For decades, the U–Pb isotope system of zircon is routinely used in geosciences for the determination of intrusion ages of felsic magmatic rocks. It is, however, well-known that zircon and the related U–Pb isotope system may be influenced by secondary hydrothermal alteration and new (hydrothermal) zircon growth. As a result, age determination may yield meaningless or alteration ages. An example is given here for intensely altered late Paleozoic granitoid rocks from the Muruntau area, Central Kyzyl Kum, Uzbekistan. Pb–Pb single grain evaporation and U–Pb SHRIMP isotope systematics of zircon from six granitoid rocks yield roughly consistent results defining ages ranging between 288 and 303Ma. At first glance, these ages may be understood as intrusion ages of the magmatic rocks hosting the zircon. However, there is an apparent contradiction between the sequence of the measured U–Pb SHRIMP ages and the relative age sequences established in the field. Evaluation of data from comprehensive mineralogical and petrological studies reveals that the U–Pb system of magmatic zircon is complicated by inheritance in I-type granitoids, recrystallization, new hydrothermal zircon growth, and subsequent secondary alteration. Widespread albitization of the granitoids led to the formation of new, U-rich, hydrothermal zircon forming overgrowths on older grains or even new whole single crystals. Later alteration events caused direct and (apparent) reverse U–Pb isotope discordances. These late disturbances of the U–Pb system are mainly due to widespread sericitization. Consequently, precise concordant U–Pb zircon SHRIMP ages around 290–294Ma defined for U-rich, largely undisturbed crystal areas constrain the timing of albitization rather than that of the intrusion. The reliability of the concordant U–Pb SHRIMP ages measured on U-rich zircon is confirmed by a U–Pb SHRIMP age of hydrothermal monazite (292±8Ma). This monazite associates with extremely U-rich zircon (U–Pb SHRIMP age: 291±3Ma) in a sample of the Murun granite recovered beneath the giant Muruntau gold deposit by super-deep drilling. Obviously, detailed mineralogical studies are important for a correct interpretation of zircon ages when hydrothermal alteration and late re-mobilization of zirconium and uranium were significant as often observed in areas with intense ore mineralization. The extensive Au mineralization found at Muruntau is similar in age to albitization but apparently somewhat younger. Likewise, the intrusion age of Permian granites is at least somewhat older than the hydrothermal zircon ages confirmed by U–Pb ages of monazite. It is, however, difficult to constrain further granite intrusion ages based on presently available data.

Results of U-Pb (SHRIMP II) and Rb-Sr dating of sillimanite schists in southwestern Transbaikalia are presented, which testify to high-temperature metamorphism in the Late Mesozoic. The metamorphism might have been related to the formation of metamorphic cores or tectonoplutonic activity accompanied by the formation of rift structures in Transbaikalia and Mongolia.

The U-Pb (SHRIMP-II) age of zircons from garnet-spinel peridotite nodules in Cenozoic alkali basalts of the Vitim Plateau, Transbaikal region were determined. Most of the zircons are euhedral and subhedral prismatic crystals with an elongation of 1.5–2.0. Fragments of crystals and nearly equant crystals with rounded edges are present as well. Rounded or irregular cores are observed in some grains. None of the zircons yielded an age that would correspond to the time of basalt eruption (21–2.35 Ma or younger). The youngest dates range from 135.2 ± 2.7 Ma to 141 ± 3 Ma (Early Cretaceous). Both concordant values and the lower intersection of discordia with concordia (138.8 ± 5.7 Ma) are within this age interval. The upper intersection corresponds to 1891 ± 26 Ma. A considerable part of the concordant values are grouped within the intervals (164.6 ± 1.6)–(183.4 ± 2.0) and (264.0 ± 7.3)–(295.7 ± 0.76) Ma (Early-Middle Jurassic and Early Permian, respectively). The older concordant values fall in the interval 1462 ± 19 to 1506 ± 4 Ma (Mesoproterozoic). Proterozoic age was obtained for cores of composite zircon grains. Zircons pertaining to all age intervals are enriched in REE relative to chondrite (except La). The chondrite-normalized REE patterns are positively sloped with an increase in contents from LREE to HREE. The LREE and HREE contents and the depth of the Eu minimum tend to increase with age. In composite zircons of Proterozoic age, cores are somewhat enriched in REE. It has been suggested that crystallization of zircon as a separate phase in peridotites extremely depleted in Zr was related to a low degree of partial melting. The melt that formed in the intergranular space and that was repeatedly enriched in Zr was not extracted from the solid framework of rock and crystallized in situ under the changed thermodynamic conditions in the upper mantle. The occurrence of zircons of several age intervals in peridotites testifies to the multistage evolution of the upper mantle and recurrent partial melting under various physicochemical conditions.

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.

Crystalline schists of the El’gakan unit (Nyukzha River) were affected by penetrative (volume) replacement by plagiogneisses and granite-gneisses (Lc1) and were then transformed into a polymigmatite complex with successively developing leucosomes Lc2, Lc3, and Lc4. After a thrust-nappe structure was formed in response to collision processes, a new generation of granite veins was produced (Lc5), and then tonalite gneisses Lc 6 avt and branching migmatites with leucosomes Lc 6 all were formed along strike-slip fault zones. Zircons from granite-gneisses Lc1 were classified into four types (populations) based on SHRIMP II data. Type I (rhythmically zonal cores) were dated at 2960 and 3010 Ma, which is correlated with the age of the magmatic (predominantly volcanic) protolith. Types II and III were dated at 2703 Ma, which corresponds to granitization under amphibolite-facies conditions and the origin of the Stanoi granite-gneiss. This event is correlated with granulite metamorphism and ultrametamorphism over the whole territory of the Dzhugdzhur-Stanovoi folded area. The most widely spread type IV of the zircons has an age of 1915 Ma, which corresponds to the metamorphism coeval with overthrusting and, hence, with the collision of the Stanovoi plate and a margin of the Siberian Platform. Concentrations of REE, U, and Th and the Th/U ratio were determined to systematically decrease from type I to IV of the zircons (except their type III, whose Th/U ratio increases to >1). Zircons from Lc5 have a concordant age of 139 Ma, which is comparable with the age of the Late Stanovoi granites. The compositional changes from the older cores to younger rims of zircons from Lc5 are analogous to those mentioned above for zircon from Lc1. The concordant age of zircons from Lc 6 avt is 127–130 Ma. Their Th/U ratio increases from cores (<1) to rims (>1), which suggests that melt may have appeared when Lc 6 avt was formed. ICP-MS analyses of 53 rock samples reveal differences in the character of the trend (increase/decrease) and magnitude of the changes in the concentrations of trace elements in the distinguished granitization and migmatization series; correlations were revealed between the concentrations of elements and composition of the rock groups. For example, the development of Lc1 was associated with enrichment in Rb, Sr, Ba, LREE, Th, Zr, and Hf at depletion in Nb, Ta, U, and HREE relative to the original rocks. The leucosomes of the Lc2, Lc3, and Lc4 migmatites are depleted in all of these elements except LILE, which is thought to be explained by infiltration-controlled granitization with volume replacement and partial melting at the development of vein leucosome and the subsequent mobilization of the melts together with residues. The different signs of the changes in the LREE and LILE concentrations is unusual for anatectic processes and can be modeled by equilibrium or disequilibrium melting.

The structure and composition of accessory zircons from the tonalites of the Vyg River, southeastern Karelia, were investigated. Their local U-Pb SHRIMP dating yielded ages between 3127±15 and 3146±25 Ma. It was shown that the zircons consist of three zones, a central part containing solid and melt inclusions and zoned magmatic and metasomatic shells. The obtained ages correspond to the magmatic and metasomatic stages of zircon crystallization. In general, the zircons have elevated contents of LREE (up to 867 ppm La), which were mainly accumulated in the outer metasomatic shell. Apatite and CO2 inclusions are widespread. Orthoclase, orthopyroxene, ilmenite, galena, quartz, and bastnaesite were identified in a solid inclusion in one zircon core using a CAMSCAN MX 2500 electron microscope. The presence of bastnaesite accentuates the relation of LREE with a CO2-rich fluid. It was shown that REE content is not correlated with U, Th, and U/Th ratio.

Geochemical and geochronological studies of the main types of granitoids of the Angara-Vitim batholith (AVB) and granites of the Zaza complex in western Transbaikalia were carried out. U-Pb (SHRIMP-II) and Rb-Sr dating yielded the age of autochthonous gneiss-granites of the Zelenaya Griva massif (325.3±2.8 Ma), quartz syenites of the Khangintui pluton (302.3±3.7 Ma) and intruding leucogranites of the Zaza complex (294.4±1 Ma), monzonites of the Khasurta massif (283.7±5.3 Ma), and quartz monzonites of the Romanovka massif (278.5±2.4 Ma). The U-Pb and Rb-Sr dates show that the Late Paleozoic magmatism in western Transbaikalia proceeded in two stages: (1) 340–320 Ma, when predominantly mesocratic granites of the Barguzin complex, including autochthonous ones, formed, and (2) 310–270 Ma, when most AVB granitoids formed. We suggest that at the early stage, crustal peraluminous granites formed in collision geodynamic setting. At the late (main) stage, magmatism occurred in postorogenic-extension setting and was accompanied by the formation of several geochemical types of granitoids: (1) typical intrusive mesocratic granites of the Barguzin complex, similar to those produced at the first stage; (2) melanocratic granitoids (monzonitoids, quartz syenites), which were earlier dated to the early stage of the AVB evolution; (3) leucocratic medium-alkali (peraluminous) granites of the Zaza intrusive complex; and (4) some alkali-granite and syenite intrusions accompanied by alkaline mafic rocks. The diversity of granitoids that formed at the late stage of magmatism was due to the heterogeneous composition of crust protoliths and different degrees of mantle-magma participation in their formation.