Age of Granulites of the Cheremshanskaya Unit (Sharyzhalgai Uplift,Siberian Craton,Russia): New Data

Doklady Earth Sciences - The present paper presents new U–Pb and Sm–Nd age data obtained for granulites of the Cheremshanskaya Unit enclosing metamorphosed volcano-sedimentary sulfide...     

Peculiarities of the oxygen isotope ratio in precious opals

This paper presents the results of the δ¹⁸O study of the precious opals from Primor’e (Raduzhnoe deposit), Australia, and Ethiopia and the modern opals from the hydrotherms of the Mendeleev Volcano (Kunashir Island, Kuril Islands). It is established that the oxygen isotope ratio in opals may serve as a criterion for the estimation of their formation temperature. The low-temperature sedimentary opals are relatively enriched in the heavy oxygen isotope independently of the sedimentary or volcanic host rocks. Examples are the Australian and Slovakian opals of the A-type. The hydrothermal opals are enriched in the light oxygen isotope, which depends on the precipitation temperature. The higher the temperature, the lighter the oxygen isotope ratio of the precipitating opal is and the closer it is to that of the hydrothermal fluid.     

Geochemistry of stable oxygen and hydrogen isotopes in minerals and corundum-bearing rocks in northern Karelia as an indicator of their unusual genesis

The paper presents newly obtained data on the oxygen and hydrogen isotopic composition of minerals in corundum occurrences and their host rocks in northern Karelia. Minerals in the Khitoostrov and Varaka corundum-bearing zones have extremely low δ¹⁸O (lower than ?26‰) and δ D (lower than ?215‰), which suggest that the mineral-forming process involved glacial waters and that the minerals preserve the isotopic ratios of their protolith. Aluminous corundum plagioclasites were produced by high-pressure Svecofennian (1.9-1.8 Ga) metamorphism of Paleoproterozoic rocks that had been metasomatized with the involvement of meteoric waters during the Guronian glaciation epoch.     

Hydrothermal precious opals of the Raduzhnoe deposit,north Primorye: The nature of the opalescence

This paper reports the results of the study of hydrothermal precious opals. It was established that, in terms of their nanostructure and formation conditions, the hydrothermal opals differ from exogenic precious opals. They are made up of small globules and do not show any structuring of their nanoparticles on the basis of closest packing, which is typical of exogenic precious opals. During their formation, these opals were subjected to pneumatolytic annealing—they experienced the impact of a high-temperature vapor under elevated pressure. The influence of the thermal effect led to the formation of two-dimensional photonic band gaps in the chaotic opal matrix. These bands are composed of sheets whose cells were formed owing to the thermal effects according to the principle of “Benard cells.” Precisely these structured blocks and thin films are responsible for the spectral dispersion of light and iridescence.     

New isotopic data on Late Cenozoic age and mantle origin of gem zircon and corundum from placers of Primorye,Russia

The paper shows that sapphire (corundum) and hyacinth (zircon) from the Primorye placers have genetic relationships with the Late Cenozoic within-plate basalts of mantle origin. The alternative hypothesis of their derivation from the Early-Middle Cretaceous S-type granites does not fit the evidence presented here. This conclusion is based on the age determinations (less than 15 m.y., zircon SHRIMP and Fission-Track analyses) and oxygen isotope compositions (δ¹⁸O SMOW = 4–6.5 vol %) of clastic zircon and corundum from two sapphire districts of Primorye: Marevka, occurring in the Marevka and Kedrovka river basins in the north, and Shkotovo located in the area of upper Arsenievka and Steklyanukha Rivers in the south.     

Garnet inclusions in alkali basalts of Primorye: Deep-seated xenoliths or cognate megacrysts?

Detailed study of garnet and garnet-pyroxene inclusions from alkali basalts of Primorye allowed us to distinguish two heterogeneous mineral assemblages: early (primary) and late (secondary) ones. The former reflects the initial stage of rock formation and consists of garnet, clinopyroxene, orthopyroxene, plagioclase, and ilmenite. The latter, orthopyroxene-plagioclase-spinel assemblage replaces garnet. The transformation of garnet typically accompanies the change in the garnet assemblage to a higher temperature pyroxene-plagioclase-spinel assemblage owing to its entrainment in high temperature magma. The study of the chemical composition and textural relationships of minerals, together with the examination of similar mineral assemblages from alkali basalts around the world and mafic-ultramafic intrusions, led us to the conclusion that the garnet inclusions of Primorye are typical xenoliths and xenocrysts representing fragments of tholeiite intrusions of an ophiolite complex that were transformed in the host basaltic magma.     

Transformation of garnet megacrysts captured by alkali mafic magma

The results of research of symplectites from the Shavaryn-Tsaram (Hangaj plateau, Mongolia) and Bartoj (Dzhida basaltic field, Russia) alkali basaltic rocks are presented. The symplectite compositions and structures were studied, and the physical and chemical parameters at which primary megacrysts were transformed into secondary mineral assemblages were defined. It is established that both garnet megacrysts and garnet-clinopyroxene aggregates were formed at pressures of 10–13 kbar and temperatures over 1300°C. The transformation of garnet into minerals of the secondary assemblage is considered as solid state water assisted resorption of garnet at a depth corresponding to pressures of 4–8 kbar and temperatures ranging from 1000 to 1300°C. The kelyphitic rims on the garnet megacrysts resulted from melting of the megacrysts at the contact with the hosting alkali basaltic rock.     

U–Pb age and geochemistry of zircon inclusions in sapphire: Alkali basaltic source of gems in placers of Primorye

This paper reports the first results of U–Pb SHRIMP dating of zircon inclusions in sapphire from the Nezametninskii placers in the north of Primorye. The obtained age of paragenesis of these minerals (12.2 ± 0.2 Ma) is almost consistent with the age of erupted alkali basalts in the region, thus evidencing their genetic relation. According to the geochemistry data on trace elements, the dated zircon was crystallized from an alkali basaltic magma or its later trachyte/phonolite derivatives.