Dunites from the hyperbasite massif of the Main Ural Fault zone (Polar Urals): mineralogy, geochemistry and formation conditions

Abstract

Introduction: In the Main Ural Fault (MUF) area of the Polar Urals, dunite harzburgite bodies of uncertain genesis are localised in the Upper Proterozoic sediments of the Nyarovey series of the western tectonic zone. They are either marginal continental riftogenic formations of the Lower Middle Ordovician or tectonic erosion klippes of the dunite-harzburgite complex of the ophiolite Syum-Keu massif located to the east. The aim of the study was to determine the conditions for the formation of hyperbasite bodies based on mineralogical and geochemical studies of dunites from the massif near Ingilor Lake. Methodology: We carried out petrographic, petrogeochemical, and mineralogical studies of dunites using precision methods. We studied the structural features and compositions of ultramafic rocks and minerals (chromespinelides and olivines) which clearly indicate mantle and crustal processes. Results and discussion: By their petrochemical features, dunites differ from similar rocks from stratified intrusive bodies, as they are mantle rocks. Typically, they have two types of deformational structures: protogranular and porphyroclastic, which correspond to the conditions of mantle and crustal formation. Due to the low concentration of rare earth elements, values of calcium-aluminium ratio, significant variations in compositions, and high chromium content of primary mantle chromespinelides, ultramafic rocks are most likely apoharzburgite rocks, which are not typical for mantle riftogenic continental hyperbasites. These features make them similar to ultramafic rocks of oceanic and supra-subduction zones and dunites from large ophiolite complexes of the Southern and Polar Urals, including the SyumKeu complex. Metamorphic transformations shown in chromespinelide zoning and olivine structures occurred at temperatures of 520–640°C. Conclusions: The results suggest that the dunite harzburgite bodies in the MUF zone of the Polar Urals are of a klippe nature.