Geodynamic conditions of massive sulfide formation in the Magnitogorsk megazone

Research subject. Volcanism, rock geochemistry, geodynamics, and massive sulfide formation in the Magnitogorsk megazone (MMZ) of the Southern Urals in the Middle Paleozoic.Materials and Methods. Across the largest part of the massive sulfide deposits under investigation, the authors conducted route studies, including geological surveys of individual ore fields and quarries of deposits, core samples of deep wells and transparent sections. Representative analyses of petrogenic and microelements were performed using wet chemistry and ICP-MS in analytical centers in Russia and Europe. Along with the authors’ data, analytical materials published by Russian and foreign researchers were used. Geodynamic reconstructions were carried out taking into account regional data on gravics, thermal field, magnetometry, and seismic stu dies, including «Urseis-95».Results. The geodynamic reconstructions established that the main elements of the paleostructure of the Southern Urals in the Devonian were the subduction zone of the eastern dip and asthenospheric diapirs that penetrated into the «slab-window», which determined the type of volcanic belts, the composition and volume of volcanic rocks of pyrite-bearing complexes, and ore matter of pyrite deposits. The following geodynamic zones in the MMZ were identified: 1 – polychronous accretion prism; 2 – frontal and developed island arcs (D1e2–D2ef1); 3 – zone of back-arc spreading (D1e2); 4 – rear island arc (D2ef1).Conclusions. All investigated zones and ore areas are characterized by an autonomous development of volcanism, a special deep structure and a different composition, as well as by a different volume of massive sulfide deposits that vary in the Cu and Zn ratios and Pb, Ba, Au amounts. In the MMZ volcanic complexes, three groups of plume source basalts are distinguished. The results can be used in predictive-estimation and search operations for massive sulfide mineralization.

Platinum Group Elements in the South Transbaikalian Basanites (First Data)

—Study of the behavior and distribution of platinum group elements (PGE), along with other data, is necessary for geodynamic reconstructions. There are almost no PGE data for Transbaikalia, one of the large regions of Russia. This work presents the first data on the contents and behavior of PGE in the Cenozoic intraplate alkali basaltoids of southern Transbaikalia. The total PGE contents are 20–40 ppb. The PGE pattern of the studied basanites is similar to those of mafic OIB, including the Hawaiian ones, and rocks of large igneous provinces: tholeiitic basalts of the Siberian Platform and basaltoids of the West Siberian Plate. Based on this similarity and on the intraplate location of the South Transbaikalian basanites, we have concluded that the basanitic melts formed under mantle plume impact.

Pressure-to-depth conversion models for (ultra-)high-pressure metamorphic rocks: review and application

<p>Pressure estimated from metamorphic rocks is one of the main tools for geodynamic reconstructions. The pressure-temperature path of UHP metamorphic rocks typically shows a linear increase of P and T followed by a rapid drop of Pressure at near-constant temperature. The geological history can be reconstructed by using the metamorphic pressure as a proxy for depth. Researchers often base their geodynamic reconstruction on a simple linear mapping of pressure to depth, by considering that the pressure is the weight of the overlying column of rock or lithostatic pressure. In recent years, an increasing corpus of evidence demonstrates that rocks can experience pressures that deviate from the lithostatic state on the order of GPa. These deviations can be at the scale of the orogen (Petrelli and Podladchikov, 2002), the outcrop (Jamtveit et al., 2018; Luisier et al., 2019); or even at the grain-scale (Tajcmanova, 2015). Thus, these studies raise the concern that metamorphic pressures may not be reliable proxies for depth, and therefore could not be used for geodynamic reconstructions. The objective of this contribution (1) to review the various models proposed in the literature for metamorphic pressure, (2) to formulate analytical models with simple assumptions that can be used to convert metamorphic pressure to depth even in the case where pressure deviates significantly from the lithostatic pressure. We use our pressure-to-depth conversion models to estimate the depth of ~60 samples from various orogens worldwide. The prediction of the different models varies widely. Some models predict depth as deep as 160km for specific samples, while other models predict depth $<75$ km (i.e. deepest depth of the Moho) for all data points.  We discuss the limits of applicability and the geodynamic implications of each model. </p>

Towards understanding the roll-back subduction of narrow oceanic domains: inferences from the modelling of Carpathians subduction zone

<p>Numerous subduction systems in the Meditteranean realm are derived from the subduction of narrow oceanic domains, which are too narrow to generate the means of a fully coupled two-dimensional thermo-mechanical numerical model that takes into account the visco-elasto-plastic properties of different lithospheric domains. The results show that the narrow extent of the Ceahlau-Severin Ocean commonly assumed by paleogeographic reconstruction cannot generate roll-back upon subduction, in particular for models that must assume that slabs do not penetrate the 660 km discontinuity. Therefore, we propose that the subduction of the Carpathians system must have an inherited component from a previous orogenic evolution, which will ensure sufficient slab-pull to generate roll-back in the Carpathians realm. The model is constrained by recent results in terms of mantle structure and geodynamic reconstructions, while multiple compositional, thermal distribution and geometrical scenarios are tested in successive models. In all of our models, roll-back is achieved, which indicates that the proposed inherited component can sufficiently explain the roll-back subduction of the aforementioned narrow oceans. The subducting oceanic slab does not penetrate the 660 km discontinuity, this is in agreement with seismic tomographic results from various Mediterranean subduction zones. The exact onset and dynamics of the roll-back are mostly controlled by the thermic age of the ocean and the convergence kinematics of the continental slabs. An outlook on possible future improvements to the model, such as taking into account pre-existing rheological weakness zones in the lithosphere, is discussed and the opportunity of a seismo-thermo-mechanical modelling to investigate the seismic cycle in the Vrancea-zone is highlighted.</p>

"Diffusive" border of isotopic reservoirs of indian and pacific morb types beneath Kamchatka

The convergence zones of lithospheric plates in the Northwest Pacific are the boundaries of the two main isotopic domains of the Earth - the Indian and Pacific MORB types, separated be cold oceanic lithosphere. This configuration limits of their interaction by special geodynamic environments - the influence of deep plume sources or the distraction of the subducted slab and intrusion of the oceanic asthenosphere into the subcontinental mantle wedge. The latter mechanism is reconstructed in the Central Kamchatka Depression on the basis of geological, geochemical, and high-precision (double-spike) lead isotopic data. The role of the oceanic asthenosphere in magma generation in the zones of convergence of oceanic and continental lithospheres is a poorly studied but not unique phenomenon that must be considered under geodynamic reconstructions and the creation of new, more realistic models of the juvenile continental crust formation.

Serpentines as the indicators of mesozoic peridotites metamorphic and geodynamic transformations in the Internal Ukrainian Carpathians

Formulation of the problem. Peridotites of ophiolite complexes,being the fragments of the oceanic upper mantle that have undergone several stages of partial melting, brought to the surface by tectonic movements, also have undergone metamorphic transformations almost immediately after its formation. Because of serpentinization, the mineral composition of the rocks became more complicated. The analysis of the final structure and composition of apoperidotites allows obtaining data for geodynamic reconstructions regarding the stage of their formation. Review of previous publications. It has been determined that serpentines are the most common secondary minerals of peridotites of the Uholskyi complex in the Ukrainian Carpathians, and the processes of serpentinization took place at a depth of 40–50 to 100 km (?) at T = 450–600 °C and P = 13–16 kbar (Stupka O., 2013). The study of serpentinites of the Main Ural Fault (Panas'yan L. et al., 2014) revealed that high alumina and high chromium serpentinites have ultrabasic protoliths formed in the mantle, and medium alumina and low chromium varieties – the protoliths of the basic composition which were born in the conditions of the crust. Based on the study of serpentinites in the orogenic Qinling belt (China), researchers (Wu K. et al., 2018) determined their mantle origin: magnetite-enriched antigorite serpentinites were formed as a result of the interaction of serpentinized apoperidotites of mantle protoliths with molten rock in the subduction channel. Purpose. We have investigated secondary serpentines in order to reconstruct the geodynamic conditions of the formation and transformations of the peridotites (Uholskyi complex), localized in the Marmarosh rocky zone in the Internal Ukrainian Carpathians, and are most widely spread in the interfluve of Velyka and Mala Uholka-rivers. Methods. The work is based on the results of geological observations of the Uholskyi complex rocks in natural outcrops, as well as petrographic, mineralogical (including X-ray diffraction, thermal and microprobe analysis), and geochemical studies. Results. The paper presents the study results of serpentinized apoperidotites of the ophiolite Uholskyi complex in the Internal Ukrainian Carpathians. Serpentinized apoperidotites (T2–K1?) form olistoliths in the Soimulska olistostrome-conglomerate strata of the Lower Cretaceous age. The investigated serpentines are the rock-forming minerals of lizardite and antigorite serpentinites. Lizardite serpentinites are characterized by lenticular-looped textures formed by α-lizardite and non-altered chrome-spinellids. Antigorite serpentinites, recognized by striped-shale textures, contain antigorite, β-lizardite and magnetite. Lizardite serpentinites are characteristic of the regressive metamorphism of the greenschist facies upper part, and antigorite serpentinites are a typical formation of the progressive metamorphism of the lower greenschist – upperlower epidote-amphibolite facies. Regressive metamorphism occurred under geodynamic conditions of spreading and the progressive ones – under subduction conditions between the terrains of Dacia and Tisza, which led to the closure of the Transylvanian-Mureş Paleocean. It has been concluded, that the protoliths of lizardite serpentinites were the primary mantle rocks of the ultrabasic (restitic) composition, and the protoliths of the antigorite serpentinites were the lithospheric rocks of the basic composition. Conclusions. The complex study of serpentinized apoperidotites of the Uholskyi complex makes it possible to reconstruct the metamorphic transformations of the primary mantle protoliths and to determine the stages of lithosphere formation within the fold-nappe structures. The obtained results can be used for prediction of serpentinite mineralization.

Geodynamic Evolution and Metallogeny of Archaean Structural and Compositional Complexes in the Northwestern Russian Arctic

This paper highlights the geodynamic evolution of the early Precambrian rock associations in the northwestern part of the Russian Arctic where the rocks are exposed in the Kola region (northeastern Baltic Shield). The evolution is shown to predetermine the metallogenic potential of the area. It is emphasized that the Earth’s evolution is a non-linear process. Thus, we cannot draw direct analogies with Phanerozoic time or purely apply the principle of actualism, which is still widely used by experts in Precambrian geology to study the premetamorphic history of ancient deposits. In both cases, the principles should be adjusted. This article provides a novel technique for reconstructing geodynamic regimes of protolith formation in the early Precambrian. The technique identifies changing trends in geodynamic regimes during the formation of the Archean structural and compositional complexes in the Kola region. These trends fit into the earlier suggested general scheme of their formation, thus enhancing its reliability. The metallogeny of the ore areas is specified. The results of the geodynamic reconstructions explain most of the location patterns of minerals within the Kola region. Thus, the authors consider the metallogenic forecast based on geodynamic reconstructions to be a promising trend for further research.