Age of the Vishnyakovskoe deposit of rare-metal pegmatites (East Sayan): U-Pb geochronological study of manganotantalite

The U-Pb age of the manganotantalite from rare-metal pegmatites of the Vishnyakovskoe deposit (East Sayan Belt) has been assessed at 1838 ± 3 Ma. The acquired data indicate the pegmatites of this deposit and associated granites of the Sayan complex belong to the postcollision South Siberian igneous belt (1.88–1.84 Ga), which stretches along the southwestern frame of the Siberian Craton by more than 2500 km, from the Yenisei Ridge to the Aldan Shield. Formation of this igneous belt is related to joining (starting from about 1.9 Ga BP) of the series of continental microplates and island arcs to the Siberian Craton; this led to final stabilization of the craton at about 1.8 Ga BP.     

Age stages of metamorphism at the Kitoi Sillimanite schist deposit, southeastern Prisayan’e

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 age of high-pressure metasomatism in shear zones during collision-related metamorphism in the Lapland granulite belt: The Sm-Nd method of dating the paragenesises from sillimanite-orthopyroxene rocks of Por’ya Guba nappe

The Sm-Nd isotope-chronological method is used for the first time to investigate the whole rock and rock-forming minerals from metasomatic sillimanite orthopyroxene rocks in the Por’ya Guba nappe of the Lapland granulite belt. As a result isochrone values of the age are obtained, which allows estimate the minimum time intervals that have passed from the moment of formation of the investigated rocks. They indicate that peak conditions of collision metamorphism in fluid-permeable shear zones of the LGB occurred in the Paleoproterozoic Svekofennian age when high-temperature high-pressure sillimanite-orthopyroxene metasomatic rocks were formed. The obtained data make it possible to assume that, with regard to the measurement errors, high-pressure metasomatism did not last over 10 Ma, and this process did not coincide in time with older medium pressure metamorphism.