The Nui Chua layered peridotite-gabbro complex as manifestation of Permo-Triassic mantle plume in northern Vietnam

New data on the age, composition, formation conditions, and ore-geochemical specialization of the Nui Chua layered peridotite-gabbro complex are reported. They evidence that the complex resulted from the Permo-Triassic mantle plume activity in northern Vietnam (southern framing of the Yangtze Platform). Two series of mafic and ultramafic rocks differing in ore productivity—layered (PGE-Cu-Ni) and pegmatoid (Fe-Ti-V)—have been recognized within the complex. The first estimates of the composition of their parental melts have been obtained.     

Comparison of petrology and isotope geochemistry of the Bulka peridotite–gabbro massif and granitoids of the Kyzykchadra complex (West Sayan)

The study provides new petrologic and isotope geochemical data for rocks of the 465 ± 5 Ma Bulka massif (Borodina et al., 2011). The primary amphibole from granitoid stocks cutting across the layered series of the massif yielded an Ar–Ar age of 415.9 ± 3.7 Ma. The rocks of the Bulka massif have ¹⁴³Nd/¹⁴⁴Nd ratio of 0.513243 and εNd (Т) values of +12.00. The granitoids have ¹⁴³Nd/¹⁴⁴Nd ratios between 0.512919 and 0.512961 and εNd (Т) values between +8.03 and +9.25. The Nd isotope composition indicates that the rocks of the Bulka massif and granitoids were derived from a depleted mantle source. Depletion of the rocks of the massif in LILE, LREE, and HFSE over LILE is inherited from the mantle source, which has geochemical signatures of N-MORB and subduction-related components. Granitoids are metaluminous I-type granites, which were probably generated either by differentiation of intermediate to mafic mantle-derived magmas or by melting of metabasites. The rocks of the granitoid stocks are characterized by enrichment in LILE and LREE and depletion in HFSE over LILE, which suggests derivation from arc-related parental magmas.     

Geochemical features and geodynamic setting of formation of the Lukinda dunite–troctolite–gabbro massif (southeastern framing of the Siberian Platform)

The Lukinda dunite–troctolite–gabbro massif in the Selenga–Stanovoy superterrane on the southeastern framing of the Siberian Platform was earlier considered Precambrian. The performed ⁴⁰Ar/³⁹Ar dating of the massif plagioclase yielded an Early Permian age (285 ± 7.5 Ma). The main specific petrochemical features of the intrusion rocks during their crystallization differentiation are an increase in SiO₂ and CaO contents and a decrease in FeO_tot content, with TiO₂ content remaining low and showing minor variations. A specific geochemical feature of the Lukinda massif ultrabasite–basites is a slight domination of LREE over HREE, with (La/Yb)_N= 1.0–8.2. The depletion of the massif rocks in LILE (except for Sr and Ba), REE, and HFSE suggests that the massif formed on an active continental margin.     

Middle–Late Devonian island-arc volcanosedimentary complexes in northwestern Rudny Altai

A volcanosedimentary complex of Middle–Late Devonian deposits in northwestern Rudny Altai is described. Analysis of sedimentologic processes, the kind of manifestation and specific composition of volcanism, and petrographic and chemical compositions of sedimentary rocks permitted reconstruction of paleogeographic and geodynamic settings, which confirmed the hypothesis of the island-arc nature of the Rudny Altai zone in the Middle–Late Devonian. Also, an alternative variant of its rift nature is considered. The rift buildings are referred to as local isolated complexes confined to positive paleovolcanic structures rising above the bottom in the fore-arc basin. The denudated part of the rift system is formed mainly by slope facies. Clastics of rift buildings, together with fragments of silicite and felsic-volcanics layers, compose widespread olistostrome breccias. We have established the humid type of lithogenesis, which, together with minor sea level fluctuations, favors karst processes and the formation of red-colored and bauxite-like rocks. The coeval volcanics composing lava flows and tuff layers are mainly rhyolites and are often of ultrafelsic composition.     

New data on the composition and age of picritoid and alkaline-basite complexes of the Northern Mongolian segment of the Central Asian fold belt

The new petrochemical and isotope-geochronological characteristics for picritoid and associated alkali-basaltic complexes of the Northern Mongolian Belt (NMB) are given. They are composed in small intrusions, dykes, sills, and volcanic covers with rocks of a higher potassium and titanium content. The studied areas with these units belonged to the western part of the NMB, which stretches sublatitudinally from Uureg Nuur Lake in the west to the south of Trans-Hovsgol Region in the east. By the petrochemical parameters, the complex corresponds to subalkaline basites and lamprophyres, including micaceous plagioperidotites and picrodolerites. The obtained values of Ar-Ar age (342–319 Ma B.P.), together with the known data on other areas in this complex where these rocks are distributed, evidences for a relatively wide (Middle-Late Paleozoic) interval of its origination.     

Strike-slip tectonics and subalkaline mafic magmatism in the Early Paleozoic collisional system of the western Baikal region

We discuss strike-slip tectonics as the key agent in the formation of the Early Paleozoic (Caledonian) collisional system of the western Baikal region. This tectonic setting implies existence of local syncompressional extension, with the ensuing conditions for mantle drainage and magmatism. Lower-middle crust collisional complexes exposed in the Olkhon area of the western Baikal region provide a record of synmetamorphic subalkaline-mafic magmatism associated with the early synorogenic collapse of the Olkhon collisional system, a part of the Central Asian collisional-accretionary belt.     

Detailed Holocene climate record from the carbonate section of saline Lake Tsagan-Tyrm (West Baikal area)

We present results of a complex study of evaporite sediments from one of the small saline lakes with carbonate sedimentation in the Ol’khon area and substantiate their high significance for paleoclimatic reconstructions. The mineral composition of the bottom sediments was studied by XRD analysis, IR spectroscopy, scanning electron microscopy, elemental analysis (SR-XFA), etc. By decomposition of the complex XRD profiles of carbonate minerals into individual peaks by Pearson VII function, we identified carbonate phases in each sample and determined their proportions. A high-resolution carbonate record has been obtained for the first time for the lacustrine sediments. It bears the information about the stratigraphic distribution of Mg-calcites (a continuous series of structurally disordered low- to high-Mg calcites, up to Ca-dolomites), in which the amount and proportions of phases with different Mg contents are controlled by Mg/Ca, salinity, and total alkalinity of the lake water changing depending on the climatic cycles and lake level fluctuations. Comparison of the carbonate record for the Holocene section dated by the radiocarbon (¹⁴C) method with results of lithological, diatom, and palynological analyses, data on stable isotopes (δ¹⁸O and δ¹³C), and the distribution of some geochemical indicators of climatic changes permitted the reconstruction of the intricate evolution of the Lake Tsagan-Tyrm basin, which was controlled by the regional climate from the Atlantic period to the present time. The directed change in various characteristics of essentially carbonate sediments evidences that the Ol’khon regional climate has become more arid in recent 6.5 kyr. Moreover, drastic frequent changes of climate and, correspondingly, the Lake Tsagan-Tyrm water level in different periods of its existence have been revealed. The widespread saline and brackish lakes in southern East Siberia, Mongolia, and North China and the highly informative (in terms of paleoclimate and paleolimnology) carbonate sediments might help to recognize the general tendencies of paleoclimate changes and local fluctuations in Central Asia.     

Isoferroplatinum mineral assemblage from the Burgastain Gol placer (Western Mongolia)

Platinum group element (PGE) mineral assemblage has been discovered in the gold placers along the Burgastain Gol and Iljgen Gol (Western Mongolia). It includes isoferroplatinum (Pt₃Fe) grains with inclusions of cooperite (PtS), laurite-erlichmanite (RuS₂-OsS₂), cuprorhodsite-malanite (CuRh₂S₄-CuPt₂S₄), irarsite-hollingworthite (IrAsS-RhAsS), and bowieite (Rh₂S₃).It has been established that the isoferroplatinum assemblage was generated from a volcanoplutonic picrite complex in the Ureg Nuur area, which is widespread in the central part of the Harhiraa accretionary terrane. According to composition, the PGE mineral–Cr-spinel assemblage was referred to as the Ural-Alaskan type.     

The alkaline and carbonatitic rocks of Gorny Altai (Edel'veis complex) as indicators of Early Paleozoic plume magmatism in the Central Asian Fold Belt

The minor intrusions of the Edel'veis alkaline–carbonatite complex are bounded by the spurs of the North Chuya Ridge in southeastern Gorny Altai. According to Ar–Ar isotope data, the complex formed in the Middle Cambrian (～507 Ma). All of its components (alkali clinopyroxenite–melanogabbro–alkali syenite + Ca-carbonatite) occur in only one pluton. Silicate igneous rocks are equivalent in silica content and alkalinity to potassic alkaline and subalkalic mafic rocks. Apatite-phlogopitic Ca-carbonatites are enriched in P₂O₅ (up to 3.6 wt.%), Sr (～2500–5500 ppm), and REE (up to ～2000 ppm) and are, presumably, of liquation genesis. A PREMA-type plume component was a predominant magma source for the complex (?_Nd(T) = +6.56 to +6.85). According to isotope data (⁸⁷Sr/⁸⁶Sr(T) ～ 0.7032–0.7039; δ¹⁸O ～ 7.5–14.9‰; δ¹³C ～ –2.7 to –8.4‰), the fractionation of the melts was accompanied by their crustal contamination. The trace-element composition of the rocks suggests that the complex developed on a continental margin and its development was accompanied by late-collisional rifting and the mixing of moderately depleted (PREMA) and enriched suprasubductional lithospheric mantle (EM I or EM II) with continental crust. It is presumed that the alkaline and carbonatite complexes in the western Central Asian Fold Belt are of primary plume origin and form a LIP within this belt together with other associations produced by Early Paleozoic (510–470 Ma) magmatism.     

Early Mesozoic lamprophyres in Gorny Altai: petrology and age boundaries

The Mesozoic Chuya dike complex was recognized by R.V. Obolenskaya based on the similar mineral composition of dikes and their age characteristics. Lamprophyres occur along the large Terekta–Tolbonur and Kurai–Kobda shear zones. The Chuya complex was studied by the example of two areas, South Chuya and Yustyd, with different levels of erosional truncation. The dikes of the first area are localized in the South Chuya Ridge, where they cut Cambrian–Ordovician metamorphic rocks, and the dikes of the Yustyd area occur in the Devonian terrigenous blackshale deposits of the Yustyd trough. The dikes of these areas differ in structures, textures, the degree of carbonatization, and mineral composition. The performed studies of rocks and minerals confirmed that the dikes of both areas belong to the same complex. They helped to establish the regularities of the lithologic composition of the entire complex and its local areas and to substantiate the recognition of areas not only from their geologic position but also by the composition, structures, and textures of rocks and their mineral composition. Geochronological data show two stages of the complex formation: 236–234 and 250–242 Ma. The results of studies also demonstrate that the lamprophyres and coeval syenites of the Tarkhata massif are fractionates of the same parental melt and can be united into a hypabyssal-plutonic complex. Comparison with other Permo-Triassic lamprophyre complexes showed that the wide variations of the composition of the Chuya rocks and its trend as well as the geochemical anomalies are specific features of complexes of high-K lamprophyres.