Vratislav Hurai

Metamorphic evolution and fluid composition of garnet-clinopyroxene amphibolites from the Tatra Mountains, Western Carpathians

Relics suggesting an early high-pressure eclogite facies stage have been identified in the garnet and clinopyroxene-bearing amphibolites of the Tatra Mountains, in the Variscan basement of the Western Carpathians, Slovakia. In these rocks, primary omphacite (Cpx I) has been wholly converted to symplectites of diopside (Cpx II) and plagioclase. Apart from minor inclusions in the garnet cores, amphibole and plagioclase are secondary minerals, formed in the kelyphitic rims between garnet and clinopyroxene. Several generations of amphibole (pargasite, hornblende, cummingtonite, actinolite) are evidence of a transformation down to greenschist facies conditions. Thermobarometric calculations from mineral inclusions in the garnet cores yield 670–700 °C and 10–15 kbar, recording the initial path from upper amphibolite to eclogite facies conditions. The attainment of the eclogite facies stability field is inferred from the composition of a “reconstructed” omphacite (Jd36), implying a minimum peak-pressure of 15–16 kbar. Conditions of around 650 °C and 8–10 kbar record the post-eclogite breakdown and partial re-equilibration in the amphibolite facies region. n nThe earliest fluid inclusions contain high-density nitrogen-dominated, water-absent fluid. Younger are polyphase brines and two-phase H2O + N2 + CH4 inclusions with signs of heterogeneous entrapment of coexisting gas-rich and water-rich immiscible phases. Pure nitrogen (±0.5 mol.% CH4) is considered to have been the major component during the high-pressure metamorphism. In contrast, the later, aqueous inclusions are interpreted to represent retrogression-related fluids. The brines have originated by leaching and re-entrapment of saline melt inclusions observed in tonalitic-trondhjemitic layers of the amphibolite enclosing the eclogitic relics. A similar mechanism is assumed for the origin of the H2O + N2 + CH4 inclusions. These represent relics of the primary, eclogite facies-related N2 inclusions, re-equilibrated and re-entrapped during retrogression under increasing water activity. A possible source of N2 and Cl-rich fluids was primary amphibole, which decomposed during the prograde metamorphism from the amphibolite to the eclogite facies. n nThe metamorphic PT path is generally clockwise, reflecting a steep increase in pressure and temperature during burial, followed by decompression and cooling. The retrograded eclogites occur in boudins within an allochthonous unit of an inverted metamorphic sequence. Their exhumation was facilitated by tectonic transport along a ductile shear zone during the Variscan orogeny.

Origin of siderite veins in the Western Carpathians: I. P–T–X–δ13C–δ18O relations in ore-forming brines of the Rudňany deposits

Abstract Siderite–barite veins of the Rudňany ore field have precipitated from NaCl–CaCl 2 –H 2 O±CO 2 solutions with CaCl 2 weight fractions between 0.05 and 0.38, total salinities between 17 and 35 wt.%, and up to 1.8 mol% CO 2 . Homogeneous and heterogeneous trapping modes have been distinguished according to volumetric phase ratios of the fluid inclusions, which have been trapped at 175–205 °C in veiny siderite, at 200–230 °C in siderite and quartz from drusy cavities, and at 90–180 °C in the siderite–postdating quartz–sulfidic assemblage. A sequential re-equilibration technique has been developed to locate precisely vapour-saturated halite liquidus of individual brine inclusions, which normally homogenise by halite dissolution. Slopes of the vapour-unsaturated halite liquidi constrained by isochores of coexisting two-phase aqueous inclusions have been approximately three to five times steeper in natural polycomponent brines than those in the experimental NaCl–H 2 O system. Fluid pressures between 1 and 2 kbar have been determined using this method in drusy quartz, associating with siderite crystals. Oxygen isotope variations (16.8–19.9‰ V-SMOW) in siderite result from a temperature increase from ∼175 to ∼230 °C, whereas nearly constant δ 13 C values (−3.9‰ to −4.6‰ V-PDB) indicate absence of CO 2 devolatilisation. Carbon isotope composition of the parental fluid changed from −9.5‰ to −8.8‰ V-PDB, while the oxygen isotope ratios remained essentially fixed at 7.7±0.3‰ V-SMOV during formation of siderite. The calculated δ 18 O fluid value is attributed to formation water enriched in 18 O during isotopic exchange with crustal rocks at low fluid/rock ratios. The δ 13 C fluid values probably correspond to a mixture of CO 2 from dissolved matrix carbonates and that liberated by thermal decarboxylation of organic acids (acetates), which normally occur in formation waters. Leachate chemistry data have corroborated high concentrations of CaCl 2 and MgCl 2 in the ore-forming brines. High Br concentrations are similar to halite-fractionated basinal brines. Ca 2+ and K + concentrations in excess of those in normal basinal brines reflect extensive cation exchange with surrounding low- to medium-grade metamorphites, coincidental with calcite and K-feldspar dissolution, dolomitisation, and albitisation of plagioclases. High pressures of the siderite-forming fluids (1–2 kbar) must be linked with a deep circulation (∼4–8 km) of the basinal brines derived primarily from marine water, but significantly modified by interaction with low- to medium-grade Paleozoic rocks at low fluid-to-rock ratios. An anti-clockwise crystallisation PT path inferred from fluid inclusion and stable isotope data is inconsistent with the siderite precipitation during extensional tectonic regime coincidental with Permian–Triassic rifting suggested in previous models. More likely, the progressively increasing fluid pressures and temperatures could be attributed to compression and crustal thickening triggered by Jurassic subduction in the Meliatic–Hallstatt oceanic suture. The superimposed quartz–ankerite–sulfidic ores have likely crystallised from brines expelled during compression associated with a continental collision during Early to Middle Cretaceous times.

Origin of methane in quartz crystals from the Tertiary accretionary wedge and fore-arc basin of the Western Carpathians

Abstract Quartz crystals from mineralised joints of the Carpathian flysch contain fluid inclusions covering the compositional range from gaseous CH4±CO2±C2+ mixture, through CH4-saturated oil, to CH4-saturated aqueous solution. The C isotope composition of CH4 extracted from the fluid inclusions (−31 and −36‰ V-PDB) is indicative of a thermogenic dry gas released from overmature rocks, although the maturation level of the surrounding flysch corresponds to a wet-gas generating zone. Trapping PT conditions for the CH4-bearing fluids range between 130 and 205xa0°C and 0.5 and 3.7 kbar. The fluid inclusion-derived crystallisation temperatures are inconsistent with the vitrinite reflectance data. Strongly fluctuating fluid pressures during growth of the quartz crystals indicate the existence of a crack-seal mechanism with recurrent hydrostatic and lithostatic regimes. Fluid pressures exceeding the lithostatic load have been recorded in the Dukla nappe. The overpressures have been generated by fluid volume increase due to thermal decomposition of oil and kerogen to CH4 and pyrobitumen. Occurrence of oil and condensate in the CH4-bearing inclusion fluids trapped at temperatures considerably above oil window suggest a short-termed, transient post-orogenic heating, rather than a prolonged heating related to burial. The thermal event giving rise to the quartz crystallisation has been coincidental with regional collapse and uplift of the Western Carpathians.

Eclogite-hosting metapelites from the Pohorje Mountains (Eastern Alps): P-T evolution, zircon geochronology and tectonic implications

Phase-equilibrium modelling, geothermobarometry, ion-microprobe dating and mineral chemistry of zircon have been used to constrain the P-T-t evolution of metapelitic kyanite-bearing gneisses from the ultrahigh-pressure (UHP) metamorphic terrane of the Pohorje Mountains in the Eastern Alps. These eclogite-hosting rocks are part of the continental basement of the Austroalpine nappes. Based on calculated phase diagrams in the system Na2O-CaO-K2O-FeO-MgO-MnO-Al2O3-SiO2-H2O (NCKFMMnASH) and conventional geothermobarometry, the garnet-phengite-kyanite-quartz assemblages of gneisses record metamorphic conditions of 2.2-2.7 GPa at 700-800 � C. These are considered as minima because of the potential for a diffusion-related modification and re- equilibration of the garnet and phengite during early stages of decompression. It is therefore most likely that the gneisses experienced the same peak UHP metamorphism at � 3 GPa as associated kyanite eclogites. Decompression and cooling to � 0.5 GPa and 550 � C led to the consumption of garnet and phengite, and the development of matrix consisting of biotite, plagioclase, K-feldspar � sillimanite and staurolite. Textures and phase diagrams suggest a low extent of partial melting during decompression. Cathodoluminescence images as well as zircon chemistry reveal cores encompassed by two types of metamorphic zircon rims. Ion probe U-Pb dating of three zircon cores yielded Permian (286 � 10, 258 � 7 Ma) and Triassic (238 � 7 Ma) concordia ages. The zircon rims are Cretaceous with a mean concordia age of 92.0 � 0.5 Ma and some cores gave a similar age. The Cretaceous zircons all exhibit very low Th/U ratio (,0.02) typical of metamorphic origin. In these zircons, nearly flat HREE patterns, (Lu/Gd)N ¼ 1-4, and only small negative Eu anomalies indicate formation in the presence of garnet and absence of plagioclase, which is corroborated by occurrence of Mg- and Ca-rich garnet inclusions. Therefore, these zircons are interpreted to record the Cretaceous HP/UHP metamorphism. The 92.0 � 0.5 Ma age obtained in this study agrees with that (93-91 Ma) determined earlier in the Pohorje eclogites from U/Pb zircon, Sm-Nd and Lu-Hf garnet-whole-rock dating. This implies that the eclogites and their country rocks were subducted and exhumed together as a coherent piece of continental crust. There is no evidence for a melange-like assemblage of rocks, which followed different P-T-t paths, or several subduction and exhumation cycles as proposed for some other UHP metamorphic terranes.

Hydrotectonic regime at soles of overthrust sheets: textural and fluid inclusion evidence from basal cataclasites of the Muráň nappe (Western Carpathians, Slovakia)

Abstract Newly formed, synkinematically grown minerals in basal cataclasites of the thin-skinned Muraň nappe contain primary fluid inclusions which represent samples of a hydraulic lubricant of the overthrusting nappe block. The inclusion fluid corresponds to a high-salinity brine (37–58xa0wt% NaCl eq.) of the system Na-K-Ca-Mg-Cl-SO 4 -CO 2 . Trapping temperatures between 213 and 471xa0°C are substantially higher than weak metamorphic imprint of the overlying thrust body. Strongly fluctuating inclusion fluid densities and trapping pressures between 0.2 and 5.4xa0kbar are interpreted to reflect locally supralithostatic overpressures induced by frictional heating beneath the moving nappe. The results of this study corroborate the hypothesis of Hubert and Rubey [Bull. Geol. Soc. Am. 70 (1959) 115–206], who envisaged a profound effect of pore fluid pressure at the base of moving thrust blocks.

Nitrogen-bearing fluids, brines and carbonate liquids in Variscan migmatites of the Tatra Mountains, Western Carpathians - heritage of high-pressure metamorphism

Gaseous inclusions in migmatites of the Tatra Mountains contain 5–100 mol.% nitrogen, contrasting with essentially the nitrogen-absent, mostly CO 2 -CH 4 composition of fluid inclusions in other migmatites. While CO 2 predominates in the Tatra metapelite migmatites, pure nitrogen inclusions are typical of the associated metabasite migmatites. CaCl 2 -rich brines (>30 wt.% NaCl eq.), halite, graphite and carbonate globules are the additional phases accompanying the nitrogen-rich fluids in the migmatite leucosomes. Monophase, crack-bound carbonate inclusions, consisting of calcite in metabasites and of Mg-bearing siderite in Fe-rich metapelites, are believed to represent a carbonate liquid, coexisting with and exsolving from the CO 2 -N 2 -brine fluid at temperatures between 550–700°C. Compositions and densities of the N 2 -CO 2 fluids and associated brines are similar to those in high-pressure granulites and eclogites. Therefore, these volatiles are interpreted to have been inherited from a high-pressure stage, pre-dating the migmatisation. Together with mineral textures and assemblages, the fluid-inclusion record is indicative of a substantial crustal thickening in the Western Carpathians during the Variscan orogeny. Essentially pure CO 2 -N 2 , graphite-saturated fluids in metapelites have been generated at dry conditions ( X H 2 O = 0.2-0.25), resembling those of typical granulite-facies metamorphism.

Origin of the fluids associated with granodiorite-hosted, Sb-As-Au-W mineralisation at Dbrava (Nzke Tatry Mts, Western Carpathians)

Mineral parageneses of the polymetallic, Sbrich deposit at Dúbrava has been formed during five separated stages. A fluid inclusion study demonstrates that the earliest stages with scheelite, molybdenite and arsenopyrite have been associated with immiscible CO2 (± CH4)-rich, low-saline fluids at temperatures between 300 and 400 °C and pressures as much as 2 kbar. Deposition of the main, superimposed ores, stibnite and zinckenite, has been intimately connected with circulation of aqeuous, moderately saline fluids (15.5–23.5 wt% NaCl equiv.) upon epithermal conditions. Salinity of the aqueous fluids associated with tetrahedrite is clustered around 10 wt% NaCl equiv. Quartz from the latest, barite stage has precipitated from aqueous fluids enriched in divalent cations. These fluids are believed to be genetically linked with Triassic evaporite formations preserved in the region. Temperature-salinity diagrams constructed from microthermometry data indicate influx of diluted meteorite water in the stibnite, tetrahedrite and barite stages. Isotopic data are in accordance with model. The δ18O values between −9.3‰ and +1.5‰ have been derived for water in equilibrium with quartz, coexisting with sphalerite, tetrahedrite and barite, thus confirming the participation of isotopically lighter meteoric waters in the mineral-forming solutions. The (δ18O) values between +3.3‰ and +8.5‰ estimated for the water associated with the scheelite and arsenopyrite stages, are suggestive for the majority of metamorphic and/or magmatic water in the mineral-forming, CO2-rich solutions.

Combined U/Pb and (U-Th)/He geochronometry of basalt maars in Western Carpathians: implications for age of intraplate volcanism and origin of zircon metasomatism

The age of intraplate volcanism in northern Pannonian Basin of Carpathians is revisited using a combination of zircon U/Pb, zircon (U–Th)/He and apatite (U–Th)/He dating techniques, complemented by electron microprobe (EMP) characterisation of dated minerals. A total of six maar structures and diatremes in the South-Slovakian Volcanic Field (SSVF) were dated and the obtained new ages yielded the following key findings: Two isolated maars in SE part indirectly dated by geomorphologic constraints to Late Pleistocene are actually of Pliocene (2.8xa0±xa00.2xa0Ma) and Late Miocene (5.5xa0±xa00.6xa0Ma) ages. In contrast, two maars in NW part of the study area are of Late Pliocene age (4.1xa0±xa00.4 and 5.2–5.4xa0Ma), younger than the Late Miocene age (~6.5xa0Ma) inferred previously from K/Ar data on the proximal basaltic lava flows. These maars therefore belong to the second volcanic phase that was previously identified only in SE part of the SSVF. In the light of the new geochronologic data, it seems likely that the Pliocene phreatomagmatic eruptions may have occurred along extension-related, NW- and NE-trending orthogonal faults. EMP analyses and imaging revealed an extensive syn- and post-growth metasomatic replacement by dissolution-reprecipitation in the majority of zircons. Abundant silicate melt inclusions in porous metasomatised parts of the zircons are diagnostic of magmatic rather than hydrothermal metasomatism. Consistent ages of the metasomatised and non-metasomatised zones do not indicate disturbance of the U–Pb system during the metasomatism. Enrichment in U and Th loss in the metasomatised zircons are diagnostic of an increasing oxygen fugacity triggered by degassing of the volatile residual melt during the final stages of alkali basalt fractionation. Rare zircon-to-baddeleyite transformation was probably connected with lowered silica activity in carbonated basaltic magmas in south-eastern part of the study area.

U–Pb geochronology of zircons from fossiliferous sediments of the Hajnáčka I maar (Slovakia) – type locality of the MN 16a biostratigraphic subzone

Lacustrine sediments of the Hajnacka I maar of southern Slovakia contain teeth andn skeletal remains of a mammal fauna, including index Early Villanyian arvicoline rodents.n U–Pb dating of magmatic zircons extracted from the redeposited fossiliferous maarn sediments revealed a total of six volcanic events. The oldest age of 3.43 Ma wasn interpreted as that corresponding to the initial phreato-magmatic eruption that createdn the maar. Most zircons grouped around 3.06 ± 0.03 Ma, the age attributed to then catastrophic eruption that killed the mammal assemblage thriving within and around then maar studied. This age coincides with the 3.1 Ma boundary between the MN 16a and 16bn subzones of the European Mammal Neogene chronostratigraphic scale estimated from then succession of Early Villanyian palaeontological localities according to the hypsometry ofn rodent teeth. Younger zircon ages overlapped those of the neighbouring basaltic lavan flows, necks and dykes, thus recording superimposed effusions. The youngest 1.58 Ma oldn zircon defines the age limit for the final redeposition of the fossiliferous sediments inn the maar drainage lake regime.

A boundary layer-induced immiscibility in naturally re-equilibrated H2O-CO2-NaCl inclusions from metamorphic quartz (Western Carpathians, Czechoslovakia)

Naturally re-equilibrated fluid inclusions have been found in quartz crystals from alpine fissures of the Western Carpathians. Re-equilibration textures, such as planar arrangement of the decrepitation clusters as well as the quartz c- and a-axis oriented fracturing indicate explosion of fluid inclusions. The extent of fracturing, which is dependent on inclusion diameters, suggests inclusion fluid overpressures between 0.6–1.9 kb. Microthermometry data are controversial with the textures because of indicating roughly fixed initial fluid composition and density during re-equilibration, although inclusion volumes have been sometimes substantially reduced by crystallization of newly-formed quartz. It is concluded that fluid loss from re-equilibrated inclusions must have been compensated for by replacing equivalent quartz volume from cracks into parent inclusion. Such a mechanism has operated in a closed system and the re-equilibration related cracks have not been connected with mineral surface. The compositional and density differences between aqueous inclusions in decrepitation clusters and CO2-rich parent inclusions cannot be interpreted in terms of classical fluid immiscibility. Moreover, monophase liquid-filled aqueous inclusions and coexisting monophase CO2 vapour-filled inclusions in the decrepitation clusters are thermodynamically unacceptable under equilibrium metamorphic conditions. The effect of disjoining pressure resulting from structural and electrostatic forces in very thin fractures is suspected to have caused density and compositional inconsistencies between parent and cluster inclusions, as well as the unusual appearance of cluster inclusions. In high-grade metamorphic conditions, the re-equilibration probably leads to boundary layer-induced immiscibility of homogeneous H2O−CO2−NaCl fluids and to formation of compositionally contrasting CO2-rich and aqueous inclusions.