New geochronological (U-Pb, Pb-LS, Sm-Nd) studies were carried out for minerals from metamorphic rocks (aluminous plagiogneisses with sillimanite (kinzigites) and potassium shadow migmatites) to establish the sequence of metamorphic events in the Irkut block of the Prisayan marginal uplift of the Siberian platform basement. Obtained data permit the distinguishing of two main stages of regional metamorphism under the granulite and amphibolite facies conditions: 2480–2560 and 1860–1900 Ma. New age data in general are consistent with previously published zircon estimates of the Neoarchean and Paleoproterozoic ages of the granulite-facies metamorphism of the Irkut block. This gives grounds to consider the geochronological studies of garnet and monazite as promising tool for distinguishing age boundaries of metamorphic transformations in the areas of polycyclic evolution.

New data on the stratigraphy and isotopic age of supracrustal rocks from the lower part of the section constituting the northeastern limb of the Lekhta structure (northern Karelia) and their relationships with the basement are considered. Geological-petrographic, lithological-geochemical, and isotopic data are used to define three formations united into the Okhta Group. Immediate relationships between volcanics of the greenstone belt and granitoids of the basement represented by the oldest (for the Baltic Shield: 2.8 Ga) continental weathering crust after granites, are discussed. Isotopic age of volcanics and granite gneisses of the basement indicates that the Lopian supracrustal complex of the Lekhta structure was deposited in a period lasting 16 myr with duration of periods corresponding to formation of the Okhta and Pebozero groups being as long as 8 and 11 myr, respectively. In the regional stratigraphic scale, the entire Archean part of the supracrustal section in the Lekhta structure should be attributed to the Middle Lopian.

The Irkut Block with dominant rocks of the Sharyzhalgai Series and Kitoi Block with prevalence of the Kitoi Series rocks are the main structures of the Presayanian basement elevation of the Siberian Craton. Two stages of metamorphism, Neoarchean (2.6–2.7 Ga) and Paleoproterozoic (1.85–1.87 Ga), were established for the granulitic complexes of these blocks. The rocks of the Kitoi sillimanite schist deposit composed of sillimanite, andalusite-sillimanite, and garnet-sillimanite schists and gneisses underwent by intense ultrametamorphic transformations which led to the formation of sillimanite- and garnet-bearing plagioclase and feldspar migmatites, and also granites, sienites, and granite-pegmatites. The geochronological study of melanocratic schists and leucocratic plagiogneisses-the typical rocks of the Kitoi deposit-showed the manifestation of metamorphism only at the Archean and Proterozoic boundary (2450–2550 Ma).

The geological structure, age, and genesis of sedimentary-volcanogenic, metamorphic, and metasomatic rocks from the Terskii greenstone belt fringing the southern Imandra-Varzuga structure in the southeastern Kola Peninsula are discussed with defining main stages in endogenic activity of the region in the Late Archean and Early Proterozoic. The U-Pb method (SHRIMP-II, ID-TIMS, and Pb-LS techniques) was used to determine the age of volcano-sedimentary rocks of the Imandra Group as well as that of magmatic and superimposed metamorphic and metasomatic processes. The basic-intermediate metavolcanics of the Imandra Group are dated at 2.67 Ga, which corresponds to the Lopingian Gimol'skii Superhorizon (Late Archean). The Archean metavolcanics were subjected to Early Proterozoic regional metamorphism 2.1 Ga ago and metasomatic processes in the period of 1.85 to 1.77 Ga ago. The obtained data indicate multistage evolution of rock formation in the Terskii greenstone belt located in the southern flank of the Imandra-Varzuga structure in the Kola Peninsula.

The petrologic and isotopic-geochronologic study of basement rocks that were penetrated by a deep borehole in the marginal part of the East European Platform revealed that its section overlain by the Vendian-Paleozoic sedimentary cover is Early Proterozoic in age and largely consists of aluminous migmatized biotite, biotite-cordierite, and biotite-cordierite-sillimanite gneisses, which are intruded by granites, plagiogranites, and metatonalites. The lower part of the section is dominated by amphibole schists and amphibolites with subordinate nonmetamorphosed dolerite dikes and pegmatite veins. By metamorphism parameters ( T = 630–680°C, P = 2–4 kbar), the metamorphic complex may be considered as the shallowest one, compared with other Early Proterozoic complexes, developed at least in the southwestern part of Fennoscandia. The progressive decrease in the mineral-formation pressure observed in the Lower Proterozoic metamorphic rocks southward, away from the Karelian Craton is likely explained by the tectonic transport (thrusting) of the Svecofennides over the margin of the Karelian Craton and their subsequent deeper erosion near the craton. The magmatic crystallization of metamorphic palgiogranites, penetrated at depths of 925–928 and 1004 m, is estimated by the U-Pb ID-TIMS method on zircons to occur 1860 ± 9 Ma ago. It is shown that by their age, the REE composition, and isotopic-geochemical characteristics, these rocks are close to the plagiogranites formed in the southeastern extremity of the Svecofennnian belt in the present-day northern Ladoga region and the Karelian Isthmus. No rocks, which could be correlated by their lithology with the Archean rocks of the Karelian Craton, are found.

Based on particular examples, this paper considers the capabilities and constraints of the step-leaching Pb-Pb dating (PbSL) of metamorphogenic minerals. It was shown that stepwise leaching allows the separation of fractions enriched in uranogenic and thorogenic Pb isotopes, which can be used for the determination of mineral ages and the time of crystallization of coexisting equilibrium and, occasionally, disequilibrium monazites. The presence of monazite in a mineral is indicated by a high Th/U ratio similar to that of monazite. The main limitation of the method is related to the presence in minerals of both disequilibrium domains of the mineral matrix and disequilibrium monazite microinclusions. By the example of minerals studied, we discussed three scenarios for the development of the U-Th-Pb isotopic systems of metamorphogenic minerals.

The isotopic-geochronological studies of zircons from granites of the Borovoe, Makinsk, and Zhukei massifs located in the eastern part of the Precambrian Kokchetav median massif revealed that they were formed during the relatively brief period from 431 to 423 Ma ago, which allowed them to be united into the Early Silurian Borovoe Complex.

Local isotopic and geochemical studies of the zircons from metasomatites of the Terskii greenstone belt allowed us to determine two stages of metamorphism (2680 and 2025 My) and two stages of metasomatosis (2600 and 1800 My). Almost all the zircons were either metasomatic or affected to some degree by metasomatic processes caused by the enrichment of zircons in light rare earth elements, Th, U, Sr, and Ba, and flattening of the Ce anomaly.

Thermodynamic regime of culmination phase of high-grade metamorphism of the Umba nappe (Lapland allochthon) was studied, and peak metamorphic monazite was dated. Based on calculation of end member reactions, the metamorphic assemblages of aluminous gneisses from the upper and lower parts of the nappe are close to equilibrium. The metamorphic conditions of the rocks are estimated to be about 800°C and 7 kbar in its upper part and 9 kbar in its lower part. The formation of orthopyroxene-sillimanite aggregates points to increasing pressure and temperature at the prograde stage of PT path, whereas cordierite rims mark the onset of decompression and cooling. The pressure difference of 2–2.5 kbar identified by thermobarometric methods between aluminous gneisses from the upper and lower parts of the Umba nappe corresponds to a depth gradient about 7.5 km, which agrees with approximate thickness of the Umba nappe. The study of the eclogitelike rocks developed after the rocks of the Paleoproterozoic Kolvitsa gabbronorite massif made it possible to trace the P-T evolution of metamorphsim: the temperature peak of granulite stage (11 kbar, 860°C) was followed by pressure increase (up to 14 kbar and more), and then decompressional cooling due to the exhumation of the Por’ya Guba nappe together with the Kolvitsa Massif. The U-Pb monazite age of 1904 ± 3 Ma obtained for aluminous gneisses from the upper part of the Umba nappe corresponds within error to the timing of metamorphic events determined from metamorphic zircon in the anorthosites of the Kolvitsa Massif (1907 ± 2 Ma) and zircon from aluminous gneisses in the melange zone (1906 ± 3 Ma). These isotopic data confirm the conclusion of simultaneous high-pressure granulite metamorphism in the upper and lower portions of the Umba nappe.

Several isotopic methods (U-Pb, Sm-Nd, Rb-Sr, and K-Ar) were applied to different rock-forming and accessory minerals to decipher the chronology of events in a separate segment of the Belomorian mobile belt. Enderbites intruded supracrustal rocks at 2.73 Ga and granodiorites were emplaced at 2.41 Ga. Immediately afterwrads, a permeable schistosity zone was formed along the enderbite-granodiorite contact. Isotopic data indicate that this zone served as a pathway for heat and fluid. The retrograde stage of regional metamorphism and subsequent cooling continued from 1.89 Ga till ∼ 1.46 Ga.

Рассмотрены особенности строения, состава и возраст раннепалеозойских гранитоидных и габбро-гранитных ассоциаций, слагающих Кожуховский и Дудетский батолиты северной части Кузнецкого Алатау. В составе Кожуховского батолита, расположенного в Алатауском вулканоплутоническом поясе, выделяются породы толеитового, известково-щелочного и субщелочного типов, формирование которых происходило в два этапа. Ранними являются гранитоиды тылинского кварцдиорит-тоналит-плагиогранитного комплекса ( 530 млн. лет, Тылинский массив, толеитовый тип), возникшие в островодужной геодинамической обстановке. На втором этапе ( 500 млн. лет) в аккреционно-коллизионной геодинамической обстановке сформировались мартайгинский кварцдиорит-тоналит-плагиогранитный комплекс (Кожуховский массив, известково-щелочной тип) и краснокаменский монцодиорит-сиенит-граносиенитовый комплекс (Краснокаменский массив, субщелочной тип). В Дудетском батолите, расположенном в Алтае-Кузнецком вулканоплутоническом поясе, наиболее широкое развитие получили интрузивные породы субщелочного ряда (малодудетский монцогаббро-монцодиорит-сиенитовый и карнаюльский граносиенит-лейкогранитный комплексы), в меньшей степени щелочного ряда (верхнепетропавловский щелочно-габброидный карбонатитсодержащий комплекс), формирование которых происходило в возрастном интервале 500–485 млн. лет. Nd-изотопные исследования показали, что породы Кожуховского батолита имеют преимущественно субдукционные источники исходных расплавов ( Nd = +4.8 +4.2). Субщелочные породы Дудетского батолита обнаруживают широкие вариации изотопных параметров. Nd-изотопный состав монцодиоритов и монцогаббро малодудетского комплекса ( Nd = +6.6), в совокупности с повышенной щелочностью пород и высокими содержаниями Nb и Ta, свидетельствует о доминирующем вкладе расплавов обогащенного мантийного источника и участии деплетированного мантийного субстрата. Сиениты этого комплекса характеризуются более низкими значениями параметров Nd (+3.2 +1.9), что, возможно, связано с плавлением метабазитов, образованных из обогащенного мантийного субстрата. Некоторый вклад корового материала при магмогенерации отмечается для пород карнаюльского комплекса, имеющих более низкие содержания Nb и Ta, по сравнению с породами малодудетского комплекса, при близком значении Nd (+3.6).

The paper reports geological, chemical, and geochronological data on the Early Paleozoic granitoid and gabbro-granite associations, which compose the Kozhukhovskii and Dudetskii batholiths in the northern part of the Kuznetsk Alatau. The Kozhukhovskii batholith located in the Alatau volcanoplutonic belt is made up of tholeiitic, calc-alkaline, and subalkaline rocks that were formed in two stages. The first stage corresponded to the formation of granitoids of the Tylinskii quartz diorite-tonalite-plagiogranite complex (∼530 Ma, Tylinskii Massif, tholeiitic type) in an island arc setting. The second stage (∼500 Ma) produced the Martaiga quartz diorite-tonalite-plagiogranite complex (Kozhukhovskii Massif, calc-alkaline type) and the Krasnokamenskii monzodiorite-syenite-granosyenite complex (Krasnokamenskii Massif, subalkaline type) in an accretionary-collisional setting. The Dudetskii batholith is situated in the Altai-Kuznetsk volcanoplutonic belt and contains widespread subalkaline intrusive rocks (Malodudetskii monzogabbro-monzodiorite-syenite and Karnayul’skii granosyenite-leucogranite complexes) and less abundant alkaline rocks (Verkhnepetropavlovskii carbonatite-bearing alkaline-gabbroid complex), which were formed within the age range of 500–485 Ma. Our Nd isotopic studies suggest mainly a subduction source of the rocks of the Kozhukhovskii batholith (εNd from + 4.8 to + 4.2). Subalkaline rocks of the Dudetskii batholith exhibit wide isotopic variations. The Nd isotopic composition of monzodiorites and monzogabbro of the Malodudetskii Complex (εNd = + 6.6), in association with the elevated alkalinity and high Nb and Ta contents of these rocks, testifies to the predominant contribution of an enriched mantle source at the participation of a depleted mantle source. The lower εNd (from + 3.2 to + 1.9) in its syenites possibly indicates their generation through melting of metabasic rocks derived from enriched mantle protolith. The rocks of the Karnayul’skii Complex have lower Nb and Ta contents at similar εNd (+3.6), which suggests some crustal contribution to their formation.

Interaction of metamict zircon with solutions of various composition (H 2 O, 1 m NaCl + 0.5 m HCl, 1 m NaCl, 1 m NaHCO 3 , 0.1 m Na 2 CO 3 , 1 m Na 2 CO 3 , 2 m Na 2 CO 3 , and 1 m NaOH) was examined at temperatures of 25–800°C, pressures of 1 and 5 kbar, and exposure times of 3, 7, and 14 days. The choice of fluid compositions was determined by the fact that aqueous, chloride, and carbonate solutions are the principal components of postmagmatic, metamorphic, and metasomatic fluids that can affect zircon in nature. Occurring in contact with these solutions, metamict zircon congruently and incongruently dissolves competing with the sorption of elements, crystallization of newly formed phases, and the restoration of the crystal structure. The intensities of these processes are controlled by the temperature, pressure, exposure time, and the pH and composition of the solution.

The Ladoga, Russia, and adjacent Sulkava, southeastern Finland, metamorphic complexes are the two largest “granulite” provinces of the Svecofennian domain. In this area, the domain is composed of outer and inner zones. Sulkava is situated in the inner zone, which principally can be compared to the accretionary arc complex of Southern Finland. Ladoga is situated in the outer zone, which is correlated with the accretionary arc complexes of central and Western Finland. The complexes contain different metamorphic assemblages, which are caused by the different composition of the sedimentary protoliths: the rocks of the Sulkava metamorphic complex are higher in Al and K than those of the Ladoga Complex. Pb-Pb step leaching dating was used to determine the age of prograde sillimanite from both complexes. The dates thus obtained constrain metamorphic peaks for the Sulkava and Ladoga complexes at 1799 ± 19 Ma and 1878 ± 7 Ma, respectively, which is consistent with the U-Pb monazite ages of gneisses from both of the complexes. The differences in the ages of the metamorphic minerals from these complexes reflect the Early Svecofennian (1.89–1.86 Ga) and Late Svecofennian (1.83–1.79 Ga) metamorphic stages in the Fennoscandian Svecofennides.