The Alps as part of a larger system of Circum-Mediterranean orogens
Meliatic blueschists and their detritus in Cretaceous sediments: new data constraining tectonic evolution of the West Carpathians
Swiss Journal of Geosciences volume 112, pages 55–81 (2019)
The Western Carpathian Meliata Superunit (Meliaticum) includes a heterogeneous group of units—the blueschist-facies Bôrka Nappe and the very low-grade chaotic complexes—polygenous mélange containing material derived from various tectonic settings (Jaklovce Unit) and Jurassic oceanic sediments with olistostrome bodies (Meliata Unit s.s.). The high pressure/low temperature (HP/LT) metamorphism, development of tectonic mélanges and synorogenic sediments took place simultaneously during the Middle–Late Jurassic in connection with closing of the Meliata Ocean. We present some new data concerning composition of variegated mélanges related to the subduction–accretion processes of the Meliata Ocean and its continental margins. The polygenous mélange contains a mixture of fragments of HP/LT up to unmetamorphosed sedimentary and volcanic rocks, including blueschist-facies radiolarites. New electron microprobe chemical age data of monazites from metasediments of the Bôrka Nappe cluster in two peaks—earliest Cretaceous ages are interpreted in terms of post-exhumation, renewed burial during formation of the Meliatic accretionary wedge. The mid-Cretaceous ages might record the thermal relaxation during the thick-skinned nappe stacking and exhumation of the neighbouring Veporic metamorphic dome. In addition, we describe two distinct types of tourmalines and epidotes occurring in the Bôrka blueschists that document the prograde HP and retrograde LP metamorphic events. Detritus of the blueschist-facies rocks appears for the first time in the heavy mineral spectra of the Barremian–Aptian platform limestones. These limestones occur as clasts, together with glaucophanite pebbles, in the Albian–Cenomanian flysch formations of the Pieniny Klippen Belt (Klape Unit). We also review occurrences of other blueschist and ophiolitic erosional products in Cretaceous clastic formations and suggest that all have a Meliatic provenance.
In terms of the plate tectonics theory, the blueschist facies metamorphic units, along with synorogenic wildflysch and ophiolite-bearing mélange complexes, play a crucial role in deciphering tectonic evolution of subduction–accretion zones and collisional orogenic systems. Fragments of high pressure and relatively low temperature (HP/LT) metamorphosed, predominantly oceanic magmatic and sedimentary rocks are usually found in the axial zones of collisional orogens where they indicate position of fossil plate boundary, i.e. suture. Specific structural settings and processes are necessary to preserve at least partly the HP/LT metamorphic assemblages during their return to the Earth’s surface. These include either a rapid, nearly isothermal exhumation by the corner flow and extension in the rear parts of accretionary wedges (e.g. Platt 1986, 1993), or more commonly a slow, buoyancy-driven exhumation within the subduction channel where heating is suppressed by the refrigerating effect of the cold descending slab (e.g. Ernst 1988).
In the Alpine–Carpathian mountain belt, the majority of (U)HP/LT units are related to the Paleogene subduction of the Pennine oceanic lithosphere and attached continental slivers. These are widespread in the Western Alps (e.g. Pleuger et al. 2007; Weber et al. 2015 and references therein); eastward they crop out only in tectonic windows from below the Austroalpine basement sheets—the Tauern window in the central Eastern Alps (Kurz et al. 1998; Nagel et al. 2013) and the Rechnitz–Kőszeg and Bernstein windows at their eastern extremity (Koller 1985; Ratschbacher et al. 1990). No eastward continuation of this Paleogene blueschist belt can be found at the present erosional surface of the Western Carpathians.
Following the fundamental works by Thöni and Jagoutz (1993), Neubauer (1994), Froitzheim et al. (1996) and Dallmeyer et al. (1996), the Late Cretaceous tectonism and eclogite facies metamorphism in the Austroalpine units of the Eastern Alps have been attributed to collisional processes resulting from closure of different oceanic domains—the (Neo)Tethys-related Meliata or Meliata–Hallstatt Ocean (Gawlick et al. 1999; Faupl and Wagreich 2000; Neubauer et al. 2000; Frisch and Gawlick 2003; Froitzheim et al. 2008 and many others). According to Schmid et al. (2004, 2008), the Eoalpine high-pressure belt represents a dividing element of former northern (lower plate) and southern (upper plate) margins of Triassic Meliata Ocean along an intracontinental suture zone continuing westward from the Meliata Ocean embayment, possibly related also to a Jurassic strike-slip wrench zone (Frank and Schlager 2006). Similarly, Janák et al. (2004) and Stüwe and Schuster (2010) ascribed the development of the Eoalpine eclogites to an intra-Austroalpine subduction of gravitationally unstable continental lithosphere due to its densification by Permian magmatic underplating of gabbroic bodies and post-extension thermal thickening of the mantle lithosphere.
Sharing an analogous structural position, Late Cretaceous age and tectonometamorphic evolution, the Veporic metamorphic complex of the Central Western Carpathians was correlated with the Eastern Alpine Eoalpine metamorphic units by Janák et al. (2001). The Cretaceous metamorphism of the Veporic Superunit was explained by post-collisional crustal thickening due to stacking of thick-skinned thrust sheets in the footwall of the Upper Jurassic blueschist- and ophiolite-bearing Meliatic units (Plašienka 1991, 1997; Plašienka et al. 1997; Janák et al. 2001; Plašienka in Froitzheim et al. 2008). In this conception, the Meliata and related units, including the high-pressure Bôrka Nappe, represent the accretionary complex in an allochthonous position rooted between the prowedge Central Carpathian (Austroalpine) basement-cover thrust stack and the retrowedge Internal Carpathian Pelso Megaunit with the South-Alpine and Dinaric links (Transdanubian and Bükk units, respectively—cf. Haas et al. 1995).
The overall structure, tectonic division and evolution of the Western Carpathians has been reviewed in several papers, e.g. Andrusov (1968), Maheľ (1986), Plašienka et al. (1997), Froitzheim et al. (2008), or Hók et al. (2014). However, views on tectonics of the southern Carpathian zones and, particularly, on the positions, relationships and palaeogeographic settings of the Meliatic and related units remain controversial. The fundamental question concerns the origin of the Meliata Ocean either as an independent Triassic back-arc basin developed inboard the active European margin during northward subduction of Palaeotethys (e.g. Stampfli and Borel 2002; Stampfli and Kozur 2006; Stampfli and Hochard 2009), or as the north-western embayment of Neotethys (e.g. Haas et al. 1995; Frisch and Gawlick 2003; Schmid et al. 2008; Missoni and Gawlick 2011). Consequently, these different models result in two different interpretations of Jurassic subduction of the Triassic Meliata Ocean: (1) It represents the result of subduction below an active margin of variously interpreted “southern continent” (Adria or the Pelso Megaunit including the Bükk element as its eastern promontory, Tisia terrane, or an island arc—e.g. Channell and Kozur 1997; Faryad and Henjes-Kunst 1997; Plašienka et al. 1997; “unknown block” of Lexa et al. 2003); (2) It is the result of subduction underneath the obducted Jurassic ophiolite nappes with the Meliata-type mélange complexes at their base (Dinaric model—Schmid et al. 2008; Missoni and Gawlick 2011).
Diverse opinions exist also about the relations of the Late Jurassic closure of the Meliata Ocean to the Cretaceous Carpathian orogeny. The following models have been proposed: (1) the Meliatic units were emplaced during an older Wilson cycle (Cimmerian orogeny according to Kozur 1991; Kozur and Mock 1997; Lexa et al. 2003) that is not related to the Cretaceous–Paleogene Alpidic cycle; (2) they represent an obduction-related Middle–Upper Jurassic fold-thrust belt with sequential development of frontal trench-like synorogenic basins (Neotethyan Belt of Missoni and Gawlick 2011); (3) the Cretaceous orogeny was driven by continuing post-obduction subduction of the Austroalpine lower crust and mantle lithosphere attached to the sinking Meliata slab until the Late Cretaceous (Stampfli and Hochard 2009; Handy et al. 2010); or analogously (4) The Meliatic complexes formed during a similar subduction scenario following collision of Central Carpathian domain and the southern continental block (Adria-related Pelso) that drove the post-collision shortening of the central Carpathian zones (Plašienka 1991; Plašienka in Froitzheim et al. 2008). Additional uncertainties concern the palinspastic settings of the Turnaic and Silicic cover nappe units overlying the Meliatic elements. They have been assigned to the lower Austroalpine plate (i.e. northern margin of Meliata—Kovács et al. 1989, 2011; Kozur 1991; Kozur and Mock 1997; Less 2000; Schmid et al. 2008), to the upper plate (southern margin—Hók et al. 1995; Rakús 1996; Neubauer et al. 2000; Lexa et al. 2003; Csontos and Vörös 2004; Dallmeyer et al. 2008), or emplaced by the sinistral strike-slip translation with or without the attached Meliatic units (Frank and Schlager 2006; Kövér and Fodor 2014; Kövér et al. 2018).
To resolve ambiguous interpretations of the position and evolution of Meliatic complexes, new stratigraphic, sedimentological, structural and metamorphic data are indispensable. Meliatic rocks crop out only in scattered occurrences of both tectonic mélanges and chaotic sedimentary units with very complex internal structure; therefore any new data may provide crucial information for constraining their tectonometamorphic evolution and relationships to adjacent units.
In this paper, we present new results regarding the composition of a polygenous mélange unit that involves also fragments of blueschist-facies radiolarites, new monazite age data from the HP Bôrka Unit, and manifestation of blueschist detritus in the Lower Cretaceous clastic sediments of several Western Carpathian units. These findings are discussed in the context of tectonometamorphic evolution of the Meliatic subduction/accretion complexes, including their exhumation and erosion revealed by the sedimentary record. In the present paper, we do not address the recently widely discussed problems of regional relationships and correlation of the Meliatic units and their paternal Meliata Ocean with similar occurrences in the Alps, Dinarides and particularly in north-eastern Hungary. In this aspect, the reader is advised to the topical works by Kövér et al. (2018) and Plašienka (2018), and to references therein.
Our study concerns two remote, but mutually related areas—the Meliatic units in the southern Carpathian zones, and the Klape and related units in the so-called Peri-Klippen Zone adjoining the classic Pieniny Klippen Belt at the boundary between the Central and External Western Carpathians (Fig. 1). While the first area exposes the blueschist-facies metamorphic rocks affiliated with Meliatic units in their original tectonic position, their erosional products in the form of blueschist pebbles and detrital blue amphiboles occur in Cretaceous flysch formations of the second region.
The investigated area in the Slovak–Aggtelek Karst of the Slovakian–Hungarian borderland represents a transitional zone between the southern Austroalpine units of the Central and the South Alpine–Dinaric units (Transdanubian and Bükk, respectively) of the Internal Western Carpathians occurring to the south (Figs. 1, 2). The lowermost structural position is occupied by the Upper Austroalpine Gemer Superunit (Gemericum) composed of mostly low-grade Lower Paleozoic volcano-sedimentary formations overlapped by Pennsylvanian, Permian and Lower–Middle Triassic, chiefly continental and shallow-marine clastic formations (e.g. Ivanička et al. 1989; Vozárová and Vozár 1988). At the contact with the underlying basement and cover rocks of the Vepor Superunit (Veporicum) along the Lubeník thrust fault (Fig. 2), the partially independent Gemeric zone is formed by the Ochtiná Unit. This unit is composed of Mississippian mafic volcanites, shales and carbonates (including magnesite bodies; cf. Vozárová 1996).
The Gemeric basement-cover sheet is overridden by a stack of cover nappes involving the Meliatic, Turnaic and Silicic units from bottom to top (Fig. 2). Based on rock composition and metamorphic characteristics, the Meliata Superunit (Meliaticum) was differentiated into three types of units (Lačný et al. 2016): (1) HP/LT Bôrka Unit in the lower structural position, which is overridden by and partly imbricated with (2) ophiolite-bearing polygenous and serpentinitic mélanges of the Jaklovce Unit, and (3) sedimentary complex with olistostrome bodies almost devoid of ophiolitic material (Meliata Unit s.s. or Meliata Olistostrome Formation—Mello et al. 1997).
The high-pressure metamorphism and paleotectonic significance of blueschist facies rocks affiliated with the Meliata Unit s.l., currently assigned to the Bôrka Nappe (Leško and Varga 1980; Mello et al. 1998), were for the first time described by Reichwalder (1970, 1982). Their detailed petrological studies were later completed by Faryad (1995a, b, 1999), Faryad and Henjes-Kunst (1997); Faryad and Hoinkes (1999) and Mello et al. (1998). It was shown that the Meliatic complexes consist of assorted mélanges with ophiolitic material, deep-water sediments, olistostromes, blocks and slices with distinct composition and metamorphic histories.
The tightly imbricated and dismembered Bôrka Nappe embraces metasediments and metavolcanics of Permian and Triassic age, and possibly also of Jurassic age (Mello et al. 1998). The approximate age assignment is based on the lithological correlations only, since all metasediments are devoid of determinable fossils due to the HP/LT metamorphic recrystallization and ductile deformation. From the lithotectonic point of view, the unit was divided into two complexes. The lower complex rocks are consisting of Upper Paleozoic volcano-sedimentary formations like siliciclastic conglomerates, sandstones, shales and 266 Ma old rhyolites (SHRIMP zircon ages; Vozárová et al. 2012) and Lower Triassic quartzites, slates and calcareous schists. The upper structural complex starts with probably lower Anisian pale marbles and dolostones. The first, syn-rift volcanism appears in the upper part of the carbonate complex in a form of mixed carbonate-volcaniclastic material including basaltic lapilli, hyaloclastites and peperites, followed by massive and pillowed basaltic submarine lava flows. The mafic volcanic rocks, associated with abyssal sediments, show a geochemical evolution from back-arc basalts (BABB) to the enriched and normal mid-oceanic ridge basalts (E-MORB and N-MORB, respectively) associated with abyssal sediments (Ivan 2002a, b; Faryad et al. 2005). Black, phyllitic and siliceous slates containing some olistostrome bodies form the upper metasedimentary complex of the Bôrka Nappe of the inferred Late Triassic, and possibly also Jurassic age (Mello et al. 1998). Tectonic fragments derived from the Variscan, possibly Gemeric basement (gneisses and amphibolites—Faryad 1988; Faryad and Frank 2011), overprinted by blueschist-facies metamorphism, indicate intimate relationships of the Bôrka Unit with the continental margin. Owing to continental basement slices and continental character of the lower pre-rift complex and its lithological similarities with the southern Gemeric Permian—Lower Triassic cover, the Bôrka Unit is palaeogeographically interpreted as fragment of the distal passive continental margin facing the Meliata Ocean which rifted in the Anisian (Mello et al. 1998; Plašienka 1998, 2018).
Detailed microstructural, petrological and geochronological investigations of the HP/LT Bôrka Nappe were carried out by a number of authors (Maluski et al. 1993; Dallmeyer et al. 1996, 2008; Faryad 1995a, b, 1999; Faryad and Henjes-Kunst 1997; Mello et al. 1998; Faryad and Hoinkes 1999; Árkai et al. 2003; Faryad et al. 2005; Vozárová et al. 2008; Putiš et al. 2011a, 2014, 2015; Németh et al. 2012; Li et al. 2014). The maximal P–T conditions of HP/LT metamorphism were detected in glaucophane-bearing metabasalts (Faryad 1995a; Faryad and Hoinkes 1999). Textural relationships reveal that the Bôrka Nappe experienced a prograde burial path with increasing P–T conditions from ca 0.5 GPa and 350 °C up to maximum pressures of at least 1.2 GPa and temperatures up to 460 °C. These were calculated from the association of glaucophane, epidote, albite, jadeite-rich Na-pyroxene, phengite, paragonite and garnet. Comparable P–T conditions were estimated for the metapelites, using glaucophane, chloritoid, paragonite, phengite, albite, chlorite, Na-pyroxene and garnet (Faryad 1995b). However, the HP metamorphic assemblages are not uniformly distributed within the whole unit, some parts record slightly lower P–T conditions, and others show exhumation-related, greenschist facies re-equilibration. Ivan (2002a, b, 2007) distinguished several lithotectonic units of the Bôrka Nappe and characterized their metamorphic histories and geochemical signatures.
The HP/LT metamorphic conditions were attained at ca 155 Ma (mean of 39Ar/40Ar ages of phengitic micas; Maluski et al. 1993; Dallmeyer et al. 1996, 2008; Faryad and Henjes-Kunst 1997). However, these white mica ages might record early stages of cooling during slow exhumation within the subduction channel as well, since the peak metamorphic temperatures reached ca 460 °C, i.e. at the upper limit of closure temperatures of K–Ar system in phengites. On the other hand, Ar/Ar dating of phengites from the lower complex of the Bôrka Nappe, which is composed of Upper Paleozoic siliciclastic and acid volcaniclastic formations, yielded slightly older ages around 172 Ma (Faryad and Henjes-Kunst 1997). Monazite electron microprobe (EMPA) chemical age dating yielded ages of 167 ± 12 Ma up to 190 ± 16 Ma, though with a wide scatter, for the underlying south Gemeric Paleozoic rocks (Vozárová et al. 2008, 2014; Hurai et al. 2015). However, the chemical monazite dating should be treated with caution, since for example the EMPA age spectra from the Gemeric albitite metasomatites scatter between 185 and 78 Ma, whereas LA-ICPMS 206Pb/238Pb isochron lower intercept from the same monazites yielded 139 ± 1 Ma, which corroborates the supposed involvement of the Gemericum in the thrust stacking processes. Consequently, the meaning of monazite ages older than ca. 160 Ma is unclear and their interpretation as a distinct tectono-thermal event might be misleading (Hurai et al. 2015).
Mélanges of the Jaklovce Unit contain fragments of basalts, serpentinites, acid volcanites, radiolarites of Middle–Upper Triassic and Jurassic age, and various Triassic shallow- and deep-water carbonates (Mock et al. 1998; Ivan et al. 2006, 2009; Ivan and Méres 2009; Putiš et al. 2011a, b; Németh and Radvanec 2014). All these blocks and slices are embedded in a Jurassic radiolarite-shaly-marly-sandy matrix, showing variably strong tectonic overprinting of the chaotic sedimentary complexes.
A distinct mélange type is represented by the typical tectonic, ophiolite-bearing mélanges. In places, they consist of brecciated serpentinized ultramafics enclosed in a strongly sheared serpentinite matrix (Jaroš et al. 1981), whereas at other sites the mélange also includes other fragments of the ophiolite suite—various types of basalts, dolerites and associated oceanic sediments like Middle–Upper Triassic radiolarites and pelagic limestones, embedded in shaly matrix. We propose the term Bretka Complex for the Meliatic ophiolite-bearing mélanges (see Fig. 3), which occur at several sites (Jaklovce, Dobšiná, Bretka, Čoltovo, Hodkovce).
The Meliata Unit s.s., (type locality indicated in Fig. 2), is a sedimentary unit of Jurassic deep-water deposits with olistostromes and olistoliths of various Triassic carbonates and Ladinian radiolarites, containing only little ophiolitic material (Mock et al., 1998 and references therein; Aubrecht et al. 2012). It should be noted that Jurassic radiolarites of the Meliata and Jaklovce units are distinctly different from the Triassic radiolarites. They contain a considerable amount of terrigenous admixture and are associated with flysch-type sediments instead of oceanic basalts.
After scraping off, rocks of the Meliata Unit s.s. escaped deeper burial in the accretionary wedge and presently form the structurally highest parts of the Meliatic complexes (Fig. 3) affected by very low- to low-grade (anchizonal or sub-greenschist) metamorphic transformation reaching 280–350 °C at 250–500 MPa. K–Ar ages of white mica concentrates of low-grade slates scatter between 178 and 115 Ma, with a peak at ca 145–140 Ma (Árkai et al. 2003; Kövér et al. 2009). This prograde event was coeval with retrograde mylonitic overprinting of the Bôrka blueschist phyllites. Slightly higher metamorphic pressure estimates from the overriding Turnaic nappes (Kövér et al. 2009; Kövér and Fodor 2014) might indicate out-of-sequence thrusting within the developing accretionary wedge.
The Gemeric and Meliatic units are overridden by the Turňa Superunit (Turnaicum or Tornaicum—cf. Vozárová and Vozár 1992; Lačný et al. 2016 and references therein). The very low-grade to low-grade Turnaic units are composed of Upper Paleozoic to Lower Triassic clastic sequences, lower Anisian platform carbonates and post-Pelsonian deep-water pelagic limestones and shales. In part they are out-of-sequence imbricated with the underlying Meliatic complexes, both belonging to the accretionary complex (Árkai et al. 2003; Kövér et al. 2009; Lačný et al. 2016).
The Silica Superunit (Silicicum) is represented by the classic Silica Nappe in the investigated area, whose allochthonous position was first recognized by Kozur and Mock (1973). The Silica Nappe flatly overlies the imbricated Meliatic–Turnaic units with a marked metamorphic and structural discordance (Reichwalder 1982). The base of the nappe is formed by lenses of Upper Permian to Lower Triassic evaporites followed by Lower Triassic clastics and Middle to Upper Triassic, dominantly carbonate platform complex. The thin Jurassic sequence is terminated by Upper Jurassic pelagic deposits with terrigenous admixture and carbonate olistostromes containing clasts of Middle Jurassic radiolarites (Sýkora and Ožvoldová 1996; Rakús 1996; Rakús and Sýkora 2001). Although generally considered as not metamorphosed, varying and in places fairly high conodont colour alteration indices (CAI 1.5–5.5; cf. Gawlick et al. 2002; Havrila 2011) indicate that the Silica Nappe might represent an amalgamated unit composed of several fragments that experienced different thermal overprint.
Analyses of all minerals including monazites were performed by Cameca SX-100 microprobe at the Department of the Electron Microanalysis, State Geological Institute of Dionýz Štúr, Bratislava. Monazite dating requires special measurement conditions since the calculated age strongly depends on the precise measurement of Pb, U, Th, and Y. For monazite analysis, beam conditions of 15 kV and 100 nA and varying counting times, depending on the measured element: Pb 150 s, Th 45 s, U 75 s, Y 45 s and all other elements 25–35 s. The elements were calibrated using synthetic or natural standards: all 14 REE elements were calibrated from synthetic phosphates, P was calibrated from apatite, Ca and Si from wollastonite, Al from corundum, Pb from galena, Th from ThO2, and U from UO2. Thorium, U, Pb, Y, and P were measured with LPET (large PET), and REE with LLIF (large LIF) and Si, Al with TAP analysing crystal. The beam diameter was typically 3–5 μm.
The measurement is complicated by the presence of various interferences among the X-ray lines. We used ThMα1, UMβ1, PbMα1, and YLα X-ray lines. The interferences between PbMα1–YLγ1 and UMα1–ThMβ1 were corrected by empirically measured correction coefficients. Interferences between REE X-ray lines were also corrected. The accuracy of monazite dating is therefore related to the monazite secondary standards, whose variability is constrained by SHRIMP analyses. The following monazite standards were used: granite from Veikola, Finland (1825 Ma), pegmatite from Madagascar (495 Ma), gneiss-migmatite from Dürstein/Wachau, Austria (341 Ma), granite from Aalfang, Austria (327 Ma) and monzogranite from Nakane, Japan (77 Ma). Before measuring monazites of unknown age, we first measured all standards, each at least with 20 points. The ± 5–7 Ma deviations for each monazite age standard are considered as a good precision. The statistical approach of Montel et al. (1996) was applied for the resulting age determination. The program DAMON (Konečný et al. 2004) was then used for the age recalculations, histograms, and isochron plots.
The composition of tourmaline was established with CAMECA SX-100 electron microprobe in wavelength-dispersion mode, at the State Geological Institute of Dionýz Štúr, Bratislava. The analytical conditions were: accelerating voltage 15 kV, beam current 20 nA, and beam diameter of 3 to 5 μm. The tourmaline samples were analyzed with the following standards: wollastonite (SiKα. CaKα), TiO2 (TiKα), Al2O3 (AlKα), pure Cr (CrKα), pure V (VKα), fayalite (FeKα), rhodonite (MnKα), forsterite (MgKα), willemite (ZnKα), pure Ni (NiKα), albite (NaKα), orthoclase (KKα), BaF2(FKαα) and NaCl (ClKα). Lower detection limits of the measured elements varied between 0.01 and 0.05 wt. %; V, Cr, Mn, Zn, Ni, F, and Cl were also below their respective detection limits. The analytical data were normalized according to the PAP procedure (Pouchou and Pichoir 1985).
Procedures of separation and analyses of the heavy mineral concentrates from turbiditic sandstones of Cretaceous flysch formations are described in detail in the paper by Bellová et al. (2018).
Composition of Meliatic polygenous mélanges
We have investigated in detail a distinctive narrow strip of Meliatic rocks that appears on the northern slopes of the Tri peniažky Hill SW of Jelšava town and at the nearby locality Nandráž (Fig. 2; Table 1), where they are squeezed between the underlying Gemeric Upper Paleozoic clastic sediments and overriding Middle–Upper Triassic carbonate formations of the Turňa Nappe. The mélange complex is characterized by a strongly imbricated to completely disintegrated structure with slivers of both non-metamorphic and HP/LT metamorphosed rocks. Variously sized, mixed slices and blocks of almost all types of Meliatic complexes described above occur together in the studied small area.
Unmetamorphosed to very low-grade rocks (Meliata Unit s.s. and/or the Jaklovce Unit) are represented by radiolarites affected only by diagenesis (Fig. 4a, b), serpentinites with preserved original peridotitic structure (pseudomorphs after pyroxene, olivine and spinel) (Fig. 4j–l), and part of light crystalline limestones with layers of green shales. Pale red radiolarites are characterized by a massive texture and irregular dense network of tiny quartz veins (Fig. 4c, d).
Rocks containing index blueschist-facies metamorphic minerals like paragonite (Pg), garnet (Grt), glaucophane (Gln), piemontite (Pmt) and chloritoid (Cld) are typical for the HP/LT Bôrka Nappe (mineral abbreviations according to Whitney et al. 2010). They include radiolarites with Grt, Gln and Pmt (Fig. 4e–i), chlorite-sericite phyllites with Pg and Cld and various lithoclasts, which represent the matrix of the polygenous mélange complex in this area (Fig. 5a–f), and crystalline limestones intercalated with thin layers and fragments of grey to black sericite–chlorite schists with relics of paragonite (Fig. 5g–l; for chemical analyses see Online resource 1).
Despite poorly exposed area, there is apparently no order in distribution of these tightly packed slices and blocks exhibiting contrasting lithologies and metamorphic overprint. Therefore, we interpret this complex as a tectonic mélange, which probably formed at the base of a developing accretionary wedge and incorporated HP/LT fragments exhumed from the subduction channel, as well as non-metamorphic to low-grade rocks, partly also unconsolidated sediments, derived from the Meliatic complexes forming the accretionary prism.
Blueschist-facies metaradiolarites and associated metabasalts
In addition to the Tri peniažky locality described above (Fig. 4e–i), radiolarites metamorphosed under blueschist facies conditions have been found in the Bôrka Nappe near the villages Honce and Štítnik (Fig. 2, Table 1). They form blocks (from centimetres to several metres in diameter) in black phyllitic matrix, but they have been found also as individual blocks together with metabasalts transformed to glaucophanites. Macroscopically dark reddish-brown colour and conchoidal fractures are typical for these metaradiolarites. They are also accompanied by blocks of various types of pelagic sediments transformed to phyllites with paragonite and/or chloritoid and HP/LT metamorphosed basalts with well-preserved hyaloporphyric, intersertal or ophitic phantom textures, and strongly depleted N-MORB geochemical signatures. Examples of various textures of blueschist-facies metabasalts from several localities (Figs. 1, 2, Table 1) are presented in Fig. 6a–f.
Radiolarites metamorphosed under HP conditions are microscopically very fine-grained rocks with dominant quartz with dispersed Fe–Mn oxide ore pigment. Radiolarians are still discernible as oval-shaped phantoms. The presence of garnets, sodium pyroxenes and sodium amphiboles are typical features (Fig. 6g–l). Na-pyroxenes have been found only as inclusions in garnets, ca. 5 μm in size. Aegirine or manganoan aegirine are prevailing components in composition of Na-pyroxenes (Ae 62–75 mol%, Jd 5–22 mol%, Quad 15–20 mol%, Mn = up to 0.180 p.f.u.) (Fig. 7a). Compositions of Na-amphiboles vary between magnesio-riebeckite and manganoan magnesio-riebeckite (Fig. 7b). The spessartine component is prevailing in the composition of garnets (75–78 mol%), andradite (15–19 mol%) and grossular (5–8 mol%) are present in smaller amount, whereas almandine (0–2 mol%) and pyrope (0–0.7%) are negligible (Fig. 7c).
Furthermore, radiolarites originally metamorphosed under the blueschist facies conditions and thereafter retrogressed at greenschist facies have been found in the Bôrka Nappe in the surroundings of Dobšiná town (Fig. 2). Besides prevailing quartz and pigment of Fe-oxides, some Na-actinolite, chlorite and epidote are present. The HP/LT metamorphic stage is detectable by locally preserved glaucophane cores in Na-actinolite crystals or aggregates of green aegirine.
The majority of other radiolarites of the Meliatic Superunit, outside of the Bôrka Nappe and associated mélanges, do not bear any sign of HP/LT metamorphism with one exception—veinlets of magnesioriebeckite in radiolarites have been found near Jaklovce village as an indication of elevated pressure between greenschist and blueschist facies metamorphic conditions (Ivan and Méres 2009; Putiš et al. 2011a).
Dual monazite age groups from the Bôrka Nappe
The Middle–Upper Triassic up to Jurassic? fine-grained siliciclastic metasediments form the sedimentary matrix of the Bôrka Nappe in which blocks of various pelagic metasediments, metacarbonates and metabasalts are emplaced (Mello et al. 1998). The pelagic metasediments are represented by metasiltstones, albitic phyllites, sericite–chlorite phyllites (Fig. 8a–f), chloritoid schists, radiolarites with glaucophane, metasilicites with phosphates, metasilicites with basaltic volcanogenic admixture and metacarbonates. Relics of older paragonite in cores of younger muscovite/sericite are often preserved in sericitic phyllites. Numerous post-kinematic idiomorphic porphyroblasts of monazites (30–500 μm in size) were observed and analyzed in these phyllites. Monazites possess a typical oscillation zonation which is reflected in their chemical composition by relatively lower content of Ce2O3 (27% wt%) and higher content of Nd2O3 (19.2 wt%), Sm2O3 (3.7 wt%) and ThO2 (4.8 wt%) in the cores of the monazite crystals. On the other hand, there are lower values of Nd2O3 (16 wt%), Sm2O3 (0.7 wt%) and ThO2 (0.1 wt%) and increased values of Ce2O3 (36.6 wt%) at the crystal rims (for chemical analyses see Online resource 7).
The EMPA dating of monazites from the locality Honce near Štítnik village (sample OS-07-128) provided two age groups: (1) 147 ± 17 Ma and (2) 89 ± 18 Ma (ages calculated by the statistical method of Montel et al. 1996; Fig. 9a1, a2). We have obtained comparable monazite ages also from the chlorite-sericite phyllites at the locality Hrádok near Jelšava (sample OCH 2B; for chemical analyses see Online resources 9 and 10), which yet again fall into two statistical groups (Fig. 9b1, b2): (1) 139 ± 13 Ma and (2) 97 ± 5 Ma.
Two types of tourmaline and epidote
Two tourmaline generations were found in metasediments of the Bôrka Nappe in the surroundings of town Dobšiná (Fig. 2; sample OS-07-147, locality Dobšiná–Stirn; Fig. 10a–l). The first generation of homogeneous tourmaline (Tur1–dravite) does not contain any inclusions. The second, younger generation (Tur2–schorl) typically comprises numerous inclusions of the greenschist facies minerals and also includes Tur1 crystals (Fig. 10d). The metamorphic origin of both types is revealed by their position in the field of tourmalines in Ca-poor metapelites and tourmaline-quartz rocks in the classification diagram of Henry and Guidotti (1985) (Fig. 11c).
Based on microprobe analyses (Online resource 5), Tur1 differs from Tur2 (Figs. 10g–i, 11a–c) by a higher content of Al2O3 (33–35 wt%), and lower contents of FeO (< 8 wt%) and MgO (< 6 wt%). Weight contents of Tur2 amount to 25–27% of Al2O3, 10–14.5% of FeO and 6–7.5% of MgO. Differences in chemical composition of both types of tourmaline are reflected in their structure. Compared to Tur2 tourmalines, the Tur1 generation is characterized by a higher number of vacancies and higher Mg/(Mg + Fe) ratio. The Tur2 type differs from Tur1 in the lower content of Al, Mg and vacancies. Both tourmaline types belong to the group of alkaline tourmalines and their chemical compositions vary in the range of the dravite–schorl continual series. Tur1 shows a homogeneous chemical composition and represents dravite, whilst schorl is prevailing in rims of Tur2.
In the same sample (OS-07-147), two generations of epidote group minerals—clinozoisite subgroup were identified. By its composition, Ep1 corresponds to clinozoisite to epidote and Ep2 to epidote (Fig. 12a; Armbruster et al. 2006). Epidotes of the first generation (Ep1) are homogeneous and without inclusions (Fig. 10e). The second generation (Ep2) always contains Ep1 as inclusions, together with quartz, albite, titanite, Fe-oxides. Ep1 is often surrounded by a thin (less than 3 μm) transitional zone of Ep1-2. The chemical composition of Ep1 differs from that of Ep2 by showing a higher content of Al2O3 (23–33 wt%) and a lower content in Fe2O3 (1.1–7.7 wt%) and MnO (< 0.5 wt%). Content of Al2O3 varies between 21–23 wt%, Fe2O3 between 11–16.5 wt% and MnO between 0.5 and 1.3 wt% in Ep2. Compared to Ep1, the transitional zone (Ep1-2) shows higher contents of Fe2O3 (7.7–11 wt%), MnO (4–6 wt%) and REE (ΣREE 7–11 wt%).
The content of Al apfu in Ep1 is higher than in Ep2, while contents of Fe apfu and Mn apfu are lower (Figs. 10j–l, 12b). Contents of Fe apfu and Mn apfu are highest and Al lowest in the transitional zone (Ep 1–2). This zone is specific also by the content of REE 0.20–0.34 apfu.
The earliest manifestation of detrital glaucophane in “Urgonian” limestones
The Cretaceous conglomerates of the Pieniny Klippen Belt contain pebbles of variable sedimentary, magmatic and metamorphic rocks (for the reviews see Mišík and Sýkora 1981; Mišík and Marschalko 1988; Mišík et al. 1980, 1981; Birkenmajer 1988). HP/LT, blueschist-facies metamorphic rocks are represented by: (1) pebbles of Upper Jurassic glaucophanites in the Albian–Cenomanian Upohlav-type conglomerates of the Klape Unit (see below); and as (2) terrigenous clastic admixture in pebbles of Barremian–Aptian shallow-water, “Urgonian”-type bioclastic limestones that also occur in the Upohlav conglomerates. We have studied pebbles of “Urgonian” limestones with the clastic admixture from localities in the Klape and Manín units—Figs. 1, 13, Table 1.
In the “Urgonian” limestone pebbles, we have identified benthic foraminifera of genus Orbitolina and planktonic foraminifera of genus Hedbergella (Barremian–Cenomanian), as well as fragments of reef-building bivalves, echinoid particles, algae, corals and other shallow-water organisms in the limestone pebbles (Fig. 13a, b). Lithoclasts are represented by glaucophanites, serpentinites, HP/LT phyllites, amphibolites, acid and mafic volcanic rocks, sandstones, quartzites and carbonates older than Barremian. Detrital minerals include glaucophane, paragonite, muscovite, garnet, chloritoid, allanite, spinel, rutile, calcium amphibole (titanium magnesio-hastingsite), quartz, albite, tourmaline, zircon, biotite, chlorite and barite (Fig. 13c–g). Locally, the amount of blue amphiboles in the heavy mineral concentrates attains 47%. Except of rounded zircon grains, all other heavy minerals are sharp-edged.
According to their composition, the blue sodium amphiboles belong to the series glaucophane—ferro-glaucophane. The composition of detrital white micas complies with the complete substitution series of K-Na micas with the end members muscovite and paragonite. Paragonite forms centres of mica flakes replaced by muscovite in the rims. The composition of the garnets indicates their provenance from: (i) low-grade metasediments (phyllites; analyses No. 17, 22, 23 and 47 in Online resource 4); and (ii) from serpentinites (high content of andradite and grossular; analysis No. 38 in Online resource 4). The composition of spinels corresponds to the chromite—magnesio-chromite series.
The probable age range of the studied “Urgonian” limestones from pebbles of the Upohlav conglomerates is constrained by the rich orbitolinid foraminiferal microfauna. The limestones with terrigenous admixture contain numerous large-sized flat to low conical tests, which probably belong to species Palorbitolina? lenticularis (Blumenbach) (sample 9179; Fig. 14e–i). The precise systematic classification is not possible due to a missing embryonic apparatus of the orbitolinid tests. However, their rock-forming abundance indicates the so-called “Orbitolina episode” in development of the “Urgonian” platform during the early Aptian (Vilas et al. 1995). This could also explain a mass presence of the palorbitolinids without embryonic apparatus since the increased terrigenous influx and nutrification resulted in preference for an asexual reproduction. Therefore, the terrigenous- and orbitolinid-rich limestones could be assigned to Palorbitolina lenticularis Zone, which according to Schroeder et al. (2010) corresponds to late Barremian—early Aptian interval, i.e. around 125 Ma. This age is also indicated by the species Dictyoconus cf. arabicus Henson (Fig. 14j), which is mentioned from the late Barremian with presumable transition to early Aptian (Baud et al. 1994). Orbitolinid tests show a high concentration of spinel grains, which confirm a certain preferences of agglutinated foraminifera to use heavy minerals in construction of the shell material (Waśkowska 2014).
Besides detrital limestones, pebbles of the Orbitolina-bearing limestones with low content in terrigenous admixture are present as well (sample 20444). They contain a different type of orbitolinids with highly conical tests, larger protoconch and more distinctly developed subepidermal cells in marginal zone. Some axial sections of orbitolinids display a subembryonal zone and deuteroconch with alveoli, which are a typical generic feature of Mesorbitolina (Fig. 14a–d). Based on this type of embryonic apparatus, such orbitolinids could belong to species Mesorbitolina texana (Roemer) and M. cf. parva (Douglass). These species are considered to represent index orbitolinid taxa of the late Aptian biozones (sensu Schroeder et al. 2010). Orbitolinids are associated with other foraminiferal species, such as Charentia cuvillieri Neumann, Debarina hahounerensis Fourcade, Melathrokerion cf. valserinensis (Brönnimann & Conrad), Haplophragmoides cf. globosus Lozo, Ammobaculites cf. subcretaceous Cush & Alex, Pseudochrysalidina sp., Bolivinopsis sp., Pseudocyclammina sp. and Conorboides sp. Dasycladal algae are represented by species Neomeris cretacea Stenmann. In general, the above mentioned species Palorbitolina? lenticularis (Blumenbach), Mesorbitolina texana (Roemer) and M. cf. parva (Douglass) are datable for the late Barremian—Aptian age of the spinel- and glaucophane-bearing limestones.
It should be noted that these “Urgonian”-type Barremian–Aptian limestones are considerably different from other “Urgonian” limestones occurring in several Western Carpathian units (Mišík 1990; Michalík and Soták 1990; Michalík 1994). Beside differences in the fossil content and facies, it is the presence of siliciclastic admixture what is the most distinctive feature, because other common “Urgonian” limestones are devoid of it. The only exception is the Barremian–Aptian bioclastic–siliciclastic limestone formation of the Tatric cover succession in the Malé Karpaty Mts. However, these clastic limestones are characterized by the stable zircon-rutile-tourmaline (ZRT) heavy mineral association without any chrome spinels or blue amphiboles. The siliciclastic admixture was obviously derived from local sources in the Tatric crystalline basement and/or recycled from the Lower Triassic sandstones (Jablonský et al. 1993).
Detrital blue amphiboles in mid-Cretaceous synorogenic clastic formations
Amphiboles belong to the group of less stable and resistant heavy minerals (Pettijohn 1941, 1975; Morton 1984). They are especially sensitive to intrastratal dissolution during diagenesis and their occurrence in older sediments may be a matter of special diagenetic conditions rather than their presence or absence in source rocks. Higher amounts of blue amphiboles in psammitic rocks then may indicate either a random local input, or limited preservation of otherwise widespread minerals.
Because of instability and low resistance, there are more findings of glaucophanite rock pebbles in exotics than blue-amphibole grains dispersed in the sediment (cf. Ivan and Sýkora 1993; Ivan et al. 2006; vs. Sýkora et al. 1997, von Eynatten and Gaupp 1999; Bellová et al. 2018). Blue amphiboles described from the Albian–Cenomanian flysch sandstones are represented mostly by glaucophane to ferroglaucophane. Crossite was also commonly reported as the blue amphibole in the exotics (e.g. Ivan and Sýkora 1993; von Eynatten and Gaupp 1999), but this amphibole name was abolished later and is no more in use (Leake et al. 1997). Formerly, the glaucophanite exotic pebbles were considered to be different from glaucophanites of the Bôrka Unit because of differences in the lawsonite content (Šímová and Šamajová 1982; Šímová 1985) and in the protolith (Faryad and Schreyer 1997; Faryad 1998). However, radiometric dating revealed the same Late Jurassic age of the blueschist pebbles (155 Ma—Dal Piaz et al. 1995) and more thorough analyses showed that the differences are minimal (e.g. Ivan et al. 2006) and hence the exotic source area was most likely formed by units derived from the Neotethyan Meliata Ocean.
Recently, Bellová et al. (2018) have reported a high amount of blue amphiboles in mid-Cretaceous turbiditic sandstones of the Klape Unit in the Middle Váh River Valley of western Slovakia (Fig. 1). Besides blue amphiboles (ca 20%), the heavy mineral concentrates contain abundant chrome spinels (46%), stable ZRT association (14%), epidote (5%), apatite (5%), garnet (4%) and some monazite and kyanite grains. Amphibole composition corresponds to glaucophane and ferroglaucophane.
Notes to the tectonometamorphic evolution of the Meliatic units
Based on the review of existing information, presented in the introductory chapter, and newly achieved metamorphic petrological and geochronological data, we discuss the possible scenario for the onset, course and termination of subduction of the Meliata Ocean. Our new EMPA monazite age data corroborate the time relationships of blueschist and superimposed greenschist facies metamorphism in the Bôrka Nappe. Two distinct tectonometamorphic stages postdating the HP metamorphism are indicated by older ages (1) around 145–140 Ma that were attained under greenschist-facies conditions and could indicate welding of the Meliatic and Turnaic units into a united accretionary complex that subsequently overrode the Gemeric Superunit (Fig. 15). This event is expressed by coeval upright folding and imbrication during emplacement of the Meliatic–Turnaic nappe stack over the Gemeric basement-cover complexes (Lačný et al. 2016). Consequently, the older monazite ages postdate by some 10–15 Myr the peak blueschist-facies metamorphic ages of the Bôrka Unit clustering around 155 Ma. The subsequent thrusting-related exhumation of the Meliatic units is recorded by the 135 Ma age of metamorphic perovskite and zircon (U–Th)/He cooling ages at 130–120 Ma (Putiš et al. 2014, 2015; Li et al. 2014). The resulting burial of the Gemeric complexes is revealed by low-grade metamorphism at 350–400 °C and 400–600 MPa dated to ca 140–120 Ma (Hurai et al. 2008, 2015; Vozárová et al. 2008, 2014). Subsequently, the Gemeric and overlying Meliatic complexes were affected by development of a cleavage fan related to thrusting over the foreland Veporic Superunit.
In the time interval of 130–90 Ma, the main phase of the Central Western Carpathian thick-skinned thrust stacking occurred. According to the inferred scenario of tectonometamorphic evolution of the Gemeric and Veporic units (see Jeřábek et al. 2012; Plašienka et al. 2016; Plašienka 2018 and references therein), at first the Gemeric basement sheet with passively carried Meliatic–Turnaic units was thrust over the southern Veporic zones between ca 130–120 Ma. Afterwards, the Veporic crustal-scale wedge propagated outward from the Meliata collision zone and overrode the Fatric basement attached to the foreland Tatric plate, concurrently with detachment of the Fatric sedimentary cover between 100 and 90 Ma. This process was finalized by emplacement of the detached Fatric cover nappes (Krížna and related units; e.g. Plašienka 2003, 2018; Prokešová et al. 2012) above the Tatric cover (Fig. 15). The crustal-scale shortening and thickening of the rear Veporic part of the propagating orogenic wedge was accompanied and followed by orogen-parallel unroofing and gradual exhumation of the Veporic metamorphic dome, along with the overlying Gemeric–Meliatic–Turnaic thrust pile (e.g. Vojtko et al. 2016). Consequently, the younger monazite age cluster (2) in the range 95–90 Ma might record thermal relaxation and elevated thermal gradient related to exhumation processes of the neighbouring Veporic metamorphic complex.
The distinctive poikiloblastic tourmalines (Tur2, see Sect. 3.4) are typical constituents of some rare eclogitic and eclogite-related rocks (Altherr et al. 2004; Marschall et al. 2006; Konzett et al., 2012; Broska et al. 2015), where they were interpreted as being formed in the decompression (exhumation) phase of originally high-pressure to ultra high-pressure (HP/UHP) metamorphosed rocks (Marschall et al. 2006). Considering the higher content in Mg and Al and the lower content in Fe (Fig. 10g–i), the Tur1 generation likely formed during the subduction-related HP/LT metamorphic alteration of pelagic sediments. In contrast, Tur2 might have crystallized during exhumation under greenschist facies (LP/LT) metamorphic conditions, as indicated by a comparatively higher portion of Fe and lower amounts of Mg and Al, and a number of poikiloblastic inclusions of quartz, epidote, Fe-oxides and titanite. These features point to a rapid, fluid-assisted blastesis of Tur2.
The genesis of two epidote generations is interpreted in a similar way like that of the two tourmaline generations. Tourmaline Tur 2 always includes relics of Ep2 only and the same inclusions as found in Ep2. Accordingly, the first generation (Ep1) might have originated in the blueschist-facies metamorphic conditions, whereas blastesis of the second generation (Ep 2) was connected with the superimposed metamorphosis in the greenschist facies, which affected the studied metasediments during exhumation of the Bôrka Nappe.
Distribution of erosional products of the Meliatic blueschists
The investigation of the sedimentary response to exhumation and thrusting processes offers additional hints for palaeotectonic interpretations. Therefore, the provenance studies of synorogenic clastic formations play an essential role in the reconstruction of the structure and tectonic evolution of ancient orogenic belts, because the sedimentary record may preserve information about composition, age and exhumation history of areas and units that were subsequently to a large extent, or even completely eroded and do not participate on the present erosional surface any more (e.g. Dickinson 1988).
The blueschist-facies metamorphic rocks are usually intimately related to the dismembered ophiolite suites, therefore their erosional products deposited in clastic sediments of neighbouring sedimentary basins occur jointly. This applies to both the coarse-grained material in conglomerates, as well as to heavy mineral assemblages in sandstones and siltstones. The Albian–Cenomanian conglomerates containing the exotic ophiolite- and blueschist-derived pebbles occur in several Central Western Carpathian units (Fatric, Tatric and in the largest extent in the Klape Unit in the Peri-Klippen Zone) and were thoroughly studied by a number of authors (e.g. Šímová 1982, 1985; Ivan and Sýkora 1993; Dal Piaz et al. 1995; Faryad and Schreyer 1997; Ivan et al. 2006 and references therein).
A more problematic issue concerns the recognition of blueschist-derived heavy mineral fractions in fine-grained clastic sediments. While the most indicative heavy mineral derived from ophiolitic ultramafic and mafic magmatic rocks—the chrome spinel is relatively resistant and sometimes abundant (e.g. Jablonský et al. 2001 and references therein; see also Bellová et al. 2018), the blue sodium amphiboles as the most significant evidence of blueschist-facies sources are less resistant and might be reduced or completely erased by post-sedimentary diagenetic processes like intrastratal solution. Nevertheless, they could be preserved under appropriate conditions. If so, the detrital blue amphiboles, glaucophane in particular, provide essential information about the time of the blueschists exhumation and surface erosion.
In the Western Carpathians, the first evidence of detrital chrome spinels comes from the Valanginian and Hauterivian (i.e. 135–130 Ma) siliciclastic turbidites of the Fatric Zliechov Basin located north of the developing Veporic–Gemeric–Meliatic collisional thrust stack. Turbidites with massive input of chrome spinels (up to 90% of heavy mineral concentrates—Jablonský et al. 2001) were fed from southern sources, therefore derivation of chrome spinels could be directly related to erosion of Meliatic ophiolite-bearing mélange complexes that were in a piggyback position above the Gemeric sheet thrust over the southern Veporic zones (cf. Figure 7 in Plašienka 2018). However, no blue amphiboles or other high-pressure minerals have been reported from these turbidites yet. It is worth noting that ophiolite detritus in sediments appeared considerably later here than in the Northern Calcareous Alps, where the earliest presence of detrital chrome spinels was documented from the Kimmeridgian deposits (Gawlick et al. 2015), i.e. some 20 Myr earlier than in the Carpathians. In the Transdanubian Gerecse Mts (Fig. 1), a massive input of chrome spinels into the Lower Cretaceous coarsening-upward flysch sequence started by the end of Berriasian (i.e. at 140 Ma; Császár and Árgyelán 1994; Árgyelán 1996, 1997).
The manifestation of detrital chrome spinel and blue amphibole in pebbles of the “Urgonian”-type shallow-marine organogenic limestones with siliciclastic admixture was first documented by Mišík (1979—up to 47% of blue amphiboles in the heavy mineral fraction) and Mišík et al. (1980). However, these atypical “Urgonian” limestones do not occur in a primary position, but only as pebbles in the mid-Cretaceous wildflysch conglomerates of the Tatric and Fatric units, including the Peri-Klippen Klape Unit (Figs. 1, 15). In the hundreds of metres thick Albian–Cenomanian Klape flysch complex they occur in association with various blueschist-facies pebbles. Most probably, the “Urgonian” pebbles were derived from ephemeral biohermal build-ups fringing the emerged parts of Meliatic units. Considering their late Barremian—early Aptian age, the exhumation to the surface and first erosion of the Meliatic HP metamorphic rocks is dated to around 125 Ma, which coincides with the above reported thrusting-related exhumation and cooling ages (Putiš et al. 2014). The “Urgonian” limestone pebbles with detrital chrome spinels and serpentinite fragments, but lacking blue amphiboles, occur in the Albian flysch conglomerates of the Tatric sedimentary cover succession and Fatric Krížna Nappe (Mišík et al. 1980, 1981), as well as in the Transdanubian Gerecse Mts (Császár and Árgyelán 1994; Császár 1997). Wagreich et al. (1995) studied analogous clasts of “Urgonian”-type limestones with ophiolite detritus from the Upper Cretaceous Gosau conglomerates of the Northern Calcareous Alps. They found a few blue amphibole grains in two samples.
Always in association with detrital chrome spinel and locally with chloritoid, the blue amphiboles occasionally occur in the mid-Cretaceous (Albian–Cenomanian; i.e. from ca 110 Ma onward) synorogenic flysch formations of the Klape Unit in the Peri-Klippen Zone in western Slovakia (Mišík et al. 1980; Mišík and Sýkora 1981; Jablonský et al. 2001). Nevertheless, there are at least two localities, where the content of blue amphiboles in the heavy mineral concentrates is extraordinary—39% (Sýkora et al. 1997) and 20% (Bellová et al. 2018)—asterisks (1) and (2) in Fig. 1, respectively. The substantial enrichment at these two localities can be explained either by a transient exposure of the parental blueschists in the source area, or by the syn- or post-sedimentary impoverishment in majority of the host turbiditic sandstone beds, or both. The sporadic occurrence of blue amphiboles was described also from the Upper Cretaceous Gosau-type sandstones in the Peri-Klippen Zone (Wagreich and Marschalko 1995). It seems probable that these were recycled from older mid-Cretaceous sandstones of the Klape Unit (Plašienka and Soták 2015). Occurrences of chrome spinels and blue amphiboles in Cretaceous clastic formations of the Eastern Alps were reviewed and discussed e.g. by Winkler (1996).
Of special importance is “mosaic” (poikiloblastic) tourmaline revealed in the exotics-bearing psammites (Bellová et al. 2018), but also in tourmalinite pebbles occurring in the Cretaceous exotic conglomerates (Bačík et al. 2008). They were so far not detected in pre-Cretaceous Mesozoic clastic sediments of the Western Carpathians (Aubrecht and Krištín 1995; Aubrecht 2001). Nevertheless, they were registered also in the Meliatic glaucophanite-bearing complexes (see Sect. 3.4 above). Consequently, the appearance of these tourmalines provide a clue to their provenance and due to their better preservation potential, they are an important complementary indicator of presence of HP/UHP metamorphics in the source areas.
In addition to clasts of dismembered, but apparently complete ophiolite suite, pebbles of blueschist-facies metabasalts were encountered also in the Upper Cretaceous Gosau-type conglomerates (Hovorka et al. 1990). They occur in a narrow slice separating two partial structures of the Silicic Stratená Nappe, close to the present northernmost surface exposures of allochthonous Meliatic units (Fig. 1).
In this contribution, all findings of ophiolite- and blueschist-derived clastic material in the Cretaceous flysch deposits are regarded as having been derived from the Meliatic complexes in the southern Western Carpathian zones. Argumentation in support of this opinion was presented by Plašienka (1995a, 1997, 1998, 2012, 2018; Plašienka and Soták 2015) and the possible mechanism how it could have happened was proposed by Plašienka (1995b, 2018), Kissová et al. (2005) and Prokešová et al. (2012). Nevertheless, this view has not been accepted unambiguously. Instead, the concept of the exotic Pieniny cordillera that prevailed for many decades is still maintained by some authors (e.g. Aubrecht in Bellová et al. 2018). The puzzling concept of an exotic ridge was based on the massive occurrence of extremely variable pebble material, part of them of unknown provenance (summarized e.g. by Mišík and Marschalko 1988), which were designated as exotic. With the introduction of the plate tectonic theory, the source area was interpreted as a temporarily elevated compressional ridge at an active continental margin, variously explained as a subduction mélange complex or imbricated outer edge of an accretionary prism (e.g. Mišík, 1979; Mišík and Sýkora 1981), or else as the magmatic arc (Birkenmajer 1988). The ridge should have been located at the outer Tatric, i.e. Austroalpine margin facing the Triassic–Jurassic oceanic realm subducted during the Late Jurassic, but the exotic pebble material appeared as late as in mid-Cretaceous synorogenic wildflysch formations, diminished during the latest Cretaceous and then the exotic ridge was fully eliminated by underthrusting beneath the Central Carpathian thrust wedge. However, this scenario resembles a “deus ex machina” solution, since it is in a severe contradiction with the tectonostratigraphic evolution of the outer Tatric (i.e. Austroalpine) margin, and the Western Carpathians as a whole (e.g. Plašienka 1995a). Although the exotic pebble material was not re-evaluated comprehensively after the detailed analyses of Mišík and his co-authors from 1970 and 1980-ties, occasional studies in last two decades, such as presented and quoted in this contribution (e.g. Kissová et al. 2005; Ivan et al. 2006), are bringing still new arguments supporting derivation of this material from the southern Carpathian zones, without the need of an enigmatic exotic cordillera.
The paper presents a collection of new results concerning composition of variegated mélanges related to the subduction–accretion–collision processes of the Meliata oceanic domain and its continental margins, provides data about the atypical blueschist-facies radiolarites as components in these mélanges, presents the new EMPA monazite ages of HP rocks of the Bôrka Nappe that cluster in two peaks, describes two distinct types of tourmalines and epidotes occurring in the Bôrka blueschists, and reviews and interprets occurrences of the blueschist- and associated ophiolite-derived detritus in Cretaceous clastic sediments of especially the Klape Unit. From our interpretation of gathered data, the following conclusions can be drawn:
The Western Carpathian Meliata Superunit (Meliaticum) consists of three subunits that show in part independent characteristics concerning their composition and metamorphic evolution, but on the other hand they share the common structural position and evolution. In part they are mixed together to form polygenous mélange complexes.
So far not recognized blueschist-facies radiolarites, as a newly discovered component of Meliatic mélanges, are described for the first time in the Carpathians. They occur in polygenous mélange complex together with clasts of unmetamorphosed radiolarites and various other rocks, thus they register mixing processes during the mélange formation.
Two measured EMPA monazite age clusters from the Bôrka Unit culminate at around 145–140 Ma and 95–90 Ma. The first set coincides with the peak of K–Ar age spectra of fine-grained white micas from the very low-grade Meliata Unit s.s., therefore it could have been related to the additional burial of Meliatic units below the Turnaic nappes in the final phase of formation of the Meliata accretionary complex, and during its ensuing thrusting over the Gemeric foreland. The second age group might be tentatively ascribed to the thermal relaxation related to the initial stages of exhumation of the adjacent Veporic metamorphic dome associated with increased heat and metamorphic fluid flux.
Peculiar shallow-marine “Urgonian” limestones (Barremian–Aptian) contain sometimes high amounts of siliciclastic material including detrital chrome spinel and blue amphibole and thus provide evidence of the earliest recognized erosion of the Meliatic blueschists. These limestones are only present as pebbles in mid-Cretaceous flysch conglomerates. Nevertheless, they are associated with pebbles of glaucophanites and other blueschist-facies rocks implying their common provenance.
Scattered, but locally high concentrations of detrital blue amphiboles in the heavy mineral fractions from mid-Cretaceous turbiditic sandstones are always associated with the ophiolitic detritus dominated by chrome spinels. At the same time, the synorogenic turbiditic sequences, especially of the Klape Unit, contain huge bodies of unsorted conglomerates that contain pebbles of various HP/LT metamorphosed rocks which exhibit many common characteristics with the in situ appearances of blueschist-facies rocks in the Meliatic units, first of all the identical Upper Jurassic metamorphic age. Moreover, pebbles and detrital grains of unusual poikiloblastic tourmalines from the Cretaceous conglomerates and sandstones of the Peri-Klippen Klape Unit show the same characteristic as tourmalines from the Meliatic blueschists occurring in situ.
Summing up, the Meliatic provenance of the blueschist erosional products in the mid-Cretaceous synorogenic clastic formations appears to be the most plausible solution of the long-term discussed problem of source areas of this “exotic” material.
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The authors appreciate the financial support from the Grant Agency for Science, Slovakia (projects VEGA 1/0085/17 and 2/0034/16) and from the Slovak Research and Development Agency (projects APVV-0212-12 and APVV-17-0170). The authors are indebted to László Fodor and an anonymous reviewer for their valuable comments and suggestions that helped to improve the scientific content a readability of the earlier version of the manuscript. The same applies to the editorial comments of Edwin Gnos and Stefan Schmid. The analyzed samples are stored at the Department of Geology and Palaeontology and Department of Geochemistry, Faculty of Natural Sciences, Comenius University in Bratislava, and at the Earth Science Institute of the Slovak Academy of Sciences in Banská Bystrica.
Editorial handling: E. Gnos.
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Plašienka, D., Méres, Š., Ivan, P. et al. Meliatic blueschists and their detritus in Cretaceous sediments: new data constraining tectonic evolution of the West Carpathians. Swiss J Geosci 112, 55–81 (2019). https://doi.org/10.1007/s00015-018-0330-7
- Meliata Ocean
- High pressure/low temperature metamorphism
- Monazite dating
- Heavy minerals