A.M. Stanevich

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.

Palaeomagnetism and U-Pb dates of the Palaeoproterozoic Akitkan Group (South Siberia) and implications for pre-Neoproterozoic tectonics

Abstract We present new geochronological and palaeomagnetic results from the late Palaeoproterozoic Akitkan Group in South Siberia. The zircon U–Pb conventional age of the rhyodacite from the upper part of the group is 1863±9 Ma and the age of the dacite from the lower part of the group is 1878±4 Ma. Palaeomagnetic study of sedimentary and some igneous rocks from the upper part of the group isolated a high-temperature characteristic component (D=193°, I=19°, k=51, α95=7°) which is supported by two of three applied conglomerate tests. However, the third intra-formational conglomerate test demonstrates a contaminating overprint of uncertain nature for a part of our collection. The analysis of data suggests that this overprint occurred at time when the geomagnetic fields direction was similar to that at the time of the deposition. Therefore the corresponding palaeomagnetic pole (22.5 °S, 97.4 °E, dp=1.5°, dm=2.8°) may be considered as representative for the deposition time. Palaeomagnetic study of the sediments in the lower part of the Akitkan Group isolated a stable primary remanence (D=189°, I=8°, k=111, α95=5°) supported by positive intra-formational conglomerate and fold tests. The palaeomagnetic pole (30.8 °S, 98.7 °E, dp =2.5°, dm=5.0°) is nearly coeval with the 1879 Ma Molson B pole from the Superior craton. We used these two poles to compare the relative position of Siberia and the Superior craton in the late Palaeoproterozoic. It is different from their reconstruction around 1000 Ma. This demonstrates their relative movements in the Mesoproterozoic.

Testing the snowball Earth hypothesis for the Ediacaran

Ediacaran Siberia was at tropical paleolatitudes when the glacigenic strata of the Goloustnaya Formation (Baikal Group, Siberia) were deposited at sea level. The presence of such deposits (at tropical latitudes) is at the core of the snowball Earth hypothesis, which is generally accepted for the previous Cryogenian glaciations. To test this hypothesis for the Ediacaran Period, we determined concentrations of platinum group elements (PGE) in the transitional unit between glacigenic conglomerates and postglacial cap carbonates of the Goloustnaya Formation. We speculate that if oceans were completely covered by ice during the glaciation, the ice prevented accumulation of PGE-rich cosmic dust and micrometeorites during that period, i.e., the snowball Earth stage. Such particles would have accumulated rapidly on the ocean floor at the ice-melting event, providing a geochemical signal; however, unlike the previous Cryogenian glaciations, this signal is at a background level, and we conclude that either the Ediacaran glaciation did not reach the snowball stage, or it was of very short duration.

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.

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

New data about structure and time of formation of the Khamar-Daban terrane: U-Pb LA-ICP-MS zircon ages

This paper provides new data on the age of detrital zircons from metaterrigenous rocks of the Khamardaban Group (Kornilovskaya and Shubutuiskaya formations) which crown the succession of the Khamar-Daban terrain. It was established that the accumulation of the protoliths of both formations occurred in the interval between the Late Riphean and Early Ordovician. In this case, there is a difference between sequences of Shubutuiskaya and Kornilovskaya formations due to a sharp change in a provenance area and depositional conditions, which is a consequence of the change in the paleogeodynamic environment. In addition, this indicates the tectonic juxtaposition of these sequences and probability of accumulation of deposits of the Shubutuiskaya Formation within Dzhida island arc system.

Mineralized biota from the neoproterozoic Uluntui Formation

First finds of microfossils from the silicified microphytolitic carbonates of the Uluntui Formation in the southwestern Baikal region are described. They are represented by remains of the cyanobacterial community that formed microphytolites. These microfossils are accompanied by acritarchs probably belonging to green algae. The silicified microfossils are sufficiently well preserved despite complete and repeated recrystallization of host primary carbonate rocks. Main phases of secondary mineralization are related to hydrothermal-metasomatic processes, which were likely provoked by collisional events on the southern flank of the Siberian Craton in the Silurian.

Rift–Related Sediments of the Passive Continental Margin of the Paleo-Asian Ocean (Baikal Segment)

The geological position, composition, and age of detrital zircons of sedimentary deposits of the Nugan Formation of the Western Baikal region underlying the Golousta Formation of the Baikal series of Ediacaran age have been studied. The formation of both stratigraphic units due to the same sources of detrital material, located within the southern flank of the Siberian Craton, has been proved. The deposits of the Nugan Formation have been demonstrated to mark the rifting stage of the formation of the passive margin of the Paleo-Asiatic Ocean: their accumulation occurred in the Late Cryogenian during the interval 720–640 Ma.