Home – Home – Jornals – Russian Geology and Geophysics 2015 number 9

New data are presented on the geologic structure, age, petrogeochemical composition, and conditions of formation of the Late Proterozoic Meteshikha ultramafic-mafic pluton of the Ikat complex. Mafic rocks are the main rocks of the massif, whereas ultramafic rocks are secondary; both of them correspond to two intrusive phases. The first phase includes a layered rock series enriched in intercumulus amphibole, which varies in composition from olivine gabbro to leucocratic gabbro-anorthosite; the second is composed of wehrlite, plagiowehrlite, and olivine clinopyroxenite. Mineralogical, petrographic, geochemical, and isotope studies show that the rocks of both phases crystallized from the same mantle melt; note that the PT -conditions of their formation were considerably different. We suppose that they were separated in the intermediate chamber during fractional crystallization and the accumulation of early minerals (olivine and, probably, clinopyroxene) in the lower part of the chamber. Using the COMAGMAT software, we have found the composition of the parental melt for the rocks of the first phase - normal tholeiitic basalt with 0.2-0.5 wt.% water, which might have crystallized at 3.0-3.5 kbar and the oxygen activity controlled by the QFM buffer. The differentiated series is characterized by gradual depletion with Cr and Ni and enrichment with Sr, Ti, Cu, and REE during the evolution of melt. The REE patterns for the massif rocks have a similar low-fractionation trend with domination of light lanthanides over heavy ones and (La/Yb) _N = 1.25-2.75. Multielement spectra are characterized by negative anomalies of K, Th, Nb, and Zr and positive anomalies of Ba, U, Sm, and Sr. The geochemical characteristics of the rocks are similar to those of the tholeiitic basalts of present-day island arcs. Studies show that the Meteshikha massif formed in the subduction setting of the active margin of the Siberian continent in the Late Riphean (809 Ma).

We summarize results of geological, geochronological, petrogeochemical, and isotope-geochemical (Sr-Nd) studies of Late Vendian-Early Paleozoic granitoid batholiths in Eastern Tuva (Kaa-Khem, East Tannu-Ola, Khamsara, etc.). Analysis of geochronological (U-Pb, Ar-Ar) data has shown that the Late Vendian-Early Paleozoic granitoids in Eastern Tuva formed in several stages in the time interval 562-450 Ma and at different geodynamic stages of the regional evolution: island-arc (562-518 Ma) and accretion-collision (500-450 Ma), with the latter stage characterized by more intense granitoid magmatism. Diorite-tonalite-plagiogranite associations with different petrogeochemical parameters are the most widespread in the region. Petrogeochemical studies of the Late Vendian-Early Paleozoic plagiogranitoid associations have revealed high- and low-alumina varieties reflecting different conditions of formation of parental melts. At the island-arc stage of the regional evolution, only low-alumina plagiogranites of tholeiitic ( M -type) and calc-alkalic ( I -type) series formed. Their parental melts were generated at 3-8 kbar through the partial melting of N-MORB-type metabasalts in equilibrium with amphibole restite. Isotope-geochemical studies have shown positive e _Nd values (6.9-6.3) and low Sr isotope ratios (( ⁸⁷Sr/ ⁸⁶Sr) ₀ = 0.7034-0.7046). The lower (as compared with the depleted mantle) e _Nd values and specific petrogeochemical composition (positive Nb-Ta and negative Ti anomalies) of the plagiogranites reflect the subduction nature of metabasic substratum and the subordinate role of ancient crustal material. At the accretion-collision stage of the regional evolution, high- and low-alumina plagiogranitoids of calc-alkalic series (I-type) formed. The high-alumina plagiogranitoids are products of melting of N-MORB-type metabasalts in equilibrium with garnet restite at ³15 kbar in the lower part of the collisional structures, and the low-alumina ones formed through the melting of metabasites in equilibrium with amphibole restite at ≤8 kbar in the upper part of the same structures. The Sr-Nd isotope data for the high- and low-alumina plagiogranitoids generated at the accretion-collision stage show that the rejuvenation of rocks is accompanied by the decrease in e _Nd (from 6.2 to 3.4) and the increase in their model Nd age T _Nd(DM) (from 0.73 to 0.92 Ga) and ( ⁸⁷Sr/ ⁸⁶Sr)0 (0.7036-0.7048). This points to the essentially metabasic composition of the parental substratum, as in the case of the island-arc plagiogranitoids, and the progressive supply of ancient crustal material to the magma chamber.

Palaeoseismological studies were performed within the Yaloman graben (Gorny Altai). Five Quaternary sections with coseismic deformation structures (seismites) have been recognized in the lower coarse of the Malyi Yaloman River. Traces of ancient earthquakes are localized at two levels (Late Pleistocene-Holocene). The most likely mechanisms of the seismite formation are brittle failure, liquefaction, and fluidization. The types of coseismic deformations and their sizes suggest that the Yaloman graben was the locus of prehistoric earthquakes with M ≥ 5-7, although modern-day seismic activity consists of smaller-magnitude earthquakes. This should be taken into account in assessing the seismic hazards during construction of gas pipeline to China and tourism infrastructure facilities.

A brief summary of different viewpoints on the sedimentation model for the Achimov rock unit in West Siberia is presented. The differences in the concepts of the rock unit structure have led to ambiguous approaches to mapping of the boundaries of its area. Our study of the clinoform structural model for Neocomian section yielded geological and geophysical data substantiating the positions of the western and eastern boundaries of the Achimov rock unit.

Granite pegmatites are a unique natural object that makes it possible to study magmatic processes that lead to the formation of ore-generating media and systems. This paper summarizes modern views on phase transformations in aqueous silicate systems at parameters close to those of the transition from magmatic to hydrothermal crystallization. Comparison of phase diagrams and the results of study of pegmatite-forming media permits making conclusions about the crystallization of the water-saturated magmas of miarolitic granite pegmatites. The fluid regime of aqueous granite systems of simple composition, not enriched in fluxing components, is determined mainly by magma degassing or the supply of volatiles with flows of transmagmatic fluids. These processes cause the separation of essentially carbon dioxide or essentially hydrous fluid. During the crystallization of such magmas, crystallization from silicate melt is separated in PT -space and, possibly, in time from the crystallization from aqueous or mixed carbon dioxide-aqueous super- and subcritical solutions. The evolution of chambers of water-saturated granitic and pegmatitic magma enriched in F, B, and alkali metals presupposes the formation of a heterogeneous mineral-forming medium in which crystallization occurs in the magmatic melt at high-temperature stages; as temperature decreases, crystallization can proceed in hydrous fluid, hydrosilicate, and/or hydrosaline simultaneously. Hydrothermal crystallization can also take place in a heterogeneous medium consisting of aqueous solutions of different salinities and vapor or water-carbon dioxide gas mixture. The relationship between different fluid regimes during the evolution of volatile-saturated granitic and pegmatitic magmas determines the variety of postmagmatic rocks accompanying granite massifs.

This paper presents the first geochemical data on Cr-spinels from ultramafic rocks of the Alkhadyr terrane, which were obtained on a representative collection of samples using modern research methods. The compositional data on melt inclusions allowed the identification of three generations of Cr-spinels on the basis of their morphology, composition, and relationships with the rock-forming minerals. Different types of geochemical zoning were recognized in heterogeneous Cr-spinel grains. The composition of parental melt and crystallization temperatures of the minerals in ultramafic rocks were derived from the compositional data on Cr-spinels and trapped melt inclusions.

Experimental data on Fe-CaCO3 interaction at 6 GPa and 1273-1873 K are presented. The system models the hypothetical redox interaction in subducting slabs at the contact with the reduced mantle and a putative process at the core-mantle boundary. The reaction is accompanied by carbonatite melt formation. It also produces Fe3C and calcium wüstite, which form solid or liquid phases depending on experimental conditions. In iron-containing systems at 6 GPa, calcium carbonate melts in the range 1473-1573 K, which is consistent with aragonite disappearance from complex carbonate systems. The composition of calcium carbonate liquid is not influenced by metallic Fe. It corresponds to nearly pure CaCO3. Along the mantle adiabat or at slightly higher temperatures, nearly pure CaCO3 coexists with metallic iron or calcium wüstite. This hypothesis explains the coexistence of metallic iron and carbonate inclusions in lithospheric and superdeep diamonds.

We consider mineral assemblages and mineralogical and geochemical peculiarities of hypogene gold from the Khaak-Sair multistage low-sulfide gold-quartz ore occurrence in listwänites. Three productive substages of Au- and Ag-mineral formation have been recognized on the basis of mineralogical studies: gold-sulfosalt-sulfide-quartz, gold-mercury-quartz, and gold-selenide-telluride-sulfide-quartz. These substages were characterized by the following sequences of mineral formation: (1) ultrahigh-fineness gold → high-fineness gold → argental gold (medium- and low-fineness gold) → electrum + Ag-bearing and argental fahlores (up to 50 wt.% Ag) ± acanthite ± hessite; (2) high-fineness gold → Hg-bearing and mercurian gold → mercurian electrum → mercurian kustelite → Au-bearing mercurian silver; and (3) high-fineness gold → mercurian gold → mercurian electrum + naumannite + Te-bearing naumannite + fischesserite + tiemannite + hessite + coloradoite + Ag-bearing minerals of the galena-clausthalite series (up to 6 wt.% Ag) ± Se-cinnabar ± Se-imiterite. Productive mineral assemblages of the ore occurrence formed in the hypabyssal facies (depth ~1.5 km, P ~0.5 kbar) on the background of a temperature decrease from 290 to 160 °C and variations in f (O ₂), f (S ₂), f (Se ₂), and f (Te ₂).

In this paper, the solution of two-dimensional Maxwell’s equations is considered using the Laguerre transform. Optimal parameters of the difference schemes for the equations are obtained and presented. Numerical values of these optimal parameters are given. Second-order difference schemes with the optimal parameters provide an accuracy of the solution of the equations that is comparable to the accuracy of the solution using fourth-order schemes. It is shown that, when using the Laguerre transform, the number of optimal parameters can be reduced compared to the Fourier transform. This reduction leads to a simplification of the difference scheme and a reduction in the amount of computation, i.e., to efficiency of the algorithm.

Computer inversion of measurement data based on the equation of particle transport is a topical component of the modern nuclear-geophysical technologies. In this work we have elaborated an approach and iterative methods for this inversion according to the gamma-gamma method data. In the framework of the “successive approximations by characteristic interactions” approach, typical interactions have been established and iterative methods for the solution of two problems have been elaborated, namely, the problem of evaluation of the formation density and the problem of evaluation of two parameters - the formation density and the mud cake thickness. The numerical experiments confirm the theoretical concepts of the properties of iterative processes and convergence of methods.