A Rb-Sr mineral isochron has been obtained for the alkali granites from the Katun’ Massif (Northeastern Tuva). This isochron corresponds to an age of 456.6 ± 1.4 Ma and to the initial ratio of Sr isotopes 0.70424±13. The points obtained for the nepheline syenites of the same massif and for the albites of the Aryskan deposit also fall on this isochron. All these data, together with K-Ar datings for the alkali granites of the Sayan and Sangilen zones, indicate that taphrogenic intracontinental alkaline magmatism was widespread in southern Siberia as early as the Late Ordovician. At the end of the Silurian – the beginning of the Devonian and in the Middle Carboniferous, the alkalic magmatism in the region was of local occurrence. The traditional views of the one-cycle formation of the alkaline rocks and of their strictly Devonian or Permian-Triassic age contradict the literature data now available.

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