An Ordovician Age of the Dunite–Wehrlite–Clinopyroxenite Banded Complex of the Nurali Massif (Southern Urals,Russia): U–Pb (SHRIMP) Zircon Age Data

The data on the geochemistry and geochronology of zircons from wehrlites and clinopyroxenites of the dunite–wehrlite–clinopyroxenite banded complex that lies at the base of the crustal section of the ophiolite complex of the Nurali massif are presented. The obtained U–Pb age of the banded complex of 450 ± 4 Ma differs markedly from the previous age data. According to REE distribution patterns zircons from ultramafic rocks are attributed to the magmatic type and they indicate the age and supposed genetic similarity of the above rocks with lherzolites and dunites from the mantle section of the Nurali massif.     

Tectonic Settings of Palaeoproterozoic Uranium Deposits of the Jiao–Liao–Ji Belt in Eastern Liaoning Province, China

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First Finds of Native Iridium in Chromite–Platinum Ledge Ore and Placers of Zonal Clinopyroxenite–Dunite Massifs of the Urals

Doklady Earth Sciences - The results of studying the aggregates of native iridium from chromitites of the Svetloborskii and Sosnovskii clinopyroxenite–dunite massifs of the platinum-bearing...     

A Tectonic Model of the Transform Fault of the Verkhoyansk–Chukotka Mesozoids

Doklady Earth Sciences - In this work we considered the origin of the Verkhoyansk–Kolyma, Chukotka, and Southern Anyui belts, which are part of the Verkhoyansk–Chukotka folded area. It...     

Geochemical Types of Granitoids of the Mongol—Okhotsk Belt and Their Geodynamic Settings

The following geochemical types of granitoids have been investigated in the Mongol-Okhotsk belt:tholetitic,palingenic calc-alkaline,latitic,plumasitic and arpaitic rare-metal granites.Plagiogranites of the tholeiitic series occur within the Mongol-Okhotsk suture,indicating a subduction environment.The calc-alkaline granitoids responsible for the batholith-like intrusions and their formation are related to collision environments.The latest granitoids of the latite series and rare-metal granites came into existence after the collision of continental masses,providing evidence of intraplate magmatism.     

Mesozoic–Cenozoic volcanism and geodynamic events in the Central and Eastern Arctic

The patterns and history of Mesozoic–Cenozoic plume magmatism in the Arctic is considered in relation with suprasubduction volcanism and geodynamic events. The Mesozoic–Cenozoic magmatic history of the area includes seven stages, distinguished by correlation of ages and compositions of volcanics associated with mid-ocean rifting, plumes, and subduction; three of seven stages correspond to global events at 230–200 Ma, 130–120 Ma, and 40–30 Ma. The reported study supports the inferred inverse correlation between plate velocities and amount of subduction-related volcanism. The gained knowledge is used for paleotectonic reconstructions in the Arctic.     

Geochronology of the Gabbro–mafic Microgranular Enclaves–granite Associations in the Gejiu District, Yunnan Province and Their Geodynamic Significance

Many igneous rocks distribute in Gejiu tin polymetallic ore-field at Yunnan province, rocks including basalt, gabbro, mafic microgranular enclaves, granites (porphyritic granite and equigranular granite) and akaline rocks. The ages of the granites and akaline rocks which are considered to have genetic connecting with the mineralization have been comfirmed, but the gabbro-mafic microgranular enclaves-granite assemblage’s ages are still unknown. By means of LA-ICP-MS zircon U-Pb dating, the data of Shenxianshui equigranular granite, the mafic microgranular enclave in Jiasha area, the host rock of the mafic microgranular enclaves and the Jiasha gabbro are around ~80 Ma. Besides the above mentioned data, a group of new ages at ~30 Ma were discovered in this study, which is from gabbro and mafic microgranular enclaves. Based on the previous data and the new data gained this time, we suggest the major geochronology framework of the magmatism and mineralization events in Gejiu area is ~80 Ma, which is consistent with the Late Cretaceous magmatism and mineralization events in the whole southeast Yunnan and west Guangxi area and they were suggested to belong to the same geotectonic setting in late Yenshannian. And the new ages of the ~30 Ma obtained in this study is considered to represent a responding to the complicate tectonic evolution history of the Tibetan orogenic events in Cenozoic.     

Ore Mineralization in Ultramafic and Metasomatic Rocks of the Ospin–Kitoi Massif,Eastern Sayan

In the Ospin–Kitoi ultramafic massif of the Eastern Sayan, accessory and ore Cr-spinel are mainly represented by alumochromite and chromite. Copper–nickel mineralization hosted in serpentinized ultramafic rocks occurs as separate grains of pentlandite and pyrrhotite, as well as assemblages of (i) hexagonal pyrrhotite + pentlandite + chalcopyrite and (ii) monoclinal pyrrhotite + pentlandite + chalcopyrite. Copper mineralization in rodingite is presented by bornite, chalcopyrite, and covellite. Talc–breunnerite–quartz and muscovite–breunnerite–quartz listvenite contains abundant sulfide and sulfoarsenide mineralization: pyrite, gersdorffite, sphalerite, Ag–Bi and Bi-galena, millerite, and kuestelite. Noble metal mineralization is represented by Ru–Ir–Os alloy, sulfides, and sulfoarsenides of these metals, Au–Cu–Ag alloys in chromitite, laurite intergrowth, an unnamed mineral with a composition of Cu₃Pt, orcelite in carbonized serpentinite, and sperrylite and electrum in serpentinite. Sulfide mineralization formed at the late magmatic stage of the origination of intrusion and due to fluid–metamorphic and retrograde metasomatism of primary rocks.     

Deep-Seated Xenolith of the Platinum-Bearing Websterite from the Paleoproterozoic Volchetundra Gabbro–Anorthosite Massif in the Kola Region

Doklady Earth Sciences - The results of studying of the deep-seated platinum-bearing xenolith discovered in the Volchetundra gabbro-anorthosite massif in the Kola Region are presented. It is shown...     

Tectonic Framework and Evolution of the Dabie Mountains in Anhui, Eastern China

The Dabie Mountains are believed to be a collisional orogenic belt between the Yangtze amd Sino-Koreancontinental plates. It is composed of the foreland fold-thrust zone, the subducting cover and basement of theYangtze continental plate, the coesite- and diamond-bearing ultra-high pressure metamorphic zone and themeta-ophiolitic melange zone in the subducting basement, the fore-arc flysch nappe and the back thrust zoneoccurring respectively on the southern and northern margins of the Sino-Korean continental plate and the in-herited basin with molassic deposits on the northern margin. When the palaeo-Dabie oceanic plate subductednorthward in the Early Palaeozoic, volcanic arc and back arc basin probably formed on the southern margin ofthe Sino-Korean continental plate. The Sm / Nd isotopic dating of the strata and eclogite which were drawn in-to the foreland fold-thrust zone indicates that the intense collision of the two continental plates took place inthe Early Mesozoic.     

Formation of Water-Bearing Defects in Olivine in the Presence of Water–Hydrocarbon Fluid at 6.3 GPa and 1200°C

The main trends of water dissolution in Fe-bearing olivine have been investigated in the olivine–H₂O–hydrocarbon fluid system in experiments at a pressure of 6.3 GPa, a temperature of 1200°C, and hydrogen fugacity ( fH₂) buffered by the Mo–MoO₂ equilibrium. The content and contribution of ОH defects of different types in Fe-bearing olivines depend on the composition of reduced fluids in the system. As the fraction of hydrocarbons in the fluid increases, the H₂O content in olivine crystals decreases from 900 to 160–180 ppm, while the ОН absorption peaks become lower at high frequencies and occupy a larger part of the infrared spectrum in the low-frequency region. According to the experimental results, even the deepest seated mantle olivines with OH defects were not equilibrated with a fluid rich in light alkanes or oxygenated hydrocarbons.     

Mesozoic–Cenozoic Tectonic Evolution and Uplift in Pamir: Application of Fission Track Thermochronology

The Pamir Plateau can be divided into three secondary tectonic units from north to south: the North, the Middle and the South Pamir Blocks. The North Pamir Block belonged to the southern margin of Tarim-Karakum, thermochronological study of the Pamir structural intersection indicates that accretion of the Middle Pamir Block to the Eurasian Continental Margin and its subduction and collision with the North Pamir Block occurred in the Middle–Late Jurassic. Due to the Neo-Tethys closure in the Early Cretaceous, the South Pamir Block began to collide with the accretion(the Middle Pamir Block) of the Eurasian Continental Margin. Affected by the collision and continuous convergence between the Indian Plate and the Eurasian Plate since the Cenozoic, Pamir is in a multi-stage differential uplift process. During 56.1–48.5 Ma, North Pamir took the lead in uplifting, that is, the first rapid uplift in the Pamir region began there. The continuous compression and contraction of the Indian and Eurasian plates during 22.0–15.1 Ma forced the Pamir tectonic syntaxis to begin its overall uplift, i.e. Pamir began to enter the second rapid uplift stage in the Early Oligocene, which lasted until the Middle Miocene. During 14.6–8.5 Ma, South Pamir was in a rapid uplift stage, while North Pamir was in a relatively stable state, showing asymmetry of tectonic deformation in the Pamir region in space. Since 6.5 Ma, Pamir began to rapidly uplift again.     

Age of the Raygorodok Au-bearing Gabbro—Monzodiorite Massif (Northern Kazakhstan)

Accessory zircon from the polyphase Raygorodok stock (Northern Kazakhstan) has been dated by local U–Pb analysis (SIMS and LA-ICP-MS). This Au-bearing intrusion has been dated to 442–447 Ma, suggesting its emplacement at the very end of the Ordovician and Early Silurian, allowing the stock’s correlation with the Stepnyak Complex of small intrusions. Thus, control of small intrusions of the Stepnyak type over gold mineralization has been corroborated permitting their use as a regional prospecting characteristic.     

Tectonic Evolution of Tertiary Basins in circum-Pacific Belt of China and Their Geodynamic Setting

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Geochronology and Geochemical Characteristics of the Early Mesozoic Tangquan Pluton in Southwestern Fujian and Its Tectonic Implications

The Tangquan granodioritic pluton in Dalian County, southwestern Fujian, China, which extends in a NE direction with an exposed area of about 130 km2, used to be considered a product of Early Cretaceous magmatism. The present study suggests for the first time that the pluton was formed in the Early Jurassic by using multiple methods for isotopic dating, which give zircon U-Pb ages of 186.8 Ma and 179.0 Ma, Rb-Sr isochron age of 162.02±4.5 Ma, and biotite 40Ar/39Ar plateau age of 158.1±0.7 Ma. The cooling rate for the pluton was relatively low (4.76℃/Ma) during the early stage (183-162 Ma) because of the compressional environment. It was emplaced in a higher cooling rate (50℃/Ma) in an extensional environment during the later stage (162-158 Ma). The granodiorites are metaluminous-peraluminous, relatively enriched in Na2O and depleted in K2O, and characteristic of I-type granites of crust-mantle mixed sources. They are moderately enriched in Rb, Th, Hf and LREE, and depleted in Ti, Nb, Ta and Sr, and     

Hydrocarbons in the Basement of the South China Sea (Vietnam) Shelf and Structural–Tectonic Model of their Formation

Geotectonics - The paper considers the origin of hydrocarbon accumulations within the Pre-Cenozoic basement of the Vietnam shelf. It is shown that the formation of hydrocarbon deposits is...     

Change in the Geodynamic Settings in the Pacific‒Eurasia Transition Zone at the End of the Early Cretaceous

Doklady Earth Sciences - An attempt to provide a new explanation for the change in the geodynamic regime in the Early Cretaceous is presented below. It accounts for data on a number of global...