Structural framework of the Bashkirian anticlinorium, SW Urals

The Bashkirian anticlinorium of the southwestern Urals shows a much more complex structural architecture and tectonic evolution than previously known. Pre-Uralian Proterozoic extensional and compressional structures controlled significantly the Uralian tectonic convergence. A long-lasting Proterozoic rift process created extensional basement structures and a Riphean basin topography which influenced the formation of the western fold-and-thrust-belt with inversion structures during the Uralian deformation. A complete orogenic cycle during Cadomian times, including terrane accretion at the eastern margin of the East European platform, resulted in a high-level Cadomian basement complex, which controlled the onset of Uralian deformation, and resulted in intense imbrication and tectonic stacking in the subjacent footwall of the Main Uralian fault. The Uralian orogenic evolution can be subdivided into three deformation stages with differently oriented stress regimes. Tectonic convergence started in the Late Devonian with ophiolite obduction, tectonic accretion of basin and slope units and early flysch deposits (Zilair flysch). The accretionary complex prograded from the SE to the NW. Continuous NW/SE-directed convergence resulted finally in the formation of an early orogenic wedge thrusting the Cadomian basement complex onto the East European platform. The main tectonic shortening was connected with these two stages and, although not well constrained, appears to be of Late Devonian to Carboniferous age. In the Permian a final stage of E–W compression is observed throughout the SW Urals. In the west the fold-and-thrust-belt prograded to the west with reactivation of former extensional structures and minor shortening. In the east this phase was related to intense back thrusting. The East European platform was subducted beneath the Magnitogorsk magmatic arc during the Late Paleozoic collision. The thick and cold East European platform reacted as a stable rigid block which resulted in a narrow zone of intense crustal shortening, tectonic stacking and high strain at its eastern margin. Whereas the first orogenic wedge is of thick-skinned type with the involvement of crystalline basement, even the later west-directed wedge is not typically thin-skinned as the depth of the basal detachment appears below 15 km and the involvement of Archean basement can be assumed.     

Upper Riphean and Vendian sandstones of the Bashkirian anticlinorium

The analysis of lithogeochemical data on the Upper Riphean and Vendian sandstones from the Bashkirian anticlinorium showed that sandstone associations formed in a passive sluggish tectonic regime in the middle Late Vendian were replaced by associations accumulated in the more active tectonic settings. This is well seen in the SiO₂-K₂O/Na₂O and (Fe₂O₃^* + MgO)-TiO₂ diagrams reflecting the particular and median compositions of psammites. The lithochemical characteristics of sandstones were examined to determine the compositional variation of rock complexes eroded on paleodrainage areas. Quartz-rich sedimentary, metasedimentary, and metamorphic rocks, as well as felsic igneous rocks prevailed in the paleodrainage areas throughout the entire Late Riphean and Early Vendian, while the main sources of clastic material in the Late Vendian were igneous intermediate and basic rocks. With allowance made for the previous comparative-lithological data and some other materials, significant similarity in the position and orientation of compositional fields of psammites from the middle and upper levels of the Asha Group (Bashkirian anticlinorium) with fields of psammites from different syncollisional (flysch and molasse) basins in the SiO₂-K₂O/Na₂O, K₂O/Na₂O-SiO₂/Al₂O₃, F1–F2 and other diagrams suggests that the middle Late Vendian (beginning from the Basa level) was marked by a variation in tectonic/geodynamic settings of sandstone accumulation and in composition of the eroded paleodrainage systems. The revealed trend agrees well with concept of the existence of the Late Riphean-Vendian Pechora paleocean.     

Peculiarities of the structure and evolution of the Riphean Ai volcanic complex,South Urals

The Ai volcanic complex is a part of the Ai Formation, which begins the stratotypical Riphean section in the South Urals and lies on the Archean Taratash metamorphic complex. New geochemical and isotope data were obtained for the volcanic rocks. The dominant porphyritic plagioclase and pyroxene trachibasalts associated with dacites are characterized by higher contents of alkalis and titanium, which is typical of rift volcanism. However, other geochemical data, e.g., decreased Ni contents, are beyond of this scheme. The U-Pb (SHRIMP) age of zircons from dacites is 1415 ± 11 Ma.     

Neoproterozoic riftogenic subalkali basites of the Central Urals: Geochemical specifics of clinopyroxene

Trace-element distribution in clinopyroxenes of different generations was used to decipher the intricate melt fractionation history of the trachybasalts, trachyandesites, and gabbroids from different rift-related magmatic complexes, which were formed during evolution of the Neoproterozoic passive margin of the East European craton (western slope of the Central Urals). It was established that chromian and high-magnesian cores of the early Cpx phenocrysts in the trachybasalts are not xenogenic, but represent relict minerals that were formed at the early stage of fractionation of a high-Mg melt. The fact that the trachybasalts and trachybasaltic andesites contain high-Mg Cpx with trace-element patterns similar in shape and element abundance indicates their formation via differentiation of a common melt. However, their subsequent evolution was different: trachybasaltic andesites were subjected to significant crustal contamination, which was recorded in composition of late Cpx. All subalkali basites were presumably formed from comparatively deepseated melts, because, according to thermodynamic calculations, early cores in the gabbroids crystallized at temperature more than 1200°C and pressure between 10 and 14–15 kbar, i.e., at depths of approximately 35–50 km. Some differences observed between trace-element composition of Cpx from the studied subalkali basatoids and gabbroids are inconsistent with their derivation from a common source, but similarity of their Cpx in many characteristics undoubtedly indicates close depths and compositions of their sources. Geochemical peculiarities of Cpx in the Neoproterozoic rift rocks from the western slope of the Central Urals testify that they were derived from melts formed by relatively low degree partial melting with garnet in residue. The geochemical specifics of clinopyroxenes from the Neoproterozoic riftogenic subalkali basites of the Central Urals and basaltoids from the Paleozoic Tagil structure of the same region showed that this island-arc system reflects the composition of melt source and its reworking by mantle fluids, which were different in the course of plume-lithosphere interaction and suprasubduction processes.     

Berdyaush pluton of rapakivi granites,South Urals: New data on the geological structure and geodynamic evolution

The new version of the geological structure of the Berdyaush pluton (a single intrusion of rapakivi granites in the Urals) presented in this paper is significantly distinct from the previous structural schemes. Rapakivi granites compose no more than 10–20% of the area of the pluton and they are widespread only in its northeastern and southwestern flanks. The contacts between gabbro (I phase), hybrid syenodiorites (II phase), and rapakivi granites (III phase) are transitional, metasomatic. The hybrid syenodiorites and rapakivi granites are formed after gabbroic rocks as a result of their intense thermal and metasomatic transformation by the deep fluids. The driving force of this process could be the unilateral compression of the Berdyaush pluton resulting from formation of the eastward continental rift in the beginning of the Middle Riphean.     

Mantle sources and magma evolution of the Rooiberg lavas,Bushveld Large Igneous Province,South Africa

We report a new whole-rock dataset of major and trace element abundances and ⁸⁷Sr/⁸⁶Sr–¹⁴³Nd/¹⁴⁴Nd isotope ratios for basaltic to rhyolitic lavas from the Rooiberg continental large igneous province (LIP). The formation of the Paleoproterozoic Rooiberg Group is contemporaneous with and spatially related to the layered intrusion of the Bushveld Complex, which stratigraphically separates the volcanic succession. Our new data confirm the presence of low- and high-Ti mafic and intermediate lavas (basaltic—andesitic compositions) with >?4 wt% MgO, as well as evolved rocks (andesitic—rhyolitic compositions), characterized by MgO contents of <?4 wt%. The high- and low-Ti basaltic lavas have different incompatible trace element ratios (e.g. (La/Sm)_N, Nb/Y and Ti/Y), indicating a different petrogenesis. MELTS modelling shows that the evolved lavas are formed by fractional crystallization from the mafic low-Ti lavas at low-to-moderate pressures (~?4 kbar). Primitive mantle-normalized trace element patterns of the Rooiberg rocks show an enrichment of large ion lithophile elements (LILE), rare-earth elements (REE) and pronounced negative anomalies of Nb, Ta, P, Ti and a positive Pb anomaly. Unaltered Rooiberg lavas have negative εNd_i (??5.2 to ??9.4) and radiogenic εSr_i (6.6 to 105) ratios (at 2061 Ma). These data overlap with isotope and trace element compositions of purported parental melts to the Bushveld Complex, especially for the lower zone. We suggest that the Rooiberg suite originated from a source similar to the composition of the B1-magma suggested as parental to the Bushveld Lower Zone, or that the lavas represent eruptive successions of fractional crystallization products related to the ultramafic cumulates that were forming at depth. The Rooiberg magmas may have formed by 10–20% crustal assimilation by the fractionation of a very primitive mantle-derived melt within the upper crust of the Kaapvaal Craton. Alternatively, the magmas represent mixtures of melts from a primitive, sub-lithospheric mantle plume and an enriched sub-continental lithospheric mantle (SCLM) component with harzburgitic composition. Regardless of which of the two scenarios is invoked, the lavas of the Rooiberg Group show geochemical similarities to the Jurassic Karoo flood basalts, implying that the Archean lithosphere strongly affected both of these large-scale melting events.     

Crystallisation sequence and magma evolution of the De Beers dyke (Kimberley,South Africa)

We present petrographic and mineral chemical data for a suite of samples derived from the De Beers dyke, a contemporaneous, composite intrusion bordering the De Beers pipe (Kimberley, South Africa). Petrographic features and mineral compositions indicate the following stages in the evolution of this dyke: (1) production of antecrystic material by kimberlite-related metasomatism in the mantle (i.e., high Cr-Ti phlogopite); (2) entrainment of wall-rock material during ascent through the lithospheric mantle, including antecrysts; (3) early magmatic crystallisation of olivine (internal zones and subsequently rims), Cr-rich spinel, rutile, and magnesian ilmenite, probably on ascent to the surface; and (4) crystallisation of groundmass phases (i.e., olivine rinds, Fe-Ti-rich spinels, perovskite, apatite, monticellite, calcite micro-phenocrysts, kinoshitalite-phlogopite, barite, and baddeleyite) and the mesostasis (calcite, dolomite, and serpentine) on emplacement in the upper crust. Groundmass and mesostasis crystallisation likely forms a continuous sequence with deuteric/hydrothermal modification. The petrographic features, mineralogy, and mineral compositions of different units within the De Beers dyke are indistinguishable from one another, indicating a common petrogenesis. The compositions of antecrysts (i.e., high Cr-Ti phlogopite) and magmatic phases (e.g., olivine rims, magnesian ilmenite, and spinel) overlap those from the root zone intrusions of the main Kimberley pipes (i.e., Wesselton, De Beers, Bultfontein). However, the composition of these magmatic phases is distinct from those in ‘evolved’ intrusions of the Kimberley cluster (e.g., Benfontein, Wesselton water tunnel sills). Although the effects of syn-emplacement flow processes are evident (e.g., alignment of phases parallel to contacts), there is no evidence that the De Beers dyke has undergone significant pre-emplacement crystal fractionation (e.g., olivine, spinel, ilmenite). This study demonstrates the requirement for detailed petrographic and mineral chemical studies to assess whether individual intrusions are in fact ‘evolved’; and that dykes are not necessarily produced by differentiated magmas.

     

The stability of ortho- and clinopyroxenes,olivine, and garnet in kimberlitic magma

It is generally accepted that the composition of ultrabasic nodules and their quantitative proportions do not significantly change during their transportation with kimberlitic magma to the Earth’s surface. We performed an experimental study of the relative stability of olivine, garnet, and pyroxenes in kimberlite melt at high pressure and temperatures (4 GPa, 1300–1500 °C). The study has shown that the loss in weight of minerals and, correspondingly, the rate of their dissolution in kimberlite melt differ considerably. The following sequence of the dissolution rates of minerals has been established: Cpx ≥ Opx > Gar > Ol. Pyroxenes are characterized by the most rapid dissolution, and olivine is the most stable mineral. The assumption is made that clinopyroxenites and websterites disintegrate more rapidly than dunites and lherzolites in kimberlitic magma.     

Magnetite-orthopyroxene symplectites in gabbros of the Urals: A structural track of olivine oxidation

The chemical compositions of magnetite-orthopyroxene symplectites (MOS) and rock-forming minerals—olivine (Ol), clinopyroxene (Cpx), and magnetite(Mt)—have been studied in 20 samples of olivine-bearing rocks in the Urals, including troctolite, olivine gabbro, and gabbronorite. MOS are orthopyroxene (Opx) monocrystals up to 500 μm in size containing myrmekite-like magnetite intergrowths up to 20–30 μm in width. According to the microprobe examination, the dark-colored minerals are characterized by a high Fe mole fraction F = Fe/(Fe + Mg) = 0.20–0.50, whereas F = 0.33–0.65 is typical of the bulk rock compositions. The plagioclase varies in composition from An₉₀ to An₅₀. No significant compositional difference has been established between the MOS and rock-forming minerals. The F _opx and F _Ol are closely correlated (linear trend, r = 0.97); F _Ol/F _Opx is ～1.2. Similarly, a positive correlation between F _Opx and F _Cpx is noted (linear trend, r = 0.90); F _Opx/F _Cpx is ～1.2. The crystallization temperature of the Ol-Opx-Cpx assemblage is roughly estimated at 700–800°C. A high positive correlation (r = 0.95) is established between the TiO₂ contents in the magnetites from the MOS (Mt1) and host rock (Mt2). The Mt1/Mt2 ratio reaches ～0.8, implying that Mt1 contains somewhat less TiO₂ than Mt2. Hence, the rock-forming and MOS minerals make up an equilibrium assemblage. As follows from the structural pattern, symplectites were formed as products of the reaction between olivine and oxygen in the solid state with the entire volume of the rock involved in the oxidation; i.e., the distance of the diffusion was significant. Free oxygen appeared as a product of the dissociation of the water penetrating into the hot gabbro and ultramafic rocks at the initial stage of the tectonic extension and high-temperature hydration. According to the redox state of dunite coexisting with gabbro, the oxygen fugacity is estimated at +2.7 log units of fO₂ relative to the QFM buffer. The structure and products of the olivine oxidation were eventually obliterated in the course of the hydration.     

Structural stages and evolution of the Urals

Five main structural and historical stages are established in the territory of the Urals: 1) Archean-Paleoproterozoic, a time of formation of the Volgo-Uralia subcontinent and its amalgamation with the other blocks of the craton of Baltica; 2) Riphean-Vendian (Meso- and Neoproterozoic), а stage that was finished with formation of Timanides; 3) Paleozoic-Early Mesozoic stage, corresponding to the development of the Uralides; 4) Mid-Jurassic-to Miocene platform stage; 5) Pliocene-Quaternary neo-orogenic stage. In this paper stratigraphic data are discussed, schemes of the structural zonation are presented, and the problems of the structural geology and geodynamics of sedimentary and magmatic complexes are discussed in a chronological order. Ideologically, the paper is based on plate and plume tectonics, in their modern versions.     

Melt inclusions in olivine phenocrysts in unaltered kimberlites from the Udachnaya-East pipe,Yakutia: Some aspects of kimberlite magma evolution during late crystallization stages

The results of a complex study of melt inclusions in olivine phenocrysts contained in unaltered kimberlites from the Udachnaya-East pipe indicate that the inclusions were captured late during the magmatic stage, perhaps, under a pressure of <1 kbar and a temperature of ≤800°C. The inclusions consist of fine crystalline aggregates (carbonates + sulfates + chlorides) + gas ± crystalline phases. Minerals identified among the transparent daughter phases of the inclusions are silicates (tetraferriphlogopite, olivine, humite or clinohumite, diopside, and monticellite), carbonates (calcite, dolomite, siderite, northupite, and Na-Ca carbonates), Na and K chlorides, and alkali sulfates. The ore phases are magnetite, djerfisherite, and monosulfide solid solution. The inclusions are derivatives of the kimberlite melt. The complex silicate-carbonate-salt composition of the secondary melt inclusions in olivine from the kimberlite suggests that the composition of the kimberlite melt near the surface differed from that of the initial melt composition in having higher contents of CaO, FeO, alkalis, and volatiles (CO₂, H₂O, F, Cl, and S) at lower concentrations of SiO₂, MgO, Al₂O₃, Cr₂O₃, and TiO₂. Hence, when crystallizing, the kimberlite melt evolved toward carbonatite compositions. The last derivatives of the kimberlite melt had an alkaline carbonatite composition.     

Olivine: a monitor of magma evolutionary paths in kimberlites and olivine melilitites

Petrographic and chemical criteria indicate that the overwhelming majority of olivines in kimberlites are probably cognate phenocrysts. The implied low volume of xenocryst olivines requires that primitive kimberlite magmas are highly ultrabasic liquids. Two chemically distinctive olivine populations are present in all of the kimberlites studied. The dominant olivine population, which includes large rounded olivines and smaller euhedral crystals, is Mg-rich relative to late-stage rim compositions. It is characterized by a range in 100 Mg/(Mg + Fe) and uniform Ni concentration, reflecting Rayleigh-type crystallization during magma evolution. The most Mg-rich of these olivines are considered to be similiar to those in the mantle source rocks. The second compositional population, generally very subordinate, though markedly more abundant in the megacrystrich Monastery kimberlite, is Fe-rich relative to rim compositions. This group of olivines crystallized from evolved liquids in equilibrium with iron-rich megacrysts, both entrained by the kimberlite magma during ascent. Differences between the chemical fields of Fe-rich olivines in Group I and Group II kimberlites point to relatively deeper derivation of the latter suite. Olivine chemistry can be used to characterize kimberlite magma sub-types, and may prove to be a useful tool for evaluating the diamond potential of kimberlites.     

Typomorphism and parasteresis of Yushkinite, the Pai-Khoi anticlinorium

Yushkinite found in quartz-calcite hydrothermal veins in the Pai-Khoi Anticlunorium (the middle reaches of the Silova-Yakha River) is associated with fluorite, sphalerite, and sulvanite and occurs as fine-lamellar aggregates. The mineral is pinkish purple, with perfect cleavage parallel to (0001). The Moh hardness is lower than 1. In reflected light, yushkinite is anisotropic, with strong bireflectance. It is uniaxial and optically negative. Yushkinite was discovered and approved by the Commission on New Minerals and Mineral Names, International Mineralogical Association, in 1983. Twenty years later, reexamination of yushkinite and associated minerals gave rise to the discovery of a new carbonate phase and specification of the physical properties and chemical composition of yushkinite.     

Porphyry Deposits of the South Urals: Rhenium Distribution

Please refer to the attachment(s) for more details     

Zirconology of miaskites from the Ilmeny Mountains,South Urals

It is shown that the replacement and long evolution of miaskitic zircons led to the formation of two main age groups: 420–380 Ma (I) and 260–240 Ma (II). The age of miaskites is estimated at 440–445 Ma. Zircons I bear traces of fragmentation, dissolution, and replacement; they have “flat” REE patterns typical of metasomatic (hydrothermal) types, which is caused by allochthonous nature of the studied miaskites. Zircons II with differentiated REE patterns are similar to magmatic varieties, but have metamorphic origin. Mineralogical–geochemical and age characteristics of zircons in combination with structural–compositional features of miaskites define their metasomatic nature. The origin of the early zircon generations was related to the Ordovician rifting, while late generations were formed during shear deformations at the final stage of the evolution of the Uralian orogen.     

Calculated diffusion coefficients and the growth rate of olivine in a basalt magma

Concentration gradients in glass adjacent to skeletal olivines in a DSDP basalt have been examined by electron probe. The glass is depleted in Mg, Fe, and Cr and enriched in Si, Al, Na, and Ca relative to that far from olivine. Ionic diffusion coefficients for the glass compositions are calculated from temperature, ionic radius and melt viscosity, using the Stokes-Einstein relation. At 1170°C, the diffusion coefficient of Mg²⁺ ions in the basalt is 4·5.10^?9 cm²/s. Comparison with measured diffusion coefficients in a mugearite suggests this value may be 16 times too small. The concentration gradient data and the diffusion coefficients are used to calculate instantaneous olivine growth rates of 2–6.10^?7 cm/s. This is too slow for olivine to have grown in situ during quenching. Growth necessarily preceded emplacement such that the composition of the crystals plus the enclosing glass need not be that of a melt. The computed olivine growth rates are compatible with the rate of crystallization deduced for the Skaegaard intrusion.     

Origin of carbonatite magma during the evolution of ultrapotassic basite magma

Clinopyroxene phenocrysts in fergusite from a diatreme in the Dunkel’dyk potassic alkaline complex in the southeastern Pamirs, Tajikistan, and from carbonate veinlets cutting across this rock contain syngenetic carbonate, silicate, and complex melt inclusions. The homogenization of the silicate and carbonate material of the inclusions with the complete dissolution of daughter crystalline phases and fluid in each of them occur simultaneously at 1150?1180°C. The pressures estimated using fluid inclusions and mineral geobarometers were 0.5–0.7 GPa. The behavior of the inclusions during their heating and their geochemistry are in good agreement with the origin of carbonate melts via liquid immiscibility. Carbonatite magma was segregated at the preservation of volatile components (H₂O, CO₂, F, Cl, and S) in the melt, and this resulted in the crystallization of H₂O-rich minerals and carbonates and testifies that the magma was not intensely degassed during its ascent to the surface. The silicate melts are rich in alkalis (up to 4 wt % Na₂O and 12 wt % K₂O), H₂O, F, Cl, and REE (up to 1000 ppm), LREE, Ba, Th, U, Li, B, and Be. The diagrams of the concentrations of incompatible elements of these rocks typically show deep Nb, Ta, and Ti minima, a fact making them similar to the unusual type of ultrapotassic magmas: lamproites of the Mediterranean type. These magmas are thought to be generated in relation to subduction processes, first of all, the fluid transport of various components from a down-going continental crustal slab into overlying levels of the mantle wedge, from which ultrapotassic magmas are presumably derived.