Contents
Volume 18, ¹ 2, 1999
ContentsVolume 18, ¹ 2, 1999 |
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Russian Academy of Sciences in Geologic Research in the Russian Far East 3
Tectonics and Metallogeny
N.A.Shilo. The problem of the early history of the Earth, and its metallogenic effect 9
L.M.Parfenov, V.G.Vetluzhskikh, G.N.Gamyanin, Yu.V.Davydov, A.V.Deikunenko, A.V.Kostin, V.M.Nikitin, A.V.Prokopyev, A.P.Smelov, V.M.Supletsov, V.F.Timofeev, V.Yu.Fridovsky, A.I.Kholmogorov, Ya.V.Yakovlev. Metallogenic zonation of the territory of Sakha Republic 18
S.I.Sherman, K.Zh.Seminsky, A.V.Cheremnykh. Destructive zones and fault-block structures of Asia 41
Geophysics
R.Z. Tarakanov, O.K. Omelchenko. Peculiarities of the velocity structure of tectonosphere from data on the South Kamchatka-Yakutia geotraverse 54
N.M.Levashova, M.N.Shapiro. Paleomagnetism of Upper Cretaceous island-arc complexes, Median Range of Kamchatka 65
Magmatism
F.P.Lesnov, A.V.Travin, Yu.N.Lebedev. Geological, structural and K-Ar isotopic evidence for polychronicity of mafic - ultramafic complexes in the ophiolites of the Central Asia Fold Belt 76
N.V.Berdnikov, N.P.Romanovsky. Granitoids of the Khungari and Upper Udomi Groups of the North Sikhote-Alin in the Anyui bowing area: inclusions in minerals, and the problems of formation and ore potential 86
Mineralogy and Geochemistry
I.A.Tararin, D.W.Lee, W.S.Kim. Chromitite of the ophiolite assemblage from the Hunter Fracture Zone (North Fiji Basin) 94
L.N.Khetchikov, V.V.Ratkin, V.V.Malakhov. Ilvaite from skarn-polymetallic deposits of the Dalnegorsk ore region (Primorie) 102
V.I.Beryozkin, A.P.Smelov. Geochemistry of magmatic rocks from the greenstone belts of the Olyokma area, Aldan Shield 112
Stratigraphy
D.I.Agapitov, L.I.Mitrofanova. Stratigraphy of Neogene deposits from the Khatyrka basin (Chukotka) 123
Anniversaries
For V.F.Bely's 70th birth anniversary 134
N.A.Shilo
The problem of the early history of the Earth, and its metallogenic effects
“The Model of the Early History of the Earth», suggested by P.S.Klark, Jr, K.K. Turekyan, and L.Grossman, is analyzed; on the basis of the author’s earlier hypothesis of the Solar system formation from a spiral thermoplasma cloud with a whirlwind structure, a conclusion is drawn on the way of condensation of protosolar matter into a solid phase, which served as material for the formation of the Earth.
L.M.Parfenov, V.G.Vetluzhskikh, G.N.Gamyanin, Yu.V.Davydov, A.V.Deikunenko, A.V.Kostin, V.M.Nikitin, A.V.Prokopyev, A.P.Smelov, V.M.Supletsov, V.F.Timofeev, V.Yu.Fridovsky, A.I.Kholmogorov, Ya.V.Yakovlev
Metallogenic zonation of the territory of Sakha Republic
A base for metallogenic zonation is discrimination of a metallogenic belt that incorporates all deposits and ore shows formed in a specific geodynamic environment, such as island-arc, active continental margin, passive continental margin, sedimentary basin of a certain type, rift, continental block collision, etc. The belts stretch for thousands of kilometers attaining a width of hundreds of kilometers, and are marked by a particular time of formation. They are distinguished by metallogenic zones, ore districts, deposits, and ore shows. The metallogenic units of the eastern part of the Siberian Platform and Verkhoyano-Kolyma orogenic area have been characterized. Metallogenic zones and finer units have been defined in the basement of the Platform and in a collage of terranes of the Verkhoyano-Kolyma orogenic area. Metallogenic belts of these assemblages can only be deduced from paleotectonic reconstructions, which is a subject of further investigations.
S.I.Sherman, K.Zh.Seminsky, A.V.Cheremnykh
Destructive zones and fault-block structures of Asia
Small-scale geological and structural mapping was carried out for destructive zones of the lithosphere (DZL) of the world. Larger-scale maps were compiled for destructive zones of Central Asia and its adjacent regions which vary in regimes of geodynamic development. It is shown that the structural base of the DZL is formed by faults and fault-block structures. Their formation is regular and occurs according to a non-linear pattern, which relates the average sizes of the blocks Lbl or fault lengths Lf with their number Nbl,f by L=A / Nñ. It is revealed that the type of a stress state predetermines structuring of the destruction process. In Central Asia, DZLs being a result of geodynamic activity at deep levels may also serve as an ‘instrument’ for assessing the intensity of deep geodynamic processes.
R.Z. Tarakanov, O.K. Omelchenko
Peculiarities of the velocity structure of tectonosphere from data on the South Kamchatka-Yakutia geotraverse
A verified version of a P-wave velocity cross-section was constructed from the long South Kamchatka-Yakutia geotraverse (about 3000 km) on the basis of observations of seismic station networks. Two-dimensional velocity cross-sections stipulated on this geotraverse on an irregular network are smoothly approximated by using an apparatus of convex splines and unit expansion. Verification of velocity cross-sections is made by an iterative method of “shooting” in consort with a point-by-point comparison of the theoretical travel time curves constructed by a ray method with the empirical ones. The velocity structure peculiarities are presented for the entire depth of the tectonosphere (up to 700 km). The geotraverse crosses structures with different histories of tectonic development: from a seismic focal zone outcrop in the South Kamchatka area through the Okhotsk Sea northern part and the activated volcanic belt in the coastal continental part of Asia to Mesozoic-Cenozoic structures in the Yakutia area. The constructed version of the velocity cross-section differs from Jeffrey’s model averaged for the entire Earth. The cross-section is characterized by much lower velocities (up to 0.1-0.15 km/sec) in the South Kamchatka volcanic region and beneath the coastal continental area and also between the known layers of higher velocities, whose roofs lie at depths of about 400 and 660 km. Over a depth range of 250-480 km it is characterized by higher velocities than in Jeffrey’s model. No considerable continuation (deeper than boundaries of seismic manifestations) of the high-velocity layer identified with a seismic focal zone is observed on the given geotraverse. The constructed velocity cross-section can be used for calculation of travel time curves and for different geotectonic constructions.
N.M.Levashova, M.N.Shapiro
Paleomagnetism of Upper Cretaceous island-arc complexes, Median Range of Kamchatka
Late Cretaceous island-arc complexes were sampled for paleomagnetic study in the northern part of the Median Range of Kamchatka. After thermal demagnetization involving 12-18 steps, either a reversed characteristic remnant magnetization component (ChRM) was distinguished or remagnetization circles were defined in most samples. The fold test is positive, and the ChRM is likely to be primary. Magnetization trend of dec.=105.5°, inc.=-67.0°, a95=4.1° for these rocks corresponds to a paleolatitude of 49.7° ± 5.6° N. This value is by 20°-22° lower than the Late Cretaceous North American reference values, and a northward transportation of the studied tectonostratigraphic terrain and the island arc to which it had originally belonged, is strongly indicated. The observed paleolatitudes approximately coincide for all other Late Cretaceous island-arc complexes from Kamchatka and the southern part of the Koriak Highlands. We conclude that all these complexes were formed within the same island arc which had originated 20°-25° more to the south from the continental margin in the Campanian and then had been moving until the emplacement in the Paleocene. The northward drift of the island arc coincided with volcanic activity, and the related subduction zone also had to drift with a quickly moving oceanic plate, most probably with the Kula plate. This implies that the subduction zone dipped oceanward and was consuming the oceanic lithosphere, presumably the oceanic periphery of the North American plate.
F.P.Lesnov, A.V.Travin, Yu.N.Lebedev
Geological, structural and K-Ar isotopic evidence for polychronicity of mafic - ultramafic complexes in the ophiolites of the Central Asia Fold Belt
Space and time relationships between ultramafites and mafites in ophiolite associations of the Central Asia Fold Belt have been considered as exemplified by the Naran mafic-ultramafic massif (West Mongolia). According to geological and structural data, the massif consists of three structural elements: 1 - protrusion of ultramafic rocks; 2 - intrusive of mafic rocks breaking through it, and 3 - mainly pyroxenite contact-reaction zone located along their boundary. This is indicative of different time of formation of the above elements, i.e., of the massif polychronicity. Additional evidence concerning the massif polychronicity has been obtained by isotopic studies based on the modified K-Ar method. The behavior of the K-Ar isotopic system has been studied, exemplified by 17 whole rock and monomineral samples from ultramafic and mafic rocks. Based on the isochrone created for monomineral samples of clinopyroxene and plagioclase from gabbronorites of the mafic intrusive and websterites of the contact zone, it was established that the mafic intrusive had formed in a range of 745±100 million years (Late Riphean). The isotopic system of the rocks and minerals in the ultramafic protrusion became disturbed to some extent due to secondary processes; this made it impossible to determine their age. Judging by the fact that ultramafic rocks are broken through by a mafic intrusive and that clinopyroxene from lherzolite represented in them is essentially enriched in radiogenic Ar, it can be assumed that the protrusion of ultramafic rocks had formed before 745±100 million years, i.e., before Late Riphean. The advanced K-Ar method allows us to obtain satisfying results when dating mafic-ultramafic complexes from whole rock and monomineral samples of clinopyroxenes and plagioclases given their good preservation and thorough sampling.
N.V.Berdnikov, N.P.Romanovsky
Granitoids of the Khungari and Upper Udomi Groups of the North Sikhote-Alin in the Anyui bowing area: inclusions in minerals, and the problems of formation and ore potential
The paper compares thermobarogeochemical characteristics of granitoids from the Khungari and Upper Udomi Groups, making up massifs exposed in the vicinity of the Anyui bowing of the North Sikhote-Alin fold area. The paper aims at outlining thermobarogeochemical criteria of granitoid ore potential in the region. It is shown that the ore-free granites from the Khungari Group are mainly distinguished by a carbon dioxidic-methane composition of fluid in inclusions. Quartz from the tin-bearing granitoids of the Upper Udomi Group is dominated by water-saline inclusions of K-Na cation specialties. Computations were made of granitoid P-T crystallization. Geodynamic models for their formation are discussed which may make realization of their ore potential possible.
I.A.Tararin, D.W.Lee, W.S.Kim
Chromitite of the ophiolite assemblage from the Hunter Fracture Zone (North Fiji Basin)
Chromitite dredged with the rocks of the ophiolite assemblage from the Hunter Fracture Zone shows a distinct layering. The first layer consists of a serpentinized harzburgite with relics of olivine and orthopyroxene. The middle layer is represented by orthopyroxenite with accessory Cr-spinel. A low Al2O3 content of orthopyroxene and a comparatively high Cr/(Cr+Al) ratio of Cr-spinel from the rocks reveal a high degree of its alumina-depletion and suggest that Cr-spinel is similar in composition to chromite of peridotite from an island-arc system. The third layer is represented by norite consisting of plagioclase and amphibolized orthopyroxene with a varying amount of ore chromite from 4-5 to 60 vol.%. Microprobe data are evidence that ore chromite is depleted in Cr2O3 and enriched in MgO and Fe2O3, as compared to accessory spinel from orthopyroxenite of layer 2. Textural and structural peculiarities of ore chromite (atoll and skeleton crystals of ore chromite that are hosts for orthopyroxene and plagioclase of the norite layer), significant differences in composition of accessory and ore chromite suggest that ore magma origination involves processes of magmatic differentiation of the parent ultramafic source accompanied by liquid immiscibility. Ore magma may be preserved for some time after consolidation of ultrabasites and form in them intrusions with chromite replacing silicate minerals.
L.N.Khetchikov, V.V.Ratkin, V.V.Malakhov
Ilvaite from skarn-polymetallic deposits of the Dalnegorsk ore region (Primorie)
This paper shows regularities in distribution of ilvaite in skarn-polymetallic deposits of the Dalnegorsk ore region. The chemical composition of ilvaite and its change in space and time have been studied by chemical, microX-ray-spectral, and Mössbauer methods. Conditions of ilvaite formation and factors responsible for its concentration pattern in different ore bodies and deposits are discussed.
V.I. Beryozkin, A.P.Smelov
Geochemistry of magmatic rocks from the greenstone belts of the Olyokma area, Aldan Shield
Similarities and differences in major, trace and rare earth element composition of the metamorphosed komatiite-tholeiitic and calc-alkalic volcanics from the Olondo, Tungurcha and Subgan greenstone belts (GSB) have been established. Metamorphosed dunites, peridotites, gabbro, diorites, and anorthosites that could be genetically related to volcanics have also been investigated. The geochemistry of the volcanics has been compared with that of standard rocks. The nature of magmatism depends on the geodynamic conditions of the origin and evolution of the GSB.
D.I.Agapitov, L.I.Mitrofanova
Stratigraphy of Neogene deposits from the Khatyrka basin (Chukotka)
Under discussion are the problems of stratigraphic division and correlation of Neogene deposits, widespread in the subaerial part of the Khatyrka basin and making up the upper portion of its sedimentary cover. The Neogene rocks were penetrated by 23 parametric, prospecting and structural holes, and are partly exposed on the surface. The Neogene sections are distinguished by deposits of the Lower Miocene, non-stratified Lower-Middle Miocene, Middle Miocene, non-stratified Middle and Upper Miocene, Lower Pliocene, and non-stratified Upper Pliocene – Lower Pleistocene. A draft is suggested of a correlation stratigraphic scheme for Neogene deposits, and discrimination is made of ‘horizons’ as taxonomical units linking lithologo-stratigraphic subdivisions of Cenozoic sedimentary sections of all structural-and-facial zones and subzones of the sedimentary cover.