present the first results of a comprehensive isotope-geochemical study of dolomitic carbonatites of the Mal'dzhangarka massif located in the southeast of the Billyakh melange zone (southeastern periphery of the Anabar Shield). Zircon grains separated from core samples from a depth of 6-30 m have a three-phase structure. All of them were trapped from the host metamorphic rocks and mark the age of the main stage of high-gradient metamorphism in the region, 2027 +/- 9 Ma. Pyrochlore containing 38-705 ppm U and 5-21 ppm radiogenic Pb, with weak metamictization of the crystal lattice and an undisturbed U-Pb system, made it possible to estimate the concordant age of rare-metal mineralization in the massif, 167 +/- 4 Ma, which is probably close to the crystallization age of the host carbonatites. The estimated age corresponds to one of the stages of kimberlite-carbonatite magmatism on the eastern slope of the Anabar dome. The Re-Os isotope system of pyrite from superposed late carbonate-sulfide veinlets in the carbonatites testifies to a close (within the error of determination) age, 179 +/- 14 Ma, and a low initial Os isotope ratio, which indicates the contribution of mantle material to the formation of this isotope system.

The paper presents newly acquired isotope-geochemical and U–Pb isotope zircon dating (SHRIMP) results on four posttectonic granitoid massifs in the southeastern part of the Karelian Granite–Greenstone Province (GGP) in the Fennoscandian Shield. The massifs are located near the Hautavaara Structure, in the southeastern part of the Mesoarchean (3.05–2.85 Ga) Vedlozero–Segozero Greenstone Belt, which is confined to the western margin of the Vodlozero crustal block with a Paleoarchean (TNdDM > 3.2 Ga) prehistory. All four massifs (Hautavaara, Chalka, Shuya, and Nyalmozero) were shown to have similar structural–tectonic settings, were emplaced nearly simultaneously (at 2745–2740 Ma), and display variations in the rock compositions that were predetermined by differences in the composition of the magma sources and the conditions of their derivation. The Hautavaara Massif in the central part of the structure and the Chalka Massif on its western margin are made up of moderately alkaline high-Mg granitoids (sanukitoids), whose initial diorite melts were derived by melting the lithospheric mantle metasomatized in an active-margin setting at 3.00–2.90 Ga. The Shuya granodiorites and Nyalmozero leucogranites, which are confined to the eastern flank of the structure, yield highly fractionated HREE patterns (Dyn/Ybn = 3.5 to 5.14), negative εNdT = –0.9 to –2.8, and were produced by melting a Mesoarchean crustal source at various depths. This source was similar to the 3.05- to 2.90-Ga felsic volcanics in the Hautavaara Structure. The Shuya granodiorites contain elevated Cr and Ni concentrations, suggesting that the melts were generated in the crust with the involvement of mafic magma, which was likely coeval with the primitive sanukitoids. The melting of the continental lithosphere at mantle and crustal levels in the Karelian GGP in the latest Neoarchean are thought to have occurred in an extensional environment during collapse of the collisional orogen, in accordance with the model (Laurent et al., 2014).

Isotope study of ore-bearing rocks at Fe–Mn rift deposits of the Atasu and Zhezdy (Dzhezdy) ore districts in Central Kazakhstan has been carried out for the first time. The Atasu felsic effusive rocks (Ushkatyn-1 deposit), synchronous to ore genesis, were formed at the boundary between the Frasnian and Famennian (373 ± 4 Ma). The Zhezdy ore-bearing gravelites began to accumulate not earlier than the end of the Early Devonian (after 400 Ma), but before the felsic volcanism impulse at the turn of 370 Ma. The data obtained can be indicative of asynchronous Fe–Mn deposition in the Zhezdy and Atasu ore districts.

The article presents the first U-Pb (SHRIMP–II) zircon age data on dikes cutting dunite of the Kamenushinsky dunite–clinopyroxenite–gabbroic massif in the Ural Platinum Belt. The dikes are made up of gabbro-pegmatite, hornblendite, and granite, the field relations between which suggest their subsequent intrusion in the dunites. Their geochemical features show that they belong to two different magmatic series. The gabbro–pegmatite is the derivatives of the dunite–clinopyroxenite–gabbro series, while hornblendites are the derivatives of the gabbro series. At the same time, the granite corresponds to anatectic granites developed within the Platinum Belt. U-Pb zircon dating of the gabbro yielded an age of 418.3 ± 4.5 Ma, which can be considered as the upper age limit of the dunite–clinopyroxenite–gabbro series. The age of the hornblendite of 421.0 ± 2.4 Ma is close to the age of the majority of the gabbroic rocks of the Ural Platinum Belt. The U-Pb zircon dating of the granite yielded a relatively young age of 384.9 ± 0.5 Ma. This testifies the continuation of the intrusive magmatic activity within the Ural Platinum Belt after the formation of the youngest volcanic rocks of the Tagil volcanic arc, the gabbro–diorite–granite intrusions of the Auerbakh Complex, and significantly expands the time range of magmatism associated with partial melting of the gabbro protolith directly within the belt.

—We performed geological, geochronological, geochemical, and isotope-geochemical studies of igneous rocks of the Ust’-Ignok gabbrodiorite massif in the Urik–Iya graben of the Siberian craton and summarized the obtained and published data on early Proterozoic mafic igneous rocks in the South Siberian postcollisional magmatic belt. It has been established that the Ust’-Ignok massif is composed of rocks of the continuous series from biotite gabbro via gabbrodiorites and diorites to quartz diorites. U–Pb zircon dating of quartz diorites of the Ust’-Ignok massif yielded an age of 1836 ± 10 Ma, i.e., the massif rocks might have originated at the final stage of the formation of the South Siberian postcollisional magmatic belt. The rocks of the Ust’-Ignok massif are of normal and medium alkalinity. All igneous rocks from gabbro to quartz diorites show distinct negative anomalies of Nb–Ta and Ti in their multielement patterns, and their εNd(T) values vary from +0.3 to –0.9. The geochemical indicator ratios in the gabbroids point to insignificant contamination of their source with continental-crust material and to their formation through the melting of an enriched lithospheric-mantle source. Gabbrodiorites–quartz diorites of the Ust’-Ignok massif resulted, most likely, from the fractional crystallization of gabbroids. Analysis of the geochemical and isotope characteristics of mafic igneous rocks of the South Siberian postcollisional magmatic belt shows that most of them resulted from the melting of the subcontinental lithospheric mantle with suprasubductional geochemical features. This mantle might have formed during subduction processes preceding the formation of the Siberian craton.

Introduction The aim of our study was precise U-Pb dating by SHRIMP of zircons megacrysts from 7 pipes within five kimberlite fields of different ages and located from the center to outward of NE part of the Siberian platform, Russia. The genesis of ‘kimberlitic’ zircon is controversial. The favoured hypothesis is that the zircons are the products of metasomatic processes in the upper mante. U-Pb ages of ‘kimberlitic’ zircons have been interpreted as representing the timing of kimberlite emplacement, based on the assumption that the U-Pb system is continuously reset under upper mantle conditions prior to transport to the surface (Davis et al., 1980). We have analyzed zircons from following kimberlite pipes: 325 let Yakutii and Zapolyarnaya (Verkhnemunskoye field), Druzhba (Chomurdakhskoye field), Malokuonamskay and Anomaliya 20/85 (Kuranakhskoye field), Hrizolitovaya (Molodinskoye field), Ruslovaya (Kuoykskoye field).

Perovskite is a relatively common melt-precipitated mineral of kimberlite and related mantle rocks (Chakmouradian, 2000).Besides perovskite is also a well-known mineral which has proved amenable to U-Pb dating of the kimberlite emplacement.The main problems and promises of U-Pb SIMS dating of perovskite are detail described in many articles (Ireland,

The chapter presents new geochronologic results of various isotope techniques (U-Pb SIMS and Re-Os TIMS) along with their comparison with already published data. The main intrusions of the Norilsk district are demonstrated to be emplaced almost simultaneously with two possible magma intrusion pulses at 254 +/- 4 and 244 +/- 4 Ma (U-Pb SIMS), assuming c. 10 Ma duration of igneous activity. This is corroborated by sulfides Re-Os dating (245-250 Ma), suggestion synchroneity of intrusion and the ore formation. Some Permian and Carboniferous zircon xenocrysts have been found along with Precambrian grains (c. 1.9 and 2.7 Ga), while no Devonian xenocrysts has been revealed. A group of 145-150 Ma old mafic rocks has also been discovered: those nature and relation to the ore-bearing Norilsk intrusions yet to be studied. The geochronologic study suggests, that: (1) ore-bearing massifs belong to the early emplacement phase (250-255 Ma); (2) ore-bearing massifs contain xenogenic Palaeozoic zircons, pointing to important role of the host-rocks; (3) Late Triassic igneous activity (225-230 Ma) has not affected ore systems.

Проведено U-Pb датирование минералов группы пирохлора из редкометалльных месторождений ильмено-вишневогорского комплекса Южного Урала. Для измерения возраста отдельных кристаллов пирохлора была использована новая методика локального U-Pb датирования на вторично-ионном масс-спектрометре SHRIMP-II, разработанная в ЦИИ ВСЕГЕИ (г. Санкт-Петербург). U-Pb датирование высокоурановых пирохлоров (с содержанием окислов урана более чем 2.5 мас. %) проводилось с помощью лазерной абляции и масс-спектрометра с ИСП-МС (DUV-19 и LA-ICP MS). U-Pb система изученных образцов пирохлора свидетельствует о многоэтапном формировании редкометалльной ниобиевой минерализации ильмено-вишневогорского комплекса. Наиболее древний возраст (378 ± 4.9 млн лет) фиксируется изотопными U-Pb системами уранпирохлоров ранних карбонатитов Потанинского месторождения. Этот этап рудообразования, возможно, отражает завершающие стадии кристаллизации щелочно-карбонатитовой магматической системы. Следующие этапы рудообразования широко проявлены на Вишневогорском (230 ± 1.5 млн лет) и позднее на Потанинском (217.2 ± 1.9 млн лет) месторождениях и, вероятно, связаны с ремобилизацией и переотложением щелочно-карбонатитового и редкометалльного вещества на постколлизионном этапе эволюции карбонатитовых комплексов Урала.

Results of this study of titanite samples collected from silicate rocks and apatite-nepheline-(sphene) ores from Paleozoic polyphase alkaline nepheline syenite complexes of the Khibiny and Lovozero massifs revealed the possibility of their in-situ U-Pb dating using sensitive high-resolution ion microprobe SHRIMP-II with an accuracy of 1.0-1.5%, which is comparable with that of U-Pb zircon analysis. Employing different approaches to age determination of the formation of the U-Pb system of titanites, the combined isochrons and mixing lines were plotted from the data obtained from the differentiated complex samples (121 analyses of five Khibiny samples and 52 analyses of one Lovozero sample) and apatite-nepheline ores (120 analyses of five Khibiny samples and 88 analyses of three Lovozero samples). They indicate synchronous crystallization of titanite in silicate rocks throughout the complexes: 374.1 +/- 3.7 Ma for the Khibiny massif and 380.9 +/- 4.5 Ma for the Lovozero massif, and attest to the later formation of phosphate-rare-metal ores: 371.0 +/- 4.2 and 361.4 +/- 3.2 Ma, respectively. The relatively delayed ore mineralization specific to the Lovozero massif can be accounted for the significantly lower volumes of magmatic melt and ore fluid involved, different thermal conditions, and the pattern of the investigated mineralization. As such, the obtained U-Pb data from titanite make it possible to limit significantly the time interval (most likely, not exceeding 15-20 Ma) comprising the evolution and activity of the ore-magmatic system of major agpaitic complexes, which is probably associated with plume magmatism. (C) 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

The results of local U–Th–Pb-SIMS-SHRIMP-II analysis of the collection of pyrochlores from Nb rare-metal deposits of the Ilmeny–Vishnevogorsky alkaline complex are discussed. Independently of the chemical composition, the host rocks, and the stage of carbonatite formation, the studied pyrochlores are characterized by an almost undisturbed U–Pb isotope system. This allowed to estimate the age of ore mineralization on three deposits (Vishnevogorsky, Buldymsky, and Potaninsky) with an accuracy of ±1–6 m. y. The U–Pb system of the studied pyrochlore samples suggests three epochs of the mineral formation: 380, 230–240, and 216–218 Ma. Convergence of the results obtained by the different methods of U–Pb dating for one of the samples from our collection (±0.5–1.0 Ma) allows to use it as an in-house standard for local dating.

The Olkhon terrane is a part of the Early Palaeozoic accretionary-collisional system in the northern Central Asian Orogenic Belt (CAOB). The terrane was produced by an Ordovician collision as a collage of numerous chaotically mixed tectonic units composed of rock complexes of different ages originated in different tectonic settings. The pre-collisional history of the terrane is deciphered using new data on zircon ages and chemistry of rocks from several complexes. The oldest Olkhon rocks are the 1.87–1.83 Ga granulite and gneissic granites of the Kaltygey complex, which is an exotic Palaeoproterozoic tectonic slice. The next age group consists of the Ust-Zunduk orthogneisses (807 ± 9 Ma) and the Orso amphibolites and gneisses (792 ± 10 and 844 ± 6 Ma). Samples of both complexes have negative εNd(t) values. The Ust-Zunduk and Orso complexes can have formed in active margins of continents or in crustal blocks other than southern Siberia. The Ediacaran subduction-related rocks of the Olkhon complex may have formed in an island arc setting within the Palаeo-Asian Ocean (PAO). The protolith of schists after volcanic rocks has an age of 637 ± 4 Ma and shows positive ɛNd(t) values. The Ediacaran/Cambrian Tonta mafic granulites (ca.545 Ma), with OIB affinity and slightly positive ɛNd(t), were derived from an enriched mantle source and may represent a fragment of an oceanic island. The Cambrian Shebarta gneisses after continental-arc greywackes with negative ɛNd(t) values were deposited in a back-arc basin of a microcontinent within the PAO, between 530 and 500 Ма. The Cambrian Birkhin metamorphics after PAO mature island-arc rocks have U-Pb ages of ca. 500–490 Ma and positive ɛNd(t) values. All pre-collisional complexes in the Olkhon terrane have their analogues among the rocks formed during main events in the northern CAOB history. Thus the reconstructed milestones in the Olkhon terrane history appear to be an echo of events in the CAOB northern segment.