Electron microprobe dating of monazite in granite from Kanbao area, Hebei province.

The geochronological data documented in previous studies imply that metamorphosed monzogranites with garnet within unit Xi'agong located in Kangbo area could be displaced in Mesoproterozoic era. We analyzed monazite grains separated from the monzogranite samples with garnet using EPMA Th-U-total Ph dating method to determine the ages of these samples, and the ages are calculated from the analytical data using the methods offered by Suzuki and Adachi et al.. The new obtained monazite Th-U-total Ph ages yield four peak ages of 252Ma, 265Ma, 281Ma and 322Ma respectively, the results show that fine-grained monzogranites with garnet in unit Xi'agong were not formed in proterozoic era but in Permian, which is consistent with the late Permian collisional model between the North China and the Siberian plates. The fine-grained monzogranite samples with garnet in unit Xi'agong are characterized by their peraluminous nature, indicating the disappear of Mongolia ocean, these monzogranitoids were formed in multi-episodes of magma activities during collision and orogenic processes between the North China and the Siberian plates,which provides important chronological evidence for the closure of the Paleo-Asian Ocean.