Zircons of the Permian age (280–290 Ma) from intrusive magmatic rocks in Riphean structure of the Southern Urals

Research subject. Magmatic complexes that are developed in the lower (volcanogenic-sedimentary) part of the Ai Formation of the Lower Riphean of the Taratash anticline in the Southern Urals. Results. For the frst time, the Lower Permian SHRIMP dating (288.6 ± 3.1 Ma by U-Pb method on zircon from monzogabbro) was obtained for a dike cutting the basalts of the Lower Paleozoic (420–450 Ma) age. Conclusions. On the eastern slope of the Urals there is a chain of massifs which are close in the age. The chain belongs to the Lower Permian Stepninsky monzogabbro-granosyenite-granite complex, represented by the Uiski, Vandyshevski, Biryukovski and Stepninski intrusions with the age of 281 ± 2, 280 ± 2 and 286 ± 2 Ma, correspondingly (U-Pb method after zircons, SHRIMP-II, VSEGEI) and earlier obtained dates 281 ± 4 Ma (Rb-Sr isochrone) and 283 ± 2 Ma (isotope Pb-Pb method after zircons). The Stepninsky complex was described earlier as plume-dependent. The monzogabbro dike, described in this paper, although being at a considerable distance from the Sepninsky complex, is situated at a strike of the chain of the stepninsky intrusions, is close to them by the composition and age and can be ascribed to the same plume episode. The idea of the plume character of the complex was ехрressed by us relatively long ago based on a superimposed character of the chain of the intrusions over the earlier, collisional Uralian structures. As for the geochemical character (monzogabbro) the dike conforms with one of two standard trends of the Stepninsky complex – monzonite (monzogabbro, monzodiorites, syenites).

Polychronous zircons of volcanics of the Navysh complex of the Lower Riphean Ai Formation (Southern Urals)

The volcanics of the Navysh complex of the Lower Riphean Ai Formation in the Southern Urals are well studied petrochemically and dated by several methods. In 2013 zircons from a trachybasalt porphyrite (sample 2152) gave a concordant SHRIMP date 1752±11 Ma, which was used as a fundamental for the lower boundary of the Riphean with no special arguments against it. The later attempts to repeat this date for the Navysh volcanics were not successful: the collected zircons were either more ancient (> 2500 Ma), or more young (< 500 Ma). From the beginning, the zircons with such ages were regarded as xenogenic or secondary metasomatic, or belonging to paleozoic dykes intruding the Riphean volcanics. However, the clearly expressed mineralogical properties of the Paleozoic zircons and their frequent presence in volcanics, not dykes, led to a conclusion that the zircons and Navysh volcanics, containing them, and exposed within the area of development of the Ai Formation, are polychronous. To support this conclusion, the authors studied in more detail the zircons of the Navysh trachybasalts, developed in the Ai Formation. The main conclusion, obtained from this new data, was that the volcanics attributed to the Navysh complex, form a polychronous system, including both the Lower Riphean (1750 Ma) and Paleozoic (450 Ma) rocks. The zircons of these age groups differ in their mineralogical and geochemical properties supporting the idea that they belong to different primary sources which may be due to repeating plume processes, which partly reanimated – heated and melted-rocks of the previous cycle and/or created new sources of melts.

The sources of zircons in clastic rocks of the Riphean deposits of the Urals

New age determinations of detrital zircons of sandstones augmented the possibilities of interpretation of their provenance. This interpretation is often restricted by a formal comparison of age-and-composition characteristics of detrital crystals with any very distant model objects. A different situation arises when the role of a source of a detritus is claimed by local objects. The analysis of SHRIMP and TIMS - datеs of zircons and U and Th concentrations in them, and also a comparison of histograms of primary zircons from Riphean volcanics and rocks of the Taratash complex on one hand and the detrital zircons from the sandstones of Vendian (Asha series) and Lower Riphean (Ai Formation) on the other, have shown that the age variations of sources and clastics are comparable in many aspects. It means that the age characteristics of primary zircons from the Riphean volcanics and rocks of the Taratash complex as sources of zircon clastics for the Riphean and Vendian sandstones in the Southern Urals are regulated by processes of resedimentation, though the influence of distant sources is not excluded.

The first results of U-Pb datings of detrital zircons from the basal deposits of the Upper Precambrian of the Subpolar Urals

In the north of the Timan-Ural region, in contrast to its southern regions, there are no Lower Riphean deposits. The question of the presence of Middle Riphean stratons here remains open, since their age is accepted conditionally. In the Subpolar Urals, the Puiva Formation, which lies at the base of the Upper Precambrian section of this region, belongs to the Middle Riphean. The first U-Pb results of the LA-SF-ICP-MS dating of detrital zircons from terrigenous deposits of the Puiva Formation give reason to limit the lower age interval of their formation abroad about 900 million years ago. The obtained dating of the detrital zircons of their obviously basal terrigenous sediments for the Timan-Northern Ural Upper Precambrian, taking into account already available data on the age of the Upper Precambrian strata of this region, indicate that the lower age limit of the Timan continental margin of the Baltic is close to the boundary of the Middle and Late Riphean.

The Lower Precambrian in the structure of paleozoic in the Subpolar Urals

In many polymetamorphic complexes of the Urals, rocks with Early Precambrian age marks have been established. But only with respect to the two polymetamorphic complexes located on the western slope of the Southern Urals: the Taratash and Aleksandrov, their indisputable belonging to the Archean-Paleoproterozoic section is recognized. They are framed by weakly metamorphosed Lower Riphean sediments and reliable geochronological data are obtained from them, which unambiguously indicate the Early Precambrian age of rock metamorphism. The available Early Precambrian age datings for other Ural polymetamorphic complexes (with the prevailing number of Late Precambrian and Paleozoic age values) are interpreted differently. Therefore, their attitude to the Lower Precambrian section has been disputed by many researchers. In the article, for the first time, based on the results of mass U-Pb dating of metamorphic zircons from the gneiss of the Nyrtin polymetamorphic complex of the Subpolar Urals, taking into account the available data, the Paleoproterozoic age of the earliest stage of rocks metamorphism (2127 ± 31 Ma) is substantiated. This gives grounds to assert that the complex under consideration, as well as the Taratash and Alexandrov complexes of the Southern Urals, belongs to the Lower Precambrian formations involved in the structure of Uralides.

Conditions for the formation of Fe‒Mg metasomatic carbonates in the Lower Riphean terrigenous-carbonate sediments of the Southern Urals

Fe–Mg carbonate metasomatites in the limestones of the Suran suite of the Lower Riphean in the Avzyansky ore district of the Bashkir meganticlinorium are represented by large deposits of Fe-magnesite (Ismakaevo deposit) and breinerite stocks (Bogryashka deposit). The metasomatic zonality is represented by a series of limestone — dolomite — Fe-magnesite (breinerite). Ferrous magnesite contains up to 8 mol. % FeСO3. In breinerite iron saturation varies from 10 to 45 mol. % FeСO3. The metasomatic fluid was a brine of Ca, Na, Mg chlorides with an impurity of Fe, and was connected with the remobilization processes of evaporite brines buried in sediments of the Lower Riphean. The salinity and homogenization temperature of fluid inclusions in magnesites are in the range of 20–26% eq. NaCl and 200–240°C, and in breinerites — 10–15% eq. NaCl and 140–190°C, respectively. The interaction of fluid with terrigenous rocks in the tectonically active zone of the Mashak riftogenic graben led to the enrichment of various ligands and more active hydrothermal redeposition in the metasomatic products of medium and heavy lanthanides. During the migration of fluid through the limestone unite in the cooling process, Fe-magnesite of the Ismakaevo deposit was formed in the frontal zone, and the breinerite of Bogryashka deposit — in the rear zone adjacent to the shale source of brine.