Home – Home – Jornals – Russian Geology and Geophysics 2019 number 1

Geological, geochronological, and geochemical isotope studies are carried out for metamorphosed volcanic rocks and dolerites of the Maltsevka sequence of the Elash Group in the Biryusa block of the Siberian craton. It is found that mafic igneous rocks (dolerites and basaltic andesites) are close in composition to intraplate basalts. Flat or slightly fractionated REE patterns ((La/Yb) _n = 1.3-2.3) and positive ε_Nd( T ) values of +3.7 and +4.1 are observed. It is assumed that the depleted asthenospheric mantle and, possibly, plume mantle were the sources of these rocks, while the lithospheric mantle had no significant effect. Meta-andesites of the Maltsevka sequence belong to the tholeiitic series and have high La, Th, and U contents. Pronounced negative Nb and Ti anomalies are observed in the multielement patterns of these rocks, along with negative ε_Nd( T ) values of -4.6. It is assumed that meta-andesites resulted from the late Archean crustal melting with the participation of the mantle material. Metarhyolites prevalent in the Maltsevka sequence are divided into two groups similar in REE composition to А-type and I-type granites. A-type metarhyolites show high contents of Zr, Y, Nb, Th, and REE (except for Eu) and positive ε_Nd( T ) values of +2.2 and might have resulted from the melting of a source with geochemical isotope parameters close to those of mafic igneous rocks of the Maltsevka sequence. I-type metarhyolites have low contents of Y, Yb, Zr, and Nb but high contents of Th and show negative ε_Nd( T ) values of -3.7. They might have resulted from the melting of lower crustal diorite-tonalite rocks with addition of juvenile mantle material to the magma generation area. U-Pb zircon dating of metarhyolites of the Maltsevka sequence corresponding to A- and I-type granites showed that they are close in age, 1872 ± 10 and 1874 ± 10 Ma, respectively, which agrees with the age estimated earlier for granitoids of the Sayan complex of the Biryusa block. The similar ages and structural positions, along with the localization within the same structure, made it possible to unite volcanic rocks of the Elash Group and granitoids of the Sayan complex of the Biryusa block into a Paleoproterozoic volcanoplutonic association. The rocks of the association form the Sayan-Biryusa volcanoplutonic belt stretching for about 300 km along the zone of junction of the Biryusa block of the Angara fold belt and the Archean Tunguska superterrane of the Siberian craton. The belt is part of the large Paleoproterozoic South Siberian postcollisional magmatic belt formed at the final formation stage of the Siberian craton, when it was possibly part of the Paleoproterozoic Columbia supercontinent.