Mineralogical Features of Ultramafic and Mafic Rocks of Syum-Keu Ophiolite (Polar Urals, Russia): Implications for Petrology and Paleogeodynamics

Abstract

The Syum-Keu massif is the northernmost ophiolite complex of the Ural mobile belt. It differs from other massifs of the Polar Urals due to the prominent distribution of lherzolites in the upper mantle section. This feature aligns it more closely to some massifs in the southern part of the belt (Kraka). Thus, a comparison of the ultramafic rock compositions in these massifs is highly relevant. Thus, comparing the compositions of ultramafic rocks from these massifs is highly relevant and is one of the primary objectives of this study. Our second objective is to study the microstructural features of ultramafic rocks from the upper mantle, as they can indicate modes of subsolidus processes that played a key role in the formation of this massif. Our study utilizes optical microscopy, assessments of bulk rock composition using X-ray fluorescence and ICP-MS, as well as mineralogical methods, such as scanning electron microscopy with energy dispersive spectroscopy and electron backscattered diffraction, for the microstructural analysis of peridotites. In addition to ultramafic rocks from the upper mantle section, the composition and mineralogy of mafic rocks from the crustal section were studied. The microstructural analysis of ultramafic rocks indicates their two-stage evolution. The first is associated with plastic flow under the upper mantle conditions dominated by the olivine slip along the {0kl}[100] system, while the second reflects formation in the lower crust, with lower-temperature deformation along the {110}[001] slip system. Comparing the mineralogy of the Syum-Keu peridotites to lherzolite massifs in the Southern Urals reveals a significant difference in accessory Cr-spinel composition; the former show elevated iron content (Fe trend), indicating intense crustal metamorphism. Similarly, amphiboles in Syum-Keu ultramafic rocks exhibit a significant crustal (metamorphic) component, while the same minerals in the Kraka massif suggest a mantle (magmatic) origin. Mafic rocks in the Syum-Keu massif also typically display a high degree of metamorphism. The obtained results generally corroborate prior findings on a longer evolution of the upper mantle ultramafic rocks of the Syum-Keu massif compared to those of the Kraka massif. Our results are also consistent with the suprasubduction nature of these ultramafic rocks. Our findings can be utilized in further studies of the microstructure and composition of ophiolites from the Polar Urals to provide a more detailed characterization of the partial melting conditions of the mantle source, the plastic flow of peridotites, and their interaction with melts and fluids.