Home – Home – Jornals – Russian Geology and Geophysics 2023 number 12

Radiometric dating of zircon grains from crustal xenoliths in kimberlites of the Yakutian diamond province show that most of the Daldyn and Markha terranes were formed in Paleoarchean but preserve some fragments of Eaoachean and possibly even Hadean crust recycled in several tectonothermal events. The oldest zircons were crystallized about 3.2 Ga and recrystallized during later activity stages of 2.9, 2.8-2.7, and 1.9 Ga, whereby they gained radiogenic hafnium produced by ¹⁷⁶Lu decay in the rock. The degrees crust rework and the tectonothermal stages varied across the Anabar tectonic province. The earliest events of 3.2 and 2.9 Ga left record in zircons from kimberlites in the Nakyn field, while the signatures of the 2.7 Ga activity are best pronounced in zircons from kimberlites sampled in the Upper Muna and Nakyn fields. On the other hand, zircons from lower crustal mafic granulite xenoliths in the Daldyn and Alakit-Markha kimberlites lack traces of the earliest crust history and only evidence of the last 1.9 Ga event, which remained mute in xenolith samples from the Upper Muna field. Zircons from felsic granulite and metadiorite xenoliths in the Udachnaya kimberlite, which represent middle and upper crust, show a peak at 2.6 Ga besides that of 1.9 Ga. The synthesized available data support several previous inferences: vertical and lateral heterogeneity of the crust in the Yakutian diamond province; absence of linkage between the crust recycling degree and major collisional zones of the Siberian craton; absence of the separate Markha terrane. Correlation of age peaks corresponding to thermal events in the crust history of the Anabar tectonic province with those of large igneous provinces allows a hypothesis that the revealed tectonothermal events may be related with the activity of superplumes.

The Early Devonian Altai-Sayan rift system (ASRS) has spread to the structures of East and West Sayan, Kuznetsk Alatau, and Mongolian Altay. Its largest fragments are the Tuva, Delyun-Yustyd, Kan, Agul, and Minusa basins as well as depressions in northwestern Mongolia. The paper summarizes the geologic, geochemical, and Sr-Nd isotope characteristics of the ASRS mafic rocks represented by nappes of moderately alkaline and alkali basalts and their subvolcanic and intrusive rock analogues. They are present in all magmatic associations and are divided into low-Ti (TiO₂ = 0.2-2.2 wt.%) and high-Ti (TiO₂ = 2.2-4.3 wt.%) subgroups. These rocks are characterized by wide variations in Sr isotope characteristics (ε_Sr(T) = -16 to +30). High-Ti mafic rocks are common at the southern segment of the ASRS; they show a weak positive Ta-Nb anomaly (La/Nb = 0.8-1.1) and are relatively enriched in LREE ((La/Yb)_N = 6-14) and radiogenic Nd (ε_Nd(T) = 3.8-8.7). Low-Ti varieties are confined to the northwestern segment of the ASRS; they are enriched in Ba but depleted in Th, U, Nb, Ta (La/Nb = 1.2-2.2), Zr, Hf, LREE ((La/Yb)_N = 3-7), and radiogenic Nd (ε_Nd(T) = 2.0-6.0). Taking into account the existence of different terranes, which were combined in the structure of the Altai-Sayan folded area during accretion (ca. 500-480 Ma), we propose a model suggesting different environments of magma formation at the southern and northwestern segments of the ASRS and the relationship of magmatism with a mantle plume within the ASRS. In composition the plume corresponds to the sources of high-Ti magmas. The effect of the melted lithospheric mantle of different compositions beneath different groups of terranes led to the observed isotope-geochemical heterogeneity of mafic rocks within the ASRS, in particular, the absence of high-Ti mafic rocks from the Minusa basin.

Rock samples from the Khamaryn-Khural-Khiid combustion metamorphic (CM) complex, including cristobalite clinker, ferroan tridymite-sekaninaite and cristobalite-fayalite paralavas, which are rock types new to the complex, as well as clinker xenoliths in melilite-nepheline paralava, have been studied in terms of chemistry and mineralogy. The obtained data on rock-forming, minor, accessory, and rare phases (silica polymorphs, cordierite-group minerals, fayalite, Fe and Ti oxides, ferrosilite, etc.) have implications for the formation conditions and processes of the CM rocks. The Raman spectra of sekaninaite, indialite, ferroindialite, mullite, and anhydrous Fe-Ca-Mn phosphate, presumably from the graftonite group, have several specific features. The diversity of mineral assemblages in the CM rocks is due to heterogeneous lithology of the sedimentary protolith and to local effects in the multistage history of the Khamaryn-Khural-Khiid complex. According to geochemical data, all CM rocks of the complex are derived from the Early Cretaceous Dzunbain Formation, their protolith molten to different degrees. The cristobalite clinker and tridymite-sekaninaite and cristobalite-fayalite paralavas were produced by partial melting of pelitic rocks containing different amounts of iron in a wide temperature range. The formation of mullite developed from dehydration-dehydroxylation and incongruent partial melting of amorphous pelitic matter. Large-scale crystallization of mullite in clinker, occurred from the high-silica potassic aluminosilicate melt at >850 °C. Combustion of subsurface coal seams heated the overburden to >1050 °C or locally to >1300-1400 °C (melting point of detrital quartz) or even, possibly, to >1470 °C corresponding to the stability field of β-cristobalite. Melilite-nepheline paralava was formed by incongruent melting of silicate (pelitic) and carbonate (calcite) components of marly limestone under elevated CO₂ partial pressure. Oxygen fugacity ( ƒ _O2) during combustion metamorphism changed from strongly reducing conditions favorable for crystallization of Fe phosphides (barringerite, schreibersite) and metallic iron from silica-undersaturated melts parental to melilite-nepheline paralava to high ƒ _O2 values that can maintain the formation of hematite in Fe-rich CM rocks.

The geochemical and mineralogical aspects of the distribution of rare-earth elements (REE) in ferruginous deposits (FD) and bottom sediments of the southeastern Laptev Sea are analyzed. The FD are dominated by biomorphs developed after polychaete tubes. The REE patterns depend on the proportion of ore and non-ore substances. The ore substance is represented mainly by iron hydroxides (limonite). It is a product of suboxic diagenesis enhanced by bioturbation and determines the slight enrichment of FD in MREE and cerium deficiency (Ce_an = 0.94). The non-ore substance comes from terrigenous sediments and has an (alumino)silicate composition. It controls the scandium content and is the source of REE mineral grains, among which monazite-(Ce) prevails. The sediments demonstrate a common similarity in REE patterns to shales and suspended material transported to the Laptev Sea by the Lena River, with elevated LREE and MREE contents and the value of Ce anomaly almost equal to unity (Ce_an = 1.06). In the sediments, REE minerals occur mostly in the silt fraction (<63 μm in size). The high content of organic matter (C_org of up to 2.15%) of predominantly terrigenous origin (OM_ter of up to 85%) in the sediments explains the oxygen deficiency and weak diagenetic mineral formation with low accumulation of trace elements, including REE, in the FD. The total REE content in the FD is lower than that in the sediments (on average, 173 ppm against 206 ppm).

The Bazhenov Formation of the Western Siberian oil-and-gas province is currently one of the key objects of potential growth in oil reserves and production. Here we present results of a geochemical study of the Bazhenov Formation according to the methodology developed at Trofimuk Institute of Petroleum Geology and Geophysics, Novosibirsk (IPGG SB RAS) for rocks highly enriched in organic matter. The features of the group and hydrocarbon compositions of the bitumoids that are filling open and closed pores of the Bazhenov Formation rocks from the North-Surgut region were considered. It has been established that most of the naphthides are in a free (phase-segregated) or sorbed state in a closed pore space. The open pores contain the most mobile components of bitumoids, redistributed from closed pores during the primary migration and remained after the secondary migration of naphtides from the Bazhenov Formation to the Berriasian-lower Aptian reservoirs (A and B strata groups). The methodology used in this work allows determining intervals with increased open porosity and oil saturation in the section of the Bazhenov Formation (silicites and carbonates) by the volume of pore space occupied by open-pore bitumoids.

The Bachat earthquake ( M = 6.1) with the epicenter coordinates of 54.29° N, 86.17° E occurred on June, 18 2013 near the Bachatsky coal strip mine, is the world’s largest earthquake induced while mining solid minerals. More than 5000 aftershocks were registered and the spatial volumetric structure of the aftershock area was investigated. The Bachat coal field is located in the Salair zone of the Kuznetsk Depression and is represented by a brachysynclinal fold of a very complex structure characterized by the damage and fracturing behaviors of rocks. The main event is confined to the coal-mine pit wall, with the greatest density of aftershocks observed in its middle portion. In the cross section, aftershocks form a wide area with its deepened portion shifted towards the Kuznetsk Depression, while large faults bounding the depression dip down under the Salair Ridge. In the exposed pit wall, at a depth of 4 km, the activated area resembles a rhombus whose horizontal diagonal line runs across the entire length of the mine pit and decreases both in upward and downward directions. The area is seismically activated to a depth of 6 km, with more intense activation of rock mass observed within the 1-3 km depth interval. Results of the study of the mechanisms of aftershock sources revealed a disagreement between the stress state of rock mass of the Bachatsky open-pit coal mine modeled from the mechanisms of aftershocks, and the mainshock mechanism of the Bachat earthquake.

The paper presents data on the contents of major and trace elements in garnet xenocrysts from kimberlites of the highly diamondiferous V. Grib pipe (1100 grains) and weakly diamondiferous TsNIGRI-Arkhangelskaya pipe (446 grains). We have established that the high diamond potential of the V. Grib kimberlite pipe is due to several factors related to the composition and structure of the lithospheric mantle represented by kimberlite: (1) a “cold” regime, with a heat flow of 36-38 mW/m²; (2) a thick “diamond window” (70-102 km), with the depth level of the lower boundary of the lithospheric mantle estimated at >200 km; (3) the high degree of preservation of diamond-bearing peridotites under the P-T conditions of diamond stability despite the high degree of impregnation of the lithospheric-mantle rocks by high-temperature silicate melts. The low diamond content of the TsNIGRI-Arkhangelskaya kimberlite pipe as compared with the V. Grib pipe is due to the following factors: (1) a more intense heat flow in the lithospheric mantle, 38-42 mW/m²; (2) a thinner “diamond window”, 10-60 km, with the depth level of the lower boundary of the lithospheric mantle estimated at <200 km; (3) weak impregnation of the rocks of the middle and lower lithospheric mantle by CHO fluid/melt, which might have induced diamond formation; (4) minimum preservation of diamond-bearing peridotites in the lower lithospheric mantle, partly because of the possible impregnation of this zone by high-temperature silicate melts.

The digital elevation model (SRTM03) of the central part of the Lower Amur province is analyzed. The Limuri-Amgun ring structure is identified on the basis of a set of features, and it is also the location of the Pilda-Limuri and Kherpuchin-V’yunsky gold-ore regions, as well as the Albazino deposit, known for large industrial gold deposits and many small deposits and ore occurrences. The distribution of linear and ring elements of the relief is analyzed and geologically interpreted, and its spatial relationship with gold deposits and ore occurrences is shown. The relationship between the morphological severity of the ring structure and the deep structure of the region is shown. It is revealed that the structure is magmatic and characterized by a high concentration of intrusive bodies. According to the reference data on the age and composition of the igneous complexes of the region, the formation of the ring structure is associated with the processes of neoformation and transformation of the Earth’s crust and upper mantle during the end of the Late Cretaceous and the beginning of the Paleocene in a suprasubduction geodynamic setting.