N.L. Dobretsov

Opening and Tectonic Evolution of the Paleo-Asian Ocean

The Paleo-Asian ocean is defined by units located between the Russian (East European), Siberian, Tarim, and Sino-Korean (North China) continents. The study of the composition, age, and structural position of island-arc magmatic rocks, ophiolites, and high-pressure meta-morphic assemblages and their mutual correlations made it possible to identify similarities and differences in the evolution of the Paleo-Asian and Paleo-Pacific oceans. The initial stage of the evolution of the Paleo-Asian ocean defined its opening at 900 Ma, whereas opening of the Paleo-Pacific took place at 750 to 700 Ma. Closing of the Paleo-Asian ocean in the Carboniferous (NE branch) and the Permian corresponds to the main stage of reorganization and reopening of the Paleo-Pacific. The maximal opening of the Paleo-Asian ocean occurred after or simultaneously with the first accretion-collision event at 600 to 700 Ma, resulting from the collision of microcontinents and the Siberian continent. Vendian-Early Cambrian boninite-bearing isla...

Meso- and Cenozoic Tectonics of the Central Asian Mountain Belt: Effects of Lithospheric Plate Interaction and Mantle Plumes

This paper reviews and integrates new results on: (1) the Late Paleozoic and Mesozoic evolution of Central Asia; (2) Cenozoic mountain building and intramontane basin formation in the Altay-Sayan area; (3) comparison of the tectonic evolutionary paths of the Altay, Baikal, and Tien Shan regions; (4) Cenozoic tectonics and mantle-plume magmatic activity; and (5) the geodynamics and tectonic evolution of Central Asia as a function of the India-Himalaya collision. It provides a new and more complete scenario for the formation of the Central Asian intracontinental mountain belt, compared with the generally accepted model of the “indentation” of the Indian plate into the Eurasian plate. The new model is based on the hypothesis of a complex interaction of lithospheric plates and mantle-plume magmatism. Compilation and comparison of new and published structural, geomorphological, paleomagnetic, isotopic, fission-track, and plume magmatism data from the Baikal area, the Altay, Mongolia, Tien Shan, Pamir, and Tibe...

Tectonic Setting and Petrology of Ultrahigh-Pressure Metamorphic Rocks in the Maksyutov Complex, Ural Mountains, Russia

The Maksyutov metamorphic complex is the first locality where coesite pseudomorphs in garnet were described. The importance of this discovery was not understood until ultrahigh-pressure (UHP) metamorphism was independently recognized in the Dora Maira Massif of the western Alps and the Western Gneiss Region of Norway. The coesite pseudomorphs are significant because they suggest that the lower unit of the Maksyutov complex probably underwent UHP metamorphism at depths greater than 80 km in a paleosubduction zone. The Maksyutov complex, situated in the southern Ural Mountains of Russia, forms an elongate N-S belt along the boundary between the European and Russian plates. The complex contains two superimposed tectonic unitsa lower eclogite-bearing schist unit that underwent high-pressure (HP) to UHP metamorphism and an upper meta-ophiolite unit subjected to blueschist/greenschist-facies metamorphism. The lower unit lithologies range from quartzofeldspathic, to graphite-rich, to mafic-ultramafic composition...

Mantle Plumes and Their Geologic Manifestations

This paper interprets the conditions of plume formation with a focus on interpretations of origin at the core-mantle boundary. We also discuss important geological manifestations including formation of alkaline-basalt oceanic islands, kimberlite pipes, and plateau-basalt fields. The paper concentrates on the Siberian traps that formed between 249 and 245 Ma, one of the worlds largest known plateau-basalt fields, and on the magmatic zonation and ore mineralization associated with the field and that are interpreted as forming from a Siberian mantle plume. One part of the field consists of undifferentiated low-K tholeiite basalts in the Tunguska syncline and contains only minor associated ore deposits. Major, unique Cu-Ni and PGE deposits are associated with differentiated alkaline basalt and with mafic-ultramafie intrusions in the northwestern Siberian platform. Angara-Ilim-type magnetite deposits occur in the southern part of the plateau-basalt province and are related to interaction of mafic magmas with overlying carbonate and evaporite sedimentary rocks. Permian and Triassic alkaline granitoid and bimodal magmatic rocks occur in fold-and-thrust belts that frame the southern, southwestern, and northwestern margins of the main trap area. The paper also presents new geochemical and geochronological data for A-granites for the Taymyr region that are synchronous with traps. For both the southern and northern framing areas, porphyry Cu deposits interpreted as derived from a mantle-crust system are associated with subalkaline and alkaline granitoids.

Serpentinitic Melanges Associated with HP and UHP Rocks in Central Asia

Serpentinitic mélanges containing eclogites, blueschists, and jadeitic rocks have been studied within the following foldbelts: Borus (West Sayan), Chagan-Uzun (Gorny Altai), Chara (East Kazakhstan), Kokchetav (North Kazakhstan), and Maksyutov (Southern Urals). The West Sayan (Borus belt) and Gorny Altai (Chagan-Uzun belt) are Caledonian foldbelts formed after collisions of paleo-seamounts with primitive island arcs, which resulted in the exhumation of eclogites and jadeitic rocks. The Chara belt of Hercynian age was formed during the Late Carboniferous-Permian collision of the Siberian and Kazakhstan continents and represents a strike-slip zone comprising serpentinitic mélanges and HP units of different ages. In Caledonian time, the mélanges represent an accretionary wedge at the margin of the Kazakhstan continent. The Kokchetav and Maksyutov foldbelts of, respectively, Caledonian and Hercynian ages resulted from collision of microcontinents with island arcs and formation of sheeted complexes incorporating large lenses of UHP-HP rocks and less serpentinitic mélanges. Two types of serpentinitic mélange occur in all suture zones formed as a result of collision of island arcs with seamounts or other island arcs. Type I mélange contains HP rocks in antigorite or olivine-talc-antigorite schists associated with high-temperature mantle peridotites, which can be regarded as part of the hanging wall of a paleosubduction zone. Type II mélange is a part of an obducted oceanic crust and contains blocks and inclusions of LP and LT metamorphic rocks associated with island arc and/or seamount terranes. The suture zones formed by continent/island arc or microcontinent collision (the Kokchetav megamélange and the lower unit of the Maksyutov complex) comprise no large sheets of serpentinitic mélange, but only subordinate lenses of mélanges with HP rocks (the Maksyutov lower unit) or small and sporadic ultramafic lenses inside a tectonic mélange with a metasedimentary matrix.

Comparison of the Kokchetav and Dabie Shan Metamorphic Complexes: Coesite- and Diamond-Bearing Rocks and UHP-HP Accretional-Collisional Events

The Kokchetav and Dabie Shan complexes are typical examples of ultrahigh-pressure metamorphic complexes (UHPM) and are important units of the largest suture zones within the Eurasian continent. The Dabie Shan complex is located in the center of a long Permian-Triassic high-pressure (HP) belt between the Sino-Korean and Yangtze cratons. Other members of this belt are the Sulu region of of NE China, the Imjingang belt in Korea, the Sangun and Marginal Hida belts in Kyushu, the Spassk zone in the Sikhote-Alin of the Russian Far East, and the Bikou, Animaqing, Ailaoshan, and Lancang belts in China bounding the western margin of the Yangtze craton. The Kokchetav complex is located in the center of the largest Early Paleozoic HP belt in Asia, which includes the North Qilian complex, the Kekesu and Atbashi zones of the Tien Shan, and the Aktyuz and Makbal areas in the North Kyrgyz Range. The structure of the Kokchetav complex is interpreted as a mega-melange zone that consists of seven tectonic units separated b...