GEOCHEMISTRY OF THE VOLCANIC ROCKS OF DAWANGZI ROCK FORMATION IN NORTHERN DAXINGANLING MOUNTAINS

The Dawangzi rock formation of Wolegen rock group in Xiongguan area, northern Daxinganling Mountains, is composed of mainly meta-clastic rocks with metarhyolite and meta-trachyandesite. The zircon U-Pb dating of the trachyandesite by ICP-MS yields the ²⁰⁶Pb/²³⁸U age of 443.8-783.8 Ma, with the zircon U-Pb concordia age of 463.7±2.1 Ma, and the weighted mean age of 463.6±6.4 Ma(n=10, MSWD=0.073). The tested zircons show developed oscillation rings, with the Th/U ratio generally ranging from 0.42 to 1.76, indicating a magmatic origin. This age represents the forming age of the trachyandesite, which is believed to be formed in Middle Ordovician, consistent with the formation time of regional Dawangzi rock formation (Cambrian-Early Silurian). The trachyandesite, belonging to alkali-calcic quasi-aluminous rocks, is characterized by unobvious Eu anomaly, depletion of HFSEs and enrichment of LILEs. The rocks have the features of crust-mantle mixed source, which was formed in the continental margin environment and related to the subduction of Paleo-Asian Ocean crust.