Oceanic igneous rocks throughout the Altai-Sayan Fold Belt (ASFB) in central-southern Siberia are often considered to be late Precambrian–early Paleozoic accreted elements of oceanic crust – often of uncertain paleogeographic or geodynamic origin. We explore the role of suprasubduction zone settings in the formation of different ASFB terranes. Our study offers a non-accretionary perspective on the tectonomagmatic development of basalt-bearing units in the ASFB on the example of the forearc terrane of the Ediacaran–early Cambrian Tannuola-Khamsara island arc (herein termed Sayan-Tuvan forearc zone). We describe the geochemistry, structural geology, and stratigraphic relations of basalts of the Aldynbulak, Uttug-Khaia, and Chingin formations, which are integral parts of the Sayan-Tuvan forearc zone. The Aldynbulak basalt samples mainly fall in the compositional fields of ocean island basalts and enriched mid-ocean ridge basalts (E-MORB) and likely derived from a deep mantle source. The Uttug-Khaia and Chingin basalts are N- and E + T-MORB-like basalts, carrying forearc geochemical signatures. Specifically, the Chingin Formation contains boninite dikes and is associated with a boninite-bearing ophiolite. Boninites are commonly associated with forearc magmatism and thus a forearc formation setting is likely. Tectonic and stratigraphic considerations imply that the Aldynbulak basalts formed first, followed by the Uttug-Khaia and later the Chingin basalts and boninites. A schematic model, involving decompression melting of the mantle, is proposed for the development of the studied forearc basalt suites that are linked with the growth of the Tannuola-Khamsara island arc system 580–540 million years ago.

In rocks from mafic-ultramafic massifs distributed in the territory of Tuva, accessory minerals, with the exception of chromospinel and partly minerals of platinum group elements, have almost not been studied. Some accessory sulfides and arsenides of Ni, Cu, Co, and Fe from rocks and chromitites of the Idzhim, Ergak, Agardag, Nizhne-Tarlashkinsk, Malokoptinsk, Brungansk, and Mazhalyk massifs, as well as from ultramafic bodies of the Kaa-Khem ophiolite zone, were analyzed in single grains. In this connection, we investigated the species affiliation, chemical composition and other properties of accessory minerals from serpentinized olivine clinopyroxenites on the example of the ultramafic Bezymyannyi massif (Tyva Republic, Russia). It was found that the accessory minerals in this massif are represented by millerite, hizlewoodite, nugget copper, pentlandite, pyrrhotite, chalcosite, bornite, pyrite, as well as chromospinel, magnetite, andradite, and zircon. Nugget nickel and iron-nickel copper were found for the first time on the territory of Tuva. It was determined that Cu concentration in grains of nugget copper varies within 93-98 wt. %, and Ni content- within 0.9-7.2 wt. %. The content of Fe impurity in iron-nickel copper can reach 7 wt. %. The formation of nugget copper presumably occurred in the process of fluid infiltration, which formed rims along the periphery and in the microcracks of millerite and hizlewoodite grains. Pseudomorphoses of nugget copper over hizlewoodite sometimes preserved its angular microinclusions. In this process, the Ni content within each of the nugget copper outcrops was kept approximately at the same level, while it varied in the range of 0.9-7.2 wt% in its different outcrops. The formation of nugget nickel copper could be accompanied by removal of hydrogen sulfide and partially Ni from the reaction zone. Another mechanism of appearance of nugget copper phenocrysts is possible- it was released in the process of influence of copper-containing fluid on magnetite grains, in which its microinclusions were formed. In this case, the nugget copper was enriched with some amount of not only Ni, but also Fe, and the concentration of the latter in nugget copper could reach 7 wt. %, which allows us to call it iron-nickel nugget copper. In accordance with the petrogenetic model based on the study of a large number of mafic-ultramafic massifs located in Tuva, we consider the Bezymyannyi massif as a fragment of the contact-reaction zone of a larger, but very poorly eroded polygenic maficultramafic body, which is part of the West Tuva area of the ophiolite association. Based on this model, we assume that olivine clinopyroxenites of the Bezymyannyi massif and the micromineralization of sulfides and nugget copper detected in them were formed as a result of magma-metasomatic interaction of the mafic melt and its fluids containing Cu with earlier restitogenic ultramafics that composed the protusia.

The Tes-Khem site is one of the main research sites of the pre-collision tectonomagmatic history of the early Caledonids of the Agardag subzone. The Agardag subzone is located in the southern part of one of the regions of Russia - the Republic of Tyva, near the border with Mongolia. This subzone is a back-arc structure that is part of the East Tuvan back-arc zone of the Early Caledonids. Spatially closely associated basalts of similar age, but of different petro- and geochemical composition, are represented within the Tes-Khem site. Basalts are found in the Kuskunnug formation and in serpentinite melange. The Kuskunnug basalts have an E-MORB-like composition. Basalts from inclusions in melange are divided into two types. The first type is similar to the Kuskunnug basalts, but is characterized by an ultra-titan and OIB-like composition (Ultra-Ti basalts). The second type is ultra-potassium basalts (Ultra-K basalts). The Kuskunnug basalts and Ultra-Ti basalts could have been formed from a deep mantle source at the level of garnet peridotite at different degrees of its partial melting (5-15% and 2%, respectively). According to the lithological composition of the host strata, geochemical and Sm-Nd isotopic compositions, the Kuskunnug and Ultra-Ti basalts are similar to the Aldynbulak basalts of the Sayan-Tuva fore-arc zone. Probably, both those and other basalts were formed in the same geodynamic conditions at the stage of the subduction initiation. The actualistic method allows us to assume that the genesis of Ultra-K basalts corresponds to the potassium rear-arc basaltoids of Kamchatka and Japan, which develop under conditions of back-arc stretching. In addition, basalts with N-MORB-like characteristics, composing the Chonsair strata and inclusions in the melange in the area of the Agardag massif, represent, in our opinion, the lava complex of the Agardag back-arc ophiolites. Summarizing our assumptions, all the diversity of genetic types of basalts of the Agardag subzone can be explained as follows: 1) about 570 Ma ago, during the initiation of subduction from a deep, enriched source at 2 and 5-15% degrees, respectively, of partial melting of garnet peridotite OIB-like Ultra-Ti and E-MORB-like Kuskunnug basalts were formed; 2) in the initial stage of subduction, Agardag ophiolites, including Chonsair basalts, were formed in the process of back-arc spreading; 3) under the stationar subduction regime, moderate titanium, relatively high-alumina, ultra-potassium basalts were formed in the rear of the Tannuola-Khamsara island arc, under conditions of back-arc stretching. The results of this study, due to the limited number of analyzed samples, are considered preliminary.

We examined PT parameters, geochemical peculiarities, and fluid sources of the Ulug-Sair ore occurrence attributable to class of intrusion-related gold deposits and according to ore mineral assemblages corresponding to Au-Bi type with wide Bi minerals (AgBiTe, Bi2Te2Se, Cu3,07BiS3, Bi), tellurides (Au and Ag), Se-tellurides (Ag and Bi), and selenides (Au, Ag, and Hg). We identified that ‘pre-gold’ quartz-tourmaline veins were deposited using an aqueous Mg-Na-K-chloride fluid with a salinity of 8–10 wt % NaCl eq. At 325–370 °C; host breasts were formed due to a CO2-water fluid containing CH4 and N2, with a salinity of 0,18–6,1 wt % NaCl eq. at least 200–400 °C. Gold-bearing mineral assemblages were formed at P ~ 0,75–1,0 kbar (~ 2,3–3 km) due to CO2-water chloride (Na-K±Fe, Mg) fluid with CH4, Na2SO4, and Na2B2O5, and salinities 1,7–12,5 wt % NaCl eq. during the decreasing temperatures from 360 up to 115 °C (gold-sulfide-quartz veins – 360–130 °С, and gold-telluride-sulfide-quartz veins – 330–115 °C) and variations fO2, fS2, fSe2, and fTe2. The isotopic composition of δ34SH2S fluid (-0,7…+2,5 ‰) indicates the juvenile or magmatic origin of fluid and ore elements. The δ34OH2O fluid indicates that, at an early substage, the formation of ore occurrence involved a fluid of magmatic or metamorphic origin (+7,3…+11,4 ‰), and, in the later substage, it mixed with meteoric waters (-2,3…+9,1 ‰).

This short dictionary is the first attempt to systematize information about the Tuvan names of things and phenomena related to geology, mining, and resources obtained after processing of mineral raw materials. Here we find a very brief Russian-Tuvan geological dictionary containing 131 terms, of which 49 terms are proposed for use for the first time, including 16 terms as additional to those already available in the above dictionaries. In the brief Russian-Tuvan-English geological dictionary, terms related to chemical elements account for about 8 percent of their total amount, to metals and their alloys about 15 percent, to minerals about 15 percent, to mineral aggregates, rocks, and ores about 42 percent, and to other terms about 20 percent. The 11 chemical elements consist of ten metals and one nonmetal.

Ulug-Sair Au-Bi-Te-Se mineralization is one prospect for native Au in the Western Tuva, and its origin remains debated. Mineralization consists of gold–sulfide–quartz veins in the host sedimentary rocks (conglomerates, siltstones, shales), quartz–tourmaline, and quartz–carbonate–sericite–altered rocks. To determine its origin, we examined the mineralogical–geochemical features, formation conditions, and fluid sources of the Ulug-Sair ore. A mineralogical–geochemical investigation outlines two substages with Au: an early gold–sulfide–quartz with pyrite, chalcopyrite, galena, gold, and electrum; and a late gold–telluride–sulfide–quartz, characterized by the presence of Bi-bearing minerals (AgBiTe, Bi2Te2Se, Cu3BiS3, Bi), tellurides (Au and Ag), Se-tellurides (Ag and Bi), and selenides (Au, Ag, and Hg). The paragenesis of Au–Ag tellurides, and fluid inclusion study data (microthermometry, Raman spectroscopy, LA-ICP-MS, and crush leach analysis (gas and ion chromatography, ICP-MS) in quartz showed that quartz–tourmaline-altered rocks were formed by an aqueous Mg–Na–K-chloride fluid with a salinity of 8–10 wt % NaCl eq. at 325–370 °C, whereas the host quartz–carbonate–sericite-altered rocks were formed from CO2–H2O fluid containing CH4 and N2, with a salinity of 0.18–6.1 wt % NaCl eq. at 200–400 °C. Gold-bearing mineral assemblages were formed at P ~ 0.75–1.0 kbar (~2.3–3 km) due to CO2–H2O chloride (Na–K ± Fe, Mg) fluid with CH4, Na2SO4, and Na2B2O5, and salinities 1.7–12.5 wt % NaCl eq. at temperatures decreasing from 360 up to 115 °C (gold–sulfide–quartz veins—360–130 °C, and gold–telluride–sulfide–quartz veins—330–115 °C), and variable fO2, fS2, fSe2, and fTe2. Results of the investigation of the isotope composition of S in pyrites indicates the magmatic origin of the fluid (δ18SH2S fluid from −0.4 to +2.5‰). The stable O isotope data in quartz indicates that, at an early substage, the formation of ore involved a fluid of magmatic and metamorphic origin (from +8.2 to +11.6‰), and, in the later substage, multiple sources of hydrothermal fluids (from +3.1 to +10.4‰), including magmatic-derived, metamorphic-derived, and meteoric waters. These data, in conjunction with structurally controlled mineralization, point towards similarities of the Ulug-Sair ore system with orogenic gold deposits.

The article presents the results of 30 determinations of isotopic age in 18 zircon grains performed by the LA-SF-ICP-MS method from four bulk samples of olivine gabbronorite (K-1, K-2), olivine gabbro (K-5) and plagioverlite (K-7) selected in the central part of the Kalbakdag mafic-ultramafic massif, which was previously considered as a typical layered intrusion. This massif is located in the central part of the territory of Tuva within the Ondum island-arc subzone of the Early Caledonids and breaks through the metavulcanogenic-terrigenous strata of the Late Neoproterozoic - Lower Cambrian. The rocks of the massif are characterized by moderate magnesium content, low alkali content, very low titanium and phosphorus content. The content of rare earth elements in them is lower than NMORB at (La/Yb)n = 0.52-1.71, positive anomalies of K, Sr, Zr, Hf, Eu of low intensity and negative anomalies of Nb and Ta are noted on spiderograms. The dated zircons, in addition to differences in morphological and optical properties, are characterized by significant variations in isotopic age, that is, they are polychronous. The entire collection of dated zircons is conditionally divided into four "populations" (clusters): Neoarchean-Paleoproterozoic (2747-1862 Ma, zircons from samples K-2 and K-5), Neoproterozoic (799-648 Ma, zircons from samples K-1 and K-5); Cambrian-Devonian (525-383 Ma, zircons from samples K-2 and K-7) and Carbonian-Permian (325-279 Ma, zircons from sample K-5). Zircons from the first and second "populations" are proposed to be considered as a xenogenic phase introduced into the parent melts of the gabbroids of the Upper Mantle protolith massif. They are to varying degrees "rejuvenated" very ancient juvenile zircons that were previously present in this protolith and preserved in ultramafic restites formed on it. In turn, zircons from the Cambrian-Devonian "population" are proposed to be considered as a syngenetic phase, the age of which corresponds to the time of formation of gabbroids of this array. The concordant age of syngenetic zircons was 477 +/- 5 Ma. Zircons from the Permian "population" have been identified as an epigenetic phase formed during infiltration into rocks of an array of fluids separated from late granitoid melts. According to the totality of available data, the Kalbakdag massif is considered by us as a polygenic association of spatially converged fragments of earlier protrusion of hybridized restitogenic ultramafites (plagioverlites and pyroxenites) and mesoabyssal gabbroid intrusive (olivine and non-olivine gabbro and gabbronorites) introduced in the Early Paleozoic.

For the first time, for the territory of Tuva, relict and epigenetic zircons from restitogenic harzburgites and chromitites of the Agardag chromite-bearing ultramafic massif were discovered and sold by U-Pb by the isotope method. An array located in the southeastern part of Tuva near Lake Shara-Nur, located on the southwestern flank of the ophiolite South Tuva mafic-ultramafic area. The lens-shaped body of the massif has a length of about 20 km with a maximum width of 4 km. Its long axis is oriented in a northeast direction. The massif has steep (75-80 degrees in NW) tectonic contacts with host metaterrigenous-volcanic Riphean-Lower Cambrian (?) Formations and is considered by us as a protrusion. The massif is composed to varying degrees with serpentinized harzburgites and dunites, as well as antigorite and more rare antigorite-chrysotile serpentinites. In the near-contact zones of the massif, serpentinites are intensively schistose. The massif is intruded with gabbro rods, gabbro-diorites, gabbro-diabases, diabase and basalt porphyrites stocks and dikes. Zircons isolated from large-sized (similar to 20 kg) samples of harzburgites and chromitites are represented by relict and epigenetic genetic varieties. Their short-prismatic crystals sometimes have rounded edges due to resorption, a relatively low intensity of the cathodoluminescent glow to its complete absence, and also often disturbed oscillatory zoning. The sub-concordant and concordant values of the U-Pb isotope age of relict zircons range from 885-392 Ma. It is assumed that these age variations are associated with a partial diffusion loss of radiogenic lead, which caused the uneven "rejuvenation" of the U-Pb isotopic systems of very ancient juvenile zircons located in the upper mantle protolite during its partial melting with the formation of harzburgite and dunite restites, as well as chromite deposits associated with them. The observed close values of the isotopic age of zircons from harzburgites and from chromitites suggest that both formed approximately at the same time. A few zircon grains from harzburgites, which showed concordant age values in the range of 293-276 Ma, are considered by us as an epigenetic phase. Their formation, as expected, was caused by the infiltration of fluids, which were released by melts, which formed later granitoid intrusions, cutting the ultramafic massif, and also framing it with metaterigenic-volcanogenic formations. Evidence of the infiltration of such fluids is found in the chromitites of the mass of Uvarovite - Kemmerite veins, the formation of which requires the addition of silica.

НоВые ДаННые о ВозРаСТе И ВеЩеСТВеННом СоСТаВе каРа-оССкоГо ПеРИДоТИТ-ПИРокСеНИТ-ГаББРоНоРИТоВоГо маССИВа (ТУВа) монгуш а

Presents the results of detailed studies of the chemical composition of the main (olivine, orthopyroxene, clinopyroxene, plagioclase and amphibole) and secondary (scapolite, magnetite, ilmenite) minerals of channel samples of olivine gabbronorite Callbackmessage array. The gabbroids of this massif crystallized, probably, during the upward motion of the mafic melt, which was a structured suspension. The latter consisted of mixing and acquiring a wavy arrangement of phenocrysts of olivine, orthopyroxene, clinopyroxene, and plagioclase immersed in the bulk, consisting of subparallel oriented long axis and "streamlined" porphyritic phenocrysts of plagioclase laths and because of this, with trachytoid texture. The result of mixing porphyroid inclusions in the process of upward movement of the melt-suspension was that in being in close proximity to each other pyroxene inclusions evidence of their chemical equilibrium, that is, the consistency of their compositions. Banded texture gabbroids Callbackmessage array are likely the result of late-magmatic processes that are not associated with intra crystallization-gravitational differentiation of mafic melt.

The study of fahlores is one of the most effective areas of mineralogical research that allows deciphering the conditions for formation of ore deposits, which in its turn is the scientific basis for their search, exploration and rational mining. The main aim of the study was to reveal mineralogical-geochemical features and evolution of the composition of fahlores, as well as features of the ore-forming hydrothermal fluids that formed the Ak-Sug gold-molybdenum-copper-porphyry deposit. Research methods: field studies, detailed mineralogical studies with ore mineralization in polished sections (polished sections) with Olympus polishing microscope and electron microscope MIRA 3 LMU (Tescan Orsay Holding) combined with Xray microanalysis systems INCA Energy 450+Xmax-80 and INCA Wave 500 (Oxford Instruments Nanoanalysis Ltd). Results. We revealed variations in composition of the fahlores group minerals of the Ak-Sug gold-molybdenum-copper-porphyry deposit, which are represented by the minerals of the tennantite-tetrahedrite series. The latter are developed as part of three mineral associations corresponding to three generations. For the fahlores, a latent smooth zoning is characteristic, due to an increase in the Sb content to the outer zones. Evolution of compositions of the I generation fahlores: Cu-tennantite -> Fe-tennantite -> Zn-tennantite, generation: Cu-tennantite -> Zn-tennantite -> Zn-tennantite-tetrahedrite. Fahlores of the III generation is represented by Zn-tennantite-tetrahedrite. The presence of high-copper tennantite, Zn-tennantite, Zn-tennantite-tetrahedrite, and mineralogical features of the Ak-Sug ore deposits indicate a relatively increased oxidation potential of ore-forming hydrothermal fluids. The main factors of ore deposition were a change in the oxidation-reduction character, variations of f.Se2, fTe2 and temperature decrease of ore-bearing fluid.

Өдүген или Өдүген-Тайга — известное среди тувинцев название местности. Для оленеводов же Тоджинского кожууна Тувы Өдүген-Тайга является неотъемлемой частью их культуры и быта. Однако этот топоним отсутствует на топографических и любых других картах как данного района, так и всей Тувы и прилегающих сопредельных территорий. Почему топоним отсутствует, где именно он расположен, какова его этимология и как он соотносится с древнетюркским Отюкеном? В статье представлены результаты краткого анализа опубликованных топонимических, этимологических и фольклорных сведений о местности под названием “Өдүген” (Одуген), приведены сведения информантов из числа местных жителей Тоджинского кожууна, которых автор опрашивал в 2017 г. Проведена корреляция полученных сведений с независимыми от них геологическими данными.Установлено, что местность Өдүген исторически охватывала горные области Центральной Азии, на которых расположены проявления позднекайнозойского вулканизма. К этим областям относятся большая часть территории горных систем Восточного Саяна и Хангая. Исторические и фольклорные сведения о расположении данного топонима и геологические данные о наиболее молодом вулканизме указывают на одни и те же местности в Восточном Саяне и Хангае. Этимология слова филологами делится на три составные, которые образованы присоединением к тюркской глагольной основе ‘өт’ (в значении ‘испражняться’), отыменного аффикса ‘(а)к’ (өт+(а)к > өдек: навоз; шлак, лава [вулканические]; ...) и форманта ‘(а)н’, дающего имя собственное: өт+(а)к+(а)н > өдүген. Таким образом, автор полагает, что “Өдүген” обозначал территорию, содержащую продукты вулканических извержений — вулканические лавы и шлаки — “испражнения” Земли.

Odugen or Odugen-Taiga is a well-known territorial name among Tuvans. For the Todzhu reindeer herders of Tuva, Odugen-Taiga is an integral part of their culture and way of life. However, this toponym is absent on topographic or any other maps of the area, the whole of Tuva and adjacent territories. The article deals with the four-fold question of why the toponym does not appear on maps, where exactly it is actually located, what its etymology is and how it is relate to the ancient Turkic Otuken. We present a brief analysis of the published toponymical, etymological and cultural sources about the area called ‘Өдүген’ (Odugen), adding to it the information provided by the locals of the Todzhu district, who were interviewed in 2017. The obtained information is then tested against the hard geological data. It is proved that historically the area named Odugen covered the mountain regions of Central Asia, which showed manifestations of late Cenozoic volcanism. These areas include most part of the mountain systems of Eastern Sayan and Khangai. Historical and folk information about the location of this toponym and geological data on the youngest volcanism both point to the same areas in the Eastern Sayan and Khangai. Linguists have discovered three elements in the etymology of the word, which was formed by joining the Turkic verbal stem ‘өt’ (meaning ‘to defecate’), noun affix ‘(a)k’ (өt+(a)k > өdek: manure; slag, lava [volcanic]; ...) and proper name formant ‘(a)n’: өt+(a)k+(a)n > өdugen ~ odugen. Thus, we hold that “Odugen” meant the area containing the products of volcanic eruptions — volcanic lava and slag — the “excrement” of the Earth.

The rare abundance of natural iodine-bearing minerals of the chlorargyrite-bromargyrite series, their water solubility, comparatively small sizes, mineral forms (single micrograms, crusts, incrustations, micron-sized crystals), small hardness 1,5-2,5, instability for an electron beam impact in microprobe analysis caused insufficient knowledge of their mineralogical-geochemical features. The aim of the research is to determine genesis and mineralogical-geochemical features of iodine-bearing minerals of the chlorargyrite-bromargyrite series in oxidized ores in the Khaak-Sair and Tardan-2 gold-quartz ore occurrences case study. Methods: field studies, detailed mineralogical studies using ore mineralization in polished sections (polished microsections) using scanning electron microscopes MIRA 3 LMU (Tescan Orsay Holding) combined with Xray microanalysis systems INCA Energy 4 50+Xmax-80 and INCA Wave 500 (Oxford Instruments Nanoanalysis Ltd) and Vega 3 (Tescan) with EDA (Oxford Instruments X-act). Results. The authors have identified and described the hypergenic iodine-bearing minerals of the chlorargyrite-bromargyrite series in the oxidized ores of the Khaak-Sair and the Tardan-2 gold-quartz ore occurrences. It is confirmed that the iodine-bearing differences of the Khaak-Sair in the AgCl-AgBr series are represented by bromian iodian chlorargyrite (up to 8,16 wt. % I), chlorian iodian bromargyrite (up to 11,11 wt. % I), iodian chlorian bromargyrite (up to 13,94 wt. % 1) and iodian bromargyrite (up to 15,45 wt. % I), on the Tardan-2-bromian iodian chlorargyrite (up to 4,29 wt. % I), chlorian iodian bromargyrite (up to 10,16 wt. % D. The AgCl-AgBr halides in these ore occurrences are represented as their solubility decreases: AgCl -> AgBr -> Agl. Iodine content increases from chloride phases to more bromide phases within AgCl-AgBr series of the Khaak-Sair and the Tardan-2, because the parameters of face-centered cubic lattice gradually increase from AgCI to AgBr and the limits of Agl solid solutions in AgCl-AgEr series are very limited due to the fact that Ag iodides differ in structure from its chlorides and bromides.

Article presents the author’s presentation at International Science Conference devoted to 100th jubilee "United Tuva in united Russia: history, modernity, futures” (July 3-4, 2014, Kyzyl). On the basis of archival material it provides most important episodes in the history of discovery and extraction of gold deposits in the territory of Tuva in the XIX - early XX centuries.

The study provides new petrologic and isotope geochemical data for rocks of the 465 ± 5 Ma Bulka massif (Borodina et al., 2011). The primary amphibole from granitoid stocks cutting across the layered series of the massif yielded an Ar–Ar age of 415.9 ± 3.7 Ma. The rocks of the Bulka massif have 143 Nd/ 144 Nd ratio of 0.513243 and εNd (Т) values of +12.00. The granitoids have 143 Nd/ 144 Nd ratios between 0.512919 and 0.512961 and εNd (Т) values between +8.03 and +9.25. The Nd isotope composition indicates that the rocks of the Bulka massif and granitoids were derived from a depleted mantle source. Depletion of the rocks of the massif in LILE, LREE, and HFSE over LILE is inherited from the mantle source, which has geochemical signatures of N-MORB and subduction-related components. Granitoids are metaluminous I-type granites, which were probably generated either by differentiation of intermediate to mafic mantle-derived magmas or by melting of metabasites. The rocks of the granitoid stocks are characterized by enrichment in LILE and LREE and depletion in HFSE over LILE, which suggests derivation from arc-related parental magmas.

The paper reports original isotopic and geochemical data on Early Precambrian lavas in the Ozernaya Zone in Mongolia. According to their normalized trace-element patterns, the rocks are classified into the following groups: (1) rocks similar to N-MORB; (2) rocks similar to E-MORB; (3) basalts enriched in trace elements, with HFSE minima; and (4) basalts depleted in trace elements, with HFSE minima. All of the lava types could be produced in an island arc-backarc basin system. The magmatic rocks of group (1) were likely formed in a spreading backarc basin, and those of group (2) were likely generated within the influ- ence zone of a hotspot or were derived from heterogeneous upper mantle domains. The lavas of group (3) seem to be fragments of an ensimatic, relatively primitive island arc. The basalts and basaltic andesites of group (4) were likely produced by mixing melts of groups (1) and (3). The fact that lavas of groups (1) and (4) sometimes intercalate within a single stratigraphic section suggests that the extension and subduction zones were closely spaced and operated simultaneously. The magmas of groups (1), (2), and (3) were derived from different mantle sources, which possessed different ratios of trace elements and were different in isotopic composition.