Chaotic complexes of the Meliata Unit: biochronology, lithostratigraphy and geochemistry of a mélange near Čoltovo (Western Carpathians, Slovakia)

The supposed Meliata suture divides the Austroalpine-related Central Western Carpathian (CWC) units from the Internal Western Carpathian (IWC) units of the Adria and/or Dinaridic affinity (Transdanubian and Bükk, respectively). The tightly imbricated and dismembered Triassic–Jurassic complexes of the Meliata Unit (Meliaticum) record the subduction/accretionary processes connected with the opening, expansion, subduction and closure of the Neotethyan Meliata Ocean in the southern Western Carpathian zones. The Meliaticum exhibits a lithologically variable composition. In general, three main partial units or complexes can be distinguished: i) the blueschist-facies Bôrka Nappe representing the distal continental margin exhumed from the subduction channel during the Late Jurassic and subsequently thrust over the foreland lower CWC plate; ii) variable chaotic, in part ophiolite-bearing mélange complexes; iii) relatively coherent Jurassic successions of deep-marine hemipelagic and distal flysch deposits with olistostrome bodies which include Triassic carbonate blocks revealing the late Mid-Anisian (Pelsonian) breakup of the Meliata Ocean. This study focuses on the examination of the chaotic complexes of the Meliata Unit at the crucial Čoltovo locality. Due to the poor outcrop conditions caused by soil and debris cover and tectonic reworking typical for the Meliaticum, heavy equipment was used to expose the bedrock and establish the relationships between various components. Based on lithological study and biostratigraphy of radiolarians, the Čoltovo locality represents a chaotic mélange structure of Lower Jurassic clastic pelagic deposits and Middle Jurassic olistostromes with cherty intercalations and blocks of Middle Triassic limestones, terrigenous siltstones and variegated radiolarites, Upper Triassic red radiolarites, siltstones and basic volcanics.

Geochemical analyses were undertaken on various types of sedimentary rocks. Majority of the samples fall within the fields of the pelagic environment affected by terrigenous input from the marginal continental area and a continental slope/rise environment, where they intercalate the radiolaritic shales. In the provenance discrimination diagram, most of the sample's plot fall in the fields of mature quartzose, i.e. continental terrigenous provenance. Basic volcanics plot in the border between the within-plate tholeiites with volcanic arc basalts and MORB. According to primitive mantle normalized REE patterns and other immobile trace elements, samples indicate a typical oceanic crust origin.

Čoltovo locality records gradual spreading and deepening of the Meliata Ocean which was the NW branch of the Neotethys. First phase of deep-water sedimentation is represented by distal terrigenous clastics and radiolarite sedimentation with significant volcanic and tectonic activity during the Middle–Upper Triassic. The second, Jurassic phase, points to the formation of olistostromes in connection to the subduction process. In its current form the Meliatic complexes are fragmented into allochthonous blocks/sheets, comprising the Meliatic Basin and the northern continental margin, together incorporated into subduction–accretion mélanges.

 

Acknowledgements: The research was supported by following projects: APVV-17-0170, APVV-21-0281 and VEGA 1/0435/21.