Magmatic and Xenogenic Zircons from Granitoids of the Kaa-Khem Batholith as Age Markers of the Crust in the Junction Zone of the Tannu-Ola Island Arc and the Tuva–Mongolian Microcontinent (Eastern Tuva)

––We present the results of study of the geologic structure, petrologic composition, and age of plagiogranitoid associations in the east of the Kaa-Khem batholith (Eastern Tuva). The batholith is located in the junction zone of the Tannu-Ola island arc (TIA) of Vendian–early Cambrian age and the Precambrian rocks of the Tuva–Mongolian microcontinent (TMM). Plagiogranitoids of this region formed in an accretion–collision setting in the period 490–450 Ma. Three stages of formation of plagiogranitoid associations have been established (~489, ~476, and ~450 Ma). The plagiogranitoid associations in the east of the Kaa-Khem batholith do not differ in petrologic composition and age from the plagiogranitoid associations (500–450 Ma) in the west. Xenogenic zircons in the studied plagiogranitoids of the eastern Kaa-Khem batholith have ages of 2335–517 Ma. Their ages are divided into several clusters (524–517, 549–536, 615–586, 684–647, 739–735, 810–794, 842–827, and 2335 Ma) reflecting the heterogeneity of the crust in the TIA–TMM junction zone. The wide range of ages and the abundance of xenogenic zircons in plagiogranitoids of the eastern Kaa-Khem batholith testify to the contribution of older crustal sources to the granite formation at all stages of accretion–collision processes (from 490 to 450 Ma). The much narrower age range of xenogenic zircon (616–474 Ma) and its low contents in coeval plagiogranitoids of the western Kaa-Khem batholith are consistent with their localization within the TIA and the relative homogeneity of the crust in the area of felsic-melt generation. In the west, the influence of older crustal sources was insignificant and manifested itself only at the final stage (~450 Ma) of accretion–collision processes.