LAUSR

Abstract Knowing the protoliths of eclogites is important to understanding the nature and process of orogeny. It has been debated whether the eclogites of the Marun-Keu complex, Russian Polar Urals represent a fragment of metamorphosed oceanic crust or a fragment of metamorphosed continental crust. Here, we focus on eclogites from the garnet- peridotite-eclogite unit in the complex at the Mica Hill. The eclogites have high MgO contents (mostly greater than 8 wt%) and Mg# (Mg/[Mg + Fe2+] = 0.7–0.8), their Al2O3 contents vary from 16 wt% to 26 wt%, and CaO contents vary from 10 wt% to 15 wt%, respectively. TiO2 contents less than 0.5 wt% and a large variation of chromium concentrations (282–1745 ppm). The eclogites are enriched in the light rare earths (LREE), have positive Eu anomalies (δEu = 1.0–2.3), and varying eNd (t) from −3 to +2. Such geochemical features are evidence that the protoliths of these eclogites are troctolite and gabbro, which may have formed in a rift environment of a continental margin. Using the SHRIMP method to date zircons two groups of ages were obtained: 500–490 Ma and 370–360 Ma, respectively. The older population is interpreted as protolith formation age while the later population is interpreted as the metamorphic age of the eclogite facies. So the eclogites are likely the result of high pressure metamorphism due to subduction of the newly formed Eastern European continental margin under the Syum-Keu intra-oceanic arc, and not metamorphosed ophiolite (Paleo-Urals oceanic crust).