The Early Baikalian crystalline complex in the basement of the Dzabkhan microcontinent of the Early Caledonian orogenic area, Central Asia

Fragments of continental blocks or microcontinents are represented in the Early Caledonian orogenic area of Central Asia (or Early Caledonian superterrane); the largest of these are the Dzabkhan and Tuva-Mongolian microcontinents, with Early and Late Precambrian crystalline basements, respectively. In the linkage zone of these microcontinents, crystalline rocks of the Tarbagatai and Songino blocks that are considered as units of the Early Precambrian ensialic basement of the superterrane are also known. They are composed of strongly metamorphosed rocks formed during the Early Baikalian orogeny about 790 to 820 Ma. U-Pb zircon dating and Nd isotope studies revealed, within the northwestern Dzabkhan microcontinent, the Dzabkhan-Mandal zone of crystalline rocks associated with the Riphean crust-forming process. The age of the gneiss substrate of this zone is estimated as 1.3 to 0.86 Ga. An early episode of metamorphism is dated at about 856 ± 2 Ma. The data available so far indicate a heterogeneous structure of the Dzabkhan microcontinent basement represented by Early Precambrian and Early and Late Baikalian crystalline formations.     

Neoarchean sanukitoid magmatism in the Kola region: Geological, petrochemical, geochronological, and isotopic-geochemical data

New geological, isotopic-geochronological, petrochemical, and isotopic geochemical data were obtained on the Porosozero and Kolmozero sanukitoid intrusions in the Kola region. The Porosozero differentiated intrusion was formed in four phases successively emplaced during approximately 60 Ma. Phase 1 consists of a gabbrodiorite-quartz monzodiorite-granodiorite-granite series. The zircon ages of granodiorite and quartz monzodiorite from the Porosozero are 2733 ± 6 and 2734 ± 4 Ma, respectively. Phase 2 of the intrusion comprises biotite leucogranites and aplite and leucoplagiogranite veins. The zircon age of the leucogranite is 2712 ± 6 Ma. Phase 3 consists of lamprophyre dikes of odinite-spessartite-vogesite composition. The emplacement age of the lamprophyres is constrained by the age of magmatic zircon from an odinite dike: 2680 ± 10 Ma. The age of the metasomatic zircon is 2629 ± 8 Ma. Phase 4 is composed of the youngest pegmatite veins. The Rb-Sr isochron age of the phase-1 rocks is 2724 ± 74 Ma. The zircon age of granitoids from the Kolmozero is 2736 ± 4 Ma. The rocks of the sanukitoid intrusions affiliate with the calc-alkaline series, have Mg# = 0.45?0.60, are enriched in Ba, Sr, K, P, and LREE, and contain elevated concentrations of Cr and Ni. Sm-Nd isotopic data on sanukitoids from both intrusions suggest that they were derived from a mantle source enriched in LILE and LREE and having ?_Nd(2740) from +1.02 to +0.36. It was melted approximately 140 Ma after its origin [T(DM) = 2.9?2.8 Ga]. The rocks of the Porosozero and Kolmozero are proved to be similar to magmatic sanukitoid series of Archean and Phanerozoic age whose genesis was controlled by mantle-crustal interaction in suprasubduction environments at active continental margins. Elevated concentrations of Ag and Au in rocks from the Porosozero make it metallogenically promising in terms of precious metals.     

Early Precambrian crystalline complexes of the Central Asian microcontinent: Age,sources, tectonic position

In the Early Caledonian superterrane of Central Asia, an accretionary orogen of mosaic structure, pre-Riphean Baidaragin and Bumbuger complexes are exposed in the Baidarik block of the Dzabkhan microcontinent. Zircon dating on ion-ion SHRIMP II microprobe and Nd isotopic-geochemical systematics are used to establish protolith age for Neoarchean orthogneisses of the Baidaragin complex, age constraints for accumulation of Lower Proterozoic metasediments of the Bumbuger Complex and provenance of sedimentary material. The results of isotopic dating facilitate correlation of the Baidarik block crystalline complexes with basement formations of North Eurasian ancient cratons. Possible position and migration path of the Dzabkhan microcontinent during the Early Proterozoic transformation of supercontinents Columbia-Rodinia-Pangea are considered based on interpretation of paleomagnetic data.     

Sources of the Late Riphean carbonatite magmatism of Northern Transbaikalia: Geochemical and isotope-geochemical data

New major and trace element and Nd-Sr isotope results are reported for the carbonatites of the Veseloe and Pogranichnoe occurrences, Northern Transbaikalia. The carbonatites from both these occurrences are enriched in Sr, Ba, LREE, Th, U, and depleted in Ti, Cr, and V relative to primitive mantle. As compared to the “average dolomite carbonatite”, the rocks from the Northern Transbaikalia have higher contents of Ni, Cr, and low contents of Ba, Ti, and V. The rocks are characterized by ⁸⁷Sr/⁸⁶S in the range of 0.7037–0.7043 and ɛNd from + 0.6 to + 2.05. Obtained geochemical and isotope data indicate that the carbonatites were derived from moderately depleted source with a contribution of enriched component.     

New data on Late Riphean volcanism in northern part of the Yenisey ridge

New species of organic fossils are described from the Precambrian Valday series on the Onega peninsula. This ancient Precambrian faunal locality was discovered in 1973; the first fossils described from it were pteridinians, assigned somewhat tentatively to the species Pteridinium simplex Gürich (AN SSSR, Izvestiya ser. geol., no. 12, 1974). The present article describes scyphomedusas assigned to a new genus and species (Albumares brunsae Fedonkin), typical Pteridinium simplex Gürich, Dickinsonia costata Sprigg, and new arthropod genera and species — Onega stepanovi Fedonkin and Vendomia menneri Keller. The species assemblage found shows that the Valday series on the Onega peninsula is of the same age as the Pound sandstones of Ediacara province in Australia. —Authors.     

Sources of granitoids in the Tuva-Mongolian microcontinent and surrounding structures: Evidence from Pb, Nd, and O isotopic systematics

Newly obtained data on Pb isotopic ratios in feldspars from Early Proterozoic granitoids of the Sangilen block of the Tuva-Mongolian microcontinent and Caledonian structures surrounding it, considered together with earlier data on the O and Nd isotopic systems, indicate that ancient (approximately 2 Ga) crustal material mixed in the sources of granites of the Tuva-Mongolian microcontinent with younger juvenile material. Positive ɛ_Nd values of granitoids from the Bashkimugor and Chgargalant massifs are accounted for by processes of crustal contamination during the interaction of the melt with crustal material. Similar Nd isotopic characteristics of granitoids in the Khoromnug pluton were caused by the melting of the Late Riphean crust. In granitoids of the massifs in the Kaakhem and Eastern Tannuola zones of the surrounding Caledonian structures, the involvement of juvenile material from oceanic crust increases away from the Sangilen block. Granites in the junction zone between the Tuva-Mongolian microcontinent and surrounding structures display evidence of the presence (up to 10–20%) of an ancient crustal component, and the melting history of granitoids in the Eastern Tannuola zone is dominated by an Early Paleozoic juvenile component in combination with material similar to the Vendian ophiolites of the Agardag-Erzin zone. An increase in the δ¹⁸O value, the ²⁰⁶Pb/²⁰⁴Pb ratio, and the T_Nd(DM) values within a single complex (from older to younger granitoid phases) is explained by the systematic involvement of crustal material in the melting processes.     

Sources of the allothigenic quartz in the Riphean basal complexes, southern urals

Using an instrumental technique, we carried out a direct comparison of quartz from the Riphean sandstones, sandy fractions from fragments in the Riphean conglomerates and Archean crystalline rocks, which represent the basement inlier of the Russian Platform within the Western Urals (Taratash anticlinorium). It is shown that clastic quartz in the Riphean basal complexes was mainly related to denudation of the Lower Proterozoic platformal cover, whose rocks occur as fragments in the Riphean conglomerates. The probable contribution of eroded crystalline rocks into the Riphean sediments was presumably very insignificant.     

Late Cretaceous-Paleocene magmatic complexes of central Kamchatka: Geological settings and compositional features

A comparative analysis of the Late Cretaceous-Paleocene volcanism was conducted for four areas of Kamchatka: the Pravyi Tolbachik-Levaya Shchapina-Adrianovka interlfuve (the northern part of the Tumrok Range), the area south of the Ipuin River and Mt. Khrebtovaya (the northern Valaginsky Range), the area of Mt. Savul’ch (the upper reaches of the Kitil’gina River, northern Valaginsky Range), and the Kirganik-Levaya Kolpakova interfluve (the Sredinny Range). New petrochemical, geochemical, and isotopic data on the volcanic rocks from these areas are reported. The examination of this material, together with already published data on volcanic and plutonic rocks of similar composition and age, made it possible to establish the following: (1) the considered basaltoids are ascribed to the subalkali basalt-trachyandesite series with transition toward a meymechite-picrite rock association; (2) the alkali content in the rocks of the Valaginsky-Tumrok-Sredinny ranges increases simultaneously with the increase of the Rb content, while the contents of HFSE and radioactive elements decrease and then again increase. Two trends are identified in the Yb_n-Ce_n diagram: a positive trend spanning most of the volcanic and plutonic rocks and a negative trend defined by the data points of the meymechite-picrite association. The first trend reflects the rock evolution during crystallization differentiation, while the second trend was produced by different degrees of melting of initial protolith. The possible geodynamic reconstructions of this volcanism are discussed as well.     

Sedimentary complexes of the cover of the Dzabkhan continental block: Different sedimentary basins and source areas

The geochemical and Sm–Nd isotope characteristics of Late Precambrian and Early Cambrian sandstones previously related to the sedimentary cover of the Dzabkhan continental block are reported. It is established that the Riphean and Vendian sedimentary rocks of the Ul’zitgol’skaya and Tsaganolomskaya Formations were accumulated within the Dzabkhan continental block as a result of recycling of the terrigenous deposits formed at the expense of destruction of basement rocks and younger granite. The formation of terrigenous rocks of the Bayangol’skaya Formation after a gap in sedimentation occurred in the sedimentary basin, where only the Late Riphean formations of the juvenile crust, probably of the Dzabkhan–Mandal block were the sources, without the contribution of the ancient crustal material. The Tsaganolomskaya and Bayangol’skaya Formations were formed in different sedimentary basins and cannot be related to the same complex.     

Epigenesis of Middle Riphean Rocks in the Bashkir Meganticlinorium,Southern Urals: Timing of Alterations and Geological Implications

The Rb–Sr and K–Ar characteristics of whole-rock argillite samples from the Middle Riphean Yurmatinian Group have been studied. The timing of final rock alteration was estimated at 525 ± 30 Ma by the Rb–Sr geochronometer. In the section near the Bol'shoi Avzhyan Settlement, the rocks experienced alteration up to the stage of deep epigenesis. The K–Ar data indicate that rock alterations continued after the Rb–Sr system conservation and was accompanied by a gradual gain of K. The great temporal gap between alterations of Middle and Lower Riphean rocks was established in the southern Urals. This gap may be explained either by the affiliation of sampled rocks to different tectonic units or by principal errors in stratigraphic correlations.     

Late Archean magmatic complexes of the Azov terrane,Ukrainian Shield: Geological setting,isotopic age,and sources of material

Geochemical, isotopic-geochemical, and geochronological information was obtained on magmatic rocks from the Saltychan anticlinorium in the Azov domain of the Ukrainian Shield. The rocks affiliate with the calc-alkaline series and a high-Mg series. The rocks of these series notably differ in concentrations of trace elements and REE and range from gabbro to granodiorite-quartz diorite in composition. The NORDSIM ionprobe U-Pb zircons ages of rocks belonging to the Obitochnen Complex and having both elevated and normal mg# correspond to 2908–2940 Ma. The Osipenkovskaya intrusion has an age of 2855 ± 19 Ma. The most alkaline North Obitochnen intrusion was emplaced in the Proterozoic, at 2074 ± 11 Ma. The age of the amphibolite metamorphism of the host gneisses is reliably dated at 3120–3000 Ma. The model Sm-Nd ages of the intrusive rocks do not exceed 3150 Ma. According to geochemical evidence, the parental melts of the magmatic rocks were derived from mantle domains variably enriched in lithophile elements. The results obtained by studying the Sm-Nd isotopic system corroborate the conclusion drawn from geochemical evidence that most of the melts were derived from the mildly enriched mantle, practically without involvement of ancient crustal material. The mantle became enriched in LREE at approximately 3000 Ma, which corresponds to the age of metamorphism of the supracrustal rocks. This process was separated from the derivation of the melts by a time span of 70–80 Ma. The relative age of the intrusive rocks and their variable composition can be most adequately explained by a contribution of heat and material from a plume to the derivation of the parental melts of these rocks.     

Paleoproterozoic sequences and magmatic complexes of the Losevo suture zone of the Voronezh crystalline massif: Geological position,material composition,geochemistry, and paleogeodynamics

In order to resolve the contradictions associated with uncertainty in the identification of the material composition, subdivision, and conditions of formation of the Paleoproterozoic intrusive, metavolcanogenic, and metasedimentary sequences of the Losevo suture zone of the Voronezh crystalline massif, this work presents geological, petrographic, petrochemical, and geochemical features of these sequences. The stratigraphic and magmatic scheme of the central part of the Losevo suture zone is clarified. In particular, the Paleoproterozoic Losevo Series is divided into two sequences: Strelitsa (marginal sea) and Podgornoe (island arc). A new hypabyssal Novo-Voronezh metagabbro-diabase complex, comagmatic to metatholeiites of the Podgornoe sequence, is distinguished. The isotope age of the Strelitsa sequence is assumed to be 2172 ± 17 Ma on the basis of the results of age dating of zircon cores from the Usman plagiogranites, intruding this sequence. The upper age boundary of the Strelitsa sequence corresponds to the age of premetamorphic gabbro of the Rozhdestvenskoe complex, comagmatic to metavolcanites (2120 ± 11–2158 ± 43 Ma). The age of the Usman plagiogranite complex is clarified. On the basis of geological-structural and petrographic-mineralogical analyses of metavolcanogenic rocks, lithological analysis of metasedimentary formations, and new geochemical data obtained from metavolcanites and metamorphosed deposits, the pattern of paleogeodynamic evolution of the Losevo suture zone in the first half of the Paleoproterozoic is proposed. The next stages are distinguished: (1) intrusion of tholeiites of transition T-MORB type in spreading zones and deposition of terrigenous strata in the marginal sea basins; (2) intrusion of Nb-depleted tholeiites and plagiorhyolites, the geochemical characteristics indicating their formation in the subduction setting; (3) intrusion of gabbroids of the Rozhdestvenskoe complex; (4) formation of an island arc synchronously with stage 2, tholeiitic and calc-alkaline (Podgornoe sequence) volcanism; (5) intrusions of gabbro-diabases, subsynchronous to volcanism, of the Novovoronezh complex and diorite-granitoides, crystallization of granitoides of the Usman complex; (6) a break in sedimentation and formation of molasses of the Voronezh (Somovo) Formation.     

Mudrocks from the Siberian Hypostratotype of the Riphean and Vendian: Chemistry,Sm–Nd Isotope Systematics of Sources,and Formation Stages

Lithology and Mineral Resources - The chemical composition and Nd isotope systematics were obtained for mudrocks (mudstones) from sections of the Siberian hypostratotype of the Riphean and Vendian...