Stages of the Early Proterozoic Lower Crustal Growth in the Central Asian Orogenic Belt with Reference to the Baidarik Terrane

The structure of the Central Asian Orogenic Belt (CAOB) is determined by a combination of Neoproterozoic and Paleozoic paleoceanic and island-arc complexes and blocks with Precambrian basement. The Dzabkhan terrane, one of the largest blocks in the central segment of the CAOB, was initially considered as a microcontinent with an Early Precambrian basement. It included the Baidarik, Tarbagatai, Songino and proper Dzabkhan terranes, which are attributed to the Early Precambrian “cratonic” terranes. At present, the Early Precambrian complexes were established only in the northwestern Baidarik terrane and the Ider complex of the Tarbagatai terrane. Their folded structure was formed at ~ 1855 Ma, which is typical of the basement of ancient Northern Eurasia craton. The Early Proterozoic charnockitoids of the Baidarik terrane with an age of 1854 ± 5 Ma contain granulite-facies xenoliths. Previously, these xenoliths were considered as belonging to the Bumbuger Complex, which hosts the charnockitoid massif. However, zircons from two-pyroxene granulite xenoliths yielded an age of 1850 ± 4 Ma, which almost coincided with the age of 1854 ± 5 Ma determined for host charnockites. These data showed that charnockitoids generated at the lower crustal depth brought up the fragments of the Early Proterozoic lower crust to the amphibolite-facies metamorphic zone. The obtained new age data on zircon (2413 ± 12 Ma) extracted from two-pyroxene quartz diorite (enderbite) xenolith in these charnockitoids provided new insight into Early Proterozoic granulite facies metamorphism, which reflects the stages of the lower crustal growth in the geological evolution of the Early Precambrian complexes of the Baidarik terrane.