Based on a comparative study of geochemistry of metavolcanics and metasediments of two large terranes, Baikal-Muya and Khamar-Daban-Ol’khon, as well as of the Baikal-Patom passive margin and Olokit accretionary wedge, we have recognized volcanosedimentary series accumulated in the settings of island arcs of different maturities and fragments of volcanosedimentary complexes of back-arc and fore-arc basins. Metabasalts of the Medvezhy and Tyya Formations in the basement of passive-margin sequence and the Olokit Group are similar in geochemistry to plateau basalts and mark the beginning of rifting on the platform periphery. The abundance of metavolcanics and turbidites in the Olokit Group permits this structure to be considered an accretionary wedge of the Baikal-Muya island arc. According to the metavolcanics composition, the Baikal-Muya terrane formed in the environment of oceanic ensimatic island arcs and back-arc and inter-arc basins with the minimum amounts of sediments and contains ophiolite slices. The geochemistry of metavolcanics and metasediments of the Ol’khon, Talanchan, and Slyudyanka complexes evidences their formation in the environment of ensialic back-arc sediment-rich basin (Slyudyanka, Ol’khon, and Svyatoi Nos series), mature island arc (Anga-Talanchan paleoarc, Anga and Talanchan Groups), and fore-arc basin (Khangarul’ Group). According to chemistry and evolution history, all these complexes must be assigned to the Khamar-Daban-Ol’khon terrane.

At the Center of Isotope Studies of the A.P. Karpinsky Russian Geological Research Institute, the structure and isotope composition of zircons from two granitoid complexes, the age of their sequential growth zones, and the hosted inclusions have been studied using a SHRIMP-II ion mass spectrometer. The zircons consist of deformed cores with crystalline melt inclusions and of shells: inner, with glassy, partly devitrified inclusions, and outer metamorphogene, with fluid inclusions. Judging from the zircon zoning, crystallization of melts of both complexes proceeded in several stages: (1) The generation of melts and the beginning of zircon core growth (505 and 493 Ma) were synchronous with the overthrusting in the Ol’khon region; (2) The rapid ascent of melts (the inner shell, 479 and 475 Ma) together with the host rocks was caused by upthrust faulting and shear dislocations; (3) The metamorphogene shell (456 Ma) reflects the second stage of metamorphism. At the same time, the Shara-Nur migmatite–granite complex corresponds in composition, structures, and textures to syncollisional K-granites, whereas the differentiated Khaidai gabbro-diorite–diorite–granodiorite–granite complex is close in geochemical features (similar to those of the Anga sequence metavolcanics) and the mantle (juvenile) source of substance to the recent island-arc magmatism. It is suggested that the Caledonian island-arc magmatism was close in time to the accretion of the sediments of back-arc basin (Ol’khon Group) to the continental margin, on the one hand, and to the island-arc block, on the other.

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Comparative study of geological and isotope-geochemical features of the Early Paleozoic granitoids of the Khamar-Daban Ridge and Olkhon Island located in the Baikal region has revealed their close age and composition. Besides, they were referred to as syncollisional S-type formations derived from gneiss-schistose substratum of metamorphic sequences. Granitoids of the Solzan massif in the Khamar-Daban Ridge, as well as the Sharanur complex on Olkhon Island, occur in the autochthonous and allochthonous facies. They primarily consist of migmatites, plagiogranites, gneiss granites, and K–Na-granites. The magmatic rocks of the Sharanur complex include subalkaline granosyenites and quartz syenites spatially proximal to K–Na-granites. In the north of the island we investigated alkaline syenites which might be related to the Budun massif of basic rocks. On Olkhon Island in the Tashkiney valley, the surveyors recognized the geochemical type of pegmatoid rare-metal granites bearing beryllium mineralization. As was found, they are distinguished from Be-muscovite and spodumene pegmatites of the Khamar-Daban by high Rb, Cs, Sn, Nb, Ta, and W but low Li concentrations, which is probably due to Li-enrichment in the protolith of the Kornilova Formation relative to the Olkhon sequence. This points to the inheritance of the protolith composition at all stages of syncollisional granite formation. The geochemical study has shown similarity of calc-alkaline and subalkaline granitoids of the Khamar-Daban Ridge and Olkhon Island and their affinity in age and average composition of the regional continental crust. In addition, it has revealed the evidence for the existence of the Olkhon–Khamar-Daban block occurring as a single terrane in the Baikal region.

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