Nd-Sr Isotopic Study of Magmatic Rocks and 40Ar/39Ar Dating of the Mafic Dike of the Proterozoic Ulan-Sar’dag Ophiolite Mélange (Southern Siberia, East Sayan, Middle Belt, Russia)

We report the first radiogenic Nd-Sr isotope data in the magmatic rocks island-arc ophiolite assemblage from the middle branch of the East Sayan ophiolite complexes to better constrain geodynamic processes in this segment of the CAOB in southern central Siberia. The magmatic rocks belong to the following geochemical types: (1) Ensimatic island-arc boninites; (2) island-arc assemblage; (3) enriched basalts of mid-ocean ridges; and (4) oceanic island-like basalts. The boninites have a positive value εNd (T), which is generated from a depleted mantle source (N-MORB). The island-arc assemblage has negative or slightly positive εNd (T) and was formed from an enriched mantle source due to the subduction of terrigenous rocks. The source of the terrigenous material was most likely the rocks of the Archean TTG (Trondhjemite Tonalite Granodiorite) complex of the Gargan block. Isotopic ratios for E-MOR and OIB-like basalts are characterized by positive or slightly negative values of εNd (T). The mafic dike, which crosscut ophiolite rocks, corresponds to OIB-like basalts. The values of εNd (T), measured 87Sr/86Sr and I (Sr), in the mafic dike correspond to the EM I mantle source. The E-MOR and OIB-like basalts appear to be formed in late-stage asthenospheric mantle melting via the decompression melting processes. The obtained isotope geochemical data for the E-MOR and OIB-like basalts probably indicate the mixing of island-arc melts with asthenospheric melts. We undertook 40Ar/39Ar dating of the mafic dike, which crosscut the ophiolite unit. The mafic dike has a whole-rock 40Ar/39Ar weighted mean plateau age of 799 ± 11 Ma. The dating constrains the minimum age of the ophiolite and island-arc magmatism in the region.

Petrogenesis, W metallogenic and tectonic implications of granitic intrusions in the southern Great Xing’an Range W belt, NE China: insights from the Narenwula Complex

Extensive magmatism in NE China, eastern Central Asian Orogenic Belt, has produced multi-stage granitic plutons and accompanying W mineralization. The Narenwula complex in the southwestern Great Xing’an Range provides important insights into the petrogenesis, geodynamic processes and relationship with W mineralization. The complex comprises granodiorites, monzogranites and granite porphyry. Mafic microgranular enclaves are common in the granodiorites, and have similar zircon U–Pb ages as their host rocks (258.5–253.9 Ma), whereas the W-bearing granitoids yield emplacement ages of 149.8–148.1 Ma. Permian granodiorites are I-type granites that are enriched in large-ion lithophile elements and light rare earth elements, and depleted in high field strength elements and heavy rare earth elements. Both the mafic microgranular enclaves and granodiorites have nearly identical zircon Hf isotopic compositions. The results suggest that the mafic microgranular enclaves and granodiorites formed by the mixing of mafic and felsic magmas. W-bearing granitoids are highly fractionated A-type granites, enriched in Rb, Th, U and Pb, and depleted in Ba, Sr, P, Ti and Eu. They have higher W concentrations and Rb/Sr ratios, and lower Nb/Ta, Zr/Hf and K/Rb ratios than the W-barren granodiorites. These data and negative ϵHf(t) values (–6.0 to –2.1) suggest that they were derived from the partial melting of ancient lower crust and subsequently underwent extreme fractional crystallization. Based on the regional geology, we propose that the granodiorites were generated in a volcanic arc setting related to the subduction of the Palaeo-Asian Ocean, whereas the W-bearing granitoids and associated deposits formed in a post-orogenic extensional setting controlled by the Mongol–Okhotsk Ocean and Palaeo-Pacific Ocean tectonic regimes.

Mapping the complexity of Transform Margins

Transform margins are a function of the pre-existing crustal architecture (pre-transform) and the interplay of syn- and post-transform geodynamic processes. We use a suite of geospatial databases to investigate four transform margins: East Africa (Davie Deformational Zone, DDZ), Equatorial Africa, and the South African and Falkland (Malvinas) margins (Agulhas-Falkland Fracture Zone, AFFZ). The East African margin is the most complex of the four. This is a consequence of Late Jurassic - Early Cretaceous transform motion affecting highly heterogeneous crust, and post-transform deformation that varies along the margin. Equatorial Africa most closely adheres to traditional definitions of “transform margins”, but actually comprises two principal transform systems - the Romanche and St. Pauls, dictated by the pre-transform distribution of mobile belts and West African craton. All four margins are spatially associated with volcanism, and each exhibits narrow uplifts associated with transpression or transtension. But the causal relationship of these features with transform processes differ. Volcanism along the East African margin is pre- and post-transform. Syn-transform volcanism on the AFFZ is spatially limited, with the AFFZ possibly acting as a conduit for magmatism rather than as a causal driver. Transform margins are varied and complex and require an understanding of pre-, syn- and post-transform geodynamics.

Thermochemical structure and evolution of cratonic lithosphere in central and southern Africa

The thermochemical structure of the subcontinental mantle holds crucial information on its origin and evolution that can inform energy and mineral exploration strategies, natural haz-ard mitigation and evolutionary models of the Earth1−4. However, imaging the fine-scale thermochemical structure of continental lithosphere remains a major challenge. Here we combine multiple land and satellite datasets via thermodynamically-constrained inversions to obtain a high-resolution thermochemical model of central and southern Africa. Results reveal diverse structures and compositions for cratons, indicating distinct evolutions and responses to geodynamic processes. While much of the Kaapvaal lithosphere retained its cra-tonic features, the western Angolan-Kasai shield and the Rehoboth block have lost their cra-tonic keels. The lithosphere of the Congo Craton has been affected by metasomatism, increas-ing its density and inducing its conspicuous low-topography, geoid and magnetic anomalies. Our results reconcile mantle structure with the causes and location of volcanism within and around the Tanzanian Craton, whereas the absence of volcanism towards the north is the result of local asthenospheric downwellings, not to a previously-proposed lithospheric root connecting with the Congo Craton. Our study offers greatly improved integration of man-tle structure, magmatism and the evolution and destruction of cratonic lithosphere and lays the groundwork for new evolutionary models and exploration frameworks for the Earth and other terrestrial planets.

Kinematic Boundary Conditions Favouring Subduction Initiation at Passive Margins Over Subduction at Mid-oceanic Ridges

We perform numerical modelling to simulate the shortening of an oceanic basin and the adjacent continental margins in order to discuss the relationship between compressional stresses acting on the lithosphere and the time dependent strength of the mid-oceanic ridges within the frame of subduction initiation. We focus on the role of stress regulating mechanisms by testing the stress–strain-rate response to convergence rate, and the thermo-tectonic age of oceanic and continental lithospheres. We find that, upon compression, subduction initiation at passive margin is favoured for thermally thin (Palaeozoic or younger) continental lithospheres (<160 km) over cratons (>180 km), and for oceanic basins younger than 60 Myr (after rifting). The results also highlight the importance of convergence rate that controls stress distribution and magnitudes in the oceanic lithosphere. Slow convergence (<0.9 cm/yr) favours strengthening of the ridge and build-up of stress at the ocean-continent transition allowing for subduction initiation at passive margins over subduction at mid-oceanic ridges. The results allow for identifying geodynamic processes that fit conditions for subduction nucleation at passive margins, which is relevant for the unique case of the Alps. We speculate that the slow Africa–Europe convergence between 130 and 85 Ma contributes to the strengthening of the mid-oceanic ridge, leading to subduction initiation at passive margin 60–70 Myr after rifting and passive margin formation.

GEOMORPHOLOGICAL RISKS AND ASSESSMENT OF ECOLOGICAL-GEOMORPHOLOGICAL SITUATIONS OF MINING REGIONS OF ARID ZONE OF KAZAKHSTAN

The importance of the impact of emerging ecologic-geomorphological situations in areas of active mining development of Kazakhstan on the life of the population is difficult to overestimate in conditions of a dry climate, water scarcity and the use of outdated technologies for the extraction of solid minerals. This article examines the ecologic-geomorphological situations of mining areas of arid platform-denudation plains of Kazakhstan and gives their characteristics, which is based on an analysis of the leading natural and anthropogenically caused geodynamic processes, their spatial coverage, ratios and comparative assessment as an indicator of geomorphological risk within selected morphogenetic types of relief.

Lamprophyres, lamproites and related rocks as tracers to supercontinent cycles and metallogenesis

Proterozoic to Cenozoic lamprophyres, lamproites and related rock types hold a unique potential for the investigation of processes affecting mantle reservoirs. They originated from primary mantle-derived melts that intruded both cratons and off-craton regions, that were parts of former supercontinents - Columbia, Rodinia and Gondwana-Pangea. Well-known for hosting economic minerals and elements such as diamonds, base metals, platinum-group elements and Au, they are also significant for our understanding of deep-mantle processes, such as mantle metasomatism and mantle plume-lithosphere interactions, as well as large-scale geodynamic processes, including subduction-related tectonics, supercontinent amalgamation and break-up, respectively. This Special Publication presents an overview of the state-of-the-art and recent advances as achieved by individual research groups from different parts of the world, and outlines future research directions. Mineralogical, geochemical, geochronological and isotope analyses are used to decipher the complex petrogenetic and metallogenetic evolution of these extraordinary rocks, and unravel a complete history of tectonic events related to individual supercontinent cycles. The Special Publication including this introductory chapter also deals with some issues related to the classification of these rocks.

Ways to improve the reliability of interpreting the results of geophysical surveys performed for the local forecast and prospecting for solid minerals: a geodynamic approach

Background. The geodynamic approach is effectively used in regional forecasting and prospecting works. However, its application for local forecasting and prospecting for solid minerals is limited and sometimes impossible. One of the key problems of local forecasting and prospecting for minerals (solid, liquid, gaseous) is the presence of non-standard (flickering) geophysical anomalies, which complicates the interpretation of the results of geophysical surveys performed at different times at different stages of geological exploration. The article is devoted to clarifying the possibility of using geodynamic research in local forecasting and prospecting for solid minerals on the basis of attracting the latest scientific knowledge from the field of studying the spatio-temporal patterns of the development of modern geodynamic processes and their influence on the Earth’s geophysical fields. Aim. To increase the reliability of interpreting the results of geophysical surveys performed for local forecasting and prospecting for solid minerals.Materials and methods. The research was carried out on the basis of a comprehensive analysis of literature data, fund materials and the results of many years of the author’s own research in the fields of modern geodynamics and prospecting and exploration geodynamics. The initial data were based on the monitoring data of various Earth’s geophysical fields (deformation, seismic, and surface magnetic).Results. A geodynamic reason for the appearance of flickering anomalies in the Earth’s geophysical fields (in particular, magnetic) has been established, and a mechanism for their formation under the influence of modern geodynamic processes has been proposed. The possibility of using the geodynamic approach in the prospecting for solid minerals has been expanded, and ways to increasing the efficiency of local searches have been proposed.Conclusions. The research demonstrates the possibility of using geodynamic studies in local prospecting for solid minerals, which helps to increase the reliability of the results of interpretation of geophysical survey data, and, as a result, to reduce the overall financial and time costs involved with searching for mineral deposits.

3D Modeling of Stability of Mobile Jack-Up Rigs Under Seismic Impacts

The work is devoted to the study of the influence of external mechanical impacts on the stability of jack-up floating drilling rigs. Ensuring safety when working on jack-up floating drilling rigs (jack-up rigs) is one of the most important tasks at the initial stages of offshore hydrocarbon field development. The aim of the work is simulation of the stability of the rigs, which is in the operating mode, both in the absence of external effects acting on it, and when it is affected by various geodynamic processes. Calculation of jack-up rig stability in sea soil with complex viscoelastoplastic rheology under external mechanical influences was carried out using the finite element method. As a result of the work, the spatial distributions of stress and strain fields were determined both in the jack-up rig itself and in the area of the soil in the vicinity of its supports, in an equilibrium and disturbed state. The results of equilibrium static calculations made it possible to analyze the zones of localization of stresses and deformations in the jack-up rig with respect to stability: the structural elements that are most dangerous from the perspective of stability loss were identified. The analysis of the soil area in the vicinity of the jack-up rig supports made it possible to estimate the typical dimensions of the stress concentration zones arising during the jack-up installation and draw conclusions about the requirements on details and scalability of the model of mechanical properties needed for geomechanical modeling at specific objects. The obtained results of dynamic calculations - the response of theconsidered system to external influences simulating remote seismic processes - made it possible to study the process of loss of jack-up rig stability. The critical values of the parameters describing external influences were determined: in particular, the amplitudes of surface seismic waves leading to the loss of stability of the jack-up rig were found. The obtained results made it possible to formulate an algorithm for predicting seismic hazard during drilling in offshore conditions, based on numerical modeling of mechanical processes at specific locations.