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Geological, petrological, geochemical, and isotope data from the Yenisei Ridge indicate three stages of rifting and attendant within-plate magmatism at 750, 700, and 670 Ma. The igneous rocks of the three stages are, respectively, metarhyolite-basalt, trachybasalt-trachyte, and alkali ultramafic (alkali picrite) associations. Magmatism was concurrent with terrigenous deposition of the Neoproterozoic Upper Vorogovka, Chingasan, and Chapa Groups. The volcanosedimentary complexes were deposited in narrow rift-like graben along faults. The earlier consolidated flanking uplifts of the graben experienced granitoid magmatism synchronously with rifting and within-plate volcanism. The respective plutonic events produced granitoid intrusions of the Ayakhta (760–750 Ma), Kutukas (690–700 Ma), and Middle Tatarka (~700 Ma) alkaline complexes, and the later (about 650–670 Ma) alkali ultramafic Chapa complex of carbonatites and metasomatites. Basalts and alkaline rocks are chemically similar to ocean-island and continental-rift basalts which have been reliably attributed to mantle plumes. Neoproterozoic rifting and within-plate magmatism were possibly related to the plume activity responsible for the breakup of Rodinia. These events in the Yenisei Ridge appear to be coeval with rifting and within-plate magmatic processes in other continental blocks which may have been parts of the Rodinia supercontinent.

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The article reviews the history, main scientific fields and major results of the studies performed by the Laboratory of Petrology, Geochemistry and Ore Genesis and the Laboratory of Geology and Magmatism of Ancient Platforms. ЭВОЛЮЦИЯ СИБИРСКОГО КРАТОНА, ПЕТРОГЕНЕЗИС И АЛМАЗОНОСНОСТЬ МАНТИЙНЫХ МАГМАТИЧЕСКИХ СИСТЕМ T h e 6 5 t h A n n i v e r s a r y o f t h e I n s t i t u t e o f t h e E a r t h ' s C r u s t , S B R A SП. И. Дорогокупец, К. Н. Егоров Инcтитут земной коры CО PАН, Иpкутcк, РоccияАннотация: Рассмотрена история, основные научные направления и основные научные результаты исследований за период 2009-2013 гг. в лаборатории петрологии, геохимии и рудогенеза и лаборатории геологии и магматизма древ них платформ.

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The Onot and Bulun terranes are confined to the Sharyzhalgai Uplift in the southwestern margin of the Siberian craton. They consist of alternating blocks and nappes of Paleoarchean tonalite-trondhjemite-granodiorite complex and supracrustal metasedimentary-volcanogenic rocks of greenstone belts (GSB). The lower part of the Onot GSB is made up of a bimodal association of aporhyolite microgneisses with subordinate amphibolites, while the upper part consists of amphibolites associated with banded iron formation, metapelites, dolomitic marbles, and magnesites. The Urik GSB in the Bulun block comprises three rock associations:(1) garnet amphibolites and amphibolites alternating with kyanite-bearing mica schists and quartzite schists;(2) garnet-bearing biotite and amphibole crystalline schists with tectonic lenses of garnet amphibolites;(3) biotite and amphibole-biotite orthogneisses and biotite plagiogneisses. The microgneisses (metarhyolitoids) of the Onot belt are correlated with within-plate volcanic rocks and A-type granites. The composition of the amphibolites corresponds to high-Mg low-Ti tholeiitic basalts. The formation of metavolcanic rocks of the Onot GSB was related to the rifting of the Paleoarchean continental crust, which is supported by the formation of felsic metavolcanic rocks from an ancient tonalite source and by the geochemical signatures of crustal contamination of metabasalts. The amphibolites of the Urik GSB are subdivided into three petrogeochemical types. The first and second types correspond to high-and low-Mg tholeiitic basalts and have practically flat multielement patterns. The amphibolites of the third type correspond to subalkaline leucobasalts. Two types of orthogneisses are comparable with intermediate-acid volcanic rocks of the andesite-dacite and adakite series. In terms of geochemistry, the metamafic volcanic rocks of the Urik GSB represent the rocks of the oceanic crust. Oceanic settings of their formation are confirmed by an association of metavolcanic rocks with abyssal distal siliceous-argillaceous deposits. The formation of two types of intermediate-acid metavolcanic rocks of andesite-dacite and adakite associations, as well as garnet-bearing paraschists, was presumably related to subduction settings.

We present results of study of mineral assemblages and PT-conditions of metamorphism of mafic garnet–two-pyroxene and two-pyroxene granulites in the Early Precambrian metamorphic complex of the Angara–Kan terrane as well as the U–Pb age and trace-element and Lu–Hf isotope compositions of zircon from these rocks and the zircon/garnet REE distribution coefficients. The temperatures of metamorphism of two-pyroxene granulites are estimated as 800–870 to ~900 °C. Mafic garnet–two-pyroxene granulites contain garnet coronas formed at 750–860 °C and 8–9.5 kbar. The formation of the garnet coronas proceeded probably at the retrograde stage during cooling and was controlled by the rock composition. The age (1.92–1.94 Ga) of zircon cores, which retain the REE pattern typical of magmatic zircon, can be taken as the minimum age of protolith for the mafic granulites. The metamorphic zircon generation in the mafic granulites is represented by multifaceted or soccerball crystals and rims depleted in Y, MREE, and HREE compared to the cores. The age of metamorphic zircon in the garnet–two-pyroxene (~1.77 Ga) and two-pyroxene granulites (~1.85 and 1.78 Ga) indicates two episodes of high-temperature metamorphism. The presence of one generation (1.77 Ga) of metamorphic zircon in the garnet–two-pyroxene granulites and, on the contrary, the predominance of 1.85 Ga zircon in the two-pyroxene granulites with single garnet grains suggest that the formation of the garnet coronas proceeded at the second stage of metamorphism. The agreement between the zircon/garnet HREE distribution coefficients and the experimentally determined values at 800 °C suggests the simultaneous formation of ~1.77 Ga metamorphic zircon and garnet. Zircon formation by dissolution/reprecipitation or recrystallization in a closed system without exchange with the rock matrix is confirmed by the close ranges of 176Hf/177Hf values for the core and rims. The positive εHf values (up to +6.2) for the zircon cores suggest that the protolith of mafic granulites are derived from depleted-mantle source. The first stage of metamorphism of the mafic granulites and paragneisses of the Kan complex (1.85–1.89 Ga) ended with the formation of collisional granitoids (1.84 Ga). The second stage (~1.77 Ga) corresponds to the intrusion of the second phase of subalkalic leucogranites of the Taraka pluton and charnockites (1.73–1.75 Ga).