U-Pb geochronology, geochemistry and tectonic implications ofdiorite from Nangnimsan of Mehe in northern Da Hinggan Mountains

LA-ICP-MS zircon U-Pb dating indicates that the weighted average age of diorite from Nangnimsan,Mohe in the northern Da Hinggan Mountains in Heilongjiang Province is 516.2± 1.7 Ma， as a result of magmatic intrusion of Early Cambrian. Ceochemistry analysis indicates that the rocks belong to high-K calc-alkaline series,with low content of Si02( 52.24%-53.64% ) and high Mg ( MgO = 5.67%-6.56% , Mg# = 61.05-65.46).The rocks are characterized by the enrichment of LREE and LILE (e.g. Rb, Ba, Pb) , and relative depletion in HREE and HFSE (e.g. Ta, Nb, P , Ti) , and with a high content of Sr (442.3-622.3)×10-6, Yb (2.49-3.42)×10-6 and Y (25.64-35.49) ×10-6,and slightly negative Eu anormaly (δEu = 0.60-0.81) . Based on Rb/Sr (0.15-0.25) and Rb/Nb (8.27-15.12) , it is concluded that the primary magma of diorite was derived from the mixture of the crust and mantle. Combined with background of regional tectonic evolution, it is suggested that diorite was formed in the extensional environment after the collage of the ECM with the Siberian.