21_6

Volume 21, 6, 2002

T.M. Okuneva.Biostratigraphy of the Triassic of the Far East and Trans-Baikal area 3

G.B. Udintsev, R. Gerzonde, D.Ye. Teterin, G.V. Shenke, A.F. Beresnev, K. Gol, N.A. Titaeva, A.N. Fenogenov, N.A. Kurentsova, I.A. Roshchina/i>The geology of Pyotr I Island, and the probable dynamics of development of the West Antarctic in the Bellingshausen Sea 31

A.A. Sorokin, S.I.Dril. The Yankan ophiolite complex of the Mongolian-Okhotsk fold belt: petrology and geodynamic position 46

V.N. Patrikeev. Lithological and tectonic delamination of the crust in the Northwest Pacific 61

I.A.Tararin, S.K.Zlobin, G.M.Kolesov, E.M.Sedykh, V.M.Chubarov, T.M.Filosofova. Island-arc magmatism of the Hunter Fracture Zone (North Fiji Basin, Pacific Ocean) 78

G.S. Mirzekhanov, Ye.V. Kurbatov. Granulometry and morphology of gold of man-made placers in the Russian Far East 99

M.M. Konstantinov, A.K. Bystrovа, V.Yu. Fridovskiy. Stratigraphic levels of localization of gold and silver deposits in the Verkhoyansk and Central Kolyma metallogenic provinces 105
Discussion
L.V. Eirish. The "stratigraphic" levels of gold-bearing deposits 112
Yu.I. Bakulin. The reasons for confinement of endogenic mineralization to stratigraphic levels 116
Chronicles
G.L. Kirillova. IV International Symposium, Project 434 UNESCO: results and prospects 120
N.А. Bogdanov, О.V. Chudaev. Role of International Community in the study of geology of the Pacific area 123
Index of papers published in the journal in 2002 125

T.M. Okuneva
Biostratigraphy of the Triassic of the Far East and Trans-Baikal area

Detailed descriptions of the main sections are offered, which as they are, are published for the first time, and evidence for the biostratigraphy of the Triassic in the Transbaikal region, Upper, Middle and Lower Amur region, West Priokhotye and the southern part of North Priokhotye, and in the South Sikhote-Alin is presented. The description of the sections is accompanied by a layer-by-layer listing of ammonoids and bivalves. Basing on these faunal groups, material and paleontological characteristics of zones, subzones and layers with fauna are given for the whole study area. The ammonoids of the Lower Triassic in the 6 earlier known zones were supplemented with paleontological characteristics; in the Middle Triassic, new names for fauna-bearing layers for the Upper Anisian were suggested. The Striatosirenites kinasovi new zone was established in the base of the Lower Norian; layers holding Dittmaritoides were identified in the Middle Norian; and Arcestes colonus –bearing layers, in the Upper Norian. The first find of Mesohimavatites indigiricus in the unique section in the West Priokhotye on the Tugurskiy Bay coast is noticeable. Basing on it, layers of the same name were discriminated in the upper part of the Middle Norian. Levels with Cyrtopleuritidae? were conventionally designated in the Middle Norian, and levels with Neoprotrachyceras? and Protrachyceras were conventionally defined in the Carnian stage. In the bivalve chart, 6 layers holding them were discriminated for the first time, in the Lower Triassic, assemblages of Norian and Rhaetian bivalves were complemented, and layers holding Costatoria napengensis were discriminated in the Rhaetian.

A.G.B. Udintsev, R. Gerzonde, D.Ye. Teterin, G.V. Shenke, A.F. Beresnev, K. Gol, N.A. Titaeva, A.N. Fenogenov, N.A. Kurentsova, I.A. Roshchina
The geology of Pyotr I Island, and the probable dynamics of development of the West Antarctic in the Bellingshausen Sea

The oceanic crust in the West Antarctic – Bellingshausen and Amundsen Seas, is characterized by numerous fracture zones and seamounts. The origin and nature of these structures are still unclear. The morphology, deep structure and petrology of the basement rocks are the key to an understanding of the geodynamics of the West Antarctic. The effect of the morphostructure of the basement on the transport of sediments supplied from the West Antarctic is enormous. The relationship between the fracture zones, volcanic seamounts and islands, and the sediment transportation by suspension flows were the subject of joint Russian-German investigations in 1998-2001. The data obtained suggest a destructive development of continental margins of the Bellingshausen and Amundsen Seas as an alternative to ideas about their development in the process of subduction of oceanic plates under continent.

A.A.Sorokin, S.I.Dril
The Yankan ophiolite complex of the Mongolian-Okhotsk fold belt: petrology and geodynamic position

Basing on geological and petrologo-geochemical data, a tectonically disjointed ophiolite complex of Middle Paleozoic (?) age is recognized within the western part of the Yankan terrain of the Mongolian-Okhotsk fold belt (the Upper Amur region), which subsequently got the name after the latter. The following constituent parts are reconstructed in its composition: metamorphosed peridotites (apoharzburgite serpentinites of melange); a cumulative banded complex (serpentinite peridotites, cumulative gabbroids); a complex of homogeneous gabbroids; a dyke, and a volcanogenic-sedimentary complexes. Among metabasites of the dyke and volcanogenic-sedimentary complex, varieties close to BABB type are most common. They are represented in the form of fragments in the serpentinite melange, by diabases of the dyke complex, and by a predominant part of metavolcanics of the Dzhalinda suite. This type of metavolcanics is characterized by low and moderate HFSE contents (primarily, Nb) in comparison with LILE and LREE. Only an insignificant part of metavolcanics from the Dzhalinda suite shows a relation to the OIB type magma source, while the greenstone rocks of the Shakhtaun suite correspond to N-MORB compositions. Basing on the petrologo-geochemical analysis, we came to the conclusion that the ophiolite complex was formed in the back-arc spreading basin.
In the modern structural plan, the above associations are tectonically superposed in the accretion complex.
The oceanic crust in the West Antarctic – Bellingshausen and Amundsen Seas, is characterized by numerous fracture zones and seamounts. The origin and nature of these structures are still unclear. The morphology, deep structure and petrology of the basement rocks are the key to an understanding of the geodynamics of the West Antarctic. The effect of the morphostructure of the basement on the transport of sediments supplied from the West Antarctic is enormous. The relationship between the fracture zones, volcanic seamounts and islands, and the sediment transportation by suspension flows were the subject of joint Russian-German investigations in 1998-2001. The data obtained suggest a destructive development of continental margins of the Bellingshausen and Amundsen Seas as an alternative to ideas about their development in the process of subduction of oceanic plates under continent.

V.N. Patrikeev
Lithological and tectonic delamination of the crust in the Northwest Pacific

This paper offers the results of the study of the Earth’s crust by the seismic reflection method on the DSS submeridional profile in the DELP experiment carried out in the northwestern basin of the Pacific Ocean. This evidence proves that the crust is layered. Layers 1 and 2 are characterized by lithological, and Layers 3A and 3B, tectonic lamination. Two boundaries of strong structural disturbances have been identified in the crust. The upper boundary coincides with the roof of Layer 3 and manifests itself in the change of angles of dip of faults from subvertical in Layer 3A to 20-25° in Layer3B. This gives evidence of the change in the state of dilatation deformation of rocks at this boundary and a decrease in the rock strength in Layer 3B.
The lower boundary corresponds to the roof of the contrasting horizon and manifests itself in the flattening of oblique fault zones in Layer 3B as far as the horizontal. This is related to the transition of the medium into a cataclastic state.

I.A.Tararin, S.K.Zlobin, G.M.Kolesov, E.M.Sedykh, V.M.Chubarov, T.M.Filosofova
Island-arc magmatism of the Hunter Fracture Zone (North Fiji Basin, Pacific Ocean)

It has been established that two different-age structural stages, a melange zone and the overlying volcanic cover, participate in the geological structure of the Hunter Fracture Zone being the southeastern continuation of the Vanuatu island-arc system. These structural stages are characterized by different island-arc volcanism. The island-arc magmatic products of these stages have been first described in detail from the petrological and geochemical standpoint. Oceanic and island-arc metabasalts and metadolerites metamorphosed under the greenschist facies belong to the upper part of the ophiolite section of this region, and modern boninites, island-arc tholeiites, and subalkaline basaltoids of K-Na series, non-metamorphosed or poorly altered under the zeolite facies, are common in the melange zone. Metabasites of the ophiolite complex are suggested to be formed during the Late Eocene-Early Miocene stage of the Vityaz island-arc system; and modern boninites, island-arc tholeiites and subalkaline basaltoids, during the Late Pliocene-Recent stage of development of the Vanuatu island-arc system. A thick complex of island-arc formations of the volcanic cover represented by island-arc tholeiitic basalts, volcanites of the bimodal calc-alkaline series, and oceanic tholeiites of the spreading structures is formed on the ophiolite basement of the melange zone in the Late Pliocene-Recent stage of magmatism. Close spatial and temporal relationships in one region of contrast-titanium island-arc complexes is shown to be caused by processes of oblique subduction accompanied by transform dislocations along the Hunter Fracture Zone. Conditions of oblique subduction, probably, result in break of continuity of the subducted oceanic plate and ascent of the poorly depleted mantle material giving rise to subalkaline basaltoids.

G.S. Mirzekhanov, Ye.V. Kurbatov
Granulometry and morphology of gold of man-made placers in the Russian Far East

The granulometric and morphological peculiarities, and the pattern of distribution of gold in the man-caused formations of placer deposits depend on different industrial and geological factors.
The dimension of gold in the dump complex is of a wide spectrum, including also native gold. This allows the use of traditional flushing equipment for its repeated flushing. Incidentally, the actual gold loss during flushing is less than the calculated technological values.

M.M. Konstantinov, A.K. Bystrovа, V.Yu. Fridovskiy
Stratigraphic levels of localization of gold and silver deposits in the Verkhoyansk and Central Kolyma metallogenic provinces

The urgent problem of reproduction of the mineral-raw material base of noble metals can be solved, on the one hand, by application of new technologies for exploration, and on the other, by nonconventional approaches to the analysis of geological evidence. The latter can involve the analysis and finding of patterns of distribution of gold and silver deposits in a stratigraphic section conventionally reckoned as hydrothermal and related to much later geological events. The experience of such an analysis has been realized in the study of the Verkhoyansk and Central Kolyma metallogenic provinces encompassing the largest gold and silver deposits.