Home – Home – Jornals – Russian Geology and Geophysics 2010 number 6

This is a snapshot of the today's views of the Earth with its geospheres and terrestrial and extraterrestrial triggers of its dynamics and energy sources. Along with the presented brief historic outline of the planetary evolution, these data can make a basis for creating in the future a truly global model of the Earth's dynamics and evolution.

The paper is a synthesis of models for basic geodynamic processes (spreading, subduction transient into collision, mantle plumes) in relation with the Earth's evolution and regularly changing geodynamic parameters. The main trends and milestones of this evolution record irreversible cooling of the Earth's interior, oxidation of the surface, and periodic changes in geodynamic processes. The periodicity consists of cycles of three characteristic sizes, namely, 700-800 Myr global cycles, 120, 90, and 30 Myr smaller cycles, and short-period millennial to decadal oscillations controlled by changing Earth's orbital parameters and, possibly, also by other extraterrestrial factors. Major events and estimates of mantle and surface temperatures, heat flow, viscosity, and the respective regimes of convection and plume magmatism have been reported for the largest periods of the Earth's history: Hadean (4.6-3.9 Ga), Early Archean (3.9-3.3 Ga), Late Archean (3.3-2.6 Ga), Early Proterozoic (2.6-1.9 Ga), Middle Proterozoic (1.9-1.1 Ga), Neoproterozoic (1.1-0.6 Ga), and Phanerozoic with two substages of 0.6-0.3 and 0.3-0 Ga.
Current geodynamics is discussed with reference to models of spreading, subduction, and plume activity. Spreading is considered in terms of double-layered mantle convection, with focus on processes in the vicinity of mid-ocean ridges. The problem of mafic melt migration through the upper mantle beneath spreading ridges is treated qualitatively. Main emphasis is placed on models of melting, comparison of experimental and observed melt compositions, and their variations in periods of magmatic activity (about 100 kyr long) and quiescence. The extent and ways of interaction of fluids and melts rising from subduction zones with the ambient mantle remain the most controversial. Plume magmatism is described with a "gas torch" model of thermochemical plumes generated at the core-mantle boundary due to local chemical doping with volatiles (H₂, CH₂, KH, etc.) which are released from the metallic outer core, become oxidized in the lower mantle, and decrease the melting point of the latter. The concluding section concerns periodicities in endogenous processes and their surface consequences, including the related biospheric evolution.

The Jinping-Song Da rift structure joining the Emeishan Large Igneous Province is composed of Permian high- and low-Ti volcanic and volcanoplutonic ultramafic-mafic associations of different compositions and genesis. High-Ti picrites, picrobasalts, basalts, and dolerites are enriched in LREE and depleted in HREE and show low Al₂O₃/TiO₂ ratios (~4), commensurate ε_Nd (T) values (+0.5...+1.1) and low (Th/Nb)PM ratios similar to those of OIB-enriched mantle source. The established geochemical characteristics evidence that the parental melts of these rocks were generated from garnet lherzolite at the depths of garnet stability (~200 to 400 km). Later, high-Mg low-Ti volcanics (komatiites, komatiitic basalts, and basalts) and associating small peridotite-gabbro massifs and komatiite-basalt dikes were produced as a result of ~20% partial melting of depleted water-poor (?0.03 wt.% H₂O) peridotite substratum from the hottest upper part of mantle plume at relatively shallow depths (100-120 km). The LREE-depleted komatiites and komatiitic basalts are characterized by low (Ce/Yb) _CH values, ¹⁸⁷Re/¹⁸⁸Os = 0.05-1.2, ⁸⁷Sr/⁸⁶Sr = 0.704-0.706, positive ε_Nd (T) values (+3...+8), γ_Os = -0.5...+0.9, and strong negative anomalies of Ba, K, and Sr on the spidergrams. The scarcer LREE-enriched komatiites, komatiitic basalts, and basalts vary greatly in chemical composition and values of ε_Nd (T) (+6.4...-10.2), ⁸⁷Sr/⁸⁶Sr (0.706-0.712), and γ_Os (+14.8...+56), which is due to the different degrees of crustal contamination of parental magmas. The Rb-Sr isotopic age of basaltic komatiite is 257 ± 24 Ma. The Re-Os age determined by analysis of 12 komatiite samples is 270 ± 21 Ma. These data agree with the age of flood basalts of the Emeishan Large Igneous Province. The komatiite-basalt complex of the Song Da rift is still the only Phanerozoic PGE-Cu-Ni-complex of this composition. The geochemistry of accompanying Cu-Ni-PGE-ores confirms their relationship with komatiite-basaltic magmatism.

The paper reports the results of integrated geological, petrological, geochemical, and geochronological studies of the Tastau igneous ring complex in the Zaisan orogen of eastern Kazakhstan. Interaction between felsic and mafic magmas has been studied. Hybrid rocks are represented by gabbros and diorites injected into a granitic magma chamber. They occur as dikes and pillow-like and globular mafic bodies variously disintegrated and mixed with host granitoids. The age of synplutonic rocks is 242±20 Ma (U/Pb zircon dating), which is, with regard to analytical error, substantially younger than it was presumed.
Mechanisms of interaction between felsic and mafic magmas have been studied. They include mechanical (mingling) and chemical (mixing) inreractions, which produce composite mixtures and hybrid rocks. The ratios of mafic to felsic components involved in the formation of intermediate rocks were calculated from major elements by regression analysis and tested with regard to rare and trace elements. The model for mingling includes quenching of the mafic melt when it is injected into the granitic magma chamber, decomposition of crystalline fragments, dispersion of fragments and crystals in the magma chamber under conditions of rapid turbulent flow, and enrichment of felsic magma with femic components to produce monzonitic magmas.

In this paper I present results of IR spectroscopic measurements of water solubility in Al-bearing periclase and ferropericlase (Mg# = 88) synthesized at 25 GPa and 1400-2000?C. The IR spectra of their crystals show narrow absorption peaks at 3299, 3308, and 3474 cm^-1. The calculated H₂O contents are 11-25 ppm in periclase (Al₂O₃ = 0.9-1.2 wt.%) and 14-79 ppm in ferropericlase (Al₂O₃ = 0.9-2.9 wt.%). Ferropericlase contains more H₂O and Al₂O₃ than periclase at 1800-2000?C. I suggest that addition of Al₂O₃ does not influence the solubility of water in ferropericlase but can favor the additional incorporation of Fe₂O₃ into the structure. The incorporation of Fe³⁺ into ferropericlase increases water solubility as a result of iron reduction to Fe²⁺. It is shown that water has limited solubility in ferropericlase from mantle peridotite; therefore, ferropericlase cannot be considered an important hydrogen-bearing mineral in the lower mantle.

This work is a review of the modern concepts of the accumulation of Fe and Mn in the Baikal bottom sediments and the regularities of formation of iron-manganese (including phosphate-bearing) nodules in the lake. Special attention is given to the probable participation of hydrothermal water in this process and the genesis of ancient nodules deeply buried in the Baikal sediments. The possible relationship between the shore ores and the nodules in the bottom sediments in the adjacent Baikal regions has been first shown.

Gold ore districts in the Siberian (North Asian) craton and bordering terranes have been studied. Studies showed the long duration of gold concentration processes (Early Cambrian to Late Mesozoic and Cenozoic) and the influence of structural geologic, magmatic, and metallogenic factors on the formation of ore districts. The largest Late Mesozoic (J-K) accumulations of gold deposits in southeastern Russia were discovered in the Aldan-Stanovoi Shield and on the northern margin of the Argun superterrane in the Aldan (Yakutia), Balei (Transbaikalia), and Gonzha (Upper Amur area) ore-placer districts.
The geologic-geophysical positions of these three districts have been compared. All of them are situated in zones of influence of variously trending long-lived deep faults, bordered by large Precambrian uplifts, and spatially (paragenetically) related to local magma chamber domes of Late Mesozoic (J-K) intrusive, subvolcanic, and extrusive-effusive bodies, dikes, and terrigenous-pyroclastic blankets. The areas of Jurassic-Cretaceous volcanoplutonic rocks are related to the influence of the East Asian sublithospheric "superplume".
All this confirms the important ore-controlling role of large long-lived deep faults (in the form of global and regional gravity gradient zones) in the distribution of highly productive precious-metal ore-magmatic systems. This suggests that the junctions between gravity gradient zones of different trends and ranks are important to the identification of gold prospects in metallogenic prediction studies and small-scale prospecting. The Archean-Proterozoic age and the great occurrence depth of the tectonic zones suggest that extensive long-lived mobile zones (before the post-Cambrian breakup of the Siberian craton) significantly affected further evolution of the orogenic belts bordering the craton and their metallogeny, including the distribution of precious-metal deposits.

The former stratigraphic chart of the Cambrian in the Turukhansk-Irkutsk-Olekma region of the Siberian Platform was compiled in 1986 and approved in 1988. Large amounts of data were obtained from new wells drilled in the western and northern parts of the region during the preceding 20 years. Modifications necessitated by new data entail an indefinitely continuous series of revised facies zonation, recognition of new facies areas, regions and zone, as well as definition of a regional early Mayan stratigraphic hiatus. A new local stratigraphic division is the Olenchima Member (base of the Evenk Formation) underlain by a hiatus. The term Kochumdek Formation is no longer used in the Bakhta region, and its lithologically distinct subformations (Yasenga, Moktakon, Mara, and Abakun) were raised to formation rank. The Olenchima Formation is no longer used in the Baikit zone; but instead, the Litvintsevo Formation was established here by correlation.

The paper discusses the evolution of the order Stromatoporoidea in the epicontinental sedimentary basin of the Siberian Platform and Taimyr during the Ordovician and Silurian. Specimens of the oldest genus, Priscastroma , were found in the middle of Middle Ordovician sediments. This genus is represented by the species Priscastroma . gemina Khrom., which has two forms, A and B. Tracing the emergence of new genera over time, we identified two distinct branches in stromatoporoid evolution.
The ancestor of the first branch is P. gemina f. A., which gave rise to the genus Cystostroma . The latter is the ancestor of two subbranches with predominant horizontal skeletal elements. The subbranches differ only in tissue microstructure. The genera Stromatocerium, Dermatostroma , and Aulacera display dense fibrous microstructure, whereas the genus Rosenella and its descendants display dense microstructure. The genus Lophiostroma , with a lamellar-fibrous tissue, may be a dead branch of evolution.
The ancestor of the second branch is P. gemina f. B., which gave rise to the genus Labechia and its descendants. This branch has a dense tissue, with predominating vertical skeletal elements.
Ordovician stromatoporoids from Siberia were compared with those from other basins of the world. Comparison shows that all the Ordovician genera from the epicontinental basin of the Siberian Platform and Taimyr originated here. Thus, this basin was one of the centers of stromatoporoid origin.

The three-dimensional shear velocity lithospheric structure at depths from 0 to 70 km beneath the southern Baikal rift system and its surroundings has been imaged by inversion of P -to-SV receiver functions from 46 digital stations operated in two teleseismic international projects in southern Siberia and Mongolia. The receiver functions were determined from teleseismic P waveforms and inverted to obtain depth dependences of S velocities at each station, which were related to tectonic structures. The computed vertical and horizontal sections of the 3D shear velocity model imaged a transition from relatively thin crust of the southern Siberian craton to thicker crust in the folded area south and southeast of Lake Baikal, with a local zone of thin crust right underneath the South Baikal basin. The velocity structure beneath the Baikal rift, the mountains of Transbaikalia, Mongolia, and the southern craton margin includes several low-velocity zones at different depths in the crust. Some of these zones may record seismic anisotropy associated with mylonite alignment along large thrusts.

A numerical experiment has been applied to explore the potentialities and the limitations of the inversion of IP-affected TEM responses of a two-layer earth with a polarizable layer above (Model 1) or below (Model 2) a nonpolarizable layer. The IP effects have been incorporated into the models via a Cole-Cole complex frequency-dependent conductivity. One of us computed synthetic in-loop and coincident-loop transient responses with added Gaussian noise, and the other performed single and joint inversion of the two sets of pseudoexperimental data. Model 1 turns out to be advantageous over Model 2 in TEM applications and gives a good fit of the Cole-Cole parameters (chargeability, IP time constant, and exponent) even in the absence of a priori information. In the case of Model 2, the lack of a priori information causes problems as to recognize which layer is polarizable, and the fit of the Cole-Cole parameters is generally worse. The layer thicknesses and resistivities are rather accurate in both groups of models, irrespective of whether a priori information is available. As the upper layer increases in thickness ( H ₁), the fit of its parameters ever improves in both models, while the parameters of the lower layer, on the contrary, contain a greater error. Joint inversion of in-loop and coincident-loop transients improves the fit in most cases.
Relative rms error (σ_rel) does not depend on the upper layer thickness for Model 1 but decreases as H ₁ increases in the case of Model 2. The error in joint inversion is times that in single inversion, which means that additional criteria other than σ_rel may be useful to estimate the inversion quality.