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Russian Geology and Geophysics

2013 year, number 10

1.
INTERPLAY OF MAGMATISM, SEDIMENTATION, AND COLLISION PROCESSES IN THE SIBERIAN CRATON AND THE FLANKING OROGENS

N.L. Dobretsov, M.M. Buslov, J. De Grave, E.V. Sklyarov
Keywords: Microcontinents, island arcs, tectonics, geodynamics, orogens, sedimentary basins, accretionary-collisional zones, Central Asian orogen, Siberian craton, Kazakhstan–Baikal continent

Abstract >>
The interplay of geodynamic and sedimentation processes in the Central Asian orogen and the Siberian craton is discussed in several aspects: (1) general tectonics of the Central Asian orogen, (2) correlation of deposition and collision events, (3) comparison of deposition history and sediment sources on the northern and eastern margins of the Siberian craton, and (4) history of the Central Asian orogen (Altaids) and formation of Early Mesozoic sedimentary basins. Chemical and isotope compositions and geochronology of Neoproterozoic–Paleozoic sedimentary sequences indicate deposition synchronicity in basins of different types, within both the craton and the orogen. Thus geodynamic models of deposition in separate basins provide reliable evidence of the history of orogens flanking the Siberian craton. The study has confirmed the existence of the Vendian–Early Paleozoic Charysh–Terekta–Ulagan–Sayan–Olkhon strike-slip suture between the continental-margin complexes of Siberia and Kazakhstan, with the crust of juvenile and mixed types, respectively. Late Paleozoic large-scale strike-slip faulting deformed the previous tectonic framework and caused tectonic mixing of the older structures on different margins. This superposed deformation makes it difficult to decipher the paleogeography, paleotectonics, and paleogeodynamics of the Central Asian orogen.
																								



2.
EARLY EVOLUTION OF THE PALEOASIAN OCEAN: LA–ICP-MS DATING OF DETRITAL ZIRCON FROM LATE PRECAMBRIAN SEQUENCES OF THE SOUTHERN Margin OF THE SIBERIAN CRATON

D.P. Gladkochub, A.M. Stanevich, A.M. Mazukabzov, T.V. Donskaya, S.A. Pisarevskii, G. Nicoll, Z.L. Motova, T.A. Kornilova
Keywords: Detrital zircon, sedimentary rocks, Neoproterozoic, Paleoasian ocean, Siberian craton

Abstract >>
We present U–Pb (LA–ICP-MS) data on detrital zircon from the Late Precambrian terrigenous rocks of the Baikal Group and Ushakovka Formation, western Cisbaikalia (southern flank of the Siberian craton). The sources of clastic material for the studied sediments are interpreted. The youngest group of detrital zircon grains from the upper Baikal Group and Ushakovka Formation permits assigning these sediments to the Vendian. The lack of Mesoproterozoic detrital zircon in most of the analyzed samples confirms the hypothesis of a global (~1 Gyr) break in endogenic activity within the southern flank of the Siberian craton through the Precambrian. The abundance of Neoproterozoic zircon in sandstones from the upper horizons of the Baikal Group and the Ushakovka Formation might be due to the shrinkage of the ocean basin as a result of the convergence of the craton with the microcontinents and island arcs within the Paleoasian ocean.
																								



3.
LATE PRECAMBRIAN TERRIGENOUS ROCKS OF THE ANAMAKIT–MUYA ZONE OF THE BAIKAL–MUYA BELT: GEOCHEMISTRY AND RESULTS OF LA–ICP-MS DATING OF DETRITAL ZIRCONS

N.V. Dmitrieva, E.F. Letnikova, M.M. Buslov, A.I. Proshenkin, H. Geng
Keywords: Late Precambrian sedimentary rocks, LA–ICP-MS dating, Nd dates, sedimentation conditions

Abstract >>
A comparative geochemical characteristics of Late Precambrian sedimentary rocks (Ust’-Kelyana and Tuluya rock units) in the Anamakit–Muya zone of the Baikal–Muya belt is given, and the conditions of their sedimentation are considered. The first results of U–Pb (LA–ICP-MS) dating of detrital zircons and Sm–Nd isotope data on the Tuluya unit deposits are presented. Petrogeochemical study showed that the studied sediments are first-cycle rocks similar in composition to terrigenous island-arc sediments. The low contents of Th, Rb, Zr, Hf, and LREE and high contents of Co, Ni, Sc, V, Cr, and Fe2O3* in the sandstones of the Ust’-Kelyana unit evidence that these rocks are similar to oceanic-arc deposits. In contrast, the enrichment of the Tuluya unit rocks in Zr, LREE, Th, Rb, and Nb indicates their similarity to deposits of continental island arcs or active continental margin. Isotope-geochronological studies of the Tuluya rock unit showed the mixing of detrital material resulted from the erosion of Neoproterozoic island-arc igneous rock associations (625–700 Ma), like those in the Karalon–Mamakan zone (Yakor’ and Karalon Formations), and more ancient associations, like the Kelyana (812–824 Ma) and/or Dzhaltuk Groups. Judging from the minimum age of detrital zircon, the lower boundary of sedimentation corresponds to 0.6 Ga.
																								



4.
THE VENDIAN PASSIVE CONTINENTAL MARGIN IN THE SOUTHERN SIBERIAN CRATON: GEOCHEMICAL AND ISOTOPIC (Sr, Sm–Nd) EVIDENCE AND U–Pb DATING OF DETRITAL ZIRCONS BY THE LA-ICP-MS METHOD

E.F. Letnikova, A.B. Kuznetsov, I.A. Vishnevskaya, S.V. Veshcheva, A.I. Proshenkin, H. Geng
Keywords: Vendian–Cambrian, sedimentary sequences, isotope composition, geochemistry, detrital zircons, Siberian Craton

Abstract >>
Geochemical and isotopic (Sm–Nd and Sr) studies of deposits of the Baikal and Oselok Groups in the southern Siberian Craton and LA-ICP-MS U–Pb dating of detrital zircons show that they accumulated in passive continental-margin settings in the Vendian. The time limits of sedimentation were assessed on the basis of Sr chemostratigraphy of carbonate deposits of the Baikal Group and LA-ICP-MS U–Pb dating of detrital zircons in first-cycle terrigenous deposits of the Oselok Group. The main provenances for rocks of these groups were constant. These were rocks of the cover and basement of the Siberian Craton. Tuffite horizons in upper portions of the groups are the only sign of Late Vendian activation of this block, which is reflected in changes of geochemical indices of terrigenous rocks and their younger Sm–Nd model ages.
																								



5.
EARLY CARBONIFEROUS PALEOGEOGRAPHY OF THE NORTHERN VERKHOYANSK PASSIVE MARGIN AS DERIVED FROM U–Pb DATING OF DETRITAL ZIRCONS: ROLE OF EROSION PRODUCTS OF THE CENTRAL ASIAN AND TAIMYR–SEVERNAYA ZEMLYA FOLD BELTS

A.V. Prokopiev, V.B. Ershova, E.L. Miller, A.K. Khudoley
Keywords: Siberian continent, Verkhoyansk passive margin, Carboniferous, detrital zircon, paleogeography

Abstract >>
The first U–Pb dating of detrital zircons from the Lower Carboniferous sandstones in the frontal part of the northern Verkhoyansk fold-and-thrust belt showed that detrital zircon age spectra for the Lower Visean (Krestyakh Formation) and the Upper Visean–Serpukhovian (Tiksi Formation) rocks are quite different. The Early Visean sandstones contain up to 95% detrital zircons of Precambrian age, while those of Late Visean–Serpukhovian age, only 55%. The shape of age distribution plots of Precambrian zircons for both samples is similar, indicating that reworking of terrigenous sediments of the Krestyakh Formation or the same sources dominated in Early Visean time (crystalline basement of the craton, eroded Meso- and Neoproterozoic sedimentary complexes, and igneous rocks of Central Taimyr) contributed significantly to the accumulation of the Late Visean–Serpukhovian deposits. In the rocks of the Tiksi Formation, 45% of detrital zircons are of Paleozoic age, while 24% are Early Paleozoic, with prevailing Cambrian and Ordovician ages. Possible provenance areas with abundant igneous rocks of this age could be the Taimyr–Severnaya Zemlya and Central Asian fold belts extending along the northern, western, or southwestern margins of the Siberia. The presence of Middle–Late Devonian zircons is thought to be related to the erosion of granitoids of the Yenisei Ridge and the Altai–Sayan region. Early Carboniferous detrital zircons probably had a provenance in igneous rocks of the Taimyr–Severnaya Zemlya fold belt, on the assumption that collision between the Kara block and the northern margin of the Siberian continent had already occurred by that time. In Early Visean time, sedimentation occurred in small deltaic fans, likely along steep fault scarps that formed as a result of Middle Paleozoic (Devonian–Carboniferous) rifting. The clastic material came from small rivers that eroded the nearby area. Late Visean–Serpukhovian time was marked by a sharp increase in the amount of clastic material and by the appearance of detrital zircons coming from new provenance regions, such as fold belts extending along the northern and southwestern margins of the Siberian continent. A large river system, which was able to transport clastic material over large distances to deposit it in submarine fans on the northern Verkhoyansk passive continental margin, had already existed by that time.
																								



6.
SYNMETAMORPHIC GRANITOIDS (~490 MA) AS ACCRETION INDICATORS IN THE EVOLUTION OF THE OL’KHON TERRANE ( western Cisbaikalia)

T.V. Donskaya, D.P. Gladkochub, V.S. Fedorovskii, A.M. Mazukabzov, M. Cho, W. Cheong, J. Kim
Keywords: Granites, quartz syenites, U–Pb geochronology, geochemistry, accretion, collision, Early Paleozoic, Ol’khon terrane, Central Asian Fold Belt

Abstract >>
We present geological, structural, and geochemical data on synmetamorphic granitoids from the Tutai and South Ol’khon plutons of the Ol’khon terrane (Central Asian Mobile Belt) with an estimation of the U–Pb zircon age of the Tutai granites. The structural and petrological data suggest the synfolding and synmetamorphic origin of the granitoids. The U–Pb zircon age of the Tutai granites (488.6 ± 8.0 Ma) almost coincides with the previously estimated age of quartz syenites from the South Ol’khon pluton (495 ± 6 Ma). The plutons occupy the same position in the regional structure. The granitoids underwent final deformations and metamorphism at 464 ± 11 Ma. The Tutai pluton consists of moderately potassic granites, whereas the South Ol’khon pluton is made up of quartz syenites and granites. The geochemical characteristics of granites from both plutons (low Y and Yb contents, fractionated REE patterns) indicate their formation under conditions of garnet crystallization in deep crustal restite. The higher Y and Yb contents of the South Ol’khon quartz syenites as compared with those of the granites suggest the lack of equilibrium between the quartz syenite magmas and garnet parageneses during their formation or evolution. The Tutai and South Ol’khon granites were derived from quartz–feldspar crustal rocks, whereas the South Ol’khon quartz syenites might have originated from a mixed (crust–mantle) source. It is presumed that the granitoids formed within accretion-thickened crust. Early accretion, which has been first identified in the region, affected not only the Pribrezhnaya zone (the zone of the Tutai and South Ol’khon plutons) but also the entire Anga–Satyurty megazone of the Ol’khon terrane. The accretion ended with the convergence and oblique collision of the Ol’khon terrane and Siberian continent, when strike-slip tectonics became ubiquitous.
																								



7.
RESTITIC ULTRAMAFIC ROCKS IN THE EARLY CALEDONIAN COLLISIONAL SYSTEM OF WESTERN CISBAIKALIA

A.S. Mekhonoshin, A.G. Vladimirov, V.G. Vladimirov, N.I. Volkova, T.B. Kolotilina, E.I. Mikheev, A.V. Travin, D.S. Yudin, V.V. Khlestov, S.V. Khromykh
Keywords: Collisional tectonic activity, restitic ultramafic rocks, metamorphism, deformations, synkinematic granites, U/Pb and Ar/Ar isotopic dating, thermochronology, Olkhon region, western Cisbaikalia, Russia

Abstract >>
Early Caledonides in the Olkhon region of western Cisbaikalia, being part of the folded framing of the Siberian craton, are a unique geologic object for studying processes of mantle–crust interaction at deep levels of the Earth’s crust. This paper describes restitic ultramafic bodies and boudins spatially confined to faults (blastomylonite sutures), as well as synkinematic granites related to amphibolite facies of metamorphism. Estimates are given for the PT -conditions of metamorphic rocks from the folded framing of the ultramafic bodies, the chemical and mineral compositions of ultramafic rocks, blastomylonites and synkinematic granites, and the results of U–Pb and Ar–Ar isotopic dating. Particular attention is paid to the thermal history of tectonic exposure of the ultramafic bodies as relics of the paleo-oceanic crust in the Early Caledonian collisional system of western Cisbaikalia.
																								



8.
STRUCTURE AND EVOLUTION OF THE EARTH’S CRUST IN THE REGION OF JUNCTION OF THE CENTRAL ASIAN FOLD BELT AND THE SIBERIAN PLATFORM: SKOVORODINO–TOMMOT PROFILE

A.N. Didenko, A.S. Efimov, P.A. Nelyubov, A.S. Sal’nikov, V.S. Starosel’tsev, B.F. Shevchenko, M.V. Goroshko, V.A. Gur’yanov, N.G. Zamozhnyaya
Keywords: Reference depth profiles, deep structure, tectonics, paleoreconstructions, Central Asian Fold Belt, Siberian Platform

Abstract >>
An integrated geological and geophysical study was performed to investigate the region of junction of the eastern part of the Central Asian Fold Belt and the Siberian Platform in the Skovorodino–Tommot 3-DV reference profile line (52°–60° N, 122°–129° E), where the belt is separated from the Aldan–Stanovoi Shield of the platform by a series of deep faults. The main results are as follows: Seismic, density, and geoelectric characteristics of rocks were obtained and used to determine (refine) the intracrustal boundaries of tectonic structures; large-block structure of the Earth’s crust, caused by mantle faults, and the difference between the layered structure of the crust for the shield and fold regions were established; and available paleomagnetic data were used to perform palinspastic reconstructions for 180 and 140 million years, the most productive metallogenic epoch in the region, coeval with collision processes at the closure of the Mongol-Okhotsk paleobasin.
																								



9.
TECTONICS AND GEODYNAMICS OF GORNY ALTAI AND ADJACENT STRUCTURES OF THE ALTAI–SAYAN FOLDED AREA

M.M. Buslov, H. Geng, A.V. Travin, D. Otgonbaatar, A.V. Kulikova, Ming Chen, S. Glorie, N.N. Semakov, E.S. Rubanova, M.A. Abildaeva, E.E. Voitishek, D.A. Trofimova
Keywords: Collision, accretion, subduction, Hercynides, Caledonides, Salairides, strike-slips, overthrust nappe, Central Asian folded area, Siberian continent, Gondwana, Altai–Sayan folded area, Gorny Altai, Paleo-Asian Ocean

Abstract >>
Packages of Late Paleozoic tectonic nappes and associated major NE-trending strike-slip faults are widely developed in the Altai–Sayan folded area. Fragments of early deformational phases are preserved within the Late Paleozoic allochthons and autochthons. Caledonian fold-nappe and strike-slip structures, as well as accompanying metamorphism and granitization in the region, are typical of the EW-trending suture-shear zone separating the composite Kazakhstan–Baikal continent and Siberia. In the Gorny Altai region, the Late Paleozoic nappes envelop the autochthon, which contains a fragment of the Vendian–Cambrian Kuznetsk–Altai island arc with accretionary wedges of the Biya–Katun’ and Kurai zones. The fold-nappe deformations within the latter zones occurred during the Late Cambrian (Salairian) and can thus be considered Salairian orogenic phases. The Salairian fold-nappe structure is stratigraphically overlain by a thick (up to 15 km) well-stratified rock unit of the Anyui–Chuya zone, which is composed of Middle Cambrian–Early Ordovician fore-arc basin rocks unconformably overlain by Ordovician–Early Devonian carbonate-terrigenous passive-margin sequences. These rocks are crosscut by intrusions and overlain by a volcanosedimentary unit of the Devonian active margin. The top of the section is marked by Famennian–Visean molasse deposits onlapping onto Devonian rocks. The molasse deposits accumulated above a major unconformity reflects a major Late Paleozoic phase of folding, which is most pronounced in deformations at the edges of the autochthon, nearby the Kaim, Charysh–Terekta, and Teletskoe–Kurai fault nappe zones. Upper Carboniferous coal-bearing molasse deposits are preserved as tectonic wedges within the Charysh–Terekta and Teletskoe–Kurai fault nappe zones. Detrital zircon ages from Middle Cambrian–Early Ordovician rocks of the Anyui–Chuya fore-arc zone indicate that they were primarily derived from Upper Neoproterozoic–Cambrian igneous rocks of the Kuznetsk–Altai island arc or, to a lesser extent, from an Ordovician–Early Devonian passive margin. A minor age population is represented by Paleoproterozoic grains, which was probably sourced from the Siberian craton. Zircons from the Late Carboniferous molasse deposits have much wider age spectra, ranging from Middle Devonian–Early Carboniferous to Late Ordovician–Early Silurian, Cambrian–Early Ordovician, Mesoproterozoic, Early–Middle Proterozoic, and early Paleoproterozoic. These ages are consistent with the ages of igneous and metamorphic rocks of the composite Kazakhstan–Baikal continent, which includes the Tuva-Mongolian island arc with accreted Gondwanan blocks, and a Caledonian suture-shear zone in the north. Our results suggest that the Altai–Sayan region is represented by a complex aggregate of units of different geodynamic affinity. On the one hand, these are continental margin rocks of western Siberia, containing only remnants of oceanic crust embedded in accretionary structures. On the other hand, they are represented by the Kazakhstan–Baikal continent composed of fragments of Gondwanan continental blocks. In the Early–Middle Paleozoic, they were separated by the Ob’–Zaisan oceanic basin, whose fragments are preserved in the Caledonian suture-shear zone. The movements during the Late Paleozoic occurred along older, reactivated structures and produced the large intracontinental Central Asian orogen, which is interpreted to be a far-field effect of the colliding East European, Siberian, and Kazakhstan–Baikal continents.
																								



10.
VENDIAN–EARLY CAMBRIAN ISLAND-ARC PLAGIOGRANITOID MAGMATISM IN THE ALTAI–SAYAN FOLDED AREA AND IN THE LAKE ZONE OF WESTERN MONGOLIA ( geochronological, geochemical, and isotope data)

S.N. Rudnev, V.P. Kovach, V.A. Ponomarchuk
Keywords: Vendian–Early Cambrian island-arc plagiogranitoid magmatism, geochronology, geochemistry and isotope geochemistry, Central Asian Fold Belt, Altai–Sayan folded area, Lake Zone of western Mongolia

Abstract >>
We generalize results of geological, geochronological, geochemical, and isotope-geochemical studies of the Vendian–Early Cambrian island-arc plagiogranitoid magmatism in the Altai–Sayan folded area and in the Lake Zone of western Mongolia. Based on these data, we analyzed the scales of development of plagiogranitoid magmatism, studied the petrologic composition and isotope characteristics of granitoids, and established the main sources of plagiogranitoid-generating melts and the leading mechanisms of formation of Early Caledonian juvenile crust.
																								



11.
PETROLOGY AND GEOCHEMISTRY OF GABBRO AND PICRITES FROM THE ALTAI COLLISIONAL SYSTEM OF HERCYNIDES: EVIDENCE FOR THE ACTIVITY OF THE TARIM PLUME

S.V. Khromykh, A.G. Vladimirov, A.E. Izokh, A.V. Travin, I.R. Prokop’ev, E. Azimbaev, S.S. Lobanov
Keywords: Gabbro–picrite associations of fold belts, mantle plumes, plume–lithosphere interaction

Abstract >>
We present petrological, isotopic, and geochronological data on gabbro–picrite plutons from the Altai collisional system of Hercynides (eastern Kazakhstan). The geological, geochemical, and geochronological data suggest that these rocks are indicators of the activity of the Tarim plume. The gabbro and picrites formed in two stages (~293 and 280 Ma) in an acid-to-basic succession, explained by a model for the interaction of thermochemical plumes with the lithosphere. Early igneous activity, which gave rise to subalkalic gabbro plutons, reflects the first interaction between the ascending plume and the lithosphere, with low-melting sublithospheric protoliths. Further interaction was characterized by the sublithospheric spread of the plume head with intense heating of the lithospheric base and possible intrusion of deep melts, which resulted in the formation of Cu–Ni–PGE gabbro–picrite plutons in the Altai collisional system and Northwest China.
																								



12.
GEOLOGIC POSITION, AGE, AND PETROGENESIS OF PLAGIOGRANITES IN NORTHERN RUDNY ALTAI

M.L. Kuibida, N.N. Kruk, O.V. Murzin, S.P. Shokal’skii, N.I. Gusev, T.I. Kirnozova, A.V. Travin
Keywords: Collisional magmatism, Carboniferous diorite–tonalite–plagiogranite complexes, geochemical parameters, Rudny Altai

Abstract >>
The geologic position, age, petrologic composition, and petrogenesis of mesoabyssal plagiogranites in northern Rudny Altai, dated earlier to the Early–Middle Devonian, are considered. The Middle Carboniferous (322–318 Ma) age of granitoids has been substantiated by isotope-geochronological data (U–Pb zircon dating and Ar–Ar amphibole and biotite dating). Geologic-structural studies showed that the intrusion of granitoids took place at the time when compression was changed by sinistral faulting. This led to the conclusion that the granitoids formed at the peak of the collision between the Siberian and the Kazakhstan paleocontinents. Geochemical and isotope studies showed that most of the analyzed plagiogranites belong to high-alumina (continental) type and resulted from the deep melting (~15 kbar) of metabasic substrates compositionally similar to N-MORB (judging from results of geochemical modeling and the Nd isotope composition). The presence of plagiogranites of low-alumina (oceanic) type in the postgranite dike series testifies to the melting of the Rudny Altai heterogeneous crust at different depths during its collisional thickening.