Search

High-grade epithermal Au–Ag veins of the Vodorazdelnaya district in far eastern Russia are hosted by Early Cretaceous volcanic rocks of the Tytylveem belt. The largest deposit is Zone 37, a 1-km long, northeast-striking vein that is up to 35-m wide, containing at least 32 t Au. The main shoot consists of crustiform—colloform banded quartz-chalcedony-adularia-sulfide veins, of low-sulfidation style. The vein is cut by late-mineral rhyolite sills and dikes. Zone 37 vein is dated at 117 ± 2 Ma, overlapping within error with the cross-cutting rhyolites (121–119 Ma) and with intrusions of the nearby Ilirney granitic pluton (119–117 Ma). The volcanic host rocks are dated at 121–115 Ma. At September Northeast, 15 km west-northwest of Zone 37, mineralisation consists of veins up to 4 m wide, with crustiform banded quartz ± chalcedony ± adularia ± chlorite, and Zn-Pb-Cu sulfides, of intermediate sulfidation style, in a 1-km-long, north-northeast striking trend. High grades are associated with gold dendrites in fine-grained quartz bands and with minor late-stage tellurides. The veins were disrupted by intrusion of rhyolite dikes and associated phreatic breccias. Some breccias host mineralisation as vein clasts and minor matrix sulfides. The Tytylveem belt is preserved below a regional unconformity that forms the base to the overlying Late Cretaceous (106–77 Ma) Okhotsk-Chukotka Volcanic Belt (OCVB). Intrusive rocks attributed to OCVB magmatism were emplaced at about 96 Ma and are associated with overprinting hydrothermal alteration, sub-economic veins and thermal resetting of some vein adularia ages at Zone 37, to 91–97 Ma.

First data on oxygen isotopic composition in phenocrysts in volcanic rocks from Okhotsk–Chukotka volcanic belt (106–78 Ma, North-Eastern Russia) together with Sr and Nd isotopic data gives additional geochemical evidence on belt segmentation. Calculated δ18Omelt in calc-alkaline magmas of belt vary from normal-δ18O (from +5.2 to +6.9‰) consistent with mantle-like signatures of radiogenic isotopes. Low-δ18O (from +4.7 to +4.8‰) silicic magmas were found in Western–Okhotsk segment of the belt interpreted as assimilation of hydrothermally altered rocks from pre-caldera rifted basement. The highest δ18O values were obtained for Chukotka segment of the belt reaching +10.8‰ and these are correlated with elevated 87Sr/86Sr due to assimilation of relatively mature high-δ18O continental crust. Comparison with Kamchatka volcanic belt is performed.

The article presents new data on geology, mineralogy, and geochemistry of porphyry-copper, gold-silver, and silver-polymetallic ore occurrences in the Kongin magmatic zone of the Omolon terrane and granitoids, associated with it. It has been demonstrated that petrology and the age of granitoids are similar to those in the OCVB Penzhina segment. Mineralization of the Kongin zone is very close to that of the OCVB, differing in relatively high Mo in porphyry-copper ores and prevalence of silver-polymetallic mineralization over gold-silver.

New data are obtained on the age of the epithermal gold-silver mineralization of the Kubaka deposit (Omolon Massif, Northeast Russia). Our geological observations in the open-pit mine of the Zokol zone of the deposit allow us to conclude that the mineralization formed between the eruptions of Upper Devonian ore-hosting volcanites of the Kedon series (zircon U-Pb age ~ 370 ± 2.5 Ma, SHRIMP) and Lower-to-Middle Jurassic trachybasalts and picrobasalts of the Omolon Complex (40Ar/39Ar plateau age of 179 ± 8 Ma) without the vein penetrating into the Carboniferous Corba Formation. The 40Ar/39Ar plateau age for a white adularia from the productive quartz vein yielded 299 ± 3.5 Ma (MSWD = 0.91, p = 0.43, 66% of 39Ar released in four steps). Our results contradict a commonly accepted concept of “polychronous” ore formation (during the Carboniferous and Cretaceous) or long-term mineralization at the Kubaka deposit, and likely indicate a high probability of at least an Upper Carboniferous upper age limit of epithermal gold mineralization.

The Ul’ya flora comes from the Amka Formation of the Ul’ya Depression, located in the Okhotsk–Chukotka volcanic belt (North-East Russia). This flora includes ~50 species, among which conifers predominate. Ferns and angiosperms are also diverse. The Ul’ya flora is characterized by high endemism and by the presence of numerous Early Cretaceous relicts (Hausmannia, Podozamites, Phoenicopsis, Baiera, Sphenobaiera). Four new endemic species of conifers from the Ul’ya flora are described: Elatocladus amkensis Golovneva, sp. nov., Araucarites sheikashoviae Golovneva, sp. nov., Elatocladus gyrbykensis Golovneva, sp. nov. and Pagiophyllum umitbaevii Golovneva, sp. nov. Two-lobed leaves of Sphenobaiera are assigned to S. biloba Prynada based on their epidermal structure. Because of its systematic composition the Ul’ya flora is correlated with the Coniacian Chaun flora of Central Chukotka, with the Coniacian Aliki flora from the Viliga–Tumany interfluve area, and with the Coniacian Kholchan flora of the Magadan Region. The U-Pb age of zircon (ID-TIMS method) from plant-bearing tuffites within the Amka Formation at the Uenma River is 86.1 ± 0.3 Ma. Thus, Coniacian age (most likely the end of the Coniacian, near the Coniacian/Santonian boundary) is assigned to the Ul’ya flora and plant-bearing pyroclastic deposits of the Amka Formation on the basis of paleobotanical and isotopic data.

This paper reports the first results of U–Pb SHRIMP dating of zircon inclusions in sapphire from the Nezametninskii placers in the north of Primorye. The obtained age of paragenesis of these minerals (12.2 ± 0.2 Ma) is almost consistent with the age of erupted alkali basalts in the region, thus evidencing their genetic relation. According to the geochemistry data on trace elements, the dated zircon was crystallized from an alkali basaltic magma or its later trachyte/phonolite derivatives.

This study provides SHRIMP-RG data on zircons from garnet gedritites, the products of retrograde metamorphism of eclogite-like rocks constituting belonging to the basement of the Omolon Massif. The earliest episode recorded by oscillatory-zoned cores having high HREE and Ti contents occurred at 3.25–3.22 Ga (Paleoarchean) and is interpreted to represent an upper age limit of a metamorphic or magmatic protolith. One zircon core with a pronounced negative Eu anomaly yielded a concordant age of 2.6 Ga, which is interpreted to mark a Neoarchean episode of granite formation. The studied population of zircons provides the most distinct record of a Paleoproterozoic (1.9 Ga) event, which is marked by formation of garnet gedritites under amphibolite-facies conditions. This event is recorded by transparent recrystallization rims of preexisting large zircon grains and small newly-formed grains, which are characterized, compared with their cores, by lower crystallization temperatures and one order of magnitude lower concentrations of U, Th, and HREE, and the presence of garnet micro-inclusions.

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.

The South Anyui suture zone consists of late Paleozoic–Jurassic ultramafic rocks and Jurassic–Cretaceous pre-, syn-, and postcollisional sedimentary rocks. It represents the closure of a Mesozoic ocean basin that separated two microcontinents in northeastern Russia, the Kolyma-Omolon block and the Chukotka block. In order to understand the geologic history and improve our understanding of Mesozoic paleogeography of the Arctic region, we obtained U-Pb ages on pre- and postcollisional igneous rocks and detrital zircons from sandstone in the suture zone. We identified four groups of sedimentary rocks: (1) Triassic sandstone deposited on the southern margin of Chukotka; (2) Middle Jurassic volcanogenic sandstone that was derived from the Oloy arc, a continental margin arc, along the Kolyma-Omolon block, south of the Anyui Ocean, a sample of which yielded no pre-Jurassic zircons and a single peak at 164 Ma; (3) suture zone sandstone that yielded Late Jurassic maximum depositional ages and likely predated the collision; and (4) a Mid-Cretaceous syncollisional sandstone that had a maximum depositional age of 125 Ma. These rocks were intruded by postkinematic plutons and dikes with ages of 109 Ma and 101 Ma that postdate the collision. We present a seismic-reflection line through the South Anyui suture zone that indicates south-vergence of thrusting of the Chukotka block over the Kolyma-Omolon block, opposite of most existing models and opposite of the vergence in the Angayucham suture zone, the postulated along-strike equivalent in Alaska. This suggests that Chukotka and Arctic Alaska may have different pre-Cretaceous histories, which could solve space problems with existing reconstructions of the Arctic region. We combine our detrital zircon data and interpretations of the seismic line to construct a new GPlates model for the Mesozoic evolution of the region that decouples Chukotka and Arctic Alaska to solve space problems with previous Arctic reconstructions.

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 pre-Cenozoic kinematic and tectonic history of the Arctic Alaska Chukotka (AAC) terrane is not well known. The difficulties in assessing the history of the AAC terrane are predominantly due to a lack of comprehensive knowledge about the composition and age of its basement. During the Mesozoic, the AAC terrane was involved in crustal shortening, followed by magmatism and extension with localized high-grade metamorphism and partial melting, all of which obscured its pre-orogenic geological relationships. New zircon geochronology and isotope geochemistry results from Wrangel Island and western Chukotka basement rocks establish and strengthen intra- and inter-terrane lithological and tectonic correlations of the AAC terrane. Zircon U–Pb ages of five granitic and one volcanic sample from greenschist facies rocks on Wrangel Island range between 620±6 and 711±4 Ma, whereas two samples from the migmatitic basement of the Velitkenay massif near the Arctic coast of Chukotka yield 612±7 and 661±11 Ma ages. The age spectrum (0.95–2.0 Ga with a peak at 1.1 Ga and minor 2.5–2.7 Ga) and trace element geochemistry of inherited detrital zircons in a 703±5 Ma granodiorite on Wrangel Island suggests a Grenville–Sveconorwegian provenance for metasedimentary strata in the Wrangel Complex basement and correlates with the detrital zircon spectra of strata from Arctic Alaska and Pearya. Temporal patterns of zircon inheritance and O–Hf isotopes are consistent with Cryogenian–Ediacaran AAC magmatism in a peripheral/external orogenic setting (i.e. a fringing arc on rifted continental margin crust).

The tectonomagmatic evolution of eastern Chukotka, NE Russia, is important for refining the onset of Pacific plate subduction, understanding the development of the Amerasia Basin, and constraining Arctic tectonic reconstructions. Field mapping and strategic sample collection provide relative age constraints on subduction-related continental arc magmatism in eastern Chukotka. Ion microprobe U–Pb zircon ages provide absolute constraints and identify five magmatic episodes (c. 134, 122, 105, 94 and 85 Ma) separated by three periods of uplift and erosion (c. 122–105, 94–85 and post-85 Ma). Volcanic rocks in the region are less contaminated than their plutonic equivalents which record greater crustal assimilation. These data, combined with xenocrystic zircons, reflect the self-assimilation of a continental arc during its evolution. Proto-Pacific subduction initiated by c. 121 Ma and arc development occurred over c. 35–50 myr. Crustal growth was simultaneous with regional exhumation and crustal thinning across the Bering Strait region. Ocean–continent subduction in eastern Chukotka ended at c. 85 Ma. The timing of events in the region is roughly synchronous with the inferred opening of the Amerasia Basin. Simultaneous arc magmatism, extension and development of the Amerasia Basin within a back-arc basin setting best explain these coeval tectonic events.

Sample localities, details of the analytical methods, data tables and the full discussion of the results of electron back-scatter diffraction studies of quartz lattice preferred orientations.

Despite the exposures of Precambrian and Paleozoic rocks and the accretionary tectonic history of the northern Pacific (northeastern Asia, Alaska, and Kamchatka), it is likely that a considerable portion of the lower crust of the continental margins is much younger and was generated by Cretaceous postaccretion magmatic events. Data on xenoliths suggest that Late Cretaceous and Paleocene mafic intrusions and cumulates of calc-alkaline magmas may become more important with increasing depth. This conclusion is based on the petrological and geochronological investigation of lower-middle crustal xenoliths borne by mantlederived alkali basalt lavas and U-Pb dating of zircon cores from the igneous rocks of the region. We studied deep mafic xenoliths of granulites and gabbroids (accounting for

The Sokhatnyi intrusion is an example of a later Mesozoic deferential massif referred to the gabbro-monzodiorite type in Northeastern Russia; according to geological data, it precedes high-alumina granitoids of the Yano-Kolymskaya folded system. It is shown that formation of the layered series of the massif is caused by the crystallized and gravitational fractioning of the high-alumina olivine basalt with increased potassium alkalinity in shallow water media. The boundary facies of the massif are represented by manzogabbroidnorites and monzodiorites. The U-Pb age determinations of zircon (SHRIMP-II) from the taxitite striped biothite gabbro in the lower boundary facies showed 148 ± 1 million years. Thus, taking into account the geological relations and geochronological data on the Sokhatinyi gabbro-monzodiorite, a differential intrusion was formed within the same age period as granite batholites of the Main belt.

A suite of mainly spinel peridotite and subordinate pyroxenite xenoliths and megacrysts were studied in detail, enabling us to characterize upper mantle conditions and processes beneath the modern North American–Eurasian continental plate boundary. The samples were collected from 37-Ma-old basanites cropping out in the Main Collision Belt of the Chersky Range, Yakutia Republic (Russian Far East). The spinel lherzolites reflect a mantle sequence, equilibrated at temperatures of 890–1,025 °C at pressures of 1.1–2 GPa, with melt extraction estimated to be around 2–6 %. The spinel harzburgites are characterized by lower P–T equilibration conditions and estimated melt extraction up to 12 %. Minor cryptic metasomatic processes are recorded in the clinopyroxene trace elements, revealing that percolating hydrous fluid-rich melts and basaltic melts affected the peridotites. One of the lherzolites preserves a unique melt droplet with primary dolomite in perfect phase contact with Na-rich aluminosilicate glass and sodalite. On the basis of the well-constrained P–T frame of the xenolith suite, as well as the rigorously documented melt extraction and metasomatic history of this upper mantle section, we discuss how a carbonated silicate melt infiltrated the lherzolite at depth and differentiated into an immiscible carbonate and silicate liquid shortly before the xenolith was transported to the surface by the host basalt. Decreasing temperatures triggered crystallization of primary dolomite from the carbonate melt fraction and sodalite as well as quenched glass from the Na-rich aluminosilicate melt fraction. Rapid entrainment and transport to the Earth’s surface prevented decarbonatization processes as well as reaction phenomena with the host lherzolite, preserving this exceptional snapshot of upper mantle carbonatization and liquid immiscibility.

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 (

U-Pb SHRIMP-dating of zircons from twenty five intrusions representative of the Main granitoid batholith belt and associated dike swarms (Yano-Kolyma gold bearing province, North East of Asia) are mostly ∼150 ± 3 Ma (Kimmeridgian-Tithonian). Two less widespread impulses of magmatism dated at 160–155 Ma and 146–143 Ma representing the full range of ages present in the Main belt. Paleoproterozoic (∼1.8 Ga) inherited zircons were found in three intrusions from the south-western part of the belt where Precambrian crust of the North-Asia craton is inferred to underlie it.

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).

Mantle xenolith bearing olivine melanephelinites from the Okhotsk sector of the Okhotsk–Chukotka Volcanic Belt (OCVB), northeastern Russia, occur as small isolated volcanoes emplaced within massive late Early to Late Cretaceous subduction-related calc-alkaline rocks. The xenoliths are typical mediumto fine-grained anhydrous mainly spinel lherzolites that are strongly to weakly foliated with intensive to minor recrystallization to equigranular texture. The primitive mantle normalized whole-rock REE have flat patterns or patterns with slightly elevated light REE (LREE) ((La/Yb)N 1⁄4 0.48–1.38). The REE in clinopyroxenes have systematically decreasing normalized abundances from Sm to La, implying that the LREE enrichments in the whole-rock REE patterns are attributed to circulation of minor intergranular fluids or melts. Equilibration temperatures and pressures calculated for the Viliga samples are in the range of 1050–1160 8C and 15–21 kbar, respectively. Ca diffusion rates in olivine reveal a rapid transport to the surface (2–6 days) of these peridotites. Model calculations have shown that the fertile lherzolites can be produced by 2–9% batch melting, whereas the depleted peridotites require 15% batch melting of a primitive source. The cessation of the interaction between the palaeo-Pacific plate and the NE Russian margin at c. 87 Ma apparently caused a ‘piecemeal’ collapse of the former followed by intrusion and ascent of olivine melanephelinitic magma, which entrained xenoliths from the asthenospheric mantle of the subducted plate during the Pliocene through the generated window(s). Moreover, clinopyroxenes that have low Sr/Sr and high Nd/Nd and plot in and above the mid-ocean ridge basalt (MORB) field are consistent with an upwelling asthenospheric mantle through the window(s) created by the ‘piecemeal’ collapse of the palaeo-Pacific plate. Mantle xenoliths in alkali basalts emplaced in massive subduction-related calc-alkaline rocks are important rocks, as they could provide information about the nature of the lithospheric mantle wedge and the geochemical and metasomatic processes that occur in such tectonic settings. Fluids released from sediments of a downgoing subduction slab could cause modal metasomatism in the lithospheric mantle wedge, forming hydrous phases such as amphibole and phlogopite (Peacock 1993). Migration of melts produced by melting of a downgoing subduction slab can also affect the lithospheric mantle wedge, causing crypticmetasomatism, as has been demonstrated by Kilian & Stern (2002) for the Cerro Fraile mantle xenoliths in southernmost Patagonia and by Kepezhinskas et al. (1996) for the Valovayam Volcanic Field in the northern segment of Kamchatka arc xenoliths. The mantle xenoliths from the Viliga River volcanic field in the northern Okhotsk region, NE Siberia (Fig. 1), are hosted by olivinemelanephelinites and basanites and occur at two localities (Kananyga River and Aliki River areas) on top of calc-alkaline rocks up to 1550 m above sea level, which are related to the subduction of the palaeo-Pacific slab (Zonenshain et al. 1990). The age of the undersaturated lavas ranges from 10.7 to 7 Ma (Akinin 1997, 2005; Apt et al. 1998). A limited number of mantle xenoliths from the region have been studied by Akinin et al. (1997) and Apt et al. (1998). Here we present a summary of the petrology, mineralogy and geochemistry of the ultramafic xenoliths from both localities and discuss the implications of these data with respect to the subduction slab and mantle wedge processes.

U–Pb ages, in conjunction with oxygen isotope and hafnium isotope geochemistry of zircons from granitic and siliciclastic crustal xenoliths from Cenozoic alkali basalts on Zhokhov Island (De Long Archipelago, Russian Arctic), provide new insights about the island’s subsurface geology. Zircons from granitic gneiss xenoliths yield 206Pb/238U ages ranging from 600 to 660 Ma, similar to protolith ages of granite intrusions and orthogneisses in Protouralian–Timanian magmatic basement of the northern Urals as well as to rocks that form the basement of Arctic Chukotka and Wrangel Island, thus suggesting continuity between these three regions. Depleted mantle-like Hf and O isotopic signatures in the dated zircons suggest juvenile crust that originally formed in the Neoproterozoic and was later reworked during the Paleozoic and Mesozoic. Sandstone xenoliths contain detrital zircon (DZ) populations that establish their depositional age as younger than Permian and reveal similarities to DZ populations of Permian and Triassic strata of Taimyr and Chukotka.

Current geochronological data on the Okhotsk-Chukotka volcanic belt (OCVB) and relevant problems are discussed. The belt evolution is suggested to be modeled based on 40Ar/39Ar and U-Pb dates more useful in several aspects than common K-Ar or Rb-Sr dates and methods of paleobotanical correlation. Based on new 40Ar/39Ar and U-Pb dates obtained for volcanic rocks in the OCVB northern part, the younger (Coniacian) age is established for lower stratigraphic units in the Central Chukotka segment of the belt, and the eastward migration of volcanic activity is shown for terminal stages of this structure evolution.

Nephelinites and basanites of the Enmelen volcanic field, Chukchi Peninsula, Russia, contain upper mantle xenoliths of relatively calcium- and magnesium-rich spinel lherzolites, pyroxenites, and megacrysts. The phase assemblages of the lherzolites require equilibration near 1.5 GPa, and calculated equilibration temperatures for most inclusions are in the range 850–1030 °C. These temperatures are similar to those calculated for lherzolite inclusions from other Bering Sea localities (Nunivak Island and Seward Peninsula) and are higher than temperatures expected for likely conductive geotherms beneath these volcanic fields. The relatively high temperatures may be the result of magma intrusion into the mantle lithosphere and consequent perturbation of the geotherm shortly before entrainment of the xenoliths in basalt. Two Enmelen lherzolites equilibrated at higher temperatures (1230–1240 °C) and provide further evidence for heating due to intrusive magmas. Some spinel lherzolite inclusions have flat rare earth element patterns and major and trace element abundances close to that of the bulk silicate earth. Based on the occurrence of similar fertile peridotites at Nunivak Island and Seward Peninsula, near-primitive mantle compositions appear to be common in the upper mantle beneath the Bering Sea. These peridotites may represent recent additions to the mantle lithosphere from mantle plumes related to the volcanism. Other Enmelen inclusions are relatively light rare earth element-enriched group I lherzolites metasomatized by a silicate melt, group II pyroxenites precipitated from a variety of melts, and augite megacrysts with convex-upward rare earth element patterns consistent with precipitation from the host basalts at high pressures.

[1] To test existing models for the formation of the Amerasian Basin, detrital zircon suites from 12 samples of Triassic sandstone from the circum-Arctic region were dated by laser ablation-inductively coupled plasma-mass spectrometry (ICP-MS). The northern Verkhoyansk (NE Russia) has Permo-Carboniferous (265–320 Ma) and Cambro-Silurian (410–505 Ma) zircon populations derived via river systems from the active Baikal Mountain region along the southern Siberian craton. Chukotka, Wrangel Island (Russia), and the Lisburne Hills (western Alaska) also have Permo-Carboniferous (280–330 Ma) and late Precambrian-Silurian (420–580 Ma) zircons in addition to Permo-Triassic (235–265 Ma), Devonian (340–390 Ma), and late Precambrian (1000–1300 Ma) zircons. These ages suggest at least partial derivation from the Taimyr, Siberian Trap, and/or east Urals regions of Arctic Russia. The northerly derived Ivishak Formation (Sadlerochit Mountains, Alaska) and Pat Bay Formation (Sverdrup Basin, Canada) are dominated by Cambrian–latest Precambrian (500–600 Ma) and 445–490 Ma zircons. Permo-Carboniferous and Permo-Triassic zircons are absent. The Bjorne Formation (Sverdrup Basin), derived from the south, differs from other samples studied with mostly 1130–1240 Ma and older Precambrian zircons in addition to 430–470 Ma zircons. The most popular plate tectonic model for the origin of the Amerasian Basin involves counterclockwise rotation of the Arctic Alaska–Chukotka microplate away from the Canadian Arctic margin. The detrital zircon data suggest that the Chukotka part of the microplate originated closer to the Taimyr and Verkhoyansk, east of the Polar Urals of Russia, and not from the Canadian Arctic.

Seismic re×ection and/or refraction studies reveal re×ective middle and lower crust and a sharp Moho (∼32 km depth) beneath a broad region of the Bering Shelf between Alaska and northeast Russia. Basalt ×ows on Saint Lawrence Island of the late Cenozoic Bering Sea basalt province contain upper mantle and crustal xenoliths that include maµc cumulate rocks and lesser pyroxene-bearing gneisses that equilibrated at ∼4–6 kbar. The gneissic xenoliths are interpreted as intrusive rocks that acquired deformation and/or recrystallization fabrics during granulite facies metamorphism. Three gneissic xenoliths from two sites yielded zircons that were dated by the UPb method with the SHRIMP II (sensitive high resolution ion microprobe). Zoned prismatic zircons of magmatic origin yield ages mostly ca. 85–90 Ma. Rounded, nonzoned zircons from other samples are likely metamorphic in origin and yield mostly ∼64 Ma ages. No older ages were obtained. More abundant gabbroic xenoliths are interpreted to represent maµc magmas emplaced into the middle to lower crust during this same approximate time span. The oldest surface rocks on Saint Lawrence Island include Paleozoic-Mesozoic shelfal units of the Brooks Range (once deposited on Precambrian basement) but xenolith age data suggest that such older rocks, if ever volumetrically important in the deeper crust, could have been reconstituted and remobilized during younger thermal and/or magmatic events. Conversely, Late Cretaceous to Paleocene magmatic rocks are likely increasingly important with depth in the crust. A similarly young age is inferred for the development of seismically imaged re×ective crust and (by inference) the Moho beneath the Bering Shelf.

The Okhotsk-Chukotka volcanic belt is part of an extensive late Early to Late Cretaceous Andean-style magmatic arc that spans the entire eastern margin of the Asian continent. The belt itself stretches 3000 km from the Chukotka Peninsula to the Uda River and comprises ∼1.2 x 10 6 km 3 of volcanic rock over a 500,000 km 2 area. Despite its size and regional tectonic significance, the time span of magmatic activity is poorly constrained and the subject of significant debate, mostly in the Russian literature. In this paper, we provide new geochronologic control on the timing of inception and cessation of magmatism for the Arman and Maltan-Ola volcanic fields. These field localities were chosen because they are well studied, relatively accessible, and contain floral assemblages that have been used to correlate volcanic sequences at the regional scale. The majority of the volcanic sequence was emplaced between 85.5 ′ 1.3 Ma and 74.0 ′ 1.2 Ma, as shown by 17 new 4 0 Ar/ 3 9 Ar ages. The Coniacian-Santonian to Campanian age range indicated is 15 m.y. younger than the Albian to early Cenomanian age range given by a synthesis of floral stratigraphic, K-Ar, and Rb-Sr geochronologic data. The calc-alkaline part of the volcanic section spans an apparent age range of 85.5 ′ 1.3 Ma to 80.7 ′ 0.8 Ma. Capping basalts were emplaced between 77.5 ′ 1.1 Ma and 74.0 ′ 1.2 Ma and exhibit a within-plate geochemical signature, which we attribute to a temporally and geochemically distinct, possibly extension-related, phase of magmatism. The apparent northwestward migration of the arc front from the interior (seaward) zone (Taigonos Peninsula, Magadan batholith) in Albian-Cenomanian time to the Arman and Maltan-Ola volcanic fields in Coniacian-Santonian to Campanian time may be explained by shallowing of the subducting paleo-Pacific (Kula?) oceanic plate. The flat-lying nature of these volcanic rocks and the within-plate geochemical affinity of the capping basalt unit are inconsistent with prevailing tectonic models for the cessation of arc magmatism and formation of the Sea of Okhotsk which require the collision of a microcontinental block or oceanic plateau with the northeast Asian margin in the Late Cretaceous.