Riphean–Vendian–Cambrian Magmatism of the Mankhambo Block (Subpolar Urals): Geochemical Typification, Correction of Geodynamic Concepts, and the Role of Plume–Lithosphere Interaction

Abstract

The geochemical typification of magmatic rocks of the Mankhambo block (southern part of the Lyapinsky anticlinorium), which is confined to the junction zone of the Ural orogen with the Timanide–Cadomide belt relics, was performed based on the generalization of abundant isotope-geochemical data, application of modified discriminant diagrams, and analysis of Y/Nb ratio. The magmatic rocks mark two stages of the geodynamic evolution of this structure. The first stage (RF_2–3) is characterized by the tholeiitic magmatism (subvolcanic basic rocks of the Shchokuryinsky and Moroinsky complexes). According to composition and source type (Y/Nb = 3–5, E-MORB), these rocks can be correlated with riftogenic plume-related series of the East European passive margin. Magmatism of the second stage (650–500 Ma) was related to the evolution of divergent transform continental margin. This stage involved the subsequent formation of calc-alkaline magmatic series (653–608 Ma) (Sys’insky and Parnuksky complexes) and bimodal basalt–rhyolite association (569–554 Ma) (Sablegorsky and Laptopaisky complexes), which vary from tholeiitic to calc-alkaline rocks. High values of the Y/Nb ratio (5–8) in the basic rocks of the Sablegorsky complex suggest that their generation was related with intensification of transform movements, when plume-related basic magma (E-MORB-type source) interacted with more depleted slab-derived component. The rhyolites of the Sablegorsky complex mark the incipient generation of the A-type rhyolite–granite association (Y/Nb = 0.2–1), which was continued (522–490 Ma) by the granites of the Mankhambo and Ilya-Iz massifs. Their genesis may be related to the differentiation of magmas from two possible sources. A-granites with Y/Nb = 2.0 (phase I of the Mankhambo massif) were generated from E-MORB-type source. Granites of phase II (Y/Nb = 0.2–1) were derived from heterogeneous sources with the participation of OIB-type component. In general, the rocks of the Vendian–Cambrian stage, in comparison with older rocks, are characterized by the highest degree of plume-lithosphere interaction, with contribution of subduction-modified lithospheric mantle. The role of crustal contamination increases from the rhyolites of the Sablegorsky and Laptopaisky complexes to the granites of the Mankhambo massif. The presence of relict zircons and the “crustal” Sr and Hf isotope signatures indicate the contribution of ancient crustal material in their protolith. The genesis of the rhyolite–granite association may be associated with the emplacement of the “Mankhambo” plume. The plume role in the magma generation of rocks of the Mankhambo block increases with decreasing age.