Zircon U-Pb ages and Hf isotopes of granites in Longzhen area and their geological implications
龙镇地区花岗岩锆石U-Pb年龄和Hf同位素及地质意义

Longzhen granites, which locate in the boundary area of Great Xing'an Range and Lesser Xing'an Range Mts,belong to the A-I granite belt between the Xing'an and Songnen blocks. The rock assemblages of this area are generally similar, mainly comprising granodiorite and monzonite granite, with geochemical characteristics of high-K calc-alkaline I-type granite series. The U-Pb ages obtained by zircon LA-ICP-MS dating indicate that the Zhengdashan, Molabushan and Chaoyanglinchang plutons were emplaced at 320~315Ma,169±3Ma and 187~171Ma, respectively. The emplacement of these granites was related to the block-amalgamation during the closure of the Paleo-Asian Ocean in Paleozoic and the tectonic evolution of Paleo-Pacific in Mesozoic. Zircon Hf isotopic features indicate that the source rocks of the two episodes of granites are igneous rocks which originated from depleted mantle of Neoproterozoic-Phanerozoic time. Combined with previously published data of Sr-Nd-Hf isotopes, it is concluded that the time of crustal growth of Xing'an Block was mainly in the Neo-Proterozoic and Phanerozoic, which is consistent with that of the Songnen Block. However, it is younger than that of Ergunna Block (Middle- to Neo-Proterozoic),that is, these blocks have different evolution histories.