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Zircon U-Pb ages and Hf isotopes of granites in Longzhen area and their geological implications

ZHANG YanLong,GE WenChun,GAO Yan,CHEN JingSheng and ZHAO Lei College of Earth Sciences,Jilin University,Changchun 130061,China  
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-alkalineⅠ-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.
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