Geochronology and geochemistry of the Fangcheng Neoproterozoic alkali-syenites in East Qinling orogen and its geodynamic implications

作     者：BAO ZhiWei WANG Qiang BAI GuoDian ZHAO ZhenHua SONG YaoWu LIU XiaoMing 

作者机构：Key Laboratory for Metallogenic Dynamics Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou 510640 China Key Laboratory of Isotope Geochronology and Geochemistry Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou 510640 China Henan Institute of Geological Survey Zhengzhou 450007 China Key Laboratory of Continental Dynamics Department of Geology Northwest University Xi'an 710069 China 

出 版 物：《Chinese Science Bulletin》 (CHINESE SCIENCE BULLETIN)

年 卷 期：2008年第53卷第13期

页      面：2050-2061页

核心收录：

学科分类：070902[理学-地球化学] 0709[理学-地质学] 07[理学] 

基　　金：the National Natural Science Foundation of China (Grant Nos.40472032,40721063 and 40672070) the National Basic Research Program of China (Grant No.2006CB403504) the Innovative Project of Chinese Academy of Sciences (Grant No.KZCX2-YW-128) 

主　　题：黑花岗石 地球化学 锆石 测定年代 

摘      要：Finding of Neoproterozoic syenites at Fangcheng in the northern Qinling region of the East Qinling orogen provides an important constraint on timing of tectonic transformation to extensional regime. The alkaline pluton consists mainly of nepheline syenite,aegirine syenite,and alkali-feldspar syenite. The syenites are of intermediate (SiO2=54%―62%),rich in alkali (K2O+Na2O=12%―15%),aluminum (Al2O3=16.81%―23.26%) and large ion lithophile elements (LILE),without any obvious Nb,Ta,Zr,and Hf anoma-lies. The Fangcheng syenites are geochemically characterized by relative enrichment of LREE,minor differentiation of HREE,significant negative Eu anomalies (Eu=0.13―0.23),less negative εNd(t) values of -1.37 to -3.90,young Nd model ages of 1364 to 1569 Ma,and high zircon saturation temperatures of 915 to 1044℃. The syenitic magmas probably originated from small proportion melting of upper mantle in an extensional regime of intraplate-anorogenic tectonic setting,and have been slightly contaminated by crustal materials during ascending and/or emplacement. LA-ICP-MS zircon U-Pb dating yields ages of 844.3±1.6 Ma (MSWD=0.86),suggesting that the Fangcheng alkaline syenites formed in the early Neo-proterozoic. They are the oldest Neoproterozoic alkaline rocks ever recognized in the Qinling orogen as well as in South China. This implies that the tectonic regime of the Qinling region would have transformed from post-collisional stretch to intraplate-anorogenic extension no later than 844 Ma.