The Ureg Nuur Pt-bearing volcanoplutonic picrite–basalt association in the Mongolian Altay as evidence for a Cambrian–Ordovician Large Igneous Province

The paper discusses geological, mineralogical, petrographic, and geochemical data on the Ureg Nuur volcanoplutonic association of high-Mg volcanic and subvolcanic rocks located among Vendian–Cambrian accretionary structures in the Mongolian Altay. These rocks have a high potassium alkalinity (K₂O/Na₂O up to 1.2), are enriched in LILE and Sr, and have negative Zr–Hf and Nb anomalies in multielement spectra; this confirms the suprasubduction type of the source of melts. The geological setting and established age (512.4 ± 6.1 Ma, ³⁹Ar–⁴⁰Ar dating of biotite phenocrysts) evidence picritic magmatism at the accretionary stage of the development of the Altay fragment of the Paleoasian ocean. This indicates a large igneous province related to a mantle plume.     

The Triassic stage of mafic magmatism in the Dzhugdzhur–Stanovoi superterrane (southern framing of the North Asian craton)

With U-Pb zircon dating, the ages of the Ul'degit (228 ± 1 Ma) and Chek-Chikan (203 ± 1 Ma) mafic massifs were determined. These massifs were earlier considered to form at the Early Precambrian stage of the geologic evolution of the Dzhugdzhur–Stanovoi superterrane. In geochemical features the igneous rocks of the massifs show relation with a within-plate source, on the one hand, and are similar to igneous rocks of subduction zones, on the other. They might have formed after subduction, which caused the intrusion of gabbroids of the Lucha massif (248 ± 1 Ma) and diorites of the Tok-Algoma complex (238 ± 2 Ma), followed by the fracturing of the subducted plate.     

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.     

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.     

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.     

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.     

Molecular research on the systematically challenging smoothhound shark genus Mustelus: a synthesis of the past 30 years

The species-rich genus Mustelus (smoothhounds) of the shark family Triakidae is one of the most bio-economically important groups of elasmobranchs in the world’s oceans. Despite the commercial value of Mustelus, the systematics of the group remains largely unresolved and there is no global review or synthesis of knowledge about the conservation status and conservation genetics of smoothhounds across all oceanic regions. Here, we analysed published studies as well as grey literature to gain insight into the biogeographic, ecological and behavioural factors that shape genetic diversity in smoothhounds, and we identify critical knowledge gaps. From a series of molecular phylogenetic studies it can be inferred that the genus Mustelus is paraphyletic and that the aplacental species evolved secondarily from the placental species of the genus. The increasing availability of genetic data aids in disentangling systematic issues, such that more meaningful morphological characters can be chosen for use in practical field-identification keys for co-occurring smoothhounds. An integrative taxonomic approach to the genus Mustelus may offer the best chance of recording and protecting the biodiversity of these sharks. Furthermore, it is evident that different smoothhound species exhibit unique gene-flow patterns, suggesting varying rates within species and hence that species-level conservation approaches would be most appropriate. Molecular studies have advanced our understanding of smoothhound biology (including reproductive traits), ecology and evolution. While many knowledge gaps remain, a crucial lesson from this review is that, when doing assessments on a molecular level, it is important to place genetic results in a broader context, by assimilating biological and ecological data, if definitive conclusions are to be drawn.