Abstract
Phase equilibrium modelling of metagranitoids and metapelites in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O system was used to characterize Variscan and Alpine metamorphism in the crystalline basement of the Vepor Unit, West Carpathians. The calculated P–T conditions range between 570–670 °C and 6–8.5 kbar for the Variscan and 430–600 °C and 5–11 kbar for the Alpine event. These two events show contrasting metamorphic field gradients and P–T evolutions, indicating 22–27 °C km−1 during the retrograde Variscan metamorphism and 15–18 °C km−1 during the prograde Alpine metamorphism. The prograde Alpine metamorphism is associated with the Early Cretaceous overthrusting of the southern Gemer Unit, which resulted in apparently contemporaneous burial and horizontal ductile spreading of the underlying Vepor basement. The argon diffusion modelling was used to interpret the existing Variscan, Alpine and mixed 40Ar/39Ar cooling ages, and to constrain the T–t evolution and Alpine thermal overprint. Constructed Alpine metamorphic isograds and isotherms show horizontal P–T gradients resulting from heterogeneous exhumation of the deeper parts of the Vepor basement along two narrow belts during later folding. Here the structurally deeper metapelites form large-scale anticlinal cusp-like structures that separate structurally higher metagranitoids.
The distribution of metamorphic isograds is commonly used to identify metamorphic field gradients associated with large-scale tectonic processes, such as collision, subduction or extension of the continental crust. Because of the strong influence of the temperature on the metamorphic reaction kinetics (Spear 1993), metamorphic isograds are interpreted as the traces of isograd surfaces related to certain thermal conditions. Although early thermal numerical models suggested that the peak pressure and temperature conditions are not attained synchronously (England & Thompson 1984; Thompson & England 1984), the distribution of metamorphic isograds, showing spatial variations in the peak temperature conditions, remains an important observation that can help to identify dominant tectonothermal regimes (Todd & Engi 1997). Until recently, only fragments of P–T paths could be obtained from conventional thermobarometry; however, recent advances in thermodynamic modelling (Powell et al. 1998; Connolly 2005) allow reconstruction of more complete parts of P–T evolutions. Mapping of metamorphic field gradients and the deciphering of P–T evolutions can contribute to our understanding of burial and exhumation of either coherent crustal blocks or their constituent parts (Štípská et al. 2006).
In this study, we use P–T sections, calculated with the Perple_X thermodynamic software (Connolly 2005), to distinguish prograde Alpine and retrograde Variscan P–T evolutions and metamorphic field gradients within the polymetamorphic Vepor basement in the Central West Carpathians. The phase-equilibrium modelling of Alpine metamorphic conditions, combined with field structural analysis and reassessment of an existing 40Ar/39Ar dataset, is used to discuss the mechanisms of Cretaceous burial and subsequent heterogeneous exhumation of the Vepor basement.
Geological setting
The Central West Carpathians consist of three major crustal units, from south to north, the Gemer, Vepor and Tatra (Fig. 1). The differences and similarities in lithological content and tectonometamorphic history of the three units suggest an affinity to two major crustal blocks, the northern and the southern block, separated by a crustal-scale thrust zone. The southern block, represented by the Gemer Unit, comprises volcano-sedimentary sequences of Early Palaeozoic age, exhibiting Variscan low- to medium-grade metamorphism (Faryad 1991), and Carboniferous–Triassic cover. The Gemer Unit is overlain by a Jurassic subduction–accretionary complex of the Meliata Ocean (Faryad 1995; Faryad & Henjes-Kunst 1997). The northern block, represented by the Tatra and Vepor units, comprises Variscan high-grade metamorphic rocks, Carboniferous granitoids and Permian–Early Cretaceous cover. Both the blocks are tectonically overlain by the Turňa, Silica, Choč and Krížna nappes (Fig. 1), which are composed of the Late Palaeozoic–Cretaceous platform and rift-related sedimentary sequences (Plašienka et al. 1997, and references therein). During the Cretaceous, the overall convergence of the region led to juxtaposition of the three crustal units, emplacement of the above-mentioned nappes and, consequently, to tectonometamorphic reworking of both blocks. However, the grade of the Alpine metamorphic overprint differs significantly in the two blocks. The southern block exhibits very low-grade to greenschist Alpine metamorphic conditions (Faryad 1997), whereas the northern block exhibits a northward decrease in metamorphic conditions from upper greenschist- to amphibolite-facies conditions in the Vepor Unit (Vrána 1966, 1980; Plašienka et al. 1999; Janák et al. 2001; Koroknai et al. 2001) to prehnite–pumpellyite-facies conditions in the Tatra Unit (Krist et al. 1992).
Tectonic map of the Central West Carpathians and tectonic sketch of the Vepor Unit based on Geological Map of ČSSR 1:200 000 (1963–1964) with subdivision into four tectonostratigraphic units: Vel'ký Bok Cover (VBC) and Foederata Cover (FC) units; Gemer–Vepor Contact Zone (G/VCZ); Král'ova Hol'a Complex (KHC); Hron Complex (HC). Locations of investigated petrological samples and structural profile A–A′ are indicated. Numbers 1–38 represent locations of published 40Ar/39Ar ages (Table 1).
The Vepor Unit
The overall structure of the Vepor Unit is characterized by alternation of the NE–SW-trending belts with different lithologies (Fig. 1). The basement consists of two complexes exhibiting different lithology and structural position (Klinec 1966). The structurally lower Hron Complex (Fig. 2) is formed of paragneisses, micaschists and amphibolites, whereas the Král'ova Hol'a Complex hanging wall is formed of Upper Devonian migmatites and anatectic orthogneisses, Lower Carboniferous S-type granitoids and Upper Carboniferous tonalites (Bibikova et al. 1988, 1990; Michalko et al. 1998). The division of the Vepor basement into two lithotectonic complexes (Klinec 1966) was later questioned, mostly because of the differences in the lithology and degree of Alpine tectonometamorphic reworking of the southern and northern parts of the Hron Complex (Fig. 1). Nevertheless, the mutual structural relationship of the two dominant lithologies within the Vepor Unit is identical to that of the Tatra Unit and to that of the eastern segment of the Vepor Unit (Fig. 1), exhibiting lower degree of Alpine reworking (Janák 1994; Jacko et al. 1996). Consequently, the inverted crustal structure (granitoids overlying metasediments) within both units argues for an inherited Variscan crustal stratification (Bezák et al. 1997).
NNW–SSE-oriented structural profile A–A′ across the Vepor Unit (location shown in Fig. 1). Structural features: SV, Variscan metamorphic foliation; SA1, Alpine metamorphic foliation; LA1, Alpine lineation; SA2, axial planes of FA2 folds; LA2, axis of FA2 folds; DA1, first Alpine deformation event; DA2, second Alpine deformation event. Stereograms are lower hemisphere Schmidt equal area plots.
The ages of the Variscan and Alpine tectonometamorphic events were established by the U–Pb and 40Ar/39Ar ages. The U–Pb magmatic zircon ages from the Král'ova Hol'a Complex migmatites and granites as well as hornblende 40Ar/39Ar cooling ages from the Hron Complex amphibolites indicate that Variscan metamorphism occurred between 370 and 340 Ma (Král' et al. 1996; Michalko et al. 1998; Putiš et al. 2001). The intrusions of Carboniferous granitoids (Bibikova et al. 1988, 1990) were followed by minor post-orogenic Late Permian–Early Triassic A-type granites and subalkaline volcanites (Petrík et al. 1995; Kotov et al. 1996; Putiš et al. 2000). The Alpine metamorphic event is defined by numerous hornblende, muscovite and biotite 40Ar/39Ar cooling ages, which show two distinct age populations ranging between 115–105 Ma and 87–80 Ma (see Table 1 for 40Ar/39Ar data and references).
Summary table of published 40Ar/39Ar data
Variscan metamorphism in metapelites of the Hron Complex reached kyanite grade (Méres & Hovorka 1991; Kováčik 1993; Table 2), although the northern part of this complex also contains remnants of Variscan eclogites (Janák et al. 2007). Janák et al. (2001) established three Alpine metamorphic zones within the southern part of the Hron Complex: (1) chloritoid + chlorite + garnet; (2) garnet + staurolite + chlorite; (3) staurolite + biotite + kyanite. Corresponding P–T estimates range from 500 °C and 7 kbar to 620 °C and 10 kbar. Variscan metamorphism of the Král'ova Hol'a Complex reached conditions of partial melting indicating 680–730 °C and 4–6 kbar (Siman et al. 1996). Subsequent Alpine metamorphism is associated with greenschist-facies reworking at 400–500 °C and 5 kbar (Vrána 1980).
Selected previous P–T estimates for Variscan and Alpine metamorphic events, recorded in the various rock complexes of the Vepor Unit
The southern part of the Vepor Unit, the Gemer–Vepor Contact Zone, is formed by schists and meta-arkoses or metasandstones. These rocks have traditionally been related to the Variscan basement (Klinec 1966; Vrána 1966); however, pollen analysis suggests both Early and Late Palaeozoic depositional ages (Planderová & Vozárová 1978; Klinec & Planderová 1981). Alpine metamorphism of the Gemer–Vepor Contact Zone is characterized by the presence of chloritoid–kyanite schists (Vrána 1964) with estimated P–T conditions of 500–590 °C and 4.5–7.7 kbar (Lupták et al. 2000; see also Table 2). The area locally exhibits subsequent HT–LP metamorphic overprint (Vozárová 1990), related to intrusion of the Late Cretaceous (82–75 Ma) Rochovce I-type granite (Hraško et al. 1998, 1999; Poller et al. 2001).
Two metamorphosed cover successions, each with a distinct lithostratigraphy, have been recognized in the Vepor Unit (Biely 1964). The Triassic para-autochthonuous metasediments of the Foederata Cover (Rozlozsnik 1935) overlie the Gemer–Vepor Contact Zone and the Král'ova Hol'a Complex. Alpine metamorphic conditions of the Foederata Cover did not exceed 380 °C and 4.5 kbar (Lupták et al. 2003; see also Table 2). Permian to Early Cretaceous allochthonous metasediments of the Vel'ký Bok Cover (Biely 1964), exhibiting low-grade Alpine metamorphic overprint (Plašienka et al. 1989; Lupták et al. 2003), occur at the northern margin of the Vepor Unit.
Structural record within the Vepor Unit
Three main deformational events were identified in the basement of the Vepor Unit and two in the Permian to Early Cretaceous cover. The earliest deformation DV, recorded exclusively within the Vepor basement, is interpreted as pre-Alpine, whereas the later two deformations DA1 and DA2, also affecting the cover, are interpreted as the first and second Alpine phases (Fig. 2).
The earliest deformation DV is associated with the development of high-grade metamorphic schistosity SV in the Hron Complex and Gemer–Vepor Contact Zone, and high-grade orthogneiss fabric and migmatite layering in the Král'ova Hol'a Complex. Where unaffected by later deformation, the SV fabric generally shows east–west trends.
In the Hron Complex and Gemer–Vepor Contact Zone, the DA1 deformation event is characterized by transposition of the previous fabric into new penetrative metamorphic foliation SA1, which is axial planar to the locally preserved isoclinal folds. In the Král'ova Hol'a Complex, the DA1 event is represented by a heterogeneously developed deformation fabric defined by shape-preferred orientation of micas and elongated quartz aggregates. In the Foederata and Vel'ký Bok Cover, the SA1 low-grade metamorphic fabric is mostly parallel to the bedding, being axial planar to centimetre- to kilometre-scale isoclinal folds (Plašienka 2003). With the exception of the Hron Complex, where the SA1 fabric dips towards the NW and SE at shallow to steep angles, the SA1 fabric is subhorizontal and/or gently dipping to the east or south (Fig. 2). The SA1 fabric bears ENE–WSW-trending LA1 stretching lineation defined by alignment of feldspar, quartz, and/or micas. LA1 is locally subparallel to the axis of the isoclinal FA1 folds.
During the DA2 deformation event, large-scale folds developed in the Hron Complex and Gemer–Vepor Contact Zone, and crenulation cleavage affected the DA1 high-strain domains in the Král'ova Hol'a Complex. Generally, the FA2 folds have subvertical NE–SW- to ENE–WSW-trending axial planes and subhorizontal axis. In the southern Hron Complex, the SA2 axial planar cleavage exhibits large-scale inverse fan structure (Fig. 2). In the Foederata and Vel'ký Bok Cover, the DA2 deformation resulted in the development of metre-scale upright folds.
Petrography
The rocks studied here are: (1) paragneisses and micaschists (with semipelitic to pelitic compositions) of the Hron Complex, (2) metagranitoids (variably deformed granodiorite–tonalite) of the Král'ova Hol'a Complex and (3) schistose metasediments (originally sandstones, shales and volcaniclastic rocks) of the Gemer–Vepor Contact Zone (see Fig. 1 for locations). Two metamorphic mineral assemblages (M1 and M2), as well as a primary igneous (Ign) mineral assemblage (Table 3), were identified on the basis of the textural relations and chemical compositions (mineral abbreviations after Kretz 1983).
Relict igneous (Ign) and metamorphic (M1 and M2) mineral assemblages distinguished on the basis of textural relations and mineral compositions
Paragneisses and micaschists of the Hron Complex
The Hron Complex paragneisses–micaschists consist of garnet, biotite, chlorite, muscovite, plagioclase, quartz and accessory ilmenite, titanite, epidote, allanite, zircon, monazite and tourmaline. Two varieties of garnet, muscovite, biotite and plagioclase are related to metamorphic stages M1 and M2 (Table 3). Garnet (I) represents cores of large garnet porphyroblasts, whereas garnet (II) either envelops garnet (I) or occurs in the matrix as small single grains. Garnet (I) and (II) can be further distinguished on the basis of different amounts of opaque mineral inclusions or compositional differences, visible in the back-scattered electron (BSE) images (Fig. 3). Muscovite (I) is coarse-grained (0.2–1 mm) and differs from the fine-grained muscovite (II) in the matrix in composition (see below). Biotite (I) is coarse-grained and dark brown and is characterized by the presence of exsolution rutile needles. It is commonly replaced by chlorite. In contrast, biotite (II) is fine-grained and lighter brown. Large plagioclase (I) porphyroblasts are overgrown by fine-grained muscovite (II), whereas small plagioclase (II) grains are muscovite-free.
BSE images showing two varieties of garnet and relevant compositional profiles. Linescans are indicated in each image. (a, b) Hron Complex paragneiss sample PP207. Garnet (I) contains biotite (I), plagioclase (I), ilmenite, and quartz inclusions. White rectangle in (a) corresponds to the BSE image in Figure 5. (c, d) Hron Complex micaschist sample PP482. Garnet (I) is present as relics enclosed by garnet (II). (e, f) Král'ova Hol'a Complex metagranitoid sample PP150 shows igneous garnet (I) enveloped by metamorphic garnet (II). (g, h) Gemer–Vepor Contact Zone schist sample BL1 shows idioblastic garnet (I) rimmed by garnet (II).
Two samples of paragneiss (PP207, PP473) and one of micaschist (PP482) from different locations in the Hron Complex (Fig. 1) were selected for detailed petrological study because of the clear textural relations between the M1 and M2 metamorphic mineral assemblages. Paragneiss sample PP207 is characterized by the presence of two varieties of garnet (Fig. 3a) and by biotite (II) overgrowing chlorite (Fig. 4a). The core garnet (I) is rich in biotite, ilmenite, plagioclase and quartz inclusions, whereas the inclusion-poor rim garnet (II) contains only ilmenite and quartz. Sample PP473 is rich in plagioclase and contains only one variety of garnet (c. 0.3 mm), related to the first metamorphic stage M1, which is intensely fractured and partly replaced by both biotite (II) and chlorite (Fig. 4b). Sample PP482 is characterized by high muscovite and chlorite contents and by relics of garnet (I), enclosed by garnet (II) (Fig. 3c). Only one variety of biotite, almost completely replaced by chlorite, is related to the first metamorphic stage M1.
Photomicrographs of representative textural relations. (a) Hron Complex paragneiss sample PP207 shows biotite (II) overgrowing chlorite. (b) Hron Complex paragneiss sample PP473 shows two varieties of muscovite and biotite. Biotite (II) and chlorite overgrow fractured garnet. (c) Král'ova Hol'a Complex metagranitoid sample PP150 shows plagioclase (I) porphyroblast replaced by muscovite (II), biotite (I) containing exsolution rutile needles, two varieties of muscovite and biotite, and garnet (II) within plagioclase (I) porphyroblast. (d) Gemer–Vepor Contact Zone schist sample BL1 reveals two varieties of biotite and garnet.
Metagranitoids of the Král'ova Hol'a Complex
Five Král'ova Hol'a Complex metagranitoid samples (PP150, PP28, PP225, PP229 and MM136) from the central part of the Vepor Unit (Fig. 1) were selected for detailed petrological study, because they contain garnet and provide clear textural relations between Ign and M2 mineral assemblages. Common minerals in metagranitoids are biotite, muscovite, chlorite, clinozoisite, plagioclase, quartz and accessory zircon, apatite, monazite, alanite, titanite, rutile and ilmenite. Some samples also contain garnet and relics of K-feldspar. The K-feldspar phenocrysts, plagioclase (I), tabular muscovite (I) and dark brown tabular biotite (I) are interpreted as primary igneous phases (Table 3). Large biotite (I) grains are characterized by the presence of exsolution rutile needles (Fig. 4c). Plagioclase (I) porphyroblasts are replaced by fine-grained muscovite (II) and clinozoisite. Samples PP150 and PP28 contain porphyroblastic garnet, which is formed by partly corroded garnet (I) cores, probably of igneous origin, and idioblastic rims, formed by metamorphic garnet (II) (Fig. 3e). The metamorphic mineral assemblage, related to the M2 metamorphic stage (Table 3), comprises small plagioclase (II) grains, chlorite intergrowing with biotite (II), clinozoisite and small garnet (II) grains, occurring within or adjacent to plagioclase porphyroblasts (Fig. 4c).
Schists of the Gemer–Vepor Contact Zone
The Gemer–Vepor Contact Zone schists consist of biotite, muscovite, chlorite, plagioclase, quartz and accessory magnetite and ilmenite. Some samples also contain garnet, clinozoisite, tourmaline and cordierite, the last resulting from a local HT–LP metamorphic overprint (Vozárová 1990). Two samples of Gemer–Vepor Contact Zone schists (BL1 and PP304) were selected for detailed petrological study because of the clear textural relations between the M1 and M2 metamorphic mineral assemblages. Two varieties of biotite and one variety of plagioclase can be distinguished optically. Biotite (I) is idiomorphic and dark brown, whereas biotite (II) is anhedral and light brown (Fig. 4d). Two textural varieties of garnet, described in these rocks by Korikovsky et al. (1990), were detected only in sample BL1. Idioblastic garnet (I) in the core is enveloped by garnet (II), which exhibits abundant quartz and ilmenite inclusions (Figs 3g and 4d⇑). Sample PP304, containing only the garnet (II) variety, exhibits relics of chlorite, which are overgrown by biotite (II).
Chemical composition of the minerals
The minerals were analysed using the CAMSCAN 54 SEM with energy-dispersive spectrometers and microanalytical system LINK ISIS 300 at the Institute of Petrology and Structural Geology, Charles University, Prague, and the CAMECA SX 50 electron microprobe with four wavelength-dispersive spectrometers at the Institute of Mineralogy, Technische Universität Stuttgart. Mineral formulae and ferric/ferrous iron ratios were calculated on the basis of 12 oxygens and eight cations for garnet and 11 oxygens for micas. Representative analyses of minerals, exhibiting two compositional varieties and/or used in thermodynamic or conventional thermometry calculations, are presented in Table 4 (garnet) and Table 5 (muscovite, biotite).
Representative microprobe analyses of garnet from the Hron Complex, Král'ova Hol'a Complex and Gemer–Vepor Contact Zone
Representative microprobe analysis of muscovite and biotite from the Hron Complex, Král'ova Hol'a and Gemer–Vepor Contact Zone
The two varieties of garnet (Fig. 3, Table 4) usually exhibit sharp compositional transition (Fig. 5). Apart from garnet (I) in the Král'ova Hol'a Complex metagranitoids (sample PP150 and PP28 in Table 4), interpreted as igneous on the basis of textural relations and the high Mg and Mn contents, garnets (I) in the Hron Complex paragneisses–micaschists and the Gemer–Vepor Contact Zone schists (samples PP207, PP473, PP482 and BL1 in Table 4 and Fig. 3) exhibit similar compositions with high Fe and Mg contents. In addition, these garnets exhibit similar compositional trends, manifested by a minor decrease in Mg and XMg (XMg=Mg/(Mg+Fe)), and a minor increase in Mn towards the rim. Garnet (II) is enriched in Ca at the expense of the Fe content and exhibits pronounced compositional zoning, manifested by a rimward increase in Fe, Mg and XMg, and a decrease in Ca and Mn (Fig. 3, Table 4).
Two detailed compositional profiles across the garnet (I)–garnet (II) boundary from the Hron Complex paragneiss sample PP207 (see Fig. 3a for location of the BSE image).
The two textural varieties of biotite mostly reveal identical composition with Ti contents ranging between 0.08 and 0.15 atoms per formula unit (a.p.f.u.) and XMg between 0.3 and 0.5 (Table 5). The biotite (I) composition, established in biotite inclusion enclosed in plagioclase (I) within garnet (I) (sample PP207; Fig. 3a), exhibits a higher Ti content (0.29 a.p.f.u.) and lower XMg (0.22).
Chlorite is present in most samples and its XMg value varies between 0.33 and 0.48.
In the Hron Complex paragneisses–micaschists and Král'ova Hol'a Complex metagranitoids, the two textural varieties of muscovite differ in their Si and Na contents. The Si and Na contents in muscovite (I) are in the range of 3.00–3.11 a.p.f.u. and 0.04–0.21 a.p.f.u., respectively, whereas muscovite (II) with higher proportion of phengite has Si contents of 3.11–3.32 a.p.f.u. and Na contents of 0.02–0.14 a.p.f.u. (Table 5). Only one textural variety of muscovite in the Gemer–Vepor Contact Zone schists contains 3.06–3.33 a.p.f.u. of Si and 0.04–0.15 a.p.f.u. of Na.
The two textural varieties of plagioclase, present in most samples from the Hron Complex and Král'ova Hol'a Complex, exhibit identical albite composition with less than 5 mol% of anorthite component. The original plagioclase (I) composition (An 23–24 mol%) was established in porphyroblasts of the Hron Complex paragneiss (sample PP473) or in inclusions within garnet (I) (sample PP207) (Fig. 3a). In the Gemer–Vepor Contact Zone schists, the plagioclase (I) with 13–28 mol% An is locally replaced by plagioclase (II) of albite composition. It is not clear whether the newly formed albite and clinozoisite in the Gemer–Vepor Contact Zone schists are part of the M2 metamorphic mineral assemblage, or if these are late-stage alteration products.
P–T conditions
The textural relations within the studied rocks demonstrate the polymetamorphic nature of the Vepor basement complex. The first metamorphic event (M1) was recognized in the Hron Complex paragneisses–micaschists and in the Gemer–Vepor Contact Zone schists. It is characterized by the presence of garnet (I), coarse-grained muscovite (I) and biotite (I), and plagioclase (I) porphyroblasts (Table 3). The second metamorphic event (M2) identified in all the investigated rock complexes is represented by relatively Ca-rich garnet, phengite muscovite (II), albite plagioclase (II), chlorite, ± biotite (II), and ± clinozoisite. Considering the existing geochronological data for the Vepor Unit, especially the 40Ar/39Ar ages (Table 1), the recognized M1 and M2 events relate to Variscan and Alpine regional metamorphic events, respectively.
To interpret the phase relations and metamorphic evolution of the studied rocks, the P–T conditions of the two metamorphic events were estimated by means of phase equilibrium modelling and the P–T section approach (Powell et al. 1998; Connolly & Petrini 2002). The P–T sections were calculated using the thermodynamic software Perple_X (Connolly 2005), with the internally consistent thermodynamic dataset of Holland & Powell (1998; 2002 upgrade). The bulk-rock compositions used in the modelling were determined by three methods. (1) The whole-rock composition obtained by X-ray fluorescence (XRF) analysis was used to model P–T conditions for the older (M1) metamorphic event, as well as for the younger (M2) metamorphic event in those samples exhibiting complete M2 overprint. (2) The modified whole-rock composition was used for the M2 event in samples preserving a significant modal proportion of the M1 phases. Modification of the whole-rock composition involved exclusive removal of garnet (I), as the other mineral phases of the M1 assemblage re-equilibrated their compositions (plagioclase (I) and biotite (I)) or represent a minor modal portion of the rock (muscovite (I) in the sample PP207). (3) The estimated equilibration volume (Stüwe & Powell 1995) was used for the M2 event in samples with abundant pre-M2 minerals, which inhibited modification of the whole-rock composition. The equilibration volume, covering a representative modal portion of the M2 mineral assemblage, was derived from the BSE images using image analysis. For conversion to the proportions of oxides, the chemically zoned garnets were approximated by their mean composition; this did not affect our modelling results because of the low modal content of garnet (<1 modal%). The resulting bulk-rock compositions are indicated in each P–T section (Figs 6 and 7⇓).
P–T sections calculated in the MnNCKFMASH system with excess of water and quartz for the Hron Complex paragneiss–micaschist samples; bulk-rock compositions are indicated in the top right corner of each P–T section. (a) M1 event in the sample PP207; (b) M2 event in sample PP207; (c) M1 event in sample PP473; (d) M1 and M2 events in sample PP482. Compositions of garnet (I) and (II) are plotted by using isopleths of XMg, grossular and spessartine content in garnet. The white squares show garnet (I) core and rim compositions; the wide grey arrows show garnet (II) growth P–T path indicated by core-to-rim compositional change.
P–T sections calculated in the MnNCKFMASH system with excess of water and quartz for the Král'ova Hol'a Complex (KHC) and Gemer–Vepor Contact Zone (G/VCZ); bulk-rock compositions are indicated in the top right corner of each P–T section. (a) M2 event in the Král'ova Hol'a Complex metagranitoid samples PP28, PP150, PP225, PP229 and MM136; (b) M1 event in the Gemer–Vepor Contact Zone schist sample BL1; (c) M2 event in the Gemer–Vepor Contact Zone schist sample BL1; (d) M2 event in the Gemer–Vepor Contact Zone schist sample PP304. Compositions of garnet (I) and (II) are plotted by using isopleths of XMg, grossular and spessartine content in garnet. The white squares show garnet (I) core and rim compositions; the wide grey arrows show garnet (II) growth P–T path indicated by core-to-rim compositional change.
As for the whole-rock composition, mineral chemistry (particularly the high Mn content in garnets) and modal proportion of the mineral phases, all P–T sections were calculated in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (MnNCKFMASH) system with excess water. Titanium was not included in our calculations because of its low content (<0.5 wt%) and the minor role of Ti-bearing phases, such as rutile (within biotite (I)) and ilmenite, in the metamorphic reactions in question. Although the manganese content in the analysed rocks is also low, it was included in modelling because of its striking effect on the stability of garnet (Tinkham et al. 2001). In addition to the plagioclase solution model by Newton et al. (1980), we used the solution models of White et al. (2001) for NCKFMASH end-member phases expanded by the Mn end-members for biotite, chlorite, garnet, cordierite and staurolite (Tinkham et al. 2001).
As an independent method (different solution models and thermodynamic data from those used in P–T section calculations), the conventional garnet–biotite thermometer of Kleemann & Reinhardt (1994) was employed to evaluate our phase equilibrium modelling results (Table 6). Garnet rim–biotite analyses used for the calculations are listed in Tables 4 and 5.
Temperature estimates calculated for M1 and M2 metamorphic events using garnet–biotite thermometer of Kleemann & Reinhardt (1994)
Paragneisses and micaschists of the Hron Complex
Two P–T sections, constructed to estimate the P–T conditions of sample PP207, were modelled using the whole-rock composition for the M1 event and modified whole-rock composition for the M2 event (see above). For the M1 event P–T section, the observed equilibrium mineral assemblage (Table 3) corresponds to the five-variant Grt–Bt–Ms–Pl–Qtz–H2O field (Fig. 6a). Peak P–T conditions for the M1 event were established using the garnet (I) core composition and isopleths of XMg (0.11), grossular (5 mol%), and spessartine (5 mol%) component (Table 4). The estimated P–T conditions of c. 680 °C and c. 8.5 kbar are in good agreement with the results of garnet–biotite thermometry, which yielded 660 °C (Table 6). This temperature was calculated using garnet (I) enclosing biotite (I) (Fig. 3a, Tables 4 and 5). The M2 assemblage in sample PP207 (Table 3), exhibiting biotite overgrowths of chlorite, indicates a prograde transition from the four-variant Grt–Bt–Ms–Chl–Pl–Qtz–H2O field into the five-variant Grt–Bt–Ms–Pl–Qtz–H2O field (Fig. 6b). The P–T increase from 540 °C and 9 kbar to 600 °C and 11 kbar, is suggested by a rimward increase of XMg (from 0.055 to 0.085) and a decrease of the grossular (from 24 to 17 mol%) and spessartine (from 3 to 1 mol%) contents within garnet (II) (Fig. 3b, Table 4). Garnet–biotite thermometry, using the garnet (II) rim and matrix biotite (II), yields 550 °C for the M2 event (Table 6).
Based on the textural observations in sample PP473 (Fig. 4b), the peak metamorphic mineral assemblage (Grt–Bt–Ms–Pl–Qtz) relates to the M1 metamorphic event (Table 3). The corresponding P–T section (Fig. 6c) was calculated using the whole-rock composition. For the isopleths of XMg, grossular and spessartine component, the garnet (I) core composition (XMg=0.165, Grs 4 mol%, Sps 8 mol% in Table 4) plots into the four-variant Grt–Bt–Ms–Chl–Pl–Qtz–H2O field and records peak M1 P–T conditions of c. 580 °C and c. 6.5 kbar.
As minerals formed during the M1 event (Table 3) are rarely preserved in sample PP482 (see garnet (I) in Fig. 3c), the whole-rock composition was used to determine the P–T conditions for both metamorphic events (Fig. 6d). The stability of biotite, together with the chemical composition of garnet (I), restricts the P–T range for the M1 event to two possible fields: the four-variant Grt–Bt–Ms–Chl–Pl–Qtz–H2O and the three-variant Grt–Bt–Ms–Chl–St–Pl–Qtz–H2O fields. The actual lack of chlorite (I) and staurolite in this sample can be explained by an intense M2 overprint. Considering both possible fields and the garnet (I) composition (Grs 10 mol%, Sps 6 mol%), the P–T estimate for the M1 event is in the range of 6.5–7.5 kbar and 580–600 °C (dashed ellipse in Fig. 6d). Although this estimate remains problematic because of the lack of chlorite and staurolite and the lower XMg in garnet (I), it is similar to the M1 estimate from nearby sample PP473 (Fig. 6c; see Fig. 1 for locations). For the M2 event in sample PP482, the observed mineral assemblage (Table 3) corresponds to the five-variant Grt–Chl–Ms–Pl–Qtz–H2O field (Fig. 6d). An increase in XMg (from 0.061 to 0.083), together with a decrease in spessartine (from 12 to 8 mol%) and grossular (from 19 to 18 mol%) contents towards the garnet (II) rim, suggests a prograde metamorphic path and an increase in P–T conditions from 535 °C and 6.5 kbar to 555 °C and 7 kbar. As garnet (II) represents c. 6 modal% of the rock, considerable changes in the effective bulk-rock composition must have occurred during the garnet growth. Therefore, the P–T section, calculated from the whole-rock composition, characterizes only the initial stages of the garnet (II) growth and, consequently, the outermost rim-composition of garnet (II) (Table 4) was not taken into consideration (see Gaidies et al. 2006).
Metagranitoids of the Král'ova Hol'a Complex
Because of the large proportion of relict igneous phases (Fig. 4c, Ign in Table 3) in metagranitoids, the P–T sections for the M2 event were calculated using the estimated equilibration volume. As the studied metagranitoid samples (PP28, PP150, PP225, PP229, and MM136) exhibit nearly identical bulk-rock composition, their garnet (II) compositions (Table 4) were plotted into a single P–T section (Fig. 7a). The observed M2 mineral assemblage (Table 3) corresponds to the three-variant Grt–Bt–Ms–Chl–Czo–Ab–Qtz–H2O field. Using the garnet (II) compositions, the estimated P–T conditions are in the range of 430–450 °C and 5.5–6.5 kbar (sample PP225) to 460–480 °C and 7.5–8.5 kbar (sample PP150). A prograde metamorphic P–T path is suggested by the garnet (II) compositional zoning (Fig. 7a). The garnet–biotite thermometry, used for the garnet (II) rim and matrix biotite (II) (Tables 4 and 5), yields temperatures ranging between 430 °C (sample PP225) and 530 °C (sample PP150) (Table 6).
Schists of the Gemer–Vepor Contact Zone
P–T conditions of sample BL1 (Table 3) were calculated using the whole-rock composition for the M1 event and modified whole-rock composition for the M2 event. Although biotite (I) and garnet (I) are the only preserved M1 phases in this sample, the presence of former muscovite, chlorite and plagioclase is suggested by the composition of garnet (I), which plots into the four-variant Grt–Bt–Ms–Chl–Pl–Qtz–H2O field (Fig. 7b). Peak P–T conditions established using the garnet (I) core composition (XMg=0.134, Grs 5 mol%, Sps 10 mol% in Table 4) indicate c. 580 °C and c. 6 kbar. The observed M2 mineral assemblage in sample BL1 corresponds to the four-variant Grt–Bt–Ms–Chl–Pl–Qtz–H2O field (Fig. 7c). An increase in XMg (from 0.086 to 0.093), together with a decrease in the spessartine (from 9 to 3 mol%) and grossular (from 29 to 26 mol%) contents, towards the garnet (II) rim indicates a prograde metamorphic path and an increase in P–T conditions from 520 °C and 8 kbar to 540 °C and 9 kbar. Garnet–biotite thermometry, applied for the garnet (II) rim and matrix biotite (II), indicates a temperature of 560 °C for the M2 event (Table 6).
As a result of the strong M2 overprint, relics of biotite (I), revealing identical composition to biotite (II), are the only sign of the M1 event in sample PP304 (Table 3). Therefore, the whole-rock composition was used to calculate the P–T conditions for the M2 event. Considering the biotite (II) overgrowths of chlorite and using the garnet compositional isopleths, the P–T conditions of the M2 event indicate transition from the four-variant Grt–Bt–Ms–Chl–Pl–Qtz–H2O field to the five-variant Grt–Bt–Ms–Pl–Qtz–H2O field (Fig. 7d). A prograde P–T path, indicating an increase in the P–T conditions from 510 °C and 8 kbar to 530 °C and 9 kbar, is suggested by the rimward increase of XMg (from 0.073 to 0.087), the decrease in spessartine (from 21 to 13 mol%) and the constant grossular (26 mol%) content. This P–T estimate is consistent with the results of garnet–biotite thermometry, indicating a temperature of 525 °C for the M2 event (Table 6).
Re-evaluation of the existing 40Ar/39Ar dataset
Heterogeneity of the 40Ar/39Ar ages obtained from the Vepor basement is commonly explained by the complex P–T–t history during the Variscan and Alpine tectonometamorphic events (Maluski et al. 1993; Kováčik et al. 1996; Král' et al. 1996). Nevertheless, there appear to be systematic variations in the age spectra across the Vepor basement, which, using numerical modelling of argon diffusion (Lister & Baldwin 1996), can be directly related to a particular T–t history. The aim of this section is to demonstrate regional differences in the thermal history of the Vepor basement rocks, pointing to Alpine heterogeneous exhumation of the original Variscan crustal stratification.
There are three types of apparent age spectra within the existing 40Ar/39Ar dataset (see Table 1 for references), with the following features: (1) well-developed plateaux (Cretaceous ages between 80 and 87 Ma); (2) either well-developed plateaux or low-temperature release of excess argon, followed by broad pseudo-plateaux encompassing the rest of 39Ar (Palaeozoic ages older than 340 Ma); (3) a stepwise increase in the apparent ages (reaching a maximum between 85 and 180 Ma) (Fig. 8a and b). The integrated ages, obtained in the last case, clearly represent mixtures between the Cretaceous and Palaeozoic age populations.
Argon diffusion modelling in hornblende. (a, b) Examples of hornblende 40Ar/39Ar apparent age spectra showing mixed ages from the Hron Complex: (a) sample 0033/1 from Kováčik et al. (1996); (b) sample KLB-8 from Král' et al. (1996). (c) Modelled hornblende 40Ar/39Ar apparent age spectra calculated using the MacArgon software (Lister & Baldwin 1996). Spectra 1–4 indicate different P–T conditions during the 200 Ma of tectonic quiescence were obtained by using dashed T–t paths 1–4 in (d). (d) Possible T–t range for the Vepor Unit with T–t paths 1–4 used for argon diffusion modelling in hornblende. Closure temperatures for hornblende, muscovite and biotite, as well as fission tracks for apatite, are indicated in the figure.
The spatial distribution of the existing hornblende, muscovite and biotite 40Ar/39Ar ages reveals a systematic pattern across the Vepor basement. The centre of the southern Hron Complex exhibits a concentration of Cretaceous plateau ages, which, for hornblende, changes to mixed ages along strike to the NE and SW (Fig. 9). In the northern Hron Complex, Palaeozoic plateau ages prevail over the mixed age spectra for hornblende, whereas muscovite and biotite exhibit Cretaceous plateau ages. The 40Ar/39Ar spectra from the Král'ova Hol'a Complex exhibit Palaeozoic plateau ages for hornblende and Cretaceous or mixed ages for micas.
Tectonic sketch with subdivision into complexes following Figure 1. Estimated Alpine metamorphic conditions and the existing 40Ar/39Ar ages (see Fig. 1 for locations and Table 1 for references) are shown. Idealized isotherms of peak Alpine temperatures based on distribution of our P–T estimates, existing 40Ar/39Ar ages, and microstructural variations within the Král'ova Hol'a Complex (Jeřábek et al. 2007) were plotted on the map. Alpine metamorphic zones identified in the southern belt of the Hron Complex by Janák et al. (2001) are shown for comparison.
Applying the concept of closure and blocking temperatures, the existing 40Ar/39Ar spectra can be re-evaluated in terms of Alpine thermal overprint. Argon closure temperatures (i.e. temperatures below which the mineral grain is effectively closed to argon diffusion; McDougal & Harrison 1988), define minimum Alpine temperatures for rocks exhibiting Cretaceous plateau ages. On the other hand, argon blocking temperatures (i.e. temperatures above which significant argon loss will occur in the time frame of ongoing diffusion; Lister & Baldwin 1996), must be taken into consideration when estimating maximum Alpine temperatures for rocks exhibiting Palaeozoic plateau and mixed ages.
To explain the development of the stepwise increase in the apparent age spectra for mixed ages, we have simulated the effects of possible P–T–t histories on the argon solid-state diffusion and 40Ar/39Ar apparent age spectra, using the MacArgon software and associated MacSpectrometer (Lister & Baldwin 1996). The diffusion domain parameters and the P–T–t history must be specified for the purposes of modelling. Because our argon diffusion modelling is based on the published dataset of argon age spectra and detailed sample description (grain size, mineral defects and compositional variations) is usually not available, the experimentally derived diffusion parameters of McDougal & Harrison (1988) had to be complemented by some simplifying assumptions, as follows: (1) the development of the stepped age spectra is related to partial argon loss from a single mineral phase of homogeneous chemical composition; (2) the diffusion is homogeneous, intra-granular and restricted to a 40 μm wide cylindrical domain. The possible P–T–t history for the Vepor basement is determined by the following assumptions: (1) the Variscan crystallization of hornblende and micas (onset of their argon isotopic system) was followed by cooling; (2) despite the fact that the region exhibits a record of Permian postorogenic and Jurassic rifting (Plašienka 1995; Plašienka et al. 1997), the Permian–Early Cretaceous sedimentation in the area suggests c. 200 Ma of relative tectonic quiescence between the end of Variscan magmatism (c. 300 Ma, Bibikova et al. 1990) and the onset of Cretaceous convergence (c. 100 Ma, Table 1); (3) the whole crustal column was uplifted during the declining stages of Alpine orogeny, as indicated by the Late Cretaceous apatite and zircon fission-track ages (Král' 1982; Koroknai et al. 2001).
Our diffusion modelling suggests a critical dependence of the 40Ar/39Ar spectral pattern on the ambient temperatures (and hence depths) attained after the Variscan event. For mixed ages, the best match between simulated and observed 40Ar/39Ar apparent age spectra occurs for P–T–t histories that involve high ambient temperatures during the 200 Ma of tectonic quiescence (compare the real and modelled hornblende spectra and the corresponding T–t paths in Fig. 8).
Although stepped age spectra can also develop during a shorter period of residence in the argon partial retention zone (that portion of the stable crust where the temperatures are insufficient to completely reset the argon isotopic system; Baldwin & Lister 1998) (e.g. Permian and Jurassic thermal spikes related to rifting or the Cretaceous tectonometamorphic event), the temperature interval for which the stepped age spectra develop is then very small (c. 10 °C). In general, the greater the period of residence in the argon partial retention zone, the greater is the temperature interval (difference between closure and blocking temperature) for which the stepped age spectra develop. Consequently, the widespread occurrence of stepped age spectra across the Vepor basement argues for a long period of residence in the argon partial retention zone.
Blocking temperatures, calculated for the 200 Ma of quiescence, constrain the ambient temperatures above which the stepped age spectra develop and below which the Palaeozoic ages are preserved. Our phase equilibrium modelling suggests a temperature increase during the Alpine event. However, unless the Alpine overprint reached the argon closure temperatures, the original 40Ar/39Ar apparent age spectra did not experience significant modification. Thus, the occurrences of stepped age spectra suggest that the magnitude of the Alpine temperature increase did not exceed the difference between closure and blocking temperatures and the rocks remained at elevated temperatures for a relatively short period of time before the final Late Cretaceous cooling.
For a cooling rate of 5–50 °C Ma−1 across the closure temperature and 200 Ma of residence time in the argon partial retention zone, the argon closure and blocking temperatures are 490–531 and 415 °C for hornblende, 337–378 and 267 °C for muscovite, and 301–333 and 240 °C for biotite (see Lister & Baldwin 1996). These closure and blocking temperatures have been used to approximate the ambient temperatures, attained by samples from the Vepor basement, during the Alpine event. The spatial distribution of the estimated ambient temperatures, together with our P–T estimates and microstructural variations across the Král'ova Hol'a Complex (Jeřábek et al. 2007), has been used to construct idealized Alpine isotherms across the Vepor basement (Fig. 9).
Discussion
Polymetamorphic record within the Vepor basement
The distinct chemical composition and zoning of Variscan (M1) and Alpine (M2) garnets were used to interpret the P–T evolution during these two metamorphic events. The question arises as to the extent to which the composition of garnet (I) was affected by Alpine metamorphism. The compositional profile across the garnet (I)–garnet (II) boundary in the Hron Complex paragneiss sample PP207 (Figs 3a and 5⇑) exhibits a sharp compositional transition, without diffusion-related smoothing for all the elements except manganese, suggesting very limited internal diffusion at medium temperatures (Alpine temperatures of 540–600 °C). Nevertheless, the preservation of such a sharp compositional transition may also indicate relatively short exposure to the Alpine peak P–T conditions.
The Variscan garnet (I) core-to-rim compositional changes plotted on the calculated P–T sections (Figs 6 and 7⇑) follow a garnet consumption trend (decrease in modal proportion) and indicate retrograde P–T evolution. The chemical composition of the garnet (I) core plots on the calculated P–T section far above the predicted garnet-in line (Figs 6 and 7⇑). Such gaps in the P–T path can be explained by diffusional homogenization of the garnet (I) during Variscan peak metamorphism, which, however, has been subsequently affected by Variscan retrograde diffusional modification. The absence of more complete P–T paths for the Alpine garnets, plotting above the garnet-in line and exhibiting prograde growth zoning, can be explained using the results of our argon diffusion modelling. The modelling suggests that certain domains of the Vepor basement had already experienced temperatures near the closure and blocking temperatures for the argon isotopic system in hornblende (c. 490–531 and 415 °C) prior the onset of Cretaceous metamorphism.
P–T estimates derived from the phase equilibrium modelling range between 570–670 °C and 6–8.5 kbar for the Variscan and 430–600 °C and 5–11 kbar for the Alpine metamorphism and show good correlation with the results of garnet–biotite thermometry. Our P–T estimates for Alpine metamorphism are consistent with the majority of published P–T data. However, for the Variscan metamorphism in the Hron Complex, they exhibit considerably higher values (see Table 2). Because our Variscan P–T estimates are similar to those from the neighbouring Tatra Unit (Krist et al. 1992; Janák et al. 1996), where only a low-grade Alpine overprint occurs, this discrepancy can be explained by problems related to application of conventional thermobarometry in polymetamorphic domains.
The phase equilibrium modelling of the M1 and M2 mineral assemblages indicates systematic differences between the Variscan and Alpine metamorphism. The estimated P–T ranges for the retrograde Variscan and the prograde Alpine events exhibit contrasting metamorphic field gradients of 22–27 °C km−1 and 15–18 °C km−1, respectively (Fig. 10). The relatively hot Variscan metamorphic field gradient is a transient feature typical of the late exhumation stages of the hot orogenic lower crust, commonly reported for Variscan rocks of the adjacent Bohemian Massif (e.g. O'Brien 2000; Tajčmanová et al. 2006; Košuličová & Štípská 2007). In contrast, the Alpine metamorphic field gradient is considerably cooler and corresponds to geothermal gradients, commonly reported for the Alpine collisional belts (Hoinkes et al. 1999, and references therein).
Summary diagram of P–T conditions and P–T path segments specified for each of the studied samples. The peak P–T estimates for Variscan (M1) event (grey crossed squares and grey dashed ellipse) and fragments of prograde Alpine (M2) P–T paths (black arrows) are shown together with the resulting geothermal gradients of 15–18 °C km−1 for the Alpine and 22–27 °C km−1 for the Variscan event. Previously published P–T estimates of Janák et al. (2001) from the southern Hron Complex and Gemer–Vepor Contact Zone are marked by crosses (conventional thermobarometry) and squares (average PT THERMOCALC software, version 2.7). Rock complexes indicated in the figure are the Hron Complex (HC), Král'ova Hol'a Complex (KHC), and Gemer–Vepor Contact Zone (G/VCZ).
The starting points of the Alpine P–T path vectors depicted in Figure 10 suggest that the studied samples were located at different crustal depths prior to burial, but shared a common field geotherm. The preserved P–T path segments exhibit near-contemporaneous increases in pressure and temperature, indicating that the peak P–T conditions correspond to metamorphic conditions at maximum pressures (Pmax). Consequently, the metamorphic field gradient of 15–18 °C km−1 at peak-burial corresponds to an instantaneous field geotherm, which is slightly lower than a standard geothermal gradient (Ranalli 1995), implying minor crustal thickening and stretching of the isograds. The northernmost sample PP482 deviates from the field geotherm of 15–18 °C km−1 (Fig. 10) and exhibits a significantly higher initial temperature. This difference can be explained by a thermal anomaly related to the Jurassic rifting documented in the vicinity of the northern edge of the Vepor Unit (Plašienka 1995).
Interpretation of the burial P–T history
Phase equilibrium modelling allowed us to reconstruct segments of the Alpine prograde P–T path by using the garnet core-to-rim compositional changes. The fractionation of the bulk-rock composition, which could affect garnet growth zoning by depletion of some elements (Mn, Fe) from the matrix (Gaidies et al. 2006), can be neglected as the modal proportion of garnet in the studied samples is low (c. 1.5 modal%). The garnet compositional changes indicate an increase in temperature and pressure of 20–60 °C and 1–1.5 kbar, respectively (Fig. 10).
The prograde compositional zoning of the Alpine garnets is traditionally attributed to burial of the Vepor Unit, related to its southward underthrusting below the supracrustal Gemer Unit (e.g. Kováčik et al. 1996; Plašienka et al. 1999; Janák et al. 2001) during the north–south shortening of the region (Lexa et al. 2003). Nevertheless, the thickness of the overriding rock pile is unclear and is disputed, and depends on the interpretation of the status of the Gemer–Vepor Contact Zone. If this contact zone is considered to be part of the Vepor cover, then the Alpine P–T conditions of c. 550 °C and 8 kbar (Lupták et al. 2000; and this work) suggest a c. 28 km thick overburden (e.g. Plašienka et al. 1999; Lupták et al. 2000). However, if the Gemer–Vepor Contact Zone is considered to be part of the Vepor basement, which we prefer on the basis of its polymetamorphic record, then the overburden is much thinner (c. 15 km), being constrained by the metamorphic conditions of the Foederata Cover, indicating 380 °C and 4.5 kbar (Lupták et al. 2003).
The magnitudes and slopes of the P–T path vectors in Figure 10 should be interpreted with caution, as they were obtained from different lithologies. Nevertheless, real magnitudes of Alpine burial can be assessed for the pre-Alpine surface sediments (c. 350 °C and 4 kbar in the Foederata Cover; e.g. Lupták et al. 2003) and for the deepest parts of the Vepor basement. In the deep southern Hron Complex, the magnitude of Alpine burial displacement probably corresponds to the modelled P–T path vector, indicating a 60 °C and 1.5 kbar P–T increase (sample PP207). This is suggested by the presence of numerous hornblende stepped age spectra in this belt, which, according to our argon diffusion modelling, implies that the total temperature increase in the Hron Complex during the Alpine event must have been less than the difference between the argon closure and blocking temperature for hornblende (c. 75 °C). A downward decrease in the Alpine burial displacement (Fig. 11a) is also documented in the Král'ova Hol'a Complex metagranitoids (Figs 7a and 10⇑); however, the magnitude of the P–T path vectors in the Král'ova Hol'a Complex metagranitoids is lower than that of the deeper Hron Complex (Fig. 11a).
A model of vertical shortening and horizontal ductile spreading based on differential burial along a vertical crustal profile of the Vepor Unit. (a) Differential vertical displacement shows downward decrease in magnitude of burial. The wide grey lines represent positions of particles following homogeneous deformation. Approximate position of argon partial retention zone for hornblende estimated for the period of tectonic quiescence of 200 Ma is indicated. Filled and open circles in (a) and (b) show final (peak-pressure) and initial depths derived from the pressure estimates of Lupták et al. (2003) (Foederata Cover) and this work (Král'ova Hol'a Complex and Hron Complex). The arrows demonstrate relative vertical displacement of the samples. The difference between modelled and total magnitudes of burial for the Král'ova Hol'a Complex samples (PP225, PP229, PP150 and MM136) is explained by deformation partitioning as illustrated in sketch 2 in (b). (b) Sketch of the three burial stages of vertically distributed crustal domains within the Vepor Unit, illustrating heterogeneous accommodation of deformation. Differential burial of the three domains corresponds to vertical shortening being proportional to horizontal stretching.
The hypothesis of downward decrease in magnitude of Alpine burial implies that the internal part of the Vepor basement had to accommodate significant vertical shortening and horizontal stretching. This observation is consistent with the synmetamorphic (synburial) development of the subhorizontal mylonitic fabric SA1 in the Vepor basement. Consequently, the vertical shortening and horizontal east–west (orogen-parallel) ductile stretching of the Vepor basement may have been coeval with northward overthrusting of the Gemer Unit (Fig. 11b).
Distribution of the Alpine isograds and isotherms
Our interpretation of the Vepor basement burial history suggests that the Alpine metamorphic isograds developed subhorizontally and subparallel to the synmetamorphic deformation fabric SA1. This is supported by the structurally downwards increase in the P–T conditions and associated decrease in 40Ar/39Ar ages. If the Alpine metamorphic isograds originally developed subhorizontally, then the post peak-metamorphic tectonic events must have deformed the isograds to explain their current partially steep disposition.
Our structural field observations indicate that the Alpine synburial fabric SA1 is folded and heterogeneously reworked by a steep axial cleavage SA2, which chiefly developed within the NE–SW-trending belts of the Hron Complex. Moreover, the southern belt of the Hron Complex exposes rocks recording the highest Alpine metamorphic conditions (see also Vrána 1964; Plašienka et al. 1999; Janák et al. 2001). Janák et al. (2001) documented an internal metamorphic zonation within this belt, marked by east–west-trending metamorphic isograds, indicating a southward decrease in the metamorphic grade (see Fig. 9 for the location of the zones). Our P–T data suggest that a similar decrease in the metamorphic grade also occurs towards the north, where the P–T estimates from the southern Hron Complex and neighbouring central part of the Král'ova Hol'a Complex exhibit a difference of up to 150 °C and 5 kbar (Fig. 9). The spatial distribution of our Alpine P–T data is in good agreement with the regional variations of the existing 40Ar/39Ar ages (Fig. 9), pointing to the folding-related redistribution of the Alpine isograds and isotherms (Fig. 12).
Block diagram showing a model of heterogeneous exhumation of buried thermal stratification of the Vepor basement. The bulges elevated above the contemporary erosional surface represent an imaginary surface of the 450 °C isotherm. (For colour coding of the rock complexes see Fig. 2.)
Significance of the orogen-parallel extension in the Vepor Unit for the Cretaceous evolution of the Central West Carpathians: alternative models
Based on the existing deep seismic transect 2T and available structural data, Tomek (1993) and Plašienka et al. (1997) proposed that burial of the Vepor Unit is associated with the crustal-scale thrust sheet stacking of the Central West Carpathians. According to this geometry, the Veporic sheet was surrounded by two strongly non-coaxial crustal-scale thrust zones (Plašienka 1993, 2003) and, as described here, experienced synconvergent orogen-parallel stretching. A similar tectonic scenario has been recently proposed by Indares et al. (2000), who described orogen-parallel stretching, perpendicular to thrusting, within the rheologically weaker units bounded by strong thrust sheets. This model, however, fails to explain the deformation compatibility problem, which can be eliminated only if the synconvergent orogen-parallel stretching is compensated by extrusion of weakly constrained lateral foreland (Seyferth & Henk 2004). Nevertheless, the evidence for laterally unconfined boundary conditions of the Cretaceous Central West Carpathians convergent system (see Tapponnier et al. 1982; Ratschbacher et al. 1991a) remains poorly defined (Frank & Schlager 2006). Therefore, the synburial lateral escape model for the Vepor basement must be considered with caution.
An alternative explanation of orogen-parallel stretching is associated with a model of synconvergent exhumation of deeply buried crustal rocks (e.g. Selverstone 1988; Ratschbacher et al. 1991a, b; Mancktelow & Pavlis 1994; Fügenschuh & Schmid 2005). This concept was favoured by Plašienka et al. (1999) and Janák et al. (2001), who suggested that the Vepor Unit evolved as a metamorphic core complex during Cretaceous growth of the Central West Carpathians orogenic wedge.
In contrast, we are of the opinion that the spatial variations in the Alpine metamorphic grade across the Vepor basement resulted from DA2 folding of the synburial deformation fabric SA1 (Fig. 12). Differential exhumation occurred mainly along two belts, where structurally deeper paragneisses–micaschists of the Hron Complex separate higher metagranitoids of the Král'ova Hol'a Complex by forming large-scale FA2 anticlines (Fig. 2). A similar model of differential exhumation of lower crustal rocks within the hinge zones of crustal-scale anticlines has been proposed for the Namche Barwa region on the basis of field documentation and numerical modelling (Burg et al. 1997; Burg & Podladchikov 1999).
The previously described, low-angle extensional shearing, related to exhumation and tectonic unroofing of the Vepor Unit (Hók et al. 1993; Plašienka 1993; Plašienka et al. 1999; Janák et al. 2001), is restricted in our model to the contact between the Vepor and Gemer units. Similarly to the exhumational model for the Tauern Window of Mancktelow & Pavlis (1994), this low-angle detachment fault zone could operate synchronously with synconvergent large-scale folding.
On the other hand, because of the low resolution of existing geochronological data, the incomplete pattern of Alpine metamorphic isograds and the lack of knowledge of Cretaceous plate configuration in the Central West Carpathians, we cannot rule out the hypothesis of rapid switches between contraction and extension. This model may involve (1) north–south contraction during the Early Cretaceous Central West Carpathians thrust sheet stacking, followed by (2) east–west orogen-parallel crustal extension localized within the ductile Vepor Unit and resulting in the development of a detachment zone between the Vepor and overlying Gemer units and (3) Late Cretaceous north–south contraction resulting in the formation of upright FA2 folds. A tectonic switching model, thermally and rheologically justified by the calculations of Thompson et al. (2001), is currently being proposed in many regions (e.g. Collins 2002a, b; Schulmann et al. 2002). Based on modern and ancient examples, tectonic switching occurs when a slab retreat induces an upper plate extension, transiently followed by a flat subduction (or slab flip) and crustal thickening (Collins 2002a). If the tectonic switching model is applied to the Central West Carpathians region, then the driving force may be represented by the south-dipping lithospheric subduction operating during Cretaceous time (Plašienka 2003).
Further structural work and tectonic modelling may be required to distinguish between these models.
Conclusions
The polymetamorphic record in the Vepor basement related to the Variscan and Alpine regional metamorphic events has been characterized using phase equilibrium modelling. The Variscan metamorphism indicates P–T conditions of 570–670 °C and 6–8.5 kbar, retrograde P–T evolution and a metamorphic field gradient of 22–27 °C km−1, corresponding to the late exhumation stages of the hot Variscan orogenic lower crust. In contrast, the Alpine metamorphism indicates P–T conditions of 430–600 °C and 5–11 kbar, prograde P–T evolution and a metamorphic field gradient of 15–18 °C km−1, characteristic of Alpine collisional belts.
The existing 40Ar/39Ar cooling ages from the Vepor basement have been re-evaluated in terms of thermal history and Cretaceous temperature overprint. The examination of existing argon age spectra, by means of the argon diffusion modelling, suggests that certain parts of the Vepor basement had already been located deep in the crust before the onset of the Cretaceous tectonometamorphic event.
During the Cretaceous, the Vepor Unit was thrust under the Palaeozoic metasediments of the Gemer Unit, resulting in apparently contemporaneous burial and orogen-parallel ductile spreading of the Vepor Unit.
Spatial variations in P–T estimates and Palaeozoic to Cretaceous 40Ar/39Ar ages indicate that the post-burial isotherms were deformed during subsequent large-scale folding. The folding is responsible for differential exhumation of the Vepor basement, where the structurally deeper metapelites separate structurally higher metagranitoids by forming large-scale anticlines.
Acknowledgements
This work was supported by research grants from the Czech Science Foundation (GACR 205/03/1490), the Charles University Science Foundation (GAUK 373/2004) and the MSM project No. 0021620855. Internal funding of CNRS UMR 7517 to O.L. is gratefully acknowledged. Two anonymous reviewers are thanked for their critical comments. M. Krabbendam is thanked for his careful editorial work. The valuable comments of J. Konopásek and V. Janoušek are appreciated.
- © 2008 The Geological Society of London
References
Alpine burial and heterogeneous exhumation of Variscan crust in the West Carpathians: insight from thermodynamic and argon diffusion modelling
