S. A. Kasatkin

The nature of the continental crust of Sikhote-Alin as evidenced from the Nb isotopy of Rocks of Southern Primorie

The study of the Nb isotopy of rocks of the conti� nental crustal is an effective tool for evaluating the nature of mechanisms of crustforming processes and ages of their manifestation (1). In the first approxima� tion, the Nd model isotope age characterizes the age of formation of the continental crust from the mantle source or, in other words, transformation from oceanic crust to continental. The Nd isotope systematics of sedimentary rocks of upper levels of the continental crust allows us to estimate the average model age and the possible paleogeographic source areas, as well as to determine the lower age limit of sedimentation if there are no paleontological data. Moreover, the use of two� stage Nd model ages of granitoides along with the geochemical and petrological data obtained allow us to obtain data about the deep levels of the continental crust (levels of generation of the granitic melt). In some cases, the combination of these approaches makes it possible to reconstruct the chemical compo� sition and the nature and mechanisms of formation of the continental crust of separate blocks (terranes) and large fold belts (2).

Nd Isotope Systematics in Metamorphic Rocks of the Southern Russian Far East

Sm–Nd isotopic–geochemical investigations are widely used in the study of metamorphic sequences for estimating the age of continental crust formation and the lower age limit of protoliths of metamorphic com� plexes [1]. When the protolith is of primary magmatic origin, the Nd model age reflects in the first approxi� mation the onset of the continental crust formation from the mantle source or, in other words, the time of transformation of the oceanic crust into the continen� tal crust. The Nd isotope systematics of metamorphic terrigenous sedimentary rocks offers the opportunity to estimate the average model age and likely paleogeo� graphic provenances, as well as the lower age limit of sediment accumulation. In this communication, we discuss the data of the Nd isotopic composition in metamorphosed rocks from the three largest blocks located in the southern Far East areas: Matveevka–Nakhimovka, Sergeevka, and Anyui. It should be noted that the studied com� plexes were traditionally considered as representing inliers of the Early Precambrian crystalline basement and only recently have they been regarded as Neogean complexes that were formed by accretion, collision, or postcollision processes [2, 3]. The available geochro� nological data provide no grounds for answering the following question: what is the lag between metamor� phic transformation of rocks and their geological age? The Nd isotope measurements performed for meta� morphic complexes of the southern Far East bring us nearer to assessment of the lower age limits of their formation and to determination of the isotope crust composition for the whole region. The Khanka superterrane is located in southwestern Primor’e region (Fig. 1). It is formed by several ter� ranes of various origins. The southern and northern parts of the superterrane represent fragments of the passive margin of a craton and Early Paleozoic oro� genic belt, respectively. The last of them comprises the Matveevka and Nakhimovka metamorphic terranes that differ from each other in the constituting rocks and degree of their metamorphic transformation [6]. The most intensely metamorphosed rocks of the Matveevka terrane are united into the Iman Group, which is traditionally subdivided into the Ruzhino (diopside–calcite and forsterite–calcite marbles, biotite and biotite–cordierite gneisses with intercala� tions of quartzites and marbles), Matveevka (biotite– sillimanite and biotite–garnet–cordierite gneisses with quartzite and marble intercalations), and Tur� genevka (biotite–amphibole gneisses, crystalline schists, and amphibolites) formations. In the Nakhi� movka terrane, metamorphic rocks are united into the Ussuri Group, which consists of the Nakhimovka (biotite and biotite–amphibolite gneisses with lenses of marbles and amphibolites) and Tat’yanovka (biotite, diopside, and muscovite–graphite crystalline schists) formations. The relationships between the Iman and Ussuri groups remain unknown.

Geochemistry of rocks in the Anuy metamorphic dome, Sikhote-Alin: Composition of the protoliths and the possible nature of metamorphism

The paper presents geological, geochemical, and isotopic data on metamorphic rocks in the Anuy block (dome) in the Northern Sikhote-Alin and the surrounding sedimentary rocks of the Samarka accretionary prism. The geochemistry and isotopic composition of the amphibolite-facies metamorphic rocks (variably migmatized gneisses and crystalline schists) in the Anuy block and unmetamorphosed Jurassic-Cretaceous sediments surrounding the block are proved to be similar. All of them corresponded to the erosion products of the transitional-type crust (mature island arcs and active continental margins), have similar major- and trace-element compositions, and Nd model ages of 1.25–1.4 Ga. The geochemistry and isotopic parameters of metapelites in the Anuy block are principally different from those of analogous rocks in the Khanka Massif (the latter rocks are erosion products of the mature crust and have a Nd model age of 1.7–1.9 Ga). The metabasites, which are found as beds and lenses in gneisses and crystalline schists in the Anuy block and among sedimentary rocks surrounding the block, have a composition corresponding to oceanic basalts of the N- and E-MORB types. Based on the synthesis of geological, geochemical and isotopic data it was suggested that the Anuy block could be not a fragment of the basement of an ancient continent (as was believed previously) but rather a complex of the Early Cretaceous granite-metamorphic core of the Cordilleran type.

Composition and formation settings of cretaceous terrigenous rocks of the West Sakhalin terrane

The source areas and formation settings of Cretaceous rocks of the West Sakhalin terrane are established on the basis of the composition of terrigenous rocks. Sedimentation along the continent–ocean boundary was accompanied by large-scale left-side transform slides of the Izanagi Plate relative to the Eurasian continent with a minor role of volcanic processes. The ensialic island arc, granite-metamorphic rocks of the mature continental margin, and fragments of ancient accretionary prisms with ophiolites and cherts were the sources of clastic material.

First evidence for the Middle Triassic volcanism in South Primorye

A detailed study of a relatively well-exposed fragment of the Barabash Formation in the southern part of the Voznesenka terrane is carried out to specify the geodynamic settings of the Permian volcanogenic and volcanogenic-sedimentary complexes in South Primorye. It is established that the basaltic flows juxtaposed in the studied sequence originated from sharply different sources. The geochemical characteristics indicate that the basalts from the sequence base were presumably derived by melting of oceanic lithospheric mantle or asthenosphere, while the source of the overlying basalts was lithospheric mantle reworked by a subduction process. The basalts are subsequently overlain by tuffaceous–terrigenous and terrigenous rocks and limestones with remains of Capitanian (Middle Permian) fauna. Accessory zircons extracted from the tuffaceous–terrigenous rocks yield an U–Pb concordant age of 233.3 ± 3.3 Ma (Middle Triassic Ladinian Stage) for the youngest zircon population. The obtained data lead us to conclude that the Barabash Formation is a tectonostratigraphic rather than stratigraphic unit and may be a fragment of the Triassic accretionary wedge. The obtained data cast doubt on the accepted assignment of this unit to the Voznesenka terrane. It is more logical to include it in the Laoelin–Grodekov terrane, which represents a fragment of the Late Paleozoic active continental margin. This suggests that the boundary between these blocks should be specified and the timing of the final stage of amalgamation of the Laoelin–Grodekov terrane with the terranes of the Bureya–Khanka orogenic belt should be revised.

GEOCHEMISTRY AND PETROGENESIS OF VOLCANIC ROCKS AND THEIR MANTLE SOURCE IN THE EAST VIETNAM SEA AND ADJACENT REGIONS IN THE CENOZOIC

The East Vietnam Sea is one of the largest marginal basins in western Pacific Ocenan, formed by breaking of continental margin in the Late Mesozoic. Geochemical data of the Miocene - Pleistocene bazanic samples collected in the East Sea and neighboring areas show two major eruption trends that reflect the formation and development of the region. The early eruption event is characterized by low alkaline, TiO2, Na2O, K2O and P2O5, and high SiO2 group, comprising olivine and tholeiitic bazans. The later eruption demonstrates high alkaline, TiO2, Na2O, K2O and P2O5, and low SiO2 group, mainly generated by central-type volcanic eruptions, consisting of alkaline olivine and olivine bazans. Distinctive geochemistry of the volcanic rocks within the East Vietnam Sea and adjacent areas is illustrated by wide range of Magnesium index (Mg#= 35-75). At the values of Mg#>65, the relation between Mg# and major oxides is unclear. In contrast, Mg# 65 (Olivine differentiation) the isotope ratios start changing. The primitive components are computed based on the principle of olivine compensation. The computed results show that the critical pressure for Tholeiite melting was estimated from ~11.97-20.33 Kb (ca. 30 - 60 km deep) and the Alkaline melting pressure varies from ~16.87-34.93 Kb (corresponding to the depths of ~60 km to 100 km). The continuous range of melting pressures suggests two trends of tholeiitic and alkaline eruptions occurr at various depths in the same magmatic source. Hight temperature and melting pressure of the primitive magma are dependent on partial melting pressure. Possibly, this process was triggered by the asthenosphere intrusion resulted from the closure of the Neo-Tethys following the India - Eurasia collision. This event has not only made the mantle hotter and easily melted but also triggered the opening of the marginal seas, including the East Vietnam Sea.

Paleozoic Granitoids of the Southern Part of the Voznesenka Terrane (Southern Primorye): Age, Composition, Melt Sources, and Tectonic Settings

This paper presents data on the geological position, geochemistry, age, and isotopic characteristics of the granitoids of the southern part of the Voznesenka terrane, Southern Primorye (Muraviev–Amursky Peninsula and its vicinities). All of the studied granitoids were formed in three stages: the Ordovician, Silurian, and Permian. The Silurian and Permian ages of the granitoid intrusions have been previously determined (Ostrovorussky Massif, 432–422 Ma, and 250 ± 4 Ma, early and late associations, respectively; Sedanka massif, 261 ± 3 Ma). The granites of the Artem and Nadezhdinsky massifs define an U–Pb zircon age of 481 ± 6 and 452 ± 4 Ma, respectively. The geochemical and isotope data show mainly the crustal nature of the granitoids. Their formation was related to melting of relatively immature rocks of the continental crust (mafic–intermediate volcanic rocks). The Nd isotope composition of the granitods (TNd(DM–2) = 1.3 Ga) indicates the absence of the mature ancient crust at the basement of the southern Voznesenka terrane. The maximum contribution of mantle sources to the granite formation is recorded in the Permian associations. A comparison of the peaks of intrusive magmatism in the southern part of the Voznesenka terrane and adjacent territories suggests that the formation of the granitoids of the Muraviev–Amursky Peninsula and its vicinities was caused by the interaction of continental blocks with two oceanic basins: the Paleoasian (and its fragments) and Paleopacific ones.

The Investigation Results of the April 12, 2014, M = 4.5 Primorye Earthquake (Far Eastern Russia)

Primorye is a region with quite moderate shallow seismicity which has been insufficiently investigated so far in this respect. Based on the obtained instrumental data of regional seismic networks and macroseismic data collected in southwestern Primorye on the crustal earthquake with M = 4.5 occurred on April 12, 2014, we have first succeeded in determining the hypocenter parameters and the focal mechanism of the mainshock of this shallow earthquake and estimating the hypocenter parameters of the following aftershock.

Fluid-Controlling Significance of the Nosappu Fracture Zone and Conditions for the Formation of Methane Fluxes and Gas Hydrates (Sea of Okhotsk Region)

It has been observed that the intensity of underwater gas flares unexpectedly increased after the deep-focus (625.9 km) earthquake that occurred in the Sea of Okhotsk on August 14, 2012. In this regard, we have analyzed the data resulting from interpretation of the focal mechanism for the strike-slip earthquakes which occurred in the Benioff seismic zone of the subducting Pacific Plate within the Sea of Okhotsk region over the period from 1977 to 2010. The NNW sinistral and NE dextral faults are found to form a conjugate system due to the WNW stress field. We have established that the dextral faults are mostly common at a depth of about 200 km along the Kuril Islands extension, while the sinistral ones are concentrated in the Nosappu Fracture Zone and traced to the NNW down to a depth of 680 km. The area of the gas flare discharge and gas hydrate accumulations have the same (NNW) direction. Thus, we have revealed that the Nosappu Fracture Zone appears to be a structure which controls fluid fluxes, providing permeability of the subducting slab of the Pacific Plate for ascending fluids from the lower mantle.

Cretaceous Strike-Slip Dislocations in the Dalat Zone (Southeastern Vietnam)

Studies of ductile dislocations of the Jurassic terrigenous deposits and faulting kinematics of the Cretaceous injective structures in the Dalat zone show that the volcano-plutonic belt of southeastern Vietnam was formed under NE-striking sinistral dislocations affected by NNW transpression. This deformation took place in similar conditions as for the Cretaceous structure of the eastern margin of Asia. Due to the Indo-Eurasian collision, since the Cenozoic the Indo-Sinian Block has been displaced toward the southeast by a distance of more than 500 km from its primary position, causing transformation of the previously formed Cretaceous structures. Thus, the Cenozoic tectonogenesis in southeastern Vietnam should be studied with regard to the previous Cretaceous tectonic deformation.