CLOSURE TIME OF NENJIANG OCEAN: Constrains from Zircon Geochronology and Geochemistry of the Early Permian Dawusu Monzogranite

Nenjiang Ocean is an important branch of the eastern section of Paleo-Asian Ocean, and its closure time is of great significance to define the tectonic evolution of the eastern section of Central Asian orogenic belt. The paper studies the petrology, chronology and geochemistry of the Permian rock mass in Dawusu area, northern Daxinganling Mountains, and discusses its tectonic background, which provides new evidence for the constrains on the closure time of Nenjiang Ocean and tectonic evolution in northern Daxinganling Mountains. The zircon U-Pb geochronology results show that the weighted mean age of monzogranite is 292±4 Ma, corresponding to Early Permian. The monzogranite belongs to weak peraluminous Ⅰ-type granite, with the REE distributions in right-inclined pattern, enriched LILEs (Rb, Ba and K) and depleted HFSEs(Nb, P and Ti), showing the characteristics of post-orogenic granites, which may be formed in extensional environment. Geochemically, the rock shows moderately negative Eu anomaly(δEu=0.46-0.47), with the Th/U ratio of 5.61-5.94, Nb/Ta ratio of 7.98-8.14, Ti/Zr ratio of 6.11-6.36, and low Mg^# value(22-23), indicating that it was formed in the partial melting of lower crust, with a high degree of crystallization differentiation occurred during the ascent. Based on the published regional chronological data, the Carboniferous-Permian magmatism can be divided into three stages: 360-350 Ma, 345-292 Ma and 289-260 Ma. By comparing the spatiotemporal distribution, lithological assemblage and geochemical characteristics of magmatic rocks of each stage, it is concluded that the magmatic rocks of the three stages show a continuous change of subduction-postcollision-postorogenic extension, which records the closure process of Nenjiang Ocean. It is believed that Dawusu monzogranite is the product of the second-stage magmatism, formed in the extensional environment after the collision between Erguna-Xing'an block and Songnen block, which defines that Nenjiang Ocean should be closed at the end of Early Carboniferous.