T.V. Donskaya

Petrology, geochronology, and tectonic implications of c. 500 Ma metamorphic and igneous rocks along the northern margin of the Central Asian Orogen (Olkhon terrane, Lake Baikal, Siberia)

A significant portion of the continental crust of northern Eurasia is thought to have formed during the evolution of the Central Asian Orogenic Belt at the time of accretion of continental terranes and island arcs. Records of this event are well preserved within the Siberian craton–Central Asian Orogenic Belt transition zone in Lake Baikal region, particularly in the Olkhon terrane. Our results establish granulite-facies conditions for peak metamorphism in the Olkhon terrane, and indicate that the granulites were derived from island arc mafic volcanic rocks and back-arc basin sediments. Sensitive high-resolution ion microprobe dating of metamorphic zircons from two mafic granulites yielded 238U/206Pb ages of 507 ± 8 and 498 ± 7 Ma, and magmatic zircons from syntectonic syenite yielded an age of 495 ± 6 Ma. The main metamorphic event occurred at about 500 Ma, and was probably related to collision of the Barguzin microcontinent with the Siberian craton. Ages from 535 to 2750 Ma for detrital zircon cores in early Palaeozoic metasediments of the Olhkon terrane were obtained. Archaean ages of detrital zircons in such metasediments suggest that the Barguzin microcontinent was originally part of the Aldan Province of the Siberian craton that was detached in late Mesoproterozoic, and reattached to the craton during early Palaeozoic collision.

A one-billion-year gap in the Precambrian history of the southern Siberian Craton and the problem of the Transproterozoic supercontinent

Available geochronological data substantiate the existence of an apparent ca. one billion year gap in geological activity in the southern part of the Siberian craton. The duration of the gap is about 0.8 to 1.1 Ga in the Sayan Uplift and at least 0.9 Ga in the Baikal Uplift. We suggest that the absence of major geological activity in this interval might be due to the southern margin of Siberia occupying an internal position within a Transproterozoic supercontinent, that is, a fragment of Nuna that did not disperse until the late Neoproterozoic breakup of Rodinia. The absence of Mesoproterozoic–early Neoproterozoic sedimentary successions in southern Siberia could possibly be explained by their removal by erosion. Ediacaran subsidence following the breakup of Rodinia may reflect the solidification of magma chambers that fed Neoproterozoic mafic dike swarms. We suggest that a combination of these factors (dike emplacement and erosion) has a significant influence on global tectonics, controlling the uplift and subsidence of ancient cratons.

Palaeoproterozoic to Eoarchaean crustal growth in southern Siberia: a Nd-isotope synthesis

Abstract Nd-isotope analyses from 114 rock samples are reported from the southern part of the Siberian craton to establish a first-order crustal formation scheme for the region. The Nd-isotope data show considerable variability within and among different cratonic units. In many cases this variability reflects differing degrees of mixing between juvenile and older (up to Eoarchaean) crustal components. The fragments of Palaeoproterozoic juvenile crust within the studied segment of the Siberian craton margin have Nd-model ages of c. 2.0–2.3 Ga. Voluminous Palaeoproterozoic granites (c. 1.85 Ga) were intruded into cratonic fragments and suture zones. These granites mark the stabilization of the southern Siberian craton. The complexity in the Nd data indicate a long history of crustal development, extending from the Eoarchaean to the Palaeoproterozoic eras, which is interpreted to reflect the amalgamation of distinct Archaean crustal fragments, with differing histories, during Palaeoproterozoic accretion at 1.9–2.0 Ga and subsequent cratonic stabilization at 1.85 Ga. Such a model temporally coincides with important orogenic events on nearly every continent and suggests that the Siberian craton participated in the formation of a Palaeoproterozoic supercontinent at around 1.9 Ga.

Proterozoic basic magmatism of the Siberian Craton: Main stages and their geodynamic interpretation

Geological data on the Precambrian basic complexes of the Siberian Craton and their isotopic age are considered. The three main episodes of Precambrian basic magmatism of Siberia correspond to certain stages of the geodynamic evolution of the craton and the Earth as a whole. In the Late Paleoproterozoic, largely in the south and the north of the craton, the basic rocks were emplaced against the background of post-collision extension, which followed the preceding collision-accretion stage responsible for the formation of the craton. In the Mesoproterozoic, primarily in the north of the craton, basic magmatism was controlled by dispersed within-plate extension apparently caused by the impact of a mantle plume. Neoproterozoic basic magmatism widespread in the southern and southeastern parts of the craton was caused by rifting, which accompanied breakdown of the Rodinia supercontinent and opening of the Paleoasian ocean along the southern margin of the Siberian Craton.

Rare metal granites of the Katugin complex (Aldan shield): Sources and geodynamic formation settings

Isotope–Geochemical Sm–Nd studies of the Early Proterozoic alkaline granites of the Katugan complex (Aldan shield) were carried out. The unique Katugan rare metals (Ta, Nb, Zr, Y, and REE) deposit is confined to these granites. Parent melts of the granites are of mantle–crustal nature.

Precambrian sedimentation in the Urik-Iya Graben, southern Siberian Craton: Main stages and tectonic settings

The Paleoproterozoic sedimentary and volcanic-sedimentary sequences of the Urik-Iya Graben at southern flank of the Siberian Craton have been studied. Based on the isotopic U-Pb LA-ICP-MS dating of detrital zircons contained in the clastic fraction of the studied rocks, three main extension stages accompanied by sedimentation are recognized; each stage is characterized by certain types of sediments and conditions of their accumulation. The oldest rocks (Ingashi Formation) mark early extension events (∼1.91−1.87 Ga), which were caused by collapse of the orogen that arose due to collision of the Biryusa and Sharyzhalgai blocks. The basin formed as a result of extension is regarded as an aulacogen. Granitoids of the Sayan Complex were emplaced in the cratonic lithosphere at the final stage of the first extension stage. The second stage of extension started ∼1.75 Ga ago as a response to the effect of the inferred mantle plume on the lithosphere of the Siberian Craton. It was accompanied by deposition of the Daldarma Formation. Stress inversion took place at the final stage (∼1.70 Ga), and an intracratonic fold zone arose at the place of the paleoaulacogen. The third extension stage (1.65−1.60 Ga) corresponds to the time of molasse accumulation in pull-apart basins (Yermosokha Formation). The final stage of rifting was marked by emplacement of granitoids (Chernaya Zima Complex, 1.53 Ga), which completed the active tectonic events in the region. Afterward, the Urik-Iya Graben transformed into a stable intracratonic domain. The available data allow us to revise the tectonic history of the Urik-Iya Graben. In light of new evidence, this structural unit may be interpreted as a long-evolving paleoaulacogen. The series of revealed sedimentation settings reflects the formation of a consolidated continental lithosphere at the southern flank of the Siberian Craton.

Fragment of the Early Paleozoic (∼500 Ma) island arc in the structure of the Olkhon Terrane, Central Asian fold belt

The volcanic (basaltic, basalt andesitic, andesitic, and rhyolitic) porphyric rocks of the Tsagan-Zaba complex are studied in the Olkhon composite terrane of the Central Asian foldbelt. The concordant U-Pb (SHRIMP-II) age of single zircon grains from rhyolites (492 ± 5 Ma) may be interpreted as the period of formation of the Tsagan-Zaba complex. The volcanic rocks of this complex are characterized by clear suprasubduction geochemical features and positive ɛNd(t) values. The similar ages, compositions, and ɛNd(t) values of the studied volcanic rocks and gabbroic rocks of the Birkhin pluton allow us to combine them into a common Birkhin volcano-plutonic association, which may be considered as a fragment of a section of the mature island arc of ∼500 Ma in age. The gabbroic rocks may be interpreted as the middle part of this section, whereas the volcanic and volcanosedimentary rocks belong to its upper part. The section was disintegrated 470–460 Ma ago, when the Early Paleozoic island arc was accreted to the southern flank of the Siberian craton in the course of the oblique collision and became a part of the Olkhon composite terrane.

Microfossils from the Arymas and Debengda Formations, the Riphean of the Olenek Uplift: Age and Presumable Nature

Studied assemblages of diverse organic-walled microfossils separated from the Arymas and Debengda formations of the Olenek Uplift include several paleobiological groups of microorganisms. Sufficiently large morphotypes of the first group are identified with remains of cyanobacteria. Morphotypes of variable spiral structure, which dwelt in association or in symbiosis with cyanobionts, are attributed to the same bacterial community. The other group includes a series of different acritarch genera whose characters suggest their affinity with green algae of the order Desmidiales. It is very likely that this group coexisted on siliciclastic shoals with large ancestral forms of the present-day brown algae. Several microfossil taxa have been known before from the Neoproterozoic deposits only. With due regard for the relatively gradual accumulation of sedimentary succession lacking large hiatuses and for the regular series of K-Ar dates characterizing three Riphean formations of the Olenek Uplift, it is possible to suggest that there was the Arymas-Debengda-Khaipakh cycle of long-lasted, almost uninterrupted sedimentation within the time span of 1250–900 Ma. It is also admissible that age ranges of some Late Precambrian microfossils are much larger than their distribution intervals postulated formerly.

Age and sources of late precambrian sedimentary sequences of the Southern Baikal Region: Results of the U-Pb LA-ICP-MS dating of detrital zircons

The first data on the age of detrital zircons are given for Late Precambrian terrigenous rocks of the Baikal Group and Ushakovka Formation of the southern flank of the Siberian Craton. The ages obtained for 348 zircons cover the Paleoarchean to Late Ediacaran period, demonstrate the dynamics of change of sources of the clastic material in the sedimentation basin, and mark the changes of the Late Precambrian tectonic regimes. The age of the youngest group of detrital zircons extracted from the rocks of the Kachergat Formation allows us to restrict the upper age limit of accumulation of the rocks of the Baikal Group to the Late Ediacaran (Late Vendian).

A period of global uncertainty (Blank Spot) in the Precambrian history of the southern Siberian Craton and the problem of the transproterozoic supercontinent

Two peaks of tectonomagmatic activity (1.90‐1.85 and 0.78‐0.74 Ga ago) are noted in the Precambrian history of the southern Siberian Craton. The first peak corresponds to the formation of the Siberian Craton [1]; the second peak, to rifting processes in the marginal zone of the craton at the stage of breakdown of Rodinia [2, 3], which completed by opening of the Paleoasian ocean. The time span between these peaks remains a blank spot in the geological history of the southern Siberian Craton. According to traditional views (see review in [4]), this period was characterized by continuous sedimentation at the craton margin (Fig. 1) with sporadic outbursts of volcanic activity recorded in sections of the Chaya, Medvezh’ya, and Khota formations and emplacement of subvolcanic intrusions (Chaya, Angaul, and Nersa dike complexes). However, the age of volcanic rocks and dike swarms, which could indirectly constrain the age of sedimentary rocks, remained indefinite. To fill this gap, we performed geochronological study of volcanic and dike complexes presumably corresponding [5, 6] to the aforementioned period of uncertainty (~1.9‐0.7 Ga). The results obtained would seemingly attract interest only for specifying regional stratigraphic schemes and chronology of igneous rocks. However, the new geochronological data taken together and their implications for chronostratigraphy of sedimentary sequences in the southern Siberian Craton yielded a surprisingly impressive result. At the very beginning, we would like to point out specially that areas insufficiently studied in terms of geochronology, e.g., the Baikal‐Patom Highland, have been omitted from our consideration. The Baikal and marginal uplifts of the Siberian Craton basement were the key targets of this study. The