International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

Volume 72 no. 2 / April 2021

Volume 72 no. 2 / April 2021

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Articles in this issue

  • Three-dimensional magnetotelluric model along seismic profile 2T: An improved view on crustal structure in central Slovakia (Western Carpathians)

    Abstract: We present the crustal-scale geophysical model based on the magnetotelluric method focused on 3-D model- ling of the seismic 2T profile crossing the major Western Carpathian tectonic units in central Slovakia. The results of the 3-D modelling show substantial improvement in previous 2-D models of deep crustal structure in central Slovakia, mainly of the physically distinct tectonic segments and major geo-electrical regional structures like the zone of the Carpathian Conductivity Anomaly, which indicates the occurrence of the large-scale shear zone in the contact zone of the European platform and Inner Western Carpathians. High detail geo-electrical data in 3-D magnetotelluric (MT) cross section also allowed a better interpretation of other conductive anomalies. In the final integrated interpretation (combi- nation of 3-D geo-electrical model, gravity data and seismic reflectors), it is shown that frontal part of the Inner Western Carpathians plate exhibits the transpressional tectonic style of the back-thrust Outer Western Carpathians (Flysch Belt) and Pieniny Klippen Belt units over the progressing Inner Western Carpathian thrust wedge. These back-thrusts form the southern branch of the accretionary structural fan – a large-scale transpressional flower structure typical mainly but not only for oblique convergent regimes. The southernmost segment of the profile with high whole-crust conductivity due to a higher heat flow caused by young volcanic activity indicates partial melting in the middle and lower crust.
  • From subduction to collision: Genesis of the Variscan granitic rocks from the Tatric Superunit (Western Carpathians, Slovakia)

    Abstract: Granitic rocks from the Core Mountains of the Tatric Superunit (Western Carpathians, Slovakia) were dated by means of the sensitive high-resolution ion microprobe (SHRIMP) zircon U–Th–Pb method. The dated granitic rocks yielded a broad interval of the Concordia ages from 365 ± 5 Ma to 332 ± 3 Ma and largely invalidated the former hypothesis of a duality/antagonism in emplacement ages of the Variscan S- and I-type granites (Mississippian vs. Pennsylvanian) in the Central Western Carpathians (CWC). Generally, the obtained magmatic ages cluster in two separate intervals reflecting different stages of orogeny. The older, Famennian–Tournaisian event (365–350 Ma) was related to subduction of the Rheic Ocean, whereas the younger, mostly Visean event (348–332 Ma) was associated with collisional melting. The Th/U ratios of analysed zircons are compatible mainly with their magmatic origin (Th/U > 0.2), while the lower ones (Th/U 
  • Carboniferous mafic metavolcanic rocks in the Northern Gemeric Unit: Petrogenesis, geochemistry, isotope composition and tectonic implication

    Abstract: The paper presents whole rock chemical composition and Sr–Nd isotope data from selected metabasic rocks from the Mississippian and Pennsylvanian sequences of the Northern Gemeric Unit. The analysed metabasic rocks belong to the subalkaline magmatic series with Nb/Y ratios ranging from 0.03 to 0.21. They fit into the low-Ti tholeiitic series, characterized by TiO2 contents of less than 2.5 wt. % and Ti/Y ratios below 500. Petrological and chemical signatures show the separation of the Group I (Pennsylvanian) from the Group II (Mississippian) metabasalts, which is ­supported by biostratigraphical data of the surrounding metasediments. The Group I metabasalts display higher contents of Zr, Th, Rb and U, Pb, Zn, Ni compared to the metabasalts of Group II and conversely lower contents of Nb, Ta and V. The chondrite normalized rare earth elements curves show a uniform pattern, with rare earth element enrichment and no or weak positive or negative Eu anomalies (0.88–1.23 vs. 0.89–1.17). The (Tb/Yb)N ratios from 1.36–1.62 in the Group I or 0.92–1.55 in Group II are symptomatic of spinel-bearing peridotite mantle source. Based on trace and rare earth element distribution patterns, the Group I metabasites correspond to the N-MORB/E-MORB field and the Group II metabasites shift significantly towards the BABB and CAB fields. The Sr/Nd isotope systematics confirmed depleted mantle isotopic signatures, with minor influences from crustal sources and affected by fluid-related subduction meta­somatism. All the studied samples have positive εNd(0) ranging from 7.92 to 8.68 for Group I and from 4.59 to 10.52 for Group II metabasalts. The 87Sr/86Sr(0) values vary between 0.7053–0.7081 and between 0.7052–0.7076, respectively, and 0.7109 for basaltic andesite.
  • Magnetic fabrics in the turbidite deposits of the Central Carpathian Paleogene Basin in relation to sedimentary and tectonic fabric elements

    Abstract: Anisotropy of magnetic susceptibility and anisotropy of anhysteretic remanent magnetization were studied in the Oligocene turbidites of the weakly deformed Central Carpathian Paleogene Basin. In order to decide whether the magnetic fabric can be related to deposition from a paleoflow or to incipient weak tectonic deformation we compared magnetic fabrics in individual intervals of the Bouma sequence with sedimentary structures and tectonic brittle mesostructures (joints). In the Ta–Te intervals we observed a good correlation between maximum susceptibility axes and SW(W)–NE(E) oriented paleoflows. Within convoluted and slump folded sandstones the AMS fabric coincides with the orientation of soft-sediment deformation structures. These features suggest the sedimentary origin of the AMS fabric. Three types of AARM sub-fabric were distinguished. The AARM type 1 represents magnetic foliations parallel either to NNW–SSE or to NE–SW oriented joints with magnetic lineations distributed along the joins. In the AARM type 2 magnetic foliations remained parallel to the bedding while magnetic lineations group around the joint to bedding intersections. The AARM type 3 coincides with the AMS fabric. Magnetic and microscopic analyses indicate that the AARM fabrics are connected to magnetite associated with subordinate ferrimagnetic iron sulphides. Both minerals occur in a sub-microscopic size and formed most likely during late diagenesis through the alteration of pyrite, possibly accompanied by burial clay transformation processes. The growth of the authigenic ferrimagnetic minerals was conditioned by combined effects of the sedimentary petrofabric, lithology and stress conditions during the inversion of the basin in the Early to Middle Miocene.
  • Manganese-rich carbonate and phosphate concretions from the Subsilesian Unit of the Outer Western Carpathians (Czech Republic): Composition and unique selenium weathering products

    Abstract: This research focuses on two new occurrences of sedimentary concretions in the Subsilesian Unit of the Outer Western Carpathians (Czech Republic) from non-calcareous clays and claystones of the Paleocene to the Eocene age. Powder X-ray diffraction study proved heterogenous matrix, varying in content of siderite, Mn-rich siderite, Ca-rich rhodochrosite and fluorapatite. Electron microscopy revealed microsparitic carbonates with indistinct zoning. According to the geochemical and stable isotope clues the concretions originated in medium to highly reducting environment during early diagenesis. Stable isotope δ13C values (−11.3 and −4.8 ‰ PDB) and δ18O (1.8 and 1.0 ‰ PDB) correspond well to early diagenetic marine carbonates and part of the CO2 was derived from oxidation of biological material. Weathering of concretions has been on the surface and along fissures. While siderite-rich concretions produce goethite, rhodochrosite-­rich concretions produce a cellular structure of todorokite, birnessite and buserite on rims of carbonate cores. Fissure mineralisation contains association of goethite accompanied by native selenium and clausthalite. The origin of the selenium minerals is interpreted as products on the redox barrier between Mn2+/Mn4+ and/or Fe2+/Fe3+, where selenites and selenates are highly soluble but native selenium and selenides are extremely insoluble.