Yakchi chert–volcanogenic Formation—fragment of the Jurassic accretionary prism in the Central Sikhote-Аlin,Russian Far East

The Yakchi chert–volcanogenic formation is differentiated at the base of the stratigraphic succession in the Khor-Tormasu subzone of the Central Sikhote-clin structural–formational zone or the Samarka terrane of the Jurassic accretionary prism. The paper considers the results of biostratigraphic study of its deposits and petrogeochemical studies of its basalts. A tectonically disrupted sequence of the Yakchi Formation is restored on the basis of fossil conodonts and radiolarians, and its Late cermian–Middle Jurassic age is determined. The authors interpret the resulting stratigraphic succession in terms of changing depositional settings on the moving oceanic plate and recognize events of the ocean history recorded in it. Chert accumulated on the oceanic plate in pelagic canthalassa/caleopacifica from the Late cermian through to the Middle Jurassic. Deposition of siliceous claystone in the Late cermian–Early Triassic reflects the decline in productivity of radiolarians and a long anoxic event in Panthalassa. Chert accumulation resumed in the Triassic and persisted in the Jurassic, and it was interrupted by the eruption of basalts of different nature. Formation of the Middle–Late Triassic oceanic intraplate basalts likely occurred on the thick and old oceanic lithosphere and that of the Jurassic basalts on the thin and newly created lithosphere. In the Middle Jurassic, chert accumulation was replaced by accumulation of tuffaceous siltstone at a subduction zone along the csian continental margin. The middle Bathonian–early Callovian age of this siltstone closely predates accretion of the Yakchi Formation. The materials of the upper layer of the oceanic plate that formed over 100 million years in different parts of the ocean and on the lithospheric fragments of different ages were accreted to the continental margin. The bulk of the accreted material consists of oceanic intraplate basalts, i.e., fragments of volcanic edifices on the oceanic floor. accretion of this western part of the Khor-Tormasu subzone occurred concurrently with accretion of the southeastern part of the Samarka subzone in Primorye, which clarifies the paleotectonic zonation of the Central Sikhote-Alin accretionary prism. The cataclastic gabbroids and granitoids, as well as the clastic rocks with shallow-marine fossils in the Khor-Tormasu subzone, are considered as possible analogues of the Okrainka-Sergeevka allochthonous complex.     

First Data on the Age (U–Pb,SHRIMP-II) and Composition of Zircon from the Unique Yarega Oil–Titanium Deposit,South Timan

Doklady Earth Sciences - Isotope–geochemical study of zircon from leucoxene–quartz sandstone of the Yarega deposit in South Timan was conducted for the first time using the Sensitive...     

Eastern segment of the Kiselevka-Manoma terrane (Northern Sikhote Alin): Paleomagnetism and geodynamic implications

The comprehensive geological-geophysical study of the lower Cretaceous volcanosedimentary rocks of the Kiselevka block of the Kiselevka-Manoma lithotectonic terrane made it possible to reach the following conclusions: (1) The composition of the volcanogenic rocks and the lithology of the sediments of the Kiselevka block indicate their formation in a within-plate oceanic setting; the petro- and geochemical characteristics of the studied volcanic rocks are similar to those of the Hawaiian hot spot. (2) The distinguished characteristic component of the natural remanent magnetization of the volcanosedimentary rock complex of the Kiselevka block yields a positive fold test and age similar to that of the rocks. According to the orientation of this characteristic component, the paleolatitudes of the rock formation (18° ± 5° N) and the coordinates of the paleomagnetic pole (Plat = 18.6°, Plong = 222.4°, dp = 5.2, dm = 9.1°) of the Kiselevka Block were determined. (3) The kinematic reconstructions based on the obtained and published data indicate that, (1) in the Valanginian-Albian, the Kiselevka Block migrated northwestward with the Izanaga Plate at a velocity of 15–20 cm/yr, passing over 5 thou. km up to the Eurasian margin (the Korean Peninsula); (2) in the Albian-Campanian, the block, as a fragment of the Kiselevka-Manoma accretionary wedge, moved along the Eurasian transform margin with a velocity of 4–5 cm/yr to its present-day position, where it was integrated into the continental plate.     

Mantle sources of the Cenozoic volcanic rocks of the Lake Kizi region in the eastern Sikhote Alin

The Middle Cenozoic lava sequence of the Lake Kizi region was studied. It characterizes the activity of sources in the Northern zone of the eastern Sikhote Alin: a Middle Eocene pulse of slab-related magmatism and prolonged injection of magmas from the sublithospheric convecting mantle in the Late Oligocene. Low contents of high field strength elements (Nb and Ta) with low Nb/Ta, Ce/Pb, and Nb/La and high K/Nb ratios and a low (⁸⁷Sr/⁸⁶Sr)₀ of 0.703399 were determined in a Middle Eocene dacite with an age of ∼43.5 Ma. Three phases of Late Oligocene volcanic eruptions were distinguished: (1) basaltic andesites (29–27 Ma), (2) basaltic trachyandesites and trachyandesites (27–24 Ma), and (3) andesites (∼23 Ma). The lavas of the first and third phases showed low Ce/Pb, Nb/La, and Ba/La and high K/Nb ratios, which are also characteristic of supraslab processes. The lavas of the second phase are shifted with respect to these ratios toward ocean island basalt compositions. The entire Late Oligocene volcanic sequence falls within a narrow range of the initial strontium isotope ratios, (⁸⁷Sr/⁸⁶Sr)₀, from 0.703661 to 0.703853. Such ratios are characteristic of volcanic and subvolcanic rocks with ages of ∼37, 31–23, and ∼16 Ma over the whole region of the Tatar Strait coast.     

Geochemistry of metasomatic zircons from the Terskii greenstone belt

Local isotopic and geochemical studies of the zircons from metasomatites of the Terskii greenstone belt allowed us to determine two stages of metamorphism (2680 and 2025 My) and two stages of metasomatosis (2600 and 1800 My). Almost all the zircons were either metasomatic or affected to some degree by metasomatic processes caused by the enrichment of zircons in light rare earth elements, Th, U, Sr, and Ba, and flattening of the Ce anomaly.

     

Paleomagnetism and accretionary tectonics of northern Sikhote Alin

The results of the paleomagnetic investigation of the sediments pertaining to the Silasinskaya Formation of the Kiselevka–Manoma terrane within the Sikhote Alin orogenic belt are presented. The ancient prefolding magnetization component is revealed: Decs = 271.7°, Incs = 52.2°, Ks = 13.5, and a ^95s = 5.1° (positive fold and reversal tests); and the coordinates of the corresponding paleomagnetic pole for ~103 ± 10 Ma are calculated: Plat = 26.3°, Plong = 70.5°, dp = 4.8°, and dm = 7.0°. As a result of this study, the geodynamical settings and paleolatitudes of the formation of three objects in the northern part of Sikhote Alin orogen are established: (a) the Kiselevskaya Formation of the Kiselevka–Manoma terrane was formed 133 Ma ago at 19° N under the seamount condition on the Izanagi Plate; (b) the Silasinskaya Formation of the Kiselevka–Manoma terrane was formed 103 Ma ago at 35° N under the oceanic island arc conditions; and (c) the Utitskaya Formation of the Zhuravlevsk–Amur terrane was formed 95 Ma ago at 54° N in the active continental margin conditions. It is found that the transform continental margin of Eurasia developed in the time interval from 105 to 65 Ma ago in the regime of a left-lateral submeridional shear from 30° to 60° N. The complete attachment of the studied rocks of the Kiselevka–Manoma terrane to the Eurasia’s margin (to the Zhuravlevsk–Amur terrane) occurred at the boundary of 60–70 Ma. Simultaneously, the sense of the displacement in the submeridional shears changed from left-lateral to right-lateral with the formation of pullapart type basins (Lake Udyl’).     

Volcanic rocks of the Khabarovsk accretionary complex,southern Far East Russia

The results of study of the volcanic rocks of the Khabarovsk accretionary complex, a fragment of the Jurassic accretionary prism of the Sikhote Alin orogenic belt (the southern part of the Russian Far East), are presented. The volcanic rocks are associated with the Lower Permian limestones in the mélange blocks and Triassic layered cherts. The petrography, petrochemistry, and geochemistry of the rocks are characterized and their geodynamic formation conditions are deduced. The volcanic rocks include oceanic plume basalts of two types: (i) OIB-like intraplate basalts formed on the oceanic islands and guyots in the Permian and Triassic and (ii) T(transitional)-MORBs (the least enriched basalts of the E-MORB type) formed on the midoceanic ridge in the Permian. In addition to basalts, the mélange hosts suprasubduction dacitic tuff lavas.     

Petrogeochemical conditions and geodynamic settings of volcanic rocks in the Kiselyovka–Manoma accretionary complex (Russian Far East)

The Kiselyovka–Manoma accretionary complex formed at the end of the Early Cretaceous during subduction of the Pacific oceanic plate underneath the Khingan–Okhotsk active continental margin along the east of Eurasia. It is composed of Jurassic–Early Cretaceous oceanic chert, siliceous mudstone, and limestone that include a significant amount of basic volcanic rocks. The known and newly obtained data on the petrogeochemistry of the Jurassic and Early Cretaceous basalt from various parts of the accretionary complex are systemized in the paper. Based on the comprehensive analysis of these data, the possible geodynamic settings of the basalt are considered. The petrogeochemical characteristics provide evidence for the formation of basalt in different parts of the oceanic floor within the spreading ridge, as well as on oceanic islands far from the ridge. The basalts of oceanic islands are mostly preserved in the accretionary complex. The compositional variations of the basalts may be controlled by the different thickness of the oceanic lithosphere on which they formed. This is explained by the varying distances of the lithosphere from the spreading zone.