Volume 24, 5, 2005
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Volume 24, 5, 2005 |
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Ye.V. Kolesov
Summary magnetostratigraphic scale of the Paleozoic in northeastern
This paper summarizes for the first time the results of
paleomagnetic investigations of many years (1976-2000) of Paleozoic assemblages
which are widely developed in northeastern Russia. Most of the studied sections
are rich in the remains of various organisms (coral, foraminifera, ostracod,
brachiopod, trilobite, graptolite, and conodontoforide). Some of these sections
are regional stratotypes and parastratotypes of the boundaries of systems,
sections, stages, and also of the suites distinguished there. More than 100
exposures located in the four subregions of the north-eastern part (Omulyovka
and Prikolyma uplifts, Oloi zone of the Alazeya-Oloi eugeosyncline system, and
the north-eastern margin of the Omolon massif) have been studied. They occupy
the age interval from Ordovician up to Permian inclusive. After a thorough
laboratory study, only 33 outcrops proved suitable for magnetostratigraphic
constructions. On their basis, summary magnetetostratigraphic sections were
constructed for each of the above mentioned subregions. Then, based on
biomagnetostratigraphic evidence, these sections were compared with each other,
which resulted in the summary magnetostratigraphic scale of the Paleozoic of
northeastern Russia. This scale is compared with the General stratigraphic scale
(GSS), General magnetostratigraphic scale of polarity (GSSP), and the scale of
isotopic age. Based on this scale, one can make division and correlation of
Paleozoic assemblages in northeastern Russia, and also conduct similar
procedures with coeval deposits from other regions of Russia and elsewhere in
the world. This scale is also suggested to be used for age determination of
rocks in the sections where paleontological characteristics are not sufficiently
informative for these experiments. One of the pluses of this scale is a
possibility to specify and add the General magnetostratigraphic scale of
polarity (MSSP).
Ye.Ye. Chernov,
D.V. Kovalenko, G.V. Ledneva
Paleomagnetism of geological complexes of the Kuyul Range (Central
Koryakia).
Paleomagnetic investigations of geological complexes of the Kuyul Range (Central
Koryakia) were conducted. Three components of remnant magnetization were
distinguished. The relatively low-temperature component (A) is "postfold". The
high-temperature components (B and C) are possibly "prefold". If so, the studied
complexes formed at the near-equatorial latitudes. The spreading axes were
oriented sublatitudinally. Possibly, the collision between the investigated
complexes and the continent was attended by a strike-slip component.
Petromagnetic investigations show a strong degree of alterations of magnetic
minerals.
M.Ye.Melnikov,
I.N.Ponomareva, D.D. Tugolesov, V.Kh.Rozhdestvensky
Drilling results of ferromanganese cobalt-rich crusts on the
guyots of the Magellan Seamounts (the Pacific Ocean)
The results of ferromanganese Co-rich crusts investigation are presented in the
paper based on shallow boreholes drilled with the help of the submersible unit
GBU-1.5/4000 on the guyots Fedorov and Alba (Magellan Seamounts, NW Pacific).
The results are of special value due to the unique possibility to study a drill
core of crusts and underlying rocks in situ with a strictly coordinated point,
which is not really available in the course of dredging. Sections along drilling
profiles are given. It is shown that the average thickness within subhorizontal
and gently sloping surfaces is significantly higher than that on the relatively
steep slopes. The variability of crust thickness is determined by the
variability of their section composition. The reduction of crust thickness is
the result of omission of certain layers in the section. An increase in crust
thickness is usually determined by an anomalous increase in the thickness of the
lower layers I-1 and/or I-2. Generally, the variability of thickness within
specific layer is not large.
Possible paleoreconstructions of ore forming conditions based on the obtained
sections are demonstrated. Cases of crust thickness reduction caused by
overlapping by fresh carbonate sediments are considered, as well as a wide
development of buried nodule horizons in the area.
A conclusion was made on the wide applications of shallow holes drilling for the
purposes of ferromanganese crust ores investigation on the seamount slopes.
V.N. Senachin
"Free mantle surface" anomalies of the Okhotsk Sea region, and
their relation to deep processes
Great isostatically compensated density anomalies were found by a number of
scientists (Watanabe et al., 1977; Yoshii, 1973 et al., a.o.) in the upper
mantle of deep sea basins of marginal seas in East Asia. The estimation of "free
mantle surface" anomalies in the Okhotsk Sea region was aimed at the definition
of the origin of the given density anomalies. The results of this research have
shown that the Okhotsk Sea region is characterized by general increase in the
depth of the free mantle surface with an amplitude about 0.5 km, and against the
background of this increase, regional anomalies of the greater and lesser depth
are observed which are linked with the structures marked in the bottom relief.
These anomalies are, probably, formed in the process of destruction of the
earth's crust thus resulting in the reduction of its thickness. They can be
caused by lateral temperature anomalies in the lithosphere which are formed
under the basins as a result of conductive heating, fluid uplift and magmatism
into the crust. The ubiquitous deepening of the free mantle surface points to
the presence of a large upper mantle high density body probably distributed
beyond the boundaries of the Okhotsk Sea into the adjacent parts of continent
and ocean. This high density body is almost completely isostatically compensated.
Therefore, the deep isostasy mechanism with a low mantle compensation level
takes place in the active margins of East Asia.
V.Ye. Strikha
Early Jurassic granitoids of the Chagoyan and Shimanovsk
massifs of the Mamyn block, Amur superterrane (Upper Priamurye)
This paper offers the results of investigation of the composition of
rock-forming minerals and geochemical features of Early Jurassic granitoids in
the Chagoyan and Shimanovsk massifs (Mamyn block of the Amur superterrane).
The results of the investigation showed that the rock-forming minerals of
granitoids had crystallized from melts of normal and increased alkalinity in
different redox settings under different fluid regime of halogens, and in
conditions of hyp- and mesoabyssal facies of depth. In terms of fluid
composition, the Shimanovsk granitoids are close to rare metal granitoids, and
they formed in conditions of lesser oxygen fugacity as compared with the
Chagoyan rocks, and at high relative fugacity fHF/fH2O è fHCl/fH2O and lower
fH2O. The distinct differences in the composition of fluid of the Chagoyan and
Shimanovsk rocks, which co-exists with biotite, together with their geochemical
and petrochemical features give evidence of the autonomous character of
formation of their initial melts.
The diversity of Early Jurassic intrusive rocks are to a great extent attributed
to the composition of the initial substratum. The source for the first-phase
rocks of the Chagoyan massif was most likely garnet-bearing biotite amphibolites
as distinct from the second-phase granitoids, which are characterized by a more
leucocratic source with mixed amphibolite-metagraywacke composition. For the
Shimanovsk granitoids, the initial substratum was evidently crystalline schists
with an admixture of carboniferous matter.
The formation of Early Jurassic granitoids occurred in a collision environment,
probably, during collision between the Amur superterrane and the Siberian craton.
B.F. Palymsky
The principles of the structure-and-matter classification of
geological formations
The offered general classification of different-type geological formations is
based on the structure-and-matter principles using the evidence of northeastern
Russia. Six taxonomic levels are outlined: formation type (sedimentary, magmatic,
metamorphic, and ore geological formations) - formation family - formation group
- formation kind (geological formation proper) - variety - formation individual
(formation formed in a specific region, at a certain time, and geologically
mapped as stratigraphic divisions, magmatic and metamorphic complexes).
E.L.
Shkolnik
The principal tasks of geology in the first half of the XXI century
V.T. Frolov?s work "The science of geology: philosophical analysis" considers in
detail the situation taking place at present in the geological science in the
context of gnoseological analysis and determination of scientific tasks, aims,
and real potential of geology in this century. Having the same opinion and
supporting most of V.T. Frolov?s evaluations and viewpoints, the reviewer does
not agree with him in one crucial issue in principle -geology implications in
new time, and, correspondingly, the ways of getting out of the crisis
experienced at present by geology in our country. The most important problem is
not drawing the history of the Earth; rather it is the development of genetic
models for discovery of mineral deposits that do not crop out at the day
surface.