Vol 27, No 1 (2019)

Reduced CO2 fluid as an agent of ore-forming processes: a case study of dolomite-replacement skarns at the Yoko-Dovyren massif)
Simakin A.G., Kislov E.V., Salova T.P., Shaposhnikova O.Y., Nekrasov A.N.

The paper presents newly obtained geochemical data on outer-contact rocks and carbonatereplacement skarns of the Yoko-Dovyren layered ultramafic-mafic intrusion in the northern Baikal area. The rocks initially contained CO2-rich fluid with a high oxygen fugacity (up to NNO + 3–4), which was generated by the partial decomposition of dolomite and by reactions between SiO2 and carbonates. The skarn blue diopside is enriched in Pt (up to 0.2 ppm) and V (300 ppm), and the wollastonite zone of the skarns contains elevated Re concentrations (up to 0.4 ppm). The REE pattern of the contact-zone quartzite is identical to the REE patterns of phlogopite-bearing lherzolites from the lower contact part of the Yoko-Dovyren massif. These geochemical features of the rocks of the intrusion may be explained by the transfer and redeposition of material by reduced H2O-CO2 fluid. According to thermodynamic calculations, a reaction between H2O-CO2 fluid and high-Mg olivine at a subsolidus temperature of T = 950оC and pressure P = 2 kbar should result in a decrease in the oxygen fugacity to QFM – 2 and, hence, generate much CO. According to the calculations, a low oxygen fugacity (close to QFM + 0.7) can also be maintained by pyrrhotite oxidation with H2O and CO2 fluid components under cumulus P-T parameters. As a result of these reactions, the fluid should enrich in Pt extracted from magmatic sulfides, and this Pt can be redeposited in rocks, including those composing the skarn zones.

Петрология. 2019;27(1):4-18
Ga Mafic dikes and sills of Northern Fennoscandia: Petrology and Crustal evolution
Erofeeva K.G., Stepanova A.V., Samsonov A.V., Larionova Y.O., Egorova S.V., Arzamastsev A.A., Kovalchuk E.V.

New petrographic, geochemical, and Sm-Nd isotopic data provide constraints on the petrogenesis of ca. 2400 Ma gabbronorite, picrodolerite and dolerite mafic sills and dikes in the Liinakhamari and Sorvaranger areas, Kola–Norwegian terrane, Northern Fennoscandia. The sills are differentiated. Their chilled margins are composed of porphyritic picrodolerite with olivine (Fo92-81) and clinopyroxene (Mg# = 0.80–0.85) phenocrysts enclosed by quenched groundmass with intergrows of fan-shaped branched plagioclase grains and clinopyroxenes. The bottom of the sills are usually composed of cumulative olivine gabbronorite upsection followed by olivine gabbro, gabbro, and quartz-bearing dolerites. Picrodolerite and dolerite dikes are close to chilled margins and evolved quartz-bearing dolerites from the upper parts of picrodolerite sills, correspondingly, in terms of mineral and chemical composition. The distribution of trace elements in sills sections is caused by fractionation crystallization of picrodolerite magma with a leading role of gravity settling of olivine phenocrysts. Variations of neodymium isotopic composition in the cumulative gabbronorites in the lower (εNd from –0.25 to +0.82) and dolerites in the upper (εNd from –0.85 to –2.4) parts of the sills are probably related with an addition of crustal contaminant into the magma after phenocrysts crystallization. Doleritic dikes have more radiogenic neodymium isotopic composition (εNd = –0.10) in comparison with dolerites of sills that suggests lower degrees of crustal contamination in dikes. Evaluation of conditions of phenocrysts crystallization using mineral thermobarometers and modelling of crystallization using alphaMELTS indicate that the main stage of the evolution of ca. 2400 Ma mafic melts was probably related with high degrees of fractional crystallization and crustal contamination in the upper crust at P =1–4 kbar. Ca. 2400 Ma mafic intrusions in the Kola–Norwegian terrane are similar with coeval mafic dikes in Troms area in the Northern Fennoscandia, basaltic komatiites of Vetreny Belt and doleritic dikes of Kostomuksha area in the Karelian Craton in terms of petrographic, geochemical and Sm-Nd isotopic characteristics. It suggests that studied mafic sills and dikes in the Kola–Norwegian terrane could be considered as a component of ca. 2400 Ma large igneous province in the Fennoscandia.

Петрология. 2019;27(1):19-46
Tectonic position of the Early Neoproterozoic–Early Paleozoic metamorphic belts within the Tuva–Mongolian terrane of the Central Asian Orogenic Belt
Kozakov I.K., Salnikova E.B., Anisimova I.V., Azimov P.Y., Kovach V.P., Plotkina Y.V., Stifeeva M.V., Fedoseenko A.M.

The Tuva–Mongolian terrane (TMT) of the Central Asian Orogenic Belt is a composite structure with a Vendian–Cambrian terrigenous-carbonate cover. The formation of the northern part of TMT is marked by the granitoids of the Sumsunur Complex with an age of 785 ± 11 Ma. The Sangilen and Khan-Khukhay blocks of its southern part also form a composite structure, which originated during Early Paleozoic(500–490 Ma) low-moderate pressure regional metamorphism reaching amphibolites-granulite facies. The earlier high-pressure metamorphism was established in the Moren Complex of both the blocks. In the Sangilen block, this metamorphism reached conditions of kyanite-garnet-biotite-orthoclase subfacies of amphibolites facies (temperature ~750oC, pressure 9–10 kbar). The upper age limit of this metamorphism is determined by granites with an age of 536 ± 6 Ma, which cut across migmatized biotite gneisses of the Moren Complex. The latter are intruded by the granitoids of the Ortoadir pluton, which were previously dated at 521 ± ± 12 Ma (U-Pb method, TIMS). Its emplacement predated the Early Paleozoic low-moderate pressure metamorphism, the timing of which is constrained by syn- and postmetamorphic granitoids with ages of 496 ± 4 and 489 ± 3 Ma. The age of 513 ± 4 Ma established for the granitoids of the Ortoadir Complex in the Khan-Khukhay Block more accurately constrains the lower age boundary of collision processes. This determined the amalgamation of the fragments of the high-pressure metamorphic belt with basement and carbonate-shelf cover units of the Tuva–Mongolian terrane, as well as the upper age boundary of early metamorphism. The timing of the main mappable structure of the Khan–Khukhay Block and low-moderate pressure regional metamorphism is marked by the synmetamorphic granitoids with an age of 505 ± ± 2 Ma. In general, the metamorphic rocks of the Sangilen, Khan–Khukhay, and Kaakhem blocks can be considered as fragments of the Late Ediacaran high-pressure metamorphic belt, which were amalgamated to the western margin of TMT within 515–505 Ma, after emplacement of the granitoids of the Ortoadir Complex, and were reworked by regional Early Paleozoic low-moderate pressure metamorphism.

Петрология. 2019;27(1):47-64
Migration of radiogenic helium in the crystal structure of sulfides and prospects of their isotopic dating
Yakubovich O.V., Gedz A.M., Vikentyev I.V., Kotov A.V., Gorokhovskii B.M.

The migration of helium from the crystal lattices of sulfides (pyrite, pyrrhotite, chalcopyrite, bornite, and sphalerite) and sulfosalts (tennantite and tetrahedrite) was studied. It was shown that helium occurs in submicrometer inclusions of uranium- and thorium-bearing minerals. The curves of helium thermal desorption from the sulfide and sulfosalts were obtained by the step-heating method and analyzed on the basis of the single-jump migration model. The interpretation of these data led to the conclusion on the possibility of the U-Th-He dating of pyrite. It was shown that the migration parameters of helium in the other sulfides and sulfosalts are not suitable for their potential use as U-Th-He mineral geochronometer. Based on a comparison of data on helium migration in various minerals, it was suggested that high helium retentivity in some sulfides and arsenides (pyrite and sperrylite) is related to the type of their crystal lattice, packing density, and specific electric resistivity.

Петрология. 2019;27(1):65-86
Genesis of manganese ore occurrences of the Olkhon Terrane
Sklyarov E.V., Lavrenchuk A.V., Starikova A.E., Fedorovskii V.S., Khromova E.A.


Geological and mineralogical data are reported on the manganese occurrences of the Olkhon terrane (Western Baikal region), which are localized in metadolerites of the Ustkrestovsky Complex, high-temperature mafic hornfels, granites, calcitic marbles and calciphyres, and occasionally are developed as separate veins in gneiss granites or small lenses in quartzites. Most of them are made up of high-temperature mineral assemblages (Opx + Cpx + Pl + Ilm ± Grt± Bt ± Amp), the main manganese carriers in which are ferrorhodonite
(33–36 wt % MnO), orthopyroxene (6–12 wt % MnO), and ilmenite (3–16 wt % MnO). Obtained data are in conflict with traditional concepts that these rocks are gondites (manganese-rich metamorphosed sediments) or that manganese flux in carbonate sediments was related to the volcanic activity that occurred simultaneously with sedimentation at about 500 Ma. The diversity of manganese occurrences was produced by metasomatic processes that occurred almost simultaneously with regional metamorphism and emplacement of subalkaline mafic bodies during collisional tectonogenesis (about 470 Ma).

Петрология. 2019;27(1):87-104
Comparison of the compositions and microstructures of Terrestrial and Lunar impact glasses: samples from The Zhamanshin Crater and Luna 16, 20, and 24 missionse
Gornostaeva T.A., Mokhov A.V., Kartashov P.M., Bogatikov O.A.

The paper presents pioneering data on the comparative study of impact glasses from the Zhamanshin crater and lunar regolith (delivered by the Luna 16, 20, and 24 probes). The data were acquired using analytical techniques of ultrahigh spatial resolution. Many of the melt and condensate impact glasses, both terrestrial and lunar, are similar in inner structure and composition, which were controlled primarily by the physics of the impacts and similar compositions of the targets.

Петрология. 2019;27(1):105-119

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