U-Pb (SIMS SHRIMP-II) age of volcanic rocks from the Tulukuev caldera (Streltsov Uranium-Ore Cluster,Eastern Transbaikalia)

The precision dating (U-Pb local by zircons, SHRIMP-II) of volcanic rocks in the unique uranium-bearing structure of Transbaikalia is performed for the first time. The basic conclusions are as follows. The volcanic activity in the Tulukuev caldera covers the period of not less than 30–35 mln years, within the period from (not later than) 162 to 128 mln years. Two stages of caldera evolution are established: the early (trachydacite-basalt) stage up to 154 mln years and the late (trachybasalt-rhyolite) stage from 142 to 128 mln years, with a 10 mln year break, which caused the deep erosion of the lower layer. Three phases of rhyolite magmatism are substantiated. The first one, 142 mln years, is the ejection of ignimbrites (microfelsitic rhyolites); the second one, 137–135 mln years, is the outflow of lavas of sanidine-morion rhyolites and subvolcanic and ring dyke intrusions. The third phase, 128 mln years, is connected with the occurrence of cesium-bearing perlites in the southwestern part of the caldera. The age of the granite-porphyries of the Krasnokamensk stock almost coincides with the precision data of the age of the uranium ores [4]. It is found that zircons from the granite-porphyries within the ore field of the Argunsk deposit have an anomalously high content of uranium. This fact can additionally testify to the time-and-spatial closeness of magmatism and processes of ore formation.     

Paleoproterozoic basaltoids in the North Baikal volcanoplutonic belt of the Siberian craton: age and petrogenesis

The oldest igneous rocks in the Paleoproterozoic (~1.88–1.85 Ga) North Baikal postcollisional volcanoplutonic belt of the Siberian craton are the basaltoids of the Malaya Kosa Formation (Akitkan Group). The youngest are the composite (dolerite–rhyolite) and doleritic dikes cutting the granitoids of the Irel’ complex and the felsic volcanic rocks of the Khibelen Formation (Akitkan Group). The position of Malaya Kosa basaltoids in the Akitkan Group section and published geochronological data on the felsic volcanic rocks overlying Malaya Kosa rocks suggest that their age is ~1878 Ma. The rhyolites from the center of a composite dike were dated by the U–Pb zircon method at 1844 ± 11 Ma, and the dolerites in the dikes are assumed to be coeval with them. Malaya Kosa basaltoids correspond to high-Mg tholeiites and calc-alkaline andesites, whereas the dolerites in the dikes correspond to high-Fe tholeiites. Geochemically, these basaltoids and dolerites are both similar and different. As compared with the dolerites, the basaltoids are poorer in TiO₂ (an average of 0.89 vs. 1.94 wt.%), Fe₂O₃¹ (9.54 vs. 14.71 wt.%), and P₂O₅ (0.25 vs. 0.41 wt.%). However, these rocks are both poor in Nb but rich in Th and LREE, ε_Nd(T) being negative. According to petrographic and geochemical data, they derived from compositionally different sources. It is assumed that the basaltoids originated from subduction-enriched lithospheric mantle, whereas the dolerites originated from refractory lithospheric mantle metasomatized by subduction fluids. The isotopic and geochemical features of mafic rocks in the North Baikal belt are well explained by their formation during crustal extension which followed subduction and collision in the region. The early stages of postcollisional extension evidenced the melting of subduction-enriched lithospheric mantle with the formation of parent melts for Malaya Kosa basaltoids. At the final stages of the formation of the North Baikal belt, during the maximum crustal extension, Fe-enriched melts rose to the surface and generated the dolerites of the dikes.     

U–Pb ages of detrital zircons from Paleozoic metasandstones of the Gelnica Terrane (Southern Gemeric Unit, Western Carpathians, Slovakia): evidence for Avalonian–Amazonian provenance

U–Pb (SHRIMP) determinations on detrital zircons from the Early Paleozoic Gelnica Terrane metasandstones and their Permian overlap sediments of the Inner Western Carpathian Southern Gemeric Unit define five age populations based on age-probability plots. The metasandstones were sampled for detrital zircons from six stratigraphic levels, four of them in the Late Cambrian/Ordovician Gelnica Terrane metasandstones and the two in Permian envelope sequence. The data set includes 84 U–Pb ages for individual detrital zircons. These ages are combined with the previously dated inherited zircons from the associated metavolcanites (n?=?31). The majority of the pre-Permian detrital and inherited zircons (95%) belong to the three main populations: population A—the Paleoproterozoic/Neoarchean ages ranging from 1.75 to 2.6?Ga; population B—the Mesoproterozoic ages with the range of 0.9 to 1.1?Ga; population C—the Neoproterozoic ages, ranging from 560 to 807?Ma. The detrital zircon age spectrum from the basal Permian sediments reflects the strong recycling from the underlying Gelnica Terrane, with the presence of the dominant Precambrian C and B populations (94% of total), including the minor populations A. The range of the detrital zircon ages from the Late Permian sandstones is wider, with additional population D, ranging from 497 to 450?Ma and population E with a time span from 369 to 301?Ma. Within the Late Permian detrital zircon assemblage, the Proterozoic population A?+?B?+?C form only 25% of total. The detrital zircon data suggest that the Gelnica Terrane belongs to the peri-Gondwanan terrane with a source area located on the northwestern margin of Gondwana close to Amazonia. This terrane should have travelled a long distance in the Phanerozoic times.     

Deep-seated pegmatites of the Emiytas mafic-ultramafic complex on Big Lyakhov Island,New Siberian Islands,and their age: <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar and SHRIMP data

Pegmatites of the Emiytas basic-ultrabasic metamorphic complex have a granodiorite-granite composition. Their genetic relations with the host amphibolites follow from the low initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7028 and from the P-T conditions (650°C and 10 kbar), which are close to those of the host rocks. Amphibole, biotite, and muscovite megacrysts analyzed by the ⁴⁰Ar/³⁹Ar method yielded plateau ages of 209.7 ± 0.9, 203.0 ± 0.9, and 178.5 ± 1.5 Ma, respectively. The former two minerals contain excess Ar, whereas the K-Ar system of the muscovite is undisturbed. The cooling of the complex to the closure temperature of this system was likely controlled by its exhumation to a shallower depth level. Zircons from the Emiytas pegmatites occur as polyhedral equant crystals with weakly contrasting sectorial zoning, very low concentrations of U (4–8 ppm in the enriched domains), and low Th/U ratios (0.002–0.003), which suggest that the mineral crystallized at significant depth in the presence of fluid. Tentative SHRIMP II measurements (five analyses) yielded a zircon age of 202 ± 17 Ma. The applying a specialized approach to the analysis of young low-U zircons on an ion probe is discussed. In spite of the small number of analyses, new geochronologic data leave no doubt that the crystallization age of the pegmatites is Late Triassic-Early Jurassic and invalidate earlier ideas that the Emiytas complex is Precambrian-Early Paleozoic. This conclusion makes the Emiytas amphibolites to be one of the various oceanic and suprasubduction complexes related to the Mesozoic South Anyui suture, which is important for reconstructions of the tectonic evolution of the East Siberian Arctic shelf.     

Zircon geochronology of the Mashak volcanic rocks and the problem of the age of the lower-middle Riphean boundary (Southern Urals)

In the type sections of the Riphean within the Bashkirian mega-anticlinorium (Southern Urals), the Mashak Formation represents a basal unit of the Middle Riphean erathem. The formation comprises throughout its area of distribution the alternation of volcanic, volcano-sedimentary, and sedimentary sequences and is divided into the lower, middle, and upper subformations. The volcanic rocks containing zircons (four samples, rhyodacite and rhyolite collected at Mashak, Berezyak, and Bolshoi Shatak ranges) are largely confined to the lower subformation. Analyses were performed using a SHRIMP II methodology, with special attention to the mineralogical characteristics of zircons, including their habit, morphology, preservation, and inclusions. All zircons show similarities in their mineral chemistry and geochemistry, which are indicative of the geochemical affinity of the volcanic rocks. At the same time, all zircon grains are characterized by specific typological parameters, which may equally reflect the parameters involved in the development of such volcanic rocks under different conditions. The integrated U-Pb age of zircons (SHRIMP II, VSEGEI, St. Petersbrug) from the four samples is 1383 ± 3 Ma. On the basis of the age of the Berdyaush gabbro-granitoid intrusion (up to 1410 Ma), the most likely age of this boundary is 1400 Ma, which is equated to the Calymmian and Ectasian of the International Stratigraphic Scale.     

Unusually high thermal stability of the Uranium-Xenon isotopic system in nonmetamict zircons

Doklady Earth Sciences -     

The Late Paleozoic geodynamics of the West Transbaikalian segment of the Central Asian fold belt

New data testifying to Late Paleozoic tectonometamorphic processes at the West Transbaikalian segment of the Central Asian Fold Belt have been obtained. Zircon dating (SHRIMP-II) of highly metamorphosed rocks showed that the processes took place at 295.3 ± 1.6 Ma. Based on these data, the Late Paleozoic ages of granitoids of the Angara–Vitim areal pluton (340–280 Ma) and some dike complexes in Transbaikalia (300–280 Ma), and the Late Paleozoic age of some carbonate-terrigenous strata dated earlier to the Early Paleozoic, we have substantiated the significant role of Hercynian tectogenesis in the consolidation of the regional continental crust. We have also shown that the Late Paleozoic endogenous events and accompanying sedimentation processes were related to the geodynamic conditions governed by the changing parameters of the subsidence of the Mongol-Okhotsk oceanic subduction slab beneath the Siberian continent. Changes in the slope and rate of the slab subsidence resulted in A-subduction conditions in the distal part of the suprasubduction plate, which led to the formation of accretion-collisional orogen and the Angara–Vitim areal pluton.