N. D. Sergeeva

Zirconology of Navysh volcanic rocks of the Ai Suite and the problem of the age of the Lower Riphean boundary in the Southern Urals

185 Volcanic rocks of the Navysh Complex occur in the base of the Lower Riphean Ai Formation of the Burzyan Series within the Bashkirian meganticlinorium (South ern Urals). They represent practically the only object suitable by the material–geological and geochronolog ical criteria for estimation of the age of the lower bound ary of not only the Ai Formation, but the whole strato typical Riphean section of the Southern Urals. Trachybasalts prevail in the composition of Navysh volcanic rocks; vein and subvolcanic bodies are observed as well. The peculiarities of structure, com position, metasomatic and dynamic alterations of vol canogenic and sedimentary rocks of the Burzyan Series are considered in detail in [1–3]. Complex application of K–Ar, Rb–Sr, and U–Pb methods demonstrated that the age of volcanic rocks of the Navysh Complex is 1615 ± 45 Ma; the age of their transformations resulting from bostonization is 630 ± 60 Ma. Furthermore, the mineralogical and geochemical properties of zircons, the presence of dif ferent inclusions and melt inclusions among them, the existence of successive generations, the participation of corrosion and crushing, the redistribution of impu rities, and the influence of metamict nature were described in detail in [2]. In addition to significant variations of the age isotope ratios for most zircons (from 670 to 1490 Ma), fractions with concordant ages of 1350 ± 15 Ma were distinguished. For many years this dating of Navysh volcanic rocks was used for substantiation of the lower age boundary of the Riphean in different stratigraphic schemes including the stratigraphic scheme of the Russian Precambrian [4] and the scale of geological time suggested by W.B. Harland et al. [5] at the level of 1650 Ma. Development of modern methods of isotope investigations provided the possibility to specify the age “content” of the dating obtained and to estimate its agreement with the geological information on the age boundaries of Riphean sections of the Southern Urals more precisely. Zircons from the new samples of volcanic rocks of the Navysh Complex were studied by the SHRIMP methodology [6]. The results were dis couraging. In two samples from the Ushat Brook (Fig. 1), zircons from basaltoids, which were included in the Navysh volcanogenic complex by all previous researchers, were dated as 441.8 ± 8.2 and 437 ± 11 Ma; this level was obtained in other areas of the Bashkirian meganticlinorium [7]. Ages of zircons from 11 samples ranged within 1350–1400 Ma provid ing evidence for the Mashak (Middle Riphean) level of their formation. This result not only complicated the task, but contradicted the entire amount of accumu lated knowledge as well. The studies were continued due to the newly obtained data of the structure of crys tals reflecting their polygenetic nature. This conclu sion was supported by both the structural and age peculiarities of crystals (Fig. 2) extracted from fine grained porphyric andesite occurring on the left bank of the Malyi Navysh River at the base of the NW slope of the Efremov Mountains (Sample 1392, Fig. 1).

New paleomagnetic data from Late Neoproterozoic sedimentary successions in Southern Urals, Russia: implications for the Late Neoproterozoic paleogeography of the Iapetan realm

We present the results of paleomagnetic study of Ediacaran sedimentary successions from the Southern Urals. The analysis of the sedimentary rocks of the Krivaya Luka, Kurgashlya and Bakeevo Formations reveal stable mid-temperature and high-temperature remanence components. Mid-temperature components were acquired during Devonian (Bakeevo Formation) and Late Carboniferous–Early Permian remagnetization events. The high-temperature components in Kurgashlya and Bakeevo Formations are interpreted to be primary, because they are supported by a positive conglomerate test (Bakeevo Formation) and magnetostratigraphic pattern (Kurgashlya Formation). The high-temperature component in the Krivaya Luka Formation is interpreted to be a Late Ediacaran overprint. Our new paleomagnetic poles together with some previously published Ediacaran poles from Baltica and Laurentia are used herein to produce a series of paleogeographic reconstructions of the opening of the Iapetus Ocean.

A Devonian >2000-km-long dolerite dyke swarm-belt and associated basalts along the Urals-Novozemelian fold-belt: part of an East-European (Baltica) LIP tracing the Tuzo Superswell

Abstract Two dolerite dyke swarms are recognized along and paralleling the Ural Mountains, Russia. The Uralian swarm is 1400-km long (2300-km long if traced from its inferred plume centre). Further north, the Pay-Khoy swarm can be traced through the Pay-Khoy–Novaya Zemlya fold belt for a distance of c. 250 km (800-km long if traced from its inferred plume centre). An Upper Devonian age for volcanism associated with the Pay-Khoy swarm is well constrained by isotopic data. A Devonian age for the Uralian swarm was until now supported mainly by broad geological field relationships between the dykes and host rocks, i.e. dykes locally cut Proterozoic, Ordovician to Devonian sedimentary rocks, but never Carboniferous sequences. Rare isotopic age determinations also support an Upper Devonian age for the weakly altered dykes. Herein, a new precise U–Pb baddeleyite age determination of 377.2 ± 0.9 Ma is reported from a large (>50-m wide) gabbro–dolerite dyke cutting the uppermost Proterozoic in the Middle Urals. This result confirms a predicted Upper Devonian age for the dyke and gives additional support to the Devonian age of the Uralian swarm. Given this precise U–Pb age, the Uralian swarm is correlated with the Pay-Khoy swarm, Devonian basalts which are widely distributed across the East European (Baltica) Craton, as well as alkaline magmatic rocks and kimberlites of more restricted distribution. Collectively, this Devonian magmatism can be combined into a Kola–Dnieper Large Igneous Province (LIP) with two proposed mantle plume centres: (1) a southern centre at the intersection of the Dnieper-Donets aulacogen and Uralian swarm and (2) a northern centre in the Barents Sea, at the convergence between the Pay-Khoy dyke swarm and an unnamed swarm extending from the northern coast of the Kola Peninsula, as well as converging rift/graben zones. Furthermore, this Kola–Dnieper LIP of the East European (Baltica) Craton, with its two plume centres, is approximately coeval with the Yakutsk–Vilyui LIP and its plume centre on the eastern side of the Siberian Craton. These two LIPs were in close proximity at that time and their three plumes may represent a superplume derived from an active part of a single deep mantle LLSVP (Large Low Shear Wave Velocity Province), the so-called Tuzo superswell.

Archean metabasic rocks at the base of the Riphean of the Bashkirian Meganticlinorium (Southern Urals)

835 The Ai Formation at the base of the Lower Riphean in the Bashkirian Meganticlinorium (Southern Urals) overlies on metamorphic rocks of the Taratash Archean–Early Proterozoic Complex with erosional and angular unconformity, and, in some places, with the weathering crust at the base, which was observed in boreholes and excavations and mainline pits [1–3]. Along the eastern wing of the Taratash anticline, the contact between the Ai Formation and the Taratash Complex is complicated by dislocations, which are indicated by greenschist mylonites. The contact zone includes gabbro dolerite dykes intruding the Ai For mation. In some cases, the age of these intrusive bod ies has been established and, as a rule, they are younger than the Ai Formation. Transgressive contact between the Ai Formation and underlying rocks of the Taratash Complex was not previously observed directly in out crops or new excavations [3]. We described the contact on the left bank of the Misaelga River (55°35′10.5′′ N, 059°44′44.4′′ E), 8 km to the NE from the Arshinka Village (Fig. 1). The following rocks are revealed in the road excavation along the gas pipeline (outcrop 3664) from the east to the west (from the bottom) (Fig. 2).

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.

Zirconology of pyroxenites from the Kiryabinka pyroxenite-gabbro complex (Southern Urals)

In this study we discuss the problem of dating the Kiryabinka complex. The data collected on zircons from pyroxenites of the Kiryabinka polyphase pyroxenite-gabbro complex can help address a number of controversial issues regarding the Precambrian geology of the Southern Urals. First, the age of the complex (T = 680 ± 3.4 Ma) can be assigned within the late Riphean (RF4, Arshinian) or the middle Neoproterozoic (Cryogenian). The available zircon dates from gabbroic and granitoid rocks in the western flank of the Southern Urals (Berdyaush, Akhmer, and Barangul massifs) are supplemented with a new age of ultramafic rocks, the differentiates of a basaltic magma, which further corroborate the conclusion about the Upper Riphean age of the country rocks.

Uranium-lead age of zircons from granites and the substrate of the Mazara massif (Southern Urals)

The U–Pb dating of zircons from the Mazara granite massif (Southern Urals) by means of the SHRIMP II technique resulted in the conclusion that the zircons were constituted of two key generations belonging, respectively, to the substrate and to the intrusion proper. The age of the granite substrate presented by zircon cores is estimated at 1527–1548 Ma; the final stage of the substrate evolution is characterized by a concordant date of 1388 ± 16 Ma. The latter estimate is much closer to the Mesoproterosoic stage of magmatism. The information on the formation of granites of the Mazara intrusion is mainly contained in the outer parts of the crystals, with two stages distinguished within the formation history. The early and final stages are determined by the datings of 746.6 ± 24.3 and 709.1 ± 5.2 Ma, respectively (both estimates conform to the Neoproterozoic).

The Riphean Arsha Group of the South Urals: A Problem of the Geodynamic Origin of Rock Associations

The geodynamic origin of volcanic and sedimentary rocks of the Upper Precambrian Arsha Group of the western slope of the southern Urals is considered. It is shown that both rock types are riftogenic/plume in origin. The problem of the spatial and temporal correlation of the Arsha and Karatau groups requires further study.

Polychronous Zirconology of Navysh Volcanics of the Ai Formation (Southern Urals)

In order to resolve the age of Navysh volcanics (NV), which is usually attributed to the Lower Riphean of the Ai Formation, we have used geochronological, petrologic, and mineralogical methods of zirconology, apart from the SHRIMP isotopic data of single zircon grains. Moreover, TIMS isotope age analyses have been conducted, the results of which can be regarded as both controlling and providing the most correct information. The TIMS and SHRIMP data make it possible to suggest a polychronous character of the NV, which include not only Riphean, but also Paleozoic groups of volcanics. In this situation, an assessment of the scales of such polychroneity of NV and, correspondingly, of the Ai Formation as a whole becomes urgent.

Mineralogy, U–Pb (TIMS, SHRIMP) age, and rare-earth elements in zircons from granites of the Mazara Massif, South Urals

The paper reports the results of mineralogical, geochemical, and geochronological (TIMS and SHRIMP) study of heterogeneous zircons from granites of the Mazara Massif, South Urals. Obtained data revealed the Mesoproterozoic age (1550–1390 Ma) of a granite protolith and the Neoproterozoic age of their formation (745–710 Ma). In the La–Sm/La diagram, the zircons of the massif occupy an intermediate position between the fields of magmatic and metasomatic (hydrothermal) zircons. This “intermediate” field is proposed to ascribe to the late magmatic zircons, which provides more reliable characterization of zircon formation throughout the entire crystallization history of a granite melt, up to the appearance of genetically metamict metasomatic hydrozircons.