Jurassic-Cretaceous tectonometamorphic story of the Meliata Unit: U-Th-Pb dating of monazites from blueschist-facies Bôrka Nappe and mélanges

The junction between the Central and Internal Western Carpathians is characterized by the occurrence of the Meliata Superunit (Meliaticum). The area represents a transitional zone between the Central (Austroalpine) and the Internal Western Carpathians (South-Alpine, Adria or Pelso Megaunit). The lowest structural position is occupied by the Upper Austroalpine basement-cover Vepor and Gemer Superunits, overridden by a series of Inner Western Carpathians cover nappes involving the Meliatic, Turnaic and Silicic units.
Meliata Unit (Meliaticum) represents one of the most crucial elements and at the same time significant tectonic unit of the Western Carpathians. In general, it is considered to be a remnant of the closure of the Neotethys-related Meliata Ocean, which took place during the Triassic–Jurassic period. The Meliaticum was subdivided into three distinct types of units, with this subdivision being based on rock composition and metamorphic characteristics. I) The HP/LT Bôrka Nappe located in the lower structural position is considered to be a transitional member between Meliatic oceanic domains and the Austroalpine Gemer Unit; II) Ophiolite-bearing, serpentine, and polygenous mélanges and III) a low-grade sedimentary complex consisting of olistostrome bodies, only with relics of ophiolitic material – Meliata Unit s.s.
In our research, we focused on comprehensively dating of rocks belonging to the Bôrka nappe and mélange complexes, taking into careful consideration the surrounding units and their similar tectono-metamorphic history. During the dating process, our emphasis was put on monazite, which is one of the most prevalent accessory minerals in metamorphic rocks under a variety of P-T conditions.
The method applied for monazite dating is predicated on the precise determination of the Th, U and Pb contents by means of the chemical isochron method (CHIME) (Konečný et al., 2018). The analysis of the monazites was performed using a Cameca SX-100 microprobe at the Department of Electron Microanalysis, State Geological Institute of Dionýz Štúr (Bratislava, Slovakia). For the analysis of monazite, a beam current of 100 nA and an accelerating voltage of 15 kV were used. The statistical approach of Montel et al. (1996) was applied for the resulting age determination. The shape and position of monazite crystals can also represent information, which is why they have attracted considerable attention. If this information can be linked to particular processes or events, monazite can offer a detailed account of the geological history of the host rocks.
We completed 503 analyses of monazite grains in total. The 16 samples in our study were collected from 8 different locations, primarily from the highly metamorphosed rocks of the Bôrka nappe, but also from mélange complexes.
The monazites that we dated do not show single metamorphic events, but they display a nice generation of Alpine monazites within the 180–80 Ma age range with four main peaks (Figure 1). This finding suggests the presence of a continual series of events associated with the Meliata Unit, which exhibit a high degree of correlation with previously documented events. The subduction, exhumation and the subsequent formation of an accretionary complex with nappe stacking of the Western Carpathians are recorded by the monazite ages.
The Bôrka Nappe reached its peak of blueschist facies metamorphism around 155–160 Ma (e.g. Faryad & Henjes-Kunst, 1997; Figure 1). The 135 Ma age of metamorphic perovskite and zircon (U–Th)/He cooling ages at 130–120 Ma supports the exhumation of the Meliatic units (Figure 1) due to thrusting and exhumation (e.g. Putiš et al., 2014). This event is probably connected with low-grade metamorphism of the Gemeric complexes, which occurred during burial below the Meliatic–Turnaic nappe stack, dated to c. 140–120 Ma (e.g. Vozárová et al., 2014). Central Western Carpathians underwent significant nappe stacking between 130 and 90 Myr ago (Figure 1), as evidenced by the superposition of the Gemer complexes over the Vepor Unit (e.g. Plašienka et al., 2016), with the Meliatic-Turnaic accretionary wedge being located at the top.
The subsequent change from compression to extension regime is associated with the unroofing of the Veporic core complex. The extensional tectonic regime during the sliding of the overburden complexes caused the collapse of the Meliatic-Turnaic accretionary complex (Figure 1) is and also structurally associated with the blastesis of monazites in Vepor Unit (97 ± 4 Ma; Bukovská et al., 2013).