T. V. Romanyuk

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).

First results of U-Pb dating of detrital zircons from basal horizons of Uralides (Polar Urals)

The most modern and the most selfconsistentideas about the structure of the Uralian thrustfold beltare reported in [1]. This book develops insights byN.P. Kheraskov [2] who believed that there are two agegroups whose units are widespread in the Urals; theyare the Uralides and the ProtoUralides (below, PreUralides).

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 isotope dating (LA-ICP-MS) of detrital zircons from sandstones of the low Cambrian Brusov formation of the Southeastern White Sea region: A constraint for the lower age limit of the beginning of the Arctida-Baltica collision

The basement of the peripheral structures of the northeastern part of the East European Craton (EEC) is formed of relicts of the folded zone (Fig. 1) that is called the Timanides orogen [1] or Pre Uralides– Timanides orogen [2, 3] in the published literature. The total age interval of the crystalline rock complexes in the structure of relicts of the Pre Uralides Timanides orogen (over 250 isotope dating results) is constrained by approximately 510–750 Ma [4, 5]. The orogen relicts are overlain by Upper Cambrian and Lower Ordovician clastical strata composed mainly of products of erosion of Pre Uralides–Timanides crys talline rock complexes (Fig. 2, plot 10) [5]. The age of clastical strata overlying the relicts of the Pre Uralides–Timanides orogen reliably constrains the upper time limit of orogen existence.

Testing the Models of Late Vendian Evolution of the Northeastern Periphery of the East European Craton Based on the First U/Pb Dating of Detrital Zircons from Upper Vendian Sandstones of Southeastern White Sea Region

1073 The basement of the Northeastern Periphery of the East European Craton (EEC) is composed of relics of the fold belt called the “Timan orogen” [1] or “Pre Uralides Timanides orogen” [2] in publications of the past 20 years. The age interval of crystalline complexes of the orogen is estimated at 510–750 Ma [3]. The tec tonic nature of the orogen is still disputable: it is inter preted as either an accretionary structure, which evolved in multiple stages at the active Timan margin of Baltica (ancient carcass of the EEC) in the second half of the Neoproterozoic [1], or a collisional unit formed at the Precambrian–Cambrian boundary as a result of collision between the Bol’shezemel’skaya active margin of Arctida and the Timan passive margin of Baltica [2]. Based on the results of dating for detrital zircons (dZr) from sandstones of the Upper Vendian Tamitsa Subformation, Southeastern White Sea Region (SWSR), we will test these versions of the tec tonic origin.

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.

The First Results of Study of Hydrocarbon Biomarkers and Hydrocarbons of a Diamond-like Structure in the Riphean, Vendian, and Lower Cambrian Rocks of the Katanga Saddle

The results of geochemical study of samples from Riphean–Lower Paleozoic rocks enriched in organic matter (the rocks most likely parental for oil) from the southern part of the Siberian Platform are reported.