FORMING AGE AND GEOCHEMISTRY OF THE VOLCANIC ROCKS FROMMANKETOUEBO FORMATION IN OROQEN QI, DAXINGANLING MOUNTAINS

The zircon U-Pb dating result indicates that the volcanic rocks from Manketouebo Formation in Oroqen area of Daxinganling Mountains were formed in the Late Jurassic (152.9±1.7 Ma) based on the analysis of chronology and geochemical characteristics of the rocks. The volcanic rocks are characterized by rich Si and alkali, poor Ca and Mg, low Mg^# and very low Cr, Co and Ni contents, with enrichment of LILEs (K, Rb and Ba) and LREE, and depletion of HFSEs (Nb, Ta and Ti) in trace elements. The volcanic rocks have the characteristics of crust source, with Rb/Sr value of 0.66 averagely, much higher than N-MORB(0.03), OIB(0.047) and E-MORB(0.033).The average TiZr value of 13.94 is consistent with the range of crust-derived magma (< 20), indicating that the primary magma of volcanic rocks originated from partial melting of crustal rocks. According to the rock geochemical characteristics and regional existing research data, it is considered that the formation of volcanic rocks in the study area is related to the southeastward subduction of Mongolian-Okhotsk Ocean.