Calcite microstructures recording polyphase deformation history of the Meliata Unit

Abstract

<p>The geological structure of the Western Carpathians is very complicated and is result of several deformation phases. The Meliata Unit (Meliaticum) as a significant part of the Western Carpathians proves existence of substantial tectonic movements. The Meliata Unit incorporates the Permian to Jurassic blueschists-facies Bôrka Nappe and the Jurassic low-grade mélange complexes with huge Triassic olistostrome bodies – the Meliata Unit s.s. Based on microstructural characteristics, the calcite is one of the most suitable minerals for study of deformation history. Calcitic metacarbonates are common elements of subduction-accretionary complexes and thus also a considerable element in rock composition of the Meliata Unit. Samples were taken from various Meliatic complexes either within the Bôrka Nappe, or as olistoliths embedded in the Jurassic mélange. Variations in deformation microstructures are clearly visible in sampled metacarbonates, what was main aspect to separate them into groups reflecting different P/T conditions. The distinguished groups more-or-less correspond to their regional occurrences and grade of metamorphosis of surrounding rocks. The first group (GI) contains relatively large calcite grains and microstructure pointing to the Grain Boundary Migration deformation mechanism, which suggests the higher temperature during dynamic recrystallization. The higher temperature is also proven by character of twin lamellas. The GI microstructures are related to the subduction processes after closure of the Meliata Ocean and exhumation of the high-pressure complexes. The second group (GII) is characterised by a significant grain size reduction and strong shape preferred orientation and thus with development of calcitic mylonite zones. They are related to forming of the Meliatic accretionary wedge. The third group (GIII) shows completely recrystallized microstructure of relatively uniform calcite grain size with sharp edges of grains. They were recrystallized in an annealing regime due to higher temperature gradient generated by a shallow granitic intrusion associated with the exhumation of the underlying Veporic metamorphic dome. The last deformation phase is marked by the bulging deformation mechanism, thus to a partial replacement of primary grains by newly formed fine-grained calcites and represent final stages of nappe emplacement.</p><p><strong>Acknowledgements</strong></p><p>Financial support from the Grant Agency for Science, Slovakia (project APVV-17-0170 & VEGA 1/0435/21) is gratefully appreciated.</p>