Rare-metal pegmatoid granites, markers of the beginning of the Hercynian within-plate stage in the Ol’khon region of the Baikal area

Abstract

On the background of Early Paleozoic precollisional, syncollisional, and late collisional igneous rocks prevailing in the region (Khaidai and Shara-Nur granitoids and Birkhin gabbroid complex), rare-metal pegmatoid granite bodies and pegmatites along the southern edge of the Ol’khon region are of particular interest. They have a Middle Paleozoic age (390-391 Ma), cut the Caledonides, and belong to different geochemical types. The Anga unit includes amazonite-containing Li-F-B pegmatites, which are also enriched in Ta, Nb, and W. In the Ol’khon Group, only one K-feldspathic body rich in Rb, Be, Nb, W, Sn, Sc, U, and Th, with large crystals of aquamarine, has been found. The compositions of granite-pegmatite bodies and accessory rare-metal minerals have been studied. The rare-metal granite-pegmatites probably form a peripheral zone of the Hercynian within-plate setting widespread in the eastern Baikal area and related to the influence of the Siberian hot spot.