Geochemistry of Early Paleozoic granitoids of the Baikal region and their geodynamic setting exemplified by the Khamar-Daban Ridge and Olkhon Island

Abstract

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.

Granites and Pegmatites of the Collisional and Intra-Plate Magmatism of the Baikal Area: Age, Mineralogical and Geochemical Types and Genesis Peculiarities

“…The mechanism of the massive formation is partial melting at high temperatures of the amphibolite facies with a slight movement of partially homogenized melt. Granites of Khamar-Daban (Solzan massive), as well as the Sharanur complex (Priolkhonye) and plagiomigmatites and plagiogranites close to them by material composition are typical syncollisional formations, with their melt source being gneisses and slates of Khamar-Daban and Olkhon metamorphic strata [10]. As in the Olkhon the migmatites and leucogranites of Khamar-Daban demonstrate developed pegmatites, which are lenticular, schlierlike, or layered bodies of orthoclase-plagioclase biotite pegmatoid rocks with a hypidiomorphicgrained structure.…”

“…In the Olkhon region of Pribaikalye (the Olkhon island, the Tashkiney valley) among the migmatized gneisses of the Olkhon strata, a zonal branched vein with apophyses of coarsemedium-grained rare-metal pegmatoid granites was found for the first time [10]. The vein is of submeridional strike and is up to 280 m long, its thickness varying from the first meters to 20-30 m (Figure 4A).…”

“…[25][26][27][28][29][30] and albite (No. [4][5][6][7][8][9][10][11], which predominate in the endocontact zones of the veins, and in their central parts the role of microcline increases, where amazonite crystals up to 10-15 cm in size are found in nests with a large-block structure. Along with feldspar and quartz, these rare-metal pegmatites contain agglomerates of zinnwaldite flakes, sometimes in accretions with tourmaline, and garnet (up to 86 mol.% of spessartin minal), magnetite, less often -muscovite and fluorite [23].…”

“…This is proven by the inheritance of granitoid compositions from their host rocks, and dependence on the composition of the crustal protolith. Comparison of geochemical characteristics of early Paleozoic granitoid rocks in the southern frame of the Siberian craton with granitoids of modern and ancient collision zones (the Himalayas and Central Spain) confirmed similarity of their composition [9][10][11]. Along with pegmatoid leucogranites and pegmatites demonstrating genetic relation to the processes of formation and evolution of early Paleozoic magmatism, vein and isometric bodies of pegmatites with rare-metal mineralization were explored in the Olkhon region of Pribaikalye (Figure 1).…”