Home – Home – Jornals – Russian Geology and Geophysics 2011 number 12

This is an overview of papers published in the present volume of Russian Geology and Geophysics (Geologiya i Geofizika), a special issue that covers presentations at the International Conference "Geodynamic Evolution, Tectonics, and Metallogeny of Orogens", held on 28-30 June 2010 in Novosibirsk (http://altay2010.igm.nsc.ru). The workshop concerned the general evolution of the Central Asian orogenic system, with a special focus on continental growth, history of oceans and continental margins, and role of plumes in accretionary-collisional tectonics and metallogeny. The discussed papers are grouped in three sections: 1. General issues of geodynamics and geodynamic evolution; 2. Role of mantle plumes in tectonics, magmatism, and metallogeny; 3. Regional tectonic and geodynamic problems of Asia.
The synthesis of data reported at the workshop demonstrates critical importance of mantle plumes for the evolution of the Paleoasian ocean and for orogenic processes in Central Asia.
In addition to three large pulses of continental growth at about 2900-2700, 1900-1700, and 900-700 Ma, three orogenic stages have been distinguished in the geological history of Eurasia: Late Cambrian-Ordovician (510-470 Ma), Late Devonian-Early Carboniferous (380-320 Ma), and Permian-Triassic (285-230 Ma). In the evolution of the Central Asian orogen, these stages were associated with events of ultramafic-mafic and bimodal plume magmatism which promoted translithospheric strike-slip faulting. Plume magmatism was an active agent in the ocean opening when the Paleotethys, Ural, Ob-Zaisan, and Turkestan basins appeared, while the Late Cambrian-Ordovician orogen was forming in Central Asia (Northern Kazakhstan, Altai-Sayan, Tuva, and Baikal areas). Closure of the Ob-Zaisan ocean and collision of the Kazakhstan-Baikal continent with Siberia in the Late Devonian-Early Carboniferous was coeval with the maximum opening of the Turkestan ocean, possibly, as a consequence of plume activity. The Tarim (285-275 Ma) and Siberian (250-230 Ma) superplume events corresponded in time to the closure of the Ural ocean and opening of the Meso- and Neotethys, as well as to major metallogenic events.

Continental crust is very important for the evolution of life because most bioessential elements are supplied from continents to oceans. In addition, the distribution of continents affects climate because they have much higher albedo than oceans, which is equivalent to that of clouds. Conventional views suggest that continental crust is gradually growing through the geologic time and that most continental crust was formed in the Phanerozoic and Late Proterozoic. However, the thermal evolution of the Earth implies that most of continental crust should be formed in the early Earth. This is "Continental crust paradox".
Continental crust comprises granitoid, accretionary complex, and sedimentary and metamorphic rocks. The latter three components originate from erosion of continental crust because the accretionary and metamorphic complexes consist mainly of clastic materials. Granitoid has two components: a juvenile component through slab-melting and a recycling component by remelting of continental materials. Namely, only the juvenile component contributes to net continental growth. The remains originate from recycling of continental crust. Continental recycling has three components: intracrustal recycling, crustal reworking, and crust-mantle recycling, respectively. The estimate of continental growth is highly varied. Thermal history implied the rapid growth in the early Earth, whereas the present age distribution of continental crusts suggests a slow growth. The former estimate based on the thermal history regards continental recycling as important, whereas the latter estimate based on the present age distribution of continental crusts regards it insignificant, suggesting that the variation of estimate for the continental growth is due to involvement of continental recycling.
We evaluated the erosion rate of continental crust and calculated secular changes of continental formation and destruction to fit four conditions: present distribution of continental crust (no continental recycling), geochronology of zircons (intracontinental recycling), Hf isotope ratios of zircons (crustal reworking), and secular change of mantle temperature. The calculation suggests some important insights. The distribution of continental crust at 2.7 Ga is equivalent to the modern amounts. The distribution of continental crust from 2.7 to 1.6 Ga was much larger than at present, and the sizes of the total continental crust at 2.4, 1.7, and 0.8 Ga became maximum. The distribution of continental crust has been decreasing since then. More amounts of continental crust were formed at higher mantle temperatures at 2.7, 1.9, and 0.9 Ga, and more amounts were destructed after then. As a result, the mantle overturns led to both the abrupt continental formation and destruction, and extinguished older continental crust. The timing of the large distribution of continental crust apparently corresponds to the timing of "icehouse" periods in Precambrian.

Variations in frequency of geomagnetic reversals through the Phanerozoic have been analyzed jointly with ⁸⁷Sr/⁸⁶Sr ratios in marine carbonate sediments. The time series of both parameters contain principal components with periods from 90 to 110 Ma and show a certain correlation. Namely, (1) both time series have five local minima spaced at similar intervals (period lengths); (2) the minima in the Δ⁸⁷Sr/⁸⁶Sr curve follow in time those of the reversal frequency, with a lag from 12 Myr in the Ordovician to 38 Myr in the Cretaceous; (3) the rate of heat transfer from processes at the core-mantle boundary (in D´ ´ layer) which control the Earth's geological life was from 7 to 25 cm/yr in the Phanerozoic. This rate approaches the observed velocities of horizontal plate motion and the predicted mantle convection rates.

There were two key stages in the history of Paleozoids that formed in the place of the Paleoasian ocean, one in the Cambrian-Ordovician and the other in the Permian-Triassic. Both time spans were characterized by a combination of similar geodynamic, magmatic, and geomagnetic events: closure and opening of oceanic basins, intense plume magmatism associated with Earth's core cooling, and absence of geomagnetic reversals (superchrons). Three superchrons about 490-460, 260-300, and 124-86 Ma correlate with major events of plume magmatism. Plume reconstructions have to be updated for the period 490-460 Ma, which corresponded to the third superchron and was marked by ocean opening. The previous superplume, about 800-740 Ma, requires further justification but fits the global periodicity with 240 Ma major cycles and smaller ones of 120 (or also 30) Ma.
In the Late Cambrian-Ordovician, large-scale accretion and collision events acted, in similar tectonic settings, upon the vast territory that currently extends from the Polar Urals to Lake Baikal (and was times larger in the past). As a result, Gondwanian microcontinents (Kokchetav, Altai-Mongolian, Tuva-Mongolian, etc.) and island arcs joined into the Kazakhstan-Tuva-Mongolian system. The formation of the Late Cambrian-Ordovician orogen in Central Asia was synchronous with the opening of the Ural, Ob-Zaisan, Turkestan, and Paleotethys oceans. The plume pulses (520-500 and 490-460 Ma) might have been responsible for the opening of new oceans, accelerated amalgamation of terranes, and synchronicity in geodynamic events from the Urals to Transbaikalia.

Several complexes are recognized within the Sora porphyry Cu-Mo deposit: plutonogenic, porphyry (ore-bearing), and dike. They formed since Ordovician till Devonian at the collision, postcollisional, and rift stages of the regional evolution, respectively. Magmatism was manifested at the deposit synchronously with intraplate magmatism, which was widespread within Kuznetsk Alatau and was initiated by the Altai-Sayan mantle plume. In structural position and geochemical characteristics the dike complex is similar to the intraplate complexes in adjacent regions. It formed after the development of the Sora ore-magmatic system including the plutonogenic and porphyry complexes with similar geochemistry and metallogeny.
According to the models for the relationship of mantle plumes with ore-magmatic systems, the plutonogenic and porphyry complexes of the Sora deposit developed at the stage of the thermal plume effect on lithosphere, which caused its melting and, as a result, calc-alkalic magmatism. A change of the collision and postcollisional geodynamic regime by the rift one favored the ascent of plume melts, which then participated in the formation of intraplate structures, in particular, the dike complex of the Sora deposit.

The Koshrabad massif, referred to as the Hercynian postcollisional intrusions of the Tien Shan, is composed of two rock series: (1) mafic and quartz monzonites and (2) granites of the main phase. Porphyritic granitoids of the main phase contain ovoids of alkali feldspar, often rimmed with plagioclase. Mafic rocks developed locally in the massif core resulted from the injections of mafic magma into the still unconsolidated rocks of the main phase, which produced hybrid rocks and various dike series. All rocks of the massif are characterized by high f (Fe/(Fe + Mg)) values and contain fayalite, which points to the reducing conditions of their formation. Mafic rocks are the product of fractional crystallization of alkali-basaltic mantle melt, and granitoids of the main phase show signs of crustal-substance contamination. In high f values and HFSE contents the massif rocks are similar to A -type granites. Data on the geochemical evolution of the massif rocks confirm the genetic relationship of the massif gold deposits with magmatic processes and suggest the accumulation of gold in residual acid melts and the rapid formation of ore quartz veins in the same structures that controlled the intrusion of late dikes. The simultaneous intrusion of compositionally different postcollisional granitoids of the North Nuratau Ridge, including the Koshrabad granitoids, is due to the synchronous melting of different crustal protoliths in the zone of transcrustal shear, which was caused by the ascent of the hot asthenospheric matter in the dilatation setting. The resulting circulation of fluids led to the mobilization of ore elements from the crustal rocks and their accumulation in commercial concentrations.

The Mesozoic Chuya dike complex was recognized by R.V. Obolenskaya based on the similar mineral composition of dikes and their age characteristics. Lamprophyres occur along the large Terekta-Tolbonur and Kurai-Kobda shear zones. The Chuya complex was studied by the example of two areas, South Chuya and Yustyd, with different levels of erosional truncation. The dikes of the first area are localized in the South Chuya Ridge, where they cut Cambrian-Ordovician metamorphic rocks, and the dikes of the Yustyd area occur in the Devonian terrigenous blackshale deposits of the Yustyd trough. The dikes of these areas differ in structures, textures, the degree of carbonatization, and mineral composition. The performed studies of rocks and minerals confirmed that the dikes of both areas belong to the same complex. They helped to establish the regularities of the lithologic composition of the entire complex and its local areas and to substantiate the recognition of areas not only from their geologic position but also by the composition, structures, and textures of rocks and the mineral composition. Geochronological data show two stages of the complex formation: 236-234 and 250-242 Ma. The results of studies also demonstrate that the lamprophyres and coeval syenites of the Tarkhata massif are fractionates of the same parental melt and can be united into a hypabyssal-plutonic complex. Comparison with other Permo-Triassic lamprophyre complexes showed that the wide variations of the composition of the Chuya rocks and its trend as well as the geochemical anomalies are specific features of complexes of high-K lamprophyres.

Magnetotelluric soundings (MTS) in the Kyrgyz Tien Shan along 74? and 76?E profiles reveal conductors in the crust which delineate the boundaries of the At-Bashi accretionary-collisional zone and the Issyk-Kul microcontinent. Correlated to earthquake converted-wave patterns ( v _P) along the MANAS profile collected in 2007, the geoelectric model for the At-Bashi zone lends support to the hypothesis that the position and dip of large thrust sheets, as well as the way and direction of exhumation of eclogites in this zone, are similar to those in northwestern China. Petrological analysis, geothermobarometry, and elastic P -wave velocities measured in laboratory on lower-crust and upper-mantle xenoliths indicate that at the time when the xenoliths were dragged to the surface at ~70 Ma, the Moho was 20 km shallower than now (35 km against 55 km) and the heat flux was 20 mW/m² higher (80 against 60 mW/m²).

On the basis of stratigraphical and geological data, paleogeographical and palinspastic reconstructions of the Kazakhstan Paleozoides were made, their multistage geodynamic evolution was considered, and their tectonic zonation was substantiated. The main stages are described: the initiation of the Cambrian and Ordovician island arcs; the development of the Kazakhstan accretionary-collisional composite continent in the Late Ordovician as a result of continental subduction and the amalgamation of Gondwana blocks with the island arcs (a long granitoid collisional belt also formed in this period); the development of the Devonian and Carboniferous-Permian active margins of the composite continent; and its tectonic destruction in the Late Paleozoic.
In the Late Ordovician, compensated terrigenous and volcanosedimentary complexes formed within Kazakhstania and developed in the Silurian. The Sakmarian, Tagil, Eastern Urals, and Stepnyak volcanic arcs formed at the boundaries with the Ural, Turkestan, and Junggar-Balkhash Oceans. In the late Silurian, Kazakhstania collided with the island arcs of the Turkestan and Ob'-Zaisan Oceans, with the formation of molasses and granite belts in the northern Tien Shan and Chingiz. This was followed by the development of the Devonian and Carboniferous-Permian active margins of the composite continent and the inland formation of the Early Devonian rift-related volcanosedimentary rocks, Middle-Late Devonian volcanic molasse, Late Devonian-Early Carboniferous rift-related volcanosedimentary rocks, terrigenous-carbonate shelf sediments, and carbonaceous lake-bog sediments, and Middle-Late Carboniferous clastic rocks of closed basins. In the Permian, plume magmatism took place on the southern margin of the Kazakhstan composite continent. It was simultaneous with the formation of red-colored molasse and the tectonic destruction of the Kazakhstan Paleozoides as a result of a collision between the East European and Kazakhstan-Baikal continents.

The Chinese Altay, a key portion of the Central Asian Orogenic Belt (CAOB), is dominated by variably deformed and metamorphosed sedimentary rocks, volcanic rocks, and granitic intrusions. Its Early Paleozoic tectonic setting has been variously considered as a passive continental margin, a subduction-accretion complex, or a Precambrian microcontinent, and two representative competing tectonic models have been proposed, i.e., open-closure versus subduction-accretion. Recent studies demonstrate that the high-grade metamorphic rocks previously considered as fragments of a Precambrian basement have U-Pb zircon ages (predominantly 528 to 466 Ma) similar to those of the widely distributed low-grade metasedimentary rocks named as Habahe Group in the region, and all these metasedimentary rocks were predominantly deposited in the Early Paleozoic. Petrological evidence and geochemical compositions suggest that these metasedimentary rocks were probably deposited on an active, not a passive continental margin as previously proposed. The detrital zircons of sediments and igneous zircons from granitoids including the inherited ones (mainly 543-421 Ma) mostly give positive ε _Hf ( t ) values, suggesting significant contributions from mantle-derived juvenile materials to the lower crust. A modeling calculation based on zircon Hf isotopic compositions suggests that as much as 84% of the Chinese Altay is possibly made up of "juvenile" Paleozoic materials. Thus, available data do not support the existence of a Precambrian basement but rather indicate that the Chinese Altay is a huge subduction-accretion complex in the Paleozoic. The U-Pb zircon dating results for granitoids indicate that magmatism was active continuously from Early to Middle Paleozoic, and the strongest magmatic activity took place in the Devonian, coeval with a significant change in zircon Hf isotope compositions. These findings, together with the occurrence of chemically distinctive igneous rocks and the high-temperature metamorphism, can be collectively accounted for by ridge-trench interaction during the accretionary orogenic process.

Granites from the Tunka pluton of the Sarkhoi complex located in the eastern Tunka bald mountains (East Sayan) have been dated at the Middle Ordovician (462.6±7.8 Ma) by LA ICP MS.
The granites of the Sarkhoi complex within the studied area cut a fold-thrust structure consisting of deformed fragments of the Vendian (Ediacaran)-Early Cambrian cover of the Tuva-Mongolian microcontinent (Upper Shumak metaterrigenous formation and Gorlyk carbonate formation). The red-colored conglomerates and sandstones of the Late Devonian-Early Carboniferous(?) Sagan-Sair Formation overlie the eroded surface of the Tunka pluton granites in the eastern Tunka bald mountains. The Sagan-Sair Formation, in turn, is overlain along a low-angle thrust by a group of tectonic sheets, which comprises the volcanic and carbonate sediments of the Tolta Formation, biotitic schists, and plagiogneisses with garnet amphibolite bodies. Two nappe generations have been revealed on the basis of the described geologic relationships, the Middle Ordovician age of the Tunka pluton granites, and numerous Late Paleozoic Ar-Ar dates of syntectonic minerals from the metamorphic rocks in the area. The first thrusting stage was pre-Middle Ordovician, and the second, Late Carboniferous-Permian. The Lower Paleozoic thrust structure resulted from the accretion of the Tuva-Mongolian microcontinent to the Siberian Platform. The Late Paleozoic nappes resulted from intracontinental orogeny and the reactivation of an Early Paleozoic accretionary belt under the effect of the Late Paleozoic collisional events.

According to the new geological, geochronological, and structural data, the Tunka bald mountains (East Sayan) have a nappe structure, which formed in the Late Carboniferous-Early Permian. The deformations have been dated by the ⁴⁰Ar-³⁹Ar method on the basis of syntectonic micas and amphiboles, whose structural and spatial positions have been determined in oriented thin sections. The geometric analysis of macro- and microtextures has revealed three development stages of the deformation structures, which followed one another in progressive deformation. The first (thrust-fault) stage (316-310 Ma) comprised a group of north-verging thrust sheets. At the second (fold deformation) stage (305-303 Ma), they were folded. The third (strike-slip fault) stage (286 Ma) comprised high-angle shears, along which V-shaped blocks were squeezed westward from the most compressed areas. All the structures developed under N-S-trending compression. The thrusting in the Tunka bald mountains was coeval with the major shear structures in the eastern Central Asian Fold Belt (Main Sayan Fault, Kurai, Northeastern, and Irtysh crumpled zones, etc.). Also, it was simultaneous with the formation of continent-marginal calc-alkalic and shoshonite series (305-278 Ma) as well as that of the alkali and alkali-feldspar syenites and granites (281-278 Ma) of the Tarim mantle plume in the Angara-Vitim pluton located near and east of the studied region. Thus, the simultaneous development of the Late Paleozoic structures, active-margin structures, and plume magmatism in southern Siberia might have resulted from the global geodynamic events caused by the interaction between the tectonic plates which formed the Central Asian Fold Belt.

We have established that the terrigenous deposits of the Haisuin Formation and metamorphic deposits of the Shutkhulai block are similar in geochemical characteristics to the rocks of the Oka Group. The volcanics of the Sarkhoi Group and, to a lesser extent, the crystalline deposits of the Gargan block and rocks of the Dunjugur ophiolite complex served as sourcelands for the studied deposits. The terrigenous deposits of the Oka Group and Haisuin Formation and the pararocks of the Shutkhulai block accumulated in the same sedimentary basin localized on the margin of the Tuva-Mongolian massif in the setting of an island-arc system.

The tectonic structure of the junction of the eastern Central Asian Fold Belt and the Siberian Platform, along with the deep structure of the Earth's crust and lithosphere in this region, has been described on the basis of new-generation geological and geophysical data (seismic, geoelectric, and space-structural studies as well as new-generation geological maps), combined with new interpretation techniques (processing of the previous data by special software). The data suggest the existence of oblique collision during the convergence of the tectonic plates and, correspondingly, tectonic units composing these plates, when the Mongol-Okhotsk paleobasin closed. Such a scenario within the Aldan-Stanovoi Shield is evidenced by areas of syn- and postcollisional magmatism, with their deep-level and geochemical characteristics, and by the presence of a Late Mesozoic fold-thrust zone. Deep "traces" of these tectonomagmatic events, detected in the course of geological and geophysical modeling, are manifested as inclined deep boundaries between the crustal and lithospheric blocks. On the Earth's surface they correspond to large fault systems: Dzheltulak, North and South Tukuringra, Gilyui, and Stanovoi. It has been found that the influence of collision decreases northward with distance from the junction of the eastern Central Asian Fold Belt and the Siberian Platform (Dzheltulak and North Tukuringra transcrustal faults).