Vol 31, No 1 (2023)

The Udokan volcanic plateau (UVP), like other areas of the Late Cenozoic volcanic province of Central Asia, formed in the interval from the Middle Miocene to the Pleistocene. Its products have high alkalinity and vary in composition from alkaline picrobasalts and basanites to alkaline trachytes. Compositional changes were controlled by two differentiation trends, which corresponded to different conditions of the original magmas generation. Rocks with low SiO₂ contents (<45 wt. %) were formed from melts of low melting degrees that arose under conditions of elevated pressures and temperatures. Formation of rocks corresponding to the composition range 45–61 wt. %. % SiO₂ was associated with the differentiation of basalt melts, which arose at shallower depths and at lower temperatures. The geochemical characteristics of the UVP basaltoids make them similar to OIB-type basalts. They are also close in Sr, Nd, and Pb isotopic composition, corresponding to the parameters of a moderately depleted mantle, which is close to the composition of oceanic basalt sources corresponding to the mantle of deep mantle plumes. The corresponding mantle component is present in the sources of other volcanic regions of the Late Cenozoic intraplate volcanic province of Central Asia, which indicates the involvement of the lower mantle plume in the formation of these regions.

The tectonic evolution of the Siberian cratonic margins offers important clues for global paleogeographic reconstructions, particularly with regard to the complex geological history of Central Asia. The Yenisey Ridge fold-and-thrust belt at the western margin of the Siberian Craton forms part of the Central Asian Orogenic Belt (CAOB) and is a key to understand the Precambrian tectonic evolution of the Siberian Craton and crustal growth in the CAOB. Understanding the tectonic evolution of the Yenisei Ridge is crucial for solving the debate related to the role of the Siberian Craton within the Rodinia supercontinent assemblies and breakup with the opening of the Paleoasian ocean. Here we report new data on the petrogenesis, tectonic settings, thermodynamic conditions, metamorphism and protolith ages for compositionally contrasting rocks of the Garevka metamorphic complex, obtained from the results of mineralogical-petrological, geochemical, and isotope-geochronological studies. Possible models and geodynamic settings for their formation are discussed. Based on the results of U-Pb dating of zircons, two new pulses of Neoproterozoic endogenous activity at the western margin of the Siberian craton were established, associated with Grenville (930–900 Ma) and (880–845 Ma) post-Grenville Valhalla (880–845 Ma) accretion-and-collision processes. These episodes of regional crustal evolution are correlated with the synchronous successions and similar style of rocks along the Arctic margin of Rodinia and supports the spatial proximity of Siberia and North Atlantic cratons (Laurentia and Baltica), which is consistent with the proposed Neoproterozoic paleogeographic reconstructions of the Rodinia configuration.

The aegirine end-member (NaFe³⁺Si₂O₆) in clinopyroxenes resulted from incorporation of Fe³⁺ into the mineral structure effects the accuracy of reconstruction of the P-T conditions in the high-grade metamorphic rocks and also allows evaluation the redox conditions of their formation. As a rule, the content of this end-member in clinopyroxenes is evaluated based on the crystal chemical recalculations of microprobe analyses. However, in some publications on eclogites, the results of recalculations of clinopyroxenes were compared with the data of Mössbauer spectroscopy. Significant difference was revealed between the measured and calculated Fe³⁺/ΣFe ratios, that can significantly affect the results of geothermometry. This paper presents the results of the Mössbauer spectroscopy measurements of clinopyroxene fractions separated from three samples of garnet-clinopyroxene granulites from the Udachnaya kimberlite pipe. The ratios Fe³⁺/ΣFe = 0.22–0.26 measured in clinopyroxenes correspond to 6–10 mol. % aegirine. These estimates are in good agreement with the values obtained for clinopyroxenes from the same samples by the recalculation of microprobe analyzes using the charge balance method. Following to this conclusion, we believe that crystal chemical recalculations of microprobe analyzes of clinopyroxenes from non-eclogitic rocks make it possible to correctly estimate the Fe³⁺ content in them. Similar recalculation of microprobe analyzes of clinopyroxenes from crustal xenoliths from other localities, as well as from ferrobasalts of the continental flood basalts provinces, ferrodolerite dikes, and gabbroid xenoliths (similar in bulk chemical composition to many lower-middle-crustal xenoliths) revealed significant amounts of previously unaccounted aegirine in them (up to 13 and 4–9 mol. %, respectively), that unleashes the potential for the reconstruction of redox conditions in many rocks.