––Geological, petrological, isotope-geochemical (40Ar/39Ar analysis, petrochemical data, and geochemistry of trace elements and REE), and mineralogical researches testify that the formation of the early Paleozoic basalt and picrite complexes of the West Siberian Plate basement was related to the development of the Cambrian subduction zone of the Paleo-Asian Ocean. Using the compositions of clinopyroxenes and amphiboles (and also programs of computational modeling), we have established the Р–Т conditions of formation of early Paleozoic picrite complexes. Crystallization of clinopyroxenes began at significant depths (25–20 km) and at high temperatures (1300–1275 °C). Olivine might have formed at elevated pressures (8–7 kbar) and temperatures of 1540–1490 °C. Amphiboles formed, most likely, at 6.1–4.5 kbar and much lower temperatures, 1105–1060 °C. Petrochemical analysis and data on trace elements and REE in the rocks of the studied early Paleozoic complexes in West Siberia testify to their intricate formation involving magmatic systems with basalt (island arc and back-arc basins), picrite, and shoshonite (and also WPB type) melt characteristics. Taking into account the similar geochemical characteristics of the early Paleozoic basaltoids and picrites of the West Siberian Plate basement and the Kamchatka volcanics, we suggest that a considerable part of the studied ancient complexes formed by the model implying (as in the case of the Sredinnyi Ridge in Kamchatka) the action of enriched magmatic systems during the development of a destructive window (“slab-window”) under rupture of subducted plate on the background of common island arc magmatism.

Геолого-петрологические, изотопно-геохимические (40Ar/39Ar анализ, данные по петрохимии и геохимии редких, редкоземельных элементов) и минералогические исследования свидетельствуют о том, что формирование раннепалеозойских базальтовых и пикритовых комплексов фундамента Западно-Сибирской плиты связано с развитием кембрийской (520 495 млн лет) субдукционной зоны Палеоазиатского океана. Использование составов клинопироксенов и амфиболов (а также программ расчетного моделирования) позволило установить РТ параметры формирования раннепалеозойских пикритовых комплексов. Кристаллизация клинопироксенов началась на значительных (25-20 км) глубинах и при высоких (1300-1275°C) температурах. В условиях повышенных (8-7 кбар) давлений при температурах 1540-1490°C могло идти формирование оливина. На уровне 6.1-4.5 кбар и при значительно более низких температурах (1105-1060°C) наиболее вероятно образовывались амфиболы. Петрохимические исследования, а также данные по редким и редкоземельным элементам в породах рассмотренных раннепалеозойских комплексов Западной Сибири свидетельствуют о сложности их формирования при участии магматических систем с признаками базальтовых (островных дуг и задуговых бассейнов), пикритовых, шошонитовых (а также типа WPB) расплавов. В целом, учитывая сходство геохимических характеристик раннепалеозойских базальтоидов и пикритов фундамента Западной Сибири с особенностями вулканитов Камчатки, предполагается, что формирование значительной части рассмотренных древних комплексов происходило по модели, связанной (как и в случае Срединного хребта на Камчатке [Перепелов, 2014]) с действием обогащенных магматических систем в ходе развития деструктивного окна («slab-window») при разрыве субдуцированной плиты на фоне развития обычного островодужного магматизма.

The carried out researches with the help of 40Ar/39Ar analysis have shown a coincidence of formation (about 490 million years) of Paleozoic picrite and basalt complexes of the basement of the Western Siberian Plate. The data on petrochemistry, geochemistry of rare and rare-earth elements, and also on compositions of clinopyroxenes, testifies to formation of picrites of borehole № 11 (Chkalovsky area) as a result of action of intra plate magmatic systems of oceanic islands - OIB. Calculations on the basis of clinopyroxene compositions speak about considerable parameters of crystallization of picrite porphyrite minerals: 1215-1275 °C, 4.5-8 kbar. As a whole, it is established that considered picrite basalt complexes were formed with the assistance of enriched plume magmatic systems in intra plate conditions of ancient ocean near to its active margin.

40Ar/39Ar analysis showed a simultaneous (at about 490 Ma) formation of the Paleozoic picrite and basalt complexes of the West Siberian Plate basement. The petrochemistry, trace and REE geochemistry, and composition of clinopyroxene indicate the formation of the picrite of well no. 11 (Chkalov area) as a result of intraplate magmatism of the OIB type. Calculations based on the compositions of clinopyroxene allowed crystallization of minerals of porphyric picrite at 1215–1275°C and 4.5–8 kbar. In general, it has been found that the picrite basalt complexes considered were formed from enriched igneous plume systems under intraplate conditions near the active margin of the ancient ocean.

Актуальность. Изучение расплавных включений в минералах является одним из наиболее достоверных путей получения прямой информации о характере магматических процессов, позволяет построить модель образования интрузий и проследить эволюцию физико-химических условий кристаллизации магматических комплексов. Цель: получить достоверные сведения о составе расплава, что дает возможность проследить эволюцию физико-химических условий образования пород платиноносного дунит-верлитового массива Медек. Объекты: расплавные включения в хромшпинелидах дунитов и верлитов Медекского базит-ультрабазитового массивов. Методы. Поиск расплавных включений проводился в монофракциях акцессорных хромшпинелидов из дунитов и верлитов массива. Полированные препараты просматривались под микроскопом, и отбирались зерна, содержащие многофазные микровключения определённой формы, размера (10–30 мкм) и положения внутри зерна. Далее проводились высокотемпературные эксперименты. Выбор температурного режима (1280–1300 °С) определялся задачей перевести включения в расплавленное состояние, затем его закалить, и полученное стекло проанализировать на рентгеновском микроанализаторе и сканирующем микроскопе. Результаты. В результате анализа стекол гомогенизированных расплавных включений установлено, что состав родоначальной магмы, из которой кристаллизовались дуниты массива, отвечал пикритоидному расплаву (MgO 13,36 мас. %), который эволюционировал до низкокалиевого пикробазальтового расплава, наиболее близкого по петрохимическим характеристикам к данным по расплавным включениям в хромитах из океанических комплексов. Геотермобарометрические расчеты и моделирование с помощью программ COMAGMAT и PETROLOG свидетельствуют о становлении пород массива при давлении 3–5 кбар и температурном диапазоне 1400–1230 °С. Расчетные данные и результаты высокотемпературных экспериментов показали хорошую сходимость.

The relevance of the research. The study of melt inclusions in minerals is one of the most reliable ways of obtaining direct information on the nature of magmatic processes, allows building a model of formation of intrusions and tracing the evolution of physico-chemical conditions of crystallization of magmatic complexes. The main aim of the research is to prove the magmatic origin of rocks of this massif, which makes it possible to consider the distribution of noble metals in melts, based on the available experimental data. Objects: melt inclusions in Cr-spinel from dunites and wehrlites Medek basite-ultrabasite massifs. Methods. The search for melt inclusions was carried out in accessory Cr-spinel sample isolated from dunites and wehrlite, to determine the nature of the high temperature experiments. For this purpose, the samples were taken (about 50-100 grains of Cr-spinel fraction 0,2-0,5 mm) and placed in a graphite microcontainer with internal dimensions in the first millimeters. Micro containers were tightly closed with graphite covers. Thus, the Cr-spinel in these containers were at temperatures above 1000 degrees C under reducing conditions as a result of the reaction of air oxygen with graphite. The experiments were carried out at high temperatures on the basis of existing methods of investigation of melt inclusions. For maximum transformation of the melt in inclusions into homogeneous glass, quenching into water was carried out. The choice of the temperature regime (1280-1300 degrees C) was determined primarily by the fact that the main task of high-temperature experiments was to melt the contents of inclusions and to obtain glass during hardening, which was then analyzed on a microprobe and a scanning microscope. Result. Conducted research of homogenised melt inclusions glass showed that crystallization of Cr-spinel and containing wehrlite of the ultrabasic Medek massif (East Sayan) were involved in low potassium picrobasalt of melts of normal alkalinity, of the most similar petrochemical characteristics to the data on melt inclusions in Cr-spinel from oceanic complexes. The calculations were made for various mineralogical barometers and different models of crystallization on the basis of compositions of glasses of heated melt inclusions in Cr-spinel in the COMAGMAT program, which showed that the pressure in formation of massive rocks was about 4 kbar (2,8-4,7 kbar); the combined use of mineralogical thermometers and calculations on the basis of glass compositions of heated melt inclusions in Cr-spinel according to PETROLOG and COMAGMAT programs allows us to speak about the formation of dunites from picritoids (MgO 17,85 wt. %) of the melt, since the crystallization of olivine at 1380 degrees C, the formation of wehrlite occurred mainly from picrobasalt (MgO 13,36 wt. %) of the melt with the crystallization of olivine starting from 1280-1275 degrees C, but in much smaller quantities than clinopyroxene, the intensive formation of which was at lower temperatures (starting from 1235-1230 degrees C); a comparative analysis of the results of computational modeling of melt and mineral compositions with data on melt inclusions and real olivines and clinopyroxenes indicates their consistency, as evidenced by the fact of the location of melt inclusions along the calculated trends of melt change, that is, experimental data on inclusions on the one hand confirm the reliability of the calculations, and on the other - this ratio shows that the inclusions are not an accident, but record the natural and theoretically justified evolution of the melt.

Vitreous basalts and plagioclase lapilli from Tolbachik Volcano studied by high-temperature gas chromatography reveal features of a fluid regime uncommon to suprasubduction melts. Prominent depletion in volatiles confirms the anomalous behavior of the Tolbachik fluid systems. Vitreous basalts contain minor amounts of water (0.16-0.27 wt.%) and carbon dioxide (95-440 ppm). New data on volatiles in the Tolbachik plagioclase lapilli show very low contents of CO2 and total gas (exclusive of H2O) and enrichment in reduced fluids (CO and CH4) relative to the basalts. In general, analysis of basalts and plagioclase lapilli from different eruptions trace a progressive increase in reduced fluids (CO and CH4) and decrease in CO2 and total gas from past to present events. The concentrations of CO2 decrease, while those of CO and CH4 in basalts and plagioclase lapilli increase systematically with an increase in FeO/MgO ratios and K2O contents in the lavas and in anorthite component in plagioclase. (C) 2017, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

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 degrees C and then olivine formation continued to 1280 degrees C during the evolution of melts. (C) 2016, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

New data on silicate melt inclusions in accessory Cr-spinel have given an insight into the physicochemical conditions of petrogenesis of dunites of the Guli massif (northern Siberian Platform). Studies of the inclusions demonstrate the contribution of high-Mg (16-22 wt.% MgO) alkali-picritic and picrobasaltic melts to the crystallization of these ultrabasic rocks. During the intrachamber crystallization of the Guli massif dunites, the composition of magmatic systems evolved from picrite meimechite (with olivine forming at 1500-1380 degrees C and Cr-spinel forming at 1420-1360 degrees C) to picrobasalt. Comparison with well-known associations of ultrabasic igneous rocks has shown that melt inclusions in Cr-spinel from the Guli massif dunites are similar to inclusions in olivine phenocrysts from meimechites in the contents of most petrochemical components and in the distribution of indicator trace and rare-earth elements. In general, the new information on melt inclusions testifies that the Guli massif dunites formed with the participation of high-temperature high-Mg melts similar in composition to meimechite magmas. (C) 2016, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

В результате анализа расплавных включений в акцессорных хромшпинелидах установлены физико-химические параметры кристаллизации дунитов. Экспериментальные, аналитические исследования включений прямо свидетельствуют о формировании рассмотренных дунитов из высокомагнезиальных расплавов, близких по своему петрохимическому составу и высокотемпературным характеристикам к меймечитовым магмам. Устанавлена последовательная эволюция состава магматических систем в ходе внутрикамерной кристаллизации дунитов массива от пикрит-меймечитового (с формированием оливинов при 1590–1415°С и образованием хромшпинелидов при 1405–1365°С) к пикробазальтовому и базальтовому.

This paper presents the first geochemical data on Cr-spinels from ultramafic rocks of the Alkhadyr terrane, which were obtained on a representative collection of samples using modern research methods. The compositional data on melt inclusions allowed the identification of three generations of Cr-spinels on the basis of their morphology, composition, and relationships with the rock-forming minerals. Different types of geochemical zoning were recognized in heterogeneous Cr-spinel grains. The composition of parental melt and crystallization temperatures of the minerals in ultramafic rocks were derived from the compositional data on Cr-spinels and trapped melt inclusions.

On the basis of analysis of molten inclusions in chrome-spinelide, physicochemical parameters of dunite crystallization were defined. Experimental, analytical studies directly indicate that dunite was formed from high-temperature melts close in petrochemical composition and high-temperature characteristics to meimechite magmas. Successive evolution of magmatic systems compositions in a course of intra-chamber crystallization of dunite was established: from picriteâ “meimechite (with olivine formation at 1590–1415°C and chrome-spinelide crystallization at 1405–1365°C) to picrate-basalt and basalt.

Study of Early Cambrian sedimentary-volcanogenic complexes hosting the Kyzyl–Tashtyg pyrite deposit in Eastern Tuva has revealed cyanobacteria and algae that existed in zones of ore-forming hydrothermal systems similar in characteristics to present-day “black smokers” at ocean bottoms. Along with archaeocyaths and Cyanophyta from the host sedimentary rocks and microfossils from basalt amygdules, various cyanobacteria, monocyatheans, and sponge spicules have been found in ferrosiliceous deposits and metasedimentary rocks. Scanning electron microscopic, mineralogical, and thermobarogeochemical studies helped to reconstruct their hydrothermal environment.

Ion microprobe data for minerals from the eclogites of the Atbashi Range (South Tianshan) constrain the distribution of trace (Rb, Sr, Ba, Cr, V, Zr, Hf, Nb, Ta, U, Th, and Y) and rare-earth elements (REE) in zoned garnets and mineral inclusions in them. This study showed that garnets from the Atbashi eclogites are the main hosts for heavy REE; epidotes are important hosts for REE, Y, Sr, Th, and U; and omphacites are depleted in almost all trace elements compared with the bulk-rock compositions. Garnet, as well as epidote and omphacite inclusions exhibit systematic rimward depletion in a number of trace elements, which is related to the depletion of the rock matrix in these elements during crystallization. Deviations from this trend, including the enrichment of garnet rims in HREE and strong variations in the REE contents of garnets and mineral inclusions, can be explained by metamorphic reactions involving the destabilization of REE-bearing minerals. Our data suggest that the mobility of trace elements under eclogites-facies conditions is mainly controlled by the stability of certain minerals.