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In situ garnet Lu/Hf and mica Rb/Sr geochronology provides new insights on Eo-Alpine metamorphism in the Veporic Superunit of the Western Carpathians

Published online by Cambridge University Press:  16 October 2026

Jaroslaw Majka Open the ORCID record for Jaroslaw Majka [Opens in a new window] ,
Christopher J. Barnes  and
Marian Janák
Jaroslaw Majka*
Affiliation:
Department of Earth Sciences, Uppsala University , Uppsala, Sweden Faculty of Geology, Geophysics and Environmental Protection, AGH University of Krakow , Kraków, Poland
Christopher J. Barnes
Affiliation:
Department of Earth Sciences, Uppsala University , Uppsala, Sweden Institute of Geological Sciences, Kraków Research Centre, Polish Academy of Sciences, Kraków, Poland Department of Earth and Environmental Sciences, The University of British Columbia Okanagan, Kelowna, Canada
Marian Janák
Affiliation:
Earth Science Institute, Slovak Academy of Sciences, Bratislava, Slovakia
*
Corresponding author: Jaroslaw Majka; Email: jaroslaw.majka@geo.uu.se
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Abstract

Eo-Alpine metamorphism in the Western Carpathians is mostly recorded by low-to-medium grade rocks of the Veporic Superunit. However, the multimethod geochronological data from this key unit to understand Alpine evolution of the Western Carpathians are scarce. To better understand the timing of burial (subduction), peak conditions and post-peak exhumation, we applied in situ garnet Lu/Hf and mica Rb/Sr geochronology. Garnet yielded a somewhat imprecise, yet valid and geologically meaningful, age of ∼116 Ma interpreted as a record of subduction. In contrast, biotite Rb/Sr ages of ∼91 Ma and ∼75 Ma record resetting in the near-peak conditions upon the earliest stages of exhumation and fluid-aided re-equilibration, respectively. A white mica Rb/Sr dating also reflects Alpine disturbance of the isotopic system. The geochronologic data presented herein, together with the other already published geochronologic results, allow us to formulate a coherent subduction-exhumation model for the Veporic Superunit in an intracontinental setting. The upper plate for this subduction system would be a continental block belonging to Adria. We postulate that the rocks in question were subducted due to prior thinning of the continental crust, much as in the Eastern Alps, where the Lower Central Austroalpine units were subducted beneath the Adria plate. Altogether, these temporal and tectonic similarities demonstrate that the major allochthonous units in the Western Carpathians could indeed be equivalents to those defined in the Eastern Alps.

Keywords

CarpathiansAlpsgeochronologyVepor Mtsintracontinental subduction

Information

Type
Original Article
Information
Geological Magazine , Volume 163 , 2026 , e58
Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
Copyright
© The Author(s), 2026. Published by Cambridge University Press

1. Introduction

The Western Carpathians, especially their central domain, form a sequence of allochthons resembling the Austroalpine thrust sheets of the Eastern Alps. Among the Western Carpathian allochthons, the Veporic Superunit is commonly correlated with the Lower Central Austroalpine units of the Eastern Alps (Figure 1; Froitzheim et al. Reference Froitzheim, Plašienka, Schuster and McCann2008). These units bear unique evidence for Eo-Alpine high-pressure (HP) to ultrahigh-pressure (UHP) metamorphism along a steep geothermal gradient attributed to intracontinental subduction (Janák et al. Reference Janák, Froitzheim, Lupták, Vrabec and Ravna2004, Reference Janák, Petrik, Plašienka, Froitzheim and Konečny2025; Stüwe and Schuster, Reference Stüwe and Schuster2010; Miladinova et al. Reference Miladinova, Froitzheim, Nagel, Janák, Fonseca, Sprung and Münker2022). In this context, the Veporic Superunit represents a key puzzle piece in the orogenic history of the Alps-Carpathians megaorogenic belt (e.g. Janák et al. Reference Janák, Plašienka, Frey, Cosca, Schmidt, Lupták and Méres2001, Reference Janák, Petrik, Plašienka, Froitzheim and Konečny2025; Schmid et al. Reference Schmid, Fügenschuh, Kissling and Schuster2004; Plašienka, Reference Plašienka2018) to link the two major European mountain belts (Plašienka and Bielik, Reference Plašienka and Bielik2024). Despite the key position of the Veporic Superunit within the tectonic framework of the Western Carpathians and the entire Alps-Carpathians belt, the geochronological information of its metamorphic evolution is relatively limited. The two available geochronological studies that targeted metamorphic monazite revealed similar ∼97–96 Ma ages interpreted to reflect prograde-to-peak stages of metamorphism (Bukovská et al. Reference Bukovská, Jeřábek, Lexa, Konopásek, Janák and Košler2013; Janák et al. Reference Janák, Petrik, Plašienka, Froitzheim and Konečny2025). The other published studies have focused mostly on the cooling of the entire Veporic thrust sheet. The ages obtained in the course of those studies range between 90 and 55 Ma (e.g. Janák et al. Reference Janák, Plašienka, Frey, Cosca, Schmidt, Lupták and Méres2001; Putiš et al. Reference Putiš, Frank, Plašienka, Siman, Sulák and Biroň2009; Vojtko et al. Reference Vojtko, Králiková, Jeřábek, Schuster, Danišík, Fügenschuh, Minár and Madarás2016). In this paper, we present an in situ geochronologic approach to date Eo-Alpine metamorphism of a key outcrop of the Veporic Superunit. We provide pioneering in situ Lu/Hf garnet and Rb/Sr biotite ages, confirming Cretaceous (Aptian to Santonian) timing of prograde-to-retrograde metamorphism due to intracontinental subduction in the Western Carpathians. The new geochronological information permits further speculation on the close similarities between the Western Carpathians and the Eastern Alps tectonic histories.

Figure 1.

A map of Alpine belt in Europe, modified from Wikimedia Commons 358 (https://commons.wikimedia.org/wiki/File:Tectonic_map_Mediterranean_EN.svg; username: 359 Woudloper; Creative Commons Attribution-Share Alike 1.0 Generic license: 360 https://creativecommons.org/licenses/by-sa/1.0/deed.en). The broad study area is marked by a black rectangle, referring to the map to the right of it. The latter shows a distribution of medium- to high-grade metamorphic complexes, on the sketch map of principal tectonic units of the Eastern Alps and Western Carpathians and their foreland (modified after Janák et al. Reference Janák, Plašienka, Frey, Cosca, Schmidt, Lupták and Méres2001, Reference Janák, Petrik, Plašienka, Froitzheim and Konečny2025). Abbreviations: PAFZ – Peri-Adriatic fault zone; VB – Vienna basin; RFZ – Rába fault zone; HDFZ – Hurbanovo–Diósjenő fault zone; MHFZ – Mid-Hungarian fault zone. A red circle marks the sampling site and the study area.

A map of the Alpine-Carpathian belt in Europe, highlighting tectonic units and fault zones.
Figure 1. Long description

Panel A: A broad map of the Alpine-Carpathian belt in Europe. The map shows various tectonic units, including the Eurasian plate, African plate, Iberian plate, and the Alpine fold and thrust belt. Key locations such as the Calabrian arc, Dinarides, Hellenides, Apennines, and the Western Carpathians are labeled. The map also includes symbols for major faults, thrust faults, strike-slip faults, and normal faults. Panel B: A detailed map of the Eastern Alps and Western Carpathians. This map highlights the distribution of medium- to high-grade metamorphic complexes and principal tectonic units. Key areas such as the Bohemian Massif, Molasse Zone, Northern Calcareous Alps, and the Western Carpathians are labeled. Fault zones such as the Giudicarie fault zone, Peri-Adriatic fault zone, Vienna basin, Rába fault zone, Hurbanovo-Diósjenő fault zone, and Mid-Hungarian fault zone are marked. A red circle indicates the study area and sampling site.

2. Geological setting

The Veporic Superunit forms a metamorphic core complex with its deepest exposed rock sequences having undergone Eo-Alpine metamorphism (e.g. Lupták et al. Reference Lupták, Janák, Plašienka, Schmidt and Frey2000; Janák et al. Reference Janák, Plašienka, Frey, Cosca, Schmidt, Lupták and Méres2001; Jeřábek et al. Reference Jeřábek, Faryad, Schulmann, Lexa and Tajčmanová2008). It tectonically overlies the Tatric allochthon, while it is overthrust by the Gemeric thrust sheet (Figure 1). The Veporic crystalline rocks are represented by a metasedimentary succession with minor metavolcanics and orthogneisses intruded by a Variscan granitoid. In more detail, the entire superunit is divisible into several subunits comprising (1) generally amphibolite facies orthogneisses, migmatitic parganeisses and garnet-bearing mica schists of the Muráň, Ostrá and Klenovec complexes, (2) the greenschist facies metasedimentary Hladomorná dolina Complex, (3) the Kráľova hoľa granitoid plutonic complex, and (4) slightly to not metamorphosed, Pennsylvanian-Permian and Triassic sedimentary successions, covering the Variscan basement (Figure 2). The Veporic Superunit is polymetamorphic and bears a record of Variscan medium-pressure metamorphism of Barrovian type, with 22–27°C km−1 geothermal gradient reaching up to amphibolite facies conditions (Jeřábek et al. Reference Jeřábek, Faryad, Schulmann, Lexa and Tajčmanová2008, Reference Jeřábek, Lexa, Schulmann and Plašienka2012), and a subsequent Eo-Alpine metamorphic event. Eo-Alpine metamorphism, related to the Cretaceous orogeny (e.g. Vrána, Reference Vrána1964; Vozárová, Reference Vozárová1990, Méres and Hovorka, Reference Méres and Hovorka1991; Plašienka et al. Reference Plašienka, Janák, Lupták, Milovský and Frey1999; Lupták et al. Reference Lupták, Janák, Plašienka, Schmidt and Frey2000, Reference Lupták, Janák, Plašienka and Schmidt2003; Janák et al. Reference Janák, Plašienka, Frey, Cosca, Schmidt, Lupták and Méres2001; Jeřábek et al. Reference Jeřábek, Faryad, Schulmann, Lexa and Tajčmanová2008) shows increasing P-T conditions from low-grade metamorphism in the metasediments of the Permian-Triassic cover (Lupták et al. Reference Lupták, Janák, Plašienka and Schmidt2003) up to 12–14 kbar and 600°C in the deepest exposed levels (Janák et al. Reference Janák, Plašienka, Frey, Cosca, Schmidt, Lupták and Méres2001, Reference Janák, Petrik, Plašienka, Froitzheim and Konečny2025, Jeřábek et al. Reference Jeřábek, Faryad, Schulmann, Lexa and Tajčmanová2008), reflecting a steep metamorphic field gradient of ∼15°C km−1 characteristic for continental subduction zones (Peninston-Dorland et al. Reference Penniston-Dorland, Kohn and Manning2015).

Figure 2.

Geological sketch map of the eastern part of the Veporic Superunit and overlying units. Metamorphic zones and isotherms according to Janák et al. (Reference Janák, Plašienka, Frey, Cosca, Schmidt, Lupták and Méres2001) and Jeřábek et al. (Reference Jeřábek, Faryad, Schulmann, Lexa and Tajčmanová2008). 1 – chloritoid + chlorite + garnet zone, 2 – garnet + staurolite + chlorite zone, 3 – staurolite + biotite + kyanite zone. Taken from Janák et al. (Reference Janák, Petrik, Plašienka, Froitzheim and Konečny2025), modified.

Geological sketch map of the eastern part of the Veporic Superunit and overlying units.

3. In situ mica Rb/Sr and garnet Lu/Hf geochronology – methodology

Geochronological analysis of both garnet (Lu/Hf) and mica (biotite and muscovite; Rb/Sr) was performed at the Fipke Laboratory for Trace Element Research (University of British Columbia Okanagan, Kelowna, Canada). Analysis was performed using an ESL NWR 193 excimer laser ablation (LA) apparatus with a TwoVol3 ablation cell connected to an Agilent 8900 inductively coupled plasma reaction cell mass spectrometer (ICP-MS/MS). The Lu/Hf geochronology setup follows the protocol of Larson et al. (Reference Larson, Cottle, Button, Dyck, Lihter and Shrestha2024) that is based on Simpson et al. (Reference Simpson, Gilbert, Tamblyn, Hand, Spandler, Gillespie, Nixon and Glorie2021) and Wu et al. (Reference Wu, Yang, Wang, Roberts, Niu, Wang, Yang and Wu2024). The Rb/Sr geochronology setup follows the protocol of Larson et al. (Reference Larson, Button, Shrestha and Camacho2023) that is based on Zack and Hogmalm (Reference Zack and Hogmalm2016) and Redaa et al. (Reference Redaa, Farkaš, Gilbert, Collins, Wade, Löhr, Zack and Garbe-Schönbergf2021).

Garnet and mica were both ablated in situ in standard thin sections (30 µm thickness). Sites for laser ablation were selected away from grain boundaries to avoid ablating into other matrix minerals in the subsurface surrounding the minerals of interest. The mica was also selected with visible cleavage planes to reduce potential delamination of silicate sheets if ablating (sub-)parallel to the c-axis. Laser ablation of mica was carried out using a 5 Hz repetition rate and fluence of ∼3.5 J/cm3. Laser ablation of garnet was conducted with a 10 Hz repetition rate and a fluence of ∼4.0 J/cm3. For both, a 10 second laser warm-up preceded each ablation, followed by 30 seconds of ablation, and 15 seconds of post-ablation signal washout (totalling 25 seconds of washout time that includes the succeeding 10 second laser warm-up). Two pulses of the laser are done prior to commencement of the analytical session. The ablation cell used He as a carrier gas at a flow rate of 350 mL/min, mixed on-line with N2 at a flow rate of 4 mL/min, and Ar nebulizer gas at 0.85 mL/min. For Lu/Hf geochronology, NH3 was the reaction cell gas at a flow rate of 25%. For Rb/Sr geochronology, N2O was used in the reaction cell at a flow rate of 25%. All parameters for Lu/Hf and Rb/Sr geochronology can be found in Tables S1 and S2.

The following reference materials were used for Lu/Hf geochronology: SRM NIST610 and SRM NIST612 (Jochum et al. Reference Jochum, Weis, Stoll, Kuzmin, Yang, Racze, Jacob, Stracke, Birbaum, Frick, Günther and Enzweiler2011), Högsbo garnet (1029 ± 1.7 Ma; Romer and Smeds, Reference Romer and Smeds1994), GWA-1 garnet (1267.0 ± 3.0 Ma; Ribeiro et al. Reference Ribeiro, Kirkland, Smit, Musiyachenko, Korhonen, Evans, Rankenburg, McDonald, Glorie, Gilbert, Goemann, Belousov, Oalmann, Clark and Makin2024), GWA-2 garnet (934.7 ± 1.4 Ma; Ribeiro et al. Reference Ribeiro, Kirkland, Smit, Musiyachenko, Korhonen, Evans, Rankenburg, McDonald, Glorie, Gilbert, Goemann, Belousov, Oalmann, Clark and Makin2024) and Skaha garnet (72.4 ± 0.6 Ma; Larson et al. Reference Larson, Dyck, Cottle, Canam and Milkowski2025). All reference materials and unknowns were ablated using a beam diameter of 150 µm, with the exception of Högsbo garnet that was analysed with a beam diameter of 80 µm. The reference materials were programmed in blocks between sequences of 15 unknown analyses. The list of analytes and their dwell times are found in Table S1.

The following reference materials were used for Rb/Sr geochronology: SRM NIST610 and SRM NIST612 (Jochum et al. Reference Jochum, Weis, Stoll, Kuzmin, Yang, Racze, Jacob, Stracke, Birbaum, Frick, Günther and Enzweiler2011), 1B phlogopite (990 ± 6 Ma; Camacho et al. Reference Camacho, Lee, Fitzgerald, Zhao, Abdu, Jenkins, Hawthorne, Kyser, Creaser, Armstrong and Heaman2012), 1O phlogopite (986 ± 5 Ma; Camacho et al. Reference Camacho, Lee, Fitzgerald, Zhao, Abdu, Jenkins, Hawthorne, Kyser, Creaser, Armstrong and Heaman2012), La Posta biotite (89.2 ± 1.0 Ma, 91.6 ± 1.2 Ma; Zack and Hogmalm, Reference Zack and Hogmalm2016; Rösel and Zack, Reference Rösel and Zack2022), MA1 muscovite (in-house dates of c. 360–340 Ma; Kellett et al. Reference Kellett, Larson and Skipton2024), MicaMg nanopowder (519.4 ± 6.5 Ma, 519.7 ± 1.3 Ma; Hogmalm et al., Reference Hogmalm, Zack, Karlsson, Sjöqvist and Garbe-Schönberg2017; Rösel and Zack, Reference Rösel and Zack2022) and MicaFe nanopowder (299 ± 4 Ma, 305.4 ± 2.0 Ma, 307.6 ± 0.4 Ma; Grove and Harrison, Reference Grove and Harrison1996; Redaa et al. Reference Redaa, Farkaš, Gilbert, Collins, Löhr, Vasegh, Forster, Blades, Zack, Giuliani, Maas, Baldermann, Dietzal and Garbe-Schönberg2022; Rösel and Zack, Reference Rösel and Zack2022). All reference materials and unknowns were ablated with a beam diameter of 70 µm, and all reference materials were programmed in blocks separating sequences of 15 unknown analyses. The list of analytes and their dwell times are found in Table S1.

The geochronological ratios from the raw measured data are calculated with Iolite software (v. 4.10; Paton et al. Reference Paton, Hellstrom, Paul, Woodhead and Hergt2011) using in-house data reduction schemes for both Lu/Hf and Rb/Sr (Larson et al. Reference Larson, Button, Shrestha and Camacho2023, Reference Larson, Cottle, Button, Dyck, Lihter and Shrestha2024). The data reduction schemes include background subtraction and correction for both instrument drift and downhole fractionation of the raw data (Paton et al. Reference Paton, Woodhead, Hellstrom, Hergt, Greig and Maas2010). SRM NIST610 was used as the primary calibration reference material for both Lu/Hf and Rb/Sr geochronological data. The time-integrated profiles of each laser ablation analysis were monitored to remove data anomalies due to, e.g. subsurface inclusions, fractures, mineral intergrowths or poor ablation behaviour of the mineral (e.g. mica delamination). Unstable ablation signals were excluded from the final dataset. Results were exported from Iolite and processed further to correct pulse/analog (P/A) detector conversion and matrix corrections. The P/A factors for Lu/Hf geochronology were computed by comparing measured 176Lu/176Hf against the expected value of 0.47714 for SRM NIST612. The P/A factors for Rb/Sr geochronology were calculated by comparing 87Sr/86Sr against 87Sr/86Sr88 (denoting 86Sr calculated from measured 88Sr). Matrix corrections were performed using Högsbo garnet for Lu/Hf and 1B phlogopite for Rb/Sr. The isochron dates for both reference materials were calculated using IsoplotR (Vermeesch, Reference Vermeesch2018). The resulting date was converted to 176Hfrad/176Lu and 87Srrad/87Rb values based on their respective decay constants (0.00001867 Myr−1 from Söderlund et al. Reference Söderlund, Patchett, Vervoort and Isachsen2004 for 176Lu; 0.000013972 Myr−1 from Villa et al. Reference Villa, De Bièvre, Holden and Renne2015 for 87Rb) and then corrected to the ratio that corresponds to the expected dates of the two materials. The resulting correction factor was applied to 176Lu/177Hf and 87Rb/86Sr values for all other secondary reference materials and unknowns. Isochrons for the reference materials and unknowns as well as single-spot dates (where necessary) were computed from the final ratios using IsoplotR. The reported isochron dates incorporate dispersion from measurements, excess dispersion of the analytical session, uncertainties of the decay constants and propagated uncertainties from the reference material used for matrix correction.

The Lu/Hf secondary reference materials yielded the following isochron dates using the default initial 176Hf/177Hf value in IsoplotR: Högsbo garnet: 1030 ± 11 Ma (MWSD: 2.4; n: 23); GWA-1 garnet: 1284 ± 36 Ma (MWSD: 1.4; n: 23); GWA-2 garnet: 943 ± 49 Ma (MWSD: 1.8; n: 23); Skaha garnet: 68 ± 8 Ma (MWSD: 0.9; n: 23). The Rb/Sr secondary reference materials provided the following isochron dates using initial 87Sr/86Sr values pertinent to each reference material: 1B phlogopite: 990 ± 8 Ma (MWSD: 2.3; n: 21) with an initial 87Sr/86Sr value of 0.7035 (Camacho et al. Reference Camacho, Lee, Fitzgerald, Zhao, Abdu, Jenkins, Hawthorne, Kyser, Creaser, Armstrong and Heaman2012); 1O phlogopite: 969 ± 7 Ma (MWSD: 1.8; n: 19) with 0.7035 (Camacho et al. Reference Camacho, Lee, Fitzgerald, Zhao, Abdu, Jenkins, Hawthorne, Kyser, Creaser, Armstrong and Heaman2012); La Posta biotite: 98 ± 2 Ma (MWSD: 0.9; n: 21) with 0.7049 (Rösel and Zack, Reference Rösel and Zack2022); MA1 muscovite: 370 ± 3 Ma (MWSD: 20; n: 21) with 0.7500; MicaMg: 519 ± 4 Ma (MWSD: 13; n: 21) with 0.7261 (Rösel and Zack, Reference Rösel and Zack2022); MicaFe: 313 ± 2 Ma (MWSD: 7.3; n: 21) with 0.7261 (Rösel and Zack, Reference Rösel and Zack2022).

The garnet chemical data were calculated from the background-subtracted counts per second (CPS) using the trace element data reduction scheme in Iolite. The primary reference material was defined as SRM NIST610, and Al was used as the internal standard with a fixed value at 11.98 wt% for recalculating the unknown garnet results. This approach was chosen as Al wt% does not significantly vary between the four major garnet endmembers in metapelitic rocks, and not all cations of the garnet were measured. Major and trace element chemical data of the mica were calculated using a mineral chemistry recalculation spreadsheet of Barnes et al. (Reference Barnes, Zack, Bukała, Rösel, Mark and Schneider2024), which uses the major and trace element background-subtracted CPS of secondary mica and unknowns along with SRM NIST610 and SRM NIST612 as the primary reference materials for computation (see Barnes et al. Reference Barnes, Zack, Bukała, Rösel, Mark and Schneider2024 for explanation of the spreadsheet). This spreadsheet is used in order to allow all major and trace elements to be variable for the mica due to the complexity of substitutions that can affect all cation sites of the mica.

4. Results

4.1. Petrography, mineral chemistry

The dated sample (V-9) is a garnet-bearing mica schist from the staurolite + biotite + kyanite zone of the Ostrá Complex, i.e. representative of the highest-grade Eo-Alpine metamorphic zone in the southern Veporic Superunit (Figure 2). The same sample was investigated by Janák et al. (Reference Janák, Petrik, Plašienka, Froitzheim and Konečny2025) in terms of metamorphic P-T conditions and Th-U-Pb dating of monazite (see Janák et al. Reference Janák, Petrik, Plašienka, Froitzheim and Konečny2025 for details).

The Alpine metamorphic assemblage includes garnet, white mica (muscovite and paragonite), biotite, chlorite, plagioclase, quartz and minor tourmaline and staurolite in the matrix (Figure 3). Accessory minerals are rutile, ilmenite, zircon, allanite (REE-epidote) and monazite. The peak assemblage is represented by garnet, phengite, paragonite, biotite and rutile, formed at 12–14 kbar and 560–600°C (Figure 6).

Figure 3.

A microscopic view of the dated garnet mica schist in plane-polarized light (a); BSE image of idiomorphic garnet containing quartz and rutile (b); X-ray chemical maps showing distribution of Ca, Mg and Mn (c–e), and WDS chemical profile showing prograde, one-step chemical zoning in the garnet (f).

Microscopic images and chemical maps of garnet mica schist.

Garnet is idioblastic, containing inclusions of quartz, ilmenite and allanite (REE-epidote) in the cores and rutile in the rims (Figure 3). Garnet composition is almandine (69 to 75 mol%) with 12 to 23 mol% grossular, 5 to 12 mol% pyrope and 1 to 3 mol% spessartine end members. Garnet is zoned with the almandine and pyrope components increasing, and spessartine, grossular and Fe/(Fe+Mg) ratio decreasing from core to rim; an oscillatory zoning with Ca enrichment and Fe depletion is observed in the rim (Figure 3c–f). Overall, garnet zoning recorded by major elements suggests a prograde growth during a single metamorphic event and only minor retrogression Figure 3). White mica compositions range from phengite (up to Si = 3.21 apfu) to muscovite and paragonite. Biotite shows annite to phlogopite composition with Mg/(Mg+Fe) ratio = 0.5–0.6, TiO2 content varies from 1.5 to 1.6 wt%. Plagioclase compositions range from An17 to An20. Staurolite is Fe-rich with Fe/(Fe+Mg) ratio = 0.8 and 2.5 wt% ZnO. Staurolite is a retrograde phase, not belonging to the peak metamorphic assemblage but formed during decompression (Figure 6). Thus, the studied rock was formed at a quite steep geothermal gradient, definitely cooler than in a Barrovian regime.

4.2. In situ garnet Lu/Hf geochronology

Ninety-three results of in situ garnet Lu/Hf geochronology were accepted from one hundred and ten analyses. A positive correlation between Ca (2.7–4.6 wt%) and Ti (177–550 µg/g) is recognized in the chemical data; however, there are no apparent correlations with garnet chemistry and Lu/Hf geochronological data (Table S2). Overall, the garnets contain low amounts of Lu and Hf, with averages of 1.3 and 0.1 ppm, respectively. The resulting 175Lu/176Hf ratios are low, ranging from 0.14 to 42.60, but most values are below 6.91. The resulting isochron of all data yields a date of 116 ± 48 Ma (MSWD: 1.4), with the high uncertainty due to the low Lu concentration in the garnets (Figure 4).

Figure 4.

Results of in situ garnet Lu/Hf geochronology are presented as an isochron. The uncertainties of the ellipses are 2SE. The isochron is anchored to an initial 176Hf/177Hf value of 0.2829 ± 0.003.

A scatter plot showing the relationship between two variables related to garnet geochronology.

4.3. In situ mica Rb/Sr geochronology

For biotite, sixty-four results from seventy analyses were accepted for statistical evaluation. Compared to the white mica, the biotite results are more dispersed and overall provide much higher values of 87Rb/86Sr (16.01–1135.18) and 87Sr/86Sr (0.7180–2.0767). The biotite results appear to provide an unanchored isochron date of 87 ± 1 Ma (MSWD: 2.4; 0.6951 ± 0.0025). However, the MSWD is slightly greater than 1, and the initial 87Sr/86Sr value that is below the non-radiogenic solar system value of 0.6994 (Compston et al. Reference Compston, Berry, Vernon, Chappell and Kaye1971), suggests a mixed population. Calculating single-spot dates for the individual results using an initial 87Sr/86Sr value of 0.7 provides a dispersion of dates from 99 ± 21 Ma to 63 ± 11 Ma. Weighted averages calculated from the ten oldest and ten youngest are 91 ± 2 Ma (MSWD: 0.5) and 75 ± 3 Ma (MSWD: 0.9), respectively (Figure 5). The choice of an initial 87Sr/86Sr value of 0.7 implies that the model ages should be understood as maximum estimates, as the true initial value is likely >0.7. However, the older dates correspond to higher 87Rb/86Sr (291.94 vs. 133.47) and 87Sr/86Sr (1.0715 vs. 0.8395) and are less sensitive to the initial 87Sr/86Sr value chosen. For example, an initial 87Sr/86Sr value of 0.72 would result in weighted averages of 87 ± 2 Ma and 41 ± 6 Ma. We opt to report the dates using 0.7, as we can conclude the weighted averages are maximum dates instead of choosing an arbitrary initial value that could be above or below the true value, resulting in uncertainty if the dates are artificially younger or older than the true geological ages. Along with the Rb/Sr isotope variations, the weighted averages also show correlations with mineral chemistry. The older dates compared to the younger dates are higher in average Li (155 vs. 123 µg/g), Na (2059 vs. 1470 µg/g), Rb (679 vs. 555 µg/g), and Ba (432 vs. 318 µg/g), and lower in Mg (7.1 vs. 7.4 wt%), Co (68 vs. 83 µg/g), and Sr (12 vs. 19 µg/g). Notably, the older dates also have higher 87Rb/86Sr (291.94 vs. 133.47) and 87Sr/86Sr (1.0715 vs. 0.8395). See Table S3 for the full dataset.

Figure 5.

Results of in situ biotite Rb/Sr geochronology. The results are presented as an isochron for biotite (a). The biotite data are also presented as (b) an array of single-spot dates. The two isochrons are unanchored, and the resulting initial 87Sr/86Sr values are reported. The biotite single-spot dates were calculated using an initial 87Sr/86Sr value of 0.700 ± 0.001. The ten oldest and youngest single dates were used to calculate weighted averages and are highlighted in dark purple. All uncertainties are 2SE.

Two graphs depict biotite Rb/Sr geochronology results.

Ninety analyses of white mica were performed, out of which, eighty-nine results were accepted for further statistical treatment. The eighty-nine results provide clustered and overall low values of 87Rb/86Sr (0.38–5.63) and 87Sr/86Sr (0.7252–0.7415) and a range of Fe/Al values from 0.125 to 0.015. All results provide an imprecise unanchored isochron date of 59 ± 40 Ma (MSWD: 0.6; initial 87Sr/86Sr: 0.7320 ± 0.0019; Figure 5). When divided according to the 25 highest and lowest Fe/A; values, unanchored isochrons of 151 ± 127 Ma (MSWD: 0.5; initial 87Sr/86Sr: 0.7264 ± 0.0077) and 15 ± 107 Ma (MSWD: 1.0; initial 87Sr/86Sr: 0.7330 ± 0.0029). The results suggest two potential generations of white mica with distinct dates. However, the low Rb/Sr ratios of the results that lead to the large uncertainties of the isochron dates also prescribe the isochron date to be highly sensitive of the initial 87Sr/86Sr value, thereby preventing further interpretation of the data.

5. Discussion and final remarks

The Lu-Hf in situ garnet dating results provide unequivocal evidence for Eo-Alpine metamorphism, which affected the Veporic Superunit. According to the previous works (Janák et al. Reference Janák, Plašienka, Frey, Cosca, Schmidt, Lupták and Méres2001, Reference Janák, Petrik, Plašienka, Froitzheim and Konečny2025), this metamorphic cycle occurred along a steep geothermal gradient reaching a maximum P-T conditions of 12–14 kbar and 560–600°C (Figure 6). The growth of monazite has been dated to ∼97–96 Ma (Bukovská et al. Reference Bukovská, Jeřábek, Lexa, Konopásek, Janák and Košler2013; Janák et al. Reference Janák, Petrik, Plašienka, Froitzheim and Konečny2025). According to Janák et al. (Reference Janák, Petrik, Plašienka, Froitzheim and Konečny2025), monazite formed from allanite via a prograde allanite-monazite transition. The presence of allanite and the absence of monazite inclusions in garnet thus suggest that the growth of garnet must have commenced earlier than monazite, i.e. at ∼116 Ma (Figure 6). Previous Sm-Nd dating of garnet in the mica schist yielded the age of 108.8 ± 5.6 Ma (Lupták et al. Reference Lupták, Thöni, Janák and Petrik2004). In this light, the ∼116 Ma Lu-Hf garnet age is not in conflict with the thus far obtained geochronologic data, although this age has to be taken with caution due to its large uncertainty resulting from the low Lu content in the dated garnet. Nonetheless, the Veporic Superunit must have undergone a burial to at least mid-crustal levels already in the Albian, and perhaps as early as in the Aptian. This is also corroborated by ∼116–113 Ma K/Ar and 40Ar/39Ar ages obtained for white mica from subcrops of the Veporic Superunit farther south, in Hungary (Balogh Reference Balogh1984; Koroknai et al. Reference Koroknai, Horvath, Balogh and Dunkl2001). However, the latter are likely reflecting cooling after the peak conditions, yet pointing toward Albian metamorphism. Thus, we interpret the obtained Lu-Hf date as an imprecise estimate of Albian-Aptian burial/subduction of the Veporic Superunit. The Rb/Sr dates calculated in the course of this study should be treated as the maximum age estimates. The older biotite Rb/Sr date of ∼91 Ma is interpreted to reflect likely resetting due to the thermal peak, and shows the very initial stages of cooling of the entire unit. However, the younger biotite Rb/Sr date of ∼75 Ma reflects chemical re-equilibration, likely mediated by fluids, evinced by the increase of Mg, Co, and Sr, and decrease of Li, Na, Rb and Ba with respect to the older biotite volumes. Chemical re-equilibration of the biotite coincides with in situ 40Ar/39Ar white mica ages of 77–72 Ma obtained from the same geological unit in the close vicinity of the rock investigated here (Janák et al. Reference Janák, Plašienka, Frey, Cosca, Schmidt, Lupták and Méres2001), which were interpreted as cooling ages. Together, these dates indicate that the Veporic Superunit had begun exhuming by ∼91 Ma and continued through ≤72 Ma. The Rb/Sr white mica dating results might only be taken as another indication of isotopic system disruption during Alpine metamorphism, but they cannot be easily linked to any particular process. Notwithstanding the imprecise muscovite dates, the thermochronological pattern obtained here is similar, though a bit older, to that deduced by Vojtko et al. (Reference Vojtko, Králiková, Jeřábek, Schuster, Danišík, Fügenschuh, Minár and Madarás2016) for the Kráľova Hoľa Complex (north of this study). The whole-rock Rb/Sr dates obtained by those authors are dispersed from 81 to 61 Ma. They suggest that the youngest signal reflects chloritization of biotite, i.e. a fluid-mediated re-equilibration.

Based on the multimethod geochronology presented here and in other geochronologic works published so far (Janák et al. Reference Janák, Petrik, Plašienka, Froitzheim and Konečny2025 and references therein) as well as the available P-T estimates, it could be concluded that the studied portion of the Veporic Superunit must have subducted down to at least 40–45 km already in the Aptian-Albian times, culminating most likely in the Cenomanian. The subduction must have occurred beneath another crustal domain farther south, most likely the continental Gemeric Superunit. A comparison to the tectonic scenario for the Eastern Alps proposed by Miladinova et al. (Reference Miladinova, Froitzheim, Nagel, Janák, Fonseca, Sprung and Münker2022 and references therein) would place the Veporic Superunit indeed as an equivalent to the Lower Central Austroalpine, while the Gemeric Superunit would act as the overriding allochthon, similarly to the Upper Central Austroalpine (Figure 7). The most likely candidate for a proper upper plate in such intracontinental subduction would be the Pelso block of the Adria microcontinent (Figure 7). In the Eastern Alps, the subduction was initiated in the NW foreland of the Meliata suture, most probably within a pre-existing Permian-age rift that was reactivated when convergence across the suture continued after the closing of the Meliata Ocean (Janák et al. Reference Janák, Froitzheim, Lupták, Vrabec and Ravna2004; Stüwe and Schuster, Reference Stüwe and Schuster2010). Our results indicate that the Veporic Superunit in the Western Carpathians entered the continental subduction channel probably at the same time as the Austroalpine units in the Eastern Alps but subsequently reached different peak conditions; those in the Veporic Superunit were similar to the northern margin of the Austroalpine nappes stack (Faryad and Hoinkes, Reference Faryad and Hoinkes2003; Froitzheim et al. Reference Froitzheim, Plašienka, Schuster and McCann2008; Stumpf et al. Reference Stumpf, Skrzypek and Stüwe2024).

Figure 6.

Eo-Alpine metamorphic P-T-t path of the garnet-bearing mica schist from the Veporic Superunit. The Lu/Hf and Rb/Sr geochronological data are from this study. The P-T section and monazite age data are from Janák et al. (Reference Janák, Petrik, Plašienka, Froitzheim and Konečny2025).

A phase diagram illustrating the metamorphic conditions of garnet-bearing mica schist.
Figure 6. Long description

A phase diagram illustrating the metamorphic conditions of garnet-bearing mica schist. The diagram plots pressure (P) in kilobars (kbar) on the y-axis against temperature (T) in degrees Celsius (°C) on the x-axis. Various mineral assemblages and reactions are labeled within the diagram, including garnet (Grt), biotite (Bt), muscovite (Ms), paragonite (Pa), rutile (Rt), chlorite (Chl), plagioclase (Pl), ilmenite (Ilm), staurolite (St), kyanite (ky), sillimanite (sil), and andalusite (And). The diagram features several isograds and reaction lines, indicating different metamorphic conditions. Arrows indicate the metamorphic path over time, with specific ages marked along the path. The path shows the progression from lower pressure and temperature conditions to higher pressure and temperature conditions, reflecting the metamorphic history of the rock.

Figure 7.

Reconstruction of the tectonic situation at ∼95–90 Ma during the peak-to-earliest stages of exhumation of the Veporic Superunit. After the closure of the Meliata Ocean in the south and exhumation of LT-HP rocks, the Veporic Superunit was subducted in an intracontinental setting farther north, reaching at least lower crustal levels, perhaps even the mantle depths. The main Carpathian tectonic units are compared to the potentially analogous Alpine units following Janák et al. (Reference Janák, Froitzheim, Lupták, Vrabec and Ravna2004, Reference Janák, Froitzheim, Yoshida, Sasinková, Nosko, Kobayashi, Hirajima and Vrabec2015). Likewise, the tectonic scenario is inspired by Janák et al. (Reference Janák, Froitzheim, Lupták, Vrabec and Ravna2004, Reference Janák, Froitzheim, Yoshida, Sasinková, Nosko, Kobayashi, Hirajima and Vrabec2015) and Miladinova et al. (Reference Miladinova, Froitzheim, Nagel, Janák, Fonseca, Sprung and Münker2022).

Diagram of tectonic units and their relationships during subduction.

Subduction of a continental plate beneath another continental plate commonly follows a former subduction of an oceanic plate beneath a continent (e.g. Stern and Gerya, Reference Stern and Gerya2018 and references therein). The driving force for the process is the negative buoyancy of the already subducted adjacent oceanic plate. However, in the case of the Veporic Superunit subduction, the presence of an adjacent pre-existing oceanic lithosphere has not been documented, neither in the geological record at the surface nor in the deep crustal architecture (Plašienka, Reference Plašienka2018, Soni et al. Reference Soni, Schiffer, Mazur, Mikołajczak, Müller, Potočný, Bazarnik, Hoffmann, Süle, Wegler, Fojtíkova and Majka2025). Initiation of such an intracontinental subduction without preceding oceanic plate subduction is not a fully understood process, though discussed. It has been proposed that intracontinental subduction systems may initiate due to (a) far-field tectonic effects resulting from a regional-scale convergence (e.g. Pysklywec et al. Reference Pysklywec, Gogus, Percival, Cruden and Beaumont2010; Sun et al. Reference Sun, Zhao, Malusà, Guillot and Fu2019), (b) reactivation of an already weakened continental lithosphere (e.g. Janák et al. Reference Janák, Froitzheim, Lupták, Vrabec and Ravna2004; Miladinova et al. Reference Miladinova, Froitzheim, Nagel, Janák, Fonseca, Sprung and Münker2022), (c) interaction between a plume and a continental crust that eventually triggers the mantle lithosphere foundering (Cloetingh et al. Reference Cloetingh, Koptev, Kovács, Gerya, Benies, Willingshofer, Ehlers, Andric-Tomasevic, Botsyun, Eizenhöfer, Rancois and Beekamn2021) or (d) a combination of two or all the aforementioned processes. Indeed, in the case elaborated herein, widespread Permian extension, the so-called Betliar phase, was associated with the generation of voluminous magmatism both in the Veporic and Gemeric superunits, dated to ∼265 Ma (e.g. Villaseñor et al. Reference Villaseñor, Catlos, Broska, Kohút, Hraško, Aguilera, Etzel, Kyle and Stockli2021). This process was contemporaneous with extension farther south, in the Meliata oceanic domain, as documented by a similar ∼265 Ma maximum deposition age of radiolarites (Villaseñor et al. Reference Villaseñor, Catlos, Broska, Kohút, Hraško, Aguilera, Etzel, Kyle and Stockli2021). Such weakened continental crust and presumably its mantle lithosphere in an ongoing convergence regime would likely be a locus for initiation of subduction of the Veporic Superunit beneath the Adria plate farther south. Intracontinental subduction systems, in which a continental lithospheric block is not adjacent to an oceanic lithosphere acting as a dragging force, are commonly short-term and are quickly followed by exhumation of deeply buried rocks (Yamato et al. Reference Yamato, Agard, Burov, Le Pourhiet, Jolivet and Tiberi2007). The short-lived intracontinental subduction scenario is coherent with a geochronological dataset and a deep lithospheric architecture beneath the Veporic and adjacent tectonic units (Soni et al. Reference Soni, Schiffer, Mazur, Mikołajczak, Müller, Potočný, Bazarnik, Hoffmann, Süle, Wegler, Fojtíkova and Majka2025). It is also reinforced by low-temperature thermochronological data (Koroknai et al. Reference Koroknai, Horvath, Balogh and Dunkl2001; Vojtko et al. Reference Vojtko, Králiková, Jeřábek, Schuster, Danišík, Fügenschuh, Minár and Madarás2016).

Supplementary material

The supplementary material for this article can be found at https://doi.org/10.1017/S0016756826101009

Acknowledgements

This paper is dedicated to the memory of Professor David Godfrey Gee – our mentor, friend and, most importantly, an excellent geologist, who devoted his entire career to studying ancient and active orogenic belts. The paper was supported by the National Science Centre (Poland) project no. 2021/43/B/ST10/02312 to J. Majka, the Slovak Research and Development Agency (project APVV-18-0107) and the Scientific Grant Agency VEGA (project 2/0002/24) to M. Janák. Natalia S. Mazurek is thanked for help with the figure preparation. Two anonymous reviewers are thanked for their comments.

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Figure 0

Figure 1. Figure 1 long description.A map of Alpine belt in Europe, modified from Wikimedia Commons 358 (https://commons.wikimedia.org/wiki/File:Tectonic_map_Mediterranean_EN.svg; username: 359 Woudloper; Creative Commons Attribution-Share Alike 1.0 Generic license: 360 https://creativecommons.org/licenses/by-sa/1.0/deed.en). The broad study area is marked by a black rectangle, referring to the map to the right of it. The latter shows a distribution of medium- to high-grade metamorphic complexes, on the sketch map of principal tectonic units of the Eastern Alps and Western Carpathians and their foreland (modified after Janák et al. 2001, 2025). Abbreviations: PAFZ – Peri-Adriatic fault zone; VB – Vienna basin; RFZ – Rába fault zone; HDFZ – Hurbanovo–Diósjenő fault zone; MHFZ – Mid-Hungarian fault zone. A red circle marks the sampling site and the study area.

Figure 1

Figure 2. Geological sketch map of the eastern part of the Veporic Superunit and overlying units. Metamorphic zones and isotherms according to Janák et al. (2001) and Jeřábek et al. (2008). 1 – chloritoid + chlorite + garnet zone, 2 – garnet + staurolite + chlorite zone, 3 – staurolite + biotite + kyanite zone. Taken from Janák et al. (2025), modified.

Figure 2

Figure 3. A microscopic view of the dated garnet mica schist in plane-polarized light (a); BSE image of idiomorphic garnet containing quartz and rutile (b); X-ray chemical maps showing distribution of Ca, Mg and Mn (c–e), and WDS chemical profile showing prograde, one-step chemical zoning in the garnet (f).

Figure 3

Figure 4. Results of in situ garnet Lu/Hf geochronology are presented as an isochron. The uncertainties of the ellipses are 2SE. The isochron is anchored to an initial 176Hf/177Hf value of 0.2829 ± 0.003.

Figure 4

Figure 5. Results of in situ biotite Rb/Sr geochronology. The results are presented as an isochron for biotite (a). The biotite data are also presented as (b) an array of single-spot dates. The two isochrons are unanchored, and the resulting initial 87Sr/86Sr values are reported. The biotite single-spot dates were calculated using an initial 87Sr/86Sr value of 0.700 ± 0.001. The ten oldest and youngest single dates were used to calculate weighted averages and are highlighted in dark purple. All uncertainties are 2SE.

Figure 5

Figure 6. Figure 6 long description.Eo-Alpine metamorphic P-T-t path of the garnet-bearing mica schist from the Veporic Superunit. The Lu/Hf and Rb/Sr geochronological data are from this study. The P-T section and monazite age data are from Janák et al. (2025).

Figure 6

Figure 7. Reconstruction of the tectonic situation at ∼95–90 Ma during the peak-to-earliest stages of exhumation of the Veporic Superunit. After the closure of the Meliata Ocean in the south and exhumation of LT-HP rocks, the Veporic Superunit was subducted in an intracontinental setting farther north, reaching at least lower crustal levels, perhaps even the mantle depths. The main Carpathian tectonic units are compared to the potentially analogous Alpine units following Janák et al. (2004, 2015). Likewise, the tectonic scenario is inspired by Janák et al. (2004, 2015) and Miladinova et al. (2022).

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