Serpentines as the indicators of mesozoic peridotites metamorphic and geodynamic transformations in the Internal Ukrainian Carpathians

Abstract

Formulation of the problem. Peridotites of ophiolite complexes,being the fragments of the oceanic upper mantle that have undergone several stages of partial melting, brought to the surface by tectonic movements, also have undergone metamorphic transformations almost immediately after its formation. Because of serpentinization, the mineral composition of the rocks became more complicated. The analysis of the final structure and composition of apoperidotites allows obtaining data for geodynamic reconstructions regarding the stage of their formation. Review of previous publications. It has been determined that serpentines are the most common secondary minerals of peridotites of the Uholskyi complex in the Ukrainian Carpathians, and the processes of serpentinization took place at a depth of 40–50 to 100 km (?) at T = 450–600 °C and P = 13–16 kbar (Stupka O., 2013). The study of serpentinites of the Main Ural Fault (Panas'yan L. et al., 2014) revealed that high alumina and high chromium serpentinites have ultrabasic protoliths formed in the mantle, and medium alumina and low chromium varieties – the protoliths of the basic composition which were born in the conditions of the crust. Based on the study of serpentinites in the orogenic Qinling belt (China), researchers (Wu K. et al., 2018) determined their mantle origin: magnetite-enriched antigorite serpentinites were formed as a result of the interaction of serpentinized apoperidotites of mantle protoliths with molten rock in the subduction channel. Purpose. We have investigated secondary serpentines in order to reconstruct the geodynamic conditions of the formation and transformations of the peridotites (Uholskyi complex), localized in the Marmarosh rocky zone in the Internal Ukrainian Carpathians, and are most widely spread in the interfluve of Velyka and Mala Uholka-rivers. Methods. The work is based on the results of geological observations of the Uholskyi complex rocks in natural outcrops, as well as petrographic, mineralogical (including X-ray diffraction, thermal and microprobe analysis), and geochemical studies. Results. The paper presents the study results of serpentinized apoperidotites of the ophiolite Uholskyi complex in the Internal Ukrainian Carpathians. Serpentinized apoperidotites (T2–K1?) form olistoliths in the Soimulska olistostrome-conglomerate strata of the Lower Cretaceous age. The investigated serpentines are the rock-forming minerals of lizardite and antigorite serpentinites. Lizardite serpentinites are characterized by lenticular-looped textures formed by α-lizardite and non-altered chrome-spinellids. Antigorite serpentinites, recognized by striped-shale textures, contain antigorite, β-lizardite and magnetite. Lizardite serpentinites are characteristic of the regressive metamorphism of the greenschist facies upper part, and antigorite serpentinites are a typical formation of the progressive metamorphism of the lower greenschist – upperlower epidote-amphibolite facies. Regressive metamorphism occurred under geodynamic conditions of spreading and the progressive ones – under subduction conditions between the terrains of Dacia and Tisza, which led to the closure of the Transylvanian-Mureş Paleocean. It has been concluded, that the protoliths of lizardite serpentinites were the primary mantle rocks of the ultrabasic (restitic) composition, and the protoliths of the antigorite serpentinites were the lithospheric rocks of the basic composition. Conclusions. The complex study of serpentinized apoperidotites of the Uholskyi complex makes it possible to reconstruct the metamorphic transformations of the primary mantle protoliths and to determine the stages of lithosphere formation within the fold-nappe structures. The obtained results can be used for prediction of serpentinite mineralization.