Meliata Unit Research Articles

Metamorphic Conditions of Neotethyan Meliatic Accretionary Wedge Estimated by Thermodynamic Modelling and Geothermobarometry (Inner Western Carpathians)

Metamorphic evolution of an accretionary wedge can be constrained by a reconstructed P–T conditions of the oceanic and continental margin fragments. This paper deals with the metamorphic overprinting of the Inner Western Carpathians (IWC) Meliatic Triassic–Jurassic paleotectonic units after the closure of the Neotethyan Meliata Basin. Medium to high-pressure and lower temperature conditions were estimated by Perple_X pseudosection modelling, combined with garnet–phengite, calcite–dolomite and chlorite thermometers and chlorite–phengite and phengite barometers. The Late Jurassic subductional burial to a maximum 50 km depth was estimated from the Bôrka Unit continental margin fragments at 520 °C and 1.55 GPa. This is compatible with the metamorphic peak garnet–glaucophane–phengite assemblage of blueschist facies in metabasites. The Jaklovce Unit oceanic fragments were subducted to maximum 35–40 km at 390–420 °C and 1.1–1.3 GPa. Metabasalts and metadolerites contain winchite, riebeckite, actinolite, chlorite, albite, epidote and phengite. A glaucophane-bearing metabasalt recorded an intra-oceanic subduction in blueschist-facies conditions. Rare amphibolite-facies metabasalts of this unit indicate the base of an inferred oceanic crust sliver obducted onto the continental margin wedge. The Meliata Unit oceanic/continental margin flysch calciclastic and siliciclastic metasediments suggest the burial to approximately 15–20 km at 250–350 °C and 0.4–0.6 GPa. This is indicated by a newly formed albite, K-feldspar, illite–phengite and chlorite associated with quartz and/or calcite and dolomite in these rocks. Magnesio-hastingsite to magnesio-hornblende bearing metagabbro with newly formed metamorphic magnesio-riebeckite and actinolite is an inferred detached Meliatic block tectonically emplaced in a Permian salinar mélange in the Silica Nappe hanging wall. Reconstructed P–T paths indicate variable metamorphic conditions from the medium-pressure to high-pressure subduction of the Bôrka and Jaklovce units to the Meliata Unit shallow burial in an accretionary wedge during Late Jurassic to Early Cretaceous Meliaticum evolution. Mélange blocks of Meliaticum incorporate different juxtaposed Meliatic paleotectonic units exposed in nappe outliers overlying the IWC Gemeric and Veporic superunits.

Is Cr-Spinel Geochemistry Enough for Solving the Provenance Dilemma? Case Study from the Palaeogene Sandstones of the Western Carpathians (Eastern Slovakia)

The provenance of the Proč and Strihovce sandstones is crucial for understanding the relationship between the Pieniny Klippen (PKB) and Flysch (FB) belts in the easternmost part of the Western Carpathian realm. Detrital Cr-spinels in these tectonic units were assertively interpreted as sourced from the southern sources representing the Meliata mélange. In this study, we use the geochemistry of detrital chromian spinels to identify the mafic and ultramafic source of the sediments and to compare them each other. Simultaneously, we compare their chemical compositions with those from the different Western Carpathian geological units, which could feed the Proč and Krynica basins during the Paleogene, where the Proč and Strihovce formations (fms), respectively, were deposited. Chromian spinels from the Proč and Strihovce fms exhibit similar geochemical characteristics (Cr# = 0.44–0.88 and 0.29–0.89, Mg# = 0.17–0.68 and 0.2–0.72, TiO2 = 0.0–3.67 and 0.01–2.08 wt.%, respectively). The spinels show both supra-subduction zone (SSZ) peridotite signatures and volcanic origin. Whereas volcanic spinels from the Proč Formation (Fm.) were formed under an ocean island basalt (OIB) and back-arc basin basalt (BABB) geodynamic setting, those from the Strihovce Fm. suggest a predominantly mid-ocean ridge basalt (MORB) origin. To avoid mistakes in the provenance interpretations, the data from garnet geochemistry of both formations is supplied. The analysed Cr-spinels do not absolutely overlap with spinels reported from the Meliata Unit, and their composition indicates at least two independent sources.

Meliatic blueschists and their detritus in Cretaceous sediments: new data constraining tectonic evolution of the West Carpathians

The Western Carpathian Meliata Superunit (Meliaticum) includes a heterogeneous group of units—the blueschist-facies Borka Nappe and the very low-grade chaotic complexes—polygenous melange 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 melanges 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 melanges related to the subduction–accretion processes of the Meliata Ocean and its continental margins. The polygenous melange 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 Borka 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 Borka 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.

New archaeometrical results related to Neolithic blueschist stone tools from Borsod-Abaúj-Zemplén County, Hungary

26 blueschist Neolithic polished stone tools coming from archaeological sites in Borsod-Abaúj-Zemplén County (NE Hungary) were archaeometrically analysed. All items were described and documented macroscopically and magnetic susceptibility was measured on each tool. Non-destructive XRD analyses were made on all implements to separate blueschists from other metamorphic rocks, because this technique can make a difference between the diagnostic sodic amphiboles and other amphiboles, which was confirmed by EDS/SEM in each case. Electron microprobe analyses were carried out on polished sections and selected intact axes by using “original-surface” method. Non-destructive Prompt Gamma Activation Analysis (PGAA) was performed on selected pieces. Blueschist polished stone tools were classified into five groups based on their mineral components and metamorphic evolution. Mineral chemistry analyses suggest that blueschists suffered a polyphase metamorphism. Our data were compared to the nearest blueschist outcrop in the Meliata Unit (Slovakia); it is supposed that the source area of the blueschist implements is the Meliata Unit and/or the blueschist pebbles from Cretaceous conglomerate in the Pieniny Klippen Belt.

Perovskite, reaction product of a harzburgite with Jurassic– Cretaceous accretionary wedge fluids (Western Carpathians, Slovakia): evidence from the whole-rock and mineral trace element data

Abstract Perovskite (Prv) was discovered in an abyssal harzburgite from a “mélange” type blueschist-bearing accretionary wedge of the Western Carpathians (Meliata Unit, Slovakia). Perovskite-1 formation in serpentinized orthopyroxene may be simplified by the mass-balance reaction: Ca2Si2O6 (Ca-pyroxene-member)+2Fe2TiO4 (ulvöspinel molecule in spinel)+2H2O+O2=2CaTiO3 (Prv)+2SiO2+4FeOOH (goethite). Perovskite-2 occurs in a chlorite-rich blackwall zone separating serpentinite and rodingite veins, and in rodingite veins alone. The bulk-rock trace-element patterns suggest negligible differences from visually and microscopically less (“core”) to strongly serpentinized harzburgite due to serpentinization and rodingitization: an enrichment in LREE(La,Ce), Cs, ±Ba, U, Nb, Pb, As, Sb, ±Nd and Li in comparison with HREE, Rb and Sr. The U/Pb perovskite ages at ~135 Ma are interpreted to record the interaction of metamorphic fluids with harzburgite blocks in the Neotethyan Meliatic accretionary wedge. Our LA-ICP-MS mineral study provides a complex view on trace element behaviour during the two stages of rodingitization connected with Prv genesis. The positive anomalies of Cs, U, Ta, Pb, As, Sb, Pr and Nd in Cpx, Opx and Ol are combined with the negative anomalies of Rb, Ba, Th, Nb and Sr in these minerals. The similar positive anomalies of Cs, U, Ta, ±Be, As, Sb found in typical serpentinization and rodingitization minerals, with variable contents of La, Ce and Nd, and negative anomalies of Rb, Ba, Th, Nb and Sr suggest involvement of crustal fluids during MP-LP/LT accretionary wedge metamorphism. LA-ICP-MS study revealed strong depletion in LREE from Prv-1 to Prv-2, and a typically negative Eu (and Ti) anomaly for Prv-1, while a positive Eu (and Ti) anomaly for Prv-2. Our multi-element diagram depicts enrichment in U, Nb, La, Ce, As, Sb, Pr, Nd and decreased Rb, Ba, Th, Ta, Pb, Sr, Zr in both Prv generations. In general, both Prv generations are very close to the end-member composition. In spite of low concentrations of isomorphic constituents, Prv-1 and Prv-2 display the A(La,Ce)3++B(Fe,Cr)3+=ACa2++BTi4+ heterovalent couple substitution. A decrease of ferric iron in Prv-2 indicates increasing reduction conditions during rodingitization.

Structural evolution of the Turňa Unit constrained by fold and cleavage analyses and its consequences for the regional tectonic models of the Western Carpathians

AbstractThe Turňa Unit (Turnaicum, Tornaicum) is one of the three nappe systems involved in the geological structure of the inner zones of the Western Carpathians. The unit is formed by a system of partial nappes and duplexes, which overlie the Meliata Unit s.l. and are overridden by the Silica Nappe. The Slovenská skala partial nappe in the investigated area includes clastic sediments of the mid-Carboniferous, Permian and Early Triassic age, followed by mostly deep-water Middle-Upper Triassic succession predominantly composed of carbonates. Structural analysis of cleavage planes and folds was carried out predominantly in the Lower Triassic Werfen Formation. The measured deformational structures are polygenetic and were principally formed in three successive deformation stages. The first deformation stage is represented by bedding-parallel, very low-grade metamorphic foliation that was related to nappe stacking and formation of the Mesozoic accretionary wedge during the latest Jurassic and earliest Cretaceous. The second deformation stage is represented by systems of open to closed, partly asymmetric folds with SW-NE oriented, steeply NW- or SE-dipping axial-plane cleavage. Regionally, the folded bedding planes are usually moderately SE-ward dipping, the NW-ward and subvertical dips are less common. The mesoscopic fold structures predominantly occur in the SW-NE trending anticlinal and synclinal hinge zones of large-scale folds. These structures evolved in a compressional tectonic regime with the NW-SE to N-S orientation of the maximum compressional axis. The third observed deformation stage was activated during ENE-WSW oriented shortening. This stage is chiefly represented by open, kink-type folds. Some inferences for regional structures and tectonic evolution of the area are discussed as well.

Accretionary wedge harzburgite serpentinization and rodingitization constrained by perovskite U/Pb SIMS age, trace elements and Sm/Nd isotopes: Case study from the Western Carpathians, Slovakia

Perovskite-bearing harzburgites occur in a “mélange” type blueschist-bearing accretionary wedge complex of the Inner Western Carpathians Meliata Unit in Slovakia. Although dark rounded, slightly hydrated relic “cores” of harzburgite boulders are perovskite-free, perovskite (Prv) occurrence in the surrounding serpentinites and rodingites enabled dating of hydration, resulting in two metamorphic–metasomatic Prv generations. Perovskite (1) grows parallel to relic clinopyroxene exsolution lamellae or forms randomly oriented grain clusters in serpentinized orthopyroxene (Opx 1 ) porphyroclasts, often accompanied by tiny andradite lamellae clusters, or it is partly replaced by Ti-andradite. Perovskite crystallization indicates evolving rodingitization fluids pervading the boundary between the harzburgite “cores” and Prv-free serpentinite. This strictly limited occurrence of Prv (1) within a 1 to 20-cm across-zone implies slightly postponed Prv crystallization to serpentinization by LREE(Ce,La), Ca 2+ , Ti/Fe 3+ -enriched aqueous fluids. A grain scale metasomatic mechanism partitioned Ca and Ti from the host orthopyroxene porphyroclasts, spinel (Ti) and grain-boundary pervasive fluids to Prv. In contrast, Prv (2) occurs in a 1 to 3 cm across chlorite-rich blackwall zone between hosting serpentinite and rodingite veins, thus indicating channelled rodingitization fluid flow and accompanying hydraulic fracturing. Here, Prv (2) is ingrown by chlorite and apatite. Part of this Prv (2) formed in a rodingite vein mineral assemblage composed of diopside, andradite, vesuvianite, epidote/zoisite, apatite and chlorite. Both perovskite 1 and 2 are replaced by pyrophanite along the grain rims and interiors; most likely via fluid-aided coupled dissolution–reprecipitation at increased Si–Fe–Mn–Al element solubility in rodingitization fluids pervading serpentinized harzburgite. Both Prv generations, especially Prv (2), can be partly to almost totally replaced by (Ti-) Adr. Overgrowths of spinel by andradite are occasionally observed in contact zones between the serpentinites and rodingites. LA-ICP-MS study revealed strong depletion in LREE from Prv (1) to Prv (2), and a more typically positive Eu anomaly for Prv (2). Our spider diagram depicts relative enrichment in U, Nb, La, Ce, Pr, Nd, and decreased Rb, Ba, Th, Ta, Pb, Sr, Zr in both Prv generations. The U/Pb SIMS concordia ages of Prv (1) from 3 samples range from 137 ± 1 Ma to 135 ± 1 Ma, with a mean of 135.6 ± 0.58 Ma, while Prv (2) was dated at 133.7 ± 5.4 Ma. Such negligible age differences imply a relatively short-lived rodingitization event responsible for crystallization of both Prv generations. The 143 Nd/ 144 Nd mean value of Prv (1) is 0.512153 ± 0.000017 by LA-MC-ICP-MS, thus corresponding to the initial ɛ Nd (t = 135) = − 8.2 ± 0.4 (math's mean). This suggests that the subducted and dehydrated continental crust was the main source of the interactive fluids which initiated serpentinization and rodingitization in the Neotethyan Meliatic accretionary wedge following closure of the Meliata–Hallstatt Triassic to Jurassic oceanic back-arc basin and the high-pressure metamorphism dated at ca. 160–150 Ma by 40 Ar/ 39 Ar. • Dating the fluid–rock interaction in accretionary wedge by U/Pb SIMS on perovskite • The first isotopic dating of serpentinization and rodingitization in accretion wedge • Constraining the fluid source by LA-MC-ICP-MS Nd isotopic signature of perovskite • LA-ICP-MS patterns of newly-discovered metamorphic–metasomatic perovskites • Constraining exhumation age of high-pressure rocks in the West-Carpathian wedge

The blueschist-associated perovskite-andradite-bearing serpentinized harzburgite from Dobšiná (the Meliata Unit), Slovakia

*Corresponding author Unique, perovskite(Prv)–andradite(Adr)-bearing serpentinized harzburgite fragments occur in Late Jurassic–Early Cretaceous subduction–accretionary wedge complex of the Meliata tectonic Unit (Slovakia, Western Carpathians). These rocks reflect magmatic (E1) and partial melting (E2) mantle events followed by metamorphic blueschist-facies (E3) and fluid–rock interaction (E4) events accompanied by crystallization of Prv and/or Adr. The incompletely solidified slightly depleted (residual) harzburgite composed of early-magmatic assemblage of Ol, Opx 1 , Cr-Spl ± Cpx 1 (E1 event) underwent partial melting (E2 event) under subsolidus mantle conditions. The late-magmatic aggregates of Cpx 2 , Cr-Spl, scarce Opx 2 , due to this batch melting, occur around the Opx 1 (±Cpx 1 ) porphyroclasts. The fractures and cleavage planes of the kinked Opx 1 porphyroclasts are filled by Cpx 3 as a result of subsolidus recrystallization of the Opx 1 . Cloudy lower Al Cpx 4 associated with pargasite in reaction rims between the Opx 1 and Cpx 2 , or antigorite replacing Ol, could represent the E3 high-pressure metamorphic event (dated at 160 to 150 Ma, according to published 40 Ar– 39 Ar data for “phengitic” white mica from blueschists) in serpentinites. Serpentinized zones of harzburgite contain Prv, which occurs in serpentinized Opx 1 porphyroclasts. Orthopyroxene (Opx 1 ) breakdown and formation of Prv, (Ti-rich) Adr and chrysotile appears to be postdating the closure of the Meliata back-arc basin (c. 200 to 160 Ma) and HP metamorphism. Perovskite genesis can therefore be bound to the exhumation-related serpentinization in subduction channel and emplacement of serpentinites into subduction–accretionary wedge (E4 event at 300–500 oC/5–15 kbar, at 150 to 130 Ma?). The bulk-rock trace-element patterns suggest enrichment in LREE, Ba, Cs, U, Pb, Sb and Li in comparison with HREE, HFSE, Rb and Sr. Such element mobility indicates fluid–rock interaction. Pervading aqueous fluids rich in CO 2 were transporting Ca, Ti, Fe, LREE partitioned into Prv grown along the Cpx exsolution lamellae in Opx 1 . The sources of such fluids might have been the blueschist- to greenschist-facies rocks from the Meliata melange type subduction–accretionary wedge. The late-metamorphic H 2 O–CO 2 /Si–Fe–Mn-rich fluids caused partial replacement of Prv by pyrophanite and crystallization of (Ti-rich) Adr at the expense of Cpx and Prv.

Textural and age relations of polymetamorphic rocks in the HP Meliata Unit (Western Carpathians)

Slices and tectonic blocks of amphibolite facies crustal rocks overprinted by blueschist facies metamorphism occur in the Middle-Jurassic Meliata high-pressure unit of the Western Carpathians. The Early Paleozoic age for relic muscovite is known from some micaschists with blueschist facies overpint in the eastern sector of the Meliata unit. Based on lithology, some amphibolite facies rocks are comparable with the Early Paleozoic basement rocks in the Gemeric unit. In this work, we present results of petrology and Ar–Ar and Rb–Sr geochronology from blueschist facies rocks with relicts of hornblende, muscovite, garnet, and rutile. The older amphibolite facies metamorphism was followed by retrogression prior to the blueschist facies overprint. The blueschist facies metamorphic minerals are Ca-rich garnet, glaucophane, phengite, and albite. The coarse-grained relict muscovite indicates an Early Paleozoic age using the Ar–Ar method and a Middle-Jurassic age using the Rb–Sr technique. The presence of amphibolite facies rocks within the Meliata unit is interpreted as involvment of Variscan/Pre-Variscan basement-unit rocks in the Middle-Jurassic subduction of the Meliata-Hallstatt oceanic basin.

Petrological and mineral-chemical variations in blueschist facies metasedimentary rocks from a single outcrop in the lower complex of the Bôrka Nappe (Meliata Unit, Western Carpathians, Slovak Republic)

Abstract A detailed petrographic and mineral-chemical study on metapelites from the Meliatic accretionary wedge complex (Borka Nappe, Western Carpathians, Slovak Republic) reveals the HP character of the samples using quantitative phase diagrams contoured with mineral composition, H2O mode isopleths and garnet-phengite thermometry. The presented PT pseudosections prove that small-scale differences in bulk rock composition are responsible for the variations in the mineral assemblages formed at the same PT conditions. The peak conditions indicate blueschist facies metamorphism (520–620 °C, 11–14 kbar) and are correlated with the 150–165 Ma subduction of the Mesozoic Meliata Oceanic branch of the Neotethys. Continuous decrease of P and T from peak conditions enabled the metapelitic rocks to preserve their HP assemblages. The presented HP conditions and retrograde PT path with decreasing P and T are characteristic of subduction zone tectonic settings which are in agreement with most of the geodynamic and tect...

The Meliata suture in the Carpathians: regional significance and implications for the evolution of high-pressure wedges within collisional orogens

Abstract The Meliata nappe of the Western Carpathian orogen is comprised of Triassic deep-sea metasedimentary rocks and fragments of blueschist-bearing ophiolite. These were structurally emplaced onto Permian/Triassic shelf sequences and its Variscan basement. The nappe records a succession of deformational events which formed under decreasing pressure conditions. Subduction-related burial and resulting blueschist metamorphism is dated by 40 Ar/ 39 Ar plateau ages recorded by four phengitic muscovite concentrates (160–150 Ma). These crystallized during ductile deformation characterized by predominantly coaxial NW–SE stretching. The structures were overprinted by semiductile, nonpenetrative fabric elements which formed under greenschist facies conditions and contemporaneously with fabrics which developed in footwall tectonic units. Kinematic indicators record top north to NW shear during Middle Cretaceous loading of the Meliata unit onto the Inner Carpathian nappe complex recorded by a 40 Ar/ 39 Ar whole rock phyllite age of 105.8±1.5 Ma. A subsequent southeastern sense of shear is interpreted to have resulted from extension during Late Cretaceous uplift along hinterland margins of the tectonic wedge. The Meliata unit is part of a major Late Jurassic/Early Cretaceous suture which initially extended from the Alps to the Hellenides. It has been subsequently disrupted as a result of later strike-slip faulting following Tertiary collision of the Cretaceous orogen, and was transported onto extra-Alpine European units.

Amphibole-bearing assemblages as indicators of microdomain-scale equilibrium conditions in metabasites: an example from Alpine ophiolites of the Meliata Unit, NE Hungary

The polymetamorphic evolution of metamorphosed Mesozoic ophiolitic rocks in NE Hungary was studied by microtextural and mineral chemical analyses of metagabbros and metabasalts. Both rock types preserved their original textures remarkably well. By contrast, magmatic minerals (especially clinopyroxene) were preserved only in the metagabbros. The original magmatic whole-rock and mineral chemical features had a strong influence on the formation and preservation of the various metamorphic assemblages as testified by the chemically different amphiboles (and other major minerals) formed in various microdomains. The calculated metamorphic P-T conditions are in part different for metabasalts and metagabbros. These results are in accordance with the observed petrographic features, among which the sequence of crystallisation of Na-amphibole and actinolite in the two rock types is most characteristic. Crystallisation of actinolite is followed by Na-amphibole in metabasalts, while Na-amphibole formed first in the metagabbros. Overall equilibrium conditions were not attained throughout the NE Hungarian Mesozoic ophiolite unit during Alpine metamorphic evolution. Additionally, various disequilibrium metamorphic assemblages with several generations of chemically complex amphiboles were formed even on the thin section scale. Various P-T paths during the tectonic evolution of the accretionary wedge were experienced by parts of the dismembered ophiolite sequence, which originally formed a coherent unit, and were brought back into close spatial relationship only during the post-metamorphic exhumation processes.

PETROLOGICAL AND GEOCHEMICAL FEATURES OF THE MELIATA MAFIC ROCKS FROM THE SUTURED TRIASSIC OCEANIC BASIN, WESTERN CARPATHIANS

Metabasites related to the Triassic Meliata oceanic basin occur along three tectonic zones in the Western Carpathians: (1) the Folkmar Zone, (2) the Roz?n?ava Zone (Meliata Unit, s.s.) and the (3) Bodva valley - Darno Hill Zone. The first two zones form the northern and southern boundary of the Paleozoic of the Gemericum (south-eastern Slovakia), respectively, and the latter is located south of the Meliata Unit, NE Hungary. All three zones contain metabasites. The serpentinites, radiolarian shales and cherts are considered to be complementary members of an ancient oceanic crust. Geochemical characteristics of metabasites from the Folkmar Zone indicate a MORB affinity. Metabasites from the Roz?n?ava Zone have affinity between MORB and within plate basalts, but metagabbros comparable with alkaline basalts are also present. The Darno Hill metabasites have MORB compositions and those from the Bodva valley are similar to the metabasites from the Roz?n?ava Zone. The geochemical and petrographic features of these metabasites suggest a continental rift volcanism, followed by spreading of the Triassic Meliata oceanic basin. During the Middle Jurassic, part of the oceanic crust and adjacent passive continental margin underwent a subduction-related blueschist facies metamorphism afterwards they were exhumed within a melange complex of the Meliata Unit in the southern part of Gemericum.

Very low-grade metamorphism of sedimentary rocks of the Meliata unit, Western Carpathians, Slovakia: implications of phyllosilicate characteristics

The Meliata unit represents a melange-like accretionary wedge, containing blueschist facies tectonic blocks and slices in a Triassic and Jurassic sedimentary matrix. The blueschist facies rocks are tectonic remnants of the subducted parts of the Meliata-Hallstatt branch of the Tethys. The phyllosilicate assemblages in very low-grade metapelites represent metastable disequilibrium stages which the assemblages have reached during reaction progress. Therefore, temperature and pressure values of low-T metamorphism of the sedimentary series and the late stages of decompressional cooling of blueschist facies rocks, obtained by phyllosilicate "crystallinity", chlorite thermometric and white K-mica geobarometric methods, can be regarded as semiquantitative estimates. However, results of chlorite–white mica thermobarometry suggest that local equilibrium was approached at a microscopic scale. For deciphering the age relations of prograde and retrograde events, K–Ar isotope geochronological methods were applied. The sedimentary series and related basalts of the Meliata unit experienced high-T anchizonal prograde regional metamorphism, the temperature and pressure of which can vary between ca. 280 and 350 °C and ca. 2.5 and 5 kbar. White K-mica b geobarometry suggests possible minimal pressures of ca. 1.5 to 3 kbar. The mylonitic retrogression of blueschist facies phyllites is characterised by 340 °C and 4 kbar (minimal P). The low-T prograde metamorphism was synchronous with the retrograde metamorphism of the blueschists. The ages of these two events may be between ca. 150 and 120 Ma, culminating most probably at around 140–145 Ma. Thus, the Upper Jurassic (lowermost Cretaceous) very low-grade metamorphism of the Meliata unit is younger than the subduction-related, 160–155 Ma blueschist facies event, and definitely older than the Cretaceous (100–90 Ma) metamorphism of the footwall Gemer Palaeozoic.

Two contrasting mineral assemblages in the Meliata blueschists, Western Carpathians, Slovakia

Abstract Low-grade metamorphic rocks from the Meliata unit (Western Carpathians) are characterized by the presence of typical blueschist-facies minerals. In metabasalt, an early low-pressure assemblage (<0.5 GPa at 350°C characterized by muscovite and zoisite, is followed by high-pressure glaucophane, phengite, Na-pyroxene, chlorite, clinozoisite and Al-poor titanite, indicating pressures of >1.2 GPa at 450°C Na-pyroxene shows strong compositional variations between the end-members Jd4–70, Aeg10–49 and Q17–49, respectively. Phengite has high Si content of 3.5 a.p.f.u. The zoisite with Al2Fe (100[Fetot/(−2+Altot+Fetot)]) = 3–5%, is rimmed by clinozoisite, with a maximum of 75% Al2Fe, as well as being enclosed by glaucophane. The occurrence of clinozoisite, rimming zoisite, suggests that the transformation of orthorhombic to monoclinic epidote depends not only on the temperature but also on the pressure. In the studied metabasalt, retrograde phases reflecting greenschist-facies conditions are albite and chlorite. Some neighbouring metabasites may additionally contain actinolite and biotite.

Petrological and K [sbnd]Ar and 40Ar [sbnd]39Ar age constraints for the tectonothermal evolution of the high-pressure Meliata unit, Western Carpathians (Slovakia)

The Meliata high-pressure unit, exposed in the southern part of the Western Carpathians, consists of four groups of rocks, which are characterized by different lithologies and metamorphic histories. (1) The most common are Triassic marbles, intercalated with metabasalts and glaucophane-bearing phyllites. Phengite from metabasalts and phyllites record K Ar and 40Ar 39Ar plateau ages of 152–155 Ma which are interpreted to date the high-pressure metamorphism. (2) Blueschist-facies mafic rocks and subordinate glaucophane-bearing micaschists, devoid of marbles which contain relic low-Si muscovite and newly formed high-Si phengite; the 40Ar 39Ar data obtained for the micas indicate Palaeozoic ages of ≥375–380 Ma for the early amphibolite-facies muscovite and a ≤250 Ma age for the secondary blueschist-facies phengite. (3) Glaucophane-free phyllites and phengite quartzite with low-Si and high-Si micas, respectively; textural features indicate a polyphase evolution characterized by a greenschist-facies metamorphism followed by a blueschist-facies overprint; the high-Si micas provide evidence for an early Middle Jurassic age (172 Ma) of the high-pressure metamorphism. (4) Amphibolite-facies slices and blocks, overprinted by blueschist-facies metamorphism, represent old basement rocks. Regarding the differences in lithologies, metamorphic conditions and radiometric ages of high-pressure micas (older age for continental vs. younger age for oceanic rocks), subduction of a continental wedge prior to that of oceanic crust can be assumed. This interpretation is based also on palaeogeographical reconstruction of the Mesozoic Western Tethys between the Apulian promontory and the Tisza-Bihor continental block.

Phase petrology and P‐T conditions of mafic blueschists from the Meliata unit, West Carpathians, Slovakia

Abstract Blueschists occurring as layers in calcite marbles of the Meliata unit occur along the so‐called Roznava tectonic line situated in the southern part of the Gemericum, Slovakia. Mineral assemblages and compositions from seven blueschists localities and one occurrence of amphibolite facies rocks overprinted by blueschist metamorphism were investigated. The most common minerals in the blueschists are blue amphibole, epidote and albite. Some Fe2+‐ and Al‐rich rocks also contain garnet and chloritoid, respectively. Na‐pyroxene with a maximum 50% jadeite component was also found. The blue amphiboles correspond mostly to crossite and also to glaucophane and ferroglaucophane in some samples. Almandine‐ and spessartine‐rich garnet has very low MgO content (<3 wt%). The Si content in phengite ranges between 3.3 and 3.5 pfu calculated on the basis of 11 oxygens. The zoning patterns of blue amphibole, garnet and chloritoid suggest their formation during a prograde stage of metamorphism. The P‐T conditions of metamorphism are estimated to be about 380–460° C and 10–13 kbar. Pressures of 7.5–8.5 kbar and temperatures of 350–370° C were obtained for some actinolite‐ and aegirine‐rich rocks. Apart from chlorite, other mafic minerals formed during retrograde metamorphism are biotite and occasionally also actinolite.

Petrology and phase relations of low-grade high-pressure metasediments from the Meliata Unit (West Carpathians, Slovakia)

Petrology and phase relations of low-grade high-pressure metasediments from the Meliata Unit (West Carpathians, Slovakia)