Home – Home – Jornals – Russian Geology and Geophysics 2010 number 5

The history of the Vendian-Early Paleozoic formation of protoliths of continental crust in the Gorny Altai segment of the Central Asian fold belt is considered, and their composition, isotopic characteristics, and formation mechanisms are estimated. We have established two stages of crust-forming processes in Gorny Altai: Early and Late Caledonian, with the different structures of formed geoblocks and nature and compositions of crustal protoliths.
At the Early Caledonian stage, fragments of oceanic lithosphere of basic composition (MORB, OIT, OIB) (T_Nd (DM - 2st) = 0.65-1.1 Ga) formed, as well as island arcs with andesite-basaltic and andesitic protoliths with low contents of incompatible elements (T_Nd (DM - 2st) = 0.7-0.9 Ga). At the Late Caledonian stage, the redistribution of the substance of these blocks and the external supply of material led to the formation of the heterogeneous crust of turbidite basins with an oceanic basement and andesite-dacitic upper-crustal protoliths (T_Nd (DM - 2st) varies from 0.8-0.9 Ga in the framing of the volcanic arc of Altaids to 1.4-1.6 Ga at the boundary of the Altai-Mongolian microcontinent).

We report data from the Khadarta, Khoboi, and Orso metamorphic complexes of the Olkhon terrane in the western Baikal region. High-grade rocks in the three complexes might have been derived from active continental margin rocks (island arc-backarc basin system). The backarc basin history possibly began at 840-800 Ma, according to SHRIMP-II U-Pb zircon ages of the Orso gneiss. Many tectonic units in the Olkhon terrane belonged to the active margin of the Barguzin microcontinent, which rifted off the Aldan province of the Siberian craton in the Early Neoproterozoic. The accretion of the microcontinent to the craton was accompanied by high-grade metamorphism recorded in the Khadarta and Khoboi granulites. The 507±8 Ma and 498±7 Ma SHRIMP-II U-Pb zircon ages of the latter complexes, respectively, may refer to the earliest evolution stage of the Olkhon metamorphic terrane. New data for the Olkhon terrane agree with the ages of other high-grade complexes along the southern Siberian craton (Slyudyanka, Kitoikin, Derba) and correspond to the initiation of the Central Asian orogen. With these data, the Olkhon metamorphic terrane has been interpreted as an Early Paleozoic collisional collage of fragments of the microcontinent's Neoproterozoic active margin.

New structural, petrological, chemical, isotope, and paleomagnetic data have provided clues to the Late Riphean-Paleozoic history of the Uda-Vitim island arc system (UVIAS) in the Transbaikalian sector of the Paleoasian ocean, as part of the Transbaikalian zone of Paleozoids. The island-arc system consists of three units corresponding to main evolution stages: Upper Riphean (Late Baikalian), Vendian-Lower Paleozoic (Caledonian), and Middle-Upper Paleozoic (Hercynian). The earliest stage produced the base of the system composed of Late Riphean ophiolite (971-892 Ma, U-Pb) and volcanic (837-789 Ma, U-Pb) and sedimentary rocks (hemipelagic siliceous sediments and dolerite sills), which represent the Barguzin-Vitim oceanic basin and the Kelyana island arc. The main event of the second stage was the formation of the large UVIAS structure (over 150,000 km²), which comprised the Transbaikalian ocean basin, the forearc and backarc basins, and the volcanic arc itself and consisted of many volcanotectonic units exceeding 100 km² in area (Eravnoe, Oldynda, Abaga, etc.). Lithology, stratigraphy, major-element compositions, and isotope ages of Vendian-Cambrian volcanic rocks and associated sediments indicate strong differentiation of calc-alkaline series and the origin of the island-arc system upon oceanic crust, in a setting similar to that of the today's Kuriles-Kamchatka island-arc system. The Middle-Upper Paleozoic stage completed the long UVIAS history and left its imprint in sedimentary and volcanic rocks in superposed trough basins. The rocks were studied in terms of their biostratigraphic and isotopic-age constraints, as well as major- and trace-element compositions, and were interpreted as products of weathering and tectonomagmatic rework of the UVIAS units. The synthesis of the data yielded a generalized geodynamic model of the UVIAS history.

New data testifying to Late Paleozoic tectonometamorphic processes at the West Transbaikalian segment of the Central Asian Fold Belt have been obtained. Zircon dating (SHRIMP-II) of highly metamorphosed rocks showed that the processes took place at 295.3±1.6 Ma. Based on these data, the Late Paleozoic ages of granitoids of the Angara-Vitim areal pluton (340-280 Ma) and some dike complexes in Transbaikalia (300-280 Ma), and the Late Paleozoic age of some carbonate-terrigenous strata dated earlier to the Early Paleozoic, we have substantiated the significant role of Hercynian tectogenesis in the consolidation of the regional continental crust. We have also shown that the Late Paleozoic endogenous events and accompanying sedimentation processes were related to the geodynamic conditions governed by the changing parameters of the subsidence of the Mongolo-Okhotsk oceanic subduction slab beneath the Siberian continent. Changes in the slope and rate of the slab subsidence resulted in A-subduction conditions in the distal part of the suprasubduction plate, which led to the formation of accretion-collisional orogen and the Angara-Vitim areal pluton.

The  lithospheric structure of several marginal and interior units of the eastern Central Asian fold belt has been explored in 2D geophysical models. The obtained constraints on effective parameters (density, resistivity, temperature) of lithospheric blocks and their boundaries allowed correlation of geophysical structures to tectonic settings. The geological and geophysical (including paleomagnetic) data were used jointly to model the present structure of the lithosphere along 126°E between 56°N and 40°N and to construct a palinspastic model of the same area for the latest Early Jurassic (175 Ma).

The paper presents new data on physico-chemical parameters of the Neoproterozoic-Early Cambrian plume magmatism in the Paleo-Asian Ocean. The data on clinopyroxenes show the plume-related plateau basalt magmatic systems of the Katun' paleoseamounts, which interacted with mid-ocean ridge (MOR) magmas. The Kurai paleoseamount consists mainly of plateau basalt systems, and the Agardag ophiolites represent products of OIB-type "hot-spot" within-plate magmatism. Our study of inclusions showed that the melts of the Katun' and Kurai paleoseamounts crystallized at lower temperatures (1130-1190°C) compared to the Agardag ophiolites (1210-1255°C). The petrochemical analysis of the melt inclusions showed that the Katun' and Kurai magmatic systems are different from the Mg- and Ti-richer melts of the Agardag ophiolites: The former are similar to the magmas of the Nauru Basin and Ontong Java Plateau (western Pacific), whereas the latter possess geochemical affinities to OIB-type magmatism. The rare-element compositions of the melts of the Katun' and Kurai paleoseamounts correspond to those of the Ontong Java Plateau and Nauru Basin lavas. The numerically simulated parameters of the Katun' paleoseamount primary magmas agree with the data on the magmatic systems of the Siberian Platform and Ontong Java Plateau. For the Kurai paleoseamount, the simulated results suggest interaction of deep-seated OIB-type magmatic systems with MOR ones. The Agardag ophiolites were formed in relation to mantle plume activity at the initial stages of paleo-oceanic complexes formation.

The paper discusses geological, mineralogical, petrographic, and geochemical data on the Ureg Nuur volcanoplutonic association of high-Mg volcanic and subvolcanic rocks localized among Vendian-Cambrian accretionary structures in the Mongolian Altay. These rocks have a high potassium alkalinity (K₂O/Na₂O up to 1.2), are enriched in LILE and Sr, and have negative Zr-Hf and Nb anomalies in multielement spectra; this confirms the suprasubduction type of the source of melts. The geologic setting and established age (512.4±6.1 Ma, ³⁹Ar-⁴⁰Ar dating of biotite phenocrysts) evidence picritic magmatism at the accretionary stage of the development of the Altay fragment of the Paleoasian ocean. This indicates a large igneous province related to a mantle plume.

We discuss strike-slip tectonics as the key agent in the formation of the Early Paleozoic (Caledonian) collisional system of the western Baikal region. This tectonic setting implies existence of local syncompressional extension, with the ensuing conditions for mantle drainage and magmatism. Lower-middle crust collisional complexes exposed in the Olkhon area of the western Baikal region provide a record of synmetamorphic subalkaline-mafic magmatism associated with the early synorogenic collapse of the Olkhon collisional system, a part of the Central Asian collisional-accretionary belt.

In this paper we present complex geological, petrographic, and geochronological data of the study of intermediate-composition and acid intrusive and volcanogenic rocks from the Porozhnaya massif of the South Yenisei Ridge. For the first time in the Yenisei Ridge, Devonian and Triassic U-Pb age values (SHRIMP method) have been obtained for leucogranites - 387±5 Ma and alkaline trachytes - 240±3 Ma, which allows us to attribute them to two different complexes, despite the fact that these rocks were formed within the same Severnaya riftogenic structure. Geochronological Ar-Ar data (392-387 Ma) for micas from paragneisses and leucogranitic dikes of the Yenisei suture zone, on whose extension the Severnaya riftogenic structure is located, are also given in this study. These data on Devonian tectonic-magmatic events in the South Yenisei Ridge agree well with coeval events of continental rifting - the formation of intrusive and volcanogenic rocks of the Agul graben in the Prisayan region and the Minusa basin in the Altai-Sayan folded area. The formation of alkali trachytes and alkali syenites of the Severnaya riftogenic structure, for which an age of 240±3 Ma has been established, is related to the trap magmatism of the Siberian Platform.

According to U-Pb dating, the granitoids of the Tyrma-Bureya complex in the northern Bureya-Jiamusi superterrane of the Central Asian Fold Belt are not of Paleozoic, as previously thought, but of Mesozoic age (Nizhnyaya Stoiba massif, 218±2 Ma; Talakanka and Ust'-Dikan massifs, 185±1 Ma). They formed at the early stages of collision between the North Asian and Sino-Korean cratons and the intervening Amur superterrane.

The formation environments of stratiform ore deposits in the Neoproterozoic Baikal-Patom region (BPR) have been considered. A model for the formation of the Sukhoi Log gold ore deposit in the Bodaibo zone has been put forward. The first stage is gold concentration by a chemolithotrophic bacterial community. Independently established facts suggest that bacterial communities might also have contributed to initial metal accumulation in the sediments of the Kholodnaya Pb-Zn deposit. The ore beds occur in the high-carbon sediments of the side and trough of the back-arc basin. Sedimentation (Dal'nyaya Taiga and Zhuya regional horizons) took place during the "back-arc basin - foreland basin" transition. This transition is characterized by increased sediment bioproductivity, which is clearly evidenced from the increased biophile-element content and taxonomic diversity of organic remains. Hundreds of microfossil sites in the Neoproterozoic BPR host littoral benthos (cyanobacteria and brown algae) and plankton (green algae). Most microfossils in the outer shelf, on the basin side, and in its trough belong to chemolithotrophic bacteria. These bacteria are assumed to have accumulated metals in the vent field of the back-arc basin. Studies showed the ability of microorganisms (bacteria, algae, fungi, etc.) to accumulate Fe, Mn, Au, Pb, Zn, and other metals. Bacterial communities are particularly important for metal accumulation in the vent fields of rift zones and areas of arc volcanism. All these conditions were observed in the Neoproterozoic BPR.