Okhotsk-Chukotka Volcanogenic Belt Research Articles

On the Likely Palaeoelevation of the Turonian – Coniacian Arman Flora Site (North-Eastern Asia)

The Turonian - Coniacian (Late Cretaceous) Arman Flora (North-eastern Russia, Magadan District) exhibits a high diversity of fossil angiosperms (28 dicot species). Based on their physiognomy, palaeoclimate variables were estimated showing that the flora experienced a humid warm-temperate climate without any pronounced seasonality. Palaeoclimate variables estimated for the coastal plain floras (Penzhina, Kaivayam and Tylpegyrgynai floras, North-eastern Russia) are the same (within analytical errors) as those estimated for the Arman Flora that existed within uplands of the Okhotsk-Chukotka volcanogenic belt. This conclusion implies that the elevation of the intermontane basin of the Arman Flora biotope was not high, presumably not more than 600 m - and not sufficient to be reflected in palaeoclimate experienced by the Arman Flora.

Ginkgophytes of the Ul’ya flora (the Ul’ya depresiion, the Okhotsk-Chukotka volcanogenic belt)

The Ul’ya flora comes from the Coniacian volcanogenic deposits of the Amka Formation (the Ul'ya depression, southern part of the Okhotsk-Chukotka volcanogenic belt). Ginkgoaleans are diverse in this flora and represented by three genera: Ginkgo, Sphenobaiera and Baiera. All specimens have no cuticle and were assigned to morphotaxa. Genus Ginkgo includes two species: G. ex gr. adiantoides (Ung.) Heer with entire leaves and G. ex gr. sibirica Heer with dissected leaves. Genus Sphenobaiera also consists of two species: S. ex gr. longifolia (Pom.) Florin with 4–8 leaf lobes and S. ex gr. biloba Prynada with two leaf lobes. Genus Baiera is represented by new species B. lebedevii Golovn., sp. nov.Leaves of this species are 25–30 cm long and 13–16 cm wide, narrowly wedge-shaped with flat slender petiole, dichotomously dissected 4–5 times into linear segments 3–6 mm wide with 6–12 veins. The length of ultimate segments is equal to about a half of leaf length. Leaves attached spirally to ovoid short shoots about 2 cm long. Among the Late Cretaceous floras similar diversity of ginkgoaleans was recorded only in the Turonian-Coniacian Arman flora from middle part of the Okhotsk-Chukotka volcanogenic belt (Herman et al., 2016). Four species of ginkgoaleans from the Ul’ya flora (except G. ex gr. adiantoides) are considered as the Early Cretaceous relicts.

Late Cretaceous Kholokhovchan Flora of Northeastern Asia: Composition, age and fossil plant descriptions

The Kholokhovchan Flora comes from tuffaceous – terrigenous deposits of the Vetvinskaya Member (Chalbugchan Group) in the Penzhina and Oklan rivers interfluve, Northeastern Russia. The depositional environment of the plant-bearing deposits is interpreted to have been a freshwater lake. The Kholokhovchan Flora hosts 42 fossil plant species belonging to Marchantiopsida, Polypodiopsida, Ginkgoales, Leptostrobales, Bennettitales, Pinales and Magnoliopsida. It is characterised by diverse angiosperms, less diverse conifers and ferns, by the presence of relatively ancient Sphenobaiera, Phoenicopsis and Pterophyllum together with advanced Late Cretaceous Taxodium, Glyptostrobus and angiosperms, among which platanoids are quite diverse. The Kholokhovchan Flora is most similar to Penzhina and Kaivayam floras of the Anadyr-Koryak Subregion and Arman Flora of the Okhotsk-Chukotka volcanogenic belt (Northeastern Russia) and should be dated as Turonian–Coniacian. The Kholokhovchan Flora, that populated volcanic plateaus and intermontane valleys, are characterised by a mixture of ancient “Mesophytic” plants with typical Late Cretaceous “Cenophytic” taxa. This peculiar composition probably reflects a gradual penetration of new angiosperm-dominated plant assemblages into older floras: during the Late Cretaceous, “Cenophytic” assemblages migrated along river valleys and other disturbed habitats into the interior of Asia, eventually occupying volcanogenic uplands, and in places replacing the “Mesophytic” fern-gymnospermous communities that existed there. Two new angiosperm species, as well as four the most characteristic conifers of the Kholokhovchan Flora, are described: Cupressaceae gen. et sp. indet. cf. Widdringtonites sp., Taxodium cf. olrikii, Taxodium sp., Glyptostrobus sp., Ettingshausenia vetviensis sp. nov. and Parvileguminophyllum penzhinense sp. nov.

New localities of Cenozoic alkali basalts with mantle xenoliths in Northern Okhotsk Region (Seimkan field)

A new occurrence of Cenozoic alkali basaltic magmatism (Seimkan field) is studied in northern Okhotsk Region, northeastern Russia. The occurrence is located within the Cretaceous Okhotsk–Chukotka volcanogenic belt, on the Sea of Okhotsk Plate. K–Ar dating reveals the Paleocene (about 60 Ma) and Eocene (about 50 Ma) ages of the basanites, the oldest ages of all known Cenozoic alkali basalts in northeastern Russia. The petrography and geochemistry of the basanites are discussed and compared with alkali basalts from other occurrences of northeastern Russia. The physicochemical conditions of magma generation and crystallization are estimated. The emplacement of the Seimkan basanites is related to the structural reworking of the Sea of Okhotsk Region in the Paleocene.

The Agan epithermal gold-silver deposit and prospects for the discovery of high-sulfidation mineralization in northeast Russia

The Arman volcanotectonic depression (VTD) containing the Agan deposit is distinguished as the most promising area for the discovery of high-sulfidation (HS) epithermal Au deposit during prospecting in the Central Okhotsk ore district of the Okhotsk-Chukotka volcanogenic belt (OChVB). Studies reveal that the volcanic rocks of the Agan deposit strongly differ from those of the reference HS-type epithermal deposits. It was found that quartz-alunite metasomatites in the ore field are characterized by low Au content and Sn content two orders of magnitude higher than those of Cu and Mo. The pair-correlation coefficients are Kcor (Au-Sn) = 0.73 and Kcor(Au-Cu) = 0.22. The ore bodies of the Agan deposit do not contain enargite and luzonite—the main indicator minerals for Au productive HS-type mineralization; porous (“vuggy”) quartz is weakly manifested. In terms of the mineral complex, the epithermal mineralization revealed in the metasomatites of the deposit is close to the intermediate sulfidation type. At the same time, this mineralization, in many of its features, is similar to the mineralization developed in siliceous and quartz-alunite lithocaps, which are formed above degassing intrusions. In this setting, HS-type ore-bearing fluids either are not formed in the system or do not reach epithermal depths.

Cretaceous ultramafic volcanism in the Sredinnyi Range, Kamchatka

Late Mesozoic metapicrites and metapicrobasalts occur in all apoterrigenous rock sequences in Kamchatka’s Sredinnyi crystalline massif, forming sheets, flows, and sills ranging between a few and 100 m or greater in thickness. Metapicrites possess geochemical parameters that are intermediate between mid-oceanic ridge tholeiites and island-arc tholeiites, which is typical of volcanic rock complexes in marginal basins. The host rocks are terrigenous rock sequences of the Kikhchik series and its metamorphosed analogues, viz., the Kolpakova, Kamchatka, and Malka series, which were formed in a common continental-margin sedimentary basin during the late Mesozoic sedimentation cycle with the same source, which was a province in the eastern margin of the Asian continent. The extension of continental crust in the Cretaceous sedimentary basin and intersections by faults of the same age as that of the Okhotsk-Chukotka volcanogenic belt, combined to produce basite and ultrabasite volcanism. Emplacement of mantle material (the ascent of mantle plumages) accompanied by deep hydrogen-rich fluids into the basement of the crust from volcanogenic terrigenous deposits of the marginal basin produced considerable temperature increases in the crust, as well as magmatic replacement of volcanogenic sedimentary rock sequences with the subsequent generation of magma chambers and ascent of acid volcanic and granitoid rocks into the upper crust.

New angiosperms from the Late Cretaceous Ust’-Emuneret flora of central Chukotka

New angiosperms from the Emuneret Formation of the Enmyvaam River basin are described: Menispermites lebedevii sp. nov., Cissites reticulatus sp. nov., Viburniphyllum emuneretum sp. nov., and Dicotylophyllum stipulare sp. nov. These species are characteristic elements of the Ust’-Emuneret floristic assemblage, which occur from the volcanogenic deposits of the late stage of the development of Okhotsk-Chukotka volcanogenic belt. The floristic assemblage includes 57 species of fossil plants. It is characterized by dominant angiosperms, subdominant conifers, and subordinate ferns, liverworts, ginkgoaleans, and czekanowskialeans. Macrofloristic study indicates the Santonian-Early Campanian age of plant-bearing deposits.

Geochemistry of metabasites of the Kolpakov Group of the Sredinny crystalline Massif in Kamchatka

Metabasites (amphibolites, garnet amphibolites, and basic crystalline schists) compose numerous sheeted bodies (often highly boudined) from a few to 100 meters thick in the plagiogneisses and migmatites of the Kolpakov Group. Chemically, they are reconstructed as basalts and picrites that were metamorphosed, as host terrigenous rocks, under the kyanite-sillimanite subfacies of the amphibolite facies (t = 620–650°C, Ps = 5.9–6.9 kbar). Metabasites are dominated by amphibolites and basic crystalline schists distributed throughout the entire section of the Kolpakov Group, whereas garnet amphibolites are more typical of the upper parts of the group, where they are intercalated with amphibolites, basic crystalline schists, plagiogneisses, and quartzites. Metaultrabasites (plagioclase-free amphibolites) occur much more rarely as small boudins up to few meters in size. According to U-Pb SHRIMP zircon dating, the plagiogneiss protolith age corresponds to the end of the Early-Late Cretaceous (90–100 Ma), which is similar in age to the weakly metamorphosed terrigenous deposits of the Kikhchik Group of the Sredinny Range. This allows us to consider the terrigenous rocks of these groups as isofacial sedimentary rocks. The same age (Early-Late Cretaceous boundary) was taken for protoliths of metabasites forming interbeds among metaterrigenous deposits of the Kolpakov Group. The interval of 100−90 Ma coincides with the beginning of the formation of the Okhotsk-Chukotka volcanogenic marginal-continental belt in East Asia. It is shown that the Kolpakov Group possesses the geochemical features of tholeiitic basalts of different geodynamic settings and comprises both typically island arc (low-Ti) and oceanic (moderate to high-Ti) tholeiites associated with ultrabasic volcanic rocks—picrites. Such a chemical peculiarity of basic rocks is typical of the marginal-continental extension zones (pull-apart basin) that were initiated on the sialic crust. It is obvious that similar geodynamic setting of the basite magmatism existed for the Sredinny Range of Kamchatka. The ascent of the mantle matter beneath the extension zone of the continental crust of the sedimentary basin and its intersection by faults that formed simultaneously with the Okhotsk-Chukotka volcanogenic belt served as the beginning of the basite volcanism in the sedimentary basin. They provided an intense fluid effect and a temperature increase in the crust with subsequent granitization and metamorphism of volcanogenic-terrigenous deposits and, finally, the development of the modern structure of the Sredinny Kamchatka Massif. The intense Late Cretaceous basite volcanism and associated granitoid magmatism in Kamchatka were presumably caused by the ascent of mantle plumes bearing hydrogen fluids.

The Late Cretaceous Aarya flora of the northern Okhotsk region and phytostratigraphy of the lower part of the Okhotsk-Chukotka volcanogenic belt section

The Zarya flora comes from volcanogenic sedimentary rocks of the Zarya and Parnyi formations that correspond to the basal part of the section of the Okhotsk-Chukotka volcanogenic belt in the Omsukchan district (Magadan oblast, Russia). The revision of its taxonomic composition resulted in identifying approximately 25 species of horsetails, ferns, cycads, ginkgoalens, czekanowskians, conifers, and angiosperms. The Zarya flora is characterized by a combination of the Early Cretaceous relicts (Hausmannia, Birisia, Sphenobaiera, Phoenicopsis, Nilssonia, Podozamites) and typical Late Cretaceous taxa (Taxodium, Sequoia, Menispermites, Dalembia, Trochodendroides, Cissites, Terechovia, Platanaceae). Among all the paleofloral assemblages of the Okhotsk-Chukotka volcanogenic belt, the Zarya flora is the most similar to the Turonian-Coniacian Arman flora of the Magadan region, which indicates their synchronism and floral unity. The Chingandzha flora of the Omsukchan area, which comes from the same stratigraphic level as Zarya flora, differs substantially from the latter in its taxonomic composition. It is conceivable that the Chingandzha flora was confined to a large river valley which was connected to coastal lowlands. The plant remains of the Arman flora with many mountain relicts could be buried in sediments of intermountain troughs isolated from coastal lowlands. Araucarites ochotensis sp. nov. is described.

New data on the composition and age of the Ust’-Emuneret flora from the Enmyvaam River basin (Central Chukotka)

The Ust’-Emuneret floristic assemblage from the Emuneret Formation developed in the Enmyvaam River basin (Okhotsk-Chukotka volcanogenic belt) is described. The age of flora-bearing deposits is debatable, being previously determined in the interval of the Cenomanian to Campanian. According to the new data, the assemblage includes 57 species of fossil plants with dominant angiosperms, subdominant conifers, and subordinate ferns, liverworts, ginkgoaleans, and czekanowskialeans. The Ust’-Emuneret flora is characterized by the presence of advanced forms among different plant groups, which occur in Santonian-Campanian and younger floras of northeastern Russia, and by the presence of relict ginkgoaleans and czekanowskialeans. The flora is compared with other floras from neighboring regions, which provides grounds for the inference on its most probable Late Santonian age (probably, including the Early Santonian and initial Campanian).

Геданский флористический комплекс из верхнемеловых отложений хольчанской свиты Охотско-Чукотского вулканогенного пояса

The Gedan floristic assemblage occurs from upper layers of the Kholchan Formation of the Okchotsk-Chukotka volcanogenic belt (OCVB). The locality is situated at the Gedan River in the middle part of the Arman River basin. The Gedan assemblage is composed of 6 taxa: Cladophlebis sp., Sphenobaiera sp., Ginkgo ex gr. adiantoides (Ung.) Heer, Taxodium amguemensis (Efimova) Golovn., Metasequoia sp., Pagiophyllum sp. The similarity of the Gedan floristic assemblage with the Karamken and the Khirumki floristic assemblages from the Kholchan Formation of the Okhotsk sector of the OCVB allows us to join them in the Kholchan flora. This flora is distinct from more ancient Arman flora, which dated as the Turonian-Coniacian and from younger Ola flora, which dated as the Santonian-early Campanian. The age of the Kholchan flora is estimated as the Coniacian on the basis of stratigraphic position, presence of Podozamites, Metasequoia and Quereuxia and also isotopic data. This flora is equivalent with the Chaun flora of Central Chukotka, with the Aleeki flora from the Villigha and Toomahni Rivers interfluve and with the Ulya flora from the southern part of the Okhotsk-Chukotka volcanogenic belt.

Энмываамский флористический комплекс из верхнемеловых отложений Центральной Чукотки

The Envymaam floristic assemblage occurs from basaltic rocks of the Envymaam Formation in the Envymaam River basin, Central Chukotka segment of the Okhotsk-Chukotka volcanogenic belt. This assemblage consists of 18 taxa: Hepaticites sp., Coniopteris tschuktschorum (Krysht.) Samyl., Phoenicopsis ex gr. angustifolia Heer, Cupressinocladus enmyvaamensis Golovn., sp. nov., Araliaephyllum philippoviae Golovn., sp. nov. Among them, two species are fi rstly described. The Envymaam assemblage is characterized by presence of the Early Cretaceous relicts (Phoenicopsis), by rarity of angiosperms and by small sizes of their leaves. Age of the Envymaam floristic assemblage is determined as the late Campanian on the basis of systematic composition, palynological, paleomagnetic and isotopic data. Direct continuity between the Envymaam and the Ust’-Emuneret floristic assemblages from the subjacent Emuneret Formation it isn’t observed that is caused probably by the poor composition of the Envymaam floristic assemblage. Palynological assemblages from the Envymaam, Ust’-Emuneret, Ola and Arkagala Formations are very similar.

Янский флористический комплекс из верхнемеловых вулканогенных отложений Северного Приохотья

The Yana floristic assemblage occurs from tuffs and tuffites of the upper part of the Ola Formation (the Campanian) of the Okhotsk-Chukotka volcanogenic belt (OCVB). The locality is situated in the upper part of the Yana River basin (Magadan Region). The Yana assemblage is composed of 18 taxa: Polypodiophyta: Cladophlebis sp., Mygdykitia ochotica Golovn. et Shczep., gen. et sp. nov. Cycadopsida: Ctenis paljavaensis Philipp. Czekanowskiopsida: Phoenicopsis ex gr. angustifolia Heer Pinopsida: Picea yanensis Golovn. et Shczep., sp. nov., Piсea sp., Pityostrobus sp., Pityolepis sp. 1, 2, Pityocladus sp., Cupressinocladus sp., Elatocladus anorovii Golovn. et Shczep., sp. nov. Magnoliopsida: Quereuxia angulata (Newb.) Krysht. ex Baik., Trochodendroides sp. 1, 2, Dicotylophyllum sp. 1—3. Among them, three species and one genus are firstly described. According to general systematic composition the Yana floristic assemblage is similar to the Ola flora (the Campanian) from the Arman River and the Maltan River basins and has some common taxa with the Chaun flora (the Coniacian) of the Central Chukotka. But the Yana assemblage does not contain typical species of the Ola and the Chaun floras and differs from them by some endemic species. The Yana floristic assemblage comes from coarse-grained weakly stratified tuffites, whereas the plant remains of the Ola flora are usually connected with fi ne-grained bedded volcanogenic-terrigenous lacustrine deposits. It is possible to assume, that the Yana floristic assemblage reflects the vegetation of upland slope habitats, whereas the Ola flora characterizes the vegetation of lowlands near the lakes at volcanic plateau.

Новые данные о позднемеловых флорах Ульинского прогиба (Западное Приохотье)

New investigation of fossil plants from volcanic-sedimentary deposits of the Amka Formation in Ulya River basin (southern part of the Okhotsk-Chukotka volcanogenic belt, Northeastern Russia) shows, that the Arinda, Uenma, Ust-Amka and Gyrbykan floristic assemblages from diff erent localities of this formation have very close systematic composition. We propose to joint these assemblages in the single regional flora, which is named the Ulya flora. The Ulya flora consists of almost 40 species. The majority of them are represented by new undescribed taxa. In this flora gymnosperms (Phoenicopsis ex gr. speciosa Heer, Ginkgo ex gr. adiantoides (Ung.) Heer, G. ex gr. sibirica Heer, Sphenobaiera sp., Podozamites sp., Elatocladus spp., Araucarites sp., Sequoia sp., Metasequoia sp., Cupressinocladus sp., Ditaxocladus sp., Pityophyllum sp., Pityostrobus sp.) predominate. Ferns (Asplenium dicksonianum Heer, Arctopteris sp., Cladophlebis spp. and several undescribed taxa) and angiosperms (Trochodendroides spp., undetermined Platanaceae, Dicotylophyllum spp., Quereuxia angulate (Newb.) Krysht. ex Baik.) are not abundant. This flora is characterized by presence of the Early Cretaceous relicts (Phoenicopsis, Sphenobaiera and Podozamites), by rarity of angiosperms and by high endemism. On the base of comparison of the Ulya flora with other floras from middle and northern parts of the Okhotsk-Chukotka volcanogenic belt, the age of the Ulya flora is estimated as the Coniacian.

Холоховчанский флористический комплекс позднего мела Охотско-Чукотского вулканогенного пояса (Северо-Восток Азии)

Results of comprehensive study of the Kholokhovchan floral assemblage collection is summarized. These plant fossils were collected in 1978 by E. L. Lebedev from volcanogenic deposits in Penzhina and Oklan rivers interfluve, North-Eastern Russia. This assemblage was previously known as a list of Lebedev’s preliminary identifi cations only. He had suggested that the Kholokhovchan assemblage is correlative to the latest Albian — early Turonian Grebenka flora from the Anadyr River middle reaches. However, our study demonstrates that the Kholokhovchan assemblage is most similar to the presumably the Turonian-Coniacian Arman flora of the Okhotsk-Chukotka volcanogenic belt and, therefore, should be dated as the Turonian-Coniacian or Turonian.

Аунейский флористический комплекс из верхнемеловых отложений Охотско-Чукотского вулканогенного пояса

The Aunei floristic assemblage comes from volcanic-sedimentary deposits of the Aunei unit. The locality is situated in the middle course of the Eropol River (Anadyr River basin). This area belongs to the Anadyr segment of the Okhotsk-Chukotka volcanogenic belt (Belyi, 1977). The Aunei floristic assemblage consists of 21 species. Among them gymnosperms (Phoenicopsis ex gr. angustifolia Heer, Podozamites sp., Elatocladus zheltovskii Philipp., Elatocladus sp., Araucarites subacutensis Philipp., Sequoia sp., Metasequoia sp., Cupressinocladus sp., Pityophyllum sp., Pityostrobus sp., Pityolepis sp.) predominate. Ferns (Tchaunia tchaunensis Samyl. et Philipp., Kolymella raevskii Samyl. et Philipp., Cladophlebis tschuktschorum Philipp.) and angiosperms (Trochodendroides microphylla Philipp., Trochodendroides sp. 1, 2, Dicotylophyllum nanum Golovn., Dicotylophyllum sp. 1, 2, Quereuxia angulata (Newb.) Krysht. ex Baik.) are not abundant. This assemblage is characterized by presence of the Early Cretaceous relicts (Phoenicopsis and Podozamites), by rarity of angiosperms and by small sizes of their leaves. Taxonomic composition of the Aunei floristic assemblage most closely resembles the composition of the Chaun flora, which comes from the Coniacian deposits of the Chaun Group of Central Chukotka. From 21 species of the Aunei assemblage 14 species are common with the Chaun ones. Among them there are the characteristic endemic plants of the Chaun flora: Tchaunia tchaunensis, Kolymella raevskii, Araucarites subacutensis, Elatocladus zheltovskii, Trochodendroides microphylla. The great systematic similarity of these floras indicate that they were even-aged and belonged to the single regional Chaun flora, which was distributed not only at Central Chukotka but also at area of the Anadyr segment of the Okhotsk-Chukotka volcanogenic belt. Age of the Aunei floristic assemblage is also determined as the Coniacian.

Новый вид Pseudotorellia Florin (Ginkgoales) из верхнемеловых отложений Лено-Вилюйской впадины (Восточная Сибирь)

Новый вид Pseudotorellia Florin (Ginkgoales) из верхнемеловых отложений Лено-Вилюйской впадины (Восточная Сибирь)

Позднемеловая флора из вулканогенных образований северного Приохотья (Охотско-Чукотский вулканогенный пояс)

Assemblage of fossil plants from the Gydra, Yum and Kananyga Formations (the Villigha and Toomahni Rivers interfluve, Okhotsk-Chukotka volcanogenic belt) are joined in the Aleeki flora. The systematic description of main species is given and floristic and phytostratigraphical analysis of this flora is carried out. New species Lobifolia alikensis Golovn. et Shczep. is described. The Aleeki flora is typical flora of the Okhotsk-Chukotka volcanogenic belt. It contain few angiosperms and significant amount relic elements among ferns (Hausmannia, Lobifolia), cycadophytes (Heilungia), czekanowskiales (Phoenicopsis) and ginkgoales (Sphenobaiera, Ginkgo ex gr. sibirica). In consequence of its stratigraphical position the Aliky flora is slightly younger than the Chingandzha flora (the Turonian-Coniacian) and it is compositionally similar with the Chaun flora (Coniacian). On this basis the age of the Aleeki flora is estimated as the Coniacian. The common taxa of the Aleeki and Chingandzha floras are Coniopteris tschuktschorum, Asplenium dicksonianum, Cladophlebis inaequipinnulata, Birisia sp., Ginkgo ex gr. adiantoides, G. ex gr. sibirica, Sequoia, Metasequoia, Menispermites, Dalembia and Trochodendroides. The common genera of the Aleeki and Chaun floras are Coniopteris, Asplenium, Arctopteris, Cladophlebis, Ginkgo, Sphenobaiera, Heilungia, Phoenicopsis, Picea, Sequoia, Metasequoia, Menispermites, Dalembia and Trochodendroides. Besides that the Aleeki flora contains some species, which were believed as endemic of the Chaun flora: Tchaunia lobifolia, Cladophlebis grandis and Araucarites subacutensis. The Aleeki and Chaun floras are now considered as contemporaneous regional floras, which were distributed in the northern part of the Okhotsk-Chukotka volcanogenic belt.

Age and lithotectonic correlations of igneous complexes of the Nyavlenga volcanoplutonic structure, Northeast Russia

According to paleofloral and Rb-Sr isotope data, the Nyavlenga structure was rapidly formed in the initial Albian (Buor-Kemyus phytostratigraphic horizon). Igneous rocks of this structure belong to the calcalkaline series characteristic of the early development stage of the Okhotsk-Chukotka volcanogenic belt (OCHVB), owing to which it differs distinctly from the bimodal riftogenic complex of the Omsukchan Graben (Silyap Horizon). The early development stage of the OCHVB outer zone (Buor-Kemyus, Arman, and Amka horizons) was accompanied by multiple volcanic cycles (andesitic → dacitic-rhyolitic associations) with the formation of corresponding structures. There is a need for elaborating a special nomenclature for identifying volcanic cycles and related structures.

New age data on the magmatic rocks from the western sector of the Okhotsk-Chukotka volcanogenic belt

New age data are reported for the magmatic rocks from the western flank of the Okhotsk-Chukotka volcanogenic belt consisting of the Ul’ya and Kuidusun volcanic zones. Four U-Pb SHRIMP zircon dates within 106-90 Ma were obtained for the upper part of the Emanrin Formation and the lower part of the Ul’ya Group of the Ul’ya volcanic zone. Large Verkhneallakh and Atarbai subvolcanic intrusions from the Kuidusun volcanic zone were dated using the Rb-Sr method at 115–107 Ma and 85 Ma, respectively. U-Pb dating of zircon microsamples from adamellites and diorites of the Sizindzha Massif yielded, respectively, ages of 91 and 90 Ma. New geochronological data indicate that the Selitkan-Sungari and Okhotsk-Chukotka volcanogenic belts are coeval and can be united in a common Okhotsk-Sungari system of volcanic belts and zones (megabelt).