Sr-Nd-Pb isotopic systematic and geochronology of ultramafic alkaline magmatism of the southwestern margin of the Siberian Craton: Metasomatism of the sub-continental lithospheric mantle related to subduction and plume events

© 2020 To provide new insights into the origin and evolution of ultramafic lamprophyres (UMLs) and their mantle source, we examined two UML (aillikite and damtjernite) occurrences of different ages in the western portion of the Siberian Craton (Ilbokich and Chadobets). New age, mineral and rock geochemistry, along with Sr–Nd–Pb–C–O isotope data was obtained. Our new 206Pb/238U perovskite age (399 ± 4 Ma) confirms the previously published Early Devonian age of the Ilbokich aillikite. Rb[sbnd]Sr isochron and 40Ar/39Ar dating yielded a Middle Triassic age (243 ± 3 Ma and 241 ± 1 Ma, respectively) for the Chadobets aillikites, indicating post-Trap emplacement of these rocks. Both UMLs are characterized by incompatible elements, including light rare earth element (LREE) enrichments (La is up to ×200 chondrite concentration), and strong fractionation of REEs ((La/Yb)n: 33–84). Despite the close geochemical affinity of both UMLs, the Nd isotopic compositions of aillikites, as well as the Pb isotopic composition of Chadobets and Ilbokich UMLs, do not overlap and are distinctly different from each other. The initial Sr and Nd isotopic compositions of the Ilbokich UMLs fall in within a narrow 87Sr/86Sr0 range (0.7032–0.7042) and εNd(T) (4.03–3.97). Chadobets UMLs have a similar Sr isotopic signature (87Sr/86Sr0: 0.7031–0.7043) and a more depleted Nd isotopic signature (εNd(T) 4.09–5.08). The initial Pb isotope compositions of the Chadobets UMLs are moderately radiogenic, ranging between 206Pb/204Pb = 18.4–19.0, 208Pb/204Pb = 38.3–38.8, and are characterized by a narrow 207Pb/204Pb ratio between 15.5 and 15.6. The Ilbokich Pb isotope compositions are less variable and range between 206Pb/204Pb = 18.0–18.4, 208Pb/204Pb = 37.8–38.4 and 207Pb/204Pb ratios between 15.5 and 15.6. The oxygen isotopic composition of carbonate from both UMLs is characterized by highly variable δ18O values from +12.1 and up to +20.5‰ (SMOW). The isotopic composition of δ13C values range from −1.3‰ to −7.1. Based on the minor impact of crustal contamination in both aillikites, it is inferred that their radiogenic isotope composition reflects a mantle source signature. The mantle source of the Chadobets aillikites is likely to include carbonatitic magma as a metasomatic agent. In contrast, phlogopite-rich metasomes within the lithospheric mantle could have contributed more significantly to the Ilbokich aillikites. These metasomes could be formed during the Caledonian orogeny, which did not only affect the southwestern boundary of the Siberian Craton, but also expanded to the craton interior. This study provides additional support for the evolution of the south-western portion of the Siberian SCLM, ranging from mantle containing phlogopite enrichment domains during the Early Devonian to hydrous-phase reduced mantle in the Triassic due to the thermal impact of the Siberian Traps

Assessing the Influence of Environmental Parameters on Amur Tiger Distribution in the Russian Far East Using a MaxEnt Modeling Approach

A better understanding of which biological and anthropogenic parameters are strong predictors of suitable habitats for tigers will help address conservation planning in those areas, which is crucial for maintaining connectivity and preventing further population fragmentation. The aim of this study was to develop a spatial model based on a number of environmental and anthropogenic variables as well as tiger presence data from a 2005 large-scale winter survey to predict Amur tiger distribution within its range in the RFE. Modeling the geographic distribution of Amur tigers required an application of the MaxEnt algorithm using a dataset of 1027 tiger track records and a set of environmental variables, such as distance to rivers, elevation and habitat type, and anthropogenic variables, such as distance to forest and main roads, distance to settlements and vegetation cover change. The models were divided into two groups based on elevation and habitat type. Elevation (AUC = 0.821) appeared to be a better predictor of habitat suitability for tigers than habitat type (AUC = 0.784)

Early Palaeoproterozoic granulite-facies metamorphism and partial melting of eclogite-facies rocks in the Salma association, eastern Fennoscandian Shield, Russia

The Salma-type Archaean eclogites exposed along the northwestern boundary of the Belomorian Eclogite Province in the eastern Fennoscandian Shield formed as a result of the Mesoarchaean–Neoarchaean subduction and collision. The common protoliths of the Salma-type subduction-related eclogites were oceanic layered gabbro and volcanic-sedimentary assemblage. The eclogite-facies pillow lavas and associated alumina-siliceous sediments that fill interpillow space and intercalate with lava flows are the main objects of our work. The kyanite-garnet–phengite–quartz rocks formed after alumina-siliceous sediments contain fluid inclusions trapped in large relic quartz grains. The fluid inclusions yielded an isochore that corresponds to PT-conditions of a beginning of the Salma oceanic rock subduction from the seafloor level that generally confirms the sedimentary provenance of these rocks. The alumina-siliceous sediments underwent the eclogite-facies metamorphism at pressure no lower than 21 kbar and temperatures of 650–750 °C and transformed into kyanite-garnet–phengite–quartz rocks. During exhumation under granulite-facies conditions at temperatures up to 900 °C and pressure down to 9 kbar, eclogite facies metasediments underwent partial melting accompanied by disequilibrium breakdown of phengite + quartz association with formation complex polymineralic pseudomorphs consisting of feldspars, biotite, muscovite, kyanite, corundum, and dumortierite. U-Pb dating of Th-rich igneous zircon from melted metasedimentary and mafic rocks using the LA-ICP-MS and TIMS methods yielded the time of granulite facies event accompanied by partial melting processes at ~2.45 Ga. After this, zircon underwent fluid-induced alteration, causing partial dissolution followed by precipitation of new Th–poor zircon and zircon rims around ancient grains at ~1.9 Ga ago © 2021 Elsevier B.V