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  • Author or Editor: Balázs Horváth x
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A detailed petrographic and mineral-chemical study on metapelites from the Meliatic accretionary wedge complex (Bôrka 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 tectonic reconstructions for the area.

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Four paleosol layers indicating wet and moderate periods and five loess layers indicating dry and cold climate were separated by different methods. The following climate cycle model, based on the development of the sediment sequence created by the influence of climatic, geologic and geomorphologic phenomena, was established by detailed paleomagnetic studies (e.g. anisotropy of magnetic susceptibility (AMS), isothermal remanent magnetization (IRM), frequency dependence of magnetic susceptibility (κFD), etc.):

  1. A well-foliated magnetic fabric predominantly built up by multi-domain ferromagnetic minerals (magnetite, maghemite) was developed during the semi-arid (350–400 mm/y) and cold loessification period of the Pleistocene. The magnetic fabric can reflect the direction of dust deposition and/or the paleoslope.
  2. The accumulation period of dust was followed by the more humid (650 mm/y) pedogenic period indicated by the enrichment of superparamagnetic minerals and by the disturbed or inverse magnetic fabric developed during pedogenesis by different processes (e.g. leaching and/or bioturbation).
  3. The third period following the pedogenic period is the humid erosional phase indicated by the finely layered reworked loess. The magnetic fabric built up by multi-domain ferro- and superparamagnetic minerals is characterized by better-aligned directions of principal susceptibilities than in the wind blown material. Sheet wash and other waterlogged surface processes appeared in the fabric of these layers. This process is possibly connected to sudden, rare yet significant events with high precipitation and absence of vegetation.
  4. The cycle was closed by the beginning of the next dust accumulation period.
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