Геохимическая и Sr-Nd-Pb изотопная характеристики щелочных пород и карбонатитов Белозиминского массива (Восточный Саян)

Abstract

Belaya Zima alkaline carbonatite complex (East Sayan, Russia) is a multiphase intrusion of the central type. The carbonatite stock is exposed in the central part of the complex, which is composed of different mineralized carbonatite. Alkaline silicate rocks form ring-shaped bodies surrounding the central carbonatite and are represented by melteigites and ijolites, dyke-shaped bodies of alkaline syenites. The article presents the results of Pb/Pb dating, petrological and geochemical studies, including radiogenic (Nd, Pb and Sr) isotopic data. The age value of 631 ± 11 Ma, obtained from the Pb/Pb isochron line for all rocks of intrusion, is consistent with the previous age determinations [643 ± 3 Ma, Yarmolyuk et al. 2005 and 645 ± 6 Ma, Doroshkevich et al., 2016; 645 ± 6 Ma, Salnikova et al., 2019]. The age is in the range of 700-600 Ma, the interval of the formation of alkaline carbonatite complexes located along the edge of the Siberian craton (Aldan-Stanovoy shield, Yenisei Ridge, East Sayan). Emplacement of the complexes is associated with the final stage of the break-up of the Rodinia supercontinent [Yarmolyuk et al., 2005]. Melteigites are more magnesium (Mg # = 28–35) in comparison with ijolites (Mg # = 24–30) and alkaline syenites (Mg # = 15–29), which proves the lowest degree of differentiation of the former. The low total Mg # of the Belaya Zima alkaline silicate rocks, their low nickel and chromium concentrations confirm the fact that olivine crystallization has completed. Early crystallization of perovskite in melteigites led to depletion of the residual melt in REEs and Nb and formation of REE-depleted ijolites and nepheline syenites. There are the evolution trend of HFSE and REEs from early calcite carbonatite to calcite-dolomite and ankerite carbonatites. The calcite and calcite-dolomite carbonatites have the highest contents of Nb-Ta and Zr-Hf and reflect the early crystallization of pyrochlore and zirconolite. Late ankerite carbonatites are enriched in REEs in comparison with calcite and calcite-dolomite carbonatites, are REEs are concentrated in rare earth carbonates and monazite (Ce). Isotopic data (87Sr/86Sr = 0.702672–0.703125 and eNd (Т) = 3.14–4.97) for rocks of the Belaya Zima complex indicate that primary melts were formed from a heterogeneous, moderately depleted mantle source. The high concentration of incompatible elements in the rocks (Sr and Nb, La/Sm ratio), the presence of carbonatites in the complex indicate that the possible mantle metasomatic agent had a carbonate initial composition. This conclusion is confirmed by the results of melt inclusions study in minerals from the Belaya Zima ijolites [Andreeva et al., 2007]. A high Gd/Yb ratio (up to 7) in the rocks indicates that they were derivatives of primary melts formed by low degree of partial melting of a garnet-bearing source.