Mongol Okhotsk Fold Belt Research Articles

FIRST LA-ICP-MS APATITE FISSION-TRACK AGES FROM THE SIBERIAN PLATFORM BASEMENT (NEPSKO-BOTUOBINSKAYA ANTECLISE)

We present the first results of LA-ICP-MS apatite fission-track dating (AFT) from nine core samples from wells, that exposed the top of the crystalline basement of the Siberian platform within the Nepsko-Botuobinskaya anteclise on the ~2 km depth. Obtained AFT ages belong to three clusters with mean values of 200, 140, and 60 Ma. A thermal event of ~200 Ma is widespread throughout almost the entire Siberian platform and reflects the stage of its Early Jurassic uplift. Reset of the fission-track geochronometer in apatite ~140 Ma correlates with tectono-thermal event marking the final stage of the Mongol-Okhotsk fold belt collision. The youngest AFT ages of ~60 Ma, on the one hand, may reflect the early (crypto) stage of the Baikal rifting, and, on the other hand, be a consequence of the high content of uranium in apatite.

Age and Geodynamic Setting of the Lower Jurassic Sandstones in the Onon Fragment of the Aga Terrane, Mongol–Okhotsk Fold Belt

Age and Geodynamic Setting of the Lower Jurassic Sandstones in the Onon Fragment of the Aga Terrane, Mongol–Okhotsk Fold Belt

The Age of Gold Mineralization of the Elga Deposit, Mongol–Okhotsk Fold Belt: 40Ar/39Ar Geochronological Constraints

The Age of Gold Mineralization of the Elga Deposit, Mongol–Okhotsk Fold Belt: 40Ar/39Ar Geochronological Constraints

The Age and Sources of Protoliths of Metasedimentary Rocks in the Eastern Tukuringra Terrane of the Mongol–Okhotsk Fold Belt: Results of U–Th–Pb, Lu–Hf, and Sm–Nd Isotope Studies

The Age and Sources of Protoliths of Metasedimentary Rocks in the Eastern Tukuringra Terrane of the Mongol–Okhotsk Fold Belt: Results of U–Th–Pb, Lu–Hf, and Sm–Nd Isotope Studies

Tectonic Nature of the Ul’ban Terrane (Mongol–Okhotsk Fold Belt): Results of U–Pb and Lu–Hf Isotope Studies of Detrital Zircons

The U–Pb and Lu–Hf isotope studies of detrital zircons from Mesozoic metasedimentary rocks of the Sorukan and Naldinda formations of the Ul’ban terrane are performed for the first time. It is shown that Early Precambrian zircons are almost completely absent in these rocks, while Early Mesozoic zircons with Paleozoic, Neoproterozoic, and Mesoproterozoic Hf model ages dominate. Based on the data obtained, it is assumed that the Ul’ban terrane is a part of the accretionary complex of the northern framing of the Amur superterrane and is neither a depression superimposed on the Paleozoic formations of the Mongol–Okhotsk belt nor an accretionary complex in the southern framing of the North Asian Craton, as was believed earlier.

Ages and Sources of Sedimentary Rocks of the Lan Terrane in the Mongol-Okhotsk Fold Belt: Results of Zircon U-Pb and Lu-Hf Isotope Studies

The paper presents the results of U-Pb and Lu-Hf isotope studies of detrital zircons from the terrigenous the Lan 410 Ma (Lower Devonian), Terrane in the eastern part of the Mongol-Okhotsk fold belt. The lower age rocks limits are for the Nivakh formation ~ for the Verkholan series ~332 Ma (Mississippian), for the Ugokhan series ~239 Ma (Middle Triassic). The evidences were obtained for the Lower Triassic (or Middle-Late Triassic) age of the sediments assigned to the Ugokhan series previously classified as Upper Paleozoic. These data indicate that the Lower Mesozoic sedimentary complexes in the structure of this belt are developed much wider than is currently believed. The main sources of clastic material for the Devonian, Carboniferous, and Triassic sedimentary rocks of the Lan terrane were mainly igneous and metamorphic complexes of the North Asian craton framing, along with island arcs. The Lan Terrane can be considered as a fragment of the accretionary complex formed in front of the southeastern margin of the North Asian craton.

Triassic Stage in the Evolution of Mesozoic Sedimentogenesis of the Mongol–Okhotsk Fold Belt

Triassic Stage in the Evolution of Mesozoic Sedimentogenesis of the Mongol–Okhotsk Fold Belt

Age and Sources of Dzhagdy Terrane Metasedimentary Rocks in the Mongol–Okhotsk Fold Belt: Detrital Zircon U–Pb and Lu–Hf Isotopic Data

Ditrital zircon U-Pb and Lu-Hf isotopic data are reported for metaterrigenous deposits of Dzhagdy Terrane in the Mongol–Okhotsk Fold Belt. The youngest zircon populations in metasedimentary rocks of the Dzheskogon and Nekter formations is concluded to be of the Middle–Late Triassic and Early Jurassic Age, suggesting that the formations are Early Mesozoic and not Carboniferous, as was previously assumed. We have made the assumption that the Dzheskogon, Nekter, and Bochagor formations of the Dzhagdy Terrane are not a single sedimentary sequence, but a set of tectonic slices consisting of Late Paleozoic and Early Mesozoic rocks of differing genesis. This raises the question on whether the terrane under consideration is a fragment of an accretionary prism. Detrital zircon U-Pb geochronological and Lu-Hf isotopic data coupled with whole-rock Sm-Nd isotopic data from previous studies indicate that the sedimentary basin was mainly sourced from the continental Amur Superterrane (from the south in present-day coordinates). Material replenishment from the southern framing of the North Asian Craton (from the north in present-day coordinates) was either minimal or absent.

40Ar/39Ar Age of Gold Ore Metasomatites of the Albyn Deposit, Mongol–Okhotsk Fold Belt

The paper reports the results of 40Ar/39Ar geochronological studies of the muscovite–quartz–albite metasomatites of the ore bodies and host rocks of the Albyn deposit located in the Selemdzha–Kerbi structural zone of the eastern part of the Mongol–Okhotsk Fold Belt. To a first approximation, the age of the hydrothermal ore process responsible for the formation of the Albyn deposit can be estimated within an interval of 131–130 Ma. At the same time, we cannot exclude the existence of an earlier stage of this process at ~135 Ma, although this assumption requires further substantiation. A close age estimate (~134–130 Ma) was obtained earlier for the ore metasomatites of the Malomyr deposit located in the same structural zone. There are no data on the magmatic manifestations of close age within the considered region, which makes it impossible to relate the ore mineralization of the Albyn deposit with magmatic processes. At the same time, the age of the thermal event superimposed on the host rocks of the Afanasiev Formation is 131 ± 3 Ma, which likely indicates the regional nature of this event. We suggest that a significant role in the mobilization, redistribution of ore matter, and formation of the Albyn deposit was played by large-scale deformations accompanied by hydrothermal activity.

Comparative Geochemical Characteristics and Genesis of Large Polyphase Plutons in the Cores and on the Periphery of the Mongol–Okhotsk Fold Belt Magmatic Areas of Different Ages

The early Mesozoic Baga-Khentei pluton is a part of the Daurian–Khentei batholite that was formed under the impact of the Mongolian plume on the lower horizons of the continental crust in the collision compression zone that appeared during closing of the Mongol–Okhotsk Ocean. Mantle and crustal melt sources were involved in the formation of the batholith and its margins. The Ikh-Narotin-Khid massif tends to the boundary of the rift region on the periphery of the Late Mesozoic area. The petro-geochemical similarity of its rocks to the composition of host gneisses suggests that the latter belong to the most likely substrate in the formation of palingenic granitoids of the calc-alkaline series. The differences in the rock compositions of large plutons are shown in minor differentiation of the Baga-Khentei massif, which is likely to be determined by the anatexis conditions and the subsequent formation of melts in the collision compression setting, while the granites of the Ikh-Narotin-Khid massif were formed under extension, which favored deep differentiation of magmatic melts.

Age and Sources of Metasedimentary Rocks of the Tokur Terrane of the Mongol–Okhotsk Fold Belt: Results of U–Pb and Lu–Hf Isotope Studies

This paper presents the results of U–Pb (LA-ICP-MS) and Lu–Hf isotope studies of detrital zircons from the presumably Permian meta sedimentary rocks of the Tokur terrane. It has been established that the grains of detrital zircons from the meta sedimentary rocks of the Tokur and Ekimchan formations have a similar age distribution, which indicates the similar (or equal) age of these formations. The youngest zircons in these rocks has a U–Pb age of 326–323 Ma, which determines the lower age limit of the protolith of the rocks. The upper age limit of the formations corresponds to 254–251 Ma, which is the age of the Late Permian granitoids intruded into these formations. The main sources of zircon in the meta sedimentary rocks of the Tokur terrane are the igneous and metamorphic massifs of the southeast framing of the North Asian Craton. The Tokur terrane can be considered as a fragment of the Paleozoic accretionary complex, formed near the southeastern margin of the North Asian Craton.

Age and Sources of Detrital Zircons from Jurassic Conglomerates of Strelka Depression (Northern Framing of the Mongol–Okhotsk Fold Belt)

This paper presents the results of U–Th–Pb geochronological study of detrital zircons from cement of the Jurassic conglomerates of the Strelka Depression extending in the sublatitudinal direction along the border between the southern margin of the Selenga-Stanovoy superterrane and the Mongol–Okhotsk fold belt. It has been shown that Paleoproterozoic and Neoarchean zircons overwhelmingly dominate in the conglomerate cement. This indicates that the main input of the material to the sedimentation basin was carried from the Southeast border of the North Asian craton (from the north in modern coordinates). In our opinion, the Strelka Depression formed after the completion of orogenic processes associated with the formation of the Mongol–Okhotsk structure.

40Ar/39Ar Age of Gold Mineralization of the Albyn Deposit (Eastern Part of the Mongol–Okhotsk Fold Belt)

40Ar/39Ar Age of Gold Mineralization of the Albyn Deposit (Eastern Part of the Mongol–Okhotsk Fold Belt)

Geochemical Features of Early Mesozoic Metabasalts of the Western Part of the Tukuringra Terrane, Mongol–Okhotsk fold belt

The paper presents the results of geochemical and Sm–Nd isotope study of the metavolcanic rocks of the western part of the Tukuringra Terrane, one of the largest terranes of the Mongol–Okhotsk fold belt. It is shown that among the metavolcanics of the Teplokluchevskaya, Garmakan, and Algaja formations there are varieties similar to OIB and N-MORB, yet no rocks revealing features characteristic of the subduction origin are found. Being of Early Mesozoic age, the studied metabasalts are the youngest rocks within the Mongol–Okhotsk belt. The Lower Mesozoic deposits of the western part of the Tukuringra Terrane, including the volcanics of the OIB and N-MORB types, may have been relics of the residual ocean basin. The data obtained reflect the absence of subduction processes in the Late Triassic–Middle Jurassic at the final stages of the formation of the Mongol–Okhotsk fold belt. The closure of the Early Mesozoic basin was probably due to large-scale shear displacements.

Age and Sources of Lower Mesozoic Metasedimentary Rocks of the Un’ya-Bom Terrane in the Mongol–Okhotsk Fold Belt: Results of U–Pb Geochronological (LA-ICP-MS) and Sm–Nd Isotope Studies

The results of the U—Th—Pb geochronological studies indicate that in the Un’ja-Bom terrane the youngest peaks on the relative probability curves of ages are 207 and 212 Ma for zircons from the metasiltstone of the Kurnal formation and from the metasiltstone of the Amkan formation. Similar age estimates for the youngest peaks were obtained for the detrital zircons from the metasedimentary rocks of the Tukuringra terrain of the eastern part of the Mongol-Okhotsk belt, which indicates that the Lower Mesozoic flyschoid complexes in the structure of this belt are developed much wider than is currently believed. The results obtained in our U—Th—Pb geochronological and Sm—Nd isotope geochemical studies suggest’s that the sedimentary rocks material was carried to the sedimentation basin mainly from the continental massifs of the Amur superterrane.

The Structure of the Mongol-Okhotsk Fold Belt and the Problem of Recognition of the Amur Microcontinent

Abstract —The paper presents a new understanding of the geologic composition and geodynamic evolution of the Mongol–Okhotsk Fold Belt. It considers the issues related to recognition and substantiation of the Amur composite microcontinent (Amuria superterrane). We analyze the latest data on the geologic composition, age, and paleomagnetism of the Neoproterozoic–Paleozoic complexes, such as the Argun terrane and neighboring structures of Transbaikalia and Mongolia, as one of the key elements of Amuria. In particular, we have refined the age of a number of Precambrian and Paleozoic stratified and magmatic stages and demonstrated the absence of an Archean–Paleoproterozoic crystalline basement. Using a set of our own paleomagnetic and paleontological data, we have substantiated the equatorial position of the Argun terrane in the close proximity to Siberia at 560–525 Ma. The results of our study and the performed analysis of available geological data on the Argun terrane and neighboring Transbaikalia and Southeastern Asia territories point to the fallacy of previous arguments about the Amur composite microcontinent as a single tectonic unit, whose collision led to the formation of the folded structures of the Mongol-Okhotsk belt. This conclusion is of crucial importance for reconstructing the geodynamic evolution of the eastern part of the Central Asian Fold Belt in the Neoproterozoic, Paleozoic, and Mesozoic.

Distribution of Mineralization, Ages, and Sources of the Malomyr Gold Deposit, Eastern Part of the Mongol–Okhotsk Fold Belt

A reliable estimate is obtained for the gold mineralization age for the Malomyr deposit (eastern part of the Mongol–Okhotsk Fold Belt), one of the most well-known in the Russian Far East. The obtained data indicate that the age of the hydrothermal ore process that led to the formation of the Malomyr deposit can be estimated at ~134–130 Ma and the age of postore dikes is 110–104 Ma. At the same time, there are no data on magmatism manifesting itself within the studied region 134–130 Ma ago, making impossible to link the ore mineralization of the Malomyr deposit to magmatic processes. We think that dislocation processes accompanied by hydrothermal activity played a significant role in the mobilization, redistribution, and formation of the Malomyr deposit, which was supported by the results of structural studies. The first results of Rb-Sr and δ34S studies indicate that the ore material came from both crustal and mantle sources.

Geochemical Features and Sources of Metasedimentary Rocks of the Western Part of the Tukuringra Terrane of the Mongol–Okhotsk Fold Belt

This work presents the results of geological, geochemical, Sm–Nd isotope-geochemical studies of metasedimentary rocks of the Teploklyuchevskaya, Garmakan, and Algaja formations of the Tukuringra Terrane of the eastern part of the Mongol–Okhotsk fold belt, as well as U–Th–Pb geochronological (LA-ICP-MS) studies of detrital zircons from these rocks. It is established that the lower age boundary of formation of the protolith of metasedimentary rocks of the Teploklyuchevskaya Formation is about 243 Ma (Middle Triassic); those of the Garmakan and Algaja formations are ~175 Ma (Lower–Middle Jurassic boundary) and ~192 Ma (Lower Jurassic), respectively. This makes it possible to correlate the Teploklyuchevskaya, Garmakan, and Algaja formations with the youngest sedimentary complexes of the eastern part of the Mongol–Okhotsk fold belt. In terms of geochemistry, the protoliths of metasedimentary rocks of the above-mentioned formations are the most similar to sedimentary rocks of island arcs and active continental margins. The source terrigenous material was transported from the southern frame of the Mongol–Okhotsk fold belt. It is not improbable that Lower Mesozoic deposits of the western part of the Tukuringra Terrane, in particular, and the eastern part of the Mongol–Okhotsk fold belt, as a whole, are relics of residual basins, preserved in “gaps” in the collision zone between the southern margin of plates of the North Asian Craton and the Amur Superterrane.

Age, geochemistry, and sources of clastic materials and accumulation settings of the Glubokin Formation (Eastern Transbaikalia)

We report the results of geochemical, Sm–Nd isotope–geochemical, and U–Pb detrital zircon geochronological studies of metaterrigenous rocks of the Glubokin Formation assigned provisionally to the Upper Riphean. This formation is developed where the Mongol–Okhotsk belt is almost completely squeezed by the surrounding continental structures and, hence, bears important information on its evolution. The obtained results suggest the following conclusions: (1) the Glubokin Formation is Early Pennsylvanian or post-Early Pennsylvanian in age, not Riphean as previously suggested; (2) the Glubokin Formation belongs to the Mongol–Okhotsk Fold Belt rather than to the Argun continental massif, as suggested in the existing schemes of structural zoning; (3) the volcanogenic-terrigenous deposits of the Glubokin Formation were precipitated in a back-arc basin setting above the subduction zone subsiding beneath the southeastern margin of the North Asian Craton; and (4) the main sources of clastic material for the Glubokin Formation were igneous and metamorphic complexes of different ages from the southeastern margin of the North Asian Craton.

Middle Triassic age of metarhyolite of the Bondikha Formation, Argun Continental Massif, Central Asian Fold Belt

Research works carried out for the first time made it possible to determine the age of one of the key geological complexes in the Argun Continental Massif (Central Asian Fold Belt). It has been found that the metarhyolite of the Bondikha Formation is not Middle–Late Riphean, as previously thought, but is of Middle Triassic age (242 ± 6 Ma). Based upon the geochemical characteristics of the volcanics and regional geology of the Bondikha Formation, it can be assumed that they formed during one of the stages of subduction processes in the history of the Mongol–Okhotsk Fold Belt.