The paper presents the main results of isotopic-geochronological and mineralogical-geochemical studies at the territory of the Ilmeny State Reserve over 2010–2020. A summarized age scheme is given for various rock types of the Ilmenogorsk polymetamorphic complex. The textural-structural features of milonites and main directions of metasomatic processes in the rocks of the complex are shown.

In the article present results of U-Pb — dating of zircons and petrologic-geochemical study of garnet-biotite plagiogneises and quartz-plagioclase-amphibole granofels host rocks samples from Vishnevogorsky sequence of the oldest rocks of the southern Urals Eastern domain are presented. U-Pb-dating of zircons were obtained by ion microprobe (SHRIMP II). The maximum age of the substrate plagiogneises Vishnevogorsky sequence not younger than 2700 Ma, and granulitic metamorphism plagiogneises falls on the Proterozoic age range 1740–2220 Ma. The dates obtained for plagiogneises and granofels Vishnevogorsky sequence reflect all major (PR1–P1) age stages of the Urals development. Many of the dated events are manifested only in the zircons generation and are not reflected in the mineral paragenesises of the studied rocks.

The results of U–Pb zircon dating, as well as the petrologic-geochemical study of samples of zircon-bearing garnet–biotite plagiogneiss and quartz—plagioclase–amphibole granofels from the Vishnevogorsky sequence of the oldest rocks of the Eastern domain of the Southern Ural are presented. The zircon ages were obtained using an ion microprobe (SHRIMP II). The maximum age of the substrate of plagiogneises in the Vishnevogorsky sequence is not younger than 2700 Ma and the granulite metamorphism of plagiogneiss falls in the Proterozoic age range of 1740–2220 Ma. The dates obtained for plagiogneisses and granofels of the Vishnevogorsky sequence reflect all major (PR 1 –P 1 ) age stages in the development of the Urals. Many of the dated events have manifested only in the zircon generations and are not reflected in the mineral parageneses of the studied rocks.

Syenitic migmatites of the contact of the Ilmenogorsky miaskite block are characterized by high (>500 ppm) contents of REEs, which are mainly hosted in fuorapatite and titanite of the leucosome. Apatite exhibits zoned and sectorial composition with decreasing REE and Si contents and increasing Cа, P, and F contents from central parts to the margins indicating the change in physicochemical conditions of mineral formation. The La/Nd ratios of REE-bearing fuorapatite indicate that apatite-1 formed from a F-bearing fuid in contrast to apatite-2 formed from an alkaline, F–CO2–SO3-rich fuid during albitization. The REEs and F released by alkaline metasomatism accumulated in late generation of REE–F-bearing titanite. Migmatization of rocks was accompanied by involvement of an alkaline F–CO2–SO3-bearing fuid.

The formation time of 410.5 ± 1.1 Ma (zircon U/Pb SHRIMP) and the duration of rock crystallization (2–2.5 Ma) were determined for gabbro from the Nurali massif. The gabbro zircons showed a complicated polyphase structure. A new polygenous type of zonality including traces of primary growth and of the processes of crushing, dissolution, and substitution was discovered in the zircons. The formations of gabbro and ultramafites (lherzolites) of the massif are separated by a time break of 30–35 Ma determined by the genetic discontinuance.

The results of study of rocks of the Ilmenogorsk miaskite block - mylonitized miaskites, “sandyites” and monzonitic rocks, which were found for the first time, are presented in the paper. We studied textural-structural features of rock and their mineral and chemical composition. The composition of petrogenic elements in rocks were determined by atomic absobtion method, rare earth, rare and trace elements - ICP-MS method. Microprobe analyses compositions of minerals were made on the scanning electron microscope REMMА-202 M with energy-dispersive console Link systems LZ with Si-Li detector. Correlation of data was performed using the program “Magellanes”. These data allowed us to establish the degree of transformation of rocks and multistage metasomatic processes. The mineral assemblages indicate amphibolite facies of metamorphism, which was accompanied by participation of the F-bearing fluid and formation of specific accessory minerals - highly aluminous titanite and allanite. The highly variable Al contents of these minerals is evidence of high alkalinity at the presence of the fluorine in the fluid. The minerals of the banalsite-stronalsite group in sandyites points to a wide range of temperatures during metasomatic processes. Alteration of rocks results of the change of their chemical composition: the ТiO2, MgО, СаО, total Fe, and LREE contents increased and SiO2, Al2O3, and К2O contents decreased. The rocks are characterized by high REE contents in contrast to the host mylonitized miaskites and the high contents of lithophile elements (Rb, Ba, Sr, Th) and low contents of Co, Cu, W, Ni, Cr, and Pb. Our data indicate the metasomatic origin of the studied rocks. The increase in the contents of trace and rare earth elements from milonitized miaskites to sandyites and monzonitic rocks reflect their mobility and significant role of assimilation of the continental crust. The mobility of these elements and their redistribution in the rocks increase during active influence of fluid and growth in it the contents of alkalis and fluorine. Thus, sandyites of the Ilmenogorsky miaskite block are metasomatic rocks which produced after mylonitized miaskites in the linear tectonic zones during late postcollisional shear stage. The monzonitic rocks are ortho-rocks and were probably formed after country diorites. They kept relics of primary structures, but were transformed simultaneously with sandyites under influence of multistage tectonic-metamorphic processes.

Изученные (U-Pb-метод, SHRIMP II) цирконы из рутиловых эклогитов максютовского комп- лекса (д. Шубино) образуют три возрастных кластера. Цирконы неопротерозойские (561 ± 10 млн лет), фиксируют образование эклогитов, субстратом для них были диабазовые, габбро-диабазовые тела. Древние цирконы неоархейские (2884 ± 36 млн лет), отвечают мантийному субстрату, который был неоднократно (2303 ± 12; 2008 ±18; 1626 ± 59 млн лет) преобразован. Цирконы раннего ордовика-раннего карбона (433-477 ± 6; 340 ± 40 млн лет) отражают наложенные процессы, соответствующие высокобарическому метаморфизму.

The age data (U–Pb, SHRIMP II) of zircons from rutile eclogites of the Maksutov Complex (MC) (village Shubino, Southern Urals) were subdivided into three age groups. The Neoproterozoic zircons (561 ± 10 Ma) recorded the formation stage of eclogites, the protolith of which was diabase and gabbro-diabase bodies. The Neoarchean ancient zircons (2884 ± 36 Ma) belong to the mantle substratum, which was repeatedly transformed (2303 ± 12, 2008 ± 18, 1626 ± 59 Ma). Zircons of early Ordovician–early Silurian age (433–477 ± 6 Ma; 340 ± 40 Ma) recorded superimposed processes, corresponding to high-pressure metamorphism.

The age trend (SHRIMP U/Pb) of the evolution of zircon is obtained for the first time in lherzolites of the Nurali Massif. Zircons are subdivided into groups by the crystallomorphological and geochemical features. These specific features in zircon development are confirmed by the age dates. Precambrian dates (no younger than 1190 Ma) correspond to mantle sources of the lherzolite block. The Early Silurian (445–448 and 439–440 Ma) wass the time of lherzolite magmatism of 10–15 Ma in duration. The Middle Devonian (382.9 ± 8.7 Ma) corresponded to postmagmatic processes related to the effect of gabbro–diorite intrusions crowning in the Nurali Massif.

Zircons in serpentinites from Nyashevo massif of the Ilmenogorskii complex were dated for the first time by means of the SHRIMP technique. The maximum date of 1892 ± 23 Ma for the zircons accounts for the minimum age of their mantle substrate probably constituting the restite residue. The date is comparable to those for metamorphic rocks of the Selyankino group, as well as of fenite–sand amphibolites of the Ilmenogorskii complex. The Upper Ordovician age limit of 443 ± 12 Ma is adequate for formation of the massif and conforms to the age of the Buldym massif and miaskites. The Early Permian dates of zircons (275.8 ± 2.1 Ma) represent late shear processes in the Ilmenogorskii complex.

Aluminoenstatite was found in the rocks of the Karabash and Nyashevo massifs (South Urals). The features of its chemical composition (high Al contents), along with textural-structural peculiarities of the rocks, indicate a range of its formation conditions. The high Al2 O3 contents (15.3– 20.0 wt. %), elevated Fe contents, and noticeable Na contents in aluminoenstatite from mafic-ultramafic Karabash massif may reflect the upper mantle–lower crust formation conditions. The high Al2 O3 (15.6–16.2 wt. %) and Ca contents in aluminoenstatite from mafic mylonites formed after from serpentenites of the Nyashevo massif are evidence of tectonometamorphic transformation conditions in the middle–lower crust.

It is shown that the replacement and long evolution of miaskitic zircons led to the formation of two main age groups: 420–380 Ma (I) and 260–240 Ma (II). The age of miaskites is estimated at 440–445 Ma. Zircons I bear traces of fragmentation, dissolution, and replacement; they have “flat” REE patterns typical of metasomatic (hydrothermal) types, which is caused by allochthonous nature of the studied miaskites. Zircons II with differentiated REE patterns are similar to magmatic varieties, but have metamorphic origin. Mineralogical–geochemical and age characteristics of zircons in combination with structural–compositional features of miaskites define their metasomatic nature. The origin of the early zircon generations was related to the Ordovician rifting, while late generations were formed during shear deformations at the final stage of the evolution of the Uralian orogen.

Dating of zircon (SHRIMP) from dunite and harzburgite of the Karabash massif was carried out for the first time. Relics of ancient crystals (1940 ± 30 Ma in harzburgite, 1860 ± 16 Ma in dunite) provide evidence for the Paleoproterozoic age of the protolith. The morphological peculiarities of zircon crystals allow us to assume differentiation of the magmatic source 1720 m. y. ago. The major variety of zircons indicates stages of metamorphic evolution in the Neoproterozoic (530–560 Ma) and Early–Late Ordovician (440–480 Ma).