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The paper presents data on the miaskite syenites of the Lovozero massif, pulaskites of the Khibiny massif, rocks of the larvikite–lardalite series of the Kurga massif, and subalkaline volcanics, which are preserved as remnants in the roof of the massifs. The studied rocks are characterized by a low agpaitic coefficient of <1, the absence of minerals typical of peralkaline rocks (eudialyte, aenigmatite, etc.), and the presence of zircon. The morphological features and chemical composition of zircon from miaskite of the Lovozero massif syenite indicate that the mineral is of magmatic nature. The crystallization age of the miaskites was dated on zircon at 373 ± 5 Мa. The isotope-geochemical characteristics of rocks of the subalkaline series indicate that the miaskites of the Lovozero massif are of mantle origin, show no indications of their crustal contamination, and were produced during the evolution of ankaramite melt. The pulaskites of the Khibiny massif were formed according to an analogous scenario, except their assimilation with crustal material, whose proportion did not exceed, according to model calculations, 10%. [ABSTRACT FROM AUTHOR]

The estimation of crystallization and metamorphic reworking ages of mafic rocks in the polycyclic Precambrian areas is a difficult problem. Magmatic baddeleyite can be partially or completely replaced by polycrystalline zircon within a wide range of temperature and pressures, from greenschist to granulite facies. Evaluation of the age of each phase of the zircon–baddeleyite aggregates can provide information on both the age of the magmatic crystallization and metamorphism. U-Th-Pb (SHRIMP-II) and U-Pb (ID-TIMS) geochronological studies were carried out for single baddeleyite grains and zircon–baddeleyite aggregates from gabbronorites ("drusites") of the Ambarnsky massif (Belomorian Province, Fennoscandian Shield). The petrological studies indicate the simultaneous growth of coronas at the olivine–plagioclase boundary and zircon rims around baddeleyite. U-Pb (ID-TIMS) dating of single baddeleyite grains yielded 2411 ± 6 Ma crystallization age of gabbronorites of the Ambarnsky massif. U-Pb (ID-TIMS) dating coupled with the discrete chemical abrasion give an age of 1911 ± 35 Ma for metamorphic zircon rims. The obtained results indicate that coronitic textures in the gabbronorites were formed 500 million years later than the magmatic crystallization of rocks as a result of the granulite-facies metamorphism that was probably related to the Lapland-Kola orogeny. [ABSTRACT FROM AUTHOR]

The paper presents data on the geological structure of the Kichera zone of the Baikal–Vitim belt (BVB) at the boundary between the marginal part of the Siberian craton and the Barguzin–Vitim superterrane of the Central Asian Orogenic Belt. Early Neoproterozoic (Early Baikalian) and Late Neoproterozoic (Late Baikalian) structures and complexes are identified and characterized in the Kichera zone of the BVB. Data are presented on the geochemistry of the rocks and on their U–Pb isotope age (zircon, SIMS and ID-TIMS) and on the Nd isotope characteristics of rocks from various parts of the Kichera zone, including representative rock association of the Nyurundukan migmatite–tonalite–metabasite complex with MORB-type tholeiites and tholeiites with intraplate geochemical features. It is shown that the sources of the Early Neoproterozoic complexes of the Kichera zone, which were metamorphosed at 0.76–0.74 Ga as a result of accretion events in the marginal part of the craton, were dominated by Early Precambrian recycled crustal material. The Late Neoproterozoic complexes typomorphic of the Kichera zone were formed in the Cryogenian–Ediacaran (720–545 Ma) from prevailing juvenile sources. Our data suggest that the metabasites of the Nyurundukan complex were formed in an environment of segmented troughs of the pull-apart paleorift system of the Kichera zone and can be compared with a reduced complex of continental-margin ophiolites transformed at 630 ± 7 to 615 ± 3 Ma. The destruction of the ancient continental crust of the craton ended with the formation and exhumation of deep rocks in the Late Ediacaran, the emplacement of adakite granites of the postcollisional geochemical type, and the formation of grabens filled with a terrigenous complex. The juvenile and riftogenic crust produced during the Late Neoproterozoic tectonic evolution of the Kichera rift zone does not show any features of mature continental-type crust. [ABSTRACT FROM AUTHOR]

The possibility of the application of high-uranium self-irradiation-damaged metamict zircon for U-Pb geochronological studies (ID-TIMS) after preliminary high-temperature annealing and acid treatment was demonstrated using zircon from the Li–F granites of the Turga massif (Eastern Transbaikalia). Optimal conditions of high-temperature annealing and acid treatment of zircon with high self-irradiation α-dose can be selected to remove completely a metamict phase and preserve sufficient quantities of the mineral for U-Pb (ID-TIMS) studies. [ABSTRACT FROM AUTHOR]

The chemical compositions of Phanerozoic basalts of all known geodynamic settings (mid-ocean ridges, oceanic plateaus, oceanic islands, island arcs, fore-arc and back-arc basins, continental rifts, large igneous provinces) are compared with Archean basalts, including basalts of greenstone belts and high-grade gneiss complexes. Linear discriminant analysis has shown that the Archean basalts differ significantly from Phanerozoic basalts in terms of the content of the least mobile major and trace elements. In this regard, the geochemical systematics of Phanerozoic basalts in most cases cannot be used to reconstruct the geodynamic settings of Archean basalts. The results obtained are illustrated by the example of the Mesoarchean basalts of the Olondo fragment of the Tokko–Khani greenstone belt. [ABSTRACT FROM AUTHOR]

We studied the petrography, mineralogy, and geochemistry of the Paleoproterozoic (2.06 Ga) granites of the Katugin massif (Stanovoy suture zone), which hosts the combined rare-metal Katugin deposit. Three groups of granites were distinguished: (1) biotite ( Bt) and biotite-riebeckite ( Bt-Rbk) granites of the western block of the massif; (2) biotite-arfvedsonite ( Bt-Arf) granites of the eastern block; and (3) arfvedsonite ( Arf), aegirine-arfvedsonite ( Aeg-Arf), and aegirine ( Aeg) granites of the eastern block. The Bt and Bt-Rbk granites of the first group are mainly metaluminous and peraluminous rocks with rather high CaO contents and the minimum F contents among the granites described here. It was suggested that the granites of this group could be derived from a source dominated by crustal rocks with a small addition of mantle materials. These granites probably crystallized from a metaluminous-peraluminous melt with elevated CaO and moderate F contents. Melts of such compositions are least favorable for the crystallization of ore minerals. The Bt-Arf granites of the second group are mainly peralkaline and show high contents of CaO and Y and low contents of NaO and F. A mixed mantle-crust source was proposed for the Bt-Arf granites. The initial melt of the Bt-Arf granites could have a peralkaline composition with elevated CaO content and moderate to high F content. The Arf, Aeg-Arf, and Aeg granites of the third group are enriched in ore mineral and were classified as peralkaline granites with very low CaO contents, elevated Na2O and F contents, and usually very high contents of Zr, Hf, Nb, and Ta. Based on the geochemical and isotopic data, it was supposed that the source of the granites of the third group could be derivatives of basaltic magmas produced in an OIB-type source with a minor addition of crustal material to the magma generation zone. It was suggested that the primary melt of this granite group could be a peralkaline CaO-poor and F-rich silicic melt, which is most favorable for the crystallization of ore minerals. Based on the analysis of the geochemical characteristics of the three granite groups and their relationships within the Katugin massif, a qualitative model of its formation was proposed. According to this model, the Bt and Bt-Rbk granites of the western block crystallized first, followed by the Bt-Arf granites of the eastern block and, eventually, the Arf, Aeg-Arf, and Aeg granites enriched in ore minerals. [ABSTRACT FROM AUTHOR]

Late Quaternary geyserites in the Ol'khon area, Baikal rift zone, are a geologically and petrologically unique rocks, which may provide information on the character of hydrothermal processes during the development of modern rift zones. The geyserites of this area are characterized by the presence of minerals atypical of such rocks: highly crystalline graphite (which is usually formed at high temperatures) and α-carbyne (which is a natural carbon modification that has been previously found only among products of experiments). Other newly formed minerals of the geyserites are equilibrium chalcedony, talc, and tremolite. This suggests that the rocks were formed under conditions significantly different from those under which classic geyserites are produced. Geyserites in the Ol'khon area were most probably produced by silica-saturated solutions rich in certain ore components (Fe, Mn, Sb, As, and Y) and heated to temperatures of about 400°C. [ABSTRACT FROM AUTHOR]

Data obtained on the Khangai zonal magmatic areole in the Central Asian Orogenic Belt indicate that accessory zircons in alkaline granitoids have crystallogenetic characteristics that generally reflect specifics of the evolution of the parental alkaline magmas. The late differentiation of these magmas was characterized by an increase in the concentrations of trace and rare-earth elements, water, and fluorine in the residual melts and aqueous salt-bearing fluids. Their action was associated with local transformations of the zircon, first and foremost, with local enrichment of zircon crystals in fluid inclusions and in crystalline inclusions of sulfides an other ore minerals. This disturbed the Pb isotopic composition of the zircon, for example, led to its enrichment in common (admixture) Pb and thus caused significant errors and uncertainties in the U-Pb zircon dates. Our recently obtained data indicate that one of the most efficient methods of preparing accessory zircons enriched in common Pb to their U-Pb dating is their preparatory treatment with acids (Makeev, 1981; Mattinson, 1994, 1997, 2005). The application of this technique makes it possible to rid the zircons of phases enriched in common Pb and usually to obtain reliable geochronologic data. The method of preparatory acid treatment of zircons is thus best suitable for geochronologic studies of granitoids of elevated alkalinity. [ABSTRACT FROM AUTHOR]

The Early Caledonian Central Asian Orogenic Belt hosts fragments of continental blocks with Early and Late Precambrian crystalline basement. One of the structures with an Early Precambrian basement was thought to be the Dzabkhan microcontinent, which was viewed as an Early Precambrian 'cratonal terrane'. The first geochronologic data suggest that the basement of the Dzabkhan microcontinent includes a zone of crystalline rocks related to Late Riphean tectonism. Geological, geochronological (U-Pb zircon dates), and Nd isotopic-geochemical data were later obtained on the northwestern part of the Dzabkhan microcontinent. The territory hosts the most diverse metamorphic complexes thought to be typical of the Early Precambrian basement. The complexes were determined to comprise the Dzabkhan-Mandal and Urgamal zones of high-grade metamorphic rocks. Gabbrodiorites related to the early metamorphic episode and dated at 860 ± 3 Ma were found in the Dzabkhan-Mandal zone, and the gneiss-granites marking the termination of this episode were dated at 856 ± 2 Ma. The granitoids of the Dzabkhan batholith, whose emplacement was coeval with the termination of the late high-grade metamorphic episode in rocks of both zones, have an age of 786 ± 6 Ma. Similar age values were determined for the granitoids cutting across the Late Precambrian rocks of the Songino and Tarbagatai blocks, which mark the stage when the mature Late Riphean continental crust was formed. The Late Riphean magmatic and metamorphic rocks of the Dzabkhan microcontinent were found out to have Nd model ages mostly within the range of 1.1-1.4 Ga at ɛ(T) from +1.9 to +5.5. The Nd model age of the metaterrigenous rocks is 2.2−1.3 Ga at ɛ(T) from −7.2 to +3.1. The results of our studies provide evidence of convergence processes, which resulted in the Late Riphean (880−780 Ma) continental crust in Central Asia. Simultaneously with these processes, divergence processes that were responsible for the breakup of Rodinia occurred in the structures of the ancient cratons. It is reasonable to suggest that divergence processes within ancient continental blocks and Rodinia shelf were counterbalanced by the development of the Late Riphean continental crust in the convergence zones of its surrounding within established interval. [ABSTRACT FROM AUTHOR]