E. N. Gorozhanina

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).

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�

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.

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.

First results of U–Pb dating of detrital zircons from the Ordovician clastic sequences of the Sol-Iletsk Block, East European Platform

The first LA–ICP–MS U–Pb isotopic ages of detrital zircons from the Ordovician sandstones of the Sol–Iletsk Block (well 2–Ordovician), located at junction of the East European Platform with the Pre-Caspian Basin and the Pre-Uralian foredeep, are presented. Two detrital zircons with well-defined ages of 561 ± 4 and 570 ± 5 Ma were found in sample K15–501. They confirm the Ordovician age of the sandstones, which earlier had been defined on the basis of seismic–stratigraphic and lithological correlations. The age distribution of the detrital zircons indicates the significant role of Late Precambrian rocks as provenance sources. However, those rocks still remain unknown in the Early Precambrian basement of the Volga–Ural part of the EEP.

The Navysh graben-rift of the South Urals as a fragment of the Early Proterozoic aulacogen

1052 The South Urals represents a southern segment of the Late Paleozoic folded structure that was formed by collision between the eastern margin of Laurussia and the Kazakhstan continent. During this process, Upper Precambrian sequences that accumulated at the aula cogen stage in development of the East European Platform along its eastern (Uralian) periphery were involved in fold–thrust deformations of the foreland. These Upper Precambrian sequences are rested upon the crystalline Taratash Complex, which is composed of the oldest rocks in the South Urals being dated back to 1780–2700 Ma (or even 3.5 Ga, according to some estimates [1]). The rocks of the complex crop out in the northern axial zone of the Bashkir megaanticlino rium, where they are considered as forming an inlier of the Volga–Urals segment of the East European Plat form basement. The sedimentary and volcanic rocks of the Navysh Subformation of the Ai Formation con stitute the oldest unmetamorphosed unit in the Bash kir megaanticlinorium. They overlie the erosional sur face and unconformity crystalline rocks of the Taratash Complex. The transgressive contact between the Navysh Subformation and the Taratash Complex is established in mine workings and well sections [2]. The contact is complicated by tectonic fractures marked by greenschist milonites and gabbro–dolerite dikes. In the outcrops, the transgressive contact between the Ai Formation and the underlying Taratash Complex was never observed in this region [3]. It was first described on the left side of the Misaelga River 8 km northeast of the village of Arshinka. This section was proposed to serve as a reference section for the limitotype (stratotype) of the Riphean lower boundary in the South Urals [4]. The boundary is marked by Ai volcanics, which have been dated by the U–Pb method (SIMS SHRIMP II) at 1752 ± 11 Ma [5]. Detrital zircons extracted from quartz sandstones of the Ai Formation sampled near its transgressive con tact with the Taratash Complex (55°32′34.74′′ N, 59°41′54.84′′ E) yielded ages of 3625 ± 53 to 1891 ± 23 Ma (U–Pb dating by the LA ICP MS method) [6]. These estimates are consistent with the Early Riphean age of the Navysh Subformation.

First results of U/Pb dating of detrital zircons from middle Riphean sandstones of the Zigalga Formation, South Urals

The results of U/Pb dating of detrital zircons from sandstones of the Zigalga Formation, which is the base level of the Middle Yurmatu Group of the Bashkir uplift, southwestern Urals, are presented. The U/Pb ages of detrital zircons from sandstones of the Lower, Middle, and Upper Riphean are compared.

First U-Pb age of detrital zircons from sandstones of the Upper Emsian Takaty formation of the Western Urals with regard to the problem of primary sources of the Uralian diamond placers

Several alternative points of view currently exist on the origin of the primary sources of diamonds from the Cenozoic Western Urals placers. Some researchers suppose that their economic diamond resource potential is related to diamonds from tuffisitic facies of the mantle kimberlites-lamproites or impact structures. Other researchers suggest that diamonds originated from the eroded sandstones of the Upper Emsian Takaty Formation of the Lower Devonian, which represents ancient (fossil) placers or intermediate reservoirs. It is assumed that these reservoirs collected diamonds from worn kimberlite bodies, which were located in the Urals or on the East European platform (EEP). This paper presents the first U-Pb (LA-ICP-MS) age of detrital zircons from quartz sandstones of the Takaty Formation, which spans a range from 1857.5 ± 53.8 to 3054.0 ± 48.0 Ma. The absence of detrital zircons younger than 1.86 Ga excludes that the structural complexes of the Uralian, Fennoscandian, and Sarmatian EEP parts were the provenance areas that supplied the clastic material to the sedimentary basin, which accumulated the Takaty Formation. The similar age of our zircons and ancient crystalline complexes of the Volga-Uralian EEP part allows consideration that it was a single provenance area. If we assume that the diamond resource potential of the Western Urals is completely or partly related to the ancient diamond placers from the Takaty Formation, then the intermediate diamond reservoirs from its structure originated due to redeposition of destruction products of primary diamond-bearing rocks of the Volga-Uralia area. Thus, within the Volga-Uralian part of the EEP basement, we may expect identification of a previously unknown stage of kimberlite formation, which is significantly older than that responsible for the diamond resource potential of the Arkhangel’sk province.

The paleogeodynamic conditions of redeposition of conodont elements in the late devonian-early carboniferous sediments of the Southern Urals

The peculiarities of conodont fauna redeposition of the Upper Devonian-Lower Carboniferous sediments from different structural-facial zones of the Southern Urals (the Zilair megasynclinorium on the western slope of the Southern Urals (paleocontinental sector) and the Magnitogorsk-Bogdanovsk graben in the central part of the Magnitogorsk megasynclinorium on the eastern slope (paleooceanic sector)) have been studied. The regularities of conodont fauna redeposition in the flysch foredeep and riftogenous depression (of graben) formed in the back of the volcanic arc after volcanism termination have been described.

New data on the structure of the southern part of the Cis-Uralian foredeep according to results of seismic work

New data on the structure of the Mrakovsk depression in the southern part of the Cis-Uralian trough was obtained as a result of reinterpretation of the seismic sections of regional profiles no. 10 and no. 14. This data was compared with results of recent seismic studies in the Orenburg Cis-Uralian region. In the western part of this depression, structures formed by listric type faults were established. Permian reef massifs are connected with these structures. Such structures also occur in the Orenburg Cis-Uralian region. In the central part of the depression, a structure analogous to the Perovsk‐Kornilov ridge in the Orenburg Cis-Uralian region is revealed. It is shown that the structure is limited from the west and east by strike-slip faults. The eastern part of the trough is a continuation of the front folding zone buried under the Upper Permian molasse sediments. It has a complicated structure, and it needs separate investigation. For this reason, the eastern part of the trough is not considered in this work. In the tectonic relation, the Mrakovsk depression is a part of the Cis-Uralian trough. A chain of oil deposits localized in the reef barrier massifs of SakmarianArtinskian and Asselian age occurs along the western flank scarp of the trough in the Bashkirian Cis-Urals [1]. Along the eastern flank, several oil deposits are connected with the thrusts and upthrusts formed during the formation of the fault-thrust Urals belt [2]. Beloglinskoye deposit was discovered in the central part of the Mrakovsk depression in Middle Devonian sediments. Regional seismic works along profiles no. 10 and no. 14 were performed in the Beloglinka area within the Mrakovsk depression in 1990‐1992 (Figs. 1‐3). Interpretation of these profiles was performed in Houston (United States) in accordance with an agreement of cooperation between the firm PGI and the Open JointStock Company Bashneftegeophysika. Seismic material of the profiles was interpreted according to the following programs: “pre-stack depth migration,” “lateral velocity analysis,” etc. Wave correlation was performed, and seismic‐geologic sections were constructed. Due to the existence of the interference waves, there is a multivariance of the interpretation of the seismic picture, especially with fault detection. In this work we suggest one possible interpretation. The initial stage of the Cis-Uralian trough occurred in the Late Carboniferous and was connected with the formation of the Urals folded system [2, 4‐7]. Three zones are distinguished in its structure: western, central, and eastern. The western zone includes the western flank scarp of the trough and adjacent regions of the eastern slope of the Russian platform. The central zone corresponds to the most submerged part of the Cis-Uralian trough. The eastern zone is the zone of thrusts (eastern flank of the trough and adjacent Ural foreland folds). The Cis-Uralian trough belongs to the foredeep (foreland basins) type [8] with an asymmetrical cross section. Passive and active flanks are distinguished in their structure. The active one is a fold and thrust flank. The opposite flank, which is remote from the orogen, is the passive flank. The passive flank of the foreland basin is complicated by normal faults. The reasons for the formation of the fault structure of the trough’s remote flank are discussed in the literature [9]. It is assumed that one of the reasons is connected with the influence of a load from the thrust wedge side onto the platform’s edge [10]. In the central zone, a region of uncompensated downwarping filled with deep-water sediments and subsequently filled with flysch and molasse occurs [7].