Okhotsk-Chukotka Volcanic Belt Research Articles

Provenance analysis for Middle Eocene sediments in the West Kamchatka sedimentary basin (Tigil area)

The paper presents the results of reconstruction of Middle Eocene provenances for the West Kamchatka sedimentary basin (WKSB) corresponding to the Tigil area. It has been established that the early (Eocene) evolution stage of WKSB was marked by the deposition of terrigenous sediments in intermontane depressions followed by the accumulation of shallow-marine sediments after transgression. In terms of the composition, sandstones of the Middle Eocene Snatol Formation correspond to graywackes. With respect to the geochemistry of sandstones, their provenances were confined to an active continental margin and island arc. The mineral composition of the heavy fraction suggests an alternating dominance of felsic and mafic rocks in the provenances. Dating of the clastic zircon from sandstones of the Snatol Formation by the LA-ICP-MS method revealed a wide variation range of their age. The most significant peak is close to the age of calc-alkaline magmatism in the Okhotsk–Chukotka volcanic belt. This fact provides insight into the Eocene paleogeography: the major rock provenances were located in the Okhotsk–Chukotka volcanic belt and the eastern Achaivayam–Valagin island arc. Local sources of clastic material were represented by the Utkholok and Kinkil volcanic belts.

Isotopic age of flora-bearing beds from the Amka Formation stratotype, Okhotsk-Chukotka volcanic belt

U-Pb SHRIMP-dating of zircons from flora-bearing volcanic rocks of the Amka Formation stratotype (Ul'ya depression, Okhotsk-Chukotka volcanic belt) yield weighted mean 206Pb/238U age of 85.5 ±2 Ma (Santonian to Coniacian stage). This isotope dating is consistent with inferred Coniacian age of Ul’ya flora from the Amka Formation.

Tectonics and metallogeny of the junction zone between the North Asian craton and Pacific tectonic belt

The tectonics and metallogeny of the junction zone between the North Asian craton and Pacific tectonic belt are considered. This zone is characterized by a wide variety of structures superposed on the metamorphic basement, which was formed in the course of a multistage geologic development of the craton from the Precambrian to the Cenozoic. They are related to the craton evolution and its response to the collision and subduction processes in the adjacent orogenic belt, processes in the passive and active continental margins, and plume magmatism. The geological structure of the region includes blocks of metamorphic rocks of the Aldan–Stanovoi shield, Paleoproterozoic volcanogenic troughs, Mesoproterozoic–Neoproterozoic and Early Paleozoic structures of the platform cover, Late Paleozoic volcanic and terrigenous troughs, structures of the Late Mesozoic Okhotsk–Chukotka volcanic belt of the active continental margin, and Late Cretaceous riftogenic structures formed in response to plume magmatism. In total, six metallogenic epochs are recognized in the development of ore mineralization: Archean–Early Paleoproterozoic, Late Paleoproterozoic, Mesoproterozoic, Neoproterozoic, Late Paleozoic, and Late Mesozoic. The minerageny of the junction zone between the craton and Pacific belt is highly diversified, being characterized by distinct evolution in time and space. Each development stage features its own set of mineral resources.

40Ar/39Ar ages and geochemical characterization of Cretaceous bentonites in the Nanushuk, Seabee, Tuluvak, and Schrader Bluff formations, North Slope, Alaska

Diagenetically altered volcanic ash deposits (bentonites) found in Cretaceous terrestrial and marine foreland basin sediments have the potential to be used for chronostratigraphy and subsurface correlation across Alaska's North Slope. Detailed age and geochemical studies of these volcanogenic deposits may also shed light on the tectonic evolution of the Arctic. Though these bentonites have been previously studied, there are few published results for regional bentonite ages and geochemistry due to challenges of dating weathered volcanic ash. We analyzed mineral separates from cored bentonites recovered from wells in the National Petroleum Reserve Alaska. The analyses confirm that an intense period of volcanic ash deposition on Alaska's North Slope began by the late Albian and persisted throughout the Cenomanian, an interval of rapid progradation and aggradation in the Colville basin. These results also add to a sparse record of radioisotopic ages from the Nanushuk Formation. A bentonite preserved in delta plain sediments in the upper Nanushuk Formation dates to 102.6 ± 1.5 Ma (late Albian), while a bentonite near the base of the overlying Seabee Formation was deposited at 98.2 ± 0.8 Ma, in the early Cenomanian. The two ages bracket a major flooding surface at the base of the Seabee Formation near Umiat, Alaska, placing it near the Albian-Cenomanian boundary (100.5 Ma). Several hundred feet up-section, the non-marine Tuluvak Formation contains bentonites with 40Ar/39Ar ages of 96.7 ± 0.7 to 94.2 ± 0.9 Ma (Cenomanian), several million years older than previously published K–Ar ages and biostratigraphic constraints suggest.Major and trace element geochemistry of a sub-sample of six bentonites from petroleum exploration wells at Umiat show a range in composition from andesite to rhyolite, with a continental arc source. The bentonites become more felsic from the late Albian (∼102 Ma) to late Cenomanian (∼94 Ma). A likely source for the bentonites is the Okhotsk-Chukotka Volcanic Belt (OCVB) of eastern Siberia, a continental arc which became active in the Albian and experienced episodes of effusivity throughout the Late Cretaceous. Chronostratigraphically anomalous 40Ar/39Ar ages coincide with peaks of magmatic activity in the OCVB, suggesting that these anomalously old ages may be due to magmatic contribution of xenocrysts or recycling of detrital minerals from older volcanic events. An alternative explanation for the chronostratigraphically anomalous ages is mixing of bentonites with detrital sediment derived from unroofing and erosion of metamorphic rocks in the Brooks Range, Herald Arch, and Chukotka throughout the mid to Late Cretaceous.

Growing conditions of the Ust’-Emuneret flora and the Santonian-Campanian climate of Chukotka

New data on climatic conditions of the Ust’-Emuneret flora growth in the Enmyvaam River basin, Okhotsk-Chukotka volcanic belt, are reported. A great diversity of woody dicot leaves in the studied flora permits for the first time for this region the reconstruction of quantitative paleoclimatic parameters using the CLAMP analysis. On the basis of the available results, the Santonian-Campanian climate of the northern Okhotsk-Chukotka volcanic belt and the adjacent Anadyr-Koryak region is analyzed.

Paleomagnetism of the Late Cretaceous ignimbrite from the Okhotsk-Chukotka Volcanic Belt, Kolyma-Omolon Composite Terrane: Tectonic implications

New Late Cretaceous paleomagnetic results from the Okhotsk-Chukotka Volcanic Belt in the Kolyma-Omolon Composite Terrane yield stable and consistent remanent directions. The Late Cretaceous (86–81Ma) ignimbrites from the Kholchan and Ola suites were sampled at 19 sites in the Magadan area (60.4°N, 151.0°E). We isolated the characteristic paleomagnetic directions from 16 sampled sites using an alternating field demagnetization procedure. The primary nature of these directions is ascertained by dual polarities and positive fold tests. A tilt-corrected mean direction (D=42.8°, I=84.7°, k=46.0, α95=10.0°) yields a paleomagnetic pole of 66.7°N, 168.5°E (A95=18.8°) which appears almost identical to the 90–67Ma pole reported from the Lake El’gygytgyn area of the Okhotsk-Chukotka Volcanic Belt (Chukotka Terrane). This consistency suggests that the Kolyma-Omolon Composite Terrane and Chukotka Terrane has acted as a single tectonic unit since 80Ma without any significant internal deformation. Accordingly, we calculate a combined 80Ma characteristic paleomagnetic pole (Long.=164.7°E, Lat.=68.0°, A95=10.9°, N=12) for the Kolyma-Omolon-Chukotka Block which falls 16.5–17.5° south of the same age poles from Europe and East Asia. We ascribe this discrepancy in pole positions to tectonic activity in the area and infer a southward displacement of 1640±1380km for the Kolyma-Omolon-Chukotka Block with respect to the North American and Eurasian blocks since 80Ma; more than 260km of it is attributed to tectonic displacement in the Arctic Ocean due to the opening of the Canadian Basin.

Новый вид рода Sequoia Endl. (Cupressaceae) из позднемеловых отложений Северо-Востока России

New species Sequoia ochotica Yudova et Golovn. (Pinopsida, Cupressaceae) from the Turonian-Coniacian deposits of the Arman and Chingandzha Formations of the Okhotsk-Chukotka volcanic belt is described based at morphological features of leaves and shoots. Two other Late Cretaceous species of this genus: S. minuta Sveshn. from the Vilyui River basin of Eastern Siberia and S. tenuifolia (Schmalh.) Sveshn. et Budants. from the New Siberian Islands have comparable shoot morphology, but these species were described based at epidermal features.

Petrology and geochemistry of the Cretaceous granitoid magmatism of Central Kamchatka, exemplified by the Krutogorova and Kol’ intrusive complexes

The problem of the geochemical classification of granitoid magmatism in the zone of interaction of oceanic and continental plates is considered in this paper by the example of Mesozoic granitoids of the Krutogorova and Kol’ intrusive complexes of the Sredinny Range, Kamchatka. Based on new geological, petrological, and geochemical data (including the Sr, Nd, and Pb isotope systematics of rocks), it was shown that the protoliths of the granitoids were volcanic-terrigenous sequences accumulated within a Cretaceous marginal basin in the eastern Asian continent. The granitoids crystallized at ∼80 Ma (SHRIMP U-Pb age) under the conditions of the andalusite-sillimanite depth facies corresponding to a pressure of approximately 2 kbar and induced contact metamorphism in the host sequences, which are made up of sediments with sheetlike bodies of mafic and ultramafic volcanics (Kikhchik Group and its metamorphic analogues of the Kolpakova, Kamchatka, and Malki groups). The lower age boundary of sedimentation of the host sequences and the time of basic volcanism coincide with the beginning of the formation of the Okhotsk-Chukotka volcanic belt. Such a correlation is not accidental and reflects a genetic connection between the processes of magmatic activation in the continental-margin sedimentary basin and the formation of the continental margin volcanic belt in eastern Asia. The development of basic volcanism in the sedimentary basin accompanied by the ascent of deep fluids resulted in the entrainment of crustal materials into magmatic processes and the formation of crustal magma chambers, the activity of which was manifested by the eruption of intermediate and silicic lavas and emplacement of shallow granitoid intrusions of considerable areal extent. These intrusions induced contact metamorphism in the enclosing volcanosedimentary complexes. The subsequent Eocene (60-50 Ma) collision processes related to the obduction of the oceanic segment of the crust of the transitional zone onto the Asian continental margin resulted in the tectonic piling of the rocks of Central Kamchatka and strong crustal thickening, which was favorable for its metamorphic alteration reaching the kyanite-sillimanite depth level of the amphibolite facies under the influence of a thermal front and deep fluids affecting lower crustal zones. The Eocene regional metamorphism caused not only metamorphic transformations, migmatization, and granitization in the sequences of the Sredinny Range, which underwent only contact hornfels formation during the first stage, but also metamorphism, migmatization, and extensive foliation in the igneous rocks of the Kol’ and Krutogorova complexes, which were transformed into gneissic metagranites.

Distribution and transport processes of lithogenic material from the Amur River revealed by the Sr and Nd isotope ratios of sediments from the Sea of Okhotsk

Primary productivity is very high in the Sea of Okhotsk, most likely because of terrigenous input from the Amur River that includes dissolved matter and suspended matter. To reveal the transport and deposition processes of the fluvial materials in the northwestern continental shelf of the Sea of Okhotsk, the Sr and Nd isotopes of the detritus in the surface sediments and suspended matter were investigated. The regional variations of the radiogenic isotopes indicate that the detritus in the continental shelf has three main sources: Amur River detritus, with a high 87Sr/86Sr ratio (0.711–0.715) and relatively low εNd value (−7.6 to −7.3); volcanic detritus derived from the Okhotsk-Chukotka volcanic belt to the north of the Okhotsk Sea, with a low 87Sr/86Sr ratio (0.703) and high εNd value (+7 to +8); and detritus from the sedimentary rocks north of Sakhalin Island, with a high 87Sr/86Sr ratio (0.709–0.710) and low εNd value (−10.2 to −9.2). The results indicate that the Amur River detritus is dispersed across the northwestern continental shelf and further transported southward along the east coast of Sakhalin while flowing out of the shelf. Around the northeast coast of Sakhalin Island, the coarse-grained detritus from the north region of Sakhalin Island is distributed only in the narrow region of the shallow depth area and is not transported to the shelf slope. The Amur River detritus flowing out of the shelf is mixed with the Okhotsk-Chukotoka volcanic material transported from the northern area of the Okhotsk Sea by the Okhotsk Sea intermediate water (OSIW). The transport processes of the Amur River detritus in this area are supported by previous studies of turbidity that indicated that the dense shelf water (DSW) entrains resuspended sedimentary particles on the northwestern continental shelf and transports them to the OSIW.

Geochemistry and genesis of metabasites from the Kolpakova Group of the Sredinnyi Range in Kamchatka

The data on the Sm, Nd, Rb, Sr, and Pb isotope compositions obtained for sedimentary and igneous rocks from the Sea of Okhotsk [1] and recent informa� tion on zircons from metaterrigenous rocks and gran� itoids of the Sredinnyi Range crystalline massif in Kamchatka [2–5] imply the existence of an epiconti� nental crust beneath this basin and influx of terrige� nous material constituting sedimentary sequences of the Sredinnyi Range from the eroded mature conti� nental crust. The new data on volcanogenic rocks from the ter� rigenous sequences of the Sredinnyi Range indicating that deposition of terrigenous sediments was accom� panied by intense volcanism synchronous with the ini� tiation of the formation of the Okhotsk–Chukotka Volcanic Belt supplement to our knowledge of this region [6]. The wide distribution of mafic and ultra� mafic volcanites in terrigenous sections suggests the existence of a magmatic chamber beneath the sedi� mentary basin, which served presumably as a source of heat and fluids responsible for granitization and meta� morphism of rocks constituting the Sredinnyi Range of Kamchatka. In this communication, we present new data on the geochemistry and mineralogy of metabasites from the Kolpakova Group of the Sredinnyi Range massif in Kamchatka with discussion of a possible mechanism of their formation among terrigenous sequences of the Cretaceous epicontinental marginal basin. The Kolpakova Group of apoterrigenous meta� morphic rocks over 2500 m thick crops out at the base of the observable section of the Sredinnyi Range mas� sif, where it consists of the lower, middle, and upper lithological units. The lower unit, which is most wide� spread in the massif limits, is composed of migmatized kyanite–garnet–biotite ± staurolite ± muscovite and garnet–biotite–muscovite plagiogneisses and migma� tites. The middle unit is largely represented by biotite and garnet–biotite plagiogneisses with migmatization slightly less intense as compared with that in the lower unit. The upper unit crops out in the northern part of the massif on the right side of the Zolotaya River head and left side of the Srednyaya Andrianovka River. It is composed of alternating amphibolites, garnet amphibolites, clinopyroxene–amphibole (frequently with garnet) crystalline schists, garnet–amphibole, garnet–biotite–amphibole, garnet–biotite, and kyan�

Mineral types of hydrothermal alteration zones in the Dukat ore field and their relationships to leucogranite and epithermal gold-silver ore, northeastern Russia

The paper considers the localization of potassic and propylitic hydrothermal alteration zones in the domal volcanic-plutonic structure controlling the position of the Dukat ore field with the eponymous unique epithermal Au-Ag deposit. Comprehensive mineralogical and geochemical data on rocks and minerals in hydrothermal alteration zones and associated intrusions have shown that quartz-jarosite-sericite, quartz-pyrite-sericite, and quartz-adularia-chlorite alterations were formed with the participation of fluid flows related to a fingerlike projection of a high-K leucogranite porphyry intrusion with large phenocrysts. These hydrothermal alterations developed in the rifted graben under conditions of divergent plate boundaries, whereas quartz-clinozoisite-calcite, epidote-chlorite, and garnet-calcite-chlorite alterations were linked to K-Na leucogranite intrusive bodies and developed under conditions of convergent plate boundaries reactivated as a result of formation of the marginal Okhotsk-Chukotka volcanic belt. Phase separation and coagulation of specific portions of ascending fluids resulted in the formation and stabilization of small-sized particles of native silver and other ore components, which enabled involvement in flows of secondary geothermal solutions and ore-forming fluids. The Sr, Nd, and Pb isotopic compositions of rocks and minerals from the hydrothermal alteration zones, associated intrusions, and economic orebodies at the Dukat deposit indicate that their components have been derived from the juvenile continental crust, which was altered in pre-Cretaceous periods of endogenic activity. The components of gangue minerals of potassic and propylitic hydrothertmal alterations and associated intrusions have been taken from deep sources differing in 87Sr/86Sr and 143Nd/144Nd at similar U/Pb and Th/Pb ratios. Chalcophile lead in products of hydrothermal activity and melanocratic inclusions in leucogranite has been taken from regions with elevated U/Pb and Th/Pb ratios.

Petrography, geochemistry, and Hf‐Nd isotope evolution of drill core samples and target rocks from the El'gygytgyn impact crater, NE Chukotka, Arctic Russia

AbstractEl'gygytgyn in northeast Chukotka (Russia) is a 3.6 Ma, 18‐km‐diameter impact structure. The impact crater was recently drilled in the framework of a project sponsored by the International Continental Scientific Drilling Program (ICDP). Target rocks at the El'gygytgyn area are dominated by the felsic members of the Late Cretaceous Okhotsk‐Chukotka Volcanic Belt (OCVB). Such a target lithology is unique among terrestrial impact craters, thereby providing the opportunity to study shock metamorphism in siliceous volcanic rocks. Here, we present a petrographic, geochemical, and isotopic study of the section of the drill core underneath the lacustrine sediments, extending from ∼316 m to 517 m below the lake bottom (blb). The drill core stratigraphy includes ∼80 m of suevite and a cross section through a volcanic suite, which consists of (1) a middle section (∼390–423 mblb) with dominant felsic tuffs and a few mafic members, and (2) a welded rhyolitic‐dacitic ignimbrite (∼423–517 mblb). The melt fragments embedded in the suevite are interpreted as being impact‐related by comparison with impact glasses from the crater and in opposition to the target rock, which does not include similar melts. A suevitic dyke crosscuts the lower section of the core at the depth 471.40 mblb. Evidence for shock metamorphism is concentrated in the upper 10 m of the drill core and almost limited to the suevitic breccia. The geochemical and isotope (Nd and Hf) composition of samples from the target and the drill core reveals relationships to the “Berlozhya magmatic assemblage” (BMA) arguing for similar source magmas. The primitive upper mantle (PUM)‐normalized trace element plot of rocks investigated here confirms a subduction‐related signature, as previously proposed for rocks from both OCVB and BMA.

Conditions of the formation of hypabyssal granitoid intrusions using the example of the Sizindzha Massif, the Okhotsk-Chukotka volcanic belt

The Sizindzha hypabyssal intrusive massif located in the Kuidusun volcanic zone of the Okhotsk-Chukotka volcanic belt has been mapped and studied in detail. The complex of textural-petrographic features indicates the in situ formation of granitoids by the successive replacement of the host sedimentary rocks of the Verkhoyansk Complex and the medium-grained diorites (more rarely, gabbro) by later granites. It was established that the oldest fine-grained intermediate rocks observed as xenoliths in all the granitoids are in situ relicts of the molten substrate.

Ore formation at the Kupol epithermal gold-silver deposit in northeastern Russia deduced from fluid inclusion study

The Kupol epithermal gold-silver deposit-the largest of this type of mineralization in northeastern Russia-is situated in the outer zone of the Okhotsk-Chukotka volcanic belt. The results of thermobarogeochemical study of fluid inclusions in quartz from ore veins at the Kupol deposit are compared with the data on the Dvoinoi and Arykvaam deposits. The study of aqueous extracts from fluid inclusions revealed that the chemical compositions of ore-forming fluids at the Dvoinoi and Kupol deposits are similar in most elements. The only substantial difference is that fluids from the Kupol deposit are considerably enriched in sulfate, as is characteristic of the alunite-subtype of epithermal high-sulfidation mineralization. The salinity of aqueous solutions filling inclusions in amethyst and quartz from ore veins at the Kupol and Dvoinoi deposits is two-three times higher than the salinity of fluid inclusions from the barren veins at the Arykvaam occurrence. The data obtained support the hypothesis put forward earlier that fumaroles and solfataras played a part in ore deposition at the Kupol deposit.

Geological structure of the Kashevarov Trough (central Sea of Okhotsk)

This paper reports the composition and age of rocks dredged from the Kashevarov Trough (central Sea of Okhotsk) during cruise 41 of the R/V Akademik M.A. Lavrentyev in 2006. It was found that the Late Cretaceous and Eocene volcanics from the Kashevarov Trough and Okhotsk-Chukotka volcanic belt, struc� tures of which are traceable in the Sea of Okhotsk, have similar petrographic and geochemical features. The Cenozoic sedimentary cover consists of three differentag e complexes: (1) the late Oligocene (~28.2-24.0 Ma); (2) the terminal late Oligocene-early Miocene (24.0-20.3 Ma); (3) the terminal late Pliocene-early Pleis� tocene (2.0-1.0 Ma). The upper Oligocene-lower Miocene sediments were deposited in relatively shallow� water settings, whereas the late Pliocene-early Pleistocene complex was formed in deeper environments, which was probably determined by tectonic processes. The geological data indicate that the Kashevarov Trough and the surrounding underwater rises represented in the Oligocene-early Miocene a single shelf zone of the Sea of Okhotsk, which is underlain by a structurally integral Mesozoic basement and is now subsided to depths of 800-1000 m.

Secondary quartzites and their relation to gold mineralization at the Svetloe deposit, Ul’ya Trough, Okhotsk-Chukotka volcanic belt

Hydrothermally altered rocks and their relation to gold mineralization at the Svetloe deposit in the Ul’ya Trough, western sector of the Okhotsk-Chukotka volcanic belt, are studied. The rock composition and style of mineralization are typical of acid-sulfate type deposits. The hydrothermally altered rocks of the deposit are medium-temperature secondary quartzites with developed alunite facies. The upper structural level in the massif of secondary quartzites is made up of a sheetlike body 30 km2 in area and up to 300 m thick with gentle (5–10°) periclinal bedding relative to the arcuate axis of the volcanic range. Below the sheet, there are a number of local steep mostly carinate zones and small (a few hundreds of square meters) multilayered trough-shaped structures, the foot of which is located 100–150 m under the secondary quartzite sheet. All structures of the lower level are restricted to the volcanic range pericline, marked by elevated thicknesses of monoquartzites, and have a gradual transition to the sheetlike part of the massif. The massif of secondary quartzites shows symmetrical zoning. The central zone of the sheetlike body is composed of alunite quartzites framed by dickite varieties. Down the section, the latter grade into hydromica and hydromica-montmorillonite argillic rocks. Monoquartzites form axial zones in the structures of the lower level. In the blanketlike part of the deposit, monoquartzites occur as rare thin lenses and interlayers spatially related to local structures of the lower level. Gold mineralization at the deposit associates with porous monoquartzites. Gold precipitated after quartzite formation during infilling of pores and cavities with colloform silica with barite, and hypogene jarosite.

Evolution of calc-alkaline magmas of the Okhotsk-Chukotka volcanic belt

Petrological, geochemical, and isotope geochronological aspects of the evolution of calc-alkaline magmatism were investigated in the Western Okhotsk flank zone, the Okhotsk segment, and the Eastern Chukchi flank zone of the Okhotsk-Chukotka volcanic belt (OCVB). The OCVB is a tectonotype of continental margin volcanic belts comprising much greater volumes of felsic ignimbritic volcanics compared with mature island arcs (MIA, Kuril-Kamchatka and Aleutian) and the Andean continental margin. The volcanic rocks of continental margin volcanic belts (OCVB and Andean belt) are enriched in K, Ti, and P compared with the rocks of MIA and show a trend toward the field of high-potassium calc-alkaline series. Primitive andesite varieties (Mg# > 0.6) were not yet found in the OCVB, but there are relatively calcic varieties unknown in Andean-type structures and a significant fraction of moderately alkaline rocks, which are not typical of MIA. Variations in trace and major element characteristics in the basalts and andesites of the OCVB were interpreted as reflecting the competing processes of assimilation/mixing and fractional crystallization during the evolution of the parental basaltic magma. Significant lateral variations were established in the composition of the mantle sources of calc-alkaline magmas along the OCVB over more than 2500 km. The initial isotopic ratios of Sr, Nd, and Pb in the volcanics of the Okhotsk segment are relatively depleted and fall near the mixing line between PREMA and BSE. The magma source of the Western Okhotsk flank zone is most enriched and approaches EMI, whereas that of the central and eastern Chukchi zones contains an admixture of the EMII component. The geochronological characteristics of all the main stages of OCVB magmatism were comprehensively studied by U-Pb SHRIMP and ID-TIMS zircon dating (86 samples) and 40Ar/39Ar analysis (73 samples). In general, a discontinuous character was established for the OCVB magmatism from the middle Albian to the early Campanian (106–77 Ma). The volcanism is laterally asynchronous. There are several peaks of volcanism with modes at approximately 105, 100, 96, 92.5, 87, 82, and 77 Ma. The Coniacian-Santonian peaks correspond to the most extensive stages of the middle and late cycles of felsic volcanism. A decreases and a hiatus in magmatic activity were reconstructed for the end of the Cenomanian and the beginning of the Turonian. The volcanism was terminated by plateau basalts with ages of 76–78 Ma, which mark a change in the geodynamic setting from frontal subduction to the regime of a transform margin with local extension in zones normal to the slip direction. A catastrophic character of eruptions with rather narrow ranges of volcanism (<2 Myr) were established taking into account new reliable age estimates for some individual large calderas. The accumulation rate of volcanic materials in such structures was up to 0.15–0.36 km3/yr and even higher.

Phenocrysts in silicic volcanic rocks in the northern part of the Okhotsk-Chukotka Belt and their crystallization conditions

Microprobe analyses of phenocrysts in silicic volcanic rocks from the northern part of the Okhotsk-Chukotka Volcanic Belt are reported. The parameters of the state of the magmatic system evaluated using techniques of mineralogical thermobarometry indicate that the phenocrysts crystallized at 750–1050°C. The most realistic results are likely yielded by the QUILF program package and the amphibole-plagioclase equilibrium. The calculated temperatures are sometimes higher than the average estimates made for other volcanic areas worldwide with the use of analogous mineral assemblages. The likely explanation of this is a relatively low water pressure in the parental silicic magmas of the Okhotsk-Chukotka Volcanic Belt (OCVB), which, in turn, suggests that the protolith did not contain much hydrous phases. The lithostatic pressure under which the amphibole-bearing assemblages crystallized was evaluated at 1–4 kbar, which corresponds to depths of 4–15 km. The Mg# of the biotite and amphibole exhibits obvious spatial variations, likely due to variations in the oxygen regime in the feeding magmatic chambers. The $f_{O_2 }$ level calculated for the frontal zone of the Central Chukotka Segment (CCS) varies between NNO + 1 and NNO + 3, and those from Anadyr Segment (AS) and the rear zone of the Central Chukotka Segment (CCS) are between QFM − 1 and NNO + 1. The possible factors that most efficiently controlled the oxygen fugacity in the silicic magmas seem to have been variations in the contents of highly reduced phases (such as organic carbon) in the crustal protolith. At a relatively homogeneous bulk composition of the silicic volcanics, the compositions of phenocrysts of Fe-Mg silicates can significantly vary, and this makes it possible to utilize these phenocrysts as an additional tool for the differentiation and correlation of volcanic sequences.

The genus Heilungia Prynada (Cycadopsida) in the Late Cretaceous floras of the Okhotsk-Chukotka volcanic belt (Northeastern Russia)

New finds of Heilungia Prynada are described from the Turonian-Coniacian volcanogenic sediments of the Chaun Series (central Chukchi Peninsula) and Kananyga Formation (northern coast of the Sea of Okhotsk) of Okhotsk-Chukotka volcanic belt. The accomplished revision of newly described and earlier described species of cycadophytes with marginal fan-shaped venation has resulted in the union of the genera Heilungia and Butefia. The occurrence of members of Heilungia is traced in the Cretaceous deposits of the Okhotsk-Chukotka volcanic belt.

The diachronous formation of the Enmyvaam and Amguema-Kanchalan volcanic fields in the Okhotsk-Chukotka volcanic belt (NE Russia): Evidence from isotopic data

The diachronous formation of the Enmyvaam and Amguema-Kanchalan volcanic fields in the Okhotsk-Chukotka volcanic belt (NE Russia): Evidence from isotopic data