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Vol 31, No 1 (2023)
Articles
Conditions of Rock Formation and Magma Sources of the Late Cenozoic Udokan Volcanic Plateau
Abstract
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.
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Staurolite in Metabasites: P-T-X Conditions and the Ratios of Petrogenic Components as a Criterion of the Appearance of Staurolite
Abstract
In addition to the widespread Fe-Mg staurolite, typical for medium-temperature high-alumina metapelites, there are a number of finds of magnesian staurolite in metamorphosed mafic rocks – metabasites. Based on thermodynamic modeling and analysis of the mineral formation patterns, the most significant factors of the staurolite formation in metabasites have been revealed. For the formation of staurolite in metabasites, in contrast to staurolite in low- and medium-pressure metapelites, medium- and high-pressure conditions of metamorphism are necessary. An increase in the proportion of carbon dioxide in the composition of the water-carbon dioxide fluid has practically no effect on staurolite-forming mineral reactions, but leads to their shift to lower temperatures and higher pressures. Al, Fe, Mg, Ca are critical petrogenic rock components for the formation of magnesian staurolite, the contents and ratios of which primarily determine the stability of staurolite in metabasites. To understand the regularities of mineral formation, it seems appropriate to divide metabasites into subgroups of predominantly magnesian, iron-magnesian, and ferruginous protoliths. Based on this division, three petrochemical modules are proposed in the form of the ratio of rock-forming components: MgO/CaO, CaO/FM, Al2O3/FM, based on which it is possible to predict the appearance of staurolite in the basic rock when the corresponding P-T conditions of metamorphism are reached.
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Evidence of Grenville and Valhalla Tectonic Events at the Western Margin of the Siberian Craton from Rocks of the Garevka Complex (North Yenisei Ridge)
Abstract
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.
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The Monticellite-Bearing Rocks of the Krestovskaya Intrusion: the Genesis According to Melt Inclusion Study
Abstract
The investigation of monticellitolites and olivine-monticellite rocks from the Krestovskaya Intrusion shows that the principal minerals (olivine and monticellite) contain higher amount of MgO than same minerals in olivinites and kugdites of the Intrusion. In the studied rocks olivine contains 90–93 mol. % Fo and monticellite has 41.6–42.3 mol. % Fo whereas in olivinites and kugdites olivine and monticellite contains 86–87 mol. % Fo, and 37.2–41.2 mol. % Fo, respectively. Melt inclusion study in minerals of monticellite rocks evidenced that the monticellite rocks of the Krestovskaya Intrusion were formed due to mixing melts different in composition and volatiles: K-rich high-iron low-alumina kamafugitic melt and Na-rich high-magnesia high-alumina picritic one. Minerals crystallized at high temperatures in the following sequence: perovskite I (1250–1230°C) → perovskite II (≥1200°C) ↔ olivine (>1200°C) → monticellite (>1150°C). Perovskite I in monticellite rocks, as well as olivine in olivinites, crystallized from K-rich high-iron (Mg # = MgO/(MgO + FeO) – 0.37), low-alumina kamafugitic melt. During crystallization of late perovskite II in monticellite rocks, the melt became more magnesian (Mg# = 0.41) and richer in Na2O and Al2O3, which is intermediate in composition between kamafugite and alkali picritoid. Olivine in monticellite rocks crystallized from melts similar in composition to melilitite, having a K-rich composition with Mg # = 0.39, whereas monticellite formed from a heterogeneous high-magnesian Si-undersaturated melt, which is highly enriched with volatile components (including H2O) and salts. The crystallization of minerals was accompanied by accumulation of volatile components in the mixing melts and as a result by the further processes of silicate-carbonate liquid immiscibility under 1250–1190°C and by multiphase carbonate-salt immiscibility under below 1190°C. In the latter event, the separated carbonate melt began to decompose into simpler immiscible fractions: alkali-sulfate-carbonate, alkali-phosphate-carbonate and calcio-carbonate.
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Aegirine-Bearing Clinopyroxenes in Granulites from Xenoliths of the Udachnaya Kimberlite Pipe, Siberian Craton: Comparison of the Results of Mössbauer Spectroscopy and Electron Micropobe Analysis
Abstract
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.
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