Continental Massif Research Articles

Diverse magmatism along the northern margin of Tarim during the Ediacaran: Transition from Rodinian dispersing to Gondwana assemblage

The Ediacaran igneous rocks and sedimentary sequences play a key role for our understanding the transition from dispersing of the Rodinian continents to the assemblage of the Gondwana. On the other hand, some continental massifs at the marginal area of the Rodinia and Gondwana could preserve solid evidences for deciphering the geodynamic background from the break-up of Rodinia to the assemblage of Gondwana. In this contribution, we report field observations, petrography, ages and systematic geochemistry of the diverse Ediacaran igneous activities in the northern margin of the Tarim Block, which was favored as a marginal continental massif in the Rodinia configuration by most geologists. The Ediacaran igneous rocks along the northern margin of Tarim include the Baicheng and Yangxia granites, Tiemenguan syenite, Kurle mafic dykes and the basalt layers in the Zamoketi Formation (northeastern Tarim) and the Sugetbrak Formation (northwestern Tarim). Precise zircon UPb dating revealed that these distinct rocks were broadly coeval with crystallization/eruption ages of 630–600 Ma. Bulk-rock elemental and SrNd isotope compositions as well as zircon LuHf isotope compositions demonstrate that the Baicheng and Yangxia granites were derived from Neoarchean mafic lower crust and underwent intensive fractionation before their emplacements. The Tiemenguan syenites have potassic andesitic compositions, in combination with their pronounced enriched NdHf isotopes and extremely high Zr saturation temperatures (∼900 °C), we argue that their possible shoshonitic primary magmas were derived from previously metasomatized subcontinental lithospheric mantle source (amphibole-bearing) and then, coupling with assimilation and fractional crystallization (AFC) effects in magma chamber and/or en route to the emplacement space. This can account for their intriguing geochemical features. The ca. 615 Ma Zamoketi and Sugetbrak basalts and the slightly younger porphyrite dykes (ca. 580 Ma) cutting the Zamoketi basalt layer, show typical OIB-like geochemical signatures, they were most likely derived from a depleted asthenospheric mantle source with variable fractionation of olivine and pyroxene and negligible crustal contamination before eruption. On the whole, the geochemistry of the diverse Ediacaran igneous rocks unambiguously demonstrates that they were genetically related to an extensional environment. In combination with the significant passive continental features of the Ediacaran-Cambrian sedimentary sequences, we consider that these Ediacaran igneous rocks were the latest phase of igneous activity related to the dispersing of the Tarim at the margin of the Rodinia supercontinent.The dispersing of the Rodinian supra-continent was diachronic with the assemblage of the Gondwana. Late Tonian to Ediacaran dispersing of the Tarim from northern margin of Australia induced the opening of the Proto-Tethys Ocean. Then the initiation of the southward subduction of the Proto-Tethys Ocean at ca. 530 Ma (Fortunian) led to the Tarim assemblage to Gondwana at ca. 440 Ma.

Age and Sources of Melts of Metavolcanic Rocks of the Djagdagle Formation of the Northwestern Part of the Bureya Continental Massif, Central Asian Orogenic Belt

Age and Sources of Melts of Metavolcanic Rocks of the Djagdagle Formation of the Northwestern Part of the Bureya Continental Massif, Central Asian Orogenic Belt

Black Sea Morphotectonic Nature

Abstract The presence of oceanic crust on the Black Sea bottom indicates that this sea basin was a section - a Pontic Ocean gulf at the northern margin of the Phanerozoic Tethys Ocean. The late Phanerozoic subduction of the Tethys oceanic crust under the European continental massif in this area took place in the subduction zone, whose modern traces are found today south of the Crimean Peninsula. On the surface of the subducted oceanic crust were numerous fragments of the destroyed continental crust on the northern margin of the Gondwana continent. These relatively lighter than oceanic crust crustal fragments (or a significant part of them) were not subducted. They accumulate in the immediate vicinity of the subduction zone on the unsub ducted remnants of oceanic crust. Such is the case with the Black Sea. It is not an imposed subcontinental basin, but a last modern relict of the northern margin of the Late Phanerozoic Tethys Ocean.

The Geotectonic Peculiarities of the North Caspian Permian Salt-Bearing Basins (Kazakhstan)

This article examines the geotectonic and sedimentary features of the Upper Devonian–Carboniferous–Permian deposits of the North Caspian basin, represented by deposits of marine Paleozoic-isolated carbonate platforms formed during the subsidence of the basement on the passive continental margin. The top is covered by thick salt-bearing Kungurian deposits from the end of the Early Permian epoch. The formation of carbonate platforms is associated with a major tectonic restructuring of the basin at the turn of the Caledonian and Hercynian eras, when the Paleo-Tethys Ocean was formed and isolated carbonate islands began to grow in an open marine environment. The central part of the depression experienced a long and gradual subsidence that spanned the entire Paleozoic era and the beginning of the Mesozoic era. In the south and east, from the Devonian to the Permian periods, barriers were formed in the form of island carbonate massifs that separated the North Caspian basin from the Paleo-Tethys Ocean. During the formation of the salt-bearing basin, these barriers limited water exchange and ensured a one-way influx of sea water from the open ocean. As a result, at the end of the Permian period, thicker salts accumulated; however, during the collision of the continental massifs, an invasion of many kilometers of redbeds occurred. They initially stopped salt accumulation; however, gradually, in the north of the Caspian Sea during Roadian times, the salt accumulation continued. The post-Roadian time is associated with the influx of large quantities of redbed sediments, which caused gravitational instability in the underlying salt, and salt tectonics began with the formation of domal structures.

ИСТОЧНИКИ И ПАЛЕОГЕОДИНАМИЧЕСКАЯ ОБСТАНОВКА НАКОПЛЕНИЯ ТЕРРИГЕННЫХ ОТЛОЖЕНИЙ НИЖНЕКЕМБРИЙСКОЙ БЫСТРИНСКОЙ СВИТЫ АРГУНСКОГО МАССИВА

The article presents the results of mineralogical, geochemical, isotopic, and geochemical (Sm-Nd) studies on siltstones from the upper Bystraya subformation of the Argun continental massif, as well as U-Pb dating of detrital zircon grains from them. The main goal of the research was to identify the main sources of clastic material, reconstruct the paleogeodynamic environment of accumulation, and constrain the lower age limit for the formation of terrigenous rocks of the upper Bystraya subformation. The mineralogical composition of the studied rocks indicates their accumulation in an environment associated with subduction processes. The trace element composition of siltstones from the subformation suggests the presence of rocks of mixed composition in the source area in terms of their silica content. Based on Sm-Nd isotopic and geochemical data, it was established that the main sources of clastic material for siltstones of the stratigraphic unit were Paleoproterozoic rocks and (or) younger rocks whose formation is associated with the reworking of the Paleoproterozoic continental crust. Most of the zircons from the studied sample of siltstones of the upper Bystraya subformation are Neo- and Paleoproterozoic in age. Their sources are probably Neo- and Paleoproterozoic igneous rocks widespread within the Argun massif in China. The age of the youngest zircons (556 ± 9 and 566 ± 10 Ma) isolated from siltstones of the upper Bystraya subformation was used to constrain the lower limit of their accumulation in the middle of the Ediacaran.

The Upper Paleozoic on the northern margin of the Tarim minor continent as a non-classical foredeep trough (evolution, tectonics, comparison)

New data on the stratigraphy and structure of the Pre-Kokshaal foredeep (Kyrgyz and Chinese Tianshan), fragments of which have been preserved on the northwestern edge of the Tarim continental massif, now in the Ferghana, Kokshaal and Kharke ridges, are presented. The foredeep emerged when the Turkestan ocean closed and the South Tianshan collisional structure built at the end of the Carboniferous and the beginning of the Permian, about 310 - 290 Ma. The next parts of the Pre-Kokshaal foredeep may be highlighted. 1) The inner zone, which is superimposed on the distal part of the former slope of the Tarim continent and filled with Moscowian – Kasimovian turbidites, then folded and thrusted to the southeast. 2) To the south, outer zone, which corresponds to the proximal part of the slope, where the same events and structures developed here later, in the Gzhelian – Asselian. Analogues of these mostly flysch zones are usually not included as parts of the Pre-Ural, Prealpine and other classic foredeeps, where they are completely captured into the thrust complexes. 3) Marginal zone of the Pre-Kokshaal trough was loaded with thick clastic deposits only in the Gzhelian, but already at the beginning of the Permian its subsidence compensated, and the flysch was covered by Asselian limestones. After the Asselian thrusts in the Ferghana Ridge and less significant deformations in the east, development of the foredeep towards the south apparently stopped under the influence of the Tarim mantle plume, which facilitated the separation of the mantle slab and led to thickening and heating of the Tarim continental crust. After the Hercynian collision completed, in the Late Cenozoic the Pre-Kokshaal Upper Paleozoic foredeep experienced intracontinental transverse compression and surface shortening, when it completely disappeared in the east of the Tien Shan, but supplemented from the south by a new Cenozoic molasse trough.

Composition and Age of Sources of the Lower–Middle Cambrian (?) Terrigenous Rocks from The Ernichny Formation in the Argun Massif, Eastern Part of the Central Asian Fold Belt

The paper presents the results of the mineralogical and geochemical studies of sandstones and siltstones of the Ernichny formation of the Argun series in the Argun continental massif, and the results of U–Pb (LA‑ICP-MS) dating for the detrital zircons from these rocks. It is established that the youngest detrital zircons from sandstone of the Ernichny Formation have age of 549–570 Ma. A maximum on the relative probability curve of zircon age corresponds to 566 Ma. These data determine the lower age limit of their accumulation on the border of the Ediacaran and Paleozoic age. According to the results of U–Pb dating of detrital zircon, the majority of detrital zircons from sandstones of Ernichny Formation are characterized by Neo- and Paleoproterozoic ages. The sources of zircons were Neo- and Paleoproterozoic igneous and metamorphic rocks that were widely developed within the Argun continental massif. In turn, the geochemical features of the terrigenous rocks of the Ernichny Formation, together with the presence of the poorly sorted and rounded clastic material in the studied samples, as well as the presence of interlayers of gravelstones, enabled us to establish that they were formed in a subduction-related setting.

The First Data on the Age of Metamorphic Rocks of the Synchuga Block of the Jiamusi Continental Massif (Central Asian Orogenic Belt)

The First Data on the Age of Metamorphic Rocks of the Synchuga Block of the Jiamusi Continental Massif (Central Asian Orogenic Belt)

A Mesoproterozoic to Jurassic history of continental eclogites from the Guatemala Suture Zone–implications for a peri-Amazonian ancestry

Cretaceous continental eclogites of the Chuacús Complex (Guatemala Suture Zone) contain an extensive record spanning more than 1.1 Ga. In this study, we combine U-Pb zircon geochronology, Sm-Nd and Lu-Hf isotope systematics, and geochemistry with field observations to determine the origin of eclogite protoliths. Mafic precursors are multiple comprising Ectasian to Middle Jurassic rocks originated in convergent, within-plate, and continental rift settings. Eclogites with estimated ca. 1.3 Ga protolith ages display chemical and isotopic features suggesting that mafic precursors formed in an extensional arc setting. These rocks are part of a complexly folded gneiss sequence (El Chol unit) that underwent pervasive perturbation of the zircon U-Pb system during a polymetamorphic evolution. One generation of OIB-like protoliths yield crystallization ages of 1.1–1.0 Ga, were derived from an enriched mantle source, and belong to a bimodal igneous assemblage (Cubulco unit) related to intraplate magmatism. An additional eclogite with OIB characteristics is part of a considerably younger bimodal suite (Agua Caliente unit) emplaced at ca. 228 Ma during a renewed pulse of within-plate activity. The youngest eclogite protoliths crystallized at ca. 168–162 Ma and represent E-MORB-type metabasites derived from a depleted mantle source and contaminated with variable amounts of crustal material. Hence, the geological record of the Chuacús eclogites includes evidence for: (1) extensional magmatism in a pre-Rodinian continental arc setting, (2) Stenian intraplate magmatism compatible with global large igneous province activity around Rodinia, (3) the initial breakup of western equatorial Pangea during the Late Triassic, and (4) the consequent Middle Jurassic rifting of Middle America terranes from the northwestern Gondwana margin. These data imply that the internal continental massif of the Guatemala Suture Zone may have evolved from a peri-Amazonian crust in the same way as other basement inliers of Mexico and the Northern Andes formerly adjoined within Pangea.

THE FIRST DATA ON THE AGE OF METAMORPHIC ROCKS OF THE SYNCHYGA BLOCK OF THE JIAMUSI CONTINENTAL MASSIF (CENTRAL ASIAN OROGENIC BELT)

The paper presents the results of U–Th–Pb geochronological studies of zircons from metamorphic rocks of the Amur group of the Synchyga Block of the Jiamusi continental massif. It is shown that the garnet-biotite-sericite schists of the Amur group have a primary sedimentary origin. The lower depositional age of the protolith can be determined by the age of the youngest zircons from group with Neoproterozoic ages ~832 Ma. The upper age boundary of protolith, apparently, corresponds to the age of metamorphic zircons ~533 Ma. Thus, the schists of the Amur group of the Synchyga Block are of Neoproterozoic in age rather than Early Precambrian, as was accepted earlier.

AGE AND PROVENANCE OF THE ARGUN SERIES SEDIMENTARY ROCKS OF THE ARGUN CONTINENTAL MASSIF: LA-ICP-MS U-Pb AGES OF DETRITAL ZIRCONS

For the first time there were obtained LA-ICP-MS U-Pb age data on detrital zircons from sediments of the Bystraya and Ernichnaya formations of the Argun series of the northwestern Argun continental massif. The results obtained lead to the following conclusions: (1) the Bystraya series combines heterochronous Low Cambrian carbonate rocks and Upper Proterozoic terrigenous deposits; (2) the age of the youngest group of detrital zircons extracted from sediments of the Ernichnaya formation allows us to restrict the lower age limit to the Ediacaran (556±11 Ma), which does not contradict a standard stratigraphic age; (3) terrigenous materials entering the basin may come from the Argun continental massif. The inferred source areas of clastic materials are the Neo- and Paleoproterozoic magmatic and metamorphic formations, widespread across the Argun massif. Besides, the deposits we studied contain a smaller amount of the Mesoproterozoic zircons. Their provenance still remains unknown since the Argun massif does not include any Mesoproterozoic magmatic rocks with sufficiently accurate geochronological age estimates.

AGE AND SOURCES OF DETRITAL MATERIAL FOR THE METASEDIMENTARY ROCKS OF THE NILAN TERRANE OF THE MONGOL-OKHOTSK OROGENIC BELT

The paper presents the first results of U-Th-Pb (LA-ICP-MS) isotopic dating of detrital zircons from the Paleozoic (?) metasedimentary rocks of the Nilan Terrane of the Mongol-Okhotsk orogenic belt to refine the age estimates and determine their probable correlation with earlier dated sediments of this belt. It has been stated that the metasedimentary rocks of the Olga, Salarin and Rovnen formations and of the Sivak formation of the Nilan terrane are characterized by the almost complete absence of the Early Precambrian zircons, slightly negative ε Nd(t) values, and Mesoproterozoic t Nd(DM) and t Nd(С) values. The isotope dataset obtained shows that most of the detrital sediments were carried from continental massifs of the Amur superterrane.

The first Sm-Nd isotopic data for metamorphic rocks of the Gudjal block in the eastern part of Bureya continental massif

It is the first time that there have been Sm-Nd isotopic geochemical studies conducted into the metamorphic rocks of the Gudjal block considered as a basement protrusion of the Bureya continental massif. It has been found that biotite and garnet-amphibole schists of the Gudjal formation are characterized by strongly negative εNd(0)=–15.7…–17.2 values and Paleoproterozoic values of Nd model age of tNd(DM)=2.4–2.1 Ga. Similar isotopic parameters are also typical of gneissic biotite granodiorites: εNd(0)=–18.7, tNd(DM)=1.8 Ga. Out of orthorocks of the Gudjal block studied, the Neoproterozoic orthogneisses and orthoamphibolites of the Tulovchikha formation and the Neoproterozoic gneiss-granites of the Nyatyngran complex of the Paleoproterozoic model age may be considered as a probable substrate for melts.

Sources of Sediment Clasts and Depositional Environment of Sedimentary Rocks of the Daur Series of the Argun Continental Massif

Sources of Sediment Clasts and Depositional Environment of Sedimentary Rocks of the Daur Series of the Argun Continental Massif

ИСТОЧНИКИ КЛАСТИЧЕСКОГО МАТЕРИАЛА И УСЛОВИЯ НАКОПЛЕНИЯ ОСАДОЧНЫХ ПОРОД ДАУРСКОЙ СЕРИИ АРГУНСКОГО КОНТИНЕНТАЛЬНОГО МАССИВА

The paper presents the results of geochemical and isotopic-geochemical (Sm-Nd) studies of the Upper Riphean (?) (in the Stratigraphic Scale of Russia) sedimentary rocks of the Urulungui and Dyrbylkey formations of the Daur Series in the Argun continental Massif, as well as U-Th-Pb and Lu–Hf isotopic studies of detrital zircons from these formations. Rocks of the Urulungui and Dyrbylkey formations differ significantly in mineral composition and content of the main elements, which is probably due to the degree of chemical weathering of the source rocks. Analysis of concentrations of trace elements in sedimentary rocks of the Urulungui and Dyrbylkey formations enabled to establish that they were formed in a subduction-related setting. The presence of conglomerate-breccias, conglomerates, gravelstones, fragments of igneous and volcanic rocks, a weak degree of roundness of clastic material, as well as abundant Neoproterozoic detrital zircons in the formations is evidence of tectonic and igneous activities in the region during sedimentation. Sedimentary rocks of the Urulungui Formation are characterized by two-stage model ages TNd(DM2) = 2.0–2.1 Ga, and the Dyrbylkei Formation – TNd(DM2) = 1.6–1.8 Ga according to the results of Sm-Nd isotopic-geochemical studies. The abundant Neoproterozoic zircons in sandstones of the Urulungui and Dyrbylkey formations suggest that Neoproterozoic igneous rocks widely developed within the western Argun continental Massif were the main source of clastic materials. At the same time, the presence of Early Precambrian detrital zircons is evidence that Early Precambrian rocks were also present in the source area. However, Early Precambrian igneous and metamorphic associations whose age would be reliably determined from geochronological data have not yet been found so far within the Argun continental Massif, and thus it is not out of the question that rocks of the Daur Series form a «detached block» intruded in the Argun Massif.

Geochemical and Nd‐Hf Isotopic Constraints on the Petrogenesis of an Archean Granitoid in the Erguna Massif, NE China

AbstractCrystalline basement and Precambrian crustal growth of the continental massifs within the Central Asian Orogenic Belt (CAOB) are still pending problems. Our geological and geochemical investigations identified an Archean (2606 Ma) granitic pluton in the Biliya area of the Erguna Massif. The Neoarchean granitoids show high and positive in‐situ zircon ∊Hf(t) (+0.3 to +10.0), whole‐rock ∊Nd(t) (+4.8) and whole‐rock ∊Hf(t) values (+2.1). They are characterized by high Y + Ce + Zr + Nb (> 220 ppm) and Zr contents and could be classified as A‐type granites. These granitoids are characterized by high Zr saturation temperatures (TZr) (796–836°C). They were derived from partial re‐melting of juvenile mafic lower crust in an intracontinental back‐arc extensional environment. This newly identified Neoarchean granitic pluton may represent the crystalline basement of the several continent massifs within the CAOB, and their high ∊Hf(t)–∊Nd(t) values may also indicate the occurrence of lateral crustal growth events in these massifs during the Neoarchean.

Review on geology and mineralization of North Korea (I: Precambrian)

AbstractIn this review, we introduce the geology and mineralization of the northern part of the Korean Peninsula based on recently published data in North Korea. Geological characteristics and events are described in order of geological period and the representative mineral deposits are presented according to the geologic period to show the relationship between the geology and mineralization. In particular, we rearrange the stratigraphy of the Precambrian basement and reset the timing of intrusion and mineralization based mainly on the reported zircon U–Pb age data. The Precambrian geotectonic provinces of North Korea contain continental cores and massifs, active continental margins, and rift zones. The Precambrian strata between the eastern North China Craton (NCC) and North Korea are newly correlated considering rearranged stratigraphy in this review. The Precambrian strata of North Korea are characterized by the depletion of the Paleoproterozoic to Mesoproterozoic strata compared to the NCC from 1.8 to 1.5 Ga. It may indicate that the northern part of the Korean Peninsula was the passive continental margin environment during this period. The common occurrences of bimodal igneous suites (mafic‐felsic and alkaline‐subalkaline) are also characteristic of North Korea. The bimodal magmatism is well explained by the progressive tectonic evolution from oceanic crust‐continent subduction (2.0–2.2 Ga) and island arc–continent collision (~1.9 Ga) to post collisional extension (1.82–1.87 Ga) in the NCC including North Korea. The Precambrian geological similarities between Northeastern China and North Korea would have resulted in the same or similar mineralization. For example, Algoma type banded iron formations of the Fe deposits of the Anshan–Benxi area in China and Musan Fe deposit in North Korea were formed during the Neoarchean and Paleoproterozoic. Sedimentary exhalative type Pb–Zn and metasedimentary type magnesite deposits were formed within the Jiao‐Liao‐Ji belt extending from the Liaoning Rift Zone of China (Pb–Zn deposits in the Qingchenzi orefield and magnesite deposits in Dashiqiao magnesite belt) to the Macheollyeong Rift Zone of North Korea (Geomdeok Pb–Zn deposit and Daeheung–Ryongyang magnesite deposits) during Paleoproterozoic. The geotectonic, geological and metallogenic similarities between Northeastern China and North Korea indicate that the Precambrian continental massifs of North Korea might be evolved in accordance with the NCC evolution. This review is expected to be helpful for understanding the geotectonics, geology, and related mineralization of the northern part of Korean Peninsula and correlating with those of the NCC.

Early Paleozoic magmatic events in the Bureya Continental Massif, Central Asian Orogenic Belt: Timing and tectonic significance

The Bureya Massif is one of the largest continental massifs in the eastern Central Asian Orogenic Belt. The reconstructions of tectonic scenarios for the evolution of this belt are mainly based on geochronological and geochemical data. However, relevant data from the Bureya Massif are scarce. Here, we report whole-rock geochemistry, zircon U-Pb geochronology, and Lu-Hf isotope data for Early Paleozoic granite intrusions to constrain the timing of the magmatic and tectonic processes in the Bureya Massif. Geochronological data yielded ages of 495 ± 5, 486 ± 8, 471 ± 6, 453 ± 2, and 426 ± 6 Ma for the granitoids in the Bureya Massif. The chemical composition of the Early Paleozoic granitoids mostly showed affinities to A-type (A2-subtype) granites. Furthermore, they have zircon εHf(t) values of −5.3 to +5.7, and two-stage model ages tHf(C) of 1.52–0.95 Ga. These results suggest that the parental magmas of the Early Paleozoic granitoids were derived either from melting of a dominantly Neo- and Mesoproterozoic crustal source. Early Paleozoic magmatism in the Bureya and Songnen–Zhangguangcai Range massifs was characterized by similar timing and identical Hf-in zircon isotope compositions. Additionally, both arrays contain early Neoproterozoic granitic intrusions and Early Cambrian and Ordovician sedimentary units, suggesting that they were part of a single continental massif at the beginning of the Cambrian or even in the Neoproterozoic. In this case, the tectonic nature of the Early Paleozoic magmatism in these two massifs should be the same and was probably related to the docking of the Jiamusi Massif.

ГРАНИТОИДЫ АЛТАХТИНСКОГО КОМПЛЕКСА ВОСТОЧНОЙ ЧАСТИ БУРЕИНСКОГО КОНТИНЕНТАЛЬНОГО МАССИВА ЦЕНТРАЛЬНО-АЗИАТСКОГО СКЛАДЧАТОГО ПОЯСА: ВОЗРАСТ, ГЕОХИМИЧЕСКИЕ ОСОБЕННОСТИ, ГЕОДИНАМИЧЕСКАЯ ИНТЕРПРЕТАЦИЯ

New data are reported on the age, composition and formation conditions of the Late Triassic granitoids of minor intrusions from the eastern part of the Bureya continental massif of the Central Asian fold belt, and their geodynamic interpretation is given. U-Pb geochronological studies on zircons indicate that the age of rocks of the Altakhta syenite-leucogranite complex is in the 235–224 Ma range, which intrusion proceeding in two phases. The formation of magmatites is associated with the first phase (235.3 ± 2.9–230 Ma), which by their geochemical features are similar to А1-type intraplate granitoids and by REE concentrations to subduction- and collision-related granitoids. Geochemical characteristics of А2-type and А2 + I & S-type granites (226–224 Ma) suggest their formation in a syncollisional setting. The evolution of the chemical composition of the Altakhta granitoids indicates a change in the geodynamic setting type, when the opposing (orthogonal) movement of the plates at the convergent boundary (subduction) changed to their sliding past each other (transform continental margin).

Age and Provenance of the Daur Series Sedimentary Rocks (Riphean), Argun Continental Massif: Results of U–Th–Pb and Lu–Hf Isotope Studies of Detrital Zircons

The first results of U–Th–Pb and Lu–Hf isotope studies of detrital zircons from sedimentary rocks of the Daur Series (Argun continental massif of the Central Asian Fold Belt) are presented. It is established that the youngest detrital zircons from sandstones of the Urulungui and Dyrbylkei formations of the Daur Series have the ages of 899 and 771 Ma, respectively. These data determine the maximum depositional age of the Daur Series and agree with the current ideas about the Late Riphean age of this series. The majority of Late Riphean detrital zircons from sedimentary rocks of the Daur Series are characterized by Early Precambrian Lu–Hf model ages. In addition, the Archean and Early Proterozoic detrital zircons are present in the studied rocks. This suggests either the presence of ancient (Early Precambrian) recycled material in the sedimentary rocks of the Daur Series or the fact that the studied part of the Argun massif is an “exotic block,” whose origin requires additional research.