U–Pb (LA-ICP-MS) geochronological studies of detrital zircon from rocks of the Early Proterozoic Khargitui Formation of the Sarma Group, distributed in the central part of the Baikal uplift of the Siberian craton basement and included into the structure of the Sarma zone of the Akitkan orogenic belt, are carried out. It is established that the Khargitui Formation comprises the rocks formed in different time intervals: >2.7, 2.15–1.95, and <1.7 Ga. It is shown that Meso- and Neoarchean age peaks (2.7–3.1 Ga) are characteristic of zircon from leucocratic gneisses, which indicates the formation of gneiss protoliths after 2.7 Ga due to the destruction of Archean granitoids in the Sarma zone of the Akitkan orogenic belt and igneous rocks in the Anabar superterrane basement of the Siberian craton. The time interval of 2.15–1.95 Ga corresponds to the accumulation of protoliths of mica–quartz schists, which is substantiated by the age of the youngest zircon grains of 2150 Ma and the age of metamorphism of these rocks of 1.95 Ga. The presence of an age peak at 1833 Ma and younger zircon grains aged in the range of 1675–1785 Ma in the sample of mica–quartz schist taken from another area of rock outcrops assigned to the Khargitui Formation indicates that the protoliths of these rocks accumulated after the formation of the Akitkan orogenic belt and the South Siberian postcollisional magmatic belt. The results obtained indicate that the rocks assigned to the Khargitui Formation of the Sarma Group of the Western Baikal region cannot be considered as a single stratigraphic unit, since they formed at different stages of the evolution of the Akitkan orogenic belt. It is shown that, based on recent data, the deposits of the Khargitui Formation should be divided into complexes of different ages (stratigraphic units), each of which corresponds to a certain stage of the geological evolution of the territory.

--We carried out a detailed geological, geochronological, geochemical, and isotope study of diorites from a dike located in the central part of the Baikal uplift of the Siberian craton. The geochemical and isotope data obtained for diorites of the studied intrusion were compared with coeval mafic and intermediate igneous rocks of the southern part of the Siberian craton. The U-Pb (ID-TIMS) baddeleyite age of 1862 +/- 7 Ma has been estimated for diorite from a dike located in the area of the Onguren Village. The obtained data are the first reliable age determination for the Paleoproterozoic mafic and intermediate igneous rocks of the Baikal uplift, which are part of the South Siberian postcollisional magmatic belt. The dike is of NE strike. The dike rocks correspond in chemical composition to diorites and are highly differentiated varieties (mg# = 36.5-37.4). There are no significant variations in the composition of diorites in the marginal and central parts of the dike. The rocks are characterized by low contents of TiO2, P2O5, and Nb, high contents of Th, Zr, Ba, and LREE, and low negative values of epsilon Nd(T) (-5.9 to -6.2). We assume that the diorites formed from the enriched subcontinental lithospheric mantle. The obtained geochemical and isotope data show similar negative values of epsilon Nd(T) from -4.3 to -11.6 for most of Paleoproterozoic mafic and intermediate igneous rocks of the South Siberian postcollisional magmatic belt within the Aldan Shield, the Baikal uplift, and the Irkut block of the Sharyzhalgai uplift. These rocks correspond in geochemical features to rocks resulted from the melting of subduction-modified lithospheric mantle sources in the postcollisional extension setting at the final Paleoproterozoic stage of formation of the Siberian craton.

Приводятся результаты петрографических, геохимических и геохронологических (LA-ICP-MS) исследований раннепротерозойских терригенных пород (песчаников и сланцев) верхней части иликтинской свиты сарминской серии Байкальского выступа фундамента Сибирского кратона. Петрохимические и петрографические характеристики позволяют рассматривать песчаники иликтинской свиты как полимиктовые и граувакковые песчаники, а сланцы как граувакковые алевролиты и алевропелитовые аргиллиты. Минеральный и химический состав пород иликтинской свиты указывает на преобладание пород кислого состава в области их источника. Установлено, что породы иликтинской свиты не были подвержены раннепротерозойскому региональному метаморфизму, а вторичные изменения в породах отражают в разной степени проявленную метаморфическую переработку, связанную с раннепалеозойскими коллизионными событиями. На основании данных о возрасте регионального метаморфизма (1.95—1.98 млрд лет), а также с учетом возраста прорывающих породы иликтинской свиты гранитоидов (1.86—1.91 млрд лет), время накопления иликтинской свиты можно оценить в узком диапазоне 1.91—1.95 млрд лет. Полученные данные по возрасту детритовых цирконов из песчаников иликтинской свиты не противоречат сделанным выводам, в частности в том, что самые молодые зерна детритового циркона обнаруживаются в диапазоне 1955—1993 млн лет. В результате проведенных исследований было установлено, что основными источниками детритового материала для терригенных пород иликтинской свиты могли являться магматические и метаморфические породы раннепротерозойского Акитканского орогенного пояса при некотором добавлении материала из пород прилегающего к нему с востока Алданского супертеррейна. Анализ геодинамических условий осадконакопления в совокупности с петрографическими и геохимическими характеристиками пород позволили сделать вывод, что породы иликтинской свиты сарминской серии могли накапливаться как молассоидные образования в осадочных бассейнах, сформированных после образования Акитканского орогенного сооружения. Терригенные породы, детритовые цирконы, ранний протерозой, Акитканский орогенный пояс, Сибирский кратон New petrographic, geochemical, and geochronological (LA-ICP-MS) data have been obtained for the Early Proterozoic upper Ilikta Formation of the Sarma Group within the Baikal uplift, a basement inlier of the Siberian craton. The Ilikta Formation consists of polymictic and graywacke sandstones and shales of graywacke siltstone and silty-pelitic mudstone varieties, identified from major-element chemistry and petrography. The mineralogy and chemistry of the samples indicate their origin from a felsic source. The Ilikta Formation rocks eluded the Early Proterozoic regional metamorphism but underwent secondary alteration to different grades associated with early Paleozoic collisional events. The Ilikta Formation was deposited during the 1.91–1.95 Ga interval, as constrained by the ages of regional metamorphism (1.95–1.98 Ga) and granitic intrusion (1.86–1.91 Ga). These estimates are consistent with dating of detrital zircons from the Ilikta Formation sandstone which showed the youngest ages of 1955–1993 Ma. The detrital material for the Ilikta Formation deposition was apparently derived from the igneous and metamorphic rocks of the Early Proterozoic Akitkan orogen, and some amount of material additionally came from the adjacent Aldan superterrane located in the east. The reconstructed geodynamic environment of sedimentation, together with the petrography and chemistry of rocks, allow interpreting the Ilikta Formation of the Sarma Group as molasse accumulated in extension basins after the formation of the Akitkan orogen.

—We present new data on the chemical composition of the late Precambrian rocks in the upper part of the regional Balaganakh stratigraphic horizon (the Nugan Formation at the Baikal segment of the Sayan–Baikal–Patom belt (SBPB) and the Bugarikhta Formation of the Balaganakh Group at the Patom segment of the SBPB) and the U–Pb (LA-ICP-MS) age of detrital zircons from the Bugarikhta Formation. It has been established that the rocks of the Nugan and Bugarikhta formations resulted from the destruction of igneous and metamorphic rocks. The contents and ratios of trace and rare-earth elements in the rocks of the Nugan and Bugarikhta formations indicate a predominance of felsic igneous rocks in the provenance and the presence of rocks of mafic and intermediate compositions at the source of the terrigenous rocks of the Nugan Formation. The U–Pb age of detrital zircons from all rocks of the upper part of the regional Balaganakh Horizon is close to the age of the rocks in the basement of the southern margin of the Siberian Platform, which suggests that this basement was the main source of clastic material in the sedimentation basins. It is argued that the “pre-glacial” (pre-Marinoan) terrigenous deposits of the Nugan and Bugarikhta formations, as well as the deposits of the Ipsit Formation at the Sayan segment of the SBPB, accumulated in the Late Riphean (ca. 720–640 Ma), probably in postrift basins that formed immediately after the separation of Siberia from Laurentia at the early stages of the opening of the Paleoasian Ocean.

ABSTR A C T Several published Mesoproterozoic paleogeographic reconstructions suggest proximity of northern Laurentia and southern Siberia. However, the apparent absence of the traces of the ca. 1.27 Ga giant Mackenzie magmatic event in southern Siberia was somewhat contradictory to this hypothesis. Here we present geochronological, miner-alogical, geochemical, Nd isotopic, and paleomagnetic data from the Srednecheremshansk dyke-shaped intru-sion, which was recently found in the Sharyzhalgay uplift of the southern part of the Siberian craton and which can be related to the Mackenzie event. The plagioclase-bearing peridotite of this intrusion yielded U-Pb (ID-TIMS) baddeleyite concordia age of 1260 +/- 3 Ma, which is interpreted as the time of their emplacement. This age is close to the previously published baddeleyite age of gabbro from the same intrusion (1258 +/- 5 Ma). The Srednecheremshansk intrusion is composed of ultramafic and mafic rocks. The main rock-forming minerals of the intrusion are olivine, orthopyroxene, clinopyroxene, phlogopite, and plagioclase in various proportions. Sulfide mineralization of the intrusion is represented by pentlandite nodules. The chemical composition of Sredne-cheremshansk ultramafic and mafic rocks correspond to subalkaline peridotite gabbro and gabbro. These rocks are characterized by negative epsilon Nd(t) values range from-6.3 to-6.9. Ultramafic and mafic rocks have similar geochemical and isotopic characteristics, indicating their generation from a single subcontinental lithospheric mantle source. The chemical composition of the Srednecheremshansk intrusion is similar to those of the ca. 1.27 Ga Muskox mafic-ultramafic intrusion of the Mackenzie Large Igneous Province in northern Laurentia. Paleo-magnetic data permit a variety of possible Laurentia-Siberia reconstructions in Mesoproterozoic. The combina-tion of geochronological, geochemical, and paleomagnetic data allows the relation of the Srednecheremshansk intrusion with the Mackenzie magmatic event.

Widespread 720 Ma magmatism has been linked with the break-up of Rodinia and the onset of the Sturtian 'Snowball Earth' event. We report a new U-Pb baddeleyite age from the Baikal dyke subswarm in southern Siberia which increases the known extent of the 720 Ma Irkutsk LIP and confirms a previous hypothesis that the Baikal and Sayan subswarms converge at the southern tip of the Irkutsk promontory. Together they define a mantle plume centre with direct links to the 720 Ma Franklin plume centre of northern Laurentia, thus constraining the paleo-reconstruction of southern Siberia and northern Laurentia. It is inferred that this combined 720 Franklin - Irktusk LIP event is associated with the breakup of southern Siberia from northern Laurentia during fragmentation of the Rodinia supercontinent. Expansion of 720 Ma magmatism into Siberia greatly increases the scale of the Franklin-Irktusk LIP.

— The paper presents detailed geological, petrographic, geochemical, and isotope data on the dolerites from dykes of the Baikal dike subswarm in the central part of the Baikal basement inlier of the Siberian craton. The main geochemical and isotope criteria were identified for classifying the dolerites that are similar in geological–structural position and mineral composition into three geochemical groups. Provisional age estimates of the dolerites are presented. The dolerites of the first and, perhaps, second groups compose Neoproterozoic (715 Ma) dikes. The first group includes medium- to coarse-grained dolerites, which form thick (more than 5–10 m) dikes. The dolerites of this group are characterized by low concentrations of Th (0.6–2.1 ppm) and Nb (3.3–9.2 ppm) and by ɛNd(T) = ‒0.5 to ‒3.9. Considered together, the geochemical and isotope data show that these dolerites could be produced as a result of the melting of a mantle source produced by mixing mantle components close in composition to oceanic plateau basalts and subduction-related subcontinental lithospheric mantle. The second group includes fine-grained dolerites from thin (1–5 m) dikes, including dikes located in contact with the dikes of the first group. The dolerites of the second group are characterized by higher concentrations of Th (3.0–5.3 ppm) and Nb (9.8–21.1 ppm) and by ɛNd(T) = –5.3 to ‒6.0. Geochemical and isotopic data on the dolerites of the second group indicate that continental crustal material was added to the mantle source unified for the dolerites of first and second groups. The third group includes medium-grained dolerites, which make up individual dikes that are contrastingly different in geochemical and isotope characteristics from the Neoproterozoic dolerites of the first and second groups. Dolerites of this group show low concentrations of Th (0.6–1.6 ppm) and Nb (2.7–5.1 ppm) at low values of (Th/La) pm (0.29–0.71) and 143 Nd/ 144 Nd (0.511223‒0.511544), which indicates that these dolerites may have been generated as a result of the melting of the subcontinental lithospheric mantle enriched in subduction components. Dolerites of the third group exhibit geochemical characteristics close to those of Paleoproterozoic (1.84 Ga) dikes of the South Siberian post-collisional magmatic belt.

The paper presents the first U-Pb (LA-ICP-MS) dating results for detrital zircons from the quartzsericite-chlorite schist of the Anai formation within the Baikal salient of the Siberian platform basement. During the study the detrital zircons showed a major age peak at 1.86 Ga, which indicates that the rocks of the Anai formation were accumulated after the development of magmatic rocks of the South Siberian post-collisional magmatic belt with an age of 1.88‒1.84 Ga. This fact makes it possible to reconsider a point of view on the belonging of the Anai formation to the section of the Paleoproterozoic Sarma group whose rocks were intruded by granitoids of the South Siberian magmatic belt. It is shown that the rocks of the Anai formation can be considered as age and facial equivalents of the Proterozoic sediments of the Purpol formation of the Patom zone. Taking into account the Anai formation sediments intruded by sills and dikes of the Neoproterozoic (~720 Ma) dolerites, it can be concluded that the rocks of this formation were presumably accumulated in the Early Neoproterozoic time.

Petrographic, geochemical and U-Pb geochronological research studies are done into cataclastic granitoids of the southeastern part of the Irkutsk block of the Sharyzhalgay basement uplift of the Siberian craton – the uplift which is considered southern part of the Tungus superterrane according to most of tectonic schemes. The representative sample of these granitoids corresponds to granodiorite in its geochemical properties and is characterized by high contents of Al 2 O 3 , Th, Sr, Ba, low concentrations of K 2 O, Nb, Y, Yb, a highly fractionated rare earth elements pattern (La n /Yb n =284), and the absence of europium anomaly. U-Pb geochronological studies of zircon from cataclastic granodiorite were carried out independently by two methods: SIMS and LA-ICP-MS, which showed good consistency of the results. The U-Pb age of the cores of zircon grains with magmatic zoning corresponds to 2893±19 Ma (SIMS method) and 2889±16 Ma (LA-ICP-MS method). These results can be interpreted as the age of the Archean granodiorite protolith. The rims of zircon crystals with the Archean cores, as well as the individual zircon crystals with parallel zoning, yielded ages of 1855±6 Ma (SIMS method) and 1864±5 Ma (LA-ICP-MS method), which record the time of granodiorite transformations. The age of about 1.86 Ga corresponds to the main Early Proterozoic stage of metamorphism, migmatization and magmatism, which show their widespread occurrence in the Sharyzhalgay basement uplift. This age estimate together with the previously published ages for metamorphic and their synchronous magmatic events in the Sharyzhalgay uplift allowed concluding that the Tungus superterrane joined the earlier formed core of the Siberian craton in the time interval 1.85–1.88 Ga. The final stage in the Siberian craton formation is the development of the South Siberian post-collisional magmatic belt, intersecting all large Early Precambrian blocks of the southern part of the Siberian craton, already united into a single structure.

—We present results of petrographic, geochemical, and isotope-geochemical (Sm–Nd) studies of the Paleoproterozoic terrigenous rocks of the Urik–Iya graben, which formed during three successive stages of extension. We have established that these rocks are both petrogenic (Ingashi and Daldarma formations) and lithogenic (Ermosokha Formation) sediments. It is concluded that the rocks in the lower and, partly, middle parts of the Urik–Iya graben section (Ingashi Formation and Lower Daldarma Subformation) resulted mostly from the disintegration of felsic igneous rocks. The terrigenous rocks in the middle part of the section (Upper Daldarma Subformation) might have formed through the disintegration of both felsic and mafic igneous rocks. The rocks in the upper part of the section (Ermosokha Formation) probably formed from the underlying terrigenous rocks of the Ingashi and Daldarma formations. The Nd model age (2.3–2.5 Ga) estimated for the rocks of the three studied sections points to a predominance of rocks of the Neoarchean and Paleoproterozoic upper continental crust in the provenance.

The paper presents a petrographic, geochemical, and Sm-Nd isotopic data on the gneisses from different tectonic zones of the Olkhon terrane of the Central Asian Orogenic Belt, as well as the composition of garnets and the age of zircons in these metaterrigenous rocks. The garnet-biotite gneisses of the Anga-Sakhurta zone, as well as the garnet-bearing and garnet-free gneisses (granulites) of the Chernorud zone may result from metamorphism of immature terrigenous rocks of polymictic or greywacke compositions similar in geochemical characteristics to rocks of continental arcs. At the same time, the gneisses of these zones show both similarities and some differences in geochemical and isotopic characteristics, including variations in ɛNd(T) values from –0.2 to –9.0, which may indicate different proportions of one or another source in their protoliths. The age of most detrital zircons in the gneisses of the Chernorud and Anga-Sakhurta zones corresponds to 530–1000 Ma, and the youngest detrital zircons have an age of 522–537 Ma. The geochemical and geochronological data on the gneisses of the Chernorud and Anga-Sakhyurta zones suggests that the protoliths of these gneisses could be formed from the same sources of predominantly Neoproterozoic age. We assumed that gneiss protoliths could initially be sediments of the continental slope of Neoproterozoic composite terrane assembled to the Siberian Platform at 600‒610 Ma. The transport of clastic material being gneiss protoliths from this composite terrane, took place towards the Paleo-Asian Ocean. The youngest zircons with an age of about 530 Ma could be generated from igneous complexes intruding the Neoproterozoic composite superterrane. The garnet-biotite gneisses of the Krestovskaya zone are similar in chemical composition to immature graywacke sandstones, The ɛNd(T) value in these gneisses is –3.7. Detrital zircons in the gneisses of the Krestovskaya zone form age peaks at 780–820 and 498–515 Ma. Based on geochemical and geochronological data we assume that these gneisses could be formed in an intracontinental basin formed at the orogenic stage during accretionary-collisional events at the amalgamation the Orso block and the Birkhin island arc into the Krestovskaya zone.All terrigenous rocks being gneiss protoliths were metamorphosed at 460–510 Ma under granulite or amphibolite facies associated with accretionary and collisional events, which led to the formation of the Early Paleozoic Olkhon composite terrane.

The paper presents the results of the detailed structural analysis and 40Ar/39Ar dating of the deformations in the Golets Vysochaishii deposit area. The geological and structural studies showed the folded-deformation sequence from large lying folds through axial-plane cleavage formation and cleavage to quartz-filled fractures. The 40Ar/39Ar dating of syntectonic sericite yielded the ages of two Hercynian tectonic impulses – 340 and 320 Ma. This stage is characterized by the earlier-known successive complication of the Early Paleozoic fold-nappe structure of the area. Analysis has been made on four samples taken from different sites and characterized by different sets of deformational structures. The 40Ar/39Ar age of 340 Ma reflects the formation of small-size folds as a result of interlayer sliding and sliding along the cleavage axial planes. The 40Ar/39Ar age of 320 Ma reflects the formation of discretely manifested crenulation cleavage and low-Au, low-sulfidation quartz veins oriented obliquely to the elements of bedding and axial-plane cleavage.

The paper presents the results of the detailed structural analysis and 40Ar/39Ar dating of the deformations in the Golets Vysochaishii deposit area. The geological and structural studies showed the folded-deformation sequence from large lying folds through axial-plane cleavage formation and cleavage to quartz-filled fractures. The 40Ar/39Ar dating of syntectonic sericite yielded the ages of two Hercynian tectonic impulses – 340 and 320 Ma. This stage is characterized by the earlier-known successive complication of the Early Paleozoic fold-nappe structure of the area. Analysis has been made on four samples taken from different sites and characterized by different sets of deformational structures. The 40Ar/39Ar age of 340 Ma reflects the formation of small-size folds as a result of interlayer sliding and sliding along the cleavage axial planes. The 40Ar/39Ar age of 320 Ma reflects the formation of discretely manifested crenulation cleavage and low-Au, low-sulfidation quartz veins oriented obliquely to the elements of bedding and axial-plane cleavage.

Linear or lens-like carbonate (marble) and carbonate-silicate bodies among gabbro and amphibolites within the Krestovsky subterrane of the Olkhon composite terrane (West Baikal Area) are identified as dikes. The dikes commonly dip almost vertically, range in thickness from 20 cm to a few meters, and are up to 100 m long. The Olkhon marble dikes quite often coexist with dolerite dikes and/or granite veins and show signatures of emplacement synchronously with the igneous bodies. The marble dikes differ from mantle carbonatites in mineralogy and chemistry and thus may be derived from sedimentary carbonate rocks molten during collisional events. The origin of the Olkhon carbonate and carbonate-silicate dikes may be explained with two possible geodynamic scenarios. They may be derived either from Neoproterozoic carbonate sediments upon the Early Precambrian basement of a cratonic block which was involved in collisional events, or from abundant carbonate sedimentary material in an island-arc terrane. Large-scale melting of silicate and carbonate rocks was maintained by heat released from mantle mafic magma intruding into the lower crust. The batches of both crustal (carbonate and granitic) and mantle (mafic) melts intruded late during the collision in a strike-slip tectonic setting.

The paper presents the results of the detailed structural analysis and Ar-40/Ar-39 dating of the deformations in the Golets Vysochaishii deposit area. The geological and structural studies showed the folded-deformation sequence from large lying folds through axial-plane cleavage formation and cleavage to quartz-filled fractures. The Ar-40/Ar-39 dating of syntectonic sericite yielded the ages of two Hercynian tectonic impulses - 340 and 320 Ma. This stage is characterized by the earlier-known successive complication of the Early Paleozoic fold-nappe structure of the area. Analysis has been made on four samples taken from different sites and characterized by different sets of deformational structures. The Ar-40/Ar-39 age of 340 Ma reflects the formation of small-size folds as a result of interlayer sliding and sliding along the cleavage axial planes. The Ar-40/Ar-39 age of 320 Ma reflects the formation of discretely manifested crenulation cleavage and low-Au, low-sulfidation quartz veins oriented obliquely to the elements of bedding and axial-plane cleavage.

The article describes the fold-thrust structure of the Golets Vysochaishy deposit located at the Baikal-Patom Upland in the Marakan-Tunguska megasyncline. The latter is composed of terrigenous-carbonate carbonaceous rocks metamorphosed in greenschist facies conditions. The deposit is detected in the hanging wing of the asymmetric Kamenskaya anticline. In a cross section, the anticline is an S-shaped structure extending in the latitudinal direction. The main feature of the Golets Vysochaishy deposit is the development of interlayer sulfidization zones (pyrite, pyrrhotite), including gold-bearing ones. Its gold-ore zones tend to occur in layered areas of interlayer sliding in the rocks of the Khomolkhinskaya suite.Four structural markers revealed within the deposit area are indicative of repeated deformation processes: (1) sublatitudinal folding, cleavage of the axial surface and its subsequent transformation into schistosity; (2) crenulation cleavage; (3) interlayer sliding and rock breakdown with interlayer drag folds, parallel microfractures and polished slickensides; (4) large quartz veins and veinlets that cross cut the main structural elements in plan.