Home – Home – Jornals – Russian Geology and Geophysics 2023 number 10

Paleoproterozoic mafic associations of the Irkut block from the Sharyzhalgai uplift are gabbro-dolerite dikes and small gabbronorite and monzodiorite massifs, which formed at 1.87-1.84 Ga and were coeval with granitoids and basite intrusions of the South Siberian magmatic belt (SSB). All the Paleoproterozoic mafic associations of the Irkut block are characterized by the presence of biotite and alkali feldspar, enrichment in K₂O, LILE, Th, and light REE, highly fractionated multielement spectra with sharp Nb and Ti depletion, and extremely low ε_Nd( T ) from -5.1 to -10.1. In these compositional features, they are similar to mafic complexes in the central and eastern parts of the SSB (the Baikal uplift and the western Aldan shield). Their geochemical and isotopic characteristics did not result from crustal contamination but point to derivation from the subcontinental lithospheric mantle (SCLM) enriched by reaction with felsic subduction-related and OIB-like mafic melts formed at a low degree of melting. The geochemically contrasting Paleoproterozoic gabbronorites in the Onot block of the Sharyzhalgai uplift are marked by depletion in K₂O, Ba, LILE, Th, and light REE, weak depletion in Nb, and higher ε_Nd( T ) from -0.3 to -1.4. The gabbronorites indicate not only an increase in the contribution of a depleted source to their genesis but also the heterogeneity of the subcontinental lithospheric mantle below the south margin of the Siberian Craton. The formation of enriched SCLM domains throughout the South Siberian belt was mainly the result of Archean subduction-related metasomatic processes. The wide distribution of Paleoproterozoic mafic complexes with subduction geochemical signatures and negative ε_Nd( T ) on most early Precambrian cratons is due to global change in the composition and an increase in the heterogeneity of the subcontinental lithospheric mantle toward the end of the Archean.

In this paper, we present a description of the characteristics of the Lotmvara-II sill, which is a representative of the Serpentinite Belt (SB) composed of a series of shallowly emplaced ultrabasic intrusive bodies. The Paleoproterozoic SB complexes were derived from a large-scale mantle plume of komatiitic melt. The sill consists predominantly of fine-grained (locally nearly micrograined) harzburgites with subordinate zones of dunites and orthopyroxenites, located in the central and marginal parts, respectively. It formed from an Al-undepleted komatiitic magma of extremely high Mg content and may represent a near-surface laccolithic “ridge.” In general, the sill is comparatively homogeneous and does not have distinct zoning in the distribution of Mg# values in rock compositions (Mg# = 84.2-88.9, average 86.7). Detailed studies show that olivine, chromian spinel, and ilmenite are the most strongly magnesian in the central part of the body. The maximum values of Mg# equal to 90.7-91.4 in the compositions of olivine at the center of the sill are interpreted as “centers of initial crystallization”. The low values of Mg# equal to 73.4-76.4 are attributed to manifestations of the recurrent generation of olivine. The values of Mg# of orthopyroxene in the sill are within the range 84.6 to 92.3. Orthopyroxene grains in a porphyritic texture are surrounded by a rim of calcic amphibole (autometasomatic in origin); they do not differ compositionally from normal grains. The Zn content of the chromian spinel generally decreases toward the marginal parts of the sill. There is an insignificant degree of magmatic differentiation in the sill with respect to the principal components, but incompatible elements (REE and HFSE) locally show increased levels of their relative enrichment, which is reflected in the nature of the mineral associations described. Thus, the sill has a cryptic zonal structure, which is consistent with its overall crystallization from the center to the edges. The data gathered suggest the presence and significant development of volatile components, halogens, CO₂, and especially magmatic H₂O, which are capable of strongly lowering the liquidus and reducing the density and viscosity of the high-magnesium melt, thereby improving its mobility during ascent from the mantle to the near-surface level of the crust. An increase in ƒ_O2 is observed during in situ subvolcanic crystallization of the sill, as noted earlier in the related complexes of the belt. The relatively small volume of the komatiitic magma in the sill crystallized fairly quickly, resulting in unusual mineral intergrowths. Thus, the Lotmvara-II sill is a novel member in the Serpentinite Belt-Tulppio Belt (SB-TB) in the Paleoproterozoic SB-TB megastructure of the Fennoscandian Shield.

Preservation of soft-bodied organisms as casts is common in the Ediacaran fossil record and extremely rare in Cambrian rocks. Among the factors, limiting fossilization of soft tissues, there is bioturbation-sediment disturbance by burrowing organisms. It is the emergence of burrowing metazoans and associated sediment bioturbation that is thought to be one of the major causes for the disappearance of Ediacaran soft-bodied organisms from the fossil record around ~540 Ma. Here, we study an assemblage of fossils preserved as casts in the Fortunian carbonates of the Olenek Uplift (northeastern Siberian Platform) in association with a typical Fortunian ichnoassemblage. The overall morphology and preservation of the fossils reveal that they comprise microbially induced sedimentary structures and soft-bodied holdfasts. The latter are vaguely reminiscent of some Ediacaran holdfasts, though it is unlikely that they include their phylogenetic descendants. Three-dimensional preservation of the studied fossils was caused by authigenic crystallization of calcite and its further early-diagenetic dolomitization. Our study confirms the critical importance of specific environmental conditions ensuring preservation of soft-bodied organisms as three-dimensional molds and casts. This unique interplay of environmental factors became rare in the Cambrian Period, which was caused by intensification and expansion of bioturbation in marine basins.

Dataset on the Middle Devonian volcanic-carbonate-terrigenous sediments of Salair and the Kuznetsk trough, submerged northward under the Mesozoic-Cenozoic West Siberian geosyneclise (WSG) cover, have been presented. Mainly deep water sediments composed of thin- and fine-grained clastic material (with predominance of tephroids) and thin-, fine-clastic biodetrital limestones with slope slide breccia, as well as turbidites and peculiar “conglomerate-like” limestones (paleoseismites) were found. Sedimentary and geodynamic environments have been reconstructed for the Middle Devonian strata. Predominantly andesitic composition of pyroclastic ashes with fewer values of felsic and basic volcanic components have been determined. The structures related to the continental margin environments of the Siberian continent, the continental slope and its basement have been found. According to sedimentary and geochemical data, the paleogeodynamic environment within the studied region was relatively stable and generally corresponded to the island arc conditions. The paleoclimate of the source area changed from semi-humid in the late Emsian-early Eifelian (Malaya Salairka age) to aridic in the middle Givetian (Kerlegesh age). Basin sediments formed in the deep water oxic environments with good water aeration, excepting euxinic environments found in the lowermost part of studied succession.

A continuous history of annual air temperatures in the Russian Altai for the past 2000 years has been reconstructed for the first time by integrating climate archives stored in the bottom sediments of four lakes in Gorny Altai (Teletskoye, Kucherla, Lower Multa, and Middle Multa). The integrated chronology comprises all known climate events of the two millennia time span: Roman warm epoch (~200 to 500 AD), Dark Age cold epoch (~500 to 750 AD), medieval warming (~750 to 1300 AD), and Little Ice Age (~1300 to 1850 AD). The events distinguished in the integrated climate reconstruction agree perfectly in number and time with the events reconstructed from other climate proxies for the Altai and adjacent areas of West Siberia, Tyva, and West Mongolia. Therefore, the temperature patterns represent a common course of the 2000-year climate evolution over a large part of Central Asia. Similar synchronicity is observed with the available quantitative climate reconstructions for the Northern Hemisphere and its different regions. It means that the climate change in Central Asia over the past two millennia has followed the general global scenario.

Magmatism manifestations in the transition zone from the continental margin of Kamchatka to the Kurile island arc and some geophysical parameters of the subducted oceanic plate of the Northwestern Pacific are considered. The presence of the Miocene coastal volcanic (Pribrezhny) complex at the base of the South Kamchatka volcanic belt contributes to intense crustal processes causing caldera-forming eruptions in the Holocene. The Northern Kuriles are characterized by initiation of areal volcanism associated with crustal fault zones. Anomalous is the absence of volcanism on Shumshu island proximal to Kamchatka. Seismic tomography data reveal a high seismic velocity anomaly below it, which can explain this phenomenon. Based on the presented data an assumption is made about existence of a seafloor elevation on the slab, whose submergence led to disintegration of the melting regions responsible for generation of volcanism.

This study continues analysis of the new seismic tomographic structure of the suprasubduction complex of the central zone of Kamchatka, obtained from the dense local networks data of 2018-2020, and is devoted to the analysis of the velocity structure in the Malko-Petropavlovsk fracture zone margins and around them. The seismic tomographic model involves about 98,000 P - and S -wave travel times from 2963 local earthquakes from August 2018 to July 2020. The resolution of this model makes it possible to trace the feeding systems of volcanoes of the South Kamchatka and East Volcanic Belt to the slab surface, as well as to identify subvertical structural faults. To construct the orientations of the compression and extension axes we used the foci mechanisms of 41 earthquakes with М ≥ 4.5 from the catalog of the International Seismological Center for the period 1979-2019. Along the Malko-Petropavlovsk fracture zone, the Avacha transform fault is clearly traced in the geometry and mutual arrangement of velocity anomalies almost throughout the entire depth of the model. Comparison of seismic anomalies with a map of the directions of the compression and extension axes distribution from the earthquake foci mechanisms showed the correlation between the change in the value of the velocity anomalies along the Avacha transform fault with the axes direction change by almost 180°. A near-surface low-velocity anomaly to the depths of 25-35 km was found along the western border of the Malko-Petropavlovsk zone under the southern tip of the Sredinny Ridge. This anomaly probably marks the axes junction zone boundary of the ancient volcanic front along the Sredinny Ridge and the modern active Eastern Volcanic Belt, which formed as a result of the Kronotsky paleoarc accretion. To the west from the Sredinny Ridge southern tip, another low-velocity anomaly was revealed. This anomaly was traced to a depth of ~150 km, has a contrasting southern boundary confirmed by the distribution of the compression and extension axes directions by the earthquake foci mechanisms and apparently marks the southern boundary of the West Kamchatka block.

In the Bouvet Island region (South Atlantic), a hotspot operates in the region of the triple junction of midocean ridges. On the basis of laboratory modeling data, the structure of the conduit of a thermochemical plume melting out in the mantle from the core-mantle boundary is presented. The thermal power of the Bouvet thermochemical plume is determined from the volume of uplifted and erupted rocks above the lower topographic level. To determine the mass flow rate of the melt for the plume, a topographic profile is used in a section perpendicular to the Bouvet hotspot trajectory and passing through the Bouvet plume. The thermal power of the Bouvet plume is 1.7 · 10¹⁰ W. Based on the obtained power, the plume diameter is d = 10-13 km. The Bouvet plume belongs to intermediate-power plumes. Such plumes are diamondiferous, because their eruption on the surface transports the melt from a depth of > 150 km, at which diamond is stable. The Bouvet plume trajectory originates in South Africa. Initially, the melt erupted on the cratonic surface through a diatreme. Next, the plume was preserved in the region of the drifting oceanic lithosphere and became no longer diamondiferous. The following morphostructures of the triple junction region with contrasting types of magmatic systems are distinguished according to petrological and geochemical data: MOR and the Bouvet volcanic island, which results from the plume activity. For the Bouvet region, K₂О (0.5%) and Н₂О (up to 0.9%) are identified in the composition of the deep magmatic melt. There is enrichment in H₂ up to 100 ppm (up to 50 ppm in the Mid-Atlantic Ridge (MAR)) and in CH₄ up to 12 ppm (up to 1 ppm in the MAR). Thus, it is suggested by the specific features of the melt composition that the Bouvet Island plume is thermochemical. This paper also presents a diagram showing free-convective flows in the asthenosphere in the Bouvet Triple Junction region. Sections are constructed on which the association between the convective structure and bottom morphostructures in the Bouvet region is identified. Large-scale asthenospheric flows are responsible for the formation of MOR. Convective rolls at the top of the asthenosphere account for the formation of the Bouvet and Moshesh transform faults. The Bouvet plume is under the influence of the ascending upper-mantle flow confined to the MOR axis and locally intensifies the ascending flows of the asthenospheric rolls.