves in Mučín section are present within the fine tuff (Ft) close to the boundary with the sandy tuff layer (Ct). Localization of fossil leaves at Lipovany is vaguely described as a bedded tuff or tuffite located in the upper part of sandpit (Němejc andKnobloch, 1969, 1973;Sitár and Kvaček, 1997). Despite imprecise 40 Ar/ 39 Ar data from Mučín fine grained tuff, the result from this study confirm late Ottnangian age of both fossil leaves associations. Then, the floral assemblages from both studied localities document Ottnangian climatic patterns as were presented by original authors (e.g. Kuthan, 1963;Němejc andKnobloch, 1969, 1973). These localities together with Ipolytarnóc can again be used as parastratotype of the Ottnangian regional stage as already suggested by Němejc and Knobloch (1973) and Hably (1985). Hence, previous works in which the Lipovany and Mučín fossil assemblages are interpreted as a late Eggenburgian must be reconsidered (e.g. Erdei et al., 2007;Kučerová, 2009;Sitár and Kvaček, 1997;Vass and Elečko, 1992). Similarly, the interpretation of Márton et al. (2007), that rainforest vegetation from the studied localities is most probably younger than the swamp vegetation of the Salgótarján Formation is most likely not correct. The numerous papers described sediments and coal seams of Salgótarján Fm. in the overburden of the studied tuff (e.g. Bartkó, 1985;Kuthan, 1963;Pálfy et al., 2007;Vass and Elečko, 1992). The most probable paleoenvironmental scenario is that the terrestrial sediments of the Bukovinka/ Zagyvapálfalva Fm. formed a paleosurface overgrown by a humid subtropical forest as indicated by leaf assemblage (Hably, 1985;Kučerová, 2009;Němejc and Knobloch, 1973;Sitár and Kvaček, 1997). The catastrophic volcanic activity destroyed this ecosystem. The ash-fall deposits together with the ignimbrites buried the existing flora and fossil tracks and protected them against decay. Silica-rich hydrothermal fluids associated with the volcanic activity petrified tree trunks, which are common within the studied tuff and within the underlying Bukovinka/Zagyvapálfalva Fm. (e.g. Bartkó, 1985;Sitár and Kvaček, 1997;Vass and Elečko, 1992). Deposition of the Salgótarján Fm. followed, and was possibly affected by volcanism which triggered change in the local climate, morphology and edaphic conditions. In any case, climatic conditions remained subtropical, but floral assemblages changed to swamp forests in the Salgótarján Fm. (e.g. Nagy, 2005;Němejc, 1963;Planderová in Vass and Elečko, 1992). These climatic conditions represent the beginning of Miocene Climatic Optimum (Böhme, 2003;Sitár and Kvaček, 1997).
It should be noted that deposition of Salgótarján Fm. took place during latest Ottnangian and earliest Karpatian (Pálfy et al., 2007;this study). Presence of Sphenolitus belemnos (Holcová, 2001) within Salgótarján Fm. must be interpreted as reworked from older strata. The Karpatian marine transgression in this area began around 17.3Ma, similarly as in the Vienna Basin (Harzhauser et al., 2019). Dating of this event in Vienna Basin was set to 17.23±0.18Ma (Roetzel et al., 2014), what was recalibrated using the constant of Renne et al. (2011) to 17.29± 0.18Ma (Table 1).

CONCLUSION
The new plagioclase and biotite 40 Ar/ 39 Ar ages of 17.49±0.54Ma and 17.28±0.06Ma, respectively, from the fossiliferous Lipovany tuff indicate that the volcanic eruption event took place during the latest Ottnangian up to the Ottnangian/Karpatian regional stage boundary   40 Ar/ 39 Ar data together with magnetostratigraphy (end of C5Dn chron) supports this regional stage boundary.
The data supplements correlation of the fossil flora assemblage from the Ipolytarnóc, Mučín and Lipovany sites. In this area, the terrestrial environment was overgrown by subtropical rain forest which existed before the volcanic eruption. This environment was buried during a volcanic event associated with the formation of pyroclastic flows. The presented catastrophic event conserved these important fossil sites. After the eruption a subtropical swamp forest developed and lead to the deposition of Salgótarján Fm. Obtained data indicate that the deposition of Salgótarján Fm. is younger, with an age of about 17.3Ma.