This is a synopsis of the available data on crustal carbonatites, including their temporal and spatial distribution, mineralogy, geochemistry, and stable isotope (δ18O and δ13C) patterns. Crustal carbonatites are intrusive rocks containing >50 vol. % carbonate minerals and ≤20 wt. % SiO2, which crystallize from partial melts of primary sedimentary carbonate rocks in the lower crust. They commonly occur as dykes in high-grade metamorphic complexes, bear silicate minerals typical of metasomatic environments, show isotopic and geochemical signatures of carbonate sediments or transitional varieties to mantle-derived carbonatites, and are emplaced during tectonic activity in strike-slip, rifting, or postcollisional extension settings. Partial melting of carbonate material in the crust and intrusion of melt batches to shallower crust levels is possible provided that primary carbonate sediments are present in the lower crust while the melting region is heated up by underplated mantle mafic magma and is fluxed sufficiently with H2O-rich fluids.

Specific gabbro with the main rock-forming mineral of fassaite—alumina-rich (up to 12 wt

Model calculations were used to estimate the compositions of melts during fractional crystallization corresponding to the formation of the Malyi Zadoi massif, which is located in the Irkut block of the Sharyzhalgai uplift in the southwest of the Siberian craton. It is shown that the gabbronorites of the massif are comagmatic to the plagioperidotites and olivine gabbronorites. The estimates obtained for the composition of the model melts are used to characterize the composition of the mantle source of the parental melt. The geochemical characteristics led us to suggest that the parental melt of the Malyi Zadoi massif was formed by melting an enriched source, a conclusion consistent with isotope data that indicate that the mantle Sm/Nd ratio decreased in the Archean. The probable source of the parental melt could consist of depleted lithospheric mantle material metasomatized by felsic melts coming from rocks of a subducting oceanic plate.

––The paper presents new data on the chemical and mineral compositions of ultramafic rocks and various gabbro of the Birkhin massif in the Ol’khon region. Porphyric phenocrysts and zoned grains of clinopyroxene have been first found and studied in gabbronorite, which made it possible to reconstruct the entire melt crystallization trend from ultramafic to gabbro parageneses. Similar clinopyroxene trends have been established for clinopyroxenites and subvolcanic ankaramites, whose bodies and dikes have been recently discovered within the massif and in its environment. The total petrological data show that the magnesian high-Ca ankaramite melt corresponds in composition to the assumed primary melt for the Birkhin massif.

The paper summarizes major and trace-element compositions and Sm–Nd isotope data on metabasites (amphibolites) and gabbroids of the Onot granite–greenstone block in the Sharyzhalgai basement uplift, southwestern Siberian craton. The Onot block consists of tectonically combined nappes of the Paleoarchean tonalite–trondhjemite–granodiorite (TTG) complex and the metasedimentary-volcanic complex of the greenstone belt (GB). The Mezoarchean (∼2.88 Ga) metabasalts of the greenstone belt and Paleoproterozoic (∼1.86 Ga) gabbronorites and vein gabbros were formed at rifting and postcollisional extension, respectively. The Archean metabasites of the greenstone belt and enclaves in the TTG complex compositionally correspond to low-Ti tholeiitic basalts and basaltic andesites. The basaltic rocks are characterized by flat REE patterns [(La/Sm) n = 0.9–1.9], depletion in Nb relative to Th and La (Nb/Nb* = 0.4–1.1), and a wide range of mostly positive ε Nd (T) values (from +5.2 to –1.0). The enrichment of the basaltic andesite in incompatible elements, its Eu minimum, and negative ε Nd (T) values resulted from contamination by Paleoarchean TTG gneisses, that form the basement of GB. The Paleoproterozoic gabbronorites have high Mg# and extremely low concentrations of Ti and incompatible elements. The rocks are characterized by low (Nb/Y) PМ (0.8–1.0), negative ε Nd (T) values (from 0 to –1.4), and weak enrichment in Th and LREE relative to Nb. The vein gabbros have low (La/Sm) n , positive ε Nd (T) values of +2.8 and +0.3, and a negative Nb anomaly (Nb/Nb* = 0.3–0.4). The trace element-composition of the amphibolites, gabbronorites, and gabbros and the results of geochemical modeling indicate that the parental melts were derived mainly from weakly depleted mantle sources. The Nd isotope composition of the Paleoproterozoic gabbroids resulted from the evolution of the heterogeneous Archean lithospheric mantle. Variations in the isotope and trace-element composition of the amphibolites reflect the initially depleted nature of the Mezoarchean mantle and its metasomatic alteration by fluids/melts, which occurred before its melting at ∼2.88 Ga. The geochemical and Nd isotopic characteristics of gabbronorites and gabbros indicate that the lithospheric mantle had become progressively more heterogeneous by the Paleoproterozoic due to preceding Archean processes. The variable depletion of both the Archean and the Paleoproterozoic mafic rocks in Nb relative to Th and La may be explained by mantle metasomatism and does not reflect the geodynamic settings of the mafic magmatism.

Linear or lens-like carbonate (marble) and carbonate-silicate bodies among gabbro and amphibolites within the Krestovsky subterrane of the Olkhon composite terrane (West Baikal Area) are identified as dikes. The dikes commonly dip almost vertically, range in thickness from 20 cm to a few meters, and are up to 100 m long. The Olkhon marble dikes quite often coexist with dolerite dikes and/or granite veins and show signatures of emplacement synchronously with the igneous bodies. The marble dikes differ from mantle carbonatites in mineralogy and chemistry and thus may be derived from sedimentary carbonate rocks molten during collisional events. The origin of the Olkhon carbonate and carbonate-silicate dikes may be explained with two possible geodynamic scenarios. They may be derived either from Neoproterozoic carbonate sediments upon the Early Precambrian basement of a cratonic block which was involved in collisional events, or from abundant carbonate sedimentary material in an island-arc terrane. Large-scale melting of silicate and carbonate rocks was maintained by heat released from mantle mafic magma intruding into the lower crust. The batches of both crustal (carbonate and granitic) and mantle (mafic) melts intruded late during the collision in a strike-slip tectonic setting.

The Olkhon terrane in the Western Baikal area accommodates four types of carbonate-silicate mixtures: injection (protrusion), metamorphic-boudinated, mingling, and tectonite marble mélange. The outcrops of injection mélange consist of a carbonate matrix with inclusions of native silicic rocks found in the immediate vicinities, commonly cover large areas and lack any distinct linearity in the map view. Mélange of the metamorphic boudinage type comprises diopsidite and tremilote-diopsidite fragments in a dolomitic or calcite-dolomitic matrix. Its origin is apparently due to tectonism and related metamorphism of quartz sandstones in Neoproterozoic strata on the passive margin of the Siberian craton. Mingling mélange appears as calcite marble or carbonate-silicate (calciphyre) veins with metadolerite and granite inclusions of different sizes. The veins formed by intrusion of carbonate and silicate melt batches and subsequent fragmentation of silicate rocks that crystallized earlier. Marble tectonites localized in narrow zones record the late phase of ductile marble injection.

The small volcanic islands of the central Greater Kuril Chain (GKC) produce strong explosive eruptions, two of which occurred in the recent years. The latest eruption took place at Raikoke volcano, which occurred on June 21–25 (2019) and was one of the largest on the Kuril Islands in the 21st century. This study presents mineralogical, petrographic and geochemical data on the air fall ash and pyroclastic density current deposits to elucidate the causes of high explosivity of this eruption. The magma involved in the Raikoke volcanic eruption is likely to have had a mantle origin and basaltic composition. Prior to forming a near-surface reservoir, the magma fractionated (i.e. due to crystallization of olivine and pyroxene) at depths of about 26 km, which is close to the Moho discontinuity in the middle part of GKC. The presence of amphibole among the 2019 pyroclastic minerals indicates that the magma contained significant (i.e. no less than 4 wt%) amounts of water at depth. This magma was degassed prior to intrusion into a shallower reservoir. The magma in the shallow reservoir had high crystallinity and contained viscous felsic residual melts, which prevented the development of effusive eruptions or Strombolian explosions. It is likely that magmatic fluid bubbles were still present, but their abundance was insufficient for lava fragmentation in the conduit. This led to low degree of vesiculation of the erupting melts. The results of this study demonstrate that the explosivity of the eruption is most likely related to the interaction in the volcanic edifice between meteoric waters and the magma, which was largely degassed and contained significant amounts of crystals and felsic melt (i.e. had high viscosity and low mobility).

The rocks of the Olkhon terrane experienced high-grade regional metamorphism reaching granulite facies. In addition to regional metamorphism, they were subjected to contact metamorphism and autometamorphism (mafic dikes). Three types of regional metamorphism have been distinguished. A terrane collage north of the Orso Zone revealed two stages of regional metamorphism: early-stage granulite metamorphism ( Т = 750–900°С, Р = 8–9 kbar, around 500 Ma) and late-stage amphibolite and low–amphibolite facies ( Т = 550–710°С, Р = 4–6 kbar, 460–470 Ma). The Orso microterrane separating the Krestovsky island-arc subterrane from a collage of other microterranes contains high-pressure mineral assemblages (up to 10 kbar), which are characterized by the presence of high-calcium garnet in metapelites. The Orso microterrane and Krestovsky subterrane are characterized by relatively high-pressure path of metamorphism. An extended zone of contact high-temperature metamorphism related to the emplacement of gabbro of the second phase of the Ustkrestovsky Complex has been mapped in the Krestovsky subterrane. The temperature of formation of two-pyroxene hornfels after metaporphyrites of the Birkhin volcanoplutonic association is estimated at 750–850°С. Beerbachites were formed by autometamorphism of subvolcanic mafic bodies, which compose the northern part of the Tazheran composite massif (syenite, Ne-syenite, subalkaline gabbro), dikes in the Birkhin gabbro massif, as well as dikes and separate blocks in marble mélange. The temperature of autometamorphism during formation of beerbachites is estimated from two-pyroxene geothermometer as 700–1000°С.

Dismembered ophiolites in the Early Paleozoic Olkhon terrane, a part of the Baikal collisional belt in the southern periphery of the Siberian craton, occur as fault-bounded blocks of ultramafic and mafic rocks from a few meters to hundreds of meters in size. The ultramafic rocks are mainly dunite–harzburgite peridotites with gradual transitions between the lithologies, as well as moderate amounts of enstatitite, wehrlite, and clinopyroxenite, but no lherzolite. Most peridotites have strongly depleted chemistry and a mineralogy corresponding to the harzburgite type usual for ophiolites of suprasubduction zones (SSZ). The mafic rocks are leuco- to melanocratic gabbros with different relative percentages of clinopyroxene, olivine, and plagioclase, which enclose thin layers and lenses of clinopyroxenite and anorthosite. They bear back-arc basin geochemical signatures, a setting inferred for the Neoproterozoic southern Siberian craton. The gabbroic rocks are of two geochemical groups; most of their trace-element patterns show Ta-Nb minimums and Sr maximums common to suprasubduction zone ophiolites. Judging by the Ol + Opx + Chl + Chr mineral assemblages, the Olkhon peridotites underwent low amphibolite and amphibolite regional metamorphism at 500–650 °C. The occurrence of the ultramafic and mafic bodies is consistent with formation in an accretionary wedge metamorphosed during a collisional orogeny. The mantle and crustal parts of the Olkhon ophiolite suite apparently were incorporated into the terrane during the frontal collision of perio-oceanic structures with the Siberian craton. Then, in a later oblique collision event, they became dismembered by strike-slip faulting into relatively small bodies and fault blocks exposed in the present erosional surface.

We announce the second edition of the Aerospace geological map of the Olkhon Region (Baikal, Russia), scale 1:40 000, which was published in 2017. The map has been considerably revised and updated, and its changes are critical for correct understanding of the regional geology, tectonics and geodynamics. Only a small number of its printed copies have been released, and therefore the map may not be available for all interested specialists. The electronic version of the map is available for studying and/or printing (see the link to its pdf file in the paper’s supplement). The pdf file is about 68 MB, i.e. small compared to the original map (more than 5 GB), but the quality is maintained. The map does not show the base layer due to the terms of the licenses owned by the companies and satellite owners.

We announce the second edition of the Aerospace geological map of the Olkhon Region (Baikal, Russia), scale 1:40 000, which was published in 2017. The map has been considerably revised and updated, and its changes are critical for correct understanding of the regional geology, tectonics and geodynamics. Only a small number of its printed copies have been released, and therefore the map may not be available for all interested specialists. The electronic version of the map is available for studying and/or printing (see the link to its pdf file in the paper’s supplement). The pdf file is about 68 MB, i.e. small compared to the original map (more than 5 GB), but the quality is maintained. The map does not show the base layer due to the terms of the licenses owned by the companies and satellite owners.

This paper reports the results of thermodynamic modeling of the formation of the Birkhin volcanoplutonic association by means of geochemical thermometry. The obtained liquid line of descent for the magma chamber of the Birkhin Massif indicates that the melt evolved into the field of subalkaline composition. The melts of the volcanics of the Tsagan-Zaba complex are identical to the melts in the magma chamber in both major components and REEs. It was concluded that the Birkhin and Tsagan-Zaba complexes are comagmatic. A scenario was proposed for the development of the Birkhin volcanoplutonic association involving multiple eruptions of volcanic rocks during its formation. It was shown that the intermediate rocks are not derivatives of basic magmas, but were produced by interaction of basic volcanic rocks with felsic intrusions cutting them.

The possibility of reconstructing the geological chronicle by identifying the peculiarities in the variations of the Earth’s magnetic field associated with the reversals is one of the fundamental applications of paleomagnetism. The most detailed records of reversal events whose duration is, on average, one to ten thousand years have been recognized from the results of studying flood basalts of the large igneous provinces. At the same time, recent publications report the facts that are interpreted as a record of a geomagnetic reversal in the intrusion bodies. Inter alia, these data have been obtained for the relatively thin Ergalakh dolerite sills in the Norilsk region of the Siberian trap province which are supposed to have recorded the «Ivakinsky-Syverminsky» reversal corresponding to the Permian–Triassic boundary. The interpretation is based on the hypothesis of slow cooling of the intrusion during which its apical parts are magnetized during the Ivakinsky epoch of reversed polarity whereas the central parts acquire magnetization after the reversal during the Syverminsky time corresponding to normal polarity. In this paper, we consider the results of mathematical modeling to discuss the validity of these assumptions and the potential eligibility of subvolcanic intrusions as a source of information for studying geomagnetic reversals. It is shown that the duration of their cooling including the interval of the most probable magnetization is a few orders of magnitude shorter than the duration of the reversal transitions, whereas the presence of the components with normal and reversed polarity is most likely to be due to the effect of self-reversal.

Представлены результаты термодинамического моделирования методом геохимической термометрии процесса формирования бирхинской вулканоплутонической ассоциации. Построенная траектория эволюции состава расплава в магматической камере Бирхинского массива показывает, что расплав эволюционирует в область субщелочного состава. Составы расплавов вулканитов цаган-забинского комплекса совпадают с расплавом камеры как по петрогенным, так и по редкоземельным компонентам. Сделан вывод о комагматичности бирхинского и цаган-забинского комплексов. Предложен сценарий развития бирхинской вулканоплутонической ассоциации, предполагающий многократные излияния вулканических пород в ходе становления вулканоплутонической ассоциации. Показано, что породы среднего состава не являются дифференциатами базитовой магмы, а являются результатом взаимодействия вулканических пород основного состава с прорывающими их кислыми интрузиями.

Geological and mineralogical data are reported on the manganese occurrences of the Olkhon terrane (Western Baikal region), which are localized in metadolerites of the Ustkrestovsky Complex, high-temperature mafic hornfels, granites, calcitic marbles and calciphyres, and occasionally are developed as separate veins in gneiss granites or small lenses in quartzites. Most of them are made up of high-temperature mineral assemblages (Opx + Cpx + Pl + Ilm ± Grt ± Bt ± Amp), the main manganese carriers in which are ferrorhodonite (33–36 wt % MnO), orthopyroxene (6–12 wt % MnO), and ilmenite (3–16 wt % MnO). Obtained data are in conflict with traditional concepts that these rocks are gondites (manganese-rich metamorphosed sediments) or that manganese flux in carbonate sediments was related to the volcanic activity that occurred simultaneously with sedimentation at about 500 Ma. The diversity of manganese occurrences was produced by metasomatic processes that occurred almost simultaneously with regional metamorphism and emplacement of subalkaline mafic bodies during collisional tectonogenesis (about 470 Ma).

The possibility of reconstructing geologic events by identifying patterns in variations of the geomagnetic field related to reversals is one of the fundamental applications of paleomagnetism. The most detailed records of reversal events, whose duration averages 1–10 thous. years, are known from studies of flood basalts of large igneous provinces. At the same time, there have been recent publications presenting facts interpreted as records of geomagnetic reversals in intrusive bodies. Specifically, such data were obtained for relatively thin dolerite sills of the Ergalakh complex in the Norilsk region of the Siberian trap province that supposedly recorded the Permian-Triassic “Ivakin-Syvermin” reversal. This interpretation is based on the hypothesis of a slowly cooling intrusion, in which its apical parts magnetized in the Ivakin epoch of reversed polarity and the central parts—after the reversal in the Syvermin epoch of normal polarity. In this paper, using results of mathematical modeling, we discuss the validity of such assumptions and the potential attractiveness of subvolcanic intrusions for studies of geomagnetic reversals. We demonstrate that the duration of their cooling, including the most probable interval of magnetization is several orders of magnitude less than the duration of reversal transitions, and that the most probable cause of the occurrence of both polarities is the self-reversal effect.