Subduction–exhumation cycle recorded by calcite deformation microstructures: blueschist-facies metacarbonates and kinematic implications for deformation of the Meliata Unit (Western Carpathians)

Abstract

The Meliata Unit (Meliaticum) is a tectonic superunit of the Western Carpathians that incorporates the blueschists-facies Bôrka Nappe and the low-grade metasediments and polygenetic mélange, Meliata Unit s.s., both occurring as scattered tectonic slices overlying the Gemeric Superunit. Calcitic marbles were sampled in a wide area within the Bôrka Nappe and blocks embedded in Jurassic oceanic sediments (Meliata Unit s.s.). Based on the microstructural evaluation and electron backscatter diffraction analysis (EBSD), the carbonates of the Bôrka Nappe experienced differential post-subduction P–T–D paths related to a collisional/exhumation setting following closure of the Neotethys-related “Meliata Ocean”. Variations in the calcite deformation microstructures were used to distinguish three principal microstructural groups. The first group (G1) contains large columnar and lobate calcite grains (≥ 1 mm) reflecting peak P–T conditions during subduction of the Meliata oceanic lithosphere. The second group (G2) exhibits dynamic recrystallization of the original G1 grains resulting in grain size reduction (< 0.5 mm) and shape-preferred orientation related to exhumation and formation of the accretionary complex. The third group (G3) shows a ‘foam’ microstructure with a uniform grain size (0.4–1 mm), sharp grain boundaries and triple junctions. The G3 microstructure may have been caused by a static recrystallization at elevated temperatures postdating the main deformation, and it is restricted to peripheries of the underlying Veporic metamorphic dome and probably is not associated with the Meliata sequences nor its tectono-metamorphic evolution. The corrected kinematic sections indicate dominantly ESE–WNW-trending lineations suggesting top-to-WNW kinematics of the Meliata subduction–exhumation process for G1 and G2 microstructures, and Gemeric–Veporic E–W orogen-parallel stretching for G3.