Zirconology of dunite of the Nizhnii Tagil massif (middle Urals)

Doklady Earth Sciences -     

Physicochemical conditions of crystallization of dunites of the Nizhnii Tagil Pt-bearing massif (Middle Urals)

Studies of primary multiphase silicate inclusions in accessory Cr-spinels from the fine-grained dunites of the Nizhnii Tagil Pt-bearing massif reveal their similarity to melt inclusions trapped by chromite during its growth. The analyzed Cr-spinels with multiphase silicate inclusions differ in composition from ore chromites of the same massif and from chromites (with melt inclusions) from ultramafic oceanic complexes but are similar to Cr-spinels in dunites from Pt-bearing alkaline ultramafic massifs (Konder and Inagli). According to petro- and geochemical data on heated multiphase silicate inclusions, the studied Cr-spinels crystallized with the participation of subalkalic picrobasaltic melts similar to the magmas of the Konder Pt-bearing massif and having almost the same chemical composition as tylaites. The differences between the compositions of olivines formed within the multiphase silicate inclusions and of the rock-forming minerals show that the studied Cr-spinels formed from an intercumulus liquid melt in the olivine crystal interstices during the cumulate crystallization of most of the Nizhnii Tagil massif dunites in the intrusive chamber. Numerical modeling based on the compositions of heated multiphase silicate inclusions in accessory Cr-spinels demonstrates that olivines and Cr-spinels from the studied dunites crystallized at 1430 to 1310 °C and then olivine formation continued to 1280 °C during the evolution of melts.     

Zirconology of ultrabasic rocks of the Karabash massif (Southern Urals)

Dating of zircon (SHRIMP) from dunite and harzburgite of the Karabash massif was carried out for the first time. Relics of ancient crystals (1940 ± 30 Ma in harzburgite, 1860 ± 16 Ma in dunite) provide evidence for the Paleoproterozoic age of the protolith. The morphological peculiarities of zircon crystals allow us to assume differentiation of the magmatic source 1720 m. y. ago. The major variety of zircons indicates stages of metamorphic evolution in the Neoproterozoic (530–560 Ma) and Early–Late Ordovician (440–480 Ma).     

Archean U-Pb isotope age of zircon from dunite of the Nizhny Tagil massif (the Uralian Platinum Belt)

Doklady Earth Sciences - Material identity of dunite from zoned-type massifs of the Aldan shield (Kondyor, Chad and others) and dunite “cores” from the Uralian Platinum Belt (i.e., UPB)...     

Structure and formation mechanism of the Nizhny Tagil dunite-clinopyroxenite massif,Central Urals

The results of the structural study of the Nizhny Tagil platiniferous massif in the Central Urals are presented. This is a classic massif of the Ural-Alaskan-type zonal dunite-clinopyroxenite complexes. The massif is characterized by a nearly concentric vertical planar internal structure conformable to petrographic zoning (layering). The primary ultramafic rocks are distinguished by adcumulative protogranular structure with relict euhedral olivine protocrysts and distinct linear orientation of minerals, which was formed as a result of magmatic flow. The deformational linear and planar structures conformable to the early structural elements were formed in the process of subsequent coaxal high-temperature ductile flow. At this stage, dynamometamorphic zoning is formed, expressed in the change of the protogranular microstructure typical of the inner portion of the massif by porphyroclastic and mosaic microstructures in its marginal part. The country rocks underwent conjugate high-temperature metamorphism with the formation of hornfels and kytlymite. The structure of the massif is considered to be a result of dynamic differentiation in the course of magmatic flow followed by high-temperature coaxal flow during intrusive and diapiric ascent of rocks to the crustal level.     

Zirconology of serpentinites from Nyashevo massif (Southern Urals)

Zircons in serpentinites from Nyashevo massif of the Ilmenogorskii complex were dated for the first time by means of the SHRIMP technique. The maximum date of 1892 ± 23 Ma for the zircons accounts for the minimum age of their mantle substrate probably constituting the restite residue. The date is comparable to those for metamorphic rocks of the Selyankino group, as well as of fenite–sand amphibolites of the Ilmenogorskii complex. The Upper Ordovician age limit of 443 ± 12 Ma is adequate for formation of the massif and conforms to the age of the Buldym massif and miaskites. The Early Permian dates of zircons (275.8 ± 2.1 Ma) represent late shear processes in the Ilmenogorskii complex.     

Conditions of crystallization of olivine shonkinites in the Inagli massif (Central Aldan)

The olivine shonkinites localized among dunites and alkali gabbroids in the northern part of the alkaline ultrabasic Inagli massif (northwestern part of Central Aldan) have been studied. The obtained data on the chemical and trace-element compositions of the rocks and minerals and the results of melt inclusion study showed that the olivine shonkinites crystallized from alkaline basanite melt enriched in Cl, S, CO2, and trace elements. Clinopyroxene crystallized at 1180-1200 °C from a homogeneous silicate-salt melt, which was probably separated into immiscible silicate and carbonate-salt fractions with temperature decreasing. The composition of the silicate fraction evolved from alkaline basanite to alkaline trachyte. The carbonate-salt fraction had an alkaline carbonate composition and was enriched in S and Cl. The same trend of evolution of clinopyroxene-hosted melts and the igneous rocks of the Inagli massif suggests that the alkali gabbroids, melanocratic alkali syenites, and pulaskites formed from the same magma, which had a near-alkaline basanite composition during its crystallization differentiation. The geochemical studies showed that the olivine shonkinites and glasses of homogenized melt inclusions in clinopyroxene grains have similar contents of trace elements, one or two orders of magnitude higher than those in the primitive mantle. The high contents of LILE (K, Rb, and Sr) and LREE in the olivine shoshonites and homogenized inclusions suggest the enriched mantle source, and the negative anomalies of HFSE and Ti are a specific feature of igneous rocks formed with the participation of crustal material. The slight depletion in HREE relative to LREE and the high (La/Yb)_n ratios in the rocks and inclusion glasses (10.0-11.4 and 4.7-6.2, respectively) suggest the presence of garnet in the mantle source.     

Autometasomatic processes in granitoids of Neroysk-Patok massif (Circumpolar Urals)

The granitoids of the Neroysk-Patok massif exhibit autometasomatic zones in which potassium enrichment is followed by that of sodium, and later, by acid leaching. Such processes affected the country rocks, as well, and are attributed to the action of postmagmatic solutions. The concentration of rare nietal minerals is closely related to the activity of such solutions. —R.C. Epis     

Nickel-sulfide ores in ultrabasites of Khabarnyy massif (South Urals)

The mineralized area (fig. 1) lies inside a large dunitic body. The sulfides are small phenocrysts of pentlandite (with chalcopyrite and pyrrhotite); chromite is generally present and metasomatic magnetite is locally abundant, as replacer of the sulfides. A genetic connection of the sulfides with the dunites is indicated. There is no evidence of any epigenetic segregation of the ore minerals and hence no reason to expect presence of economic ores in this particular part of the massif. — V.P. Sokoloff     

The nature of grain boundaries in dunite from zoned complexes of the Urals

Doklady Earth Sciences -     

Nature of zircon in lherzolites (Uzyansky Kraka massif,Southern Urals)

Doklady Earth Sciences -