Abstract:Tadong group is a set of high greenschist facies - amphibolite facies metamorphic rock series, which is considered to be part of the Precambrian metamorphic basement of “Songnen – Zhangguangcailing block”. But the time of its formation has been uncertain. Using zircon U-Pb (LA-MC-ICP-MS)test method, the study was conducted on the chronology of the biotite monzogneiss of the Zhudundian formation in the Tadong rock group. In 105 tests, 104 effective measurement points were between 239 and 2690Ma. The age spectrum shows many age groups: 2690 ~ 2679Ma ( n = 2), 2509 ~ 2304Ma (n = 8), 2287 ~ 2173Ma ( n = 5), 1924 ~ 1836Ma (n = 4), 1428 ~ 1352Ma (n = 2), 1171 ~ 1017Ma (n = 3), 997 ~ 929Ma (n = 12), 922 ~ 822Ma (n = 40), 812 ~ 793Ma (n = 14), 745 ~ 738Ma ( n = 2), 279 ~ 239Ma (n = 8). Early Precambrian zircons accounted for ~ 18%, Grenville orogenic period zircons accounted for ~ 53%, Rodinia supercontinent cracking stage zircons accounted for ~ 14%. The age concordant of the zircons shows that the Precambrian zircons all lost Pb to varying degrees. Two age segments, 239-1356ma and 1806-2576ma, were respectively processed for data processing, and it was found that the incongruent offline intersection of the former was 476±84 Ma, while the incongruent offline intersection of the latter was 1711±210 Ma, which may reflect two important tectonic thermal events. Zircons of late Paleozoic age are characterized by magmatic zircons whose Th/U ratio is generally greater than 0.1. However, their complex CL image features, including cloud, flow, wedge and fir leaf-like rings, have regular shapes, indicating the result of modification for later thermal events. Based on previous research results and regional geological construction characteristics, it is suggested that the main body of the Tadong rock group was formed in the Neoproterozoic period of ~ 750Ma, which may be a response to the cracking of the Rodinia supercontinent and may have undergone structural transformation events of ~ 516Ma, ~ 476Ma and ~ 426Ma. It may be a structural complex rock at the end of the early Paleozoic period, which was influenced by strong thermal events in the later period of ~ 260Ma. The existence of a large number of early Precambrian detrital zircons supports the understanding of the existence of the ancient basement of the “ Songnen - Zhangguangcailing block ”.
