N. B. Kuznetsov

The Cambrian Baltica-Arctida Collision, Pre-Uralide-Timanide Orogen, and Its Erosion Products in the Arctic

Researchers have obtained in the last decade a considerable amount of new data on various aspects of the geological setting of high-latitude regions of the Arctic and northern regions of Russia. These data have made it possible to revise the tectonic scenario of the Late Precambrian‐Cambrian evolution of the northeastern and eastern framing of the East European Craton (EEC) [3]. Collision of two Late Precambrian continents— Baltica (Neoproterozoic skeleton of the East European Craton) and Arctida (Fig. 1)—at the Late Precambrian/Cambrian boundary is the key issue in this scenario. The Late Precambrian Arctida continent incorporated the following structures: blocks of an ancient sialic crust located now in the Arctic sector (Barents, Kara, and Novosibirsk blocks); the Arctic Alaska‐ Chukchi block; some fragments of the Innuitian foldbelt (the block of the northern parts of Peary Land and Ellesmere Island); and the Lomonosov Ridge block (Fig. 2). The blocks were separated and accreted at the Arctic periphery of northeastern Eurasia or North America owing to the origination of several differentaged spreading centers (systems) in the Late Mesozoic and Early Cenozoic. This process was accompanied by the opening of the Eurasian, Amerasian (Canadian), and other basins with the oceanic crust in the North Arctic Ocean. The multistage retrospective “closure” of oceanic basins served as a basis for the initial reconstruction of the Arctida continent [15]. In this model, Arctida existed as an autonomous large massif of the continental crust up to the Devonian and incorporated all sialic blocks mentioned above except the Barents block [15]. In later models, the Barents block was also included in Arctida. Moreover, Arctida was shown as an autonomous continent that was separated from other ancient massifs with the continental crust until the Late Precambrian/Cambrian boundary [1, 3] when the Bol’shaya Zemlya active margin of Arctida) collided with the Timan passive margin of Baltica. The collision of continents produced the new composite Arcteurope continent [1]. The pre-Uralide‐Timanide collisional orogen appeared in the Baltica‐Arctida collision zone [3]. Pre-Ordovician complexes of the northeastern and eastern framing of the East European Craton (Timanides and pre-Uralides) make up the Central Uralian Uplift and the basement of the Timan‐Pechora basin, respectively. In works of N.S. Shatsky, N.P. Kheraskov, and A.S. Perfil’ev, these complexes are subdivided into two large groups characterized by significant compositional distinctions (Fig. 3).

The first results of U/Pb dating and isotope geochemical studies of detrital zircons from the neoproterozoic sandstones of the Southern Timan (Djejim-Parma Hill)

This report presents the first results of U/Pb dating, isotope-geochemical, and geochemical studies of detrital zircons from the Neoproterozoic clastic rocks of the Southern Timan. Sixty-one zircon grains were treated, including 51 from red-colored sandstones and 10 grains from aleurosandstones of the Djejim Formation of the southern Chetlas-Djejim zone (Djejim-Parma Hill). It was found that the U/Pb-ages of zircons from the rocks of the Djejim Formation, varied from ∼2.97 to ∼1.20 Ga. The studies of microelement composition in 47 grains (of 61 U/Pb isotope ages obtained), on the basis of several empirical regularities found formerly, show that the detrital zircons had originated from “granites” (22 grains), “diorites” (12 grains), or their volcanic analogues, or more rarely, from “syenites” and “basites” (5 and 8 grains, respectively). The Lu/Hf isotope system of zircons allows one to estimate the model ages (TDMC) of the substrate magmatic rocks being parental to the zircons considered. In particular, Archean zircons are characterized by ∼2.84–3.36 Ga model ages of magmaforming rocks. For some of the grains, their model ages (∼2.84 Ga) are close to those of zircons as such (∼2.7–2.8 Ga), which points to the juvenile character of the substrate from which the parent magma of the zircons treated was fused. For Proterozoic (to Middle Riphean) zircons, the Lu/Hf isotope system allows one to estimate the model age of the substrate of their parental rocks within ∼2.00–3.36 Ga, which shows that these rocks were formed under the recycling of the Archean and Early-Proterozoic crust. The ages obtained for detrital zircons, as well as model ages of the substrate of the corresponding parental magmatic rocks, are quite comparable to the age of crystalline complexes of the ancient framework of the East European Platform (EEP), formed in the course of the Archean, Early-Proterozoic, and Early-Middle Riphean tectonomagmatic events. This permits us to conclude that the Neoproterozoic detrital complexes of the Timan were formed owing to the erosion of earlier Neoproterozoic and Early Precambrian complexes constituting the Neoproterozoic Baltica continent, presenting complexes of the passive margin of this continent. A variety of ages of detrital zircons from sandstones and aleurosandstones from the Djejim Formation of Djejim-Parma Hill, and of the estimates of magmatic rocks parental to these zircons, may be characterized as a Baltic Provenance signal.

The First U-Pb (LA-ICP-MS) isotope data of detrital zircons from the basal levels of the Riphean stratotype

Attempts at dating detrital zircons from Late Pre� cambrian terrigenous rocks of the Western Urals have already been made. In (1-3), the age data of detrital zircons from Upper Middle-Upper Riphean and Lower and Upper Vendian (Upper Vendian-Middle Cambrian) deposits were summarized. This work pre� sents the first results of U-Pb LAICPMS isotope dating of detrital zircons, extracted from sandstones of the basal horizons of the Riphean stratotype sequence of the Southern Urals (Ai Formation, Burzyan Group of the Bashkir Meganticlinorium). The Bashkir meganticlinorium, a major structural unit of the structure of the Uralian foldthrust belt is located in the western segment of the Southern Urals (Fig. 1a). The Bashkir Meganticlinorium is composed of unevenly dislocated Paleozoic rocks. Within the Meganticlinorium, PreOrdovician complexes, divided distinctly into two units, are exposed on the surface. The lower horizons of these complexes are represented by Early Precambrian granitoides, amphibolites, and gneisses with relics of granulite� facies rocks (4, 5). This rock assemblage is referred to the Taratash metamorphic complex. In the northeast� ern part of the Bashkir Meganticlinorium, the Taratash complex composes the core of the Taratash Anticline. The rocks of the Taratash complex are over� lapped with stratigraphic and structural unconformity by a complex of terrigenous, terrigenouscarbonate, carbonate, and rarely volcanogenic and volcano� genic-sedimentary rocks with a total thickness of up to 12-15 km (4, 6, 7). The basal part of the complex is represented by conglomerates. The large lower part of this complex is the typical Riphean Erathem sequence (4, 6-8). The upper part is represented by the stratum of alternating aleurolites, sandstones, and rare arkose, and polymictic conglomerates (up to 1500 m thick) are referred as Upper Vendian (4, 7) or Upper Ven� dian-Middle Cambrian Asha (9).

The first finds of skeletal fossils in the Kuk-Karauk formation of the Asha Group (Southern Urals) and their significance for determining the beginning of the Pre-Uralian-Timanian orogeny

The Bashkirian Uplift (BU) is the largest element of the southern part of the zone of the Central Ural uplift, composed predominantly of Precambrian com� plexes. The Bashkirian Uplift extends for about 330 km at a width of up to 100 km (Fig. 1). In the north, west, and south, the Bashkirian Uplift is bor� dered by Paleozoic strata of the Western Uralian Megazone. In the east the BU is bordered along the Zyuratkul fault by irregularly dislocated and metamorphosed Paleoproterozoic and Precambrian complexes of the Uraltau uplift. The Precambrian formations of the BU are charac�

First Results of Isotopic Dating of Detrital Zircons from the Clastic Rocks of the Pre-Uralides-Timanides Complexes: Contribution in the Late Precambrian Stratigraphy of the Enganepe Uplift, Western Polar Urals

The upper horizons of the Earth’s crust in the eastern and northeastern framing of the East European platform consist of two units. The upper unit is made up of Late Precambrian and younger (mainly sedimentary) complexes; the lower unit consists of Late Precambrian complexes termed in total as Pre-Uralides‐Timanides [1, 2]. The Pre-Uralides‐Timanides form uplifts and anticlinoriums extending as an uninterrupted chain along the western slope of the Urals. They are also exposed at Timan and the Kanin Nos peninsula. Age analogs of the Pre-Uralides‐Timanides complexes occur as fragments on Kildin Island, in the Rybachii, Srednii, and Varanger peninsulas, on Paikhoi (Amderma region) and Vaigach Island, and in the southern part of the Novaya Zemlya archipelago. These complexes were recovered by several boreholes in the Pechora plate and, according to geophysical data, are traceable in the Barents Sea shelf [2, 3].

Geochemical and Lu-Hf (LA-ICP-MS) systematic of detrital zircons from lower neoproterozoic Lemeza Sandstones, Southern Urals

1200 In our previous communication [1], we presented geochronological data on detrital zircons (dZr) from sandstones of the Lemeza Subformation of the Karat� avian Zil’merdak Formation (standard of the Upper Riphean in northern Eurasia) developed in the Bash� kir Anticlinorium of the Southern Urals that borders the East European Platform (Fig. 1). From a total of 79 analyses, 66 qualified measurements yielded 206 Pb/ 207 Pb ages ranging from 3070 ± 27 to 1817 ± 59 Ma (Fig. 2a). The occurrence of two distinct differ�

Structural Patterns of Pre-Uralides in the Enganepe Uplift (Polar Urals) As a Reflection of the Cambrian Collision of Baltica and Arctida

The East European Platform (EEP) is bordered by the Ural foldbelt of Hercynides on the east and the Pechora Plate (PP) with the adjacent Barents Sea shelf in the northeast. The upper horizons of the Earth’s crust in the Urals, PP, and Barents shelf are characterized by a two-level structure. The lower stratigraphic‐structural level consists of Late Precambrian complexes (Pre-Uralides‐Timanides), whereas the upper level is composed of Postcambrian complexes. In terms of the composition and structure, PreUralide‐Timanide complexes are subdivided into two groups [4]. The first group (southwestern Pre-Uralides‐ Timanides) is mainly represented by sedimentary rocks. These rock complexes outcrop in Timan and compose a part of the adjacent PP basement. In the southeast, the complexes are traced in the Urals (Kvarkush anticlinorium and Bashkir Uplift). In the northwest, they outcrop in the northeastern Baltic Shield (Kildin Island and Rybachii, Srednii, and Varanger peninsulas) and take part in the structure of adjacent areas of the Barents Shelf. The second group (northeastern Pre-Uralides‐ Timanides) includes not only sedimentary rocks, but also volcanic and volcanosedimentary complexes, granitoids, gabbroids, and rare ophiolites. These complexes make up the northeastern part of the PP basement and outcrop within several positive structures in the northern part of the western slope of the Urals (e.g., the Enganepe Uplift). The available arguments allow correlation of Late Precambrian complexes of the Wedel Jarlsberg Land (WJL) on southwestern Spitsbergen with the Pre-Uralide‐Timanide complexes. The

Geochemical and Lu/Hf isotopic (LA-ICP-MS) signature of detrital zircons from sandstones of the basal levels of the Riphean stratotype

This paper presents the results of selective study of trace elements (29 analyses) and the Lu/Hf isotopic system (41 analyses) in preliminarily dated (U-Pb) detrital zircons (dZrs) from sandstones of the Ai Formation of the Burzyan Group of the Bashkirian Anticlinorium, which compose the basal horizons of the typical Riphean section of the Southern Urals. The statistically processed trace-element patterns of dZrs showed that “diorites” were dominant over “syenites” among the source rocks of dZrs. The rock types estimated by trace-element patterns for the cores and rims of two large grains (“diorite” and “syenite”) coincided. The analysis of the Lu/Hf isotopic system of dZrs revealed a wide dispersion of the ɛHf value from +7.1 to −20.1 at the TDMC model age of the substrate from 2.25 to 3.95 Ga. Four grains (in one case with the core and rim studied) from the population of the large transparent cherry zircons (TCZ) are characterized by the “syenitic” rock type and extremely ancient TDMC values of 3.22, 3.45, 3.64, 3.66, and 3.75 Ga at ages of zircons of 2486, 2784, 2873, 1977, and 1984 Ma, respectively. Two “dioritic” grains from the TCZ population have significantly distinct parameters: 2.37 and 2.51 Ga at 2049 and 2057 Ma, respectively. It is evident that this specific population of dZrs was formed with a significant contribution of very ancient crustal material, which became active under “syenitic” magmatism and provided the TDMC value of >3.5 Ga. Numerous juvenile dZrs form a compact cluster, which correspond to the rocks of the southern part of the Volga-Sarmatian orogen (age of 2.1–2.0 Ga, TDMC = 2.1–2.4 Ga). The complexes of the entire the Volga-Uralia, the Volga-Sarmatian orogen, and adjacent areas could be the provenance areas for the Ai sandstones in contrast to the northeastern areas of the East-European Platform with dominant “granitic” source rocks and TDMC values lower than 3.5 Ga.

The Cambrian Pre-Uralide-Timanide Orogen: Structural Evidence for Its Collisional Origin

At present, there are two alternative standpoints on the northeastern fringing of the Late Precambrian– Early Paleozoic evolution of the East European Platform: accretionary and collisional. According to the accretionary concept [1], the Timan margin of the Baltia continent (Precambrian framework of the East European Platform) successively amalgamated island arcs, microcontinents, relicts of basins underlain by the oceanic crust, and other structures. Contrary to this concept, other researchers, myself included, share the collisional hypothesis, according to which the Precambrian–Cambrian transition was marked by collision between the Late Precambrian Timan passive margin of the Baltia continent with the Bolshezemel’skaya active margin of the Arctida continent [2]. The Pechora– Ilych–Chiksha zone of faults traceable in the basement of the Pechora Plate and its continuations in the Barents Sea shelf and Kvarkush Anticlinorium in the Urals represent a relict of the collisional suture (Fig. 1). The main structural consequence of collision was the formation of the Cambrian collisional divergent orogen of pre-Uralides–Timanides, the axial part of which is marked by the suture. The southwestern limb of the orogen is largely composed of dislocated sedimentary complexes of the Late Precambrian passive margin of the Baltia continent, which were thrust over its adjacent areas, while the northeastern limb is mostly represented by complexes of the Bol’shezemel’skaya active margin of Arctida and its rear zones that were involved into large-amplitude overthrust displacements toward the latter. This communication is dedicated to structural analysis of relict complexes formerly constituting the pre-Uralides–Timanides of the Timan– Pechora–Barents Sea region and neighboring areas (Fig. 2). Relicts of the southwestern limb of the orogen. The structural paragenesis of pre-Uralide–Timanide complexes constituting the southwestern limb of the orogen has been studied in the Varanger, Srednii, and Rybachii peninsulas. Some information on the structure of the pre-Uralides–Timanides in the southwestern limb of the orogen can be derived from the structural analysis of the Kanin Kamen Range and the near-Timan part of the Pechora Plate basement based on seismic materials. In all these areas, the pre-Uralide–Timanide complexes are characterized by a distinct southwestern vergence of fold–thrust dislocations and the approximately NW– SE strike of mesostructural linear elements (hinges of different-scale asymmetrical folds, mineral and deformation-related linearity).

First results of U–Pb dating of detrital zircons from the Upper Ordovician sandstones of the Bashkir uplift (Southern Urals)

The first results of U–Pb dating of detrital zircons from Upper Ordovician sandstones of the Bashkir uplift in the Southern Urals and U–Pb isotopic ages available for detrital zircons from six stratigraphic levels of the Riphean–Paleozoic section of this region are discussed. It is established that the long (approximately 1.5 Ga) depositional history of sedimentary sequences of the Bashkir uplift includes a peculiar period lasting from the Late Vendian to the Emsian Age of the Early Devonian (0.55–0.41 Ga). This period is characterized by the following features: (1) prevalence of material from eroded Mesoproterozoic and Early Neoproterozoic crystalline complexes among clastics with ages atypical of the Volga–Urals segment of the East European Platform basement; (2) similarity of age spectra obtained for detrital zircons from different rocks of the period: Upper Vendian–Lower Cambrian lithic sandstones and Middle Ordovician substantially quartzose sandstones.