Geologiya I Geofizika, 2002, V 43, N 10, October.
TECTONICS
SILURIAN DEPOSITION IN EAST SIBERIA AND THE ABSENCE OF STRONG
EUSTATIC FLUCTUATIONS
E. V. Artyushkov and P. A. Chekhovich
889
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
(<$E"~">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
ccompanied by rapid tectonic uplift or subsidence of the crust, which can be
interpreted as a specific type of tectonic movements on platforms.
Silurian, tectonic movements, eustatic fluctuations, fringing seas, numerical
modeling, East Siberia
PETROLOGY, GEOCHEMISTRY, AND MINERALOGY
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
912
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.
Siberian and Deccan Traps, multiple intrusive series, layered gabbroid,
monzonitoid, paleorift system
GEOCHEMICAL FEATURES OF ROCKS OF VOLCANOPLUTONIC ASSOCIATIONS
OF THE DUKAT GOLD-SILVER DEPOSIT
M. N. Zakharov, R. G. Kravtsova, and L. A. Pavlova
924
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<$E+->1.4 Ma, and that of leucogranites of the second phase, at
80<$E+->2 Ma. All magmatic formations of the uplift belong to the petrochemical
calc-alkalic series. Two groups of rocks which differ in the magnitude of serial
index (<$Esigma>) have been recognized: monzo- and leucogranitic. The first
group contains monzogranites (<$Esigma>-2.41), diorites, amphibolic diorite
porphyrites, and postmineral Na-basalts. The second group contains leucogranites
(<$Esigma>-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.
Geochemistry, petrochemistry, REE and trace elements, igneous rocks
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
936
Diamond crystallization and character of phase formation in the systems K2CO3-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.
Diamond, high pressure, crystallization, carbonate-silicate systems
GEOPHYSICS
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 947
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 radiophysical methods. High-precision measurements of amplitudes and
phases of radio signals are provided by new recording equipment presented in the
paper.
Earthquake precursors, geophysical anomalies, electromagnetic monitoring
VARIATIONS IN ELECTRICAL RESISTANCE OF DUNITES ASA FUNCTION OF
TEM PERATURE AND SERPENTINIZATION DEGREE (by the example of the Urals)
V. V. Bakhterev and L. D. Bulykin
956
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 800°C 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/R<$E->>>0) in the temperature
ranges of impurity-bearing (150-300 °C) and pure (350-600 °C) serpentinites.
Serpentinization of dunites, temperature, electrical resistance, activation
energy