Introduction

Studies of clastic sedimentary rocks are crucial for paleotectonic reconstructions as they can provide information about source areas which have been eroded and recycled during tectonic processes. Furthermore, in the absence of fossils and other stratigraphic means the detrital-zircon geochronology helps to estimate a maximum age of deposition (e.g. Fedo et al. 2003; Dickinson and Gehrels 2009; Spencer et al. 2016 and references therein). The Western Carpathian belt consists of a collage of the pre-Alpine terranes, amalgamated during the Variscan orogeny and subsequently deformed during the Alpine orogeny. These repeated tectono-thermal events lead to destruction and recycling of continental or oceanic crust fragments. The overlapping siliciclastic sediments document well such events.

The Carboniferous and Permian basins reflected largely a compressional tectonic regime of the Variscan orogeny during the continent–continent collision in its final stages (Ziegler and Stampfli 2001; von Raumer et al. 2003, 2009, 2013; Stampfli et al. 2013 and references therein). Fragments of the Variscan crust, coupled with their late- and post-Variscan cover, have been amalgamated, together with the dominantly carbonate Mesozoic sequences, in the Cretaceous Western Carpathian’s nappe system. The Western Carpathians have been traditionally divided into external and internal structural zones. The main differences between them consist in their rock complexes, in age of the main Alpine deformational and metamorphic events and in the intensity of their effects.

The internal structural zone includes several northward-stacking crust-scale superunits, with representative crystalline basement and their Late Palaeozoic–Mesozoic cover, and several overlying nappe systems (Andrusov 1968 and references therein; Andrusov et al. 1973; Maheľ 1986; Biely et al. 1996a, b; Plašienka et al. 1997; Rakús et al. 1998 and references therein). The Alpine deformations reflect the high pressure–low temperature Middle/Late Jurassic subduction and the Early/Middle Cretaceous collision events followed by nappe stacking. Typical post-nappe sedimentary formations are represented by less preserved Late Cretaceous (Senonian) sediments and mostly Paleogene and Neogene to Quaternary sedimentary deposits that rest on the pre-Late Cretaceous nappe structure.

The external structural zone comprises flysch-dominated Mesozoic and Paleogene formations, virtually missing pre-Mesozoic complexes and a negligible occurrence of post-nappe sedimentary cover. Generally, this zone is represented by a system of rootless nappes, i.e. sedimentary sequences detached from their basement and incorporated into the north verging nappes largely from the Late Oligocene to Middle Miocene (Birkenmajer 1985; Biely et al. 1996a and references therein).

The relics of the Late Palaeozoic sedimentary basin fillings are preserved only in the Western Carpathian internal structural zone, as a part of principal crust-scale superunits, from N to S. They are: Tatricum, Veporicum and Gemericum, as well as several cover nappe systems: Fatricum, Hronicum, Meliaticum with the Bôrka Nappe, Turnaicum and Silicicum (Fig. 1).

Fig. 1
figure 1

(modified after Biely et al. 1996b)

Simplified geological sketch map of the Western Carpathians. a Schematic map of the in the Hronicum Pennsylvanian–Permian sequence in the Malé Karpaty Mts with location of detrital-zircon sample (modified from Vozár in Vozárová and Vozár 1988); b schematic geological map of the Hronicum Pennsylvanian–Permian sequence on the northern slopes of the Nízke Tatry Mts., showing sample locations (modified after Vozár in Biely 1992); c cross section through the principal tectonic units of the Western Carpathians with position of the groups of nappes (modified after Hók et al. 2014). LML Lubeník–Margecany line

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The present study is focused on the Late Paleozoic sequence of the rootless Hronicum Unit and the subsequent text is devoted to this tectonic unit. To discover characteristic features of the presumed Hronicum basement, we took five samples from the Pennsylvanian–Permian siliciclastic sediments for the in situ U–Pb SHRIMP detrital-zircon dating (Fig. 1a, b). The recognition of the Carboniferous–Permian siliciclastic provenance could help to assess of the Hronicum homeland location. The acquired zircon age spectra are important for further tectonic constraints and correlation of former position of the internal Western Carpathian Alpine tectonic units. This paper is focused on the potential changes of the provenance source areas in response to changes in geotectonic regime and on comparison of detrital-zircon age spectra with zircon data from the other peri-Gondwanan terranes.

Geological setting and lithostratigraphic entities of the Hronicum Unit

The Hronicum Unit (Fig. 1a, b) has been defined as a rootless nappe system (or middle Subtatricum according to Andrusov 1968) composed of the uniform Carboniferous–Permian volcano-sedimentary sequence, differentiated Oberostalpine-type of Triassic and only locally preserved Jurassic–Early Cretaceous sediments (Biely et al. 1996a and references therein). Recently, the Hronicum is interpreted as a system of piled-up partial nappes and tectonic slivers, whose geometric relationships from one mountain to another are unsettled (Havrila 2011). The allochthonous position of Hronicum rootless nappes relative to basement implies their ultra-Veporicum origin, presumably from the presumed homeland area between the Veporicum and Gemericum units (cfr. Biely and Fusán 1967; Rakús et al. 1998). Generally, the Hronicum nappes rest directly on the Fatricum Unit, with exception of the Veporicum zone where they overthrust the Veporicum cover directly (Biely and Bezák 1997). The southernmost tectonic slivers of the Hronicum relics were recognized near the Veporicum/Gemericum tectonic contact, along the Lubeník–Margecany Line (LML; Fig. 1c).

The original Hronicum basement is misplaced. The dominant arcosic character of the Late Palaeozoic terrestrial siliciclastic sediments, associated with continental tholeiites, indicates an extensionally sedimentary basin located on the continental crust. This opinion is also supported by the findings of some mylonitized granitoid tectonic slices below the basal part of the lower partial Hronicum Nappe (Andrusov 1936; Vozárová and Vozár 1979). The 310–340 Ma ages of the source area is indicated by the 40Ar/39Ar cooling ages of detrital mica from sandstones (Vozárová et al. 2005). This suggests that the formation of the Ipoltica Group (IG) sedimentary basin is younger than 310 Ma.

The Late Paleozoic volcanic and sedimentary rocks, forming the IG succession, built up the basal part of the multi-nappe Hronicum Unit that occurs in almost all central Western Carpathian mountain ranges. At these places, they overthrust the Tatricum and Veporicum basements and their Mesozoic cover complexes, as well as the Mesozoic succession of the Fatricum superficial nappe. On the basis of the most completely preserved successions on the northern slopes of the Nízke Tatry Mts., the IG was defined as composed of two lithostratigraphic units; (1) the Nižná Boca Formation (Pennsylvanian) and (2) the Malužiná Formation (Permian) (Vozárová and Vozár 1981, 1988).

Carboniferous–Permian lithostratigraphic features

Nižná Boca Formation (NBF) is the lowermost in the IG lithostratigraphic unit occurring at the tectonic base of the Hronicum partial nappes. Macroflora from the NBF upper part indicates the uppermost Pennsylvanian, Kasimovian–Gzhelian (ICS Stratigraphic Chart 2016) that correlates with the Stephanian, according to Regional Stratigraphic Scale of Central Europe (Menning and Hendrich 2012). It was documented by Sitár and Vozár (1973) on the basis of well-preserved relics of Asterotheca miltoni, Asterotheca arborescens, Cordaites palmaeformis and Callipteridium gigas. Equally, the same stratigraphic range was proved by palynology (Planderová 1979). The NBF sequences are tectonically variably truncated at the base of the Hronicum nappe thrust plane. Their tectonic remnants are best preserved at the northern slopes of the Nízke Tatry Mts. (up to 400 m thick).

Generally, the NBF represents a cyclical clastic sequence with a distinct tendency of coarsening upwards. Numerous small repeating sedimentary cycle fining-upward is the most typical feature. Graded bedded grey sandstones and fine-grained conglomerates with numerous erosive contacts are associated with minor mudstones, as well as layers rich in plant detritus, indicating a fluvial–lacustrine deltaic association. Sequences of fine-grained sandstones, alternated with mudstones and shales of grey to black colour correspond to lacustrine lithofacies. A multiple vertical alternation of fluvial and lacustrine cycles and their unification in the two-regional coarsening upward cycles indicate mutual prograding from lacustrine delta to fluvial environment. Synsedimentary subaerial volcanism is represented, first of all, by abundant redeposited dacite volcanoclastic material mixed with the non-volcanic detritus. Thin layers of dacite tuffs are less frequent and small lava flows are exceptional.

Malužiná Formation (MF) has gradual transition from the underlying NBF Late Mississippian succession. The MF principal features are: (1) thick sequence of red beds, comprising a complex of cyclical alternation of variegated conglomerates, sandstones and shales; (2) rift-related andesite–basalt volcanism of a continental tholeiitic magmatic affinity; (3) three regional megacycles fining upwards (500–700 m thick) arranged above each other (III. order cycles). Principally, the MF sediments have been deposited in fluvial and fluvial–lacustrine environments, in permanently arid and semi-arid climate. Basal parts of each of the megacycles consist of channel-lag and point-bar deposits, associated laterally and vertically with flood plain and natural levee sequences. On the other hand, the upper parts of the megacycles are characterized by a playa, ephemeral lake and scarce inland sabkha lithological associations. Lenses of dolomites and gypsum were recognized in the uppermost part of the megacycles at some places. Thin caliche/calcrete horizons are frequent within the top part of the lower-order cycles.

The differences among megacycles are first of all, in the frequency of the first order coarse-grained sedimentary cycles containing conglomerates and coarse-grained sandstones. Major frequency of coarser sedimentary cycles is presented within the first megacycle (58%), while less amount has been observed in the second one (31%) and minimum in the third megacycle (6%). Also, the change of the sandstone/shale ratios is characteristic; 3.5 for the first megacycle and 2.2 and 1.2 for second and third ones, respectively. Essential part of the MF succession is the andesite–basalt continental tholeiitic volcanism that occurred within two eruption phases (Vozár 1977, 1997; Dostal et al. 2003; Vozár et al. 2015). Thin interbedded mafic lavas are common in the first and third megacycles (first and second eruption phase).

The Autunian–Saxonian microflora assemblages, described by Planderová (1973) and Planderová and Vozárová (1982) from the MF first and second megacycles, correspond to the Cisuralian–Guadalupian stratigraphic range, according to the Global Stratigraphic Scale (ICS 2016) and the stratigraphic correlations by Izart et al. (1998). This assumption is supported by the Pb/U dating from uranium-bearing layers that have been found within the second megacycle (263 ± 11 Ma, Rojkovič 1997).

The important magnetostratigraphic data, confirming the position of the Illawarra Reversal magnetic horizon, were obtained from the upper part of the second megacycle. Menning (1995, 2001) proved this reversal magnetic event at the boundary of the 265 Ma. On the basis of magnetostratigraphic results, as well as the Thuringian microflora described by Planderová (1973), the third megacycle is considered to be of the Lopingian age.

Commonly, the grade of regional metamorphism did not exceed the very low-grade boundary (according to pumpellyite + prehnite + quartz assemblage—Vrána and Vozár 1969; illite crystallinity indices from pelites—Plašienka et al. 1989; Šucha and Eberl 1992). Illite crystallinity indices are also changed at the base of the Hronicum thrust plane and suggest temperature of the low-grade greenschist facies (illite crystallinity indices, unpublished data).

Analytical method

Zircons have been extracted from rocks by standard grinding, heavy liquid techniques and magnetic separation. The rock-forming minerals were studied by the electron microprobe (CAMECA SX-100), in laboratory of the Geological Survey of Slovak Republic, Bratislava. The internal structure of individual zircon crystals and shapes were examined with cathodoluminescence (CL) imaging by SEM and optic microscopy.

In situ U–Pb analyses were performed using sensitive high-resolution ion microprobe (SHRIMP-II) in the Centre of Isotopic Research (CIR) at VSEGEI, applying a secondary electron multiplier in peak-jumping mode following the procedure described by Williams (1998) and Larionov et al. (2004). Primary beam size allowed the analysis of ca. 27 × 20 µm area. The 80-µm-wide ion source slit, in combination with a 100-µm multiplier slit, allowed mass-resolution M/ΔM ≥ 5000 (1% valley); hence, all the possible isobaric interferences were resolved. The following ion species were measured in the sequence: 196(Zr2O)–204Pb–background (ca. 204.5 AMU)–206Pb–207Pb–208Pb–238U–248ThO–254UO. Four mass-spectra for each analysis were acquired. Each fifth measurement was carried out on the TEMORA-1 Pb/U standard (Black et al. 2003). The 91500 zircon (Wiedenbeck et al. 1995) was applied as “U-concentration” standard. The obtained results have been processed by the SQUID v1.12 (Ludwig 2005a) and ISOPLOT/Ex 3.22 (Ludwig 2005b) software, with decay constants of Steiger and Jäger (1977). Common lead correction was done on the basis of measured 204Pb/206Pb. The ages given in text, if not additionally specified, are 207Pb/206Pb for zircons older than 1.2 Ga, and 206Pb/238U, for those younger than 1.2 Ga. The errors are quoted at 1σ level for individual points and at 2σ level in the Concordia diagram, for the Concordia ages or any previously published ages discussed in the text. For interpretation purposes, the probability density (ISOPLET/Ex 3.75, Ludwig 2012) was constructed using 206Pb/238U ages for zircon younger than 1.2 Ga and 207Pb/206Pb ages for zircons older than 1.2 Ga. The Kolmogorov–Smirnov (K–S) statistic test was adopted from Guynn and Gehrels (2010) and this test has been used for the comparison of detrital-zircon age distributions.

In this study, we follow the time-scale calibration of International Chronostratigraphic Chart (2016) in order to compare geochronological data from detrital zircons with fossil-bearing sedimentary units and tectono-thermal events.

Sample characteristics

Five samples have been processed for zircon dating from the Carboniferous–Permian Hronicum sandstones. Four samples were collected from the northern slopes of the Nízke Tatry Mts., and one sample from the Malé Karpaty Mts (Table 1). Special attention has been paid to outcrops from the northern slopes of the Nízke Tatry Mts., along the Ipoltica Valley and its tributaries that form the composite lithostratotype of the Ipoltica Group. As the Malužiná Formation represents the dominant part of the Hronicum Late Palaeozoic sequence, four samples from this formation have been collected. Modal composition and petrofacial parameters of the studied sandstones are given in Table 2.

Table 1 List of the studied Carboniferous–Permian Hronicum sandstone samples with location
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Table 2 Modal composition and petrofacial parameters (in percentages) of the studied Carboniferous–Permian sandstones from the Hronicum Unit
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Generally, sandstones from the both formations contain a clastic detritus derived from: (1) granitoids, migmatites and gneisses; (2) low-grade metamorphic rocks, represented by different type of phyllites, metaquartzites, lydites, metasandstones and scarcely by greenschist; (3) dacite and less frequent andesite–basalts and their volcaniclastics, synsedimentary as well as reworked (Ďurovič 1971; Vozárová 1981; Vozárová and Vozár 1988; Vďačný et al. 2013). It was supported as well, by heavy minerals studies of Vďačný and Bačík (2015).

The NBF sandstones are rich in polycrystalline quartz and lithic grains, and corresponding to a lithic sandstone composition. On the other hand, the MF sandstones resemble to arcose arenites, rich in feldspars and monocrystalline quartz. This tendency has also been observed in the studied sandstones (Table 2). In the NBF sandstone sample (NTZ-1), feldspar/lithic (F/L) and monocrystalline/polycrystalline quartz (Q m /Q p ) ratios are < 1 (~  0.77). On the contrary, Q m /Q p ratios in the MF sandstones are > 1 (1.3–2.2), while the F/L ratios are highly variable (1.4–22). An exception is the sample NTZ-4, from the basal part of the third megacycle that is rich in quartz (Q t 89%) and poor in feldspars (F 3%) with a total absence of lithic grains (L sm). All the studied sandstone samples show a low index of mineral maturity (Im = Q t /F − ranging from 0.82 to 1.17), with exception of the sample NTZ-4 (Im = 8.1). The plagioclase/alkali feldspar ratios (P/K) are nearly equal in the all samples (1.15–2) that clearly indicate the dominance of plagioclases. Volcanic fragments prevail among lithic grains that is also indicated by L v /L sm ratios (3.3 and 2.5–2.9) in the NBF as well as in the second megacycle of the MF sandstones.

Results of U–Pb SHRIMP zircon dating

Zircon characteristics

Zircon grains are mostly in size range of 200–400 µm [Suppl. 1a, b, c, d, e in the Electronic Supplementary Material (ESM) only]. Generally, the majority of the zircons have a prismatic shape and pink colour. They are clear and transparent, euhedral, long- and short-prismatic, with dominant aspect ratios of 1:4 which, but stubby crystals, with aspect ratios of 1:2, are also present. Overwhelming majority of the zircon grains display magmatic oscillatory growth zoning (regular or irregular), concentric and less of marginal sector type (ESM_1a, b, c, d, e; ESM = Electronic Supplementary Material). The mode of the compositional zoning varies widely in bimodal successions of richer and poorer trace-element bands. In some zircon grains, the modifications of their internal texture with discontinuity of growth oscillatory zoning (mosaic and convolute texture) have been observed. Frequently, the regular growth zoning is interrupted by the textural disconformities. A particular type of zoning is represented by the patchy texture. CL images (ESM_1a, b, c, d, e) show that some grains are composite, with inherited cores surrounded by a new growth rims. Xenocrystic cores are commonly separated from their rims by irregular surfaces, which truncate internal zoning of subrounded cores. The oldest detrital-zircon grains preserve the abrasion and fracturing caused by erosion and reworking by sedimentary processes. They are subrounded or rounded in shape.

Zircon age data

Nižná Boca Formation

The NBF zircon population has been separated from sample NTZ-1 (Fig. 1b; ESM_2a), total 48 grains have been analysed. The Concordia diagrams are given in Fig. 2. Most of the analyses yielded the Late Pennsylvanian/Early Cisuralian ages in the range of 287–309 Ma (21 grains; ~ 44% of the total; ESM_2a) with prominent peak at 296 Ma. The average Concordia calculated from these data at 95% confidence level gives the age of 296.6 ± 2 Ma (Fig. 2b). On the probability density plot, further two minor peaks at 478 and 550 Ma are notable (see the summarizing Fig. 7). They correspond to five grains ranging from 465 to 486 Ma (~ 11% of the total) and six grains in the time-span from 494 to 551 Ma (~ 13% of the total), respectively. Only two zircon grains (~ 4% of the total) show the Mississippian ages of 320 ± 4 Ma and 341 ± 6 Ma. Other zircon grains demonstrate Precambrian ages: Ediacaran (561 ± 8 Ma, 571 ± 10 Ma, 619 ± 9 Ma; ~ 8% of the total), Tonian (896 ± 13 Ma, 988 ± 13 Ma; ~ 4% of the total) and Mesoproterozoic (1224 ± 15 Ma; ~ 2% of the total), as well as the seven Paleoproterozoic–Neoarchean grains (~ 14% of the total), with ages ranging from 1943 to 2723 Ma (Fig. 2a; ESM_2a).

Fig. 2
figure 2

Concordia plots of detrital zircons dated from the sample NTZ-1: a Concordia showing the all detrital-zircon ages; b selected sector of the Concordia relevant to calculation of the maximum depositional age of the Nižná Boca Fm. The curve is based on 238U/206Pb ages up to 1.2 Ga and 207Pb/206Pb ages for older ages (here and further on)

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The youngest Late Pennsylvanian/Early Cisuralian detrital-zircon population demonstrates low-to-moderate U and Th contents (157–912 and 43–472 ppm, respectively). The majority of 232Th/238U ratios are moderate, between 0.25 and 0.57, fitting well the zircons of igneous origin.

Similarly, the Mississippian as well as Ordovician and Cambrian detrital-zircon populations have the textural and chemical features that are characteristic for magmatic crystallization process.

Malužiná Formation

Detrital zircons of the MF sandstones have been studied from four samples that were collected up at different lithostratigraphic horizons in the northern slopes of the Nízke Tatry Mts. (samples NTZ-2, NTZ-3, NTZ-4) and Malé Karpaty Mts. (sample MK-31). Largely, the samples were collected at the basal part of the three sedimentary megacycles (Fig. 1a, b).

First megacycle

The detrital-zircon assemblage from the basal part of the MF first megacycle has been extracted from NTZ-2 sample (Fig. 3a, b; ESM_2b). Forty-six analytical spots spread broadly 206Pb/238U age cluster between 291 and 2689 Ma. However, comparing with the NBF zircon assemblage (sample NTZ-1), the Mississippian/Latest Devonian ages dominate sharply in this sample (27 grains; ~ 59% of the total) that is clearly demonstrated by a distinct peak in the probability density plot (see the summarizing Fig. 7). Among those, the Early Mississippian ages overcome, ranging from 345 to 359 Ma (16 grains; ~ 35% of the total). Nine grains (~ 20%) correspond to the Upper Devonian (Famennian), extending between 360 and 371 Ma. Only two grains are represented by the younger Mississippian ages, namely, 322 ± 2 and 329 ± 2 Ma (~ 4%).

Fig. 3
figure 3

Concordia plots of detrital zircons dated from the sample NTZ-2: a Concordia showing the all detrital-zircon ages; b selected sector of the Concordia showing the time interval 200–600 Ma

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Of the ten Early Palaeozoic grains two correspond to the 458 and 485 Ma ages and further eight are in the range of 491–508 Ma. Remaining eight grains are Precambrian: two are Ediacaran (556–569 Ma) and six in the range 2015–2689 Ma.

Second megacycle

The detrital-zircon assemblages were separated from two samples: the sample NTZ-3 (Nízke Tatry Mts.) and the sample MK-31 (Malé Karpaty Mts.) (Fig. 1a, b). These samples record a similar provenance age distribution as the sample NTZ-1, with prominent age peaks at 295 and 297 Ma, respectively. Concordia age diagrams are given in Figs. 4 and 5.

Fig. 4
figure 4

Concordia plots of detrital zircons dated from the sample NTZ-3: a Concordia plot showing the all detrital-zircon ages; b selected sector of the Concordia showing the Concordia ages of two prominent detrital-zircon populations

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Fig. 5
figure 5

Concordia plots of detrital zircons dated from the sample MK-31: a Concordia plot showing the all detrital-zircon ages from the sample MK-31; b selected sector of the Concordia showing the time interval 200–600 Ma

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Altogether, in the sample NTZ-3, 42 grains have been analysed (ESM_2c). As mentioned above, the dominant detrital-zircon population (15 grains; 36% of the total) is represented by ages in the range 284–299 Ma that coincide with the 294 ± 2 Ma Concordia age (Early Cisuralian; Asselian/Sakmarian; Fig. 4b), as well as with 295 Ma leading peak at the relative probability plot (see the summarizing Fig. 7). A subordinate peak at 353 Ma characterizes the zircon population ranging between 346 and 356 Ma, with the 352 ± 3 Ma Concordia age (7 grains; 17% of the total), corresponding to the Early Mississippian (Tournaisian). Minority of the detrital-zircon grains shows Ordovician (one grain of 461.2 ± 4.6 Ma) or Cambrian ages (4 grains in the range 511–517 Ma). The Precambrian ages are relatively frequent. There are six Neoproterozoic grains with dominating Ediacaran ages (four grains in the range 545–606 Ma) and fewer Cryogenian–Tonian ages (665 ± 7 and 762 ± 8 Ma, respectively). Eight detrital-zircon grains yielded Paleoproterozoic/Neoarchean ages in the range of 1965–2843 Ma. Only one grain corresponds to Mesoproterozoic (1240 Ma).

Total 47 grains have been analysed in the sample MK-31 (Fig. 5; ESM_2d). In one grain from this population two analyses have been done (together 48 spots). These analyses gave 361.5 ± 4 Ma age at the rim and 1889 ± 8 Ma age in the xenocrystic core (ESM_1d). This fact clearly documents that this old inherited zircon grain was entrained by the Late Devonian magma. Within the MK-31 detrital-zircon population the majority of the recorded ages are in the range 285–309 Ma (25 of 48; ~ 53%), with a prominent peak at 298 Ma (see the summarizing Fig. 7). The most significant peak on the relative probability plot straddles the Pennsylvanian/Cisuralian boundary and confirms a dominance of the Cisuralian zircon grains (19 of 48). Less protruding older age clusters are in the range 345–361 Ma (four grains) and 446–465 Ma (three grains), as well as in the range 491–541 Ma (five grains). The oldest age clusters are represented by Ediacaran 547–578 zircon ages (six grains) and Paleoproterozoic 1802–2372 Ma zircon ages (five grains).

Third megacycle

Forty-one grains from sample NTZ-4 have been analysed: this is a quartz-rich sandstone from the basal part of the MF third megacycle (Fig. 1b; ESM_2e). The majority of the obtained ages range from 332 to 363 Ma (13 of 41 grains), with a prominent peak at ca. 360 Ma that is revealed at the Concordia diagram as well as at the relative probability plot (Figs. 6, 7). Less prominent age clusters are in the range 293–307 Ma (4 of 41 grains), 438–484 Ma (4 of 41 grains) and 495–538 Ma (9 of 41 grains). The oldest detrital-zircon ages span over Neoproterozoic–Archean interval (11 of 41 grains). Three of them are in the range of 543–612 Ma and correspond to Ediacaran. Eight grains yielded 2007–2974 Ma ages falling within the Paleoproterozoic to Mesoarchean range.

Fig. 6
figure 6

Concordia plots of detrital zircons dated from the sample NTZ-4: a Concordia plot showing the all detrital-zircon ages from the sample NTZ-4; b selected sector of the Concordia showing the time interval 200–650 Ma

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Fig. 7
figure 7

Comparison of probability density curves showing detrital-zircon ages from the Hronicum Pennsylvanian–Permian succession

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Discussion

Zircon data and depositional age

The Ipoltica Group detrital-zircon age data demonstrate two major zircon populations recognized by prominent peaks and weighted means. The younger population ranges from 288 to 309 Ma, with the prominent peak at 295 Ma (Fig. 7). This age cluster corresponds to the earliest Cisuralian and/or straddles the boundary of latest Pennsylvanian/earliest Cisuralian. The second major detrital-zircon population is recorded in the time range 345–371 Ma, with a prominent peak at 356 Ma (Fig. 7). This age cluster is linked with the Mississippian/Devonian boundary, corresponding to the Tournaisian–Famennian. Although zircons of these two prominent populations are present in the all studied samples their relative proportions within the individual samples are quantitatively different. The Pennsylvanian/Cisuralian assemblage dominates in the Nižná Boca Formation (sample NTZ-1) as well as at the basal part of the Malužiná Formation’s second megacycle (samples NTZ-3 and MK-31). On the contrary, in the sandstones from the first and third megacycles of the Malužiná Formation (samples NTZ-2 and NTZ-4), the Tournaisian–Famennian zircon assemblage is principal. Notably, the overwhelming zircon majority from the both dominant zircon populations manifests clearly their magmatic origin. This is expressed by the presence of well-developed oscillatory growth zoning as well as the high values of the 232Th/238U ratios. The K–S statistic test confirms that the samples MK-31, NTZ-1 and NTZ-3 passed the test for 95% confidence; the P values higher than 0.05, i.e. ranging from 0.464 to 0.950 (ESM_3a, b). On the other hand, the samples NTZ-2 and NTZ-4 are similar to each other (P values 0.338). The sample NTZ-4 is slightly similar to the sample NTZ-3 (P value 0.056). D values exhibit reciprocal differences in all studied samples. It is supposed that the differences in P values between two mention groups of compared samples are caused by the differences in proportionality of the two prominent detrital-zircon ages. Despite all, the studied samples could have been shed from the same source region. Small differences result from the individual zircon age populations presented in each sample in different frequency relations.

Majority of the Pennsylvanian/Cisuralian detrital zircons in the NBF and MF second megacycle sandstones correspond well to their mineral composition that is relatively rich with the volcaniclastic lithic fragments (samples NTZ-1, NTZ-3 and MK-31; Table 2). On the other hand, the feldspathic MF first megacycle sandstones (sample NTZ-2; Table 2) are distinguished by the prominent Mississippian/Devonian detrital-zircon assemblage. These compositional changes in zircon distribution, reflect most likely horizontal movements in the source area or ascending of an intrabasin transfer zone. Contrasting to this, the MF third megacycle sandstones (sample NTZ-4; Table 2) are characterized by the relatively high mineral maturity, quantified by the 89% detrital quartz grains in composition. Such a rapid change in mineral maturity at the base of third megacycle could be a consequence of a break in sedimentation and restructuring within the sedimentary system caused by a deformation pulse, which induced faulting, uplift and erosion or break in sedimentation. Such a type of tectonic movement may have been synchronous with the Guadalupian geomagnetic “Illawarra Reversal” event (265 Ma—Menning 1995, 2001; Wardlaw et al. 2004—close to the boundary Wordian–Capitanian at ca. 266 Ma) that can be related to the so-called “Mid-Permian Episode” of Deroin and Bonin (2003). This first-order magnetostratigraphic time marker has been documented within the upper part of the MF second megacycle (Vozárová and Túnyi 2003), in the underlier of the MF third megacycle. Thus, the remarkably diffuse patterns of zircon ages recorded in the sample NTZ-4 (Fig. 6), could be a consequence of a long-lived reworking and inferred mineral maturity. Within this zircon assemblage, the Early Palaeozoic and Precambrian ages are dominant (59% of the total), but very scattered. The only significant cluster occurs in the range of 332–363 Ma, with a prominent relative probability peak at 361 Ma. This Mississippian/Devonian zircon age population is much more similar to that reported from the MF first megacycle sandstones (Fig. 7).

A youngest detrital-zircon age is commonly used as a maximum limit for the age of deposition. This is applicable only for the estimation of the Nižná Boca Formation maximum sedimentation age, by the presence of the dominant latest Pennsylvanian/earliest Cisuralian zircon population. This Pennsylvanian/Cisuralian zircon population was consequently redeposited into the all lithostratigraphic horizons of the Malužiná Fm. red beds. The youngest NBF detrital-zircon grains (2 from 15) give the age of 288 ± 4 Ma that corresponds to the Early Cisuralian. Other 13 grains are scattered between 293 and 306 Ma, with the 297 Ma weighted average. As the volcanic lithoclasts are prevailing in the composition of the NBF sandstones, the Late Pennsylvanian/Early Cisuralian zircon provenance source is supposed to be the synsedimentary volcanism. This cluster of zircon ages perhaps signalizes the protracted magmatic events, approximately from 306 to 293 Ma. The probability density age peak at 297 Ma, proves the maximum sedimentation age of the Nižná Boca Fm. to the end of Asselian or overlaps the boundary Asselian/Sakmarian (Fig. 8). This is implied also by macrofloral assemblage, mainly by the occurrence of the Callipteridium gigas Gutbier that was described by Sitár and Vozár (1973) in the upper part of the Nižná Boca Fm. Similarly, Broutin et al. (1999) suggested that fossils of the continental Autunian could be regarded as characteristic of the Latest Gzhelian to the Early Sakmarian time interval. It was ascribed to a sequence of sandstones with bituminous black shales in the Autun Basin of the French Massif Central. The beginning of the Malužiná Fm. deposition is associated with the post-Asselian time, the most likely during the Sakmarian. Thus, the sediments of the first and second MF megacycles could have persisted from the Middle–Late Cisuralian up to Middle Guadalupian, corresponding with the position of the Illawarra Reversal (Vozárová and Tunyi 2003). The following MF third megacycle sediments could have been a post-Illawarra in age, consistent with the Lopingian epoch (Fig. 8). The 260 Ma 87Rb/86Sr age of the basalt sample from the second eruption phase in the MF third megacycle confirms this interpretation (Vozárová et al. 2007). The Autunian–Saxonian microflora assemblages within the MF first and second megacycle sediments and the Thuringian microflora, proved by Planderová (1973, 1979) within the MF third megacycle, are in a good agreement with this interpretation. This suggestion is also confirmed by a strong change in climatic conditions near the NBF and MF lithostratigraphic boundary. While the NBF sediments are grey to black, typical for humid/semi-humid climate, the MF deposits are represented by the semi-arid/arid red beds in all occurrences. The climax of this aridization trend was recognized in the upper part of the second megacycle and is emphasized by the presence of thin evaporite horizons, caliche and pedogenic dolomite nodules. In the European Permian basins, such a strong aridization effect is found near the Sakmarian–Artinskian boundary (Roscher and Schneider 2006) that fits well the proposed stratigraphic inferences.

Fig. 8
figure 8

Stratigraphic scheme of the Hronicum Late Paleozoic sedimentary succession tabulated on the chronostratigraphic correlation chart, based on detrital-zircon age data. The time-scale calibration follows the ICS Chart (2016). IR Illawarra Reversal magnetic horizon, B basic volcanics and volcaniclastics of tholeiitic magmatic trend, D calc-alkaline dacites and their volcaniclastics

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Provenance

As was previously proposed, the prominent Late Pennsylvanian/Early Cisuralian detrital-zircon ages are clearly related to the NBF synsedimentary volcanism. This is confirmed by the presence of the dacitic volcaniclasts that are abundant in the studied sandstone samples (Table 2). The second prominent zircon age cluster is the Mississippian/Devonian ranging from 345 to 371 Ma (Fig. 7). Magmatic rocks with the equal ages are decidedly widespread in the Western Carpathian crystalline basement (WCCB). Recent SIMS zircon data (Kohút et al. 2008, 2009, 2010; Burda et al. 2011; Broska et al. 2013; Vozárová et al. 2016) and monazite chemical data (Finger et al. 2003; Petrík and Konečný 2009) of I- and S-type granitoids from the WCCB indicate ages between 367 and 353 Ma. Similarly, previous TIMS multigrain zircon analysis (Bibikova et al. 1988; Shcherbak et al. 1990) and single grain evaporation ages (Kráľ et al. 1997; Kohút et al. 1997; Poller and Todt 2000; Gaab et al. 2005) indicate nearly synchronous magmatic events in the range 345–356 Ma. SIMS zircon dating from the WCCB high-grade orthogneisses yielded identical ages (371–363 Ma SHRIMP data, Putiš et al. 2008, 2009; 357–363 Ma single evaporation ages; Poller et al. 2000).

Detrital monazite ages, with the dominant apparent ages ranging 340–371 Ma from the MF sandstones of the Malé Karpaty Mts., fit the same time span as the obtained zircon ages and confirmed derivation from the Western Carpathian granitoids (Vozárová et al. 2014). Exceptional are the monazite ages that have been obtained from the MF dacite pebbles of the Nízke Tatry Mts. by Demko and Olšavský (2007) and Olšavský (2008). That magmatic monazite displayed the peak at 360 and 340 Ma with the age dispersion from 310 to 370 Ma and weighted average of 342 ± 12 Ma (22 analyses). All these data are consistent with petrofacial analyses of the Hronicum Permian sandstones in the all Western Carpathian occurrences, indicating the provenance of dissected magmatic arc at an active continental margin (Vozárová and Vozár 1988; Vozárová 1990; Vďačný et al. 2013). Mixing of magmatogenic and volcanogenic detritus, associated with a small amount of low- to medium-grade metamorphic clasts, is common. A dissected magmatic arc tectonic setting was defined by Dickinson and Suczek (1979), Dickinson (1985, 1988) and Ingersoll (1990) by similar petrofacies. Also, the mineral composition of the MF sandstones clearly indicates dominance of plagioclases above K-feldspars, with low K/P ratios ranging from 0.5 to 0.6. Correspondingly, the chemical composition of the MF sandstones reflects the prominent intermediate, tonalitic source that is typical for the magmatic arc tectonic setting (Vďačný et al. 2013).

Smaller clusters among the Hronicum detrital-zircon assemblages are represented by the Cambrian–Ordovician age spanning 446–540 Ma. These data create low peaks in probability density plots, but their time span is wide, with principal peaks at 467 and 503 Ma (Fig. 7). It is necessary to note the dominance of the Cambrian over Ordovician detrital-zircon ages. The Cambrian to Ordovician and lesser Silurian Concordia magmatic ages, with older 525–470 and younger 480–440 Ma ages have been defined in layered amphibolites and orthogneisses of the WCCB Tatricum and Veporicum Units (SHRIMP data Putiš et al. 2008, 2009; single grain evaporation; Gaab et al. 2005; chemical monazite ages; Janák et al. 2002), as well as from the Southern Gemericum acid volcanics (Vozárová et al. 2010, 2016). Thus, the Hronicum Ordovician–Cambrian detrital-zircon assemblage could be, undoubtedly, derived from the WCCB crust. These results fully confirm the previous considerations about the root zone of the Hronicum Nappes (Biely and Fusán 1967; Rakús et al. 1998).

The oldest detrital-zircon populations hold the remarkable features of a manifold recycling, also indicated by their rounded shape. Dominant are two groups of the Precambrian ages, Ediacaran in the range of 545–612 Ma, and Paleoproterozoic–Neoarchean spanning 1.8–2.8 Ga (Fig. 7). The Tonian–Stenian (762–988 Ma, three grains from 224) and the Mesoproterozoic zircon ages (~ 1.2 Ga, two grains from 224) are insignificant. Similar SHRIMP zircon ages have been obtained from the WCCB gneisses, migmatites and igneous rocks where they occur as the zircon inherited cores (Putiš et al. 2008, 2009; Broska et al. 2013 and references therein).

Generally, the presented detrital-zircon ages specify the provenance of the Hronicum Carboniferous–Permian sediments from the Variscan WCCB crust, with the direct link to the Armorican/Cadomian fragments of the European Variscides that has been proved by the study of the Veporicum and Tatricum crystalline rocks by Putiš et al. (2008, 2009). The oldest Hronicum detrital-zircon population resembles the Ediacaran and Paleoproterozoic–Neoarchean zircon ages derived from the peri-Gondwanan Cadomian Belt (545–612 Ma) and the West African Craton provenance (1.8–2.8 Ga). The absence or scarcity of the Mesoproterozoic zircon ages is typical (Fig. 7). Commonly, all the European Armorican terranes are devoid of Mesoproterozoic zircons and have also been interpreted to indicate the West African Craton provenance (Fernández-Suárez et al. 2002, 2014; Linnemann et al. 2008, 2014; Zeh and Gerdes 2010; Drost et al. 2011; Henderson et al. 2016 and references therein).

Contamination of the Variscan WCCB crust by recycling of Cadomian Belt, Eburnian and Archean West African Craton crust is evident. Correspondingly, the SHRIMP detrital-zircon ages from the Early Paleozoic Rakovec Terrane (Fig. 9) of the Northern Gemericum Unit (NGU) also indicate the affinity to the Cadomian Belt (550–650 Ma) and the West African Craton (1.8–2.7 Ga) (Vozárová et al. 2013). Finally, the Late Devonian–Early Mississippian tectono-thermal evolution of the NGU Rakovec Terrane is indicated by the 355 Ma peak of the detrital-zircon ages from the Mississippian syn-collisional basin (the Ochtiná Group sequence; Vozárová et al. 2013). This is identical to the second prominent peak of the Hronicum Pennsylvanian–Permian detrital-zircon ages. The only difference is the absence of the Late Pennsylvanian/Early Cisuralian detrital zircon within the NGU detrital-zircon population, whereas this zircon population is one of the prominent in the considered Hronicum sandstones (Fig. 9). On the contrary, the Southern Gemericum Early Paleozoic Gelnica Terrane and its Permian cover revealed completely different detrital-zircon populations (Vozárová et al. 2012). The zircon clusters dominate the Avalonian–Cadomian arc-derived 560–670 Ma ages and significant older, Tonian–Stenian 0.9–1.1 Ga, Middle Paleoproterozoic 1.8–2.4 Ga and Archean 2.6–2.8 Ga zircon ages. Typical is the absence of the Late Devonian–Early Mississippian zircons in the pre-Upper Permian sediments (Fig. 9). Distribution of the Proterozoic–Archean and Tonian–Stenian detrital zircons and deficiency of the Mesoproterozoic grains indicate more similarity to the Saharan Metacraton than with the West African Craton provenance (Vozárová et al. 2016).

Fig. 9
figure 9

The Northern and Southern Gemericum detrital-zircon ages are taken from Vozárová et al. (2012, 2013)

Normalized probability density curves showing the detrital-zircon ages from the Hronicum, Northern and Southern Gemericum successions.

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The Western Carpathian Carboniferous–Permian depositional basins underwent the compression regime during the final stages of the Variscan Orogeny (Vozárová 1998; Vozárová et al. 2009 and references therein). Climax of the Variscan collisional events is reflected by the deep-water Late Tournaisian/Visean turbidites within the NGU Ochtiná Group, containing prominent the Upper Devonian/Lower Mississippian detrital-zircon population. These intrasutural or fore-deep depositional troughs were evolved not only in the Eastern Alpine–Western Carpathians realm (Nötsch–Veitsch–North Gemeric zone after Neubauer and Vozárová 1990) but the relics of the equivalent sequences were described in the wider domain of the Circum-Pannonian Region related to the sequential suturing of the Variscan orogenic belt (Veitsch–Nötsch–Szabadbattyán–Ochtiná zone after Ebner et al. 2008). The primary basement of the Mississippian Ochtiná zone is partly uncertain. However, there are some indications that the NGU oceanic and arc-related Rakovec and Klátov terranes were amalgamated, or partly diachronous to the onset of the Ochtiná Group accumulation (Vozárová 1996, 1998). The Mississippian foredeep could evolved partly diachronous to the main metamorphic and magmatic events in the Alpine–Carpathian Variscan Belt that formed the WCCB crust. The Western Carpathian crystalline basement has been interpreted as a part of the Mediterranean Crystalline Zone (Flügel 1990; Vozárová 1998; Ebner et al. 2008 and references therein) or the Proto-Tatricum (Broska et al. 2013) as a part of the Galatian superterrane defined by Stampfli et al. (2011) and Stampfli (2012). Broska et al. (2013) considered the Late Devonian–Early Mississippian arc-related I-type magmatism as the main indication of the Paleotethys subduction beginning at the southern active margin of the Proto-Tatricum.

Detrital material derived from the Late Devonian/Early Mississippian magmatic arc has been an integral part of the sedimentary filling in the Middle/Late Mississippian NGU Ochtiná Group, as well as in the Hronicum Pennsylvanian/Permian sedimentary troughs. These depositional tracks are reflected by the presence of the Late Devonian–Early Mississippian detrital-zircon assemblages that were recognized within the both successions (Vozárová et al. 2013 and present paper). The main differences between these basins are related to the time of their formation and their tectonic relation to the Variscan orogenic belt. The beginning of the NGU Ochtiná basin is dated to the late Tournaisian–Visean and the opening of the Hronicum basin to the latest Moscovian–Kasimovian. Position of the NGU Ochtiná basin implies that has been formed upon a subducted or underthrusted plate during the first stage of collision as the peripheral foredeep. On the contrary, the Hronicum Pennsylvanian–Permian basin followed the post-orogenic transpressional/transtensional regime. This has been formed as the retroarc foreland basin located behind the magmatic arc on the continental crust.

Conclusions

The new U–Pb detrital-zircon ages data from the Hronicum Pennsylvanian–Permian sandstones have been obtained. The conclusions may be summarized as follows:

  1. 1.

    U–Pb SHRIMP detrital zircon ages revealed two dominant populations, first, in the range of 285–309 Ma, with a prominent peak at 295 Ma and the second spanning 345–371 Ma, with a prominent peak at 356 Ma. Only small age clusters are represented by the Cambrian–Ordovician ages in the range of 540–446 Ma. Among the scattered Precambrian detrital-zircon grains, two age groups are dominant; the Ediacaran in the range of 545–612 Ma, and the Paleoproterozoic–Neoarchean in the range of 1.8–2.8 Ga.

  2. 2.

    The probability density age peak at 297 Ma permits to shift the maximum sedimentation age of the Nižná Boca Fm. until the end of Asselian. Thus, the beginning the Malužiná Formation deposition has been determined at least in Sakmarian. The sediments of the first and second MF megacycles could have been accumulated from the Middle Cisuralian up to Middle Guadalupian, restricted by the time of the Illawarra Reversal magnetic horizon (~ 265 Ma) foundation. The third MF megacycle sediments could have been a post-Illawarra in age, mainly relative to the Lopingian. Their high mineral maturity points out to the vague break in sedimentation that could be after, or nearly synchronous with the Guadalupian geomagnetic Illawarra Reversal event.

  3. 3.

    Preferential detrital-zircon sources of the Hronicum Pennsylvanian–Permian sediments were the Cisuralian synsedimentary rift-related volcanism and the Late Devonian/Early Mississippian magmatic arc. Dominant magmatic character of zircons is supported by their composition, morphology and oscillation growth zonality. The zircon age spectra support the close relations with the Armorican terranes and derivation from the Cadomian Belt, which were associated with the West African Craton during the Neoproterozoic and Cambrian time.

  4. 4.

    The continental Hronicum Pennsylvanian/Permian depositional trough was created in the Variscan post-orogenic stages, as the transtensional/extensional retro-arc depositional basin originated on the continental crust, behind the active continental margin.