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Russian Geology and Geophysics

2002 year, number 10

1.
SILURIAN DEPOSITION IN EAST SIBERIA AND THE ABSENCE OF STRONG EUSTATIC FLUCTUATIONS

E. V. Artyushkov and P. A. Chekhovich
Keywords: silurian, tectonic movements, eustatic fluctuations, fringing seas, numerical modeling, East Siberia
Pages: 893-915

Abstract >>
The concept of time-dependent eustatic fluctuations has been largely recognized, and a number of such fluctuations, with amplitudes of 20 to 100 m and durations of 1 to 3 Ma (third-order cycles), have been inferred for the Phanerozoic. Eight cycles of this type were distinguished for the Silurian on the basis of water-level curves for different regions. East Siberia in Silurian time was occupied by a large sea basin whose fill has been well documented in many sections. Some Silurian sections show slow deposition for 10 to 20 Ma at shallow water depths (~10 m), which rules out significant third-order eustatic cycles. The amplitude of these cycles in Silurian time could not exceed ~10-20 m, as follows from analysis of fluctuation curves and sedimentation rates. Earlier data on East Baltic regions did not show large eustatic cycles in the Cambrian and early Ordovician, which, along with the results from the Silurian sections of Siberia, casts doubt on the existence of rapid and strong eustatic fluctuations over the greatest part of the Phanerozoic. Considerable rapid sea depth changes occurred in a number of Cambrian and Silurian deposition basins at a relatively stable water level. Therefore, the slow subsidence of these basins was accompanied by rapid tectonic uplift or subsidence of the crust, which can be interpreted as a specific type of tectonic movements on platforms.
																								



2.
MULTIPHASE PHENAI MATA INTRUSIVE COMPLEX (Deccan Trap Province, India) AND ITS ANALOGS ON THE SIBERIAN PLATFORM

O. V. Koroleva, B. V. Oleinikov, and V. V. Zolotukhin
Keywords: siberian and Deccan Traps, multiple intrusive series, layered gabbroid, monzonitoid, paleorift system
Pages: 916-928

Abstract >>
This paper reports new data on petrology and geochemistry of gabbroids and quartz monzonites from the multiphase Phenai Mata Intrusive Complex (PMIC). The complex is situated in the northwest of the Deccan Trap Province in the Narmadan Rift Zone. It is made up chiefly of trap-intruding layered gabbros and related quartz monzonites. These both rocks are intruded by dikes of basites of tholeiite composition. The gabbroids are rich in Mg and poor in K. The final-stage basites are distinguished by higher contents of SiO2, TiO2, K2O, and incompatible elements and are comparable with basites of paleorift zones of the Siberian Platform.
Original material was used to compare differentiates of tholeiite series from two largest provinces of manifestation of basaltic magmatism of ancient platforms: Deccan and Siberia. Comparison of element distribution shows that monzonitoids of different types of occurrence have in general similar chemical composition and the PMIC rocks are very close to quartz syenite-porphyries of composite dikes of the Chara-Sin' Dike Swarm (CSDS). Similarity between the geodynamic settings of intrusions from the CSDS (Vilyui Paleorift System) and the PMIC (Narmada Rift Zone) as well as their geochemical resemblance suggest a common mechanism for generation of intermediate and silicic rocks in tholeiite-basalt magmatic series on ancient platforms.
																								



3.
GEOCHEMICAL FEATURES OF ROCKS OF VOLCANOPLUTONIC ASSOCIATIONS OF THE DUKAT GOLD-SILVER DEPOSIT

M. N. Zakharov, R. G. Kravtsova, and L. A. Pavlova
Keywords: geochemistry, petrochemistry, REE and trace elements, igneous rocks
Pages: 929-939

Abstract >>
Study was given to the rocks of volcanoplutonic association of the Dukat gold-silver deposit, northeastern Russia. The Dukat plutonic uplift, with which the deposit is associated, is a result of intrusion of a two-phase pluton into a series of Lower Cretaceous ignimbrites. It consists of early-phase monzogranites and late-phase leucogranites. The Rb-Sr age of radiodacite ignimbrites is estimated at 99.1 1.4 Ma, and that of leucogranites of the second phase, at 80 2 Ma. All magmatic formations of the uplift belong to the petrochemical calc-alkalic series. Two groups of rocks which differ in the magnitude of serial index () have been recognized: monzo- and leucogranitic. The first group contains monzogranites (-2.41), diorites, amphibolic diorite porphyrites, and postmineral Na-basalts. The second group contains leucogranites (-2.02), Lower Cretaceous ignimbrites, and subalkalint K-basalts.
The monzogranites and leucogranites of the Dukat plutonic uplift are geochemically specialized for Ag, Sn, B, and REE. As a rule, there is no correlation between these and rock-forming elements. The index of accumulation of concentrations, IAC, is 17.9 in mozongranites and 19.7 in leucogranites. Carrier minerals and concentrators of ore elements and REE are accessory minerals (orthite, tourmaline, and ore minerals). The highest contents of volatiles (F, B, Cl, S), REE, Ag, Sn, and, to a lesser extent, Pb and Zn are recorded in ignimbrites and explosive breccias of rhyodacite composition, occurring at the top of the intrusion. Ignimbrite IAC is 69.5. Maximum concentrations of Au are noticed in explosive breccias of diorite porphyrites. Data on distribution of volatiles and REE in rocks of volcanoplutonic associations of the Dukat Au-Ag deposits are reported for the first time.
It is supposed that mineralization formed under the effect of a deep-seated fluid containing F, B, Cl, S, and other gas mineralizers. The most probable source of ore-bearing fluids is a long-lived local chamber of basaltic magma far from the granitoid pluton.
																								



4.
PHASE FORMATION AND DIAMOND CRYSTALLIZATION IN CARBON-BEARING ULTRAPOTASSIUM CARBONATE-SILICATE SYSTEMS

A. F. ShatskII, Yu. M. Borzdov, A. G. Sokol, and Yu. N. Pal'yanov
Keywords: diamond, high pressure, crystallization, carbonate-silicate systems
Pages: 940-950

Abstract >>
Diamond crystallization and character of phase formation in the systems K 2CO3-C, K2CO3-SiO2-C, and K2CO3-Mg2SiO4-C were studied at 6.3 GPa, 1650 °C for 40 h using the multianvil split-sphere equipment. The SiO2/K2CO3 and Mg2SiO4/K2CO3 ratios were chosen as variable parameters. The degree of graphite-to-diamond transformation and rate of diamond growth on seeds have been determined as a function of these ratios. Composition domains have been revealed in which spontaneous diamond nucleation and diamond growth on seeds proceed. As the concentrations of silica and forsterite in the systems K2CO3-SiO2-C and K2CO3-Mg2SiO4-C, respectively, increase to 10 wt.%, the diamond formation becomes more intense. Given a further increase in contents of SiO2 or Mg2SiO4, this intensity gradually decreases until the complete termination of spontaneous nucleation and then, diamond growth on seeds. The conditions were created under which diamond was crystallized from a potassium carbonate-silicate melt, including the main components of deep-seated ultra-potassium fluxes, together with coesite in the system K2CO3-SiO2-C and with forsterite in the system K2CO3-Mg2SiO4-C.
																								



5.
ELECTROMAGNETIC MONITORING IN THE BAIKAL RIFT ZONE, SEISMICITY TRIGGERING, AND EARTHQUAKE PRECURSORS

A. M. Popov, V. M. Akulov, N. N. Klimov, Yu. B. Bashkuev, M. G. Dembelov, A. N. Maksimovich, and A. V. Mashanov
Keywords: earthquake precursors, geophysical anomalies, electromagnetic monitoring
Pages: 951-959

Abstract >>
Many extraterrestrial geophysical anomalies are interpreted as earthquake precursors. However, the origin of these anomalies has been debated, and their physical relationship with earthquakes remains unclear. Many anomalies are attributed to lunar-solar-terrestrial interactions or to atmospheric effects and have no apparent relation to earthquake nucleation. Correlation of extraterrestrial pulses with earthquake origin times may be due to their triggering effect on tectonic processes in the lithosphere. Detection of these anomalies requires continuous monitoring of geophysical fields both in the upper (atmosphere) and lower (lithosphere) half-spaces, primarily with controlled-source radio-physical methods. High-precision measurements of amplitudes and phases of radio signals are provided by new recording equipment presented in the paper.
																								



6.
VARIATIONS IN ELECTRICAL RESISTANCE OF DUNITES AS A FUNCTION OF TEMPERATURE AND SERPENTINIZATION DEGREE (by the example of the Urals)

V. V. Bakhterev and L. D. Bulykin
Keywords: serpentinization of dunites, temperature, electrical resistance, activation energy
Pages: 960-967

Abstract >>
Electrical-resistance measurements, thin-section, chemical, thermal differential, and thermal gravity analyses were applied to dunites of different serpentinization degrees (from 0 to 100%, at 1-4% interval) from the Nizhnii Tagil dunite-clinopyroxenite massif in the Ural PGE-bearing belt (Solovieva Gora). The obtained functions show the temperature-dependent behavior of electrical resistance in the range of 20 to 800oC and a dependence of conductance activation energy on the degree of early serpentinization of dunites. The serpentinization-dependent activation energy shows no resistance dependence at normal temperature but is proportional to resistance temperature coefficient lgR0 (R0 is equal to resistance at 1/R0) in the temperature ranges of impurity-bearing (150-300 oC) and pure (350-600 oC) serpentinites.