RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 7, ES5003, doi:10.2205/2005ES000188, 2005

Discussion

[40]  This study proved that the Polar Ural region exposes the ophiolite fragments dated about 670 million years. Some ophiolites mapped in the largest ophiolite allochthons were dated Cambrian (about 490 Ma) or somewhat older. The results of our study and the geochemical data obtained by Dushin [1989, 1997] and by Scarrow et al. [2001] suggest that the oldest structural features of the Polar Ural region contain the relics of encialic volcanic arcs and their slopes, as well as those of the bottoms of marginal sea basins. The presence of the volcanic rocks of different compositions, including adakite and boninite, suggests that these rocks are not purely supersubduction products but are also the products of some atypical subduction which had been accompanied by the collision of the spreading ridges with the arc, by the sealing of the subduction zone, and by the origin of asthenospheric windows. The paleomagnetic data obtained by A. N. Didenko and the authors of this paper [Didenko et al., 2001] suggest that the Enganepe volcanic arc was located at that time not far from the edge of the Baltic plate and might have been connected with the Kadoma volcanic arc.

[41]  Our isotopic and geochronological data suggest that the metamorphic volcano-sedimentary rocks of the Kharbei Block had accumulated in pre-Vendian time. The fragments of the rocks of the ophiolitic suite occur as olistoliths in the olistostrome layers and seem to be even older. The rocks of the Kharbei High transformed to metamorphic rocks in the time interval of 460 pm 10 million years, which corresponds to the Middle Ordovician. That time witnessed the widespread Finmark or Early Caledonian orogeny in the Scandinavian caledonides, the obduction of the ophiolites, granite formation, and metamorphism. This time was extremely characteristic of the Central Asian Belt as a whole. That was the time of the ophiolite obduction, granite formation, and metamorphism. This time was extremely typical of the Central Asian Belt as a whole. It witnessed the ophiolite obduction and the mass formation of granite gneiss domes in West Mongolia, in the Baikal, Altai-Sayan, Sangilen, and other regions. The data obtained in this study confirm the existence of the Vendian and Late Riphean ophiolites, as well as of the rocks of the volcanic arcs and marginal basins, associated with them, in the Polar Ural region. It appears that the structural features of the active continental margin were surrounded in Late Riphean and Vendian time by the oceanic basin which might connect the paleooceanic formations of the Paleoatlantic ocean (Yapetus) and those of the Paleoasian ocean. The age of the metamorphism of the metamorphic rock complexes in the Polar Ural area suggests that part of the nappe structure of this region had been formed as early as the pre-Middle Ordovician time, and that the Cadomian and Early Caledonian, as well as the Late Caledonian (Hercynian) are brought to contact one another.

[42]  The schematic tectonic map of the region (see Figure 1) shows that the pre-Devonian rock complex does not include the rocks emplaced during the formation of the mature continental crust, except for some rock complexes of the Kharbei Block; this contradicts the view of the Early Ordovician destruction of the older continental basement and justifies the model of the continuous development of the paleooceanic region.

[43]  The ophiolites dated Cambrian-Early Ordovician, similar to the Voikar-Synya ophiolites, were found in the Scandinavian Caledonides (in the areas of the Karmoy, Bomlo, and Stord islands and in the areas of the Bergen, Solund, Stavfegen, and Scalvaier island arcs) and in the Appalachian Mountains (the Humber Arm and Hei Bei allochthons of the Newfoundland Island), that is in the structural features of the Paleoatlantic (Yapetus) Ocean. It should be noted that the episodes of collision, obduction, and granite formation in the listed regions coincide in many respects with the epochs reconstructed for the eastern part of the modern Siberian margin of the Paleoasian Ocean [Khain, 1989; Khain et al., 2003]. On the other hand, some Cambrian-Early Ordovician ophiolites are known in the structural features of the western (in modern coordinates) Ural-Kazakhstan margin of the Paleoasian Ocean, namely, in the North Kazakhstan and North Tien Shan areas, as the Maikain-Kyzyltass and Kirgiz-Terskei ophiolite zones. All of these data suggest one scenario for the development of the Paleoasian Ocean, including the Ural-Kazakhstan margin and the margin of the Paleoatlantic Ocean.

[44]  The evolution history of the Paleoasian ocean shows some differences in the evolution of its western and eastern segments (in modern coordinates). Its western (Tarim-Kazakhstan) margin remained passive throughout the Neoproterozoic time, some rifting environments having been reconstructed for it, too. Active continental margins began to develop in the eastern part of the ocean (present day reference frame) as far back as the beginning of the Neoproterozoic. This is proved by the fact that the Siberian continent and the central Mongolian microcontinent are surrounded by the Late Riphean-Vendian ophiolite belts. The new data suggest the ophiolite rock complexes dated 1000, 830, 700-670, and 570 million years, the fragments of which occur now as allochthons or are exposed from under the younger sedimentary cover [Khain et al., 2002, 2003; Pfander et al., 2002]. Associated with the ophiolite complexes are the island-arc volcanics and the sedimentary rocks of back-arc basins. The existence of this Circum-Siberia Belt proves that the Siberian Continent was separated from the other continents by some oceanic space or by a strait, the Paleoural Ocean might have acting as this potential strait. The Paleoasian and Paleoatlantic oceans (Yapetus) might have been connected in Neoproterozoic time by the Polar Ural suture.

[45]  The important period of the ocean formation was the time interval of 650-510 million years. This period of time was characterized by the high complication of the structure of ocean margins. This time witnessed the dying off of the large system of volcanic arcs, the closure of the sea basins associated with them, and the accretion of the resulting segments to the edges of the continents and microcontinents, and the obduction of the early ophiolites. All of these events took place at the background of the generation of new subduction zones and the opening of new marginal basins. Two main periods of this activity were dated 590-570 Ma and 530-540 Ma. As the result of these processes the Paleoasian ocean was transformed at the mid-Cambrian to the intricate system of basins with the oceanic crust, island-arc systems, and microcontinents with a terrigenous-carbonate rock cover. At that time the structure of both parts of the paleoocean was similar to the modern situation east and north of Australia.

[46]  The period of time from the end of the Cambrian to the beginning of the Ordovician was marked by the opposite processes in the development of the different segments of the Paleoasian Ocean.

[47]  During the Late Cambrian and Ordovician the western part of the Paleoatlantic ocean was represented by the active margin of the West Pacific ocean, where the opening of basins with oceanic crust took place, and systems of volcanic arcs were formed. In the east that time was marked by the collision of the island arcs and microcontinents and by the closure of the interarc basins. These processes resulted in the origin of two types of regions: amagmatic subduction-accretion regions with flyschoid sedimentation (Gornyi and Mongolian Altai and West Sayan) and collision-obduction regions (Tuva, the Eastern Sayan region, West and North Mongolia, and the Baikal region), distinguished by the maximum concentration of microcontinents. Throughout the Late Cambrian to Early Ordovician time, these regions experienced the collisions of their median ridges and volcanic arcs with the microcontinents and continents, which were accompanied by the obduction of their ophiolites, by the intensive crustal and crustal-mantle magmatism, high-temperature metamorphism, and shear deformation.

[48]  The new data obtained recently in the Polar Ural region confirm the existence of the long-lived Paleoasian ocean which had existed there at least from the time of 1100 million years. Throughout this long history its western Ural-Kazakhstan and eastern Siberian-Mongolian margins (present day reference frame) developed in different ways, yet having some similar periods in their geologic histories. In the light of the new data, the Ural Mountain Belt seems to be a heterogeneous building. Its southern and intermediate segments evolved following the western, Kazakhstan, evolution version, whereas the evolution history of its Polar Ural segment was similar to that of the Mongolia-Sayan-Yenisei segment of the Asiatic Belt.


RJES

Citation: Khain, E. V., A. A. Fedotova, E. V. Bibikova, E. B. Salnikova, A. B. Kotov, K.-P. Burgat, V. P. Kovach, and D. N. Remizov (2005), The Neoproterozoic and Early Paleozoic geological history of the Ural-Kazakhstan margin of the Paleoasian Ocean using new isotopic and geochronological data obtained for the Polar Ural region, Russ. J. Earth Sci., 7, ES5003, doi:10.2205/2005ES000188.

Copyright 2005 by the Russian Journal of Earth Sciences

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